Investigation of the Oxygen Supply and Distribution in a Bubbling Fluidized Bed by Using Natural Ilmenite for Oxygen Carrier Aided Combustion

Abstract

The application of oxygen carrier materials has one of its origins in the chemical looping combustion processes, which aim at inherent CO2 sequestration by realizing fuel oxidation in the absence of combustion air. This concept can be transferred to conventional fluidized bed combustion processes, the so-called oxygen carrier aided combustion. The replacement of the conventionally applied bed material with an oxygen carrier focuses mainly on large-scale circulating fluidized bed combustion plants. The main stated performance improvements include the reduction of carbon monoxide emissions even at lower excess air ratios and the enhancement of the combustion efficiency. Nevertheless, the ability of shifting oxygen in space and time is of certain interest especially in small-scale bubbling fluidized bed systems. Imperfect fuel mixing, limited residence time in the dense bed zone, as well as limited system complexity often mitigate their combustion performance. Process conditions in bubbling fluidized beds differ from those prevalent in circulating beds regarding bed material particle size distribution, fluidization velocities, or particle volume fraction in the bed. Within this study, the widely used oxygen carrier ilmenite serves as bed material in a laboratory bubbling fluidized bed combustion. The focus was on the oxygen distribution profiles and their shift by ilmenite into the bed during methane combustion. The variation of the excess air ratio, fluidization velocity, and bed height reveal a significant increase of the in-bed CO2 yield (max. 55%) within ilmenite experiments. Moreover, the oxygen conversion profile confirms the shift of the oxygen supply by ilmenite from the lower reactor part to the reduction dominated zone. The amount of shifted oxygen increases at lower excess air ratios. Thus, full conversion is not achieved as kinetics and limited residence time in the bed inhibit the fuel conversion.