Varied directions of heat flow and emission of volatiles impact evolution of products in pyrolysis of wet and dry pine needles

In pyrolysis of biomass via furnace heating, directions for travel of volatiles and heat flow are opposite. This is drastically different from simultaneously heating surface/inner core of a biomass particle with microwave heating. Changing direction of heat flow and volatiles affects volatiles-char interactions and evolution of pyrolytic products. This was investigated herein by pyrolysis of wet and dry pine needles via furnace or microwave heating at 600 °C. The results indicated that microwave heating induced intensive cracking of outer/inner structures of pine needles. This promoted gasification, decreased production of biochar/bio-oil, diminished formation of both light and heavy organics in bio-oil. Microwave heating also accelerated aromatization, forming biochar of higher C/O and C/H ratios and thermal stability. Additionally, formation of “hot spot” in pyrolysis of wet pine needles via microwave heating was evidenced by the phenomenon of decomposition of CaCO3 to CaO in pyrolysis, which also contributed to enhanced aromatic degree of biochar. Besides, pyrolysis of wet pine needles via microwave heating also generated developed pore structures in biochar (151.4 m2 g−1), as the inherent water and carbonaceous organics were heated simultaneously. Hydrogen bonding from inherent water was largely cracked only above 500 °C. Prolonged retention time of H2O reacted with carbonaceous intermediates, generating more aliphatic structures. Additionally, microwave heating also induced formation of unique morphology like massive thorn-like particulate matters on biochar.