Carbon Capture Begins : An Analysis Of Post-Combustion Carbon Capture Storage (CCS)

Research

The last article of this series looked into the Pre-combustion CCS and the recent efficiency levels reported. When it comes to CCS the first assumption is that carbon capture is only possible post combustion of the participating fuel.  But, the previous article proved there are many other possibilities.The figure below shows the different stages CC can be done. This article looks into the post combustion CCS. With the previous knowledge of the pre-combustion this would be easier to understand.

There are several methods of how post combustion CCS is done. Several well known methods are discussed here. They are, i) Chemical absorption ii) Physical absorption iii) Adsorption iv)  Membranes and v) Cryogenics separation.

First method we look at is chemical absorption. This is using chemical solvents in separating CO2. Some of the solvents commonly used used are Ammonia, Monoethanolamine Solutions (MEA)diethanolamine (DEA),  methyldiethanolamine (MDEA) and  diisopropanolamine (DIPA), The target of using a solvent is to create weakly bonded intermediate compound. Chemical absorption is more suited for industrial flue gases since a relatively pure CO2 stream could be captured and produced. Studies show that when  MEA solutions are used up to 90% of the capture is possible out of flue gas. So far, the most well established method to remove CO2 is chemical absorption with packed columns.

Next method is Physical absorption. This involves the use of Henry’s law in absorbing CO2 into a solution (thttp://www.800mainstreet.com/9/0009-006-henry.html).  DEPG (Selexol™)  and Methanol (Rectisol®) are common solvents used in this process. These are also known as gas treating solvents and generally used in acidic gas removal. It is an important observation that at low pressures the physical solutions become impractical due to compression of the gas for physical absorption becoming expensive.

The adsorption method is a heterogenous process unlike the above  absorption methods. The capability of solids to selectively adsorb CO2 into the imperfections on the external surfaces is put to use in this method. Adsorption largely depends on the pressures(more dominant) and temperatures of the flue gas, the size of the pores and the energy of interactions between CO2 and the sorbent.

Membrane separation is also a technique that has been known for few decades. Porous or non porous membranes can be used in the separation. Membrane method is used commonly in the petrochemical industry. Hydrophobic microporous hollow-fibre (HF) or hollow fiber membrane contactor (HFMC)  method  is one of the membrane types used in the membrane separation. The commonly known materials like polypropylene (PP) polytetrafluorethylene (PTFE) and polythene (PE), are used to make these membranes.  An absorption liquid is also used alongside the membrane in wetted absorption methods. This liquid could be NaOH Having to remove impure compounds like H2S is a disadvantage of the membrane method.

Cryogenic fractionation is the final method in the list. This method is popular because the captured CO2 stream is at high pressures. The CO2 concentration is also higher than 90%. The disadvantage of this method is the higher energy required for the refrigeration.

All the above methods are collectively researched based on the assumption that the consumption of fossil fuels will continue to the next three decades. It is our belief also that while expanding the use of  renewable sources, effective carbon capturing methods should also be widely researched.