CO2 capture is the separation of CO2 from emissions sources or the atmosphere. From emissions sources, CO2 is recovered in a concentrated stream that is amenable to sequestration or conversion. CO2 capture technologies are a current area of significant interest, and technologies are needed which allow for viable CO2 capture in conjunction with fossil fuel use.
Generally speaking, CO2 capture can be conducted as a post-combustion or pre-combustion process. Post-combustion processes are based on chemical absorption in relatively low temperature, low CO2 partial pressure conditions such as those typically found in flue gas.
Typical absorbents used are amines and carbonates. In a typical process, the flue gas contacts the chemical absorbent in sorption column, the CO2 is transferred from the flue gas to the absorbent, and there are two out-going flows from sorption column; a cleaned gas-stream with low CO2 content and a stream containing water, absorbent and CO2. After the absorption process, the absorbent and the CO2 are separated in a regeneration column. When heated, the absorbents ability to retain CO2 is reduced, resulting in regeneration of the absorbent, which can then be re-used. The CO2 leaves the regeneration column as a gas stream of high CO2 purity.
Carbon dioxide capture as described above is relatively energy intensive. Because CO2 is present at dilute concentrations and low pressure, high volume of gas must be treated. A relatively low CO2 capture capacity of the absorbent also has tremendous impact. For example, the CO2 capture capacity of commercial liquid amine processes is about 0.68 mole/kg. Since the capacity is low, either large reactors or frequent regeneration is necessary. Additionally, trace impurities such as sulfur dioxide, nitrogen oxides, and particulate matter in the flue gas can degrade sorbents and reduce the effectiveness of certain CO2 capture processes. Large parasitic loading may also result, as compressing captured or separated CO2 from atmospheric pressure to pipeline pressure (about 2,000 psia) represents a large auxiliary power load on the overall power plant system.
A more effective alternative is pre-combustion CO2 capture. Pre-combustion CO2 capture relates to gasification plants, where fuel is converted into gaseous components by applying heat under pressure and chemically decomposing the fuel to produce synthesis gas (syngas), which is composed of hydrogen (H2), carbon monoxide (CO), and minor amounts of other gaseous constituents. The syngas may then be processed in a water-gas-shift reactor, which converts the CO to CO2 and increases the CO2 and H2 molecular concentrations to about 40 percent and 55 percent, respectively. The CO2 can then be captured from the synthesis gas prior to combustion in a combustion turbine. Because the CO2 is relatively concentrated in the synthesis gas stream, separating the CO2 becomes much more effective, as the high partial pressure and high chemical potential improves the driving force for various types of separation and capture technologies. However, typically used pre-combustion technologies involve solvent absorption of CO2 at low temperature and relatively high pressure, followed by a decrease in pressure for CO2 release and recovery. This process results in a low temperature, low pressure CO2 stream, and the required cooling and subsequent reheating of the fuel stream necessary for combustion decreases the plant thermal efficiency and increases cost. Additionally, solvent-based pre-combustion CO2 removal processes may be sensitive to water content in the synthesis gas stream, and require water removal prior to CO2 capture for effective operation.
The energy efficiency of pre-combustion capture would be significantly improved in Integrated Gasification Combined Cycle (IGCC) processes if the sorbent were operational at moderate or high temperatures. Currently, water-gas shift reactors in IGCC processes elevate gas streams to 200 to 300° C., so as to transform synthesis gas to CO2, H2, and steam. As discussed above, these gases must be cooled before current CO2 removal technologies can be used. Thus, there are considerable advantages in developing sorbents for CO2 capture at moderate to hot gas temperatures, as the hot gas remaining after CO2 removal can be directly introduced to the turbine systems. If CO2 can be removed from the gas stream directly after the water-gas shift reactor, a pure H2 stream can be obtained at high temperatures for various applications. Aside from use in IGCC applications, sorbents for moderate to hot gas temperatures also can be useful for chemical and metallurgical applications.
Some minerals will undergo thermodynamically favorable reactions with CO2, separating it from a gas stream and forming a stable, chemically bonded product. These have been investigated largely for mineral carbonation, where carbon dioxide is chemically reacted with alkaline and alkaline-earth metal oxide or silicate minerals to form stable solid carbonates for long-term CO2 sequestration. Using magnesium hydroxide (Mg(OH)2) for the formation of magnesium carbonate (MgCO3) is among the mineral processes which have been investigated. Decomposition of the formed carbonates with temperature has also been investigated for cycles utilizing regenerable sorbents, however, in the typical cycle where regeneration of the carbonate loaded sorbents is limited solely to thermal decomposition through increased temperature, these sorbents have a disadvantage in that decreased reactivity tends to result from multiple absorption/regeneration cycles. It would be advantageous if a mineral based sorbent such as Mg(OH)2 could be used in a cyclic absorption/desorption process designed to maintain sorbent reactivity. It would be further advantageous if the sorbent were operational at moderate or high temperatures.
Accordingly, it is an object of this disclosure to provide a process utilizing a regenerable Mg(OH)2 sorbent for the separation of CO2 from a gaseous stream for the production of a moderate or high temperature CO2-depleted gas stream.
Further, it is an object of this disclosure to provide a process utilizing a regenerable Mg(OH)2 sorbent for the separation of CO2 from a gaseous stream where the regenerable sorbent maintains activity over a number of absorption/desorption cycles.
Further, it is an object of this disclosure to provide a process utilizing a regenerable Mg(OH)2 sorbent for the separation of CO2 from a gaseous stream using steam regeneration of the Mg(OH)2 sorbent followed by a polishing process to maintain sorbent activity.
Further, it is an object of this disclosure to provide a process utilizing a regenerable Mg(OH)2 sorbent for the separation of CO2 from a gaseous stream using steam regeneration followed by a polishing process, where the polishing process utilizes water liberated during the CO2 absorption process such that water requirements are reduced.
Further, it is an object of this disclosure to provide a process utilizing a regenerable Mg(OH)2 sorbent for the separation of CO2 from a shifted syngas stream comprised of H2, H2O, and CO2 in a pre-combustion process.
Further, it is an object of this disclosure to provide a process utilizing a regenerable Mg(OH)2 sorbent for the separation of CO2 from a shifted syngas stream comprised of H2, H2O, and CO2 prior to H2 combustion in an IGCC plant, utilizing steam generated by the IGCC plant.
Further, it is an object of this disclosure to provide a process utilizing a regenerable Mg(OH)2 sorbent for the separation of CO2 from a gaseous stream in a manner tolerant to H2O in the gaseous stream, avoiding requirements for dehydration prior to CO2 separation.
Further, it is an object of this disclosure to provide a process utilizing a regenerable Mg(OH)2 sorbent for the separation of CO2 from a gaseous stream at warm gas temperatures, avoiding the necessity for cooling and subsequent reheat of the gaseous stream for pre-combustion CO2 removal.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.
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