Small Boiler Emission Norms: The 2025 Dust Limits and How They Are Enforced
Since 15 May 2025, every industrial boiler in India outside the power sector has had to meet a particulate limit set by its size and fuel: 500 mg/Nm³ below 2 TPH, and as low as 100 above 10 TPH. State Boards have moved from notices to consequences — Rajasthan now withholds Consent to Operate from boilers that fail it, and Delhi-NCR has a 50 mg/Nm³ limit with closures behind it. Here is what the rules say, and what it takes to pass them.
10 min read
The short version
The Environment (Protection) Amendment Rules, 2023 — notification G.S.R. 373(E), dated 16 May 2023 — replaced the particulate standards for industrial boilers with one table, set by steam capacity and by fuel. The rules gave industry two years, and they came into force on 15 May 2025. Since then a 5 TPH coal-fired boiler has had to keep its stack below 150 mg/Nm³ of dust — a figure many older dust collectors were never designed to reach.
For two years that was easy to ignore. It no longer is. On 12 March 2026 the Rajasthan State Pollution Control Board told its regional offices to give non-complying units a last opportunity of 60 days to upgrade their pollution control equipment, against a time-bound action plan and a bank guarantee — and not to renew Consent to Operate for any unit that does not comply. In Delhi-NCR the limit for listed industries is now 50 mg/Nm³, and CPCB has directed the NCR State Boards to close units that have not ordered the equipment to meet it.
If your boiler is above 2 TPH and its only dust collector is a cyclone, it is worth finding out where your stack stands before your consent comes up for renewal.
The limits, by size and fuel
The notified table has three capacity bands and two fuel columns. Below 2 TPH, the limit is 500 mg/Nm³ for every fuel. From 2 TPH to below 10 TPH, it is 150 mg/Nm³ for coal, pet coke, furnace oil, LDO and LSHS, and 250 mg/Nm³ for agro-residue and bagasse. At 10 TPH and above, it is 100 mg/Nm³ for the same fossil fuels and 250 mg/Nm³ for agro-residue and bagasse.
Those numbers are a large step down. The standards they replaced ranged from 150 to 1,200 mg/Nm³ depending on boiler size. The final rules were milder than the 2022 draft in one place only: the draft had proposed 50 mg/Nm³ for fossil-fuel boilers of 10 TPH and above, and the notified figure is 100.
The fuel column matters more than it first appears. In the 2 to 10 TPH band, a husk-fired boiler has a limit of 250 mg/Nm³ and the same boiler burning coal has 150. A plant that switches from biomass to coal — or adds coal to the fuel mix in a lean season — moves to the stricter column without changing a single piece of equipment. The fuels a boiler may burn are normally written into its consent, so a fuel change is a consent change, and the dust collector has to be checked against the new limit before it is made, not after.
Who the rules cover
The 2023 rules apply to industrial boilers burning coal or lignite, pet coke, bagasse or other agro-fuels, furnace oil, LDO or LSHS, and to boilers in bagasse and agro-fuel power plants. Thermic fluid heaters and hot air generators are covered too, on an equivalent fuel consumption basis — so a thermic fluid heater is held to the band of the boiler that would burn the same amount of fuel. Captive power plants burning coal or lignite are the exception: they follow the thermal power plant standards of S.O. 3305(E), dated 7 December 2015.
Particulate matter is not the only limit. A separate notification, G.S.R. 96(E) of 29 January 2018, sets 600 mg/Nm³ of SO2 and 300 mg/Nm³ of NOx for boilers burning fuels other than agro-residue and natural gas. Boilers on agro-based fuel and natural gas have no SO2 or NOx limit under it, though they still have to meet the stack height criteria.
Your State Board can be stricter than the national table, and your consent is the document that binds you. Read the conditions in it — the stack, its height, the fuel, the control equipment and the monitoring frequency — rather than relying on the national figures alone.
What mg/Nm³ means, and why excess air does not help
A limit in mg/Nm³ is milligrams of dust per normal cubic meter of flue gas — the gas volume corrected to standard temperature and pressure, so that a hot, expanded gas cannot look cleaner than it is. The rules also fix the oxygen basis: measurements are normalized to 6% oxygen on a dry basis for solid fuels, and 3% for liquid fuels.
That second correction closes the obvious loophole. Blowing more air through the furnace dilutes the dust in each cubic meter, but the normalization scales the reading back up — normalized value equals measured value multiplied by (21 minus the reference oxygen) divided by (21 minus the measured oxygen). A boiler running at high excess air gains nothing on the dust reading, and loses efficiency in the process.
Stack height is set alongside the limits: the greater of H = 14Q^0.3, with Q the SO2 emission in kg/h, and H = 74Q^0.27, with Q the particulate emission in tonnes per hour — and never less than 11 m for boilers below 2 TPH, or 30 m for larger ones.
Why a cyclone alone often does not pass
Many small boilers still run with a cyclone as their only dust collector. A cyclone spins the gas and throws dust to its wall. It works well on coarse particles and progressively worse as particles get smaller, and fly ash from coal and especially biomass carries a large share of fine particles. Those are what pass straight through to the stack.
At 500 mg/Nm³, below 2 TPH, a well-designed multicyclone — many small cyclones in parallel, which spin the gas harder and catch finer dust — may be enough, depending on the fuel's ash and how cleanly it burns. At 150 or 100 mg/Nm³ the fine fraction has to be removed as well, and that is the job of a fabric bag filter or an electrostatic precipitator. The usual arrangement keeps the multicyclone as a first stage: it takes the coarse load and catches sparks before they reach the filter bags.
Wet scrubbers are the other route, and they suit gas that is sticky or acidic. They also make a wet effluent that has to be settled and treated, which is a new problem rather than no problem.
Before you fit a bag filter
A bag filter is not a box bolted onto the flue. Four things decide whether it works on your boiler. The first is draft: a filter adds resistance to the gas path, so the ID fan and its motor have to be checked for the extra pressure drop, or the furnace goes positive and the boiler loses output. The second is temperature: the filter fabric has a maximum operating temperature, and below the flue gas dew point — common during start-up — moisture and acid condense on the bags and blind them, so start-up needs a bypass or a warm-up sequence.
The third is sparks. Husk, wood and other biomass carry burning particles into the flue, and a spark reaching the bags can start a fire; a multicyclone or spark arrestor ahead of the filter is not optional on biomass. The fourth is ash: the filter's hoppers collect fine, dusty ash that has to be emptied, conveyed and stored without becoming the next complaint.
Then measure. A stack test by a laboratory your State Board accepts, at the boiler's normal load, tells you where you actually stand; the fuel's ash content tells you how much dust the collector has to remove. Sampling ports and a safe platform to reach them belong on the stack anyway — they are part of what the Board will ask to see.
What enforcement looks like now
Rajasthan's office order of 12 March 2026 is a useful picture of how State Boards are handling the rules. Units that do not comply but hold a valid Consent to Operate get a last opportunity of 60 days to upgrade their air pollution control equipment, against a time-bound action plan and a bank guarantee. Consent is not renewed for units that do not comply. Where a unit still fails, the order directs action as per law.
Delhi-NCR has gone further. CAQM Direction No. 98 of 20 February 2026 set a particulate limit of 50 mg/Nm³ for the 17 categories of highly polluting industries, Red category medium and large industries, food processing and textile units operating boilers or thermic fluid heaters, and metal industries with furnaces, across the NCR. It replaced the 2022 directions, and after an amendment in July 2026 applies to all of them from 1 October 2026. At the end of October 2026, CPCB directed the State Boards of Delhi, Haryana, Uttar Pradesh and Rajasthan to issue show-cause notices to units that had neither placed work orders for pollution control equipment nor obtained an adequacy assessment or exemption — and to close those that could not show evidence within five days, with revocation of consent, disconnection of electricity and sealing of premises available to them.
The NCR districts of Rajasthan and Uttar Pradesh fall under both regimes. Outside the NCR, the national table and your consent are what apply — and the direction of travel is the same everywhere.
A practical order of work
Start with your consent and your capacity band: find the limit that actually applies, for the fuel you actually burn. Get a stack test at normal load. If you are within the limit with margin, keep the test report, keep the collector maintained, and re-test after any fuel change. If you are not, have the collector, the ID fan, the flue gas temperature and the space around the boiler assessed together, because a filter that the fan cannot pull through is not a solution.
Do it before the consent renewal, not because of it. A time-bound action plan backed by a bank guarantee is a far worse position to negotiate a retrofit from than a plan of your own.