CARBON CAPTURE BY ALGAE
- Concentration increase in C02 in the atmosphere leads to changes in climate.
- The production, decomposition, and accumulation of biomass play a crucial role in the global carbon cycle.
- Carbon capture from photosynthesis, carbon deposition in the soil and oceans, and carbon release from biological and geological sources should be in equilibrium for the maintenance of a well-balanced ecosystem.
- Due to carbon emissions from the combustion of fossil fuel and biomass, and reduced carbon uptake as a result of global deforestation and the loss of arable land this equilibrium has been disturbed.
- The environmentally sustainable way to reduce greenhouse gas emissions associated with energy production is to generate energy from carbon-neutral or reduced-carbon-emission sources.
- Biomass products can be converted into energy-dense, liquid-storage fuels.
- There are several reasons for the greater biomass yields of algae versus land plants.
- Advantages of microalgae-based biofuels are
- greater production yields and
- available land area (compared with terrestrial crops)
- algae’s ability to capture CO2 as bicarbonate in ponds,
- reducing atmospheric CO2 emissions;
- and reduced competition for land, particularly arable land used for food production.
CARBON CAPTURE BY ALGAE
- Aquatic carbon capture and biomass production systems have the ability to capture CO2 in ponds in a nongaseous form as bicarbonate to fertilize the algal growth.
- The dominant form of CO2 in water is bicarbonate given at moderate pH and temperature (below 30 degrees).
- Algae can concentrate bicarbonate in the cell due to the presence of active bicarbonate pumps within them.
- The bicarbonate is dehydrated by carbonic anhydrase. This results in the capture of CO2 through Calvin-cycle
- Its ultimate form is algal biomass.
- 6 – 2 grams of CO2 are captured for every gram of algal biomass produced.
- Industrial sources of CO2 can be harvested using algal ponds.
- The injection of power plant flue gases (have high CO2 concentrations, ranging from 10% to 20%) into algal ponds upgrades algal biomass yields by three times.
- The efficiency of CO2 capture by algae vary according to the state of the algal physiology, pond chemistry, and temperature.
- Carbon-dioxide capture efficiencies as high as 80% to 99% are achievable by the production of algal biomass under optimal conditions.
- An algal pond of 3600 acres would be sufficient to capture 80% of the plant’s CO2 emissions during daylight hours with a biomass productivity rate of 20 grams dry weight per square meter per day.
- Locating algal ponds near CO2 point sources gives several potential costs- and energy-saving advantages.
- Integrated power plant–algal pond facilities:
- reduce the costs of CO2 transportation
- produce limited waste heat from the power plant for warming ponds in the winter
- The major drawback is CO2 capture by biomass production is not workable in the dark.
CARBON SEQUESTRATION BY ALGAE
- CO2 must be isolated over long-time intervals to have the greatest impact on greenhouse gas accumulation.
- A lower risk strategy for mitigating carbon dioxide is to chemically convert CO2 into stable liquids or solids.
- Another strategy for isolating carbon is to bury it as biomass.
- Permanent burial of total algal biomass in deep geologic formations and burial of extracted or processed carbon-rich fractions from algal biomass are the two possible algal carbon isolating systems.
- The drawback of the direct burial of total algal biomass is that inorganic nutrients would also be buried with the algal biomass. Burying of inorganic elements is not sustainable.
- Burying only the neutral lipid or hydrocarbon fraction of the algal biomass is an alternative approach.
- More than 75% of the mass of neutral lipids (TAG) is carbon, TAGs represent rich sources of captured carbon in cells.
- Triacylglycerols extraction can be injected into geologic formations to seal the carbon in place.
- TAGs do not contain elements other than carbon, hydrogen, and oxygen so the inorganic nutrients which are required for algal growth can be recycled and not buried with the carbon.
- Laboratory studies suggest that carbon capture by algae is a viable strategy for mitigating CO2 emissions from anthropogenic sources.
- Life-cycle analyses suggest the costs for capturing carbon and producing liquid biofuels from algae may approach the costs of producing petroleum-based fuels in the next 5 to 10 years