WO2010123449A1

WO2010123449A1 - A boiler equipped with cooled baffles in the flue passage

Info

Publication number: WO2010123449A1
Application number: PCT/SE2010/050435
Authority: WO
Inventors: Lennart Nordh; Karl-Erik Brink; Anders VICTORÉN
Original assignee: Metso Power Ab
Priority date: 2009-04-24
Filing date: 2010-04-21
Publication date: 2010-10-28
Also published as: SE533545C2; EP2422133A1; SE0950276A1

Abstract

The present invention concerns a boiler (1) with a circulation system for boiler water which comprises water pipes (3) arranged to circulate boiler water or steam through the pipes with a cooled partition (12) arranged to cool a selected, exposed part of the steam boiler (1). In accordance with the present invention, the cooled partition (12) comprises a number of outer tubes (13), each of which has an inner tube (14) arranged in the outer tube (13), and which outer tube (13), at an upper end (15), has an open connection to a pipe (3) for cooling media, in which pipe a flow is established. Adjacent outer tubes (13) may appropriately be connected with heat-conducting connectors (40) over at least 70% of the axial length of the outer tubes and embedded in a fireproof material (41) that forms the outer surface of the partition.

Description

A BOILER EQUIPPED WITH COOLED BAFFLES IN THE FLUE PASSAGE

TECHNICAL AREA

The present invention concerns a boiler with a circulation system for boiler water or steam, which boiler has thin partitions in the flue for redirecting and/or cooling flue gases.

BACKGROUND TO THE INVENTION

In steam boilers in which combustible material is combusted in a furnace, hot flue gases are formed that are conducted out of the steam boiler while the hot flue gases emit thermal energy to the boiler water circulating in a circulation system for the boiler. A prior art method of transferring thermal energy from hot flue gases to boiler water in a small steam boiler of an older type involves using a Field's steam boiler system from the late 19^th century in this steam boiler. To increase the heating area, vertical tubes (Field's tubes), closed at the lower ends, are used in this. They are suspended from the furnace roof. Inside these tubes are inner circulation tubes which are open at both ends. When the boiler is in operation, heat applied to the outer gap between the concentrically arranged tubes will achieve a steam/water mixture that has a lower density than the water in the inner tube. This creates powerful natural self-circulation in these tubes which effectively increases the heat-absorbing area in the boiler. Heat is thus supplied to the stationary water volume above the furnace roofs sheet metal from these Field's tubes. These Field's tubes are freely exposed in the furnace.

Field's tubes were also used as heat-absorbing surfaces for superheating steam, preferably with forced circulation. Examples of such prior art solutions are shown in patents from the early 20th century, for example GB 153780 & GB8151 (1915) and GB 19472 (1913).

Regardless of how the heat is transferred to the boiler water or steam, the flue gases, on their way out of the steam boiler, are conducted through ducts in which parts of the boiler are subject to extreme stress, hi particular for superheaters of steam, these heat-absorbing surfaces are subjected to extreme temperatures. In boilers that are fired with biofuel, coal or peat, a conventional superheating temperature of around 540°C is achieved in practice. However, the preferred temperature is up to 600°C. The designs have to be able to cope with problems of expansion and hot gas corrosion that the superheater surfaces are subjected to. The aim of the present invention is to offer an improved design of heat-absorbing partitions in a boiler. These partitions can easily be installed in the boiler without special designs to absorb expansion problems. These partitions can also be cooled and protected in their most exposed parts in an optimum manner. In this way, it is possible to absorb the heat in the boiler's flue in an improved manner and also achieve a higher practicable superheating temperature.

Another aim is to obtain a partition that can easily be installed as an upgrade to a boiler, whereby the partition can be installed in the flue to redirect the flue gases to the extent required in order to optimise combustion.

Yet another aim is to use the partitions to build up separate spaces in the boiler's flue in which it is possible to fire the boiler using a less corrosive fuel.

DESCRIPTION OF THE INVENTION

The invention concerns a boiler with a circulation system for boiler water or steam, the flow of which can only be driven via self-circulation, or in which pumps may exist to facilitate the flow. The circulation system comprises water pipes or steam pipes arranged in such a way that, when the steam boiler is in operation, they can circulate boiler water through the pipes in a circuit in which water passes from the steam dome in the outer down tube to the furnace and convection parts and in which water and steam pass from the furnace and convection parts up to a steam dome in which steam is separated from the circuit. The steam can then be conducted from the steam dome directly or via intermediate superheaters to heat-absorbing partitions in accordance with the invention.

The boiler comprises a thin- walled partition in the flue arranged to project from the walls of the flue and designed to guide the flue gases in the flue past these partitions without passing through these partitions. The partition is cooled and comprises a number of outer tubes integrated in the partition arranged in the plane of the partition and an inner tube placed in the outer tube. The outer tube has an upper end with an open connection to one of the water pipes for boiler water and a sealed lower end. The inner tube has an upper open end connected to a water pipe for boiler water and a lower open end at the outer tube's sealed lower end. In some embodiments, the inner tube's upper open end is located at the outer tube's upper end and they are connected to the same water pipe or flow for boiler water. The tubes in the partition may extend in either a vertical or a horizontal direction downwards from the point at which the outer tube is connected to a water pipe.

In a preferred embodiment, the axial length of the outer tubes constitutes at least 70% of the partition's projection length from the walls of the flue.

It is appropriate also for adjacent outer tubes in the partition to be connected with heat- conducting connectors over at least 70% of the axial length of the outer tubes. These connectors may appropriately consist of sheet metal or flat steel that is preferably welded to the outer tubes. Alternatively, they may be anchored in some other way without welding the tubes, which are often pressure-classified.

To protect the outer tubes further, they may be embedded in a fireproof material that forms the outer surface of the partition.

Where the partition walls are to function as boiling surfaces, the liquid that circulates through the pipes to cool the outer tubes is used as the heat-absorbing medium, which liquid is conducted from a steam dome either directly or via other boiling surfaces or comprises newly- added liquid that replaces that which is evaporated as steam.

Where the partition walls are to function as superheaters for steam, the steam that circulates through the pipes to cool the outer tubes is used as the heat-absorbing medium, which steam is conducted from a steam dome either directly or via other superheaters.

Where access is required to the flue in the position of the partition, it is appropriate to arrange in the inner tube an internal concentric through tube which passes through the entire inner tube and emerges at the other sealed end of the outer tube. Via this through tube it is possible to connect a source for the addition of additive to the flue gases via the partition or connect a control unit for connection to a thermocouple arranged at the other sealed end of the outer tube, which thermocouple is exposed to the flue gases.

According to another embodiment, a flue gas duct formed from the walls of the boiler may comprise a separating partition wall that separates two parts of the flue gas duct from each other. In such case, the partition wall may be at least partially constructed of a cooled partition in accordance with the present invention, i.e. a cooled partition comprising a number of integrated outer tubes with an inner tube arranged in the outer tube, which outer tube, at an upper end, has an open connection to one of the water pipes for boiler water and a sealed lower end, in which connection the inner tube has an upper open end at the outer tube's upper end and a lower open end at the outer tube's sealed lower end and the cooled device extends at least partially in a vertical direction downwards from the point at which the outer tube is connected to the water pipe or steam pipe.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 shows in outline a section of a boiler, here a recovery boiler;

Figure 2a shows a cooled partition in a first embodiment of the invention;

Figure 2b shows the partition in Figure 2a in a view from below;

Figure 3 a shows a cooled partition in a second embodiment of the invention;

Figure 3b shows the partition in Figure 3a in a view from below;

Figure 4 shows the flow of cooling water through a tube used in the partition;

Figure 5 shows a cooled partition in a third embodiment of the invention;

Figure 6 shows a cooled partition in a fourth embodiment of the invention;

Figure 7 shows a cooled partition in a fifth embodiment of the invention;

Figure 8 shows a cooled partition in a sixth embodiment of the invention;

Figure 9 shows a cooled partition in a seventh embodiment of the invention curved in a cylindrical shape;

Figure 10 shows a cooled partition in an eighth embodiment of the invention with a curved cylindrical shape.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows a boiler 1 that comprises a furnace 6 in which combustion takes place. The boiler 1 may, as shown in the figure, be a recovery boiler in which black liquor BL is sprayed into the base of the furnace while combustion air is supplied at various levels. Air is normally supplied in a recovery boiler with primary air P_AIR down towards the base and secondary air S_AI_R slightly higher up, but still below the point of supply of black liquor, and finally tertiary air at one or more levels, T_AIR-_I and TA_IR-_2S above the black liquor supply. It is critical for combustion in the furnace of a recovery boiler that the upward flow of flue gases is not too fast towards the outlet, for which reason the various air supplies may take place crosswise to influence the dwell time in the various zones and with an increasing proportion of air (up to and beyond the stoichiometric ratio) while passing through the flue. However, the boiler may be any other boiler in which production of hot water and/or steam takes place, for example FB (Fluidised Bed) boilers, power boilers, combined heat and power boilers or standard steam boilers, which are fired with either fuel or waste.

The boiler 1 has a circulation system 2 for boiler water, including any pumps. The circulation system comprises water pipes 3, 4 arranged in such a way that, when the steam boiler 1 is in operation, they can circulate boiler water through the pipes 3, 4 in a circuit in which water and steam pass, in a heated liquid HL, from the various heat-absorbing surfaces 12a-12d of the furnace 6 up to a steam dome 5 in which steam is separated from the circuit and water CL runs back towards the furnace 6 and its heat-absorbing surfaces to pass by them again. The steam ST may then be conducted from the steam dome to other heat-absorbing surfaces to produce superheated steam.

In Figure 1, the reference number 3 indicates water pipes in which water and steam HL rise towards the steam dome 5, while the reference number 4 designates water pipes in which water CL runs down from the steam dome 5 towards the area of the furnace 6. The downward water pipes 4 may appropriately be arranged separate from the furnace 6, for example on the outside of the steam boiler 1, so that no heat is supplied to water running from the steam dome on its way down. Down in the area of the furnace 6, the water may then be used to absorb thermal energy and transfer the thermal energy to the steam dome. The circulation of the boiler water in the pipes is often caused by powerful self-circulation that is driven by the strong heat generated in the furnace 6. Low-density water mixed with steam rises in the pipes while water with a higher density from the steam dome 5 runs down. If the pipe network is extensive and has a high pressure drop, pumps may also be installed to facilitate circulation if self-circulation is not sufficient.

In addition to the circulation circuit for boiler water, the boiler also comprises circulation of steam ST from the steam dome 5, which steam is conducted to heat-absorbing surfaces, here 12a, 12b and 12c, in which the steam is heated to superheated steam SST. This superheated steam is often conducted to a steam turbine for production of electricity or for another use of the steam. Figure 1 shows a superheater, fired separately with its own burner EB, with superheating surfaces in the form of partitions 12a for steam. These partitions 12a project from the flue's walls and enclose a separate furnace for the burner EB. The flue gases are here forced down and out of this space by passing over the partitions' inner edges and are mixed with flue gases from the boiler's principal combustion. Figure 1 also shows a number of cooled partitions 12b that form deflection walls for the flue gas in the flue gas duct. The figure also shows that the partitions 12c and 12d may be arranged as heat-absorbing partitions in a reversing shaft 7 for the flue gases, dividing the flue gas flow into parallel areas. The figure shows a superheater 12c for steam, suspended from the roof of the reversing shaft, and boiling surfaces 12d projecting subsequently from the walls of the reversing shaft. All these partitions 12a, 12b, 12c and 12d are appropriately made of cooled partitions 12 in accordance with the invention and have here an increased service life on account of their ability to resist heat.

A cooled partition 12 embedded in fireproof material will now be explained with reference to Figures 2a and 2b. The cooled partition 12 is shown in Figure 2a connected to a pipe 3 for flowing steam ST. The cooled partition 12 comprises a number of outer tubes 13 and an inner tube 14 arranged in each of the outer tubes 13. The outer tube 13 has, at an upper end 15, an open connection to the outgoing steam pipe 3 for superheated steam SST and a sealed lower end 16. The inner tubes 14 have an upper open end 17 that is connected to an incoming steam pipe 3 for steam ST and a lower open end 18 at the outer tubes' sealed lower end 16. hi the embodiment in accordance with Figure 2a, the inner tube 14's lower end 18 is located at the outer tube 13's sealed lower end 16. The cooled partition 12 may extend in either a vertical or a horizontal direction from the point at which the outer tubes 13 are connected to an incoming steam pipe 3. The axial length of the outer tubes 13 appropriately constitutes at least 70% of the partition's projection length from the walls of the flue, hi order to create a coherent partition, adjacent outer tubes 13 in the partition are connected with heat-conducting connectors 40 over at least 70% of the axial length of the outer tubes. These may be welded to the outer tubes or otherwise anchored to them as also shown in Figure 2a. These connectors 40 appropriately consist of sheet metal or flat steel that is preferably welded to the outer tubes. This produces a single thin-walled partition with optimum cooling on the most exposed parts of the partition, i.e. at the outer end of the partition, corresponding to the lower end in Figure 2. Figures 3a and 3b show the same partition 12 but the outer tubes 13 are embedded in a fireproof material 41 that forms the outer surface of the partition. Such embedding takes place in applications in which the partition is exposed to highly corrosive environments at high temperature.

The function of the cooled partition 12's outer tubes in an appropriate embodiment in which they are cooled with boiler water is shown in Figure 4. Here, the outer tubes 13 and the inner tube 14 are connected to the same pipe 3 through which boiler water flows; self-circulation is used in each outer tube 13. When the cooled tube 13 extends downwards into an area with hot flue gases, water that is located in the gap between the outer tube 13 and the inner tube 14 will be mixed with steam and have a lower density than the water located inside the inner tube 14. The steam/water mixture therefore rises in the gap between the outer tube 13 and the inner tube 14. The water located inside the inner tube 14 has a higher density and will fall instead. Part of the boiler water that flows in pipe 3 will then be sucked down in the inner tube 14 and will subsequently return upwards, absorbing thermal energy from the area around the cooled device 12. The boiler water in the water pipe 3 flows from left to right in the figure, as shown by the arrows. The normal water flow rate in the water pipe 3 is in the range 0.3 to 1.5 m/s. The water flow rate established in the downward tube 14 is 0.5 to 2.5 m/s and in the outer gap between tubes 13 and 14 it is 0.2 to 1.0 m/s.

Another embodiment of the partition is shown in Figure 5, where the incoming steam pipe 3 for steam ST is arranged coaxially in a collection chamber for the outgoing pipe 3 for superheated steam SST. In this way, it is possible to have a compact design of the connections to the inner tubes 14 and the outer tubes 13.

Yet another embodiment of the partition 12 is shown in Figure 6, where the partition is built with a connection equivalent to that shown in Figure 4, which is preferably used if boiler water with natural circulation is used as the cooling medium for the tubes 13.

Yet another embodiment of the partition 12 is shown in Figure 7; it is equivalent to that shown in Figures 3a and 3b. The difference here is that adjacent outer tubes 13 in the partition are connected with a number of heat-conducting connectors 40a in the form of short stays or clamps. These stays may be welded to the outer tubes or otherwise anchored to them. Embedding with fireproof material 41 produces a single thin-walled partition with protection against hot gas corrosion and optimum cooling on the most exposed parts of the partition, i.e. at the outer end of the partition, corresponding to the lower end in Figure 7.

Figure 8 shows an embodiment in which at least some of the tubes are provided with a through tube 21 that has been passed through the outer wall of the water pipe 3 and through some of the inner tubes 14 of the cooled partition 12. In the second tube from right in the figure, the through tube 21 connects, with its lower end, to the outer tube 13 of the cooled partition 12. The through tube 21 consists, in this case, of an element that may be used to add an additive to the steam boiler from a source Ch or to suck out flue gases for sampling. Without the cooled partition 12, the hot flue gases would act on this element unimpeded. The cooled partition 12 can now cool the through tube and contribute to increasing its service life. In the first tube from right in the figure, contact lines run from a control unit CU to a thermocouple 20 via the through tube 21. In such case, the thermocouple is kept cooled using the cooled device 12.

Figures 9-10 show other alternatives in accordance with the invention, where the thin-walled partition 12 is curved into a cylindrical shape and may be arranged, for example, in outlets of cyclones hi the flue. Figure 9 shows a circular partition equivalent to Figures 2a and 2b, and Figure 10 is equivalent to Figures 3 a and 3b.

The embodiments shown in the figures may be used hi boilers of different types and, in principle, everywhere there is a desire to install a partition to deflect flue gases hi the flue or to divide the flue gas flow into areas and where the partition requires cooling. The thin-walled partitions may also be bent into shapes other than a strictly flat structure or circular structure, for example an L-shaped partition structure.

Claims

1. A boiler (1) with a furnace (6) and a circulation system for heat-absorbing media arranged in the flue from the steam boiler, which circulation system (2) comprises pipes (3, 4) arranged in such a way that, when the boiler (1) is in operation, they can circulate heat-absorbing media through the pipes (3, 4) in a circuit and which boiler

(1) comprises thin- walled partitions in the flue, which partitions are arranged to project from the walls of the flue to guide the flue gases through the flue past these partitions without passing through these partitions, c h a r a c t e r is e d in t h a t the partition forms a deflection wall (12a, 12b) for the flue gas in the flue gas duct, so that the flue gases are forced over the inner edges of the partitions, and comprises a number of outer tubes (13) integrated in the partition and arranged in the plane of the partition and that the outer tubes (13) are embedded in a fireproof material (41) that forms the outer surface of the partition, where each outer tube (13), at a first end (15), has an open connection to a first pipe (3) for heat-absorbing media and a sealed second end (16), and where an inner tube (14) is arranged in the outer tube (13) and the inner tube (14) has a first open end (17) connected to a second pipe (3) for heat- absorbing media and a second open end (18) at the outer tube (13)'s sealed second end (16), and where heat-absorbing media circulate between the first and second pipe to cool the tube arranged inside the partition.

2. A boiler (1 ) in accordance with claim \, c h a r a c t e r is e d in th « * the axial length of the outer tubes (13) constitutes at least 70% of the projection length of the partition from the walls of the flue.

3. A boiler (1) in accordance with claim 2, c h a r a c t e r i s e d i n t h a t adjacent outer tubes (13) in the partition are connected with heat-conducting connectors (40) over at least 70% of the axial length of the outer tubes.

4. A boiler (1) in accordance with claim 3, c h a r a c t e r i s e d i n th a tthe connector (40) consists of sheet metal or flat steel that is preferably welded to the outer tubes.

5. A boiler (1) in accordance with any of the preceding claims, characterised in that the heat-absorbing medium that circulates through the pipes (3, 4) to cool the outer tubes (13) consists of liquid and is subjected to boiling in the tube.

6. A boiler (1) in accordance with any of the preceding claims, c h aracterised in t h a t the heat-absorbing medium mat circulates through the pipes (3, 4) to cool the outer tubes (13) consists of steam conducted in pipes from a steam dome (5) and is subjected to superheating in the tube.

7. A boiler (1) in accordance with any of the preceding claims, ch aracterised in thatthe inner tube (14) comprises an internal concentric through tube (21) which passes through the entire inner tube and emerges at the second sealed end of the outer tube (13).

8. A boiler (1) in accordance with claim 7, c h aracteri sed in thatthe concentric through tube (21) is connected to a source (Ch) for the addition of additive to the flue gases via the partition.

9. A boiler (1) in accordance with claim 7, characterised in thatihe concentric through tube (21) is connected to a control unit (CU) for connection to a thermocouple (20) arranged at the outer tube (13)'s second sealed end, which thermocouple is exposed to the flue gases.