SCRUBBER PROBLEMS: Most scrubbers for incinerators handling vapor or liquid wastes are of packed bed design. Variations include spray towers, trayed towers and venturi scrubbers. Acid gases are usually the target of scrubber operation. Water chemistry is usually simple, but there are always surprises.
Scale Deposition: Definite solubility limits exist for all minerals in water. The limits are composition and temperature dependent. Scrubbers remove acidic components from incinerator exhaust gas using absorption in water, and a neutralizing chemical such as NaOH or calcium carbonate are used to convert the acid to its salt (NaCl, Na2SO4, etc.). These salts, combined with whatever minerals (hardness) came with the fresh water feed stream, affect the scaling potential of the circulating solution. Once the solubility limit is exceeded, the salts begin coming out of solution on any available surfaces, such as the packing or other scrubber internals, eventually blocking flow. Deposition can be avoided by keeping a low level of dissolved salts in the scrubber liquid (increase liquid blowdown rate, use only caustic for neutralization or replace hard scrubber makeup water with deionized water).
Source of problem: Operating with excessive dissolved solids.
Hypochlorite: Sodium hypochlorite (NaOCl) is a product of neutralizing Cl2 with NaOH. The reaction is 2NaOH + Cl2 ==> NaCl + NaOCl + H2O. NaOCl attacks glass, so improperly designed FRP shell construction or FRP circulating pipes can be ruined as the glass fibers are consumed. In addition, if the scrubber blowdown liquid is later acidified (as in a waste water treatment plant) the NaOCl breaks down, releasing chlorine gas!
Source of problem: Poor FRP fabrication; improper waste water treatment.
Thermal Damage: Most incinerator flue gas scrubbers are constructed of the least expensive acid proof materials available. These are usually FRP, polypropylene or similar and are sensitive to high temperatures. Prior to scrubbing, the hot flue gas must be cooled. Safety items include high temperature switches in the quenched gas duct, quench liquid low-flow switches, etc. Should these safeties be bypassed, a quench system failure (like a damaged pump or water spray tip) can result in burned or melted scrubber internals.
Source of problem: Inadequate maintenance of flue gas quench system.
Salt Particles / SO3 aerosols: Scrubbers designed to remove acid gases typically are not very good at removing small particles. Organic liquid wastes containing salts are fed directly to the flame zone of an incinerator burner in order to achieve stable combustion. The high temperature in this area can vaporize the salt compounds. When the salt vapor cools to furnace temperature it condenses to droplets, which further cool in the gas quench section. Result: very small salt particles. Similar particle size can be seen when burning any waste containing sulfur. The SO3 from the furnace forms tiny droplets of H2SO4 when quenched with water prior to scrubbing. This "aerosol" acts like a fine particulate and passes through the acid gas scrubber. A high energy scrubber, electrostatic precipitator, or filter must be used in these cases.
Source of problem: Waste combustion forms small particles; additional equipment must be specified.
Pump Cavitation: Most flue gas scrubbers recirculate the scrubbing liquid in order to reduce the rate of blowdown to water treatment. Circulating pump cavitation can sharply reduce the rate of scrubbing liquid available and damage the pump.
Source of problem: Incorrect pump specification, faulty pump suction piping design, poor scrubber sump design or foreign matter clogging the pump suction.
Channeling and flooding: Incinerator flue gas scrubbers often incorporate beds of random packing. If the scrubbing liquid is not well distributed across the bed, the incoming gas can flow preferentially up one side of the scrubber, reducing acid gas removal efficiency. If gas flow is too high the downward flow of liquid can be slowed, flooding parts of the bed, increasing gas pressure drop and reducing scrubbing efficiency.
Source of problem: Improper scrubber design or possible fouling of the scrubber packing.