Constraints of biomethane generation yield and carbon isotope fractionation effect in the pathway of acetotrophic with different coal-rank coals

•The biomethane yield is related to the maceral contents rather than coal rank.•Values αCO2-CH4 and εC show biomethane mainly generated from acetotrophic.•Carbon isotope variations of coal, acetate, methane, and carbon dioxide are illustrated.•The anaerobic methane oxidation, multi-stage degradation, and substrate consumption jointly control carbon isotope fractionation in biodegradation.