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Georgia Tech Develops Ultra Low-Emissions Combustor

(Left) A traditional combustor mixes fuel and air before injection into the combustion chamber. (Right) Tech’s system injects the fuel and air separately into the combustor. Click to enlarge.

Georgia Tech researchers have developed a new combustor (the combustion chamber where fuel is burned to power an engine or gas turbine) designed to burn fuel in a wide range of devices with ultra low emissions of nitrogen oxide (NOx) and carbon monoxide (CO).

The new Stagnation Point Reverse Flow Combustor, originally developed for NASA, burns fuel with NOx emissions of less than 1 parts per million (ppm) and CO emissions of less than 10 ppm, significantly lower than emissions produced by other combustors.

The device has a simpler design than existing state-of-the-art combustors and could be manufactured and maintained at a much lower cost, making it more affordable in everything from jet engines and power plants to home water heaters.

The project’s initial goal was to develop a low emissions combustor for aircraft engines and power-generating gas turbines that must stably burn large amounts of fuel in a small volume over a wide range of power settings (or fuel flow rates).

Existing combustors premix fuel with a large amount of swirling air flow prior to injection into the combustor to reduce emissions. This requires complex and expensive designs, and the combustion process often excites instabilities that damage the system.

The Georgia Tech combustor burns fuel in low temperature reactions that occur over a large portion of the combustor. By eliminating all high temperature pockets through better control of the flow of the reactants and combustion products within the combustor, the device produces far lower levels of NOx and CO and avoids acoustic instabilities that are problematic in current low emissions combustors.

The Georgia Tech design eliminates the complexity associated with premixing the fuel and air by injecting the fuel and air separately into the combustor while its geometry forces them to mix with one another and with combustion products before ignition occurs.

The reactants are injected at the center of the open end of the combustor, and flow towards its closed end, where the velocity must be zero—and hence, the velocity of the reactants must decrease as the flow approaches the closed end.

Click to enlarge.

This establishes a low velocity region in the vicinity of the closed end that can help to stabilize the combustion process [Stagnation Point]. Since the generated products and burning gas pockets can not leave the combustor through its closed end, they must reverse their flow direction and exit the system through the annular opening around the injection system [Reverse Flow].

As the stream of hot products laden with radicals flows out of the combustor, it must mix with the incoming reactants. Mixing with hot products increases the reactant temperature and the presence of radicals in the resulting mixture should lead to reduced ignition temperatures.

The combination of stagnation point and reverse flow allows the combustor to operate stably at lower inlet temperatures and/or lower fuel-air ratios, and, thus, produce lower NOx emissions.

The project was funded by the NASA University Research Engineering Technology Institute (URETI) Center on Aeropropulsion and Power and Georgia Tech.




It isn't too clear but it looks as if the air flow has to be turned 180 degrees twice to apply this in a jet engine. If so, that has to take energy away.

But perhaps I miss the point. And for static power turbines it might not matter.


These emission figures are excellent.
If every water heater in the country used this method, we would have much cleaner air.
If you used this in a gas, steam or stirling for hybrid autos, you would reduce polution significantly.

Rafael Seidl

The principle employed here is dilution of the reaction gases with combustion products (chiefly CO2 and H20). This reduces local rates of heat release because the inert components absorb it like sponges. This reduces local temperatures. In a steady state combustor, it also stabilizes the flame because the flame front never has enough energy to dart around.

Similar phenomena are observed in reciprocating engines with exhaust gas recirculation. In that case, there is a propagating flame but the speed of the flame front is reduced. In extremis, with a lot of control effort, you can even get the mixture to ignite in many places almost simultaneously and no discernible flame front ever developes (these approaches are collectively referred to as HCCI).

All of these approaches reduce NOx, typically at the expense of a increase in incomplete combustion products, chiefly hydrocarbons and carbon monoxide. Those can be cleaned up with an oxidation catalyst, though.

Gas turbines, steam and stirling engines have all been tried in mobile applications, without commercial success. One big problem with continous combustion is that at the small scales required for e.g. an automobile, the available materials limit peak process temperatures in the component that delivers the mechanical power to ~1100 deg C. That means you pay a huge penalty over reciprocating engines in terms of the core Carnot efficiency, a function of the ratio of absolute peak to ambient temperature (in deg Kelvin). Continous combustion engines also suffer from relatively poor response to load transients, a key issue in automobiles.

However, this new combustor could be very attractive for distributed electricity generation, especially if it is combined with district heating and absorption chillers for summer a/c. By keeping primary NOx low, you can afford to let such systems run for many hours at a time in residential neighborhoods without having to resort to expensive NOx aftertreatments like SCR.

Paul Dietz

However, this new combustor could be very attractive for distributed electricity generation, especially if it is combined with district heating and absorption chillers for summer a/c.

A SOFC topping cycle might be a better bet for that; SOFC integrates nicely with microturbines, by allowing the current density in the stack to be greatly increased.



I agree the absorption cooling with an SOFC would be a good idea. You would have distributed generation while getting efficient cooling. This would take some of the 3X peak off the grid usage in summer that is now caused by compression cooling.

Rafael Seidl

Regarding SOFCs: all true, but I suspect they are still rather more expensive than the more traditional alternative of a small steam-only or co-generation plant.


Commercial SOFCs with turbine cogeneration are cost competitive now. United Tech and others have been selling them for years. The absorption cooling is a new twist, but it would just be a system adaptation. This would be for large buildings like hospitals where uninterupted power and lower cost heating and cooling would all be desirable.

Since this is a car site, you could add that moving the tech towards lower cost stacks
could be applied in Vehicle to Grid applications.


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Joseph, take your meds.

Rafael, while I would agree with your first premise about Carnot efficiency and continous combustion engines, your second objection smacks of "Baby, gonna shut 'em down" thinking.

If we are talking serial hybrid gensets, then the poor "pedal to the metal" , a.k.a., response to load transients, seems an irrelevant concern. Am I missing something?


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