Post-combustion CO2 capture refers to removal of CO2 from an exhaust gas stream from the combustion of a fuel in a power plant, boiler, blast furnace or similar combustion environment. In the terms of this article, post-combustion may also refer to the capture processes used on any post-process gas stream even when applied on a gas stream from an industrial process that does not actually involve combustion, such as hydro-carbon reformation processes.
CO2 removal from non-combustion emission sources is usually referred to by specific professional terms (such as natural gas processing or natural gas sweetening when referring to removal of excess CO2 from natural gas streams) or by specific process terms (such as the amine absorption process).
The world's first gas power plant with CO2 capture i Lubbock, Texas (1980).
The most common method for separating CO2 from a gas stream in use today is chemical absorption using alkaline solvents. The flue gas passes through an aqueous alkaline solvent, and since CO2 is acidic it is bound to the solvent. The CO2-rich solvent trickles to the bottom of the absorber, and is transported to a separation unit (stripper). The rich solution is heated to 100 - 140°C (depending on the exact solvent in use). This reverses the absorption process, releasing most of the CO2 to form a pure stream for compression and transport. The lean solvent is transported back to the absorber for reuse.
The absorption takes place in an absorption tower (scrubber). The exhaust from a boiler or turbine is usually well above efficient absorption temperature, and needs to be cooled to between 40 and 60°C. After cooling, the gas enters at the bottom of the absorber, while the solvent is pumped to the top. The gas passes through the solvent as the gas moves upwards and the solvent moves downwards. Packing inside the absorption tower provides a largest possible contact area for gas an liquid.
When CO2 is captured from fossil fuel power plants, the energy used in the absorption/regeneration process leads to an efficiency penalty for the overall plant. Most of the energy use comes from the generation of heat and steam that is used in the regeneration process. In addition, some energy is required to power fans to support the gas flow though the absorption tower and to power solvent pumps.
Simplified model of a post- combustion capture unit.
(Source: Nyhetsgrafikk.no / ZERO)
Flue gas from fossil fuel combustion contains other acidic gases such as SOx and NOx which also react with the solvent. If these gases are not removed before the absorption proses, they will form heat stable salts, clogging the packing in the scrubber and resulting in extra consumption of the solvent. Therefore the flue gas must be pretreated to remove most of the NOx and SOx before absorption. Ash and soot in the flue gas will also clog the absorber and must be removed. Removal of acidic compounds and soot is usually done at power plants without CO2 capture, to meet other environmental demands, but additional removal of sulphurs may be necessary to meet carbon capture requirements.
CO2 absorption is most efficient when the partial pressure of CO2 is high. As absorption capture usually happens at near atmospheric pressure, higher CO2 content increases the capture efficiency. Novel power plant designs have been suggested where the capture process happens at higher pressure, thus increasing the CO2 partial pressure. It may also possible to recycle some of the flue gas back to the combustion phase to increase CO2 content before capture. This may be a solution to increase efficiency of CO2 capture from gas turbines, as turbine exhausts have low CO2 content compared to boiler exhausts.
Choosing the right solvent is important to reduce the energy penalty of the capture process. A good absorbent can bind large amounts of CO2 quickly, and has a low desorption temperature. The lower the temperature where solvent regeneration happens, the lower the energy cost of CO2 capture. The solvents must also have a low byproduct formation and low decomposition rates.
At atmospheric pressure, amine solvents have proven themselves to be most effective. Amines are alkaline organic compounds and derivatives of ammonia. A number of different amines have been tested as CO2 absorbents, and several different amine capture systems are commercially available. Different amine systems are tailored for different capture conditions, such as gas stream pressure and temperature and CO2 content. Amine CO2 separation has many industrial applications today.
Ammonia may also be used to capture CO2. Ammonia is far more volatile than amines, and therefore capture has to happen at lower temperature to avoid vaporization of the solvent. Lower temperature leads to a lower reaction time. Therefore, a bigger absorber is needed, and the process is best suited for flue gas with high CO2 content.
To remove CO2 using ammonia, the flue gas must be cooled to between 0 and 10°C before entering the scrubber. On the other side, far less energy is needed in the regeneration phase. sing ammonia may have significant energy use benefits over amines, but the process is not yet commercially mature for CO2 capture.
Main article: Membranes
Membranes are materials with selective permeability, allowing only the desired molecules to pass through it. Membranes are in commercial use for removal of CO2 from high pressure gas streams. The membranes that are available today are only efficient at high flue gas pressure and CO2 concentration.
Membranes more suitable for capture of CO2 from a typical flue gas streams are being developed. One option is a hybrid membrane/solvent system. In these systems, membranes are used to provide a very high surface area for reaction between flue gas and solvent. Other membrane systems are also being developed and may become commercially available for full scale flue gas CO2 capture in the furture.
Solid sorbents have been proposed as an alternative to the liquid sorbents used in current capture systems. Among the materials being considered for full scale CO2 capture application are sodium and potassium oxides and carbonates. CO2 separation is achieved by passing hot flue gas through the solid sorbent, which forms carbonates (or bicarbonates), thus absorbing the CO2. Increasing the temperature in the sorbent then causes regeneration, releasing the CO2.
Adsorption is a chemical reaction where atoms or molecules accumulate on the surface of a material, different from absorption here the substance diffuses into the solvent to form a solution. This process also has potential as a method for capturing CO2. The process has been used for CO2 removal from synthesis gas for hydrogen production, but it is still on the experimental stage for CO2 removal from flue gases. Based on experiments and models it has been concluded that full scale CO2 capture from flue gases may be possible.