Fuel production via microalgal CO2 fixation
One of the most serious environmental problems today is that of global warming, caused primarily by the heavy use of fossil fuels. In Japan, large amounts of CO2 are released into the atmosphere from electric power plants and industry. The CO2 generated by these large point sources could potentially be recovered with relative ease through the use of an established technology such as chemical absorption. The enormity of the amounts of potentially recoverable CO2 would however necessitate the development of technologies for sequestering or, more favorably, utilizing this CO2.
Photosynthetic micro algae are potential candidates for utilizing excessive amounts of CO2, since when cultivated these organisms are capable of fixing CO2 to produce energy and chemical compounds upon exposure to sunlight. The derivation of energy from algal biomass is an attractive concept in that unlike fossil fuels, algal biomass is rather uniformly distributed over much of the earth's surface, and its utilization would make no net contribution to increasing atmospheric CO2 levels.
Hydrogen is regarded as a potential energy source of the future, since it is easily converted to electricity and bums cleanly. Hydrogen is currently produced by fossil fuel-based processes which emit large amounts of CO2, and relatively smaller amounts of other air pollutants such as sulphur dioxide and nitrogen oxides. Biological H2 production has thus recently received renewed attention owing to urban air pollution and global warming concerns.
The use of micro algae as sources of liquid fuels is an attractive proposition from the point of view that micro algae are photosynthetic renewable resources, are of a high lipid content, have faster growth rates than plant cells, and are capable of growth in saline waters which are unsuitable for agriculture. While the lipid content of micro algae, on a dry cellular weight basis varies between 20 and 40 %, lipid contents as high as 85 % have been reported for certain micro algal strains. Botryococcus braunii, is a unique micro algal strain, having a long-chain hydrocarbon content of between 30 and 40% (dry weight basis), which is directly extractable to yield crude oil substitutes. Both physical and chemical processes are applicable in the production of liquid fuels from algal strains of high lipid content. These processes include direct lipid extraction in the production of diesel-oil substitutes, transesterification in the formation of ester fuels, and hydrogenation in the production of hydrocarbons (3). Oily substances are also produced via liquefaction of micro algal biomass through thermochemical reactions under conditions of high pressure and temperature.
Micro algae posses several attractive characteristics:
1) Costs associated with the harvesting and transportation or micro algae are relatively low, in comparison with those of other biomass materials such as trees, crops, etc.
2) By virtue of their relatively small sizes, micro algae can be easily chemically treated.
3) Algae can be grown under conditions which are unsuitable for conventional crop production.
4) Micro algae are capable of fixing CO2 in the atmosphere, thus facilitating the reduction of increasing atmospheric CO2 levels, which are now considered a global problem.
Micro algal oils are produced through either biological conversion to lipids or hydrocarbons or thermochemical liquefaction of algal cells.
Micro algae as biological sources of lipids and hydrocarbons
Micro algae contain lipids and fatty acids as membrane components, storage products, metabolites and sources of energy. Algal fatty acids and oils have a range of potential applications. Algal oils posses characteristics similar to those offish and vegetable oils, and can thus be considered as potential substitutes for the products of fossil oil
