EP4792068A1

EP4792068A1 - A device and a method for providing air into a combustion furnace and a combustion furnace

Info

Publication number: EP4792068A1
Application number: EP24794482.0A
Authority: EP
Inventors: Jokke JANTUNEN
Original assignee: Andritz Oy
Current assignee: Andritz Oy
Priority date: 2023-10-13
Filing date: 2024-10-11
Publication date: 2026-08-19
Also published as: CN121969866A; WO2025078729A1; UY40936A; AU2024360325A1; KR20260079429A

Abstract

The invention relates to a device (10) for providing air into a combustion furnace (18) which device (10) comprises a casing (11) encapsulating a distributing chamber (20) and a feeding duct (21) extending outwards from the distributing chamber (20); an inlet (12) for receiving air into the distributing chamber (20) from an air source; an outlet (13) at the end of the feeding duct (21) for exiting air out from inside the casing (11); an inlet valve (19) arranged at the inlet (12) and being adjustable to control air entering through the inlet (12) into the distributing chamber (20); a damper (14) being arranged inside the feeding duct (21) and being movable from a restricted position to an open position or therebetween to adjust the flow of air exiting through the outlet (13). In the device (10) according to the invention the damper (14) comprises a first restricting element (15a) and a second restricting element (15b) arranged one on the other at a distance from each other inside the feeding duct (21) such that at least portion of the air coming from the distribution chamber (20) to the feeding duct (21) can flow to the outlet (13) through a gap (GP) between the first restricting element (15a) and the second restricting element (15b). The invention relates also to a combustion furnace (18) and a method for providing air into a combustion furnace (18).

Description

A DEVICE AND A METHOD FOR PROVIDING AIR INTO A COMBUSTION

FURNACE AND A COMBUSTION FURNACE

Field of the invention

The present invention relates to a device and a method for providing air into a combustion furnace. Furthermore, the present invention relates to a combustion furnace.

Background of the invention

Black liquor in a pulp mill is burned in a recovery boiler furnace in one hand to utilize lignin extracted from wood ships in digester to produce energy for the mill and in the other hand to recover the pulping chemicals to recycle them in cooking to keep additions in minimum. The combustion of black liquor is a sensitive process where the balance of firing temperature and pressure, nozzle angle and type and liquor quality need to be maintained. To achieve optimal burning conditions in the recovery boiler’s furnace also suitable amount of combustion air must be present. This is achieved by applying controllable air feeding devices provided at the air inlets of the furnace. Traditionally, an air feeding method is applied where the combustion air is fed into the furnace in two-phases by feeding primary air with the black liquor to be burned and secondary air with the downstream flue gases. However, a modern recovery boiler has today typically primary, secondary, tertiary and quaternary air levels to ensure even as possible combustion, and therethrough low emissions and complete burning of the fuel. Known devices and methods for providing air into boiler furnaces are described, for instance, in documents US5,699,746 A, US 6,024,028 A, US 7,681 ,508 B and US 7,665,458 B2.

Combustion air is introduced into the furnace through the nozzles from air ducts surrounding the boiler. Typically, the air ducts have kind of damper for regulating air flowing through each nozzle. Air passes through an air guide and then through the nozzle into the furnace. Amount of combustion air fed into the furnace is controlled based on flue gas residual oxygen. Its content of 2-3 % is desirable to avoid such draft losses and deposits that are hard to clean with soot blowers. Enough high residual oxygen level ensures also that carbon monoxide emissions remain as low as possible. To reach these aims it is thus important that air flow through the feeding nozzles into the furnace is appropriately controllable, and preferably as even and smooth as possible.

Furthermore, one purpose of the air feeding is to ensure appropriate cooling of air-feeding equipment (i.e., air feeding nozzles) at the air openings being typically made of steel or refractory. Radiation heat of the furnace raises the temperature of the nozzle very high. Reactive and corroding substances such as sodium hydroxide present in the furnace entrain within turbulent gases at the nozzle area. Structure around the nozzle area is not capable of transmitting heat from the nozzle by conduction effectively. Therefore, the nozzle can reach a temperature that is higher than desired. When the nozzle is at high temperature, and reactive and corrosive substances are present, the nozzle quickly becomes corroded. This, naturally, shortens maintenance intervals required to ensure smooth and uninterrupted recovery boiler operation.

Due to the above-mentioned challenges and drawbacks relating to the presently known technology there is need to further improve airflow control of a recovery boiler combustion furnace. Thus, rethinking and redesigning the air distribution device accordingly would provide additional value over the currently known air feeding devices, combustion furnaces and applied air providing methods thereof.

Summary of the invention

The above-described drawbacks and challenges that relate to the known solutions are overcome by the device and method according to the invention because the air coming into the device, at first, enters in to a distribution chamber wherein the air flow steady before it continues its flowing out from the device through a feeding duct, and wherein it enters in to the distribution chamber through an inlet opening wherein its entering is controlled by an adjustable inlet valve, and wherein in the device the air flow is fed after its entering to the distribution chamber in to a feeding duct in which the air flow is further controlled by a damper to achieve even and smooth airflow before it enters into the furnace through the outlet at the end of the feeding duct. More specifically, the device according to the invention is characterized by what is described in the independent claim 1 , combustion furnace what is described in the independent claim 16, and the method what is described in the independent claim 19. The dependent claims 2-15 describe advantageous embodiments of the device, claim 17 and 18 advantageous embodiments of the combustion furnace, and the dependent claims 20 and 21 advantageous embodiments of the method according to the invention.

An advantage of the device, combustion furnace and method according to the invention is that it simplifies the construction, is usable on all air openings, has more precise control over airflow, has less pressure loss over control device and always achieves appropriate air cooling to whole airport opening despite register operation position. The new device also achieves a significantly smoother air flow to the combustion furnace in the different adjustment positions of the device. These solutions aim to ensure cooling of the air opening feed nozzles independent of the running situation of the combustion furnace, which in turn enables longer lifetimes and longer maintenance intervals for the nozzles. Due to this, extended period of continuous running for a recovery boiler’s combustion furnace is achieved. The duration of air nozzles of the furnace is one limiting factor for driving cycles of the combustion furnace. Thus, this highly improves the productivity and cost-effectiveness of black liquor treatment process at a recovery boiler.

Furthermore, it should be notified that the device and method according to the invention can be also applied for feeding air into combustion furnaces of other types of boilers than recovery boilers, e.g., into combustion furnaces of power boilers.

Brief description of the drawings

In the following, the invention will be described in more detail with reference to the appended drawing in which: Fig. 1 shows an embodiment of a device according to the invention seen obliquely from side,

Fig. 2 shows a longitudinal cross-section of the device shown in the figure 1 seen obliquely from side,

Fig. 3 shows the damper and the casing back wall of the device shown in the figures 1 and 2 seen obliquely from side, and

Fig. 4 shows a longitudinal cross-section of the embodiment shown in the figures 1 -4 when it is equipped with a sweeping element fitted through its second arm-part and second restricting element.

Detailed description of some advantageous embodiments

A device depicted in figures 1 -4 is an example embodiment of a device 10 according to the invention. It may be assembled in any kind of a combustion furnace which includes suitable air inlet openings such as e.g., combustion furnace of a recovery boiler. Typically, for instance in case of recovery boiler, several devices are assembled in each furnace, so that one device is assembled in each air inlet opening. These may be placed in different airfeeding levels of the furnace, i.e., e.g., at primary, secondary, tertiary and quaternary air levels.

The dimensions and shape of the device shown in the figures 1 -4 is only exemplary. Therefore, the size and shape of the device 10 may vary according to type, size, shape and construction of the furnace air-inlet opening into which the device is fitted. However, the shape of the device is preferably such that its height is greater than its width as the case is with the embodiment shown in the figures 1 -4. This is because such dimension is considered most advantageous in simulations made. The simulations have proved that by using such dimensions velocity distribution over the crosssection is most even and thus, among others, vibration caused by the flow velocity differences and turbulence remain low. However, it has not been recognized any significant difference between oval or angular shapes of the cross-section. Hence, both types are possible and can be chosen e.g., depending on the shape of the air-inlet opening of the furnace.

Also, for instance, adjustability of the flow rate of air or gas that is fed through device may vary in different embodiments. In some embodiments the adjusting range may be 0 to -100 % i.e., so that in the first extreme setting airflow can be totally closed by the device and that a second extreme setting the air flow fed into the device can flow almost freely from inlet 12 to the outlet 13 of the device (at least some minimal losses are always present in this kind of device). However, in many cases it is preferred that minimum airflow through the device is e.g., 10 % to 40% of the maximum air flow that is fed through the device. Also, the maximum proportion may be limited to be e.g., between 70 to 95 %.

When the embodiment of the device shown in the figures 1 -4 is assembled to a combustion furnace 18 shown in the figures it may be fitted at least partly into a ring channel covering the air-inlet opening 23 of the combustion furnace walls 22. However, the device 10 shown in the figures 1 -4 can be fitted in any other kind of air inlet opening of a recovery boiler combustion furnace or that of other kind combustion furnace. Thus, the combustion furnace may be, for instance, power boiler where the fuel being burnt may be any biofuels, such as e.g., bark, wood chips, sawdust, herbaceous pellets, or agricultural crops, and/or fossil fuels like coal, crude oil, or natural gas.

In the embodiment of the device shown in the figures 1 -4 the device 10 comprises a casing 11 encapsulating a distributing chamber 20 and a feeding duct 21 extending outwards from the distributing chamber 20. The device has also an inlet 12 for receiving air into the distributing chamber 20 from an air source and an outlet 13 at the end of the feeding duct 21 for exiting air out from inside the casing 11 .

At the inlet 12 there may be an adjustable inlet valve 19 arranged to control air entering through the inlet 12 into the distributing chamber 20. The inlet valve 19 can be e.g., kind of an adjustable flap, or a shutter by means of which the open area of the inlet 12 can be adjusted to restrict the air flowing into the distributing chamber 20 from an air source in appropriate manner. The inlet valve may be also arranged outside the device, e.g., into the incoming air duct of the air source that is connected to the inlet 12. However, in some embodiments there may not be inlet valve at all.

Inside the feeding duct 21 there is a damper 14 being movable from a restricted position to an open position or therebetween to adjust the airflow exiting through the outlet 13. The damper 14 comprises a first restricting element 15a and as second restricting element 15b. However, in some embodiments the damper may comprise third or further restricting elements.

Principally, the damper 14 is capable to restrict the air flow through in the feeding duct 21 from about 0 to 100% (zero restriction is difficult to reach in practice, since there is always present some drag in any kind of conduit wherein some fluid is flowing). Thus, in preferred embodiments, the restriction typically varies between 10 to 90%. The adjustment is in the embodiment of figures 1 -4 arranged such that an area of the open crosssection of the feeding duct 21 within the distance covered by the portion of the damper 14 being inside the feeding duct 21 remains mainly constant regardless of the position of the damper 14. This is achieved by shaping the outer surfaces of the first restricting element 15a and the second restricting element 15b such that they conform the shape of the respective opposite inner surface of the feeding duct 21 within the distance inside which the position of the damper 14 can be changed. In such embodiments where the damper has a first restricting element and a second restricting element (and possible further restricting elements) are such that they can be adjusted independently from each other, this may concern each of them independently from the others.

In the embodiment shown in the figures 1 -4 position control the first restricting element 15a and the second restricting element 15b have been accomplished by means of a first arm-part 16a and a second arm-part 16b. These have been connected to the first restricting element 15a and the second restricting 15b respectively. As it is shown best in the figure 3 the first arm-part 16a and the second arm-part 16b extend sealably from outside to inside the casing 11 for moving the first restricting element 15a and the second restricting element 15b from outside the casing 11. To achieve appropriately sealed sliding bushing into the casing 11 the casing back wall 17 includes a first sliding bushing 17a and a second sliding bushing 17b through which the first arm-part 16a and second arm-part 16b are fitted inside the casing 11 slidably. For achieving air-tight sealing the first sliding bushing 17a and the second sliding bushing 17b may comprise e.g., suitable gaskets, or the airtightness may be achieved by other suitable means. In those embodiments which have more than two restricting elements and respective arm-parts and sliding bushings may be arranged correspondingly.

As it is shown in the figure 4 the first arm-part 16a and the first restricting element 15a are hollow to form continuous conduit 24a from rear end of the first arm-part 16a to the front end of the first restricting element 15a. In this embodiment there is also similar continuous hollow conduit 24b formed through second arm-part 16b and the second restricting element 15b. Thus, it is possible to fit additional appliances and instruments from outside the distribution chamber 20 to the outlet 13 of the device 10 through continuous hollow conduits 24a and/or 24b. During the time when the conduits 24a and/or 24b are not used they may have been blocked from their end at the rear end of the first arm-part 16a and/or the second arm-part 16b (which ends are those locating outside the back wall 17 of the distribution chamber 20).

In the figure 4 it is further depicted a sweeping appliance 25 for cleaning the outlet 13. In this case, it has been fitted through the conduit 24b at the second arm-part 16b and the second restricting element 15b. In this embodiment, the sweeping appliance 25 can be alternatively fitted through the first arm-part 16a and the first restricting element 15a to clean the outlet 13 respectively. However, there may be also such embodiments where only the first arm-part (or the second arm-part) and the first restricting element (or the second restricting element) includes such hollow conduit. In case of embodiment of the figures 1 -4 the first restricting element 15a and the second restricting element 15b have been arranged one on the other at a distance from each other such that at least portion of the air coming from the distribution chamber 20 can flow through the gap GP between them to the outlet 13. Thus, the damper 14 of the embodiment shown in the figures 1-4 forms between the first restricting element 15a, the second restricting element 15b and the feeding duct 21 a first airflow passage AFP1 , second airflow passage AFP2 and the third airflow passage AFP3.

In the embodiment of the figures 1 -4 the feeding duct 21 has a top part 21a, an intermediate part 21 b and a bottom part 21 c. Thus, the first airflow passage AFP1 is formed between the first restricting element 15a and the top part 21 a, the second airflow passage AFP2 is formed between the first restricting element 15a, the second restricting element 15b and the intermediate part 21 b, and the third airflow passage AFB3 is formed between the second restricting element 15b and the bottom part 21 c. Therefore, the proportion of the air being flowable through the first airflow passage AFP1 and/or the third airflow passage AFP3 is adjustable by moving the damper 14. In this embodiment the first restricting element 15a and the second restricting element 15b are connected fixedly to each other. However, in such embodiments wherein these have been arranged to be movable independently from each other, the first airflow passage and the third airflow passages may have different area of cross-section and thus provide asymmetric airflow through the feeding duct. This provides further possibilities to control the device which may, in many cases, help to further improve the combustion process in the furnace and hence aid to achieve still lower emissions for the furnace.

When the damper 14, in the embodiment of figures 1 -4, is in the restricted position the proportion of the air flowing through the first airflow passage AFP1 and/or third airflow passage AFP3 may be e.g., in between 0 - 40 % of the total airflow. When the damper 14 is in open position, the proportion of the air flowing through the first airflow passage AFP1 and/or third airflow passage AFP3 is e.g., in between 40-70 % of the total airflow. The proportion of air flowing through the second airflow passage AFP2 is, in the embodiment shown in the figures 1 -4, constant since in it the first restricting element 15a and the second restricting element 15b is fixedly attached to each other such that gap GP is formed between them. Thus, in different embodiments of this kind, the gap GP between the first restricting element 15a and second restricting element 15b may vary. Typically, the gap GP has been chosen so that the constant proportion air flowing through the second airflow passage AFP2 is in between 10 - 60 % of the total airflow. However, in some embodiments, it is preferably in between 20 to 40% of the total airflow.

When using the method of providing air into a combustion furnace 18, the method comprises at least following method steps:

-providing a device 10 for providing air into a combustion furnace 18 at the combustion furnace 18 such that the feeding duct 21 is connected to an air inlet opening 23 of the combustion furnace 18,

-connecting an air source to the inlet 12 of the device 10,

-controlling the damper 14 of the device 10 to control the airflow coming into the distribution chamber 20 and flowing out from the device 10 through the feeding duct 21 .

In an embodiment of a method wherein the device 10 is such that it has an inlet valve 19, the inlet valve 19 and the damper may be controlled such a way that 0 to 100 % of the airflow coming into the distribution chamber 20 flows out from the device 10 through the feeding duct 21. This is accomplished by limiting the airflow coming through the inlet 12 by the inlet valve 19 such that pressure drop occurs as little as possible in the feeding duct 21. Advantage of such airflow control is that in that way minimum turbulent in the airflow flowing through the feeding duct 21 is achieved. For improving the airflow control, the volume flow of the air and/or air-pressure at least at one point within the feeding duct 21 may be determined by using mathematical modelling and/or by measurement by applying suitable measurement sensors. However, an improved airflow control may also be based on testing and measuring some indirect parameter which value is dependent on the way and/or amount of combustion air provided into the furnace.

In the embodiment according to figures 1 -4, the first restricting element 15a and the second restricting element 15b have been shaped such that at least the leading edge and the middle-portions have shape of a symmetrical airfoil. Thus, with a damper 14 having this kind of first restricting element 15a and the second restricting element 15b so called “Low-Drag” and/or “Low- pressure loss” process is achieved. Thus, such an aerodynamic shape reduces turbulent backflow after damper 14 keeping the airflow even and connected to adjustment surfaces of the feeding duct 21 thorough opening 13. In case of the embodiment of the figures 1 -4 the leading edge is smooth towards incoming air flow as it can be seen from the figure 3. However, the tail end is blunt to ensure that it does not extend into the furnace through the outlet 13 when the damper 14 has been adjusted to the closed position. An advantage of the blunt tail end is also that it would not be vulnerable for burning and/or wear and thereby increase turbulent backflow later during its lifetime. Nevertheless, in some other embodiments of this kind also the tail end may be somehow smoothly shaped having e.g., the shape of tail end of a symmetrical airfoil. In such case the feeding duct may be shaped correspondingly (i.e. e.g. made longer such that restricting element having such tail end do not extend outside the outlet in any of its positions).

In some embodiments of the device the damper may include location control arrangement integrated in the structures of the damper to ensure accurate and reliable control. The damper or one of its first restricting element and/or second restricting element and/or further restricting element may alternatively or in addition include any kind of cleaning arrangement (e.g., such as described above or configured somehow differently) for ensuring cleanliness of the outlet and/or other interior wall surfaces of the device (at least at the area of the feeding duct close to the outlet) for ensuring as stable and turbulent free air flow as possible through the device all the time during the devices lifetime. Furthermore, in embodiments of the device, being applied in recovery boilers, the device may be designed such that in pulp chemical recovery processes a good outlet edge cooling flow is achieved. In such device even airflow around furnace air feed nozzle keeps furnace air feed nozzle temperature-controlled prolonging it’s lifetime and preventing scaling buildup around air feed nozzle internal parts.

Thus, the device, combustion furnace and method of the invention is not limited to the embodiments described above but can vary within the scope of the appended claims.

Claims

1. A device (10) for providing air into a combustion furnace wherein the device (10) comprises

-a casing (11 ) encapsulating a distributing chamber (20) and a feeding duct (21 ) extending outwards from the distributing chamber (20),

-an inlet (12) for receiving air into the distributing chamber (20) from an air source,

-an outlet (13) at the end of the feeding duct (21 ) for exiting air out from inside the casing (11 ),

-a damper (14) being arranged inside the feeding duct (21 ) and being movable from a restricted position to an open position or therebetween to adjust the airflow exiting through the outlet (13), and wherein the damper (14) comprises a first restricting element (15a) and a second restricting element (15b) arranged one on the other at a distance from each other inside the feeding duct (21 ) such that at least portion of the air coming from the distribution chamber (20) to the feeding duct (21 ) can flow to the outlet (13) through a gap (GP) between the first restricting element (15a) and the second restricting element (15b).

2. The device (10) according to claim 1 wherein the first restricting element (15a) and the second restricting element (15b) has shape of a symmetrical airfoil.

3. The device (10) according to claim 1 or 2 wherein the device (10) comprises an inlet valve (19) arranged at the inlet (12) and being adjustable to control air entering through the inlet (12) into the distributing chamber (20).

4. The device (10) according to claim 3 wherein the device (10) is capable to restrict the air flow through in the device (10) from 0 to 100 %.

5. The device (10) according to any of the preceding claims wherein an area of the open cross-section of the feeding duct (21 ) within the distance covered by the portion of the damper (14) being inside the feeding duct (21 ) remains mainly constant regardless of the position of the damper (14).

6. The device according to any of the preceding claims wherein the first restricting element (15a) and the second restricting element (15b) are movable from the restricted position to the open position or therebetween independently from each other.

7. The device (10) according to any of the preceding claims wherein the damper comprises a first arm-part (16a) connected to the first restricting element (15a) and the second arm-part (16b) connected to the second restricting element (15b) and wherein the first arm-part (16a) and the second arm part (16b) extending sealably from outside to inside the casing (11 ) for moving the first restricting element and the second restricting element from outside the casing (11 ).

8. The device according to claim 7 wherein the first arm-part (16a) and the first restricting element (15a) are hollow to form continuous conduit (24a) from rear end of the first arm-part (16a) to the front end of the first restricting element (15a).

9. The device according to claim 7 or 8 wherein the second arm-part (16b) and the second restricting element (15b) are hollow to form continuous conduit (24b) from rear end of the second arm-part (16a) to the front end of the second restricting element (15a).

10. The device (10) according to any of the preceding claims, wherein the device (10) comprises a first airflow passage (AFP1 ), a second airflow passage (AFP2) and a third airflow passage (AFP3) formed in between the first restricting element (15a), the second restricting element (15b) and the feeding duct (21 ).

11. The device according to claim 10 wherein the feeding duct (21 ) has a top part (21 a), an intermediate part (21 b) and a bottom part (21 c), and wherein

-the first airflow passage (AFP1 ) is formed between the first restricting element (15a) and the top part (21 a),

-the second airflow passage (AFP2) is formed between the first restricting element (15a), the second restricting element (15b) and the intermediate part (21 b), and

-the third airflow passage (AFB3) is formed between the second restricting element (15b) and the bottom part (21 c).

12. The device according to claim 10 or 11 wherein proportion of the air flowing in the first airflow passage (AFP1 ) and the third airflow passage (AFP3) is adjustable by moving the damper (14).

13. The device according to claim 12 wherein the proportion of the air flowing through the first airflow passage (AFP1 ) and/or third airflow passage (AFP3) is in between 0 - 40 % of the total airflow coming in to the into the feeding duct (21 ) when the damper (14) is in the restricted position.

14. The device according to claim 12 or 13 wherein the proportion of the air flowing through the first airflow passage (AFP1 ) and/or third airflow passage (AFP3) is in between 40-70 % of the total airflow coming in to the into the feeding duct (21 ) when damper (14) is in open position.

15. The device according to any of claims 12-14 wherein the proportion of the air flowing through the second airflow passage (AFP2) is 10 - 60 % of the total airflow coming in to the into the feeding duct (21 ).

16. A combustion furnace (18) comprising at least one device (10) according to any of claims 1 to 15.

17. A combustion furnace according to claim 16 wherein the combustion furnace (18) is a combustion furnace (18) of a recovery boiler.

18. The combustion furnace (18) according to claim 16 or 17 wherein the device (10) is fitted at least partly into a ring channel covering the air-inlet (23) of the combustion furnace (18).

19. A method of providing air into a combustion furnace (18) comprising method steps of

-providing a device (10) according to any of claims 1 to 15 at the combustion furnace (18) such that the feeding duct (21 ) is fitted at least partly through an air inlet (23) of the combustion furnace (18) such that air can be fed into the combustion furnace (18) through the device (10),

-connecting an air source to the inlet (12) of the device (10),

-controlling the damper (14) of the device (10) to control the airflow coming into the distribution chamber (20) and flowing out from the device (10) through the feeding duct (21 ).

20. The method according to claim 19 wherein the device comprises the inlet valve (19) according to claim 2, and wherein the inlet valve (19) and the damper (14) is controlled such that 0 to 100 % of the airflow coming into the distribution chamber (20) flows out from the device (10) through the feeding duct (21 ).

21 . The method according to any of claims 19 or 20 wherein the volume flow of the air and/or air-pressure at least at one point within the feeding duct (21 ) is determined by using mathematical modelling and/or by measurement.