Experts flag dilution of NOx emission limit for coal-fired power stations

The SC on July 8 relaxed limits for coal-fired power stations commissioned between December 2003 and 2016 — to 450 mg / Nm3 from 300 mg / Nm3

By Soundaram Ramanathan
Published: Monday 13 July 2020

Power stations commissioned between December 2003 and 2016 can emit 450 milligram / normal cubic metre (mg / Nm3 ) oxides of nitrogen (NOx), up from 300 earlier, the Supreme Court said July 8, 2020. Experts have cautioned against further relaxation.

A joint committee of the Union power and environment ministries in August 2019 recommended watering down emission limits.

The relaxation, however, will not affect the deadline to meet the December 2015 pollution norms, which is up till December 2022 for most units. 

The order 

The 300 mg / Nm3 limit was according to a December 2015 notification by the Ministry of Environment, Forest and Climate Change (MoEF&CC).

The Ministry of Power had proposed revising it up because meeting 300 mg / Nm3 and below was not possible at varying load. The Central Pollution Control Board (CPCB) was asked to examine it further by MoEF&CC earlier.


To verify the claim of load-affecting emissions, CPCB in collaboration with National Thermal Power Corporation Ltd (NTPC) monitored emissions at seven units of four thermal power plants between February 13 and April 2, 2019.

Five of them complied with NOx emission standards of 300 mg / Nm3 at full load. Some units did not comply during partial load operations even after combustion modification.

Combustion modification is a NOx-control technique where no new investment is required to set up pollution control systems and marginal reductions can be achieved. Bharat Heavy Electricals Ltd, India’s leading supplier of boiler components, assured that NOx emission level of 450 mg / Nm3 could be achieved by combustion modification.

The joint committee thereby decided that it would recommend the revision of NOx limits from 300 mg / Nm3 to 450 mg / Nm3 for plants installed between January 1, 2004 and December 31, 2016.

Coal-fired power stations with capacity of 121,340 megawatt were commissioned, which is 58 per cent of the total installed utility-based coal-fired capacity.

The Union power ministry has been seeking relaxation in NOx emission limits for new plants from 100 mg / Nm3, citing performance issues in the pollution control technology. However, this was not agreed by the joint committee in-principle.

The top court, according to the joint committee recommendations, asked NTPC to provide details of its pilot project where it found performance issues with technology.

But there was no mention on NOx standards upward revision — there was an ambiguity on the norms among a majority of stations.

With the Supreme Court July 8 order, power stations will not have to install further technologies like selective non-catalytic reactors to bring emissions below 300 mg / Nm3.  The technology installation cost about 10-20 lakhs / MW.

What experts say

“Coal-fired power plants are some of the most polluting industries in the country. They account for over 60 per cent of the total PM emissions from all industry, as well as 45 per cent of the SO2, 30 per cent of oxides of nitrogen and over 80 per cent of the mercury emissions. So even as we continue using coal, India’s thermal power sector must clean up its act. This is absolutely non-negotiable,” Sunita Narain, director-general, Centre for Science and Environment (CSE), said. 

An industrial expert who did not wish to be named, said: “For plants set up after 2016, NOx remains 100 mg / Nm3. I think that DOOSAN Group will demonstrate the efficacy of NOx through selective catalytic reactors by March 2021 in the Hardua Ganj (UP) project and followed later by a few others like Ghatampur by Larsen & Toubro.”

CSE, a New Delhi-based policy research advocacy organisation, released a report Coal-based Power Norms: Where do we stand today in April 2020, which had mentioned poor compliance with the norms by the sector.

Narain added: “Our assessment finds that even after seven years since the notification, and after the agreed five-year extension given to this sector in 2017, most of the total installed coal-fired capacity will not be compliant with the crucial sulphur dioxide (SO2) standards by 2022.”

Furthermore, there is little information in the public domain about compliance with particulate matter (PM) or NOx standards and there was no direction to the thermal power plants that they must meet the crucial water standards, which would make the sector more responsible on its usage.


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