Wet Fgd

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ABSORBER SYSTEM GYPSUM HANDLING SYSTEM GAS SYSTEM LIMESTONE SYSTEM

In the absorber the flue gases are washed counter currently with a limestone/gypsum suspension for the removal ofSO2, SO3, HCl and HF by forming gypsum (CaSO4) as the main product and consuming limestone as absorbent.
The limestone suspension enters the absorber sump where it is mixed with the gypsum slurry. Fromthe absorber sump the slurry mix is recirculated by recycle pumps and distributed by spray nozzles to produce very fine suspension droplets. A very effective gas-liquid contact is realised byatomising the slurry in the spraying levels. Each operating recirculation pump is connected to its own slurry spray bank.

CaCO3+SO2+2H2O+1/2O2CaSO4.2H2O+CO2
• ABSORPTION • REACTION WITH LIMESTONE •OXIDATION

ABSORPTION
The sulphur oxides SO2 and SO3 in the flue gas dissolve in the water of the slurry droplets according to: SO2 + H2O H2SO3 SO3 + H2O H2SO4 This H2SO3 respectively H2SO4 must beneutralized quickly in order to maintain efficient SO2 and SO3 absorption.

REACTION WITH LIMESTONE
The H2SO3, H2SO4 and HCl reacts with the limestone from the fine limestone particles containedin the falling suspension droplets according to:
CaCO3 + H2SO3 CaSO3 + CO2 ↑ + H2O CaCO3 + H2SO4 CaSO4 + CO2 ↑ + H2O CaCO3 + 2HCl CaCl2 + CO2 ↑ + H2O The above mentioned reactions are ionic andhence they occur in the solution. The reaction with limestone is completed in the absorber sump. The slurry residence time ensures a sufficient degree of conversion.

OXIDATION
The air, which isblown into the absorber sump is needed to oxidize the sulphite to sulphate:

Oxidation:

2 CaSO3 + O2 2 CaSO4
CaSO4 + 2H2O CaSO4 . 2H2O

Crystallisation:

Oxidation and crystallization occursmainly in the absorber sump. The retention time in the absorber sump is long enough to form good system gypsum crystals (CaSO4 x 2H2O).

In the absorber sump, gypsum is constantly produced. To...