Claims:CLAIMS

1. A Circulating Fluidized Bed Combustion (CFBC) boiler (1) comprising
- a first boiler pass (2) containing a combustion chamber (3) in a lower section (4) of the first boiler pass (2) in which a fluidized bed of solid fuel (5) and recycled solids (6) is to be formed by combustion air (7) blown into the combustion chamber (3), and first convection heating coils (8) in an upper section (9) of the first boiler pass (2),
- a solids separation system (10) installed at an outlet (11) of the first boiler pass (2) designed to collect solids leaving the first boiler pass (2) and return them to the combustion chamber (3) as recycled solids (6), and
- a second boiler pass (12) containing second convection heating coils (13, 14, 15),
wherein erosion protection plates (23, 23’) are mounted on the first (8) and/or second convection heating coils (13, 14, 15), characterized in that the erosion protection plates (23, 23’) are removably fixed to the first (8) and/or second convection heating coils (13, 14, 15) by bolted connections (24).

2. The CFBC boiler according to claim 1, characterized in that the erosion protection plates (23, 23’) are fixed to the first (8) and/or second convection heating coils (13, 14, 15) by means of generally U-shaped clamps (25) that encompass the respective convection heating coil (8; 13, 14, 15) and the respective erosion protection plate (23), and that are closed by a counter-plate (26) secured to the clamp (25) by screw nuts (27).

3. The CFBC boiler according to claim 2, characterized in that the screw nuts (27) are lock nuts.

4. The CFBC boiler according to claim 1, characterized in that the erosion protection plates (23) comprise a main plate (28) extending along the respective heating coil (8; 13, 14, 15) and an extension plate (29) being fixed to one end of the main plate (28) and extending the main plate (28) in a plane parallel to the main plate (28), so as to overlap with the main plate (28) of a neighboring erosion protection plate (23’).

5. The CFBC boiler according to claim 4, characterized in that the erosion protection plates (23) are arranged on the respective coils (8; 13, 14, 15) in such a way that a gap (30) is formed between the main plates (28) of the neighboring erosion protection plates (23, 23’), wherein the gap (30) has a length (L1) of at least 3.5 %, preferably at least 4 %, of the length (L2) of the neighboring erosion protection plates (23, 23’) at a temperature of 30 °C.

6. The CFBC boiler according to claim 4, characterized in that one of the bolted connections (24) fixing at least one of the erosion protection plates (23, 23’) to the respective convection heating coil (8; 13, 14, 15) is arranged in the region of the extension plate (29).

7. The CFBC boiler according to claim 1, characterized in that the bolted connections (24) are arranged on the erosion protection plates (23, 23’) at regular spacing intervals.

8. The CFBC boiler according to claim 1, characterized in that a majority of the erosion protection plates (23, 23’) of the first (8) or second convection heating coils (13, 14, 15) has a length (L2) of at least 600 mm.

9. The CFBC boiler according to claim 1, characterized in that the erosion protection plates (23, 23’) are made of stainless steel and/or carbon steel.

Dated this 27th day of July, 2020.

(SOUMEN MUKHERJEE)
IN/PA - 214
Applicants’ Agent
, Description:CFBC Boiler with Bolted Erosion Protection Plates

Prior Art

The invention relates to a Circulating Fluidized Bed Combustion (CFBC) boiler with erosion protection plates according to the preamble of claim 1.

In conventional power plants fuel is burnt in a combustion chamber of a boiler and the resulting heat is transferred to water in order to create steam driving turbines for power generation.

A particular type of boiler are CFBC boilers, in which combustion occurs in a so-called fluidized bed formed by a continuous stream of combustion air supplied to the bottom of the combustion chamber, that suspends a mixture of solid fuel and ash particles within the combustion chamber. The gas cushion between the solids allows the particles to move freely, giving the bed a liquid-like (fluidized) characteristic. Heat from the combustion process boils water in convection heating coils turning it into high-energy steam.

Due to the high velocities of the stream of combustion air (typically 4-5m/s) that are necessary to maintain the fluidized bed, the flue gas exiting the first boiler pass comprising the combustion chamber and first convection heating coils contains a large amount of entrained ash and unburnt fuel particles. For this reason a solids separation system, usually a hot or cold cyclonic separator, is used to free the flue gas from these particles before it enters the second boiler pass containing additional second convection heating coils. The separated particles are returned as recycled solids into the combustion chamber.

Due to the working principle of CFBC boilers the convection heating coils are highly stressed components: The coils are filled with pressurized water and/or high pressure steam and have to resist large temperature differences between the switched-off and the operational state of the facility. Further, the coils are prone to erosion due to particles entrained in the flue gas that impact on the coils with high speed and high temperatures. For these reasons it is known in the prior art to mount erosion protection plates on the convection heating coils in order to shield them from erosion and to achieve a longer service life of the coils, since any failure and/or replacement of a single heating coil requires to switch off the complete facility and thus results in large financial losses.

The erosion protection plates known in the prior art are mounted to the convection heating coils by welding. Each of the elongated erosion protection plates is fixed to the respective convection heating coil by a number of generally U-shaped clamps embracing the coil. The welding has to be carried out on site, connecting the arms of the U-shaped clamps to both sides of the erosion protection plate.

For a good protection of the coils it is important that the erosion protection plates are hold firmly to the coil in particular at their ends and that neighboring plates overlap in order not to leave a gap in which the coil would be unprotected.

With regards to the erosion protection plates known in the prior art it was observed frequently by service engineers that the erosion protection plates failed due to distortion and/or rupture, leading to unprotected coils failing due to erosion after only a short period of time.

Although the design of the erosion protection plates made provisions for an expansion gap between adjacent erosion protection plates, that was to be covered by an extension plate, the workers on site often had rendered these expansion gaps useless when mounting the plates. When welding the ends of the plates to a clamp either the expansion gap was obstructed by welding material, or both adjacent erosion protection plates were welded to the same clamp. Hence, it is a disadvantage of the erosion protection plates known in the prior art that there is a risk to incorrectly mount the plates to the coils that inevitably leads to a failure of plates and the coils in use, and that therefore highly-skilled, expensive labor forces are required to correctly mount the plates on site also involving substantial amount of time.

Disclosure of the invention

It is therefore the object of the present invention to provide a CFBC boiler with a simplified maintenance and a longer service life of its convection heating coils.

This object is achieved by a CFBC boiler with the features of claim 1.

Hereby, a Circulating Fluidized Bed Combustion (CFBC) boiler is provided that comprises:
- a first boiler pass containing a combustion chamber in a lower section of the first boiler pass in which a fluidized bed of solid fuel and recycled solids is to be formed by combustion air blown into the combustion chamber, and first convection heating coils in an upper section of the first boiler pass,
- a solids separation system installed at an outlet of the first boiler pass designed to collect solids leaving the first boiler pass and return them to the combustion chamber as recycled solids, and
- a second boiler pass containing second convection heating coils.
In the boiler erosion protection plates are mounted on the first and/or second convection heating coils. According to the invention the erosion protection plates are removably fixed to the first and/or second convection heating coils by bolted connections.

Using bolted connections to fix the erosion protection plates to the heating coils simplifies mounting of the plates on site and also reduces the risk of incorrect mounting. Since neighboring erosion protection plates cannot be bonded to each other in error when using bolted connections, it is ensured that the plates remain movable with respect to each other to compensate for thermal expansion. Moreover, the bolted connections simplify replacement of individual erosion protection plates.

Preferably, the erosion protection plates are fixed to the first and/or second convection heating coils by means of generally U-shaped clamps that encompass the respective convection heating coil and the respective erosion protection plate, and that are closed by a counter-plate secured to the clamp by screw nuts. In this case, the counter-plate provides for a bearing face for the erosion protection plate. In particular no direct bolted connection of the erosion protection plates to any other component is required. The erosion protection plates are held on the convection heating coils in a form-fitting manner, enclosed by the U-shaped clamps and the counter-plates. This arrangement results in less stress and distortion in the components due to thermal expansions since the relative movement of the components is less strictly inhibited.

Particularly preferably the screw nuts securing the counter-plate to the clamp are lock nuts. Thereby unintentional loosening of the screw nuts is inhibited. A lock nut within the meaning of this application may for example be a single, self-locking nut or it may comprise a screw nut and a second counter-nut.

In preferred embodiments of the invention the erosion protection plates comprise a main plate extending along the respective heating coil and an extension plate being fixed to one end of the main plate and extending the main plate in a plane parallel to the main plate, so as to overlap with the main plate of a neighboring erosion protection plate. The extension plate allows to cover an expansion gap between neighboring erosion protection plates, such that the convection heating coils are protected seamlessly. The extension plate is preferably shop-welded to the main plate, such that no welding on site is necessary.

It is further preferred that the erosion protection plates are arranged on the respective coils in such a way that a gap is formed between the main plates of the neighboring erosion protection plates, wherein the gap has a length of at least 3.5 %, particularly preferred at least 4 %, of the length of the neighboring erosion protection plates at a temperature of 30 °C. It has proven to be advantageous to choose the length of the gap by far larger than the thermal expansion to be expected by the erosion protection plates in order to disentangle the movement of coils, erosions protection plates and their mounting further. The erosion protection plates are preferably made of stainless steel and/or carbon steel.

It is especially preferred that one of the bolted connections fixing at least one of the erosion protection plates to the respective convection heating coil is arranged in the region of the extension plate. Holding the erosion protection plate with one clamp at its extension plate results in an improved guidance of the neighboring protection plates at their joint.

Preferably, the bolted connections are arranged on the erosion protection plates at regular spacing intervals. Thereby, the mounting pattern of the erosion protection plates is simplified and mounting of the plates is thus further facilitated.

In preferred embodiments a majority of the erosion protection plates of the first or second convection heating coils has a length of at least 600 mm. Due to the improved compensation of thermal expansion by mounting the plates according to the invention, it has become feasible to use longer erosion protections plates of lengths larger than 600 mm. Using longer plates results in a lower number of plates to be mounted and thus facilitates the mounting process further.

Further advantageous embodiments are described in the following description and the dependent claims.

The invention is explained in more detail below with regard to the embodiments shown in the attached drawings.

Brief description of the drawings

Fig. 1 shows in a block diagram a schematic illustration of a CFBC boiler according to the invention,
Figs. 2a, 2b show schematically prior art erosion protection plates mounted to convection heating coils by welding in a side view and in a transverse section, respectively,
Fig. 3 shows schematically in a side view erosion protection plates mounted to convection heating coils by bolted connections according to the invention,
Figs. 4a, 4b show schematically transverse sections of the mounted erosion protections plates according to the cutting lines B-B and A-A of Fig. 3, respectively.

Embodiments of the invention

In the drawings, identical parts are always provided with the same reference signs and are therefore usually referred to only once.

Fig. 1 the general setup of a CFBC boiler according to the invention is shown.

The CFBC boiler 1 comprises:
- a first boiler pass 2 containing a combustion chamber 3 in a lower section 4 of the first boiler pass 2 in which a fluidized bed of solid fuel 5 and recycled solids 6 is to be formed by combustion air 7 blown into the combustion chamber 3, and first convection heating coils 8 in an upper section 9 of the first boiler pass 2,
- a solids separation system 10, preferably a cold cyclone, installed at an outlet 11 of the first boiler pass 2 designed to collect solids leaving the first boiler pass 2 and return them to the combustion chamber 3 as recycled solids 6, and
- a second boiler pass 12 containing second convection heating coils 13, 14, 15.

In the depicted embodiment the first convection heating coils 8 comprise water and/or steam from a steam drum 21 which is further heated in order to create high pressure steam 22 to drive a steam turbine (no shown). The first convection heating coils 8 – that are only schematically shown in Fig. 1 – may thus for example be part of an evaporator and/or a superheater of the CFBC boiler. The first convection heating coils 8 may be located for example at the screen delimiting the combustion chamber 3 or in the freeboard.

The second convection heating coils 13 may be part of the economizer, pre-heating the feed water for the steam drum 21. The second convection heating coils 14 and 15 are used as pre-heating coils for combustion air 7.

The flue gas exiting the second boiler pass 12 passes through a precipitator 16, e.g. an electric precipitator, for further cleaning, and through an ID fan 17 before it leaves the facility through stack 18. The ash from the combustion chamber 3 and the particles precipitated in precipitator 16 are conveyed to an ash storage 19.

In addition to the solid fuel 5 usually a sorbent 20 is fed to the combustion chamber in order to reduce SOx emissions. A typically used sorbent is limestone.

Figs. 2a and 2b show erosion protection plates 230 according to the prior art as a comparative example. The erosion protection plates 230 are mounted to the convection heating coils 8, 13 by welding. Each of the elongated erosion protection plates 230 is fixed to the respective convection heating coil 8, 13 by a number of generally U-shaped clamps 250 embracing the coil. This welding has to be carried out on site and connects the arms of the U-shaped clamps 250 to both sides of the erosion protection plate 230.

The erosion protection plate 230 comprises a main plate 280 and an extension plate 290 that covers a gap 300 formed between the main plates 280 of neighboring erosion protection plates 230. As can be seen in Fig. 2b when providing for a clamp 250 in the region of the extension plates 290, it is important to only weld the extension plate 290 to the clamp 250, but not to the main plate 280 of the neighboring erosion protection plate 230. Otherwise the gap 300 becomes useless in compensating thermal expansion. Moreover, welding material entering the gap 300 during welding may obstruct the gap 300 an inhibit compensation of thermal expansion. Thus, particularly clean and correct welding is of major importance when mounting the erosion protection plates 230 of the prior art.

Figs. 3, 4a and 4b show an embodiment of the invention that solves this problem. According to the invention the erosion protection plates 23, 23’, that are mounted on the first 8 and/or second convection heating coils 13 are removably fixed to the first 8 and/or second convection heating coils by bolted connections 24.

The erosion protection plates 23, 23’ are fixed to the first and/or second convection heating coils 8, 13 by means of generally U-shaped clamps 25 that encompass the respective convection heating coil 8, 13 and the respective erosion protection plate 23, and that are closed by a counter-plate 26 secured to the clamp 25 by screw nuts 27. Preferably, the screw nuts 27 are lock nuts.

The erosion protection plates 23 of the embodiment comprise a main plate 28 extending along the respective heating coil 8, 13 and an extension plate 29. The extension plate 29 is fixed to one end of the main plate 28 and extends the main plate 28 in a plane parallel to that of the main plate 28. The extension plate 29 is arranged, so as to overlap with the main plate 28 of the neighboring erosion protection plate 23’. In addition, one of the bolted connections 24 fixing at least one of the erosion protection plates 23, 23’ to the respective convection heating coil 8, 13 is arranged in the region of the extension plate 29.

The extension plate 29 is preferably shop-welded to the main plate 28, though generally also a one-piece design would be conceivable.

According to the embodiment shown in Figs. 3 and 4 the erosion protection plates 23 are arranged on the respective coil 8, 13 in such a way that a gap 30 is formed between the main plates 28 of the neighboring erosion protection plates 23, 23’. The gap 30 has a length L1 of at least 3.5 % of the length L2 of the neighboring erosion protection plates 23, 23’ at a temperature of 30 °C. Preferably, a majority of the erosion protection plates 23, 23’ of the first (8) or second convection heating coils (13, 14, 15) has a length L2 of at least 600 mm. Preferably, the erosion protection plates 23, 23’ are made of stainless steel and/or carbon steel.

The bolted connections 24 are arranged preferably on the erosion protection plates 23, 23’ at regular spacing intervals.

List of reference signs

1 CFBC boiler
2 First boiler pass
3 Combustion chamber
4 Lower section of first boiler pass
5 Solid fuel
6 Recycled solids
7 Combustion air
8 First convection heating coils
9 Upper section of first boiler pass
10 Solids separation system
11 Outlet of first boiler pass
12 Second boiler pass
13, 14, 15 Second convection heating coils
16 Precipitator
17 ID fan
18 Stack
19 Ash storage
20 Sorbent
21 Steam drum
22 Steam
23, 23’, 230 Erosion protection plates
24 Bolted connections
25, 250 Clamp
26 Counter-plate
27 Screw nut
28, 280 Main plate
29, 290 Extension plate
30, 300 Gap

L1 Length of gap
L2 Length of neighboring erosion protection plates
I Spacing intervals