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Supercritical Water Oxidation vs Supercritical Water Gasification: Which Process Is Better for Explosive Wastewater Treatment?

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journal contribution
posted on 04.02.2015, 00:00 by Jinli Zhang, Jintao Gu, You Han, Wei Li, Zhongxue Gan, Junjie Gu
2,4,6-Trinitrotoluene (TNT), as a representative component of explosive wastewater, is treated in supercritical water gasification (SCWG) and supercritical water oxidation (SCWO) using molecular dynamic simulations based on ReaxFF reactive force field as well as density functional theory (DFT). The detailed reaction processes, important intermediates and products distribution, and kinetic behaviors of SCWG and SCWO systems have been analyzed at the atomistic level. For the SCWG system, TNT is activated by water cluster or H radical and the N atom is mainly converted into NH3 more than N2 through two significant intermediates NOH and C–N fragment. In addition to water cluster and H radical, the TNT is activated by O2 in the SCWO system. Besides, the N atom is transferred into N2 more than other N-containing products after 750 ps simulation. Combined with the calculated cracking energy of the bonds in TNT, SCWG can accelerate its degradation and is easier for C–N bond breaking or changing through other reactions because of its low cracking energy (69.6 kcal/mol in thermal decomposition and 59.0 kcal/mol in SCWG). In addition, a large amount of H2 molecules is produced in SCWG, which is a meaningful way of transforming waste to assets. On TNT degradation, SCWO with inadequate O2 that can be treated as partial oxidation reaction (SCWPO) can combine the advantages of SCWG and SCWO (with enough O2) to convert TNT into CO2, H2O, as well as H2 and NH3 with high economic value. Finally, a kinetic description is performed whose activation energies (17.6 and 18.4 kcal/mol) are theoretically consistent with experimental measurements.