Experiments on ReaxFF molecular dynamics simulation of O₂/H₂O combustion of Zhundong demineralized char

The oxy-coal combustion steam system of near-zero emissions efficiently uses coal resources and reduces CO₂ emissions. It uses demineralized coal as fuel that reacts with pure oxygen and steam within a high-pressure combustion chamber. Demineralization modifies the chemical structure of coal and changes its reactivity. However, the evolution of abundant short-lived free radicals during char combustion cannot be easily observed and detected via experiments.

The combustion mechanism of the Zhundong demineralized char (ZDDChar) was comprehensively analyzed herein. ¹³C solid-state nuclear magnetic resonance (¹³C-NMR) was used to chemically characterize the carbon molecular structure of ZDDChar and develop its macromolecular structure model. Combustion microreaction of ZDDChar was simulated under high steam concentration, oxygen concentration, temperature, and pressure via the molecular dynamics simulation of the reaction force field molecular dynamics (ReaxFF MD). As a result, the O₂/H₂O combustion mechanism of ZDDChar was elucidated at the molecular level, thereby addressing the experimental limitations.

Results revealed that ZDDChar has a molecular formula of C₈₀H₂₈O₁₆N. Its macromolecular structural model was characterized by the predominance of thick cyclic aromatic hydrocarbons, with oxygen-containing functional groups primarily in the form of hydroxyl, carboxyl, carbonyl, and pyran groups. Contrary to the conventional ZDDChar–air combustion, O₂/H₂O combustion was characterized by C+H₂O and C+O₂ reactions that competed with each other. High concentrations of H ions and OH radicals promoted the C+H₂O gasification reaction in a positive direction. The competition between O, H, and OH radicals for reactive sites within ZDDChar varied under different atmospheres. High pressures considerably promoted the C+O₂ reaction compared with the C+H₂O reaction. In addition, the combustion of char was enhanced at high pressures, and the combustion reaction rates increased at high temperatures. The depth of the char burning degree was directly proportional to the duration of combustion. The combustion reaction of ZDDChar under the aforementioned four conditions was initiated by the rupture of a side-branch chain and the decomposition of an unstable aliphatic ring, followed by the ring-opening and ring-breaking of thick ring aromatic hydrocarbons with larger bond energy into small molecular fragments. The small molecule fragments interacted with the surrounding reactive free radicals to form stable combustion products.

The ReaxFF MD simulation of O₂/H₂O combustion of ZDDChar offered unique insights into the combustion of ZDDChar at the microscopic level. In addition, successive bond-breaking and bond-forming processes during char combustion could be observed via this simulation, and the evolution of combustion intermediate products could be elucidated. It also provides a foundation for students to develop a comprehensive understanding of the coal combustion mechanism and offers a theoretical basis for the development and application of subsequent combustion technologies.