Market to Market - Wisconsin Renewables - Iowa Public Television

In April 2006, Norswiss Farms of Rice Lake, WI, a 1,100 cow dairy farm, began the operation of an 848 kW CHP system operating on anaerobic digester gas from cow manure. The CHP system, installed by Microgy, Inc., a subsidiary of Environmental Power Corporation, is driven by a Jenbacher JGS316 GS-BL biogas engine with JW/IC heat recovery to maintain the temperature of the digester tank. The CHP system operates 24/7 and exports all of the generated electricity to the grid. This system will annually generate over 6,500,000 kWh, enough energy for 600+ homes. The anaerobic digester was installed at Norswiss Farms to assist in the reduction of animal waste problems associated with manure disposal on farms, odor reduction and the potential revenue benefit resulting from the sale of electricity. The methane conversion to energy process begins as the cow manure is collected and piped to a heated complete mix anaerobic digester where microbes break down organic material in an environment devoid of oxygen. Within the digester tank, waste decomposes over time into a variety of products, including biogas rich in methane (CH4). The methane is then piped through a scrubber where moisture is removed and the gas is cleaned. The biogas is then sent to the Jenbacher engine where waste heat from the engine's jacket water and intercoolers is sent to a heat exchanger for hot water production that is then piped back to the digester to maintain the 750,000 gallon digester tank in the Thermophilic temperature range (120°F - 140°F). In addition to the cow manure, food waste oils are also used in the digestion process to help balance the gas quality and flow. The retention time of the manure while in the digester averages 20 days. The slurry exiting the digester gets separated; the liquid goes into a nearby lagoon to be used as fertilizer and the fibrous solids and are used for animal bedding. Norswiss Farms is expected to save over $70,000 per year on animal bedding, a by-product of the anaerobic digestion process, additional savings due to increased cow comfort, a decrease in animal mortality and a better overall manure management plan, and revenue earnings from the sale of electricity to the utility grid.