EP0188183B1 - Process and device for recovering thermal energy from the exhaust gases of thermal-power stations - Google Patents

Abstract

In a steam generating system, steam condensate extracted from steam used to reheat boiler water in heaters (H1, . . . , H4) is revaporized in the recuperators-vaporizers (RV1, . . . , RV3) by heat exchange contact with the flue gas from boiler (B). The steam thus formed is recycled to contribute to the reheating of the water fed to the boiler.

Description

The present invention relates to a process for recovering thermal energy from burnt gases, more particularly in thermal power stations.

The invention also relates to the device for implementing the method and in particular to a new industrial device hereinafter abbreviated as "recuperator-evaporator" intended for the recovery of thermal energy, also called sensible heat, from the gases resulting from the combustion of fossil fuels (natural gas, coal, lignite, ...) from thermal power plants, abbreviated as "burnt gases", by its transfer to condensers from heaters, condensates which are evaporated.

The term “thermal power station” should be understood to mean any installation for converting thermal energy into mechanical energy by means of condensable fluid performing a thermodynamic cycle, in particular thermal fossil fuel power stations.

The known state of the art or prior art is shown in Figures 1 and 2.

FIG. 1 represents the diagram of a conventional thermodynamic cycle of a power plant, the turbine driving an electric generator not shown. The complete water cycle is shown. It passes successively into the boiler (CH) where it is successively heated until boiling, vaporized and superheated; it then expands in the "high pressure" turbine (THP), then is reheated (RS), then expands in the "low pressure" turbine (TBP), then condenses in the condenser (C), hence the condenser water extraction pump sends it to successive "low pressure" heaters (R "R ₂ , R ₃ and R ₄ ) where it is heated by steam drawn from the turbine (TBP).

The water then arrives at the degasser (DG) then at the food tank (B), at the outlet of which the food pump (PA) brings it to the high pressure of the cycle and returns it to the boiler after passage through the three heaters successive "high pressure" (P _s , R ₇ and R _B ).

The part of the cycle to which the invention relates is essentially that of "low pressure" heaters. Patent EP-0 032 641 shows in its figures 1 and 3 such parts of the cycle.

The invention also relates to the cycle of burnt gases.

Figure 2 shows schematically the combustion gas circuit.

Atmospheric air (A), for example at 20 ° C, enters the air heater (RA) where it is heated, for example to 285 "C, by the burnt gases; it then enters the boiler (CH ) where it burns the fuel and leaves in the form of burnt gases (GB), for example at 330 ° C, to enter the air heater; these burnt gases are cooled there by atmospheric air (A) which they heat up, then are released to the atmosphere.

Corresponding to the previous temperatures given by way of example, the temperature of the burnt gases is 120 "C. In practice the temperature of the burnt gases discharged into the atmosphere is between 115 ° C and 185 ^c C. A quantity of significant energy is thus lost, corresponding to the sensible heat of the burnt gases.

The temperature of the burnt gases leaving the heater is conditioned by several factors. The air heater being a gas / gas exchanger, has poorer exchange coefficients than those where a fluid is liquid or those where a fluid changes state; taking into account the large variations in air temperature at the passage of the exchanger, of the order of 150 to 250 degrees Celsius, the air heater is a large device whose cost would be prohibitive if it had to modify (respectively raise and lower) the temperature of the air flows much more to substantially reduce energy loss.

However, it is the consideration of the phenomena of water condensation, possibly acidified by sulfur oxides resulting from the combustion of sulfur fuels, in the burnt gases which will determine the minimum admissible temperature for the burnt gases. This condensation is mainly avoided because of the danger of corrosion and the cost of the heater if it had to be made of special materials resistant to sulfuric acid. In air heaters, condensation phenomena on cold walls are favored by the low temperature of the ambient air entering it, whose temperature is much lower than the dew point of the burnt gases; this dew point is a function of the humidity of the burnt gases, which depends on the hydrogen content of the fuel; it is in many cases between 60 and 70 ° C. The presence of sulfur oxides, however, raises this dew point. Given this phenomenon of condensation on cold walls, the average temperature of the outgoing burnt gases must be significantly higher than the theoretical dew point.

Another element to take into account is the draft of the chimney rejecting the burnt gases to the atmosphere; a high temperature is favorable to the draft of the chimney (reduction in the blowing power of the air) and to the dispersion of the burnt gases in the atmosphere.

On the other hand, constraints relating to environmental protection currently lead to (partially) desulfurization of the burnt gases, in order essentially to reduce the acidity of the rains.

Common desulfurization processes include washing the flue gases with chemical solutions; it is desirable for this operation that the burnt gases do not enter at a too high temperature (in particular that at the outlet of the air heaters) in the desulfurization installation. FR-2,534,150 describes a desulphurization plant for burnt gases where these gases are previously cooled to around 80 ° C in a heat exchanger which, after desulphurization, heats them up to send them to the chimney. However, reheating desulfurized burnt gases is not essential; the chimney and its fan can be sized for relatively cold gases; these burnt gases can also be discharged by means of atmospheric refrigerants with natural draft, which can disperse the burnt gases in the atmosphere much more advantageously with regard to environmental protection than the conventional chimneys of power plants. . Patent DE-2 453 488 relates to the rejection of burnt gases purified by natural draft cooling towers.

FR-2 043 956 discloses a device comprising a steam turbine and a boiler for supplying the turbine. In this device, the drinking water is heated by a succession of heaters. Part of the food water being reheated is also reheated in economizers, each time being sent back a little further into the stream of food water being reheated. In these economizers, the water is brought into heat exchange contact with the burnt gases from the boiler.

The use of these economizers does not reduce the amount of steam taken from the turbine for the heater associated with the economizer. The performance of this type of device is low.

The present invention aims to improve the performance of the assembly.

The invention aims in particular to recover the sensitive energy of the burnt gases, between 115 to 185 ° C, between their exit from the air heater and the dust collector and their entry into the desulphurization plant or their rejection to the chimney.

Its purpose is also to alleviate the drawbacks limiting the temperature of the gases burned at the outlet of the air heaters and to obviate the phenomena of condensation on cold walls and the need for large exchange surfaces required by gas / gas exchangers. .

These objects are achieved by the invention as defined in the claims mentioned at the end of this description.

FIG. 3 represents the part of the cycle of FIG. 1 comprising the "low pressure" heaters (R, to R ₄ ) where recuperators-evaporators according to the invention have been incorporated, in this case three devices: RE "RE ₂ and RE _3. These recuperators and evaporators are steel boxes containing a bundle of tubes, the tubes being internally traversed by the burnt gases which pass successively in RE ₃ , RE ₂ and RE, while cooling.

The REs also receive a fraction of the condensates from the heaters (R "R ₂ , R ₃ ) and evaporate them on contact with the tubes through which the burnt gases pass; the vapor formed is returned to the heaters from which the condensates originated.

In the example shown in Figure 3, RE ₃ is associated with R ₃ , RE ₂ with R ₂ and RE ₁ with R ₁ , and the burnt gases arrive from the air heater (RA) at 180 ° C.

The recuperators-evaporators therefore transfer thermal energy to the heaters essentially in the form of latent heat of evaporation; this supply of energy by the burnt gases results in a reduction in the withdrawal of steam from the turbine for the heater associated with the recuperator-evaporator.

A greater flow of steam is therefore available to the turbine to relax there by converting its internal energy into mechanical work. The quantities of steam drawn for the heaters R "R ₂ and R ₃ mentioned in FIG. 3 are effectively substantially lower than those mentioned in FIG. 1.

The efficiency of the thermodynamic cycle is thus improved from 1 to 1.5%.

If the gases at the outlet of the air heater are only at 120 ° C, only two recuperators-evaporators can be placed in the cycle of FIG. 1, as shown in FIG. 4.

It is the condensation temperatures in the heaters and the temperature of the burnt gases that determine the choice of heaters that can be associated with recuperators-evaporators.

Preferably, the injection of the residual energy of the gases burned in the cycle water will be done in a staggered fashion, in as many stages as there are heaters, the energy injection taking place at a level relatively low in temperature than the burnt gases, the hottest burnt gases being associated with the hottest condensate, the coldest burnt gas, with the coldest condensate.

In the succession of heaters considered from the condenser to the boiler, there is a heater from which association is no longer possible because the temperature of its condensates is close to or higher than that of the burnt gases, making it impossible to transfer thermal energy from the burnt gases to the heaters.

In the succession of heaters, it is possible that a heater is not associated with a recuperator-evaporator, while the previous heater and the following heater are; this will essentially be the case when the heat exchange provided for at this heater is too low for the capitalized energy gain due to the improvement in the thermodynamic efficiency of the cycle to offset the investment costs of a recuperator-evaporator.

Finally, it is possible that the first heater (s) from the condenser are not coupled with a recuperator-evaporator, in particular because the heat exchange would be too low, as mentioned above; as the thermodynamic gain is all the lower the lower the temperature and the pressure at the heater, the economic interest in the placement of recuperator-evaporator decreases when one approaches the condenser, and could no longer justify the installation of a recuperator-evaporator. On the other hand, lowering the temperature of the burnt gases below the acid dew point could lead to the use of more expensive materials, leading to investment costs which would no longer be offset by the reduction in costs. production due to the improvement of the cycle efficiency by a recuperator-evaporator where the temperature of the burnt gases would drop below the dew point. The invention also applies to cases where all the heaters are at the same pressure on the water side, except for pressure drops; in these cases there is only one pump P located at the outlet of the condenser and there is no longer any distinction between "low pressure" and "high pressure" heaters.

The technology of recuperators-evaporators is related to that of evaporators.

Figures 5 and 6 show in section, respectively parallel to the length and perpendicular to the length, a recuperator-evaporator.

This device consists externally of a cylindrical box with a horizontal axis, consisting of a ferrule 1 and two bottoms 2 and 3. These bottoms together with the tube plates 4 and 5 determine the "smoke boxes" 6 and 7. In 6 the burnt gases (GB) arrive at the recuperator-evaporator; in 7, they come out.

The tube plates 4 and 5 are connected together by the "smoke tubes" 8, allowing the sealed passage of the burnt gases from the smoke box 6 to the smoke box 7. The diameter of the tubes will generally be between 25 and 100 mm, for example 1 "1/2.

The tubes are grouped in superimposed layers, for example twenty-five layers of thirty-five tubes, inscribed, in section according to FIG. 6, in a rectangle.

The parallelepipedic bundle of tubes 8 is closed laterally by partitions 9 and 10, and is surmounted by a distribution tank 11 the bottom of which is pierced with a multitude of holes ensuring watering of the tubes 8 by water, partial condensate a heater, arriving at the top of the box through the orifice 12.

In contact with the tubes 8 heated internally by the burnt gases (GB), the water partially evaporates and the vapor formed escapes through the orifices 16 and 17 located above the water level 14, adjusted by the valve 15 .

To ensure sufficient irrigation flow, a pump 13 takes the condensate from the bottom of the recuperator-evaporator and recycles it through this device.

The materials constituting the elements in contact with water (condensates and steam), that is to say the ferrules 1, the partitions 9 and 10, the tank 11, the internal structural elements (supports, spacers,. ..) are similar to those used for heaters, especially for example steel.

As for the tubes 8 and the tube plates 4 and 5, they must be able to withstand on one side the aggressiveness of the burnt gases. Their material may be the same material as that of the partitions; but if the burnt gases are particularly corrosive, their material may be even more noble, such as stainless steel or a nickel-based alloy.

If the gases are very aggressive and exhibit condensation phenomena of acid solutions, the tubes could be made of ceramic material, for example, or glass or fluorinated organic polymer. The material of the tubes and the plates can also be composite, for example stainless steel covered with a layer of polytetrafluoroethylene, internally for the tubes, and on the face of the smoke boxes for the tube plates. The smoke boxes could possibly be covered internally with a resin, an organic polymer, protecting their metal walls against corrosion by burnt gases.

When the temperature of the burnt gases is lowered below the dew point, the recuperator-evaporator has the advantage of partially desulfurizing the burnt gases.

The external face of the tubes being in contact with a fluid having an intense phase change, in this case an evaporation in the form of boiling, the corresponding coefficients of heat transfer are excellent, better than with a gas and better than with a liquid. This feature of the heat exchange significantly reduces the dimensions of the device compared, in particular, to a gas / gas exchanger such as an air heater.

As for the heat transmission on the internal face of the tubes, it can be increased, for example, by an internal fin, increasing the contact surface and the turbulence of the gases.

It is noted that the fluid in contact with the gas burned through the walls of the tubes is at a relatively high temperature, that of the condensates of the heaters, much higher than that of the ambient air, and that, consequently, the phenomena condensation on a cold wall are reduced or even avoided.

In the case of FIG. 3, the power gain is of the order of magnitude of 1.5 MW at the shaft of the turbine, the latter driving a generator delivering on the electrical network 125 MW.

In the case of Figure 4, the gain is 0.35 MW at the turbine shaft.

Such gains are significantly higher than the cost of recuperators-evaporators capitalized over the life of the power plant.

As a variant, two or more successive recuperators-evaporators can be integrated into a single device, with a smoke box 18 common to the two recuperators-evaporators located between them. FIG. 7 represents such a variant integrating two recuperators-evaporators.

The integration of two successive devices can be further advanced, as shown in Figure 8, where there is no longer a smoke box intermediate; the tubes 8 of such a double recuperator-evaporator have a length corresponding to the assembly of the two devices, that is to say a length equal to the sum of the lengths of the tubes of the two recuperator-evaporators of FIG. 7, all other things being unchanged. In the double apparatus of FIG. 8, the two recuperators-evaporators are separated by a sealed tube plate 21 not allowing the passage of fluid between the spaces 22 and 23.

Claims (9)

1. Process of recovering of the thermal energy of the flue gas of a power station with steam turbine, in which station steam is taken from the turbine to heat feed water fed to the boiler, said flue gas being put in heat exchange contact with a vapor condensate exclusively coming from said taken steam, said condensate thus being revaporized and the steam thus formed being recycled to the boiler to contribute to the heating of the feed water.

2. Process in accordance with claim 1, characterized in that the heat exchange with the flue gas is made in several stages, with each corresponding to a stage of heating the water fed to the boiler, the steam condensates being at a temperature level relatively little below the one of the flue gas, the hottest flue gas being associated with the hottest condensates and the coldest flue gas with the coldest condensates.

3. Process in accordance with claim 2, characterized in that the steam condensate comes from the "low pressure" part of the circuit of heating of water fed to the boiler and that the steam formed by the heat exchange with the flue gas is returned to said "low pressure" part to contribute to the heating of the water fed to the boiler.

4. Device for the application of the process in accordance with any of claims 1 to 3, usable in a steam turbine power station in which steam is taken from the turbine to heat the water fed to the boiler (CH), said station comprising for that purpose a set of heaters (R₁...R₄), characterized in that it consists in at least one recuperator-vaporizer (RE₁...RE₃) in which the flue gas exchanges its heat with a steam condensate from a heater (R,...R₃) cited hereabove and coming exclusively from steam taken from the steam turbine.

5. Device in accordance with claim 4, characterized in that the recuperator-vaporizers (RE,...RE₃) are casings containing at least two smoke boxes (6, 7) and one nest of tubes (8) which convey inside the flue gas which goes successively in the recuperators-vaporizers (RE₃, RE₂ and RE₁) whilst cooling.

6. Device in accordance with claim 5, characterized in that the recuperators-vaporizers (RE,...RE₃) receive a fraction of the condensates of the heaters (R₁, R₂, R₃) and vaporize them in contact with the tubes conveying the flue gas, the steam formed being returned to the heaters whence the condensates came.

7. Device in accordance with claim 5, characterized in that at least two successive recuperators-vaporizers are built into a single apparatus.

8. Device in accordance with claim 7, characterized in that two successive recuperators-vaporizers are built into a single apparatus with a smoke box common to two recuperators-vaporizers and located between them.

9. Device in accordance with claim 5, characterized in that the length of tubes (8) corresponds to whole of at least two successive recuperators-vaporizers and that the various recuperators-vaporizers are separated by a sealed tube plate (21) not allowing the passage of fluid between the spaces of heat exchange of each recuperator-vaporizer.

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