|The AFEX process. Click to enlarge.|
A patented pre-treatment process developed by Michigan State University professor Bruce Dale can reduce the cost of fermenting cellulosic biomass to ethanol by reducing pre-treatment processing steps and eliminating the need to add additional nutrients to support fermentation. A paper on the work is published in the current issue of the Proceedings of the National Academy of Sciences.
The AFEX (ammonia fiber expansion) pretreatment process uses ammonia to make the breakdown of cellulose and hemicellulose in plants 75% more efficient than when conventional enzymes alone are used. Cellulose in plants must be broken down into fermentable sugars before they can be turned into biofuel.
Doctoral student Ming Lau and I have shown that it’s possible to use AFEX to pretreat corn stover (cobs, stalks and leaves) and then hydrolyze and ferment it to commercially relevant levels of ethanol without adding nutrients to the stover. It’s always been assumed that agricultural residues such as corn stover didn’t have enough nutrients to support fermentation. We have shown this isn’t so.—Bruce Dale
Currently, pretreating cellulose with acid is a common way to break the material down into fermentable sugars. But after acid pretreatment, the resulting material must be washed and detoxified. That removes nutrients, leading to the mistaken idea that crop waste lacks the necessary nutrients, Dale said.
Cellulosic material pretreated with the AFEX process doesn’t have to be washed or detoxified, allowing ethanol to be created from cellulose without added nutrients or other steps.
Washing, detoxifying and adding nutrients back into the pretreated cellulose are three separate steps. Each step is expensive and adds to the cost of the biofuel. Breaking down cellulose into fermentable sugars cost effectively has been a major issue slowing cellulosic ethanol production. Using AFEX as the pretreatment process can dramatically reduce the cost of making biofuels from cellulose.—Bruce Dale
The research also shows that the chemical compounds created when the stover goes through the AFEX process can improve the overall fermentation process, according to Lau. “This is at odds with the general perception that these compounds are detrimental and should be removed.”
The AFEX process. In the AFEX process, which operates under relatively mild conditions, biomass is heated (~100 °C) with concentrated ammonia. Rapid pressure release ends the treatment. Ninety-nine percent of the ammonia is recovered and reused, while the remainder serves as a nitrogen source downstream for fermentation.
The AFEX process decrystallizes cellulose, depolymerizes hemicellulose and depolymerizes, and remove lignin from cellulose/hemicellulose. AFEX minimizes degradation of the sugar in the biomass.
The next step could be a pilot plant, Dale said, perhaps at MBI International. MBI, a subsidiary of the MSU Foundation, partners with universities and companies to commercialize technology. Dale said that they are working to make AFEX technology fit with the needs of several companies, including Mascoma, which plans to open a cellulosic ethanol plants in Michigan.
This research is supported by the Great Lakes Bioenergy Research Center and the MSU Research Foundation. Dale’s research also is supported by the Michigan Agricultural Experiment Station.
The Role of Cellulosic Conversion (Dale, 2007)