Fuel Properties of Torrefied Biomass from Sapindus Pericarp Extraction Residue under a Wide Range of Pyrolysis Conditions

In this work, a novel biomass, the extraction residue of Sapindus pericarp (SP), was torrefied by using an electronic oven under a wide range of temperature (i.e., 200–320 °C) and residence times (i.e., 0–60 min). From the results of the thermogravimetric analysis (TGA) of SP, a significant weight loss was observed in the temperature range of 200–400 °C, which can be divided into the decompositions of hemicellulose (major)/lignin (minor) (200–320 °C) and cellulose (major)/lignin (minor) (320–400 °C). Based on the fuel properties of the feedstock SP and SP-torrefied products, the optimal torrefaction conditions can be found at around 280 °C for holding 30 min, showing that the calorific value, enhancement factor and energy yield of the torrefied biomass were enhanced to be 28.60 MJ/kg, 1.36 and 82.04 wt%, respectively. Consistently, the values of the calorific value, carbon content and molar carbon/hydrogen (C/H) ratio indicated an increasing trend at higher torrefaction temperatures and/or longer residence times. The findings showed that some SP-torrefied solids can be grouped into the characteristics of a lignite-like biomass by a van Krevelen diagram for all the SP-torrefied products. However, the SP-torrefied fuels would be particularly susceptible to the problems of slagging and fouling because of the relatively high contents of potassium (K) and calcium (Ca) based on the analytical results of the energy dispersive X-ray spectroscopy (EDS).