DACs generally operate by pushing air past a sorbent chemical which binds with carbon dioxide but allows other molecules to pass unimpeded. For example, one of the earliest sorbents employed was a calcium hydroxide solution, which strongly binds with CO2 to create calcium carbonate. The captured CO2 is then unbound from the sorbent, purified and concentrated for use in industrial applications. Of course this is often easier said than done. With the calcium carbonate method (which is derived from the Kraft process), the material must be separated from the solution, dried, and then carbonized at 700 degrees C.
This however reveals the Achilles heel of DACs: their cost. A 2015 study from the National Academies estimated costs of around $400 to $1,000 per ton of CO2 extracted at that time. With nations needed to collectively pull 5 billion tons of carbon out of the atmosphere, every year until 2050, to remain within the bounds of the Paris Climate Accord, doing so with just DACs would prove economically infeasible. The associated energy costs needed to carry out these chemical processes (estimated at 12 gigajoules of electricity per ton of CO2 captured) would be equally staggering.
"Direct air capture could become a major industry if the technology matures and prices drop dramatically," Professor Chris Field, former co-chair of the Intergovernmental Panel on Climate Change's (IPCC), and Dr Katharine Mach, director of the Stanford Environment Facility, wrote in a 2017 Science article. "Direct air capture might require much less land but entail much higher costs and consumption of a large fraction of global energy production."
Of course, that's not stopping a number of entrepreneurs from trying. In fact, efforts over the past few years have already seen DAC sequestration costs drop to between $94 and $232 per ton, according to the journal Joule.