Abstract
Recently straw-bitumen-pellets have been proposed as an alternative fuel. This work presents a mathematical model for steady state combustion of straw-bitumen-pellets in a bubbling fluidized bed. The combustor is divided into three zones; dense bed, splashing zone and freeboard. Important processes including volatile segregation, char comminution and elutriation, bed particles ejection; and post-combustion in splashing zone and freeboard have been considered and simplified. Submodels for hydrodynamic, volatile release and combustion, and char consumption have been implemented. Energy balance for splashing zone and freeboard has been set to predict axial temperature profile. Model results demonstrate that about 53% of volatiles combustion and about 62% of the total heat release take place within the bed. The fraction of heat released in the splashing zone is about 33% whereas the remainder portion (7%) releases in freeboard beyond the splashing zone. The ejected sand particles; however, recover back to the bed about 83% of heat released in the two latter zones. The model yields the axial profiles of different species concentrations in the two bed phases (bubble and emulsion), in the splashing zone and in the freeboard. Moreover, the model predicts the axial temperature profile in the splashing and the freeboard zones that characterizes by two maxima. A small peak is built in the splashing zone with a small overheating as the ejected sand particles recover the great part of released heat. Another maximum arises in the freeboard where the flux of the ejected particles turns out to be very few and its impact on gas temperature becomes insignificant. The second maximum has relatively much higher overheating temperature. The influences of operating variables on the combustion performance have been evaluated as well. In particular, the maximum overheating in freeboard gets higher with decreasing excess air factor, with lowering bed temperature and with increasing fluidization velocity. Bed temperature among the others has the highest impact on combustion performance. An acceptable agreement are found between the predicted and the measured concentrations and temperature profiles.
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