Patent Description:
The present invention also relates to a thermal plant for generating steam comprising said recovery steam generator.

Thermal plants for generating steam normally comprise a recovery steam generator, which is connected to a source of hot flue-gases. The source of hot flue-gases may be a gas turbine or an industrial plant.

Recovery steam generators normally comprise a flue-gases flowing chamber and a steam circuit supplied with water and extending at least partially within the flue-gases flowing chamber so as to exploit the heat of the flue-gases to generate steam; the steam circuit comprises in sequence at least one evaporation section and one superheating section.

The temperature control in the steam circuit is essential to avoid temperature rises beyond nominal conditions. In addition, the temperature control mode is essential to optimise the heat exchange between the flue-gases and the steam circuit and to optimise the efficiency of the plant thermal cycle. In particular, the temperature control in the superheating section has an important effect on improving heat transfer. The superheating section consists of several heat exchange banks and is arranged in the hottest section of the flue-gases flowing chamber. Non-efficient thermodynamic processes in one or more banks of the superheating section affect the heat transfer in the banks downstream of the superheating section with obvious disadvantages.

The temperature control in the steam circuit is particularly important when the steam generator operates with low flue-gas flow rates (i.e. system low load situations) or excessive flow rates (i.e. peak system load situations), in particular climatic conditions wherein outside temperatures are high (such as in summer) or during the system start-up steps.

Optimising the heat exchange results in an increase in the efficiency of the steam generator.

It is therefore an aim of the present invention to make a highly efficient steam generator.

Some solutions have already been implemented and disclosed in documents <CIT>, <CIT>, <CIT>, <CIT>, <CIT> and <CIT>. However, such solutions are characterised by complex and bulky structures that are not efficient enough.

In accordance with such aims, the present invention relates to a recovery steam generator as claimed in claim <NUM>.

Thanks to a controlled injection of at least one heat transfer fluid into the connecting pipes between the superheating banks, the temperature of the steam in the superheating section can be controlled. The type and process parameters of the injected heat transfer fluid have an effect on the efficiency of the process and heat recovery.

It is also an aim of the present invention to implement a highly efficient thermal plant.

According to these aims, the present invention relates to a thermal plant as claimed in Claim <NUM>.

Further characteristics and advantages of the present invention will become clear from the following description of a non-limiting embodiment thereof, with reference to the figures of the attached drawings, wherein:.

In <FIG> reference number <NUM> denotes a thermal plant for steam generation.

The plant <NUM> shown in <FIG> is schematically represented and is not complete in all its parts.

In the non-limiting embodiment herein described and shown, the plant <NUM> is configured to produce electric energy and therefore the steam generated is used to generate electricity, as we shall see shortly in detail.

A variant not shown provides that the plant <NUM> is configured to produce thermal energy, that is useful for example in district heating applications.

The plant <NUM> comprises a gas turbine unit <NUM>, a steam turbine unit <NUM>, a steam generator <NUM>, and a tank <NUM>.

The gas turbine unit <NUM> is the first motor of the plant <NUM> and may be power-supplied by any fuel.

The gas turbine unit <NUM> is connected to a generator <NUM> and comprises a compressor <NUM>, a combustion chamber <NUM> and a gas turbine <NUM>.

The steam turbine unit <NUM> is coupled to a respective generator (not shown in the enclosed figures) and comprises at least one steam turbine (not shown).

The steam generator <NUM> recovers the residual heat from the combustion flue-gases generated by the gas turbine unit <NUM> and produces steam to be supplied to the steam turbine unit <NUM>.

In particular, the steam generator <NUM> comprises a flue-gases flowing chamber <NUM>, an inlet hood <NUM>, a steam circuit <NUM> and a chimney <NUM>.

The flue-gases flowing chamber <NUM> extends along a longitudinal axis A and is provided with an inlet <NUM> and an outlet <NUM>.

In the non-limiting example herein described and shown, the flue-gases flowing chamber <NUM> extends along an axis A that is arranged, in use, substantially horizontally.

According to a variant not shown, the flue-gases flowing chamber may extend along an axis that is arranged, in use, substantially vertically.

The inlet <NUM> of the flue-gases flowing chamber <NUM> is supplied with flue-gases from the gas turbine <NUM>. The flue-gases flow into the inlet hood <NUM> and the flue-gases flowing chamber <NUM> substantially following a feed direction D.

The outlet <NUM> of the flue-gases flowing chamber <NUM> is connected to the chimney <NUM>, through which flue-gases are discharged into the atmosphere.

The steam circuit <NUM> is schematically represented in <FIG>. Substantially, the steam circuit <NUM> is supplied with water, preferably from the tank <NUM>, and extends at least partially within the flue-gases flowing chamber <NUM> so as to exploit the heat of the flue-gases to generate steam.

The water in the tank <NUM> is preferably demineralised and is mostly water from a condenser (not shown) connected to the steam turbine unit <NUM>.

In the steam circuit <NUM>, the water from the tank <NUM> is transformed into steam. The feed direction V of the water and steam within the steam circuit <NUM> is substantially opposite to the direction D.

The steam circuit <NUM> comprises at least one evaporation section <NUM> and at least one superheating section <NUM>, which is arranged downstream of the evaporation section <NUM> along the feed direction V.

In the non-limiting example herein described and shown, the steam circuit <NUM> further comprises an economiser section <NUM>, arranged upstream of the evaporation section <NUM> along the feed direction V. The economiser section <NUM> is optional and may not be present.

Each section comprises respective heat exchange banks suitably configured to optimise the heat exchange between the flue-gases flowing in the flue-gases flowing chamber <NUM> and the water and steam flowing in the steam circuit <NUM>.

In <FIG> two alternative configurations of the superheating section <NUM> of the steam generator <NUM> are represented.

In both configurations, the superheating section <NUM> is characterised by the steam circulation having a temperature higher than the saturation temperature.

In other words, superheated steam circulates in the superheating section <NUM>.

With reference to <FIG>, the superheating section <NUM> comprises a plurality of superheating banks <NUM> arranged in series.

The superheating banks <NUM> shown in <FIG> are supplied with the steam coming from the evaporation section <NUM> and are normally referred to as SH (SuperHeating) banks in technical jargon. The last superheating bank <NUM> supplies the steam turbine unit <NUM>. In the non-limiting examples of <FIG> there are three superheating banks <NUM>.

In the configuration shown in <FIG>, the superheating section <NUM> comprises further superheating banks <NUM> arranged in series with each other, normally referred to as RH (ReHeating) banks in technical jargon, which are supplied with steam coming from the steam turbine unit <NUM>. Preferably, the steam supplied to the superheating banks <NUM> is steam coming from the high-pressure stage of a steam turbine (not shown).

The last superheating bank <NUM> supplies the steam turbine unit <NUM>. Preferably, the last superheating bank <NUM> supplies a medium pressure stage of a steam turbine (not shown).

Sometimes, the superheating banks <NUM> are also referred to as re-superheating banks to emphasise the fact that they are supplied with steam which has been already superheated by the superheating banks <NUM> and coming from the steam turbine unit <NUM>.

Here and hereinafter the term superheating bank is intended to identify all the heat exchange banks of the superheating section <NUM> wherein superheated steam circulates irrespective of the origin of the steam circulating through them. The superheating banks <NUM> are connected to each other by means of a plurality of connecting pipes <NUM> (only one of which is visible in a side view). In the optional configuration of <FIG>, the superheating banks <NUM> (two in total) are connected to each other by a plurality of connecting pipes <NUM> (only one of which is visible in a side view).

In other words, each superheating bank <NUM> is connected to the adjacent superheating bank <NUM> by a plurality of connecting pipes <NUM> and, if present, each superheating bank <NUM> is connected to the adjacent superheating bank <NUM> by a plurality of connecting pipes <NUM>.

Preferably, the superheating banks <NUM> and <NUM> extend in respective planes orthogonal to the extension axis A of the flue-gases flowing chamber <NUM>. In <FIG> the configuration of <FIG> is shown according to a view from above, which makes the connecting pipes <NUM> visible.

With reference to <FIG>, the steam circuit <NUM> comprises a plurality of first injection pipes <NUM>, configured to inject a first heat transfer fluid into each of the connecting pipes <NUM>, and a plurality of second injection pipes <NUM>, configured to inject a second heat transfer fluid into each of the connecting pipes <NUM>.

The first injection pipes <NUM> are connected to a first manifold 32a, and the second injection pipes <NUM> are connected to a second manifold 32b. The flow rate of the first heat transfer fluid supplied to the first manifold 32a is adjusted by a first valve 33a under the control of a control device <NUM>, and the flow rate of the second heat transfer fluid supplied to the second manifold 32b is adjusted by a second valve 33b under the control of the control device <NUM>.

The first heat transfer fluid is preferably different from the second heat transfer fluid.

In the non-limiting example herein described and shown, the first heat transfer fluid is steam and the second heat transfer fluid is water.

It should be understood that other heat transfer fluids may also be used, such as carbon dioxide CO<NUM>.

The steam supplied to the connecting pipes <NUM> must have a pressure greater than the pressure of the steam circulating in the connecting pipes <NUM> to ensure a correct injection.

Therefore, the steam may be drawn from a steam source at a suitable pressure or may be drawn at any point in the steam circuit <NUM> upstream of the two superheating banks <NUM> connected by the connecting pipes <NUM> into which the steam is injected.

For example, the steam may be drawn at any point in the steam circuit <NUM> at the outlet of the evaporation section <NUM>.

Water may be taken from a dedicated source or tank <NUM> or from any point in the steam circuit <NUM> arranged upstream of the evaporation section <NUM>.

For example, water may be drawn from any point in the steam circuit <NUM> in the economiser section <NUM>, if present.

According to a variant not shown, the steam circuit <NUM> comprises a plurality of injection pipes, configured to inject a single heat transfer fluid into each of the connecting pipes. In this case as well, the flow rate of the supplied heat transfer fluid is adjusted by a respective valve under the control of the control device. In this case as well, the heat transfer fluid may be steam, water or, for example, carbon dioxide CO<NUM>.

Referring to <FIG>, each connecting pipe <NUM> is fluidically connected to a respective injection pipe <NUM> and a respective injection pipe <NUM>.

The connecting pipes <NUM> extend along a path, which comprises at least an inlet portion <NUM>, an outlet portion <NUM> and an intermediate portion <NUM> arranged between the inlet portion <NUM> and the outlet portion <NUM>.

The inlet portion <NUM> is connected to a respective manifold pipe of the tube bundle (non-visible) defining the superheating bank <NUM> arranged upstream along the direction V, while the outlet portion <NUM> is connected to a respective manifold pipe of the tube bundle (non-visible) defining the superheating bank <NUM> arranged downstream along the direction V.

The intermediate portion <NUM> is planar. In other words, the intermediate portion <NUM> extends substantially along a plane, preferably orthogonal to the extension plane of the superheating banks <NUM>.

The inlet portion <NUM> and the outlet portion <NUM> preferably have respective curved sections <NUM><NUM> to allow the connection between the manifold pipe of the tube bundle defining the superheating bank <NUM> and the intermediate portion <NUM>.

In the non-limiting example herein described and shown, the curved sections <NUM><NUM> connect portions of the connecting pipe <NUM> arranged at <NUM>°.

Preferably, the intermediate portion <NUM> of the connecting pipe <NUM> follows a path wound around itself so as to substantially define a loop, preferably rectangular.

The intermediate portion <NUM> comprises, in sequence, a first rectilinear section 41a connected to the inlet portion <NUM>, a substantially U-shaped section 41b, a second rectilinear section 41c, and a final curved section 41d connected to the outlet portion <NUM>.

Preferably, the injection pipes <NUM> and <NUM> fit in respective injection pipes <NUM> at respective connecting points <NUM><NUM> arranged substantially in the intermediate portion <NUM> of the connecting pipe <NUM> at a predetermined distance.

Preferably, downstream of each connecting point <NUM><NUM> along the steam flow direction V the connecting pipe <NUM> has a respective localised enlargement of the flowing section. In other words, downstream of the connecting points <NUM><NUM> along the steam flow direction V the connecting pipe <NUM> is provided with respective bulges <NUM><NUM>.

Each bulge <NUM><NUM> is defined by an initial portion 50a 50b, wherein there is a gradual radial increase in the flowing section starting from the initial section of the connecting pipe <NUM> to a maximum value, by a central portion 51a 51b, wherein the flowing section is constant and at the maximum value, and by an end section 52a 52b, wherein there is a gradual return of the flowing section from the maximum value to the initial value of the connecting pipe <NUM>.

Advantageously, the bulges <NUM><NUM> mitigate the thermal shock effects due to the temperature change caused by the injection of the first fluid and the second fluid.

Furthermore, the bulges <NUM><NUM> promote mixing of the steam circulating in the connecting pipes <NUM> with the first fluid and the second fluid.

Preferably, the injection pipe <NUM> of the first fluid substantially fits in the curved section <NUM> of the inlet portion <NUM>, while the injection pipe <NUM> fits in the rectilinear section 41c of the intermediate portion <NUM>.

Referring to <FIG>, the injection pipe <NUM> preferably has a nozzle <NUM>, preferably L-shaped, so that it can be positioned substantially at the centre of the connecting pipe <NUM>. The centre of the connecting pipe <NUM> means a central position within the connecting pipe as shown in <FIG>. Such position allows an optimal mixing of the first fluid supplied with the injection pipe <NUM>.

More preferably, the opening <NUM> of the nozzle <NUM> has a profile defined so that the ratio of the profile perimeter of the opening <NUM> to the flowing area of the opening <NUM> is greater than an optimum reference value. This increases the mixing of the steam flow and the flow of the first injected fluid having, in most cases, different temperatures.

According to a further variant not shown, the steam circuit comprises additional injection pipes configured to inject one or more heat transfer fluids into each of the connecting pipes <NUM> connecting the further superheating banks <NUM> of the configuration shown in <FIG> under the control of the control device. Basically, the structure of the connecting pipes <NUM> and the additional injection pipes is similar to that shown in <FIG> and <FIG>.

In use, when the control device <NUM> deems it necessary to change the temperature or the flow rate of the steam circulating in the connecting pipes <NUM> (or also in the connecting pipes <NUM> according to the above-described variant), it adjusts the control valves 33a 33b so as to supply appropriate flow rates of the first fluid and, possibly, of the second fluid.

In particular, the control device <NUM> is configured to open the adjustment valves 33a 33b under specific operating conditions of the steam generator <NUM>; for example at peak load or low load times, or during summer periods.

Obviously, the same considerations made for the connecting pipes <NUM> may be applied to the connecting pipes <NUM>. Therefore, if required, the control device <NUM> may adjust the supply of one or more heat transfer fluids in the connecting pipes <NUM> as well.

Advantageously, the proposed solution enables a quick and effective increase in the plant performance. Furthermore, the presence of a plurality of injecting pipes <NUM>, <NUM> capable of injecting one or more heat transfer fluids directly into the connecting pipes <NUM>, <NUM> between the superheating banks <NUM>, <NUM> allows to act effectively and rapidly.

Claim 1:
Recovery steam generator (<NUM>) comprising:
a flue-gases flowing chamber (<NUM>) extending along a longitudinal axis (A) and provided with an inlet (<NUM>) and an outlet (<NUM>);
a steam circuit (<NUM>) supplied with water and extending at least partially within the flue-gases flowing chamber (<NUM>) so as to exploit the heat of the flue-gases to generate steam; the steam circuit (<NUM>) comprising:
• a superheating section (<NUM>) comprising at least two superheating banks (<NUM>; <NUM>) arranged in series with each other;
• a plurality of connecting pipes (<NUM>, <NUM>) configured to connect the at least two superheating banks (<NUM>; <NUM>) in series with each other;
• at least a plurality of first injection pipes (<NUM>; <NUM>) configured to inject a first heat transfer fluid into each of the connecting pipes (<NUM>; <NUM>, <NUM>);
• a control device (<NUM>) configured to regulate the flow rate of the first heat transfer fluid supplied via the first injection pipes (<NUM>; <NUM>);
the recovery steam generator (<NUM>) being characterized by the fact that the first injection pipes (<NUM>) are connected to a first manifold (32a) provided with a first valve (33a) and by the fact that the control device (<NUM>) is configured to adjust the first valve (33a) to control the flow rate of the first heat transfer fluid fed to the first manifold (32a).