Cheaper, Cleaner Combustion

Even outfitted with the best available technologies, gas-turbine power plants and industrial boilers emit significant amounts of nitrogen oxides, precursors to smog and acid rain. But better results could come from a new combustor that’s so simple, it’s been described as a “teacup with a straw.”

Researchers at Georgia Tech say their simple design – in which fuel goes down the “straw,” air mixes from a donut-shaped inlet around the straw, and combustion gases flow out of the “teacup’s” sides (see image) – slashes nitrogen oxides emissions to as low as one part per million (the best existing technologies emit nine parts per million), and also produces less carbon monoxide pollution.

The reduction in emissions is done without complex and costly equipment that mixes air and fuel, or systems to cleanse pollutants from the exhaust. The device was invented at Georgia Tech by a team headed by aerospace engineer Ben Zinn.

Controlling NOx emissions costs industry billions of dollars. And of course the pollutants that do get emitted worsen air quality and poison waterways. “If someone can show a combustor that can get to two parts per million NOx and not require [exhaust scrubbers] on the power plant, you have made a big step to save capital costs, reduce the cost of electricity, and reduce pollution,” says Richard Dennis, turbine technology manager at the U.S. Department of Energy’s National Energy Technology Laboratory in Morgantown, WV.

Essentially, the Georgia Tech design minimizes areas of very high temperatures within the combustion chamber and also minimizes so-called “back-mixing” of combustion gases with hot areas – zones where the nitrogen oxides form. Instead of premixing fuel with air, the shape of the combustion chamber efficiently forces the air and fuel to mix and guides the combustion process.

If the simple device works as Zinn believes it will, the question becomes: Why didn’t anyone think of it sooner? “That’s a very good question,” Zinn says. A Georgia Tech startup, Plum Combustion, is working to commercialize the technology. Zinn says he’s already talking to potential customers about the device, including gas-turbine makers and even manufacturers of ordinary gas water heaters.

And while the new design could, according to Zinn, surpass conventional gas-turbine combustors in reducing nitrogen oxide emissions, it could have its biggest impact in industrial boilers used for heating and powering industrial facilities, where emissions are generally even higher.

“We were surprised since it looks so simple – just a teacup with a straw in it,” says the Department of Energy’s Dennis. While he emphasizes the need for more study, he adds: “But the reason it seems to work so well, when you flow fuel and air into that column and it combusts, everything is just forced right out, it appears that you don’t get as much back-mixing. If you do that fast enough, you avoid some NOx issues.”

One way industry removes nitrogen oxides today is to add a catalytic system with ammonia-injection in the exhaust stream to break down the NOx into nitrogen and water. But this is expensive, and adds pressure to the system, reducing a power plant’s efficiency. And if too much ammonia is added, it can escape and create other pollutants.