Mathematical modelling of straw combustion in a moving bed combustor: A two dimensional approach

Abstract

Combustion of wheat straw in Serbia is a promising way of energy conversion. Straw is renewable and environmentally friendly energy source that can be burned in direct combustion process to provide electricity and heat. Despite its apparent simplicity, direct combustion is a complex process from a technological point of view.Conventional combustion equipment is not designed for burning agricultural residues. Devices for agricultural waste combustion are still in developing phase, which means that there is no good enough design solution on the world market yet. In order to construct a boiler and realise optimal combustion conditions, it is necessary to develop a mathematical model for biomass combustion.In this study, an original numerical model for the surface combustion of straw in a moving bed is proposed and relevant equations solved using original programme realised in C++, to simulate heterogeneous characteristics of the burning process. Two-dimensional mathematical model includes: moisture evaporation, straw pyrolysis, gas product combustion and char combustion.The model provides detailed information on burning processes in moving bed, which is otherwise very difficult to obtain by conventional experimental techniques. The model provides information concerning temperature front propagation, reaction front rate, as well as remaining gas species composition in bed.An important result of performed simulations is the time needed for the complete burnout of the moving bed. This time strongly depends on operating conditions, such as primary air flow, moving bed height and combustion chamber temperature. Total burnout time is decisive information for the design of grate furnaces, since it is needed for the estimation of grate length and size of the entire furnace. Thus, the presented model can be used to perform investigations of different furnace conditions and may be used for the optimisation of existing furnaces and development of new ones.Simulated moving bed profile, bed temperature as well as volatiles and char composition as a function of moving bed length using various air flows are shown in this work.The combustion model is validated by comparing numerical results with experimental and numerical results available in the literature, and there is a good agreement for different processes in a moving bed.