Direct Air Capture with Pore Space Storage (DACPS)

Basics of DACPS

 DACPS (dăk-p-s) involves using an industrial plant to capture CO₂ gas from atmospheric air and then compressing it to a supercritical (semi-liquid) state.
 The supercritical CO₂ is stored long-term by injecting it deep (more than about 800 meters) underground into tiny pore spaces in a layer of sandstone or other sedimentary rock.

 Below a depth of 800 meters, the temperature and pressure is greater than CO₂'s critical temperature and pressure of 31^oC (87.8^oF) and 73.8 bar (1071 psi), thus keeping CO₂ in the supercritical (semi-liquid) state and does not revert back to gas.

DACPS has six basic steps:

1. Extract CO₂ gas from the atmosphere in a plant where fans blow air through an air contactor and CO₂ molecules attach to either a liquid solvent or a solid sorbent (the two main types of DAC plants).
2. Release the CO₂ from the solvent or sorbent using heat and/or water. The solvent/sorbent is reused.
3. Liquify extracted CO₂ by compressing and cooling.
4. Transport the liquified CO₂ via pipeline to underground injection wells.
5. Inject the CO₂ deep into the ground into a sandstone or other permeable reservoir which is overlain by an impermeable layer such as shale (the caprock or topseal). Semi-liquid CO₂ underground — being more buoyant (less dense) than surrounding groundwater and rock — tends to float upward toward the land surface through any openings in the overlying rock layers.
6. Trapped under the caprock, the CO₂ flows laterally, outward from the injection well, fingering through the sandstone reservoir layer until there are only disconnected globules of CO₂ in the sandstone pores and further advancement at the leading edge of the CO₂ plume ceases.

 DACPS and direct air capture with carbon mineralization storage (DACCM) differ only in the mechanism that stores the CO₂ in rock. DACPS stores the CO₂ by physical trapping of the semi-liquid CO₂ in the pore spaces of rock (typically sedimentary rock such as sandstone or limestone). DACCM stores CO₂ by a chemical reaction in which semi-liquid CO₂ reacts with basalt (or similar igneous rock) to form solid rock (limestone). . .
 DACCS (direct air capture with carbon sequestration) is the traditional, widely used — general — expression that refers to underground storage of DAC-captured CO₂, without regard to the mechanism that traps the injected CO₂ underground. So, DACCS refers to both DACPS and DACCM.
 Distinguishing between DACPS and DACCM — rather than lumping them together as DACCS — helps in understanding the geographic distribution of DAC projects, evaluating CO₂ leakage potential and storage durability, and development of public policy.

Read more

 DACPS is viewed by many as a tool that is necessary for the world to reach net zero during the transition from fossil fuels to energy sources that have low or zero greenhouse gas emissions. DACPS is widely criticized, however, for the potential to prolong the use of fossil fuels and the potential for hindering the scale-up of low-emissions energy sources such as solar.

 Globally, there are large-scale DAC plants in operation that capture CO₂ for use in products (direct air capture and use, DACU). And there are large-scale DAC plants that store CO₂ underground using carbon mineralization (DACCM). But there is not yet a large-scale DAC plant in operation that stores CO₂ in underground pore space (DACPS). This may change in 2025, however, when the oil company, Oxy, is expected to start up its new DACPS plant in west Texas (USA). The plant, named STRATOS, is designed to capture 500,000 tons of CO₂ per year. . .
 Injecting CO₂ underground is nothing new for the oil industry as it has injected CO₂ into underground pore space since the 1940s in enhanced oil recovery (EOR) projects. Most CO₂ used in EOR has come from naturally occurring CO₂ gas deposits. Since 1972, increasingly more CO₂ for EOR is coming from gas captured from the exhaust flues of industrial plants such as natural gas processing plants and ammonia fertilizer plants. DAC could supply EOR if economics are favorable.
 In addition to STRATOS, nine other large-scale DACPS projects worldwide are in the planning stage (view table). Many DACCS projects that may use pore space storage (see CDR.fyi, search on DACCS) have pre-sold carbon credits for future delivery to buyers who are trying to meet their companies' net zero goals by the year 2050 — a deadline consistent with many national net-zero goals. Investors and government incentives in the form of grants or tax credits help drive many of projects.

Read more

 Injecting CO₂ underground can cause earthquakes if injected with a high enough pressure to fracture rock underground. Semi-liquid CO₂ stored underground can leak to the land surface through an injection well or an old oil or gas well in the storage area if CO₂ degrades the steel well casing. CO₂ can also leak to the land surface if a flow pathway through a topseal layer of rock is dissolved by carbonic acid that is created when CO₂ dissolves in groundwater.

 DACPS requires a significant amount of energy to capture and compress CO₂. The amount of energy used in DACPS to capture and compress one ton of CO₂ is currently estimated to be 2.17 MWh/tCO₂, dropping to 1.44 MWh/tCO₂ by year 2050.
 For a 1-million-ton per year DAC plant, the 1.44 MWh/tCO₂ equates to the annual energy use of 96,000 homes (assuming each home uses an average of 3.0MWh of energy annually). The energy would need to come from a low- or zero-carbon source for a DACPS project to be carbon negative (i.e., remove more carbon dioxide than the project produces). A cradle-to-grave life-cycle assessment of a project is necessary for verifying the project will be carbon negative.

 DACPS costs include the costs of: CO₂ capture, compression, transport, underground storage, and monitoring. The total cost of removing and storing one ton of CO₂ using DACCS (i.e., DACPS or DACCM) in 2020 was estimated to be between approximately €175 to €400 per ton of CO₂ removed. The estimate projected the cost to drop to roughly the €100 to €250 range by the year 2050 due to gains from technology advances and improved economy of scale.

 The number of tons of CO₂ removed from the atmosphere and stored in a DACPS project can be measured using a flow meter at the injection wellhead.
 Monitoring is aimed at detecting leaks of CO₂ from the underground reservoir. Monitoring methods include monitoring wells, seismic mapping of the underground CO₂ plume, and CO₂ gas detectors installed in soil or on the land surface. Government regulations typically require an approved monitoring plan prior to approving a permit for a CO₂ injection well.
 Globally, at least ten different MRV protocols have been developed for DACPS.

 CO₂ stored in a geological formation (including DACPS projects) is considered to be virtually permanent (at least 10,000 years).

 DACPS, as well as the other geologic methods of CDR, are considered to be "novel" (new unproven) methods of CDR — compared to conventional, proven CDR methods (e.g., biochar, bioenergy with carbon capture and storage, afforestation).
 A scenario has been described for the year 2050 in which the novel methods annually supply about 2 gtCO₂ of the total annual 10 gtCO₂ of CDR that will be needed then in order to limit global warming to 2^oC above pre-industrial (circa year 1850) levels.
 To visualize the potential scale of DACPS development in 2050, if DACPS supplies one-fourth (0.5 gtCO₂ per year) of the annual 2 gtCO₂, then 500 DACPS plants — each with 1 million gtCO₂ annual removal capacity — will be needed.