- October 5, 2026
- Posted by: Aceget
- Category: RECD & Emission Control Devices
A textile processing unit in Panipat already did the hard part. Its 1010 kVA generator, too large for any other compliance route in the National Capital Region, carries a certified retrofit emission control device. Now the company’s finance team has spotted something else: a piped natural gas line runs along the industrial road, and a dual fuel kit could replace a large share of the set’s diesel with cheaper gas.
The plant head asks a simple question. If we convert to dual fuel, can we keep the RECD? Will the two systems work together, and will we still be compliant?
The answer is yes, an RECD can be used after a dual fuel conversion, and in some situations it makes good sense. But the two systems were usually designed separately, and they meet in the engine’s air path and exhaust stream. Gas changes how much soot reaches the filter, how hot the exhaust runs and what unburned fuel the catalyst sees. A few interface checks make the difference between two systems that cooperate and two systems that fight.
This guide sets out where the two routes stand in regulation, why a site might want both, what changes in the exhaust after conversion, and the six questions to settle before you run them together.
Two Routes, One Generator
In NCR, CAQM Direction No. 76 of September 2023 framed DG compliance largely by engine power:
| Engine power band | Broad route under the direction |
|---|---|
| 19 kW to 125 kW | Dual fuel |
| 125 kW to 800 kW | Dual fuel or RECD |
| Above 800 kW | RECD |
The bands are in kW, while most generator records list kVA. At a typical 0.8 power factor, kW is roughly 0.8 times kVA. Rules and interpretations have been revised before, so always check the current direction and your State Pollution Control Board’s position. Aceget’s DG set emission regulations page summarises the framework, and our comparison of RECD vs dual fuel kit sets out the trade-offs between the two routes.
The table shows that dual fuel and RECD are presented as alternatives in the middle band. They are not mutually exclusive in engineering terms. A dual fuel engine still burns diesel, and an RECD can treat its exhaust. The question is whether combining them is useful, and how to do it properly.
How a Dual Fuel Kit Works, Briefly
Most retrofit dual fuel kits for DG sets work by fumigation. Natural gas is metered into the engine’s intake air, usually through a mixer or injector upstream of the turbocharger or in the intake manifold. The engine draws in a lean mixture of air and gas. The diesel injection system still delivers a reduced quantity of diesel, which ignites from compression and in turn ignites the gas.
The share of energy supplied by gas is called the substitution rate. It varies with load: low at light load, higher at mid to high load, and limited at the top end by knock and exhaust temperature. Many kits cut back gas or switch to diesel-only operation when gas pressure drops, exhaust temperature climbs too high or a fault is detected. Our explainer on dual fuel generators and how they work covers the principle in more depth, and Aceget’s dual fuel kit page describes the hardware.
Why a Site Might Want Both
Combining a dual fuel kit with an RECD is not the default for every site. It makes sense in four situations.
1. The RECD is mandatory and dual fuel is a cost decision
This is the Panipat case. A set above 800 kW needs an RECD. Adding dual fuel does not change that requirement; it is purely a fuel cost decision. The question is not whether to keep the RECD but how to make the two work together.
2. Diesel fallback happens often
When the gas supply is interrupted, during pipeline maintenance, a pressure drop or a fault, the kit returns the engine to full diesel operation. In that mode, the engine is an ordinary diesel again, with ordinary diesel soot. An RECD covers those hours. For a site where gas interruptions are common, this can be a strong argument.
3. Residual particulate still matters
Even in dual fuel mode, the engine still burns the pilot diesel, and particulate emissions, though usually reduced, are not eliminated. For sites close to homes, schools or hospital wards, the extra reduction an RECD provides may be worth having. Our article on RECDs for hospitals discusses sensitive receptors.
4. A transition plan
Some sites fit an RECD now and plan a gas connection later, when a pipeline reaches the area. Others fit dual fuel now but expect to need an RECD if rules change. Understanding how the two interact protects either plan.
What Changes in the Exhaust After Conversion
Gas alters the exhaust in ways that matter to an RECD. The direction of each change depends on the operating mode.
| Exhaust property | Diesel-only mode | Dual fuel, light load | Dual fuel, mid to high load |
|---|---|---|---|
| Soot reaching the filter | Normal diesel level | Often only slightly lower, because substitution is low | Usually much lower |
| Unburned hydrocarbons, including methane | Low | Can be high, as lean gas mixture burns incompletely | Moderate, falling as combustion improves |
| Carbon monoxide | Low to moderate | Often higher | Moderate |
| Exhaust temperature | Normal for load | Similar or slightly higher | Often higher than diesel at the same load |
| Lubricating oil ash | Unchanged | Unchanged | Unchanged |
Two items stand out. First, soot falls sharply at higher substitution rates, which means the filter loads more slowly. Second, unburned methane and other hydrocarbons rise, especially at light load. That unburned gas is often called methane slip, and it is a known issue for dual fuel engines. Research such as the study on methane slip reduction in conventional dual-fuel natural gas diesel engines and the work on emissions from dual fuel diesel and natural gas vehicles describe how slip varies with load and injection strategy.
The Six Interface Questions
When an RECD and a dual fuel kit share an engine, these are the points where they meet. Settle each before running the combined system.
Question 1: Does the RECD’s backpressure upset the kit’s calibration?
Fumigation kits meter gas in proportion to the air the engine draws in. Exhaust backpressure affects how well the engine breathes, which in turn affects air flow and the air-to-gas ratio. A filter that is clean on the day the kit is tuned will be partly loaded a month later, with slightly higher backpressure.
What to do: commission the dual fuel kit with the RECD already in place, so the kit is tuned on the real exhaust system. Ask the kit supplier whether their control adapts to changes in air flow, or whether retuning is needed after RECD cleaning or replacement. Keep total backpressure within the engine maker’s limit at all times. Our explainer on how exhaust backpressure affects a DG set describes the limits.
Question 2: Can the catalyst handle unburned gas?
A diesel oxidation catalyst is designed to oxidise carbon monoxide and the heavier hydrocarbons in diesel exhaust. Methane is a very stable molecule and needs much higher temperatures to oxidise than most diesel hydrocarbons. A standard diesel catalyst will typically convert only a limited share of methane slip at normal DG exhaust temperatures.
There is a second effect. Hydrocarbons and carbon monoxide that do oxidise on the catalyst release heat. If unburned fuel levels are high, that heat raises the temperature inside the device. In normal operation this is modest. If the kit malfunctions and floods the exhaust with unburned gas, the catalyst and filter could see a temperature rise beyond normal.
What to do: ask the RECD vendor whether their catalyst has been assessed with dual fuel exhaust, what methane conversion to expect and what maximum temperature the device tolerates. Ask the kit supplier what protections cut gas on misfire or abnormal exhaust temperature. Do not assume the RECD will solve methane slip.
Question 3: How will regeneration behave with less soot?
Lower soot in dual fuel mode is good news for the filter. Passive regeneration may be easier because exhaust temperatures at mid to high load are often higher. But a set that spends long periods at light load in dual fuel mode may produce soot that is wetter with unburned hydrocarbons, and a filter that loads slowly may also see fewer clear regeneration events.
What to do: review the backpressure trend over the first few months of combined operation. Expect a slower rise. If the trend climbs and does not fall after high-load runs, investigate.
Question 4: Where will the kit’s exhaust temperature sensor sit?
Many dual fuel kits watch exhaust temperature and reduce gas if it climbs too high, protecting turbochargers and valves. The RECD also has a temperature sensor, usually at its inlet. If the two are installed by different contractors, it is easy to end up with sensors in poor positions, or with one blocking access to the other.
What to do: agree sensor positions in a single drawing. The kit’s protective sensor should normally sit close to the engine or turbocharger outlet, upstream of the RECD, so that the device does not delay or mask the reading. The RECD’s sensors sit at its own inlet and outlet.
Question 5: How will the monitoring systems talk?
The RECD’s monitoring records backpressure and temperature. The kit’s controller records substitution rate, gas pressure and fault events. On their own, neither tells the whole story.
What to do: ask whether the two systems can share data or at least export logs with matching timestamps. Being able to see substitution rate alongside backpressure and inlet temperature makes troubleshooting much faster.
Question 6: Are approvals, warranties and compliance clear?
Approval for the device comes through CPCB’s System and Procedure for Emission Compliance Testing of RECDs, while dual fuel kits follow their own approval route. The combination may not have been tested as a pair.
What to do:
- Ask the RECD vendor to confirm in writing that dual fuel operation does not void their warranty or approval documentation
- Ask the kit supplier the same about the RECD
- Ask your State Pollution Control Board how the combined configuration should be recorded in your consent or registration
- Keep both sets of documents together for inspections
Our article on whether an RECD can be added to a generator with an existing DPF or SCR covers a related question about stacking systems, and how to read an RECD test certificate explains what approval documents contain.
Safety Where Gas Meets Hot Exhaust
Adding a gas train to an engine that already has a hot RECD on its exhaust brings a few safety points that neither supplier may raise on their own.
- Separation. Gas piping, regulators and solenoid valves should be routed well away from the RECD housing and hot exhaust pipework. A small gas leak near a hot surface is a serious hazard.
- Ventilation. In enclosed plant rooms, ventilation must cover both engine heat, which the RECD adds to if any part of it sits inside, and the possibility of gas accumulation. Follow the gas distribution company’s and the kit supplier’s ventilation guidance.
- Leak detection. Gas detectors in the generator room, interlocked with the gas shut-off valve, are a sensible addition where they are not already required.
- Hot work. Any later work on the RECD that involves cutting or welding near the gas train needs gas isolation, purging where applicable, a hot work permit and a fire watch.
- Misfire protection. Unburned gas reaching a hot catalyst can oxidise there and raise device temperature. Confirm that the kit cuts gas promptly on misfire, loss of diesel pilot or abnormal exhaust temperature.
- Emergency shutdown. Make sure the operator knows how to isolate gas and stop the engine from a safe position.
The Cost View: Running Both Systems
A combined installation has running costs from both sides, and it is worth listing them together before deciding.
| Cost item | From the RECD | From the dual fuel kit |
|---|---|---|
| Routine inspection | Backpressure trend review, sensor checks | Gas train inspection, leak checks, calibration |
| Periodic service | Ash cleaning at intervals set by hours and oil use | Mixer, regulator and valve service as per supplier |
| Monitoring | Device data platform, if subscription based | Kit controller logs, if subscription based |
| Fuel | No direct effect, beyond a small backpressure penalty | Gas purchased, diesel saved |
| Downtime | Filter removal for cleaning | Recalibration after engine or exhaust changes |
On a set with long running hours and good gas substitution, fuel savings usually dominate the picture. On a set that runs only a few hundred hours a year, the savings shrink and the fixed costs of two systems weigh more heavily. Run the numbers with your actual hours and load profile. Our article on tips to reduce diesel generator fuel consumption has further ideas on the fuel side.
Which Should Be Fitted First?
Sites often ask about the order of works.
| Situation | Suggested order | Reason |
|---|---|---|
| RECD already fitted, dual fuel planned | Inspect and clean or regenerate the RECD, then fit and tune the kit with the RECD in place | Kit is calibrated on the real exhaust system |
| Dual fuel already fitted, RECD planned | Fit the RECD, then retune the kit | Added backpressure may change air flow and substitution |
| Both planned together | Fit the RECD first, then the kit, with one commissioning plan | Single set of baseline readings and sensor positions |
In every case, record baseline readings: backpressure at a fixed load, exhaust temperature at the turbocharger outlet and at the RECD inlet, and substitution rate. These baselines are what you compare against later.
When One System Is Enough
Combining the two is not always worthwhile. Consider these cases:
- A set in the middle band with reliable gas, mostly high-load running, and no sensitive receptors nearby. Dual fuel alone may meet requirements and deliver savings. An RECD adds cost with limited benefit.
- A set in the middle band with no gas supply on the horizon. An RECD alone is the practical route.
- A set above 800 kW. An RECD is required. Dual fuel is optional and justified only by fuel economics.
- A set in the lowest band. Dual fuel is the directed route; an RECD may add little unless local conditions require it.
Our guide on RECD cost in India and the article on gas generators vs diesel generators help with the economic side.
A Worked Example: Textile Processing Unit in Panipat
This illustrative case follows the plant from the opening.
The starting point
- 1010 kVA set, roughly 808 kW, above the 800 kW threshold, so an RECD route applies
- RECD fitted 14 months earlier, with backpressure and temperature monitoring
- Set runs about 2,200 hours a year at an average of 65% load during grid shortages and evening peaks
- PNG pipeline available at the gate
The dual fuel decision
The finance team estimated a meaningful fuel cost saving at the site’s load pattern, with gas substitution rates highest during the long mid-load runs. The kit supplier proposed a fumigation system with gas cut-off on high exhaust temperature, low gas pressure and misfire detection. Aceget’s 1010 kVA dual fuel kit page shows the type of system involved.
How the interface questions were settled
| Question | Resolution |
|---|---|
| Backpressure and calibration | RECD regenerated with a long loaded run, backpressure recorded, then kit tuned with RECD in place; supplier agreed to check calibration after each RECD cleaning |
| Catalyst and methane | RECD vendor confirmed device temperature limits and stated that methane conversion would be limited; kit protections set to cut gas on abnormal exhaust temperature |
| Regeneration | Monitoring trend to be reviewed monthly for six months |
| Sensor positions | Single drawing prepared; kit thermocouple at turbocharger outlet, RECD sensors at device inlet and outlet |
| Monitoring | Both systems export logs with synchronised clocks; plant engineer reviews them together |
| Approvals | Written confirmations from both suppliers; SPCB informed of the configuration; documents filed together |
Six months later, illustrative
The backpressure trend rose more slowly than in the previous diesel-only period, consistent with lower soot. On two occasions, gas supply dropped and the kit returned the engine to diesel-only mode for several hours; the RECD handled those runs as normal. Visible smoke at the stack fell, and the fuel bill came down. Textile sites face other site-specific issues such as lint around the set; our guide to RECDs for textile mills covers those.
Maintenance When Both Systems Run Together
| Task | Frequency | Notes |
|---|---|---|
| Review backpressure trend with substitution rate | Monthly | Look for unexplained rises |
| Check kit calibration | After any RECD cleaning or replacement | Backpressure change can shift air flow |
| Inspect gas train for leaks | As per kit supplier and gas safety rules | Safety first |
| Check sensor condition and positions | Quarterly | Both systems |
| Review diesel-only running hours | Monthly | High fallback hours mean more soot to the filter |
| Engine service | As per engine maker | Oil, filters, injectors, valve clearances |
| RECD ash cleaning | As the trend indicates | Lubricating oil ash is unchanged by gas |
The RECD maintenance guide covers the device routine in more detail.
Questions Owners Ask About Dual Fuel and RECDs
Can I keep my RECD after converting my generator to dual fuel?
Yes, in most cases. The RECD continues to treat the exhaust, and it covers diesel-only operation whenever gas is unavailable. Tune the dual fuel kit with the RECD in place and confirm warranties with both suppliers.
Does dual fuel reduce soot reaching the RECD?
Usually yes, especially at mid to high loads where gas substitution is highest. At light load the reduction may be small.
Will the RECD remove methane slip from a dual fuel engine?
Only partly. Standard diesel oxidation catalysts convert methane poorly at typical generator exhaust temperatures. Methane slip should be managed through the kit’s calibration and the engine’s operating range.
Does an RECD affect a dual fuel kit’s performance?
It can affect air flow through backpressure, which may change the air-to-gas ratio. Commission the kit with the RECD fitted and check calibration after RECD cleaning.
Do I need both in NCR?
It depends on engine power. Above 800 kW, an RECD route applies; dual fuel is optional. In the 125 to 800 kW band, either route was allowed under Direction 76. Check current rules with CAQM and your SPCB.
Which should I fit first?
If both are planned, fit the RECD first and then tune the kit. If one is already fitted, adjust the other around it and record fresh baseline readings.
Two Systems, One Plan
A dual fuel kit and an RECD can share a generator comfortably, provided they are treated as one system rather than two separate purchases. Tune the kit on the real exhaust, understand that the catalyst is not a cure for methane slip, place sensors on a single drawing, read the two sets of data side by side and keep approvals tidy. Done that way, the plant gets lower fuel bills from the gas and dependable particulate control from the filter, including on the days the gas does not arrive.
Aceget supplies both CPCB type-approved RECDs and dual fuel kits, and can plan them together for engines from 25 kVA to 2500 kVA. For the size in this article’s example, see the 1010 kVA DG set RECD. To plan a combined installation, contact Aceget’s engineers, or compare ratings on the retrofit emission control device page.