# Carbon capture and utilization

Carbon capture and utilization (CCU) is the process of capturing carbon dioxide (CO$_2$) from industrial processes and transporting it to where one intends to use it in industrial processes.
As of 2022, around 73% of the CO$_2$ captured annually is used for enhanced oil recovery. Around 1% of captured CO$_2$ is used as a feedstock for making products such fertilizer, e-fuels, concrete and reactants for various chemical synthesis.
There are several additional considerations to be taken into account. As CO$_2$ 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 CO$_2$ removal, while those that involve construction materials can be more effective.
The profitability of CCU depends partly on the carbon price of CO$_2$ being released into the atmosphere. Carbon capture and utilization may offer a response to the global challenge of significantly reducing greenhouse gas emissions from major stationary (industrial) emitters.

## Definition and distinction

Carbon capture and utilization (CCU) is defined as capturing CO$_2$ from industrial processes and transporting it via pipelines to where one intends to use it in industrial processes. The pipelines are pressurized as the only option for transporting the CO$_2$ over long distances.
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 efuel.
CCU and CCS are sometimes discussed collectively as carbon capture, utilization, and sequestration (CCUS).

## Sources of carbon

CO$_2$ is typically captured from fixed point sources in heavy industry such as petrochemical plants. CO$_2$ captured from these exhaust stream itself varies in concentration. A typical coal power plant will have 10-12% CO$_2$ concentration in its flue gas exhaust stream. A biofuel refinery produces a high purity (99%) of CO$_2$ with small amount of impurities such as water and ethanol. The captured CO$_2$ contains impurities and the CO$_2$ transported through pipelines will contain impurities, such as ammonia, N$_2$ , H$_2$S , C$_2$$^+$, CO$^+$, O$_2$$^+$, NO$_x$, SO $_x$$^+$ and arsenic. Hydrogen can cause hydrogen embrittlement, and water can cause corrosion in steel pipes.$^{:424}$
The separation process itself can be performed through separation processes such as absorption, adsorption, or membranes.

## Examples of technology and application
### CO$_2$ electrolysis

CO$_2$ 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 CO$_2$ would constitute a very environmentally sustainable practice. For example, CO$_2$ can be captured and converted to carbon-neutral fuels in an aqueous catalysis process. It is possible to convert CO$_2$ in this way directly to ethanol, which can then be upgraded to gasoline and jet 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 CO$_2$ and H$_2$. 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 CH$_3$OH. 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. Synthesis of methanol from carbon dioxide is done through a hydrogenation reaction in the presence of a catalyst. Commonly used catalysts are copper, zinc, and palladium. These reactions are typically performed under high pressure conditions to shift the reaction equilibrium towards the methanol product via Le Chatelier's Principle.
Dimethyl Ether
Dimethyl Ether has shown promise as a carbon neutral fuel as a potential alternative to diesel fuel. Dimethyl Ether has typically been synthesized from a dehydration reaction of methanol in the presence of an acid catalyst, but researchers have recently developed a one step method to convert carbon dioxide into dimethyl ether using a bifunctional catalyst and similar conditions to the synthesis of methanol from syngas.

### Chemical synthesis

As a highly desirable C$_1$ (one-carbon) chemical feedstock, CO$_2$ 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 CO$_2$ as a feedstock. Chemical synthesis is not a permanent storage/utilization of CO$_2$, as aliphatic (straight chain) compounds may degrade and release CO$_2$ 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 dioxide also could be used in chemoenzymatic processes to synthesize starch without cells. In nature starch is usually synthesized within cells from carbon dioxide via photosynthesis. In cell-free synthesis, 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 be converted to six carbon sugar units and finally polymerize into starch. Compared to photosynthesis, which involves sixty biochemical reactions, cell-free synthesis needs eleven steps. This means cell-free synthesis can be faster than photosynthesis. The synthesis rate is 8.5 times that of corn starch, and the absorbance rate of carbon dioxide is more efficient than that of plants. This method is still developing, and the first publication on the topic was only in 2021, so there are still some problems. First, this method needs significant energy inputs, just as plants need sunlight. If the electricity used is not produced cleanly, large carbon dioxide emissions will still result. Moreover, high costs present a barrier to commercialization.
In 2023, an international team of researchers at the University of Sydney and the University of Toronto developed a new acid-based electrochemical process for the conversion of CO$_2$ captured from emission sources or directly from air.

### Enhanced oil or gas recovery

In enhanced oil recovery, the captured CO$_2$ 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%.
Carbon Sequestration with Enhanced Gas Recovery (CSEGR) is a process in which CO$_2$ is injected deep in the gas reservoir and as a result, at the gas wells which are some distance away, methane (CH$_4$) is produced. This process by active injection of CO$_2$ 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 CO$_2$ 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 CO$_2$ can be removed from the air per 1 tonne of dry algal biomass produced, though this number actually varies depending on the species. The CO$_2$ captured will be stored non-permanently as the biofuel produced will then be combusted and the CO$_2$ will be released back into the air. Microalgae biofuels are considered to be a part of the third generation of biofuels, being an alternative energy source for fossil fuels without the disadvantages accompanying first and second generation biofuels. This technology is not mature yet. Current microalgal culture systems have not been designed for high throughput biomass growth and carbon capture. Raceways, high-rate algal ponds, and photobioreactors are the most widely used for microalgal cultivation at a large-scale. The limitations of these systems are related to microalgal growth requirements. Ponds are operated at narrow depth to ensure sufficient light distribution and thus need a large land surface.

## Environmental impacts

Pipelines can fail through either ductile fracture and/or a brittle fracture.$^{:425}$
As of 2015, 16 life cycle environmental impact analyses had 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).
Life-cycle assessments (LCA) are not standardized as studies that perform them use different assessment methodologies and parameter that change the results of the LCA. Enhanced methodology guidelines and standardization of practice are necessary to better gauge and compare the impact of the various CCU technologies.

## Regulation

In the US, Federal Energy Regulatory Commission (FERC) and the Surface Transportation Board (STB) exercise jurisdiction. The Corps of Engineeers may issue nationwide permits.