De-sulphur oxide (SOx) solutions are imperative for coal-based power plants to comply with the Ministry of Environment, Forest and Climate Change’s environmental norms prescribed for thermal power plants (TPPs). While several technology solutions are avÂailable to help TPPs meet the prescribed norms, flue gas desulphurisation (FGD) is one of the most commoÂnly used methods for SOx control. FGD desÂcribes a set of technologies used to remoÂve SOx from the exhaust flue gases of fossil fuel power plants. They include a control device that absorbs and remoÂves SOx by using the alkaline reagent or sorbent to produce a solid compound. Notably, the majority of de-SOx systems, at various stages of installation across NTPC LiÂmited’s power plants, are wet-limestone based FGDs. In these systems, SOx is oxidised using limestone as a reÂagent to produce gypsum, which is subsequently reÂmoved as a by-product. ApÂart from limestone, seawater and amÂmoÂnia can be used as reagents in wet FGDs. Dry abÂsorbent injection (DSI) is another de-SOx solution, mostly suitable for small-sized TPPs.
Wet FGD
Wet FGD is the one of the most popular and well-suited de-SOx technologies for Indian coal-based power plants. Wet FGD comprises four main processes – flue gas handling, reagent (limestone) handling and preparation, absorption and oxidation, and secondary water and gypsum handling. Depending on the reagent used, an FGD can be classified as seawater based, ammonia based or liÂmestone based. Limestone-based wet FGD systems can remove 90 to 99 per cent of SOx and are the dominant choice for TPPs. These systems are the most verÂsatile, being suitable for units of any size, and are favoured for their relatively low cost and the production of a markeÂtable by-product (gypsum). Notably, the first FGD system was installed at NTPC’s 500 MW Vindhyachal Stage V project in 2018 and was based on wet limeÂstone technology. NTPC projects whÂere wet limestone-based FGD technology is being implemented include the 1,320 MW Solapur super TPP, the 1,320 MW Tanda Stage II project, the 500 MW UnÂchahar project, and the 1,320 MW Meja power project.
Seawater-based FGD systems use seawater as a reagent, and are most suitable for coastal power plants. They do not require any extra chemicals to remove SOx – the natural alkalinity of seawater can remove up to 99 per cent of SOx. 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. Coastal Gujarat Power LiÂmited, 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/Semi-dry FGD
In dry and semi-dry FGD systems, the SOx reacts with limestone particles to form sulphite in a humid environment. Broadly, dry and semi-dry FGD proceÂsses include furnace/duct sorbent inÂjection using sodium/calcium-based reaÂgeÂÂnÂts, and spray drier absorber teÂchnologies using slaked lime or limestone as reagent. Dry FGD systems are more cost-effective for small-sized power plants. As compared to wet scrÂubbers, these systems consume about 60 per cent less water. Dry/Semi-dry FGD technologies also have a removal efficiency of 70-98 per cent. However, while these systems entail a lower caÂpex, their opex is on the higher side as compared to othÂer FGD technologies. Hindalco IndusÂtries Limited is setting up semi-dry FGD (circulating fluidised bed scrubbers) systems at its 150 MW unit at Mahan Aluminium at Singrauli, Madhya PradeÂsh. The contract for executing the project was awarded to ISGEC Heavy EngiÂneering Limited in September 2020.
DSI Systems
Another post-combustion de-SOx technology solution is DSI technology. It is preferable for small units lying in the range of 60-250 MW. The cost of the reagent in this technology is relatively higher than that in wet limestone and ammonia-based FGD. Units running on low plant life factors and with low remaining operating lives (seven to nine years) are preferable for DSI. It has a SOx removal efficiency of 50-60 per cent, which is sufficient to meet the sulphur diÂoxide emission norms when these emissions are in the range of 800-1,000 mg per Nm3. DSI uses calcium-based or sodium-based sorbents to remove sulphur dioxide. DSI offers advantages such as a lower capital cost and a smaller installation footprint. It also reduces emissions of other acidic gases and heavy metals such as mercury. It is quicker to install and commiÂssion. While conventional wet limestone FGD takes over two years to install, DSI takes only 12-14 months to be up and running.
NTPC has commissioned DSI-based FGDs in four units of Dadri Stage-I. Moreover, DSI system erection work is at advanced stages in two units of TanÂda (Stage I).
Issues and challenges
There are various challenges in the implementation of FGDs in the power sector. The availability of domestic FGD technology suppliers/vendors is limited and many leading engineering, procurement and construction contractors/ sub-vendors may reach saturation point due to parallel ongoing FGD projects. The overbooking of suppliers has resulted in an increase in manufacturing time for FGD equipment.
Further, progress in the execution phase has been slow, and has decelerated further due to the CoÂvid-induced pandemic. On the financing front, alongside difficulties in the sanctioning and granting of funds for FGD retrofits from various financial institutions, especially to existing power plants listed as non-performing or streÂssÂed assets, the industry has also faced fluctuations in terms of capex from unit/capacity-wise benchmarked values. Notably, the steep increase in steel prices witnessed in recent months is expected to significantly impact the cost of FGD systems.
On the technical front, many of the older coal-based units are also likely to face challenges due to non-availability of space for installing FGD systems. Apart from this, the lack of availability of quality limestone and its long-distance tranÂsportation cost is a challenge in FGD exÂeÂcuÂtion. Besides, unutilised gypsum, which is the waste produced from the FGD plants is also a concern area. GyÂpsum production is expected to be 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 installÂing wet lime FGD). It will vary based on the PLF range (usually 55-80 per cent) and the sulphur content (usually 0.32 per cent) in the coal.
Overall, the efficiency of FGD systems is likely to be affected at lower loads as the system requires a certain level of reagent flow, water flow and flue gas temperature. Besides, any increase in auxiliary coÂnsumption because of FGDs could impact overall plant availability.
Net-net, the selection of a suitable SOx control technology is essential to achiÂeve the desired results and objectives. The choice of deploying de-SOx solutioÂns depends on the SO2 removal efÂfiÂciÂenÂcy requirement, the remaining life of the unit, the PLF, caÂpiÂtal and operating coÂsts, reliability and spaÂce constraints, the supply chain, ash scÂaÂlability and disposal, and the impleÂmeÂntation schedule. Based on these considerations, with proÂper planning, deÂvelopers need to carefully assess whÂat the most appropriate technology woÂuld be. In addition, addressing challenÂges pertaining to eqÂuipÂment availability, skilled manpower and finance may help TPPs meet the emission standards sooner.