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].
1.4.2 Energy and H2 Supply
Another consideration is the energy required to capture and convert CO2, which must certainly be derived from renewable sources (Figure 1.5) [38]. If this energy comes from fossil oils, much more CO2 will be emitted than separated. Fortunately, the renewables now account for over 25% of global power output (hydro: 16%, wind: 5%, PV: 2%), [1] and the costs of PV and wind power become even lower than that of fossil fuels (natural gas and coal) (Figure 1.6) [39]. Thus, electricity from renewable sources can be converted into H2 by water electrolysis, which can be performed on an industrial scale. Nevertheless, H2 produced by electrolysis systems (2.5–6 US$/kgH2) is at present more expensive than that from current industrial production based on conventional fossil sources, like natural gas reforming and coal gasification (<1 US$/kg H2) [40, 41].
Figure 1.5 Low‐carbon electricity generation by source in 2017.
Source: Data from explore energy data by category, indicator, country or region (IEA) [38].
Figure 1.6 Levelized cost of energy comparison: Renewable energy versus conventional generation.
Source: Data from Lazard.com, Lazard's levelized cost of energy analysis [39].
1.5 Political Aspect: Tax
The future prospects for CO2 utilization on large scale will mainly depend on policy support. The carbon tax, a fee imposed on the burning of carbon‐based fuels (coal, oil, gasoline, and natural gas), is one policy for reducing the use of fossil fuels. To reduce CO2 emissions, as many as 29 countries have implemented carbon taxes as of 2019. Tax rates, including energy taxes, differed according to use and fuel type
