Researchers test sweet sorghums for best ethanol production

October 11, 2010

AMARILLO - The continued demand for cleaner-burning fuels has Texas AgriLife Extension Service and Texas AgriLife Research specialists working to determine which varieties of sweet sorghum will produce the most ethanol.

Dr. Brent Bean, an agronomist with AgriLife Extension and AgriLife Research, recently harvested sweet sorghum plots grown at the AgriLife Research farm near Bushland. Bean's plots are a part of a federal Sun Grant project examining the production of biofuels.

The research plots allow the comparison of different varieties and seeding rates of sweet sorghum, Bean said. With the use of a sorghum mill, which squeezes the juice from the sorghum stalks, he can take measurements that ultimately allow him to see how much ethanol would be produced on a per-acre basis.

Bean said the trial is made up of new sweet sorghum hybrids or varieties developed by AgriLife Research sorghum breeder Dr. Bill Rooney. Varieties are planted under both dryland and irrigated conditions. In addition to comparing varieties, Bean is also comparing five seeding rates or seed populations planted.

In each of the 30-foot-long variety plots, 10 feet of the row is hand-harvested with a machete, he said. The stalks are bundled together and taken out of the field where they are weighed, then six plants from each harvested sample are run through the sorghum mill.

The juice from the six plants is collected and placed in an instrument called a refractometer, which measures the amount of light refracted in units called Brix. This Brix value is highly correlated to sugar content of the juice, Bean said, adding it is the sugar from sorghum that is converted into ethanol.

In 2009 studies, it was estimated that irrigated sweet sorghum produced 200 gallons of ethanol per acre, while the dryland sweet sorghum made about 80 gallons of ethanol per acre. Bean said this was assuming that 65 percent of the juice could be extracted from the sorghum.

He said it is possible that a commercial mill might approach 95 percent recovery of juice, which would greatly increase the potential amount of ethanol produced.

A byproduct of removing the juice from the sorghum is the plant material left over, Bean said. This material is called bagasse, and can be placed back on the field as crop residue, used as cattle feed or possibly burned at an ethanol plant as an additional energy source.

The final step to the harvest testing process, Bean said, is to take a sample from each 10 feet of row and run it through a limb chopper. The chopped material is weighed to get a fresh weight and then dried to get the dry weight, which will help determine the percent of moisture of each sample.

"By doing that, we can calculate how much either dry matter or fresh matter we had on a per-acre basis," Bean said. "By knowing that and also knowing our sugar content from our juice, we can calculate ethanol on a per-acre basis."

Texas A&M AgriLife Communications

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