Negative Emissions Technologies (NETS): Feasibility Study - Technical Appendices

Appendix 6. DACCS

Liquid Solvent DACCS

Configuration

Carbon Engineering’s process is split into 4 key stages^{[23], [52]}: 1.) the air contactor unit, where ambient CO₂ absorbs into the hydroxide KOH solvent to form a K2CO3 salt; 2.) the pellet reactor, where the carbonate salt reacts with another solvent (Ca(OH)₂) in a fluidised bed to form CaCO₃, which in turn regenerates the KOH solvent; 3.) the calciner, where CaCO₃ is heated to 900degC and thermally decomposes to release the captured CO₂, which is subsequently compressed to 150 bar ready for storage; 4.) the slaker, where the Ca(OH)₂ solvent is regenerated by hydrating the CaO exiting the calciner. This cyclic configuration enables continued operation. The process heat and power demands are met through onsite natural gas combustion within a NGCC unit, where the CO₂ emissions are captured.

There are alternative process configurations where the NGCC unit has been replaced with grid electricity^[23], with the most ambitious being a fully electrified system which meets all heat and power demands. This comes at the expense of a high electricity penalty (1,535 kWh_el/tCO₂)^[52], but at the benefit of downsizing several process units.

Loading capacity

The CO₂ loading capacity on the solvent must be limited to a maximum concentration of 30%, due to the corrosive nature of hydroxide bases. This in turn reduces the CO₂ flux potential for absorption. This is reflected by the fact that the CO₂ flux of a strong NaOH base is ~0.52 molCO₂.min^-1.m^-3, which is an order of magnitude lower than compared to solid amine adsorbents. In order to maximise this loading capacity, Brentwood XF12560 packing is used to increase surface contact area^[51].

Temperature

The sorbent regeneration temperature is considerably high (at 900degC), which can at present only be achieved through natural gas combustion^[23]. However, there is scope to investigate alternative low carbon fuels via the Dreamcatcher project^{[4], [5]}.

Energy requirement

In the baseline case, where all energy demands are met through the NGCC unit, 2.45 MWh/tCO₂ of natural gas is required. If instead electrical demands are met through the grid, then natural gas usage drops to 1.46 MWh/tCO₂ coupled with heat recovery and 366 kWh/tCO₂ of electricity^[23]. The majority of this power demand (70%) is allocated to the CO₂ compressors^[48]. There is also the instance of having a fully electrified system, which requires 1,535 kWh/tCO₂ of electricity^[52].

Modularity

Compared to solid absorbent DACCS, the modularity of Carbon Engineering’s capture units is poor, with capture capacities having to be greater than 10 ktCO₂/year^[51]. Therefore, it is preferential to deploy liquid solvent DACCS at larger scales.

New research

The reaction kinetics of the solvent are being improved through biomimetic catalysts (e.g., using carbonic anhydrase, which hydrate and dehydrate CO₂ orders of magnitude faster than amines or water)^[51].

Solid Adsorbent DACCS

Configuration

When regenerating the sorbent bed to release CO₂, either Temperature Swing Adsorption (TSA), Pressure Swing Adsorption (PSA), or Vacuum Swing Adsorption (VSA) is used. The benefit of TSA is that it exhibits lower steam requirements (0.2–0.4 kg steam/kgCO₂) and is cheap, whilst PSA experiences quicker CO₂ adsorption/desorption times, but at the expense of higher costs and safety^[51]. VSA provides similar benefits to PSA and is safer; however, it is the most expensive option^[51].

Climework’s pilot plant is described in Fasihi et al’s work^[52]. Ambient air is drawn into modular contactor units using fans, where CO₂ and moisture adsorb onto the solid surface of a special cellulose fibre that is supported by amines; thus enabling CO₂ capture and providing sufficient water for onsite use (0.8–2 t/tCO₂). The remaining air in the adsorbent bed is then purged by reducing pressures or inserting steam into the system. The CO₂ is then released by heating to temperatures of 80-120degC, compressed, and is ready for storage or utilisation. Finally, the sorbent bed is cooled to ambient conditions before re-use. A whole adsorption/desorption cycle takes between 4-6hrs^[52]. A key benefit of this process is its ability to use a wide range of sorbents and low temperature heat sources, such as waste heat, geothermal energy, and heat pumps. However, there are also drawbacks associated with building the large surface area structures that exhibit low-pressure drops, as they exhibit high capital costs^[48].

Global Thermostat follows a similar process, with the key differences being a lower regeneration temperature requirement (85–95 degC), the adsorbent being an amino-polymer, and the full cycle time being shorter (~100s)^[52]. The heat and electricity requirements are also lower (please see Table 14).

Loading capacity

Compared to liquid hydroxide solvents, the use of solid adsorbents enables much higher CO₂ loading by weight (3.53 mol CO₂ min^-1.m^-3). Therefore, the flux of CO₂ adsorption is less limited^[51]. A hierarchical adsorbent pore structure of micro and mesopores is used to maximise adsorption^[51].

Temperature

The temperatures required to regenerate the sorbent (80-120degC) are significantly lower than compared to liquid DACCS^[51]. This is due to adsorbent bonds formed between CO₂ and the sorbent surface being weaker than the chemical bonds formed during absorption.

Energy requirement

For the Climeworks process, the electrical demands are lower than that of Carbon Engineering (200–300 kWh/tCO₂), to supply the fan and control systems. However, heat demands are greater (1.5 -2.0 MWh/tCO₂)^[52]. Despite this, the temperature of the heat is much lower and hence is easier to source.

Modularity

Climeworks’ technology is provided in small modular units, with the maximum potential of one unit being 50 tCO₂/year. This modularity helps the technology be manufactured and deployed at scale^[51].

New research

There’s bountiful research investigating alternative adsorbents that show potential in reducing thermal energy demands. This includes metal–organic frameworks (MOFs), zeolites, activated carbons, etc. In terms of design, a moving bed adsorber is being considered over the more traditional fixed bed, which helps reduce pressure drops and cycle times^[51].

Other DACCS

Cryogenic DACCS

Takes advantage of the sublimation point of CO₂. The CO₂ extracted from the air is converted into a solid or sublimated to produce a high purity gas stream^[51].

Moisture/humidity swing adsorption (MSA)

MSA uses anionic exchange resins to capture and evolve CO₂. These sorbents will bind to CO₂ in arid conditions and evolve CO₂ when contacted with water, which has the potential to decrease energy requirements but at the expense of increased water consumption^[51]. After CO₂ is removed, the system is heated to 45 degC to dry the resin sheets. The electrical energy demands range from 316-326 kWhel/tCO₂, depending on whether a fan is used to draw in ambient air within the contactor^[52].

Electro-swing adsorption

In this process CO₂ binds to a polyanthraquinone-carbon nanotube composite upon charging and is released upon discharge, eliminating the need for thermal energy, and producing a high purity CO₂ stream^[51].

Molecular filters

Nano sized molecular filters are used to capture CO₂ from the air powered by solar energy. The technology is expected to only require 333 kWhel/tCO₂ of electricity, where pure CO₂ is delivered at 100 bar, at a cost of 14 €/tCO₂^[52].

Alternative feedstocks

Manufactured alkaline feedstocks (e.g., MgO) and aqueous amino acids could absorb CO₂, where CO₂ is regenerated by crystallization of an insoluble carbonate salt with a guanidine compound^[51].

DAC pilot projects

Liquid solvent DAC

Carbon Engineering is the only company active in liquid solvent-based DAC, who use an aqueous KOH solvent to capture carbon. At present, they have a demonstration and pilot plant capturing a combined 1,365 tCO₂/year, which costs 132-191 £/t CO₂^[227]*. The company’s goal is to establish broad commercial deployment of synthetic fuels produced from captured carbon and green hydrogen.

Solid sorbent DAC

Climeworks is the most well-known solid sorbent-based DAC company, who use an adsorbent made of special cellulose fibre supported by amines to capture carbon^[52]. Their first demonstration project was in 2014, in collaboration with Audi and Sunfire, where ambient CO₂ was captured and converted to synthetic diesel^[52]. Since then, an additional 14 projects have been undertaken, most of which utilise the captured carbon to either produce e-fuels, aid plant growth in horticultural sites, or carbonate beverages. Their first CO₂ storage project was a pilot plant located in Iceland, capturing 50tCO₂/year in 2017. This site was scaled up in capacity in 2021, to provide 4,000 tCO₂/year of negative emissions at a cost of 411-494 £/tCO₂^[51]*. This latest project is named Orca and is the largest operating DACCS facility in the world.

Global Thermostat uses an amino-polymer adsorbent to capture CO₂^[52]. The company has pilot and commercial demonstration plants operating since 2010 in the United States, which provide a combined capture capacity of 1500 tCO₂/year.

Smaller scale DAC companies include Antecy, a Netherlands based company who have developed a detailed DAC design in collaboration with Shell that utilises a K2CO3 adsorbent, and Hydrocell Ltd, a Finnish company that has built a 1.387 tCO₂/year pilot unit that exhibits low regeneration temperatures (70–80 degC) and is a NETs producer of water (1.9 t/tCO₂)^[52].

Moisture Swing Adsorption (MSA)

MSA DAC is less developed, and only has two small-scale companies developing the technology: Skytree (founded in the Netherlands) and Infinitree (founded in the United States). Skytree are building upon electrostatic absorption and moisturising desorption, and Infinitree utilise an ion exchange sorbent. Early markets for both companies are urban farming projects, for which captured CO₂ is used to assist in plant growth^[52].

*Values converted from USD to GBP using conversion of 1 USD = 0.83 GBP.