The Torrefaction of Agricultural and Industrial Residues: Thermogravimetric Analysis, Characterization of the Products and TG-FTIR Analysis of the Gas Phase

Danijela Urbancl (), Deniz Agačević, Eva Gradišnik, Anja Šket, Nina Štajnfelzer, Darko Goričanec and Aleksandra Petrovič
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Danijela Urbancl: Faculty of Chemistry and Chemical Engineering, University of Maribor, Smetanova 17, 2000 Maribor, Slovenia
Deniz Agačević: Faculty of Chemistry and Chemical Engineering, University of Maribor, Smetanova 17, 2000 Maribor, Slovenia
Eva Gradišnik: Faculty of Chemistry and Chemical Engineering, University of Maribor, Smetanova 17, 2000 Maribor, Slovenia
Anja Šket: Faculty of Chemistry and Chemical Engineering, University of Maribor, Smetanova 17, 2000 Maribor, Slovenia
Nina Štajnfelzer: Faculty of Chemistry and Chemical Engineering, University of Maribor, Smetanova 17, 2000 Maribor, Slovenia
Darko Goričanec: Faculty of Chemistry and Chemical Engineering, University of Maribor, Smetanova 17, 2000 Maribor, Slovenia
Aleksandra Petrovič: Faculty of Chemistry and Chemical Engineering, University of Maribor, Smetanova 17, 2000 Maribor, Slovenia

Energies, 2025, vol. 18, issue 17, 1-19

Abstract: Four biomass residues–rosemary pomace, rosemary cake, grape seed and apple pomace–were torrefied at 250, 350 and 450 °C, and the physical, chemical and structural changes were characterized. The mass and energy yield decreased with increasing torrefaction temperature; the lowest mass (~10.4%) and energy yield (~10.6%) were observed for rosemary cake torrefied at 450 °C. The HHV increased the most for all feedstocks at 350 °C, with rosemary cake reaching a peak value of 36.4 MJ/kg at 350 °C. Ash content increased with temperature due to organic mass loss, while volatiles decreased and fixed carbon increased in most samples. The FTIR spectra showed the progressive loss of hydroxyl, carbonyl and C–O functionalities and the appearance of aromatic C=C bonds, indicating the formation of the biochar. TGA and DTG analyses revealed that the torrefied samples exhibited higher initial and maximum temperatures for decomposition, confirming improved thermal stability. The TGA-FTIR analyses of gas emissions during pyrolysis and combustion showed that the emissions of CO 2 , CH 4 , NO x and SO 2 decreased with increasing degree of torrefaction. Overall, 350 °C was optimal to maximize energy density. The results show that agro-industrial residues can be effectively converted into sustainable biofuels, which offer the dual benefit of reducing waste disposal problems and providing a renewable alternative. In practice, such residues could be used for decentralized power generation in rural areas, co-combustion in existing power plants, or as feedstock for advanced bioenergy systems.

Keywords: torrefaction; fuel; biowaste; thermogravimetric analysis; energy yield; mass yield (search for similar items in EconPapers)
JEL-codes: Q Q0 Q4 Q40 Q41 Q42 Q43 Q47 Q48 Q49 (search for similar items in EconPapers)
Date: 2025
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