Although billions of dollars are being spent on biofuels research, one piece of the puzzle — a cost-efficient, effective way to convert biomass into cellulosic ethanol — is missing.
The starch in corn grain easily converts to sugars that are fermented by yeast into corn ethanol. The whole-plant conversion to cellulosic ethanol, however, isn't as easy, says Matt Digman, USDA-ARS ag engineer at the U.S. Dairy Forage Research Center at Madison, WI.
Cellulose is a main component of a plant's cell wall and does a good job protecting the five- and six-carbon sugars within, he says. A pretreatment process is needed to soften that wall.
Bruce Dale, a Michigan State University chemical engineer, has developed an ammonia fiber expansion (AFEX) pretreatment process. “The process works on all types of cellulosic materials. It works best on grasses and crop residues, but even on some woody species,” Dale points out.
Even so, enzymes still need to be developed to break through to the sugars quickly and cost-effectively. A number of companies are working to produce enzymes, but different enzymes are needed for different feedstocks.
In addition, the yeast process used to ferment the sugars into alcohol will only utilize six-carbon sugars. Purdue University researchers are working on a way to use five-carbon sugars as well, Digman says.
“Cellulosic ethanol has some challenges to overcome,” the ag engineer adds.