Influence Mechanism of Acidic Mineral Components on the Reaction Behavior of Ion-Exchangeable Calcium during Coal Pyrolysis

• • Acid washing (HCl and HCl-HF) reduced ash content but also decreased organic element contents; H/C atomic ratios for YL-De, YL-HCl, YL-De-Ca, and YL-HCl-Ca were 0.61, 0.64, 0.58, and 0.59, respectively, indicating structural changes that affect pyrolysis reactivity. • • Loading ion-exchangeable Ca2+ increased thermal weight loss rate in the 500–550 °C range, shifting the DTG peak temperature from 530 °C to 514 °C, demonstrating enhanced cracking ability and thermal reactivity. • • At 600 °C pyrolysis, kaolinite dehydroxylation to metakaolin occurred; the presence of acidic minerals promoted hydrogen transfer to volatiles, and the coexistence with Ca2+ increased aliphatic hydrocarbon content in tar from 13.60% (YL-De-Ca) to 21.98% (YL-HCl-Ca), a 61.6% relative increase. • • Acidic mineral components inhibit the adverse effect of ion-exchangeable Ca2+ on tar lightening, as evidenced by lower small aromatic ring content (<6 rings) in char from Ca-loaded coal, which is critical for improving tar quality and downstream processing.