The MicroTurbines capture methane biogas produced by the wastewater treatment process, cleanly burning it to generate electricity and heat that is used to maintain the necessary temperature in the digester.
The project wasn’t on the agenda for Rick Ashling, superintendent of the wastewater treatment plant. He wasn’t looking for a new system. Instead, Alliant Energy went looking for him—or, more accurately, for someone like him who fit the bill. The trigger for the project was compliance with Minnesota’s Conservation Improvement Program.
“Alliant was looking for energy credits, so they approached the city engineer,” Ashling begins the story. In Minnesota, explains Alliant’s turbine expert Alan Rutkowski, a fund is set aside for conservation improvements. “It’s a state mandate: Utility revenue is set aside.”
Mandated Conservation
The Conservation Improvement Program was implemented to ensure that utility companies invest in cost-effective programs, with the goals of promoting awareness of energy conservation and its effect on the environment, reducing utility bills for residential and business customers, generating innovations in development of energy-efficient technology, and promoting energy resource development. Under the plan, public utilities in Minnesota are required to invest a portion of their state revenue in projects designed to reduce customer consumption of electricity and natural gas and improve efficient use of energy resources, according to the Minnesota Department of Commerce. By state law, regulated electric utilities must invest 1.5% of their state revenue in CIP. Regulated natural gas utilities must invest 0.5%.
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| The city inaugurated its new sysem with a ribbon cutting. |
Each utility is allowed to develop a unique conservation plan, which is reviewed and approved by the commerce department commissioner, who looks for cost-effective projects that reach a wide range of the utility’s customers from residential to commercial, industrial and agricultural. Projects that emphasize energy- and demand-saving goals, implement cost-effective renewable technologies, and expand natural gas energy savings for commercial and industrial users are encouraged. Special projects addressing the needs of low-income customers are required.
According to Jan Scott (president of the Dubuque, IA-based Unison Solutions Inc., responsible for installation and maintenance of the units), wastewater treatment plants, landfills, and food processors—or any facility with an anaerobic digester—make good candidates for this kind of distributed generation project where methane biogas can be converted into heat and electricity. Albert Lea’s wastewater treatment plant fit the parameters. After considering several types of projects, Rutkowski was particularly pleased to find in Albert Lea a project that allowed them to save both electricity and gas.
“We’re a large energy consumer, which is just what they were looking for,” acknowledges Ashling, who adds that the plant is “always looking for innovation ... but we made them prove the value of this.” Alliant Energy, based in Madison, WI, proved not only to Ashling, but also to the city that the application could benefit the plant as well as meet CIP requirements.
He put together a proposal featuring two MicroTurbines, keeping within the limitations of available funding. However, Rutkowski had in reserve an alternate proposal featuring four microturbines, just in case. “It’s more efficient, and it doesn’t double the cost to double in size.” The city recognized the potential for additional savings, whereupon it purchased the additional two microturbines with separate funds. “The city manager thought if two was good, four was better,” Ashling simplifies. “We have enough gas product to handle four units, so it made sense. They have a small footprint so they don’t take up much space.” Outside, a microturbine takes up about 2½ feet by 5 feet, with 3 feet of space required between units. They don’t take up much more space inside, as Scott points out, citing an application in Sheboygan, WI, where 10 turbines fit within a 13-foot-by-19-foot room.
Ashling indicates the plant still has room for more, which it might need someday, since the city has expressed interest in industrial development: “We’re designed for 12 million flow; there’s plenty of room for industrial growth!”
Although the city purchased two units outright, the other two are owned by Interstate Power and Light, which is also responsible for maintenance through the five-year life of the project. At the end of the fifth year, Rutkowski says IPL will sell the equipment to the city at a discount.
Scope and Benefits
Capstone MicroTurbine, launched in 1998 and now an industry leader, holds 74 patents and has accumulated over 12 million hours of operation. The MicroTurbine is based on jet engine technology with the addition of patented air bearings and state-of-the-art electronics. With only one moving part, it is nearly maintenance-free, providing a reliable, environmentally beneficial solution for power generation. Capstone MicroTurbines are available in sizes of 30 kW and 65 kW for biogas-fueled applications; the 65 kW can come with an integrated stainless steel exhaust heat exchanger.
Four microturbines, a Cain heat exchanger and Vane Tech gas conditioning equipment all were installed by Unison Solutions, which is a Capstone MicroTurbine distributor and service provider. The work began in the summer of 2003 and they were online by April 2004. Scott explains that Unison frequently serves as project manager with similar turnkey projects.
A compendium of key requirements for installation begins with sufficient quantities and quality of gas. “The first step is to test the gas for sulfur, siloxanes, and methane percentage,” advises Scott. “Skipping this step causes problems for a lot of people.
“Wastewater plants are sized on flow: million gallons per day and a specific amount of waste,” Scott continues. “To be viable for a project like this, they must have an anaerobic digester in which bacteria creates a consistent supply of methane gas.”
Projects target facilities that are still flaring gas: i.e., burning off excess gas instead of using it. Scott advises customers with flared gas to run a spreadsheet or consult Capstone’s financial models in order to determine the gas quantity and various maintenance factors. However, he believes that “with natural gas prices going up, people are looking at options. This system is a good choice.” Scott advises leaving the flare in case the system shuts off. “It’s a living, breathing system. It’s not static. You want to leave the flare for maintenance, safety, and system balance issues.”
Next on the list, he continues, comes site selection for mounting the equipment. The turbines must be mounted on a concrete pad, but can be located inside or out. Scott prefers an outdoor location because the turbines perform better in cool weather; however, because they operate without the noise, vibration, and maintenance issues associated with reciprocating engines, they are conducive to interior locations. Scott cautions that installers have to be cognizant of local codes.
You do have to remain connected to the grid if using a biogas digester, but Scott explains that the MicroTurbine system reduces reliance on the utility company, thereby lowering utility bills.
Once installed, the project entails using methane biogas generated by bacteria during the wastewater treatment process to power the MicroTurbines that in turn generate electricity. Reciprocally, the exhaust heats the water used for biogas generation. “It was installed as an energy-saving process,” Steve Jahnke, Albert Lea city manager, told the Albert Lea Tribune. “It gives us the ability to use the methane gas already generated at the plant. We’re able to take a waste product and use it for something beneficial.”
As Scott summarizes, they take raw gas and produce usable fuel from it. The Albert Lea plant produces 4.5 million gallons of waste per day. The biogas produced per day reaches 75,000 cubic feet at 60% methane with 1,000 ppm of H2S (hydrogen sulfide). “Gas cleanup is a huge issue,” he emphasizes. “When you take biogas, it raises issues of H2S.”
H2S and organic sulfur need to be removed from biogas to prevent corrosion of equipment. H2S can also inhibit the effective removal of siloxanes, which are present in most digester and landfill gas. A chemical commonly used in industrial products such as lubricants and in personal care products like cosmetics, siloxane easily finds its way into water, compounding performance issues. It’s one of the major causes of microturbine, boiler, and engine damage. When biogas containing siloxane is combusted in a gas turbine, silica (SiO2) collects within the equipment, resulting in lower power generation capacity. Unison provides filtration systems to remove siloxane and various other technologies, such as chemical scrubbing, to remove sulfur.
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| Use of the system results in energy as well as the reduction of energy costs. |
Methane gas produced by the digester is directed from the flare on the way to a gas compression skid. The system uses a condensate removal to protect the equipment. Moisture is removed to prevent condensation within the compressor and gas is delivered to the sour gas fuel kit particulate-free with a relative humidity lower than 25%.
It then enters the MicroTurbines, which typically output 108 kW, as well as a 560°F exhaust that is channeled into the exhaust-to-water heater exchanger. That helps maintain the digester at a steady 95°-98°F so it can continue generating bacteria. “The hot water off the exhaust keeps the digester running at the right temperature to continue producing methane,” Ashling details. “We use less natural gas by using supplemental heat for the digestive process.”
Heat is just one of the byproducts of the process; electricity is the other. As Ashling says, use of the MicroTurbines results in two benefits: energy generation and the ensuing reduction of subsequent energy costs. “We spend $30,000 a month on energy in kilowatt use,” Ashling reveals. “The MircroTurbines can help reduce that.” As Rutkowski explains, the amount of energy produced is considerably less than the unit consumes. The four 30-kW Capstone MicroTurbines generate 2,500 kWh per day at peak production and 28,000 Btu per day. Currently, the project is saving six therms of natural gas per hour, or 52,560 therms per year.
Ashling indicates that the plant runs all four turbines most of the time. “We make it a priority. We manage the operation to keep them running as much as possible.”
Maintaining the Flow
Under the five-year agreement, Alliant covers the cost of maintenance, which is performed by Unison. Ashling is content with the arrangement but says he’s preparing for the future when less support is available. “Unison is slowly training us in maintenance and operation. Our operators take readings now—it’s routine. It’s the operator’s job to check and maintain this type of equipment. The duties required to maintain the turbines are very similar to what we’re doing now, so we shouldn’t have any added problems.”
Scott concurs that the maintenance is pretty straightforward. “They’re turbines; there’s no oil, no coolants—just air and fuel filters. There’s not a lot to it. Keep it spinning at 96,000 rpm, and it’s pretty happy.”
He accedes that the plant’s technicians are accustomed to similar equipment and capable of doing routine maintenance, although Unison monitors and troubleshoots remotely. “There’s a daily checklist of items they monitor. The steady, ongoing maintenance is fairly routine.”
Rewards, Recognition And Re-creating Success
“Turbines are relatively new technology,” Rutkowski estimates: eight to 10 years. They run “very clean” with emissions so low they can be licensed in California. Ironically, despite its success, the Minnesota program has been reduced, with the remaining funding available only for maintenance. “We aren’t able to do any other projects under this program,” Rutkowski commiserates. Based on the success of the Albert Lea project, however, he did install 10 units at a wastewater treatment plant in Wisconsin.
Ashling went on a speaking tour in 2004 to promote the benefits of the system but said dejectedly that other power companies weren’t offering “good deals,” which diminished customer interest.
Alternately, response from state organizations was wholeheartedly positive. Albert Lea’s wastewater treatment plant won the Minnesota Government Reaching Environment Achievements Together award in 2004. Established in 1993 by the Interagency Pollution Prevention Advisory Team, the governor’s award acknowledges all government programs and projects that show outstanding achievement in pollution control. MnGreat, as it is known, honors organizations that exemplify environmental leadership by preventing, reducing or eliminating the generation of waste and pollution at the source, and avoiding practices that merely transfer pollutants from one environmental medium to another.
In July, Unison began a similar project in Iowa. The client had used a digester with a flare for 13 years. Although flares do destroy impurities, they don’t operate to the same level as the MicroTurbine. At this site, the gas will be “cleaned, compressed and used in a boiler formally fueled by natural gas,” Scott relates. The market is becoming more active, he says, likely due in part to steep rises in gasoline and natural gas prices. “It’s not the same market, but it sensitizes people. There’s an awareness factor that translates to commercial and industrial applications. There’s a transition to generation.”
He congratulates Alliant for being at the forefront of that transition. “Wisconsin and Minnesota are progressive on emissions, and Alliant has a real passion for distributed generation. Nobody has done what they’ve done. They believed in it, and they made an investment in Capstone.” As Ashling sees it, Alliant also made an investment in Albert Lea, and he intends to make sure it pays off.
Indianapolis, IN-based LORI LOVELY writes on transportation and technical subjects.
DE - November/December 2006
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