The world
has paid attention towards renewable energy sources to overcome environmental
issues and limited availability of fossil issues. Therefore use of algal
biomass is recognized a one of best solution. The production of biofuel algae
as the raw material is considered referred as the third generation fuel
production. It is able to minimize or eliminate the negative impacts due to the
first and second generation fuel production. Algal biomass production does not
compete with food production in the world like first generation fuel and it
does not threat to the food security.
Algae biomass
production does not involve with agricultural lands. It harvests high yield per
unit area of land. Freshwater is not essential for algae production. Sea water
and wastewater also can be used.
Both microalgae and
microalgae are used for the algal fuel production. Microalgae are single celled
algae that live as individual or colony. Oil extracted from microalgae is used
for biodiesel production. Macroalgae with high sugar content in cell wall is
used for bioethanol production via fermentation. Macroalgae are digested in an
anaerobic digester for methane production.
Algae culture: It is the growth of algal biomass.
Macroalgae are grown directly on the open sea. But most people tend to collect
macroalgae from the sea and estuary rather than cultivation. Thereby it threats
the ecosystem balance. Microalgae are grown in the specifically designed
systems available in the land. It has to be supplied externally nutrients,
light, carbon dioxide, water and stable temperature. Wastewater is provided in
the form of nutrients. Open ponds and photo bioreactor are currently used for
microalgae cultivation.
Open ponds: It is constructed in the shape of
circular or rectangular (raceway). It may large shallow or deep pond according
the depth. The depths of open ponds varies from 1-100cm. water is mixing by
using paddles or other mechanical devices. The bottom of raceways ponds are
made of concrete or lined with plastic. Due to the low cost of plastic lined,
raceways ponds may expand from few feet to thousand meters in length. Circular ponds are oldest method of
algal cultivation. A centrally located rotation arm is used to mix the water.
Advantages of open
ponds:
1) Low per capital cost and operating
cost
2) Low energy requirement
Disadvantages of open
ponds:
1) Open pond system is less successful
in the temperate countries which experience seasonal changes than tropical
countries which do not experience seasonal changes. Because algae cannot grow
during winter season so algae cultivation is restricted summer.
2) Sunlight causes the loss of significant
amount of water due to the evaporation. Large surface area of pond expose to
the sunlight. Therefore large amount of water is needed to operate the open
pond system.
3) Open pond system produce low algal
density per unit compare to the photo bioreactor.
4) They are highly vulnerable to
contamination by native species. Algae may subject to predatory attacks in the
open pond system.
5) Light is not equally distributed
throughout the open ponds because algae growth in the top layer of the water
column restricts the light penetration to the bottom.
6) Loss of Carbon dioxide into the
atmosphere from the ponds cannot be controlled. Carbon dioxide tends to expel
into atmosphere through diffusion. As the temperature rises, it decreases the
solubility of gas in the water.
Photo bioreactor: they are the closed system of algal
cultivation. Algae are grown in the closed vertical vessels which are made of transparent material such
as stainless stain, glass or polyethylene. It helps light to penetrate through
it. But use of polythene is less durable and sensitive to environmental
changes. Conditions within the reactors can be controlled. It leads to harvest
maximum yield per unit of land area. The
photo bioreactor can be either operated as batch or continuous mode.
Advantages:
1) Photo bioreactor produce maximum
yield per unit area because it offer larger area to volume ratio for the algal
growth compare to the open ponds.
2) As algae are grown in closer system,
it restricts the evaporation of water.
Water requirement for the closer system is reduced.
3) It save the space as it use vessel in
the vertical position to grow so minimum land area can be used. Even it can be
established in indoors.
Disadvantage:
1) Its capital cost and operative cost
is high.
2) As it is operated under controlled
conditions, it requires good care and maintenance.
3) Technicians should have proper
knowledge on the system.
Algae harvesting: After cultivation, increasing of
concentration of algal biomass refer to as the algae harvesting. Nearly 20-30%
of production cost has to be spending on the harvesting process. Centrifugation, gravity filed, flotation
and filtration are the available techniques for algae harvesting. Although centrifugation gives high algae cell
density, it is energy intensive. Therefore approach like gravity settling is
the best solution for the algal biofuel production. Algae biomass is deposited
at the bottom of the settling tank under the process of flocculation. It can be
used with or without chemical flocculants. But use of chemical flocculants is
more efficient in biomass settling.
Algae processing: It is the process of the oil
extraction from the algal biomass. Here two methods can be used. Algae biomass
can be used directly for biofuel production or oil that is extracted from the
alga biomass can be used for biofuel production. There are two methods
available for extraction of oil form the algae biomass including solvent
extraction and cell disruption to release the oil substances. Algal biomass is dried first. Then later
solvent extraction is used to separate the oil content form the algae. This oil
can be used for biodiesel or green diesel production. Algal biomass can be
disgusted in an aerobic digester to produce methane. It can be fermented into
bioethanol by using microorganisms such as Yeast and enzymes.
Biodiesel derived from algae: Oil content varies according to the
type of algae. Therefore algae with the high oil content are selected to grow.
Biodiesel is produced from algal oil and methanol by esterification. Biodiesel
is similar in composition to fossil diesel.
Green diesel derived from algae: it is produced by using a refinery
method. The large oil molecules found in the algae are breakdown into small
hydrocarbon chain that can be used directly in the diesel engine at elevated
temperature and elevated pressure in the presence of catalyst.
Bioethanol derived from algae: Here alga with high sugar content
is used. Pretreatment is done to release the cellulose from the cell wall and
breakdown them into fermentable sugar. It can be done using physical, chemical
and biological methods. Use of biological method is relatively more beneficial.
It does not need energy and chemical. But microorganisms that involve in
breakdown of cellulose may not specific to cellulose they may degrade the
glucose and fructose. Yeast is added externally and it produce methanol through
fermentation. Then sucrose is breakdown into glucose and fructose by invertase.
These glucose and fructose is converted into ethanol by zymase. Then ethanol is
subjected to distillation. Because bioethanol still contains a large amount of
water. So the mixture of water and ethanol is heated. The boiling point of
ethanol is lower than that of the water. So ethanol is converted into vapor
phase before water. It is condensed to spate from the water. It helps to
increase the concentration of ethanol and clean it.
Benefits of algal fuel:
1) It does not threat to the food
production in the world. It need minimum land requirement compared to the
biofuel production from food crops. It does not compete with nutrients
available for agriculture because wastewater can be used to provide nutrients.
2) It absorbs considerable amount of
carbon dioxide for biomass production. So it helps to carbon sequestration.
Thereby it reduces the greenhouse gas effect and global warming.
3) Use algae as a raw material for
biofuel production can be used to produce a verity of fuels.
4) Algae secretes oil compound that can
be used directly as ingredient in bio fuel production so there is no technical
challenges to pretreatment of lignocellulose biomass.
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