Direct air capture, measured heat in

One solid sorbent, used for ten cycles without being replaced, captures carbon dioxide from air at about 400 ppm and releases it as a concentrated stream, with a cyclic working capacity and a measured heater energy that are both past the vapor-promoted resin process cited here.

Taking carbon dioxide out of air is only useful if the heat to get it back off the sorbent is small and the sorbent keeps its capacity. Equilibrium uptakes and tabulated heats of adsorption leave out the sensible heat of the bed and the water that real air carries. The grade is carbon dioxide collected from a humid 400 ppm feed, divided into the electrical energy that actually went into the heater, on one sample, ten times.

Wang, Qu, Ding, Webley, and Li report a primary-amine resin whose carbon dioxide uptake is 1.11 mmol g⁻¹ at 20 °C and 400 ppm. In their vapor-promoted desorption cycles the working capacity stayed at 0.9–1.0 mmol g⁻¹ over nine cycles. They calculate a thermal energy of 10.4 MJ per kilogram of CO₂ for the solar-heated prototype, and they give 12–15 MJ kg⁻¹ for temperature-vacuum swing on the same resin, including 0.5–1 MJ kg⁻¹ of electrical energy for evacuation. The abstract reports carbon dioxide of 97.7% purity together with water, and recovery of 98% of the adsorbed CO₂ in the sunlight-driven prototype. Desorption in that process is driven near 100 °C by the vapor, without a vacuum pump.

A heat of adsorption in kilojoules per mole is not this measurement. Their 10.4 MJ kg⁻¹ already includes sensible heat and the latent heat of water, which is why it sits well above a bare reaction enthalpy.

At least 1.00 g of dry sorbent is packed in a tube. The dry mass is recorded before cycle 1. The feed is air, or synthetic air, at 1.00 ± 0.05 bar and 25 ± 2 °C, with the inlet carbon dioxide measured at 400 ± 40 ppm and the relative humidity at 50 ± 10%. Inlet concentration, humidity, and flow are measured, not assumed.

Each cycle is adsorption until the outlet has returned to 95% of the inlet concentration, or for a shorter preregistered time, then desorption. Desorption heats the bed from 25 ± 2 °C to a final temperature of at most 120 °C. The product gas is collected until the outlet carbon dioxide returns to within 5% of the inlet value. Water is condensed. The carbon dioxide collected is the integral of measured outlet flow and measured dry-gas concentration. Purity is the mole fraction of carbon dioxide in that collected gas on a dry basis.

Heater energy is the electrical energy delivered to the heater from the start of the ramp until collection stops, measured by a watt-hour meter or by integrated voltage and current. No blank heat is subtracted. A tabulated enthalpy does not count.

Cycle 1 is reported and not graded. Cycles 2 through 10 are graded. The same sorbent charge is used for all ten cycles. It is not reloaded, washed, or regenerated outside the measured heater step. The ten cycles finish within 14 days.

Working capacity is the moles of carbon dioxide collected in desorption, divided by the initial dry mass. Energy is the heater energy divided by the mass of carbon dioxide collected. Both are averages over cycles 2 through 10.

Over cycles 2 through 10, the average working capacity is at least 1.50 mmol of CO₂ per gram of dry sorbent, the average heater energy is at most 6.0 MJ per kilogram of CO₂ collected, and the average dry-gas purity is at least 95 mol% CO₂. The bed returns to 25 ± 2 °C between cycles, and the desorption temperature never exceeds 120 °C.

1.50 mmol g⁻¹, 6.0 MJ kg⁻¹, 95 mol%, 120 °C, ten cycles, 1.00 g, and 14 days are pass marks defined here. The capacity mark is above the 0.9–1.0 mmol g⁻¹ cyclic capacity and above the 1.11 mmol g⁻¹ uptake of the resin cited above. The energy mark is below the 10.4 MJ kg⁻¹ calculated for that vapor-promoted prototype. Neither number is a claim that no other paper has printed a larger equilibrium uptake.

• A tube packed with glass beads of the same bed volume, taken through one adsorption and desorption under the same flows and the same temperature program, collects less than 5% of the carbon dioxide collected in the candidate’s cycle 2. If it does not, the analyzer or a leak is producing the signal, and the sorbent is not graded.
• The inlet concentration is measured on every cycle. A feed that drifts outside 400 ± 40 ppm voids that cycle.
• Flow is measured at the outlet during desorption. An assumed flow fails the attempt.
• The watt-hour measurement is checked the same week against a resistive load of known power. The check is reported.

Any solid sorbent, including amines, salts, and frameworks. A co-adsorbed water layer, if it is picked up from the humid feed and not added as liquid water or steam from outside. Electrical heat. A tube, a heater, a flow meter, and a carbon dioxide analyzer.

A feed above 1,000 ppm carbon dioxide, including flue gas. Liquid amine scrubbing. Adding steam, purge gas, or vacuum and omitting their energy. Subtracting a sensible-heat blank so that only a reaction enthalpy remains. Reporting an equilibrium uptake in place of the carbon dioxide collected on cycles 2 through 10. Replacing or reloading the sorbent between graded cycles. A desorption above 120 °C. A sample below 1.00 g dry mass.

Over cycles 2 through 10, the average working capacity is at least 1.20 mmol g⁻¹ and the average heater energy is at most 8.0 MJ kg⁻¹, with dry-gas purity at least 95 mol% and the same temperature cap. 1.20 mmol g⁻¹ and 8.0 MJ kg⁻¹ are pass marks for this lesser bar. They are past the cited cyclic capacity and past the cited 10.4 MJ kg⁻¹, and they are not the full mark. A high capacity at an unmeasured enthalpy is not partial success.

Resin uptake of 1.11 mmol g⁻¹ at 20 °C and 400 ppm, cyclic working capacity of 0.9–1.0 mmol g⁻¹, calculated thermal energy of 10.4 MJ kg⁻¹, the 12–15 MJ kg⁻¹ comparison, 97.7% purity, and 98% recovery: Wang, Qu, Ding, Webley, and Li, Nature Communications, 2024, 15, 9745, doi:10.1038/s41467-024-53961-4 (opens in a new tab). The 1.50 mmol g⁻¹, 6.0 MJ kg⁻¹, 1.20 mmol g⁻¹, 8.0 MJ kg⁻¹, 95 mol%, 120 °C, 1.00 g, and 14-day requirements are pass marks defined here. Open point: other sorbents have been reported with equilibrium uptakes, at 400 ppm, above 1.11 mmol g⁻¹. Those uptakes are not cyclic heater-energy measurements on a humid feed, so this problem does not treat them as having cleared the joint bar. A later paper that does report both numbers under a comparable humid cycle should move the marks.