A trio of cellulosic ethanol plants in the final stages of construction in Iowa and Kansas are being threatened by an Environmental Protection Agency proposal aiming to cut federal renewable-fuel mandates.

Officials at Poet-DSM, Dupont Chemical and Abengoa Bioenergy say the plants—each worth $200 million to $250 million—have taken more than a decade to develop and build. According to Poet Bioenergy, the leading U.S. ethanol producer, the industry's duty to fulfill EPA's Renewable Fuel Standard (RFS) is the reason they are being built.