Carbon Capture and Storage:-The way forward
We rarely witness evolution on a time frame short enough for human life to take notice as these changes usually occur over many lifetimes which results in a gradual drift in the creature’s DNA to survive. The above line is quite dramatic in its own right. Isn’t it? But what if I say that there is a word hidden in plain sight that poses a serious question
| If your answer is “survive” then congrats as you are one of the few who know about the severity of the topic that is going to be discussed and if your answer is something else then let me explain to you by sharing the story of some of our friends from the animal kingdom. In the late 1700s for the very first time, humans noticed a change in the physical appearance of the peppered moth as its colour went from speckled white and black to complete black. What brought about this change you may ask? The answer lies in humanity’s quest for rapid industrialization and ignorance towards mother nature. At first, everyone thought it to be an isolated and rare case but the human’s influence over the environment grew and so did the cases. The cases went from being rare and isolated to the new normal in just under 100 years. These moths had to adapt themselves to live in this newly industrialised world, one stained by soot. |
To evolve, or to die
The rate at which we have been spewing pollutants have increased since the old days of the industrial revolution. Our CO2 emissions have increased from 1600 million metric tonnes to 36000 million metric tonnes (about a 2150% increase) since 1865 and despite our best efforts that number is not showing any signs of declining. Human population and development are continuing to outpace our efforts to curb these emissions. The progress we have made recently with the adoption of renewable energy is encouraging especially now that the effects of climate change are becoming more and more noticeable. But renewables alone won’t be able to lower CO2 emissions at the rate which is needed.
Various Ludacris projects had been envisioned from planting trees in Sahara to using Aerosols to block out the sun in specific regions but none have been viable enough to be implemented.
All this begs an interesting question: -
“What if we could stop carbon
emissions from entering the atmosphere in the first place by capturing it from
the pollution sources like a coal-powered steam plant or cement factory and
then storing it underground ...or better yet find a use for it?”
The answer is ‘Carbon Capture and Sequestration’ which honestly has been around for a few years but it’s been slow to take off, that is until now.
CCS
The method of collecting excess carbon dioxide (CO2), moving it to a disposal facility, and depositing it elsewhere so that it does not enter the atmosphere is known as carbon capture and storage (CCS). CO2 is typically collected from large point sources, such as a cement factory or a biomass power plant, and deposited in a carbon storage facility.
CCS is a three-step process that includes the following steps: -
v The capture of CO2 from plants or industrial
processes.
v Transport of the
captured compressed CO2 with the help of pipelines.
v Underground injection and geological sequestration (i.e., storage) of CO2 into deep underground rock formations. These formations are often a mile or more beneath the surface and consist of porous rock that holds the CO2. Overlying these formations are impermeable, non-porous layers of rock that trap the CO2 and prevent it from migrating upward.
CAPTURE
Point sources, such as vast fossil
fuel or biomass energy installations, natural gas electric power generating
plants, businesses with significant CO2 emissions, natural gas refining,
synthetic fuel plants, and fossil fuel-based hydrogen manufacturing plants, are
the most efficient sources for capturing CO2.
Broadly, three different
configurations of technologies for capture exist:
v Post-combustion- In post-combustion capture, the CO2 is removed after combustion of the fossil fuel—this is the scheme that would be applied to fossil-fuel-burning power plants. Here, carbon dioxide is captured from flue gases at power stations or other large point sources. The technology is well understood and is currently used in other industrial applications, although not at the same scale as might be required in a commercial-scale power station. Post-combustion capture is most popular in research because existing fossil fuel power plants can be retrofitted to include CCS technology in this configuration.
v Pre-combustion - The
technology for pre-combustion is widely applied in fertilizer, chemical,
gaseous fuel (H2, CH4), and power production. In this type, firstly the fossil
fuel is partially oxidized, mainly by gasification process to form syngas (CO
and H2). Then the CO from the resulting syngas undergoes a water-gas shift
reaction with steam (H2O) and is shifted into CO2 and more H2 gas. The
resulting CO2 can be captured from a relatively pure exhaust stream. The H2 can
now be used as fuel; the carbon dioxide is removed before combustion takes
place.
The reactions involved are as follows: -
Coal → CO +
H2
{Gasification}
CO + H2O
→ H2 + CO2
{Water-gas shift}
CH4 + H2O
→ CO + H2
{Reforming}
There are several advantages and disadvantages of using this process. The main advantage is the high concentration of CO2 with high pressure resulting in the increased driving force for separation. The disadvantage is that it is only applicable to new power plants because the capture process has to be an integrated part of the combustion process.
v Oxyfuel combustion
– In this method, the fuel is burned in presence of pure oxygen instead of air
resulting in CO2 separation efficiency of close to 100% given that the fuel and
oxygen be free of contaminants. Power plant processes based on oxyfuel
combustion are often referred to as "Zero Emission" cycles because
the CO2 stored is not a fraction removed from the flue gas stream (as in the
cases of pre-and post-combustion capture) but the flue gas stream itself. Some of the main disadvantages are that a turbine needs to
be developed for high-efficiency gas power plants and that air separation is
quite an energy demanding.
Fig: - Carbon Capture Technologies
In all the above capture
technologies we have talked about the separation of CO2 from the flue gases but
how does this separation take place. Which mechanisms are responsible?
The following are the mechanisms proposed for carbon separation: -
Membrane separation
Absorption
Multiphase absorption
Adsorption
Chemical looping combustion
Calcium looping
Cryogenic
To be fair, among all the above-mentioned processes the most dominant one i.e., the one that has been implemented so far industrially is Absorption or ‘Carbon Scrubbing using Amines’.
In this process, the flue gas flows through an absorption tower or scrubber. The flue gas comes in contact with amine (usually monoethanolamine) mixed with water which binds with carbon dioxide in a relatively weak carbon bond. The bound carbon dioxide is transported to another tower known as a stripping tower where the solvent is heated which separates carbon dioxide and amine in a process known as regeneration. The amine is then reused to absorb more carbon dioxide. The flue gas which is usually at about 80 degree Celsius is cooled to 40 degree Celsius before absorption.
About two-thirds of the total cost of CCS is made up by the capture and separation process, making it the bottleneck to the industry-wide deployment of CCS technologies. Hence, to optimize a CCS process we need to optimize just the capture and separation process as the transport and storage steps of CCS are rather mature technologies.
TRANSPORTATION
v Once the CO2 has been captured and separated, it has to be transported to suitable storage sites. This is done most likely by pipelines which is by far the most mature market technology available today. Pipelines are also the cheapest form of transport of large volumes of CO2. Before transporting, the gaseous CO2 is typically compressed to a pressure above 8 MPa to avoid two-phase flow regimes and increase the density of the CO2, thereby making it easier and less costly to transport.
v Ships can also be
utilized for the transportation of liquified CO2 where pipelines are not
feasible. In certain instances, like distant locations, transport by ships may
be economically more viable than laying down the framework for the pipelines.
The liquified CO2 can be transported over larger distances just like LPG albeit
at a lower pressure of 0.7MPa but this currently takes place on a small scale
due to limited demand and supply.
v Road and rail tankers also are ‘technically’ feasible options but they are seldom employed due to the uneconomical nature of their operation.
Fig: - Transport of captured CO2
by different methods.
SEQUESTRATION
Various sites have been conceptualized for the permanent storage (sequestration) of captured CO2 ranging from geological formations, deep oceans, saline aquifers and tar sands.
Fig: - Different CO2 Sequestration techniques.
A. Geological Storage – It is also referred to as geo-sequestration and this method involves injecting CO2 in its supercritical form directly underground geological formations. Oil fields, gas fields, saline formations, unmineable coal seams, and saline-filled basalt formations have been suggested as storage sites as they are comprised of various trapping mechanisms (physical and chemical alike) which would act as barriers for the CO2 from escaping to the surface.
·
Unmineable
coal seams are mostly favoured because the CO2 molecules have a high affinity
to get adsorbed and absorbed onto coal surface which in turn releases the
previously absorbed methane and the methane can be recovered using enhanced coalbed
methane recovery techniques. The sale of the aforementioned coalbed methane can
be used to adjust a portion of the total cost for the CO2 storage but if the
resulting methane is burned then the whole point of sequestering the original
CO2 would be rendered ineffective.
· Captured CO2 is also very effective in the tertiary extraction of crude oil from oilfields. This process is called ‘Enhanced Oil Recovery’ and this process is gaining momentum in recent years. The CO2 is injected into the reservoir to push additional oil to a production wellbore, or other gases that dissolve in the oil to lower its viscosity and improves its flow rate. This process is not a carbon-neutral process as CO2 is released when the recovered oil is burned.
B. Deep ocean CO2 sequestration - The direct injection of CO2 into the ocean has the potential to minimise peak CO2 concentrations and their rate of increase in the atmosphere. However, it is predicted that 15–20 per cent of the CO2 pumped into the ocean would escape back into the atmosphere over hundreds of years if this process is used. Because of its longer estimated preservation time (thousands of years vs. hundreds of years for oceanic injection), the geologic injection may be preferable to oceanic injection. If CO2 reacts underground to form carbonate minerals, even longer residence periods could be obtained, minimising the risk of escape into the atmosphere.
C. Saline aquifers are large, deep formations of porous rocks. These are essentially porous sandstones and limestones with a lot of brine water in their pore space. CO2 disposal into brackish (saline) aquifers from stationary sources (e.g., fossil-fuelled power plants) has been proposed as a potential way to reduce greenhouse gas emissions into the atmosphere. The CO2 is compressed to very high pressure, about 95 bar or higher, and then pumped into saline aquifers, where the CO2 replaces the aquifer water in the porous space.
D. Tar-sand CO2 sequestration - The deep-sea oil-bituminous sand bed is pumped with compressed CO2 at 200 bar and 400°C. CO2 can exist as a supercritical fluid with a specific gravity of between 0.6 and 0.8 at a depth of 600–1000 m. In the saline forming water, supercritical CO2 is buoyant and can rise until it comes into contact with a cap. Bitumen is soluble in CO2 and turns into a liquid, making it easy to extract from unminable bituminous seams.
CHALLENGES AHEAD IN CSS
Commercialization is without a doubt the most difficult aspect of CCS implementation. CCS necessitates the purchase of capital-intensive, long-term properties. Those properties include CO2 transport pipelines and geological storage resources, which cost hundreds of millions of dollars to appraise, instal, and expand, in addition to the catch facility. In most markets, the utility CCS offers, carbon abatement, has little or low demand. Although capture technologies have been well developed and proven, their use in most industries has been minimal, increasing the potential risk. Most jurisdictions lack rules governing CO2 geological storage, posing an enforcement danger.
THE CURRENT STATE OF CCS
CCS
technologies have been in development for a long time, and a range of
programmes are currently underway around the world. CCS is being established in
several nations, including the United States, Canada, Brazil, the United
Kingdom, Germany, Spain, Norway, Sweden, Algeria, Saudi Arabia, India,
Australia, China, Indonesia, China, Taiwan, Hong Kong, Japan, and South Korea.
The Global CCS Institute's 2017 Global Status of CCS study listed 37
large-scale CCS facilities as of September 2017, a decrease of one project from
its 2016 Global Status of CCS report. Twenty-one of these schemes is either
operational or in the planning stages, capturing more than 30 million tonnes of
CO2 each year. By far the most CCS programmes are located in North America. In
the UK, five CCS schemes have been proposed, but none have progressed beyond
the planning stage, let alone begun operations.
STATE OF CCS IN INDIA
The
Indian Department of Science and Technology has developed a national programme
on CO2 storage research and issued a request for proposals in August 2020 to
fund CCS research, production, pilot, and demonstration projects. This is part
of the ACT campaign, for which India has pledged one million euros to assist
Indian participants. In India, a small-scale CCS plant is already operational.
A factory in Thoothukudi's industrial port captures CO2 from its coal-fired
furnace and uses it to produce baking soda. It would save 60,000 tonnes of CO2
from being released into the atmosphere per year.
CONCLUSION
Finally, carbon sequestration is a vast and significant subject. It's critical in the fight against climate change's consequences. To reverse the impacts of climate change, carbon capture and recycling must be combined with innovative renewable energy generation technologies to ensure that no more carbon is emitted into the environment. Furthermore, cleaner agricultural practices, reforestation, and afforestation are successful ways to restore natural sequestration processes that occurred before humans tampered with the ecosystem.
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