BioCycle June 2010, Vol. 51, No. 6, p. 24
Scale matters when evaluating the economic feasibility of anaerobic digestion systems. An ongoing project on a small Minnesota farm is testing the parameters.
TYPICALLY the average Minnesota dairy farm is too small to economically justify a digester. Most of the current commercially available technology is geared towards larger livestock operations, but in Minnesota, over 90 percent of dairy operations have 300 cows or less. Appropriately sized, reliable technology could open anaerobic digestion to a greater number of facilities that could benefit from managing waste sustainably and generating renewable energy.
In 2005, the Minnesota Project, a nonprofit based in St. Paul, Minnesota, set out to find anaerobic digestion technology that could scale down and prove cost-effective for the average sized dairy. Jer-Lindy Dairy Farm in Brooten, Minnesota, owned by Jerry and Linda Jennissen, was selected to test the technology. The farm encompasses 240 acres, with 215 Holsteins, 125 of which are milking cows, producing about 3,000 gallons of manure each day.
“We had three main reasons why we wanted to participate in the project,” says Jerry Jennissen. “The first was we wanted to create an additional revenue stream for the farm and ensure its future success. Secondly, we wanted to create a better manure storage system for our farm and finally we thought the project could help protect our farm against any future environmental concerns through regulation. We feel we have accomplished the last two, but are still working on the first reason.”
Three nationwide RFP processes solicited bids from engineering firms to design and install the digester system and electrical generation equipment. A handful of engineering firms were selected to visit Jer-Lindy Dairy and submit a site-specific bid for further evaluation. After a two-year search, Genex Farm Systems in Shawano, Wisconsin, and Logan, Utah-based Andigen, LC, were selected to install Jer-Lindy’s system. Construction started on the $450,000 project in the fall of 2007. The farm began producing electricity from biogas in May 2008.
About 70 percent of the initial pilot project was funded through grant dollars from various government agencies. The rest of the project was financed using low-interest loans available through the Minnesota Department of Agriculture. The Minnesota Project played a major role in finding funding opportunities, says Ryan Stockwell, former director of energy and agriculture at the Minnesota Project.
The Jer-Lindy system is comprised of a 33,000 gallon induced blanket reactor (IBR) digester operating at temperatures between 104° and 106°F. Total volume going into the system each day is about 7,000 gallons, which equates to a retention time of a little under five days, explains Ed Watt, president of Andigen, which designed and built the digester. An IBR is a high rate anaerobic digestion system patterned after an upflow anaerobic sludge blanket (UASB) reactor but modified to handle higher solids concentrations, Watt says. Digesters are composed of one or more above ground vertical column tanks that can operate at either mesophilic or thermophilic temperatures.
Manure is scraped twice a day from the barns to a cross channel where it is flushed to a reception pit and fed to the digester. The system initially used recycled effluent from the digester to flush the flume of manure into the reception pit, producing a slurry of 6 to 8 percent solids. A mixer pump in the reception pit keeps the manure in suspension.
The manure slurry is piped at a steady rate into the bottom of the IBR. A heat exchanger, which uses waste heat captured from the generator, heats the manure to 106° to 115°F as it passes through the pipes. The IBR naturally forms a thickened area of sludge in the lower portion of the tank that contains high concentrations of bacteria. Bacteria in this sludge blanket digest the manure, forming biogas. Biogas attaches to the solids, causing the material to float up through the sludge blanket in the tank.
Near the top of the tank is a submerged septum or partition that provides a means of separating the gas from the solids. An opening in the center of the cone shaped septum allows the biogas to rise and exit out the top of the vessel. The solids tend to sink back down the tank after the gas is knocked off. Effluent also passes through the opening in the septum and exits the tank via a pipe located above the septum.
The system does not employ any mechanical or hydraulic mixing. If the tank is mixed, the bacteria gets flushed out with the effluent, Watts explains. Not mixing the tank results in more bacteria per cubic foot.
The biogas is composed of about 65 percent methane and under 1,000-ppm of hydrogen sulfide (H2S). It passes through a water bubble bed that removes particulates and some of the carbon dioxide and H2S in the gas. “It is a very effective means of removing simple contaminates from the biogas,” says Joe Borgerding, owner of RJ Electric LLC, the project’s electrician and mechanic. A gas conditioning system was not installed due to economic considerations.
The biogas fuels a GM 350-cubic-inch engine that can generate up to 40-kW of electricity. Initially the system produced enough biogas to run the generator at 50 percent capacity. Any power not used to operate the pumps, digester and separation equipment is sold to the grid. Excess gas, or gas produced when the engine is not running, is flared.
Effluent exiting the tank is passed through a Fan screw press to separate the liquid from the solids. “We went with a little bigger separator not because of capacity but because we wanted to get the material drier since it was going to be used for bedding,” says Rolly Meinke, Genex’s representative responsible for new product development.
Liquid from the separator was stored in a 1,200-gallon poly tank. Any excess liquid that did not fit into the tank was gravity fed to a lagoon.
INCREASING BIOGAS PRODUCTION
Once the digester was operating, Jer-Lindy was approached by AMPI, a national milk processor, that was looking for a digester to handle some of their cheese whey permeate. “They were seeing increasing costs to land apply the waste,” Stockwell says.
After receiving the necessary permits from the Minnesota Pollution Control Agency, Jer-Lindy started codigesting the cheese whey. “It was only a few thousand gallons added every day but it actually made a considerable impact,” Stockwell explains. “Within 24 to 48 hours there was a significant increase in gas production and within a week they were at capacity of the genset.” Adds Jennissen: “We tripled our gas production by adding whey.” The total volume of cheese whey waste going through the system varies, but is around 10 percent by volume.
It is not known exactly how much biogas the system is producing. The digester was installed with a meter that was too small for the production it achieved, Borgerding says. “It was never expected that the digester would reach the volume it is at. We are able to produce the full 40-kW and flare off a large amount of gas in the summer.”
The Agricultural Research Utilization Institute (AURI), a nonprofit whose mission is to strengthen the rural Minnesota economy, is providing technical assistance to the project, measuring the quality of the biogas coming out of the digester (methane content and H2S) and looking at the characteristics of the codigestion substrates. “We have been helping Jer-Lindy determine what is of value to put into the digester and suggesting different feeding rates,” explains Jen Wagner-Lahr, AURI project development director.
The addition of cheese whey, along with chiller water obtained from the milking parlor, eliminates the need to recycle the liquid effluent. The cheese whey is put through the 1,200-gallon tank and brings the solids content of the manure down to 6 to 8 percent. Chiller water is added as required.
Eliminating the use of the effluent liquid had several positive effects. The effluent had a tendency to increase the alkalinity in the digester, Menike says. Some of the H2S formed during digestion also gets into the effluent. Sending that effluent back through the digester a second time raises H2S levels in the biogas.
The Jer-Lindy project has encountered problems keeping the engine running around the clock. To keep costs down the first unit installed was a remanufactured engine converted to run on biogas. “We had probably 3,000 hours on the engine and it failed,” Menike says. The supplier replaced the failed engine with another remanufactured engine. It too ran about 3,000 hours and failed.
The problems are caused by hydrogen sulfide and water vapor in the biogas forming an acid that corrodes the soft metals in the engines, Borgerding explains. The engines also were not designed to accommodate variable BTU levels. Biogas quality and volume vary based on ambient temperature along with the type and volume of codigestion substrate. Typically waste products are stored in the reception pit for about 12 hours prior to being fed to the digester. During this period bacteria start to form and begin the decomposition process. During the winter, substrates entering the reception pit are very cold, typically about 40°F. The cold temperatures inhibit the production of the bacteria, resulting in lower gas volumes and lower methane content.
Genex purchased a new GM 350, which is now installed at the project. The new engine has a lower compression ratio, higher temperature range and operates at higher RPMs. This unit is expected to last three to four times longer with some optimization, says Borgerding.
He is working on the design of a new engine that will reduce the extreme engine wear caused by the acid levels. “I have also found a way to increase the combustion efficiency for the poor BTU levels of the biogas and a way to operate the engine in cases of low gas volume and high gas volume,” he says. The new design can start generating gas at 5-cfm and handle flows up to 50-cfm.
The new engine will also be larger, enabling the dairy to produce additional electricity for sale to the grid. The exact details of the new engine are proprietary. Borgerding is hoping to commercialize it for use by other biogas projects.
AURI is also providing a cost share grant to tackle the issues of the generator set upgrade. “The idea is to come up with an idea of how to retrofit an engine that will run more reliably and more efficiently,” Wagner-Lahr says. “The whole system has to be not only feasible technically and economically but also feasible in terms of a management issues for a small-scale dairy. Our funding is going to cover 50 percent of the R&D costs of this project.” This phase of the project is being done in conjunction with several private and public partners.
Minnesota’s net metering law has set its ceiling at 40 kW. Under net metering, Jer-Lindy receives a higher rate for selling electricity to the grid, Stockwell explains: “This made a huge difference in the economics.” But it also required undersizing the genset, which added difficulty to the project. From a technology standpoint, engines in this size class are not designed to stand up to the rigors of 24/7 operations running on biogas.
Installation of a larger, more robust genset along with the addition of codigestion inputs that produce more biogas will push electricity production over the 40-kW ceiling. The dairy will then be required to negotiate rates to sell electricity back to the grid under a separate power purchase agreement, which will likely pay lower rates. But the local electric utility, Stearns Electric Association, has been an active, supportive partner of the project.
“There is a general consensus that projects are not going to be economically feasible until we have higher net metering rates,” Wagner-Lahr says. She notes the Jer-Lindy economic situation is unique due to the grants awarded at the beginning of the project.
One additional challenge that needs to be addressed is the capital issue, says Stockwell. He sees the potential for AMPI and others food processors to help with some of the initial upfront capital obligations. Investing in dairy digester projects would essentially be akin to investing in alternative waste handling processes.
“AMPI and others are obviously seeing rising costs of dealing with regulation for land application of these waste materials and increased costs for shipping the waste,” he says. “It is in their interest to find an alternative and it seems that on-farm digesters have real potential.”
The Jennissens are still committed to making the project work economically for their farm. “We still have a lot of hope that we can make this project become profitable to our farm,” says Jerry Jennissen. “Joe Borgerding and his genset idea remains to be the number one way to create a revenue stream. It will not only make our system more profitable but also provide an opportunity for future operations of a similar size.”
Diane Greer is a Contributing Editor to BioCycle.