Numerical modeling of small-scale biomass straw gasifier

A 3-D numerical model of a two-stage 900-kWth gasifier built by Vidir Biomass, Manitoba using the computational fluid dynamic code Fluent 6.2 was developed to predict the details of the flow, gasification and thermal gradients within this small-scale straw gasifier. This gasifier is unique in that it uses large round 1000 kg bales as the fuel and precipitates the silica in the secondary chamber to avoid fouling of the convection section. The geometry and mesh of the gasifier were generated using GAMBITRTM 2.4, a 3-D solid modeling function provided with Fluent. Boundary conditions during the operation of a two-stage gasifier were implemented in the numerical model. The flow field is assumed to be a steady-state, turbulent, reacting continuum field that could be described locally by general conservation equations. The governing equations for gas-phase fluid momentum, heat transfer, thermal radiation, and particle-phase transport were solved using the finite difference method implemented in Fluent. All materials including gas species and solid biomass particles were assigned appropriate properties. The properties of the gas species including density, viscosity, thermal conductivity, and specific heat capacity vary with the local gas phase temperature. The ideal gas law for density and the mass-weighted mixing law for viscosity, thermal conductivity and heat capacity were used to model the local mixture properties. Gas-phase reactions were assumed to be limited by mixing rates as opposed to chemical kinetic rates. Gaseous reactions were calculated assuming local instantaneous equilibrium. The straw fuel bed was modeled as flow through a porous media. Once the appropriate boundary conditions of the gasifier were developed and applied to the model, the flow pattern, distribution of temperature and gas composition in the gasifier was predicted throughout the primary and secondary chamber of the gasifier. A 1-D equilibrium model was also used to model straw gasification with the biomass fuel represented by the chemical formula, CHaOb. A steady state operation, thermodynamic equilibrium, and complete conversion of the solid bio-fuel to gas were assumed in the equilibrium model. This model was used to compare to the 3-D gasification model for validation. The 3-D base case was also validated using the gasifier, including gas-phase measurements. A stoichiometric model using the mass and energy balance was also developed to verify the syngas compositions predicted by either the equilibrium model or the 3-D model to ensure mass and energy balance.;Process parameters such as moisture content, porosity, bed height, excess air ratio and composition of biomass on the gasifier were then investigated to find an optimal controller. The simulations have proved to be useful to designers who are using the model to optimise the air system design. Of importance is to use the model results to develop an appropriate primary and secondary air control to react to changes in fuel composition and moisture content. The results show that maintaining an appropriate primary to secondary air ratio is critical to the operation of the gasifier as the pressure drop through the porous bed varies as the fuel is being gasified.;Then 3-D numerical results were compared to the 1-D model, and experimental data obtained using a 900-kWth gasifier indicated good agreement with the 1-D model and experimental data.