Performance evaluation of ultra-low emission transformation of dust removal equipment in coal-fired power plants?

The new environmental protection policy requires that the smoke emission concentration of coal-fired power plants should be less than 5mg/Nm3, reaching the ultra-low emission standard. In order to meet the standard requirements, a coal-fired power plant in Heilongjiang Province adopted different routes to carry out ultra-low emission transformation, systematically studied different transformation routes, analyzed the technical principles and characteristics of each transformation route, and tested the performance of the transformed equipment. The test results show that the smoke emission concentration of Unit 2 is

3.46mg/Nm3, and the soot emission concentration of Unit 3 after transformation is 3.85mg/Nm3. Both transformation routes can meet the emission limit requirements of 5mg/Nm3, and the equipment runs reliably and stably.

In order to make the coal-fired units meet the ultra-clean emission demand of 5mg/m3 or 10mg/Nm3, the current transformation methods are mainly to improve the efficiency of existing dust collectors, the technical transformation of desulfurization and dust removal integration, or to increase the combined scheme of wesp transformation.

A coal-fired power plant in Heilongjiang has carried out ultra-low emission transformation for its units 2 (1×350MW) and 3 (1×600MW). Unit 2 boiler is Hg-1170/17.4-ym1subcritical coal-fired boiler. The original dust collector is a double-chamber four-field electrostatic precipitator, and each boiler is equipped with two dust collectors. The power supply is three-phase power frequency power supply, and the desulfurization process adopts limestone-gypsum desulfurization process, one for each furnace.

The boiler of Unit 3 is a π-type drum boiler with subcritical primary reheat and full suspension structure, Hg-2030/ 17.5-YM9. The original dust removal device is a two-chamber four-field electrostatic precipitator, and the power supply of the precipitator is a three-phase power frequency power supply. The original desulfurization system adopts limestone-gypsum wet desulfurization technology, with one furnace and one tower.

The ultra-low emission reconstruction route of Unit 2 adopts the combined route of the second, third and fourth electric field pulse power supply reconstruction of electrostatic precipitator and wesp after desulfurization tower; The ultra-low emission transformation route of Unit 3 adopts the combined route of high-frequency power supply transformation of the first and second electric fields of electrostatic precipitator, pulse power supply transformation of the third and fourth electric fields and integrated transformation of desulfurization and dust removal.

At present, the evaluation of ultra-low emission transformation route is mainly discussed from the aspects of feasibility, economic benefits and environmental benefits. There are few tests and verifications of environmental protection equipment for ultra-low emission transformation, and most of them are single-circuit or single-machine. This paper systematically introduces the different technical routes of transformation of 300MW and 600MW units, tests the performance of the environmental protection equipment after transformation, and analyzes the test data, which fully verifies the feasibility of the transformation route.

1 reconstruction route of ultra-low emission of smoke and dust

The capacity of active electrostatic precipitator units accounts for about 79.9% of the coal-fired power plants that have been put into operation. The original design of dust collector is generally about 50 ~ 100 mg/nm3. The transformation methods of electrostatic precipitator include high frequency power supply transformation, pulse power supply transformation, moving electrode transformation, electric bag transformation, low temperature transformation, flue gas conditioning transformation and so on. After the transformation, the dust emission concentration at the outlet of the dust collector can usually be controlled at 20 ~ 30%.

30mg/Nm3, therefore, the ultra-clean emission requirements of 5mg/Nm3 or 10mg/Nm3 can no longer be met by only reforming the electrostatic precipitator, and the subsequent environmental protection facilities need to be reformed on the basis of improving the efficiency of the original electrostatic precipitator. The transformation method is usually the integrated technical transformation of desulfurization and dust removal in desulfurization tower, or adding wesp after desulfurization system.

1. 1 Improved technology of electrostatic precipitator

At present, the most widely used technology to improve the efficiency of electrostatic precipitator is power supply transformation technology. Usually, the combination of high frequency power supply and pulse power supply is adopted. The high frequency power supply adopts the energy conversion form of "power frequency AC"-"DC"-"inverter AC"-"boost rectifier"-"high frequency DC", and finally a pulsating DC waveform of 4 ~ 40 kHz can be obtained.

High frequency power supply has the advantages of high working frequency, low output ripple, high average voltage and current, high conversion efficiency and high power factor. High-frequency power supply saves energy while ensuring charging intensity, and is suitable for treating high-concentration dust. It is difficult for high frequency power supply to remove ultrafine dust particles larger than resistance. Therefore, the high frequency power supply is most suitable for the conversion of the first and second electric fields. The first and second electric field sources of the unit 3 are switched in this way.

The transformation of pulse power supply for electrostatic precipitator mostly adopts the form of DC superimposed pulse. On the basis of DC high voltage provided by DC power supply, high voltage pulses are superimposed. DC superimposed pulse power supply has the advantages of high voltage rise rate (μs level), short pulse voltage duration, high peak electric field intensity, high dust removal efficiency, and suppression of back corona phenomenon. Pulse power supply is more expensive and more suitable for the final electric field transformation.

The second, third and fourth electric fields of the cell 2 and the third and fourth electric fields of the cell 3 are switched in this way.

1.2 integrated desulfurization and dust removal technology

Desulfurization towers in coal-fired power plants are mostly in the form of reverse spray towers, and fine dust in flue gas can be captured and removed by droplets through the absorption zone or by demisters. In the absorption zone, the dust in the flue gas comes into contact with liquid droplets, and is mainly captured by inertial collision, interception and Brownian diffusion.

The factors affecting the dust removal efficiency in the absorption zone mainly include the flow field in the tower, spray density, liquid-gas ratio, droplet atomization and so on. At present, the methods to improve the efficiency of absorption zone mainly include increasing spray layer, reforming the original spray layer, increasing alloy tray, increasing or optimizing guide plate, replacing nozzles or increasing the number of nozzles. Installing alloy trays or baffles at the desulfurization entrance can optimize the flow field in the tower, mainly by reforming or increasing the original spray layer.

Spray coating can improve spray density and liquid-gas ratio.

Changing nozzles or increasing the number of nozzles can improve the atomization effect. The demisting zone mainly relies on gravity and inertial impact to separate liquid droplets from flue gas. Demister can be divided into flat plate, roof and tube bundle. The stages used by demisters are mostly 1 ~ 4. Generally speaking, the greater the number of stages, the higher the demisting efficiency, but the increasing range is lower and lower, and the pressure loss and cost increase accordingly.

Tube bundle demister is mainly composed of tube bundle cylinder, speed increaser, separator, confluence ring and diversion ring. Fine droplets and particles condense and gather under the condition of high-speed movement, thus separating from the gas phase. Tube bundle demister is usually used as the first stage demister.

At present, most of the ultra-low emission reconstruction technologies are to remove the original demister and add 3 ~ 4 demisters. The first stage mist eliminator adopts tube mist eliminator, and the second stage to the fourth stage adopts roof mist eliminator, and 57 can ensure that the droplet concentration at the outlet is below 30mg/Nm3. The integrated transformation process system of desulfurization and dust removal is simple, the daily operation and maintenance are convenient, the transformation period is short, and the operation cost and investment cost are lower than wesp. Number three.

The desulfurization tower of the unit was reformed by adopting the integrated technology of desulfurization and dust removal, and the spray layer was added, the air distribution device was added, all nozzles in the spray layer were replaced, and the tubular demister and the three-stage roof-type high-efficiency demister were set up.

1.3 wet electrostatic precipitator technology

Wesp is arranged between desulfurization facilities and chimneys to remove fine particles such as flue gas, limestone and gypsum glue from saturated wet flue gas after desulfurization. The charging principle is the same as that of dry electrostatic precipitator. In wesp, a continuous water film is formed on the dust collecting electrode, and the flowing water flushes the captured dust into the ash hopper and discharges it with the water. The running resistance is small, and the removal effect of fine particles and heavy metal particles is good, which is affected by the change of coal type.

Speak louder.

Wesp can remove dust and fog drops at the same time, and because there is no vibration device, it will not produce secondary dust. According to the flue gas flow mode, wesp can be divided into tubular type and radial type. The tubular wesp anode plate is arranged parallel to the airflow direction, and the radial wesp anode plate is arranged perpendicular to the airflow direction. Wesp runs reliably and stably, and can ensure the flue gas emission concentration of 5mg/Nm3.

Below, but on the basis of the original environmental protection facilities, we need to add a set of devices. The system is more complex, the maintenance workload is greater, the transformation cycle is longer, the floor space is larger, and the investment cost and operation cost are higher. A wesp was added in the renovation of Unit 2.

2 performance test

2. 1 test method

The performance test of environmental protection facilities for ultra-low emission reconstruction of units 2 and 3 was carried out. Unit 2 tested the performance of electrostatic precipitator and wesp, and Unit 3 tested the performance of electrostatic precipitator and desulfurization tower. When the unit load is greater than or equal to 90%, select the test conditions. The test standard is based on DL/T4 14-20 12 Technical Specification for Environmental Monitoring of Thermal Power Plants and GB/T 1 157-66.

Sampling method of gaseous pollutants GB/T 1393 1-2002 Performance test method of electrostatic precipitator GB/T 2 1508-2008 Performance test method of coal-fired flue gas desulfurization equipment GB/T 15 187-2008. The test sites are selected at the inlet and outlet of the dust collector, the inlet and outlet of the desulfurization tower and the inlet and outlet flue section of wesp. There are four flues at the inlet and outlet of the dust collector, and the desulfurization tower enters and exits.

Each port has 1 flue, and each entrance and exit of wesp has 1 flue. See Figure 1 and Figure 2 for the test location map.

Fig. 2 Test position of device 3

When the boiler load, dust collector and desulfurization tower are running stably, the grid distribution method is used in each section, and the flue gas quantity, temperature, oxygen content and humidity are measured at the same time, and the measured flue gas quantity is converted into the flue gas quantity in standard state, dry basis and 6%O2. The smoke and dust samples were collected by isokinetic sampling method, the filter cartridge was used for sampling at the inlet of the dust collector, and the filter membrane was used for sampling at the outlet of the dust collector, desulfurization tower and wesp inlet and outlet.

Dry and weigh the filter cartridge and filter membrane respectively, and calculate the smoke concentration according to the weight gain of the filter cartridge before and after sampling and the sampling volume of standard temperature and pressure. Calculate body resistance and dust removal efficiency. See table 1 for detailed test items, instruments and methods. Please refer to formulas (1) ~ (4).

Table 1 test items, instruments and methods

Calculation formula of smoke concentration:

Where: c is the converted smoke concentration, mg/Nm3；; G2 is the final weight of filter cartridge and filter membrane, g; G 1 is the initial weight of filter element and filter membrane, g; Vnd is the sampling volume of standard temperature and pressure, l; α is the measured air excess coefficient; 1.4 is the air excess coefficient of 6%O2.

Calculation formula of dust removal efficiency:

2.2 test results

The resistance of electrostatic precipitator of unit 2 meets the performance guarantee value, and the dust removal efficiency meets the performance guarantee value, but the outlet smoke concentration does not meet the equipment performance guarantee value. The main reason is that the smoke concentration at the entrance of electrostatic precipitator is greater than that at the design entrance. The body resistance, dust removal efficiency and outlet flue gas concentration of wesp of Unit 2 all meet the equipment performance guarantee value. The dust concentration at the entrance of wet electrostatic precipitator is lower than that at the exit of electrostatic precipitator.

17.85mg/Nm3, and this part of dust is mainly removed by desulfurization tower. The specific test results are shown in Table 2 and Table 3. The test shows that the ultra-low emission requirement of soot emission concentration ≤5mg/Nm3 can be achieved after the ultra-low emission transformation of Unit 2, and the transformation route is feasible and the effect is good.

The body resistance, dust removal efficiency and outlet flue gas concentration of the electrostatic precipitator of Unit 3 meet the requirements of the equipment performance guarantee value, and the absorption tower resistance and outlet flue gas concentration of Unit 3 meet the requirements of the performance guarantee value. The specific test results are shown in Table 4 and Table 5. The test shows that unit 3 can meet the ultra-low emission requirements of smoke and dust emission concentration ≤5mg/Nm3 after ultra-low emission transformation, and the transformation route is feasible and the effect is good.

Table 2 Performance Test Results of Electrostatic Precipitator for Unit 2

Table 3 wesp Performance Test Results of Unit 2

Table 4 Performance Test Results of Electrostatic Precipitator for Unit 3

Table 5 Performance Test Results of Desulfurization System of Unit 3

3 Conclusion

In order to meet the emission requirements of less than 10mg/Nm3 or 5mg/Nm3, it is necessary to upgrade the dust removal equipment of active units, and integrate the desulfurization and dust removal tower or add wesp devices. Through the experimental study on the ultra-low emission reconstruction performance of units 2 and 3 in a coal-fired power plant in Heilongjiang Province, it is shown that wesp reconstruction combination mode and electricity quantity are added to the pulse power supply of electrostatic precipitator.

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