Analyses of an entrained-bed coal gasifier using a CFD model coupled with chemical reaction kinetics

A three-dimensional numerical model for the coal combustion and gasification of an entrained-bed gasification reactor has been developed by integrating the commercial computational fluid dynamic software FLUENT with the detailed kinetics modeling software CHEMKIN. The injection system of discrete-phase fuel is employed in the simulation model. The transport exchange of mass, momentum, and energy between different phases are taken into account in the numerical model. The physical models include the turbulence model, the radiation model, and the pyrolysis model. Nine homogeneous and heterogeneous reaction equations are considered for the coal combustion and gasification processes. The proposed numerical model, which accounts for the turbulent combustion by employing the mixture-fraction approach coupled with the flamelet model, has been validated by the experimental data. Four parameters, including the wall temperature, the feeding temperature, the water addition and blend with biomass, have been examined to investigate the effects of operating conditions on the gasification process. The numerical results obtained from the present study show that a lower wall temperature leads to more production of carbon monoxide and hydrogen. It also results in a lower outlet temperature. Increasing the feeding temperature leads to a better gasification performance and a slight increase of the outlet temperature. There exists an optimal rate for the water addition in enhancing the hydrogen production. Blending the coal with biomass may lead to higher hydrogen generation and lower carbon-monoxide production in comparison with the pure coal gasification. The resulted outlet temperature is lower. An entrained-bed gasification reactor installed with a multi-stage-feeding system is also examined. The numerical result shows that the gasification performance can be improved by introducing the second feeding stage, since the extent of the temperature rising at the second feeding stage is smaller than that at the first feeding stage.