Selective Product Enhancement in an Auger Reactor: Pyrolysis of Pine Bark through In Situ Recirculation of Pyrolysis Vapors

In biomass pyrolysis, final product selectivity is governed not only by major reaction conditions like temperature and heating rate but also by complex vapor–solid interactions and secondary reactions. Yet, the influence of internal flow configuration on pyrolysis vapor remains poorly understood in continuous pyrolysis systems. This study aims to evaluate how controlled vapor–solid interactions via changes in the vapor outlet port location affect the distribution and transformation of pyrolysis products. Experiments were performed in a continuous laboratory-scale auger reactor, processing pine bark at highest treatment temperatures (HTT) of 600, 700, and 800 °C. The reactor featured five independently heated zones and six selectable vapor outlet ports, enabling three vapor flow modes: parallel flow (PF, conventional cocurrent flow operation) and two counterflow (CF) configurations to systematically manipulate vapor–solid contact. Results showed that one of the CF configurations, where vapors passed through the coldest (the incoming) biomass zone before exiting, enhanced vapor condensation on incoming biomass and promoted secondary reactions, leading to up to a 15.5% relative increase in biochar yield compared to PF. The increase in biochar yield was accompanied by an increase in fixed carbon yield, and H₂ and CH₄ yields, indicating intensified thermal cracking and polymerization of pyrolysis vapors. In contrast, the CF configuration involving vapor recirculation without interaction with the coldest zone favored external condensation and achieved the highest bio-oil recovery. The PF configuration exhibited the lowest char yield and the highest unaccounted carbon fraction due to poor vapor condensation at elevated outlet temperatures. These findings demonstrate that the manipulation of vapor–solid interactions serves as a critical parameter for steering pyrolysis pathways toward targeted product enhancement, offering a scalable approach for optimizing biochar, gas, and bio-oil yields through in situ vapor recirculation.