Response surface methodology applied to spent coffee residue pyrolysis: effect of temperature and heating rate on product yield and product characterization

This work evaluated the pyrolytic products from spent coffee residue pyrolysis in a fixed-bed reactor. The effects of the heating rate and temperature on the main pyrolytic products were investigated using a factorial experimental design. The average product yields obtained were liquid (49.45 wt%), char (26.45 wt%), and gas (24.05 wt%). Temperature was the most significant factor affecting product yield. Maximum devolatilization occurred at 823 K and 80 K/min, the same conditions that minimized the char yield. The spent coffee ground (SCG) decomposition occurred mainly due to a dehydration reaction, followed by decarboxylation. The heating rate and temperature directly influenced the production of phenols, and the formation of esters and ketones was favored at 723 K. Statistical analysis of the biochar element composition showed a carbon enrichment and a decrease in oxygen, hydrogen, and nitrogen contents at higher temperatures due to strong devolatilization. Char polarity and aromaticity were strongly affected by the temperature. The C/N ratios of the SCG chars (18.4-20.7) indicated that these chars are possible soil additives.