Fig. 1 illustrates the lime-gypsum method that uses limestone as an absorbent. When the absorbent absorbs and removes sulfur dioxide (SO₂) from exhaust gas, gypsum is collected as a byproduct. The lime-gypsum method consists of the following major reactions and steps:

<Reaction formulae>

 (Absorption step)
   CaCO3 + SO2 + (1/2)H2O->CaSO3.(1/2)H2O + CO2
 (Oxidation step)
   CaSO3.(1/2)H2O + (1/2)O2 + (3/2)H2O->CaSO4.2H2O

<Process>

(1) Absorption step

Exhaust gas is sent from the boiler to the cooler. The cooled gas flows into the absorber, and produces a gas-liquid contact with lime slurry that absorbs sulfur dioxide (SO₂). The gas is thus desulfurized and discharged from the exhaust stack.

(2) Lime supply

To use lime as an absorbent, limestone slurry is made before being supplied to the absorber.

(3) Oxidation step

Calcium sulfite (CaSO₃.1/2H₂O) is a reaction by product of the absorber. At the oxidation tower, the air oxidizes the calcium sulfite into gypsum slurry.

(4) Gypsum collection

Gypsum slurry is extracted from the oxidation tower, and enriched to a predetermined concentration. The gypsum slurry is then dehydrated for storage.

[Effective use of the byproduct; gypsum]

The byproduct gypsum from the exhaust gas desulfurizer is collected by the abovementioned process, and is used (as raw material) in cement, in building materials (boards), for soil improvement, and other purposes.

Note that gypsum production per watt-hour of power generation varies depending on unit operating conditions and SOx removal efficiency. In fiscal 1996, our company produced gypsum of 9-10 t/GWH with 1% of sulfur content in fuel. The total gypsum produced by the exhaust gas desulfurizer, in fiscal 1996, was about 83,500 t.