Ash Transformation during Fixed-Bed Combustion of Agricultural Biomass with a Focus on Potassium and Phosphorus

Abstract

In this study, ash transformation during fixed-bed combustion of different agricultural opportunity fuels was investigated with a special focus on potassium (K) and phosphorus (P). The fuel pellets were combusted in an underfed fixed-bed pellet burner. Residual ashes (bottom ash and slag) and particulate matter were collected and 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, almost all P (>97%) was found in residual-/coarse ash fractions, while K showed different degrees of volatilization, depending on fuel composition. During combustion of poplar, which represents Ca–K-rich fuels, a carbonate melt rich in K and Ca decomposed into CaO, CO2, and gaseous K species at sufficiently high temperatures. Ca5(PO4)3OH was the main P-containing crystalline phase in the bottom ash. For wheat straw and grass, representing Si–K-rich fuels, a lower degree of K volatilization was observed than for poplar. P was found here in amorphous phosphosilicates and CaKPO4. For wheat grain residues, representing P–K-rich fuels, a high degree of both K and P retention was observed due to the interaction of K and P with the fuel-bed constituents, i.e., char, ash, and slag. The residual ash was almost completely melted and rich in P, K, and Mg. P was found in amorphous phosphates and different crystalline phases such as KMgPO4, K2CaP2O7, K2MgP2O7, and K4Mg4(P2O7)3. In general, the results therefore imply that an interaction between ash-forming elements in a single burning fuel particle and the surrounding bed ash or slag is important for the overall retention of P and K during fuel conversion in fixed-bed combustion of agricultural biomass fuels.

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

“…The low temperature of a standard ashing test (550 °C) would stabilize higher quantities of carbonates, sulfates, and chlorides in the condensed phase compared to the experiments, possibly causing a lower mass closure due to the higher volatilization of elements from carbonates, sulfates, and chlorides above 1100 °C. The ash yield from the WS experiment was slightly above 100%, which was attributed to char remaining in the ashes . In addition, significant concentrations of HCl (median 14–58 ppm) and SO 2 (median 109–345 ppm) were measured in the flue gas during the experiments with sewage sludge mixtures, which are not accounted for in the mass balance.…”

“…SHS mixtures were the most effective at reducing the slagging tendency of SS, with the amount and durability of the slag decreasing with the share of SH. The experiment with pure WS suggested a significant risk of slag-related operational issues, and the experiment had to be cut short because of slag buildup on the burner cup . The WSS mixtures produced lower shares of slag, and the slag durability was decreased when compared to the original fuels SS and WS.…”

“…Schematic overview of the underfeed pellet burner (20 kW nominal) in the reference boiler, ash sampling points, flue gas measurements, and temperature and pressure measurements. Adapted from Hedayati et al 46 Copyright 2022 American Chemical Society. density and heating value.…”

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

“…37 Further, ashes where K 2 MgP 2 O 7 and K 4 Mg 4 (P 2 O 7 ) 3 have been identified have been associated with problematic ash melting behavior. 12,15,39 Finally, the variation in the share of network formers (SiO 2 , PO 2.5 ) to total ash oxides in the mixture is represented by molar ratio δ (eq 4).…”

“…Although most of the biomass fuels included in this study had high γ values (median ∼ 0.74), P-rich biomasses such as cereal grains (γ =0.14–0.25), rapeseed meal (γ = 0.45), or dried distiller’s grain with solubles (γ = 0.19) have much lower ratios. ,, Based on the melting characteristics of the CaO-K 2 O-P 2 O 5 and MgO-K 2 O-P 2 O 5 systems, the liquidus temperatures for compositions close to a stoichiometric composition of K 2 MgP 2 O 7 and K 4 Mg 4 (P 2 O 7 ) 3 are several hundred degrees lower than the respective system with CaO . Further, ashes where K 2 MgP 2 O 7 and K 4 Mg 4 (P 2 O 7 ) 3 have been identified have been associated with problematic ash melting behavior. ,, Finally, the variation in the share of network formers (SiO 2 , PO 2.5 ) to total ash oxides in the mixture is represented by molar ratio δ (eq ). …”