INTERNATIONAL JOURNAL OF ENERGY RESEARCH Int. J. Energy Res., 22, 1351—1364 (1998) NUMERICAL COMPUTATION OF A CIRCULATING FLUIDIZED BED COMBUSTOR LU HUILIN*, BIE RUSHAN, YANG LIDAN, ZHAO GUANGBO AND TAN XIU Department of Power Engineering, Harbin Institute of Technology, Harbin, People+s Republic of China SUMMARY A mathematical model to describe a circulating fluidized-bed combustor is presented. A modified two-phase model which was used in the bubbling fluidized-bed combustor is considered to simulate the dense zone of the bottom section. For the upper section of the bed the momentum and energy-balance equation are used to predict the temperature and velocity profiles for the gas and the particles. The model performs mass balances for the chemical gas species (O  ,H  O, CO, CO  and SO  ) with consideration being given to the last for retention by limestone particles. The model is applied to typical conditions of a circulating atmospheric fluidized-bed boiler and the simulation results show the expected trends.  1998 John Wiley & Sons, Ltd. KEY WORDS circulating fluidized-bed boiler; numerical simulation; combustion; SO  retention 1. INTRODUCTION Circulating fluidized-bed boilers have a number of advantages over conventional bubbling fluidized-bed boilers and have become an important topic of research. The empirical design of the circulating fluidized-bed boiler furnace practiced for many years promoted studies to understand the processes involved in such an equipment. Nowadays, due to strict pollution regulation and the necessity of high performance in the circulating fluidized-bed boiler as well as the development of powerful computers, research programmes have commenced. Investigations on high-velocity fluidization in the riser have been conducted by many investigators (see e.g. Yerushalmi and Avidam, 1985; Kwauk et al., 1987; Rhodes and Geldart, 1987). These models allowed the identification of the circulating fluidized-bed flow structure with a denser zone at the bottom and a fast bed above, and with a higher particle concentration near the wall region and dilute particles flowing in the centre which formed a core-annular structure. Berruti and Kalogerakis (1989) modelled the circulating fluidized bed using a core-annular theory which was applied to a small diameter riser used in catalytic processes. Kunii and Levenspiel (1990) proposed a model for the hydrodynamics which is concerned with the three different phases (dilute particles entrained, ascending and descending clusters) with mass transfer between them. The key parameter in this model is the decay constant for the fall off of bulk density of solid particles with height in the freeboard. Wei{ et al. (1987) developed a mathematical model which describes coal combustion in a circulating fluidized bed, based on the mass and energy balances for a compartment and considered eight chemical species. However, no details are given about solid concentration distribution in the reactor and no special treatment is given to the bottom region. Actually the solid concentrations are much higher in the *Correspondence to: Professor Lu Huilin, Department of Power Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China CCC 0363-907X/98/151351—14$17.50 Received 30 April 1998  1998 John Wiley & Sons, Ltd. Accepted 29 May 1998