Carbon dioxide capture technology for coal-fired power plants

2.2 Carbon dioxide capture processes 2.2.1 Organic amine solvent absorption method The amine compound absorption methods mainly include the thermal potassium alkali method (Benzophenol method, arsenic alkali method, steric hindrance method, etc.) and the alkoxy alcohol amine method (MEA method, DEA method, MDEA method, etc.). This is one of the most widely used processes in China, accounting for 70%. Its advantages are: high absorption efficiency and a relatively simple process ； The disadvantage is: high energy consumption for regeneration ； The mass transfer area is small ； Foaming, mist entrainment, and bubbling will occur; direct contact between gas and liquid leads to significant corrosion of the equipment ； It will produce environmental pollutants. Since a single amine absorbent cannot simultaneously meet the requirements of high absorption efficiency and low regeneration energy consumption, a series of improvements have been made to organic amines: ① Use of composite amines: By adding some primary amines to MDEA, its carbon dioxide absorption rate can be increased. The MDEA-based decarburization method offers advantages such as low energy consumption, high gas purification efficiency, solution stability, low volatility, and minimal corrosion to carbon steel equipment; as a result, it is adopted by many ammonia and methanol production plants. ②Use of activators: Activators currently under investigation for activating tertiary alcohol amines include piperazine PZ, DEA, MEA, enamines, and 2,3-butanedione. The carbon dioxide absorption capacity of amine solvents is enhanced upon the addition of these activators. The patented AP-814 absorbent developed by Dao Company is a specially formulated MDEA solution; it is said to have a higher capacity for absorbing CO2, thereby reducing the regeneration load on amine treatment units. ③Steric amines: Introducing certain groups with steric effects into amine molecules can significantly improve the decarburization and desulfurization efficiency of absorbents. The most commonly used steric amine cited in the literature is AMP (2-amino-2-methyl-1-propanol). This method offers a high absorption rate and easy desorption, but it has a high vapor pressure and is relatively expensive. 1) MEA method: Reaction of MEA with CO2: (1) The MEA method has been extensively studied and successfully applied to CO2 recovery in chemical plants. However, MEA technology has drawbacks such as high costs, slow absorption, low absorption capacity, large amounts of absorbent required, high equipment corrosion rates, degradation of amines by other flue gas components, and high energy consumption during absorbent regeneration. 2) Activated MDEA method: (2) (3) Equation (2) + Equation (3) yields: (4) The German company BASF developed the activated MDEA decarburization process (aMDEA method) by adding a certain amount of activator to an MDEA aqueous solution. In the early 1970s, it was industrialized in the United States and Germany, and widely used in decarbonization devices in ammonia synthesis plants. In the 1990s, after the French Elf Group improved this process, it was also applied to natural gas purification, mainly for treating natural gas with very low H2S levels but high CO2 levels. The activated MDEA method for decarbonization offers advantages such as low energy consumption, high gas purification efficiency, solution stability, low volatility, and virtually no corrosion to carbon steel equipment, which is why it is adopted by many ammonia and methanol plants. 2.2.2 Adsorption method The adsorption method is further divided into temperature-swapped adsorption (TSA) and pressure-swapped adsorption (PSA); it relies on the selective and reversible adsorption of CO2 by solid adsorbents from the raw material gas mixture to separate and recover CO2. The adsorbent adsorbs CO2 at high temperatures (or high pressures), and releases CO2 upon cooling (or reducing pressure). Commonly used adsorbents include natural zeolites, molecular sieves, activated alumina, silica gel, and activated carbon. Currently, the pressure swing adsorption process is widely used in industry. The advantages of this method are: a simple process, low energy consumption, and strong adaptability. Its disadvantages are: limited adsorption capacity, a large amount of adsorbent required, frequent adsorption and desorption, and high requirements for automation. The drawback of the adsorbent itself is that it poses significant limitations for industrial application, which is why its use is limited. 2.2.3 Ion solution absorption method: Ionic liquids are salts composed of an organic cation containing a nitrogen heterocycle and an inorganic anion; they represent a new type of solvent that has become popular in the chemical industry due to their environmental friendliness. Ordinary ionic liquids can only dissolve carbon dioxide through simple physical dissolution, so their solubility is not very high. Later, scientists introduced –NH2 groups into ionic solutions, thereby creating functional ionic liquids. The cations in such ionic liquids contain –NH2 functional groups; under normal temperature and pressure, the saturated concentration of CO2 in these ionic liquids is as high as 7.4% (by mass), which is close to the theoretical molar fraction of 0.5 mol CO2/mol•L‑1. The advantages of this method are: low energy consumption for regeneration, good thermal stability, and the ability to design cations and anions ； It exhibits better selectivity for CO2, has a high absorption capacity, good efficiency, and is green and clean. The disadvantages are: high cost (ionic liquids are expensive) and high viscosity (which reduces the carbon dioxide absorption rate). 2.2.4 Membrane separation method The membrane separation method utilizes membranes made of certain polymeric materials to separate gases based on the differences in their permeability to various gases. The membranes used for CO2 gas separation are mostly made of cellulose acetate, polysulfone, polyamide, etc. In recent years, new membrane materials with excellent properties have been emerging; for example, polyimide membranes, phenoxyl-modified membranes, diaminopoly sulphone composite membranes, polycarbonate composite membranes containing diamines, and low-molecular-weight acrylate impregnated membranes all exhibit superior CO2 permeability. Recently, some inorganic membranes made of silica, zeolite, and carbon have also been developed. It is one of the carbon dioxide capture technologies that is developing most rapidly in the world today. The advantages of this method are: low investment, low energy consumption, compact equipment, and easy maintenance. The disadvantage is that it is difficult to obtain high-purity carbon dioxide. Our country has made certain progress in the technology of capturing carbon dioxide using membrane separation. The carbon dioxide membrane separation unit developed by the Dalian Institute of Chemical Physics is already capable of capturing carbon dioxide from low-quality natural gas. Recently, this system was successfully deployed at the Hainan Fushan oil field of CNPC and passed the acceptance tests, becoming China’s first carbon dioxide capture system using membrane separation technology. 2.2.5 Membrane absorption method The membrane absorption method is a combination of membrane separation and chemical absorption. Common ones include hollow fiber microporous membrane contactors. The structure of a membrane contactor is similar to that of a shell-and-tube heat exchanger: the mixed gas flows on the outside of the membrane, while the absorbent solution flows on the inside; the gas and the absorbent solution flow in opposite directions through the hollow fiber microporous membrane elements to facilitate absorption. The high permeability of the membrane and the high absorption capacity of the absorbent solution are its unique advantages. The advantages of this method are: a stable mass transfer interface, a large specific surface area, high mass transfer efficiency, low energy consumption, a small device size, and great operational flexibility. Compared with traditional tower absorbers, it offers advantages such as a high packing density, a stable gas-liquid contact interface, and no foaming or liquid flooding. The disadvantages include: a relatively complex manufacturing process, high costs, issues with membrane wettability, and problems with the compatibility between the absorbing solution and the membrane. For situations with low throughput and low concentration, the membrane separation-solvent absorption coupling technique has advantages. However, the membrane absorption method is still at the laboratory stage. The presence of NOx, SO2, and dust in actual flue gas may also have a negative impact on the carbon dioxide absorption process. Additionally, the interaction between the absorbent solution and the membrane material requires further research, especially regarding the effect of the absorbent solution on membrane properties before and after CO2 absorption during system operation. 2.2.6 ECO2 technology In recent years, the technology of using ammonia water to wash flue gas to remove CO2 has attracted attention worldwide. The American company Powerspan has developed the ECO2 capture process, which uses ammonia to capture CO2 from power plant flue gases. BP’s alternative energy company, Powerspan, is developing and testing its CO2 capture technology based on ammonia. The next step will be to commercialize this technology for use in coal-fired power plants. This technology involves using ammonia water to wash flue gas in order to remove CO2. The reaction and process flow are as follows: The absorption of CO2 by ammonia water solution is a gas-liquid absorption process accompanied by chemical reactions, and it plays an important role in the purification of raw gases for ammonia synthesis, the production of ammonium bicarbonate, and the alkali industry. Its advantages are: high load capacity, no corrosion issues, low energy consumption for regeneration, no degradation in flue gas environments, low operating costs, and the by-products have certain economic value. The disadvantages are: the ammonia concentration cannot be too high, it is prone to explosion, and it generates environmental pollutants. 2.2.7 Low-temperature distillation method: The low-temperature distillation method takes advantage of the difference in boiling points between CO2 and other gas components; it involves liquefaction at low temperatures followed by distillation to separate CO2 from these other gases. Due to the large equipment investment, high energy consumption, and poor separation efficiency of this method, its cost is high. The distillation process is mainly used to improve the crude oil recovery rate. During oil extraction, injecting CO2 into the oil reservoir can increase oil recovery rates. However, as the oil recovery rate increases, associated gas is also produced. Low-temperature distillation is primarily used to separate and purify CO2 from associated gas in oil fields, so as to reinject it into the oil wells for reuse. According to calculations by Dutch research institutions, coal-fired power plants that do not use decarbonization technologies have an efficiency of 38%, with CO2 emissions of 0.95 kg/(kW•h) ； After separating CO2 using low-temperature distillation, the plant’s efficiency dropped to 26%, and CO2 emissions were reduced to 0.14 kg/(kW·h). At present, the use of low-temperature distillation to recover CO2 from flue gas is still in the theoretical research stage. This method is mainly used to recover CO2 from associated gas in oil fields. The more typical processes are the three-tower and four-tower processes developed by Ryan Holmes of the American company Koch Process (KPS); the entire process includes ethane recovery, methane removal, as well as the recovery of additives and CO2. This method can produce liquid CO for pipeline transport, but it has high energy consumption, poor separation efficiency, and high costs. This method is applicable to situations with high carbon dioxide content, such as the recovery of CO2 from associated gas in oil fields. In future IGCC designs or CO2 recycling systems, low-temperature distillation is worth considering due to the high concentration of CO2 in the flue gas.