(nicotinamide adenine dinucleotide phosphate), essential for the assimilation of
inorganic carbon and for oxygen production (Williams and Laurens 2010). This
process begins in two photosystems (I and II), where pigments such as chlorophyll
are responsible for absorbing mainly photons and transferring energy to an
electron-accepting substance (located in the thylakoid membranes). From this stage,
the excited chlorophyll recovers 6 lost electrons, where the energy is used for the
water photolysis, also referred to as Hill reactions (Heldt and Piechulla 2011). By
removing the light electrons, water molecules decompose into H
+ ions, releasing
oxygen atoms to form the gaseous O 2 molecule, a significant product of microalgae
metabolism. Figure 2 shows the schematic representation of water photolysis and
oxygen generation during photosynthesis in a microalgae eukaryotic cell. This is an
important aspect of photosynthesis, because all the oxygen generated in the process
comes from the water photolysis (Barber 2017). The reaction can be described, in
chemical terms, as follows (Eq. 1):
2H 2 O À! O 2 þ 4H
þ
þ 4e
À
ð1Þ
Additionally, the theoretical and realistic conversion efficiencies of water photolysis can be obtained by biological estimates, in terms of quantum efficiency, i.e.,
through the energy fraction of absorbed photons, or calculated from the solar
Table 3 Critical issues in oxycombustion systems
Parameter
Technical barrier
Oxygen supply
An oxycombustion plant requires large amounts of high-purity
oxygen. The only option available on the market is ASU, which
requires intense energy demand, operating expenses (OPEX), and
capital expenditure (CAPEX)
Cost
The technology is expensive. Demand for electricity can increase
plant cost by 70–80%
Scale-up
Although there is an oxygen–air separation process commercially
available, it has not been deployed at the scale required for large
power plants applications
Energy integration
Steam required for regeneration can only be extracted at conditions
defined by the power plants steam cycle. Additionally, mitigation
can result in the generation of significant quantities of waste heat.
Energy integration can improve plant efficiency
Auxiliary power for CO 2
mitigation
Auxiliary power is also required to operate CO 2 mitigation
technologies. This decreases the power plant’s net electrical
generation and significantly reduces net power plant efficiency
Mechanical integration
Any CO 2 mitigation system must fit within the boundaries of the
power plant. This is a significant barrier when dealing with
existing plants that have fixed layouts and limited open space
Flue gas pollutants
Constituents of the combustion exhaust gases, mainly sulfur, can
damage the equipment and reduce its useful life
Water usage
A significant amount of water is used in current technologies for
cooling during CO 2 compression
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inorganic carbon and for oxygen production (Williams and Laurens 2010). This
process begins in two photosystems (I and II), where pigments such as chlorophyll
are responsible for absorbing mainly photons and transferring energy to an
electron-accepting substance (located in the thylakoid membranes). From this stage,
the excited chlorophyll recovers 6 lost electrons, where the energy is used for the
water photolysis, also referred to as Hill reactions (Heldt and Piechulla 2011). By
removing the light electrons, water molecules decompose into H
+ ions, releasing
oxygen atoms to form the gaseous O 2 molecule, a significant product of microalgae
metabolism. Figure 2 shows the schematic representation of water photolysis and
oxygen generation during photosynthesis in a microalgae eukaryotic cell. This is an
important aspect of photosynthesis, because all the oxygen generated in the process
comes from the water photolysis (Barber 2017). The reaction can be described, in
chemical terms, as follows (Eq. 1):
2H 2 O À! O 2 þ 4H
þ
þ 4e
À
ð1Þ
Additionally, the theoretical and realistic conversion efficiencies of water photolysis can be obtained by biological estimates, in terms of quantum efficiency, i.e.,
through the energy fraction of absorbed photons, or calculated from the solar
Table 3 Critical issues in oxycombustion systems
Parameter
Technical barrier
Oxygen supply
An oxycombustion plant requires large amounts of high-purity
oxygen. The only option available on the market is ASU, which
requires intense energy demand, operating expenses (OPEX), and
capital expenditure (CAPEX)
Cost
The technology is expensive. Demand for electricity can increase
plant cost by 70–80%
Scale-up
Although there is an oxygen–air separation process commercially
available, it has not been deployed at the scale required for large
power plants applications
Energy integration
Steam required for regeneration can only be extracted at conditions
defined by the power plants steam cycle. Additionally, mitigation
can result in the generation of significant quantities of waste heat.
Energy integration can improve plant efficiency
Auxiliary power for CO 2
mitigation
Auxiliary power is also required to operate CO 2 mitigation
technologies. This decreases the power plant’s net electrical
generation and significantly reduces net power plant efficiency
Mechanical integration
Any CO 2 mitigation system must fit within the boundaries of the
power plant. This is a significant barrier when dealing with
existing plants that have fixed layouts and limited open space
Flue gas pollutants
Constituents of the combustion exhaust gases, mainly sulfur, can
damage the equipment and reduce its useful life
Water usage
A significant amount of water is used in current technologies for
cooling during CO 2 compression
13 Biofuels from Microalgae …
279