Experimental Study on the Effect of NO Reduction by Tar Model

Energy & Fuels 2009, 23, 4099–4104

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Experimental Study on the Effect of NO Reduction by Tar Model Compounds Chun-yuan Liu,† Yong-hao Luo,*,† Jia Duan,‡ Rui-zhi Zhang,† Li-yuan Hu,† and Yi Chen† School of Mechanical and Power Engineering, Shanghai Jiao Tong UniVersity, Shanghai 200240, People’s Republic of China, and Shanghai Pudong Project DeVelopment Company Limited, Shanghai 201203, People’s Republic of China ReceiVed February 22, 2009. ReVised Manuscript ReceiVed June 3, 2009

An experimental study on NO reduction by biomass gasification tar had been carried out on the flow reaction system. Tar is so complex that tar model compounds were selected to reduce NO. Those model compounds were benzene, toluene, styrene, and phenol. According to the thermogravimetry-mass spectrometry (TG-MS) analyses of pyrolysis and gasification of rice straw, corn stalk, and corncob, these four compounds are main compositions of biogas tar. Experimental results show that the four model compounds have different effects on NO reduction, and we study their NO reduction effect individually in this paper to shed light on the reburning effect of the main species of tar. NO reduction efficiency, η, is obtained from fuel-lean to fuel-rich conditions of each fuel in the temperature range of 900-1400 °C. In general, phenol leads to a better performance under the experiment conditions, and NO reduction efficiency is maintained at almost 30-40% and changes little with temperature. For benzene, NO reduction efficiency is correspondingly low under the temperature ranges. Toluene and styrene, which have a hydrocarbon substituent attached to the benzene ring, perform better than benzene. The NO reduction efficiency of toluene is low at lower temperatures but increases a lot with increasing temperature, and the fuel-rich condition is helpful for reducing NO. NO reduction efficiency of styrene increases a little with temperature and changes a lot with the bulk equivalence ratio under higher temperatures (1100-1400 °C), but the maximum efficiency occurred at almost the same bulk equivalence ratio, φ ) 1.27. For benzene and phenol, fuel-lean conditions favor their performance. The results will instruct further research and engineering application of biogas reburning.

1. Introduction Coal-fired power plants are a major source of NOx, and controlling the emission of the pollutant is the key issue to use coal as the energy supply. It is especially important in China because a coal-based energy structure has been imposed as a significant threat to the environment, with coal consumption amounting to 60% of the total energy consumption in China.1 The reburning process has been demonstrated to be an effective technique to remove NOx from exhausts of both stationary and mobile sources. Its application is expected to be feasible in most kinds of combustion systems only if different reburn fuels are used under appropriate operating conditions. A lot of combustibles can be used as reburning fuels. The most common reburning fuel is natural gas. Although it is relatively expensive, it has caused a lot of interest in the last several years.2,3 There are many hydrocarbon fuels that are also efficient in NO reduction.4-8 * To whom correspondence should be addressed. Telephone: 86-02134206267. E-mail: [email protected]. † Shanghai Jiao Tong University. ‡ Shanghai Pudong Project Development Co. Ltd. (1) Chang, J. L.; Wu, C. Z. Renewable Sustainable Energy ReV. 2003, 7, 453–486. (2) Dagaut, P.; Lecomte, F. Combust. Sci. Technol. 1998, 139, 329– 363. (3) Bilbao, R.; Millera, A.; Alzueta, M. U. Ind. Eng. Chem. Res. 1994, 33, 2846–2852. (4) Dagaut, P.; Lecomte, F.; Chevailler, S.; Cathonnet, M. Fuel 1999, 78, 1245–1252. (5) Glarborg, P.; Alzueta, M. U.; Dam-Johansen, K.; Miller, J. A. Combust. Flame 1998, 115, 1–27.

Recently, there has been an increasing interest in biomass reburning9-14 because it has some advantages over other reburn fuels, such as being renewable and CO2-neutral. Biomass reburning includes direct and indirect reburning. Biomass direct reburning may change the quality of fly ash, so that the fly ash cannot be used further for the cement industry. Biomass indirect reburning or biomass gasification for reburning, in which the biogas is used as the reburn fuel, does not change the quality of fly ash10 and has a high NOx reduction efficiency close to natural gas reburning.11 The main combustible compositions of biogas are CO, H2, CH4, and other C2-C3 hydrocarbons. The mechanism and experimental research on NO reduction by these permanent gases have been investigated a lot in recent years. For example, the non-hydrocarbon fuels in biogas, CO and H2, (6) Dagaut, P.; Lecomte, F.; Chevailler, S.; Cathonnet, M. Combust. Flame 1999, 119, 494–504. (7) Dagaut, P.; Luche, J.; Cathonnet, M. Fuel 2001, 80, 979–986. (8) Dagaut, P.; Luche, J.; Cathonnet, M. Combust. Flame 2000, 121, 651–661. (9) Vilas, E.; Skifter, U.; Jensen, A. D.; Lo´pez, C.; Maier, J.; Glarborg, P. Energy Fuels 2004, 18, 1442–1450. (10) Berge, N.; Kallner, P.; Oskarsson, J.; Rudling, L. Joule III Programme, Clean Coal Technology R&D. Operational Problems, Trace Emissions and Byproducts Management for Industrial Biomass Cocombustion; European Commission: Brussels, Belgium, 1999; Vol. 5, pp 665-694. (11) Rud¨igger, H.; Kicherer, A.; Greul, U.; Spliethoff, H.; Hein, K. R. G. Energy Fuels 1996, 10, 789–796. (12) Glarborg, P.; Kristensen, P. G.; Dam-Johansen, K.; Alzueta, M. U.; Millera, A.; Bilbao, R. Energy Fuels 2000, 14, 828–838. (13) Dagaut, P.; Lecomte, F. Energy Fuels 2003, 17, 608–613. (14) Fan, Z. L.; Zhang, J.; Shen, C. D.; Lin, X. F.; Xu, Y. Q. Energy Fuels 2006, 20, 579–582.

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have the ability to reduce NO,12 and the effect of simple hydrocarbons in biogas, such as methane, ethane, ethylene, acetylene, propane, and propene, on reburning is proven by either experiment or simulation.4-8,12-14 While in the research of reburning by biogas, most researchers use synthetic biogas and focused on the effect and mechanism of NO reduction by the permanent gas compositions,11-14 the effect and mechanism of NO reduction by tar is scarce. Apart from causing environmental hazards, tar, generated in the biomass gasification process, is known to create processrelated problems in the end-use devices, such as fouling, corrosion, erosion, and abrasion.15,16 Before the gas can be introduced into the gas engine, the tar content has to be reduced to low values. The cleaning systems nowadays are too expensive to be used in small-scale applications; in addition, the energy in the tar is lost when it is removed. Therefore, until now, tar is still the bottleneck of the commercialization of biomass gasification technology.17 Greul et al.11 carried out the reburning experiment using coal pyrolysis gas with and without tar. The results indicate that coal pyrolysis gas with tar is more effective in NO reduction. There is research that indicates that oxidation of tar produces light C1-3 hydrocarbons,18,19 which can reduce NO. Because of this, we tried to put forward a new method to directly use the biomass gasification tar,20 that is, using biogas that contains the tar species as reburning fuel in the coal-fired burners to cut down the emission of NOx. Thereby, no extra treatment to remove tar is needed in the biomass gasification process. It is a new way to make use of tar and at the same time to remove another pollutant, NO. It can also simplify the design of the gasifier because there are no serious limitations on tar content. In previous research, Duan and Luo20 compared the NO reduction efficiency by biogas with and without tar. The results indicate that biomass gasification tar can promote NO reduction. We further carry out the reburning experiment by biogas tar. In view of the complexity of biomass gasification tar, model compounds are usually used to investigate the process involving tar. NO reduction by tar is the total effect of all tar species; in addition, different tar species may produce different results. Therefore, we choose four model compounds to study their NO reduction effect to shed light on the NO reduction character of the main species of tar, which are benzene (C6H6), toluene (C6H5CH3), styrene (C6H5C2H3), and phenol (C6H5OH). According to the experiments of thermogravimetry-mass spectrometry (TG-MS) analyses of pyrolysis and gasification of rice straw, corn stalk, and corncob,21,22 benzene, toluene, styrene, and phenol are present in relatively large extent and they are the main compositions of biogas tar.22 The four compounds cover the primary, secondary, and tertiary products of tar in the biomass gasification process, and they are usually selected as tar model compounds in the literature. For example, benzene, (15) Jun, H.; Kim, H. Renewable Sustainable Energy ReV. 2008, 12, 397–416. (16) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. Biomass Bioenergy 2003, 24, 125–140. (17) Maniatis, K. In Progress in Thermochemical Biomass ConVersion; Bridgwater, A. V., Ed.; Blackwell Science: Oxford, U.K., 2004; pp 1-31. (18) Jess, A. Fuel 1996, 75, 1441–1448. (19) Brezinsky, K. Prog. Energy Combust. Sci. 1986, 12, 1–24. (20) Duan, J.; Luo, Y. H. Energy Fuels 2007, 21, 1511–1516. (21) Chen, Y.; Duan, J.; Luo, Y. H. J. Anal. Appl. Pyrolysis 2008, 83, 165–174. (22) Duan, J. Experimental study of NO reduction by biomass gasification tar. Dissertation, Institute of Thermal Energy Engineering, Shanghai Jiao Tong University, Shanghai, China, 2008.

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Figure 1. Schematic diagram of the flow reaction system.

as the simplest aromatic hydrocarbon, has been studied in several papers as a tar model compound18,19,23,24 and is also the important intermediate during cracking of polycyclic aromatic hydrocarbons (PAHs). The other three model compounds in our paper are also one-ring aromatic hydrocarbons but have different substituents attached to the phenyl, which are methyl (CH3), ethylene (C2H3), and hydroxyl (OH). Among these, toluene18,19,23,24 and phenol25-27 have been studied a lot for their conversion as tar model compounds and styrene was selected as a model compound in this paper because the unsaturated substituent, ethylene (C2H3), was different from other substituent structures and is also the main composition of tar.16 Therefore, the objective of the present work is to perform an experimental study of NO reduction by these tar model compounds (benzene, toluene, styrene, and phenol) in the temperature range of 900-1400 °C and from fuel-lean to fuelrich conditions (φ from 0.53 to 2). The results will instruct the reburning experiment by biogas with tar in the future research and application. 2. Experimental Section 2.1. Experimental Setup. The experimental setup (Figure 1) was composed of a feeding system, a reaction system, and a continuous analysis system. A corundum tube was employed as the reactor, which was 10 mm in inner diameter and 1 m in length. It could be endured as high as 1700 °C. The heated zone was 300 mm in length, and an example of the temperature profile obtained for a system temperature of 1200 °C is shown in Figure 2, with a central zone of approximately 100 mm in length, where the temperature can be considered constant. This zone is seen as the reaction zone. The electric furnace with a SiC tube as the electrothermal element supplied heat to the reactor and could reach an upper limit of 1600 °C. A Pt-Rh thermocouple measured the temperature in the electric furnace. A single-chip microcomputer was the temperature controller, and the precision of the temperature control was (3 °C. The feeding system provided the reaction gases. NO, high-purity N2 (g99.99%), and compressed air were supplied from the pressured gas cylinders. High-purity N2 was balanced gas as well as carry gas of the model compounds. The four tar model compounds, benzene (g99.5%), toluene (99.5%), styrene (g99.0%), and phenol (g99.5%), are high-purity reagent. To ensure their different inlet concentrations, N2 carried the model compound from the saturators at different volume flows to the reactor. In view of the vapor pressure of model compound being very low, expecially phenol (it is solid at room temperature, and the saturated concentration in nitrogen is approximately 0.5 mg/m3 at room temperature), (23) Coll, R.; Salvado´, J.; Farriol, X.; Montane´, D. Fuel Process. Technol. 2001, 74, 19–31. (24) Taralas, G.; Kontominas, M. G. Energy Fuels 2005, 19, 87–93. (25) Hu, X. L.; Hanaoka, T.; Sakanishi, K.; Shinagawa, T.; Matsui, S.; Tada, M.; Iwasaki, T. J. Jpn. Inst. Energy 2007, 86, 707–711. (26) Ei-Rub, Z. A.; Bramer, E. A.; Brem, G. Fuel 2008, 87, 2243–2252. (27) Polychronopoulu, K.; Bakandritsos, A.; Tzitzios, V.; Fierro, J. L. G.; Efstathiou, A. M. J. Catal. 2006, 241, 132–148.

Effect of NO Reduction by Tar Model Compounds

Energy & Fuels, Vol. 23, 2009 4101 The bulk equivalence ratio, φ, is defined as

φ)

(fuel %/O2 %)inlet (fuel %/O2 %)stoi

(10)

The nominal gas residence time, τ, is defined as

τ)

VreactorT0 V˙0Treactor

(11)

The subscripts inlet, outlet, and stoi mean the inlet and outlet of the reactor and the stoichiometric combustion, respectively. Vreactor is the volume of the reaction zone, and V˙0 is the volume flow of the reaction gas before entering the reactor. Figure 2. Temperature profile inside the heated zone of the flow reactor at 1200 °C.

as well as it is solid at room temperature, the saturators were placed in an electronic constant temperature water bath pot, which can be heated to 99 °C. The UV spectrometer (BRU741, made by B&W Tek, Inc.) was used to quantify the accurate concentration of model compound. Several rotameter flowmeters, manufactured by Changzhou Shuanghuang Thermo-technical Instrument Co., Ltd., were used to measure and control the gas flow. A Kane-May QUINTOX flue gas analyzer was used to analyze the exhausted gas continuously. The accuracy of the measurement of NO was (5 ppm for NO % < 100 ppm and (5% for NO % > 100 ppm. The measured field was 0-5000 ppm, and the resolution factor is 1 ppm. A tampon filter was placed at the sampling inlet to clean the sampling gas because soot was detected in the experiment. 2.2. Experimental Conditions. The temperature covered 900-1400 °C, and the interval was 100 °C. A constant flow rate of 175 L/h was used, which corresponds to a gas residence time τ of the reaction zone between 85 and 130 ms depending upon the temperature. The initial concentration of NO was kept at 1100 ppm. According to the equations of complete combustion as follows, the initial concentration of each fuel should be different to keep the bulk equivalence ratio, φ, at the same range, from 0.53 to 2.0, in our experiments. Therefore, the inlet concentration of each fuel was fixed as follows: C6H6 %inlet, 2290 ppm; C6H5CH3 %inlet, 2000 ppm; C6H5OH %inlet, 600 ppm, C6H5C2H3 %inlet, 1140 ppm. That is, the bulk equivalence ratio of each fuel increased from φ < 1 to φ > 1 with the oxygen concentration decreased. In other words, the experimental conditions varied from fuel lean to fuel rich.

C6H6 + 7.5O2 f 6CO2 + 3H2O

(1)

φ ) 7.5(C6H6 %/O2 %)inlet

(2)

C6H5CH3 + 9O2 f 7CO2 + 4H2O

(3)

φ ) 9(C6H5CH3 %/O2 %)inlet

(4)

C6H5OH + 7O2 f 6CO2 + 3H2O

(5)

φ ) 7(C6H5OH %/O2 %)inlet

(6)

C6H5C2H3 + 10O2 f CO2 + 4H2O

(7)

φ ) 10(C6H5C2H3 %/O2 %)inlet

(8)

The NO reduction efficiency, η, is defined as

η)

NO %inlet - NO %outlet × 100% NO %inlet

(9)

3. Results and Discussion 3.1. Benzene. NO reduction efficiency was lower than other model compounds under most of the conditions in our experiment. From Figure 3, we can see that NO reduction efficiency first increased a little with temperature and then decreased. However, the turning point of temperature varied from different bulk equivalence ratios, φ. For φ ) 0.55, NO reduction efficiency obtained the maximum of 38.1% at 1200 °C and then decreased; while for φ ) 2.0, NO reduction efficiency reached a peak of 22.82% at 1000 °C and then decreased. With φ increased, the tturning became lower and the corresponding maximum NO reduction efficiency, ηmax, decreased as well (see Table 1). Oxidation chemistry of benzene indicates that benzene oxidation is initiated at about 1000 K,28 independent of the stoichiometry. Brezinsky19 summarized the mechanism of benzene oxidation: oxidation of benzene evolves into the oxidation of phenyl, which follows a gradual course C6 f C5 f C4, and a great deal of ethane and acetylene forms in C4 hydrocarbon oxidation. Therefore, the products of benzene pyrolysis and oxidation are CO, CO2, H2O, and some C2 hydrocarbons, among which there are NO reducing agents. In addition, benzene cracking is competing with polymerization, and with an increase in the temperature, polymerization behavior occurs easily and consumes a lot of light hydrocarbons and radicals; therfore, NO reduction by these light hydrocarbons and radicals will be weakened. When under fuel-rich conditions, polymerization will occur at a lower temperature. From panels 4-6 of Figure 3, we also see that the NO reduction efficiency of benzene increased evidently when the bulk equivalence ratio decreased at these higher temperatures (1200, 1300, and 1400 °C), which means that fuel-lean conditions favor NO reduction for benzene. Experimental and simulation results in ref 28 indicate that, as temperature increases, the rate of consumption of benzene strongly depends upon the oxygen concentration. Their experimental results also show that adding 100 ppm NO can promote the pyrolysis of benzene in fuel-lean conditions.28 The reason is that there is a lot of HO2 radicals in fuel-lean conditions and NO reacts with HO2 through the reaction NO + HO2 f NO2 + OH, thus changing the composition of the radical pool, that is, converting the unreactive HO2 radical into OH. Thereby, the presence of NO may significantly promote benzene oxidation under lean conditions. Under stoichiometric or fuel-rich conditions, the concentration of the HO2 radical is smaller and the promoting effect of NO by this mechanism diminishes. Therefore, more light gases produced in the fuel-lean conditions by benzene (28) Alzueta, M.U.; Glarborg, P.; Dam-Johansen, K. Int. J. Chem. Kinet. 2000, 32, 498–522.

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Figure 3. Comparison of NO reduction by tar model compounds: (1) 900 °C, (2) 1000 °C, (3) 1100 °C, (4) 1200 °C, (5) 1300 °C, and (6) 1400 °C.

oxidation can reduce NO, which may be the reason why fuellean conditions favor NO reduction for benzene. Among the four tar model compounds, the NO reduction efficiency of benzene is the lowest. Benzene is considered the most stable aromatic hydrocarbon compound in biogas, and its reaction activity is bad. To produce simple hydrocarbon molecules or radicals, the benzene ring should be open first to form phenyl. From Table 2, the active energy of reaction 1 C6H6

f C6H5 + H is the largest, which is perhaps the reason for benzene having a low NO reduction efficiency. In comparison to benzene, the NO reduction efficiency of toluene, styrene, and phenol is higher. From the molecular structure, we can see that there are substituents attached to the benzene ring for these compounds, which are methyl (CH3), ethylene (C2H3), and hydroxyl (OH). It needs a lower active energy for these radicals to break up from the benzene ring than to open the ring and

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Table 1. ηmax and the Turning Point of the Temperature of Benzene φ

tturning (°C)

ηmax (%)

0.55 0.75 0.83 0.95 1.14 1.43 1.63 2.0

1200 1200 1200 1100 1100 1100 1000 1000

38.1 37.0 32.73 25.82 24.73 23.55 23.09 22.82

Table 2. Comparison of the Active Energy number

reaction

E (kJ/mol)

1

C6H6 f C6H5 + H

2

C6H6 + O f C6H5 + OH

3

C6H6 + O2 f C6H5 + HO2

4

C6H5OH + O f C6H5O + OH

5

C6H5OH f C6H5O + H

38329

6

C6H5C2H3 f C6H6 + C2H2

22429

7

C6H5CH3 f C6H5CH2 + H

37329

8

C6H5CH3 f C6H5CH3

30329

46029 6228 26628 3129

then produce phenyl (C6H5+). Table 2 lists the active energy of the reactions mentioned above. In comparison to reaction 1, other active energies of the pyrolysis reactions 4-8 are comparatively small from Table 2. 3.2. Phenol. The NO reduction efficiency of phenol was efficient under the temperature range, increased a little when the temperature increased from 900 to 1000 °C, and then changed little with the temperature. From the results, we can see that NO reduction efficiency is a little higher when the bulk equivalence is low, especially in panels 1 and 2 of Figure 3, which means that fuel-lean conditions are helpful to reduce NO. In fact, phenol is an important intermediate during the combustion of aromatics.30,31 Brezinsky et al.32 studied the oxidation of phenol from fuel-lean to fuel-rich conditions at φ ) 0.64, 1.03, and 1.73. Experimental results show that CO, CH4, C2H2, C2H4, C4H6, C5H6, C6H6, and a little amount of C3 hydrocarbons, such as propylene and propyne, are the main products of phenol oxidation. With the increase of the bulk equivalence ratio, the concentrations of these products become lower.32 Therefore, the NO reduction efficiency decreased. For benzene and phenol, the NO reduction efficiency is higher in fuel-lean conditions than that in fuel-rich conditions. We analyze this further from the perspective of active energy. From Table 2, reactions 2 and 3 are oxidizing reactions for benzene, which need a lower active energy in contrast to reaction 1. Therefore, when oxygen is enough, reaction 2 and 3 happen more rapidly than reaction 1 and produce free radicals that can react with NO. Therefore, the NO reduction efficiency of benzene in oxygen-rich conditions is higher than that in oxygenlean conditions. The bond C-O is strong in phenol and not (29) National Institute of Standards and Technology (NIST). NIST Chemistry WebBook. http://webbook.nist.gov/chemistry/. (30) Horn, C.; Roy, K.; Frank, P.; Just, Th. 27th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1998; pp 321338. (31) He, Y. Z.; Mallard, W. G.; Tsang, W. J. Phys. Chem. 1988, 92, 2196–2201. (32) Brezinsky, K.; Pecullan, M.; Glassman, I. J. Phys. Chem. A 1998, 102, 8614–8619.

easy to replace or eliminate hydroxyl (OH) from the benzene ring. We can see this from Figure 2 that the active energy of reaction 5 is high, which means a low reaction activity for phenol in fuel-rich conditions. Reaction 4 needs lower active energy; therefore, hydroxyl (OH) is easy to break up from the benzene ring when there is oxygen. In addition, bond O-H is easy to break up as well. Therefore, when oxygen is enough, C6H5OH + O f C6H5O + OH will happen and produce simple hydrocarbons and free radicals. This is the reason that phenol has more ability of NO reduction in fuel-lean conditions than in fuel-rich conditions. 3.3. Toluene. The NO reduction efficiency of toluene was low at lower temperatures but increased a lot with temperature. For example, when the temperature increased from 1100 to 1200 °C, NO reduction efficiency increased 23% at φ ) 0.55 and 32.54% at φ ) 2.0. The research on toluene oxidation and pyrolysis33-35 indicates that high temperature enhances the conversion of toluene in both the pyrolysis and oxidation conditions. The products formed in toluene oxidation and pyrolysis play important roles in NO reduction. Under oxidation conditions, toluene can follow the pathway suggested by Brezinsky:19 oxidation of alkyl benzene evolves into oxidation of phenyl or benzene, which follows a gradual course similar to benzene mentioned above. At low pressure and high temperature, toluene decomposes and produces molecular weights lower than that of C7H8, such as CH3, C2H2, C3H3, C4H2, C4H3, C5H5, C6H6, and C7H7.34,35 Through the hydrocracking, hydrodealkylation, and dry reforming reactions, those products of toluene oxidation and pyrolysis, especially H2, CO, CH3, CH4, and C2H2, have the ability to reduce NO. The fact that high temperature enhances the conversion of toluene into intermediates capable of reducing NO can explain the effect of temperature. NO reduction efficiency of toluene increased with the bulk equivalence ratio under the experimental conditions, which means that the high content of oxygen is not favored for NO reduction and is more evident at higher temperatures. According to our previous research,22 the NO reduction mechanism by toluene (NRT model) had been set up and also proven by the flow reaction experiment. The NRT model indicates that pyrolysis of toluene tends to yield HCCO and C2H radicals. The reaction activity of these two radicals is higher than CHi (i ) 1, 2, and 3) radicals. Simulation results22 indicated that radicals HCCO and C2H (ethynyl) play a key role in NO reduction by toluene through reactions HCCO + NO f HCNO + CO and C2H + NO f HCN + CO. When the oxygen content increased, C2H and HCCO are oxidized through reactions O + HCCO f H + 2CO and C2H + O2 f HCO + CO, thus making the NO reduction efficiency lower. In addition, the NO reduction mechanism by tar is a little different from the simple hydrocarbons, in which CHi (i ) 0, 1, 2, and 3) and HCCO are considered key reduction agents.5-8 3.4. Styrene. NO reduction efficiency of styrene increased with the temperature from 900 to 1300 °C, and the maximum amplify is between 1000 and 1100 °C. For example, NO reduction efficiency increased 42.27%, at φ ) 1.27 between 1000 and 1100 °C, while the efficiency at 1400 °C is lower than that at 1300 °C. From panels 3-6 of Figure 3, NO reduction efficiency changed so much with the bulk equivalence ratio under higher temperatures (1100, 1200, 1300, and 1400 °C). However, the maximum efficiency occurred at almost the same bulk equiva(33) Bounaceur, R.; Costa, I. D.; Fournet, R. Int. J. Chem. Kinet. 2005, 37 (1), 25–49. (34) Smith, R. D. J. Phys. Chem. 1979, 83 (12), 1553–1563. (35) Smith, R. D. Combust. Flame 1979, 35, 179–190.

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lence ratio, φ ) 1.27. This result is in accordance with the NO reduction experiments with varying the initial styrene concentration in our other experimental research.22 From the results, we can see that the optimal NO reduction occurs under a little fuelrich conditions. Cracking and polymerization are co-occurring during pyrolysis and oxidation of aromatic hydrocarbons; therefore, NO reduction by the pyrolysis intermediates is competing with the polymerization. It is really a very complex course. Soot was detected in most of the experiments, especially under higher temperatures. However, there is no soot detected in the NO reduction experiment of phenol. The polymerization and sooting behavior of the different tar model compounds mainly depend upon the temperature, gas residence time, and bulk equivalence ratio. There is research that indicates that soot is also effective in NO reduction;36-38 therefore, NO reduction efficiency of each model compound is the whole effect of light gases and intermediates, products of pyrolysis and oxidation of model compound, and soot, product of model compound polymerization. Although the oxidation chemistry of aromatic compounds is of significant theoretical and practical interest, the oxidation behavior of aromatic compounds is atypical as well as difficult compared to other hydrocarbons, because of the characteristics of the aromatic ring and its rupture. Therefore, we just analyze the NO reduction, concentrating on the light gases and radicals, and did not quantify the soot formed. However, the effect is the total effect of light gases and soot. 4. Conclusions From the experimental results, we can see that the NO reduction by the tar model compound is feasible and reburning (36) Aarna, I.; Suuberg, E. M. Fuel 1997, 76, 475–491. (37) Commandre´, J. M.; Stanmore, B. R.; Salvador, S. Combust. Flame 2002, 128, 211–216. (38) Illa´n-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C.; Calo, J. M. Energy Fuels 1993, 7, 146–154.

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by biogas with tar is an economic way to make use of tar and avoid the complex tar-removing process. The following conclusions were obtained from this paper: In general, phenol leads to better performance under the experimental conditions; NO reduction efficiency keeps at almost 30-40% at 900-1400 °C and changed little with temperature. For benzene, NO reduction efficiency is correspondingly low under the temperature ranges, while toluene and styrene, which have hydrocarbon substituents attached to the benzene ring, perform better than benzene. The NO reduction efficiency of toluene is low at lower temperatures but increases a lot with temperature, and fuel-rich conditions are helpful to reduce NO. The NO reduction efficiency of styrene increases with temperature and changes a lot with the bulk equivalence ratio under higher temperatures (1100-1400 °C), but the maximum efficiency occurred at almost the same bulk equivalence ratio, φ ) 1.27. For benzene and phenol, fuel-lean conditions favor their performance. Despite the uncertainties in the chemistry in NO reduction by aromatic compounds, the experimental results will instruct our further research on biogas reburning, which will have a bright future in a rapid energy-demanding society. NO reduction by tar is the total effect of all tar species, and the presence of all tar compositions in the biogas-reburning process should be much more complex. The mechanism and experimental study of reburning by tar or a tar model compound is significant and needs further research for the application of reburning by biogas with tar. Acknowledgment. The authors gratefully acknowledge the support from the National High Technology Research and Development Program of China (863 Program) (2008AA05Z312). EF9001567