Carbon Intensity of Algae Biofuels

Carbon Intensity of Algae Biofuels

Written by Green Chip Stocks
Posted August 5, 2009 at 3:00PM

Sometimes the content on Green Chip Stocks is driven by what our readers are asking. That's the case with today's blog.

I recently received the following question from reader identifying himself only as Dwayne:

I just read your article on growing algae to produce bio-diesel. I was wondering about the relative amounts of CO2 input/output, i.e. has anyone done the math to figure out how much CO2 algae take in to produce a gallon of diesel fuel, compared to the amount of CO2 produced when that same gallon is burned. If the amount released from burning is equal to or less than the amount algae consume, then this is a win/win situation. If the amount released by burning is greater than the amount consumed, then we have a problem.

Perhaps you could address this in another article?

Thank you.

At first glance this can seem like a complex question. Honestly, it took me a few seconds to wrap my head around it.

In reality, the answer is very simple and, in my opinion, further strengthens the case for mass-adoption of algae biofuels.

With all biofuels, the amount of carbon dioxide released when burned is equal to the amount of carbon dioxide consumed when the feedstock was grown.

That means for corn-based ethanol, the amount of CO2 released when one gallon is burned is about the same amount of carbon absorbed by the plants that grew the corn to make that one gallon.

Verbose, I know.

For algae, it takes about one ton of CO2 to make two tons of algal material. It's difficult to say how much biodiesel or ethanol two tons would make because different companies are achieving different results.

Taking average conversion numbers, about 190 gallons of fuel (both biodiesel and ethanol) can be made from those two tons of algae.

When burned, those 190 gallons will emit about 10.5 pounds of CO2 each. This is better than the 20 pounds that comes from each gallon of gas burned in an internal combustion engine.

So the short answer is: algae biofuel, when burned, releases the same amount of CO2 that was used to grow it, which is still less than burning gas. So it's a winner already.

Enter Complexity

But because the CO2 used to grow algae can come right from power plant combustion, that 10.5 pounds of carbon is the result of more than just the liquid fuel being burned, greatly lowering the carbon intensity.

If you have an algae cultivation system attached directly to a coal-burning power plant, emissions can be reduced by some 40%. So the resultant emissions when the algae biofuel is burned come not only from powering the car, but also from burning coal to produce hundreds of thousand of kilowatt-hours.

This effectively makes algae a carbon sink.

Of course, the exact numbers depend on numerous factors, such as the conversion rate of algae to fuel and exactly how much electricity was produced relative to the CO2 absorbed by the algae.

No matter the exact carbon intensity for algae biofuels, their ability to absorb CO2 from other energy production operations makes it one of the best to date. No other feedstock can absorb more carbon dioxide from external sources during the growth cycle.

Hope this answers the question,


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