An Anaerobic Alternative

Undefined mixed cultures traditionally used to produce methane through anaerobic digestion show potential for creating higher-value carboxylates
By Erin Voegele | May 20, 2011

Anaerobic digestion systems have long been used to produce methane, a renewable form of natural gas. The microbial communities found in anaerobic digesters, known as undefined mixed cultures, essentially degrade organic materials into chemical precursors that are eventually converted into methane by methanogens, a group of microorganisms found in those undefined mixed cultures.

While producing methane through anaerobic digestion systems can be economical in some configurations in the U.S., such as those treating waste water or burning the resulting methane to provide heat and power to a plant or other industrial facility, the economics can be more difficult to manage in other situations; specifically, in situations where the renewable methane is used to create electricity that is fed to the grid.

This is economically challenging because the methane used to produce electricity must compete with fossil-based natural gas used to fuel power plants. Low natural gas prices have made it hard for renewable methane to compete economically.

However, researchers at Texas A&M and Cornell Universities are developing respective anaerobic biotechnologies that inhibit methane production in undefined mixed cultures. Instead, the cultures produce higher value molecules, known as carboxylates, which are chemical precursors to alkanes found in fuels, such as gasoline and Jet A-1. The resulting carboxylates feature a significantly higher value than methane.

According to Mark Holtzapple, Texas A&M professor of chemical engineering, methane is currently valued at approximately $4 per million Btu. In comparison, drop-in biofuels that can be manufactured from carboxylates are worth nearly $20 per million Btu. Terrabon Inc., which is currently working to commercialize a carboxylate platform developed by Holtzapple and his team at Texas A&M, further estimates that methane production results in approximately $80 to $90 of value per dry ton of feedstock introduced into the system, while the carboxylate platform creates between $250 and $400 in value from the same amount of feedstock.

Research and Development
Holtzapple has been working on developing a carboxylate platform for more than a decade. “We wanted to come up with an economical way to convert biomass into fuels and chemicals,” he says. “In my view, the beauty of this is there is no sterility required and you do not have to purchase enzymes.” This significantly reduces the capital costs and operating expenses when compared to many other advanced biofuel production technologies.

There are several ways research teams are working to inhibit methane production in undefined mixed cultures, including the use of inhibitors and manipulation of heat and pH in a digester. Holtzapple’s team is using a methane inhibitor known as iodoform. “It’s a methane analog,” he says. “It has a structure similar to methane. What it does is it binds to the active site of enzymes that release methane and plug them up.”

Holtzapple and his team built a pilot-scale carboxylate facility on the Texas A&M campus in 2000. The plant can process approximately 100 pounds of feedstock per day. “The purpose of the pilot-plant on campus is to go all the way to ketones,” he says. “We can make a few kilograms of ketones. From that, we’ve actually made gasoline and jet fuel. By converting the ketones to alcohols you can covert the alcohols to gasoline and jet fuel.”

Researchers at Cornell University are working to develop a different carboxylate platform. According to Lars Angenent, an associate professor of environmental engineering at Cornell, his team has been working to develop its technology for approximately three years.

To date, some of Angenent’s research has focused on studying the microbiology communities found in existing anaerobic digestion systems. Undefined mixed cultures contain thousands of microbial species that work together in a food web, he says, noting his team is interested in learning what types of microbes are in those systems, what they are doing, how they work together, and how the system can be improved.

The team recently conducted a study of nine full-scale anaerobic digestion systems treating brewery wastewater. Samples were collected from each system once a month for a year, resulting in 112 samples. According to Angenent, there have been important technological advances in gene sequencing in recent years that have allowed his team to generate 400,000 sequences of a particular gene from the microbes to characterize them.

“We found about 5,000 different species of microbes in these reactors,” he says. “They were all working together to treat waste and eventually make methane.” The research determined that each of the nine anaerobic digestion systems featured its own unique community structure, which didn’t change much over time. Data gathered from the samples was then analyzed with operational data provided by the brewery, which included pH and temperature. The ultimate goal of the study and Angenent’s research is to alter microbial communities in a way that inhibits methane production.

While the technology developed by Texas A&M features a methane inhibitor, Angenent and his team are working to inhibit methane by adjusting the pH within the digester. A pH of 5.5 has been shown to mitigate methane production by the communities, he says. According to Angenent, pilot-scale evaluations of process could begin within three to four years.

Scaling Up
While Angenent and his team at Cornell continue to work towards pilot-scale production, Terrabon is working to commercialize the carboxylate production technology developed at Texas A&M.  Terrabon, which has exclusive license to the carboxylate platform developed by Holtzapple, is already operating a demonstration-scale plant and has plans to break ground on a commercial facility late this year or early next.

The demonstration-scale plant became operational in early 2009, says Terrabon Director and CEO Gary Luce. There are essentially two components to the plant he says: an upstream portion that takes in waste materials; and a downstream process that converts the resulting carboxylates into renewable gasoline and JP8 jet fuel.

The facility can take in approximately 40 to 50 tons of biomass material per month. It currently processes food waste collected from a Texas A&M cafeteria, local hospitals and several grocery stores. According to Luce, the carboxylate technology employed by the plant is very similar to traditional anaerobic digestion systems.

“[In methane production] you basically start with biomass,” Luce says. “A mixed culture breaks down the biomass [into carbohydrates and sugars] using its own set of enzymes. Then those get further digested into organic acids, and the organic acids ultimately get eaten by methanogens that make methane and CO2. That one step right before the methane is where these organic acids—or carboxylate acids—are being produced by this mixed culture. What our process does is it follows that same biological process, which you would find inside a cow….What we’ve done is create through chemical engineering, process design and optimization [a way to] control temperature, pH, and mixing through fermentation tank design to optimize the maximum yield of these organic acids.”

That’s what the front end of the technology does, Luce explains. “Then we effectively convert those acids to salts so we can get them to dissolve in water and separate it out from the mush that is still [in the digester]—that is the biomass that hasn’t been digested,” he says. “Then we concentrate that down and effectively thermally convert those organic acids into ketones…From there we use a traditional chemical engineering catalyst to convert those secondary alcohols into gasoline and JP8. The upstream is the biological system and the back end of our system is just a traditional chemistry system. The IP [intellectual property] and know-how is kind of how we think those two systems [work] together.”

In addition to using additives to inhibit methane production, Terrabon’s process also uses a timing mechanism to mitigate methane. “It takes awhile for methanogens to organize themselves,” he says. “We run our system at a very small liquid residence time. Because the system is being flushed so quickly, it’s not being held up in there. It actually doesn’t give time for the methane to be produced.” The scale of the system can also impact methane production. As the anaerobic digestion industry began producing bigger and bigger reactors they found that the reactors would get “stuck.” In other words, they wouldn’t turn the carboxylates into methane. “What we’ve done is captured some natural tendencies of how systems operate and then wrapped it around chemistry to make renewable fuels,” Luce says.

Terrabon currently expects to have a commercial-scale system operational by 2013, and already has strong development partners on board. “We’ve got two large strategic partners invested in us, Waste Management Inc. and Valero Energy Corp.,” Luce says.

Finding a Home
There are several key factors that make the carboxylate platform an attractive biorefining technology. Perhaps most importantly, it is economical. “Because it does not require sterility or enzymes, the cost is very attractive compared to other process options,” Holtzapple says. The process is also flexible in terms of both feedstock and final products. “With the carboxylate platform you can produce almost any base chemical currently made from petroleum or natural gas,” he adds. “In addition, you can make every basic category of hydrocarbons.”

Early on in its commercial development, Holtzapple says he expects the technology to take in waste products as feedstock. This includes municipal solid waste and sewage sludge. “But, eventually I think it will be scaled up to use energy crops for agricultural residues,” he says.

There are several operational configurations under which the carboxylate platform may prove economically competitive. While Terrabon intends to construct a new, free-standing plant that will utilize waste feedstock sourced from Waste Management, Angenent notes that there is also potential to co-locate carboxylate production systems with existing biorefineries, where waste biomass and wastewater could be taken in as feedstock. In addition, it may also be possible to retrofit existing anaerobic digestion systems to produce carboxylates rather than methane.

Although Angenent notes that standalone plants are likely to be constructed in the future, he thinks co-location strategies would be advantageous. “The biggest value for the dollar is if you have a biorefinery concept where you…get the most value out of the biomass that you have going into a plant,” he says. For example, existing wet mill corn ethanol plants typically produce a significant amount of waste water. Feeding that waste into a carboxylate production facility would allow a plant owner to convert waste nutrients found in the water into a biofuel, resulting in a new revenue stream. The carboxylate platform would also allow the resulting clean water to be recycled back into the ethanol plant.

With all the potential the carboxylate platform is showing, there is surprisingly little research going into the technology right now. “I want to predict that this is going to be a hot topic and a big area, but it’s not at this point,” he said. The lack of ongoing research in the platform might be tied to the fact that research money for methane production technologies has slowed significantly in recent years, and the two technologies are similar enough that they often get lumped together. A great deal of research interest has been focused on synthetic biology routes to biofuels. “There is nothing wrong with that, but I think it is also very scientifically exciting to [develop] new ideas on the same angle with these undefined mixed cultures,” says Angenent, noting that he will continue to work to bring more interest to this exciting technology.

Author: Erin Voegele
Associate Editor, Biorefining Magazine
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