Control of ammonia slip below 2 ppmv in a high dust SCR installation is operationally distinct from low dust or tail-end arrangements because the catalyst is exposed to raw flue gas immediately downstream of the economizer. In that location, particulate loadings commonly reach 10–45 g/Nm³, and the catalyst must simultaneously maintain NOₓ reduction, resist particle impingement, and avoid accumulation of ammonium bisulfate. The dominant reduction reaction is 4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O over V₂O₅-WO₃/TiO₂. Slip evolves when local NH₃/NOₓ ratio exceeds stoichiometric availability at the active sites, driven by flow stratification, channel blockage, reagent maldistribution, or catalyst deactivation. In high dust service, the controlling process variables include gas temperature, SO₂-to-SO₃ conversion, dust chemistry, catalyst pitch, and ammonia injection grid design. Published data for the precise slip response of individual biomass co-fired configurations remain limited.

Ammonia Slip Formation Is Governed by Stoichiometric Ratio Imbalance and Local Flow Maldistribution

At the catalyst face, NH₃/NOₓ molar ratio must be maintained between 0.90 and 1.05 to keep outlet slip below 2 ppmv at design NOₓ conversion. Values above 1.05 increase ammonia breakthrough; values below 0.90 reduce NOₓ removal. Fly ash in high dust units erodes catalyst leading edges and creates localized channel plugging that shifts velocity profile by more than ±15 % of design face velocity. Injection grid lances require balancing to ±2 % of design ammonia flow in each zone through calibrated control valves and differential pressure transmitters. Nozzle discharge velocity should remain between 15 m/s and 25 m/s; lower values permit ash ingestion, while higher values cause jet penetration and catalyst erosion.

What Measurement Technologies Function Reliably Above 30 g/Nm³ Particulate Loading?

Continuous ammonia slip measurement must distinguish vapor-phase NH₃ from particulate-bound ammonium salts and moisture interference. Extractive analyzers require heated filters and blowback to delay blinding; in situ optical methods are affected by signal attenuation and lens fouling. Measurements used for compliance should be certified under EN 15267-3 and subject to QAL2 procedures under EN 14181:2014. The response time defines achievable feedback control speed. Extractive analyzers with 30–120 s sample transport and conditioning delay may miss rapid valve transients; in situ TDLAS with 1–5 s response can capture such excursions. The table compares configurations used in high dust ducts.

Alkali and alkaline earth metal poisoning in high dust SCR units creates a spatial deactivation pattern that cannot be corrected by increasing total ammonia flow without increasing slip. Potassium chloride and potassium sulfate condense on catalyst surfaces during biomass or agricultural residue co-firing; potassium ions displace ammonia adsorption on Brønsted acid sites and can lower activity K/K0 by 0.2–0.4 over 8,000–16,000 h depending on fuel. Calcium sulfate and sodium oxide may blind pore mouths, further reducing effective surface area. Injection trimming must therefore compensate for measured activity gradients across the catalyst, not simply for flow-weighted NOₓ load.

Ammonia Injection Grid Balancing, Nozzle Discharge Velocity, and Purge Air Design

High dust ammonia injection grids are arranged across the duct upstream of the catalyst face. Tuning should produce an ammonia distribution with relative standard deviation below 5 % at the catalyst inlet. Valve hysteresis below 1 % and actuator deadband below 0.5 % are required to preserve low slip under load changes. Continuous purge air flow at 20–40 L/min per lance prevents fly ash ingress, but excessive purge dilutes local NH₃ and depresses reduction efficiency. Nozzle diameter and spacing are selected to limit jet momentum relative to main flow while maintaining coverage in zones with high edge velocity.

When Flue Gas Temperature Drops Below the Ammonium Bisulfate Dew Point During Low Load Operation

Ammonium bisulfate condensation in high dust SCR units commonly occurs between 260°C and 310°C, depending on SO₃ and NH₃ partial pressures. At low boiler load, reducing ammonia injection to maintain slip below 2 ppmv is preferred over chasing NOₓ removal, because ABS formation consumes ammonia and produces a falsely low slip reading followed by later release. Air heater fouling increases pressure drop; sootblowing and air heater cleaning cannot fully remove deposits below 150°C. Economizer bypass may maintain catalyst inlet temperature above 320°C, but operational boundaries include fabric filter thermal limits and induced draft fan capability.

Continuous compliance under EN 14181:2014 QAL2 requires parallel reference measurements at three load levels and a variability test for calibration function validity. The reference method ISO 17179:2016 uses heated sampling with impinger analysis and demands uncertainty below 20 % of the limit value. High dust units require additional probe maintenance because fly ash accumulates in sample lines and adsorbs ammonia, causing underestimation. The control system should reject slip signals coinciding with filter blowback or purge air interruption because these events produce artificial ammonia spikes with no change in injected reagent flow.