Influence of Water on Nitrous Oxide Adsorption and Desorption on Coals

Abstract

Oxygen-enriched coal-combustion flue gas geologic sequestration in unmineable coal seams is a promising option to mitigate gaseous pollutant emissions and recover coalbed methane (CH4). Water (H2O) always exists in practical coal seams and shows significant impact on fluid adsorption, desorption, diffusion, and flow behaviors. Thus, this study mainly addressed the influence of H2O on nitrous oxide (N2O) adsorption and desorption, a typical component that existed in oxygen-enriched coal-combustion flue gas, on different rank coals. Results indicate that the N2O adsorption equilibrium and kinetic behaviors within moist coals can be well-predicted using the Sips model and the simplified bidisperse kinetic model, respectively. The H2O dramatically hinders the adsorption and diffusion capability of N2O within coals, which is attributed to the fact that H2O molecules occupy pore channels, induce coal matrix swelling, and compete with N2O molecules for the same adsorption sites, particularly the oxygen-containing functional groups. The H2O impairs the physisorption and chemisorption of N2O on coals. Furthermore, the H2O enhances the chemical transformation among the four main nitrogen-containing species within coals, that is, pyridine-N, pyrrole/pyridone-N, quaternary-N, and oxide-N. Particularly, the abundant −H groups of H2O molecules inhibit the decomposition of pyrrole/pyridone-N and accelerate the transformation of pyridine-N, while −OH groups of H2O molecules promote the formation of oxide-N. To sum up, the practical N2O geologic sequestration should focus on the effects of in-situ H2O within coal reservoirs.