Modelling of biofuel combustion in fixed beds

Licentiate thesis, 2006

Fixed bed combustion is common is small to medium scale heat production in grate furnaces and domestic boilers. Modelling of these fuel beds can give a better understanding of the conversion process inside them, and provide a tool for analyzing the performance of furnaces and problems related to the fuel bed. The complexity of the task is however significant and there are at present no models considering all phenomenon related to a bed in a grate furnace. To give reasonable computational times the models depend on simplifications. One simplification often used is the neglect of gradients within individual fuel particles. The influence of this approximation has been investigated in this work. A bed model and a single particle model are presented. The bed model is a one-dimensional model describing the conversion of a batch of biofuels ignited at the top while combustion air is fed from below. All conversion stages are considered; drying, devolatilisation, heterogeneous and homogeneous reactions. The particle model is two-dimensional and describes heating, drying and devolatilisation of a particle inside the bed. The surface temperature, given by a simulation with the bed model, is used as boundary condition. The bed and particle model are linked to each other to account for intra-particle gradients in the conversion of the fuel bed. The linking is carried out in an iterative manner in which the bed and particle model are run in series. The bed model provides the boundary condition for the particle model which in turn gives input parameters to the bed model to reflect the impact of intra-particle gradients. The influence of these gradients on ignition rate, temperature profiles and release of moisture and volatiles is investigated.