Minimizing Superheater Corrosion

To generate electricity from steam efficiently, HRIs must heat the steam to at least 700°F (371°C). This temperature results in more fireside corrosion in MSW-fired boilers than in regular boilers. Corrosion in refuse boilers is related to the high chlorides in MSW. While an RDF processing system can remove some of the material containing chlorides, removing chloride-containing material in the RDF processing system is not a realistic means to prevent boiler corrosion. High-nickel-alloy superheater tubes (e.g., Inconel 825) minimize superheater corrosion in addition to protecting the furnace from overloading and providing

FIG. 10.9.12 Mounting tubes vertically in a horizontal superheater section to prevent particle velocity increases. (Reprinted, with permission, from A.J. Licata, R.W. Herbert, and U. Kaiser, 1988, Design concepts to minimize superheater corrosion in municipal waste combustors, National Waste Processing Conference, Philadelphia, 1988 [New York: ASME].)

FIG. 10.9.12 Mounting tubes vertically in a horizontal superheater section to prevent particle velocity increases. (Reprinted, with permission, from A.J. Licata, R.W. Herbert, and U. Kaiser, 1988, Design concepts to minimize superheater corrosion in municipal waste combustors, National Waste Processing Conference, Philadelphia, 1988 [New York: ASME].)

FIG. 10.9.13 Boiler design criteria for corrosion and erosion control. (Reprinted, with permission, from Licata, Herbert, and Kaiser 1988.)
FIG. 10.9.14 Rapper boiler superheater headers. (Reprinted, with permission, from Licata, Herbert, and Kaiser 1988.)

rugged furnace walls. Hydrogen chloride corrosion begins by penetrating a slag layer on the superheater tubes. The tubes must be kept clean by soot blowers or mechanical rapping. Chlorides in hot gases become corrosive and can destroy a superheater.

With improved superheater designs, the operating superheater temperature can be increased from 750 to 825 or 900°F (Licata, Herbert, and Kaiser 1988). This temperature can be achieved when gas velocities are kept between 15 and 18 ft/sec to minimize the erosion caused by the impact of the particles. In addition, tubes should be liberally spaced to mitigate the increase in velocity as ash buildup occurs. Figures 10.9.12 and 10.9.13 show the recommended superheater design criteria for velocities and temperatures.

Another design improvement is the elimination of the harmful effects of soot-blowing by steam or air which damages the protective oxide film, creates hot spots from nonuniform cleaning, and reentrains ash into the flue gas. Rapping rather than blowing can eliminate these effects (Licata, Herbert, and Kaiser 1988). Figure 10.9.14 shows pneumatically actuated mechanical rappers that allow deposits to slide down the tube surfaces into the hoppers below.

The boiler design should also protect against stratification (which can result in reduced atmosphere quality) by forcing the flue-gas stream to make a 180° turn before entering the superheater (see Figure 10.9.12). When the excess air level is maintained at 80 to 85%, high levels of CO concentration caused by incomplete combustion can be prevented. Another recommended feature is a ceramic lining for the postcombustion zone. This lining provides a 1-sec (minimum) residence time for flue gases at temperatures in excess of 1800°F (980°C) before they enter the superheater section.

An increased soot removal frequency and innovative cleaning techniques can minimize the secondary formation of dioxins and furans. Cleaner tubes have fewer fly ash particles on which dioxins and furans can form and allow more heat to be transferred away from flue gases. This heat transfer further cools the gases below the 450°F (250°C), which is conducive to dioxin and furan formation. Additionally, minimizing the production of precursors in the furnace by maximizing combustion efficiency helps decrease secondary dioxin and furan formation.

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