A new study suggests that overall the CO2 emissions attendant to producing biofuel from algae may be worse than those from corn, canola (rape-seed) or switch grass. The main problem is the use of carbon dioxide brought from elsewhere in "gas-bottles" and inputs of fertilizer, particularly nitrogen and phosphorus. According to a Life-cycle analysis, the land-based crops all were found to sequester more carbon than that incurred in growing them, while the contrary was true for growing algae, meaning that replacing fossil fuels by algal fuels could cause an overall increase in carbon emissions.
Not surprisingly, the report just published in the prestigious American Chemical Society journal, Environmental Science and Technology, has put the cat among the pigeons, since there are many companies gearing-up to produce algal biofuels. The US Algal Biomass Organisation has claimed that the study contained "faulty assumptions" and was based on "grossly outdated data".
Now, I am a fan of growing algae not the least of which because to do so means that far more fuel might be produced per unit area than is the case from the above mentioned land-based crops, as algae have a better photosynthetic yield; there is no need to use freshwater since algae grow well (even better) on saline waters or wastewaters, thus preserving an already endangered resource; you can put the tanks on any land (even deserts), so there is no need to compromise food-production in a competition over the same arable land to grow food-crops or fuel-crops; they might be used to clean CO2 from the smokestacks of power-stations fired from e.g. gas or coal; they might be used to clean wastewaters of nitrogen and phosphorus.
On closer inspection, the report is in fact very positive about growing algae, particularly in the latter two respects. Read positively, the data are only in opposition to making fuel from algae if nitrogen and phosphorus nutrients are added in their mineral forms, and if the CO2 has to be injected into the system (transported as a compressed gas) as made mainly by the process of steam reforming methane, along with most of the world's available hydrogen:
(Overall) CH4 + 2H2O --> CO2 + 4H2.
That H2 is used to make nitrogen (ammonium sulphate and nitrate) fertilizer by reacting it with N2 via the Haber Bosch process to make ammonia (NH3), and so there is in a way a symbiosis between the production of CO2 and NH3. The phosphorus would likely come from mining "rock phosphate", which requires energy too.
However, the figures in this "cradle to farm gate" analysis (i.e. they do not include the energy costs of processing the algae or other biomass into fuel per se) show that if the production of algae is combined with a wastewater treatment strategy, so that N and P are removed from it by the algae (an otherwise energy intensive procedure), and fed with CO2 from smokestacks, most of the environmental burdens attendant to growing algae are offset (i.e. an algae production plant, a power station and a sewage-works should all be placed in mutual proximity). Of three possible municipal wastewater effluents evaluated as a source of N and P, the most effective was source-separated urine with a very high content of these elements, in which case growing algae became more environmentally beneficial than the land-based crops.
Even if there remains some dispute over the exact figures used, what the study does highlight is the importance of developing an integrated paradigm of production and recycling for algal fuel production as I stressed before in the context of rare metals such as are required to maintain the electronics and solar power industries.
"Environmental Life Cycle Comparison of Algae to Other Bioenergy Feedstocks," By Andres F. Clarens, Eleazer P. Ressurreccion, Mark A. White and Lisa M. Colosi, Environ. Sci. Technol., 2010, 44, 1813.