Process for improving heat recovery from geothermal water

Abstract

An improved process for the recovery of heat from geothermal water by adding calcium hydroxide in a controlled manner to transform silica dissolved in the water into suspended calcium silicate hydroxide thereby reducing silica deposition and enabling more heat to be recovered from the water.

Description

[0001] PROCESS FOR IMPROVING HEAT RECOVERY FROM GEOTHERMAL WATER

[0002] TECHNICAL FIELD

[0003] The invention relates to an improved process for the recovery of heat from geothermal water. In particular, the invention relates to the treatment of geothermal water with calcium hydroxide in a controlled manner to transform silica dissolved in the water into suspended calcium silicate hydroxide thereby reducing silica deposition which enables more heat to be recovered from the geothermal water and more electricity generated than would otherwise be possible.

[0004] BACKGROUND OF THE INVENTION

[0005] Geothermal energy offers an attractive renewable resource for base-load electricity generation. However, the conversion efficiency of heat in underground superheated water in a geothermal resource into electricity is only about 12-16%. This is essentially due to the thermodynamic constraints associated with the steam / water flash and the conversion of heat energy into work through Carnot cycle turbines. The separated geothermal water (brine) emanating from the steam / water separation stage following the steam flashing process, represents some two thirds of the mass flow from the reservoir. This very high volume and mass flow of water stream, typically 200 to 3000 tonnes per hour, which is at a temperature of about 120-150 °C depending on the flash pressure, is a significant source of lower grade heat energy. The water is usually supersaturated in silica. Some of this heat energy is captured in the heat exchangers of downstream binary cycle plants and converted to electrical energy. The binary cycle heat exchange exit temperatures are typically about 110-100 °C, but are often higher depending on the level of problematic dissolved silica in the water, particularly for high enthalpy fields. This cooler water, now further supersaturated in silica, is then ideally reinjected to recharge the reservoir and also to return other potentially problematic dissolved entities, such as arsenic and boron, to the reservoir to avoid them entering the environment.

[0006] The precipitation of silica from separated geothermal brine that is supersaturated in dissolved silica, and the consequent formation of a silica scale deposit in pipes, heat exchangers and reinjection wells, is a major problem in geothermal resource utilisation worldwide. This scale severely compromises the heat energy recoverable in binary cycle electricity generation and increases equipment and field maintenance. Silica precipitation typically prevents the recovery of heat energy from geothermal water below about 150 to 100 °C, depending on the level of silica supersaturation, for industrial and consumer direct heating applications.

[0007] Previous attempts to address this problem include operating the steam / water flash at a high temperature to keep a high separated brine temperature of about 130 to 150 °C or greater, thereby reducing the extent of silica supersaturation and the propensity for initiating and continuing silica deposition. However, this also reduces the amount of steam produced in the flash and hence reduces the amount of electricity generated from the steam phase.

[0008] Another approach is to retard the silica polymerisation process by reducing the pH of the water to about pH 5 or lower to increase the length of time for the induction period, which is a precursor to silica polymerisation, deposition and scale formation. However, low pH can cause corrosion of steel pipework and other plant equipment. Furthermore, any subsequent reduction in temperature of the separated water, such as necessarily occurs across a binary cycle or industrial heat exchanger for direct heating applications, readily facilitates additional silica deposition and silica scaling inside the heat exchanger tubes. This silica scale compromises the efficiency of heat transfer across the heat exchange surfaces and reduces the amount of heat energy that can be exchanged to the binary cycle working fluid and therefore the amount of electricity generated and heat recoverable for direct heating.

[0009] Silica deposition and scaling necessitates regular shutdowns and cleaning of the intractable silica deposits from heat exchanger surfaces. The silica also progressively precipitates in reinjection wells and the local geologic substructure, which over time compromises the reinjection process and can lead to the need to ream out the well, carry out environmentally undesirable down well chemical cleaning, or possibly drill a new well.

[0010] The formation of calcium silicate hydrate from sodium silicate solutions with high concentrations of dissolved silica entities, by the addition of calcium hydroxide in a Ca:Si mol: mol ratio of about 1 : 1 or higher, together with a necessary reduction in pH by the addition of acid to the calcium hydroxide, has been reported by Johnston et al. (NZ 537747). Also, it has been reported that calcium silicate hydrate with a Ca:Si mol: mol ratio of about 1: 1 has been formed from geothermal water by the addition of Ca2+and OH' ions in a Ca:Si mol: mol ratio of about 1: 1 (see for example, Johnston et al., GRC Bulletin, 2018, 1240-1250). While the use of calcium ions and hydroxide ions for this purpose has shown merit, greater effectiveness and efficiencies are needed to prevent silica deposition in geothermal water and consequent silica scaling as the water is cooled for heat recovery and electricity generation.

[0011] As part of their further investigations, the inventors have now found that significant advantages can be achieved by controlling the form and amount of calcium hydroxide used. The inventors have found that by further lowering the silica saturation index and reducing the Ca2+demand through reducing the Ca:Si mol:mol ratio to about 0.4-0.8, process improvements can be achieved.

[0012] It is therefore an object of the invention to provide a method for improving heat recovery and electricity generation from geothermal water, or to at least provide a useful alternative to existing methods. SUMMARY OF THE INVENTION

[0013] In a first aspect of the invention there is provided a process for recovering heat from geothermal water comprising:

[0014] (i) treating a stream of geothermal water containing dissolved silica by adding particulate calcium hydroxide, where the amount of calcium hydroxide added gives a Ca:Si mol: mol ratio below 1, to transform at least some of the dissolved silica into calcium silicate hydroxide hydrate particles suspended in the stream, and to simultaneously increase the pH of the stream to at least 8; and

[0015] (ii) passing the stream through a heat exchanger to recover heat energy from the stream.

[0016] In certain embodiments of the invention, the amount of calcium hydroxide added in step (i) is sufficient to provide a stoichiometric excess of hydroxide ions relative to the hydroxide content of the calcium silicate hydroxide hydrate.

[0017] In certain embodiments of the invention, the Ca:Si mol:mol ratio is in the range 0.3 to 1.0, preferably in the range 0.4 to 0.6.

[0018] In certain embodiments of the invention, the pH of the stream is increased from about 6.5 to 8.5 to about 9 to 12 on treatment with the calcium hydroxide.

[0019] In certain embodiments of the invention, the silica saturation index of the stream is reduced to less than 1 on treatment with the calcium hydroxide.

[0020] In certain embodiments of the invention, the silica is in the form of HaSiC ' or H4SiO4, or a combination thereof.

[0021] In certain embodiments of the invention, the calcium hydroxide is in the form of a suspension or slurry in water, preferably in a concentration of up to about 15 wt %.

[0022] In certain embodiments of the invention, the heat exchanger has an inlet stream temperature in the range 100-200 °C, typically 120-150 °C, and an outlet stream temperature less than 80 °C, more preferably less than 50 °C.

[0023] In certain embodiments of the invention, the process further includes generating electricity from the heat recovered from the heat exchanger. Preferably, the heat exchanger is a binary cycle heat exchanger.

[0024] In certain embodiments of the invention, the process further includes recovering heat energy from the water for direct heating applications.

[0025] In certain embodiments of the invention, the process further includes reinjecting the treated stream into ground.

[0026] In certain embodiments of the invention, the amount of arsenic in the calcium silicate hydroxide hydrate is less than 20 mg kg-1.

[0027] In certain embodiments of the invention, the calcium silicate hydroxide hydrate particles flow through plant equipment as a suspension in the geothermal water without deposition of calcium silicate hydroxide hydrate or of silica on surfaces of the plant equipment. In certain embodiments of the invention, the calcium silicate hydroxide hydrate particles are separated continuously from the geothermal water.

[0028] BRIEF DESCRIPTION OF THE FIGURES

[0029] Figure 1 is a schematic diagram of a conventional geothermal process for generating electricity from the heat energy of a geothermal resource.

[0030] Figure 2 is a schematic diagram showing integration of the process of the invention involving injection of calcium hydroxide into a geothermal water flow and the separation of calcium silicate hydroxide hydrate particles following the binary cycle plant.

[0031] Figure 3 shows the rapid reduction of dissolved silica (SiCh) content (open circles) and the consequent rapid increase in pH (crosses) of the water, following calcium hydroxide addition and the consequent formation of calcium silicate hydroxide hydrate.

[0032] Figure 4 shows the increase in pH of geothermal water following calcium hydroxide addition as a function of the Ca:Si ratio.

[0033] Figure 5 shows the consequent rapid reduction in the Silica Saturation Index SSI in the treated geothermal water with time, due to the corresponding rapid decrease in dissolved silica (SiC ) content and the rapid increase in pH following calcium hydroxide addition and calcium silicate hydroxide hydrate formation. The horizontal dashed line represents a value of SSI=1, above which silica scaling occurs. The Ca:Si ratio is 0.8. The data are from laboratory results using synthetic geothermal water with a SiC concentration of 1000 mg kg-1SiO2 and a starting pH=8.5.

[0034] Figure 6 shows the residual silica content of the geothermal water after addition of calcium hydroxide particulates and the formation of calcium silicate hydroxide hydrate.

[0035] Figure 7 shows the Silica Saturation Index SSI values for the pilot plant operation at the Wairakei geothermal resource over six hours. The silica content of the incoming geothermal water (raw brine) to the pilot plant and the geothermal water exiting the pilot plant following the addition of particulate Ca(OH)2 slurry and the formation of calcium silicate hydroxide hydrate (after CaSil process) were measured at about 15 minute intervals over this six hours of continuous operation. From these measurements, the SSI was determined for both geothermal water streams and the SSI values plotted against time. The dotted line is where the SSI=1, above which silica scaling occurs.

[0036] Figure 8 shows the Silica Saturation Index SSI and the Silica Saturation Temperature SST, values for Wairakei, Kawerau and Mokai geothermal water (brine) at reinjection particular temperature of 85 °C for comparison purposes. Figure 8 Left shows the SSI values for the incoming geothermal water and the exiting geothermal water following the addition of particulate Ca(OH)2 slurry and the formation of calcium silicate hydroxide hydrate for each of the three resources. The solid horizontal line is where the SSI=1, above which silica scaling occurs. Figure 8 Right shows the SST values at which silica scaling will take place for the incoming and exiting geothermal waters, for these three geothermal resources respectively.

[0037] Figure 9 shows the dependence of the arsenic content in calcium silicate hydroxide hydrate on the Ca:Si ratio used in the formation of calcium silicate hydroxide hydrate from synthetic geothermal water at a laboratory scale, and from pilot plant operation using Wairakei geothermal water.

[0038] Figure 10 shows the arsenic content in ppm (mg kg-1) of calcium silicate hydroxide hydrate material produced from synthetic and natural geothermal water with dissolved silica contents of 500 and 1000 mg kg-1SiC , and also the effect on the arsenic content of the material of the addition of NaOH to the water to provide further hydroxide ions during the calcium silicate hydroxide hydrate formation.

[0039] DETAILED DESCRIPTION

[0040] Definitions

[0041] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which the inventions belong. Although any assays, methods, devices and materials similar or equivalent to those described herein can be used in the practice or testing of the invention, various assays, methods, devices and materials are now described.

[0042] It is intended that reference to a range of numbers disclosed herein (for example 1 to 10) also incorporates reference to all related numbers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example 2 to 8, 1.5 to 5.5 and 3.1 to 4.7) and, therefore, all sub-ranges of all ranges expressly disclosed herein are expressly disclosed. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner.

[0043] As used in this specification, the words "comprises", "comprising", and similar words, are not to be interpreted in an exclusive or exhaustive sense. In other words, they are intended to mean "including, but not limited to".

[0044] Any reference to prior art documents in this specification is not to be considered an admission that such prior art is widely known or forms part of the common general knowledge in the field.

[0045] The term "geothermal water" means water that is heated or superheated within the earth by natural phenomena to a temperature ranging from about 30 °C to about 350 °C, and includes all steam, water, and water vapour, and any mixture of any of them that has been heated by a natural phenomenon. The term "stoichiometric excess" means an excess amount of one or more reactants in relation to the amount required in a chemical reaction to form a particular chemical compound.

[0046] The term "silica" means SiC in all its amorphous, crystalline and hydrated solid forms.

[0047] The term "dissolved silica" means all Si-containing species dissolved in water, including but not limited to, silicic acid H4SiO4, silicate anions H3SiO4', H2SiO42' and HSiC ', and polymeric forms thereof. It will be understood that "dissolved silica" is an industry-standard term referring to the above Si-containing species and does not refer to solid SiC which is insoluble in water.

[0048] The term "calcium silicate hydroxide hydrate" means any species having the formula CaxSiOy(OH)z.wH2O. Such species may include minor or trace amounts of adsorbed or chemically bound anions and cations typically present in geothermal water, for example AsCh' , HS-, HSb2S4-, H3BO3, B(0H)4, Mg2+, Na+, K+, and Li+.

[0049] The term "silica saturation index" or "SSI" means the ratio of the concentration of silica in solution at a supersaturated level to the solubility of amorphous silica under the same conditions (particularly temperature and pH).

[0050] The invention

[0051] The invention relates to the treatment of geothermal water with calcium hydroxide in a controlled manner to provide calcium ions and sufficient hydroxide ions at a low Ca:Si mol: mol ratio to react with dissolved silica species in geothermal water to form calcium silicate hydroxide hydrate particulates. The excess hydroxide ions simultaneously increase the pH of the geothermal water thereby reducing the SSI to SSI less than or equal to 1 and avoids silica deposition taking place when the water is cooled down to about 40 °C or lower. The ability to reduce the temperature of the water from, for example, 150 °C to 40 °C or lower enables substantially more heat energy to be recovered from the geothermal water and also more electricity to be generated than would otherwise be possible.

[0052] The calcium silicate hydroxide hydrate has a variable stoichiometry with regard to the content of Ca2+, Si4+, O2', and OH' ions and H2O molecules in the structure. The stoichiometry of the calcium silicate hydroxide hydrate material formed depends on the reaction chemistry, the amount of each of the Ca2+, Si4+, O2', and OH' ions and the requirement for charge neutrality. This is governed by the amount of calcium hydroxide added in relation to the dissolved silica in the geothermal water, as specified by the Ca:Si mol:mol ratio used. The amount of hydroxide ions that is required to form a particular calcium silicate hydroxide hydrate at Ca:Si mol:mol ratios of 0.4-0.8 is less than the amount provided by the calcium hydroxide with the excess hydroxide ions serving to simultaneously increase the pH of the treated geothermal water. Collectively, the reduction in dissolved silica in the treated geothermal water and the associated increase in pH, reduces the SSI to substantially lower values than are otherwise achievable. This enables the geothermal water to be cooled to much lower temperatures, down to about 40 °C or lower and therefore substantially more heat energy to be recovered and electricity to be generated. This is not currently possible using existing industry practices.

[0053] The reduced Ca:Si mokrnol ratio also minimises the unwanted precipitation of calcium carbonate from geothermal water.

[0054] The invention provides a process for recovering heat from geothermal water comprising:

[0055] (i) treating a stream of geothermal water containing dissolved silica by adding particulate calcium hydroxide, where the amount of calcium hydroxide added gives a Ca:Si mol: mol ratio below 1, to transform at least some of the dissolved silica into calcium silicate hydroxide hydrate particles suspended in the stream and to simultaneously increase the pH of the stream to at least 8; and

[0056] (ii) passing the stream through a heat exchanger to recover heat energy from the stream.

[0057] Conventional geothermal process

[0058] Geothermal hot water dissolves mineral components from the subsurface rocks and is generally considered as a dilute brine supersaturated in dissolved silica. The main chemical species and their respective compositions for geothermal waters in New Zealand resources are shown in Table 1. Note, the values presented in Table 1 are typical values that can vary depending on natural changes in the geothermal resource and the geothermal water compositions over time, and the specific wells that were sampled. The composition of geothermal water in New Zealand is similar to geothermal waters internationally.

[0059] Table 1: The major chemical species and their respective concentrations in mg kg-1(parts per million by weight) typically present in geothermal waters for various New Zealand geothermal resources.

[0060] A conventional geothermal process is shown in Figure 1. Superheated geothermal water at high pressure in the geothermal reservoir (1), typically about 1-4 km below ground level (2), is piped (3) to the steam / water separator (4) where the pressure is reduced and a portion of the high temperature water flow is flashed into steam. This high pressure steam (5) drives a turbine and generator to produce electricity (6). The condensed steam (7) from the turbine is piped (8) into the cooler separated water flow (11). The resulting separated water flow (9) from the steam / water separator is supersaturated in dissolved silica and is piped to the binary cycle electricity generation unit (10). The resulting cooler separated water flow (11), together with the steam condensate flow (8), is reinjected through a well (13) into the outer cooler area of the geothermal reservoir (14). The dashed line shows silica precipitation and silica scale formation (12) from the separated water which is supersaturated in silica, into the pipework (9, 11), the binary cycle plant heat exchangers (10) and the reinjection well (13).

[0061] Geothermal process of the invention

[0062] Figure 2 shows a modified geothermal process integrating the process of the present invention. The processes labelled (1) to (8) are the same as for the conventional process shown in Figure 1. Integration of the process of the invention involves the continuous addition of calcium hydroxide (15) to the hot separated water preferably close to the steam / water separation stage (4). The calcium silicate hydroxide hydrate particles form very rapidly and are carried by the geothermal water flow (12) through the pipework (9) and the binary cycle heat exchanger (10), and are continuously separated as the calcium silicate hydroxide hydrate product (16).

[0063] The cooler treated geothermal water flow is combined with the steam condensate (8) and reinjected (13) into the reservoir (14). Silica precipitation and silica scale formation in (9), (10), (11), (12), (13), (14) are prevented. Flash steam plants

[0064] Geothermal water at a temperature between about 140 °C and 370 °C and under pressure is piped to the surface up through a production well. About one third of the mass flow of the water is flashed into steam under a pressure drop, driving a turbine which generates electricity. As the steam cools it condenses back into water and is returned to the reservoir. This process involves a Carnot cycle engine where heat energy is converted to work. The efficiency depends on the temperature differential as the steam is condensed across the turbine. This imposes a significant thermodynamic limit on the amount of electricity that can be generated (typically about 40-45%) excluding heat losses. The overall efficiency for electricity generation from the steam phase from the superheated geothermal water in the reservoir is about 12-15%. In practice, overall efficiency is lower due to losses in the system.

[0065] Binary cycle plants

[0066] Hot geothermal water following steam / water separation (about two thirds of the mass flow from the wells) is usually reinjected back into the reservoir, but can instead be directly obtained from a geothermal well and used to generate further electricity before being reinjected. The separated hot water, typically at a temperature of 130-140 °C, is passed through a heat exchanger where its heat is transferred to a liquid (such as iso-butene, pentane, iso-pentane, ammonia, water, or mixtures thereof) that boils at a lower temperature than geothermal water. When that liquid is heated it turns to a high pressure vapour, which drives turbines for electricity generation.

[0067] The extent to which the separated hot water can be cooled and hence the amount of heat energy that can be extracted and electricity generated, is severely limited by silica deposition and silica scale formation which occurs when the dissolved silica in the water is above the equilibrium saturation level, i.e. immediately after the steam / water flash process. Typically cooling to only about 100 °C is possible, or higher, depending on the level of supersaturated silica in the water. However, this results in a silica saturation index SSI>1 where problematic silica deposition and scale formation takes place in the binary plant heat exchangers, pipes and reinjection wells. This requires regular shut downs, costly and aggressive mechanical and chemical cleaning and maintenance, and the costly drilling of new reinjection wells. The restricted cooling range and the thermodynamic constraints of the Carnot (binary) cycle severely limit the amount of heat energy that can be extracted from this separated geothermal water flow and the amount of electricity generated by the binary plant accordingly. It is important to keep the SSI equal to or less than 1 to prevent silica deposition. The binary cycle plant typically increases the overall efficiently of geothermal electricity generation by about 5-7% bringing the total efficiency to 17-20%. In contrast to conventional binary cycle plants, the present invention enables further cooling of the separated hot water to about 60-80 °C thereby enabling more electricity generated by binary cycle technology, or to about 30 °C using Kalina cycle technology, by keeping the SSI equal to or less than 1 at these cooler temperatures. This is not achievable with current geothermal resource utilisation practices for geothermal water supersaturated in silica. However, this is achieved according to the invention by eliminating silica deposition through the rapid capture of dissolved silica and the formation of calcium silicate hydroxide hydrate thus removing silica from the water, together with a simultaneous increase in pH. The calcium silicate hydroxide hydrate formed is typically in particulate form, but desirably the particles do not bind to themselves, and most importantly they do not deposit as a scale on the surfaces of plant pipework and equipment or reinjection wells.

[0068] Direct heat use

[0069] Direct heat applications involve using geothermal heat directly (without a heat pump or electricity generation plant) for such purposes as heating buildings, industrial processes, domestic heating, greenhouses, aquaculture, public baths and pools. The efficiency in recovering and using the available heat energy in a separated geothermal water flow through heat exchangers in direct heating applications is about 90% or greater, which is substantially more than the 12-20% that can currently be harnessed from the resource for electricity generation. Direct heating applications can in principle use high and moderate to low (as low as 30 °C, but in practice this may be limited to about 40-60 °C) temperature geothermal waters. However, silica deposition compromises the use of geothermal water that is cooled below about 150 °C and severely restricts or prevents its use completely in cooling the water below about 100 °C. This means that the considerable amount of heat energy that is potentially available in separated geothermal water cannot be harnessed and is essentially wasted when reinjected back to the reservoir. Since the present invention enables cooling of separated hot water to about 20-30 °C without silica deposition because the SSI is reduced and maintained at equal or less than SSI=1 at these lower temperatures, substantially more heat energy (up to several times more) can be extracted for direct heating applications from the same water flow.

[0070] Reinjection

[0071] Reinjection of the cooler (spent) geothermal water is necessary to replenish geothermal reservoirs. Otherwise, reservoirs decline significantly. If reservoir levels drop too much due to the extraction of water and steam, underground cavities can form that may collapse leading to sink holes potentially posing severe risks to infrastructure and safety. Reinjection is also needed to dispose of unwanted toxic gases as well as the greenhouse gas carbon dioxide, and other undesirable species dissolved and suspended in the brine. The deposition of silica in rock structures and in reinjection wells, especially perforated well pipes, is a major problem that necessities costly and aggressive chemical and mechanical cleaning and the drilling of new reinjection wells.

[0072] The present invention avoids silica deposition in reinjection well pipework and reinjection wells. Since the invention enables water to be reinjected into reservoirs at much cooler temperatures (down to about 30 °C), polymer-based piping (suitably pressure-rated) can be used to pipe the water to reinjection wells.

[0073] Silica deposition from geothermal water

[0074] One of the limitations of geothermal water usage is the damage caused by the deposition of minerals, notably silica, dissolved in the underground geothermal water reservoir. Geothermal water sourced from an underground rock reservoir contains dissolved chemicals leached from the minerals in the rocks that it contacts underground. Certain species, such as silica, dissolve to their saturation solubility at the hot underground temperatures. When the hot geothermal water is brought to the surface and cooled in the steam / water separation stage of a binary cycle heat exchanger, the water becomes supersaturated in dissolved silica and hence there is a strong driving force for the silica to precipitate as heat energy is recovered from the hot water and the water cools. The precipitated silica forms an intractable sinter or scale deposit on the surfaces of the machinery and pipes in the geothermal plant.

[0075] This scale can cause significant damage by blocking valves, narrowing of pipes and damaging effects on other sensitive equipment such as turbine blades in electricity generation. Scale has a thermal insulating effect causing efficiency decreases in the transfer of heat from the geothermal water in heat exchangers of a binary cycle plant and for direct use applications. Silica scaling can completely block pipes, especially perforated pipes used in reinjection wells, and can build up on rock structures in reinjection wells requiring the drilling of new reinjection wells at significant cost. Removal of scale is intensive and expensive. Pipes and equipment are often discarded as cleaning is too costly. Reinjection wells often have to be treated with large quantities of hydrofluoric acid and other acids or alkalis to remove scale, or new wells have to be drilled. Furthermore, scale removal often requires plant shutdowns, thereby significantly reducing the efficiency of geothermal plants and associated revenue generation.

[0076] There are two commonly used solutions for dealing with the problem of scale build up. The first is to keep the water in the steam / water separator at a suitably high temperature, typically at least 120-160 °C. At these temperatures, scale is less likely to form or forms more slowly. However, at a higher temperature, plants do not run as efficiently as is theoretically possible. In binary cycle plants, the temperature of the water exiting heat exchangers must also be kept high, and consequently the temperature gradient across the exchangers is reduced in order to minimise deposition in the heat exchangers, pipe work and re-injection wells. This severely limits the amount of heat energy that theoretically can be recovered and electricity generated from the geothermal water. Scale deposition not only restricts the flow of geothermal water through the heat exchanger, it also reduces the heat flow across the heat exchanger surfaces, thereby further reducing the thermal energy which can be used to generate electricity. Direct heat use applications can be completely impeded due to the need to retain a high reinjection temperature.

[0077] The other commonly used way to reduce scale formation is to add acid or sequestering agents to the water as it flows through the geothermal plant. The lowering of pH levels by acid addition slows the reaction in which silica precipitates and the scale is formed, hopefully long enough for the water to be reinjected into the ground before any silica scale forms in large amounts. However, the use of acids increases the likelihood of corrosion in steel pipe work, heat exchangers and process equipment. In practice silica deposition and scaling still occur. To protect the turbines in a steam plant, the steam is scrubbed by spraying cold water to remove acid droplets or gas. The introduction of cold water into the steam significantly reduces the effectiveness of the geothermal energy generated by a turbine. Sequestering agents are often costly. Both the use of sequestering agents and acids only delay scale formation. Scale still forms.

[0078] Addition of calcium hydroxide

[0079] The addition of calcium hydroxide as a particulate slurry in water provides 1 mole of calcium Ca2+ions and 2 moles of OH' hydroxide ions per mole of calcium hydroxide. When mixed into a geothermal water flow, these ions react rapidly with dissolved silica entities, principally HaSiC ', before they are able to polymerise, precipitate and form a silica scale. A non-stoichiometric calcium silicate hydroxide hydrate is produced as particulates which do not stick together or adhere to metal surfaces and deposit as a scale in pipes, equipment and heat exchangers. The calcium silicate hydroxide hydrate particulates flow through pipework and heat exchangers with the geothermal water as a dilute suspension without settling out or adhering to the metal surfaces. The calcium silicate hydroxide hydrate can be separated as a product having a number of beneficial applications.

[0080] The 2 moles of OH' ions relative to the 1 mole of Ca2+ions for a Ca:Si ratio of less than 1 are in excess of the amount required for the calcium silicate hydroxide hydrate product to form. The pH of the treated water therefore increases accordingly.

[0081] The calcium hydroxide is preferably added as a suspension or a slurry of calcium hydroxide particles in water, at slurry concentrations up to about 15 wt % solids. It is important for the slurry to be a free flowing suspension for a continuous process. The addition of the suspension or slurry and the formation of calcium silicate hydroxide hydrate can be carried out in water temperatures up to 100 °C at atmospheric pressure and above 100 °C and up to about 230 °C under elevated pressure conditions.

[0082] The reaction kinetics are predominately first order. The majority of the H3SiO4' ions are captured and react to form calcium silicate hydroxide hydrate within the first 10 seconds. The reaction proceeds essentially to completion over about 5 minutes.

[0083] The reaction front of the calcium silicate hydroxide hydrate formation on the calcium hydroxide particle surface progresses through the calcium hydroxide particle to effect complete conversion of the calcium hydroxide particle into a calcium silicate hydroxide hydrate particle. This progressively and rapidly removes the dissolved silica entities from the geothermal water as calcium hydroxide dissolves and at the same time increases the pH of the geothermal water to collectively prevent silica deposition. The 2: 1 OH':Ca2+mol : mol ratio to supply the Ca2+and OH' ions for the formation of calcium silicate hydroxide hydrate provides an excess concentration of OH' ions relative to Ca2+ions required for the calcium silicate hydroxide hydrate formation reaction and product. The excess of OH' ions results in a simultaneous increase of the pH (alkalinity) of the geothermal water, typically to about pH 9.5 or higher.

[0084] The particles of calcium hydroxide act as nucleation centres for the formation of calcium silicate hydroxide hydrate by providing a localised high concentration of calcium and hydroxide ions to facilitate the rapid formation of calcium silicate hydroxide hydrate on the surface and through the calcium hydroxide particles.

[0085] The calcium hydroxide particulate slurry can be added to the geothermal water at any point following steam / water separation.

[0086] The Ca2+ions can also be provided by a soluble calcium salt such as calcium chloride and the hydroxide ions can be provided by the addition of a soluble metal hydroxide, for example sodium hydroxide. The addition of Ca2+ions independently, can be used to adjust the Ca :Si ratio if required. However, this approach does not provide nucleation centres for the calcium silicate hydroxide hydrate particles.

[0087] The calcium silicate hydroxide hydrate formed in this process typically has an average particle size in the range of 0.1 to 100 microns. The particles can loosely agglomerate to form entities of several millimetre in size. However, there is no chemical bonding between the particles and a scale deposit cannot form.

[0088] The stoichiometric composition of the calcium silicate hydroxide hydrate material formed depends on the Ca :Si ratio which is determined by the amount of calcium hydroxide added to the dissolved silica-containing geothermal water, and can be represented by the general reaction :

[0089] H3SiO4' + xCa2++ 2xOH~ - CaxSiOy(OH)z.wH2O + OH’i -z) The value of x is determined by the desired Ca:Si ratio used in the reaction, i.e. the amount of Ca(OH)2 used relative to the dissolved Si (silica) content of the geothermal water, which also determines the amount of hydroxide ions added. The oxide and hydroxide contents of the calcium silicate hydroxide hydrate are in turn determined by the structural arrangement, chemical bonding and electroneutrality of the particular calcium silicate hydroxide hydrate formed. The water is both hydrogen-bonded to the surface of the nanoplates in the calcium silicate hydroxide hydrate nanostructure and is also present in the pores formed by the nanoplates within the nanostructure.

[0090] The use of a stoichiometric excess of hydroxide ions when adding Ca(OH)2 to the geothermal water simultaneously increases pH and reduces the SSI of the treated water. This enables the removal of the dissolved silica species, predominately HaSiC ', before they can polymerise and deposit silica scale. Unreacted silica remains in solution and no longer has any chemical driving force to polymerise and deposit silica scale.

[0091] The rapid reaction of dissolved silica entities and consequent reduction in dissolved silica content due to the formation of calcium silicate hydroxide hydrate, together with the rapid increase in pH of the water, upon the addition of calcium hydroxide are shown in Figure 3.

[0092] The dissolved silica content in the water decreases rapidly by about 70% over about 5-10 seconds and by about 90% over 1 minute from the initial silica content. After about 5 minutes, the reaction is essentially complete. This rapid decrease is due to the high concentration of Ca2+and OH' ions surrounding the rapidly dissolving calcium hydroxide particles. The formation of the calcium silicate hydroxide hydrate particles is similarly rapid (Figure 3).

[0093] The pH of the water following calcium hydroxide addition similarly increases rapidly over about 10 seconds from about pH = 8.5 to pH = 10.7, up to about pH = 11.4 after 1 minute, and about pH = 11.7 after 5 minutes (Figure 3).

[0094] The rate of decrease in dissolved silica in the water correlates well with the rate of increase in the pH of the water thereby demonstrating the close chemical linkage between these two effects that result specifically from the use of calcium hydroxide in preventing silica deposition (Figure 3).

[0095] The rate of removal of silica (HsSiOT) from the geothermal water is however slightly faster than the rate of increase in pH because the OH- ions are weakly hydrogen bonded to the calcium silicate hydroxide hydrate particle surface.

[0096] The extent of the increase in the pH depends on the Ca:Si mol: mol ratio. With increasing Ca:Si ratio the final pH of the geothermal water following calcium hydroxide addition increases accordingly. The rapid rate of increase in pH following calcium hydroxide addition is essentially the same for each Ca:Si ratio, as shown by the same shape profile of the respective pH curves. This is shown for Ca:Si mokmol ratios of 0.2, 0.3, 0.4, 0.6 and 0.8 from the pilot plant operation using geothermal water from the Wairakei resource in New Zealand (Figure 4).

[0097] Silica saturation index (SSI)

[0098] Silica saturation index (SSI) is the ratio of the concentration of silica in solution at a supersaturated level to the solubility of amorphous silica under the same conditions.

[0099] The SSI is dependent on temperature and pH of the silica-containing solution. A silica- containing solution, for example geothermal water, containing silica dissolved to a supersaturated level where SSI >1, at a particular temperature and pH, will precipitate silica until sufficient silica is removed until SSI=1. Silica does not precipitate from a silica- containing water with SSI<1. As the pH of a silica-containing solution is increased for a particular temperature, the silica solubility increases. Hence, for a particular silica-containing water where SSI>1 due to cooling the water and hence where the silica would normally precipitate, such precipitation can be prevented by the simultaneous increase in pH to a level where the SSI becomes less than or equal to 1.

[0100] For SSI=1, the dissolved silica content in geothermal water is the same as for amorphous silica solubility and the system is in chemical equilibrium. For values of SSI>1, dissolved silica will polymerise and precipitate from the geothermal water to form silica scale. For values of SSI<1, the dissolved silica will remain in solution in the geothermal water and silica scale is not formed.

[0101] The temperature-dependent solubility of amorphous silica at saturated vapour pressure of water must first be calculated using formula (1):

[0102] -731 (1) l°gC = — ^— + 4.52 where C is the solubility of amorphous silica (mg kg-1) and T is temperature (K).

[0103] An equilibrium between silicic acid (H4SiO4) and silicilate ions (H3SiO4‘) occurs when silica dissolves in water. The equilibrium is dependent on the pH of the solution which gives silicic acid the characteristics of a weak acid. The dissociation constant for this behaviour is calculated according to formula (2) : log Ki = — -2-5-4915.36 * 10“6* T2(2) where Ki is the dissociation constant.

[0104] The solubility of amorphous silica calculated using formula (1) must be adjusted for temperature and pH using formula (3) : where S is the adjusted silica solubility (mg kg'1), pH is the pH of water, Ki is the first dissociation constant for H4SiO4 in equilibrium with HsSiC ', and YH3sio is the Activity Coefficient of HsSiC '.

[0105] In order to compare solutions with different compositions, temperatures, pH and silica concentrations, the SSI is used. SSI is a dimensionless characteristic figure, representing the ratio of the concentration of dissolved silica relative silica solubility of a solution calculated according to formula (4) : where Csiiica is the silica concentration (mg kg-1) and Csiiica solubility is the silica solubility (mg kg-1).

[0106] An SSI below or equal to 1 will not initiate silica precipitation, deposition or scale formation, whereas an SSI value above 1 will silica cause precipitation and silica scale formation. The corresponding temperature at which an SSI of 1 is achieved is the silica saturation temperature (SST).

[0107] Silica solubility decreases with decreasing temperature. Therefore, as heat energy is extracted from hot geothermal water and the water consequently cooled, the SSI increases to SSI>1 and silica precipitates out as an intractable silica scale blocking pipework, heat exchangers and reinjection wells. This removal of silica from the water by precipitation and silica scale formation continues until a value of SSI= 1 is reached.

[0108] However, for a particular temperature, the silica solubility increases rapidly with increasing pH above about pH=8. The inventors have found that, not only does the particulate calcium hydroxide added to the geothermal water capture dissolved silica in the water and transform it to finely suspended calcium silicate hydroxide hydrate, but also the excess hydroxide ions from the calcium hydroxide causes an increase in pH of the treated water effectively reducing the SSI to < 1 thereby increasing the solubility of the silica and preventing its propensity to precipitate and form a silica scale as the water is cooled. Figure 5 shows the rapid decrease in the SSI of the geothermal water with time following the addition of calcium hydroxide to the water. This rapid decrease in SSI with time (Figure 5) correlates directly with the rapid reduction of dissolved silica content and the corresponding rapid increase in pH of the treated geothermal water with time (Figure 3) in accordance with the theory and associated formulas (1)— (4) above. The data presented in Figure 3 and Figure 5 were generated from the laboratory scale continuous process detailed in Example 3 using synthetic geothermal water. The specific conditions were: SiC concentration = 1000 mg kg-1SiC , starting pH of the water = 8.5, a Ca :Si mok mol ratio = 0.8, and at room temperature 20 °C.

[0109] This dual effect, i.e. conversion of dissolved silica to calcium silicate hydroxide hydrate that removes dissolved silica from the geothermal water, and pH increase that increases the solubility of the silica in the water, collectively decrease the SSI, enabling more heat to be extracted from geothermal water without encountering the problems caused by silica precipitation and deposition as a scale when the water is cooled to temperatures significantly below those that are possible with current industry practice.

[0110] Overall, the simultaneous rapid removal and reduction of dissolved silica (HaSiO^) in the geothermal water through the formation of calcium silicate hydroxide hydrate, together with the rapid increase in pH through the excess hydroxide ions provided by the calcium hydroxide (Figure 3) substantially reduces the SSI of the water (Figure 5). Silica precipitation and silica scale formation are substantially reduced or even prevented entirely.

[0111] The data in Figure 5 confirm the SSI is rapidly reduced from a very high SSI=9 at which silica rapidly precipitates to SSI=1 in about 1-2 minutes, at and below which silica cannot precipitate and deposit as an intractable scale.

[0112] Example 7 analyses pilot plant data for Wairakei geothermal water and shows a reduction in SSI. The results show that the SSI of the incoming geothermal water flow (Raw Brine, Figure 7) at 95 °C, was initially 1.4 and progressively increased to about 2 during the 6 hour run as the compositon of the incoming geothermal water changed. Some fluctuations within this period were observed. These SSI values are all significantly above SSI=1 confirming that silica scaling will occur in this incoming water.

[0113] Most importantly, the results clearly show that the addition of the particulate Ca(OH)2 slurry and the formation of calcium silicate hydroxide hydrate according to the process of this invention, substantially reduces the SSI of the treated geothermal water to a value of SSI=0.15 (Figure 7). This is due to the rapid formation of calcium silicate hydroxide hydrate which captures and remove dissolved silica from the treated geothermal water together with the simultaneous increase in pH of the water. The SSI=0.15 is substantially less than 1 and silica scaling has been eliminated.

[0114] Even though the SiC content in the feed geothermal water flow varied by about 30%, the SSI of the treated geothermal water was essentially constant at about 0.15 (Figure 7). This further shows the robustness of the chemistry and technology of the invention, and its applicability to eliminate silica scaling from geothermal waters (brines).

[0115] Example 8 looks at both SSI and SST (Silica Saturation Temperature) using pilot plant data for Wairakei, Kawerau and Mokai geothermal waters for a common reinjection temperature of 85 °C for comparative purposes. The measurements show that the SSI value for the incoming geothermal water stream (raw brine) are 1.8 for Wairakei, 2.3 for Kawerau and 2.8 for Mokai (Figure 8 left). These are considerably above SSI= 1 showing that silica scale readily forms from these waters at the temperature of 85 °C for these incoming streams. The corresponding SSI values for these waters following addition of the particulate Ca(OH)2 slurry and the formation of calcium silicate hydroxide hydrate are substantially lower, being 0.2 for Wairakei, 0.7 for Kawerau and 0.8 for Mokai (Figure 8 left). These substantially lower SSI values clearly show that by practising this invention silica scaling is prevented. The corresponding SST values which are the respective temperatures below those that silica scaling occurs for these incoming water streams, are presented in Figure 8 right. These show that for the Wairakei water silica scaling will occur below 125 °C, Kawerau water below 160 °C and Mokai water below 180 °C. For the exiting streams following the addition of particulate Ca(OH)2 slurry and the formation of calcium silicate hydroxide hydrate, the SST values have been lowered substantially to 42 °C for Wairakei, 70 °C Kawerau and 75 °C for Mokai water. These lower SST values clearly show that substantially more heat energy can be recovered from geothermal water and therefore more electricity generated than was otherwise possible.

[0116] Ca:Si ratio

[0117] The Ca:Si mokmol ratio governs the stoichiometry and the nanostructural and chemical properties of the calcium silicate hydroxide hydrate formed.

[0118] The Ca:Si mol: mol ratio determines the amount calcium hydroxide required in relation to the amount of dissolved silica in the geothermal water, to control formation chemistry and the properties of the calcium silicate hydroxide hydrate. The minimum Ca:Si ratio that facilitates the formation of calcium silicate hydroxide hydrate is about 0.3. The preferred ratio is about 0.4-0.8, more preferably about 0.4-0.6. This ensures the development of the nanostructure, surface area and pore volume of the calcium silicate hydroxide hydrate that determine its chemical and physical properties and therefore its uses.

[0119] The change in pH of the geothermal water following calcium hydroxide is dependent on the Ca:Si ratio (Figure 4). With increasing Ca:Si ratio, the amount of calcium hydroxide added to the geothermal stream is increased accordingly. The amount of excess hydroxide ions, and hence the pH, correspondingly increase. The rapid increase in pH for different Ca:Si ratios and the shape of the pH vs. time curves for the different Ca:Si ratios are similar (Figure 4), confirming that the same chemical processes are taking place at each Ca:Si ratio. The residual dissolved silica concentration in the water following calcium hydroxide addition and the formation of calcium silicate hydroxide hydrate for geothermal waters of different dissolved silica concentrations, e.g. 600, 800 and 1200 mg kg-1SiC , for a particular Ca:Si ratio, are similar (Figure 6). This shows the importance of the Ca:Si ratio in determining the amount of calcium hydroxide that is required to reduce the dissolved silica concentration in the water, increase the pH and hence reduce the SSI to prevent silica deposition. In addition, the amount of calcium silicate hydroxide hydrate formed is similar for the different Ca:Si ratios from about 0.3-0.6 due to the variable stoichiometric nature of the calcium silicate material and the different silica concentrations in the geothermal water (Table 2).

[0120] The concentrations of calcium ions and hydroxide ions immediately surrounding the calcium hydroxide particles are higher than those respectively in the bulk geothermal water, than would be the case if they were supplied in total amount separately by a calcium salt with a high solubility, e.g. CaCl2, and the hydroxide ions were provided by solution of highly soluble alkali metal hydroxide, e.g. NaOH. This further illustrates the importance of using calcium hydroxide particulates as a source of Ca2+and OH' ions for the nucleation and formation of calcium silicate hydroxide hydrate and providing the excess OH' ions to increase the pH of the treated geothermal water.

[0121] Control of calcium silicate formation reaction

[0122] The residual silica content of the geothermal water after addition of calcium hydroxide particulates and the formation of calcium silicate hydroxide hydrate for starting dissolved silica concentrations of 600, 800 and 1200 mg kg'1SiO2, temperatures of 50 °C and 70 °C, and after reaction times of 30, 40, 75 and 300 seconds, are shown in Figure 6. The Ca:Si ratio is 0.6. Each data point is shown by a vertical bar. The starting dissolved silica concentrations of 600, 800 and 1200 mg kg-1SiC , are shown by the respective dashed lines. These experiments were performed in the laboratory in a continuous reactor using a synthetic geothermal water where sodium silicate was used to provide the dissolved silica. The pH of the resulting water was adjusted to pH 8 to mimic geothermal water. The reaction was quenched after 30, 40, 75 and 300 seconds respectively. The calcium silicate hydroxide hydrate was rapidly filtered off and the residual treated water analysed for dissolved silica, and the pH measured.

[0123] The vertical bars for 30 seconds reaction time for each of the silica concentrations and temperatures respectively show a very rapid decrease in dissolved silica concentrations in the treated water due to the rapid formation of calcium silicate hydroxide hydrate. This is consistent with Figure 3. The dissolved silica content after about 30 seconds is about 400- 500 mg kg-1SiC across the different starting silica concentrations and temperatures. There was a much slower decrease in silica concentration in the treated water over the next 300 seconds, again showing the same trend as in Figure 3. The residual silica concentration in the treated water after 300 seconds reaction time is generally about 250-300 mg kg'1SiC .

[0124] For a particular initial silica concentration, there is little difference between the rate of silica removal and hence the rate of calcium hydroxide hydrate silicate formation between the 50 °C and the 70 °C reaction conditions. This shows that the availability of Ca2+and OH' ions from the calcium hydroxide particles are more important in controlling the rapid kinetics of the calcium silicate hydroxide hydrate formation reaction than the effect of temperature. This observation, together with the shape of the curve in Figure 3, suggests the formation of calcium silicate hydroxide hydrate is essentially chemically controlled and is approximately 1storder in kinetics. Improved heat and electricity recovery

[0125] The polymerisation of dissolved silica and resulting precipitation of silica increases as the temperature of geothermal water decreases. According to the present invention, dissolved and polymeric silica can be captured by adding solid particulate calcium hydroxide and forming calcium silicate hydroxide hydrate from the dissolved silica entities in the geothermal water. In this process, the calcium silicate hydroxide hydrate rapidly forms as fine micron-sized particles that remain in suspension. The fine particles of the calcium silicate hydroxide hydrate flow cleanly through the downstream pipes and heat exchangers carrying the geothermal water. They do not agglomerate or adhere to metal surfaces and there is no deposition of scale. This is because the chemical and structural properties of the calcium silicate hydroxide hydrate are significantly different from those of silica. Since the problem of silica precipitation and silica scale formation on equipment services is avoided by the process of this invention, the temperature of the water stream can be lowered much further than is currently possible, before any silica precipitation occurs. It therefore becomes possible to extract substantially more heat from the geothermal water stream, via heat exchangers for example, without encountering the problem of unwanted silica precipitation and silica scale formation.

[0126] The process of the invention also enables significantly more electricity to be generated from a geothermal resource because more heat can be recovered from the hot geothermal water flow via the heat exchangers of a binary cycle electricity generation plant, through the ability to cool the hot water to lower temperatures than is presently possible in existing geothermal plants.

[0127] Depending on the dissolved silica content in supersaturated geothermal water and the temperature dependence of the silica solubility and typical pH valuies of the water, separated geothermal water can currently only be cooled to about 150-100 °C in conventional geothermal plants before silica deposition and scale formation takes place, which progressively blocks heat exchangers, pipework and reinjection wells. For high silica content waters, the lower temperature limit can be as high as 180 °C. This constraint on the extent of cooling severely limits the amount of heat that can be recovered for direct heating applications and the amount of electricity that can be generated from the separated water in geothermal resource utilisation. In principle, a binary cycle electricity generating plant can be operated efficiently with a heat exchanger exit temperature down to about 80 °C, which is below the temperature where silica deposits and silica scale forms extensively.

[0128] The present invention prevents silica deposition and enables more electricity to be generated from the water that currently exits binary cycle heat exchangers in geothermal operations. This can be achieved by interfacing a new binary cycle electricity generating plant in tandem downstream from the existing binary plant. The additional installed electricity generation capacity in megawatts (MW) and the annual generation in gigawatt hours (GWh) made possible, are shown in Example 5 and Table 3 for a representative selection of currently operated New Zealand and international geothermal resources with different water flow rates and dissolved silica concentrations. The respective new binary plant heat exchanger inlet temperature is that of the existing binary plant heat exchanger exit temperatures for each location. The heat exchanger exit temperature of the new binary plant is set at about 90 °C which represents a workable thermodynamic efficiency and economic temperature. In principal, this could be lower, down to about 60 °C, depending on the design and size of the heat exchanger system, as economic considerations allow. The additional generation capacity and hence the the annual generation for each resource is significant. The data show that additional electrticity generation capacity for the New Zealand and international resources are significant, and represent about a 20-50% increase in the electricity generation capacity of the binary cycle plants processing the water upstream of the heat exchanger for a new binary plant.

[0129] Alternatively, for a greenfields geothermal installation, the binary cycle plant can be designed for a heat exchanger inlet temperature being the temperature of the hot separated geothermal water, typically 150-100 °C or higher, and a heat exchanger exit temperature of 60-90 °C.

[0130] The prevention of silica deposition using the process of the invention similarly enables the geothermal water flow to be further cooled to about 30 °C and substantially more heat energy to be recovered from the water for direct heating applicatiions. Consideration of thermodynamic efficiency, heat exchange size and associated economics, suggests that the practical lower temperature is about 40-60 °C.

[0131] The additional installed heat recovery capacity in MW and the annual recoverable heat in GWh for direct heating applications, made possible by the process of the invention, for a representative selection of currently operated New Zealand and international geothermal resources with different water flow rates and dissolved silica concentrations is shown in Example 6 and Table 4. The respective direct heat exchanger inlet temperatures are those of the existing binary plant heat exchanger exit temperatures. The amount of recoverable heat energy is shown for direct heat exchanger exit temperatures of 90 °C, 60 °C and 40 °C for each geothermal resource (Table 4). The data show that the amount of heat energy that can be recovered is substantial, and increases progressively with the decrease in the direct heat exchanger exit temperature. It is not possible to recover this amount of heat energy from geothermal water by any other means, due to the silica deposition problem. The invention prevents silica deposition and uniquely enables the recovery of substantial amounts of heat energy from the water.

[0132] Although in principle the water can be cooled to lower temperatures, e.g. down to about 30 °C, thermodynamic efficiency and cost considerations suggest this is likely to be uneconomic. The data show that additional heat recovery capacity increases with decreasing heat exchanger exit temperature, and can be up to 79 MW for the New Zealand resources (Nga Awa Purua) and 91 MW for the Dieng resource when the water is cooled to 40 °C. Overall, these amounts of energy are substantial, particularly when the water is cooled to 40 °C. They are much greater than the amount of electricity (electrical energy) generated by the binary cycle plants processing the water upstream of the heat exchanger for direct heating. This opens up considerable opportunities for heat recovery by the invention claimed here, than is otherwise possible.

[0133] Prevention of corrosion

[0134] The increase in pH of the geothermal water following the addition of calcium hydroxide and the formation of calcium silicate hydroxide hydrate, shifts the chemical potential of the dissolution of iron i.e. corrosion, into the region of chemical passivation. This reduces the propensity of corrosion of steel pipework and equipment carrying this geothermal water.

[0135] In addition, the alkaline pH of this geothermal water similarly reduces acid degradation of the cement casings in reinjection wells.

[0136] Control of arsenic uptake

[0137] Geothermal water can contain low levels of dissolved arsenic species, typically AsCh3' in concentrations up to about 2-10 mg kg-1As, but in some fields this can be up to 100 mg kg-1As, which can adsorb onto silica and calcium silicate hydroxide hydrate particle surfaces. It is important to limit the uptake of arsenic onto calcium silicate hydroxide hydrate to below the acceptable limits of arsenic for applications of the calcium silicate hydroxide hydrate product.

[0138] One advantage of the process of the invention is the control of the uptake of arsenic on the calcium silicate hydroxide hydrate surface by reducing the Ca:Si ratio and further increasing the pH during the calcium silicate hydroxide hydrate formation reaction.

[0139] The Ca:Si ratio strongly influences the uptake of arsenic by calcium silicate hydroxide hydrate particles as shown by the laboratory and pilot plant data in Figure 9. There is essentially a linear relationship between the arsenic content of the calcium silicate hydroxide hydrate and the Ca:Si ratio, with the uptake of arsenic decreasing with decreasing Ca:Si ratio. (Figure 9). The linear relationship holds for increasing silica concentration and increasing temperature. The arsenic uptake decreases slightly with increasing temperature (Figure 9) and increases with increasing dissolved silica content in the geothermal water (Figure 10). The increase in pH due to the excess hydroxide ions in the calcium silicate hydroxide hydrate formation, increases the negative charge on the surface of the calcium silicate hydroxide hydrate particles providing an electrostatic barrier to limit the uptake of the AsCh3' ions. The further increase in hydroxide ions through the addition sodium hydroxide or a similar base, increases the negatively charged electrostsatic barrier and reduces the extent of arsenic uptake further. This is particularly important for for higher dissolved silica geothermal water (Figure 10).

[0140] The extent of the reduction in the uptake of dissolved arsenic species in geothermal water by calcium silicate hydroxide hydrate during formation, that can be achieved by reducing the Ca:Si ratio in the calcium silicate hydroxide hydrate formation reaction, is shown in Figure 10. The data are for geothermal waters containing 500 and 1000 ppm (mg kg-1) dissolved silica each with 6 ppm (mg kg'1) dissolved arsenic that is present largely as the AsOs3' anion, for Ca:Si ratios of 0.2, 0.3, 0.4, 0.6 and 0.8 respectively. These show that the As content (uptake) of the calcium silicate hydroxide hydrate is reduced markedly as the Ca:Si ratio is decreased from 0.8 to 0.4, with only small further reductions for 0.3 and 0.2 ratios. Although the amount of As taken up by the calcium silicate hydroxide hydrate is greater for that formed from water containing 1000 ppm dissolved silica, compared with water containing 500 ppm dissolved silica, the pattern of reduction in the As uptake with decreasing Ca:Si ratio is similar (Figure 10). Also, with the addition of hydroxide ions to the geothermal water along with calcium hydroxide in the formation of calcium silicate hydroxide hydrate, the data show the uptake of As similarly reduces with decreasing Ca:Si ratio, but the extent of As uptake is significantly reduced further with additional hydroxide ions.

[0141] At the preferred Ca:Si ratio of 0.4, the arsenic content of the calcium silicate hydroxide hydrate is desirably low at about 17 mg kg-1for water with 500 mg kg-1dissolved silica, and slightly lower at about 15 mg kg-1As for for water with 1000 mg kg-1dissolved silica (Figure 8).

[0142] Collectively these data show that the arsenic uptake for calcium silicate hydroxide hydrate during formation form geothermal water, can be reduced by reducing the Ca:Si ratio and by increasing the pH by the further addition of hydroxide ions (Figure 8). This is a significant advantage of the process of the invention. It is important that calcium silicate hydroxide hydrate products recovered from geothermal water and used for various applications are low in arsenic.

[0143] Example 9 describes an experiment where the uptake of dissolved arsenic species in synthetic geothermal water containing 500 and 1000 mg kg-1SiC and 6 mg kg-1arsenic was measured for the calcium silicate hydroxide hydrate products. The results are shown in Figures 9 and 10.

[0144] The data and results presented in Figure 9 clearly show essentially a linear relationship between the arsenic content of the calcium silicate hydroxide hydrate product and the Ca:Si ratio used in the reaction to form the product. There is a close correlation between the laboratory data for 500 mg kg-1(ppm) SiC water for the synthetic geothermal water in the laboratory study and the pilot plant data for the 550 mg kg-1(ppm) SiC Wairakei geothermal water. There appears to be a slight increase in arsenic content of the calcium silicate hydroxide hydrate product for the higher 1000 mg kg'1(ppm) SiC water in the laboratory study.

[0145] The invention is further described with reference to the following examples. It will be appreciated that the invention as claimed is not intended to be limited in any way by this example.

[0146] EXAMPLES

[0147] Example 1: Laboratory scale batch process

[0148] A synthetic geothermal water solution was prepared by mixing 11.53 g of sodium silicate solution (waterglass) (26% SiCh content, grade D) is into 5L of distilled water in a plastic beaker, making a 600 mg kg-1SiC solution. Since sodium silicate solution is very alkaline, the pH was adjusted downwards to that of geothermal water, typically about pH 6.5- 8.5, via the dropwise addition of 2M HCI. For a 0.4 Ca:Si ratio, 1.48 g of Ca(OH)2 was thoroughly mixed with distilled water to make 59.2 g total solution mass. The Ca(OH)2 suspension was then added to the pH adjusted sodium silicate solution under vigorous mixing. After 5 minutes of mixing, a volume of the cationic flocculant Separ Chemie PK311 was added to provide a flocculate concentration of 4 mg kg-1. The solution was left to mix for another minute before mixing was stopped. Calcium silicate hydroxide hydrate particles formed within a few seconds. After about 5 minutes, the solid material was separated by Buchner filtration, plug washed twice with distilled water and once with ethanol before being dried at 120 °C overnight to provide a dry calcium silicate hydroxide hydrate powder.

[0149] Example 2: Laboratory scale batch process for determining rate of formation of calcium silicate hydroxide

[0150] General procedure

[0151] A 2L batch of sodium silicate at a concentration of either 600, 800 or 1200 mg kg-1SiC solution as required was prepared in a plastic beaker by diluting D grade sodium silicate (containing approximately 33.2 wt% SiC ) with distilled water necessary to provide the particular dissolved SiC concentration. The large 2L batch was made to ensure a consistent concentration of SiC across the different timeframe for the same temperature and same Ca:Si ratio conditions used. The sodium silicate solution was heated in a water bath to the desired temperature of 50 °C or 70 °C). An 300mL aliquot of this solution was taken, and pH adjusted to 8.5 using 2M HCI. The reaction was conducted in a smaller water bath placed on top of stirring magnetic hot plate. A slurry 1.78mL of 10 wt% Ca(OH)2 particulates was added to the centre of the vortex. At the required time, the reaction was stopped, and the calcium silicate hydroxide hydrate particulates separated by Buchner vacuum filtration. A sample of the filtrate was collected and analysed for Si and Ca quantity using the UV-Visible spectroscopy molybdate method and Atomic Absorption Spectroscopy. The calcium silicate hydroxide hydrate filter cake was washed with distilled water followed by ethanol, dried at 120 °C in an oven overnight and analysed for Si and Ca using an electron microscope equipped with energy dispersion X-ray spectroscopy.

[0152] Specific example: Ca:Si ratio 0.4

[0153] Sodium silicate (33.2 wt%, 4.82 g) was placed in a plastic beaker and distilled water was added to make 2000 g total of solution containing 800 mg kg-1SiC . The solution was heated in a water bath to 70 °C. A 300 mL sample of the solution was placed into another plastic beaker equipped with magnetic stir bar, which was then placed in a water bath over a stirring (500 rpm) hot plate. The pH was adjusted to 8.5 using 2M HCI. To this solution, 1.78 mL of 10 wt% slurry of Ca(OH)2 in distilled water was added to the middle of the vortex. The reaction was left to stir for 30 seconds before it was quickly pass through a Buchner vacuum filtration system to separate the calcium silicate hydroxide hydrate product. An aliquot of 10 mL of the filtrate was then taken before the filter cake was washed with distilled water and ethanol and then analysed for Si and Ca quantity using the UV-Visible spectroscopy molybdate method and Atomic Absorption Spectroscopy respectively. The results for the reduction in dissolved silica by the invention are shown in Figure 6. Similar experiments were carried out where the reaction was stopped after 40, 75 and 300 seconds and for all the four time intervals for solutions with starting silica concentrations of 600 and 1200 800 mg kg-1SiC . The results are all shown in Figure 6.

[0154] Example 3: Laboratory scale continuous process for 5 L hr1geothermal water flow rate.

[0155] A synthetic geothermal water solution and Ca(OH)2 suspension were prepared on the basis of the information presented in Table 2 (see Example 4) scaled to a geothermal water flow rate of 5 L hr1. Dissolved silica concentrations of 600, 800, 1000 and 1200 mg / kg were used which represent typical concentrations of dissolved silica in supersaturated geothermal waters. Ca:Si mol:mol ratios of 0.3, 0.4 and 0.6 were used for each silica concentration (Table 2). The synthetic geothermal water solution and calcium hydroxide suspension were pumped individually using peristaltic pumps to a small T-junction where they were mixed. Calcium silicate particles formed within a few seconds. Where necessary, an in-line static mixer immediately downstream of the T-junction can be used to improve mixing. The ratio of the flow rates was the same as the volume ratio of the synthetic geothermal water solution and Ca(OH)2 suspension. The suspension of calcium silicate hydroxide hydrate particles was collected in a large vessel and a flocculant added. The calcium silicate hydroxide hydrate product was filtered, washed and dried in the same manner as described in Example 1.

[0156] The above processes were carried out at room temperature and also at elevated temperatures up to about 70 °C by immersing the solutions in a water bath. These data and results further demonstrated the application of the invention in a continuous process over the range of silica concentrations typical of geothermal waters in New Zealand and internationally, and the range of preferred Ca:Si mokmol ratios.

[0157] The data presented in Figure 3 and Figure 5 were generated from the process at the following specific conditions: SiC concentration = 1000 mg kg-1SiC , starting pH of the water = 8.5, Ca:Si mokmol ratio = 0.8, and at room temperature 20 °C.

[0158] Example 4: Pilot scale continuous process

[0159] A continuous operation PLC controlled pilot plant was designed, constructed and operated at two different New Zealand geothermal resources, Wairakei and Kawerau, processing up to about 3 tonnes per hour of geothermal water provided by a feed from a main pipeline. The input temperature of the water was typically about 100-130 °C. In-line flow meters, automated valves, pressure sensors and temperature sensors were used to monitor and control the process. The Wairakei geothermal water has a silica concentration of about 550 mg kg-1 SiC and the Kawerau geothermal water has a silica concentration of about 800 mg kg-1SiC .

[0160] A 2.5 wt % suspension of Ca(OH)2 was prepared and contained in a stirred tank. The mass flow of suspension required to provide the selected Ca:Si ratio, determined using the information in Table 2, was pumped directly into the hot geothermal water feed line immediately before an in-line static mixer. Calcium silicate hydroxide hydrate particles were observed to form in a few seconds through samples taken close behind the Ca(OH)2 slurry injection point, and flowed freely as a dilute suspension through an in-line heat exchanger to replicate the heat recovery in a Binary Cycle plant or for direct-heating applications. The subsequently cooled calcium silicate hydroxide hydrate - geothermal water flow was flashed to atmospheric pressure, and a measured amount of Separ Chemie PK311 cationic flocculant to provide a 3 parts per million concentration in the treated geothermal water, was pumped into the flow. The calcium silicate hydroxide hydrate particles were separated continuously using a lamellar separator followed by a press filter to provide a filter cake that can be used directly or dried to a powder and used.

[0161] This successfully demonstrated the process of the invention at a pilot plant scale using the chemical process unit operations, equipment and process control systems which would be used in a full scale commercial operation.

[0162] Table 2: The amount of calcium hydroxide required for the reaction with silica in geothermal water to form calcium silicate hydroxide hydrate for different initial silica concentrations and Ca:Si ratios. These are normalised for a geothermal water (brine) flow of 1 tonne per hour.

[0163] Example 5: Additional electricity generation

[0164] The respective amounts of additional electricity that can be generated from the geothermal water flow that exits the binary cycle plant heat exchanger in an existing geothermal electricity generation operation, as a result of being able to cool the water to 90 °C which is currently not possible in existing binary cycle electricity generation, were calculated from available operational data from three New Zealand and two international resources and the attributes of the invention. The data are shown in Table 3. These amounts of additional electricity generation depend on the flow rate of the geothermal water, the dissolved silica concentration and the temperature of the water exiting the binary plant. At present this electricity generation potential is wasted, as the water exiting the current binary plant is reinjected into the ground.

[0165] Table 3: Electricity generation capacity and annual electricity generation for three representative New Zealand and two international operational geothermal resources. The assumed binary plant efficiency is r| = 10% with 8322 operational hours (equals 95 % running time) over 1 year. Example 6: Additional heat recovery

[0166] The respective amounts of additional heat energy that can be recovered from the geothermal water flow that exits the binary cycle plant heat exchanger in an existing geothermal electricity generation operation, as a result of being able to cool the water to much lower temperatures than is currently possible, were calculated from available operational data from four New Zealand and two international resources and the attributes of the invention. The data are shown in Table 4. These amounts of additional heat energy depend on the flow rate of the geothermal water, the dissolved silica concentration and the temperature of the water exiting the binary plant.

[0167] Table 4: Additional heat recovery capacity and annual additional heat energy recovered for four representative New Zealand and two international operational geothermal resources. An availability of 95 % (equals 8322 operational hours) is assumed over 1 year.

[0168] Example 7: Reduction in Silica Saturation Index - Pilot Plant data for Wairakei geothermal water

[0169] The reduction in the Silica Saturation Index SSI, was measured at 15 minute intervals over a 6 hour period during a routine pilot plant operation at the Wairakei geothermal resource. The data and results are shown in Figure 7. The geothermal water temperature was 95 °C and a 0.4 Ca:Si mol:mol ratio was used. The SiC content of the water was measured at the start of the run and this value was used to set the particulate Ca(OH)2 slurry dosing level for the 0.4 Ca:Si mol:mol operating condition. Although it was observed that the the SiO2 content of the geothermal water slowly increased over the 6 hour period due to natural fluctuations and changes in the water composition, the Ca(OH)2 dosing level was not changed. This meant that in practice the Ca:Si ratio used, actually decreased progressively during the run (Figure 7). The objective was to test the robustness of the calcium silicate hydroxide hydrate formation chemistry and to determine if such fluctuations in the SiCh content of the water affected the SSI.

[0170] Example 8: Silica Saturation Index and Silica Saturation Temperature - Pilot Plant data for Wairakei, Kawerau and Mokai geothermal waters

[0171] The Silica Saturation Index SSI, and the Silica Saturation Temperature SST, were determined for the incoming geothermal water stream and the exiting stream following the addition of particulate Ca(OH)2 slurry and the formation of calcium silicate hydroxide hydrate, during pilot plant operation at the Wairakei, Kawerau and Mokai geothermal resources (Figure 8). For comparison purposes a reinjection temperature of 85 °C was used. These are three different resources at distinctly different geographic locations and are not connected. The geothermal waters have different temperatures, brine chemistries and silica concentrations. A common reinjection temperature was chosen.

[0172] Example 9: Reducing the arsenic uptake

[0173] The uptake of dissolved arsenic species in synthetic geothermal water containing 500 and 1000 mg kg-1SiC and 6 mg kg-1arsenic was measured for the calcium silicate hydroxide hydrate products formed at Ca:Si mokrnol ratios of 0.2, 0.3, 0.4, 0.6 and 0.8, respectively.

[0174] The results for geothermal water at 1000 mg kg-1SiC at room temperature and 500 mg kg-1SiC at 60 °C are shown in Figure 9. The results for a similar set of measurements from the operation of the pilot plant using Wairakei geothermal water containing about 550 mg kg-1SiC and 6 mg kg-1arsenic, and at 90°C for the same Ca:Si ratios, are also shown in Figure 9.

[0175] Figure 10 shows the effect on the uptake of arsenic species, of the addition of further hydroxide ions to the Ca(OH)2 slurry during the formation of calcium silicate hydroxide hydrate at a Si:OH' mol: mol ratio of 0.1 for synthetic geothermal water containing 500 and 1000 mg kg-1SiC at room temperature, for the same Ca:Si mol: mol ratios.

[0176] The arsenic uptake data in Figure 9 for synthetic geothermal water was obtained by the following methodology. For each Ca:Si ratio, a 3 L bulk solution was prepared by diluting a 33.2 wt% sodium silicate solution to provide either the 500 or 1000 mg kg'1content as required. A volume of sodium arsenate solution was added to provide a dissolved arsenic content of 6 mg kg'1As. The pH was adjusted to 8.5 using sodium hydroxide to represent geothermal water. A 500 mL volume of the synthetic geothermal water was heated to the desired temperature. The amount of a 2.5 wt% suspension of Ca(OH)2 to provide the particular Ca:Si mol: mol ratio was added to the geothermal water with rapid stirring to form the calcium silicate hydroxide hydrate product. The product was separated by filtration, dried and analysed for its Ca, Si and As content by Atomic Absorption spectroscopy.

[0177] For Wairakei geothermal water containing 550 mg kg-1SiC and 8 mg kg-1As, and at a temperature of 90 °C, calcium silicate hydroxide hydrate was similarly formed at the different Ca:Si ratios by the addition of the required amount of a 2.5 wt% Ca(OH)2 suspension. The product was separated by filtration, dried and analysed for its Ca, Si and As content by Atomic Absorption spectroscopy.

[0178] The arsenic uptake data in Figure 10 was produced by the same methodology as used for Figure 9, using synthetic geothermal water containing 500 and 800 mg kg-1respectively and at room temperature. However, the required amount of sodium hydroxide solution to give a Si:OH' mol:mol ratio of 0.1 was added to the 2.5 wt% Ca(OH)2 suspension, which was then added to the synthetic geothermal water. The resulting calcium silicate hydroxide hydrate products were similarly separated, dried and analysed for their Ca, Si and As contents.

[0179] Although the invention has been described by way of example, it should be appreciated that variations and modifications may be made without departing from the scope of the invention as defined in the claims. Furthermore, where known equivalents exist to specific features, such equivalents are incorporated as if specifically referred in this specification.

Claims

CLAIMS1. A process for recovering heat from geothermal water comprising:(i) treating a stream of geothermal water containing dissolved silica by adding particulate calcium hydroxide, where the amount of calcium hydroxide added gives a Ca:Si mol: mol ratio below 1, to transform at least some of the dissolved silica into calcium silicate hydroxide hydrate particles suspended in the stream, and to simultaneously increase the pH of the stream to at least 8; and(ii) passing the stream through a heat exchanger to recover heat energy from the stream.

2. A process as claimed in claim 1, wherein the amount of calcium hydroxide added in step (i) is sufficient to provide a stoichiometric excess of hydroxide ions relative to the hydroxide content of the calcium silicate hydroxide hydrate.

3. A process as claimed in claim 1 or claim 2, wherein the Ca:Si mol:mol ratio is in the range 0.3 to 1.0.

4. A process as claimed in any one of claims 1 to 3, wherein the Ca:Si mol:mol ratio is in the range 0.4 to 0.6.

5. A process as claimed in any one of claims 1 to 4, wherein the pH of the stream is increased from about 6.5 to 8.5 to about 9 to 12 on treatment with the calcium hydroxide.

6. A process as claimed in any one of claims 1 to 5, wherein the silica saturation index of the stream is reduced to less than 1 on treatment with the calcium hydroxide.

7. A process as claimed in any one of claims 1 to 6, wherein the silica is in the form of HsSiO^ or H4SiO4, or a combination thereof.

8. A process as claimed in any one of claims 1 to 7, wherein the calcium hydroxide in is in the form of a suspension or slurry in water.

9. A process as claimed in claim 8, wherein the calcium hydroxide in the suspension or slurry is present at a concentration of up to about 15 wt %.

10. A process as claimed in any one of claims 1 to 9, wherein the heat exchanger has an inlet stream temperature in the range 100-200 °C.

11. A process as claimed in claim 10, wherein the inlet stream temperature is in the range120-150 °C.

12. A process as claimed in any one of claims 1 to 11, wherein the outlet stream temperature is less than 80 °C.

13. A process as claimed in claim 12, wherein the outlet stream temperature is less than 50 °C.

14. A process as claimed in any one of claims 1 to 13, further including generating electricity from the heat recovered from the heat exchanger.

15. A process as claimed in any one of claims 1 to 14, further including recovering heat energy from the water for direct heating applications.

16. A process as claimed in any one of claims 1 to 14, wherein the heat exchanger is a binary cycle heat exchanger.

17. A process as claimed in any one of claims 1 to 16, further including reinjecting the treated stream into ground.

18. A process as claimed in any one of claims 1 to 17, wherein the amount of arsenic in the calcium silicate hydroxide hydrate is less than 20 mg kg-1.

19. A process as claimed in any of the claims 1 to 18, wherein the calcium silicate hydroxide hydrate particles flow through plant equipment as a suspension in the geothermal water without deposition of calcium silicate hydroxide hydrate or of silica on surfaces of the plant equipment.

20. A process as claimed in any of the claims 1 to 19, wherein the calcium silicate hydroxide hydrate particles are separated continuously from the geothermal water flow.

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