Some U.S. biofuel mandates can't be reached, according to a new report from the National Research Council. At least not without major technological innovations or policy changes.

The "cellulosic" biofuel mandates contained in the current Renewable Fuel Standard (RFS2), amended in the 2007 Energy Independence and Security Act, want to see consumption of - by 2022 -- 16 billion gallons of cellulosic biofuels produced from wood, grasses, or non-edible plant parts.

The Act demands that by 2022 the consumption of renewable fuels should also include 15 billion gallons of conventional biofuels, mainly corn-grain ethanol; 1 billion gallons of biomass-based diesel; and 4 billion gallons of advanced renewable biofuels, other than ethanol derived from cornstarch.

In each case there's a specific demand that the fuel in question should achieve a certain lifecycle GHG threshold, referring to total emissions from planting to harvesting and on to distribution and ultimate use in a vehicle.

The NRC report, entitled "Renewable Fuel Standard: Potential Economic and Environmental Effects of US Biofuel Policy," allows that conventional biofuels and biomass-based diesel fuel can probably meet consumption mandates.

Cellulosic biofuels, however, are different because there are countless economic, policy, environmental, and social barriers to be met first. It's still a developing industry, and to see it through to meeting the mandate would demand federal budget outlays that may or may not be available. As well, the report says, the likely economic and environmental effects are at best "mixed."

"A key barrier to achieving RFS2 is the high cost of producing biofuels compared to petroleum-based fuels and the large capital investments required to put billions of gallons of production capacity in place," the report says. "As of 2010, biofuel production was contingent on subsidies, tax credits, the import tariff, loan guarantees, RFS2, and similar policies. These policies that provide financial support for biofuels will expire long before 2022 and cannot provide the support necessary for achieving the RFS2 mandate."

Right now, there simply aren't any refineries in the U.S. for converting cellulosic biomass to fuel, at least none that are commercially viable.