What is the Circular Carbon Economy? Carbon Capture and Reuse

The circular carbon economy sees “carbon” as a type of resource, while CO₂ is regarded as a byproduct of the carbon utilization process. We need to implement adequate resource and waste management, apply the 4R principles of reduce, reuse, recycle, remove, and also create a closed circular economic model to maintain a safe habitat for people, while guaranteeing quality of life. The article will introduce the circular carbon economy and how to implement the 4Rs and carbon capture, utilization, and storage (CCUS).

What is the Circular Carbon Economy?

The carbon cycle is a natural circulation system that maintains climate stability on Earth. CO₂ produced through activities such as biological respiration, volcanic eruptions, burning of fuel, and microbial decomposition are absorbed by plants and algae. When these plants and algae die, CO₂ will be buried underground or sink to the seabed again, balancing the amount of CO₂ present in the atmosphere. However, the enormous amount of CO₂ emitted by people after the Industrial Revolution has upset the balance of this cycle. Carbon is considered a resource used for economic activities, which also generates CO₂, a type of hazardous waste, in the process.

In the traditional linear economic model, people make use of carbon by extracting and burning carbon-rich fossil fuels such as oil, coal and natural gas. Ultimately, waste from this process is discharged into the atmosphere in the form of CO₂, and such a cradle-to-grave economic model exacerbates the greenhouse effect that in turn causes climate change.

To sustain a linear economic model, the key to resolving climate change is to minimize the use of resources as much as possible. This means that people’s quality of life will be affected. To guarantee a safe climate and maintain a comfortable living environment, people have proposed the concept of a circular carbon economy, using the circular economy as a reference. Through practices such as reuse, waste reduction, and zero-waste, carbon can be kept in a closed resource utilization system to lower environmental harm.

The circular carbon economy consists mainly of the 4R method of reduce, reuse, recycle, and remove, which is different from the zero waste hierarchy in the circular economy. These four approaches are equally important and rely on one another, thus resource investment and solution implementation are not prioritized.

Please refer to the article: WHAT IS ZERO WASTE? ZERO WASTE PRINCIPLES AND EXAMPLES. For more information on zero waste hierarchy.

In terms of reduction measures, carbon use can be reduced via increasing energy utilization efficiency and using alternative energies. Reuse measures involve the production of new products without consuming energy. Recycling involves the conversion of CO₂ into new products through energy consumption. Remove involves taking waste that cannot be reduced, reused or recycled to create a natural or artificial carbon sink, in order to move the CO₂ out of the closed resource utilization system. Although it cannot generate new value, this CO₂ will be prevented from impacting the environment.

What are the 4Rs of the Circular Carbon Economy?

The 4Rs of circular carbon economy are: Reduce, reuse, recycle, and remove. They form the cornerstone of the circular carbon economy.

The reduction measures include increasing energy production and transport efficiency to lower the amount of fuel needed for electricity generation power; increasing the power generation capacity of renewable energy sources such as hydropower, nuclear power, wind power, tidal power, solar power, and geothermal power; reducing carbon usage in power generation projects.

Reuse measures include using CO₂ to increase greenhouse crop yields; feeding it to algae to enrich the food of oysters or salmon; converting it into carbonate that can be used as a concrete raw material for construction purposes; using it to make carbonated drinks; using it as a refrigerant gas; using it to increase oil recovery rates.

Recycling measures include converting CO₂ to fuels such as methane and methanol using electrical or thermal energy; and producing chemical fertilizers, plastics, and synthetic rubber.

Removal of CO₂ can be via natural or artificial means. Examples include wetland conservation; engaging in low till agriculture and cover planting trees; direct capture of CO₂ from the air; storage of CO₂ in systems that can retain it for centuries such as building materials or the earth’s crust.

Reduce: Mitigate Carbon Entering the Atmosphere

The “reduction” measures in the circular carbon economy involve decreasing the amount of CO₂ escaping into the atmosphere at the source and lowering the amount of CO₂ that must be managed and captured. Currently, the following methods are implemented:

Increase Transport Efficiency

According to estimates by the International Energy Agency (IEA), more than 20% of global emissions can potentially be reduced by improving energy efficiency in transport. In terms of public road transport, lowering the weight of vehicles and enhancing the efficiency of engines, as well as increasing the market share of EVs will maintain the CO₂ emissions level of 2018 despite an increase in the number of vehicles. In the future, if vehicle production stagnates or declines, CO₂ emissions will be gradually lowered.

In the transportation sector, the aviation industry is the most in need of improvement. The use of more efficient engines and improvements in aircraft design help to enhance efficiency. However, due to the long delivery times and high cost of aircraft and the fact that groundbreaking innovations in electric motors and fuel cells will not occur in the near future, replacing short-distance air transport with high-speed rail is a much more economical and affordable alternative, because its energy efficiency is 11 times that of aircraft. In areas with high-speed rail systems such as Eurostar, China Railway High-speed, and Japan’s Shinkansen, air transport usage has declined by over 40%.

Increase the Efficiency of Buildings and Home Appliances

Currently, the technology is available to improve the energy efficiency of cooking, water, space heating, and cooling equipment. Many countries also offer incentives through taxes or subsidies. In particular, improving thermal insulation alone could reduce global energy consumption by half, and the use of district cooling equipment could also reduce cooling energy consumption by 50%.

Improve Industrial Equipment

Although industrial activities are increasing with the growing population and spending power, they are shifting away from heavy industry with higher energy use intensity to light industry with lower energy use intensity. Light industry is the key to reducing CO₂ emissions, and it can be realized through new machines with increased electric motor efficiency and decreased waste heat emissions. For heavy industry with higher energy use intensity, the main measures include increasing the material recovery rate for scrap metals, waste glass, and waste plastics to reduce CO₂ emissions.

Renewable Energy

Renewable energy refers to energy generation methods with an endless supply of raw materials including geothermal, wind, tidal, solar, and bio-power generation. Currently, the two most sophisticated and promising technologies are solar power and wind power, while bio-fuel power generation can compensate for the instability of solar and wind power due to weather conditions. In the process of renewable energy generation, no CO₂ will be emitted, making it an integral part of many circular carbon economy measures. For example, the process of purifying and recycling CO₂ into different products requires a lot of energy, and the use of renewable energy will increase the number of options available.

Nuclear Power

Nuclear power involves the use of uranium. Although it is a non-renewable energy source, no CO₂ is discharged during the power generation process. Barring accidents, it is the most reliable and stable source of low-carbon energy. In 2019, it became the second largest source of low-carbon electricity in the world, second only to hydropower. Nuclear power can be used to support inconsistent renewable energy sources. Besides electricity generation, heat energy from nuclear power can also be used for industrial and residential heating, seawater desalination or hydrogen production, thereby reducing CO₂ emissions on multiple fronts.

Reuse: Capturing and Reinjecting CO₂

“Reuse” measures in the circular carbon economy utilize CO₂ directly or via low energy intensity methods to produce other new products for people:

Oil Recovery

CO₂ is injected into the rock formations that originally stored oil to force the otherwise difficult-to-recover oil to flow to production wells, thereby increasing oil production while retaining CO₂ in the rock formations. The amount of CO₂ contained inside the rock formation may be more than the amount of CO₂ generated by burning the additional recovered oil.

Building Materials

Injecting CO₂ during the concrete mixing process produces calcium carbonate which forms part of the filler for bonding and curing purposes. Concrete infused with CO₂ not only offers higher performance than traditional concrete but can also store CO₂ in a stable material that lasts for centuries.

Fertilizer (Urea)

Urea is made by reacting ammonia with CO₂, a major by-product of the ammonia production process. Urea is used in the soil, introducing nitrogen to the roots of crops and releasing CO₂ into the soil in the process, which will eventually be released back into the atmosphere.

Refrigeration, Manufacturing of Beverages, and Other Commercial Applications Such As Promoting Plant Growth

CO₂ is also used for other commercial applications including refrigeration, beverage manufacturing, and promoting the growth of plants or algae. Although the demand for CO₂ in these applications is increasing, it offers less benefit for the circular carbon economy compared to the three applications mentioned above.

Recycle: Transforming CO₂ Into New Products

The “recycling” part of the circular carbon economy involves the mixing and processing of CO₂ with other materials via energy or high temperature to produce new products or energy.

Synthetic Fuel

Since CO₂ is a very stable molecule, a lot of external energy is needed to convert it into fuel. Currently, the most mature method is to combine CO₂ with electrolytic hydrogen to form hydrocarbon fuels such as methane to replace fossil fuels.

Synthetic Chemicals

CO₂ can be used in carbon-based chemical materials for producing plastics, fibers, and synthetic rubber. Most of the carbon in these products is derived from fossil fuels. However, synthetic chemicals require high energy and water use intensity, hence renewable energy and site selection become extremely important.

Construction Aggregates

Waste materials such as iron slag or coal powder produced by power plants or industrial processes can be reacted with CO₂ to create construction aggregates, after undergoing separation and high-temperature/high-pressure purification.

Remove: Carbon Capture and Sequestration

Although reducing, reusing, and recycling facilitates the effective management and utilization of carbon resources and waste, there is already excess CO₂ in the current circular carbon economy, and reusing or recycling it merely temporarily stores it in other products such as fuels or fertilizers. After the products are used, the CO₂ will be released back into the circulation system. Other approaches are also available to store CO₂ for a long time, such as remaking it into building materials, etc. Nonetheless, this method cannot sufficiently sequester a large amount of CO₂, due to limited demand. Consequently, CO₂ must be sequestered by applying a removal technique that removes it directly from the carbon cycle system.

Geological Storage

CO₂ from the atmosphere or waste collection systems is injected into rock formations deep underground, which are not water permeable and can prevent CO₂ leakage.

Afforestation

Afforestation converts CO₂ into O₂ throughout the plants’ life cycle and removes CO₂ from the atmosphere. In particular, since mangroves grow in wetlands, they are less easily decomposed by microorganisms after death. Instead, they store CO₂ for a long time in the form of biochar. The carbon stored per unit area this way is 3-5 times that of tropical forests. Hence, planting mangroves and protecting wetlands is extremely important as a means of CO₂ removal.

Carbon Capture, Utilization and Storage (CCUS)

CCUS technology evolved from CCS (CO₂ Capture, Storage). Compared to CCS, which involves simple capture and sequestration of carbon, CCUS has the added functional of utilization, to fully embody a circular carbon economy. CCUS technology is explained below through the process of capture, transport, utilization, and storage.

CO₂ Capture

People capture CO₂ mainly from coal or gas-fired power plant emissions, the atmosphere, and the ocean.

The smokestack of a coal-fired or gas-fired power plant is the ideal place to collect CO₂ because the flue gases contain a large amount of CO₂. Since the 1950s, people have installed amine solution tanks in the smokestacks to capture CO₂, which forms soluble carbonate in the amine solution, which can be released through heating and pressure.

Direct air capture removes CO₂ directly from the atmosphere and uses a potassium hydroxide solution to react with CO₂ in the air, producing potassium carbonate. Calcium hydroxide is then reacted with the potassium carbonate to produce calcium carbonate for carbon sequestration purposes. Alternatively, a fan is used to force air through a filter, the CO₂ captured can later be released by heating the filter. Since direct air capture relies on air contact surface area, a large plot of land is needed.

The CO₂ concentration in the ocean is dozens of times that in the atmosphere. The C CO₂ absorption process from seawater is similar to seawater desalination. After drawing in seawater, a small amount (roughly 1%) of acid-base liquid containing bicarbonate is separated through osmosis. The remaining seawater from which CO2 has been removed is directly returned to the sea, and the acid-base liquid can be purified to create CO₂.

CO₂ Transport

The CO₂ captured will undergo pressurization to turn it into a liquid that can be stored in cylinders or transported via pipelines. CO₂ pressurization and transport consume energy, hence using renewable energy from the reduction measures is crucial to this stage.

CO₂ Utilization

After collecting CO₂, it is reused or recycled and turned into fuels, chemicals, or construction and commodity materials, to complete a circular carbon economy.

CO₂ Storage

Apart from reuse, CO₂ can also be compressed to form a supercritical fluid (SCF), a fluid that combines the properties of gases and liquids simultaneously. The SCF is injected into suitable geological structures 800m underground for permanent storage. Most of these sequestration sites are empty offshore oil wells to minimize energy consumption from additional drilling into the ground.

Why Is the Circular Carbon Economy Important to Us?

Because it offers the possibility of maintaining current economic development and quality of life while avoiding threats from climate change to our living environment, a circular carbon economy has become a become an overwhelming important goal. It can minimize the use of organic carbon resources, preventing them from being converted into CO₂ and released into the atmosphere. At the same time, removal measures can convert CO₂ in the atmosphere back into organic carbon or store it in the earth’s crust. The ultimate goal is to restore GHG concentration in the atmosphere to pre-industrial revolution levels, which is consistent with the carbon reduction guidelines formulated by the IPCC to offset humanity’s impact on climate.