Agricultural biomass is considered a prospective renewable feedstock in the energy conversion units. Despite its widespread availability, agricultural biomass is often underutilized in the thermochemical processes due to its high ash content. These types of biomass usually contain a relatively high share of potassium (K), silicon (Si), and phosphorus (P). The presence of these elements in the fuel can cause ash-related issues like slagging, deposit formation, fine particle emissions, etc. However, extracting these elements during thermochemical conversion processes can increase the economic viability of using agricultural biomassas a feedstock. There is still a lack of detailed understanding regarding ash-transformation reactions in entrained flow conversion of such biomassas sortments. Therefore, detailed knowledge regarding ash-transformation pathways during the thermochemical conversion of agricultural biomass can target the aim of reducing or eliminating ash-related issues together with recovering valuable Si- and K-P-rich compounds.

The main objectives of this work were, therefore, to 1) investigate the ash transformation pathways of Si and P during entrained flow combustion of different types of Si- and P-rich agricultural biomass and 2) investigate the potential of extracting valuable Si- and K-P compounds with high purity from the gas phase and/or residual ashes formed during entrained flow conversion of different agricultural biomass. Experiments were conducted within a lab-scale laminar drop tube furnace (DTF) at 1200 and 1450 °C in combustion conditions (using air) and in pyrolysis conditions (using N2). Three different agricultural biomass types were used, namely, rice husks, grass, and draff (i.e., beer brewing residue). Rice husks represent Si-rich fuel with minor amounts of P, K, and Ca. The grass fuel represents K-Si-rich fuel with moderate amounts of P, Ca, and Mg. Draff, on the other hand, represents a P-rich fuel with a relatively high share of Si and a moderate to minor Ca, Mg, and K content. The produced residual materials, i.e., coarse ash (> 1 μm), fine particle (< 1μm) fractions, and chars after the experiments were morphologically and chemically characterized by SEM-EDS, XRD, ICP-AES, IC, and CHN-analysis. The interpretation of experimental findings and theoretical assessment of ash transformation were facilitated through TECs.

The combustion experiments using a laminar drop tube furnace revealed distinctive ash transformations across the studied agricultural biomass. For all fuels, Si was entirely retained in the coarse ash (> 1 μm) fractions, whereas P was found mainly in coarse ash fractions and in a minor to moderate amount in the fine particle (< 1 μm) ash fractions.

For the rice husks, most of the minor ash-forming elements (i.e., K, Ca, Mg, and P) present in the fuel were retained at the inner surface of the chars during the pyrolysis/devolatilization experiments conducted in the DTF, which were later incorporated into a melt found in the coarse ash fractions after combustion experiments. Consequently, Si-rich molten spheres were formed at the inner surface of the coarse ashes. The share of molten ash increased in the coarse ash fractions with the combustion temperature. Concerning grass, the retained P in the residual coarse ash fractions was found in a K-Ca-Mg-rich phosphosilicate melt and crystalline Ca9MgK(PO4)7(OH), Ca5(SiO4)0.3(PO4)2.7(OH)0.7, and Ca7(SiO4)2(PO4)2. The phosphosilicate melt was most likely formed through the initial formation of a K-rich silicate melt, with subsequent incorporation of Ca, Mg, and P. As for draff, the P retained in the coarse ash fractions was identified as Ca-Mg-rich phosphosilicate melt and crystalline Ca3Mg3(PO4)4. This phosphosilicate melt most likely originated from the phytate-derived Ca-Mg-phosphates that melt and interact with Si-rich particles/sites.

TECs predicted that extracting pure Si-containing compounds (i.e., SiC(s)) from the gas phase during the entrained flow conversion of rice husks would require very high temperatures in the flame (i.e., around 2000 °C) to volatilize a moderate amount of Si present in the rice husks. Additionally, it would require a pyrolysis cooling atmosphere and relatively high surface temperatures (i.e., around 1500 °C) to form potentially valuable Si-containing compounds. These conditions are challenging to achieve in practice. However, the experimental investigations showed the possibility of extracting relatively pure silica from the residual coarse ash fractions collected after the entrained flow combustion of rice husks. TECs did not predict the probability of forming pure Si and/or K-P-containing compounds from the gas phase for grass fuel. Regarding draff fuel, both TECs and experimental investigations indicated that a surplus of P to Si and cations in the fuel can facilitate the formation of valuable K-phosphates and/or H3PO4 from the gas phase at lower surface temperatures (i.e., <400 °C). Moreover, the coarse ash fractions obtained after the combustion of grass and draff primarily contained different phosphosilicate melts. The plant availability of P in such melts needs to be evaluated.

The findings derived from this work offer valuable insights into the ash transformation of phosphorus (P) and silicon (Si) during the thermochemical conversion of agricultural biomass with varying ash compositions under entrained flow conditions. The obtained knowledge could be used to propose efficient measures to mitigate the associated ash-related problems and to propose interesting pathways to extract valuable Si- and K-P-rich components during the thermal conversion of agricultural biomass in entrained flow reactors.