A number of recent reports discuss the state of the art and challenges of various technologies and approaches1. Given the importance of emissions capture in mitigation scenarios, a long list of the technological pathways that have been developed over the past decades and are built with capture are becoming a reality, and at scale, as we have seen in the previous chapter. This section discusses various approaches that can be taken to CCS,2 highlighting three key distinctions used to differentiate capture processes and related activities.
The first distinction is whether the capture of emissions is achieved through a natural process or an industrial one. As part of the global carbon cycle, carbon exchanges continuously occur among oceans, land and the atmosphere. Within this cycle, carbon is absorbed by oceans and taken up by plants for photosynthesis, a portion of which is fixed in living organisms and released back into the atmosphere after decomposition or combustion. Changes in the composition of soil, in the occupation of land and in CO2 concentrations in oceans, to name only a few factors, all have impacts on the quantity of carbon naturally captured every year. The implication is that any action or policy that results in changes to these factors, notably through land use modifications or forest management, including planting trees, has an impact on net CO2 quantities present in the atmosphere.
In contrast, industrial capture results from the application of human-made technologies. This is arguably the main type of approach being discussed within carbon capture terminology and a long list of technologies and processes have been developed to make it effective, economically viable and energy efficient. In general, although industrial capture processes are typically more efficient than natural capture, the latter has the advantage of being able to run directly on sunlight, even though it can also be used in more industrial settings when valuable by-products can compensate low CO2 capture productivity.
Similarly, the storage of the captured gases can be natural or industrial. Natural storage takes place through the accumulation of organic matter in long-lived plants like trees, as well as in anaerobic environments like soils, bogs, lakes, or other bodies of water. Not surprisingly, there are considerable uncertainties about the long-term stability of these reservoirs. Industrial storage takes place through injection into underground salines or empty fossil-fuel geological structures, or through the transformation into stable forms such as carbonates.
The second distinction is whether the capture occurs through preventing emissions at a point source or through removing gases already present in the atmosphere to decrease their concentration. Overwhelmingly, the technological options available or explored are tied to capturing carbon from flue gas streams at point sources, as the result of either power generation, industrial transformation, or fuel and fertilizer production. Power generation from coal, for instance, has seen a few commercial-scale facilities being put into operation around the world, while power generation from natural gas is more costly but functions along the same principles. Although much less efficient, power or heat generation from biomass holds the promise of the added benefit of the natural capture of CO2 already realized by feedstock once it enters the power plant, in theory providing negative emissions. This bioenergy with carbon capture and storage (BECCS), used for heat in industry or for electricity or hydrogen production, is at the core of negative emissions technologies in the modelling presented in this Outlook.
Although power generation is often most visible, fuel production and capture from industrial processes are the dominant sectors where capture occurs or is in development. In the case of hydrogen, most of the current production comes from the reforming of natural gas; four industrial facilities around the world are coupled with capture installations. The capture of CO2 occurs both from the steam methane reforming operation itself and from the burning of fuel to provide heat for the reforming unit. Natural gas processing also results in CO2, from the use of energy at processing facilities and because unprocessed natural gas often contains CO2. In fact, the first commercial plant to begin CCUS in 1972 was a natural gas processing plant (Global CCS Institute 2016). Furthermore, CO2 is captured from the fermentation process in bioethanol production.
Industrially, CO2 capture also occurs in chemical production (e.g., ammonia and ethylene), fertilizer production, and through the capture of emissions from waste to produce energy. Other processes could also lead to capture. Cement production, for instance, results in CO2 emissions both through the burning of fuel for its heat needs and through the calcination of limestone. While heat can come from low-emitting sources, the process emissions remain and can be captured. In the iron and steel industry, the transformation of iron ore for use in steelmaking also results in emissions.
Another approach consists in removing CO2 from the atmosphere through DAC.3 Although DAC occurs naturally, as mentioned above (for instance, through photosynthesis), it can also be achieved through the formation of metal carbonates or with sorbents. Several processes exist for DAC with sorbents (liquid or solid), either with absorption or adsorption and then treating the sorbent to detach the CO2.
Finally, the third important distinction is whether the CO2 captured is valued and used or is instead stored, as reflected in CCS (carbon capture and storage) vs. CCU (carbon capture and utilization) terminology. Different combinations of these three distinctions lead to several pathways in the CCUS discussion, although the current preferred option respecting this third distinction is through enhanced oil recovery (see section 12.2 below).Figure 12.1 summarizes the capture pathways. At point sources, capture technologies are usually broken down into three categories: post-combustion, pre-combustion and oxyfuel combustion. Post-combustion capture occurs through chemical absorption or using membranes after fuel combustion. Pre-combustion capture first involves the gasification of the fuel, which is transformed into CO and H2. The CO is then made to react with steam to produce CO2, allowing its separation and the combustion of H2 to generate energy. The process has the advantage of leading to high concentrations of CO2 in the flue gas stream, as well as the production of a carbon-free fuel (H2). The third option is oxy-fuel combustion, which is similar to the post-combustion process but combusts the fuel with pure oxygen, increasing the concentration of CO2 in the flue gas and preventing NOx and SOx components in the stream. The downside is the significant amount of energy required to generate the pure oxygen. Each of these capture technologies can be coupled with several separation methods, including absorption, adsorption, membrane separation, chemical looping, cryogenic distillation, and hydrate-based separation (Ghiat and Al-Ansari 2021).
Figure 12.1 – Various CO2 capture pathways #
As concerns the third distinction (storage vs. use of the CO2 captured), in theory all pathways provide the option of either storing or using the CO2 captured, or both. In practice, preferences are often linked to economic factors, with utilization being preferred whenever possible. Storage-only options occur mainly in deep saline formations or in depleted oil and gas reservoirs. There are various utilization possibilities, including direct utilization (e.g., using CO2 as a refrigerant to improve energy efficiency), chemical conversion (e.g., into fuels or fertilizer), biological conversion (e.g., by microalgae into carbon compounds), and mineral carbonation (e.g., to produce carbonate blocks instead of cement in the construction industry). Finally, enhanced-oil recovery and coal-bed methane recovery, each of which uses CO2 to facilitate the extraction process, are both a utilization and a storage option as the CO2 remains sequestered in the formations after its use.
Footnotes
1 See, for example: IPCC 2005; Royal Society and Royal Academy of Engineering 2018; Global CCS Institute 2016; Global CCS Institute 2020; Vega et al. 2020; Pilorgé et al. 2020.
2 Although we discuss the distinction between CCS, CCU and CCUS below, for the sake of simplification we use CCS as a general term throughout this chapter, as sequestration will play a central role in reaching net-zero emissions.
3 See, for example: Keith, D.W. et al. 2018