---
title: "Alkali metals in ash"
description: "Alkali metals — primarily sodium (Na) and potassium (K) — are the dominant drivers of low-melting fouling in biomass, waste-to-energy and certain coal boilers. Alkali compounds (KCl, NaCl, K₂SO₄, Na₂SO₄) melt or soften at temperatures (650–900 °C) lower than typical convective-pass tube-metal temperatures, so they arrive at the tube surface partly molten and bond tenaciously."
canonical_url: "https://sylio.co/glossary/alkali-metals-in-ash"
last_updated: "2026-06-28T02:29:41.247Z"
---

**Alkali metals** — primarily sodium (Na) and potassium (K) — are the dominant drivers of low-melting fouling in biomass, [waste-to-energy](/glossary/waste-to-energy) and certain coal boilers. Alkali compounds (KCl, NaCl, K₂SO₄, Na₂SO₄) melt or soften at temperatures (650–900 °C) lower than typical convective-pass tube-metal temperatures, so they arrive at the tube surface partly molten and bond tenaciously.

## Where alkali concentration is high

<table>
<thead>
  <tr>
    <th>
      Fuel
    </th>
    
    <th>
      Approximate alkali-in-ash range
    </th>
  </tr>
</thead>

<tbody>
  <tr>
    <td>
      Wood (clean stems)
    </td>
    
    <td>
      Low (1–5%)
    </td>
  </tr>
  
  <tr>
    <td>
      Bark, <a href="/glossary/hog-fuel">
        hog fuel
      </a>
    </td>
    
    <td>
      Medium (5–15%)
    </td>
  </tr>
  
  <tr>
    <td>
      Straw and <a href="/glossary/straw-agricultural-residue-firing">
        agricultural residues
      </a>
    </td>
    
    <td>
      High (10–25%)
    </td>
  </tr>
  
  <tr>
    <td>
      <a href="/glossary/bagasse">
        Bagasse
      </a>
    </td>
    
    <td>
      Medium-high
    </td>
  </tr>
  
  <tr>
    <td>
      <a href="/glossary/waste-to-energy">
        MSW / RDF / SRF
      </a>
    </td>
    
    <td>
      High (variable)
    </td>
  </tr>
  
  <tr>
    <td>
      Coal
    </td>
    
    <td>
      Low
    </td>
  </tr>
</tbody>
</table>

## Operational consequences

- [Low-melt sticky ash](/glossary/low-melt-sticky-ash) bonding to superheater and economiser tubes
- Accelerated [tube wastage](/glossary/tube-erosion-tube-wastage) from corrosive deposits
- [SCR catalyst poisoning](/glossary/catalyst-poisoning) by alkali species
- Bed-material agglomeration in [BFB](/glossary/bfb-boiler) and [CFB](/glossary/cfb-boiler) boilers

## Cleaning

Active [sonic-horn](/glossary/sonic-horn) cleaning prevents fresh alkali-rich deposits from consolidating into bonded slag, which is the only practical mitigation short of fuel substitution.

## Related terms

- [Low-melt sticky ash](/glossary/low-melt-sticky-ash)
- [Chloride-induced corrosion](/glossary/chloride-induced-corrosion)
- [Catalyst poisoning](/glossary/catalyst-poisoning)
- [Bagasse](/glossary/bagasse)
- [Straw / agricultural-residue firing](/glossary/straw-agricultural-residue-firing)