Using agricultural and food-processing waste products, Hometown BioEnergy is helping the Minnesota Municipal Power Agency meet state-mandated renewable energy standards while also providing a valuable fertilizer for area farmers and solid biomass as a fuel for other facilities. Courtesy: Avant Energy Inc.
We’ve all heard the phrase “one man’s trash is another man’s treasure.” Hometown BioEnergy (HTBE) offers a case in point. The plant uses vegetable-processing waste and livestock manure in an anaerobic digestion process to produce biogas for use in engine generator sets. The agricultural trash is a real treasure for the company.
Avant Energy Inc.—the energy management company that developed, oversaw construction, and continues to manage operation of the plant for Minnesota Municipal Power Agency (MMPA)—sees tremendous opportunities to redefine the term “waste.” It believes bioenergy can help utilities, large waste producers, food processors, and communities create value from waste streams and reduce overall energy costs. Its development team supports clients in the planning, design, and construction of both conventional and renewable power generation facilities.
While some people may think that the plant is utilizing vital resources that could otherwise be used to fertilize area farmland, it actually increases options for the agricultural industry. Not only does it help manage disposal of the waste, but its biological process also produces a liquid fertilizer—rich in nutrients such as nitrogen, phosphorus, and potassium—that is then sold back to local farmers to spread on their land. In addition, undigested biomass is dried to create a solid fuel, which can be sold to other biomass and coal-fired facilities to fuel their boilers.
A Flexible Design
Variety may be the “spice of life,” but it doesn’t always make for the best “seasoning” at biomass plants. When dealing with agricultural waste products, though, variety is also a fact of life. Materials are frequently exposed to weather, which means rain, sun, and wind will all significantly affect moisture content. If waste is stockpiled for any length of time prior to delivery, quality can degrade. Farm management practices—such as cleaning schedules, herd size, square footage per animal, and base bedding material—can also affect waste quality.
One of Avant’s key objectives when designing the HTBE facility was maintaining flexibility in the allowable feedstocks. Tolerating variety enables the plant to manage large seasonal changes in waste volumes from suppliers in the region. In addition, it allows the company to accept wastes that become available on a sporadic basis with fewer operational challenges.
The process begins with feedstock delivery. The mixture of local agricultural and food processing waste includes sweet corn silage, food waste, livestock waste, and waste oils (all sourced from within a 60-mile radius). The material is transported to HTBE via truck and delivered to the plant’s receiving hall. The biomass is weighed, mixed, and heated in feeding modules to prepare it for the anaerobic digestion process.
“The percentages of each type of material varies; however, on average, the plant sees approximately 50% higher solid wastes and 50% liquid wastes,” said Kelsey Dillon, vice president BioPower for Avant Energy.
Inside the digesters, bacteria work together in the absence of oxygen to decompose organic material and produce biogas. Biogas typically contains about 55% to 70% methane, 30% to 45% carbon dioxide, and trace amounts of other gases, including sulfur in the form of hydrogen sulfide (H2S). As H2S is corrosive, it must be removed before combustion. At HTBE, the H2S is removed from the biogas through microbial processes in gas cleaners as the product leaves the anaerobic digesters.
Three large, white, dome-shaped gasholders—each with a 9,000-cubic-meter capacity—store the biogas for future use in one of four 2-MW Cummins Power Generation C2000 N6C engine gensets (Figure 1). The onsite gas storage allows dispatch flexibility to provide on-peak electricity, maximizing revenue potential. The electricity produced is supplied directly to the local distribution system serving the City of Le Sueur.
|1. Green energy. Four Cummins Power Generation engine gensets utilize the biogas to generate electricity for the local distribution system. Courtesy: Avant Energy Inc.|
Heat recovery is another important part of the process. Cooling water from the engine water jackets is the main source of reclaimed energy. The heated water is fed into an insulated storage tank and then used in the gas cleaner, feeding module, electric generation, and in-floor heating systems.
HTBE also recovers energy from the engine exhaust, which is used by the post-digestion system. By utilizing the heat recovery system, HTBE believes it extracts every ounce of value possible from its process. The heat is supplied to the plant’s belt dryer, thereby reducing the moisture content of the solid by-product. This makes it more valuable as a renewable solid fuel for combustion offsite in biomass and coal-fired burners. It is also safe to reuse the separated solids as a bedding material for livestock in place of more expensive sawdust and wood shavings.
Although the plant’s 8-MW capacity may seem small compared to the average utility power plant, it is one of the largest facilities of its kind. The plant is permitted to process up to 45,000 dry tons of waste per year.
A Community Asset
Developing a power plant of any type comes with challenges. For HTBE, the technology is not particularly familiar to most people in the U.S., so its first challenge was educating the community about the project.
Avant wanted local residents to understand that a facility of this kind is a long-term asset. HTBE provides important benefits to the local community by collecting and processing local wastes, and creating a renewable source of electricity that flows directly into the host community’s power system. The company shared project information with the public early on in the development phase. It hosted meetings with community leaders, held open houses for local residents, and offered regular construction updates.
In addition, Avant placed an emphasis on hiring locally. Although the plant only has seven permanent employees, local contractors, businesses, farmers, and other area residents all contributed to construction and startup in various ways, and continue to support operational activities. The plant also uses some seasonal employees.
A Rapid Timeline
Minnesota winters offer a multitude of challenges, especially for construction companies. Nonetheless, the project broke ground in December 2012 with a one-year goal for completion.
“The aggressive construction schedule for the plant was a significant challenge, but we worked weekends and long hours. We also had good management and strategies in place to complete the project in the one year timeframe,” Dillon said.
Avant managed the construction, hiring a variety of firms to perform mechanical, electrical, structural, and building work. Xergi A/S—a Denmark-based company that designs and builds biogas plants—delivered the anaerobic digestion technology, and Barr Engineering Co. provided engineering support for the project. Biomass loading began in October 2013 and operations commenced on schedule two months later.
“As is the case with all power plants in a northern climate, especially an extreme climate like Minnesota, freeze protection is always a key priority. We started operation last winter during one of the coldest on record, but worked to successfully solve what can be very pesky freeze problems,” said Benjamin Simmons, asset manager for HTBE.
Enthusiasm for Renewables
Minnesota was a logical location for designing and building HTBE. The state has one of the nation’s most aggressive renewable energy standards, requiring all utilities to generate at least 25% of their electricity from renewable energy sources by 2025.
Wind is the clear leader in the state’s renewable portfolio. It produced 15.7% of the in-state generation in 2013, placing Minnesota fifth nationally based on production percentages. Given the state’s latitude, many people may be surprised to learn that Minnesota has annual solar radiation similar to parts of Florida and Texas, but that potential remains largely untapped. In 2013, however, the Minnesota Legislature passed an omnibus energy bill requiring public utilities to generate 1.5% of their energy from the sun, so solar is likely to gain some ground in the future.
But agriculture remains one of the state’s largest industries, so a plant that utilizes farm waste is a natural fit. HTBE filled that niche, and its success could lead to development of similar plants elsewhere. For utilities mandated to meet renewable energy standards, biogas’s dispatch flexibility offers clear advantages over wind and solar resources.
“We see strong potential for more BioPower plants and are pursuing project opportunities throughout the country. BioPower is an especially attractive renewable option due to its reliable and dispatchable nature, but BioPower is also more complex than wind or solar and requires more logistical coordination during development and operations. Areas with large amounts of food or agricultural waste and a local utility with a renewable appetite make good candidates for BioPower,” said Dillon.
As a unique waste solution for rural areas and a reliable renewable energy resource, HTBE is the worthy recipient of a POWER Top Plant award. ■
— Aaron Larson is a POWER associate editor (@POWERmagazine, @AaronL_Power).