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Biogas production: from a covered lagoon digester and utilization in a microturbine.

The total manure produced by dairy cows in the state of California is 30,000,000 metric tons (33,000,000 short tons) per year. As presently handled, dairy manure produces undesirable odors, biogas, and nutrient overloads resulting in air and water pollution. California Polytechnic State University (Cal Poly) has devised a system at its dairy to capture methane, a naturally emitted manure biogas, and convert it into a usable energy source to create electric power. By incorporating a covered lagoon digester system with a microturbine electric generation system, manure's undesirable byproducts can be transformed into valuable assets for a dairy farmer. If the total manure produced by dairy cows in the state of California could be converted to methane, the theoretical annual energy production would be 20 trillion British thermal units (BTU) which would be enough to power a 200-megawatt power plant.

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At the Cal Poly dairy, about 90 percent of the manure is deposited on concrete, flushed through a solids separator, and pumped into a 14,400-cubic-meter (4-million-gallon) covered lagoon digester. Approximately 300,000 to 400,000 liters (79,000 to 106,000 gallons) of flushed manure containing .5 percent solids and 4000 milligrams per liter (.1 ounce per gallon) of chemical oxygen demand (COD) are loaded daily into the digester resulting in a hydraulic retention time of 40 days. After exiting the lagoon, the COD of the effluent has been reduced to 1000 milligrams per liter (.04 ounces per gallon) resulting in much less odor. As the manure is anaerobically digested by bacteria located at the bottom of the lagoon, up to 127 cubic meters (400 cubic feet) of biogas is produced daily and collected beneath a special floating cover. This biogas, containing 70 percent methane, is then piped to the gas handling system where it provides fuel for a 30-kilowatt microturbine electric generator. The microturbine has been operating since July 1, 2002.

The table on the following page summarizes the results of this testing, showing biogas utilized, microturbine kilowatt setting, and kilowatt-hours generated. The data can be divided into roughly one-month periods, during which times the microturbine power setting was maintained at specific kilowatt levels. During the first part of July 2002, when the power was set at 15 kilowatts, the average daily running time was 3.6 hours, the biogas consumption was 44 cubic meters per day, and the net electrical output was just over 28 kilowatt-hours. When the power setting was increased to 20 kilowatts later in July, the daily running time dropped slightly to 3.4 hours, the biogas consumption increased to almost 54 cubic meters per day, and the net kilowatt-hours increased dramatically to 47 kilowatt-hours per day. When the setting was raised to 25 kilowatts, the average daily running time dropped dramatically to 2.4 hours, accompanied by a drop in biogas consumption to 38 cubic meters per day and a net electrical production of only 31 kilowatthours per day. The microturbine was then reset to 15 kilowatts, and the resulting daily hours, biogas, and net electricity were only 1.6 hours, 22 cubic meters, and 14 kilowatt-hours, respectively.

The initial low biogas production was due to leaks in the cover, which were subsequently repaired, and varying levels in the lagoon depth, which was also corrected. As also shown in the table, the biogas production eventually climbed to 127 cubic meters per day last March. The microturbine setting was at 15 kilowatts for the 15-hour duration of operation, and 143 kilowatt-hours were produced. Emissions resting of the microturbine exhaust indicated a NOX level of under 3 ppm, a major consideration in siting power plants in California.

The overall goal of this project is to recover as much energy as possible from the waste of the dairy cows and re-incorporate that energy back into the Cal Poly electrical grid for the benefit of the dairy. Cal Poly's dairy produces a monthly electric bill of almost $3,000, which results in a yearly expenditure of nearly $35,000. When the microturbine electric generation system is operating at its optimal performance rate, the predicted energy production of the microturbine system is just over 52,000 kilowatt-hours per year. This energy, at a rate of $0.12 per kilowatt-hour, is worth about $6,240, which is about 18 percent of the dairy's yearly energy costs.

In addition to the above electrical output, when operating at the optimal run time of 15 hours per day, the exhaust heat energy produced by the microturbine can be recovered at a rate of 100,000 BTUs per hour and is worth about $3,300 annually when compared to the average $6 per 1 million BTU cost of natural gas. The total economic benefits of the microturbine electric generation system, including electricity and process heat, can then be estimated to be worth approximately $10,000. The environmental benefits of a covered lagoon digester system, although not quantified in a measurable economic value, are considerable. By virtue of the cover capturing the gas emissions from the dairy manure, both odor and the release of green-house gases are considerably reduced.

Methane is a very potent greenhouse gas and is 21 times stronger than carbon dioxide. The methane emissions from the dairy at Cal Poly can be calculated as being almost 45 kilograms (100 pounds) per day or 18.3 metric tons (18 long tons/16 short tons) per year and is therefore equivalent to over 380 metric tons (345 short tons/386 long tons) of carbon dioxide per year. It is anticipated that in the future greenhouse gas credits will be available to the dairy farmer, and the farmer will be paid for capturing methane from a dairy lagoon.
Microturbine Performance Summary

 Hours/day Average kW KW.h/day
Time period of microturbine [m.sup.3]/day net
 operation setting biogas energy

July 1-9, 2002 3.6 15 44 28
July 25-Aug. 8, 2002 3.4 20 54 47
Aug. 9-18, 2002 2.4 25 38 31
Aug. 19-Sept 19, 2002 1.6 15 22 14
October 2002 1.4 15 13 12
November 2002 1.6 15 20 14
December 2002 2.7 15 29 25
January 2003 4.0 15 32 36
February 2003 3.7 20 34 47
March 2003 15 15 127 143


ASAE member Douglas W. Williams is a professor in the BioResource and Agricultural Engineering Department, California Polytechnic State University, San Luis Obispo, CA 93402 USA; 805-756-6153, fax 805-756-2626, dwwillia@calpoly.edu.

Diana Gould-Wells is an instructor in the Civil and Environmental Engineering Department, California Polytechnic State University, San Luis Obispo, CA 93402 USA.

The authors gratefully acknowledge the California State University Agricultural Research Initiative and the Western Regional Biomass Energy Program for providing funding for purchase, installation, and initial operation of the equipment utilized at the Cal Poly covered lagoon digester.
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Author:Williams, Douglas W.; Gould-Wells, Diana
Publication:Resource: Engineering & Technology for a Sustainable World
Geographic Code:1USA
Date:Apr 1, 2004
Words:1140
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