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A peek into California's method of picking green fuels

Agriculture.com Staff 09/04/2015 @ 9:32am

Pick a number, any number.

At the risk of downplaying two years of hard work by the staff of the California Air Resources Board, that's essentially what the Board did last Thursday when it approved regulations that rank corn-based ethanol as slightly more polluting with greenhouse gases than gasoline. Eventually, if those numbers remain in California's new "low carbon fuel standard," corn-based ethanol from the Midwest could be locked out of a market for fuels in the state.

As board member Daniel Sperling pointed out just before the vote, the staff reached a major achievement just by managing to compare all types of fuel with the same measurement.

We measure electricity in kilowatts hours, for example, and gasoline and ethanol in Btus (British thermal units). CARB converted everything to megajoules, a measure of energy that equals about 948 Btus. And there are lots of greenhouse gases -- water vapor, carbon dioxide, methane, nitrous oxide, ozone. Whatever a fuel puts out when it's burned, CARB converted all of the emissions to the equivalent of carbon dioxide.

To rank the carbon footprint of fuels, CARB started with a "life cycle analysis" of the energy it takes to produce and use a fuel. Those are the direct emissions. In the case of gasoline, for example, CARB looked at how much greenhouse gas was released in pumping and extracting crude oil, in refining gasoline and transporting it to gas stations, and how much is released when burned in vehicles. That life cycle analysis gave California gasoline a "carbon intensity value" of just under 96 grams of carbon dioxide equivalent per megajoule of energy.

The life cycle analysis for ethanol was similar, measuring greenhouse gases associated with growing corn, from fertilizer, fuel and other inputs. Then emissions from biorefineries were measured. Finally, tailpipe emissions were included. The average for Midwest corn-based ethanol was about 69 grams of CO2 per megajoule, well under gasoline.

That's all good science that generated little controversy.

But in the case of ethanol, and no other fuels, CARB's staff tried to estimate the indirect effects of carbon dioxide that's released when new cropland is brought into production somewhere else to offset acres of corn grown for ethanol.

CARB used the Global Trade Analysis Program (GTAP) developed at Purdue University to project the effects of increased corn production for ethanol on land use worldwide. Because some tropical rain forest and grasslands are expected to be cleared to grow crops to make up for lost food production, the staff at the Air Resources Board concluded that ethanol adds another 30 points to its carbon intensity value. Add those 30 points to ethanol's carbon intensity rating of 69 and it winds up a little higher than gasoline, at more than 99 grams of carbon dioxide equivalent for every megajoule of energy in the fuel. (A megajoule is about 948 British thermal units, or roughly the energy from just over a tenth of a gallon of ethanol).

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