Method for chemically conditioning a heat-transfer fluid in a circuit of a power generation plant

Abstract

The invention relates to a method for chemically conditioning a heat-transfer fluid in a circuit of a power generation plant, the circuit (CS) comprising a hot source (GV) and a cold source (COND), each made of a metal alloy including iron, the method including the steps of: A) injecting reducing species and acid-base species into the heat-transfer fluid in order to define, at a given temperature, a hydrogen potential – reference redox potential pair; B) injecting a reducing species at the inlet (MP1) of the hot source in order to reduce the redox potential, thus increasing the solubility of the various phases of the iron in the heat-transfer fluid; and C) removing, at the inlet (MP2) of the cold source, the reducing species injected in step B), thus increasing the redox potential of the heat-transfer fluid, and reducing the solubility of the various phases of the iron in the heat-transfer fluid.

Description

[0001] DESCRIPTION

[0002] TITLE: PROCESS FOR THE CHEMICAL CONDITIONING OF A HEAT TRANSFER FLUID IN A CIRCUIT OF AN ENERGY PRODUCTION PLANT

[0003] Technical field of the invention

[0004] The invention relates to the field of chemical conditioning of a heat transfer fluid in a circuit of an energy production plant.

[0005] The invention may, for example, relate to a circuit of a heat production plant conveying a heat transfer fluid in single-phase form (liquid) between a hot source and a cold source. The invention may also relate to a secondary circuit of an electricity production plant conveying a heat transfer fluid in two-phase form between a hot source (steam generator) and a cold source (condenser).

[0006] Technical background

[0007] Figure 1 shows a schematic of a secondary circuit CS of a power generation plant.

[0008] This secondary circuit comprises different components, namely a hot source, here a steam generator, a cold source, here a condenser, a pipe connecting the steam generator to the condenser, integrating one or more turbines intended to be driven by the flow of heat transfer fluid (in the form of steam generated by the steam generator) and another pipe connecting the condenser to the steam generator and integrating a pump to drive the heat transfer fluid (in liquid form from the outlet of the condenser) and heaters.

[0009] The heat transfer fluid is usually water in a power generation plant.

[0010] In this type of circuit, we encounter phenomena of thinning of the walls in certain places and phenomena of deposition of material on the walls of the circuit in other places.

[0011] The phenomena of thinning of the walls of a circuit are known as dissolution and erosion-corrosion, preferentially located in the portions of the circuit carrying a heat transfer fluid in single-phase vapor form (pipe connecting the steam generator to the condenser and turbine). In the portions of the circuit carrying a heat transfer fluid in single-phase liquid form, corrosion also contributes to the thinning of the walls and the generation of species in the aqueous phase. The phenomena of material deposition on the walls of a circuit are known as fouling or clogging, mainly located in the single-phase liquid circuit portions (pipe connecting the condenser to the steam generator and pump) and two-phase liquid-vapor (steam generator, condenser).Fouling refers to precipitation of oxides or metals on the entire walls, for example of the steam generator, from species present in the aqueous phase, whereas clogging refers to localized precipitation on thermohydraulic singularities, for example fluid passages in tube plates or distribution plates.

[0012] The term dissolution-precipitation refers to a chemical action which modifies the passive film likely to be present on the wall by reducing its thickness by dissolution (which activates corrosion) or by increasing it by precipitation (which slows corrosion).

[0013] The term erosion-corrosion refers more to a mechanical action which removes part of the oxide and which has the consequence of accelerating the corrosion of the metal (loss of the passive film).

[0014] To limit these phenomena, which are harmful in the long term to the proper functioning of the electricity production plant, chemical conditioning of the heat transfer fluid is carried out. Thus, as visible in Figure 1, the secondary circuit includes a means of chemical conditioning, generally installed at the outlet of the condenser.

[0015] This chemical conditioning method allows the injection of reducing and acid-base species into the pipe connecting the condenser to the steam generator (liquid phase) to define a reference solubility of the different phases of Iron in the heat transfer fluid. Iron is in fact an element present in the different types of alloy constituting the different components of the secondary circuit.

[0016] The injection of reducing and acid-base species allows a hydrogen potential - redox potential (pH-Eh) pair to be adjusted to a given reference value. It should be remembered that the hydrogen potential (pH) measures, by the logarithm of the activity of the solvated proton, the acid-base character of the water, and that the redox potential (Eh), measures the oxide-reducing character of the water with respect to the species present in the water or the materials in contact with it (oxidation reaction of a metal towards an oxidized form, change in the oxidation state of a solvated chemical species, etc.).

[0017] The interest in adjusting the value of the couple (pH-Eh) can be understood with the support of figure 2.

[0018] Figure 2 shows the Pourbaix diagram (Eh as a function of pH) of iron in water at 150 °C and atmospheric pressure. The two lines (a) and (b) represent the thermodynamic stability domain of water (outside these lines (a) and (b), water decomposes by oxidation-reduction to give dioxygen or dihydrogen as the case may be). Depending on the values ​​taken by the hydrogen potential (pH) and the redox potential (Eh) of the solution, different oxides and aqueous species will appear. For example, at a temperature of 150 °C, magnetite (FeaO^) can only form at pH values ​​between 6 and 14 and Eh values ​​between -0.3 V and -1.2 V, i.e. in basic and reducing environments. This defines, for the given temperature, a stability domain for magnetite (possible form or degree of oxidation for iron in water).

[0019] Generally speaking, a Pourbaix diagram shows three domains for a metal: a corrosion domain where the metal oxidizes quickly, an immunity domain where the metal does not corrode and a passivity domain where corrosion is prevented by the formation of an oxide layer.

[0020] In the context of the chemical conditioning carried out on the heat transfer fluid of a secondary circuit of an electricity production plant, we therefore seek to define an Eh-pH couple in the passivity domain of a given oxide (magnetite in our example) among all the possible oxides of the metal concerned (Iron) to protect against corrosion.

[0021] The stability domain of the oxide is, however, relatively wide and leaves a wide range of possible values ​​for the pH - Eh couple. This is all the more true since, in a secondary circuit of an electricity production plant, the pressure is typically around 70 bars which makes it possible to extend the stability domain of water (in particular, curve (a) is shifted downwards compared to the representation in figure 2).

[0022] To set the most appropriate pH-Eh pair value, we seek in parallel to limit the solubility of the oxide thus formed.

[0023] Figure 3 shows the solubility diagram of magnetite at 150°C as a function of pH. This diagram shows that a minimum solubility of magnetite in water exists and is obtained for a pH between 9 and 11.

[0024] This range of pH values ​​reported on the Pourbaix diagram in Figure 2 makes it possible to define a more restricted domain for the redox potential Eh, approximately between -500mV and -900mV (at 150°C). In practice, Eh is typically set to a value of -600mV for a temperature of 25°C.

[0025] It is therefore this double condition on the stability domain of a given oxide (figure 2) and its minimum solubility (figure 3) which makes it possible to define an optimum pH-Eh couple. The value of this couple evolves with the temperature therefore when the heat transfer fluid moves within the circuit, it should be noted that a classical conditioning is typically taken, at 25°C, such that (pH, Eh) = (10, - 600mV). This limits the phenomena of dissolution-precipitation and erosion-corrosion of the walls of the components of the secondary circuit.

[0026] In practice, and as previously specified, the pH-Eh pair is adjusted by injecting reducing and acid-base species into the liquid phase (monophase).

[0027] In a secondary circuit (which operates in two-phase), ammonia (NH3), ethanolamine (C2H7NO) or morpholine (C4H9NO) can be chosen to adjust the hydrogen potential pH and hydrazine to adjust the redox potential Eh. All these species are soluble and volatile and can therefore be present in both the liquid and vapor phases encountered in the secondary circuit.

[0028] Figure 4 schematically represents what happens in the secondary circuit when the heat transfer fluid is in motion, in terms of temperature (top), the solubility of an element M (typically the Iron contained in the alloys of the different components of the secondary circuit) in the chemically conditioned heat transfer fluid (middle) and the thickness of material deposit or thinning (dissolution of oxides, erosion-corrosion) which results on the walls of the secondary circuit (bottom).

[0029] Thus, on the top curve, we note that the heat transfer fluid alternately sees its temperature increase in the steam generator (hot source) then decrease in the condenser (cold source) and remain relatively constant in the pipes.

[0030] On the right curve in the middle, we note that the CM solubility, eqof element M decreases with temperature (the composition in reducing and acid-base species determined by the quantity injected of these species by the chemical conditioning means of the secondary circuit remains constant, but the pH and the Eh vary with the temperature). In this case, we note on the middle curve that the CM solubility of element M in the heat transfer fluid decreases in the steam generator (hot source) then increases in the condenser (cold source) and remains relatively constant in the pipes.

[0031] This reflects the fact that the material flow of element M is directed from the heat transfer fluid to a wall in the steam generator (hot source). This is therefore where the secondary circuit is likely to encounter precipitation phenomena involving fouling and / or clogging. Conversely, a material flow of element M is directed from a wall to the heat transfer fluid in the condenser (cold source). This is therefore where the secondary circuit is likely to experience dissolution and / or erosion phenomena at its walls, also activating the corrosion of these walls containing element M (as a reminder, typically Iron). On the bottom curve, we also note the location of the deposit (hot source) and the location of corrosion (cold source).

[0032] The chemical conditioning currently in operation makes it possible to greatly limit the phenomena of dissolution-precipitation (fouling, clogging) and also erosion-corrosion.

[0033] However, these phenomena nevertheless exist and remain observable over long time scales, but nevertheless shorter than the operating times of electricity production plants.

[0034] The comments made previously in the case of a secondary circuit of an electricity production plant can be generalized to various power plant circuits.

[0035] One objective of the invention is therefore to reduce or even eliminate deposits clogging or fouling the walls of a circuit in an energy production plant.

[0036] Another objective of the invention is to be able to further limit the erosion-corrosion of the metal alloys present in such a circuit.

[0037] Furthermore, another objective is to reduce the quantity of toxic products used in such a circuit.

[0038] Summary of the invention

[0039] To solve at least one of the aforementioned objectives, the invention proposes a method for chemically conditioning a heat transfer fluid in a circuit of an energy production plant, the circuit comprising the following components: a hot source a cold source

[0040] - a first pipe connecting the outlet of the hot source to the inlet of the cold source

[0041] - a second pipe connecting the outlet of the cold source to the inlet of the hot source GV, each of said components being made of a metal alloy comprising iron, the method comprising a step consisting of:

[0042] A) injecting reducing and acid-base species into the second pipe to define, at a given temperature, a reference hydrogen potential - redox potential pair in the heat transfer fluid, characterized in that the method comprises the following additional steps:

[0043] B) inject a reducing species at the inlet of the hot source to reduce the redox potential of the heat transfer fluid, thereby increasing the solubility of the different phases of iron in the heat transfer fluid, and

[0044] C) remove, at the inlet of the cold source, the reducing species injected in step B) or inject, at the inlet of the cold source, an oxidizing species to compensate for the effects of the reducing species injected in step B), this increasing the redox potential of the heat transfer fluid and reducing the solubility of the different phases of Iron in the heat transfer fluid.

[0045] The method according to the invention may comprise at least one of the following additional steps, taken alone or in combination:

[0046] - during step B), a quantity of reducing species is injected ensuring that the solubility of the different phases of Iron over the entire area of ​​the circuit going from the inlet of the hot source to the inlet of the cold source is greater than the solubility of the different phases of Iron at the inlet of the hot source;

[0047] - the reducing species injected at the inlet of the hot source in step B) is hydrazine or dihydrogen, preferably dihydrogen;

[0048] - the oxidant injected into the heat transfer fluid at the inlet of the cold source in step B) is dioxygen;

[0049] - step A) consists of imposing a pH between 9 and 11 and a redox potential less than or equal to -550mV, for example between -650mV and -550mV, at a temperature of 25°C;

[0050] - the acid-base species injected during step A) is chosen from ammonia (NH3), ethanolamine (C2H7NO) or morpholine (C4H9NO) or a mixture thereof;

[0051] - the reducing species injected during step A) is hydrazine (N2H4);

[0052] - step A) is carried out at the outlet of the cold source;

[0053] - the circuit is a secondary circuit of an electricity production plant in which the hot source is a steam generator and the cold source a condenser, said circuit conveying a two-phase heat transfer fluid, in vapor form in the first pipe and in liquid form in the second pipe;

[0054] - the circuit is a circuit of a heat production plant, said circuit carrying a single-phase heat transfer fluid in liquid form.

[0055] Brief description of the figures

[0056] Other objects and characteristics of the invention will appear more clearly in the description which follows, made with reference to the appended figures, in which: [Fig. 5] is a representative diagram of a secondary circuit of an electricity production plant as provided for implementing the method according to the invention;

[0057] [Fig. 6] represents what happens in the secondary circuit shown in Figure 5 when the heat transfer fluid is moving there, in terms of temperature (top), the solubility of an element M, Iron in this case (middle), and the thickness of the material deposit or thinning which results on the walls of the secondary circuit (bottom)

[0058] [Fig. 7] provides the evolution of the redox potential (Eh, on the ordinate) as a function of the dihydrogen concentration (on the abscissa, logarithmic scale) for several values ​​of the dihydrogen pressure (curves mainly oriented vertically, lowest pressure on the left and highest on the right) and for several values ​​of the temperature (curves mainly oriented horizontally, lowest temperature at the top and highest at the bottom);

[0059] [Fig. 8] represents the evolution of the solubility of Iron in the heat transfer fluid (water) of the secondary circuit of the installation represented in [Fig. 5] as a function of the temperature, this evolution therefore being based, in accordance with the framework of the invention, on two operating points: a low temperature operating point PFBT and a high temperature operating point PFHT in the case of a reference conditioning based on ammonia;

[0060] [Fig. 9] represents the evolution of the solubility of Iron in a heat transfer fluid (water), always with a reference chemical conditioning based on ammonia, as a function of the temperature and this, for different pH values ​​and for each pH, ​​for different values ​​of the dihydrogen concentration;

[0061] [Fig. 10] is a representation similar to that of Figure 9, but for a reference chemical conditioning based on ethanolamine;

[0062] [Fig. 11] is a representation similar to that of Figure 9, but for morpholine-based reference chemical conditioning.

[0063] Detailed description of the invention

[0064] The following description is made in support of the attached figures 5 to 11.

[0065] Figure 5 is a representative diagram of a secondary circuit of an electricity production plant as intended to implement the method according to the invention.

[0066] This secondary circuit CS includes in particular the various conventional components shown in support of Figure 1. Thus, the secondary circuit CS includes a steam generator GV, a condenser GOND, a first pipe CA1 connecting the outlet of the steam generator GV to the inlet of the condenser GOND, said first pipe CA being intended to convey the heat transfer fluid in the form of steam and, a second pipe CA2 connecting the outlet of the condenser COND to the inlet of the steam generator GV, said second pipe CA2 being intended to convey the heat transfer fluid in the form of liquid. It should be remembered that each of the components of the secondary circuit is made of a metal alloy containing Iron.

[0067] This secondary circuit CS also includes a chemical conditioning means MCC in accordance with that of the prior art. This chemical conditioning means MCC makes it possible to implement, in accordance with what is currently carried out in the state of the art, a chemical conditioning step A) consisting of injecting reducing and acid-base species into the second pipe to define, at a given temperature, a reference hydrogen potential - redox potential (pH-Eh) pair in the heat transfer fluid. This MCC means is generally located at the outlet of the condenser COND (cold source) to which it can then be assimilated. Consequently, step A) is then generally carried out at the outlet of the condenser.

[0068] However, the secondary circuit CS further comprises a means MP1 for introducing a product into the secondary circuit, this means MP1 being located at the inlet of the steam generator GV (hot source). Furthermore, the secondary circuit CS further comprises a means MP2 for introducing or extracting a product from the secondary circuit CS, this means MP2 being located at the inlet of the condenser COND (cold source).

[0069] The following steps can then be implemented.

[0070] A step B) consisting of injecting a reducing species at the inlet of the steam generator GV, to reduce the redox potential (-AEh) of the heat transfer fluid and therefore increasing the solubility of the different phases of Iron in the heat transfer fluid. It is understood that this step B) is implemented using the MP1 means located at the inlet of the steam generator, inlet to which said MP1 means can be assimilated.

[0071] A step C) consisting of removing, at the inlet of the condenser, the reducing species injected in step B), this increasing the redox potential (+AEh) of the heat transfer fluid and therefore reducing the solubility of the different phases of Iron in the heat transfer fluid. Alternatively or in addition, step C) may also consist of injecting, at the inlet of the condenser, an oxidizing species to compensate for the effects of the reducing species injected in step A). ​​In all cases, it is understood that this step C) is implemented using the MP2 means located at the inlet of the condenser, an inlet to which said MP2 means can be assimilated. The implementation of steps B) and C) makes it possible to reduce the deposits fouling or clogging the walls of the secondary circuit.

[0072] Furthermore, advantageously, during step B) it is possible to inject a quantity of reducing species so that the solubility of the different phases of Iron over the entire area of ​​the secondary circuit going from the inlet of the steam generator GV to the inlet of the condenser GOND is greater than the solubility of the different phases of Iron at the inlet of the steam generator.

[0073] This also helps to limit the dissolution-precipitation and erosion-corrosion of the metal alloys present in the secondary circuit.

[0074] A situation which makes it possible to both reduce or even eliminate deposits clogging or blocking the walls of the secondary circuit while limiting the erosion-corrosion of the metal alloys present in this circuit is explained with the support of figure 6.

[0075] Figure 6 schematically represents what happens in the secondary circuit CS shown in Figure 5 when the heat transfer fluid is moving there, in terms of temperature (at the top), the solubility of an element M (Iron in this case) (in the middle) and the thickness of material deposit or thinning (dissolution-precipitation, erosion-corrosion) which results on the walls of the secondary circuit (at the bottom).

[0076] Thus, on the top curve of Figure 6, we note that the heat transfer fluid alternately sees its temperature increase in the steam generator (hot source) then decrease in the condenser (cold source) and remain relatively constant in the pipes. This top curve of Figure 6 is identical to the corresponding one in Figure 4.

[0077] On the right curve of Figure 6, we note that there are two operating points PFHT, PFBT, unlike what happens for the corresponding curve of Figure 4. The transition from one operating point to the other is carried out by implementing step B) and step C). For each operating point, the solubility CM, eq of element M decreases with temperature.

[0078] On the middle curve of figure 6, we note, just before the steam generator inlet (point A), that the solubility CM of element M in the heat transfer fluid is that provided by the chemical conditioning of step A). ​​We are then in a completely classic operating point.

[0079] Step B) is then implemented at the inlet of the steam generator (hot source), which has the effect of immediately increasing the solubility of element M in the heat transfer fluid (point B). We then move from a first, conventional PFBT operating point to a second, PFHT operating point.

[0080] The solubility of element M then decreases in the steam generator (hot source) with increasing temperature, but with values ​​linked to this second operating point up to the outlet of the steam generator (point C).

[0081] The solubility remains substantially constant in the first pipe up to the condenser inlet (point D).

[0082] At the condenser inlet, step C) of the method according to the invention is implemented. The solubility of element M in the heat transfer fluid then decreases immediately (point E). We then move from the second PFHT operating point to the first, conventional PFBT operating point.

[0083] Then, the solubility of element M in the heat transfer fluid increases in the condenser until its outlet (point F) with the decrease in temperature.

[0084] This solubility ultimately remains substantially constant in the second pipeline up to point A.

[0085] Thus, between point D (condenser inlet) and point A (steam generator inlet), we are in a first, classic PFBT operating point, which is the one shown in Figure 4. On the contrary, between point A (steam generator inlet) and point D (condenser inlet), we are in a second PFHT operating point, radically different from the first operating point due to the introduction of a reducing species in the secondary circuit at the steam generator inlet.

[0086] On the bottom curve of Figure 6, and unlike the corresponding curve of Figure 4, we note that there is then neither deposition of material (no precipitation) at the level of the steam generator (hot source) nor thinning (no dissolution) at the level of the condenser (cold source). This means that the precipitation of element M in the steam generator becomes impossible and that the dissolution of element M in the condenser also becomes impossible.

[0087] In the case where there is a partial overlap of solubilities between the two states PFHT and PFBT (contrary to what is represented in figure 6) for example because the quantity of reducing species introduced at the inlet of the steam generator is not sufficient, the efficiency is then not maximum. Nevertheless, this is still of interest for the secondary circuit because there remains a reduction in the quantity of material displaced between the condenser (dissolution, erosion) and the steam generator (precipitation, but also deposition which corresponds to a sedimentation of solid particles on the walls) compared to a classic conditioning at a single operating point.

[0088] From a practical point of view, it is important to determine the quantity of reducing species to be introduced at the inlet of the steam generator during step B) of the process according to the invention in order to change the redox potential as desired.

[0089] By definition, the evolution of the redox potential depends directly on the evolution of the concentration of dihydrogen in the heat transfer fluid (chemical species electrochemically equivalent to all reducing species, including hydrazine, of formula N2H4 is equivalent to 2 H2). We can for example refer to figure 7 which shows this link for different values ​​of temperature (roughly horizontal lines) ranging from 25°C to 300°C and for different values ​​of dihydrogen pressure (roughly vertical lines, lowest pressure on the left and pressure increasing towards the right) ranging from 10' 3bar to 10 bar. By definition also, the concentration of dihydrogen will depend on the additional mass flow rate of dihydrogen added or removed from the heat transfer fluid, taking into account the mass flow rate of heat transfer fluid circulating in the secondary circuit. The mass flow rate of heat transfer fluid present in the secondary circuit depends on the installation concerned, but it is known.

[0090] We will first present an example for a given installation and therefore a given (known) heat transfer fluid mass flow rate.

[0091] Example

[0092] In this example, the classical chemical conditioning (step A) is carried out with ammonia and hydrazine. The addition of reducing agent at the steam generator inlet (at a temperature of 200 °C in this example) decreases the redox potential from -519 mV / ENH to -553 mV / ENH. At the condenser inlet (at a temperature of 30 °C in this example), the redox potential is increased from -535 mV / ENH to -514 mV / ENH. With these conditions, no precipitation of iron oxides will occur in the steam generator since the solubility of the oxides remains constantly higher than that of iron in the heat transfer fluid.

[0093] Hydrazine can be chosen as a reducing agent added at the steam generator inlet.

[0094] But, we can also provide another molecule with a reducing character, including dihydrogen. Indeed, as can be seen with the values ​​provided above as an example, the variation in redox potential being low at each passage from one operating point to another, the quantity of dihydrogen necessary to achieve this variation at the inlet of the steam generator is also low. Concretely, a variation of -34mV of the redox potential at the temperature of 200 °C (i.e. passage from -519mV / ENH to -553mV / ENH) is obtained by adding dihydrogen to the heat transfer fluid in order to increase the concentration of dihydrogen dissolved in the heat transfer fluid (water) by +52.4 pmol.kg -1 (equivalent to 1.3 cm 3 TPN. kg -1of gaseous dihydrogen, TPN designating normal temperature and pressure conditions - this equivalence is intended to give the volume of gaseous dihydrogen under TPN conditions which would have to be introduced into the heat transfer fluid to obtain +52.4 pmol.kg -1 of hydrogen dissolved in the heat transfer fluid). The link between the desired evolution of the redox potential and the concentration of hydrogen to be added is made by Figure 7.

[0095] The addition of dihydrogen at such (very low) concentrations is advantageous because it will most of the time be spontaneously expurgated (step C) of the process according to the invention) from the heat transfer fluid at the condenser without the addition of oxidizing species. Indeed, the condenser typically provides vacuum pumping. However, for installations not having vacuum pumping at the condenser, a simple addition of an oxidant such as dioxygen (step C) of the process according to the invention), in our example at a concentration of +26.2 pmol.kg -1 of oxygen, will eliminate the hydrogen introduced into the secondary circuit during step A) and this, before the fluid passes through the circuit again.

[0096] Figure 8 shows more clearly what happens with this conditioning example. Figure 8 represents the solubility of iron in the heat transfer fluid as a function of temperature when following the heat transfer fluid through a cycle in the secondary circuit shown in Figure 5. Two PFHT and PFBT curves are shown for a high-temperature operating point and a low-temperature operating point, respectively. Points A to F are the same as those shown in Figure 5 (middle).

[0097] At point A, we are just before the steam generator inlet but before the injection of the reducer into the heat transfer fluid.

[0098] At point B, the reducer has been injected and we are just after the steam generator inlet.

[0099] Between points A and B, we therefore observe the effect of adding the reducer according to step B) at the inlet of the steam generator which results in a drastic increase in the solubility of iron in the heat transfer fluid.

[0100] Then, between point B and point C, we move from the steam generator inlet to its outlet. We see that the solubility decreases with the rise in temperature within the steam generator. At the outlet of the steam generator, the temperature then decreases until the inlet of the condenser and this describes the passage from point C to point D (first pipe).

[0101] The reducer is then removed in accordance with step C) of the process at the condenser inlet, which results in the transition from point D to point E and therefore a drastic reduction in the solubility of iron in the heat transfer fluid.

[0102] The condenser lowers the temperature and at the outlet, we arrive at point F.

[0103] Finally, from point F, we return to point A by going from the condenser outlet to the steam generator inlet (second pipe).

[0104] End of example.

[0105] Of course, beyond this example, it will be possible to work with ammonia (NH3) under other conditions. We can refer to the chart in Figure 9 which provides the solubility of iron in the heat transfer fluid (on the ordinate) as a function of the temperature (on the abscissa), for different values ​​of the hydrogen potential (pH), in this case pH = 9, pH = 9.5 and pH = 10 (taken at 25°C each time) and, for each aforementioned pH value, for different dihydrogen concentrations, respectively 1, 10 and 100 pmol / kg. Figure 9 makes it possible to adapt the quantities of reducing agent and / or oxidant to be used for conditioning at two operating points according to the invention, depending on the particular operating conditions of the secondary circuit (temperatures within the steam generator and the condenser, acid-base conditioning used on the installation, etc.).The conversion of the dihydrogen concentrations shown in Figure 9 into redox potential can be done using Figure 7.

[0106] It is also possible to use other classic conditioning methods than that carried out with ammonia (NH3), and therefore with ethanolamine (C2H7NO) or morpholine (C4H9NO) to adjust the pH, while hydrazine can be kept to adjust the redox potential (Eh).

[0107] The charts given in Figure 10 and Figure 11 correspond to that provided in Figure 9, respectively for ethanolamine (Figure 10) and morpholine (Figure 11). These figures make it possible to adapt the quantities of reducing agent and / or oxidant to be used for conditioning at two operating points according to the invention, depending on the particular operating conditions of the circuit. The conversion of the dihydrogen concentrations indicated in these figures into redox potential can also be carried out using Figure 7.

[0108] It should be noted that an additional and significant effect of the two-point operating conditioning according to the invention is expected on the precipitation of oxides in so-called "single-pass" steam generators, i.e. steam generators that vaporize all of the water they receive. By delaying and / or moving the area of ​​occurrence of oxide precipitation towards high temperatures in the steam generator, these oxides will have a high probability of being entrained in the steam rather than being fixed on the various walls. Indeed, in "single-pass" steam generators, the total transformation of liquid water into steam inevitably leads to an increase in the concentration of aqueous species, and ultimately, the reaching of a solubility limit for each element before the heat transfer fluid leaves the steam generator.Entraining the solids formed in the steam (solubility limit reached) is also a way of limiting fouling and / or clogging, even if in this particular case of “single pass” steam generators, it is impossible to completely avoid the precipitation of oxides in the water vaporization zone.

[0109] The preceding description, made with reference to a secondary circuit of an electricity production plant, also concerns a heat production plant, in which the circuit carries a single-phase heat transfer fluid in liquid form between the hot source (for example a boiler, or a heat exchanger recovering heat from a heat source) and the cold source (for example a heat exchanger).

Claims

CLAIMS 1. Method for chemically conditioning a heat transfer fluid in a circuit of a power generation plant, the circuit (CS) comprising the following components: - a hot spring (GV), - a cold source (COND), - a first pipe (CA1) connecting the outlet of the hot source (GV) to the inlet (MP2) of the cold source (COND), - a second pipe (CA2) connecting the outlet of the cold source (COND) to the inlet (MP1) of the hot source (GV), each of said components being made of a metal alloy comprising iron, the method comprising a step consisting of: A) injecting reducing and acid-base species into the second pipe (CA2) to define, at a given temperature, a reference hydrogen potential - redox potential (pH-Eh) pair in the heat transfer fluid, characterized in that the method comprises the following additional steps: B) inject a reducing species at the inlet (MP1) of the hot source to reduce the redox potential (Eh) of the heat transfer fluid, this increasing the solubility of the different phases of Iron in the heat transfer fluid, and C) remove, at the inlet (MP2) of the cold source, the reducing species injected in step B) or inject, at the inlet (MP2) of the cold source, an oxidizing species to compensate for the effects of the reducing species injected in step B), this increasing the redox potential (Eh) of the heat transfer fluid and reducing the solubility of the different phases of Iron in the heat transfer fluid.

2. Method according to claim 1, characterized in that during step B), a quantity of reducing species is injected ensuring that the solubility of the different phases of Iron over the entire area of ​​the circuit going from the inlet of the hot source to the inlet of the cold source is greater than the solubility of the different phases of Iron at the inlet of the hot source.

3. Method according to one of the preceding claims, characterized in that the reducing species injected at the inlet of the hot source in step B) is hydrazine or dihydrogen, preferably dihydrogen.

4. Method according to one of the preceding claims, characterized in that the oxidant injected into the heat transfer fluid at the inlet of the cold source in step C) is dioxygen.

5. Method according to one of the preceding claims, characterized in that step A) consists of imposing a pH between 9 and 11 and a redox potential less than or equal to - 550mV, for example between -650mV and -550mV, at a temperature of 25°C.

6. Method according to one of the preceding claims, characterized in that the acid-base species injected during step A) is chosen from ammonia (NH3), ethanolamine (C2H7NO) or morpholine (C4H9NO) or a mixture thereof.

7. Method according to one of the preceding claims, characterized in that the reducing species injected during step A) is hydrazine (N2H4).

8. Method according to one of the preceding claims, characterized in that step A) is carried out at the outlet (MCC) of the cold source (COND).

9. Method according to one of the preceding claims, characterized in that the circuit is a secondary circuit of an electricity production plant in which the hot source is a steam generator and the cold source a condenser, said circuit conveying a two-phase heat transfer fluid, in vapor form in the first pipe and in liquid form in the second pipe.

10. Method according to one of claims 1 to 8, characterized in that the circuit is a circuit of a heat production plant, said circuit conveying a single-phase heat transfer fluid in liquid form.

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