Using an oxygen-carrier as bed material for combustion of biomass in a 12-MWth circulating fluidized-bed boiler

The initial experiences of using an oxygen-carrying metal oxide, ilmenite, in the 12-MWth circulating fluidized bed (CFB) boiler/gasifier system at Chalmers University of Technology are presented. The rationale for the addition of ilmenite to the solids inventory is that ilmenite has the ability to alternately take up and release oxygen, and thereby improve the distribution of oxygen throughout the furnace. As a consequence, less air is needed to maintain low emissions from carbon monoxide (CO) and unreacted hydrocarbons (HC) during the combustion of volatile-rich fuels, such as biomass. One of the conducted experiments involved only the boiler, and the reference case corresponded to operation solely with silica-sand as the bed material, while in an additional three cases, ilmenite in various amounts was added to make up to 40 wt.% of the total bed inventory. During the experiments, the concentrations of CO and nitric oxide (NO) in the convection path of the boiler were measured. The addition of ilmenite to the silica-sand decreased the concentrations of CO and NO by 80% and 30%, respectively. Additional experiments were performed in which a concentrated stream of raw gas produced in the indirect gasifier was injected into the freeboard of the boiler. In one experiment, only silica-sand was used, while 12 wt.% ilmenite was added to the bed material in a separate experiment. The concentrations of CO and HC were measured at three different heights in the boiler and at nine positions over a cross-section of the furnace. The concentrations of CO and total HC in the furnace cross-section during concomitant gasification operation were reduced by the addition of ilmenite.