The technology for the
elimination of sulphur dioxides from flue gasses has been available for
more than twenty years. For large thermo-energetic complexes the wet
calcite process has become the method of choice. The great advantage of
this process is its high efficiency (up to 95%) and the cheap absorption
material, ground limestone (CaCO3). The product of the desulphurisation of the flue gasses is gypsum (CaSO4 X 2H2O), which does not present an environmental hazard and can be, if adequately processed, used in construction.
Wet Calcite Flue Gas Desulphurisation Process
The
process is based on the absorption of the sulphur dioxide from the flue
gasses into the suspension, where it bonds with the calcite to form a
stable product (gypsum). The neutralisation material is calcite. The
differences of the partial pressures of SO2 in the flue gas and the
fluid cause the SO2 to enter the suspension spray, where it hydrates
producing sulphuric acid which breaks down very rapidly.
SO2
(g)
→ SO2 (aq)
SO2 (aq) + H2O → H2SO3
H2SO3 → H+ + HSO3ˉ
HSO3ˉ → H+ + SO3²ˉ
In melting down the limestone (CaCO3), the hydrogen
ions are neutralized, carbon acid is produced which breaks down into
water and carbon dioxide, and the latter leaves the liquid phase and is
exhausted along with the flue gasses.
CaCO3 ((aq)
+ H+ → Ca2+ + HCO3ˉ
HCO3ˉ + H+ → H2O + CO2
The excess of calcium ions in the suspension reacts
with the hydrogen-sulphite and sulphite ions that are products of the
disproportionation of the sulphuric acid. The thus produced sulphite
ions oxidise into sulphate ions in the absorber, with the oxygen present
in the flue gasses, and with an intensive supply of air and mixing of
the suspension in the reaction chamber.
CaCO3 + 2H+ + 2HSO3ˉ → Ca(HSO3)2 + H2O + CO2
Ca(HCO3)2+ + 2H+ + SO3²ˉ → CaSO3 × 1/2H2O + 3/2H2O +
2CO2
CaSO3 × 1/2H2O + 1/2O2 + 3/2H2O → CaSO4 × 2H2O
Parallel to the oxidation of the sulphite into
sulphate, the crystallisation of the gypsum also takes place. It is
important to remove formed, rough crystals (this occurs in the
hydrocyclone wreath) and to return the minuscule basic cores into the
scrubber. The flue gasses contain, in addition to sulphuric compounds,
acidic compounds like hydrochloric acid (HCl) and hydrofluoric acid
(HF). The chlorine and flour bond to the calcium while the CO2 exits via
the exhaust.
CaCO3 + 2HCl → CaCl2 + H2O + CO2
CaCO3 + 2HF → CaF2 + H2O + CO2