Photo: NREL.

Biodiesel — the cleaner-burning vegetable-based oil that can be substituted for ordinary petroleum diesel — is getting a lot of press these days. That’s not too surprising: alternatives to oil tend to get a lot of attention when fuel prices are rising, which they’re certainly doing right now.

Perhaps the biggest piece of recent policy news is Washington state’s new renewable fuels standard, passed just last month, which mandates that 2 percent of the diesel sold in the state must be biodiesel by the end of 2008.

That got me thinking — why just 2 percent? Couldn’t we do better than that?

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Well, maybe so. But perhaps not by a whole lot.

The problem is really one of scale: We use an awful, awful lot of petroleum in the Northwest. Huge amounts. So much, in fact, that there’s just not enough cropland for biodiesel to make much of a dent in our petroleum habit.

The average Washington resident uses about 10 gallons of petroleum-based highway fuels each week. There are more than 6 million of us, so all together we burn about about 3.2 billion gallons of gasoline and diesel each year. And that doesn’t include the petroleum we use for jet fuel, chemical feedstocks, road oil, asphalt, boats, etc.

How much of that demand could biodiesel meet? Between 2003 and 2004, national production (PDF) of biodiesel tripled, from about 25 million gallons to some 75 million gallons, largely because of a $1 per gallon federal tax credit. So the entire annual national output of biodiesel could have fueled the cars and trucks of Washington state for … wait for it … about 9 days.

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Obviously, current biodiesel production is far lower than its potential. Still, the most optimistic estimate I’ve seen is that Washington state farmers could produce about 100 million gallons of canola-based biodiesel each year, or about 3 times as much as the renewable fuel standard would mandate. That much biodiesel could offset about 11 days worth of highway fuel use. And producing that much canola oil would take about 880,000 acres — nearly one-fifth of the state’s harvested cropland (PDF).

So clearly, as great as biodiesel is, it’s just a small piece of a bigger puzzle.

Just to be clear: I’m not gunning for biodiesel here. I’m not a hater. I think biodiesel has clear global-warming benefits, and just as importantly can help clean up truck and bus exhaust in center cities.

And while I do think there are some concerns about using food crops to power rush hour — a habit that may have some counter-productive effects over the long term — experimentation with this sort of thing is nifty. And I’m all for R&D for even more productive biodiesel crops (algae, anyone?).

But mostly the biodiesel numbers reinforce my belief that there’s no silver bullet that will solve our petroleum dependence. It’s going to take a whole lot of strategies developed in parallel.

We should forge ahead with biodiesel — but as we do so, we should be sure to keep the real scale of the task in mind.