Micro-reaction Mechanism Study of the Biomass Thermal Conversion Process using Density Functional Theory

Abstract: Biomass, or bio-energy, is one of the most important alternative energies because of environmental concerns and the future shortage of fossil fuels. Multi-scaled bioenergy studies have been performed in the division of Energy and Furnace Technology, which included studies of macroscopic systems such as systems and reactors, modeling of computational fluid dynamics (CFD), and atomic/molecular level studies. The present thesis focus on the atomic/molecular level that based on quantum chemistry methods.The microscopic structure study of biomass is the first and an important step for the investigation of the biomass thermal conversion mechanism. Cellulose, hemicellulose, and lignin are the three most important components for biomass. The atomic interactions among these three main components were studied, including the hydrogen bond linkages between cellulose and hemicellulose, and the covalent bond linkages between hemicellulose and lignin.The decomposition of biomass is complicated and includes cellulose decomposition, hemicellulose decomposition, and lignin decomposition. As the main component of biomass, the mechanism of cellulose pyrolysis mechanism was focused on in this thesis. The study of this mechanism included an investigation of the pathways from cellulose to levoglucosan then to lower-molecular-weight species. Three different pathways were studied for the formation of levoglucosan from cellulose, and three different pathways were studied for the levoglucosan decomposition. The thermal properties for every reactant, intermediate, and product were obtained. The kinetics parameters (rate constant, pre-exponential factor, and activation energy) for every elementary step and pathway were calculated. For the formation of levoglucosan, the levoglucosan chain-end mechanism is the favored pathway due to the lower energy barrier; for the subsequent levoglucosan decomposition process, dehydration is a preferred first step and C-C bond scission is the most difficult pathway due to the strength of the C-C bonds.The biomass gasification process includes pyrolysis, char gasification, and a gas-phase reaction; Char gasification is considered to be the rate-controlling step because of its slower reaction rate. Char steam gasification can be described as the adsorption of steam on the char surface to form a surface complex, which may transfer to another surface complex, which then desorbs to give the gaseous products (CO and H2) and the solid product of the remaining char. The influences of several radicals (O, H, and OH) and molecules (H2 and O2) on steam adsorption were investigated. It was concluded that the reactivity order for these particles adsorbed onto both zigzag and armchair surfaces is O > H2 > H > OH > O2. For water adsorbs on both zigzag and armchair carbon surfaces, O and OH radicals accelerate water adsorption, but H, O2, and H2 have no significant influence on water adsorption.It was also shown that quantum chemistry (also known as molecular modeling) can be used to investigate the reaction mechanism of a macroscopic system. Detailed atomic/molecular descriptions can provide further understanding of the reaction process and possible products.