Chemical and Petroleum Engineering, Vol. 31, Nos. 5-6, 1995

MATERIALS SCIENCE

SUPERALLOYS FOR FLUE GAS DESULFURIZATION UNITS

G. Grossmann and D. Agarval'

UDC 669.15-194.56

In the treatment of flue gas in fuel-fired power generating stations, the most widely used method is wet gas desulfurization (WGD) by the injection of lime slurry. In various critical zones of the absorber, serious corrosion problems arise; these problems can be solved by the use of superalloys or nonmetallic coatings with extremely high corrosion resistance. In Fig. 1 we show schematically a WGD system using lime slurry; the most highly corrosive zones (critical zones) of the absorber are indicated by asterisks. Since the various zones of an WGD unit differ in the degree of corrosivity of the medium, different metals and nonmetals must be used. However, WGD systems are massive and require large quantities of materials in their construction; therefore, it is economically inefficient to use plates with a thickness greater than 6.35 nma (1/4"). The optimal solution, from the standpoint of economic efficiency and complete reliability of the equipment, has become the use of thin, tack-welded sheets of superalloys such as Nicrofer 5716 hMO (alloy C-276). In Table 1 we show the nominal chemical compositions of highly corrosion-resistant alloys produced by Krupp VDM AG that are recommended for critical zones of the WGD unit that is defined by the parameters listed in Table 2. Alloy 59 is a new N i - C r - M 0 alloy that has been demonstrated to be highly corrosion-resistant in laboratory tests in a great variety of media. This alloy has undergone commercial tests in many coal-fired generating stations. In Table 3 we present comparative corrosion data on the Alloys 59 and 22 in various corrosive media. It will be seen that the Alloy 59 is more corrosion-resistant than the Alloy 22 and hence more suitable for critical zones of absorbers in coal-fired power stations and in waste incinerators. Alloy C-276 has demonstrated its worth during the course of 15 years of use in the most corrosive zones of WGD systems of coal-fired power stations and waste incinerators in the United States and Europe. It has been found to be resistant to overall and pitting corrosion in WGD systems in which the main corrosive agents are sulfur and sulfuric acid in combination with chlorides, fluorides, and other components. The C-276 alloy, together with alloy C, has been used successfully for more than 50 years in various fields of industry throughout the world and has provided a solution for serious corrosion problems in both oxidizing and nonoxidizing media. Alloy 22 is a relatively new material for WGD conditions. This alloy is similar in chemical composition to the C-276 (see Table 1). According to the results of certain specialized laboratory tests (unrelated to the media encountered in WGD units), alloy 22 has a considerably higher resistance to pitting corrosion than alloy C-276. However, the overall corrosion rate of alloy 22 in sulfuric acid of various concentrations, at temperatures that are typical for WGD systems, is approximately 2-5 times the rate for the C-276 alloy. In an article presented by specialists of the firm Krupp VDM AG at a conference in West Germany in March 1989, it was shown that after testing for 10,000 h, the C-276 alloy did not exhibit any indications of corrosion, whereas the alloy 22 was subject to shallow pitting. Under other corrosive conditions, the alloy 22 was found to be more resistant than the C-276. Alloy C-4 is similar in composition to C-276, with lower contents of iron and tungsten. This alloy is structurally stable after extended exposure to a temperature of about 650~ confirming its high resistance to intergranular corrosion (IGC). Experience in the use of this alloy in Germany has shown that it is suitable for such elements of WDG units as the absorber inlet, the mixer for the caustic fusion cake, and the absorber sumps. Krupp VDM AG, Germany. Translated from Khimicheskoe i Neftyanoe Mashinostroenie, No. 6, pp. 26-29, June, 1995.

330

0009-2355/95/0506-0330512.50 9

Plenum Publishing Corporation

TABLE 1 Alloy Nicrofer 5923 hMo (59) Nicrofer 5716 hMoW (C-276) Nicrofer 5621 hMoW (22) Nicrofer 6616 hMo (C-4) Nicrofer 6020 hMo (625) Nicrofer 4823 hMo (G-3) Nicrofer 3127 hMo (31) Cronifer 1925 hMo

Manufacturer's catalog number 06059 10276 06022 06455 06625 06985 08031 08925

! " Mo 16 16 I4 16

Contents of elements, % W Fe !"--C -1 0,005"" 3,5 5 I 0.005 3 3 0.005 -2 , 0,005 I [

Ni 59 57 58 66

Cr 23 16 2I I6

63

22

9

-

48 31 25

23 27 2I

7 6,5 6,5

----

3

i0,010

Other --

0,2 V 0,2 V 0,3Ti 3,4 .',b

19 i 0.010 0,3 Nb; 2 Cu Remainder ! 0.006 0,2 N; 1,3 Cu " i 0.008 0,2 N; 0,9 Cu

Alloy 625 has a lower content of molybdenum in comparison with the C-276 and has a varying resistance in WGD systems. Since the corrosion resistance of this alloy is analogous to that of the C-276, a complete changeover has been made in W G D units to the use of C-276. Alloy G-3 has a lower content of nickel and contains only 7% molybdenum; hence it is less expensive than the C-276, and it has found useful applications in the zones of the WGD units where the corrosion conditions are less severe. Alloy 31 is a new F e - N i - C r - M o alloy produced by Krupp VDM AG. It is analogous to the 625 alloy in corrosion resistance but is less expensive. This high-chrome, completely austenitic alloy, containing 6.5 % molybdenum and doped with 0.2% nitrogen, has a high corrosion and erosion resistance in acidic chloride and oxidizing chloride media. Owing to its high pitting equivalent, it is extremely resistant to pitting and crevice corrosion. Cronifer 1925 h M o alloy, an iron-base alloy, is similar to G-3 alloy in its corrosion resistance in many media. The 6% molybdenum and 0.2% nitrogen contents of this alloy increase its corrosion and erosion resistance, and its low cost makes it economical for use in the less corrosive zones of WGD units. It is being used successfully in W G D systems in Europe and is currently under test in the United States. Krupp V D M AG manufactures these alloys for use in WGD systems in the form of sheet, strip, plate, rod, pipe, and welding materials. The company achieves high quality of metal production by the use of highly efficient equipment and strict observance of the proper technology and interoperational criteria - - considerably more stringent than commercial standards. The ability of the firm to produce cold- and hot-rolled sheet and plate with widths up to 2500 m m in lengths of 8000 mm provides unique advantages for consumers and manufacturers: - R e d u c e d costs in the manufacture of equipment. -Less waste material. - F e w e r weld seams (and hence less influence of welding defects); in turn, this facilitates inspection and maintenance operations. In addition, time is saved in erection, the cost of the structures is reduced, and future operating costs will be lower. Apart from the cost saving in fabrication due to the less extensive welding operations, costs related to cutting sheets and dressing the edges are also reduced. The ability to produce sheets in the width required by the consumer also reduces the cost of erection. The manufacture of a stack with a height of 148 m in Louisville (USA) using sheets of the alloy Nicrofer 5716 hMoW (C-276 alloy) with a width of 2340 mm, using tack-welding technology, confirmed a 50% saving of fabrication costs in comparison with the version in which the sheets have a width of 1220 mm. Thus, the use of wide sheets may result in a significant reduction of total design cost of the structure. The "tack-welding technology" requires little or no special tooling or equipment, and special training of personnel is not necessary. This method is described in detail in [1]. The general principle of "tack-weld technology" is illustrated in Fig. 3. Detailed technology has also been worked out for preparing the surface and for trimming and clamping the sheets, the parameters of spot welding and rivet welding have been established, and procedures have been defined for the investigation and testing of welded joints and for welding regimes in maintenance operations. Krupp VDM AG offers a large number of weldable alloys and can provide industrial expertise on W G D systems; the firm can also offer assistance in the performance of any maintenance operations, restoration of old structures, or the design of new structures (starting with the concept of turnkey operation); the firm has a bank of corrosion data and results from inspections by experts. Other facilities of the firm include a corrosion laboratory with technical expertise in engineering solutions with special emphasis on the solution of corrosion problems in application to W G D units; technical documentation is available on all aspects of industrial process technology. 331

TABLE 2 pH

Element of unit

Mole fraction (• 1000) of chlorides, ppm[ Temperature, ~

Alloy

i

Inlet to scrubber (wet/dry zones)

< I-=4

5--100

50-150

Outlet line for dry or heated gas

< 1--4

1--30

_50--100

Outlet line of wet gas

< 1---4

1-30

50--55

Flue

< 1--4

1--30

50--100

Absorber and internals

< 4-7

1-30

50---55

Heater pipes

< 1---4

1-30

50--90 '

Humidifiers (dependingon location)

< 1---4

1--30

50-90

Blind flanges of humidifier

< I.---4

1--50

50-90

3--7

1--50

25--55

Pumps, pipelines, valves, settlers, demisters, heads

Nicrofer 5923 hMo Nlcrofer 5716 hMoW Nicrofer 5621 hMoW Nicrofer 6616 hMo Nicrofer 5923 hMo Nicmfer 5923 hMoW Nicrofer 5621 hMoW Nicrofer 5923 hMo Nicrofer 5716 hMoW Nicrofer 5621 hMoW Nicrofer 5923 hMo Nicrofer 5716 hMoW Nicrofer 5621 hMoW Nicrofer 3127 hMo Cronifer 1935 hMo Nicrofer 5923 hMo Nicrofer5716 hMoW Nicrofer 5621 hMoW Nicrofer 5923 hMo Nicrofer 5716 hMoW Cronifer 1925 hMo Nicrofer 5923 hMo Nicrofer 5716 hMoW Cronifer 1925 hMo

TABLE3 Test method ASTM G-28, Method A ASTM G-28, Method B 10% H2SO4, boiling, 14 days 70,000 ppm CI-, pill (H2SO4), boiling, 21 days 1.5% HCI, boiling, 14 days 10% solution of FeC13.6H20, 85~ 72 h

Overall corrosion rate of alloy (mm/yr) 22 59 0,9 0,6 0,17 0,1 0,46 0,14 0,08

0,008

0,68 0,75

0,2 0,1

Note: Critical temperature for pitting in 10% solution of FeC13'6H20 was 85~

for Alloy 22, and above 85~

for

Alloy 59.

Krupp VDM AG guarantees its superalloy materials. The technical staff can provide assistance in determining the most economical material for WGD systems, and will propose an overall materials concept for the entire project. Some of the main reasons why the use of C-276 alloy as a liner is effective in solving corrosion problems in WGD systems are its complete reliability, ease of application of the sheets under plant and field conditions, repairability, and ease of fabricating new structures and renovating old structures. It should be noted that the "tack-weld technology" has been used in power stations in the United States since 1980, giving reliable operation of inlet and outlet piping, humidifier blades, and piping for the lime slurry feed. More than 1000 metric tons of the C-276 alloy are in service in power stations in the United States and West Germany. The advantages of the wide sheets produced by Krupp favor the introduction of this technology for new designs in European countries. Economic considerations dictate the need for complete reliability of WGD systems, since unplanned shutdowns due to corrosion or other problems are very costly for the power station. Experience has shown that in critical zones of WGD units, metal sections and elements prepared from corrosion-resistant metallic materials have operated reliably for more than 15 Years. It has also been established (on the basis of data from operations in Germany) that some nonmetallic liners, while they are low 332

Fig. 1. Scheme of WGD system (typical U.S. technology): 1) inlet of contaminated gas to absorber*; 2) absorber; 3) upper part of line with spray heads; 4) lime slurry feed line; 5) collector of lime slurry for recycle; 6) tank for lime slurry (caustic fusion cake); 7) humidifier (seal of humidifier*); 8) bypass line; 9) mixer outlet line*; 10) demister; 11) deflection blade; 12) heaters, heat exchangers, or bypass heater*; 13) lower part of stack*; 14) stack* (asterisks denote the zones of most severe corrosion).

G LPS Fig. 2. Arrangement of sheets, supplied by Krupp VDM AG, with lined stack: 1) concrete; 2) carbon steel; 3) Nicrofer 5716 hMoW (C-276 alloy); LPS) lower part of stack; B) base; I) eight segments of eight narrow sheets; II) 17 segments of wide sheets; III) five segments of wide sheets.

in capital cost, are not completely reliable. Hence, economic considerations do not always determine the selection of materials for each individual design. 333

I

tl

2

3 ~

/,i,

5

,<

f

i

Fig. 3. Basic scheme of application of Nicrofer 5923 hMo alloy liner: 1) carbon steel; 2) bead of intermittent weld; 3) alloy sheet, thickness 1.6 ram; 4) bead of gas-tight weld; 5) spot welding or rivet welding. The calculation method used most widely and accepted in industry is an equivalent or total annual calculation of the cost of various design solutions for WGD systems with different operating costs during the term of service of the unit. The method uses the principle of cash flow calculation. Krupp VDM AG can assist in evaluating and determining the total annual cost of equipment, based on data furnished by power stations. The same as in all economic calculations, certain assumptions must be made, and information must be available on the credit percentage rate, the cost of primary capital, amortization, tax laws, insurance, the costs of operation with either planned or unplanned shutdowns, the payroll and other costs for operating personnel, and so on. By varying the initial assumptions (if parameters are formulated for specific cases), the costs of alternative variants can be determined, so that the most economical and effective solutions can be selected in planning maintenance or reconstruction operations or in designing new WGD systems. In conclusion, let us note that the research division of Krupp VDM AG is constantly developing new alloys and working out methods to improve product quality. The new alloys Nicrofer 3127 hMo (Alloy 31) and to some extent the alloy Nicrofer 5923 hMo (alloy 59) offer great promise for WGD systems and other fields of application. Krupp VDM AG is the world leader in supplying metals with high corrosion resistance; this position is based solely only on the unique capabilities of the firm for supplying alloys in a wide range of dimensions, including superwide sheet and plate. Other factors in the firm's success are the technical assistance that it provides in combination with competitive prices and on-time supply of metal products, as well as expertise in manufacturing and welding, corrosion data, information on operating experience, guarantees based on extensive studies of the features of industrial processes, and strict observation of engineering regulations, more stringent than the usual commercial standards.

REFERENCES 1.

334

T. Hoffmann, M. Rokel', and V. Kherda, "Investigation of weldability of new nickel-base superalloy Nicrofer 5923h Mo," Khim. Neft. Mashinostr., No. 1, 26-30 (1995).