Mechanistic Study on the Impact of the Occurrence of Various Basic Mineral Species on Spontaneous Combustion Propensity of Alkali‐Rich Low‐Rank Coal

Abstract

Spontaneous combustion of low‐rank coal, especially alkali‐rich coal, is a key issue that determines the safety of storage and transportation. To understand the role of different forms of Alkali and alkaline earth metals (AAEMs) on coal spontaneous combustion, AAEMs in raw coal were sequentially eluted by water, ammonium acetate (NH4OAc) and hydrochloric acid (HCl) solution. The occurrence forms and mineralogical characteristics of AAEMs in raw coal and its derived coal samples were investigated. Further, the spontaneous combustion tendency of raw coal and derived coal samples was measured by programmed heating process and the chemical structure parameters were characterized by thermogravimetric analysis and differential scanning calorimetry (TGA‐DSC) and in situ infrared spectroscopy. The results show that demineralization mitigates the spontaneous combustion tendency by the increasing the ignition temperature, confirming the enhancement of inherent mineral on low‐temperature oxidation. In situ infrared spectroscopy results suggest that acid‐soluble and ion‐exchangeable AAEMs have greater catalytic impact on the oxidation reaction of coal than water‐soluble AAEMs. Specifically, the complete removal of AAEMs by acid‐washing treatment has the most obvious effect on the change of oxygen‐containing functional groups, which greatly reduces the generation rate of carboxylic acid, ester and anhydride groups during the low temperature oxidation. Moreover, calcium elements including ion‐exchangeable calcium and HCl‐soluble calcium can promote the oxidation capacity of coal at low‐temperature, which were verified by loaded coal samples.