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Biomass Suspension Combustion: Effect of Two-Stage Combustion on NOx Emissions in a Laboratory-Scale Swirl Burner Weigang Lin,* Peter A. Jensen, and Anker D. Jensen Department of Chemical and Biochemical Engineering, Technical UniVersity of Denmark, Building 229, Lyngby DK-2800, Denmark ReceiVed June 20, 2008. ReVised Manuscript ReceiVed September 22, 2008
A systematic study was performed in a suspension fired 20 kW laboratory-scale swirl burner test rig for combustion of biomass and co-combustion of natural gas and biomass. The main focus is put on the effect of two-stage combustion on the NO emission, as well as its effect on the incomplete combustion. When twostage combustion was applied, the NO emission level can be significantly reduced. The experimental results show that an optimal first-stage combustion stoichiometry (λ1) exists, at which a minimum NO emission can be achieved. An optimal stoichiometry of around 0.8 in the fuel-rich zone exists with respect to minimizing NO emissions. When using wood and straw as co-firing fuels, 15-25% of the fuel-N is converted to NO. Straw appears to give the lowest conversion of fuel-N to NO. The results indicate that the optimal stoichiometry in the fuel-rich (λ1) zone for gaining the lowest NO may result from the homogeneous reaction, by comparing the NO emissions when firing natural gas with NH3 addition and co-firing natural gas and biomass. The experimental results also show no significant increase of incomplete combustion of gas and char by applying optimized two-stage combustion.
Introduction The consequences of carbon dioxide (CO2)-induced global warming cause major concern worldwide. The consumption of energy produced with fossil fuels is the major factor that contributes to the global warming. Biomass is a renewable energy resource and has a nature of CO2 neutrality. In the last 2 decades, the amount of biofuels consumed for power generation has increased significantly. In the United States, already more than 9 GWe capacity of power generation based on biomass has been installed.1,2 In the European Union, it is predicted that the biomass-based power generation capacity will increase from 7.8 GWe in 2001 to 26.5 GWe in 2010.3 In Denmark, more than 1.3 million tons of wood and 750 000 tons of straw were used in power plants in the year 2001.4 According to the Danish energy policy, the part of biomass in the energy supply pattern will continue to expand in the future. The use of wood and straw in Denmark will reach 9 million tons (105 PJ) by the year 2030 according to the plan “Energy 21”.5 Until 2002, Denmark has 50 combined heating and power (CHP) plants operating on wood chips, 25 on wood pellets, and 75 straw* To whom correspondence should be addressed. Telephone: +4545252835. Fax: +45-45882258. E-mail:
[email protected]. (1) Bain, R. L.; Amos, W. P.; Downing, M.; Perlack, R. L. Highlights of Biopower Technical Assessment: State of the Industry and the Technology, National Renewable Energy Laboratory, Golden, CO, April 2003; p 39. (2) Veringa, H. J. Advanced Techniques for Generation of Energy from Biomass and Waste, Energy Research Center of the Netherlands (ECN), Petten, The Netherlands, 2005; p 24. (3) Insights, B. The Future of Global Biomass Power Generation: The Technology, Economics and Impact of Biomass Power Generation, Strategic Market Report, Research and Markets, Dublin, Ireland, 2004. (4) Schultz, G. Securing of supply in short and longer term of wood and straw. In 2001 Task Meeting of IEA Bioenergy Task 30, Denmark, 2001. (5) Larsen, I. Renewable EnergysDanish Solutions, Danish Energy Authority, Copenhagen, Denmark, 2003.
fired plants.6 Many of the biomass-fired plants are grate boilers, but suspension firing of biomass have been used recently. There has then been an increasing demand to have a higher flexibility when firing biomass fuels in terms of load flexibility while maintaining high power efficiency, and biomass suspension firing can fulfill these requirement. An example of a co-firing power plant is the Danish Avedøreværket unit 2 (AVV2). The AVV2 unit, with ultra-super-critical (USC) steam parameters is a multifuel power plant for co-firing natural gas, oil, and pulverized wood pellets with a net electricity efficiency of up to 47%.7 However, suspension combustion of biomass is still in an initial stage, and problems, such as ash deposition, corrosion, emission, and ash disposal may arise. Among the problems, this work will investigate the NO emission and combustion efficiency when co-firing wood, straw, and natural gas. It is known that when co-firing the woody biomass and natural gas on the AVV2 boiler, the emission of NO is normally at a low level. However, it occurs that the NO emission may exceed the emission limit level occasionally when the SCR unit is not in operation.8 Also, firing or co-firing biomass may result in a fast deactivation of SCR catalysts.9,10 The replacement or regeneration of the SCR catalyst is costly. Thus, it is desirable to apply primary methods for minimizing NO emissions. The staged combustion has been proven an effective primary method to reduce the NOx emissions in the suspension combus(6) IEA Energy Policies of IEA CountriessDenmark 2002 Review. Organisation for Economic Co-operation and Development (OECD)/ International Energy Agency (IEA), 2002; p 132. (7) Ottosen, P.; Gullev, L. Avedøre unit 2sThe world’s largest biomassfuelled CHP plant. News from DBDH, 2005. (8) Jensen, J. P. Personal comunication, 2006. (9) Zheng, Y. J.; Jensen, A. D.; Johnsson, J. E. Appl. Catal., B 2005, 60 (3-4), 253–264. (10) Kling, A.; Andersson, C.; Myringer, A.; Eskilsson, D.; Jaras, S. G. Appl. Catal., B 2007, 69 (3-4), 240–251.
10.1021/ef8004866 CCC: $40.75 2009 American Chemical Society Published on Web 02/27/2009
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and biomass27-30 and co-firing biomass and natural gas,31 no systematic results with respect to the effect of two-stage combustion on the NOx emissions and burnout behaviors are reported. The main objective of this work is to study experimentally the emissions of NOx and CO, as well as the unburned carbon in fly ash from combustion of biomass and co-combustion of biomass and natural gas, with a focus on the influence of twostage combustion and optimal first-stage stoichiometry. Experiments were performed in a laboratory-scale 20 kW swirl burner firing wood, straw, and natural gas for providing knowledge of how to reduce NOx emissions without the cost of combustion efficiency. Experimental Section
Figure 1. Diagram of the test rig for co-combustion of natural gas and biofuels.
tion system.11-23 In addition to the aerodynamic air staging (low NOx burner), two-stage combustion, in which fuel-rich and fuellean zones of combustion are segregated through external divided air ports,13 is widely applied. It has been reported that the minimum NOx level is observed with two-stage combustion for firing gas11,12,18 and firing coal in the suspension combustion system.14,15,22 The degree of NOx reduction by two-stage combustion and the optimal stoichiometry of the fuel-rich zone considerably depend upon the type of fuels. However, it was reported that the unburned carbon content in the fly ash increases when measures are taken for reducing the emission of NO in pulverized coal-fired boilers.22,24-26 However, to the knowledge of the authors, no detailed information is available with respect to the influence of two-stage combustion on the NO emissions and unburned carbon in fly ash for suspension combustion of biomass and co-combustion of biomass and natural gas. Although limited investigations are published on co-firing coal (11) Martinzp, F. J.; Dederick, P. K. NOx from fuel nitrogen in twostage combustion. In the 18th International Symposium on Combustion, The Combustion Institute, Pittsburgh, PA, 1977; pp 191-198. (12) Takagi, T.; Tatsumi, T.; Ogasawara, M. Combust. Flame 1979, 35, 17–25. (13) Wendt, J. O. L. Prog. Energy Combust. Sci. 1980, 6 (2), 201–222. (14) Chen, S. L.; Heap, M. P.; Pershing, D. W.; Martin, G. B. Influence of coal composition on the fate of volatile and char nitrogen during combustion. In the 19th International Symposium on Combustion, The Combustion Institute, Pittsburgh, PA, 1982; pp 1271-1280. (15) Chen, S. L.; Heap, M. P.; Pershing, D. W.; Martin, G. B. Fuel 1982, 61 (12), 1218–1224. (16) Beer, J. M. Chem. Eng. Sci. 1994, 49 (24A), 4067–4083. (17) Wendt, J. O. L. Combust. Sci. Technol. 1995, 108 (4-6), 323– 344. (18) Mao, F. H.; Barat, R. B. Combust. Flame 1996, 105 (4), 557–568. (19) Li, S. C.; Williams, F. A. Combust. Flame 1999, 118 (3), 399– 414. (20) Makino, K. IFRF Combust. J. 2000, article number 200007. (21) Smith, D. J. Power Eng. 2001, 105 (10), 71–74. (22) Ikeda, M.; Makino, H.; Morinaga, H.; Higashiyama, K. JSME Int. J., Ser. BsFluids Therm. Eng. 2004, 47 (2), 180–185. (23) Man, C. K.; Gibbins, J. R.; Witkamp, J. G.; Zhang, J. Fuel 2005, 84 (17), 2190–2195. (24) Walsh, P. M. Energy Fuels 1997, 11 (5), 965–971. (25) Ikeda, M.; Kozai, Y.; Makino, H. JSME Int. J., Ser. BsFluids Therm. Eng. 2002, 45 (3), 506–511. (26) Huang, L. K.; Li, Z. Q.; Sun, R.; Zhou, J. Fuel Process. Technol. 2006, 87 (4), 363–371.
Laboratory-Scale 20 kW Test Rig. Experiments were carried out in a suspension fired 20 kW laboratory-scale swirl burner test rig, whose diagram is shown in Figure 1. The system consists of a fuels and air dosing part, a combustion unit, a flue gas processing part, and sampling, analytical, and data acquisition system. The combustion air is divided into three streams: primary air for feeding the solid fuel, secondary air to form the swirling flow of the burner, and tertiary air for the two-stage combustion. Each air stream is controlled by a needle valve and a rotameter. Natural gas is from the distribution grid. Ammonia can be added to the natural gas stream to simulate fuel nitrogen. The solid fuels are stored in a container, below which a screw feeder is weighed continuously by a balance. The core part of the test rig is a vertical oriented furnace, with a swirl burner located at its top. The schematic of the furnace is illustrated in Figure 2. The inner diameter of the furnace is 315 mm, with a total height of 1.85 m. The furnace wall is made of refractory materials, which can tolerate a temperature of up to 1600 °C. The fuels are fired downward. A detailed description of the swirl burner is given elsewhere.32 Eight ports are situated on one side of the furnace, where thermocouples can be inserted to monitor the temperature distribution in the furnace chamber. Two windows are installed in the furnace, one at the near-burner region and the other in the bottom of the furnace, which allow us to observe directly the flame structure. To study the effect of two-stage combustion, the tertiary air stream can be injected at different locations: from the top ports, from port 7, and from port 6, where a neck is installed to separate the two combustion zones as shown in Figure 2. A water-cooled particle-sampling probe with a conic tip can be inserted from the furnace bottom. The probe is connected to a hot porous ceramic filter with a heated pipe line to avoid water condensation. The amount of gas withdrawn by the sampling system is controlled in such a way that the isokinetic sampling is obtained. The exit of the furnace is followed by a large, low-pressure drop gas-solid separator. After the separator, the flue gas is cooled down in a heat exchanger and vented to a chimney. The flue gas is sampled through port 8 near the furnace exit. After the gas conditioning system, the concentrations of O2, CO2, CO, NO, and SO2 in the flue gas is analyzed by a series of continuous gas analyzers. The signals of measured temperatures, gas concentrations, and weight of biofuels in the feeding container are continuously monitored and logged to a computer by a data acquisition system. (27) Spliethoff, H.; Hein, K. R. G. Fuel Process. Technol. 1998, 54 (1-3), 189–205. (28) Annamalai, K.; Thien, B.; Sweeten, J. Fuel 2003, 82 (10), 1183– 1193. (29) Damstedt, B.; Pederson, J. M.; Hansen, D.; Knighton, T.; Jones, J.; Christensen, C.; Baxter, L.; Tree, D. Proc. Combust. Inst. 2007, 31 (2), 2813–2820. (30) Wu, C.; Tree, D.; Baxter, L. Proc. Combust. Inst. 2007, 31 (2), 2787–2794. (31) Casaca, C.; Costa, M. Combust. Sci. Technol. 2003, 175 (11), 1953– 1977. (32) Lin, W.; Jensen, P. A.; Jensen, A. D. Manuscript prepared.
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Figure 2. Illustration of the furnace, in which the swirl burner is located on the top, and the position of the tertiary air injection. Table 1. Proximate and Ultimate Analyses of Biofuels Used (on a Delivered Basis) fuel
beech saw dust
straw pellet
Proximate Analysis moisture wt % 9.04 ash wt % 0.61 volatile wt % 76.70 fixed carbon (by difference) wt % 13.65 lower caloric value MJ/kg 16.44
8.65 4.76 69.87 16.72 15.76
Ultimate Analysis wt % 45.05 wt % 5.76 wt % 39.41 wt % 0.13 wt % 0.01
42.88 5.65 37.51 0.49 0.06
carbon hydrogen oxygen (by difference) nitrogen sulfur
Fuels. The fuels used in this study are natural gas and two biofuels, beech saw dust (wood) and pulverized wheat straw pellet (straw). The proximate and ultimate analyses of the two biofuels and the chemical composition of natural gas are listed in Tables 1 and 2, respectively. The major difference of the composition between the two fuels is the content of ash and nitrogen. Straw has a higher ash content than wood. The nitrogen content of straw is nearly 4 times of that of wood. The particle size distributions of wood and straw, determined by sieve, are shown in Figure 3, indicating a large particle size of wood. From the size distribution, the median diameters of the wood and straw powder are calculated to be 0.28 and 0.16 mm, respectively. Experimental Conditions. In this set of experiments, the emphasis is put on how two-stage combustion will affect the NO
Figure 3. Particle size distribution of the beech saw dust and pulverized straw pellets.
emission and the degree of burnout of combustibles (e.g., the CO emission and unburned carbon in fly ash). The experiments performed include firing of natural gas with NH3 addition, co-firing of natural gas and wood, co-firing of natural gas and straw, cofiring of wood and straw, and firing of wood solely. Before an experiment is performed, the furnace is heated by firing natural gas for at least 6 h to reach a stable condition. In varying the fraction of tertiary air, the tangential flow to the swirl burner is adjusted to keep the swirl number approximately equal to 2. When (co)firing biomass, the oxygen level at the furnace exit is kept to about 4%, which
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Figure 4. Effect of first-stage combustion stoichiometry on the NO emission at similar conditions with and without adding NH3 to gas flames ([, λtot ) 1.1; 9 and 2, λtot ) 1.15). Table 2. Natural Gas Composition CH4 C2H6 C3H8 iC4H10 nC4H10 C5H12 nC5 C5+ N2 CO2 lower caloric value density
mol % mol % mol % mol % mol % mol % mol % mol % mol % mol % MJ/Nm3 kg/Nm3
89.06 6.08 2.47 0.39 0.54 0.11 0.08 0.05 0.29 0.91 39.624 0.8243
Figure 5. Comparison of the effect of the first-stage stoichiometry on the conversion of fuel-N to NO in firing natural gas with NH3 dosing (this work, NH3,in ) 325 ppm) and in firing ethylene with the addition of CH3NH3 (with inlet nitrogen concentration of 950 ppm) in a twostage turbulent flow combustor (adopted from Mao and Barat18).
corresponds to an air ratio of approximately 1.25. When firing natural gas, the oxygen level in the exit is around 2%, which is in a similar range of gas-fired power plants. The range of applied operating parameters in the experiments is listed in Table 3.
Results Addition of NH3 to Natural Gas Flame. The results of the influence of two-stage combustion on the NO emission from NH3-dosed natural gas flames are shown in Figure 4. The figure presents two sets of results performed at similar conditions, as well as the NO emission level of natural gas flame without the addition of NH3. The results illustrate the same trend with respect to the effect of the first-stage combustion stoichiometry on the NO emissions when NH3 is added. A minimum NO emission is observed with an optimal value of λ1 around 0.8. The NO level seems not to be affected directly by λ1 for the case without NH3 addition, as indicated by the results in Figure 4. The NO level reaches the maximum at the λ1 close to unity, probably because of the higher temperature in the near burner region at λ1 ≈ 1, observed in some experiments. The results are generally in agreement with many studies of gas-phase hydrocarbon combustion with the addition of nitrogen compounds, which showed minimum NO levels at λ1 from 0.7 to 0.83.11,12,18 However, it seems that the optimal value of λ1 depends upon the type of fuel and nitrogen-containing compound as seen when comparing the present results to those from Mao and Barat.18 Figure 5 shows a comparison of the effect of two-stage firing on fuel-N conversion to NO from natural gas with the addition of NH3 by the present work and by firing ethylene with the addition of CH3NH3.18 It is shown that a minimum of the conversion of nitrogen compounds to NO occurs for both cases, although the reactor configuration and the gas mixing behaviors may be different. In addition, it is observed that the optimal value of λ1 for combustion of C2H4
Figure 6. Effect of two-stage combustion on the NO emission levels when co-firing wood/gas (wood share of 50% on a thermal basis) and straw/gas (straw share of 44% on a thermal basis), with a total combustion stoichiomtry of 1.25.
+ CH3NH3 is lower (around 0.7) than that for combustion of CH4 + NH3, probably because of the different reaction routes of the conversion to NO. Co-firing Biomass and Natural Gas. The effect of twostage combustion was tested by co-firing natural gas and two biofuels: beech saw dust (wood) and pulverized wheat straw pellet (straw). Figure 6 presents the experimental results of the effect of the first-stage combustion stoichiometry on the NO emission when co-combustion of wood/natural gas and straw/natural gas. For co-firing of wood and gas, a minimum of NO emission is observed at a λ1 value of 0.8. It is noticed that the NO emission can be reduced to a level as low as about 55 ppm and, in comparison to one-stage combustion, a reduction of nearly 60% can be obtained. For co-firing straw and gas, it appears that a NO minimum exists at a λ1 value of 0.8-0.9. The NO emission can be reduced by more than 75% compared to one-stage combustion. Because the nitrogen contents of wood and straw are different, it is difficult to compare the results in Figure 6 directly. Thus, a comparison of the conversion of fuel-N to NO for co-firing wood/gas and straw/gas with respect to the effect of two-stage combustion is shown in Figure 7. A similar trend is revealed for the co-combustion of natural gas with the two biofuels. However, the conversion of the fuel-N to NO is higher for cofiring wood than for co-firing straw. It should be emphasized
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Figure 7. Comparison of the conversion of fuel-N to NO for co-firing natural gas and saw dust and co-firing natural gas and straw powder. Figure 9. Effect of two-stage combustion on conversion of fuel-N to NO when co-firing wood/straw (wood share of 60% on a thermal basis) and when firing wood.
Figure 8. Effect of two-stage combustion on the NO emission level when co-firing wood/straw (wood share of 60% on a thermal basis) and when firing wood. Table 3. Experimental Conditions Applied in the Study on Two-Stage Combustion NG + NH3 fuel share (% on a thermal basis) λtot λ1 third air location power (kW)
1.1-1.15 0.65-1.1 P6 23
NG + wood
NG + straw
wood + straw
wood
50/50
56/44
60/40
100
1.25 0.7-1.25 P6 24
1.25 0.75-1.25 P6, P7 24
1.25 0.6-1.25 P6, P7 20
1.23-1.27 0.65-1.25 P6, P7, Ptop 22-24
that the contribution of NO formation from natural gas combustion is not excluded in the calculation of the conversion of fuel-N to NO in Figure 7, which may give a slightly higher conversion. Co-firing Wood and Straw. The influence of two-stage cofiring of wood and straw is presented in Figure 8. For a comparison, the results of 100% wood firing are also shown in Figure 8. It is seen that the behavior of the NO emission when cofiring the two biofuels is somewhat different from co-firing natural gas and wood or straw and also from firing wood alone, although an optimal λ1 for NO emission exists. Unlike the other co-firing, in which the NO level sharply increases when λ1 is higher than 0.9, the NO level increases progressively but no sharp increase is observed when λ1 is higher than 0.9, when co-firing wood and straw. It should be noticed that the NO levels from one-stage combustion (far right point in Figure 8) are the same from cofiring wood/straw and from firing wood, although the nitrogen content in straw is much higher than that in wood. Thus, the conversion of the fuel-N to NO is lower when co-firing wood/ straw than firing wood, as shown in Figure 9. The results indicate that a synergistic effect for NO reduction may occur
Figure 10. Effect of the location of the tertiary air injection on NO emission when firing wood (λtot ) 1.23-1.27).
when co-firing wood and straw, which will be discussed in a later section. Effect of the Location Where the Tertiary Air Stream Is Injected. The effect of the location of the tertiary air injection on the NO emission is examined by the introduction of the tertiary air stream from four locations: port 6, the default location of the tertiary air injection and 910 mm below the burner mouth; port 7, located 225 mm downstream of port 6; and 50 and 450 mm below the burner mouth by two injection probes from the top of the furnace, parallel to the direction of gas flow. It should be emphasized that the tertiary air stream is in a cross-flow to the main gas stream in the furnace for ports 6 and 7, which will result in a better mixing of the two gas streams than the parallel flow from the top probes. The experimental results of firing wood are presented in Figure 10. It is shown that the NO level decreases for λ1 < 1 when shifting the tertiary air from port 6 to port 7. Both the mixing behaviors between the tertiary air stream and main gas stream and the residence time in the fuel-rich zone may contribute to the difference.22 On the basis of the geometric parameters, gas flow, and the total stoichiometry, the values of average gas residence time in the fuel-rich zone at different values of λ1 are estimated and listed in Table 4. It should be mentioned that the tertiary air stream cannot mix with the main flow immediately when injecting the tertiary air from the furnace top. The actual residence time for these two cases is too complicated to be estimated accurately in such a
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Figure 11. Effect of two-stage combustion on CO emission when cofiring different fuels and on the unburned carbon in fly ash when firing wood.
Figure 12. Effect of the tertiary air injection location on CO emission when firing wood.
Table 4. Average Gas Residence Time in the Fuel-Rich Zone in Two-Stage Combustion average residence time in the fuel-rich zone (s) port\λ1 Ptop5 Ptop45 P6 P7
0.6 0.24 2.14 4.32 6.27
0.7 0.20 1.83 3.70 5.37
0.8 0.18 1.60 3.24 4.70
0.9 0.16 1.42 2.88 4.18
1 0.14 1.28 2.59 3.76
1.1 0.13 1.17 2.36 3.42
1.2 0.12 1.07 2.16 3.13
simple way. Because of the difference in flow mixing patterns, the injection of the tertiary air from ports 6 and 7 may be considered as two-stage combustion with overfiring air in a boiler, while the injection of the third air from the ports in the furnace top may be regarded to simulate the overburner air in a combustion chamber. CO Emission Level and Unburned Carbon in the Fly Ash. The effect of two-stage combustion on the emission of CO for co-firing wood and natural gas and firing wood is shown in Figure 11. The CO emission level in co-combustion of wood/ gas is, in general, much lower (2000 ppm), when the tertiary air is injected from port 7 in the case of λ1 being lower than unity. In contrast, The CO level is lower than 50 ppm, when injecting the third air stream from the top ports. An important parameter that governs the CO emission in the suspension combustion is the excess air level. When the tertiary air injection is shifted downstream in two-stage combustion, the residence time in the secondary stage will be shortened.
Figure 13. Temperature profiles in two-stage combustion of wood at different first stage stoichimetry (tertiary air injection from port 6, λtot ) 1.26).
Obviously, there is not enough time for the large amount of CO entering the secondary stage to be burned completely. Temperature Profiles in the Furnace. The effect of twostage combustion on the temperature profiles in the furnace when firing wood is shown in Figure 13. It is indicated that the highest temperature occurs at port 3. In the region near the burner (port 1), the temperature is lower than at port 3. The temperatures are decreased downward. Such temperature profiles are observed for co-firing wood/gas and straw/gas. When firing natural gas, the highest temperature occurs at port 1. The difference is due to the fact that natural gas ignites as soon as it enters the furnace, while biomass particles experience first devolatilization and the volatile ignition near port 2, downstream of port 1. It is noticed from visual observation that there is no further delay of volatile ignition when co-firing wood compared to co-firing straw, although the particle size of wood is larger that that of straw, suggesting a same order of magnitude of particle heating rate and devolatilization rate for wood and straw. The figure also shows a higher temperature at port 7 at low λ1 than at high λ1. This indicates that more combustibles burn at the secondary (33) Chen, S. L.; Pershing, D. W.; Heap, W. P. Bench-scale Evaluation of Non-US Coals NOx Formation under Excess Air and Staged Combustion Conditions, Energy and Environmental Research Corporation, Irvine, CA, 1981; p 80.
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Figure 14. Comparison of the effect of two-stage combustion firing natural gas with NH3 addition, co-firing wood/natural gas, and firing wood in the swirl burner.
Figure 15. Comparison of the effect of two-stage suspension combustion of different fuels on NO emissions (9, this work; [, 2, and O, Chen et al.;15 ×, Spliethoff et al.27).
stage (downstream of port 6) at lower λ1, i.e., more reducing condition at the first stage. Discussion Comparison of NH3-Dosed Natural Gas Flame and Co-firing of Wood/Gas. In many studies, nitrogen-containing compounds are added to gaseous fuel combustion to simulate the conversion of the fuel-N to NO.11,12,18,34 By this method, it is easier to model the conversion process and understand the mechanisms. However, the results obtained may be different from those obtained when firing solid fuels at similar conditions. Figure 14 shows a comparison of the influence of two-stage combustion of natural gas with NH3 dosing, co-firing wood/ natural gas, and wood firing on the NO emission profile. In the figure, the equivalent inlet concentrations of nitrogen-containing compounds (defined as the NO concentration in the dry flue gas corrected to 6% of O2 when all inlet N compounds are converted to NO) are 325, 175, and 340 ppm for NH3-dosed natural gas flame, co-firing wood/natural gas, and firing wood, respectively. It is observed from Figure 14 that the NO emission profiles with respect to the fuel-rich stage stoichiometry (λ1) are similar for co-firing wood/gas and firing wood. In contrast, the gradient (34) Sullivan, N.; Jensen, A.; Glarborg, P.; Day, M. S.; Grcar, J. F.; Bell, J. B. Combust. Flame 2002, 131 (3), 285–298.
Lin et al.
of the NO emission with respect to λ1 is higher for the NH3dosed natural gas flame than for firing or co-firing wood. The difference in the NO emission profiles for the two cases may result from exclusion of the heterogeneous reaction when firing natural gas. The heterogeneous reactions may have two effects on wood co-firing. In the devolatilization stage, part of the volatile-N in the biofuel may be released as N2 instead of NH3 or HCN because of the catalytic effect of alkali and alkaline earth compounds,35 which are present in biofuels. In addition, the char combustion is slower than volatile combustion, and the biomass char has a high reactivity toward reduction of NO.36,37 The reduction of NO by char occurs at reducing conditions as well as at oxidizing conditions. The heterogeneous reduction of NO may be the main reason for the lower NO level at high λ1 in the co-firing experiments. It is noticed that the optimal values of λ1 are almost the same, and the minimum values of NO emission are close for all three cases. This may indicate that the homogeneous reactions play a major role in the minimal NO level observed at two-stage combustion. At high λ1, NO will be formed by direct oxidation of fuel-N, which is mostly released with volatile at high temperatures. At very low λ1, a large fraction of N-containing compounds, such as HCN and NH3, released from devolatilazation are not oxidized at the first stage and enter the second stage to oxidize to NO when mixed with the tertiary air. Both experimental work38 and modeling work39 confirmed that HCN and NH3 levels increase at the exit of the first stage. Synergistic Effect for NO Reduction of Co-firing. In coal combustion, it has been reported that a synergistic effect on blending is observed,40 which is indicated by a difference between the actual performance of the blend and that predicted by the addition of the performance of individual fuels and the blend ratio. In Figure 8, a lower level of NO is shown than expected when co-firing wood and straw, when considering the additive property from co-firing wood/gas and straw/gas in Figure 6. The synergistic effect on NO emission is also reported when blending two coals with very different characteristics in suspension combustion.41,42 With the co-firing of coal and straw in a pulverized coal boiler, the conversion of fuel-N to NO decreased with an increasing straw share, which was explained by the high-volatile content in straw.43 It is well-known that two-stage combustion is more effective in reducing NO for the high-volatile coal than the low-volatile coal.20 In the present work, it is not probable that the synergistic effect on NO emission is caused by the volatile content because both wood and straw have a high-volatile content with the same order of magnitude. One of the possible reasons may be the difference in the reactivity of char for reduction of NO. A (35) Ohtsuka, Y.; Wu, Z. H.; Furimsky, E. Fuel 1997, 76 (14-15), 1361– 1367. (36) Sorensen, C. O.; Johnsson, J. E.; Jensen, A. Energy Fuels 2001, 15 (6), 1359–1368. (37) Garijo, E. G.; Jensen, A. D.; Glarborg, P. Energy Fuels 2003, 17 (6), 1429–1436. (38) Chen, S. L.; Heap, M. P.; Pershing, D. W. Bench-scale NO emissions testing of world coals: Influence of particle size and temperature, The 1982 Joint Symposium on Stationary Combustion NO Control, Palo Alto, CA, 1982; pp 35.1-35.19. (39) Jensen, A.; Johnsson, J. E. Chem. Eng. Sci. 1997, 52 (11), 1715– 1731. (40) Majid, A. A.; Paterson, N.; Reed, G. P.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2005, 19 (3), 968–976. (41) Haas, J.; Tamura, M.; Weber, R. Fuel 2001, 80 (9), 1317–1323. (42) Rubiera, F.; Arenillas, A.; Arias, B.; Pis, J. J. Fuel Process. Technol. 2002, 77, 111–117. (43) Pedersen, L. S.; Nielsen, H. P.; Kiil, S.; Hansen, L. A.; DamJohanesn, K.; Kildsig, F.; Christensen, J.; Jespersen, P. Fuel 1996, 75 (13), 1584–1590.
Biomass Suspension Combustion
Energy & Fuels, Vol. 23, 2009 1405
Table 5. Effect of Inlet Fuel-N on the NO and CO Emissions at Optimal λ1 fuel
inlet fuel-N (mol N kJ-1)
λ1, opt
conversion of fuel-N to NO at optimal λ1 (%)
NO emission at optimal λ1 (ppm at 6% O2)
CO emission at optimal λ1 (ppm at 6% O2)
co-firing wood/gas (50/50%) gas with 325 ppm NH3 addition firing wood co-firing straw/gas (44/56%) co-firing straw /wood (40/60%)
2.72 4.82 5.43 9.85 12.21
0.82 0.83 0.85 0.8 0.77
31 26 24 15 17
55 86 81 95 128
33 0 125 7 94
previous investigation showed that the reactivity of the char from straw is higher that that from wood.37 In addition, the difference in the mineral content in wood and straw may influence the char yield at suspension combustion conditions. The difference in particle size distribution of the two biofuels may also affect the aerodynamics in the near burner region, which may influence the NO formation and destruction routes. In addition, the volatile-N distribution caused by the catalytic effect of ash constituent summarized by Glarborg et al.44 may also contribute to the synergistic effect. More work is needed for understanding the synergistic effect. Effect of Fuel Type on NO Emission When Using Two-Stage Combustion. To understand the effect of two-stage combustion on the NO emission using different fuels, a comparison of this work and the work in the literature15,27 is shown in Figure 15. It is shown that a minimum of NO emission occurs for all fuels, except for the anthracite, in which the NO emission increases with an increasing value of λ1. This agrees with the previous hypothesis that the NO minimum of two-stage combustion is mainly caused by the homogeneous reactions. The figure also demonstrates that the optimal value of λ1 varies in a range of 0.7-0.85. The main results in the present work, regarding the influence of nitrogen content per unit power input of fuels on the NO and CO emissions, as well as on the conversion of fuel-N to NO at the optimal first stage combustion stoichiometry, are summarized in Table 5. It can be seen that a low NO emission is obtained by applying two-stage combustion at an optimal firststage stoichiometry. At such conditions, the CO level is at a low level. In addition, it seems that, in general, the conversion of fuel-N to NO decreases with an increase in the input fuel-N, as reviewed by Glarborg et al.44 Conclusions From the experimental study of two-stage suspension combustion of biomass and co-firing gas/biomass, the following conclusions can be drawn: (44) Glarborg, P.; Jensen, A. D.; Johnsson, J. E. Prog. Energy Combust. Sci. 2003, 29 (2), 89–113.
Two-stage combustion can significantly reduce the NO emission. An optimal stoichiometry of around 0.8 in the fuelrich zone exists with respect to minimizing NO emissions. When using wood and straw as co-firing fuels, 15-25% of the fuel-N is converted to NO. Straw appears to give the lowest conversion of fuel-N to NO. In addition, the CO emission can be kept at a low level at the optimal λ1. The results of the influence of two-stage combustion on NO emission when firing natural gas with NH3 addition and cofiring natural gas and biomass indicate that the optimal stoichiometry in the fuel-rich (λ1) zone for gaining the lowest NO may result from the homogeneous reaction (volatile combustion). The difference in NO profiles with respect to λ1 of NH3-dosed natural gas flame and biomass firing is probably caused by the heterogeneous reactions involving char in NO formation and destruction. A synergistic effect occurs when co-firing two biofuels, wood and straw, with respect to NO emission, with a lower conversion of fuel-N to NO than those of individual biofuels. The synergistic effect may be caused by the reactivity of the biofuel char toward NO reduction, the catalytic effect of ash toward the volatile-N distribution, and the difference in near burner aerodynamics because of the particle size distribution of the two biofuels. The experimental results show that NO emission levels can be significantly lowered when co-firing natural gas and biomass without increasing incomplete combustion of gas and solids by applying optimized two-stage combustion. Acknowledgment. The financial support of the present work by the companies Energinet.dk, Dong Energy A/S, and Vattenfall A.D. (contract number 6526) is gratefully acknowledged. This work is also a part of the Combustion and Harmful Emission Control (CHEC) Research Center funded by the Technical University of Denmark, the Danish Technical Research Council, the European Union, the Nordic Energy Research, Dong Energy A/S, Vattenfall A.B., F L Smidth A/S, and the Public Service Obligation. EF8004866
