Uncovering the True Nature of NOx Production in Heat Engines

[Clausius2]

As a future engineer, my proffessors teach us to be concerned about pollutant emissions. I always hear about NOx production inside heat engines and turbomachinery. But little is explained about the true nature of this problem.

Usually, we are told that NOx is produced at high temperatures, when Fuel to Air ratio is nearly stochiometric. I want the reasons of a chemicist about that. Why those conditions enhance NOx generation?. Which are the general conditions for NOx formation inside a combustion chamber?

Thanks.

 

[gravenewworld]

I'll take a stab at this. Let's say you have oxygen and nitrogen present in an engine or something so you have the reaction

N2 + O2------>NOx


Inside an engine the temperature is really high so thermodynamically

G=H-TS. A higher temperature will lower delta G, meaning the reaction can occur spontaneously more easily. Kinetically, using the Arrhenius equation
K=Ae^-Ea/RT

you can see a larger temperature will increase the rate constant of the reaction.

Larger temperatures should push the reaction further thermodynamically and kinetically.

 

[Clausius2]

You're explanation makes sense. But is TOO general. It could be employable with several reactions. The NOx reactions seem to have a particular behavior when T is high enough and with stochiometric Fuel to Air ratio in spark plug engines. In fact, FAR approx 1 are avoided for that reason in spite of being the FAR of maximum indicated mean effective pressure. I have never heard the opinion of a chemicist about that. Usually engineers don't get into much depth when explaining it.

Thanks for trying it.

 

[Bystander]

a_O2 or f_O2plotted as a function of "FAR" may prove illuminating --- activity/fugacity of oxygen (and excited species) is going to be low at rich conditions and high at lean burns. Low activity leads to low NOX production, and high to high.

 

[Clausius2]

a_O2 or f_O2plotted as a function of "FAR" may prove illuminating --- activity/fugacity of oxygen (and excited species) is going to be low at rich conditions and high at lean burns. Low activity leads to low NOX production, and high to high.

That sound good, but I don't know what is "a_O2 nor f_O2".

 

[Bystander]

Activity --- fugacity.

 

[Bystander]

Activity --- fugacity. You can call it partial pressure (approximation) --- calculate it as if you had complete combustion for each FAR.

 

[Gokul43201]

Simply put, N2, being fairly inert takes a large activation energy to react with anything, including oxygen. This activation is provided by high temperatures when the engine is throttled at heavy load conditions. At lower temperatures, the reaction rate between N2 and O2 is small compared to the rate at which CO2 is produced from combustion of the fuel. So, at lower temperatures CO2 (and also hydrocarbon, I think) emissions dominate.

I've heard that a slightly leaner than ideal (~15 to 17) A/F ratio produces most NOx. From the argument of activity (Bystander) a leaner ratio should result in more NOx, but I'm sure at a very lean mix, the NOx emissions go down. I believe this is because of the lower temperatures that are associated with increasingly leaner mixes.