Progress in Energy and Combustion Science 38 (2012) 156e214 Contents lists available at SciVerse ScienceDirect Progress in Energy and Combustion Science journal homepage: www.elsevier.com/locate/pecs Review Oxy-fuel combustion of pulverized coal: Characterization, fundamentals, stabilization and CFD modeling Lei Chen, Sze Zheng Yong, Ahmed F. Ghoniem* Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307, USA article info abstract Article history: Oxy-fuel combustion has generated significant interest since it was proposed as a carbon capture technology Received 13 January 2011 for newly built and retrofitted coal-fired power plants. Research, development and demonstration of oxy- Accepted 27 August 2011 fuel combustion technologies has been advancing in recent years; however, there are still fundamental Available online 14 December 2011 issues and technological challenges that must be addressed before this technology can reach its full potential, especially in the areas of combustion in oxygen-carbon dioxide environments and potentially at elevated Keywords: pressures. This paper presents a technical review of oxy-coal combustion covering the most recent experi- Carbon capture mental and simulation studies, and numerical models for sub-processes are also used to examine the Oxy-fuel combustion Coal differences between combustion in an oxidizing stream diluted by nitrogen and carbon dioxide. The Heat transfer evolution of this technology from its original inception for high temperature processes to its current form for Flame stabilization carbon capture is introduced, followed by a discussion of various oxy-fuel systems proposed for carbon CFD capture. Of all these oxy-fuel systems, recent research has primarily focused on atmospheric air-like oxy-fuel combustion in a CO2-rich environment. Distinct heat and mass transfer, as well as reaction kinetics, have been reported in this environment because of the difference between the physical and chemical properties of CO2 and N2, which in turn changes the flame characteristics. By tracing the physical and chemical processes that coal particles experience during combustion, the characteristics of oxy-fuel combustion are reviewed in the context of heat and mass transfer, fuel delivery and injection, coal particle heating and moisture evap- oration, devolatilization and ignition, char oxidation and gasification, as well as pollutants formation. Operation under elevated pressures has also been proposed for oxy-coal combustion systems in order to improve the overall energy efficiency. The potential impact of elevated pressures on oxy-fuel combustion is discussed when applicable. Narrower flammable regimes and lower laminar burning velocity under oxy-fuel combustion conditions may lead to new stability challenges in operating oxy-coal burners. Recent research on stabilization of oxy-fuel combustion is reviewed, and some guiding principles for retrofit are summarized. Distinct characteristics in oxy-coal combustion necessitate modifications of CFD sub-models because the approximations and assumptions for air-fuel combustion may no longer be valid. Advances in sub-models for turbulent flow, heat transfer and reactions in oxy-coal combustion simulations, and the results obtained using CFD are reviewed. Based on the review, research needs in this combustion technology are suggested. Ó 2011 Elsevier Ltd. All rights reserved. Contents 1. Introduction . ........................159 1.1. Carbon capture technologies for coal-fired power plants . ...........................159 1.2. Focus and methods . ..........................................161 1.3. Brief history of oxy-fuel combustion technologies . .....................................161 1.3.1. Oxy-fuel combustion for high temperature processes . ...........................161 1.3.2. Oxy-fuel combustion for CCS . ..........................................162 1.4. Oxy-coal combustion systems . ..........................................163 1.4.1. Atmospheric oxy-coal combustion systems with flue gas recycle . ...........................163 1.4.1.1. Air Separation Unit (ASU) . ......................................163 * Corresponding author. Tel.: þ1 617 253 2295; fax: þ1 617 253 5981. E-mail address: [email protected] (A.F. Ghoniem). 0360-1285/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.pecs.2011.09.003 L. Chen et al. / Progress in Energy and Combustion Science 38 (2012) 156e214 157 1.4.1.2. Carbon dioxide purification unit (CPU) . .......................164 1.4.1.3. Flue gas recycle (FGR) system . .......................164 1.4.2. Pressurized oxy-coal combustion systems . .......................164 1.4.3. Performance of the oxy-coal combustion systems . .......................165 2. Thermodynamics, transport and chemistry in oxy-coal combustion . .....................166 2.1. Heat transfer . .......................166 2.1.1. Radiative heat transfer . .......................166 2.1.2. Convective heat transfer . .......................167 2.1.3. Matching of combustion temperature . .......................168 2.1.4. Matching of radiative and convective heat transfer distribution . .......................171 2.2. Fuel delivery and injection . .......................172 2.2.1. Pulverized coal transport . .......................172 2.2.2. Coal-water slurry . .......................172 2.3. Heating and moisture evaporation of coal particles . .......................173 2.4. Coal devolatilization and char formation . .......................175 2.5. Ignition of coal particles . .......................176 2.5.1. Theoretical analysis of homogenous ignition delay . .......................176 2.5.2. Experimental studies on coal particle ignition delay in O2/N2 and O2/CO2 ...................................................177 2.5.3. Burning of volatiles . .......................179 2.6. Oxy-char combustion . .......................179 2.6.1. Kinetics of char oxidation in oxy-fuel combustion . .......................180 2.6.2. Effect of diffusivity . .......................181 2.6.3. Effect of gasification reactions . .......................183 2.6.4. Effect of heat capacity . .. .......................185 2.6.5. Effect of pressure . .......................185 2.6.6. Summary . .......................186 2.7. Pollutants formation . .......................187 2.7.1. Fly ash formation . .......................187 2.7.2. Ash deposit/slag formation . .......................188 2.7.3. NOx formation . .......................188 2.7.3.1. Nitrogen chemistry . .......................188 2.7.3.2. Lower NOx emission intensities in oxy-coal combustion . .......................189 2.7.3.3. Reasons for lower NOx emission intensities . .......................190 2.7.4. SO2 formation ................................................................................. .......................191 2.7.5. CO concentration and emission . .......................191 2.8. Summary.............................................................. .............................. .......................192 3. Stabilization of oxy-coal combustion . ............................................193 3.1. Flammability under oxy-fuel conditions . .......................193 3.2. Laminar flame propagation . ..