Ash Fusion During Combustion of Single Corn Straw Pellets

Abstract

This investigation identified conditions at which corn straw ash melts and examined how this phenomenon affects the combustion of char residues. Corn straw was pelletized in cylinders and was burned at elevated temperatures in the range of 1200–1400 °C, and at different air flow velocities. The pellets were inserted in a preheated furnace, where they were subjected to moderately high heating rates. Their combustion behavior was observed with cinematography, thermometry, and thermogravimetry. Upon insertion in the furnace, the pellets devolatilized and formed volatile envelope flames, upon extinction of which, the chars experienced concurrent heterogeneous combustion and ash fusion. Residues were assessed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), coupled to energy dispersive spectroscopy (EDS). The burnout times of the volatiles and the chars decreased drastically with increasing temperature. At 1300 °C and 1400 °C, the remaining ash underwent complete melting and the final structures of the pellets collapsed to molten pools. At 1400 °C, all of the chlorine and most of the potassium were released into the gas phase. The straw ash was identified as a high-density silicate melt. Although surface ash melted completely, it flowed to the base of the pellet. Therefore, it did not significantly hinder the oxidation of the carbonaceous char. Hence, to increase the likelihood of complete corn straw carbon burnout and of ash melting and flowing to the bottom of the furnace, operating temperatures higher than 1300 °C, in conjunction with mild air flow rates, are recommended.