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De-SOx technologies play a crucial role in mitigating sulphur dioxide (SO2) emissions from industrial sources, particularly in coal-based power plants. Among these technologies, wet flue gas desulphurisation (FGD) is one of the most popular and well-suited methods for Indian thermal power plants (TPPs). As the nation transitions towards a low-carbon economy, investments in cleaner technologies like FGD systems will play a pivotal role in mitigating air pollution and safeguarding the environment. However, challenges persist in the widespread adoption of De-SOx technologies. These include high capital costs, technical complexities and the need for continuous maintenance and operational optimisation. Addressing these challenges requires collaborative efforts among government agencies, industries, research institutions and technology providers.
In September 2022, the Ministry of Environment, Forest and Climate Change extended the deadlines for TPPs to implement SOx reduction equipment by two years, pushing the new compliance deadlines to December 2026, based on the location of the TPP. Category A (within 10 km of Delhi-NCR and cities with over 1 million population) has an extended deadline of December 31, 2024. Category B (within 10 km of critically polluted areas or non-attainment cities) has a new deadline of December 31, 2025, and Category C has until December 31, 2026. TPPs set to retire by December 31, 2027, are exempt from SO2 emission norms upon submission of an undertaking to the Central Pollution Control Board and the Central Electricity Authority (CEA). Those retiring by December 31, 2022 (Category A) and December 31, 2025 (Categories B and C) are exempt from norms for parameters other than SO2 emissions. Non-compliant TPPs face environmental penalties of Re 0.20 per unit (0-180 days), Re 0.30 per unit (181-365 days) and Re 0.40 per unit (366 days and beyond).
According to data from the CEA, as of December 2023, FGDs have been commissioned and are operational for 26 units, totalling 11,590 MW in capacity. Furthermore, bids have been awarded for 225 units, totalling 101,970 MW in capacity. Sector-wise, bids have been awarded for 131 units in the central sector, 48 in the state sector and 46 in the private sector. Additionally, a notice inviting tenders has been issued for 27,445 MW of capacity across 96 units.
Limestone-based wet FGD stands out as the top choice among de-SOx technologies for Indian coal-based power plants. These systems effectively remove 90-99 per cent of SOx emissions, making them the preferred option. They are valued for their adaptability to units of different sizes, cost-effectiveness as well as the production of a marketable by-product, gypsum. A significant milestone in the adoption of wet limestone technology was the installation of the first FGD system at NTPC Limited’s 500 MW Vindhyachal Stage V project in 2018. NTPC has undertaken substantial measures to tackle SOx emissions by implementing lime-based wet FGD systems and dry sorbent injection (DSI)-based FGD in eight units, totalling 2,990 MW.
Seawater-based FGD systems utilise seawater as a reagent, making them particularly well-suited for coastal power plants. Seawater-based FGD involves low capital cost, low auxiliary power consumption, requires no reagent, generates no by-product while clearing up effluent discharge, provides easy seawater availability and treatment and has a lesser tariff impact. However, it is not viable for other plants.
These cost-effective technologies are ideal for small to medium-sized power plant units and projects with water scarcity issues. They reduce water consumption by approximately 60 per cent compared to wet scrubbers. Dry/Semi-dry FGD technologies achieve removal efficiencies between 70-98 per cent. However, they have higher operational expenses despite lower initial capital costs. GE Power has completed the reliability run and full load operations, spanning 14 days, for India’s first semi-dry FGD system project. Undertaken for Hindalco Industries Limited (1x150MW), this project is for its Aditya aluminium plant in Lapanga, Sambalpur, Odisha.
DSI technology offers an effective de-SOx solution for small units (60-250 MW), despite higher reagent costs compared to wet limestone and ammonia-based FGD systems. It is suitable for plants with low life factors (seven to nine years), meeting SOx emission norms (50-60 per cent removal efficiency, 800-1,000 mg per Nm3). Unlike wet limestone FGD, DSI can be installed and commissioned in 12-14 months. Notably, NTPC has chosen DSI for its Dadri power plant, with the erection work for DSI systems in advanced stages for two units of NTPC’s Tanda Stage I (4×110 MW).
A jet bubbling reactor operates by injecting finely atomised limestone slurry into the flue gas stream, reacting with SO2 to form less harmful compounds like calcium sulphite and calcium sulphate. Using a bubbling fluidisation technique, air introduced at the reactor’s bottom causes the limestone slurry to form bubbles, enhancing reactant mixing and process efficiency. This design offers benefits such as high efficiency, flexibility, compactness and low maintenance needs. Nabha Power Limited employs the jet bubbling reactor technology from Chiyoda Corporation.
The cost of FGD systems depends on various factors such as unit size, configuration and layout constraints. It is also influenced by desired SO2 removal efficiency, chimney layout and corrosion protection choices. Other cost drivers include demand for FGD equipment, site-specific conditions, inflation, steel prices and external events like the Covid-19 pandemic. Commissioning schedules, gas flow requirements and duct length also impact costs. These factors collectively determine the financial investment needed for FGD technology in industrial applications.
Reducing operations and maintenance (O&M) costs in FGD systems requires addressing various challenges. Optimising booster fan flow to meet emission norms is crucial, ensuring efficient SO2 removal while minimising energy consumption. Adjusting the liquid-to-gas ratio by controlling the number of pumps and intake gas volume optimally matches the load pattern of sulphur and power generation, enhancing process efficiency. Additionally, integrating air compressor operations with main plant sources eliminates the need for redundant systems, reducing maintenance complexity and costs. Similarly, connecting the cooling water system to the main plant sources streamlines operations and decreases maintenance efforts. Implementing variable frequency drive (VFD) controls for certain fans post-commissioning enables better control and energy efficiency. Upgrading water pumps to VFD-based systems based on header pressure optimisation enhances performance and reduces wear and tear. Further, transitioning gypsum bleed pumps from manual startup to continuous operation ensures smoother processes and minimises manual intervention. Addressing these challenges systematically can significantly contribute to reducing O&M costs in FGD systems while maintaining optimal performance.
According to an IIT Delhi study, there is a significant decrease (65-85 per cent) in SO2 levels within 10-40 km of coal-based TPPs with FGD implementation, while sulphate reduction in ambient air is about 12 per cent within 200 km. However, FGD implementation leads to increased carbon dioxide (CO2) emissions per unit of power generated due to the release of CO2 during desulphurisation processes. This could result in at least a 1 per cent rise in coal consumption and auxiliary power consumption in TPPs, leading to higher greenhouse gas emissions. FGD installations also increase water consumption by about 0.5 m3 per MWh and require limestone transportation, which generates CO2 and environmental impacts from limestone mining. Additionally, FGD installations require a significant capex of about Rs 2 trillion, leading to electricity tariff increases nationwide. Wet limestone-based FGD systems cost around 14 million per MW, resulting in an electricity tariff increase of approximately Re 0.71 per kWh.
Several challenges persist in facilitating the seamless implementation of FGD systems, with space constraints and limited vendor availability as primary concerns. On the technical front, older coal-based units face hurdles due to space limitations for FGD system installation. Additionally, the scarcity of quality limestone and the associated high transportation costs pose execution challenges. The management of unutilised gypsum, a by-product of FGD plants, also presents a notable concern.
Looking ahead, with the market offering various emission control technologies, it becomes imperative to carefully select the appropriate SOX control technology to effectively achieve desired results and objectives. Moreover, addressing challenges such as high capital costs, technical complexities and operational requirements can aid power generators in installing FGD systems.
Overall, the primary focus for Indian thermal-based power generation companies is to adhere to the new emission standards. To meet SOx regulations, developers have increasingly turned to FGD systems. Hence, extending the timeline for FGD installation, implementing a graded action plan for immediate FGD installations at TPPs in highly polluted regions and initiating a phased manufacturing programme for FGD units are essential steps in this direction.
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