Filtration and separation for industrial carbon capture, transport, and storage
Novel filtration and separation products and a deep understanding of material science and fluid contamination characteristics are needed to reduce the Opex of carbon capture.
Lara Heberle and Julien Plumail
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In addition to electrification, hydrogen, and other clean energy technologies, large-scale carbon capture, utilisation, and storage (CCUS) is critical to achieving net-zero 2050 goals. These goals were set forward by the International Energy Agency (IEA) in 2021 as a challenging path to restrict global temperature rise to 1.5°C. One of the key aspects of the plan is to limit emissions from point-source industrial emitters that produce elevated levels of CO₂, which are often hard to abate. These industries include cement, lime, steel, and aluminum production, bioenergy, refineries, chemicals, natural gas and coal power plants, pulp and paper, and waste-to-energy.1
Looking at the carbon capture value chain, there are a range of technologies at widely varying technical readiness levels (TRL). The most mature carbon capture technology, which is currently used in most industrial carbon capture installations, is chemical absorption, where a solvent selectively binds with the CO₂ in one column called the absorber and regenerates in a secondary regenerator column where the CO₂ is released. Solvent-based absorption technology is well known and has been used extensively in natural gas treating plants such as in amine sweetening processes. Other carbon capture technologies at lower TRLs include physical absorption, adsorbents, oxyfuel combustion, cryogenics, calcium or chemical looping, and membranes.
Once CO₂ is captured, it is typically dehydrated, compressed into a dense or supercritical phase for easier transport, then transported via pipeline or ship. It can be utilised in material production, enhanced oil recovery, or other processes or stored in depleted reservoirs or saline formations.
Which capture technologies are favourable highly depends on process economics, often cited in units of $/ton CO₂. Because CO₂ does not have an intrinsic value, installations are driven by credits and regulations. This drives the industry to seek the lowest expense-proven solution and actively pursue technologies that offer cost reduction and increased equipment lifetime.
Solving the contaminant challenge
In the critical-to-decarbonise industrial sectors, CO₂ is typically captured after a combustion process. Therefore, flue gas feed streams entering CO₂ capture processes can contain an elevated level of combustion byproduct contaminants. These feed contaminants can increase process operating expenses by (1) increasing the need for water replacement in wash systems and direct contact coolers, (2) increasing the frequency of solvent, membrane, or adsorbent replacement, (3) for solvent-based processes, causing amine emissions in the flue gas outlet from the absorber, and (4) fouling critical process equipment such as heat exchangers, reboilers, compressors, and absorber internals, thereby reducing process efficiency, increasing energy requirements, and requiring more frequent maintenance.
Additionally, contaminants can be generated during the carbon capture process. For instance, corrosion byproducts, solvent degradation compounds, and heat-stable salts can build up over time in solvent loops. Similarly, in downstream process steps, lube oil and solid contaminants can be introduced into the concentrated CO₂ stream. These contaminants also increase operating expenses by contaminating successive stages of equipment, leading to off-specification pipeline contents, and can plug reservoirs.
For each of these problems related to contaminants, reliable filtration and separation steps are critical to maintaining low operating expenses. Filtration and separation products for solvent clean-up are well known due to decades of experience with gas treatment. However, other applications, such as feed treatment before CO₂ capture processes, solvent emission prevention, and downstream, including dense-phase CO₂ purification are less known, emerging applications in this sector. Pall applications in solvent clean-up, feed treatment, and solvent emission prevention are shown in Figure 1, with detail in Table 1. Applications downstream and in dense-phase CO₂ purification are shown in Figure 2, with details in Table 2.
With a solvent-based CO₂ capture process, the process efficiency and operating expenses of the entire unit hinge on the cleanliness of the solvent and equipment. On a positive note, recommended filtration and separation steps are well-studied due to the longevity of these processes in gas processing plants.
Solid feed contaminants such as fine fly ash particulates (as small as <1 µm diameter) that can bypass feed pretreatment steps due to their small size can build up and foul the lean/rich heat exchanger, the reboiler, the absorber internals and require more frequent solvent change-out over time. Contaminants can also alter the surface tension of the solvent, causing an increased tendency to foam and increased foam stability, requiring the use of anti-foam. Finally, fine particulates can form aerosol nuclei, which contribute to solvent emissions, resulting in solvent losses out of the absorber vent, as found from tests at the post-combustion carbon capture plant at Niederaussem.² Corrosion products from stainless steel and similar equipment can also precipitate in the rich side of the solvent loop into solid particulates such as iron compounds, causing similar issues.
To remove these solids, particulate filtration of the solvent is recommended at a minimum of 10% slipstream. The target level for solids after filtration is 1-5 ppmw. Five or 10 µm-rated absolute particle filters are recommended, based on the diameter of the solid particulates.
It is important to understand the differences between how particulate filters are rated. Nominal ratings are arbitrarily assigned by the filter manufacturer, and there is no regulation for the value of the nominal ratings to indicate the performance of removing certain particle sizes. In contrast, absolute particle filter ratings must meet rigorous ‘ISO or ASTM’ standards. The absolute rating of a particle filter directly corresponds to the largest diameter of particle that the filter will allow through – all larger particulates will be captured. An example of the difference between solvent cleanliness after using no filter, a nominally rated filter, and an absolute-rated filter is shown in Figure 3.
Rich side filtration is commonly recommended to remove precipitated corrosion like iron sulphide and to protect the lean/rich heat exchanger. Significant improvements in the removal of solvent contaminants have been demonstrated using Pall absolute-rated filters, with extensive data proving the removal of precipitated corrosion products and process equipment protection from the gas treating industry.³
Lean filters can also be added to the process scheme to prevent fine particulates from entering the absorber. Lean filtration is particularly recommended for polishing and removing adsorbent fines if there is a carbon bed on the lean solvent side.
Carbon beds are often installed to remove solvent degradation products and have been found to remove some metal ions. Degradation products such as organic acids, formed by the solvent degrading through oxidative and thermal mechanisms, can be corrosive, cause foaming, solvent losses, and reduced absorber capacity. Metals are common from internal metallurgy and can catalyse amine degradation. Not all activated carbon targets the same contaminants, so the product must be selected carefully to ensure that it does not prematurely plug.
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