Ethanol from Sweet Sorghum

I.C. Anderson
Iowa State University

Stalks of sweet sorghum contain fermentable sugars in the sap equal to 400-600 gallons of ethanol per acre,.or about twice that from corn grain. Sweet sorghum is the crop that communities used to grow to make sorghum syrup (molasses). The varieties adapted to Iowa have a mature seed head at time of harvest. The head uses much sugar to produced grain so the stalk yield of sugar is relatively low. We use varieties adapted to southern U.S. that are very late up here so they have an immature head, grow to eights of 10-12 feet. At harvest during September a crop weights 80,000 lbs/acre and is 78% water and 22% dry weight which is much wetter than corn silage.

There are two problems with harvesting and fermenting the sugars to ethanol. First, a weight of 40 tons per acre in itself almost precludes transport to a central facility for fermentation. Secondly, the sugars in the stalk began to “sour” upon harvest or from freezing in the field. We have been successful in overcoming these two problems by spraying the forage in the forage chopper as it comes off the chopping cylinder with sulfuric acid. As the forage goes along the cross auger it is sprayed with a yeast inoculum.

A combination of the cross auger, and the large paddle fan throwing the silage up into the forage wagon thoroughly mixes the forage-acid-yeast which is very essential. The acid lowers the pH to about 4.3 which inhibits bacterial contamination, on the plants, from converting sugar to lactic acid. The presence of lactic acid in the forage inhibits yeast from fermenting sugars to ethanol. The forage is put in a lined and covered pit silo where fermentation is complete in a few days. The ethanol is stable in the pit silo for many months. In summary, we have learned a lot about the production and fermentation of sweet sorghum during the past 12 years. Maximum sugar yield occurs with about 75 lbs N/acre. The hotter and dryer the summer the greater the sugar yield. Our greatest sugar yield was in southern Iowa in 1988 when corn in the experiment yielded 24 bushels/acre.

In the silo the acre of forage still weighs 80,000 lbs with a bulk density of 0.7 which equals 1800 cubic feet or a truckbed 10 feet wide, 8 high, and 22 long. Transport to a central facility would be costly and dangerous. We have attempted various methods of distilling ethanol from the silo, but none are doable or economical for an on-farm process. We continue to try to develop an economical on-farm portable system for distilling the ethanol. Our goal is to produce a product that is 70-90% ethanol, which can be done with simple and relative inexpensive sieve plate towers, and this product could be transported to a central facility for conversion to fuel grade ethanol. (8/00)