Simulation Research on the Fixed-Bed Gasification Process in a Two-Stage Combined Gasifier

The novel two-stage gasification process that is combined entrained-flow for first stage and fixed-bed for second stage was shown in the study. The benefit of the second stage was to recycle the sensible heat from first stage syngas and enhance the energy utilization. The fixed-bed gasification reaction of second stage was individually investigated by numerical simulation. The simulated results were validated well with the experimental data in terms of gas concentration, gas flow rate, and carbon conversion. The distributions of velocity, temperature, gas concentration, solid mass fraction, and carbon conversion during the gasification process were analyzed. The influences of coal amount and particle size in the second stage on gasification reactivity were also discussed in the study. Results showed that the gas temperature was reduced and the effective gas concentration increased when first stage syngas flowing through the second stage coal layer. The increase of coal amount of second stage showed a more significant heat recovery, but the heating rate and ultimate carbon conversion would be reduced. The increase of particle size resulted in the decreasing of effective gas concentration and gas-solid reaction rate, which was attributed to the change of specific surface area and the increase heat transfer resistance from the particle surface to inside.