Proceedings of CUChE Alumni Symposium 2022
On “Circular Economy on Sustainable Basis: The Role of Chemical Engineers”
CUChEAA ISBN: 978-81-954649-1-3
December 2022 P a g e | 26 Volume 2, Issue 1
of hydrogen and carbon monoxide. The carbon
monoxide is further mixed with steam in a second
reactor (“shift reactor”) to produce additional hydrogen
along with CO
2
. The resultant stream of hydrogen and
CO
2
can further be separated into individual
constituents. Hydrogen is a carbon-free energy carrier so
if the CO
2
is captured and stored, it can be utilised to
produce heat and power. This hydrogen produced in the
reformer integrated with CCS is called ‘blue hydrogen’
#
.
The CO
2
concentrations from the shift reactor are
typically 15-60% by volume on a dry basis. The primary
stage of fuel conversion step is costly and involved than
post-combustion capture; however, the relatively high
concentration of CO
2
and high pressures are more
favourable for CO
2
separation.
# Hydrogen can also be generated using electrolysis of
water utilising renewable energy sources, which is
termed ‘green hydrogen’. Both blue and green hydrogen
can be utilised to decarbonise the power and industrial
sectors.
2.1.3 Oxy-Combustion Capture
Oxyfuel combustion systems combust fuels in the
presence of oxygen instead of air to produce a flue gas
which is mainly comprised of water vapour and CO
2
rather than CO
2
, N
2
, and water. This results in flue gas
streams with high concentrations of CO
2
(>80% CO
2
by
volume) and the volume of flue gas is considerably low.
The oxygen purity required for oxyfuel combustion is
~95-99%. The water vapour is removed from the flue
gas through cooling and compression. Furthermore,
additional treatment may be required to remove
contaminants and non-condensable gases (i.e., nitrogen)
from the flue gas prior to the final CO
2
disposal.
Theoretically, oxyfuel combustion systems can capture
the CO
2
but the requirement for supplementary gas
treatment processes to remove impurities (e.g., sulphur
and nitrogen oxides) reduces the capture efficiency to
~90%. Although large-scale oxygen separation systems
are commercially available, oxyfuel combustion for CO
2
capture has not been deployed at an industrial scale to
date and is only in the demonstration stage.
2.2 CO
2
Compression and Purification
Captured CO
2
from absorption (or other processes) is a
low-pressure stream and contains water and oxygen.
These contaminants can cause corrosion in the
downstream equipment and pipelines. The low-pressure
saturated CO
2
stream is routed to dehydration to remove
the moisture and then sent to the oxygen removal unit to
meet the specification of CO
2
which is suitable for
transportation.
In all CCS schemes, the most important and energy-
intensive step is compression. It requires a large amount
of power to increase the pressure of the CO
2
to achieve
the transport and injection conditions. Captured CO
2
goes through low pressure first and then a high-pressure
compression system based on the requirement of the
injection pressure. The high-pressure compression
utilises part compression followed by pumping in the
dense phase region (supercritical). These are generally
multistage integrally geared centrifugal machines.
2.3 CO
2
Transportation
Most of the high-pressure CO
2
pipelines operate in the
dense phase and carry large volumes of CO
2
from pure
stream sources (such as power plants and hydrogen
reformers) to the ultimate destination at the injection
well. The pipelines are mainly carbon steel. The
corrosion rate of carbon steel in dry supercritical CO
2
is
low, hence it is important to remove the moisture from
CO
2
to a minimum level in the conditioning step.
2.4 CO
2
Injection
Injection of captured CO
2
is the final and very important
step of the CCS technical chain. CO
2
can be injected into
deep saline aquifers, depleted hydrocarbon reservoirs, or
deep un-minable coal seams. The injection pressure
could be in the region of 150- 200 barg based on the
characteristics of the reservoir.
Therefore, to apply the CCS process it is essential that
the following aspects are assessed to decarbonise the
industrial sector.
• CO
2
Capture Process - post-combustion, pre-
combustion, oxy-combustion.
• Combustion fuel - natural gas, coal, oil, biomass
etc.
• Site location - distance from a potential injection
storage site, pipeline length
• Cooling system - air cooling, water cooling
• Means of CO
2
Transport - onshore/offshore
pipeline, ship tankers, etc.
• Physical state of CO
2
for transport - vapour
phase, dense phase
• Storage type - depleted oil or gas fields, saline
aquifers, EOR etc
• Sink type - onshore, offshore.
In the following sections, the above processes have been
explored further to understand which are more suitable
for the power and industrial sectors.
3. Power Plants
3.1 Overview of Industry
Across the world, power plants mostly burn fossil
fuels such as coal, oil, and natural gas to produce
electricity. A steam turbine generator is generally used
for coal or oil-based power stations, which convert
mechanical energy to electrical energy. It is estimated
that more than 85 percent of electricity produced in the
world utilises steam turbines (Rackley, 2017). In the
natural gas-fired plants, a combustion turbine is
employed, which utilises the dynamic pressure of the
high-pressure fuel gas and is mixed with air in the
combustion chamber to operate the turbine.
Combined Cycle Gas Turbine (CCGT) plants (also
called cogeneration plants or CHP (combined heat and
power plants) use a gas turbine, a steam boiler, and a
steam turbine (See figure 3) to generate electricity. The
hot exhaust gas (temperature approx. >550°C) from the
gas turbine exchanges heat with water to produce steam
which in turn produces electricity in the steam turbine
generator (STG). The combined cycle improves the