The Environmental Protection Agency on Monday proposed tweaks to its Renewable Fuel Standard that would allow biofuels to be processed at more than one location and would allow cellulosic ethanol to be made from poplar and willow trees.

EPA also proposed new quality standards and environmental performance guidelines for biofuel blends containing 16 to 83% ethanol, and the agency is taking comments on the proposal for the next 60 days before it is approved.

The Renewable Fuel Standard, or RFS, was enacted in 2007 and designed to cut greenhouse gas emissions and boost use of advanced fuels such as cellulosic ethanol. However, critics say regulatory delays have contributed to slow growth in next-generation fuels.

EPA proposed adding hybrid poplar trees and willow trees as materials approved for production of cellulosic biofuel. DuPont and Poet already produce small amounts of cellulosic ethanol made from crop waste such as corn stalks.

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The change that would allow biofuel makers to partially process a feedstock at one location and further process into an biofuel at another location would “increase the economics and efficiency for the production of biofuels, particularly advanced and cellulosic fuels that have the lower carbon footprints,” the EPA said in a summary of the proposal.

“This is a rule we really needed to have. We’re delighted to see it,” Michael McAdams, president of the Advanced Biofuels Association, said of the proposal.

A full copy of the 374-page proposed rule is available here.