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10.3 Flue Gas Desulfurization 291

Table 10.3 The most important FGD systems with regenerable sorbents
Process Sorbent/principle End/by—product
Wellman—Lord Sodium sulfite (Na 2 SO 3 ) Concentrated SO 2
Bergbau Forschung/Uhde Activated carbon Concentrated SO 2
Linde SOLINOX Physical absorption (amine) Concentrated SO 2
Spray-dry scrubbing Sodium carbonate (Na 2 CO 3 ) Elemental sulfur
MgO process Magnesium oxide (MgO) Concentrated SO 2

Wellman-Lord process [23] is the most widely used wet renewable sorbent FGD
process. Sodium carbonate, Na 2 CO 3 is the actual sorbent and NaHSO 3 is the
product of reaction.

Na 2 CO 3 þ SO 2 þ H 2 O ! 2NaHSO 3 ð10:14Þ

The sulfur is released again as SO 2 , which is extracted as a mixture of
approximately 85 % SO 2 and 15 % water. The mixture can be further processed to
sulfuric acid.

NaHSO 3 ! Na 2 SO 3 þ SO 2 þ H 2 O ð10:15Þ
1
SO 2 þ = 2O 2 þ H 2 O ! H 2 SO 4 ð10:16Þ
Some sodium sulfite/sulfate is formed as a by-product; make-up soda or trona is
needed to balance this loss. Make-up Na 2 CO 3 (soda) or Na 2 CO 3 · NaHCO 3 (trona)
is necessary to compensate the loss of sodium from the system with the CaSO 3 /
CaSO 4 product. Therefore, an actual process includes a more concentrated sodium-
based liquid with limestone/lime scrubbers, combined with precipitation and
regeneration in separate devices.

10.3.2 Steam Reactivation of Calcium Based Sorbents

Since the molar volume of CaSO 3 or CaSO 4 is higher than molar volume of CaO or
Ca(OH) 2 , the volume of sulfation products shall be greater than the volume of
sorbent consumed. This result in a progressive pore blockage that gradually elim-
inates the access of SO 2 to the active CaO surface. Due to this pore eliminating
phenomena, only a fraction of the CaO is utilized in sulfation. The low conversion
rate of sorbent results in not only a waste of resource and low efficiency in FGD, but
also increased particulate contaminants in the gas stream and consequently the
increased burden for the particulate separation units and sludge treatment.