Biomass '09: Agricultural anaerobic digestion trends on the rise; more potential exists

By Lisa Gibson
Posted July 15, 2009, at 5:15 p.m. CST

National trends in anaerobic digestion of agricultural manure have increased between 2000 and 2007 from fewer than 50 million kilowatt hours per year to more than 200 million kilowatt hours per year, according to Dan Stepan, senior resource manager with the Energy & Environmental Resource Center in Grand Forks, N.D., and a presenter at the organization's Biomass �09: Power, Fuels, and Chemicals Workshop Wednesday.

A key niche for the process is converting biomass materials to methane gas. In the U.S. this year, 98 anaerobic digesters are using dairy farm manure, 19 use hog manure, three use manure from caged layers, two from ducks and one each from boilers, beef and mixed manure, Stepan told the crowd.

"But there's still potentially a large untapped resource," he said. The potential biogas to energy production from swine farms is more than 3.1 billion kilowatt hours per year, he showed in a graph, and the potential from dairy farms is more than 3.3 billion. About half of the country's waste water treatment facilities have anaerobic digesters, but only 19 percent use the biogas, Stepan said.

Anaerobic digestion is an old technology. "By �old,' I mean really old," Stepan said. The process was used in Assyria in the 10th century to heat bath water and has been used in the U.S. for the past 100 years to treat municipal and industrial waste and wastewaters. According to Stepan, it's an attractive solution for several reasons: the high water content of many biomass materials make them impractical for combustion; drying costs to achieve a combustible condition exceed the value of energy recovered by combustion; and anaerobic digestion produces valuable fuel gas.

But the process does have its challenges with different feedstocks, Stepan said. "Siloxanes are a unique attribute of municipally-derived biogas," he said. The volatile silicon-based compound is used today in personal care products and paints, among other products. It can be found in municipal digester and landfill biogas at high concentrations and forms silica, or glass, when it's combusted. Accumulated silica damages engine cylinders, turbine blades, exhaust heat exchangers and piping. The typical processes to control Siloxane include refrigeration at less than 40 degrees Fahrenheit followed by activated carbon; advanced refrigeration to minus 25 degrees F; and selexol liquid absorption, Stepan said.

Hydrogen sulfide is another gas produced during anaerobic digestion and control techniques include chemical, physical and biological processes. EERC has developed a proprietary sulfide control process with a blend of ingredients that minimizes the production of hydrogen sulfide, kills the bacteria that produces it, and scavenges any that is produced. It also possesses long-term control effects and comes at a low cost, Stepan said. EERC will demonstrate its sulfide control technology capabilities during anaerobic digestion of dairy manure on the Haubenschild Farm Dairy in Princeton, Minn. The project will take place over the next 2� years and consists of three phases: lab screening experiments, bench-scale testing and pilot-scale demonstration, Stepan said. Lab screening experiments are taking place now and bench-scale digester design activities have been initiated.

Haubenschild Farm has a 500,000 gallon digester that uses the manure from 850 cows, Stepan said. The process produces 72,500 cubic feet per day of biogas with a methane content of 60 percent. The combined-heat-and-power unit, made with a diesel engine and an electrical generator, generates enough electricity for the farm's operations plus 60 homes and enough heat for the digester and all other buildings on the farm, Stepan said. The digested manure is used as fertilizer, which saves an estimated $40,000 a year, he added.