Molecular dynamics investigation on supercritical water oxidation of a coal particle

Abstract

The supercritical water oxidation (SCWO) is a technology with very low environmental impact and significant economic benefits. In present paper, the micro-scale dynamics and the kinetic reaction mechanism of the SCWO of a coal particle is investigated by the reactive force-field (ReaxFF) method for the first time. Comparisons between experimental and analytical results on the activation energy show the coal particle model and the ReaxFF MD can successfully reproduce the SCWO process. Results on the atomistic scale conversion ways of O2, CO2, H or H2 and H2O show the SCWO is not simply the gasification plus the combustion. In SCWO, the direct combustion of the coal is weakened apparently due to the existence of the water, the oxidation reaction between the water or H radicals and the oxygen to generate OH plays an important role in coal conversion. The oxidation between carbon structures and OH to generate CO2 and H2O has substituted for the direct combustion reaction with the oxygen to be the dominant way in coal conversion. This process contains the characteristics of both combustion and gasification process. The role of water molecules is something similar to the catalysis. It is converted to OH radicals first in a series of gasification and oxidation processes, then the OH radicals turn into H2O in the following oxidation reactions. After that, the effect of reaction conditions and the transition of N on the SCWO process are studied. Comparing to the single supercritical water gasification, N2 and NO are found to be the extra products in SCWO. CN, CHN and CHON are the dominant N-containing products before they are converted to N2 and NO. The results in present work give insight into the oxidation mechanism of the coal particle, and the SCWO can effectively reduce the pollution emission in a much cleaner way compared with the conventional coal-fired mode.