---
title: "DeNOx"
description: "DeNOx is the collective term for post-combustion NOx-reduction technologies on industrial flue gas. The two dominant options are Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR). Both rely on a reagent — ammonia or urea — that reacts with NOx to produce nitrogen and water."
canonical_url: "https://sylio.co/glossary/denox"
last_updated: "2026-06-28T02:29:37.346Z"
---

**DeNOx** is the collective term for post-combustion NOx-reduction technologies on industrial flue gas. The two dominant options are [Selective Catalytic Reduction (SCR)](/glossary/selective-catalytic-reduction) and [Selective Non-Catalytic Reduction (SNCR)](/glossary/selective-non-catalytic-reduction). Both rely on a reagent — ammonia or urea — that reacts with NOx to produce nitrogen and water.

## Why DeNOx is mandatory

NOx is a regulated pollutant under the [Industrial Emissions Directive (IED)](/glossary/industrial-emissions-directive), [MATS](/glossary/mats-us-mercury-and-air-toxics), [EPA NSPS](/glossary/epa-nsps), [TA Luft 2021](/glossary/ta-luft-2021) and most national emission codes. Limits for coal-fired power stations and large [WtE](/glossary/waste-to-energy) plants are usually 100–200 mg/Nm³ on a 30-day average, with stricter site-specific BAT-AEL values from BREF revisions.

## Choice of technology

<table>
<thead>
  <tr>
    <th>
      Factor
    </th>
    
    <th>
      Favours SCR
    </th>
    
    <th>
      Favours SNCR
    </th>
  </tr>
</thead>

<tbody>
  <tr>
    <td>
      Reduction efficiency required
    </td>
    
    <td>
      > 70%
    </td>
    
    <td>
      30–60%
    </td>
  </tr>
  
  <tr>
    <td>
      Plant size
    </td>
    
    <td>
      Large
    </td>
    
    <td>
      Small / medium
    </td>
  </tr>
  
  <tr>
    <td>
      Capital available
    </td>
    
    <td>
      Higher
    </td>
    
    <td>
      Lower
    </td>
  </tr>
  
  <tr>
    <td>
      Space available
    </td>
    
    <td>
      More
    </td>
    
    <td>
      Less
    </td>
  </tr>
  
  <tr>
    <td>
      Catalyst cost tolerance
    </td>
    
    <td>
      Yes
    </td>
    
    <td>
      Avoid
    </td>
  </tr>
  
  <tr>
    <td>
      Fuel chemistry
    </td>
    
    <td>
      Predictable
    </td>
    
    <td>
      Variable
    </td>
  </tr>
</tbody>
</table>

Many plants run combined systems: SNCR provides bulk reduction, SCR polishes to meet permit limits.

## Related terms

- [Selective Catalytic Reduction (SCR)](/glossary/selective-catalytic-reduction)
- [Selective Non-Catalytic Reduction (SNCR)](/glossary/selective-non-catalytic-reduction)
- [NOx reduction efficiency](/glossary/nox-reduction-efficiency)
- [Ammonia injection grid](/glossary/ammonia-injection-grid)