EP0640198A1

EP0640198A1 - Heat recovery boiler with induced circulation.

Info

Publication number: EP0640198A1
Application number: EP93908787A
Authority: EP
Inventors: Alfred Dethier
Original assignee: Cockerill Mechanical Industries SA
Current assignee: Cockerill Mechanical Industries SA
Priority date: 1992-05-08
Filing date: 1993-04-30
Publication date: 1995-03-01
Anticipated expiration: 2013-04-30
Also published as: WO1993023702A1; DK0640198T3; DE69311549D1; US5575244A; GR3024652T3; ATE154426T1; EP0640198B1; RU94046035A; DE69311549T2; AU3946793A; ES2104144T3; JPH07506662A; RU2124672C1; BE1005793A3

Abstract

PCT No. PCT/BE93/00092 Sec. 371 Date Dec. 28, 1994 Sec. 102(e) Date Dec. 28, 1994 PCT Filed Apr. 30, 1993 PCT Pub. No. WO93/23702 PCT Pub. Date Nov. 25, 1993The invention relates to a boiler (1) wherein at least one steam generating circuit comprises an ejector (25) capable of providing induced circulation of water in the boiler in normal operating conditions. The corresponding water/steam separation reservoir (17) is arranged at any height with respect to the outlet collector (manifold) (15) of the evaporator device (11) of said circuit.

Description

Induced circulation heat recovery boiler

Subject of the invention

The invention relates to a heat recovery boiler in which the circulation of water is ensured without recourse to the thermo-siphon effect.

It also relates to a method for the optimal use of such a boiler, for example in a power plant. Technological background

Heat recovery boilers necessarily include means for ensuring the circulation of fluids. They find industrial application in so-called combined cycle power plants, as well as in so-called cogeneration plants, for the simultaneous production of electricity and steam.

^' They can be used in other conventional applications.

Such boilers are used to recover the large amount of heat contained in the flow of exhaust gases from a gas turbine and to transform water into steam. This is then itself used in a steam turbine which drives an alternator. State of the art

The boilers are supplied with water by means of a feed pump. They include one or more steam generation circuits, each comprising an evaporator device and a water / steam separation tank. These are connected to each other by pipes where water begins to circulate, followed by a water / steam mixture. Several steam generation circuits can be installed in a boiler in order to supply steam with different pressures and thus improve the overall efficiency of the installation.

The heat exchanges between the gases coming from the gas turbine and, at the beginning the water, then then the water / steam mixture circulating in the boiler take place at the level of the evaporator device. The latter consists of finned tube circuits mounted vertically or horizontally depending on the case, and installed in a flow of hot gases originating, for example, from a gas turbine. Conventionally, during operation, each device the evaporator is supplied with water from the corresponding water / vapor separation flask via a so-called inlet manifold, onto which the inlets of the tubes constituting this evaporator device are welded and a so-called outlet manifold which collects the water / steam mixture obtained. This outlet manifold is connected to the same separation tank, thus creating a closed circuit.

The number of tube circuits connecting the so-called inlet and outlet manifolds depends on the size and operating conditions of the boiler.

The pressure drop of the water between the inlet and outlet manifolds of the evaporator device depends in particular on the configuration of the pipes. According to different variants, the tubes of the evaporator device can be arranged either vertically or horizontally.

There are essentially two types of boilers depending on the type of water circulation in the circuits. We speak of "natural" circulation or by thermo-siphon effect, when the water circulates in the boiler thanks to the difference in density of the water when it passes from the liquid phase to the gaseous phase. Boilers with natural circulation are for example described in patents US-A-2,031,423 and US-A-2,702,026.

By US-A-2,257,358 there is known a steam generator device with circulation by thermo-siphon in which a qualified ejector device constituted by two co-axial conduits and not otherwise described is mounted at the outlet of a separator tank water / steam to accelerate the thermo-siphon effect.

The device described comprises two independent circuits in which the water to be heated circulates in horizontal tubes from bottom to top while the gases from combustion circulate from top to bottom, said separator tank being mounted above the boiler.

As indicated, a circulation is carried out by thermo-siphon under the combined effect of a "natural" and artificially accelerated circulation.

It should be noted that the qualified ejector device is relatively basic and cannot be regulated. It is also mounted in the return line (down) of one of the circuits coming from the separator tank. Patent application EP-A-0357590 describes a boiler with horizontal tubes operating on the basis of natural water circulation, without the application of a circulation pump, thanks to the thermo-siphon effect.

The water circulates in a loop between the flask and the evaporator device in the various pipes. It descends from the balloon in an unheated branch and rises there in a heated branch where it is in the form of a water / vapor mixture, the evaporator device being inserted in the "rising" branch. In normal operation, the driving force of circulation reaches a maximum value determined by the difference in height between the tank and the outlet manifold of the evaporator device.

Thus, to obtain a sufficient driving force for example of 1 Kg / cm ² , between the collectors, it is necessary to have above the outlet manifold a column of water about 10m high, which requires significant bulk.

In addition, the value of the pressure drop in normal operation is not predetermined to comply with the thermal stability and flow requirements in the boiler, which require, according to. desired pressures maximum circulation rate. This circulation rate depends on the value of the force drive and that of the pressure drop in a given circuit.

As the motive force obtained by natural circulation is weak, it is necessary to have one _; large number of parallel tube circuits in the evaporator device to reduce the pressure drop. The structure of the collectors is therefore all the more complicated. The diameter of the tubes must also be larger in order also to reduce the pressure drop. The circulation rate of a boiler is the average number of revolutions that a drop of water must make in the evaporative circuit before completely vaporizing and thus leaving the circuit. This rate remains limited in boilers with natural circulation given the low driving forces involved. In addition, as the flow rate may be too low in certain tube circuits, this can result in a loss of overall performance and high risks. corrosion of these tubes by precipitation on the internal wall of all the salts contained in the water, following the total evaporation of the small amount of water included in this circuit.

A start-up phase of the water circulation is necessary and can be carried out in different ways, for example by the action of an ejector possibly coupled to an additional pump, and mounted in a bypass line and which would be used only for starting, by injecting gas into the risers or by connecting the inlet and outlet manifolds of the evaporator device.

Boilers of the type described are relatively bulky and their performance depends largely on their configuration.

We speak of forced or assisted circulation when the circulation of water in the boiler is created by one or more so-called circulation pumps arranged between the tank and the collectors. Usually, the tubes of the evaporator device are then arranged horizontally.

Circulation pumps consume energy and sometimes require significant maintenance costs. Aims of the invention

An essential object of the invention is to combine the advantages of boilers with natural circulation and forced circulation, while not having the disadvantages. The object of the invention is to provide compact heat recovery boilers, that is to say with a height of water in the rising branch above the outlet manifold which may be of no importance.

It also aims to provide boilers that do not require the use of a circulation pump to circulate the water in the circuits of tubes constituting the evaporator device.

It also aims to provide such boilers in which the pressure drop of the water between the inlet manifold and the outlet manifold of the evaporator device can be chosen at a predetermined value according to the stability criteria desirable for the boiler. In particular, it is intended that in such boilers, the pressure drop is not determined solely by the height of the water column in the rising branch of the boiler.

A further object of the invention is to provide such boilers in which circulation is ensured by an economical device, more reliable since it is less complex and requires little maintenance costs.

A final object of the invention is to make it possible to limit the number of evaporator circuits and to select tubes of small diameter, less sensitive to thermal stresses, and to obtain a simpler construction of the collectors, which have fewer tube connections. and can also be smaller and lower water volume in the evaporator, resulting in improved dynamic behavior and reduced time constants.

Essential elements of the invention “The subject of the invention is a heat recovery boiler comprising one or more steam generation circuits, optionally at different pressures, each comprising

- a water / vapor separation tank, - an evaporator device with finned tubes arranged horizontally in a flow of hot gases,

- descent and ascent pipes ensuring communication between the flask and the evaporator device, via an inlet manifold and an outlet manifold.

In the boiler of the invention, at least one steam generation circuit includes an ejector capable of ensuring an induced circulation of water in the boiler during normal operation, the corresponding water / steam separation tank then being able to be arranged at a any height relative to the outlet manifold of the evaporator device of this circuit.

Thanks to the ejector, the induced circulation of water can be maintained in a regular manner.

Preferably, each steam generation circuit includes an ejector capable of ensuring the induced circulation of water in the boiler during normal operation. In such a case, the boiler can then be devoid of a circulation pump.

The ejector is preferably placed on a supply line.

Each steam generation circuit comprising an ejector can be provided with means for ensuring a minimum flow rate of this ejector during the start-up phase of the boiler. It may be an auxiliary starter pump provided on a line mounted in diversion between a point of the downpipe and a point of the supply line located upstream of the ejector. Alternatively, the balloon of the steam generation circuit concerned can be provided, in its water zone, with a device capable of allowing its emptying during the start-up phase.

It goes without saying that the two solutions can coexist in the same steam generation circuit.

Advantageously, each ejector is provided at its conical nozzle with a movable needle. This needle allows adjustment of the characteristics of the ejector.

According to a preferred embodiment, the difference in height between the tank of a steam generation circuit and the outlet manifold of the corresponding evaporator device is zero.

Advantageously, according to the invention, the tank of a circuit can be disposed at a height less than that of the outlet manifold of the corresponding evaporator device.

The invention also relates to a method for the use of a boiler as described above in which the following steps are carried out: - water is introduced into the evaporator device and the flask of at least one steam generation circuit, by means of a feed pump, up to a so-called start level; - a means is actuated to ensure a minimum flow allowing the operation of the ejector during the start-up phase;

- the boiler is heated;

- the supply pump is actuated again so as to allow the ejector to ensure the circulation induced during normal operation of the boiler. Preferably, during the start-up phase, a movable needle is introduced into a part of the ejector, and it is actuated in order to regulate the flow of water as required. Brief description of strings

The invention will be better understood by referring to the accompanying drawings, in which FIG. 1 is a schematic view of a first embodiment of a boiler with induced circulation according to the invention; Figure 2 is a schematic view of a second embodiment of an induced circulation boiler according to the invention, and Figure 3 is a sectional view of an ejector provided with a needle which can be used in a boiler according to the invention. Detailed description of the figures

Figure 1 shows a schematic view of a boiler 1 according to the invention arranged between a gas turbine and a steam turbine not shown, as for example in a power plant.

The following description will refer to a boiler used in such an application, but it should be understood that it will not go beyond the scope of the invention by applying it to traditional boilers.

The boiler 1 is supplied by means of a reservoir 3 and a supply pump 5. The supply line 7 is provided with a control valve 9 which can be actuated according to the water requirements of the boiler 1. A evaporator device 11 consisting of finned tubes arranged horizontally in a hot gas exhaust channel 12 is conventionally provided. In FIG. 1, three circuits of finned tubes are shown in parallel, but in practice, thanks to the invention, it is possible to limit ourselves to 200 to 300 circuits, which is a small number compared to boilers with state-of-the-art natural circulation, which usually comprises around 800 circuits.

This evaporator device 11 comprises classique¬ ment an inlet manifold 13 and an outlet manifold 15. The ^r both are connected to a separation tank water / steam inlet 17. The manifold 13 is connected to the water zone of said balloon 17 via a so-called down pipe 19, while the outlet manifold 15 is connected to the vapor zone of the balloon 17 by a so-called up pipe 21. A pipe 23 for the departure of the steam from the balloon 17 is provided at the top in the vapor zone.

An ejector 25 is placed at the intersection of the supply line 7 and the downpipe 19. Before starting the boiler, water is introduced into the evaporator device 11 and into the tank 17 by means of the pump d 'power 5, up to a so-called start level. When the water level in the balloon 17 reaches a few tens of centimeters, the regulating valve 9 is closed. The supply water is then used as the working fluid, it passes through the ejector 25 with a certain pressure drop increasing its speed, which induces a suction of water in the downpipe 19 and therefore the circulation movement some water. For this reason, we speak of induced circulation in the boiler.

The feed water / water mixture from the flask is discharged to the inlet manifold 13 with a determined overpressure. The ejector continues to operate continuously during the normal market of the boiler, that is to say from the moment when the flow of the working fluid entering it reaches a certain value.

During the start-up period, the supply water flow will be zero. However, an ejector can only operate if it has a minimum flow rate.

The closing of the regulation valve 9 is in principle necessary to ensure a correct start: before the boiler works, there is no water consumption. It is therefore necessary to avoid overfilling the balloon 17, to prevent water from flowing to the steam outlet pipe 23, which would be inadmissible.

However, it is necessary to ensure water circulation at start-up in the evaporator / balIon device circuit, in order to heat all of the elements uniformly. Depending on the site conditions, this circulation can be obtained in different ways.

A first possibility is illustrated in FIG. 1. A branch line 27 can be provided on the downpipe 19, terminating on the supply line 7 upstream of the ejector 25. On this line 27, a pump is then provided. starting aid 29 and an auxiliary valve 31; the latter is opened when the valve 9 is closed. The pump 29 temporarily ensures the circulation of the working fluid from the water coming from the balloon 17. This pump 29 may be of low capacity.

Alternatively, as shown in Figure 2, one can provide a pipe 33 for emptying the balloon 17 with possible recycling of water to the tank 3 or to a condenser not shown, or a pure and simple evacuation of the water.

The drop in water level in the balloon 17 will induce a call for water which will force the regulating valve 9 to open and the supply pump 5 to ensure a motor flow, which will allow normal operation of the 'ejector 25. In this case, the valve 9 therefore remains open even at start-up, and it is possible to admit feed water into the boiler without risk of drowning the latter. When the starting circulation is established, the boiler 1 can be heated, either by starting the gas turbine, or by operating the smoke registers (not shown), depending on the installation. The first bubbles of vapor will quickly form in the lower part of the evaporator device 11, pushing the water towards the balloon 17 via the rising pipe 21. The water level in the balloon will therefore increase. It will then gradually decrease, depending on the steam produced and sent to the user. When the level has returned to a normal value, feed water must be introduced into the boiler 1, in quantities equal to the steam produced: the control valve 9 is fully open and the ejector 25 then works in steady state normal. The starting circuit can then be cut.

Note that for each variation in temperature or flow rate of the hot gases entering the boiler, there corresponds a variation in the steam flow rate and therefore an identical variation in the feed water flow rate, controlled by the regulating valve 9.

FIG. 3 is a detailed view of an improved ejector according to the invention. It conventionally comprises a body 35, a suction flange 37, a mixing zone 39, a diffuser 41 and a conical nozzle 43. The latter is advantageously provided with a movable needle 45. During the start-up phase, the needle 45 is introduced inside the conical nozzle 43, which makes it possible to limit the flow of working fluid while maintaining the induced flow capacities of the ejector 25.

In normal operation of the boiler 1, the needle 45 is withdrawn from the nozzle 43 and the ejector 25 operates according to its initial characteristics.

In the boilers of the invention, either an ejector can be used. standard, i.e. an improved ejector such as that shown in FIG. 3.

A concrete example of a boiler according to the invention is described below (but not shown in the figures).

Typically, a current combined cycle power plant comprises one or two gas turbines of 100 and 500 M, each equipped with a heat recovery boiler with two pressure levels, producing high pressure steam (about 80 to 100 kg / cm ² ) and low pressure steam (about 8 to 10 kg / cm ² ), feeding a steam turbine with two pressure levels with a power of 100 to 150 MW.

Each boiler has two steam generation circuits, each provided with three heat exchangers, namely an evaporator, an economizer and a superheater, and a water / steam separation tank.

The two steam generation circuits are independent and each of them can operate in induced circulation according to the invention. It will not, however, depart from the scope of the invention if, while a given steam generation circuit of a boiler comprises an ejector which ensures the induced circulation of the working fluid, another circuit of the same boiler operates according to another type of circulation, for example forced circulation by means of a circulation pump.

In the example given, for each steam generation circuit operating in induced circulation, the pressure drop in the evaporator device will be chosen as a function of the flow and heat exchange stabilities, ie 3 to 5 kg / cm ² . The finned tubes will then be of small diameter (approximately 32 to 38 mm). The volume of water in the evaporator device can also be relatively small (around 10 to 15 m3). This capacity will be sufficient to accept the transfer of water from the evaporator device during start-ups.

The sheets constituting the balloon may be reduced in thickness (approximately 30 to 50 mm), allowing high gradients of temperature and / or pressure. In this way, the start-up time of the boiler can be very short, and able to adapt to the very short start-up times of gas turbines. The dynamic behavior of the boiler is significantly improved, with reduced time constants. The traffic rate can also be chosen with a high safety margin.

Claims

claims

1. Heat recovery boiler (1) comprising one or more steam generation circuits, optionally at different pressures, each comprising

- a water / steam separation tank (17),

- an evaporator device with finned tubes arranged horizontally in a flow of hot gases, - down pipes (19) and up pipes (21) ensuring communication between the flask (17) and the evaporator device (11), via a inlet manifold (13) and an outlet manifold (15), characterized in that at least one steam generation circuit comprises an ejector (25) capable of ensuring an induced circulation of water in the boiler (1 ) in normal operation, and in that the corresponding water / vapor separation tank (17) is disposed at any height relative to the outlet manifold (15) of the evaporator device (11) of this circuit.

2. Boiler according to claim 1, characterized in that each steam generation circuit comprises an ejector (25) capable of ensuring the induced circulation of water in the boiler (1) in normal operation.

3. Boiler according to claim 2, characterized in that it does not have a circulation pump.

4. Boiler according to any one of claims 1 to 3, characterized in that 1 ^» ejector (25) is placed on a supply line (7).

5. Boiler according to any one of claims 1 to 4, characterized in that each steam generation circuit comprising an ejector (25) further comprises means for ensuring a minimum flow allowing the operation of this ejector (25) during the start-up phase of the boiler (1).

6. Boiler according to claim 5, characterized in that an auxiliary starting pump (31) is provided on a line (27) mounted in diversion between a point of the downpipe (19) and a point of the line feed (7) located upstream of the ejector (25).

7. Boiler according to claim 5, characterized in that the balloon (17) of said steam generation circuit is provided in its water zone with a device (33) capable of allowing the emptying of this balloon (17) during the start-up phase.

8. Boiler according to any one of the preceding claims, characterized in that each ejector (25) is provided, at its conical nozzle (43) with a needle (45) movable.

9. Boiler according to any one of claims 1 to 8, characterized in that the difference in height between the tank (17) of a steam generation circuit and the outlet manifold (15) of the evaporator device (11) corresponding is zero.

10. Boiler according to any one of claims 1 to 9, characterized in that the balloon (17) of a steam generation circuit is disposed at a height ^* lower than that of the outlet manifold (15).

11. Method for using a boiler (1) according to claim 1, characterized in that the following steps are carried out:

- Water is introduced into the evaporator device (11) and the tank (17) of at least one steam generation circuit, by means of a feed pump (5), and this up to a so-called start level;

- A means is actuated to ensure a minimum flow allowing the operation of the ejector (25) during the start-up phase;

- the boiler is heated (1);

- the supply pump is operated again (5) so as to allow the ejector (25) to ensure the circulation induced during normal operation of the boiler (1).

12. Method according to claim, 11, characterized in that during the start-up phase, a movable needle (45) is introduced into a part of

The ejector (25) consisting of a conical nozzle (43) and that it is actuated in order to regulate the flow of water as required.