Save with slurry

The ingenuity of a qualified diesel mechanic and ex-sugarcane farmer from KwaZulu-Natal Shelby Tyne, now CEO of his consultancy Biogas Power, could provide dairy and pig farmers with an economical biogas digester system using slurry on their farms to generate electricity, save money and reduce their carbon footprint.
Issue date : 27 February 2009

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The ingenuity of a qualified diesel mechanic and ex-sugarcane farmer from KwaZulu-Natal Shelby Tyne, now CEO of his consultancy Biogas Power, could provide dairy and pig farmers with an economical biogas digester system using slurry on their farms to generate electricity, save money and reduce their carbon footprint.

Dairy and pig farmers around the country flush animal waste into one or more small dams at least once a day. While some farmers have found ways to utilise this nutrient-rich slurry as fertiliser for crops and pasture, others say slurry takes up valuable space. Shelby Tyne first came to the attention of Farmer’s Weekly some years ago in an article about how he and his father-in-law, Derrick Hilton, were using an innovative chicken manure, biogas electricity system (24 August 2007). Since then, Shelby and Derrick have set up the renewable-energy consultancy Biogas Power to design renewable energy systems and assist people and companies in setting them up.

The cost of heating large volumes of water in an electric geyser runs into thousands of rand a month, but by using the biogas produced by the slurry, farmers can save that money.A prototype of one of Shelby’s biogas power systems is currently on trial on the dairy farm Scotston Trust Farm owned by Peter Clowes near Underberg, in the KwaZulu-Natal Midlands. It easily demonstrates how these systems can save money. The system is relatively simple and the prototype costs about R12 000. Refinement may still be needed, which might change construction and installation costs, as would adapting the design to suit a farmers’ specific needs.

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How the system works
“The prototype system has a 6,5m x 6,5m raft floating on the surface of the slurry dam,” explains Shelby. “The raft’s square frame is made of steel tubing. A reinforced polypropylene tarpaulin, attached to the frame, forms leak-proof contact with the surface of the slurry dam. Biogas, released from the surface of the slurry under the raft, is captured under the tarpaulin. It then flows via a hose to a large, airtight accumulator bag to provide as constant a flow of gas as possible.”
Another hose runs from the biogas accumulator bag to a flame-heated 200 geyser in the dairy. The methane feeds the gas burner 24 hours a day, preheating cold water to between 50°C and 60°C. This preheated water then passes to a conventional 400â„“ electric geyser where it’s further heated to the required temperature of 80°C.

Peter says when the electric geyser heated the water from cold 24 hours a day at a cost of 20c/kWh, for an average of 30,4 days a month, his electricity bill for the geyser alone was about R5 150 a month. While the exact electricity cost-saving benefits of a biogas-powered, water preheating system have yet to be quantified, Shelby is convinced eliminating the need to heat cold water to the required 80°C saves 50% to 65% on the farmer’s monthly electricity bill.

A tentative estimate is savings should pay the system off within six to eight months. “This is a prototype system, so a modification such as better insulation in the preheating system will add efficiency,” says Shelby.

“While the system is relatively simple, the process of biogas production is complex. Prospective users must ensure the natural processes in the slurry dam producing biogas are not threatened in any way.” The Biogas Power company website explains how the biogas is a product of anaerobic digestion in a stable-temperature environment. The anaerobic digestion of organic matter such as animal waste mixed with water from dairies and piggeries consists of a series of reactions enabled by a mixed group of bacteria. In the process, organic matter is digested to eventually release large volumes of methane and carbon dioxide, together with smaller volumes of other gas such as hydrogen, nitrogen, water vapour and hydrogen sulphide.

The digestive processes of bacteria in the anaerobic environment at the bottom of a slurry dam must be kept at a dynamic equilibrium or it will stop, or turn sour.
In the prototype system, methane and hydrogen is captured. Methane must form at least 55% of all the gas burnt to achieve the hottest and cleanest flame.

Environmental benefits
According to George Tivchev, a biogas consultant who works with Shelby, methane is 27 times more harmful to the environment than carbon dioxide. The only other consideration in Shelby’s system is a gas burner ring needs to be modified to burn biogas because commonly available liquefied petroleum gas (LPG) burners aren’t suited to the lower, calorific value of biogas. The biogas burner also has to be brushed every few days to remove harmless, sulphur powder build-up, a byproduct of hydrogen in biogas.

“Burning biogas to preheat water is the most acceptable and efficient use of methane,” says George. Shelby says the prototype system currently produces between 12m3 of biogas a day. “If more rafts are placed on the dam surface, more gas will be captured and stored to be available to preheat water. Excess biogas could be used in gas burners for staff to cook meals. As long as the slurry dam is kept topped up with water and animal waste, and anaerobic digestion in the dam is kept going, a never-ending supply of renewable energy should be produced.”
Eskom has shown interest in Shelby’s prototype because when it’s eventually more widely available, it will help Eskom achieve its goal of reducing electricity consumption by 10% nationally.

Contact Biogas Power in KwaZulu-Natal on (031) 781 1981 or 083 642 8229 or e-mail [email protected] In the Eastern Cape contact Rob Cloete on (046) 684 1187 or 083 678 5563 or e-mail [email protected]     |fw