NO Emission Behavior in Oxy-fuel Combustion Recirculated with Carbon Dioxide Jeong Park,* ,² June Sung Park, ² Hyun Pyo Kim, ² Jeong Soo Kim, ² Sung Cho Kim, ² Jong Geun Choi, ² Han Chang Cho, ‡ Kil Won Cho, ‡ and Heung Soo Park ‡ School of Mechanical & Aerospace Engineering, Sunchon National UniVersity, 315 Maegok, Suncheon, Jeonnam 540-742, Korea, and Energy Team, Research Institute of Industrial Science and Technology, #32 Hyoja-dong, Nam-gu, Pohang 790-330, Kyungbuk, Korea ReceiVed July 4, 2006. ReVised Manuscript ReceiVed October 25, 2006 A numerical study is conducted to grasp the flame structure and NO emissions for a wide range of oxy-fuel combustion (covering from air-blown combustion to pure oxygen combustion) and various mole fractions of recirculated CO 2 in a CH 4 -O 2 /N 2 /CO 2 counterflow diffusion flame. Special concern is given to the difference of the flame structure and NO emissions between air-blown combustion and oxy-fuel combustion w/o recirculated CO 2 and is also focused on chemical effects of recirculated CO 2 . Air-blown combustion and oxy-fuel combustion without recirculated CO 2 are shown to be considerably different in the flame structure and NO emissions. Modified fuel oxidation reaction pathways in oxy-fuel combustion are provided in detail compared to those in air-blown combustion without recirculated CO 2 . The formation and destruction of NO through Fenimore and thermal mechanisms are also compared for air-blown combustion and oxy-fuel combustion without recirculated CO 2 , and the role of the recirculated CO 2 and its chemical effects are discussed. Importantly contributing reaction steps to the formation and destruction of NO are also estimated in oxy-fuel combustion in comparison to air-blown combustion. Introduction About 85% of the world’s commercial energy needs are supplied by fossil fuels, and hence, CO 2 capture and storage, including its reuse, presents an opportunity to achieve significant reduction in greenhouse gas emissions from fossil energy use. Approximately one-third of all CO 2 emissions due to human activity come from fossil fuels used for generating electricity. Currently, there are three main approaches for capturing CO 2 from combustion of fossil fuels, namely, precombustion capture, postcombustion capture, and oxy-fuel combustion. In oxy-fuel combustion, nearly pure oxygen (instead of air) is used for combustion that would result in a flue gas composed of mainly CO 2 and H 2 O as well as a small concentration of inert gases and nitrogen (due to air infiltration that happens in practice) and excess O 2 . Hence, the concentration of CO 2 in the flue gas can be greatly increased. As a result, only simple gas purification is required to capture CO 2 in this process. In this combustion mode, if fuel is burnt in pure oxygen, the resulting flame temperature would be excessively high. Hence, the CO 2 rich flue gas could be recycled to the combustor to reduce the flame temperature and make it similar to that in the air case. This recycle combustion process also has a further benefit in suppressing NO x formation. 1,2 The NO x formation in real oxy-fuel burners is due to nitrogen contamination in the fuel stream and/or air leaks. 3 While the study covered the sensitivity of NO production due to effects of air infiltration, fuel contamination, flame radiation, and aerodynamic straining, the recycle combustion process on NO x formation was not provided. From a practical point of view, it may be desirable to have higher intermediate soot without affecting the soot emission level because of the enhancement of the heat transfer by radiation. This is because the enhanced radiant fluxes could decrease the flame temperature and NO x concentrations. 4,5 Li and Williams 6 found that NO x emission could be decreased in counterflow partially premixed flames by reducing CH con- centrations and, hence, the contribution of the prompt mecha- nism into the NO x formation process. It was also shown in methane-air flames 7 and highly preheated H 2 -air flames 8 diluted with CO 2 that chemical effects with recirculated CO 2 were mainly caused by the reaction CO 2 + H f CO + OH, and this affects flame structure and thermal NO considerably, since the reaction also competed with the principal chain branching reaction H + O 2 f O + OH for H-atom. It was also recognized that these chemical effects modified reaction path- ways, and these could contribute to the formation and destruction of prompt NO. The present study is conducted numerically to grasp the formation and destruction of NO in oxy-fuel combustion recirculated with CO 2 . The computation covers all the range from air-blown combustion to pure oxygen combustion. An * To whom correspondence should be addressed. Tel.: +82-61-750- 3533. Fax: +82-61-750-3530. E-mail : jeongpark@sunchon.ac.kr. ² Sunchon National Universit. ‡ Research Institute of Industrial Science and Technology. (1) Croiset, E.; Thambimuthu, K. Can. J. Chem. Eng. 2000, 78, 402- 407. (2) Tan, Y.; Douglas, M. A.; Croiset, E.; Thambimuthu, K. Fuel 2002, 81, 1007-1016. (3) Sung, C. J.; Law, C. K. Proc. Combust. Inst. 1998, 27, 1411-1418. (4) Feese, J. J.; Turns, S. R. Combust. Flame 1997, 109, 266. (5) Beltrame, A.; Porshnev, P.; Merchan, W.; Saveliev, A.; Fridman, A.; Kennedy, L. A.; Petrova, O.; Zhdanok, S.; Amouri, F.; Charon, O. Combust. Flame 2001, 124, 295-310. (6) Li, S. C.; Williams, F. A. Combust. Flame 1999, 118, 399. (7) Hwang, D. J.; Park, J.; Oh, C. B.; Lee, K. H.; Keel, S. I. Int. J. Energy Res. 2005, 29, 107-120. (8) Park, J.; Kim, K. T.; Park, J. S.; Kim, J. S.; Kim, S.; Kim, T. K. Energy Fuels 2005, 19, 2254-2260. 121 Energy & Fuels 2007, 21, 121-129 10.1021/ef060309p CCC: $37.00 © 2007 American Chemical Society Published on Web 12/16/2006