Ash Formation during Combustion of Rice Husks in Entrained Flow Conversion Conditions

This study investigates the detailed ash transformation process during the combustion of rice husks in entrained flow conditions. The experiments were conducted in a lab-scale drop tube furnace at 1200 and 1450 degrees C in pyrolysis/devolatilization (using N-2) and combustion (using air) conditions. The detailed ash transformation process during the different fuel conversion stages in combustion (i.e., devolatilization and char combustion) was investigated by comparing the results obtained in the pyrolysis/devolatilization experiments with the combustion experiments. The resulting residual chars, ashes, and particulate matter (PM) were collected and characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), inductively coupled plasma atomic emission spectroscopy (ICP-AES), ion chromatography (IC), and CHN analyses. Furthermore, the obtained results were interpreted via thermodynamic equilibrium calculations (TECs). For all investigated conditions, Si, Ca, and Mg were retained entirely in the coarse ash and char fractions (>1 mu m). Meanwhile, K and P were found in coarse ash/char fractions and fine particulate fractions (<1 mu m). A moderate, at 1200 degrees C, to high share, at 1450 degrees C, of the detected K and P was found in the fine particle fractions after combustion. The majority (>95%) of the detected S and Cl were volatilized during the experiments. The study showed an accumulation of minor ash-forming elements (i.e., K, Ca, Mg, P) on the inner part of rice husk chars, initiating melt formation during the char combustion stage. The identified melt at 1200 degrees C after combustion was rich in Si with minor amounts of K, Ca, Mg, and P. The share of molten ashes was increased at 1450 degrees C compared to that at 1200 degrees C. Overall, the results presented in this work reveal detailed insights into the ash transformation processes taking place in different parts of the fuel during the combustion of rice husks in entrained flow conditions.