JERZY DORA1, MIECZYSŁAW A. GOSTOMCZYK2, MACIEJ JAKUBIAK2, WŁODZI-MIERZ KORDYLEWSKI2, WŁODZIMIERZ MISTA3, MONIKA TKACZUK2

PARAMETRIC STUDIES OF THE EFFECTIVENESS OF NO

OXIDATION PROCESS BY OZONE

1DORA Power Systems, Wilczycka 8, 51-361 Wrocław, Poland 2Wrocław University of Technology, Institute of Power Engineering and Fluid Mechanics,

W. Wyspiańskiego 27, 50-370 Wrocław, Poland 3 Institute of Low Temperature and Structure Research, Polish Academy of Sciences,

Okólna 2, 50-422 Wrocław, Poland

The process of NO pre-oxidation by ozone was studied in a laboratory apparatus using air as the carrier gas. Ozone was produced in dry air streams using a dielectric barrier dis-charge (DBD) nonthermal plasma reactor. The temperature of the process was varied from 17 to 170 C. The stoichiometric ratio of O3/NO was in the range of 0.83.8 and the residence time varied from 4.3 to 8 s.

Badanie procesu utleniania NO ozonem przeprowadzono w skali laboratoryjnej uży-wając powietrza jako gaz nośny dla NO. Ozon wytwarzano z osuszonego powietrza z wyko-rzystaniem generatora ozonu typu DBD. Zapewniono możliwość regulacji temperatury w reaktorze, która była w zakresie od 17 do 170 C. Stosunek molowy O3/NO był w zakresie 0.83.8, a czas przebywania w reaktorze zmieniał się od 4,3 do 8 s.

Corresponding autor, e-mail: wlodzimierz.kordylewski@pwr.wroc.pl

1. INTRODUTION

The most abundant gaseous pollutants released to the atmosphere from coal-fired power plants are sulfur dioxide (SO2) and nitrogen oxides (NOx). It is believed to have a ma-jor contribution to acid rains and smog formation. In order to reduce the devastating effects of release of SO2 and NOx from stationary sources E.U. countries took legislative actions to re-strict the emissions of these gases. In 2001 the EU adopted the Directive on the limitation of certain pollutant from large combustion plants (LCP) to control SO2 and NOx emissions. This Directive sets also the limit for NOx emission for boilers larger than 200 MWt to 200 mg/m3 of dry flue gas [1].

Following the accession to the EU Poland has agreed to adopt the European Commu-nity policy on the environment protection and sustainable development. For the Polish heat and power generating industry being almost completely coal dependent the most important consequences result from the EU policy concerning the reduction of emission of certain at-mospheric pollutants, among which reduction of NOx emission below 200 mg/m3 (6% O2) after 2015 is the biggest challenge.

The commercial methods for NOx emission reduction in power plants may be classi-fied into two groups: primary or combustion modification-based technologies (low-NOx com-bustion systems) and secondary or flue-gas treatment-based technologies (post-combustion methods). The primary methods are based on modification of the combustion process to pre-vent generation of NOx above the actual standards [2]. In Poland the primary methods of NOx control became very common because they have made possible to fulfil domestic standards of NOx emission (500600 mg/m3) at acceptable costs [3]. Experience of German power plant industry showed that the limit of NOx emission 200 mg/m3 (6%O2) from coal-fired power plants could be fulfilled by the primary methods only for the lignite-fired new large boilers [4]. It is rather unlikely to fulfil this limit for the bituminous-fired plants, therefore addition-ally the post-combustion methods must be used.

Depending upon the pollutant concentration and the flow rate of the polluted gas, NOx can be abate by post-combustion methods like selective catalytic reduction (SCR) or recover-ing (adsorption, and absorption) processes. One major drawback of recovering processes is that these need a post-treatment to reactivate or clean up waste materials, such as solid ad-sorbent or liquid waste (water or organic solvent). SCR is regarded as the best available tech-nology (BAT) and actually is the major post-combustion method for control of NOx emission in coal-fired power plants. In Europe SCR has got commercial status for coal-fired power plants in the eighties [5]. SCR is capable of achieving the desired reductions, however, high cost and/or technical limitations caused by unique boiler configurations often make stand-alone SCR a less than optimum solution. Therefore, Hybrid Post Combustion NOx Control systems can be used which consist of a combination of selective non-catalytic reduction (SNCR) with SCR. These Hybrid systems can offer substantial benefit in enhanced perform-ance and they have achieved NOx emission reductions as high as 95%. However, the related technologies (SCR and SNCR) are considered to be expensive and not environmentally friendly because they require the use of NH3 as the reducing factor.

An alternative approach to control both the NOx and SO2 emissions are techniques for simultaneous NOx/SO2 removal in existing wet or semi-dry flue gas desulfurization (FGD) installations. These techniques are under development mainly in the U.S.A., where the appli-cation of SCR technology, particularly in high-sulfur coal-fired power plants, have met some obstacles [6]. This method consist of two stages: in the first stage NOx is oxidized to highly soluble NO2, N2O4, N2O3 and N2O5 (further denoted by NOy), and in the second stage prod-ucts of oxidation are absorbed by caustic scrubbing. Because in Europe, for SO2 removal from flue gas the most widely process in use is limestone desulfurization in wet scrubbers, an inte-

gration of this process with NOx oxidation to simultaneous control of SO2/NOx/Hg0 could won commercial acceptance [7].

In the paper a results of preliminary experimental study of NOx oxidation by ozone in the laboratory apparatus is presented.

2. GAS-PHASE PREOXIDATION OF NO IN FLUE-GAS

The most often used oxidizing agent for NO pre-oxidation is ozone, which is produced from oxygen or air in non-thermal plasma (NTP) generator (ozonizer). The oxidation of NOx using ozone is a naturally occurring process in the atmosphere.

For example, the BOC Group, Inc. has developed and patented the low temperature oxidation technology called LoTOx, in which ozone is generated on demand from gaseous oxygen in the exact amount required for oxidation of the NOx in the free-standing corona dis-charge reactor (CDR) [8]. Then the ozone is injected into flue gas stream where it reacts with NO and NO2. The basic chemical reactions in gas-phase NOx oxidation are:

NO + O3 = NO2 + O2 2NO2 + O3 = N2O5 + O2

These higher nitrogen oxides (N2O5, and/or N2O3) are highly water soluble and are efficiently scrubbed out with water as nitrous and nitric acids or with caustic solutions as nitrates or ni-trite salts. By combining this technology with Wet Scrubbing System, the operator can reduce also particulates, SOx and NOx all in a single vessel. The LoTOx System is very selective for NOx removal, oxidizing only the NOx and therefore efficiently using ozone, without causing any significant SOx oxidation.

Powerspan company proposed new, cost-effective, patented Electro-Catalytic Oxida-tion technology, known as ECO®, which is designed to simultaneously remove NOx, SO2, fine particulate matter (PM2.5), acid gases such as hydrogen fluoride (HF), hydrochloric acid (HCl), and sulfur trioxide (SO3), mercury (Hg), and other metals from the flue gases of coal-fired power plants, especially from high-sodium lignite-fired combustion [9]. The core of the ECO® technology is a dielectric barrier discharge (DBD) reactor composed of cylindrical quartz electrodes residing in metal tubes. Electrical discharge through the flue gas produces reactive O and OH radicals,

O2 + e O + O + e H2O + e OH + O + e

O + H2O 2OH which react with flue gas components (NOx, SO2…) to oxidize and transform it into acid mist (HNO3, H2SO4…) at the low temperature range of 65150 C. The oxidized compounds are subsequently removed in a downstream scrubber and wet electrostatic precipitator.

In the both methods highly soluble (Tab. 1) products of NO oxidation (NO2 and higher oxides) are absorbed and neutralized in caustic scrubbers (e.g. the limestone scrubbing in LoTOx). Mitsubishi Heavy Industry has proposed improvement of the scrubbing stage by the wet catalytic absorption process for more efficient NO2 capture in a limestone scrubbing me-dium [10]. The catalyst was iodine:

2NO2 + 2I- 2NO2

- + I2 (liquid) Iodine ions were regenerated via sulfite ions:

I2 + SO3

-2 + H2O 2I- + SO4-2 + 2H+

The efficiency of the oxidation-absorption techniques of flue-gas NOx removal is

strongly dependent on parameters of the oxidation process: temperature, retention time, mix-ing and water dispersion. Belco Technologies Corporation has developed and patented an apparatus for controlling NOx emissions [11], in which flue gas is preliminary cooled to 6070 C by water injection (0.6710.7 m3 H2O per 1000 m3 flue gas), NOx is oxidized by ozone and the products of oxidation are removed by water flowing down the walls of the scrubber. For the ratio of O3/NO in the range of 0.53.5 mol/mol and the residence time τres 0.510 s NO concentration was reduced below 50 mg/m3 and the efficiency of flue gas clean-ing was better than 90%.

Table 1. Selected properties of nitrogen oxides

Property NO N2O NO2 N2O3 N2O4 N2O5 Atomic mass, g/mol 30.006 44.013 46.005 76.01 92.011 108.01

Liquid 1.3 1.2228 1.443 1.4 1.443 Density at 298 K, kg/m3 Vapor 1.34 1.8 3.4

Melting temperature, C - 163.6 - 90.86 - 11.2 -00.1 - 11.2 41 Boiling temperature, C - 151.7 - 88.48 21.1 3.0 21.1C decompose

Enthalpy, kJ/mol + 82.05 - 35.05

Color colorless colorless tawn celeste liquid transparent white

powder Solubility in water1, g/dm3 0.032 0.111 213.0 500.0 213.0 500.0 1solubility of SO2: 27 g/dm3 at 50 C and 5.8 g/dm3 at 90 C

The technology for removing NOx from flue gases involving injection of microwave activated oxidizing compounds such as aqueous H2O2 solution into gas steam was described by Gravitt et.al. [12]. This method employs microwave energy to produce highly active forms of oxidizing compounds that react simultaneously with NOx in gaseous streams to form com-pounds that can be readily removed by conventional pollution abatement processes such as wet scrubber system. During microwave activation process the temperature of H2O2 droplet is elevated very quickly (0.053 s) from ambient up to the boiling point of H2O2, which is 152 oC under standard conditions with fast evaporation and formation of very reactive free radi-cals like hydroxyl and hydroperoxy radicals (OH and HO2). The proposed process is very effective and at least about 8090% of NO is oxidized into higher valence nitrogen oxides for approximately stoichiometric ratio (H2O2: NO ≈ 1:1 mol/mol)

The properties of nitrogen oxides (Tab. 1) became a basis for the development of new technologies of flue gas cleaning called “Multi-Pollution Control” [10]. For example, Gostomczyk and Krzyżyńska [13] have examined the effectiveness of simultaneous removal of NOx, SO2 and Hg0 from the flue gas using combinations of caustic sorbent (Ca(OH)2) and different oxidizers (O3, H2O2, NaOCl and Ca(OCl)2) in the pilot plant installation. They showed the ability of simultaneous reduction of NOx and SO2 emissions below 200 mg/m3 and Hg0 <1 μg/m3 at 6%O2 in flue gas.

The review showed that multi-pollutant control appears to be possible using oxidation-absorption processes when the flue-gas pre-oxidation of NO is the first stage. However, the technologies are still in the developing stage and require further investigations.

3. EXPERIMENAL

3.1. APPARATUS AND PROCEDURE

The experimental studies on the effectiveness of NO oxidation by ozone and absorp-tion of the oxidation products in caustic washers were conducted in the laboratory apparatus. The scheme of the laboratory set up was shown in Fig. 1. The oxidizing reactor (4) was a spi-ral reactor made of a cuprum tube having a length of 8.6 m and a diameter of 8 mm. Dried air was used as the carrier gas (flow rate was 125 dm3/h, the velocity of gas inside the reactor was 2.5 m/s and the corresponding residence time was τres = 3.44 s). The carrier gas was doped with NO diluted in N2. Ozone (26 mg/min) was blown into the carrier gas before the spiral reactor inlet from the ozone generator (12). The products of oxidation were removed from the carrier gas in the washers (5) with a considerable excess of CaCO3 or NaOH in water solution. From the washers (5) gas was flowing through the washer (6) (containing the solu-tion of KI for removal of ozone residue) and through the gas dryer (7) (cotton wool) to the gas analyzer (8). Sulfur dioxide was not added to the carrier gas because the temperature below 230 C ozone practically doesn’t oxidize SO2 to SO3 [14]. Ozone was produced from air in the dielectric barrier discharge (DBD) generator (12). The ozone concentration was in the range of 26 g of O3 per 1 m3 of air. Nitrogen oxide was delivered into the carrier gas from a steel bottle (10), where it was stored diluted in N2 (approx. 2.5% of NO from Messer) under pressure 20 MPa. The spiral reactor (4) was put into a container filled with oil. The oil temperature was adjusted and maintained in the range of 20170 C by an electric heater and a temperature controller Rm combined with a thermocou-ple PT 100.

The volumetric flow rate of the carrier gas (approx. 125 dm3/h) was measured by a ro-tameter R1. The volumetric flow rate of air to the ozone generator (12) was controlled by the mass flow controller ERG 1 NPSb BETA-ERG Sp. z o.o. (11). The volumetric flow rate of NO (diluted in N2) was controlled by the mass flow controller GFC17 model, AALBORG INSTRUMENTS & CONTROLS Inc. (9).

compressed air P

1 Z1

R1

Z2

14

11

12

13

Z4

R2

9

10

32 4 5 6 7 8

A

RmZ3

R3

NO

OA

Fig. 1. Scheme of the experimental, 1, 2, 3 and 13 – T-connectors, 4 – spiral reactor, 5 – NO2 absorber, 6 – KI

washer, 7 – gas dryer, 8 – gas analyzer, 9 – mass flow controller, 10 – NO bottle, 11 – mass flow controller, 12 –

O3 generator, 14 – KI washers, Z1 Z4 – valves, R1R3 – flow meters, P – pressure controller, Rm – heater and temperature controller, OA – ozone analyzer

The NOx concentration in the carrier gas was measured by the gas analyzer (8) GA 40

model, Eljack Electronic, based on electrochemical sensors. The additional washer (6) with KI solution was used for protecting the gas analyzer against residual ozone in the carrier gas. An amount of O3 injected into the carrier gas was controlled dividing the flow from the ozonizer (12) into two flows: the first was directed to the spiral reactor (4) and the second to the three washers (14) with KI solution to measure the absorbed O3 by the starch-iodine method [15].

The effectiveness of the NO oxidation was defined as:

NO = (1 – [NO]red / [NO]ref) · 100% where [NO]ref and [NO]red denote the reference and reduced NO concentrations in the carrier gas after the drier (7) measured by the gas analyzer (8) when the ozone generator (12) was switched off (ref) and switched on (red), respectively.

3.2. OZONE GENERATOR CHARACTERISTICS

A laboratory DBD (Dielectric Barrier Discharge) type ozone generator (12) was used in the experiment. The generator was fed by dried air at constant volumetric flow rate 20 dm3/h. The electric discharge was ignited by employing AC (60 kHz) high voltage in the range of 1015 kV. The generator was operating at the ambient temperature (1721 C) and atmospheric pressure.

Because the ozone generator was a prototype made by a small engineering company DORA Power Systems some its characteristics were evaluated. Figure 2 shows the ozone concentration change in air depending on the volumetric airflow through the ozone generator for its electric power approx. 15 W.

0

1

2

3

4

5

6

7

8

0 50 100 150 200 250 300

Volumetric flow of air, dm3/h

Ozo

ne c

once

ntra

tion,

g/m

3

Fig. 2. Ozone concentration vs. volumetric flow rate of air

Dielectric barrier discharge is a low energy electric discharge with non-thermal ioniza-tion, however, how long as the plasma reactor is operating in air, the formation of NOx is un-avoidable [16]. Because of the purpose of these studies the knowledge of this additional NOx injection with ozone was essential, therefore a mass spectrometer QMS-200 OmniStar from Balzers with SEM detectors was used to evaluate the NO and NO2 concentrations. The ozone concentration in air was measured using the ozone analyzer BMT 964 BT model, BMT MESSTECHNIK GMBH. The nitrogen oxides concentrations appeared to be two orders lesser than the concentration of O3 (Tab. 3).

Table 3. Range of O3, NO and NO2 of concentrations in air after the ozone generator

Specie NO NO2 O3

Concentration, ppm 200500 100200 30 00070 000

4. RESULTS AND DISCUSSION

4.1. INFLUENCE OF THE STOICHIOMETRY The effect of the concentration of ozone added on the efficiency of NO oxidation was studied varying the ozone flow rate and keeping the remaining parameters constant:

- airflow: 95 dm3/h, - [NO]ref: 180 ppm, - temperature of the spiral reactor Tr: 18 C.

The results of measurements are shown in Fig. 3.

0102030405060708090

100

0.0 0.2 0.4 0.6 0.8 1.0Stoichiometric ratio of O3/NO, mol/mol

Effe

ctiv

enes

s, %

Fig. 3. Effectiveness of NO removal by ozone vs. the stoichiometric ratio of O3/NO

According to the oxidation reaction, NO reacts with ozone in one to one stoichiome-

try:

NO + O3 = NO2 + O2

The resulted relationship in Fig. 3 is approximately in agreement with the stoichiometry of NO oxidation due to a relatively long residence time res, which was about 7.5 s.

4.2. INFLUENCE OF THE RESIDENCE TIME The residence time res is an important factor if the method is going to be used for re-moval of NO from flue gas in power plants. In the experiment the residence time was varying by changing the volumetric flow rate of the carrier gas through the spiral reactor in the range:

- velocity of gas in the reactor: 1.072.54 m/s, - residence time res: 8.0643.385 s.

Flow rates of NO and O3 and the excess of O3 were kept constant: - flow rate of O3: 1.097 mgO3/min, - flow rate of NO: 0.584 mgNO/min - O3/NOref: 1.14 mol/mol,

but the NO concentration varied due to changes of the volumetric flow rate of the carrier gas (Tab. 4). The influence of the residence time in the spiral reactor on the effectiveness of NO removal from the carrier gas by ozone was shown in Fig. 4.

Table 4. The effectiveness of NO removal by ozone vs. the residence time res

[NO]ref [NO]red Gas ve-locity res NO

ppm mg/m3 ppm mg/m3 m/s s % 245 328 40 53.6 1.07 8.07 83.67 196 263 40 53.6 1.31 6.55 79.59 169 228 40 53.6 1.56 5.52 75.76 123 165 57 76.4 2.05 4.20 53.66 98 131 67 89.7 2.54 3.40 31.63

0

10

20

30

40

50

60

70

80

90

100

3 4 5 6 7 8

Residence time, s

Effe

ctiv

enes

s, %

Fig. 4. Effectiveness of NO removal by ozone vs. the residence time in the reactor

4.3. INFLUENCE OF TEMPERATURE

Because the rate of ozone decay is strongly dependent on the temperature, therefore its influence on the oxidation process should be examined. The temperature within the spiral reactor Tr was selected: 17, 50, 150 and 170 C in these studies, (170 C is in the range of the flue gas temperature from the coal-fired boilers). The flow rate before the spiral reactor was kept unchanged 125 + 8.5 +1.2 = 134.7 dm3/h. For the mean, reference concentration of NO: 136 mgNO/m3 (208 mgNO2/m3) the ratio O3/NOref was 0.897 mol/mol.

The results are shown in the Fig. 5, where the line with nodes () represents the effectiveness NO calculated for the [NO]red concentrations as measured. The effectiveness of NO oxidation by ozone was significantly reduced at elevated temperatures. For the mean ref-erence concentration of NO: 136 mgNO/m3 and for the ratio O3/NOref = 0.897 mol/mol the effectiveness of NO removal from the carrier gas varied from 57 to 44% in the temperature range 50150 C, respectively. The best effectiveness of NO removal (approx. 76%) was achieved at the temperature 17 C and for = 0.887 mol/mol. The least effectiveness (approx. 33%) occurred at the temperature 170 C, when O3/NOref = 0.818 mol/mol.

20

30

40

50

60

70

80

0 20 40 60 80 100 120 140 160 180

Temperature, °C

Effe

ctiv

enes

s, %

Fig. 5. Dependence of direct () and corrected (■■■) effectiveness of NO removal by ozone on temperature

A considerable reduction of the effectiveness of NO oxidation by ozone with the in-

crease of the temperature can be explained by decrease of the residence time res in the spiral reactor due to the volumetric flow rate rise when the temperature Tr increased (Tab. 5). The second important factor is decay of ozone at an elevated temperature [17].

To compensate the effect of volumetric flow rate increase and reduction of residence time res on the effectiveness ηNO a correction factor was introduced:

= res,17 / res,Tr

where res,17 and res,Tr denote the residence time at the temperature 17 C and Tr, respectively (Tab. 5).

Table 5. The volumetric flow rate of gas and the residence time in the spiral reactor vs. the temperature

Tr, C Flow rate, 105, m3/s

Gas veloc-ity, m/s

res, s Correction factor

17 50 100 150 170

3.742 4.167 4.813 5.458 5.716

1.324 1.475 1.70 1.93 2.02

6.50 5.83 5.06 4.45 4.3

1.0 1.115 1.285 1.461 1.512

Assuming an approximate proportional relationship between [NO]red and the residence

time the correction factor was used for the correction of [NO]red and recalculation of the effectiveness ηNO (Tab. 6). The estimated in such a way effectiveness ηNO, corr is presented in Fig. 5 (line with ■■■ nodes). The mean value of the effectiveness of NO removal in the range of the temperature of 50150 C was 60% after correction.

Table 6. Effectiveness of NO removal by ozone vs. temperature in the reactor for O3/NO = 0.8180.887 mol/mol

Tr res [NO]ref [NO]red Corrected

[NO]red ηNO

Corrected ηNO, corr

°C s ppm mg/m3 ppm mg/m3 ppm mg/m3 % %

17 6.50 207 277 50 67 - - 75.86 75.68 50 5.83 203 272 88 118 79.8 107 56.65 63.16 100 5.06 209 280 100 134 78.5 105 52.15 67.01 150 4.40 206 276 116 155 80.3 108 43.69 63.83 170 4.30 220 295 148 198 97.9 131 32.73 49.49

4.4. INFLUENCE OF NO CONCENTRATION IN THE CARRIER GAS

Usually, the effectiveness of reduction of NOx emission is better when its concentra-

tion is higher, which is also true in this case because the rate of NO oxidation is directly pro-portional to the concentration of NO. The influence of the NO concentration in the carrier gas on the effectiveness of NO removal by ozone was investigated at the ambient temperature 19 C. The volumetric flow rate of the carrier gas was 125 dm3/h and the flow rate of ozone (in air) was 8.5 dm3/h. The content of NO in the carrier gas was varied changing the volumetric flow rate of NO (diluted in N2) from 0.3 to 1.2 dm3/h (Tab. 4). The results of the measure-ments are shown in Fig. 6.

Table 4. Influence of NOref concentration in the carrier gas on the effectiveness of NO removal by ozone [NO]ref [NO]red O3/NOref NO

ppm mg/m3 ppm Mg/m3 mol/mol % 210 281 40 54 1.00:1 81.00 164 220 39 52 1.28:1 76.22 106 142 46 62 1.97:1 56.60 55 74 25 33 3.79:1 55.40

50

55

60

65

70

75

80

85

90

50 100 150 200 250 300

NO concentration, mg/m3

Effe

ctiv

enes

s, %

Fig. 6. Effectiveness of NO removal by ozone vs. NO concentration in the carrier gas

5. CONCLUSIONS

The parametric studies conducted in a laboratory apparatus confirmed great potential of the process of NO oxidation by ozone to removal of NO from flue gas. However, there are several factors which influence this process when applied in coal-fired boilers for reduction of NOx emission. The obtained results lead to the following conclusions:

a) The highest rate of NO oxidation by ozone was observed at the ambient temperature (1720 C).

b) Increase of the temperature of the carrier gas caused reduction of the effectiveness of NO removal by ozone due to diminish of the residence time and destruction of ozone at the elevated temperature.

c) The effectiveness of NO oxidation by ozone appeared to be sensitive to the residence time; approximately 6 s of the contact time was required to reach the effect of NO re-moval related to the stoichiometric ratio of O3/NO.

d) The effectiveness of NO removal by ozone diminished when NO concentration in the carrier gas was decreased.

SYMBOLS

ηNO – effectiveness of the NO oxidation, % Tr – temperature of the spiral reactor, C res – residence time in the spiral reactor, s - correction factor.

REFERENCES

[1] Directive 2001/80/EC of the European Parliament and of the Council, 2001. [2] WILK R., Low-emission combustion, Gliwice, Wyd. Politechniki Śląskiej, 2002. [3] KORDYLEWSKI W., Spalanie i paliwa, Wrocław, Wyd. Politechniki Wrocławskiej, 2008 (in Polish). [4] Hein K.R.G., Jager G., Results of combustion modification for the reduction of NOx-emission, FACT – Vol. 10: Combustion Modeling and Burner Replacement Strategies, ASME, 1990. [5] Selective catalytic reduction (SCR), long-term experience and test procedures, Thermal Generation Study Committee, 20.03 THERNOX, UNIPEDE, Paris, 1997. [6] GOSTOMCZYK M.A., JÓZEWICZ W., Simultaneous Control of SO2, NOx and mercury emissions from coal-fired boilers, Combined Power Plant Air Pollution Control–MEGA Symposium, Washington DC, U.S., May 19-22 (2003). [7] KUCOWSKI J., LAUDYN D., PRZEKWAS., Energetyka a ochrona środowiska, War-szawa, WNT, 1997. [8] HWANG S-Ch., et al., US Patent, 2003, No. 6649132. [9] LARNON Ch. R., et al., Electro + catalytic oxidation technology applied to mercury and trace elements removal from flue gas, Conference on Air Quality II, McLean, VA, Septem-ber 20, 2000. [10] ELLISON W., Chemical process design alternatives to gain simultaneous NOx removal in scrubbers, POWER_GEN International, Las Vegas , December 9-11, 2003. [11] HSIEH J., et al., US Patent, 2007, No. 7214356. [12] GRAVITT A.C., ISAAC T.L., US Patent, 2002, No. 6423277. [13] GOSTOMCZYK M.A., KRZYŻYŃSKA R., Multi-pollutant control from pulverized coal-fired boiler OP-430, U.S. EPA - U.S. DOE EPRI - A&WMA Power Plant Air Pollutant

Control "MEGA" Symposium, Baltimore, U.S., August 28-31 (2006). [14] MOC Y.S., LEE H.J, Fuel Proc. Techn., 2006, 87, 591. [15] THORP C.E., Ind. Eng. Chem. Anal. Ed., 1940, 12, 209. [16] MISTA W., KACPRZYK R., Catal. Tod., 2008, 137, 345. [17] OZONEK J., Analiza procesów wytwarzania ozonu dla potrzeb ochrony środowiska, Lublin, PAN, 2003.

BADANIA EFEKTYWNOŚCI PROCESU UTLENIANIA NO OZONEM

JERZY DORA, MIECZYSŁAW A. GOSTOMCZYK, MACIEJ JAKUBIAK, WŁODZI-MIERZ KORDYLEWSKI, WŁODZIMIERZ MISTA, MONIKA TKACZUK

Zgodnie z Dyrektywą Unii Europejskiej LCP [1], dotyczącą ograniczenia emisji nie-

których zanieczyszczeń do powietrza z dużych źródeł spalania paliw, w roku 2016 zacznie w Polsce obowiązywać nowy limit na emisję NOx (< 200 mg/m3 dla 6 %O2). Spowoduje to, że dotychczas stosowane w polskiej energetyce metody pierwotne ograniczania emisji NOx będą niewystarczające. Z tego powodu większość krajowych elektrowni stanie przed koniecznością zakupu drogiej inwestycyjnie i eksploatacyjnie technologii SCR lub posiadającej szereg wad technologii SNCR. Poszukuje się dlatego innych, mniej kosztownych niż SCR, metod ograni-czania emisji NOx. Obiecującą alternatywą w stosunku do metody SCR jest grupa metod jed-noczesnego usuwania NOx, SO2 i rtęci z zastosowaniem instalacji z mokrym lub pół-suchym odsiarczaniem spalin (IOS), w których NO utlenia się do wyższych tlenków przed reaktorem IOS. Konieczność utleniania NO wynika z jego znikomej rozpuszczalności w stosunku do NO2 i N2O5, które doskonale rozpuszczają się w wodzie. Obecnie badanych jest wiele rodza-jów utleniaczy NO, z punktu widzenia ich efektywności i przede wszystkim ze względu na koszt. Jednym z bardziej obiecujących utleniaczy NO jest ozon, który nie musi być transpor-towany, ani magazynowany, można go generować bezpośrednio przy kotle.

Celem pracy jest zbadanie wpływu warunków występujących w instalacji kotłowej na efektywność usuwania NOx ze spalin z zastosowaniem utleniania NO ozonem i wychwyty-waniem produktów utleniania w roztworach kaustycznych. Badania nad wpływem parame-trów na proces utleniania NO ozonem przeprowadzono w skali laboratoryjnej. Ozon wytwa-rzano z powietrza z wykorzystaniem generatora ozonu typu DBD (ang. Dielectric Barier Di-scharge). Gaz nośny dla NO stanowiło powietrze, do którego dodawano tlenek azotu z butli, żeby zapewnić jego udział około 200 ppm. Reaktorem utleniającym był reaktor typu prze-pływowego wykonany ze zwiniętego spiralnie przewodu miedzianego. Zapewniono możli-wość regulacji temperatury w reaktorze, która była w zakresie 17170 C. Produkty utlenia-nia były usuwane z gazu nośnego za reaktorem w baterii płuczek Drechslera zawierających wodną zawiesinę CaCO3 lub NaOH. Efektywność procesu utleniania NO ozonem określono na podstawie wskazań analizatora spalin umieszczonego za płuczkami absorbującymi.

Potwierdzono eksperymentalnie, że dla czasów przebywania res ponad 6 s efektyw-ność ubywania NO z gazu nośnego w wyniku utleniania ozonem jest w przybliżeniu zgodna ze stechiometrią reakcji NO z O3. Jeżeli jednak skracano czas kontaktu, to obserwowano znaczne zmniejszenie efektywności usuwania NO z użyciem ozonu, do 50% dla res = 4 s (dla stosunku molowego: 1,14 mol O3/mol NO). Stwierdzono znaczący wpływ temperatury na efektywność utleniania NO ozonem: wzrost temperatury procesu powodował obniżenie efek-tywności utleniania NO, co prawdopodobnie ma związek z szybszym rozkładem ozonu w wysokich temperaturach oraz ze wzrostem strumienia objętości gazu w reaktorze, czyli krót-szym czasem kontaktu. Próbowano wyeliminować wpływ tego drugiego czynnika przez wprowadzenie współczynnika korekcyjnego. Na tej podstawie oceniono efektywność metody na 60% w zakresie temperatury 50150 C. Wykazano również, że im wyższa koncentracja NO w gazie nośnym tym lepsza efektywność procesu utleniania NO ozonem.