PLASMA FOR REMEDIATION OF VOLATILE ORGANIC COMPOUNDS

NON-THERMAL ATMOSPHERIC PRESSUREPLASMA FOR REMEDIATION OF VOLATILE

ORGANIC COMPOUNDS

A thesis submitted to The University of Manchester for the degree of

Doctor of Philosophyin the Faculty of Engineering and Physical Sciences

2012

ZAENAB ABD ALLAHTHE SCHOOL OF CHEMICAL ENGINEERING AND ANALYTICAL

SCIENCE

Contents

1 Introduction to the environmental problem and technological plasmas 28

1.1 The environmental problem of air pollution. . . . . . . . . . . . . . . . . 28

1.1.1 Global warming and greenhouse gases. . . . . . . . . . . . . . . 29

1.1.2 Acid rain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

1.1.3 Ground level ozone . . . . . . . . . . . . . . . . . . . . . . . . . 30

1.2 Volatile organic compounds (VOCs) . . . . . . . . . . . . . . . . . . . . 30

1.3 Removal of VOCs from gas streams. . . . . . . . . . . . . . . . . . . . . 31

1.4 Principles of technological plasmas . . . . . . . . . . . . . . . . . . . . . 33

1.5 Non-thermal plasma generation, properties and application . . . . . . . . 34

1.6 Types of plasma discharges . . . . . . . . . . . . . . . . . . . . . . . . . 36

1.7 Direct current (DC) discharge . . . . . . . . . . . . . . . . . . . . . . . 36

1.7.1 Corona discharge plasma reactors . . . . . . . . . . . . . . . . . 37

1.7.2 Radio frequency (RF) and microwave discharges . . . . . . . . . 38

1.7.3 Dielectric barrier discharge (DBD) . . . . . . . . . . . . . . . . . 38

1.7.4 Surface discharge plasma reactor (SD) . . . . . . . . . . . . . . . 40

1.8 Packed-bed plasma reactor characteristics and advantages for air pollutantremediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

1.8.1 Pellet materials and dielectric constant . . . . . . . . . . . . . . . 42

1.8.2 Pellet size and shape . . . . . . . . . . . . . . . . . . . . . . . . 43

1.8.3 Advantages and applications of packed-bed plasma reactors . . . 43

2

1.9 Literature review for dichloromethane and methyl chloride decompositionusing plasma. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

1.10 Aims and objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

1.11 Structure of the thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

2 Methodology 47

2.1 Experimental arrangements . . . . . . . . . . . . . . . . . . . . . . . . . 47

2.2 Packed-bed plasma reactors . . . . . . . . . . . . . . . . . . . . . . . . . 48

2.2.1 Single stage packed-bed plasma reactor . . . . . . . . . . . . . . 48

2.2.2 Multiple packed-bed plasma reactor . . . . . . . . . . . . . . . . 50

2.3 Spectroscopic diagnostic techniques using an FTIR spectrometer . . . . . 51

2.3.1 Principles of IR spectroscopy . . . . . . . . . . . . . . . . . . . 52

2.3.2 FTIR spectrometer components . . . . . . . . . . . . . . . . . . 53

2.3.3 The advantages of FTIR spectroscopy . . . . . . . . . . . . . . . 58

2.4 Sampling, spectral analyses and concentration calculations . . . . . . . . 58

2.4.0.1 Signal to noise ratio, spectral resolution and limit ofdetection for the FTIR spectrometer . . . . . . . . . . . 62

2.4.1 Optical emission spectroscopy . . . . . . . . . . . . . . . . . . . 65

2.5 Gas delivery system and the calculations of the gas flow rates . . . . . . . 66

2.6 Error calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

2.6.1 The error in concentrations calculation. . . . . . . . . . . . . . . 68

2.6.2 Error caused by DCM temperature change . . . . . . . . . . . . . 69

2.7 Electrical measurements and calculations . . . . . . . . . . . . . . . . . 70

2.8 Time required to reach a steady state plasma in nitrogen and argon gasstreams generated in a packed-bed plasma reactor . . . . . . . . . . . . . 72

3

2.8.1 Dichloromethane decomposition in nitrogen and argon plasmaover time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

2.8.2 Nitrogen and argon plasma emissions over time in a packed-bedplasma reactor. . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

2.8.3 Consumed power and plasma temperature over time . . . . . . . 74

2.8.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

3 Remediation of dichloromethane, CH2Cl2 using non-thermal plasma gener-ated in a packed-bed plasma reactor 77

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

3.1.1 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . 78

3.2 Influence of oxygen concentration on the removal efficiency of DCM. . . 79

3.2.1 The effect of oxygen concentration on the formation of plasmaend products. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

3.3 Influence of initial dichloromethane concentrations on the removal efficiency. 85

3.3.1 The effect of initial dichloromethane concentration on the forma-tion of plasma end products. . . . . . . . . . . . . . . . . . . . . 85

3.4 Influence of energy density on the removal efficiency of DCM. . . . . . . 87

3.4.1 Influence of energy density on the formation of plasma end productfrom dichloroethane decomposition. . . . . . . . . . . . . . . . . 89

3.5 Influence of the plasma residence time on the removal efficiency of DCM. 90

3.6 Influence of background gas on the removal efficiency of DCM . . . . . . 91

3.6.1 Argon plasma influence on the removal efficiency of dichloromethane 92

3.7 Reaction pathway for the decomposition of dichloromethane in non-thermal plasma reactor. . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

3.8 Initial kinetic simulation work . . . . . . . . . . . . . . . . . . . . . . . 97

3.9 Comparison between the results obtained in this investigation for oxygenconcentration influence and other work. . . . . . . . . . . . . . . . . . . 100

3.10 Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 102

4

4 Remediation of Methyl Chloride, CH3Cl using non-thermal plasma gener-ated in a packed-bed plasma reactor 104

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

4.2 Influence of oxygen concentration and initial methyl chloride concentrationon the removal efficiency of methyl chloride . . . . . . . . . . . . . . . . 105

4.2.1 Influence of oxygen and initial methyl chloride concentration onplasma by-product formation. . . . . . . . . . . . . . . . . . . . 108

4.3 Influence of energy density on the removal efficiency . . . . . . . . . . . 111

4.3.1 Energy density effect on plasma by-product formation . . . . . . 112

4.4 Influence of plasma residence time on the removal efficiency . . . . . . . 117

4.5 Reaction pathway for the decomposition of methyl chloride in non-thermalplasma reactor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

4.6 Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 120

5 The effect of adding alkene on the destruction of DCM in non-thermal plasmagenerated in a packed bed reactor 122

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

5.2 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

5.3 The influence of adding varying amounts of propylene to a gas streamcontaining 500 ppm of dichloromethane on the removal efficiency ofdichloromethane in non-thermal plasma. . . . . . . . . . . . . . . . . . . 124

5.3.1 The effect of added propylene concentration on the formation ofplasma end products. . . . . . . . . . . . . . . . . . . . . . . . . 127

5.3.2 Reaction pathway for the decomposition of propylene in non-thermal plasma reactor. . . . . . . . . . . . . . . . . . . . . . . . 129

5.4 Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 131

5

6 Multiple packed-bed plasma reactor 133

6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

6.1.1 Experimental set up . . . . . . . . . . . . . . . . . . . . . . . . . 134

6.2 Multiple packed-bed plasma reactor for dichloromethane remediation . . 136

6.2.1 The energy efficiency parameter β for the multiple packed-bedplasma reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

6.2.2 The formation of plasma end products as a result of dichlorometh-ane decomposition in a multiple packed-bed plasma reactor . . . 141

6.3 The effect of adding propylene on the removal efficiency of dichlorometh-ane in a multiple packed-bed plasma reactor. . . . . . . . . . . . . . . . . 144

6.3.1 The decomposition of dichloromethane with the addition of 500ppm propylene using a multiple packed-bed plasma reactor. . . . 144

6.3.2 The decomposition of dichloromethane with the addition of 1000ppm propylene using a multiple packed-bed plasma reactor. . . . 148

6.3.3 The energy efficiency parameter β for dichloromethane decom-position in a multiple packed-bed plasma reactor with the additionof 1000 ppm propylene to the gas stream. . . . . . . . . . . . . . 151

6.4 In situ IR absorption measurements for the decomposition of dichloro-methane using a multiple packed-bed plasma reactor. . . . . . . . . . . . 152

6.4.1 Comparison of dichloromethane measured in situ and in-line usinga multiple packed-bed plasma reactor. . . . . . . . . . . . . . . . 156

6.4.2 The detection of new species with in situ measurements using amultiple packed-bed plasma reactor. . . . . . . . . . . . . . . . . 157


6

7 Behaviour of nitrogen oxides in non-thermal plasma packed-bed reactor 160

7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160

7.2 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160

7.3 Formation of nitrogen oxides in non-thermal plasma with nitrogen andoxygen gas mixture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

7.4 The effect of adding chlorinated hydrocarbons on the formation of nitrogenoxides in non-thermal plasma generated in a single stage packed-bedplasma reactor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162

7.5 The effect of adding propylene alone compared with a mixture of dichloro-methane and propylene on the formation of nitrogen oxides in non-thermalplasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167

7.6 The influence of the number of plasma cells on the formation of nitrogenoxides as plasma by product during the decomposition of dichloromethanein a nitrogen-oxygen plasma. . . . . . . . . . . . . . . . . . . . . . . . . 171

7.7 A comparison of NOx behaviour in all the studied conditions in air plasma. 173

7.8 Reaction pathway for the formation of nitrogen oxides in non-thermalplasma reactor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175


8 Summary, conclusions and further work 177

8.1 Further work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

9 Appendix 202

The total word count of this thesis is 58460 words.

7

List of Figures

1.1 A sketch of the Earths annual energy balance illustrating the incoming

radiation from the sun and the radiated energy from earth surface. All

the numbers are in W m−2, the width of arrows is proportional to their

importance. This sketch is taken from Kiehl et al. (1). . . . . . . . . . . . 29

1.2 Examples of naturally generated plasma, sun, aurora and lightning. Pic-

tures are taken from (en.wikipedia.org) on 26-11-2011 . . . . . . . . . . 34

1.3 Several DC discharges as a function of the properties of the applied voltage

and discharge current. Taken from (2) and (3) . . . . . . . . . . . . . . . 37

1.4 Two examples of pulsed corona plasma reactors. (a) is a wire and pipe

reactor and (b) is a wire and plate reactor . . . . . . . . . . . . . . . . . 38

1.5 Discharge development in a DBD reactor. (a, b and c) shows the discharge

development, while (d and e) show the effect of increasing the dielectric

conductivity and decreasing the gap. Taken from (4) . . . . . . . . . . . 39

1.6 Some examples for DBD plasma reactors . . . . . . . . . . . . . . . . . 40

1.7 An example of surface discharge plasma reactor . . . . . . . . . . . . . . 41

1.8 Examples of packed-bed plasma reactors . . . . . . . . . . . . . . . . . . 41

2.1 A schematic diagram of the experimental inlet system which has been used

for all the investigations carried out in this study. . . . . . . . . . . . . . 48

2.2 A velocity profile distribution in a funnel . . . . . . . . . . . . . . . . . . 49

2.3 A photo and a sketch for the single stage plasma reactor . . . . . . . . . 50

2.4 Photograph and sketch of the modified single stage packed-bed plasma

reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

2.5 Photograph and sketch of the multiple packed-bed plasma reactor. Three

similar neon sign power supplies with a frequency of 20 kHz were used. . 52

8

2.6 Dichloromethane fundamental modes of vibration. Adapted from Shiman-

ouchi, 1972 (5). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

2.7 An example FTIR spectrum for about 500 ppm of dichloromethane in

nitrogen showing the vibrational modes. A multiple pass optical gas cell

with a 5.3 m pathlength was used. . . . . . . . . . . . . . . . . . . . . . 55

2.8 A schematic diagram for the layout of a FTIR spectrometer . . . . . . . . 55

2.9 An example of FTIR interferogram . . . . . . . . . . . . . . . . . . . . . 56

2.10 An example of the steps taken via the FTIR spectrometer to obtain an

absorbance spectrum for a gas stream of 500 ppm DCM, 3 % oxygen

and nitrogen. A multiple pass optical gas cell with 5.3 m pathlength and

spectral resolution of 1 cm−1 were used. . . . . . . . . . . . . . . . . . . 57

2.11 Specac multiple pass gas cell and a sketch demonstrating the multiple

reflections to obtain a 5.3 m pathlength . . . . . . . . . . . . . . . . . . 59

2.12 An example of areas selection when calculating the concentration of (A)

DCM and (B) CO. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

2.13 The integrated area between 787 and 647 cm−1 for (a) an experimental

spectrum of DCM at unknown concentration measured with 5.3 meter

optical pathlength and a resolution of 2 cm−1. (b) standard spectrum for

1 ppm dichloromethane measured with 1 meter optical pathlength and a

resolution of 0.1 cm−1. . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

2.14 Examples of the different options for setting the spectral baseline for

integrating the area beneath each peak. . . . . . . . . . . . . . . . . . . 61

2.15 An example FTIR spectrum for about 1000 ppm CO with a resolution of 1

cm−1. A multiple pass optical gas cell with a 5.3 m pathlength was used. . 62

2.16 An example of the spectrum used to calculate the limit of detection. (a)

shows the background noise with a resolution of 1 cm−1, while (b) shows

the HCN spectrum for 1 ppm with a resolution of 0.1 cm−1. Both spectra

are for 5.3 m optical pathlength. . . . . . . . . . . . . . . . . . . . . . . 64

2.17 An example HCN limit of detection calculation. The intercept point

between HCN peaks heights and the minimum accepted signal to noise

ratio. HCN limit of detection is about 15 ppm. . . . . . . . . . . . . . . . 64

9

2.18 Nitrogen plasma emission . . . . . . . . . . . . . . . . . . . . . . . . . . 66

2.19 Simplified sketch of the gas delivery system . . . . . . . . . . . . . . . . 66

2.20 Standard and experimental spectra for 500 ppm CO. The standard spec-

trum with a resolution of 0.1 cm−1 is in blue and the experimental spectrum

with a resolution of 2 cm−1 is in red. The black spectrum presents the

subtraction of the two spectra. . . . . . . . . . . . . . . . . . . . . . . . 69

2.21 Voltage and current waveforms measured using a pico scope. . . . . . . . 70

2.22 Power wave form over one pulse. . . . . . . . . . . . . . . . . . . . . . . 71

2.23 The removal efficiency of 500 ppm of dichloromethane in nitrogen and

argon plasma over thirty minutes starting from the moment of initiating

plasma. Gas streams with 0 and 1 % oxygen in a total flow rate of 1 L

min−1 were used. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

2.24 UV visible emission from argon plasma after about fifteen minute of initi-

ating plasma in a packed-bed plasma reactor . . . . . . . . . . . . . . . 74

2.25 The intensity of excited nitrogen molecules and argon atoms in packed-bed

plasma is measured over thirty minutes. Total flow rate of 1 L min−1 and

an energy density of about 1000 J L−1 were used. . . . . . . . . . . . . . 75

2.26 Deposited energy consumption and the temperature of the reactor body

for nitrogen plasma as a function of time. Measurements were taken every

minute from the moment of initiating plasma and up to 30 minutes . . . . 75

3.1 Plasma inlet system which had been used to investigate the influence of a

variety of parameters on the remediation of dichloromethane. . . . . . . . 79

3.2 FTIR spectra for 500 ppm dichloromethane after plasma reactor (a) ni-

trogen plasma without adding oxygen (b) nitrogen plasma with adding

3 % oxygen to the gas stream. A multiple pass optical gas cell with a

5.3 m pathlength, a total flow rate of 1 L min−1 and an energy density

of about 1000 J L−1 were used. Measurements were taken about 0.75

seconds downstream of the plasma reactor. . . . . . . . . . . . . . . . . 80

10

3.3 The removal efficiency of 500 ppm of dichloromethane in nitrogen non-

thermal plasma as a function of oxygen concentrations in a packed-bed

plasma reactor. A total flow rate of 1 L min−1 and an energy density of

about 1000 J L−1 were used. . . . . . . . . . . . . . . . . . . . . . . . . 81

3.4 Carbon and chlorine balance for the decomposition of 500 ppm of dichloro-

methane as a function of oxygen concentration in nitrogen non-thermal

plasma. A total flow rate of 1 L min−1 and an energy density of about 1000

J L−1 were used. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

3.5 The influence of oxygen concentration on the formation of CO, CO2 and

NOCl as plasma end products for DCM decomposition in nitrogen non-

thermal plasma. A total flow rate of 1 L min−1 and an energy density of


3.6 The removal efficiency of dichloromethane as a function of initial dichloro-

methane concentration and two oxygen concentrations of 0 and 3 % in a

packed-bed plasma reactor. A total flow rate of 1 L min−1 and an energy

density of about 1000 J L−1 were used. . . . . . . . . . . . . . . . . . . . 86

3.7 The influence of initial dichloromethane concentration on the formation

of HCN and HCl in nitrogen non-thermal plasma. A total flow rate of 1 L

min−1 and an energy density of about 1000 J L−1 were used. . . . . . . . 87

3.8 The influence of initial dichloromethane concentration on the formation of

CO, CO2 and NOCl in nitrogen non-thermal plasma with the addition of 3

% oxygen. A total flow rate of 1 L min−1 and an energy density of about

1000 J L−1 were used. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88


thermal plasma as a function of energy density and two oxygen concentra-

tions of 0 and 3% in a packed-bed plasma reactor. A total flow rate of 1 L

min−1 was used. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

3.10 Energy density influence on the formation of plasma end products from

the decomposition of 500 ppm dichloromethane in a packed-bed plasma

reactor. (a) nitrogen plasma with 0 % oxygen and (b) nitrogen plasma with

3 % oxygen. A total flow rate of 1 L min−1 was used. . . . . . . . . . . . 90

11


thermal plasma as a function of residence time of the gas in plasma. Two

oxygen concentrations of 0 and 3% and a fixed energy density of about

1000 J L−1 were used. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

3.12 FTIR spectra for about 590 ppm of dichloromethane in argon plasma

without adding oxygen to the gas stream. A multiple pass optical gas cell

with a 5.3 m pathlength, a total flow rate of 1 L min−1 and an energy

density of about 1000 J L−1 were used. Measurements were taken about

0.75 seconds downstream of the plasma reactor. . . . . . . . . . . . . . . 92

3.13 FTIR spectra for about 590 ppm of dichloromethane in argon plasma with

adding 5 % oxygen to the gas stream. A multiple pass optical gas cell with

a 5.3 m pathlength, a total flow rate of 1 L min−1 and an energy density

of about 1000 J L−1 were used. Measurements were taken about 0.75

seconds downstream of the plasma reactor. . . . . . . . . . . . . . . . . 93

3.14 A comparison of the removal efficiency of dichloromethane in nitrogen and

argon plasma as a function of oxygen concentration. A total flow rate of 1

L min−1 and an energy density of about 1000 J L−1 were used. . . . . . . 94

3.15 A comparison of the simulation and experimental results for the removal

efficiency of dichloromethane in nitrogen plasma as a function of oxygen

concentration. A total flow rate of 1 L min−1 was used. . . . . . . . . . . 99

3.16 A comparison of the simulation and experimental results for the influence

of residence time on the removal efficiency of dichloromethane in nitrogen

and 3 % oxygen gas mixture. A total flow rate of 1 L min−1 was used. . . 99

3.17 The influence of oxygen concentration on the formation of CO and CO2 in

nitrogen plasma. A comparison of the simulation and experimental results. 100

3.18 A comparison of the simulation and experimental results for the influence

of residence time on the formation of CO and CO2 in nitrogen and 3 %

oxygen gas mixture. A total flow rate of 1 L min−1 was used. . . . . . . . 101

3.19 A comparison between Fitzsimmons et al. plasma reactor design and the

reactor used in this work. . . . . . . . . . . . . . . . . . . . . . . . . . . 102

12

4.1 FTIR spectra for methyl chloride after plasma reactor (a) nitrogen plasma

without adding oxygen (b) nitrogen plasma with adding 3 % oxygen to the

gas stream. A multiple pass optical gas cell with a 5.3 m pathlength, a

total flow rate of 1 L min−1 and an energy density of about 2280 J L−1

were used. Measurements were taken about 0.75 second downstream of

the plasma reactor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

4.2 The removal efficiency of 500, 600, 700 and 1000 ppm of methyl chloride

in nitrogen non-thermal plasma as a function of oxygen concentration in a

packed-bed plasma reactor. A total flow rate of 1 L min−1 and an energy


4.3 Carbon monoxide formation as a function of oxygen concentration and

the initial methyl chloride concentration in a nitrogen non-thermal plasma

generated in a packed-bed plasma reactor. A total flow rate of 1 L min−1

and an energy density of about 2280 J L−1 were used. . . . . . . . . . . . 109

4.4 Carbon dioxide formation as a function of oxygen concentration and the

initial methyl chloride concentration in a nitrogen non-thermal plasma

generated in a packed-bed plasma reactor. A total flow rate of 1 L min−1

and an energy density of about 2280 J L−1 were used. . . . . . . . . . . . 109

4.5 Carbon balance as a function of oxygen concentration and the initial

methyl chloride concentration in a nitrogen non-thermal plasma generated

in a packed-bed plasma reactor. A total flow rate of 1 L min−1 and an

energy density of about 2280 J L−1 were used. . . . . . . . . . . . . . . . 110

4.6 The removal efficiency of 500 and 1000 ppm of methyl chloride in nitrogen

non-thermal plasma as a function of energy density and two oxygen con-

centrations of 0 and 3% in a packed-bed plasma reactor. A total flow rate

of 1 L min−1 was used. . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

4.7 Carbon balance for the decomposition of 500 and 1000 ppm of methyl

chloride as a function of energy density in nitrogen non-thermal plasma. A

total flow rate of 1 L min−1 was used. . . . . . . . . . . . . . . . . . . . 113

4.8 Chlorine balance for the decomposition of 500 and 1000 ppm of methyl

chloride as a function of energy density in nitrogen non-thermal plasma. A


13

4.9 The influence of the removal efficiency of 500 and 1000 ppm of methyl

chloride on the formation of HCN and HCl in nitrogen non-thermal plasma.

A total flow rate of 1 L min−1 was used. . . . . . . . . . . . . . . . . . . 114

4.10 Hydrogen cyanide concentration as a function of energy density and two

methyl chloride initial concentrations of 500 and 1000 ppm in nitrogen

non-thermal plasma. A total flow rate of 1 L min−1 was used. . . . . . . . 115

4.11 Hydrogen chloride concentration as a function of energy density and two

methyl chloride concentrations of 500 and 1000 ppm in nitrogen non-

thermal plasma. A total flow rate of 1 L min−1 was used. . . . . . . . . . 115

4.12 Carbon monoxide concentration as a function of energy density and two


thermal plasma with the addition of 3 % oxygen. A total flow rate of 1 L

min−1 was used. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

4.13 Carbon dioxide concentration as a function of energy density and two


thermal plasma with the addition of 3 % oxygen. A total flow rate of 1 L

min−1 was used. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

4.14 The influence of the removal efficiency of 500 and 1000 ppm of methyl

chloride on the formation of CO and CO2 in nitrogen non-thermal plasma

with the addition of 3 % oxygen. A total flow rate of 1 L min−1 was used. 117

4.15 The removal efficiency of 1000 ppm of methyl chloride in nitrogen non-

thermal plasma as a function of residence time and oxygen concentration

in a packed-bed plasma reactor. . . . . . . . . . . . . . . . . . . . . . . 118

5.1 Ozone and alkene double bond reaction. . . . . . . . . . . . . . . . . . . 122

5.2 FTIR spectra for 1000 ppm propylene after plasma reactor (a) nitrogen

plasma without adding oxygen (b) nitrogen plasma with adding 5 % oxygen

to the gas stream. A multiple pass optical gas cell with a 5.3 m pathlength,

a total flow rate of 1 L min−1 and an energy density of about 1820 J L−1

were used. Measurements were taken about 0.75 seconds downstream of

the plasma reactor. Spectral resolution is 1 cm−1. . . . . . . . . . . . . . 124

14

5.3 Dichloromethane decomposition in non-thermal plasma with 300, 500 and

1000 ppm of propylene as a function of oxygen concentration. A total flow

rate of 1 L min−1 and an energy density of about 1820 J L−1 were used. . 125

5.4 Dichloromethane decomposition in non-thermal plasma with 0, 500 and

1000 ppm of propylene as a function of oxygen concentration. A total flow

rate of 1 L min−1 and an energy density of about 1820 J L−1 were used. . 126

5.5 Propylene decomposition in non-thermal plasma with and without the

presence of 500 ppm DCM as a function of oxygen concentration. A total

flow rate of 1 L min−1 and an energy density of about 1820 J L−1 were used.126

5.6 FTIR spectra for nitrogen plasma exhaust of 1000 ppm propylene and 500

ppm DCM (a) without adding oxygen (b) with adding 5 % oxygen to the

gas stream. A multiple pass optical gas cell with a 5.3 m pathlength, a


were used. Measurements were taken about 0.75 seconds downstream of

the plasma reactor. Spectral resolution is 1 cm−1. . . . . . . . . . . . . . 128

5.7 CO, CO2 and CH2O formation as a function of initial propylene con-

centration in air gas stream containing 500 ppm of dichloromethane.

Non-thermal plasma was generated in a packed-bed plasma reactor with

a total flow rate of 1 L min−1 and an energy density of about 1820 J L−1. 129

5.8 Comparison between FTIR spectra for 500 ppm propylene after the plasma

reactor with nitrogen gas stream and 100 ppm of methyl chloride. . . . . 131

6.1 A schematic diagram of the experimental inlet system which has been used

to investigate The remediation of DCM in a multiple packed-bed plasma

reactor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

6.2 Design and a photograph of the multiple packed bed plasma reactor with

three plasma cells powered by three identical neon sign power supplies . 136

6.3 In-line FTIR spectra for 500 ppm DCM and 1% oxygen in nitrogen plasma

as a function of cells number in a multiple packed-bed plasma reactor. A

multiple pass optical gas cell with a 5.3 m pathlength, a total flow rate of

1 L min−1 and a spectral resolution of 1 cm−1 were used. . . . . . . . . . 137

15

6.4 The decomposition of 500 ppm of dichloromethane in nitrogen non-thermal

plasma as a function of oxygen concentration and the number of plasma

cells of a multiple packed-bed plasma reactor. A total flow rate of 1 L

min−1 was used. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

6.5 Carbon monoxide formation as a function of oxygen concentration and

the number of plasma cells of a multiple packed-bed plasma reactor. A


6.6 Carbon dioxide formation as a function of oxygen concentration and the

number of plasma cells of a multiple packed-bed plasma reactor. A total

flow rate of 1 L min−1 was used. . . . . . . . . . . . . . . . . . . . . . . 143

6.7 Nitrosyl chloride formation as a function of oxygen concentration and the

number of plasma cells of a multiple packed-bed plasma reactor. A total

flow rate of 1 L min−1 was used. . . . . . . . . . . . . . . . . . . . . . . 143

6.8 FTIR spectra for a mixture of 500 ppm DCM and 500 ppm propylene with

the addition of 1% oxygen in nitrogen plasma as a function of cells number

in a multiple packed-bed plasma reactor. A multiple pass optical gas cell

with a 5.3 m pathlength, a spectral resolution of 1 cm−1 and a total flow

rate of 1 L min−1 were used. . . . . . . . . . . . . . . . . . . . . . . . . 145

6.9 The decomposition of 500 ppm DCM in nitrogen non-thermal plasma with

the addition of 500 ppm propylene as a function of oxygen concentration

and cells number in a multiple packed-bed plasma reactor. A total flow

rate of 1 L min−1 was used. . . . . . . . . . . . . . . . . . . . . . . . . . 146

6.10 A comparison of the decomposition of 500 ppm DCM in nitrogen non-

thermal plasma with and without the addition of 500 ppm propylene as

a function of oxygen concentration and the number of plasma cells in a

multiple packed-bed plasma reactor consisting of three plasma cells. A


6.11 The decomposition of 500 ppm propylene in nitrogen non-thermal plasma

as a function of oxygen concentration and cells number in a multiple

packed-bed plasma reactor. A total flow rate of 1 L min−1 was used. . . . 147

16

6.12 The decomposition of 500 ppm DCM in nitrogen non-thermal plasma with

the addition of 1000 ppm propylene as a function of oxygen concentra-

tion and plasma cells number in a multiple packed-bed plasma reactor

consisting of three plasma cells. A total flow rate of 1 L min−1 was used. . 149

6.13 The decomposition of 1000 ppm propylene in nitrogen non-thermal plasma

as a function of oxygen concentration and cells number in a multiple

packed-bed plasma reactor. A total flow rate of 1 L min−1 was used. . . . 149

6.14 A comparison of the decomposition of 500 ppm DCM in nitrogen non-

thermal plasma with and without the addition of 1000 ppm propylene as

a function of oxygen concentration and the number of plasma cells in a

multiple packed-bed plasma reactor consisting of three plasma cells. A


6.15 In situ measurements using a multiple packed-bed plasma reactor which

consists of three plasma cells. One power supply was used to power the

three cells at the same time. . . . . . . . . . . . . . . . . . . . . . . . . . 154

6.16 An in situ FTIR spectrum after the first plasma cell in a multiple packed-

bed plasma reactor which consists of three plasma cells. A pathlength of

10 cm, a flow rate of 1 L min−1 and an oxygen concentration of 5 % were

used. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

6.17 DCM removal efficiency as a function of plasma cells number and oxygen

concentrations measured immediately after each plasma cell in a multiple

packed-bed plasma reactor consisting of three plasma cells. A total flow


6.18 Comparison of the in situ and in line measurements of the DCM removal

efficiency using one plasma cell in a multiple packed-bed plasma reactor

as a function of oxygen concentration. An energy density of 1470 and 1494

J L−1 were used for the in situ and in line experiments respectively. A flow


6.19 The absorbance of the new peak as a function of oxygen concentration. In

situ measurements were taken after the first plasma cell. An energy density

of about 1470 J L−1 was applied. . . . . . . . . . . . . . . . . . . . . . . 158

17

7.1 FTIR spectrum for nitrogen oxides formation in nitrogen plasma with the

addition of 3 percent oxygen to the gas stream. A multiple pass optical gas

cell with a 5.3 m pathlength, a total flow rate of 1 L min−1 and an energy


7.2 Nitrogen oxides concentrations in nitrogen plasma generated in a packed-

bed plasma reactor as a function of oxygen concentrations. . . . . . . . . 162

7.3 FTIR spectra for plasma exhaust with the presence of DCM and CH3Cl in

nitrogen plasma. Spectrum (a) presents plasma exhaust for 500 ppm DCM

and 3% oxygen. Spectrum (b) presents plasma exhaust for 500 ppm CH3Cl

and 3% oxygen. A total flow rate of 1 L min−1 and an energy density of


7.4 Nitric oxide concentration as a function of oxygen concentrations and the

addition of 500 ppm DCM and CH3Cl respectively to the plasma inlet gas

stream. A total flow rate of 1 L min−1 and an energy density of about 1820

J L−1 were used. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

7.5 Nitrogen dioxide concentration as a function of oxygen concentrations and

the addition of 500 ppm DCM and CH3Cl respectively to the plasma inlet

gas stream. A total flow rate of 1 L min−1 and an energy density of about

1820 J L−1 were used. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

7.6 Nitrous oxide concentration as a function of oxygen concentrations and

the addition of 500 ppm DCM and CH3Cl respectively to the plasma inlet

gas stream. A total flow rate of 1 L min−1 and an energy density of about

1820 J L−1 were used. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

7.7 FTIR spectra for plasma exhaust with the presence of DCM and C3H6

in nitrogen plasma. Spectrum (a) presents plasma exhaust for 1000 ppm

C3H6 and 3% oxygen. Spectrum (b) presents plasma exhaust for a mixture

of 500 ppm DCM and 1000 ppm C3H6 with 3% oxygen. A total flow rate

of 1 L min−1 and an energy density of about 1820 J L−1 were used. . . . . 168

7.8 Nitric oxide concentration as a function of oxygen concentrations and the

addition of 1000 ppm propylene alone and a mixture of 500 ppm DCM

and 1000 ppm propylene. A total flow rate of 1 L min−1 and an energy


18

7.9 Nitrogen dioxide concentration as a function of oxygen concentration and

the addition of 1000 ppm propylene alone and a mixture of 500 ppm DCM

with 1000 ppm propylene. A total flow rate of 1 L min−1 and an energy


7.10 Nitrous oxide concentration as a function of oxygen concentration and the




7.11 Nitric oxide concentration in a multiple packed-bed plasma reactor as a

function of oxygen concentrations and the addition of 500 ppm dichloro-

methane. A total flow rate of 1 L min−1 was used. . . . . . . . . . . . . . 172

7.12 Nitrogen dioxide concentration in a multiple packed-bed plasma reactor

as a function of oxygen concentrations and the addition of 500 ppm di-

chloromethane. A total flow rate of 1 L min−1 was used. . . . . . . . . . . 172

7.13 Nitrous oxide concentration as a function of oxygen concentration and the




7.14 NOx concentration in air plasma. (a)air plasma no VOCs, (b) 500 ppm

DCM in air plasma, (c) 500 ppm CH3Cl in air plasma, (d) 1000 ppm C3H6

in air plasma and (e) mixture of 500 ppm DCM and 1000 ppm C3H6. A


were used. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

19

List of Tables

1.1 Examples of air pollutants, where: HFCs are hydrofluorocarbons, PFCsare perfluorocarbons, SF6 is sulphur hexafluoride and HAPs are hazardousair pollutants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

1.2 Health and environmental problems for several examples of air pollutants 29

1.3 Physical properties for methyl chloride and dichloromethane . . . . . . . 32

1.4 Primary processes in non-thermal plasma. . . . . . . . . . . . . . . . . . 35

2.1 The limit of detection in ppm for all the species in the gas stream for theexperiments carried out. . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

3.1 The decomposition efficiency of 590 ppm dichloromethane and the con-centrations of all species exist in the plasma exhaust, in ppm. as a functionof oxygen percentages in argon plasma . . . . . . . . . . . . . . . . . . . 94

3.2 A comparison between Fitzsimmons et al. results and the results obtainedwith the carried out investigations. . . . . . . . . . . . . . . . . . . . . . 102

6.1 DCM removal efficiency in air plasma using a multiple packed-bed plasmareactor consisting of three plasma cells. Energy density (E) and energyefficiency parameter (β ) are calculated. . . . . . . . . . . . . . . . . . . 140

6.2 The Energy efficiency parameter, β , for the decomposition of 520 ppmof dichloromethane using one, two and three plasma cells with differentoxygen concentrations. . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

6.3 DCM removal efficiency in air plasma (%) with the addition of 1000 ppmpropylene using a multiple packed-bed plasma reactor consisting of threeplasma cells. Energy density (E) and energy efficiency parameter (β ) arecalculated. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

20

6.4 The Energy efficiency parameter, β , for the decomposition of 510 ppmof dichloromethane with the addition of 1000 ppm propylene to the gasstream and using one, two and three plasma cells with different oxygenconcentrations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

6.5 The removal efficiency of 500 ppm DCM in air plasma using a multiplepacked-bed plasma reactor consisting of three plasma cells connected toone power supply. Energy density (E) and energy efficiency parameter (β )are calculated. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156

7.1 The concentrations of NO, NO2 and N2O in ppm with the presence of 500ppm DCM as a function of oxygen concentration in nitrogen plasma . . . 164

7.2 The concentrations of NO, NO2 and N2O in ppm with the presence of 500ppm methyl chloride as a function of oxygen concentration in nitrogenplasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

9.1 Integration limits for the detected species in the plasma exhaust of di-chloromethane decomposition in nitrogen-oxygen plasma. . . . . . . . . 205

9.2 The removal efficiency of about 500 ppm of dichloromethane as well asthe concentrations of all species in the plasma exhaust as a function ofoxygen concentration in nitrogen plasma generated using a packed-bedplasma reactor. Concentrations are in ppm . . . . . . . . . . . . . . . . . 206

21

Abstract

Non-thermal plasma generated in a dielectric barrier packed-bed reactor has been usedfor the remediation of chlorinated volatile organic compounds. Chlorinated VOCs areimportant air pollutant gases which affect both the environment and human health. Thisthesis uses non-thermal plasma generated in single and multiple packed-bed plasmareactors for the decomposition of dichloromethane (CH2Cl2, DCM) and methyl chloride(CH3Cl). The overall aim of this thesis is to optimize the removal efficiency of DCM andCH3Cl in air plasma by investigating the influence of key process parameters. This thesisstarts by investigating the influence of process parameters such as oxygen concentration,initial VOC concentration, energy density, and plasma residence time and background gason the removal efficiency of both DCM and CH3Cl. Results of these investigations showedmaximum removal efficiency with the addition of 2 to 4 % oxygen to nitrogen plasma.Oxygen concentrations in excess of 4 % decreased the decomposition of chlorinated VOCsas a result of ozone and NOx formation. This was improved by adding an alkene, propylene(C3H6), to the gas stream. With propylene additives, the maximum remediation of DCMwas achieved in air plasma. It is thought that adding propylene resulted in the generationof more active radicals that play an important role in the decomposition process of DCMas well as a further oxidation of NO to NO2. Results in the single bed also showed thatincreasing the residence time increased the removal efficiency of chlorinated VOCs inplasma. This was optimized by designing a multiple packed-bed reactor consisting ofthree packed-bed cells in series, giving a total residence time of 4.2 seconds in the plasmaregion of the reactor. This reactor was used for both the removal of DCM, and a mixtureof DCM and C3H6 in a nitrogen-oxygen gas mixture. A maximum removal efficiency ofabout 85 % for DCM was achieved in air plasma with the use of three plasma cells and theaddition of C3H6 to the gas stream. Nitrogen oxides are air pollutants which are formed asby-products during the decomposition of chlorinated VOCs in plasmas containing nitrogenand oxygen. Results illustrate that the addition of a mixture of DCM and C3H6 resulted inthe formation of the lowest concentration of nitric oxide, whilst the total nitrogen oxidesconcentrations did not increase. A summary of the findings of this work is presented inchapter eight as well as further work. To conclude, the maximum removal efficiency ofdichloromethane was achieved in air plasma with the addition of 1000 ppm of propyleneand the use of three packed-bed plasma cells in series. The lowest concentration of nitricoxide was formed in this situation.

Declaration

No portion of the work referred to in the thesis has been submitted in support of anapplication for another degree or qualification of this or any other university or otherinstitute of learning.

23

Copyright

1. The author of this thesis (including any appendices and/or schedules to this thesis)owns any copyright in it (the Copyright) and he has given The University ofManchester the right to use such Copyright for any administrative, promotional,educational and/or teaching purposes.

2. Copies of this thesis, either in full or in extracts, may be made only in accordancewith the regulations of the John Rylands University Library of Manchester. Detailsof these regulations may be obtained from the Librarian. This page must form partof any such copies made.

3. The ownership of any patents, designs, trade marks and any and all other intellectualproperty rights except for the Copyright (the Intellectual Property Rights) and anyreproductions of copyright works, for example graphs and tables (Reproductions),which may be described in this thesis, may not be owned by the author and may beowned by third parties. Such Intellectual Property Rights and Reproductions cannotand must not be made available for use without the prior written permission of theowner(s) of the relevant Intellectual Property Rights and/or Reproductions.

4. Further information on the conditions under which disclosure, publication andexploitation of this thesis, the Copyright and any Intellectual Property Rights and/orReproductions described in it may take place is available from the Head of Schoolof (insert name of school) (or the Vice-President) and the Dean of the Faculty ofLife Sciences, for Faculty of Life Sciences candidates.

24

Acknowledgements

Many thanks go to my supervisor Dr Philip Martin for all the support, help, encour-agement and understanding throughout my PhD. Thanks for his kindness and sociabilitywhich made me feel welcome in a new country. I was always able to turn up to his officewithout an appointment to discuss anything.

Thanks also to Professor J Christopher Whitehead for all the help, explanations anddiscussion with regards to CHEMKIN modeling. Thanks for Professor Hugh Coe for hisvaluable assistance with the ozone chemistry.

Thanks must also go to my dear friends David Sawtell and Andrew Campen for allthe help they provided in the experimental work, numerous work related discussions andreading through my thesis. Many thanks for David and Martin Hehn for the help withLaTex. It did really help a lot in easing the writing up of this thesis. Thanks for encouragingme and getting me started with LaTex. Thanks for Vasili Kasyutich, Robert Holdsworth,Adam Higginson, Ali Arafeh, Raja Ibrahim and Ben Embley for all the help they provided.Thanks for John Riley, Loris Doyle, Paul Rothwell, Roy Kershaw and Gary Burns formuch appreciated technical assistance. Thanks also to TDL Sensors Ltd for the loan ofequipment.

Thanks to the Capacity Building Project of the British Council and the Higher Institutefor Environmental Research, University of Tishreen, Ministry of Higher Education in Syriafor a studentship.

Finally, a huge thanks, love and gratitude to my family. My brothers, sisters and myprecious Mum, thank you very much for all your support and advice. To my late Dad,without him I would never have made it to this stage. I am forever thankful for his support,guidance and love.

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Nomenclature

AC Alternating currentAl2O3 AluminaBaTiO3 Barium titanate◦C Degree celsiusCH4 MethaneCH2O FormaldehydeCH2Cl2 Dichloromethane, methylene chlorideCH3Cl Chloromethane, methyl chlorideCCl4 Carbon tetrachlorideCO Carbon monoxideCO2 Carbon dioxideCOCl2 PhosgeneC2H4 EthyleneC3H6 Propylene, propeneC6H6 BenzeneC7H8 CycloheptatrieneDBD Dielectric barrier dischargeDC Direct currentDCM Dichloromethane, Methylene chlorideFTIR Fourier transforms infraredHAPs Hazardous air pollutantsHFCs HydrofluorocarbonsHz HertzHCl Hydrogen chlorideHCN Hydrogen cyanideH2O Water vapourHCOOH Formic acidHNO3 Nitric acidIR InfraredkV KilovoltkHz KilohertzK KelvinKBr Potassium bromidekJ kilojoulemm Millimeter

26

MCT Mercury cadmium tellurideMFC Mass flow controllerN(2P), N(2D), N(4S) Electronically excited states of atomic nitrogenN2 (A3 Σu

+) Electronically excited states of molecular nitrogenNOx Nitrogen oxidesNO Nitric oxideNOCl Nitrosyl chlorideN2O Nitrous oxideNH3 AmmoniaNO2 Nitrogen dioxideO3 OzoneO(1D), O(3P) Electronically excited states of atomic oxygenODS Ozone depleting substancesPFCs PerfluorocarbonsPM10 Particulate matter with a diameter of 10 micrometresPM2.5 Particulate matter with a diameter of 2.5 micrometresppm Parts per millionSOx Sulphur oxidesSF6 Sulphur hexafluorideUV Ultra-violetRF Radiofrequencyε Dielectric permittivityVOCs Volatile organic compoundsµm Micrometres SecondSNR Signal to noise ratioTLV Threshold level valueTe Electron temperatureTh Heavy particles temperature

27

Chapter 1

Introduction to the environmental problem andtechnological plasmas

1.1 The environmental problem of air pollution.

Air pollution is caused by the presence of harmful material in the atmosphere. Thedamaging effect of some materials for human health and property, vegetation and theenvironment, classifies them as air pollutants. These materials could be discharged to theatmosphere through natural sources such as plants and volcanic eruptions or by severalhuman made activities. As it is impossible to control the natural sources of air pollutants,the attention of researchers are directed toward effort to control human made pollutants.Man made actions which produce air pollutants are mainly related to industrial andtransport processes. These process are necessary to maintain the standard of living forhumans, so they cannot be stopped. Controlling air pollution emissions is mainly achievedby process modifications, exhaust treatments or a combination of both.

Some examples of air pollutants are categorized in table 1.1.

Table 1.1: Examples of air pollutants, where: HFCs are hydrofluorocarbons, PFCs areperfluorocarbons, SF6 is sulphur hexafluoride and HAPs are hazardous air pollutants.

Air Pollutants Examples

Acid gases SOx, NOx, HClGreen house gases COx, CH4, NxOy, O3, HFCs, PFCs, SF6

Volatile organic compounds (VOCs) C7H8, C6H6 and chlorinated hydrocarbons.Ozone depleting substances (ODS) CCl4, CFCs and HAPs

Toxic gases mercury, dioxinRadioactive gases isotopes of carbon, iodine, caesium, radonParticulate matter PM10 and PM2.5

Some examples of health and environmental problems for COx, NOx, O3 and VOCsare presented in table 1.2.

Greenhouse gases, acid rain and ground level ozone are major environmental problems.

28

Table 1.2: Health and environmental problems for several examples of air pollutants

Gases Health problems Environmental problems

COx Headaches, dizziness Greenhouse effectNOx Eye and throat irritations Acid rainO3 Headaches, coughing, eye and throat irritations Greenhouse effect and vegetation damage

VOCs Headaches, dizziness, carcinogenic Enhance greenhouse effectand ground level O3

Figure 1.1: A sketch of the Earths annual energy balance illustrating the incoming radiationfrom the sun and the radiated energy from earth surface. All the numbers are in W m−2,the width of arrows is proportional to their importance. This sketch is taken from Kiehl etal. (1).

1.1.1 Global warming and greenhouse gases.

The earth’s surface absorbs heat from solar radiation and radiates thermal emissionsto the atmosphere. The greenhouse effect is the process of greenhouse gases absorbingthe radiated energy from the earth, and reflecting it back to the earth’s surface as well asto space. This process is vital for keeping the surface of the earth warm and maintainingthe earth’s energy balance. However, increasing the concentrations of greenhouse gasesby human activities causes an increase in the levels of radiation retained in the earthatmosphere, leading to global warming (6). Figure 1.1 shows a sketch of the Earths annualenergy balance illustrating the incoming radiation from the sun and the radiated energyfrom earth surface.

The main gases which are affected by human activities are carbon dioxide (CO2),

29

methane (CH4), nitrous oxide (N2O), ozone, hydrofluorocarbons (HFCs), perfluorocarbons(PFCs), and sulphur hexafluoride (SF6). The main anthropogenic sources of these gasesare: industrial combustion, chemical and petrochemical production, metal production, roadtransport, energy production and landfilling of solid waste.

1.1.2 Acid rain

Acid rain is a term which describes the deposition of oxides on the earth’s surface.This process occurs by wet depositions through rain, snow, fog, or dry deposition of gasesand particles on the earth surface. Acids in the atmosphere come from natural sourcessuch as volcanic emissions, forest fires and decay of vegetation. The main anthropogenicacid sources are power stations which use fossil fuel, factories and vehicles which mainlyproduce sulphur oxides (SOx) and nitrogen oxides (NOx). These acids can be carried bywind for long distances. Excess concentrations of sulphuric and nitric acids are harmfulfor human health, soil, plants, surface water and aquatic animals (6).

1.1.3 Ground level ozone

The formation of ozone in the troposphere is a health hazard which cases severalproblems. Chest pain, coughing, airway irritation and scarring of lung tissue. Ground levelozone photochemical formation is promoted by the presence of VOCs and NOx on sunnydays. The emission of VOCs and NOx to the atmosphere by natural sources or humanmade sources such as industrial emissions and vehicles exhaust result in increasing theproduction of ozone at ground level. As well as health problems, ozone harms vegetationand ecosystems. Controlling the emissions of NOx and VOCs help in reducing ozoneformation (6).

This work concentrates on the remediation of VOCs from gas exhausts.

1.2 Volatile organic compounds (VOCs)

Volatile organic compounds (VOCs) are chemical compounds consisting mainly ofcarbon, hydrogen and other elements such as chlorine, fluorine and sulphur. VOCs wouldbe liquids or solids under specific temperatures and pressures, but due to their low boiling

30

point the vapour pressure of these substances is very high at room temperatures. Forexample, methyl chloride (CH3Cl) has a boiling point of -24.2 ◦C and a vapour pressure of490 kPa at 20 ◦C (7; 8).

Volatile organic compounds are air pollutants which affect both the environment andhuman health. There are a very large number of VOCs which are considered as airpollutants which contribute to environmental and climate change. VOCs are associatedwith the formation of fogs, acid rain, global warming and ozone formation (7–15).

VOCs effect on human health differs according to the VOC type as well as the leveland duration of exposure. The health effects vary from eye, noise and throat irritation,headache and dizziness, to more serious health problems such as miscarriages, damagingthe immune system of infants and children, causing damage to liver, kidney and centralnervous system, as well as causing cancer (8; 11; 16–19).

Some volatile organic compounds are produced from natural sources including emis-sions from plants and natural forest fires. The VOCs which are harmful for both envir-onment and human health are primarily caused by man-made activities. Oil refineries,petrol storage and distribution, vehicle exhausts, solvents usage and manufacturing, surfacecoating and painting, as well as many other industrial processes are the main sources ofanthropogenic VOCs (7; 11–13; 20–22).

1.3 Removal of VOCs from gas streams.

There are several traditional ways to remove VOCs from a gas stream. However,these techniques suffer from a variety of problems. Thermal oxidation has the problem ofgenerating NOx and other harmful by-products. It also has a high insulation cost. Catalyticoxidation has the problems of deactivation with time, poisoning of the catalyst by lead,sulphur and halogens and the disposal of contaminated used catalyst. Materials such asactivated carbon, zeolite and polymer can be used to absorb the VOCs. This techniquerequires the disposal of used adsorption materials, it also causes a pressure drop in thesystem. Biofiltrations and membrane separation suffer from pressure drop, sensitivity totemperature, plugging and humidity (8; 23–27).

Plasma could be an alternative method for VOC removal from gas streams. Plasmahas been shown to have a positive effect on the reduction of diesel exhaust pollutants aswell as several volatile organic compounds. Non-thermal plasma generated at atmospheric

31

pressure and room temperature gives several advantages for air pollution control. Workingat atmospheric pressure reduces the cost related to vacuum pumps and system modificationsto maintain the required pressure. No heating is required for the non-thermal plasmaprocess; it is generated at room temperature. This technique is easy to run and control. Aplasma system is easy to switch on and off and does not require a long time to be ready tobe used compared with thermal techniques. (28–36).

The focus of this work is the remediation of chlorinated VOCs. Chlorinated VOCs areserious air pollutant gases which affect both the environment and human health (7; 16;17). Methyl chloride (CH3Cl) and dichloromethane (CH2Cl2) were used as examples ofchlorinated VOCs, presenting a simple form of chlorinated VOCs with one and two chlorineatoms respectively. Dichloromethane and methyl chloride are released to atmosphere via avariety of industrial processes such as oil refining; surface coating, painting and printing;plastics manufacturing; pharmaceuticals production, vehicle exhaust and biomass burning(7; 14; 20–22; 37–39); power stations and iron and steel manufacturing plants whichuse coal as fuel (40–42); incineration of municipal solid waste, especially plastics whichcontain chloride. Chemical plants use methyl chloride as solvent during the production ofbutyl rubber (43); pulps and paper mills (37) are other potential sources. Table 1.3 showsthe physical properties for dichloromethane and methyl chloride. Taken from (44).

Table 1.3: Physical properties for methyl chloride and dichloromethane

Methyl chloride Dichloromethane

Chemical formula CH3Cl CH2Cl2Appearance Colourless gas with a faint sweet odour Colourless liquidMolar mass 50.49 g/mol 84.93 g/mol

Density 2.22 kg/m3 (0 ◦C) 1.33 g/cm3, liquidBoiling point -24.2 ◦C 39.6 ◦C

Vapour pressure 490 kPa (20 ◦C) 47 kPa (20 ◦C)

Both dichloromethane and methyl chloride contribute mainly to indoor air pollutioncausing several health problems such as headache, nausea, dullness, dizziness, pulmonaryirritation, effects on the central nervous system. Excessive exposure can cause abortion,affect the birth weight and causing cancer (16; 17; 19). The threshold level value (TLV)for dichloromethane and methyl chloride is 500 and 200 ppm over eight hour respectively(data supplied by Sigma Aldrich).

Dichloromethane and methyl chloride enter the environment as liquid or gas. Whenreleased to the environment as a liquid, the majority of these particular solvents evaporate

32

due to their low boiling point. Some of the released liquid makes its way to the groundwaterharmfully affecting plants, animals and humans. The majority of DCM and methyl chloridereleased to the atmosphere are in the gas phase. Although the direct contribution of DCMand methyl chloride to ozone depletion is small, the chlorine radicals generated whenthey break down in the air contributes to ozone depletion and the formation of smog(12; 45; 46).

Before investigating the use of non-thermal plasma for the removal of chlorinatedVOCs, a background theory of plasma generation, types, properties and applications isdescribed in the following section.

1.4 Principles of technological plasmas

The states of matter are generally regarded as solids, liquids and gases. Plasma iswidely referred to as the fourth state of matter. Plasma is a gas which is either fully orpartially ionized. Applying a high electric field to a gas leads to the ionizationation ofthe gas atoms and molecules. Ions, electrons and radicals are generated as a result of thisionization; a gas in a such state is known as plasma (28; 47; 48).

Plasma forms 99 % of the universe in many forms such as solar corona, nebula andsolar wind. Our sun is a huge plasma ball. Plasma also appears naturally on the earth asaurora and lightning. Aurora appears about 100 kilometers above the earth’s surface andmostly towards the poles. The sun sends streams of charged particles. This solar radiationinteracts with the molecules and atoms in the earth atmosphere causing them to be ionized.The earth’s magnetic field traps these particles which get more dense near the poles leadingto the generation of aurora. Lightning occurs in storms when an accumulation of chargedparticles inside clouds cause a large potential difference between the clouds and earth.This huge potential difference leads to the formation of discharges between clouds andearth. This discharge stream consists of accelerated electrons which cause the air in thispath to be ionized, creating a highly conductive plasma path between the clouds and earth(28; 47).

As well as naturally generated plasma, artificial plasma can be produced using differenttypes of plasma reactors which are described later in this chapter. There are two types ofartificial plasmas, thermal equilibrium hot plasmas and non-thermal or non-equilibriumplasmas.

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Figure 1.2: Examples of naturally generated plasma, sun, aurora and lightning. Picturesare taken from (en.wikipedia.org) on 26-11-2011

Thermal plasmas involve the use of high power plasma generators to create widespreadionization and high temperatures in the gas bulk. The applied power is injected into allplasma particles almost simultaneously, giving an equally high temperature for electronsand heavy particles (Te = Th ≈ 10 000 K) at the same time leading to the formationof plasma in thermal equilibrium (47; 49). Thermal plasmas are mostly generated atatmospheric and higher pressure conditions. The two main characteristics of thermalplasmas are a very high energy density, and the production of a large number of varietyof active species, which leads to numerous industrial applications for thermal plasmas.The high energy density of thermal plasma which ranges from 100 W cm−3 to above10 kW cm−3, produces a high temperature energy source which is used in a variety ofindustrial applications such as plasma cutting, welding, deposition, melting, refining, wastetreatment, vaporizing, spraying, arc furnaces and many other high temperature applications.Thermal plasma is also used as a source of chemically active species in plasma chemicalsynthesis and to produce high purity materials (28; 35; 47; 49; 50).

1.5 Non-thermal plasma generation, properties and ap-plication

Non-thermal plasma, also known as cold plasma, is a non-equilibrium plasma, in whichthe energy is delivered primarily to the electrons instead of the kinetic energy of gas atomsand molecules. Thus, the gas remains cold whilst the electrons gain high energy andtemperature (Te) which range from 10 000 K to 100 000 K which is equivalent to 1-10 eV.The heavy particles in the gas discharge have a temperatures (Th) ranges from 300 to amaximum of 1000 K. (Te >> Th). The energetic electrons are responsible for initiatingthe chemical reactions in the plasma (28; 47; 49; 51).

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In general, the generation of non-thermal plasma requires the following elements: twoelectrodes, high voltage power supply, dielectric material, gap between the electrodes anda flow of gas through this gap. Applying an electric field between the reactor electrodesleads to the generation of free electrons, providing these electrons with a very high energyand accelerating the electrons. These highly energetic fast moving electrons collide withthe atoms and molecules in the gas and cause them to be ionized, excited or dissociated asshown in the following reactions (28; 47; 49; 52–54).

Table 1.4: Primary processes in non-thermal plasma.

Primary process / 10−8 seconds

Excitation e−+A−−→ A∗+ e−

Ionization e−+A−−→ A++ e−+ e−

Dissociation e−+A2 −−→ 2A+ e−

Attachment e−+A2 −−→ A−2Dissociative attachment e−+A2 −−→ A−+ADissociative ionization e−+A2 −−→ A++A+2e−

Electronic decomposition e−+AB−−→ A+B+ e−

Charge transfer A++B−−→ A+B+

These processes are referred to as the primary processes in plasma, and usually takearound 10−8 seconds to be performed. Electrons, ions, excited atoms and moleculesare produced as a result of these primary processes. These active species will undertakenumerous recombination reactions between each other such as: ion and ion, radical andradical and radical+ neutral, leading to the formation of the final plasma products. Theseprocesses are known as the secondary processes in plasma and have a timescale of about10−3 seconds.

The production of active species using non-thermal plasmas at temperatures closeto room temperature allows various applications for surface treatments. Some of theseapplications are not possible using conventional methods which need high temperaturesto initiate the required chemical reactions, as some materials such as plastics can notstand high temperatures. Some examples of surface treatment using plasma are thin filmdeposition, plasma etching of semiconductors, cleaning, decontamination and sterilizationof medical equipment and packaging, and many other applications. Non-thermal plasma isalso used for fuel reforming such as the generation of hydrogen from methane (33; 50; 55;56).

The ability of non-thermal plasma’s to produce active species which initiate a chainof chemical reactions in gas components at atmospheric pressure and room temperature,

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makes the plasma a very promising tool for environmental applications. It also requiresless energy to be generated compared with thermal plasma as only the electrons getheated, not the total gas bulk. Some examples of the use of non-thermal plasma forenvironmental applications are: diesel exhaust cleaning, water treatment, the control ofair pollutants produced as a result of paint spray, paper mills, food and wood processingplants, pharmaceutical plants and many other applications (33; 35; 50; 56).

Atmospheric non-thermal plasma has been used in this thesis for the removal of VOCs.

1.6 Types of plasma discharges

Non-thermal plasmas can be generated using a variety of plasma reactors such as:pulsed corona discharge, dielectric barrier discharge DBD, surface discharge (SD), radiofrequency (RF), microwave and packed-bed plasma reactors. These discharges can also becategorized depending on the behaviour of the electric field, into three types of discharges,these are the DC discharge, AC discharge and pulsed discharge. A brief description ofsome these plasma reactors is presented in this section.

1.7 Direct current (DC) discharge

Depending on the electrical properties of applied voltage and current, several types ofplasmas can be generated. Changing the current and voltage leads to the change from onedischarge type to another. Figure 1.3 shows the dependence of several plasma dischargeson the change in discharge current and applied voltage.

At the applied voltage is increased a Townsend discharge is generated with low currentflow. The electric field in this region is adequate to provide the electrons, initially presentin the gas, with the necessary energy for ionizing neutral atoms and creating extra chargedparticles. The generated secondary electrons ionize more neutrals and sustain the discharge.By increasing the discharge current, the voltage drops and a transition to corona dischargetakes place. This discharge occurs at the points of high electric field such as sharp points,edges and wires. A transition to sub-normal glow and normal glow discharge is obtainedwith a further decrease of the voltage and an increase of the current. In the glow dischargeregion, the electrons collide with the gas molecules and atoms producing highly excitedatoms and molecules where electrons have been excited to a higher electronic energy level.

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Figure 1.3: Several DC discharges as a function of the properties of the applied voltageand discharge current. Taken from (2) and (3)

However, molecules tend to return back to the lower and more stable energy level, emittingphotons at specific wavelengths in the UV-visible region. In the normal glow region, thevoltage is independent from the current. The current density is constant at the cathodesurface. Only a fraction of the cathode is in contact with the plasma in this discharge. Withincreasing the current the entire cathode surface is covered by the discharge. Increasing thecurrent and voltage at this point causes the transition to an abnormal glow discharge, whichis much brighter than the normal glow discharge. With further increase in the current andvoltage, an irreversible transition of glow-to-arc takes place. Arc discharge occur at lowapplied voltage and high discharge current.

1.7.1 Corona discharge plasma reactors

Corona discharges can be observed around high voltage power lines or ships mast’sduring a lightning storms. Generating a corona discharge artificially requires a strongelectric field and two electrodes. One of the electrodes should be a small diameter wire,needle or sharp edge. Figure 1.4 shows two examples of corona discharge plasma reactors.(a) presents a wire and pipe plasma reactor and (b) presents a wire and plate plasma reactor.The electric field could be generated using a direct current (DC), alternating current (AC)or pulsed DC. The pulsed corona discharge is generated using a pulsed power supplywhich provides a short duration pulse voltage. The use of a pulsed power supply reducesthe energy consumption by a factor of five comparing with the use of a DC power supply.The discharge mode in a pulsed corona discharge is a streamer which makes it suitable

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Figure 1.4: Two examples of pulsed corona plasma reactors. (a) is a wire and pipe reactorand (b) is a wire and plate reactor

for pollutants removal. The main advantages of pulsed corona discharge are the relativelybig discharge gap of about 10 cm which make it good for large scale applications and forreducing the pressure drop through the reactor. The other advantage is the low capitalcost. Pulsed corona discharge has wide applications for air pollution treatment such as theremoval of NOx, SO2 and VOCs (28; 35; 47; 50; 57; 58).

1.7.2 Radio frequency (RF) and microwave discharges

Radio frequency power can be coupled to plasma through three main ways. Firstly,using an oscillating magnetic field to generate an inductively coupled discharge. Secondly,an oscillating electric field is also used to generate a capacitively coupled discharge. Finallyboth a magnetic and an electric field could be used together to generate a quasi-opticalor microwave discharge. Both RF and microwave discharges are generated by a highfrequency electromagnetic field. RF frequencies range between 1 - 100 MHz with the mostused frequency of 13.56 MHz which has been allocated by the international communicationauthorities to avoid interferences with communication channels. The wavelength of theRF discharge ranges between 3 - 300 meters. The microwave frequency ranges between300 MHz - 10 GHz with the most commonly used frequency of 2.45 GHz. The microwavewavelengths are in the centimeters range (28; 47; 50).

1.7.3 Dielectric barrier discharge (DBD)

The dielectric barrier discharge consists of two planar or cylindrical electrodes and atleast one insulating layer between the electrodes. Applying a strong electric field across

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Figure 1.5: Discharge development in a DBD reactor. (a, b and c) shows the dischargedevelopment, while (d and e) show the effect of increasing the dielectric conductivity anddecreasing the gap. Taken from (4)

the discharge gap between the electrodes leads to the formation of electron avalanchesfrom the cathodes toward the anode. Streamer avalanches are formed on the surface ofthe anode’s dielectric barrier. These streamers bridge the gap moving towards the cathode.Figure 1.5(a) and (b) show the formation of electron and streamer avalanches respectively.In the case of small size streamers, a large number of filaments (microdischarges) arerandomly distributed in the discharge gap between the electrodes. However, if the sizeof these avalanches is big enough, they interfere with each other forming a wide positivecharge area. The dielectric material acts to prevent the formation of arcs and streamersbetween the electrodes. Increasing the electrical conductivity of the dielectric material,limits the current flow to the system and spread the discharge over the entire electrodearea. Figure 1.5(d) shows the effect of increasing the dielectric conductivity on limiting theformation of streamers. Another way to suppress the formation of streamers is by reducingthe discharge gap, as shown in Figure 1.5(e). A smaller gap lowers the electric field andlimits streamer propagation through the distance between the electrodes.

The material of the dielectric layer could be glass, quartz, ceramic, or a thin coating ofenamel or polymer over the electrodes. Figure 1.6 shows examples of different configur-ations of DBD reactors. The gap between the electrodes ranges between 0.1 to 100 mmin which the plasma is generated. An alternating current (AC) voltage is applied acrossthe electrodes. The discharge mode is either glow discharge or microdischarges. Someof the main applications of DBD plasma reactors are ozone generation, chemical vapourdeposition, surface modification and pollution control (28; 35; 47; 50; 54; 59).

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Figure 1.6: Some examples for DBD plasma reactors

1.7.4 Surface discharge plasma reactor (SD)

This type of plasma reactor requires the use of a dielectric barrier material for the samepurpose as for the DBD reactors. Figure 1.7 presents an example of SD plasma reactor.A reactor tube of Pyrex, quartz or alumina is used as the insulating material. A coaxialmetal wire coil is used as the inner electrode where the high voltage is applied. An outerelectrode which is usually grounded is wrapped, taped or coated on the outer surface ofthe dielectric barrier tube. Plasma is generated as a thin layer on the dielectric surfacenear the surface electrodes as shown in Figure 1.7. There are a variety of applicationsfor this type of plasma such as: ozone generation, medical equipment sterilization andair pollutant removal. The SD is very effective in producing ozone as a result of the freeplasma zone where the ozone can be accumulated without being destroyed by the plasmadischarge. A strong non-thermal plasma could be generated using SD which makes it veryeffective for the decomposition of air pollutants such as NOx, SOx, alcohols, aldehydesand hydrocarbons (35; 47; 56; 60–63).

The reactor which was used in this work is a packed-bed plasma reactor. The comingsection explains the important parameters and advantages for a packed-bed reactor.

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Figure 1.7: An example of surface discharge plasma reactor

Figure 1.8: Examples of packed-bed plasma reactors

1.8 Packed-bed plasma reactor characteristics and advant-ages for air pollutant remediation

A packed-bed reactor is a DBD reactor which consists of two electrodes and dielectricbeads which fill the space between the electrodes. This type of reactor could be designedwith or without a dielectric layer separating the electrodes from the packed-bed. Figure 1.8shows two examples of packed-bed plasma reactors. Figure 1.8(a) presents a packed-bedreactor where the dielectric beads are the insulating material between the two electrodes.While figure 1.8(b) shows a packed-bed reactor where the Pyrex tube presents an extrainsulating layer in addition to the beads between the electrodes. Applying a high voltageto the electrodes leads to the generation of a very strong electric field on the contact pointsbetween the pellets themselves and between the pellets and the electrodes. This intenseelectric field at the contact points of the pellets leads to the breakdown of the gas nearthe contact points and the formation of microdischarges which leads to the generation ofplasma in the reactor (28; 35; 47; 64).

The most important parameters to be considered when choosing the packing pellets are

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the pellet material, dielectric constant, size and shape (28; 65).

1.8.1 Pellet materials and dielectric constant

A variety of pellet materials have been used as packing beads in plasma reactors such asglass, quartz, aluminum oxides and ferroelectrics. Some examples of the ferroelectric beadsare barium titanate (BaTiO3), magnesium titanate (MgTiO4), calcium titanate (CaTiO3),strontium titanate (SrTiO3), and lead titanate (PbTiO3). The use of different pellet materialsaffect the plasma discharge as a result of a different dielectric constant for each material.Pellets materials can be non-catalytic or catalytic. In the case of catalytic material surfacereactions between the gas and the catalyst take place. It is important to choose the packingmaterial with a suitable dielectric constant for the application of the plasma reactor. Forexample, using a packed-bed plasma reactor for ozone generation requires a packingmaterial with a dielectric constant less than 10. For pollutant removal a pellet with adielectric constant more than 2000 is usually used (28; 47; 65).

As well as the dielectric constant the type of packing bead material could influence thechemical reactions taking place in the plasma region. For example when silica and quartzwere used as a dielectric material in a packed-bed plasma reactor for ozone generation bySchmidt-Szalowski et al. (66) the results showed that using silica resulted in a better ozonegeneration. This is due to the absorption of oxygen molecules by silica which provides anextra route for ozone generation as shown in the following reaction (66; 67):

O+O2(ad) −−→ O3(ad) −−→ O3 (R1.1)

In this thesis, BaTiO3, which is a non-catalytic material, has been chosen as a dielectricin the packed-bed plasma reactor for gas pollutant abatement. The dielectric constant ofBaTiO3 ranges between 2000 to 10000 (28; 35; 65). Such a high dielectric constant isrequired to prevent arcing between the electrodes. Several studies have reported the useof BaTiO3 as a dielectric in plasma reactors for air pollution treatment. Yamamoto et al.

(68–70) have used BaTiO3 as a dielectric material for decomposing VOCs in a packed-bedplasma reactor. Takaki et al. (71; 72) investigated the decomposition of DCM and C2F6

using a packed-bed plasma rector packed with BaTiO3 beads. Ogata et al. (73; 74),Sugasawa et al. (75), Fitzsimmons et al. (76), Wallis et al. (77; 78), Harling et al. (79–83)and Hill et al. (84; 85) they have investigated the decomposition of different types of VOCsin a packed-bed plasma rector using BaTiO3 as the dielectric material. Non of these studieshave reported any surface reactions between the barium titanate beads and the species inthe gas phase.

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1.8.2 Pellet size and shape

Pellet size affects the number of contact points between the pellets and between thepellets and the electrodes. Increasing the bead size leads to less contact points betweenbeads which leads to fewer microdischarges in the plasma region. Increasing the beadsize also affects the void volume in the packed-bed area. A bigger void volume requireshigher voltage to generate a breakdown in the gas and to generate plasma. Pellet shapealso has an important effect on the reactor performance. Pellets with sharp edges causethe formation of a high local electric field at the edges leading to the formation of highlyenergetic electrons. The number of contact points are also affected by the pellet shape(47; 65; 71; 74). The pressure drop in the packed-bed is also affected by the shape and thesize of the pellets. for example, using hollow pellets such as a hollow cylinder reducesthe pressure drop in the packed-bed. Using smaller size pellets causes a bigger pressuredrop. The pressure drop, ∆P, in packed-bed reactors can be estimated using the followingequation (65).

∆P =

150(1−ε)µgdpUgρg

+1.75dpε3

(ρgU2g )(1−ε)

× Dgc

(1.1)

Where ε is the void fraction, µg is gas viscosity, dp is packing pellet diameter, Ug is thegas superficial velocity, ρg is the gas density, D is the bed depth and gc is the gravitationalconstant (1 kg m s−2).

It is essential to select the right properties of material, size, shape and dielectric constantfor the dielectric beads to suit the required applications.

1.8.3 Advantages and applications of packed-bed plasma reactors

The use of a packed bed plasma reactors provide the advantages of a uniform distribu-tion of gas flow and gas discharge in the reactor. Also, this type of reactor could easily bemodified to include catalyst with the pellets in the same packed-bed or as a separate stagedownstream of the plasma region. Packed-bed plasma reactors have two major applications,ozone generation and air pollutant removal (86; 87). As for pollutant abatement, the packedbeds reactors were first used in collecting particles (88). Later it has been used for VOCdecomposition (68; 69; 73; 76; 77; 84; 89–93), odour removal (94), and CO2 reduction(95). The combination between catalyst and ferroelectric pellets could reduce the amountof some harmful plasma products such as NOx and ozone (65; 77; 79; 92; 96).

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This work uses packed-bed plasma reactor with barium titanate dielectric material forthe remediation of dichloromethane and methyl chloride.

1.9 Literature review for dichloromethane and methyl chlor-ide decomposition using plasma.

Several studies have investigated the decomposition of dichloromethane (DCM) inplasma. Sugasawa et al. (75) investigated the decomposition of dichloromethane in apacked-bed plasma reactor. They found that adding oxygen to a N2-DCM gas stream causedless decomposition of dichloromethane. Huang et al. (97) studied the decomposition ofdichloromethane in a wire-in tube pulsed corona reactor. They also found that increasingthe oxygen concentration decreased the removal efficiency of dichloromethane. Penetranteet al. (98) looked into the decomposition of dichloromethane using electron beam andpulsed corona plasma reactors at room temperature and atmospheric pressure in bothnitrogen and a dry air gas mixture. They found that dichloromethane decomposition ismore efficient in nitrogen than it is in a dry air gas mixture. Yamamoto et al. (68) examinedthe effect of several plasma parameters on the removal efficiency of dichloromethane inboth an AC ferroelectric pellet reactor and a nanosecond pulsed corona reactor. They foundthat increasing the residence time or applied voltage leads to increasing the destructionefficiency of dichloromethane. Li et al. (99) used a radio frequency plasma reactor fordichloromethane decomposition in nitrogen and argon at low pressure of 1 to 5 Torr.They investigated the effect of several plasma operation parameters on the decompositionefficiency of dichloromethane. They showed that the destruction of dichloromethanedecreased by increasing the total gas flow rate or by increasing the feed concentrationof dichloromethane. The decomposition efficiency of dichloromethane in their resultswas higher with argon plasma compared with nitrogen plasma. Fitzsimmons et al. (76)investigated the effect of oxygen concentration on the removal efficiency of 500 ppm ofdichloromethane in a non-thermal plasma generated in a packed-bed plasma reactor. Theyfound a maximum DCM removal efficiency in nitrogen plasma with the addition of 2 to 3% of oxygen.

Hsieh et al. (100) have investigated the decomposition of methyl chloride by using anRF plasma reactor and a gas mixture of argon, CH3Cl and oxygen. Their system workedat a low pressure of about 20 Torr with temperatures of about 440 ◦C and a total flowrate of 100 cm3 m−1. They investigated the effect of oxygen, input power and the feed

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concentration on the decomposition of methyl chloride using a RF plasma reactor. Theywere able to achieve a 99.99 % of CH3Cl decomposition in their system.

These studies have investigated the use of plasma for dichloromethane and methylchloride remediation. However, a complete and clear understanding of the effect ofdifferent plasma parameters on the decomposition process for chlorinated VOCs is yetto be established. Most of the previously mentioned studies have found that addingoxygen to the gas stream resulted in either decreased the removal efficiency of chlorinatedVOCs or a maximum decomposition with small oxygen concentrations of about 3 %. Forenvironmental applications, it is more practical to use air plasma rather than controllingoxygen concentrations to about 3 percent. It is also preferable to use atmospheric pressurerather than low pressure which require larger running cost for the plasma system.

This work investigate the removal efficiency of dichloromethane and methyl chloridein an atmospheric pressure plasma. It aims to determine the maximum removal efficiencyof chlorinated VOCs in air plasma.

1.10 Aims and objectives

In this thesis, non-thermal plasma generated in packed-bed plasma reactors has beenused to investigate the remediation of chlorinated volatile organic compounds. ChlorinatedVOCs are important air pollutant gases which affect both the environment and humanhealth. Two chlorinated VOCs are investigated in this work, methyl chloride (CH3Cl) anddichloromethane (CH2Cl2). They were used as examples of chlorinated VOCs, presentinga simple form of chlorinated VOCs with one and two chlorine atoms respectively.

The overall aim of this thesis is to optimize the removal efficiency of methyl chlorideand dichloromethane in air plasma by investigating the influence of key process parameters.

The objectives of this thesis are to:

• Investigate the influence of oxygen concentration, initial VOC concentration, energydensity, plasma residence time and background gas on the removal efficiency ofdichloromethane and methyl chloride.

• Investigate the influence of alkene additives on improving chlorinated VOCs decom-position and reducing nitrogen oxides generation.

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• Investigate the influence of using a multiple packed-bed plasma reactor on theremoval efficiency of dichloromethane, propylene and a mixture of both of them in anitrogen-oxygen plasma.

• Investigate the use of in situ measurements for the detection of short lived chemicalspecies in the plasma in order to further understand the mechanism of the chemicalreactions taking place in and after the plasma region.

1.11 Structure of the thesis

This thesis starts with a methodology chapter, where the plasma system, spectroscopictechniques, analysis methods and calculations are explained. Chapters three and fourpresent the investigation of the influence of process parameters such as oxygen concentra-tion, initial VOC concentration, energy density, plasma residence time and backgroundgas on the removal efficiency of dichloromethane and methyl chloride respectively. Theinfluence of adding alkenes on the removal efficiency of dichloromethane is discussed inchapter five. Chapter six reports an investigation of the use of a multiple packed-bed plasmareactor for the decomposition of dichloromethane and propylene. In situ measurementsthrough the multiple packed-beds are also investigated in this chapter. This thesis thenproceeds to investigate the formation of nitrogen oxides as plasma by products during thedecomposition of VOCs in nitrogen-oxygen gas mixture. A summary of the work andresults obtained in this thesis are discussed in chapter eight as well as further work.

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Chapter 2

Methodology

The purpose of the work presented in this chapter is to describe the experimental systemused for the remediation of VOCs. The design and characteristics of all the plasma systemcomponents are explained. The spectroscopic analysis and the methods used for calculatingthe concentration of chemical species in the gas stream are discussed. Calculations forlimits of detection, signal to noise ratio, the flow rates, errors and applied power are alsodiscussed. The time required to obtain a steady state plasma is investigated.

2.1 Experimental arrangements

Figure 2.1 shows a sketch of the experimental set up. Different gas cylinders wereused depending on the experiments that were carried out. The gases (BOC) were used asdelivered, and consisted of air (79 % nitrogen, 21 % oxygen), nitrogen (99.998 %), oxygen(99.99%) and argon (99.999%). Propylene (99.95%) and methyl chloride (99.99%) gaseswere also used. Mass balance calculations using Dalton’s law and the Antoine equationhave been made so as to adjust the gas flow through MKS 247 mass flow controllers.These calculations are explained later in this chapter. The flow rate of the carrier gas andthe temperature of the liquid DCM (99.6 %) in the bubbler were controlled to obtain therequired concentration of DCM in the inlet gas stream of the reactor. The liquid DCM wascooled down to around -20 ◦C by placing the bubbler in a coolant of 50% water and 50%antifreeze and using a HAAKE EK45 chiller unit. Cooling the DCM reduces it’s vapourpressure from 0.47 bar at 20 ◦C to 0.065 bar at -20 ◦C, which make it easier to obtain aDCM concentration of about 500 ppm. Neon sign power supplies providing a high voltage,which could be varied using a variac, were used to power the reactors. A voltage probe(TES TEC HVP - 15HF) and current probe (Pearson(TM) current monitor model 411)have been used to characterize the supplied power. Plasma is generated using a single, or amultiple cell packed-bed plasma reactor, at atmospheric pressure. The inlet system hasbeen designed to allow the sampling of the gas stream before and after the plasma reactor.An arrangement of valves has been used to control the direction of the gas flow. In lineFTIR analyses has been made using a multiple pass optical gas cell providing an opticalabsorption pathlength of 5.3 meters. Gas leaving the reactor has to travel 1 meter of pipes

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Figure 2.1: A schematic diagram of the experimental inlet system which has been used forall the investigations carried out in this study.

before reaching the multiple pass optical gas cell. An optical emission spectrometer wasused to collect emission from plasma. Spectra have been obtained with a Bruker Equinox55 FTIR spectrometer and an Ocean Optics USB 2000 UV-Vis spectrometer.

2.2 Packed-bed plasma reactors

This section explains the design and characteristics of the two packed-bed plasmareactors which have been used throughout this study. Both a single stage, and a multiplepacked-bed plasma reactor, have been designed and used.

2.2.1 Single stage packed-bed plasma reactor

Figure 2.3 shows a photo and a sketch of the single stage plasma reactor. The singlestage packed-bed plasma reactor consists of a Pyrex glass tube with an internal diameterof 31 mm and about 20 cm long. Two stainless steel pipes ending with funnels coveredwith stainless steel meshes have been used as electrodes. The funnels achieve a good gasdistribution in the packed-bed area. Figure 2.2 illustrate the distribution of the gas velocityprofile in the pipe and the funnel. When the gas comes out of the pipe to a wider part thevelocity of the gas is reduced. The gas stream line would diverge and get distributed all

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Figure 2.2: A velocity profile distribution in a funnel

over the wide part of the funnel. This diffusion of the gas stream in the funnel leads toa better distribution of the gas over the entire volume of the packed-bed plasma reactor(101–103).

Barium titanate BaTiO3 beads, from CATAL international Ltd., with a diameter of 1.4- 2.8 mm were used as the packed-bed dielectric filling material. Barium titanate beadscan be produced by heating barium carbonate BaCO3 and titanium dioxide TiO2 to around1000 ◦C (104). BaTiO3 is commonly used as a dielectric material in plasma reactors forpollutant removal because of its high dielectric constant. Packing pellets of a dielectricconstant higher than 2000 are usually used for gaseous pollutant remediation(65). Bariumtitanate has a dielectric constant of between 2000 to 10000 in a temperature range between20 to 120 ◦C (65; 105–107). The dielectric constant is affected by the operating conditions,such as temperature, applied frequency and applied electric field strength (65). The totalpacked bed volume was about 24.9 cm3, while the void volume was about 16 cm3, allowinga residence time of 0.95 s with a total flow rate of 1 L min−1.

Some attempts to modify this design to enable in situ measurements have been carriedout. An extra Teflon tube was manufactured to hold two meshes perpendicular to theaxis in the centre of the tube, leaving an empty space of about 5 mm between the twomeshes. This empty space was aligned with two KBr windows in the reactor glass tubingbody. The aim of this modification was to create an area free of beads in the packed-bedplasma region to allow in situ measurements by passing an infra-red beam through thisarea. Figure 2.4 shows a photo and a sketch of the modified single stage plasma reactor.Teflon and stainless steel meshes have been tested on the barrier of the separation area.Meshes with big, small, circular and rectangular holes have been used. The separation areabetween the two meshes has been changed between 2 to 5 mm. All the attempts to generate

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Figure 2.3: A photo and a sketch for the single stage plasma reactor

plasma with this modification in the packed-bed plasma reactor were not successful. Thiscould be due to the manufacturing of the Teflon tube, as the two meshes were not exactlyparallel to each other within the tube. Also the Teflon meshes were not manufacturedsimilarly with the same number and position of holes in both of them. This modificationhad to be discarded.

2.2.2 Multiple packed-bed plasma reactor

A multiple packed-bed plasma reactor consisting of three plasma cells in series wasdesigned. The design of this reactor was a modification of similar reactors reported byHubner et al. (108) and Harling et al. (82). Figure 2.5 shows a sketch and photo of themultiple packed-bed plasma reactor. The reactor body is made of perspex with dimensionsof 45× 12× 7 cm. The reactor consists of three plasma cells. Each cell has two rectangularstainless steel mesh electrodes with a dimensions of 10 × 5 × 0.1 cm. Barium titanatebeads with a diameter of 1.5-2.5 mm were used as a dielectric material, filling the spacebetween the two electrodes in each cell. Each plasma cell has a dimension of 10 × 5× (0.5 - 2.5) cm giving a gas residence time of 1.4 s in each cell at a flow rate of 1 Lmin−1, and a total residence time of 4.2 s in the plasma regions of the reactor. Plasma cells

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Figure 2.4: Photograph and sketch of the modified single stage packed-bed plasma reactor

were powered by three identical neon sign power supplies each providing high voltagealternating current at a frequency of 20 kHz. The input voltage was controlled using avariac. The packed-bed cells have been designed to allow a change in the distance betweenthe electrodes from 0.5 to 2.5 cm; leading to an increase or a decrease in the packed-bedvolume. KBr windows with a diameter of 4 cm and a thickness of 5 mm were installed atseveral parts allowing in situ measurements via FTIR spectroscopy across the 10 cm widthof the reactor.

The gas mixture entered the reactor through a funnel and exited through another funnel.These funnels were added to the reactor body to insure a good gas distribution through thereactor and to prevent any back flow or gas accumulation at the end of the reactor body.

2.3 Spectroscopic diagnostic techniques using an FTIRspectrometer

The main spectrometer which has been used to identify and quantify the chemicalspecies in the plasma system is an FTIR spectrometer (Bruker Equinox 55).

The Fourier transform infra-red (FTIR) spectrometer is a well known and establishedanalytical spectroscopic technique. An FTIR spectrometer has been used in this work tocharacterize the chemical species in the gas stream before and after the plasma reactor.

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Figure 2.5: Photograph and sketch of the multiple packed-bed plasma reactor. Threesimilar neon sign power supplies with a frequency of 20 kHz were used.

In this section, the principles of infra-red spectroscopy, instrument components, theadvantages of FTIR spectrometers, spectrum analyses, limit of detection and signal tonoise ratio are explained.

2.3.1 Principles of IR spectroscopy

An infra-red spectrum occurs as a result of the transitions between quantanized vi-brational energy states (109). For the vibration of a molecule to be infra-red active itshould produce a change in the dipole moment (110; 111). Molecules with a number ofatoms N have 3N degrees of freedom. Translational motion of the whole molecule in threeperpendicular axes (x, y and z) uses three degrees of freedom. The rotational motion ofnon-linear molecules about x, y and z axes uses another three degrees of freedom. therotational motion of a linear molecule uses two degrees of freedom as there is no rotationabout the internuclear axes. As a result, the fundamental modes of vibration is given by3N-6 for a non-linear molecule and 3N-5 for a linear molecule (110). The main moleculewhich has been investigated in this work is dichloromethane (CH2Cl2). For this non-linearmolecule with N=5, there are nine fundamental modes of vibration which are shown in

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figure 2.6. Four of these vibrational modes (N-1) are stretching vibrations and five of them(2N-5) are bending vibrations (5). All these vibrational modes are infra-red active exceptnormal mode 5, the H-C-H out-of-plate twisting, is IR inactive due to the symmetry of thismode. Figure 2.7 shows an example FTIR spectrum for dichloromethane indicating thevibrational modes.

When an infrared beam passes through a sample gas, some of the beam wavelengthsinteract with the molecules in the gas sample. The molecules will absorb some of theincident infrared beam I0. The transmitted beam I after the sample is then detectedand analysed. The transmitted beam (I) varies with the length of the sample (l) and theconcentration of the sample (c). Beer Lambert Law describes the relation between theincident radiation and the transmitted radiation as follow (112).

I = I0exp−εcl (2.1)

Where: ε is the molar absorption coefficient. Since the concentration, c, of the sample isoften unknown, an expression which is linear with the concentration is needed.

A =− ln(II0) = εcl (2.2)

where A is the absorbance of the sample at a given wavenumber. Further detailed explana-tion of spectrum analyses and chemical species concentration calculations are discussed ina subsequent section.

2.3.2 FTIR spectrometer components

The key components in any Fourier Transform infra-red spectrometer are an infra-redsource, interferometer, optics and detectors. Figure 2.8 presents a sketch of the basicarrangement of a FTIR spectrometer. The infra-red source produces a broad range of infra-red radiation of different wavelengths. Usually a rod of rare earth oxide or a resistivelyheated silicon carbide rod, known as a Globar, is used as an IR source after it has beenheated to an orange glow (112).

The interferometer of FTIR spectroscopy is based on the two beam interferometerdesigned by Michelson in 1881. It depends on the interference between two beam withdifferent path lengths to create an interferogram. The infra-red beam emitted by the sourceis directed to a beam splitter which partially reflects some of this beam to a fixed mirror

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Figure 2.6: Dichloromethane fundamental modes of vibration. Adapted from Shimanouchi,1972 (5).

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Figure 2.7: An example FTIR spectrum for about 500 ppm of dichloromethane in nitrogenshowing the vibrational modes. A multiple pass optical gas cell with a 5.3 m pathlengthwas used.

Figure 2.8: A schematic diagram for the layout of a FTIR spectrometer

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Figure 2.9: An example of FTIR interferogram

and partially transmits some of the beam to a movable mirror. The two split beams reflectback to the beam splitter where they interfere with each other because of their differentpath lengths caused by the movable mirror. This interference between the two beamsproduces an interferogram. The more frequencies that combine, the more complex is theinterferogram (109; 112). A typical FTIR interferogram is shown in figure 2.9. The FTIRspectrum can be extracted from the FTIR interferogram via a mathematical process knownas a Fourier Transform. Figure 2.10 presents an example of FTIR spectral measurementstaken after the plasma reactor for a 500 ppm DCM and 3 % oxygen in nitrogen plasma.Part (a) shows the background and signal spectra, (b) shows the transmittance spectrumand (c) shows the absorbance spectrum.

Optical components are usually a set of reflecting mirrors which collect and collimatethe radiation emitted by the IR source. The collimated beam passes through the interfero-meter and then to the reflecting mirror which focuses the beam to the sample compartmentand then refocuses it to the detector.

Infra-red detectors are divided into two main types, thermal detectors and quantumdetectors. The most common type of thermal detector is the pyroelectric detector which ismade from deuterated triglycine sulphate (DTGS). This detector works at room temperatureand has a combination of high speed, good sensitivity and low cost. For the quantumdetector, the most common is the photoconductive mercury cadmium telluride (MCT)detector. This detector is usually cooled to 77 K using liquid nitrogen (109). Both types ofdetector have a rapid response which is required for FTIR. MCT detectors have a highersensitivity compared with DTGS detectors. Window materials for the detectors and gascells must be made from salts such as potassium bromide (KBr) and sodium chloride

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Figure 2.10: An example of the steps taken via the FTIR spectrometer to obtain anabsorbance spectrum for a gas stream of 500 ppm DCM, 3 % oxygen and nitrogen. Amultiple pass optical gas cell with 5.3 m pathlength and spectral resolution of 1 cm−1 wereused.

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(NaCl), which are transparent in the infra-red region of the spectrum.

2.3.3 The advantages of FTIR spectroscopy

The main advantage of the Fourier Transform method over the traditional dispersive IRsystems is the reduced measurement time from minutes to fractions of a second. This isbecause all the wavelengths are measured simultaneously rather than sequentially. This isusually referred to as the multiplex (Fellgett) advantage. In a dispersive IR spectrometereach point of the spectrum is recorded separately by scanning sequentially over the spectralrange. In this case the majority of time is spent recording the baseline rather than thesignal. Also, the infra-red radiation power from the IR source to the sample and then tothe detector is higher for FTIR spectrometer. This advantage is called the throughput orJacquinots advantage (111). The signal to noise ratio for an FTIR spectrometer is higherthan that of a dispersive spectrometer. As a result of combining the values of Fellgetts andJacquinots advantages, a FTIR spectrometer is about 2000 times more sensitive than IRdispersive spectrometers (109). Beam scattering in a FTIR spectrometer is negligible, asthe sample is placed behind the modulating interferometer.

2.4 Sampling, spectral analyses and concentration calcu-lations

An FTIR spectrometer (Bruker Equinox 55) was used to identify and determine theconcentration of components in the plasma reactor inlet and outlet gas streams. The gasstreams were passed alternately into a multiple pass optical gas cell (Specac Ltd) with aWhite-type mirror arrangement producing multiple light passes between an arrangement ofreflecting mirrors to give an optical pathlength of 5.3 meters (111). Sampling was carriedout about 0.75 seconds after the plasma reactor. Figure 2.11 shows a photo and a sketchof the multiple pass gas cell. FTIR analyses have been obtained at room temperature andatmospheric pressure at a spectral resolution of 1 or 2 cm−1. Liquid nitrogen cooled MCTor DTGS detectors were used to detect the infra-red beam after being passed throughthe gas sample, and to record absorption spectra for molecules in the sample. Afterobtaining a spectrum for the gas sample, standard spectra from Pacific Northwest NationalLaboratories (PNNL) data base (113) in addition to other literature (76; 114–116) havebeen used to identify the molecules in the sample. Concentrations of each species were

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Figure 2.11: Specac multiple pass gas cell and a sketch demonstrating the multiplereflections to obtain a 5.3 m pathlength

determined by integrating the area under each peak using Brukers OPUS software and thencomparing with standard spectra produced by Pacific Northwest National Laboratoriesfor 1 ppmv at atmospheric pressure, one meter path length and 0.1 cm−1 resolution anda 25 ◦C temperature (113). In the case of overlapping peaks, as is the case with carbonmonoxide (CO) in the range of 2050 to 2240 cm−1 and nitrous oxide (N2O) in the range of2170 to 2260 cm−1, parts of the spectra which are out of the overlapping area were used.In the case of calculating the concentrations of CO, half the absorbance spectrum for CObetween 2050 and 2144 cm−1 was used to calculate the area beneath the spectrum andthen compared with the area of the standard spectrum in the identical wavenumber range.The same method is applied for any overlapped molecular spectra. Figure 2.12 illustratesthe area measured for DCM and CO.

An example of calculating the concentration of dichloromethane measured in a multiplepass optical gas cell with an optical pathlength of 5.3 meters is explained. Figure 2.13shows the integration area for experimental and standard spectra for dichloromethane.The experimental spectra is measured for x ppm with 5.3 meter optical pathlength, whilethe standard spectrum is measured for 1 ppm and 1 meter pathlength. A and B presentsthe area beneath each peak measured between 787 and 647 cm−1 using the integration

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Figure 2.12: An example of areas selection when calculating the concentration of (A) DCMand (B) CO.

function in the OPUS software, (A = 66.9, B = 0.0231). To make the standard spectrumarea comparable with the experimental spectrum, the area B was multiplied by 5.3. Theconcentration x was calculated as follow:

x = A/(B×5.3) = 66.9/(0.0231×5.3) = 546ppm (2.3)

The concentrations of all the other species detected in the gas stream were calculated usinga similar method. The integration methods in OPUS software provide several options forsetting the baseline for integrating the spectral area. Figure 2.14 shows some of thesemethods. In this work, the baseline for each peak was chosen separately to give the bestmatch with the baseline of the spectrum.

Measurements were made for the gas stream before and after the plasma reactor. Theconcentrations of the VOC entering and leaving the reactor were calculated as explainedpreviously. After that, the removal efficiency (η) of the investigated VOC was calculatedas follows:

η =(Cin−Cout)×100

Cin(2.4)

Where: Cin and Cout are the inlet and outlet concentrations of the investigated VOC inppm. For example, for about 546 ppm of dichloromethane entering the plasma rector, andabout 400 ppm exiting the reactor, the removal efficiency of dichloromethane is: (546 -400) × 100 / 546 = 26.7 %.

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Figure 2.13: The integrated area between 787 and 647 cm−1 for (a) an experimentalspectrum of DCM at unknown concentration measured with 5.3 meter optical pathlengthand a resolution of 2 cm−1. (b) standard spectrum for 1 ppm dichloromethane measuredwith 1 meter optical pathlength and a resolution of 0.1 cm−1.

Figure 2.14: Examples of the different options for setting the spectral baseline for integrat-ing the area beneath each peak.

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Figure 2.15: An example FTIR spectrum for about 1000 ppm CO with a resolution of 1cm−1. A multiple pass optical gas cell with a 5.3 m pathlength was used.

2.4.0.1 Signal to noise ratio, spectral resolution and limit of detection for the FTIRspectrometer

The signal to noise ratio (SNR) is an indicator of the quality of spectra. For a signalto be clearly distinguishable from the background noise, the SNR should be greater thanfour (109; 110). An example spectrum for about 1000 ppm of carbon monoxide, taken at aresolution of 1 cm−1 using the FTIR spectrometer is presented in figure 2.15. The SNR inthis example is SNR = 1.5

0.2 = 7 ·5.

Spectral resolution is often used as a measure of the performance of a spectrometer. Asthe absorption of molecules take place over a range of frequencies, rather than at a singlefrequency, a high resolution allows absorbance peaks to be resolved and reduce the spectraloverlapping. Spectral resolution gives the accuracy in which the separation of the differentwavelengths for an absorption molecule occur. The resolution of a FTIR spectrometer isrelated to the inverse of the maximum distance travelled by the moving mirror. Increasingthe resolution leads to more data points to be measured by the interferogram and thusbetter accuracy. However, a high spectral resolution is not always preferable as increasingthe resolution causes less IR beam energy from the source to reach the detector. Thisleads to a weak signal and an increase in the baseline noise and consequently a reduced

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signal to noise ratio. Further increase of the resolution leads to a very weak signal which isindistinguishable from the background noise. Therefore, it is advisable not to measure thespectra of compounds with relatively broad bands at high resolution. Resolutions of 1 or 2cm−1 have been used in this work. Signal to noise ratio can be enhanced by increasing thenumber of scans through the sample. However, increasing the number of scans requires alonger sampling time, which can be impractical.

As has been mentioned earlier, for the signal to be clearly distinguished from thenoise, the signal should be four times greater than the noise. A signal which does notachieve this condition is not reliable. The limit of the detection for all the species in thegas stream has been calculated. An example calculation for the HCN limit of detection ispresented. Figure 2.16 shows (a) a background spectra taken for nitrogen gas stream beforeintroducing any chemicals to the system and (b) a standard spectrum of HCN. The standardspectrum of HCN is taken from Pacific Northwest National Laboratories spectra PNNL(113). As the standard spectrum is measured for 1 ppm HCN with a 1 meter pathlength,the spectrum was multiplied by 5.3 to give the equivalent spectrum in a 5.3 meter opticalpathlength which is used in this work. As the noise has positive and negative values, theroot mean square for the noise (RMS) was calculated. The root mean square for HCNpeak heights is also calculated for different HCN concentrations. Figure 2.17 shows thenoise RMS value, the minimum signal which should be four times greater than the noiseRMS and the peak height for different concentrations of HCN. The point at which the peakheight line intercepts with the minimum signal height line gives the limit of detection forHCN. This calculation shows that the limit of detection for HCN is about 15 ppm. Thesame methods were followed for calculating the limit of detection for all species in the gasstream which are IR active.

Table 2.1 shows the minimum limit of detection for all the species in the gas stream ofthe experiments carried out in this study.

This limit was not always achieved. The limit of detection for species in the gas streamcould differ from one set of experiments to another according to the value of the baseline noise. A noisy background was found when some particles from the reactor exhaustdeposited over the mirrors and windows of the optical pass gas cell. Regular cleaning forthe optical cell mirrors and windows were carried out during this study. Noisy baselineswere also noticed with in situ measurements where the MCT detector was placed close tothe high voltage transformer. This position caused electrical interference with the signaland resulted in a high noise RMS value.

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Figure 2.16: An example of the spectrum used to calculate the limit of detection. (a) showsthe background noise with a resolution of 1 cm−1, while (b) shows the HCN spectrum for 1ppm with a resolution of 0.1 cm−1. Both spectra are for 5.3 m optical pathlength.

Figure 2.17: An example HCN limit of detection calculation. The intercept point betweenHCN peaks heights and the minimum accepted signal to noise ratio. HCN limit of detectionis about 15 ppm.

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Table 2.1: The limit of detection in ppm for all the species in the gas stream for theexperiments carried out.

Chemical components Limit of detection (ppm)

DCM 24CH3Cl 28C3H6 30CCl4 3HCl 20HCN 15CO 16CO2 3

NOCl 1COCl2 5

NO 13NO2 4N2O 1

2.4.1 Optical emission spectroscopy

This technique measures electronic spectra in the UV-visible region. The electricfield generated between the electrodes in the plasma reactor provides the electrons ingas molecules and atoms with high energy. This gained energy produces highly excitedmolecules where electrons have jumped to a higher electronic energy level. However,molecules tend to return back to the lower and more stable energy level, emitting photonsat specific wavelengths in the UV-visible region (110). An optical emission spectrometerdetects photon emission in this region. Spectra obtained provide information about speciespresent within the plasma and their electron excitation, which in turn enables the calculationof vibrational and electron temperature within the plasma. The UV-visible emission spectrafrom the plasma reactor were obtained with an Ocean Optics USB 2000 spectrometerwhich consists of a lens connected to a detector via fibre optic cable. The lens was placedat a distance of about 10 cm from the plasma reactor. The integration time was set to 500ns and the spectra were averaged over 50 scans. Peaks have been identified using standardtables which have been stated by Pearse (117)and also by Striganov (118) , The NISTdatabase has also been used to identify the peaks (113; 119). These references providethe information on the excited molecules or atoms that exist in the examined plasmasaccording to the specific wavelength and intensity for each species. Figure 2.18 shows anexample of an optical emission spectrum for a nitrogen plasma.

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Figure 2.18: Nitrogen plasma emission

Figure 2.19: Simplified sketch of the gas delivery system

2.5 Gas delivery system and the calculations of the gasflow rates

MKS mass flow controllers were used to control the concentration of gases enteringthe plasma reactor. These mass flow controllers were sent, at the start of this study, to themanufacturer for calibration. Experiments carried out for this study, use several gases witha wide range of concentrations. Mathematical calculations depending on a mass balancewere carried out to determine the flow rate set for each mass flow controller. An examplecalculation for a total flow rate of 1 L min−1 for a gas mixture of 500 ppm dichloromethane,1 percent oxygen and remainder nitrogen is presented.

Figure 2.19 shows air (21 % O2 and 79 % N2) and nitrogen flow through three mass

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flow controllers A, B and C. Mass flow controller (A) controls the amount of air and hencethe concentration of oxygen in the reactor feed stream. Nitrogen flow through (C) bubblesthrough liquid DCM, controlling the amount of dichloromethane entering the plasmareactor. The flow rate in (B) was changed to maintain a total flow rate of 1 L min−1 forthe reactor inlet gas. Mass balance calculations were carried out to determine the requiredflow rate of gases in each gas line. Over all material balance: A+B+C = L = 1Lmin−1

Dichloromethane balance: x C = 500 × 10−6 × 1

Where x is the mole fraction of the dichloromethane in the gas. x is calculated usingDalton’s law of partial pressure:

x =pi

ptot(2.5)

Where pi is the partial vapour pressure of dichloromethane which is calculated usingthe Antoine equation, and ptot is the total pressure equal 760 mmHg (atmospheric pressure).The Antoine equation:

log10(pi) = a−(b÷ (T + c)) (2.6)

Where T is dichloromethane temperature of -20 ◦C, a, b and c are Antoine coefficients fordichloromethane, where: a is 7.11464, b is 1152.41 and c equals 232.442 (120).

Oxygen overall balance: 0 ·21 ×A = y× (1)

Where y is the mole fraction of oxygen in the feed gas. Solving these equations givesthe flow rates in each of the mass flow controllers A, B and C. Mass flow controllers arecalibrated for air and nitrogen, corrections factors should be taken in considerations whenusing other gases.

Similar calculations have been made for different percentages of oxygen, dichloro-methane concentration and total flow rate. FTIR measurements showed about 500 ppmof dichloromethane in the gas stream. Oxygen and nitrogen concentrations cannot bemeasured using the FTIR spectrometer as they are not infra-red active.

2.6 Error calculations

There are two main types of errors occurring in this work. The first one is the error indetermining concentration. This error comes from the difference in the resolution betweenthe standard and experimental spectra. The second error comes from the change in the

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dichloromethane temperature in the bubbler leading to a change of the concentration ofDCM entering the plasma reactor. As for the standard deviation errors, measurementsfor a standard sample of 500 ppm CO have been repeated in the exact conditions ofresolution, pathlength, temperature and pressure. Fifty spectra were recorded and thestandard deviation of the measured concentration was less than 1 %, therefore these errorshave been ignored.

2.6.1 The error in concentrations calculation.

To find the error taking place when comparing the standard spectra with a resolution of0.1 cm−1 with the experimental spectra with a resolution of 1 or 2 cm−1, the followinginvestigations were carried out. A standard gas sample containing (500 ± 1.5%) ppm ofcarbon monoxide in nitrogen was analysed using the Bruker Equinox 55 FTIR spectrometerwith a resolution of 2 cm−1. Fifty runs of gas measurements using the FTIR spectrometerwere made. The concentration of carbon monoxide in each run was calculated using themethod explained above. Equation 2.3 was used to measure CO concentrations. Theaverage value of CO concentration which is equal to 474.8 ppm was used to calculate theerror taking place in the concentration measurements. The standard sample contains (500± 1.5%) ppm of CO, while the average measured concentration via FTIR spectrometer is474.8 ppm. The error was calculated as an average between the upper and lower limit asfollows:

Upper limit error= ((500+1.5%)−474.8)×100(500+1.5%) = 6 ·44%

Lower limit error= ((500−1.5%)−474.8)×100(500−1.5%) = 3 ·59%

Average error= 6.44+3.592 = 5 ·02− ≈ 5%

These calculations show an error of about 5 percent in the concentration measurements.This error is mainly due to the resolution difference between the standard spectra obtainedfrom Pacific Northwest National Laboratories (PNNL) data base (113) and the measuredspectra using the FTIR spectrometer. This resolution difference affects the value of theintegrated area beneath each peak, leading to an error in the concentration calculations.Figure 2.20 illustrates this difference, standard spectrum for 500 ppm CO with a resolutionof 0.1 cm−1 is shown in blue and experimental spectrum for about 500 ppm CO with aresolution of 2 cm−1 is in red. The black spectrum is the subtraction of the two spectra.Due to the difference between the two spectra as a result of resolution difference, thesubtraction spectrum is not a straight line.

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Figure 2.20: Standard and experimental spectra for 500 ppm CO. The standard spectrumwith a resolution of 0.1 cm−1 is in blue and the experimental spectrum with a resolution of2 cm−1 is in red. The black spectrum presents the subtraction of the two spectra.

2.6.2 Error caused by DCM temperature change

The liquid DCM in the bubbler was cooled down to around -20 ◦C by placing thebubbler in a coolant of 50% water and 50% antifreeze and using a HAAKE EK45 chillerunit. The chiller unit has a sensitivity limit for temperature change. The temperaturefluctuates between ±1◦C resulting in a change of the concentration of DCM enteringthe plasma reactor. The effect of temperature change on DCM concentration has beencalculated using Dalton’s law equation (2.5) and Antoine equation (2.6). Substitutingdichloromethane temperatures T = - 19 and -21 ◦C into the Antoine equation and thencalculating the concentration of DCM, x, gives concentrations of 530 and 470 ppm re-spectively. A difference of about 30 ppm occurs when the dichloromethane temperaturefluctuates within two degrees range. This difference resulted in an error of about 6 percent.

Error = 30×100500 = 6%.

Thus, for DCM calculations, the experimental error is the sum of the concentrationerror and the temperature change error, 5 +6 = 11%. However, for the plasma by-product,the error comes from the concentration error only, which is equal to 5 percent.

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Figure 2.21: Voltage and current waveforms measured using a pico scope.

2.7 Electrical measurements and calculations

Plasma reactors were powered using two types of power supplies. The first one is aHansen neon sign power supply with a frequency of 20 kHz. The applied voltage couldbe changed using a key on the side of the transformer. Turning the key clockwise up to360◦increases the output voltage of the transformer. The second type is also a neon signtransformer with a frequency of 20 kHz which has been attached to a variac to change theinput voltage and thus, the output voltage is changed. A high voltage probe (TES TECHVP - 15HF) and a current probe (Pearson(TM) current monitor model 411) connectedto a Pico scope (Pico Technology ADC-216) have been used to characterize the suppliedpower. Both the power supplies provide pulses of voltages and currents. Pulse repetitionis 100 pulses per second with pulse duration of 10 ms. Figure 2.21 shows an example ofvoltage and current waveforms.

The average power delivered to the plasma reactor is calculated by multiplying voltageand current and integrating over one pulse duration.

Pt =∫ t

0

√V 2

t × I2t dt (2.7)

Where P is the deposited power measured in watts, t is the pulse duration in seconds, Vis the voltage applied to the plasma reactor measured in volts, and I is the input currentmeasured in amperes. Figure 2.22 shows the power measured over one pulse duration

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Figure 2.22: Power wave form over one pulse.

of 0.01 seconds. The power graph was integrated over one pulse duration using Origin’sintegration tool. The integration area for this example was 0.2479, dividing this area over0.01 seconds gives a power equal to about 25 watts.

The specific input energy (SIE) which is the energy deposited in the gas per unit volumewas used in all the investigations carried out in this study. SIE calculated by dividing themeasured energy per second (W) over the gas flow rate with a unit of L s−1.

SIE(JL−1) =P(Js−1)

Q(Ls−1)(2.8)

For example, using a total flow rate of 1 L min−1 and a deposited power of 25 watt givesan energy density of SIE = 25(Js−1)

1/60(Ls−1)= 1500 J L−1

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2.8 Time required to reach a steady state plasma in nitro-gen and argon gas streams generated in a packed-bedplasma reactor

When starting to generate plasma in the reactor, experiments showed that the plasmabecomes more intense with time until it gets to a stable point. This point is wherethe gas temperature and consumed power are constant meaning that their influence onVOCs decomposition is stable. This is mainly due to thermal effects. Before starting theexperiments the reactor and the dielectric material is at room temperature which could beanything between 20 to 1 ◦C in winter time. Initiating the plasma causes the temperature ofthe dielectric material and the reactor body to rise up to around 75 ◦C (348 K). The rise inthe gas temperature is due to the occurrence of hot spots in the contact points between thedielectric beads. Strong micro discharges appear at the bead’s contact points (121). Heattransfer take place between these micro discharges and the passing gas stream resultingin a rise of the temperature of the plasma reactor. As these strong micro discharges arevery small and well distributed over the packed bed volume, the temperature in the plasmareactor does not get very hot.

Temperatures in the range of 350 K do not have any significant effect on the plasmachemistry. Harling et al. (79) have found that a packed-bed plasma reactor temperatureof higher than 150 ◦C (423 K) is required to enhance the decomposition efficiency ofdichloromethane. Burns et al. (122) have reported that the rate coefficient of electronattachment to dichloromethane increased with temperatures in the range of 467 K. Thethermal decomposition of dichloromethane take place at temperatures in the range of 800K and above (123).

Experiments have been carried out to investigate the time which is needed to generateplasma of a steady state with nitrogen and argon gas streams. This has been determinedby the removal efficiency of dichloromethane, the intensity of plasma emissions, and theconsumed energy over time in a packed bed plasma reactor. A steady state is reached whenrelatively constant values for DCM decomposition, intensity of the plasma emissions, andthe applied power were obtained.

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Figure 2.23: The removal efficiency of 500 ppm of dichloromethane in nitrogen and argonplasma over thirty minutes starting from the moment of initiating plasma. Gas streamswith 0 and 1 % oxygen in a total flow rate of 1 L min−1 were used.

2.8.1 Dichloromethane decomposition in nitrogen and argon plasmaover time

The removal efficiency of dichloromethane in nitrogen and argon plasma was measuredevery minute from the moment of initiating plasma until 30 minutes. Figure 2.23 showsthe removal efficiency of 500 ppm of dichloromethane in nitrogen and argon plasma overthirty minutes. Gas streams with 0 and 1 % oxygen in a total flow rate of 1 L min−1 wereused. Results show that a constant value of dichloromethane decomposition was achievedin all test situations after about 15 minutes from initiating plasma.

2.8.2 Nitrogen and argon plasma emissions over time in a packed-bed plasma reactor.

The Ocean Optics USB 2000 UV-Visible emission spectrometer was used to measureplasma emissions over time. Figures 2.18 and 2.24 show examples of nitrogen and argonplasma emissions respectively.

The intensity of excited nitrogen molecule N2(0-0) and argon atoms at 337 nm and763.5 nm wavelengths respectively were measured every minute from the moment of

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Figure 2.24: UV visible emission from argon plasma after about fifteen minute of initiatingplasma in a packed-bed plasma reactor

initiating plasma to thirty minutes, see figure 2.25. Results show that after about tenminutes of plasma generation, the intensity of excited nitrogen molecules and argon atomslevelled off.

2.8.3 Consumed power and plasma temperature over time

The consumed power over thirty minutes in a nitrogen plasma has been measured aswell as the temperature of the outside packed-bed plasma reactor body. The reactor bodyis a Pyrex glass tube with a thickness of about 1.2 mm. It is thought that the temperatureinside the reactor is of 2 to 3 degrees higher than the temperature on the outside tube. Itwas not possible to verify whether the gas in the reactor was getting heated to about 77◦C after 15 min or is it just the barium titanate beads which get heated. The temperatureof the plasma exhaust gas at a distance of about one meter from the reactor was about 20◦C. The temperature of the reactor body was measured using an RS thermometer attachedto the outer surface of the packed-bed reactor. The in-line gas analyses using the FTIRspectrometer takes place at a 1.5 m distance from the plasma reactor. Results showed thatboth the consumed energy and the temperature of the plasma reactor levelled off afterabout fifteen minutes of plasma. Consumed power increased with time up to about 17 Wwith fifteen minutes. Increasing the time after that did not result in changing the deposited

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Figure 2.25: The intensity of excited nitrogen molecules and argon atoms in packed-bedplasma is measured over thirty minutes. Total flow rate of 1 L min−1 and an energy densityof about 1000 J L−1 were used.

Figure 2.26: Deposited energy consumption and the temperature of the reactor body fornitrogen plasma as a function of time. Measurements were taken every minute from themoment of initiating plasma and up to 30 minutes

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power. Temperature rose from 2 ◦C to about 75 ◦C from the moment of initiating plasmaand up to about fifteen minutes respectively.

The previous experiments showed that about fifteen minutes was required to ensuregetting a standard state plasma. It is thought that the thermal effect is the main reason forthe need of 15 min to obtain a constant characteristic of plasma. Such plasma behaviourhas been noticed by several groups. Martin et al (124) reported that 15-20 min was neededto get an equilibrium before starting recording the data. Yamamoto et al. (68) reportedthat plasma took 30 min to get to a station point of plasma reactor temperature. Hsieh et

al.(100) reported the need for about twenty minutes to achieve a stable plasma. Chavadejet al. (125) showed that the plasma system reached a steady state after 30 minutes.

2.8.4 Summary

This section described the methods used for this work. The plasma system, spectro-scopic techniques, sampling, analysis methods and calculations are explained. In thisstudy, all the plasma measurements were taken after about twenty minutes from initiatingplasma in the packed-bed plasma reactor. A time of ten minutes was allowed between themeasurements to make sure that the gases in the multiple pass optical gas cell are fullyreplaced with the new concentrations of gases.

The removal efficiency of dichloromethane using non-thermal plasma generated in apacked-bed plasma reactor is investigated in the next chapter. The influence of severalprocess parameters on dichloromethane decomposition is discussed.

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Chapter 3

Remediation of dichloromethane, CH2Cl2 usingnon-thermal plasma generated in a packed-bed plasmareactor

3.1 Introduction

This chapter investigates the use of non-thermal plasma technology for the decom-position of dichloromethane as an example of a chlorinated volatile organic compound.Chlorinated VOCs are air pollutants that affect both the environment and human health.Dichloromethane is released to atmosphere via a variety of industrial processes such as oilrefining; surface coating, painting and printing; plastics manufacturing; pharmaceuticalsproduction; vehicle exhaust; and biomass burning (7; 14; 20–22; 37–39).

This work starts by repeating the investigation carried out by Fitzsimmons et al. (76)into the influence of oxygen concentration on DCM removal efficiency and the formationof plasma end products using a packed-bed plasma reactor. A comparison between theresults obtained in this study and Fitzsimmons et al.’s results is discussed.

This work proceeds into investigating the effect of a variety of other parameters, whichwere not investigated by Fitzsimmons et al., such as DCM initial concentration, energydensity, residence time and background gas on the removal efficiency of DCM in non-thermal plasma. Initial kinetic simulation work for the decomposition of dichloromethanein non-thermal plasma was carried out and is presented at the end of this chapter.

The aim of this chapter is to identify the influence of key process parameters on theremoval efficiency of dichloromethane in a non-thermal plasma generated in a packed-bedplasma reactor. With the investigation carried out, it was then possible to optimize theparameters which play a key role in dichloromethane remediation to achieve a maximumremoval efficiency in air plasma.

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3.1.1 Experimental setup

Figure 3.1 presents the inlet system used in these experiments. MKS mass flowcontrollers were used to control the concentration of the gases entering the plasma reactor.Nitrogen or argon gas was bubbled though a liquid DCM with a purity of 99.6 %. The flowrate of nitrogen and the temperature of the liquid DCM in the bubbler were controlled toobtain about 500 ppm DCM in the inlet gas stream of the reactor. The liquid DCM wascooled down to around -20 ◦C by placing the bubbler in a coolant of 50% water and 50%antifreeze and using a HAAKE EK45 chiller unit. The calculations of the gas flow rateswere discussed in the methodology chapter. Gate valves were used to change the directionof the gas flow as required. Oxygen concentration varying from 0 to 21 percent was addedto the gas stream using an air cylinder with nitrogen plasma and oxygen cylinder withargon plasma. The BOC gases were used as delivered, and consisted of air (79% nitrogen,21% oxygen), oxygen (99.99%), nitrogen(99.998%, oxygen free) and argon (99.999%).A Hansen neon sign power supply providing a high voltage of about 14 kV pk-pk at afrequency of 20 kHz was used to generate plasma. A Tektronix 1000 to 1 TES TEC HVP- 15HF voltage probe and Pearson (TM) current monitor (model 411) have been used tocharacterize the supplied power. The plasma was generated using a single stage packed-bedplasma reactor with a packed bed volume of 24.9 cm3 allowing a residence time of 0.95s. Plasma reactor feed gas and products were analysed using an FTIR spectrometer asexplained in the methodology chapter. The inlet system was designed to allow bypassingthe plasma reactor to monitor the input gas at any stage of the experiment.

Measurements were carried out after about 15 minutes after initiating the plasma in thereactor to insure that a steady state was achieved. This was discussed in the methodologychapter. Most experiments were repeated at least three times and an average result ispresented.

An example in-line FTIR spectra for dichloromethane decomposition products with 0and 3% of added oxygen is shown in figure 3.2 The main plasma end products are shown.

Results and discussion

This chapter investigates the influence of process parameters such as oxygen concentra-tion, initial dichloromethane concentration, energy density, residence time and backgroundgas on the removal efficiency of dichloromethane. A comparison of the effect of oxygenconcentration results in this work with Fitzsimmons et al. (76) is discussed.

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Figure 3.1: Plasma inlet system which had been used to investigate the influence of avariety of parameters on the remediation of dichloromethane.

3.2 Influence of oxygen concentration on the removal ef-ficiency of DCM.

These experiments have been carried out using the inlet system showed in figure 3.1.A total flow rate of 1 L min−1 allowing a residence time of 0.95 s for the gas streamin the plasma region and an energy density of about 1000 J L−1 were used. A gasstream consisting of an initial concentration of approximately 500 ppm dichloromethanein nitrogen was used. Oxygen concentration varying from 0 to 21% was added to the gasstream.

Figure 3.3 shows the influence of oxygen concentration on the removal efficiencyof dichloromethane. Results show a maximum DCM decomposition with 2 to 4 % ofadded oxygen to the gas stream. After that increasing the oxygen concentration resulted indecreasing the removal efficiency of dichloromethane (126).

In nitrogen plasma without the addition of oxygen, dichloromethane decomposedmainly as a result of reactions with nitrogen atoms as well as electron impact and otheractive species reactions with DCM. The enhancement in the decomposition efficiencyof dichloromethane with the addition of small oxygen concentration to the gas stream isdue to the participation of active oxygen atoms and molecules as well as OH radicals in

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Figure 3.2: FTIR spectra for 500 ppm dichloromethane after plasma reactor (a) nitrogenplasma without adding oxygen (b) nitrogen plasma with adding 3 % oxygen to the gasstream. A multiple pass optical gas cell with a 5.3 m pathlength, a total flow rate of 1 Lmin−1 and an energy density of about 1000 J L−1 were used. Measurements were takenabout 0.75 seconds downstream of the plasma reactor.

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Figure 3.3: The removal efficiency of 500 ppm of dichloromethane in nitrogen non-thermalplasma as a function of oxygen concentrations in a packed-bed plasma reactor. A totalflow rate of 1 L min−1 and an energy density of about 1000 J L−1 were used.

attacking dichloromethane molecules. Increasing the oxygen concentration more than 4percent resulted in decreasing the removal efficiency of dichloromethane.

Increasing the oxygen concentrations in the plasma gas stream results in the form-ation of more oxygen atoms due to electron attachment. Electron attachment to O2 ismore favorable compared with electron attachment to dichloromethane (127). However,dichloromethane decomposition via electron attachment is a very slow reaction and is nota significant reaction for DCM decomposition (76; 127–129). The reaction pathway forthe decomposition of dichloromethane in non-thermal plasma reactor is fully explained ina further coming section.

Fitzsimmons et al. (76) found a similar relationship for DCM decomposition innitrogen-oxygen gas mixture. They found that the mean electron energy was almost thesame in pure nitrogen plasma and in air plasma with less the 1 % difference. This impliesthat the nature of electron impact processes are not affected by the addition of 21 % oxygento the plasma gas stream. They showed that the reduction of DCM decomposition withincreasing the oxygen concentration over 3 % is due to the formation of NOx and ozone athigher oxygen concentrations.

The formation of ozone and NOx consumes oxygen and active nitrogen atoms and

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molecules leading to less decomposition of dichloromethane. Ozone has a low reactionrate with most VOCs comparing with oxygen radicals (130–132). As a result of ozoneand NOx formation at oxygen concentrations higher than 4 %, the destruction efficiencyof DCM decreased. Other studies have found similar results for the influence of oxygenconcentration on chlorinated hydrocarbons decomposition in plasma (133; 134).

Ozone as an end plasma product was not detected in Fitzsimmons et al. nor in thecurrent work. This does not mean that it has not been produced in the plasma. It couldbe consumed in the plasma between the pulses, or decomposed after the plasma reactorbefore the gas gets to the FTIR spectrometer. However, nitrogen oxides were formedwith the addition of oxygen and their concentration increased with increasing the oxygenconcentration. A full investigation of nitrogen oxides formation in plasma is discussed in asubsequent chapter.

3.2.1 The effect of oxygen concentration on the formation of plasmaend products.

The main plasma end products for dichloromethane decomposition in nitrogen plasmawithout oxygen additions are HCN, HCl and CCl4. Small concentrations of CO, CO2, N2Oand COCl2 were also detected indicating the presence of oxygen in the system. This couldbe due to a small amount of water vapour in the inlet system as well as some impuritiesin the nitrogen and dichloromethane gases, it also could be from barium titanate. About80 % of the decomposed carbons were detected after the plasma reactor, mainly as HCN.The 20 % of missing carbon is thought to be due to the formation of soot in the plasmareactor (100). Soot deposition in the packed-bed region over the dielectric beads and theinner side of the glass tube was observed.

The effect of soot deposition on the chemistry taking place in the plasma region isminimal. The frequency of radical reaction with soot particles is of several magnitudesless than that of the reaction of these radicals in the gas phase (135). The interactionbetween radicals and soot particles is mainly oxidation of the soot by O and OH radicalsforming CO and H2 molecules. NO2 can be adsorbed on the surface of the soot andremains there if the temperature is less than 333 K. However, for temperatures more than350 K, the desorption of NO, HNO2 and CO may take place (136; 137). Dorai et al. (135)has found that soot deposition in a plasma discharge with temperatures of 453 K leadsto an improvement of NOx remediation by converting NO to HNO2. They also foundthat soot deposition leads indirectly to increasing the amount of OH radicals. This in

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turn enhances the decomposition of hydrocarbons in the gas stream. They also found thatelectron attachment to soot has a negligible effect on the plasma physics and chemistry. Asfor the experiments carried out, the gas temperature ranges between 340 and 350 K. Thedesorption of NO, HNO2 and CO by soot are minimal in this temperature range. Reactorcomponents were cleaned regularly to get rid of soot deposition. Oxygen plasma was alsoused to oxidize and remove carbonated depositions.

From the chlorine mass balance, HCl, CCl4 and COCl2 were the main chlorine productsdetected after the plasma reactor. Figure 3.4 shows 40 % of missing chlorine in the chlorinebalance, this is probably due to the formation of chlorine molecules (Cl2) which cannot bedetected using FTIR spectroscopy. The chlorine balance was calculated as follow:

Chlorine balance % = ([HCl]+ 4[CCl4]+ 2[COCl2]+2[CH2Cl2out])× 100 / 2[CH2Cl2in]

With the addition of oxygen to the gas stream, the main plasma end products fromdichloromethane decomposition are CO, CO2, NOCl, and small concentrations of COCl2.Nitrogen oxides and N2O were also formed. HCN, HCl and CCl4 were not detected afterthe addition of oxygen to the gas stream. This is due to the decomposition of these productsby the reaction with active oxygen and OH radicals.

The carbon balance presented in figure 3.4 shows 45 to 50 % missing carbon withthe addition of 2 to 4 percent of oxygen to the gas stream. This is due to the increasedformation of soot with increasing the decomposition of DCM which takes place at 2 to 4percent of added oxygen. Carbon balance in the case of adding oxygen to the gas streamwas calculated as follow:

Carbon balance % = ([CO]+[CO2]+[CH2Cl2out])× 100 / [CH2Cl2in]

The only chlorine component detected after the plasma reactor with the addition ofoxygen is NOCl. NOCl concentrations account for a small amount of the decomposedchlorine in the reactor. The missing chlorine is probably due to the formation of chlorinemolecules which is FTIR inactive.

Figure 3.5 shows the influence of oxygen concentration on the formation of CO, CO2

and NOCl. A total flow rate of 1 L min−1 and an energy density of about 1000 J L−1 wereused. Results showed that adding oxygen to the gas stream resulted in a fast increase ofCO formation. Increasing the oxygen concentration did not lead to further increase ofCO concentration. This is due to the oxidation of CO to CO2 with higher percentages ofoxygen in the gas stream. CO2 concentration increased linearly with an increase of theoxygen amount in the plasma as a result of CO oxidation. NOCl concentrations increased

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Figure 3.4: Carbon and chlorine balance for the decomposition of 500 ppm of dichloro-methane as a function of oxygen concentration in nitrogen non-thermal plasma. A totalflow rate of 1 L min−1 and an energy density of about 1000 J L−1 were used.

Figure 3.5: The influence of oxygen concentration on the formation of CO, CO2 and NOClas plasma end products for DCM decomposition in nitrogen non-thermal plasma. A totalflow rate of 1 L min−1 and an energy density of about 1000 J L−1 were used.

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with increasing the oxygen concentration up to 9 %. After that, increasing the oxygenpercentage did not have a clear effect on NOCl concentrations, which levels off at 30 to35 ppm. This is due to the link between NO formation (138), CH2Cl2 decomposition andNOCl formation. As NO concentration increased and CH2Cl2 decomposition decreasedwith increasing the oxygen concentration, NOCl concentration levelled off.

NO+Cl−−→ NOCl (R3.2)

3.3 Influence of initial dichloromethane concentrationson the removal efficiency.

The effect of dichloromethane concentration in the feed gas on the removal efficiencyof dichloromethane and the formation of plasma end products in a packed-bed plasmareactor are investigated in this section. Figure 3.6 shows the influence of initial concentra-tions of dichloromethane varying between 100 and 1000 ppm on the removal efficiencyof dichloromethane in non-thermal plasma generated in a packed-bed plasma reactor.Experiments were carried out in nitrogen plasma without the addition of oxygen andwith the addition of 3 percent oxygen to the gas stream. Results show a linear decreasein the removal efficiency with increasing dichloromethane concentration in the feed gaswith both 0 and 3 percent of added oxygen and a constant flow rate. Increasing the feedconcentration of dichloromethane requires larger amount of active species in the plasmato carry on the decomposition process. Active species in the plasma could be increasedby changing plasma parameters such as energy density and residence time. In the sameplasma reactor and plasma conditions of energy density and residence time, increasingthe initial concentration of dichloromethane leads to less decomposition. For practicalapplications, it is important to identify the amount of dichloromethane which could beremoved in the plasma system with different feed concentrations.

3.3.1 The effect of initial dichloromethane concentration on the form-ation of plasma end products.

The main plasma end products for dichloromethane decomposition in nitrogen plasmawere HCN and HCl. A small concentration of CCl4 was also detected. The main plasmaby-products with the addition of oxygen to the gas stream were CO, CO2, NOCl, NOx, and

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Figure 3.6: The removal efficiency of dichloromethane as a function of initial dichlorometh-ane concentration and two oxygen concentrations of 0 and 3 % in a packed-bed plasmareactor. A total flow rate of 1 L min−1 and an energy density of about 1000 J L−1 wereused.

N2O. In this section, the influence of initial dichloromethane concentration on HCN, HCl,CO, CO2 and NOCl formation is investigated. Nitrogen oxides formation in the plasmawith the presence of chlorinated hydrocarbons is fully discussed in chapter seven. TheCCl4 concentration in nitrogen plasma without the addition of oxygen was too small forany investigation to be carried out effectively.

Figure 3.7 shows the influence of dichloromethane concentration in the feed gas onthe formation of HCN and HCl as plasma end products. A total flow rate of 1 L min−1

and an energy density of about 1000 J L−1 were used. Results show that increasingthe initial concentration of dichloromethane leads to a linear increase of HCN and HClconcentration in plasma products. HCN and HCl concentration is directly linked to thedecomposed amount of dichloromethane in plasma. As the decomposed concentration ofDCM increased from about 65 ppm to 410 ppm from an initial DCM concentration of 100to 1000 ppm respectively, carbon and chlorine concentration increased accordingly leadingto the formation of more HCN and HCl.

Figure 3.8 shows the influence of initial dichloromethane concentration on the forma-tion of CO, CO2 and NOCl in nitrogen plasma with the addition of 3 percent oxygen to thegas stream. A total flow rate of 1 L min−1 and an energy density of about 1000 J L−1 were

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Figure 3.7: The influence of initial dichloromethane concentration on the formation ofHCN and HCl in nitrogen non-thermal plasma. A total flow rate of 1 L min−1 and anenergy density of about 1000 J L−1 were used.

used. Results show that the concentration of CO2 and NOCl increased linearly with increas-ing initial concentration of dichloromethane from 100 to 1000 ppm. CO concentrationsincreased in a logarithmic shape with increasing initial concentrations of dichloromethane.The formation of CO, CO2 and NOCl depends on the amount of carbon and chlorine in theplasma. Increasing the initial concentration of dichloromethane increases the amount ofcarbon and chlorine produced in the plasma as a result of dichloromethane decomposition.This leads to the formation of higher concentrations of CO, CO2 and NOCl with increasingfeed concentration of dichloromethane.

3.4 Influence of energy density on the removal efficiencyof DCM.

The effect of increasing the energy density on the removal efficiency of dichloromethaneis investigated in this section. Energy density was increased from 0 to about 3000 J L−1.An initial concentration of 500 ppm of dichloromethane with both 0 and 3% oxygen wasused. Figure 3.9 presents the removal efficiency of 500 ppm of dichloromethane as afunction of energy density and two oxygen concentrations of 0 and 3%. A total flow rate

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Figure 3.8: The influence of initial dichloromethane concentration on the formation of CO,CO2 and NOCl in nitrogen non-thermal plasma with the addition of 3 % oxygen. A totalflow rate of 1 L min−1 and an energy density of about 1000 J L−1 were used.

of 1 L min−1 was used. Deposited power was increased by increasing the input voltageusing a variac before the transformer.

Results for 0 and 3% oxygen showed that the removal efficiency of dichloromethaneincreased with increasing the energy density. The removal efficiency of dichloromethaneincreased linearly with increasing the energy density in the case of nitrogen plasma withoutthe addition of oxygen to the gas stream. The increase of the removal efficiency was in alogarithmic shape with the addition of 3 percent oxygen to the gas stream. A completeremoval efficiency was achieved with about 3000 J L−1 in the case of adding 3 % oxygento the gas stream.

Increasing the energy density leads to greater collision frequency of electrons and DCMmolecules, resulting in the formation of more excited species in the plasma, which leads toan increase of the decomposition of dichloromethane.

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Figure 3.9: The removal efficiency of 500 ppm of dichloromethane in nitrogen non-thermalplasma as a function of energy density and two oxygen concentrations of 0 and 3% in apacked-bed plasma reactor. A total flow rate of 1 L min−1 was used.

3.4.1 Influence of energy density on the formation of plasma endproduct from dichloroethane decomposition.

The main plasma by-products for dichloromethane decomposition in nitrogen plasmawithout the addition of oxygen to the gas stream were HCN and HCl. With the addition ofoxygen to the gas stream, the main detected plasma by-products were CO, CO2, NOCland nitrogen oxides. The influence of energy density on the formation of HCN, HCl,CO, CO2 and NOCl is investigated in this section. Figure 3.10 shows the formation ofplasma end products from the decomposition of 500 ppm of dichloromethane as a functionof applied energy density. The formation of HCN and HCl from the decompositionof dichloromethane in nitrogen plasma and the formation of CO, CO2 and NOCl fromdichloromethane decomposition in nitrogen-oxygen plasma as a function of energy densityis presented in this figure. Results show an increase in the formation of all plasma endproducts with increasing the energy density from 0 to 3000 J L−1. Increasing the energydensity to about 3000 J L−1, resulted in increasing the decomposition of dichloromethaneto about 90 % in nitrogen plasma and to about 100 % with the addition of 3 % oxygento the gas stream. This increase of the decomposition of dichloromethane leads to theproduction of greater amounts of carbon and chlorine in the plasma which in turn result inmore formation of plasma products such as HCN, HCl, CO, CO2 and NOCl.

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Figure 3.10: Energy density influence on the formation of plasma end products from thedecomposition of 500 ppm dichloromethane in a packed-bed plasma reactor. (a) nitrogenplasma with 0 % oxygen and (b) nitrogen plasma with 3 % oxygen. A total flow rate of 1 Lmin−1 was used.

3.5 Influence of the plasma residence time on the removalefficiency of DCM.

The residence time of the gas in the plasma region was controlled by changing thetotal flow rate of the gas stream entering the plasma reactor. To investigate the effect ofresidence time on the removal efficiency of dichloromethane and the formation of plasmaend products in a packed-bed plasma reactor, flow rates ranging between 0.5 to 2 L min−1

were used. These flow rates give a residence time ranging between 1.95 to 0.49 secondsrespectively.

Figure 3.11 shows the removal efficiency of 500 ppm of dichloromethane as a functionof residence time and oxygen concentration. Two cases were investigated, nitrogen plasmawithout the addition of oxygen to the gas stream, and nitrogen plasma with the addition of3% oxygen.

Results show that increasing the residence time of the gas in the plasma region resultedin increasing the removal efficiency of dichloromethane in nitrogen plasma with andwithout the addition of oxygen. Increasing the residence time of the gas components inthe plasma region, results in the formation of more active species per unit volume in theplasma. This in turn increases the number of reactions and collisions with dichloromethanein plasma leading to more removal efficiency.

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Figure 3.11: The removal efficiency of 500 ppm of dichloromethane in nitrogen non-thermalplasma as a function of residence time of the gas in plasma. Two oxygen concentrations of0 and 3% and a fixed energy density of about 1000 J L−1 were used.

Results show that doubling the plasma residence time did not double the removalefficiency of dichloromethane. This indicates that although plasma residence time is a veryimportant factor in VOC removal it is not the only factor which play a role in increasing theremoval efficiency. Other factors related to the interaction between the chemical reactionstaking place in the plasma play a significant role in the VOC removal process.

3.6 Influence of background gas on the removal efficiencyof DCM

The effect of the carrier gas on the removal efficiency of dichloromethane is investigatedin this section. This work investigates the removal efficiency of dichloromethane inatmospheric pressure non-thermal plasma generated in packed-bed plasma reactor. A gasstream consisting of argon as a carrier gas, dichloromethane and oxygen was used. Acomparison between the results obtained with nitrogen and argon plasma is discussed.

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Figure 3.12: FTIR spectra for about 590 ppm of dichloromethane in argon plasma withoutadding oxygen to the gas stream. A multiple pass optical gas cell with a 5.3 m pathlength,a total flow rate of 1 L min−1 and an energy density of about 1000 J L−1 were used.Measurements were taken about 0.75 seconds downstream of the plasma reactor.

3.6.1 Argon plasma influence on the removal efficiency of dichloro-methane

This work investigates the removal efficiency of about 590 ppm of dichloromethane inargon plasma generated in a packed-bed reactor with the addition of oxygen concentrationsranging between 0 and 21 %. A total flow rate of 1 L min−1 and an energy density of about1000 J L−1 were used. Figures 3.12 and 3.13 show FTIR spectra for the plasma productsfor the decomposition of about 590 ppm dichloromethane in argon with 0 and 5 % oxygenrespectively.

The main plasma end product in the case of argon plasma were HCl and CCl4. Smallconcentrations of CO, CO2, HCN and COCl2 were detected indicating the presence ofoxygen and nitrogen in the plasma. This is due to some impurities in the inlet system aswell as a possible leak. With the addition of oxygen to the gas stream, the main plasma endproducts were CO, CO2, HCl and COCl2. Low concentrations of HNO3 and NO2 werealso detected.

The destruction of DCM as well as the concentration of the plasma products whilevarying the oxygen percentage from 0 to 21 % has been performed. Table 3.1 shows the

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Figure 3.13: FTIR spectra for about 590 ppm of dichloromethane in argon plasma withadding 5 % oxygen to the gas stream. A multiple pass optical gas cell with a 5.3 mpathlength, a total flow rate of 1 L min−1 and an energy density of about 1000 J L−1 wereused. Measurements were taken about 0.75 seconds downstream of the plasma reactor.

removal efficiency of dichloromethane and the concentration of the plasma end products.

In argon plasma, the removal efficiency of dichloromethane was about 70 percent withoxygen concentrations ranging from 0 to 7 percent. Increasing the oxygen concentrationfrom 7 to 21 percent increased the removal efficiency of dichloromethane from 70 to about90 percent respectively. This did not match the work carried out by Falkenstein et al.

(133). They found a maximum decomposition for trichloroethylene in argon plasma withthe addition of 0.3 % of oxygen. Increasing the oxygen concentrations more than 0.3 %reduced the removal efficiency of trichloroethylene.

Figure 3.14 shows a comparison between the removal efficiency of dichloromethane innitrogen and argon plasma as a function of oxygen concentration. The decomposition ofdichloromethane in argon plasma takes place as a result of dissociation, collisions withelectrons and excited argon (Ar∗) as well as reacting with excited atoms and moleculessuch as Cl, H, and CH2Cl. Adding oxygen to the gas stream leads to the formation ofexcited oxygen atoms and molecules as well as the formation of OH radicals. These activeradicals participate in the destruction of dichloromethane and the formation of plasma endproducts. A full discussion of dichloromethane destruction in argon plasma is discussedby Li et al. (99).

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Table 3.1: The decomposition efficiency of 590 ppm dichloromethane and the concentra-tions of all species exist in the plasma exhaust, in ppm. as a function of oxygen percentagesin argon plasma

O2 % DCM DCM % HCl COCl2 CO CO2 CCl4 HCN

0 178 70 190 7 106 4 12 321 195 67 19 5 112 72 195 67 17 4 115 83 188 68 15 4 119 84 182 69 19 4 125 85 176 70 23 5 133 87 177 70 28 5 145 910 120 80 45 5 155 1013 119 83 46 6 167 1115 78 87 48 4 166 817 91 88 51 5 167 1021 66 89 54 6 168 12

Figure 3.14: A comparison of the removal efficiency of dichloromethane in nitrogen andargon plasma as a function of oxygen concentration. A total flow rate of 1 L min−1 and anenergy density of about 1000 J L−1 were used.

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Figure 3.14 shows that adding up to 7 % oxygen to argon plasma did not increase theremoval efficiency of dichloromethane. The amount of active oxygen and OH radicals wasnot enough to influence the removal efficiency of dichloromethane. Oxygen concentrationsin excess of 7 % were required to increase the removal efficiency of dichloromethane.

Compared with nitrogen plasma, the dichloromethane decomposition in argon plasmadid not show a maximum decomposition with 2 to 3 % of added oxygen. More removalefficiency of dichloromethane was achieved in argon plasma compared with nitrogenplasma with oxygen concentrations in excess of 5 percent.

The decrease in the decomposition of dichloromethane in nitrogen plasma with increas-ing the oxygen concentrations is attributed to the formation of nitrogen oxides and ozonewhich consumes active nitrogen and oxygen species. In argon plasma, ozone is still formedby combination between active oxygen atoms and molecules. However, no active oxygenatoms and molecules were consumed by reacting with nitrogen to form nitrogen oxides.Also, the number of excited argon atoms was not consumed in forming any plasma byproducts. The ionization potential and excited energy are lower for argon compared withnitrogen, this leads to a higher plasma density and energy when argon is used resultingin a higher collision frequency with dichloromethane molecules compared with nitrogenplasma. For these reasons, the removal efficiency of dichloromethane in argon plasma isgreater than in nitrogen plasma.

Carbon monoxide was the main carbon produced product in argon plasma. It accountedfor about 32 % of the decomposed carbon. Carbon monoxide concentration increased withincreasing oxygen concentration up to 13 %. After that CO concentration levelled off andincreasing the oxygen concentration did not result in the production of more CO. This isdue to the oxidation of CO with high oxygen concentrations.

Hydrogen chloride was the main chlorinated product produced in argon plasma withoutthe addition of oxygen. HCl concentrations accounted for about 23 % of the decomposedchlorine. Adding oxygen to the gas stream resulted in a fast drop in the concentration ofHCl due to reactions between HCl and active oxygen atoms and molecules. The majorityof decomposed chlorine is converted to molecular chlorine Cl2 which is FTIR inactive.

Our findings support the chemical reaction scheme for dichloromethane decompositionproposed in the literature. Several studies have investigated the reaction pathway forCH2Cl2 destruction in plasma and thermal processes (76; 77; 98; 123; 127; 139). Asummary of the main reactions for CH2Cl2 decomposition in plasma are discussed in thissection.

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3.7 Reaction pathway for the decomposition of dichloro-methane in non-thermal plasma reactor.

In nitrogen plasma, the decomposition of dichloromethane is initiated due to electronimpact with CH2Cl2 molecules in the gas stream. The electron impacts decompose CH2Cl2and generate active chlorine atoms via the following reactions.

e−+CH2Cl2 −−→ CH2Cl+Cl+ e− (R3.3)

e−+CH2Cl2 −−→ CHCl2 +H+ e− (R3.4)

However, the dissociative electron attachment to dichloromethane is very slow with arate coefficient of around 6.5 × 10−13 cm3 s−1 with temperatures of 298 K.

In the case of using nitrogen as a carrier gas, energetic electrons in the plasma collidewith nitrogen molecules leading to the formation of active nitrogen atoms and molecules(71; 98; 127).

e−+N2 −−→ 2e−+N(4S)+N+ (R3.5)

e−+N2 −−→ 2e−+N(2D)+N+ (R3.6)

e−+N2 −−→ 2e−+N+2 (R3.7)

e−+N2 −−→ e−+N(4S)+(N(4S) or N(2D) or N(2P)) (R3.8)

As a result of these initial reactions, active chlorine, hydrogen and nitrogen atoms andmolecules are formed. The decomposition of DCM proceeds after that due to the reactionbetween DCM molecules and the formed active species in the plasma discharge.

CH2Cl2 +Cl−−→ CHCl2 +HCl (R3.9)

CH2Cl2 +Cl−−→ CH2Cl+Cl2 (R3.10)

CH2Cl2 +H−−→ CH2Cl+HCl (R3.11)

CH2Cl2 +H−−→ CHCl2 +H2 (R3.12)

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CH2Cl2 +N2∗ −−→ CH2Cl+Cl+N2 (R3.13)

CH2Cl2 +N∗ −−→ CH2Cl+NCl (R3.14)

Due to these reactions and further reactions, end products such as hydrogen chloride(HCl), carbon tetrachloride (CCl4) and hydrogen cyanide (HCN) were formed.

The addition of oxygen to the gas stream leads to the formation of the radicals O(3P),O(1D) and O2(

1∆) by electron impact (133; 139; 140). Electron attachment to oxygen ismore favorable compared with electron attachment to dichloromethane.

e−+O2 −−→ O(3P)+O(3P)+ e− (R3.15)

e−+O2 −−→ O(3P)+O(1D)+ e− (R3.16)

e−+O2 −−→ O2(1∆)+ e− (R3.17)

The formation of these active oxygen radicals allows for faster DCM removal, mainlyvia the following reactions.

CH2Cl2 +O(1D)−−→ CH2Cl+ClO (R3.18)

CH2Cl2 +O(3P)−−→ CHCl2 +OH (R3.19)

These reactions generate OH radicals which in-turn contribute to decomposing DCM .

CH2Cl2 +OH−−→ CHCl2 +H2O (R3.20)

Further reactions and oxidations of the reaction products take place in the plasma leadingto the formation of plasma end products such as carbon monoxide (CO), carbon dioxide(CO2) and phosgene (COCl2) in argon plasma. In nitrogen plasma, nitrogen oxides NOx

and nitrosyl chloride (NOCl) are also formed.

3.8 Initial kinetic simulation work

A chemical kinetic package, CHEMKIN II, was used to support the experimental work.The model used in this work is an adaptation of the reaction pathway originally reportedby Fitzsimmons et al.(76; 141). Fitzsimmons et al. developed a reactions scheme for thedecomposition of dichloromethane in nitrogen-oxygen plasma. This model assumes auniform processing of the gas in the plasma region. It also assumes that the formationof active species takes place at each half cycle of the microdischarge current pulses. A

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fresh feed of the active species is added at each half cycle. The chemistry is initiated andprogresses until the end of the pulse. For example, in a discharge with a frequency of 20kHz and a plasma residence time of 0.9 seconds, there are 18000 cycles. This means thereare 36000 Pulses for integration, with 0.25000E-04 s time interval for each pulse. Theconcentration of the plasma end products is produced at the end of the process.

Two files are required for running CHEMKIN II, an input file and a parameter file. Theinput file contains all the chemical reactions which are assumed to take place in the plasmaregion. The rate constants are calculated using the modified Arrhenius equation (142):

K = ATnexp−EaRT (3.1)

Where A is a temperature independent constant, T is the temperature of the gas (K), n is aconstant. The unit for the preexponential factor is mol-cm-s-K. The activation energy Ea isin (cal/mol) and R is the universal gas constant (cal/K mol). A, n and Ea should be specifiedin the input file. The other required file is the parameter file. This file contains all theconditions of the plasma discharge such as pressure, temperature, initial gas concentrations,residence time, discharge frequency and active species concentration. Active speciesconcentration was a best guess depending on energy distribution function. A list of theproposed chemical reactions for DCM decomposition can be seen in the appendix. Thislist is adapted from Fitzsimmons et al. work (76; 141).

The influences of oxygen concentration and plasma residence time on the removalefficiency of dichloromethane were investigated. A comparison between the model and ex-perimental results are shown in figures 3.15 and 3.16. Figure 3.15 shows the decompositionof 500 ppm of dichloromethane as a function of oxygen concentration. A gas temperatureof 348 K, residence time of 0.95 seconds and a total flow rate of 1 L min−1 were used.Simulation results show a maximum dichloromethane decomposition with the addition of1 percent oxygen to the gas stream. Increasing the oxygen concentrations decreased theremoval efficiency of dichloromethane. These results are in a good agreement with theexperimental results.

The influence of plasma residence time on the decomposition of dichloromethane ina gas mixture of nitrogen and 3 percent oxygen was simulated. The residence time wasvaried between 0.49 and 1.95 seconds. Figure 3.16 shows a comparison between theexperimental and modelling results. Results show that the simulation results are in a goodagreement with the experimental results.

However, the model results for the concentration of the plasma end products such as

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Figure 3.15: A comparison of the simulation and experimental results for the removalefficiency of dichloromethane in nitrogen plasma as a function of oxygen concentration. Atotal flow rate of 1 L min−1 was used.

Figure 3.16: A comparison of the simulation and experimental results for the influence ofresidence time on the removal efficiency of dichloromethane in nitrogen and 3 % oxygengas mixture. A total flow rate of 1 L min−1 was used.

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Figure 3.17: The influence of oxygen concentration on the formation of CO and CO2 innitrogen plasma. A comparison of the simulation and experimental results.

CO and CO2 was not in an agreement with the experimental results. Figure 3.17 shows acomparison between the model and the experimental results for CO and CO2 concentrationas a function of oxygen concentration.

A comparison of the influence of plasma residence time on the concentrations of CO andCO2 obtained in the model with experimental results is presented in Figure 3.18. Resultsdo not show an agreement between the experimental and simulation results. However, theconcentration of CO and CO2, increased with increasing the residence time for both thesimulation and experiments.

More work is required to improve the simulation model, in order to obtain a betteragreement for the formation of plasma end products with the experimental results.

3.9 Comparison between the results obtained in this in-vestigation for oxygen concentration influence and otherwork.

This work started by repeating Fitzsimmons et al. (76) investigations for the effect ofoxygen concentration. Fitzsimmons et al. investigated the effect of oxygen concentration

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Figure 3.18: A comparison of the simulation and experimental results for the influence ofresidence time on the formation of CO and CO2 in nitrogen and 3 % oxygen gas mixture.A total flow rate of 1 L min−1 was used.

on the removal efficiency of 500 ppm of dichloromethane in a non-thermal plasma gen-erated in a packed-bed plasma reactor. They used a total flow rate of 1 L min−1 givinga residence time of 0.25 seconds for the gas stream in plasma region. An AC powersupply providing a voltage of 15 kV peak to peak and a frequency of 10 kHz were used inFitzsimmons experiments. Barium titanate beads were used as the dielectric material inthe packed-bed plasma reactor.

Table 3.2 shows a comparison between Fitzsimmons et al. results and the results foundin this investigation. The removal efficiency of DCM and the main plasma end productswith three oxygen concentrations of 0, 3 and 21 % are presented in this table. Resultsfor DCM removal efficiency show a similar shape of maximum decomposition at 3 %of added oxygen. However, there is a huge difference between the removal efficiency ofDCM achieved in this work compared with Fitzsimmons et al. results. This difference isdue to the longer residence time of about 0.70 seconds in the plasma region, as well as thedifference in the design of the packed-bed plasma reactors used in both studies.

Fitzsimmons et al. packed-bed plasma reactor design does not allow a good distributionof the gas though the entire volume of the packed-bed. The majority of the gas streampasses through the middle part of the reactor, through a small plasma volume. Figure3.19 illustrate the gas flow through Fitzsimmons reactor and the reactor used for this

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Table 3.2: A comparison between Fitzsimmons et al. results and the results obtained withthe carried out investigations.

O2 DCM decomposition / % Main plasma end products

Fitzsimmons et al. 0 18 HCN3 20 CO, CO2, COCl2, NO2 and N2O

20 12 Same as with 3 % O2

Current investigations 0 53 HCN, HCl, CCl43 75 CO, CO2, NOCl, NOx and N2O

21 54 Same as with 3 % O2

Figure 3.19: A comparison between Fitzsimmons et al. plasma reactor design and thereactor used in this work.

investigation. In the reactor used, the funnels at the inlet and outlet of the packed-bed areaallow for better distribution of the gas over the entire volume of the plasma region. Theeffect of residence time on the removal efficiency of DCM was discussed in a previoussection.

3.10 Summary and conclusions

The influence of several process parameters on the removal efficiency of dichlorometh-ane in an atmospheric pressure non-thermal plasma generated in a packed-bed plasmareactor were investigated in this section. The main findings and conclusions for theseinvestigations are:

• The removal efficiency of dichloromethane in atmospheric pressure non-thermalplasma generated in a packed-bed plasma reactor with nitrogen carrier gas has amaximum at 2 to 4 % of added oxygen.

• The removal efficiency of dichloromethane in nitrogen plasma decreased with in-creasing oxygen concentrations above 5 percent as a result of the formation ofnitrogen oxides and ozone.

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• Increasing the initial concentration of dichloromethane in the feed gas results indecreasing the removal efficiency of dichloromethane in the packed-bed plasmareactor with both nitrogen and a mixture of nitrogen and oxygen gas streams.

• Increasing the energy density in the plasma system increased the removal efficiencyof dichloromethane in both nitrogen and a mixture of nitrogen and oxygen gasstreams.

• A longer residence time for the gas stream in the plasma increased the removalefficiency of dichloromethane as a result of increasing the amount of active speciesgenerated in the plasma which leads to increasing the number of collisions. However,the increase in the decomposition of dichloromethane was not in the same form asthe increase in the residence time. This shows that residence time is not the onlyparameter which plays a role in the plasma process.

• Dichloromethane destruction in argon plasma is greater than in nitrogen plasma withoxygen concentration higher than 5 percent. This is due to the higher energy anddensity of argon plasma compared with nitrogen plasma.

• Increasing the oxygen concentration more than 7 % in argon plasma results inincreasing the removal efficiency of dichloromethane.

• The concentration of plasma end product such as HCl, HCN, CO, CO2 and NOClincreased with increasing the initial concentration of dichloromethane or the appliedenergy density.

Before optimizing the investigated process parameters, the influence of these parameterson the removal efficiency of another chlorinated VOC is investigated. Next chapterinvestigates the influence of several process parameters on the removal efficiency of methylchloride.

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Chapter 4

Remediation of Methyl Chloride, CH3Cl usingnon-thermal plasma generated in a packed-bed plasmareactor

4.1 Introduction

This section investigates the removal efficiency of methyl chloride (CH3Cl) as anotherexample of chlorinated VOCs using non-thermal plasma. As explained in chapter one,chlorinated VOCs are serious air pollutants that affect both the environment and humanhealth. Methyl chloride is one of the simplest chlorinated VOCs which has been released tothe atmosphere via a variety of industrial processes such as power stations and incinerationof municipal solid waste especially plastics which contain chloride (40; 43).

In this work, a non-thermal atmospheric pressure plasma generated in a packed-bedplasma reactor in a nitrogen-oxygen gas mixture is used. The effect of a variety of plasmaand gas mixture parameters on the removal efficiency of methyl chloride as well as theformation of plasma end products were investigated. A neon sign power supply providinga frequency of 20 kHz and about 14 kV pk-pk was used to power the plasma reactor.Applied power was varied by using a variac to control the input voltage of the powersupply. The inlet system explained in chapter three was also used for the work carried outin this chapter. Methyl chloride (99.99%), nitrogen (99.998 %) and oxygen (99.99%) gaseswere used as supplied from BOC. Gas flow was controlled using MKS 247 mass flowcontrollers. Gas analyses were carried out using in line FTIR spectroscopy both before andafter the plasma reactor to identify the chemical species in the gas stream and to calculatetheir concentrations. As there is no correction factor for methyl chloride flow throughthe mass flow controllers, a series of initial experiments were carried out using the FTIRspectrometer to establish the flow settings which give certain concentrations of methylchloride in the reactor feed stream. An example of FTIR spectra for methyl chloride afterthe plasma reactor with 0 and 3 % of added oxygen is presented in figure 4.1 The mainspecies detected after the plasma were HCl and HCN when no oxygen was added to thegas stream. While CO, CO2, NO, NO2, N2O and NOCl were detected when oxygen wasadded to the system.

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Figure 4.1: FTIR spectra for methyl chloride after plasma reactor (a) nitrogen plasmawithout adding oxygen (b) nitrogen plasma with adding 3 % oxygen to the gas stream. Amultiple pass optical gas cell with a 5.3 m pathlength, a total flow rate of 1 L min−1 andan energy density of about 2280 J L−1 were used. Measurements were taken about 0.75second downstream of the plasma reactor.

This chapter presents:

• An investigation of the influence of oxygen concentration, methyl chloride initialconcentration, energy density and plasma residence time on the removal efficiencyof methyl chloride as well as the formation of plasma end products.

• A discussion of methyl chloride reactions in nitrogen and nitrogen-oxygen plasma.

4.2 Influence of oxygen concentration and initial methylchloride concentration on the removal efficiency ofmethyl chloride

This work describes the investigation of the effect of adding different oxygen concen-trations ranging between 0 to 21 % to the gas stream on the destruction efficiency of methyl

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Figure 4.2: The removal efficiency of 500, 600, 700 and 1000 ppm of methyl chloridein nitrogen non-thermal plasma as a function of oxygen concentration in a packed-bedplasma reactor. A total flow rate of 1 L min−1 and an energy density of about 2280 J L−1

were used.

chloride. The effect of the initial concentration of methyl chloride entering the plasmareactor on the removal efficiency of CH3Cl is also investigated.

Figure 4.2 shows the removal efficiency of 500, 600, 700 and 1000 ppm of methylchloride as a function of oxygen concentration in a packed-bed plasma reactor.

The removal efficiency of methyl chloride for all the tested concentrations was verysmall in nitrogen plasma without the addition of oxygen to the gas stream. The decom-position of CH3Cl in nitrogen plasma takes place as a result of electron attachment aswell as reacting with excited atoms and molecules produced in plasma such as N, N2 (A3

Σu+), CH3 and Cl. A removal efficiency of about 15 to 19 % was found when the initial

concentration changed between 500 to 1000 ppm of CH3Cl. This is possibly due to thelow electron attachment rate constant for methyl chloride at temperatures less than 500 K(143). This slow reaction leads to the formation of small amounts of chlorine atoms andCH3 in the plasma and so a less contribution of these active species in the decompositionprocess of methyl chloride in nitrogen plasma.

Adding oxygen to the gas stream enhanced the decomposition efficiency of methylchloride. This is due to the participation of active oxygen atoms and molecules in attackingmethyl chloride molecules as shown in later reactions R4.28, R4.29 and R4.30. Active

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radicals such as OH, ClO and HO2 also contribute to methyl chloride consumption. Aswell as these reactions, HCl molecules, produced as a result of chlorine atoms reactionwith CH3Cl, are decomposed when oxygen was added to the plasma system releasingactive chlorine radicals as shown in reaction R4.21. These chlorine radicals participate in afurther decomposition of methyl chloride (100; 144).

OH+HCl−−→ H2O+Cl (R4.21)

A maximum decomposition of methyl chloride has been found with all the investigatedinitial concentrations at 2 to 3 % of added oxygen. The removal efficiency decreased afterthat with an increase of the oxygen concentration. Similar results have been found wheninvestigating the effect of oxygen concentration on the removal efficiency of DCM inchapter three. The decrease of the removal efficiency of methyl chloride with increasing theadded concentration of oxygen to the gas stream occurs for the same reasons as explainedin chapter three. Increasing the oxygen concentration leads to the formation of ozone aswell as the formation of NOx in the plasma. The formation of these species consume alarge amount of the active oxygen and nitrogen atoms and molecules which otherwisewould have reacted with the VOC presents in the plasma and decomposed it (76; 133; 134).NOx formation in plasma is discussed in an upcoming chapter.

Increasing the initial concentration of methyl chloride resulted in a gradual decrease ofthe removal efficiency. Similar results were also found for the effect of initial concentrationon the decomposition of DCM in chapter three. These results are in line with previouswork by Indarto et al. (145). They investigated the influence of the initial concentrationof chlorinated hydrocarbons CCl4 and CHCl3 on their conversion in a gliding-arc plasma.Their results showed a gradual decrease in CCl4 and CHCl3 conversion with increases ofthe initial concentration.

Increasing the concentration of methyl chloride requires a larger amount of activespecies to carry on the decomposition process. To increase the amount of active speciesin the plasma, more energy density is required. Thus, in the same plasma conditions,increasing the initial concentration of methyl chloride leads to less decomposition. It isvery important to identify the amount of VOC which could be removed in the plasmasystem with different initial VOC concentration.

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4.2.1 Influence of oxygen and initial methyl chloride concentrationon plasma by-product formation.

The main plasma by-products for methyl chloride decomposition in nitrogen plasmawere HCN and HCl. However when oxygen was added to the gas stream the main plasmaby-products were CO, CO2, NOx, and N2O. A small concentration of about 27 ppm ofNOCl was also detected. In this section, the influence of oxygen and initial methyl chlorideconcentration on CO, and CO2 formation is investigated. NOx and N2O formation in theplasma with the presence of chlorinated hydrocarbons are fully discussed in chapter seven.The NOCl concentration was too small for any investigation to be carried out effectively.

Figures 4.3 and 4.4 show the effect of oxygen and initial CH3Cl concentration onthe formation of CO, and CO2 respectively. A total flow rate of 1 L min−1 and anenergy density of about 2280 J L−1 were used. Results show that increasing the oxygenconcentration from 0 to 3 % resulted in a large increase of CO concentration in the exhaust.Increasing the oxygen concentration further did not lead to an increase of CO concentration.This is due to increasing CO oxidation in nitrogen plasma in the presence of high oxygenconcentrations. CO2 concentration changed in a similar form to the removal efficiency ofCH3Cl with increasing oxygen concentration. A peak of CO2 was found between 2-3 %of added oxygen. After that increasing the oxygen concentration resulted in decreasingthe concentration of CO2. This indicates that CO2 formation is mostly dependent on thedecomposition of carbon intermediates which are generated in the plasma as a result ofmethyl chloride decomposition.

As for the influence of the initial concentration of CH3Cl on CO and CO2 formation,increasing the initial concentration of CH3Cl from 500 to 1000 ppm caused an increaseof the concentration of CO in the plasma reactor. Increasing the initial concentration ofCH3Cl from 500 to 700 ppm, increased the CO2 concentration. However for 700 and1000 ppm of CH3Cl, CO2 exhaust concentration was almost the same. With higher feedconcentrations of methyl chloride, more hydrocarbons and chlorinated hydrocarbons areformed in the plasma. The further decomposition of these intermediates in the plasmaleads to the formation of higher concentrations of CO and CO2 (100).

A carbon balance for the plasma system is presented in figure 4.5. Carbon balance wascalculated as follow:

Carbon balance % = ([CO]+[CO2]+[HCN]+[CH3Clout])× 100 / [CH3Clin]

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Figure 4.3: Carbon monoxide formation as a function of oxygen concentration and theinitial methyl chloride concentration in a nitrogen non-thermal plasma generated in apacked-bed plasma reactor. A total flow rate of 1 L min−1 and an energy density of about2280 J L−1 were used.

Figure 4.4: Carbon dioxide formation as a function of oxygen concentration and the initialmethyl chloride concentration in a nitrogen non-thermal plasma generated in a packed-bedplasma reactor. A total flow rate of 1 L min−1 and an energy density of about 2280 J L−1

were used.

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Figure 4.5: Carbon balance as a function of oxygen concentration and the initial methylchloride concentration in a nitrogen non-thermal plasma generated in a packed-bed plasmareactor. A total flow rate of 1 L min−1 and an energy density of about 2280 J L−1 wereused.

With 1 to 5 % of added oxygen to the gas stream and with all the investigated con-centrations of methyl chloride, there was about 10 to 20 % of missing carbon which hasnot been detected after the plasma reactor. At these oxygen concentrations the optimumdecomposition of methyl chloride is taking place. This amount of missing carbon isthought to be due to the formation of soot in the plasma reactor (100). Soot deposition inthe packed-bed region over the dielectric beads and the inner side of the glass tube wasobserved when the maximum decomposition of methyl chloride took place.

In the case of nitrogen plasma with no oxygen addition, there was more carbon detectedafter the reactor compared with the initial carbon entering the plasma reactor. This couldbe due to the decomposition of soot depositions on the surface of the beads and the innerside of the reactor body which took place during previous experiments (146). There is alsoa small amount of CO2 detected from the lab atmosphere in the small gap between theplasma cell and the FTIR detector. This extra amount of carbon is within an error of about5 %.

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Figure 4.6: The removal efficiency of 500 and 1000 ppm of methyl chloride in nitrogennon-thermal plasma as a function of energy density and two oxygen concentrations of 0and 3% in a packed-bed plasma reactor. A total flow rate of 1 L min−1 was used.

4.3 Influence of energy density on the removal efficiency

The effect of increasing the energy density on the removal efficiency of CH3Cl isinvestigated in this section. Energy density was increased from 0 to about 3000 J L−1.Initial concentration of 500 and 1000 ppm of methyl chloride with 0 and 3% oxygen wereused. Figure 4.6 presents the removal efficiency of 500 and 1000 ppm of CH3Cl as afunction of energy density and two oxygen concentrations of 0 and 3%. A total flow rateof 1 L min−1 was used. Power was increased by increasing the input voltage using a variacbefore the transformer.

Results for 0 and 3% oxygen showed that, the removal efficiency of CH3Cl increasedlinearly for both these situations with increasing the energy density. Increasing the energydensity leads to more collisions frequency resulting in the formation of more excited speciesin the gas which leads to increasing the decomposition of methyl chloride. However, a bigdifference was noticed in the case without the presence of oxygen and with the additionof 3 % oxygen. In the case of 500 and 1000 ppm CH3Cl without the addition of oxygen,the removal efficiency ranged between 0 to a maximum of about 21% with increasing theenergy density from 0 to about 3000 J L−1. The removal efficiency with both the initial

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concentrations was almost the same.

In the case of adding 3% oxygen to the gas stream the removal efficiency rangedbetween 0 to 56% and 0 to 48% for 500 and 1000 ppm of CH3Cl respectively. This bigdifference of removal efficiency is due to the reactions responsible for decomposing methylchloride in the plasma reactor. As explained earlier, in the case of nitrogen plasma withoutthe addition of oxygen, methyl chloride is decomposed due to electron collisions andreaction with active nitrogen atoms and molecules produced in the plasma. However, theelectron attachment rate constant for methyl chloride at temperatures less than 500 K isvery small (143) which affects the removal efficiency of methyl chloride. With the additionof oxygen to the gas stream, active oxygen atoms and molecules as well as OH radicalsincrease the decomposition of methyl chloride.

4.3.1 Energy density effect on plasma by-product formation

The main plasma by-products for methyl chloride decomposition without the additionof oxygen to the gas stream were HCN and HCl. HCN was the main carbon containingproduct which accounted for almost all the reacted carbon. While HCl accounted for 80to 85 % of decomposed chlorine in the plasma system. This is due to the formation ofchlorine molecules which cannot be detected using FTIR spectroscopy. Figures 4.7 and4.8 show the carbon and chlorine balance for 500 ppm and 1000 ppm of methyl chloridedecomposition respectively as a function of energy density.

Figures 4.10 and 4.11 show the concentration change for HCN and HCl without theaddition of oxygen as a function of energy density and the initial CH3Cl concentration.No HCN was detected with less than 600 J L−1 of energy density. An energy densityof about 1200 J L−1 was required to start detecting HCl. Although there was a smalldecomposition of CH3Cl of about 4 % at about 500 J L−1 and up to 10 % for 1200 J L−1,no HCN and HCl were detected below 500 and 1000 J L−1 respectively. This could bedue to the limit of detection for HCN and HCl in the FTIR spectrometer. To achieve asignal to noise ratio of above four, about 55 ppm of HCN and 34 ppm of HCl was required.Although it was possible to detect a 46 ppm of HCN this measurement is not accurate asthe signal to noise ratio was less than four. A full explanation about the limit of detectioncan be found in the methodology chapter. Once HCN and HCl have been detected, theirconcentration increased linearly with increasing the energy density, indicating a linearincrease of the decomposition of CH3Cl as has been found in the previous section. MoreHCN and HCl were formed with increasing the initial concentration of methyl chloride

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Figure 4.7: Carbon balance for the decomposition of 500 and 1000 ppm of methyl chlorideas a function of energy density in nitrogen non-thermal plasma. A total flow rate of 1 Lmin−1 was used.

Figure 4.8: Chlorine balance for the decomposition of 500 and 1000 ppm of methylchloride as a function of energy density in nitrogen non-thermal plasma. A total flow rateof 1 L min−1 was used.

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Figure 4.9: The influence of the removal efficiency of 500 and 1000 ppm of methyl chlorideon the formation of HCN and HCl in nitrogen non-thermal plasma. A total flow rate of 1 Lmin−1 was used.

from 500 to 1000 ppm. Figure 4.9 shows that the formation of HCN and HCl is directlylinked to the decomposed amount of methyl chloride in plasma. As the amount of carbonand chlorine produced from CH3Cl decomposition was more with 1000 ppm of methylchloride feed compared with 500 ppm, more HCN and HCl were formed with 1000 ppm.

With the addition of oxygen to the gas stream, the main detected plasma by-productswere CO, CO2 and NOx. A small concentration of NOCl was also detected with a maximumof about 10 ppm. The influence of energy density on the formation of CO and CO2 isinvestigated. NOx generation is fully discussed in a further chapter. Figures 4.12 and4.13 show the concentration change for CO and CO2 with the addition of 3 % oxygenas a function of energy density and the initial CH3Cl concentration. Results showed thatCO and CO2 concentrations increased linearly with the increase of the energy density.Increasing the energy density leads to greater collision frequency of electrons and DCMmolecules resulting in the formation of more excited species in the gas which leads toincreasing the decomposition of methyl chloride (100; 146).

The measured concentrations of CO and CO2 were more with a 1000 ppm initialconcentration of CH3Cl compared with 500 ppm initial concentration. Figure 4.14 showsthe relation between methyl chloride decomposition and the formation of CO and CO2.Again, this is due to the decomposition of more amounts of methyl chloride in the case ofusing 1000 ppm compared with using 500 ppm. The decomposition of methyl chloride

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Figure 4.10: Hydrogen cyanide concentration as a function of energy density and twomethyl chloride initial concentrations of 500 and 1000 ppm in nitrogen non-thermal plasma.A total flow rate of 1 L min−1 was used.

Figure 4.11: Hydrogen chloride concentration as a function of energy density and twomethyl chloride concentrations of 500 and 1000 ppm in nitrogen non-thermal plasma. Atotal flow rate of 1 L min−1 was used.

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Figure 4.12: Carbon monoxide concentration as a function of energy density and twomethyl chloride concentrations of 500 and 1000 ppm in nitrogen non-thermal plasma withthe addition of 3 % oxygen. A total flow rate of 1 L min−1 was used.

Figure 4.13: Carbon dioxide concentration as a function of energy density and two methylchloride concentrations of 500 and 1000 ppm in nitrogen non-thermal plasma with theaddition of 3 % oxygen. A total flow rate of 1 L min−1 was used.

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Figure 4.14: The influence of the removal efficiency of 500 and 1000 ppm of methyl chlorideon the formation of CO and CO2 in nitrogen non-thermal plasma with the addition of 3 %oxygen. A total flow rate of 1 L min−1 was used.

leads to the formation of CO and CO2 as end product.

4.4 Influence of plasma residence time on the removal ef-ficiency

The residence time of the gas in the plasma region was changed by changing the totalflow rate of the gas stream entering the plasma reactor. To investigate the effect of residencetime on the removal efficiency of methyl chloride in a packed-bed plasma reactor, flowrates ranging between 0.5 to 2 L min−1 were used. These flow rates give a residence timeranging between 1.95 to 0.49 seconds respectively.

Figure 4.15 shows the removal efficiency of 1000 ppm of methyl chloride as a functionof residence time and oxygen concentration. Two cases were investigated, nitrogen plasmawithout the addition of oxygen to the gas stream and with the addition of 3% oxygen.

Results show that increasing the residence time in the plasma region resulted in increas-ing the removal efficiency of methyl chloride. Increasing the residence time of the gascomponents in the plasma region, results in the formation of more active species in theplasma which react with methyl chloride and leads to more decomposition taking place.

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Figure 4.15: The removal efficiency of 1000 ppm of methyl chloride in nitrogen non-thermalplasma as a function of residence time and oxygen concentration in a packed-bed plasmareactor.

Results show that increasing the plasma residence time by factors of 2, 3 and 4 did notincrease the removal efficiency of methyl chloride in the same form. For example doublingthe residence time from 0.5 second to 1 second increased methyl chloride decomposition,with 3 percent oxygen, from about 25 to 35 percent respectively rather than doublingthe decomposition to 50 %. Similar results were found in chapter three for the influenceof residence time on dichloromethane decomposition. As explained in chapter three,although plasma residence time is a very important factor in VOC removal it is not theonly factor which plays a role in increasing the removal efficiency. Other factors related tothe interaction between the chemical reactions taking place in the plasma play a significantrole in the VOC removal process.

4.5 Reaction pathway for the decomposition of methylchloride in non-thermal plasma reactor.

Several studies have investigated the chemical kinetic mechanism for methyl chloridedecomposition (100; 123; 144; 147–153). This section discusses the main reactions of theactive species with methyl chloride in nitrogen plasma with and without the addition of

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oxygen to the gas stream.

Initiating plasma in nitrogen and methyl chloride gas mixture leads to the formation ofactive radicals and excited states such as N(2P), N(4S), N(2D), N2 (A3 Σu

+), CH3, H andCl as a result of electron collisions. When oxygen is added to the system, active oxygenatoms and molecules such as O(3P), O(1D), O2(

1∆) are formed in the plasma. As wellas active oxygen and nitrogen molecules and atoms, active radicals such as OH, ClO andHO2 are also formed in the plasma. The decomposition of methyl chloride in plasma takeplace as a result of reactions between methyl chloride molecules and some or all theseactive species.

Methyl chloride decomposition in nitrogen plasma is initiated due to the electroncollisions and the reaction with active nitrogen radicals in the plasma region. Electronattachment with methyl chloride R4.22 is a very slow reaction especially with gas temper-atures less than 500 k (143; 147). The gas temperature in the carried out experiments formethyl chloride decomposition in non-thermal plasma ranges between 330 to 380 k.

e−+CH3Cl−−→ CH3 +Cl− (R4.22)

As a result of this slow reaction, small amounts of chlorine atoms and CH3 are initiallyformed in the plasma and so a less contribution of these active species in the decompositionprocess of methyl chloride in nitrogen plasma.

CH3Cl+N−−→ products (R4.23)

CH3Cl+N2(A3Σu

+)−−→ products (R4.24)

CH3Cl+CH3 −−→ CH2Cl+CH4 (R4.25)

CH3Cl+Cl−−→ CH2Cl+HCl (R4.26)

CH3Cl+Cl−−→ CH3 +Cl2 (R4.27)

In the case of nitrogen, oxygen and methyl chloride gas mixture, active oxygen atomsand molecules play a significant role in CH3Cl destruction and the formation of plasmaend products such as CO, CO2 and NOCl.

CH3Cl+O−−→ CH2Cl+OH (R4.28)

CH3Cl+O(1D)−−→ CH3 +ClO (R4.29)

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CH3Cl+O2 −−→ CH2Cl+HO2 (R4.30)

Extra active species such as OH, ClO, HO2 and H are formed as a result of theseinitial reactions (100; 144). These active species react with CH3Cl and cause furtherdecomposition via the following reactions.

CH3Cl+OH−−→ CH2Cl+H2O (R4.31)

CH3Cl+ClO−−→ CH2Cl+HOCl (R4.32)

CH3Cl+HO2 −−→ CH2Cl+H2O2 (R4.33)

CH3Cl+H−−→ CH2Cl+H2 (R4.34)

CH3Cl+H−−→ CH3 +HCl (R4.35)

Further reactions for the initial reaction products, either with each other, or with activeradicals in the plasma leads to the formation of plasma end products such as HCl, NOCl,CO and CO2.

4.6 Summary and Conclusions

This work is the only work which fully investigates the decomposition of methyl chlor-ide in atmospheric pressure non-thermal plasma generated in a packed-bed plasma reactor.This chapter contains the investigations of the influence of several process parameters onthe decomposition of methyl chloride in a packed-bed plasma reactor. The main findingsare:

• Oxygen concentration in nitrogen-oxygen plasma affects the removal efficiency ofmethyl chloride in a packed-bed plasma reactor. The optimum remediation of methylchloride was achieved with the addition of 2-3 % of oxygen to the gas stream.

• Increasing the initial concentration of methyl chloride in the feed stream resulted indecreasing the removal efficiency of CH3Cl in a packed-bed plasma reactor.

• Increasing the energy density in the plasma system increased linearly the removalefficiency of methyl chloride.

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• A longer residence time for the gas stream in the plasma increased the removalefficiency of CH3Cl. However, the increase in the decomposition of CH3Cl was notof the same form as the increase in the residence time. This indicates that althoughplasma residence time plays a very important role in CH3Cl decomposition it is notthe only parameter which play a role in the plasma process.

• The concentration of end-products from the plasma increased with increasing theinitial concentration of CH3Cl, increasing the energy density and increasing theresidence time in the plasma.

• The concentrations of HCN, HCl and CO2 are linked directly to the amount ofCH3Cl that has been decomposed in the plasma.

The influence of oxygen concentration on the removal efficiency of chlorinated VOCscan be enhanced by additives such as alkenes. The next chapter presents an investigationof the influence of adding propylene on the removal efficiency of dichloromethane.

Plasma residence time can be increased by using a multiple packed-bed plasma reactor.The use of multiple packed-bed plasma reactor for dichloromethane removal is investigatedin chapter six of this thesis.

As increasing the energy density increased the cost of the plasma system, no furtherinvestigation of this parameter was carried out. The concentration of the chlorinatedVOCs in the feed gas cannot be controlled, as these concentrations differ according to theindustrial process which produces these VOCs in the process exhausts. This parameterwas not investigated further.

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Chapter 5

The effect of adding alkene on the destruction of DCM innon-thermal plasma generated in a packed bed reactor

5.1 Introduction

The removal efficiency of dichloromethane and methyl chloride, presented in chaptersthree and four, showed a maximum removal efficiency at two to four percent of addedoxygen to the gas stream. After this point the destruction of dichloromethane and methylchloride decreased with increasing oxygen concentration. As discussed previously, increas-ing the concentration of oxygen in the plasma region leads to the formation of ozone via arecombination between oxygen atoms and molecules, and the formation of nitrogen oxides(NOx). As a result, less active oxygen and nitrogen atoms and molecules are taking part inthe nitrogen and oxygen based chemistry for the decomposition of chlorinated VOCs.

The aim of this study is to enhance the removal efficiency of chlorinated VOCs in airplasma. To improve the decomposition of chlorinated VOCs, ozone and/or NOx formationhad to be controlled. Air plasma leads inevitably to the formation of NOx in the gas stream.Ozone formation in air plasma is also unavoidable. However, it is possible to decomposeozone by reacting with alkene carbon-carbon double bonds leading to the formation ofOH and H radicals. Ozone reacts with alkenes forming an energy rich ozonide, whichdecomposes to form carbonyl compound and Criegee intermediates as shown in figure5.1. The excited Criegee intermediates then decompose forming OH and H radicals.(67; 154–161).

OH is a key radical for fast destruction of chlorinated VOCs in plasma (77; 99). It isthought that the formation of OH and H radicals leads to an increase in the destruction ofchlorinated VOCs with increasing oxygen concentration up to that of air.

Figure 5.1: Ozone and alkene double bond reaction.

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Previous work by Shin et al. (162) have found that adding alkenes such as propylene(C3H6) and ethylene (C2H4) to plasma gas stream resulted in reducing NO formation.Propylene additives resulted in better NO reduction compared with ethylene. Other studiesby Dorai et al. (163; 164) and Penetrante et al. (165) have reported the important role ofpropylene additives on NOx remediation in plasma gas stream. Hill et al. (85) reportedthe positive effect of propylene additives on enhancing the decomposition of propaneusing non-thermal plasma. Propylene decomposes easily in plasma, producing OH and Hradicals, which in turn help the decomposition of other hydrocarbons in the gas stream.Due to the positive effect of propylene on both NOx reduction, and improving hydrocarbondecomposition, propylene has been used to investigate the effect of adding alkenes to thegas stream for improving the removal efficiency of chlorinated VOCs and NOx reduction.

This chapter investigates the influence of propylene additives on the removal efficiencyof dichloromethane, and the formation of plasma end products. The removal efficiency ofpropylene in non-thermal plasma is also investigated.

5.2 Methodology

The same experimental arrangement as in chapter three was used in this chapter. Arange of propylene levels were added to the gas stream. The effect of adding about 300,500 and 1000 ppm of propylene on the removal efficiency of DCM in a non-thermalatmospheric pressure plasma generated in a packed-bed plasma reactor is investigated. Thedecomposition of propylene in plasma is discussed. The influence of propylene additiveson the formation of plasma end products is also investigated.

A nitrogen gas stream of about 500 ppm DCM with a total flow rate of 1 L min−1,allowing a residence time of 0.95 seconds for the gas stream in the plasma region wasused. Oxygen concentration ranged between 0 to 21 percent and the energy densitywas about 1820 J L−1. Gases were used as supplied from BOC, propylene (99.95%),nitrogen (99.998%) and oxygen (99.99%). Gas analysis was carried out using in line FTIRspectroscopy both before and after the plasma reactor to identify the chemical species inthe gas stream and to calculate their concentrations.

Measurements were carried out about 20 minutes after initiating the plasma in thereactor to ensure that a steady state was achieved. This was discussed in the methodologychapter. Most experiments were repeated at least three times and averages of the resultsare presented.

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Figure 5.2: FTIR spectra for 1000 ppm propylene after plasma reactor (a) nitrogen plasmawithout adding oxygen (b) nitrogen plasma with adding 5 % oxygen to the gas stream. Amultiple pass optical gas cell with a 5.3 m pathlength, a total flow rate of 1 L min−1 andan energy density of about 1820 J L−1 were used. Measurements were taken about 0.75seconds downstream of the plasma reactor. Spectral resolution is 1 cm−1.

Example in-line FTIR spectra for propylene decomposition products with 0 and 5% ofadded oxygen is shown in figure 5.2 The main plasma end products are shown.

5.3 The influence of adding varying amounts of propyl-ene to a gas stream containing 500 ppm of dichloro-methane on the removal efficiency of dichlorometh-ane in non-thermal plasma.

This section presents the investigation of adding a range of propylene levels to the gasstream on dichloromethane decomposition. Figure 5.3 shows the influence of propyleneand oxygen concentrations on the removal efficiency of dichloromethane. Propyleneconcentrations of about 300, 500 and 1000 ppm were added to a gas stream containing 500ppm of dichloromethane. Results show a maximum DCM decomposition with 2 to 4 % ofadded oxygen to the gas stream in the cases of adding 300 and 500 ppm of propylene to thegas stream. However, adding 1000 ppm of propylene to the gas stream resulted in linear

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Figure 5.3: Dichloromethane decomposition in non-thermal plasma with 300, 500 and1000 ppm of propylene as a function of oxygen concentration. A total flow rate of 1 Lmin−1 and an energy density of about 1820 J L−1 were used.

and fast increase in the dichloromethane removal with increasing oxygen concentration upto 5 percent. Increasing the oxygen concentration after that resulted in a slower increase ofdichloromethane removal up to about 75 % with air plasma (166).

Figure 5.4 shows a comparison of the removal efficiency of dichloromethane withoutthe addition of propylene and with the addition of 500 and 1000 ppm. Results show that theaddition of 300 and 500 ppm of propylene to the gas stream resulted in less dichloromethanedecomposition compared with the case of without the addition of propylene. Results alsoshow that the decomposition of 500 ppm of dichloromethane with the addition of 500ppm propylene was higher than with adding 300 ppm propylene. The destruction of DCMincreased significantly with the addition of about 1000 ppm of propylene compared withthe addition of 300 or 500 ppm. The optimum decomposition of dichloromethane wasobtained with air plasma and the addition of 1000 ppm propylene to the gas stream.

To explain this behaviour, it is important to investigate the decomposition of propylenein non-thermal plasma with initial concentrations of 300, 500 and 1000 ppm. Figure 5.5shows the decomposition of 300, 500 and 1000 ppm of propylene in the plasma. Theremoval efficiency of propylene in nitrogen plasma without the addition of oxygen wasabout 55%. Adding oxygen to the reactor inlet gas stream of propylene and nitrogen,

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Figure 5.4: Dichloromethane decomposition in non-thermal plasma with 0, 500 and 1000ppm of propylene as a function of oxygen concentration. A total flow rate of 1 L min−1

and an energy density of about 1820 J L−1 were used.

Figure 5.5: Propylene decomposition in non-thermal plasma with and without the presenceof 500 ppm DCM as a function of oxygen concentration. A total flow rate of 1 L min−1

and an energy density of about 1820 J L−1 were used.

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causes a better decomposition of C3H6. A removal efficiency of about 75 to 80 % wasachieved when oxygen was added. Increasing the oxygen concentration did not seem tohave an effect on the decomposition of C3H6.

Previous results show that adding low concentrations of propylene to the gas streamdecreased the destruction of DCM compared to the situation without the presence of C3H6.However, with higher propylene concentrations, the decomposition of DCM increased withan increase of the oxygen concentration as presented in figure 5.3 and 5.4. The decreaseof the decomposition of DCM at 300 and 500 ppm of propylene is thought to be due tothe consumption of active radicals in the plasma such as N, O, H and OH by reaction withpropylene. Figure 5.5 shows a decomposition efficiency of propylene of about 75 to 85%with the presence of DCM. The decomposition of propylene in plasma with DCM was onthe range of 75 to 85% with any tested concentration of propylene. This means that at lowpropylene concentrations the active species in the plasma were consumed by propylenewhile small concentrations of OH and H radicals are produced as a result of propylenedecomposition. As for the higher concentrations of propylene, more OH and H radicalswere formed, which seems to be enough to cover the consumption, and to increase thedecomposition of DCM.

Comparing the decomposition of propylene in non-thermal plasma with and withoutthe presence of 500 ppm of dichloromethane, shows that propylene decomposition at anyconcentration increased with the presence of DCM as shown in figure 5.5. Propylenedecomposition in the presence of DCM increased by about double compared with no DCMin nitrogen plasma. This is possibly due to the reaction of propylene with chlorine radicals(167):

C3H6 +Cl−−→ C3H5.+HCl (R5.36)

5.3.1 The effect of added propylene concentration on the formationof plasma end products.

Figure 5.6 shows FTIR spectra for the plasma reactor exhaust of 1000 ppm propyleneand 500 ppm dichloromethane in nitrogen plasma with adding 0 and 3 percent oxygen tothe gas stream. The main species detected in the plasma exhaust with no oxygen is addedare HCN and HCl. The detection of a small concentration of CO indicates the presence ofa small concentration of oxygen in the system. This may be due to some impurities in the

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Figure 5.6: FTIR spectra for nitrogen plasma exhaust of 1000 ppm propylene and 500ppm DCM (a) without adding oxygen (b) with adding 5 % oxygen to the gas stream. Amultiple pass optical gas cell with a 5.3 m pathlength, a total flow rate of 1 L min−1 andan energy density of about 1820 J L−1 were used. Measurements were taken about 0.75seconds downstream of the plasma reactor. Spectral resolution is 1 cm−1.

inlet system or from BaTiO3 beads. The main chemical species which have been detectedin the plasma exhaust with the addition of oxygen to the gas stream are CO, CO2, CH2O,ClNO, NOx, N2O and the unreacted DCM and C3H6.

The main plasma end products in the presence of oxygen are CO, CO2 and CH2O. Theyaccounted for almost all the reacted carbons in the cases of addition of 300 and 500 ppm ofpropylene. However there was about 20 % unaccounted carbon in the case of adding 1000ppm propylene to the gas stream. This amount of missing carbon is thought to be due tothe formation of soot in the plasma reactor (100). Soot deposition in the packed-bed regionover the dielectric beads and the inner side of the glass tube was observed when adding1000 ppm of propylene to the gas stream. Figure 5.7 shows the influence of several initialconcentrations of propylene on CO, CO2 and CH2O formation. The feed gas consists ofair with 500 ppm dichloromethane. Increasing the initial concentration of propylene from0 to about 1000 ppm caused a linear increase of the concentration of CO, CO2 and CH2Oin the plasma reactor. With higher feed concentrations of propylene, more hydrocarbonsare formed in the plasma. The further decomposition of these intermediates in the plasmaleads to the formation of higher concentrations of CO, CO2 and CH2O. The formation ofnitrogen oxides is discussed in a subsequent chapter.

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Figure 5.7: CO, CO2 and CH2O formation as a function of initial propylene concentrationin air gas stream containing 500 ppm of dichloromethane. Non-thermal plasma wasgenerated in a packed-bed plasma reactor with a total flow rate of 1 L min−1 and anenergy density of about 1820 J L−1.

5.3.2 Reaction pathway for the decomposition of propylene in non-thermal plasma reactor.

Several studies have investigated the reaction pathway for C3H6 destruction in plasmaand thermal processes (85; 124; 168–171). A summary of the main reactions for C3H6

decomposition in plasma are discussed in this section.

The decomposition of propylene in nitrogen plasma without the addition of oxygentakes place as a result of reactions with nitrogen atoms forming ethylene and hydrogencyanide. (168):

C3H6 +N−−→ C2H4 +HCN+H (R5.37)

The ethylene that is formed can then be decomposed via reaction with nitrogen atomsproducing more HCN.

C2H4 +N−−→ HCN+CH3 (R5.38)

The decomposition of propylene proceeds after that due to reactions with H and CH3

(170; 172):

C3H6 +H−−→ C3H5 +H2 (R5.39)

C3H6 +CH3 −−→ CH4 +C3H5 (R5.40)

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HCN and HCl were detected as plasma end products in this work.

The addition of oxygen to the gas stream leads to the formation of the radicals O(3P),O(1D) and O2(

1∆) by electron impact (133; 139; 140). The formation of these activeoxygen radicals allows for faster propylene decomposition, mainly via the followingreactions (162; 165):

The reduction of C3H6 in the presence of oxygen takes place via reacting with Oradicals as follows:

C3H6 +O−−→ C2H5 +HCO (R5.41)

C3H6 +O−−→ CH2CO+CH3 +H (R5.42)

C3H6 +O−−→ CH3CHCO+H+H (R5.43)

C3H6 +O−−→ C3H5 +OH (R5.44)

The presence of oxygen in the plasma leads to the formation of ozone. Ozone reacts withC3H6 forming OH and H radicals (67; 154–156).

C3H6 +O3 −−→ CH3CHOO+CH2O (R5.45)

CH3CHOO−−→ CH3 +CO+OH (R5.46)

CH3CHOO−−→ CH3 +CO2 +H (R5.47)

After these initial reactions OH is formed. OH radicals rather than O atoms became themain radical that consumes C3H6 (162; 165).

C3H6 +OH−−→ C3H6OH (R5.48)

C3H6 +OH−−→ C3H5 +H2O (R5.49)

Further reactions and oxidation of the reaction products take place in the plasma leadingto the formation of plasma end products such as CO, CO2, NOx and formaldehyde (CH2O).These species were detected as plasma end products in this work.

It was not possible to investigate the influence of propylene additives on the removalefficiency of methyl chloride, as the methyl chloride absorbance spectrum overlaps withpropylene and hydrogen cyanide spectra as illustrated in figure 5.8. Measuring the concen-tration of methyl chloride is not possible in this case.

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Figure 5.8: Comparison between FTIR spectra for 500 ppm propylene after the plasmareactor with nitrogen gas stream and 100 ppm of methyl chloride.

Kinetic simulations of the reactions taking place when propylene was added to theplasma were too involved for the remit of this thesis. Further work to model this situationusing CHEMKIN II package is required.

5.4 Summary and Conclusions

Under some conditions, adding propylene to a gas stream containing 500 ppm ofdichloromethane enhanced the decomposition of dichloromethane in nitrogen-oxygennon-thermal plasma.

• Adding 1000 ppm of propylene to the gas stream, improved the removal efficiencyof dichloromethane especially with oxygen concentrations higher than 5 percent.

• The removal efficiency of propylene in nitrogen-oxygen plasma with the presenceof dichloromethane was significantly higher compared with the situation of nodichloromethane in the gas stream.

• The optimum removal efficiency for dichloromethane in air plasma was achievedwith the addition of 1000 ppm of propylene to the gas stream.

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• Adding 300 and 500 ppm of propylene did not enhance the decomposition ofdichloromethane in non-thermal plasma. Such propylene concentrations actuallysuppressed the removal efficiency of dichloromethane. This is due to consumingthe active species in the plasma by propylene decomposition, while the producedOH and H radicals formed as a result of propylene decomposition, were not largeenough to cover the consumption and participate in increasing the destruction ofdichloromethane.

As many industrial gas exhausts contains chlorinated and non-chlorinated hydrocarbons atthe same time, it is important to investigate the destruction of a mixture of these VOCs innon-thermal plasma. To conclude, the aim of this chapter of achieving the best possibleremoval for VOCs in air plasma was accomplished. Alkene and DCM removal efficiencyenhanced significantly with the presence of both of them in air plasma.

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Chapter 6

Multiple packed-bed plasma reactor

6.1 Introduction

Previous investigations of the effect of residence time on the removal efficiency ofdichloromethane and methyl chloride have shown that increasing the residence time causeda linear increase in the decomposition of both dichloromethane and methyl chloride.Increasing the residence time of the gas components in the plasma region resulted in theformation of more active species in the plasma per unit volume. This in turn increases thenumber of reactions and collisions with dichloromethane or methyl chloride in the plasmaleading to improved removal efficiency.

The residence time of gases in the plasma region could be increased using one or moreof the following three methods. First, reducing the flow rate, as investigated previouslyin chapters three and four, results in increasing the plasma residence time. However, forindustrial gas exhaust applications; this method is not practical as there are large volumesof gas requiring treatment. Another way to increase the residence time is increasing thepacked-bed volume. Again this method has limitations, as increasing the packed-bedvolume causes an increase of the break down voltage required to initiate and generateplasma in the reactor. Alternatively, the residence time of VOC in plasma could be increasedby increasing the number of plasma cells which the gas flows through. Increasing thenumber of plasma cells does not have any of the disadvantages which the previous twomethods have.

Several studies have reported the use of a multiple cell plasma reactor for air purificationapplications. Harling et al. (82) had reported the use of a multiple plasma discharge forthe destruction of ethylene and toluene. They used a multi plasma cell reactor containingthree packed-beds in series to achieve a 95 % reduction of the ethylene and 72 % reductionof toluene. Hubner et al. (108) also reported the use of a five stage packed-bed plasmareactor for ethylene treatment. They reported an almost complete removal of ethylene fromthe gas stream.

Chavadej et al. (173) reported the use of a four stage corona discharge for the removalof ethylene. They found that increasing the stage number increased the removal efficiency

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as a factor of increasing the residence time and after 0.38 s a complete conversion wasachieved.

None of these studies involved the removal of chlorinated hydrocarbons using a multiplestage plasma reactor. The reason behind designing this multiple packed-bed plasma reactoris to investigate the effect of plasma residence time and the accumulative effect of plasmacells number on the destruction efficiency of the VOCs. Also, to allow in-situ measurementsafter each bed.

The aims of this chapter are:

• To investigate the influence of the number of plasma cells on the removal efficiencyof dichloromethane.

• To study the influence of the number of plasma cells on the removal efficiency of amixture of dichloromethane and propylene.

• TO evaluate the energy efficiency parameter (β ) for the decomposition of dichloro-methane in a multiple packed-bed plasma reactor.

• To assess the effect of the number of plasma cells on the formation of plasma endproducts.

• To carry out in situ measurements through the KBr windows after each plasma cell.

• To compare in situ and in line measurements for the decomposition of dichlorometh-ane.

6.1.1 Experimental set up

Figure 6.1 presents a schematic of the experimental arrangements for the work describedin this chapter. Gases were used as supplied; nitrogen (99.998%), oxygen (99.99%) andpropylene (99.95%). Dichloromethane was introduced to the gas stream using a bubblerwith nitrogen as the carrier gas. The flow rates of the gases were controlled by mass flowcontrollers. A total flow rate of 1 L min−1 was used at atmospheric pressure. A multiplepacked-bed plasma reactor consisting of three cells in series was used. Figure 6.2 shows

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Figure 6.1: A schematic diagram of the experimental inlet system which has been used toinvestigate The remediation of DCM in a multiple packed-bed plasma reactor.

a photograph and a sketch of the multiple packed-bed plasma reactor. The gas mixtureentered the reactor through a funnel and exited through another funnel. These funnelswere added to the reactor body to insure a good gas distribution through the reactor andto prevent any back flow or gas accumulation at the end of the reactor body. The reactorbody is made of Perspex with dimensions of 45 × 12 × 7 cm. The reactor consists ofthree plasma cells. Each cell has two stainless steel mesh electrodes with dimensions of10 × 5 × 0.1 cm. Barium titanate beads with a diameter of 1.5-2.5 mm were used as adielectric material, filling the space between the two electrodes in each cell. Each plasmacell has a dimension of 10 × 5 × 0.9 cm giving a gas residence time of 1.4 s in each cellat a flow rate of 1 L min−1, and a total residence time of 4.2 s in the plasma regions ofthe reactor. Plasma cells were powered by three identical neon sign power supplies eachproviding high voltage alternating current at a frequency of 20 kHz. The input voltage wascontrolled using a variac for each power supply. The packed-bed cells have been designedto allow a change in the distance between the electrodes from 0.5 to 2.5 cm; leading to anincrease or a decrease in the packed-bed volume. KBr windows with a diameter of 4 cmand a thickness of 5 mm were installed at several parts allowing in-situ measurements viaFTIR spectroscopy across the 10 cm width of the reactor. FTIR measurements were alsocarried out in-line using a multiple pass cell with 5.3 meters pathlength.

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Figure 6.2: Design and a photograph of the multiple packed bed plasma reactor with threeplasma cells powered by three identical neon sign power supplies

6.2 Multiple packed-bed plasma reactor for dichlorometh-ane remediation

The decomposition of dichloromethane in a multiple packed-bed plasma reactor asa function of the number of plasma cells and oxygen concentration is discussed in thissection. A gas mixture of about 500 ppm DCM in nitrogen plasma and a total flow rateof 1 L min−1 have been used. Oxygen concentrations were varied between 0 to 21 %.Spectra were collected in-line after the plasma reactor using FTIR spectroscopy. Figure6.3 shows FTIR spectra for 500 ppm dichloromethane and 1% oxygen in nitrogen plasmaas a function of cells number in a multiple packed-bed plasma reactor. The main speciesdetected by FTIR are NOCl, CO, CO2, N2O, NOx and unreacted DCM. In the case ofusing three cells in series, a small amount of HCN and water were detected. HCN was notdetected after adding more oxygen concentrations to the gas stream. The water presencewas due to some impurities, as well as a small leak in the system.

The removal efficiency of DCM when one, two and three plasma cells were used iscalculated and presented in figure 6.4. Results show that the decomposition of DCMincreased with increasing the cell number. In the case of using one and two plasma

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Figure 6.3: In-line FTIR spectra for 500 ppm DCM and 1% oxygen in nitrogen plasma as afunction of cells number in a multiple packed-bed plasma reactor. A multiple pass opticalgas cell with a 5.3 m pathlength, a total flow rate of 1 L min−1 and a spectral resolution of1 cm−1 were used.

cells the decomposition of DCM has a maximum destruction with 1 to 3 % of addedoxygen, after that increasing the oxygen concentration reduced the removal efficiency ofdichloromethane. In the case of using three plasma cells in series, the destruction of DCMwas at a maximum with 0 and 1 % of added oxygen. After that the destruction of DCMdecreased slightly with increasing the oxygen concentration. These results are similarto the findings in chapter three. As explained in chapter three, increasing the oxygenconcentration leads to the formation of ozone and NOx, which consumes the active speciesin the plasma resulting in less dichloromethane destruction.

Results show that the removal efficiency of dichloromethane increased with increasingthe number of plasma cells of a multiple packed-bed plasma reactor. Increasing the numberof plasma increases the residence time of the gas components in the plasma region. Thisresults in the formation of more active species in the plasma, leading to an increase ofthe number of reactions and collisions with dichloromethane in the plasma, which causeshigher removal efficiency.

The removal efficiency of dichloromethane did not increase linearly with increasingthe number of plasma cells. The increase of the removal efficiency was not a result of anadditive nor a cumulative effect. For example, in the case of 1 % of added oxygen to the

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Figure 6.4: The decomposition of 500 ppm of dichloromethane in nitrogen non-thermalplasma as a function of oxygen concentration and the number of plasma cells of a multiplepacked-bed plasma reactor. A total flow rate of 1 L min−1 was used.

gas stream, an initial DCM concentration of 520 ppm entered the first plasma cell with aresidence time of 1.4 s. The DCM removal efficiency using one cell is about 62 % whichmeans 322 ppm of DCM has been decomposed. If each plasma cell is going to decomposea fixed amount of DCM, this means that the use of two plasma cells should completelyremove the 520 ppm of DCM from the gas stream.

In the case of a cumulative effect, each plasma cell should reduce the DCM concen-tration entering the cell by 62 %. For example, with 520 ppm of DCM entering the firstplasma cell, a 62 % destruction of DCM takes place leaving about 198 ppm of DCM toenter the second cell. With a 62 % of destruction to the 198 ppm taking place in the secondplasma cell, a DCM concentration of about 75 ppm should enter the third cell. With athird 62 % DCM removal to take place in the third cell, the concentration of the DCMleaving the plasma reactor should be about 29 ppm. The total DCM removal efficiencyusing three plasma cells in series with a total residence time of 4.2 s should be about 95 %.Experimental results did not support any of the two theories. The total removal efficiencyof dichloromethane with 1 % oxygen using three plasma cells in series was not 100 % nor96 %. Results showed a 86 % removal efficiency.

This means that, although the residence time of the gas stream in the plasma region is a

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very important factor in increasing the dichloromethane removal efficiency, it is not theonly factor which is playing a role in increasing the removal efficiency. The gas streamentering the second plasma cell has different components to the gas stream entering thefirst plasma cell. This means that the chemical reactions taking place in the second plasmacell are affected by the new chemical species which were produced in the first cell leadingto an unexpected pattern of increase of the removal efficiency of DCM. Harling et al. (82)found similar behaviour for the decomposition of ethylene and toluene in a multiple plasmadischarge reactor process.

6.2.1 The energy efficiency parameter β for the multiple packed-bedplasma reactor

The energy efficiency of the multiple packed-bed plasma reactor was investigated inthis section. The energy efficiency parameter, β , was calculated for each plasma cell withdifferent oxygen concentrations. β allows the comparison of the energy efficiency of thedecomposition of volatile organic compounds in any plasma reactor (76; 82; 140; 174).

[X ] = [X0]exp(−Eβ

)(6.1)

β =−E/ ln[X ]

[X0](6.2)

[X0] is the initial concentration of the investigated VOC entering the plasma reactor. [X] isthe concentration of the VOC leaving the plasma reactor and E is the energy density (JL−1). The energy density E was calculated by dividing the measured power by the gas flowrate as explained in the methodology chapter. Error in calculating the (β ) parameter arisefrom errors in calculating the concentration of the investigated VOC. It has been estimatedto be around 5 percent.

Table 6.1 shows all the values used for calculating the energy efficiency parameter forthe decomposition of 520 ppm of dichloromethane in air plasma using a multiple packed-bed plasma reactor. Dichloromethane concentration entering and leaving the plasma reactoris presented as [X0] and [X] respectively, the energy density (E) and the energy efficiencyparameter (β ) calculated for using one, two and three plasma cells are presented.

Table 6.2 shows the energy efficiency parameter for dichloromethane decompositionusing one, two and three plasma cells with oxygen concentrations ranging between 0 and21 percent.

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Table 6.1: DCM removal efficiency in air plasma using a multiple packed-bed plasmareactor consisting of three plasma cells. Energy density (E) and energy efficiency parameter(β ) are calculated.

Plasma Cells X0 / ppm X / ppm DCM / % E / J L−1 β (±5%) / J L−1

One cell 520 342 34 1494 3570Two cells 520 240 54 2940 3802

Three cells 520 151 71 4422 3576

Table 6.2: The Energy efficiency parameter, β , for the decomposition of 520 ppm of di-chloromethane using one, two and three plasma cells with different oxygen concentrations.

Energy efficiency parameter, β (±5%), / J L−1

O2 / % One cell Two cells Three cells

0 1689 3412 22721 1544 2442 22713 1535 2531 26685 1750 2704 2636

10 2837 3389 345915 2832 3510 377921 3570 3802 3576

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Results show that, with any oxygen concentration, it is more energy efficient to use oneplasma cell compared with using two or three plasma cells. However, with air plasma, theenergy efficiency parameter was almost the same using one or three plasma cells. Theseresults did not match Harling et al.’s (82) findings. They found that using three plasmacells is more energy efficient than using one or two plasma cells. They reported a largedifference of the energy efficiency parameter when using one cell and when using threeplasma cells. The energy efficiency parameter decreased from 8 × 105 J L−1 using oneplasma cell to 2000 J L−1 with three plasma cells. However, they barely obtained a 0.1percent toluene removal using one plasma cell. The removal efficiency of toluene increasedto 72 percent using three plasma cells. Between one and three cells, the removal efficiencyincreases by 720 times, and this is the reason for the large decrease in the energy efficiencyparameter. In this current work, the removal efficiency of dichloromethane increased byabout double by using three plasma cells in air plasma compared with using one plasmacell.

Although the chemical species investigated by Harling et al. (toluene) and the currentwork (dichloromethane) are different, the main reason for the huge difference in the energyefficiency parameter is due to the plasma reactor design. The reactor design plays a veryimportant role in insuring a good distribution of the gas in the plasma region leading toa better decomposition of the pollutants in the gas stream. Their reactor design sufferedfrom a similar problem as that for Fitzsimmons et al.’s reactor design (76), which wasexplained in chapter three of this thesis. Harling et al. reactor design does not allow a gooddistribution of the gas though the entire volume of the packed-bed. The majority of thegas stream passes through the middle part of the reactor, through a small plasma volume.While in the reactor used for the current investigations, the funnels at the inlet and outletof the multiple packed-bed reactor allow for better distribution of the gas over the entirevolume of the plasma region. This shows the importance of a good design of the plasmareactor. A good distribution of the treated gas stream over the entire volume of the plasmaregion enhances the decomposition efficiency of the VOCs in plasma.

6.2.2 The formation of plasma end products as a result of dichloro-methane decomposition in a multiple packed-bed plasma re-actor

The main plasma by-product for dichloromethane decomposition in a multiple packed-bed plasma reactor with nitrogen plasma was HCN. HCl was not detected in this reactor,

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Figure 6.5: Carbon monoxide formation as a function of oxygen concentration and thenumber of plasma cells of a multiple packed-bed plasma reactor. A total flow rate of 1 Lmin−1 was used.

this is due to the presence of oxygen from impurities and leaks, in the system. Previousinvestigations showed that HCl decomposes with the addition of oxygen to the gas stream.CO, CO2, NOCl, NOx, and N2O are the main plasma by-products with the addition ofoxygen to the gas stream. In this section, the influence of oxygen and the number ofplasma cells on the concentration of CO, CO2 and NOCl formation are investigated. Theformation of nitrogen oxides in the plasma with the presence of chlorinated hydrocarbonsis fully discussed in chapter seven. Figures 6.5, 6.6 and 6.7 show the influence of thenumber of plasma cells as well as oxygen concentration on the formation of CO, CO2 andNOCl respectively.

CO concentration increased with increasing the oxygen concentration up to 3 percent.Increasing the oxygen concentration more than 3 percent did not result in a further increasein CO concentration. This is due to the oxidation of CO at higher oxygen concentrations.NOCl formation also increased with increasing the oxygen concentration up to 10 percentand then levelled off. CO2 formation increased with increasing the oxygen concentrationin the gas stream. Similar behaviour for CO and CO2 formation was found in the previousinvestigations of this study.

As for the influence of the number of plasma cells on the formation of plasma endproduct, The concentration of CO, CO2 and NOCl increased with increasing the number

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Figure 6.6: Carbon dioxide formation as a function of oxygen concentration and thenumber of plasma cells of a multiple packed-bed plasma reactor. A total flow rate of 1 Lmin−1 was used.

Figure 6.7: Nitrosyl chloride formation as a function of oxygen concentration and thenumber of plasma cells of a multiple packed-bed plasma reactor. A total flow rate of 1 Lmin−1 was used.

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of plasma cells. This increase is a result of increasing the residence time of the gasstream in the plasma. As explained earlier, increasing the number of plasma cells givesan increase in the residence time of the gas components in the plasma region. Thisresults in the formation of more active species in the plasma, leading to an increase in thenumber of reactions and collisions with dichloromethane in the plasma, which causes moredichloromethane decomposition. This increase of the decomposition of dichloromethaneleads to the production of greater amounts of carbon and chlorine species in the plasmawhich in turn result in more formation of plasma products such as CO, CO2 and NOCl.

6.3 The effect of adding propylene on the removal effi-ciency of dichloromethane in a multiple packed-bedplasma reactor.

As shown in chapter five, the addition of about 1000 ppm of propylene enhanced theremoval efficiency of dichloromethane, whilst adding 500 ppm suppressed the removalefficiency of DCM in a single stage packed-bed plasma reactor. In this section the effectof adding 500 and 1000 ppm propylene on the decomposition of DCM using a multiplepacked-bed plasma reactor consisting of three plasma cells in series is investigated. Acomparison of the results obtained in this section with the results presented in chapter fiveis made.

6.3.1 The decomposition of dichloromethane with the addition of 500ppm propylene using a multiple packed-bed plasma reactor.

A gas mixture of 500 ppm DCM and 500 ppm propylene, reminder nitrogen, to give1 L min−1 flow rate was introduced to a multiple packed-bed plasma reactor. Oxygenconcentrations were varied between 0 and 21 %. Gases were analysed after the plasmareactor with an FTIR spectrometer. Figure 6.8 shows FTIR spectra for a gas mixture of500 ppm DCM and 500 ppm propylene with 1 percent oxygen using one, two and threeplasma cells respectively. The main species detected after the plasma reactor were HCN,CO, CO2, NOCl and nitrogen oxides.

Figure 6.9 presents the removal efficiency of DCM when one, two and three plasmacells were used as a function of oxygen concentration and the addition of 500 ppm

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Figure 6.8: FTIR spectra for a mixture of 500 ppm DCM and 500 ppm propylene withthe addition of 1% oxygen in nitrogen plasma as a function of cells number in a multiplepacked-bed plasma reactor. A multiple pass optical gas cell with a 5.3 m pathlength, aspectral resolution of 1 cm−1 and a total flow rate of 1 L min−1 were used.

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Figure 6.9: The decomposition of 500 ppm DCM in nitrogen non-thermal plasma with theaddition of 500 ppm propylene as a function of oxygen concentration and cells number ina multiple packed-bed plasma reactor. A total flow rate of 1 L min−1 was used.

propylene to the gas stream. The DCM removal efficiency increased with increasing thenumber of plasma cells. However comparing these results with the results obtained withoutthe addition of 500 ppm propylene show that the decomposition of DCM reduced with theaddition of 500 ppm propylene with any number of plasma cells. These results support thefindings in chapter five. Figure 6.10 presents a comparison of the removal efficiency ofDCM with and without the addition of 500 ppm of propylene to the gas stream.

Results show that the addition of 500 ppm of propylene to a gas stream containing500 ppm of DCM did not enhance the removal efficiency of DCM. This was expected, assimilar results have been found when a single stage packed-bed plasma reactor was used.As discussed in chapter five, the addition of low concentrations of propylene consume theactive species in the plasma by reacting with the propylene, while lower concentrationsof OH and H radicals are produced as a result of propylene decomposition. Figure 6.11shows that propylene decomposition in plasma ranged between 70 to an almost completeremoval using one to three plasma cells respectively.

The produced OH and H radicals, as a result of propylene decomposition in plasma, arenot enough to cover for the consumption of the active species in the plasma by propylenedecomposition. This leads to a reduction of the decomposition of dichloromethane in the

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Figure 6.10: A comparison of the decomposition of 500 ppm DCM in nitrogen non-thermalplasma with and without the addition of 500 ppm propylene as a function of oxygenconcentration and the number of plasma cells in a multiple packed-bed plasma reactorconsisting of three plasma cells. A total flow rate of 1 L min−1 was used.

Figure 6.11: The decomposition of 500 ppm propylene in nitrogen non-thermal plasmaas a function of oxygen concentration and cells number in a multiple packed-bed plasmareactor. A total flow rate of 1 L min−1 was used.

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plasma system.

Although the removal efficiency of DCM with the addition of 500 ppm propylene wasless than without the addition of propylene, the removal efficiency still increased withincreasing the number of plasma cells as shown in figure 6.10.

As explained earlier, increasing the number of plasma cells gave an increase to theresidence time of the gas components in the plasma region. This results in the formationof more active species in the plasma leading to an increase in the number of reactionsand collisions with dichloromethane in the plasma, which causes more dichloromethanedecomposition.

6.3.2 The decomposition of dichloromethane with the addition of 1000ppm propylene using a multiple packed-bed plasma reactor.

As previously investigated in chapter five, the addition of 1000 ppm propylene to thegas stream increased the decomposition of DCM after 5 % of added oxygen using a singlestage plasma reactor. In this section, the effect of adding 1000 ppm propylene to the gasstream on the removal efficiency of DCM using a multiple packed-bed plasma reactor isinvestigated. A gas mixture of 500 ppm DCM and 1000 ppm propylene, reminder nitrogen,giving a 1 L min−1 flow rate was introduced to a multiple packed-bed plasma reactor.Oxygen concentrations were varied between 0 to 21 %. The gas stream was analysedbefore and after the plasma reactor using FTIR spectrometer.

Figure 6.12 presents the removal efficiency of DCM with the addition of 1000 ppmpropylene using one, two and three plasma cells as a function of oxygen concentration.Results show that a small decrease of the decomposition of dichloromethane occurredwith the addition of 1000 ppm propylene and using one plasma cell. To explain thisdecrease in the removal efficiency of dichloromethane, the decomposition of the 1000 ppmpropylene in the plasma system was calculated. Figure 6.13 shows the decomposition of1000 ppm propylene in the multiple packed-bed plasma rector. With one plasma cell, thedecomposition of propylene was between 40 to 50 percent. This means that only about400 to 500 ppm of propylene was decomposed in the plasma. Previous investigations ofthe effect of adding 500 ppm of propylene to the gas stream on the removal efficiency ofdichloromethane, showed that this concentration is not enough to participate in enhancingdichloromethane decomposition. The decomposition of 500 ppm propylene, consumed the

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Figure 6.12: The decomposition of 500 ppm DCM in nitrogen non-thermal plasma with theaddition of 1000 ppm propylene as a function of oxygen concentration and plasma cellsnumber in a multiple packed-bed plasma reactor consisting of three plasma cells. A totalflow rate of 1 L min−1 was used.

Figure 6.13: The decomposition of 1000 ppm propylene in nitrogen non-thermal plasmaas a function of oxygen concentration and cells number in a multiple packed-bed plasmareactor. A total flow rate of 1 L min−1 was used.

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Figure 6.14: A comparison of the decomposition of 500 ppm DCM in nitrogen non-thermalplasma with and without the addition of 1000 ppm propylene as a function of oxygenconcentration and the number of plasma cells in a multiple packed-bed plasma reactorconsisting of three plasma cells. A total flow rate of 1 L min−1 was used.

active species in the plasma and did not generate enough OH and H radicals to cover thisconsumption and participate in further dichloromethane decomposition.

The decomposition of DCM with the addition of 1000 ppm propylene using two andthree plasma cells, increased linearly with increasing the oxygen concentration up to5 %. After that, increasing the oxygen concentration up to 21 % resulted in a slightincrease of dichloromethane decomposition. The results for two and three plasma cellssupport our findings in chapter five, of the influence of adding 1000 ppm propylene onthe removal efficiency of dichloromethane. Figure 6.13 shows that the decomposition of1000 ppm propylene using two and three plasma cells was in the range of 70 to 90 percentrespectively.

A comparison of the decomposition efficiency of DCM with and without the additionof 1000 ppm propylene is presented in Figure 6.14. Results show that dichloromethanedecomposition in a multiple packed-bed plasma rector, increased with the addition of 1000ppm propylene and oxygen concentrations higher than five percent. A dichloromethaneremoval efficiency of about 85 percent was achieved in air plasma with the addition of1000 ppm propylene using three plasma cells.

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6.3.3 The energy efficiency parameter β for dichloromethane decom-position in a multiple packed-bed plasma reactor with the ad-dition of 1000 ppm propylene to the gas stream.

The energy efficiency parameter (β ) for the decomposition of dichloromethane withthe addition of 1000 ppm propylene using a multiple packed-bed plasma reactor wasinvestigated.

Table 6.3 shows all the values used for calculating the energy efficiency parameterfor the decomposition of 510 ppm of dichloromethane with the addition of 1000 ppmpropylene in air plasma. Dichloromethane concentration entering and leaving the plasmareactor is presented as [X0] and [X] respectively, the energy density (E) and the energyefficiency parameter (β ) calculated for using one, two and three plasma cells are presented.

Table 6.3: DCM removal efficiency in air plasma (%) with the addition of 1000 ppmpropylene using a multiple packed-bed plasma reactor consisting of three plasma cells.Energy density (E) and energy efficiency parameter (β ) are calculated.

Plasma Cells X0 / ppm X / ppm DCM / % E / J L−1 β (±5%) / J L−1


Three cells 510 77 85 4788 2524

Table 6.4 shows the energy efficiency parameter calculated for using one, two and threeplasma cells for the decomposition of dichloromethane with the addition of 1000 ppmpropylene to the gas stream. The β parameter for the decomposition of dichloromethanewith oxygen concentrations ranging between 0 and 21 percent was calculated.

Comparing the energy efficiency parameter values shows that, with oxygen concen-trations ranging between 0 and 5 %, it is more energy efficient to use one plasma cellcompared with using two or three plasma cells. Increasing the oxygen concentration morethan 5 percent makes the system more energy efficient with using two plasma cells. Resultsshow that using a third plasma cell did not further reduce the energy efficiency parameter.This finding is very important, as it calls into question the benefit of adding a third plasmacell for the decomposition of dichloromethane.

As the energy efficiency parameter decreases with increasing the removal efficiency

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Table 6.4: The Energy efficiency parameter, β , for the decomposition of 510 ppm ofdichloromethane with the addition of 1000 ppm propylene to the gas stream and using one,two and three plasma cells with different oxygen concentrations.

Energy efficiency parameter, β (±5%), / J L−1

O2 / % One cell Two cells Three cells

0 2155 4918 63801 1609 2524 23173 1776 3323 35545 2563 2981 29757 2894 2822 2883

10 2806 2599 270215 3646 2458 261321 4219 2528 2524

of the VOC in the plasma system. It is important to identify the number of plasma cellswhich could be used, while the plasma system is still an energy efficient system.

6.4 In situ IR absorption measurements for the decom-position of dichloromethane using a multiple packed-bed plasma reactor.

In situ absorption measurements of plasma processes are very important for providinginformation of the chemical species generated in the plasma region. This helps in under-standing the chemical reactions taking place in the plasma, which lead to the decompositionof VOCs and the formation of plasma end products. This understanding of the chemicalmechanism provides information of the best ways to modify the plasma system in order toachieve a better VOC removal, as well as reducing the harmful plasma end products.

As it was not possible to carry out in situ absorption measurements through the plasmaregion generated in the packed-bed plasma reactor, as explained in the methodologychapter, analysis was carried out on the gas stream immediately after it exits the plasmaregion, rather than downstream . The in-line measurements were taken about one meterdownstream of the plasma reactor with a time delay of about 0.75 seconds. This delaybetween the moment the gas exits the plasma and the moment it reaches the measuringpoint could cause some change of the DCM concentrations and hence the calculated

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removal efficiency. These in situ measurements could also help with detecting some ofthe short-lived species which are recombined or decomposed after the plasma reactor andbefore they reach the measuring point. Identifying these products will help in furtherunderstanding the mechanism of the chemical reactions taking place in the plasma.

These experiments are focused on comparing the removal efficiency of DCM measuredin situ immediately after the plasma region and in-line after about 0.75 seconds fromexiting the plasma region. This work also investigates the presence of any chemical specieswhich are generated in the plasma but reacted by the time they reach the measuring pointdownstream .

A gas mixture of about 500 ppm DCM in nitrogen plasma and a total flow rate of 1 Lmin−1 have been used. Oxygen concentrations were varied between 0 to 21 %. At first,experiments were carried out using one neon sign power supply providing a high voltage ofabout 14 kV pk-pk at a frequency of 20 kHz connected to the three plasma cells at the sametime. Figure 6.15 illustrates the multiple packed-bed plasma reactor arrangements withone power supply providing power to the three cells at the sane time. The mid-infraredbeam from the FTIR spectrometer was directed through the reactor via the KBr windowsimmediately after each plasma cell. The pathlegth of the beam through the reactor is 10 cm.The beam exiting the reactor was collected and directed to an external MCT detector usingoff axis parabolic mirrors. Spectra obtained were analysed using Bruker’s OPUS softwareto calculate the concentrations of all the species in the gas phase, as was explained in themethodology chapter.

When carrying out these in situ measurements, two problems were faced. First, CO2

measurements cannot be reliable as the IR beam travels through an open area outside theplasma reactor. CO2 is present normally in the air and its concentration in the lab wasaffected by the number of students in the lab at the time of the experiment. The secondproblem was an abnormality in the spectra background between 2700 and 3500 cm−1.This means that any species which have a vibration mode in this region cannot be analysed.Figure 6.16 shows an example spectrum after the first plasma cell illustrating these twoproblems. As can be seen, CO2 concentration was less than the concentration recordedin the back ground spectrum. This could be due to more students in the lab at the timewhen the back ground spectrum was taken comparing with when this spectrum was taken.The abnormality in the spectrum between 2700 and 3500 cm−1 wavenumbers could bedue to electric field interference between the transformer and the external MCT detectoras they were placed close to each other. It also could be due to some kind of particulatedeposited on the KBr windows or the reflecting mirrors. The presence of water in the

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Figure 6.15: In situ measurements using a multiple packed-bed plasma reactor whichconsists of three plasma cells. One power supply was used to power the three cells at thesame time.

room’s atmosphere presented another difficulty for accurately measuring the concentrationsof NO2, NOCl and NO. For these reasons, the only investigated species in this section isdichloromethane.

Figure 6.17 shows the removal efficiency of DCM immediately after each plasma cellin a multiple packed-bed plasma reactor consisting of three plasma cells. The removalefficiency of DCM after the first plasma cell has a maximum of about 63 % with 1 to 3% of added oxygen. The decomposition decreased slightly after that with the increaseof oxygen concentration. The removal efficiency of DCM using two and three plasmacells connected to the same power supply at the same time had also a maximum with 1to 3 % of added oxygen. Increasing the oxygen concentration more than 3 % resulted inreducing the DCM removal efficiency. The lowest DCM removal efficiency was whenthree plasma cells were used, compared with using one or two plasma cells. Measuring theenergy density when one, two and three plasma cells were used showed that the energydensity was reduced with an increased number of plasma cells. This was expected as theenergy was distributed among the three cells rather than being concentrated on one plasmacell. Table 6.5 presents the removal efficiency of DCM, the energy density and the energyefficiency parameter with the use of one, two and three plasma cells at 21 % of addedoxygen. Results show that it is more energy efficient to use one plasma cell than using twoor three plasma cells connected to the same power supply at the same time.

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Figure 6.16: An in situ FTIR spectrum after the first plasma cell in a multiple packed-bedplasma reactor which consists of three plasma cells. A pathlength of 10 cm, a flow rate of1 L min−1 and an oxygen concentration of 5 % were used.

Figure 6.17: DCM removal efficiency as a function of plasma cells number and oxygenconcentrations measured immediately after each plasma cell in a multiple packed-bedplasma reactor consisting of three plasma cells. A total flow rate of 1 L min−1 was used.

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Table 6.5: The removal efficiency of 500 ppm DCM in air plasma using a multiple packed-bed plasma reactor consisting of three plasma cells connected to one power supply. Energydensity (E) and energy efficiency parameter (β ) are calculated.

Plasma Cells X0 / ppm X / ppm DCM/ % E / J L−1 β (±5%) / J L−1


Three cells 500 410 18 960 4837

Decreasing the energy density applied to each plasma cell, reduces the collision fre-quency in the plasma, resulting in less formation of the excited species in the plasma. Thisresults in a decrease in the decomposition of dichloromethane.

The plan was to repeat these in situ measurements with three similar power supplies,each of them powering one of the three plasma cells, similar to the experiments present atthe start of this chapter. However, these experiments were not possible at the time for safetyreasons. Some modifications of the experimental set up and the electrical connections wererequired to enclose all the experimental parts in a safety box with interlocks. Unfortunately,these modifications were not done on time, so these experiments had to be left for furtherwork.

6.4.1 Comparison of dichloromethane measured in situ and in-lineusing a multiple packed-bed plasma reactor.

A comparison of the detected dichloromethane concentration exiting the first plasmacell, and the concentration detected downstream at the measuring point, (with a time delayof about 0.75 seconds), is presented in figure 6.18. As using two and three plasma cellsconnected to the same power supply resulted in a less energy density compared with wheneach cell is connected to a separate power supply, a comparison between the in situ and inline measurements in this case are invalid.

The measurements of the removal efficiency of DCM in situ and in line were almostthe same between 0 to 5 % of added oxygen to the gas stream. The decomposition ofDCM measured with more than 5 % oxygen present was more with in situ measurementscompared with in line measurements. The in line measurements were carried out at adistance of about a meter downstream of the plasma reactor with a time delay of about0.75 seconds.

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Figure 6.18: Comparison of the in situ and in line measurements of the DCM removalefficiency using one plasma cell in a multiple packed-bed plasma reactor as a function ofoxygen concentration. An energy density of 1470 and 1494 J L−1 were used for the in situand in line experiments respectively. A flow rate of 1 L min−1 was used.

Results indicates that a recombination among the active radicals in the plasma productcontinue to take place after the gas has exited the plasma region, which causes an increaseof the DCM concentration detected downstream . One of these reactions could be betweenCH2Cl and chlorine molecules Cl2 as shown in reaction R6.50.

CH2Cl+Cl2 −−→ CH2Cl2 +Cl (R6.50)

A comparison of the concentration of DCM after the second and the third plasma cellwith downstream measurements is not possible, as the energy density was extremelydifferent in these two cases. Further investigations for the concentrations of the otherplasma by-products are required to establish a full understanding of the reactions whichmay continue to take place after the plasma region.

6.4.2 The detection of new species with in situ measurements using amultiple packed-bed plasma reactor.

This section investigates the formation of short-lived chemical species which aregenerated in the plasma region and continued to react with active radicals after the plasmaand before reaching the measuring point downstream . The in situ measurements which

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Figure 6.19: The absorbance of the new peak as a function of oxygen concentration. Insitu measurements were taken after the first plasma cell. An energy density of about 1470J L−1 was applied.

were made with an FTIR spectrometer, of the decomposition of dichloromethane in anitrogen-oxygen plasma showed a new absorbance peak at 1380 - 1430 cm−1 which wasnot detected with in line measurements. Infrared absorption in this range could be dueto C-O-H bending, O-H bending, CH2 and CH3 deformation (115). The intensity of thispeak increased with increasing oxygen concentration, indicating that it is more likely tobe C-O-H or O-H bending. Figure 6.19 shows a comparison of the peak absorbance withoxygen concentration. It is not possible to confirm whether it did or did not appear in theplasma before adding oxygen as some oxygen impurities exist in the inlet system at alltimes. This peak is still unassigned.

Also, it would not be possible to detect any new species which have a vibrational modein the region were the abnormality was occurring in the spectrum.


Results presented in this chapter show that using a multiple packed-bed plasma reactorenhanced the removal efficiency of both dichloromethane and a mixture of propylene anddichloromethane. A maximum removal efficiency of dichloromethane in air plasma wasachieved under certain conditions.

• Increasing the number of plasma cells, increases the removal efficiency of dichloro-methane, mainly as a result of increasing the residence time. The energy efficiencyparameter should be taken into consideration when using more than one plasma cell.

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• The removal efficiency of about 500 ppm of dichloromethane increased with theaddition of 1000 ppm of propylene to the gas stream. A maximum removal efficiencyof about 85 % was achieved in air plasma with the use of three plasma cells.

• It is more energy efficient to decompose a 500 ppm of DCM in air plasma with theaddition of 1000 ppm propylene using two plasma cells in series compared withusing one plasma cell or less propylene concentration.

• The residence time in the plasma region is a very important factor in increasing theremoval efficiency of VOCs. However, it is not the only factor which plays a rolein increasing the removal efficiency of the VOC in a multiple packed-bed plasmareactor. The plasma product produced after one plasma cell affect the reactionswhich are taking place in the next plasma cell.

• The formation of plasma end products increased with increasing the number ofplasma cells.

• In situ measurement is very important as it helps with further understanding themechanism of the chemical reactions taking place in and after the plasma region.This helps in establishing the optimum conditions for VOCs removal in plasma.

Results presented in chapters three to six showed that, about 85 percent removalefficiency of dichloromethane in air plasma generated in packed-bed plasma reactors canbe obtained. However, air plasma leads unavoidably to the formation of nitrogen oxides.Next chapter discuss the behaviour of nitrogen oxides in non-thermal plasma.

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Chapter 7

Behaviour of nitrogen oxides in non-thermal plasmapacked-bed reactor

7.1 Introduction

Previous chapters presented the successful use of non-thermal plasma generated ina nitrogen-oxygen gas mixture to decompose VOCs. However, the use of nitrogen andoxygen gas mixture in a plasma to remove VOCs leads inevitably to the formation ofnitrogen oxides (77; 165; 175–177). Nitrogen oxides are air pollutants which are amongstthe causes of several environmental problems such as formation of ozone, acid rain andglobal warming (9; 178).

The formation of nitrogen oxides in a non-thermal plasma is affected by the presenceof volatile organic compounds in the gas stream. This chapter investigates the formation ofnitrogen oxides with the decomposition of dichloromethane, methyl chloride, propylene,and a gas mixture of dichloromethane and propylene in non-thermal plasmas generatedin a single stage packed-bed plasma reactor. The formation of nitrogen oxides using amultiple packed-bed plasma reactor consisting of three plasma cells is also discussed.

When a single stage packed-bed plasma reactor was used, the inlet system used inchapters three, four and five was used for these investigations. For the use of a multiplepacked-bed plasma reactor, the inlet system explained in chapter six was used.

7.2 Methodology

The formation of nitrogen oxides in a single stage packed-bed plasma reactor wasdescribed using the experimental set up explained in chapter three. A Neon sign powersupply providing a frequency of 20 kHz and about 14 kV pk-pk was used to power theplasma reactor. Nitrogen (99.998 %) and oxygen (99.99%) gases were used as suppliedfrom BOC. Gas flow was controlled using MKS 247 mass flow controllers. Gas analysiswas carried out using in line FTIR spectroscopy both before and after the plasma reactor.

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Figure 7.1: FTIR spectrum for nitrogen oxides formation in nitrogen plasma with theaddition of 3 percent oxygen to the gas stream. A multiple pass optical gas cell with a 5.3m pathlength, a total flow rate of 1 L min−1 and an energy density of about 1820 J L−1

were used.

Measurements were carried out about 20 minutes after initiating the plasma in thereactor to ensure that a steady state was achieved. This was discussed in the methodologychapter. Most experiments were repeated at least three times and averages of the resultsare presented.

7.3 Formation of nitrogen oxides in non-thermal plasmawith nitrogen and oxygen gas mixture

Figure 7.1 shows an example FTIR spectrum for plasma exhaust of nitrogen and 3 %oxygen with a total flow rate of 1 L min−1 and energy density of about 1820 J L−1. Asseen in the FTIR spectrum, NO, NO2 and N2O are detected. The spectrum shows thepresence of some amount of water in the system. It is thought to be due to some impuritiesin the inlet system. Figure 7.2 presents the concentration of NO, NO2 and N2O generatedin nitrogen plasma as a function of oxygen concentration.

Results show that the formation of NO and NO2 increased with increasing the oxygenconcentration. While the concentration of N2O increased with increasing the oxygen

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Figure 7.2: Nitrogen oxides concentrations in nitrogen plasma generated in a packed-bedplasma reactor as a function of oxygen concentrations.

concentration up to 3 percent then levelled off at about 120 ppm.

7.4 The effect of adding chlorinated hydrocarbons on theformation of nitrogen oxides in non-thermal plasmagenerated in a single stage packed-bed plasma reactor.

Chapters three and four of this thesis have shown that non-thermal plasma couldbe efficiently used for the remediation of dichloromethane and methyl chloride in anitrogen-oxygen plasma. However, the presence of dichloromethane and methyl chloridein nitrogen-oxygen plasmas affects the formation of nitrogen oxides.

This section investigates the effect of CH2Cl2 and CH3Cl on nitrogen oxide generationin plasma. 500 ppm of DCM and CH3Cl have been added separately to the gas stream.Figure 7.3 shows FTIR spectra for samples collected after the plasma reactor. Spectrum(a) presents plasma exhaust for an inlet gas stream of 500 ppm dichloromethane and 3%oxygen in nitrogen plasma. Spectrum (b) presents plasma exhaust for an inlet gas streamof 500 ppm methyl chloride and 3% oxygen in nitrogen plasma. The concentrations ofNO, NO2 and N2O were calculated as a function of oxygen concentration in the plasma.

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Figure 7.3: FTIR spectra for plasma exhaust with the presence of DCM and CH3Cl innitrogen plasma. Spectrum (a) presents plasma exhaust for 500 ppm DCM and 3% oxygen.Spectrum (b) presents plasma exhaust for 500 ppm CH3Cl and 3% oxygen. A total flowrate of 1 L min−1 and an energy density of about 1820 J L−1 were used.

Tables 7.1 and 7.2 present the concentrations of NO, NO2 and N2O with the presence of500 ppm dichloromethane and 500 ppm methyl chloride respectively. Figures 7.4, 7.5 and7.6 show a comparison of the concentration of NO, NO2 and N2O as a function of oxygenconcentration and the presence of 500 ppm of DCM and methyl chloride respectively.

The concentration of NO, NO2 and N2O are almost the same with the addition ofdichloromethane and methyl chloride. These results show that chlorinated hydrocarbonshave the same effect on NOx formation in a nitrogen and oxygen plasma.

Results show that NO concentration was reduced with the presence of DCM and methylchloride in nitrogen and oxygen plasma, while NO2 and N2O concentration increased.Futamura et al. (175) have reported a similar result. They found that the addition ofchlorinated air pollutants such as CH2Cl2, CCl4 and trichloroethylene (C2HCl3) to aplasma inlet gas mixture of nitrogen and oxygen suppressed NO formation and remarkablyincreased NO2 selectivities.

Adding chlorinated hydrocarbons to nitrogen-oxygen plasma leads to the formation ofchlorine monoxide (ClO) mainly via reactions of oxygen atoms with dichloromethane and

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Table 7.1: The concentrations of NO, NO2 and N2O in ppm with the presence of 500 ppmDCM as a function of oxygen concentration in nitrogen plasma

O2 % NO NO2 N2O

0 0 0 231 17 63 1172 33 105 1363 49 147 1464 59 169 1485 74 189 1437 133 225 1609 155 255 170

10 151 262 16913 195 306 17315 227 339 17217 234 344 17321 226 330 175

Table 7.2: The concentrations of NO, NO2 and N2O in ppm with the presence of 500 ppmmethyl chloride as a function of oxygen concentration in nitrogen plasma

O2 % NO NO2 N2O

0 0 2 361 29 62 1332 47 107 1353 51 147 1444 65 178 1505 98 193 1567 142 235 1619 157 267 164

10 164 279 16713 200 295 16615 223 319 16917 236 341 16421 225 330 159

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Figure 7.4: Nitric oxide concentration as a function of oxygen concentrations and theaddition of 500 ppm DCM and CH3Cl respectively to the plasma inlet gas stream. A totalflow rate of 1 L min−1 and an energy density of about 1820 J L−1 were used.

Figure 7.5: Nitrogen dioxide concentration as a function of oxygen concentrations and theaddition of 500 ppm DCM and CH3Cl respectively to the plasma inlet gas stream. A totalflow rate of 1 L min−1 and an energy density of about 1820 J L−1 were used.

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Figure 7.6: Nitrous oxide concentration as a function of oxygen concentrations and theaddition of 500 ppm DCM and CH3Cl respectively to the plasma inlet gas stream. A totalflow rate of 1 L min−1 and an energy density of about 1820 J L−1 were used.

methyl chloride in plasma.

O(1D)+CH2Cl2 −−→ CH2Cl+ClO (R7.51)

O(1D)+CH3Cl−−→ CH3 +ClO (R7.52)

Oxygen atoms react also with chlorine molecules to form ClO.

Cl2 +O−−→ ClO+Cl (R7.53)

The reaction of chlorine atoms with ozone and oxygen molecules also leads to theformation of ClO.

Cl+O3 −−→ ClO+O2 (R7.54)

Cl+O2 −−→ ClO+O (R7.55)

Chlorine monoxide reacts with NO and converts it to NO2 leading to less NO and moreNO2 compared to concentrations without the presence of chlorinated hydrocarbons.

NO+ClO−−→ NO2 +Cl (R7.56)

Futamura et al. (175) and Harling et al. (176) have proved using experiments andsimulations respectively that halogen oxides (such as ClO) are responsible for the increaseof the conversion of NO to NO2 in plasma.

Nitric oxides also react with NCO and NCl, which are formed in the plasma as a resultof DCM and methyl chloride decomposition, to form N2O (179; 180).

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7.5 The effect of adding propylene alone compared witha mixture of dichloromethane and propylene on theformation of nitrogen oxides in non-thermal plasma

Chapter five of this thesis investigated the decomposition of propylene and dichloro-methane using a non-thermal plasma generated from nitrogen and oxygen gas streams.The formation of nitrogen oxides in the presence of propylene, and a mixture of propyleneand DCM are investigated in this section.

In this section, the effect of adding 1000 ppm propylene alone and a mixture of 500ppm DCM and 1000 ppm propylene on nitrogen oxides generation in nitrogen and oxygengas stream were carried out. Oxygen concentration was varied between 1 and 21% andan energy density of about 1820 J L−1 was applied. Figure 7.7 Shows FTIR spectra forsamples collected after the plasma reactor. Spectrum (a) presents the plasma exhaust foran inlet gas stream of 1000 ppm propylene and 3% oxygen in nitrogen plasma. Spectrum(b) presents the plasma exhaust for an inlet gas stream of 500 ppm DCM and 1000 ppmpropylene with 3% oxygen in nitrogen plasma. The concentrations of NO, NO2 and N2Owere calculated. Figures 7.8, 7.9 and 7.10 show a comparison of the concentrations ofNO, NO2 and N2O respectively as a function of oxygen concentration and the additionof 1000 ppm propylene alone and a mixture of 500 ppm DCM and 1000 ppm propylenerespectively.

Comparing the results with and without adding propylene to a nitrogen-oxygen gasstream, shows that adding 1000 ppm of propylene to the gas stream reduces the formationof nitric oxide and nitrous oxides. However, the concentrations of nitrogen dioxidesincreased slightly with the addition of 1000 ppm propylene to the gas stream.

Adding propylene to the gas stream results in the formation of hydroperoxy radicals(HO2) via reactions of hydrocarbon intermediates produced from C3H6 oxidation as shownin the following two reactions (165):

CH3O+O2 −−→ CH2O+HO2 (R7.57)

HCO+O2 −−→ CO+HO2 (R7.58)

The HO2 radical is the main radical that converts NO to NO2 (162; 165).

NO+HO2 −−→ NO2 +OH (R7.59)

This reaction form OH radicals which in turn react with NO and NO2 forming nitrous

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Figure 7.7: FTIR spectra for plasma exhaust with the presence of DCM and C3H6 innitrogen plasma. Spectrum (a) presents plasma exhaust for 1000 ppm C3H6 and 3%oxygen. Spectrum (b) presents plasma exhaust for a mixture of 500 ppm DCM and 1000ppm C3H6 with 3% oxygen. A total flow rate of 1 L min−1 and an energy density of about1820 J L−1 were used.

Figure 7.8: Nitric oxide concentration as a function of oxygen concentrations and theaddition of 1000 ppm propylene alone and a mixture of 500 ppm DCM and 1000 ppmpropylene. A total flow rate of 1 L min−1 and an energy density of about 1820 J L−1 wereused.

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Figure 7.9: Nitrogen dioxide concentration as a function of oxygen concentration and theaddition of 1000 ppm propylene alone and a mixture of 500 ppm DCM with 1000 ppmpropylene. A total flow rate of 1 L min−1 and an energy density of about 1820 J L−1 wereused.

Figure 7.10: Nitrous oxide concentration as a function of oxygen concentration and theaddition of 1000 ppm propylene alone and a mixture of 500 ppm DCM with 1000 ppmpropylene. A total flow rate of 1 L min−1 and an energy density of about 1820 J L−1 wereused.

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and nitric acids respectively (162; 165):

NO+OH−−→ HNO2 (R7.60)

NO2 +OH−−→ HNO3 (R7.61)

RO2, organic peroxy radicals, are formed as a result of C3H6 reaction with O and OHradicals. RO2 reacts with NO converting it into NO2 (124; 162; 163; 181).

NO+RO2 −−→ NO2 +RO (R7.62)

These reactions show that adding propylene to a nitrogen-oxygen plasma reduces theconcentration of nitric oxides by further oxidation into nitrogen dioxide. Futamura et

al.(175) and Martin et al. (124) have reported that NO to NO2 conversion increased withthe addition of hydrocarbons to the plasma inlet gas stream. The addition of propyleneresulted in reducing the concentrations of N2O. This is due to decomposition of N2O byreaction with CH3O and CN, which are produced in the plasma as a result of propylenedecomposition (182; 183).

Several studies have reported the effect of adding hydrocarbons to a plasma gas streamon the reduction of NOx (162), (163), (165). Shin et al. (162) and Dorai et al. (163)have carried out kinetic modelling studies to investigate the effect of hydrocarbons on theoxidation of NO to NO2. Shin et al. found that propene is more effective in oxidizing NOcompared with other hydrocarbons such as methane (CH4) and ethylene (C2H4). Dorai et

al. proved that propylene has a significant effect on the reduction of NOx, particularly NOconcentrations in plasma.

Adding a mixture of 500 ppm dichloromethane and 1000 ppm propylene to a nitrogen-oxygen plasma, resulted in further reduction of nitric oxide generation, no nitric oxideswhere detected with 1 to 3 % of added oxygen. More formation of NO2 occurred in thissituation, this is due to the further oxidation of NO to NO2 as a result of the formation ofboth halogen oxides (such as ClO) and hydrocarbon radicals RO2.

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7.6 The influence of the number of plasma cells on theformation of nitrogen oxides as plasma by productduring the decomposition of dichloromethane in a nitrogen-oxygen plasma.

The formation of nitrogen oxides as a plasma by product during dichloromethanedecomposition in a multiple packed-bed plasma reactor is investigated in this section. Theexperimental set up, explained in chapter six, is used for this investigation. A multiplepacked-bed plasma reactor consisting of three cells in series was used. Figure 6.2 showsa photograph and a sketch of the multiple packed-bed plasma reactor. Plasma cells werepowered by three identical neon sign power supplies each providing high voltage alternatingcurrent at a frequency of 20 kHz. Measurements were carried out in line using an FTIRspectrometer with a multiple pass cell of 5.3 meters pathlength.

Figures 7.11, 7.12 and 7.13 show the formation of nitric oxide, nitrogen dioxide andnitrous oxide in a multiple packed-bed plasma reactor as a function of oxygen concentrationand the addition of 500 ppm dichloromethane.

Results show that the concentrations of NO, NO2 and N2O increased with using twoplasma cells compared with using one plasma cell. However, the concentration of nitrogenoxides decreased with using three plasma cells.

Explaining the behaviour of nitrogen oxides in a multiple packed-bed plasma rectoris not straight forward. Several factors play a role in the formation and reduction ofnitrogen oxides in the multiple packed-bed plasma reactor. The chemistry taking placein the first plasma cell is similar to that discussed earlier in this chapter. However, thechemistry taking place in the second and third cell is completely different. As the feedgas components for the second cell are a mixture of chemical species which were formedin the first plasma cell. A similar situation occurs for the third plasma cell. To be ableto establish the chemical mechanism for the generation of nitrogen oxides in a multiplepacked-bed plasma reactor, all the species in the feed gas for each cell need to be identifiedand quantified. This shows the importance of the in situ measurements which were carriedout in chapter six. More work is required to enhance the in situ measurements, in order toanalyze all the chemical species in the gas stream.

171

Figure 7.11: Nitric oxide concentration in a multiple packed-bed plasma reactor as afunction of oxygen concentrations and the addition of 500 ppm dichloromethane. A totalflow rate of 1 L min−1 was used.

Figure 7.12: Nitrogen dioxide concentration in a multiple packed-bed plasma reactor as afunction of oxygen concentrations and the addition of 500 ppm dichloromethane. A totalflow rate of 1 L min−1 was used.

172

Figure 7.13: Nitrous oxide concentration as a function of oxygen concentration and theaddition of 1000 ppm propylene alone and a mixture of 500 ppm DCM with 1000 ppmpropylene. A total flow rate of 1 L min−1 and an energy density of about 1820 J L−1 wereused.

7.7 A comparison of NOx behaviour in all the studied con-ditions in air plasma.

As the aim of this work is to achieve the best removal efficiency of VOCs in airplasma, a comparison of nitrogen oxides concentration with the previously discussedconditions using a single stage packed-bed plasma reactor is presented. Figure 7.14 showsthe concentrations of NO, NO2 and the total amount of nitrogen oxides (NO + NO2 +N2O) with the conditions: (a) air plasma, (b) air plasma with the addition of 500 ppmDCM, (c) air plasma with the addition of 500 ppm methyl chloride, (d) air plasma with theaddition of 1000 ppm propylene and (e) air plasma with the addition of a mixture of 500ppm DCM and 1000 ppm propylene. Results show that nitric oxide concentration was atmaximum without the addition of any VOCs to the plasma system. The addition of anyof the investigated VOCs (dichloromethane, methyl chloride, propylene and a mixture ofdichloromethane and propylene) reduced the concentration of nitric oxide. The lowestconcentration of NO was formed with the addition of a mixture of dichloromethane andpropylene. A reduction of about 48 percent in the concentration of nitric oxide took placeunder these conditions compared with air plasma.

173

Figure 7.14: NOx concentration in air plasma. (a)air plasma no VOCs, (b) 500 ppm DCMin air plasma, (c) 500 ppm CH3Cl in air plasma, (d) 1000 ppm C3H6 in air plasma and(e) mixture of 500 ppm DCM and 1000 ppm C3H6. A total flow rate of 1 L min−1 and anenergy density of about 1820 J L−1 were used.

Nitrogen dioxide concentration increased with the addition of any of the investigatedVOCs to the gas stream. The largest concentration of NO2 was formed with the additionof a mixture of dichloromethane and propylene. An increase of about 38 percent in theconcentration of nitrogen dioxide took place under these conditions compared with airplasma. Comparing the 48 % reduction in NO concentration with the 38 % increase inNO2 concentration in the case of adding a mixture of dichloromethane and propylene toair plasma, shows that not all the decrease in nitric oxide concentration is due to furtheroxidation of NO to NO2. Nitric oxide concentration is also reduced through reactions withplasma intermediates such as: NCO, HCCO, Cl and Cl2 to form CO2, HCN and NOCl(184–186).

The total amount of nitrogen oxides, increased with the addition of chlorinated hydrocar-bons to air plasma. The addition of propylene alone reduced nitrogen oxides concentrationcompared with air plasma. While the total amount of nitrogen oxides with the addition ofa mixture of dichloromethane and propylene was almost the same as that with air plasma.

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7.8 Reaction pathway for the formation of nitrogen ox-ides in non-thermal plasma reactor.

Several studies have investigated the chemical kinetic mechanism for the formation ofnitrogen oxides (152; 165; 175; 176; 187; 188). A brief explanation of the main chemicalreactions which leads to the formation of nitrogen oxides in plasma is discussed in thissection.

Excited nitrogen and oxygen atoms and molecules such as N(2P), N(4S), N(2D), N2

(A3 Σu+), O(3P), O(1D) and O2(1∆) are formed in the plasma as a result of electron impact.

The presence of these species in the plasma lead to the formation of nitrogen oxides. Itis known that, nitrogen and oxygen plasma will unavoidably lead to the formation ofNO, NO2 and N2O (77; 165; 175; 176). Nitrogen oxides are mainly formed through thefollowing reactions:

N+O+M−−→ NO+M (R7.63)

N(2D)+O2 −−→ NO+O (R7.64)

N+O3 −−→ NO+O2 (R7.65)

Further oxidation of NO, produce NO2:

O(3P)+NO+M−−→ NO2 +M (R7.66)

NO+NO+O2 −−→ NO2 +NO2 (R7.67)

NO+O3 −−→ NO2 +O2 (R7.68)

N2O is also a common plasma by-product if nitrogen and oxygen are present in plasma.

N∗2 +O2 +M−−→ N2O+O+M (R7.69)

O(1D)+N2 +M−−→ N2O+M (R7.70)

As well as the formation of NO in the plasma, reverse reactions for converting NO tomolecular nitrogen N2 takes place(165).

NO+N(4S)−−→ N2 +O (R7.71)

175


This section investigated the formation of nitrogen oxides as plasma by-products ofVOCs decomposition in non-thermal plasma generated in a nitrogen-oxygen gas mixture.Results showed that the formation of nitric oxides is reduced by about 50 percent comparedwith nitrogen-oxygen plasma without the addition of any VOC.

• Nitric oxide generation in nitrogen-oxygen plasma is reduced with the addition ofany of the following VOCs to the gas stream: dichloromethane, methyl chloride,propylene and a mixture of dichloromethane and propylene.

• The addition of a mixture of dichloromethane and propylene resulted in the form-ation of the lowest concentration of nitric oxide, while the total nitrogen oxidesconcentrations did not increase.

• The reduction of nitric oxides in nitrogen-oxygen plasma with the addition of severalVOCs is mainly due to further oxidation of nitric oxide to nitrogen dioxide.

As the presence of the VOCs in the plasma helps the oxidation of NO to NO2, moreNO2 was produced while the total amount of nitrogen oxides stayed almost the same aswithout adding VOCs to the gas stream. However, it is known that the removal of NO2 froma gas stream using catalyst, such as a lean NOx catalyst, NOx traps, and selective catalyticreduction (SCR), is easier than the removal of NO using catalyst (35; 165; 189–195)especially with the presence of hydrocarbons (124; 165; 194; 196–202).

NO2 +hydrocarbons+ catalyst−−→ N2 +CO2 +H2O (R7.72)

Although nitrogen oxides were formed as a result of VOCs remediation in non-thermalplasma, it is possible to remove the formed nitrogen oxides species using a variety ofcatalysts. Catalysts could be combined with the plasma reactor (181; 203–207) or couldbe placed after the plasma reactor (36; 165) Catalysts were not used in this study, however,the use of catalyst to remove NOx has been well researched and investigated by severalgroups (36; 165; 190–194; 196; 201–204; 208–215).

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Chapter 8

Summary, conclusions and further work

The primary aim of this work was to identify the influence of several parameterson the removal efficiency of chlorinated VOCs in an atmospheric pressure non-thermalplasma generated in a packed-bed plasma reactor. With the investigation carried out, itwas then possible to optimize the parameters which play a key role in chlorinated VOCsremediation to achieve a maximum removal efficiency in air plasma. This work also aimsto reduce the formation of nitrogen oxides which are formed as plasma by-products forVOCs decomposition.

Investigating the influence of oxygen concentration, initial VOC concentration, energydensity and plasma residence time on the removal efficiency of dichloromethane andmethyl chloride resulted in the following findings:

• The removal efficiency of dichloromethane and methyl chloride in atmosphericpressure non-thermal plasma generated in a packed-bed plasma reactor with nitrogen-oxygen gas mixture has a maximum with 2 to 4 % of added oxygen.

• The removal efficiency of dichloromethane and methyl chloride in nitrogen-oxygenplasma decreased with increasing oxygen concentrations above 5 percent as a resultof the formation of nitrogen oxides and ozone in the plasma. Such decrease of thedecomposition with high oxygen concentrations is expected to take place with anytype of VOCs as a result of reducing the active species in the plasma region.

To optimize the influence of oxygen on the removal efficiency of chlorinated VOCs, inorder to achieve the maximum VOC remediation in air plasma, a propylene additive wasused.

• Increasing the initial concentration of dichloromethane and methyl chloride in thefeed gas results in decreasing their removal efficiency in a packed-bed plasma reactorwith nitrogen and oxygen gas mixture.

For practical applications, the concentration of the chlorinated VOCs in the feed gas cannotbe controlled, as these concentrations differ according to the industrial process whichproduces these VOCs in the process exhausts. This parameter was not investigated further.

177

• Increasing the energy density in the plasma system increased the removal efficiencyof dichloromethane and methyl chloride in a packed-bed plasma reactor with nitrogenand oxygen gas mixture. The decomposition of VOCs in any plasma system isexpected to increase with increasing the energy density as a result of increasing theactive species generated in the plasma volume.

As increasing the energy density in the plasma system increases the cost of the system, nofurther investigation of this parameter was carried out.

• A longer residence time for the gas stream in the plasma increased the removalefficiency of dichloromethane and methyl chloride; this is due to increasing theamount of active species generated in the plasma which leads to increasing thenumber of collisions.

To enhance the influence of plasma residence time on the remediation of chlorinated VOCs,a multiple packed-bed plasma reactor consisting of three plasma cells was used. Increasingthe residence time of the gas stream in the plasma region is expected to have a positiveeffect in enhancing the removal efficiency of all types of air pollutants.

The addition of propylene to the gas stream and the use of a multiple packed-bedplasma reactor, resulted in the following findings:

• Adding 1000 ppm of propylene to the gas stream, improved the removal efficiencyof dichloromethane especially with oxygen concentrations higher than 5 percent.Lesser propylene concentrations did not improve dichloromethane remediation innon-thermal plasma.

• An optimum removal efficiency of about 75 % for dichloromethane in air plasmawas achieved with the addition of 1000 ppm of propylene to the gas stream.

• The removal efficiency of propylene in a nitrogen-oxygen plasma in the presenceof dichloromethane was significantly higher compared with the situation of nodichloromethane in the gas stream.

• Increasing the number of plasma cells, increases the removal efficiency of dichloro-methane, mainly as a result of increasing the residence time.

178

• The residence time in the plasma region is a very important factor in increasing theremoval efficiency of VOCs. However, it is not the only factor which plays a rolein increasing the removal efficiency of the VOC in a multiple packed-bed plasmareactor. The plasma product produced after one plasma cell, affect the reactionswhich are taking place in the next plasma cell.

• A maximum removal efficiency of about 85 % for about 500 ppm of dichloromethanewas achieved in air plasma with the use of three plasma cells and the addition of1000 ppm propylene to the gas stream.

In the case of the formation of nitrogen oxides as plasma by-product of chlorinatedVOCs decomposition in plasma:

• Nitric oxide generation in a nitrogen-oxygen plasma is reduced with the additionof any of the following VOCs to the gas stream: dichloromethane, methyl chloride,propylene and a mixture of dichloromethane and propylene.

• The addition of a mixture of dichloromethane and propylene resulted in the form-ation of the lowest concentration of nitric oxide, while the total nitrogen oxidesconcentrations did not increase. Adding alkenes to plasma gas stream is expected toenhance the decomposition of other VOCs in the gas stream as well as reducing theformation of NO. However, the optimum molar ratio between the added alkene andthe other VOCs in the gas stream might differ.

To conclude, the maximum removal efficiency of dichloromethane was achieved in anair plasma with the addition of 1000 ppm of propylene and the use of three packed-bedplasma cells in series. The lowest concentration of nitric oxide was formed in this situation.This work add a better understanding of the decomposition of chlorinated VOCs in non-thermal plasma in general and packed-bed reactors in particular. The obtained results applyfor all types of non-thermal plasma reactors which are working under similar conditions ofapplied power, temperature, pressure, residence time and initial VOC concentrations. Ithighlights the important effect of VOCs mixtures on the decomposition of these mixturesas well as the reduction of harmful by-products such as NOx.

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8.1 Further work

The main area which requires further work is the in situ measurement of the plasmaprocess. In situ measurements are very important for identifying and quantifying thechemical species formed in the plasma. This information helps in further understandingthe mechanism of the chemical reactions taking place in the plasma region. It also helpsin optimizing the plasma system and conditions to achieve complete VOC removal usingnon-thermal plasma.

As the gas exhaust from industrial processes contains a mixture of volatile organic com-pounds, it is important to investigate the decomposition of a mixture of VOCs, simulatingindustrial gas exhaust in non-thermal plasma. This work showed that the decompositionof both propylene and dichloromethane in a gas mixture containing both of them using apacked-bed plasma reactor was enhanced significantly compared with the decompositionof each of them when treated alone. Further investigations of the decomposition of otherchlorinated VOCs such as methyl chloroform (C2H3Cl3), trichloroethylene (C2HCl3) andtetrachloroethylene (C2Cl4) would be useful to check whether higher chlorinated VOCsfollow the same decomposition scheme as methyl chloride and dichloromethane.

Kinetic simulation is another area for further development. Modelling the decompos-ition of methyl chloride, dichloromethane, propylene; and a mixture of propylene anddichloromethane is important for supporting the experimental results and understandingthe breakdown pathways of the VOCs in plasma.

Further investigations of the addition of catalysts to the packed-bed plasma reactor forNOx reduction would be advantageous. Catalysts can be added downstream of the plasmareactor or mixed with the dielectric beads in the same region.

Lots of studies have reported the ability of achieving a good decomposition of differenttypes of VOCs using non-thermal plasma technology. However, plasma technology isyet to be applied for treating the exhaust of industrial plants. A compact plasma systemwhich achieves a good decomposition of mixtures of VOCs and gets rid of the harmfulby-products, such as NOx and COx at the same time is still to be carried out. All the studieswhich have been carried out so far are for laboratory scale experiments and for shortrunning times. Plasma reactors which can treat the huge volumes of industrial processesexhaust and run continuously are still to be tested.

180

Bibliography

[1] J. T. Kiehl and K. E. Trenberth, “Earth’s annual global mean energy budget,” Bulletin

of the American Meteorological Society, vol. 78, pp. 197–208, 1997.

[2] G. Francis, The Glow Discharge at Low Pressure. Springe, 1956.

[3] H. Conrads and M. Schmidt, “Plasma generation and plasma sources,” Plasma

Sources Science and Technology, vol. 9, no. 4, p. 441, 2000.

[4] Y. Kim, M. S. Cha, W.-H. Shin, and Y.-H. Song, “Characteristics of dielectric barrierglow discharges with a law- frequency generator in nitrogen,” Journal Of the Korean

Physical Society, vol. 43, pp. 732–737, 2003.

[5] T. Shimanouchi, Tables of Molecular Vibrational Frequencies. (Consolidated

Volume 1). Us National Bureau of Standards National Standard Reference DataSeries, United States. National Bureau of Standards, 1972.

[6] C. Baird, Environmental Chemistry. New York: W. H. Freeman and Company,second ed., 1995.

[7] R. Hester and R. Harrison, eds., Volatile Organic Compounds in the Atmosphere.Cambridge: Royal Society of Chemistry, 1995.

[8] N. D. Nevers, Air pollution control engineering. McGraw - Hill Book Inc., 1995.

[9] S. Metcalfe and D. Derwent, Atmospheric Pollution and Environmental Change.Hodder Arnold, an imprint of Hodder Education, a member of the Hodder HeadlineGroup, 338 Euston Road, London NW1 3BH, 2005.

[10] Q. U. A. R. Group, “Diesel vehicle emissions and urban air quality,” Tech. Rep.Second Report, Public and Environmental Health, December 1993.

[11] O. of Air Quality Planning and Standards, “Our nation’s air status and trendsthrough 2008,” tech. rep., U.S. Environmental Protection Agency, Office of AirQuality Planning and Standards, Research Triangle Park, North Carolin., 2008.

[12] U. S. E. P. Agenc, “EPA National Air Pollutant Emission Trends 1900 1998,” tech.rep., Office of Air Quality Planning and Standards, Research Triangle Park, NC2771, 2000.

181

[13] U. S. E. P. Agency, “National air pollutant emission trends, procedures document,1900-1996,” tech. rep., Office of Air Quality Planning and Standards, ResearchTriangle Park NC 27711, 1998.

[14] C. Leigh, “Air quality - an overview,” in Fuels Emissions and Air Quality, pp. 52–107, Society of Operations Engineers, 2001.

[15] J. Dewulf and H. Van Langenhove, “Anthropogenic volatile organic compoundsin ambient air and natural waters: a review on recent developments of analyticalmethodology, performance and interpretation of field measurements,” Journal of

Chromatography A, vol. 843, no. 1-2, pp. 163–177, 1999.

[16] B. P. Bell, P. Franks, N. Hildreth, and J. Melius, “Methylene-chloride exposure andbirth-weight in Monroe County, New-York,” Environmental Research, vol. 55, no. 1,pp. 31–39, 1991.

[17] H. Taskinen, M. L. Lindbohm, and K. Hemminki, “Spontaneous-abortions amongwomen working in the pharmaceutical-industry,” British Journal of Industrial Medi-

cine, vol. 43, no. 3, pp. 199–205, 1986.

[18] W. B. Bunn, T. W. Hesterberg, P. A. Valberg, T. J. Slavin, G. Hart, and C. A. Lapin,“A reevaluation of the literature regarding the health assessment of diesel engineexhaust,” Inhalation Toxicology, vol. 16, no. 14, pp. 889–900, 2004.

[19] C. W. M. Bodar, F. Berthault, J. H. M. de Bruijn, C. J. van Leeuwen, M. E. J. Pronk,and T. G. Vermeire, “Evaluation of EU risk assessments existing chemicals (ECRegulation 793/93),” Chemosphere, vol. 53, no. 8, pp. 1039–1047, 2003.

[20] T. Elbir, B. Cetin, E. Cetin, A. Bayram, and M. Odabasi, “Characterization ofvolatile organic compounds (VOCs) and their sources in the air of Izmir, Turkey,”Environmental Monitoring and Assessment, vol. 133, pp. 149–160, 2007.

[21] H. Guo, S. C. Lee, P. K. K. Louie, and K. F. Ho, “Characterization of hydrocarbons,halocarbons and carbonyls in the atmosphere of Hong Kong,” Chemosphere, vol. 57,no. 10, pp. 1363–1372, 2004.

[22] R. Friedrich and S. Reis, Emissions of Air Pollutants. Springer, 2004.

[23] K. Urashima and J.-S. Chang, “Removal of volatile organic compounds from airstreams and industrial flue gases by non-thermal plasma technology,” Dielectrics

and Electrical Insulation, IEEE Transactions on, vol. 7, pp. 602 –614, oct 2000.

182

[24] T. K. Poddar, S. Majumdar, and K. K. Sirkar, “Removal of VOCs from air bymembrane-based absorption and stripping,” Journal of Membrane Science, vol. 120,pp. 221–237, 1996.

[25] J. Cha, V. Malik, D. Bhaumik, R. Li, and K. Sirkar, “Removal of VOCs from wastegas streams by permeation in a hollow fiber permeator,” Journal of Membrane

Science, vol. 128, pp. 195–211, 1997.

[26] R. S. Juang, S. H. Lin, and M. C. Yang, “Mass transfer analysis on air stripping ofVOCs from water in microporous hollow fibers,” Journal of Membrane Science,vol. 255, pp. 79–87, 2005.

[27] B. Ozturk and D. Yilmaz, “Absorptive removal of volatile organic compoundsfrom flue gas streams,” Trans IChemE, Part B, Process Safety and Environmental

Protection, vol. 84(B5), p. 391398, 2006.

[28] A. Fridman, Plasma Chemistry. Cambridge University Press, 2008.

[29] T. Oda, “Non-thermal plasma processing for environmental protection: decomposi-tion of dilute VOCs in air,” Journal of Electrostatics, vol. 57, no. 3-4, pp. 293–311,2003. 4th International Conference on Applied Electrostatistics (ICAES 2001) OCT08-10, 2001 DALIAN, PEOPLES R CHINA.

[30] R. McAdams, “Prospects for non-thermal atmospheric plasmas for pollution abate-ment,” Journal of Physics D-Applied Physics, vol. 34, no. 18, pp. 2810–2821, 2001.Conference on the Future of Technological Plasmas MAR 19-21, 2001 BRIGHTON,ENGLAND.

[31] X. Feng and C. J. Rong, “Application of non-thermal plasma technology for indoorair pollution control,” Environmental Informatics Archives, vol. 2, pp. 628–634,2004.

[32] EPA, “Using non-thermal plasma to control air pollutants,” tech. rep., 2005.

[33] A. Mizuno, “Industrial applications of atmospheric non-thermal plasma in envir-onmental remediation,” Plasma Physics and Controlled Fusion, vol. 49, no. 5A,pp. A1–A15, 2007. 13th International Congress on Plasma Physics MAY 22-26,2006 Kiev, UKRAINE.

[34] L. A. Rosocha and R. A. Korzekwa, “Advanced oxidation and reduction processes inthe gas phase using non-thermal plasmas,” Advanced Oxidation Technology, vol. 4,no. 3, 1999.

183

[35] H. H. Kim, “Non-thermal plasma processing for air pollution control: A historicalreview, current issues, and future prospects,” Plasma Processes and Polymers, vol. 1,no. 2, pp. 91– 110, 2004.

[36] T. Yamamoto, C. L. Yang, M. R. Beltran, and Z. Kravets, “Plasma-assisted chemicalprocess for NOx control,” IEEE Transactions on Industry Applications, vol. 36,no. 3, pp. 923–927, 2000.

[37] R. Corporation, “Locating and estimating air emissions from sources of methylenechloride,” tech. rep., Radian Corporation Post Office Box 13000 Research TrianglePark, North Carolina 27709, Dallas Safriet Emission Inventory Branch U. S. En-vironmental Protection Agency Research Triangle Park, North Carolina 27711,February 1993.

[38] A. McCulloch, M. L. Aucott, T. E. Graedel, G. Kleiman, P. M. Midgley, and Y.-F.L, “Industrial emissions of trichloroethene tetrachloroethene and dichloromethanereactive chlorine emissions inventory,” Journal Of Geophysical Research, vol. 104,pp. 8417–8428, 1999.

[39] J. M. Lobert, W. C. Keene, J. A. Logan, and R. Yevic, “Global chlorine emis-sions from biomass burning: Reactive chlorine emissions inventory,” Journal Of

Geophysical Research, vol. 104, pp. 8373–8389, 1999.

[40] A. McCulloch, M. L. Aucott, C. M. Benkovitz, T. E. G. G. Kleiman, P. M. Midgley,and Y.-F. Li, “Global emissions of hydrogen chloride and chloromethane from coalcombustion and industrial activities reactive chlorine emissions inventory,” Journal

Of Geophysical Research, vol. 104, pp. 8391 – 8403, April 20 1999.

[41] F. Keppler, D. B. Harper, T. Rockmann, R. M. Moore, and J. T. G. Hamilton,“New insight into the atmospheric chloromethane budget gained using stable carbonisotope ratios,” Atmospheric Chemistry and Physics, vol. 5, p. 3899391, 2005.

[42] W. C. Keene, M. A. K. Khalil, D. J. E. III, A. McCulloch, T. E. Graedel, J. M.Lobert, M. L. Aucott, S. L. Gong, D. B. Harper, G. Kleiman, P. Midgley, R. M.Moore, C. Seuzaret, W. T. Sturges, C. M. Benkovitz, V. Koropalov, L. A. Barrie, andY. F. Li, “Composite global emissions of reactive chlorine from anthropogenic andnatural sources: Reactive chlorine emissions inventory,” Journal Of Geophysical

Research, vol. 104, pp. 8429–8440, 1999.

[43] M. Solid Waste Association of North America Silver Spring, Polyvinyl Chiloride

Plastics in Municipal Solid Waste Combustion Impact Upon Dioxin Emissions - A

184

Synthesis of Views. National Renewable Energy Laborator, Solar Energy ResearchInstitute, 1993.

[44] P. Linstrom and W. Mallard, eds., NIST Chemistry WebBook, NIST Standard Refer-

ence Database Number 69, http://webbook.nist.gov, (retrieved February 28, 2012).

National Institute of Standards and Technology, Gaithersburg MD, 20899.

[45] K. R. G?rtler and K. Kleinermanns, “Photooxidation of exhaust pollutants. ii:Photooxidation of chloromethanes : degradation efficiencies, quantum yields andproducts,” Chemosphere, vol. 28, pp. 1289–1298, 1994.

[46] R. Grtler, U. Mller, S. Sommer, H. Mller, and K. Kleinermanns, “Photooxidation ofexhaust pollutants: Iii. photooxidation of the chloroethenes: Degradation efficien-cies, quantum yields and products,” Chemosphere, vol. 29, no. 8, pp. 1671 – 1682,1994.

[47] A. Fridman and L. Kennedy, Plasma Physics and Engineering. Taylor and Francis,2004.

[48] Y. Eliezer and S. Eliezer, The fourth state of matter. Adam Hilger, BristolandPhiladelphia, 1989.

[49] W. Manheimer, L. E. Sugiyama, and T. H. Stix, eds., Plasma Science and the

Environment. American Institute of Physics, 1997.

[50] C. Tendero, C. Tixier, P. Tristant, J. Desmaison, and P. Leprince, “Atmosphericpressure plasma: A review,” Spectrochimica Acta Part, vol. 61, pp. 2–30, 2006.

[51] E. T. Protasevich, Cold non-equilibrium plasma, generation, properties, applica-

tions. Cambridge International Science Puplishing, 1999.

[52] H. H. Kim, G. Prieto, K. Takashima, S. Katsura, and A. Mizuno, “Performanceevaluation of discharge plasma process for gaseous pollutant removal,” Journal of

Electrostatics, vol. 55, no. 1, pp. 25 – 41, 2002.

[53] B. Eliasson and U. Kogelschatz, “Nonequilibrium volume plasma chemical pro-cessing,” Plasma Science, IEEE Transactions on, vol. 19, pp. 1063 –1077, dec1991.

[54] M. Lieberman and A. Lichtenberg, Principles of plasma discharges and materials

processing. Wiley-Interscience, 1994.

185

[55] J. R. Roth, Industrial Plasma Engineering Volume 2: Applications to Nonthermal

Plasma Processing. Institute of Physics Publishing Bristol and Philadelphia, 2001.

[56] A. Bogaerts, E. Neyts, R. Gijbels, and J. Van der Mullen, “Gas discharge plasmasand their applications,” Spectrochimica Acta Part B, vol. 57, pp. 609–658, 2002.

[57] J.-S. Chang, P. Lawless, and T. Yamamoto, “Corona discharge processes,” Plasma

Science, IEEE Transactions on Plasma Science, vol. 19, pp. 1152 –1166, dec 1991.

[58] S. Masuda, S. Hosokawa, X. Tu, and Z. Wang, “Novel plasma chemical technologies- PPCP and SPCP for control of gaseous pollutants and air toxics,” Journal of

Electrostatics, vol. 34, p. 415 438, 1995.

[59] U. Kogelschatz, “Filamentary, patterned, and diffuse barrier discharges,” IEEE

Transactions on Plasma Science, vol. 30, no. 4, pp. 1400 – 1408, 2002.

[60] T. Oda, R. Yamashita, K. Tanaka, T. Takahashi, and S. Masuda, “Analysis of low-temperature surface discharge plasma products from gaseous organic compounds,”Industry Applications, IEEE Transactions on, vol. 32, pp. 1044 –1050, sep/oct 1996.

[61] S. Masuda, E. Kiss, K. Ishida, and H. Asai, “Ceramic based ozonizer for high speedsterilization,” in Industry Applications Society Annual Meeting, 1988., Conference

Record of the 1988 IEEE, vol. 2, pp. 1641 –1646, oct 1988.

[62] T. Oda, T. Takahashi, H. Nakano, and S. Masuda, “Decomposition of fluorocarbongaseous contaminants by surface discharge-induced plasma chemical processing,”Industry Applications, IEEE Transactions on, vol. 29, pp. 787 –792, jul/aug 1993.

[63] T. Oda, R. Yamashita, T. Takahashi, and S. Masuda, “Atmospheric pressure dis-charge plasma decomposition for gaseous air contaminants-trichlorotrifluoroethaneand trichloroethylene,” Industry Applications, IEEE Transactions on, vol. 32, pp. 227–232, mar/apr 1996.

[64] H. H. Kim, H. Kobara, A. Ogata, and S. Futamura, “Comparative assessment ofdifferent nonthermal plasma reactors on energy efficiency and aerosol formationfrom the decomposition of gas-phase benzene,” IEEE Transactions on Industry

Applications, vol. 41, no. 1, pp. 206–214, 2005. ESA-IEEE/IAS/EPC Joint Meeting2003 JUN 24-27, 2003 Little Rock, AR.

[65] H. L. Chen, H. M. Lee, S. H. Chen, and M. B. Chang, “Review of packed-bed plasmareactor for ozone generation and air pollution control,” Industrial & Engineering

Chemistry Research, vol. 47, no. 7, pp. 2122–2130, 2008.

186

[66] K. Schmidt-Szaowski, “Catalytic properties of silica packings under ozone synthesisconditions,” Ozone: Science & Engineering, vol. 18, pp. 41–55, 1996.

[67] P. O. R. Group, “Ozone in the United Kingdom,” Tech. Rep. Fourth Report, 1997.

[68] T. Yamamoto, K. Ramanathan, P. A. Lawless, D. S. Ensor, J. R. Newsome, N. Plaks,and G. H. Ramsey, “Control of volatile organic-compounds by an AC energizedferroelectric pellet reactor and a pulsed corona reactor,” IEEE Transactions on

Industry Applications, vol. 28, no. 3, pp. 528–534, 1992.

[69] T. Yamamoto, “Decomposition of toluene,trichloroethylene, and their mixture usinga BaTiO3 packed-bed plasma reactor,” Advanced Oxidation Technology, vol. 1,no. 1, pp. 67–78, 1996.

[70] T. Yamamoto, “Optimization of nonthermal plasma for the treatment of gas streams,”Journal of Hazardous Materials, vol. 67, no. 2, pp. 165–181, 1999.

[71] K. Takaki, J. S. Chang, and K. G. Kostov, “Atmospheric pressure of nitrogen plasmasin a ferro-electric packed bed barrier discharge reactor - Part I: Modeling,” IEEE

Transactions on Industry Applications Transactions on Dielectrics and Electrical

Insulation, vol. 11, no. 3, pp. 481–490, 2004.

[72] K. Takaki, K. Urashima, and J. S. Chang, “Ferro-electric pellet shape effect on C2F6

removal by a packed-bed-type nonthermal plasma reactor,” IEEE Transactions on

Plasma Science, vol. 32, no. 6, pp. 2175–2183, 2004.

[73] A. Ogata, K. Mizuno, S. Kushiyama, and T. Yamamoto, “Methane decompositionin a barium titanate packed-bed nonthermal plasma reactor,” Plasma Chemistry and

Plasma Processing, vol. 18, pp. 363–373, 1998. 10.1023/A:1021897419040.

[74] A. Ogata, N. Shintani, K. Mizuno, S. Kushiyama, and T. Yamamoto, “Decomposi-tion of benzene using a nonthermal plasma reactor packed with ferroelectric pellets,”IEEE Transactions on Industry Applications, vol. 35, no. 4, pp. 753–759, 1999.

[75] M. Sugasawa, G. Annadurai, and S. Futamura, “Reaction behavior of toluene-dichloromethane mixture in nonthermal plasma,” Industry Applications, IEEE

Transactions on, vol. 45, pp. 1499 –1505, july-aug. 2009.

[76] C. Fitzsimmons, F. Ismail, J. C. Whitehead, and J. J. Wilman, “The chemistry ofdichloromethane destruction in atmospheric-pressure gas streams by a dielectricpacked-bed plasma reactor,” Journal of Physical Chemistry A, vol. 104, no. 25,pp. 6032–6038, 2000.

187

[77] A. E. Wallis, J. C. Whitehead, and K. Zhang, “The removal of dichloromethanefrom atmospheric pressure air streams using plasma-assisted catalysis,” Applied

Catalysis B-Environmental, vol. 72, no. 3-4, pp. 282–288, 2007.

[78] A. E. Wallis, J. C. Whitehead, and K. Zhang, “The removal of dichloromethanefrom atmospheric pressure nitrogen gas streams using plasma-assisted catalysis,”Applied Catalysis B-Environmental, vol. 74, no. 1-2, pp. 111–116, 2007.

[79] A. M. Harling, H. H. Kim, S. Futamura, and J. C. Whitehead, “Temperature depend-ence of plasma-catalysis using a nonthermal, atmospheric pressure packed bed; thedestruction of benzene and toluene,” Journal of Physical Chemistry C, vol. 111,no. 13, pp. 5090–5095, 2007.

[80] A. M. Harling, A. E. Wallis, and J. C. Whitehead, “The effect of temperatureon the removal of DCM using non-thermal, atmospheric-pressure plasma-assistedcatalysis,” Plasma Processes and Polymers, vol. 4, no. 4, pp. 463–470, 2007.

[81] A. M. Harling, H.-H. Kim, S. Futamura, and J. C. Whitehead, “Temperature depend-ence of plasma-catalysis using a nonthermal, atmospheric pressure packed bed; thedestruction of benzene and toluene,” The Journal of Physical Chemistry C, vol. 111,no. 13, pp. 5090–5095, 2007.

[82] A. M. Harling, D. J. Glover, J. C. Whitehead, and K. Zhang, “Novel methodfor enhancing the destruction of environmental pollutants by the combination ofmultiple plasma discharges,” Environmental Science & Technology, vol. 42, no. 12,pp. 4546–4550, 2008. PMID: 18605584.

[83] A. M. Harling, D. J. Glover, J. C. Whitehead, and K. Zhang, “The role of ozone inthe plasma-catalytic destruction of environmental pollutants,” Applied Catalysis B:

Environmental, vol. 90, no. 1-2, pp. 157 – 161, 2009.

[84] S. L. Hill, C. Whitehead, and K. Zhang, “Plasma processing of propane at hyper-atmospheric pressure: Experiment and modelling,” Plasma Processes and Polymers,vol. 4, no. 7-8, pp. 710–718, 2007. Times Cited: 2 10th International Symposium onHigh Pressure, Low Temperature Plasma Chemistry SEP 04-08, 2006 Saga, JAPAN.

[85] S. L. Hill, H. H. Kim, S. Futamura, and J. C. Whitehead, “The destruction ofatmospheric pressure propane and propene using a surface discharge plasma reactor,”Journal of Physical Chemistry A, vol. 112, no. 17, pp. 3953–3958, 2008. TimesCited: 1.

188

[86] H. L. Chen, H. M. Lee, and M. B. Chang, “Enhancement of energy yield for ozoneproduction via packed-bed reactors,” Ozone: Science & Engineering, vol. 28, no. 2,pp. 111–118, 2006.

[87] A. B. Murphy and R. Morrow, “Glass sphere discharges for ozone production,”IEEE Transactions On Plasma Science, vol. 30, no. 1, pp. 180–181, 2002.

[88] A. Mizuno, Y. Yamazaki, H. Ito, and H. Yoshida, “Ac energized ferroelectric pelletbed gas cleaner,” IEEE Transactions on Industry Applications, vol. 28, no. 3, pp. 535–540, 1992. 1990 Annual Meeting of The Industry Applications SOC of IEEE Oct07-12, 1990 Seattle, Wa.

[89] C.-L. Chang and T.-S. Lin, “Decomposition of toluene and acetone in packeddielectric barrier discharge reactors,” Plasma Chemistry and Plasma Processing,vol. 25, no. 3, pp. 227–243, 2005.

[90] T. Yamamoto, “VOC decomposition by nonthermal plasma processing - a newapproach,” Journal of Electrostatics, vol. 42, no. 1-2, pp. 227–238, 1997.

[91] M. Sugasawa, G. Annadurai, and S. Futamura, “Nonthemal plasma chemical pro-cessing of mixed VOCs,” Conference Record of the 2005 IEEE Industry Applications

Conference, Vols 1-4, pp. 2918–2923, 2005. 40th Annual Meeting of the IEEE-Industry-Applications-Society OCT 02-06, 2005 Hong Kong, PEOPLES R CHINA.

[92] A. E. Wallis, J. C. Whitehead, and K. Zhang, “The removal of dichloromethanefrom atmospheric pressure air streams using plasma-assisted catalysis,” Applied

Catalysis B-Environmental, vol. 72, no. 3-4, pp. 282–288, 2007.

[93] T. Yamamoto and S. Futamura, “Nonthermal plasma processing for controllingvolatile organic compounds,” Combustion Science and Technology, vol. 133, no. 1-3,pp. 117–133, 1998.

[94] I. K. Ahmad, A. E. Wallis, and J. C. Whitehead, “The plasma destruction of odorousmolecules: Organosulphur compounds,” High Temperature Material Processes,vol. 7, no. 4, pp. 487–499, 2003.

[95] K. Jogan, A. Mizuno, T. Yamamoto, and J. S. Chang, “The effect of residence timeon the CO2 reduction from combustion flue gases by an AC ferroelectric packed-bedreactor,” IEEE Transactions on Industry Applications, vol. 29, no. 5, pp. 876–881,1993.

189

[96] P. A. Gorry, J. C. Whitehead, and J. Wu, “Adaptive control for NOx removal innon-thermal plasma processing,” Plasma Processes and Polymers, vol. 4, no. 5,pp. 556–562, 2007.

[97] L. Huang, K. Nakajo, S. Ozawa, and H. Matsuda, “Decomposition of dichlorometh-ane in a wire-in-tube pulsed corona reactor,” Environmental Science & Technology,vol. 35, no. 6, pp. 1276–1281, 2001.

[98] B. M. Penetrante, M. C. Hsiao, J. N. Bardsley, B. T. Merritt, G. E. Vogtlin, A. Kuthi,C. P. Burkhart, and J. R. Bayless, “Decomposition of methylene chloride by electronbeam and pulsed corona processing,” Physics Letters A, vol. 235, no. 1, pp. 76–82,1997.

[99] C. T. Li, W. J. Lee, C. Y. Chen, and Y. T. Wang, “CH2Cl2 decomposition byusing a radio-frequency plasma system,” Journal of Chemical Technology and

Biotechnology, vol. 66, no. 4, pp. 382–388, 1996.

[100] L.-T. Hsieh, W.-J. Lee, C.-Y. Chen, Y.-P. G. Wu, S.-J. Chen, and Y.-F. Wang,“Decomposition of methyl chloride by using an RF plasma reactor,” Journal of

Hazardous Materials, vol. 63, no. 1, pp. 69 – 90, 1998.

[101] R. B. Bird, W. E. Stewart, and E. N. Lightfoot, Transport phenomena. John Wiley& Sons, second edition ed., 2002.

[102] G. K. Batchelor, An introduction to fluid dynamics. Cambridge University Press,1967.

[103] B. S. Massy, Mechanics of fluids. Chapman & Hall, sixth edition ed., 1989.

[104] S. S. Ryu, S. C. Park, S. P. Lee, D. S. Sinn, S. K. Lee, and D. H. Yoon, “Method formanufacturing dielectric ceramic BaTiO3 powder, and multilayer ceramic capacitorobtained by using the ceramic powder,” 2010.

[105] W. D. Kingery, D. R. Uhlmann, and H. K. Bowen, Introduction to ceramics, 2nd

ed. New York Chichester: Wiley-Interscience, 1976. Previous ed. by W.D. Kingery,1960. Bibl. - Index.

[106] H. S. Nalwa, ed., Handbook of Low and High Dielectric Constant Materials and

Their Applications. Elsevier Inc, 1999.

[107] W. Kanzig, “History of ferroelectricity 1938-1955,” Ferroelectrics, vol. 74, no. 1,pp. 285–291, 1987.

190

[108] J. M. Hubner and J. Ropcke, “On the destruction of volatile organic compoundsusing a dielectric pellet bed reactor,” Journal of Physics: Conference Series, 3rd

International Workshop on Infrared Plasma Spectroscopy, vol. 157, 2009.

[109] P. R. Griffiths and J. A. de Haseth, Fourier Transform Infrared Spectrometry. JohnWiley & Sons, Inc., second edition ed., 2007.

[110] C. Banwell and E. McCash, Fundamentals of molecular Spectroscopy,. McGraw -Hill Book Company, fourth edition ed., 1994.

[111] J. M. Chalmers, ed., Spectroscopy in Process Analysis. Sheffield Academic PressLtd, 2000.

[112] P. Atkins, Physical Chemistry. Oxford University Press, sixth ed., 1998.

[113] S. W. Sharpe, T. J. Johnson, R. L. Sams, P. M. Chu, G. C. Rhoderick, and P. A.Johnson, “Gas-phase databases for quantitative infrared spectroscopy,” Applied

Spectroscopy, vol. 58, no. 12, pp. 1452–1461, 2004.

[114] R. H. Pierson, A. N. Fletcher, and E. S. Gantz, “Catalog of infrared spectra forqualitative analysis of gases,” Analytical Chemistry, vol. 28, no. 8, pp. 1218–1239,1956.

[115] G. Socrates, infrared Characteristic Group Frequencies. Chichester: John Wiley &Sons, second edition ed., 1994.

[116] P. M. Chu, F. R. Guenther, G. C. Rhoderick, and W. J. Laffert, “The NIST quantitat-ive infrared database,” Journal of Research of the National Institute of Standards

and Technology, vol. 104, no. 59, pp. 59–81, 1999.

[117] R. Pearse and A. Gaydon, The Identification of Molecular Spectra. fourth edition ed.,1976.

[118] A. Striganov and N. Sventitskii, Tables of Spectral Lines of Nutral and Ionized

Atoms. 1968.

[119] Online, “National Institute of Standards and Technology (NIST), Atomic SpectraDatabase 78, (cited 22/02/2012)(http://www.nist.gov/pml/data/asd.cfm),” 2012.

[120] R. Perry and D. Green, Perry’s Chemical Engineers’ Handbook. McGraw-Hill, 7thedition ed., 1997.

191

[121] F. Holzer, F. D. Kopinke, and U. Roland, “Influence of ferroelectric materials andcatalysts on the performance of non-thermal plasma (ntp) for the removal of airpollutants,” PLASMA CHEMISTRY AND PLASMA PROCESSING, vol. 25, no. 6,pp. 595–611, 2005.

[122] S. J. Burns, J. M. Matthews, and D. L. McFadden, “Rate coefficients for dissociativeelectron attachment by halomethane compounds between 300 and 800 k,” The

Journal of Physical Chemistry, vol. 100, no. 50, pp. 19436–19440, 1996.

[123] W. P. Ho, R. B. Barat, and J. W. Bozzelli, “Thermal-Reactions of CH2Cl2 inH2/O2 Mixtures - Implications for Chlorine Inhibition of CO Conversion to CO2,”Combustion and Flame, vol. 88, no. 3-4, pp. 265–295, 1992.

[124] A. R. Martin, J. T. Shawcross, and J. C. Whitehead, “Modelling of non-thermalplasma after treatment of exhaust gas streams,” Journal of Physics D-Applied

Physics, vol. 37, no. 1, pp. 42–49, 2004.

[125] S. Chavadej, W. Kiatubolpaiboon, P. Rangsunvigit, and T. Sreethawong, “A com-bined multistage corona discharge and catalytic system for gaseous benzene re-moval,” Journal of Molecular Catalysis A: Chemical, vol. 263, no. 1-2, pp. 128 –136, 2007.

[126] Z. Abd Allah, D. Sawtell, V. Kasyutich, and P. A. Martin, “FTIR and QCL dia-gnostics of the decomposition of volatile organic compounds in an atmosphericpressure dielectric packed bed plasma reactor,” Journal of Physics: Conference

Series, vol. 157, no. 1, 2009.

[127] B. M. Penetrante, M. C. Hsiao, J. N. Bardsley, B. T. Merritt, G. E. Vogtlin, A. Kuthi,C. P. Burkhart, and J. R. Bayless, “Identification of mechanisms for decompositionof air pollutants by non-thermal plasma processing,” Plasma Sources Science and

Technology, vol. 6, no. 3, p. 251, 1997.

[128] I. Szamrej, W. Tchrzewska, H. Ko, and M. Fory, “Thermal electron attachmentprocesses in halomethanes part I. CH2Cl2, CHFCl2 and CF2Cl2,” Radiation Physics

and Chemistry, vol. 47, no. 2, pp. 269 – 273, 1996.

[129] J. A. Ayala, W. E. Wentworth, and E. C. M. Chen, “Thermal electron attachmentrate to carbon tetrachloride, chloroform, dichloromethane, and sulfur hexafluoride,”The Journal of Physical Chemistry, vol. 85, no. 26, pp. 3989–3994, 1981.

192

[130] R. Atkinson, D. Baulch, and e. a. Cox, RA, “Evaluated kinetic and photochemicaldata for atmospheric chemistry .3. IUPAC subcommittee on gas kinetic data evalu-ation for atmospheric chemistry,” Journal of physical and chemical reference data,vol. 18, no. 2, pp. 881–1097, 1989.

[131] D. Baulch, R. Cox, and e. a. Crutzen, PJ, “Evaluated kinetic and photochemical datafor atmospheric chemistry .1. codata task group on chemical-kinetics,” Journal of

physical and chemical reference data, vol. 11, no. 2, pp. 327 – 496, 1982.

[132] R. G. W. Norrish and R. P. Wayne, “Photolysis of ozone by ultraviolet radiation.2. photolysis of ozone mixed with certain hydrogen-containing substances,” Pro-

ceedings of the Royal Society of London. A. Mathematical and Physical Sciences,vol. 288, no. 1414, p. 361, 1965.

[133] Z. Falkenstein, “Effects of the O2 concentration on the removal efficiency of volatileorganic compounds with dielectric barrier discharges in Ar and N2,” Journal of

Applied Physics, vol. 85, no. 1, pp. 525–529, 1999. Times Cited: 24.

[134] H. R. Snyder and S. K. Anderson, “Effect of air and oxygen content on the dielectricbarrier discharge decomposition of chlorobenzene,” IEEE Transactions on Plasma

Science, vol. 26, no. 6, pp. 1695–1699, 1998.

[135] K. H. Rajesh Dorai and M. J. Kushner, “Interaction between soot particles and nox

during dielectric barrier discharge plasma remediation of simulated diesel exhaust,”J. Applied Physics, vol. 88, no. 10, p. 6060, 2000.

[136] M. Kalberer, K. Tabor, M. Ammann, Y. Parrat, E. Weingartner, D. Piguet, E. Rssler,D. T. Jost, A. Trler, H. W. Gggeler, and U. Baltensperger, “Heterogeneous chemicalprocessing of (no2)-n-13 by monodisperse carbon aerosols at very low concen-trations,” The Journal of Physical Chemistry, vol. 100, no. 38, pp. 15487–15493,1996.

[137] K. Tabor, L. Gutzwiller, and M. J. Rossi, “Heterogeneous chemical kinetics of no2

on amorphous carbon at ambient temperature,” The Journal of Physical Chemistry,vol. 98, no. 24, pp. 6172–6186, 1994.

[138] R. Wilkins, Jr, and I. Dodge, M.C.and Hisatsune, “Kinetics of nitric oxide catalyzeddecomposition of nitryl chloride and its related nitrogen isotope exchange reactions,”Journal of Physical Chemistry, vol. 78, pp. 2073 – 2076, 1974.

193

[139] S. A. Vitale, K. Hadidi, D. R. Cohn, and L. Bromberg, “Evaluation of the reactionrate constants for chlorinated ethylene and ethane decomposition in attachment-dominated atmospheric pressure dry-air plasmas,” Physics Letters A, vol. 232, no. 6,pp. 447–455, 1997.

[140] B. M. Penetrante, M. C. Hsiao, J. N. Bardsley, B. T. Merritt, G. E. Vogtlin, P. H.Wallman, A. Kuthi, C. P. Burkhart, and J. R. Bayless, “Electron-beam and pulsedcorona processing of carbon-tetrachloride in atmospheric-pressure gas streams,”Physics Letters A, vol. 209, no. 1-2, pp. 69–77, 1995.

[141] C. Fitzsimmons, Use of non-thermal plasma systems for VOC destruction and

air quality improvement. PhD thesis, University of Manchester. Department ofChemistry, 1999.

[142] H. S. Fogler, Elements of Chemical Reaction Engineering. Pearson Education,fourth edition ed., 2006.

[143] P. Datskos, L. Christophorou, and J. Carter, “Temperature-enhanced electron at-tachment to CH3Cl,” Chemical Physics Letters, vol. 168, no. 3-4, pp. 324 – 329,1990.

[144] S. B. Karra and S. M. Senkan, “A detailed chemical kinetic mechanism for the oxid-ative pyrolysis of chloromethane,” Industrial & Engineering Chemistry Research,vol. 27, no. 7, pp. 1163–1168, 1988.

[145] A. Indarto, J.-w. Choi, H. Lee, and H. K. Song, “Treatment of CCl4 and CHCl3emission in a gliding-arc plasma,” Plasma Devices and Operations, vol. 14, no. 1,pp. 1–14, 2006.

[146] S. Futamura, A. Zhang, G. Prieto, and T. Yamamoto, “Factors and intermediatesgoverning by product distribution for plasma chemical processing,” in Industry Ap-

plications Conference, 1996. Thirty-First IAS Annual Meeting, IAS ’96., Conference

Record of the 1996 IEEE, vol. 3, pp. 1818 –1825 vol.3, oct 1996.

[147] D. Pearl and P. Burrow, “Thermal decomposition and the apparent dissociativeattachment cross section of heated methyl-, ethyl- and t-butyl-chloride,” Chemical

Physics Letters, vol. 206, no. 5-6, pp. 483 – 487, 1993.

[148] R. Yildirim and S. M. Sedan, “Formation of high molecular weight byproductsduring the pyrolysis and oxidative pyrolysis of CH3Cl,” Industrial & Engineering

Chemistry Research, vol. 34, pp. 1842–1852, 1995.

194

[149] T. Tran and S. M. Senkan, “Coke formation in the pyrolysis and oxidative pyrolysisof methane and methyl chloride,” Industrial & Engineering Chemistry Research,vol. 33, pp. 32–40, 1994.

[150] S. Miyazaki and S. Takahashi, “Spectral study of reactions of active nitrogen withhalogenated hydrocarbons,” Mem. Def. Acad. Math. Phys. Chem. Eng, vol. 13, 1973.

[151] J. J. Orlando, “Temperature dependence of the rate coefficients for the reaction ofchlorine atoms with chloromethanes,” International Journal of Chemical Kinetics,vol. 31, no. 7, pp. 515–524, 1999.

[152] J. T. Herron, “Evaluated Chemical Kinetics Data for Reactions of N(2 D), N(2 P),and N2 (A3 Σu

+) in the Gas Phase,” Journal of Physical and Chemical Reference

Data, vol. 28, no. 5, pp. 1453–1484, 1999.

[153] R. Atkinson, D. Baulch, R. Cox, J. Crowley, J. Hampson, R.F, J. Kerr, M. Rossi,and J. Troe, “Summary of evaluated kinetic and photochemical data for atmosphericchemistry,” pp. 1–56, 2001.

[154] Roger and Atkinson, “Gas-phase tropospheric chemistry of organic compounds: areview,” Atmospheric Environment, vol. 41, Supplement, no. 0, pp. 200 – 240, 2007.

[155] R. Atkinson and S. M. Aschman, “OH Radical Production from the Gas-Phase Reac-tions of 0 8 with a Series of Alkenes under Atmospheric Condition,” Environmental

science & technology, vol. 27, no. 7, pp. 1357 –1363, 1993.

[156] P. Warneck, Chemistry of the natural atmosphere. Academic Press, second edi-tion ed., 2000.

[157] P. Neeb, F. Sauer, O. Horie, and G. K. Moortgat, “Formation of hydroxymethylhydroperoxide and formic acid in alkene ozonolysis in the presence of water vapour,”Atmospheric Environment, vol. 31, no. 10, pp. 1417 – 1423, 1997.

[158] R. Atkinson, S. M. Aschmann, J. Arey;, and B. Shorees, “Formation of OH Radicalsin the Gas Phase Reactions of O3 With a Series of Terpenes,” Journal of Geophysical

Research, vol. 97, pp. 6065–6073, 1992.

[159] A. R. Rickard, D. Johnson, C. D. McGill, and G. Marston, “OH yields in the gas-phase reactions of ozone with alkenes,” Journal of Physical Chemistry A, vol. 103,no. 38, pp. 7656–7664, 1999.

195

[160] J. D. Fenske, A. S. Hasson, S. E. Paulson, K. T. Kuwata, A. Ho, and K. N. Houk,“The pressure dependence of the OH radical yield from ozone-alkene reactions,”Journal of Physical Chemistry A, vol. 104, no. 33, pp. 7821–7833, 2000. TimesCited: 48.

[161] R. S. Disselkamp and M. Dupuis, “A temperature-dependent study of the ozonolysisof propene,” Journal of Atmospheric Chemistry, vol. 40, no. 3, pp. 231–245, 2001.Times Cited: 4.

[162] H.-H. Shin and W.-S. Yoon, “Hydrocarbon effects on the promotion of non-thermalplasma NO-NO2 conversion,” Plasma Chemistry And Plasma Processing, vol. 23,no. 4, pp. 681–70, 2003.

[163] R. Dorai and M. J. Kushner, “Effect of propene on the remediation of NOx fromengine exhausts,” Society of Automotive Engineer, 1999.

[164] R. Dorai and M. J. Kushner, “Consequences of propene and propane on plasmaremediation of nox rajesh dorai and mark j. kushner,” J. Applied Physics, vol. 88,no. 6, p. 3739, 2000.

[165] B. M. Penetrante, R. M. Brusasco, B. T. Merritt, W. J. Pitz, G. E. Vogtlin, K. M.C.,H. H. Kung, C. Z. Wan, and K. Voss, “Plasma-assisted catalytic reduction of NOx,”(San Francisco, CA), Society of Automotive Engineere Fall Fules and LubricantsMeeting, 1998.

[166] Z. A. Allah, D. Sawtell, R. Ibrahim, V. Kasyutich, and P. Martin, “Study of di-chloromethane decomposition in dielectric barrier discharge plasma reactors usingadvanced spectroscopic diagnostics techniques,” in 19th International Symposium

on Plasma Chemistry, Ruhr University, Bochum, Germany, 2009.

[167] J. Pilgrim and C. Taatjes, “Infrared absorption probing of the Cl + C3H6 reaction:rate coefficients for HCl production between 290 and 800 K,” J. Phys. Chem. A,vol. 101, pp. 5776 – 5782, 1997.

[168] G. Paraskevopoulos and C. A. Winkler, “Rates of the reactions of active nitrogenwith ethylene and propylene,” The Journal of Physical Chemistry, vol. 71, no. 4,pp. 947–951, 1967.

[169] R. Wilk, N. Cernansky, W. Pitz, and C. Westbrook, “Propene oxidation at low andintermediate temperatures: A detailed chemical kinetic study,” Combustion and

Flame, vol. 77, no. 2, pp. 145 – 170, 1989.

196

[170] C. K. Westbrook and F. L. Dryer, “Chemical kinetic modeling of hydrocarboncombustion,” Progress in Energy and Combustion Science, vol. 10, no. 1, pp. 1–57,1984.

[171] W. Makulski, J. GawYowski, and J. Niedrlelskl, “Dissociation of propylene ex-cited by the impact of low-energy electrons in the gas phase,” Journal of Physical

Chemistry, vol. 85, pp. 2950–2955, 1981.

[172] A. C. Kinsman and J. M. Roscoe, “A kinetic analysis of the photolysis of mixturesof acetone and propylene,” International Journal of Chemical Kinetics, vol. 26,no. 1, p. 191200, 1994.

[173] S. Chavadej, K. Saktrakool, P. Rangsunvigit, L. L. Lobban, and T. Sreethawong,“Oxidation of ethylene by a multistage corona discharge system in the absence andpresence of Pt/TiO2,” Chemical Engineering Journal, vol. 132, no. 1-3, pp. 345 –353, 2007.

[174] B. M. Penetrante, M. C. Hsiao, J. N. Bardsley, B. T. Merritt, G. E. Vogtlin, andP. H. Wallman, “Electron beam and pulsed corona processing of volatile organiccompounds in gas streams,” Pure and Applied Chemistry, vol. 68, no. 5, pp. 1083–1087, 1996.

[175] S. Futamura, A. Zhang, and T. Yamamoto, “Behavior of N2 and nitrogen oxides innonthermal plasma chemical processing of hazardous air pollutant,” IEEE transac-

tions on industry applications, vol. 36, no. 6, pp. 1507 –1514, 2000.

[176] A. M. Harling, J. C. Whitehead, and K. Zhang, “NOx formation in the plasmatreatment of halomethanes,” Journal of Physical Chemistry A, vol. 109, no. 49,pp. 11255–11260, 2005.

[177] K. Takaki, T. Sasaki, S. Kato, S. Mukaigawa, and T. Fujiwara, “NOx removal fromengine exhaust gas using multipoints DBD and pulsed streamer discharges with SOSpulse generator,” in Power Modulator Symposium, 2002 and 2002 High-Voltage

Workshop. Conference Record of the Twenty-Fifth International, pp. 575 – 578,june-3 july 2002.

[178] O. o. A. Q. P. Clean Air Technology Center and Standards, “Nitrogen Oxides (NOx)Why and How They are Controlled,” tech. rep., U.S. Environmental ProtectionAgency, 1999.

197

[179] W. Jones and L. Wang, “CARS investigation of the kinetics of reactions of NCOwith NO,” Journal of Applied Spectroscopy, vol. 38, pp. 32–36, 1993.

[180] M. Clyne, A. MacRobert, and L. Stief, “Elementary reactions of the NCl radical.Part2.-Kinetics of the NCl + ClO and NCl + NO reactions,” Journal of the Chemical

Society, Faraday Transactions 2, vol. 81, 1985.

[181] B. S. Rajanikanth and A. D. Srinivasan, “Pulsed plasma promoted absorp-tion/catalysis for NOx removal from stationary diesel engine,” IEEE Transaction on

Dielectrics and Electrical Insulation, vol. 14, no. 2, pp. 302–311, 2007.

[182] N. Sanders, J. Butler, L. Pasternack, and J. McDonald, “CH3O(X2E) productionfrom 266 nm photolysis of methyl nitrite and reaction with NO,” Chemical Physics,vol. 49, 1980.

[183] D. Safrany and W. Jaster, “The effect of additives upon the reaction of cyanogen withactive nitrogen: reactions of carbon atoms, CN radicals, and the chemiluminescentreaction of C2N radicals with atomic oxygen,” Physical Chemistry, vol. 72, pp. 3318– 3322, 1968.

[184] M. Lin, Y. He, and C. Melius, “Theoretical aspects of product formation from thenco + no reaction,” Journal of Physical Chemistry, vol. 97, pp. 9124 – 9128, 1993.

[185] S. Carl, Q. Sun, L. Vereecken, and J. Peeters, “Absolute rate coefficient of the hccoplus no reaction over the range t=297-802 k,” Journal of Physical Chemistry, A,vol. 106, pp. 12242 – 12247, 2002.

[186] W. DeMore, S. Sander, D. Golden, R. Hampson, M. Kurylo, C. Howard, A. Rav-ishankara, C. Kolb, and M. Molina, “Chemical kinetics and photochemical datafor use in stratospheric modeling. evaluation number 12,” JPL Publication, NASA,vol. 4, pp. 1 – 266, 1997.

[187] M. F. Golde, “Reactions of N2 (A3 Σu+),” International Journal of Chemical

Kinetics, vol. 20, no. 1, pp. 75–92, 1988. Times Cited: 51.

[188] J. F. Noxon, “Active nitrogen at high pressure,” Journal of Chemical Physics, vol. 36,pp. 926–941, 4 1962.

[189] H. Hamada, Y. KintaichiI, and e. a. Sasaki, M, “Selective reduction of nitrogenmonoxide with propane over alumina and HZSM-5 zeolite - effect of oxygen andnitrogen-dioxide intermediate,” Applied Catalysis, vol. 70, no. 2, pp. 15 –20, 1991.

198

[190] F. Radtke, R. Koeppel, and A. Baiker, “Harmful by-products in selective catalyticreduction of nitrogen-oxides by olefins over alumina,” Catalysis Letters, vol. 28,pp. 131–142, 1994.

[191] J. Yan, M. C. Kung, W. M. H. Sachtler, and H. H. Kung, “Co/Al2O3 Lean NOx

Reduction Catalyst,” Journal of Catalysis, vol. 172, no. 1, pp. 178 – 186, 1997.

[192] T. Maunula, Y. Kintaichi, M. Inaba, M. Haneda, K. Sato, and H. Hamada, “Enhancedactivity of In and Ga-supported sol-gel alumina catalysts for NO reduction byhydrocarbons in lean conditions,” Applied Catalysis B: Environmental, vol. 15,no. 3-4, pp. 291 – 304, 1998.

[193] S. Broer and T. Hammer, “Selective catalytic reduction of nitrogen oxides by com-bining a non-thermal plasma and a V2O5-WO3/TiO2 catalyst,” Applied Catalysis B:

Environmental, vol. 28, no. 2, pp. 101 – 111, 2000.

[194] H. Miessner, K.-P. Francke, and R. Rudolph, “Plasma-enhanced HC-SCR of NOx inthe presence of excess oxygen,” Applied Catalysis B: Environmental, vol. 36, no. 1,pp. 53 – 62, 2002.

[195] Z. Liu and S. L. Woo, “Recent advances in catalytic DeNOx science and technology,”Catalysis Reviews, vol. 48, no. 1, pp. 43–89, 2006.

[196] C. Yokoyama and M. Misono, “Catalytic reduction of NO by propene in the presenceof oxygen over mechanically mixed-metal oxides and Ce-ZSM-5,” Chemistry And

Materials Science, vol. 29, no. 1-2, pp. 1–6, 1992.

[197] C. Yokoyama and M. Misono, “Catalytic reduction of nitrogen oxides by propene inthe presence of oxygen over cerium ion-exchanged zeolites : II. Mechanistic studyof roles of oxygen and doped metals,” Journal of Catalysis, vol. 150, no. 1, pp. 9 –17, 1994.

[198] K. A. Bethke, C. L. Kung, Y. M. C. B., and K. H. H., “The role of NO2 in thereduction of NO by hydrocarbon over Cu-ZrO2 and Cu-ZSM-5 catalysts,” Catalysis

Letters, vol. 31, no. 2-3, pp. 287–299, 1995.

[199] H. Hamada, “Selective reduction of NO by hydrocarbons and oxygenated hydro-carbons over metal oxide catalysts,” Catalysis Today, vol. 22, no. 1, pp. 21 – 40,1994.

199

[200] M. Misono and K. Kondo, “Catalytic removal of nitrogen monoxide over rare-earthion-exchanged zeolites in the presence of propene and oxygen,” Chemistry Letters,vol. 6, no. 6, pp. 1001–1002, 1991.

[201] T. Oda, T. Kato, and e. a. Takahashi, T, “Nitric oxide decomposition in air byusing non-thermal plasma processing - with additives and catalyst,” in IAS ’96 -

Conference Record Of The 1996 IEEE Industry Applications Conference, Thirty-first

IAS Annual Meeting, vol. 3, pp. 1803–1807, 1996.

[202] A. Mizuno, R. Shimizu, A. Chakrabarti, L. Dascalescu, and S. Furuta, “NOx removalprocess using pulsed discharge plasma,” Industry Applications, IEEE Transactions

on, vol. 31, pp. 957 –962, sep/oct 1995.

[203] H. Kim, K. Tsunoda, and e. a. Katsura, S, “Reduction of NOx in diesel exhaust withmethanol over alumina catalyst,” in IAS ’97 - Conference record of the 1997 IEEE

industry applications conference / thirty-second IAS annual meeting, no. 1-3 in 1-3,pp. 1937–1941, 1997.

[204] A. Mizuno, K. Shimizu, K. Yanagihara, K. Kinoshita, K. Tsunoda, H. Kim, andS. Katsura, “Effect of additives and catalysts on removal of nitrogen oxides usingpulsed discharge,” in Industry Applications Conference, 1996. Thirty-First IAS

Annual Meeting, IAS ’96., Conference Record of the 1996 IEEE, vol. 3, pp. 1808–1812 vol.3, oct 1996.

[205] B. S. Rajanikanth and V. Ravi, “Removal of nitrogen oxides in diesel engine exhaustby plasma assisted molecular sieves,” Plasma Science and Technology, vol. 4, no. 4,p. 1399, 2002.

[206] Y. S. Mok and Y. J. huh, “Simultaneous removal of nitrogen oxides and particulatematters from diesel engine exhaust using dielectric barrier discharge and catalysishybrid system,” Plasma Chemistry and Plasma Processing, vol. 25, no. 6, pp. 625–639, 2005.

[207] J. O. Chae, J. W. Hwang, K. Y. Kim, J. H. Han, H. J. Hawang, V. I. Demidiouk,C. H. Son, and J. K. Kim, “Reduction of the particulate and nitric oxide from thediesel engine using plasma chemical hybrid system,” Physics of Plasmas, vol. 8,pp. 1403–1411, 2001.

[208] K. A. Bethke, M. C. Kung, B. Yang, M. Shah, D. Alt, C. Li, and H. H. Kung, “Metaloxide catalysts for lean NOx reduction,” Catalysis Today, vol. 26, no. 2, pp. 169 –183, 1995. Selective Reduction of Nitrogen Oxides by Hydrocarbons.

200

[209] M. Tabata, H. Tsuchida, K. Miyamoto, T. Yoshinari, H. Yamazaki, H. Hamada,Y. Kintaichi, M. Sasaki, and T. Ito, “Reduction of NOx in diesel exhaust withmethanol over alumina catalyst,” Applied Catalysis B: Environmental, vol. 6, no. 2,pp. 169 – 183, 1995.

[210] G. Dinelli, L. Civitano, and M. Rea, “Industrial experiments on pulse coronasimultaneous removal of NOx and SO2 from flue gas,” Industry Applications, IEEE

Transactions on, vol. 26, pp. 535 –541, may/jun 1990.

[211] K. P. Francke, H. Miessner, and R. Rudolph, “Plasmacatalytic processes for envir-onmental problems,” Catalysis Today, vol. 59, no. 3-4, pp. 411 – 416, 2000.

[212] H. Kim, K. Tsunoda, and e. a. Katsura, S, “A novel plasma reactor for NOx controlusing photocatalyst and hydrogen peroxide injection,” IEEE Industry Application

Society, vol. 1-3, pp. 1937–1941, 1997.

[213] S. Yoon, A. G. Panov, R. G. Tonkyn, A. C. Ebeling, S. E. Barlow, and M. L. Balmer,“An examination of the role of plasma treatment for lean NOx reduction over sodiumzeolite Y and gamma alumina: Part 1. Plasma assisted NOx reduction over NaY andAl2O3,” Catalysis Today, vol. 72, no. 3-4, pp. 243 – 250, 2002.

[214] S. Yoon, A. G. Panov, R. G. Tonkyn, A. C. Ebeling, S. E. Barlow, and M. L. Balmer,“An examination of the role of plasma treatment for lean NOx reduction over sodiumzeolite Y and gamma alumina Part 2: Formation of nitrogen,” Catalysis Today,vol. 72, no. 3-4, pp. 251 – 257, 2002.

[215] R. G. Tonkyn, S. E. Barlow, and J. W. Hoard, “Reduction of NOx in synthetic dieselexhaust via two-step plasma-catalytic treatment,” Applied Catalysis B: Environ-

mental, vol. 40, p. 20721, 2003.

201

Chapter 9

Appendix

Awards

• Best oral presentation award in the School of Chemical Engineering and AnalyticalScience Post Graduate Conference 2009, The University of Manchester.

• Young Scientists Best Paper Award 2009 at the 19th International Symposium onPlasma Chemistry at the Ruhr University, Bochum in Germany.

• Best poster award at the Technological Plasma Workshop TPW 2009 which washeld in Glasgow,UK.

Publications

Published articles

• FTIR and QCL diagnostics of the decomposition of volatile organic compoundsin an atmospheric pressure dielectric packed bed plasma reactor. Z Abd Allah, DSawtell, V L Kasyutich, P A Martin, 3rd International Workshop on Infrared PlasmaSpectroscopy, Journal of Physics: Conference Series 157, 1 (2009).

• Study of dichloromethane destruction in dielectric barrier discharge plasma reactorsusing advanced spectroscopic diagnostics techniques. Z. Abd Allah, D. Sawtell,R.K. Ibrahim, V.L. Kasyutich, P.A. Martin, 19th International Symposium on PlasmaChemistry, Ruhr University, Bochum, Germany: Conference Series 19, 551 (2009).

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Oral presentations

• Decomposition of volatile organic compounds (VOCs) using non-thermal plasmatechnology for air pollution abatement. Zaenab Abd Allah, School of ChemicalEngineering and Analytical Science Post Graduate Conference 2009, The Universityof Manchester, UK.

• Non-thermal atmospheric pressure plasma for VOC remediation. Zaenab Abd Allah,Technological Plasma Workshop TPW 2012, Manchester, UK.

Poster presentation

• Initial work with non-thermal plasmas produced by a dielectric packed-bed reactorfor the decomposition of volatile organic compounds. Zaenab Abd Allah, DavidSawtell, Vasili Kasyutich, Philip Martin, Technological Plasma Workshop TPW2007, Queens University, Belfast, UK.

• Decomposition of Volatile Organic Compounds Using Non-Thermal Plasmas Pro-duced by a Dielectric Packed Bed Reactor. Zaenab Abd Allah, David Sawtell,Vasili Kasyutich, Philip Martin, 3rd International Workshop on Infrared PlasmaSpectroscopy 2008 (IPS 2008), University of Greifswald, Greifswald, Germany.

• Spectroscopic diagnostics of the decomposition of volatile organic compounds in anatmospheric pressure non-thermal plasma reactor. Zaenab Abd Allah, David Sawtell,R. K. Ibrahim, Vasili Kasyutich, Philip Martin, Technological Plasma WorkshopTPW 2008, Open University, Milton Keynes, UK.

• The effect of alkene additives on the decomposition of volatile organic compoundsin an atmospheric pressure dielectric packed bed plasma reactor. Zaenab Abd Allah,David Sawtell, Philip Martin, Technological Plasma Workshop TPW 2008, OpenUniversity, Milton Keynes, UK.

• Study of dichloromethane destruction in dielectric barrier discharge plasma reactorsusing advanced spectroscopic diagnostics techniques. Z. Abd Allah, D. Sawtell,R.K. Ibrahim, V.L. Kasyutich, P.A. Martin, 19th International Symposium on PlasmaChemistry, Ruhr University, Bochum, Germany (2009).

203

• Initial investigation of the effect of multiple packed bed plasma reactors on thedestruction efficiency of volatile organic compounds. Zaenab Abd Allah, David. A.G. Sawtell, Raja. K. Ibrahim, Adam Higginson, Philip. A. Martin, TechnologicalPlasma Workshop TPW 2009, Glasgow, UK.

• In-situ and in-line spectroscopic diagnostic techniques for the decomposition ofvolatile organic compounds using non-thermal plasma reactors. Zaenab Abd Allah,David. A. G. Sawtell, Raja. K. Ibrahim, Vasili L. Kasyutich, Philip. A. MartinAdvances in Process Analytics and Control Technology APACT 2010, Manchester,UK.

• The effect of multiple packed bed plasma reactors as well as additives on thedestruction efficiency of dichloromethane. Zaenab Abd Allah, Philip. A. Martin,Technological Plasma Workshop TPW 2011, Bristol, UK.

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Tables

Table 9.1: Integration limits for the detected species in the plasma exhaust of dichloro-methane decomposition in nitrogen-oxygen plasma.

Species Integration limits / (cm−1)

DCM (1) 783.2 726DCM (2) 1285.5 1235CO 2144.33 2050CO2 2386.19 2300ClNO 1830 1770HCN 3390 3220HCl 2854 2718COCl2 870.27 806.76CCl4 803 786.86NO 1960 1878.5NO2 1652 1562N2O 2270 2223.88

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Table 9.2: The removal efficiency of about 500 ppm of dichloromethane as well as theconcentrations of all species in the plasma exhaust as a function of oxygen concentrationin nitrogen plasma generated using a packed-bed plasma reactor. Concentrations are inppm

O2% DCM DCM% ClNO CO CO2 N2O NO2 NO CCl4 HCl HCN COCl2

0 261 53 0 47 2 23 0 0 10 80 106 51 183 67 4 117 19 117 63 172 142 74 11 113 24 136 105 333 142 75 17 118 28 146 147 494 155 72 19 119 33 148 169 595 168 70 22 118 37 143 189 747 169 70 27 120 48 156 225 133

10 235 58 29 114 63 143 262 15113 241 57 32 116 77 142 306 19515 250 55 34 120 87 147 339 22717 260 53 36 124 98 143 344 23421 257 54 35 119 102 113 294 226

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Documents

PLASMA FOR REMEDIATION OF VOLATILE ORGANIC COMPOUNDS