Carbon capture and utilization

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.[14] For example, CO₂ can be captured and converted to carbon-neutral fuels in an aqueous catalysis process.[15][16] It is possible to convert CO₂ in this way directly to ethanol, which can then be upgraded to gasoline and jet fuel.[17][18]

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.

A proven process to produce a hydrocarbon is to make methanol. Traditionally, methanol is produced from natural gas.[19] Methanol is easily synthesized from CO₂ 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 CH₃OH. 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.[20][21] 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.[22] 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.[23]

Dimethyl Ether

Dimethyl Ether has shown promise as a carbon neutral fuel as a potential alternative to diesel fuel. Dimethyl Ether is 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.[24]

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,[25] urea,[25] and PVC.[26] 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.[27] 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.[26] 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".

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.[28] 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 CO₂ captured from emission sources or directly from air.[29]

In enhanced oil recovery, 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%.[26]

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.[30]

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.[31] 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.[31]

Fuels can be produced from algae

A study has suggested that microalgae can be used as an alternative source of energy.[32] 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.[33] 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. 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.[34] This technology is not mature yet.[35] 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.[36]

An approach that is also proposed as a climate change mitigation effort is to perform plant-based carbon capture.[37] 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.[38] However, the claims on the efficacy of plant-based carbon capture for climate change mitigation has received a fair amount of skepticism.[39]