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1lth New GeothermalWorkshop 1989

MATERIAL PROBLEMS OF GEOTHERMAL POWER PLANTS:A PHILIPPINE EXPERIENCE

Rogelio Datuin and Felicito M. Gazo

NationalPower Corporation, Republic of the

A B A

Material problems, such as scaling,corrosion, sludge deposition,microbiological fouling and algalformation, affect the equipment andpower generation o r geothermal powerplants in the Philippines. Higher operational expenses are incurred toprevent or check their occurrences, through cooling water treatment chemicals, plant rehabilitation andother preventive maintenance works.

This research study deals withprobable causes of those material prob 1 ems and researches on

to solve or lessen their occurrence. It also deals with

on lime treatment program,steam washing program and other issuesof geothermal operation, such as sludgedisposal, steam supply and quality.

INTRODUCTION

Geothermal steam is the most viableenergy source in the Philippines basedon separate studies made by the WorldBank and an Italian energy consultingfirm on the Philippine energy potential. The Philippine geothermal energy reserve, is estimated to excaed

4,431 probablereserve, MW have been tested and only 894 MW or less than have beenharnessed. This means that geothermalenergy, which has a resource potential equivalent to 2 billion barrels oil

year can more than adequately coverthe equivalent 263 million barrels oil energy requirement of thePhilippines by the year

Geo t herma 1 deve t is verypromising in the Philippines since the country happens to be situated in anarea called the circum-Pacific "Ring of

Within this "ring" are numerous dormant and active volcanoes whosesubsurfaces smolder with intensegeothermal heat. Because of theextensive vulcanism in the country and the presence of major faults traversingmost of the major islands, the presenceof thermal areas of varying magnitudes and intensities is expected, thus providing the right forpresence of geothermalyossibi it

OPERATIONS NPC POWERPLANTS : EXISTING SYSTEM

The National Power Corporation (NPC)of the Philippines produces bulkelectric power from operation of four(4) major geothermal power plants inTiwi, Mak-Ban, Leyte (Tongonan), andSouthern Negros (Palinpinon). Based onthe 1988 NPC Annual Report, theseplants contribute (orthe installed plant capacity of 5,782

in the country.

At present, NPC geothermal power plants use the open recirculating cooling water system wherein water iscontinuously

Since thecooling tower is exposed to theatmosphere, make-up water must be addedto replace the water being evaporated from the tower, aside from water lostthrough leaks and intentionally discharged (by blowdown) to maintain anacceptable level of salinity oralkalinity in the recirculating water.

The salient basic operations of NPCgeothermal power plants are described below. From a well drilled tapping

reservoir, hot water andsteam pass through an elaborate system of valves and pipes. Throughseparators, steam is separated from water. Impurities like mud are alsoseparated and discharged along with hotwater into the disposal canals. Some ofthe excess hot water are reinjected back to reservoirs. The resultingmechanical energy drives a generator, which produces electric power. Electricity is stepped up by atransformer several and then transmitted to a substation fordistribution to franchise holders(electric power retailers). From theturbine, the steam goes to thecondenser where it is blended and cooled by the cooling water and wherethe condensable gases are discharged through the ejector or gas compressorinto the atmosphere.

Condensation of steam and the actionof the ejector create a vacuum in thecondenser which draws water from the cooling tower basin through the main cooling pipeline. The condensed water is collected in the hot well from which it is pumped up to the cooling tower.

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The condensed water, which has beencooled in this tower. is collected inthe cold where the coolingwater to the the oil cooler,the generator exciter coolers andother auxiliaries, taken. Excesswater is reinjected back to reservoirs.

Inherent in t h e geothermal steam arehydrogen sulfide and carbon dioxidegases which constitute ofcondensables. The non-condensable gases

1% by weight of steam, areremoved by the gas ejector system orscrubbed the tower fan whileothers are dissolved hot water inthe cooling tower.

COMMON MATERIAL PROBLEMS OF GEOTHERMALPOWER PLANTS

Water passing through theexchanger equipment of coolingwater system contains and transportsmaterial problems. These problems aresimilar regardless of the size and typeof cooling tower used. They vary onlyin degree depending on the water used,and the type of materialsthat come into contact with the coolingwater

Material problem8 occur in all water-carrying lines and aystem componentsthat come into contact with the coolingwater. These material problems areclassified into 1) scales, 2) sludge or

deposits, 3) corrosion, 4)microbiological. growths, and 5) turbine

depositions.

Scales are coatings ofpredominantly inorganic materials dueto supersaturation of water-solubleminerals. interfere with heattransfer thereby theefficiency of heat exchangers. Scale formation depends on water temperature,alkalinity or acidity, and ofocale-forming materials in the coolingwater. Typical consist of

carbonate. calcium sulfate, calcium phosphate, silica. and

si icate

Sludge or deposits are formedby suspended solids circulatingwater and accumulated materials inheat-exchanger They are alsocaused by products and theby-products of on between

and contaminants.Sludge occur or areproduced artificially. They arepresent n the atmosphere.from in the cooling tower ororiginate from man-made throughchemical addition in the system.decrease efficiency of heatexchanger through deterioration of the equipment.

deposits occur in geothermal. power plants due to increased of poor-quality and

contaminated water, waste recycling, high water temperatures and heat transfer and system operation for longer periods between cloanings. Their formation is affected by watervelocity, temperature, equipment design, poor start-up techniques andplant operation controls. Typical.classes of sludges are lime-soda ashsoftening sludge, coagulation-softeningsludge, and alum-coagulation sludge.

Corrosion causes deterioration andperforation of equipment resulting tolost product, inefficient operation anddowntime for maintenance andreplacement, and increased production

Corrosion varies with chemicalcomposition, temperature and velocityof geothermal steam: geothermalsource, and power cycle chosen,Geothermal brines contain hydrogen ion

chloride. hydrogencarbon dioxide, ammonia, sulfate, andoxygen which produce significantcorrosive effects on metal1iccomponents of the plant. The corrosionof metals destroy costly equipment andresults in heavy deposition ofcorrosion products, considerabledowntime, and costly production losses.

The amount and rate of corrosion areaffected by:

1) Presence of dissolved whichincreases electrical conductivity ofwater through increased electronflow between metal Components andwater.

2) Higher amount of solidsresulting in higher conductivity andcorrosion rate!.

3) Acidity which increases dissolution rate of base metal and oxide film formation an the metal surfaces.

4) Water velocity which brings tothe metal, carry corrosion productsand erode metal surfaces o rprotective films.

5) of every 25" toup to which thecorrosion rates.

Microbial growth which promotesformation of corrosion and

by-products of organisms.

Microbiological growths are createdby large and microscopic organisms. such algae, fungi. slime, and bacteria. They grow within a coolingsystem, all pipes and heat-exchanger They plug water

which to metaldeterioration and cause under deposit corrosion and destruction ofmaterials of the cooling tower.Micr b 1 i f 3.ing eitin the heat anddischarge canals.

Turbine depositions are due to the build-up of hardchemical deposits as temperature

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falls with expansion, thereby reducing the output of the turbine and upsettingits dynamic balance.

failure due to resonantvibration results from blade fracturescaused by metal fatigue. This is not a specific weakness of turbines, but canbe by build-up ofchemical deposits on the blades andalteration of their natural resonant vibration frequency. Scale depositions on steam parts affect unit efficiency, capacity and The impact ofdeposits on turbine performance dependson their thickness, location andsurface A layerof deposit can substantially increase friction loss at the steam-flowboundary. If a portion of the deposit.flakes off, leaving an even rougher surface, friction loss increases andefficiency loss may greater than forthe amount of deposit distributed uniformly. Mechanical hazards ofdeposit formation are governed entirelyby the loss of unit capacity.

CHEMISTRY OF THE GEOTHERMAL ENVIRONMENT

Geothermal water exists eithercorrosive or acidic water or slightlyalkaline water which depooi tsprotective scales of metal oxides, silica or carbonate. Geothermal steam contains carbon dioxide, hydrogen sulfide, ammonia, traces of boric acidand other minor constituents, which limits the of certain constructionmaterials in geothermal power plants. Soluble gases, like carbon dioxide and

sulfide, in the steam causecorrosion of metals and alloys theseplants.

Steam is generated from geothermalhot water by steam generating equipment. separator and flasher.If these equipment are wetsteam with impurities will enter the turbine and serious troubles.When steam generating system operating under overload condition, steam with salt and iron impuritieswill also enter the turbine and causesufficient damages. Steam impurities entering the turbine cause valve sticking, erosion, corrosion. downloadwith scaling, or corrosioncracking of which areserious troubles in the plant.

Turbine manufacturers sat of steam preventcorrosion troubles in the plant. Total dissolved solids in steam should belower than 5 ppm, chlorides, andiron should bo lower thancondensable (less than byweight) and dry so not tocause trouble in t h e

OBSERVATIONS

1) Sludge and accumulationproblems are experienced in thecooling tower basins and turbine

blades, respectively, of NPCgeothermal power plants. Sludgeconsists of sulfur and iron oxidewhich are typical corrosion products of geothermal power plant operation. High concentrations of toxic metalssuch as arsenic, mercury, boron, lead, chromium, zinc, aluminum,calcium, magnesium, sodium andpotassium are also present in thesludge (see Table No. Theoccurrence of these metalsattributed to high concentration of hydrogen sulfide, increased amountof dissolved solids in the steamcondensate, acidic nature of coolingwater, and use of proprietary water

Scalestreatment chemicals.consisting of silica and iron oxide, are found in the steam pipes, hot

condenser and air coolers.

Corrosion is prevalent in NPCgeothermal power plants due to thehighly corrosive cooling water.

3) Microbiological problems rarelyoccur in the Mak-Ban and Leyte

due to biocide treatment andacidity of cooling water used whichinh b the growth ofmicroorganisms. Microbiologicalproblems, however, affect thecooling towers of the andSouthern Negros geothermal powerplants. They attack the wooden partsof the tower and usually onthe decks or louvers ofcooling that are incontact th sunlight. Temperatureand of the water are alsoideal for microbial growth. Thepresence of organic and inorganicsalts from improper additionfouling control agents or biocidesensures of nutrients andenhances the favorable environmentfor ological growth (seeTable No. 2 for algae analysis).

4) Geothermal brines from NPCgeothermal power plants containmetala, non-metals and gases

Table No. Brines have inherent corrodents which cause corrosion.Chloride. sodium, , potassium,ca ium, and otherconstituents are whenentering the plant . . Theseformation of scales.and deposits.

5) Turbine deposits consist of iron,silica, sulfur and chloride. Scalecarry-over in deposita onsteam-part5 affects efficiency, capacity, and reliability ofturbine unit . Li teraturc studiesrevealed that a deposit thickness ofonly 3 mils inch) willdecrease stage efficiency by 3 to 4per cent and reduce flow capability by 1 per cent. The immediate effectof turbine deposits are notyet detected in NPC geothermalplants, although, turbine bladefailures occurred in Tiwi in 1984and 1985 due to resonant vibration.It was opined that this trouble

Datuin and Gazo

resulted from blade fractures causedby metal fatigue and bythe build-up of chemical deposits on the blades. The turbine deposits also altered the blades’ naturalresonant vibration frequency causing breakdown and unexpected shutdowns in the plant.

SOLUTIONS TO MATERIAL PROBLEMS OF NPCPOWER PLANTS

Annually, spends huge operationalcosts for different types of watertreatment which have the sametarget results in all geothermal powerplants. Furthermore, proprietary treatment programs more expensiveand corrosive than the baseline water treatment program which causticsoda as corrosion inhibitor. Typicalcosts of water treatment usedin NPC geothermal power plants areshown in Table No. 4. Experiencesshowed that corrosion inhibitors account for the highest share of watertreatment in NPCgeothermal power plants (see Table No.

Corrosion Treatment.

Corrosion can beminimized by:

prevented or

a) Corrosion inhibitors which reduce orstop corrosion by interfering with the corrosion mechanism through formation of a thin coating with no affect on heat transfer.

the withpolyethylene coating. coaltar, fiberglass-reinforced plastic (FRP), or other anti-corrosion coating materials.

Maintenance of of cooling water(steam condensate and domesticwater) at to 7.

d) blowdown of water flowinginto the small orifice of pipeline.

e) Changing the metal

f) environment through formation of a protective film ofcalcium on the metalsurface using the natural calcium and alkalinity in the water.

Removal of corrosive oxygen from thewater by mechanical or chemical de-aeration.

The most practical and cheapest wayof control is throughformation of a protective film ofcalcium carbonate on the metal. surfaceusing the natural calcium andalkalinity in the water. A uniformcoating of calcium carbonate on the surfaces will. physicallysegregate the metal from the corrosiveenvironment. Theoretically. controlled deposition of carbonatecan a film thick enough to interfere with the corrosion mechanism,

transfer.affect heat

This is done through lime treatment,method which raises the calcium content, alkalinity. of thecooling water system at a cheaper pricethan caustic soda. Lime treatment program conducted in geothermalpower plants results

and promising duesubstantial decrease in water treatmentcosts without the operation of the plant as a whole. (see Table No.

The savings derived during the limetreatment program isof the of proprietarychemicals already inCorrosion rates during the limetreatment program are a l s o generallycomparable or than those using

treatment Projectedsavings to he generated by NPCgeothermal power planta during limetreatment program are shown in TableNo. 7.

2. Treatment of Deposit

Dispersants can be used to break upthe smaller particles and them suspended in thecooling water. They also prevent

formation and enableremoval from via blowdownand filtration.

Clarifiers and other solidseparators reduce the amounts ofsuspended debris, anddissolved minerals and organics inmake-up water, thereby mechanically removing the foulants.

Fine or sidestreamfiltration removes substantialconcentrations of foulants from theentire recirculating water loopregardless of the water source. A 1%

5% passing of the circulating water through a suitable filter canbe very effective in keeping lowersuspended solid levels Clothfilters or screens at the coolingtower air intakes can reduceairborne contamination.

can be removed by injectionsmall rubber balls into and

through heat-exchanger tubes duringplan t operation. These balls w i p ethe clean as they throughand repeat the scouring action onthe tube surfaces. Highly abrasive coated balls can be used f o r veryhard deposits but not with softalloys. The process can be performedcontinuously or at periodicintervals, as needed.

Plant systems can also use plugs orbrushes in plastic holders forscouring of steam Condenserwater flow pushes the throughthe to wipe deposits from theheat-transfer surfaces. Reversingthe water flow repeats the cleaning action.

3. Treatment

Frequent blowdown controlsmicrobiological growths, thisis not practical when expensive

are used during treatment.Alternateneeded to inhibit growth ofmicroorganisms by tering thepermeability of microbe cell interfering with vital life processes. Microbiological plate counts shouldalso be done at least twice per month to check the effectiveness of biocidesand to optimize the concentration andfrequency of feeding.

In the treatmentcalcium hypochlorite (70%chlorine), a

biocide to lime causticsoda, , is regularly during highincidence ofgrowths in the louvers and wood supports of the cooling tower. Whenevercalcium hypochlorite is sufficientto control microbiological inthe system, an alternate microbiocide is used to chlorine toprevent the existingmicroorganism.

4 . Installation of Steam Scrubbers

Sludge problems can bethrough instal.

of steam scrubbers in steam supplysystem. In the absence scrubbers,the pipelines as the scrubberitself as the steam is transported fromthe wells to the plant. In wet fields,however, separation of from wateris imperfect if the are near thegenerating plant. With smallerdistance, the scrubbing effect ofpipelines is lessened, and ofsolubles reach the turbine. A steamscrubber good and clean

of steam to tha plant, but itcannot fully eliminate thecondensable gases, ties and otherfactors ribut to sformation. Usually, the Installation ofa steam scrubber entails highinvestment cost.

5. Sludge Disposal

Toxic components cooling towersludge, A s , Fe, can beinsolubilized by mixing sludge with suitable proportion of Portland cemantto fix heavy precipitate theheavy metal hydroxides, andadsorbed into cement hydrates. Leachatetests indicate that. the optimum dosage for fixation is 1 part cement to 3parts sludge. This is based on theconcentrations of leached ascompared to pollution regulation andintegrity of cement block. A t therecommended treatment ratio ofcement cost is 200 forevery cubic meters drums) ofsludge processed.

Proper disposal of sludge is done by:

Manual removal from cooling tower

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basins into suitable suchas drums for easy to adisposal The pond has to beadequately located and designed toprevent any potential danger to thepeople, the river systems andadjacent lands for agricultural production.

Treatment with to fixhazardous pollutants present in thesludge and filling into alined pond. use concrete1iner is another safeguardto impede the exfiltration o rhazardous substances.

c) Pond to include coveringwith plastic material when not inuse; and concrete of thepond with appropriate thickness when it is already full.

Steam Washing Program

Italian Experience

Since 1959, ENEL-Italy has beensuccessfully implementing a turbinedescalinp program for lowering

rates and decreasingincidences of turbine blade breakdownsand plant shutdowns in its geothermalpower plants. These plants flush outthe aggressive steam corrodents through spaced of analkaline solution of water and causticsoda in the steam header. This is donsabout before the axial

scrubber orpreferably at the jet well head.

Experience

PGI (NPC Steam Supplier) is alsoimplementing own steam washingprogram in Tiwi and Mak-ban to preventscale build-up and corrosion in steam

This is done through spaceinjection of water downstream of the

scrubber. Initialresults show that the program ispromising, whereby savings and increased generation are being

TOPICS

1) showed that dewatered sludge consisting to 35%solids a heating value 3,000to 10,000BTU per lb. Sludge can beused for landfilling, disposed in

ocean or incinerated with high consumption and energy

recovery. Sludge can beintegrated with other solid-wastecombustion facilities, sludge digestion can recover methane gasfor in turbine and engines,arid can be combined with other waste minerals to form alternate fuels.

2) scale deposition on turbine blades, titanium shrouds with

lite should beinstalled in the stapes of turbineblades that are usually affected.

Datuin and Gaao

3) In Japan, sludge from geothermal power plants are synthesized intozeolite under mild hydrothermal conditions, through addition ofaluminum ions and/or using differenttypes of sludge. Sludge areconverted into zeolites by changingthe molar ratios of startingmaterials and using proper organicammonium ions. The prepared zeolites are used for petroleum purification (Sato, et. al,

REFERENCES

Sheldon , "Cooling-WaterTreatment for Control of Scaling,Fouling, and Corrosion" PowerMagazine, s. 1 - S24

1988 National Power Corporation AnnualReport

Gazo, Felicito M., Lime Treatment Program: An Alternate Cooling Water Treatment Program for Geothermal Power Plants in the Philippines NPCOccasi ona 1 Paper

Gazo, Felicito M. and Perez, CordeliaM . , Technical Study on Prevention andSolution t o Material Problems of NPCGeothermal Power Plants NPCUnpublished Paper

Shair, Salem "Selecting Biological Fouling Inhibitors for Cooling WaterSystems" Plant Engineering Magazine

Sato, T., Kurita, et. al., "Preparationof ZSM-4 and -5 Zaolites by UsingSludge Obtained from Geothermal PowerPlant" Chemical Letters, TheChemical Society of Japan, pp. 123-126

Reeber, Robert "Coatings inGeothermal Energy Production"Thin Solid F i l m s , 72, 33-47

T. and Hraithwaite,"Corrosion Control in GeothermalSystems" ( Energy151-160

Allegrini Benvenuti, G.Corrosion Characteristics and

Geothermal Power Plant Protection (Collateral) Processes of Abrasion,Erosion and Scaling"

Special Issue, PartI,

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Table No. 5 Typical Costs of Water Treatrent Chericals Used i nNPC Geotherral Power Annual Costs)

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No. 6 Results of Trialand Previous

i n

P L A N T: PARTICULARS LEYTE :: Treatrent Period :

3 A: .:: Treatrent Costs

: L i reI Proprietary: Chericals

54 72.13 :

: Corrosion Rate (N i l s per year) .: L i r e 20.88 21.51 39.7 :: Proprietary

: Chemicals 52.50 37.82 15.6 :: INCREASE

(DECREASE] 24.1 :(60.23) (43.15) 154.5

Capacity 37.5 37.5 55

SAVINGS

I

flak-Ban :or :

: Leytr ,013 ,002or 84,622 :

: Negros ,001or 94,122

:

ANNUAL SAVINGS OF NPC (WITHOUT 1.74 .