Effects of circulating ash and dolomite on mercury transition during coal step conversion of pyrolysis and combustion

In aim to study the influence of circulating ash used as heat carrier and dolomite used as catalyst on mercury transition during the step conversion of coal pyrolysis and combustion, five experiments with different operating conditions were conducted in this study, including four pyrolysis experiments of raw coal, coal/ash, coal/ dolomite, coal and ash/dolomite, and combustion experiment of pyrolysis solid product. By changing the experimental conditions including final pyrolysis temperature, heating rate, coal-ash ratio and dolomite type, the transition path and final mercury distribution in the step conversion process were comprehensively analyzed. Experiments results indicated that mercury completely released out from coal when the final pyrolysis temperature was 600 degrees C. The individual addition of ash could promote the mercury release from coal at lower temperature. Dolomite could catalyze the oxidation of Hg0 in gas phase, and the modified dolomite had better catalytic capacity for Hg0 oxidation than natural dolomite. The addition of circulating ash and natural dolomite both had a certain catalytic effect on Hg0 emission peaks during the coal pyrolysis. Increasing the coal-ash ratio was conducive to the oxidation of Hg0 in pyrolysis gas. During the direct combustion of coal, the content of Hg0 released into the gas phase could reach to 69.97%. By adding ash, ash/natural dolomite and ash/modified dolomite to the coal pyrolysis process, the content of Hg0 in pyrolysis gas decreased to 58.99%, 34.77% and 31.30%, respectively. The research is able to offer the theoretical foundation for mercury contaminants' removal and process optimization of the step conversion technology.