Electrostatic precipitator is generally a dust removal device which uses DC negative high voltage to ionize gas, produce corona discharge, and then charge the dust, and separate the dust from the gas under the action of strong electric field force. It is characterized by high dust removal efficiency, generally above 99%, and the highest design efficiency can reach 99.99%. Generally, the dust concentration at the outlet of the dust collector can be guaranteed to be 50- 100 mg/m3, and the resistance loss is small. Generally it is 49- 196 Pa, so the power consumption of the fan is low. According to the flue gas volume per hour 1000m3, the power consumption is about 0.2-0.8 kW·h, the flue gas volume is large, the adaptability to flue gas concentration is good, and the operating cost is low. However, it has a large one-time investment and steel consumption, a large floor space, high requirements for manufacturing, installation and operation, and is sensitive to flue gas temperature changes, and its application scope is limited by dust specific resistance. According to the data [1], the most suitable specific resistance range of electrostatic precipitator is104-5×10/kloc-0 (-).
The four electrostatic precipitators in Shen13th phase are composed of mechanical parts in Czech Republic and electrical parts in East Germany. Due to unreasonable factors such as design, manufacture and installation, the operation parameters have been poor since it was put into production, and the design parameters have never been reached. After repeated research and discussion by engineers and experts, as well as the technical transformation of electromechanical control system, although it has been improved, it still fails to meet the rated operating parameters. Especially in recent years, with the aging of equipment, the operating parameters have been unstable, which often appear: flashover occurs when the secondary voltage is low or even close to zero or rises to a lower voltage; The secondary current cannot rise and keep running at low current or the secondary current is unstable and swings violently. According to the operation and maintenance experience and technical analysis of our factory for many years, the reasons and countermeasures affecting the operation parameters of the third phase electrostatic precipitator in our factory are discussed as follows.
2. Cause analysis of influencing operation parameters:
2. Influence of1back corona on operation parameters:
The most suitable dust specific resistance range of electrostatic precipitator is104-5×10/kloc-0 (-), while the dust specific resistance of our factory is10/1-1kloc. The so-called back corona refers to the partial discharge phenomenon caused by the dust layer with high specific resistance deposited on the surface of the dust collector. When the specific resistance of dust exceeds the critical value1010 (-), the performance of electrostatic precipitator will decrease with the increase of specific resistance. When the specific resistance exceeds1012-,it is difficult to achieve the ideal effect by using conventional electrostatic precipitator. This is because if the dust deposited on the dust collector is a good conductor, it will not interfere with the normal corona discharge, and if it is dust with higher specific resistance, the charge will not be released easily. As the dust layer deposited on the dust collector becomes thicker, it becomes more difficult to release the charge. At this time, on the one hand, because the dust layer fails to release all the charges, its surface still has the same polarity as the corona electrode, so it repels the charged dust later. On the other hand, due to the slow charge release in the dust layer, a large potential gradient is formed between the dust. When the electric field intensity in the dust layer is greater than its critical value, local breakdown will occur between the pores of the dust layer, and positive ions with opposite polarity to the corona electrode will be generated, and the generated positive ions will move to the corona electrode to neutralize the negatively charged particles in the corona region. As a result, the current is greatly increased and the voltage is reduced. The operation parameters are unstable, and the electrostatic dust removal performance is obviously worse.
When the performance of electrostatic precipitator exceeds the critical value1010 (-), it decreases with the increase of specific resistance, which can also be demonstrated according to ohm's theorem: voltage drop when current passes through dust with a certain resistance.
△U=j * Rs= j *póR (V)[2]
Where: j-current density in dust layer (A/cm)
Ω r-dust layer thickness (cm)p- specific resistance (-)
The voltage acting between the electrodes is ug = u-△ u = u-j pó r (v).
U- voltage applied by electrostatic precipitator
As can be seen from the above formula, if the specific resistance of dust is not too high, the influence of voltage drop in the dust layer deposited on the dust collecting electrode on the space voltage Ug can be ignored. However, with the increase of specific resistance, if it exceeds the critical value1010 (-), the voltage △U in the dust layer will become very large, which will lead to a certain degree of local breakdown of the dust layer and spark discharge, that is, a case study of the main reasons affecting the operating parameters of electrostatic precipitator.
Back corona phenomenon.
To sum up, the most obvious influence of back corona on volt-ampere characteristics is:
A) reducing the spark discharge voltage to reduce the secondary voltage;
B) A stable corona trap is formed, which leads to sudden change or discontinuity of current and instability of the sum of operating parameters.
C) The maximum corona current is greatly increased. When spark discharge is about to happen, the secondary current is several times the normal current.
The measures to prevent and weaken the back corona are [3]: try to reduce the dust specific resistance so that the dust layer is not broken down. The main methods are as follows:
The flue gas is quenched and tempered. (including: wetting treatment; Chemical conditioning treatment)
Adopt high temperature electrostatic precipitator.
Adopt wide spacing electrostatic precipitator.
4) Using high voltage pulse power supply system is the most effective means to completely eliminate back corona and solve the problem that high specific resistance dust is not easy to collect. Its simple principle is to add pulse voltage with short action time on the basis of DC voltage. DC voltage is critical corona onset voltage, and pulse voltage ionizes gas to generate corona current. This power supply method can control corona current by changing the frequency and width of pulse voltage without reducing the electric field voltage. The product of corona current density and specific resistance of the dust layer deposited on the dust collector is always lower than the breakdown voltage of the dust layer, thus completely avoiding the back corona phenomenon. At the same time, the energy consumption of electrostatic precipitator will be greatly reduced, which has great economic benefits. The United States, Japan, Denmark and other countries have already successfully operated and confirmed the actual use effect. It is the development and application direction of electrostatic precipitation in China.
The dust specific resistance of Shenyi dust collector is101-1013-,which belongs to high specific resistance dust and is not conducive to dust removal. During operation, back corona often occurs in electric field, which seriously affects the improvement of operation parameters due to frequent discharge. According to this situation, combined with solving other problems of dust collectors in our factory, the #5 and #8 electrostatic precipitators were reformed several years ago. The homopolar distance is increased from 300mm to 400 mm, and the working voltage is increased from 30KV to about 45KV. At the same time, the high-voltage microcomputer control is adopted, which improves the operation parameters, prevents and weakens the back corona phenomenon to a great extent, but it has not been completely eliminated. #6 and #7 electrostatic precipitators have not been modified. With the aging of equipment, not only corona phenomenon occurs from time to time, but also corona wire hypertrophy and dust accumulation on anode plate are exposed, which seriously affects the stability and improvement of operating parameters and needs to be comprehensively reformed in the future.
2.2 Influence of corona wire hypertrophy and anode plate dust accumulation on operation parameters:
The thinner the corona wire, the stronger the corona. However, because a small amount of dust particles in the ion zone around the corona electrode are positively charged, they move to the negative corona electrode and deposit on the corona wire. If the dust has strong adhesion and is not easy to be rapped down, the dust on the corona wire becomes more and more concentrated, that is, the corona wire becomes thicker, which greatly reduces the corona discharge effect, which is corona wire hypertrophy. Dust with strong adhesion sometimes accumulates on the anode plate. In both cases, the operating parameters will be significantly reduced. The main reasons are as follows:
1) When the dust collector is under low load or stops running, the temperature and dew point of electrostatic dust removal are low, and water or sulfuric acid condenses between dust particles and between dust particles and electrodes to dissolve its surface. When the dust collector runs again, the dissolved substances solidify or crystallize, resulting in great adhesion.
2) Due to the adhesion caused by the nature of dust, explore the use of suitable coal to solve it.
3) Some polar plates and polar lines are seriously corroded, and the dust adsorbed on the surface is not easy to be removed by vibration. Although some cathode wires were replaced after shutdown, the corroded anode plate could not be replaced until it was overhauled.
4) Air leakage makes cold air enter the electric field from the inspection door, flue, expansion joint and insulating sleeve. This will not only increase the flue gas treatment capacity, but also cause condensed water due to temperature drop, which will cause dust accumulation and hypertrophy of corona electrode and creep corrosion of insulating sleeve.
5) Insufficient rapping strength or failed rapping will lead to hypertrophy of corona wire and dust accumulation on anode plate, which will affect the rise of current and voltage. In our daily practice, we find that when the current and voltage are obviously reduced and the adjusted microcomputer does not work, the electric field is suspended for a few minutes.
(Vibrating continues to run) The current and voltage increase obviously after re-input, and the operation parameters return to the original state after a few minutes, which fully shows that the vibrating intensity is not enough. 1998, in order to solve the problem that the two electric fields of anode rapping are easily blocked, the #6 electrostatic precipitator was reformed by two-way rapping. After long-term operation observation, not only the blocking failure is obviously reduced, but also the corona wire hypertrophy and anode plate dust deposition are greatly improved.
2.3 Influence of corona blocking on operation parameters:
When the dusty gas passes through the electric field space, the dust particles collide with the free ions in it and become charged, so there are two forms of charges in the electrostatic precipitator-ionic charge and particle charge. Therefore, corona current is the movement of gas ions on the one hand, and dust particles on the other, but the size and mass of dust particles are much larger than those of gas ions, so the movement speed of gas ions is several hundred times that of dust ions (the average speed of gas ions is 60- 100 m/s, while the speed of dust ions is less than 60 m/s). The corona current formed by dust ions only accounts for 1-2% of the total corona current. With the increase of dust concentration in flue gas, the number of dust ions also increases, which makes the corona current formed by dust ions smaller, but the space charge formed by dust ions is larger, which is close to that formed by gas ions, which seriously inhibits the generation of corona current, makes dust particles unable to get enough charge, and greatly reduces the secondary current. If the dust concentration is too high, the main reasons are as follows:
1) The dust concentration in the flue gas is high. According to our observation for many years, due to the different coal quality and high dust concentration, the current and voltage of the three-stage electrostatic precipitator are sometimes affected to varying degrees, especially the current of the first and second stages decreases obviously, and the dust removal effect becomes worse; The electrostatic precipitator under the same working conditions does not make any adjustment to the high-voltage microcomputer electronic control system and the vibration microcomputer electronic control system, and sometimes the current and voltage are very high and the ash hopper quantity is normal, which shows that the dust concentration in the flue gas has a great influence on the operating parameters of the electrostatic precipitator.
2) With the increase of flue gas velocity (electric field wind speed), corona blockage will occur to varying degrees. The design flue gas velocity of the three-stage electrostatic precipitator is1.159 m/s. If the flue gas velocity exceeds this parameter, it will inevitably affect the increase of current and voltage in operation. Electrostatic precipitator operates under negative pressure. When the joint of the machine body is not tightly sealed and leaks air, cold air will enter the electric field from the outside, which will increase the speed of flue gas passing through the electrostatic precipitator and increase the amount of flue gas staying in the electric field per unit time, leading to corona blockage in different degrees and worsening the operating parameters.
In order to reduce the influence of high dust concentration in flue gas, a few years ago, the corona wire of the third phase electrostatic precipitation was changed from sawtooth wire to barbed wire suitable for collecting high dust concentration through overhaul, and the corona blockage phenomenon was obviously reduced after the transformation. However, in recent years, with the aging of the dust collector, due to other reasons, the maintenance cycle of the dust collector has not been arranged in time, and the increase of air leakage has not been completely controlled, resulting in the increase of corona blockage, and the secondary current sometimes drops obviously in operation, even making the current tend to zero.
2.4 Influence of boiler exhaust gas temperature and pressure on operation parameters:
The temperature and pressure of flue gas affect the corona initial voltage, the electric field intensity on the corona electrode surface, the space charge density near the corona electrode and the effective mobility of molecular ions. Some effects of temperature and pressure on the performance of electrostatic precipitator can be analyzed through the change of flue gas density.
T = T ò=ò0 * T0/T *P/P0/P0 (kg/m3) [4]
Ω 0 —— Smoke density at t0 and P0 (kg/m3)
T0—— temperature in standard state (273 k)
P0- atmospheric pressure in standard state (10 1325pa)
T—— actual temperature of flue gas (K)
P—— actual flue gas pressure (pa)
As can be seen from the above formula, this parameter decreases with the increase of temperature and the decrease of pressure. When the parameters decrease, the corona start voltage begins to corona.
At the same time, the electric field intensity and spark discharge voltage on the surface of corona electrode should be reduced so that the secondary voltage does not rise. This is because when the gap decreases, the effective mobility of ions increases due to the decrease of the number of collisions with neutral molecules, because under certain applied voltage conditions, this will lead to the decrease of the space charge density near the corona electrode and the increase of the average current of the dust collector electrode. The decrease of space charge density near corona electrode leads to a certain corona current on the surface of corona electrode, and the electric field intensity is low. Therefore, in order to maintain a certain average corona current density on the anode plate, it is necessary to reduce the applied voltage, which leads to the decrease of operating parameters.
The highest flue gas temperature of Shenyang 1 No.3 boilers can reach about 180℃, and the optimum operating temperature of electrostatic precipitator is 140℃- 150℃. Running at this high temperature will directly affect the increase of secondary voltage and secondary current of electrostatic precipitator. However, the flue gas pressure has little effect after the preliminary test, and reducing the temperature of boiler exhaust gas is beneficial to improve the operating parameters of electrostatic precipitation.
2.5. The influence of high voltage short circuit on operation parameters:
High voltage short circuit directly affects the operating parameters of electrostatic precipitator. After the high voltage is completely short-circuited, the secondary current I2 rises, the secondary voltage U2=0, and the corresponding electric field loses its dust removal function. In order to prevent short-circuit current from burning the electric field or damaging the rectifier transformer, the corresponding control cabinet must be stopped immediately. It can be seen that high voltage short circuit has the greatest influence on the operating parameters of electrostatic precipitator. The phenomena and causes of high voltage short circuit mainly include the following aspects:
1) When the secondary current I2 rises and the secondary voltage U2 drops (sometimes U2=0), there is a great suspicion of high voltage short circuit; When the changing value of I2. U2 is not large, which is due to the change of flue gas conditions, which leads to the increase of load and the decrease of external loop voltage drop, or the improper position of the secondary output tap of the whole transformer and the decrease of electric field insulation. At this time, we should find out the reason of insulation decline from the electric field, adjust the boiler operation condition or change the position of the secondary tap of the rectifier transformer.
2) When U2 drops sharply, the secondary ammeter and the secondary voltmeter swing in the opposite direction, that is, the secondary voltmeter instantly drops to zero, while the secondary ammeter rises sharply. At this time, it is mostly because the cathode wire or anode plate in the electric field is broken or welded, and the different polar distances change or even short circuit when the flue gas flows (at this time, I 2 reaches the meter head, U2=0), and the noise changes with time, the temperature rises, and the equipment is safe.
3)I2 is greater than the normal value, U2=0, and the hand does not swing. The main reasons are:
(1) In the electric field, the plates and wires are completely short-circuited or short-circuited by dust, and the high-voltage cable is grounded.
(2) The electric field or the cathode insulating porcelain insulator is seriously affected with damp, or the water ingress reduces the insulation even to zero. Water inflow leads to the discharge of cathode insulation suspender in operation, and carbonization completely loses insulation effect, resulting in high voltage short circuit. The high-pressure porcelain bottle is broken.
(3) Transformer failure.
Due to the aging of some equipment, the phenomenon of low or even zero electric field insulation or high voltage cable breakdown to the ground often occurs in the operation of Shenyi Phase III electrostatic precipitator, which seriously affects the current and voltage parameters in the operation of electrostatic precipitator, and it is urgent to replace some equipment through overhaul.
2.6 The operating environment of microcomputer control cabinet and the influence of insufficient booster and variable capacitance of electrostatic precipitator on operating parameters:
The surrounding environment of microcomputer control cabinet directly affects the correct execution and feedback control of electronic control elements in microcomputer. If there is too much dust in the electronic control components, it will inevitably affect the heat dissipation and cause the temperature to rise, thus sending signals by mistake, which will seriously affect the current and voltage parameters in operation. The third phase of ESP was put into operation and installed because the distribution room was not tightly sealed. During the operation of electrostatic precipitator, a large amount of dust enters the distribution room, which seriously affects the correct action of microcomputer control system. Although regular cleaning is strengthened, it is far from meeting the needs of microcomputer operation. At present, the environment of #6, #7 and #8 ESP power distribution rooms is still very poor in operation, except that the environment of #5 ESP power distribution room has been obviously improved after overhaul, which is in urgent need of thorough rectification and closure.
The step-up transformer of electrostatic precipitator has great influence on the operation parameters. Because the mechanical part of Shenyi ESP is made in Czech Republic, and the electric control cabinet and step-up transformer are made in East Germany, there is no strict matching calculation in the design. The dust collecting area of electrostatic precipitator is too large, which is equivalent to 300 thousand sets in China, and the capacity of booster transformer is small. When the capacity of step-up transformer is large enough, the load change has little effect on its output voltage; Otherwise, when the capacity of the step-up transformer is insufficient, the load change has a great influence on its parameters. Because the step-up transformer does not match the body capacity when it is designed, the step-up transformer has a small capacity, so when the current rises, the transformer itself rectifies the silicon pile and resistor.
The resistance and high voltage cable have large voltage drop, which reduces the voltage of the electric field, so that the voltage and current of the electric field cannot be improved and the parameters can not meet the rated requirements.
The solution is to widen the pole distance and reduce the dust collection area (realized by #5 and #8 electrostatic precipitators), but this method is also limited by the maximum allowable voltage of the transformer. When the voltage reaches the rated 55KV, the transformer has overcurrent. Therefore, the fundamental solution is to replace the large-capacity step-up transformer.
3. Conclusion: From the above analysis, it can be seen that the main reasons that affect the current operating parameters of the first and third phases of ESP are:
Dust is bigger than resistance. The exhaust temperature is high.
Corrosion, deformation and spacing change of some plates and wires.
The vibration is not strong enough.
Aging of high voltage cables; Body wear and air leakage; Some heat preservation boxes have air leakage, rain leakage and insufficient heat preservation.
The capacity of step-up transformer is insufficient, and the operation parameters can't reach the rated value.
The distribution room is not tightly sealed, and the microcomputer running environment is poor.
4. Measures and countermeasures: In view of the current situation, the measures and long-term countermeasures should be:
Select suitable coal, burn reasonably, and reduce the exhaust temperature.
Take the overhaul as an opportunity to replace corroded and deformed polar plates, polar wires and unqualified high-voltage cables, completely eliminate air leakage, and put them into an incubator for heating. Completely solve the problem of poor sealing of #6, #7 and #8 power distribution rooms.
All adopt wide-spaced double-sided vibration (#5 and #8 adopt wide-spaced vibration, #6 adopts double-sided vibration). Replace large-capacity step-up transformer or adopt high-voltage pulse power supply.