CFD Modeling of Biomass Thermal Conversion in A Novel Solar-Thermal Reactor

Abstract

The excessive utilization of fossil fuels during the past five decades resulted in a gradualdepletion of reserves and caused a serious negative impact on the environment, such as air pollution, climate change and acid rain. To sustain and meet the world’s energy demand, bioenergy is being extensively researched for sustainable production of clean and renewable fuels. Biomass pyrolysis, which is one of the main bioenergy technologies, is a highly endothermic process, where the primary products are a pyrolytic liquid phase (bio-oil), a solid phase (bio-char), and a gas (non-condensable gases). To minimize the energy demand for biomass pyrolysis, a hybrid system combining solar-thermal with biomass is a promising and cost-effective option for large-scale implementation. This thesis focuses on the thermochemical conversion of biomass via fast pyrolysis for biofuels (liquid bio-oil and gas) and biochar production in a novel solar-thermal conversion reactor. The study is purely theoretical and mainly involves developing and solving a Computational Fluid Dynamic (CFD) model using ANSYS Fluent software package, to predict the flow hydrodynamics, heat transfer, reactions, and products of pyrolysis. A flow hydrodynamic model (solid/gas velocity, pressure, and solid distribution) was firstly created and solved using the Eulerian- Eulerian approach, with constitutive equations based on Kinetic Theory of Granular Flow (KTGF). This was followed by a sensitivity analysis to assess the effect of meshing, drag models and turbulence on the model predictive capabilities. Consequently, a reliable hydrodynamic model was endorsed and validated using preexisting experimental data obtained at the AUS labs using an advanced Particle Image Velocimetry (PIV) measuring technique. The validated model was then upgraded by incorporating heat transfer, drying and devolatilization of the pyrolysis reaction equations, to envisage the performance of the proposed novel reactor design. The predicted bio-oil, bio-char and non-condensable gases were found to be qualitatively and quantitatively valid, thus lending strong support to the feasibility of the proposed concept. It is hoped that the model will be a cornerstone for future development and scale-up of the proposed novel solar-thermal reactor.