Thermal radiation heat transfer and biomass combustion in a large-scale fixed bed boiler

The main focus of this work is to investigate the performance of some simple radiation models used in the thermal radiative heat transfer calculations in a 55 MWe fixed bed boiler with wet wood-chips as the fuel. An optically thin approach, Rosseland approximation, and the P₁-approximation were utilised in the investigation. A new optimised version, as it comes to computational speed, of the exponential wide band model (EWBM) is used. The initial calculations showed that the optically thick approach failed, The optically thin approach actually gave the best prediction of the temperature, if the mean beam length (L_m) was chosen carefully. The P ₁-approximation gave less good predictions than the best optical thin case, but it could be the best engineering model to use if little was known about the mean beam length. The conclusion is that the optically thin model is sensitive to the chosen mean beam length (L_m) used in the EWBM. The P₁-approximation is almost insensitive to the choice of L _m, due to the consideration of radiation self-absorption, where the different predicted values of the incident radiation compensate for different values of L_m. For the same reason, the use of other solution techniques, such as DOM or DTM, may lead to the same conclusion, i.e., the insensitivity of the choice of L_m.