MATHEMATICAL-MODELING OF FLUIDIZED-BED COAL GASIFIERS

In this review, previously reported mathematical models for fluidized-bed coal gasifiers are critically examined. Various rate processes occurring in the gasifier as well as reported pilot-scale studies are also briefly reviewed. Development of a detailed mathematical model is limited by inadequate understanding of many important aspects such as secondary decomposition and combustion of volatiles, primary products of char combustion, etc. Few reported kinetic investigations of the char gasification of chars have considered the inhibitive effects of the reaction products. Pilot-scale gasification studies reported in the literature are scarce and few report experimental data in sufficient detail for model validation purposes. A steady-state isothermal model for a bubbling fluidized-bed coal gasifier is developed assuming instantaneous devolatilisation and char combustion and finite rate char gasification. The effects on model predictions of assumptions regarding the products of combustion/decomposition of the volatiles, products of char combustion and the extent of water-gas shift equilibrium in the emulsion phase predictions are analysed by comparing the predictions with experimental data from three pilot-scale gasifiers. It is concluded, that due to the relatively short residence time available for the particles in a pilot-scale gasifier, the contribution to carbon conversion of the slow char gasification reactions is comparatively small. Its performance, therefore, is mainly controlled by the fast coal devolatilisation, volatiles combustion/decomposition and char combustion reactions. The need for detailed investigation of these phenomena in the context of fluidized-bed gasification is emphasised.