Ash transformation processes during entrained flow combustion of brewer's spent grains with a focus on phosphorus

Ash transformation processes during the entrained flow combustion of brewer's spent grains (BSG), a P-rich grain- and seed-based agricultural biomass, were investigated in a 150 kW horizontal powder burner operating at ∼1400 °C. Coarse ash particles (> 1 μm) were obtained from several locations within the furnace and heat exchanger zone. The collected ash samples were characterized using energy-dispersive X-ray fluorescence (ED-XRF), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Complementary thermodynamic equilibrium calculations were conducted to interpret experimental findings. The results showed that the majority of P was retained in the residual coarse ash fractions (> 1 μm) as Ca-Mg-rich phosphosilicate melt. Coarse ash samples collected near the flame zone exhibited intensified interaction between partially molten Ca-Mg-phosphates and Si-rich particles, promoting an increased melt fraction in the condensed phases. These insights deepen the understanding of ash transformation during entrained flow combustion of P-rich grain- and seed-based agricultural biomasses. They also carry practical implications for slag mitigation, system design, and phosphorus recovery from residual ash fractions.