Battling those other emissions: nitrogen oxides

By Cat Hofacker|February/March 2021

Researchers at MIT have drawn up the blueprint for a refrigerator-sized ceramic cylinder that, if it works as proposed on tomorrow’s passenger aircraft, would all but eliminate their emissions of nitrogen oxides, a family of pollutants from aviation and other sources that aggravates asthma and damages the atmosphere.

Researchers were inspired by a type of catalytic converter on diesel trucks, in which engine exhaust mixes with an ammonia solution and passes through a honeycombed sheet of ceramic, often an aluminum and silicon compound called zeolite. It’s chemically treated with iron or copper salts that catalyze a reaction among the hydrogen and nitrogen in the ammonia and the nitrogen oxides in the combustion gases. The reaction turns the oxides into water vapor and less dangerous nitrogen gas.

The researchers describe their proposed aviation version of such a catalytic converter in the January issue of the journal Energy & Environmental Science. Instead of passing exhaust through a honeycomb structure, they would pass the exhaust through a 2.2-meter-long cylinder formed by rolling up a sheet of zeolite that’s been folded to create pleats. Researchers call this device a catalyst, and its cylindric shape makes it easier to fit aboard an aircraft while providing “a lot of surface area through which to pass the flow” of exhaust, explains Steven Barrett, lead author of the paper.

But because the exhaust must now pass through a catalyst, thrust would be reduced slightly. So Barrett says the concept would be best suited for one of today’s hybrid-electric conceptual designs that move the engines to the aircraft’s belly and supplement thrust with small electrically driven propellers on the wings. In the preliminary configuration outlined in the paper, generators positioned behind each of the jet engines would create electricity for the motors that drive the propellers.

A valve near the engine or engines would spray an ammonia solution into the exhaust stream, catalyzing the conversion of the nitrogen oxides into nitrogen gas and water vapor, which would travel down the aircraft’s tailpipe and be expelled into the atmosphere.

“The key downside of this setup” is if it were installed on a hybrid-electric aircraft roughly the size of a Boeing 737, “you probably end up increasing fuel burn by about half a percent compared to what it otherwise would have been,” Barrett says. That’s due to the weight of the tank to hold the ammonia solution.

But he believes this tradeoff would be worth it. While some aircraft concepts will seek to reduce or eliminate carbon dioxide emissions, “there are other emissions that we also need to zero out for a long-term sustainable aviation sector,” Barrett says. “We need a solution that both addresses climate change and addresses the air pollution issue.”

The researchers are refining the design of their proposed plane.