FR3005143A1 - THERMAL INSTALLATION FOR THE PRODUCTION OF ELECTRICITY BY COMBUSTION - Google Patents

Abstract

The invention relates to a thermal installation (1) for producing electricity by combustion, comprising: - a boiler (12); a flue gas discharge duct (100); - a main circuit (13) on which are arranged successively a steam generator (10) in heat exchange with the boiler (12), at least one steam expansion turbine (14a, 14b), a condenser (16), a degassing cover (17) and a liquid compressor (15); - power generation means (140) driven by the at least one turbine (14a, 14b); the installation being characterized in that it further comprises a device (30) putting in heat exchange the flue gas discharge duct (100) and the main circuit (13) between the condenser (16) and the cover degassing (17). The invention further relates to a device for improving the performance of a thermal installation and a thermal transfer method according to a thermodynamic cycle called "Rankine".

Description

GENERAL TECHNICAL FIELD The present invention relates to an improved thermal plant for the production of electricity. STATE OF THE ART Current large-scale power plants are installations in which thermal energy (supplied by combustion fumes, a nuclear reactor, a solar-concentrating furnace, etc.) is conventionally converted into electricity. via a turbine and an alternator changing the state of a two-phase fluid (which is almost exclusively water when the order of magnitude of the megawatt is exceeded for the supplied electrical power) following the "Rankine cycle" . The so-called "thermal" or "flame" plants use a fuel (oil, gas, fuel oil, biomass, waste, etc.) burned in a fireplace in the presence of air, possibly enriched with oxygen. A boiler allows the transfer of heat from the combustion fumes to heat and vaporize the water. In optimized installations, this cycle can comprise the following stages (the values indicated below are illustrative and not limiting): - compression (110 bar) of liquid water (called "feedwater") by a pump of liquid water; - Heating and evaporation of this water in the boiler; overheating of the steam produced (500 ° C); - Relaxing this steam in one or more high-pressure turbines (to lower the temperature to 8 bar), producing mechanical energy; - reheating of water vapor (300 ° C); - expansion in one or more low pressure turbines, still producing mechanical energy; - condensation of steam at a "cold source"; - slight compression of liquid water by a pump called "condensate"; - Passage of water in a degassing tarpaulin at a pressure slightly above atmospheric pressure (for example 1.2 bar); - recompression of water at 110 bar, etc.

The degassing tarpaulin has the important function of evacuating traces of dissolved oxygen in the feedwater. The principle of this cover is based on Henry's Law, which states that the maximum concentration of a gas in solution, in equilibrium with an atmosphere containing this gas, is proportional to the partial pressure of this gas at this point. Oxygen dissolves badly in the hot water, a fraction of the water vaporizes in the tank, causing the oxygen out of the circuit. In order for the liquid water to have a sufficient temperature for the tank to play its role, it is usual to use heat from a steam stream extracted from one or the other of the turbines. At the outlet of the tarpaulin, it is also common to preheat the water before sending it into the boiler, always with steam extracted from a turbine. Indeed, it is known to those skilled in the art that the liquid water must be heated as much as possible by steam extracted from the turbines.

The encyclopedia "Power Plant Engineering", a reference book on large thermal installations by Black and Veatch, even teaches that the complete preheating of water by steam is "ideal" from a thermodynamic point of view. To further reformulate, the optimization of a Rankine cycle 30 therefore requires preheating the condensed water downstream of a steam turbine as much as possible by means of steam extracted from this turbine.

This encyclopedia thus proposes architectures in which the liquid water leaving the condenser is preheated a first time by the steam extracted by the low pressure turbine, then a second time at the level of the tank by the steam extracted from the high pressure turbine, then a third time at the outlet of the compressor again with steam extracted from the high pressure turbine. RANKINE's theory of maximizing electrical output would indeed require that steam be continuously extracted "throughout the turbine" to continuously heat the liquid water until it boils. The person skilled in the art therefore seeks today to maximize the number of steam extractions from the turbines in order to gradually heat up this liquid water. Very large installations (> 500 MWe) are known with up to twelve steam extractions at various points of the turbines. The water thus reaches 100 ° C by the sole influence of the extracted steam. However, it is difficult to extract steam at even higher temperatures since each extraction of the turbine (especially when the extraction is upstream of the turbine) locally degrades the flow of steam and alters the production of mechanical energy. It is thus known to use the residual heat of the fumes at the outlet of the boiler to further preheat the liquid water before sending it into the boiler for evaporation. This is generally done by using an "economizer", which is an expensive exchanger designed to withstand the combustion fumes and possibly the acidic compounds they may contain (S03, HCl, acetic acid, etc.). Another economizer may also allow the recovery of residual heat from the flue gases to preheat the combustion air (the fresh air charged with oxygen that is injected into the firebox). However, if the fumes are in contact with a cold spot, acid condensation may occur on the walls of the economizer, which will slowly but surely corrode it. This condensation is variable according to many parameters (fuel composition, water vapor content, presence of alkalis, etc.) but usually occurs at 60 to 90 ° C.

As outdoor air is by definition variable in temperature, it is common (and particularly when burning biomass) to preheat - again with steam extracted from the low pressure turbine - combustion air up to at about 100 ° C before sending it to the economizer.

In doing so, the economizer is protected from corrosion between the air and the fumes, but the fumes then come out at temperatures that are frequently at 140/160 ° C, hence a potential of still exploitable energy. This energy potential is even more important in cases where: - the combustion air is enriched in oxygen, or even consists of pure oxygen (which is often used for incinerators), because in fact it is dangerous to overheat air with a high oxygen content since it can then attack the metals, or even burn them; - the fuel introduced into a boiler is "heavy" (that is to say, it has a large mass compared to that of the air necessary for its combustion: coal, biomass, waste ... in opposition to natural gas (CH4) or at the extreme of pure hydrogen H2), because the mass flow rate of the fumes (sum of the mass flow rates of combustion air and fuel) is then much greater than that of air of combustion, which means that it has more energy to transmit than the air can absorb, or in other words, that the fumes will drop less in temperature than the air will rise in temperature for the same power transferred fumes to the air. If in the vicinity of the installation, there is a need for low temperature heat, for example an urban network with water at 80 ° C, it is then known to those skilled in the art to put a last exchanger in fumes to supply this external heat network (cogeneration).

But this scenario is rare, because often these facilities are built away from residential areas to minimize environmental risks (fire, etc.) or societal constraints (residents do not want to support the truck traffic, or see the factory).

On the other hand, and particularly in the case of biomass, these installations are designed for "nominal" conditions: fuel with well-defined characteristics, outdoor air at 15 ° C, heat requirement of a steam customer (case of cogeneration) of XT / hour of steam at such pressure, etc. The operational reality is naturally different from the nominal case. The various automations make it possible to compensate for the irregularities of a certain number of these parameters, but it is found that the outlet temperature of the fumes always fluctuates by about twenty degrees. The design must necessarily incorporate this and provide a "safety margin" to prevent the fumes come out too cold with a fuel that could introduce acids (a simple bottle of PVC plastic inadvertently thrown into the fuel will generate hydrochloric acid) and can corrode an element of the installation. Those skilled in the art also know how to greatly cool these fumes in order to cause the condensation of the water vapor contained in the fumes. This is called "condensing" boilers. If we can value (heat network) energy at a temperature below the dew point of the fumes (typically 50 to 60 ° C) then we recover a significant amount of heat, minimizing corrosion problems. Indeed, the acids will condense but will be very diluted by water which will reduce the corrosive potential. Thus, either the flue gas temperature remains very high, or its complete condensation is caused, but provided that a suitable cold source is available. In all cases, the smoke energy can not be used for electricity.

It would therefore be interesting to be able to further increase the overall performance of these power generation facilities regardless of their environment, while avoiding the risk of corrosion.

PRESENTATION OF THE INVENTION According to a first aspect, the invention relates to a thermal installation for producing electricity by combustion, comprising: - a boiler; - a flue gas exhaust duct; a main circuit on which are successively arranged a steam generator in heat exchange with the boiler, at least one steam expansion turbine, a condenser, a degassing tank and a liquid compressor; - Means for generating electricity driven by the at least one turbine The installation being characterized in that it further comprises a device putting in heat exchange the flue gas discharge pipe 20 and the main circuit between the condenser and the degassing tank. The installation according to the invention is advantageously completed by the following characteristics, taken alone or in any of their technically possible combination: the device comprises a heat transfer fluid circuit on which are arranged: a first heat exchanger engaging the flue gas discharge duct and heat transfer fluid circuit; a second heat exchanger putting into engagement the main circuit and the heat transfer fluid circuit. - The device further comprises a probe disposed in the flue gas duct near the first heat exchanger, and means for regulating the coolant temperature at the first heat exchanger according to smoke parameters measured by the probe; - The regulating means comprise a pump regulating the flow of heat transfer fluid in the first exchanger and / or a three-way valve 5 regulating the proportion of heat transfer fluid from the first exchanger transmitted to the second exchanger; in which the parameters measured by the probe are at least one characteristic parameter of a corrosion potential and / or a flue gas temperature, the coolant temperature at the first exchanger being regulated so that the temperature of the fumes at the level of the first exchanger is greater than the acidic dew point of the fumes; the heat transfer fluid has a temperature of between 70 ° C. and 110 ° C. at the outlet of the first exchanger when the device is in nominal operation; - The heat transfer fluid is liquid water or steam, at a pressure between 0.6 bar and 1.5 bar, the first exchanger being an evaporator and the second exchanger being a condenser; - The heat transfer fluid is liquid water at a pressure of at least 3 bar; - The flue gas exhaust duct comprises smoke treatment means, the device being disposed at the outlet of the flue gas treatment means; the degassing tarpaulin is fed by a first branch of the main circuit for extracting steam from a turbine; the installation comprises at least two turbines including a first turbine and a second turbine, the main circuit comprising a second branch for extracting steam from the second turbine, the second branch being in heat exchange with the circuit at the level of a first preheating exchanger disposed on the main circuit between the condenser and the device. the steam circuit comprises a third branch for extracting steam from the first turbine, the third branch being in heat exchange with the main circuit at a second preheating exchanger disposed on the main circuit downstream of the tank; degassing; - The at least one second exchanger of the device engages the heat transfer fluid circuit and the main circuit between the first preheating exchanger and the degassing cover; the device comprises a second additional exchanger putting into engagement the coolant circuit and the main circuit between the condenser and the first preheating exchanger. According to a second aspect, the invention proposes a device for improving the performance of a thermal plant for producing electricity by combustion, the installation comprising a boiler; a flue gas exhaust duct; a main circuit on which are successively arranged a steam generator in heat exchange with the boiler, at least one steam expansion turbine, a condenser, a degassing tank and a liquid compressor; electricity generating means driven by the turbines; the device being characterized in that it puts in heat exchange the flue gas exhaust duct and the main circuit between the condenser and the degassing tank. According to a third aspect, a thermal transfer process according to a "Rankine" thermodynamic cycle implemented by a thermal plant for producing electricity by combustion comprising a boiler; a flue gas exhaust duct; a main circuit on which are arranged successively a steam generator in heat exchange with the boiler, at least one steam expansion turbine, a condenser, a degassing tank and a liquid compressor; the method being characterized in that it comprises steps of: - evaporation of liquid water in water vapor in the circuit at the steam generator; 5 - expansion of the water vapor in the at least one turbine; condensation of the water vapor in liquid water in the condenser; - Preheating the liquid water at a device in heat exchange with the flue gas discharge circuit; degassing the liquid water in the degassing tarpaulin; 10 - compression of the liquid water by the compressor. PRESENTATION OF THE FIGURES Other characteristics, objects and advantages of the invention will emerge from the description which follows, which is purely illustrative and nonlimiting, and which should be read with reference to the accompanying drawings, in which: FIG. diagram of a thermal installation according to the prior art; FIG. 2 is a diagram of one embodiment of a thermal installation according to the invention. DETAILED DESCRIPTION General Architecture FIG. 1 represents a thermal power plant 1 for producing electricity according to the prior art, in accordance with the optimized embodiment described above. FIG. 2 represents an installation 1 according to the invention, corresponding in particular to the installation represented in FIG. 1, equipped in addition with the device 30 for improving energy performance. However, it will be understood that the invention is not limited to this embodiment. This installation 1 (known or according to the invention) uses the energy of a combustion, whether of gas, fuel oil or petroleum, but also waste or biomass, which will be discussed in more detail later on. . The installation 1 comprises a boiler 12 having a focal point 120, in which the fuel is burned. Such foci are known to those skilled in the art, and it will be understood that the invention is not limited to any particular geometry or dimensioning. The boiler 12 is in heat exchange with a main circuit 13 at at least one steam generator 10. This main circuit 13 is a circuit in which water circulates under the liquid or gaseous states (steam of water). The position of the elements on this circuit 13 will be defined according to the direction of flow of the liquid / vapor water: for example, "upstream" means in the reverse direction of the flow. The steam generator 10, in which water (at very high pressure) vaporizes vapor, is followed on the circuit 13 of a superheater 11, also located at the boiler, closest to the hearth 120. The superheater 10 is, for example, a tube gas / gas exchanger for heating the temperature up to 500 ° C. This overheating prevents condensation of the steam during its turbining. At the outlet of the boiler 12, a flue gas exhaust duct 100 receives and transports the flue gas to a chimney 113 where it is discharged into the atmosphere. Prior to this discharge, the flue gases are treated via flue gas treatment means 110, which generally comprise a cyclone 111 (for the recovery of solid dust in the flue gases) and a dewatering fan 112 (also known as a draft fan). The flue gas treatment means 110 may also comprise filters and chemical units. At the beginning of the duct 100, despite the heat transfer to the superheater 11 and then to the steam generator 10, the flue gases are still very hot.

A first economizer 18 as described above is disposed at the inlet of the duct 100, and makes it possible to preheat the liquid water entering the steam generator 10. At the output of the first economizer 18, a second economizer 21 can engage the duct 100 with a combustion air duct 20. This second duct 20 allows the extraction of atmospheric fresh air charged with oxygen for injection into the furnace 120 of the boiler 12. With the second economizer 21, this combustion air is preheated so as to reduce the amount of energy consumed to bring this air to the combustion temperature.

The combustion air may be atmospheric air, but also pressurized air and / or air enriched with oxygen, or even pure oxygen. The superheated steam circulating in the circuit 13 is successively expanded in at least one turbine, and advantageously at least two turbines (or groups of turbines): a first turbine 14a (high pressure turbine) then a second turbine 14b (low turbine Pressure). The turbine or turbines 14a, 14b are connected to one or more electricity generators 140 (the turbines 14a, 14b may, for example, share a common axis, or even be in the same casing, the two turbines described being then simply two stages (one stage consisting of a row of rotor blades) of the same turbine). The number of turbines depends on the power: up to 20 to 30 MWe there is usually only one, which must be divided beyond that for size issues (the number of turbines / stages can be much higher together). In the following description we will take the example of two independent turbines, but it will be understood that the invention is not limited to this configuration, and that the presence of a turbine is sufficient to implement the Rankine cycle. . Between the first and second turbines 14a, 14b, the steam may be reheated (not shown). The twice-expanded vapor, still in the gaseous state (or two-phase but mostly gaseous) is condensed in the condenser 16, at a temperature between 10 and 60 ° C. At the outlet of the latter, a condensate pump 150 operates a slight rise in pressure. Before returning to the steam generator 10, the liquid water can undergo several rise in temperatures which will be detailed below. In all cases, it passes through a degassing cover 17 at which it releases the dissolved oxygen it contains, then undergoes a high compression (typically 110 bar) at the compressor 15.

Preheating the liquid water The plant 1 according to the invention differs from the optimized installations known in the manner of preheating the liquid water before vaporization.

In particular, as can be seen in FIG. 2, it comprises a device 30 for improving energy performance which puts in heat exchange the flue gas exhaust system 100 and the main circuit 13. But compared with conventional economizers ( in particular the first economizer 18), this device 30 is particularly distinguished in that it is disposed on the circuit 13 between the condenser 16 and the degassing cover 17. This device 30 advantageously heats the condensates at a temperature of 80 ° C. at 100 ° C and partially replaces one or more of the steam extractions of the turbines 14a, 14b (via the branches 130a 130b, 130c which will be described in more detail later in this description). This may seem paradoxical at first glance, since, as explained above, the thermodynamic ideal consists of a preheating of the liquid water by the sole energy of the sampled vapor.

The device 30 is however not intended to provide all the energy necessary for condensate heating and thus be in contradiction with the conclusions of the experts who sought to optimize the Rankine cycle. Indeed, by providing the condensate heat that would otherwise be rejected to the atmosphere and lost (since as we will see the device 30 can lower the temperature of the fumes of another 20 ° C or 40 ° C without increasing the risk of corrosion), it is avoided to extract a certain amount of steam (around 3 bar) of the turbines 14a, 14b. This steam remains in the turbines 14a, 14b and allows, by fully relaxing, an increase in electricity production greater than the loss due to the fact that the new cycle deviates slightly from the perfect thermodynamic cycle. The device 30 is in this respect contrary to the traditional teaching that states that the optimization of a Rankine cycle requires maximum preheating of the condensed water downstream of a steam turbine 15 by means of steam. extracted from this turbine. From an energy point of view, lowering the flue gas by 40 ° C allows additional recovery of up to 2% of the thermal power of the boiler, whereas heating the condensates up to 100 ° C may require a total of 3 at 4% of this power (depending on the size of the installation). Calculations show that the expansion up to 0.1 bar of non-extracted steam provides 0.4% more electricity. This may seem small, but insofar as the typical electrical efficiency of a large biomass combustion plant is only one-third, the relative increase in electricity generation compared to the prior art is then greater than 1.2%, which is ultimately very profitable for large installations (several tens or even hundreds of thousands of euros per year depending on size). Particularly advantageous embodiments of the device 30 will be described later. As explained, the latter does not replace the preheating of the water by means of steam extraction, which is why, as can be seen in FIG. 2, the installation 1 always includes a supply of the degassing tarpaulin 17 via a first branch 130a of the main circuit 13 for the extraction of steam from one of the turbines 14a, 14b. In addition, the installation 1 comprises a second branch 130b for extracting steam from the second turbine 14b and / or a third branch 130c for extracting steam from the first turbine 14a, the second branch 130b being in exchange thermal circuit with the circuit 13 at a first preheating exchanger 131a and / or the third leg 130c being in heat exchange with the circuit 13 at a second preheating exchanger 131b. The first and second exchangers 131a, 131b are disposed on either side of the degassing cover 17 as shown in FIG. 2. The device 30 is advantageously disposed at the outlet of the first preheating exchanger 131a (in other words just before degassing tarpaulin 17). It therefore makes it possible to limit the extraction of steam to a temperature level higher than that of the device 30, in particular the extraction via the first branch 130a (hence the more vapor retained in the second turbine 14b). However, it will be understood that depending on the temperature and pressure levels of the main circuit 13 and the configuration of the turbines 14a, 14b, it may be more advantageous to place the device 30 upstream of the first preheating exchanger 131a, or even to remove the latter (as well as the second branch 130b). As a numerical example for a biomass plant, the first preheating exchanger 131a can mount the condensates at 90 ° C with steam extracted at about 1.5 bar, and then the degassing tank 17 is heated to 120 ° C with water. steam at 3 bar. Due to the particularly high temperature obtained at the outlet of the first exchanger 131a, it becomes more advantageous to place the device 30 upstream of the latter, so that it is the extraction via the second branch 130b rather than the first one. 130c branch that can be reduced. The present installation is thus not limited to any particular configuration, it is only important that the device 30 be disposed between the condenser 16 and the degassing cover 17. Furthermore, it will be noted that the extraction of steam on the second branch 130b, is usually "unregulated". This means that the pressure of the steam extracted from the second turbine 14b is not constant. It varies for example as a function of the outside temperature which influences the temperature of the condenser 16 and therefore the pressure thereof. The pressure variations of the condenser "up" then the turbine 14b causing pressure variations at the branch 130b. Conversely, the pressures of the extractions of high pressure turbines such as the first turbine 14a are "regulated" at a constant pressure, so as to guarantee their use in a downstream process. The "unregulated" outputs disturb less dynamic flows in the turbine (hence more mechanical power) and are therefore used to heat the condensate. This then results in the temperature of the condensates leaving the first preheating exchanger 131a is not constant and will vary for example from 75 to 95 ° C depending on the day. On the other hand, the degassing cover 17 will be kept permanently at the same temperature (120 ° C. in the example) by a heat booster of a "regulated" outlet at the level of the extraction of the first branch 130 a. It will be described later an architecture of the device 30 particularly suitable for the case of "unregulated" extractions of steam. To sum up, in the advantageous embodiment represented by FIG. 2 (corresponding to a standard installation in which the temperature of the liquid water at the outlet of the first preheating exchanger 131a is of the order of 60-70 ° C.), the liquid water at the outlet of the condenser 16 is successively preheated, before reaching the steam generator 10, by: - the first preheating exchanger 131a, - the device 30, - the degassing cover 17, - the second preheating exchanger 131b, - the first economizer 18. Device for improving energy performance The device 30 may be a conventional economizer (similar to the first economizer 18) in which the liquid water of the circuit 13 passes, but alternatively it may have a more complex architecture. complex (visible in Figure 2), which makes it particularly effective in recovering the energy of the fumes while preventing any risk of corrosion.

In this preferred architecture, the device 30 comprises its own heat transfer fluid circuit 31 on which are arranged: a first exchanger 32 putting into engagement the flue gas discharge circuit 100 and the heat transfer fluid circuit 31; at least one second heat exchanger 33 engaging the main circuit 13 and the heat transfer fluid circuit 31. This separate circuit 31 (which is preferably a water circuit, but other heat transfer fluids are conceivable) presents all the Firstly, the advantage of preventing any contamination of the very pure water of the circuit 13 by compounds coming from the combustion fumes. In addition, it can serve as a thermal "buffer" in the event of abnormally low temperature and / or abnormally corrosive composition of the fumes (for example in the event of a plastic bottle falling into the fuel), so as to prevent the condensation of 25 compounds that could damage the conduit 100 and more particularly the device 30. For this, the device 30 further comprises a probe 35 disposed in the flue gas duct 100 near (preferably upstream) of the first exchanger 32, and means 34 for regulating the temperature of the coolant at the first exchanger 32 as a function of the parameters of the fumes measured by the probe 35.

Many embodiments are possible for the temperature control means 34, but two examples will be described in particular. In a first embodiment, the means 34 consist of a pump regulating the flow of coolant in the first exchanger 32. The idea is to slow down, or even cut, the flow of water in the circuit 31 in case of risk of corrosion. This has the effect of reducing the amount of thermal energy transferred via the device 30, resulting in an increase in the temperature of the water of the circuit 31 at the level of the first exchanger 31, fumes which decrease less in temperature, and a eliminating the risk of condensation. This is the embodiment shown in FIG. 2. As an alternative to controlling the flow rate via the pump, it is possible to use a three-way valve regulating the proportion of coolant from the first heat exchanger 32 transmitted to the second heat exchanger. 33. In other words, when the three-way valve is completely closed 100% of the fluid leaving the first exchanger 32 is sent to the second exchanger 33, and when it is fully open 100% of the fluid loop in the first exchanger 32 without back through the second exchanger 33. All intermediate ratios are possible between these two positions. Thus the three-way valve allows - in case of risk of corrosion - water in the first exchanger 32 to circulate on itself at a constant rate, hence its rapid increase in temperature. The regulating means 34 may comprise both a three-way valve and a controllable pump for even more precise management of the fluid temperature. Indeed, the two compounds most likely to cause corrosion are HCl (hydrochloric acid) and especially H2SO4 (sulfuric acid). In flue gases, HCl is present as such (hydrogen chloride), but sulfuric acid is present as SO2 (sulfur dioxide) and SO3 (sulfur trioxide) which can react with H2O to form H2SO4 ( as well as H2SO3, sulfurous acid, in lesser proportions) in complex reactions. S03 is the most dangerous, as SO2 is a neutral gas that only attacks the metal at high temperatures. The aforesaid gases including secondary residues from the combustion (oxidation) of fuels containing chlorine and sulfur. For example, wheat straw is known to have high levels of chlorine.

As long as they are gaseous, there is no problem, but they can condense at higher temperatures than water vapor: this is known as the acidic dew point, which will lead acid droplets and pitting corrosion. It will be appreciated that alkalis (sodium Na, potassium K) aggravate the problems by reacting with acids, forming dusts less than one micron in diameter which will settle. The dew point of the acid is mainly given by the SO 3 content. It is to be distinguished from the dew point temperature of the water, which is given by the partial pressure of the water vapor. For example, a content of 4 mg / Nm 3 of SO 3 leads to an acidic dew point of 94 ° C, hence the known practice of never going below 120 ° C. This margin of safety is no longer necessary thanks to the control loop formed by the probe 35 and the pump 34. The parameters measured by the probe 35 are thus at least one characteristic parameter of a corrosion potential and / or a flue gas temperature (advantageously both, it will be understood that the probe 35 may designate a set of probes), the coolant flow rate in the circuit 31 being regulated so that the temperature of the fumes circulating around the first exchanger 32 remains higher at the point of acidic dew smoke.

By characteristic parameter of a corrosion potential is meant any parameter relating to the composition of the fumes which makes it possible to estimate the risk of corrosion and the acidic dew point. In particular, this parameter may be a hydrogen potential, or the measurement of the concentration in one or more chemical elements. For example, MetsoTM offers analyzers that measure a concentration of chlorine, a concentration of sulfur, and estimating the risk of corrosion from the ratio S / CI concentrations. If the acidic dew point of the fumes is much lower than their current temperature, the pump 34 can be rotated at maximum speed to use the heat of the fumes as widely as possible to preheat the liquid water of the circuit 13. If it is found that the risk of corrosion worsens (arrival of a polluted fuel), "pollution is allowed to pass" by slowing down the flow rate / increasing the rate of recirculation of the fluid (depending on the control means 34 selected) and raising the temperature of the first heat exchanger 32. Its temperature then greatly exceeds the acidic dew point, and this prevents any condensation, or even allows to re-evaporate the first acid condensates that would have formed. The device 30 may include data processing means which generates the instructions sent to the pump 34 based on the measurements received from the probe 35 and pre-established rules. The device 30 is designed as "end of cycle", i.e., it is intended to be disposed in line 100 after all other economizers 18, 21 and just before the chimney 113; that is, when the smoke is at its minimum temperature. The device 30 is preferably disposed at the outlet of the flue gas treatment means 110. It may, however, be disposed before or after a dewatering fan 112. In order to limit the consumption of this fan 112, it is desirable that the first exchanger 32 is not bulky to reduce the pressure drop. It is preferentially a "tube" type exchanger. To reduce the diameter of these tubes without sacrificing the heat exchange capacity, an advantageous solution to be provided is that the heat transfer fluid in the circuit 31 is liquid water at a pressure of between 0.6 bar and 1.5 bar, advantageously a pressure slightly greater than or equal to atmospheric pressure. At temperatures of 70 ° C to 110 ° C provided in the circuit 31, the water vaporizes (the first exchanger 32 is an evaporator). This is true when the pump 34 is in nominal operation, i.e. the flow rate is not reduced because of the risk of corrosion. If the first exchanger 32 is an evaporator, the second exchanger 33 is a condenser at which the vapor condenses in contact with the liquid water of the circuit 13 at still low temperature. The pump 34 must then be disposed on the liquid return leg of the circuit 31. Alternatively, the heat transfer fluid may be liquid water (without phase change) at a pressure of at least 3 bar, advantageously about 9 bar. . In addition, knowing that the "dangerous" zone in the fumes is around 60/90 ° C, the first exchanger 32 is advantageously designed to allow sufficient mixing of the fumes so that they do not cool too much locally (since the liquid water in the return circuit 31 to the first heat exchanger 32 is at a low temperature level) but lowers slowly in average temperature (homogeneously). This all the more makes it possible to have low pressure losses and therefore not to have a high electricity consumption for the dewatering fan 112. In the case of a "non-regulated" steam extraction via the 30 second branch 130b, a preferential way of optimizing the device 30 is to have two second exchangers 33 (not shown) of the device 30 connected in parallel on either side of the first preheating exchanger 131a. In other words, we add a second additional exchanger 33 'between the condenser 16 and the first preheating exchanger 131a.

When the steam of the second branch 130b is at a high temperature / pressure, (therefore at a temperature greater than that of the circuit 31), it becomes more advantageous to inject the heat of the device 30 to the circuit 13 at the second exchanger additional 33 '. It is then the second branch 130b that will see the steam flow therethrough decrease (instead of the first branch 130a) since the condensate will get hotter to the first preheating exchanger 131a. Conversely, if the second branch 130a is at a temperature lower than that of the circuit 31, then the heat remains injected just before the cover 17 (at the level of the second exchanger 33 initial) so as to reduce the need for steam called on the first branch 130a. It is easy to understand that automatisms (function of temperature measurements at various points of the main circuit 13) make it possible to optimize the heat injections in these three exchangers 33, 33 'and 131a, so as to maximize electricity production. Improvement of an existing installation According to a second aspect, the invention relates to the device 30 for improving the performance of a thermal plant 1 for generating electricity by combustion alone. Indeed, as explained above, there are many large installations for which it would be very profitable to increase the electrical efficiency even if only one percent. This device 30 makes it possible to modify an existing installation in order to obtain the yield increases previously highlighted, without any significant structural change in this installation. 3005 143 22 It is sufficient to install the first exchanger 32 at the end of the duct 100, the second exchanger 33 at the inlet of the degassing cover 17, and to connect them with the circuit 31. Then, without any intervention, the flow of extracted vapor at level 5 of a steam extraction (in particular of the branch 130c) of the existing installation will be decreased automatically (since the liquid water arriving at the tarpaulin will be hotter) and this non-extracted steam will go to relax in the second turbine 14b, to obtain the increase of electric production. Any previously discussed improvements can be used. It is possible in particular to include in the conduit 100 the probe 35, thanks to which the device 30 also makes it possible to better control the risk of corrosion.

Process According to a third aspect, the invention relates to a heat transfer process following a "Rankine" type thermodynamic cycle implemented by a thermal plant 1 for producing electricity by combustion as described previously (equipped with the device 30). . This method comprises the usual steps of an optimized Rankine cycle, but the preheating of the water uses the device 30: evaporation of liquid water in water vapor in the circuit 13 at the level of the steam generator 10; Expansion of the steam in the at least one turbine 14a, 14b; condensation of the water vapor in liquid water in the condenser 16; preheating the liquid water at the device 30 in heat exchange with the flue gas exhaust duct 100); degassing the liquid water in the degassing tank 17 heated by the first branch 130a; - Compressing the liquid water by the compressor 15.

3005 143 23 This cycle can of course integrate the additional preheating mentioned above, in particular thanks to the first and / or second preheating exchangers 131a, 131b, and the first economizer 18.

The method may further comprise overheating (at superheater 11), or even additional resheating between the two turbines 14a and 14b.

Claims (16)

REVENDICATIONS1. Thermal plant (1) for producing electricity by combustion, comprising: - a boiler (12); a flue gas exhaust duct (100); a main circuit (13) on which are successively arranged a steam generator (10) in heat exchange with the boiler (12), at least one steam expansion turbine (14a, 14b), a condenser (16), a degassing cover (17) and a liquid compressor (15); - power generation means (140) driven by the at least one turbine (14a, 14b); the installation being characterized in that it further comprises a device (30) putting in heat exchange the flue gas discharge duct (100) and the main circuit (13) between the condenser (16) and the cover degassing (17). 2. Installation according to claim 1, wherein the device (30) comprises a coolant circuit (31) on which are arranged: - a first exchanger (32) engaging the flue gas duct (100) and the coolant circuit (31); at least one second exchanger (33) engaging the main circuit (13) and the coolant circuit (31). 3. Installation according to claim 2, wherein the device (30) further comprises a probe (35) disposed in the flue gas exhaust duct (100) near the first heat exchanger (32), and control means ( 34) of the heat transfer fluid temperature at the first exchanger (32) as a function of flue gas parameters measured by the probe (35). 4. Installation according to claim 3, wherein the regulating means (34) comprises a pump regulating the coolant flow in the first heat exchanger (32) and / or a three-way valve regulating the proportion of heat transfer fluid from the first exchanger (32) transmitted to the second exchanger (33). 5. Installation according to one of claims 3 and 4, wherein the parameters measured by the probe (35) are at least one characteristic parameter of a corrosion potential and / or a flue gas temperature, the heat transfer fluid temperature at the level of the first heat exchanger (32) being regulated so that the flue gas temperature at the first exchanger (32) is greater than the acidic dew point of the flue gases. 6. Installation according to one of claims 2 to 5, wherein the coolant has a temperature between 70 ° C and 110 ° C output of the first exchanger (32) when the device (30) is in nominal operation. 7. Installation according to one of claims 2 to 6, wherein the heat transfer fluid is liquid water or steam, at a pressure between 0.6 bar and 1.5 bar, the first exchanger (32) being an evaporator and the second exchanger (33) being a condenser. 8. Installation according to one of claims 2 to 6, wherein the heat transfer fluid is liquid water at a pressure of at least 3 bar. 9. Installation according to one of the preceding claims, wherein the flue gas duct (100) comprises flue gas treatment means (110), the device (30) being disposed at the outlet of the flue gas treatment means ( 110). 10. Installation according to one of the preceding claims, wherein the degassing cover (17) is fed by a first leg (130a) of the main circuit (13) for the extraction of steam from a turbine (14a, 14b). 11. Installation according to one of the preceding claims, comprising at least two turbines (14a, 14b) including a first turbine (14a) and a second turbine (14b), the main circuit (13) comprising a second branch (130b). for extracting steam from the second turbine (14b), the second leg (130b) being in heat exchange with the circuit (13) at a first preheating exchanger (131a) disposed on the main circuit (13) between the condenser (16) and the device (30). 15 12. Installation according to claim 11, wherein the steam circuit (13) comprises a third branch (130c) for extracting steam from the first turbine (14a), the third branch (130c) being in heat exchange with the main circuit (13) at a second preheating exchanger (131b) disposed on the main circuit (13) downstream of the degassing cover (17). 13. Installation according to one of claims 11 and 12, in combination with one of claims 2 to 8, wherein the at least 25 a second exchanger (33) of the device (30) engages the fluid circuit coolant (31) and the main circuit (13) between the first preheating exchanger (131a) and the degassing tank (17). The plant of claim 13, wherein the device (30) comprises a second additional heat exchanger (33 ') engaging the heat transfer fluid circuit (31) and the main circuit (13) between the condenser (16) and the first preheating exchanger (131a). 15. Device for improving the performance of a thermal plant (1) for producing electricity by combustion, the installation comprising a boiler (12); a flue gas exhaust duct (100); a main circuit (13) on which are successively arranged a steam generator (10) in heat exchange with the boiler (12), at least one steam expansion turbine (14a, 14b), a condenser (16), a degassing cover (17) and a liquid compressor (15); power generation means (140) driven by the at least one turbine (14a, 14b); the device (30) being characterized in that it puts in heat exchange the flue gas discharge duct (100) and the main circuit (13) between the condenser (16) and the degassing tank (17). 16. Heat transfer process according to a thermodynamic cycle called "Rankine" implemented by a thermal plant (1) for producing electricity by combustion comprising a boiler (12); a flue gas exhaust duct (100); a main circuit (13) on which are successively arranged a steam generator (10) in heat exchange with the boiler (12), at least one steam expansion turbine (14a, 14b), a condenser (16), a degassing cover (17) and a liquid compressor (15); power generation means (140) driven by the at least one turbine (14a, 14b); the method being characterized in that it comprises steps of: evaporating liquid water into water vapor in the main circuit (13) at the steam generator (10); - Relaxing the water vapor in the at least one turbine (14a, 14b); condensation of the water vapor in liquid water in the condenser (16); preheating the liquid water at a device (30) in heat exchange with the flue gas exhaust duct (100); degassing the liquid water in the degassing tank (17); - Compression of the liquid water by the compressor (15).