Thermal power plants (TPPs) are some of the key emitters of pollutants such as SOx, NOx, and particulate matter (PM). In order to curb emissions from TPPs, power plants operators are increasingly installing appropriate emission control technologies such as flue gas desulphurisation (FGD), selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR) and electrostatic precipitators (ESPs). In addition, operators are embracing efficient and effective operations and maintenance (O&M) strategies to improve power plant performance, maintain equipment health and minimise wasteful emissions.
The Ministry of Environment, Forest and Climate Change (MoEFCC) notified the revised emission norms in December 2015. As per these, the permissible level of SOx emissions for plants installed after January 1, 2017 is 100 Nm3. It is 600 Nm3 for plants with a capacity of less than 500 MW and installed before December 31, 2003; and 200 Nm3 for plants with a capacity of more than or equal to 500 MW, installed before December 31, 2016 .
Meanwhile, power plants commissioned between 2003 and 2016 were previously required to cap their NOx emissions at 300 mg per Nm3. This limit was later raised to 450 mg per Nm3. For plants commissioned 2017 onwards, the limit for NOx emissions has been set at 100 mg per Nm3. For older plants commissioned prior to December 2003, the NOx limit has been set at 600 mg per Nm3.
The PM emission standards in the revised norms are also based on the installation dates of the plants. A limit of 100 mg per Nm3 has been set for plants commissioned till December 31, 2003, while for plants commissioned between January 1, 2004, and December 31, 2016 it is 50 mg per Nm3, followed by 30 mg per Nm3 for plants commissioned January 1, 2017 onwards.
In April 2021, the MoEFCC extended the timelines for TPPs to comply with the emission norms by one to three years, and announced the categorisation of plants into three groups through the Environment (Protection) Amendment Rules, 2021. Category A TPPs, comprising plants within a 10 km radius of the National Capital Region or cities with a population of over a million, are required to meet the emission norms by December 2022. Category B TPPs, comprising plants within a 10 km radius of critically polluted areas or non-attainment cities, have to meet the norms by 2023. Category C includes the remaining plants, which have been given an extension till 2024. In addition to the extension of timelines, the new rules will levy an environment penalty on non-retiring TPPs for non-compliance beyond the timeline. The maximum fine for defaulting plants under Category A is 20 paise per unit, whereas it is 15 paise per unit for plants in Category B and 10 paise per unit for plants in Category C.
Later, in September 2022, the MoEFCC extended the deadlines for TPPs to install equipment for cutting sulphur emissions by two years. Based on their location, the TPPs now have time up to December 2026 to comply with the emission norms.
Emission control technologies
SOx control: Wet-FGD-based on limestone is one of the most frequently used technologies for reducing SOx. About 90 per cent of SOx is removed effectively by wet FGD systems. FGD can be categorised as seawater based, ammonia-based or limestone based, depending on the reagent employed. Emerging FGD systems are mostly based on wet limestone technology. A large number of TPPs, including the 1,320 MW Solapur Super TPP, the 1,320 MW Tanda Stages II project, the 500 MW Unchahar project, the 2,000 MW Simhadri super TPP Stage I and II projects, the 1,500 MW IGSTPP Jhajjar project, the 1,980 MW Sipat Super TPP Stage I project, and the 1,320 MW Meja Power project are installing wet FGD technology to control SOx emissions.
Dry sorbent injection (DSI) is another post-combustion SOx removal technique, which is preferred for plants with unit sizes in the range of 60-250 MW. Notably, NTPC has opted for DSI at its Dadri power plant. DSI system erection work is at advanced stages in two units of NTPC’s Tanda Stage I (4×110 MW).
According to CEA data, as of January 2023, FGDs have been commissioned and are operational for 22 units with a total capacity of 9,280 MW. Meanwhile, bids have been awarded for 219 units aggregating 100,430 MW in capacity. Sector-wise, bids have been awarded for 135 units in the central sector, 46 in the private sector and 38 in the state sector. Apart from this, the notice inviting tender (NIT) has been issued for 25,475 MW of capacity spanning 83 units.
NOx control: By utilising techniques such as combustion modification processes, NOx outflows may be reduced to 300-600 mg per Nm3. However, improved control methods, such as SCR and SNCR, are needed to further reduce emission levels by 100-300 mg per Nm3. NTPC Limited has awarded NOx combustion modification tenders for over 20 GW of capacity in an effort to reduce NOx emissions. Additionally, 29 units with a combined capacity of 8.2 GW are at various stages of commissioning for combustion modification works. Furthermore, 28 units totalling about 13 GW, including three Jhajjar units and two Dadri units, have completed combustion modification.
While SCR and SNCR are established technologies for NOx emission reduction with respect to low ash coal, they have not yet been cleared for the abrasive and high ash Indian coal. NTPC is conducting several pilot tests and studies to determine if SCR technology is appropriate for Indian coal.
PM control: The majority of TPPs currently have indigenous ESPs for controlling PM emissions. These have been commercially successful. However, to meet the stricter emission standards, improved ESP systems are needed. The CEA has published a thorough proposal explaining the strategy for the augmentation of ESPs for SPM control up to 2024.
Emission reduction with proper O&M
Effective tracking of the operating parameters of power plants is useful for the early discovery of deviations and flaws in power plant performance. New-age technologies such as data analytics, internet of things, machine learning and artificial intelligence are useful in identifying these deviations in performance. These solutions minimise energy losses, increase cycle efficiency, limit unplanned interruptions, eliminate unnecessary preventive maintenance and improve the overall plant performance.
With more efficient O&M practices, power plants will generate more electricity while utilising less coal for each unit of power produced. This will result in minimal production of undesirable emissions. Efficient O&M of old and ineffective power facilities improves the efficiency of power generation and gradually reduces carbon dioxide emissions.
To conclude, TPPs have been sluggish in complying with emission standards. As such, the new deadline extension has come as a huge relief for developers. However, rigorous adherence to the implementation schedule is required, especially in light of India’s commitment to net zero emissions. Overall, efficient and well-defined O&M practices coupled with the installation of emission control technology will lead to a significant reduction in TPP emissions.