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284 Lawrence K. Wang et al.

efficiencies (99+%) and because it has very high turndown capabilities. The liquid
redox system is considered by some (58,59) to be the best available control technology
for geothermal power plants. The process employs a nontoxic, chelated iron catalyst
that accelerates the oxidation reaction between H S and oxygen to form elemental sul-
2
fur. The oxidation process is
H S + 0.5 O →S (elemental sulfur) + H O
2 2 2
As implied by its generic name, liquid redox, all of the reactions in the process occur
in the liquid phase in spite of the fact that the above reaction is a vapor-phase reac-
tion. In the process, the NCG is contacted in a wet scrubber (shown in Fig. 17) with
the aqueous, chelated iron solution where the H S is absorbed and ionizes into sul-
2
fide and hydrogen ions. This process is presented as follows:
+
H S (vapor) + H O → 2 H + S 2−
2 2
The dissolved sulfide ions then react with chelated ferric ions to form elemental sul-
fur:
S 2− + 2 Fe 3+ → S (elemental sulfur) + 2 Fe 2+

The solution is then contacted with air in an oxidizer, where oxygen (in air bubbles)
is absorbed into the solution and converts the ferrous ions back to the active ferric
state for reuse as follows:

0.5 O (vapor) + H O + 2 Fe 2+ → 2 Fe 3+ + 2OH −
2 2
Combining the above three reactions yields the following reaction,

H S + 0.5 O → S (elemental sulfur) + H O
2 2 2
As illustrated in Fig. 17, the air emission stream (containing H S) enters the wet
2
scrubber’s absorption section, where it is contacted with the scrubbing solution (LO-
CAT solution) and where the H S is converted to elemental sulfur. The partially
2
reduced solution then circulates to the oxidation section where it is contacted with air,
which reoxidizes the iron. The exhaust air from the oxidation section and the sweet
NCG from the absorption section are exhausted to the atmosphere.
In the conical portion of the vessel (see Fig. 17), the sulfur will settle into a slurry of
approx 15 % (by weight). A small stream is withdrawn from the cone and sent to a
vacuum belt filter, where the sulfur is further concentrated to approx 65% (by weight)
sulfur. Some units stop at this stage and sell the sulfur cake as a fertilizer. If molten
sulfur is required, the cake is reslurried and melted as shown in Fig. 17.
Although an innovative wet scrubbing system (i.e., the liquid redox system) has
slightly high capital cost, it is very inexpensive to operate. Operating costs usually
range between $0.20/kg of H S to $0.25/kg of H S (48).
2 2
Example 26
Wet scrubbing using lime/limestone is one of the feasible processes for flue gas desulfu-
rization (32). Discuss the following:
1. The process description, performance, and future of the wet scrubbing flue gas desul-
furization process.
2. The chemical reactions of the wet scrubbing flue gas desulfurization process.