The scope of the hydrogen sulfide (H2S) abatement program at The Geysers Geothermal Power Plant and the measures currently under way to reduce these emissions are discussed. The Geysers steam averages 223 ppm H2S by weight and after passing through the turbines leaves the plant both through the gas ejector system and by air-stripping in the cooling towers. The sulfide dissolved in the cooling water is controlled by the use of an oxidation catalyst such as an iron salt. The H2S in the low Btu ejector off gases may be burned to sulfur dioxide and scrubbed directly into the circulating water and reinjected into the steam field with the excess condensate. Details are included concerning the disposal of the impure sulfur, design requirements for retrofitting existing plants and modified plant operating procedures. Discussion of future research aimed at improving the H2S abatement system is also included

Allen, G. W.

Mccluer, H. K.

English

NASA Technical Reports Server

I. 
THE HYDROGEN SULFIDE EMISSIONS ABATEMENT PROGRAM 
AT THE GEYSERS GEOTHERMAL POWER PLANT 
G. W. Allen and H. K. McCluer 
Pacific Gas and Electric Company 
San Ramon, California 
This paper describes the scope of the hydrogen sulfide (HzS) 
abatement program a t  The Geysers Geothermal Power Plant and 
the measures  currently under way to reduce these emissions. 
Geysers steam averages 223 ppm H2S by weight and after passing 
through the turbines leaves the plant both through the gas ejector 
system and by air-stripping in the cooling towers. 
solved in the cooling water is  controlled by the use of an oxidation 
catalyst such as  an iron salt. The HzS in the low Btu ejector off 
gases may be burned to sulfur dioxide and scrubbed directly into 
the circulating water and reinjected into the s team field with the 
excess condensate. Details a r e  included concerning the disposal 
of the impure sulfur, design requirements for retrofitting existing 
plants and modified plant operating procedures. 
future research aimed a t  improving the H2S abatement system is 
also included. 
The 
The sulfide dis- 
Discussion of 
INTRODUCTION 
While geothermal energy has been developed for the production of electric 
power in several locations throughout the world, the only commercial develop- 
ment in operation in the United States is a t  The Geysers, about 80 miles north 
of San Francisco. From Pacific Gas and Electric Company's initial 12 mega- 
watt (MW) unit, which commenced operation in 1960, nine additional units have 
been constructed. The total capacity is  now 396 MW, which makes The Geysers 
the largest  geothermal electric power plant in the world. 
tion presently under way will increase net output to about 900 MW by the end 
of 1977. 
proven and developed. The facilities and operations a t  The Geysers Power 
Plant have been well described previously (Refs. 1 through 4). Thus, only 
those aspects pertinent to the hydrogen sulfide emissions abatement program 
will be discussed herein. 
Design and construc- 
There a r e  plans for continuing expansion as steam reserves  a r e  
In contrast to most of the world's geothermal resources, the wells a t  The 
Geysers produce slightly superheated steam. 
amounts, usually less  than 1. percent by weight, of noncondensible gases, mostly 
carbon dioxide. 
This steam contains small 
The ranges of concentrations of the noncondensible gases 
319 
https://ntrs.nasa.gov/search.jsp?R=19750012787 2020-03-22T23:13:30+00:00Z
measured at The Geysers a r e  presented in Table 1. 
components of the atmosphere, and their concentrations that we have measured 
in ambient a i r  at The Geysers are not considered harmful. However, hydrogen 
sulfide (H2S) is  an air pollutant that P G and E, or  any other user  of geothermal 
steam, i s  required to control under regulations of the State of California Air 
Resources Board (ARB) and also the local a i r  pollution control districts (APCD). 
The APCD regulations applicable a t  The Geysers generally state that no single 
emission source shall exceed 1000 par ts  per  million by volume (ppmv) of any 
sulfur compound expressed as sulfur dioxide (SO2). The ARB H2S standard is 
0. 030 ppmv for ambient a i r .  Since this concentration is approximately the odor 
threshold for HzS, a s  a rule of thumb one can say that if one can smell HzS, the 
ARB standard is being exceeded. 
These gases are natural 
The generating units at The Geysers use slightly superheated s team at 
about 100 psig, and after passage through the turbine, the low pressure steam 
is condensed in direct  contact condensers operating a t  3-5 inches of mercury,  
absolute. Figure 1, the power cycle diagram for the proposed Unit 15, is 
typical of the units of The Geysers. 
steam a r e  continuously removed from the condenser by the two-stage gas 
ejector system and a r e  vented to the atmosphere through stacks from 60-85 feet 
above ground level, depending on the unit. However, since the steam contacts 
the cooling waters directly in the condenser, about two-thirds of the H2S dis- 
solves in the cooling waters and i s  then carr ied to the cooling tower. 
Most of the noncondensible gases in the 
The cooling waters, in passing through the cooling tower, a r e  partially 
air-stripped, so the tower exhaust a i r  contains on the order of one ppm, H2S. 
Some of the H2S (on the order of 30 percent) is naturally oxidized to elemental 
sulfur in the cooling tower and eventually returns to the steam field with the 
surplus condensate. Though the emissions from the cooling towers are well 
within permissible levels, i t  is still  desirable to minimize these emissions 
f rom the power generating units to reduce the H2S concentrations in the ambient 
a i r  outside of The Geysers Power Plant. 
abatement program included the releases f rom both the gas ejector stacks and 
the cooling towers. 
Thus, P G and E's H2S emissions 
11. DESCRIPTION O F  THE ABATEMENT PROGRAM 
P G and E began investigating methods to control the H2S emission from 
These investigations have included litera- The Geysers Power Plant in 1971. 
ture  searches,  parametric evaluations, bench-scale testing (both in the P C 
and E laboratories at the Department of Engineering Research and at The 
Geysers), and, at the present time, unit-scale testing at Units 1, 2, and 4. 
A. Cooling Tower Emissions 
The bench-scale testing at  The Geysers investigated four methods to con- 
trol  the emissions from the cooling towers by reacting the H2S to elemental 
sulfur (So) in the water phase before it could be air-stripped. These were: 
(1) Direct injection of SO2 into the cooling waters to oxidize the H2S to S o  
by the Claus reaction 
320 
2H2S t SO2 - 35 0 t 2H20 
(2)  Simultaneous injection of both SO2 and air .  
( 3 )  Addition of a metal catalyst, iron, to the cooling waters to promote 
direct  oxidation of the H2S to So by the oxygen dissolved into the 
circulating cooling waters in the cooling towers: 
0 2H2S t O2 - 2s t 2H20 
( 4 )  Addition of Cataban, a chelated iron compound manufactured by the 
Rhodia Corporation, to catalyze the direct  oxidation of H2S by the 
di s s olved oxygen. 
AI1 four of these methods reduced the HzS emissions from the bench-scale 
test apparatGs. The third approach, addition of a metal catalyst to the circulat- 
ing cooling waters, was chosen for unit-scale testing, since this method appears 
to be best  adapted for operations at The Geysers Power Plant. 
of the tests of methods to control cooling tower HzS emissions a r e  being made 
at Units 1 and 2. These two units, of nominal generating capacities of 12. 5 
and 14 MW, are the oldest and smallest of all the units installed a t  The Geysers. 
However, their design and layout a re  similar to all subsequent units in oper- 
ation. 
confidence. 
is also underway a t  Unit 1 (Ref .  5). 
The majority 
This permits us to scale up to any desired s ize  with a high degree of 
A pPogram to study corrosion by the metal-containing condensate 
Several different metals will catalyze the oxidation of H2S in the water 
phase (Ref .  6 ) .  
Geysers: iron, nickel, and copper; copper was the least  effective of the three 
It was found that iron concentrations of approximately 30 m g / l  and nickel con- 
centrations of 1. 5 to 2 m g / l  could reduce the total HzS emissions f rom the 
cooling towers by 90 percent o r  more. 
tion catalysts is  now a matter of relative economics and their effects, if any, 
on the long-term operation of the units, 
t e r m  testing of the iron oxidation catalyst at Unit P have shown that there is  
a definite increase in the corrosion rate for  this unit. 
this may have on equipment service life i s  still under evaluation, Additional 
corrosion rate monitoring equipment is being installed to expedite the testing 
of alternate catalysts and the effectiveness of corrosion inhibitors. 
Three of these were experimentally investigated a t  The 
The choice between these two oxida- 
Preliminary results f rom the long- 
The overall effect that 
The use of the metal catalysts markedly increases the suspended solid 
content of the circulating waters,  both f rom the catalyst itself and the elemental 
sulfur which is produced. Work has been done on minimizing the solids buildup 
in the cooling tower basin and in removing these solids from the excess con- 
densate that is returned to the steam field. 
is rapid sand filtration. 
more  than 90 percent of the suspended solids and perform consistently with 
little attention, an important factor in The Geysers * operation. 
The method of choice at  this time 
Pilot  tests have shown that sand filtration can remove 
32 1 
B. Gas Ejector Emissions 
Two methods to control the emissions of the H2S in the noncondensible 
In one gases vented from the gas ejector system are  still  under evaluation. 
method, the H2S is burned to SO2 which is then scrubbed into the cooling waters 
to maintain the SO2 concentration in the products of combustion that would be 
vented to the atmosphere below the 1000 ppm, regulation. 
gases will burn since they contain enough methane and hydrogen to sustain 
combustion (150-200 Btu/cu f t ) .  
eventually either is injected into the steam field or  i t  can be neutralized with 
lime if the waters a re  too acidic for the reinjection well casing o r  cooling water 
system components. The marginal flammability and pulsating nature of the 
gases from the ejectors posed some challenging engineering designs. However, 
a full-scale burner/scrubber system has been designed for Unit 4 (26 MW). 
An oxidation catalyst feeder system and a sand filtration system have also been 
engineered for the tests at Unit 4, incorporating all the design features to date, 
and construction has started. 
the design of a full-scale abatement system for Unit 11 (106 MW). 
The noncondensible 
The SO2 scrubbed into the circulating waters 
The data obtained a t  this unit will be used in 
In the other method, H2S emissions from the ejectors a r e  controlled by 
scrubbing the noncondensible gases with a water solution containing a metal 
catalyst so  that the H2S is dissolved and oxidized to So. 
scrubbing method will also be performed a t  Units 1 and 2 this year. 
Tests of the H2S 
C. Sulfur Disposal 
The solids collected in the rapid sand filter include elemental sulfur with 
some metal catalyst and fine rock dust carried up the well by the steam. 
Studies show that i t  is  not economical to refine the recovered sulfur to com- 
mercial  purity; the cost of the refined sulfur from The Geysers would be higher 
than that of byproduct sulfur from natural gas desulfurization plants. 
we have not produced large enough quantities of the solids to evaluate their 
possible direct  application to soils o r  conversion to sulfuric acid for fert i l izers.  
Other practical applications will also be considered. Until a practical appli- 
cation for these solids can be developed, i t  is  planned to dispose of the solids 
in a landfill. 
ity of constructing a landfill disposal site at The Geysers for these solids, 
Should it be necessary to neutralize the cooling waters that have been used to 
scrub out the SO2 f rom burning H2S in the noncondensibles, the calcium sulfate/ 
sulfite formed would also be disposed of in the landfill. 
To date 
P G and E has retained a consulting firm to evaluate the feasibil- 
111. CONCLUSIONS 
W e  expect that the control methods described herein will reduce the H2S 
emissions f rom a unit to permissible levels and will control them so that they 
alone will not exceed the ARB ambient air quality standard for H2S a t  ground 
level beyond a few hundred feet f rom a unit. 
successful test operation this year,  a program for installing H2S control equip- 
ment on ten existing units, as well as future units, has been proposed to 
regulatory agencies. 
In anticipation of a period of 
322 
It should be noted that the control methods described above were 
selected on the basis of our existing knowledge and experience; they are not 
necessarily the optimum that could ultimately be developed for use at The 
Geysers. There is considerable effort in progress in the world today on the 
development of processes and equipment to control emissions of sulfur com- 
pounds from industrial equipment. Thus, we anticipate that further research 
will be performed on H2S emissions control a t  The Geysers. 
interest  that we now see i s  the addition of nonmetallic catalysts to the circula- 
ting cooling waters. A second a rea  of interest  is the removal of the HzS from 
the steam upstream of the turbine. 
herein should provide the necessary HzS emissions control for further develop- 
ment of this important, nonfossil-fueled power generating resource. 
One area of 
However, the control methods described 
1. 
2. 
3 .  
4. 
5. 
6 .  
REFERENCES 
A. W. Bruce, "Eng'ineering Aspects of a Geothermal Power Plant, 
Paper for the United Nations Symposium on the Development and 
Utilization of Geothermal Resources, P isa ,  Italy, September 22- 
October 1, 1970. 
J. P. Finney, F. J. Miller, D. B. Mills, "Geothermal Power Project of 
Pacific Gas and Electric Company at The Geysers, California, Summer 
meeting of the IEEE Power Engineering Society, San Francisco, 
California, July 1972. 
J. P. Finney, "Design and Operation of The Geysers Power Plant, I '  in 
Geothermal Energy, P. Kruger and C. Otte, Stanford University 
P res s ,  1973. 
P. Matthew, "Geothermal Operating Experience at Geysers Power Plant, 
Journal of the Power Division, Proceed'mgs of the ASCE, Vol.  99, 
No. P02, November 1973. 
F. J. Dodd, "Effects of Hydrogen Sulfide Abatement Program on Corrosion 
at The Geysers, 
Western States Corrosion Seminar, Pomona, California, May 1974. 
National Association of Corrosion Engineers, 
E. S. Snavely and F. E. Blount, "Rates of Reaction of Dissolved Oxygen 
with Scavengers in Sweet and Sour Brines, Corrosion, Vol. 25, No. 10, 
October 1969. 
32 3 
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The hydrogen sulfide emissions abatement program at the Geysers Geothermal Power Plant

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