Investigation of 3D flow and heat transfer in solid-fuel grate combustion: Measures to reduce high-temperature degradation

Undue deterioration of grate material can lead to increased maintenance costs in solid fuel furnaces. Excessive grate temperatures and reducing conditions at the grate surface are conditions that increase the probability of material degradation. To investigate measures to avoid these conditions, and thus reduce grate degradation, an unsteady, three-dimensional numerical model is employed to predict char combustion near the surface of a grate for a variety of scenarios. The parameters studied include the inlet air flux, the thickness of the ash layer on the grate, and the shape of the grate itself; both the size of the passages through the grate and the spacing between them. Predictions indicate that reducing the inlet air flux can decrease the peak temperature in the grate. However, it should be noted that overall furnace operation is sensitive to the inlet air flux, particularly considering NO_X emissions, which limits its use as a controlling factor. The presence of an ash layer on the grate surface has a beneficial effect; it promotes the dispersion of air across the grate surface, hindering reducing conditions while increasing the mean conversion rate near the grate. Increasing the spacing between passages, i.e. reducing the porosity of the grate, causes the peak temperature of the grate to increase. Furthermore, reducing conditions occur when the distance between passages exceeds 30 mm. Decreasing the size of the passages through the grate reduced the peak temperatures experienced by the grate for all scenarios examined. Smaller passages are expected to improve furnace performance in all scenarios, and the minimum passage size should therefore be limited only by the ash-clogging propensity of the target application.