Decarbonisation Technology - November 2023 Issue

Euent

Liquid/dense-phase carbon dioxide

Feed

Very cold system

CO clean-up

Compression & dehydration

Refrigeration unit

Figure 5 Cryogenic carbon capture

are listed in Figure 4 , along with some typical membrane configurations used to maximise the CO₂ recovery and purity. Distillation – cryogenic or high-pressure-based carbon capture Distillation of CO 2 from various streams, such as flue gas from combustion power plants, process heaters, and combustion generator turbines, is accomplished using a combination of compression to medium to high pressures (500-3,000 psig) and cooling of the fluid to very low temperatures, possibly even to cryogenic temperatures near -50ºF. A simplified block diagram of a system is presented in Figure 5 . “ As DAC technologies mature and sources of low-cost energy becomes available, DAC may become comparable to point source CO₂ capture ” Direct air capture Rather than capturing higher concentrations of CO₂ directly from emission point sources, direct air capture (DAC) is an interesting option that can allow emitters of CO₂ to offset their emissions by removing the CO₂ using DAC. This may allow some facilities to continue to emit CO₂ emissions as regulations tighten by capturing an equivalent amount of CO₂ directly from the atmosphere. DAC today captures less than 1 Mt CO₂/year, but carbon drawdown from the atmosphere is considered to be critical to address the 1.5C outlined in the Paris agreement. The liquid solvent DAC system consists of the contactor and the calciner. In the contactor, air is pushed or pulled through packing material. The packing material is soaked with a KOH solution, which reacts with CO₂ in the air to form potassium carbonate (K₂CO₃) in the solution.

The solution then goes to a regeneration reactor, where anionic exchange with calcium hydroxide (Ca(OH)₂) produces calcium carbonate (CaCO₃) and regenerates the KOH solution. The regenerated solution is then pumped back to the contactors. The solid DAC system with adsorption cycles can use temperature, pressure, humidity, or a combination of these factors to regenerate and saturate the sorbent. Temperature swing adsorption involves adsorbing CO₂ at a given temperature until the target capacity is reached and then regenerating the bed with heat (such as steam). Pressure swing adsorption uses elevated pressure to adsorb CO₂ and then reduces pressure to release it. These systems are fast, but have increased capital costs due to generating and handling pressurising gases. Vacuum swing adsorption adsorbs CO₂ at atmospheric pressure and then reduces pressure to release it. These systems reduce cost and hazards but require industrial vacuum use. Humidity or moisture swing adsorption favours the adsorption of water over CO₂ and regenerates the sorbent by introducing water to the system. These various approaches offer flexibility in controlling the regeneration and saturation phases and can be adapted to different environments and conditions. As DAC continues to evolve, new technologies are being developed that do not fit neatly into the categories of solid or liquid systems. DAC is currently very energy intensive, which can impact the economics drastically. As DAC technologies mature and sources of low-cost energy becomes available, DAC may become comparable to point source CO₂ capture. “There are currently 18 DAC plants operating worldwide, capturing almost 0.01 Mt CO₂/year, and a 1 Mt CO₂/year DAC plant is in advanced development in the United States. In the Net Zero Emissions by 2050 Scenario, direct air capture is scaled up to capture almost 60 Mt CO₂/year