ReaxFF simulation of the effect of CO₂/H₂O atmosphere on char conversion during pressurized oxy-fuel combustion process

Pressurized oxy-fuel combustion technology, regarded as one of the main directions for CO₂ capture in coal-fired power plants in the future, has been widely studied by academics in recent years. Among these studies, the wet-cycle pressurized oxy-fuel combustion process, in which the recirculating flue gas contains H₂O, has been considered to have better economic potential. The effect of a CO₂/H₂O atmosphere on the pressurized oxy-fuel combustion of char fragments in the pressure range of 4-10 MPa was investigated using reactive molecular dynamics (ReaxFF MD). The contributions of oxidation, CO₂ gasification, and H₂O gasification to char conversion were quantified through an atomic labeling method, and the mechanisms by which pressure and the CO₂/H₂O atmosphere influence char conversion were explored. It was shown that the increase in H₂O inhibited char conversion, resulting in a decrease in carbon conversion by approximately 10% at lower pressures and 1% at higher pressures, while the increase in CO₂ facilitated char conversion, leading to an increase in carbon conversion by approximately 6%-9% with CO₂ across the range of pressures studied under the combustion conditions of the mixed char/O₂/CO₂/H₂O system. The carbon conversion of char was found to increase by 2%-14% with increasing pressure, with the enhancement becoming more pronounced with increasing H₂O and decreasing CO₂. It was observed that pressurization increased the contribution of gasification to char consumption but decreased the contribution of oxidation. This phenomenon was most pronounced in the 30%-CO₂ atmosphere, where carbon conversion was enhanced by 14% and the contribution of oxidation was decreased by 3% with increasing pressure, while the contribution of gasification was elevated by 17%. A competitive relationship was identified between oxidation, CO₂ gasification, and H₂O gasification. At 4 MPa and 40%-H₂O/60%-CO₂, as H₂O concentration increased, the contribution of oxidation decreased by 22.4%, and the contribution of CO₂ gasification decreased by 5.5%. Conversely, as CO₂ concentration increased, the contribution of oxidation decreased by 10%, and the contribution of H₂O gasification decreased by 12%. The competition between oxidation and gasification was reflected in the competition between gasification products and char for O₂, while the competition among gasifications was reflected in the competition for active sites.