Carbon Capture, Utilisation & Storage (CCUS)

They are a group of technologies for capturing of CO₂ from large and stationary CO₂ emitting sources, such as fossil fuel-based powerplants and other industries. CCUS also involves the transport of the captured CO₂ to sites, either for utilisation in different applications or injection into geological formations or depleted Oil & Gas fields for permanent storage and trapping of CO₂.

Neef for Carbon Capture (CCUS) Technologies

1. Necessary to decarbonise hard to electrify sectors: CCUS offers only known technology for decarbonising the hard to electrify and CO₂ intensive sectors such as steel, cement, oil & gas, petrochemicals & chemicals and fertilisers.
2. Hydrogen economy: CCUS is expected to play a critical role in enabling hydrogen economy through production of blue hydrogen (i.e., coal gasification based hydrogen production with CCUS) based on India’s rich coal endowments.
3. Sustenance of existing emitters: Nearly two-thirds of India’s 144 mtpa crude steel capacity and 210 GW of coal-based power capacity have an age of less than 15 years and cannot be wished away or stranded and need to be made sustainable by retrofitting with CO2 capture and disposition infrastructure. 

Carbon Capture Technologies

There are three broad categories of technologies for Capturing CO₂:

1. Post-combustion technologies: CO₂ is separated from the flue gas after combustion. Fossil fuels like coal, oil, natural gas etc. are burnt in the presence of air. Hence, the flue gas is rich in N₂ and the CO₂ percentage typically varies between 3-15%. Since the partial pressure in CO₂ in the flue gas is quite low, very high-volume chemical solvent (amine) circulation is required for CO₂ capture. This makes post-combustion technologies energy and cost intensive.
2. Pre-combustion technologies: This involve removing CO₂ through upstream treatment of fossil fuels before combustion. Major difference between pre-combustion & post-combustion is that the former is favoured in cases where the gas stream has a higher partial pressure of CO₂, such as in gasification of fossil fuels, natural gas based H₂ production or sour gas processing. Since no chemical bonds need to be broken for solvent regeneration, the thermal energy penalty is much lower. The regeneration of physical solvent is primarily achieved by reducing pressure.
3. Oxy-fuel combustion technologies: While post & pre-combustion carbon capture technologies have been commercially established, oxy-fuel combustion technologies are still in the development stage. Oxy-fuel combustion represents an emerging novel approach to near zero-emission. It is accomplished by burning the fuel in pure oxygen (O₂) instead of air (O₂ & N₂). The flue gas stream would be primarily composed of water & CO₂, rather than N₂. High-purity CO₂ can be recovered by condensation of water.

Direct Air Capture (DAC): DAC directly captures dilute CO₂ (at 415 ppm) from the air and may also emerge as a form of carbon capture that has wide applicability, as it is independent of the source and concentration of the emission stream.

However, DAC is still in early stages and the economics and scale of operations are yet to be established.

CO₂ Capture technologies

Solvent-based absorption: Solvent based CO₂ capture processes have been used for processing natural (sour) gas, combustion flue gas and Fischer-Tropsch (FT) synthesis products. The fundamental principle on which solvent-based CO₂ capture technologies work is selective absorption of CO₂ over other gaseous constituents.  

The CO₂  present in the feed/process gas is first selectively absorbed in an absorber using a solvent (physical  or chemical), the CO₂ lean gas exits the absorber. The CO₂ rich solvent is sent to a stripper type configuration where CO₂ is released from the solvent and the lean solvent is regenerated for reuse. 

Solvent based CO₂ capture technologies are classified into:

1. Physical solvent-based absorption: CO₂ gets physically dissolved in the solvent. This method performs well at higher partial pressure of CO₂. Ex. Higher gas stream pressure and CO₂ concentration in Syngas of gasifiers and SMRs make physical absorption-based capture more suitable. 
2. Chemical solvent-based absorption: CO₂ reacts with solvent chemically. This method is better suited for gas-streams having low concentration and partial pressure of CO₂ due to the high chemical affinity to CO₂ to amine/carbonate based chemical solvents and faster rate kinetics.  Ex. In Low CO₂ partial pressures in the flue gas of coal-fired power plants make amine based chemical absorption preferred technique. Common solvents used as: Amine based solvents, Non-Aqueous Solvents, Carbonate-based solvents etc. While primary and secondary amines (such as MEA, DGA, AEE, DEA) have higher reaction rates and lower CO2 carrying capacities, tertiary, and polyamines (such as MDEA and piperazine) have lower reaction kinetics and higher CO2 carrying capacities. 
3. Adsorption: In this process, CO₂ molecules selectively adhere to the surface of adsorbent material and form a film due to difference in diffusivities and heat of absorption values for feed gas stream components. This method is suitable for gas streams with moderate to high pressure and moderate CO2 concentration such as SMR flue gas or BF gas.
4. Cryogenic separation: This process like conventional distillation process, except that it involves separation of components from a gaseous mixture (instead of liquid) based on the difference in their boiling points. This technology is preferred in cases where cost of power is low. This technology provides a unique advantage by generating additional hydrogen without increasing the amount of feedstock (natural gas)/producing the same quantity of hydrogen with lower natural gas consumption. 
5. Microalgae based carbon capture: Microalgae utilise the sparsely concentrated CO2 from atmosphere via Carbon Concentrating Mechanism (CCM) and thus are well-qualified for CO2 capture from a more concentrated stream of flue gas. Microalgae use CO₂ as a nutrient for cultivation of microalgae. Microalgae can be cultivated in saline water systems as well and do not compete with food crops for arable land for cultivation. Due to faster growth cycle of microalgae, they can typically entrap 10-50 times more CO₂ compared to terrestrial plants. They can also deacidify the seawater or wastewater used for their cultivation. This technology is, however, in its nascent stage.

End use of captured CO₂ can be either utilisation or permanent storage. 

CO₂ UTILISATION TECHNOLOGIES

Rising interest in CCUS as a decarbonisation solution across industries, there is an also a need to look at CO₂ utilisation pathways and technologies that are most appropriate for India. Some proven technologies for utilisation of captured CO₂ are:

1. Enhanced Oil Recovery (EOR): CO₂ is used in EOR to produce low-carbon oil from maturing oil fields. EOR can help India towards residual oil extraction that is environmentally sustainable and economically feasible.
2. Green Urea: Urea production from green ammonia can utilise a significant part of CO₂. India’s production of ammonia is primarily based on imported LNG. 
3. F&B applications: CO₂ can be utilised in applications such as carbonated drinks, dry ice and modified atmosphere packing. However, scales are quite small compared to the volume of CO₂ generation.
4. Building Materials (Concrete & Aggregates): Utilising CO₂ for producing building materials (aggregates & concretes) is likely to be the most attractive and feasible option. CO₂ can be used both during concrete curing & aggregate formation. 
5. Chemicals (Methanol & Ethanol): CO₂ can be used production of chemicals such as methanol and ethanol at commercial scales. 

Methanol is a low carbon hydrogen carrier that can support applications like fuel substitution and act as intermediate to produce various speciality chemicals like acetic acid, MTBE, DME and formaldehyde producing products like adhesives, foams, plywood subfloors etc. CO₂ hydrogenation process is used to convert captured CO₂ into methanol.

Ethanol can be produced by ethylene hydration or biological processes using H₂, CO and CO₂ by biological gas fermentation process. Ethanol can be blended with Petrol to reduce fuel import bill.

6. Polymers: CO₂ can be converted into various polymers such as polyether carbonates, polycarbonates, diphenyl carbonate, cyclic carbonates etc. A polymer product of CO₂ named AirCarbon has found multiple applications (Laptop packaging, cell phone casings, furniture etc.)

CO₂ Storage Options

1. Enhanced Oil Recovery (EOR): Ex. In India, Mumbai High, Assam shelf, Krishna Godavari basin & Cambay basins are prominent sites for storage of CO₂. In CO2 EOR, compressed CO2 is injected into the reservoir. At high densities, CO2 is readily miscible with oil. It swells the oil and reduces its viscosity, thereby driving it away from rock formations and towards the production wells. It is estimated that 3.4 Gt of storage is available in India for CO₂ storage. 
2. Enhanced Coal Bed Methane Recovery (ECBMR): In this method, CO₂ is injected into unmineable coal seams under supercritical conditions. The CO2 injected is accumulated in the coal cleats in a dense gas phase. This CO2 is adsorbed and absorbed in the coal. Since CO₂ has a higher affinity for coal than CBM, it pushes the coal bed methane towards production wells, thus enhancing its primary recovery. Similar to CO₂ EOR, ECBMR can help in permanently storing CO2 and the recovered methane can also help offset the cost of carbon captureThe potential for ECBMR is localized in the eastern region due to the presence of major coalfields. These can be storage clusters for industries that are close to the coalfields, such as steel and power plants.
3. CO₂ storage in Deep Saline Aquifers: Captured CO₂ can be permanently stored in deep saline aquifers. Deep saline aquifers consist of porous rock formation that contains high quantities of unusable saltwater. Salt/mineral content is very high in this water rendering it unusable for human use. Brine water is called formation liquid and it is trapped by an impermeable rock called caprock. However, compared to EOR or ECBMR, injection of CO₂ in deep saline aquifers has no economic benefit.  
4. CO₂ storage in Basalts: Basaltic rocks constitutes divalent cations of Ca, Mg and Fe. They can react with CO2 dissolved in water to form stable carbonate minerals and thus can offer a safe CO2 sequestration method. Compared to saline aquifers, basalt rocks offer faster reaction kinetics due to abundance of iron, calcium and magnesium oxides. Abundance of basalts on Earth’s surface is the reason interest in CO₂ storage R&D programs in basalts.