Good Neighbors

Lately, cellulosic ethanol plant startups have basked in the bioenergy limelight. While less celebrated, anaerobic digesters are adding value to operations and seem to be trending in blueprints.
By Katie Fletcher | November 19, 2014

When ethanol plants can provide excess sugar to neighboring anaerobic digesters (AD) and get something as good as cake out of the transaction, residing in the same neighborhood simply makes sense. Colocating these bioenergy sources seems to be trending amongst both traditional and cellulosic ethanol plants.

Early U.S. commercial-scale corn ethanol plants left AD out of initial design because the concept wasn’t mature, but nowadays, digesters are used to streamline the production process by converting waste streams inherent to the ethanol production process into renewable energy. Ever since AD colocation has become a viable and valuable option, digesters have been making their way onto blueprints and into upgrade plans of ethanol plants. “Anaerobic digestion has been fine-tuned and modified over the millennia, and it’s a shame to not take full advantage of that,” says Evan Chrapko, CEO of Himark Biogas.

Nontraditional ethanol plants are approaching AD as not optional, but rather, a necessity. “Bioenergy technologies, such as cellulosic ethanol and anaerobic digestion, are being colocated to minimize external energy consumption and reduce emissions, making the overall solution more sustainable,” says Chris Standlee, executive vice president global affairs with Abengoa Bioenergy.

Abengoa’s cellulosic ethanol plant in Hugoton, Kansas, is joined by Poet-DSM and DuPont in a series of cellulosic ethanol plants that came on line this year, incorporating digesters at ground-level construction. Iogen is another player working on AD in the cellulosic space. “It’s natural it would be part of the project investment,” says Pat Foody, executive vice president of advanced biofuels with Iogen Corp.

In fact, Foody believes the only time to decide to add AD to a cellulosic ethanol plant is at conception because of the need to determine where the soluble residues are going to go before construction. “I think there is a huge amount of integration that occurs,” Foody says. “The natural thing is the AD is integrated into the design at the onset. I personally couldn’t see that a bolt-on would really work commercially, it’s so integrated.”

Some traditional, commercial-scale plants have been successful in implementing AD down the road, while others have not. Western Plains Energy LLC’s 50-MMgy corn ethanol plant in Oakley, Kansas, implemented a Himark AD system onsite. The roughly $40 million installation consists of four digesters and a tertiary lagoon replacing natural gas used to heat the plant’s boilers. Another Himark project is Growing Power Hairy Hill LP, which is the first integrated biorefinery in Canada. In September, Calgren Renewable Fuels’ 55-MMgy ethanol plant installed a DVO-designed digester built by Andgar in Pixley, California.

Sharing the Stage

On the cellulosic side, AD integration has always been the intent. Although the applications are just now beginning or nearing biogas production, they are expected to add value to their respective operations. Poet-DSM’s Project Liberty in Emmetsburg, Iowa, is said to have the nation’s largest digester, which, to date, has received little coverage as it shares the stage with one of the country’s first cellulosic ethanol facilities that is in its final stage of commissioning. “The AD is a known technology, but this unit is very large and has many different subprocesses,” says Rod Pierson, vice president of operations at Poet. “The Poet Research team had done a great job of developing expertise that will benefit in the start of the unit.”

DuPont’s digester, collocated with its cellulosic ethanol plant in Nevada, Iowa, is smaller than Project Liberty’s. “Our anaerobic digester is a relatively small component of our cellulosic ethanol plant, simply meant to treat recycled water, not a major project or source of energy in its own right,” says Wendy Rosen, global public affairs lead with DuPont Industrial Biosciences. Due to the operation’s infancy, the company has little other public information to share at this time.

Abengoa decided AD was worth integrating with its cellulosic ethanol facility after contracting with an independent laboratory to conduct bench-scale studies of pilot-plant-generated wastewater to determine the effectiveness of both anaerobic and aerobic digestion. “These studies concluded that anaerobic digestion would be most suitable to produce the treated wastewater quality required by the cellulosic ethanol process,” Standlee says.

Abengoa’s cellulosic facility is located where water resources are limited, so conserving overall water use is important. “The long-term viability of the facility depends on minimizing water use and discharge,” Standlee says.

Nontraditional Nuances

Whether traditional or nontraditional, digesters are implemented for similar reasons, although the drive to implement AD at cellulosic plants seems to be stronger. Some of the byproduct from starch plants can be processed into DDGs for sale into feed markets. Cellulosic ethanol plants do not have this option, however, the materials that remain after distillation at a cellulosic plant can be fed into a digester. Abengoa’s 5.5 million-gallon digester’s primary influent streams derive from pretreatment steam condensation and stillage evaporator condensate. “Individual influent process streams are mixed and pH adjusted before entering the AD,” Standlee says.

Abengoa’s AD effluent is temperature adjusted and biochemical oxygen demand (BOD) is digested further in an aerobic digester. Aerobic digester effluent is then filtered and sterilized with UV light before returning to the enzymatic process. Waste sludge from both the AD and aerobic reactors is combined. Once the AD microbes digest the influent BOD and produce biogas, the biogas is dehydrated, compressed and routed to a multifuel boiler. This boiler drives a turbine connected to a power generator at the plant supplying approximately 25 MMBtu per hour from the produced steam. The digester saves the facility money by recovering some of the energy from the wastewater contaminants, and the biogas will save an estimated $750,000 a year, according to Standlee.

Project Liberty’s digester, developed by Netherland-based Paques, is modeled after a small-scale AD operation at Poet’s pilot plant in Scotland, South Dakota. The 1 million-gallon digester tank is fed a liquid stream from the whole stillage that contains residual organic materials. “The organic material is fed into the reactor where the methanogens convert the organic material to methane (biogas), the biogas is then purified so it can be used to displace natural gas,” Pierson says. “Seed granular methogenic sludge will be transferred into the reactors in November, and organics will begin to be fed shortly after with an extended ramp-up for the sludge to acclimate to the feedstock.”

The biogas is used to fuel existing boilers and dryers. The steam that is not used at Project Liberty is sent to Poet’s adjacent starch plant to displace natural gas use.

Traditional Distinctions

Corn ethanol plants also use digesters as a way to replace energy sources with biogas, but feedstocks differ. Traditional plants require some feedstock outside of the ethanol production process. “The main challenge and the first thing you need to address is the quantity and quality of the waste that you have available to feed the thing,” Chrapko says.

Corn plants can source feedstocks from various waste sources. “The large number of dairies in our area led us to consider using biogas as a substitute for natural gas,” says Lyle Schlyer, president of Calgren Renewable Fuels. “Food waste from local plants will be taken at a later date.”

Andgar Corp.’s installation of DVO’s digester design will take in 55,000 gallons of waste per day into a 73-foot by 175-foot by 16-foot digester. This digester design is different from upright continuous mixed tanks. DVO’s digester is a patented, two-stage mixed plug flow process in a concrete vessel partly below grade. Eric Dvorak, business development and design engineer with DVO, says it functions like a garden hose. “Similar to a garden hose, you put the water in on one end, it has to travel all the way down to the other end and finally come out, so it’s a set distance that material has to travel and if you put it in at a set rate you’re going to have a set retention time,” he says.

The liquid height in the digester vessel is about 14 feet, but the top of the digester is about 16 feet. The two feet of air space across the entire digester is where the biogas collects and will come out a tube as it gets sent to the ethanol plant, Dvorak says.

The biogas will offset 7,000 MMBtu of natural gas per month helping fuel a duct burner, which resides between the discharge of the turbine and the inlet of the steam generator. The electricity and steam cogeneration unit will be fueled partially with waste from Four J Farm Dairy, the closest source of manure. “They also have plenty of farm land upon which to use the nutrient-rich liquid from the digester,” Schlyer says.

Calgren is also separating the fiber from the digester and trucking it back to Four J Farm Dairy, where it likely will be used primarily as cattle bedding material. Excess fiber can be sold as a fertilizer or a peat moss replacement, Dvorak says. DVO has this byproduct, “Magic Dirt,” currently being sold through Walmart stores.

This integrated strategy of receiving waste from the dairy farm and looping it back as fertilizer or other byproducts is something Himark is also working on. The company’s patented, integrated biorefinery design is used at its 10.5-MMgy Growing Power Hairy Hill project. Besides producing ethanol from high-starch wheat, DDGs are fed to cattle at the nearby feedlot in exchange for manure, which is fed to the digester to produce electricity and steam for the plant. The digester is also fed large amounts of residential organic waste from the Alberta area. GPHH will additionally produce 30,000 tons of premium biobased fertilizer per year.

Trevor Nickel, general manager with Himark, says it was a no-brainer to combine AD and ethanol production, but actual implementation was less so. “It was apparent fairly early on that the thermal energy generated when you have an energy production facility, like a digester, doesn’t have a home it makes a lot of sense to use that in an ethanol plant,” Nickel says. “Not only could you find a home for that thermal energy, you could find a home for the distiller’s grains.”

One Step Further

Another reason operations are co-locating digesters is the high value for renewable identification numbers (RINs).  One of the reasons Himark’s AD operation was added at WPE’s ethanol plant was derived from a business decision based on the company’s projections that the D5 RIN value would be extremely high. Whether or not that ends up being the case, it will not affect the technology and other benefits in efficiencies, Chrapko says.

Foody and his colleagues at Iogen are looking for ways to generate RINs from both the biogas and cellulosic ethanol. Iogen includes AD in all of its initial designs, and is taking it one step further by working with sources of biogas from cellulosic plants and other sources to provide the market with an alternative to compressed natural gas for RIN generation. “We have been looking at increasing the yield of RINs from biomass and the first and most basic way to get RINs is to make ethanol,” Foody says.

Iogen is evaluating converting methane into hydrogen, and then transferring that hydrogen to a refinery to add to the finished fuel to generate RINs and Low Carbon Fuel Standard credits. “What’s unique about this process is all the pieces are demonstrated, there is no additional process technology—it’s really an approach for routing biogas into a qualifying application,” Foody says. “All the parts are already demonstrated, it’s just the contracting path and the EPA approval that has not yet taken place.”

AD technology has developed over the millennia, and both traditional and nontraditional ethanol plants have been taking advantage of the opportunities biogas can add to operations. Whether moving digesters and ethanol plants into the same neighborhood is a good idea boils down to economics. Many are proving it’s a decision worth making. “I think that every ethanol plant out there deserves or needs to have one of these biogas plants,” Chrapko says. “If not only from an environmental point of view, but definitely from an economic point of view.”

Author: Katie Fletcher
Staff Writer, Biomass Magazine