Air Quality Control

Steps to curb emissions by TPPs

The coal-fired power generation is considered as one of the major sources of environmental pollution in India. According to industry estimates, the current emission levels in the country by thermal power plants (TPPs), of sulphur dioxide (SO2) is in the range of 800-1,100 mg per Nm3, while that of nitrogen oxides (NOx) is between 450-750 mg per Nm3 and of mercury (Hg) is 0.008-0.018 mg per Nm3.

The government has, thus, been taking necessary action to contain environmental pollution from TPPs. In December 2015, the Ministry of Environment, Forest and Climate Change (MoEF&CC) came out with standards for sulphur oxide (SOx), NOx, mercury (Hg) and suspended particulate matter emissions by TPPs. These standards were proposed to be implemented by all the existing units by 2022, whereas the plants commissioned after December 31, 2016 were required to meet the standards right from the date of commissioning.

In order to meet the revised standards by the stipulated deadline, new emission control systems, including flue gas desulphurisation (FGD) systems, systems based on selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR) need to be installed and retrofitted at power plants. A quick look at these technologies as well as the challenges in implementation…

  FGD systems

FGD systems remove SO2 from flue gas produced by boilers, furnaces and other sources. As per the new norms, SOx emissions have to be brought down to 600 mg per Nm3 for units that are 15-25 years old. For a five to seven-year old unit, the emission level needs to be reduced to 300 mg per Nm3, and the latest commissioned units need to limit their SOx emissions to 100 mg per Nm3.

A detailed phasing plan was prepared by the Central Electricity Authority (CEA) to outline the roadmap for the installation of FGD systems progressively by 2022. The FGD phasing plan covers 414 coal-based units with 160,092 MW of capacity, that is around 70 per cent of the total coal-based power capacity. Further, installation of FGD and other pollution control equipment at TPPs in Delhi NCR, near large habitations and in critically polluted areas, was prioritised with the date of implementation brought forward to 2021. According to the CEA’s data, so far (as of May 2020), FGDs have been commissioned and are operational for 1,740 MW of capacity – two units of NTPC Dadri project (2×210 MW) as well as two units at CLP India’s Haryana project (2×660 MW). For NTPC Limited, the total thermal capacity for award of FGD exceeds 62 GW. Of this, FGD tenders for more than 59 GW have already been awarded, while FGD systems for around 2 GW of capacity are under implementation. Besides, NTPC Limited is also providing consultancy services to state gencos in Uttar Pradesh, Punjab and Haryana as well as NTPC’s joint venture companies.


The technologies available to reduce NOx emissions are classified as primary methods, which involve the combustion process modification to limit the formation of NOx; and secondary methods, which involve NOx reduction by reaction with reagent. The secondary methods are of two types, SNCR and SCR.

NTPC has done pilots to assess the efficacy of various NOx reduction technologies and methods. According to its trials, it has been found that NOx reduction achievable from combustion modification is around 750 mg per Nm3 to 450 mg per Nm3. For SNCR technology, NOx reduction is in the range of 20 per cent to 30 per cent only. Further, it was found that while a reduction in NOx of around 80 per cent is achievable, it is not consistent due to part load operation of power plants. Hence, the issue of review and relaxation of the existing NOx emission norms is currently under review with the MoEFCC to modify the current limit of 100 mg per Nm3 for NOx.

As per the de-NOx action plan of NTPC, combustion modification has already been implemented at five units of 2,480 MW capacity. Combustion modification contracts have been awarded/under execution at 42 units with 18,080 MW capacity. Further, de-NOx contracts for three units totalling 1,000 MW are currently under tendering.

Key concerns and challenges

A major concern facing the industry in the implementation of air quality control systems is the technological limitation in implementing DeNOx systems. Further, space constraints in the existing projects for the set-up of FGD systems is a challenge. Another challenge has been the high capex involved, at around Rs 4 million–Rs 9 million per MW for FGDs. This, in turn, leads to higher tariff as the generation cost increases by 25-30 paise, affecting the consumers. The availability of quality limestone, which is a necessary raw material for FGDs, is another challenge. There are concerns with regard to its transportation as well, given that new railway and road infrastructure has to be created. Further, quality gypsum for disposal is also a concern as a readily available market for by-product is not available. Though the cement industry uses gypsum, its utilisation is only 5 per cent, that is, 20 million tonnes, out of a total production of 400 million tonnes of gypsum at TPPs.

More importantly, the tight schedule with a deadline of December 2022 for the implementation of FGDs is a Herculean task, more so in the context of the current Covid-19 crisis. The lack of indigenous suppliers for key equipment and systems required for FGD systems has posed a concern due to supply chain disruptions caused by Covid-19 lockdown. TPPs are also facing a liquidity crunch due to the demand slowdown.


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