(FWIW, I'm willing to bet the energy efficiency will be excruciatingly low. No mining operation can break even if the ore is not rich enough. If you want surface carbon, biomass may be the only sufficiently concentrated source, and not a particularly efficient one.)
"We've been attacked by the intelligent, educated segment of the culture." -- Pastor Ray Mummert, Dover, PA, during an attempt to introduce creationism -- er, "intelligent design", into the Dover Public Schools.