Studies in char gasification-I A lumped model

The gasification of char in a steam-oxygen fluidized bed was studied. The char was assumed to be composed of base carbon and ash. The gaseous compounds found were limited to CO, CO₂, H₂, H₂O, O₂ and CH₄. As the oxygen and hydrogen were assumed not to co-exist, the bed was divided into a shallow combustion zone where the carbon combustion would take place and a gasification zone where no oxygen would be found. A lumped model for the gasification zone was formulated. The thickness of the combustion zone was assumed to be negligible compared to the total height of the reactor. All chemical changes in the gasification zone were described by three reactions: steam gasification, carbon hydrogasification and water-gas shift reaction. The gases and the solids in the gasification zone were assumed to be at uniform temperature. From the results of the model calculations it was concluded that one should use the fairly complicated empirical kinetic expressions developed by Johnson for reliability over a wide range of pressures and residence times. The effects of the reactor pressure, the char residence time and the amount of oxygen in the feed on the gasifier performance were analysed.