Desah’matron. 19 (1976) 325-336 @ Ekevier Scientific Publisb~ Company, i\m~t.erdam - &int.ed in The Netherlands

325

GEOTHERMAL ENERGY RECOVERY PROCESS

L. Awerbuch, T.E. Lindemuth, S.C. May and A.N. Rogers Ressawh am2 E%gineeriq, BECHTE’L CCRX?R.4TIO?i, San _+ancisco, Cff Lif. 94 7 t 3

1. SUMMARY

The follouing paper discusses a novel process for the production of both power and water from geothermal brines. This resource is found in many Parts Of the world, and especially in Mexico, Japan, Italy, New Zealand, Iceland, and the USA. While geothermal brines have been used for centuries for domestic purposes (space heating, etc.), only recently have attempts been made to produce power and water. Power plants using brine, such as the 75 MUe station in Cierro Prieto. B.C. Mexico, number less than a dozen. worldwide. Water production from geothermal brine is, for the most part, in the experimented stage. The United States Department of the Interior, Bureau of Reclamation, has been conducting desalting pilot plant investi- gations for the last four years near Holtville, California, USA.

Successful exploitation of this new resource requires the development of new techniques and equipment. The state of the art may be said to parallel sea water desalting efforts of 10 to 15 years ago. This paper outlines one new process that shows promise for the successful utilization of the geothermal resource.

2. BACKGROUND

The recent energy crisis stimulated international interest in the develop- ment of viable alternatives to Fossil fuels. Among the energy sources which have been proposed are solar energy, wind power. and geothermal energy. The last is particulary attractive since it is the most concentrated form of so-called “zero-cost” energy and it is not subject to diurnal variations or to the whims of the weather.

As with all new technologies, geothermal power requires the solution of difficult engineering problems, among them corrosion and the deposition of solids on the power plant components. When hot brines are flashed to lower pressures to produce steam for a turbine. a small but significant quantity of solids is carried in the form of a spray suspended in the steam which is released. These suspended droplets may be removed almost completely by properly designed mist eliminators. This procedure, however, is of little value in removing volatile contaminants such as silica, boric acid, H2S. ammonia, and CO

-2’ The last three are corrosive to various plant components;

silica and bori acid plate out on turbine blades, leading to loss of capacity and sometimes component fai 1 ures. A process recently conceived and patented -by Bechtel (ref. 11, isoiates the power Plant components from the above corrkive soluble gases and scale-forming &pars by exposing them only to pure steam, generated by transferring heat from flashed geothermal vapor to distilled water, This is shown schematically in Figure 1.

The pure steam 1s fed to the high pressure ports of the turbine, Additional motive steam is generated by flashing the distilled water to lower pressure. This additional steam is delivered to the intermediate pressure ports of the turbine.

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GEOTHERX~L ENERGY RECOVERY PROCESS 327

One of the novel features of the Bechtel invention is the preheating of condensed turbine exhaust in the Direct Contact Recovery Unit at the bottom of Figure 1. The condensate is sprayed into one side of a chamber where it contacts hot vapor generated on the other side of a partia? wall in the sanuz chamber. The contacting action raises the temperature of the condensate a few degrees in each stage. The condensate is pumped into successively hotter chambers (stages), moving from right to left in Figure 1, finally leaving the Direct Contact Recovery Unit at an elevated temperature.

The hot vapor required for this process is generated spontaneously by the flashing of the geothermal brine. which flows in the opposite direction (from left to right in the figure). As the brine moves from state to stage, each one at a successively lower pressure , a portion of it flashes into steam which, in turn, condenses on the distillate sprayed into that stage. As many in this audience will readily notice, the multistage flashing and heat recovery from the hot brine is similar to the widely used MSF technique for sea water desalination. This is a good example of the type of technology transfer now being used in the exploitation of the geothermal resource.

The flowsheet is readily adapted to the generation of power from a very concentrated geothermal brine such as that of the Niland field in the Imperial Valley of Southern California. Figure 1 indicates that a portion of the stages to dilute the brine to just below its saturation COnCentratiOn, thus preventing crystallization within the system.

At the other extreme, a dilute geothermal brine can generate power and can deliver a fraction of its water content in the form of a purified distillate without danger of crystallization during the flashing process. A flow sheet for this option is included showing that a geothermal power plant generating 50 MWe net can deliver roughly 6 million gallons (22,700 m3) per day of product water.

In a study conducted by Bechtel under U. S. Government contract, several alternative techniques have been investigated for the generation of power from a geothermal heat source of high temperature and moderate to high salinity, among them---

1. feeding the flashed geothermal steam directly to the turbine;

2. transferring heat from geothermal flashed vapor to an organic fluid to generate motive vapor for the turbine;

3. flashing the brine through a multistage flash (MSF) plant through which the organic fluid is pumped inside conventional tubing. This fluidSheated by MSF heat recovery, then drives the turbine,

The direct use of geothermal vapor in a turbine , although simple, may be undesirable from the standpoint of corrosion and scale build-up on turbine blades and associated equipment. The last two alternatives listed above present problems arising from the difficulties of containing the organic fluid as well as the flatmmbilitv of some of the fluids under consideration. In addition, the costs of organic vapor turbines generally exceed the cost of an equivalent steam turbine. In contrast, the design concept described in this report incorporates a conventional steam turbine and condenser, which “see” nothing but the vapor of double-distilled water.

328 L. AWERBUCH ET AL

The design concept is based on conservative parameters_ Operating temper- atures and fluid concentrations are well within the solubility range of the typical brines studied here. The geothermal brine does not contact heat exchanger tubing. as in some of the alternative concepts considered. Consequent?y, should precipitates form as a result of maloperation, their effect would be minimal, and the deposits can be readily removed from the large, openflash chambers.

3_ ‘PROCI SS ANALYSIS

The following section discusses the performance of the Bechtel energy recovery system for two types of geothermal brine, typical of sites in the Imperial Valley in California, U.S.A. High salinity brine, such as found near Niland, California, will be discussed for the generation of power only, since the total dissolved solids content of this brine is so high that even a small amount of water removal will cause precipitation of salts. This brine is attractive for power generation, h&ever. due to its high initial temoerature of 550°F [288OC). Lower salini tv brine. 1 i ke that at Heber, California, shows the potential for both wate; and power production. Since this brine has a lower initial temperature. more brine is required per KM of power production; and therfore. the economics of a dual purpose operation become more attractive. The chemical analysis of these brines is shonn In Table 1.

Table 1 Properties o-f Geothermal Brines

Constituent Concentration (m/L)

High Salinity Low Salinity (Kfland Area) (Keber Area)

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Silica (so21

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sra OF Ions

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GEOTHERMAL ENERGY RECOVERY PROCESS 329

3.2 POWER ONLY CYCLE

Figure 2 presents the florsheet of a power plant to deliver 50 MWe of net 8,600 gpm (1950 m /hr)

~~fk-e of 800 psig (5.6 Kg/C 2 f geothermal brine at 5500F (287OC) and a ) enters the wellhead separator, where i t

flashes down to a pressure of 180 psig (l-3 Kg/CM2). The 796,000 lbs/$our (360.000 Kg/hr) of steam thus gere.ated is sent to the evaporator, where the steam condenses, delivering Its heat to boil distilled water at a temperature of 35OoF (176°C)_ The steam released by this boiling process provides motive power to the turbine of the power plant. Additional motive steam is generated, but at a lower pressure, by flashing the distilled water in a second vessel to its final temperature of 215OF (lOl°C). deliv- ering the resulting steam to the intermediate pressure inlet port of the turbi ne.

The steam released from the brine in the wellhead Separator wi?? contain the major POrtiOn of the noncondensible gases dissolved in the geotherm+)l brine: C02. NH3. and H S. wi?? interfere with hea P

These gases are corrosive. In addition, they transfer by blanketing the heat transfer Surfaces.

To remve them t0 the greatest extent practicable, 10% of the steam generated in the wellhead separator is vented to the atmosphere.

A Portion of the generated steam which is condensed, serves to rinse the demister of the wellhead separator, the balance is combined with separator blowdown and delivered to the first chamber, stage # 1 of the direct contact heat recuperator (DCHR), at 408OF (208OC). As the brine enters, it flashes down to 155 wig (1.8 Kg/CM21 converting a portion of its water content to water vapor. The residual brine enters the next chamber, stage 4 2, which is maintained at a still lower pressure; and again, a portion of its water content is flashed into vapor. This flashing process is repeated from stage to Stage Until finally, the residual brine is discharged as blowdown from the last chamber, stage il 13.

In the meanwhile, the turbine exhaust. which has condensed in the power plant condenser, is sprayed into stage t 13 behind a baffle in that chamber- The flashed vapor generated in stage # 13 condensed on the COO? spray Of condensate so aS to augment its volume slightly and raise its temperature. It is apparent that the liquid falling to the bottom is a fairly pure distilled water Since the vapor has passed through an efficient demister prior to condensing on the water spray. This partially heated product is Sprayed into stage # 12, where it further heated. The purified liquid proceeds upstream through the OCHR, countercurrent to the stream Of flash- ing brine. The condensing-heating process is repeated from stage to Stage, until the fairly pure product emerges from stage # 1 at a temperature Of 340°F (17loC). From there it proceed3 to the evaporator previously discussed. entering it at 340°F (176OC), only 10 (6oC) below its saturation temperature-

AS a result of the recovery of heat by direct contact condensation in the OCHR, the them1 efficiency of the plant has been increased substantially. There is a Secondary advantage to this improved thermal efficiency The condenser and cooling tower are reduced in size and cost since the energY lost must ultimately be rejected to the atmosphere- In addition, a de- crease in heat rejection is accompanied by a decrease in pump and blower energy consumed by the cooling circuit-

330

A concentrated brine of the composition shown in Table 1 will approach saturation as it traverses the DCHR, To avoid problems associated with crysta'ilization within this unit, roughly one-third of the purified water leaving the low pressure flasher is injected into the hot brine.of stage S! 8. As a result, the composition of plant blowdown is practically ident with that of the geothermal brine entering the DCHR.

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CEOTHEFBUL ENERGY RECOVERY PROCESS 331

3.3 PCWER-PLUS-WATER CYCLE

A geothermal brine of lower temperature produces less power per pound Of brine than does a higher temperature feed in a similar power plant cycle. The flysheet Of Figure 3 delivers 50 MHe of net power from 21,000 gpm (4750 W /N) of geothermal brine leaving the we?? at 3BGoF (193oC). The overall Power plant cycle is aknost identical with that of Figure 2, differ- ing Principally in the ~%i~rn temperatures of the wellhead separator, the evaporator. and the DCHR, :

The assumed feed composition corresponds to that of the wells at Neber, near the lower end of California’s Imperfal Valley (see Table 1). The brine of Niland contains about 25% by weight of dissolved salts, roughly seven times the concentration of sea water. In contrast, the Heber feed used in this section contains approximately 1.4% dissolved salts. Con- sequently, the design concentratjon ~$11 be far below the saturation limit in the plant blowdown. Thus, the midstream dilution indicated in Figure 2 (Stage 8 of the DCHR) is not required here, permitting the production of almost 6 million gallons per day of high quality distilled water in addition to the 50 Mlie of power without the consumption of any additional energy and with no major changes in plant design.

4. PROCESS EQUU%EENT

fn order to provide the reader with a comparison of these processes to a conventional power plant, preliminary equipment designs have been done. These designs were used by Bechtel to predict the potential cost effective- ness of the processes. Tables 2 and 3 show the estimated dimensions for major equipment for both processes. The vessels for welJhead separator, evaporator. low pressure flasher, and condenser are of rather conventfonal design_ The direct contact heat recuperator is unusual and is shown in sectional views in figure 4.

5. C~PARrSON WITH OTHER PROCESSES

In a study conducted by Bechtel Corporation , under Ll. 5. Government contract, a number of ftowsheets have been examined for the generation of power from geothermat brine. ln this section. several of the more interesting alter- natives are compared with the patented Bechtel process.

5.? SIMPLE SRINE FLASH

The steam evolved by brine flashed in the nellhead separator can be sent directly to the high pressure port of the turbine. The flashed brine can then be flashed further, down to an ultimate temperature of 215OF (lO”ioC), and this supplementary steam delivered to the intermediate pressure port of the turbine. This is a simple system comprising few pieces of equSpment. Its thermal eff9ciancy is favorable: Each pound of Niland-type brine at 55OoF (ZBloC) yields a net output of 12.0 watt-hours/lb of brine.

The principle drariback of this type of flowsheet is the exposure of the turbine and condenser to droplets of saline material carried along with

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GEOTHERMAL ENERGY RECOVERY PROCESS 333 -

TABLE 2

Overall Dimensions of Major High-Temperature Equipment

We1 1 head separator Evaporator Lcm-pressure flasher Direct contact heat recuperator -

Vessel #l - Stages #l-8. incl. Vessel t2 - Stages #9-13. incl.

Condenser

Overal 1 Length

4D Ft. 42 Ft. 32 Ft.

96 Ft. 60 Ft. 44 Ft.

Diameter

6 Ft. 12 Ft. 12 Ft.

12 Ft. 12 Ft. 9 Ft. High x

72 Ft. Wide

TABLE 3

Overal 1 Dimensions of Major Low-Temperature Equipment

Overal 1 Lenqth Diameter _ .

Wellhead separator Evaporator tow-pressure flasher Direct contact heat recuperator -

Vessel #l - Stages fl-4 Vessel $2 - Stages 85-9 Vessel 83 - Stages #9-72 Vessel 14 - Stages #13-l?

Condenser

80 Ft. 84 Ft. 64 Ft.

73 Ft. 73 Ft. 73 Ft. 92 Ft. 44 Ft.

6 Ft. 12 Ft. 12 Ft.

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19 Ft. Wide

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the steam which is generated_ Even if the steam is stripped of its partic- ula&e matter by ZL dcsrister, it will carry gaseous caRpaunds - asssonia, carbon dioxide, and hydrogen sulfide - and the volatile solids, sifica and boric acid_ Ttle gases are highly corrosive; the silica sod boric acid deposit on tmbine blades_

5.2 fLASHED BRIRE/BINARY FLUID

The turbine and condenser can be shielded from contact with brine or corrosfve vapor by exposing them only to the vapor of an organic binary fluid_ The fluid, in turn. is heated by passage through tubing bathed in the vapor of flashed geothemal brine. In one system analyzed here, the geothermal btine is flashed in four successive stages through which the organic liquid flows inside heat exchanger tubing_

The heated organic fluid is tJw?n vaporized and delivered to the turbine. frcne which Jt is discharged to the condenser_ The cooled condensate mat be reheated to Sts baiting point in the heat excbqer tubing. The tosses associated with the reheat process, wasting valuable geothermal engery, 1-r the overat energy effectiveness to 9-S watt-hours per pound of brine.

5.3 XULTISTAGE FLASH/BINARY FLULO

This cycle flashes the geothermal brine down almost to ambient temperature. The vapor thereby released. serves to reheat the organic vapor, flowing thruugh tubes heated in the flashing chambers. to its boiling point- Thus. the heat contained in the brine, discharged at about 215Of f?Ol°C). in the flowsheet of Section 5.2. is recovered a17 the way down to the final brine blowdown tcsaperature of about 1MoF (54’C)_ The energy effectiveness is improved to 11.0 watt-hours per pound of geothermal fluid, approaching the 11.7 watt-hour vatue of the patented Bechtet process.

These thermdynamic calculations are based on the use of isobutane as the binary fluid. Isobutane has been the preferred working flu-fd in a number of parer- cycle studies because of its favorable physical properties_ A further frqrrovement, however, in the efficiency of energy conversion can be achieved by adopting isopentane as the binary fiuid. The temperature of the Niland-type brine elevates the isopentane into its supercritical region, raising the averalt net energy efficfency of the system to about 15 watt- hours per pound of geothermal brine.

The chief disadvantage of either the isobutane or the isopentane cycle arises frost the need to contain these materials at etevated temperatures and pressures_ Careful attention is required to the development of suit- able seals and packings. In addition, these organics are highly flanrnable and can generate explosive vapors_

In contrast to the alternates discussed above, the patented 6echtet process circu’tates only distitted water through the power plant circuit.

386 L. r\WERBUCH ET AL

The turbine contacts only pure steam. The plant components. although large, can be fabricated in existing shops by conventional procedures. The heart of the Bechtel process, namely, the direct contact heat recup- erator, is simi!ar to coimrercial multistage flash desalination design, nsdified per the novel features of the patent. The Bechte? design yields a highly satisfactory 11.7 watt-hours per pound of geothermal brine. Finally. it is capable not only of generating power, but also producing substantial quantities of double-distilled water in the same equipment and without the .zonsumption of any additional energy.

6. ECONOMIC ANALYSIS

While the details of a cost analysis cannot be published at this time, the following general results can be considered:

a. The instaIled cost of both the high and low salinity cycle plants should be nearly equal to that of fossil fueled plants of the same capacity.

b. Power costs for both cycles arecomparable to current costs for oil and coal fired generation. As the value of these two latter fuels increases, the attractiveness of the geothermal processes will increase.

c. For the low salinity cycle, the value c: pure water produced provides a cost benefit of up to 5 miils per KW hour over fossi 1 fueled systems.

REFERENCES

1. LEON AUERBUCH AND CHARLES T. DRANEY. Geothermal Energy Recovery Process, U.S. Patent No. 3953972, May 4. 1976.

2. BECHTEL CORP. FOR ERDA, Conceptual Design of Commercial. 50 MWe (net) Geothermal Power Plants at Heber and Niland, California. July 1976.