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Energy & Fuels 2007, 21, 3160–3164
Experimental Study of Oxy-Fuel Combustion and Sulfur Capture in a Mini-CFBC L. Jia,*,† Y. Tan,† C. Wang,‡ and E. J. Anthony† Natural Resources Canada, CETC-Ottawa, 1 Haanel Dr., Ottawa, ON, Canada K1A 1M1, and School of Energy & Power Engineering, North China Electric Power UniVersity, Baoding City, Hebei ProVince, The People’s Republic of China ReceiVed May 10, 2007. ReVised Manuscript ReceiVed July 3, 2007
Oxy-fuel technology uses effectively pure oxygen for fossil fuel combustion in order to obtain a highly concentrated CO2 stream, suitable for direct compression and sequestration. It is an effective technology to reduce greenhouse gas emissions to the atmosphere from large point sources such as power generation plants. Oxy-fuel FBC technology has the combined advantage of producing high CO2 concentration flue gas and allowing excellent fuel flexibility. In addition, with external cooling of the recirculated solids, the flue gas recirculation ratio can be reduced. CETC-Ottawa has carried out oxy-fuel fluidized bed combustion with flue gas recirculation on its modified mini-CFBC. The mini-CFBC has an internal diameter of 100 mm and internal height of 5000 mm. Both bituminous and sub-bituminous coals were fired. Limestone was premixed with coal and fed to the mini-CFBC. Recirculated solids were cooled in the return leg of the mini-CFBC. The bed temperature was controlled at about 850 °C, while the oxygen concentration in the primary gas was ∼25% and in the secondary gas was ∼50%. With flue gas recycle, the CO2 concentration in the flue gas reached 82–90%. Sulfur capture efficiency and CO and NOx concentrations were also measured and were all at acceptable levels. The transition from air firing to oxy-fuel firing was a fast and relatively smooth process, and operation of the mini-CFBC under oxy-fuel firing conditions was similar to that of air firing.
Introduction Power production from fossil fuel combustion results in the emission of greenhouse gases, of which the dominant contributor is CO2. Oxy-fuel combustion is one of the technologies that can produce effectively pure carbon dioxide (CO2) from fossil fuel-fired power plants, by using pure oxygen instead of air for the combustion process. As a result, the flue gas stream consists mainly of CO2 and water vapor. In order to control the combustion temperature, part of the flue gas is recycled to the combustion chamber. Until recently, most of the research activities in oxy-fuel combustion have concentrated on pulverized coal combustion. These studies have shown that oxy-fuel can be adopted for retrofitting existing power plants as well as for new units.1–4 Buhre summarized the literature on oxy-fuel combustion of pulverized coal and discussed a number of operational concerns, including heat transfer, environmental issues, ignition, and flame stability.5 * To whom correspondence should be addressed. E-mail:
[email protected]. † CETC-Ottawa. ‡ North China Electric Power University. (1) Douglas, M.; Chui, E.; Tan, Y.; Lee, G. K.; Croiset, E.; Thambimuthu, K.V. Oxy-fuel combustion at the CANMET Vertical Combustor Research Facility, Proceedings of the 1st National Conference on Carbon Sequestration, Washington, DC, May 14–17, 2001. (2) Wang, C. S.; Berry, G. F.; Chang, K. C.; Wolsky, A. M. Combustion of pulverized coal using waste carbon dioxide and oxygen. Combust. Flame 1988, 72, 301–310. (3) Zheng, L.; Tan, Y.; Wall, T. Some thoughts and observations of oxy-fuel technology developments, Proceedings of the 22nd International Pittsburgh Coal Conference, Pittsburgh, PA, September 12–15, 2005. (4) Zheng, L.; Clements, B.; Douglas, M. Simulation of an Oxy-Fuel Retrofit to a Typical 400 MWe Utility Boiler for CO2 Capture, Proceedings of the 26th International Technical Conference on Coal Utilization and Fuel Systems, Clearwater, FL, March 5–8, 2001.
Fluidized bed combustion (FBC) technology has been used in the combustion of various fuels such as coal, coke, municipal solid waste (MSW), wood, and other biomass. FBC technology has excellent fuel flexibility, inherently low NOx production, and the potential to achieve SO2 emissions reductions of 90% or more by in situ addition of limestone. These inherent advantages suggest that oxy-fuel FBC combustion technology has great potential for both the utility and industrial markets. In circulating fluidized bed combustion (CFBC), fuel is combusted in a hot bed of solid particles (sorbent and ash) fluidized by combustion air that is introduced from below through a series of nozzles. CFBC operates at gas velocities high enough to entrain a large portion of the solids (4–8 m/s), which are then separated from the flue gas and recycled back to the fluidized bed combustor. The solid circulation provides an effective means, in addition to flue gas recycle, of controlling the combustion. Oxy-fuel CFB combustion can also use external solid heat exchangers to extract heat from the circulating solids to maintain combustor temperature. In consequence, oxy-fuel CFBC can significantly reduce the amount of recycled flue gas or alternatively can permit much higher oxygen concentration in the combustor. This allows the economics of oxy-fuel firing in CFBC boilers to be improved over PC or stoker firing because recirculated gas flow can be reduced significantly.6,7 In a preliminary engineering design study, Jukkola indicated that, (5) Buhre, B. J. P.; Elliot, 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, 283–307. (6) Marion, J.; Bozzuto, C.; Andrus, H.; Chamberland, R. Greenhouse gas emissions control by oxygen firing in circulating fluidized bed boilers: Phase 1 - A preliminary systems evaluation, PPL Report No. PPL-03-CT009, 2003.
10.1021/ef7002359 CCC: $37.00 2007 American Chemical Society Published on Web 09/18/2007
Oxy-Fuel Combustion
Energy & Fuels, Vol. 21, No. 6, 2007 3161 Table 1. Analysis of Coals proximate analysis, wt % (dry) moisture, wt % (as analyzed) ash volatile matter fixed carbon ultimate analysis, wt % (dry) carbon hydrogen nitrogen sulfur ash oxygen (by difference) heating value (MJ/kg)
Eastern bituminous (EB)
Highvale
1.08 8.86 35.78 55.56
10.39 19.17 33.76 47.07
77.81 5.05 1.49 0.95 8.86 6.04 32.51
59.78 3.49 0.76 0.22 19.17 16.58 23.27
Experimental Section
Figure 1. CETC-Ottawa’s mini-CFBC.
for an oxy-fuel-fired 210 MWe CFBC,8 the boiler island occupied about 51% of the area of the air-fired CFB with the same capacity. Similarly, the oxy-fuel CFBC occupied about 56% of the building volume. Since there appear to be no special barriers to building such units, oxy-fuel CFBC is clearly a nearterm option for CO2 mitigation technology. Alstom has successfully demonstrated oxy-fuel CFBC technology in its Connecticut research center with local oxygen concentration up to 70% and global oxygen concentration up to 50%.6,7 However, Alstom used bottled CO2 instead of the recycled flue gas in these pilot-scale tests. Tan et al. reported earlier oxy-fuel CFBC combustion results in CETC-O’s miniCFBC with flue gas recycle.9 However, the highest CO2 concentration in the flue gas was limited to about 57% due to leakage in the system. In the present work, oxy-fuel CFB combustion was conducted in a mini-CFBC with flue gas recirculation. Two coals, a bituminous and a sub-bituminous one, were used. Limestone was added at a nominal Ca/S ratio of 2 to evaluate sulfur capture under oxy-fuel CFB combustion conditions. Concentrations of CO2, SO2, CO, O2, and NOx were monitored throughout the tests. Normal mini-CFB combustion tests (air firing) were also performed with the same coals to provide a basis for comparison. (7) Marion, J.; Bozzuto, C.; Nsakala, N.; Liljedahl, G. Evaluation of advanced coal combustion and gasification power plants with greenhouse gas emission control, Topical Phase-I, DOE-NETL Report, under Cooperative Agreement No. DE-FC26-01NT41146, Prepared by ALSTOM Power Inc., May 15, 2003. (8) Jukkola, G.; Liljedahl, G.; Nsakala, N.; Morin, J.; Andrus, H. An ALSTOM vision of future CFB technology based power plant concepts, Proceedings of 18th ASME International Conference on Fluidized Bed Combustion, Toronto, Ontario, Canada, May 2005. (9) Tan, Y.; Jia, L.; Hughes, R.; Lu, D.; Anthony, E. J. Oxy-fuel combustion with recycled flue gas on a mini CFB combustor, The Clearwater Coal Conference (The 31st International Technical Conference on Coal Utilization & Fuel Systems), Clearwater, FL, May 2006.
Mini-CFBC. Oxy-fuel CFB combustion tests were conducted in CETCOttawa’s mini-CFBC (Figure 1). The mini-CFBC consists of a riser, a cyclone, a return leg, and two feeders. The riser has an internal diameter of 100 mm and internal height of 5 m (from the distributor to the center line of the transition piece leading to the cyclone). It is equipped with both ash removal and flue gas analysis systems. The flue gas is continuously analyzed for NOx, SO2, CO, CO2, and O2. The SO2, CO, and CO2 are determined by nondispersive infrared analyzers, while O2 analysis is based on the paramagnetic principle. NOx is measured with a chemiluminescence analyzer. Solids pass from the combustor through a transition section into a cyclone and down through the return leg to re-enter the combustor directly above the bottom bed section. Four stock ceramic-insulated band heaters surround the bottom section of the combustor to help achieve appropriate combustion temperatures quickly. These heaters are computer-controlled and, if necessary, can be run during operation to help maintain isothermal conditions in the dense bed zone. Solid feed rates of up to 15 kg/h can be maintained. Bed temperature is in the range 750–950 °C. Superficial gas velocity is up to 8 m/s. The mini-CFBC has been extensively modified for oxy-fuel CFB combustion. Modifications include the addition of an oxygen supply line and a flue gas recycle train. Fuel, Limestone, and Bed Material. Eastern bituminous coal and Highvale coal (a sub-bituminous coal) were used. Table 1 gives the analysis of the two coals. Both coals were crushed to -5 mm. For sulfur capture tests, Havelock limestone was used when Eastern bituminous coal was fired. Table 2 gives the analysis of the Havelock limestone. The particle size of the limestone was in the range 0–0.5 mm. In all cases, sand of size 0.15–0.35 mm was used as initial bed material. Operating Conditions. The mini-CFBC was started in air-firing mode. When the bed temperature reached the desired level, the air supply was shut off. The flue gas recycle blower was turned on. Oxygen was then supplied to the mini-CFBC. The transition from the airfiring mode to the oxy-fuel-firing mode occurred easily and took only a few minutes. The mini-CFBC was operated at nominal bed temperature of 850 °C. Superficial gas velocity was about 4 m/s. Global oxygen concentration during oxy-fuel combustion periods was ∼28–30%. Results and Discussion The first combustion tests carried out here used Highvale coal, since it is a low-sulfur fuel (0.22%) and does not require
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Table 2. Analysis of Havelock Limestone CaO MgO SiO2 Al2O3 Fe2O3 Na2O K2 O MnO TiO2 Cr2O3 P2O5 SO3 V2O5 SrO BaO NiO LOF sum
53.99 0.59 1.23
