Developments in geothermal energy in Mexico—Part twenty-two. Causes of erosion of the rotor blades in a geothermal turbine

The principal causes of erosion which affected the blades of the two last stages of a 37.5 MW_e geothermal turbine are analyzed. The steam piping system, auxiliary equipment (flashers, dehumidifiers, etc.) and the turbine itself were carefully inspected. The design of the turbine and lay out of the piping system have been analyzed and evaluated. The moving blades were tested for cracks using the magnetic particle method. Also, the natural frequencies of the blades have been measured. It was concluded that the blades of the two last stages should be replaced and the steam path of the low pressure part of the turbine must be modified to avoid similar problems in the future. Several simple modifications of the piping system were recommended as well.     

Developments in geothermal energy in Mexico—part thirty. Conclusion of the corrosion in mexican geothermal wells project

This paper is complementary to part 19 of this series, in which partial results from the joint IIE/CFE studies were presented. The objective was to define the specifications for steel used in geothermal well construction in Mexico and to characterize and control identifiable corrosion inducing factors. The complete results of corrosion testing in wellhead pressure chambers, down hole chambers and in an autoclave simulation system are included. Also shown are chemical, mechanical and metallographic studies on steels commonly used in Mexican geothermal wells, as well as the main conclusions.     

Developments in geothermal energy in Mexico—part eighteen. History of the exploration of the cerro prieto geothermal field

Exploration of the geothermal field of Cerro Prieto started in the 1960's, including the drilling of wells at depths from 1000 m down to 1300 m, which discharged a large amount of geothermal fluid at high pressure. A simple model of the field indicated a suitable productive area of less than 1 km². More wells were drilled in the 1970's, resulting in an expected suitable productive area of 4 km². In 1986, the known suitable productive area was 25 km². The potential of the field was 1000 MW of power, which ensured the continuous production of the 620 MW already installed [1]. Currently, deep drilling is progressing on the eastern parts of the field, including wells 450 m deep which will eventually lead to a better knowledge of the extent and potential of the field through pressure, temperature and production measurements obtained from exploration wells.     

Developments in geothermal energy in Mexico—Part twenty. Technical advantages and disadvantages of geothermal well-head units

The technical advantages and disadvantages resulting from locating a steam turbine unit close to a geothermal well-head are presented. The short steam piping system has implications in the design of a well-head turbine. It is concluded that there are some technical differences between large geothermal turbines and well-head units, which may be decisive in the selection of geothermal plant.     

Developments in geothermal energy in Mexico—part thirty-one. Prediction of geothermal cement durability from field tests

Cement data reported in 1985 by the Geothermal Cements Task Group of The American Petroleum Institute (API) Committee 10 on cements standardization are presented. In order to obtain adequate cement compositions for geothermal environments, the API and National Bureau of Standards (NBS), U.S.A. and the Comisión Federal de Electricidad (CFE), together with the Instituto de Investigaciones Eléctricas (IIE), Mexico, have completed a programme to evaluate 16 cement compositions in the Cerro Prieto geothermal field in both down-hole baskets and in surface chambers.     

Developments in geothermal energy in Mexico—part nineteen. Corrosion in Mexican geothermal wells

The Instituto de Investigaciones Eléctricas and the Comisión Federal de Electricidad have initiated a collaborative study to define the specifications of steels for use in geothermal well construction in Mexico. Tests have been designed to characterize and control identifiable factors affecting corrosion. The study includes three main areas of activity: (a) studies of cases of material failure from several Mexican fields were made; (b) studies of general, localized and stress corrosion of sample coupons exposed to geothermal fluid were made in wellhead pressure chambers; (c) laboratory tests are being carried out under controlled hydrodynamic conditions.     

Developments in geothermal energy in Mexico—part thirty-four. Numerical steady-state simulation of the steam supply system of the cerro prieto geothermal plant

The philosophy used in the mathematical modelling of the steam supply network of Cerro Prieto geothermal power stations is described. In order to form the simulation model of this system, mathematical models were developed for: the surface equipment such as separators, pressure control stations, steam turbines, low and high pressure interconnections, purges and wells. With the above mentioned models a computer programme was produced which operating conditions in the steam supply system.     

Developments in geothermal energy in Mexico—part forty. The future for geothermal energy in Mexico

In Mexico there are many potential geothermal energy sources with a wide range of temperatures. At present high temperature geothermal energy is used to generate electricity. Considerable experience has been accumulated in the exploitation of geothermal energy. Lower grade geothermal heat could be used to generate electricity with organic Rankine cycle plants. There is little industrial exploitation of lower temperature geothermal resources at present even though in the centre of Mexico areas of geothermal activity coincide with industrial zones. However, this situation could change with the use of suitable heat exchangers and heat pumps, particularly absorption systems to supply process heat with very little pollution.     

Developments in geothermal energy in Mexico—part thirteen: the operation of surface equipment for geothermal fluid conduction at Cerro Prieto I

At Cerro Prieto I 34.5 PJ_e have been generated and 334 million tonnes of geothermal fluid have been produced. The length of the present system of 300 mm (12 in) to 350 mm (14 in) dia. steam pipelines is 24 km. That of the water pipeline system of 200 mm (8 in) to 400 mm (16 in) dia. pipe is 46.5 km. A total of 32 wells, 41 separators, 51 silencers, 7 steam driers, and 290 200 mm (8 in) to 400 mm (16 in) dia. valves are in operation. Corrosion in the pipelines and other surface equipment has not been a serious problem. The principle maintenance problem has been the cleaning of silica scaling and sediments in pipelines, equipment and channels. Frequent maintenance is necessary in order to maintain the capacity of water pipelines, separators and channels. The well liners and in some cases the deposits themselves have been affected by scaling. Even the small amounts of silica that are carried by the separated steam cause scaling of turbine blades, reducing their capacity.     

Developments in geothermal energy in Mexico—Part thirty-seven. Procedure to diagnose production abatement problems in geothermal wells

A procedure to diagnose cases of production decrease in geothermal wells is presented. Most commonly, a production decrease in geothermal wells is due to: surface pipeline scaling, mechanical damage in the wellbore, entrance of cooler fluids to the producing reservoir layer and reservoir and well pipeline scaling. The procedure, which is presented as a decision diagram, is based in chemical and well production data. It is able to identify the above mentioned causes for a given well. This procedure was successfully applied to 17 wells from the Cerro Prieto geothermal field. In addition, a silica deposition rate parameter, Rd, was designed. It can be used as a forecasting tool for well scaling. It is proposed that this parameter is important in deriving suitable production strategies to minimise the effect of silica scaling processes in the reservoir.     

Developments in geothermal energy in Mexico—part twenty-five: Operating experience with a geothermal wellhead turbine

After several months of operation of a wellhead 5 MW geothermal turbine, an incident occurred due to the failure of the governor oil pipe joint. The oil tank drained out causing damage to the turbine bearings. Heavy scaling of the first blading stage of the turbine was observed, particularly in the nozzles. Recommendations for the design and remote control operation of the wellhead unit are discussed with reference to the faults found in this case.     

Developments in geothermal energy in Mexico—part thirty-five. Direct contact and surface condensers in geothermal electric power plants

At present, direct contact condensers are the most widely used type in geothermal plants around the world. They have technical and economic advantages over surface condensers. The latter are only used where there are environmental problems associated with the release of polluting gases to the atmosphere, principally hydrogen sulphide.The non-condensable gas chemistry is an important factor in the decision to select direct contact or surface condensers. When plant emissions are strictly limited, surface condensers are used. In the Cerro Prieto geothermal field a total capacity of 620 MW_e has been installed. The field is in a large open valley with constant winds. This provides very good dispersion of the hydrogen sulphide released. However, further increases in installed capacity will need to incorporate control of hydrogen sulphide releases. Thus the Comisión Federal de Electricidad (CFE) has plans to build future plants with surface condensers and sulphur recovery systems.     

Developments in geothermal energy in Mexico—part twenty-nine: Scaling studies at the Cerro Prieto geothermal field

The high silica content (over 900 p.p.m.) of Cerro Prieto geothermal brine causes problems of scaling at the electric power facility. The scaling tendency is a direct function of the silica saturation ratio. Scaling rates of 1–10 mm per year were experienced at low saturation ratios (SR) < 2 and scaling rates of 100–400 mm per year at (SR) > 3. Pilor plant tests satisfactorily predict scaling rates in commercial equipment. Amorphous silica deposited in process equipment can be dissolved by NaOH, KOH and NH₄HF₂. The use of chemical products to remove scale from well liners is a good method of control. The removal of colloidal silica by using 20–30 p.p.m. of lime has been developed.     

Developments in geothermal energy in Mexico—Part twenty six: Experimental assessment of an ammonia/water absorption cooler operating on low enthalpy geothermal energy

Mexico possesses large amounts of geothermal brine at temperatures which are too low to enable electricity to be generated efficiently and economically. Most of the geothermal fields in Mexico are located near important agricultural areas. Perishable food losses in Mexico, resulting from inadequate handling and cold storage facilities, vary from 35 to 50%. In order to prove the technical feasibility of operating heat-driven absorption cooling systems on low enthalpy geothermal energy, a prototype ammonia/water absorption cooler was installed in the Cerro Prieto geothermal field, where the ambient temperatures exceed 40°C and the cooling water temperatures reach 30°C. The unit has operated successfully with evaporative cooling loads exceeding the design value. The experimental data obtained will provide an excellent basis for the design of large scale heat-driven absorption refrigeration systems.     

Developments in geothermal energy in Mexico—part thirty-nine. Extraction of useful heat and silica removal from geothermal brine utilizing liquid fluidized bed heat exchangers

Liquid fluidized bed heat exchangers can be utilized for the recovery of heat from geothermal brine in order to avoid scaling. At the same time, these heat exchangers remove silica from the geothermal brine. A new parameter, obtained from the bed expansion and referred to as the porosity of transition, is defined. This parameter contributes to a better understanding of systems utilizing liquid fluidized bed heat exchangers. From the model proposed by Goto [J. Phys. Chem.60, 1007–1008 (1956)] for silica polymerization and modified by Axtmann [Geothermics15, 185–191 (1986)] for silica removal, the rate of reaction constant is expressed as a function of silica equilibrium monomer concentration (solubility).     

Development in geothermal energy in mexico—part twenty seven: The potential for geothermal organic rankine cycle power plants in Mexico

Mexico possesses large amounts of geothermal energy. Samples from over 800 geothermal surface phenomena indicate that only 1.1% have an estimated reservoir temperature greater than 200°C. Current practice in Mexico is to produce power from such reservoirs using an open flash steam cycle. It is estimated that 4.8% of geothermal resources are in the temperature rangefrom 140 to 200°C which is a suitable range in which to operate Organic Rankine cycle power plants. Organic Rankine cycle power plants have been built in a range of sizes from 10kW_e to 45 MW_e. They have considerable potential for increasing the production of electricity from Mexico's geothermal resources. Organic Rankine cycle plants are discussed together with the conditions for their economic operation.