The U.S. Environmental Protection Agency on Monday proposed changes to its Renewable Fuel Standard (RFS) — a program authorized under the Energy Policy Act of 2005 that seeks to reduce greenhouse gas emissions by expanding use of renewable fuels. The proposal, which will be open for comments from the public for 60 days after its publication in the Federal Register, would allow biofuel producers to partially process renewable feedstocks at one facility and further process them into renewable fuels at another facility.

This, the EPA argues, “will provide the opportunity for increasing the production and use of renewable fuels by allowing the market to operate in the most efficient and economical way.”

“This would increase the economics and efficiency for the production of biofuels, particularly advanced and cellulosic fuels that have the lower carbon footprints,” the agency said in a statement.

The U.S. is currently the world’s largest ethanol fuel producer, generating more than twice as much as Brazil, which is the second-largest.  In 2015, over 350,000 barrels of fuel ethanol — most of which was made using corn as the main feedstock — were produced in the country, and nearly 332,000 barrels of it were consumed domestically.

The EPA also called for amended quality standards for biofuel blends containing up to 83 percent ethanol. These mixtures of gasoline and ethanol are currently used in “flex fuel vehicles (FFVs)” — automobiles whose engines are designed to run on more than one kind of fuel.

“FFVs currently represent about 8 percent of the U.S. passenger vehicle fleet. EPA is revising its gasoline regulations to make it clear that E16-E83 fuel blends are not gasoline, and hence not fully subject to gasoline quality standards,” the EPA said. “However, we will also be putting in place standards that ensure the quality and environmental performance of this fuel.”

In addition, the EPA also proposed including hybrid poplar trees and willow trees among materials used for the production of cellulosic biofuels.

The proposed tweaks come just days after the publication of a controversial study whose authors argued that biofuels were not inherently carbon neutral.

The study, which was supported by the American Petroleum Institute, was published last week in the journal Climate Change. The authors stated that corn-based biofuels burned under the EPA’s RFA were far worse for the climate than fossil fuels — something they said the agency failed to detect because of a critical error in the way greenhouse gas emissions from biofuels is calculated.

“When it comes to the emissions that cause global warming, it turns out that biofuels are worse than gasoline,” lead author John DeCicco said in a statement. “So the underpinnings of policies used to promote biofuels for reasons of climate have now been proven to be scientifically incorrect.”