A review on coal pyrolysis and gasification: understanding the chemistries and influence of operating conditions

Abstract Pyrolysis and gasification are two popular technologies that have been applied for coal utilization for centuries. In recent times, their applications for sustainable coal utilizations have attracted significant interest in hydrocarbon-based chemical synthesis, synthetic natural gas production, and carbon material synthesis. However, despite their long history in the coal industry, the underlying chemistries governing the pyrolysis and gasification processes are not well understood. This has contributed to why these technologies have yet to witness widespread commercialization in the coal industry. Therefore, the chemistries of coal pyrolysis and gasification were discussed in this review. It was found that the extent of primary pyrolysis reactions depends on hydrogen because hydrogen stabilizes the free radicals, and its absence slows down or stops the reactions. Operating conditions such as temperature and reactive atmospheres directly affect the pyrolytic reactions and the characteristics of the products, while pressure and particle size affect the mass transfer of volatile species and influence the extent of secondary pyrolysis reactions. In gasification, the composition of the syngas is principally determined by the water-gas-shift and methane-reforming reactions, while operating conditions such as temperature, catalysts, and choice of gasifying agents are used to control the characteristics of the syngas, dictated by the intended application.