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Published February 15, 2010, 11:23 AM

Methane moves to front burner

CROOKSTON, Minn. — Methane gas may be getting more attention soon as a renewable energy source and a way to contain odor issues, which have plagued ranchers situated near towns and cities.

By: Matt Bewley, Agweek

CROOKSTON, Minn. — Methane gas may be getting more attention soon as a renewable energy source and a way to contain odor issues, which have plagued ranchers situated near towns and cities.

“One of the things I’ve wanted since 9/11 was to get our country independent of foreign energy,” says Christo Robberts, director of the manufacturing management program at the University of Minnesota-Crookston. “For me, methane is about the only way we can get there with an environmental friendliness and for something that we can sustain into the future.”

The other biofuel

Methane has been the also-ran in the biofuels race, despite a years-long record of being used in some interesting ways to save energy and reduce odor and greenhouse gases. It does not compress easily, so it cannot be retrofitted onto vehicles the way propane units can. But it does have its strengths.

It can heat farm shops during the winter and fuel gasoline generator sets to supply electricity, all while making use of bio-wastes such as manure and food garbage and reducing the odors normally associated with them.

Robberts thinks the possibilities for methane are worth studying “because we can do so much with so little.”

“I think we can do more with methane than what we can do with ethanol, and the energy balance in methane is much more positive than what it is with ethanol,” Robberts says.

He says that, ounce for ounce, methane can deliver more energy than ethanol. The processes used to create a given amount of ethanol energy require more energy than the fuel can deliver. The same is true of methane, but Robberts says that methane is a lot closer to delivering the same amount of energy required to create it.

“There’s always energy loss, but it’s a much higher-yielding process than what we have with ethanol,” he says.

He points out that he is all for ethanol and supports its ability to provide transportation fuel.

“The conversion, although it’s not a perfect conversion, it is better than not converting at all,” he says.

Basic digesters

Methane digesters use anaerobic or oxygen-free decomposition to convert manure into methane gas. Without oxygen, bacteria in the manure decompose the organic matter and produce a biogas composed of about two-thirds methane and one-third carbon dioxide.

The most common type of methane digester, the plug flow digester, also is the simplest in design and therefore popular in rural areas of poor countries, where the drawn-off gas can be used for cooking and to heat water.

These are long trenches, usually about 15 feet, dug into the ground and filled with animal manure and sometimes food waste. It is capped with an airtight plastic tarp with enough slack in it to allow for inflation with the biogas. Sealed into this flexible tent is a PVC pipe used to extract the gas and deliver it to the home.

Each day, animal manure is fed into one end of the digester, pushing the processed matter toward the opposite end, where an equal amount is drawn off to maintain the proper level inside the digester tank. The processed matter is high-nutrient slurry that can be applied as fertilizer on crops.

Programs in poor countries are installing small units, about 1 yard wide by 5 yards long, for around $300. Larger, more complex systems are in use in the United States.

Digester diets

When cows eat grass, they convert it inside their four stomachs into usable energy. All ruminants have this efficient energy conversion system, but the resulting manure has little to offer in the way of stored energy. To some extent, the same is true of horse manure.

Both typically contain a certain amount of fiber, which is important to the methane digester’s chemical process, but qualify as no better than low-grade feedstocks. Their digestive systems are too efficient.

But Robberts says there are other types of manure that could be put to good use.

“If we look at pig manure, you have a much higher energy value,” he says. “And if we look at human waste streams, those are a very much higher stream.”

He says he would like to route city waste streams as a primary feed source into methane digesters. Unfortunately, the less-fibrous textures of swine and human waste generally inhibit the naturally induced “stirring” inside the digester.

This is where the cow and horse manure comes in.

“If we add the cow manure or the horse manure or any of the manures with a lot of fiber in it, the actual fiber itself acts as a way in which we can move the whole fluid,” Robberts says. “It will stir itself.”

Vegan digester

Part of the energy quotient in manure also could be based on fat content.

“One thing with a methane digester; it works better when you have high-

energy substrates; something that’s got a lot of fat in there,” Robberts says.

He is considering oilseeds as a feedstock, particularly some canola varieties. He says some of the seeds with higher fat content would “work wonderfully in a digester.”

He admits that he’s not sure whether the perfect kind of canola exists, as varieties destined for human consumption may be overshadowing the inedible, higher-energy varieties in research.

“We might have some canolas out there that are even higher-yielding in oil that we’re not utilizing because we cannot eat it. But for an energy source, we can bring in crops that we don’t even know about that will be high-yielding oil crops; totally unsuited for human consumption, but will work very well in a digester.”

He says that means there is a lot of valuable research that can be done on high-yielding oil crops.

Farm scale

So what could a farm-scale methane digester do for a farming operation?

“If we look at a farm-level digester, the goal is to produce energy,” Robberts says. “With methane, you can run an internal combustion engine, so you can run a pump there, or a generator.”

It can be used for heating, too. Both concepts have been in use at a landfill, according to assistant professor Paul Aakre, who has been studying ag power and machinery for several years at the Crookston campus.

“The best example of a methane digester in use around here right now, in my judgment, is down at the Fargo (N.D.) landfill,” he says.

They draw methane gas through pipes driven down into the soil at the 40-year-old landfill. They clean the gas and pipe some of it to a nearby Cargill processing plant, where it is used to run a steam boiler for heat. The rest goes into the landfill’s transfer building, where it is used for both steam heat and running a big Caterpillar gas engine to generate electricity. This power is supplemented with wind and solar energy.

“That’s quite a project down there, now,” Aakre says. “They’ve been doing that now for two or 2½ years.”

The Cat engine runs 24 hours a day and has been subjected to a certain amount of methane corrosion, which led to a recent $70,000 overhaul.


One solution that methane might offer would be to turn hog and cattle feedlot operations into better-smelling, more self-sustained neighbors, Robberts says.

Owners of hog farms, for instance, “might want to put a cattle facility next to it . . . and then put all that stuff into a methane generator,” he says. “We would have gotten rid of two very, very bad problems and ended up with anincredible energy source.”

But there’s a lot of research left before this can be put to effective use.

“My vision is we should be doing the research to be able to design the things for a specific application on a farm that would have minimum cost involved,” Robberts says. “It’s no good having a thing save you $10,000 if it’s costing you $50,000 every year.”

He says that when the British installed simple digesters in Nepal in the 1960s, they refined their designs over time and ended up with a simple but efficient digester.

“They were nonmechanical totally,” he says. “They were self-sustaining, and they ran for years with a minimum of input.”

He likens the design to a septic tank system, except that gases and overflow are being captured and used. But on-farm applications tend to be more complex.

“It’s not a turn-key operation,” Aakre says. “With most of these digesters, there are things that go wrong. The pH gets too high or too low, or something else happens, you lose some heat.”

He says that there probably are fewer than 10 digesters in the cattle-rich state of Wisconsin. Most of them have been in operation for less than five years, and some have had problems.

“It’s an old technology, but if it was an easy technology, there’d be a lot of digestion equipment,” Aakre says. “But it does take a lot of the stink out of the manure.”