Biomass fuels: Animal wastes

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1.2 Animal Wastes
Animal wastes include manures, renderings, and other wastes from livestock finishing operations. Although animal wastes contain energy, the primary motivation for biomass processing of animal wastes is mitigation of a disposal issue rather than generation of energy. This is especially true for animal manures. Animal manures are typically disposed of through land application to farmlands. Tightening regulations on nutrient management, surface and groundwater contamination, and odor control are beginning to force new manure management and disposal practices.

Biomass technologies present attractive options for mitigating many of the environmental challenges of manure wastes. The most common biomass technologies for animal manures are combustion, anaerobic digestion, and composting. Moisture content of the manure and the amount of contaminants, such as bedding, determine which technology is most appropriate.

With over 300,000 head of cattle (more than any other county in Washington) Yakima County has more than adequate volumes of cattle manure necessary for a biomass project. These cattle collectively produce roughly 4,500,000 tons of raw manure per year; and this does not include Yakima County’s significant populations of sheep and other livestock.

The dairy industry in particular is well suited to biomass-to-energy opportunities because of the large volume of manure that a milking cow produces, and because dairy operations have automated and frequent manure collection processes. Yakima County is the largest producer of dairy products of any county in the State, and the dairy populations within the County include approximately 75,000 to 85,000 active milking cows on about 80 separate dairies.

1.2.1 Dry Animal Manure
Dry animal manure is produced by feedlots and livestock corrals, where the manure is collected and removed only once or twice a year. Manure that is scraped or flushed on a more frequent schedule can also be separated, stacked, and allowed to dry. Dry manure is typically defined as having a moisture content less than 30 percent. Dry manure can be composted or can fuel a biomass-to-energy combustion project. If all the manure from Yakima County’s 300,000 head of cattle was collected and dried, it would amount to approximately 550,000 tons of biomass fuel per year. This is more than adequate for a commercial-scale, centralized biomass combustion project.

Animal manure does have value to farmers as fertilizer, and a biomass-to-energy project would need to compete for the manure. Some dairy and livestock operations would not be interested in disposing of their manure at a biomass-to-energy project, especially if they were responsible for the cost of transportation and a tipping fee. However, the total volume of manure produced in Yakima County exceeds the amount of fertilizer required for the farmlands, and Nutrient Management Plans are beginning to limit the over-fertilization of farmlands. Therefore, although there are competitive uses for the manure and low-cost disposal options at this time, manure disposal is going to become more costly over time, and the demand for alternative disposal options, including biomass-to-energy, will only increase.

1.2.2 Wet Animal Manure (Dairy Manure Slurry)
Wet animal manure is typically associated with larger and more modern dairy operations that house their milking cows in free-stall barns and use a flush system for manure collection. The combination of free-stall barns and manure flushing collects all of the milking cow manure with every milking cycle, two or three times a day. The manure is significantly diluted through the addition of the flush water, but after separation of some of the flush water, the slurry is an excellent fuel for biomass-toenergy processing through anaerobic digestion technology.

The technology of anaerobic digestion is described in greater detail in Section 3.3 of this report, but in terms of biomass fuel, a manure slurry concentration of about 6 percent solids is ideal. The average full-size, 1,400-pound milking cow produces about 112 pounds, or 13.5 gallons, of raw manure every day with a 12.5 percent solids concentration as excreted. Dilution with flush water to a 6 percent solids concentration for anaerobic digestion results in an average 28 gallons per milking cow per day. This is a significant volume of manure slurry, over 51 million gallons per year for a 5,000-head dairy. Yakima County has more than enough dairy manure slurry to supply a commercial-scale anaerobic digestion project.

For optimal anaerobic digestion, the manure slurry must be collected and transferred to the digester continuously, as the manure will begin to break down and release its energy immediately upon excretion. A digester located on the dairy site would use piped systems to accomplish the transfer of the slurry directly from the manure collection system to the digester. However, for a large-scale, centralized digester facility processing wastes from multiple dairies across the County, transportation of slurry at 6 percent concentration is prohibitive. A typical manure truck can carry a maximum of 4,000 gallons, which would equate to 40 truckloads every day for a moderate-sized, 5,000-head dairy. To mitigate the transportation issue, centralized digestion requires that the manure slurry be concentrated at the dairy and then rediluted at the digester.

Denmark has led the world in centralized anaerobic digestion, and other European countries have successfully implemented centralized digestion as well. Their success demonstrates that the logistics of manure transport can be overcome and that centralized digestion is technologically viable. Manure for these facilities is routinely hauled an average of 1 to 12 miles in vacuum tanker trucks with 3,000- to 8,000- gallon capacities.

1.2.3 Other Animal Wastes
Renderings, fats, and other wastes from animal finishing can also be used in a biomass-to-energy project. These wastes typically have value for refeeding or other applications. One potentially valuable use of these wastes is in the production of biodiesel. Biodiesel is typically manufactured by blending methanol (produced by anaerobic digestion or other technologies) with vegetable or corn oils. However, animal renderings can replace the oils. Threemile Canyon Farms, described in Section 4 of this report, is considering such a biodiesel production facility.

by Yakima County Public Works, Solid Waste Division
From 'Review of Biomass Fuels and Technologies', 2003

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