Algae, The Fuel Source

Microalgae, like higher plants, produce storage Lipids in the form of triacyglycerols (TAGs). Comparatively algae produce more oil than any other oilseeds which are currently in use. Many microalgal species can be induced to accumulate substantial quantities of lipids, often greater than 60% of their biomass.
 Algae produce oil, and because of their growth rate and yields, they could produce significantly higher quantities than other energy crops. Some estimates suggest that microalgae are capable of producing up to 15,000 gallons of oil per hectare a year. This could be converted into fuels, chemicals and more.

Microalgae, specifically, possess several attractive characteristics in the context of energy and biofuels:

• They provide much higher yields of biomass and fuels, 10-100 times higher than comparable energy crops.
• They can be grown under conditions which are unsuitable for conventional crop production.
• Microalgae are capable of fixing CO2 in the atmosphere, thus facilitating the reduction of increasing atmospheric CO2 levels, which are now considered a global problem.
• Algae biofuel is non-toxic, contains no sulfur, and is highly biodegradable.

The following is the list of fuels that can be obtained from algae:
•    Biodiesel
•    Ethanol
•    Hydrogen
•    Methane
•    Biomass – where algae biomass is directly used for combustion
•  Other hydrocarbon fuel variants, such as JP-8 fuel, gasoline, biobutanol etc.

The U.S. Department of Energy’s Aquatic Species Programme (ASP), undertook over a decade of research (between 1978 and 1996), and found that algae were only economically viable as a biofuel at oil prices of more than $60 a barrel.


Whatever be the final energy product/s, the following represent the stages involved in the algae to fuel process:



Cultivation of Algae

Like plants, algae use the sunlight for the process of photosynthesis. Photosynthesis is an important biochemical process in which plants, algae, and some bacteria convert the energy of sunlight to chemical energy. Algae capture light energy through photosynthesis and convert inorganic substances into simple sugars using the captured energy.

Algae cultivation is an environmentally friendly process for the production of organic material by photosynthesis from carbon dioxide, light energy and water. The water used by algae can be of low quality, including industrial process water, effluent of biological water treatment or other waste water streams.

There are two main methods of cultivation
I.    Open Ponds
II.   Photobioreactors

Algae harvesting

Separating algae from its medium is known as harvesting. Harvesting methods depends primarily on the type of algae. The high water content of algae must be removed to enable harvesting. The most common harvesting processes are flocculation, microscreening and centrifugation. These must be energy-efficient and relatively inexpensive so selecting easy-to harvest strains is important. Macroalgae harvesting employs manpower whereas, microalgae can be harvested using microscreens, centrifugation, flocculation or by froth flotation.


Algae Oil Extraction

Oil extraction from algae is a hotly debated topic currently because this process is one of the more costly processes which can determine the sustainability of algae-based biodiesel.
In terms of the concept, the idea is quite simple: Harvest the algae from its growth medium (using an appropriate separation process), and extract the oil out of it. Extraction can be broadly categorized into two methods:

Mechanical methods which include

• Expression/Expeller press
• Ultrasonic-assisted extraction
• Chemical methods

Chemical methods which include

• Hexane Solvent Method
• Soxhlet extraction
• Supercritical fluid Extraction

Transesterification

Algal oil, like other vegetable oils, is highly viscous, with viscosities ranging 10–20 times those of diesel fuel. Chemical conversion of the oil to its corresponding fatty ester appears to be a promising solution to the high viscosity problem.

The traditional method to produce biodiesel from algae oil is through a chemical reaction called transesterification. Under this method, oil is extracted from the algal biomass and it is processed using the transesterification reaction to give biodiesel as the end-product. This is a fairly simple and well-understood route to produce biodiesel from vegetable oils, and it can be used equally well for biodiesel from algae as well.


Ethanol from Algae

Ethanol from algae is possible by converting the starch (the storage component) and Cellulose (the cell wall component). Put simply, lipids in algae oil can be made into biodiesel, while the carbohydrates can be converted to ethanol. Algae are the optimal source for second generation bioethanol due to the fact that they are high in carbohydrates/polysaccharides and thin cellulose walls. Many macroalgae species are rich in carbohydrates and cellulose and hence could make a more suitable feedstock for ethanol than microalgae. The primary methods of ethanol production from algae are discussed below.

• Fermentation of algae biomass
• Fermentation of algae extract left over after extraction of oil
• Fermentation of syngas

It is also possible to derive ethanol from the algae left-over after extraction of oil (that is, the de-oiled algae cake). This de-oiled algae cake can be converted into ethanol through fermentation of the extract. This gives rise to the interesting possibility of producing both biodiesel and ethanol from algae!

De-oiled Algae Cake

Deoiled algae cake is a source of nutrients for humans and animals, because the cake of many algal species has high protein content, sometimes as high as 50 to 60% of dry matter. Except for sulphur-containing amino acids (methionine and cystine), the essential amino acid content in many algal species is favourable for the nutrition of farm animals. Algae are also a rich source of carotene, vitamin C and K, and B-vitamins.

Applications of the de-oiled cake include

• Algae cake to ethanol
• Algae cake to methane through anaerobic digestion
• Algae cake to electricity - There are many ways to generate electricity from biomass using thermochemical pathways. These include directly-fired or conventional steam approach, co-firing, pyrolysis and gasification.
• The press cake can be used as fertilizer, without conversion. However, it might not be the best use. The algae cake will probably be more valuable as animal feed than as fertilizer.
• Meal and protein options when retained in watery phase:

      
Fermentation to alcohol (including ethanol), making CO2 available for recycle
Anaerobic digestion to methane for power production and  effluent recycle
Food source for aquaculture (fish, shrimp)