Thermodynamic Equilibrium Study on the Melting Tendency of the K-Ca-Mg-P-Si-O System with Relevance to Woody and Agricultural Biomass Ash Compositions

Abstract

A major challenge in the combustion of biomass fuels is the heterogeneity of ash-forming elements, which may cause a wide range of ash-related problems. Understanding the melting tendency of the coarse ash fractions is necessary to mitigate agglomeration and slagging. This work aims to evaluate the melting tendency of the K-Ca-Mg-Si-P-O system by use of thermodynamic equilibrium calculations. The formation of condensed phases were systematically assessed in a combustion atmosphere, varying temperatures, and composition. Compositional ranges were based on fuel ash data extracted from the Phyllis 2 database. The speciation and degree of polymerization of phosphates, silicates, and melts were evaluated and indicated a systematic variation in composition. The melt fraction was predicted as a function of temperature and composition. The melting tendency was modeled for three systems, i.e., a P-dominated, a Si-dominated, and a mixed Si-P system. Four ratios between K 2 O, CaO, MgO, SiO 2 , and P 2 O 5 were found to have a large effect on the melting tendency of the ash mixtures: the ratio between network formers (SiO 2 , P 2 O 5 ), K 2 O to total network modifiers, CaO to CaO + MgO, and the ratio of network formers to total ash oxides. This modeling approach showed qualitative agreement with ash-related issues seen in previous lab-scale experiments in bubbling fluidized bed and fixed bed combustion. Practical implications of the results are discussed from the perspective of fuel design with the aim of preventing ashrelated problems. This study presents a novel method of applying thermodynamic equilibrium calculations for a broad range of compositions and shows potential for predicting ash-related issues related to the melting of coarse ash fractions.

Ash Transformation during Fixed-Bed Co-combustion of Sewage Sludge and Agricultural Residues with a Focus on Phosphorus

“…Melt formation in all the experiments could be ascribed to the melting behavior of the individual fuel ashes. Excessive slagging during WS combustion due to K silicate formation and partial melt formation by carbonate melting during the combustion of Ca-K-rich fuels (SH) were described and discussed previously. ,, For SS, the initiation of slag could be correlated to a sintering effect of Fe-rich regions on the particle surface. , However, the chemical composition in the slag fractions showed a high degree of incorporation of ash-forming elements from both individual fuels in the respective mixtures. Furthermore, the distinctive regions in the slag cross sections indicate an immiscibility gap in the ash mixture, with a clear partitioning of Si and P between the two melts.…”

“…Furthermore, the distinctive regions in the slag cross sections indicate an immiscibility gap in the ash mixture, with a clear partitioning of Si and P between the two melts. Previous studies indicate that high shares of Ca in the ash would be incorporated in a P-rich slag and promote the formation of a secondary Si-rich slag …”

“…The dominance of orthophosphates indicates that the ash transformation reactions of P in these ash mixtures are dominated by substitution reactions primarily involving K + , Ca 2+ , Mg 2+ , Fe 2+ , Fe 3+ , and Al 3+ . These substitution mechanisms are expected given that the ash composition of the individual fuels and the fuel mixtures is cation-rich and most P originates from an inorganic association in the SS fuel. , Because of the large share of melt in the residual ash fractions, these substitution reactions are likely to have occurred through liquid–liquid interactions. However, the formation of K-containing phosphates may be restricted because of the immiscibility gap in the system, as the partitioning of Si and P may limit the interaction between the two systems.…”

“…Excessive slagging during WS combustion due to K silicate formation and partial melt formation by carbonate melting during the combustion of Ca-K-rich fuels (SH) were described and discussed previously. 28 , 46 , 52 For SS, the initiation of slag could be correlated to a sintering effect of Fe-rich regions on the particle surface. 27 , 29 However, the chemical composition in the slag fractions showed a high degree of incorporation of ash-forming elements from both individual fuels in the respective mixtures.…”

“…Previous studies indicate that high shares of Ca in the ash would be incorporated in a P-rich slag and promote the formation of a secondary Si-rich slag. 52 …”