Carbon Capture
As I study for Imperial College London's interview, I might as well share my knowledge on carbon capture methods (something that I wrote in my UCAS Personal Statement!).
There are many "colours" to carbon:
1. Brown Carbon
Brown carbon compounds refer to the organic compounds formed from the incomplete combustion of biomass, typically from plants and wood. These brown carbon are known to absorb sunlight, contributing to atmospheric warming and influencing climate change. They produce complex mixtures of organic compounds with light-absorbing properties. They are sourced from wildfires, agricultural burning, and biomass burning (or in other words, brown carbon = dust).
2. Black Carbon
Black carbon is composed primarily of carbonaceous particles from incomplete combustion processes, such as burning of fossil fuels and biomass. These particles, like brown carbon, are very effective at absorbing sunlight and are responsible for atmospheric warming, thus they are a significant contributor to urban air pollution and climate change. Sources include coal-fired power plants, diesel engines, and wood stoves.
3. Blue Carbon
Blue carbon, like the ocean blue, refers to carbon captured by coastal and marine ecosystems, such as mangroves, seagrasses, and salt marshes. These ecosystems are highly efficient at absorbing and storing carbon. They can sequester carbon at rates 10 to 100 times faster than in-land greenery.
4. Green Carbon
Green carbon (you've probably guessed it...), refers to the carbon captured and stored by terrestrial ecosystems, including forests, grasslands, and soil. Green carbon is absorbed through photosynthesis and stored in plant biomass and soil organic matter.
Now that we know the different "colours" of carbon, let's take a look at carbon capture!
Carbon capture methodologies are essentially techniques used to capture carbon dioxide (CO2) emissions from sources such as power plants, industrial facilities, and even directly from the atmosphere.
For power plants and industries, there are primarily two types of carbon capture:
1. Pre-Combustion Carbon Capture
As the name suggests, this method involves capture carbon dioxide before the fuel is burned. It begins with converting the fuel (most often coal, natural gas, or biomass) into syngas through gasification. By feeding the fuel into a gasification reactor, where it operates at high temperatures (700C-1,500C), and pressures ranging from atmospheric to high pressures (435psi), the fuel is heated to evaporate its moisture content. Then, pyrolysis occurs where the fuel is broken down into gaseous, liquid, and solid residues (char). The gasification occurs when char reacts with limited oxygen or steam to form syngas. The end product (syngas) include a mixture of gases, mainly hydrogen, carbon monoxide, along with carbon dioxide, methane, nitrogen, etc.
The syngas is then treated through cooling and quenching (reduces temperature of syngas), particulate removal (cyclones, wet scrubbing, electrostatic precipitators for purification), removal of water vapor (scrubbing and cooling to prevent formation of acids), sulfur compound removal (hydrodesulfurization, physical absorption to eliminate sulfur oxides).
The crucial step would be the carbon dioxide removal, where chemical solvents (monoethanolamine) are used to selectively absorb CO2 from syngas, pressure swing adsorption which uses adsorbents to capture CO2 under high pressure then releasing it at low pressure, and membrane separation.
2. Post-Combustion Carbon Capture
This process takes place after combustion (well, duh), which produces flue gas containing CO2, nitrogen, water vapor, etc. The CO2 is separated from the flue gas using methods like chemical absorption with amines, membranes, and cryogenic distillation (cooling flue gas to liquefy and separate CO2).
Comparison: Pre VS. Post
As you can see the difference between the paragraph lengths, pre-combustion is more complex than post-combustion carbon capture. And when there is higher complexity, the cost rises.
Other Carbon Capture Methods
3. Direct Air Capture
Direct Air Capture (DAC) extracts CO2 directly from the atmosphere using chemical sorbents or solid materials.
One common method uses a solution of amines, which reacts with CO2 to form a bicarbonate. The solution is then heated, causing the CO2 to be released and recaptured in a separate steam. (But this process is energy-intensive!). An application of DAC is seen in enhanced oil recovery to extract additional oil from existing reservoirs, where the CO2 is injected into depleted oil fields to improve pressure and enhance oil flow. (Professor Psarras from UPenn specialises in this, FYI.)
However, DAC energy requirements are intensive and there is a definite need for renewable energy, since using fossil fuels to supply the energy requirements offsets our efforts in carbon capture.
But then, the question arises,
Where do we store this carbon?
Now that we've extracted the carbon from our syngas, flue gas, and atmosphere, where do we store it? Right now, two mainstream ideas include:
1. Carbon Sequestration
This involves injecting CO2 into underground geological formations, such as depleted oil and gas fields, deep saline aquifers, or unmineable coal seams.
The CO2 is compressed into supercritical state. It occurs when the CO2 is at a temperature and pressure above its critical point, where it possess properties between those of a gas and a liquid.
In supercritical state, the CO2 behaves like a liquid in terms of density but flows like a gas. The high density allows it to dissolve more CO2 per unit volume, making it more efficient for transport over long distances. Supercritical CO2 can be compressed more easily than gaseous CO2, allowing for easier containment in pipelines and reducing leakage risks. The dense phase also minimizes volume, making it more cost-effective to transport large quantities. Other than that, there is also lower transport pressure.
Moving on with our sequestration, the CO2 is injected underground, where it is trapped by impermeable geological layers above the storage site, preventing upwards migration.
2. Cement Production
CO2 can be injected into the concrete mix before or during the curing process. The CO2 reacts with the calcium hydroxide present in the cement to form calcium carbonate, which is a stable mineral and permanently stores CO2.
Cement production is one of the largest sources of industrial CO₂ emissions, largely due to the thermal decomposition of limestone (CaCO₃ → CaO + CO₂) at high temperatures. By using CO₂ in cement production, it is possible to enhance sustainability and potentially sequester carbon.
Alright, I think I've squeezed all my brain juice on this matter. Moving on!
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