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Biofuel: some numbers

What's the most energy-efficient crop source for ethanol?

Posted by David Roberts at 12:14 PM on 07 Feb 2006

Read more about: energy | biofuels | ethanol

Biofuel is the hot topic lately in the green blogosphere. There's legitimate dispute about the political and environmental wisdom of plant-based fuels, but at the very least everyone should be starting from a valid, shared set of numbers (oh, to dream).

In an attempt to offer up such numbers, I'm going to ... rip off somebody smarter than me. Namely, Lester Brown, founder of the Worldwatch Institute, founder of the Earth Policy Institute, and author of the recently released Plan B 2.0, which is the best big-picture summary of our environmental situation I've ever read (and I'm only 2/3 through it!). The entire thing can be downloaded for free from EPI's site.

There are two key indicators when evaluating various crops for biofuel: fuel yield per acre and net energy yield of the biofuel, minus energy used in production and refining. This table (taken from Chapter 2) compares crops based on the first indicator:

Ethanol and Biodiesel Yield per Acre from Selected Crops
Fuel Crop Fuel Yield (gallons)
  Sugar beet (France) 714
  Sugarcane (Brazil) 662
  Cassava (Nigeria) 410
  Sweet Sorghum (India) 374
  Corn (U.S.) 354
  Wheat (France) 277
  Oil palm 508
  Coconut 230
  Rapeseed 102
  Peanut 90
  Sunflower 82
  Soybean 56 (author's estimate)

Note: Crop yields can vary widely. Ethanol yields given are from optimal growing regions. Biodiesel yield estimates are conservative. The energy content of ethanol is about 67 percent that of gasoline. The energy content of biodiesel is about 90 percent that of petroleum diesel.

Here's what the book has to say about the second indicator, net energy yield:

For net energy yield, ethanol from sugarcane in Brazil is in a class all by itself, yielding over 8 units of energy for each unit invested in cane production and ethanol distillation. Once the sugary syrup is removed from the cane, the fibrous remainder, bagasse, is burned to provide the heat needed for distillation, eliminating the need for an additional external energy source. This helps explain why Brazil can produce cane-based ethanol for 60¢ per gallon.

Ethanol from sugar beets in France comes in at 1.9 energy units for each unit of invested energy. Among the three principal feedstocks now used for ethanol production, U.S. corn-based ethanol, which relies largely on natural gas for distillation energy, comes in a distant third in net energy efficiency, yielding only 1.5 units of energy for each energy unit used.

Another perhaps more promising option for producing ethanol is to use enzymes to break down cellulosic materials, such as switchgrass, a vigorously growing perennial grass, or fast-growing trees, such as hybrid poplars. Ethanol is now being produced from cellulose in a small demonstration plant in Canada. If switchgrass turns out to be an economic source of ethanol, as some analysts think it may, it will be a major breakthrough, since it can be grown on land that is highly erodible or otherwise not suitable for annual crops. In a competitive world market for crop-based ethanol, the future belongs to sugarcane and switchgrass.

The ethanol yield per acre for switchgrass is calculated at 1,150 gallons, higher even than for sugarcane. The net energy yield, however, is roughly 4, far above the 1.5 for corn but less than the 8 for sugarcane.

For the most part, I'll leave readers to do what they will with these numbers. But one thing seems quite clear: Corn-based ethanol is a friggin' boondoggle. It's just about the worst source for ethanol, requiring enormous acreage and producing very little energy relative to energy inputs. The recent enthusiasm for ethanol among the powers-that-be in the U.S. has much more to do with massive corporate subsidies than any genuine interest in a sustainable energy future. I'm guessing Bush's talk about switchgrass, etc. is largely to provide cover for these subsidies.

Soy biodiesel not a long-term solution either

For the same reasons you cite corn ethanol as a boondoggle, soybean based biodiesel is as well.  Now, I drive a car that runs on soy biodiesel and it isn't really a problem with biodiesel making up less than 1% of the diesel fuel market. But if the growth trend continues for both ethanol and biodiesel, switchgrass and algae will be the only real economic and environmentally sustainable solutions.  

As a supplement to your article, algal biodiesel isn't commercially available but also shows promise of providing from 10-15K gallons per acre.  A couple pilot projects are underway:

Toronto Star: Algae farms make case for Kyoto Turning Emissions Into Fuel With Algae
USA Today: Algae - like a breath mint for smokestacks


What is the yield per acre of hemp, and why is it not incluced in this study?

The Pecking Order

Guess the feedstock pecking order for biodiesel & ethanol are as follows:

Biodiesel, the best feedstock appear to be algae (assuming the damn process works that is), jatropja and allied waste crops, palm oil, coconut, rapeseed, and all the is simply politically sponsored feedstock (guess America is big in corn)

For ethanol, I think sugarcane is probably the best feedstock, and corn possibly the worst

Just some thoughts

If you folks have time, see also The Biodiesel WWW Encyclopedia for some more info on all these

Narsi, BPO

Solar Power with electric vehicles is way better

If we cover an acre roof top with solar electric panels here in cloudy Wisconsin it will generate 300,000 kWh per year.  Put that directly into an electric vehicle that gets 3 miles per kWh... and it will go 900,000 miles.

No land is needed, no water, no tilling, no soil loss, no farmer, no biorefinery, no transporation.. and the car's battery is the energy storage device.

We are barking up the wrong tree!



Um, these numbers say nothing about relative production costs. (And Journey to Forever has a longer list of oilseed yields.)

One of the reasons (besides subsidies) that soy biodiesel exists is that the oil was initially a by-product of the much more abundant and valuable meal. And soy is what farmers in the Midwest are accustomed to growing in rotation with corn.

Lester Brown seems to love sugar beets. But he clearly has not studied its economics, which are poor. The industry survives here in Europe only thanks to government support. (Unlike cane, for example, there is no bagasse that can be used to co-generate electricity.) A couple of years ago, the USDA's Economic Research Service looked once again at sugar-based ethanol in the United States, and declared it uneconomic (even with the $0.51/gallon blenders' credit).

And don't forget, sugar beets are a row crop, which means they exert a heavy toll on the soil. A lot of soil is displaced when it is harvested, also. Here in France, the highway authorities have to erect signs out in the northern French countryside every autumn, warning motorists to watch out for slippery patches of mud (and squashed beets) that cover the road in places.

Finally, the net (non-fossil) energy yield for switchgrass -- still only produced in pilot-scale plants -- may be much higher than for corn, and half as good as sugarcane, but the process is much more capital-intensive, which drives up its unit costs considerably.

These are only my personal opinions.

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