Does combustion have to produce gaseous CO2?

In summary: Most fuels are composed of hydrogen and carbon only. Oxygen from the atmosphere is used to burn the fuel. Natural combustion is going to produce water vapor and carbon dioxide, along with traces of other compounds depending on how the fuel is burned. That's chemistry.Now, if you want to carry a separate tank of oxidizer around with you in addition to the fuel, like in a rocket, you can have some different combustion products, depending on the fuel and the oxidizer chosen. However, most oxidizers are dangerous to handle and obviously more expensive than just sucking oxygen out of the air.In summary, you are trying to understand why it is so hard to create energy from fossil fuels
  • #1
Fooality
196
42
I'm not a chemist, but I'm interested in it. What I am trying to understand is why its so hard to get energy from fossil fuels without being able to sequester the carbon released in some kind of liquid form. I understand how piston engines use the expansion of gases (through combustion) to work, but I recent learned about these Stirling engines:
https://en.wikipedia.org/wiki/Stirling_engine
This engine, as I understand it, simply uses a difference in heat between two sources two work. This means that any exothermic reaction, even if it produces no gases, could drive it. What I am wondering is if the exothermic reactions from fossil fuels could output only liquid waste (containing the carbon), or must always produce gaseous CO2... And if the latter is true, why?

Thanks for any responses.
 
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  • #2
What would the liquid containing carbon be? And how much energy would the reaction produce?

Also, for any thermodynamic cycle, efficiency is a function of temperature difference (the larger the temperature difference, the more efficiency the cycle).
 
  • #3
russ_watters said:
What would the liquid containing carbon be? And how much energy would the reaction produce?

Also, for any thermodynamic cycle, efficiency is a function of temperature difference (the larger the temperature difference, the more efficiency the cycle).

As to the answer to both, I don't know. But when I look around, I see a vast amount of petrochemical reactions, that do all kinds of things. Yet when I look at carbon sequestration, I see these exotic things like storing CO2 in tanks and freezing it, but no mention of this kind of thing. What I'm trying to figure out is if there is a general, show stopping scientific principle that makes it impossible to throw in some chemical herbs and spices to fossil fuel combustion (or maybe oxidation is better word?), get a substantial amount of heat out to drive a Stirling engine, with only liquid or solid (at room temperature) carbon bi-products. I'm just curious if this is even possible.
 
  • #4
Well, CO2 is not liquid or solid at ordinary atmospheric pressures and temperatures. CO2 gas skips over the liquid phase entirely when it is cooled sufficiently and turns directly into a solid, which is called dry ice.

In order to make liquid CO2 at ordinary temperatures, quite a bit of pressure must be present:

https://en.wikipedia.org/wiki/Carbon_dioxide

Liquid CO2 can only form when the pressure is above about 5 atmospheres and the temperature is above -57°C. For warmer temperatures, much higher pressure is required to keep CO2 in the liquid phase.

And no, there is no secret magic recipe of 11 herbs and spices which can change the physics of CO2.
 
  • #5
SteamKing said:
Well, CO2 is not liquid or solid at ordinary atmospheric pressures and temperatures. CO2 gas skips over the liquid phase entirely when it is cooled sufficiently and turns directly into a solid, which is called dry ice.

In order to make liquid CO2 at ordinary temperatures, quite a bit of pressure must be present:

https://en.wikipedia.org/wiki/Carbon_dioxide

Liquid CO2 can only form when the pressure is above about 5 atmospheres and the temperature is above -57°C. For warmer temperatures, much higher pressure is required to keep CO2 in the liquid phase.

And no, there is no secret magic recipe of 11 herbs and spices which can change the physics of CO2.

Of course. That's why I'm asking if there can exist a separate chemical reaction which can produce bi-products as a a different chemical that contains the carbon, which is liquid at room temperature. While also producing energy.
 
  • #6
Fooality said:
Of course. That's why I'm asking if there can exist a separate chemical reaction which can produce bi-products as a a different chemical that contains the carbon, which is liquid at room temperature. While also producing energy.
No.
 
  • #7
Why?

And to be clear here, I am hearing you say that there is no chemical reaction which takes fossil fuels as an input, produces substantial amounts of heat, but does not release CO2. What I am asking is why is that?
 
  • #8
Because most fuels are composed of hydrogen and carbon only. Oxygen from the atmosphere is used to burn the fuel. Natural combustion is going to produce water vapor and carbon dioxide, along with traces of other compounds depending on how the fuel is burned. That's chemistry.

Now, if you want to carry a separate tank of oxidizer around with you in addition to the fuel, like in a rocket, you can have some different combustion products, depending on the fuel and the oxidizer chosen. However, most oxidizers are dangerous to handle and obviously more expensive than just sucking oxygen out of the air.

The Titan II ICBM used a hydrazine-based fuel with an oxidizer of "red fuming nitric acid". Sounds like lovely stuff to handle. Hydrazine (N2H4) contains no carbon, and neither does the nitric acid oxidizer. Both substances are highly toxic and dangerous to handle.
 
  • #9
SteamKing said:
Because most fuels are composed of hydrogen and carbon only. Oxygen from the atmosphere is used to burn the fuel. Natural combustion is going to produce water vapor and carbon dioxide, along with traces of other compounds depending on how the fuel is burned. That's chemistry.

Now, if you want to carry a separate tank of oxidizer around with you in addition to the fuel, like in a rocket, you can have some different combustion products, depending on the fuel and the oxidizer chosen. However, most oxidizers are dangerous to handle and obviously more expensive than just sucking oxygen out of the air.

The Titan II ICBM used a hydrazine-based fuel with an oxidizer of "red fuming nitric acid". Sounds like lovely stuff to handle. Hydrazine (N2H4) contains no carbon, and neither does the nitric acid oxidizer. Both substances are highly toxic and dangerous to handle.

Okay, now I'm learning. (Again, I'm not a chemist, just a curious laymen) What I'm hearing you say is that the basic air-combustion process is super easy, and relatively safe, so that's why its so popular. But its going to produce CO2. And that's why these carbon sequestration researchers are so focused on it, anything else would be less viable industrially.

But I'm still wondering - is it physically possible to have some fancy oxidizer (or cocktail of things) that works with something like octane or coal, but wraps up much of the carbon in some other output chemical that's a liquid or solid while releasing heat? Or is there some general physical law which prevents that or makes it unlikely?
 
  • #10
Fooality said:
Okay, now I'm learning. (Again, I'm not a chemist, just a curious laymen) What I'm hearing you say is that the basic air-combustion process is super easy, and relatively safe, so that's why its so popular. But its going to produce CO2. And that's why these carbon sequestration researchers are so focused on it, anything else would be less viable industrially.

But I'm still wondering - is it physically possible to have some fancy oxidizer (or cocktail of things) that works with something like octane or coal, but wraps up much of the carbon in some other output chemical that's a liquid or solid while releasing heat? Or is there some general physical law which prevents that or makes it unlikely?
If there is, nobody's found an oxidizer which is safe, cheap, and easy to adapt to current engine technology. The stuff you get out using this fancy oxidizer may be nastier to handle than CO2, which is what plants need to survive.
 
  • #11
Fooality said:
As to the answer to both, I don't know. But when I look around, I see a vast amount of petrochemical reactions, that do all kinds of things. Yet when I look at carbon sequestration, I see these exotic things like storing CO2 in tanks and freezing it, but no mention of this kind of thing. What I'm trying to figure out is if there is a general, show stopping scientific principle that makes it impossible to throw in some chemical herbs and spices to fossil fuel combustion (or maybe oxidation is better word?), get a substantial amount of heat out to drive a Stirling engine, with only liquid or solid (at room temperature) carbon bi-products. I'm just curious if this is even possible.
No, what you are suggesting is not possible. The energy from burning hydrocarbons comes from the bond energy differences between the hydrocarbon and the products of combustion. In order to "burn" something, you combine it with oxygen. Hence: CO2. Anything that you combine it with that would create a more complex liquid would still be a hydrocarbon and would have unburnt energy in it. It would be a lower energy output reaction.
But I'm still wondering - is it physically possible to have some fancy oxidizer (or cocktail of things) that works with something like octane or coal, but wraps up much of the carbon in some other output chemical that's a liquid or solid while releasing heat? Or is there some general physical law which prevents that or makes it unlikely?
There might be, but the one we use now is available everywhere, for free, so there isn't any possibility that anything else could work as well for as little money.
 
  • #12
Fooality said:
But I'm still wondering - is it physically possible to have some fancy oxidizer (or cocktail of things) that works with something like octane or coal, but wraps up much of the carbon in some other output chemical that's a liquid or solid while releasing heat? Or is there some general physical law which prevents that or makes it unlikely?

What other people here already said is this: take methane for instance, and burn it with air:

[itex]CH_4 + 2O_2 \rightarrow 2H_2O + CO_2[/itex] +55.5MJ/kg

In the ideal case, this reaction gives you 55.5 MJ/kg of burned methane. Anything you do to modify the above reaction will significantly lower the heat output. It turns out that it is more energy efficient to try to remove the carbon dioxide from the air after complete combustion. The cheapest way is to do nothing and let nature do its work. For instance, natural carbonation occurs between CO2 and naturally occurring rock like calcium oxide. The reaction is:

[itex]CO_2 + CaO \rightarrow CaCO_3 (s)[/itex] -0.179 MJ/kg

This reaction costs energy (0.179 MJ/kg), but much less than the original combustion reaction, and it is free if you let nature do its work for you. Unfortunately, it is also a very slow reaction. With the current rate of CO2 addition, we need much faster removal.

With carbon capture, one option is to let CO2 be absorbed into liquid potassium carbonate (with some additional stuff to convince CO2 that this is a really good idea). This absorption costs approximately 0.0015 MJ/kg of energy, which is very cheap. Of course, you still need to make the potassium carbonate, the additives, the conversion plant, the infrastructure then there is the storage of the potassium, so there are some additional (energy) cost.
 
  • #13
bigfooted said:
naturally occurring rock like calcium oxide

No such thing as naturally occurring calcium oxide, it is way too reactive.
 
  • #14
russ_watters said:
No, what you are suggesting is not possible. The energy from burning hydrocarbons comes from the bond energy differences between the hydrocarbon and the products of combustion. In order to "burn" something, you combine it with oxygen. ...
bigfooted said:
What other people here already said is this: take methane for instance, and burn it with air:

[itex]CO_2 + CaO \rightarrow CaCO_3 (s)[/itex] -0.179 MJ/kg
...
This reaction costs energy (0.179 MJ/kg), but much less than the original combustion reaction, and it is free if you let nature do its work for you. Unfortunately, it is also a very slow reaction. With the current rate of CO2 addition, we need much faster removal.

With carbon capture, one option is to let CO2 be absorbed into liquid potassium carbonate (with some additional stuff to convince CO2 that this is a really good idea). This absorption costs approximately 0.0015 MJ/kg of energy, which is very cheap. Of course, you still need to make the potassium carbonate, the additives, the conversion plant, the infrastructure then there is the storage of the potassium, so there are some additional (energy) cost.

Thanks to both of you for really informative posts. The idea I'm sort of getting from you russ, is that CO2 is really sort of a minimum energy state for those elements to be in, and really anything more complicated will contain some energy in the bonds, so consequently it absolutely can't release as much energy as a simple combustion reaction...so there really is kind of a grand principle at play.

I hear you both saying that no such industrially feasible option exists, which doesn't surprise me. Fossil fuels still seem like the most bang for the buck in terms of energy and current infrastructure, so I'm sure if such an equivalent energy solution existed people would have jumped on it long ago, to make big oil and Al Gore happy.

What I am impressed with (bigfooted) is the seemingly low energy cost (setting industrial cost aside) of the technology you mentioned. Googling about it, I see a fair amount of research is going into this sort of chemical sequestration, with multiple methods:
http://sequestration.mit.edu/pdf/Anusha_Kothandaraman_thesis_June2010.pdf
Yet still, they seem to be all working directly with the CO2, never an alternative reaction. They don't just mix the hydrocarbons with the sequestering chemical and oxidizer. I wonder if the heat associated with the combustion makes alternative reactions unlikely due to what russ was talking about, and these reactions that sequester carbon are somehow more delicate.
 
  • #15
Borek said:
No such thing as naturally occurring calcium oxide, it is way too reactive.
Limestone is mostly calcium carbonate, CaCO3, which can be converted to quicklime, CaO, by heating it and driving off carbon dioxide.
 
  • #16
SteamKing said:
Limestone is mostly calcium carbonate, CaCO3, which can be converted to quicklime, CaO, by heating it and driving off carbon dioxide.

How does it change the fact there is no naturally occurring CaO?
 
  • #17
Borek said:
How does it change the fact there is no naturally occurring CaO?
Mebbe he got confused about what rocks contain what chemicals ...
 
  • #18
Borek said:
How does it change the fact there is no naturally occurring CaO?
I thought native lime (naturally occurring CaO) was quite common, trapped in rock, and that due to surface erosion, the CaO on the surface is reacting with CO2 to limestone.

google...google...
natural occurring lime is mentioned here:
https://en.wikipedia.org/wiki/Lime_(material)
mineral carbonation due to CaO is mentioned here:
https://en.wikipedia.org/wiki/Carbon_sequestration#Mineral_Carbonation

Well, not very conclusive actually... But I'm not a geologist and (Borek), I think you know more about chemistry than I do, so maybe I understood this wrong.
 
  • #19
While CaO can be present in nature wherever there is a chance CaCO3 was heated by natural processes, it is not stable - it reacts pretty fast with whatever is present (most often just water and carbon dioxide). But as the processes that can produce it are quite rare amount of the CaO available at any given point on the Earth can be safely ignored when talking about natural sequestration of carbon.

No idea what the list of oxides in the mineral carbonation section of the carbon sequestration article is intended to mean, it lists also K2O, which is even more reactive (and production of which requires much higher temperatures that CaO). My bet is that it is just a variant of explaining things without following real chemistry, for example we can talk about magnesium silicate as if it was a product of the MgO +SiO2 reaction - which doesn't occur, but some natural processes can follow this stoichiometry.
 
  • #20
Hey, can any of you recommend a book for me to understand this better? I have no experience in chemistry, but I have calculus and linear algebra and other math. My interests are in energy issues, engines, and other uses of chemistry in mechanical settings (like automotive).
 
  • #21
Any serious general chemistry book should give you a good foundations. First thing you will need is the Hess law (and more generally thermodynamics), but you won't be able to get the details without knowing a bit about chemistry in general.
 
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  • #23
bigfooted said:
more mechanical engineering type of chemistry (combustion in engines, etc) you can find in the excellent Princeton summer school lectures (grad/postgrad level):

http://www.princeton.edu/cefrc/combustion-summer-school/
http://www.princeton.edu/cefrc/combustion-summer-school/lecture-notes/
http://www.princeton.edu/cefrc/combustion-summer-school/archived-programs/

Thanks for that! My undergrad was in computer science though, so I may have to get grounded in the basics before I go too deep into grad/postgrad stuff. But I find I can still pick up a thing or too even if I'm missing some fundamentals...
 
  • #24
co2 is carbon and oxygen.Oxygen is an oxidixer and you need it to burn stuff.Carbon is the fuel.Add heat and the two will undergo chemical changes and come together to co2.BUT if you found some other oxdixer and fuel and burn it in a container free of o2,Perhaps...
 
  • #25
SteamKing said:
CO2+CaO→CaCO3(s) -0.179 MJ/kg
This reaction costs energy (0.179 MJ/kg), but much less than the original combustion reaction, and it is free if you let nature do its work for you.
SteamKing said:
Limestone is mostly calcium carbonate, CaCO3, which can be converted to quicklime, CaO, by heating it and driving off carbon dioxide.
Something sounds odd to a non-chemist here.
Generally, if a reaction "costs energy", why would "nature do its work for you"?
If this reaction ( CO2+CaO→CaCO3 ) "costs energy", why does the reaction "CaCO3, which can be converted to quicklime, CaO, by heating" need the "by heating"? Surely heating supplies energy, so would promote the first reaction, rather than the second?
I know there are reactions like the Haber process where heat is used simply to increase the rate of a reaction, despite the reaction being exothermic and needing to use pressure to force the reaction in the required direction. Are you saying that here?

Is the " - 0.179 MJ/kg " in an equation like this supposed to be the amount of heat given out by the reaction (or -ve, taken in during the reaction, as implied by the "costs energy") or is it a loss of internal energy, making the reaction exothermic, as I think is common in chemical equations?

And if we can't persuade the chemists to put good honest phlogiston directly into their equations, can any of them give me a clue as to how I can know which version people are using when they write equations like that? (ie. how do I tell whether the +/- kJ are being released from chemical bonds and heating up the products, or whether something odd is happening and it's an endothermic reaction?)
 
  • #26
Merlin3189 said:
Something sounds odd to a non-chemist here.
Generally, if a reaction "costs energy", why would "nature do its work for you"?
If this reaction ( CO2+CaO→CaCO3 ) "costs energy", why does the reaction "CaCO3, which can be converted to quicklime, CaO, by heating" need the "by heating"? Surely heating supplies energy, so would promote the first reaction, rather than the second?
I know there are reactions like the Haber process where heat is used simply to increase the rate of a reaction, despite the reaction being exothermic and needing to use pressure to force the reaction in the required direction. Are you saying that here?

Is the " - 0.179 MJ/kg " in an equation like this supposed to be the amount of heat given out by the reaction (or -ve, taken in during the reaction, as implied by the "costs energy") or is it a loss of internal energy, making the reaction exothermic, as I think is common in chemical equations?

And if we can't persuade the chemists to put good honest phlogiston directly into their equations, can any of them give me a clue as to how I can know which version people are using when they write equations like that? (ie. how do I tell whether the +/- kJ are being released from chemical bonds and heating up the products, or whether something odd is happening and it's an endothermic reaction?)
You have inadvertently attributed bigfooted's post to me:

bigfooted said:
What other people here already said is this: take methane for instance, and burn it with air:

CH4+2O2→2H2O+CO2CH_4 + 2O_2 \rightarrow 2H_2O + CO_2 +55.5MJ/kg

In the ideal case, this reaction gives you 55.5 MJ/kg of burned methane. Anything you do to modify the above reaction will significantly lower the heat output. It turns out that it is more energy efficient to try to remove the carbon dioxide from the air after complete combustion. The cheapest way is to do nothing and let nature do its work. For instance, natural carbonation occurs between CO2 and naturally occurring rock like calcium oxide. The reaction is:

CO2+CaOCaCO3(s)CO_2 + CaO \rightarrow CaCO_3 (s) -0.179 MJ/kg

This reaction costs energy (0.179 MJ/kg), but much less than the original combustion reaction, and it is free if you let nature do its work for you. Unfortunately, it is also a very slow reaction. With the current rate of CO2 addition, we need much faster removal.

With carbon capture, one option is to let CO2 be absorbed into liquid potassium carbonate (with some additional stuff to convince CO2 that this is a really good idea). This absorption costs approximately 0.0015 MJ/kg of energy, which is very cheap. Of course, you still need to make the potassium carbonate, the additives, the conversion plant, the infrastructure then there is the storage of the potassium, so there are some additional (energy) cost.

Most of what you find in nature is limestone (CaCO3), from which quicklime (CaO) may be made by heating it in a kiln. Quicklime is a key material used to make cement, and the quicklime used in cement is made by heating crushed limestone in a kiln. If you take the quicklime out of the kiln, let it cool, and expose it to air, the quicklime will react with the CO2 in the air and turn back into CaCO3.
 
  • #27
Ooops! Sorry, Steamking. (And Bigfoot.)
Some sort of slip in cutting and pasting.
 
  • #28
Fooality said:
Of course. That's why I'm asking if there can exist a separate chemical reaction which can produce bi-products as a a different chemical that contains the carbon, which is liquid at room temperature. While also producing energy.
SteamKing said:
No.
Oh yes.
C6H12O6->2CH3CH(OH)COOH
Liquid at room temperature (if racemic) and producing energy.

Not much, mind you. Anaerobic fermentation produces something like 1/20 of the energy released by breathing the same substrate to gaseous CO2.
There is a reason why all multicellular animals need to breathe oxygen for at least parts of their lifecycle.
 
  • #29
A major problem with lighter than air ships is the loss of mass resulting from the combustion of the fuel powering the engines. This lass of mass makes the lighter than air craft difficult to control during descent. The product of the combustion of the fuel is largely CO2 gas and H2O vapor. Since petrol is largely (CH2)n, effort has gone into condensing the H2O vapor to replace the lost fuel mass, but only has been rewarded with moderate success. If the CO2 could simply be fixed as the petrol is consumed, it would be a major boon to lighter than air craft.
 
  • #30
Take organic chemistry first
 
  • #31
You could think of oxidizing alkanes only partially, e.g. to formic acid. However, formic acid is toxic and also volatile. So in praxis not much easier to store than liquid CO2.
An exotic possibility would be the reaction 5C+3/2 O2 +CaCO3 -> Ca C6O6, the latter being known as calcium rhodizonate. A solid, very appreciated in analytic and forensic chemistry. Would be interesting to know the enthalpy of the reaction.
 
  • #32
Simply put CO2 and H2O are the thermodynamically stable products obtained when burning hydrocarbons. Of course multiple intermediates exist but these are all radicals and highly reactive. In order to burn hydrocarbons in a controlled fashion, think of biochemical processes. These require highly complex supramolecular machineries to convert sugars into CO2 and H2O aerobically and lactic acid or ethanol anaerobically making use of relatively stable intermediates. I'm not even sure if it would be possible to halt these reactions in the presence of O2 to get something like an ethanol product. Probably more radical chain reactions would be induced. So producing anything else from hydrocarbons in the presence of oxygen than CO2 and H2O is unlikely.
 
  • #33
I think certain metals can be "burned" (perhaps oxidized in a solution, or whatever). Aluminium comes to mind.

Of course we need to make the aluminium first, which takes energy. Energy from other sources (nuclear power, solar) could be concentrated for use as fuel in aircraft or the like, or CO2 could be made into octane (gasoline) which would be carbon neutral when it was subsequently burned.

These technologies are all relatively expensive currently.
 

1. What is combustion?

Combustion is a chemical reaction that occurs when a fuel (such as gasoline, wood, or natural gas) combines with oxygen in the air to produce heat, light, and new chemical compounds.

2. Does combustion always produce gaseous CO2?

No, combustion does not always produce gaseous CO2. The production of CO2 depends on the type of fuel being burned and the amount of oxygen available. Incomplete combustion can result in the production of other gases such as carbon monoxide (CO) or soot (particulate matter).

3. Why is CO2 produced during combustion?

CO2 is produced during combustion because it is a byproduct of the chemical reaction between the fuel and oxygen. The carbon atoms in the fuel combine with the oxygen atoms in the air to form CO2.

4. Can combustion produce other greenhouse gases besides CO2?

Yes, combustion can produce other greenhouse gases besides CO2. Methane (CH4) and nitrous oxide (N2O) are also produced during combustion, and they have a higher global warming potential than CO2.

5. Is it possible to reduce the production of CO2 during combustion?

Yes, it is possible to reduce the production of CO2 during combustion. This can be achieved by using cleaner and more efficient fuels, improving combustion processes, and implementing technologies such as carbon capture and storage to capture and store CO2 emissions.

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