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Developments in geothermal energy in Mexico—Part fourteen. Environmental aspects of geothermal systems
Heat Recovery Systems & CliP Vol. 7, No. 4, pp. 365-374, 1987 Printed in Great Britain.
0890-4332/87 $3.00 +0.00 Pergamon Journals Ltd
DEVELOPMENTS IN GEOTHERMAL ENERGY IN MEXICO--PART FOURTEEN. ENVIRONMENTAL ASPECTS OF GEOTHERMAL SYSTEMS S. ML~CADO Divisibn de Estudios de Ingenieria, Instituto de lnvestigaciones El~-icas, M~xico
F. J. BERMF~O Comisi6n Federal de Electricidad, Cerro Prieto, M~xico
and H. FEaN^m>EZ Departamento de Gcotermia, Divisi6n Fuentes de Ener#a, Instituto de Investigaciones El,~'ctricas, Cuernavaca, M;:xico (Received 12 February 1987) AImraet---Geothermal fields have a number of potential pollutants such as brines, gases, noise, heat and steam dispersion and accidental spills. Additional problems arise from subsidence, induced seismicity, scenic changes and land use. Measures to control their adverse effects on the environment are discussed with particular reference to the Cerro Prieto geothermal field at Mexicali, B.C. Mexico. Methods to reduce biolosieally induced corrosion in the cooling water system of a geothermal power plant are also discussed.
INTRODUCTION
Geothermal fields have a number of potential pollutants which include brines, gases, noise, heat and steam dispersion and accidental spills. Additional problems can arise from subsidence, induced seismicity, scenic changes and land uses. Fortunately technology exists to control and reduce the adverse effect of almost all these geothermal pollutants. One of the most significant problems is that of biologically induced corrosion in the cooling water systems of geothermal power plants. The overall strategy for reducing the adverse environmental impact of pollutants also includes plans for the use of geothermal wastes. Industrial, agricultural and aquiculture water uses are currently under development. Installations for commercial salts extraction and demonstration plants for waste heat usage are already under construction. The precise ecological impact of the pollutants is specific to a particular reservoir and the local environmental characteristics. The measures taken for controlling geothermal pollutants vary from one country to another depending on government regulations, bureaucratic procedures and long-term cultural heritage factors, etc. In this paper the problems will be discussed with reference to the Cerro Prieto geothermal field at Mexicali, B.C. Mexico. EVALUATION
OF
POLLUTANTS
AND
THEIR
CONTROL
There are high and low impact problems. The high ones arise from the gases which are released to the atmosphere and the high salinity brines discharged to ponds, lakes and the sea or reinjected underground. The low impact problems include noise, heat and steam dispersion, subsidence, induced seismicity, land use, scenic changes and accidental spills. The potential pollutants and ecological effects are listed in Table I for the various types of reservoirs. "The controls used to overcome these problems in various geothermal fields throughout the world are listed in Table 2. The Cerro Prieto geothermal field at Mexicali, B.C. Mexico has a water dominant reservoir. Some of these controls have been used at Cerro Prieto with good results. 365
366
S.
MERCADO
et al.
Table I. Potential pollutants and ecological effects on geothermal systems Water dominant
Contaminant Gases Brine Noise H e a t dispersion
Subsidence I n d u c e d seismicity Scenic Land use
x x x x x x
Reservoir t y p e Low Steam enthalpy
Geopressurized
Hot d r y rock
N.A. x x x x x
---x -?
x -x x x -
-x -x x x
x
x
x
x
x
x
x
x
x
x
Table 2. Control methods used to limit environmental impact Controls used
Contaminant Gases Brine disposal
Noise Heat Subsidence Induced seismieity Scenic and land use Accidental spills
Chemical treatment, Stretford process, HiS combustion, use of H202, catalysis, upstream reboiler, injection, etc. Reinjection, salt extraction, evaporating ponds, and channeling to the sea or to lakes. Twin silencers for brine--steam mixture discharges, and thermal insulation for steam lines. Thermal insulation of pipes and surface equipment. Adjustment of production, structure repair and reinjection. Adjustment to fluid injection. Flora replacement and g r e e n h o u s e s . Blowout preventors, casing repair and plugging of wells.
SO 2
Table 3. Chemical analysis of Cerro Prieto brine Chemical Na K Ca Li Mg Sr Zn HCO~ SiO2 CI pH
Total dissolved solids content (ppm) 10,735 3042 416 29 0.5 3.8 0.5 5.9 1050 21,300 5.58
Brine
At Cerro Prieto the fluid extracted (steam and brine) by deep wells is separated in cyclone separators. Steam is utilized to feed the turbine. At the same time, brine is discharged to a 16 km 2 evaporation pond where it is evaporated and concentrated for the extraction of mineral compounds, mainly potash. This is utilized in the production of imported fertilizers. The original area of the pond (1973-1984) was 8 km 2 for an installed capacity of 180 MW and a brine extraction rate of 3000 ton h-'. The present pond area is 16 km 2 for a generating capacity of 620 MW and an estimated extraction rate of geothermal fluids of 1 x l0 s m 3 y r - I of which up to 60% is waste brine. Other applications for waste fluids are being developed at the Cerro Prieto geothermal field including an absorption refrigeration system, a greenhouse, a fish hatchery and a chicken farm, all of them operating from geothermal waste fluids. Also, an amount of waste brine will be reinjected in the near future. A typical analysis of the Cerro Prieto brine is shown in Table 3. Gases Air pollution is a major problem caused by geothermal energy exploitation. At the Cerro Prieto geothermal field air pollution is mainly caused by H2S emissions. This is a severe contaminant which also attacks the power plant itself, causing metal corrosion including damage to electrical equipment and power lines. At a higher concentration, H2S attacks concrete and may be a serious hazard for human life. Some of these emissions do not originate from the power plant but occur naturally from bubbling pools and fumaroles.
Developments in geothermal energy in Mexico---XIV
367
Table 4. Steam chemical impurities at turbine inlet (units 1 and 2) Condensate impurity type
Concentration (ppm)
CaCO 3 F¢ SiO2 CI
0.0 0.05 0.2 1.0
so,
O.Ol
Na CO t H2S NH4 pH
0.4 14,000 1500 I l0 7.3
In geothermal exploitation, HeS emissions occur when drilling the wells, in the testing and measurement of wells and in almost every exploitation step, including plant operation. Other gaseous pollutants in the geothermal fluids in addition to H2S are released to the atmosphere when the geothermal fluids reach the surface. At the Cerro Prieto I power station some steps were taken in order to control the emissions: stacks were installed at a higher position than first planned. The fluid discharge at the greater heights helped to control the H2S pollution on site due to the north winds which carry away the H:S and other contaminants. Also a gas pipeline was constructed from the power plant to the evaporation pond (more than I km distance). The pipeline incorporated blowers to transfer the gases towards the pond. These subsequently were taken out of service after being extensively corroded [1]. Thus pollution on site was reduced. However it is not possible to completely eliminate gaseous pollution at the plant surroundings, since the contaminants are partially dissolved by the cooling water in the direct contact condensers and ejector coolers and then released by the cooling tower. Several methods have been tested for controlling HeS emissions at Cerro Prieto II and III power plants including a burner scrubber and an upstream reboiler [1]. In order to further control HzS emissions, future power plants at Cerro Prieto will be provided with surface condenser cooling systems. Research is being carried out to extract sulfur or sulfur compounds from HzS for commercial purposes. The steam of Cerro Prieto I has a gaseous content of 1.5% by weight. This is the highest one in the field. The gaseous content for Cerro Prieto II and III is less than 1% (typical well analyses are shown in Table 4). A gaseous content less than 0.7% is expected for Cerro Prieto IV. The main gases present in the geothermal fluids are: CO2 (95%), HeS (4%), NH3 (0.7%), and minor contents of hydrogen, methane, ethane, propane and butane. Continuous HzS monitoring is currently being carried out. The monitoring equipment includes several fixed units distributed on the geothermal field near the wells, at the evaporating pond, in the surrounding areas near the plant, as well as a mobile unit. Data obtained from this monitoring equipment show that, to date, H2S dispersion has caused minimal effects on the area. Some measurements have reported only 3 ppb in some surrounding points. Figure 1 shows the results of typical dispersions obtained from the operation of Cerro Prieto I, II and III (total 620 MW), with partial control of H2S. The minimum air pollution index at the surrounding areas of the plants is an important factor. No~
Noise contamination occurs at the Cerro Prieto geothermal field when making seismic surveys. The following activities involve noise; drilling works, cleaning or testing wells, flow measurements, steam vented on safety valves or regulation systems, construction and use of access roads and plant. A loud noise is caused by the direct discharge to atmosphere required for cleaning the well pipe and the production strata. Currently a large twin concrete-base glass-fibre stack silencer which eliminates the spraying of salt and reduces the noise index (60 dB compared to 130 dB for direct atmospheric discharge) is utilized in all wells. The steam flow regulation system of each unit is provided with steel and concrete silencers which are located far away from the plant (approx. 300 m). The turbine noise is similar to the noise X.L$. ?/4--E
368
S. MERCADOet
al.
Fig. 1. Cerro Prieto H2S dispersion (ppb).
produced by a conventional fossil fuel power plant. At the Cerro Prieto I unit, the high noise level is caused by the steam-gas ejectors. At the Cerro Prieto II unit, a turbocompressor for gas extraction was utilized, noticeably reducing the noise index. Heat and steam
The heat rejection at Cerro Prieto is through wet cooling towers. Steam dispersion is through silencers and open hot water channels. The estimated heat rejection to the atmosphere is approximately 2.3 GW for the actual capacity of 620 MW. The plants are located in an open valley. Thus the environmental impact is minimal since the dispersion takes place in a very large area. Subsidence
Subsidence is expected when large quantities of fluids are removed from geothermal reservoirs. Recent surveys in Cerro Prieto showed small local uplift and subsidences at the geothermal field and at the Mexicali Valley mainly caused because of the deep natural fracture crossing the field (the San Andreas fault system) which produced the geothermal underground reservoir. A maximum subsidence of 62 mm during the period 1977-1979 for the Cerro Prieto geothermal field has been reported. The largest geothermal subsidence reported in the world is 4.5 m [2] for the Wairakei geothermal field, New Zealand, between 1964 and 1974. Induced seismicity
Due to its relation to fault zones, the occurrence of earthquakes in the Cerro Prieto geothermal area is very common. They normally occur twice a year for the field and the Mexicali Valley. This has not changed during the exploitation of the geothermal field. Reports have shown no increase or increasing incidence. Land use
The Cerro Prieto geothermal field is located on the west portion of the Mexicali Valley. It lies only l0 m above sea level on the delta sediments of the Colorado River which consist of a very large fiat extention. The main part of the field was saline with many hot spring and boiling pools in which the ecological impact only includes scenic changes because it was not usable for crops. On this side is the Cerro Prieto I power station with outputs of 180 MW. The Cerro Prieto II and III power stations, each with outputs of 220 MW, are on the east part of the field, where farming was the main activity. This is now used for power generation.
Developments in geothermal energy in Mexico---XlV
369
The cotton crops on these lands were very poor, and at present the farmers use the irrigation water on other lands with better production. Fortunately, the valley is very large, and many lands are not used because the irrigation water is not enough. The total area used for geothermal activities is 31~km2 including the evaporating pond which has an area of 16 km'. Microearthquake studies for Cerro Prieto have been done in order to delimit the geothermal reservoir and determine some reservoir parameters. At present, there is a continuous monitoring for passive seismicity. In the near future, when reinjection is on line, a comparison and evaluation of seismicity increases caused by reinjection will be carried out. BIOLOGICALLY I N D U C E D C O R R O S I O N IN THE COOLING WATER SYSTEMS OF POWER PLANTS Cooling water systems in geothermal power plants involve different problems than those with conventional thermoelectric plants. For instance, the cooling system of the Wairakei geothermal power plant uses fresh water from the Waikato river in a direct contact low-level condenser and without recirculation. The H2S existing in the steam phase is therefore dissolved into the water and no corrosion problems have been reported. The geyser plants in California utilize a cooling system with an induced draft tower. Sulphur precipitation has been observed in the sump of the tower. The waste condensate is reinjected and the system operates without any apparent corrosion problems even to the extent that some plants utilize aluminum pipes. The Ahuachapan plant, El Salvador, also utilizes a cooling system with an induced draft tower. No problems have been reported, due perhaps to the low H2S content of the gases. At the Hatchobaru plant, Japan, a similar situation has been reported. At Larderello, Italy, natural draft towers have not presented any problems after several decades in operation. New plants will start operating in the near future and will be provided with induced draft concrete towers with plastic backfilling. The Krafla plant, Iceland, has a cooling system with an induced draft tower which has been operated at only one quarter of its capacity. Problems were reported because of pH values as low as 3. This is corrected by adding NaOH twice a week. At the Olkaria plant, Kenya, and the McBan plant, Philippines, similar problems have been reported. Operating experience at Cerro Prieto Cerro Prieto I power station units 1 and 2 (37.5 MW each) started operating in 1973 and both have been operating continuously since then except for preventive maintenance and a problem at the cooling tower. The system characteristics are shown in Figs 2 and 3. steom
water to rl turbine 8 Jl generator II heat [[
woter~"~"~ in~t _ ~ t~ ~- I ~
H-" ~
~I I I II (.e'=-/-~--~.il I"1 II II I ~.~1 I I I I
,,o,o.oer,
" -
._r-? I /__~, steam I/~ inlet V(~
¢l'~nnel~iiJ
"- °
I o"er condenser I~:~::=:l~'Z2j:::::::=~-~::Jcooler I111 III I
I
Illff IIIII
--
Jl
Fig. 2. Main condenser and aflcr-coolcn (units I-IV, O~'m Priclo).
3"10
S. M£RCAOOet
al.
water
12320 tonne h"1 32oC ---- ~
water 410 tonne h"l goslsteom and air
10 700 tonne h"~
T 10 tonne h"1
24tonne steO%r
h-I
water
20tonne
1190 tonne h"1 steam 6.4 bar
h"1 9.4 tonne h"t gOS~ air and steam
direct contact condenser 0.11 bar
286 tonne h'~ 0.35 bar
/
1.04 bar
~.5.8°C
45.3 °C
6f~.6°C
to cooling tower
12 600 tonne h.I 46°C
hot well
Fig. 3. Heat balance of cooling system (units I-IV).
The cooling water system for the 37.5 MW units consists of a vertical cylindrical barometric jet condenser with a load of 286 ton h- ~, operating at 0.11 bar absolute. The cooling water design inlet temperature is 32°C, handling 10,700 ton h -m of cooling water. Other important data are: 9.6m height x 6.7 m dia. shell, 12 m height x 2 m dia. tail pipe, 1.5 m dia. cooling water inlet pipe, 3.6 m dia. steam inlet pipe, 0.7 m dia. gas outlet pipe, 130 ton assembly weight. Each two-stage condenser has three gas ejectors, a barometric jet condenser type inter- and after-cooler. Some important data are: 39 kgh -~ dry gas extracting capacity, 0.1 bar absolute suction pressure, 6.4 bar steam pressure, 44 ton h-~ steam consumption and 1600 ton h-~ cooling water at 32°C (see Figs 2 and 3). There are two concrete channels (hot and cold water) per unit, two pumps for the cooling tower and two pumps for the condenser. An induced draft cooling with six cells made of redwood and treated with chromates and copper arsenide has been installed. The original design of the plant did not provide protection for the cooling system. However, some system components were protected during the plant construction, but many were left without protection against the action of corrosive geothermal agents. Protection
The four concrete channels (hot and cold water) between the condensers and the cooling towers were covered with coaltar epoxy to a thickness of 20 ram. These channels are 120 m long with a
6 pH 4 /-----45°C :L 5" ( heated)
2 0 1
I1:00
12!oo
13!o0
14!o0
15!00
IS!O0 h
Time of day June 8, 1973
Fig. 4. pH changes of cooling water sample in laboratory (sample water from cooling plant system).
Developments in geothermal energy in Mexico--XIV
371
5000 2400 180G SO4
ppm
1200
600
start o~_mtmn .
j/
/ start treatment ~,rf-" ( plant in operation)
~-~piont stand by-~/
'. . . . . . . . . . .
without treatment 2 6 I0 14 18 22 26 30 4 6 12 16 20 24 28 2 6 10 14 113 22 2650 April May June Day of month (1973)
Fig. 5. Sulphate change in cooling water (unit If).
rectangular cross section of 2.20 m by 1.50 m. The hot well and the sump of the tower were also covered with this epoxy, making a total area of 11,000 m 2. The carbon steel water tail pipes of the jet condenser of the first unit were covered with epoxy resin. On the other hand, the same components of the second unit were left without protection. The bottom parts of the mild steel condensers were covered with 316 ss. The upper parts were covered with epoxy resin. Pumps, pipes and screws of the cooling tower are made of stainless steel. The protections described above were considered excessive for handling "distilled" water or low concentration water (Table 4). Therefore, the material selected for the hydrogen generator and the turbine oil cooler was aluminium. The first unit was operated and tested for 15 days. A light precipitation of sulphur compounds occurred during the first days of operation. However, it was not considered a problem. Nevertheless at the end of the test, when examining the equipment, a great number of punctures and perforations were found in the pipes of the turbine oil coolers. Organic structures and sulphur clusters were detected in the punctured areas. The same kind of corrosion was expected to be found at the generator hydrogen coolers but, though some corrosion was observed, there were no perforations due to the thickness of the wall pipe. Once the unit was ready for further operation, the system was protected with a phosphate base inhibitor. Continuous measurements of pH chemical determinations and analyses of H:S were made (Figs 4-9).
~
Ca(0H);~dositicQtion ,tort NQOHdosification
500 4OO
Ca(OH)2 :500 or NoOH
pH
i2oo 0
biocide dosificotion"--7 65kg each time 4 ~ A April
I00 A A ~&~AA
May Day of month {1973)
June
Fig. 6. pH change in relation with NaOH and Ca(OH) 2 dosification (unit II).
372
S. Mr~cADo
et
ol.
600 dition of Co(OH)2
500 E
400
=
)
SOO
o
200 100
without treatment
C--..~p_,tn,_l_._,,o_~by__--L-I ~.j"*"'of,,at, oddit~n Ca(OH)2 April
May Day of month (1973)
June
Fig. 7. Total hardness alteration by adding Ca(OH)=.
SOOO 7OOO 6000 ul :~, SO00
-
4ooo
~) SO00 2000 1000
l~plont in stand by---I
April
May Day of month (i973)
June
Fig. 8. Conductivity changes in cooling water (unit II).
300
plant start of operation 200
CI ppm plant in operation IOO
p-io'n; m sta n"d-b
April
"
y
~
-
-
*
May Day of month (1973)
Fig. 9. Chloride changes in cooling water.
~
June
Developments in geothermal energy in M e x i c o - - X I V
373
Table 5. Microbiological analysis of cooling water Count
( × 10-') Bacteria Acromobacter species Aerobacter species Flavobacterium species Bacillus species Proteus species Pseudomonus species Thiobacillus thiooxidans Thiobacillus denitrificans Others Total bacteria count
1973
1986
20 30 10 15 10 i Present Present 9 95
Present Present Present Present Present Present Present Present Present <5
Fungi Saccharomyces species
0.015
Present
A more copious precipitation of sulphur and sulphur compounds (Fig. 5) caused a milky consistency of the water. Heavy precipitation on the screens and battens of the cooling tower were observed when the pH value fell below 3. Because an inhibitor had been utilized, corrosion at the unprotected parts of carbon steel and aluminium was not expected. However the pH had to be increased by adding flakes or a concentrated solution of NaOH directly into the water channels. About 200 kg per day per unit were needed. Since plant operation in such conditions was impossible to maintain for a long period, laboratory studies were carried out in order to determine what was causing the lowering of the pH and the heavy sulphur precipitation. The studies reported that H2S was changed to sulphur and other compounds due to bacterial action which currently could be eliminated by adding chlorophenol biocide and quartenary amines. However, for this specific case, it did not solve the problem. Further laboratory studies reported that the proliferation of bacteria (106 colonies ern -3, Table 2) was due to the inhibitor. Thus the utilization of the inhibitor was immediately suspended and in a few days, following a complete change of water (usually 0.1 m 3 s-~ are wasted) and adding biocide in excess, the pH was finally controlled. In order to decrease the use of biocides, several compounds were tested with continuous and intermittent dosification. The latter was reported to be the best method for controlling bacteria formation to less than 5 x 104 colonies cm -3. Present treatment
At the present time, biocides from different manufacturers are used (quaternary-amides, bisthiocyanates and occasionally chlorophenols) in an attempt to utilize as much as possible the biodegradable kind to avoid water pollution. Currently 100 kg of bioeides are added once every ten days to each unit. The amounts required are double or triple during the summer time due to the fact that the air temperature is higher resulting in higher bacteria proliferation. At the present time, utilizing this method, the bacteria problem is under control (Table 5). Problems with concrete channels
Although pH and bacteria problems were under control, the roofs of the concrete channels were still attacked. This was due to higher anaerobic bacteria proliferation in HzS saturated steam environment at temperatures in the range of 30--45°C. The coal tar epoxy was attacked first and the concrete later on, registering a 5 em penetration as well as a loosening of gravel. After three years of operation it was necessary to repair the channel roofs by adding sulphate resistant concrete and by protecting them with a PVC plate of I mm thickness. At zones at the sump tower and below the channel water line no loosening of the concrete nor coal tar epoxy protection was observed. Corrosion in barometric tail pipes and carbon steel valves
After two years of operation, carbon steel pipes were destroyed by corrosion (bacteria attack)
374
S. M~CADO et al.
and then replaced with stainless steel and glass fibre. Carbon steel valves were also attacked by corrosion; they were repaired and then protected with high temperature epoxy resin. Turbine and generator oil and hydrogen cooler tubes
Heat exchanger tubes for cooling turbine oil were made of aluminium. After operating for 15 days the pipes showed a great number of punctures and perforations and were replaced by 304 stainless steel. However, after a few months' operation, some pipes exhibited punctures. Further studies showed a higher bacteria proliferation in the absence of water flow through the pipes. This was due to the existence o f two heat exchangers of which only one operated continuously. The problem was solved through the simultaneous operation of the two heat exchangers and when the unit had to stop for any reason, the remaining water was drained and the pipes dried. The generator hydrogen cooler pipes had pitting but no perforations. To avoid any risks they were replaced by titanium pipes and, to date, they have not been attacked. Cooling tower
The first Cerro Prieto I cooling tower packing was made of redwood treated with chromates and copper arsenate. Traces of chromate, arsenic and copper in the cooling water may indicate leaching of these substances. If the process continues the wood packing may be left unprotected. Therefore in later cooling towers plastic packing has been used. Acknowledgements--The authors wish to thank Dr Christopher Heard for his advice and reviewof the paper. Thanks are
also due to Mrs Maria Eugenia Calder6n for her excellenttyping. REFERENCES 1. A. Marion, F. Bermejoand P. Per6z, Developmentsin geothermalenergy in Mexico--part thirteen: The operation of surfaceequipmentfor geothermalfluidconductionat Cerro Prieto I. Heat RecoverySystems& CHP, 7, 273-284 (1987). 2. W. $tilwell, W. Hall and J. Tawhai, Ground movementin New Zealand geothermal fields. Proc. 3rd U.N. Syrup. on the Developmentand Utilization of Geothermal Resources, Vol. 2, pp. 1427-1434. San Francisco (1975).
0890-4332/87 $3.00 +0.00 Pergamon Journals Ltd
DEVELOPMENTS IN GEOTHERMAL ENERGY IN MEXICO--PART FOURTEEN. ENVIRONMENTAL ASPECTS OF GEOTHERMAL SYSTEMS S. ML~CADO Divisibn de Estudios de Ingenieria, Instituto de lnvestigaciones El~-icas, M~xico
F. J. BERMF~O Comisi6n Federal de Electricidad, Cerro Prieto, M~xico
and H. FEaN^m>EZ Departamento de Gcotermia, Divisi6n Fuentes de Ener#a, Instituto de Investigaciones El,~'ctricas, Cuernavaca, M;:xico (Received 12 February 1987) AImraet---Geothermal fields have a number of potential pollutants such as brines, gases, noise, heat and steam dispersion and accidental spills. Additional problems arise from subsidence, induced seismicity, scenic changes and land use. Measures to control their adverse effects on the environment are discussed with particular reference to the Cerro Prieto geothermal field at Mexicali, B.C. Mexico. Methods to reduce biolosieally induced corrosion in the cooling water system of a geothermal power plant are also discussed.
INTRODUCTION
Geothermal fields have a number of potential pollutants which include brines, gases, noise, heat and steam dispersion and accidental spills. Additional problems can arise from subsidence, induced seismicity, scenic changes and land uses. Fortunately technology exists to control and reduce the adverse effect of almost all these geothermal pollutants. One of the most significant problems is that of biologically induced corrosion in the cooling water systems of geothermal power plants. The overall strategy for reducing the adverse environmental impact of pollutants also includes plans for the use of geothermal wastes. Industrial, agricultural and aquiculture water uses are currently under development. Installations for commercial salts extraction and demonstration plants for waste heat usage are already under construction. The precise ecological impact of the pollutants is specific to a particular reservoir and the local environmental characteristics. The measures taken for controlling geothermal pollutants vary from one country to another depending on government regulations, bureaucratic procedures and long-term cultural heritage factors, etc. In this paper the problems will be discussed with reference to the Cerro Prieto geothermal field at Mexicali, B.C. Mexico. EVALUATION
OF
POLLUTANTS
AND
THEIR
CONTROL
There are high and low impact problems. The high ones arise from the gases which are released to the atmosphere and the high salinity brines discharged to ponds, lakes and the sea or reinjected underground. The low impact problems include noise, heat and steam dispersion, subsidence, induced seismicity, land use, scenic changes and accidental spills. The potential pollutants and ecological effects are listed in Table I for the various types of reservoirs. "The controls used to overcome these problems in various geothermal fields throughout the world are listed in Table 2. The Cerro Prieto geothermal field at Mexicali, B.C. Mexico has a water dominant reservoir. Some of these controls have been used at Cerro Prieto with good results. 365
366
S.
MERCADO
et al.
Table I. Potential pollutants and ecological effects on geothermal systems Water dominant
Contaminant Gases Brine Noise H e a t dispersion
Subsidence I n d u c e d seismicity Scenic Land use
x x x x x x
Reservoir t y p e Low Steam enthalpy
Geopressurized
Hot d r y rock
N.A. x x x x x
---x -?
x -x x x -
-x -x x x
x
x
x
x
x
x
x
x
x
x
Table 2. Control methods used to limit environmental impact Controls used
Contaminant Gases Brine disposal
Noise Heat Subsidence Induced seismieity Scenic and land use Accidental spills
Chemical treatment, Stretford process, HiS combustion, use of H202, catalysis, upstream reboiler, injection, etc. Reinjection, salt extraction, evaporating ponds, and channeling to the sea or to lakes. Twin silencers for brine--steam mixture discharges, and thermal insulation for steam lines. Thermal insulation of pipes and surface equipment. Adjustment of production, structure repair and reinjection. Adjustment to fluid injection. Flora replacement and g r e e n h o u s e s . Blowout preventors, casing repair and plugging of wells.
SO 2
Table 3. Chemical analysis of Cerro Prieto brine Chemical Na K Ca Li Mg Sr Zn HCO~ SiO2 CI pH
Total dissolved solids content (ppm) 10,735 3042 416 29 0.5 3.8 0.5 5.9 1050 21,300 5.58
Brine
At Cerro Prieto the fluid extracted (steam and brine) by deep wells is separated in cyclone separators. Steam is utilized to feed the turbine. At the same time, brine is discharged to a 16 km 2 evaporation pond where it is evaporated and concentrated for the extraction of mineral compounds, mainly potash. This is utilized in the production of imported fertilizers. The original area of the pond (1973-1984) was 8 km 2 for an installed capacity of 180 MW and a brine extraction rate of 3000 ton h-'. The present pond area is 16 km 2 for a generating capacity of 620 MW and an estimated extraction rate of geothermal fluids of 1 x l0 s m 3 y r - I of which up to 60% is waste brine. Other applications for waste fluids are being developed at the Cerro Prieto geothermal field including an absorption refrigeration system, a greenhouse, a fish hatchery and a chicken farm, all of them operating from geothermal waste fluids. Also, an amount of waste brine will be reinjected in the near future. A typical analysis of the Cerro Prieto brine is shown in Table 3. Gases Air pollution is a major problem caused by geothermal energy exploitation. At the Cerro Prieto geothermal field air pollution is mainly caused by H2S emissions. This is a severe contaminant which also attacks the power plant itself, causing metal corrosion including damage to electrical equipment and power lines. At a higher concentration, H2S attacks concrete and may be a serious hazard for human life. Some of these emissions do not originate from the power plant but occur naturally from bubbling pools and fumaroles.
Developments in geothermal energy in Mexico---XIV
367
Table 4. Steam chemical impurities at turbine inlet (units 1 and 2) Condensate impurity type
Concentration (ppm)
CaCO 3 F¢ SiO2 CI
0.0 0.05 0.2 1.0
so,
O.Ol
Na CO t H2S NH4 pH
0.4 14,000 1500 I l0 7.3
In geothermal exploitation, HeS emissions occur when drilling the wells, in the testing and measurement of wells and in almost every exploitation step, including plant operation. Other gaseous pollutants in the geothermal fluids in addition to H2S are released to the atmosphere when the geothermal fluids reach the surface. At the Cerro Prieto I power station some steps were taken in order to control the emissions: stacks were installed at a higher position than first planned. The fluid discharge at the greater heights helped to control the H2S pollution on site due to the north winds which carry away the H:S and other contaminants. Also a gas pipeline was constructed from the power plant to the evaporation pond (more than I km distance). The pipeline incorporated blowers to transfer the gases towards the pond. These subsequently were taken out of service after being extensively corroded [1]. Thus pollution on site was reduced. However it is not possible to completely eliminate gaseous pollution at the plant surroundings, since the contaminants are partially dissolved by the cooling water in the direct contact condensers and ejector coolers and then released by the cooling tower. Several methods have been tested for controlling HeS emissions at Cerro Prieto II and III power plants including a burner scrubber and an upstream reboiler [1]. In order to further control HzS emissions, future power plants at Cerro Prieto will be provided with surface condenser cooling systems. Research is being carried out to extract sulfur or sulfur compounds from HzS for commercial purposes. The steam of Cerro Prieto I has a gaseous content of 1.5% by weight. This is the highest one in the field. The gaseous content for Cerro Prieto II and III is less than 1% (typical well analyses are shown in Table 4). A gaseous content less than 0.7% is expected for Cerro Prieto IV. The main gases present in the geothermal fluids are: CO2 (95%), HeS (4%), NH3 (0.7%), and minor contents of hydrogen, methane, ethane, propane and butane. Continuous HzS monitoring is currently being carried out. The monitoring equipment includes several fixed units distributed on the geothermal field near the wells, at the evaporating pond, in the surrounding areas near the plant, as well as a mobile unit. Data obtained from this monitoring equipment show that, to date, H2S dispersion has caused minimal effects on the area. Some measurements have reported only 3 ppb in some surrounding points. Figure 1 shows the results of typical dispersions obtained from the operation of Cerro Prieto I, II and III (total 620 MW), with partial control of H2S. The minimum air pollution index at the surrounding areas of the plants is an important factor. No~
Noise contamination occurs at the Cerro Prieto geothermal field when making seismic surveys. The following activities involve noise; drilling works, cleaning or testing wells, flow measurements, steam vented on safety valves or regulation systems, construction and use of access roads and plant. A loud noise is caused by the direct discharge to atmosphere required for cleaning the well pipe and the production strata. Currently a large twin concrete-base glass-fibre stack silencer which eliminates the spraying of salt and reduces the noise index (60 dB compared to 130 dB for direct atmospheric discharge) is utilized in all wells. The steam flow regulation system of each unit is provided with steel and concrete silencers which are located far away from the plant (approx. 300 m). The turbine noise is similar to the noise X.L$. ?/4--E
368
S. MERCADOet
al.
Fig. 1. Cerro Prieto H2S dispersion (ppb).
produced by a conventional fossil fuel power plant. At the Cerro Prieto I unit, the high noise level is caused by the steam-gas ejectors. At the Cerro Prieto II unit, a turbocompressor for gas extraction was utilized, noticeably reducing the noise index. Heat and steam
The heat rejection at Cerro Prieto is through wet cooling towers. Steam dispersion is through silencers and open hot water channels. The estimated heat rejection to the atmosphere is approximately 2.3 GW for the actual capacity of 620 MW. The plants are located in an open valley. Thus the environmental impact is minimal since the dispersion takes place in a very large area. Subsidence
Subsidence is expected when large quantities of fluids are removed from geothermal reservoirs. Recent surveys in Cerro Prieto showed small local uplift and subsidences at the geothermal field and at the Mexicali Valley mainly caused because of the deep natural fracture crossing the field (the San Andreas fault system) which produced the geothermal underground reservoir. A maximum subsidence of 62 mm during the period 1977-1979 for the Cerro Prieto geothermal field has been reported. The largest geothermal subsidence reported in the world is 4.5 m [2] for the Wairakei geothermal field, New Zealand, between 1964 and 1974. Induced seismicity
Due to its relation to fault zones, the occurrence of earthquakes in the Cerro Prieto geothermal area is very common. They normally occur twice a year for the field and the Mexicali Valley. This has not changed during the exploitation of the geothermal field. Reports have shown no increase or increasing incidence. Land use
The Cerro Prieto geothermal field is located on the west portion of the Mexicali Valley. It lies only l0 m above sea level on the delta sediments of the Colorado River which consist of a very large fiat extention. The main part of the field was saline with many hot spring and boiling pools in which the ecological impact only includes scenic changes because it was not usable for crops. On this side is the Cerro Prieto I power station with outputs of 180 MW. The Cerro Prieto II and III power stations, each with outputs of 220 MW, are on the east part of the field, where farming was the main activity. This is now used for power generation.
Developments in geothermal energy in Mexico---XlV
369
The cotton crops on these lands were very poor, and at present the farmers use the irrigation water on other lands with better production. Fortunately, the valley is very large, and many lands are not used because the irrigation water is not enough. The total area used for geothermal activities is 31~km2 including the evaporating pond which has an area of 16 km'. Microearthquake studies for Cerro Prieto have been done in order to delimit the geothermal reservoir and determine some reservoir parameters. At present, there is a continuous monitoring for passive seismicity. In the near future, when reinjection is on line, a comparison and evaluation of seismicity increases caused by reinjection will be carried out. BIOLOGICALLY I N D U C E D C O R R O S I O N IN THE COOLING WATER SYSTEMS OF POWER PLANTS Cooling water systems in geothermal power plants involve different problems than those with conventional thermoelectric plants. For instance, the cooling system of the Wairakei geothermal power plant uses fresh water from the Waikato river in a direct contact low-level condenser and without recirculation. The H2S existing in the steam phase is therefore dissolved into the water and no corrosion problems have been reported. The geyser plants in California utilize a cooling system with an induced draft tower. Sulphur precipitation has been observed in the sump of the tower. The waste condensate is reinjected and the system operates without any apparent corrosion problems even to the extent that some plants utilize aluminum pipes. The Ahuachapan plant, El Salvador, also utilizes a cooling system with an induced draft tower. No problems have been reported, due perhaps to the low H2S content of the gases. At the Hatchobaru plant, Japan, a similar situation has been reported. At Larderello, Italy, natural draft towers have not presented any problems after several decades in operation. New plants will start operating in the near future and will be provided with induced draft concrete towers with plastic backfilling. The Krafla plant, Iceland, has a cooling system with an induced draft tower which has been operated at only one quarter of its capacity. Problems were reported because of pH values as low as 3. This is corrected by adding NaOH twice a week. At the Olkaria plant, Kenya, and the McBan plant, Philippines, similar problems have been reported. Operating experience at Cerro Prieto Cerro Prieto I power station units 1 and 2 (37.5 MW each) started operating in 1973 and both have been operating continuously since then except for preventive maintenance and a problem at the cooling tower. The system characteristics are shown in Figs 2 and 3. steom
water to rl turbine 8 Jl generator II heat [[
woter~"~"~ in~t _ ~ t~ ~- I ~
H-" ~
~I I I II (.e'=-/-~--~.il I"1 II II I ~.~1 I I I I
,,o,o.oer,
" -
._r-? I /__~, steam I/~ inlet V(~
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"- °
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I
Illff IIIII
--
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Fig. 2. Main condenser and aflcr-coolcn (units I-IV, O~'m Priclo).
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water
12320 tonne h"1 32oC ---- ~
water 410 tonne h"l goslsteom and air
10 700 tonne h"~
T 10 tonne h"1
24tonne steO%r
h-I
water
20tonne
1190 tonne h"1 steam 6.4 bar
h"1 9.4 tonne h"t gOS~ air and steam
direct contact condenser 0.11 bar
286 tonne h'~ 0.35 bar
/
1.04 bar
~.5.8°C
45.3 °C
6f~.6°C
to cooling tower
12 600 tonne h.I 46°C
hot well
Fig. 3. Heat balance of cooling system (units I-IV).
The cooling water system for the 37.5 MW units consists of a vertical cylindrical barometric jet condenser with a load of 286 ton h- ~, operating at 0.11 bar absolute. The cooling water design inlet temperature is 32°C, handling 10,700 ton h -m of cooling water. Other important data are: 9.6m height x 6.7 m dia. shell, 12 m height x 2 m dia. tail pipe, 1.5 m dia. cooling water inlet pipe, 3.6 m dia. steam inlet pipe, 0.7 m dia. gas outlet pipe, 130 ton assembly weight. Each two-stage condenser has three gas ejectors, a barometric jet condenser type inter- and after-cooler. Some important data are: 39 kgh -~ dry gas extracting capacity, 0.1 bar absolute suction pressure, 6.4 bar steam pressure, 44 ton h-~ steam consumption and 1600 ton h-~ cooling water at 32°C (see Figs 2 and 3). There are two concrete channels (hot and cold water) per unit, two pumps for the cooling tower and two pumps for the condenser. An induced draft cooling with six cells made of redwood and treated with chromates and copper arsenide has been installed. The original design of the plant did not provide protection for the cooling system. However, some system components were protected during the plant construction, but many were left without protection against the action of corrosive geothermal agents. Protection
The four concrete channels (hot and cold water) between the condensers and the cooling towers were covered with coaltar epoxy to a thickness of 20 ram. These channels are 120 m long with a
6 pH 4 /-----45°C :L 5" ( heated)
2 0 1
I1:00
12!oo
13!o0
14!o0
15!00
IS!O0 h
Time of day June 8, 1973
Fig. 4. pH changes of cooling water sample in laboratory (sample water from cooling plant system).
Developments in geothermal energy in Mexico--XIV
371
5000 2400 180G SO4
ppm
1200
600
start o~_mtmn .
j/
/ start treatment ~,rf-" ( plant in operation)
~-~piont stand by-~/
'. . . . . . . . . . .
without treatment 2 6 I0 14 18 22 26 30 4 6 12 16 20 24 28 2 6 10 14 113 22 2650 April May June Day of month (1973)
Fig. 5. Sulphate change in cooling water (unit If).
rectangular cross section of 2.20 m by 1.50 m. The hot well and the sump of the tower were also covered with this epoxy, making a total area of 11,000 m 2. The carbon steel water tail pipes of the jet condenser of the first unit were covered with epoxy resin. On the other hand, the same components of the second unit were left without protection. The bottom parts of the mild steel condensers were covered with 316 ss. The upper parts were covered with epoxy resin. Pumps, pipes and screws of the cooling tower are made of stainless steel. The protections described above were considered excessive for handling "distilled" water or low concentration water (Table 4). Therefore, the material selected for the hydrogen generator and the turbine oil cooler was aluminium. The first unit was operated and tested for 15 days. A light precipitation of sulphur compounds occurred during the first days of operation. However, it was not considered a problem. Nevertheless at the end of the test, when examining the equipment, a great number of punctures and perforations were found in the pipes of the turbine oil coolers. Organic structures and sulphur clusters were detected in the punctured areas. The same kind of corrosion was expected to be found at the generator hydrogen coolers but, though some corrosion was observed, there were no perforations due to the thickness of the wall pipe. Once the unit was ready for further operation, the system was protected with a phosphate base inhibitor. Continuous measurements of pH chemical determinations and analyses of H:S were made (Figs 4-9).
~
Ca(0H);~dositicQtion ,tort NQOHdosification
500 4OO
Ca(OH)2 :500 or NoOH
pH
i2oo 0
biocide dosificotion"--7 65kg each time 4 ~ A April
I00 A A ~&~AA
May Day of month {1973)
June
Fig. 6. pH change in relation with NaOH and Ca(OH) 2 dosification (unit II).
372
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600 dition of Co(OH)2
500 E
400
=
)
SOO
o
200 100
without treatment
C--..~p_,tn,_l_._,,o_~by__--L-I ~.j"*"'of,,at, oddit~n Ca(OH)2 April
May Day of month (1973)
June
Fig. 7. Total hardness alteration by adding Ca(OH)=.
SOOO 7OOO 6000 ul :~, SO00
-
4ooo
~) SO00 2000 1000
l~plont in stand by---I
April
May Day of month (i973)
June
Fig. 8. Conductivity changes in cooling water (unit II).
300
plant start of operation 200
CI ppm plant in operation IOO
p-io'n; m sta n"d-b
April
"
y
~
-
-
*
May Day of month (1973)
Fig. 9. Chloride changes in cooling water.
~
June
Developments in geothermal energy in M e x i c o - - X I V
373
Table 5. Microbiological analysis of cooling water Count
( × 10-') Bacteria Acromobacter species Aerobacter species Flavobacterium species Bacillus species Proteus species Pseudomonus species Thiobacillus thiooxidans Thiobacillus denitrificans Others Total bacteria count
1973
1986
20 30 10 15 10 i Present Present 9 95
Present Present Present Present Present Present Present Present Present <5
Fungi Saccharomyces species
0.015
Present
A more copious precipitation of sulphur and sulphur compounds (Fig. 5) caused a milky consistency of the water. Heavy precipitation on the screens and battens of the cooling tower were observed when the pH value fell below 3. Because an inhibitor had been utilized, corrosion at the unprotected parts of carbon steel and aluminium was not expected. However the pH had to be increased by adding flakes or a concentrated solution of NaOH directly into the water channels. About 200 kg per day per unit were needed. Since plant operation in such conditions was impossible to maintain for a long period, laboratory studies were carried out in order to determine what was causing the lowering of the pH and the heavy sulphur precipitation. The studies reported that H2S was changed to sulphur and other compounds due to bacterial action which currently could be eliminated by adding chlorophenol biocide and quartenary amines. However, for this specific case, it did not solve the problem. Further laboratory studies reported that the proliferation of bacteria (106 colonies ern -3, Table 2) was due to the inhibitor. Thus the utilization of the inhibitor was immediately suspended and in a few days, following a complete change of water (usually 0.1 m 3 s-~ are wasted) and adding biocide in excess, the pH was finally controlled. In order to decrease the use of biocides, several compounds were tested with continuous and intermittent dosification. The latter was reported to be the best method for controlling bacteria formation to less than 5 x 104 colonies cm -3. Present treatment
At the present time, biocides from different manufacturers are used (quaternary-amides, bisthiocyanates and occasionally chlorophenols) in an attempt to utilize as much as possible the biodegradable kind to avoid water pollution. Currently 100 kg of bioeides are added once every ten days to each unit. The amounts required are double or triple during the summer time due to the fact that the air temperature is higher resulting in higher bacteria proliferation. At the present time, utilizing this method, the bacteria problem is under control (Table 5). Problems with concrete channels
Although pH and bacteria problems were under control, the roofs of the concrete channels were still attacked. This was due to higher anaerobic bacteria proliferation in HzS saturated steam environment at temperatures in the range of 30--45°C. The coal tar epoxy was attacked first and the concrete later on, registering a 5 em penetration as well as a loosening of gravel. After three years of operation it was necessary to repair the channel roofs by adding sulphate resistant concrete and by protecting them with a PVC plate of I mm thickness. At zones at the sump tower and below the channel water line no loosening of the concrete nor coal tar epoxy protection was observed. Corrosion in barometric tail pipes and carbon steel valves
After two years of operation, carbon steel pipes were destroyed by corrosion (bacteria attack)
374
S. M~CADO et al.
and then replaced with stainless steel and glass fibre. Carbon steel valves were also attacked by corrosion; they were repaired and then protected with high temperature epoxy resin. Turbine and generator oil and hydrogen cooler tubes
Heat exchanger tubes for cooling turbine oil were made of aluminium. After operating for 15 days the pipes showed a great number of punctures and perforations and were replaced by 304 stainless steel. However, after a few months' operation, some pipes exhibited punctures. Further studies showed a higher bacteria proliferation in the absence of water flow through the pipes. This was due to the existence o f two heat exchangers of which only one operated continuously. The problem was solved through the simultaneous operation of the two heat exchangers and when the unit had to stop for any reason, the remaining water was drained and the pipes dried. The generator hydrogen cooler pipes had pitting but no perforations. To avoid any risks they were replaced by titanium pipes and, to date, they have not been attacked. Cooling tower
The first Cerro Prieto I cooling tower packing was made of redwood treated with chromates and copper arsenate. Traces of chromate, arsenic and copper in the cooling water may indicate leaching of these substances. If the process continues the wood packing may be left unprotected. Therefore in later cooling towers plastic packing has been used. Acknowledgements--The authors wish to thank Dr Christopher Heard for his advice and reviewof the paper. Thanks are
also due to Mrs Maria Eugenia Calder6n for her excellenttyping. REFERENCES 1. A. Marion, F. Bermejoand P. Per6z, Developmentsin geothermalenergy in Mexico--part thirteen: The operation of surfaceequipmentfor geothermalfluidconductionat Cerro Prieto I. Heat RecoverySystems& CHP, 7, 273-284 (1987). 2. W. $tilwell, W. Hall and J. Tawhai, Ground movementin New Zealand geothermal fields. Proc. 3rd U.N. Syrup. on the Developmentand Utilization of Geothermal Resources, Vol. 2, pp. 1427-1434. San Francisco (1975).