Removal efficiency

Removal efficiency is the fraction of a target pollutant removed by a process or control device. In its simplest form, it is calculated from inlet and outlet pollutant rates: removed amount divided by inlet amount. A system with 100 units entering and 10 units leaving has 90 percent removal efficiency.

The term is used for NOx removal across SCR and SNCR systems, SO2 removal across scrubbers, acid gas control, odour control, and sometimes particulate control. For particulate collectors, collection efficiency is often the more specific term, but the underlying mass-balance idea is similar.

Measurement context

Removal efficiency must be tied to a measurement basis. Concentration alone can mislead if gas flow changes. A rigorous calculation uses mass rate or normalised concentration with defined oxygen, moisture, temperature, and flow assumptions. For batch or destruction systems, destruction and removal efficiency may also account for chemical transformation, not just capture.

Sampling uncertainty matters. Low inlet concentration can make a high percentage number unstable. A device may show high removal efficiency but still fail an outlet concentration limit if the inlet loading is very high. Conversely, low inlet loading can make a good device appear inefficient because the difference is near the detection limit.

Operating implications

Removal efficiency is useful for diagnosing process health. Falling SCR removal can mean catalyst deactivation, poor ammonia distribution, low temperature, catalyst pluggage, or excess ammonia slip constraints. Falling scrubber removal can mean poor reagent feed, liquid distribution, scaling, or gas bypass. For dust collectors, falling efficiency can indicate leaks, re-entrainment, or overloaded cleaning systems.

Acoustic cleaning context

Sonic horns do not chemically remove pollutants. Their effect on removal efficiency is indirect: they keep active surfaces, gas passages, hoppers, and collection areas available. In SCR service, cleaner catalyst faces help maintain NOx removal at lower pressure drop. In ESP or baghouse service, cleaner internals help maintain particulate collection and reduce re-entrainment.

Measurement context

Removal efficiency depends on where the inlet and outlet samples are taken, whether the gas flow is corrected to the same basis, and whether the measurement captures steady operation or short cleaning events. A collector can show high average efficiency while still producing short particulate spikes during rapping, pulse cleaning, start-up, or hopper upsets. For this reason, efficiency should be read alongside mass loading, opacity, continuous particulate trend, differential pressure, and process state.

In acoustic cleaning applications, the aim is often indirect. The cleaner may improve removal efficiency by preventing re-entrainment, keeping catalyst or filter surfaces open, or reducing the ash load that reaches a final collector. It may also temporarily increase dust movement inside the system. Commissioning should therefore compare baseline and post-installation measurements over a representative operating period, not only during a short clean test.

Related terms

• Collection efficiency
• Selective Catalytic Reduction
• NOx reduction efficiency