At cruising altitude, airplanes constantly emit a flow of nitrogen oxides (NOx), gaseous chemical compounds that remain in the atmosphere and produce ozone and fine particles, aerosols, which cause air quality degradation and climate change.
In a study published late last year in the journal Energy & Environmental Science, scientists at the Massachusetts Institute of Technology (MIT) are proposing the use of an ammonia-based selective catalytic reduction (SCR) system, which can result in an approximately 95% reduction in NOx emissions.
The new concept, which, according to the authors, is capable of eliminating “the problem of aviation air pollution”, is inspired by the emission control systems already used in trucks and other vehicles for diesel land transport. These devices have a post-combustion emission control system to reduce the NOx generated by the engines.
The operation of airplanes without pollutant emissions
Today’s aircraft designs feature jet engines attached below each wing. Each engine receives a turbine that, in turn, drives a propeller to move the airplane through the air, while the exhaust flows through the rear, which prevents the use of emission control devices, which could interfere with the thrust of the engines.
In the hybrid-electric, or turboelectric, design proposed by MIT scientists, the aircraft’s power source remains a conventional gas turbine, yet integrated into the cargo hold. This turbine drives an electric generator (also in the hold) to drive the propellers or fans mounted on the plane’s wing.
SCR was previously unviable, when the mass flow rates in the core were an order of magnitude higher than diesel engines in trucks, but the current technological trend, which produces smaller engine cores with also lower mass flow rates in the core flow, presents new opportunities for emission control.