How Cellulosic Ethanol Works

That's not corn! Nope, the latest alternative fuel that may someday power your cars relies on biomass such as cut wood as its starting point. See more green science pictures.
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The United States used an average of 20.7 million barrels of petroleum daily in 2007, more than any other country, and more than half of which was imported [sources: EIA, EIA]. The country, however, is hoping to change that scenario, and U.S. energy policy is sending a clear message: Import less petroleum and use more renewable fuels. The latest law behind the message, the Energy Independence and Security Act of 2007, requires the country to replace 36 billion gallons of its annual petroleum use with renewable fuel by 2022 [source: RFA].

Corn ethanol and cellulosic ethanol rank among the fuels that will make up the difference. We know corn ethanol. What is the new stuff? It's alcohol made from tough plant stems, leaves and trunks instead of supple starch. So far, refineries around the world can produce it only on a demonstration scale. It's more complicated to make than corn ethanol. If it were sold at the pump now, it would be more expensive than both corn ethanol and gasoline.

The U.S. government has invested in cellulosic ethanol research and refineries, including $1.3 billion for the Department of Energy to dispense in 2009 [source: Waltz (2009)]. The funding has seeded commercial plants, which could open in the United States as early as 2010.

Driving on cellulosic ethanol as opposed to pure gasoline has environmental benefits, and it could eventually be cheaper than other liquid fuels, depending on the price of corn and oil.

Read on to learn what this fledgling fuel has to offer.

Cellulosic vs. Starch Ethanol

You can make ethanol from many plant sugars. Cellulose and starch are just two examples. No matter what you start with, the ethanol production process takes polysaccharides, or complex sugars, from the plant, breaks them into single sugars and converts them into ethanol.

The differences between starch and cellulosic ethanol start with the plants. In the United States, starch ethanol is made from corn kernels. Cellulosic ethanol, however, starts with cellulose, the most abundant carbon-containing material on the planet, and hemicellulose. Plants make 100 billion tons (91 billion metric tons) of cellulose every year [source: Campbell].

Starch is how plants store energy, so it's easy to break down. Enzymes throughout the natural world, such as alpha-amylase in our mouths, can break starch into glucose.

Cellulose and hemicellulose resemble plant armor. Inside cell walls, they're tangled with a third tough material, lignin, which makes plants woody.

If starch melts in your mouth and cell walls resist degradation, then it makes sense that starch is easier to convert into ethanol. When starting with starch, refineries grind corn kernels and add common amylase enzymes, which break the starch into glucose. Yeast then converts the glucose into ethanol.

When starting with cellulosic biomass, ethanol production is slower and more complicated. Grinding the plants is just the beginning. Refineries add acid to unweave hemicellulose, cellulose and lignin -- lignin is in the way, since it isn't fermentable. Next, acid breaks down hemicellulose into four component sugars. Then cellulose is freed, but enzymes must break it into glucose. Now, refineries are stuck with five sugars to convert to ethanol. Glucose is easy, but the others aren't. Microbes that naturally ferment all five sugars poorly tolerate bioreactors, so refineries need engineered microbes or a microbe potpourri. Toxin buildup, incomplete conversions and slow enzymes all complicate the process and lower the ethanol yield.

Another advantage of corn is its predictable amount of starch whereas cellulose and hemicellulose contents vary by the plant [source: Waltz (2008)]. On the other hand, cellulosic ethanol dangles some environmental benefits. It can turn waste, not food, into ethanol. When crops such as switchgrass are farmed for cellulose, they use less fertilizer and water than corn [source: NREL]. If researchers can learn to fully release and ferment the sugars in cellulosic biomass, it will make more ethanol per volume of plant than corn kernels [source: Aden].

Read on to learn how tree trunks become fuel.

Making Cellulosic Ethanol

Cellulosic ethanol production from start to finish
Cellulosic ethanol production from start to finish
Image courtesy DOE Biomass Program

Cellulosic ethanol starts with cellulosic biomass. Almost every stem, leaf and tree trunk in the world qualifies, from farm wastes to grass clippings and recycled newspaper [source: BlueFire]. Farmers also can grow energy crops for cellulosic biomass, including switchgrass, and some trees. They're called energy crops because they grow densely in a small area and don't need much else.

The first step in production is transporting the plants to the refinery. The refinery would then make ethanol either biochemically or thermochemically.

We'll explain the biochemical method first. Let's say we're in corn country in the Midwestern United States. After harvest, farmers bundle dry corn stalks and cobs littering the field and truck them to a nearby refinery. The debris is ground into bits.

The bits go to pretreatment, where they steep in hot sulfuric acid. Cellular walls and contents dissolve. The acid pushes lignin out of the way to free hemicellulose, then decomposes hemicellulose into its four sugars: xylose, mannose, arabinose and galactose. Cellulose is now freed.

Scientists call the next step cellulose hydrolysis. Here, the acid is washed off, and the mixture goes to tanks with enzymes called cellulases, which turn cellulose into glucose.

Now, we have a soup of sugars: glucose, plus hemicellulose's four sugars. Their concentrations and the microbes used in the next tank for fermentation, depend on the plant species you started with.

Next comes separation. Stillage -- everything that's not alcohol -- settles to the bottom of a tank and is sent for processing and reuse. The alcohol stays on top and goes to distilling, which purifies it to fuel grade [source: DOE].

Making cellulosic ethanol thermochemically poses benefits. The thermochemical method converts lignin and gets the most ethanol from wood or any plant [source: NREL]. Thermochemical conversion has drawbacks, too, including expensive catalysts and tar buildup that has to be cleaned.

While refineries order the thermochemical steps differently, they start by drying the plants. Next, the plants are burned into a synthesis gas, or "syngas," made of carbon monoxide (CO) and hydrogen (H2). Because gasification also forms tar and sulfur, which interfere with making ethanol, the gases go to a tar reformer, which chemically converts these obstructions into more syngas. The gas is cleaned again to get closer to pure CO and H2, then compressed and run across a metal catalyst. The catalyst builds the gases back up into molecules of choice: ethanol or hydrocarbons that are similar to gasoline. Separations remove the ethanol [source: NREL].

For either method, the ethanol is trucked to special stations to mix with gasoline, arriving at gas stations afterward.

Cellulosic Ethanol Emissions

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.

Benefits of Cellulosic Ethanol

For the average city-dweller's health, E85 emissions confer a mixed blessing, not necessarily an outright benefit. They contain less carbon monoxide than gasoline, which can aggravate heart disease. They also cut nitrogen oxides, those notorious perpetrators of acid rain, smog and haze, to half the level of gasoline [source: Yanowitz]. Lastly, E85 lowers emissions of two cancer-causing substances, benzene and 1,3-butadiene, below gasoline but dramatically boosts two other likely carcinogenic substances, acetaldehyde and formaldehyde. U.S. government-funded researchers concluded that the total toxicity for tailpipe emissions from gasoline and E85 is the same [source: Yanowitz].

The biggest benefit of making ethanol from cellulose is the inexhaustibility and convenience of cellulosic biomass. It's more available than corn or any other source of ethanol, or for that matter, any existing source of fuel. When done wisely, cellulosic ethanol production can get rid of waste and make fuel. If you're of the anti-greenhouse gas persuasion, its production and burning releases less greenhouse gas than gasoline. It has other environmental and clean-air benefits, which you read about in the last section. If you dislike oil drilling, oil importation, want an alternative at the gas pump when oil or corn prices increase or think there's a limit to how much fuel corn can make, cellulosic ethanol provides an alternative.

That's the good news. What about the bad news?

Concerns about Cellulosic Ethanol

As you can see, E85 costs less at this gas station in Belmont, Wis., thanks to a tax credit that eventually would also benefit cellulosic ethanol.
As you can see, E85 costs less at this gas station in Belmont, Wis., thanks to a tax credit that eventually would also benefit cellulosic ethanol.
Mark Hirsch/Getty Images

"Critics are plentiful out there," says Andy Aden, a process engineer who studies cellulosic ethanol production at the National Renewable Energy Laboratory in Golden, Colo.

One concern is the expense of production. Presently, cellulosic ethanol costs more to produce by the gallon than corn ethanol, which is more expensive than gasoline. In the United States, the price of corn ethanol is lowered artificially, thanks to a $0.51 tax credit for the entities who blend it with gasoline, which would also help cellulosic ethanol. Critics point to what most cellulosic ethanol refineries can't do: produce enough ethanol to sell to gas stations.

Optimistic researchers say production costs will drop as the production steps are revised. The U.S. Department of Energy set a benchmark for cellulosic ethanol to be competitive with gasoline by 2012, at a production cost of $1.33 per gallon. "We're still trying to hit that," says Aden. Four companies, which plan to open commercial-scale refineries in the United States in the next four years, at least see profit in trying.

"If you're going to produce biofuel from biomass on a very large scale, where is the land going to come from to grow it?" asks Aden, summarizing another concern about cellulosic ethanol. Some groups envision switchgrass spreading over the country like a weed and charge that energy crops will compete with food crops. Food and cellulosic biomass can come from the same plant if corn is food and the stalks are cellulosic biomass. Using "urban green," or yard waste to make ethanol requires no new land. Even in a planting scenario, switchgrass can grow on plots of soil too small or degraded to support food crops, where it will restore nutrients, says Aden.

Other critics consider rainforests. If developed countries supplement oil use with cellulosic ethanol, they'll export demand for cellulose to developing countries that have big forests. Those countries would make the fastest money by clear-cutting. However, it's not necessary to hack fresh trees for cellulose when common activities like farming, managing for forest fires and paper milling produce cellulosic biomass that's otherwise treated as waste. Governments can also regulate if and how rainforests are farmed.

Low fuel mileage is also a downfall for ethanol. Even if it costs less at the pump, your car will travel 25 percent fewer miles on a tank of E85 than on gasoline, says Aden.

The benefits and drawbacks are worth weighing, but cellulosic ethanol has to clear technical and economic hurdles before it shows up at a gas pump near you.

Keep reading for more links to the future of green technology.

Related HowStuffWorks Articles


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