Influence mechanism of AAEMs on the pyrolysis of coal density-separated fractions: Insights from combining TGA, in-situ Raman spectroscopy and in-situ EPR technique

In this study, in-situ Raman and in-situ Electron Paramagnetic Resonance (EPR) spectroscopy combining thermogravimetric analysis (TGA) were developed to investigate the effects of alkali and alkaline earth metallic species (AAEMs) on the evolution of Zhundong coal (a typical AAEMs-rich coal) density-separated fractions including <1.40 g/cm³, 1.40–1.45 g/cm³, 1.45–1.50 g/cm³ and >1.50 g/cm³ during pyrolysis. The inherent AAEMs in the Zhundong coal mainly exist as Na and Ca. For occurrence characteristics of AAEMs, the relative amount of ion-exchangeable AAEMs is close between density-separated fractions, and the water-soluble and HCl-soluble AAEMs mainly exist in the >1.50 g/cm³ fraction. The pyrolysis weight loss and the maximum mass loss rate (R_max) decrease with the increases of the fraction's density. The chemical structure and the occurrence characteristics of AAEMs of density-separated fractions have a combined effect on their pyrolysis characteristics. At the devolatilization stage of the pyrolysis, water-soluble AAEMs promote the release of active components, accelerate the formation of more stable bonds between AAEMs and char matrix and inhibit the release of the 1–2 aromatic rings in char especially for the <1.40 g/cm³ fraction. In this stage, the formation of the cross-linking structures and 3–5 aromatic rings especially for the >1.50 g/cm³ fraction, and the coupling of free radicals especially for the <1.40 g/cm³ fraction are promoted. At the aromatization polymerization stage, the effects of water-soluble Na/K are obvious, especially for the <1.40 g/cm³ fraction. The divalent AAEMs all can inhibit the condensation of aromatic rings and improve the reactivity of stable free radicals, especially for water-soluble divalent AAEMs in the >1.50 g/cm³ fraction. Good correlations between pyrolysis reactivity and in-situ chemical structure were found and established. It was expected to direct the coal utilization based on integrated cascade stages.