[{"data":1,"prerenderedAt":473},["ShallowReactive",2],{"site-header-common":3,"site-footer-common":59,"site-navigation-common":86,"resources-blog:esp-back-corona-high-resistivity-ash":113},{"id":4,"extension":5,"footer":6,"meta":50,"navbar":51,"stem":57,"__hash__":58},"common\u002Fcommon.yml","yml",{"tagline":7,"legalTitle":8,"cookieSettingsLabel":9,"links":10,"sections":11},"Acoustic cleaning intelligence for industrial fouling, soot, ash, dust and build-up.","Legal","Cookie preferences",[],[12,24,36,48],{"title":13,"links":14},"Product",[15,18,21],{"label":16,"to":17},"How it works","\u002F#how-it-works",{"label":19,"to":20},"Application fit","\u002F#fit",{"label":22,"to":23},"ROI calculator \u002F Coming soon","\u002F#economics",{"title":25,"links":26},"Applications",[27,30,33],{"label":28,"to":29},"Boilers and heat transfer","\u002Fapplications\u002Fboilers-heat-transfer",{"label":31,"to":32},"SCR catalyst cleaning","\u002Fapplications\u002Fscr-catalyst-cleaning",{"label":34,"to":35},"Dust collection","\u002Fapplications\u002Fdust-collection",{"title":37,"links":38},"Resources",[39,42,45],{"label":40,"to":41},"Blog","\u002Fresources\u002Fblog",{"label":43,"to":44},"Glossary","\u002Fglossary",{"label":46,"to":47},"Buyer's guide","\u002Fresources\u002Fblog\u002Facoustic-cleaning-system",{"title":8,"links":49},[],{},{"links":52,"action":56},[53,55],{"label":25,"to":54},"\u002F#applications",{"label":16,"to":17},{"label":22,"to":23},"common","rzKP9GVWGTSjN3RTtCIYeR1oX7iQIvlngM7hHP108rc",{"id":4,"extension":5,"footer":60,"meta":80,"navbar":81,"stem":57,"__hash__":58},{"tagline":7,"legalTitle":8,"cookieSettingsLabel":9,"links":61,"sections":62},[],[63,68,73,78],{"title":13,"links":64},[65,66,67],{"label":16,"to":17},{"label":19,"to":20},{"label":22,"to":23},{"title":25,"links":69},[70,71,72],{"label":28,"to":29},{"label":31,"to":32},{"label":34,"to":35},{"title":37,"links":74},[75,76,77],{"label":40,"to":41},{"label":43,"to":44},{"label":46,"to":47},{"title":8,"links":79},[],{},{"links":82,"action":85},[83,84],{"label":25,"to":54},{"label":16,"to":17},{"label":22,"to":23},{"id":4,"extension":5,"footer":87,"meta":107,"navbar":108,"stem":57,"__hash__":58},{"tagline":7,"legalTitle":8,"cookieSettingsLabel":9,"links":88,"sections":89},[],[90,95,100,105],{"title":13,"links":91},[92,93,94],{"label":16,"to":17},{"label":19,"to":20},{"label":22,"to":23},{"title":25,"links":96},[97,98,99],{"label":28,"to":29},{"label":31,"to":32},{"label":34,"to":35},{"title":37,"links":101},[102,103,104],{"label":40,"to":41},{"label":43,"to":44},{"label":46,"to":47},{"title":8,"links":106},[],{},{"links":109,"action":112},[110,111],{"label":25,"to":54},{"label":16,"to":17},{"label":22,"to":23},{"id":114,"title":115,"author":116,"body":117,"description":431,"extension":432,"meta":433,"navigation":434,"path":435,"primaryKeyword":126,"publishedAt":436,"secondaryKeywords":437,"seo":442,"sources":445,"stem":467,"summary":468,"updatedAt":436,"__hash__":472},"blog\u002Fresources\u002Fblog\u002Fesp-back-corona-high-resistivity-ash.md","ESP back corona and high-resistivity ash: why your precipitator loses collection","Sylio",{"type":118,"value":119,"toc":420},"minimark",[120,144,151,156,173,176,232,235,239,252,259,267,270,273,277,280,288,296,304,308,321,329,332,350,354,357,384,387,391,404,407,410,414,417],[121,122,123,127,128,133,134,138,139,143],"p",{},[124,125,126],"strong",{},"ESP back corona"," is one of the few precipitator faults in which more indicated current can accompany worse collection. The transformer-rectifier set appears electrically active, yet useful voltage is constrained, particle charging weakens and stack ",[129,130,132],"a",{"href":131},"\u002Fglossary\u002Fopacity","opacity"," or ",[129,135,137],{"href":136},"\u002Fglossary\u002Fparticulate-matter","particulate matter"," rises. That apparent contradiction is a clue: the current is passing through a high-resistivity ash layer in a way that works against the ",[129,140,142],{"href":141},"\u002Fglossary\u002Felectrostatic-precipitator","electrostatic precipitator",".",[121,145,146,147,143],{},"This is an ash chemistry and temperature problem. It is not a plate-cleaning problem, and acoustic cleaning does not fix it. That distinction separates back corona from hopper accumulation, rapper-related release and inlet deposits, which are covered in the ",[129,148,150],{"href":149},"\u002Fresources\u002Fblog\u002Fesp-hopper-ash-buildup-and-re-entrainment","ESP hopper ash buildup and re-entrainment diagnostic",[152,153,155],"h2",{"id":154},"the-ash-resistivity-window-is-indicative-not-universal","The ash resistivity window is indicative, not universal",[121,157,158,162,163,167,168,172],{},[129,159,161],{"href":160},"\u002Fglossary\u002Fresistivity","Resistivity"," describes how strongly the collected dust layer opposes electrical conduction. Once a charged particle reaches a ",[129,164,166],{"href":165},"\u002Fglossary\u002Fcollecting-electrode","collecting electrode",", some charge must drain through the layer to the grounded plate. If it drains too quickly, the particle is held weakly and can return to the gas by ",[129,169,171],{"href":170},"\u002Fglossary\u002Fre-entrainment","re-entrainment",". If it drains too slowly, charge and voltage build across the layer.",[121,174,175],{},"Published thresholds do not line up perfectly. The US EPA cost manual associates loose holding and severe re-entrainment with resistivity below about 10⁸ Ω cm, and says back corona is usually prevalent above about 2 × 10¹¹ Ω cm. US government training material and recent journal work describe roughly 10⁸ to 10¹¹ Ω cm as a broadly workable range. Other studies use different boundaries because ash composition, test method, temperature, moisture and gas chemistry differ.",[177,178,179,195],"table",{},[180,181,182],"thead",{},[183,184,185,189,192],"tr",{},[186,187,188],"th",{},"Indicative resistivity at ESP conditions",[186,190,191],{},"Likely behaviour",[186,193,194],{},"Main risk",[196,197,198,210,221],"tbody",{},[183,199,200,204,207],{},[201,202,203],"td",{},"Below about 10⁸ Ω cm",[201,205,206],{},"Charge drains readily",[201,208,209],{},"Weak adhesion and re-entrainment",[183,211,212,215,218],{},[201,213,214],{},"About 10⁸ to 10¹⁰ or 10¹¹ Ω cm",[201,216,217],{},"Charge drains without collapsing plate hold",[201,219,220],{},"Generally favourable collection",[183,222,223,226,229],{},[201,224,225],{},"Above about 10¹¹ Ω cm",[201,227,228],{},"Charge drains slowly through the ash layer",[201,230,231],{},"Restricted voltage, sparking and back corona",[121,233,234],{},"These are diagnostic bands, not acceptance limits. A resistivity result is incomplete without the test temperature, water vapour concentration, gas composition and method. A dry laboratory sample can behave very differently from freshly collected ash in its actual flue gas.",[152,236,238],{"id":237},"how-back-corona-forms","How back corona forms",[121,240,241,242,246,247,251],{},"In normal negative ",[129,243,245],{"href":244},"\u002Fglossary\u002Fcorona-discharge","corona discharge",", the ",[129,248,250],{"href":249},"\u002Fglossary\u002Fdischarge-electrode","discharge electrodes"," generate negative ions. Those ions charge suspended particles, and the electric field drives them towards the grounded plates. The resulting ion current then passes through the collected ash layer.",[121,253,254,255,143],{},"With high-resistivity fly ash, that current produces a growing voltage drop across the layer. When the local field becomes strong enough, gas in pores or cracks within the ash breaks down. The dust surface begins generating positive ions, opposite to the ions used for normal charging. This reverse ionisation is ",[129,256,258],{"href":257},"\u002Fglossary\u002Fback-corona","back corona",[121,260,261,262,266],{},"The positive ions move into the inter-electrode space, neutralise part of the negative particle charge and distort the useful field. Particle migration towards the plate slows. Severe back corona can also force the controls to hold a lower secondary voltage or create excessive sparking. The result is lower ",[129,263,265],{"href":264},"\u002Fglossary\u002Fcollection-efficiency","collection efficiency",", even though secondary current may rise sharply after the back-corona onset point.",[121,268,269],{},"That last behaviour matters during diagnosis. Low voltage with high or rapidly increasing current is not evidence that useful corona power has improved. On a voltage-current curve, back corona can appear as a knee where voltage stops rising normally while current continues to increase. Under constant-current control, falling voltage and power may be the clearer symptom. The current beyond the onset point includes reverse ion activity that does not help collect ash.",[121,271,272],{},"Back corona should not be confused with corona suppression, sometimes called space-charge quenching. A very high inlet dust concentration can consume available ions and restrict current before particles are adequately charged. That condition more often presents as high voltage with unusually low current in the heavily loaded inlet field. Back corona originates in the resistive layer on a collecting plate and can produce abnormally high current at constrained voltage. Both reduce particle migration, and both can occur in the same precipitator, but they call for different first actions. Reducing inlet loading or correcting flow distribution addresses suppression. Changing resistivity addresses back corona.",[152,274,276],{"id":275},"why-temperature-sulphur-and-moisture-change-resistivity","Why temperature, sulphur and moisture change resistivity",[121,278,279],{},"Fly ash conducts through two broad paths. Surface conduction depends on material adsorbed onto the particles, especially water and acidic species. Volume conduction occurs through the particle material itself and becomes more important at high temperature. Their combined effect often produces a resistivity curve that rises to a peak and then falls, rather than moving in one direction across the whole temperature range.",[121,281,282,283,287],{},"For coal fly ash, government training material places a common high-resistivity region at roughly 120 to 175°C. A study of 60 industrial ash samples reported resistivity maxima across roughly 100 to 200°C. Those figures illustrate the shape of the curve, not a universal temperature window. Mineral composition and flue gas chemistry can move the peak substantially, so the distinction between a ",[129,284,286],{"href":285},"\u002Fglossary\u002Fhot-side-esp-cold-side-esp","hot-side and cold-side ESP"," matters.",[121,289,290,291,295],{},"Sulphur affects the surface path. Low-sulphur fuel often produces less SO3 available to adsorb on the ash, which can raise resistivity in a cold-side ESP. Fuel sulphur alone does not predict the result. Furnace conditions, ",[129,292,294],{"href":293},"\u002Fglossary\u002Fso2-so3-conversion","SO2 to SO3 conversion"," across an SCR catalyst, ash alkalinity, air-heater behaviour and upstream capture all influence how much conditioning species reaches the precipitator.",[121,297,298,299,303],{},"Moisture can also lower surface resistivity, particularly at cooler conditions. But lowering temperature or adding water is not automatically safe. Moving metal surfaces towards the ",[129,300,302],{"href":301},"\u002Fglossary\u002Facid-dew-point","acid dew point"," can introduce sulphuric acid condensation, corrosion, sticky ash and downstream handling problems. The correct temperature is therefore an engineered operating range based on the measured ash curve and dew-point margin.",[152,305,307],{"id":306},"diagnose-the-electrical-problem-before-choosing-a-remedy","Diagnose the electrical problem before choosing a remedy",[121,309,310,311,315,316,320],{},"Start with the emissions outcome, then work upstream. EPA guidance identifies outlet PM, opacity, secondary voltage, secondary current and corona power as primary ESP performance indicators. Trend them by load and fuel condition. Opacity is useful for timing, but it is affected by particle size and optical properties, so it is not a direct substitute for ",[129,312,314],{"href":313},"\u002Fglossary\u002Fmass-loading","mass loading",". Where fitted, a PM ",[129,317,319],{"href":318},"\u002Fglossary\u002Fcems","continuous emissions monitoring system"," gives a stronger emissions measure.",[121,322,323,324,328],{},"Next, compare every ",[129,325,327],{"href":326},"\u002Fglossary\u002Fesp-field-bus-section","field and bus section",", not only the total power. Record secondary voltage, current and spark rate at stable load, then obtain voltage-current curves under a controlled procedure. A similar change across several fields following a fuel or temperature change supports a resistivity diagnosis. One section with repetitive arcing, no voltage or an abnormal response can instead indicate a close clearance, broken electrode, dirty insulator or another local fault.",[121,330,331],{},"Measure ash resistivity at representative gas temperature and moisture where possible. If testing is done in a laboratory, reproduce the intended flue gas conditions and document sample history. Compare the result with fuel sulphur, ash chemistry, inlet temperature, moisture, SO3 availability and the timing of the performance loss. A number measured on dry stored ash is not enough to specify conditioning.",[121,333,334,335,339,340,344,345,349],{},"Finally, rule out mechanical and gas-flow causes. A failed ",[129,336,338],{"href":337},"\u002Fglossary\u002Fesp-rapper","ESP rapper",", high ",[129,341,343],{"href":342},"\u002Fglossary\u002Fesp-hopper","ESP hopper",", electrode misalignment, poor gas distribution or ",[129,346,348],{"href":347},"\u002Fglossary\u002Fsneakage","sneakage"," can also reduce collection or raise opacity. A rapper-correlated puff points towards dust release. A back-corona voltage-current signature points towards electrical breakdown in the ash layer. Similar stack symptoms do not make the faults interchangeable.",[152,351,353],{"id":352},"what-actually-fixes-high-resistivity-ash","What actually fixes high-resistivity ash",[121,355,356],{},"The corrective action must change ash conductivity, move the process away from the damaging part of its resistivity curve, or provide more collection margin.",[358,359,360,367,373],"ul",{},[361,362,363,366],"li",{},[124,364,365],{},"Flue gas conditioning."," SO3 or sulphuric acid conditioning can create a more conductive surface on high-resistivity fly ash. Water, sodium compounds and other agents are used for some duties. Selection and dose require representative testing and a controlled plant trial. More reagent is not automatically better, because excess acid species can increase corrosion, acid mist, fouling or ash-handling difficulty.",[361,368,369,372],{},[124,370,371],{},"Temperature adjustment."," Cooling and humidification may lower surface resistivity and reduce gas volume when there is safe margin above condensation limits. Moving to genuinely hot-side operation can lower resistivity through volume conduction, but a hot-side ESP is a major process and equipment arrangement, not a small set-point change. Temperature, dew point, gas volume and material limits must be assessed together.",[361,374,375,378,379,383],{},[124,376,377],{},"More effective collecting area."," Increasing ",[129,380,382],{"href":381},"\u002Fglossary\u002Fspecific-collection-area","specific collection area",", adding a field or reducing gas flow provides more residence time and plate area for the reduced particle migration rate. This can recover collection margin, but it compensates for the difficult duty rather than changing ash chemistry.",[121,385,386],{},"Modern power controls, intermittent energisation or pulse systems can sometimes operate closer to the useful voltage while limiting back-corona current. They are mitigation measures, not proof that the resistivity problem has disappeared. The acceptance test remains improved field electrical behaviour together with lower outlet PM or opacity under comparable load and fuel conditions.",[152,388,390],{"id":389},"where-acoustic-cleaning-does-not-apply","Where acoustic cleaning does not apply",[121,392,393,394,398,399,403],{},"A ",[129,395,397],{"href":396},"\u002Fglossary\u002Fsonic-horn","sonic horn"," changes none of the variables that create back corona. It does not lower resistivity, add SO3 or moisture, move the gas temperature, restore useful voltage, or add collecting area. An ",[129,400,402],{"href":401},"\u002Fglossary\u002Facoustic-cleaner","acoustic cleaner"," therefore cannot fix ESP back corona or high-resistivity ash, and it never replaces the plate or discharge-electrode rappers.",[121,405,406],{},"Its legitimate ESP role is narrower. Acoustic cleaning can help prevent dry, friable, loosely bonded ash from accumulating in hoppers and on suitable inlet-side gas-distribution devices, provided the released material has a clear discharge route. It is weak or useless against sticky, wet, molten, sintered or hard-bonded deposits.",[121,408,409],{},"Never install a horn on the outlet field. Acoustic energy there can release collected dust where little or no downstream collection length remains, worsening re-entrainment and opacity. If the evidence points to back corona, adding acoustic energy is the wrong intervention regardless of mounting position.",[152,411,413],{"id":412},"the-bottom-line","The bottom line",[121,415,416],{},"ESP back corona begins inside a high-resistivity ash layer, where electrical breakdown creates reverse-polarity ions and reduces useful particle charging. Temperature, moisture, sulphur chemistry and ash composition determine the risk, so diagnose it with representative resistivity data, field-by-field electrical behaviour and emissions trends.",[121,418,419],{},"The fixes are flue gas conditioning, a justified temperature change, more collection margin or an appropriate electrical mitigation. Acoustic cleaning does not belong on that list. Keep its ESP role to suitable dry-ash hoppers and inlet devices, and keep horns away from the outlet field.",{"title":421,"searchDepth":422,"depth":422,"links":423},"",2,[424,425,426,427,428,429,430],{"id":154,"depth":422,"text":155},{"id":237,"depth":422,"text":238},{"id":275,"depth":422,"text":276},{"id":306,"depth":422,"text":307},{"id":352,"depth":422,"text":353},{"id":389,"depth":422,"text":390},{"id":412,"depth":422,"text":413},"High-resistivity ash can cause ESP back corona, lower collection and raise opacity. Learn how to diagnose it and which chemistry and design fixes work.","md",{},true,"\u002Fresources\u002Fblog\u002Fesp-back-corona-high-resistivity-ash","2026-07-20",[438,439,440,441],"high resistivity fly ash","precipitator collection efficiency","flue gas conditioning","ESP performance loss",{"title":443,"description":444},"ESP back corona and high-resistivity ash","Diagnose ESP back corona caused by high-resistivity ash. See how temperature, moisture and SO3 affect collection, and which fixes work.",[446,449,452,455,458,461,464],{"title":447,"url":448},"US EPA: Monitoring by Control Technique, Electrostatic Precipitators","https:\u002F\u002Fwww.epa.gov\u002Fair-emissions-monitoring-knowledge-base\u002Fmonitoring-control-technique-electrostatic-precipitators",{"title":450,"url":451},"US EPA: Air Pollution Control Cost Manual, Chapter 3, Electrostatic Precipitators","https:\u002F\u002Fwww.epa.gov\u002Fsites\u002Fdefault\u002Ffiles\u002F2020-07\u002Fdocuments\u002Fcs6ch3.pdf",{"title":453,"url":454},"AirKnowledge: Air Pollution Control, Particulate Matter","https:\u002F\u002Fairknowledge.gov\u002FILT\u002FPERM242\u002FCurrent\u002FCI\u002F03PERM242_Handout_PM_Control.pdf",{"title":456,"url":457},"Journal of Scientific and Industrial Research: Studies on Factors Influencing Fly Ash Resistivity from Electrostatic Precipitator with Reference to India","https:\u002F\u002Fnopr.niscpr.res.in\u002Fbitstream\u002F123456789\u002F12605\u002F1\u002FJSIR%2070%289%29%20795-803.pdf",{"title":459,"url":460},"Journal of Aerosol Science: An Experimental Study on the Performance of a Single Discharge Wire-Plate Electrostatic Precipitator with Back Corona","https:\u002F\u002Fdoi.org\u002F10.1016\u002FS0021-8502(98)00064-0",{"title":462,"url":463},"Fuel: Measurement and Prediction of Fly Ash Resistivity over a Wide Range of Temperature","https:\u002F\u002Fdoi.org\u002F10.1016\u002Fj.fuel.2017.12.047",{"title":465,"url":466},"US Department of Energy OSTI: Particulate Control Highlights, Research on Electrostatic Precipitator Technology","https:\u002F\u002Fwww.osti.gov\u002Fbiblio\u002F6982696","resources\u002Fblog\u002Fesp-back-corona-high-resistivity-ash",[469,470,471],"Back corona is electrical breakdown inside a high-resistivity ash layer. Reverse-polarity ions reduce useful particle charging and raise emissions.","Resistivity depends on ash chemistry and flue gas temperature, moisture and SO3, so representative testing and field-by-field electrical data matter.","Fixes change chemistry, temperature or collection margin. Acoustic cleaning cannot fix back corona and belongs only at dry-ash hoppers or the inlet side.","LLSS2sX2ZLzgboc71IFVNwcIpDX_Gar-xRAgs1fSBWs",1784564593968]