Features

Applied Research Labs’ project could keep
tiny pollution particles out of the air

Researchers at the Applied Research Laboratories (ARL) at The University of Texas at Austin are using a concept called acoustic agglomeration to make the air emissions from coal-fired power plants cleaner.

This acoustic energy isn’t Eric Clapton playing “Layla” unplugged. The volume is a head-banging 140-156 decibels, which is “rocket-blast takeoff loud,” said Doug Meegan, an ARL research scientist on the project.

Done at the right volume and at the right frequencies, acoustic agglomeration knocks very small fly ash particles into bigger ones, which are captured by the pollution control devices used in coal-fired power plants. 

Doug Meegan, a research scientist at Applied Research Laboratories, led development of technology that uses sound to reduce pollutants from the smokestacks of coal-fired power plants.

The smaller particles—those less than 10 microns—typically elude the control devices and get into the air. Ten microns is about the diameter of a fifth of a human hair or the low end of the size of pollen.

Meegan and the ARL team have been developing the technology with TXU Power, an arm of the Dallas-based TXU Corporation, which is funding the project. TXU Power generates electricity that is delivered to more than 2.5 million people in Texas.

“Burning coal to produce electricity results in a byproduct of ash, much like burning wood leaves ash in your fireplace,” said Bill Muston, research and development manager at TXU Electric Delivery.  “Power plants in the U.S. have particulate control devices that capture, typically, over 99 percent of all the ash. What they don’t capture is that fine particulate because it’s just harder.

“Nevertheless, it can cause some issues with complying with regulations. We thought acoustic technology might be an approach to enhance existing controls and improve at fairly modest cost.”

It’s the economics

Such a technology would help energy companies like TXU by allowing them to more cleanly burn coal—the cheapest, most abundant and dirtiest fossil energy source—with what amounts to a tweak of the current process.

Acoustic agglomeration has been studied for decades, but an economically sound version that would allow wide-scale deployment hasn’t been found.

TXU’s participation has made a difference, Meegan said.

“We’ve always been driven by the industry needs,” he said. “We’ve always had an eye on how to make it better, cheaper and more efficient.”

In the chimney

Particles less than 10 microns in diameter are hard to trap by pollution control devices.

Acoustic agglomeration is more than getting the particles to move around. It’s getting them to move around in certain ways so that the smaller particles are more likely to bump into the larger ones.

The big particles tend not to move around, Meegan said.

“They have mass and inertia so the sound field will just move by big particles,” he said.

The sound, however, gets the smaller particles to move around in something of a lockstep with the sound waves, which is called entrainment.

“So now you’ve got this situation where the big particles are relatively stationary and the little particles are moving around a lot so they’ll tend to have collisions with each other,” Meegan said. “When these particles are relatively close together in these dense clouds of ash, you’re going to stimulate a lot of collisions very rapidly.

“The particles bind together because of electrostatic forces,” he said, “or if the particles are still molten as they are when they come out of the boiler, they may actually fuse together if they touch.”

What’s the frequency?

In its research, the ARL team has made several discoveries about acoustic agglomeration that make it more effective and more efficient.

Meegan and the team found it’s critical to have the proper selection of frequencies.

This illustration shows sound waves forcing smaller particles to stick to bigger ones, the process called acoustic agglomeration.

“Just by being off by as little as 10 percent can cut the effectiveness in half,” Meegan said.

The ARL researchers discovered there has to be active feedback for constantly adjusting the frequency of the sound.

“You’ve got to have a procedure that has active feedback where you’re constantly adjusting the frequency, monitoring what it’s doing to the fly ash-particle distribution and adjusting to optimize that,’ he said. “We found right there that you can basically double the efficiency of acoustic agglomeration.”

They also found that by modulating the frequency in a warble tone that goes up and down in pitch like a siren can double the rate of agglomeration. Meegan has a patent on that process.

“You put those two together and we found a way to quadruple the efficiency of acoustic agglomeration,” he said. “With the same amount of acoustic power, and therefore electrical power, we’re basically able to get four times the agglomeration.”

Extreme environment

Those discoveries were breakthroughs toward an economically feasible acoustic agglomeration product. But they still had put it in a package that would withstand the extreme environment of a power plant chimney.

“You’re wearing a hard hat, steel-toed boots, long pants and shirt,” he said. “It’s 100 degrees out in the Texas summer. You’re wearing safety eyewear. You’re wearing earplugs. In some places you’re also wearing a respirator.

“The device has to survive those conditions.”

The ARL team is developing a sound source that seems to do just that. It blasts out tones between 140 and 156 decibels and is capable of withstanding high temperatures and corrosive gasses in the exhaust.

What can you build?

The project involves physics, acoustics and engineering and it found someone with a background in the three areas. Meegan studied physics throughout his academic career and as a Ph.D. student at The University of Texas at Austin, he was in the lab of mechanical engineering Professor Mark Hamilton.

The ARL team conducted tests on the acoustic agglomeration system at TXU’s 800-megawatt Martin Lake power plant in northeast Texas.

At ARL, they are serious about the “Applied” part of the name.

“When a Ph.D. starts here they want to know what you can build,” Meegan said.

That was OK with Meegan.

“As a graduate student, I was taking machine shop classes so I know how to use lathes and milling machines and I know how to build things,” he said. “I also like the parts of physics you can see and hear and touch. I was always turned off by subatomic physics and the more modern physics that seemed like it didn’t impact my everyday life.

“I was always interested in acoustics because of my interest in music. I was a classically trained percussionist. I played in ensembles and jazz bands throughout college. Anything that was related to acoustics and sound was of interest to me.”

ARL and TXU hope to have a product version of the technology ready to go in anywhere from 18 months to five years.

“The interesting point now is that we know it will work, we know we can produce sound sources and do testing at power plants and have the equipment work,” Meegan said. “Where we are now, it’s an economics problem.”

In any other place, except maybe, a launch pad at blast off, the device’s earsplitting loudness could be a problem. Not here.

“It’s already very loud in this part of the power plant and all the infrastructure is there to contain the noise,” Meegan said. “It’s not an issue.”

Tim Green

Photo of Doug Meegan: Tim Green
Additional photos: Applied Research Labs

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Applied Research Laboratories