A critical review on direct catalytic hydrogasification of coal into CH4: catalysis process configurations, evaluations, and prospects

Coal catalytic hydrogasification (CCHG) is a straightforward approach for producing CH₄, which shows advantages over the mature coal-to-CH₄ technologies from the perspectives of CH₄ yield, thermal efficiency, and CO₂ emission. The core of CCHG is to make carbon in coal convert into CH₄ efficiently with a catalyst. In the past decades, intensive research has been devoted to catalytic hydrogasification of model carbon (pitch coke, activated carbon, coal char). However, the chemical process of CCHG is still not well understood because the coal structure is more complicated, and CCHG is a combination of coal catalytic hydropyrolysis and coal char catalytic hydrogasification. This review seeks to shed light on the catalytic process of raw coal during CCHG. The configuration of suitable catalysts, operating conditions, and feedstocks for tailoring CH₄ formation were identified, and the underlying mechanisms were elucidated. Based on these results, the CCHG process was evaluated, emphasizing pollutant emissions, energy efficiency, and reactor design. Furthermore, the opportunities and strategic approaches for CCHG under the restraint of carbon neutrality were highlighted by considering the penetration of “green” H₂, biomass, and CO₂ into CCHG. Preliminary investigations from our laboratories demonstrated that the integrated CCHG and biomass/CO₂ hydrogenation process could perform as an emerging pathway for boosting CH₄ production by consuming fewer fossil fuels, fulfilling the context of green manufacturing. This work not only provides systematic knowledge of CCHG but also helps to guide the efficient hydrogenation of other carbonaceous resources such as biomass, CO₂, and coal-derived wastes.