Effect of circulating ash on sulfur conversion characteristics in the coal polygeneration process

•More than 30% of the total sulfur content was retained in the circulating ash.•Sulfur retention capacity of Fe2O3 was superior to that of CaO during pyrolysis.•FeS was easily oxidized to Fe2O3, while CaS tended to be oxidized to CaSO4.•Sulfur in ash released less sulfurous pollutants than char during combustion.

Sulfur transformation in the polygeneration process of Xinzhou high-sulfur coal employing a circulating ash heat carrier was studied using a fixed bed reactor. Circulating ash was effective in capturing the gaseous sulfur, along with ~ 80% of H2S, 59% of COS, 55% of CS2, and 48% of SO2. Sulfur mass balance calculations revealed that 30.88% of the total sulfur content was retained in the circulating ash at 600 °C. The sulfur retention capacity of the ash was promoted by an increase in temperature, an increase in the ash to coal mass ratio, and a decrease in ash particle size. Sulfur captured in the circulating ash released lower quantities of SO2 than the sulfur remaining in the char during combustion. Furthermore, CaO and Fe2O3 played a dominant role in the sulfur retention of the circulating ash, and the sulfur retention capacity of Fe2O3 was superior to that of CaO during pyrolysis. X-ray diffraction results indicated that the sulfur captured by CaO and Fe2O3 was converted into CaS and FeS. However, FeS was easily oxidized to Fe2O3, accompanied by the emission of SO2, while CaS tended to be oxidized to CaSO4 during combustion.

Graphical abstractCirculating ash was effective in capturing the gaseous sulfur, along with 80% of H2S, 59% of COS, 55% of CS2, and 48% of SO2. By using the circulating ash in the polygeneration process, significant quantities of the sulfur-based gases could be fixed into the ash, thus reducing the environmental effects of such gases.TG-MS analysis of the gaseous sulfur compounds released during pyrolysis.Figure optionsDownload full-size imageDownload as PowerPoint slide