Fast Pyrolysis of Dead Trees Yields Bio OilS. Himmelstein | May 04, 2017
More than 40 million acres of forest in the western U.S. have been decimated by the mountain pine beetle, leaving stands of dead trees that can fall at any moment or add fuel to a wildfire. Land managers need a remedy to deal with millions of the precarious dead giants, but harvesting for lumber is not feasible because the infestation stains the wood and causes the tree to crack on the inside.
A University of Washington team is fine-tuning a fast pyrolysis process to remove beetle-killed trees and use them to make renewable transportation fuels or high-value chemicals. The general process involves heating small pieces of organic material in an oxygen-free chamber at about 500 degrees Celsius, until the solid material becomes a vapor. As the vapor rises and moves into other chambers, it cools and becomes a dark brown bio oil.
The beetle-impacted trees are a good fit for making bio oil, partly because the entirety of a tree becomes extremely dry when it is killed by an infestation. That makes for a simpler fast-pyrolysis process, because it isn’t necessary to first dry the wood before heating it to extreme temperatures
The researchers developed a system that can efficiently break down woodchip-sized pieces, though the team has successfully turned an entire log into bio oil. Other fast pyrolysis systems must use small wood pellets 1 to 2 millimeters in length, which often adds an extra step of grinding larger pieces down to the appropriate size before converting them to bio oil.
Woodchips are placed on a rotating surface and a hot stainless steel plate moves down from above, crushing the wood. The woodchips become hot from direct contact with the metallic surface, and the chemical transformation from solid to vapor begins.
The method could be used in mobile pyrolysis units so dead trees can be processed on site, saving on transportation costs associated with moving large pieces of wood out of the forest. Mobile units placed on a small flatbed truck are already being used for standard wood-to-oil processing, and improvements could make the process more efficient and cost-effective.