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Manure Digesters Shed a Different Light on Horse Power

By Suzanne Bush

As the world's demand for energy gallops ahead of suppliers' ability to produce it, rising prices are causing more than gas pains for consumers. Few aspects of daily life remain untouched by the twin demons of pollution and skyrocketing fuel costs. Will we run out of air before we run out of the fuels that are polluting the air? Will corn deliver us from our dependence on foreign oil? While experts disagree about where to look for new energy, whether ethanol is a feasible alternative to oil and how to assess the environmental impact of the internal combustion engine, most concur that there is not going to be a single solution.

The complexity of the dilemma is daunting. But it has not deterred some innovative thinkers who have been whittling the problem into more manageable dimensions by simplifying it. They've gone back to the farm, one of the basic units of human enterprise. And they found a way to solve two problems—a surfeit of animal waste and a shortage of energy—at once, by adding combustion. Manure digesters might not seem glamorous, but they are bringing BTUs and common sense to the quest for energy.

Manure to Energy

Brubaker Farms in Lancaster County, PA recently fired up a manure digester that generates more than 4,000 kilowatt hours of electricity per day, or enough to power more than 150 homes, according to Dennis Wolff, Pennsylvania's Agriculture Secretary. The digester is fed by manure from the 700 cows on the dairy farm. In a state where farming is a key industry, the idea of managing manure by converting it to energy is promising, even though there are some hurdles to overcome. It confers a whole new meaning to horsepower, when you consider the possibilities as the state's equine industry staggers, along with other farm operations, under the weight of thousands of tons of manure daily.

The digester at Brubaker Farms was built by RCM Digesters of Berkley, CA. Eric Fast, an environmental scientist at RCM, explains that in combusting the manure, excess heat is produced along with electricity. "Excess heat is diverted to other places on the farm," he says. "The first use of the power generated is on site, and the excess goes to the power grid."

And the digester reduces pollution by eliminating the lagoons that previously held the manure. "The lagoons would produce methane," Fast says, "and there have been studies done that methane is many times more detrimental to the environment than carbon dioxide." He says the digester is a 90-foot diameter tank, sixteen feet underground.

Larger Scale Projects for More Power Output

"We know our technology is capable of cleanly and efficiently burning horse muck," says Craig Harting, Chief Operating Officer of Global Green Solutions. His company is looking at a project in Florida, where the equine industry needs to dispose of more than 100,000 tons of excess equine waste annually.

One of the issues in these projects is cost-effectiveness. A minimum volume of waste is required to make the economics work out, as there are significant capital costs for building digesters. "We think the number of horses required would be on the order of 20,000, but since our process works on multiple feed stocks, a combination of horse muck, urban wood waste, trimmings, etc. would work. In many cases you can find a combination of materials. Typically biomass plants use material within a 50 mile radius."

He says the ideal situation would utilize the steam from combustion as well as the electricity. "The best application would be one where there was an industrial company that was a heavy process steam user," he explains. "The most efficient way to generate steam is through co-generation where you're generating steam and electric power." Co-generation adds about 15 percent to conversion efficiency of a digester. "You lose efficiency if you generate only steam or only electricity," he says.

The kinds of plants Global Green Solutions builds are significantly larger in output and capacity than the one at Brubaker Farms. "The smallest plant size would generate 13 megawatts a year," he says. That would be enough to supply residential use for about 8,000 homes. The plant would require about 10 acres, with most of the space taken up by the place where trucks unload the material.

Waste Not, Want Not

At the Brubaker Farms facility, the digester's solid waste is put back to work as animal bedding and the excess is sold for mulch. Harting says that the process of converting horse manure and other waste into energy is straightforward, but making sure the process doesn't generate more pollution is critical. "It's actually a very clean process. When you combust anything you get nitrous oxides and ash. Our process is emission efficient. There is an ash stream, but you capture all of that through filters."

He says that the ash byproduct isn't wasted. "Horse muck has about five to seven percent ash," he says. "But it is a good addition for mulching soil." And there's no need to separate manure from the bedding, regardless of whether the bedding is straw or shavings.

Pennsylvania Governor Ed Rendell wants the state to become energy independent, and part of his vision is for consumers to be able to choose renewable sources of energy. "Energy generated from this source is renewable by any definition," Harting says, even though it is not necessarily a panacea. "The challenge is you can't produce renewable energy from any source and have it competitive with coal." But the cost comparison doesn't take into account the reality of the volume of waste that has to go somewhere. "If the alternative is landfilling waste," Harting says, a strict dollar-for-dollar cost comparison isn't reasonable. "The places where this works are places where there's some initiative or change in the law, where people want to do something."

A Crisis on Many Levels

The components of the crisis—and by any measure the fuel situation is a crisis—are so tightly woven into daily life that it's impossible to pull any thread without disturbing the whole fabric. Growing corn for ethanol has affected the supply and cost of food grains. Gallon-for-gallon, ethanol contains less energy than gasoline. Plowing fields, and growing and harvesting corn for ethanol take more energy than the ethanol will eventually yield. Yet ethanol plants are multiplying even as consumers face dramatic increases in food costs. Despite the drawbacks, there don't seem to be many voices questioning the logic of the stampede to ethanol.

Creating the energy-independent future that people like Rendell envision won't happen without significant investments of political will, community support and innovative conservation programs.

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