Abstract: The principle, characteristics and application scope of various CO2 separation technologies are introduced, and the methods are compared. The application of chemical absorption method in the capture and purification of CO2 from cement kiln tail gas was described in detail, and the application of CO2 separation technology under oxygen-enriched combustion was discussed. The prospect of CO2 separation technology is also discussed.
Key words: CO2 separation technology; cement kiln tail gas; capture and purification; Oxygen-enriched combustion
1 Introduction
With the energy crisis and the greenhouse effect becoming more and more serious, how to control the emission of CO2 and how to recycle and utilize it has become a worldwide concern [1]. The efficient separation of CO2 is the key to CO2 capture and recovery. CO2 separation technology is the research focus of scholars from all over the world, and many separation methods have been proposed, such as absorption separation method, adsorption separation method, membrane separation method, cryogenic distillation method, chemical circulation combustion method, electrochemical method, hydrate method, oxygen-enriched combustion method and so on. The emission of CO2 from
cement industry accounts for about 20% of the total emission of CO2 in China [2]. In order to reduce the emission of CO2 in cement industry, improve the level of cement production technology and energy efficiency, and realize the advanced and green manufacturing in cement industry is the development direction of cement industry. A large amount of CO2 emissions into the atmosphere not only cause the greenhouse effect, but also a huge waste of carbon resources [3]. According to the Cement Technology Roadmap 2050 jointly developed by the International Energy Agency (IEA) and the Cement Sustainability Initiative (CSI) in 2009, CCS technology alone needs to contribute 56% to achieve 18% CO2 emission reduction potential in the global cement industry by 2050. The remaining 44% comes from increased energy efficiency, use of alternative fuels, and in cement kiln flue gas is the development direction of cement industry [5].
CO2 separation technologies
are classified according to the principle, power and carrier of separation. CO2 separation technologies mainly include absorption separation method, adsorption separation method, membrane separation method, cryogenic distillation method, chemical circulation combustion method, electrochemical method, hydrate method, etc.
2. According to different absorbents, it can be divided into physical absorption method and chemical absorption method. The
physical absorption method is to dissolve and absorb CO2 under specific conditions (such as pressure, temperature, etc.) By using the solubility of CO2 in solvents changing with pressure and temperature. Then the operating conditions (such as depressurization, heating, etc.) Are changed to release CO2 and regenerate the solvent. The whole process obeys Henry's law. This method is suitable for flue gas with high CO2 partial pressure, and requires that the adsorbent has high solubility, good selectivity, high boiling point, no corrosion, no toxicity and stable performance. The commonly used absorbents for physical absorption are methanol, polyethanol dimethyl ether, N-methyl-2-pyrrolidone, etc.
Chemical absorption refers to the use of CO2 for chemical reaction to form a complex intermediate compound, and then by changing the conditions (such as heating, depressurization or inert gas purging, etc.). The CO2 in the absorption liquid rich in CO2 is desorbed, and the absorbent is regenerated. Typical chemical absorbents include ammonia, hot alkaline solutions, monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), diisopropanolamine (ADIP), methyldiethanolamine (MEDA), and diglycolamine.
2. Because of the inherent properties of CO2 , such as molecular space structure and molecular polarity, some adsorbents have stronger adsorption capacity for CO2 than other components in the mixed gas. Therefore, when the mixed gas passes through the adsorbent bed at a certain pressure, the adsorbent will selectively adsorb the strong adsorbate CO2 component, while the hard-to-adsorb component is discharged from the outlet of the adsorbent bed. Adsorption method can be divided into temperature swing adsorption (TSA), pressure swing adsorption (PSA) and vacuum adsorption. The adsorbent adsorbs CO2 at high temperature (or high pressure), and CO2 is desorbed after cooling (or depressurization). CO2 is separated by periodic temperature (or pressure) changes. Commonly used adsorbents are natural zeolite, molecular sieve, activated alumina, silica gel and activated carbon.
2. The driving force of membrane separation is the pressure difference between the two sides of the membrane. Under the condition of pressure difference, the gas component with high permeability preferentially permeates the membrane to form a permeation gas flow, while the gas component with low permeability forms a residual gas flow on the inlet side of the membrane. The two gas flows are led out separately to achieve the purpose of separation.
Membrane-based absorption is a new membrane separation process which combines membrane technology with gas absorption technology. A hollow matrix membrane is used as a support, so that the contact area of gas and absorption liquid is remarkably increased (about 600-1200 mZ/m3 ), and the entrainment phenomenon caused by direct contact of gas and liquid is overcome. It has the advantages of stable mass transfer interface, large specific surface area, high mass transfer efficiency, low energy consumption, small device volume and large operation flexibility. Hydrophobic microporous hollow fiber is usually used as the membrane, which plays the role of gas-liquid two-phase membrane in the mass transfer process. The gas diffuses from the gas phase on one side of the membrane to the liquid phase on the other side through the micropores of the membrane, and is absorbed by the liquid phase. The membrane has no selectivity for the gas itself, and the absorbent plays a key role in the selectivity of the components.
2. For the mixed gas with high CO2 content, this method is more economical and reasonable, and the liquid CO2 product can be obtained directly by compression, condensation and purification. The mixture with low CO2 content needs to be compressed and cooled several times to cause the phase change of CO2 . O that the CO2 is concentrated and separated from the mixed gas such as the flue gas and the like. Low temperature separation includes direct distillation, double column distillation, adding additives and controlling freezing. Direct distillation leads to the troublesome formation of CO2 solids in the distillation tower. This process is primarily used to improve crude oil recovery. During oil recovery, CO2 is injected into the reservoir to improve oil recovery.
2. comprising an air reactor and a fuel reactor in series. The metal undergoes an oxidation reaction with oxygen in the air in the air reactor to become an oxygen-carrying state in the form of a metal oxide, and then the fuel and the metal oxide undergo a reduction reaction in the fuel reactor. CO2 and H2O are generated for recycling. The main advantage of CLC is that the technology is based on two-step chemical reaction, which realizes the cascade utilization of chemical energy and has higher energy utilization efficiency. The air reactor mainly emits N2 , which will not pollute the air; The fuel is combusted under the catalysis of the oxygen carrier, the temperature is low (600-1200 deg C), and no nitrogen oxide is generated; The gas discharged from the fuel reactor is mainly CO2 and steam, and high-purity CO2 can be separated by simple condensation without consuming too much energy. Ishida [7] estimates that this technology can increase the thermal efficiency of power plants to 50% -60%, while the CO2 emission rate is reduced to
0.2. Molten carbonate is a pasty corrosive agent, which is difficult to manufacture and operate. The SO2 in the flue gas can also poison the cell, and the quarantine of the electrolyte and the degradation of the electrode are also serious problems in the high-temperature flue gas environment. Solid-state electrolytes, on the other hand, operate at lower temperatures than molten carbonate batteries, are easier to handle, have significantly fewer corrosion problems, and have a longer service life than molten carbonate. Therefore, the separation of CO2 from flue gas using solid electrolyte membrane in combination with molten carbonate is one of the promising directions.
2. The equilibrium pressure of different gases forming hydrate at the same temperature is very different, so the gas with lower equilibrium pressure can form hydrate by controlling the pressure, and then the required gas can be separated. Glew et al. [12] conducted a series of studies on the hydrate method. Hydrate method is a new separation technology based on many years of experimental research on hydrate. This separation technology has great potential in the separation and concentration of CO2 from power plant flue gas or IGCC syngas. Beside, that hydrate formed by the final separation of the CO2 mix gas is almost pure CO2 hydrate and can be directly sealed in the form of hydrate. 2.the
air is compressed to separate oxygen which is used as an oxidant for combustion, and the CO2 generated after the combustion is dedusted, dried and compressed to obtain the product CO2 . If the volume fraction of CO2 in the combustion products is more than 90%, no separation is required.This method can make the separation and absorption of CO2 and the treatment of SO2 easier, reduce the emission of NOx, and at the same time, the recirculation of flue gas makes the exhaust volume of the combustion device much lower than that of the traditional way, and reduce the comprehensive emission of pollutants. The disadvantage is that the increased SO2 will corrode the equipment, and the technology is not yet mature. This method is suitable for the separation of CO2 from flue gas [13-14]. 2.9 Comparison
of
several separation methods According to the development in recent years, the absorption method is the most mature and widely used CO2 separation technology, the pressure swing adsorption method is being gradually promoted, and the membrane method and oxygen-enriched combustion method are being developed. However, there are still many shortcomings. In practical application, the appropriate method can be selected according to the purity requirements of raw gas and CO2 product gas. See Table 1 for the specific comparison of each method:
Table 1 Comparison of
advantages and disadvantages of main CO2 separation methods 3 Application
of CO2 separation technology in cement industry 3. CO2 separation and purification is the core process. At present, chemical solution absorption and pressure swing adsorption have been applied in the capture and purification of CO2 from cement kiln tail gas. PSA method has been eliminated by some cement plants, and oxygen-enriched combustion method is under research and has not yet been industrialized. 3.2 Application status
of
CO2 separation technology in cement industry 3.2. Cement kiln tail gas not only contains CO2 , N2 , O2 , H2O , SOx, NOx, dust, HCl, HF and other pollutants. The existence of impurities will increase the cost of capture and separation, so before the tail gas enters the absorption tower, it needs to be pretreated, including water washing cooling, water removal, electrostatic precipitation, desulfurization and denitrification. After pretreatment, the flue gas enters the absorption tower, the temperature of the absorption tower is kept at 40 ~ 60 ℃, CO2 is absorbed by the absorbent, and then the flue gas enters a water washing container to balance the water in the system and remove the solvent droplets and solvent vapor in the gas, and then leaves the absorption tower. The rich solvent, which has absorbed CO2 , is pumped through a heat exchanger to the top of the Regeneration Tower. The absorbent is regenerated at a temperature of 100 to 140 ° C and a pressure slightly higher than atmospheric pressure. The water vapor is returned to the Regeneration Tower via condenser and the CO2 leaves the Regeneration Tower. The regenerated alkali solvent is pumped back to the absorber through a heat exchanger and cooler. The flow chart of the chemical absorption method is shown in Figure 1.
Flow chart
of chemical absorption method 3.2. According to Maddox [16], CO2 partial pressure and CO2 content before and after treatment are used as references to select the absorption solution. Hot potassium carbonate absorption solution is more suitable for CO2 partial pressure of 1-7 bar. When the partial pressure of CO2 in the gas is less than 1standard atmospheric pressure, the alcohol amine aqueous solution is a better choice. Leci and Goldthorpe [17] evaluated that the hot potash method also requires lower concentrations of SOx and NOx in the flue gas entering the system, approximately 1 ppm. Therefore, for the decarburization of cement kiln tail gas, the alcohol amine absorption solvent is used as the absorption liquid, and the kiln tail gas needs to be pretreated by desulfurization and denitrification before entering the absorption tower. The research results of
research institutions [18-21] show that it is feasible to use ammonia solution as absorption liquid to remove CO2 from flue gas. The ammonia absorption liquid has that advantage of low cost of the ammonia, high removal efficiency, low regeneration energy consumption, combined removal of CO2 , S0x and NOx in the flue gas in theory, low investment and low cost; The unit CO2 removal cost is much lower than that of the conventional amine method and the like. However, there are still some problems to be solved if the ammonia absorption liquid is to be completely applied in the cement industry, such as the leakage and explosion prevention of ammonia in operation, how to control the high volatility of ammonia absorption and regeneration, the research on the absorption and removal of CO2 by high concentration ammonia, and the improvement of the absorption and removal of CO2 by ammonia; Study on combined removal of CO2 , SOx and NOx in flue gas.
In addition, efficient composite absorption solvents and new ion absorption solvents are also research hotspots, which are currently under research and development.
3. The combustion products are mainly CO2 and water vapor, in addition to excess oxygen to ensure complete combustion, as well as oxidation products of all components in the fuel, inert components in the fuel or air leaked into the system, etc. After being condensed by cooling water steam, the CO2 content of the flue gas is 80-98%. In this way, the CO2 with high concentration can enter the pipeline for storage after compression, drying and further purification. Flow chart of CO2 capture from oxygen-enriched combustion flue gas is shown in Fig. 2:
4 Conclusion and prospect
CO2 in kiln gas or flue gas can be separated by absorption, adsorption, membrane and other methods. The greenhouse effect caused by CO2 can be alleviated by storing or recycling CO2 . Although there are many methods to separate CO2 , some of them are still in the research stage and need further verification. As far as the current research progress is concerned, membrane separation is more economical, and the key lies in the development of membranes with high selectivity and permeability. Although the chemical absorption method has been industrialized, it shows the disadvantages of high cost, high energy consumption and low efficiency. The absorption efficiency of the chemical absorption solvent needs to be further improved. The membrane separation-absorption combination method has low energy consumption and good absorption effect. If the problem of its complex process is solved, it is expected to be used in industrial production. The capture and purification of CO2 from
cement kiln tail gas is still in the initial stage, and the efficient CO2 separation, capture and purification technology needs to be further studied.
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