Production of Ethanol and Other Chemicals from Corn Stover and Sorghum

Grant# 02-01
Principal Investigator: I. C. Anderson
Organization: Department of Agronomy, Iowa State University
Technical Area: Efficiency and Renewable Energy

Background and Significance:
We have had much experience in the laboratory and with 1500-gallon tanks for the anaerobic digestion of corn stover and mixtures of stover with swine manure to produce methane. Anaerobic digestion involves two groups of microbial organisms, those that convert cellulose to acetic acid (acid bacteria) and those that use acetic acid to produce methane (methanogenes). We can easily inhibit the methanogenes and use the acid bacteria to produce chemicals such as glycerol, lactic acid, ethanol and acetic acid. We have used metabolic inhibitors that preferentially alter the pathway toward the desired product, but too much acetic acid leaks through to lower the pH and inhibit the bacteria so that only low levels of the product are formed. We have found that with a pH controller we can pump in a stirred suspension of limestone to neutralize acetic acid as it forms. We will concentrate on using bisulfate inhibition to produce glycerol, a high value product from corn stover.

For fuels and chemicals from field crops we have used sweet sorghum for ethanol production. Sweet sorghum while being harvested with a forage chopper was acidified and inoculated with yeast and then ensiled. The crop contains about 75% moisture and weighs 80,000 pounds per acre with the liquid portion containing 15% sugar equal to about 800 gallons of ethanol per acre after ensiling. We have tried various methods of distilling the ethanol, but most were not feasible or were energy intensive. We have available at the Iowa Energy Center’s Biomass Energy Conversion (BECON) facility a new type of distillation system consisting of a heated 800-gallon drum, with rotating paddles, attached to a distillation column. After distillation, the forage will be ensiled with an inoculum of lactic acid bacteria for silage or used as a feedstock for anaerobic digestion.

Project Objectives:
The first goal is to use the acid bacteria of anaerobic digestion to produce glycerol and acetic acid in high enough concentrations to make their isolation economical. Corn stover would be the feedstock and the remaining material would be anaerobically digested to methane. This will be demonstrated in 400-gallon tanks at the BECON facility and later published as part of a journal paper.

The second objective is to demonstrate the distillation of ethanol from fermented sweet sorghum silage. The spent forage would be evaluated as potential silage for cattle feed and as an anaerobic digestion feedstock for methane production. This second objective will also be demonstrated to the public at the BECON facility with posters and examples of the materials.

Plan of Work:
For production of chemicals from corn stover we will use the 400-gallon acid tank under conditions such as high temperatures (45°C) and with pH control for producing a bacterial inoculum for laboratory studies on producing glycerol, acetic acid, and ethanol. We will use various glycolytic blocks such as bisulfite and bromoform. The acid tank will be connected to the larger methane-producing tank (1600-gallon) to allow normal two-stage anaerobic digestion. The acid tank will be fed ground stover and water manually as needed according to the percent solids in the acid tank. A sewage sludge inoculum will be added to both tanks at the beginning. Temperature of the acid tank will be slowly increased to an optimum temperature for acid production and to decrease mathanogenes in the acid tank.

Results of laboratory studies should be helpful in adjusting conditions in the acid tank for producing larger concentrations of desired chemicals. Commercially, about one-quarter of the liquid in the acid tank could be withdrawn each day through a screen for isolation of chemicals, or the acid tank could be operated separately and the waste fed to a methane-producing tank.

For sweet sorghum we will harvest and ferment to ethanol as usual. Fermented forage will be loaded into a drum and heated until temperature indicates that the distillation is finished. We will measure the amount of electricity used for the distillation. Samples of the spent forage will be investigated as to its value as silage for cattle and as a feedstock for anaerobic digestion.