Removal of H/sub 2/S from geothermal steam. Semi-annual report, April 1-October 31, 1975 Page: 7 of 30
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Bureau of Mims nn Morgantown', We^t. Virginia- using iron oxide and fly ash
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is a typical dry process.v .
Since most solid sorbents for H2S removal are essentially metal oxides in
nature, thennodynamic studies on the potential metal oxides have been
conducted.' Metal oxides studied include ZnO, FeO, Fe^, FegO^, Sn02»
energy change for reaction of these metal oxides with H2S are given in
Appendix I. Special emphasis has been placed on ZnO-H2S adsorption and regen-
eration. Typical chemical reactions involved in this sorption-regeneration
cycle are:
Free energy change of reactions, he^t of reactions, and equilibrium constants
at various temperature of these reactions have been calculated and are shown
in Table 1 and Figure 1. The equilibrium H2$ concentration, based on a
typical geothermal steam with 200 ppm of H2S as a feedstock, is 0.029 ppm,
which is much lower than the allowable H2S concentration for steam-turbine
electric generators. Thus, using ZnO as a sorbent to remove H2S from geo-
thermal steam is thermodynamically feasible. However, as shown in Figure
1, the formation of ZnSO^ is favored over formation of ZnO when ZnS is
regenerated with oxygen or air at low temperatures. Temperatures higher than
1200°C are required for regeneration in accordance with reaction (2).
At these temperatures, the absorbent rapidly loses its capacity to absorb
H2S (the loss of activity of ZnO in regeneration has been observed in
laboratory experiments and will be discussed later).
Other metal oxides studied possess the same characteristics as ZnO.
No significant problems exist in adsorption of H2S, but complete regeneration
is difficult. Difficulties in restoring sorption capacities may be thermo-
dynamically attributed to the preferential formation of sulfates rather than
oxides.
ZnO + H2S = ZnS + H20
ZnS + 3/2 02 = ZnO + S02
ZnS + 202 = ZnSO^
0)
(2)
(3)
It is postulated that calcined mixtures of metal oxides may provide
crystal stability sufficient for repeated sorption-regeneration cycles.
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Li, C.T. Removal of H/sub 2/S from geothermal steam. Semi-annual report, April 1-October 31, 1975, report, October 31, 1975; Richland, Washington. (https://digital.library.unt.edu/ark:/67531/metadc1101267/m1/7/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.