# What does it mean when “a gas is dilute in CO₂”?

I'm reading an article about carbon sequestration, and I came across a strange phrase: "The flue gas is dilute in $\ce{CO2}$". I can't quite ascertain its meaning from the context. I think it means that the $\ce{CO2}$ is in low concentration in the flue gas, but I'm no chemistry expert.

Does this have a technical meaning in chemstry when a gas is dilute in another substance?

• Thank you for including the article, my guess is that they are saying there is "not enough" $CO_2$ from what I can read, judging by the line following which reads "...resulting in... Low $CO_2$ partial pressure." The context is "barriers to implementation" which I assume... means not helping us scrub it. – Leonardo Sep 17 '12 at 3:40
• @Leonardo: I added the link to the article. The phrase is included at the top of table 1, under "Barriers to implementation". – Paul Sep 17 '12 at 3:43
• Yes, kind of counter-intuitive that there is more $CO_2$ before combustion rather than after. Looking at the chart it seems to have all the answers but I have never looked into the process which is why I am only commenting. I think a good question would be about this process itself and why is there more carbon dioxide before combustion, and this is coal burning right? This is looking at the middle flow-chart for pre-combustion. – Leonardo Sep 17 '12 at 3:55

In this case, it's comparing post-air-combustion $\ce{CO2}$ capture with pre-combustion capture, and with post oxy-combustion capture.

In the two latter cases, the $\ce{CO2}$ stream is very highly concentrated. However, with air-combustion, the flue gases contain a lot of nitrogen and NOx; hence the $\ce{CO2}$ is diluted in the flue gas, compared to the other two methods, where it's concentrated.

And as to Leonardo's supplementary question in the comments, about why the pre-combustion stream has higher concentrations of $\ce{CO2}$ than the post-air combustion stream:

the comparison here isn't between two points in the same process. It's between analagous points in two completely different processes.

With post-combustion capture, the $\ce{CO2}$ is stripped out after the hydrocarbons (coal, gas, oil) have been burnt in air.

Whereas with pre-combustion capture, the hydrocarbon goes through a gasifier/shift process, resulting in a stream of $\ce{H2}$ mixed with $\ce{CO2}$. And at this point, the $\ce{CO2}$ is stripped out, and the $\ce{H2}$ is burnt.

And that's why the $\ce{CO2}$ concentration is higher in the pre-combustion capture process, than in the post air-combustion capture process.