Sustainability

Published on May 8th, 2008 | by greengrok

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Part 2: There Are Good and Bad Biofuels

cornstalksToday’s post is by Dr. Bill Chameides, dean of the Nicholas School of the Environment and lead author of the forthcoming blog The Green Grok. This is the second post in a 2-part series on biofuels.

Last week’s topic was why corn ethanol is an environmental loser.

But are all biofuels losers? No. Some can be winners. One of those is called cellulosic ethanol.

What Is Cellulosic Ethanol?

All ethanol — whether it is corn or cellulosic — is the same chemical compound: C2H5OH. You might recall from elementary chemistry courses that the “OH” group at the end of the formula indicates that the compound is an “alcohol.” Alcohols can have varying numbers of carbon atoms. Alcohol with two carbon atoms is called “ethanol.” The other alcohols are generally too toxic to be ingested, and thus ethanol has been the libation of choice down through the ages. (Ethanol used as fuel is rendered nonpotable.)

So corn ethanol and cellulosic ethanol don’t signify different types of ethanol, but rather the different material (or feedstocks) used to produce them.

Why Cellulosic Ethanol Can Be an Environmental Winner

Corn ethanol is produced from kernels — actually only a small part of the corn kernels — the sugars and starches. Herein lies one of the limitations of corn ethanol. You see, sugars and starches comprise a tiny fraction of the corn plant’s mass — about 2-15%. Because only a small fraction of a plant is used to make corn ethanol, the amount you can produce is limited.

Cellulosic ethanol is a different story. Most of the dry biomass — as much as 80% — is typically made up of cellulosic material — the stuff that makes the plant sturdy. So you can make a lot of ethanol using a plant’s cellulose instead of its sugars and starches. (By the way, even if the cellulosic material comes from corn, we still call it “cellulosic ethanol.” Corn ethanol is made solely from the sugars and starches of the corn kernel.)

The Major Advantage of Cellulosic Ethanol

Our guts are unable to digest cellulose, so we typically throw away that part of crops. A lot of it is left on the field or disposed of as agricultural waste. For corn, the cellulosic material includes the corn stover — the leaves and stalk — and the cob.

Remember what made corn ethanol such an environmental negative? A main reason is that it requires that land being used to grow food (or left as forests or grassland) be converted to growing an energy crop. And that leads to lots of global warming pollution.

This is not a problem for cellulosic ethanol — we can simply use the agricultural waste from food crops to make the ethanol and thereby avoid all those emissions.

Why We Can’t Fill Our Tanks With the Cellulosic Stuff … Yet

Unfortunately, right now, producing cellulosic ethanol on an industrial scale is too expensive. Unlike converting a plant’s sugars and starches to corn ethanol, making cellulosic ethanol requires that we first break down the cellulosic material. But because this material is what makes a plant sturdy, the atoms in these compounds are strongly bonded together and that makes them hard to break apart. The processes we have available today to do this are too expensive to make cellulosic ethanol commercially competitive.

But that will likely change. Scientists and engineers are working to make a commercially viable form of cellulosic ethanol. Some are developing new chemical processes; others are trying to genetically engineer new microbes that can “ferment” cellulose into ethanol like normal microbes that ferment sugars into ethanol. (The U.S.Department of Energy is helping fund six biorefineries.)

Cellulosic Ethanol Could Help Cut U.S. Global Warming Pollution

By my own estimates, agricultural and forest wastes could supply as much as 35 billion gallons of ethanol per year, saving up to 76 megatons of global warming emissions per year. (These results are somewhat larger than but consistent with other recent estimates (e.g., see Smith et al. 2004).) Such savings would cut a little less than 5% of all our heat-trapping pollution and about 15% of the emissions from the transportation sector.

By mid-century, cellulosic ethanol could supply as much as 86 billion gallons of ethanol, saving a little more than 180 megatons of global warming pollution per year — or almost 12% of America’s total global warming pollution and about 35% of the emissions from the transportation sector.

These are significant numbers. But to reach such levels we would need to grow bioenergy crops such as switch grass. Such cultivation, in turn, would require converting lands for this purpose, and that could raise some of the problems discussed in last week’s post.

The Bottom Line of Biofuels: There Are Winners and Losers

The saying “waste not, want not” applies to biofuels. The best biofuels are made from agricultural or forests wastes or from plants cultivated on degraded or marginal lands. The product from such feedstocks — cellulosic ethanol — is where we should be directing our entrepreneurial energies.

Dr. Bill ChameidesRead more about Dr. Bill Chameides, Dean of the Nicholas School of the Environment and Earth Sciences at Duke University.

See also:

CleanTechnica: Gene from Cow’s Stomach Engineered to Create More Affordable Biofuel

Gas 2.0: Mascoma Update — Cellulosic Ethanol Company Adds $10 Million From Marathon Oil

CleanTechnica: First Sustainable Ethanol to Mass Market?



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  • http://aguanomics.com/ David Zetland

    Nice post on the technology. Of course, we would be closer to cellulosic if the congress had not decided (after appropriate bribe giving) on corn as *the* input of choice. The best energy policy is one that is technology neutral. Tax carbon and then let the inventors find the cheapest way to solve the problem.

  • Todd

    I am no expert, but don’t all those leftover corn stalks and other farming “wastes” actually protect the soil from leaching of minerals (and chemicals) and also replenish the soil as they decompose?

    Yes, corn ethanol might be the worst biofuel, but burning any fuel (fossil, cellulosic or other) is not good for the reason mentioned above and because I envision a scenario where all the remaining grasslands and forests on Earth are turned into farms to grow fuel. No thanks.

    I think the mantra of the environmental community should be to push for *drastically* higher fuel economy standards.

    I know this article was just comparing the corn v. cellulosic, but efficiency should always be in the discussion.

  • William

    Corn cellulosic is used as silage for animal feed. How many other tons of animal feed did you include?
    How specifically did you calculate the gallons from waste? Specific numbers and sources of materials. Most of the numbers I have seen on switch grass require planting of marginal lands and using current crop lands?

  • http://nicholas.duke.edu/ Erica Rowell

    Dr. Chameides responds.

    Todd, you are absolutely right. Corn stover and other agricultural waste do serve a purpose. They help prevent erosion, and the products of their decomposition replenish the soil with much needed nutrients.

    In order for the production of cellulosic ethanol from agricultural waste to be sustainable, you have to be sure to leave some of the waste behind — studies suggest at least about 30%. Most estimates of how much ethanol can be produced from agricultural waste assume some non-negligible percentage of the waste is left on the soil.

    And you’re right about efficiency. It’s key.

    William,
    For methodology, check out DOE, USDA and NRDC reports. Ours was similar.

    Our first estimate was based just on waste. Anything used for silage is not waste.

    Our flat-out, maximum estimate for the future would use all ag byproducts available plus cultivation of switch grass grown on marginal land, which could have environmental consequences.

  • Uncle B

    I like the Algae method of producing bio-diesel, but no one will tell what nutrients are required, Why?
    I would like to see a fair, unbiased analysis of hemp and and its’ favorable characteristics for oil food and fiber production,now that modern labs can easily police illegal crops.
    Cellulosic ethanol looks good. Can cellulose other than corn be used?
    Large gas plants that use animal manure to produce methane seem good to me, we feed the corn, we eat the meat, we drive cars with the gas, we fertilize the fields with the sludge to grow corn again. If we can use some of the cornstalks from the fields to produce ethanol fuel that’s even better!
    Desert solar/electric power may cost a lot up front, but it is renewable, sustainable, reliable although intermittent, but we could charge our car batteries when it is available to level the load to the grid, not so hard to do!
    Could there be crops that would produce more cellulose quickly from poor land? A weed maybe? I see lots of room for development in the cellulose arena.
    All of the above mentioned methods and a few more I don’t know about, would diversify the fuel supply to prevent another monopolistic situation, something capitalism is not immune to. better legislation concerning monopolistic schemes would help democracy unhitch from exploitative capitalism.

  • http://www.enterprisingenergy.com/ scott

    I found this post very informative and interesting, I have not studied bio fuels as much as I would like to have done, so it is good to get a couple of clear and simple posts to help out!

    thanks!

  • Jonathan Hansen

    Currently I am doing a project for my science class for a grade and this is part of my project. We are very concerned about our environment so I honestly think that ethanol is the way to go because we can control it. if the ground is a major concern to some people all we have to do is have a controlled area even if it comes to growing for fuel. And if that becomes a problem we will get to that but while everyone argues abotu wether this is good or bad we our environment is going down the drain. look at the now while looking at the future.

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