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Old 11-28-2007, 12:00 AM   #1
kevinthenerd
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Acetylene as a fuel

Acetylene can burn like hell because its enthalpy of formation is actually positive. It's inherently unstable, and defining a stoichiometric air/fuel ratio for it isn't exactly telling the whole story since it'll burn in an inert environment in the right temperature and pressure.

(Correct me if I'm wrong on this.)

Anyway, all this leads to me to wonder whether acetylene can be used as an injected additive in a very high performance engine. Water could perhaps be used to get the temperatures down to what modern metals can handle. Pressures would be insane, and mechanical failure would truly be the only limit to how much torque you could make at any particular rpm. I'm willing to bet that you'd actually want to make the torque up high just to keep the engine from blowing up. The accelerative forces would cancel some of the compressive forces to keep the rod from buckling in compression. With the same torque, you make more horsepower up top anyway, and average horsepower is what really matters.

(By the way, for the torque-vs-horsepower debate, read this first: http://kevinthenerd.googlepages.com/torque_vs_hp.html )


I'm thinking this could be used in bracket racing where the rules are damn-near open.
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Old 11-28-2007, 01:41 AM   #2
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Re: Acetylene as a fuel

Big thing most people will miss with the torque vs horsepower thing.

As gearing increases the torque, it also reduces the rpm range.

Many will miss the scale on the x axis, but it shows the root of the problem. Drivability suffers when very gearing is used to compensate for a lack of torque. In the extreme case the engine can't reach across the next gear change.
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Old 11-28-2007, 01:56 AM   #3
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Re: Acetylene as a fuel

I think you might be confusing terms. Enthalpy is simply the heat content. All fuels have enthalpy, or they wouldn't be fuels.

The bottom line is this; fuels burn and give off BTUs. Acetylene is no different. Once its ignited, it gives up its energy, period.

Can you be more specific in your theory?
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Old 11-28-2007, 10:37 AM   #4
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Re: Acetylene as a fuel

Quote:
Originally Posted by curtis73
I think you might be confusing terms. Enthalpy is simply the heat content. All fuels have enthalpy, or they wouldn't be fuels.

The bottom line is this; fuels burn and give off BTUs. Acetylene is no different. Once its ignited, it gives up its energy, period.

Can you be more specific in your theory?
In a rough sense, enthalpy is energy, yes. How it differs from "energy" is whether we're talking about constant pressure or constant volume, but for here, they can be discussed as the same thing.

If you wanted to make octane from pure carbon and dihydrogen (the diatomic stable form of hydrogen) you would need to remove energy. Thus, octane has a negative enthalpy. Its "enthalpy of formation" is negative because it takes a negative amount of energy to form it. Its chemical energy is lower than the carbon and hydrogen it's made of.

The same is true for CO2, alcohols, and just about every other fuel out there. However, to make acetylene, you actually need to add energy. This energy can be released when it burns. You get more than just the energy of combining the hydrogen and carbon with atmospheric oxygen to form CO2 and H2O; you get the energy of breaking up the acetylene too. I'm not a chemistry major, but I would guess this is why it burns so hot.

So, if acetylene releases a lot of energy from more than just simple combustion, I wonder whether it could be used in a race engine, assuming the rules allow it. As I said, water could be used to absorb energy to keep the motor from running too hot.

Any input would be appreciated.
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Old 11-28-2007, 06:30 PM   #5
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Re: Acetylene as a fuel

The term you're looking for is "activation energy". It's the amount of energy (usually supplied as heat) required to break the initial bonds and free up the molecules to react with their environment.

In the acetylene case (technical name is Ethyne) it's the carbon-carbon triple bond that's responsible for the high activation energy.

But most of the reason acetylene burns so hot is due to the oxygen usually supplied at the same time. When burnt in air it smokes a lot (all that carbon) and the almost 80% nitrogen that doesn't burn transports a lot of the heat away before high temps can be reached.
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Old 11-28-2007, 07:17 PM   #6
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Re: Acetylene as a fuel

Quote:
Originally Posted by kevinthenerd
If you wanted to make octane from pure carbon and dihydrogen (the diatomic stable form of hydrogen) you would need to remove energy. The same is true for CO2, alcohols, and just about every other fuel out there. However, to make acetylene, you actually need to add energy. This energy can be released when it burns. You get more than just the energy of combining the hydrogen and carbon with atmospheric oxygen to form CO2 and H2O; you get the energy of breaking up the acetylene too. I'm not a chemistry major, but I would guess this is why it burns so hot..
Firstly, that first sentence is where your logic finds its flaw. It takes a great deal of energy input to make octane. If you were able to get energy out of octane both while making it and while burning it, we would have perpetual motion.

it doesn't matter how much energy it takes to make acetylene. The only energy we see is what's released when it burns. It doesn't matter what happened before you put it in the car, what comes out during combustion is the only energy we can discuss as part of the reaction.

A good example is propane and gasoline. Both have vaguely similar BTU contents, but the propane is so much easier to refine since it takes very little distillation to get it... but we don't count that energy in the car. All the car sees is how many BTUs push on the piston.

It also takes energy to make hyrdrocarbons. In the case of crude oil, it has been billions of years of pressure and heat. In the case of alcohols, we borrow energy from sugar and the fungus that eats it, but energy is input to make alcohols.

When you do a chemical experiment in a lab to measure its energy content, you don't count the amount of calories you burned carrying the beaker.

The bottom line is this... a chemical like gasoline, acetylene, propane, alcohol, or kerosene carries energy in the form of potential energy. When you combust it with oxygen, it releases it. You get out the same BTUs as it took to create it. Period. If that weren't the case (if for instance you GOT energy by creating alcohol AND when it was combusted) we would have mastered perpetual motion. Think of it like a bucket of water. Combustion is like emptying the bucket and using the water. Then to refill the bucket, you have to add water. The same is true of combusting a fuel. If you burn gasoline or acetylene into H2O and CO2, it would take the same amount of energy to return the H2O and CO2 back to fuel as you got out of it when you burned it. That's just a chemical fact.

Imagine a rock on a hill. If you roll it down hill, it gives off energy. If you want to return it to the top of the hill, you have to put in as much energy as you got out. (actually much more since much of the energy will be lost to friction and sound)

Now... back to the topic at hand. Acetylene only burns so hot because you inject oxygen into the reaction. When you start an Acetylene torch without oxygen it burns VERY slowly with an orange, sooty flame. It has similar energy properties to LDPE which is what they use to make plastic cups. It doesn't burn fast enough on its own to be an effective automotive fuel. It might work in a very low-rpm genset application, or on large engines that spin very slowly, but I would imagine that anything more than 1000-1500 rpms might be too quick for the acetylene to burn. Flame front speeds could be increased with higher compression, but I don't know what acetylene's threshold of ignition is; that is to say its "octane" rating.
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Old 11-28-2007, 11:14 PM   #7
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Re: Acetylene as a fuel

Quote:
Originally Posted by curtis73
Firstly, that first sentence is where your logic finds its flaw. It takes a great deal of energy input to make octane. If you were able to get energy out of octane both while making it and while burning it, we would have perpetual motion.

it doesn't matter how much energy it takes to make acetylene. The only energy we see is what's released when it burns. It doesn't matter what happened before you put it in the car, what comes out during combustion is the only energy we can discuss as part of the reaction.

A good example is propane and gasoline. Both have vaguely similar BTU contents, but the propane is so much easier to refine since it takes very little distillation to get it... but we don't count that energy in the car. All the car sees is how many BTUs push on the piston.

It also takes energy to make hyrdrocarbons. In the case of crude oil, it has been billions of years of pressure and heat. In the case of alcohols, we borrow energy from sugar and the fungus that eats it, but energy is input to make alcohols.

When you do a chemical experiment in a lab to measure its energy content, you don't count the amount of calories you burned carrying the beaker.

The bottom line is this... a chemical like gasoline, acetylene, propane, alcohol, or kerosene carries energy in the form of potential energy. When you combust it with oxygen, it releases it. You get out the same BTUs as it took to create it. Period. If that weren't the case (if for instance you GOT energy by creating alcohol AND when it was combusted) we would have mastered perpetual motion. Think of it like a bucket of water. Combustion is like emptying the bucket and using the water. Then to refill the bucket, you have to add water. The same is true of combusting a fuel. If you burn gasoline or acetylene into H2O and CO2, it would take the same amount of energy to return the H2O and CO2 back to fuel as you got out of it when you burned it. That's just a chemical fact.

Imagine a rock on a hill. If you roll it down hill, it gives off energy. If you want to return it to the top of the hill, you have to put in as much energy as you got out. (actually much more since much of the energy will be lost to friction and sound)

Now... back to the topic at hand. Acetylene only burns so hot because you inject oxygen into the reaction. When you start an Acetylene torch without oxygen it burns VERY slowly with an orange, sooty flame. It has similar energy properties to LDPE which is what they use to make plastic cups. It doesn't burn fast enough on its own to be an effective automotive fuel. It might work in a very low-rpm genset application, or on large engines that spin very slowly, but I would imagine that anything more than 1000-1500 rpms might be too quick for the acetylene to burn. Flame front speeds could be increased with higher compression, but I don't know what acetylene's threshold of ignition is; that is to say its "octane" rating.
When I talk about making acetylene and making octane, I'm not starting with CO2 and H2O. I'm starting with C and H2. You can burn C and O2 to make CO2, and you can burn H2 and O2 to make H2O. CO2 and H2O both have a negative enthalpy of formation.

When you burn octane, you're losing the energy required to make it. You gain back energy when the hydrogen and carbon combine with atmospheric oxygen to make water and carbon dioxide, and this results in a net gain. In the case of acetylene, you're gaining twice. You're gaining the energy of making CO2 and H2O, and you're gaining the energy you previously lost making the acetylene.

As far as perpetual motion... I'm ignoring entropy and the second law of thermodynamics here. If you can somehow make a smooth burn in an insulated container, in order to minimize irreversibilities, it won't matter much anyway.
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Old 11-29-2007, 03:03 AM   #8
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Re: Acetylene as a fuel

I see what you're saying, but there is no net gain. Its a wash... zero. If it takes 10 joules to make it, you get 10 joules when you combust it. Energy is energy. You can't end up with a net gain because that would be creation of energy that isn't there.
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Old 11-29-2007, 10:04 AM   #9
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Re: Acetylene as a fuel

Quote:
Originally Posted by curtis73
I see what you're saying, but there is no net gain. Its a wash... zero. If it takes 10 joules to make it, you get 10 joules when you combust it. Energy is energy. You can't end up with a net gain because that would be creation of energy that isn't there.
When you break apart octane into H2 and C, it requires exactly as much energy as it would have taken to make it from H2 and C. When you break octane into H2O and CO2, however, you first lose the energy to break the octane in to H2 and C, and then you gain the energy from making H2O and CO2 from the loose H2 and C.

hf (kJ/mol):
Octane: -249950 (liquid)
Octane: -208450 (gas)
Carbon dioxide: -393520
Water: -241820 (vapor)
Water: -285830 (liquid)
Acetylene: +226730


(Don't forget to balance the equation if you want to figure this out. I'm feeling lazy. For ease of calculation, let's assume that the reactants and products go in and come out at the standard state.)
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Old 11-29-2007, 01:30 PM   #10
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Re: Acetylene as a fuel

Quote:
When you break octane into H2O and CO2, however, you first lose the energy to break the octane in to H2 and C, and then you gain the energy from making H2O and CO2 from the loose H2 and C.
I see the other flaw... in chemical reactions you simply meausure the net delta H. The equation doesn't show C8H18 breaking up into 4C2 and 9H2, then recombining with O2. The activation energy (spark) begins combustion, which does not break it down into H and C, it begins the combination of oxygen with the HC molecule. You don't split it up in to componenents and recombine, its just one reaction. The (unbalanced and simplified) equation looks like this:

C8H18 + O2 + EA = H2O + CO2 + heat

what you are suggesting is that the equation look like this:

C8H18 + heat = 8C + 9H2 and THEN
C + H2 + O2 + EA = H20 + CO2 + heat

... and that's just not the case.

Its all one reaction. You don't get a net gain, you simply unlock the heat stored in the bond of octane and the atoms settle into their end compounds. Its not a double reaction. Your enthalpy numbers only apply if that is the reaction that is being created. If you separate acetylene into H and C and then combine the H with O, your enthalpy will be that of a hydrogen combustion reaction. Period. You can't count the separation of acetylene because its not included in the reaction.

Combustion is combustion. Its a single equation, so the only enthalpy numbers you can use are those for THAT reaction, not any other reactions that could or might occur. Burning acetylene means you get the enthalpy involve with burning acetylene, not the enthalpy that you get from separating acetylene then burning its components.
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Old 11-29-2007, 05:37 PM   #11
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Re: Acetylene as a fuel

Aside from all the technical info I can't read about the fuel. The challenge is always the air (oxygen) in engines producing power from limited cfm by size,rpm,pressure.

Am I missing something here besides possibly taking up a touch less (insignificant amount) space?
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Old 11-29-2007, 08:16 PM   #12
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Re: Acetylene as a fuel

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Am I missing something here besides possibly taking up a touch less (insignificant amount) space?
No, not really. But remember that gaseous fuels displace more air than liquid fuels.
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Old 12-02-2007, 03:51 PM   #13
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Re: Acetylene as a fuel

Quote:
Originally Posted by curtis73
what you are suggesting is that the equation look like this:

C8H18 + heat = 8C + 9H2 and THEN
C + H2 + O2 + EA = H20 + CO2 + heat

... and that's just not the case.
It's not necessarily how it happens, but it's how it works out mathematically in order to get useful data, yes.

Let's say air travel is free. I can fly from Orlando to Boston and then to Denver and get the same final net change in altitude as I can by flying straight from Orlando to Denver. Now, let's say I know the difference in altitudes between Orlando and Boston, and I know the difference in altitudes between Boston and Denver. I have no good way of directly measuring the altitude difference between Orlando and Denver, but the sum of the two other differences should yield the net effect.

Now, let's say my layover, Boston, is some neutral thermodynamic state. If I can figure out how much energy it takes to get there and how much energy it takes to get away from there, I can take the sum of the two and go from there.

Mathematically speaking, energy is a linear operator, and it is conservative. You can add and subtract it, and the total comes out the same.

Let's give a little proof. Let's do the combustion of octane. I know from one of my favorite books, Internal Combustion Fundamentals by John B. Heywood, that gasoline has an energy density of roughly 45 MJ/kg. (I use this in personal calculations all the time.) Gasoline isn't exactly the same thing as octane, but they're close. I can only show this with octane because gasoline is very complex.

First of all, I need you to know that I'm assuming that both the products and the reactants are vapors. I'm also assuming that they're entering and exiting at a standard state of 25 C at 1 bar. The system is adiabatic except for the heat loss we're measuring.

Let's make a table of the values we'll need. These are the enthalpies of formation, at the standard state, measured in kJ/kmol.

H2 = 0 (stable diatomic molecule)
O2 = 0 (stable diatomic molecule)
N2 = 0 (stable diatomic molecule)
C = 0 (stable atom)
H2O (vapor) = -241 820
CO2 = -393 520
C8H18 = -208 450

We'll also need the molecular weights if we plan to do anything with mass.

H = 1
C = 12
N = 14
O = 16

Let's balance the equation.

C8H18 + 12.5 (O2 + 3.76 N2) = 8 CO2 + 9 H2O + 47 N2

The enthalpy balance for heat going out is

Q = (H products) - (H reactants)

Q = [(-208450*1)] - [ (-393520*8) + (-241820*9) ] = 5 116 090 kJ/kmol

Now, to express this in terms of kJ/kg, we divide by molar mass

M(C8H18) = 8*12 + 18*1 = 114

(5116090 kJ/kmol)/(114 kg/kmol) = 44.878 MJ/kg

which is about equal to the value in Heywood's text
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Old 12-03-2007, 11:53 PM   #14
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Re: Acetylene as a fuel

But you're still missing my point...

Who cares about enthalpy of formation? All you see is the reaction in the chamber. Period. The only thing we need to know is BTUs for both fuels. It doesn't matter how much enthalpy you get in or out when you make or break; how many joules or BTUs get released from combusting it?
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Old 12-04-2007, 12:01 AM   #15
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Re: Acetylene as a fuel

Quote:
Originally Posted by curtis73
But you're still missing my point...

Who cares about enthalpy of formation? All you see is the reaction in the chamber. Period. The only thing we need to know is BTUs for both fuels. It doesn't matter how much enthalpy you get in or out when you make or break; how many joules or BTUs get released from combusting it?
Enthalpy of formation is useful if you're trying to pass a chemistry exam. But as a fuel the calorific value is all that matters.

Acetylene has approx 50 MJ/kg
Propane has approx 50 MJ/kg.

There is no benefit.
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