Energy Fuels 2010, 24, 4144–4152 Published on Web 07/13/2010
: DOI:10.1021/ef100299n
Nitric Oxide Emissions during Coal and Coal/Biomass Combustion under Air-Fired and Oxy-fuel Conditions Scott A. Skeen,† Benjamin M. Kumfer, and Richard L. Axelbaum* Consortium for Clean Coal Utilization, Washington University in Saint Louis, Saint Louis, Missouri 63130. †Present address: Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94550. Received March 14, 2010. Revised Manuscript Received June 3, 2010
Nitric oxide (NO) emissions are measured during the combustion of sub-bituminous Powder River Basin coal and coal/sawdust mixtures under air-fired and oxy-fuel conditions in a once-through laboratory-scale (30 kWth) unpiloted combustor. In this system, flame attachment to the burner appeared to be the most important parameter for obtaining the lowest possible NO emissions. Oxy-coal conditions in which air was replaced with a 30% O2/70% CO2 mixture in both the primary and secondary oxidizer streams resulted in a 20% reduction in NO. When the O2 concentration in the primary and secondary streams was varied, an optimum set of conditions resulting in minimal NO formation was found. Varying the adiabatic flame temperature by altering the secondary oxidizer gas composition had little effect on NO emissions, provided that the temperature was sufficient to maintain a burner-attached flame. Sawdust co-firing in one scenario resulted in flame instability and increased NO formation. When stable flame conditions were achieved, the effect of sawdust co-firing on NO emissions was dependent upon the sawdust particle size, with larger particles having little effect on NO emissions and smaller particles reducing NO emissions under air-fired conditions but not under oxy-fuel conditions.
of oxy-coal combustion have been performed, and a review of the work prior to 2005 is available by Buhre et al.3 Increased energy costs are anticipated in an oxy-coal power plant equipped for carbon capture and storage (CCS) in large part because of the parasitic load of the air separation unit and the CO2 compression and purification unit. Nevertheless, oxy-fuel combustion results in a reduced flue gas volume, enabling higher boiler thermal efficiencies, and has demonstrated the potential to reduce NOx emissions by as much as 60-75% and SOx emissions by as much as 27%, suggesting that postcombustion treatment equipment can be scaled back, improving the benefit/cost ratio.4,5 A common low-NOx strategy for pulverized coal boilers is oxidizer staging. For wall-fired units, a burner is typically configured with two separate gas streams that supply insufficient air for complete combustion: a primary oxidizer (PO), in which the coal particles are entrained, and a secondary oxidizer (SO) that surrounds the PO and usually exits through swirl vanes to aid in flame stabilization and mixing. A third flow (overfire air or oxidizer) can be introduced some distance downstream to consume the remaining fuel (usually char) under fuel-lean conditions. This strategy results in a reducing region between the burner and the overfire oxidizer, where NOx can be converted to molecular nitrogen. The effectiveness
Introduction One of the most pressing environmental challenges facing our world is the threat of climate change because of the presence of excess greenhouse gases, such as carbon dioxide (CO2), in the atmosphere. As a major contributor to global CO2 emissions, coal-fired power plants are receiving enormous attention. One of the most promising methods to reduce CO2 emissions from coal power plants involves capturing the CO2 present in the exhaust gases for long-term geologic storage, enhanced oil recovery (EOR), or enhanced coal bed methane (ECBM) production. Under conventional air-fired conditions, the concentration of CO2 in the exhaust gases is relatively low (10-20%) because of the presence of N2 from air. At such low concentrations, CO2 capture requires postcombustion separation processes that consume a significant amount of energy with no potential benefit to the combustion process. However, oxy-coal combustion can produce exhaust gas CO2 concentrations of up to 95%1, enabling capture by more direct methods while potentially improving combustion efficiency and reducing pollutant emissions. Oxy-coal combustion involves replacing the oxidizer air with a combination of oxygen (g95 vol % O2) and recycled flue gas (RFG). Since proposed as a means to produce CO2 for EOR,2 many laboratory-scale, pilot-scale, and computational studies investigating the design and operational issues
(3) Buhre, B. J. P.; Elliott, L. K.; Sheng, C. D.; Gupta, R. P.; Wall, T. F. Oxy-fuel combustion technology for coal-fired power generation. Prog. Energy Combust. Sci. 2005, 31 (4), 283–307. (4) Lu, Y.; Chen, S.; Rostam-Abadi, M.; Varagani, R. K.; ChatelPelage, F.; Pranda, P.; Bose, A. C. Techno-economic study of the oxycombustion process for CO2 capture from coal-fired power plants. Proceedings of the 22nd International Pittsburgh Coal Conference, Pittsburgh, PA, Sept 12-15, 2005; pp 277/1-277/15. (5) Chatel-Pelage, F.; Varagani, R.; Pranda, P.; Perrin, N.; Farzan, H.; Vecci, S. J.; Lu, Y.; Chen, S. Applications of oxygen for NOx control and CO2 capture in coal fired power plants. Therm. Sci 2006, 10 (3), 119– 142.
*To whom correspondence should be addressed: Department of Energy, Environmental & Chemical Engineering, One Brookings Drive, St. Louis, MO 63130. Telephone: 314-935-7560. Fax: 314-935-7211. E-mail: axelbaum@ wustl.edu. (1) Chen, J.-C.; Huang, J.-S. Combustion efficiency and CO2 emission from O2/N2, O2/CO2, and O2/RFG coal combustion processes. Environ. Eng. Sci. 2007, 24 (3), 353–362. (2) Abraham, B. M.; Asbury, J. G.; Lynch, E. P.; Teotia, A. P. S. Coal-oxygen process provides carbon dioxide for enhanced recovery. Oil Gas J. 1982, 80 (11), 68–70, 75. r 2010 American Chemical Society
4144
pubs.acs.org/EF
Energy Fuels 2010, 24, 4144–4152
: DOI:10.1021/ef100299n
Skeen et al.
of staged combustion can be influenced by the level of oxidizer deficiency in the burner (i.e., the depth of staging), the flame stand-off distance, and the near burner temperature, which can influence the stand-off distance and the rate of coal devolatilization. Deeper staging in general is desirable for lower NOx emissions; however, if there is insufficient air in the near burner region, temperatures can become too low, leading to greater flame stand-off distances. This can lead to SO air entrainment and increased NOx formation,6 flame instabilities, late coal devolatilization, and reduced boiler efficiencies and char burnout.5,7 Considering these factors, it has been recognized that oxyfuel combustion provides opportunities for further reductions in NOx emissions because (1) molecular nitrogen has largely been removed from the system, (2) some NOx will be recycled through the flame region, where it will have additional opportunities to be reduced to N2,8-11 and (3) independent control of oxygen and RFG concentrations enables a decoupling of temperature and stoichiometry in the near burner and postflame regions.6,12-16 Early oxy-coal studies8-11 identified three pathways for NOx reduction enabled by flue gas recycle in oxy-fuel combustion, namely: (1) high CO2 concentrations may lead to more CO (because of either gasification of char, char þ CO2 f 2CO, or thermal dissociation) available to reduce NOx to N2 via the reaction NO þ CO = 0.5N2 þ CO2, which can be catalyzed at the char surface; (2) recycled NOx may interact with nitrogen species released during devolatilization (volatiles N); and (3) recycled NOx may be reduced to N2 by hydrocarbon species released during devolatilization when passing through the flame zone. Okazaki and Ando8 investigated the relative influence of these mechanisms by simulating
the effects of volatiles with CH4 and NH3 and char with anthracite in separate experiments. It was concluded that increased CO2 concentrations contributed to less than 10% of the observed NOx reduction, while reactions between recycled NOx and volatiles N accounted for 50-80%, and reactions between recycled NOx and hydrocarbon species in the flame accounted for 10-50% of the observed reduction. The influence of the oxygen concentration (g20 vol %), as enabled by independent control of oxygen and RFG, was demonstrated by Hu et al.17,18 at variable temperatures and equivalence ratios in an electrically heated furnace. Emissions of NOx on a mass basis were shown to be only slightly dependent upon oxygen concentrations g50 vol % and were comparable to results obtained in air when pure oxygen was used. Liu and Okazaki19 proposed a novel oxy-fuel scenario, in which the oxygen concentration was reduced relative to air (
