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A digestible idea

Anaerobic digestion could boost ethanol plant profits by millions of dollars — and make corn ethanol “greener.”

That’s the finding of a new AURI-sponsored study that showed the feasibility of making methane and fertilizer from thin stillage, dissolved corn solids left over from ethanol production. “Ethanol plants could potentially become energy independent if all the energy in thin stillage could be captured in the form of methane,” says study author David Rein of Rein & Associates, a wastewater engineering company based in Moorhead, Minn.

Anaerobic digestion could add $10 million to the bottom line of a 50-million-gallon ethanol plant, Rein estimates. Digestion could also conserve water, earn valuable carbon credits, and boost corn ethanol’s energy balance by offsetting fossil-fuel use, he says.

Anaerobic digestion is a microbial process that produces methane and carbon dioxide, or “biogas,” from organic materials. Biogas is a natural-gas substitute that can be burned in a furnace or used to power a turbine for electricity.

Today, anaerobic digestion is used in many food and ag-processing industries, and for municipal- wastewater treatment. Sugar beet processors, for example, digest wastewater and use the methane to run their dryers. “It’s proven technology that’s been around a long time and is widely used,” Rein says.

Some ethanol plants already use small digesters, called methanators, to clean up their wastewater. But the ethanol industry is not yet using anaerobic digestion to generate power, says Michael Sparby, AURI project director. That could change as ethanol plants seek renewable alternatives to natural gas, he says. “I’m hearing a lot of interest in stillage digestion.”

Energy from coproducts

Stillage is the slurry of corn solids and water left over after corn starch has been fermented and distilled. Ethanol plants separate whole stillage into distiller’s grains, a nutritious animal feed, and thin stillage, which is water and dissolved solids. Usually, thin stillage is concentrated through evaporation and added to the distiller’s grains.

All of these ethanol coproducts “contain significant amounts of energy that could potentially be recovered in the form of biogas, through anaerobic fermentation,” Rein says.

AURI tested both whole stillage and thin stillage digestion, with support from the Minnesota Corn Growers Association, Otter Tail Power Co., Otter Tail Ag Enterprises and the City of Fergus Falls. The research was done at the Fergus Falls Wastewater Treatment Plant, which operates a municipal sludge digester. Methane produced in the digester fuels the plant’s boilers, which can run on either natural gas or biogas. Currently, only a fraction of the digester’s capacity is being used, Rein says.

In a full-scale demonstration, whole stillage was added to the city’s digester to supplement wastewater sludge. The demonstration was a great success, Rein says. With the addition of whole stillage, which Rein calls an ideal feedstock, the digester generated enough biogas to completely satisfy the plant’s fuel needs.

Thin stillage tested

AURI also tested thin-stillage digestion. That may be a more attractive option for ethanol plants, Sparby says, because it preserves distiller’s grains, an important livestock feed and a major revenue source. It also saves the cost of evaporating thin stillage and drying the solubles.

A successful pilot-scale test was run from June 4 through Oct. 15, 2007 at the Fergus Falls waste treatment center. The 10,000-gallon, continuous-feed digester ran very well on thin stillage, Rein says, producing 5.4 cubic feet of methane per pound of organic material. During the 15 to 20 days in the digester, more than 80 percent of the organic matter in the stillage was converted to biogas.

The digestion process also purifies the stillage water, which can then be filtered and recycled. Water conservation is becoming an increasingly important issue for the ethanol industry, Sparby says.

Preventing ‘in-digestion’

One of the problems associated with anaerobic digestion is the formation of “struvite,” a hard, scaly compound that builds up in the digester and fouls the equipment.

Struvite is composed of magnesium, phosphate and ammonia — all important plant nutrients. Thin stillage contains very high concentrations of these chemicals, Rein says. “Struvite is a threat to effective digester operation,” but also “presents a significant opportunity for fertilizer recovery.”

By harvesting magnesium, phosphate and other nutrients from thin stillage before it goes to the digester, ethanol plants could produce more biogas and generate another commercial product, Rein says. “It’s the struvite recovery that makes it workable.”

In experiments at the Fergus Falls water treatment plant, a portable struvite pilot plant removed 89 percent of magnesium from thin stillage, and more than two-thirds of phosphate and ammonia, Rein says. Good quality struvite pellets were produced, which could be sold as a slow release 5-21-1 fertilizer.

The dirt-like biosolids left after anaerobic digestion also make good renewable fertilizer, Rein adds, supplying about 160 pounds of nitrogen per ton.

New products to sell

A 50-million-gallon corn ethanol plant produces about 500,000 gallons per day of thin stillage. By digesting it, the plant could generate 3.2 million cubic feet of methane per day, Rein says. That would be enough to displace about two-thirds of the plant’s natural gas needs. At a natural gas price of $8 per thousand cubic feet, the methane would be worth about $25,000 per day, he says, or $9 million per year.

Also, a 50-million-gallon ethanol plant could harvest 10 tons per day of struvite, which commands up to $1,500 per ton as turf-grass fertilizer, Rein says. The plant could also market 14 tons per day of biosolids, which have a $100-plus per acre nitrogen value and improve soil tilth like manure.

Net income from digestion, not counting capital costs, could reach $28,000 per day or $10 million per year, Rein estimates. And as carbon-trading markets develop, green credits could provide yet another revenue stream, Sparby says. “If you could get most of your energy needs, plus fertilizer and water for recycling, the payback on an anaerobic digester could be pretty quick,” Sparby says — as fast as five years.

Minnesota plant interested

Today, no U.S. ethanol plants use thin stillage as the sole feedstock for anaerobic digestion, Sparby says. A few are digesting whole stillage or combining thin stillage and other organic material. An Idaho ethanol plant, for example, is building a full-scale digestion system that will run on thin stillage plus manure.

In Minnesota, Otter Tail Ag Enterprises, a 55-million-gallon corn dry mill in Fergus Falls is interested in stillage digestion. The company began making ethanol in early 2008 and is now looking for renewable alternatives to natural gas. “Our goal is to be a low cost producer and reduce our fossil fuel consumption,” says CEO Kelly Longtin.

This year, Otter Tail will spend about $13 million on natural gas — its second-largest operating expense after corn. “We don’t see natural gas prices going down a lot either,” Longtin says.

He was pleased with AURI’s stillage-digestion trial results. “We like the amount of biogas it produces and the amount of renewable fertilizer that would come off it,” he says. “That’s a real opportunity, especially when we see what’s happened to fertilizer prices.”

Stillage digestion would also qualify for more carbon credits than other types of renewable power, such as biomass combustion, he says. “Carbon credits could be a very important piece of this. That market in the past has traded at $1.50 to $4 per ton.”

On the down side, he says, anaerobic digestion is “a biological process that can get disrupted. You can ‘kill’ a digester.” And digesters have high up-front costs. Longtin estimates it would cost at least $20 million to build a full-scale digester for the plant.

But overall, Longtin says, “we’re excited about the results of the digestion study.”