By 2030, India intends to reduce the emission intensity of its GDP by 30-35 per cent from the 2005 level. In this context, the Ministry of Environment, Forest and Climate Change, issued new environmental norms in December 2015 aimed at reducing emission of suspended particulate matter (SPM), SOx, NOx and mercury at thermal power plant (TPP). Considering the anticipated water scarcity in the future and the importance of water conservation, norms for specific water consumption by TPP were also notified for the first time.
The existing stations were given a deadline of December 2017 and plants under construction were to comply with the norms by January 2017. The norms impacted an existing coal-based capacity of around 193 GW and under construction capacity of around 66 GW and required installation of specific pollution control equipment at TPPs.
However, the implementation of the new norms within a short period of two years was a very challenging task given the requirement for uninterrupted power supply in the country. In view of this, the Ministry of Power issued instructions to the Central Electricity Authority (CEA) to prepare a phasing plan. It was decided that regional power committees, which are the coordinators for power supply at the regional level, would be the most suitable forum to formalise the phasing plan. In view of the above, special meetings were held between the regional power committees and the stakeholders. After a number of deliberations, the earliest practical feasible plan extending up to December 2022 was prepared for the installation of flue gas desulphurisation (FGD) systems and other pollution control equipment at the identified coal-based units.
A look at the technologies being deployed for emission control and the progress in implementation so far…
SOx control
FGD involves the removal of sulphur dioxide (SO2) from flue gas. Sulphur content in Indian coal ranges from 0.25 per cent to 0.5 per cent while in imported coal it is more than 0.6 per cent. Coal with 0.5 per cent sulphur, generates SO2 in the range of 1,500–2,000 mg per Nm3. SO2 emission results in acid rain, corrosion of buildings and structures, and deterioration of human health.
By far the most commonly used FGD technology is wet scrubber. In a wet scrubber, a reagent such as limestone or lime in slurry form, perhaps with additives, reacts in a spray tower with the oxides of sulphur to form calcium sulphite, which is oxidised to form calcium sulphate or gypsum. This technology, however, requires large quantities of water. Water usage can be reduced by using semi-dry scrubbers, such as spray dry scrubbers (SDSs) or circulating dry scrubbers, or dry scrubbing technologies.
For FGD installation, the CEA has identified 414 units aggregating 161.52 GW of capacity to be non-compliant. Of these, around 350 units (128.7 GW) will complete the installation of FGD systems in 2021 and 2022 as per the phasing plan.
As of June 2018, FGD systems have been commissioned for 1,820 MW of capacity across three units in two private sector power plants – Tata Power’s 500 MW Trombay TPP and CLP India’s 2×660 MW Mahatma Gandhi TPP, which is currently under renovation and modernisation. Meanwhile, bids have been awarded for nine other thermal power units aggregating 3,320 MW of capacity, including NTPC’s 3×500 Indira Gandhi super TPP and 2×490 Dadri TPP.
Further, tenders have been issued for 26,465 MW of capacity across 54 units. Tender specifications have been finalised for another 27,760 MW of capacity across 58 units. Apart from this, feasibility studies have been completed for 127 units totalling 46,800 MW and have begun for 140 units totalling 44,962 MW.
PM emission control
The emission control norms notified by the MoEFCC in December 2015 have considerably lowered the particulate matter (PM) emission limits for coal-based power plants. As against the earlier range of 150-350 mg per Nm3, the new PM emission limits range between 30 and 100 mg per Nm3, depending on the year of commissioning of thermal power projects (TPPs). The PM limit is 100 mg per Nm3 for projects commissioned before 2003, 50 mg per Nm3 for those commissioned between 2003 and 2016, and 30 mg per Nm3 for TPPs commissioned in 2017 and beyond.
PM emissions can be controlled pre-combustion, in-combustion, and post-combustion in coal-based power plants. Pre-combustion control can be achieved by selecting the right type of coal and by washing the coal, while in-combustion control is carried out by optimising combustion and injecting sorbents into the flame zone. There are various methods for post-combustion control of PM 2.5 emissions, the most common being the installation of electrostatic precipitators (ESPs) and fabric or baghouse filters.
An ESP is a device that electrostatically separates particles from the flue gas stream while imposing minimal pressure loss on the stream. Unlike PM removal devices such as cyclonic collectors, scrubbers and baghouse filters, high gas-stream pressure loss and associated high draught fan energy consumption is avoided when using an ESP. Most of the country’s power plants have already installed ESPs. However, with the narrowing of PM emission norms, there are concerns regarding the efficacy and operational performance of the existing ESPs.
As per the CEA, 220 units aggregating 63 GW are required to be compliant with the new norms with the installation/upgradation of ESPs by 2022. Of these, about 12,000 MW of units will upgrade their ESP systems between 2018 and 2020. Further, 200 units (53.4 GW) out of 231 units (65.9 GW) will complete the upgradation of ESPs in 2021 and 2022.
NOx control
Coal-based power plants account for a significant share in the annual NOx emissions. TPPs are responsible for an estimated 30 per cent of annual NOx emissions from India’s industrial sector. NOx emissions from TPPs were unregulated until recently. In fact, during 1996-2010, it increased dramatically by over 97 per cent, with an average annual growth of about 5 per cent.
The 2015 standards require TPPs installed before December 31, 2003 to limit their NOx emissions at 600 mg per Nm3. The standards are aligned with the global standards and are more stringent for newer plants. The NOx emission levels are set at 300 mg per Nm3 for plants installed between December 31, 2003 and December 31, 2016; and at 100 mg per Nm3 for those installed after December 31, 2016. Compliance with the new norms is expected to help achieve 70 per cent NOx emission reduction by 2026–27.
At the new TPPs, in order to meet the 100 mg per Nm3 standard, plants will have to utilise selective catalytic reduction (SCR) technology, which injects ammonia into flue gas to reduce NOx in the presence of a catalyst. SCR has been proven as an effective method globally to reduce NOx emissions from coal-fired power plants. However, older plants aiming to achieve the 300 mg per Nm3 standard can potentially attain these levels through a combination of in-furnace, combustion control and selective non-catalytic reduction (SNCR) technologies. SNCR is a simpler post-combustion control system, which can help achieve reliable NOx reductions ranging from 25 per cent to 50 per cent, and can be installed within a regular plant outage schedule. SNCR systems do not require a catalyst, but their effectiveness is dependent upon sufficient reaction time within a narrow flue gas temperature window and adequate mixing of the reagent with the flue gas. Other non-catalytic technologies include low NOx burners and over-fire air systems.
Water pollution control
The revised norms required all existing plants to achieve a specific water consumption of 3.5 m3 per MWh by December 2017. Further, plants with once-through cooling were required to install cooling towers. Another gazette notification in January 2016 mandated the use of treated sewage water from sewage treatment plants if it is available within a 50 km radius of a TPP. All plants set up after January 2017 are required to operate at a water consumption level of 2.5 m3 per MWh and achieve zero discharge.
Power plant owners, therefore, need to recycle waste water and look at installing zero liquid discharge (ZLD) systems. A ZLD system involves a range of advanced waste water treatment technologies to recycle, recover and reuse the treated wastewater, thereby ensuring that there is no discharge of wastewater into the environment. While ZLD systems have higher operating costs, investments are justified by a high recovery of water (90-95 per cent) and several by-products from salt.
Conclusion
In a significant policy development, the government has recently issued a notification stating that investments in the installation of emission control technologies will be considered for tariff pass-through. Further, in a case related to pollution in the Delhi-NCR region, the Supreme Court has mandated the installation of flue gas desulphurisation (FGD) technology in the region by December 2019. The government has also directed central PSUs NTPC and the Damodar Valley Corporation to comply with the standards by 2021.
That said, a few issues continue to slowdown the implementation process. For instance, there are currently no proven technologies for NOx emission control. Further, there is a need for greater technological innovation and research and development on the wastewater management front. The availability of limestone, a key raw material for FGD systems, is a concern given its multiple uses. Financing the pollution control equipment of stressed power projects is also a challenge.
While extended timelines for implementing the new norms should give some respite to TPPs, tracking compliance progress and tearing down technical and non-technical barriers would be important to attract investments and facilitate a smoother transition towards cleaner energy.