Carbon Dioxide - Is It Chamets?
350 words, from the original, 2683 words
Rav Shlomo Zalman Auerbach, zt”l Paskens that carbon dioxide produced by Chametz fermentation [bread, beer, or whisky] is not Chamets and is permitted during Pesach. [Halichos Shlomo (Moadim, Nissan-Av, 4:3 and footnote 14) & Hagadah of Rav Auerbach (page 309)] 

Some suggest that just as alcohol distilled from Stam Yaynom is forbidden because the distillation process does not actually alter the final product from the original, so too carbon dioxide is in no significant manner altered from the original Chametz from which it originated.  

Rivash [255, cited in ShA OCh 123:24] proves this from the Mishna Machshirim 2:1-2, which differentiates between condensation in a bathhouse, which remains unchanged from its source, and condensation in a cave which is deemed to be a new entity. The problem is that we do not know what are the salient considerations that differentiate these two examples. It may be that the heat is not what is relevant, but the large volumes of visible vapour produced by the heat. In this case, carbon dioxide which is recovered in large volumes will retain its identity as the Chametz it is derived from.

The Gemara [Avodah Zara 66b-67a, cited in Shulchan Aruch Y.D. 108:4-5] describes dough or bread that absorbs non-Kosher wine flavour when placed on a wine barrel and is thereby prohibited. We see that taste of prohibited foods can forbid a Kosher food even when it is only via an invisible low temperature vapour.

It appears that ultimately what concerns the Halacha is whether there is any residual taste from the non-Kosher.

Accordingly, earlier inefficient distillation processes produced a product that did have a residual taste of the non-Kosher original, whereas modern efficient distillation provides an end product that is utterly devoid of any taste of the original.

Rabbi Dovid Cohen, representing the CrC, surmises that Rav Auerbach ruled that carbon dioxide produced from Chamets is permitted simply because the carbon dioxide about which he was providing a Halachic ruling, had absolutely no taste of Chamets.
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I should like to add the following:
We may dry meat by placing it above the stove on which we are cooking milk. Even though the meat is enveloped by the steam rising from the boiling milk. 
This is not Kosher AFTER the fact, BeDiEved; no, this is permitted in the first instance. When you want to dry meat and ask your Orthodox Rabbi, “May I hang my meat above the stove where I am cooking milk?”  The rabbi, if he is ruling from the Shulchan Aruch, will advise you that it is permitted.  The only Halachic consideration he will warn you of is to ensure that the meat hangs high enough so that the steam reaching the meat is less than Yad Soledes.

I believe it is a fortiori, if the steam from boiling milk is not milk, then certainly the gas from Chamets fermentation is not Chamets.