Effect of foreign minerals on sulfur transformation in the step conversion of coal pyrolysis and combustion

•The coal with larger particle size has more sulfur residue in char.•Fe2O3 mainly affects the sulfur migration from gas to solid during coal pyrolysis.•MgO, NaOH, and KOH have no sulfur fixation effect.•Sulfur fixation effect of CaO is lower than that of Fe2O3 in pyrolysis stage.•CaSO4 as the combustion products of CaS is more stable and can be fixed in ash.

To explore the sulfur transformation in the grading utilization of coal, the forms and distributions of sulfur at different operation parameters and foreign minerals were investigated in coal pyrolysis with solid heat carrier and combustion, respectively. The results show that the increasing pyrolysis temperature increases the sulfur content in the gaseous phase. For the coal with larger particle size, more sulfur will residue in char. The change of the mass ratio of quartz sand to coal and heating rate have little effect on the sulfur distribution in gas, tar and char products. After 3 wt% CaO is introduced to the raw coal, sulfur in gas and tar decreases from 29.68 and 12.16 wt% of the raw coal to 22.31 and 10.66 wt%, respectively. Fe2O3 mainly affects the migration of gaseous sulfur to solid phase sulfur during coal pyrolysis. MgO, NaOH, and KOH have no sulfur fixation effect. The sulfur-fixed efficiencies for 3 wt% CaO and 3 wt% Fe2O3 are 17.17 and 38.16 wt%, respectively. When the contents of CaO and Fe2O3 increase, the sulfate sulfur and sulfide sulfur in the char increase, and the sulfur fixed in ash which obtained from the combustion of char increases. The inhibitory effect of CaO on the secondary release of SO2 in the combustion stage is stronger than that of Fe2O3. This paper provides a basic reference for the removal of sulfur during the step conversion of coal pyrolysis and combustion.