Experimental and Modeling Studies of Torrefaction of Spent Coffee Grounds and Coffee Husk: Effects on Surface Chemistry and Carbon Dioxide Capture Performance

Torrefaction of biomass is a promising thermo-chemical 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 degrees 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 degrees 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 degrees 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 degrees C, the mass and energy yield values of the torrefied biomass samples declined remarkably (97.3% at 250 degrees C to 67.5% at 300 degrees C for CH and 96.7% at 250 degrees C to 75.1% at 300 degrees C for SCG). The SCG-derived torrefied biomass tested for CO2 adsorption at 25 degrees 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 CO2 molecules under a typical post-combustion scenario.