(418i) Mathematical Modeling of the Physicochemical Evolution of Biomass Particles during Pyrolysis and Gasification

Gasification has been identified as an energy-efficient, environmentally-friendly and economically-feasible technology to partially oxidize biomass into a gaseous mixture of syngas consisting of H₂, CO, CH₄ and CO₂. High-quality syngas can be further used to catalytically synthesize liquid fuels and produce hydrogen. Multi-scale physical and chemical changes at both particle and reactor levels occur during biomass pyrolysis and gasification. At a biomass particle level, the intra-particle reactions, heat and mass transfer, and structural evolution occur during pyrolysis and gasification when the volatiles are released. The yield and composition of the products from a reactor depend on these physicochemical changes of the biomass particles. This research is to develop a transient mathematical model to describe the physical and chemical changes of biomass particles during pyrolysis and gasification at different reaction environments. The mathematical model consists of four sub-models to describe heat transfer, mass transfer, reactions and structural evolution, respectively. A heat transfer sub-model is used to determine the time-temperature profile of a biomass particle with varying physicochemical properties under different reaction environments. A temperature-dependent reaction kinetics sub-model is then used to determine the volatilization rates and predict the dynamic elemental composition of the particle. A mass transfer sub-model is used to describe the intra-particle transport of the volatiles. A structural evolution sub-model is used to predict the changes of porosity, surface area and particle size of the biomass particle that are caused by the volatilization during pyrolysis and gasification and significantly affect the heat and mass transfer of the particle. The model will be validated by the experimental data of dynamic mass, elemental composition and physical properties of biomass particles during pyrolysis and gasification. The dynamic mass of the particles during pyrolysis and gasification are measured by a thermogravimeter (TGA). The elemental compositions of the biomass samples collected at different stages of pyrolysis and gasification are analyzed by an elemental analyser. The porous structure of the biomass particles including pore size, pore volume and surface area is studied by using a Micromeritics ASAP 2020 analyzer. Direct observation of the porous structure is performed using a Carl Zeiss Auriga-BU FIB FESEM microscope and Carl Zeiss Libra 120 plus TEM microscope.