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A simple kinetic model is developed for biomass gasification in a bubbling fluidized bed (BFB) with steam as the fluidizing gas. The biomass pyrolysis is described by a two-step kinetic model in which the primary pyrolysis is modeled by three parallel first-order reactions producing noncondensable gas, tar (bio-oil), and char, and the secondary pyrolysis is modeled by a first-order reaction representing homogeneous thermal cracking of tar. In addition to the yields of pyrolysis products that are often modeled as lumped species, the proportions of major compounds in the pyrolysis gas are predicted based on CHO elemental balances. By incorporating homogeneous and heterogeneous biomass gasification reactions, a seamless kinetic model of a BFB gasifier is developed. An ideal reactor model is used for the BFB gasifier assuming perfectly mixed solids and plug flow of the gas phase. This predictive model is a useful tool to relate biomass gasification product yields and composition to key process operating parameters such as biomass ultimate analysis, reactor temperature, and steam-to-biomass ratio. Predictions of the gasifier model are in good agreement with experimental data from the literature.

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Sorption-enhanced steam reforming, assisted by membrane separation of H2 in a fluidized bed reactor, is simulated numerically based on a kinetic two-phase model. A residence time distribution function method is implemented to account for CO2 capture in continuous operation. The effects of operating pressure, total gas feed rate, solid recycle rate, fresh sorbent feed rate, effective membrane area, and permeate pressure on the performance of a continuous fluidized bed reactor are investigated. A CH4 conversion of >91% for operation at 0.6 MPa and 550 °C is predicted to be possible with the assistance of the sorbent and membranes. The reforming performance is very sensitive to the effective surface area of membranes. A sorbent fraction of >0.7 (by mass) is necessary to achieve a product with H2 selectivity of >98%, free of CO and CO2, for realistic membrane effectiveness factors. Adding fresh sorbent or increasing the sorbent mass fraction improves the H2 productivity for a moderate solids recycling rate. Effective CO2 capture rate depends greatly on the sorbent feed rate.