XPS study on the change of carbon-containing groups and sulfur transformation on coal surface

Two Chinese coals, Liuzhi (LZ) and Zunyi (ZY), were pyrolyzed in a fixed bed reactor under N2 and H2 atmospheres from 400 °C to 700 °C. X-ray Photoelectron Spectroscopy (XPS) was used to study the changes of carbon-containing groups and sulfur transformation on the surface of coals and their chars. Under N2 atmosphere, on the ZY chars surface, the contents of O=C-O, C-H or C-C groups have a decreasing trend and the one of C=O or O-C-O, C-O is increasing compared with that of raw coal at all temperatures. For LZ coal, the changes of O=C-O, C=O or O-C-O, C-O contents are similar to ZY coal, however the one of C-H and C-C content is contrary to that of ZY coal. Under H2 atmosphere, for ZY coal, not only O=C-O content, but also C=O and O-C-O, C-O contents have a decreasing trend, while C-H and C-C contents increase compared with raw coal; for LZ coal, except for C=O and O-C-O content has a decreasing trend, and other carbonyl groups have the same trend as that under N2 atmosphere. And for these coals, with the temperature increasing, the S/C ratio on the char surface increases under N2 atmosphere, however the one decreases at the temperatures from 400 °C to 600 °C and increases much at 700 °C under H2 atmosphere. This indicates that under both atmospheres, sulfur can transfer from inner to the surface during pyrolysis, but hydrogen can react with sulfur-containing radicals, so the S/C is lower than that under nitrogen.