Prediction of polycyclic aromatic hydrocarbon species formation using chemical reaction engineering and chemical kinetics models: Effects of pyrolysis parameters and biomass composition

Biochar, a valuable byproduct of biomass pyrolysis, offers significant potential for various applications, but the formation of toxic polycyclic aromatic hydrocarbons (PAHs) during its production remains a critical concern. This study introduces a novel approach by employing the Chemical Reaction Engineering and Chemical Kinetics (CRECK) model to predict the fraction of specific PAH species formed at the core stage of slow pyrolysis. This contrasts with prior studies that focused on species data measurements taken at the reactor outlet from reaction product exit routes under high-temperature (840–1800 K) for either gasification or combustion conditions but not for slow pyrolysis. The CRECK Solid Biomass (CRECK-S-B) model was applied to coffee husk and corncob biomass at heating rates of 2, 10, 20, and 30 °C min−1 and temperatures ranging from 350 to 750 °C. The model's predictions, which generate tar species varying in composition and quantity depending on the heating rate, were used as input for the OpenSmoke gas-phase batch reactor model, with results validated against experimental data. Key findings reveal that the 450–550 °C range favors PAH formation, with the maximum naphthalene (C10H8) fraction (0.0011) occurring at 450 °C, 3000 s, and 10 °C min−1, strongly influenced by naphthyl (C10H7) concentration. To minimize PAH content in biochar, we recommend maintaining temperatures above 500 °C, using a low heating rate (2 °C min−1), and extending biomass residence times beyond 50 min. This study underscores the need for comprehensive experimental studies to further investigate the combined effects of pyrolysis parameters on PAH formation and to optimize biochar production.