STUDIES ON FLUIDISED BED COMBUSTION OF COAL AND MATHEMATICAL MODELLING OF FLUIDISED BED COMBUSTOR

Abstract

Fluidized bed combustion as a method of improving boiler design has been under development for a number of years,taking the advantage of the presence of fluidized solids to decrease the combustion temperature, to increase heat transfer coef-ficients, and to operate on low grade coals, Fluidized bed combustion of coal with its potential for reducing nitrogen 7 I oxide emissions and for control of sulfur oxide emissions by the addition of limestone or dolomite to the bed has witneesed a keen interest in recent years. In the present work, a mathematical model based on two phase theory of fluidization as proposed by Davidson and Harrison (3), which can predict the performance of fluidized coal combustors, has been developed. The bed is consisting of two phases, a bubble phase and an emulsion phase. The emulsion phase is assumed to be completely mixed whereas bubble phase is regarded to be in plug flow, All chemical reactions are assumed to be taking place in the emulsion phase. The reaction at char particle surface is assumed to be diffusion controlled whereas the gas phase reaction of CO to CO2 is assumed to be kinetically controlled. Expressions are derived to account for the effects of elu-triation and particle size distribution in coal feed, bed and elutriant, The formulation of model includes heat and mass transfer effects, A computer program has been developed in FORTRAN-IV and computer results obtained are plotted on CI LCuMP-1037 plotter. The program requires a maximum'ClIT time of 36 min-..:,es on .DBOsystem-2050 computer. Given the bed dimensions, superficial air velocity, size distribution and' analysis of coal feed and the properties of the inert bed material, the model can predict the concentration and temperature distributions throughout the bed, the combustion efficiency, carryover heat lose, the carbon load of the bed, the size distribution of solids in the bed and elutriEnt and the composition of the exit gas.