Unless a major technological breakthrough occurs in the next few years, a U.S. government push to put 16 billions of gallons of cellulosic biofuel into gas tanks annually by 2022 will be a bust, hints a new report. 

The push comes from the congressionally mandated Renewable Fuel Standard. Of the mandated total of 36 billion gallons from a mix of biofuels, the corn-derived ethanol target of 15 billion gallons is doable, the report says. 

But a big part of the standard — 16 billion gallons of cellulosic biofuels from non-edible plant material such as cornstalks and switchgrass — is unlikely to be met, Wallace Tyner, an agricultural economist at Purdue University, told me Tuesday.

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"The technologies are just not advanced enough to be commercial, they are not cheap enough yet to be commercial, and we are going to have to invest more in R&D if we want to accelerate the pace," he said.

Tyner co-chaired the National Academy of Sciences report requested by Congress on the potential economic and environmental effects of U.S. biofuel policy.

"We're not saying don't do biofuels, we're not saying that it is a bad thing to do, we're just saying it is not going to happen in today's environment unless big things change," he said.

Breakthroughs needed
Currently, no commercially viable biorefineries exist for converting cellulosic biomass to fuel. That's daunting given that it took 30 years to go from zero to 200 plus plants producing more than 15 billion gallons of corn-ethanol, Tyner noted.

"Here we are in 2011 and we have 11 years to get to 2022 and build 16 billion gallons with a technology that's costlier and riskier, a feedstock that's costlier, and it is just not likely to happen," he said.

Breakthroughs are needed in every pathway to produce cellulosic biofuel.

For example, Tyner explained that one technique with a lot backing called fast pyrolysis, which breaks down biomass with heat, produces unstable oils that can't be further refined to gas, diesel, and jet fuel.

Other techniques such as gasification have been done for decades, but it remains too expensive to be commercially viable due to capital costs and catalysts used in the process.

Questionable environmental impacts
And even if technological breakthroughs drive down costs and make cellulosic biofuel commercially viable, a question remains whether or not its use will impact land use or help curb greenhouse gas emissions implicated in global climate change.

Using leftover corn stalks or wood chips from sawmills have no impact on land use and are a net positive for greenhouse gas emissions, Tyner noted, since those materials are already being generated. 

But the impacts other feedstock such as switchgrass and miscanthus is uncertain. Tyner said that preliminary research suggests these crops sequester carbon as they grow, some of which gets locked up in the soil via the root system.

"Those, I think, are going to come out OK," he said, "be we don’t know for sure."

Other biofuel crops, notably corn-derived ethanol, has been lambasted over the years for competing with food for people and livestock as well as having questionable impact on greenhouse gases. 

"For every bushel of corn that you feed to an ethanol plant instead of a hog that hog still has to be fed," Tyner noted, "so more corn or corn substitute has to be grown somewhere else in the world."

This process, the report notes, could involve clearing perennial vegetation such as forests that lock up carbon in trees and soils. Even though biofuels are considered carbon neutral, the loss of forest may offset this gain.

The Environmental Working Group, which has long opposed government mandates for corn-based ethanol, applauded the findings of the new report for showing the negative impact of ethanol subsidies on the environment.

"The new report provides more evidence that corn ethanol production continues to raise food prices around the world and harms the planet by releasing more greenhouse gases than regular gasoline," the group said in statement. 

Promoters of biofuels have long seen corn-derived ethanol as a bridge to more environmentally-friendly cellulosic ethanol, but the new report suggests that bridge is unlikely to be crossed by 2022. 

Further complicating progress on cellulosic ethanol is regulatory uncertainty, which hobbles investment in R&D, noted Tyner. After all, there's not even a guarantee the fuel standard will be around in the future or enforced. 

"You put all those uncertainties into the basket and overlay that with today's financial condition where venture capital is nothing like it was a few years ago and it is going to be hard" to find investors," he said.

More on biofuels:

• Race for better biofuels heats up
• Is algae biofuel too thirsty?
• Venture capitalist eyes cellulosic ethanol
• Energy bill a boon for ethanol, and a challenge

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John Roach is a contributing writer for msnbc.com.