Three-stage combined dust removal process for reducing dust concentration in flue gas emission of coal-fired boiler

Abstract

The invention discloses a three-stage combined dust removal process for reducing dust concentration in flue gas emission of a coal-fired boiler, which adopts a three-stage combined dust removal control technology for flue gas emission control of a unit: a'high frequency + pulse 'combined high-efficiency low-consumption dry-type first-stage electric dust removal technology is adopted, so that the dust collection efficiency of an electrostatic dust collector is improved; high-dust flue gas is reduced to 20 mg / Nm < 3 > or below through primary dust removal; the flue gas is desulfurized by a two-stage series absorption tower to wash fine dust in the flue gas so as to realize secondary dust collection; and finally, carrying out three-stage dust removal on desulfurization slurry fog drops and fine smoke dust carried by the flue gas through wet type electric dust removal, so that the flue gas dust emission of the unit is less than or equal to 5mg / Nm < 3 >, and an ultralow emission control index is achieved. According to the three-stage combined dust removal process for the dust concentration in flue gas emission of the coal-fired boiler, dust emission is reduced, air is purified, and the environment is protected.

Description

Technical Field

[0001] The invention relates to a three-stage combined dust removal process for reducing dust concentration in smoke emissions from coal-fired boilers, belonging to the technical field of dust control in smoke emissions from coal-fired boilers. Background Art

[0002] During the flue gas emission process of the coal-fired power unit in the city steel plant, the electrostatic precipitator of the 300MW generator set is a dry type electrostatic precipitator, which is powered by an industrial frequency power supply. Its output high-voltage DC voltage is stepped up by a silicon rectifier transformer and then rectified by a single-phase half-wave. The output DC power is a half-sine wave with a low average voltage. Due to the limitations of the industrial frequency power supply, the dust concentration of the dry type electrostatic precipitator in the existing process is 40-70mg / m 3 After the limestone / gypsum desulfurization tower and absorption tower, the general smoke dust emission concentration is controlled at 14-20mg / m 3 , and its emission effect met the emission requirements at that time.

[0003] In order to respond to environmental protection requirements, the total outlet dust content is less than 5mg / m 3 Building on the existing dust removal equipment for the two 300MW units (dry, dual-chamber, five-field electrostatic precipitators and limestone / gypsum-based, single-tower desulfurization), the urgent task was to improve the efficiency of the dry electrostatic precipitators and desulfurization absorber towers, thereby reducing dust emission concentrations. In this context, a three-stage combined dust removal process and equipment upgrades were proposed. Summary of the Invention

[0004] The present invention aims to provide a three-stage combined dust removal process for reducing the dust concentration in the flue gas emissions of coal-fired boilers, thereby reducing dust emissions, purifying the air and protecting the environment.

[0005] The purpose of this invention is to reduce the total dust content at the outlet of a coal-fired 2*300MW unit boiler to less than 5mg / m 3 Based on the existing 2*300MW unit dust removal equipment (dry double-chamber five-field electrostatic precipitator and limestone / gypsum method one-tower absorption tower desulfurization), the dry double-chamber five-field electrostatic precipitator power supply is transformed into a high-frequency power supply, and a high-frequency superposition pulse power supply is used in the last 4-5 electric fields of the electrostatic precipitator electric field smoke and air duct to improve the dust removal efficiency of the dry electrostatic precipitator and achieve the first-stage dust removal; after the existing desulfurization absorption tower, another absorption tower is added, so that the boiler exhaust gas passes through two-stage series absorption towers, and the slurry in the absorption tower is used to wash the fine dust in the flue gas for secondary dust collection; the second-stage dust removal and desulfurization effect is achieved; in order to control the final total outlet dust content, a double-chamber two-field electrostatic wet precipitator is added at the flue gas main inlet, and the power supply still uses a high-frequency power supply. The flue gas finally passes through the wet electrostatic precipitator to carry out three-stage dust removal on the desulfurization slurry droplets and fine smoke dust carried in the flue gas, and finally meets the ultra-clean emission requirements, and the total flue gas outlet dust emission is less than 5mg / m3 .

[0006] The present invention provides a three-stage combined dust removal process for reducing the dust concentration in the flue gas emissions of coal-fired boilers through the research and development of raw fuel combustion technology, waste generation control technology, and collection and classification control technology. The three-stage combined dust removal control technology is adopted in the unit flue gas emission control: (1) First-stage "high frequency + pulse" combined high-efficiency and low-consumption electric dust removal: improve the dust collection efficiency of the first-stage dust removal, i.e., the electrostatic precipitator, and reduce the dust concentration of the boiler flue gas to 20 mg / Nm through the first-stage dust removal. 3 (2) The fine dust in the flue gas is washed by two-stage series absorption tower desulfurization to achieve secondary dust collection: including "double-tower double-circulation" limestone-gypsum wet tower desulfurization, dust removal primary absorption tower and secondary absorption tower; by washing the fine dust in the flue gas and performing secondary dust collection, the dust concentration of the boiler exhaust gas is reduced to 8 mg / Nm 3 (3) Finally, a three-stage high-efficiency and low-consumption wet electrostatic precipitator is used: the flue gas carrying desulfurization slurry droplets and fine smoke dust are subjected to three-stage dust removal by the wet electrostatic precipitator, and finally the unit flue gas dust emission is reduced to ≤5mg / Nm 3 , achieving ultra-low emission control indicators.

[0007] The above three-stage combined dust removal process is described in detail as follows: Step one: Use the first-stage dust removal electrostatic precipitator control technology: Developed a "high frequency + pulse" combined high-efficiency and low-consumption electrostatic precipitator technology, which solved the problem of high energy consumption and low efficiency of industrial frequency voltage dust collection, and successfully achieved a new breakthrough of "high frequency + pulse" superimposed voltage of 140kV, which increased the dust collection efficiency by 30% and reduced the power consumption of electrostatic dust removal by 37.6%.

[0008] The scheme of the control technology of the first-stage dust removal electrostatic precipitator is as follows: 1) Since the spiral wire is easily affected by the smoke system and short-circuited, this project replaced the cathode wires in the last four and five electric fields of the electrostatic precipitator smoke duct from spiral wires to sturdy wave-shaped wires, reducing the grounding failure rate of the electrostatic precipitator electric field caused by spiral wire breakage and improving the operational reliability of the electrostatic precipitator electric field.

[0009] 2) The power supplies for the first, second, third, fourth, and fifth electric fields in the ESP smoke duct have been replaced with high-frequency power supplies. This increases the smooth DC voltage of the electric field, improving ESP efficiency and reducing energy consumption. Furthermore, a pulsed power supply is superimposed on the high-frequency power supply for the fourth and fifth electric fields. This pulsed high-voltage power supply significantly increases the peak voltage and peak corona power of the electric field. The extremely narrow, high-energy pulses effectively overcome the flashover voltage limitations of conventional DC power supplies, increasing the peak voltage to over 140 kV (with the pulsed power supply voltage superimposed on the fundamental DC voltage, the peak electric field voltage can reach 140 kV, with an upper limit of 162 kV). The output peak current is increased from a few amperes to 200 amperes, improving dust removal efficiency and being particularly suitable for fine dust. The charge concentration generated by the pulsed power supply is hundreds of times greater than that of conventional DC power supplies, significantly increasing the charge of the dust, especially for PM2.5 fine dust. Under equivalent operating conditions, dust emissions can be reduced by over 50%.

[0010] 3) Changing the low-voltage vibration of the original electric field pole line and plate to "compound power vibration" is more conducive to collecting plate dust and improving dust removal efficiency.

[0011] Step 2: Secondary dust removal "double tower double circulation" limestone-gypsum wet desulfurization and dust removal control. The specific plan is as follows: 1) The double-tower double-circulation process technology is adopted: the desulfurization efficiency of the first-stage tower is controlled at 80-90%, and the SO2 concentration at the outlet of the first-stage absorption tower is controlled to 500-700 mg / Nm 3 The secondary tower is designed with a desulfurization efficiency of 95%, and the SO2 emission concentration at the secondary tower outlet is controlled to be lower than 35 mg / Nm 3 The following increases the desulfurization capacity of the unit and the desulfurization margin. As the flue gas passes through the two-stage absorption tower, the slurry washes the fine dust in the flue gas and repeatedly collects the dust, effectively reducing the dust concentration in the flue gas.

[0012] The dual-tower, dual-circulation design connects the towers to a total of eight circulation pumps, with four pumps in each of the primary and secondary absorber towers, corresponding to each of the absorber's four spray layers. The new tower's circulating pumps are equipped with smaller motors, allowing for more flexible operation. This allows for lower operating costs during periods of low load and low sulfur content by reducing the number of circulating pumps in operation.

[0013] 2) The "double tower double circulation" desulfurization control process is equipped with a double tower with 8 layers of atomized slurry spraying. The first and second absorption towers are respectively equipped with 4 layers of spraying layers, each equipped with a spraying device. The circulating pump pumps the slurry from the lower slurry pool to the spraying layer, and sprays it through the nozzle to form atomized droplets with fine particles and high reaction activity. The flue gas passes through the 8 layers of atomized slurry in the double tower, which can fully contact and wash the original flue gas with the slurry, reducing the original flue gas to 6500 mg / Nm 3 SO2 is stably controlled at 35mg / Nm 3, ensuring 99.62% desulfurization efficiency and dust removal efficiency, and increasing the adaptability of the desulfurization system to raw coal types.

[0014] 3) The "dual-tower, dual-circulation" desulfurization control process allows for independent pH control of the absorption and oxidation processes. This enhances oxidation at low pH values ​​and absorption at high pH values, improving both oxidation and absorption efficiency and saving energy.

[0015] 4) The "dual-tower, dual-circulation" desulfurization control process extends the limestone dissolution time. In particular, the low pH operation of the first tower accelerates limestone dissolution, reduces the calcium-sulfur ratio, and improves limestone utilization.

[0016] Step 3: Three-stage dust removal wet electrostatic precipitator control technology: To ensure the smoke dust 5 mg / Nm 3 To meet emission standards over a long period of time and take into account the environmental protection needs of removing Hg and SO3 aerosols, this solution adds wet electrostatic precipitator after desulfurization as a control measure for droplets and smoke. The specific operations are as follows: 1) A dual-chamber, dual-electric field wet dust collector is installed after desulfurization (the exhaust channel is dual-channel, with two electric fields installed in each channel) to ensure dust collection reliability; 2) Use 4 high-frequency power supplies to provide power to different areas, reducing energy consumption for dust collection; 3) Use plate-type horizontal wet dehumidification to reduce smoke resistance and improve the reliability of wet dehumidification operation; 4) Synergistically remove gypsum droplets, SO3 aerosols and heavy metal Hg to ensure that the total exhaust dust concentration is effectively controlled at 5mg / Nm 3 Within.

[0017] Beneficial effects of the present invention: (1) The present invention has developed an energy-saving and emission-reduction technology that is suitable for coal-fired power plants in urban steel plants through a series of research and practice on energy-saving and emission-reduction technologies for coal-fired units. It adopts the ultra-low emission technology of "double-tower desulfurization" + "three-stage dust removal" to solve the problem of ultra-clean dust emission from coal-fired units with a dust concentration of ≤5mg / Nm 3 and solve the technical problems of the industry; and achieve major technological innovations in the development, application and promotion of industry technologies.

[0018] (2) It makes full use of the effects of high-frequency power supply and pulse power supply on the electric fields at all levels in the electrostatic precipitator, thereby improving the overall dust collection efficiency of the electrostatic precipitator, especially making a positive contribution to the collection of fine dust below PM2.5; this technology breaks through the application of "high-frequency + pulse" combined high-efficiency power supply in electrostatic precipitators. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a schematic diagram of the structure of a four-channel, double-chamber, five-electric-field electrostatic precipitator in the existing process; Figure 2 This is a three-stage combined dust removal process diagram for reducing dust content in flue gas emitted by coal-fired boilers according to the present invention; Figure 3 This is the voltage waveform analysis diagram of high-frequency power supply and industrial frequency power supply; Figure 4 Secondary voltage waveforms for pure DC power supply and intermittent power supply.

[0020] In the figure, 1 is the boiler, 2 is the air preheater, 3 is the electrostatic precipitator, 4 is the primary absorption tower, 5 is the secondary absorption tower, 6 is the wet electrostatic precipitator, 7 is the chimney, 8 is the induced draft fan, and 9 is the flue. DETAILED DESCRIPTION

[0021] The present invention is further illustrated below by way of examples, but is not limited to the following examples.

[0022] Introduction to existing technology: A certain plant has two 300MW generator sets, each boiler is equipped with a four-channel double-chamber five-field electrostatic precipitator (such as Figure 1 As shown), it is a high-efficiency dust removal flue gas purification equipment, which is responsible for the purification of smoke dust after coal powder combustion in the boiler. With the improvement of environmental protection requirements, the safe, stable and efficient operation of electrostatic precipitators is particularly important. The electric field layout of the 300MW generator set in this embodiment is as follows Figure 1 As shown: The electrostatic precipitator consists of two parts: the main body and the power supply. The main body includes the dust collector shell, ash hopper, discharge electrode, dust collecting electrode, air flow distribution device, vibration cleaning device, insulator and insulation box. Figure 1 The figure shows a dual-chamber, five-field electrostatic precipitator. From bottom to top, they are field 1, field 2, field 3, field 4, and field 5. The corona wires for fields 1, 2, and 3 use barbed wire, while those for fields 4 and 5 use spiral wire. The electrostatic precipitator has four channels: A, B, C, and D. Channels A and B are combined and fed into induced draft fan #1. Channels C and D are combined and fed into induced draft fan #2.

[0023] Working Principle of a Dry Electrostatic Precipitator: An electrostatic precipitator is a flue gas purification device that removes particulate dust from flue gas emitted by coal-fired boilers, significantly reducing the amount of dust discharged into the atmosphere. It is a crucial environmental protection device for improving environmental pollution and air quality. Its working principle is as follows: When dust particles in flue gas pass through a high-voltage electrostatic field, they collide with positive and negative ions and electrons between electrodes, becoming charged (or charged through ion diffusion). Under the influence of the electric field, the dust particles, charged with electrons and ions, move toward the oppositely charged electrodes and adhere to them. Vibration, etc., causes the dust on the electrodes to fall into the ash collection hopper, purifying the flue gas passing through the electrostatic precipitator and protecting the atmosphere and the environment.

[0024] Simply put, the entire power supply process for an electrostatic precipitator involves 380V power being supplied to the primary winding of a rectifier transformer. One end of the secondary winding's two terminals is connected to the anode plate (which is grounded), and the other end is connected to the cathode line within the electric field through a damping resistor. This creates a strong electrostatic field between the anode and cathode plates and the cathode line when power is applied, attracting dust particles from the flue gas. The clean flue gas is then transported to the chimney through an induced draft fan and discharged into the atmosphere, achieving the dust removal effect. The secondary voltage of the entire precipitator is controlled by the primary voltage. That is, the primary line voltage is 380V. By adjusting the conduction angle of the thyristor (SCR) through a controller, the primary voltage is adjusted, which indirectly changes the secondary output voltage of the rectifier transformer. The rectifier transformer itself contains a rectifier circuit composed of numerous rectifier diodes or silicon stacks. Its function is to boost the AC power input from the primary winding and rectify it into DC power, which is then input into the electric field. This creates a strong electromagnetic field within the electric field, attracting dust particles and achieving the dust removal effect.

[0025] Limitations of power-frequency power supplies for dry electrostatic precipitators: Electrostatic precipitator theory shows that the greater the velocity ω of charged particles in an electric field, the higher the efficiency of the ESP. The velocity is proportional to the square of the electric field strength, which is proportional to the applied voltage. Therefore, generally speaking, the higher the operating voltage of the electric field, the more efficient the ESP.

[0026] The 300MW generator set electrostatic precipitator is a dry type electrostatic precipitator, powered by an industrial frequency power supply. Its output high-voltage DC voltage is boosted by a silicon rectifier transformer and then rectified by a single-phase half-wave. The output DC power is a half-sine wave with a low average voltage. The highest peak voltage is the highest voltage during electric field flashover, while the valley voltage is relatively low. The average voltage is often much lower than the peak voltage. Although the two 300MW generator sets were equipped with the most advanced and efficient dust removal flue gas purification equipment at the time and the high voltage power supply used the Xijiang Jiahuan industrial frequency high voltage DC power supply, due to the limitations of the industrial frequency power supply, the dust emission concentration before the transformation was 40-70mg / m 3 After passing through the desulfurization absorption tower, the smoke emission concentration is less than 20mg / m 3 The general smoke emission concentration is 14-20mg / m 3 Its emission effect meets the emission requirements at that time. However, according to the latest ultra-low emission smoke concentration index, the total outlet is less than 5mg / m 3 The dust removal effect of this dry electrostatic precipitator cannot ultimately guarantee the emission requirements, so it is very necessary to transform the dry electrostatic precipitator.

[0027] In view of the above problems, the improvement scheme of the present invention is adopted to make improvements, and the specific improvement process is as follows: 1. The specific transformation process of the development of the "high frequency + pulse" combined high-efficiency and low-consumption electrostatic precipitator technology is as follows: Based on the analysis of common faults of dry electrostatic precipitators and the limitations of power frequency power supply for dry electrostatic precipitators introduced in the first level of ultra-low emission control technology, we started to implement technical transformation from these two aspects. In order to reduce the frequent disconnection of the cathode spiral wires of the fourth and fifth electric fields of the dry electrostatic precipitator, which causes the electric field to be grounded and unable to increase the voltage, resulting in reduced electrostatic dust removal efficiency, we found a cathode wire with high mechanical strength, adaptable to the electric field after dust removal, and good dust removal effect on fine dust: This embodiment uses the corrugated wire produced by Fujian Longjian Environmental Protection Co., Ltd.; specifically: A. Transformation of electrostatic precipitator The structure of the original 300MW unit electrostatic precipitator: the first, second and third electric fields use barb wires, and the fourth and fifth electric fields use spiral wires. The advantages of barb wires are low corona inception voltage, generally 15KV, strong discharge, not easy to break, high mechanical strength, and a strong discharge type corona wire. It is suitable for configuration in the first few electric fields of the electrostatic precipitator and is not prone to failure. Due to the low working voltage, it is not suitable for capturing fine dust in the rear electric field. At the same time, its current density is large. If it is installed in the rear electric field, it will consume more electricity. Therefore, spiral wires are used for the fourth and fifth electric fields. According to the operation of the electrostatic precipitator of the 300MW generator set, the fourth and fifth electric fields have a relatively high failure rate of spiral wire breakage. There are problems such as easy dust accumulation, no fixed discharge point, uneven discharge, frequent field grounding, voltage failure, and poor dust removal effect.

[0028] The present invention selected the aforementioned corrugated wire produced by Fujian Longjian Environmental Protection Co., Ltd. during the transformation. The corrugated wire uses a small arc end discharge and has the advantages of high operating voltage, low current density, good wind speed adaptability, energy saving, and good reliability. It is particularly suitable for use in low dust concentration electric field areas. In combination with the above analysis, the electrostatic precipitator body is overhauled; the four and five electric field spiral wires of the electrostatic precipitator are completely removed and replaced with corrugated wires. The cathode spiral wires of the existing four and five electric fields are replaced with corrugated wires. This will effectively reduce the faults of electrostatic precipitator disconnection and voltage failure, improve the reliability of electrostatic precipitator use, and correspondingly improve the dust reduction effect of electrostatic precipitator.

[0029] B. Comparison of the effects of power frequency power supply and high frequency power supply for electrostatic precipitator Electrostatic precipitator theory shows that the greater the velocity ω of charged particles in an electric field, the higher the efficiency of the electrostatic precipitator. The velocity is proportional to the square of the electric field strength, and the electric field strength is proportional to the voltage applied within the field. Therefore, in general, the higher the operating voltage of the electric field, the higher the efficiency of the electrostatic precipitator.

[0030] like Figure 3 As shown in the following figure: The voltage waveform analysis diagram of high-frequency power supply and industrial frequency power supply shows that the high-frequency power supply has a more obvious corona effect on the electric field and a better dust removal effect. Figure 4The secondary voltage waveforms of pure DC power supply and intermittent power supply are provided. The pulse power supply has intermittent output and high instantaneous output voltage, which can avoid back corona and spark discharge, increase pulse energy, and increase charging time from milliseconds to microseconds, which can greatly improve the dust removal effect.

[0031] The electrostatic precipitator power frequency power supply (that is, the power supply before the electrostatic precipitator is modified) has the highest peak voltage, which is the highest voltage when the electric field flashes over, while the valley voltage is relatively low, and the average voltage is often much lower than the peak voltage; the electrostatic precipitator high-frequency power supply outputs 400kHZ high-frequency power, which provides a smooth DC power supply after rectification, and the voltage can rise to the highest output voltage when the electric field flashes over. Since the output voltage of the high-frequency power supply is higher than the output voltage of the power frequency power supply, the dust removal efficiency of the electrostatic precipitator is improved.

[0032] The high-frequency power supply has strong adaptability to working conditions and can provide the dust collector with various voltage waveforms ranging from close to pure DC to large pulsation amplitudes. For various specific working conditions, it can provide the most suitable voltage waveform, thereby improving dust removal efficiency.

[0033] The high-frequency power supply for electrostatic precipitators provides a near-pure DC waveform to the dust collector. Compared to 50 / 60Hz high-frequency rectifiers, the output voltage ripple of this high-frequency power supply when operating in pure DC is typically less than 5%, significantly less than the 35%-40% ripple of a power-frequency power supply. Its flashover voltage is high, with an average operating voltage up to 1.3 times that of a power-frequency power supply and an operating current up to twice that of a power-frequency power supply. This allows for more power input within the same electric field, effectively improving dust collection efficiency.

[0034] C. Effect of electrostatic precipitator pulse power supply (1) The pulsed high-voltage power supply can significantly increase the peak voltage of the electric field and the peak corona power. The extremely narrow high-energy pulse effectively breaks through the flashover voltage limit of the conventional DC power supply. The peak voltage can be increased to more than 140kV, and the output peak current is increased from a few A to 200A.

[0035] (2) Improve dust removal efficiency, especially for fine dust. The charge concentration excited by the pulse power supply is hundreds of times that of conventional DC power supply, which greatly increases the charge of the powder, especially for PM2.5 fine dust. Under the same working conditions, dust emissions can be reduced by more than 50%.

[0036] (3) Strong adaptability to working conditions and effective suppression of back corona. When powered by a pulse power supply, the average current is small, which reduces the charge accumulation in the dust layer and thus reduces the occurrence of back corona. In addition, the average voltage and current and peak voltage and current of the pulse power supply are adjustable separately, which greatly improves the adaptability of coal types and dust, and has a wide range of adaptability to harsh working conditions such as high resistivity dust.

[0037] (4) High efficiency and energy saving. The pulse unit is responsible for dust charging. Its power supply time is short and it adopts an energy feedback mechanism. Most of the energy during pulse boosting will be sent to the energy storage capacitor for recovery and can be used for the next pulse. The basic DC high-voltage unit only needs to maintain the corona inception voltage of the electrostatic precipitator electric field, thereby improving the efficiency of electric energy utilization and reducing the power supply while meeting the dust removal efficiency.

[0038] (5) The high-frequency high-voltage rectification equipment used in conjunction with the pulse high-voltage unit provides a nearly pure DC waveform to the dust collector. Compared with the industrial frequency 50 / 60HZ high-frequency rectification equipment, the output voltage ripple of the high-frequency power supply when it is supplied with pure DC is usually less than 5%, which is much smaller than the ripple percentage of the industrial frequency power supply of 35%~40%. Its flashover voltage is high, and the average operating voltage can reach 1.3 times that of the industrial frequency power supply, and the operating current can reach 2 times that of the industrial frequency power supply. In the same electric field, more power can be input, thereby effectively improving the dust collection efficiency.

[0039] D. After analyzing the characteristics of high-frequency power supply and pulse power supply, and the dust removal principle of electrostatic precipitator, etc.: High-frequency power supply has high nuclear power capacity for dust with high concentration, large particles and dust resistivity less than 1011Ω*cm, and has significant dust removal effect, which can reduce dust emissions by 30%; but it is not effective for ultrafine dust (PM2.5).

[0040] The pulse power supply has a high power efficiency for ultrafine dust (PM2.5) and particles with high resistivity > 1011Ω*cm, and has a significant dust removal effect, which can reduce dust emissions by 50%; Based on the existing power frequency power supply of electrostatic precipitator, all are changed to high frequency power supply, and the dust removal effect is improved from the current 50 mg / Nm 3 Reduced to 30 mg / Nm 3 , efficiency is increased by 20%, while power saving is up to 20%.

[0041] E. Upgrade and transform the low-voltage vibration / heating control system and add the "compound power vibration" function; High-frequency power supply and pulse power supply optimize the control mode of the vibration system (the electrostatic precipitator is equipped with negative vibration and positive vibration control systems), provide working power and realize the control of the vibration motor. The excellent "compound power vibration" control strategy is conducive to vibration cleaning and improves dust removal efficiency.

[0042] Secondary dust removal "double tower double circulation" limestone-gypsum wet desulfurization and dust removal control: Current situation: Before the transformation, the unit adopted a single tower design, the raw coal sulfur content was designed to be 1.5%, and the removal efficiency was designed to be 96.5%. After the flue gas passed through the 4-layer spray layer of the primary desulfurization tower, the original flue gas of 4800mg / Nm 3 The SO2 is controlled to 200mg / Nm 3If the sulfur content of raw coal exceeds the design value, it often makes it difficult to control emission indicators and the margin of raw coal selection is narrow. Taiyuan Iron and Steel's 300MW unit adopts "dual-tower dual-circulation desulfurization" control technology for SO2 control. The technical solution is designed based on a raw coal sulfur content of 2.5%, increasing the unit's adaptability to raw coal types while meeting environmental protection ultra-low emission control requirements. The specific solution is as follows: 1) The double-tower double-circulation process technology is adopted: the desulfurization efficiency of the first-stage tower is controlled at 80-90%, and the SO2 concentration at the outlet of the first-stage absorption tower is controlled to 500-700 mg / Nm 3 The secondary tower is designed with a desulfurization efficiency of 95%, and the SO2 emission concentration at the secondary tower outlet is controlled to be lower than 35 mg / Nm 3 The following increases the desulfurization capacity of the unit and increases the desulfurization margin.

[0043] According to the dual-tower dual-circulation design, the two towers are connected to a total of 8 circulation pumps. The motor capacity of the new tower circulation pump is small, and the operation mode is more flexible after the transformation. Under low load and low sulfur content, the operating cost can be reduced by reducing the number of operating circulation pumps.

[0044] 2) The "double tower double circulation" desulfurization control process is equipped with a double tower with 8 layers of atomized slurry spraying. The first and second absorption towers are respectively equipped with 4 layers of spraying layers, each equipped with a spraying device. The circulating pump pumps the slurry from the lower slurry pool to the spraying layer, and sprays it through the nozzle to form atomized droplets with fine particles and high reaction activity. The flue gas passes through the 8 layers of atomized slurry in the double tower, which can fully contact and wash the original flue gas with the slurry, and can reduce the original flue gas to 6500 mg / Nm 3 SO2 is stably controlled at 35mg / Nm 3 , ensuring a desulfurization efficiency of 99.62% and increasing the adaptability of the desulfurization system to various types of raw coal.

[0045] 3) The "dual-tower, dual-circulation" desulfurization control process allows for independent pH control of the absorption and oxidation processes. This enhances oxidation at low pH values ​​and absorption at high pH values, improving both oxidation and absorption efficiency and saving energy.

[0046] 4) The "dual-tower, dual-circulation" desulfurization control process extends the limestone dissolution time. In particular, the low pH operation of the first tower accelerates limestone dissolution, reduces the calcium-sulfur ratio, and improves limestone utilization.

[0047] The implementation of the present invention enables pollutant emission control to meet the design requirements: the smoke concentration at the dry power outlet is less than 20mg / Nm 3 , SO2 emission concentration at desulfurization outlet is less than 25mg / Nm 3 , the dust emission concentration at the wet removal outlet is less than 5mg / Nm 3 .

[0048] The "high frequency + pulse" combined high efficiency and low consumption electrostatic precipitator technology adopted in this invention improves the dust removal effect from the original smoke emission concentration of 40~70mg / Nm 3 Reduced to below 20 mg / Nm³, the emission at the outlet of the electrostatic precipitator after the transformation was 17.8 mg / Nm 3 The dust removal efficiency is 99.93%, and the operating power consumption is also greatly reduced, with significant results. The two furnaces can save 10.312 million kWh of electricity, creating an annual economic benefit of 4.74352 million yuan. Example 1

[0049] Taking advantage of the first shutdown opportunity of the ultra-low emission transformation, the dry-type electrostatic precipitator power supply devices of the two 300MW generating units were transformed. The transformation of the 1# electrostatic precipitator power supply was from May 1 to May 7, and the transformation of the 2# electrostatic precipitator power supply was from May 8 to May 14. The power supply transformation of each furnace took only 7 days, and the interval between the start-up of unit 1 and the shutdown of unit 2 was only one day. There was almost no preparation time for the two units, while the workload of each unit was very heavy. The original 20 silicon rectifier transformers, 4 high-voltage disconnecting switch cabinets, 20 original electric field high-voltage control cabinets, 4 low-voltage vibration and pressurization control cabinets, and 1 PLC control cabinet were dismantled. The original electrostatic precipitator two-phase power supply cables 2*185, totaling 40 cables, each 130 meters long, totaling 5,200 meters of cables, and the corresponding control cables were also dismantled. At the same time, it is necessary to install 20 high-frequency power supply devices, 8 pulse power supply devices, 8 high-voltage isolation switch cabinets (connected to the high-frequency power supply and pulse power supply at the same time), and lay 20 3*185 power cables of 130 meters, totaling 2,600 meters, and install 4 high-frequency power supply cabinets, 4 low-voltage vibration and pressure control cabinets, and 1 PLC control, and complete the connection and debugging of all control cables. The task is arduous, and we actively organize, from the operation procedures for construction personnel entering the factory, education and training for personnel entering the factory, to the determination of the pre-construction plan and daily construction plan, we carefully organize and work overtime. The earliest we rest is 10 o'clock in the evening, and sometimes the construction goes until 2 o'clock the next day. We work overtime and the construction tasks of the day must be completed on the same day to ensure the progress of the entire construction. After seven days and nights, we successfully completed the construction tasks before starting.

[0050] 1# electrostatic precipitator power supply transformation: complete the overhaul of the electrostatic precipitator body, replace the 4-5 electric field spiral wires, and install the corrugated wires.

[0051] Construction and commissioning: Following the maintenance schedule of the TISCO Energy Power Plant and the confirmed downtime, the engineering and technical personnel, taking into account the on-site conditions, developed a detailed plan for the entire project renovation. Through meticulous construction, planned milestone acceptance, cold and hot commissioning, and other steps, the entire project renovation was successfully completed. The main workload for the electrostatic precipitator on a single furnace is as follows: ① Remove the original 20 sets of power frequency power rectifier transformers and distribution cabinets in the power distribution room; ②Remove the original 20 sets of industrial frequency power cables; ③ 12 sets of high-frequency power supply and 8 sets of pulse power supply distribution cabinets were newly installed in the power distribution room; ④ Install 12 sets of high-frequency power supplies on the top of the dust collector; ⑤ Remove 8 isolating switch cabinets in the 4-5 electric field and install 8 new “two-input and one-output” isolating switch cabinets; ⑥ Install 8 sets of pulse power supplies on the top of the dust collector; ⑦ Re-lay 12 sets of high-frequency power supply and 8 sets of pulse power supply power cables, composite power vibration cables, communication cables, etc.; ⑧ Set up communication networks for high-frequency power supply, pulse power supply, low-voltage PLC and other equipment and lay necessary optical fibers; ⑨System debugging (cold debugging, hot debugging); After the high-frequency power supply, pulse power supply and IPEC system of No. 1 and No. 2 furnaces were put into operation, the high-frequency power supply, pulse power supply and vibration parameters were optimized and adjusted in depth with reference to the monitoring signal of the online dust monitor at the dust collector outlet. Finally, the energy-saving operation parameters of the high / low voltage equipment at all levels of the IPEC system were determined to ensure that the flue gas emissions were less than 20mg / Nm 3 Under the premise of energy conservation and environmental protection, the energy saving rate of the electrostatic precipitator electric control system is greater than 30%.

[0052] The high-voltage operating data during the test after the transformation are shown in Table 1.

[0053] Table 1

[0054] Table 1 shows the operating parameters of the high frequency electric field and pulse power supply.

[0055] Add another absorption tower after the first absorption tower. Add another set of wet electrostatic precipitator at the end of the flue gas. Figure 2As shown, the three-stage combined dust removal device includes a boiler 1, an air preheater 2, an electrostatic precipitator 3, a primary absorption tower 4, a secondary absorption tower 5, a wet electrostatic precipitator 6, and a chimney 7, connected in sequence. Denitrified coal-fired boiler flue gas enters boiler 1, passes through air preheater 2, and enters electrostatic precipitator 3 and induced draft fan 8. Electrostatic precipitator 3 uses a high-frequency power supply and a pulse power supply. Two electrostatic precipitators are arranged in parallel and are each connected to induced draft fan 8. The outlet of induced draft fan 8 is connected to primary absorption tower 4 through flue 9. The outlet of primary absorption tower 4 is connected to secondary absorption tower 5 (the secondary absorption tower has the same structure as the primary absorption tower). The outlet of secondary absorption tower 5 is connected to wet dust collector 6, and the outlet of wet dust collector 6 is connected to chimney 7. Induced draft fan 8 includes induced draft fan 1 and induced draft fan 2. Electrostatic precipitator 3 has four channels: A, B, C, and D. Channels A and B are combined and enter induced draft fan 1, while channels C and D are combined and enter induced draft fan 2.

[0056] Table 2 Dust emission concentration

[0057] After the ultra-low emission transformation facility is put into operation, the total outlet NO x Emission concentration from 100mg / Nm 3 Reduced to 25mg / Nm 3 Below, SO2 emission concentration from 100mg / Nm 3 Reduced to 15mg / Nm 3 Below, the smoke emission concentration is from 20mg / Nm 3 Reduced to 2.5mg / Nm 3 See Table 2 for details. Annually, it can reduce sulfur dioxide emissions by 3,346 tons, nitrogen oxide emissions by 1,628 tons, and smoke emissions by 232 tons.

Claims

1. A three-stage combined dust removal process for reducing dust concentration in flue gas emissions from coal-fired boilers, characterized by: A three-stage combined dust removal control technology is used to control the flue gas emissions of the unit: (1) A first-stage "high frequency + pulse" combined high-efficiency and low-consumption electrostatic precipitator is used to improve the dust collection efficiency of the first-stage dust removal, i.e., the electrostatic precipitator, and reduce the dust concentration of the boiler flue gas to 20 mg / Nm3 through the first-stage dust removal. 3 (2) The fine dust in the flue gas is washed by two-stage series absorption tower desulfurization to achieve secondary dust collection: including "double-tower double-circulation" limestone-gypsum wet tower desulfurization, dust removal first-stage absorption tower and second-stage absorption tower; by washing and collecting the fine dust in the flue gas, the dust concentration of the boiler exhaust gas is reduced to 8 mg / Nm 3 (3) Finally, a three-stage high-efficiency and low-consumption wet electrostatic precipitator is used: the flue gas carrying desulfurization slurry droplets and fine smoke dust are subjected to three-stage dust removal by the wet electrostatic precipitator, and finally the unit flue gas dust emission is reduced to ≤5mg / Nm 3 .

2. The three-stage combined dust removal process for reducing dust concentration in flue gas emissions from coal-fired boilers according to claim 1 is characterized in that: Use the first-stage dust removal electrostatic precipitator control technology: the specific method is as follows: 1) The fourth and fifth electric field spiral wires of the electrostatic precipitator were replaced with sturdy corrugated wires, which reduced the grounding failure rate of the electrostatic precipitator electric field caused by the broken spiral wires; 2) The power supply for the first, second, third, fourth, and fifth electric fields of the electrostatic precipitator is changed from the original industrial frequency power supply to a high-frequency power supply, thereby increasing the smooth DC voltage of the electric field. This increases the efficiency of the electrostatic precipitator and reduces the energy consumption of dust collection in the electric field. At the same time, a pulse power supply is superimposed on the high-frequency power supply for the fourth and fifth electric fields, and the peak voltage can be increased to over 140kV, thereby improving the collection capacity of the fourth and fifth electric fields for fine dust. 3) Changing the original low-voltage vibration of the electric field pole line and the pole plate to "compound power vibration" is more conducive to collecting the pole plate dust and improving the dust removal efficiency.

3. The three-stage combined dust removal process for reducing dust concentration in flue gas emissions from coal-fired boilers according to claim 1 is characterized in that: The specific plan for the two-stage "double-tower double-circulation" limestone-gypsum wet flue gas desulfurization and dust removal is as follows: 1) The double-tower double-circulation process technology is adopted: the desulfurization efficiency of the first-stage tower is controlled at 80-90%, and the SO2 concentration at the outlet of the first-stage absorption tower is controlled to 500-700 mg / Nm 3 The secondary tower is designed with a desulfurization efficiency of 95%, and the SO2 emission concentration at the secondary tower outlet is controlled to be lower than 35 mg / Nm 3 The following increases the desulfurization capacity of the unit and the desulfurization margin; 2) The "dual-tower, dual-circulation" desulfurization control process features eight layers of atomized slurry spraying in the dual towers. The primary and secondary absorption towers each have four spray layers equipped with internal spray devices. A circulating pump pumps the slurry from the lower slurry pool to the spray layer, where it is sprayed through nozzles, forming fine, highly reactive atomized droplets. The flue gas passes through the 8-layer atomized slurry layer of the double tower, which allows the original flue gas to fully contact and wash with the slurry; 3) "Dual-tower dual-circulation" desulfurization control process, the pH values ​​of the absorption and oxidation processes can be controlled separately, enhancing oxidation at low pH values ​​and absorption at high pH values, improving oxidation and absorption efficiency and saving energy; 4) The "dual-tower, dual-circulation" desulfurization control process extends the limestone dissolution time. In particular, the low pH operation of the first tower accelerates limestone dissolution, reduces the calcium-sulfur ratio, and improves limestone utilization.

4. The three-stage combined dust removal process for reducing dust concentration in flue gas emissions from coal-fired boilers according to claim 3 is characterized by: According to the double-tower double-circulation design, the two towers are connected to a total of 8 circulation pumps, and the first-level absorption tower and the second-level absorption tower are each equipped with 4 circulation pumps, corresponding to the 4 spray layers of the absorption tower respectively.

5. The three-stage combined dust removal process for reducing dust concentration in flue gas emissions from coal-fired boilers according to claim 1 is characterized in that: The control technology of the three-stage wet electrostatic precipitator is to add wet electrostatic precipitator after desulfurization. The specific operation is as follows: 1) A dual-chamber, two-electric-field wet dust collector is installed after desulfurization to ensure dust collection reliability; 2) Use 4 high-frequency power supplies to provide power to different areas, reducing energy consumption for dust collection; 3) Use plate-type horizontal wet removal to reduce flue gas resistance and improve the reliability of wet removal operation; synergistically remove gypsum droplets, SO3 aerosols and heavy metal Hg to ensure that the total exhaust dust concentration is effectively controlled at 5mg / Nm 3 Within.

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