The Earth's climate is undergoing a significant change characterized by global warming, which will have an important impact on the global ecosystem and social and economic development. Studies have shown that this is mainly due to the warming effect of greenhouse gases such as CO2 emitted by humans using fossil fuels. In recent decades, CO2 emissions have been increasing with economic development. In 2006, the world's CO2 emissions reached 28 billion tons, of which China accounted for 20.2%. Fossil fuels such as coal, oil, and natural gas are the main sources of CO2 emissions, and coal emits the most CO2. Coal is a relatively "dirty" energy source. Coal with the same calorific value emits much more CO2 than oil and natural gas, and is the most important source of CO2 emissions. In 2006, coal accounted for only 26% of the world's primary energy consumption, but its CO2 emissions accounted for 41.7%. This problem is particularly prominent in my country: in 2007, my country's coal consumption was 2.59 billion tons, accounting for 69.5% of my country's primary energy consumption and more than 80% of my country's CO2 emissions. Of this, 1.31 billion tons were used for power generation. In 2008, thermal power generation accounted for 80% of my country's total power generation, most of which was from coal-fired power plants. Due to its low price, abundant reserves, and easy access, coal will remain my country's main energy source for a long time to come.
Currently, there are mainly the following ways to control CO2 emissions: improve energy efficiency, use renewable energy such as wind energy, solar energy, biomass energy, and nuclear energy, and use CO2 capture technology for burning fossil fuels.
In the foreseeable future, fossil fuels will continue to be our main energy source, which requires us to adopt CO2 capture and storage technology (CCS) to reduce CO2 emissions. Thermal power plants are the most important source of CO2 emissions, and their CO2 emissions exceed 40% of the total. Due to their centralized emissions and easy control, they have become the main application objects of CO2 capture and storage technology.
CO2 capture and storage refers to the collection of CO2 emitted by power plants and then transporting it to a CO2 storage location through a pipeline. This article mainly focuses on CO2 capture technology. There are currently three main types of CO2 capture technologies:
Post-combustion capture technology
Oxygen-enriched combustion technology
Pre-combustion capture technology
Keywords: CO2 capture; thermal power plant; oxygen-enriched combustion; flue gas capture; pre-combustion capture
Post-combustion capture technology
Post-combustion capture technology is used to capture carbon in flue gas after combustion. It uses monoethanolamine (MEA) or other solutions to directly absorb CO in flue gas for capture. MEA solution is an organic chemical solvent that has been used to remove acidic gas impurities in natural gas, such as CO2, H2S, etc. more than 60 years. Its absorption of CO2 belongs to chemical adsorption, which can release CO2 under heating. Using this method to capture CO2 in flue gas can remove 75%~90% of CO2 in flue gas and obtain CO2 with a purity of 99%.
To capture CO2 in flue gas, an absorption tower and a regeneration tower need to be added to the equipment to absorb and release CO2. In addition, the steam system needs to be modified to extract steam to heat the solution and release CO2. Due to the low flue gas pressure (generally close to atmospheric pressure), low CO2 concentration (10%~15%), and huge gas flow, the capture system is large and consumes a lot of energy. The main energy loss of post-combustion capture technology lies in the regeneration of the MEA solution. It is estimated that for newly built units with CO2 capture, the efficiency will drop by about 20%~30% compared with units with the same parameters, and the energy consumed by the regeneration of MEA solution accounts for more than half of the total energy consumed. The energy required for regeneration usually comes from the low-pressure steam extraction of the turbine. Alstom has studied the CO2 capture modification of a unit in the United States, showing that 79% of the steam after the medium-pressure cylinder is used for the regeneration of the MEA solution. Because the extraction of steam prevents the unit from operating under optimal conditions, the efficiency will continue to decline.
In addition, acidic gases such as SO2 and NO2 in flue gas will react with MEA solution to generate heat-stable salts, resulting in the loss of MEA solution. Therefore, the content of acidic gases in flue gas needs to be controlled at about 10x10". This requires the modification of the desulfurization system to improve the desulfurization efficiency. As for NOX, since NOX in flue gas is mainly NO, and NO2 only accounts for about 5%, ordinary SCR system can meet the needs.
Oxygen-enriched combustion technology
Oxygen-enriched combustion technology uses oxygen production technology to pass pure oxygen and part of the recycled flue gas into the boiler for combustion, so that the CO2 concentration in the flue gas reaches more than 95%, which can be directly compressed and purified.
The equipment for capturing CO2 using oxygen-enriched combustion technology mainly includes air separation devices, flue gas recirculation devices, and CO2 compression and purification devices. The main energy loss of oxygen-enriched combustion technology lies in the separation of air to produce oxygen. The currently commonly used cooling and air separation technology consumes a lot of energy, and the required electricity accounts for about 18% of the total power generation. At the same time, due to the reduction in flue gas flow and the reduction in exhaust heat loss, the boiler efficiency can be increased by about 3%. Overall, the efficiency of the entire power plant will drop by 20%~30%. New low-cost oxygen production technologies are currently being studied, such as oxygen and ion transport membrane (OTM) technology. Once a breakthrough is made, the cost of oxygen-enriched combustion technology can be greatly reduced.
Due to the continuous circulation of flue gas, the SO2 concentration in the flue gas is 2~3 times that of air combustion. If the sulfur content of coal is high, the flue gas should be extracted after the desulfurization system to prevent equipment corrosion. If it is not high, the desulfurization equipment can be cancelled. NOX emissions will be greatly reduced under the premise of adopting low NOx combustion technology. On the one hand, it is because there is a lack of N2 in the flue gas, and no thermal NOX is generated. On the other hand, NOX can be further reduced during circulation. After CO2 is compressed and liquefied, non-condensable gases, including excess oxygen leaking into the boiler air, SO2, NOX, etc., will be separated; the pollutants can be treated according to local environmental protection requirements.
Pre-combustion capture technology is mainly used in conjunction with IGCC technology. IGCC (Integrated Gasification Combined Cycle) is an advanced technology that combines coal gasification technology with a combined cycle. The IGCC system needs to add a shift reactor, CO2 separation, and a compression purification device for CO2 capture. Coal is converted into synthesis gas, mainly composed of CO and H2, under high temperature, high pressure, and oxygen-rich environment in the gasifier: in the shift reactor, CO and water vapor in the synthesis gas generate CO and hydrogen under the action of a catalyst. Because the gas pressure is high at this time, the concentration of CO is also high, and the polyethylene glycol dimethyl ether method (Selexol) can be used to absorb CO. This method is a physical absorption method. By reducing the pressure of the solution, CO2 can be released, and the solution can be regenerated. Its energy consumption is much smaller than that of the MEA method. At the same time, due to the high gas pressure, the energy consumption of the subsequent CO2 compression process is also reduced. Some scholars have analyzed the 500 MW IGCC system and believe that after installing the CO2 capture system, the efficiency of IGCC will drop from 38.4% (HHV) to 31.2% (HHV). Among them, the conversion reactor and the compression of CO2 have the greatest impact, which reduces the efficiency by 4.2% and 2.1%, respectively. The cost of CO2 removal by this method is about 20$/T.
technical prospects
Post-combustion capture technology is the most mature technology and has been put into use. My country's first coal-fired power plant CO2 capture device - Huaneng Beijing Thermal Power Plant 3000~5000t/year CO2 capture demonstration device uses this technology. Oxygen-enriched combustion technology is currently a research hotspot, but the technology is not very mature and mostly remains in the laboratory and pilot stage. The world's largest oxygen-enriched combustion project is the 30 MW Vattenfall project built in Germany in September 2008, which uses Alstom technology. In addition, Black Hills, together with B&W, Air Liquide, and other companies, will build a 100MW oxygen-enriched combustion power plant in Wyoming, USA. The project is scheduled to be completed in 2015. Both post-combustion capture technology and oxygen-enriched combustion technology can be used to transform existing power plants. The cost of oxygen-enriched combustion technology is relatively low, but if only part of the CO2 is captured, post-combustion capture technology is more suitable. IGCC is the cleanest coal-fired technology in the world, but its high cost and immature technology limit its application. However, after installing CO2 capture, its cost increase is the least, and the CO2 removal cost is also the lowest. With the development of technology, IGCC will be widely used in the future. The disadvantage is that this technology can only be used for new power plants, and cannot be used for the technical transformation of existing power plants.
No matter which technology is used, there are certain requirements for the site. Therefore, the newly designed power plant must consider the capture of CO2, think about what technology to use in advance, reserve space for CO2 removal equipment, and find a suitable storage location nearby.
Promotion of CO2 capture technology
Although CO2 capture technology has become a research hotspot, it has not yet been promoted worldwide. This is mainly due to the following factors:
(1) Economic considerations: After CO2 capture, the efficiency of the entire power plant will drop by 20%~30%, and the cost of power generation will increase significantly. Companies that have already made profits have no motivation to capture CO2.
(2) The influence of national policies: CO2 capture must be driven by national policies. The government can consider adopting forms such as imposing a CO2 emission tax to promote the application of CO2 capture and storage technology.
(3) The influence of national policies: CO2 capture must be driven by national policies. The government can consider adopting forms such as imposing a CO2 emission tax to promote the application of CO2 capture and storage technology.
(4) Public awareness: After the adoption of CO2 capture technology, electricity prices will inevitably rise sharply. Whether it is raising electricity prices or levying carbon taxes, it needs to be recognized and supported by the public.
Building demonstration power plants is an effective measure to promote the promotion of CO2 capture technology. The EU has planned to build 12 large-scale CO2 capture demonstration power plants by 2012 to prepare for large-scale promotion around the world in 2020.
Conclusion
Three types of CO2 capture technologies for coal-fired power plants are introduced, the advantages, disadvantages and costs of various technologies are compared, and the promotion of CO2 capture technology is analyzed. Excessive cost is still the main factor restricting the development of CO2 capture technology. Comprehensive considerations should be made and integrated systems should be reasonably designed to reduce costs. For example, the generated CO2 can be used to increase the oil recovery rate of oil fields. In oxygen-enriched technology, the cold energy of imported liquefied natural gas can be used for air separation to reduce the cost of oxygen production.
