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Common Causes of Excessive NOₓ Emissions from SCR DeNOx Catalysts and Solutions
Release time:
Apr 24,2026
Selective Catalytic Reduction (SCR) is the core technology for ensuring nitrogen oxide emissions from coal-fired power plants and industrial furnaces meet regulatory standards. In actual operation, excessive NOₓ emissions are rarely caused by a single factor; they are typically the result of a combination of catalyst performance degradation, system misalignment, and adverse flue gas conditions. This article identifies the most common causes of non-compliance, their typical manifestations, and practical solutions, providing a useful reference for rapid troubleshooting and consistent compliance.
I. Insufficient Performance or Deactivation of the Catalyst Itself
The catalyst is central to the DeNOx reaction; any decline in its performance will directly result in reduced efficiency.
● Surface Coating and Pore Blockage: Fly ash and ammonium sulfate salts deposit on the surface and within the pores, covering active sites and hindering flue gas diffusion, thereby preventing the reaction from proceeding fully.
● Chemical Poisoning: Alkali metals (Na, K, As) react with active components, causing irreversible deactivation.
● High-Temperature Sintering and Aging: Prolonged operation at temperatures exceeding the design limit leads to phase transformations in the support and agglomeration of active components, resulting in a significant decrease in specific surface area and making it difficult to restore activity.
● Insufficient Loading or Severe Wear: Erosion and loss of upper-layer catalyst, missing modules, and an insufficient number of loading layers can cause the overall treatment capacity to fall short of the design value.
II. Reaction Temperature Outside the Optimal Range
The SCR reaction is highly sensitive to temperature, and deviations from the optimal temperature window directly affect efficiency.
● At low temperatures, the reaction rate slows down, leading to incomplete NH₃ adsorption and NO reduction. This results in a significant drop in DeNOx efficiency and increases the likelihood of ammonium sulfate formation, which can clog the equipment.
● Excessively high temperatures trigger NH₃ oxidation side reactions, generating additional NOₓ and accelerating catalyst aging and deactivation.
● Temperature inhomogeneity results in large cross-sectional temperature differences; reactions lag in local low-temperature zones, creating “bottleneck areas” where NOₓ levels exceed limits.
III. Ammonia Injection System Malfunctions (Most Common Causes)
Uneven ammonia injection or insufficient injection volume is the most common cause of exceedances at the site.
● Low ammonia-to-nitrogen ratio: Insufficient reducing agent prevents NOₓ from being fully reduced, resulting in efficiency that fails to meet standards.
● Clogged or damaged ammonia injection grids (AIG): Clogged nozzles, worn branch pipes, and uneven distribution cause localized “ammonia deficiency,” resulting in local exceedances even when the total ammonia volume is sufficient.
● Excessive ammonia slip: Over-injection to improve efficiency leads to air preheater clogging and corrosion, while failing to genuinely increase the effective reaction ratio.
IV. Flue Gas Flow Field and Dust Interference
Flue gas distribution and impurities can significantly reduce reaction efficiency.
● An uneven flow field with large velocity variations, the presence of vortices or dead zones, and partial flue gas “short-circuiting” (bypassing the reaction) directly lead to emissions exceeding standards at the outlet.
● Excessively high dust concentrations and high ash content rapidly coat the catalyst and clog its pores, shortening the effective reaction cycle and accelerating wear.
● SO₂/SO₃ influences high-sulfur operating conditions by promoting the formation of ammonium bisulfate, which exacerbates clogging and corrosion and reduces the effective active surface area.
V. Key Points for Rapid Troubleshooting (For On-Site Use)
1. Check the outlet NOₓ, ammonia slip, reactor pressure drop, ammonia injection rate, and inlet temperature curves.
2. Check whether the ammonia injection grating is clogged, whether the nozzles are intact, and whether flow rates are uniform across all branch pipes.
3. Observe whether the catalyst is clogged with ash, contaminated, worn, or missing.
4. Measure the temperature and flow fields across the cross-section to confirm the presence of any localized low-temperature or low-velocity zones.
5. Take samples to analyze the arsenic and alkali metal content in the catalyst to determine if poisoning has occurred.
VI. Corresponding Remedial Measures
1. For catalysts: In cases of mild ash buildup, increase soot blowing; for moderate poisoning, clean and regenerate the catalyst; for severe deactivation, replace the catalyst immediately and ensure sufficient loading.
2. Temperature Control: Maintain stable operation within the optimal range of 300–380°C and avoid frequent, significant fluctuations.
3. Optimize ammonia injection by adjusting the AIG valve opening to ensure uniform ammonia distribution across the cross-section; precisely control the ammonia-to-nitrogen ratio within a reasonable range.
4. Optimize flow patterns and dust removal by cleaning baffles and repairing flow rectifiers; improve upstream dust removal efficiency to reduce inlet dust.
5. Implement preemptive measures for high-arsenic/high-alkali fuel conditions by adding adsorbents to remove harmful components and mitigate catalyst poisoning.
VII. Summary
Excessive NOₓ emissions are fundamentally the result of a combination of mismatched reaction conditions, insufficient catalyst performance, and uneven distribution within the system. By systematically troubleshooting according to the logic of “first check ammonia injection, then examine temperature, next adjust the flow field, and finally inspect the catalyst,” compliance can be rapidly restored in most operating conditions. Stable control hinges on: rational ammonia injection, a stable temperature window, a uniform flow field, and a sound catalyst.
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2026-04-24
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