How Cellulosic Ethanol Works

Are you doing the environment a favor by driving on cellulosic ethanol instead of corn ethanol or gasoline? It's a common question with no easy answer. Scientists are still sizing up the environmental costs and benefits of cellulosic ethanol. When comparing fuels, they consider impacts over the "life" of a fuel. For gasoline, that life cycle includes mining the oil, piping or transporting it to the refinery, making gasoline, piping it to gas stations and finally being emitted by cars. For cellulosic ethanol, the life cycle is similar but begins with farming the plants.

Scientists use models to add up the impacts of a fuel over its life. But cellulosic ethanol is hard to model because so far, no one is harvesting cellulosic biomass in large amounts or commercially producing cellulosic ethanol.

We also need to decide what constitutes the "best" thing for the environment and for us. Do we want to minimize greenhouse gas, water pollution or cancer-causing agents in car exhaust? The best fuel changes with the question.

As for answers, here's what the models say. From farm to car, cellulosic ethanol releases less greenhouse gas than gasoline (86 percent less) and corn ethanol (52 percent less than gasoline) [source: Wang]. For greenhouse gases, we're talking about carbon dioxide, methane and nitrous oxide. The numbers assume that ethanol refineries run on environmentally friendly wood chips and that oil refineries run on coal.

Studies have compared emissions from cars running on ethanol and gasoline. But one thing to realize is that in the United States, no one drives on 100 percent ethanol (unless you're racing in the Indy 500). U.S. regulations require fuel alcohols to be undrinkable and diluted to 95.5 percent. Since engines don't start well in the cold on 95.5 percent ethanol, the highest percentage of ethanol fuel sold in the United States is E85, which is 85 percent ethanol and 15 percent gasoline [source: Aden]. (But cars run on E95 in sunny Brazil.)

E85 is no saintly fuel. Burning it releases toxins and pollutants, just like gasoline or diesel. The two car emissions to watch are permeation emissions -- fuel soaking through a car's pipes and hoses -- and tailpipe emissions. Permeation emissions are the worst in cars running on low-ethanol mixtures, ranging from 6 to 20 percent. They're better for pure gasoline and best for E85 [source: CRC]. Your car also matters. Cars made in 2000 are worse than those made since 2004, and you'll have no issues in cars branded "PZEV," or partial zero-emissions vehicles.