Effect of holding time on fuel properties of biochars prepared from the torrefaction of coffee residue

Decomposition reactions during torrefaction start at 563 K, as shown by thermogravimetric analysis (TGA), as determined by pretests. Based on these findings, different biochars were produced by torrefaction at 563 K, combined with increasing holding times (10–40 min). The fuel properties of biochar products were investigated by means of calorific value, ultimate analysis, true density, and scanning electron microscopy (SEM). The maximal increase in the calorific value of biochar occurred at about 20 min, which correlated positively with its carbon contents. More consistently, the true densities of the resulting biochar end products increased mostly within the holding time ranging from 10 to 20 min, which we proposed as the best process window for torrefaction of coffee residue. At longer holding time (>20 min), we observed a shrinking of biochar by SEM, making it more friable or brittle. It may involve more hemicellulose depolymerization, leading to a porous and corrugated structure.