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

## TL;DR

This study shows how changing vapor flow in a pyrolysis reactor can increase biochar and gas yields by manipulating vapor-solid interactions during pine bark pyrolysis.

## Contribution

The study introduces a scalable method to enhance pyrolysis product yields through controlled in situ vapor recirculation and vapor-solid interactions.

## Key findings

- A counterflow configuration increased biochar yield by 15.5% compared to parallel flow.
- Vapor recirculation through the coldest biomass zone promoted secondary reactions and higher fixed carbon and gas yields.
- Parallel flow resulted in the lowest char yield and highest unaccounted carbon due to poor vapor condensation.

## Abstract

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.