TL;DR
Gypsum scaling in the absorber comes from slurry density running too high or oxidation air being insufficient. Reduce slurry density to the design band and verify oxidation air flow before attempting mechanical cleaning.
What you might see
- absorber slurry density above normal operating band
- spray nozzles partially blocked reducing SO2 removal
- hard deposits visible on absorber walls at inspection
- absorber circulation pump power rising
Likely causes
Slurry density operating above the design range, increasing the probability of gypsum crystallization on surfaces
Insufficient oxidation air allowing calcium sulfite to accumulate rather than converting to gypsum
Spray nozzle wear causing uneven coverage and dry spots where scale forms preferentially
Low-pH operation dissolving scale intermittently and then re-precipitating it in unexpected locations
Required tools
- Absorber density transmitter reading from DCS
- Oxidation air blower instrumentation
- High-pressure water lance for scale removal
- Chemical-resistant gloves and respirator for absorber entry
Safety first
- Absorber vessel entry is a confined-space entry. Test for SO2 and oxygen deficiency before entry and maintain continuous gas monitoring during the work.
- FGD slurry is a mild alkali with abrasive gypsum particles. Wear chemical splash goggles and chemical-resistant gloves whenever handling slurry.
Procedure
- 1
Read the absorber slurry density from the density transmitter. Design band for most wet FGD systems is 1.08-1.14 g/cm3. Density above this range accelerates scaling.[1]
- 2
Increase absorber blowdown rate to bring density into the target range. Direct the blowdown to the gypsum dewatering system.
- 3
Verify that the oxidation air blower is running and the flow is at or above the design stoichiometric ratio. Insufficient oxidation converts less sulfite to sulfate, promoting hard calcium sulfite scale.
Warning: Absorber entry for inspection requires confined-space entry permits, gas testing for SO2 and oxygen, and appropriate PPE. The absorber atmosphere can contain SO2 at hazardous concentrations. - 4
Inspect spray nozzles from the access ports during a planned outage. Replace any nozzles with cracked or enlarged orifices.
- 5
Use high-pressure water lances to remove loose scale from absorber walls during the outage. Do not use steel tools that could damage the rubber lining.
Warning: Wet FGD absorber scale contains sulfite and sulfate compounds. Wear chemical-resistant gloves, safety glasses, and a respirator during mechanical cleaning. - 6
Return to service and monitor density and SO2 removal efficiency. Adjust blowdown and limestone feed to maintain the target operating window.
Sources
Mitsubishi-Hitachi Power MHPS Wet FGD FGD (Flue Gas Desulfurization) general technical documentation, Mitsubishi-Hitachi Power
Wet FGD absorber operation and maintenance, gypsum scaling prevention, general power plant emission control references (general)
More guides for Mitsubishi-Hitachi Power MHPS Wet FGD
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