Sulfation of limestone under O2/H2O combustion conditions in circulating fluidized bed

Abstract

O2/H2O combustion is a new generation of oxy-fuel technology for CO2 capture, offering better thermodynamic and economic performance than O2/CO2 combustion. The main feature of O2/H2O combustion is that an O2-steam mixture is used as the oxidant and, as a result, H2O in the furnace can be as high as 80 %. To determine whether limestone can still be used to capture SO2 in-situ in circulating fluidized bed (CFB) boilers with such a high H2O concentration, its sulfation behavior was studied. Using a constant-temperature thermogravimetric analyzer, the influences of high H2O concentration, SO2 concentration, temperature and particle size on sulfation of limestone under O2/H2O combustion were examined. The fast sulfation stage was hardly influenced by H2O over the entire range of 0–80 %, but the sulfation rate in the slow sulfation stage improved with the increase of H2O concentration. Therefore, limestone can still be used to capture SO2 in O2/H2O combustion in CFB boilers, in spite of the high in-furnace H2O concentration. H2O is more likely to enhance the solid-state ion diffusion in the CaSO4 product layer, rather than the SO2 gas diffusion in the pores of the particles. Under conditions with 80 % H2O, when the SO2 concentration in the flue gas increased from 0.15 % to 0.45 %, the sulfation rate in the fast sulfation stage increased remarkably, while in the slow sulfation stage it barely changed. The sulfation rates in both the fast and slow sulfation stages were significantly increased with the decrease of limestone particle size from 0.45 mm to 0.075 mm. In the tested range of 840 °C–930 °C, the optimum temperature for SO2 capture under O2/H2O combustion is about 900 °C.