ABSTRACT A variety of austenitic alloys are available for use in flue gas desulfurization (FGD) systems. Alloys with Mo content varying from 2 to 16 percent have been used. The 6% Mo superaustenitic alloys perform significantly better in laboratory tests than 4% Mo stainless steels $31725, $31726 and N08904 and approach the performance of the nickel-base N06625 alloy, but fall short of the performance of the N10276 alloy. This presentation concentrates on one of the 6% Mo superaustenitic stainless steels, the N08367 alloy. The possibility of tailoring the PREN of the 6% Mo alloys is examined and the production of versions ofN08367 alloy having guaranteed minimum PREN values is described. L
aboratory corrosion data for N08367 alloy are presented. These data show that N08367 alloy is able to resist corrosion in high-chloride environments at temperatures of 66 to 71°C and pH levels as low as 2. The corrosion of alloys ranging from $31603 through N08367 to N10276 is shown to be aggravated by the addition of 1,000 ppm thiosulfate to 10,000 ppm chloride water at 66°C. The results of tests of N08367 alloy in a simulated SO2 absorber environment are described.
Fabrication of N08367 alloy for FGD applications is discussed and several FGD applications for N08367 alloy are described.
Overall, the 6% Mo alloys are shown to be cost-effective materials of construction to fill the gap between the 4% Mo austenitic stainless steels and higher-Mo nickel-base alloys.
INTRODUCTION Public demand for improved air quality led to the passage of the Clean Air Act and its subsequent amendments. These laws forced utilities to reduce their emissions of sulfur dioxide. Flue gas desulfurization (FGD) is one method for achieving this mandate. The use of lime or limestone slurries for "scrubbing" sulfur dioxide from flue gasses has become an established technology. Modem scrubbers are capable of removing in a cost-effective manner over 90 percent of the sulfur from the exhaust gas stream. For the FGD units to provide the desired improvement in air quality, they must operate reliably. Selection of suitable materials and proper fabrication are a vital part of the operability of the FGD process, and of power generation from fossil fuels.
A wide variety of metallic and non-metallic materials have been used for construction or repair of FGD equipment. Many different alloys have been employed. These have ranged from type 316L stainless steel (UNS $31603) through the various type 317 alloys ($31703, $31725, and $31726) and duplex stainless steels ($31803, $32550, etc.) to nickel-base alloys (N06625, N10276, N06022, etc.).
Factors involved in choosing materials for FGD include the composition of the fuel, the quality of the water used for scrubbing, the operating temperature, the build-up of impurities (such as chlorides) in the system, possible upset conditions, scaling or deposit buildup, and galvanic interactions.
Modem FGD units are divided into several zones which have different corrosion problems and may be built from different alloys. The most severe zones, such as the inlet and outlet duct regions, are now typically constructed using nickel base alloys like N10276. The absorber, sump, and mist eliminator zones are usually much less corrosive and lower alloy materials are frequently employed there.
6% Mo STAINLESS STEELS
Many 6% Mo superaustenitic stainless steels have been developed in recent decades. The most common of these are $31254, N08367 and N08926. More recently developed 6% Mo alloys include N08031 and $32050. In addition to their high Mo-contents, all of the modem 6% Mo superaustenitic stainless steels are marked by deliberate additions of nitrogen