ESG Economist - The wide possibilities for using CO2
After CO2 is captured, it can be stored or it can be used. In this note, we focus on CO2 utilisation.
There are wide variety of uses of CO2
Some of the technologies to use CO2 are at an early stage of development while others are in a mature stage
The wide possibilities seem promising, but is the storage of CO2 in products as desirable as in geological and/or biological sinks?
Introduction
The Paris Agreement goal is to reach net zero by 2050 and stay within a carbon budget that is aligned to a pathway to stay below 2°C degree above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5°C above pre-industrial levels. There are several ways to achieve this goal. First, limiting and mitigating CO2 and other greenhouse gas emissions. We have published several report on technologies that help reducing emissions. Examples are the use of more , , and . Second, capturing emissions from combustion such . Currently carbon capture is currently only carried out on a relatively small scale. In 2022 the world emitted 37 Gt of CO2 and only 46 Mt of it was captured via CCUS, representing 0.1%. For 2030, the IEA expects emissions to decline to 24 Gt of CO2 and the total capture capacity (including operational, under construction and planned) to reach 321 Mt , or 1.3% in a net-zero scenario. Even though it is a sharp increase in capacity compared to 2022, it is still a very small share of CO2 reduction compared to the total. Third, trapping CO2 and other emissions from the atmosphere, also called direct air capture. To reach net zero, a combination of these three possibilities is necessary. In this report we focus on the usage of captured CO2, which is the other option to storing it.
CO2 Utilisation
What is CO2 utilisation? It is an industrial process that makes an economically valuable product using CO2 at concentrations above atmospheric levels. CO2 is either transformed using chemical reactions into materials, chemicals and fuels, or it is used directly in processes such as . So to create a useful product out of something that is unwanted. The graph below on the next page shows what CO2 utilisation options are.
Biological conversion via algae
Algae are fast-growing aquatic organisms that are commonly found, such as seaweed, pond scum, and giant kelp. Blue-green algae (cyanobacteria), green algae, red algae, and diatoms are commonly referred to as microalgae. These organisms have been used for capturing carbon dioxide and sequestration (see ). The CO2 captured by microalgae via photosynthesis is stored in its biomass. The harvested microalgae could be used to produce bioenergy and other value-added products such as biogas, bio-oil, biochar, and syngas. The consumed CO2 by the microalgae is a building block of macromolecules such as lipids, proteins, carbohydrates and pigment. Using microalgae has many advantages. For a start they are highly efficient in a wide range of CO2 concentration. Moreover, they grow faster than plants and they can co-produce food, feed, biofuel and value-added products. However, there are also disadvantages. First the culture systems and downstream processing (mainly harvesting) are economically cumbersome (see ). Second, microalgae-based carbon capture technology is dependent on the species of microalgae, cultivation systems, and growth conditions (e.g., temperature, culture medium, pH, salinity, turbidity, light intensity, etc.). Optimization of algae growth also impacts the concentration of CO2 captured, for example, an increase of pH from 7.9 to 9.5 has led to a reduction of CO2 by two orders of magnitude. So algae are sensitive to other flue gas components (NOx, SOx), predation, contamination and extreme culture conditions (pH, temperature, salinity). Furthermore, microalgae need to be cultivated outdoor and, thereby, the varying environmental conditions influence their growth. Finally, most of the research and development associated with carbon capture using microalgae is still in the laboratory phase (see ). Currently, the costs are very high because of complex production economics and small production.
CO2 chemicals and fuels
The carbon (and oxygen) in CO2 can be used as an alternative to fossil fuels in the production of chemicals, including plastics, fibres and synthetic rubber. As with CO2-derived fuels, converting CO2 to methanol and methane is the most technologically mature pathway but it is also very costly. Conversion processes involving CO2, a highly stable molecule (e.g., non-reactive), remain costly especially for energy-intensive hydrocarbon products. A big chunk of cost for CO2 conversion comes from energy input, which is correlated to the change in oxidation state.
The methanol can be subsequently converted into other carbon-containing high-value chemical intermediates such as olefins, which are used to manufacture plastics, and aromatics. These are used in a range of sectors including health and hygiene, food production and processing. A special group of chemicals, polymers, are used in the production of plastics, foams and resins. The carbon in CO2 can be used in polymer production by replacing part of the fossil fuel-based raw material in the manufacturing process. Unlike the conversion of CO2 to fuels and chemical intermediates, polymer processing with CO2 requires little energy input, because CO2 is converted into a molecule with an even lower energy state (carbonate, see ).
New technology
One of the promising ideas is to use CO2 into a stable fuel that can replace fossil fuels in some applications. But most such conversion processes have had problems with low carbon efficiency, or they produce fuels that can be hard to handle, toxic, or flammable. Recently, researchers have developed an efficient process that can convert carbon dioxide into formate. This is a liquid or solid material that can be used to power a fuel cell and generate electricity. They claim that metal formate salts are very benign and stable, and are a compelling energy carrier (see ).
CO2 in building materials
CO2 can also be used in the production of building materials to replace water in concrete, called CO2 curing, or as a raw material in its constituents (cement and construction aggregates). These applications involve the reaction of CO2 with calcium or magnesium to form low-energy carbonate molecules, the form of carbon that makes up concrete. CO2-cured concrete is one of the most mature and promising applications of CO2 use, while the integration of CO2 in the production of cement itself is at an earlier stage of development (see ). Storing CO2 in building materials is for the long term and could displace emissions-intensive conventional cement. CO2-cured concrete can have superior performance, lower manufacturing costs and a lower CO2 footprint than conventionally-produced concrete. The climate benefits come mainly from the lower input of cement, which is responsible for the bulk of the costs and life-cycle emissions of concrete.
Construction aggregates (small particulates used in building materials) can be produced by reacting CO2 with waste materials from power plants or industrial processes. Among these are iron slag and coal fly ash, which would otherwise be stockpiled or stored in landfill. Producing building materials from waste and CO2 can be competitive as it offsets the cost associated with conventional waste disposal (see ).
CO2 for fertilizer and melamine products
For the production of the fertilizer Calcium Ammonium Nitrate also CO2 is used. Calcium Ammonium Nitrate can be considered as near-neutral in its effect on soil pH. It is a nitrogen fertiliser which contains equal parts of fast acting nitrate-nitrogen and longer lasting ammonium-nitrogen. This ensures a more continuous nitrogen supply to the crop and thus better efficiency of use, and also makes it suitable for unseasonal application during summer or winter. CO2 can also be used for the production of melamine products and urea based glues. Melamine is durable and its heat-resistant qualities make it perfect for producing strong plastic dinnerware, laminates, and glues.
CO2 for enhanced fuel recovery
Enhanced oil recovery (EOR)
CO2 has two characteristics that make it a good choice for enhanced oil recovery: it is miscible with crude oil, and it is less expensive than other similarly miscible fluids. What does it mean to be miscible? Imagine that you get oil on your tools while working on your car’s engine. Water will get a little of the oil off, soap and water will do a better job, but a solvent will remove every trace. This is because a solvent can mix with the oil, form a homogeneous mixture, and carry the oil away from the tool’s surface. Fluid pairs like ethanol and water, vinegar and water, and engine “degreasers” and motor oil exhibit miscibility, that is, the ability of fluids to mix in all proportions. Oil and water don’t mix as they are immiscible; and as a result, completely removing oil from tools or engine parts requires a solvent.
When we inject CO2 into an oil reservoir, it becomes mutually soluble with the residual crude oil as light hydrocarbons from the oil dissolve in the CO2 and CO2 dissolves in the oil. The physical forces holding the two phases apart (interfacial tension) effectively disappear. This enables the CO2 to displace the oil from the rock pores, pushing it towards a producing well. As CO2 dissolves in the oil it swells the oil and reduces its viscosity; affects that also help to improve the efficiency of the displacement process (see ). This works best when the CO2 density is high (when it is compressed) and when the oil contains a significant volume of light, lower carbon, hydrocarbons (typically a low-density crude oil). Below some minimum pressure, CO2 and oil will no longer be miscible.
Enhanced gas recovery (EGR)
Enhanced gas recovery is the recovery of difficult-to-recover natural gas. This includes natural gas contained in tight gas sands, shales, and coal seams. Many of these geological formations contain significant volumes of natural gas. The principle of EGR is that an injected fluid (e.g. CO2) forms a front that pushes the natural gas to the production wells.
Enhanced coalbed methane recovery (ECBM)
Enhanced coalbed methane recovery (ECBM) involves gas injection into coal to improve methane recovery, analogous to EOR. Typical injection gases include nitrogen and carbon dioxide.
If the world moves towards net-zero and away from fossil fuels, the use of CO2 for enhanced fuel recovery will decline.
Other CO2 uses
CO2 as refrigerant
CO2 has several unique thermo-physical properties making it an ideal refrigerant. CO2 is a natural cooling agent that delivers sustainable and energy-efficient refrigeration in everything from warehouses to ice machines. Solid carbon dioxide, also known as dry ice, is used for refrigeration of food because it undergoes sublimation, which means that it transitions directly from a solid to a gas without passing through the liquid phase. This process absorbs a significant amount of heat, making dry ice an effective cooling agent. made of CO2 is favoured in the shipping and transportation of frozen foods because, when it evaporates, it evaporates as a gas, not water like regular ice. This means that stored foods will not get wet in the process of transportation. However, CO2 is not much used in domestic refrigerators. This is because CO2 has a higher operating pressure than the refrigerants typically used in domestic refrigerators. The higher pressure requires more robust and expensive components, which would make domestic refrigerators more costly.
CO2 as fire extinguisher
Carbon dioxide is used in extinguishing a fire because it neither burns nor does it help in burning. It is also heavier than air, it insulates the burning substance by cutting off the supply of oxygen. Carbon dioxide extinguishes work by displacing oxygen, or taking away the oxygen element of the fire triangle. The carbon dioxide is also very cold as it comes out of the extinguisher, so it cools the fuel as well.
CO2 as extractant and uses in the food industry
CO2 extraction is one of the safest and cleanest methods and can be used to produce a variety of end products. CO2 acts like a solvent at certain temperatures and appropriate pressure. Although this method is highly expensive and complex, it meets high quality standards. In the food industry, large-scale commercial CO2 extraction plants are often found operating, for example, to decaffeinate coffee and produce essential oils from a variety of plants.
In order to derive the components of the plants or herbs, this extraction method relies on supercritical carbon dioxide. CO2 is utilized for the extraction of premium-quality essential oils, the aromatic parts of the plants like leaves, flowers and stems. For the extraction of essential oils, the only high pressure is required. CO2 extraction process occurs at moderate temperature.
CO2 has a wide variety of uses across the whole food industry. As well as being used for fizzy drinks, CO2 is used in drying to extend fruit and vegetables’ shelf-life, as dry ice for goods refrigeration in transit, stunning animals before slaughter, as well as many others. When harvested and stored, grains fruits and vegetables are susceptible to pest infestation and can result in losses. The introduction of carbon dioxide into the storage facility helps prevent this because, at a particular level, CO2 is deadly to living things thereby killing any insect or pest. The non-toxic nature of the gas makes it preferable over fumigation with chemicals.
Is utilisation a good alternative to storage?
Captured CO2 can be stored in geological sinks for up to 10,000 years depending on the sink and trapping mechanism used assuming that it is safe to do so. Storage is also possible in vegetation, soils, woody products and aquatic environments (biologic sequestration). The years of storage of CO2 in products are variable and depend on the product. The use of CO2 in most products such as fuel, plastics, refrigerant are temporary, meaning after a certain time they will be back in the atmosphere unless they are captured again. So it could be that the use of CO2 only swaps the CO2 emissions from now to the future. The use of CO2 in building materials and in enhanced fossil fuel recovery has a longer-term timeframe though, comparable to storage. But with the move away from fossil fuels, the use of CO2 for enhanced fossil fuel recovery will also decline. So we think the aim should be to lock up CO2 safely in products or storage as long as possible.
Conclusion
As we have shown above an unwanted product in decarbonizing the world such as CO2 can be very useful in producing certain products. Some of the technologies to use CO2 are at a low technical readiness levels such as the use of algae and CO2 as an energy carrier or fuel. Other technologies are available but due to a lack of production capacity they are still very expensive such as producing synthetic fuels from CO2 and renewable energy. While other technologies are in a mature phase such as the use of CO2 in making building material or methane. Especially CO2 stored in building materials will be stored for a very long time. Overall, the prospects are promising. But is CO2 stored in these products comparable with storage in geological or biological sinks? Often this is not the case. We think the aim should be to store CO2 as long as possible either in products or via geologic or biologic sequestration.