ARRANGEMENT OF HEAT RECOVERY SURFACES OF A RECOVERY BOILER

Abstract

An arrangement in a recovery boiler having a furnace for combusting waste liquor and a flue gas duct including vertical flue gas channels, at least some of which are provided with heat recovery units for recovering heat from flue gases. The first flue gas channel downstream of the furnace is provided with a reheater and one of the following heat recovery units: an economizer or a boiler bank. The reheater and the second heat recovery unit are located one after the other in the horizontal incoming direction of the flue gas, so that in a flue gas channel the flue gas flows in a vertical direction from above downwards and heats the reheater and the second heat recovery unit simultaneously. The heat recovery elements of the reheater and the second heat recovery unit may be positioned side by side in a direction that is crosswise with respect to the horizontal incoming direction of the flue gas.

Claims

1. An arrangement in a recovery boiler having a furnace for combusting waste liquor for producing chemical smelt and flue gases and a flue gas duct comprising vertical flue gas channels, at least some of which are provided with heat recovery units for recovering heat from flue gases, said heat recovery units having a width of substantially that of the flue gas duct, wherein a first flue gas channel of the vertical flue gas channels is downstream of the furnace and is provided with a reheater and a heat recovery unit, wherein the heat recovery unit is an economizer or a boiler bank, and wherein the reheater and the second heat recovery unit are arranged, with respect to a horizontal incoming direction of the flue gas one after the other so that in the first flue gas channel the flue gases flow in a vertical downwards direction through the reheater and the second heat recovery unit simultaneously and thereby heats the reheater and the second heat recovery unit simultaneously.

2. The arrangement according to claim 1, wherein the heat recovery unit is the economizer, and the reheater and the economizer are positioned in the first flue gas channel, with respect to the horizontal incoming direction of the flue gases, one after the other so that the reheater is in front of the economizer with respect to the flow of the flue gases.

3. The arrangement according to claim 1, wherein the heat recovery unit is the boiler bank, and the reheater and the boiler bank are positioned in the first flue gas channel, with respect to the horizontal incoming direction of the flue gases, one after the other so that the reheater is in front of the boiler bank with respect to the flow of the flue gases.

4. The arrangement according to claim 1, the first flue gas channel includes wall tubes and the wall tubes in fluid communication with a drum of the boiler.

5. The arrangement according to claim 1, wherein the first flue gas channel includes wall tubes in fluid communication with are coupled to a superheater in the furnace.

6. An arrangement in a recovery boiler having a furnace for combusting waste liquor for producing chemical smelt and flue gases and a flue gas duct comprising vertical flue gas channels, each of which includes a heat recovery unit configured to recovery heat from flue gases passing through the flue gas duct, wherein the heat recovery units each include heat recovery elements, wherein the flue gas channels include a first flue gas channel which is immediately downstream of the furnace and is provided with a reheater; wherein the heat recovery unit of the first flue gas channel includes at least one of an economizer and a boiler bank, wherein, in the first flue gas channel, the heat recovery unit and the reheater are positioned side by side along a direction crosswise with respect to a horizontal incoming direction of the flue gases from the furnace so that in the first flue gas channel the flue gases flow downward and simultaneously across the heat recover unit and the reheater.

7. The arrangement according to claim 6, wherein the heat recovery unit in the first flue gas channel includes the economizer.

8. The arrangement according to claim 6, wherein the heat recovery unit in the first flue gas channel includes the boiler bank.

9. The arrangement according to claim 6, wherein the first flue gas channel includes wall tubes in fluid communication with a drum of the boiler.

10. The arrangement according to claim 9, wherein the first flue gas channel is in fluid communication with a superheater in the furnace.

11. A recovery boiler including: a furnace configured to combust waste liquor and including an inlet to receive the waste liquor and an outlet for chemical smelt produced by the combustion, wherein the furnace includes walls extending vertically to confine and direct upwards flue gases generated by combustion; superheaters arranged in an upper region of the furnace; a flue gas duct adjacent and horizontally offset from the furnace, wherein the flue gas duct includes a first channel attached to the furnace and having a flue gas inlet in an upper region of the first channel and a flue gas outlet at a bottom region of the first channel, wherein the inlet receives flue gases flowing downstream from the superheaters; wherein the first channel includes a reheater and a heat recovery unit and the heat recovery unit is at least one of an economizer and a boiler bank, and wherein the reheater and heat recovery unit in the first channel are arranged to simultaneously receive flue gas flowing down through the first channel.

12. The recovery boiler of claim 11, wherein the flue gas duct further comprises a channel horizontally offset from the first channel, and is configured to receive flue gas from the flue gas outlet of the first channel and direct the flue gas upwards to an upper portion of another channel of the gas duct which is downstream of the first channel.

13. The recovery boiler of claim 11, wherein the reheater of the first channel is between the heat recovery unit and the super-heaters along a horizontal direction.

14. The recovery boiler of claim 11, further comprising wall tubes in the walls of the furnace and the wall tubes are connected to a drum to receive water and/or steam from the drum, and the heat recovery unit in the first channel is connected to the drum to direct water and/or steam into the drum.

15. The recovery boiler of claim 14, wherein the wall tubes are connected to the superheaters to direct water and/or steam flowing through the wall tubes into the superheaters.

16. The recovery boiler of claim 11, further comprising an ash hopper attached to and directly below the first channel.

17. The recovery boiler of claim 11, wherein the first channel is attached to a side wall of the walls of the furnace, and the reheater and heat recovery unit are arranged side-by-side such that a vertical plane parallel to the side wall of the furnace extends through the reheater and heat recovery unit.

18. The recovery boiler of claim 17, wherein the reheater and the heat recovery unit each have a length in a direction perpendicular to the vertical plane which is greater than a width in the vertical plane.

19. The recovery boiler of claim 17, wherein the reheater includes a plurality of reheaters and the heat recovery unit includes a plurality of heat recovery units, and at least one of the reheaters is between and adjacent to two of the heat recovery units.

20. The recovery boiler of claim 17, wherein the first channel includes wall tubes in fluid communication with at least one of a drum and the superheaters, wherein steam and/or water flows through the wall tubes and the at least one of the drum and the superheaters.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] FIG. 1 illustrates schematically a conventional recovery boiler;

[0041] FIG. 2 illustrates a preferred embodiment of the invention, where the so-called second pass of the flue gas duct of a recovery boiler is provided with a second heat recovery unit in addition to a reheater;

[0042] FIG. 3 illustrates a second preferred embodiment of the invention, where the so-called second pass of the flue gas duct of a recovery boiler is provided with a second heat recovery unit in addition to a reheater;

[0043] FIG. 4 illustrates a third preferred embodiment of the invention, where the so-called second pass of the flue gas duct of a recovery boiler is provided with a second heat recovery unit in addition to a reheater; and

[0044] FIG. 5 illustrates a fourth preferred embodiment of the invention, where the so-called second pass of the flue gas duct of a recovery boiler is provided with a second heat recovery unit in addition to a reheater.

[0045] FIGS. 2-5 use the same reference numerals as FIG. 1 where applicable.

[0046] In the embodiment presented in FIG. 2 the superheaters (T) 20 of the recovery boiler are located in the upper part of the furnace, from where the superheated steam is led via line 33 to be used at a suitable location, such as in a turbine. Further, a reheater 30 is arranged in the superheater zone. A second reheater 21 is located in the so-called second pass 22. The flue gas flows past the superheaters 20 mainly horizontally, while in the flue gas duct the flue gas flows through vertical flue gas channels in turn from above downwards and from down upwards, as shown by arrows 23. Ash hoppers 24 are provided in the lower part of the flue gas duct.

[0047] Steam enters the reheater 21 located in the second pass from a steam turbine (not shown), bled steam of which the reheater heats. The bled steam is led into the reheater 21 via line 31. From the reheater 21 the steam is led into a reheater 30 located in superheater zone, after which the heated steam is returned into the steam turbine via line 32.

[0048] In addition to the reheater, the flue gas channel, the so-called second pass 22, is provided with an economizer (E) 25. In the flue gas channel the flue gas flows vertically from above downwards and heats the reheater 21 and the economizer 25 simultaneously. With respect to the horizontal flow direction of the flue gas the reheater 21 and the economizer 25 are located one after the other. The reheater 21 and the economizer 25 extend typically to the whole width of the flue gas duct. The flue gas flows further through sequential flue gas channels and exits via a discharge opening 26. In addition to the economizer 25 the flue gas duct is provided with economizers 27 and 28. The boiler water is fed into the economizers via line 29, and after it has flown counter-currently with respect to the flue gas it is led from the economizer 25 of the so-called second pass into a drum 7 of the boiler via line 34.

[0049] When the reheater and the economizer are positioned in the second in parallel with respect to downwards flowing flue gas, the number of their tubes can be chosen more freely, since the flue gases flow past all the tubes. This gives an advantage when there is a need to change the mutual sizes of different heat recovery surfaces with respect to each other and to keep the boiler building as small as possible.

[0050] The embodiment shown in FIG. 3 relates to a recovery boiler where a boiler bank is needed. The superheaters (T) 20 of the recovery boiler are located in the upper part of the furnace, from where superheated steam is led via line 33 to be used at a suitable location, such as in a turbine. Further, a reheater 30 is arranged in the superheater zone. A second reheater 21 is located in the so-called second pass 22. The flue gas flows past the superheaters 20 mainly horizontally, while in the flue gas duct the flue gas flows through vertical flue gas channels in turn from above downwards and from down upwards, as shown by arrows 23. Ash hoppers 24 are provided in the lower part of the flue gas duct.

[0051] Steam enters the reheater 21 located in the second pass from a steam turbine (not shown), bled steam of which the reheater heats. The bled steam is led into the reheater 21 via line 31. From the reheater 21 the steam is led into the reheater 30 located in superheater zone, after which the heated steam is returned into the steam turbine via line 32.

[0052] In addition to the reheater, the flue gas channel, the so-called second pass 22, is provided with a boiler bank 40. In the flue gas channel 22 the flue gas flows vertically from above downwards and heats the reheater 21 and the boiler bank 40 simultaneously. With respect to the horizontal flow direction of the flue gas the reheater 21 and the boiler bank 40 are located one after the other. The reheater 21 and the boiler bank 40 extend typically to the whole width of the flue gas duct. In the boiler bank 40 the water at a saturated temperature coming from the drum 7 of the boiler via line 41 is boiled partly into steam 42, which is led into the drum 7.

[0053] The flue gas flows after the second pass 22 further through the sequential flue gas channels and exits via a discharge opening 26. The flue gas duct is additionally provided with economizers 43 and 44. The boiler water is fed into the economizers via line 29, and after it has flown counter-currently with respect to the flue gas it is led from the economizer 43 downstream of the so-called second pass via line 45 into the drum 7 of the boiler.

[0054] Positioning the reheater and the boiler bank in the second pass in parallel with respect to the downwards flowing flue gas provides advantages. The flue gas has in the boiler bank a certain maximum velocity, which in practice dictates the number of tubes of the boiler bank and the depth of the flue gas channel. When the boiler bank is located next to the reheater, the number of tubes in the boiler bank can be chosen more freely, since the flue gases flow also at the reheater. This provides an advantage in investment costs and electricity production in recovery boilers having a smaller need for boiler bank. The need for a boiler bank decreases at high pressure levels of live steam and at high dry solids levels of combustion liquor. The heat efficiency needed for boiling decreases as the pressure of the steam increases, the flue gas amount decreases with dryer combustion liquor. On the other hand, the feed water needs to be heated to a higher temperature, since the higher pressure simultaneously increases the saturated temperature, whereby the size of the economizer needs to the increased.

[0055] In the embodiment presented in FIG. 4 the superheaters (T) 20 of the recovery boiler are located in the upper part of the furnace, from where the superheated steam is led via line 33 to be used at a suitable location, such as in a turbine. Further, a reheater (V) 30 is arranged in the superheater zone. A second reheater 50 is located in the so-called second pass 22. The flue gas flows past the superheaters 20 mainly horizontally, while in the flue gas duct the flue gas flows through vertical flue gas channels in turn from above downwards and from down upwards, as shown by arrows 23. Ash hoppers 24 are provided in the lower part of the flue gas duct.

[0056] Steam enters the reheater 50 located in the second pass from a steam turbine (not shown), bled steam of which the reheater heats. The bled steam is led into the reheater 50 via line 51. From the reheater 50 the steam is led into the reheater 30 located in superheater zone, after which the heated steam is returned into a steam turbine via line 32.

[0057] In addition to the reheater, the so-called second pass 22 is provided with an economizer 52 so that the first flue gas channel is provided with reheater elements 50 and economizer elements 52 staggered. Thus, they are positioned side by side in a row that is crosswise with respect to the horizontal incoming direction of the flue gas. It can also be said that the elements are positioned in a row in the direction of the front wall/rear wall of the boiler. The reheater elements and economizer elements are located in the second pass in parallel with respect to the downwards flowing flue gas. In FIG. 5 the heat recovery elements 50 and 52 are positioned so that every second element is a reheater element 50 and every second is an economizer element 52. The positioning does not need to be symmetrical. It is also possible that the number of reheater elements is higher than the number of economizer elements or vice versa. The number and size of the elements is dependent on the required heat surface according to the structure of each boiler and the process conditions.

[0058] In the flue gas channel 22 the flue gas flows vertically from above downwards and heats the reheater elements 50 and the economizer elements 52 simultaneously. The flue gas flows further through the sequential flue gas channels and exits via a discharge opening 26. In addition to the economizer 52 the flue gas duct is provided with economizers 53 and 54. The boiler water is fed into the economizers via line 29, and after it has flown counter-currently with respect to the flue gas it is led from the economizer elements 52 of the so-called second pass via line 55 into the drum 7 of the boiler.

[0059] When the reheater and the economizer are positioned in the second pass in parallel with respect to the downwards flowing flue gas, the number of their tubes can be chosen more freely, since the flue gases flow past all the tubes. This gives an advantage when there is a need to change the mutual sizes of different heat recovery surfaces with respect to each other and to keep the boiler building as small as possible.

[0060] The embodiment shown in FIG. 5 relates to a recovery boiler where a boiler bank is needed. The superheaters (T) 20 of the recovery boiler are located in the upper part of the furnace, from where the superheated steam is led via line 33 to be used at a suitable location, such as in a turbine. Further, a reheater 30 is arranged in the superheater zone. A second reheater 60 is located in the so-called second pass 22. The flue gas flows past the superheaters 20 mainly horizontally, while in the flue gas duct the flue gas flows through vertical flue gas channels in turn from above downwards and from down upwards, as shown by arrows 23. Ash hoppers 24 are provided in the lower part of the flue gas duct.

[0061] Steam enters the reheater 60 located in the second pass from a steam turbine (not shown), bled steam of which the reheater heats. The bled steam is led into the reheater 60 via line 61. From the reheater 60 the steam is led into a reheater 30 located in superheater zone, after which the heated steam is returned into a steam turbine via line 32.

[0062] The superheaters (T) 20 are located in the upper part of the furnace and the reheater 60 in the so-called second pass 22. The flue gas flows past the superheaters 20 mainly horizontally, while in the flue gas duct the flue gas flows through vertical channels in turn from above downwards and from down upwards, as shown by arrows 23. Ash hoppers 24 are provided in the lower part of the flue gas duct.

[0063] In addition to the reheater, the flue gas channel, the so-called second pass 22, is provided with a boiler bank 62 so that the first flue gas channel is provided with reheater elements 60 and boiler bank elements 62 staggered. Thus, the reheater elements and the boiler bank elements are positioned side by side in a row that is crosswise with respect to the horizontal incoming direction of the flue gas. It can also be said that the elements are positioned in a row in the direction of the front wall/rear wall of the boiler. In FIG. 5 the heat recovery elements 60 and 62 are positioned so that every second element is a reheater element 60 and every second is a boiler bank element 62. The positioning does not need to be symmetrical. It is also possible that the number of reheater elements is higher than the number of boiler bank elements or vice versa. The number and size of the elements is dependent on the required heat surface according to the structure of each boiler and the process conditions.

[0064] In the flue gas channel 22 the flue gas flows vertically from above downwards and heats the reheater elements 60 and the boiler bank elements 62 simultaneously. In the boiler bank elements 62 the water at a saturated temperature coming from the drum 7 of the boiler via line 63 is boiled partly into steam that is led into the drum 7 via line 64.

[0065] The flue gas flows after the second pass 22 further through the sequential flue gas channels and exits via a discharge opening 26. The flue gas duct is additionally provided with economizers 65 and 66. The boiler water is fed into the economizers via line 29, and after it has flown counter-currently with respect to the flue gas it is led from the economizer 65 located after the so-called second pass into the drum 7 of the boiler via line 67.

[0066] Positioning the reheater elements and the boiler bank elements in the second pass parallel with respect to the downwards flowing flue gas provides advantages. The flue gas has in the boiler bank a certain maximum velocity, which in practice dictates the number of tubes of the boiler bank and the depth of the flue gas channel. When the boiler bank is located next to the reheater, the number of tubes in the boiler bank can be chosen more freely, since the flue gases flow also at the reheater. This provides an advantage in investment costs and electricity production in recovery boilers having a smaller need for boiler bank. The need for a boiler bank decreases at high pressure levels of live steam and at high dry solids levels of combustion liquor. The heat efficiency needed for boiling decreases as the pressure of the steam increases and the flue gas amount decreases with dryer combustion liquor. On the other hand, the feed water needs to be heated to a higher temperature, since the higher pressure simultaneously increases the saturated temperature, whereby the size of the economizer needs to the increased.

[0067] Although the above description relates to embodiments of the invention that in the light of present knowledge are considered the most preferable, it is obvious to a person skilled in the art that the invention can be modified in many different ways within the broadest possible scope defined by the appended claims alone.