Ash Transformation Processes during Pulverized Fuel Combustion of Rice Husks

Rice husks were combusted in a 150 kW pilot-scale powder burner connected to a horizontal ceramic-lined furnace to investigate the ash transformation processes, including deposit formation at high surface temperatures. Residual coarse ash samples (>1 μm) were collected from different positions along the furnace and heat exchanger path. Fine fly ash samples (<1 μm) were collected from the furnace outside the flame, and high-temperature deposits were collected on deposition probes having different surface temperatures. The collected samples were analyzed via scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. Additionally, thermodynamic equilibrium calculations were employed to interpret experimental results. The results showed different ash transformation processes occurring at the outer surface and inner part of the rice husks. A high share of minor ash-forming elements (i.e., K, P, Ca, and Mg) together with Si was retained in the residual coarse ash particles. The retained minor ash-forming elements were mainly incorporated in the spherical Si-rich particles with moderate amounts of K, Ca, Mg, and P that were partially molten and originated from the inner part of the rice husks. The outer surface of the rice husks primarily formed skeleton-like coarse ash particles dominated by Si. The high surface temperature deposits only contained skeleton-like coarse ash particles that were partially molten.