Effects of demineralization on the structure and combustion properties of Shengli lignite

Investigation of the effects of demineralization of inherent minerals in Shengli lignite (SL) on its structure and combustion property in the field of coal conversion is an extremely important and interesting topic. In this study, inherent minerals in SL were demineralized by hydrochloric acid (HCl) treatment. Further, the metals cations Na+, Al3+, K+, Ca2+, Mn2+ and Fe3+, corresponding to the inherent metal ions in lignite were added into the demineralized samples by the impregnation method in order to compare their effects. The surface morphology and structure parameters of the raw and demineralized lignite samples were characterized by SEM, nitrogen adsorption-desorption, FTIR, XPS, XRD and Raman spectroscopy. The combustion properties of lignite samples were investigated by thermogravimetric analysis. Demineralization led to an improvement in the amount of side chains in SL and destroyed the pore structure and aromaticity; however, it did not have obvious effects on the surface morphology of SL and the main forms of carbon and oxygen. Moreover, demineralization increased the ignition temperature and apparent activation energy, and decreased the combustion stability index of SL. Added aluminum ions resulted in a decrease in the combustion stability and reactivity of the HCl treated sample; however, the added metals cations such as Na+, K+, Ca2+, Mn2+ and Fe3+, exhibited opposite effects.