Evaluation of bottom ash slagging risk during combustion of herbaceous and woody biomass fuels in a small-scale boiler by principal component analysis

In the short- and mid-term perspective, drastic measures for the reduction of anthropogenic emissions including extensive decarbonization of the residential and industrial heating sector have to be implemented. To replace fossil fuels, solid biogenic residues and wastes will have to be increasingly utilized. Compared to clean woody biomass, these biomass assortments are commonly characterized by higher Si and alkaline metal contents recognized as major driver for low ash melting temperatures resulting in elevated risk of bottom ash slagging. To facilitate the prediction of bottom ash slagging during combustion, several fuel indices have been proposed. Based on empirical correlations with parameters relevant for slagging behavior, e.g., ash melting temperatures or slag fraction of the bottom ash, these fuel indices were subsequently enhanced and adapted for an increasing range of biomass fuel characteristics. In this study, analysis data of 26 woody and non-woody fuels and experimental data derived from two combustion test campaigns with an automatically stoked small-scale boiler were investigated through principal component analysis. Thus, the complex interdependencies between the fuel composition and the resulting bottom ash characteristics and the applicability of existing fuel indices were evaluated. The chemometric analysis highlighted that Si, Ca, K, Mg, and also the remaining Al and S in the bottom ash are crucial fuel components in the context of bottom ash melting. On this basis, the molar ratio (Si + P + K)/(Ca + Mg) was adapted and correlated with the susceptibility to slag formation which is a new parameter derived from ash content, slag fraction > 16 mm in the bottom ash, and slag category. Thus, the applicability of a newly developed fuel index was evaluated with respect to the bottom ash slagging risk during real-scale combustion. Three ranges were distinguished for the fuel index corresponding to a specific susceptibility to slag formation (i.e., low < 20 mol/g for woody biomass, elevated between 20 and 75 mol/g, and serious > 75 mol/g for straw-like fuels and blends with wood). The linear regression of the fuel index with susceptibility to slag formation exhibits a high coefficient of determination (i.e., 0.99 for woody biomass and 0.84 for straw-like fuels and their blends with wood).