SO3 formation under oxy-CFB combustion conditions

Abstract

Due to the enrichment of SO2 and H2O, SO3 formation during oxy-fuel circulating fluidized bed (CFB) combustion may significantly increase over the air fired case and this requires special attention in terms of safety consideration. In an attempt to better elucidate the formation mechanism of SO3 under oxy-fuel CFB conditions, homogenous and heterogeneous experiments were performed using a small vertical tube reactor to model the SO3 formation condition in the back pass channels and then the mechanism deduced was further validated by tests and measurements using a pilot-scale 50kWth oxy-fuel CFB combustor with wet flue gas recycle. Results show that replacing N2 by CO2 does not change the SO3 formation levels while the addition of water enhances SO3 formation. The increased O2, SO2, H2O concentrations along with increasing temperature are favorable for enhancing SO3 formation over the range of tested parameters. Fe2O3, CuO and V2O5 are shown to be able to catalyze SO2 conversion to SO3 under oxy-fuel atmosphere; of these V2O5s catalyzing ability is the strongest. Fly ash can either catalyze the SO3 formation or absorb SO3, depending on the temperature and the alkalinity of the ash. The results from the pilot plant burning bituminous coal demonstrate that SO3 concentration in the flue gas is about 4.5 times higher during oxy-fuel combustion than that under air combustion.