Q & A - Algae based CO2 Capture
1) Is there a possibility of heavy metal contamination in algae due to their presence in the flue gases?
Yes. The possibility exists because most of the absorption of toxic metals by algae is of the passive type.
2) How do the constituents other than CO2 in flue gas from power plants affect algal growth?
Sulfur oxides, particularly SO2, can have a significant effect on the growth rates and health of the microalgae. Of greatest concern is the effect SO2 has on the pH of the microalgal growth medium. When the SO2 concentration reaches 400 ppm, the pH of the medium can become lower than 4 in less than a day, which significantly affects the productivity of the microalgae. However, if the pH is maintained at 8 using NaOH, the productivity does not decrease (Matsumoto et al., 1997). Other researchers have demonstrated tolerances to sulfur oxides at approximately half of what Matsumoto and coworkers (1997) demonstrated (Brown, 1996; Zeiler et al., 1995). Nannochloris sp. (NANNO02) was found to be resistant to 50 ppm SO2, but without pH control, 300 ppm SO2 inhibited growth within 20 hours (Negoro et al., 1991).
Nitrogen oxides also comprise a significant portion of power plant flue gas. As with the sulfur oxides, nitrogen oxides can affect the pH of the algal medium, but to a lesser degree. Microalgae have been shown to tolerate and grow in a medium containing 240 ppm NOx, with pH adjustment. Brown (1996) and Zeiler and coworkers (1995) also demonstrated that microalgae are not growth inhibited by the presence of 150 ppm NOx. Negoro and coworkers (1991) found that NANNO02 grew in the presence of 300 ppm NOx after a considerable lag time. A point of interest is that the nitrogen oxides can serve as a nitrogen source for the microalgae. NOx is absorbed into the medium and oxidized into NO2 in the presence of oxygen (Negoro et al., 1991). The greater the oxygen contents of the medium, the greater the NO2 production and microalgal productivity rates (Matsumoto et al., 1997; Brown, 1996). However, the presence of elevated concentrations of oxygen results in algal photorespiration, which inhibits microalgal growth.
The effect of soot dust and ash containing heavy metals has received limited attention. Matsumoto and coworkers (1997) confirmed that when soot dust concentration is greater than 200,000 mg/m3 (0.2 g/L), algal productivity is influenced. It is rare for the soot dust concentration to reach such an elevated value since it is most commonly on the order of 50 mg/m3 (5 × 10!5 g/L). The same argument can be applied to the presence of trace heavy metals. Higher concentrations can affect algal productivity, but only in rare situations will the concentrations exceed those that will result in a significant impact.
3) Will NOx present in the flue gas serve as a nutrient, in addition to the CO2?
Flue gas supplies the carbon dioxide and has the ability to supply some of the nitrogen (from nitrogen oxide; it absorbs SO2 as well). However, it has been demonstrated that the nitrogen contribution from the flue gas is insufficient to maintain stable growth rates (Weissman and Tillett, 1992). Therefore, nitrogen (along with phosphorus and trace nutrients) needs to be added and maintained as necessary with proper engineering.
4) Can algae withstand the high temperatures in the flue gases?
In a commercial application, flue gas from the desulfurization scrubbers would be sent to the CO2 sequestration ponds for treatment. Temperatures exiting the scrubbers at many coal power stations are 140°F (60°C) and above – this could reach even upwards of 100oC. Although most organisms cannot survive at these higher temperatures, some cyanophycean algae have been shown to grow at 176°F (80°C).
Since the temperature of waste gas from thermal power stations is high, the use of thermophilic, or high temperature tolerant species are also being considered (Bayless et al., 2001). Themophiles can grow in temperature ranging from 42-100oC. An obvious advantage of the use of thermophilies for CO2 sequestration is reduced cooling costs. In addition, some thermophiles produce unique secondary metabolites (Edwards, 1990), which may reduce overall costs for CO2 sequestration. A disadvantage is the increased loss of water due to evaporation. Cyanidium caldarium, which can grow under pure CO2 is a thermophilic species (Seckbach et al., 1971). Miyairi (1995) examined the growth characteristics of Synechococcus elongatus under high CO2 concentrations. The upper limit of CO2, concentration and growth temperature for the species was 60% CO2 and 60ºC (Miyairi, 1995). Currently, an unidentified thermophilic species isolated from Yellowstone National Park has been examined by the group of researchers supported by the U.S. Department of Energy. Although less tolerant than thermophiles, some mesophiles can still be productive under relatively high temperature (Edwards, 1990). Such species also can be candidate species for the direct use of flue injection.
5) What is amount of CO2 required for algae growth?
It is estimated that approximately 2 T of CO2 will be required to produce one T of algal biomass.
- Using Closed Photobioreactors for Algae-based CO2 Capture (James Sears, 2007)
- Biodiesel Production from Algae with high carbon dioxide utilization (Hazlebeck et al., 2010)
- Coupling Waste water treatment and CO2 capture using high carbon-dioxide tolerant Algae Species: Chlorella( Hu et.al, 2010)
Peer reviewed research articles
- Biological approach using solar energy to capture CO2 while; producing H2 and high value products from algae (Skjånes et al., 2007) - Read More
- Perspectives on microalgal CO2-emission mitigation systems — A review (Ho et al., 2010) - Read More
- Evaluation of strategies for the subsequent use of CO2 (Schaefer et al., 2009) - Read More
6) Can we grow macroalgae for power plant CO2 sequestration?
Macroalgae cultivation in marine environments have been studied as a possible means of large scale CO2 sequestration, and many experts think that this avenue has good potential. A study has even estimated that a combination of micro- and macroalgae grown in open oceans could sequester between 0.7 to 3 gigatons (billion tons) of carbon per year from the atmosphere, at an estimated cost of $5 to 300 per T of carbon.
However, limited research has been done on growing macroalgae next to power plants for CO2 sequestration. These researches have suggested that there are some disadvantages in using macroalgae for CO2 sequestration, specifically in the context of the ability of macroalgae to survive in power plant flue gas. For instance, a patent by Friedlander et al, Israel Oceanographic & Limnological Research, National Institute of Oceanography (Feb 1996), suggests that Gracilaria cultures (a genus of macroalgae) did not survive more than 2-8 weeks in the power plant effluents during the one-year-long repeated experiments. The major reason was the high accumulation of copper, iron, lead and chromium from the power plant effluents as compared to concentrations in Gracilaria cultured in ambient seawater.
7) What are the major problems faced by companies implementing algae based CO2 sequestration techniques near power plants?
Some of the major problems with algae based CO2 sequestration technology are:
- The high cost of implementing the sequestration infrastructure
- The limited availability of land space near power plants for building algae systems
- There are some specific operational problems as well, which could result in significant inefficiencies. For instance, high CO2 concentrations could cause the algae suspension to become acidic, thereby stunting algae growth.
8) Can power plants use waste water from their facilities for growing algae?
Power plants may not be able to use waste water from their facilities. In the power plant industry, large volumes of water are used by cooling systems. Water is also used in the FGD (flue gas desulfurization) plant, boiler cleaning, ash transport, demineraliser plant regeneration, and water also accumulates from coal stockpile run-offs. As a consequence, the effluent from power plant industry has higher temperature and also has heavy metals such as chlorine, copper, aluminum, mercury and phosphorus. It is difficult to grow algae in these effluents. However, some strains that have tolerance to heavy metal contamination and high temperature could possibly be grown.
9) What are the methods by which flue gas can be cooled before passing it into algae systems?
Studies have found that direct cooling with no heat exchangers using cooling towers and mechanical chillers, is the most efficient and low cost cooling method of cooling the flue gas.
10) Is it necessary that algae ponds need to be constructed right next to power plants?
Construction of algae systems near power plants is mostly preferred for the reduced capital costs, which will otherwise be required for putting up the pipelines for CO2 transportation from the power plant to algae cultivation systems.
11) What is the average area required for the construction of algae ponds for each power plant?
Efforts at putting up algae-based CO2 sequestration systems near power plants are in a nascent stage. Hence only preliminary data are available regarding areas required and their attendant costs. Some initial research done at universities and during pilot projects suggest that it could take an open pond of about 8 square miles (5120 acre pond) to produce enough algae to remove carbon dioxide from a midsized — 500 MW — power plant.
12) How does algae productivity compare to other energy crops?
Unlike seasonal crops, algae can be grown year round. Since an algae crop does not result in wasted biomass, algae are generally considered to be more productive than comparable energy crops.
13) Is it necessary that algae ponds need to be constructed right next to power plants?
14) How large must an algae farm be to mitigate emissions from a typical power plant?
Based on information in the US Energy Information Administration 2006 power plant database, for the approximately 500 power plants in the US that generate and sell electricity as their primary business and use coal as the primary power source, the average facility nameplate size is 655 megawatts.
For this 'average' plant, when both the power plant and algae farm are in full operation, approximately 4 hectares of algae growing area is required to consume 40% of CO2 emissions. To achieve a 5.2% reduction in CO2 emissions, which is comparable to the 2008-2012 Kyoto Protocol overall goal, 2 acres(.8 hectare) of algae growing area would be required for the same 655 megawatt plant.
15) Which is the best way to use the algae cultivated using flue gas?
Algae that have been cultivated from flue gas can be used as biofuels, direct combustion of algae biomass or as Animal feed.