---
title: "NOx reduction efficiency"
description: "NOx reduction efficiency is the percentage of NOx removed from the flue gas by a DeNOx system, calculated as (NOx_in − NOx_out) / NOx_in. It is the headline KPI for any SCR or SNCR installation and the figure permit compliance is measured against."
canonical_url: "https://sylio.co/glossary/nox-reduction-efficiency"
last_updated: "2026-06-28T02:29:37.543Z"
---

**NOx reduction efficiency** is the percentage of NOx removed from the flue gas by a [DeNOx](/glossary/denox) system, calculated as (NOx_in − NOx_out) / NOx_in. It is the headline KPI for any [SCR](/glossary/selective-catalytic-reduction) or [SNCR](/glossary/selective-non-catalytic-reduction) installation and the figure permit compliance is measured against.

## Typical performance

<table>
<thead>
  <tr>
    <th>
      System
    </th>
    
    <th>
      Reduction range
    </th>
    
    <th>
      Typical <a href="/glossary/ammonia-slip">
        ammonia slip
      </a>
    </th>
  </tr>
</thead>

<tbody>
  <tr>
    <td>
      SCR (high-dust)
    </td>
    
    <td>
      80–95%
    </td>
    
    <td>
      2–5 ppm
    </td>
  </tr>
  
  <tr>
    <td>
      SCR (tail-end)
    </td>
    
    <td>
      90–98%
    </td>
    
    <td>
      1–3 ppm
    </td>
  </tr>
  
  <tr>
    <td>
      SNCR
    </td>
    
    <td>
      30–60%
    </td>
    
    <td>
      5–10 ppm
    </td>
  </tr>
  
  <tr>
    <td>
      Combined SNCR + SCR
    </td>
    
    <td>
      up to 99%
    </td>
    
    <td>
      2–5 ppm
    </td>
  </tr>
</tbody>
</table>

## What erodes efficiency over time

- **Catalyst masking** — fine ash blanket reducing active surface area
- **Catalyst poisoning** — chemical de-activation
- **Catalyst pluggage** — channel blockage and gas channelling
- **AIG distribution drift** — uneven NH₃/NOx mixing
- **Operating outside the temperature window** — too cool or too hot for the catalyst

## How cleaning preserves efficiency

[Sonic horns](/glossary/sonic-horn) and steam [sootblowers](/glossary/sonic-sootblower) attack masking and pluggage directly. A well-cleaned catalyst maintains 85–90% of its initial efficiency for 30,000 operating hours, against 60–70% for a poorly cleaned catalyst of the same age. The economic case for active cleaning is therefore measured in deferred catalyst replacement and avoided ammonia-over-injection cost.

## Related terms

- [Selective Catalytic Reduction (SCR)](/glossary/selective-catalytic-reduction)
- [Selective Non-Catalytic Reduction (SNCR)](/glossary/selective-non-catalytic-reduction)
- [Ammonia slip](/glossary/ammonia-slip)
- [Catalyst masking](/glossary/catalyst-masking)