Energy & Fuels 2007, 21, 1511-1516
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Effect of Biomass Gasification Tar on NO Reduction by Biogas Reburning Jia Duan,*,† Yong-Hao Luo,† Nai-Qiang Yan,§ and Yi Chen† School of Mechanical and Power Engineering and School of EnVironmental Science and Technology, Shanghai Jiao Tong UniVersity, Shanghai 200240, P. R. China ReceiVed NoVember 26, 2006. ReVised Manuscript ReceiVed February 25, 2007
Reduction of NO by biogas without tar and with tar was investigated. Flow reaction experiments were performed at high-temperature (g1000 °C), and the main parameters, such as initial oxygen concentration, bulk equivalence ratio, temperature, and initial tar content, were studied. The simulated biogas was prepared by mixing CO, CH4, H2, and CO2, and toluene was selected as a model compound as the biomass gasification tar. The moderate fuel-rich environment is favored to a high level of NO reduction by both biogas without tar and biogas with tar. The optimal bulk equivalence ratio, φ, for NO reduction by biogas with tar is larger than that of NO reduction by biogas without tar. The effect of temperature on NO reduction by biogas with model compound is stronger than that by biogas without tar. The comparison of NO reduction by biogas without tar and biogas with tar indicates that tar can either promote or limit NO reduction by biogas. The experimental results here will be helpful to the further investigation of NO reburning.
1. Introduction Biomass is one of the main renewable energies for mitigating the shortage of fossil fuel and pollution. The primary energy structure in China is mainly dependent on fossil fuels, among which coal shares almost 70%.1 With the rapid energy-demand increase in the last two decades, China has become the largest coal-consuming country in the world, which has been causing serious issues in energy sustainability and environment. So the development of biomass utilization as the energy supply has been paid more and more attention to in China. Biomass gasification application for power is an important approach to biomass utilization. However, the tar, which is the byproduct produced in the biomass gasification process, often brings forth many problems. It will condense at reduced temperature, leading to blocking, fouling in duct, valve, and engine, and polymerizing to form more complex construction in combustion, which decrease the burnout ratio of tar and causes pollution. Therefore, the removal of tar in biogas has always been considered as one of the main bottlenecks for industrializing the technology of biomass gasification. The methods for removing the tar from gasified gas can be categorized in two types depending on the location where tar is removed, either in the gasifier or outside the gasifier (secondary method).2 The former method2 consists of optimization of operating parameters, utilization of additive/catalyst, and modifications of gasifier. The latter comprise the chemical treatments of tar, such as thermal decomposition, catalytically * Corresponding author. Tel: 86 021 34205702. E-mail:
[email protected]. † School of Mechanical and Power Engineering, Shanghai Jiao Tong University. § School of Environmental Science and Technology, Shanghai Jiao Tong University. (1) Chang, J.; Leung, D. Y. C.; Wu, C. Z.; Yuan, Z. H. Renewable Sustainable Energy ReV. 2003, 7, 453-468. (2) Devi, L.; Ptasonski, K. J.; Janssen F. J. J. G. Biomass Bioenergy 2003, 24, 125-140.
cracking, and combination of them, and mechanical separation of tar. But the efficient removal of tar still remains the main technical barrier to the successful commercialization of biomass gasification technologies.3 In this study, we put forward a new approach to directly use the gasified gases with tar as reburning reductants to cut down the emission of NOx from coal-fired burners. Hereby, no extra treatment is needed to remove tar for this purpose. As we have known, NOx is one of the main air pollutants, and how to reduce its emission from the coal-fired flue gas has become an important issue. Reburning has been regarded as an effective method for restraining the produce of NOx in furnaces. A wide range of reburn fuels have application in controlling NOx, and reburn fuel properties have been found to affect the performance significantly. Natural gas is an important reburn fuel, because it contains no fuel-bound nitrogen, sulfur, or particulate matter, and it reacts fast. But natural gas is scarce and expensive in China. Pulverized coal, other hydrocarbons, refuse derived fuel, and biomass as the reburn fuel have been conducted in order to lower fuel cost. Biomass rebrurning has attracted much interest4-10 and has some advantages over other reburn fuels because biomass is renewable and CO2 neutral. (3) Maniatis, K. In Progress in Thermochemical Biomass ConVersion; Bridgwater, A.V., Ed.; Blackwell Science: Malden, MA, 2004; pp1-31. (4) Vilas, E.; Skifter, U.; Jensen, A. D.; Lo´pez, C.; Maier, J.; Glarborg, P. Energy Fuels 2004, 18, 1442-1450. (5) Berge, N.; Kallner, P.; Oskarsson, J.; Rudling, L. In Joule Programme, Volume V, Clean Coal Technology R&D, Operational Problems, Trace Emissions and By-Products Management for Industrial Biomass CoCombustion; Pierre Dechamps, Ed.; European Commission: Brussels, Belgium, 1999; Vol. V, pp 665-694. (6) Ru¨diger, H.; Kicherer, A.; Greul, U.; Spliethoff, H.; Hein, K. R. G. Energy Fuels 1996, 10, 789-796. (7) Glarborg, P.; Kristensen, P. G.; Dam-Johansen, K. Energy Fuels 2000, 14, 828-838. (8) Dagaut, P.; Lecomte, F. Energy Fuels 2003, 17, 608-613. (9) Fan, Z. L.; Zhang J.; Shen C. D.; Lin, X. F.; Xu, Y. Q. Energy Fuels 2006, 20, 579-582. (10) Harding, N. S.; Adams B. R. Biomass Bioenergy 2000, 19, 429445.
10.1021/ef060599+ CCC: $37.00 © 2007 American Chemical Society Published on Web 04/17/2007
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The present work aims at analyzing the effect of model tar compound on NO reduction by biogas reburning. 2. Experimental Section
Figure 1. Experimental setup.
Figure 2. 2. Temperature profile inside the heated zone of the reactor for a system temperature of 1400 °C.
The biomass reburning includs the direct reburning and the indirect reburning. Biomass gasification is a necessary preprocess for the indirect reburning, whereas it is unnecessary for the direct reburning. Reuse of fly ash is an important factor in evaluating the effect of biomass reburning. Biomass direct reburning may change the quality of fly ash so that the fly ash cannot be sold to cement industry. Biomass indirect reburning, or biomass gasification-for-reburning, does not change the quality of fly ash5 and has a high NOx reduction efficiency close to natural gas reburning.6 Significant progress in biomass gasification-for-reburning has been achieved over the past decade. The main combustible compositions of biogas are CO, H2, CH4, and other hydrocarbons. The non-hydrocarbon fuels, such as CO and H2, in biogas have the ability to reduce NO7. The effect of some simple hydrocarbons in biogas, such as methane, ethylene, acetylene, and cyclopropane, on biogas reburning is proved by either experiment or simulation.7-9 But most research of biomass gasification-for-reburning have focused on the effect and the mechanism of NOx reduction by the permanent gas compositions in biogas,6-9 the biomass gasification-for-reburning in the present of tar has not been wellunderstood.
2.1. Experimental Setup. The experimental setup (Figure 1) was composed of a reaction system, a gas feed system, and a continuous analysis system. The 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, and the heated zone was 300 mm in length. An example of the temperature profile obtained for a system temperature of 1400 °C is shown in Figure 2. The electric furnace with a SiC tube as electrothermal element supplied heat to the reactor and could reach an upper limit of temperature as high as 1600 °C. A Pt-Rh thermocouple measured the temperature in the electric furnace. A single-chip microcomputer was the core of the temperature controller, and the precision of temperature control was (3 °C. To prove the validity of NO reduction, tar and the model compound were carried by nitrogen from a saturator into the reactor, respectively. The tar collected from the downstream straw gasifier outlet was dissolved in dichloromethane, and the solution was filtered to remove solid residue. The solution was then dried at 35 °C in an oven for at least 48 h to evaporize dichloromethane. The mass flow rate of tar carried by nitrogen flow was 512 mg/h. The high-purity (>99.5%) toluene was selected as the model compound. Oxygen, nitric oxide, and nitrogen were also involved in the reaction. To study the effect of the model compound on NO reduction quantificationally, we used a gas of 1.01% toluene balanced by nitrogen in a pressured gas cylinder. Oxygen, nitrogen, nitric oxide, and simulated biogas were stored in four pressured gas cylinders, respectively. The oxygen and the nitrogen were both high-purity gases (>99.99%). The nitric oxide gas contained 1.01% NO, balanced by nitrogen. The simulated biogas consisting of 36.7% CO, 11.7% CH4, 16.0% H2, 33.4% CO2, and nitrogen balance were synthesized in advance. Those gases stored in pressured gas cylinders were all products of Shanghai Jiliang Reference Gas Ltd. 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. 2.2. Experimental Conditions. Table 1 lists the main variables in the experiments. In the experiments with biomass gasification tar or toluene as the only reductant, the flow ratio of the carrying gas (nitrogen) was stable. The purpose of those experiments was to prove whether NO reduction by tar or the model compound was reachable. CO, CH4, and H2 were calculated together as one part or the whole part of reburning fuel in experiments of NO reduction by simulated biogas with tar or without tar, respectively. The concentration of model compound, toluene, was calculated as that which was injected from the pressured gas cylinder.
Table 1. Variables in Experiments reductant tar T (˚C) Vtotala (NL/h) Vcarrying gasa (NL/h) tar (mg/h) toluene%inlet (ppm) (CO% + CH4% + H2%)inlet (%) O2%inlet (%) NO%inlet (ppm) a
0 °C ,101 325 Pa.
1200 60 6.3 512
toluene 1200 61 7.5
biogas without tar
biogas with toluene
1000, 1200, 1300, 1400 611
1000, 1100, 1200, 1300, 1400 611
0.7-9.9 0-5 1000
323-5160 0.7-10.3 1-5 1000
3156 1.5 207-471
1.3-4.6 200
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Figure 3. NO reduction by biomass gasification tar.
Figure 4. NO reduction by model tar compound.
Here the bulk equivalence ratio, φ, was defined as φ)
(fuel%/O2%)inlet (fuel%/O2% )stoi
(1)
Where fuel% ) CO% + CH4% + H2% + C6H5CH3%, and φ ) 0.5-2.28. The relative content of tar, γ, was defined as γ)
C6H5CH3%inlet (CO% + CH4% + H2%)inlet
(2)
The NO reduction efficiency (η) was defined as η)
NO%inlet - NO%outlet × 100% NO%inlet
(3)
Here, inlet, outlet, and stoi mean the inlet and the outlet of the reactor and the stoichiometric combustion, respectively.
3. Results and Discussion 3.1. Validity of NO Reduction by Tar and Model Compound. The tar performed a conspicuous function on NO reduction as NO%inlet < 500 ppm (Figure 3). As NO%inlet < 500 ppm, the higher inlet NO concentration contributed to a higher NO reduction. The fitted curve in Figure 3 indicated that NO reduction efficiency was almost linear with NO%inlet and increased monotonously. Experiments of NO reduction by other reburning fuel, such as natural gas, biomass, carbonized refuse derived fuel (CRFD), low rank coal, and bituminous coal, showed that NO reduction efficiency was dependent on the NO%inlet as NO%inlet 5%) concentration in biomass gasification tar species under most conditions13 and it shares 14.3% in typical composition of biomass gasification (11) Bilbao, R.; Alzueta, M. U.; Millera, A. Ind. Eng. Chem. Res. 1995, 34, 4531-4539. (12) Maly, P. M.; Zamansky, V. M.; Ho, L.; Payne, R. Fuel 1999, 78, 327-334. (13) Kinoshita, C. M.; Wang, Y.; Zhou, J. J. Anal. Appl. Pyrolysis 1994, 29, 169-181.
Figure 5. NO reduction efficiency under oxidation and pyrolysis conditions.
tar.14 Because it is less harmful than most of the other tar compounds, it is often selected as a model compound.15-18 Here, toluene was selected as the model compound. Toluene was effective on NO reduction (Figure 4). Also, the inlet oxygen concentration showed an important effect on NO reduction by toluene (Figure 4). As O2%inlet increased, the NO reduction efficiency diminished. Excess oxygen is favorable for converting reductant to carbon dioxide and water rather than forming hydrocarbon radicals conducive to NO reduction. 3.2. NO Reduction by Biogas. 3.2.1. Effect of O2%inlet on NO Reduction by Biogas without Tar. The NO reduction efficiency in the absence of oxygen increased gradually while the concentration of reductant, CO, H2, and CH4 increased, in Figure 5. The maximum increment of NO reduction by biogas without tar, from 38.4 to 60.0%, happened as the reductant concentration increased from about 5.0 to 6.5%. The increase of NO reduction diminished as the concentration of reductant increased exceeding 6.5%. The oxidation curve (O2%inlet ) 4%) reached a peak as the reductant concentration was about 5.7%, and the bulk equiva(14) Milne T. A., Evans, R. J., Abatzoglou, N. Biomass Gasifier “Tars”: Their Nature, Formation, and ConVersion; Report NREL/TP-57025357; National Renewable Energy Laboratory: Washington, D.C., 1998. (15) Taralas G., Kontominas, M. G. Energy Fuels 2005, 19, 87-93. (16) Coll, R.; Salvado´, J.; Farriol, X.; Montane´, D. Fuel Process. Technol. 2001, 74 (1), 19-31. (17) Srinakruang, J.; Sato, K.; Vitidsant, T.; Fujimoto, K. Catal. Commun. 2005, 6 (6), 437-440. (18) Juutilainen, S. J.; Simell, P. A.; Krause, O. I. Appl. Catal., B 2006, 62 (1-2), 86-92.
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Figure 6. Effect of O2%inlet. Figure 7. Positive effect of biomass gasification tar.
lence ratio, φ, was 1.1, in Figure 5. The maximum NO reduction efficiency under oxidation conditions was 77.2%, surpassing that in the absence of oxygen. In the absence of oxygen, the increase of the NO reduction needed additional consumption of the reductant. In the present of oxygen, even if the reductant concentration was low, a suitable local equivalence ratio, φ, could improve NO reduction. There was a threshold value of the reductant concentration that was 7.6% in Figure 5. Once the reductant concentration was beyond the threshold value, the NO reductions with high reductant concentration under oxidation condition would be inferior to those under pyrolysis condition. It is necessary to increase the reductant concentration to keep φ in the range of 0.5-2.0 as O2%inlet increased. There was a maximum of NO reduction efficiency for each O2%inlet curve, when O2%inlet ) 1-5% (Figure 6). Intersections of the five fit curves and vertical dash lines, AA, BB, CC, DD, and EE, in Figure 6 were helpful in predicting what would happen if O2%inlet increase with a fixed reductant concentration. Some different trends would appear. NO reduction efficiency diminished monotonously, for example, the ash AA; NO reduction efficiency increased monotonously, for example, the ash EE; NO reduction efficiency did either increase or diminish, for example, dash BB, dash CC, dash DD. The local equivalence ratio, φ, was an important factor in interpreting the complicated change of NO reduction. If the reductant concentration kept constant and O2%inlet increased, the local equivalence ratio, φ, would diminish. Only when the combustion condition was in the moderate fuel-rich condition could NO reduction realize a maximum. Not all the combustion condition, Figure 6, was in moderate fuel-rich condition, which caused the complicated change of NO reduction. 3.2.2. PositiVe Effect of Biomass Gasification Tar. Toluene, the model compound, could play a positive role in NO reduction by biogas reburning. The NO reduction efficiency evidently increased from biogas without tar to biogas with model compound (Figure 7). The relative increase of NO reduction efficiency () (ηγ)1/20 - ηγ)0)/ηγ)0) of 32, 16, 5, and 10% (Figure 7), along with the increase of the inlet global concentration of carbon monoxide, methane, and hydrogen, was appreciable. 3.2.3. Effect of φ. φ > 1 indicated that the reaction system was in the fuel-rich condition, whereas φ < 1 meant the fuellean condition. As φ was about 1.10 (Figure 8), the NO reduction efficiency reached a maximum, 80.4%, at γ ) 0. There is a more precise optimal bulk equivalence ratio, φ, for biogas
Figure 8. Effect of φ.
without tar reburning obtained in Fan’s experimental study, which is 1.15.9 The NO reduction by biogas with toluene diminished, whereas that of biogas without tar increased, as the total concentration of CO, CH4, and H2 at the inlet of reactor increase from 5.2 to 7.1% (Figure 7). The model compound, toluene, had a positive effect on NO reduction. So it could be integrated into the reductant or fuel in the reaction system. The local equivalence ratios would then be 0.50, 0.80, 1.10, and 1.40 for NO reduction by biogas without tar and 0.75, 1.20, 1.65, and 2.10 for NO reduction by biogas with model compound, as the total permanent gas concentration increased. This analysis of φ indicated that the moderate fuel-rich condition was helpful to NO reduction by biogas with tar, which was similar to NO reduction by biogas without tar. The experiments covering the range of γ ) 1/20 and φ ) 0.75, 1.20, 1.65, 2.10, 2.55 showed the optimal φ for NO reduction was either 1.20 or 1.65. Figure 8 showed an optimal φ of 1.65 for NO reduction. This optimal φ was larger than 1.1 that was the optimal φ for NO reduction by biogas without tar, which meant that more reductant was needed in the presence of model compound. The research of NO reduction by C1, C2, and C3 hydrocarbons19-21 indicates that the main pathway of (19) Glarborg, P.; Alzueta, M. U.; Dam-Johansen, K.; Miller, J. A. Combust. Flame 1998, 115, 1-27. (20) Bilbao, R.; Alzueta, M. U.; Millera, A.; Mario, D. Ind. Eng. Chem. Res. 1995, 34, 4540-4548. (21) Dagaut, P.; Luche, J.; Cathonnet, M. Combust. Flame 2004, 121, 651-661.
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Figure 9. Effect of temperature on NO reduction by biogas without tar.
the reductant is oxidation and decomposition to form simple hydrocarbon radicals and ketenyl radical. The conversion of toluene into simple hydrocarbon radicals needs much energy, and the pathway is complex. And some products with heavier molecular weights than that of C7H8 form through a polymerization reaction in toluene oxidation and pyrolysis.22-23 As a result, the utilization of toluene to reduce NO is limited. On the other hand, some reactions in toluene oxidation consume radicals, such as H, CHi22. Those reactions can compete with the reactions of NO reduction involving such radicals, and the rate of NO reduction will be limited. Additional reductant is then necessary to increase the radical pool concentration. The addition of reductant causes an increase of φ in NO reduction by biogas with model compound. 3.2.4. Effect of Temperature. High temperature was effective on NO reduction. As temperature increased from 1000 to 1400 °C, the NO reduction by biogas without tar increased (Figure 9). As a result, the maxima of the NO reduction increased from 29.7% (1000 °C) to 38.2% (1200 °C) and 56.6% (1400 °C). In the same temperature range, the NO reduction by biogas with model tar compound increased, too. This result indicated that the amplitude of the NO reduction change increased at high temperature, and high temperature was useful for NO reduction. In the presence of CO, CO reacts with NO directly7
CO + NO T CO2 + N
(4)
At high temperature, NO can be reduced by H atom directly7
H + NO T HO + N
(5)
In moderate fuel-rich conditions, the amplitude of NO reduction change by biogas with model compound, toluene, was between 13.5 and 9.5% as the reactor temperature increased from 1000 to 1200 °C (Figure 10), whereas the amplitude of NO reduction change by biogas without tar was between 8.5 and 5.1% in the same range of temperature (Figure 10). This result indicated that the effect of temperature on NO reduction by biogas with model compound was stronger than that by biogas without tar. (22) Bounaceur R.; Costa I. D.; Fournet R. Int. J. Chem. Kinet. 2005, 37 (1), 25-49. (23) Colket, M. B.; Seery, D. J. In 25th Symposium (International) on Combustion, Irvine, CA, July 31-Aug 5, 1994; The Combustion Institute: Pittsburgh, PA, 1994; pp 883-891.
Figure 10. Effect of temperature on NO reduction by biogas with model tar compound.
Figure 11. Effect of γ. Table 2. Local Equivalence Ratio φ φ O2%inlet (%)
(CO%+CH4%+ H2%)inlet (%)
γ)0
γ ) 1/40
γ ) 1/30
γ ) 1/20
3 3 3 3
3.1 4.26 5.42 6.58
0.80 1.10 1.40 1.70
1.00 1.38 1.75 2.10
1.07 1.50 1.87 2.27
1.20 1.65 2.10 2.55
Research on toluene oxidation and pyrolysis22,24-29 indicates that high-temperature enhances the conversion of toluene in both the pyrolysis condition and the oxidation condition. 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:26 oxidation of alkyl benzene evolves into oxidation of phenyl or benzene, which follows a gradual course of C6 f C5 f C4 and a great deal of ethane and acetylene forms in C4 hydrocarbon oxidation.26 At low pressure and high temperature, toluene decomposes and products with molecular weights lower than that of C7H8, such (24) Sivaramakrishnan, R.; Tranter, R. S.; Brezinsky, K. Proceedings of the Combustion Institute; The Combustion Institute: Pittsburgh, PA, 2005; Vol. 30, pp 1165-1173. (25) Sivaramakrishnan, R.; Tranter, R. S.; Brezinsky, K. Combust. Flame 2004, 139, 340-350. (26) Brezinsky, K. Prog. Energy Combust. 1986, 12, 1-24. (27) Taralas, G.; Kontominas, M. G.; Kakatsios, X. Energy Fuels. 2003, 17, 329-337. (28) Smith, R. D. J. Phys. Chem. 1979, 83 (12), 1553-1563. (29) Smith, R. D. Combust. Flame 1979, 35, 179-190.
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as CH3, C2H2, C3H3, C4H2, C4H3, C5H5, C6H6, and C7H7, are dominant.28-29 Under the experimental flue gas conditions, toluene may convert into C, CO, CH4, H2 through hydrocracking reaction, hydrodealkylation reaction, dry reforming reaction, and carbon formation reaction.30 Those products of toluene oxidation and pyrolysis, especially H2, CO, C, CH3, CH4, and C2H2, have the ability to reduce NO. That high-temperature enhances the conversion of biomass gasification tar into intermediates capable of reducing NO can explain the effect of temperature. 3.2.5. Effect of γ on NO Reduction by Biogas with Tar. The quantity of total tar formed varied under different gasification condition, which caused an unstable relative content of tar in biogas. The experiments studied four γ (γ ) 1/20, 1/30, 1/40, 0) values in stoichiometric or fuel-rich conditions (Figure 11). As γ increased, NO reduction efficiency increased, as arrow FF, or diminished, as arrow GG. Table 2 listed the local equivalence ratios of each point in Figure 11. As both φ and γ increased with a fixed (CO% + CH4% + H2%)inlet, the NO reduction diminished except that φ increased from 0.80 to 1.20. This result showed some negative effect of a model compound on biogas reburning, which meant that the model tar compound could limit NO reduction by biogas. Though the bulk equivalence ratio can explain the (30) Simell, P. A.; Hepola, J. O.; Krause, A. O. I. Fuel 1997, 76 (12), 1117-1127.
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negative effect, the mechanism of tar or model compound on NO reduction by biogas needs further investigation. 4. Conclusions In the present study, the validity of NO reduction by biomass gasification tar and a model compound, toluene, was proved experimentally. And the effect of biomass gasification tar on NO reduction by biogas reburning was investigated in comparison with NO reduction by biogas without tar. The following conclusions were obtained through this investigation: 1. Biomass gasification tar and the model compound, toluene, performed a conspicuous function on NO reduction. 2. The optimal local equivalence ratio, φ, for NO reduction by biogas with the model compound, toluene, was larger than that of NO reduction by biogas without tar. 3. The effect of temperature on NO reduction by biogas with model compound was stronger than that by biogas without tar. 4. Biomass gasification tar performs a complex effect on NO reduction by biogas reburning. Despite the uncertainties in the chemistry in NO reduction by aromatic compounds, the experiment results here are helpful to the further investigation of NO reduction by biogas reburning. Acknowledgment. The authors are grateful to the Science and Technology Commission of Shanghai Municipality for its financial support (under Grant 05dz12010). EF060599+
