P. Vangeli, Outokumpu Stainless Steel, Sweden & G. M. Carinci, TMR Stainless, USA
In many countries coal fired power plants are required by law to remove particulate matter, nitrogen oxides (NOx), sulphur dioxide (SO2), sulphur trioxide (SO3) and mercury from their boiler exhaust emissions. A combination of equipment is used, including dry electrostatic filters for particle removal, selective catalytic reduction (SCR) for NOx reduction, wet or dry flue gas desulphurization (FGD) for SO2 and mercury removal, and a wet electrostatic filter to capture small particles. In the near future, carbon dioxide capture is expected to be required.
FGD plays an important role in controlling emissions from coal fired power plants, and the materials that FGD units are made from are equally important. Stainless steels for FGD scrubbers are selected on the basis of a number of parameters, including corrosive media, operating condition, plant design and total economic aspects. Because the severity of corrosive media lessens as you move from the gas inlet to the outlet area of a unit, it becomes possible to choose a specific steel grade for each single plant that is economical.
The corrosive conditions within a scrubber are the result of a complex interaction between chlorides and fluorides, acidity, temperature and construction details, depending on the desulphurization process to be employed and the fossil fuel to be used. These factors in turn determine the severity of the corrosive media developed in the system at the various stages of SO2 capture, such as quencher liquid, absorber liquid, clean gas condensate and composition of raw gas and its condensate.
Pitting and crevice corrosion are the primary types of corrosion encountered in such desulphurization environments. Stress corrosion cracking and galvanic interactions are of secondary importance. Uniform corrosion can occur in the most hostile zones, such as the raw gas inlet ducts, which are typically made of nickel-base alloys. Resistance of stainless steel against pitting and crevice corrosion can be improved by the addition of chromium, molybdenum and nitrogen. Furthermore, nitrogen increases the mechanical strength without reducing the ductility properties, enabling designers to reduce the wall thickness of the equipment.
An estimate of the relative influence of these alloying elements on the resistance to pitting and crevice corrosion can be achieved by calculating the pitting resistance equivalent (PRE). There are different formulae to calculate the PRE number, the most common has a coefficient 16 for nitrogen, e.g. PRE = % Cr + 3.3% Mo + 16% N. The PRE number provides a ranking between different alloys, but an alternate method is to compare the resistance to pitting and crevice corrosion by using a corrosion test to determine the critical pitting temperature (CPT) and critical crevice temperature (CCT) according to ASTM G 48.
Even better ranking can be achieved by performing the tests in a solution simulating a more realistic environment, and for engineering purposes a diagram may be required, as shown in Figure 1 below.
Figure 1: A diagrammatic guideline for material selection in FGD applications
The different colours in the diagram represent different operating conditions in term of acidity and Cl-/F- concentration in the temperature range FGD scrubbers normally work in.
The assignment of steel grades is based on long-term industrial experience as well as field and laboratory testing. Other factors such as design, processing of material and workmanship will also have an influence on the material’s behaviour, but it can be used as a conservative guideline by design engineers to facilitate an economic selection of stainless steels and nickel-base alloys.
Artificial scrubber environments
To assess the corrosion resistance of various stainless steel grades, samples were exposed to artificial scrubber environments under controlled laboratory conditions1-6.
The results from a multi-partner testing programme assessed the corrosion performance of duplex and 2à‚—7 per cent Mo austenitic stainless steels in simulated flue gas environments, representative of wet SO2 scrubbers.
Both welded and un-welded coupons were exposed at 55 à‚°C and 80 à‚°C for a month to a gaseous environment containing 100 ppm SO2 and five per cent O2. The samples were continuously wetted by calcium chloride brines containing 10 000 ppm, 20 000 ppm, 30 000 ppm, 50 000 ppm and 100 000 ppm of chlorides during the duration of the test. The closed-loop water slurry was simulated with calcium chloride brines with a pH of 5.
The results of the study, shown in Table 1 showed the austenitic stainless steels 316L, 317LMN, the duplex stainless steel 2205 and the 254 SMO super-austenitic stainless steel experienced crevice corrosion-type attack under scale deposits, some pitting in the base metal and significant localized corrosion in the heat affected zone adjacent to the welds.
In contrast the nickel-base alloy C-276 experienced only very light uniform corrosion with corrosion rates not exceeding 0.015 mm/y. The 654 SMO stainless steel coupons exhibited some staining on the back but no significant etching or corrosion attack. The front surfaces of all the coupons were intact after exposure to either 55 à‚°C or 80 à‚°C brines. The apparent corrosion rate was 0.0025 mm/y for one of the coupons exposed to the 100 000 ppm chloride brine at 55 à‚°C. The corrosion rates of all the other 654 SMO coupons were below the detection level of 0.0025 mm/y.
Compared to the other alloys in this study the 654 SMO stainless steel showed some of the lowest corrosion rates after exposure to the 80 à‚°C brines. Welded coupons were also tested and the results of the visual evaluations of the front and back surfaces of the respective samples showed that none of the 654 SMO stainless steel coupons exhibited any significant corrosion attack of the welds or heat affected zone. Overall, the performance of the welded 654 SMO coupons was similar to the un-welded coupons, and both showed excellent corrosion resistance after exposure to the brines at both 55 à‚°C and 80 à‚°C.
The overall performance of the alloys in order of increasing corrosion resistance was found to be: 316L<317LMN<2205<254 SMO<654 SMO/C-276.
Field corrosion tests
Field tests7 were performed in a spray tower and in an outlet duct of an operating scrubber at a US coal fired power plant (Figure 2). In the spray tower, wet limestone and forced air oxidation were used for desulphurization of the flue gas. Welded and creviced specimens were tested for nine months in the spray tower and for eight months in the outlet duct.
Figure 2: Schematic illustration of corrosion test rack placement in the FGD process in a US coal fired power plant
The conditions in the scrubber and the outlet duct were 60 000-80 000 ppm Cl-, pH 5-5.5 and approximately 55 à‚°C and some condensates formed, by Cl- and F- concentrations unknown, PH 2 (in condensates) and 52-54 à‚°C, respectively. The test results are shown in Table 2.
The conditions during this test were relatively severe because the samples were exposed to a higher chloride concentration than in many scrubber environments.
However, 254 SMO and 2507 performed well in the spray tower and only suffered slight crevice corrosion. The reason for the relatively good performance was probably because of the high pH in the field test. Uniform corrosion occurred in the 316 stainless steel with a corrosion rate of 0.163 mm/y.
In the outlet duct, most of the attack in the base metal and weld was because of crevice corrosion under deposits. The only grades that showed pitting attack in the weld were 317LMN and 904L, whereas 254 SMO and 2205 had crevice attack under deposits in relation to the weld. Also, 316 suffered uniform corrosion, protecting it from local attack like pitting and crevice corrosion, and had a corrosion rate of 0.225 mm/y. The conditions in the outlet duct were slightly more severe compared to the spray tower, because of the formation of deposits and condensates.
Steel selection and life cycle costs
FGD plants can be divided into zones with different service conditions, and taking those conditions into consideration, an economical materials selection can be made for the absorber vessel, quencher and absorber system, demister areas, ducts, dampers and fans.
There are obvious advantages in using the high strength grades 654 SMO, 4565, 2205, and 2507 for applications where the design is based on the yield strength of the material, while 254 SMO and 4439 could be considered for other applications.
Although stainless steels are considered relatively expensive when compared with carbon steel or non-metallic concepts, cost considerations over the entire service life of a scrubber show stainless steels to be economical. If the stainless steel materials of construction are properly selected, the service life of the FGD unit should meet the service life of the power station, without any considerable maintenance cost, which is in contrast to non-metallic coatings, where regular maintenance and re-lining is part of the design concept.
Stainless steels can be used either as solid sheet or plate, which should be the main concept for new plants, or as lining (wallpapering), which is recommended as a maintenance concept.
The mechanical strength of stainless steel grades can occasionally be utilized, affecting the required wall thickness of components and also the total cost of the desulphurization unit. For example, the high content of nitrogen in 4565 implies a high yield strength, which can be used to save material and fabrication costs through reduced thickness, reduced weight and also reduced machining and welding costs.
With the exception of 654 SMO, 4565 also has better corrosion resistance than the other stainless steels investigated. Steel of type 4439 (317LMN) has frequently in the past been used for zones with less hostile environments but it has been replaced in many applications by the duplex stainless steel 2205, which is more cost effective and has a higher yield strength and similar corrosion resistance.
At Italian utility Enel’s La Spezia power plant a double loop absorber was installed8. Apart from the raw gas entry where nickel-base alloy C-22 has been applied, the whole absorber tower including internals like distributing pipes, spraying systems, sieves, collection bowl, demister support are made out of stainless steel grade 4565. Downstream, for the demister, i.e. for the wall plates of the gas outlet, 317LMN was selected.
La Spezia is the first power station where Enel specified grade 4565 for the bulk portion in a FGD system. The plant has now been in operation for more than six years without any corrosion problems. Following this positive experience, Enel has specified grade 4565 for several other FGD projects à‚— installation of new plants as well as replacement of different components like spraying systems and exhaust draft fans in existing power plants, where the originally selected material had suffered from crevice corrosion attack.
In the long-term, the most striking advantage of stainless steel construction is its reliability. The obligations of a power utility are established in laws and regulations à‚— the scrubber works or the power plant does not. With the proper selection of grade and installation, stainless steels give a reliable and long service. Furthermore, stainless steels are resistant to catastrophic failures due to excursions in operating conditions and will certainly not collapse, melt or burn like some non-metallic components or liners.
1. ‘A Nitrogen Alloyed Superaustenitic Filler Metal à‚— Properties and Application’; V. Gross, H. Heuser, T.L. Ladwein; Stainless Steel World Conference 2003, Maastricht, the Netherlands
2. ‘Field Experiences with Stainless Steels in Flue Gas Desulphurization Plants’; R. Zauter, T.L. Ladwein, W. Braun; Stainless Steel World, Vol. 12, December 2000.
3. Annual Book of ASTM Standards, ASTM G 48-03.
4. ‘An Update on Materials Selection for Flue Gas Desulfurization Control Systems’; G. Carinci, Airpol 2004, Washington, DC, USA, August 2004.
5. ‘Materials Selection and Optimization for Wet Flue Gas Desulfurization Control Systems’; G. Carinci, ‘Mega’ Symposium Conference, Baltimore, MD, USA, August,2006.
6. ‘Stainless Steel Selection for Wet Flue Gas Desulfurization Systems’; G. Carinci ‘Mega’ Symposium Conference, Baltimore, MD, USA, August, 2008.
7. ‘Stainless Steels for Flue Gas Cleaning à‚— Laboratory Trials, Field Tests and Service Experience’; B. Beckers, A. Bergquist, C-O. A. Olsson, M. Snis, and E. Torsner, Airpol 2007 Conference, Louisville, KY, USA, June 2007.
8. ‘The Construction of the Flue Gas Desulphurization Plant of La Spezia à‚— A Special Solution to Weld the High Strength Superaustenitic Steel Nirosta 4565 S”; P. Bonalumi, L. Crosta; Stainless Steel World, September 2001.
The authors would like to thank Elisabeth Torsner and Bernd Beckers of Outokumpu Stainless, Sweden for their invaluable contribution to the article.