FEW: Producing biogas from thin stillage can reduce costs, carbon footprint

By Ryan C. Christiansen | June 03, 2009
Report posted June 17, 2009, at 3:24 p.m. CST

To reduce ethanol production costs and also their carbon footprints, ethanol producers might consider using anaerobic digestion to process thin stillage to produce biogas, which can be used to replace natural gas and to provide a cleaner effluent for use during fermentation, according to Eberhard Veit, research and development manager for Eisenmann AG. Veit gave a presentation outlining Eisenmann's process at the 25th annual International Fuel Ethanol Workshop and Expo in Denver.

Eisenmann's patent-pending EtOH-TS anaerobic digestion process technology can be used to process thin stillage, a byproduct of the ethanol production process that is normally recycled to be reused during fermentation, to improve water and energy balance at an ethanol plant and to improve the performance of the plant's fermentation process, Veit said. He said the EtOH-TS process produces both biogas—which can be used in place of natural gas for drying distillers grains or used to generate electricity—and also a clean effluent, with fewer solids and organic acids, which can be recycled to be used in the fermentation process at the plant. The process removes 100 percent of the lactic acid, glycerol, and waste ethanol, as well as 94 percent of the acetic acid from the thin stillage, which is six times less than the threshold where the acid begins to inhibit yeast, he said. The process also removes as much as 78 percent of the total solids from the thin stillage, 52 percent of which are organic solids.

A typical 50 MMgy ethanol plant that produces 80 gallons per minute of thin stillage might produce as much as 24 million Btu per hour of biogas using EtOH-TS, Veit said. He said the biogas is 62 percent methane and the load on the ethanol plant's evaporator might be reduced as much as 21 percent.

Because thin stillage is an acidic byproduct, Veit said Eisenmann had to find a neutralizer to control the pH. To avoid increasing capital or operating costs, Eisenmann developed specialized ammonia-adapted microorganisms for use in its anaerobic digesters so that ammonia—a product ethanol producers already store and handle—can be used as a neutralizer. Veit said the ammonia used in the digester is recycled and reused in the effluent, providing nutrition for the yeast during fermenation. Micronutrients are also fed to the digester to aid the microorganisms with digestion.

An additional benefit to having a thin stillage anaerobic digestion system, according to Veit, is that during ethanol plant maintenance, the thin stillage tank can be dumped into the anaerobic digester to produce more biogas. Veit said the system can be used to process any mix of syrup and thin stillage. He said the payback period for installing a system is five years or less.

Eisenmann has over 50 industrial biogas systems and over 800 water treatment systems installed worldwide.