With a population of just over 9,000 people, the city of Covington in western Tennessee has made a mark on the renewables map. Its recently completed, waste-to-energy gasification plant installed by PHG Energy went on line in early September, and materials the city has been stockpiling during its development are now being fed into the system for power production.
“Yesterday, Covington was throwing wood waste and sewage sludge into the landfill,” explains Chris Koczaja, PHG Energy vice president of sales and engineering. “Today they’re mixing 80 to 90 percent woodchips with 10 to 20 percent sewage sludge and gasifying it into heat energy.”
Previously, the city was spending about $30 per ton in both tipping fees and transportation, but is already seeing positive cash flow.
The system utilizes PHGE’s proprietary 12-ton-per-day capacity downdraft gasifier to supply 6 million Btus of producer gas per hour as the central technology. The gasifier design, which has been vetted through over 40,000 hours of commercial production use, allows the city to dispose of about 10 tons of urban wood waste and 2 tons of sewer sludge each day, or 360 tons per month.
In a nutshell, PHGE’s plant design 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 power for a General Electric Clean Cycle heat-to-power Organic Rankine Cycle generator. Direct use of the ORC generator to produce power from a gasification plant was pioneered this year through a research and development project PHGE completed with General Electric.
While just bringing the project on line is exciting—commissioning should be complete by late October—the successful installation is particularly noteworthy because it will likely lead to larger projects, which have traditionally utilized updraft gasifiers.
Up with Downdraft
“For larger gasification projects, updraft gasifiers have been prevalent, as downdraft gasifiers have traditionally been limited in size and deemed not scalable,” Koczaja says. “[However] downdraft gasifiers offer easy operation and low operation and maintenance costs—very few moving parts—power density, and a cleaner gas as their output.”
The cleaner gas is a result of air being drawn downward through the feedstock, in the same direction the biomass is moving. Main reactions occur in a throat-like constriction area where the tars and volatile gases are passed through a hot bed of char and broken down into carbon monoxide and hydrogen, at a much higher temperature than in an updraft gasifier. As a result, downdraft gasifiers produce cleaner gas.
Through focused R&D, PHGE has been able to develop a line of gasifiers that maintain the benefits of downdraft while overcoming the issue of scalability.
Rather than scaling up and then looking at symptoms to try to correct them, PGHE approached the core problem—the geometry in the gasifier itself—first. “Our theoretical physicist on staff knew if he could create the right conditions inside the gasifier, he could change the fundamental design that the industry’s used over the last 100 years, in a way that would be feasible and very easy to operate,” Koczaja says.
That approach proved successful, as PHGE has built what it believes is the largest downdraft gasifier in the world at 64 tons per day and 32 million Btu/hr, located in Greenwood, Miss. Fully functional and operating just like the smaller gasifier in the Covington project, Koczaja says the company uses this “full-scale, serial #1” for feedstock testing and proof of concept. There, a vast range of feedstocks have been successfully tested, including chips from wood waste or forest residuals, purpose-grown biomass crops, chopped or shredded tires, ag waste such as corn or cotton stalks, food processing wastes, municipal or industrial sludge, coal and processed and pelletized municipal solid waste.
Feedstock flexibility is a noteworthy advantage a municipality would reap from gasification as opposed to a different technology. “New feedstocks can be introduced without having to modify components,” Koczaja says. Additionally, systems are generally easy to operate and can be stopped on a dime by personnel, all of which in Covington’s case are the city’s existing workforce, and not specialized technicians that had to be brought in. “If [Covington] needs to stop it for any reason, they can simply hit the stop button and it shuts down,” he says. “It can be started right back up.”
And perhaps the most obvious advantage is that there isn’t a time or space element, as there is when using other technologies such as landfill gas. “You have to wait, and you’re also taking up space in the landfill. [With gasification] you’re skipping that step.”
Via the Mississippi system and three other commercial installations at brick kilns, PHGE was able to prove these advantages and demonstrate the system to Covington, and Mayor David Gordon signed a contract with the company in July 2012. The total cost of the plant was $2.5 million, $250,000 of which was paid for with a grant from the Tennessee Department of Environment and Conservations. The remainder was financed through a 20-year Tennessee Municipal Bond Fund.
Confident in the scalability of its modular design, PHGE is ready to move full speed ahead with larger installations.
“Since the system design is modular, it can easily be upgraded to accommodate larger municipal or industrial facilities,” Koczaja says. “Economies of scale kick in very quickly, and savings become substantial. Other such projects now in the development pipeline are showing payback periods of three to five years. Covington is a small town, and it’s always easier and cheaper to do things bigger. If we can make it work on Covington’s scale, the economics just get better for the cost of the installed equipment.”
As for other current and future endeavors, Koczaja says PHGE is currently working with some islands in the Caribbean on energy options. “I think that is one of the biggest things that gasification can help with…they are already bringing in types of energy and filling their landfills. We’re trying to help them connect those two dots to make energy, and utilize their tremendous growing seasons and land mass to grow their energy. Those dollars going offshore to buy oil are dollars they could keep in the local economy while dramatically decreasing energy costs.”
There is one very important thing to note about gasification, Koczaja adds. “It’s here, and it’s real. It’s not conceptual with unrealistic goals and unrealistic components…it’s something, that in another month [in Covington], you’ll be able to come and touch, feel, and see that it works.”
Author: Anna Simet
Managing Editor, Biomass Magazine