This is a guest post by Sophie.
In 2009, at least 7994703000 tons (~8 billion tons) of coal were used globally (EIAa), with a resulting 13393577 tons of CO2 released as a result (EIAb) – and that’s not counting countries with no statistics. This makes coal the dirtiest energy source available.
A coal power plant doesn’t just release electricity and CO2. One of the raw materials needed for its functioning is water, and the wastewater is released back into the environment. Out of the smoke chimney comes a mixture of ash and gases collectively referred to as flue gas.
Flue gas is desulphurised by law in almost every country, so sulphates (causes of acid rain) are found only in the ppm fraction, along with nitrates. Oxygen and water are present in larger amounts, making up on average 7% and 10% (respectively) of the gas’s composition. The amount of carbon dioxide is usually between 12 and 15% (there is also some carbon monoxide), with the rest of the gas being elemental nitrogen (Chakma et al., 1995). The precise compositions vary depending on the provenance of the coal.
A look at the above compositions instills a sense of nausea: flue gas is toxic to us and other macroorganisms, yet we’re releasing millions of tons of it into the atmosphere routinely every year. But for some microalgae, that mixture is a paradise.
As demonstrated by the above article from the February 1994 issue of Popular Science, the idea of using microalgae for cleaning human mess is not new. We already looked at their potential in taking advantage of our useless dirty water (such as the wastewater from a coal plant!), but another area of research is their use for minimising the dirty components of flue gas, especially CO2.
Some microalgae thrive in areas of high CO2, with concentration of nitrates and sulphates, and in temperatures as high as 80°C – and this is without any genetic modification. There is an easy way to use these microalgae to reduce emissions from coal plants: build a microalgal bioreactor.
Flue gas comes out of the chimney at temperatures exceeding 150°C, but cooling it before it gets there is not a problem – all it needs is to be guided through a water tank (and the resulting sludge can be treated in the same way as other wastewater), and water is already pumped into a coal plant anyway.
The chilled gas can then be piped through a bioreactor containing microalgae and water. The microalgae will thrive on the sulphates and nitrates and a bloom will develop (I have done these experiments on my cultures). Even with natural light, the microalgae will use up CO2 in photosynthesis, and with artificial lighting, they will keep growing, all the while fixing the CO2 and preventing it from going out to the atmosphere.
The candidate microalgae deemed suitable for this purpose are members of the genus Spirulina. Spirulina spp. have long been known as useful microalgae (they were used by the Aztecs for food!) and they are the most cultivated microalgae today at 3000 tons/year (Pulz & Gross, 2004), so we know how to grow them. Relevant for our use is their love of sulphidic habitats (Anagnostidis & Gloubić, 1966) where they outcompete other microalgae, but they are also found in ponds and lakes. They tolerate up to 18% CO2 (de Morais & Costa, 2007), so higher than any flue gas composition. They also grow in areas of high pH (Gimmler & Degenhardt, 2001), as would be expected in a flue-gas-bioreactor. These characteristics make them ideally-suited at least for the research stage.
Microalgae can be cultivated in different ways. In extensive shallow (no more than 50cm) ponds would be the way to go for treating the coal plant’s effluent. But for flue gas treatment, the ideal reactor type would be a tubular reactor. Like the internet, this is nothing more than a series of tubes, typically stacked on top of each other (but arrangements vary).
These are ideal for use in a coal plant because they don’t need much water – unlike a plate reactor (an aquarium, basically) which has a lot of wasted space for water, a tubular reactor can have algae growing everywhere in the tube with no free space, meaning a higher abundance and more carbon captured overall. They can be built on top of the plant to take advantage of sunlight, and LED lights are enough to keep the growth going at night. They are also very space-efficient, as they can be made to be twisted to increase the surface area to volume ratio.
The only serious drawback is the cost – but desulphurisation was also expensive, and coal plants were forced to install the filters anyway.
As I said, Spirulina are the most popular microalgae, grown for all sorts of products and uses. Health food is a large market – they are used to make antioxidant pills and vitamins (Colla et al., 2007). For industry, they are used to produce hydrogen gas (Aoyama et al., 1997) and PHB (Jau et al., 2005); of course, let’s not forget biofuels. Growing them on coal plants would thus serve a double purpose: make coal a cleaner energy source (although it does nothing to help the damage and destruction from mining) and it uses otherwise useless space to make useful products [Marc's Note: I'm skeptical of the health food stuff, but you get the point].
Anagnostidis K & Golubić S. 1966. Über die Ökologie einiger Spirulina-Arten. Nova Hedwigia 11, 309-333.
Aoyama K, Uemura I, Miyake J & Asada Y. 1997. Fermentative metabolism to produce hydrogen gas and organic compounds in a cyanobacterium, Spirulina platensis. Journal of Fermentation and Bioengineering 83, 17-20.
Chakma A, Mehrotra AK & Nielsen B. 1995. Comparison of chemical solvents for mitigating CO2 emissions from coal-fired power plants. Heat Recovery Systems and CHP 15, 231-240.
Colla LM, Reinehr CO, Reichert C & Costa JAV. 2007. Production of biomass and nutraceutical compounds by Spirulina platensis under different temperature and nitrogen regimes. Bioresource Technology 98, 1489-1493.
de Morais MG & Costa JAV. 2007. Carbon dioxide fixation by Chlorella kessleri, C. vulgaris, Scenedesmus obliquus and Spirulina sp. cultivated in flasks and vertical tubular photobioreactors. Biotechnology Letters 29, 1349-1352.
EIAa. International Energy Statistics: Total Coal Consumption. Accessed on 11.01.2012.
EIAb. International Energy Statistics: CO2 Emissions from the Consumption of Coal. Accessed on 11.01.2012.
Gimmler H & Degenhardt B. 2001. Alkaliphilic and Alkali Tolerant Algae. In: Rai LC & Gaur JP (eds.). Algal Adaptation to Environmental Stresses: Physiological, Biochemical and Molecular Mechanisms.
Jau M-H, Yew S-P, Toh PSY, Cho ASC, Chu W-L, Phang S-M, Najimudin N & Sudesh K. 2005. Biosynthesis and mobilization of poly(3-hydroxybutyrate) [P(3HB)] by Spirulina platensis. International Journal of Biological Macromolecules 36, 144-151.
Pulz O & Gross W. 2004. Valuable products from biotechnology of microalgae. Applied Microbiology and Biotechnology 65, 635-648.