Cow Pies to Clean Power
Texas Panhandle feedlot operators clean out mountains of manure each time they ship a pen of beef cattle to market. Where some see a looming environmental problem, others envision a potential renewable energy source.
Panda estimates its energy costs from manure will be $2.14 per million British thermal units, compared with the national average natural gas price of $7.04 per MMBtu in early December. "We're not paying for the manure," says Bill Pentak, director of communications. Panda will, however, pay the transportation costs to move the manure from the feedlot to the ethanol plant. That will result in a big savings for feedlot operators, considering that one large feedlot operator in the area spends $350,000 each year to haul manure away, he says. Panda's Hereford plant is in the Texas Panhandle where, within a 200 mile radius of Amarillo, roughly 2.3 million head, or one-third of the nation's cattle, are fattened in feedlots annually. Each animal produces about a ton of manure a year. Now that's a Texas-sized pile of biomass. Most of that manure is used on nearby irrigated corn fields to avoid using expensive fertilizer. That could change, though, if declining aquifer levels and high energy costs for pumping water reduce irrigated acres, which would reduce fertilizer requirements and manure applications. In the past, surplus manure created disposal problems and environmental concerns. A quarter century ago, Texas A&M University and Texas Agricultural Experiment Station researchers began examining manure combustion as one use for the abundant resource. Dry, ash-free manure contains about 8,500 Btus of energy per pound. In a feedlot, however, moisture and ash content can reduce the percentage of combustible organic matter per pound of manure. TAMU researchers established that the ideal, achievable energy content is 6,500 Btus per pound, which allows for a minimum practical moisture content of 10 percent and a base line ash content of 15 percent. At 6,500 Btus per pound, manure's energy value is roughly equivalent to Texas lignite coal. The first step in optimizing its energy potential, however, is to harvest clean manure.
Proper management is required to keep moisture and ash levels low. Brent Auvermann, Texas Cooperative Extension engineering specialist, says manure absorbs and releases moisture in a dynamic fashion, and can be dried. However, once ash is in the manure it's difficult to remove. Some ash is unavoidable, but ash content can climb to 40 percent or even 60 percent when soil gets mixed into the manure. To prevent soils from contaminating manure, beef research pens at TAES were paved with fly ash. Another option for Texas feedlots is to mix the fly ash with caliche, a calcium-based subsoil, which packs hard and is used on unpaved back roads. Feedlot paving is unnecessary, however, if operators take care when harvesting manure, Auvermann says. "The machinery operation is where the action is," he says. The goal is to scrape the uncompacted manure off of a soil-surfaced feedlot, and to leave a couple of inches of highly compacted manure on top of the soil as a quasi pavement. "We have seen numbers in the order of 5,000 Btus per pound coming off a soil-surfaced feed yard, which is really very good manure," Auvermann says.
Moisture control is another issue. The Texas Panhandle receives an average of 19 inches of rainfall a year, mostly in the form of summer thunderstorms. However, during the region's infamous hot summer days enough moisture is evaporated to keep feedlots dry. Nonetheless, spring snows or rains can turn a feedlot into muck. "A good, conscientious manure harvest program helps you avoid that muck," Auvermann says. "If you leave behind a hard, smooth, well-compacted surface, it will shed rainfall readily. These management objectives that improve fuel quality also improve air quality and drainage."
Partial composting also reduces manure moisture content, says John Sweeten, resident director of the TAES in Amarillo. Piling manure in windrows for about six weeks can reduce moisture from 40 percent to 20 percent. Turning the pile once or twice during that time also helps to break down chunk sizes.
Manure at 20 percent moisture and 40 percent ash content will yield more than 4,000 Btus per pound, a value researchers believe is achievable. Panda has set a 2,758 Btu minimum content for the manure it will use in its power system. "The threshold that Panda Ethanol has set is a fairly lenient standard," Auvermann says.
Panda will be using a fluidized bed gasifier from Energy Products of Idaho, to convert manure into energy. "Fluidized bed gasification is a forgiving process," says Sweeten, who has worked on the manure power project since its inception. The gasification process can use manure with a moisture content of up to 40 percent, particle size is not as important as in some other applications and fuel quality can vary depending on the ash content.
Because of its forgiving nature, gasification was the first technology Sweeten and Kalyan Annamalai, a TAMU professor of mechanical engineering, investigated when they began their studies in the 1980s. At that time, highly sophisticated gasification technologies were not cost effective because energy prices were low, Annamalai says. Discussions with electrical utilities led him to explore the potential for cofiring manure with coal in existing boiler units.
Initially, the utilities were concerned that manure's high nitrogen content would add to nitric oxide emissions at a coal-fired power plant. Annamalai's laboratory tests, however, showed that cofiring manure with coal actually lowered nitric oxide levels. Manure is a natural urea source, he explains, which is used in some reburn processes, along with a catalyst, to reduce nitric oxide emissions. When manure burns it releases urea which combines with the nitric oxide formed by burning coal to create harmless nitrogen and water. Tests at a U.S. DOE pilot-scale facility confirmed his laboratory tests showing cofiring coal with 10 percent manure can reduce nitric oxide emissions by 80 percent, and when used in a reburn process reduce nitric oxide more than 90 percent under certain conditions.
This spring, more experiments are scheduled to test manure's performance in a large-scale boiler, which is more closely configured to commercial-scale power plants. Those tests will also measure whether manure helps to reduce mercury emissions. Manure contains a small amount of chlorine, which combines with mercury to form mercury chloride, a compound which can be removed in the power plant's scrubbers to reduce emissions, Annamalai says.
If these effects prove out in large-scale tests, cofiring and/or reburning manure with coal may become a low-cost method of reducing power plant emissions. In addition, because manure is a renewable resource, it would contribute to a net reduction in carbon emissions. "Then the problem is, we won't have enough manure," Annamalai says. If the Amarillo area's power plants were cofired using 10 percent manure, they would use up all the manure in the area, he says. An economic analysis indicates manure can only be cost effective if it's transported a maximum of 60 miles, but that could be extended to 200 miles if carbon credits could be sold on the Chicago Climate Exchange.
Annamalai continues to push his manure power investigations further. He wants to see how 5 percent manure performs in cofiring and to test manure's performance with the newer, low nitric oxide burners that are being developed. He is also working on mathematical models to examine whether high-moisture manure could be used for power. This model would call for the utilization of manure gathered in high-moisture conditions or from dairy operations that use water for manure flushing. Such a process might also be used to extract power from municipal sewage sludge. BIO
Susanne Retka Schill is a Biomass Magazine staff writer. Reach her at firstname.lastname@example.org or (701) 738-4962.