Heat transfer between a coal particle and fluidized bed during heating and devolatilization

Abstract

This paper presents an experimental and theoretical investigation of temperature in the coal particle during heating and devolatilization in hot fluidized bed (FB). A mathematical model, results of which are shown here, describes the unsteady behaviour of a coal particle during heating and devolatilization. The mathematical model is based on certain physical assumptions such as: presence of internal heat transfer resistance within the coal particle, variable thermal conductivity and heat capacity, and constant diameter of the coal particle. In order to obtain the values of parameters necessary for running the model, additional measurements were performed on released quantities of volatiles at definite temperatures. The measurements were carried out both in the FB and electrical oven, in inert (nitrogen) atmosphere, in temperature range 300-850°C. Devolatilization was isolated from combustion by conducting tests in nitrogen atmosphere with dried coal particles. Maximum discrepancy of FB reactor and oven results was 9%. The results were approximated (fitted) by a fourth-order polynomial of density vs. temperature. To test the model and to determine the unknown parameters, measurements of coal particle center temperature were conducted with a thermocouple. Two coals, a lignite and a brown coal, were tested using dried coal particles shaped into spheres of diameters 5, 7, 8 and 10 mm. FB temperature was in the range 590-710°C. On the basis of the performed experiments, a correlation for devolatilization duration vs. particle diameter was determined. The experimental and modeling results were compared. It was shown that in the first part of the process, during heating, temperature gradient within the particle was high, and later during devolatilization, it decreased.