Enhancement of ESP performance of Thermal Power Plant : a controlling measure towards Environmental Sustainability.

Transcription

1 Enhancement f ESP perfrmance f Thermal Pwer Plant : a cntrlling measure twards Envirnmental Sustainability. Abstract SN Patra & UK Sarangi. NALCO, Angul, Odisha, India patra_surendranath@yahmail.c.in Thermal Pwer plants emit fly ash as well as ther gases like Carbn dixide(co 2 ), Carbn Mnxide (CO), Sdium Oxides(NO x ), Sulpher xides( SO x ),etc. t the atmsphere and, in turn, the atmsphere gets plluted. The pllutin t this effect is dangerus t ur living sciety as well as t the plant life. The reasn is the use f fssil fuel like cal t run the thermal pwer plants. Nw-a-days. mre numbers f ld cal based pwer plants, are still in existence with high emissin. High ash cntent r deteriratin in quality f cal reserve is the majr fcus fr ur envirnmental sustainability. Arund 70% f pwer generatin is frm cal based thermal pwer plant. Alternative pwer generatin technlgies have als been adpted t reduce emissin as a majr twards envirnmental care. Air pllutin cntrl equipment fr tday s need are Electrstatic Precipitatr (ESP), Fabric Filter (FF) etc. are in greater use. But, Electrstatic Precipitatrs(ESP) are widely used all ver the wrld fr better perfrmance and better dust cllectin efficiency. It is a reliable and prven technlgy which Can effectively handle large quantities f abrasive type fly ash withut any perating prblems. Energy saving aspect n this cntrlling measures is really challenging. Fr enhancement f ESP perfrmance, prmtin f new technlgy and develpment f electrnics & materials are in cntinuus prcess f imprvement.. This paper presents the basic technlgical and peratinal cncept f ESP and perfrmance enhancement prgrams/activities being taken in Thermal pwer generatin plants, which are in accrdance with the Envirnmental Regulatins f tday s wrld. Intrductin t Electrstatic Precipitatrs An electrstatic precipitatr is a large, industrial emissin-cntrl unit. It supersedes in many way t the previusly used bag filter system and is designed t trap and remve dust particles efficiently and effectively frm the exhaust gas stream f an industrial prcess. Precipitatrs are used in these industries: Pwer/Electric Cement Chemicals Metals Paper In many thermal pwer plant plants, fly ash generated in the cal cmbustin prcess is carried

2 as dust in the ht exhaust gases. These dust-laden gases are allwed t pass thrugh an electrstatic precipitatr that cllects mst f the dust. Cleaned gas then passes ut f the precipitatr and then thrugh a stack t the atmsphere. Precipitatrs typically cllect 99.9% r mre f the dust frm the gas stream. Precipitatrs functin by electr-statically charging the dust particles in the gas stream cming ut f the biler after cal cmbustin. The charged particles are then attracted t and depsited n the cllectr plates. When enugh dust has accumulated, the cllectrs are hammered t disldge the dust, causing it t fall with the frce f gravity t hppers belw. The dust is then remved by a cnveyr, slurry frm, vacuum trapped system etc. fr dispsal r recycling. Depending upn dust characteristics, cal cmpsitin ( primarily ash cntent ) and the gas vlume t be treated, there are many different sizes, types and designs f electrstatic precipitatrs. Very large pwer plants may actually have multiple precipitatrs fr each unit Basic Principles Electrstatic precipitatin remves particles frm the exhaust gas stream f an thermal pwer plant. Often the prcess invlves cmbustin, but it can be any industrial prcess that wuld therwise emit particles t the atmsphere. The fllwing prcesses are being carried ut inside ESP: Gas distributin Unifrmly distributing the gas inside ESP Inizatin - Charging f particles by crna pwer Migratin - Transprting the charged particles t the cllecting surfaces Cllectin - Precipitatin f the charged particles nt the cllecting surfaces Charge Dissipatin - Neutralizing the charged particles n the cllecting surfaces Particle Disldging - Remving the particles frm the cllecting surface t the hpper Particle Remval - Cnveying the particles frm the hpper t a dispsal pint The majr precipitatr cmpnents that accmplish these activities are as fllws: Flue Gas Cnditining Discharge/ emitting Electrdes Cllecting electrdes/ plates Pwer Cmpnents Precipitatr Cntrls / electrnic cntrller Rapping Systems Purge Air Systems The perfrmance efficiency f ESP depends upn the ptimizatin f all the abve factrs.

3 The abve figure shws the prcess f inizatin f ash particles inside the ESP fields. The figure belw shws the cnditin f particle charging and cllectin at the cllecting plate. DEUTSCH - ANDERSON EQUATION Cllectin Efficiency = 1 - e -w. SCA where, w = Migratin velcity

4 SCA = Specific Cllecting Area Migratin Velcity The velcity with which the dust particle travel twards the cllecting electrde under the influence f electric field. Unit is m/sec Specific cllecting area Amunt f cllecting area required t be prvided t cllect dust in gas flw rate f 1 m3/s. Unit : m2/m3/sec Operatin T maximize electrstatic precipitatr efficiency a vltage cntrller usually attempts t increase the electrical pwer delivered t the field. Hwever in sme cnditins a vltage cntrller must just maintain pwer at a cnstant level. Increased electrical pwer int the electrstatic precipitatr directly crrelates with better precipitatr perfrmance, but there is a limit. If t much vltages is applied fr a given cnditin, a spark ver will ccur. During a spark ver, precipitatr perfrmance in that field will drp t zer, rendering that field temprarily ineffective. T vercme the crippling effect that spark ver has t increasing the electrical pwer in the precipitatr field, spark respnse algrithms have been develped that will interrupt pwer upn detectin f a spark, then ramp pwer back up t a high level. These respnse algrithms can greatly influence verall precipitatr perfrmance The transfrmer-rectifier rating shuld be matched t the lad impsed by the electrical field r bus sectin. The pwer supply will perfrm best when the transfrmer-rectifiers perate at 70-90% f the rated capacity, withut excessive sparking. This reduces the maximum cntinuuslad vltage and crna pwer inputs. Practical perating vltages fr transfrmer-rectifiers depend n: Cllecting plate spacing Gas and dust cnditins Cllecting plate and discharge electrde gemetry Similarly, Crna current density shuld be in the range f ma/1000 ft2 f plate area. (Calculate this using secndary current divided by cllecting area f the electrical field r bus sectin.) The actual level depends upn: Lcatin f electrical field r bus sectin t be energized Cllecting plate area Gas and dust cnditins Cllecting electrde and discharge wire gemetry The right side figure is cnditin f crna discharge inside the

5 field. Precipitatr crna pwer is the useful electrical pwer applied t the flue gas stream t precipitate particles. Either precipitatr cllecting efficiency r utlet residual can be expressed as a functin f crna pwer in Watts/1000 acfm f flue gas, r in Watts/1000 ft f cllectin area Perfrmance Imprvements: Fr imprving ESP perfrmance, the fllwing majr aspects are t be lked int fr crrective actin. Optimizing Gas Velcity Distributin Efficient precipitatr perfrmance depends heavily upn having similar gas cnditins at the inlet f each electrical field and at the inlet f each gas passage f the electrical field. Unifrmity f gas velcity is als desirable t have a gd gas velcity distributin thrughut the precipitatr. Gas Velcity Distributin in a precipitatr can be custmized accrding t the design f the precipitatr and the characteristics f the dust particles. Traditinally, precipitatrs have been designed with unifrm gas velcity distributin thrugh the electrical fields, t avid high-velcity areas that wuld cause re-entrainment. While this is still a recmmended practice, there is an advantage in sme cases t develping a velcity prfile that brings mre particles clser t the hpper.. Gas distributin arrangement at the inlet f electrstatic precipitatr.

6 The wrn-ut screen used fr unifrm gas distributin Fr imprving the unifrm distributin f flue gas inside the ESP field, the fllwing aspects are t be emphasized. Adding/imprving gas flw cntrl devices in the inlet ductwrk Adding/imprving flw cntrl devices in the inlet f the precipitatr Adding/imprving flw cntrl devices in the utlet f the precipitatr Adding a rapping system t the flw cntrl devices (where applicable) Adding/imprving hpper baffles Eliminating air leakages int the precipitatr Optimizing Crna Pwer Precipitatr crna pwer is the useful electrical pwer applied t the flue gas stream t precipitate particles. Either precipitatr cllecting efficiency r utlet residual can be expressed as a functin f crna pwer. The separatin f particles frm the gas flw in an electrstatic precipitatr depends n the applied crna pwer. Crna pwer is the prduct f crna current and vltage. Current is needed t charge the particles. Vltage is needed t supprt an electrical field, which in turn transprts the particles t the cllecting plates. In the lwer range f cllecting efficiencies, relatively small increases in crna pwer result in substantial increases in cllecting efficiency.. Optimum cnditins depend upn the lcatin f the field (inlet, center and utlet), fly ash characteristics and cmpsitin and physical cnditins (cllecting plates and discharge wires). Crna pwer levels can be ptimized by adjusting r ptimizing the field vltage. This field vltages are autmatically determined by the intelligent electrnic cntrller by prperly studying the field status such as gas vlume, gas temperature, gas velcity etc. while in peratin. Accrdingly, apprpriate crna pwer is being delivered in the field t inize the ash particles which need t be cllected befre ging t stack.

7 The abve figure shws the intermittent charging f ESP field fr better perfrmance. Intermittent charging the ESP field is the best practice fr imprving the cllectin efficiency as well as the energy saving. It als helps the equipment damage against spark initiated inside the field. The abve figure shws the current and vltage pattern at different charge rati in the ESP field. The abve figure shws the effect f back crna which deterirates the ESP perfrmance. S, crrect intermittent charging reduces the frmatin f back crna. Optimal Designing f ESP Equipments: The bjectives f equipment imprvements are t ptimize crna pwer, reduce reentrainment, and ptimize gas velcity distributin inside the precipitatr. Sme imprtant tpics t cnsider when planning equipment imprvements include: 1. Precipitatr Size : When sizing the precipitatr, it is imprtant t prvide a crss-sectin that will maintain an acceptable gas velcity. It is als imprtant t prvide fr enugh ttal discharge wire length and cllecting plate area, s that the desired specific crna current and electrical field can be applied.

8 Hrizntal and vertical design cnsideratin f ESP t meet the required efficiency 2. Minimizing Spark When the vltage applied t the electrstatic precipitatr field is t high fr the cnditins at the time, a spark ver (r will ccur. Detrimentally high amunts f current can ccur during a spark ver if nt prperly cntrlled, which culd damage the fields. A vltage cntrller will mnitr the primary and secndary vltage and current f the circuit, and detect a spark ver cnditin. Once detected, the pwer applied t the field will be immediately cut ff r reduced, which will stp the spark. After a shrt amunt f time the pwer will be ramped back up, and the prcess will start ver. Hence the space between electrdes is t be ptimally designed and t be clsely mnitred while pening the field fr inspectin and maintenance. 3 Minimizing Particle Re-entrainment: Minimizing re-entrainment f dust particles is imprtant aspect fr imprvement f precipitatr efficiency. Mst precipitatr equipment affects the re-entrainment level. The deflecting plates t be placed prperly in the path f gas stream t divert it unifrmly inside the field. Cnsequently gas velcity remains unifrm thrughut the ESP field. Cmbustin Prcess Imprvements fr Pwer Plants Cmbustin prcess cnditins mainly affect the crna pwer level. The primary cntributrs t cmbustin prcess cnditins and their effects include: Fly Ash and Flue Gas Cnditining Flue gas and fly ash characteristics at the inlet define precipitatr peratin. The cmbinatin f flue gas analysis, flue gas temperature and fly ash chemistry prvides the base fr fly ash resistivity. Typically, fly ash resistivity invlves bth surface and vlume resistivity. As gas temperature increases, surface cnductivity decreases and vlume resistivity increases.

9 In lwer gas temperature ranges, surface cnductivity predminates. The current passing thrugh the precipitated fly ash layer is cnducted in a film f weak sulfuric acid n the surface f the particles. Frmatin f the acid film (frm SO 3 and H 2 O) is influenced by the surface chemistry f the fly ash particles. In higher gas temperature ranges, vlume cnductivity predminates. Current cnductin thrugh the bdies (vlume) f the precipitated fly ash particles is gverned by the ttal chemistry f the particles. Fly ash resistivity can be mdified (generally with the intent t reduce it) by injecting ne r mre f the fllwing upstream f the precipitatr: Sulfur trixide (SO 3 ) Ammnia (NH 3 ) Sulfur Trixide and Ammnia Cnditining Systems In mst cases, a sulfur trixide cnditining system is sufficient t reduce fly ash resistivity t an acceptable level. The surce f sulfur trixide can be liquid sulfur dixide, mlten elemental sulfur, r granulated sulfur. It is als pssible t cnvert native flue gas SO 2 t SO 3. In sme instances, ammnia alne has been prven a suitable cnditining agent. It frms an ammnia-based particulate t increase the space charge. The surce f ammnia may be liquid anhydrus r aqueus ammnia, r slid urea. Finally, sulfur trixide and ammnia may be used in cmbinatin. This slutin has been successful because it can lwer fly ash resistivity and als frm ammnia bisulfate. The latter increases the adhesin f particles, and thus reduces re-entrainment lsses. Many f the precipitatrs in peratin tday were sized and designed t meet perfrmance requirements that are far belw current requirements. Time has als taken its tll n the rbust machines built many years ag. Mst units can be upgraded, repaired r rebuilt t extend their life and imprve their perfrmance. Many design experiences are there t insure the mdificatins t meet the bjectives f tday's precipitatr perfrmance. Varieties f precipitatrs have been mdified t meet mre stringent perfrmance requirements. Rapping Systems Rappers are time-cntrlled systems prvided fr remving dust frm the cllecting plates and the discharge electrdes as well as fr gas distributin devices (ptinal) and fr hpper walls (ptinal). Checking the rapping frequency and crrect peratin f each rappers including in time maintenance enhances the value additin t the ESP perfrmance

10 Cnclusin Many f the precipitatrs in peratin tday were sized and designed t meet perfrmance requirements that are far belw current requirements. Time has als taken its tll n the rbust machines built many years ag. Mst units can be upgraded, repaired r rebuilt t extend their life and imprve their perfrmance. It has the design experience t insure that mdificatins meet the bjectives f tday's precipitatr peratrs. Hundreds f precipitatrs have been mdified t meet mre stringent perfrmance requirements. Units built in the 1960s and 70s can ften be mdified t prvide 15% t 20% mre cllecting surface withut increasing the ftprint f the unit. The size f Eurpean design precipitatrs can be increased 30% and mre. In energy saving pint f view, the respnse time f a typical SCR cntrlled transfrmer rectifier can be n faster than 8.33 millisecnds. Hwever, at the higher frequency perating level f SMPS cntrller, the respnse time can be as quick as 100 micrsecnds, an rder f magnitude quicker. This quicker respnse time allws the cntrl t reduce the shrt circuit inrush current created by arcing in the precipitatr. Shrt circuits created by arcs simply send current t grund and waste pwer. By reducing these arcs, pwer is cnserved. * * * Shared Experience f peratin and maintenance f ESP.