Evolution of Hydrogen Chloride and Change in the Chlorine Functionality during Pyrolysis of Argonne Premium Coal Samples

In order to understand chlorine chemistry in coal pyrolysis, the dynamics of HCl evolution and changes in chlorine functional forms during temperature-programmed pyrolysis of eight Argonne premium coal samples have been examined with an online HCl-monitoring technique and by the Cl 2p X-ray photoelectron spectroscopy (XPS) method. The rate profiles of HCl evolved show at least three distinct peaks at 390, 520, and 600 °C, and the presence of these peaks depends strongly on the type of coal. The HCl peak at 390 °C appears with four coals alone and becomes considerably small by water washing, whereas the high-temperature peaks above 450 °C observed with almost all of the coals do not change significantly after washing. Yields of HCl up to 1000 °C are in the range of 50–90% in many cases, and the yield tends to decrease with increasing atomic Ca/Cl ratio in coal. The chlorine XPS analyses show that the chlorine in each coal is enriched at the surface and composed of inorganic and organic functional forms. The extent of the enrichment and proportion of organic chloride species increase after pyrolysis at 450 °C, whereas they decrease at high temperatures of 800 and 1000 °C. Some model experiments followed by the chlorine XPS measurements show that the reaction of HCl with carbon active sites proceeds readily at 500 °C to produce organic C–Cl forms, which release HCl again above 500 °C upon reheating in an inert gas. On the basis of the above-mentioned results, it is possible that HCl evolved below 450 °C in coal pyrolysis comes predominantly from water-soluble chlorine functional groups in coal, whereas HCl formation above 450 °C originates mainly from organic chlorides, which could be present inherently in coal and/or may be formed by secondary reactions of HCl evolved at a lower temperature with carbon active sites in the nascent char