The combustion of carbon monoxide in a two-zone fluidized bed

Abstract

The oxidation of CO to CO 2 has been studied in a two-zone fluidized bed of silica sand heated externally to 1000°C. The lower, fuel-rich, part of the bed is fluidized by a primary mixture of N 2 + CO, whereas the upper zone is fluidized by O 2 + N 2, premixed before being introduced through a sparger located roughly in the middle of the bed. First, the mixing characteristics of such a bed were studied and the cross-flow factor was measured for secondary bubbles in the upper fuel-lean zone. Then with mixing the burning of CO + O 2 occurring in the upper zone, concentration profiles were measured throughout the bed and freeboard. These studies indicate that CO and O 2 do not react in the particulate (emulsion) phase, but do react in the bubble phase as well as in the freeboard. The addition of CO 2 confirms that the strong inhibition of reaction in the particulate phase is a chemical effect and not a heat transfer effect, as has been suggested. In fact, combustion of CO is quenched in the particulate phase primarily by radicals recombinining and also by electronically excited species being deactivated on the huge available surface areas of solids, although homogeneous chain termination is also possible, as with the addition of CO 2. The implications of there being no combustion of gases within the particulate phase of a fluidized bed are discussed, especially with respect to coal combustion in these systems. The rate constant for CO and O 2 reacting in the bubble phase and freeboard was determined.