Ash transformation in single-pellet combustion and gasification of biomass with special focus on phosphorus

Abstract: The utilization of different biomass feedstocks in thermal conversion systems can contribute towards mitigation of global warming. However, the formation of different ash fractions (i.e., bottom ash, and fly ash) during thermal conversionof biomass can cause several ash-related problems such as deposit formation, slagging, and particle emissions, all of which may limit its usage as an energy source. It has been found that phosphorus (P), even in relatively low concentrations, can play a vitalrole in the abovementioned ash-related problems. However, the ash transformation reactions occurring in the thermal conversion of P-bearing biomass assortments are not fully understood and rarely described in the literature. Therefore, an understanding ofthe phenomena associated with ash transformations with a special focus on P is crucial.The overall objective was to determine the ash transformation and release of P duringsingle-pelletthermochemical conversion ofdifferent types of agricultural and forest fuelsin the low to medium temperature range (600-950 °C). Different agricultural biomasses (poplar, wheat straw, grass, and wheat grain residues), as well as forest residues (bark, twigs, and a mixture of bark and twigs) were used. Thebark and poplar fuels represent a fuel rich in K and Ca with minor P contents. The wheat straw, grass, and twigs represent a typical Si- and K-rich fuel with minor and moderate P contents. The wheat grain residues represent a typical K- and P-rich fuel witha considerable amount of Mg. The produced residual materials, i.e. chars and ashes, were characterized by SEM-EDS, XRD, and ICP-OES. The experimental results were interpreted with support from thermodynamic equilibrium calculations (TECs).The overall findings are that the majority of P (>80%) in all the studied fuels remained in the final condensed residues, and that the main fraction of P release occurred during the devolatilization stage. The chemical form of P in the residuesis strongly dependent on the relative concentrations of other major ash-forming elements such as K, Ca, and Si, as well as the type of association of P in the pure fuel. For woody-based fuels rich in Ca and K (poplar, bark, and twigs in this study), P in theash is generally found in the form of crystalline hydroxyapatite. For herbaceous fuels rich in Si and K (wheat straw and grass), P in the ash is generally found in Ca5(PO4)3OH, Ca15(PO4)2(SiO4),KCaPO4, and K-Ca/Mg phosphosilicate melts. For wheat grain residues rich in P, K, and Mg, P in the ash is found in crystalline forms K4Mg4(P2O7)3, K2MgP2O7,K2CaP2O7, and KMgPO4, as well as amorphous K-Mg/Ca phosphates.The obtained new knowledge can be used to find practical measures to mitigate ash-related problems during thermochemical conversion of P-bearing biomass fuels. It can also be used to find optimal pyrolysis process conditions to obtain biocharsuitable as alternative fuels and reducing agents in the metallurgical industry.