Summary: in this article, we give analysis to the design and model selection of acoustic cleaning system for the ESP (electrostatic precipitator) of 600MW boiler fire by lignite. We come up with the design & operation parameter for acoustic cleaning system. This article provides reference for successful application of sonic horn to remove soot deposit in ESP.
1. Case Description
In this case, the boiler unit 2*600MW is equipped with 2 ESP (2 chamber and 5 electric field). Collecting electrode is ZT24 type, barb of discharge electrode is made of stainless steel. Total dust collection area for each ESP is 115920m2. Air volume at air preheater is 3765547m3/h. Effective height of collecting & discharge electrode is 15 meters. Effective length of electric field is 22 meters. Original soot cleaning method is mechanical rapping system. For CE, it is rapping sideway. For DE, it is rapping on the top. Lignite is low heat value, high volatile matters. For chemical composition of fly ash, SiO2 is high content, Na2O is low content, SiO2+Al2O3 is higher than 85%. Soot is featured with sticky and high specific resistance. Above factors have great influence on the soot removal. Soot is easy to settle down the electrodes and difficult to clean. For this situation, mechanical rapping system can’t clean the soot effectively. During operation, if soot deposit accumulates to certain degree, series corona blocking would happen, ESP performance is badly influenced. Dust collection efficiency is greatly reduced, and gypsum production in following desulfurization process is affected.
2. Analysis for the soot cleaning problem
As lignite is used as fuel of boiler, SiO2+Al2O3 content in fly ash is higher than 80%. The soot is sticky. This is because SiO2 would volatilize under high temperature and then to condensate to form fine particles. SiO2 fines are hard and difficult to carry electron. Al2O3 is floating dust. It is very fine particle and difficult to catch. Floating dust settles down on the surface of collecting electrode and barbs of discharge electrode and forms sticky film to create back corona. This would greatly worsen the performance of ESP. If rapping force is not big enough, it would get more and more difficult to remove the soot deposit.
The requirement of minimum rapping force for different electric fields and different location of the same field is different. The closer to the last stage filed, dust is smaller and stickier, soot deposit on electrode is thinner, the stronger rapping force is required. For the same reason, the closer to the top of electrode, the stronger force is required.
3. Acoustic cleaning system design and model selection
Soot deposit in ESP is different at different area. Soot is less at bottom area, while soot is more at upper area. It is a huge work to change the mechanical rapping system structure. For this reason, we suggest to combine mechanical cleaning and acoustic cleaning to remove soot inside electrostatic precipitator.
Acoustic cleaning system installation list
Top of third field
Top of fourth field
Top of fifth field
Considering the working condition, we choose to use SQ-75 acoustic cleaning system. Frequency of acoustic wave is 75Hz±5Hz. Sound pressure at horn is 150dB±3dB. Acoustic cleaning system could generate sound source at 75Hz. Sound wave length is 4.59 meters, which is bigger than the size of obstacles. It could pass through the edge or gap between obstacles in the air. During the transmission, acoustic energy is few absorbed by the medium. The acoustic wave is non-directional and spreads evenly inside ESP.
Wave length λ and wave frequency f is inverse proportion. λ=c/f. Effective cleaning area and uniformity of acoustic cleaning system is in direct proportion to acoustic wave length. If wave frequency is higher, wave length is shorter, attenuation becomes faster, acoustic energy in unit time to clean soot deposit is smaller, cleaning performance is worse. On the contrary, cleaning is better.
Acoustic energy generated by sonic horn is in direct proportion to compressed air pressure. Compressed air pressure is higher, vibration of diaphragm is bigger, sound intensity is higher, acoustic energy is bigger, cleaning performance is better.
Cleaning force by acoustic wave could not only act on collecting electrode to make it vibrate to remove soot, moreover, it could clean soot deposit by alternating forces. The spread of acoustic wave inside ESP could generate refraction and reflection on the ESP wall and electrode plate. Acoustic energy acts on the discharge electrode, collecting electrode and ESP wall, it would cause acoustic fatigue of soot on the surface. Then soot falls down.
4. Parameter of acoustic cleaning system
(1) Effective cleaning area.
Effective cleaning area of acoustic wave is 4.5m, effective cleaning length is 15m. Optimal diameter is 3m, optimal length is 10m.
(2) Compressed air pressure: 0.4-0.7MPa.
(3) Sound intensity: ≥147dB.
(4) Sound frequency: 75Hz
(5) Max remnant soot deposit thickness on collecting electrode: ≤1.5mm.
(6) Air consumption of single sonic horn: ≤2.95m3/min. Operation time of single sonic horn is 10-20s. Instant compressed air consumption is ≤0.5-1m3. Cleaning interval is normally 0.5-2h.
(7) PLC program is adopted for the control. It could set the operation time, cleaning interval. The settings could be adjusted according to the cleaning performance. Both manual control and automatic control is provided.
5. Measures to avoid re-entrainment of dust
(1) Reasonable layout of installation of acoustic cleaning system
(2) Reasonable operation mode for different area.
(3) Reasonable settings of operation time and cleaning interval.
(4) Adjustment of sound intensity and cleaning frequency could reduce or avoid re-entrainment of dust.
The application of acoustic cleaning system effectively removes the soot deposit on collecting electrode, discharge electrode. It solves the problem of low dust collection caused by soot fouling. Before the application of acoustic cleaning system, about 7-15 days after overhaul of ESP, emission concentrate goes out of the limits.
After shutdown, field test shows that soot deposit in the third and fourth electric fields is only 1mm thickness in average. Rate of finished product of gypsum in desulfurization is improved to 80%. Cleaning performance is satisfying.
So acoustic cleaning system is an effective method to clean the ESP for boiler fired by lignite. It is low-cost solution to improve ESP dust collection efficiency.