Just a few years back, when algae started to revive as the potential solution for the emerging energy crisis, it was believed that the use of flue gas from power plants or other industries would be the main CO2 source for large scale algae cultivation.
Not a bad concept, since this is a cheap and plentiful source of carbon for the algae and helps to bring down the cost of algae-derived fuels.
Several proposals have been submitted to co-locate algae farms with e.g. coal power plants so that the flue gas doesn’t have to be transported over long distances. This sounds like a great idea, but is it really feasible on a large scale?
As more research has been performed on small scales, it has become clear that the use of flue gas does have some challenges that need to be overcome to use it for large scale algae production. One of the concerns is that flue gas contains a certain percentage of pollutants such as particulate matter, carbon monoxide, nitrogen oxides (NOx) and sulfur oxides (SOx). The contaminants are dependent on the material that has been burned.
A critical issue is the tolerance of algae to SOx and NOx. These gases are toxic to the algae growth when present in elevated concentrations. Research has shown that algae are most sensitive to the SO2 with a significant effect on the growth above 50-60 ppm (parts per million).
The toxicity levels are strain dependent and maintaining the pH with a buffer appears to help them grow under higher toxicity levels. NO can generally be tolerated up to 100 ppm, but depending on the strain, higher seems to work as well. The algae cell density matters as well, so if a continuous culture can be kept at high enough density one might get away with the higher limits.
Since the Clean Air Act Amendment in 1990, there are strict regulations in place in the US as to the levels of SO2 that can be present in industrial exhausts. This has forced industries to clean up their flue gas with a series of chemical processes and scrubbers, which remove pollutants.
In general, modern flue gas desulfurizing units (FGD’s) bring down the SOx to about 70 ppm. Some of the more tolerant strains might be ok in that, but it is still not low enough for general use in algae cultivation. Currently, the desulfurization is being further expanded to also remove toxic metals like mercury from the flue gas.
By employing state-of-the-art biotechnology and bioengineering we are currently working on adapting algal species to tolerate these higher concentrations of flue gas levels. This will shortcut the need for additional cleanup of the flue gas which is an expensive process and would make the use of flue gas for algae cultivation cost prohibitive.
Algae growth on flue gas is no longer solely being suggested for fuel production alone, but current research is now focusing on using it as a method for carbon sequestration (long term capture and storage or CO2).
In an attempt to mitigate and control “global warming”, there has been a great deal of interest and investment in efficient methods for carbon sequestration. Algae (and other plants) take up CO2 as part of their normal metabolism and generate oxygen in the process.
If algae can be adapted to tolerate the higher concentrations of flue gas, and greenhouse gasses are captured while useful biomass is generated, we will have created one more viable solution for a greener future.
Source: http://algaeforbiofuels.com/flue-gas-algae-cultivation-curse-blessing/
