By K.K. Pahuja, President, Indian Stainless Steel Development Association
Sulphur dioxide (SO2) is a toxic air pollutant that causes acid rain, haze and many health problems. As per estimates, India has overtaken China in terms of SO2 emissions despite lower levels of industrial activity and coal use. Fossil fuel-based power plants are the key source of SO2 emissions. In December 2015, the Ministry of Environment, Forest and Climate Change announced stringent emission standards to regulate nitrogen (NOx), SO2 and particulate matter emissions. These norms are more stringent than emission norms in most countries across the world. The new norms have affected over 415 thermal power plants (TPPs) across the country.
Flue gas desulphurisation (FGD) systems were introduced to control SO2 and particulate matter emissions from stack gases in TPPs. Considerable progress has been made in FGD technology. The utility industry has demonstrated the ability of FGD systems to bring down emission levels to comply with the standards, despite the wide range of coal compositions being used. FGD systems use the wet scrubbing method for removing SO2 fumes, which further form corrosive acids in absorber towers and result in acid dew point corrosion in the flue stack.
In a typical absorber chamber, the flue gas containing SO2 passes through a limestone slurry alkaline solution, which removes more than 95 per cent of the SO2 fumes. The by-product of this method is gypsum, which acts as a raw material for the cement industry.
FGD has now become the key technology to remove SO2 from the exhaust gases of boilers and is a mandatory installation in all power plants. The installation of these systems adds 7-9 per cent to the current power generation cost of Rs 7 million-Rs 8 million per MW. The reliability of systems has become increasingly important to avoid costly repairs, production loss due to non-compliance with emissions standards, and power purchase from other sources. Therefore, the proper selection of materials for the construction of these systems is of utmost importance for operational reliability.
Considerable corrosion data has been collected worldwide for alloys used in FGD systems under various conditions. Generally, metallic materials present the best solution in terms of cost and reliability. Stainless steel and nickel-based alloys offer long-time performance and low maintenance. Since the corrosion resistance of standard stainless steel grades, such as 316L, is limited, highly alloyed stainless steel grades or nickel-based alloys are preferred for the most corrosive conditions.
One of the major concerns when building scrubber units is the requirement of welded materials. Welded joints are made up of different zones. Thermal cycles induce structural modifications, filler materials induce chemical composition variations, and weld beads induce geometric variations. Welds are often the weak point in corrosion resistance. To increase the corrosion resistance of welds, stainless steel grades with improved weld ability have been developed. Some grades such as UNS N08367 have higher corrosion resistance while others have high corrosion resistance and high mechanical properties (duplex S31803/S32205 and super duplex S3255O/S32760).
These stainless steel grades are being manufactured by suppliers and are commonly used in FGD systems where the coal being burned has much higher sulphur content than Indian coal.
The comparison of the mechanical properties and corrosion resistance of these alloys indicates very high strength and high corrosion resistance of stainless steel. The selection of an alloy depends on the corrosion conditions and life cycle costing.
The use of domestically manufactured stainless steel grades mentioned in the table will reduce the overall project cost by nearly 40 per cent as compared to the current specifications of acid-resistant borosilicate bricks and high alloy clad steel plates. It can also reduce the dependence on imports of such materials, which typically have higher costs and are difficult to procure when required. Most of the materials required for FGD systems, specified in the tenders launched by central and state utilities, are not being manufactured in India. These include C276 clad plates, Ti clad plates and borosilicate bricks. The industry has to depend on high-cost imports, resulting in forex losses for the country.
Stainless steel UNS 31727 was used for the first time in the country for stack application at the Bongaigaon and Vindhyachal TPPs. The FGD system suppliers are also using stainless steel grades such as 317LMN and duplex grades UNS S32205 for some of the equipment. The majority of process sections like absorber chamber, outlet ducts and flue cans are specified in import-oriented materials, whereas the indigenously manufactured materials such as super duplex UNS 32760 and 6 per cent molybdenum could play a vital role in the faster and cost-effective execution of such large-scale projects. A Cost Comparative Index between S32760, or N08367, alloy plates and C276 cladding has been shown in the graph. Alloys like S32760 or N08367 are preferred to C276 because of their exceptional performance higher strength and lower cost. S32760 is used as a solid plate to reduce equipment weight, resulting in cost savings, compared to cladded material, which must rely on the thickness of carbon steel to provide mechanical strength.
The implementation of the revised emission standards will pose a challenge owing to stringent timelines, limited availability of suppliers and technology, shutdown for longer periods, and revenue loss during shutdowns. Therefore, it is important that end users remain sensitive to the use of indigenously available, cost-effective and competent materials such as high alloyed stainless steel. Enough experience and technical information are available for the owners of FGD systems and engineers involved in selecting and specifying the right material for construction. The Indian Stainless Steel Development Association (ISSDA) has vast technical resources to cater to the needs of FGD users, right from material selection to fabrication.