Process for improving geothermal water heat recovery

Abstract

An improved process for recovering heat from geothermal water by converting silica dissolved in water into suspended calcium silicate hydroxide by adding calcium hydroxide in a controlled manner, thereby reducing the deposition of silica and being able to recover more heat from water.

Description

TECHNICAL FIELD

[0001] The present invention relates to an improved geothermal water heat recovery process. In particular, the present invention relates to the controlled treatment of geothermal water with calcium hydroxide to convert dissolved silica to suspended calcium silicate hydroxide, thereby reducing silica deposition, which enables more heat to be recovered from the geothermal water and more power to be generated than is possible with other methods. BACKGROUND

[0003] Geothermal energy provides an extremely attractive renewable resource for base-load electricity generation. However, the efficiency of thermal conversion of subsurface superheated water in geothermal resources to electricity is only about 12-16%. This is primarily due to thermodynamic constraints on steam / water flash and conversion of thermal energy to work by a Carnot cycle turbine. After the steam flash process, geothermal water (brine) is separated from the vapor / water separation stage, which is about two-thirds of the reservoir mass flow. This water stream is very large in volume and mass flow, typically 200 to 3000 tons per hour, and at a temperature of about 120-150°C (depending on the flash pressure), and is an important source of low grade thermal energy. Silica in the water is typically supersaturated. Some of the thermal energy is captured by downstream binary cycle units' heat exchangers and converted to electrical energy. The binary cycle heat exchanger outlet temperature is typically about 110-100°C, but is often higher, depending on the content of problematic dissolved silica in the water, especially under high enthalpy field conditions. This lower temperature water, with further supersaturation of silica, is ideally re-injected into the reservoir to replenish the reservoir, while also returning other potential problematic dissolved entities (e.g., arsenic and boron) to the reservoir to avoid their entry into the environment.

[0004] Silica precipitation from separated geothermal brine supersaturated with dissolved silica and subsequent silica fouling deposition in pipes, heat exchangers, and reinjection wells is a major problem facing the worldwide utilization of geothermal resources. This fouling severely impacts recoverable thermal energy in binary cycle power generation and increases equipment and field maintenance costs. Silica precipitation typically impedes the recovery of thermal energy from geothermal water at temperatures below about 150 to 100°C (depending on the level of silica supersaturation) for industrial and consumer direct heating applications.

[0005] Previously, to address this issue, researchers attempted to operate steam / water flash evaporation at high temperatures, maintaining the temperature of the separated brine at approximately 130 to 150°C or higher. This reduced silica supersaturation and the tendency to initiate and persist silica deposition. However, this also reduced the amount of steam generated during flash evaporation, thereby decreasing the power generation of the steam phase.

[0006] Another approach is to prolong the induction period (a precursor to silica polymerization, deposition, and scaling) by lowering the water's pH to approximately pH 5 or lower, thereby slowing down the silica polymerization process. However, low pH levels can lead to corrosion of steel pipes and other equipment. Furthermore, any subsequent decrease in the temperature of the separated water (as is inevitable in binary cycles or industrial heat exchangers used for direct heating applications) can easily promote additional silica deposition and scaling within the heat exchanger pipes. This silica scaling impairs the heat transfer efficiency of the heat exchange surfaces, reducing the heat energy exchangeable to the binary cycle working fluid, thus reducing power generation and the amount of heat that can be recovered for direct heating.

[0007] Silica deposition and scaling require periodic shutdowns to remove stubborn silica deposits from heat exchanger surfaces. Silica can also gradually accumulate in reinjection wells and local geological formations, eventually impairing the reinjection process and potentially necessitating borehole enlargement, environmentally harmful downhole chemical cleaning, or even drilling new wells.

[0008] Johnston et al. (NZ 537747) reported that hydrated calcium silicate can be formed from sodium silicate solutions with high concentrations of dissolved silica by adding calcium hydroxide at a Ca:Si molar ratio of approximately 1:1 or higher, while simultaneously lowering the pH by adding acid to the calcium hydroxide. Furthermore, it has been reported that adding Ca at a Ca:Si molar ratio of approximately 1:1... 2+ and OH - ions can form hydrated calcium silicate from geothermal water with a Ca:Si molar ratio of approximately 1:1 (e.g., see Johnston et al., GRC Bulletin, 2018, 1240-1250). While the use of calcium and hydroxide ions has shown advantages, greater effectiveness and efficiency are still needed to prevent silica deposition in geothermal water, as well as silica scaling that occurs when cooling water for heat recovery and power generation.

[0009] Through further research, the inventors discovered that significant advantages can be achieved by controlling the form and amount of calcium hydroxide used. The inventors found that by further reducing the silica saturation index and lowering the Ca:Si molar ratio to approximately 0.4-0.8, the amount of Ca can be reduced. 2+The need can be addressed by process improvements.

[0010] It is therefore an object of the present invention to provide an improved method of geothermal water heat recovery and power generation, or at least to provide a useful alternative to existing methods. SUMMARY

[0011] A first aspect of the present invention provides a process for recovering heat from geothermal water, comprising: (i) treating a geothermal water stream containing dissolved silica by adding particulate calcium hydroxide, wherein the amount of calcium hydroxide added is such that the Ca:Si molar ratio is less than 1, to convert at least some of the dissolved silica to calcium silicate hydroxide hydrate particles suspended in the water stream, while increasing the pH of the water stream to at least 8; and (ii) passing the water stream through a heat exchanger to recover heat energy from the water stream.

[0012] In certain embodiments of the present 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.

[0013] In certain embodiments of the present invention, the Ca:Si molar ratio is in the range of 0.3 to 1.0, preferably in the range of 0.4 to 0.6.

[0014] In certain embodiments of the present invention, the pH of the water stream is increased from about 6.5 to 8.5 to about 9 to 12 after treatment with calcium hydroxide.

[0015] In certain embodiments of the present invention, the silica saturation index of the water stream is reduced to less than 1 after treatment with calcium hydroxide.

[0016] In certain embodiments of the present invention, the silica is in the form of H3SiO4 - or H4SiO4, or a combination thereof.

[0017] In certain embodiments of the present invention, the calcium hydroxide is in the form of a suspension or slurry in water, preferably at a concentration of up to about 15 wt%.

[0018] In certain embodiments of the present invention, the temperature of the inlet water stream to the heat exchanger is in the range of 100-200°C, typically 120-150°C, and the temperature of the outlet stream is less than 80°C, more preferably less than 50°C.

[0019] In certain embodiments of the present invention, the process further comprises generating power from the heat recovered from the heat exchanger. Preferably, the heat exchanger is a binary cycle heat exchanger.

[0020] In some embodiments of the invention, the process further includes recovering thermal energy from water for direct heating applications.

[0021] In some embodiments of the invention, the process further includes reinjecting the treated water stream into the ground.

[0022] In some embodiments of the present invention, the arsenic content in the calcium silicate hydroxide hydrate is less than 20 mg / kg. -1 .

[0023] In some embodiments of the invention, calcium silicate hydroxide hydrate particles flow through the unit equipment in the form of a suspension in geothermal water without depositing calcium silicate hydroxide hydrate or silica on the surface of the unit equipment.

[0024] In some embodiments of the present invention, calcium silicate hydroxide hydrate particles are continuously separated from geothermal water. Brief description of the attached diagram

[0026] Figure 1 This is a schematic diagram of a traditional geothermal process for generating electricity from geothermal resources.

[0027] Figure 2 This is an integrated schematic diagram illustrating the process of the present invention, which involves injecting calcium hydroxide into a geothermal water stream and separating calcium silicate hydroxide hydrate particles after a binary circulation unit.

[0028] Figure 3 The results showed that after the addition of calcium hydroxide and the subsequent formation of calcium silicate hydroxide hydrate, the dissolved silica (SiO2) content (hollow circles) decreased rapidly, followed by a rapid increase in the pH of the water (cross).

[0029] Figure 4 The change in pH of geothermal water with Ca:Si ratio after the addition of calcium hydroxide is shown.

[0030] Figure 5 The results show that the silica saturation index (SSI) in the treated geothermal water decreased rapidly over time. This is due to the corresponding rapid decrease in dissolved silica (SiO2) content, as well as the rapid increase in pH value after the addition of calcium hydroxide and the formation of calcium silicate hydrate. The horizontal dashed line represents SSI=1; above this value, silica will form scale. The Ca:Si ratio is 0.8. The data comes from laboratory experiments using synthetic geothermal water with an SiO2 concentration of 1000 mg / kg. -1 SiO2, initial pH=8.5.

[0031] Figure 6 The figure shows the residual silica content in the geothermal water after the addition of calcium hydroxide particles and the formation of calcium silicate hydroxide hydrate.

[0032] Figure 7 The Silica Saturation Index (SSI) values for the Wairakei geothermal resource pilot plant unit over a six hour period of operation are shown. The silica content in the geothermal water (raw brine) entering the pilot plant unit and the geothermal water flowing out of the pilot plant unit after the addition of the particulate Ca(OH)2slurry and formation of calcium silicate hydrate (post CaSil process) was measured every 15 minutes or so over the six hour period of continuous operation. From these measurements, the SSI for both geothermal water streams was determined and a plot of the SSI values against time was plotted. The dotted line indicates the position of SSI = 1 above which silica fouling occurs.

[0033] Figure 8 The Silica Saturation Index (SSI) and Silica Saturation Temperature (SST) values for the Wairakei, Kawerau and Mokai geothermal waters (brines) at the reinjection temperature of 85°C are shown for comparison. Figure 8 The left hand graph shows the SSI values for the inflow and outflow geothermal waters for these three geothermal resources after the addition of the particulate Ca(OH)2slurry and formation of calcium silicate hydrate. The horizontal solid line indicates the position of SSI = 1 above which silica fouling occurs. Figure 8 The right hand graph shows the SST values for silica fouling in the inflow and outflow geothermal waters for these three geothermal resources.

[0034] Figure 9 The dependence of the arsenic content in the calcium silicate hydrate on the Ca:Si ratio is shown for the formation of calcium silicate hydrate from synthetic geothermal water at laboratory scale and in the pilot plant operation using Wairakei geothermal water.

[0035] Figure 10 The arsenic content (ppm (mg kg -1 ) of calcium silicate hydrate materials produced from synthetic and natural geothermal waters with dissolved silica contents of 500 and 1000 mg kg -1 , and the effect of the addition of NaOH to the water to provide more hydroxide ions during the formation of the calcium silicate hydrate on the arsenic content in the materials. DETAILED DESCRIPTION

[0036] Definitions

[0037] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although any assays, methods, devices, and materials similar or equivalent to those described herein can be used in the practice or testing of the present application, the various assays, methods, devices, and materials are described herein.

[0038] The numerical ranges recited herein (e.g., 1 to 10) include all numerical values subsumed therein, for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9, and 10, as well as any ranges of rational values that fall within the range, for example, 2 to 8, 1.5 to 5.5, and 3.1 to 4.7. Thus, all ranges of values explicitly recited herein, whether explicitly stated or not, are considered to be expressly disclosed herein. These are only specific examples, and all possible combinations of the minimum and maximum values recited are to be considered to be expressly stated in a similar manner.

[0039] As used in the specification, "including," "containing" and like terms are not to be interpreted in an exclusive or exhaustive sense. In other words, they are not meant to exclude possible additives, but rather, to describe broadly what is meant to be included.

[0040] Any reference to prior art documents in this specification is not to be interpreted as an acknowledgement that such prior art is widely known or forms part of the common general knowledge in the field.

[0041] The term "geothermal water" refers to water heated or superheated by natural phenomena within the earth to a temperature of from about 30°C to about 350°C, including all steam, water and water vapor, and any mixture of any of these heated by natural phenomena.

[0042] The term "stoichiometric excess" refers to an amount of one or more reactants in excess relative to the amount required to form a particular compound in a chemical reaction.

[0043] The term "silicon dioxide" refers to all amorphous, crystalline and hydrated solid forms of SiO2.

[0044] The term "dissolved silicon dioxide" refers to all silicon-containing species dissolved in water, including but not limited to silicic acid H4SiO4, silicate anions H3SiO4 - , H2SiO4 2- and HSiO3 - and polymeric forms thereof. It is to be understood that "dissolved silicon dioxide" is an industry standard term referring to the above-described silicon-containing species, and not to solid SiO2that is not soluble in water.

[0045] The term "calcium silicate hydroxide hydrate" refers to any species having the chemical formula Ca x SiO y (OH) z .wH2O. Such species can include small or trace amounts of adsorbed or chemically bound anions and cations typically present in geothermal water, such as AsO3 - , HS - , HSb2S4 - , H3BO3, B(OH)4 - , Mg2+ Na + K + and Li + .

[0046] The term "silica saturation index" or "SSI" refers to the ratio of the concentration of silica in a supersaturated solution to the solubility of amorphous silica under the same conditions, particularly temperature and pH.

[0047] The Invention

[0048] The present invention involves treating geothermal water with calcium hydroxide in a controlled manner to provide calcium ions and sufficient hydroxide ions at a low Ca:Si molar ratio to react with dissolved silica species in the geothermal water to form calcium silicate hydrate particles. The excess hydroxide ions also increase the pH of the geothermal water, thereby lowering the SSI to less than or equal to 1 and avoiding silica deposition when the water is cooled to about 40°C or less. The ability to reduce the water temperature from, for example, 150°C to 40°C or less enables more thermal energy to be recovered from the geothermal water and more electricity to be generated than is possible with other methods.

[0049] The amounts of Ca 2+ , Si 4+ , O 2- and OH - ions and H2O molecules in the structure of the calcium silicate hydrate are variable in stoichiometry. The stoichiometry of the calcium silicate hydrate material formed depends on the amount of calcium hydroxide added relative to the amount of dissolved silica in the geothermal water, as indicated by the Ca:Si molar ratio used. At a Ca:Si molar ratio of 0.4-0.8, the amount of hydroxide ions required to form the particular calcium silicate hydrate is less than the amount provided by the calcium hydroxide added, and the excess hydroxide ions also serve to increase the pH of the treated geothermal water. 2+ , Si 4+ , O 2- and OH - ions and H2O molecules in the structure of the calcium silicate hydrate are variable in stoichiometry. The stoichiometry of the calcium silicate hydrate material formed depends on the amount of calcium hydroxide added relative to the amount of dissolved silica in the geothermal water, as indicated by the Ca:Si molar ratio used. At a Ca:Si molar ratio of 0.4-0.8, the amount of hydroxide ions required to form the particular calcium silicate hydrate is less than the amount provided by the calcium hydroxide added, and the excess hydroxide ions also serve to increase the pH of the treated geothermal water.

[0050] Overall, the reduction in dissolved silica in the treated geothermal water and the associated increase in pH results in a SSI value that is significantly lower than is possible with other methods. This enables the geothermal water to be cooled to a lower temperature, as low as about 40°C or less, thereby enabling significantly more thermal energy to be recovered and more electricity to be generated. This is not possible with existing industry practice.

[0051] Reducing the Ca:Si molar ratio also minimizes the unwanted precipitation of calcium carbonate in the geothermal water.

[0052] The present invention provides a process for recovering heat from geothermal water comprising: (i) treating a geothermal water stream containing dissolved silica by adding particulate calcium hydroxide, wherein the amount of calcium hydroxide added is such that the Ca:Si molar ratio is less than 1, to convert at least some of the dissolved silica to calcium silicate hydroxide hydrate particles suspended in the water stream, while raising the pH of the water stream to at least 8; and (ii) passing the water stream through a heat exchanger to recover heat energy from the water stream.

[0053] Conventional geothermal processes

[0054] Geothermal hot water dissolves mineral components in the subterranean rock, and is generally considered to be a dilute brine that is supersaturated with dissolved silica. The main chemical species and their respective concentrations in New Zealand geothermal water resources are shown in Table 1. It is noted that the values in Table 1 are typical values and can vary due to natural changes in geothermal resources and geothermal water composition over time and the specific circumstances of the sampling well. The composition of New Zealand geothermal water is similar to that of geothermal water internationally.

[0055] Table 1: Main chemical species commonly found in geothermal water from various geothermal resources in New Zealand and their respective concentrations (mg kg -1 (ppm by weight)).

[0056]

[0057] Figure 1 A conventional geothermal process is shown. High pressure superheated geothermal water in a geothermal reservoir (1), typically located about 1-4 km below ground level (2), is piped (3) to a steam / water separator (4) where the pressure is reduced and some of the high temperature water stream flashes to steam. The high pressure steam (5) drives a turbine and generator to produce electricity (6). The turbine produces condensed steam (7) which is piped (8) to a lower temperature separated water stream (11). The separated water stream (9) from the steam / water separator is supersaturated with dissolved silica and piped to a binary cycle power unit (10). The resulting lower temperature separated water stream (11) is re-injected with the steam condensate stream (8) through a well (13) to an outer cooling zone of the geothermal reservoir (14). The dotted lines indicate silica precipitation and silica scale formation (12) from the supersaturated silica separated water into the pipes (9, 11), binary cycle unit heat exchanger (10) and re-injection well (13).

[0058] The geothermal process of the invention

[0059] Figure 2 An improved geothermal process incorporating the present invention process is shown. The processes with reference numbers (1) to (8) are the same as those shown in Figure 1.Figure 1 The conventional process shown is the same. The integration of the process of the invention includes 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 stream (12) through the pipe (9) and the binary cycle heat exchanger (10) and are finally continuously separated as calcium silicate hydroxide hydrate product (16).

[0060] The cooled geothermal water stream is mixed with steam condensate water (8) and reinjected (13) into the reservoir (14). The formation of silica precipitates and silica fouling in (9), (10), (11), (12), (13), (14) is prevented.

[0061] Flash steam turbine

[0062] Geothermal water at temperatures between about 140°C and 370°C and under pressure is brought to the surface by production wells. About one third of the mass flow of water flashes into steam under pressure drop, driving a turbine to generate electricity. As the steam cools, it condenses back to water and returns to the reservoir. This process involves a Carnot cycle engine in which thermal energy is converted to work. The efficiency depends on the temperature difference as the steam condenses in the turbine. This imposes a significant thermodynamic constraint on the amount of electricity that can be generated (typically about 40-45%) not including heat losses. The overall efficiency of steam phase power generation from superheated geothermal water in the reservoir is about 12-15%. In practice, the overall efficiency is lower due to losses in the system.

[0063] Binary cycle turbine

[0064] Geothermal water after steam / water separation (about two thirds of the mass flow in the well) is usually reinjected into the reservoir but can also be taken directly from the geothermal well for further power generation before reinjection. The separated hot water is usually at a temperature of 130-140°C and, after passing through a heat exchanger, its heat is transferred to a fluid (such as isobutene, pentane, isopentane, ammonia, water or mixtures thereof) with a lower boiling point than geothermal water. The fluid, after being heated, becomes a high pressure vapor driving a turbine to generate electricity.

[0065] The degree to which the separated hot water can be cooled, and thus the amount of thermal energy that can be extracted and the amount of power that can be generated, is severely limited by silica deposition and silica fouling formation. This occurs immediately after the end of the steam / water flash process when the dissolved silica content in the water exceeds the equilibrium saturation level. Typically, the cooling temperature can only be brought down to about 100°C, or even higher, depending on the supersaturation of silica in the water. However, this results in a silica saturation index SSI > 1, leading to harmful silica deposition and fouling formation in the binary plant heat exchangers, piping, and reinjection wells. This requires regular shutdowns, expensive and erosive mechanical and chemical cleaning and maintenance, and expensive drilling of new reinjection wells. The limited cooling range and the thermodynamic constraints of the Carnot (binary) cycle severely limit the thermal energy that can be extracted from the separated geothermal water stream, and thus the power generation of the binary plant. It is critical to keep the SSI equal to or less than 1 to prevent silica deposition. Binary cycle plants typically increase the overall efficiency of geothermal power generation by about 5-7%, bringing the total efficiency to 17-20%.

[0066] In contrast to conventional binary cycle plants, the present invention is able to further cool the separated hot water to about 60-80°C, thus generating more power through binary cycle technology, or to about 30°C through Kalina cycle technology, while keeping the SSI equal to or less than 1 at the lower temperature. This is not possible with current geothermal resource utilization practices because of the supersaturation of silica content in the geothermal water. However, according to the present invention, this can be achieved by rapidly capturing the dissolved silica and forming calcium silicate hydroxide hydrates, thus removing the silica from the water while increasing the pH, to eliminate silica deposition. The calcium silicate hydroxide hydrates that are formed are typically granular, but ideally, these granules do not stick to each other, and most importantly, they do not deposit as fouling on the surfaces of the plant piping, equipment, or reinjection wells.

[0067] Direct heat utilisation

[0068] Direct heat applications are those that directly utilize geothermal energy (without the need for heat pumps or generator sets) to provide heat energy for building heating, industrial processes, home heating, greenhouses, aquaculture, public baths and swimming pools, etc. In direct heat applications, the separated geothermal water stream, after passing through a heat exchanger, has a heat recovery utilization efficiency of about 90% or higher, which is much higher than the current power generation efficiency of 12-20% from this resource. In principle, direct heat applications can use geothermal water at high, medium to low temperatures (as low as 30°C, but in practical operations it can be limited to about 40-60°C). However, silica deposition can affect the use of geothermal water cooled to below 150°C and severely limit or completely prevent its use for cooling water to below 100°C. This means that a large amount of potentially available heat energy in the separated geothermal water is not utilized and is essentially wasted when reinjected into the reservoir. Since the present invention is able to cool the separated hot water to about 20-30°C and without silica deposition, because at these lower temperatures the SSI is reduced and remains equal to or less than SSI = 1, more heat energy (up to several times) can be extracted from the same water stream for direct heat applications.

[0069] Reinjection

[0070] The cooler (used) geothermal water needs to be reinjected to replenish the geothermal reservoir. Otherwise, the reservoir water level will drop significantly. If the reservoir water level drops too much due to the extraction of water and steam, underground caverns can form that can collapse, causing sinkholes, posing a serious risk to infrastructure and safety. In addition, reinjection is also needed to deal with unwanted toxic gases, the greenhouse gas carbon dioxide, and other undesirable substances dissolved and suspended in the brine.

[0071] Silica deposition in the rock structure and in the reinjection well (especially in the perforated well pipe) is a major problem that requires expensive and erosive chemical and mechanical cleaning and drilling of new reinjection wells.

[0072] The present invention avoids silica deposition in the reinjection well pipe and in the reinjection well. Since the present invention is able to reinject the water into the reservoir at a lower temperature (as low as about 30°C), a polymer-based pipe (with an appropriate pressure rating) can be used to transport the water to the reinjection well.

[0073] Silica deposition in geothermal water

[0074] One of the limitations of geothermal water utilization is the damage caused by the precipitation of dissolved minerals, particularly silica, in the subsurface geothermal water reservoir. Geothermal water sourced from subsurface rock reservoirs contains dissolved chemicals leached from the minerals in the subsurface rock contacted. Certain materials, such as silica, will dissolve to a saturation solubility at the high temperatures in the subsurface. When the hot geothermal water is brought to the surface and cooled in the vapor / water separation stage of a binary cycle heat exchanger, the dissolved silica in the water becomes supersaturated, and thus, there is a strong driving force for silica precipitation as the thermal energy is recovered from the hot water and the water is cooled. The precipitated silica forms a hard-to-treat sinter or scale deposit on the surfaces of the geothermal unit's machinery and piping.

[0075] The scale can clog valves, narrow the piping, and have a damaging effect on other sensitive equipment, such as the turbine blades that generate the electricity, causing severe damage. The scale has a heat insulating effect, causing a reduction in heat transfer in the binary cycle unit heat exchanger and in geothermal water used in direct use applications. Silica scale can completely clog the piping, particularly perforated piping used in reinjection wells, and can build up on the rock structure of the reinjection well, requiring the drilling of a new reinjection well at great cost. The scale removal effort is large and costly. Piping and equipment are often discarded due to the high cost of cleaning. Reinjection wells often require the use of large amounts of hydrofluoric acid and other acids or bases to remove the scale, or a new well must be drilled. In addition, scale removal often requires the unit to be taken off line, greatly reducing the efficiency of the geothermal unit and the associated revenue.

[0076] There are two common solutions to the problem of scale buildup. The first is to maintain the water in the vapor / water separator at a suitably high temperature, typically at least 120-160°C. At such temperatures, scale formation is less likely or occurs at a slower rate. However, at higher temperatures, the operating efficiency of the unit is less than theoretical. In binary cycle units, the water leaving the heat exchanger must also be maintained at a higher temperature, thus requiring a reduction in the temperature gradient between the exchangers to minimize the deposition in the heat exchangers, piping, and reinjection well. This severely limits the amount of thermal energy that can be recovered theoretically and the amount of geothermal water that can be used to generate electricity. Scale deposition not only limits the flow of geothermal water through the heat exchanger, but also reduces the heat flow from the surface of the heat exchanger, further reducing the thermal energy available for electricity generation. Direct use applications can be completely blocked due to the need to maintain a higher reinjection temperature.

[0077] Another common method to reduce scale formation is to add acid or chelating agents to the water as it passes through the geothermal unit. Reducing the pH level by adding acid can slow the reaction of silica precipitation and scale formation, hopefully for a long enough time to reinject the water back into the ground before significant silica scale formation occurs. However, the use of acid increases the potential for corrosion of steel piping, heat exchangers and process equipment. In fact, silica deposition and scale formation still occurs. To protect the turbine in the steam unit, the steam is cleaned by spraying cold water to remove the acidic droplets or gas. The introduction of cold water into the steam significantly reduces the efficiency of the geothermal energy produced by the turbine. Chelating agents are generally expensive. Both the use of chelating agents and acid only delay scale formation. Scale still forms.

[0078] Addition of calcium hydroxide

[0079] Calcium hydroxide is added to the water as a slurry of particles, providing one mole of calcium Ca 2+ ions and two moles of OH - ions. When these ions are mixed into the geothermal water stream, they rapidly react with dissolved silica entities, primarily H3SiO4 - , and then polymerize, precipitate and form silica scale. The resulting non-stoichiometric calcium silicate hydrate particles do not agglomerate or stick to metal surfaces and do not deposit into scale in the piping, equipment and heat exchangers. The calcium silicate hydrate particles pass through the piping and heat exchangers with the geothermal water stream as a dilute suspension and do not settle out or stick to metal surfaces. The calcium silicate hydrate can be separated into a product with a variety of beneficial applications.

[0080] When the Ca:Si ratio is less than one, two moles of OH - ions are provided for each mole of Ca 2+ ions, in excess of the amount required to form calcium silicate hydrate product. As a result, the pH of the treated water increases accordingly.

[0081] The calcium hydroxide is preferably added as a suspension or slurry of calcium hydroxide particles in water, with a slurry concentration of up to about 15 wt % solids. For a continuous process, it is important that the slurry be a free-flowing suspension. The addition of the suspension or slurry and the formation of calcium silicate hydrate can be carried out at atmospheric pressure with water temperatures up to 100°C, at elevated pressure with water temperatures greater than 100°C and up to about 230°C.

[0082] The reaction kinetics are primarily first order. Most of the H3SiO4 - ions are captured and react to form calcium silicate hydrate within the first 10 seconds. The reaction is essentially complete in about 5 minutes.

[0083] The reaction front of calcium hydroxide particle surface generating calcium silicate hydroxide hydrate gradually progresses to the entire calcium hydroxide particle, eventually converting the calcium hydroxide particle completely into calcium silicate hydroxide hydrate particle. As the calcium hydroxide dissolves, the dissolved silica entity in the geothermal water is gradually and rapidly removed, and at the same time the pH value of the geothermal water is also raised, thus collectively preventing silica deposition. The OH - :Ca 2+ Molar ratio provides Ca 2+ and OH - ions for the generation of calcium silicate hydroxide hydrate, which makes the concentration of OH - ions excessive relative to the Ca 2+ ions required for the calcium silicate hydroxide hydrate generation reaction and product. The excess OH - ions cause the pH value (alkalinity) of the geothermal water to rise at the same time, usually to about 9.5 or higher.

[0084] The calcium hydroxide particle acts as a nucleation center for calcium silicate hydroxide hydrate by providing a local high concentration of calcium and hydroxide ions to promote the rapid formation of calcium silicate hydroxide hydrate on the surface and through the calcium hydroxide particle.

[0085] The calcium hydroxide particle slurry can be added to the geothermal water at any time after steam / water separation.

[0086] Ca 2+ ions can also be provided by soluble calcium salts (e.g. calcium chloride), and hydroxide ions can be provided by adding soluble metal hydroxides (e.g. sodium hydroxide). If necessary, Ca 2+ ions can be added separately to adjust the Ca:Si ratio. However, this method cannot provide nucleation centers for calcium silicate hydroxide hydrate particles.

[0087] The average particle size of the calcium silicate hydroxide hydrate formed in this process is usually in the range of 0.1 to 100 microns. These particles can be loosely aggregated together to form entities of a few millimeters in size. However, there is no chemical bond between the particles, so no fouling deposition is formed.

[0088] The stoichiometric composition of the calcium silicate hydroxide hydrate formed depends on the Ca:Si ratio, which is determined by the amount of calcium hydroxide added to the geothermal water containing dissolved silica, which can be represented by the following general reaction: H3SiO4 - + xCa 2+ + 2xO →Ca x SiO y (OH) z .wH2O+ OH- (2x-z)

[0089] The value of x is determined by the Ca:Si ratio required in the reaction, i.e. the amount of Ca(OH)2used relative to the dissolved Si (silica) content of the geothermal water, which also determines the amount of hydroxide ions added. The oxide and hydroxide content of the calcium silicate hydroxide hydrate is determined by the structural arrangement, chemical bonding and electrical neutrality of the particular calcium silicate hydroxide hydrate formed. Water is both bound to the surface of the nanosheets in the calcium silicate hydroxide hydrate nanostructure through hydrogen bonds and is present in the pores within the nanostructure formed by the nanosheets.

[0090] When Ca(OH)2is added to geothermal water, the use of a stoichiometric excess of hydroxide ions can simultaneously increase the pH and decrease the SSI of the treated water. This can remove dissolved silica species (mainly H3SiO4 - ) from the solution, preventing them from polymerising and depositing silica scale. Unreacted silica remains in solution and no longer has the chemical driving force to polymerise and deposit silica scale.

[0091] Figure 3 The rapid reaction of dissolved silica species and the decrease in dissolved silica content due to the formation of calcium silicate hydroxide hydrate and the rapid increase in pH of the water upon addition of calcium hydroxide is shown.

[0092] The dissolved silica content of the water rapidly decreases by about 70% in about 5-10 seconds and by about 90% in 1 minute from the initial content. The reaction is essentially complete after about 5 minutes. This rapid decrease is due to the high concentration of Ca 2+ and OH - ions around the rapidly dissolving calcium hydroxide particles. The formation of calcium silicate hydroxide hydrate particles is also rapid. Figure 3 .

[0093] The pH of the water also rapidly increases from about pH = 8.5 to pH = 10.7 in about 10 seconds, to about pH = 11.4 in 1 minute and to about pH = 11.7 in 5 minutes upon addition of calcium hydroxide. Figure 3 .

[0094] The rate of decrease in dissolved silica in the water correlates well with the rate of increase in pH of the water, demonstrating the close chemical link between these two effects, which is specifically due to the use of calcium hydroxide to prevent silica deposition. Figure 3 .

[0095] However, the rate of removal of silica (H3SiO4 - ) from the geothermal water is slightly faster than the rate of increase in pH due to the weaker hydrogen bonds between OH"ions and the surface of the calcium silicate hydrate particles.

[0096] The magnitude of the rise in pH depends on the Ca:Si molar ratio. As the Ca:Si molar ratio increases, the final pH of the geothermal water after addition of calcium hydroxide also increases. The rate of the rapid rise in pH after addition of calcium hydroxide is essentially the same for each Ca:Si molar ratio, as seen by the same shape profile of the corresponding pH curves. This is also shown for the cases of Ca:Si molar ratios of 0.2, 0.3, 0.4, 0.6 and 0.8 during the pilot plant runs in the New Zealand Wairakei geothermal resource. Figure 4

[0097] Silica saturation index (SSI)

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

[0099] The SSI depends on the temperature and pH of the silica-containing solution. Silica dissolves to a supersaturation level (SSI > 1) in a silica-containing solution (e.g. geothermal water) and at a particular temperature and pH, silica will precipitate until enough silica has been removed to reach SSI = 1. Silica will not precipitate from a silica-containing water with SSI < 1. At a particular temperature, the solubility of silica increases as the pH of the silica-containing solution increases. Therefore, for a particular silica-containing water, silica will normally precipitate as a result of the water temperature being reduced to cause SSI > 1 and this can be prevented by simultaneously increasing the pH to a level where SSI is less than or equal to 1.

[0100] When SSI = 1, the amount of dissolved silica in the geothermal water is the same as the solubility of amorphous silica and the system is in chemical equilibrium. When SSI > 1, dissolved silica will polymerise and precipitate from the geothermal water to form silica scale. When SSI < 1, dissolved silica will remain in the geothermal water solution and no silica scale will form.

[0101] First the temperature-dependent solubility of amorphous silica at water-saturated vapour pressure must be calculated using equation (1):

[0102] where C is the solubility of amorphous silica (mg kg -1 ) and T is the temperature (K).

[0103] ​When silica dissolves in water, a balance is achieved between silicic acid (H4SiO4) and silicate ions (H3SiO4 - ). This balance depends on the pH of the solution, which gives silicic acid its weak acid character. In this case, the dissociation constant can be calculated according to equation (2):

[0104] where K1 is the dissociation constant.

[0105] The solubility of amorphous silica calculated using equation (1) must be adjusted according to the temperature and pH using equation (3):

[0106] where S is the adjusted silica solubility (mg kg -1 ), pH is the pH of the water, K1 is the first dissociation constant of the H4SiO4 - and H3SiO4 - balance, is the activity coefficient of H3SiO4 - .

[0107] To compare solutions of different compositions, temperatures, pH values, and silica concentrations, the SSI can be used. The SSI is a dimensionless characteristic value that represents the ratio of the dissolved silica concentration in the solution to the silica solubility, and is calculated according to equation (4):

[0108] where C 二氧化硅 is the silica concentration (mg kg -1 ), and C 二氧化硅 solubility is the silica solubility (mg kg -1 ).

[0109] When the SSI value is less than or equal to 1, no silica precipitation, deposition, or scale formation is induced; whereas, when the SSI value is greater than 1, silica induces precipitation and scale formation. The temperature at which the SSI value reaches 1 is the silica saturation temperature (SST).

[0110] The solubility of silica decreases with decreasing temperature. Therefore, as heat is extracted from hot geothermal water, the water temperature decreases, and the SSI value increases to SSI > 1, and silica precipitates out, forming silica scale that is difficult to remove, clogging pipes, heat exchangers, and reinjection wells. This process of removing silica from water through precipitation and silica scale formation continues until the value SSI = 1 is reached.

[0111] However, at certain temperatures, the solubility of silica increases rapidly with pH (pH > 8). The inventors have found that the addition of particulate calcium hydroxide to geothermal water not only captures the dissolved silica in the water and converts it to fine suspended calcium silicate hydrate, but also the excess hydroxide ions in the calcium hydroxide cause the pH of the treated water to increase, effectively reducing the SSI to < 1, increasing the solubility of silica and preventing it from precipitating and forming silica scale as the water cools. Figure 5 The SSI of geothermal water is shown to rapidly decrease with time after the addition of calcium hydroxide to the water. The rapid decrease in SSI with time ( Figure 5 ) is directly related to the rapid decrease in dissolved silica content and the rapid increase in pH of the treated geothermal water with time ( Figure 3 ), which is consistent with the theory and associated equations (1)-(4) above. Figure 3 and Figure 5 The data shown in Figures 1-4 were generated using synthetic geothermal water using the laboratory scale continuous process detailed in Example 3. The specific conditions were: Si02concentration = 1000 mg kg -1 of water, initial pH of water = 8.5, Ca:Si molar ratio = 0.8, room temperature 20 °C.

[0112] This dual effect, the conversion of dissolved silica to calcium silicate hydrate to remove dissolved silica from the geothermal water, and the increase in pH, increases the solubility of silica in the water, together reducing the SSI, enabling more heat to be extracted from the geothermal water without encountering the problems caused by silica precipitation and deposition as scale when the water cools to temperatures well below those possible with current industrial practice.

[0113] Overall, by forming calcium silicate hydrate, the dissolved silica (H3Si04 - ) in the geothermal water is simultaneously rapidly removed and reduced, plus the excess hydroxide ions provided by the calcium hydroxide cause the pH to rapidly increase ( Figure 3 ), greatly reducing the SSI of the water ( Figure 5 ). Silica precipitation and the formation of silica scale is also significantly reduced, or even completely prevented.

[0114] Figure 5 The data in Figure 1 confirm that the SSI rapidly decreases from a very high SSI = 9 (at which point silica would rapidly precipitate) to SSI = 1 (at or below which point silica cannot precipitate and deposit as intractable scale) in about 1-2 minutes.

[0115] Example 7 analysed pilot plant data for Wairakei geothermal water and the results showed a decrease in SSI. The results showed that the initial SSI value for the incoming geothermal water stream (raw brine, Figure 7 ) was 1.4 at 95°C and increased gradually to about 2 over the 6 hour period of operation as the composition of the incoming geothermal water changed. Some fluctuations were observed during this period. These SSI values were significantly higher than SSI = 1 confirming that silica scaling would occur in this incoming water.

[0116] Most importantly, the results clearly showed that the addition of the particulate Ca(OH)2slurry and formation of calcium silicate hydrate according to the process of the present invention significantly reduced the SSI value of the treated geothermal water to SSI = 0.15 Figure 7 This was due to the rapid formation of calcium silicate hydrate which was able to capture and remove dissolved silica from the treated geothermal water while increasing the pH of the water. SSI = 0.15 is much less than 1 and silica scaling has been eliminated.

[0117] Even though the SiO2content of the incoming geothermal water stream varied by about 30%, the SSI of the treated geothermal water was essentially constant at about 0.15 Figure 7 This further demonstrates the robustness of the chemical process and technology of the present invention and its suitability for removing silica scaling from geothermal water (brine).

[0118] For comparison purposes, Example 8 used pilot plant data for Wairakei, Kawerau and Mokai geothermal waters to observe SSI and SST (silica saturation temperature) where the common reinjection temperature was 85°C. The results showed that the SSI value for the incoming geothermal water stream (raw brine) Figure 8 was 1.8 for Wairakei, 2.3 for Kawerau and 2.8 for Mokai (left). These values are much higher than SSI = 1 and indicate that these incoming geothermal water streams are prone to silica scaling at a temperature of 85°C. After the addition of the particulate Ca(OH)2slurry and formation of calcium silicate hydrate, the corresponding SSI values for these waters were significantly lower at 0.2 for Wairakei, 0.7 for Kawerau and 0.8 for Mokai Figure 8 (left). These significantly lower SSI values clearly show that silica scaling can be prevented by implementing the present invention.

[0119] Figure 8The right hand side shows the corresponding SST values, i.e. the respective temperatures below which silica fouling occurs for these inlet water streams. These results show that for Wairakei water, silica fouling occurs below 125°C, for Kawerau water, silica fouling occurs below 160°C, and for Mokai water, silica fouling occurs below 180°C. After addition of the particulate Ca(OH)2slurry and formation of the calcium silicate hydroxide hydrate, the SST values decrease significantly to 42°C for Wairakei water, 70°C for Kawerau water, and 75°C for Mokai water for the effluent water stream. These lower SST values clearly show that more heat energy can be recovered from the geothermal water, resulting in more electricity than would be possible otherwise.

[0120] Ca:Si ratio

[0121] The Ca:Si molar ratio controls the stoichiometry of the calcium silicate hydroxide hydrate formed as well as the nanostructure and chemical properties.

[0122] The Ca:Si molar ratio determines the relationship between the amount of calcium hydroxide required and the amount of dissolved silica in the geothermal water, thereby controlling the formation chemistry and properties of the calcium silicate hydroxide hydrate. The minimum Ca:Si ratio that is favourable for the formation of the calcium silicate hydroxide hydrate is about 0.3. Preferably, the Ca:Si ratio is about 0.4-0.8, more preferably about 0.4-0.6. This ensures the formation of the nanostructure, surface area and pore volume of the calcium silicate hydroxide hydrate, which determines its chemical and physical properties and, in turn, its use.

[0123] The change in geothermal water pH after addition of calcium hydroxide depends on the Ca:Si ratio Figure 4 As the Ca:Si ratio increases, the amount of calcium hydroxide added to the geothermal water stream also increases. The number of excess hydroxide ions increases and the pH also increases accordingly. The rapid increase in pH at different Ca:Si ratios and the similar shape of the pH vs. time curves at different Ca:Si ratios Figure 4 confirm that the same chemical process occurs at each Ca:Si ratio. At a particular Ca:Si ratio, the concentration of dissolved silica remaining in the water after addition of calcium hydroxide and the formation of the calcium silicate hydroxide hydrate is similar -1 for geothermal water of different dissolved silica concentrations (e.g. 600, 800 and 1200 mg kg Figure 6). This indicates that the Ca:Si ratio is important in determining the amount of calcium hydroxide required to reduce the dissolved silica concentration, increase the pH, and thus reduce the SSI to prevent silica deposition. Furthermore, due to the different stoichiometric nature of the calcium silicate materials and the different silica concentrations in geothermal water, the amount of calcium silicate hydroxide hydrate formed is similar for different Ca:Si ratios of about 0.3-0.6 (Table 2).

[0124] The concentrations of calcium and hydroxide ions around the calcium hydroxide particles are higher than the bulk geothermal water concentrations, higher than if they were all provided by a highly soluble calcium salt (e.g. CaCl2) with the hydroxide ions provided by a highly soluble alkali hydroxide (e.g. NaOH) solution. This further illustrates the importance of using calcium hydroxide particles as a source of Ca 2+ and OH - ions for the nucleation and formation of calcium silicate hydroxide hydrate, and to provide an excess of OH - ions to increase the treated geothermal water pH.

[0125] Control of calcium silicate formation reactions

[0126] Figure 6 shows the residual silica content in geothermal water after the addition of calcium hydroxide particles and the formation of calcium silicate hydroxide hydrate for initial dissolved silica concentrations of 600, 800 and 1200 mg kg -1 Si02, temperatures of 50°C and 70°C, and reaction times of 30, 40, 75 and 300 seconds. The Ca:Si ratio was 0.6. Each data point is represented by a vertical bar. The initial dissolved silica concentrations of 600, 800 and 1200 mg kg -1 Si02, respectively, are represented by dashed lines. These experiments were performed in a continuous reactor in the laboratory using 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 simulate geothermal water. The reactions were terminated after 30, 40, 75 and 300 seconds, respectively. The calcium silicate hydroxide hydrate was rapidly filtered out and the residual treated water was analysed for dissolved silica and the pH was measured.

[0127] For each silica concentration and temperature, the vertical bars for the 30 second reaction time show that the dissolved silica concentration in the treated water decreases very rapidly due to the rapid formation of calcium silicate hydroxide hydrate. This is consistent with Figure 3 the results of the previous experiments. At different initial silica concentrations and temperatures, the dissolved silica content after about 30 seconds is about 400-500 mg kg -1 Si02. Over the next 300 seconds, the rate of decrease in the silica concentration in the treated water slows down significantly, again presenting a similar trend toFigure 3 The same trend. The residual silica concentration in the treated water was typically about 250-300 mg kg after 300 seconds of reaction -1 SiO2.

[0128] There was little difference in the rate of silica removal and the rate of calcium hydroxide hydrate silicate formation at 50°C and 70°C. This indicates that the availability of Ca2+ and OH ions in the calcium hydroxide particles is more important than the temperature in controlling the fast kinetics of the calcium silicate hydroxide hydrate formation reaction. 2+ and OH - The availability of Ca2+ and OH ions is more important than the temperature in controlling the fast kinetics of the calcium silicate hydroxide hydrate formation reaction. Figure 3 The shape of the curves in Figure 6, combined with the observation that the rate of silica removal is faster at 50°C than at 70°C, indicates that the formation of calcium silicate hydroxide hydrate is essentially chemically controlled and is approximately a first order reaction.

[0129] Improved heat and power recovery

[0130] As the temperature of the geothermal water is reduced, the polymerization of dissolved silica and the resulting silica precipitation increases. According to the present invention, the dissolved and polymerized 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 forms rapidly as fine micron-sized particles and remains in suspension. The fine particles of calcium silicate hydroxide pass cleanly through the downstream pipes and heat exchangers that transport the geothermal water. They do not agglomerate or stick to metal surfaces and do not form fouling deposits. This is because the chemical and structural properties of calcium silicate hydroxide hydrate are significantly different from those of silica. Because the process of the present invention avoids the problems of silica precipitation and silica fouling in the service of the equipment, the temperature of the water stream can be much lower than is currently possible before any silica precipitation occurs. Therefore, much more heat can be extracted from the geothermal water stream through heat exchangers and the like without encountering the problems of unwanted silica precipitation and silica fouling formation.

[0131] The process of the present invention can also produce significantly more electrical power from geothermal resources because more heat can be recovered from the hot geothermal water stream by the ability to cool the hot water to a lower temperature than is currently possible through the heat exchangers of binary cycle generator sets.

[0132] Depending on the dissolved silica content of the super-saturated geothermal water, the temperature dependence of silica solubility and the typical pH of the water, the separated geothermal water from conventional geothermal power plants can only be cooled to about 150-100°C before silica will precipitate and form scale, gradually plugging the heat exchangers, pipes and reinjection wells. For high silica content water, the lower temperature limit can be as high as 180°C. This limitation in the degree of cooling severely limits the amount of heat that can be recovered for direct heating applications and the amount of power that can be generated from the separated water in geothermal resource utilisation. In principle, binary cycle power plants can operate efficiently with heat exchanger outlet temperatures as low as about 80°C, which is below the temperature at which silica precipitation and significant scale formation occurs.

[0133] The present invention prevents silica precipitation and enables more power to be generated from the water currently flowing out of the heat exchanger of a binary cycle power plant in geothermal operation. This can be achieved by connecting a new binary cycle power plant in series downstream of an existing binary plant. Example 5 and Table 3 show representative selections of New Zealand and international geothermal resources currently in operation with different water flow rates and dissolved silica concentrations, with the additional installed power generation capacity in megawatts (MW) and annual power generation in gigawatt hours (GWh). The new binary plant heat exchanger inlet temperature is the existing binary plant heat exchanger outlet temperature for each location. The new binary plant heat exchanger outlet temperature is set at about 90°C, which represents a feasible thermodynamic efficiency and economic temperature. In principle, depending on the design and scale of the heat exchanger system, this temperature can be lower, down to about 60°C, within the economic limits. The additional power generation capacity and the resulting annual power generation are very significant for each resource. The data show that the additional power generation capacity for New Zealand and international resources is very significant, meaning that the power generation capacity of a binary cycle power plant treating the water upstream of the new binary plant heat exchanger will increase by about 20-50%.

[0134] Alternatively, for greenfield geothermal plants, the binary cycle power plant can be designed with a heat exchanger inlet temperature of the temperature of the hot separated geothermal water, typically 150-100°C or higher, and a heat exchanger outlet temperature of 60-90°C.

[0135] With the present invention process preventing silica precipitation, it is also possible to further cool the geothermal water stream to about 30°C and recover more heat energy from the water for direct heating applications. Taking into account thermodynamic efficiency, heat exchanger size and associated economics, a practical lower temperature of about 40-60°C is suggested.

[0136] Examples 6 and Table 4 show the additional installed heat recovery capacity (MW) and annual recoverable heat (GWh) for direct heating applications achievable using the process of this invention for representative selections of currently operating New Zealand and international geothermal resources with different water flow rates and dissolved silica concentrations. The corresponding direct heat exchanger inlet temperatures are the outlet temperatures of existing binary unit heat exchangers. For each geothermal resource, the amount of recoverable heat energy at direct heat exchanger outlet temperatures of 90°C, 60°C, and 40°C is shown in Table 4 below. The data shows that the amount of recoverable heat energy is considerable and gradually increases as the direct heat exchanger outlet temperature decreases. No other method can recover so much heat energy from geothermal water due to silica deposition problems. This invention prevents silica deposition and uniquely enables the recovery of significant amounts of heat energy from the water.

[0137] While water could theoretically be cooled to lower temperatures, such as approximately 30°C, thermodynamic efficiency and cost considerations suggest this may not be economical. Data shows that additional heat recovery capacity increases as the heat exchanger outlet temperature decreases; Nga Awa Purua's additional heat recovery capacity reaches up to 79 MW when water is cooled to 40°C, while Dieng's reaches up to 91 MW. Overall, this energy is substantial, especially when water is cooled to 40°C. This energy far exceeds the electrical energy generated by a dual-cycle unit directly heating water upstream of the heat exchanger. This opens up a far greater opportunity for heat recovery through the invention claimed herein than could be achieved through other methods.

[0138] Prevention of corrosion

[0139] With the addition of calcium hydroxide and the formation of calcium silicate hydroxide hydrate, the pH value of the geothermal water increases, transferring the chemical potential for iron dissolution (i.e., corrosion) to the chemical passivation zone. This reduces the corrosion susceptibility of steel pipes and equipment transporting geothermal water.

[0140] In addition, the alkaline pH value of this geothermal water can also reduce the acidic degradation of cement casing in reinjection wells.

[0141] Control of arsenic uptake

[0142] Geothermal water may contain low levels of dissolved arsenic, typically AsO3. 3- Its concentration can reach up to approximately 2-10 mg / kg. -1 However, in some fields, its concentration can even reach as high as 100 mg / kg. -1As such, arsenic can be adsorbed onto the surface of silica and calcium silicate hydroxide hydrate particles. For the application of calcium silicate hydroxide hydrate products, it is important to limit the absorption of arsenic by calcium silicate hydroxide hydrate to below acceptable arsenic limits.

[0143] One advantage of the process of this invention is that the absorption of arsenic on the surface of calcium silicate hydroxide hydrate is controlled by reducing the Ca:Si ratio and further increasing the pH value during the formation reaction of calcium silicate hydroxide hydrate.

[0144] like Figure 9 Data from laboratory and pilot-scale units show that the Ca:Si ratio significantly affects the arsenic absorption by calcium silicate hydroxide hydrate particles. The arsenic content in calcium silicate hydroxide hydrate exhibits a roughly linear relationship with the Ca:Si ratio; arsenic absorption decreases as the Ca:Si ratio decreases. Figure 9 This linear relationship still holds true with increasing silica concentration and temperature. Arsenic absorption decreases slightly with increasing temperature. Figure 9 ), and increases with the increase of dissolved silica content in geothermal water. Figure 10 The excessive hydroxide ions during the formation of calcium silicate hydroxide hydrate lead to an increase in pH, which in turn increases the negative charge on the surface of the calcium silicate hydroxide hydrate particles, thus forming an electrostatic barrier that restricts the growth of AsO3. 3- Ion absorption. The addition of sodium hydroxide or a similar alkali further increases hydroxide ions, strengthening the negatively charged electrostatic barrier and further reducing arsenic absorption. This is particularly important for geothermal water with a high silica content. Figure 10 ).

[0145] Figure 10 This study demonstrates the reduction in the uptake of dissolved arsenic from geothermal water by decreasing the Ca:Si ratio during the formation of calcium silicate hydroxide hydrate. The data pertain to Ca:Si ratios of 0.2, 0.3, 0.4, 0.6, and 0.8, and the concentrations of arsenic in the water are 500 and 1000 ppm (mg / kg), respectively. -1 Dissolved silica and 6 ppm (mg / kg) -1 Dissolve arsenic (mainly as AsO3) 3- Geothermal water containing anions. These results indicate that the As content (absorption) of calcium silicate hydroxide hydrate decreases significantly when the Ca:Si ratio decreases from 0.8 to 0.4, while only a slight further decrease occurs at ratios of 0.3 and 0.2. Although calcium silicate hydroxide hydrate formed from water containing 1000 ppm dissolved silica absorbs a greater amount of arsenic compared to water containing 500 ppm dissolved silica, the trend of decreasing arsenic absorption with decreasing calcium-silicon ratio is similar. Figure 10Furthermore, during the formation of calcium silicate hydroxide hydrate, as hydroxide ions are added to the geothermal water along with calcium hydroxide, data show that arsenic absorption also decreases with decreasing calcium-silicon ratio. However, with increasing hydroxide ion concentration, arsenic absorption decreases significantly further.

[0146] When the preferred Ca:Si ratio is 0.4, for a dissolved silica content of 500 mg / kg -1 The arsenic content in the water, specifically calcium silicate hydroxide hydrate, is ideally low, approximately 17 mg / kg. -1 For dissolved silica content of 1000 mg / kg -1 The arsenic content in the water, specifically the calcium silicate hydroxide hydrate, is slightly lower, approximately 15 mg / kg. -1 ( Figure 8 ).

[0147] In summary, these data indicate that reducing the calcium-to-silicon ratio and increasing the pH value by further adding hydroxide ions can decrease the absorption of arsenic during the formation of calcium silicate hydrate from geothermal water. Figure 8 This is a significant advantage of the process of this invention. Importantly, the calcium silicate hydroxide hydrate product recovered from geothermal water and used for a variety of applications should have a low arsenic content.

[0148] Example 9 describes an experiment that measured the calcium silicate hydroxide hydrate product at concentrations of 500 and 1000 mg / kg. -1 SiO2 and 6 mg kg -1 The absorption of dissolved arsenic in geothermal water during arsenic synthesis. Results are as follows: Figure 9 and Figure 10 As shown.

[0149] Figure 9 The data and results clearly demonstrate a substantially linear relationship between the arsenic content in the calcium silicate hydroxide hydrate product and the Ca:Si ratio used in the reaction that forms the product. (Laboratory studies synthesized 500 mg / kg of geothermal water.) -1 Laboratory data for (ppm) SiO2 in water with 550 mg / kg -1 There is a strong correlation between pilot-scale data and (ppm) SiO2 in Wairakei geothermal water. In laboratory studies, for higher concentrations (1000 mg / kg)... -1 The arsenic content in the (ppm) SiO2 water, calcium silicate hydroxide hydrate product appears to be slightly increased.

[0150] The present invention will be further described below with reference to embodiments. However, the present invention is not limited in any way to these embodiments.

[0151] Example

[0152] Example 1 : Laboratory scale batch process

[0153] The method for preparing the synthetic geothermal aqueous solution is as follows: 11.53 g of sodium silicate solution (water glass) (26% SiO2 content, grade D) is mixed with 5 L of distilled water in a plastic beaker to prepare a solution containing 600 mg / kg of water. -1 SiO2 solution. Since sodium silicate solution is strongly alkaline, the pH is adjusted downwards to the pH of geothermal water by adding 2M HCl, typically around pH 6.5-8.5. For a Ca:Si ratio of 0.4, 1.48 g of Ca(OH)2 is thoroughly mixed with distilled water to make a total solution mass of 59.2 g. The Ca(OH)2 suspension is then added to the pH-adjusted sodium silicate solution under vigorous stirring. After mixing for 5 minutes, a certain amount of cationic flocculant Separ Chemie PK311 is added to achieve a flocculant concentration of 4 mg / kg. -1 Mix the solution for another minute, then stop mixing. Calcium silicate hydroxide hydrate particles will form within seconds. After about 5 minutes, separate the solid material using a Buchner funnel, wash twice with distilled water, wash once with ethanol, and then dry overnight at 120°C to obtain dry calcium silicate hydroxide hydrate powder.

[0154] Example 2: Laboratory scale batch process for determining the rate of calcium silicate hydroxide formation

[0155] General procedure

[0156] In a plastic beaker, dilute Grade D sodium silicate (containing approximately 33.2 wt% SiO2) with distilled water to prepare a 2L batch of sodium silicate solution (concentration of 600, 800, or 1200 mg / kg as needed). -1 (SiO2) to provide a specific dissolved SiO2 concentration. A 2L batch was prepared to ensure consistent SiO2 concentration over different time periods under the same temperature and Ca:Si ratio. The sodium silicate solution was heated in a water bath to the desired temperature (50°C or 70°C). An aliquot of 300 mL of this solution was taken, and the pH was adjusted to 8.5 with 2M HCl. The reaction was carried out in a small water bath placed on top of a stirring magnetic heating plate. 1.78 mL of a 10wt% Ca(OH)2 particle slurry was added to the vortex center. The reaction was stopped at the desired time, and the calcium silicate hydroxide hydrate particles were separated by Buchner vacuum filtration. The filtrate sample was collected, and the amounts of Si and Ca were analyzed using UV-Vis molybdate spectroscopy and atomic absorption spectrometry. The calcium silicate hydroxide hydrate filter cake was washed sequentially with distilled water and ethanol, dried overnight in an oven at 120°C, and the Si and Ca contents were analyzed using an electron microscope equipped with an energy-dispersive X-ray spectrometer.

[0157] Specific example: Ca:Si ratio 0.4

[0158] Sodium silicate (33.2 wt%, 4.82 g) was placed in a plastic beaker, and distilled water was added to prepare a solution with a total volume of 2000 g, containing 800 mg / kg sodium silicate. -1 SiO2. The solution was heated to 70°C in a water bath. 300 mL of the solution sample was placed in another plastic beaker equipped with a magnetic stir bar and then placed in the water bath on a stirring (500 rpm) heating plate. The pH was adjusted to 8.5 with 2M HCl. 1.78 mL of Ca(OH)2 slurry in 10 wt% distilled water was added to the center of the vortex. The reactants were stirred for 30 seconds and then rapidly passed through a Buchner vacuum filtration system to separate the calcium silicate hydroxide hydrate product. 10 mL of the filtrate was taken, and the filter cake was washed with distilled water and ethanol. The Si and Ca contents were then analyzed using UV-Vis molybdate spectroscopy and atomic absorption spectroscopy, respectively. The method of this invention reduces dissolved silica as shown below. Figure 6 As shown. Similar experiments were conducted, in which the reaction stopped after 40, 75, and 300 seconds, and for all four time intervals, the initial silica concentrations of the solution were 600 and 1200, 800 mg kg, respectively. -1 SiO2. The results are all as follows. Figure 6 As shown.

[0159] Example 3: 5 L hr -1 Laboratory scale continuous process for geothermal water flow rates.

[0160] Prepare synthetic geothermal aqueous solution and Ca(OH)2 suspension according to the information provided in Table 2 (see Example 4), and adjust to 5 L / hr. -1 The geothermal water flow rate was determined. Dissolved silica concentrations of 600, 800, 1000, and 1200 mg / kg represent typical concentrations of dissolved silica in supersaturated geothermal water. The Ca:Si molar ratios used for each silica concentration were 0.3, 0.4, and 0.6, respectively (Table 2). The synthetic geothermal aqueous solution and the calcium hydroxide suspension were pumped separately to a small T-junction for mixing using a peristaltic pump. Calcium silicate particles formed within seconds. If necessary, an online static mixer could be used immediately downstream of the T-junction to improve mixing. The flow rate ratio was the same as the volume ratio of the synthetic geothermal aqueous solution and the Ca(OH)₂ suspension. The calcium silicate hydroxide hydrate particle suspension was collected in a large container and a flocculant was added. The calcium silicate hydroxide hydrate product was filtered, washed, and dried according to the same method described in Example 1.

[0161] The above processes are carried out at room temperature, and also by immersing the solution in a water bath at a high temperature of up to approximately 70°C. These data and results further demonstrate the applicability of the present invention in continuous processes within typical silica concentration ranges and preferred Ca:Si molar ratio ranges in geothermal water in New Zealand and internationally.

[0162] Figure 3 and Figure 5 The data shown was generated under the following specific process conditions: SiO2 concentration = 1000 mg / kg -1 SiO2, initial pH of water = 8.5, Ca:Si molar ratio = 0.8, room temperature 20℃.

[0163] Example 4: Pilot scale continuous process

[0164] A continuously operating PLC-controlled pilot plant was designed, built, and operated at two different geothermal resource sites in New Zealand—Wairakei and Kawerau—capable of processing up to 3 tons of geothermal water per hour supplied via main pipelines. The input water temperature is typically around 100-130°C. Online flow meters, automatic valves, pressure sensors, and temperature sensors are used to monitor and control the entire process. The silica concentration of the Wairakei geothermal water is approximately 550 mg kg⁻¹ SiO₂, while that of the Kawerau geothermal water is approximately 800 mg kg⁻¹. -1 SiO2.

[0165] A 2.5 wt% Ca(OH)₂ suspension was prepared and placed in a stirred tank. The required suspension mass flow rate to achieve the selected Ca:Si ratio was determined based on information in Table 2 and pumped directly into the hot geothermal feed line immediately preceding the online static mixer. Samples taken near the Ca(OH)₂ slurry injection point showed that calcium silicate hydroxide hydrate particles formed within seconds and flowed freely through the online heat exchanger as a diluted suspension, replicating heat recovery in a binary cycle unit or for direct heating applications. Subsequently, the cooled calcium silicate hydroxide hydrate geothermal stream was flash-evaporated to atmospheric pressure, and a certain amount of Separ Chemie PK311 cationic flocculant was pumped into the stream to provide a concentration of 3 parts per million (ppm) in the treated geothermal water. The calcium silicate hydroxide hydrate particles were continuously separated using a layer separator, followed by a filter press to obtain a filter cake that could be used directly or dried into powder.

[0166] This successfully demonstrated the process of the invention at a pilot plant scale, employing chemical process unit operations, equipment, and process control systems that will be used in full-scale commercial operation.

[0167] Table 2. Amount of calcium hydroxide required to form calcium silicate hydrate from geothermal water under different initial silica concentrations and Ca:Si ratios. These are standardized based on a geothermal water (brine) flow rate of 1 ton per hour.

[0168]

[0169] Example 5: Additional power generation

[0170] Based on operational data from three resources in New Zealand and two internationally, and considering the characteristics of this invention, calculations were made of the additional electricity that could be generated from the geothermal water flowing out of the heat exchanger of a binary cycle generator unit during existing geothermal power generation operations, due to the ability to cool water to 90°C (which is not achievable in existing binary cycle generator units). The data are listed in Table 3. This additional power generation depends on the geothermal water flow rate, the concentration of dissolved silica, and the temperature of the water exiting the binary cycle generator unit. Currently, this power generation potential is wasted because the water exiting existing binary cycle generator units is reinjected into the ground.

[0171] Table 3: Power generation capacity and annual power generation of three representative geothermal resources and two internationally operating geothermal resources in New Zealand. Assume a binary unit efficiency η = 10% and an annual operating time of 8322 hours (equivalent to 95% operating time).

[0172]

[0173] Example 6: Additional heat recovery

[0174] Based on operational data from four resources in New Zealand and two internationally, and considering the characteristics of this invention, the amount of additional heat energy that can be recovered from the geothermal water flow exiting the heat exchanger of a binary cycle unit in existing geothermal power generation operations, due to the ability to cool water to a much lower temperature than currently available, has been calculated. The data are listed in Table 4. This amount of additional heat energy depends on the geothermal water flow rate, the concentration of dissolved silica, and the water temperature exiting the binary cycle unit.

[0175] Table 4: Additional heat recovery capacity and annual additional heat energy recovered from four representative geothermal resources and two internationally operating geothermal resources in New Zealand. Assume an annual availability of 95% (equivalent to 8322 operating hours).

[0176]

[0177] Example 7: Reduction in silica saturation index - Wairakei geothermal water pilot plant turbine data

[0178] During the conventional pilot-scale operation of the Wairakei geothermal resource, the decrease in the silica saturation index (SSI) was measured every 15 minutes over a 6-hour period. Data and results are as follows:Figure 7 As shown. The geothermal water temperature was 95℃, and a Ca:Si molar ratio of 0.4 was used. The SiO2 content in the water was measured at the start of operation, and this value was used to set the particulate Ca(OH)2 slurry dosage level under the 0.4 Ca:Si molar ratio operating conditions. Although the SiO2 content in the geothermal water was observed to increase slowly over 6 hours due to natural fluctuations and changes in water composition, the Ca(OH)2 dosage level remained unchanged. This means that during actual operation, the Ca:Si ratio used was actually gradually decreased. Figure 7 The aim was to test the robustness of the calcium silicate hydroxide hydrate formation chemistry and to determine whether such fluctuations in the SiO2 content in water would affect SSI.

[0179] Example 8: Silica saturation index and silica saturation temperature - Wairakei, Kawerau and Mokai geothermal water pilot plant turbine data

[0180] During the pilot-scale operation of geothermal units in Wairakei, Kawerau, and Mokai, after the addition of granular Ca(OH)2 slurry and the formation of calcium silicate hydroxide hydrate, the silica saturation index (SSI) and silica saturation temperature (SST) of the inflow and outflow geothermal streams were measured. Figure 8 For comparison, a reinjection temperature of 85°C was used. These three different resources are located in vastly different geographical locations and are not connected to each other. The geothermal water temperature, brine chemistry, and silica concentration all vary. Therefore, a common reinjection temperature was chosen.

[0181] Example 9: Reduction in arsenic uptake

[0182] The calcium silicate hydroxide hydrate products formed at Ca:Si molar ratios of 0.2, 0.3, 0.4, 0.6, and 0.8 contained 500 and 1000 mg kg of calcium silicate hydroxide, respectively. -1 SiO2 and 6 mg kg -1 Arsenic synthesis and absorption of dissolved arsenic substances in geothermal water.

[0183] Figure 9 It shows a concentration of 1000 mg / kg at room temperature. -1 Geothermal water containing SiO2 and 500 mg / kg at 60°C -1 Results of SiO2 geothermal water. Figure 9 The results of a similar set of measurements conducted during pilot unit operation using Wairakei geothermal water containing approximately 550 mg / kg were also shown. -1 SiO2 and 6 mg kg -1 Arsenic, at a temperature of 90℃, with the same Ca:Si ratio.

[0184] Figure 10 The results showed that the concentrations were 500 and 1000 mg / kg at room temperature. -1 In the geothermal water for the synthesis of SiO2 (with the same Ca:Si molar ratio), in the Si:OH... - The effect of adding more hydroxide ions to Ca(OH)2 slurry on the absorption of arsenic substances during the formation of calcium silicate hydroxide hydrate with a molar ratio of 0.1.

[0185] Figure 9 Arsenic uptake data from synthetic geothermal water were obtained using the following method. For each Ca:Si ratio, a 3 L bulk solution was prepared by diluting a 33.2 wt% sodium silicate solution to provide the required 500 or 1000 mg / kg arsenic. -1 The content. A certain volume of sodium arsenate solution is added to provide 6 mg / kg. -1 The dissolved arsenic content of As was determined by adjusting the pH to 8.5 with sodium hydroxide to represent the geothermal water. 500 mL of the synthetic geothermal water was heated to the desired temperature. A 2.5 wt% Ca(OH)₂ suspension (to provide a specific Ca:Si molar ratio) was added to the geothermal water, and the mixture was rapidly stirred to form a calcium silicate hydroxide hydrate product. The product was separated by filtration, dried, and its Ca, Si, and As content was analyzed by atomic absorption spectrometry.

[0186] For containing 550 mg kg -1 SiO2 and 8 mg kg -1 As geothermal water from Wairakei, at 90°C, forms calcium silicate hydrate by adding a required amount of 2.5 wt% Ca(OH)₂ suspension at different Ca:Si ratios. The product is separated by filtration, dried, and its Ca, Si, and As content is analyzed by atomic absorption spectrometry.

[0187] Figure 10 The arsenic absorption data in the data are adopted with Figure 9 Figure 9 The same method yielded results using 500 and 800 mg kg, respectively. -1 The synthetic geothermal water was determined at room temperature. However, adding the required amount of sodium hydroxide solution to a 2.5 wt% Ca(OH)₂ suspension increased the Si:OH content. - The molar ratio was brought to 0.1 before being added to the synthetic geothermal water. The resulting calcium silicate hydroxide hydrate product was also subjected to similar separation, drying, and analysis of its Ca, Si, and As content.

[0188] Although the invention has been described by way of example, it should be understood that various changes and modifications can be made without departing from the scope of the invention as defined by the claims. Furthermore, if a known equivalent exists for a particular feature, that equivalent will be considered as specifically mentioned in this specification.

Claims

1. A process for recovering heat from geothermal water comprising: (i) treating a geothermal water stream containing dissolved silica by the addition of particulate calcium hydroxide, wherein the amount of calcium hydroxide added is such that the Ca:Si molar ratio is less than 1 to convert at least some of the dissolved silica to calcium silicate hydroxide hydrate particles suspended in the water stream, while increasing the pH of the water stream to at least 8; and (ii) passing the water stream through a heat exchanger to recover heat energy from the water stream.

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

3. The process defined in claim 1 or 2, wherein the Ca:Si molar ratio is in the range 0.3 to 1.

0.

4. The process defined in any one of claims 1 to 3, wherein the Ca:Si molar ratio is in the range 0.4 to 0.

6.

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

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

7. The process according to any one of claims 1 to 6, wherein the silicon dioxide is in the form of H3SiO4 or H4SiO4, or a combination thereof. - or H4SiO4, or a combination thereof.

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

9. The process defined in claim 8, wherein the concentration of calcium hydroxide in the suspension or slurry is up to about 15 wt %.

10. The process defined in any one of claims 1 to 9, wherein the inlet water stream temperature of the heat exchanger is in the range 100-200°C.

11. The process defined in claim 10, wherein the inlet water stream temperature is in the range 120-150°C.

12. The process defined in any one of claims 1 to 11, wherein the outlet water stream temperature is less than 80°C.

13. The process defined in claim 12, wherein the outlet water stream temperature is less than 50°C.

14. The process defined in any one of claims 1 to 13, further comprising generating electricity from the heat recovered from the heat exchanger.

15. The process defined in any one of claims 1 to 14, further comprising recovering heat energy from the water for direct heating applications.

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

17. The process defined in any one of claims 1 to 16, further comprising reinjecting the treated water stream into the ground.

18. The process according to any one of claims 1 to 17, wherein the amount of arsenic in the calcium silicate hydroxide hydrate is below 20 mg kg -1 .

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

20. The process defined in any one of claims 1 to 19, wherein the calcium silicate hydroxide hydrate particles are continuously separated from the geothermal water stream.

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