Elon Musk Offers $100 Million for the Best Carbon Capture Technology

There is a huge thermodynamic barrier to any large-scale “sequestration” of CO2 underground. Carbon dioxide can be selectively absorbed from combustion gas (mostly nitrogen, oxygen, and steam) in scrubbers using alkaline solutions (usually ethanolamines or caustic), but in order to avoid excessive consumption of expensive solvents, the solvents must be “regenerated” (by heating at low pressure) to release the CO2 from the solvent, as low-pressure, nearly pure CO2 gas.

But at atmospheric pressure and (for example) 25 C, CO2 gas has a density of only 1.80 kg/m3, which means that storing 1 metric tonne underground would require a volume of 556 m3. Storing a gigatonne underground (about 1/30 of annual emissions) would require a volume of 556 cubic kilometers. It is not known whether there are enough leak-proof underground cavities whose total volume is anywhere near the volume required to store the annual CO2 emissions of the United States.

The required storage volume can be decreased by increasing the density of the CO2, which can be achieved by increasing its pressure and/or decreasing its temperature. Carbon dioxide freezes at -56.6 C (-69.8 F), but trying to force solid “dry ice” into the ground is obviously impractical.

CO2 can also be liquefied by compression and refrigeration, but there are thermodynamic limits. The boiling temperature of CO2 increases with pressure (this is true for all substances), but no substance can be liquefied above its critical temperature, which is about 31 C (88 F) for carbon dioxide. At this temperature, a pressure of 73.8 bar (1,071 psi) is needed to liquefy CO2.

Since underground temperatures increase with depth, any liquid CO2 injected into an underground cavity could be heated by the surrounding rock above the critical temperature, which would result in a rapid increase in pressure if the volume of the cavity is fixed, possibly causing a man-made earthquake. This could be avoided by compressing the CO2 to a pressure above the critical pressure (on the order of 1,200 psi or more), for which the CO2 would be a “supercritical fluid” (neither liquid nor gas) not subject to rapid increases in pressure if heated by the surrounding rock underground.

The major obstacle to CO2 sequestration is the energy required to compress CO2 from atmospheric pressure (1.01 bar or 14.7 psi) to 1,100 psi or more, a pressure ratio of about 75:1. Industrial compressors can provide a pressure ratio of between 2.0 and 2.5 per stage, meaning that compression to 1,100 psi would require at least 5 stages of compression, with water- or air-cooled heat exchangers required to remove the heat of compression between stages. Additional energy would be required to run cooling-water pumps or the fans of air-cooled exchangers.

It is estimated that the energy required to run the compressors to sequester the CO2 emitted from a natural-gas-fired power plant would consume about 20% of the power generated by the plant, and this rises to about 30% for a coal-fired power plant.

If CO2 sequestration is added to an existing coal-fired power plant, the same amount of coal used to generate 100 MWh before sequestration only produces a net 70 MWh after sequestration, meaning that for the same net power generated, the coal resources are consumed 43% faster (1 / 0.70 = 1.43), and the cost to the consumer increases by 43% (possibly more, to amortize the capital cost of the compressors and coolers). For a gas-fired plant, the gas would be consumed 25% faster (1 / 0.80 = 1.25).

Large-scale sequestration of CO2 underground is cost-prohibitive, which is why several power companies who started studying it around 2010 gave up on their projects a few years later, as economically unfeasible.

No amount of legislation or executive orders will ever change the thermodynamic properties of carbon dioxide. If the carbon dioxide concentration in the atmosphere is increasing, we just need to adapt to it (it was much higher in the past, and life on earth survived).