Molecular dynamic investigation on nitrogen migration during hydrogen production by indole gasification in supercritical water

Hydrogen production by supercritical water gasification of biomass has broad application prospects. The migration and regulation mechanism of nitrogen in supercritical water is one of the key issues in biomass utilization. As a nitrogen-containing model compound of biomass, indole was used to study the mechanism of nitrogen migration during supercritical water gasification from a molecular perspective by using reactive empirical force fields (Reaxff) combined with Density functional theory (DFT) method. The pyrrole ring cracking was the first stage and then most of the nitrogen atoms left the carbon skeleton to form ammonia precursor radicals after being attacked by H or OH free radicals in supercritical water. The overall activation energy of each pyrrole ring opening way was calculated. The dominant way for pyrrole ring opening was analyzed and the competitive relationship among the different ring-opening ways was specified by comparing the activation energies of transition states and relative energies of intermediates. Compared with pyrolysis conditions, supercritical water molecules greatly promoted the separation of nitrogen atoms from the carbon skeleton to form NH₃ and inhibited the formation of HCN. The higher temperature could facilitate the indole conversion and the hydrogen generation, but it also increased the possibility of char formation, causing part of the nitrogen to re-enter the organic structure. This study revealed the mechanism from the view of microscopic atoms, and provided theoretical support for the nitrogen regulation in supercritical water gasification of biomass.