Oil refiners shelled out a record over $2 billion to meet U.S. biofuels requirements in 2016, a 70 percent surge that helps fuel a growing debate over who should shoulder the costs for meeting environmental regulations. A Reuters review of regulatory filings from ten refiners, including Valero Energy Corp and CVR Energy Inc, showed that they paid $2.22 billion last year for compliance credits that companies use to prove they are meeting annual government requirements for volumes of biofuels to be blended with gasoline and diesel. That was 68 percent more than in 2015 and well above the previous record of $1.3 billion reached in 2013, when prices for paper credits, which trade in an opaque market, spiked to historic levels. […]