Effects of Alkaline Metals on the Reactivity of the Carbon Structure after Partial Supercritical Water Gasification of Coal

Conversion of coal, biomass, and organic waste to combustible gases H2 and CH4 in supercritical water (SCW) is an economically and environmentally favored technology. Homogeneous alkaline catalysts are commonly added to achieve high conversion efficiency, as alkalis are believed to accelerate the gasification of aromatic structures, which is the bottleneck of the whole gasification process. Nevertheless, the mechanism of how alkali metals influence the reaction of aromatic structures is still not acknowledged clearly. In this work, different pretreatments of the high-alkali metal content Zhundong coal are taken as the object to study the effect of naturally present and artificial additive alkali metals on the microcrystalline carbon structure and their reactivity with H2O during the supercritical water gasification (SCWG) process. XRD analysis was applied to reveal that alkali metals inhibit the growth of graphite-like structures during SCWG. Subsequently, the separated smaller sized aromatic structures are indicated by calculating the Fukui function values of peripheral carbon atoms to exhibit higher reactivity with H2O and the free radicals in SCW. Finally, the existence of alkali metals is proven to promote the strong chemisorption of H2O on carbon structures, which is the premise of carbon abstraction of the aromatics, via thermogravimetric analysis. The influence of different reactors and SCW is also involved in the discussion. This work is prospective for optimal utilization of the homogeneous catalysts and developing innovative catalysts.