BACK-CORONA AND ITS CONTROL IN ELECTROSTSTIC ELECTROSTS TIC PRECIPITATOR Abstract The Electrostatic Precipitator (ESP) is worldwide the most common dust collector to clean flue gases after coal fired boilers. A new patented concept to define back-corona has been introduced which enables the microprocessor controls to energize each ESP bus section individually, automatically and continuously to optimize the voltage and current to maximize the overall collection efficiency under varying process conditions. The new concept selects current and duty cycle in intermittent energization mode, based on continuos sampling and mathematical analysis of the secondary voltage from each bus section, referenced to the corona onset voltage. Maximal corona charging with minimized back-corona is achieved. This paper presents the philosop philosophy hy of the optimizing optimizing software’s, software’s, and reports reports some recent emission improvement results from operation of ESP’s, mainly after Pulverised Coal Fired boilers firing low-sulfur coals. The modular ESP control system approach The EPIC II ESP II ESP control system utilizes one EPIC II controller II controller per ESP bus section, and needs one or more Remote Terminal Unit ( RTU ) as user interface. The RTU is RTU is a portable user interface that allows the operator to read or modify data from any “node” on the system. The FläktBus is a twisted wire cable used to interconnect the “nodes”- maximum 120 - in the control system. Figure 1 shows a typical EPIC II control II control system layout with two additional options, ProMo-II an IBM PC based based software software package package for operators operators overview overview monitorin monitoring, g, logging logging and control, control, and Gateway II II - an interface to eg the existing boiler control computer (DCS).
EPIC-II
Figure Figure 1: 1:
Typi Typica call EPI EPIC C II II Con Contro troll sys syste tem, m, show showin ing g some some poss possib ible le “no “node des” s” in in an an ESP with 8 Bus Sections
All new new ProMo -II have II have modem communication possibility. This means that ESP remote control via standar standard d telep telephon hone e lines lines can can be made made - upon upon ESP owners owners deman demand d - inclu includin ding g ESP ESP 1
process diagnostics, changing and upgrading of ProMo-II software etc. Todays state of the art is to diagnose normal ESP problems via this “remote” without the necessity (or cost) for a site visit by an ESP process expert. Back-corona how it happens and its reduction of the ESP collection efficiency
Back Corona in Dust Layer on Collecting Plate
The corona current from rom the the discharge electrodes must penet penetrat rate e the flyash flyash layer layer on the collectin ting electrod rodes befor fore reaching reaching the grounded grounded plate. plate. The current through this layer generates a volta voltage ge drop drop propor proportio tional nal to the current and the resistivity of the fly ash. When the voltage drop across the fly ash layer is high enough, enough, an electrica electricall break-dow break-down n is initiated initiated in the fly ash, resulting in generation of positive positive and negative negative ions. The posit positive ive ions ions will will create create a coron corona a current that moves in the direction towards towards the discharg discharge e electrod electrodes. es. This This posi positi tive ve char charge ge coro corona na is call called ed back back-c -co orona rona beca becaus use e it move moves s in the the wrong rong,, unde undesi sire red d direction.
Back-corona decreases decreases the ESP collection efficiency in two ways: •
Some collected flyash is reentrained into the gas flow by the explosive nature of the voltage break-down in the collected fly ash
•
Positive ions generated by the voltage break-down in the fly ash layer move to the discharge electrodes thereby discharging the negatively charged particles in the gas stream.
Back-corona can be minimized in basically two ways, both with the target to reduce the voltage drop across the dust layer on the collection plates: •
Either by reducing the resistivity of the dust that causes the ionization on the collecting plates. This can be made in different ways, eg by conditioning the gas that enters the ESP with eg SO 3 or ammonia
•
Or by reducing the average current from the T/R. This can be made with the use of Semipulse ™ or full pulse technology. Semipulse ™ is an intermittent charging of a conventional T/R, so made that the average current is kept low while the peak current is still high, giving a better distributed corona along the discharge electrodes. 2 2
Related options with same target (to reduce the voltage drop across the dust layer on the collecting plates) can be conditioning with (large) amounts of moisture that will increase the dielectrical strength of the gases entering the ESP, or exchange the ESP internals to a better designed ESP electrode geometry that will give a more even current distribution.. Figure Figure 2 , shows shows a Semipu Semipulse lse™ chargi charging ng ratio ratio (CR) (CR) of 1:3, 1:3, or one used pulse pulse per three three possible (each pulse is a part of the mains half- wave). The charging ratio can be increased to cater cater for even even the highes highestt dust dust resist resistivi ivitie ties s assoc associat iated ed with with very very sever severe e backback-cor corona ona conditions. T/R Current secondary current
time
Figure 2:
Semipuls e®, operating the T/R at CR 1:3
The EPOQ EPOQ algor algorith ithm m - suppr suppres esse ses s Back-C Back-Coro orona na to increa increase se the ESP ESP colle collecti ction on efficiency A disadvantage, up to now, has been that the optimal CR and current level for maximum ESP ESP colle collecti ction on efficie efficiency ncy have have been been difficu difficult lt to determ determine ine,, becau because se they they chang change e with with changing flue gas conditions. By using optimal optimal CR, sufficient current can be maintained maintained to achieve a proper current distribution, while at same time keeping the back-corona to a minimum. The selectio selection n of optim optimal al CR and and curren currentt level level assume assume that a corre correct ct estima estimatio tion n of the presence and severity of any back-corona in any bus section of an ESP has been made. With the help of powerful microprocessors that allow fast sampling and analysis of voltage and current curves, and advanced insight into the detection criteria of back-corona, it is now possible possible to determine determine and implemen implementt the best CR and current current level, level, automatica automatically lly and continuously. In this way the CR and current level that for each individual bus section give maximi maximized zed ESP colle collecti ction on effici efficienc ency y is alway always s very very closel closely y mainta maintaine ined d - for any ESP ESP process change or variation. The outlined maximizing of the overall ESP collection efficiency is performed by a software in each individual microprocessor unit, called EPOQ (Electrostatic Precipitator Optimization of Q [Q = chargi charging ng]). ]). Typica Typically lly each each subseq subsequen uentt ESP ESP sectio section n will will have have a more more difficu difficult lt flyash than the previous field (in some unusual cases an easier flyash may be seen in a subsequent field). The continous optimization of each bus section automatically leads to the best overall collecting efficiency of the whole ESP. To further reduce some other problems caused by back-corona, a software PCR (Power Control Rapping ) has been introduced in every microprocessor controller. Altering the T/R power during the rapping will alter the electro-mechanical forces that keep the dust adhered to the collecting plates. 3
4
