FGD

Material Performance in Flue Gas Desulfurization (FGD) Systems – Experience of 42 fossil fuel-burning plants

W. L Mathay Consultant to the Nickel Institute

Pittsburgh, PA

David Gandy EPRI

Charlotte, NC

R. E. Avery Consultant to the Nickel Institute

Londonderry, NH

ABSTRACT

Recognizing the need for current material performance data, the Electric Power Research Institute sponsored a study of material performance in flue gas desulfurization systems at 42 fossil-fuel burning plants. This survey covered 12 different component areas including ductwork, absorbers, recycle tanks, slurry and spray piping, slurry pumps, slurry and spray valves, spray nozzles, mist eliminators, dampers, agitators, chimney/stacks, and other types of equipment. Materials utilized include rubber-covered carbon steel, stainless steels, nickel alloys, organic coatings, borosilicate glass blocks and plastics.

KEYWORDS

FGD, scrubbers, stainless steels, nickel alloys, other components

INTRODUCTION

There have been national and worldwide concerns regarding the control of sulfur dioxide (SO2)emissions from the combustion of fossil fuels since the early 1970’s. Control systems utilizing flue gas desulfurization (FGD) techniques, have become an increasing part of the overall installation costs of fossil fuel plants as demand for pollution control has increased. Areas within the FGD systems exposed to environments below the dew point can be quite corrosive and present a challenge in selecting the optimum materials of construction.

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Metallic materials used range from Type 316L stainless steel to nickel alloys such as Alloy C-276. Nonmetallic materials used have been predominately flakeglass coatings, rubber linings,FRP and borosilicate glass block.

Initial efforts aimed at establishing a data base on alloys being used for FGD systems resultedin the publication the Nickel Institute book, Nickel Stainless Steels and High-Nickel Alloys for Flue Gas Desulfurization Systems in 1987. A second edition was published in 1990. No further data were developed until the Electric Power Research Institute (EPRI) sponsored a study on FGD material performance in 42 installations. The survey examined 12 system component areas including ductwork, absorbers, recycle tanks, slurry and spray piping, slurry pumps, slurry and spray valves, spray nozzles, mist eliminators, dampers, agitators, chimney/stacks and other ancillary components. This paper is a brief overview of the much more detailed EPRI report, Flue Gas Desulfurization Systems – Component Material Performance and Welding, 1010913, December 2005.

FGD SYSTEMS INVOLVED IN THE SURVEY

A survey was made of 42 different installations to establish a data base for materials performance in flue gas desulfurization systems. Of the systems surveyed, there were a number of combinations involved such as spray towers with venturi quench systems, spraytowers with sieve trays, and one with packing to increase contact time between the scrubbing medium and the flue gas. There were venturi prescrubbers with various types of absorbers including weir-type absorbers, horizontal absorbers and one Jet Bubbling Reactor™ where the gases are sparged into the scrubbing medium. There were also two concrete units lined with Stebbins Tile and one fiberglass-reinforced plastic (FRP) unit.

Most of the scrubbing systems involved the use of limestone as the scrubbing medium. One system made use of dual alkalies (lime plus limestone) and a few utilized flyash with gypsum.Of the locations responding, one was in Hong Kong and the rest were in North America. Many of the absorbers were single loop (see Figure 1) with only one scrubbing section where the flue gases were quenched and one absorber section where the sulfur dioxide (SO2) was removed.It was mentioned earlier that some units use trays to increase the contact time with the scrubbing medium while others make use of venturis to aid in particulate removal.

The 4 double-loop systems (see Figure 2) that are reported in the survey each have a quench section and an absorber section separated by a bowl that collects the scrubbing liquid from the absorber. The liquid in the two sections differs in solids content and pH and may permit the use of less corrosion-resistant materials in the absorber section.

The systems generally made use of different materials of construction for scrubber inlets and outlets as well as for walls and floors. The materials that were used included carbon steel lined with stainless steels such as Type 316L, Type 317L and 317LMN and with nickel Alloys C-22™, C-276 and C-2000™. There were 5 units lined (wallpapered) with Alloy C-276, 3 clad with Alloy C-276, one solid C-276 unit and one Alloy G unit. Some usage of the duplex stainless steels such as Alloys 2205 and 255 as well as the 6% Mo stainless steels was also reported. Interestingly, many of the absorber inlet ducts were unlined carbon steel indicatingthat the ducts were operating above the dew point or were not being used because of pollution control regulations.

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Organic coatings also were used in a number of ducts, but they required periodic replacementor repair every couple of years. The stainless steels generally performed satisfactorily, as did the nickel alloys and in all there were 21 installations that reported no significant problems.

In a number of instances the data furnished by the utilities included two or more FGD units of similar design and operated under similar conditions. A total of 60 individual units were involved. The oldest two units were started 40 years ago and the six most recent were started in the 2000’s. Two thirds were started in the period of 1975 to 1990 of which one half have been rebuilt or had a major retrofit.

It should also be noted that this materials performance overview has omitted data from the EPRI report that might be of use to those interested in the FGD system operating details. Such details include plant size, fuel and scrubber medium chemistry, NOx control and equipment manufacturer for each of the individual locations.

The material experience of the main FGD component areas follows.

DISCUSSION OF COMPONENT AREAS

Absorbers

The absorbers are the vessels where SO2 is removed from the boiler flue gas by contact with the alkaline scrubber medium such as lime or limestone. The study revealed various types of absorber systems including spray towers with a venturi quench section, spray towers with sieve trays, open-spray towers, venturi absorbers, a grid-packed absorber, and a combination of venturi prescrubbers and horizontal weir-type absorbers. Many of the absorbers surveyed were single loop with one scrubbing section where the flue gases are quenched by the scrubber medium and then passed through the absorption section for SO2 removal. Figure 1 is an illustration of a typical single loop system.

Another design reported in four survey units is the dual-loop system where the quench sectionand absorber section are separated by a bowl that collects the scrubbing liquid from the absorber section. The liquid in the two sections differs in solid content and pH value and may permit the use of less corrosion-resistant materials in the absorber section Figure 2 is an illustration of a dual-loop system.

The survey included one Jet Bubbling Reactor ™ (JBR) of the Chiyoda design where the gases are sparged into the scrubbing medium. One material difference of the JBR is the extensive use of FRP for absorber internals.

The absorber materials reported in the survey are shown in Table 1 for the inlet/outlet areas and absorber walls/floors. Very often the units make use of a combination of materials such as stainless steels or nickel alloys for inlets and carbon steel lined with organic coatings such as flakeglass or rubber lining for the walls.

Most of the stainless steels are performing well. A few installations after encountering corrosion have gone to a nickel alloy replacement. The only service problems reported in nickel alloys were a few instances of leaks at welds. These were associated with fabrication

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problems rather than material deficiency. Of the 13 units with coatings in at least part of the absorber, five reported good or satisfactory performance, and eight reported variable service or coating failures requiring a metallic replacement. Two installations used concrete lined with Stebbins tile for the wet/dry zones and walls and floors.

Recycle Tanks

The recycle tanks can be separate vessels or an integral part of the absorber vessel. The tanks are used for the recirculation of the scrubber medium and slurry along with chemical additions when gypsum production is involved. Twenty two of the facilities in the study recover gypsum.

The materials reported for the recycle tanks and impellers are shown in Table 2. Coatings such as flakeglass, resins and rubber linings have been used in 23 installations with no service problems while six others required periodic “touch up” repair. Two installations have employed electrochemical protection in the recycle tanks for added corrosion protection.

Agitators or impellers are reported to be a major maintenance concern. Rubber covered carbon steel agitators continue to be used at 14 installations, even though wear of the rubber coverings necessitates periodic repair. In many instances, the rubber coverings have failed completely and been replaced by other materials.

The replacements for rubber-covered agitators have been a variety of regular and high alloy stainless steels as well as Alloy 255 duplex stainless steel. Two installations report using a cast 27% chromium impellers and another used Hastelloy™ G-30 (30% chromium) impeller, but service performance was not reported.

Ductwork

Materials used for the inlet, bypass and outlet ducts are shown in Table 3.

Flue gases in the systems in this study are normally carried directly into the absorbers by the inlet duct after leaving the electrostatic precipitators. Gas temperatures are above the dew point and range from 250oF to 400oF (121oC to 204oC) making unlined carbon steel a suitable material. However, in the portion of the duct immediately ahead of the absorber, blowback of the gas and the scrubbing medium can lead to serious corrosion. Where corrosion is a concern, nickel alloys such as Alloy C-276 or Alloy C-22™ (either solid or as linings on carbon steel) appear to be preferred.

When a bypass duct is a part of the FGD system, the gas may be used for reheating the scrubbed gas to minimize dew point corrosion of the outlet duct and to provide heat for equipment such as a hot-side precipitator. However, over one half of the installations reported not having or not using a bypass duct. Unlined carbon steel is suitable if the temperatures are above the dew point. However in seven installations, there have been corrosion problems at the outlet mixing zone requiring the use of a nickel alloy.

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As shown in Table 3, there are a variety of materials being used for the outlet duct ranging from carbon steel lined with flakeglass-filled vinyl ester and borosilicate glass blocks to regular and high alloy stainless steels and nickel alloys. Some of the organic coatings used have required periodic replacement, although one was reported to have been in service for more than 25 years. The stainless steels generally performed satisfactorily as did the nickel alloys(which were used predominantly as linings).

Chimneys/Stacks

The chimneys/stacks discharge the scrubbed flue gases from the absorber into the atmosphere and represent a major component in a FGD system. They are usually comprisedof a reinforced concrete shell with one or more liners or flues. The flues may be of acid-resistant brick, alloy sheet-lined (wallpapered) carbon steel, roll-bonded alloy clad plate, carbon steel with an organic coating or liner, or FRP. The flues are expected to resist conditions similar to those in the outlet duct. The materials reportedly used for breeching, shell and liner are shown in Table 4.

Twenty of the installations surveyed have concrete shells with acid-resistant brick liners. This rather large portion probably was influenced by the inclusion of plants more than 20 years or older. With the current trend away from reheat bypass flue gas to dry the saturated gases from the absorber and going to the wet duct/stack systems, there is a trend away from acid brick liners. The present day material stack liner trend is to Alloy C-276 roll-bonded clad plate or FRP. Borosilicate glass blocks are also a material well suited for wet duct/stack systems.

Slurry and Spray Piping

Both slurry and spray piping systems can pose problems. Slurry piping has to resist erosioncorrosion, while spray piping has to resist corrosion both on the exterior and interior. The materials used for 37 of 42 slurry pipe applications are rubber-lined carbon steel and FRP, with rubber-lined carbon steel used predominately. The Type 316 and 317 type stainless steels have been used in the other locations.

The spray piping used involves a wide array of materials including Types 316, 316L, 317, 317LM and 317LMN stainless steels, rubber-lined carbon steel, high-density polyethylene, and FRP. The stainless steels as a group and FRP are the most widely used.

Slurry Pumps

Pumps move a variety of process liquids within a FGD system such as delivering slurries to the absorbers or transferring slurries for further processing. Because the slurries have a high solid content that can cause serious erosion problems with pump components, elastomer-type linings on the metals have become standard for pump housings. Almost all of the 42 installations use rubber or elastomer-lined carbon steel for pump housings.

Pump impellers tend to be of solid alloy even though 11 installations use rubber-lined impellers. Twenty two of the installations make use of proprietary high-chromium iron for impellers. Others use Type 316 or CD 4MCu cast duplex stainless steel impellers.

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Slurry Valves

Most slurry valves used in the FGD systems were reported to be constructed of rubber-lined carbon steel, stainless steels or a combination. The exceptions were the use of Alloy C-276 at four installations.

The type of valves used was often not identified, but gate-type valves were mentioned mostoften. The only reported service problems were two liner adhesion failures and four leaks at seals.

Spray Nozzles

In the study, all installations used spray nozzles with the exception of one plant using a sieve tray. The major problems with spray nozzles appear to be plugging and erosion. The problems are minimized by the use of a fine grind of lime or limestone and proper material selection. Sixteen locations used ceramic spray nozzles, 15 used silicon-carbide nozzles while the remaining 10 used various materials such as Type 317 stainless steel, high-density polyethylene, polyvinyl fluoride, FRP, urethane, Stellite and Carborundum.

Mist Eliminators

The mist eliminators are located at the top of the absorber and are used to prevent or minimize liquid carryover into the outlet duct after the gases are scrubbed. The major concerns with mist eliminators are corrosion and plugging. Those responding reported using FRP at 14 installations, polypropylene at nine and polysulfone at seven.

Dampers

Dampers are used on the absorber inlet ducts, the absorber outlet ducts and the bypass ducts if bypass is used. Their purpose is to isolate areas for inspection or repair.

The metals used vary from carbon steel (suitable when the location is above the dew point) to various Type 317 stainless steels and Alloy C-276. Alloy C-276 is often the choice for the seal area.

SUMMARY

FGD system components surveyed comprise absorbers, recycle tanks, ductwork, chimneys/stacks, slurry and spray piping, slurry pumps, slurry valves, spray nozzles, mist eliminators, and dampers.

Absorbers

The absorbers that are most frequently used are single loop units with limestone the predominant scrubbing medium. The materials of construction, for the most part, involve a combination of materials, such as stainless steels, nickel alloys and organic-coated carbon steel (see Table 1). Coating life, as might be expected, is variable. Of particular interest is the fact that two installations involve concrete lined with Stebbins Tile in the aggressive areas.

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Recycle Tanks

These tanks handle the recirculation of the scrubbing medium where gypsum production isinvolved. The materials used are about equally divided between stainless steels, nickel alloys and rubber-coated carbon steel. The latter are widely used for impellers because of abrasion problems, even though wear is a major factor in the life of the impellers. It has been found that the use of electrochemical protection can extend the life of the tanks.

Ductwork

Inlet and outlet ductwork are used to handle the unscrubbed and scrubbed flue gases, respectively. Some portions of the inlet ducts are above the flue gas dew point and the material is usually carbon steel. However, those portions that are below the dew point and are exposed to the scrubbing medium require corrosion-resistant materials such as Alloy C-276.By-pass ducts have been used to raise the gas temperature of scrubbed flue gas. However, over half of the by-pass ducts are no longer being used because new regulations require that all flue gas be scrubbed.

Chimneys/Stacks

Many of the installations still have acid-resistant brick liners, but the trend appears to be to roll-bonded clad Alloy C-276 chimney liners or FRP

Other Components

Slurry piping and valves make use of rubber-lined carbon steel and FRP. Almost all of the installations use rubber or elastomers for pump housing and high-chromium iron or steel for impellers. Slurry valves appear to be quite variable as to composition although Alloy C-276 is mentioned frequently. Spray nozzle problems have been minimized with ceramics. Mist eliminators use FRP, polypropylene, or polysulfone. Dampers make use of carbon steel, stainless steels, or nickel alloys, depending on the conditions.

REFERENCES

1) J. D. Harrington and W. L. Mathay. Nickel Stainless Steels and High-Nickel Alloys for Flue Gas Desulfurization. Nickel Development Institute, Toronto, Ontario, Canada, 1987 and 1990

2) Flue Gas Desulfurization Systems – Component Material Performance and Welding, EPRI, Palo Alto, CA 1011913

3) Installation of Stainless Chromium-Nickel Steel and Nickel-Alloy Roll-Bonded and Explosion-Clad Plate in Air Pollution Control Equipment; Standard Recommended Practice, NACE Standard RP0199

4) Installation of Thin Metallic Wallpaper Lining in Air Pollution Control and Other Process Equipment; Standard Recommended Practice, NACE Standard RP0292

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Table 1 Materials Reportedly Used for Absorbers (Note a)

Materials Inlet/Outlet Walls/Floors

Nickel alloys (C-276, C-22™, 625, C-2000™)

26 10

High alloy stainless steels (317LMN, 317LM, 4-6% Mo)

10 12

Regular stainless steels (316L, 317L)

11 10

Duplex stainless steels (2205, 255)

2 2

Coatings (flakeglass, resins, rubber linings)

10 13

Tile, acid brick, FRP etc. 2 7

Note (a): Many plants reported more than one material for each area, for example differentmaterials for the walls and different materials for the floors.

Table 2 Materials Reportedly Used for Recycle Tanks

Materials Walls/Floors Agitators

Nickel alloys (C-276, C-22™, 625m C-2000™)

3 3

High alloy stainless steels (317LMN 317LM, 4-6% Mo)

6 4


3 7

Duplex stainless steels (2205, 255) 2 3


24 18

Tile, acid brick, FRP, etc 7 -

Cast 27% Cr stainless steel - 3

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Table 3 Materials Reportedly Used for Ductwork

Materials Inlet Outlet By pass

Nickel alloys (C-276, C-22™,625, C-2000™)

17 13 7


1 10 1


- 6 -

Duplex stainless steels (2205, 255)

- - 1


2 11 1

Tile, acid brick, FRP, glass blocks

2 3 1

Unlined carbon steel 16 - 5

Table 4 Materials Reportedly Used for Chimneys/Stacks

Materials Breeching Shell Liner

Nickel alloys (C-276, C-22™,625, C-2000™)

18 - 12


5 - -


4 - -

Coatings (flakeglass, resins) 8 - 5

Tile, FRP, glass blocks 4 - 6

Acid brick 2 2 19

Concrete - 40 -

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Figure 1Single-Loop Absorber

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Figure 2 Dual-Loop Absorber

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