What is ammonia slip and why does it matter?
Selective Catalytic Reduction (SCR) is the dominant technology for reducing nitrogen oxide emissions from combustion processes. The principle is straightforward: ammonia (NH₃) or urea is injected into the flue gas upstream of a catalyst, where it reacts with NO and NO₂ to form nitrogen and water.
In practice, perfect stoichiometric dosing is impossible. Operators deliberately overdose ammonia to ensure high DeNOx efficiency — typically targeting 90–95% NOx reduction. The excess ammonia that passes through the catalyst unreacted is called ammonia slip.
Typical ammonia slip levels range from 2–10 ppm in well-tuned systems, but can spike to 50 ppm or more during load changes, catalyst ageing, or injection system malfunctions. This residual ammonia travels downstream with the flue gas — and directly into the emission monitoring system.
How ammonia corrupts NOx measurements
The vast majority of continuous emission monitoring systems (CEMS) use chemiluminescence detection (CLD) to measure NOx. The CLD method works by reacting NO with ozone (O₃) in a reaction chamber — the resulting chemiluminescence is proportional to the NO concentration.
To measure total NOx (NO + NO₂), the sample gas first passes through a converter that reduces NO₂ back to NO. This is where ammonia becomes a problem: the molybdenum or carbon converter also oxidises NH₃ to NO. The detector cannot distinguish between NO from the original flue gas and NO produced from ammonia in the converter.
The measurement error
At 10 ppm ammonia slip and typical converter conditions, the CLD reports 5–15% higher NOx values than the actual concentration. For a plant operating near its emission limit, this phantom NOx can trigger false compliance violations — or force unnecessary reagent adjustments that waste ammonia and increase operating costs.
Why conventional filtration does not help
Standard particulate filter elements — whether borosilicate microfibre, sintered metal, or PTFE membrane — are designed to remove solid and liquid aerosol from the sample gas. They have no effect on gaseous ammonia, which passes through any mechanical filter regardless of pore size or efficiency rating.
Chemical scrubber systems (wet or dry) can remove ammonia, but they add complexity, require reagent replacement, and introduce the risk of absorbing other gas components that should be measured. For emission monitoring, any sample conditioning step must be selective — removing only the target interferent without altering the rest of the gas matrix.
The acid wash solution
Acid wash filter elements solve the ammonia problem through a chemical reaction rather than mechanical filtration. The element consists of borosilicate glass microfibre impregnated with phosphoric acid (H₃PO₄). When ammonia-laden sample gas passes through the element, the following reaction occurs:
NH₃ + H₃PO₄ → (NH₄)H₂PO₄
The resulting ammonium dihydrogen phosphate is a stable, non-volatile salt that remains trapped in the fibre matrix. The reaction is irreversible under normal operating conditions — ammonia cannot be released back into the gas stream.
Key Takeaway
Two binder variants for different temperature ranges
R+F FilterElements offers acid wash elements in two binder variants to cover the full range of SCR emission monitoring applications:
| Property | K-Type (PVDF binder) | S-Type (Silica binder) |
|---|---|---|
| Max. operating temperature | 150 °C | 480 °C |
| Typical application | Cold-side extraction, conditioned sample lines | Hot-side extraction, high-temperature probes |
| Chemical resistance | Excellent (most acids, solvents) | Excellent (all inorganic acids, flue gas components) |
| Filtration efficiency | 99.99% at 0.01 µm (Grade HE) | 99.99% at 0.01 µm (Grade HE) |
| Article number prefix | RF-AW-…-HEK | RF-AW-…-HES |
Element sizing and replacement
Acid wash elements are available in standard analyser filter sizes — 12 × 57 mm, 25 × 64 mm, and 25 × 178 mm — that fit directly into R+F FilterElements housings and most common third-party probe and filter assemblies.
The element's ammonia capacity is finite: once the phosphoric acid is consumed, the element must be replaced. Replacement intervals depend on ammonia concentration and flow rate, but typically range from 2–8 weeks in SCR applications. The element changes colour as the acid is consumed, providing a visual indication of remaining capacity.
Disposal
Spent acid wash elements contain ammonium phosphate — a common fertiliser compound. They can typically be disposed of as non-hazardous industrial waste, but always check local regulations.
Where to install acid wash elements in the sample path
Stage 1: Coarse particulate removal
A sintered metal or ceramic probe filter removes bulk particulate at the extraction point. This protects the downstream acid wash element from rapid loading.
Stage 2: Acid wash element
The RF-AW element is installed after the probe filter and before any cooler or converter. It removes ammonia while the sample gas is still hot and dry.
Stage 3: Sample cooler / dryer
The ammonia-free gas passes through the sample cooler. Without prior ammonia removal, ammonium salts would precipitate here and block the cooler.
Stage 4: Fine filtration
A final borosilicate microfibre or PTFE element provides the last barrier before the analyser, catching any particles generated during cooling.
The bottom line
Ammonia slip is an unavoidable consequence of SCR operation. But ammonia reaching your CLD analyser is not — and the measurement errors it causes are entirely preventable. A single acid wash filter element in the sample path eliminates ammonia interference, prevents ammonium salt deposits in coolers and sample lines, and ensures your NOx readings reflect reality rather than chemistry artifacts.
Need acid wash elements for your SCR monitoring system?
R+F FilterElements supplies RF-AW acid wash elements in K-type and S-type variants, available in standard analyser filter sizes. Typical delivery: 2 weeks.
