Utilities worldwide are facing increased demand for additional electricity, reduced plant emissions and greater efficiency. Part of the solution is achieved by increasing boiler temperature, pressure and coal ash corrosion resistance of the materials of boiler construction. In this paper, a new nickel-base tube alloy meeting this challenge is characterized with emphasis on its corrosion resistance. An alloy development strategy is described as well as a testing methodology to verify corrosion performance. The role of certain key elements in influencing requisite coal ash corrosion resistance is shown and the influence of several important environmental factors on the alloy of choice is discussed.
While strong environmental restrictions are requiring power companies to reduce SOx, NOx and CO2 emissions, deregulation pressures are simultaneously necessitating improvements in the thermal efficiency of all coal-fired plants. Adding to the challenge is increasing demand for more electrical capacity worldwide/1) These pressures on the utilities are spearheading the renovation of older plants and the construction of new power generating capacities to meet these challenges. A key component of the solution is increased boiler efficiency. It has been shown that the efficiency of pulverized coal- fired boilers can be increased to over 50% when ultra-supercritical steam conditions greater than 300 bar and 600°C are adopted. (2-6) In these power plants, the superheater/reheater midwall temperature can be 660°C or higher which means the well-established 9-12%Cr steels must be replaced by austenitic stainless steels with their higher creep strength and greater corrosion resistance under these conditions. (z'7) Steam conditions up to 375 bar and 700°C, being planned by both the European THERMIE project and the German MARCKO project, will lead to superheater/reheater midwall temperatures in the range of 740°C to 770°C. At these temperatures and pressures, austenitic stainless steels cannot fulfill the requirement of a 100,000 hour stress rupture strength of at least 100 MPa and corrosion resistance of less than 2 mm in 200,000 hours. (z) Nickel-base superalloys have been proposed to meet these stringent material targets. This paper summarizes the laboratory effort to develop and characterize an alloy to meet these aims.
FACTORS INFLUENCING SUPERHEATER/REHEATER CORROSION
Among the more critical components in pulverized coal-fired boilers are the superheaters and reheaters, since they are exposed to high levels of combined mechanical and thermal stress and corrosive attack. It is well documented that the corrosion rate of the superheater/reheater tubes is determined predominantly by the alkali content of the coal and the SO2/SO3 content of the combustion gas. (8-10~ The alkali content of the coal originates from sodium aluminosilicates (such as albite) and potassium aluminosilicates (such as muscovite, orthoclase and illite) and the sulfur content from pyritic sulfur, organic sulfur and sulfates present in the coal. (1~) The amount of sulfur in coals generally varies from 1 to 4 weight percent, while in rare cases the level of sulfur can reach up to 10 weight percent. (12) Sulfur is the most critical element with regard to corrosion by combustion of coal, since the presence of SOz/SO3 in the flue gas leads to the formation of highly corrosive deposits on the boiler tubes consisting of complex sulfates, such as K3Fe(SO4)3 and Na3Fe(SO4)3, which in the temperature range of 593°C to 760°C can form a liquid phase. (~3) At low temperatures, sulfur can lead to corrosion due to the formation of sulfuric acid. According to Corey et al. (8), the corrosion of superheater and reheater tubes oc
