Uneven distribution of sulfurs and their transformation during coal pyrolysis

Two Chinese coals, Liuzhi high pyrite coal with high ash content (LZ) and Zunyi high organic sulfur coal (ZY), were pyrolyzed in a fixed-bed reactor under nitrogen and hydrogen at temperature ranging from 400 to 700 °C. The effects of heat rate, temperature and gas atmosphere on sulfur transformation and sulfur uneven distribution were examined by XPS combined with traditional sulfur analysis method. The ratio of surface S to bulk S is used to describe the uneven distribution of sulfurs. It is found that oxygen is rich on the surface, while S in the bulk. The increasing ratio of surface S to bulk S with increasing temperature clearly indicates the sulfur transfer from the bulk to the char surface during pyrolysis. The ratios are higher at all temperatures studied for ZY coal than for LZ coal, which may be related to the higher ash content in LZ coal. The ratio of surface S to bulk S increases with increasing heating rate for LZ coal, while it decreases for ZY coal. In the presence of H2, the S on the surface is much lower than that under N2 and surface S in sulfidic, thiophenic and sulfoxide forms is totally disappeared for LZ coal at various temperatures and heating rates, while the surface S in thiophenic and sulfoxide forms is not totally disappeared for ZY coal, which may be related to the high rank of ZY coal. The ratio of surface S to bulk S decreases before 600 °C with increasing temperature for both coals in the presence of H2, showing that gaseous H2 can easily react with the surface S to form H2S, while above 600 °C it increases because the supply of H2 cannot match the rate of formation of HS free radicals at high temperature.