[{"data":1,"prerenderedAt":502},["ShallowReactive",2],{"site-header-common":3,"site-footer-common":59,"site-navigation-common":86,"resources-blog:catalyst-masking-vs-poisoning-vs-pluggage":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":465,"extension":466,"meta":467,"navigation":468,"path":469,"primaryKeyword":131,"publishedAt":470,"secondaryKeywords":471,"seo":474,"sources":477,"stem":496,"summary":497,"updatedAt":470,"__hash__":501},"blog\u002Fresources\u002Fblog\u002Fcatalyst-masking-vs-poisoning-vs-pluggage.md","Catalyst masking vs poisoning vs pluggage: why your SCR is losing NOx efficiency","Sylio",{"type":118,"value":119,"toc":452},"minimark",[120,125,144,152,160,164,167,264,267,271,296,299,307,320,324,330,336,339,352,356,371,379,382,385,389,400,403,416,420,423,426,429,432,436,439,442,446,449],[121,122,124],"h2",{"id":123},"four-faults-one-falling-nox-margin","Four faults, one falling NOx margin",[126,127,128,132,133,138,139,143],"p",{},[129,130,131],"strong",{},"SCR catalyst masking vs poisoning"," is not a debate about terminology. It is a maintenance decision. A ",[134,135,137],"a",{"href":136},"\u002Fglossary\u002Fselective-catalytic-reduction","selective catalytic reduction"," reactor can lose ",[134,140,142],{"href":141},"\u002Fglossary\u002Fnox-reduction-efficiency","NOx reduction efficiency"," because ash blocks its channels, because a dry layer covers active surface, because sticky ammonium salts fill pores, or because contaminants have chemically deactivated the catalyst. Those faults can all push the plant towards more ammonia injection and less emissions margin, but they do not respond to the same remedy.",[126,145,146,147,151],{},"The common control-room symptom is deceptively simple. Outlet NOx starts to rise at the old reagent rate. Operators add ammonia to hold the setpoint, then ",[134,148,150],{"href":149},"\u002Fglossary\u002Fammonia-slip","ammonia slip"," rises. Eventually the unit reaches either its NOx limit or its slip limit. That sequence proves that usable SCR performance has fallen. It does not say why.",[126,153,154,155,159],{},"Diagnosis needs four kinds of evidence together: pressure drop, NOx control margin, ammonia slip and physical inspection. Load, inlet NOx, gas temperature and reagent distribution must also be comparable. A poorly balanced ",[134,156,158],{"href":157},"\u002Fglossary\u002Fammonia-injection-grid","ammonia injection grid",", a temperature excursion or a faulty analyser can imitate catalyst deactivation without any deposit or poison being present.",[121,161,163],{"id":162},"read-the-pattern-not-one-instrument","Read the pattern, not one instrument",[126,165,166],{},"The following table is a screening tool, not a substitute for inspection or catalyst sampling.",[168,169,170,192],"table",{},[171,172,173],"thead",{},[174,175,176,180,183,186,189],"tr",{},[177,178,179],"th",{},"Failure mode",[177,181,182],{},"Pressure drop",[177,184,185],{},"NOx control and ammonia slip",[177,187,188],{},"Inspection finding",[177,190,191],{},"Correct response",[193,194,195,213,230,247],"tbody",{},[174,196,197,201,204,207,210],{},[198,199,200],"td",{},"Dry ash pluggage",[198,202,203],{},"Clear rise, often concentrated across one layer or lane",[198,205,206],{},"More reagent may be needed; outlet distribution and slip become uneven",[198,208,209],{},"Bridged or blocked channels, coarse dry ash and overloaded open lanes",[198,211,212],{},"Remove the ash, correct flow or upstream ash handling, then prevent recurrence",[174,214,215,218,221,224,227],{},[198,216,217],{},"Dry ash surface masking",[198,219,220],{},"Usually less pronounced than bulk pluggage, but it can rise as pores restrict",[198,222,223],{},"Ammonia demand creeps up, slip rises, or achievable NOx removal falls",[198,225,226],{},"Fine dry coating on leading faces and channel walls, with cells mostly open",[198,228,229],{},"Remove the loose layer and keep fresh ash from settling",[174,231,232,235,238,241,244],{},[198,233,234],{},"Ammonium bisulphate fouling",[198,236,237],{},"Can rise as sticky salt and ash restrict pores or passages, often after low-temperature operation",[198,239,240],{},"Activity falls and slip can rise; the trend often follows load and temperature",[198,242,243],{},"Tacky, smeared or glazed deposits that bind ash rather than a loose powder",[198,245,246],{},"Correct temperature, ammonia distribution and SO3 conditions; water-wash equipment or use qualified offline catalyst regeneration",[174,248,249,252,255,258,261],{},[198,250,251],{},"Chemical poisoning",[198,253,254],{},"Often stable unless a separate physical deposit is also present",[198,256,257],{},"Persistent activity loss, reagent creep and rising slip at comparable duty",[198,259,260],{},"Catalyst may look clean and open; laboratory activity and chemistry identify the poison",[198,262,263],{},"Specialist regeneration where viable, otherwise replacement",[126,265,266],{},"Pressure drop is a useful first separator, not a verdict. A flow-normalised rise supports a restriction, and bulk pluggage usually gives the stronger change. Total reactor pressure can hide a local blockage, while masking can also raise resistance if it reaches pores. Stable pressure drop excludes neither masking nor poisoning, and high pressure drop does not identify the deposit.",[121,268,270],{"id":269},"_1-dry-ash-pluggage-blocks-the-gas-path","1. Dry ash pluggage blocks the gas path",[126,272,273,275,276,280,281,285,286,290,291,295],{},[129,274,200],{}," is ",[134,277,279],{"href":278},"\u002Fglossary\u002Fcatalyst-pluggage","catalyst pluggage"," caused when ",[134,282,284],{"href":283},"\u002Fglossary\u002Flarge-particle-ash","large-particle ash",", ",[134,287,289],{"href":288},"\u002Fglossary\u002Fpopcorn-ash","popcorn ash"," or accumulated fine ash bridges channel mouths and reduces the open area available to the flue gas. It is most likely in a ",[134,292,294],{"href":293},"\u002Fglossary\u002Fhigh-dust-low-dust-tail-end-scr","high-dust SCR arrangement",", especially where inlet flow is uneven, coarse-particle screens are damaged or horizontal surfaces shed piles onto the first layer.",[126,297,298],{},"The hydraulic signature is the useful one. Reactor or layer differential pressure rises, fan demand increases and the blocked lanes force more gas through the remaining cells. That redistribution reduces residence time in the open area and can misalign NOx with the ammonia profile. The plant may then see higher outlet NOx, higher ammonia demand and local slip, but those are consequences of the blocked flow rather than proof of chemical deactivation.",[126,300,301,302,306],{},"Inspection should find a physical obstruction: bridged channel mouths, mounds on the leading face, dry debris in a ",[134,303,305],{"href":304},"\u002Fglossary\u002Fhoneycomb-catalyst","honeycomb catalyst",", or ash lodged around module frames and support beams. Map the deposit against the inlet velocity pattern.",[126,308,309,310,314,315,319],{},"Loose, dry pluggage is cleanable. Recover blocked passages with the site's approved method, correct screens, turning vanes and ash removal, then prevent recurrence. A ",[134,311,313],{"href":312},"\u002Fglossary\u002Fsteam-sootblower","steam sootblower"," provides direct force; a ",[134,316,318],{"href":317},"\u002Fglossary\u002Fsonic-horn","sonic horn"," can keep friable ash mobile before it bridges. Neither fixes poor reactor flow, and acoustic cleaning should not be expected to rescue hard-packed channels.",[121,321,323],{"id":322},"_2-dry-surface-masking-hides-active-sites","2. Dry surface masking hides active sites",[126,325,326,329],{},[129,327,328],{},"Catalyst masking"," is also physical, but it is not bulk blockage. A fine, dry ash layer covers the catalyst surface or pore mouths while much of the passage remains open. Ammonia and NOx can still enter the cell, yet less gas reaches active material. The underlying chemistry may remain sound beneath the blanket.",[126,331,332,333,335],{},"Its pressure-drop change is usually less pronounced than bulk pluggage, although pore restriction may still raise it. At comparable load, temperature and inlet NOx, the more revealing trends are rising ammonia flow to hold the same outlet NOx, increasing slip, or falling NOx removal once the slip limit prevents further reagent addition. If the ",[134,334,158],{"href":157}," has already been checked and the hydraulic profile remains reasonable, surface coverage becomes a stronger suspect.",[126,337,338],{},"The inspection distinction is texture and location. Dry masking looks like a dusty film on leading faces and channel walls, with passages still visibly open. It may be heavier in low-velocity zones or behind structural shadows. A small deposit sample should remain powdery and friable. If it is tacky, glassy, wet or hard-bonded, it is not the cleanable dry masking discussed here.",[126,340,341,342,346,347,351],{},"Cleaning can restore access when the layer is loose ash. An ",[134,343,345],{"href":344},"\u002Fglossary\u002Facoustic-cleaning-system","acoustic cleaning system"," is best used as prevention after the face is sufficiently clean, with sound keeping new particulate from settling and consolidating. It is weak against a mature bonded skin. The separate guide to ",[134,348,350],{"href":349},"\u002Fresources\u002Fblog\u002Fscr-catalyst-cleaning-sootblowers-vs-acoustic-horns","SCR catalyst cleaning methods"," covers the choice between direct-force cleaning and acoustic prevention.",[121,353,355],{"id":354},"_3-ammonium-bisulphate-is-sticky-salt-fouling","3. Ammonium bisulphate is sticky salt fouling",[126,357,358,360,361,365,366,370],{},[129,359,234],{}," is not ordinary ash masking. ",[134,362,364],{"href":363},"\u002Fglossary\u002Fammonium-bisulphate","Ammonium bisulphate",", or ABS, forms when unreacted ammonia, SO3 and water meet under deposition conditions. Risk rises when SCR temperature falls during low-load operation, when local ammonia slip is high, or when ",[134,367,369],{"href":368},"\u002Fglossary\u002Fso2-so3-conversion","SO2 to SO3 conversion"," supplies more SO3. The salt coats active sites and pore structure, then captures fly ash into a sticky mass.",[126,372,373,374,378],{},"The trend often follows operating temperature more closely than ash loading. Pressure drop may rise across the catalyst if pores and channels become restricted, but the downstream ",[134,375,377],{"href":376},"\u002Fglossary\u002Fair-heater","air heater"," can show the clearer increase because its colder surfaces favour deposition. NOx activity and ammonia slip can deteriorate at the same time. That combination can create a loop: lower activity produces more slip, and more slip supplies more reagent for ammonium salt formation.",[126,380,381],{},"Inspection should find tacky, smeared, glazed or compacted deposits rather than a loose dry blanket. Review the preceding load and temperature history, not only conditions at the time of inspection. A low-temperature period can seed ABS before the pressure-drop response becomes obvious.",[126,383,384],{},"ABS requires temperature and chemistry management. Keep the reactor within its approved operating window, correct ammonia maldistribution, limit slip and address SO3 conditions. For ABS-fouled air-heater surfaces, a planned water wash is an established removal route. Catalyst modules need a qualified offline washing or regeneration process with controlled rinsing, drying and activity testing. Soluble salts can be removed, but metal sulphation or active-component loss can prevent full recovery. Acoustic energy does not dissolve a sticky salt. Sonic horns may still move separate dry ash elsewhere in the reactor, but they are not an ABS remedy.",[121,386,388],{"id":387},"_4-chemical-poisoning-removes-active-chemistry","4. Chemical poisoning removes active chemistry",[126,390,391,394,395,399],{},[129,392,393],{},"Catalyst poisoning"," occurs when arsenic, sodium, potassium, phosphorus, lead or other contaminants react with or occupy active sites. This is ",[134,396,398],{"href":397},"\u002Fglossary\u002Fcatalyst-poisoning","SCR catalyst deactivation"," at the chemical level. A physically open catalyst can therefore lose reaction rate without a corresponding rise in pressure drop.",[126,401,402],{},"The operating pattern resembles masking: more ammonia is required for the same NOx result, slip rises, and the available control margin contracts. The decisive differences appear after physical deposits have been ruled out. Cleaning the surface does not recover performance, the decline may follow a fuel, raw-material or sorbent change, and laboratory testing finds low relative activity with the relevant chemical contamination. Visual inspection alone cannot clear or convict poisoning because the element may look normal.",[126,404,405,406,410,411,415],{},"Chemical poisoning is not reversible by cleaning. Air lancing, sootblowing and acoustic cleaning cannot restore neutralised active sites. A contaminant-specific chemical wash, where qualified, belongs to specialist regeneration rather than routine cleaning, and recovery must be verified. A specialist should test representative elements from each ",[134,407,409],{"href":408},"\u002Fglossary\u002Fcatalyst-layer-module","catalyst layer or module"," for the ",[134,412,414],{"href":413},"\u002Fglossary\u002Fcatalyst-regeneration-vs-replacement","catalyst regeneration or replacement"," decision. If regeneration cannot recover enough activity, the affected layer must be replaced.",[121,417,419],{"id":418},"a-practical-diagnostic-sequence","A practical diagnostic sequence",[126,421,422],{},"Start with normalised trends. Compare pressure drop, inlet and outlet NOx, ammonia flow, slip, gas temperature and load over equivalent operating points. An emissions number alone hides whether the controller is spending more reagent to maintain it.",[126,424,425],{},"Next, locate the hydraulic change. Layer-level pressure measurements are more useful than one total reactor value, which can hide local blockage. A sharp or lane-specific increase supports dry pluggage. A smaller change shifts attention towards masking, poisoning, distribution or measurement, but it does not settle the diagnosis.",[126,427,428],{},"Then align the event history. A rise after an ash carry-over event, damaged screen or flow change supports pluggage. A gradual reagent creep with a dusty but open face supports masking. Deterioration after prolonged low-temperature duty, accompanied by sticky deposits or air-heater pressure rise, supports ABS. A sustained loss after a fuel or feedstock chemistry change, with clean passages, supports poisoning.",[126,430,431],{},"Finally, inspect and sample. Photograph each module position, record blocked-cell coverage, note whether material is powdery or sticky, and send representative catalyst and deposit samples for analysis. Paired activity tests before and after controlled deposit removal show whether cleanable coverage hid healthy material; surface or bulk chemistry and microscopy identify poisoning. Only then should the plant choose cleaning, process correction, regeneration or replacement.",[121,433,435],{"id":434},"where-acoustic-cleaning-fits","Where acoustic cleaning fits",[126,437,438],{},"Acoustic cleaning has two legitimate roles in this diagnosis: preventing dry ash from bridging open passages and limiting a loose, dry surface mask. Both depend on the deposit being friable, weakly bonded and accessible to the sound field. The method works best after recovery cleaning, before the next layer has had time to consolidate.",[126,440,441],{},"It is weak or useless on sticky, wet, molten, sintered or hard-bonded deposits. It does not replace sootblowers where direct force is required, does not replace water washing for deposited ABS, and has no effect on completed chemical poisoning. Those limits are not edge cases. They are the boundary between a cleaning application and the wrong diagnosis.",[121,443,445],{"id":444},"the-bottom-line","The bottom line",[126,447,448],{},"An SCR losing NOx efficiency is reporting a symptom, not naming its failure mode. Rising pressure drop plus visibly blocked channels points to physical ash pluggage. Lost activity with mostly open passages and a dry surface film points to masking. A low-temperature history and sticky ammonium salt deposit point to ABS. Persistent activity loss with normal hydraulics and laboratory evidence points to chemical poisoning.",[126,450,451],{},"Match the response just as strictly. Clean and prevent dry ash pluggage or dry surface masking. Manage ABS through temperature, ammonia and SO3 control, water-wash affected air-heater surfaces, and qualify offline catalyst regeneration. Send chemically poisoned catalyst for regeneration assessment or replace it. If those four modes stay separate, pressure drop, ammonia slip and inspection evidence become a workable diagnosis instead of four reasons to turn the same cleaner up.",{"title":453,"searchDepth":454,"depth":454,"links":455},"",2,[456,457,458,459,460,461,462,463,464],{"id":123,"depth":454,"text":124},{"id":162,"depth":454,"text":163},{"id":269,"depth":454,"text":270},{"id":322,"depth":454,"text":323},{"id":354,"depth":454,"text":355},{"id":387,"depth":454,"text":388},{"id":418,"depth":454,"text":419},{"id":434,"depth":454,"text":435},{"id":444,"depth":454,"text":445},"Diagnose SCR catalyst pluggage, dry ash masking, ammonium bisulphate fouling and poisoning from pressure drop, slip and inspection evidence.","md",{},true,"\u002Fresources\u002Fblog\u002Fcatalyst-masking-vs-poisoning-vs-pluggage","2026-07-09",[398,279,472,473],"SCR losing NOx efficiency","catalyst poisoning",{"title":475,"description":476},"SCR catalyst masking vs poisoning vs pluggage","Use pressure drop, ammonia slip and inspection findings to distinguish SCR catalyst pluggage, dry masking, ABS fouling and chemical poisoning.",[478,481,484,487,490,493],{"title":479,"url":480},"US EPA - Air Pollution Control Cost Manual, Chapter 2: Selective Catalytic Reduction","https:\u002F\u002Fwww.epa.gov\u002Fsites\u002Fdefault\u002Ffiles\u002F2017-12\u002Fdocuments\u002Fscrcostmanualchapter7thedition_2016revisions2017.pdf",{"title":482,"url":483},"Oxford Academic - NOx control for high-ash coal-fired power plants in India","https:\u002F\u002Facademic.oup.com\u002Fce\u002Farticle\u002F3\u002F1\u002F24\u002F5108520",{"title":485,"url":486},"POWER - Reducing SCR Fly Ash Accumulation with Improved Reactor Inlet Airflow","https:\u002F\u002Fwww.powermag.com\u002Freducing-scr-fly-ash-accumulation-with-improved-reactor-inlet-airflow\u002F",{"title":488,"url":489},"Catalysts - Heterogeneous Catalyst Deactivation and Regeneration: A Review","https:\u002F\u002Fwww.mdpi.com\u002F2073-4344\u002F5\u002F1\u002F145",{"title":491,"url":492},"Scientific Reports - Status and development for detection and control of ammonium bisulfate","https:\u002F\u002Fwww.nature.com\u002Farticles\u002Fs41598-021-90040-w",{"title":494,"url":495},"Catalysts - Deactivation and regeneration of a tail-end SCR catalyst","https:\u002F\u002Fwww.mdpi.com\u002F2073-4344\u002F9\u002F5\u002F464","resources\u002Fblog\u002Fcatalyst-masking-vs-poisoning-vs-pluggage",[498,499,500],"A rising layer pressure drop points strongly to physical pluggage; lost NOx margin with little hydraulic change points towards masking or poisoning.","Ammonia slip shows that usable catalyst activity or distribution has deteriorated, but it cannot identify the failure mode on its own.","Clean only dry ash pluggage and surface masking. Manage sticky ABS with temperature and water washing, and poisoned catalyst with regeneration or replacement.","8V4MEC_YKloUfa503d4SDYNwf2PhpnDSZaT5EP4cd3Y",1784564594424]