---
title: "Resistivity (fly-ash)"
description: "Fly-ash resistivity is the electrical resistance of the dust layer deposited on the collecting electrodes of an ESP. It is the single most important fuel-dependent variable in ESP performance, because it controls whether the collected dust can discharge its acquired charge to ground or instead accumulates trapped charge that triggers back-corona."
canonical_url: "https://sylio.co/glossary/resistivity"
last_updated: "2026-06-28T02:29:30.836Z"
---

**Fly-ash resistivity** is the electrical resistance of the dust layer deposited on the [collecting electrodes](/glossary/collecting-electrode) of an [ESP](/glossary/electrostatic-precipitator). It is the single most important fuel-dependent variable in ESP performance, because it controls whether the collected dust can discharge its acquired charge to ground or instead accumulates trapped charge that triggers [back-corona](/glossary/back-corona).

## The resistivity window

<table>
<thead>
  <tr>
    <th>
      Resistivity (Ω·cm)
    </th>
    
    <th>
      ESP behaviour
    </th>
  </tr>
</thead>

<tbody>
  <tr>
    <td>
      Below 10⁸
    </td>
    
    <td>
      Dust discharges too quickly; <a href="/glossary/re-entrainment">
        re-entrainment
      </a>
      
       dominates
    </td>
  </tr>
  
  <tr>
    <td>
      10⁸–10¹¹
    </td>
    
    <td>
      Ideal range; standard ESP operation
    </td>
  </tr>
  
  <tr>
    <td>
      Above 10¹¹
    </td>
    
    <td>
      High risk of <a href="/glossary/back-corona">
        back-corona
      </a>
      
      ; collection efficiency collapses
    </td>
  </tr>
</tbody>
</table>

## What raises resistivity

- Low sulphur content in coal (less SO₃ to condition the ash)
- Low gas temperature near the acid dew point
- High-alkali biomass ash
- Certain cement-kiln dust compositions

## Mitigation

The classic remedy is flue-gas conditioning — injecting SO₃ or ammonia ahead of the ESP to lower ash resistivity. A complementary remedy is to keep the plate dust layer thin enough that back-corona cannot establish, which is where [sonic horns](/glossary/sonic-horn) earn their keep on high-resistivity ESPs: continuous gentle dislodging prevents the critical thickness from developing.

## Related terms

- [Electrostatic precipitator](/glossary/electrostatic-precipitator)
- [Back-corona](/glossary/back-corona)
- [Corona discharge](/glossary/corona-discharge)
- [Sonic horn](/glossary/sonic-horn)