CO2 Sequestration by Algae Reactors

Considerable hype surrounds the potential to reduce greenhouse gas emissions by using the carbon dioxide in stack gases to produce algae.  Such schemes would remove the CO2 from stack gases, bubble these gases through ponds or tubes exposed to sunlight where the algae would grow by familiar photosynthesis.  All of us have seen ponds clogged with algae during the summer and can easily imagine this being literally a ‘green’ solution that would allow us to continue burning fossil fuels.  Unfortunately, getting from here to there is not that easy.

Renewable energy is catchall term for solar energy collection and conversion to useful heat and work.  Most of us are familiar with conventional forms of renewable energy: Solar, wind, ethanol, biodiesel, wood, or hydrogen.  CO2 to algae is another such form; that is, sunlight is used to reduce CO2 to carbon, thereby allowing the re-oxidation of the carbon by combustion to CO2, then potentially recovering the CO2 by the same algae production, yielding an endless chain of combustion and recovery with no net CO2 impact.  There is no evidence at this time that such a scheme is not technically possible, but it is important to look at the fundamentals of the CO2 to algae process on the scale of the Holcomb plant to better understand how it fits into the pool of renewable energy methods from which our future will draw to maintain our way of life.

Since all renewable forms of energy come from sunlight, an important parameter is the conversion efficiency from solar radiation to heat or work of different forms of renewable energy.  Solar cells (photovoltaics) can have efficiencies from 10 to 30 percent.  Thermal collectors that heat a fluid, such as hot water heaters, have efficiencies of 50 to 80%.   Current research in photovoltaics looks toward reducing the manufacturing cost and improving efficiencies.  Biomass (ethanol, biodiesel, wood) has an average conversion efficiency of 0.1%.  Agricultural crops are in the range of 1 to 2%.  Due to the low efficiency, biomass requires long growing seasons and large crop growing areas to generate the quantities of energy needed to be economic or significant.  Sugarcane production in Brazil has an efficiency of about 8% and for this reason is an economic form of CO2 mitigation in that region.  Hydrogen as a renewable resource is produced electrolytic ally by taking electricity from a renewable generator (wind turbine, photovoltaic cell) and converting water to hydrogen and oxygen with about a 50% efficiency (overall the efficiency is 50% of the efficiency of the source of electricity, so it is 5 to 15% overall).

The strong interest in CO2 to algae results from it alleged photosynthesis conversion efficiency of as much as 12%.  This 120 times the average of all biomass; hence, the stampede of interest, research capital and press coverage.  However, the only long term study conducted by NREL of algae farms resulted in an average efficiency of about 1.3%. The theoretical maximum conversion efficiency of biomass is 12%, so the press releases purporting to achieve efficiencies at the maximum physical limit must be viewed with skepticism.  The ‘bioreactor’ technology hyped as a method of cleaning the Holcomb plant stack gases has a probable efficiency of 6%.  Translating this into surface area required for the bioreactors yields a required absorption area of around 66 square miles.  Two additional sources were found that provide data that allow the calculation of absorption area, and they both yielded similar results.

It has been estimated that the capital cost of algae bioreactors is approximately $190 per square meter.  Hence, the capital cost of CO2 sequestration by algae for one 700-megawatt plant is $31 billion.  One source projected that biodiesel would have to sell for over $800 per barrel for such a plant to be economic.  No wonder the proponents of the Holcomb plant will not include commitments to construct the algae reactors in their permit applications.

One might be led to believe that there is justification for further research and that such research might lead to an economically justified project.  Unfortunately, this is not the case.  There is no reasonable scenario that would cause either the capital cost or the efficiency of CO2 mitigation to significantly improve. Any suggestion that the CO2 to algae to biodiesel process is close to commercial implementation must be vigorously challenged as erroneous and misleading.  Further, to say that the technology is unproven is to overstate the case: the process is not feasible, and any claims to the contrary can be disproved.

The legislative supporters of the algae to biodiesel solution for the Holcomb plant greenhouse gas emissions can be faulted for not doing the necessary homework to investigate the claims of the process sponsors, but were most likely well intentioned in their desire to find a reasonable compromise that would allow construction of the plant while minimizing the health and environmental damages caused by the enormous release of CO2.  It is less clear how the Sunflower Group, that had extensive engineering and technical resources at its disposal, could have been duped so badly.

The enormity of the CO2 mitigation facility and its cost is a vivid example of the scale of the problem caused by the stack gases from a 700-megawatt facility.  This is a compelling reason that the Sierra Club fiercely opposes any new expansion of coal power plants.   Whatever sequestration or mitigation technology is proposed is going to be massive, expensive and require enormous resources that may further compound the problem of climate change, not necessarily improve it.

Here are the two additional sources for the calculation of area required for the bioreactors.


Calling it unproved technology understates its problems

By Tim C. Liebert, P.E. Chemical Engineering

ExCom member, Kanza Group


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