Development and validation of a novel process model for fluidized bed combustion: Application for efficient combustion of low-grade coal

Numerous models have been developed in Aspen Plus for the combustion of different coal types in fluidized bed reactors. However, these models are case-specific, particularly with respect to coal type and bed reactor type, implying limitations to general application of these models. Moreover, these processes were generally developed step-wisely by employing a series of model blocks to simulate fluidized bed reactors in Aspen Plus. In this study, a novel hybrid approach for modelling coal combustion has been implemented to comprehensively design a model for conversion of low-grade coal under various operating conditions. The proposed model combines sequential modelling of drying/pyrolysis (devolatilization) and combustion of coal by means of conventionally used units (RYIELD and RGIBBS), and a newly used unit (FLUIDBED) in Aspen Plus. The model validation was performed by experiments on the combustion of low-grade coal in a pilot-scale circulating fluidized bed reactor (CFBR). Experimental data were used to further calibrate the Aspen Plus model and decrease model uncertainties. The results obtained from the developed simulation model were found to be in good agreement with the experimental data. Discrepancies of less than 15% were observed, in most of the predictions of molar fractions for the resultant flue gas composition, including NO_x and SO_x, emissions which were at ppm levels. As a result, the model can easily be used for design, scale-up, and simulation of coal combustion as well as for other feedstock like biomass in fluidized bed with process optimization based on sensitivity analysis.