Modeling a fluidized bed coal gasification reactor

A steady-state model has been developed to simulate the North Carolina State University pilot-scale fluidized bed coal gasification reactor. The model involves instantaneous devolatilization of coal in the freeboard region and char combustion and gasification in the fluidized bed. A two-phase representation of the fluidized bed incorporates the phenomena of jetting, bubbling, slugging, and mass and heat transfer between phases. The model has the ability to predict both concentration and temperature profiles, the latter being useful in the prediction of possible clinker formation at hot spots within the bed. The model has been successfully used to simulate the gasification of a devolatilized Western Kentucky bituminous coal and a New Mexico subbituminous coal.; Effects of the molar steam/carbon feed ratio, molar oxygen/carbon feed ratio, fluidized bed height, and pressure on gasifier performance have been studied parametrically with the two-phase model. The molar oxygen/carbon feed ratio has significant positive effects on the reactor performance.; The molar steam/carbon feed ratio has moderate negative effects on the reactor performance. The approach to water-gas shift equilibrium is favored by lowering the steam/carbon ratio and/or raising oxygen/carbon ratio. The effects of moderate changes in bed height and pressure on the gasifier performance are small, implying that the reactor can be operated at any bed height and pressure near the targeted operating conditions without significantly affecting the gasifier output.; Organic sulfur distributions in a New Mexico subbituminous coal and a Texas lignite have been determined by a nonisothermal technique. A three-stage well-mixed bed model incorporating the hydrodesulfurization kinetics of Yergey et al. was used to correlate the evolution rate of H{dollar}sb2{dollar}S from coal or char during gasification. The predictions are generally satisfactory considering the simplicity of the model. In the absence of a suitable kinetic model the hydrolysis of COS is assumed to proceed to equilibrium in the fluidized bed. The well-mixed bed model gives an order of magnitude estimate of the COS production rate, which should generally be adequate considering the small amount of COS in the make-gas.