Post Combustion Carbon Capture
Post-combustion capture focuses directly on power plant emissions, such as flue gas. Carbon dioxide can be removed from post-combustion flue gas using regenerable solvents. The solvent most frequently encountered for CO2 capture is monoethanolamine (MEA), an amine solvent. This method is the most suitable retrofit option for existing power plants.
Post-combustion CO2 capture has been installed at about a dozen facilities worldwide. The capture process is based on chemical absorption. The captured CO2 is used for various industrial and commercial processes, e.g. the production of urea, foam blowing, carbonated beverages, and dry ice production. The captured CO2 is used as a commercial commodity. That makes absorption process, profitable because of the price realized for the commercial CO2.
The most commonly used absorbent for CO2 absorption is monoethanolamine (MEA) solvent. The cooling and heating of the solvent, pumping and compression require power input from the power plant thermal cycle, derating the thermal efficiency (heat rate) of the power plant.
In the solvent scrubbing process, the cooled flue gas is brought into contact with the solvent in the absorber at temperatures typically between 40°C and 60°C, CO2 is bound by the chemical solvent in the absorber. The flue gas is then water washed to balance water in the system and to remove any solvent droplets or solvent vapour carried over, and then it leaves the absorber.
It is possible to reduce CO2 concentration in the exit gas down to very low values, as a result of the chemical reaction in the solvent, but lower exit concentrations tend to increase the height of the absorption vessel. The rich solvent, which contains the chemically bound CO2 is then pumped to the top of a stripper (or regeneration vessel), via a heat exchanger.
The regeneration of the chemical solvent is carried out in the stripper at elevated temperatures (100140°C) and pressures not very much higher than atmospheric pressure. Heat is supplied to the reboiler to maintain the regeneration conditions. This leads to a thermal energy penalty as a result of heating up the solvent, providing the required desorption heat for removing the chemically bound CO2 and for steam production which acts as a stripping gas. Steam is recovered in the condenser and fed back to the stripper, whereas the CO2 product gas leaves the stripper. The lean solvent, containing far less CO2 is then pumped back to the absorber via the lean-rich heat exchanger and a cooler to bring it down to the absorber temperature level.
MEA has several advantages as a solvent such as its high reactivity, low cost, high absorbing capacity on a mass basis, reasonable thermal stability and thermal degradation rate. However it also has some disadvantages such as its corrosive effects. Much research has been devoted to finding or developing solvents that are superior to MEA. A better solvent would not degrade, it would work under normal flue gas outlet conditions, and it would require less energy for regeneration. Some of the ways in which alternative solvents might perform better than MEA include:
higher capacity for CO2 capture;
lower energy for regeneration;
higher absorption/desorption rates and regeneration at lower temperatures;
lower volatility and better stability;
less degradation and lower corrosivity.
The major drawback of solvent scrubbing is the cost due to the high energy requirements of the process. The energy required using MEA as a solvent can cause a 20% reduction of power generation for a pulverized fuel power plant.
In order to reduce the capital and energy cost, and the size of the absorption and regenerator (stripper) columns, new processes are being developed. One example is the membrane-absorption process, where a microporous membrane made of polytetrafluoroethylene separates the flue gas from the solvent.
The membrane allows for greater contacting area within a given volume, but by itself the membrane does not perform the separation of CO2 from the rest of the flue gases. It is the solvent that selectively absorbs CO2. The use of a gas membrane has several advantages: (a) high packing density; (b) high flexibility with respect to flow rates and solvent selection; (c) no foaming, channeling, entrainment and flooding common problems in packed absorption towers; (d) the unit can be readily transported, e.g. offshore; (e) significant savings in weight.
It is possible to design a once through scrubbing process (i.e., no regeneration step). For example, one could scrub CO2 from flue gas with seawater and then return the whole mixture to the ocean for storage. However, to-date these approaches are not as practical as those using a regenerable solvent. In the seawater scrubbing example, the large volumes of water that are required result in large pressure drops in the pipes and absorber.
Other processes have been considered to capture CO2 from power plant and industrial boiler flue gases, e.g. membrane separation, cryogenic fractionation, and adsorption using molecular sieves. Generally, these processes are less energy efficient and more expensive than the absorption methods.
Post-Combustion Capture Emerging Technologies
- Small Scale Demonstration
- Alstom Chilled Ammonia Process
- Pilot scale R & D
- RTI Dry Carbonate Process
- Lab and Bench Scale R & D
- Secondary and Tertiary Amines
- Zeolite Based Sorbents
- Amine Impregnated Dry Sorbents
- Ionic Liquids Absorption
- Enzyme based Processes
- Membrane based Processes