Combined impacts of intrinsic alkali and alkaline earth metals and chemical structure on reactivity of low-rank coal char: New explanation for the role of water-soluble AAEMs during pyrolysis and gasification

The char reactivity generally determines the overall efficiency of the whole gasification process of coal, so it is of great significance to deeply understand the factors influencing the char reactivity and the coupling mechanism between them. This study aims to investigate the combined impacts of intrinsic alkali and alkaline earth metals (AAEMs) and chemical structure of organic matter on char reactivity, and provide new evidence for confirming the controversial role of water-soluble AAEMs during char gasification. Four series of char samples prepared from Zhundong coal containing different forms of intrinsic AAEMs were isothermally gasified by air in TGA, obtaining the specific reaction-rate curves, and a reasonable evaluation index of char reactivity was established. The carbon skeleton structure of char surfaces was analyzed by Raman spectroscopy. The comprehensive discussion of chemical structure, AAEMs state and char reactivity indicates that the water-soluble AAEMs reduce the char reactivity in the early part of gasification process, which is due to the catalytic cracking of aromatic C-H caused by these substances on char surfaces during pyrolysis, while improving the char reactivity in the later part because of the original catalysis of these substances on gasification; the ion-exchangeable AAEMs significantly catalyze the opening of aromatic rings in char during gasification, especially the small rings, which weakens the reaction priority of aliphatic structures; the increasing pyrolysis temperature results in the ordering of chemical structure and the enrichment of ion-exchangeable AAEMs, and the former reduces the char reactivity, while the latter has the opposite effect; at the pyrolysis temperature of 400-600 degrees C, the enrichment of ion-exchangeable AAEMs is the main factor controlling the char reactivity, while at 700-1000 degrees C, the ordering of chemical structure plays a dominant role.