SOx Technologies

Emission control solutions for TPPs

The thermal power sector in India has been investing heavily in flue gas desulphurisation (FGD) technologies. The government’s requirement to limit sulphur oxide (SOX) emissions from thermal power plants (TPPs) by installing FGD systems is the key driver. For coal-fired power plants, several technology solutions are available to meet the SOX emission standards prescribed by the Ministry of Environment, Forest and Climate Change. Wet FGD based on limestone is one of the most commonly used methods for SOX control. SOX is oxidised to produce gypsum, which may subsequently be removed as a by-product of this post-combustion SOX removal process. Seawater and ammonia, in addition to limestone, can be employed as reagents in wet FGD. Dry and semi-dry FGD, as well as dry absorbent injections (DSIs), are some of the other post-combustion SOX control methods. Pre-combustion technologies, such as coal beneficiation, and in-combustion technologies, such as circulating fluidised bed combustion (CFBC), can also be used to lower SOX emissions.


Wet FGD is the most widely used SOX removal technology. The first FGD system based on wet limestone technology was built at NTPC’s 500 MW Vindhyachal Stage V project in 2018. NTPC’s projects that are being retrofitted with wet FGD technology include the 1,320 MW Solapur super TPP, the 1,320 MW Tanda Stage II project, the 500 MW Unchahar project and the 1,320 MW Meja power project. Wet FGD systems have over 90 per cent SOX removal efficiency. Depending on the reagent used, an FGD can be classified as seawater based, ammonia based and limestone based. Wet FGD comprises four main processes – flue gas handling; reagent (limestone) handling; and preparation; absorber and oxidation; and secondary water and gypsum handling.

Wet scrubber is by far the most widely used FGD technology. A reagent such as limestone or lime in slurry form, perhaps with additives, reacts in a spray tower with sulphur oxides to form calcium sulphite, which is then oxidised to form calcium sulphate or gypsum in a wet scrubber. This technology, however, requires large quantities of water. Water usage can be reduced by using semi-dry scrubbers, such as spray dry scrubbers or circulating dry scrubbers, and dry scrubbing technologies.

Seawater-based FGD uses seawater as a reagent. It does not require any extra chemicals to remove SOX. Seawater absorbs acidic gases like SOX because it is inherently alkaline. The effluent seawater, after reaction, flows into a seawater treatment system to complete the oxidation of the absorbed SOX into sulphate. The sulphate ion thus formed is harmless and can be put back into the sea.

FGD technologies that use limestone slurry as reagent are most versatile and suitable for units of any size. Limestone technology has a large footprint, relatively higher capex and reagent purity issues as compared to ammonia-based and dry-type FGD technologies. Meanwhile, seawater-based FGD is mostly used in coastal plants. Coastal Gujarat Power Limited, a subsidiary of Tata Power, is setting up a seawater FGD plant at the 5×830 MW Mundra thermal power station in Gujarat. The contract for setting up the FGD has been awarded to ANDRITZ. The FGD is expected to be commissioned in the third quarter of 2023 and will be the world’s largest FGD with seawater.

Dry and semi-dry FGD

In dry and semi-dry FGD systems, SOX reacts with limestone particles to form sulphite in a humid environment. Broadly, dry and semi-dry FGD processes include furnace/duct sorbent injection using sodium/calcium-based reagents, and the spray drier absorber (SDA) technology using slaked lime or limestone as reagent. An SDA system uses a roof gas disperser, a central gas disperser for dispersing flue gas, and an atomiser to spray the reagent slurry. Inside an SDA system, limestone slurry is atomised and sprayed over the flue gas to absorb SOX. The dry product thus formed is collected in an electrostatic precipitator (ESP). Small power plants can use dry FGD systems since they are more cost effective. Hindalco Industries Limited is setting up semi-dry FGD (circulating fluidised bed scrubbers) systems at its 150 MW unit at Mahan Aluminium at Singrauli, Madhya Pradesh. The contract for executing the project was awarded to ISGEC Heavy Engineering Limited in September 2020.

DSI system

DSI is another SOx removal technique used post combustion. It is especially well suited for compact units in the 60 MW-250 MW range. Because reagent costs are greater in this technique as compared to wet limestone and ammonia-based FGD, units with low plant life factors and short remaining operational lifetimes (seven to nine years) are preferred. DSI has an SOX removal efficiency of 50-60 per cent. This is sufficient to meet the SO2 emission norms in cases where these emissions are in the range of 800-1,000 mg per Nm3. DSI uses calcium-based (calcium hydroxide) or sodium-based (sodium bicarbonate) sorbents to remove SO2. It is a feasible alternative for units that cannot invest in wet and dry FGD systems. Besides, the time needed to erect and commission a DSI system is only around one year, which is much less than that for other technologies. In addition, DSI-based technologies have considerably low capex (1/4th) and very little APC (1/10th) compared to wet limestone and ammonia-based FGD technologies. The downside of DSI is that sorbent injection creates additional dust loads on ESPs, necessitating simultaneous ESP retrofitting. NTPC has chosen DSI for its Dadri power plant.

For SOx reduction, NTPC has installed a DSI system at its Dadri Stage I plant and now all the four units are meeting the emission norms. NTPC is also setting up a DSI system at Tanda TPP Stage 1 (2x110MW), Units 1 and 2.

Challenges and the way forward

One of the key priorities for Indian thermal-based power generation companies is to meet the new emission standards. To comply with the SOX regulations, developers are opting for FGD systems.

One mole of SO2 is removed through the wet lime desulphurisation process, which generates one mole of CO2, a greenhouse gas that contributes to global warming and climate change. The same is applicable to other SO2 control technologies such as DSI, semi-dry FGD and seawater FGD. Moreover, depending on the FGD technology implemented, coal consumption may also increase by up to 1 per cent due to increased APC, leading to the release of greenhouse gases. Besides, increased APC reduces the efficiency of power plants. Further, developers are concerned about the revenue loss during plant shutdowns for equipment installation, and the operational efficiency of SOx control equipment during a low-load operation of plants.

In terms of by-product generation, gypsum production is expected in the range of 14-21 million metric tonnes per annum, following the implementation of wet lime FGD in approximately 214 GW of power plants (about 90 per cent of the total capacity that is installing wet lime FGD). It will vary based on the PLF range of 55-80 per cent and the sulphur content of 0.32 per cent in coal. The FGD market is still developing in India, with limited availability of vendors and high dependence on imports from neighbouring countries. The overbooking of suppliers has resulted in an increase in manufacturing time for FGD equipment.

In terms of funding, the installation of FGD systems will require a significant outlay of funds. FGD systems will be deployed in 170 GW of current capacity, of which 10 GW has been commissioned since the Central Electricity Authority prepared the phasing plan, and about 58 GW is under development. Considering the average price of Rs 5.5 million per MW, these FGD installations are estimated to entail a total capital expenditure of around Rs 131 trillion. Private power plants will face even worse funding issues. The Covid-19 outbreak has also added to the challenges being faced in FGD equipment manufacturing and installation.

Although there are a number of emission control technologies available in the market, it is important to evaluate their efficacy in the Indian context. In addition, addressing challenges pertaining to equipment availability, skilled manpower and finance might help TPPs meet the emission standards sooner. As regulatory issues are resolved and FGD orders for substantial thermal capacity have been placed, the sector is expected to witness an increase in FGD system installation.

To conclude, it is essential to prolong the timeline for FGD installation, implement a graded action plan for immediate FGD installations at TPPs in highly polluted regions, and begin a phased manufacturing programme for FGD units.


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