Ash transformation in thermochemical conversion of different biomass resources with special focus on phosphorus

A great potential exists for increasing the use of bioenergy in thermochemical processes by utilizing agricultural biomass, forest residues, and sewage sludge that have high availability. Many of these biomass assortments have high ash contents with relatively high concentrations of ash-forming elements such as potassium (K), calcium (Ca), silicon (Si), and phosphorus (P). These elements can, during thermal conversion, cause several ash-related problems, such as deposit formation, slagging, and particle emissions. In particular, P has been found to play a vital role in such ash-related problems even at relatively low concentrations. In addition, ashes obtained from these biomass assortments could be an important source of valuable elements such as P and K. Therefore, detailed knowledge about the ash transformation and fate of P during thermal conversion of these opportunity biomass resources is of immense importance to mitigate ash-related problems and to recover valuable nutrient elements from the ash. The overall objective of this work was to determine the ash transformation and fate of P during single-pellet and fixed-bed combustion/gasification of different opportunity biomass fuels in the process temperature range of 600-1250°C. Different agricultural biomasses (poplar, wheat straw, grass, and wheat grain residues), forest residues (bark and twigs), and sewage sludge (pure and in mixtures with agricultural residues) were used. These fuels cover a wide range of overall ash compositions and different chemical associations of P in the fuel. The bark and poplar represent fuels rich in K and Ca with minor P content. The wheat straw, grass, and twigs represent typical Si- and K-rich fuels with minor to moderate P contents. The wheat grain residues (WGR) represent typical K- and P-rich fuels with a significant amount of Mg. The produced residual materials, i.e., char, different ash fractions and fine flue gas particles, were morphologically and chemically characterized by scanning electron microscopy-energy dispersive X-ray spectroscopy, X-ray diffraction, inductively coupled plasma, and ion chromatography. The interpretation of the results was supported by thermodynamic equilibrium calculations. For all fuels, a major part of the P (> 80%) was found in coarse ash fractions because the studied process conditions favored the formation of stable condensed phosphates. The thermal conversion atmosphere (i.e., gasification/combustion) only caused small effects on the P release and the speciation of the P-compounds formed. Ash transformation pathways generally lead to the formation of orthophosphates (PO43-) such as Ca5(PO₄)3(OH), CaKPO4, and Ca3(PO4)2 with the partial substitution of Ca by some cation forming elements (Fe, Mg, and/or K), as the main P containing crystalline phases. Crystalline pyrophosphate (P2O74-) compounds were also found in the residual ashes from the seed-based fuel (WGR), where P originates from phytate in the biomass. For the fuels containing a certain (sufficient) amount of Si, orthophosphates interact with silicate phases to form both amorphous and crystalline phosphosilicates. For the sewage sludge mixtures, a surplus of available K was needed to form K-bearing phosphates due to side reactions of K with Si and Al. The chemical form of P in the formed ash residues is thus strongly dependent on both the type of P association in the fuel and the relative concentrations of other major ash-forming elements, such as K, Ca, Si, and Al. For the fuels with a high (Ca+Mg)/P molar ratio (AER), i.e., for the typical wood-derived fuels bark and poplar, hydroxyapatite was the main P-containing crystalline phase found in the ash. For the studied fuels/fuel mixtures with moderate AER and a high (K+Na)/(Si+Al) molar ratio (AR), e.g., twigs, grass, wheat straw, and sewage sludge with high mixtures of agricultural residues, there was also a possibility to form alkali-bearing phosphates such CaKPO4 and K-Mg whitlockite, besides hydroxyapatite. Since these fuels contain a high amount of Si, the P can be found in both amorphous phases, i.e. phosphosilicate, and Si substituted crystalline phases, i.e. Ca10(SiO4)x(PO4)6-XOH2-x and Ca15(PO4)2(SiO4)6. For fuels with moderate AER and low AR, e.g., pure sewage sludge and sewage sludge with low mixtures of agricultural residues, K-bearing phosphates were not formed. Instead, P was found in phases such as whitlockite and phosphosilicates. For the WGR fuel with relatively low AER and high AR, K-bearing phosphates were formed in the ashes, where the P was found in crystalline K-Mg/Ca pyrophosphates and K-Mg orthophosphate, as well as amorphous K-Mg-Ca phosphates. The produced knowledge can potentially be used to, e.g., i) suggest efficient measures to mitigate ash-related problems associated with P during thermochemical conversion of opportunity biomass fuels, ii) suggest potential pathways to form plant-available phosphates directly in the thermal conversion process to enable recovery of P and K from the obtained ashes, and iii) find optimal thermal conversion process conditions to obtain bio charcoals that are suitable as alternative fuels and reducing agents in the metallurgical industry.