Naval Research Lab applying pulsed electron beam technology to reduce NOx from coal power plants; fusion research spin-off

17 November 2014

The US Naval Research Laboratory (NRL) is partnering with a power company to apply its pulsed electron beam technology to reduce NO_x emissions from coal-fired power plants. Today, industry uses Selective Catalytic Reduction to reduce NO_x, but the scrubbers are expensive to build and to operate. Dr. John Sethian, the plasma physicist leading the project at NRL, says that if NRL’s technology works as expected, it would be about 10-20% the cost of a catalytic scrubber for energy, and with no byproducts.

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The concept is to inject electron beams into the exhaust of a fossil fuel power plant and, firing them in pulses, break apart the NOx bonds. When the bonds between the nitrogen (N) and oxygen (O) atoms are ruptured, says Sethian, the atoms naturally want to combine into just pure nitrogen and pure oxygen.

NRL chemist Dr. Matthew Wolford has already proven the concept works at a small scale, using a mixture of just nitrogen and NO_x. Wolford is now testing with a more realistic sample. Actual flue gas contains multiple additional molecules which complicate the chemistry.

CO₂ wasn’t a problem, but the oxygen threw us for a loop. It’s one of these up and downs, you try not to get emotionally involved. I’m confident that we’ll solve all the problems, but I can’t guarantee it. I will guarantee, on the other hand, that we’ll figure out what’s going on.

NRL has a Cooperative Research and Development Agreement with Zerronox Corporation to pursue solutions for reducing NO_x from coal-fired power plants and other combustion-based energy sources. Further developing the pulsed electron beam and implementing a working system is additionally supported by their partnership with a large power company.

Sethian imagines splitting the exhaust into several parallel ducts, each treated with a pair of electron beams. Narrower ducts would allow the electron beam to reach all the gas more efficiently; having several systems allows for regular maintenance.

As tested at NRL, the beam works by sending a powerful pulse of electrons from a cathode through a thin foil of steel or titanium a few centimeters away.

The cathode is kept at a very high voltage; the anode is at ground, which is 0 volts. The electrons accelerate from the cathode to the anode; but the anode is so thin that it doesn’t stop the electrons—and they keep going, into the gas, and they deposit their energy into the gas. We can run them several times a second for very long duration runs.

Bonds typically break at 4 electron volts (eV) of energy. A simplistic way of thinking about it, Sethian says, is that a 400,000 volt electron beam will 100,000 bonds. “The electron beam is just a very efficient way of getting the bonds to break.” The beam has to be energetic enough to get through the foil and deposit energy all across the flue, but not so strong to be a serious safety hazard.

The electron beam makes x-rays when it stops in the gas. The higher the voltage, the more energetic the x-rays. What we’re looking for is a good, happy medium.

The NRL electron beam system, dubbed Electra, was originally developed as part of NRL’s laser fusion program. If the electrons are fired into a KrF gas (instead of into a flue gas, as with the NO_x application), they excite the molecules in the gas and produce KrF laser light. KrF lasers have unique features, such as a very smooth laser beam. This makes them ideal for driving a fusion reaction in a pea-sized pellet.

If NRL demonstrates the NO_x system works, it makes a stronger case that the KrF laser application of the electron beam technology would have the reliability needed for a fusion energy source, the researchers said.

However, achieving a system to reduce NO_x emissions is a more achievable goal than nuclear fusion. It’s already been drawn on a napkin and, says Sethian, “I think, if we lick the remaining physics and chemistry challenges, we could probably have something ready to field in about two years.”