There has been a great deal written about co-locating algae-to-biofuels projects with fossil- fuel power plants.  There is some logic to this; many power plants are seeking ways to sequester the CO2 they produce, and algae require CO2 to grow.  But things are rarely so simple.  It is important to understand that CO2 captured by algae, which are later converted into fuel and combusted, is recycled but is not sequestered.  While there is certainly value in capturing and recycling the CO2 (using it twice, in this case), two very important regulatory issues need to be addressed: (1) the regulatory requirements for power plants and other stationary sources are governed by the Clean Air Act, and are based on point-source emissions from high elevations. The use of flue gas to cultivate algae will involve non-point source emissions at ground level, which may require new regulatory policies, and (2) it is unclear what the value is of CO2 capture by algae in any carbon-credit or cap and trade framework, because the carbon may ultimately be reused and re-released to the atmosphere when algal-derived fuels are used for transportation.

Anyone contemplating an algae-to-biofuels project should also understand that there are practical limits to the amount of CO2 that can be captured by algae alone.  These are due primarily to the facts that (1) algae metabolize CO2 only during daylight hours while most large industries generate CO2 twenty-four hours a day and (2) the size of the bioreactor (and thus the amount of algae that can be grown) may be constrained by available land, heat, nutrients or water.

Frankly stated, algae can never sequester 100% of the CO2 generated by a power plant or heavy industry, although they can play a significant role.  The total weight of algae that can be supported by a given amount of CO2 is dependent on a number of factors, including the average daily sunshine, the CO2 utilization factor for a given algae species (always less than 100%) and the mass conversion ratio for the algae species (i.e., the ratio of weight of algae to weight of CO2 consumed).

We previously looked at the heat available from the Lake Hubbard power plant in Dallas for our hypothetical project.  Let’s continue with that example and calculate the amount of algae that the Lake Hubbard CO2 emissions could support:

In 2007, the Lake Hubbard facility emitted around 418,000 tons of CO2.  Taking into account the average length of day in the Dallas area, as well as published average utilization factors and mass conversion ratios, Bebout and Associates' proprietary model estimates that this will support as much as 57,000 tons of algae per year.  This represents the capture of less than 30% of the available CO2. [Recent publications have touted new algae varieties that can metabolize nearly twice as much CO2 as previously known species, so the 30% figure could theoretically be somewhat higher.]

The next question becomes, what is the total acreage of ponds that can be supported?  The answer depends on the growth rate of the algae being grown.  If we assume a maximum daily productivity of around 50g/m2/day (at the high end of growth data reported in the literature), it would take more than 1,000 acres of ponds to grow 57,000 tons of algae/year, and this to capture just under 30% of the available CO2.  Clearly, CO2 is not a limiting factor to our project.  However, project proponents should exercise some prudence when extolling the project’s ability to significantly “sequester” CO2 emissions.