>Have you had people apply to YC already with ideas in these areas?

Yes but we would love to see a lot more. We know there are lots of researchers working on this but not enough of them are starting companies.

>Can you speak to any technical shortcomings of ideas you've already seen (aside from the general cost/scale issues mentioned on the site)?

Here is a great overview of current technologies: http://www.centerforcarbonremoval.org/what-is-carbon-removal...