Wet Calcite Flue Gas Desulphurisation Process

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.

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