Computational Fluid Dynamics Modeling of Biomass Gasification in Fixed-bed Reactors Using Highly Preheated Agent

University dissertation from Stockholm : KTH Royal Institute of Technology

Abstract: Biomass gasification is considered to be one of the most promising energy recovery technologies for the widespread utilization of biomass. Mathematical models have been developed to understand the gasification process inside gasifiers. As the oldest type of gasifier, fixed-bed gasifiers have been widely studied by using zero-dimensional and one-dimensional models; however, only a limited number of two-dimensional models for this type of reactor can be found in existing literature.The primary goal of this thesis is to develop a two-dimensional computational fluid dynamics (2-D CFD) model for fixed-bed gasifiers that considers the complex kinetic mechanisms, the flow field, and a series of chemical reactions inside gasifiers. The model was evaluated for both downdraft and updraft fixed-bed gasifiers through comparison with existing data from a demonstration-scale high temperature agent gasification (HTAG) system. The results demonstrated that this model can reasonably predict the performance of fixed-bed gasifiers when high-temperature air/steam is used as the gasifying agent. The performances of fixed-bed gasifiers were also discussed under the framework of HTAG technology. This CFD model is a potentially powerful tool for analyzing the performance sensitivity of a fixed-bed gasifier, which will further aid in the design and operation of this type of system.Biomass should be produced in an ecologically and economically sustainable manner. Therefore, the optimization of the gasification process was further studied to improve gasification performance and efficiency. A zero-dimensional kinetic-free model was introduced based on an updraft fixed-bed gasifier. A thermodynamic analysis was conducted based on the first and second laws of thermodynamics for various S/B ratios and preheating temperatures of the gasifying agent. A practical operating region for the HTAG process was proposed for industrial applications.According to the results, the HTAG technology relies on an external heat source and uses super-heated air combined with steam; this results in a limited need for feedstock combustion and produces syngas with a high H2 fraction and a low tar content. Based on energy and exergy efficiency analyses from a HTAG application, plasma melting of municipal solid waste (MSW), HTAG technology is demonstrated to be preferable from environmental and energy (exergy) efficiency perspectives.

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