Since incinerators are usually installed only to satisfy emission requirements, heat recovery boilers sometimes offer the only positive payback for the project. Designed correctly, they are one of the most trouble free components in any system. But:
Steam Blanketing: Firetube boilers are most often used for incinerator heat recovery for economic reasons. Carbon steel boiler tubes are commonly used, and the tube metal temperatures are kept sufficiently low by the very rapid rate of heat removal as water flashes to steam on the outer surface of the metal. As steam bubbles form they float upward away from the tube surface, allowing fresh water to reach the tube. Excessive steam production in one area can "blanket" the area with steam, impeding water entry and allowing tube metal to approach flue gas temperature, damaging the tube. A ceramic "ferrule" sleeve at the entrance to each tube will prevent steam blanketing in this area of high gas velocity. Installation of ferrules to eliminate steam blanketing is possible, as long as gas side pressure drop does not become excessive.
Source of problem: Improper boiler tube layout or construction.
Solids Fouling: Some ash components in waste streams can have relatively low melting points. Should a molten salt droplet contact a cool boiler tube, the salt will solidify. The effect is similar to painting the tube, and the accumulation of salt cuts boiler efficiency. To avoid this, the flue gas should be cooled enough to solidify the salt droplets prior to boiler entry and soot blowers should be installed for removal of the solid salt particles. Watertube boilers with unfinned (bare) tubes in the entry zone are preferred in this service, even though the first cost of a firetube boiler might be less.
Source of problem: Improper system design.
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