USDA releases report on use of manure for energy

Manure can be used to produce energy commercially and on farms without competing with the supply needed for fertilizer, but the economics might not be beneficial to all farmers, according to a report the USDA produced for Congress titled "Manure Use for Fertilizer and for Energy."

Interest in manure-to-energy systems is growing, but implementation is scarce in the U.S. Anaerobic digestion and combustion are the most common processes used to obtain carbon dioxide and methane for electricity generation, the report says. Most digesters are on dairy and hog farms. Combustion can be beneficial to fuel large power plants with poultry litter and fed cattle manure, which have higher energy and lower moisture content. Only one combustion plant operates in the U.S., using turkey litter, and digestion systems cover less than 3 percent of dairy cows and less than 1 percent of hogs, according to the report.

Using manure for energy won't impose substantial constraints on manure for fertilizer supplies, the report says, because the technologies do not consume the nutrients that are beneficial for plant growth. In anaerobic digestion, the nitrogen, phosphorus and potassium remain in the effluent to be spread on fields. Digestion also eliminates odors and nearly eliminates pathogens, according to the report. Combustion plants burn nitrogen nutrients, but leave the phosphorous and potassium in concentrated form in the ash residues. In addition, manure-to-energy projects function in markets for fertilizer and energy and will be most economical in those areas where acquisition costs of manure are lowest, the report says. In turn, manure costs will be lowest where manure is in excess supply, with the least value as fertilizer, the report said.

Adopting digestion is costly, however, and while farmers can produce their own electricity, few can realize enough savings to justify the expense, the report said. Costs include capital, operation and maintenance, adapting existing manure handling and storage to biogas systems and the farmer's time spent learning about and maintaining the system, according to the report. Benefits are numerous, however, and include avoided costs of electricity if the biogas is used on-site for generation; avoided propane, fuel oil or natural gas purchases if heat is recovered; revenue from the sale of excess electricity to the local utility or from the sale of methane gas; avoided costs of commercial fertilizer and herbicides; avoided costs of bedding made from digested solids; and revenues from the sale of carbon credits. Farm size and location should be taken into consideration, as expenses can vary widely. Social benefits of on-farm anaerobic digestion include methane capture and the replacement of fossil fuels. Those benefits have led to proposals that support the use of manure for energy projects through state utility mandates, subsidies for capital costs and direct subsidies and credits for energy production, the report says.

By the summer of 2008, 91 commercial dairy farms were using digesters in the U.S. and another 64 had projects in the construction, design or planning phase, the report said. In addition, the EPA reported that 17 hog farms had operating digesters by that same time period, using the manure supply from 355,000 hogs. Large dairy and hog farms are more likely to adopt digesters, but it's not widespread, the report said.

Commercial combustion plants are still in their infancy in the nation, the report stated, with only one large plant in Benson, Minn., using turkey litter to produce 55 megawatts of electricity, sold to Xcel Energy. Another plant is proposed in Bozrah, Conn., and one is under construction in Hereford, Texas.

The potential for generating methane is greatest when manure is collected and stored as a liquid, slurry or semi-solid, the report said, adding that biogas potential is greatest at large dairy and swine operations because they use liquid or slurry manure.
Public support will play an important role in the widespread use of manure-to-energy systems, the report concluded.

-Lisa Gibson