Torrefaction of fruit peel waste to produce environmentally friendly biofuel

Abstract

This study torrefied Ananas comosus peel (ACp) and Annona squamosa peel (ASp) samples at different temperatures (210–300 °C) for different durations (30 and 60 min) to produce biochar as a renewable energy source. Analysis results indicated that the torrefaction temperature, not the reaction duration, had a dominant influence on the characteristics of the produced biochar. The ACp biochar had a maximum higher heating value (HHV) of 27.7 MJ/kg and maximum energy density of 1.58, which were higher than those of the ASp biochar. As the torrefaction temperature and time increased, the fixed carbon and ash contents and the fuel ratio (FR) increased gradually for both types of biochar, but the volatile matter content and energy return on investment (EROI) decreased. The hydrogen/carbon and oxygen/carbon atomic ratios of both types of biochar are comparable to those reported for lignite. Thermogravimetric and derivative thermogravimetric analyses confirmed that the thermal stability of both types of fruit waste increased considerably with the torrefaction temperature. Furthermore, the ACp biochar had a maximum EROI of 22.9 and FR of 0.36. When 10%–20% biochar was cofired with 80%–90% bituminous coal in a power plant, the FR (1.80–1.84 and 1.60–1.67)—representing combustion efficiency—approximated that of bituminous coal (1.5–2.5), but greenhouse gas (GHG) emissions decreased significantly (83.7%–94.3%). Taken together, the cofiring of ACp biochar or Asp with coal is potentially viable and economically beneficial for increasing the HHV, energy density, fixed carbon content, FR, and EROI of these materials while reducing GHG emissions, rendering these materials suitable for partially replacing bituminous coal in power plants.