Poplar-Based Bioethanol Production Awaits Spike In Price Of Crude

by Bruce Dorminey (Renewable Energy World) As the world’s fastest growing temperate tree species, poplar can add a dozen feet annually and reach maturity in as few as four years. One of its hybrids, populus x generosa, has the widest land suitability footprint of any bio-energy crop, including willow and switchgrass. But the poplar species’ malleable, rich genetics also make it ideal for maximizing its potential as a source of fuel-quality cellulosic ethanol.

Related to the cottonwood and Aspen, this white, light wood has traditionally been used for pulp, paper, and plywood veneer. In North America, it’s the leading source of chopsticks and wooden matches. But a growing number of researchers think poplar is missing its calling as a bioenergy feedstock.

“We would like to use [hybrid] Poplar bio-ethanol as a drop-in aviation fuel in the Northwest,” said Rick Gustafson, a University of Washington chemical engineer and project director of Advanced Hardwood Biofuels Northwest, a consortium of university and industry partners.

Gustafson says it grows well on marginal land that’s not productive enough for agriculture. He notes it’s not quite as easy to break down Poplar’s cellulosic sugars as agricultural residues.

“The idea is that you harvest it when you need it,” said Gustafson. “A harvester chips it onto a truck; then you take it back to the factory and dump it into the feedstock system.”

After harvest, the stand can be regenerated by vigorous re-sprouting or coppicing of the severed root stumps, says Brian Stanton, GreenWood’s plant breeder and the chief science officer at GreenWood Resources, a Portland, Oregon-based natural resource management company.

“The risk of managing a crop on a rotation of three years is much less than when you grow it for twelve or fifteen years,” said Stanton. “A density of 1500 stems per acre can grow for three years and then we cut it and send that material to the refinery.”

“Crude is now around $65 a barrel and would have to go back to $100 a barrel to make it economically sustainable,” said Tuskan. “Startups are having a hard time getting venture capital and resources to build new facilities.”

But if carbon emissions were taxed in the U.S., then these cellulosic ethanols look much better than corn-based ethanol, said Gustafson.

Gustafson says the poplar’s lignin content — a polymer that provides the glue in the tree’s cell walls — can be lower and is more reactive in poplars than other tree species, which makes it easier for bio-refiners to get at the species’ cellulosic sugars.

Poplar’s fermentable glucose sugars are mostly extracted from the tree’s cellulose. But hemi-cellulose, another polymer that gives the cell wall it strength and structure, can produce alternative fermentable sugars like xylose. Gustafson says a steam explosion pretreatment catalyzed by an acid can help break down the hemi-cellulose.

Gustafson says they would then use enzymes to break down the cellulose into individual glucose molecules. From there, its glucose molecules would be fermented. The Poplar’s lignin is itself a byproduct that can be fed to a boiler to generate electricity and steam helps offset some of the costs.