Imagine walking into a supermarket, where you’re offered a free sample of a new protein shake with a secret ingredient. After a few sips (the shake’s delicious, by the way), you learn the secret isn’t some new super-kale or the latest magical supplement — what you’re drinking is carbon dioxide that was once belched out of a steel mill smokestack. The idea of pulling CO2 out of the air and turning it into a product to be sold for profit has been attracting a lot of attention lately, and even a few dollars. And the number of startups working in carbon capture and use (CCU), also known as carbontech, has ballooned in recent years. The Circular Carbon Network, a new nonprofit initiative launched by the XPRIZE Foundation and Pure Energy Partners to catalyze more investment and commercial activity in CCU, has already tracked more than 250 companies in the space.   “There’s a huge range of products we could make out of CO2,” Marcius Extavour, executive director of the NRG COSIA Carbon XPRIZE, told GreenBiz. “So then the questions become: Which ones are the easiest to make? Which ones are cost-effective? And which ones would actually have a lower carbon ...