Extra-carbonization of biochar pellets obtained after co-torrefaction and co-pyrolysis with bituminous coal

The paper investigates the chemical and structural changes in biochar pellets resulting from the co-torrefaction and co-pyrolysis of pine wood ( Pinus sylvestris ) pellets with bituminous coal at a 1:1 ratio at 300, 500, and 700°C. At 500°C co-pyrolysis, noticeable changes occur in biochar during co-treatment with coal, as observed by Fourier-transform infrared spectroscopy (FT-IR), graphite-assisted laser desorption/ionization-Fourier transform ion cyclotron resonance mass spectrometry (GALDI-FT-ICR-MS), and Raman spectroscopy compared to individual pyrolysis. Proximate and ultimate analyses show an increase in carbon content and decreases in volatile matter, oxygen, and hydrogen in co-treated biochar pellets. FT-IR and van Krevelen plots indicate compounds with lower O/C ratios and fewer oxygen-rich functionalities. Raman spectroscopy shows that, at 300 and 500°C, co-pyrolyzed biochar pellets have slightly more structural ordering and less disorder than biochar pellets from individual pyrolysis. This suggests that co-processing allows the creation of a more ordered aromatic carbon structure. GALDI-FT-ICR-MS indicates an increase in condensed aromatic hydrocarbons at the expense of lignin-like and oxygen-rich structures compared with individual pyrolysis. Specifically, 500°C co-pyrolysis shifts biochar pellets composition toward lower O/C ratios (0–0.3); products are enriched in condensed aromatic hydrocarbons and occupy van Krevelen regions with low O/C (0–0.2) and H/C (0.4–0.8). The results demonstrate extra-carbonization effects in biochar pellets during co-pyrolysis with bituminous coal at 500 and 700°C.