SHULMAN

LOW TEMPERATURE FLASHED STEAM POWER GENERATION

GARY SHULMAN

Geothermal Power Company, Inc., 1460 West Water Elmira, New York 14905 USA

Key Words: low temperature flash steam, low temperature geothermal reservoirs, two stage separation, dual flash cycle, low pressure flash plants, Brady Hot Springs Power Plant.

ABSTRACT

Flashed steam power generation can be operated economically with geothermal reservoir temperatures below 177°C. A two stage flash steam plant in Nevada. USA, is described, designed for an initial reservoir temperature of 174°C. Reservoir temperatures as low as

and 130°C are analyzed for 10 power genera- tion.

1. INTRODUCTION

The world's great geothermal resource was first used in electric power generation at Larderello, Italy, in 1904, where boron was being mined for boric acid production from fumaroles in the area. Electric power was first generated by a small 10 generator driven by a one cylinder engine, fed with pure steam raised in a boiler heated by geothermal steam. From this early development with heat exchange boilers, the state of the art has turned to direct use of the geothermal steam in turbines.

The Lardarello wells produced dry steam with no brine, similar to The Geysers in Cove Fort in Utah, USA, and Kamojang, Java, Indonesia. The first power production from a liq- uid-dominated hydrothermal resource began in Wairakei, New Zealand in the early 1960s. Since that time, almost all geothermal power has been produced by steam turbines, with a world installed capacity of over six thousand megawatts.

Although current exploration has discovered large resource areas with temperatures below the focus for geothermal power gen- eration has been on resource temperatures of 177°C or higher. This is due to the fact that most current geothermal developers consider 177°C as the lowest resource temperature for flash steam power gen- eration. This paper will illustrate the use of lower temperatures for this purpose and show that low pressure flash plants can compete economically with more complex, higher cost binary plants for this service, and should be considered favorably by developers and utili- ties.

In 1992 a discovery well drilled at Brady Hot Springs, Nevada, USA, indicated a reservoir temperature of 174°C at a depth of 335 meters. A power sales contract was secured by the developers from Sierra Pacific Power Company, the nearest public utility, at approximately US per kwh. A geothermal power plant was designed and in- stalled for the generation of 26 This plant serves as an ex- ample of the utilization of a dual flash cycle for power production at temperatures below 177°C.

2. PROCESS

There are three basic flash steam cycles to be considered in geother- mal power generation. Listed in order of increasing cost and com- plexity they are:

single flash, non-condensing single flash, condensing

condensing

Resources with temperatures below can use these cycles for power generation. The cycles described herein are applicable to both dry steam resources and hydrothermal resources, except for dual flash which can only be done with hydrothermal systems. A dry steam resource providing 131°C steam has been used in both condensing and hybrid cycles at Cove Fort, Utah, USA. Experi- ence has shown that most resources are hydothermal; therefore we have elected to describe each cycle using that assumption.

Hydrothermal wells produce a two-phase mixture of steam and water that must be separated prior to use in these cycles. Separators can be located at the power plant or at each wellhead. These vessels generally accept the two-phase flow at a tangential inlet, thereby imparting a circular motion to the fluid in the vessel. Centrifugal forces aid in the separation of dense liquid from light vapor. The vapor is of course largely steam, but may also contain a small but significant mixture of noncondensible gases. The steam may also have small entrained water droplets which could damage turbine blading over time. Demisters are used to collect and remove water before steam enters the turbine.

2.1 Single flash, non-condensing

The single flash non-condensing cycle shown in Figure 1 is the sim- plest cycle, with steam exhausting to atmosphere through a diffuser silencer after producing useful work in the turbine. Plants of this type do not maximize the use of the resource, but they can be in- stalled at a minimum cost. Their simplicity speeds the construction and installation of skid-mounted turbine generator modules. Their use as portable units for reservoir evaluation and early cash flow in a project has been demonstrated. They may also be useful as a power source for electric drill rigs, thereby reducing drilling costs.

Some geothermal resources have a very high noncondensible gas content, which makes condensing cycles impractical. Such resources may be best served with single flash non-condensing equipment.

2.2 Single flash, condensing

The addition of acondenser more than doubles the output of a single flash plant. It also increases the cost and complexity of the plant, but the cycle remains fairly simple as shown in Figure 2. In this cycle, exhaust steam flows into a condenser after expansion through the turbine. The condenser pressure is maintained at .07 to

to maximize the expansion of the steam in the turbine, thus enhancing the power output. With low temperature resources, over half of the power developed by the turbine comes from the expansion of the steam below atmospheric pressure.

Condensers are usually fabricated from stainless steel. In a direct

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Figure 1. Single Steam Plant Schematic gas from the noncondensible gas stream. This scrubbing tower poses the hydrogen sulfide to a chemical such as sodium hydroxide or sodium hydrochlorite, creating salt water and non-hazardous sul- fur salts such as sodium sulfate or sodium sulfite. After treatment, the remaining noncondensibles are usually piped to the cooling tower for dispersion in the cooling tower plume. Recent efforts by some operators to compress and reinject the noncondensible gases con- taining hydrogen sulfide have met with some success.

2.3 Dual flash, condensing

Figure 3 shows a dual flash condensing cycle. It is a simple exten- sion of the single flash cycle which makes use of the energy remain- ing in the separated brine. By directing this brine to a low pressure separator, additional steam can be generated which can increase the total power generated by more than 50%. This additional power generation is limited by the low pressure flash separation pressure, which is generally maintained above atmospheric. There have been recent investigations into lowering the pressure to less than atmo- spheric, thereby providing additional steam for turbine expansion entirely within the vacuum range. Although technically feasible, this has not been done commercially to date.

pres- sure separation.

contact type, the steam is condensed by cooling water spray nozzles located within the vessel. Cooling water is circulated between the

the steam which condenses with this large infusion of cooling water serves as make-up water for the cooling tower.

separators,

A two-stage set of steam ejectors is used to remove the noncondensible gases from the condenser. These gases consist primarily of carbon dioxide, with traces of hydrogen sulfide. Ejectors are venturi de- vices which use high pressure steam to create a vacuum, which draws the noncondensible gases from the condenser. The ejector steam is then condensed from the mixture in small stainless steel vessels called inter- and aftercondensers.

If necessary, the stream of noncondensible gases can be treated for hydrogen sulfide removal. In many cases, a simple counterflow chemical treatment tower can be used to remove hydrogen sulfide

Hot Dual Flash Geothermal Power Plant

An example of two-stage, dual flash geothermal power generation is the 26 power plant at Brady Hot Springs, Nevada, USA. This plant used (5) production wells plus (3) reserve wells, each having an installed horsepower line shaft pump to produce 7570 liters per minute of brine per well. The wells were drilled to an average depth of 460 meters. The brine from these wells is pumped to two first stage separators, in which the steam is separated from the brine at a pressure of 4.22 The steam from each separator is then

Figure 2 9.7 Single Flash Condensing Steam Plant Schematic (9.0 net)

Aftemndem

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piped to the inlet of a condensing turbine at a pressure of 3.87 and condensed at a pressure of to generate 9

in each of the two turbine generator sets. The brine at 4.22 from the first stage separators is then admitted to a second stage low pressure separator at 2.11 for further separation of brine and steam. The steam from this unit is piped to a third turbine generator set with an inlet pressure of 1.76 and condensed at

with a generator output of 8 The brine from the separa- tor at is piped to reinjection wells approximately 1.5 kilo- meters distant from the production wells.

The total parasitic load is about 5 making a power supply to the transmission line of 21 This plant has been in operation for two years at 95% availability.

SHULMAN

4. CONCLUSION

Low temperature geothermal reservoirs well below 177°C can be operated by flash steam power generation using a dual flash cycle to significantly increase power output over single flash for a given re- source flow rate. As with any other geothermal power technology, the resource requirements increase with lower resource temperatures. At lower resource temperatures, requiring lower separation pressures, steam turbines must be designed or modified to handle larger spe- cific volumes of steam.

This can be accomplishd by two stage flash power generation with two stage steam separators, and'also with steam separators near

Figure 3. 15.1 Flash Steam Plant Schematic (14.0 Net)

3. USE OF LOW TEMPERATURE RESERVOIRS

The use of geothermal reservoirs with temperatures of and 130°C are listed below with pumped well flow requirements to support 10 of electric power generation using steam turbine inlet pressures, at atmospheric pressure, of 1.033 and con- densing to a vacuum of

Reservoir Power Output Reservoir Brine Required 170°C 10,000 976,270

150°C 1,380,300

130°C 2,329,740

mospheric pressure. In the latter case almost all power is generated in the vacuum phase of steam. In this manner, the simple, low cost design, ease of operation and maintenance, and ability to generate their own cooling water makeup, provide flash plants with a signifi- cant economic advantage compared with binary technology for low temperature power generation.

REFERENCES

DiPippo, R. (May 1988). International Development in Geothermal Power Production. Geothermal Resource Council, Bulletin. Vol. 17,

Kestin, J. (1980). Sourcebook on the Production of Electricity from Geothermal Energy. United States Department of Energy. Brown University.

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