UBC Theses and Dissertations
Modeling of biomass steam gasification in a dual fluidized bed reactor with/without lime-based CO₂ capture Hejazi, Bijan
Lime-enhanced biomass gasification in a dual fluidized bed (DFB) reactor is a promising technology that allows enhanced hydrogen production from a renewable resource with simultaneous CO₂ capture via calcium looping. In this thesis, modeling Ca-looping in a DFB biomass gasifier is broken down into different steps. Firstly, a comprehensive single particle model is developed, based on conservation of mass, energy and momentum, with two different biomass pyrolysis kinetic schemes for particles of changing thermo-physical properties. Secondly, a coupled particle and reactor model of biomass drying and pyrolysis in a bubbling fluidized bed reactor is developed to predict the yields of pyrolysis products and composition as a function of process operating parameters. Thirdly, our coupled particle and reactor model is extended to steam gasification of biomass in a bubbling fluidized bed (BFB) gasifier, and its applicability is tested by comparing predictions with independent experimental data from the literature. For steam gasification of pine sawdust at a reactor temperature of 750°C, the H₂ mole fraction in the product gas increases with increasing steam-to-biomass ratio because of the water-gas, steam methane reforming and water-gas shift (WGS) reactions. Elevating the reactor temperature reverses the exothermic WGS reaction towards more CO production and CO₂ consumption. Fourthly, the BFB gasifier model is expanded into a generic two-phase fluidized bed reactor model to evaluate the performance of the UBC dual fluidized bed gasifier under steady-state operating conditions. Finally, integrated biomass gasification with cyclic CO₂ capture in a DFB reactor is simulated by developing a model which takes into account sorbent loss of reactivity due to sintering during cyclic operation. This comprehensive reactor model is developed and tested based on a stepwise approach. Unlike previous models, this is a predictive model that minimizes reliance on empirical correlations. By coupling single particle and reactor models, biomass drying, pyrolysis and gasification are studied as a continuous process. A gap of knowledge in predicting major compounds composition in pyrolysis gas is addressed. Furthermore, the kinetic model is capable of accommodating in situ CO₂ capture during cyclic operation.
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