Saturday, May 20, 2006

Is Algal Biodiesel a Red Herring?

OK, just one more biodiesel post (it's not a biodiesel blog, I swear!)

Looking into all these issues around sustainable biodiesel, I wandered over to the Green Fuel Technologies website.

What I found made me say -- duh!

So...we know that it makes all sorts of sense to grow algae at powerplants - it cleans up emissions and captures CO2. Biodiesel people mostly assume that this oil-rich algae will be turned into biodiesel.

But no. It looks to like it is much easier to just harvest the algae, dry it out, and co-fire the plant with it...really easy with a coal plant; a bit more complicated with natural gas, but you can gasify algae and send that right on down into the turbine. Plus, the plants will get to take the credit for putting renewables on the grid, because biomass cofiring counts towards RPS requirements.

This is as opposed to selling the algae to someone who will transport it somewhere, extract the oil, and turn it into biodiesel - in which case they get no RPS credit.

So will these plants ever make more algae than they need and sell it into the biodiesel market? I did the math so you won't have to.

If the plant was burning 100% algae, it would in essence be a solar power plant.

NREL says you can get 1 quad (10^15 BTUs or 293x10^9 kWh) of biodiesel annually for every 780 square miles of land (hat tip to Mike Briggs). How much energy there is in the algae itself is not easy for me to find; to get biodiesel from algae you crush it (losing some biomass), you add some energy in the form of methanol and process heat and take some energy out in the form of glycerin.

But for hand-waving, let's use the NREL number and call that the energy yield for algae instead of biodiesel. This implies a production of 375 x 10^6 kWh per square mile.

A "typical" 400 MW combined cycle plant running at 80% capacity factor, 50% efficiency and a gasifier operating at 80% efficiency would use 7 x 10^9 kWh in fuel energy annually. It would require nearly 19 square miles of land area to grow that much algae. This is basically 30 acres per MW. For contrast, single axis PV tracking systems require about 6 acres / MW (here's an example). Now my numbers are probably +/- 50% because I use biodiesel yields a a proxy for algae yields. However you slice it, though, it's still a lot of space!

Bottom line, it is very unlikely that any power plant would grow any more than a fraction of it's fuel needs on site; and it certainly seems to make more sense to me that this algae would go right in as biomass fuel rather than being turned into biodiesel.

Now in the big picture, using algae as fuel in power plants is really cool, and is probably equally beneficial as turning it into biodiesel.

However, biodiesel advocates shouldn't pin their hopes on power plant grown algae. At least if my math is right and I'm not missing something.

7 comments:

R. Ari'el Simon said...

Hello,

I completed a research project on biofuels as a senior at a high school in the Sierra Foothills. A large portion of my paper focused on the potential of Diatomic Algal Biodiesel for at least replacement of US transportation diesel usage. Since the algae is not used for food, and can be grown in fallow lands my understanding is that is a solution to the price of food. I have not done much reading on the subject since then but am wondering what the feasibility of this application is.

Carl Lenox said...

Well, there's certainly been a lot of work on algal biodiesel by NREL and others, and more recently, some interesting work done on growing algae on powerplant sites where the CO2 rich exhaust enhances yields. That's what this post was originally referring to. There hasn't been much done as far as commercializing it at any scale, as it seems like it is difficult to get the funding required to do so.

Anonymous said...

Are your land use figures specific for algae? One of the benefits to it is that it can be grown in a vat, making it more space efficient than growing soy or other traditional biofuel crops.

Carl Lenox said...

Yes, the figures are specific to yields for algae. Growing enough algae to make any significant quantity of fuel cannot happen in a vat, it will take large outdoor spaces to do this (open ponds or "racetrack" greenhouses).

Anonymous said...

You don't grow algae on a pond or in a field- you grow it in vertical "tubes"... its like highrise apartments for plants.

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