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Background

I am trying to determine how many kg of $\ce{CO2}$ are released when burning 1 GJ of natural gas.

What I've tried

I have very little chemistry knowledge, but google led me to these two sources (Source 1:Energy Contents and Source 2:Combustion of Fuels. Based on that, I've assumed the following;

  • Natural gas has energy content of 48.2 MJ/kg
  • Which means 1 GJ of natural gas corresponds to 20.75 kg (1000/48.2)
  • Natural Gas has a 75 % carbon content

So I've (incorrectly) assumed that burning 1 GJ of natural gas will produce ~15.5 kg of $\ce{CO2}$. (0.75 * 20.75 kg)

Problem

Obviously, I am wrong, since that second source (Source 2:Combustion of Fuels) has written in the table, that;

  • 1 kg of natural gas releases 2.75 kg of $\ce{CO2}$.
  • 1 GJ of natural gas will produce 50 kg of $\ce{CO2}$.

How does that work? what am I missing?

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Let me solve the first half of it for you, and see if this gives you enough background to solve the second half on your own:

Natural gas is methane ($\ce{CH4} (g)$). The balanced chemical equation for the combustion of methane is:

$$\ce{CH4 (g) + 2O2 (g) -> CO2 (g) + 2H2O (g)}$$

We know it's balanced because the total number of atoms of each element on the left side is the same as that on the right. E.g., there are four atoms of oxygen on the left, and four on the right.

Each mole of $\ce{CH4}$ weighs 12 g (for the carbon) + 4 g (for the 4 hydrogens) = 16 g. Each mole of $\ce{CO2}$ weighs 12 g (for the carbon) + 32 g (for the 2 oxygens) = 44 g.

Since we produce 1 mole of $\ce{CO2}$ for each mole of $\ce{CH4}$, we produce 44 g of $\ce{CO2}$ for every 16 g of $\ce{CH4}$. Thus burning 1 kg of natural gas will produce:

$$\require{\cancel} 1 \,\ce{kg} \,\cancel{\ce{CH4}} \times \frac{44\, \cancel{\ce{g}} \,\ce{CO2}}{16 \,\cancel{\ce{g}} \,\cancel{\ce{CH4}}} = 2.75 \,\ce{kg} \,\ce{CO2}$$

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    $\begingroup$ Brilliant, thanks!! The equation you shared was the missing link, I never realized how heavy Oxygen was (16 g/mol). $\endgroup$ Aug 3 at 6:24
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    $\begingroup$ Happy to help. The way I look at it is not that oxygen is unusually heavy (it's still one of the lighter elements), but that hydrogen, which is the most numerous component of methane, is unusually light (it's the lightest element), which is what makes CH4 so light relative to CO2. $\endgroup$
    – theorist
    Aug 3 at 6:31
  • $\begingroup$ Natural gas is methane, but only after processing. $\endgroup$ Aug 4 at 5:29
  • $\begingroup$ @RodrigodeAzevedo Not quite. A more accurate statement would have been as follows: "The term natural gas (NG) can refer to either refined or raw (unprocessed) NG. While the former is the more common usage, both are acceptable. One can typically infer which meaning is being used from context. E.g., the OP was asking about CO2 production from combustion of NG, from which one can infer he meant refined NG, since far more CO2 is introduced into the atmosphere from the combustion of refined than raw NG. That was theorist's interpretation, which is why he equated NG it to methane." $\endgroup$
    – theorist
    Aug 4 at 6:44
  • $\begingroup$ @theorist Yes, but raw natural gas is also flared. Without a caveat, it's easy for people to assume this answer applies also to raw natural gas. $\endgroup$ Aug 4 at 8:13

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