Analysis of Operation Parameters in a Dual Fluidized Bed Biomass Gasifier Integrated with a Biomass Rotary Dryer: Development and Application of a System Model

An integrated system model was developed in UniSim Design for a dual fluidized bed (DFB) biomass gasifier and a rotary biomass dryer using a combination of user-defined and built-in unit operations. A quasi-equilibrium model was used for modelling biomass steam gasification in the DFB gasifier. The biomass drying was simulated with consideration of mass and energy balances, heat transfer, and dryer’s configuration. After validation using experimental data, the developed system model was applied to investigate: (1) the effects of gasification temperature and steam to biomass (<i>S</i>/<i>B</i>) ratio on the gasification performance; (2) the effect of air supplied to the fast fluidized bed (FFB) reactor and feed biomass moisture content on the integrated system performance, energy and exergy efficiencies. It was found that gasification temperature and <i>S</i>/<i>B</i> ratio have positive effects on the gasification yields; a H₂/CO ratio of 1.9 can be achieved at the gasification temperature of 850 °C with a <i>S/B</i> ratio of 1.2. Consumption of excessive fuel in the system at higher biomass feed moisture content can be compensated by the heat recovery such as steam generation while it has adverse impact on exergy efficiency of the system.