Waste Not, Want Not

"Value-Added Process" Produces Ethanol from Crop Waste

If you want to get more for your money, all you have to do is waste less. Nazmul Karim, Linda Henk, and other researchers at Colorado State University are applying this simple maxim to make agricultural crops more valuable, produce cleaner-burning alternative fuels, and reduce agricultural waste, all at the same time.

"Our work for the Agricultural Experiment Station is to devise processes that make use of the parts of agricultural crops that normally are thrown away," Karim says.

Karim, a professor; and Henk, a research scientist and microbiologist; in the Department of Chemical Engineering; James Linden, a professor in the Department of Microbiology, Pathology, and Immunology; and others are working on a process for producing ethanol from lignocellulosic material in agricultural waste.

"All plants, grass, trees, you name it–all the parts that aren't used for food and other products are of lignocellulosic material, which can be utilized for a higher value other than burning or destroying or letting it rot," says Karim.

Lignocellulosic material is a combination of cellulose, a complex carbohydrate that forms the main constituent of the cell wall in most plants, hemicellulose, and lignin. Lignin is a polymer that strengthens plant tissue by binding cellulose fibers. For most agricultural products, these materials are regarded as waste.

Current processes for producing ethanol use corn kernels, which makes ethanol production more expensive because corn kernels have value as a food product. Using lignocellulosic material would reduce the manufacturing cost of ethanol but only if the challenge of deriving sugars from this material is first overcome, says Karim.

Ethanol production consists of acid hydrolysis pretreatment of plant matter to break the polymers and derive sugars that can be fermented to make ethanol. Karim's team is investigating two acids–sulfuric and phosphoric acids–for pretreatment on ligno-cellulosic materials.

"Sulfuric acid does a good job deriving sugar that can be fermented into ethanol, but there are other difficulties," Karim says. Sulfuric acid leaves residue and compounds that are detrimental to the fermentation process, requiring additional purification steps.

The team also is investigating phosphoric acid pretreatment.

"Phosphoric acid pretreatment produces a little less sugar than sulfuric acid, but it does not produce some of the harmful by-products, or it produces them at greatly reduced levels. And phosphoric acid pretreatment produces phosphates, which are a beneficial by-product because they are required by microorganisms used in fermentation."

The pretreatment breaks down lignocellulosic material into xylose (a sugar), cellulose, and lignin. The cellulose is converted to glucose (also a sugar) through treatment with enzymes. Henk is investigating the use of cellulase enzymes for that purpose.

"The pretreatment breaks apart the woody structure so the cellulase enzymes can access the cellulose," Henk says. "The enzymes actually break apart the cellulose molecules, which are chains of sugars hooked together."

The sugars are destined for fermentation into ethanol. The lignin can be used as a medium for growing mushrooms–another value added by the process. A final obstacle is that xylose, a five-carbon sugar, is not as readily fermented into ethanol as glucose, a six carbon sugar. To clear this hurdle, the researchers turned to the National Renewable Energy Laboratory (a DOE facility) at Golden, Colo., that has developed a genetically modified microorganism, Zymomonas mobilis, which can break down both glucose and xylose.

"Right now, we are comparing the different methodologies to find the most efficient process," says Karim.

Karim acknowledges there are obstacles to widespread adoption of ethanol. In Colorado, oxygenated fuel with 10 percent ethanol is sold at gasoline stations, because it produces less pollution, but ethanol production currently is subsidized by the federal government because of the cost.

"Gasoline is kept at an artificially low price in this country, which makes this technology seem expensive," Karim says. "But if Americans were forced to pay the prices consumers elsewhere in the world do, it becomes more feasible."

Either way, Karim, Henk, and the others are determined to make the process work because it makes sense on so many levels.

"We've worked on this process since the late 1970s during the Carter administration," Henk says. "It will reduce our dependency on foreign oil, it will reduce carbon emissions from fossil fuels, and it will help the agricultural economy in rural America."