RECD Regeneration Explained: Active vs Passive Regeneration

A filter-based Retrofit Emission Control Device does its job by holding back soot. That same soot would choke the filter within days if nothing removed it. Regeneration is the name for removing it, by turning the trapped carbon into carbon dioxide inside the device.

Every filter-type RECD regenerates somehow. What differs is where the heat and the chemistry come from. In passive regeneration, the exhaust does the work on its own. In active regeneration, the device or the operator adds something extra to make it happen. Most real installations use a blend of the two.

This guide is the starting point for our regeneration series. It explains the two approaches in plain terms, shows how each behaves on a diesel generator rather than a truck, and helps you judge which one your DG set’s duty cycle can actually support.

Why a Filter-Based RECD Needs Regeneration at All

The core of most RECDs fitted to Indian DG sets is a ceramic wall-flow filter, the same family of technology as a diesel particulate filter (DPF). Exhaust is forced through porous channel walls. Gas passes; particles do not.

A 250 kVA set running for a few hours can push several grams of soot into the filter. That soot forms a layer on the inlet walls. As the layer thickens:

• Differential pressure (ΔP) across the device rises
• Back-pressure on the engine increases, which affects fuel use and exhaust temperature
• Spare storage in the filter shrinks, so the device gets closer to its safe soot limit

If the soot were never removed, back-pressure would eventually exceed what the engine maker allows. Our article on how exhaust back-pressure affects a DG set explains why that matters for turbochargers, exhaust valves and fuel consumption.

Regeneration resets the cycle. Soot is oxidised, ΔP falls, and the filter has room to trap more. What it does not reset is ash, the non-combustible residue that stays behind. We cover that separately in Soot vs Ash in an RECD: Why Regeneration Does Not Remove Both.

The Chemistry in One Paragraph

Soot is mostly carbon. Carbon can be oxidised by two things present in diesel exhaust:

1. Oxygen (O₂), which is plentiful in lean diesel exhaust but reacts with soot quickly only at high temperature, typically in the region of 550–650 °C.
2. Nitrogen dioxide (NO₂), which is far more reactive and can oxidise soot from roughly 250–300 °C upward, but is present only in small amounts unless a catalyst creates more of it.

Passive regeneration mostly relies on NO₂ at normal exhaust temperatures. Active regeneration mostly relies on O₂ at temperatures the engine would not reach by itself. That single difference explains almost everything else in this article. DieselNet’s overview of filter regeneration goes deeper into the reaction kinetics if you want the technical detail.

Passive Regeneration: Letting the Exhaust Do the Work

A passively regenerating RECD normally has a catalyst, either as a separate diesel oxidation catalyst (DOC) ahead of the filter, as a coating on the filter itself, or both. The catalyst converts some of the nitric oxide (NO) in the exhaust into NO₂. That NO₂ then reacts with the soot sitting on the filter walls.

How it behaves on a running DG set

• It works continuously and quietly whenever the exhaust is warm enough. There is no start, no stop and usually no display message.
• It burns soot in small amounts, so the filter never carries a large load if conditions are right.
• It needs no extra energy, no heater supply and no operator action.

The balance point

Engineers describe passive systems in terms of a balance point temperature: the exhaust temperature at which soot is being burnt as fast as the engine is producing it. Above that temperature, the filter slowly cleans itself. Below it, soot accumulates.

The balance point is not fixed. It shifts with engine-out soot, the NOx-to-soot ratio, fuel sulphur, the catalyst’s condition and the amount of ash already present. That is why we explain in RECD Regeneration Temperature: Why There Is No Universal Setpoint that a single “regeneration temperature” on a brochure is rarely the full story.

Where passive regeneration struggles

The weak point is light load. A DG set idling at 15–25% load may produce exhaust well below the balance point. Soot builds steadily. If the set is then loaded heavily, a large soot cake can burn faster than intended. Our guide on choosing an RECD for a generator running at low load deals with this problem at the selection stage.

Passive systems also depend on the catalyst staying healthy. Lubricating oil additives, high-sulphur fuel and certain contaminants can reduce catalyst activity over time. See catalyst poisoning in an RECD for the warning signs.

Active Regeneration: Adding Heat on Purpose

When the exhaust cannot clean the filter by itself, an active system raises the temperature until oxygen can burn the soot. On DG-set RECDs, three routes are common.

1. Electric heating

An electric heater warms the filter, or the gas entering it, to the point where soot ignites. Once ignition starts, the soot’s own combustion releases heat that helps the reaction travel through the filter. Some designs heat the filter while exhaust flows; others heat it with a small controlled air flow when the engine is stopped.

Electric systems need a reliable power supply sized for the heater. Our article on RECD controller and heater power supply requirements covers cable sizing, supply sources and protection.

2. Fuel-based heating

A small burner, or fuel dosed upstream of an oxidation catalyst, raises the exhaust temperature. The catalyst burns the extra fuel and the heat reaches the filter. This approach is common on vehicles but less common on retrofit DG devices, because it adds fuel handling, dosing hardware and calibration.

3. Load-assisted or engine-assisted regeneration

Some devices ask for the engine to run at a higher load for a period, which lifts exhaust temperature into the range where the device can regenerate. On a generator, that can mean running against a load bank or scheduling a test run at a meaningful load. It is simple but depends on the site’s ability to provide that load.

How an active event looks to an operator

Unlike passive regeneration, active regeneration is a discrete event. It usually:

• Starts when a trigger is reached, such as a ΔP threshold, a soot-load estimate or a run-hour count
• Shows on the controller as a status such as “Regen active” or “Heater on”
• Raises filter outlet temperature noticeably
• Ends when ΔP drops back to its clean band or a timer completes
• Writes an entry to the device’s event log

If your RECD has an operator panel, these signals are worth training staff on. Our RECD alarm response matrix and RECD event log guide show how to turn them into routine checks.

Active vs Passive Regeneration at a Glance

Factor	Passive regeneration	Active regeneration
Main oxidant	NO₂ created by a catalyst	O₂ at raised temperature
Typical working range	Normal exhaust temperature above the balance point	Filter raised to soot ignition range by heater, burner or load
Timing	Continuous, whenever exhaust is warm enough	Discrete events, triggered by ΔP, soot estimate or hours
Extra energy	None	Electricity, fuel or extra engine load
Operator visibility	Low; often invisible	High; status, alarms and logs
Best suited to	Sets that run often at moderate to high load	Sets with light, short or irregular running
Main weakness	Fails to keep up at light load	Needs supply, controls and conditions to complete a cycle
Sensitivity to fuel and oil quality	High (catalyst health)	Moderate
Typical risks	Gradual soot build-up, then a fast burn	Interrupted cycles, inhibited cycles, supply faults

The table simplifies a lot. Many devices combine both: a catalyst handles routine soot whenever the set runs warm, and an active function steps in when ΔP says passive cleaning has fallen behind.

The Hybrid Reality: Most RECDs Do Both

It is tempting to sort devices into two tidy boxes. In practice, a filter-based RECD on a DG set usually behaves as one of four types:

1. Passive only. Catalyst-coated filter, no heater. Relies on duty cycle. Simple and low-maintenance when the load suits it.
2. Passive with active backup. Catalyst does most of the work; a heater or load request clears the filter when ΔP climbs.
3. Active-led. Little or no catalyst. Regeneration happens mainly as scheduled or triggered events.
4. Off-board. The filter is removed and cleaned or regenerated elsewhere. Rare for routine soot on retrofit DG devices, but standard for ash cleaning.

The MSHA / NIOSH explanation of DPF regeneration types, written for mining equipment, uses a similar split and adds the useful idea of “active-passive” systems: equipment hot enough to burn most, but not all, of its soot during normal work.

When you compare quotations, ask each supplier which of these four categories their device belongs to on your load profile, not on a test bench. Our guide to filter-based vs electrostatic RECDs also explains why some non-filter devices do not regenerate in this sense at all.

Matching the Regeneration Strategy to the DG Set’s Duty Cycle

Regeneration choice is really a duty-cycle question. Here is how common Indian DG applications tend to line up.

Prime or long-running sets

Sets that run for many hours at 50–80% load, such as those at factories with unreliable grid supply, usually produce enough exhaust heat for passive regeneration. A catalysed design with a modest active backup tends to work well. See standby vs prime power generators for the background on duty ratings.

Standby sets with weekly test runs

Housing societies, offices and hotels often run their sets for 10–20 minutes a week at light load, plus occasional outages. That pattern rarely reaches a passive balance point. Active regeneration, or a deliberate monthly loaded run, is usually needed. Our RECD purchase planning guide for housing societies discusses how RWAs can plan for this.

Critical-backup sets

Hospitals and data centres need the set available at all times. The question is not only whether regeneration works, but when it is allowed to run and what happens if it is interrupted. We cover that in Can an RECD Regenerate While a Generator Is Supplying Building Loads? and in What Happens When a Generator Stops During RECD Regeneration?.

Short, frequent start-stop sets

Telecom sites and some commercial buildings start and stop many times a day. Each short run adds soot without much heat. These sets often need active regeneration with careful control logic. Read RECD selection for DG sets with frequent short start-stop cycles.

A quick self-assessment

Answer these four questions using your DG load logs:

1. What share of running hours is above 40% load?
2. How long is a typical run, in minutes?
3. How many starts happen per week?
4. Can the site provide a planned loaded run, or a load bank, when needed?

If most hours are above 40% load and runs are long, passive regeneration has a fair chance. If most runs are short and light, plan for active regeneration from the start.

What Operators Actually See: Passive vs Active Day to Day

The two approaches feel very different from the plant room.

On a passive-led device, the main indicator is ΔP at a known load. If it stays roughly level week after week, passive regeneration is keeping up. A steady upward creep suggests either light-load operation or ash accumulation. Logging ΔP at a reference load is the single most useful habit; see comparing RECD pressure readings at different generator loads.

On an active-led device, operators see events. They should know:

• What normally triggers a cycle
• How long a normal cycle lasts
• What the controller shows when a cycle is requested, running, complete, interrupted or inhibited
• What they should and should not do while a cycle runs

Two questions come up repeatedly with active devices. First, why a cycle sometimes refuses to start; that is explained in RECD Regeneration Inhibit Signals. Second, why a cycle starts but does not finish; see RECD Regeneration Not Completing: What Operators Should Record.

Five Misunderstandings About RECD Regeneration

“Regeneration blows the soot out of the stack.” It does not. Soot is oxidised to CO₂ inside the filter. A brief haze during regeneration can occur, but the aim is chemical conversion, not ejection.

“A passive RECD needs no attention.” It needs no regeneration action, but it still needs ΔP monitoring, periodic inspection and eventual ash cleaning. Our article on what a maintenance-free RECD claim actually means unpacks that wording.

“More regeneration is always better.” Each active cycle uses energy and adds thermal stress. Regenerating far more often than soot load requires is a sign of a sensor, settings or engine problem, not good practice. See How Frequently Should a Filter-Based RECD Regenerate?.

“After cleaning, the filter should never ask for regeneration again for months.” Not necessarily. Early requests after cleaning have specific causes, most of them fixable. We list them in Why Does an RECD Request Regeneration Soon After Cleaning?.

“Regeneration and emission testing don’t interact.” They do. A sample taken during or just after regeneration can look worse than normal operation. Our existing guide on RECD testing during regeneration explains how to handle it.

Questions to Ask Your RECD Supplier About Regeneration

Before you sign a purchase order, get written answers to these:

1. Is the device passive, active, or both on our load profile?
2. What exhaust temperature and duration does passive regeneration need to keep up?
3. What triggers an active cycle, and how long does a typical cycle take?
4. What electrical supply does the heater need, if any, and from where?
5. Can a cycle run while the set supplies the building? Can it run with the set stopped?
6. What inhibits a cycle, and how does the panel show it?
7. What happens if the engine stops mid-cycle?
8. How is ΔP measured, logged and made available to our BMS?
9. What is the expected ash cleaning interval, and who does it?
10. What operator training is included?

These questions belong in your technical RFQ. Our guide on how to write a technical RFQ for an RECD project shows where they fit, and operator training in the purchase order explains how to make training a deliverable rather than a favour.

How Regeneration Fits Into Compliance

Indian rules for in-use DG sets focus on particulate capture. CPCB’s system and procedure for RECD type approval requires a minimum PM capture efficiency of 70%, and the procedure itself defines “active regeneration” as any additional measure used to start regeneration on a periodic or continuously regenerating device. In other words, regulators expect regeneration to be part of how the device is designed and tested.

What the rules cannot do is guarantee that regeneration works on your particular set. A device that passes type approval on a test cell can still struggle on a site where the set never gets warm. That is why duty-cycle matching matters as much as the certificate. For the regulatory background, see why RECD is mandatory in India, how to verify a CPCB-tested RECD and the Central Pollution Control Board website.

Frequently Asked Questions

What is RECD regeneration?

It is the process of oxidising soot trapped in a filter-based RECD so that the filter regains capacity and back-pressure falls. Soot is converted to carbon dioxide inside the device.

What is the difference between active and passive regeneration?

Passive regeneration uses normal exhaust heat and NO₂ from a catalyst to burn soot continuously. Active regeneration adds heat through a heater, burner or extra engine load to burn soot in a defined event.

Which is better for a standby DG set?

Standby sets that run briefly at light load rarely get hot enough for passive regeneration alone. A device with active regeneration, or a planned loaded run schedule, is usually more reliable.

Does every RECD regenerate?

Filter-based RECDs do. Electrostatic and some other non-filter designs collect particles differently and do not regenerate in the same way.

Can I tell whether my RECD is regenerating?

On active devices, the controller status and temperature rise show it. On passive devices, a stable ΔP at a reference load over time is the best sign that regeneration is keeping up.

Does regeneration remove ash?

No. Ash is non-combustible and stays in the filter until it is physically cleaned.

Planning Regeneration Before the RECD Arrives

Regeneration is not a feature to read about after installation. It decides whether a filter-based RECD stays clean, whether operators trust it, and whether it keeps working five years later. Match the regeneration approach to how your set actually runs, write the operating rules into the purchase order, and give operators the signals they need.

Aceget supplies CPCB type-approved Retrofit Emission Control Devices for DG sets and helps clients choose the regeneration approach that suits their load pattern. Review our certifications, read how an RECD works, or contact our team with your DG set details for a recommendation.