At cruising altitude, airplanes emit a steady stream of NO_x into the atmosphere, where the chemicals can linger to produce ozone and fine particulates. Previous research has shown that the generation of these chemicals due to global aviation results in 16,000 premature deaths each year.

Now, MIT engineers are proposing using an ammonia-based selective catalytic reduction (SCR) system that could result in an approximately 95% reduction in NO_x emissions in exchange for a ~0.5% increase in block fuel burn. The details of the design, including analyses of its potential fuel cost and health impacts, are published in an open-access paper in the journal Energy and Environmental Science.

Efforts to improve the efficiency of aircraft propulsion systems are leading to small, power-dense engine cores with higher overall pressure ratios and combustion temperatures, which can result in higher NO_x emissions. The trend towards smaller engine cores with smaller mass flow rates in the core stream, presents new opportunities for emissions control.

Specifically, we propose and assess using a selective catalytic reduction (SCR) system that was previously infeasible when mass flow rates in the core were an order of magnitude larger than heavy-duty diesel engines for road-based applications. SCR systems would reduce NO_x emissions at the cost of increased aircraft weight and specific fuel consumption due to the pressure drop in the core stream induced by the catalyst. We quantify the effects of these trade-offs in terms of emissions reduction and fuel burn increase using representative engine cycle models provided by a major aero-gas turbine manufacturer.

Due to its size, any SCR system will likely need to be housed in the aircraft body, potentially making it most suitable for future hybrid- or turbo-electric aircraft designs. Furthermore, SCR systems require ultra-low sulfur (ULS) fuel to prevent catalytic fouling.

—Prashanth et al.

Current airplane design uses engines anchored beneath each wing. Due to this configuration, it has not been possible to use emissions-control devices, as they would interfere with the thrust produced by the engines.

In the new hybrid-electric, or turbo-electric, design, a plane’s source of power would still be a conventional gas turbine, but it would be integrated within the plane’s cargo hold. Rather than directly powering propellers or fans, the gas turbine would drive a generator, also in the hold, to produce electricity, which would then electrically power the plane’s wing-mounted, electrically driven propellers or fans.

The emissions produced by the gas turbine would be fed into an emissions-control system, broadly similar to those in diesel vehicles, which would clean the exhaust before ejecting it into the atmosphere.

This would still be a tremendous engineering challenge, but there aren’t fundamental physics limitations. If you want to get to a net-zero aviation sector, this is a potential way of solving the air pollution part of it, which is significant, and in a way that’s technologically quite viable.

This would be many, many times more feasible than what has been proposed for all-electric aircraft. This design would add some hundreds of kilograms to a plane, as opposed to adding many tons of batteries, which would be over a magnitude of extra weight.

—Steven Barrett, professor of aeronautics and astronautics at MIT and corresponding author

The paper’s co-authors are Prakash Prashanth, Raymond Speth, Sebastian Eastham, and Jayant Sabnins, all members of MIT’s Laboratory for Aviation and the Environment.

Resources

• Prakash Prashanth, Raymond L. Speth, Sebastian D. Eastham, Jayant S. Sabnisa and Steven R. H. Barrett (2021) “Post-combustion emissions control in aero-gas turbine engines” Energy Environ. Sci. doi: 10.1039/D0EE02362K