How Bi-County landfill could turn wood waste into gas energy
A proposed new technology at the Bi-County Landfill would transform wood waste into gas, and without flames or smoke.
Board members will hear a presentation next week from Aries Clean Energy about "thermal gasification" and the possibility of locating such a unit at the landfill.
“This is new technology,” said Scott McRae, project manager for a gasification plant in Lebanon, just east of Nashville. “It’s not burning. It’s not an incinerator, it’s the next step, which is gasification.”
Through a chemical process, waste materials produce a gas that can fire a steam boiler to generate electricity.
McRae said Lebanon’s unit was built at the city’s wastewater treatment plant and went online in October 2016. Each day, it uses 15 truckloads of wood – primarily ground up wooden pallets – to produce a third of the electricity needed for the sewage plant. Another third of the power is produced by solar panels.
When the $250,000 of savings on electricity costs is factored in, McRae said Lebanon has effectively “shut down the biggest energy hog” of any municipality: powering a sewer plant.
He said at this point, Lebanon's gasification operation is about “80 percent of where we want it to be” financially. The final 20 percent will include paying off the $3.5 million start-up cost and begin to turn a profit on electricity sold back to TVA, which is expected in the next three to six months.
Additionally, a byproduct of the process is a substance called biochar, which can be sold to burn in incinerators or as a soil supplement.
At Bi-County, the scale of the gasification unit would be larger, with an estimated cost of $8 to $9 million, but the benefits would also be larger.
The process could eliminate tons of wood waste currently being buried in the landfill, as well as tons of garbage and demolition debris. Tires may also be ground down for use in the unit.
The biggest cost savings would come from leachate and sludge management, according to Bi-County Director David Graham. Leachate is contaminated water that collects at the bottom of a landfill after rain water filters through the buried garbage.
“I hauled 2 million gallons of leachate last month over to Clarksville Gas & Water (wastewater treatment plant) at a cost of $20,000,” he said, adding that the figure didn’t include time and labor.
Graham had talks with Gas & Water about extending a sewer line from just beyond the Highway 374 intersection of Dover Road to the landfill to eliminate hauling the leachate with five tanker trucks. The $3.5 million cost could have quickly been erased by wiping $350,000 to $400,000 in hauling expenses off the books, he said.
However, Gas & Water wanted the landfill sewer line to tie in at the Ringgold substation. “I can’t maintain 9 miles of leachate,” Graham said.
With gasification, 100 percent of the leachate could be evaporated with no environmental harm.
Conversely, Gas & Water stopped on-site disposal of sludge, or the solid remains after wastewater treatment, following the 2010 flood. Bi-County now hauls it to the landfill, where it is released over the garbage being buried.
Because sludge is 80 percent water, Graham said the trash “can get sloppy” if too much is applied at one time.
But gasification can dry the sludge to an odorless, coffee-grounds consistency powder that can be buried or sold as fertilizer.
Total estimated annual gains were projected at more than $366,000. And additional future gains were estimated at more than $871,000.
“This is an option with a little bit of capital investment, really reduces costs at Bi-County, (and) really is environmentally friendly,” Graham said. “It keeps liability down with trucks off the road, maintenance costs go down, (and) personnel cost just went down. There’s really plenty of second- and third-order benefits to it.”
Looking to the future
Graham, who has been on the job as landfill director for only a few months, is presenting board members with several options to sustain and enhance operations.
“I’m trying to determine the best way to provide a product to our citizens at the cheapest cost and most environmentally friendly manner,” Graham said. “And I’m looking at everything."
The gasification proposal will be presented on Jan. 24.
"My vision of this place is a nonprofit, if nothing else,” Graham said. “If I can keep this thing providing a service at no taxpayer cost, then I’ve done my job.”
Mark Hicks can be reached at 931-212-7626 or on Twitter: @markhicksleaf.