Effects of chromium ion on sulfur removal during pyrolysis and hydropyrolysis of coal

The effects of impregnated Cr³⁺ on sulfur removal during pyrolysis and hydropyrolysis of coal were investigated by loading CrCl₃ into raw, demineralized and pyrite removed coal, respectively. The results indicate that Cr has no effect on the removal of pyrite. Cr affects the removal of total sulfur by forming Cr₇S₈ and affecting the removal of organic sulfur. Cr acts as the sulfur removing agent by promoting the decomposition of the unstable organic sulfur at low temperature. However, it behaves to be sulfur fixing agent between 400 and 700 °C so as to inhibit the evolution of H₂S, even in hydropyrolysis. With the increase of temperature from 700 to 1050 °C, a certain ratio of Cr₇S₈ is converted into organic sulfur during pyrolysis; however, almost all the Cr₇S₈ is reduced into Cr at 1050 °C during hydropyrolysis. And Cr significantly promotes the removal of organic sulfur at high temperature within reducing atmosphere. The XPS results indicate that the sulfur is enriched on coke surface by Cr, which is attributable to the formation of Cr₇S₈ as well as the transfer of organic sulfur from bulk to surface during pyrolysis and hydropyrolysis.