# Does combustion have to produce gaseous CO2?

1. Sep 8, 2015

### Fooality

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.

2. Sep 8, 2015

### Staff: Mentor

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. Sep 8, 2015

### Fooality

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. Sep 8, 2015

### SteamKing

Staff Emeritus
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. Sep 8, 2015

### Fooality

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. Sep 8, 2015

### SteamKing

Staff Emeritus
No.

7. Sep 8, 2015

### Fooality

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. Sep 8, 2015

### SteamKing

Staff Emeritus
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. Sep 8, 2015

### Fooality

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. Sep 8, 2015

### SteamKing

Staff Emeritus
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. Sep 9, 2015

### Staff: Mentor

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.
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. Sep 10, 2015

### bigfooted

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

$CH_4 + 2O_2 \rightarrow 2H_2O + CO_2$ +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:

$CO_2 + CaO \rightarrow CaCO_3 (s)$ -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.

13. Sep 10, 2015

### Staff: Mentor

No such thing as naturally occurring calcium oxide, it is way too reactive.

14. Sep 10, 2015

### Fooality

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. Sep 10, 2015

### SteamKing

Staff Emeritus
Limestone is mostly calcium carbonate, CaCO3, which can be converted to quicklime, CaO, by heating it and driving off carbon dioxide.

16. Sep 10, 2015

### Staff: Mentor

How does it change the fact there is no naturally occurring CaO?

17. Sep 10, 2015

### SteamKing

Staff Emeritus
Mebbe he got confused about what rocks contain what chemicals ...

18. Sep 10, 2015

### bigfooted

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.

natural occuring 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. Sep 10, 2015

### Staff: Mentor

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. Sep 10, 2015

### Fooality

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).