Char production with high-energy value and standardized properties from two types of biomass

Abstract

In this paper, a new approach to the torrefaction process is proposed. The approach consists of obtaining a single torrefied product with an energy value and standardized physicochemical properties in the range of sub-bituminous coal from two types of waste: the cotton stalk and vine pruning. A multifactorial design was used to investigate the effect of temperature, time, and reaction atmosphere on the properties of the torrefied product (char). The process was optimized according to the mass and energy yield and the increase in the higher heating value (HHV). A series of physicochemical, kinetic, and microscopic analyses were carried out on the raw and torrefied biomass to evaluate the combustible properties. Char was obtained with an energy value in the range of sub-bituminous coal and with more uniform fuel properties concerning raw biomass. The optimal torrefaction conditions for cotton stalk were 6% O2–257.8°C–60 min and 0% O2–275°C–20 min for vine pruning. The HHV of the cotton stalk and vine pruning increased from 16.88 to 22.21 MJ kg−1 and from 17.01 to 22.43 MJ kg−1, respectively. The char from both residues presented similar O/C and H/C atomic ratios, and its value decreased compared with the raw biomass. Char is hydrophobic and easy to grind. Oxygen concentration played an essential role in reducing the time and temperature of torrefaction.