Agricultural biomass is increasingly acknowledged as a versatile renewable feedstock in the energy conversion units. However, the efficient utilization of agricultural biomass in thermochemical processes could be hindered by the relatively high ash content compared to woody biomass. These types of biomasses often contain a relatively high share of silicon (Si) and phosphorus (P). The presence of these elements in the biomass can contribute to ash-related operational challenges such as slagging, fine particle emissions, and deposit formation. Beyond these challenges, the recovery of Si- and P-containing compounds as by-products during thermochemical conversion presents an opportunity to generate additional value, thereby improving the overall resource efficiency and economic viability of using agricultural biomass as a feedstock. Despite the importance of Si and P, the detailed ash transformation processes during entrained flow conversion of such biomass assortments remain inadequately understood. This knowledge is crucial for reducing or eliminating ash-related issues while unlocking pathways for recovering valuable Si- and P-containing compounds during entrained flow conversion of agricultural biomass. 

The main objectives of this work were, therefore, to 1) determine the ash transformation pathways of Si during entrained flow combustion of different types of Si- and P-rich agricultural biomass, 2) determine the ash transformation pathways of P during entrained flow combustion of different types of Si- and P-rich agricultural biomass, and 3) investigate the potential of extracting valuable Si- and P- containing compounds from the gas phase and/or residual ashes formed during entrained flow conversion of different agricultural biomass. 

The study combines lab-scale experiments in a laminar drop tube furnace (DTF) at 1200 °C and 1450 °C in combustion conditions (using air) and in pyrolysis conditions (using N2), with pilot-scale combustion experiments in a 150-kW powder burner connected to a horizontal ceramic-lined furnace. Three agricultural biomass types were selected to represent a range of Si and P concentrations in the selected fuels: rice husks representing Si-rich husks from certain cereal crops like rice and oat (i.e., Si-rich fuel with minor amounts of K, Ca, Mg, and P), grass representing Si- and K-rich herbaceous energy crops from grasses and residues from certain agricultural crops such as wheat straw and other cereal straws (i.e., K-Si-rich fuels with moderate amounts of Ca, Mg, and P), and brewer’s spent grains (BSG) representing P-rich grain- and seed-based agricultural biomass (i.e., P-rich fuels with a relatively high share of Si with moderate to minor Ca, Mg, and K content). All three fuels were investigated in the lab-scale DTF, whereas rice husks and BSG were examined in a 150-kW powder burner. The produced residual materials, i.e., coarse ash fractions (> 1 µm), fine particle fractions (i.e., PM1, <1 µm), chars, and deposits were morphologically and chemically characterized using SEM-EDS, XRD, ICP-AES, IC, and CHN-analysis. Thermodynamic equilibrium calculations (TECs) were employed to interpret experimental findings and theoretically assess ash transformation pathways.

Across all investigated fuels and combustion scales, Si was predominantly retained in the coarse ash fractions (>1 µm), indicating limited volatilization under the studied conditions. During the combustion of rice husks under both scales, Si present in the outer surface of the fuel formed skeleton-like coarse ash particles. Meanwhile, the Si present in the inner part of the fuel interacted with minor ash-forming elements (i.e., K, Ca, Mg, and P) and formed Si-rich molten spheres. Overall, the resulting coarse ash fractions were comprised of amorphous non-molten Si-rich particles, Si-rich melt with moderate to minor amounts of K, Ca, Mg, and P, and crystalline SiO2 (cristobalite). For grass, the results from the combustion experiments conducted under DTF conditions showed that the fuel inherent Si initially reacted with K to form molten K-silicates. The subsequent incorporation of Ca, Mg, and P into molten K-silicates led to the formation of K-Ca-Mg-rich phosphosilicate melt in the residual coarse ash fractions. Si was found in the residual coarse ash fractions mainly as amorphous K-Ca-Mg-rich phosphosilicate melt and crystalline SiO2 (quartz), Ca2MgSi2O7, CaSiO4, KCaSi3O9, Ca7(SiO4)2(PO4)2, and Ca5(SiO4)(PO4)2. For the BSG, the experiments conducted at both scales showed that the fuel inherent Si initially interacted with partially molten Ca-Mg-phosphates, and formed Ca-Mg-rich phosphosilicate melt. Si in the residual coarse ash fractions was identified as amorphous Ca-Mg-phosphosilicate and crystalline SiO2 (i.e., quartz and/or cristobalite). 

For all investigated fuels and conditions, P was primarily retained in the coarse ash fractions (> 1 µm) mainly in the form of orthophosphate compounds. A minor to moderate amounts of fuel inherent P identified in the fine particle (i.e., PM1, <1 µm) ash fractions, indicating partial volatilization of P during the investigated conditions. During combustion of rice husks across both scales, P was primarily retained in the residual coarse ash fractions and incorporated into Si-rich molten spheres with moderate to minor amounts of K, Ca, Mg, and P. Additionally, a moderate (≈ 20%)  to high (≈ 40%) share of P was detected in the PM1 fractions under studied combustion conditions. For grass fuel, DTF experiments showed that fuel-inherent P, together with Ca and Mg, interacted with molten K-silicates and formed K-Ca-Mg-rich phosphosilicate melt. P in the residual coarse ash fractions was found as K-Ca-Mg-rich phosphosilicate melt and crystalline Ca7(SiO4)2(PO4)2, Ca5(SiO4)(PO4)2, Ca5(PO4)3(OH), Ca2.89Mg0.1(PO4)2, and Ca9MgKPO4.  Furthermore, a minor (≈ 5%) to moderate (≈ 35%) amount of P was identified in the PM1 fractions at 1200 °C and 1450 °C, respectively. For BSG, the fuel inherent P (i.e., phytates) decomposed to partially molten Ca-Mg-phosphates, which subsequently interacted with Si-rich particles, leading to the formation of a Ca-Mg-rich phosphosilicate melt. In both DTF and powder burner experiments, P in the residual coarse ash fractions was primarily retained as amorphous Ca-Mg-phosphosilicate melt and crystalline Ca3Mg3(PO4)4. A minor (≈8%) to moderate (≈23%) share of P was also detected in the PM1 fractions under the investigated combustion conditions.

The combined results from TECs and experimental studies demonstrated the fuel-specific potential for recovering valuable Si- and P-containing compounds from gas and/or residual coarse ashes (>1 µm) during entrained flow conversion of different types of agricultural biomasses. For rice husks, TECs indicated that extracting valuable Si-containing compounds (e.g., SiC (s)) from the gas phase would require very high temperatures inside the flame (i.e., around 2000 °C) to volatilize a moderate amount of fuel inherent Si. Furthermore, it would require an inert cooling atmosphere and elevated surface temperatures around 1500 °C to form potentially valuable Si-containing compounds, which is challenging to achieve in practice. However, both lab- and pilot-scale combustion experiments showed the potential to extract relatively pure silica from the residual coarse ash fractions collected after entrained flow combustion of rice husks. In the case of grass, TECs did not indicate the possibility of forming valuable Si- and/or P-containing compounds from the gas phase. Regarding BSG fuel, TECs suggest that the surplus of P to Si and cations in the fuel can facilitate formation of valuable H3PO4 from the gas phase at lower surface temperatures (i.e.,< 400 °C). Moreover, the coarse ash fractions obtained during the combustion experiments of grass and BSG primarily contained different phosphosilicate melts. Further assessment is required to determine the plant availability of P in such melts.