MODELING OF CARBON COMBUSTION EFFICIENCY IN CIRCULATING FLUIDIZED-BED COMBUSTORS .1. SELECTION OF SUBMODELS AND SENSITIVITY ANALYSIS

The application of a specific model to coal combustion in circulating fluidized beds presents the initial difficulty of having to choose between the different submodels proposed in the literature where some important discrepancies exist. Therefore, in this work a sensitivity analysis of the carbon combustion efficiency predictions to the different hypotheses, equations, and parameters defining the different submodels has been carried out. Thus, it has been found that the hypotheses related to the radial and axial solid distributions and the possible combustion of the descending char particles in the annulus exercise an important effect on the predictions when low-reactivity coals were considered, its importance being lower with high-reactivity coals. The same occurs when modifying parameters or equations referring to the reactivity of the char or to the mass transfer around particles. However, those corresponding to the devolatilization type have no significant effect on the efficiency predictions, On the other hand, however, it has been found that the hypotheses of the type of gas flow in the dense region has a medium sensitivity on the carbon combustion efficiencies.