In response to the increasingly stringent emission standards for flue gas pollutants from thermal power plants, the emission standards for flue gas pollutants from thermal power plants have been adjusted to the gas turbine emission standards (smoke ≤ 5mg/Nm3, SO2 ≤ 35mg/Nm3, NOX≤50mg/Nm3, Hg≤0.03mg/Nm3) requirements are consistent. In view of the extremely low emission requirements of thermal power plants, it is necessary to have efficient, environmentally friendly, and energy-saving auxiliary equipment to adapt to them.
By introducing the use and economics of environmental protection equipment for flue gas pollutant control in coal-fired power stations in my country and the application of various co-processing technologies for pollutants in flue gas that have appeared and are being studied internationally, Targetedly put forward the technical solutions and route control technologies that our country should adopt to control pollutants that are of concern to the whole society in the field of thermal power.
With the increasing national requirements for air pollutant emission control, the new "Air Pollutant Emission Standard for Thermal Power Plants" (GB13223-2011) was officially implemented on January 1, 2012. The new emission standards have greatly improved the emission control requirements for smoke, sulfur dioxide, nitrogen oxides and heavy metals. The new standards stipulate that smoke particles in newly built thermal power plants are ≤20mg/Nm3, SO2≤100mg/Nm3, and NOX≤100mg/Nm3. Hg≤0.03mg/Nm3.
However, the current domestic environmental protection situation is still very serious. Some national enterprises responsible for the national economy and people's livelihood still have the responsibility to consider the social responsibility margin of emission standards for smoke, SO2, NOX and other pollutants, and set the standards for smoke, SO2, NOX, Hg Other pollutant emission standards are aligned with gas turbine emission standards, and strive to meet or exceed gas turbine power plant emission standards (smoke particles ≤5mg/Nm3, SO2≤50mg/Nm3, NOX≤50mg/Nm3, Hg≤0.03mg/Nm3).
In recent years, my country's haze phenomenon has become serious, and environmental protection requirements have become increasingly higher. This has led to the upgrade of environmental protection equipment in my country's thermal power generation field, and the requirements for environmental protection equipment for thermal power projects have become increasingly strict. Recently, China has required that the flue gas, SO2, NOX, Hg, etc. of new thermal power projects must meet gas turbine standards. This requires the environmental protection equipment of new thermal power plants to have higher active removal and removal of smoke, SO2, NOX, Hg, etc. Collaborative processing capabilities among environmental protection equipment.
In the construction process of coal-fired power stations, the operational and environmental problems caused by coal burning should be considered from a holistic perspective, and the mutual interaction and interrelated physics of various pollutants in the flue gas of coal-fired power stations should be fully understood. and chemical processes, making full use of the possible collaborative removal capabilities of existing coal-fired power plant flue gas removal equipment for smoke, SO2, NOX, Hg and other pollutants to achieve integrated treatment of pollutants and significantly reduce the environmental impact of coal-fired power plants. Pollution control costs. From the perspective of international technological development, the development of efficient and economical multiple pollutant joint removal technologies and system integration has become a hot spot.
1 Technical solution for pollutant emission control in thermal power plants
At present, in order to meet the gas turbine emission standards for thermal power plants, the main considerations are to use high-efficiency electrostatic precipitators, bag (electric bag) dust collectors, Smoke and dust removal technologies include moving plate electrostatic precipitators, low-temperature electrostatic precipitators, and limestone-gypsum wet desulfurization technology. In addition, a wet electrostatic precipitator is used to finely process the fine dust in the saturated flue gas after desulfurization, thereby achieving a higher PM2.5 control level.
Technical solutions for removal of SO2 mainly adopt high-efficiency limestone-gypsum wet desulfurization process, flue gas circulating fluidized bed semi-dry desulfurization process, etc. At present, denitration methods in thermal power plants mainly use low NOx combustion technology combined with flue gas denitration to achieve the best effect in removing NOx. The technical solution for the removal of heavy metal Hg mainly adopts the addition of oxidants (generally halogen elements, mainly CaBr2, modified activated carbon), and the synergy of SCR, ESP and FGD environmental protection equipment to achieve better mercury control effects.
1.1 Technical Plan for Emission Control of Smoke and Dust Pollutants from Thermal Power Plants
1.1.1 Control Plan for Emission of Smoke and Dust Pollutants from Thermal Power Plants
Currently, the technical plan for the emission of smoke and dust pollutants from thermal power plants has reached Gas turbine emission standards mainly consider the use of high-efficiency electrostatic precipitators, bag (electric bag) dust collectors, moving plate electrostatic precipitators, low-temperature electrostatic precipitators, etc. The high-efficiency electrostatic precipitator mainly adopts measures including high-frequency electricity and digital-analog flow field optimization. According to the current development level of domestic dust collector manufacturing technology, a double-chamber five-electric field electrostatic precipitator is selected. When the dust concentration at the entrance of the inlet dust collector is 45g/Nm3, It can control the dust emission concentration of the dust collector below 30mg/Nm3;
The domestic bag (electric bag) dust collector manufacturing technology development level, the dust removal efficiency of the bag dust collector can reach 99.99, and the dust collector outlet can be controlled The dust emission concentration is between ￡5~20mg/Nm3. Under the condition of reasonable selection of new filter materials (such as selecting PTFE base cloth to ensure the basic structure and dimensional stability of the filter material), the electric bag dust collector can fully meet the guaranteed service life of the rear bag of the electric bag dust collector and the harsher operating conditions. .
The moving plate electrostatic precipitator can use rotating brushes and moving dust collection plates to remove trapped dust to prevent corona. The moving plate system can effectively collect high resistivity dust. The dust collecting plate moves up and down at a very slow speed through the rotation of the top driving wheel. The charged dust is collected in the dust collection area; the dust attached to the plate is clamped in the non-dust collecting area. Two rotating wire brushes scrape the dust into the ash hopper.
The technical advantage of low-temperature electrostatic precipitator is that the addition of flue gas heat exchanger equipment behind the furnace further reduces the exhaust gas temperature at the rear of the boiler, and the economic efficiency of the entire unit is greatly improved; after the smoke temperature is reduced, the smoke dust is reduced The specific resistance is reduced, which improves the dust collection capacity of the electrostatic precipitator; at the same time, the flue gas volume flow rate is reduced, so that the output of the low-temperature electrostatic precipitator and its back-end flue gas flow equipment is significantly reduced, reducing the entire project investment. At present, all newly built 500MW~1050MW thermal power units in Japan basically adopt the low-temperature electrostatic precipitator process. The cooling heat exchanger of MGGH is installed before the electrostatic precipitator (ESP)
The main process flow is shown in the process flow chart 1.1 -1.
Figure 1.1-1 Flow chart of low and low temperature flue gas treatment system
my country's low and low temperature electrostatic precipitator technology has also made great progress in recent years. Low and low temperature electrostatic precipitators and power plant thermal systems Combined with the desulfurization system, it has comprehensive energy saving, water saving and environmental protection effects, and can meet the dust control requirements of the national environmental protection emission standards under the condition of burning medium and low ash coal.
Based on the construction drawing design of a 2′660MW domestic unit with medium sulfur and ash content in Inner Mongolia, the main environmental emission indicators and economic indicators of the use of low-temperature electrostatic precipitators and the use of traditional five-field electrostatic precipitators are compared. See Table 1.1-2 below:
Table 1.1-2 Comparison table of environmental emission indicators and economic indicators of low-temperature and traditional electrostatic precipitators
Serial number project low-temperature electrostatic precipitator traditional electrostatic precipitator
1 Design coal quality Inner Mongolia Baiyinhua lignite
2 Electrostatic precipitator five electric fields
3 Actual flue gas flow rate at desulfurization inlet/(m3˙h- 1)39195324405424
4 Flue gas temperature/℃90135
5 Inlet dust mass concentration/(mg˙m-3)3631
6 Dust mass concentration/ (mg˙Nm-3)2030
7Dust removal efficiency/99.9599.90
8Power consumption--------
8.1 Flue gas Heat exchanger/kW600 base value
8.2 Induced draft fan (induced draft fan) shaft power/kW-2300 base value
8.3 Total power/kW-1700 base value
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Compared with traditional electrostatic precipitators, the overall energy consumption of low-temperature electrostatic precipitators is greatly reduced. The flue gas heat exchanger of the low-temperature and high-efficiency flue gas treatment system requires equipment such as a heat medium water circulation pump, so the power consumption is higher than that of the rotary flue gas heater.
However, a cooling heat exchanger is installed in front of the electrostatic precipitator, which reduces the temperature of the flue gas entering the electrostatic precipitator, suction fan and booster fan. Although the cooling heat exchanger increases the resistance loss of the flue gas system , but the smaller flue gas volume flow slightly increases the power consumption of the suction fan; the flue gas desulfurization system not only has a small flue gas volume flow, but because the cooling heat exchanger is set in front of the dust collector, the flue gas resistance loss is also reduced, causing The fan power consumption is greatly reduced, and the shaft power is reduced. Compared with traditional dust removal, the low-temperature and high-efficiency flue gas treatment system has greatly improved environmental performance, and the dust emission mass concentration is controlled below 20mg/m3.
Calculated based on 5,500 annual utilization hours, using a low-temperature electrostatic precipitator can save 9.35
At present, low-temperature electrostatic precipitator technology occupies the mainstream position of domestic boiler main flue gas dust removal equipment due to its high economy, good technical reliability, and reasonable investment level, and other dust collector equipment is an auxiliary technical matching form.
1.1.2 The removal effect of high-efficiency limestone-gypsum wet flue gas desulfurization device on smoke and dust
Domestic desulfurization companies believe that the use of high-efficiency limestone-gypsum wet flue gas desulfurization device can remove smoke and dust The efficiency can reach about 70, but considering the deviation of coal-fired properties in each project, it is recommended that the effect of limestone-gypsum wet flue gas desulfurization device on boiler smoke and dust removal should be considered to be no more than 50. In addition, the recent environmental impact assessment approval opinions of domestic thermal power generation projects have also Follow this data.
1.1.3 Wet electrostatic precipitator refines the fine dust in the flue gas after desulfurization
Wet electrostatic precipitator (WESP) is a type of electrostatic precipitator (ESP), wet The most critical difference between the electrostatic precipitator and the usual dry electrostatic precipitator is the different dust cleaning methods. WESP uses liquid (water) to flush the surface of the dust collecting pole for dust cleaning. The liquid (water) flows down from the top of the dust collecting plate and collects dust. A uniform and stable water film is formed on the board, which takes away the particles on the board. Therefore, the working principles of WESP and dry ESP both go through three stages of charging, collection and cleaning. The working principle of dust collection and dust cleaning is shown in Figures 1.1-3 and 1.1-4.
The wet electrostatic precipitator can effectively collect fine particles (PM2.5 dust, SO3 acid mist, aerosols), heavy metals (Hg, As, Se, Pb, Cr), organic pollutants (polycyclic aromatic hydrocarbons) , dioxins), etc., there is no secondary dust, and the smoke emission can reach less than 5mg/m3. The dust collection performance of WESP has little to do with dust characteristics. It can also effectively collect dust with high viscosity or high specific resistance. It is also suitable for treating high temperature and high humidity flue gas; it is necessary to set up wastewater treatment equipment and adopt good anti-corrosion measures. . The wet electrostatic precipitator efficiency can reach about 80.
At present, there are also projects in China that use high-efficiency limestone-gypsum wet dust removal and desulfurization integrated ultra-clean emission technology, such as single-tower integrated desulfurization and dust removal deep purification technology (SPC-3D) technology, multi-layer spray layer With double trays or liquid-holding layers, the top of the desulfurization tower is equipped with high-efficiency dust collector technology, single-tower (double-tower) dual circulation is equipped with high-efficiency dust collector technology, etc. These technical forms are the latest technologies after the introduction of wet desulfurization technology in my country. After accumulation and improvement and research and development, it can not only achieve efficient SO2 removal, but also achieve refined emission control of ultra-fine dust PM2.5 after desulfurization.
At present, these technologies have emerged after the environmental protection standards of my country's thermal power units have been improved, especially after the emission standards of most domestic thermal power units have been aligned with gas turbine standards and have broken through technological bottlenecks through technological transformation. The above-mentioned Technology has also been applied in engineering, and most of them have achieved good results, but the above-mentioned technologies still need time to be further tested.
1.2 SO2 pollutant emission control plan for thermal power plants
The technical solution for SO2 removal mainly adopts high-efficiency limestone-gypsum wet desulfurization process and flue gas circulating fluidized bed Semi-dry desulfurization process, etc.
1.2.1 Flue gas circulating fluidized bed semi-dry desulfurization process
The flue gas circulating fluidized bed semi-dry flue gas desulfurization process RCFB is a gas-liquid-solid Reactive flue gas desulfurization process.
In the desulfurization tower, on the one hand, the mass transfer process from gas phase to liquid phase is carried out. The gaseous pollutants in the flue gas continuously enter the solution and react with the calcium ions in the desulfurization absorbent. On the other hand, the heat transfer of evaporation and drying is carried out. During the process, the liquid phase moisture on the particles is continuously evaporated and dried in the tower under the influence of flue gas heating, and is regenerated into solid dry desulfurization ash. The flue gas circulating fluidized bed desulfurization process has many achievements, the technology is relatively mature, and it has been commercially operated on large and medium-sized units. It can basically meet the country's new national environmental protection emission standards "Air Pollutant Emission Standards for Thermal Power Plants" (GB13223-2011).
After adopting the low-temperature flue gas circulating fluidized bed desulfurization process, taking the 2x660MW lignite unit as an example, the flue gas temperature at the inlet of the flue gas desulfurization device dropped from 150°C to 120°C, while ensuring the same operating conditions and Under the conditions of desulfurization efficiency, compared with the currently used flue gas circulating fluidized bed desulfurization process, the water volume is reduced from 180t/h to 102t/h, achieving water saving of 78t/h, the water saving rate reaches 43.3, and the water saving effect is obvious. Therefore, it is recommended to use this kind of desulfurization unit for units in areas with special water shortages to achieve better water saving effects.
1.2.2 Limestone-Gypsum Wet Flue Gas Desulfurization Process
The limestone-gypsum wet flue gas desulfurization process is currently the most widely used and technically mature flue gas desulfurization technology in the world. . This process uses cheap limestone slurry to wash the flue gas, and removes SO2 in the flue gas through heat exchange on the ship. The reaction product is gypsum. After desulfurization, the flue gas is discharged into the chimney after removing droplets through a demister. This process has wide adaptability to coal types, high desulfurization efficiency, can adapt to the requirements of large-capacity units, and has a wide adaptability to changes in SO2 concentration.
The limestone-gypsum wet flue gas desulfurization device has been put into operation in my country for many years after the technology was introduced. The reliability and safety of the process system have been recognized by users. After innovation and optimization of the process system, the process system of the desulfurization unit is more advanced than the traditional desulfurization unit, and the desulfurization efficiency is higher. Such as: multi-layer spray technology high-efficiency demister solution, multi-layer spray technology double-tray high-efficiency demister solution, single-tower dual-cycle and double-tower dual-cycle technology solutions, gyro-coupling technology centrifugal tube bundle dust removal and defogger technology, etc. Limestone -The desulfurization device of the gypsum wet flue gas desulfurization device has been significantly improved, ensuring that the desulfurization efficiency can reach about 99, which is more suitable for use in thermal power plants to improve SO2 emission standards.
The above-mentioned desulfurization technologies can guarantee extremely high SO2 removal rates. It can be seen that after a period of technological absorption and digestion, China has fully equipped with high-efficiency, low-emission desulfurization technologies.
1.3 NOX pollutant emission control plan for thermal power plants
Boiler denitrification in thermal power plants refers to the control of nitrogen oxides generated during the combustion process and the removal of nitrogen oxides from the combustion flue gas. process. At present, there are two main types of denitrification methods in thermal power plants: one is to treat it from the source and control the NOx generated during the combustion process. The main technical measures include: using low-nitrogen burners; staged combustion, controlling combustion temperature; changing the batching plan, etc.
The other type is to control the NOx emitted in the flue gas from the end. The main technical measures are: selective non-catalytic reduction (SNCR); selective catalytic reduction (SCR); SNCR/SCR Joint denitrification technology, etc. For coal-fired boilers, although low NOx combustion technology and equipment are used to control the generation of NOx, certain effects can be achieved, but the stability of the flame, combustion efficiency, control of superheated steam temperature, slagging and corrosion of the heating surface, etc. It may have an impact, and the NOx removal rate is also limited. The NOx removal rate does not exceed 60 at most, making it difficult to meet the ever-increasing environmental emission standards.
Adopt low NOx combustion technology and tail flue gas denitration to achieve full-load NOx removal technology.
2 Thermal power plant Hg and other heavy metal pollutant emission control technology
The gaseous element mercury is inactive in nature and is neither easily adsorbed nor soluble in water, making it difficult to control existing pollutants. Equipment removal. Therefore, the idea of ??mercury removal technology in thermal power plants is to promote the conversion of elemental mercury into oxidized or particulate states and take the road of composite pollution control.
At present, the main methods for removing heavy metal pollutants such as Hg include mercury removal before combustion, mercury removal during combustion, and mercury removal after combustion.
The washing rate of raw coal in my country is still low, and it is still unable to remove mercury before combustion; mercury removal during combustion is mainly to improve the combustion method to promote the conversion of mercury into oxidation state; mercury removal after combustion is currently the most important factor in coal-fired thermal power generation. The unit uses a wider range of methods. (1) Promote the conversion of elemental mercury into the particle adsorption state, and then use the dust collector to recover and remove it; (2) Promote the conversion of elemental mercury into the oxidation state, and use the water solubility of oxidized mercury to remove it in the wet flue gas desulfurization device.
In addition to the above-mentioned direct mercury removal methods, some methods of adding additives (such as CaBr2, etc.) before and during combustion can effectively improve the removal efficiency of mercury in the flue gas after combustion. In engineering applications, halogen (usually CaBr2) is often sprayed on coal conveying belts and pulverized coal pipelines. Test results of the Pleasant Prairie coal-fired power plant in the United States (600MW, burning PRB sub-bituminous coal, equipped with SCR, ESP and WFGD): After adding 25mg/kg additives to the coal, the mercury removal rate continued to be maintained at 92-97. Another newly proposed technology is to add oxidants to the bag filter membrane, which is still under exploration and research.
The flue activated carbon injection technology (ACI) is currently the most mature active mercury removal technology and has achieved good results in controlling mercury emissions from waste incinerators. This technology is to spray activated carbon into the flue before the dust collector, so that the activated carbon continuously adsorbs mercury in the flue gas during the accompanying flow process, converting the gaseous mercury into particulate mercury fixed on the adsorbent, and then uses the particulate matter emission control device to Its removal.
Currently in the United States, some ACI equipment has been put into operation. Some power plants use untreated activated carbon; some power plants use specially treated modified activated carbon to reduce the amount of activated carbon and improve mercury removal efficiency. After the Detroit Edison Power Plant (installed ESP and burned sub-bituminous coal) injected activated carbon at a rate of 48mg/Nm3 per minute, its 30-day average mercury removal efficiency reached 94;
For the control of mercury pollution in coal-fired power plants, although it has Many methods have been developed, but most are still in the research and testing stages. Currently, there are three main types of active mercury removal processes that are relatively mature and commercially available: 1. Activated carbon injection; 2. Adding oxidants (generally halogen elements, mainly CaBr2); 3. Adding oxidants supplemented by trace amounts of activated carbon injection. These processes, combined with the use of SCR, ESP and FGD, can achieve better mercury control effects.
In addition, mixed coal combustion is also a feasible process. Mixing and burning coal types with higher halogen content (especially bromine content) with coal types with lower halogen content can improve mercury removal efficiency, and there is no by-product disposal problem, which is very economical. .
3 Research on the technical route for ultra-clean emissions in my country
The development of environmental protection technology for coal-fired thermal power units in my country has formed a system of efficient flue gas treatment processes: 1. Flue gas low NOX burners and SCR flue gas denitrification process; 2. High-efficiency electric precipitator, electric bag dust collector or bag dust collector, low-temperature electric precipitator, moving plate electric precipitator; 3. High-efficiency wet flue gas desulfurization process, flue gas circulation fluidization Bed semi-dry flue gas desulfurization technology and activated coke dry flue gas desulfurization technology. According to different regions in my country, different equipment and technology combinations are adopted based on the characteristics of high-efficiency flue gas treatment technology of coal-fired thermal power units.
1) Use low NOX burner SCR, high-efficiency electrostatic precipitator, bag (electric bag) precipitator, low-temperature electrostatic precipitator or moving plate electrostatic precipitator wet flue gas desulfurization supporting wet electrostatic precipitator technology , high-efficiency limestone-gypsum wet desulfurization and dust removal integrated process;
2) Use low-NOX burner SCR, high-efficiency electric precipitator, low-temperature electrostatic precipitator, bag (electric bag) dust collector or moving plate Electrostatic precipitator is a high-efficiency limestone-gypsum wet flue gas desulfurization and heavy metal removal process. Wet electrostatic precipitator process.
The requirements for comprehensive environmental protection standards in inland and remote areas are relatively loose. The unit emissions must meet the requirements of the national environmental protection emission control standards. The coal quality has the characteristics of low calorific value, high ash, low sulfur or medium calorific value and high sulfur. , Recommendations: 1) Use low-NOX burner SCR high-efficiency electrostatic precipitator, bag (electric bag) precipitator, moving plate electrostatic precipitator limestone-gypsum wet flue gas desulfurization (equipped with high-efficiency mist eliminator) process, configure as needed Wet electrostatic precipitator technology; 2) Flue gas circulating fluidized bed boiler (or low-sulfur coal-fired boiler) Flue gas circulating fluidized bed semi-dry desulfurization process bag (electric bag) dust collector or high-efficiency electrostatic precipitator.
The characteristics of water-scarce areas are rich in coal and short of water. Unit emissions must meet the requirements of national environmental protection emission control standards. Coal quality has the characteristics of low calorific value, high ash, low sulfur or high calorific value, high ash, medium and low sulfur. Suggestion: A water-saving and efficient flue gas treatment process needs to be adopted. 1) Use low-NOX burner SCR, low-temperature electrostatic precipitator, bag (electric bag) dust collector, moving plate electrostatic precipitator, limestone gypsum-wet desulfurization device, as needed Equipped with wet electrostatic precipitator technology; 2) Circulating fluidized bed boiler (or low-sulfur coal-fired boiler) Low-temperature flue gas circulating fluidized bed desulfurization technology bag (electric bag) dust collector or high-efficiency electrostatic precipitator.
Through the research on the above technical routes, a variety of targeted technical routes for controlling pollutant emissions have been formed in China. Pollution is controlled through various technical routes such as coal quality analysis, regional location, equipment investment, and emission requirements. It is fully qualified to ensure that the comprehensive pollutant emission standards of my country's thermal power plants meet the gas turbine emission standards.
4 Conclusion
Through the above introduction and analysis, it can be seen that the current ultra-clean emission technology of flue gas from thermal power plants at home and abroad is complex and diverse, and is optimized through various combinations of environmental protection equipment according to different regions. , to further improve the removal of heavy metals such as smoke, SO2, NOX, and Hg from thermal power plants. With the passage of time and the advancement of technology, the accumulation of process technologies such as low-temperature electrostatic precipitator systems, high-efficiency wet dust removal and desulfurization integrated systems, high-efficiency limestone-gypsum wet desulfurization devices combined with wet electrostatic precipitators, and the integration of thermal power plants Removing contaminants to ultra-clean levels is technically feasible.
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