Effect of Local Hydrodynamics on the Performance of a Fluidized-Bed Gasifier

Fluidized beds are widely used for chemical processing and power generation processes. It is important to understand how the local hydrodynamics and bubbling characteristics influence the spatial distributions of gas and solid phases, heat and mass transfer, reaction rates, and ultimately the performance of the fluidized-bed reactors. In the present work, we simulated the coal gasification process in a fluidized-bed gasifier and analyzed the differences in local bubbling behaviors caused by the use of different models for gas-inert solid phase (i.e., sand) momentum exchange (beta(gas-inert)). We investigated the effect of the local bubbling behavior on the spatial distributions of coal and inert solid phases, heterogeneous reaction rates, temperatures, species distributions of coal and gas phases, and eventually on the syngas quality. In a case with higher/variable reactor temperature, we show that the segregation of the less dense coal phase in the top (due to low beta(gas-inert)) led to a high char combustion rate locally and low CO2 and H2O gasification rates. However, the cases with a higher magnitude for beta(gas-inert) resulted in a well-mixed state of inert and coal phases and led to homogeneity in the heterogeneous reaction rates, temperature, and species distributions of coal and gas phases. Overall, a well-mixed bed resulted in higher gasification efficiency, gas yield, and carbon conversion rate than the segregated bed. Further, the gasification efficiency and gas yield are found to be higher for the variable temperature reactor than the constant temperature reactor due to low steam and CO2 gasification rates in the latter case. The predictions agreed well with the corresponding measurements reported in the literature for different coal mass flow rates.