Heat recovery surfaces arrangement in a recovery boiler

Abstract

An arrangement in a recovery boiler having a furnace for combusting waste liquor and a flue gas duct comprising vertical flue gas channels, at least part of which is provided with heat recovery units for recovering heat from flue gases. The heat recovery units have a width of substantially the width of the flue gas duct, whereby downstream of the furnace the first flue gas channel is provided with a superheater. In addition to the superheater, the first flue gas channel is provided with one of following heat recovery units: an economizer, a boiler bank, or a reheater. The superheater and a second heat recovery unit are located one after the other in horizontal introduction direction of the flue gas, so that in a flue gas channel the flue gas flows in the vertical direction downwards and heats the superheater and the second heat recovery unit simultaneously.

Claims

1. An arrangement in a chemical recovery boiler having a furnace for combusting waste liquor and a flue gas duct comprising: vertical flue gas channels, at least part of which are provided with heat recovery units for recovering heat from flue gases, said heat recovery units each having a width of substantially that of the flue gas duct, wherein the vertical flue gas channels include a first flue gas channel after the furnace, and within the first flue gas channel are at least two of the heat recovery units which include a superheater and a secondary heat recovery unit, and the first flue gas channel further includes an open gas passage downstream in a flow of the flue gasses through the first flue gas channel, wherein the flue gases flow upward through the open gas passage to an upper outlet of the first flue gas channel, and the flue gases flow from the upper outlet to a next one of the vertical flue gas channels; wherein the secondary heat recovery unit is at least one of an economizer, a boiler bank and a reheater, wherein the superheater and the secondary heat recovery unit are positioned within the first flue gas channel one after the other along a horizontal incoming direction of the flue gas such that the flue gas flows downward through the first flue gas channel and thereby heats the superheater and the secondary heat recovery unit simultaneously, and wherein the secondary heat recovery unit is disposed entirely within the first flue gas channel.

2. The arrangement according to claim 1, wherein the secondary heat recovery unit is the economizer and, in the first flue gas channel, the superheater is forward of the economizer along the horizontal direction.

3. The arrangement according to claim 1, wherein the secondary heat recovery unit is the boiler bank, and, in the first flue gas channel, the superheater is forward of the boiler bank along the horizontal direction.

4. The arrangement according to claim 1, wherein the secondary heat recovery unit is a reheater and, in the first flue gas channel, the superheater and the reheater are arranged one after the other along the horizontal direction.

5. The arrangement according to claim 1, wherein the first flue gas channel includes wall tubes connected to a dedicated tube circulation system which includes a drum of the boiler for providing a steam/water mixture flow in the wall tubes, and the wall tubes are not a part of the superheater and the secondary heat recovery unit.

6. The arrangement according to claim 1, wherein the first flue gas channel includes wall tubes connected to the superheater to provide a steam flow through the wall tubes, and the wall tubes are not a part of the superheater and the secondary heat recovery unit.

7. An arrangement in a chemical recovery boiler having a furnace for combusting waste liquor and a flue gas duct comprising vertical flue gas channels, at least part of which are provided with heat recovery units for recovering heat from flue gases, wherein said heat recovery units comprise heat surface elements, wherein within a first flue gas channel after the furnace are a superheater and a secondary heat recovery unit, an open gas passage and an upper outlet, such that the flue gasses flow through the first flue gas channel sequentially downwards through the superheater and the secondary heat recovery unit, upward through the open gas passage, and through the upper outlet to a next one of the vertical flue gas channels, wherein the secondary heat recovery unit is at least one of: an economizer, a boiler bank and a reheater, and heat surface elements of the superheater are positioned side-by-side with heat surface elements of the secondary heat recovery unit along a direction transverse to a horizontal incoming direction of the flue gas, and the heat surface elements of the superheater and the heat surface elements of the secondary heat recovery unit are positioned parallel to a flow of the flue gas flowing in the first flue gas channel such that flue gas heats the superheater and the secondary heat recovery unit simultaneously, wherein the secondary heat recovery unit is disposed entirely in the first flue gas channel.

8. The arrangement according to claim 7, wherein the secondary heat recovery unit includes the economizer.

9. The arrangement according to claim 7, wherein the secondary heat recovery unit includes the boiler bank.

10. The arrangement according to claim 7, wherein the secondary heat recovery unit includes the reheater.

11. The arrangement according to claim 7, wherein the first flue gas channel includes wall tubes configured to receive a steam/water mixture flow and the wall tubes are not part of the superheater and the secondary heat recovery unit.

12. The arrangement according to claim 7, wherein the first flue gas channel includes wall tubes, and the superheater and the secondary heat recovery unit are not part of the wall tubes.

13. A chemical recovery boiler comprising: a furnace configured to combust waste liquor and direct flue gases upward; a bank of superheaters arranged in an upper region of the furnace; a flue gas duct adjacent and horizontally offset from the bank of superheaters, wherein the flue gas duct is configured to receive the flue gasses flowing from the bank of superheaters and the flue gas duct includes: flue gas channels arranged vertically in the flue gas duct and each of the flue gas channels having an upper inlet configured to receive the flue gasses, an ash hopper at a bottom portion of the flue gas channel, a heat recovery unit oriented vertically in the flue gas channel, and an open gas passage extending from the ash hopper to an upper outlet of the flue gas channel, wherein the heat recovery unit is upstream of the open gas passage along a gas path through the flue gas channel; and a first flue gas channel of the flue gas channels having the upper inlet to receive the flue gasses directly from the bank of superheaters and wherein the heat recovery unit in the first flue gas channel includes a superheater and a secondary heat recovery unit which is at least one of an economizer, a boiler bank and a reheater; wherein the gas path through the first flue gas channel flows simultaneously through the superheater and the secondary heat recovery unit, and wherein the secondary heat recovery unit is disposed entirely within the first flue gas channel.

14. The chemical recovery boiler of claim 13 wherein in the first flue gas channel the superheater is nearer to the bank of superheaters than the secondary heat recovery unit.

15. The chemical recovery boiler of claim 13 further comprising a steam drum and wherein, in the heat recovery unit of the first flue gas channel, the secondary heat recovery unit has a water inlet coupled to a water outlet of the steam drum, and a steam outlet of the superheater is configured to provide steam for a steam turbine.

16. The chemical recovery boiler of claim 13 further comprising a steam drum and wherein, in the heat recovery unit of the first flue gas channel, the secondary heat recovery unit has a steam or water outlet coupled to a steam or water inlet to the steam drum, and a steam outlet of the superheater is configured to provide steam for a steam turbine.

17. The chemical recovery boiler of claim 13 wherein the heat recovery unit for each of the flue gas channels spans a width of the flue gas duct.

18. A chemical recovery boiler comprising: a furnace configured to combust waste liquor and direct flue gases upward, wherein vertically oriented walls of the furnace include wall tubes; a bank of superheaters arranged in an upper region of the furnace; a steam drum external to the furnace and having an outlet for water which provides water for the wall tubes; a first flue gas channel adjacent and offset along a horizontal direction from the bank of superheaters, wherein the first flue gas channel is oriented vertically, is defined by walls of the first flue gas channel, has an upper inlet configured to receive the flue gases flowing from the bank of superheaters, has an upper outlet for the flue gasses and has a bottom ash hopper; a heat recovery unit entirely disposed within a region of the first flue gas channel in which the flue gasses flow downward from the upper inlet towards the bottom ash hopper, wherein the heat recovery unit is separate from the walls of the first flue gas channel and spans a width of the first flue gas channel and the heat recovery unit includes a superheater and a secondary heat recovery unit which is at least one of an economizer, a boiler bank and a reheater and wherein the first flue gas channel is configured to guide flue gases simultaneously through the superheater and the secondary heat recovery unit; and wherein the first flue gas channel includes an open gas passage in which the flue gasses flow from the heat recovery unit in an upward direction to the upper outlet, and a second flue gas channel oriented vertically and having an upper inlet configured to receive the flue gasses flowing from the upper outlet of the first flue gas channel.

19. The chemical recovery boiler of claim 18, wherein the walls of the first flue gas channel include wall tubes configured to receive a steam and water mixture flow, and the wall tubes of the first flue gas channel are not a part of the superheater and the secondary heat recovery unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates schematically a conventional chemical recovery boiler;

(2) FIG. 2 illustrates a preferred embodiment of the invention, where the so-called second pass of the flue gas duct of a chemical recovery boiler is provided with a second heat recovery unit in addition to a superheater;

(3) FIG. 3 illustrates a second preferred embodiment of the invention, where the so-called second pass of the flue gas duct of a chemical recovery boiler is provided with a second heat recovery unit in addition to a superheater;

(4) FIG. 4 illustrates a third preferred embodiment of the invention, where the so-called second pass of the flue gas duct of a chemical recovery boiler is provided with a second heat recovery unit in addition to a superheater;

(5) FIG. 5 illustrates a fourth preferred embodiment of the invention, where the so-called second pass of the flue gas duct of a chemical recovery boiler is provided with a second heat recovery unit in addition to a superheater;

(6) FIG. 6 illustrates a fifth preferred embodiment of the invention, where the so-called second pass of the flue gas duct of a chemical recovery boiler is provided with a second heat recovery unit in addition to a superheater;

(7) FIG. 7 illustrates a sixth preferred embodiment of the invention, where the so-called second pass of the flue gas duct of a chemical recovery boiler is provided with a second heat recovery unit in addition to a superheater;

(8) FIGS. 2-7 use the same reference numerals as FIG. 1 where applicable.

(9) In the embodiment of FIG. 2 the superheaters (T) 20 of the soda recovery boiler are located in the upper part of the furnace and the superheater 21 in the so-called second pass 22. The flue gas flows pass the superheaters 20 mainly horizontally, while in the flue gas duct the flue gas flows through vertical flue gas channels in turns 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.

(10) In addition to the superheater, the so-called second pass of the flue gas duct is provided with an economizer (E) 25. In the flue gas channel the flue gas flows vertically from above downwards and heats the superheater 21 and the economizer 25 simultaneously. With respect to the horizontal flow direction of the flue gas the superheater 21 and the economizer 25 are located sequentially. The superheater 21 and the economizer 25 extend typically to the whole width of the flue gas duct. The flue gas flows further through the 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.

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

(12) The embodiment shown in FIG. 3 relates to a chemical recovery boiler where boiler bank is needed. The superheaters (T) (20) are located in the upper part of the furnace and the superheater 21 in the so-called second pass 22. The flue gas flows pass the superheaters 20 mainly horizontally, while in the flue gas duct the flue gas flows through vertical channels in turns 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.

(13) In addition to the superheater, the so-called second pass of the flue gas duct is provided with a boiler bank 30. In the flue gas pass 22 the flue gas flows vertically from above downwards and heats the superheater 21 and the boiler bank 30 simultaneously. With respect to the horizontal flow direction of the flue gas the superheater 21 and the boiler bank 30 are located sequentially. The superheater 21 and the boiler bank 30 extend typically to the whole width of the flue gas duct. In the boiler bank 30 the water 33 at a saturated temperature coming from the drum 7 of the boiler is boiled partly into steam 34, which is led into the drum 7.

(14) The flue gas flows after the second pass further through the sequential flue gas channels and exits via a discharge opening 26. The flue gas duct is additionally provided with economizers 31 and 32. 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 31 downstream of the so-called second pass into the drum 7 of the boiler.

(15) Positioning the superheater and the boiler bank in the second pass next to each other 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 superheater, the number of tubes in the boiler bank can be chosen more freely, since the flue gases flow also at the superheater. This provides and 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.

(16) The embodiment shown in FIG. 4 relates to a chemical recovery boiler with a reheater. The superheaters (T) 20 and one reheater (V) 40 are located in the upper part of the furnace. Additionally, one superheater 21 is located in the so-called second pass 22. The flue gas flows pass the superheaters 20 mainly horizontally, while in the flue gas duct the flue gas flows through vertical channels in turns from above downwards and from down upwards, as shown by arrows 42. Ash hoppers 24 are provided in the lower part of the flue gas duct.

(17) In addition to the superheater 21, the flue gas channel, the so-called second pass, is provided with a reheater 41. In the flue gas channel 22 the flue gas flows vertically from up downwards and heats the superheater 21 and the reheater 41 simultaneously. With respect to the horizontal flow direction of the flue gas the reheater 41 and the superheater 21 are located sequentially. The superheater 21 and the economizer 41 extend typically to the whole width of the flue gas duct.

(18) Steam enters the reheater 41 from a steam turbine (not shown), bled steam of which the reheater heats. The bled steam is led into the reheater via line 46. From the reheater 41 the steam is led into a reheater 40, after which it is returned into the steam turbine via line 45.

(19) The flue gas flows after the second pass 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 into the drum 7 of the boiler.

(20) In the embodiment of FIG. 5 the superheaters (T) 20 of the soda recovery boiler are located in the upper part of the furnace and the superheater 51 in the so-called second pass 22. The flue flows pass the superheaters 20 mainly horizontally, while in the flue gas duct the flue gas flows through vertical flue gas channels in turns from above downwards and from down upwards, as shown by arrows 53. Ash hoppers 24 are provided in the lower part of the flue gas duct.

(21) In addition to the superheater, the so-called second pass 22 is provided with an economizer 52 so that a first flue gas channel is provided with superheater element 51 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 11/rear wall 10 of the boiler. The superheater and the economizer are positioned in the second pass in parallel with respect to the downwards flowing flue gas. In FIG. 5 the heat surface elements 51 and 52 are positioned so that every second element is a superheater element 51 and every second is an economizer element 52. The positioning does not need to be symmetrical. It is also possible that the number of superheater 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.

(22) In the flue gas channel the flue gas flows vertically from above downwards and heats the superheater elements 51 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 27 and 28. The boiler water is fed into the economizers E 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 into a drum 7 of the boiler.

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

(24) The embodiment shown in FIG. 6 relates to a chemical recovery boiler where boiler bank is needed. The superheaters (T) (20) are located in the upper part of the furnace and the superheater 61 in the so-called second pass 22. The flue gas flows pass the superheaters 20 mainly horizontally, while in the flue gas duct the flue gas flows through vertical channels in turns from above downwards and from down upwards, as shown by arrows 63. Ash hoppers 24 are provided in the lower part of the flue gas duct.

(25) In addition to the superheater, the so-called second pass 22 is provided with a boiler bank 62 so that a first flue gas channel is provided with superheater elements 61 and economizer elements 62 staggered. Thus, the superheater 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. 6 the heat surface elements 61 and 62 are positioned so that every second element is a superheater element 61 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 superheater 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.

(26) In the flue gas channel 22 the flue gas flows vertically from above downwards and heats the superheater elements 61 and the boiler bank elements 62 simultaneously. In the boiler bank elements 62 the water 33 at a saturated temperature coming from the drum 7 of the boiler is boiled partly into steam 34, which is led into the drum 7.

(27) The flue gas flows after the second pass further through the sequential flue gas channels and exits via a discharge opening 26. The flue gas duct is additionally provided with economizers 31 and 32. 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 31 downstream of the so-called second pass into the drum 7 of the boiler.

(28) Positioning the superheater 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 superheater, the number of tubes in the boiler bank can be chosen more freely, since the flue gases flow also at the superheater. This provides and 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 evaporation 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.

(29) The embodiment shown in FIG. 7 relates to a chemical recovery boiler with a reheater. The superheaters (T) 20 and one reheater (V) 40 are located in the upper part of the furnace. Additionally, a superheater 71 is located in the so-called second pass 22. The flue gas flows pass the superheaters 20 mainly horizontally, while in the flue gas duct the flue gas flows through vertical channels in turns from above downwards and from down upwards, as shown by arrows 73. Ash hoppers 24 are provided in the lower part of the flue gas duct.

(30) In addition to the superheater, the so-called second pass 22 is provided with a reheater 72 so that the first flue gas channel is provided with superheater elements 71 and economizer elements 72 staggered. Thus, the superheater elements and the reheater 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. 7 the heat surface elements 71 and 72 are positioned so that every second element is a superheater element 71 and every second is a reheater element 72. The positioning does not need to be symmetrical. It is also possible that the number of superheater elements is higher than the number of reheater 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.

(31) In the flue gas channel 22 the flue gas flows vertically from above downwards and heats the superheater elements 71 and the reheater elements 72 simultaneously. Steam enters the reheater 72 from a steam turbine (not shown), bled steam of which the reheater heats. The bled steam is led into the reheater elements via line 42. From the reheater elements 72 the steam is led into a reheater 40, after which it is returned into the steam turbine via line 45.

(32) The flue gas flows after the second pass 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 into the drum 7 of the boiler.

(33) 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.