Reducing SOx Emissions: Technologies for minimising the adverse effects of sulphur oxides

Technologies for minimising the adverse effects of sulphur oxides

Sulphur dioxide (SO2) emissions are a particularly acute problem in the power sector as well as in the country. SO2 is a corrosive gas created by the oxidation of sulphur bearing materials such as coal, oil and natural gas. Sulphur oxides are generated as a result of oxidation of the sulphur present in the coal at the combustion zone. To minimise the adverse effects of sulphur oxides on the en­vironment, many power plants and industrial facilities use de-sulphur oxide (SOX) solutions. Flue gas desulphurisati­on (FGD) is one of the key technology sol­u­tions used at thermal power plants (TPPs) to cut down emissions. It has em­er­ged as one of the key technology solutions to be adopted by TPPs to comply with the Ministry of Environment, Fore­st and Climate Change’s tightened emission norms.

Different technologies are commercially available for the reduction of SO2 emission. Commonly used technologies in the industry are wet limestone FGD, seawater-based FGD and ammonia-based FGD. Dry absorbent injection (DSI) is an­other de-SOX solution, mostly suitable for small-sized TPPs. These technologies are commercially available with abundantly available reagents for the plants. The se­lection of an appropriate FGD technology depends on technical, economic and commercial factors such as sulphur re­moval ability, reliability, space requirements and reagent availability. Economic factors such as capital cost and operating cost are also considered while selecting the FGD technology for a TPP. Power Line takes a look at the key de-SOX technologies adopted in the country…

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 and limestone based. Limestone-based wet FGD systems can remove 90-99 per cent of SOX  and are the dominant choice for TPPs. These systems are the most versatile and suitable for units of any size. They are favoured for their relatively low cost and the production of a marke­table 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 lime­stone technology. During 2021-22, the company awarded a 2,000 MW wet limestone-based FGD. NTPC is implementing wet FGD technology in 200-800 MW units aggregating 62,000 MW. NTPC projects where 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 Unchahar project, the 2,000 MW Si­m­hadri super TPP Stage I and II, the 1,500 MW IGSTPP Jhajjar project, the 1,980 MW Sipat super TPP Stage I 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 95 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. This system entails comparatively less opex and capex as the sourcing and transport of a new reagent as well as the storage and disposal of waste by-products is not required. Moreover, seawater-based FGD systems are simpler as fewer equipment and no storage and processing of reagent are required. 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.

The selection of FGD technology is done on the basis of economic, technical and commercial aspects. These include capital cost, operating cost, the efficiency to remove SO2, performance reliability, space requirement, and a proven track record. While wet limestone-based freshwater FGD is techno-economically feasible for inland power stations, amm­onia-based FGDs are not very popular because the reagent (ammonia) is considerably more expensive and hazardo­us than limestone. Moreover, there is a risk of ammonia slip, that is, ammonia releasing into the atmosphere without any reaction taking place in the FGD system, which poses environmental ha­zards. Hence, wet limestone-based FGD is a preferred option because the reagent is easily available and inexpensive and can be easily handled. Seawater-based FGD is mostly used in coastal plants.

Dry/Semi-dry FGD

In dry and semi-dry FGD systems, the SOX  reacts with limestone particles to form sulphite in a humid environment. Semi-dry/Dry flue gas desulphurisation technology is cost effective, especially for small- to medium-sized power plant units, and also very suitable for projects with limited water availability. Broadly, dry and semi-dry FGD processes include furnace/duct sorbent injection using sodium/calcium-based reagents, and spray drier absorber technologies using slaked lime or limestone as reagent. As compared to wet scrubbers, these systems consume about 60 per cent less water. Dry/Semi-dry FGD technologies also have a removal efficiency of 70-98 per cent. While these systems entail a lower capex, their opex is on the higher side as compared to other FGD technologies. Hindalco Industries Limited is setting up semi-dry FGD (circulating fluidised bed scrubber) systems at its 150 MW unit at Mahan Aluminium in Sing­ra­uli, Madhya Pradesh. The contract for ex­e­cuting the project was awarded to ISGEC Heavy Engineering Limited in September 2020.

DSI systems

Another post-combustion de-SOX  technology solution is DSI technology. It is preferable for small units in the range of 60-250 MW. The cost of the reagent in this technology is relatively higher than that of wet limestone- and ammonia-based FGD. Units running on low plant life factors and with low remaining operating life (seven to nine years) are preferable for DSI. It has an SOX  removal efficiency of 50-60 per cent, which is sufficient to meet the sulphur dioxide 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 lower capital cost and a smaller installation footprint.

It also reduces the emissions of other acidic gases and hea­vy metals such as mercury. It is quicker to install and commission. Whi­le conventional wet limestone FGD takes over two years to install, DSI takes only 12-14 months. NTPC has commissioned DSI-based FGDs in four units of Dadri Stage I (840 MW). Moreover, NTPC is implementing FGD through DSI technology in its older and small units. DSI system erection work is at advanced stages in two units of Tanda Stage I (4×110 MW).

Issues and challenges

Undoubtedly, in recent months, significant progress has been made with re­ga­rd to the ordering activity for the installation of FGD systems at power plants. State-run thermal major NTPC Limited has also planned FGD installation for around 65 GW of capacity for its 155 units. Of this, 2 per cent (1,340 MW) has been implemented and 95.5 per cent (62,280 MW) has been awarded. Mean­while, the remaining 2.5 per cent (1,454 MW) is yet to be awarded. While the ce­n­tral sector TPPs have issued tenders for the installation of FGDs, the private and state sector TPPs have been largely lagging owing to lack of clarity on tariff pass-through, funding limitations, etc.

A few challenges continue to hamper the smooth implementation of FGD systems, with the lack of space and limited vendors being the key concerns. The availability of domestic FGD technology suppliers/vendors is limited and many leading engineering, procurement and construction contractors/subvendors 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. Due to the huge gap in the demand and supply of FGD equipment, the prices of raw material and FGD components are escalating significantly.

On the technical front, many of the older coal-based units are also likely to face challenges due to the non-availability of space for installing FGD systems. Apart from this, the unavailability of quality li­mestone and its long distance transpor­tation cost are challenges in FGD execution. Besides, unutilised gypsum, which is the waste produced from FGD pl­an­ts, is a concern area. The Covid-19 out­break has also added to the challenges in FGD equipment manufacturing and installation.

Further, FGD orders envisage the retrofitting of FGD components in brownfield projects. The retrofitting of FGD components entails issues relating to conceptualisation and design. Also, a steep price escalation on FGD capital costs, difficulty in loan tie-up with the financial agencies due to uncertainty in the recovery of expenditure, and lack of experience on the FGD performance have impacted the installation of FGD systems. In addition, operation and maintenance issues with FGD include the availability of high quality limestone, utilisation/disposal of by-product, imp­act of high cycle of concentration circulating water blowdown water, dependence on coal quality, and technical expertise for addressing the challenges.

TPPs are also concerned about the recovery period of FGD costs as ageing plants will have to pass through greater amounts in tariff to discoms compared to recent plants. For producers, the uncertainty with respect to the full recovery of capital and operational expenditure, and loss of revenue during downtime are major hindrances in the installation of emission control equipment. Meanwhile, lenders refrain from financing TPPs owing to a large stressed asset portfolio in the power sector.

The way forward

Going forward, there are a number of emission control technologies available in the market and it is important to select a suitable SOX  control technology, which is essential to achieve the desired results and objectives. In addition, addressing the challenges pertaining to goods and service tax waivers and soft loans with a low rate of interest may be facilitated to the generators to help them install FGDs. Also, a special task force may be constituted for monitoring the progress and addressing the bottlenecks faced by generators during the implementation of the revised norms.

Overall, the key priority 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. Therefore, 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.