The effect of supercritical water on coal pyrolysis and hydrogen production: A combined ReaxFF and DFT study

The reaction mechanism of coal pyrolysis and hydrogen production in supercritical water (SCW) was investigated using the molecular dynamic simulations via the reactive force field (ReaxFF) method combined with the density functional theory (DFT) method. Our calculations present that the water clusters in SCW weaken the C–C bonds in aromatic rings, thus the C(ring)–C(ring) bond cracking energy decreases as much as 287.3 kJ/mol and 94.6 kJ/mol compared with that in pure coal pyrolysis and in coal pyrolysis in vapor state, respectively. After the aromatic rings break into small cyclic structures, such as quaternary rings and ternary rings, the water clusters in SCW further weaken their C–C ring bonds to induce the small cyclic rings to open. During this process, the water clusters (without any radicals) in SCW turn into H radical-rich water clusters after providing OH radicals to the cyclic rings. This is the main source for the production of hydrogen molecules in SCW–coal system. The combination of H radicals produced by coal with water clusters in SCW is another pathway which forms H radical-rich water clusters. Under the catalysis of water molecules or clusters, H radical-rich water clusters decompose into H2 and OH radicals. These OH radicals further bind with coal intermediates and result in the breaking of coal intermediates into smaller products. Therefore, the cooperative effects between SCW and coal form a virtuous circle, which greatly enhances the reaction rate of coal gasification, promotes the production of small molecules, and increases the yield of hydrogen.