Modelling of bubbling fluidised bed coal gasifiers

A previous numerical model of fluidised-bed coal gasifiers has been further improved to incorporate an overall energy balance. The improved model has been used to simulate the performance of bubbling fluidised-bed coal gasifiers of different scales. Simulations show that the predicted overall carbon conversion, operating bed temperature and concentrations of individual gas species compare well with the experimental data from three pilot-scale and a full-scale fluidised bed coal gasifier. The water–gas shift reaction, either driven by kinetics or in equilibrium in the dilute phase has significant effects on the predictions for the pilot-scale air-blown gasifiers but has little effect on a commercial-scale oxygen-blown gasifier. This is attributed to the much faster oxidation rate of H2 and CO near the distributor in the oxygen-blown commercialized gasifier than in the air-blown pilot-scale gasifiers. Results also illustrate that about 26–41% of feed oxygen is consumed in the homogeneous combustion reactions in the gasifiers simulated, the percentage of which increases with a decrease in coal rank and with an increase in operating pressure and temperature. Carbon conversions due to char gasification are significant when compared to those due to char combustion in the gasifiers simulated.

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