Comparative study of solid biofuels derived from sugarcane leaves with two different thermochemical conversion methods: wet and dry torrefaction

Abstract

This work describes the conversion of sugarcane leaves into biocoal with two thermal processes: wet torrefaction (subcritical water, 175-250 °C) and dry torrefaction (nitrogen atmosphere, 225-300 °C). The residence time was 30 min for both processes. The effects on physical and energy characteristics, including mass and energy yield, proximate and ultimate analyses, fiber analysis, higher heating value (HHV), structural parameters determined by Fourier-transform infrared spectroscopy, and O/C and H/C atomic ratios were used for comparisons. The results showed that increasing the reaction temperature lowers the mass yield; however, it also significantly improves the fuel ratio of torrefied samples. The highest HHV of wet and drytorrefied samples were 23.31 and 22.07 MJ/kg, respectively. The best removal of ash and sulfur content was obtained under wet torrefaction. Moreover, wet torrefaction was recommended as a suitable process for hemicellulose depolymerization. At 250 °C, the wet-torrefied sample had the highest fuel ratio (0.48) and was suited for biomass co-firing. The finding that wet-torrefied samples reached the same range of lignite at lower reaction temperatures than dry-torrefied samples was particularly intriguing. Torrefaction at the temperatures below 250 °C did not prove to have a statistically significant effect on the energy properties of the dry-torrefied samples. Therefore, wet torrefaction is a promising process in the thermochemical conversion of sugarcane leaves into solid biofuel.