MEL Process Chemistry
In the MEL process, lime reagent, containing calcium hydroxide (Ca(OH)2) and magnesium hydroxide (Mg(OH)2), is used to react with the SO2. As with the LSFO process, the hydroxide slurry is added to the recycle tank to replenish reagent consumed under pH control of 6 to 7.
In the scrubber, captured SO2 combines with water (H2O) to form sulfurous acid, which dissociates into hydrogen and hydrogen sulfite ions (HSO3-). The presence of magnesium in the scrubbing liquor, resulting from the addition of slaked MEL slurry to the recycle tank, promotes the build-up of magnesium sulfite (MgSO3) within the scrubber liquor. MgSO3 is a soluble alkaline salt that rapidly neutralizes (or buffers) the sulfurous acid formed from the captured SO2, according to reaction 1, to form magnesium bisulfite (Mg(HSO3)2), which also results in lowering SO2 concentration in the scrubber liquor. Thus, the driving force for the absorption of SO2 is maintained and provides the MEL FGD processes a greater capacity to absorb SO2 as compared to systems where MgSO3 is not present.
1 HSO3- + H++ MgSO3 <-> Mg(HSO3)2
The calcium component of the slaked MEL added to the recycle tank for pH control reacts with the captured SO2 in the form of Mg(HSO3)2 to precipitate calcium sulfite hemihydrate (CaSO3 • ½H2O) and regenerate MgSO3, according to reactions 2. The Mg component reacts with a portion of the absorbed SO2 to add fresh MgSO3 to make up for losses according to reaction 3.
2 Ca(OH)2 + Mg(HSO3)2 => CaSO3 • ½H2O + MgSO3 + 1½H2O
3 Mg(OH)2 + Mg(HSO3)2 => 2MgSO3 + 2H2O
A portion of MgSO3 is oxidized to MgSO4 by oxygen (O2) present in the flue gas or ambient air that comes in contact with the scrubber liquor. Some sulfate (SO4=2) in solution will precipitate as CaSO4 in the crystal lattice of the precipitated CaSO3 • ½H2O. This co-precipitation causes SO4=2 to precipitate under conditions where the scrubber liquor is unsaturated with CaSO4. Other ions that also co-precipitate in the recycle tank to a lesser extent, include carbonate (CO3=2) and magnesium (Mg++) at unsaturated conditions. The magnesium ions, which were associated with the SO4=2, are now available to pair with an available SO3=. Thus, the co-precipitation processes result in the replenishment of MgSO3 (and therefore, alkalinity) in the scrubbing liquor.
The blowdown from the scrubber is contacted with air in a sparged vessel. The CaSO3 • ½H2O is converted to CaSO4 • 2H2O and the MgSO3 is oxidized to MgSO4, according to reaction 4 and reaction 5, respectively. The CaSO4 • 2H2O precipitates in a crystalline form while the MgSO4 remains in solution. The liquor containing the dissolved MgSO4 is returned to the scrubber after dewatering the gypsum crystals. A portion of the MgSO4 returned to the scrubber proceeds to co-precipitate as CaSO4 replenishing the alkaline MgSO3, as described in reaction 4.
4 MgSO4 (liquid) + CaSO3 (solid) => MgSO3 (liquid) + CaSO4 (solid)
The CaSO4 • 2H2O crystal in the slurry effluent from the oxidizer undergoes primary and secondary dewatering. The same as for LSFO to produce the gypsum by-product cake.