Bestt Practic Bes Practices es in ES ESP Operat ion and M ainten ance for im im proved p erform ance S.Ramakrishna Senior M anager srkris rkrishna hna @ bhelrpt .c .co.in o.in
K.Rajavel Dy General M anager
[email protected] Engineering Development Centre Bharat Heavy Electr icals icals Lim Lim ited Boiler Au xiliaries Plant Plant Ranipet 6 32 406, Tam Tam ilnadu.
Particulate emission control to minimum level from thermal power plants has gained significance. All present day electrostatic precipitators are designed for emission levels of less than 50 mg/ Nm 3 . But, often we find that the emission levels are going beyond the designed designed levels. levels. This This is is due to several several facto facto rs that are not given given im port ance in the operat ion and m aintenance aintenance of the p lant. lant. It is important to understand that ESP is collecting more than 99.9 % of the ash that enters th e precipitato r. The The em iss ission going out of t he chim ney is les lesss than 0.1 % corresponds corresponds to 50 m g / N m 3 for ESP ESP inlet concent ration of 50 g/ Nm 3 . Even a small drop in collectio collectio n efficiency can lead to multi fold increase in the emission level. Hence, it is imperative that collection efficiency of the ESP has to be maintained closer or better than the design condition to achieve the desired em ission ission levels. levels. Also the ESP field operating parameters also have major effect on the performance. In this paper w e have highlight highlight ed steps in operation of ESP ESP wh ich ich can give signific significant ant im pro vem ent in the performance of ESP. Prerequisite for the better operation of the ESP is maintaining good field alignment, rapping system and ash evacuation. It is presumed that all these are m aintained aintained well. In this paper, paper, w e w ill ill discus discusss best best practices practices that are required required t o be fo llow llow ed bot h in O& M for sustaining sustaining th e em iss ission below th e designed designed level. 1.0
BEST BEST PRACTICE PRACTICESS IN OPERATIO N
1.1
Im porta nce ESP field voltage and current : Precipitation in the ESP is largely governed by ESP field voltages. The charge on the ESP particles is directly proportional to the peak voltages whereas the force on the particles towards the collecting electrode is proportional to charge on the particles and average voltages. Hence the performance of the ESP is dependant on the ESP voltage. M any tim es it b ecomes difficult t o achieve the volt ages ages in in t he ES ESP in in spite of having good inter electro electro de gaps. gaps. There There is a need to u nderstand t he reasons for such condition and t ake appro priate act act ions. The voltage in the field is dependant on the flue gas temper ature, dust concentration, ash particle size, ash resistivity, ash coating on collecting electrodes, back coron coron a conditio ns. Page 1 o f 6
Best Practices in ESP Operat ion and M ainten ance for im proved p erform ance
1.2
Flue gas tem perature: M onitoring the gas temp erature can provide useful inform ation about ESP performance. Voltage withstanding capacity of the field is directly dependant on tem perature. Higher tem perature w ill lead t o low er voltages. Follow ing graph show s the effect of tem perature on voltage.
1.3
Temperature increases the resistivity of ash particles also gives rise to back corona condition. Significant temperature changes between inlet and outlet may indicate the air inleakage prob lems. Flue gas volum e increases wit h increase in t em perat ure. Du st Concentrat ion and Particle size: Higher dust concentration norm ally affects the first tw o field operation since it leads to sparking at low er current s. Low er current s m ean lowe r peak volt ages. Generally in first few fields, the average volt ages are higher du e t o space charge created by t he ash particles. Particles create more space charge in the inter electrode gaps; hence th e voltages w ill be higher. As th e dust concentrat ion almo st reduces to less th an 0.5 % in last few fields the volt age significant ly redu ces affecting t he volt age levels and efficiency of th ese fields.
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Best Practices in ESP Operat ion and M ainten ance for im proved p erform ance 1.4
Gas Flow and its effect: The increase in gas flow increases th e em ission levels considerab ly. The graph show s th e increase in em ission levels wit h increase in gas flow .
1.5
M ethods for reducing the boiler exit tem perature:
1.6
M ill outlet tem perature to be m aintained around 90 deg C.
LRSB soot blow er and w all blow ers to be operated t o reduce the t emp erature
Low er m ill operation.
M ethods for reducing the flue gas flow:
1.5
Excess air operatio n to be m aintained w ith O2 at Econom iser out let around 3.2 to 3.5 % Air heater and du ct leakage cont rol
Op tim isation of ESP fields: Optimisation of ESP fields basically involves the setting of optimum intermittent charge ratio and base charge. The intermittent charging mode in the controller, supplies the current in pulses w hich provides a dense corona fo r a short tim e and at th e sam e tim e gives a low average current to avoid back corona. Som e of h alf cycles are skipped in th e thyristor f iring to achieve this. The pulsed current m axim um lim it is allow ed up t o 200% of the norm al m ode current in the ESP, but t he average current w ill be much lower. Interm ittent charging helps to curtail the back corona condition s. The back coro na condit ions differ fo r different field position s. The flue gas temperature, resistivity of the ash particle decides the back corona condition. Last fields of ESP experience very fin e ash part icles having high resist ivity . Also t hese particles ten d to stick to collecting electr odes. ESP voltage tend t o be very low in the order of 17 to 22 KV compared to the f irst few fields w here the voltages m ay be in the ord er of 28 KV to 32 KV. Page 3 of 6
Best Practices in ESP Operat ion and M ainten ance for im proved p erform ance Peak and valley voltages and VI characteristics of a field can give very good idea of th e function ing of the field and back coron a cond ition s. Adjusting th e charge ratio fo r achieving better peak and valley voltage can lead to significant improvement in the ESP perf orm ance. The follow ing table gives readings taken in a project bef ore and after opt im ization o f th e fields. Impr ovem ent of peak and valley voltages after changing the ration can be seen. Before optim ization Field No
After optim ization
Current In %
Voltage
Charge Peak ratio / val le y voltage
Current In %
Voltage
Charge Peak ratio / val le y voltage
1.
182
27
11
70/18
182
33
31
84/23
2.
68
30
15
55/17
68
36
61
62/23
3.
198
34
21
70/25
198
42
61
79/37
4.
197
30
31
74/28
197
36
912
81/38
5.
188
28
41
65/18
188
31
101
70/23
6.
199
27
51
72/24
199
32
141
77/30
7.
200
25
61
77/22
200
31
151
84/28
8.
200
25
61
77/22
173
30
151
70/26
Aft er opt im ising the charge ratio, the em ission levels w ere brought d ow n to less th an 45mg/ Nm 3 from earlier level of 200 m g/ Nm 3 . 1.6
Effect of higher air ingress: In som e of t he o ld ESPs it is observed t hat oxygen levels in the flue gas exceeds 9%. In addition, increase of the gas flow rate the ingress of air can lead due dilution of moisture in the flue gas as well as SOx concentration. This will have serious implication on the precipitation. Air ingress also increases the flue gas volume, resulting in poore r efficiency.
2.0
BEST M AIN TEN AN CE PRACTICES Proper m aintenance precautions and pro cedures can m ake the difference betw een a precipitator w hich o perates satisfactorily and t he ot her w ith o perational problems. M ost of the precipitator’s problem s are m echanical in nature and h ence m any of the break-down m ay be due t o poor installation and p oor m aintenance. The problem s in th e precipitato r can be br oadly classified as below ; Page 4 of 6
Best Practices in ESP Operat ion and M ainten ance for im proved p erform ance Fundam ental problem s: Gas velocity to o high, poor gas flow d istr ibutio n, aspect rat io to o small, reent rainmen t o f collected d ust, high resistivit y, inadequate rapp ing etc. M echanical problem s: Poor electrod e alignment , disto rt ed / skew ed plates, vibrating or sw inging electrodes, plugged distribution screens, shock bar bent, excessive ash deposit on electr odes, erosion of screens and rap ping shaft breakage etc. Electrical Pro blem s; Unstable electr ical contr ols, rapping m ot ors / heaters not in service, heavy sparking, Transform er t ripp ing and insulato r crack et c. One of the best maintenance practices in precipitator is to continuously monitoring of t he emission and periodically m onitoring the com ponents attributed t o th e above said problems. If the emission goes out of range, then corrective action must be planned. The corrective action may include an investigation of the reason for high emission, evaluation of the situation. Necessary follow-up action is to be taken to bring back the precipitator t o the no rm al operation w ithin the acceptable range. It has been observed over a period that the most common problems for higher emission are ash build up in hoppers due to improper ash evacuation, field misalignment and non-uniform flow distribution inside ESP chamber and unequal flow bet w een various ESP str eams.
2.1
M ON ITORING GUIDELINES
2.1 .1
START-U P PRACTICES The im port ant activities dur ing start -up are; Condu ct air-load test f or each TR set to ensure healthiness of t he fields.
2.1 .2
Insulator heaters shall be on 2 – 12 hours prior t o strat-up. Rapping system shall be in service during start-up to rem ove any settled d ust. Energise fields accord ing to established pr ocedu re
SHU T DOW N PRACTICES Rappers shall be allow ed t o oper ate for at least 8 hour s to rem ove residual dust.
2.1.3
ROU TINE OPERATION S Reviewing parameters at ESP inlet, ESP fields, rapping system, and ash evacuation system. DAILY INSPECTION AN D M AINTENAN CE Operation of dust discharge system : ensure the ash removal system is working properly Check hopper door s for air leaks
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Best Practices in ESP Operat ion and M ainten ance for im proved p erform ance
Inspection of rapper operat ion
Inspection o f TR set operat ion
W EEKLY IN SPECTION AN D M AIN TEN AN CE Thorough check of rapper operation and implement any changes in the rapping frequency, if requir ed. Availability o f heat ers in ho ppers and insulator s
Check al access doo rs for any leaks and m ake sure t hat t he leakage is arrested .
D U R IN G SEM I A N N U A L / M A JO R U N I T O V ER H A U L
2.1. 4
Em pt y all the ESP ash ho ppe rs Check and correct t he Field m is-alignm ent Inspect the internals like raping mechanisms, screens, deflector plates for wear and tear and rectify. Review th e air load readings after t he m aintenance for ensuring health iness. Check for all rapping mot ors operation Condu ct gas distr ibut ion t est, if required .
RECORD KEEPING AN D REPORTIN G Opacity repor ts and supp ort ing data
2.1.5
Electr onic records of Volt age – curren t readings of fields M aintenance and inspection r ecords w ill be kept f or 5 years and shall be easily retrievable.
QU ALITY CON TROL The cont inuous opacity m onit or shall be calibrated f or zero and span adjustm ent s.
The precipitator must be operated and maintained according to the manufacturer ’s recom m endation. Plant personnel m ust be properly trained to perform these activities with confidence. 3.0
Conclusion: The emission from the pow er plant affects the environm ent and it becomes utm ost important responsibility of the operating personnel to understand the various parameters that affect performance of the ESP and take corrective action in achieving th e desired em ission level.
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