---
title: "Economiser and backpass fouling in biomass boilers: the dP creep that steals efficiency"
description: "Learn how biomass boiler economiser fouling drives pressure drop and exit gas temperature upwards, and where online acoustic cleaning fits."
canonical_url: "https://sylio.co/resources/blog/economiser-backpass-fouling-biomass"
last_updated: "2026-06-04"
---

## Read the creep before the boiler reaches a limit

**Economiser fouling** is rarely announced by one sudden alarm. Ash accumulates through the [economiser](/glossary/economiser) and the wider [convective backpass](/glossary/convective-pass-backpass), gas-side differential pressure creeps upwards, and flue gas leaves hotter than it used to. The boiler may still make its target steam flow, but the draught system and fuel input are doing more work to get there.

This is a form of [heat-transfer surface fouling](/glossary/heat-transfer-surface-fouling) with two costs at the same time. The deposit insulates tubes, so less heat reaches the feedwater. It also occupies the gaps between tubes, so the gas path becomes more restrictive. One effect appears as exit gas temperature drift and efficiency loss. The other appears as rising dP, higher fan duty and, eventually, a load limit.

The useful question is not whether there is some ash in the backpass. There almost always is. The question is whether the rate of change is stable, whether cleaning restores the baseline, and whether the deposit is still weak enough to control online. In a biomass boiler, that last point changes markedly from the hot superheater to the cooler economiser.

## Why the cooler gas path behaves differently

Biomass ash does not have one fixed sticking temperature. Its behaviour depends on fuel chemistry, tube and gas temperature, particle size, boiler design and time on the surface. Potassium, chlorine, sulphur, silica and calcium all influence whether a deposit stays powdery, develops a liquid fraction or gains strength through sintering. That is why a single ash-fusion number cannot describe every operating condition.

Temperature still provides a useful first division. In the hot superheater region, [low-melt sticky ash](/glossary/low-melt-sticky-ash) may be partly molten or chemically adhesive. Once it lands, continued heating and reaction can produce a dense, strongly bonded layer. Experimental work on biomass ash shows that adhesion rises with sintering temperature and increases sharply around the ash deformation condition. This upstream problem is covered in more detail in the guide to [sticky ash in biomass and waste-to-energy boilers](/resources/blog/sticky-ash-biomass-waste-to-energy-boilers).

Farther downstream, gas cools through the [generating bank](/glossary/generating-bank) and economiser. Much of the entrained ash is then solid. Deposits formed mainly by particle impact and settling are more likely to be dry, porous and friable, especially when they are removed before consolidation. This does not mean every economiser deposit is easy. Alkali sulphates can strengthen with time, sticky material can fall from hotter surfaces, and moisture or condensation can turn a powder into a bonded mass. Deposit condition must be inspected, not assumed from location alone.

The same distinction applies across boiler designs, although the loading and chemistry differ. A [BFB boiler](/glossary/bfb-boiler), a [CFB boiler](/glossary/cfb-boiler) and a [hog-fuel boiler](/glossary/hog-fuel-boiler-bark-boiler) can all foul their cooler passes. Bed carryover, fuel contamination, bark content, sand, load and tube spacing change the rate and pattern. The common opportunity is that downstream ash is often intercepted while it is still less bonded than the material on the hottest surfaces.

## What rising gas-side dP is telling you

Gas-side dP, also called draught loss across a bank or pass, is the pressure difference needed to move flue gas through that section. It is not the water-side pressure drop through the economiser tubes. Keeping that distinction explicit prevents a fireside deposit problem from being confused with an internal water-treatment problem.

As [tube fouling](/glossary/tube-fouling) grows, the free area between rows decreases. Gas accelerates through the remaining gaps, turbulence and resistance increase, and the measured pressure difference rises. Deep deposits in a staggered or finned bank can be hard to see from an access door, so a slow dP trend may reveal restriction before a casual visual inspection does. If the open area becomes too small, local velocity can also increase erosion risk.

The [ID fan](/glossary/id-fan) has to overcome that added resistance. Depending on the control arrangement, the symptom may be more fan speed, a more open inlet vane or damper, higher motor current, or less draught margin at full load. The final constraint is not a universal dP number. It is the point at which the fan and furnace-pressure controls can no longer maintain the required gas flow safely. A clean-bank baseline and the slope away from it are therefore more useful than a generic alarm copied from another boiler.

## Why exit gas temperature drifts upwards

A deposit is an extra thermal resistance between hot gas and metal. As the layer thickens, the effective heat-transfer coefficient falls. Feedwater picks up less heat in the economiser and the gas carries more sensible heat downstream. At matched load and operating conditions, a rising economiser outlet or stack temperature is therefore a practical sign that heat recovery has deteriorated.

That lost heat has to be supplied somewhere. To hold steam production, the plant may burn more fuel, and auxiliary demand may rise as the ID fan works harder. Both effects worsen [heat rate](/glossary/heat-rate). The loss is easy to tolerate because it arrives gradually: a little more exit temperature, a little more fan duty, and a little less margin before cleaning. Across a long campaign, that drift is the efficiency being stolen.

Exit temperature alone is not proof of [fouling](/glossary/fouling). Fuel moisture, excess air, air leakage, feedwater inlet temperature, load and the cleanliness of upstream surfaces also move it. The diagnosis becomes stronger when exit temperature and gas-side dP rise together under comparable conditions, then both improve after effective cleaning.

<table>
<thead>
  <tr>
    <th>
      Trend
    </th>
    
    <th>
      What it can indicate
    </th>
    
    <th>
      Check before blaming deposits
    </th>
  </tr>
</thead>

<tbody>
  <tr>
    <td>
      Local backpass dP rises
    </td>
    
    <td>
      Flow area is narrowing within the measured section
    </td>
    
    <td>
      Pressure taps, transmitter zero, load and gas flow
    </td>
  </tr>
  
  <tr>
    <td>
      Exit gas temperature rises
    </td>
    
    <td>
      Economiser heat absorption is falling
    </td>
    
    <td>
      Fuel moisture, excess oxygen, air leakage and feedwater temperature
    </td>
  </tr>
  
  <tr>
    <td>
      ID fan duty rises
    </td>
    
    <td>
      Total gas-path resistance is increasing
    </td>
    
    <td>
      Dampers, fan condition and downstream equipment
    </td>
  </tr>
  
  <tr>
    <td>
      Cleaning gives only brief recovery
    </td>
    
    <td>
      Deposit remains in shadowed rows or is gaining strength quickly
    </td>
    
    <td>
      Cleaning coverage, deposit condition and ash removal below the bank
    </td>
  </tr>
</tbody>
</table>

## Separate fouling from its lookalikes

Start with trends at comparable boiler load, excess oxygen and fuel condition. Biomass moisture and ash content can vary enough to hide or imitate a cleanliness change. If the plant burns several feedstocks, tag the trend by fuel blend rather than averaging the whole campaign. A smooth rise across repeated shifts is more consistent with accumulation than a step change that follows a damper movement, transmitter fault or abrupt fuel change.

Then verify the measurement. Blow through pressure lines where the procedure allows, inspect impulse points and confirm transmitter zero. A blocked pressure tap can create a convincing false dP. Where instrumentation permits, compare pressure drop across individual banks rather than only furnace-to-stack draught. A whole-path number confirms that resistance has risen but does not locate it.

Use the next safe inspection to map the deposit through the depth of the bank. Record whether it brushes away as a dry powder, breaks as a weak crust, or has become glazed, wet, sintered or rock-like. Note plugged lanes, clean shadow zones, evidence of material falling from upstream and whether removed ash can leave the hopper below. Photographs from the same access point after each campaign are more useful than isolated descriptions such as "heavy" or "acceptable".

Finally, compare the response to existing cleaning. A sharp, repeatable dP reduction shows that at least part of the restriction remains removable. Little response may mean poor coverage, a hard deposit, misleading instrumentation or restriction elsewhere. It does not automatically mean the answer is simply more cleaning energy.

## Why this is the strongest acoustic opportunity in the boiler

The economiser and cooler backpass combine three conditions that favour online prevention: the deposit is often dry and friable, the build-up is distributed through a tube bundle, and waiting makes it harder to remove. A correctly applied [acoustic cleaner](/glossary/acoustic-cleaner) addresses those conditions with frequent, short sound pulses while the boiler remains online.

A [low-frequency acoustic cleaner](/glossary/low-frequency-acoustic-cleaner) introduces pressure oscillations into the gas volume. It does not blast one tube face like a lance. The sound field reaches around rows and into spaces that a directed jet may not cover, disturbing loose particulate and weak deposits before they consolidate. The released ash must still fall or travel to equipment that can remove it. A horn does not make the solids disappear, so hoppers and downstream collection remain part of the cleaning system.

Prevention is the important word. Published field evidence describes the economiser as a practical horn application, with frequent cycling used to stop loose ash becoming firmly adhered. A peer-reviewed boiler study likewise reports the better results on friable deposits and warns that sonic cleaning offers no absolute solution where reactions and sintering strengthen the ash.

That is why a fouled bank normally needs a baseline clean before an acoustic trial. Once the flow lanes are restored, the horn is asked to hold them, not to recover a bank that is already plugged. The broader [acoustic cleaning system guide](/resources/blog/acoustic-cleaning-system) explains the air supply, placement, controls and commissioning work around the horn itself.

## Acoustic cleaning does not replace sootblowing

The hot superheater is the honesty boundary. Sticky, partly molten and strongly sintered deposits there are not an acoustic duty. Neither are wet ash, chemically cemented material, hard upstream fragments lodged in a tight bank, or a mature deposit that has already bridged the gas lanes. Those conditions require process correction and a higher-energy or offline removal method.

A [steam sootblower](/glossary/steam-sootblower) remains necessary where bonded deposits need a concentrated jet. Acoustic horns can reduce the amount of loose ash that reaches a hardened state and may allow longer intervals between blows in suitable cooler zones. They do not remove the need for sootblowing across the boiler, and they do not replace water washing or manual cleaning when deposits have become hard. The practical distinction is about deposit and location, not a winner for every duty.

There is also a downstream limit. If a horn releases more ash than the hopper or conveying system can take away, material can accumulate elsewhere or be re-entrained. Commissioning must therefore check discharge capacity and collection performance as well as dP. Moving the restriction is not a successful clean.

## Prove the result against a clean baseline

An acoustic trial should begin with a defined zone and a recoverable baseline. Clean and inspect the target bank, confirm the pressure instruments, and record dP, economiser outlet temperature, ID fan duty, boiler load, fuel condition, excess oxygen and sootblower operation. Without matched baseline data, normal fuel variation can be mistaken for an acoustic result.

After commissioning, judge the slope rather than one favourable shift. The useful measures are whether load-normalised dP stays flatter, whether exit temperature drift slows, whether ID fan margin is preserved, and whether the required sootblower frequency falls without deposits appearing elsewhere. Inspect the bank at the next planned opportunity to confirm what the trends imply.

Acceptance criteria should be plant-specific. There is no responsible universal promise for dP reduction or efficiency recovery because boiler geometry, fuel and deposit condition govern the result. A credible project defines the starting condition, the measured zone, the comparison period and the deposit that the system is expected to control.

## The bottom line

Economiser and backpass fouling steals efficiency by two connected routes. Deposits insulate heat-transfer surfaces, sending more energy out with the flue gas, while growing into tube lanes and raising the draught loss that the ID fan must overcome. A load-normalised rise in dP together with exit gas temperature drift is the clearest operating signature, provided pressure taps, fuel changes, excess air and air leakage have been ruled out.

This cooler gas path is the strongest acoustic-cleaning opportunity in a biomass boiler because its deposits are often still dry, friable and preventable. Use sound to hold a clean bank and reduce the burden on sootblowers. Keep sootblowers and stronger cleaning for the sticky, molten, sintered and hard-bonded material that acoustic energy cannot move. The aim is not to replace every cleaning method. It is to stop the most manageable deposit from becoming the next efficiency and availability limit.