Spent coffee grounds and tea leaf residues: Characterization, evaluation of thermal reactivity and recovery of high-value compounds

Spent coffee grounds (SCGs) and tea leaf residues are investigated as a potential source of bioenergy and biochemicals and compared to several woody biomasses. The effect of initial processing (coffee roasting and tea oxidizing) on feedstock composition and thermal behavior is assessed experimentally and analytically. In addition to proximate and ultimate analyses, the materials are examined by combustion tests coupled with Fourier-transform infrared spectroscopy and thermogravimetric analysis coupled with mass spectrometry in oxidizing and inert atmospheres. The biggest differences in composition of the samples are found in the high elemental N and S contents of SCGs (2.9 wt% of N and 0.1 wt% of S on average) and especially tea leaf residues (4.5 wt% of N and 0.2 wt% of S on average). The elevated N and S values lead to NO and SO2 release during combustion. Overall, the obtained levels of reactivity and heating values (with a maximum higher heating value of 24 MJ kg(-1) for the Arabica-Robusta coffee blend) confirm the significant energy potential of both the coffee and tea samples. Mass spectrometry analysis of the studied materials reveals higher heterogeneity of macromolecular composition of coffee and tea samples compared to woody biomasses. Pyridine, pyrazine, phenols and acetic acid are the most abundant among detected species and have demonstrated beneficial characteristics for a broad spectrum of human activities. The composition characteristics and reactivity test results presented in this work promote more effective valorization and usage of these currently little used biomass residues.

Automated summary

Spent coffee grounds and tea leaf residues have significant potential as bioenergy and biochemical sources, with high nitrogen and sulfur content leading to NO and SO2 release during combustion. Mass spectrometry analysis of the studied materials reveals higher heterogeneity of macromolecular composition in coffee and tea samples.