Mechanism of sulfur migration and conversion in pyrolysis/combustion staged conversion process of high sulfur coal

Abstract

To investigate the effects of circulating ash heat carriers and dolomite catalysts on sulfur migration in the coal pyrolysis/circulating fluidized bed combustion staged conversion process, five experiments of coal pyrolysis, ash/coal co-pyrolysis, coal pyrolysis catalyzed by dolomite, ash/coal co-pyrolysis catalyzed by dolomite and combustion of pyrolysis solid products were designed and carried out. By examining the effects of pyrolysis temperature, heating rate, coal-ash ratio and dolomite species on sulfur migration, the final distribution of sulfur-containing pollutants throughout the staged conversion process was systematically analyzed. The results showed that the separate mixing of ash heat carrier at final pyrolysis temperature of 600 °C increased the residual sulfur in solid phase from 79.29 % of raw coal pyrolysis to 90.00 %. When dolomite catalyst was added individually, the sulfur released into gas phase and tar was reduced from 18.28 % and 2.43 % of the raw coal to 13.14 % and 1.94 %, respectively. And Fe additive significantly improved sulfur resistance of the dolomite catalyst. The regeneration process has little effect on the sulfur fixation capacity of natural dolomite. When ash heat carrier and dolomite catalyst were simultaneously added to the system, the sulfur fixed in ash increased from 79.29 % of the raw coal to 91.27 %, and the sulfur fixed in natural dolomite and modified dolomite accounted for 4.32 % and 4.46 % of the total sulfur, respectively. In the combustion experiments of pyrolysis solid phase products, 43.57 %, 65.57 % and 61.44 % of sulfur was retained in the solid phase residue after combustion with circulating ash, circulating ash/natural dolomite and circulating ash/modified dolomite were added, respectively. Compared with direct combustion of coal, the staged conversion process resulted in a 55 % reduction of sulfur in the boiler flue gas. This study provides a theoretical basis for the purification of sulfur-containing pollutants in the staged conversion process.