Effect of mineral removal on the structure and pyrolysis characteristics of subbituminous coal

Minerals, as a secondary component, have a significant influence on the structure and thermal reactivity of coal. In order to further clarify the effect of minerals on the structure and thermal conversion of Hefeng sub-bituminous coal (HSBC) from Xinjiang, the mineral removal of HSBC was investigated. In this paper, acid-washing treatment was used to remove minerals from HSBC. The ash composition, mineral composition, and chemical structure of HSBC and its acid-washed coals were analyzed by X-ray fluorescence spectroscopy (XRF), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). Their pyrolysis characteristics and pyrolysis products were examined by thermogravimetric infrared gas chromatography/mass spectrometry (TG-FTIR-GC/MS). Acid-washing pretreatment not only significantly alters the mineral composition of the coal, but also destroys its organic structure, resulting in decreased coal stability. Especially, hydrofluoric acid (HF), as well as hydrochloric acid and hydrofluoric acid (HCl–HF) stepwise acid-washing treatment. In addition, the pretreatment effectively increased the weight loss during coal pyrolysis by destroying the organic structure of the coal and improving the pyrolysis reactivity of coal. The pretreatment inhibits the generation of CO2 by changing the form of oxygen-containing functional groups in coal, allowing more oxygen-containing functional groups to migrate into coal tar, thereby promoting the formation of phenols, aromatics and alkanes, and improving the quality of coal tar.