The most common cleaning application for flue gases is flue gas desulphurization (FGD).These systems came into being in the rate 1960s and early 1970s because of the tightening of restriction on the release of sulfur emissions. A wide variety of alloys have been used in scrubbers, which are located between the boiler and smokestack of fossil fuel power plants to treat effluent gases and to remove SO2 and other pollutants. Typically, fly ash is removed, and the gas travels through an inlet gas duct, followed by the quencher section. next SO2 is removed in the absorber section, most often using either a lime or limestone system .A mist eliminator is employed to remove suspended droplet, and the gas proceed to the treated¨Cgas duct, reheated section, and the stack. In this way, four basic factors affect the severity and type of corrosion that occurs. They are: temperature, pH, gas saturation and erosion. About temperature, the problems caused by it excursions are primarily related to the lessening of the corrosion-resistant properties of synthetic coatings, fiberglass-reinforced plastics(FRP),and thermoplastics, possibly to the point of complete destruction at high enough temperatures. This affect metals to a lesser extant, but can make a borderline problem a serious one. The result of the reactions that take place within the scrubber is slurry with a typical pH of 4 to 5 .It allows for good absorption of SO2 and is acidic enough to reduce scale formation. Local pH values as low as 1 may exist from the concentration of chlorides entering the make up liquid with contributions from fluorides. Incase of gas saturation, the dry flue gas is not severely corrosive. However, when the gas reaches its dew point, sulfuric (H2SO4) and sulfurous (H2SO3) acids can form. In addition, hydrochloric acid (HCL) is produced because of the presence of hydrogen chloride (formed at the elevated temperatures of combustion) plus the condensing water vapor. Again, significant problems arise from the use of carbon or stainless steels. Erosion generally occurs as a result of fly ash within the gas impacting on a surface in a relatively dry area of the system or the liquid slurry impinging upon a wetted surface. In either case, areas susceptible to corrosion attack are produced.

MATERIAL SELECTION

An easily over looked but critical aspect of materials selection is the ability of the manufacturer to construct the equipment property with correct fabrication techniques. In particular, with regard to the use of high-nickel alloys, the welding recommendations of alloy producers should be precisely followed to maintain the corrosion resistance of the materials. About metals, where pH is neutral for higher, austenitic stainless steels (AISI types 304,316, and 317) perform well even at elevated temperatures. If pH is as low as 4 and chloride content is low(less than 100ppm) but temperatures are above 65°c (150°f),then Incoloy 825, Inconel 625, Hastelloy G-3 ,and alloy 904L or their equivalents are usually acceptable. When chloride content is up to 0.1¨Gand pH approaches 2, only Hastelloys C-276, G and G-3, and Inconel 625 can be successfully used. The other alloys mentioned above would be subjected to pitting and crevice corrosion .If a region is encountered with pH as low as 1 and chloride content above 0.1¨G, one of the only successful alloys acceptable is Hastelloy C-276 or its equivalent. IN terms of metal selection, the higher the molybdenum content in an alloy, the more severe the corrosive environment it can with stand in the FGD system About nonmetals , fiberglass-reinforce plastics can be used in almost any application in which temperatures do not exceed120 °c(250 °F) , regardless of whether there are high chlorides or low pHs. The best would be premium grades of vinyl-ester and polyester resins, polypropylene(PP) , chlorinated poly vinyl chloride (CPVC), and other thermoplastics can be used in such applications as mist elimination, in which temperature are suitably low for example 80°c(175°F) for PP. Rubber linings can also be used where temperatures are suitable and mechanical damage can be avoided. In case of waste incineration, the problems associated with materials for incinerator off-gas treatment equipment are similar to those used for FGD systems. Depending on the waste being burned, however, significantly higher gas temperatures as well as more varied and more highly corrosive compounds may be encountered. Materials selection for waste incineration parallels that for FGD systems to some extent, but can often be more demanding . The importance of incineration for the treatment of the domestic and industrial wastes has increased as the availability of sanitary land fills has lessened and their costs have escalated .At the same time, environmental safely regulations have limited the use of deep below-ground and sea-disposal sites for untreated wastes. Incineration provides a viable, although not inexpensive, alternative that produces scrubable gaseous and particulate contaminations from myriad of waste products. Incinerators are used to burn municipal solid wastes, industrial chemical waste, and swage sludge. In general, the off-gases can be classified according to their corrosiveness in descending order as follows: industrial chemical, municipal solid, and sewage sludge.