|Methane gas is collected from Rock Creek Dairy and piped into two CAT 3520C gas generator sets that convert the gas to electricity, which is then sold to Idaho Power Co. for distribution to the main power grid. Photo courtesy of Caterpillar Electric Power|
By Nick Kelsch, Caterpillar Electric Power
Renewable and “green” energy technologies have gained a more prominent and visible position in our daily lives. While wind turbine farms, rooftop solar panels, and electric charging stations are ubiquitous mainstays in many industrialized nations, there is one renewable technology, already well-established in Europe, which can transform North America’s agricultural heartland into the new energy frontier. Agricultural facilities, including dairy farms, are installing anaerobic digesters and gas-powered generator sets to convert animal and food waste into renewable electricity to generate power and profit. Biogas from cow manure presents a viable alternative to conventional natural gas electricity, as well as competing renewable technologies, because biogas can be produced on a continuous basis, ensuring reliable, continuous power generation. The U.S. Environmental Protection Agency estimates that 8,200 of the nation’s existing dairy and swine operations could support biogas recovery systems with the potential to generate more than 13 million MWh per year and displace about 1,670 MW of fossil-fuel generation. In addition to the generation of renewable electricity, these systems offer numerous side benefits for farm owners.
|Bettencourt Dairies installed six one-million-gallon anaerobic digesters on site to process manure at Rock Creek Dairy. Photo courtesy of Caterpillar Electric Power|
Bettencourt Dairies, one of the largest dairy farm operations in Idaho with 60,000 cows and 13 facilities, had a significant issue at one of its dairies with odor from the manure produced by its cows. Rock Creek Dairy, a Bettencourt operation with 9,000 cows, was the target of hundreds of odor complaints from local residents over a six-year period. With each cow producing about 100 pounds of manure each day, the dairy has to manage about 900,000 pounds of manure on a daily basis, while mitigating the noxious effect on the surrounding environment. “Everyone at the Department of Agriculture knew about the odor problem,” said Rick Onaindia, CFO for Bettencourt Dairies. “We knew we had to do something to address the issue.”
Bettencourt Dairies discovered a solution that would not only enable it to solve the odor and methane emissions issues, but would also generate power from its dairy operations. Six one-million-gallon anaerobic digesters were installed at Rock Creek Dairy to process manure, mitigate odors and extract methane to convert to electric power. Approximately 200,000 gallons of cow manure are collected daily, pumped through a nearly mile-long pipeline to the anaerobic digesters, and processed into clean compost used for animal bedding. It is the largest facility of its kind in the western United States.
As the manure is processed, the resulting methane gas is collected from the digesters and piped into an adjacent powerhouse containing two Cat® G3520C gas generator sets that convert the gas to electricity. The generated power is then sold to Idaho Power Company for distribution to the main grid.
Rated at 1600 ekW at 1200 rpm, the fuel system of each G3520C generator set is sized to handle higher volumes of fuel with lower methane concentrations (typically 35-to-50% methane) versus a natural gas system (>95% methane). The engine’s electronic control module automatically adjusts the air-to-fuel ratio based on real-time fuel monitoring from a gas chromatograph. This allows the system to compensate for methane content variations typical of a waste source biogas, such as cow manure. Further, individual cylinders are monitored with accelerometers to sense any abnormal combustion or detonation that is a result of a sudden change in fuel composition. The G3520C generator set utilizes a 20-cylinder, electronically controlled, lean-burn biogas engine coupled with a two-bearing synchronous generator. The biogas engines used at Rock Creek Dairy include stainless steel aftercooler cores, exhaust valves, and rear gear train bearings that remove all bright metals that may come into contact with the gaseous fuel elements that contain hydrogen sulfide. This is especially important because hydrogen sulfide, which is typical in biogas production, reacts to corrode bright metals.
The biogas cooling system is designed to operate at higher jacket water temperatures of 230° F (110° C) versus the typical natural gas system at 210° F (99° C). This is achieved to prevent condensation of sulfuric acids on internal engine components, which can form when hydrogen sulfide gas combines with water vapor in the air-fuel mixture. The elevated jacket water temperature in combination with positive crankcase ventilation also limits acidic deposits into the engine’s lube oil, further protecting components and helping to extend oil-change intervals. In addition, the G3520C is optimized for efficiency for use in parallel-to-grid applications. At Rock Creek Dairy, one of the generators runs continuously while the other unit runs variable hours following the available biogas production. Both generators feed power to a utility-grade paralleling switchgear before being sent to voltage step-up transformers and the local utility distribution network. Ultimately, it is the interface with the local electric utility that drives economies of scale for many biogas-to-energy project developers in North America.
“The interconnection with the utility was expensive, and that’s where you need consolidation of the larger dairies,” says Jay Kesting of New Energy One, the project developer and plant operator, and subsidiary of local Cat Dealer Western States Equipment Company. “It’s one of the things that holds these projects up if you don’t receive the optimal rate for the power and you are paying a lot for the interconnection. So, if you can bring dairies together in a community concept with one interconnection, you can improve your return.”
The Rock Creek Dairy system is highly efficient. In addition to converting biogas into electricity, the plant utilizes waste engine heat to keep the anaerobic digesters running at the optimal temperature. Substituting engine heat for heat generated in a boiler is yet another way for the dairy to reduce its energy costs while boosting biogas production. A third byproduct from the anaerobic digesters is a dry, odorless compost that can be used as a soil conditioner or for animal bedding.
With lessons learned from five hundred G3520C biogas generator sets installed since 2005, Caterpillar has been able to optimize the design of the generator set such that repair and maintenance schedules make feasible 10-to-20 year operating contracts on biogas fuels. “This generator set is designed to enable highly efficient and reliable operation on digester gas, delivering electricity at extremely competitive cost per kWh,” said Michael Devine, gas product & marketing manager at Caterpillar’s Electric Power Division. “At Rock Creek Dairy, the recovered heat is providing significant cost savings by avoiding the cost of a traditional heat source for a digester operation, while the sale of power is a revenue stream that offsets the costs to operate the system.”
“On the environmental side, this technology is something that I think will help us survive as a dairy,” said dairy owner, Louis Bettencourt. “There is less odor from the dairy, which is good for our neighbors, and the harmful greenhouse gases are no longer escaping into the atmosphere.”
Since August 2012, Rock Creek Dairy has been steadily ramping up biogas production, providing more than 2MW of power for distribution to the grid as part of a power purchase agreement with the local utility. Once the operation reaches full-tilt, Rock Creek Dairy’s generation capacity will double to 4 MW of power. New Energy One supplies Idaho Power with enough power to provide electricity to thousands of Idaho homes.
New Energy One and Western States are evaluating other uses for the heat from the generators, including heating greenhouses at Rock Creek Dairy. While dairies in Idaho are not currently mandated to reduce methane gas emissions, the anaerobic digester system at Rock Creek Dairy will prevent 40,000 tons of methane from being released into the atmosphere every year.
“Converting waste into energy minimizes our environmental impact and provides a consistent source of renewable energy that can be distributed to our surrounding communities,” said Onaindia. “The true value of this project is its ability to serve as a model for the Idaho dairy industry for long-term sustainability, from both the environmental and business perspectives. We want to be here for the long term, and we view this project and this technology as important to our future.”
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