Turns out, yard trimmings and sewer sludge can be used to power electricity plants.
By Paige Brettingen
Even for the most environmentally conscious residents, it’s not always easy—or possible—to recycle everything. From dead tree limbs to busted tires, most people have had to occasionally resort to using the garbage bin. But in Covington, Tenn., hard-to-recycle items are bypassing the landfill and being converted into electricity.
Though Covington has a population of just over 9,000, its biomass waste—which includes tree trimmings and sewer sludge—was adding up enough for David Gordon, who was mayor at the time, to start worrying about storage space. Also, the massive amount of waste required vehicles to transport it, putting an extra strain on the city budget.
While brainstorming solutions, Gordon learned that Covington’s 360 tons of monthly biomass waste could be repurposed thanks to a waste-to-fuel gasification system developed by Nashville-based PHG Energy.
A little over a year ago, “Covington was throwing wood waste and sewer sludge into the landfill,” says Chris Koczaja, PHG Energy’s vice president of sales and engineering. “Today they’re mixing 80 to 90 percent of woodchips with 10 to 20 percent sewage sludge and gasifying it into heat energy.”
What does that mean, exactly? To convert the matter into energy, a plant takes in scrap wood and gutter sediment as its “feedstock”—up to 12 tons of biomass each day. Then, along with air, it’s drawn downward into a constriction area. Here, tars and unstable gases pass through a hot bed of char and are broken down into carbon monoxide and hydrogen—or a clean-burning gas called “producer gas”—which then turns into heat and, subsequently, electricity. According to PHG Energy, the conversion process emits no pollution.
Covington’s $2.5 million plant—with help from a $250,000 grant from the Tennessee Department of Environment and Conservation—became operational in September 2013.
The fuel is used to power the gasification plant while also offsetting the utility bill at the city-operated wastewater treatment—a grand projected savings of $3.5 million over 20 years, not counting the cost eliminated for transporting waste from the landfill. It doesn’t stop there. The thousands of dollars in savings are being put toward improving the city’s parks and other city expenses.
Based on calculations from the U.S. Environmental Protection Agency, the carbon footprint reduction is equivalent to 75 automobile emissions for one year, or one year of emissions generated by more than 33 homes.
According to PHG Energy, while gasification plants aren’t new (the science has been around since the 1800s), what makes the Covington plant unique is its downdraft gasification system, as opposed to an updraft gasifier, which doesn’t produce energy as cleanly.
“Downdraft is the most straightforward method of gasification, and with our design, there are only about five moving parts in the system,” says Mike Webb, director of business development at PHG Energy.
News of PHG Energy’s money-saving, city-changing gasification technology is spreading. In February, officials from Portland, Ore., visited the Covington plant in an effort to find more sustainable ways to reduce its waste. “Covington is proof that gasification is here, and it’s real. It’s no longer conceptual, with unrealistic goals and unrealistic components,” says Koczaja.
In addition to the downdraft design at the Covington plant, PHG Energy has a plant in Greenwood, Miss., that is believed to be the largest downdraft gasifier in the world, taking in 64 tons of feedstock per day. The Greenwood gasifier is experimenting with new types of feedstock, including agricultural waste (corn and cotton stalks), shredded tires, and food-processing waste.
According to Webb, the Covington plant is in talks to install a larger gasifier with the capacity to accommodate five times the amount of feedstock it does now. It has even offered to accept waste from local industries and neighboring municipalities.
As if things couldn’t get any smarter, another development is in the works: PHG Energy is testing a way to cleanly gasify pellets made of municipal solid waste—otherwise known as the trash from the city’s garbage bins.
This article is brought to you by IBM’s People for Smarter Cities. TakePart is teaming up with IBM to highlight innovative ideas and creative change makers who are helping cities all around the world move forward.