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Features - Case Study

The city of Covington, Tenn., collaborates with PHG Energy and General Electric to gasify biomass and help power its wastewater treatment plant.

February 5, 2014
Lisa McKenna

Dr. David Gordon, the mayor of Covington, Tenn., has a natural curiosity when it comes to the topic of renewable energy. “I tell folks I’m a confirmed nerd,” says Gordon, who is a veterinarian by profession. “I enjoy getting into the technical and scientific part of things.”

The mayor took a serious interest in biomass gasification about four years ago, after attending a presentation on the topic at an event hosted by the Tennessee Renewable Energy & Economic Development Council (TREEDC). Gordon found the subject of biomass gasification fascinating in its own right. He also thought the technology might be useful in his own city.

“I started looking at the process of biomass gasification and thinking of how it might apply to the city of Covington,” Gordon says.

Gordon was specifically wondering about a way to decrease or eliminate the city’s monthly cost of around $11,000 to haul and landfill about 360 tons of wood waste and dewatered sewage sludge. Each day the city with a population of 9,000 collects about 10 tons of wood trimmings. The city also must dispose of two tons of dewatered sewage sludge daily from the city’s wastewater treatment plant.

“We started looking at a way to eliminate the hauling and landfill costs and at the same time produce a usable commodity or service,” Gordon explains.

In what became a four-year process, the city successfully collaborated with PHG Energy of Nashville, Tenn., and General Electric Co.’s Power & Water Division to produce a new $2.3 million facility that’s designed to do just what the Mayor hoped. It’s also a shining example of a city’s resourcefulness and collaboration that could be repeated elsewhere and for other waste streams.

Studying a solution

The mayor’s growing knowledge of biomass gasification told him that the city’s wood waste and sewage sludge might actually become a benefit to the city if these streams could be used to produce electricity, either for sale back to the local grid or for the city’s own use. Toward that end Gordon also looked into the possibility selling electricity produced through such a process back to the local provider, the Tennessee Valley Authority (TVA).

It wasn’t long before the city connected with PHG Energy (PHGE) based in Nashville, a maker of downdraft gasification systems for biomass, and the timing happened to be right for the two entities to collaborate. At the time the mayor approached PHGE, the company was conducting research on six of its own gasifiers that had been installed in 2008 at a former brick plant in Gleason, Tenn., located about two hours away from Covington. The gasifiers had been installed to offset natural gas use at the plant.

Mike Webb, director of business development for PHGE, says Mayor Gordon visited the site in Gleason looking for a way to utilize the waste material instead of putting it into the landfill. Meanwhile, PHGE was testing the use of syngas from its gasifiers to feed a 1-megawatt (MW), gas-powered Caterpillar generator it had installed, along with a gas cleaning system. “We designed it, built it, installed it, ran it and sold electricity,” says Webb of the project. The feedstock was scrap wood from a nearby wood flooring facility.

“We were cleaning up the gas,” he says, then using it to power the generator. Electricity was sold back to TVA. However after several months of studying the process, the company determined it was not ready for commercialization, Webb says.

“Running the cleaning process is too expensive,” he says, explaining that the syngas first had to be cleaned to remove tars and particulates that would otherwise harm the generator. “That really is the problem with producer gas going to a generator,” Webb observes.

In part thanks to the city of Covington’s inquiry, PHGE kept studying the matter for the better part of a year and ultimately encountered a solution that could work for its gasifiers. PHGE found that by using a standard heat exchanger and a generator employing Organic Rankine Cycle (ORC) technology—specifically GE’s Clean Cycle generator—no cleaning step was necessary.

Webb describes arriving at a sort of “aha” moment in the company’s research, when the PHGE team realized, first, that syngas from the gasifiers could be combusted to power an oil heater, and second, that the ORC kept the conversion to electricity relatively easy to accomplish.

“We looked at it and said, ‘why don’t we create heat with a thermal oxidizer and then run the ORC,’” explains Webb. The engineers at PHGE then proposed the idea to experts at GE. “They said, ‘No one has tried that, but OK.’ And it worked.”

“The lowest cost and easy to operate system we could find was the GE ORC Clean Cycle Generator,” explains Webb. He notes that while a steam generator is more efficient and can produce more electricity, steam generators also require a significantly greater amount of upkeep and maintenance. “With an ORC generator, it just sits there and runs as long as it has heating and cooling provided,” he says.

“It was, as far as either of us know, the first time someone had used the ORC directly fed with producer gas to make electricity,” says Webb. In the past he says the ORC generator has typically been used to capture waste heat in other applications.

In the PHGE-Covington setup, the gasifier produces syngas that is combusted in a thermal oxidizer to feed the ORC generator. “It’s also a great emissions control device,” says Webb of the standard thermal oxidizer, because all the tars and particulates present in the syngas are destroyed via the combustion process. “You then use that heat to power the ORC,” he explains.

Webb says there currently aren’t many downdraft gasifiers in use in the U.S., and there aren’t many companies building them. Meanwhile the ORC technology has been popular over the last decade or so, says Webb, particularly in Europe. “These two things came together at this point in time,” he observes.

Innovative collaboration

After several months of research, PHGE presented its solution to the city. “We did a lot of calculations and said, ‘This will have a positive cash flow from day one,’” says Webb.

Initially, Gordon wanted to utilize a gasifier capable of producing enough syngas to power a 1 MW generator. However as a result of PHGE’s recommendations and some utility constraints, that had to be scaled down. PHGE also determined there wasn’t a sufficient amount of feedstock to power the larger generator.

The city opted for a 125-kilowatt (KW) system that required a thermal oxidizer and a GE ORC. “When we looked at it in total, it seemed to fit the process and our fuel sources a bit better,” says Gordon.

In July 2012 the city contracted with PHGE to construct the $2.3 million waste-to-energy plant, located on city-owned property next door to its wastewater treatment plant. Construction began in April 2013 and the plant became operational in September 2013.

The resulting system comprises a downdraft gasifier from PHGE that can gasify 12 tons of feedstock per day and supply 6 million Btus of producer gas per hour. The plant also includes wood chipping and material handling, chip and sludge mixing and drying, gasification conversion of the feedstock to gas, a thermal oxidizer and an oil heater to provide heat to power the ORC generator. About half of the energy produced by the plant goes to feed the parasitic process. The rest is enough to cover half of the neighboring wastewater treatment plant’s energy needs.

“We combined three proven technologies into the system we have, so we are no longer hauling our woody waste and sewage sludge, and we are producing electricity that will essentially run half of our sewage plant,” says Gordon.

Gordon says the equipment is designed to and expected to last for about 25 years, and if it does, the city will be able to save about $4 million in excess of paying for the project. The surplus has been earmarked for development of the city’s parks system.

Feedstock options

While all of PHGE’s gasifiers run using similar technology, Webb says the company makes models large enough to power a 1 MW ORC and produce a full MW of power, “so this scales up beautifully,” he says. Though it hasn’t yet been done to that scale, says Webb, he believes it could be appropriate for regions or municipalities that are geographically distant from transmission lines or in regions where electricity is very expensive.

Webb says the gasifier also could be fed with construction & demolition (C&D) waste, which is one of the feedstocks PHGE has tested, however it must be blended with wood materials.

“Because of the chemical content of that waste, we need to mix it with a wood product so it will gasify in our machines,” he says. The company has determined that C&D waste needs to be blended in about a 50-50 mix with wood materials.

“There’s a lot of chemistries going on,” says Webb of C&D waste, which typically comprises shingles, wallboard, insulation and plastics. “To keep the chemistry consistent we’d like to blend it with wood chips of some sort,” says Webb. “We can use about 10 percent plastics in our process so we have to blend it to get to that level.”

Gordon says while he is a staunch supporter of environmental causes, this project also had to make sense financially in order for him to support it. “I’m a supporter of improving the environment, but using taxpayer money, it couldn’t just be for a warm and fuzzy feeling,” Gordon explains. “It had to make financial sense and all parts of this had to be considered to convince me to go ahead with the project.”

The permitting process took less than a year, and according to Gordon, most residents are excited about the new waste-to-energy facility. “This has really become a source of community pride because we are on the forefront of this technology.”

Furthermore Gordon says the plant is generating much interest from other communities interested in doing the same thing back home. “We have several tours scheduled on a frequent basis from other communities, and we are open to having them come look at this and see if it fits for them.”

Gordon says the city currently is looking at additional feedstock sources, too. For example a nearby industrial paper company is interested in contributing waste paper from its production processes. Gordon says he also is looking into the use of waste tires, though he admits that stream brings its own set of challenges.

Using C&D waste comprising purely wood also could be a possibility, says Gordon, as long as the material is free of metals and rock. “With those types of things going into the chippers and into the processors, that decreases the life of those machines tremendously,” he points out.

However Gordon reports the city is currently in talks with a nearby custom window and door factory interested in contributing one of its waste streams from a production process to the gasifier. “I don’t mind looking at other options if we can make sure it works and make sure it’s financially feasible,” he says.


The author is a managing editor of the Recycling Today Media Group and can be reached at