Reverse combustion: Kinetically controlled and mass transfer controlled conversion front structures

Under most conditions for reverse fixed bed combustion, transport effects of heat and gaseous species to and within fuel bed particles are important phenomena that limit the conversion rate. At present there is only one model that (1) claims to take transport limitations into account and (2) for which analytical solutions are given. Here, we present a numerical study of reverse combustion analysis on the basis of this model to show the effect of transport limitations on the reaction front structure and the consequences of these effects for the applicability and accuracy of the analytical solution procedure. A parameter set for coal combustion was used to perform the study. Results of numerical simulations indicate that the model solutions contain two limiting cases: kinetically controlled and mass-transfer-controlled conversion front structures. The kinetically controlled solutions consist of a preheat zone at the upstream side of the front and a thin reaction zone at the downstream side. The mass-transfer-controlled solutions consist of a wide reaction zone with a maximum source term situated at the upstream part of the front. The analytical solution to the model equations is shown to give correct predictions in the kinetically controlled case only. In the presence of mass transfer, the solution predicts the trend of decreasing front velocity with increasing transport limitations correctly but does not show the correct functional dependence on the parameter describing transport limitations. In addition, the predictions become highly inaccurate. This is related to the fact that the effect of transport limitations is not accounted for in the analytical solution method. Therefore, numerical methods should be used to obtain generally valid solutions.