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Comment by pbmonster | Hacker News Reader

They don't ever run those close to this hot, right? There's barely any turbine blade materials able to withstand half those temperatures.

At those temperatures, even the best alloys lose a large percentage of their strength. And also, steam this hot is incredibly corrosive.

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Gas turbines typically are limited to ~1300C to limit NOx production. However, if you have another heat source that doesn't involve combustion, that limit is taken away. And then you can run the whole thing hotter (increasing efficiency substantially).

There is no need for blades to withstand the combustion temperature, because film cooling can keep blades far cooler in high speed laminar gas flow. The challenge is that the operating gas must be dust-free or a spec of dust on the blade surface disrupts the film and causes failure.

Obviously the amount of energy wasted to pumping the film cooling gasses goes up the higher the combustion temperature is, so I assume there is still some upper limit on combustion temperature.

> Gas turbines typically are limited to ~1300C to limit NOx production. However, if you have another heat source that doesn't involve combustion, that limit is taken away.

As far as I know, combustion isn’t required for NOx formation. If you want to run hotter without producing NOx, you need to eliminate either nitrogen or oxygen. IIRC there are a couple of proposed designs for doing this: combustion in nitrogen-depleted air and chemical looping combustion.

> combustion in nitrogen-depleted air and chemical looping combustion.

Am I misunderstanding this or are both of those concepts basically "just inject pure oxygen"? And I'm no economist, but running a powerplant on rocket fuel doesn't sound that feasible to me.

Urea injection into the exhaust gas sounds more sensible to me, and even that is seems economically questionable. Fuel prices would need to be pretty high that the efficiency gains make buying urea (instead of just more fuel and running the combustion colder) worth the investment...

No-one sane wants to do combustion in a pure oxygen environment. The solution isn't that, it's to eliminate nitrogen and replace it something else. Typically CO2, because you can get it for free and already preheated by recirculating exhaust.

Ok, so you recirculate exhaust into the combustion chamber and inject new fuel.

Now what? It doesn't burn, since there is no oxidizer. The only way is to inject some. And since you don't want nitrogen, you can't inject air. So you inject pure oxygen, or something like peroxide?

Sounds expensive.

There are multiple ways to separate air into a fraction with more oxygen and a fraction with less oxygen, on site, at reasonable scale. Pressure Swing Adsorption is a common one, found in most medical oxygen concentrators.

That would need to be a fraction with no nitrogen, right? Isn't the nitrogen the problem?

Do you want it preheated? It might improve ignition, but isn't it a heat engine that gains efficiency from a large temperature gradient?

Afterburner?

That wouldn't impact blade temp.

terrible for efficiency.

Is the jet used to superheat water, or directly drive a turbine generator?

If the latter, it would be a waste of time, if the former then you just want heat, and the fan is there to provide enough oxygen to burn the fuel so why would it impact efficiency?

Edit: I was thinking literal jet engines that have indeed been used as generators, not combined cycle generators.

But nice FX.

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