TL;DR
SO2 removal falling short of the permit target is usually a slurry pH problem from poor limestone quality or a blocked spray level. Check limestone reactivity and slurry pH first.
What you might see
- outlet SO2 reading rising toward permit limit
- inlet-to-outlet SO2 removal below target percentage
- slurry pH dropping below setpoint despite normal limestone feed
- absorber level low
Likely causes
Low-reactivity limestone reducing the rate of SO2 absorption by the slurry
Slurry pH below setpoint because limestone feed is insufficient for the current SO2 load
Blocked spray nozzles reducing liquid-to-gas contact area in the absorber
Reduced absorber circulation pump output from wear or plugging
Required tools
- DCS pH and SO2 transmitter readings
- Slurry sample bottles for lab analysis
- Absorber recirculation pump current readings
Safety first
- Slurry pH adjustments require adding limestone slurry or lime, which can splash. Wear chemical goggles and gloves.
- SO2 in the flue gas path is a toxic irritant. Do not enter the absorber without a supplied-air respirator and gas monitoring.
Procedure
- 1
Read the absorber slurry pH from the pH sensor at the sump. Normal operating range is typically 5.2-5.8 pH units. Values below this range indicate under-reagent conditions.[1]
- 2
Verify the limestone feed rate against the current SO2 inlet load. Increase the limestone ball-mill output or slurry make-down pump speed if pH is low.
- 3
Pull a slurry sample and send it to the lab for limestone particle size analysis and percent solids. Fine particle size improves reactivity.
- 4
Check that all absorber recirculation pumps are running and at normal amp draw. A pump on standby that should be running reduces the liquid-to-gas ratio.
Warning: Absorber recirculation pump maintenance requires isolation of the pump and the associated spray header, followed by proper lockout/tagout before any work. - 5
Check spray nozzle coverage from the access ports if the pH and pump status are normal but removal is still low.
- 6
Review the inlet flue gas SO2 load. An increase in sulfur content in the fuel can overwhelm the reagent supply and require a higher slurry recirculation rate.
Sources
Mitsubishi-Hitachi Power MHPS Wet FGD FGD (Flue Gas Desulfurization) general technical documentation, Mitsubishi-Hitachi Power
Wet FGD SO2 removal efficiency control, limestone reactivity and slurry pH management, general power plant references (general)
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