If I have a traditional gas range-top, burning natural gas (instead of LPG or propane), what is the typical range of humidity present in the combustion byproducts immediately above the flame? The combustion process adds water to the air, and the natural gas has an inherent humidity as well. However, the combustion process also dramatically raises the temperature, thus significantly drying the air.
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$\begingroup$ Why would there be any relative humidity immediately after combustion? Temperature is higher then boiling point of water. $\endgroup$– MithoronMar 16 at 21:01
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1$\begingroup$ @Mithoron No problem as long as there still exists a saturation pressure at that temperature. For example, pure steam at 120 °C and 1 bar has a relative humidity of about 50 %. Whether this is useful information, is a different question. $\endgroup$– LoongMar 16 at 21:11
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$\begingroup$ Whoa, generally in SE we don't downvote without leaving explanations, esp. for new contributors. If the question is dumb, please explain why. FWIW, it looks like there is an answer, which is that it is undefined. $\endgroup$– Kenn SebestaMar 16 at 21:33
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$\begingroup$ Well, you just shouldn't ask about relative, but absolute or specific humidity. Relative one is highly dependent on temperature, even if it was below critical temperature. We don't really know what you want to find out here, I'm afraid. $\endgroup$– MithoronMar 17 at 0:24
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$\begingroup$ @Mithoron thanks for the clarification. I'm curious if the combustion process makes the air generally wetter or generally drier. I've taken classes in thermodynamics related to powerplants, so I've never worked with temperatures so high (or low) that we've discussed critical points as something other than curiosities. $\endgroup$– Kenn SebestaMar 17 at 0:33
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1 Answer
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Assuming ideal conditions starting from dry air, the theoretical mixture immediately after complete combustion contains $19\ \%$ $\ce{H2O}$ (molar fraction) since the oxygen from the air is replaced according to
$$\ce{CH4 + 2 O2 -> CO2 + 2 H2O}$$
However, the adiabatic flame temperature in air (not oxygen) is $2236\ \mathrm K$, which is well above the critical point temperature of $647\ \mathrm K$. Therefore, the relative humidity (i.e. the ratio of the partial pressure of water vapor to the saturation vapor pressure) is not defined at such temperatures immediately after combustion.
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$\begingroup$ "relative humidity is not defined at such temperatures immediately after combustion." Thanks for this, I had no idea. I'm going to have to read about that to get a better sense of what it even means. $\endgroup$ Mar 16 at 21:30