Pre-combustion capture refers to the removal of carbon from fuel before the combustion process. In this approach, fuel reacts partially at high pressure with oxygen or air and, in some cases, steam, to produce carbon monoxide (CO) and hydrogen (H₂). The CO then reacts with steam in a catalytic shift reactor to produce CO₂ and additional H₂. The hydrogen is used as fuel in a combined cycle plant for electricity generation and the CO₂ is then separated for sequestration. In other engineering applications, pre-combustion has been identified as a technology for residual liquid-petroleum fuel conversion where H₂, heat and power can be produced in addition to CO₂ that needs to be captured. One of the advantages of pre-combustion capture is the high CO₂ concentration of the flue stream, requiring smaller equipment size and different solvents with lower regeneration energy requirements.

Pre-combustion CO2 capture is a process in which fuel is decarbonated prior to combustion, resulting in zero carbon dioxide production during the combustion step. Coal is gasified, generally at high temperature and pressure, to produce synthesis gas. This gas mixture is then run through the water-gas shift reaction to produce H2 and CO2 at high pressure and slightly elevated temperature depending on the production. Pre-combustion CO2 capture, which refers to the separation of CO2 from H2 within this gas mixture, can then be performed to afford pure H2, which is subsequently combusted in a power plant to generate electricity.

Considerations for Pre-combustion CO2 Capture

Solid adsorbents, membrane materials, and liquid absorbers are currently under consideration as potential candidates for use in pre-combustion CO2 capture. With regard to industrial applications, pre-combustion CO2 capture systems based on CO2-absorbing solvents, which are the closest to being realized, and a number of power plants incorporating such systems are being constructed. Furthermore, the use of solid adsorbents in pressure-swing adsorption-based processes is currently under extensive investigation.

Advantages of Pre-combustion Capture

In comparison to post-combustion CO₂ capture and oxy-fuel combustion based processes, pre-combustion CO₂ capture carries a number of advantages that may be of benefit for its rapid industrial implementation. Perhaps most significantly, the gases are produced at high pressure, and the partial pressure of CO₂ in the mixture is high compared with post-combustion flue gas. As a result, regeneration of the loaded adsorbent can occur through a drop to the atmospheric pressure, which is energetically favorable and is more practical compared with a temperature or vacuum swing-based process. Furthermore, the CO₂/H₂ separation is inherently easier to perform than CO₂/N₂ or O2/N₂ separations, owing to the greater differences in the polarizability and quadrupole moment of the molecules. Thus, for purely physisorption-based separations, a greater selectivity for CO₂ over H₂ can be anticipated, which may allow next-generation pre-combustion CO₂ capture materials to be more rapidly developed than new adsorbents for post-combustion CO₂ capture or oxy-fuel combustion.

Metal-Organic Framework-Containing Membranes for Pre-combustion CO₂ Capture

 Membrane separation is an exceptionally promising strategy for pre-combustion CO2 capture. This is primarily because the high pressure of a pre-combustion gas mixture is an excellent driving force for membrane separation of CO₂ and H₂. The same properties that make metal-organic frameworks promising pre-combustion adsorbents are preserved in a membrane separation scenario. The high capacities for CO2 in bulk metal-organic frameworks are not necessarily lowered when these materials are incorporated into membranes.

Diffusion is a property of gases that is pertinent to membrane separations that is frequently not discussed for bulk nano-porous adsorbents. This important characteristic can be harnessed in CO₂/H₂ membrane separations in metal-organic frameworks. H₂ diffusion in some MOFs has been shown to be two orders of magnitude higher than the highest values recorded within a zeolite, which is a potential advantage in incorporating metal-organic framework-containing membranes into pre-combustion CO₂ capture systems. Additionally, CO₂ diffusion is slower than H2 thus preserving the selectivity of the separation materials.

Metal-Organic Frameworks as Adsorbents

Metal-organic frameworks have recently been investigated as potential next-generation adsorbents for pressure-swing adsorption-based separation of CO2 from H2. Their high surface areas afford enhanced gas adsorption capacities compared with porous solids conventionally employed in multilayer beds within current PSA systems, namely, activated carbons and zeolites, and their tunable surface chemistry is anticipated to facilitate further optimization of the material properties. Despite the opportunity for the development of metal-organic frameworks as pre-combustion CO₂ capture adsorbents relatively few reports have emerged in this regard. Although evaluation of the performance of candidate frameworks can be well approximated through the collection of high-pressure, single-component CO₂ and H₂ isotherms at near-ambient temperature. There are only a small number of examples where such experiments have been previously performed. In cases where they have been reported, the isotherms have seldom been discussed and analyzed in the context of pre-combustion capture. We consider CO₂/H₂ separations only in the context of pressure-swing adsorption based processes in which the separation of the gases is achieved by a thermodynamic equilibrium that results from the bulk adsorptive properties of the material. An alternative strategy for achieving the separation would be to make use of the difference in the kinetic diameters or diffusion properties of the two molecules in a kinetic-based separation using metal-organic framework membranes.