What is carbon capture?

The process of capturing carbon dioxide (or CO2) from the atmosphere, during power generation, and industrial processes.

What are the different categories of carbon capture?

a) Post-Combustion Carbon Capture

This category of carbon capture is concerned with capturing carbon dioxide from flue gas (the smoke coming from the factory chimney).

What are the different methods of Post-combustion carbon capture?

ai) Solvent-based carbon capture
**A solvent is a substance that can dissolve other substances. The chemical process is called absorption. This is where a liquid is soaked up into something like a sponge, cloth, or filter paper.

In carbon capture, the industry standard of solvents is amine-based solvents. The reason amine-based solvents are used is that they are the most developed solvent to date. As they have been used commercially for oil recovery since the 1950s.

An amine is a derivative of ammonia (NH3). An amine takes the structure of NH3 and removes one or more hydrogens and replaces it with another compound. Below is an image of the transformation from ammonia to a 3° amine.

Note : For this article view the letter “R” as a substitute for hydrogen.

The goal of the change is to increase the reactivity of our solvent. When we increase reaction speed we can use less energy to power our carbon capture plant.

Even though amine-based solvents are the most research-developed solvents, they require a lot of energy use, sometimes as high as 3 times the minimum theoretical energy. Making the technology very expensive and unprofitable.

**What is the process of using amine-based solvents?
**CO2 is absorbed typically using amines to form a soluble carbonate salt. The absorber operates below 60°C pressure.

This reaction is reversible and the CO2 can be released by heating the solution with the carbonate salt in a separate stripping column. The CO2 stripping occurs at 120°C and pressures ranging between 1.8 and 3 bar are shown in step 2. As the CO2-absorbent bond gets separated in the stripping column, the CO2 and absorbent are released. Usually, the CO2 is stored in the ground and the absorbent is reused in the chemical process to capture more CO2.

These solvents are thought to deliver potential benefits in CO2 capture systems, such as enhanced, increased absorption capacities, and lower regeneration heat duties.

Increased absorption leads refer to a CO2 absorption capacity. It is defined as the amount of CO2 that can be absorbed per mole of solvent (mole CO2/mole solvent). More CO2 per reaction creates a cheaper cost per ton of CO2 captured.

The main performance indicators for solvents are material selectivity and ease of regeneration. Both of which contribute to the cost of the operation. The higher the selectivity and the higher solvent regenerated the less a carbon capture solution would cost.

Selectivity, therefore, is the capability of a method to distinguish a given analyte from other substances. The analyte is our target substance in the flue gas: CO2. The higher the selectivity of our material the more likely it can absorb the CO2. The higher the selectivity of our material the more likely it can absorb the CO2. The impurities in the container might hinder the reaction.

Ease of generation is the amount of energy required to regenerate a solvent. This is to separate the bond between the solvent and CO2. After the reaction, we usually end up with less than 100% of the original solvent. The losses come as a result of impurities in the elements used (solvent, CO2, or other elements absorbed that aren’t CO2).

aii) Sorbent-based Carbon Capture

A sorbent is a material that is used to absorb liquids or gas. Solvents and sorbents have the same function. The difference is sorbents are usually solid, while solvents are usually liquid.

Sorbents are porous materials meaning they have a void within their physicals structure. When using the same absorption and separation process as the solvent-based carbon capture the CO2 is capture within the voids.

The attractive part about absorbents is they have lower regenerations energies due to lower heat capacities. Heat capacity is the number of heat units needed to raise a substance’s temperature by 1 degree. If we can raise the temperature of our sorbent we can go through the breaking of the sorbent-CO2 bond in the stripper column faster at the same temperature. Less time means less energy used, creating cheaper costs.

Controlling the pore (the void) sizing and functionalization of the absorbent have been reported to be the 2 most prominent factors affecting gas separation (referring to CO2 separating from flue gas) and uptake (the absorption process).

Pore sizing is the size of an area of space within the structure. In sorbents the larger we can make the voids the make space where CO2 can be capture. You can do this through functionalization is the process of adding new capabilities or properties to material by changing the surface chemistry of the material.

iii) Membrane-based Carbon Capture

A membrane is a selective barrier allowing some materials/elements to pass through and others do not.

Membranes are promising because they have the potential to overcome the limitation of sorbents and solvents. The main advantages of membrane-based technologies include the following:

1. Low capital cost
   The membrane requires little material to coat. It does not need additional facilities such as a large pretreatment vessel — the treatment of something with a chemical before use — and solvent storage.

2. Low operating cost
   The main operating cost for the membrane separation unit is only membrane replacement. Due to the smaller size and weight of the membrane, the cost is much lower than the conventional techniques, which replace a large amount of solvent or sorbent.

3. Simplicity and reliability
   Since the membrane does not show fast decay in performance that most likely occurs to the traditional solvents or sorbents, it can be running unattended for long periods.

4. Adaptability
   The membrane system is designed and operated to remove the required percentage of CO2 instead of the absolute quantity of CO2 removal. Variations in the feed CO2 concentration can be adjusted by varying the space velocity to keep constant product quality.

5. Design efficiency
   A membrane system can integrate several processes into one unit, such as Hg vapor removal, H2S removal, and dehydration. Traditional CO2 removal techniques have to operate these steps separately.

6. Easy for remote area
   Multiple membranes could be packed into one module to reduce size and weight, which not only increases membrane area in unit volume but also makes it easier to transport to remote locations. Simple installation is feasible at which spare parts are rare, laborers are unskilled and additional facilities (such as solvent storage, water supply, and power generation) are short in supply.

Membranes can virtually forever if you service them regularly and replace parts that wear out, like the storage tank and the faucet. Assuming that you are not constantly replacing it the typical membrane life is about 2 to 5 years, depending on the nature of the substances that are processed.

Membrane systems are not new — Selective membranes for CO2 capture have been widely developed and applied since the 1970s. There are 2 types of membrane systems. Membrane Gas Separation (MGS) and Membrane Contractor (MC). Even though I put in the post-combustion carbon capture section, it also can be used for Pre-combustion carbon capture (found later below). Below are the 2 types of membranes.

**Membrane Gas Separator (MGS)
**The performance of an MGS depends on several factors, such as the membrane material, thickness, and configuration(e.g. hollow fiber, flat sheet) as well as the module/system design.

Membrane material similar to solvents and sorbents is a mechanism within the material that makes them have a strong tendency to separate CO2 from the rest of the flue gas.

Thickness is had been found to increase membrane thickness increases microcapsule strength, whereas decrease membrane permeability. When the membrane decreases in permeability, then more substances pass through it.

As mentioned before, a configuration like hollow fiber can increase the surface area by up to 30 times. A membrane with a smaller surface area or lower permeability will impede molecular movement and thus lead to slower diffusion. The larger the surface area the quicker the reaction happens requiring less energy, reducing cost.

Selectivity, therefore, is the capability of a method to distinguish a given analyte from other substances.

Generally, the process goes as follows: gas molecules from the feed side — starting point or where the gas comes from — are absorbed by the membrane, then they diffuse across the membrane and then they desorb cause the release of the CO2 on the permeate side.

Most commercial membranes for gas separation are currently dominated by organic or polymeric membranes since they have some attractive features, e.g. lower fabrication cost compared to inorganic material, and are easy to fabricate either into flat and hollow fiber modules. These specific modules like flat and hollow fibers increase the surface area of our membrane. Higher surface area means we can capture/absorb more CO2 in the flue gas per cycle of membrane filtering.

Membrane Contractors (MC) combines membrane with conventional phase contacting operation such as absorption, and hence the benefits of both technologies can be fully utilized.

In MC, a porous (has voids) membrane is used to provide an extra area for contact between the gas and liquid phases and to avoid dispersion, which is the process of distributing substances across a wide area.

Hollow fiber membrane modules are preferred since they give up to 30 times higher surface area. On the basis, that both membranes (a hollow fiber and non-hollow fiber module) have the same absorption capacity. The one with a higher surface area has more space to capture/absorb the CO2 in the flue gas. The more you can absorb with a membrane the cheaper your cost per ton of CO2 capture.

In addition to the high surface area, the hybrid process offers other advantages, such as independent gas/liquid flow rates control without causing flooding, lack of modularity, and high cost.

Schematic of (a) membrane gas separation (MGS) and (b) membrane contactor (MC) for CO2 removal

Even though in recent years various types of materials (including inorganic and polymer-inorganic composite) have been developed as alternatives demonstrating promising features under research conditions, such as ceramic, glass, metals, zeolite, and carbon molecular sieve, but they cannot compete commercially with the current (most widely used) polymeric membranes because of the high module fabrication (production) cost.

The process of MC is very similar to MGS. The only difference between the 2 is the outcome of the reaction is the creation of a liquid or solvent.

All membranes (MGS or MC) have a trade-off between permeability — how fast molecules are passing through the membrane material — and selectivity — is the capability of a method to distinguish a target substance (CO2) from other substances. Suggesting as permeability increases selectivity decreases and vice versa.

Conclusion on Post-Combustion Carbon Capture

With membranes, it seems they only can limit, and completely stopping the production of CO2 seems probable isn’t too far away. One limitation this provides is we can maximum of net-zero on specifically on high CO2 concentration gas (15% to 20% concentration) otherwise it is less economically viable when compared to solvent and sorbent based carbon capture.

If you are just looking at an industry which is the processing of raw materials and manufactured goods in factories. Flue gases from natural gas combined cycle (NGCC) plants typically contain ~4% CO2 by volume, while compared to a CO2 concentration of 12–15% in flue gases from coal plants. Meaning that in most cases they are not economically viable for membrane use.

Advantages of Post-Combustion Carbon Capture

The advantage of post-combustion carbon capture is it can capture about 90% of the CO2 emissions. The tools/methods to capture carbon after combustion does not require fundamental changes to the processes of power and industrials plants, reducing the friction of usage.

Disadvantages of Post-Combustion Carbon Capture

The disadvantage of post-combustion carbon capture is it has a high capital investment. The energy output is reduced by at least 30%. The electricity unit generation cost can be increased by up to 140% for coal-fired plants and 60% for gas-fired plants.

Pre-combustion Carbon capture-combustion capture refers to removing CO2 from fossil fuels before combustion is completed.

This CO2 gas mixture that is created can range from 15%–50% concentration, allowing for potential uses of membranes from above. When compared to post-combustion technology, the gas mixture has a CO2 concentration of ~4–15%. The pre-combustion process allows for easier removal of the CO2 before the H2 is combusted.

Due to a higher concentration of CO2, pre-combustion capture typically is more efficient but the capital costs of the gasification process (process described above) are often more expensive than post-combustion capture methods.

The process goes as follows:
You take air and put it into an air separation unit producing almost pure oxygen. The oxygen flows into the gasifier reacting with fuel to form synthetic gas or syngas. Synthesis gas, or syngas, is a mixture of hydrogen, carbon monoxide, CO2, and smaller amounts of methane.

The syngas goes into a shift reactor and reacts with steam. The shift reactor converts carbon monoxide (CO) into hydrogen (H) and carbon dioxide. The CO2 is compressed or dehydrated which are purifying processes for natural gases, which are stored in the ground.

The hydrogen from the shift reactor gets burnt to power gas turbines and make electricity. The Heat Recovery Steam Generator (HRSG) then the excess heat from the gas turbine and turns it into steam. The steam is used to power the steam turbine to also create electricity.

The different methods of pre-combustion carbon capture include solvents, sorbents, and membranes (similar to post-combustion). The different methods are simply just ways of separating the hydrogen and CO2 from the syngas. The main compartment where each of these methods differs is how to separate the H2 and CO2 in the syngas in the shift reactor.

The methods that are being used are also the same. Pre-combustion carbon capture uses solvents, sorbents, and membranes. The key difference is the instead of trying to separate CO2 and CH4 (methane), the materials are trying to separate CO2 and H2.

In post-combustion carbon capture, when we were referring to solvents, it was about chemical solvents which are used based on the chemical reaction it creates.

In pre-combustion carbon capture, we are using physical solvents, which are used based on gas solubility. Solubility is the ability of a substance to be dissolved. This process is used as a result of the elevated CO2 partial pressure. Partial pressure refers to the pressure that a molecule is taking out of all of the pressure within a system. In this case, we are referring to CO2. Its partial pressure is value is higher in pre-combustion than post-combustion. What physical solvents can do is selectively capture CO2 in contact with a gas stream without a chemical reaction occurring.

Advantages of Pre-combustion Carbon Capture

CO2 emissions capture of 90% to 95% similar to Post-combustion, but since the syngas has a higher concentration (15–50% instead of 4 to 15%) of CO2 when comparing to post-combustion. It becomes cheaper if measured on a cost/ton basis.

Disadvantages Pre-Combustion Carbon Capture

It can only be applied to power plants and a limited type of industrial plant. This means that it can not be retrofitted — technology can be used as an add-on to some industrial and coal plants increasing friction for adoption. The energy penalty is only 20% (which is better than post-combustion carbon capture).

c) Oxy-fuel Carbon capture

Oxyfuel is where the capture of CO2 takes place after the fuel is burnt with pure oxygen instead of air. The process goes as follows:

We take the air, put it into an air separation unit, and create almost pure oxygen. Instead of using a gasifier, we use a broiler creating create heat-generating power. As an outcome of the broiler, we also create H20 and CO2, some of which are taken to control the heat into the broiler control temperature. The rest of the CO2 and H2O goes to the condensation unit.

In the unit, the CO2 can be separated by condensing of the water. Since the water was being been heated up to high temperatures, it is in its gas form: water vapor. The process of condensation takes gas and turns them into a liquid. In the process of transitioning to its liquid form, water starts to separate from CO2 Since CO2 and water are currently bonded when the water is condensed it turns into water. As an outcome, you would get CO2 that can be not collected and dehydrated, and ready for transport and storage. The H20 comes out and it can be used for hydroelectricity to power the plant.

The advantage of Oxyfuel-combustion is it can take up to 100% of CO2 (so we can get to net-zero).

Disadvantages include a higher capital investment compared to pre and post-combustion. Air separation per unit is energy-intensive (increasing cost). The process makes retrofitting — technology can be used as an additive to improve the current process — difficult. As a result, this does not have any commercial application yet.

Direct Air Carbon Capture (DAC)

The previous methods were all meant to reduce emissions of industrial plants. Since DAC captures CO2 from the atmosphere, the possible CO2 capture capacity is not restricted by emissions source — which was the case for the other 3 methods above. The general process goes as follows:

What are we doing with the carbon after we capture it?

Currently, there are 2 methods: Storage and Usage.

Carbon Capture and Storage (CCS) is where the CO2 is safely transported by road, ship, or pipeline, or a combination of the 3. Once it reaches the storage site, the CO2 of injected underground in carefully selected ore (or rocks) to store CO2. The CO2 is put several kilometers below the earth’s surface with pressure and temperature such that carbon dioxide will be in the supercritical phase.

A supercritical phase is a point in temperature and pressure where the liquid and gaseous phases of a substance merge into a single phase. This creates a dense liquid but has a viscosity like a gas. Density refers to how much mass exists in more unit volume. Liquids have a higher density per unit of volume.

There are more molecules in the CO2 in the same amount of space when compared to CO2’s gas state. Viscosity refers to a substance’s resistance to flow, so CO2 still flows like a gas. The reason you would want it in the supercritical condition is that the required storage volume is substantially less than if the CO2 were at “standard” (room)-pressure conditions.

Once injected into the ground, the CO2 goes through a process called structural storage where the CO2 is trapped under an impermeable layer of rock called the caprock. Impermeable rocks trap means that it doesn’t does let a substance pass through it. The rock is meant to prevent the CO2 from going up through the ground. As CO2 tries to keeps pushing against the caprock, the CO2 will go through mineral storage. This is a process where the CO2 binds chemically and reversibly to the surrounding rock in a process. The global capacity to store carbon dioxide is vast and lies between 5 and 30 trillion tons.

The current common storage sites for CO2 include former gas and oil fields. This happens due to enhanced oil recovery. When the CO2 is captured and stored in the ground, it flushes out oil from deposits that would otherwise be too hard to reach. It turns out that the oil that is capture once burned can release more carbon than was captured.

The process outlined above is currently labeled Carbon Capture Use and Storage (CCUS). In the process of CCUS, after the CO2 is captured, it’s used to create new materials and products. Instead of just sequestering the CO2, it’s utilized to make everything from fuel, concrete, and shoes, to cleaning products, plastics, and food.

What are Companies Working in The Space?

From brief research, I found about 80+ carbon capture companies, which I will write an article break down the technology of each one to map out different ways carbon capture. Below are 2 companies just to give you an idea about the extremes in cost.

Carbon engineering

Method: Direct Air Carbon Capture

Technology: Their technology has 4 main pieces of equipment, all of which have been widely used in large-scale industries for years. This is how our technology achieves megaton scale with low scale-up risk and improved cost estimations.

The process starts with an air contactor — a large structure modeled off industrial cooling towers. A giant fan pulls air into this structure, where it passes over thin plastic surfaces that have potassium hydroxide solution flowing over them. This non-toxic solution chemically binds with the CO2 molecules, removing them from the air and trapping them in the liquid solution as a carbonate salt.

The salt has separated the solution into small pellets in a structure called a pellet reactor, which is made from a biodegradable polymer. As the polymer break capable of being decomposed or broken down by bacteria or other living organisms. These pellets are then heated in a calciner, to release the CO2 in pure gas form. The calciner is similar to equipment that’s used at a very large scale in mining for ore processing. This step also leaves behind processed pellets that are hydrated in a slaker and recycled back into the system to reproduce the original capture chemical.

Cost/Ton of CO2 Captured: ranging from $94 to $232 per ton CO2 from the atmosphere.

Cost of Machine: $500m per plant

Potential capture of DAC plant: 1 million tons of CO2 per year per DAC Plant.

As of currently, we are emitting 50 billion tons of greenhouse gas and around 76% is CO2. This means that we are emitting 38 billion tons of CO2 every year. We would need 38,000 carbon engineering plants.

Overall Cost: That would be $19,000,000,000,000 or $19 trillion just to build the machines + if we assume the cheaper cost/ton cost it would be 3,572,000,000,000 or $3.57 trillion to capture all of that carbon per year.

ClimeWorks

Method: Direct Air Capture

Technology: Ambient air is drawn into a collector with a fan and co2 is captured on the surface of a highly selective filter material (ex. membranes) that sits inside the collector next once the filter material is full of co2 the collector (blades)closes and is heated to around 100 degrees Celsius using the Carbfix (another carbon capture company) method the captured co2 is then mixed with water and pumped deep underground where it mineralizes and can be stored permanently for thousands of years.

Business Model: Customers can subscribe to Climeworks to permanently remove carbon dioxide. The problem here is this seems a lot like donation more than a product they would sell. The carbon they capture would capture any, but it is a way for the consumer to support the company.

Cost/Ton (for consumers): There are 3 pricing models

1. Explorer: $11/month you can remove 85KG of CO2 per year. 85KG is 0.085 tones. $11/month is $132 per year. This means that you are paying $1,552.9 per ton of CO2 capture which is more than 15 times less capital efficient than Carbon Engineering

2. Discoverer: $32/month you can remove 255KG of CO2 per year. 255KG is 0.255 tones. $32/month is $384 per year. This means that you are paying $1,505.8 per ton of CO2 capture which is still more 15 times less capital efficient than Carbon Engineering.

3. Special expedition: $75/month you can remove 600KG of CO2 per year. 600KG is 0.600 tones. $75/month is $900 per year. This means that you are paying $1,500 per ton of CO2 capture which is still more than 15 times less capital efficient than Carbon Engineering.

Cost of per ton (for company): $600–$800/ton

Cost of Orca (their best plant): $15 million/plant

Potential capture of Orca (their best plant): 4,000 tons of CO2 per year per Orca Plant. As we are emitting 38 billion tons of CO2 every year, we would need. You would need 9,500,000 plants to get to net zero.

(It’s worth noting upfront that while the plant is selling its CO2, it is not covering its full costs.)

Overall Cost: 142,500,000,000,000 (or $142.5 trillion) to build all of the plants and it would cost $22,800,000,000,000 (or $22.8 billion) per year to cover sequestration and storage cost.