Numerical simulation of microwave processing of biomass materials

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University of New Brunswick
Microwave heating has attracted considerable attention during the past decade in various fields especially in biomass material processing. It offers several advantages such as rapid, volumetric and selective heating, overall higher efficiency, and easier temperature control; however, there are important issues to be understood in order to allow scale-up for higher production of renewable fuels from biomass materials. A Computational Fluid Dynamics (CFD) model of biomass microwave heating aids to understand the temperature distribution in microwave cavities. One way to address scale-up is to use CFD which allows for validation of temperature distribution at modeled scale and subsequently predicting temperature distributions, and associated energy consumption, at pilot scale. As temperature and power requirement are vital factors in making microwave processing viable, a useful CFD tool that provides this information could be invaluable for industry. The main goal of this research work is to investigate the heating behavior of biomass materials under microwave processing. A detailed CFD model is developed based on the finite volume method to describe the heat and mass transfer during the microwave processing of biomass pellets. This study presents a modeling approach for incorporating the basic fundamentals of microwave pyrolysis process in the form of source terms for mass, momentum, heat, and species into the general transport equations for nitrogen and volatiles in the gas phase and wood and bio-char in the solid phase. The model covers the complex coupling between several key elements of the process including microwave heating, pyrolysis kinetics, phase change, rapid variation in mixture properties and gas phase transport. The developed CFD model is validated through experimental trials in a custom-built microwave pyrolysis unit. The model predicts the maximum temperature, temperature rates, and temperature profiles during the process. Close agreement is obtained between the results obtained from the experiments and simulations. The results indicate that increase in microwave power level increases the maximum obtained temperature; however, the amount of absorbed power within the material decreases significantly at higher temperature levels. This insight provides an opportunity to improve the biofuel production and process condition in scale-up scenarios.