A study of misleading effects of HCl treatment in the characterization of sulfur in coal

The purpose of this study was to determine the cause of the experimentally noticed decrease of sulfate sulfur during a thermal treatment of coal. In the case of the pyrolysis of the investigated six coals of different rank and with a widely differing sulfur contents, the decrease of the amount of sulfate sulfur was often > 50% and, in some cases, > 80%. The analysis of the obtained results indicates that this effect may be attributed to the overrated values of the sulfate sulfur in coal, which are the result of certain erroneous effects of the hydrochloric acid (HCl) treatment of coal, which is used during the standard procedures for sulfate sulfur determination. It is shown that, because of the HCl treatment, a portion of the organic and/or pyritic sulfur is identified as sulfate sulfur. This effect may be expected to be more pronounced in the case of low-rank coals, as well as in coals that have a relatively large content of pyritic sulfur.