Carbon Capture and Storage
- 1 Carbon (CO₂) Capture
- 1.1 Introduction
- 1.2 Combustion
- 1.3 Types of Carbon (CO₂) Capture
- 1.4 Negative emissions
- 2 Storage Process
- 3 References
- 4 See also
- 5 External links
Carbon (CO₂) Capture
Carbon capture concerns different technologies to mitigate or eliminate CO₂ emissions into the atmosphere. CO₂ is everywhere in our daily life, besides being present in the air, it’s also generated by emissions result from the combustion of fossil fuels, biofuels, and biomass to deliver heat, or from industrial sites, such as refineries, petrochemical, power, steel, pulp, and paper plants. Another source of emissions could be from the industrial process itself. For example, hydrogen production or cement production also generate CO2. Carbon Capture Utilization and Storage (CCUS) processes significantly reduce the CO2 emissions generated.
In 2011, the global climate change summit in Cancún decided to admit CCS as an eligible technology for Clean Development Mechanism. This carbon trading mechanism of the Kyoto Protocol gives an incentive to greenhouse gas abatement projects, in developing countries. As shown in Figure 1, CCS is in the third position, after energy efficiency, and the development of renewables, in terms of CO₂ emissions reduction. Its role is around 10% at horizon 2040 but may rise to allow zero CO₂ emissions, in the second part of the century.
The 2°C scenario roadmap, indicates that up to 120 Gt of CO₂ at horizon 2050 must be captured, transported, and stored into suitable sites (Figure 2). CO₂ capture exists for a long time in the oil and gas industry as it is common to have CO₂ being present in the natural gas composition. So, in such a case, CO₂ is removed prior to gas commercialization. It means it is captured on the production site prior to the natural gas being compressed and transported.
This is the case for example in the North Sea but also in the USA, Eastern Europe, the Middle East, and Southeast Asia too. Indeed, a lot of gas fields are not exploited as their carbon dioxide concentration is above 15 %, sometimes reaching up to 80% of the natural gas composition (Figure 3).
Combustion is commonly referred to as 'burning' in the common language. Combustion is how most of the primary energy contained in fossil fuels - and biomass - is transformed into an energy vector by a chemical reaction of a fuel with, typically, atmospheric oxygen. The reaction produces heat and light in the form of glowing or a flame.
The combustion of hydrogen, that is the molecule, H₂, with the oxygen in the air, that is the molecule, O₂, produces only water, that is H₂O.
2H₂ + O₂ → 2H₂O
But combustion occurs in the majority of cases with hydrocarbons. These hydrocarbons contain predominantly atoms of carbon, C, with atoms of hydrogen, H, attached to them. If we take the example of methane, that is the molecule CH₄, which composes around 95% of natural gas, and reacts with atmospheric oxygen, the products of the combustion are carbon dioxide, CO₂, and water, H₂O. We say that the reaction is complete if there is enough oxygen available to convert all the fuel to carbon dioxide.
CH₄ + 2O₂ → CO₂ + 2H₂O
If that is not the case, if the supply of oxygen is limited, the reaction is incomplete. Some molecules of methane, CH₄, react with oxygen to form carbon monoxide, CO, and water, H₂O.
CH₄ + 3/2O₂ → CO + 2H₂O
Other molecules of methane, CH₄, again with a limited supply of oxygen, O₂, can form carbon, C, in the form of solid particulates, and water, H₂O.
CH₄ + O₂ → C + 2H₂O
Combustion processes are, in practice, designed to achieve complete combustion to avoid forming particulates of carbon C, to avoid forming carbon monoxide CO, which is a toxic pollutant, and to release as much energy as possible by converting 100% of the fuel to carbon dioxide, CO₂. This is achieved by burning fuel in an excess of oxygen.
CH₄ + 2O₂ + excess O₂ → CO₂ + 2H₂O + excess O₂
The oxygen is supplied from the air, which is made of 79% nitrogen and 21% oxygen. So, we get in the combustion gases: CH₄ plus oxygen, plus excess oxygen, and excess nitrogen, giving carbon dioxide, CO₂, water, H₂O, and the excess oxygen and the excess nitrogen, which haven’t reacted.
CH₄ + 2O₂ + excess O₂ + excess N₂ → CO₂ + 2H₂O + excess O₂ + excess N₂
To stop CO₂ from entering the atmosphere after combustion, a second process step uses technology capable of selectively transferring the CO₂ away from the combustion gases and leaving the other gases, nitrogen, oxygen, and water untouched. Or knowing that air is made of 79% nitrogen and 21% oxygen. we can separate the nitrogen, N₂, from the oxygen, O₂, in the air prior to combustion to burn the fuel. CH₄, plus oxygen plus excess oxygen gives carbon dioxide, CO₂, water, H₂O, and excess oxygen. Water can then be easily separated by condensation. 
CH₄ + 2O₂ + excess O₂ → CO₂ + 2H2O + excess O₂
Types of Carbon (CO₂) Capture
There are three types of CO₂ capture technologies. They can be applied to the large majority of industrial processes emitting CO₂ to the atmosphere. The principles are very similar when capture is added to steelworks, cement plants, oil refineries, steam methane reformers for hydrogen, chemical plants, etc.
- Post-Combustion Capture, where technologies are implemented after combustion takes place.
- Oxy-combustion, where fuel is burned in oxygen, without nitrogen.
- Pre-combustion, where carbon is removed prior to combustion, leaving a carbon-free fuel - hydrogen - to burn.
In this method, carbon dioxide is captured after the combustion process takes place. It includes various technologies Using this technology does not require making any changes to the base design of power stations. It is a fundamental difference with the other families of capture technologies, Oxyfuel Combustion and Pre-Combustion Capture. The post-combustion process consists in capturing CO₂ after combustion in the exit flue gas, by:
- A physical method (adsorption on a solid or using a permeation membrane).
- Or with a chemical one (most commonly used method).
In the context of a conventional power station. A fuel, for example, biomass or coal, or municipal waste, is combusted with air in a boiler to generate high-temperature, high-pressure steam. The steam is then used to drive a series of turbines to generate electricity. Because the combustion takes place in an excess of air to guarantee the presence of excess oxygen, the exhaust gas leaves the boiler with carbon dioxide diluted within other gases with a low concentration of typically 4-20%; the other gases being the excess oxygen left after combustion, plenty of nitrogen and water, the other product of combustion, in a gas phase. The exhaust gas then passes through a step represented here as capture. Once the CO₂ has been filtered out from the nitrogen, oxygen, and water molecules, the exhaust gas is vented to the atmosphere via the power plant stack. The CO₂ separated from the flue gas is compressed to be sent for geological storage. The capture step could capture any amount of CO₂ from the flue gas, capture levels are around 80-99%, to be as close as possible to zero residual emissions. Energy taken as electricity and heat from the power plant is required for the capture step and the compression of the carbon dioxide.
There are different technologies to separate carbon dioxide from exhaust combustion gases. They all rely on a common principle: the chemicals or materials used in Post-Combustion Capture do not react with the other combustion gases, that is nitrogen, oxygen, and water, but only with carbon dioxide. The most widely used technology is based on the property of specific chemical solvents. These chemical solvents consist of specific molecules, called amine molecules diluted in water, with the ability to absorb carbon dioxide. The following are a few examples of these technologies:
After being cleaned from NOx, SOx and particles, the combustion gas flux enters the absorber column, containing an amine-based solvent (Figure 5), generally a monoethanolamine (MEA) or diethanolamine (DEA) solvent. There, CO₂ is caught by the solvent through the surface exchanger as illustrated here. CO₂-saturated MEA can be regenerated in a stripper, and the amine solvent can be recycled into the absorber (Figure 6). After water separation, the CO₂ can be isolated and managed for reuse or storage.
Different amine-based solvents in this technique are available, but optimization is case to case. The technique can be easily deployed on existing industrial plants, allowing capturing 75 to 90 % of the CO₂ present in the combustion gas flux with a relative CO₂ purity of 90%.
Another innovative approach is to use demixing solvents and to only regenerate the CO₂-rich saturated phase while recycling the CO₂-lean phase in the absorption column (Figure 7).
Oxyfuel combustion processes use oxygen rather than air for the combustion of fuel (fuel is burned in oxygen, without nitrogen). This produces exhaust gas that is mainly water vapor and CO2 that can be easily separated to produce a high purity CO2 stream. It requires rearranging the base design of power stations by adding equipment and processes prior to the boiler where combustion takes place, as well as designing the boiler in a different way. Air enters an air separation unit where it is processed by making a cryogenic distillation of the air to be separated into its two major components, nitrogen, and oxygen. The aim here is to produce very high purity oxygen to enter the boiler of the power station together with the fuel. When fuel is burned with pure oxygen instead of air, the combustion rises to very high temperatures in the absence of nitrogen to dilute the flame and the gases. The temperatures may become so high that the materials of the combustion chamber struggle to withstand them. To limit the temperature rise, a fraction of the combustion gases is diverted back to the combustion chamber to play the role of the diluent that nitrogen would otherwise play. These combustion gases have already reacted and are inert since they don’t contain any fuel anymore. They cool down the boiler, where the combustion takes place, with fresh oxygen and fuel, and ensure that the temperatures stay at an acceptable level. The heat generated by combustion is used to raise steam to drive turbines and produce electricity. The combustion gases leave the boiler and enter a step represented here as capture. Because there is no nitrogen going through the boiler, this capture unit is much smaller than previously described for Post-Combustion Capture technologies. In a conventional combustion process, the concentrations of carbon dioxide in the combustion gases are anywhere between 4-20%, depending on the fuel and the application. Here the concentrations are much higher, reaching up to 90% of carbon dioxide, which makes it much easier to separate- especially since the rest of the combustion gases consists mainly of water. Water can be condensed into a liquid without any chemicals or complex processes, using a simple process called drying of the gas, to produce a stream of 95-99% CO₂, which is then compressed to be transported to permanent geological storage. Therefore, there is no exhaust of combustion gases to the stack. It is possible to achieve capture levels very close to 100% with this technology. Energy taken from the electricity output of the plant is required to drive the air separation process, the drying of the combustion gases, and the compression of the carbon dioxide.
Pre-combustion capture processes convert fuel into a gaseous mixture of hydrogen and CO₂. The hydrogen is separated and can be burnt without producing any CO₂. The CO₂ can then be compressed for transport and storage. The fuel conversion steps required for pre-combustion are more complex than the processes involved in post-combustion, making the technology more difficult to apply to existing power plants. Pre-combustion capture is used in industrial processes (such as natural gas processing) while its application in power generation will be via new build projects. 
Another approach for Carbon mitigation is removing carbon dioxide directly from the air around us. This is very different from traditional Carbon Capture and Storage (CCS). Air Capture absorbs carbon dioxide that's already been emitted. This is a way of trapping emissions from small mobile sources like cars, trucks, and planes, emissions that represent sixty percent of the total today.
To reach the Paris agreement’s objective, which is to limit the warming to below 1.5 degrees, conventional abatement technologies will not be sufficient. We will also need to remove around 15 GtCO₂ from the atmosphere every year by 2100. As a comparison, in 2017 we emitted 37 GtCO₂ worldwide.  This implies to remove the equivalent of 40% of our current annual emissions from the atmosphere every year. To remove carbon dioxide from the atmosphere, we need technologies that capture atmospheric CO₂ and store it permanently: the so-called Negative Emission Technologies.
Carbon Negative Technologies
There are over ten technologies for negative emissions, they include but are not limited to:
Afforestation and reforestation
Afforestation is planting trees where there were previously none, whereas reforestation is the restoration of areas where the trees have been cut down or degraded. This is one of the most feasible options, although it still has drawbacks and uncertainties. Reforestation is almost universally desirable, particularly if with native trees. One potential obstacle to afforestation is the availability of land, and the suitability of tree species with respect to bio-diversity.
Direct air capture
Direct Air Capture is the engineering of devices to suck CO₂ out of the air and then store it permanently away from the atmosphere. Sieving one out of 2500 molecules of air is, however, energy-intensive, much more so than sieving from concentrated sources, and this ultimately has a financial cost. But unlike other options involving the use of land or forests, Direct Air Capture needs very little land.
Bioenergy with CCS (BECCS)
The principal behind BECCS is that as plants grow, they encapsulate atmospheric CO₂ into biomass. In bio-fueled industries, such as power plants, pulp, and paper industries, or the sugar industry, the CO₂ released by combustion can be captured and stored instead of allowing it to return into the atmosphere.
Once the CO2 is captured and compressed, the CO2 is transported with a pipeline system or by boat to a storage site, often to be injected into an underground storage site (or geological formation), where it will be safely stored for the long term.
Although storage is one of the last steps in the Carbon Capture and Storage (CCS) process it is one of the first steps to be considered when developing the infrastructure and systems of CCS. There is no benefit to capturing CO2 unless it can be stored thus the total storage capacity and its location is an important constraint on how much CO2 can be stored and kept from leaking. Nowadays, people seemed to choose to store it on fault planes because it was proven to store the carbon well and provide less leakage than in depleted well. After storing, constant monitoring is needed to prevent leaking and for further studies about the effect of carbon injection on the earth. Seismic surveys and fracture monitoring are used to determine the condition of the carbon after being injected so that we know if the carbon has a lot of movement that can lead to earthquakes or leakage on earth.
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