Stabilizing the concentration of atmospheric CO 2 may require storing enormous quantities of captured anthropogenic CO 2 in near-permanent geologic reservoirs. Because of the subsurface temperature profile of terrestrial storage sites, CO 2 stored in these reservoirs is buoyant. As a result, a portion of the injected CO 2 can escape if the reservoir is not appropriately sealed. We show that injecting CO 2 into deep-sea sediments <3,000-m water depth and a few hundred meters of sediment provides permanent geologic storage even with large geomechanical perturbations. At the high pressures and low temperatures common in deep-sea sediments, CO 2 resides in its liquid phase and can be denser than the overlying pore fluid, causing the injected CO 2 to be gravitationally stable. Additionally, CO 2 hydrate formation will impede the flow of CO 2 (l) and serve as a second cap on the system. The evolution of the CO 2 plume is described qualitatively from the injection to the formation of CO 2 hydrates and finally to the dilution of the CO 2 (aq) solution by diffusion. If calcareous sediments are chosen, then the dissolution of carbonate host rock by the CO 2 (aq) solution will slightly increase porosity, which may cause large increases in permeability. Karst formation, however, is unlikely because total dissolution is limited to only a few percent of the rock volume. The total CO 2 storage capacity within the 200-mile economic zone of the U.S. coastline is enormous, capable of storing thousands of years of current U.S. CO 2 emissions.

A review of the literature has found a factor of 4 spread in the estimated values of the energy penalty for post-combustion capture and storage of CO 2 from pulverized-coal (PC) fired power plants. We elucidate the cause of that spread by deriving an analytic relationship for the energy penalty from thermodynamic principles and by identifying which variables are most difficult to constrain. We define the energy penalty for CCS to be the fraction of fuel that must be dedicated to CCS for a fixed quantity of work output. That penalty can manifest itself as either the additional fuel required to maintain a power plant's output or the loss of output for a constant fuel input. Of the 17 parameters that constitute the energy penalty, only the fraction of available waste heat that is recovered for use and the 2nd-law separation efficiency are poorly constrained. We provide an absolute lower bound for the energy penalty of $11%, and we demonstrate to what degree increasing the fraction of available-waste-heat recovery can reduce the energy penalty from the higher values reported. It is further argued that an energy penalty of $40% will be easily achieved while one of $29% represents a decent target value. Furthermore, we analyze the distribution of PC plants in the U.S. and calculate a distribution for the additional fuel required to operate all these plants with CO 2 capture and storage (CCS).

We describe an approach to CO 2 capture and storage from the atmosphere that involves enhancing the solubility of CO 2 in the ocean by a process equivalent to the natural silicate weathering reaction. HCl is electrochemically removed from the ocean and neutralized through reaction with silicate rocks. The increase in ocean alkalinity resulting from the removal of HCl causes atmospheric CO 2 to dissolve into the ocean where it will be stored primarily as HCO 3 -without further acidifying the ocean. On timescales of hundreds of years or longer, some of the additional alkalinity will likely lead to precipitation or enhanced preservation of CaCO 3 , resulting in the permanent storage of the associated carbon, and the return of an equal amount of carbon to the atmosphere. Whereas the natural silicate weathering process is effected primarily by carbonic acid, the engineered process accelerates the weathering kinetics to industrial rates by replacing this weak acid with HCl. In the thermodynamic limit-and with the appropriate silicate rocks-the overall reaction is spontaneous. A range of efficiency scenarios indicates that the process should require 100-400 kJ of work per mol of CO 2 captured and stored for relevant timescales. The process can be powered from stranded energy sources too remote to be useful for the direct needs of population centers. It may also be useful on a regional scale for protection of coral reefs from further ocean acidification. Application of this technology may involve neutralizing the alkaline solution that is coproduced with HCl with CO 2 from a point source or from the atmosphere prior to being returned to the ocean.

Injecting liquid CO(2) into deep-sea sediments below ca. 3 km of seawater has been suggested for the permanent storage of anthropogenic CO(2). At the pressures and temperature found below 3 km of seawater, CO(2) becomes denser than seawater and so is likely to remain permanently sequestered in the sediment. Deepwater engineering, however, is expensive and seawater depths of greater than 3 km are often only reached far from shore. Here, we consider the less expensive alternative of injecting CO(2) into marine sediments at depths shallower than required for denser-than-seawater CO(2) storage. We compare the mobility of liquid CO(2) that has been injected into deep-sea reservoirs with the mobility of supercritical CO(2) that has been injected into geologically equivalent (i.e., identical porosity, permeability, and effective stress) reservoirs with terrestrial pressure and temperature conditions. We demonstrate that buoyant liquid CO(2) with a density of about 90 % that of seawater is sufficiently immobile that it can be considered trapped by gravity and capillarity. In contrast, supercritical CO(2) under typical terrestrial conditions is highly mobile and only trapped by the appropriate confining layer in either a structural or stratigraphic trap. As a result of its very high mobility under terrestrial conditions, CO(2) injected in an unconfined formation would spread beneath the confining layer to produce a large flat cylindrical-shaped plume of pure-phase CO(2). In contrast, the less mobile CO(2) in a typical deep-sea reservoir produces a spherical-shaped plume, resulting in a pure-phase-CO(2) footprint that is much smaller than the pure-phase-CO(2) footprint formed in the confined-terrestrial reservoir.

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We strongly disagree with the statement made by Realff and Eisenberger (1) that the approach we used in our analysis of the energy requirements and of "air capture" systems is circular (2). On the contrary, our analysis is linear: i) We calculated the minimum work based on fundamental thermodynamics. ii) We then estimated second-law efficiencies using a large amount of empirical data from real processes. iii) We estimated the energy costs based on market prices.The statement by Realff and Eisenberger (1) that "the notion of minimum work does not apply" is wrong. By definition, all air capture processes start with ambient air and produce a concentrated stream of CO 2 , as well as a CO 2 -depleted airstream. As shown in our paper (2), one can precisely calculate the difference in exergy between these two end points. This exergy difference is the minimum work required by any air capture process. The fact that proposed air capture processes may have exothermic steps has absolutely no impact on the minimum work requirement; if there is an exothermic step in the process and that energy is not recovered, as is generally true for CO 2 absorption processes, that lost energy becomes a source of process inefficiency. Furthermore, if all the energy in the exothermic step were harnessed and used to drive another step in the process, and if there were no other irreversible losses through friction, for example, that air capture system would require work input exactly equal to the minimum work.We also understand that the exergy required by the process can be supplied in many forms. To keep our analysis straightforward, we chose to supply the exergy with carbon-free electricity. This has the benefits of (i) not having to worry about how much CO 2 is released by our energy source and (ii) easily determining its market price. We understand that there may be niche opportunities to provide this exergy at reduced prices. If air capture is to be deployed on a scale large enough to address climate change, however, it will need to pay market prices rather than niche prices. The idea that there are vast amounts of exergy available in the form of "cheap heat" is just not true.Although we agree that "there is no fundamental reason" for air capture second law efficiencies to be lower than those for flue gas capture, we feel we present a very strong empirical case that it is so. This analysis relies on data from real processes that incorporate a large amount of engineering experience.In our paper, we documented an empirical relationship between the second-law efficiency of separation systems and the concentration factor of those systems. It is physically possible for a new technology to break that empirical relation; however, until demonstrated experimentally, there is no reason to believe that the relation will not hold for air capture systems.

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