Combustion in a bed of solid fuel
Figure 1 shows a conceptual view of the combustion of waste above a grate. The waste, which usually contains a significant amount of moisture, is pushed over the grate by a feeder-ram and so forms a bed. The bed of waste is then heated by radiation from both the gaseous flame above the bed and the furnace’s walls. This leads to the waste being dried and igniting on the top surface of the bed, where the waste is supplied with preheated air from under the grate. The air and the combustion gas move upwards, i.e., at right angles to the direction of the slow movement of the waste. The air and gas pass through the bed within a few seconds, while the solid waste stays ∼1 h in the bed. Figure 1 also gives some details of combustion in the bed. The waste sequentially experiences an initial heat-up, evaporation of moisture, pyrolysis, gas-phase combustion, and oxidation of the char, which is all typical of the combustion of a solid fuel. As the reaction progresses, the evaporation and combustion zones, initiated at the top surface of the bed, descend towards the bottom of the bed. When the combustion process is complete, the remaining ash is cooled by the air supply and is finally discharged into a hopper. The bed’s behavior shown in Fig. 1 can be modeled as a one-dimensional transient phenomenon. Because the gradients of, e.g., temperature and concentrations of chemical species in the direction of the movement of the bed are negligible compared to those in the direction of gas flow, heat and mass transfer in the direction of movement of the bed can be ignored. Hence the one-dimensional combustor shown in Fig. 2 was built; combustion tests on cubic particles of wood were performed to simulate combustion in a real bed.