October 30, 2026
Posted by: editor
Category: RECD & Emission Control Devices

Short answer first, because that is what most people searching do RECDs reduce NOx actually want: it depends entirely on which technology is inside the device. The particulate-focused RECDs that make up most retrofits on Indian DG sets, meaning wall-flow filters, catalysed filters, oxidation catalyst plus filter combinations, electrostatic units and wet scrubbers, do not materially reduce total nitrogen oxides. Some of them, particularly catalysed designs, can change how that NOx is split between nitric oxide (NO) and nitrogen dioxide (NO2).

Real NOx reduction comes from a different family of equipment: selective catalytic reduction (SCR) with a urea reductant, exhaust gas recirculation and other engine measures, or, rarely for gensets, lean NOx traps. If a quotation promises lower NOx and none of these appear in the specification, the claim needs questioning.

This article is built as a lookup: a short NOx refresher, then the same four-line profile for each technology, then verification, a worked example and real NOx options.

Why the question keeps coming up

RECDs are sold in India primarily to meet particulate matter requirements. The Central Pollution Control Board’s procedure for emission compliance testing of retrofit devices covers in-use DG engines up to 800 kW and requires type-approved RECDs to show at least 70% PM capture efficiency. Testing is carried out by recognised agencies such as ARAI and ICAT. That framework is about PM, not nitrogen oxides.

Confusion creeps in because brochures say “reduces harmful emissions” without naming which ones, because people assume any catalyst treats every gas, and because buyers who know that new CPCB IV+ gensets control NOx assume a retrofit does the same. None of those assumptions hold automatically.

NOx basics in five minutes

What NOx actually is

NOx is shorthand for two gases: nitric oxide (NO) and nitrogen dioxide (NO2). In raw diesel exhaust, NO typically dominates, with NO2 usually a minority share of total NOx. The exact split varies with engine design, load and any aftertreatment fitted.

NOx mass is usually reported “as NO2”, counting all the NO at NO2’s molecular weight. That keeps reporting consistent but hides the real NO/NO2 split; only separate measurement of both gases reveals it.

How diesel engines form it

Most NOx in a diesel engine is “thermal” NOx. Nitrogen and oxygen from the intake air combine in the hottest zones of the combustion flame. Three conditions push it up: high peak flame temperature, plenty of available oxygen and enough time at temperature. Diesel engines run lean, with excess air, and modern high-efficiency combustion runs hot, so NOx formation is an inherent trade-off of efficient diesel operation.

This is why NOx and PM tend to pull in opposite directions in the cylinder: cooler combustion (retarded timing, recirculated exhaust) usually lowers NOx but can raise soot, and hotter, more complete combustion does the reverse.

Why it is hard to treat in diesel exhaust

Reducing NO back to nitrogen is a chemical reduction reaction. Diesel exhaust is oxygen-rich, which is an oxidising environment. The three-way catalysts used on petrol cars work because those engines run near the stoichiometric point, with very little spare oxygen. In lean diesel exhaust the oxygen wins, and a simple catalyst ends up oxidising NO to NO2 rather than reducing it.

To reduce NOx in a lean exhaust you need either a dedicated reductant that reacts selectively with NOx in the presence of oxygen (which is what SCR does with ammonia from urea), a storage-and-purge strategy (lean NOx traps), or prevention at source inside the engine. A particulate filter, however well made, is designed to capture solids. It has no reductant and no mechanism to turn NOx into nitrogen.

Do RECDs reduce NOx? A technology-by-technology lookup

Each profile below uses the same four headings. “Total NOx” means NO plus NO2 together. “NO2 share” means the fraction of that total present as NO2. Effects are described in typical, qualitative terms because real results depend on the engine, the specific design, catalyst formulation, exhaust temperature and fuel.

Uncatalysed wall-flow filter

What it targets: Solid particulate matter, mainly soot. Exhaust is forced through porous channel walls, which capture particles very efficiently, including ultrafine solids.
Effect on total NOx: Negligible. There is no catalyst and no reductant, so NOx passes through essentially unchanged.
Effect on NO2 share: Little change in most cases. Any NO2 already present in the engine-out gas may react with trapped soot at suitable temperatures, which converts some NO2 back to NO, but the total stays the same.
What to ask: How does the filter regenerate without a catalyst (active heating, burner, off-line cleaning)? If it does not rely on NO2, the NOx question is largely irrelevant to its operation.

Catalysed filter (CDPF) and DOC plus filter combinations

What it targets: Particulate matter in the filter; carbon monoxide and hydrocarbons over the catalyst once it is above light-off temperature.
Effect on total NOx: Essentially unchanged. Oxidation catalysts and catalysed coatings do not reduce NOx to nitrogen in lean exhaust.
Effect on NO2 share: This is where the story gets interesting. A platinum-group catalyst oxidises part of the NO to NO2 (2NO + O2 gives 2NO2). That NO2 is useful: it oxidises trapped soot at much lower temperatures than oxygen alone, which is the basis of passive “NO2-assisted” regeneration. As it oxidises soot, NO2 is reduced back to NO. So two processes compete. Whether the NO2 share at the tailpipe ends up higher or lower than engine-out depends on how much NO2 is made versus how much is consumed by soot. Where NO2 production exceeds what the soot can use, NO2 at the outlet can rise noticeably. DieselNet’s overview of diesel oxidation catalysts notes that this increased NO2/NO ratio is controversial because NO2 is more toxic than NO.
What to ask: Has the supplier measured NO and NO2 separately, at inlet and outlet, at representative loads? Is there a design target for limiting NO2 slip? What happens at light load, where there may be little exhaust heat to drive soot oxidation?

For the operating differences between these two families beyond NOx, such as regeneration behaviour and fuel sulphur sensitivity, see our guide to catalysed vs uncatalysed RECD filters. The DieselNet page on DPF regeneration also describes NO2 as the dominant mechanism of passive regeneration.

Partial-flow (flow-through) filter

What it targets: A share of the particulate matter. Only part of the exhaust passes through a filtering medium; the rest flows past, so capture is lower and more variable than in a wall-flow design.
Effect on total NOx: Negligible.
Effect on NO2 share: Many partial-flow designs depend on an upstream oxidation catalyst to generate NO2 for continuous soot oxidation, so they can show the same NO2 shift as catalysed wall-flow filters. An uncatalysed partial filter would show little change.
What to ask: Is there an oxidation catalyst ahead of the filter element? Has NO2 been measured across the system? The structural trade-offs are covered in wall-flow vs partial-flow filtration in RECD systems.

Electrostatic precipitator type

What it targets: Particles, which are charged by a high-voltage corona and then collected on plates or surfaces.
Effect on total NOx: Not a NOx control device. Any effect on total NOx would be expected to be small.
Effect on NO2 share: Generally small. Corona discharge can create reactive species, including ozone in some designs, and these can oxidise a portion of NO to NO2. How much depends heavily on design and operating voltage, so do not assume either way.
What to ask: Has the supplier checked for ozone and for any change in NO/NO2 split? What happens to collected soot, and how is it removed? A broader buyer comparison sits in filter-based vs electrostatic RECDs.

Wet scrubber

What it targets: Particles and water-soluble gases, by contacting exhaust with water or a scrubbing liquid.
Effect on total NOx: Usually small. NO, the dominant component, has very low solubility in water and passes through a plain water scrubber largely untouched.
Effect on NO2 share: NO2 is more reactive with water than NO, so a scrubber may remove a portion of the NO2 that is present. Because NO2 is typically only a minority share of diesel NOx to begin with, the impact on total NOx is limited. Chemically dosed scrubbers designed specifically for NOx exist in industrial settings, but they are a different, more complex class of equipment.
What to ask: What scrubbing liquid is used? Was NOx measured at inlet and outlet? Where does the spent liquid go, and is it treated? See wet scrubbing vs dry filtration for DG retrofits for the wider comparison.

Selective catalytic reduction (SCR)

What it targets: NOx specifically. A urea-water solution (often sold as AdBlue or DEF) is injected into the exhaust, decomposes to ammonia, and the ammonia reacts with NOx over a dedicated catalyst to form nitrogen and water.
Effect on total NOx: This is genuine NOx reduction. Well-designed and correctly dosed systems can achieve high conversion within their operating temperature window. Performance falls at low exhaust temperature, so light-load and short-run duty matter.
Effect on NO2 share: SCR consumes both NO and NO2. In fact, systems often perform better when an upstream oxidation catalyst raises NO2 to roughly match NO, which is why combined DOC, filter and SCR layouts exist.
What to ask: What is the temperature window? How is urea dosing controlled? What is the design limit for ammonia slip, and is there an ammonia guard catalyst? What happens to dosing during cold starts? DieselNet’s SCR technology page gives technical background on reductants, fouling risks at low temperature and ammonia slip.

Our explainers on how SCR technology reduces diesel engine emissions and AdBlue and urea systems in modern gensets go further into operation and consumables.

EGR and engine-side measures

What it targets: NOx formation inside the cylinder rather than treatment afterwards.
Effect on total NOx: Can be significant. Exhaust gas recirculation dilutes the intake charge with inert gas, lowering peak flame temperature. Injection timing changes, charge-air cooling and combustion redesign have similar aims.
Effect on NO2 share: Not a primary effect.
What to ask: These measures are normally engineered and validated by the engine manufacturer; retrofitting EGR to an in-use genset is not common practice. Altering timing yourself can raise smoke, fuel use and PM, and may affect warranty. Treat any proposal as an engine engineering project.

Lean NOx traps

What it targets: NOx, stored on an adsorbent during lean running and released and reduced during brief rich “purges”.
Effect on total NOx: Can reduce NOx, mainly demonstrated on light-duty vehicle engines.
Effect on NO2 share: Varies with design.
What to ask: Lean NOx traps need precise engine control to create rich purge events and are sensitive to fuel sulphur. They are rarely, if ever, offered for stationary DG sets. If one is proposed, ask for genset-specific field references.

Summary comparison table

Technology	Primary target	Effect on total NOx	Effect on NO2 share	Typical genset retrofit relevance
Uncatalysed wall-flow filter	Solid PM	Negligible	Little change	Common PM retrofit
Catalysed filter / DOC + filter	PM, plus CO and HC above light-off	Essentially unchanged	Can rise, depending on NO2 made vs used	Common PM retrofit
Partial-flow filter	Part of the PM	Negligible	Can rise if catalyst upstream	Used in some retrofits
Electrostatic type	Charged particles	Small	Small, design-dependent	Used in some retrofits
Wet scrubber	Particles, soluble gases	Usually small	May remove some NO2	Used in some retrofits
SCR with urea	NOx	Genuine reduction within temperature window	Consumes both NO and NO2	Specialist retrofit or factory-fit
EGR / engine measures	NOx formation	Can be significant	Not a primary effect	Factory engineering
Lean NOx trap	NOx	Reduction possible	Varies	Rare for gensets

If you take one line from this table: a device that captures particles is not, by itself, a NOx control device.

Why the NO2 share matters

If total NOx does not change, why care how it is split? Because NO and NO2 behave differently once they leave the stack.

NO2 is the more harmful of the two at the concentrations people are likely to breathe. The US EPA’s basic information on NO2 explains that breathing elevated NO2 irritates the airways and can aggravate respiratory conditions such as asthma, and that NO2 also contributes to the formation of ozone and fine particulate matter. NO released from a stack does oxidise to NO2 in the atmosphere over time, but that takes time and distance. A higher NO2 fraction at the outlet means more NO2 is present immediately, close to the source.

That matters most where exhaust can reach people before it disperses:

Basement and semi-enclosed DG rooms, where leaks from flanges or poor ventilation can let exhaust accumulate. Planning for these spaces is covered in our guide to RECD installation in basement DG rooms.
Stacks terminating near fresh-air intakes, openable windows, balconies or terraces in dense residential and commercial buildings.
Courtyards and narrow service lanes where dispersion is poor, similar to the “street canyon” concern raised for vehicle emissions.
Maintenance work around a running set, where technicians may be close to joints and sampling ports.

None of this makes a catalysed RECD unsafe; the PM reduction it provides has real health value. It does mean stack termination, separation from intakes and ventilation deserve attention, and a supplier should be able to say what their design does to NO2. For workplace exposure limits, consult your EHS adviser.

How to verify a NOx claim

If a supplier states or implies that their RECD reduces NOx, a credible claim should include the following.

Named technology. The claim should say which component reduces NOx and by what mechanism. “Advanced catalyst” is not an answer. SCR, EGR or a named NOx storage technology is.
Both NO and NO2 measured. A total NOx number on its own can hide an NO2 increase. Ask for NO and NO2 reported separately, at inlet and outlet.
Same load, same engine, same session. Before and after readings taken at different loads or on different days are not comparable, because NOx changes strongly with load. Our article on why generator load must be matched in RECD comparison tests explains why.
Stated sampling locations. Upstream and downstream ports should be identified on a drawing and placed where the flow is well mixed.
Suitable instruments with calibration records. Chemiluminescence analysers or electrochemical cells rated for NO and NO2 are typical. Some portable analysers estimate NO2 rather than measuring it directly, so check the specification. Ask for valid instrument calibration records.
Consistent units and corrections. ppm, mg/Nm3 and g/kWh are different measures, and oxygen correction or dilution can change the picture.
Certificate scope. Read exactly what the type approval or test certificate covers. Under the CPCB RECD procedure, the compliance focus is PM capture. A CPCB RECD certificate is not, by itself, evidence of NOx reduction. Our guide to reading an RECD test certificate shows where to look for scope and test conditions.
Multiple operating points. NOx performance, especially for SCR, depends on temperature. A single full-load reading says little about the 30% load your set may spend most of its life at.

If a NOx claim matters to your decision, write it into the purchase order as a measurable requirement with a defined test method, rather than accepting brochure wording. Our guidance on RECD tender specifications with measurable performance requirements covers how to phrase that.

Illustrative example: same NOx, more NO2

The following figures are illustrative only. They are chosen to make the arithmetic clear, not to represent any particular engine or product.

A DG set is tested at a steady, matched load. Upstream of a catalysed filter, the analyser reads:

Total NOx: 1,000 ppm
NO2 share: 10%

So upstream:

NO2 = 1,000 × 0.10 = 100 ppm
NO = 1,000 − 100 = 900 ppm

Downstream of the catalysed filter, on the same set at the same load, total NOx is essentially unchanged at about 1,000 ppm, but the NO2 share has risen to 35%:

NO2 = 1,000 × 0.35 = 350 ppm
NO = 1,000 − 350 = 650 ppm

What changed:

Total NOx: 1,000 → 1,000 ppm, a change of 0%
NO2: 100 → 350 ppm, an increase of 250 ppm; downstream NO2 is 350 ÷ 100 = 3.5 times the upstream level
NO: 900 → 650 ppm, a fall of 250 ppm

The 250 ppm of NO that “disappeared” was oxidised to NO2. A report showing only the NO channel could claim a 250 ÷ 900 = about 27.8% “NO reduction”: true, but misleading. A report showing only total NOx would correctly show no change but hide the NO2 rise. Only both gases give the full picture.

Two caveats. Real results can range from a small NO2 change to a large one, depending on catalyst loading, exhaust temperature and how much soot is available to consume NO2. And readings must use a consistent dry or wet basis and oxygen reference, or dilution will distort the comparison. Our article on how dilution misleads before and after measurements explains that trap.

NOx vs particulate control: keep the goals separate

A useful habit when assessing any emission claim is to ask, pollutant by pollutant, what is supposed to change and why. The answers are different for each:

PM and soot: This is what most RECDs are designed for, and what the CPCB RECD procedure measures.
CO and hydrocarbons: Can fall where the design includes an active oxidation catalyst above light-off; little effect from uncatalysed filters. See our sibling article on whether an RECD can reduce carbon monoxide and unburned hydrocarbons.
CO2: Does not fall because of an aftertreatment device. Oxidising CO, HC and soot creates small amounts of CO2, and added back-pressure can slightly raise fuel use. Our article on whether an RECD reduces CO2 emissions walks through the reasoning.
NOx: Essentially unchanged by particulate devices. Needs SCR, engine measures or a change of power source.

Keeping these four lines separate in procurement documents stops an RECD being bought for a job it was never designed to do.

What to do if you genuinely need lower NOx

Some sites do need to cut NOx: those near sensitive receptors, those under specific consent conditions, or organisations with internal environmental targets. Here are the realistic routes.

Replace with a CPCB IV+ genset

The CPCB IV+ emission norms for new genset engines up to 800 kW, notified by the Ministry of Environment, Forest and Climate Change (the draft published in 2022 proposed a 1 July 2023 start for new gensets), set limits that include NOx alongside HC, CO and PM. Meeting them generally requires a combination of engine technology and aftertreatment, which can include SCR, depending on the engine and rating. Exact limit values depend on the power band, so confirm current figures with the CPCB or MoEFCC notification and the OEM’s type approval documents before relying on any number.

Replacement is a large capital decision; our RECD vs new CPCB IV+ genset cost comparison sets out the trade-offs.

SCR retrofit for DG sets

Retrofitting SCR to an in-use genset is possible but is a specialist project. It needs adequate exhaust temperature most of the time, space for dosing, mixing and catalyst, a reliable urea supply, load-responsive controls and an ammonia slip strategy. Sets that mostly run lightly loaded or in short spells may struggle to keep the catalyst in its working window.

Gas and dual-fuel considerations

Switching some or all of the energy source changes combustion and therefore NOx. Gas engines and dual-fuel conversions have their own NOx characteristics, which can be higher or lower than diesel depending on engine design, air-fuel ratio and control strategy, so do not assume a fuel change automatically lowers NOx. Ask for measured data from comparable installations. Our comparison of an RECD vs a dual-fuel kit covers the broader trade-offs.

Operational measures

Hours that are not run produce no NOx. Load management, avoiding needless idle test runs, keeping the engine to specification and using grid supply or storage where possible all cut total emissions. They do not replace compliance obligations.

Whatever route you choose, do not bypass or remove an installed RECD in pursuit of a different emission goal. That creates a PM compliance risk and can lead to sealing and penalties under your State Pollution Control Board’s consent conditions.

FAQ

Do RECDs reduce NOx emissions from DG sets?

Most do not, in any material way. Filter-based, electrostatic and wet-scrubber RECDs are designed for particulate matter. Total NOx stays essentially unchanged. Meaningful NOx reduction needs SCR, engine measures or a different power source.

Can a catalysed DPF increase NO2?

It can. The catalyst oxidises part of the NO to NO2, which helps burn soot at lower temperatures. If more NO2 is produced than the soot consumes, the NO2 share at the outlet rises, even though total NOx stays about the same.

Does CPCB RECD certification cover NOx?

The CPCB RECD procedure focuses on particulate matter capture, with a minimum of 70% for type approval, for in-use DG engines up to 800 kW. Check the scope printed on the certificate itself, and do not treat it as evidence of NOx reduction unless NOx is explicitly covered.

Is SCR a type of RECD?

SCR is an aftertreatment technology aimed at NOx, not particulates. It can be retrofitted to some engines, but it is a separate system from a particulate RECD, with its own reductant supply, controls and temperature requirements.

Does a wet scrubber remove NOx?

A plain water scrubber may absorb some NO2, but NO, which usually makes up most of diesel NOx, is poorly soluble. The overall reduction in total NOx is typically small unless the scrubber is specifically designed and dosed for NOx.

How should I test a supplier’s NOx claim?

Ask for NO and NO2 measured separately at inlet and outlet, on the same set at the same load, with calibrated instruments, consistent units and named sampling points. Compare at more than one load if possible.

Getting the Particulate Side Right

If your compliance need is particulate matter, which it is for most DG owners asked to fit a retrofit device, the priority is a correctly sized, CPCB type-approved RECD and an honest description of what it does. Aceget’s product page describes its RECD as using a silicon carbide ceramic filter with a precious-metal coating, an oxidation catalyst stage and NO2-assisted regeneration, with up to around 96% particulate reduction depending on DG condition and fuel. It is a particulate control device, and we describe it that way.

To see the range by DG rating, visit our retrofit emission control device page. For type approval documents, see our certifications. If you want to discuss your DG set, its duty cycle or a NOx question your consent conditions raise, contact the Aceget team with your nameplate details and load profile, and we will tell you plainly what an RECD can and cannot address.

Do RECDs Reduce NOx? Why the Technology Must Be Identified