Overview of carbon dioxide capture technology

Thread Content

An overview of carbon dioxide capture technologies: There are mainly three approaches to carbon capture: pre-combustion capture, in-combustion capture (full-oxygen combustion), and post-combustion capture. I. Pre-combustion capture technology ● Pre-combustion capture refers to the separation of the carbon-containing components from the fuel before it burns, and it is primarily used in Integrated Gasification Combined Cycle (IGCC) power plants. A water-gas shift unit is introduced in IGCC power plants to react CO in the gas with water vapor to produce CO2 and H2, after which the CO2 is separated. Pre-combustion capture can overcome the drawbacks of post-combustion capture in conventional coal-fired power plants, such as high flue gas flow rates and low CO2 concentrations, and is considered one of the most promising carbon capture technology approaches for the future. The main CO2 separation technologies currently applicable for pre-combustion capture are physical absorption methods (represented by the Selexol method) and chemical absorption methods (represented by the MDEA method). ●Issues related to the type of gasifier in IGCC capture power plants, the gasification method (air/oxygen gasification), the methods for cooling and dust removal of the syngas, the CO2 separation methods, and the selection of the capture efficiency. For IGCC capture power plants using different types of gasifiers, the system configuration that yields the best technical and economic performance was determined, resulting in a capture rate of 85%-90%, which represents the lowest cost for CO2 reduction. Studies show that, at the current level of technology, when CO2 capture is taken into account in IGCC power plants, the system’s power generation efficiency will decrease by 6–11 percentage points, while the cost of electricity production will increase by 20–40% ； The power supply efficiency of IGCC capture power plants using a transport bed gasifier is 1.4–2.1 percentage points higher than that of other gasifier-based power plants, with lower power generation costs by 8–11%. In addition, the power supply efficiency, specific investment, electricity generation cost, and CO2 emission reduction cost of IGCC plants considering CO2 capture were compared with those of conventional pulverized coal PC plants. The results show that, after taking CO2 capture into account, the decrease in the power generation efficiency of IGCC plants, as well as the increase in specific investment and power generation costs, are both lower than those of PC plants. The cost of CO2 emission reduction for IGCC plants is approximately 35% lower than that for PC plants. II. Carbon capture technology in combustion – Oxygen-enriched (pure oxygen) combustion. Oxygen-enriched combustion involves using a mixture of oxygen-enriched gas (with an O2 concentration >20.947%) and some of the exhaust gas drawn back as a substitute for air in the combustion process; this increases the CO2 concentration in the exhaust gas, facilitating its compression and separation. The separated CO2 can then be directly processed and stored ； Oxygen-enriched combustion is applied in industries such as steel, glass, and cement ； Advantages: Relatively simple to modify, easy to scale up, suitable for upgrading existing units ； Theoretically, the CO2 concentration can exceed 80% ； Disadvantages: Oxygen generation equipment requires high investment, consumes a lot of energy, and has high costs. Significant advantages of pure oxygen combustion (100% oxygen enrichment): energy savings + environmental protection + integrated carbon capture. ● Energy savings: Taking natural gas combustion as an example, with air as the oxidizing agent: CH4+2O2+7.5N2→CO+2H2O+7.5N2. In pure oxygen combustion: CH4+2O2→CO2+2H2O. Nitrogen makes up about 78% of air; during normal combustion, a large amount of nitrogen does not participate in the combustion process and is released after unnecessary heating, carrying away a lot of heat and thus increasing energy consumption significantly ； In pure oxygen combustion, the oxidizing gas is pure oxygen, with almost no nitrogen present; as a result, there is no unnecessary heating, which means that pure oxygen combustion offers higher thermal efficiency and lower energy consumption ； Widely used in the steelmaking industry for ladle drying, saving 10%-50% energy compared to conventional methods ； ●Environmental protection: 95% of the pollutants in flue gas, namely nitrogen oxides, are thermal NOX (with raw material-derived and fuel-derived NOX accounting for only about 5%). This phenomenon occurs as a result of the reaction between N2 and O2 that has not fully participated in the combustion process at high temperatures; the higher the temperature, the higher the concentration of NOX ； When pure oxygen is used as the combustion aid gas, very little nitrogen enters the reactor (due to poor sealing; theoretically, it is zero), which significantly reduces the generation of thermally induced NOx, thereby greatly lowering the cost of nitrogen oxide removal ； ●Increased production: With pure oxygen combustion, the burning is complete, the flame temperature is high; the main products are CO2 and H2O. The radiation capacity is strong, and the flame radiation temperature can increase by about 100 ℃, which facilitates rapid heating of the heated object ； ●Carbon capture: The CO2 concentration in the flue gas resulting from the combustion of natural gas with pure oxygen can reach as high as 90%-95%. For a pure-oxygen combustion case at a steel plant, the technical and economic parameters are as follows: the price of oxygen is 0.36 yuan/Nm3, the price of converter gas is 0.2 yuan/Nm3, the calorific value of converter gas is 1150 kcal, and the oxygen coefficient is 0.22. Compared to the original level, 30 Nm3 of converter gas per ton of steel results in a savings of >55%. Original cost: 30×0.2=6 yuan/ton. Current cost: Fuel cost: 30×0.45×0.2=2.7 yuan; Oxygen cost: 30×0.45×0.21×0.36=1.02 yuan. Total cost: 2.7+1.02=3.72 yuan. Cost savings rate: (6-3.72)/6=38%. With an increase in baking temperature and a reduction in baking time, fuel savings exceed 55%, while cost savings exceed 38%. III. Carbon capture technologies after combustion: The main carbon capture technologies include chemical absorption, solid adsorption, pressure swing adsorption, membrane separation, and cryogenic methods. (1) In the chemical absorption method, the flue gas is pre-treated before entering the absorption tower, where it flows from bottom to top and comes into counter-current contact with the absorbent that flows from top to bottom. The alkaline absorbent reacts chemically with CO2 in the flue gas to form unstable compounds, and the decarbonized flue gas is discharged from the top of the absorption tower ； The absorbent that absorbs CO2 is the rich liquid; after being heated in a rich liquid heat exchanger, it enters the regeneration tower where the CO2 is released. The CO2 obtained after this process, along with water vapor, is cooled, and after the water is removed, high-purity CO2 gas is produced ； The absorbent after CO2 absorption is the lean solution, which flows out from the bottom of the regeneration tower; after heat exchange in the lean solution heat exchanger, it enters the absorption tower to continue absorbing CO2. Advantages: Fast absorption rate ； Large absorption capacity ； Capable of handling flue gas with low CO2 concentrations and low partial pressures ； Capable of handling large smoke volumes ； Wide range of applications. (II) Solid adsorption method: A typical solid adsorption process system consists of two reactors – one for adsorption and one for desorption. The flue gas first enters a low-temperature adsorption tower, where the adsorbent captures CO2 from the gas. The adsorbent, now enriched with CO2, is then sent to a high-temperature desorption tower (heated by steam) to release the captured CO2 ； The regenerated adsorbent is cooled in a cooler before being returned to the adsorption tower to cyclically absorb CO2 ； Advantages: No solvent is involved compared to chemical absorption methods; the process is simplified, there is no equipment corrosion, and significant energy savings and reduced consumption are achieved ； Disadvantages: A large amount of adsorbent is required, and adsorption/desorption processes need to be carried out frequently ； Generally suitable for flue gases with high CO2 concentration and high partial pressure ； Suitable for CO2 purification. (III) Pressure swing adsorption method: The pressure swing adsorption technique is based on a core principle: an adsorbent (a porous solid material) has different adsorption capacities for various components in a mixed gas at different pressures. Adsorption stage: At higher pressures, CO₂ molecules in the flue gas are preferentially and extensively adsorbed by the adsorbent, while other gas components such as nitrogen (N₂) and oxygen (O₂) are not easily adsorbed and thus pass through the adsorption tower as purified gas. Desorption stage: Once the adsorbent becomes saturated, the system pressure is reduced (or a vacuum is created) to cause the adsorbed CO2 molecules to desorb from the adsorbent. This results in a CO2 product gas with a high concentration, while the adsorbent is regenerated and can be used again in the next adsorption cycle. Generally suitable for flue gas with high pressure drop ； Suitable for CO2 purification. (IV) Membrane separation method: This method utilizes the fact that CO2 and other gases have different permeation rates through various membrane materials. Gases with relatively high permeation rates pass through the membrane and become concentrated on the permeate side, while those with slower rates remain on the retentate side, thereby achieving gas separation ； The permeation rate is related to the properties of gas molecules, the properties of the membrane, and the interaction between the permeating gas and the membrane ； Advantages: low energy consumption, no solvent evaporation, small footprint, etc ； Disadvantage: High energy consumption ； Generally suitable for flue gases with high CO2 concentration and high partial pressure ； Not suitable for handling flue gases with high flow rates ； Suitable for CO2 purification. (5) Cryogenic separation method: This method involves liquefying gases by applying pressure and lowering temperature, and utilizes the difference in boiling points between CO2 and other gases to separate CO2 ； Advantages: No use of chemical or physical absorbents; there are no issues such as absorbent corrosion ； Lower water consumption ； Liquid CO2 separated by cryogenic methods is more suitable for transportation and storage ； Disadvantage: High energy consumption ； The equipment investment is high ； Not suitable for handling large volumes of flue gas ； Generally suitable for flue gases with high CO2 concentration and high partial pressure ； Suitable for CO2 purification.