Influence of intra-particle gradients in modelling of fixed bed combustion

Conference poster, 2006

The porous media approximation, neglecting intra-particle gradients, is extensively used in modelling of fixed bed combustion. The impact of this approximation has been investigated for a batch of biofuels ignited at the top, while combustion air is fed from below. The ignition is followed by a propagation of a reaction front downwards in the bed against the air flow. As the front passes, the fuel is heated, resulting in drying and devolatilisation. Volatile gases are ignited and together with the char formed they burn as long as there is oxygen available, providing heat for propagation of the front. In a bed, where an ignition front propagates counter-current to the air flow, a very steep temperature profile is created, in which the temperature rises from ambient to more than 1200 K. This means that there will be a significant difference in the surface temperature across the height of a single particle as the width of the reaction front in most cases is just a couple of particle diameters. The internal heating of a symmetric particle leads to a two-dimensional problem, since the temperature depends both on the distance from the surface of the particle and on the height in the bed. A bed model using the porous media approximation is implemented in this work. The fuel bed is assumed to consist of spherical fuel particles of the same size and the model treats the gas and solid phases with separate energy equations. Both drying and devolatilisation are modelled with Arrhenius expressions and char conversion is modelled with an effective reaction rate accounting for both diffusion and kinetics. The concentrations of gas species are described with their respective transport equation and the homogeneous reactions are taken as the minimum of a kinetic and a mixing rate. To close the system of equations the continuity equation for the gas phase is used. The result from the porous bed model has been compared with result from the same model, where the intra-particle gradients have been taken into account by a two-dimensional particle model, below referred to as the linked model. The particle model provides information on the internal heating of the solid particles and the internal rate of drying and devolatilisation. The surface temperature of the particles, given by the porous bed model, is the boundary condition for the particle model. The bed and particle model are run in series until a converged solution has been obtained. Fuel particles of sizes between 5-40 mm, consisting of wood, and inlet gas velocities ranging from 0.05 to 0.2 m/s are used. The results show that for a fuel bed consisting of large fuel particles (>2 cm) the porous media approximation clearly influences the release of volatiles and moisture in the bed, but the temperature profile is also affected. The reaction front is wider when internal particle gradients are considered, and this leads to a less steep temperature profile and a release of volatiles in a larger section of the bed height. If gradients in the particles are neglected, volatiles and moisture are released with a high rate in a narrow part of the bed and the temperature profile is steeper. For small particles, <0.5 cm, there is no significant difference between the linked and the porous bed model. The two models show the same trends for different inlet gas flows and moisture contents of the fuel. The widening of the reaction front, caused by consideration of gradients in the particles has, however, only a minor influence on the overall conversion rate of the fuel and on the maximum temperatures of gas and solids. The relative differences in these parameters between the porous bed model and the linked bed model are not more than 15%, which is within the range of uncertainty of model parameters and scatter in experimental data.