Heliophage


Power-station-fed algae
September 16, 2007, 2:29 pm
Filed under: Interventions in the carbon/climate crisis

They sound, I’ll admit, a little less appetising than corn-fed chicken. But they might be a source of fuel for some, and there was a short article about them in The Economist‘s Tech Quarterly last week. The idea is to take effluent CO2 from power stations and bubble it through some sort of bioreactor in which there are photosynthetic algae. CO2 enrichment makes the algae grow faster. You then harvest the algae and turn them into something useful — such as biodiesel (the oil yield can be up to 30% for some algae). The Economist story highlights two companies working on this, GS Cleantech and GreenFuel Technologies. There’s more on algal biofuel in general in a recent Popular Science article.

I remember getting into a discussion about this on Synthesis, Rob Carlson’s blog a little while back. (As the biological technology archive on Synthesis shows, Rob knows a lot about this sort of stuff and keeps insightfully abreast of new developments, which is probably why he is quoted in the Economist piece). Looking back on it, I now think the discussion was somewhat at cross purposes. I was trying to think of the algae in terms of carbon capture and storage — as a way of cleaning the power station’s waste stream. Others were seeing it as a way of making biofuels better. Algal bioreactor in closeupFossil-carbon based biofuels might be cheaper to produce than whole-plant biofuels, and as such could form part of the solution for some problems, such as energy security. They would also help a little with global warming, by displacing the use of fossil fuel oils.

But I don’t think The Economist is quite right in saying that “Using photosynthesis to capture exhaust gases from power plants could reduce the emissions produced by coal-fired stations.” The emissions stay the same — it’s just that an extra loop is put into the process, potentially making things more profitable. The original electricity generator takes out of the dug-up coal the energy that plants put into it in the past, oxidising the coal into carbon dioxide in the process. Then the algae reduce that CO2 to fuel by putting put some fresh sunlight back in. Then internal combustion engines in diesel vehicles reoxidise it. If the use of this biodiesel displaces the use of fossil diesel, there’s a net effect on global warming. But all of the carbon dioxide from the original electricity generator still gets into the atmosphere eventually. The advantages from the system are the same as those for any other biofuel — there’s no real decrease in the electricity associated emissions.

There might be a way of doing something similar, though, in a genuinely carbon neutral way. If the electricity generation is done with a small CHP plant fired by using biomass — say eucalyptus — it could simultaneously generate electricity, heat for domestic or industrial use and diesel for motor vehicles. The generating plant + algal bioreactors system becomes a way of making diesel out of a biomass that is efficiently grown but not good as a liquid fuel, while at the same time producing electricity. That might me quite a nice low/medium tech locally supported generation and transportation solution, if anyone wants such a thing. Of course you’d have to show that the diesel produced was more than enough to power the trucks bringing the wood to the generating plant to start making much of an impression, and there are other ways of turning wood into vehicle fuels, through pyrolysis and the like. But in a place with enough sunlight to grow serious amounts woody biomass and drive efficient algal growth in photobioreactors such a system might be in with a chance.

Image of algae in a photobioreactor by Dan Bihn, who probably reserves all rights

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