Effect of kaolin on ash partitioning and slagging for the combustion of biomass fuels in a field-scale 250 kW grate boiler

Ash-related issues such as slagging and ash deposition in the heat transfer region of the grate boiler have been considered serious problems for safe and efficient operation. These issues must be addressed to avoid any unplanned shutdown due to failure in controlling the agglomeration and slagging during combustion. Various methods have been proposed, such as the injection of chemical additives into the grate boiler to reduce the alkali species in the bottom ash and depositions in the heat transfer region during combustion. However, the impacts of the boiler condition especially the bed conditions of the grate boiler have not been understood well in the past when adding the kaolin. Therefore, the aim of this work is to investigate, both experimentally and theoretically, the effects of blending biomass fuels with kaolin on ash-related issues in a 250 kW field-scale grate boiler. It was found that the kaolin powder performed quite effectively as an absorbent for potassium under firing conditions. The deposition propensity of fly ash was found to reduce by at least 50 % after the addition of the kaolin to the fuel mixtures. However, an increase in the degree of agglomeration was observed in the combustion chamber of the grate boiler when the kaolin was added to the fuel mixtures. High sintering was observed for the virgin wood and the recycled wood fuel blended with the kaolin at any dosages in the grate boiler. According to the elemental analysis by ICP-MS, the most dominant element found in the bottom ash, slag, and coarse fly ash was silica which captured potassium-related species and promoted the formation of slags. This also can be confirmed by the X-ray diffraction (XRD) analysis which shows that a high concentration of silica in crystalline structures were observed in both fuel mixtures. Moreover, according to the XRD analysis and thermodynamic equilibrium model prediction, adding kaolin to the feedstocks significantly influences the crystalline structure formation in the bottom ash such as the formation of Kalsilite, Leucite, Sanidine and many more.