4803 r2010 American Chemical Society pubs.acs.org/EF Energy Fuels 2010, 24, 4803–4811 : DOI:10.1021/ef100314k Published on Web 08/31/2010 Experimental Investigation of the Combustion of Bituminous Coal in Air and O 2 /CO 2 Mixtures: 1. Particle Imaging of the Combustion of Coal and Char Lian Zhang,* ,† Eleanor Binner, † Luguang Chen, † Yu Qiao, † Chun-Zhu Li, †,‡ Sankar Bhattacharya, † and Yoshihiko Ninomiya § † Department of Chemical Engineering, Monash University, GPO Box 36, Clayton Campus, Victoria 3800, Australia, ‡ Curtin Centre for Advanced Energy Science and Engineering, Curtin University of Technology, WA 6102, GPO Box U1987, Perth, WA 6845, Australia, and § Department of Applied Chemistry, Chubu University, 1200 Matsumoto-Cho, 487-8501, Kasugai, Aichi, Japan Received January 14, 2010. Revised Manuscript Received August 18, 2010 Combustion of a low-volatile bituminous coal in air versus two O 2 /CO 2 mixtures (21/79 and 27/73, v/v) was conducted at two furnace temperatures of 800 and 1000 °C in a lab-scale drop tube furnace (DTF). Through in situ photographic observation and measurement of overall coal burnout rate, CO emission profile, and unburnt char properties, a variety of distinct phenomena relating to oxy-fuel combustion has been revealed. Consistent with the literature, the significant thermal effect of CO 2 due to its large product of C p F (specific heat capacity and density) relative to that of N 2 retarded volatile ignition in the two O 2 /CO 2 mixtures. As a result, the volatiles released in O 2 /CO 2 remained as a thick protective sheath on char surface for a relatively long duration, which mainly converted into CO through partial oxidation in 21% O 2 /79% CO 2 . Increasing the O 2 fraction to 27% in CO 2 triggered the ignition/oxidation of the unburnt volatiles once their concentrations were critically accumulated on char surface in a relatively low position in the DTF. Char oxidation behavior in the late stages of the DTF was also greatly changed under oxy-fuel conditions. Due to an insufficient O 2 in char particle vicinity, the partial oxidation and even gasification of char to CO were favored during oxy-firing, which yielded less enthalpy heat and hence lowered char particle temperature substantially. Char consumption rate was, however, affected little or even slightly increased. A detailed mathematical modeling is required to quantitatively clarify the oxidation behavior of coal char in the presence of the abundant CO 2 in the DTF. Introduction Coal combustion is one of the major sources for power generation, providing approximately 37% of the electricity requirement in the world. 1 Its greenhouse gas emissions, particularly of carbon dioxide (CO 2 ), however, have been facing stringent regulations with respect to the climate change. Efforts must be made to reduce and eventually eliminate CO 2 emission in the short/medium term, thereby maintaining a sustainable utilization of coal in the carbon-constrained future. Oxy-fuel combustion is a process of burning coal in a gas stream of oxygen (O 2 ) mixed with recycled flue gas (RFG), generating a CO 2 -rich flue gas that is potentially subjected to direct sequestration/storage with minimal treatment. 1,2 Ex- tensive studies in both pilot-plant and lab scales have pointed out the pronounced influence of gas composition (air versus O 2 /CO 2 ) on coal combustion performance. The heat transfer and temperature distribution in a furnace are greatly affected by the large specific heat capacity of CO 2 . 1,3,4 Coal ignition is delayed in O 2 /CO 2 in comparison to in O 2 /N 2 with the same O 2 concentration. To match the flame/particle temperature in air, a large amount of O 2 in CO 2 , typically around 30%, is required. 1 Coal conversion rate, char properties, and reactivity are also affected by the replacement of air with an O 2 /CO 2 mixture. The influence of bulk gas, however, varies greatly with coal property and combustion facility/condition. At a given O 2 concentration, coal burnout rate in O 2 /CO 2 is slower than in O 2 /N 2. 5,6 This is not unexpected as a lower particle/ flame temperature exists in O 2 /CO 2 . A slow transfer of O 2 in CO 2 (20% less than in N 2 ) also greatly retards the char-O 2 oxidation reaction on the condition that this reaction is controlled by O 2 diffusion through an external gas boundary layer. 7 The endothermic char-CO 2 gasification reaction, as most likely occurring at high temperatures, 8,9 further makes oxy-fuel combustion complex. Knowledge for oxy-fuel combustion is still scarce. One major reason is that coal combustion is a very complex process governed by transient phenomena and a series of chemical *To whom correspondence should be addressed. Phone: þ61-3-9905- 2592. Fax: þ61-3-9905-5685. E-mail: lian.zhang@monash.edu. (1) Buhre, B. J. P.; Elliott, L. K.; Sheng, C. D.; Gupta, R. P.; Wall, T. F. Prog. Energy Combust. Sci. 2005, 31 (4), 283–307. (2) Molina, A.; Shaddix, C. R. Proc. Combust. Inst. 2007, 31, 1905– 1912. (3) Kakaras, E.; Koumanakos, A.; Doukelis, A.; Giannakopoulos, D.; Vorrias, I. Fuel 2007, 86, 2144–2150. (4) Khare, S. P.; Wall, T. F.; Farida, A. Z.; Liu, Y.; Moghtaderi, B.; Gupta, R. P. Fuel 2008, 87, 1042–1049. (5) Bejarano, P. A.; Levendis, Y. A. Combust. Flame 2000, 153, 270– 287. (6) Liu, H.; Zailani, R.; Gibbs, B. M. Fuel 2005, 84, 833–840. (7) Shaddix, C. R.; Molina, A. Proceeding of the 5th joint meeting of the US sections of the Combustion Institute, San Diego, CA, USA, March 25-28, 2007; Paper G24. (8) Rathnam, R. K.; Elliott, L. K.; Wall, T. F.; Liu, Y.; Moghtaderi, B. Fuel Process. Technol. 2009, 90, 797–802. (9) Shaddix, C. R.; Murphy, J. J. Proceedings of the 20th Pittsburgh Coal Conference, Pittsburgh, USA, Sept 15-19, 2003; CD-ROM.