Furnaces sulfur compound corrosion

Where the fuel contains sulfur compounds, sulfuric acid is ultimately formed, causing acid smutting and both hot-end (high temperature zone) and cold-end (low temperature zone) acid corrosion and fouling, and adds to the total volume of unwanted furnace area deposits. 

In general, furnace tubes and piping are made of 5Cr-1/6Mo steel for sulfur-bearing crudes when the temperature exceeds -550 F (290 C) and 9Cr-1Mo when the temperature exceeds 750°F (400°C). The temperature at which excessive corrosion of carbon steel occurs is a function of the characteristics of the sulfur compounds in the crude. This temperature can be estimated by measuring the amount of hydrogen... 

High-temperature crude corrosion is a complex problem. There are at least three mechanisms (i) furnace tubes and transfer lines where corrosion is dependent on velocity and vaporization and is accelerated by naphthenic acid (ii) vacuum column where corrosion occurs at the condensing temperature, is independent of velocity, and increases with naphthenic acid concentration (iii) side-cut piping where corrosion is dependent on naphthenic acid content and inhibited somewhat by sulfur compounds. 

After steel, probably the most important metal in refinery use at low (i.e. less than furnace) temperature is copper, usually in the form of such alloys as Copper Development Association alloys No. 443-445 (Admiralty) or CDA 715 (Monel) etc. In addition to higher heat conductivity, copper and its alloys are considered to be superior in corrosion resistance to steel in media such as dilute acids, saUne, and brackish waters, and in the presence of sulfur compounds. Because copper and its alloys have lower strength and versatility and cost more than low-carhon steels, substitution of steel by copper alloys must be justified in materials savings and/or process improvement. 

Coal is a complex and relatively dirty fuel that contains varying amounts of sulfur and a substantial fraction of noncombustible mineral constituents, commonly called ash (Chawla et al. 2011). Table 3.4 shows the common types of corrosion in coal-fired furnaces. High temperatures (above 1000 F) of the superheater/reheater favor the formation of low-melting compounds. 

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