Does carbon monoxide burn inside a fluidized bed? A new model for the combustion of coal char particles in fluidized beds

Abstract

Beds of silica sand were fluidized by mixtures of C3H8, CH4, or CO with air. Starting from cold the way such a bed behaved before it reached a steady state was observed visually. In addition, high-speed cine films were taken, as well as measurements of the loudness of the noise emitted. These beds behave in a way indicating that such hot gas mixtures at up to 1000°C do not burn in the interstices between the sand particles. Instead, combustion occurs either above the bed or in the ascending bubbles. Measurements of the aiameter (dig) of a bubble made immediately prior to ignition confirmed that the ignition temperature (Tig) of the bubble varies with dig ∝ exp( Eig RTig), so that larger bubbles ignite at lower temperatures. It proved possible to generate combustion of these gas mixtures in the particulate phase by adding Pt-coated catalyst pellets. This leads to a new model for the burning of char particles in a fluidized bed. In the model, char is first oxidized to CO with the reaction Cs + 1 2O2 → CO occurring mainly inside the pores of each particle. The resulting CO burns either above the bed or in bubbles rising up the bed, but not in the particulate phase. Considerable uncertainties exist as to the correct values of Nusselt and Sherwood numbers, as well as of, e.g., the intrinsic rate constant for the initial production of CO. However, the model is capable of predicting the temperatures observed for char particles burning in fluidized beds. Some of the problems of O2 diffusing inside the pores of a char particle and then reacting to give CO are addressed. © 1991.