Carbon capture and utilization

Carbon capture and utilization (CCU) is the process of capturing carbon dioxide ( C O₂) to be recycled for further usage. Carbon capture and utilization may offer a response to the global challenge of significantly reducing greenhouse gas emissions from major stationary (industrial) emitters. CCU differs from carbon capture and storage (CCS) in that CCU does not aim nor result in permanent geological storage of carbon dioxide. Instead, CCU aims to convert the captured carbon dioxide into more valuable substances or products; such as plastics, concrete or biofuel; while retaining the carbon neutrality of the production processes.

Captured CO₂ can be converted to several products: one group being alcohols, such as methanol, to use as biofuels and other alternative and renewable sources of energy. Other commercial products include plastics, concrete and reactants for various chemical synthesis.

Although CCU does not result in a net carbon positive to the atmosphere, there are several important considerations to be taken into account. Because CO₂ is a thermodynamically stable form of carbon manufacturing products from it is energy intensive. The availability of other raw materials to create a product should also be considered before investing in CCU.

Considering the different potential options for capture and utilization, research suggests that those involving chemicals, fuels and microalgae have limited potential for removal, while those that involve construction materials and agricultural use can be more effective.

The profitability of CCU depends partly on the carbon price of CO₂ being released into the atmosphere.

Sources of carbon

CO₂ is typically captured from fixed point sources in heavy industry such as petrochemical plants. CO₂ captured from these exhaust stream itself varies in concentration. A typical coal power plant will have 10-12% CO₂ concentration in its flue gas exhaust stream. A biofuel refinery produces a high purity (99%) of CO₂ with small amount of impurities such as water and ethanol. The separation process itself can be performed through separation processes such as absorption, adsorption, or membranes.

Another possible source of capture in CCU process involves the use of plantation. The idea originates from the observation in the Keeling curve that the CO₂ level in the atmosphere undergoes annual variation of approximately 5 ppm (parts per million), which is attributed to the seasonal change of vegetation and difference in land mass between the northern and southern hemisphere. However, the CO₂ sequestered by the plants will be returned to the atmosphere when the plants die. Thus, it is proposed to plant crops with C4 photosynthesis, given its rapid growth and high carbon capture rate, and then to process the biomass for applications such as biochar that will be stored in the soil permanently.

Examples of technology and application

CO₂ electrolysis

CO₂ electroreduction to a variety of value-added products has been under development for many years. Some major targets are formate, oxalate, and methanol, as electrochemical formation of these products from CO₂ would constitute a very environmentally sustainable practice.
For example, CO₂ can be captured and converted to carbon-neutral fuels in an aqueous catalysis process. It is possible to convert CO₂ in this way directly to ethanol, which can then be upgraded to gasoline and jet fuel.

Carbon-neutral fuel

A carbon-neutral fuel can be synthesized by using the captured CO₂ from the atmosphere as the main hydrocarbon source. The fuel is then combusted and CO₂, as the byproduct of the combustion process, is released back into the air. In this process, there is no net carbon dioxide released or removed from the atmosphere, hence the name carbon-neutral fuel.

Methanol fuel

A proven process to produce a hydrocarbon is to make methanol. Traditionally, methanol is produced from natural gas. Methanol is easily synthesized from and H₂. Based on this fact the idea of a methanol economy was born.

Methanol, or methyl alcohol, is the simplest member of the family of alcohol organic compound with a chemical formula of C H₃ O H. Methanol fuel can be manufactured using the captured carbon dioxide while performing the production with renewable energy. Consequently, methanol fuel has been considered as an alternative to fossil fuels in power generation for achieving a carbon-neutral sustainability. Carbon Recycling International, a company with production facility in Grindavik, Iceland, markets such Emission-to-Liquid renewable high octane methanol fuel with current 4,000 tonne/year production capacity.

Chemical synthesis

As a highly desirable C₁ (one-carbon) chemical feedstock, CO₂ captured previously can be converted to a diverse range of products. Some of these products include: polycarbonates (via Zinc based catalyst) or other organic products such as acetic acid, urea, and PVC. Currently 75% (112 million tons) of urea production, 2% (2 million tons) of methanol production, 43% (30 thousand tons) of salicylic acid production, and 50% (40 thousand tons) of cyclic carbonates production utilize CO₂ as a feedstock. Chemical synthesis is not a permanent storage/utilization of CO₂, as aliphatic (straight chain) compounds may degrade and release CO₂ back to the atmosphere as early as 6 months. As the use of fossil fuels decreases, removing carbon dioxide from the air is increasingly seen as a way to stop the long-term accumulation of greenhouse gases in the atmosphere. Carbon emissions and storage coupled with reductions in fossil fuel use are known as "negative emissions"., access-date=24 October 2022,

Carbon Dioxide also could use in chemoenzymatic to synthesize starch without cells. Normally in nature, it calls photosynthesis, the plant uses the cells to synthesize starch from carbon dioxide, which is the main reason planting more plants could help capture the carbon. The methods of cells free synthesize to let Carbon dioxide is reduced to methanol with an inorganic catalyst; then methanol is converted to three carbon sugar units. The three carbon sugar units will comprise six carbon sugar units and finally polymerize into starch. Compared to photosynthesis, which has sixty biochemical reactions, cell-free synthesis needs eleven steps. It means the speed of cell-free synthesis is faster than photosynthesis. The synthesis rate is 8.5 times that of corn starch, and the absorbance rate of carbon dioxide speed is more efficient than that of plants. This method is still developing, and the first publishing was only in 2021, so there are still some problems. First, this method needs a lot of energy to synthesize, like plants need solar. It means if the electricity can not use clean energy, It still emission a lot of Carbon Dioxide. The next question is whether the costs are still really high and impossible to commercialize now.

Enhanced oil recovery (EOR)

In EOR, the captured CO₂ is injected into depleted oil fields with the goal to increase the amount of oil to be extracted by the wells. This method is proven to increase oil output by 5-40%.

Enhanced gas recovery (EGR)

Carbon Sequestration with Enhanced Gas Recovery (CSEGR) is a process in which CO₂ is injected deep in the gas reservoir and as a result, at the gas wells which are some distance away, methane (CH₄) is produced. This process by active injection of CO₂ causes repressurization and methane displacement, so that the gas recovery becomes enhanced compared to water-drive or depletion-drive operations.

Carbon mineralization

Carbon dioxide from sources such as flue gas are reacted with minerals such as magnesium oxide and calcium oxide to form stable solid carbonates. These minerals can be mined, or existing brine and waste industrial minerals (including slag) can be reused. The carbonates produced can be used for construction, consumer products, and as an alternative for carbon capture and sequestration (CCS).

Approximately 1 tonne of CO₂ is removed from the air for every 3.7 tonnes of mineral carbonate produced.

Biofuel from microalgae

A study has suggested that microalgae can be used as an alternative source of energy. A pond of microalgae is fed with a source of carbon dioxide such as flue gas, and the microalgae is then allowed to proliferate. The algae is then harvested and the biomass obtained is then converted to biofuel. About 1.8 tonnes of CO₂ can be removed from the air per 1 tonne of dry algal biomass produced, though this number actually varies depending on the species. The CO₂ captured will be stored non-permanently as the biofuel produced will then be combusted and the CO₂ will be released back into the air. However, the CO₂ released was first captured from the atmosphere and releasing it back into the air makes the fuel a carbon-neutral fuel. This technology is not mature yet.

Agriculture

An approach that is also proposed as a climate change mitigation effort is to perform plant-based carbon capture. The resulting biomass can then be used for fuel, while the biochar byproduct is then utilized for applications in agriculture as soil-enhancer
Cool Planet
is a private company with an R&D plant in Camarillo, California, performed development of biochar for agricultural applications and claimed that their product can increase crops yield by 12.3% and three-fold return of investment via improvement of soil health and nutrient retention. However, the claims on the efficacy of plant-based carbon capture for climate change mitigation has received a fair amount of skepticism.

Environmental impacts

16 life cycle environmental impact analyses have been done to assess the impacts of four main CCU technologies against conventional CCS: Chemical synthesis, carbon mineralization, biodiesel production, as well as Enhanced Oil Recovery (EOR). These technologies were assessed based on 10 Life-cycle assessment (LCA) impacts such as: acidification potential, eutrophication potential, global warming potential, and ozone depletion potential. The conclusion from the 16 different models was that chemical synthesis has the highest global warming potential (216 times that of CCS) while enhanced oil recovery has the least global warming potential (1.8 times that of CCS).

See also

*Carbon capture and storage
* Climate change mitigation
*Carbon neutral fuel
*Carbon sequestration
*Greenhouse gas removal
*List of energy topics
*Low-carbon economy
* Solar Foods Ltd.

References

Further reading

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New route to carbon-neutral fuels from carbon dioxide discovered by Stanford-DTU team

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