Sweet sorghum is touted as an ideal feedstock for small-scale energy farms because of its low input cost, its sugar content and its drought tolerance. But the forage cutter header used to harvest it isn't working efficiently — plants tend to bunch up as they are fed into the crushers.
“The harvester is really the only Achilles heel at the moment,” said Tim Sharp, a former OSU professor and current operations manager for FASTech, a seller of small-scale ethanol production equipment. “We have a prototype harvester, but it's not ready for farmers yet. We really need one of our equipment companies to pick up the ball and make the system farmer-ready.”
The rest of the operation runs smoothly, Sharp says. The harvester feeds sweet sorghum into a juice extractor consisting of five in-line crushers. The juice is transferred to a container, which sits in a transport trailer. When full, the container is unloaded into the field, then transported to a large on-farm container for fermentation.
Until the harvester is perfected, a small-scale ethanol plant can run on trucked-in cow feed molasses for $130 to $156 a ton, delivered. In Oklahoma, it is available from barges in Muskogee, originating from Louisiana's sugarcane region.
Molasses is mixed with 70 percent water, yeast is added and 48 hours later, it creates a concoction similar to beer. The “beer” is transferred to a cooker. Vapor from the cooker runs through separation columns and is removed, producing 70 gallons per hour of fuel-grade ethanol.
“This is a one-step process, to go from beer to anhydrous ethanol,” Sharp said. “There is not a sieve process, just one pass through the column and you have fuel grade alcohol.”
One cooker and two separation columns will fit in a fairly small building. Capacity could be over 600,000 gallons annually, but production is limited by the energy farm's ability to store or receive its feedstock. A more realistic figure is around 180,000 gallons of ethanol annually. This would require about 300 acres of sweet sorghum and a yield of 600 gallons per acre of ethanol.
A future harvester might be equipped with a dehydrator to address the storage issue. “An acre of sweet sorghum can produce 4,000 gallons to 6,000 gallons of juice. If we can find a way to take some of the water out and get the juice to where it's about 50 percent sugar, where it's more of a syrup instead of a juice, it can be stored for a long period of time.”
Sweet sorghum varieties are available through Mississippi State University and USDA among others, but supplies would have to be ramped up dramatically if sweet sorghum were to become the feedstock of the future. “Currently, all the sweet sorghum is for boutique molasses producers. For them 50 pounds is a big seed purchase.”
The sweet spot on the sweet sorghum stalk ranges from about a foot off the ground to about 3 to 4 feet down from the top of the plant. The plant produces a seed head, which can be captured for livestock feed. The rest of the plant could be used as cattle forage or perhaps for cellulosic ethanol.