Why Do SCR Catalysts Deactivate? Poisoning, Temperature and Ammonia Slip
Poisoning, temperature and ammonia slip: sorting out the three causes and what to check first
01 · THREE CAUSES
Why do DeNOx catalysts lose performance?
A DeNOx catalyst loses performance as its active sites are blocked, covered or used up. U.S. EPA data list the causes of deactivation in selective catalytic reduction (SCR) catalysts as poisoning of active sites, thermal sintering at high temperature, plugging and fouling by ammonia-sulfur salts and particulate matter, and erosion from high gas velocity.[1] To make the causes easier to tell apart in the field, this article groups them into three lines: poisoning and physical degradation, temperature, and ammonia and ammonium salts.
As catalyst activity falls, NOx removal drops and ammonia slip rises. Once slip reaches the design value or the permitted level, new catalyst must be installed.[1] The earlier the cause is identified, the more room there is to delay replacement or handle it with partial replacement.
Normal catalystDegraded catalystFigure 1. Concept comparison of a normal catalyst module (left) and a catalyst module with dust and salts built up in its channels (right). Actual appearance and degree of deposition vary by plant.
Source: AI-assisted concept illustration
| Item | Poisoning and physical degradation | Temperature | Ammonia and ammonium salts |
|---|---|---|---|
| Typical causes | Dust erosion, channel plugging, deposits of alkalis (K, Na, Ca), heavy metals, sulfates and chlorides | Operating temperature falls below the catalyst's active range | Unreacted NH₃ reacts with SO₃, HCl and other species to form ammonium salts |
| Warning signs | Lower NOx removal, higher pressure drop across the catalyst bed | Lower NOx removal even at the same reagent dosing (most noticeable during startup and low load) | Lower NOx removal and higher slip; higher pressure drop and corrosion downstream |
| Reversible? | Alkali poisoning can be partly recovered by washing and regeneration. Thermal sintering is hard to reverse. | Often recovers once temperature is raised again, but prolonged operation at low temperature can lead to permanent deactivation | Often removed by thermal decomposition or washing |
| Main countermeasures | Upstream filtration, poison-resistant formulations, washing and regeneration | Operating temperature control, reheating, catalyst specification | Reagent dosing control, maintaining temperature, downstream cleaning |
The distinction that alkali poisoning can be partly recovered by washing and regeneration while sintering is hard to reverse follows a review paper.[3] The basis for the temperature and ammonium salt columns is explained in sections 3 and 4.
It depends on the cause. Ammonium salt deposits caused by low temperature, and deposits of water-soluble alkalis and sulfates, can sometimes be recovered by raising the temperature or by washing. Sintering and erosion, on the other hand, are hard to reverse, so identify the cause before deciding on replacement.
02 · POISONING
What causes poisoning, and how much can upstream filtration prevent?
Poisoning occurs when alkali, sulfur, chlorine and heavy metal species in the gas and dust attach to the catalyst's active sites or block its pores and interfere with the reaction. Particulate species can be reduced substantially by upstream filtration, but gaseous species are hard to stop by filtration. The key point is that upstream filtration alone does not put an end to poisoning.
SCR exposed directly to dust is damaged first
In a high-dust arrangement, where the SCR sits upstream of dust collection, dust erodes the catalyst surface and plugs the channels, raising pressure drop. On top of that, alkalis such as K, Na and Ca poison the active sites, and deposits of heavy metals, sulfates and chlorides make washing and regeneration necessary and shorten the replacement interval. Waste incinerator flue gas is cited as a typical case with high levels of alkali and alkaline earth metals.[3]
Alkalis degrade the catalyst along two main paths. One is chemical deactivation, in which the catalyst's acid sites are neutralized; the other is plugging and masking by built-up ammonium salts or ash. In vanadium-based catalysts, potassium is known to have the greatest effect.[3] Moving the SCR downstream of dust collection reduces the dust load, but the cooled flue gas may then need to be reheated.[1]
Particulate and gaseous species call for different measures
Placing equipment that removes dust first, such as a baghouse (bag filter), ahead of the catalyst reduces the amount of particulate species reaching the catalyst bed. Species that arrive as gas, by contrast, reach the catalyst bed depending on operating temperature, so catalyst formulation and temperature control have to deal with them.
| Item | Reduced by upstream filtration | Hard to stop by filtration |
|---|---|---|
| Typical species | Fly ash and fine dust, K, Na and Ca alkalis bound to dust, particulate heavy metals, unreacted sorbent, particulate salts (chlorides and sulfates) | Gaseous SO₂ and HCl, water vapor, gaseous heavy metals (such as mercury), NH₃, some fine sulfuric acid (SO₃) mist. Note: when a sorbent such as hydrated lime is injected as well, SO₂, SO₃ and HCl can also be reduced substantially in the filter cake. |
| Effect on the catalyst | Erosion of the catalyst bed, channel and pore blockage, poisoning by alkalis in the dust | Reach the catalyst bed depending on operating temperature and contribute to poisoning and ammonium salt formation |
| Main countermeasures | Upstream filtration, washing if needed | Poison-resistant catalyst formulation, operating temperature control |
Upstream filtration does not stop all alkalis. A study at a biofuel-fired plant reported a linear relationship between catalyst deactivation and exposure time to ultrafine alkali particles below 100 nm.[3] It is therefore safer to keep alkali concentration on the monitoring list even after filtration.
03 · TEMPERATURE WINDOW
What happens when operating temperature does not match the catalyst's active temperature?
When operating temperature is below the catalyst's active temperature, the reaction not only slows down but ammonium bisulfate (ABS) also condenses and blocks the catalyst pores, so NOx removal drops even further. Ammonium bisulfate is a salt formed when SO₃, moisture and NH₃ in the flue gas react.[2]
The optimum temperature varies with catalyst type and flue gas composition; EPA data give the optimum SCR temperature as about 250–427°C.[1] Flue gas leaving a waste incinerator baghouse, meanwhile, is typically around 120–180°C, while general-purpose vanadium-based catalysts are generally used at 200°C or higher. Even in tail-end arrangements, where acid gases are removed first, the flue gas is usually reheated to 170–200°C or higher for operation. There are two ways to close this gap: reheat the flue gas, or match the catalyst specification to the lower temperature. Reheating can raise operating costs significantly.[1]
Figure 2. Where baghouse operating temperature, catalyst active temperature and the ABS dew point sit relative to one another. Each range comes from a different source and is not a measurement at the same plant. Downstream of desulfurization, where SO₃ and NH₃ concentrations are low, the ABS dew point is much lower than shown. Values vary with catalyst type and flue gas composition.
Source: Baghouse operating temperature from Micro-one Co., Ltd. technical data; catalyst operating temperature from typical literature ranges; ABS dew point as reported in the literature[2]; optimum range from EPA[1]
ABS is reversible, but not if left too long
The literature reports the ABS dew point at about 280–320°C under high-sulfur flue gas conditions; downstream of desulfurization, where SO₃ and NH₃ concentrations are low, it is much lower. In tail-end SCR at waste incinerators, deposition mainly becomes a problem below roughly 200°C. Below the dew point, NH₃ and sulfuric acid vapor condense and coat the catalyst surface. The process is reversible: raising the temperature back above the dew point evaporates the deposit and activity returns. If operating temperature stays below the dew point for a long time, however, activity can change permanently.[2] Plants that operate in the low-temperature range should therefore build periodic reactivation into their plans.
04 · AMMONIA SLIP
Why does ammonia slip occur, and what does it foul?
Ammonia slip is injected ammonia that does not fully react with NOx and passes straight out with the exhaust gas. The main causes are reagent over-injection and loss of catalyst activity. EPA explains that slip can form ammonium sulfates downstream that plug or corrode equipment, and that it can be absorbed into fly ash and affect its disposal or reuse.[1]
Slip is both a cause and a result
When catalyst activity falls, slip rises, and the extra slip forms ammonium salts that foul the catalyst and downstream equipment again. To break this vicious circle, treat slip not just as an emission figure but also as an indicator of catalyst condition. Ammonium salts formed by the reaction of NH₃, SO₃ and HCl can deposit in the catalyst bed, depending on temperature and concentration.
Figure 3. Concept of the catalyst bed in an SCR reactor (left) and the inside of an enlarged catalyst channel (right). As ammonium salts build up on the channel walls, the gas flow path narrows. Actual deposit locations and amounts vary by plant.
Source: AI-assisted concept illustration
Over-injecting reagent to raise NOx removal increases chemical costs and lets unreacted ammonia escape, leading to downstream corrosion and secondary pollution. In SNCR, injection location and flue gas temperature distribution strongly influence slip, so nozzle locations and spray conditions are set by flow and temperature analysis, and injection location and dosing are controlled to follow changes in load and in furnace temperature and NOx. The principles of both methods and how to choose between them are covered in SNCR vs. SCR: What Is the Difference?.
05 · REDUCING DEGRADATION
What are the options for reducing deactivation?
There are four ways to reduce deactivation: upstream filtration, poison-resistant formulations, temperature control, and washing and regeneration. Which one works best depends on the cause of poisoning and the flue gas conditions.
- Upstream filtration: Reducing sulfur and dust beforehand reduces catalyst deactivation and downstream fouling.[1] It also lowers the burden from particulate alkalis and dust erosion.
- Poison-resistant formulations: Development continues toward catalyst formulations with better erosion and poison resistance, and toward improved geometry, pitch and layout. Research aimed at improving alkali resistance has reported using more strongly acidic supports or replacing vanadium with other metals.[3]
- Temperature control: Shorten the time the catalyst bed spends below the ABS dew point during startup, shutdown and low load. If needed, reheat or match the catalyst specification to the lower temperature.
- Washing and regeneration: Water washing or dilute sulfuric acid washing removes alkalis and sulfates. Dilute sulfuric acid removes alkalis effectively, but some of the active components, such as vanadium, can leach out and may need to be replenished.[3]
There is also a case from an SCR unit downstream of an incinerator. For catalyst operated for about 18,000 hours, water washing was reported to be more effective than 400°C heat treatment at restoring activity when removing ammonium and calcium sulfates, with only limited vanadium leaching.[3] This is the result from a single plant, so catalyst and deposit analysis is needed before applying it to other plants.
Water ordilute sulfuric acid
Figure 4. Concept of a catalyst module with deposits built up in its channels (left) and one with fewer deposits after washing and regeneration (right). Actual recovery depends on the cause of the deposits and the washing method, and some active components may be lost even after washing.
Source: AI-assisted concept illustration
There is also an entirely different approach: the catalytic filter bag approach, which places the catalyst inside the baghouse that removes dust, so dust collection and DeNOx are handled in one unit. Because it uses the existing baghouse without a separate SCR reactor, it is worth considering when installation space and equipment size are a constraint.
06 · CHECK ORDER
In what order should you check when performance drops?
Checks generally start with the items that cost least and can be checked without shutting the plant down: operating data trends, temperature history, pressure drop, gas and dust composition, and then catalyst sample analysis. The order below summarizes a typical check flow and varies with each plant's design conditions.
| Step | What to check | If you see this | Suspected cause |
|---|---|---|---|
| 1. Operating data | Inlet and outlet NOx, reagent dosing, NH₃ slip trends | Outlet NOx and slip rise together at the same dosing | Loss of catalyst activity or uneven NH₃/NOx distribution (including an injection grid check; cause identified in the next steps) |
| 2. Temperature history | Catalyst bed temperature, startup, shutdown and low-load periods | Long periods below the minimum design temperature | Ammonium salt deposits such as ABS |
| 3. Pressure drop | Across the catalyst bed and downstream equipment | Higher catalyst bed pressure drop / higher downstream pressure drop and corrosion | Channel plugging and deposits / ammonium salts from slip |
| 4. Upstream contaminants | Dust, alkalis (K, Na, Ca), SO₂, SO₃, HCl, moisture | Alkali or acid gas concentrations above design conditions | Poisoning |
| 5. Catalyst sample analysis | NOx conversion, NH₃ slip, N₂O, pressure drop, deposit composition | Alkalis or sulfates detected, lower conversion in activity tests | Confirms poison type and deposits |
| 6. Action | Higher-temperature operation, injection distribution adjustment, washing and regeneration, adding a spare layer or partial replacement | Cause and recoverability confirmed | Action chosen by cause |
At the action stage, a management plan that replaces the catalyst in portions rather than all at once helps, because replacement and disposal costs can then be spread evenly over the service life.[1]
FAQ
Frequently asked questions
How can I tell if a DeNOx catalyst has lost performance?
Suspect deactivation when, at the same reagent dosing, outlet NOx rises, ammonia slip increases and catalyst bed pressure drop changes along with them. A single indicator is not enough to identify the cause, so check temperature history and downstream equipment pressure drop as well.
Can catalyst performance lost to low temperature be recovered?
Yes. If the cause is ammonium bisulfate (ABS) deposits, raising the temperature above the dew point to decompose and evaporate them often restores activity. Long operation below the dew point, however, can lead to permanent deactivation, so a prompt response matters.
Does rising ammonia slip always point to a catalyst problem?
No. Besides loss of catalyst activity, slip also rises with reagent over-injection, uneven spray distribution and load swings. Check dosing control and spray conditions first, and if slip still rises, inspect the catalyst.
How do you decide between washing/regeneration and replacement?
By activity tests on catalyst samples and analysis of deposit composition. Alkali and sulfate deposits can sometimes be recovered by washing, but sintering and erosion are hard to reverse, and acid washing can leach out some of the active components.
Does filtering out dust upstream prevent all catalyst deactivation?
No. Particulate species are reduced substantially, but gaseous SO₂ and HCl, water vapor and NH₃ are hard to stop by filtration alone. Injecting a sorbent as well can also reduce acid gases substantially. These have to be handled through catalyst formulation and operating temperature control.
- U.S. EPA, "Air Pollution Control Technology Fact Sheet: Selective Catalytic Reduction (SCR)" (EPA-452/F-03-032), 2003.
- Zhu, Y. et al., "Ammonium-Salt Formation and Catalyst Deactivation in the SCR System for a Marine Diesel Engine", Catalysts 9(1), 21, 2019.
- Lisi, L., Cimino, S., "Poisoning of SCR Catalysts by Alkali and Alkaline Earth Metals", Catalysts 10(12), 1475, 2020.