Disturbances in Fixed-Bed Combustion

Licentiate thesis, 2007

Conversion of solid fuels in fixed beds is a common method for heat and power production. A frequent fixed-bed technique in small to medium scale is combustion in grate furnaces. Grate furnaces are widely used in the industrial energy branch and appreciated for their stability and simplicity of control. Also, the fuel flexibility makes the method economically attractive as an energy supplier to district heating systems. With increasing demands on combustion efficiency and fuel flexibility, it has been observed, though, that the fuel bed in grate furnaces often suffers from disturbances causing problems in the fuel bed and in the free-board above it, such as elevated emissions and increased material wear. To reduce these problems it is important to gain a deeper understanding of the processes in the fuel bed, both by practical investigations and modelling. An inventory of disturbances in the fuel bed of grate furnaces was carried out, in which interviews with operating personnel and recordings of fuel beds with a video camera, was made. The results show that disturbances in the fuel bed are very common. Most frequent is channelling in the fuel bed and at the bounding walls, which occur naturally at walls of packed beds and in connection with local changes in bed porosity. The inventory shows that a range of practical measures in the operation of grate furnaces amplify this phenomenon. Modelling of combustion in the free-board of a grate furnace and in the fixed fuel bed was performed. The free-board modelling was carried out with a numerical software, using a simplified bed model to describe the inlet conditions from the fuel bed. The results clearly emphasise the practicability of such a combination and effects that are difficult to foresee by measurements and observations are highlighted. A zone in the furnace that is poorly exploited by the combustion process was revealed by the computation, and an alternative design was evaluated. The first steps in creating a bed model that describes the processes in a burning bed of char were also taken. The model was implemented into a commercial software and enriched with model-specific functions. Emphasis was put on describing a two-dimensional flow, including spatial variations in flow conditions and their influence on bed porosity and shrinkage, which enables modelling of channel formation in fixed beds.