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Capture, purification, and transport of CO₂ are essential for its utilization. Table 1.3 lists several large CO₂ sources with their typical amounts and concentrations of CO₂ as well as impurities. In present CO₂ merchant market (approximately 230 Mton, US$7.7 billion), the fermentation process (i.e. bioethanol production) and ammonia production, which provide close to 100% CO₂, are predominantly CO₂ sources [5, 30]. The CO₂ generated from ethanol fermentation commercially supplies roughly 270 000 ton of CO₂ annually for EOR through pipeline from Kansas to Texas [28]. On the other hand, the production of electricity and heat accounts for 41% of global CO₂ emissions (Figure 1.4), and the transport and industrial sectors account for an additional 25% and 19%, respectively [32]. However, suitable sources of CO₂ for use in chemical transformation are limited. The gases contain various impurities, the separation of which is both energy and cost intensive. To supply CO₂ of an appropriate quality for use in chemical conversion processes, capture and separation are required (Table 1.4) [33]. The most effective CO₂ capture method as the current industrial standard is chemical absorption in an aqueous solution of an amine‐based organic compound. However, the cost (35 US$/ton) and energy consumption (2.5 GJ ton⁻¹) of amine capture must still be reduced to provide economically viable routes from carbon dioxide to fuels [34, 35].

Table 1.3 Concentration of CO₂ and contaminants from various sources.

Source: Carbon Recycling International; Capturing and Utilizing CO₂ from Ethanol: Adding Economic Value and Jobs to Rural Economies and Communities While Reducing Emissions (2017); and Greenhouse Gas Inventory Data [9, 28, 29].

Source	Amount/Mton	CO2 concentration/%	Impurities
Ethanol fermentation [28, 30]	50	99	EtOH, MeOH, H2O, H2S
Anhydrous ammonia	30	>95	NH3, CO, H2, H2O
Natural deposits	13	90–100	N2, O2, He
Power plants	4287	10–15	N2, H2O, SOx, NOx, CO
Steelmaking	266	18–20	N2, SOx, NOx, O2
Cement production [31]	220	14–33	SOx, NOx, O2
Atmosphere	3 200 000	0.04	N2, O2, SOx, NOx

Figure 1.4 CO₂ emissions from fuel combustion.

Source: Data from IEA, CO₂ emissions from fuel combustion, 2020 [32].

Table 1.4 CO₂ capture technologies.

Source: Based on Styring [33].

Capture technology	Technical principle
Chemical absorption	Chemical reaction between CO2 and absorbent by a temperature swing.
Physical absorption	Dissolution of CO2 into a liquid, the efficiency of which depends on the solubility of CO2 in the liquid.
Solid absorption	Absorption into solid absorbents, which include porous materials impregnated with amines for low‐temperature separation or other solid absorbents for high‐temperature separation.
Physical adsorption	Adsorption onto porous solids such as zeolites by a pressure or temperature swing.
Membrane separation	Permeation through a membrane with selective permeability for different gas species.

Recently, the direct capture of CO₂ from ambient air, called direct air capture (DAC), has received increasing attention [36]. One of the advantages of DAC is that it can be located anywhere, because it is unnecessary for CO₂ transport. However, from both engineering and chemistry views, there remains much room for improvements to the sorbents and processes. Additionally, thorough techno‐economic analyses of DAC processes are necessary [37].