- November 6, 2026
- Posted by: editor
- Category: RECD & Emission Control Devices
Open a filter that has been in service on a DG set for a year and you will find two kinds of deposit. One is black, light and oily to the touch. The other is pale grey or reddish-brown, dense and powdery. The first is soot. The second is ash.
They arrive together, they sit in the same filter, and they both raise back-pressure. But they leave by completely different routes. Soot can be burnt away inside the device. Ash cannot be burnt at all. It stays until someone takes the filter out and cleans it.
That single fact explains a lot of confusion on site: why a well-regenerated RECD slowly shows higher differential pressure year after year, why regeneration becomes more frequent as a device ages, and why “more regeneration” never fixes an ash problem. This article explains both residues, how to tell which one you are dealing with, and what to do about each. For the basics of how soot is burnt, start with our guide to RECD regeneration: active vs passive.
Two Residues Side by Side
| Property | Soot | Ash |
|---|---|---|
| What it is | Elemental carbon with absorbed hydrocarbons | Inorganic oxides, sulphates and phosphates of metals |
| Main origin | Incomplete combustion of fuel | Additives in lubricating oil, plus fuel and wear residues |
| Colour | Black | Grey, white, buff or rust-coloured |
| Combustible | Yes | No |
| Removed by regeneration | Yes | No |
| Rate of build-up | Fast: grams per operating hour | Slow: accumulates over hundreds or thousands of hours |
| Where it sits | A cake on the inlet channel walls | Partly on walls, mostly packed at the closed ends of inlet channels |
| Effect on ΔP | Cyclical: rises, then falls after regeneration | Permanent: raises the baseline step by step |
| Removal method | Oxidation (passive or active regeneration) | Physical cleaning off the DG set |
Read the table from top to bottom and the logic is clear. Regeneration is a chemical process that needs something to burn. Ash has already been burnt. There is nothing left in it for oxygen or nitrogen dioxide to react with.
Where Soot Comes From
Soot forms in the cylinder when fuel does not find enough oxygen to burn completely. On a diesel generator, soot output rises with:
- Light load and long idling, when combustion is cool and less efficient
- Sudden load steps that briefly over-fuel the engine
- Worn injectors and poor spray patterns
- Restricted air filters or tired turbochargers
- Poor fuel quality
Soot is the reason the filter exists. Its rate of arrival sets how often the device must regenerate, which we discuss in How Frequently Should a Filter-Based RECD Regenerate?. Engine faults that raise soot also show up as visible smoke; see why a generator emits black smoke.
Where Ash Comes From
Ash has several sources, and knowing them helps you slow it down.
Lubricating oil additives (the main source)
Every diesel engine burns a little lubricating oil. Oil passes the piston rings, travels down valve guides and leaks through turbo seals. Engine oils contain additive packages with calcium, magnesium, zinc and phosphorus compounds, which protect the engine but leave a metallic residue when burnt. In most diesel filters, oil-derived ash forms the large majority of all ash.
Fuel
Diesel itself contributes small amounts of inorganic material, and higher-sulphur fuel adds sulphate compounds. Biodiesel blends can also bring trace metals such as sodium and potassium from the production process. Our guide to biodiesel blends in an RECD-equipped generator explains why fuel specification matters here.
Engine wear and corrosion
Iron, copper, chromium and other wear metals reach the exhaust in small quantities. So do flakes of rust and scale from old exhaust pipework upstream of the RECD, which is one reason we recommend inspecting the line during a retrofit; see reusing existing exhaust pipework in an RECD retrofit.
Fuel additives
Some aftermarket fuel additives contain metals. They may claim to reduce smoke, but any metal they carry ends up as ash in the filter. We explain the wider issue in can diesel fuel additives replace an RECD?.
Why Regeneration Cannot Touch Ash
Regeneration works by oxidation. Carbon plus oxygen (or nitrogen dioxide) becomes carbon dioxide gas, which simply leaves with the exhaust. Ash is made of compounds that are already fully oxidised: metal oxides, sulphates and phosphates. Heating them to regeneration temperature does not turn them into a gas. In fact, each regeneration converts a little more soot into a little more residue, because any metals that were bound into the soot are left behind when the carbon burns away.
That is why technical literature on particulate filters consistently states that regeneration does little or nothing to remove accumulated ash, and why ash removal is treated as a separate maintenance task. DieselNet’s regeneration overview discusses this distinction, and patent literature on ash sensing exists precisely because ash cannot be cleared on board.
There is also a subtler effect. High regeneration temperatures can cause ash particles to sinter, partly fusing together. Sintered ash is harder to dislodge during cleaning. Very frequent or very hot regeneration therefore does not just fail to remove ash; it can make the eventual cleaning harder.
How Ash Changes the Way an RECD Regenerates
Ash does not sit quietly. As it builds, it changes how the device behaves in four ways.
1. It steals soot storage
Ash migrates during regeneration and packs into the plugged ends of the inlet channels, forming end plugs. Each plug shortens the usable length of the channel. A filter with a quarter of its inlet volume filled with ash can hold roughly a quarter less soot before it reaches its trigger level.
2. It raises the ΔP baseline
After a complete regeneration, a new filter returns to its clean ΔP. An ash-loaded filter returns to a higher number, because the gas must now pass through less filter wall and, in places, through an ash layer. Operators often notice this as a “clean” reading that drifts upward over months.
3. It makes regeneration more frequent
Less soot storage plus a higher starting ΔP means the trigger is reached sooner. On an active device, the cycle count per hundred running hours climbs. On a passive device, ΔP spends more time near its upper band.
4. It can mislead soot estimates
Many controllers estimate soot load from ΔP and exhaust flow. If ash is not accounted for, the controller can interpret ash restriction as soot and request regeneration that achieves very little. That pattern, frequent requests with little ΔP recovery, is one of the clearest ash signatures.
Soot Problem or Ash Problem? A Diagnostic Table
When ΔP is higher than you expect, use this table to decide which residue is responsible.
| Observation | Points to soot | Points to ash |
|---|---|---|
| ΔP falls clearly after a completed regeneration | Yes | |
| ΔP after a completed regeneration is higher than last year’s post-regeneration value | Yes | |
| Problem developed over days or a few weeks | Yes | |
| Problem developed gradually over many months | Yes | |
| Regeneration interval shortening steadily with run hours | Yes | |
| Recent run of light-load or short-run operation | Yes | |
| Engine burning noticeable oil (blue smoke, oil top-ups) | Possible | Yes |
| Black smoke upstream of the RECD | Yes | |
| Regeneration requests with little or no ΔP recovery | Yes |
The most reliable test is simple: compare ΔP at the same load immediately after a confirmed, completed regeneration against the same reading from commissioning. The gap between them is largely ash. Our guide on comparing RECD pressure readings at different generator loads explains how to make that comparison fair.
The Life of an RECD Filter in Four Stages
Thinking of the filter’s life in stages helps operators know what to expect.
Stage 1: New. No soot cake, no ash. ΔP is at its lowest. Filtration efficiency rises over the first hours as a thin soot layer forms.
Stage 2: Working. Soot loads and clears in regular cycles. Ash accumulates in tiny amounts each cycle, too small to notice week to week.
Stage 3: Ageing. Post-regeneration ΔP creeps up. Regeneration becomes more frequent. The device still performs, but with less margin.
Stage 4: Ash-limited. The filter reaches the ash level at which the supplier recommends cleaning. Waiting longer increases back-pressure on the engine and stresses the filter during regeneration, because the same soot is now packed into a smaller volume.
How quickly a filter moves through these stages depends mainly on oil consumption, oil formulation and running hours. Two identical devices on two different engines can reach Stage 4 years apart.
Cleaning Ash Out of an RECD Filter
Ash removal is a workshop or service-van job, not something done through the controller. Common approaches include:
- Thermal pre-treatment, then air cleaning. Often called “bake and blow”: the filter is slowly heated in an oven to burn off any remaining soot, then cleaned with compressed air blown in the reverse direction to the normal flow.
- Pulsed air cleaning. Short pulses of air dislodge ash from the channels into a collection system.
- Specialist wet or chemical cleaning. Used by some service providers, but only with the substrate maker’s approval, because unsuitable chemicals can damage catalyst coatings.
Whatever method is used, a good cleaning job includes inspection for cracks, melted zones and plugged channels, a flow or back-pressure check after cleaning, and a written report. Our guides to handling soot and ash removed during RECD servicing, filter core vs complete assembly replacement and auditing an RECD service report cover what to ask for.
After cleaning, a filter should behave close to Stage 1 again. If it asks for regeneration soon afterwards, something else is wrong; see Why Does an RECD Request Regeneration Soon After Cleaning?.
Slowing Ash Build-Up
You cannot prevent ash completely, but you can slow it a great deal.
- Fix oil consumption. An engine burning oil fills the filter with ash far faster. Watch top-up volumes and blue smoke. See why a generator emits blue smoke and engine oil consumption and RECD suitability.
- Use the oil grade the engine maker specifies, and ask your RECD supplier whether a lower-ash formulation is acceptable for your engine.
- Avoid metal-containing fuel additives.
- Buy fuel to specification and keep tanks clean. Ultra-low sulphur fuel reduces sulphate formation; see how ultra-low sulphur diesel improves emission control.
- Inspect upstream pipework for rust and scale during each RECD service.
- Keep the engine healthy. Turbo seal leaks and worn valve guides send oil straight into the exhaust. Our monthly maintenance checklist for diesel generators is a good routine to start with.
Keeping an Ash Ledger
Because ash builds so slowly, it is easy to miss until it becomes a problem. A simple ledger, updated at each service, makes the trend visible.
| Field | How to record it |
|---|---|
| Date and DG run hours | From the hour meter |
| ΔP at reference load after a completed regeneration | Same load every time, noted in kW |
| Regeneration count since last entry | From the controller log |
| Oil top-up volume since last entry | Litres |
| Fuel source and grade | Supplier and specification |
| Visual notes at inlet face (if accessible) | Colour, deposits, cracks |
| Action taken | None, inspection, cleaning, replacement |
Three or four entries are usually enough to show whether post-regeneration ΔP is rising steadily. That trend, not any single reading, is what tells you a clean is approaching. Our RECD asset register for multi-site DG fleets shows how to scale this across sites.
What This Means When You Choose an RECD
Ash capacity is a design property, not just a maintenance issue. Larger filter volume, wider inlet channels and good service access all make ash easier to live with. When comparing devices, ask for the expected ash cleaning interval on your engine’s oil consumption, and how the filter is removed. Our article on comparing RECD designs by ash capacity and service access gives a structured way to do that, and service clearance around an RECD explains the space you will need on site.
The MSHA / NIOSH guidance on particulate filters also notes that some filter systems are cleaned of both soot and ash with reverse-flow compressed air into a collection bag, which is a useful reminder that ash removal always involves physical handling.
Frequently Asked Questions
Does RECD regeneration remove ash?
No. Regeneration oxidises soot. Ash is non-combustible metal residue and stays in the filter until it is physically cleaned.
What is ash in a diesel particulate filter made of?
Mostly oxides, sulphates and phosphates of calcium, zinc, magnesium and other metals from lubricating oil additives, with smaller contributions from fuel, wear metals and corrosion.
How do I know if my RECD is ash-loaded?
Compare ΔP at a fixed load immediately after a completed regeneration with the commissioning value. A steadily rising post-regeneration ΔP, along with more frequent regeneration, usually points to ash.
How often does an RECD need ash cleaning?
It depends on running hours, oil consumption and filter size. Your supplier should state an expected interval for your engine. Trend data from your own logs is the best guide.
Can I clean ash by forcing extra regeneration?
No. Extra regeneration does not remove ash and can make it harder to clean by sintering it.
Does ash affect emission performance?
Ash itself is trapped, so it does not usually worsen particulate capture. Its main effects are higher back-pressure, more frequent regeneration and reduced soot capacity.
Treat Soot and Ash as Two Separate Jobs
Regeneration handles soot. Cleaning handles ash. Mixing them up leads to wasted regeneration cycles, rising back-pressure and filters that wear out early. Track post-regeneration ΔP, control oil consumption, and plan ash cleaning before the filter forces the issue.
Aceget supplies CPCB type-approved Retrofit Emission Control Devices, and our team can help you plan regeneration and ash cleaning around your DG set’s running pattern. Read our RECD maintenance guide or contact us to discuss your site.