At least 60 waste-management firms are hawking anaerobic digesters to harness the energy potential in agricultural and food processing waste. Among them is Golden, CO-based Microgy Inc. Instead of simply selling the tanks and other infrastructure of a digester, Microgy operates and maintains a system produced under license from Xergy A/S, a Danish technology provider. The first of three dairy-farm digesters came on line in June 2005, and next year the firm will shift to an owner-operator model, working with slaughterhouses, pig farms and feedlot dairies with up to 10,000 cows. Under a recently signed contract with Swift & Co., Microgy will construct four 916,000-gallon digesters and sell the resulting pipeline-quality gas to Swift's Grand Island, NE, packinghouse. After its third-quarter startup, the Swift project will generate up to 250 billion BTUs of biogas a year, the equivalent of 1.8 million gallons of heating oil.
The landscape is littered with failed digester projects, with several factors working against successful deployment. Economics may be the greatest stumbling block: most digesters use methane to power generators and sell the resulting electricity to the grid, but utility companies typically pay well below the estimated 6 cents per kW hour breakeven point. Another barrier is operations & maintenance: biogas production is not a priority at a feedlot or packinghouse, and maximized yield isn't possible if the system is not operating properly. Finally, the best technology usually isn't deployed: concrete holding tanks and mesophilic (moderate temperature) conditions result in suboptimal output. The Xergy system uses steel tanks, a thermophilic operating environment, and the same kind of controls architecture used in a contemporary food plant to improve gas yields, quality and reliability.
Steven J. Brunner is heading Microgy's technical initiative to adapt the Danish design to the North American operating environment. Brunner, vice president of engineering, has been involved in the utility, energy conservation, waste management and environmental fields for 15 years. He received his undergraduate degree in environmental studies from Dartmouth College, earned a certificate in energy management and design at Sonoma State University and was awarded an MS in building systems engineering by the University of Colorado, Boulder.
FE: Explain the relationship with Denmark's Xergy.
Brunner: While doing an anaerobic digester project in 1997, we investigated the technology and found this Danish company with 15 years experience and 30 projects operating throughout Europe. They had worked out all of the kinks and developed an advanced thermophilic process that produces higher quality methane and boosts yields by as much as five times over mesophilic systems. Our task was to transfer Danish engineering standards to the US. Xergy's technicians were involved in the construction of our first project, and we're in contact with them constantly in monitoring and managing our systems. As we've gotten better at designing, building and operating these facilities, we've developed a modular design to reduce engineering costs and tweaked it to improve efficiency.
FE: What adaptations have you made?
Brunner: By the time we built our third facility, we made adjustments to make the material feed more consistent to keep the microbes happier. If you build a larger equalization tank, for example, you can better balance the composition of the bugs' feed. Raw material variability also requires adjustments, and the Danes were surprised how much variance we have with our manure, compared to their cows. There are variations from hour to hour.
We also made changes in components such as flow meters and pumps, and the Danes have adopted some of those changes. For example, progressive cavity pumps were used at first. We were naïve about what you find in manure pits. Plastic film, 2X4s, soda bottles and tails get into a dairy's pits. We switched to rotary lobe pumps in certain locations in the system.
FE: With the Swift project, the range of raw materials will be much greater than what's available at a dairy feedlot. How will that affect the system?
Brunner: It will be a codigestion system, with a good balance of food (manure), macro- and micro-ingredients and high-buffering capacity to maintain the proper pH and temperature in the tank. The addition of substrate material boosts methane yield. A packinghouse has fat that is high quality, some that is mediocre and a lot of carbohydrates in the waste streams. We have our own empirical data and 15 years of our partner's data to help us gauge what will occur in the digester when different substrates are added.
A simple Excel spreadsheet with some macros has been used in the past to predict how much biogas and methane will be produced. It comes within 5 percent, but that can mean quite a swing. We've developed a predictive model that should provide greater accuracy and better recipe management without overgassing the system.
At the Grand Island packinghouse, we'll get manure from the corrals and the slaughter area. The plant processes 5,500 head a day. Each animal has about 50 lbs. of paunch manure, which is pretty volatile stuff. Feeding in the optimum mix of manure and other byproducts at a constant rate is critical. If you feed the bugs right, they'll reward you.
FE: How will the Grand Island project benefit Swift?
Brunner: Grand Island is a small community, but there still are issues with lagoon odor. Water discharged from the digesters will have only one-fourth the biological oxygen demand (BOD) of other waste and will help reduce odors. Biogas harvesting also reduces the amount of material that has to be land applied, which becomes more costly as the plant is forced to transport it over a wider area.
The biggest impact will be from the energy the plant receives. Initially, only three digesters will be installed, but the controls and pumping infrastructure are sized for four. The plant is going through a growth stage, and we expect the completed installation will generate close to 50 percent of the gas needed during production and all gas needs on Sunday, when hot water from the boilers is used to clean and sanitize.
FE: How well are you able to control activity in the digesters?
Brunner: The big thing for us is maintaining a continuous stream of nutrients and getting the manure up to a thermophilic temperature. It comes in at perhaps 35