A team of researchers at Arkansas State University is making headway toward solving a big obstacle in the production of cellulosic ethanol — a less-expensive way to produce the enzymes needed to break down plant cell walls. And the answer could come from a corn kernel, of all places.
“You might say we're trying to use corn for ethanol, so why are we using the corn to produce the enzymes?” said Elizabeth Hood, associate vice-chancellor for research and technology transfer at ASU, speaking at the AgTechnology Field Day at the Agricenter in Memphis.
“For us, it's a very good system because we're using enzymes in the germ, which is not part of the corn-to-ethanol process. You can very efficiently separate the germ from the starch and still use the starch in the ethanol plant, while using the germ to produce the enzyme that will digest the corn stover, rice straw or other feedstock.
And best of all, “there is a plenty of ability to not over-produce corn for ethanol, but still have enough enzyme to do what you need to do. In this case, there is no capital investment except for the harvesting equipment.”
Currently, corn grain is the primary feedstock for producing ethanol in the United States. It does not require the use of high-cost enzymes, but there are drawbacks, according to Hood.
“If we use 100 percent of the corn grain crop to make ethanol, assuming we plant 80 million acres of corn, we would reach E-10, or replace 10 percent of our transportation fuel with ethanol. Last year, about 13 percent of the corn crop went into ethanol production, which replaces around 2 percent.”
However, “when we use corn grain for ethanol, we have competition between food and fuel. It's an issue even if the corn isn't used for fuel because it's driving up the price of feed for cattle ranchers and other feeding operations. One of the solutions is to use cellulose as a feedstock for the ethanol industry.”
Cellulose for energy can be made from any plant cell from any source, notes Hood. “You don't have to grow corn or soybeans. You can mow the grass in the back yard and have cellulose.”
Costs of turning cellulose into ethanol using current enzyme technology can range from $2 to $2.93 per gallon of ethanol, which is effectively cost-prohibitive at today's fuel prices. These costs can be reduced using transgenic corn plants which contain the cellulose-eating enzymes. Applied Biotechnology Institute, in San Luis Obispo, Calif., licenses the enzyme-producing corn plants to Hood for her research.
According to Hood, part of the high cost of using enzymes is the sheer volume needed for the job. If the average yield per acre of total corn stover is 3 tons per acre, half of which is harvested, leaving the rest to satisfy requirements for no-till or to maintain soil tilth, then yield would be around 1.5 tons per acre.
“Eighty million acres of corn would produce 120 millions tons of stover, which would yield in the neighborhood of 10 billion to 11 billion gallons of ethanol, just from corn stover. We would need 1.3 million tons of enzymes annually to convert the biomass to ethanol. The volume of enzymes required to meet some of the goals we've set for replacing 10 percent of ethanol fuel is going to be a big issue.”
To get the number of enzymes needed to get to E-10 would cost $10 billion using current technology, according to Hood. “That's a prohibitive capital outlay for the enzyme companies as they are currently structured. The industrial enzymes used to break down cellulose need to be cheaper than any other enzymes ever produced for industrial use.”
One way to reduce cost is to genetically manipulate how much enzyme accumulates in the corn kernel. “Anytime we get higher amounts of enzyme per kernel, we lower the cost of our production. We have all kinds of tricks up our sleeves that we're using to do that.
“We're also working on making the enzymes more efficient. The current enzymes are pretty slow and you need lots of them. So that's part of the problem in creating cost-effectiveness.”
Using more of the corn plant to process ethanol could not only help lower costs of ethanol production through better efficiency, but provide additional income opportunities for farmers.
Hood envisions an ethanol plant situated in the middle of an area approximately 10 square miles. An area of 5 square miles around it would be planted to transgenic corn plants for enzyme production. After the enzymes are extracted, the transgenic plants are fed into the ethanol plant too.
A 1-mile buffer would separate the transgenic corn from traditional corn used for ethanol production from starch and stover. Any type of harvested biomass within a few miles of the facility could also be economically trucked to the plant.
Hood believes feedstocks could provide as much as 43 billion gallons of ethanol from 510 million tons of feedstocks. Rice hulls and similar agricultural residue could yield 1 million tons of feedstock and produce 2.5 billion gallons of ethanol; corn stover, 120 million tons and 1.2 billion gallons; municipal wood waste, 37 million tons and 3.1 billion gallons; forest residues, 44 million tons and 3.7 billion gallons; wood mills, 90 million tons and 7.6 billion gallons; and energy crops, 188 million tons and 16 billion gallons of ethanol. Energy crops could include switchgrass, elephant grass and giant reed.
“The energy proposition for farmers is the new value from current crops when non-food material is harvested; new crops dedicated to energy; enzyme production, which is a value-added crop; and there are the possibility for biomass facility co-ops.”
Hood says contract growers of the enzyme-producing corn would likely receive a 25-cent premium per bushel for identity preservation.