Third Generation of Bio-fuel basically focused on the production of Bio-diesel from Microalgae. Microalgae are single cell microorganism and it was believed to be the alternative that would change the Bio-diesel equation completely. Many of the species of Microalgae are targeted due to their high lipid content. However, the utilization of Microalgae in energy production is still in the research stage. (National Renewable Energy Laboratory (NREL), 2008. Since 1980, algae-to-Bio-fuel research has been supported greatly by governments such as Japan, United State, Germany and France. Research Program in United State is especially large.
Potentials benefits in using Microalgae as a Bio-diesel feedstock:
1. Do not use agricultural land
2. Do not compete with food productions
3. Have high oil yield (dry weight of some species of algae is as much as 60% oil)
4. Short generation time which makes it more susceptible to rapid genetic modification to improve its characteristics
5. Can be harvested almost continuously
6. The environment which it grows can be manipulated to optimize algae yield
Cultivation
Microalgae which have high mass percentage of natural oils can be cultivated typically using either of 2 methods which is using large open pond or ‘Photobioreactor’. Due to higher densities of cell, the use of photobioreactors has lower land use in comparison to open pond systems producing the same volume of fuel.
When cultivated in closed photobioreactors, the water evaporated is limited and the water extracted during the drying process can be mostly reclaimed. Besides that, the other parameters like temperature and PH of the water also can be controlled to optimize the growth of algae.
Growth of algae is enhanced by the availability of sunlight and, land and water, CO2 and nutrients. Co-locating the microalgae farm with the animal husbandry allow the direct use of manure as nutrient. Early experiments have shown that carbon dioxide rich emissions from fossil fuel plants and sewage treatment plants into beds of algae accelerate the rate at which the algae grow. In this way, algae convert the additional carbon dioxide into additional fuel. If such a system can be brought to commercial development, it would be able to reduce GHG emissions while increasing energy availability.
In terms of yield, certain species of algae can produce 80% of their body weight as oil; however, 20-50% is more common. Open pond are typically lower cost but has a lower productivity compared to photobioreactors. Table below is based on the modelling of algae production from NREL and gives an idea on the yields that are reasonable under a base case (assumptions reasonable but still challenging in near future), aggressive case (assumes identification of a strain with near optimal growth rates and lipid content) and a max case (represent near theoretical maximum based on photosynthetic efficiencies).
Harvesting
Harvesting à recovering the biomass from cultivation system. Commonly used techniques include flocculation, dissolved air flotation (DAF), centrifugation, microfiltration and decantation. Additional techniques include discrete sedimentation, membrane filtration, photo tactic auto concentration, tilapia-enhanced sedimentation, tube-settling, and ultrasonic separation. Wet biomass may be dried or dewatered. Dewatering decreases the moisture content by draining or mechanical means.
Oil Extraction and Recovery
Oil from algae can be extracted through chemical, mechanical or electrical processes to separate algae oil from cell membrane. The Triacylglycerides are the main product which goes to Bio-diesel production.
The extraction step is regarded as the most speculative in terms of large scale feasibility. Thus it is the critical area of research going forward to achieve practical algae lipid production. Existing techniques includes solvent extraction, supercritical fluid extraction, mechanical extraction, osmotic shock and sonication.
The spent biomass can be used in anaerobic digestion and power generation via gas turbine which provides power to run the plant. Besides that, it can also be used as animal feed.
Oil Conversion to Bio-Diesel
Algae oil can undergo Transesterification to produce Bio-diesel or can be hydroprocessed to renewable products depending on the slate of products desired. In some cases, the entire algae biomass is converted using thermochemical or biochemical methods like pyrolysis, gasification, liquefaction, and fermentation to produce Bio-fuels.
Figure below shows the various pathways for the processing of algae biomass.
Sustainability of Third Generation
In conclusion, estimates of the amount of oil available per acre using algae as feedstock vary widely. They reflect uncertainty about the production technology that it is in the earliest stage, but one thing that all estimates have in common is that they are greater than the amount produced by conventional feedstock. Estimates range from 5000 gallons per acre to 20000 gallons per acre. With such yields, it will be able to change the consumption pattern of petro-diesel without monopolizing a large amount of land area. However, the research on the algae based fuel is still ongoing and there is a few problems to address to if we want to produce Bio-diesel using microalgae.
Intensive research into the following problems is under way:
1. Identifying the most productive algae species
2. Discovering the best way of farming algae
3. Creating efficient way to harvest, dewater, and extract the oil.
These are significance challenges, but they are the types of challenges that are amenable to intensive research. No nation is wealthy enough to maintain generous bio-fuel subsidies indefinitely while at the same time increasing the Bio-Diesel market. If all the problems faced by the algae-based fuel are solved, it is of no doubt that it will become a fuel that drives our future!
