Torrefaction of biomass
is a promising thermochemical pretreatment
technique used to upgrade the properties of biomass to produce solid
fuel with improved fuel properties. A comparative study of the effects
of torrefaction temperatures (200, 250, and 300 °C) and residence
times (0.5 and 1 h) on the quality of torrefied biomass samples derived
from spent coffee grounds (SCG) and coffee husk (CH) were conducted.
An increase in torrefaction temperature (200–300 °C) and
residence time (0.5–1 h) for CH led to an improvement in the
fixed carbon content (17.9–31.8 wt %), calorific value (18.3–25
MJ/kg), and carbon content (48.5–61.2 wt %). Similarly, the
fixed carbon content, calorific value, and carbon content of SCG rose
by 14.6–29 wt %, 22.3–30.3 MJ/kg, and 50–69.5
wt %, respectively, with increasing temperature and residence time.
Moreover, torrefaction led to an improvement in the hydrophobicity
and specific surface area of CH and SCG. The H/C and O/C atomic ratios
for both CH- and SCG-derived torrefied biomass samples were in the
range of 0.93–1.0 and 0.19–0.20, respectively. Moreover,
a significant increase in volatile compound yield was observed at
temperatures between 250 and 300 °C. Maximum volatile compound
yields of 11.9 and 6.2 wt % were obtained for CH and SCG, respectively.
A comprehensive torrefaction model for CH and SCG developed in Aspen
Plus provided information on the mass and energy flows and the overall
process energy efficiency. Based on the modeling results, it was observed
that with increasing torrefaction temperature to 300 °C, the
mass and energy yield values of the torrefied biomass samples declined
remarkably (97.3% at 250 °C to 67.5% at 300 °C for CH and
96.7% at 250 °C to 75.1% at 300 °C for SCG). The SCG-derived
torrefied biomass tested for CO
2
adsorption at 25 °C
had a comparatively higher adsorption capacity of 0.38 mmol/g owing
to its better textural characteristics. SCG would need further thermal
treatment or functionalization to tailor the surface properties to
attract more CO
2
molecules under a typical post-combustion
scenario.