CORROSION / EROSION:
This problem is common when wastes being combusted contain acidic components or when burners / furnaces are designed with swirling flow (for high intensity operation.)
Acid Dew Point: Waste containing acidic components form combustion products which are corrosive when combined with water. Methyl Chloride contains chlorine, which forms HCl. When cooled to 120 - 150oF, the water vapor / HCl mixture in the flue gas condenses. Corrosion of refractory or vessel shell material results. Sulfur compounds burn to form SO2 and SO3 (usually 3 to 10% ends up as SO3). The SO3 combines with water vapor when flue gas cools to the dew point temperature - often between 200 and 400o F, condensing to form H2SO4. Dewpoint problems often occur at the furnace shell when the refractory / rainshield system allow a low shell temperature.
Refractory linings are relatively porous. Flue gas components penetrate to the steel shell. If the shell temperature is below the acid dewpoint of the gas, condensation will occur.
Other problem areas are any heat recovery system (air preheater) or particle removal system (baghouse) where large surface areas and reduced flue gas temperature are required. Adding heat tracing and/or external insulation may prevent the condensation.
Source of problem: Acidic flue gas is inadvertently cooled below dewpoint temperature or the materials of construction were not properly selected for the resulting acid contact.
Thermal Shock: Rapid temperature changes can cause localized stresses in refractory materials as the areas more easily heated or cooled expand or contract faster than the rest of the material. The resulting stresses overcome the molecular bonds holding the material together and pieces "spall" off. Commonly happens when a furnace is started or shutdown too quickly (see refractory guidelines in this website). Thermal shock can also be caused by spraying liquid water directly on hot refractory - usually caused by poor atomization of aqueous wastes or water quench streams. Corrosion or heat damage to water spray gun tips is a common cause.
Source of problem: Heating or cooling refractory too fast relative to its strength.
Particulate "Scrubbing": Any material can be eroded by a harder material. If ash or refractory particles strike refractory or vessel shell material, some "sanding" can take place. Excessive contact can remove enough of the base material to cause equipment failure. This is most common where waste gas with entrained particulate enters a furnace or where burner / furnace swirl is maintained using an exit choke ring.
Source of problem: Construction of the equipment of materials which are not sufficiently erosion resistant, given the waste composition. Alternately, equipment design which traps particles and "recycles" them across a refractory surface.
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