Effect of a HF–HF/HCl Treatment of 26 Coals on Their Composition and Pyrolysis Behavior

Coals contain many types of minerals. The minerals’ effect on coal pyrolysis has been studied extensively in the literature by comparing only a few raw coals (R-coals) with the corresponding acid-treated coals. This paper studies an acid treatment of 26 coals from lignite to anthracite and compares the elemental composition and pyrolysis behavior of the acid-treated demineralized coals (D-coals) with the R-coals. The acid treatment involved leaching the coals with a HF solution and then with a mixed HF/HClsolution in air at 70 °C, and the pyrolysis was carried out in a temperature range of 120–900 °C in a thermal gravimetric analysis coupled online with a mass spectrometer. The characteristic parameters of the differential mass–loss curves (differential thermal gravimetric, DTG) and the subpeaks decoupled from the DTG curves are compared and discussed. It is found that the acid treatment not only removed most of the minerals from the coals but also reduced the coals’ carbon content, increased their oxygen content, and altered their organic structure, leading to reduced mass loss in pyrolysis, especially for mid-rank coals. Some minerals removed by the acid treatment promote the coal pyrolysis in the temperature range of 400–600 °C corresponding mainly to the cleavage of C_al–C_al and C_ar–C_al bonds as well as to the decomposition of crystal water. Some of the minerals inhibited the condensation of the aromatic structure at temperatures higher than 700 °C. The reduced carbon content and increased oxygen content of D-coals are attributed to the conversion of carboxylates to carboxylic acid which promoted the pyrolysis at temperatures lower than 300 °C. The composition and pyrolysis behavior of anthracites are not significantly affected by the acid treatment.