MICROALGAE
Morton Satin, Chief Agro-Industries and Post-Harvest Management Service
Microalgae (various oleaginous species)
Description- Microalgae comprise a vast group of photosynthetic, heterotrophic organisms which have an extraordinary potential for cultivation as energy crops. They can be cultivated under difficult agro-climatic conditions and are able to produce a wide range of commercially interesting byproducts such as fats, oils, sugars and functional bioactive compounds. As a group, they are of particular interest in the development of future renewable energy scenarios. Certain microalgae are effective in the production of hydrogen and oxygen through the process of biophotolysis while others naturally manufacture hydrocarbons which are suitable for direct use as high-energy liquid fuels. It is this latter class that is the subject of this brief.
Once grown, the harvesting and transportation costs of algae species are lower that with conventional crops and their small size allows for a range of cost-effective processing options. They are easily studied under laboratory conditions and can effectively incorporate stable isotopes into their biomass, thus allowing effective genetic and metabolic research to be carried out in a much shorter time period than conventional plants.
Ecological Requirements-Microalgae represent an immense range of genetic diversity and can exist as unicells, colonies and extended filaments. They are ubiquitously distributed throughout the biosphere and grow under the widest possible variety of conditions. Microalgae can be cultivated under aqueous conditions ranging from freshwater to situations of extreme salinity. They live in moist, black earth, in the dessert sands and in all the conditions in between. Microalgae have been found living in clouds and are long known to be essential components of coral reefs. This wide span of ecological requirements plays a significant role in determining the range of metabolic products they produce.
Propagation- Microalgae can be grown in both open-culture systems such as ponds, lakes and raceways, or in highly controlled closed-culture systems, similar to those used in commercial fermentation processes. Certain microalgae are very suitable for open system culture where the environmental conditions are very specific, such as high salt or high alkaline ponds lakes or lagoons. The extreme nature of this environment severely limits the growth of competitive species, although other types of organisms may contaminate the culture. The advantages of such systems is that they are generally a low investment, very cost-effective and easy to manage. Closed-culture systems, on the other hand, require significantly higher investments and operating costs, but are independent of all variations in agro-climatic conditions and are very closely controlled for optimal performance and quality.
Open-culture systems take advantage of natural sunlight and are totally subject to the vagaries of weather unless some form of shading system is utilized. Highly controlled closed systems use photobioreactors for phototrophic culture and conventional fermenters for heterotrophic growth. The range of sophistication available for the two systems is very great, as is the associated investment. As an example, photobioreactors can vary from simple, externally-illuminated glass jars to highly engineered fermenters saturated with light transmitting fibre optic filaments to ensure even lighting to all cells and infused with specific gas mixtures to control metabolism and growth rates. Certain new photobioreactors incorporate an -type tubular design for greater cost-effectiveness and commercial efficiency.
Energy Production- In the production of energy from microalgal biomass, two basic approaches are employed depending upon the particular organism and the hydrocarbons which they produce. The first is simply the biological conversion of nutrients into lipids or hydrocarbons. The second procedure entails the thermochemical liquefaction of algal biomass into usable hydrocarbons.
Anabolic Production of Lipids and Hydrocarbons by Microalgae-Lipids and hydrocarbons can normally be found throughout the microalgal cell mass. They occur as storage product inclusions in the cytoplasm and as functional components of various membranes. In some cases, they are excreted extracellularly into the microalgal colony matrix as almost pure oleaginous globules. In certain cases, the lipid composition can be regulated through the addition or restriction of certain components in the diet. For example nitrogen or silicon starvation and other stress provocateurs may increase total lipid production.
The type and level of hydrocarbons produced is often effected by environmental factors such as light, temperature, ion concentration and pH. While it is not uncommon to find levels of 20-40% lipids on a dry basis, on occasion the quantities of lipids found in microalgae can be extraordinarily high. For example, in one particular species, Botryococcus, the concentration of hydrocarbons in the dry matter may exceed 90%, under certain conditions.
The following table gives some examples of the lipid contents of various microalgae;
LIPID CONTENT OF DIFFERENT ALGAE
Strain
% Lipid (on a dry basis)
Scenedesmus sp. 12 - 40
Chlamydomonas sp. 21
Clorella sp. 14 - 22
Spirogyra sp. 11 - 21
Dunaliella sp. 6 - 8
Euglena so. 14 - 20
Prymnesium sp. 22 - 38
Porphyridium sp. 9 - 14
Synechoccus sp. 11
Thermochemical Liquefaction of Microalgae- The convenience of microalgal harvesting and handling make it equally suitable for thermochemical processing such as liquefaction. Following a process reminiscent of the origin of petroleum products, microalgae are converted into oily substances under the influence of high temperature and high pressure. Yields in the 30-40% range of heavy-type oil can be obtained in this manner. Because of the high levels of proteinatious materials in the system, nitrogen levels leading to Nox formation have to be carefully controlled. Yields close to 50% of liquid hydrocarbon have been obtained using a very high temperature, high pressure, catalyzed hydrogenation process
Processing and Handling- In cases where the hydrocarbons are anabolically by the microalgae, direct extraction is the simplest and most effective of obtaining products. This can be effected through the employment of solvents, through the direct expression of the liquid lipids, or a combination of both methods. The thermochemical liquefaction process often results in a heavy oily or tarry material which is then separated into different fractions by catalytic cracking. As with hydrocarbons derived from other forms of renewable biomass, microalgal lipids can be converted into suitable gasoline and diesel fuels through transesterification.