esp
DESCRIPTION
electrstatic precipatatorTRANSCRIPT
WELCOME
To
A Presentation
On
Electrostatic Precipitator by S.K.Meher
Why use an ESP ?
• To control atmospheric pollution caused by fly ash .
• To meet statutory requirement of pollution control (maximum permissible 150mg/Nm cube). Preferrebly 30mg/Nm cube
• To get fly ash as a by product. to use ash as filling for mines., Cement Plant as raw meterials, bricks vas etc.
Topics to be discussed
• Principle of operation
• Construction details
• Functioning of the Controller
• Controller used at SLPP
• Other systems available
• Discussions
Principle of operation• The ash particles
move along the stream of flue gas
• We are to change their direction of travel so that they get separated from the stream of flue gas
• How ?
Principle of operation (continued)
• We impart electrical charge to the particles.
• We have a surface with opposite electrical polarity nearby.
• The particles are electrically attracted to the surface.
+
Principle of operation (continued)
• Emitting electrode is charged at high voltage DC negative.
• Collecting Electrode (plate) is at ground potential (positive).
• Electrical field is perpendicular to flue gas flow.
+ -
Principle of operation (continued)
• Emitting electrodes sprays negative ions towards positive plate (corona discharge).
• They collide with ash particles and transfer the charge to them.
• Ash particles get negatively charged.
-+
-
-
--
- -
-
-
-
--
Principle of operation (continued)
• The positive collecting plate attracts negative ash particles.
• On reaching the collecting plate, the particles get electrically neutralized and remain there.
+
-
-
-
-
-
-
-
-
-
Principle of operation (continued)
• After a layer of ash is collected on the collecting plate,it is mechanically rapped so that the ash falls into the hopper for disposal through ash handling system .
Construction
• To achieve laminar flow at lower velocity the gas path is split in several “Pass”es.
• Perforated GD screen at the entry of each pass ensure uniform velocity distribution.
• To achieve better collection efficiency several fields are provided in series in each pass.
• Many combinations are possible depending on boiler size.
Construction (passes)
• At SLPP for each boiler there are 2 passes with 6 active and 1 dummy fields each.
• Dummy fields are just vacant and provided for future addition if required.
Chimney
Boiler 1 Boiler 2
D654321
ID Fans
Construction(major parts)
• Collecting plates
• Emitting electrodes
• Rapping mechanism(collecting,emitting)
• Insulators
• High Voltage Rectiformers
• Heaters
• Ash Level Indicators
Construction(collecting plates)
• Carbon steel plates.• Each plate 13.5m H x
750mm W.• 6 such plates in a row.• There are 51 such
rows in each field.
Construction(emitting electrodes)
• Looks like a helical spring of alloy steel with hooks at both ends.
• Helix dia.30mm, wire dia. 3mm.
• Each helix is 4.5m long.
Construction(emitting assembly)
• One column consists
of 3 such helix
vertically connected.
• In one row 16 such
columns are there.
• There are 50 such
rows.
Construction(collecting & emitting)
• 50 rows of emitting electrodes are hung between 51 rows of collecting plates maintaining electrical clearance .
• Emitting electrode assembly is supported by 4 SUPPORT INSULATORS .
Construction(collecting rapping)
• Hammers for each row of collecting plates are mounted at different angles on the rapper shaft.
• When the shaft rotates all rows get hammered one by one.
Construction(emitting rapping)
• Emitting rapping is towards the top of the field.
• It is similar to the collecting rapping.
• As emitting rapping touches the live parts, an insulator is introduced in the shaft.
• This is called SHAFT INSULATOR.
Construction(synchronous programmer)
• A drum with cam & cam follower mechanism and driven by a synchronous motor gives starting command to individual rapping motors.
• This is called synchronous programmer.
• It controls all rapper motors of one pass.
• Each rap is 1 minute long.
Construction(rapping frequency)
• Collecting rappers are operated 10, 8, 6, 4,
3, 2 raps per hour respectively.
• All emitting and GD rappers are operated
at the frequency of 10 raps per hour.
Construction(High Voltage Rectiformer)
• Rectiformer is a transformer and a rectifier in one unit.
• Voltage & current feedback circuits are also there.
• 70kV DC on no load.• Each field has got its
own HVR.
Controlled LT AC
High Voltage DC
Construction(hopper heaters)
• A portion of the hopper is double walled.
• Heaters are placed between the walls to keep the ash warm so that it does not stick to the sides of the hopper.
Construction(insulator heaters)
• Shaft insulators (1 in each field) and support insulators (4 in each field) do not fall in regular gas flow path.
• They get colder and help ash accumulation on them. This reduces the field voltage.
• Heaters for individual insulators are used to keep them warm.
Construction(ash level indicators )
• Capacitance type Ash Level indicators are provided in each hopper for sensing hopper overfilled condition.
• High level trips the particular field.
Construction(ash disposal)
• Below each hopper there is a vessel called transmitter.
• The ash from the hopper is first unloaded into the transmitter.
• The ash is then pneumatically to the silos for further disposal.
Controller
Control philosophy
• To have a voltage just below the
spark-over voltage for as long as possible.
Controller(graph)
V
t
Controller(spark sensing)
• The controller measures current and calculates dI/dt .
• When this value crosses a limit a spark is sensed.
• Voltage is made zero for few milliseconds of pause time to allow ionized air to flow away.
Controller(voltage recovery)
• Voltage again rises at a fast rate to very near the spark over value (say 90%).
• Rate of rise then slows down.
• If a spark is sensed, the process repeats itself.
• If no spark is sensed, the voltage continues to rise till voltage/current limit is reached.
Controller(back corona)
• All controllers use the above method of dynamic control of voltage.
• However use of only this method is unable to tackle the BACK CORONA problem.
• Modern controllers have micro-processor based computing facilities to take care of the back corona problem.
Controller(potential difference )
• The neutralization of charged ash particles constitute a current.
• Current has to pass through the layer of ash already collected.
• Potential difference is created across the layer. (V=IR)
+-
V = I x R
Controller(ionization of trapped air)
• Due to the high resistivity of ash the potential difference in a few mm thick layer can be several kV.
• This ionizes the air trapped between the ash layers.
+ +
+
+
-
-
-
Controller(back corona)
• The positive ions are repelled by the positive plate & they come out in a stream.
• This phenomenon is BACK CORONA.
+
++
+
+ + +
+
+
+
Controller(effects of back corona)
BACK CORONA
• Neutralize charged ash particles, so they are not collected.
• Dislodges already collected particles & allows them fly away.
• Draws extra power & reduces collection efficiency.
+-+
+
+
Controller(controlling back corona)
• To reduce Back Corona IR drop has to be reduced.
• R is a property of ash which cannot be controlled.
• To reduce I, V has to be reduced which will reduce the main corona. Hence collection itself will be reduced.
Controller(intermittent charging)
• Similar to a fluorescent tube, higher voltage is required to start a corona. Once started, the corona can be maintained by a much lesser voltage.
• The field behaves like a RC circuit. Once charged, the voltage decays only gradually through the collection current.
• It can be charged again before corona stops.
Controller(intermittent charging)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
1:51:1
Controller(less average voltage)
• In conventional method of charging every half cycle, difference between the peak & average voltages is very small.
• In intermittent charging, firing of the thyristors is skipped for several half cycles & voltage is allowed to decay. While the peak voltage remains same, the average voltage comes down drastically.
Controller(less average current)
• Reduction of average voltage reduces average current which in turn reduces the IR drop across the ash layer. Thus back corona is controlled.
• The number of half cycles skipped is denoted by “charge ratio”. A charge ratio of 1:5 means firing takes place at every 5th half cycle.
Controller(charge ratio )
• Equipment used at SLPP is capable of having charge ratio upto 1:159.
• The controller has provision of finding the best charge ratio after every 2 hours through optimization mode.
• Presently fixed charge ratios are used .
Controller(base charge)
• If we use charge ratio more than 1:5, the voltage becomes too low to sustain the main corona after the 5th half cycle.
• To avoid this thyristors are fired just before 180deg from 6th half cycle onwards. This is called “BASE CHARGE”.
Controller(base charge graph)
Base Charge
0
BAPCON Controller
1 6 7 -%
R L 398706Is Limit
T/OReset
Opt HT
Accm. spark
BAPCON(symbols)
• “-” Precipitator current• “E” Precipitator voltage• “H” Sparks/min.• “o” Im Limit• “1” Is Limit• “2” S control• “3” T control• “4” Stabilizing time
BAPCON(symbols)
• “5” Under Voltage limit• “6” Charge Ratio• “7” Pulse current limit• “8” Repeat time (x 6 min.)• “9” Address• “P” Base charge set• “L” Base charge current• “ ” Peak & Valley Voltage
BAPCON(common settings)
• The following settings are common for all controllers.
• 0 Im limit 100
• 1 Is limit 200
• 2 s control 8
• 3 t control 25
• 5 Under Voltage limit 10
BAPCON(settings)
• The following settings are position dependent.
Field 1 2 3 4 5 66 C.ratio 5 7 11 21 31 41P B/C set 3 3 3 6 3 10L B/C crnt 0 0 0 2 1 4
BAPCON(alarms & indications)
Alarms:• Transformer temperature high.• Buckholtz top float.• Ash level high.• Under Voltage.Indications:• Base charge ON.• Optimum reached.• Remote.
BAPCON(trips)
• E2 Firm wire fault.
• E3 Internal fault.
• E4 kV high.
• E5 Ash level high > 10 min.
• E6 Primary Amp. High/Oil temp. high.
• E7 Buckholtz bottom.
Other systems available
• Micro-processor based controllers for rapping . When one or more field trips, rapping frequencies are automatically changed.
• Operation on the basis of opacity feedback.• Monitoring/operation through PC.• MIGI rappers.• Online diagnosis through internet.
Questions
Please !
F.A.Q.s
• Why HV is negative, not positive ?
• Practically found to be effective.
• Why emitting electrode is in spiral shape ?
• More surface area, ease of manufacture.
• Why charge - ratios are odd numbers ?
• To prevent HVR core saturation.
Thank you !