Bob Dinneen, president of the Renewable Fuels Association, a trade group, estimated that of the country’s 150 ethanol companies and 180 plants, 10 or more companies have shut down 24 plants over the last three months. That has idled about 2 billion gallons out of 12.5 billion gallons of annual production capacity. Mr. Dinneen estimated that a dozen more companies were in distress.

Ronald H. Miller, the president and chief executive of Aventine Renewable Energy, said, “The economics right now are very poor.” Aventine has suspended construction of one Nebraska plant and delayed completion of a second in Indiana.

This is not how it was supposed to be when Congress mandated in 2007 that refiners blend increasing amounts of ethanol into the country’s transportation fuel supply. The law came at a time when the country’s thirst for gasoline seemed unquenchable, and oil prices seemed only to go up.

In an effort to reduce the country’s dependence on foreign oil and to lower the greenhouse gas emissions that contribute to global warming, Congress mandated a doubling of corn ethanol use, to 15 billion gallons a year by 2015. Congress also mandated, by 2022, the use of an additional 21 billion gallons of ethanol and other biofuels produced from materials collectively known as biomass. The potential materials include corn stubble, wood chips and straw.

Congress hoped that advanced biofuels would overcome the longstanding controversies associated with corn ethanol, including the contention that its production raises food prices. Congress started small, decreeing that industry produce 100 million gallons of advanced biofuels next year and 250 million gallons in 2011. But it is becoming clear that even these modest targets will not be met.

Producing the advanced fuels entails breaking down a tough material, cellulose, that is abundant in corn cobs, wood chips and other biological waste, then converting it to liquid fuel. While scientists have proven it can be done, the cost is still high, and little if any cellulosic ethanol is being produced at commercial scale.

Carlos A. Riva, president and chief executive of Verenium, a company working to produce ethanol from sugar cane waste, said that solving the technological hurdles for this type of fuel was “not a slam dunk.” But he and other executives say they are optimistic the challenges can be overcome, and the 2011 and 2012 targets may be met a few years late.