Quantum Chemical Study on the Evolution of Sulfur Functional Groups during Char Burnout

The oxy-fuel combustion of biochar connected with carbon capture, storage, and utilization technologies is an environmentally beneficial alternative for the replacement of fossil fuels. Biochar itself consists of porously stacked layers of hydrocarbons containing several heteroatoms, such as oxygen, nitrogen, and sulfur. At present, only limited information on the combustion mechanisms for oxygen and nitrogen functionalities is available in the literature; specific information on the combustion mechanisms of sulfur-containing groups (SFGs) is lacking. In this study, we present electronic structure calculations to uncover the mechanisms of the initial oxidation reactions of SFGs. Furthermore, it is examined if the reaction mechanisms remain similar or change with increasing system size. For this purpose, we apply an automatized workflow combining reactive molecular dynamics simulations with static electronic structure calculations at different levels of theory. The results show that terminal groups such as thiols, sulfonic acids, thioketones, and S,S-dioxides follow similar reaction pathways. These SFGs are all gradually oxidized before they eventually are eliminated as SO(H) species from the carbon framework. Embedded thiophenes follow somewhat different reaction pathways that lead to the elimination of HOS· radicals or carbonyl sulfide (COS), depending on the system size. For the found oxidation channels, we report reaction and activation energies and rate constants that can be used to improve comprehensive kinetic models for the combustion of sulfur-containing biochar as a biomass-based renewable energy source.