Flue Gas Desulfurization (FGD) Wet Limestone Scrubber Calculator
Model counter-current wet limestone forced oxidation (LSFO) spray towers: SO2 absorption kinetics, L/G hydraulic ratio, limestone reagent demand, commercial gypsum yield, and oxidation air compressor loads.
1. Flue Gas Feed & SO2 Removal Target
2. Scrubber Tower Design & Reagents
Scrubber Diagnostics & Mass Balance
Absorber Operating Metrics
5 Fatal Engineering Traps in Wet Limestone FGD Scrubbers
1. Sump pH Depression & Limestone Blinding Below 5.0
Operating an FGD sump below pH 5.0 severely shifts chemical equilibrium: dissolved molecular SO₂ backpressure increases, causing instantaneous sulfur dioxide slip and collapsing removal efficiency from 98% down to <80%. When automated controllers dump excess fresh limestone slurry to compensate, the high-acidity dissolution kinetics blind the limestone surfaces with an insoluble calcium sulfite shell, producing huge piles of wasted unreacted reagent.
2. Gypsum Relative Supersaturation (>1.4 RS) & Wall Scaling
In forced oxidation scrubbers, calcium sulfate dihydrate (gypsum) precipitates continuously. If the sump slurry solids fraction drops below 10–12% wt (lack of adequate seed crystal surface area) or reaction tank residence time is cut below 12 hours, liquid-phase gypsum relative saturation surges above 1.4. This triggers catastrophic spontaneous nucleation, encrusting absorber walls, spray nozzle orifices, and mist eliminator chevrons with rock-hard scale.
3. Mist Eliminator Chevron Cementation & Stack Rainout
Absorber gas velocity (3.2–3.6 m/s) carries significant gypsum slurry droplets into the two-stage chevron mist eliminators. Washing the chevrons with recycled absorber filtrate instead of low-hardness fresh water—or failing to program bidirectional wash cycles on the underside faces—allows gypsum to cement between blade passes. Clogged blades choke gas passages, spike ID fan power by 400 kW, and cause severe droplet carryover ("stack rainout").
4. Uncontrolled Chloride Concentration & Gypsum De-rating
Chlorides from flue gas HCl dissolve into the slurry and accumulate rapidly unless purged through a dedicated FGD wastewater treatment plant. High chlorides (>20,000 mg/L) suppress limestone dissolution rates and trigger severe pitting in alloy 2205/316L internals. Crucially, commercial wallboard drywall manufacturers enforce a strict <100 ppm chloride specification; failing to wash gypsum cake de-rates thousands of tons of salable gypsum into hazardous landfill waste.
5. Oxidation Air Lance Starvation & Sulfite Slime Blindness
If forced oxidation air compressors trip or sub-surface air lances plug with scale, bisulfite ions (HSO₃⁻) fail to oxidize to sulfate (SO₄²⁻). The system reverts to producing calcium sulfite hemihydrate (CaSO₃·0.5H₂O). Calcium sulfite precipitates as sub-5 µm platelets that form a viscous, gelatinous mud that blinds vacuum dewatering belt filters, completely halting byproduct dewatering.
Chemical Reactions & Absorption Kinetics
Wet limestone forced oxidation (LSFO) is governed by two-film gas-liquid mass transfer with chemical reaction:
SO₂(g) + CaCO₃(s) + ½O₂(g) + 2H₂O → CaSO₄·2H₂O(s) + CO₂(g)
The Number of Overall Gas Phase Transfer Units (NOG) required for counter-current gas scrubbing:
NOG = ln [ CSO2,in / CSO2,out ] = -ln(1 - ηSO2)
Mass rate of SO₂ captured from the flue gas:
ṀSO2 = Qgas × (CSO2,in - CSO2,out) × 10⁻⁶ (kg/h)
Limestone reagent consumption and commercial gypsum production rates:
Ṁlimestone = [ ṀSO2 / 64.06 ] × 100.09 × (Ca/S) / (Purity / 100) (kg/h)
Ṁgypsum = [ ṀSO2 / 64.06 ] × 172.17 × 10⁻³ (tonnes/h)
Theoretical oxidation air demand (21 mol% O₂ in dry air):
Qair,ox = [ ṀSO2 / 64.06 ] × 0.5 × [ 22.414 / 0.21 ] × SRox (Nm³/h)