Running head: NON THERMAL PLASMA GASIFICATION

The latest technologies of non-thermal plasma pyrolysis of organic waste

“Review article about microwave non thermal plasma”

Mai Osama Mohamed

Cairo University

March 21, 2015

1. IntroductionIn the face of global energy crises and greenhouse gas (GHG) emissions that are ballooning more and more

with each coming year, the option of a cleaner alternative fuel which is promised to reduce our reliance on fossil fuel has finally begun to garner serious attention over the world. Hydrogen is recognized as one of the primary clean energy carrier in the future, because its combustion and oxidation are pollution free and its heat value is about three times that of gasoline per unit mass. On the other hand, Organic wastes such as rubber and plastic are among the waste materials that represent problematic wastes. To alleviate part of our energy crisis and environmental degradation, it has become imperative to make use of appropriate technologies for recovery of resources from non-conventional sources like organic waste.

Plasma is considered as the fourth state of matter and 99% of our universe is plasma. It’s electrified gas with a chemically reactive media, consisted of many constituents such as electrons, positive and negative ions, free radicals, gas atoms and molecules in the ground or any higher state of any form of excited species. There are two main categories high temperature plasma, and low temperature plasma. The high temperature plasma is known that its entire species are in a thermal equilibrium state. The low temperature is subdivided into thermal plasma, also called quasi-equilibrium plasma, which is in a local thermal equilibrium (LTE) state, and non-thermal plasma (NTP), also called non equilibrium plasma or cold plasma. The following table (1) is about the plasma classifications and its state where Te : electron temperature, Ti : Ions temperature, Tg : Gas temperature with examples about each type applications. [1]

Table 1: Classification of plasma [1]

Gasification is a thermochemical process that aims to convert the carbonaceous feedstock (organic waste) into syngas. This process is partial oxidation using air, oxygen, steam or a mixture of these as oxidizing agent at elevated temperature of the organic feed stock. Syngas is a mixture of CO, CO2, H2, CH4, trace amounts of higher hydrocarbons such as ethane and ethene, water, nitrogen (if air is used as the oxidizing agent) and various contaminants such as small char particles, ash, tars and oils. This process is done in sequential steps:

Drying: to evaporate moisture. Pyrolysis: to give gas, vaporized tars or oils and a solid char residue. Gasification: or partial oxidation of the solid char, pyrolysis tars and pyrolysis gases. [2]

The aim of the plasma is to play a catalytic role by creating reactive species needed for the different chemical reactions. Several different plasma reactors have already been developed in different research groups to reform hydrocarbons by partial oxidation or auto-thermal reforming. Only few plasma reactors have however been developed for steam reforming of hydrocarbons. Among the different plasma characteristics, we can distinguish two main categories: thermal plasma and non-thermal plasma. For thermal plasma, the electrical power injected in the

discharge is high (higher than 1 kW) and the neutral species and the electrons have then the same temperature (around 5000–10,000 K). The temperature in the reactor and the energy consumption are thus very high and the cooling of the electrodes is generally useful to reduce their thermal erosion. The use of this technology is therefore not relevant in fuel cell application for an efficient production of hydrogen in terms of energy consumption. For non-thermal plasma, the electrical power is very low (few hundreds watts): the temperature of neutral species does not change whereas the temperature of electrons is very high (up to 5000 K). In this case, the role of the plasma is notto provide energy to the system but to generate radical and excited species allowing initiating and enhancing the chemical reactions. The advantages of using non-thermal plasma are related to the lower temperature that will result in lower energy consumption and lower electrode erosion since the cooling of the electrodes is generally not necessary. In addition, the size and weight of the non-thermal plasma reactors are relatively low, which is very attractive for mobile applications. Non-thermal plasma has several advantages over traditional gasification systems due to its fast start time, low preheating requirements, low electrical power consumption, and inability to be poisoned or deactivated. Various types of non-thermal plasmas are well suited for re-forming systems because they have relatively low gas temperatures (near room temperature) and therefore the electrical energy used to generate the plasma is selectively channeled into creating active chemical species that can influence and accelerate reforming reactions. The detailed mechanism by which the plasma catalysis effect is realized in fuel-rich syngas production systems is currently being investigated. Advances in our fundamental understanding of plasma phenomenon will lead to the optimization of plasma systems to further enhance performance and make it a more attractive technology with a wider range of applications. [3, 4]

This paper aims to provide an overview of the setting up, feasibility and efficiency of the existing technologies here investigated. This state-of-the-art technology review explains the key characteristics of plasma reforming through various original approaches. The performances of some of the systems are then compared against each other and discussed.

2. Department of Mechanical Engineering, Faculty of Engineering, Chiang Mai University, Thailand.In this study, a microwave plasma reactor was used for pyrolysis of waste papers. The effects of

different argon flow rates on char and gas generation were investigated. Changes in carbon and oxygen contents from those in paper to char were significant. Char yield of over 25 % was obtained with the heating value of about 38 MJ/kg. Average gas yield and total content of combustible fraction (CO, CH4

and H2) in the gas product were 2.56 m3/kg and 36 %, respectively. The heating values of gas product and carbon conversion efficiency of the process were maximum at 6.0 MJ/m3 and 73 %, respectively. [6]

2.1 FeedstockWaste paper was the major component of combustible fraction of solid waste, accounting for

about one third of typical municipal solid wastes [6]. Waste paper is combustible, and has low contents of nitrogen and sulfur. It can be converted into gaseous fuel with less environmental pollution. It may have sufficient feedstock for waste-to-energy utilization. The samples of waste papers were obtained locally from the same source. The papers were wetted, mashed and compressed into a cylindrical mold with diameter of 20 mm and length of 40 mm. After drying in an oven at 60oC for 48 h, the mass of each compressed paper was about 5 ± 0.1 g. [6]

2.2 Pyrolysis reactorExperimental setup used in this study is shown schematically in following figure. The microwave

plasma system in this work was modified from a commercial microwave oven. Argon was used as the carrier gas. The igniter was added into the quartz reactor tube to initiate plasma when argon was excited. Sample of feedstock was placed inside a quartz reactor tube using stainless steel wire in a hang mode. Before each experimental run, the plasma system was vacuumed. Argon was fed from the bottom of the reactor tube. Flow rate was varied from 0.50 to 1.25 lpm. The microwave output power was supplied continuously at 800 W for 3 min. Successful establishment of argon plasma can be visually inspected from characteristically bright light emission. Product gas flowed to the top of the reactor tube, passing to a gas treatment unit and a sampling bag. The sampled gas was then sent for gas chromatographic analysis. The solid residue left in the reactor was carefully collected and weighed. The tests was repeated at least three times for each conditions. [6]

Figure 1: Experimental setup of the microwave plasma reactor for pyrolysis of waste paper [6]

2.3 Results and discussion

2.3.1 Raw material and charProximate and ultimate analyses of the waste paper and char are shown in Table 2. The proximate analysis gives the properties of the sample paper in mass concentration of moisture, volatile matter, fixedcarbon and ash. Release of volatile matter was found to be about 69% after microwave plasma pyrolysisprocess which was indicative of high conversion of biomass to gaseous fuel. The fixed carbon of the charwas found to significantly increase, compared to that original in the starting material. Significant degreeof carbonization appeared to take place under non-thermal plasma environment. The ultimate analysisgives the compositions in mass concentration of carbon, hydrogen, oxygen, nitrogen and sulphur. Increase in carbon content in the resulting char was inline with proximate analysis result. Hydrogen and oxygen contents were reduced. The reductions were about 80% and 77%, respectively. The volatile and char yields were listed in Table 3. They were in range between 73-74%, and 25-26%, respectively. Average char yield and its HHV were 25.95 % and 38.5 MJ/kg, respectively. [6]

2.3.2 Gas evolutionThe product gas obtained from the reaction was collected and measured. Major gas components generated were CO, H2, CH4, CO2 and O2. The most important gas species to consider for pyrolysis processes were CO, CH4, and H2. Figure 2 shows effect of carrier gas flow rate on combustible gas fractions of the product gas. Within the range of flow rates considered, CO and CH4 were not found to vary significantly with argon flow. They remained relatively stable at 16-18% and 2-4%, respectively. H2 appeared to exhibit more pronounced change with respect to carrier gas flow rate. It was found to initially increase with increasing argon flow, reaching maximum value of about 22% at flow rate of 0.75 lpm. After which, it was markedly reduced at higher carrier gas supply rate. The total content of combustible fractions in the product gas was between 28-44%. Table 3 also shows the gas yields, carbon conversion and heating values of product gas. The heating value was found to be in the range between 3.85 and 6.02 MJ/m3, showing similar pattern to change in H2 content with argon flow rate. Average gas yield and carbon conversion efficiency obtained were 2.56 m3/kg and 70.3 %, respectively. Although present in the detected product gas, other detected fraction would not be taken into account due to their low contents. A majority was undesirable tars. [6]

Table 2:Proximate and ultimate analyses of waste paper and its char [6]

Table 3: Gas and char products [6]

Figure 2: Variation in concentrations of product gas generated with argon flow rates. [6]

2.3.3 Comparison with literatureProduct gas and char obtained from plasma pyrolysis of waste paper in this study were compared againstthose obtained from other types of feedstock and plasma sources and conditions. The comparison issummarized in Table 4. Against non-thermal plasma assisted torrefaction of rice husk and cane residue[22], the char products obtained in this work were in similar magnitude, but with higher energeticcontent. In comparison with thermal plasma assisted pyrolysis of sawdust [7] and used tires [12], theproducts obtained in this work showed higher gas yield, but lower char yield. [6]

Table 4: Comparison with literature for plasma assisted pyrolysis of biomass [6]

3. Laboratoire de PhysicoChimie de I’Energstique et des Plasmas, Universitb des Sciences et Techniques de Lille, Flandres-Artois 59655 Villeneuve d’Ascq cedex, FranceThe influence of several parameters on the H, and CO yields from gasification of a coal (Blanzy) by

microwave plasma in water vapor, in a static system, is discussed. The yields of H, and CO significantly increase for low coal weights placed in the discharge illustrating that the gasification occurs on the surface of the coal layer. The time necessary for complete gasification and the gasification yields are measured for several initial weights of coal. The relative yields of H, and CO depend on the initial water vapor pressure and are independent of coal granulometry. The reactivity of a coal which was pre-reacted in the discharge is similar to that of an original coal, so a possible recycling of the solid can be envisaged.

Microwave plasma in water vapor creates radicals or ions in thermodynamical non-equilibrium, which react with coal so that the activation energy of the reaction: C + H2O H2 + CO, is greatly lowered. In a static system, previous studies were essentially concerned with the effect of the type of coal on the gasification yield in the presence of different plasma gases (Ar+ H2O, Ar). When steam is used as the plasma supporting gas in microwave discharge, radicals such as H, OH and O are generated, as well as high-energy electrons, and both reductive and oxidative conditions are provided in the plasma, indicating that steam plasma is effective for various treatmentsof materials. The main reaction products were H2, CO, CO2, CH4 and C2H2 with H2 and CO being the most important. The highest yields were obtained for lignite and kerogen (33.5 and 31.8 wt%, respectively). In the present work, only the H, and CO yields are studied. [7]

3.1 EXPERIMENTALA known weight of coal, m,, is placed in a quartz tube so that all the coal powder is in the discharge zone.

In a vacuum (~0.10 hPa), a water vapor pressure, Po, is introduced in the reactor. The discharge was then produced

over z min. After the discharge, the pressure in the reactor is P. The reaction products are then trapped at liquid nitrogen temperature; the measured pressure is P’. Helium is added to the products which are not condensed at liquid nitrogen temperature (HZ and CO) up to a pressure of 1200 hPa. The discharge tube which contains the coal is weighed before and after reaction to determine the loss in weight of coal during the reaction, m. [7]

3.2 RESULTS3.2.1 Effect of initial weight of coal

The effect of m, on H, and CO yields and on the loss in weight m is studied for “m”, values between 1 mg and 120 mg. The results are given in Table 5. The consistent relatively low maximal value of H, and CO yields which is observed for “m”, > 80 mg can be explained in two ways: the products of the reaction are not eliminated; and by the depletion of reactive species coming from the discharge in the outer layer of the coal deposition. [7]

3.2.2 Effect of discharge timeThe effect of t is studied for mO= 1, 5, 10 and 60 mg. Among the parameters described, the total pressure P

can be continuously measured. It is the sum of the partial pressures of all gaseous products and of the pressure of non-decomposed water vapor. A plot of P as a function of t (Figure 3) allows the determination of the time T* from which the gasification can be considered as complete. It is defined as the time from which the variation of P, during 10 min, is equal to 1.33 hPa. The study of the influence of the discharge time, for T values <T*, on the H, and CO yields (Table 6) leads to the following: for a given value of m, (Figure 3) the rate of gasification decreases with time and the initial rate of gasification is higher with higher m, values. The rate of gasification by defect is defined by: R = am where z = 104/91m,. (The ratio of 100/91 is from the presence of 9% ash in the coal studied.) With our operating conditions, for mO= 1 mg, gasification can be considered as complete after 15min and R=71~10%.Fo rm0=60mg, the values of T* and R are, respectively, 50 min and 8.5+ 1%. Whatever the m, and T values, H, is mainly produced rather than CO. For a given value of m,, the ratio nH2/nC0 is approximately constant when T varies and increases when m, decreases. Indeed, the spatial extent of the plasma is more important for low values of m,, so the dissociation rate of the water vapor and H, concentration increases. [7]

Figure 3: Plot of P versus t. Values of mo (mg): A, 1; B, 5; C, 10; D, 60 [7]

Table 5: Effect of initial weight of coal mo [7]

Table 6: Effect of discharge time T [7]

4.4.3 Effect of the values of the initial water vapor pressure and of granulometry.The effect of the P, value is studied for three P, values: 6.67, 13.33 and 21.33 hPa, with the last value being

the highest under our experimental conditions. The value of m, is kept constant at 5 mg. Table 7 shows that n H2, nCO, (to a smaller extent) and m increase with Po. This observation can be explained by an increase in the H and OH concentrations in the discharge zone. Four ranges of granulometry have been used: d(mm) < 0.063, 0.063 <d(mm)<0.100,0.250<d(mm)<0.500and 0.500<d(mm) ~0.800. The m, value is 10 mg. The results show an independence of the rate of gasification in H, and CO with granulometry, giving evidence that the process is little influenced by diffusion into the coal particles. [7]

Table 7: Effect of initial pressure of water vapor Po [7]

5 Department of Thermodynamics and Fluid Mechanics, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, UTM 81310, Skudai, Johor Bahru, Johor Darul T’azim, Malaysia.

A new technique to pyrolyse biomass in microwave (MW) system is presented in this paper to solve the problem of bio-oil deposition. Pyrolysis of oil palm shell (OPS) biomass was conducted in 800W and 2.45 GHz frequency MW system using an activated carbon as a MW absorber. The temperature profile, product yield and the properties of the products were found to depend on the stirrer speed and MW absorber percentage. The highest bio-oil yield of 28 wt.% was obtained at 25% MW absorber and 50 rpm stirrer speed. Bio-char showed highest calorific value of the 29.5 MJ/kg at 50% MW absorber and 100 rpm stirrer speed. Bio-oil from this study was rich in phenol with highest detected as 85 area% from the GC–MS results. Thus, OPS bio-oil can become potential alternative to petroleum-based chemicals in various phenolic based applications. [10]

5.1 Methods

5.1.1 MaterialsOil palm shell (OPS) biomass used in this experiment was acquired from palm oil mill, located in Johor state of Malaysia. Oil palm shell was grinded to size of 1.4 mm. The inherent moisture content of the OPS was about 8.5 wt. %. Coconut based activated carbon of 3 mm particle size obtained from Laju Group of Companies; Malaysia was used as MW absorber. The particles of the MW absorber were bigger in size to shun any escape of carbon particleswith vapors into bio-oil. Besides this, difference in biomass and MW absorber particle size facilitates for easy separation of MW absorber particles after pyrolysis experiments to reuse them back. Table 8 presents proximate and ultimate analysis of the OPS biomass on dry basis.Table 8: Chemical characteristics of OPS biomass on dry basis [10]

5.1.2 Experimental set-upThe schematic diagram shown in Fig. 4 represents the set-up for the MW assisted pyrolysis system. A

modified domestic MW system with maximum power of 800W and 2.45 GHz frequency was used to carry out the present research work. Quartz glass tube of 150 mm in length and 100 mm in diameter was used as reactor. This reactor was covered with three neck glass lid from the top and one neck glass lid from the bottom. The stirrer and inert gas were introduced from the central opening of three neck glass lid at the top while the other two was used for thermocouples. A thin wire mesh of 60 microns was used to support the biomass at the bottom of the reactor which

also facilitated the exit of vapors from the bottom of the reactor into the condensing unit. An anchored two-bladed stainless steel stirrer with shaft diameter of 0.008 and 0.07 m wide blades was employed. The height of the stirrerwas adjustable. The speed of the overhead stirrer was controlled via motor (90 W, 200–220 V) acquired from Tung electrical company Ltd. The stirrer speed fluctuated in the range of ±10 rpm due to MW field. But this took place in seconds because MW field in the cavity was based on duty cycle or ON/OFF mode. The two K-type thermocouples were connected to a Pico data acquisition system having an accuracy of ±1 _C and linked to a personal computerfor continuous recording of real time temperature data. Thermocouple T1 is designated for bed surface temperature and T2 represents the bed inside temperature. MW leakages during the experiments were frequently monitored by MW leakage detector model TX90 acquired from Robin Professional Test Equipment, U.K as well as to ensure a safe working environment during the experiments. [10]

Figure 4: Schematic diagram of MWpyrolysis system: (1)MWgenerator, (2)MWcavity, (3) Quartz glass reactor, (4) Stirrer, (5) Stirrer motor, (6) Thermocouples, (7) Wire mesh, (8) Temperature data acquisition system, (9) Personal computer, (10) Flue gas, (11) Bio-oil collector, (12) Condensing unit, (13) Rotameter, (14) Nitrogen gas. [10]

5.2 Outcome of new technique Pyrolysis of OPS was successfully performed under MW irradiation without any bio-oil deposition on the

reactor as well as the equipment wall. The problem of bio-oil deposition was solved by introducing inert gas from the top and exit of vapors from the bottom of the reactor. The incoming nitrogen gas from the top restricted any escape of the vapor from the top of the reactor. It was observed that the vapor could easily entrain from the bottom of the reactor. Since the condensers were directly attached to the bottom of the reactor as seen in Fig. 4, the vapors were rapidly condensed into bio-oil. Thus, the losses in bio-oil yield can be avoided due to this rapid condensation and elimination of bio-oil deposition from the reactor wall. Furthermore, cleaning of reactor and other equipment after the pyrolysis process was saved because of this new technique. There was no major problem observed in this study. This was proved by the temperature profiles and product yield as discussed in subsequent Sections. [10]

6 EconomicsNo cost effectiveness numbers were found in the public records for the FirstEnergy power plant process.

Despite the fact of lack of information, in the references investment and running cost comparison of waste air purification processes of 50000 Nm3/h for removal of volatile organic compounds of flavored processing industry. Investment cost of non-thermal plasma technique is showed in following figure. It’s clear now that economically and environmentally the non-thermal plasma techniques are the best solution. [8]

Investment cost of non-thermal plasma technique [8]

7 References[1]- Vijay Nehra, Ashok Kumar, H K Dwivedi; Atmospheric Non-Thermal Plasma Sources; International Journal of Engineering (IJE) 2, no. 1 (2008): 53-68.

[2]- A. V. Bridgwater; The technical and economic feasibility of biomass gasification for power generation; Fuel Processing Technology 74, no. 5 (1995): 631–653.

[3]- T. Paulmier, L. Fulcheri; Use of non-thermal plasma for hydrocarbon reforming; Chemical Engineering Journal 106, no.1 (2005): 59-71.

[4]- David Berry, Dushyant Shekhawat; Non-Thermal Plasma for Fossil Energy Related Applications; National Energy Technology Laboratory.

[5]- G. Petitpasa, J.-D. Rolliera, A. Darmonb, J. Gonzalez-Aguilara, R. Metkemeijera, L. Fulcheria; A comparative study of non-thermal plasma assisted reforming technologies; International Journal of Hydrogen Energy 32, no.14 (2007): 2848–2867.

[6]- Parin Khongkrapan, Patipat Thanompongchart, Nakorn Tippayawong, Tanongkiat Kiatsiriroat; Fuel gas and char from pyrolysis of waste paper in a microwave plasma reactor; INTERNATIONAL JOURNAL OF ENERGY AND ENVIRONMENT 4, no. 6 (2013):.969-974.

[7]- Djamal Djebabra, Odile Dessaux and Pierre Goudmand; Coal gasification by microwave plasma in water vapor; Fuel Processing Technology 70, no. 12 (1991): 1473–1475.

[8]-Dr. Vitas Valinčius, Dr. Viktorija Grigaitienė, Ph.D. Andrius Tamošiūnas; Report on the different Plasma Modules for Pollution Removal; Prepared by PlasTEP partner #8, Lithuanian Energy Institute, Lithuania.

[9]- Wang M.J., Huang Y.F., Chiueh P.T., Kuan W.H., Lo S.L., Microwave-induced torrefaction of ricehusk and sugarcane residues. Energy. 2012, 37, 177-184.

[10]- Zubairu Abubakar, Arshad Adam Salema, Farid Nasir Ani, A new technique to pyrolyse biomass in a microwave system: Effect of stirrer speed. Bioresource Technology. 2013, 128, 578-585.

[11]- Tang L., Huang H. Plasma pyrolysis of biomass for production of syngas and carbon adsorbent. Energyand Fuels. 2005, 19, 1174-1178.

[12]- Tang L., Huang H. An investigation of sulfur distribution during thermal plasma pyrolysis of used tires.Journal of Analytical and Applied Pyrolysis. 2004, 72, 35-40.

[13]- Zhao X., Wang M., Liu H., Li L., Ma C., Song Z. A microwave reactor for characterization ofpyrolyzed biomass. Bioresource Technology. 2012, 104, 673-678.

[14]- Shie J.L., Tsou F.J., Lin K.L., Chang Ch.Y. Bioenergy and products from thermal pyrolysis of ricestraw using plasma torch. Bioresource Technology. 2010, 101, 761-768.