BioCycle December 2009, Vol. 50, No. 12, p. 42
In La Crosse, Wisconsin, a brewery and beverage packager is redirecting biogas generated by its anaerobic digesters to a CHP unit, offsetting energy costs for a nearby hospital and its own operation.
GUNDERSEN Lutheran Health Systems and City Brewing Company, both based in La Crosse, Wisconsin, are partnering on an innovative cogeneration project fueled by biogas from the brewery’s waste treatment process. The system, which started operating in October, is expected to generate 3 million kilowatt hours (kWh) per year of electricity, equivalent to 8 to 10 percent of the power used on Gundersen Lutheran’s La Crosse and Onlaska campuses. City Brewing will employ waste heat from the system, estimated at 17,000 mmBtus/year, to warm its anaerobic digesters producing the biogas.
The bulk of the $1.5 million capital costs for the engine generator and components was paid for by Gundersen. The healthcare network received a $250,000 Wisconsin Focus on Energy grant to cover a portion of those costs.
Gundersen is a nonprofit healthcare network that serves parts of Wisconsin, Iowa and Minnesota. The network encompasses a 325-bed teaching hospital, 48 regional clinics and one of the nation’s largest multispecialty group medical practices. The cogeneration project is part of Gundersen’s ambitious energy and environmental stewardship program called Envision, conceived a few years ago when it recognized that rapidly escalating energy costs would make it increasingly difficult to deliver affordable health care. “The goal is to reduce our energy costs which ultimately reduces costs for our patients,” says Corey Zarecki, Gundersen’s efficiency improvement leader.
The aim of the multiphase Envision project is to achieve energy independence by 2014, by reducing energy consumption and offsetting the remaining energy used with renewable energy. To achieve energy independence, Gundersen is implementing energy efficiency strategies to reduce demand by 30 percent. Any new construction must decrease energy usage by 50 percent. The money saved will help finance renewable energy projects.
“The renewable energy may be generated on site, but more likely, it will come from off-site because we can only do so much on site and there are also better economics off site,” says Zarecki. “We will always use energy and buy energy from Xcel, our local utility, but we want to create enough renewable energy to offset whatever energy we are using from Xcel.”
As part of the program, Gundersen also committed to environmentally sustainable business, facility and waste management practices. “You want to reduce first,” Zarecki explains. “It’s the right thing to do, there are some nice paybacks, it gives the healthcare network credibility and the more we reduce the less we have to offset with renewables. We already hit our goal for 2009, which was a 20 percent reduction in energy use.”
FLAMES IGNITE AN IDEA
Shortly after starting discussions on the Envision initiative, a Gundersen employee driving by City Brewing noticed two gas flares, which were burning biogas produced by the company’s anaerobic digestion system. “Seeing the flares sparked the idea that maybe we could partner with these guys to harness some of the energy,” Zarecki says. City Brewing liked the idea. “If we can use the gas, that is better for the environment,” says Gerald Clements, the company’s engineering director.
Biogas at City Brewing is generated by two 500,000-gallon upflow anaerobic sludge blanket digesters. The digesters were installed by G. Heileman Brewing Company, the prior owner of the operation, when it built a new brew house in the early 1980s. Heileman was required to add the digesters to pretreat waste-water from its expanded operation prior to sending it to the La Crosse Municipal Waste Treatment plant.
City Brewing, a beverage copackager, purchased the old brewery in 1999. Today the facility produces and packages products, including beers, malt beverages, teas and juices, for a variety of companies such as Mike’s Hard Lemonade, Smirnoff Ice, Arizona Ice Tea and Samuel Adams. The packaging center has a capacity of 50 million cases a year.
Wastewater is generated when product residuals are washed down the drain and when tanks, ranging from 800 to 7,000 barrels, are cleaned between batches of beverages, Clements explains. The digesters process about 967,000 gallons of wastewater a day with COD (chemical oxygen demand) concentrations of 5,257 mg/L and TSS (total suspend solids) of 525 mg/L. The system has a 90 percent efficiency rate at removing COD.
Biogas produced by the digesters is estimated at 170 scfm, generating 89 million cubic feet/year. Methane composition ranges from 60 to 65 percent. Prior to the Gundersen partnership, City Brewing utilized some of the biogas to fuel a boiler heating the digesters in the winter. Excess biogas was flared.
Click on the figure to view it larger.
Figure 1 is a process diagram. The system is located next to the digesters on property Gundersen leases from City Brewing, which made sense for logistical and financial reasons. Gundersen is about a mile away from the digesters, Zarecki explains. Obtaining the necessary permits and right-of-ways to pipe the biogas to the network’s La Crosse campus would have been difficult.
The first phase of the project installed a single 633kW Jenbacher generator. Prior to sending the biogas to the generator to produce electricity, it is cleaned to remove moisture, carbon dioxide, hydrogen sulfide (H2S) and other impurities. Electricity produced by the system is not used directly by Gundersen but instead sent to the power grid. “We get a check from the utility provider every month,” Zarecki says.
Waste heat from the generator and the engine exhaust is captured and recycled back to City Brewing to heat the digesters. “The wastewater treatment plant boiler was removed once we knew this system was going to be put into place,” Clements says. “In the winter we get a boost in biogas production by heating the reactors a few degrees higher.”
Biogas production at City Brewing fluctuates due to seasonable variations in beverage packaging, which peaks from April to August. Flows are higher during the week than on weekends and vary during different times of the day. Gundersen considered a number of technologies, including microturbines, before settling on the Jenbacher reciprocating engine. “Based on fluctuations in the flow the engine seemed to make the most economic sense,” Zarecki says. “It also tied in well with the heat recovery.”
The system is specified to accommodate two 633kW engines to maximize gas utilization. Two smaller engines, instead of one large engine, permit more flexibility with variable gas flows, Zarecki explains. “Originally we looked at one larger engine,” he says. “But it was still too small to handle the gas at peak times.” During periods of low flow, an engine can only operate when there is sufficient gas to satisfy 50 percent or more of the engine’s capacity. Otherwise the engine needs to be shut down.
To date one engine has been installed and is operational. Flow meters are in place to collect data on actual gas production. Estimated gas flows were based on COD levels. “We wanted to get the system running, look at some of the data and then make a decision on the right size for the second engine,” Zarecki explains.
Gundersen is also thinking about alternative solutions to optimize biogas utilization given the variable flow rates. Instead of a second engine, the project team is exploring running a gas line to City Brewing’s boilers that supply process heat to the packaging plant and brewhouse. “If the gas is clean enough for the boiler to use then you actually have a more robust system,” Zarecki explains. When the engine is down for maintenance or the flow is too low to run a single engine, the gas can still be utilized by sending it to the boilers. “When the flow is too high you can run the engine full out and send any excess gas to the boilers,” he says.
An engineering study is underway to determine what is required to upgrade City Brewing’s facility boilers to burn biogas with natural gas. “They know they are going to need burner upgrades to make this happen,” Clements says. The dual fuel boilers can currently burn natural gas and oil.
One of the project’s biggest challenges was finding a system to clean the biogas. “It turns out that we have much higher levels of hydrogen sulfide (H2S) than we anticipated,” Zarecki explains. While initial sampling of the biogas stream showed the expected range for H2S, testing later in the specification process indicated much higher levels of H2S. To understand the variations, Gundersen took many samples over a period of several months. During the testing, H2S levels ranged from 2,622 parts per million by volume (ppmv) to a high of 18,706 ppmv.
Gundersen also worked with City Brewing to determine the source of the H2S. “We wanted to see if there was something in the brewing process that was unintentionally creating it,” Zarecki says.
The investigation found 10 different sources for the sulfur. Chemical reactions, which occurred when ingredients in the various beverages were mixed in the digester, produced some of the sulfur. Other sources were found in substances used to adjust product pH and clean the equipment.
Most of the traditional solutions for removing H2S from biogas could not handle high levels of H2S or were expensive, with high capital costs and/or high operating costs, Zarecki explains.
Variations in the sulfur levels made the application even more complicated. “The cleaning process had to be robust,” Zarecki says. “The sulfur levels can change from a couple of thousand ppmv to 10′s of thousands in a matter of hours, whereas in most digesters and landfills you get your number and it does not change much.”
After doing their homework, the project team discovered a new application, called SulfrStrip, for cleaning high levels of H2S from gas streams. SulfrStrip is a continuous aqueous state process that employs a catalyst to selectively remove the H2S from the biogas stream.
Changes in sulfur concentrations and flow rates are presenting some start-up challenges. Efforts are underway to optimize the clean-up process. “As with any plant, you have to find out the optimum set points for your given situation,” Zarecki says.
To date, a key lesson that Gundersen Lutheran has learned is when forming renewable energy partnerships, make sure it is structured so that it is a win-win for everyone involved. “Second, you have to have clear and constant communications, especially in the beginning of the project,” he says. Good communications have led to a better understanding of partnership opportunities. The option now under discussion to pipe cleaned biogas to City Brewing’s facility boilers offers a case in point.
Zarecki hopes this unique partnership will inspire other organizations to explore similar ventures. “At this point in our nation there are three main issues that are affecting all of us, the economy, health care and energy,” he says. “This project, and others like it, hit on all three. We not only want to lead but teach others and make a difference.”
Diane Greer is a Contributing Editor to BioCycle.