Energy Fuels 2010, 24, 910–915 Published on Web 12/09/2009
: DOI:10.1021/ef901076g
Emissions of SO2 and NOx during Oxy-Fuel CFB Combustion Tests in a Mini-Circulating Fluidized Bed Combustion Reactor L. Jia,* Y. Tan, and E. J. Anthony CanmetEnergy, Natural Resources Canada, 1 Haanel Drive, Ottawa, Ontario Canada, K1A 1M1 Received September 22, 2009. Revised Manuscript Received November 10, 2009
Anthropogenic CO2 production is primarily driven by fossil fuel combustion, and the current energy demand situation gives no indication that this will change in the near future. In consequence, it is increasingly necessary to find ways to reduce these emissions when fossil fuel is used. CO2 capture and storage (CCS) appears to be among the most promising approaches. All of the CCS technologies involve producing a nearly pure stream of CO2, either by concentrating it in some manner from the flue gases or by using pure oxygen as the combustion gas. The latter option, oxy-fuel combustion, has now been well studied for pulverized coal combustion, but to date has received relatively little attention in the case of oxy-fuel circulating fluidized bed combustion (CFBC). Recently, oxy-fuel FBC has been examined in a 100 kW pilot plant operating with flue gas recycle at CanmetEnergy. The results strongly support the view that this technology offers all of the advantages of air-fired FBC, with one possible exception. Emissions such as CO or NOx are lower or comparable to those of air firing. It is possible to switch from air firing to oxy firing easily, with oxygen concentrations as high as 60-70%, and flue gas recycle levels of 50-60%. Only sulfation is poorer, which is not in good agreement with other studies, and the reasons for this discrepancy need further exploration. However, longer tests have confirmed these findings with two coals and a petroleum coke. It also appears that changing from direct to indirect sulfation with the petroleum coke improves the sulfation, although a similar effect could not be confirmed with coal from these results.
units operated with oxy-fuel combustion using full flue gas recycle. The advantages of FBC are already well-known in terms of its ability to burn a wide range of fuels, both singly and co-fired, to achieve relatively low NOx emissions, and to remove SO2 with limestone.5 Another advantage of CFBC technology in the context of oxy-fuel firing is that external solid heat exchanger(s) can be used to extract heat from the combustion process. This allows a significant reduction of the amount of recycled flue gas required for combustion temperature control. Alternatively, this feature will permit the use of a much higher oxygen concentration in the combustor, thus improving the economics of oxy-fired CFBC over that for oxy-fuel pulverized combustion (PC). Current estimates suggest that it is possible to achieve a reduction in area of the CFBC boiler island by as much as 50%3,6,7 for oxy-fuel technology. Advantages of the technology that are more difficult to define and quantify relate to the possibility of cofiring biomass, so that with CCS, the overall combustion process may
Introduction To reduce greenhouse gas emissions from fossil fuel combustion in the utilities industry, CO2 capture and storage (CCS) appears to be among the most promising methods.1 Oxy-fuel combustion uses nearly pure oxygen instead of air for the combustion of fossil fuels, which will generate a near pure stream of CO2 ready for capture and storage. Oxy-fuel combustion has now been well studied for pulverized coal combustion, but to date has received relatively little attention for oxy-fuel circulating fluidized bed combustion (CFBC). The concept was examined over 20 years ago for bubbling FBC, but was rejected on the basis of cost.2 More recently, the boiler companies Alstom and Foster Wheeler have explored the concept using pilot-scale equipment. Alstom’s work included testing in a unit of up to 3 MW thermal in size, but did not involve recycle of flue gas.3 Foster Wheeler’s work4 also involved pilot-scale testing, using a small (30-100 kW) CFBC owned and operated by VTT (Technical Research Centre of Finland), and this work along with CanmetEnergy’s work with its own 100 kW CFBC appear to be the first such
(5) Circulating Fluidized Beds; Grace, J. R., Avidan, A. A., Knowlton, T. M. Eds.; Blackie Academic and Professional: London, UK, 1997. (6) Stamatelopoulos, G. N., Darling, S., Alstom’s CFBC Technology, Proceedings of the 9th International Conference on Circulating Fluidized Beds, in conjunction with the 45th International VGB Workshop, Hamburg, Germany, May 13-16, 2008; Operating Experience with Fluidized Bed Systems; Werther, J., Nowak, W., Wirth, K.-E., Hartge, E.-U. Eds.; pp 3-9. (7) Hotta, A., Nuorimo, K., Eriksson, T., Palonen, J., Kokki, S., CFB Technology Provides Solutions to Combat Climate Change, Proceedings of the 9th International Conference on Circulating Fluidized Beds, in conjunction with the 45th International VGB Workshop, Hamburg, Germany, May 13-16, 2008; Operating Experience with Fluidized Bed Systems, Werther, J., Nowak, W., Wirth, K.-E., Hartge, E.-U. Eds.; pp 11-17.
*Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Buhre, B. J. P.; Elliot, L. K.; Sheng, C. D.; Gupta, R. P.; Wall, T. F. Prog. Energy Combust. Sci. 2001, 31, 283–307. (2) Yaverbaum, L. Fluidized Bed Combustion of Coal and Waste Materials; Noyes Data Corp.: Park Ridge, NJ, 1977. (3) Liljedahl, G. N.; Turek, D. G.; Nsakala, N. Y.; Mohn, N. C.; Fout, T. E. Alstom’s Oxygen-Fired CFB Technology Development Status for CO2 Mitigation, 31st International Technical Conference on Coal Utilization and Fuel Systems, Clearwater, Florida, USA, May 21-25, 2006. (4) Eriksson, T.; Sippu, O.; Hotta, A.; Myohanen, K.; Hyppanen, T.; Pikkarainend, T. Oxy-fuel CFB Boiler as a Route to Near Zero CO2 Emission Coal Firing; Power Generation Europe, Madrid, Spain, June 26-28, 2007. r 2009 American Chemical Society
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Energy Fuels 2010, 24, 910–915
: DOI:10.1021/ef901076g
Jia et al. Table 1. Analysis of Fuels Eastern bituminous (EB) Kentucky petroleum coke
moisture, wt % (as analyzed) ash volatile matter fixed carbon carbon hydrogen nitrogen sulfur ash oxygen (by difference) heating value (MJ/kg)
Proximate Analysis, wt % (dry) 1.08 2.01 8.86 35.78 55.56
0.66
11.31 37.35 51.34
1.00 11.46 86.97
Ultimate Analysis, wt % (dry) 77.81 74.05 5.05 5.06 1.49 1.62 0.95 1.56 8.86 11.31 6.04 6.40
86.91 3.22 1.83 5.88 1.00 1.16
32.51
30.93
34.71
Table 2. Analysis of Limestones, wt % Havelock CaO MgO SiO2 Al2O3 Fe2O3 Na2O K2O MnO TiO2 Cr2O3 P2O5 SO3 V2O5 SrO BaO NiO LOF sum
Figure 1. Schematic of CanmetEnergy’s mini-circulating fluidized bed facility.
potentially result in a net reduction of anthropogenic CO2. In addition, FBC offers the potential for use with more marginal fuels, as premium fossil fuels come into short supply. The co-firing option offers a potentially interesting advantage of CFBC technology over conventional technology, since it is well established that FBCs can burn biomass and fossil fuels at any given ratio, ranging from 0 to 100%, thus offering the possibility of using local and seasonally available biomass fuels in a “CO2-negative” manner. The ultimate availability of premium coal for a period of hundreds of years has also recently been called into doubt, with suggestions that coal production may peak well before the end of this century.8 If such solid fuel shortages occur, fluidized bed combustion is ideally suited to exploit the many marginal coals available worldwide. CanmetEnergy began studying oxy-fuel CFB combustion in 2006, with initial results published in early 2007.9 Since then, more tests have been conducted with the focus on the effects of operating conditions on oxy-fuel CFB combustion. This paper will discuss the results generated in the CanmetEnergy’s mini-CFBC unit, particularly for SO2 and NOx emissions under different oxy-fuel CFB combustion conditions.
53.99 0.59 1.23
