Operational analysis of ash-agglomerating fluidized bed gasifiers

The operational characteristics of ash-agglomerating fluidized bed gasifiers are analyzed through a simplified approach. Based on the simple theoretical analysis, true steady state operations in ash-agglomerating fluidized bed gasifiers require that the ash concentration in the bottom ash withdrawal be equal to the steady state ash concentration dictated by the kinetics of coal combustion and gasification, and other process variables in the fluidized bed gasifiers. There are only three steady state operational modes possible depending on the coupled effects of ash agglomeration rate and the particle separation efficiency of char and ash agglomerates. Without precise knowledge of the ash agglomeration rate, the combustion and gasification kinetics, and the rate and degree of particle separation between char and ash agglomerates, experimental trial and error is required to approach the true steady state. Locating the true steady state or the lack of it may seem unimportant in a pilot plant operation, where solids samples are obtained hourly so that correctional steps can be implemented quickly, its implication for the commercial operation can be serious.