IT201900006096A1 - Recovery steam generator - Google Patents

Description

DESCRIPTION

Field of application

The invention concerns the sector of recovery steam generators.

Known art

The recovery steam generator (also known by the acronym HRSG, heat recovery steam generator) is an appliance designed to produce steam using waste heat, for example the heat contained in the exhaust fumes of an engine. The steam can be destined for energy uses or process uses depending on the needs and depending on the steam pressure. The steam pressure, in turn, essentially depends on the temperature of the hot source.

A recovery steam generator is usually made, in the known art, with the so-called "water tube" solution, that is, it is a shell and tube heat exchanger in which the hot gases pass through the shell side while the water evaporates inside the tubes. Multiple evaporation pressure levels can also be provided to optimize heat recovery. Steam generators of this type are used, for example, to recover the heat contained in the fumes of gas turbines as part of the so-called combined gas-steam cycles.

The water tube construction allows to optimize the heat recovery efficiency because it is possible to create different tube bundles at different water pressures, and / or provide a tube bundle dedicated to steam superheating. For these reasons, the construction with water pipes is often preferred in relatively large plants in which fumes are available at very high temperatures and the aim is to produce superheated steam at high pressure for energy purposes.

However, the water tube generator is not suitable for medium / small cogeneration systems. This is the case, for example, of co-generative systems used in the industrial sector which typically include a piston engine (reciprocating motion) coupled to a generator to produce electricity, and a recovery steam generator that uses the heat of the fumes from the engine to produce saturated steam intended for process uses. Cogeneration systems of this type are used in various industrial sectors, for example in the chemical and petrochemical industries, in the pharmaceutical, food, wood processing industries, etc.

A characteristic of these systems is represented by the temperature of the fumes which is not very high (eg about 450 ° C) which makes the production of high pressure and / or superheated steam not convenient; generally the steam produced with the recovery is saturated steam at medium pressure destined for process uses.

Another feature of these systems is the variability of the electrical and thermal demand. The electrical demand, on which the engine load and consequently the thermal power discharged with the fumes depend, does not always follow a trend in accordance with the steam demand. At certain moments or periods, the steam demand may exceed the quantity that can be produced with the heat recoverable from the engine fumes; or the fumes are not available because the engine is not running but a minimum quantity of steam must still be generated in order not to stop the downstream production process.

All this requires considerable flexibility of the recovery steam generator and the large water tube steam generators, developed essentially for gas-steam cycles, do not provide a satisfactory response to this need. Furthermore, in the known art, an auxiliary boiler must still be provided in parallel to the HRSG, which greatly increases the investment cost and makes the cogeneration system less attractive, especially for small or medium-sized applications.

Summary of the invention

The invention aims to provide a recovery steam generator particularly suitable for industrial applications and for the recovery of heat from the fumes of alternative internal combustion engines, which solves the problems mentioned above. In particular, the invention aims to create a recovery steam generator that is simple in construction and low in cost; that it is able to follow the fluctuations of the steam demand with respect to the electricity demand in a cogeneration system; that it has a good recovery efficiency; which can produce dry saturated steam as generally required by production processes.

The purposes are achieved with a smoke tube recovery steam generator, comprising:

a shell and a tube bundle that define a tube side and a shell side of the generator;

a smoke inlet;

a burner having a combustion chamber located inside the shell of the steam generator;

a mixing chamber communicating with the smoke inlet and the combustion chamber.

The steam generator of the invention is of the smoke tube type, in which the hot gases are fed into the tubes and the water evaporates on the shell side outside the tubes.

The smoke inlet can be connected to a smoke duct coming from an external device, such as a motor. Hereinafter, the term fumes means the fumes fed to said inlet, typically represented by the exhaust fumes of an internal combustion engine.

The steam generator includes an auxiliary fuel-fired burner, for example natural gas, which is integrated into the steam generator itself. Thanks to the mixing chamber, the burner can integrate or even replace, if necessary, the heat input of the fumes. This avoids the need for a separate boiler.

The mixing chamber can be placed at the inlet or outlet (upstream or downstream) of said combustion chamber.

In the first case, the mixing chamber allows the fumes to be introduced directly into the combustion chamber in a controlled manner; the fumes can be mixed with the fuel and the comburent of the burner at the entrance to the combustion chamber itself. For example, a preferred embodiment with a mixing chamber upstream of the combustion chamber provides a substantially toroidal body for conveying and mixing fumes at the entrance to the combustion chamber.

In the second case, the mixing chamber receives the fumes and gaseous products of combustion, downstream of the combustion chamber, and allows the two flows to be mixed before entering the pipes. For example, a preferred embodiment with a mixing chamber downstream of the combustion chamber provides a mixing chamber inside the shell of the steam generator, said mixing chamber communicating directly with a tube inlet (inlet from the tube side of the tube bundle) and with the combustion chamber.

In a preferred embodiment, the steam generator shell is an essentially cylindrical body with a horizontal axis. Preferably, the combustion chamber of the burner is also a substantially cylindrical body with a horizontal axis. More preferably, the combustion chamber is coaxial with respect to the main shell of the steam generator.

The tube bundle of the recovery steam generator can comprise one or more banks of tubes.

In a preferred embodiment, the tube bundle comprises at least a first bank of straight tubes, which are parallel to the axis of the cylindrical combustion chamber, and which are arranged above the cylindrical combustion chamber, essentially forming an arc around the chamber itself.

In an even more preferred embodiment, the tube bundle comprises said first bank of tubes and, furthermore, a second bank of tubes forming a bundle with two passages. The tubes of said second bench are preferably parallel to the tubes of the first bench and are arranged in an arc above the first bench.

The steam generator also advantageously comprises a steam separator having its own pressurized body external to said shell.

Said steam separator (or "steam drum") communicates with the shell side of the generator through at least one steam rising line and at least one liquid return line. This vapor separator, despite having a separate body, is also integrated into the steam generator, and allows the export of saturated steam, suitable for process uses, with the return of water (in the liquid state) to the evaporation chamber.

A generator according to the invention can also comprise an economizer arranged to preheat the feed water with the gases coming out from the pipe side. Preferably, said economizer is made with a bundle of pipes (or other heat exchange bodies) which are in contact with the hot fumes and which are internally crossed by the feed water. After passing through the economizer, the fumes can be discharged or treated if necessary for the removal of pollutants.

A cogeneration unit comprising a steam generator according to the attached claims is also the subject of the invention. Said cogeneration unit can comprise an internal combustion engine, preferably a reciprocating engine, connected to an electric power generator.

The invention is applied preferably, but not exclusively, in the context of cogeneration systems with electrical power of a few megawatts (typically from 1 to 5 MW of electricity) and steam production up to pressures of about 15-18 bar.

Although the preferred application concerns the production of process steam, a cogeneration unit according to the invention may possibly include a steam expander to produce energy. The steam expander is preferably of the screw type which is more convenient and easier to manage than a turbine to produce energy from saturated steam at medium pressure and / or in medium or medium-small size cogeneration systems. The screw expander is not damaged by the formation of droplets of liquid within the expanding vapor, and is therefore suitable for operation with non-superheated saturated steam.

A burner according to the invention can essentially have three operating modes.

In a first mode of operation, the heat transferred to the water comes entirely from the fumes and the auxiliary burner is not used.

In a second operating mode, the generator uses only the heat of the combustion gases produced by the burner. This operating mode allows steam to be produced even when the engine is not running.

In a third mode of operation, the generator uses both the heat of the fumes and the heat supplied by the burner. The fumes are introduced into the combustion chamber or mix with the burner gases inside the mixing chamber. The tube bundle thus receives a mixed gaseous flow including both the combustion products and the fumes. This third operating mode is the most interesting and allows you to "follow" the steam request by modulating the burner ratio to integrate, if and when necessary, the heat recovered from the fumes.

The main advantage of the recovery steam generator according to the invention is given by the flexibility in following the steam request, thanks to the integrated burner and the different operating modes that have been exemplified above. In addition, the smoke tube construction makes the generator stable and easily modulated with respect to water tube generators, for example, which can encounter a flow block if they move away from the design conditions.

Another advantage is given by the complete integration in a single appliance of the heat exchange pipes (smoke pipes), of the auxiliary burner, of the vapor separator.

The advantages will be more evident in the following, with the help of the description of a preferred embodiment.

Description of the drawings

Fig. 1 represents the functional diagram of a steam generator according to a way of carrying out the invention.

Fig. 2 is a cross section of a steam generator according to the embodiment of Fig. 1.

Fig. 3 is a cross section of the steam generator of Fig. 2.

Fig. 4 is a functional diagram of a steam generator in another embodiment of the invention.

Detailed description

Fig. 1 provides a diagram of a steam generator 1 comprising a shell 2 and a tube bundle 3 which define a tube side and a shell side of the generator itself. The tube side includes the inside of the tubes belonging to bundle 3 while the shell side includes the volume enclosed by the shell 2 around the bundle 3 itself.

Other noteworthy components of the steam generator 1 shown in Fig. 1 are: a burner 4, a steam separator 5, an economizer 6, a smoke inlet 7, an internal mixing chamber 8.

The burner 4 has a combustion chamber 9 which extends inside the shell 2. Said combustion chamber 9 communicates with the mixing chamber 8 mentioned above. Said mixing chamber 8 also communicates with the smoke inlet 7.

The burner 4 is connected (for example flanged) to the body 2 and is fed with a fuel, for example natural gas 40, and a comburent, for example fresh air 41. The air 41 is fed by a compressor 42.

The tube bundle 3 has a tube inlet 10 and a tube outlet 11. The tube inlet 10 communicates with the mixing chamber 8; the pipe outlet 1 1 communicates with an exhaust duct or a chimney 34 in which the economizer is installed 6.

The tube bundle 3 in more detail comprises two banks 31, 32 of straight tubes. The first bench 31 is fed directly (through the tube inlet 10) from the mixing chamber 8; at the outlet of the first counter 3 1 the fumes are re-directed into the pipes of the counter 32 from a chamber 33. The outlet of the second counter 32 corresponds to the pipes outlet 11 already described and communicates with the chimney 34.

The economizer 6 is essentially represented, in this example, by a series of pipes exposed to fumes and internally crossed by feed water.

The smoke inlet 7 is connected by means of a smoke duct 12 to the exhaust of an internal combustion engine 13 which drives an electric generator 14. Through the duct 12 and inlet 7, the steam generator 1 receives the fumes 15 produced by the motor 13.

It should be noted that the mixing chamber 8 communicates directly with the smoke inlet 7, with the pipe inlet 10 (inlet from the pipe side of the appliance) and with the combustion chamber 9 of burner 4. As a result of this location, the the path of the fumes 15 towards the inlet of the pipes 10 necessarily passes through the mixing chamber 8. Similarly, the combustion chamber 9 of the burner 4 also communicates with the inlet 10 of the pipes through the same mixing chamber 8, which is why the gases combustibles released by burner 4 can enter the tubes of bundle 3 passing through chamber 8.

It is therefore understood that the tube bundle 3 can be fed with the fumes 15 coming from the engine, with the burned gases coming from the burner 4, or with a mixture of both. In all cases, the hot gases exiting from the tube bundle 3 through the tubes outlet 11 pass into the chimney 34 where they transfer more heat to the feed water through the economizer 6. The cooled fumes 16 are then discharged into the atmosphere or directed to a fume treatment if foreseen.

The water side of the steam generator 1 is now described. The feed water 20 is sent under pressure with a pump 21 towards the economizer 6. A line 22 carries the preheated water, coming out of said economizer 6, towards the inside the separator 5.

The separator 5 in operation contains a certain amount of liquid water in the lower part and steam in the upper part. Said separator 5 communicates with the shell side of the generator 1 (inside the shell 2) through liquid discharge pipes 23 and steam rising pipes 24. Line 25 extracts the steam from the separator 5, for example saturated steam at a pressure of about 18 bar.

The possible operating modes can be summarized as follows.

In a first operating mode ("recovery only") burner 4 is not used. The fumes 15 from the engine 13 therefore constitute the only source of heat for the production of steam.

In a second operating mode ("integration only") the steam generator 1 works exclusively with the combustion gases of the burner 4. The combustion gases pass from the combustion chamber 9 to the mixing chamber 8 and, from this, into the tube bundle 3.

In a third operating mode ("integration recovery") the mixing chamber 8 receives both the fumes 15 from the engine and the burner burner gases. The two flows mix in chamber 8 before entering the tube bundle 3. The heat transferred to the water consequently comes partly from the recovery of the fumes and partly from the burner 4.

In the third mode of operation, we will try to make the most of the heat of the fumes 15 by using the integration via burner 4 only in case of need when the steam request cannot be fully satisfied with the heat of the fumes.

Furthermore, it should be noted that the combustion chamber 9, being itself inside the shell 2, helps to provide heat to the evaporating water.

The components which have been described with reference to the diagram of Fig. 1 are even better visible in Figs. 2 and 3 which represent a preferred embodiment.

In said preferred embodiment, the steam generator 1 is a horizontal appliance, the shell 2 is therefore an essentially cylindrical body with a horizontal axis. The combustion chamber 9 is also a cylindrical body, coaxial to the shell 2 and located in the lower part of the same. Said combustion chamber 9 has an inlet section facing the burner 4 and an outlet section 19, opposite the burner 4, which flows into the mixing chamber 8.

In Fig. 2 it can be seen that the combustion chamber 9 has a bellows wall 17 to allow thermal expansion.

In Figs. 2 and 3 the reference 18 indicates the shell side of the generator 1 which contains the evaporating water.

At the tube inlet 10 and the tube outlet 11 there are, for example, two tube plates to which the tubes that make up the benches 31 and 32 respectively are fixed.

Fig. 3 shows a preferred arrangement of the banks of tubes 31, 32. The tubes of the first bank 31 are horizontal and essentially form an arc around the combustion chamber 9. The tubes of the second bank 32 are also horizontal and arranged in an arch above the first bench 31.

The cylindrical body of the separator 5 is preferably located to the side and above the shell 2 as can be seen in particular in Fig. 3.

The figures also show the smoke exhaust duct 35 downstream of the economizer 6.

Further construction details do not need to be described as they can be established by a person skilled in the art on the basis of common knowledge.

The embodiment of Fig. 4 is another preferred embodiment and provides for a mixing chamber which is located at the entrance to the combustion chamber 9 rather than downstream of the same. In the example, said mixing chamber is essentially made of a toroidal body 80 connected to a smoke inlet 70 which receives the fumes 15 coming from the engine through the duct 12.

Said toroidal body 80 operates as a body for conveying and mixing the fumes and allows the fumes 15 to be introduced directly into the combustion chamber 9, mixing them with the fuel and air entering the chamber itself. The toroidal body 80 for example advantageously comprises openings arranged to introduce the fumes in a suitable way, for example with a swirling flow.

The internal chamber 8 can also be provided in the embodiment of Fig. 4, as illustrated, to put the combustion chamber 9 in communication with the inlet 10 of the tube bundle.

Apart from the location of the mixing chamber, the details and construction aspects of the construction of Fig. 4 can be substantially similar to those of Fig. 1 and can be made as in Figs. 2 and 3. The numerical references of Fig. 4 are therefore to be understood as analogous to those of Fig. 1 and the relative description is also applicable to them.

The realization of Fig. 4, as is clear to a person skilled in the art, can also operate in the operating modes described above, i.e. only recovery, only integration or recovery plus integration

Claims (10)

CLAIMS 1) Recovery steam generator (1) with smoke tubes, comprising: a shell (2) and a tube bundle (3) which define a tube side and a shell side of the generator; a smoke inlet (7, 70); a burner (4) having a combustion chamber (9) located in the shell (2) of the steam generator; a mixing chamber (8, 80), communicating with said smoke inlet (7, 70) and with said combustion chamber. 2) Steam generator according to claim 1, in which said mixing chamber (80) is located at the entrance of said combustion chamber, so as to allow direct entry of the fumes into the combustion chamber. 3) Steam generator according to claim 1, in which said mixing chamber (8) is located downstream of the combustion chamber, so as to allow a mixing of the fumes with the combustion products, before entering the pipes. 4) Steam generator according to one of the preceding claims in which: the shell (2) is substantially cylindrical and horizontal; the combustion chamber (9) is delimited by a substantially cylindrical body with a horizontal axis, preferably coaxial to the shell (2). 5) Steam generator according to claim 4, wherein the tube bundle (3) comprises at least a first bank (31) of straight and horizontal tubes, which are arranged above the combustion chamber (9), essentially forming an arc around to said chamber. 6) Generator according to claim 5, wherein the tube bundle (3) comprises a second bank of tubes (32), thus forming a bundle with two passages, the tubes of the second bank (32) being horizontal and arranged in an arc beyond above the first bench (31). 7) Steam generator according to one of the preceding claims, comprising a steam separator (5) having its own pressurized body external to said shell (2), in which said steam separator (5) communicates with the shell side of the generator of vapor through at least one steam rising line (24) and at least one liquid descent line (23). 8) Steam generator according to claim 6, comprising an economizer (6) arranged to preheat a feed water flow (20) with the gases leaving the pipe side, and comprising a line (22) for feeding preheated water, leaving the said economizer, towards the said vapor separator (5). 9) Cogeneration unit comprising an internal combustion engine (13), preferably a reciprocating engine, and a steam generator (1) according to any one of the preceding claims, in which the fumes inlet (7) of the steam generator is connected at the engine exhaust, to use the engine exhaust fumes (15) as a source of heat for the production of steam (25). 10) Group according to claim 9, also comprising an expander fed by at least part of the steam (25) produced in the generator, said expander being preferably of the screw type.