---
title: "Urea SNCR / aqueous-ammonia SNCR"
description: "SNCR systems use one of two principal NOx-reducing reagents: urea (CO(NH₂)₂, usually delivered as 32–50% aqueous solution) or aqueous ammonia (NH₃ at 19–29% in water)."
canonical_url: "https://sylio.co/glossary/urea-sncr-aqueous-ammonia-sncr"
last_updated: "2026-06-28T02:29:38.009Z"
---

[**SNCR**](/glossary/selective-non-catalytic-reduction) systems use one of two principal NOx-reducing reagents: **urea** (CO(NH₂)₂, usually delivered as 32–50% aqueous solution) or **aqueous ammonia** (NH₃ at 19–29% in water).

## Reagent comparison

<table>
<thead>
  <tr>
    <th>
      Attribute
    </th>
    
    <th>
      Urea
    </th>
    
    <th>
      Aqueous ammonia
    </th>
  </tr>
</thead>

<tbody>
  <tr>
    <td>
      Storage hazard class
    </td>
    
    <td>
      Non-hazardous
    </td>
    
    <td>
      Toxic / corrosive
    </td>
  </tr>
  
  <tr>
    <td>
      Required setback distance
    </td>
    
    <td>
      Modest
    </td>
    
    <td>
      Large (depends on jurisdiction)
    </td>
  </tr>
  
  <tr>
    <td>
      Permit complexity
    </td>
    
    <td>
      Lower
    </td>
    
    <td>
      Higher
    </td>
  </tr>
  
  <tr>
    <td>
      Reaction rate
    </td>
    
    <td>
      Slower (decomposes first to NH₃)
    </td>
    
    <td>
      Faster (direct NH₃)
    </td>
  </tr>
  
  <tr>
    <td>
      Reagent cost per kg-NO removed
    </td>
    
    <td>
      Higher
    </td>
    
    <td>
      Lower
    </td>
  </tr>
  
  <tr>
    <td>
      Suitability for cold furnaces
    </td>
    
    <td>
      Good
    </td>
    
    <td>
      Less good — vaporisation/distribution issues
    </td>
  </tr>
  
  <tr>
    <td>
      Solid by-product risk
    </td>
    
    <td>
      Urea solids at lance tips
    </td>
    
    <td>
      None
    </td>
  </tr>
</tbody>
</table>

## Selection drivers

Many plants choose urea for the permitting and safety advantages despite its higher reagent cost; large utilities with established ammonia handling tend towards aqueous ammonia for the lower OPEX. Both reagents produce the same [ammonia slip](/glossary/ammonia-slip) and downstream [ammonium-bisulphate](/glossary/ammonium-bisulphate) consequences when slip is high.

## Related terms

- [Selective Non-Catalytic Reduction (SNCR)](/glossary/selective-non-catalytic-reduction)
- [Ammonia slip](/glossary/ammonia-slip)
- [Ammonium bisulphate](/glossary/ammonium-bisulphate)