KINETICS OF RICE HULL CHAR BURNOUT IN A BENCH-SCALE FLUIDIZED-BED REACTOR

Rice hull, also known as husk, is a by-product of the rice milling process; it is a significant alternative energy resource for the milling industries of rice producing countries. Apart from its high energy content, the residual ash from complete combustion of rice hull at moderate temperatures in a combustor, e.g., a fluidized-bed reactor, is also a potential cement extender. The cogeneration of steam for parboil rice and for other process activities makes combustion an attractive means of disposing rice hull in the rice milling industries, which otherwise is a menace to the rice miller. In the present work, the combustion kinetics of Guyana rice hull char was studied in a bench-scale fluidized-bed reactor. The salient features of the combustion of the char were observed, and the temporal history of burnout was traced at temperatures of 973 and 1173 K. The oxygen concentrations of the inlet fluidizing gas were maintained at 2.3 and 4.6% by regulating the mixing of nitrogen and air. The rates of combustion of the residual char generated from rice hull through rapid devolatilization were determined to range from 3.18 X 10(-6) to 9.04 X 10(-6) kg/s. By fitting the conversion data with various heterogeneous reaction models, it appears that the data could be described, to some extent, by the unreacted core model with ash layer diffusion control. However, the complexity of the carbon/silica arrangement in the residual char results in uncertainties in model selection.