Modeling combustion of straw–bitumen pellets in a fluidized bed

Recently, straw–bitumen pellets have been proposed as an alternative fuel. In this paper, a comprehensive model for steady state combustion of straw–bitumen pellets in a bubbling fluidized bed is presented. The combustor is divided into three sectors: a dense bed zone, a splashing zone and a freeboard zone. Relevant phenomena including volatile release and segregation, char comminution and elutriation, bed particles ejection, and post-combustion above the bed have been taken into consideration. Energy equations for the splashing and freeboard zones have been established.Model results indicate that about 53% of volatile burns within the bed while 60% of total heat is released. On the other side, fraction of heat released in the splashing zone is about 33% whereas the remaining part (7%) is released in the freeboard zone. Ejected sand particles, however, recover back to the bed about 83% of the heat released in the two latter zones.Model calculations yield axial concentration profiles of different species along the whole length of the combustor. In particular, the model furnishes an axial temperature profile for the splashing and freeboard zones which is characterized by two maxima. A small peak occurs in the splashing zone whereas the second maximum develops in the freeboard zone with relatively much higher overheating above the bed temperature. A parametric study demonstrates that overheating in the freeboard zone may be controlled by increasing bed temperature and excess air, and decreasing fluidization velocity. Bed temperature among others is found to have the highest impact on the combustion performance. Model predictions are shown to be in good agreement with measurements.