Methane/LOX Generator?

I'm working up a "clean coal" proposal and it turns out that just about everything is available off-the-shelf but a methane/LOX electrical generator. There are guys messing around with J79 jet engines in this regard, but they throw away higher temperatures prior to entry and the exhaust temperatures don't seem to be all that low.

De Laval nozzles are really good at converting thermal energy to kinetic energy so I thought I'd ask if anyone at Armadillo had thought about the problem of using the methane/LOX rocket to generate electricity.

Old topic, but one i thought would definitely be worth reviving.
Recently as part of another project, started toying with a Methane/Lox generator concept, and am currently in the process of modeling a concept of such a system.

I was hoping someone at Armadillo could help answer a few questions, maybe even tell me how far off base i am with my concept when i get it finished.

some of the questions i have:
at the lowest throttle setting,
what is the exhaust temp?
what is the approximate exhaust velocity prior to the nozzle section?
what is the fuel burn rate at the lowest throttle setting?
Is the current design able to be scaled down?
would the engine work with a gaseous source of O2 and methane?

If you guys at Armadillo aren't able to answer because of contractual obligations, or similar reasons, i completely understand.

I'm an electrical engineer by trade, so i am still learning some of the finer details on combustion systems, and would greatly appreciate any knowledge anyone has to offer.

I'll be trying to post pics of my generator concept model here soon.

Thanks in advance.

Mike Langlais

The engineering constraints on a ground-based rocket engine are quite a bit different from aerospace, but having said that it is pretty easy to compute the numbers you want with cpropep. You'll notice the main problem with capturing the kinetic energy of the exhaust is materials strength of the turbine. You can get some distance by adding water to the combustion -- both in terms of completeness of combustion and in terms of the mass/energy (which which decreases exhaust velocity hence turbine material strength requirements), but it we're still talking Unobtanium for the turbine.

Results

Propellant composition
Code Name mol Mass (g) Composition
578 METHANE 1.0000 16.0425 1C 4H
686 OXYGEN (LIQUID) 2.0000 63.9976 2O
Density : 0.852 g/cm^3
3 different elements
C H O
Total mass: 80.040061 g
Enthalpy : -1391.30 kJ/kg

114 possible gazeous species
3 possible condensed species

CHAMBER
Pressure (atm) : 400.000
Temperature (K) : 2500.000
H (kJ/kg) : -6808.221
U (kJ/kg) : -7592.425
G (kJ/kg) : -30154.390
S (kJ/(kg)(K) : 9.338
M (g/mol) : 26.506
(dLnV/dLnP)t : -1.00218
(dLnV/dLnT)p : 1.05763
Cp (kJ/(kg)(K)) : 2.61748
Cv (kJ/(kg)(K)) : 2.26736
Cp/Cv : 1.15442
Gamma : 1.15191
Vson (m/s) : 950.43766


Molar fractions

CO 8.4876e-03
CO2 3.2267e-01
COOH 1.2937e-06
H 6.6141e-05
HCO 3.2129e-08
HO2 9.7466e-06
H2 2.7954e-03
HCOOH 8.4538e-07
H2O 6.5787e-01
H2O2 6.5337e-06
O 4.9937e-05
OH 3.2164e-03
O2 4.8192e-03

In generators of most any sort, if anything goes supersonic then You're Doing It Wrong.

It is definitely possible to run an internal combustion of whatever sort on rocket fuel. The Firestar guys down the street are doing a regeneratively cooled piston engine running on scary nitrous/fuel monopropellant. Similarly running a gas turbine generator on lox/methane would not be difficult, though you'd be wasting a lot of methane to keep the TIT low enough.

How does lox/methane fall out of clean coal? I think you'd have air/methane at the worst. That's a lot easier to work with.

The higher the speed the more of the energy is available as kinetic energy. If you want to think of it in terms of Carnot efficiency, colder the exhaust, the larger the absolute temperature ratio.



What's the largest absolute temperature ratio you get with internal combustion?



"Clean coal" entails CO2 separation.

Right, but if you put hot supersonic gas into a turbine, you're going to end up with a bunch of shockwaves and mechanical requirements that exceed existing materials.

You can pull some energy out of a rocket plume with MHD, but not an efficient proportion.

Is emitting CO considered somehow less bad than CO2, or what's the scheme for the exhaust after you've extracted the energy?

As I already pointed out:




Right. So its time to invent... any thoughts? Or do you think top men have looked at the problem hence its time to give up on high theoretic efficiencies obtainable from the de laval nozzle?


There are ways of making the combustion complete with LOX but with all that nitrogen you end up with NOx which is really nasty for a lot of reasons. There are also ways of capturing the energy. I just wanted to see if anyone thought it was obvious.

Anyone?

Burning hydrogen would be cleaner... And it should become more available. Also you can pull the heat from the exhaust pretty effectively. Supposedly the RL-10 is able to get ice forming on the bottom of the nozzle.



And zero carbon emmissons. Well, by you. Nobody bothers to calculate the carbon that goes into extracting the Hydrogen or the methane. Or the lox.

I agree hydrogen could be superior but Armadillo is building a methane engine.



Actually, all those calculations have to be done, and are, as part of the modeling of clean coal plant economics -- right up to the cost of burying the CO2. It's not pretty but its better than $100/bbl oil.

@idiom

That picture is superb!
Do you have any more like that - or can you point me to where it came from?

Ok, let me see if i can get a word in here lol
i think my objectives are slightly different than jabowery's
I've been looking at a methan/gox combustion system as opposed to lox, with the O2 pulled from the air using a multi-stage pressure swing adsorbtion system. (that's the thought anyways)Nitrogen and Argon would be removed and recovered in the first stages, and O2 in the final with the trace gases being trapped or dumped.

the methane originating from processed biogas, which iteslf would be coming from the anaerobic digestion of sewage. I've researched the methods for doing this and it's actually not that complicated.(not up front anyways, implimentation could be an entirely different story, but it has been done)

so in the case of my system, we're talking a much lower combustion rate, with hopefully minimal to zero exhaust from the generator other than CO2 and H2O (theoretically possible, though not 100% likely, I know. contaminants in the CH4 and O2 inputs can complicate things) The 'turbine' concept i've been working on is not an axial design, but would involve the centrifugal system, as the designs are much simple and more robust. (think the turbos in car's and the compressors in early jet aircraft engines.)

as for the generator exhaust.... I'm looking more into seperating out the CO2 from the H2O from the other trace gases (possibly another PSA system). the CO2 would be funneled to an Algenol bioreactor, which in turn outputs ethanol (in turn used in another generator), and the water would be used as needed. There are other parts of my system concept I have not detailedin this post, though the goal is to have very little if any output from the system other than Fertilizer, O2, H2O, and power. with the exception of developing a simple, clean burning methane and ethanol generators, everything in my system can already be found on the market. Heck, i'm sure you could also find the generators as well.

My point for asking about Armadillo's methane engine is they seem to have found a good way for getting the CH4/LOX combustion stable and efficient. and I was curious if their design could be scaled down and applicable to a methane/gox combustion system. Another thing I had considered (tell me if this seems a bad idea) after mixing the CH4/O2, compressing it prior to the combustion chamber. alternatively, the O2 could be compressed prior to the chamber with the CH4 injected there. Still learning about these systems, so any thoughts would be appreciated. (lol it certainly isn't something I do on a day to day basis as an Electrical Engineer )

thanks again

Mike

(edited to fix spelling)

First, I want to say that I think this is an excellent project that you are working on. I can't help you with the rocket turbine problems you are having, but I think you could have some degree of success by using internal combustion engines. I know people who have converted diesel engines to run on lpg, which is a variable mixture of propane and butane. I would guess that with the right type of tuning that methane could also be used to run a diesel engine. There are a number of cars manufactured today that burn petrol-ethanol mixtures and some cars can run on pure ethanol. Again, with the right type of modifications, an old petrol engine might be used to generate power from ethanol. Both of these engines could be connected to a single generator using a differential, which would reduce your hardware requirements for the project.

Just my €0.02

johno

Hey Johno,

yeah,I've seen 427cui V8s built by GM as generators that were powered by natural gas. My hope with using a turbine (centrifugal not axial) was to try and increase the simplicity of the system and reduce the mechanical losses. Also, the hope was to reduce cost of maintenance in the field.

on the same token, my original thought for the ethanol was to use a combustion engine. a separate project i have on hold would probably work well for that application. though if there is the possibility of a simpler (mechanically) system, then I'll try that route first.

I've seem that the petroleum industry uses something called an expansion turbine, or turbo-expander, that takes in the hot gas and converts the heat and velocity to mechanical energy, with the gas output being low pressure and much cooler. http://en.wikipedia.org/wiki/Expansion_turbine
lol, i ramble on i know.

I will say the idea of connecting them to a single generator via differential is an interesting thought.

I spent some time researching turbine generators a while back. My plan was to design an electric car/van, using the turbine to generate electricity onboard. I know they are more efficient, which is why I was opting for a turbine, but I was unable to find anything off the shelf that weighed less than about 8000kg iirc. I can't recall the fuel consumption or wattage of the units, but you may be able to find something that suits you if you will be producing a lot of fuel. Can you give us some idea of the scale of your project? How much fuel do you anticipate you will be producing? Is this a domestic hobby project or on a more commercial scale?

johno

Its from the now defunct Altair lander. Nasa floated those pictures around a while ago, there are bigger less cropped versions, but I can't find them at the moment.