| Thermodynamic equilibrium models and a single-particle kinetic model have been developed and applied to the gasification of biomass materials in a downdraft gasifier. Given a set of operating and input conditions, the equilibrium models can predict the temperature, gas composition and char yield at the exits of the flaming-pyrolysis (FP) zone and of the gasifier. Based on the equilibrium models, parametric studies have been conducted to assess the influences of the important operating parameters, including the air/feed ratio, moisture/feed ratio and heat loss rate, on the performance of the FP zone and of the entire gasifier. The comparison of the model predictions with the experimental data from a commercial-scale downdraft air-blown gasifier fed with wood chips confirmed that the gasifier's overall performance can be predicted reasonably well by equilibrium modeling.; To facilitate the implementation of thermodynamic equilibrium models, two highly efficient calculation methods for the carbon-hydrogen-oxygen-inert (CHOI) system have been developed. The first, an analytic method, can calculate the equilibrium gas composition in a heterogeneous CHOI system, and the second, a single-variable numerical solution based on the first method, can determine the equilibrium gas composition in a general CHOI system, either heterogeneous or homogeneous.; The pyrolysis of a single large biomass particle in the flaming-pyrolysis zone of a downdraft gasifier was simulated by a kinetic model, featuring (1) first-order Arrhenius kinetics for the pyrolysis, (2) particle heat capacity and thermal conductivity dependent on the degree of pyrolysis, and (3) a thermal environment approximating that of the FP zone. A numerical solution method based on the Crank and Nicholson method has been developed for the model. Transient temperature and density profiles, and the rate of evolution of volatiles from the particle have been obtained from simulations. Parametric studies have indicated the trends of decreasing time for completion of flaming-pyrolysis with increasing flame temperature, with increasing convective heat transfer coefficient between the flame and the particle, with decreasing particle size and with increasing exothermicity of pyrolysis reaction; these trends are all in agreement with expectations. |
