660mw ultra-supercritical cfb boiler with external bed and double-flue damper temperature control

Abstract

The application provides a 660MW ultra-supercritical CFB boiler with double-external-bed and double-flue damper temperature adjustment, and belongs to the field of ultra-supercritical CFB boilers.The existing 600MW boiler technology adopts a single external-bed temperature adjustment method, and high-temperature final-stage heating surfaces are arranged in the external bed, so that the temperature adjustment method is limited, and the role of the external bed is not fully played.The present application is characterized in that a coal feeding port is arranged on the front wall, a coal feeding port is arranged on the back wall, a coal slurry gun is arranged on the rear wall, fuel is fed from the front wall and the rear wall at the same time, the air distribution plate area is uniformly distributed in temperature, a medium-temperature superheater is arranged in the external bed, and a high-temperature superheater or a high-temperature reheater is not arranged, the material and wall thickness of the medium-temperature superheater have sufficient use temperature allowance, the external bed has sufficient adjustment capacity, and the overall and regional adjustment of the bed temperature is facilitated, and the temperature adjustment mode combining the tail double-damper temperature adjustment and the medium-temperature superheater external bed can flexibly adjust the reheated steam temperature.

Description

Technical Field

[0001] This utility model belongs to the field of ultra-supercritical CFB boilers, and in particular relates to a 660MW ultra-supercritical CFB boiler with external bed and double flue baffle temperature regulation. Background Technology

[0002] Currently, the largest circulating fluidized bed boiler in operation worldwide has a capacity of 600MW and a pressure rating of supercritical. To further improve the efficiency of power plant units, it is necessary to further increase the main steam pressure and temperature. The existing 600MW boiler technology uses a single external bed temperature control method, and the external bed is equipped with high-temperature final-stage heating surfaces, which limits the temperature control methods and does not fully utilize the role of the external bed.

[0003] Currently, all existing 600MW-class CFB boilers in the world use double air distribution plates, which have problems such as difficulty in adjustment and easy bed flipping. They must maintain a high air chamber pressure, resulting in high boiler energy consumption. Utility Model Content

[0004] In view of this, in order to solve the technical problems mentioned in the background section, this utility model proposes a 660MW ultra-supercritical CFB boiler with a medium-temperature superheater and external bed with dual flue baffles. This increases the boiler capacity to 660MW and the pressure rating to ultra-supercritical, thereby maximizing power generation efficiency and reducing coal consumption. A medium-temperature superheater is arranged inside the external bed, providing sufficient temperature regulation capability.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: a 660MW ultra-supercritical CFB boiler with external bed and double flue gas baffle temperature regulation, comprising a furnace, air duct burner, annular wind box, primary air duct, secondary air duct, water-cooled screen, screen-type high-temperature superheater, external bed, screen-type high-temperature reheater, steam-water separator, centralized downcomer, separator inlet flue gas duct screen, separator, separator outlet flue, tail wall superheater, low-temperature superheater, high-temperature economizer, low-temperature reheater, low-temperature economizer, flue gas regulating baffle, rotary air preheater, secondary air branch pipe, medium-temperature superheater, water-cooled air chamber, air distribution plate, water-cooled wall, high-pressure fluidized air heater, and air preheater bypass flue and tail flue; The furnace is connected to the separator via a separator inlet flue screen. The separator is connected to the tail flue via a separator outlet flue. The tail flue includes a front tail flue and a rear tail flue. A low-temperature reheater is arranged in the front tail flue. A low-temperature superheater and a high-temperature economizer are arranged sequentially from top to bottom in the rear tail flue. A flue gas regulating baffle is installed at the bottom of the tail flue. A low-temperature economizer is installed below the flue gas regulating baffle. A rotary air preheater is installed below the low-temperature economizer. A rotary air preheater path flue is installed in the rotary air preheater path flue. A high-pressure fluidized air heater is installed in the rotary air preheater path flue. The furnace bottom is provided with an air distribution plate, and a water-cooled air chamber is provided at the bottom of the air distribution plate. A screen-type high-temperature superheater is provided on the rear wall of the furnace, and a screen-type high-temperature reheater is provided on the front wall of the furnace. A medium-temperature superheater is arranged in the external bed, which is connected to the furnace. A rotary air preheater is connected to the primary air duct, which is connected to the air duct burner. The air duct burner is connected to the water-cooled air chamber. The rotary air preheater is also connected to the secondary air duct, which is connected to the annular air box. The annular air box is connected to the secondary air branch pipe, which is connected to the furnace in two layers.

[0006] Furthermore, the outer wall of the furnace is a water-cooled wall, and the membrane wall structure of the water-cooled wall and the air distribution plate forms a fully enclosed furnace.

[0007] Furthermore, five independent headers are installed at the bottom of the water-cooled walls on both sides, arranged in a stepped manner.

[0008] Furthermore, it includes four external beds, with a first medium-temperature superheater arranged in two of the external beds and a second medium-temperature superheater arranged in the other two external beds.

[0009] Furthermore, the separator adopts four parallel high-efficiency air-cooled cyclone separators, and a return valve is installed at the bottom of each separator. One end of the separator is connected to the separator inlet flue screen, and the other end of the separator is connected to the separator outlet flue. Each return valve is connected to an external bed through a cone valve. The external bed is connected to the furnace, and each return valve is connected to the bottom of the furnace.

[0010] Furthermore, the furnace bottom front wall is provided with 12 front wall coal feed ports, and the return leg of the return valve is provided with 4 return valve coal feed ports.

[0011] Furthermore, the bottom rear wall of the furnace is equipped with 6 coal slurry guns 28 and 6 drum slag coolers 3.

[0012] Furthermore, a first-panel high-temperature superheater and a second-panel high-temperature superheater are installed on the rear wall of the furnace.

[0013] Furthermore, the front wall of the furnace is equipped with a water-cooled screen, a first-screen high-temperature reheater, and a second-screen high-temperature reheater. Furthermore, the water-cooled wall and water-cooled screen are connected to the steam-water separator, and the steam-water separator is connected to the separator inlet flue pipe screen.

[0014] Compared with the prior art, the beneficial effects of the 660MW ultra-supercritical CFB boiler with external bed and double flue temperature regulation described in this utility model are: 1. The 660MW ultra-supercritical CFB boiler with external bed and double flue gas duct temperature regulation described in this utility model adopts an ultra-large single furnace and single air distribution plate to reduce the power consumption of the primary air fan. The use of a high-pressure fluidized air heater effectively reduces the exhaust gas temperature and improves boiler efficiency, thereby increasing power generation efficiency and reducing coal consumption. The improved main steam parameters effectively enhance the overall plant power generation efficiency, increasing the boiler capacity to 660MW, the main steam pressure to 29.4MPa, the pressure rating to ultra-supercritical, the main steam temperature to 605℃, and the reheat steam temperature to 623℃, maximizing the unit's power generation efficiency and reducing coal consumption.

[0015] 2. The front wall of this utility model is equipped with a coal feed port, the return leg is equipped with a coal feed port, and the rear wall is equipped with a coal slurry gun. Fuel is fed in from both the front and rear walls at the same time, so that the bed temperature distribution in the air distribution plate area is uniform.

[0016] 3. The external bed of this utility model is equipped with a medium-temperature superheater, but not a high-temperature superheater or a high-temperature reheater. The material and wall thickness of the medium-temperature superheater have sufficient operating temperature margin, so that the external bed has sufficient adjustment capability, which is conducive to the overall and regional adjustment of the bed temperature.

[0017] 4. This utility model adopts a single furnace and single air distribution plate structure, which can effectively reduce the lower bed pressure, reduce the primary air pressure head, reduce boiler energy consumption, and avoid the problems of bed material turnover and high lower bed pressure caused by the use of a double air distribution plate structure.

[0018] 5. This utility model adopts a temperature regulation method that combines tail-end double baffle temperature regulation with an external bed of medium-temperature superheater, which can flexibly adjust the reheat steam temperature.

[0019] 6. The bottom headers of the water-cooled walls on both sides of the furnace in this utility model are arranged in a stepped manner to prevent the problem of water-cooled wall distortion caused by the difference in expansion between the two sides of the heating surface in the middle of the water-cooled wall at the same elevation.

[0020] 7. The high-temperature superheater and high-temperature reheater of this utility model are both arranged in two stages, which is beneficial to controlling the temperature deviation of the high-temperature heating surface.

[0021] 8. This utility model sets up a bypass flue for the rotary preheater and installs a high-pressure fluidized air heater in the bypass flue, which reduces the amount of flue gas in the rotary preheater and maximizes the heat exchange effect of the rotary preheater. The high-pressure fluidized air heating significantly reduces the cold air rate of the boiler, which can effectively reduce the exhaust gas temperature and improve the boiler efficiency. Attached Figure Description

[0022] The accompanying drawings, which form part of this utility model, are used to provide a further understanding of the utility model. The illustrative embodiments of the utility model and their descriptions are used to explain the utility model and do not constitute an undue limitation of the utility model. In the drawings: Figure 1 This is a schematic diagram of the structure of the 660MW ultra-supercritical CFB boiler with external bed and double flue baffle temperature regulation according to the present invention. Figure 2 yes Figure 1 AA top view; Figure 3 yes Figure 1 BB top view; In the diagram: 1-Furnace, 2-Duct burner, 3-Roller slag cooler, 4-Front wall coal feeder, 5-Return valve coal feeder, 6-Annular wind box, 7-Return valve, 8-Conical valve, 9-Primary air duct, 10-Secondary air duct, 11-Water-cooled screen, 12-First-screen high-temperature superheater, 13-External bed, 14-First-screen high-temperature reheater, 15-Steam-water separator, 16-Centralized downcomer, 17-Separator inlet flue gas duct screen, 18-Separator, 19-Separator outlet flue gas, 20-Tail wall superheater, 21-Low temperature... 22-High-temperature economizer, 23-Low-temperature reheater, 24-Low-temperature economizer, 25-Flue gas regulating baffle, 26-Rotary air preheater, 27-Secondary air branch pipe, 28-Coal slurry gun, 29-First medium-temperature superheater, 30-Second medium-temperature superheater, 31-Water-cooled air chamber, 32-Air distribution plate, 33-Water-cooled wall, 34-Second screen type high-temperature superheater, 35-Second screen type high-temperature reheater, 36-High-pressure fluidized air heater, 37-Air preheater bypass flue, 38-Header, 39-Tail flue. Detailed Implementation

[0023] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the present invention. Furthermore, it should be noted that, for ease of description, only the parts relevant to the present invention are shown in the drawings, not all of them. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention pertains. The terminology used herein is for the purpose of describing specific embodiments only and is not intended to limit the present invention.

[0024] See Figure 1-3 This embodiment describes a 660MW ultra-supercritical CFB boiler with external bed and dual flue gas duct temperature regulation. The boiler has an overall M-shaped layout, with the furnace 1, separator 18, and tail flue 39 arranged sequentially from front to back. Specifically, it includes the furnace 1, air duct burner 2, annular wind box 6, primary air duct 9, secondary air duct 10, water-cooled screen 11, screen-type high-temperature superheater, external bed 13, screen-type high-temperature reheater, and steam-water separator 15. The components include: separator inlet flue duct screen 17, separator 18, separator outlet flue 19, low temperature superheater 21, high temperature economizer 22, low temperature reheater 23, low temperature economizer 24, flue gas regulating baffle 25, rotary air preheater 26, secondary air branch pipe 27, medium temperature superheater, water-cooled air chamber 31, air distribution plate 32, water-cooled wall 33, high pressure fluidized air heater 36, air preheater bypass flue 37, and tail flue 39. The furnace 1 is connected to the separator 18 via the separator inlet flue duct screen 17. The separator 18 is connected to the tail flue 39 via the separator outlet flue 19. The tail flue 39 adopts a double flue form, consisting of a front tail flue and a rear tail flue. A low-temperature reheater 23 is arranged in the front tail flue. A low-temperature superheater 21 and a high-temperature economizer 22 are arranged from top to bottom in the rear tail flue. A flue gas regulating baffle 25 is installed at the bottom of the tail flue 39. A low-temperature economizer 24 is installed below the flue gas regulating baffle 25. A rotary air preheater 26 is installed below the low-temperature economizer 24. A rotary air preheater bypass flue 37 is installed in the rotary air preheater bypass flue 37. A high-pressure fluidized air heater 36 is installed in the rotary air preheater bypass flue 37.

[0025] The furnace 1 is an ultra-large single-furnace structure. An ultra-large air distribution plate 32 is installed at the bottom of the furnace 1, and a water-cooled air chamber 31 is located at the bottom of the air distribution plate 32. The membrane wall structure of the water-cooled wall 33 and the air distribution plate 32 forms a fully enclosed furnace. Five independent headers 38 are installed at the bottom of the water-cooled walls 33 on both sides, arranged in a stepped manner. A first-screen high-temperature superheater 12 and a second-screen high-temperature superheater 34 are installed on the rear wall inside the furnace 1, and a water-cooled screen 11, a first-screen high-temperature reheater 14, and a second-screen high-temperature reheater 35 are installed on the front wall inside the furnace. The boiler has a total of four external beds 13. Two of the external beds 13 house first-intermediate-temperature superheaters 29, and the other two external beds house second-intermediate-temperature superheaters 30.

[0026] The separator 18 employs four high-efficiency air-cooled cyclone separators with self-balancing single-path return valves 7 installed at their bottoms. The four high-efficiency air-cooled cyclone separators are connected in parallel. One end of the separator 18 is connected to the separator inlet flue duct screen 17, and the other end is connected to the separator outlet flue duct 19. Each return valve 7 is connected to an external bed 13 via a cone valve 8. The external bed 13 is connected to the furnace 1, and each return valve 7 is connected to the bottom of the furnace 1.

[0027] The bottom front wall of furnace 1 is equipped with 12 front wall coal feed ports 4, the return leg of return valve 7 is equipped with 4 return valve coal feed ports 7, and the bottom rear wall of furnace 1 is equipped with 6 coal slurry guns 28 and 6 drum slag coolers 3.

[0028] The rotary air preheater 26 is connected to the primary air duct 9, the primary air duct 9 is connected to the air duct burner 2, and the air duct burner 2 is connected to the water-cooled air chamber 31.

[0029] The rotary air preheater 26 is also connected to the secondary air duct 10, the secondary air duct 10 is connected to the annular air box 6, the annular air box 6 is connected to the secondary air branch pipe 27, and the secondary air branch pipe 27 is connected to the furnace 1 in two layers.

[0030] One end of the centralized downcomer 16 is connected to the high-temperature economizer 22, and the other end is connected to the header 38. Its function is to introduce water from the high-temperature economizer 22 into the water-cooled wall 33.

[0031] One end of the tail-end wall-mounted superheater 20 is connected to the separator 18, and the other end is connected to the low-temperature superheater 21. The tail-end wall-mounted superheater 20 is part of 39. The water-cooled wall 33 and water-cooled screen 11 are connected to the steam-water separator 15, and the steam-water separator 15 is connected to the separator inlet flue screen 17. The superheated steam generated by the water-cooled wall 33 and water-cooled screen 11 enters the steam-water separator 15, then enters the separator inlet flue screen 17, and then enters the high-efficiency steam-cooled cyclone separator 18.

[0032] The circulation method of the 660MW ultra-supercritical CFB boiler with external bed double flue baffle temperature regulation described in this utility model includes the following steps: Step 1: The feedwater first enters the low-temperature economizer 24 and the high-temperature economizer 22 for heating, and then enters the water-cooled wall 33 and the water-cooled screen 11 in sequence to generate superheated steam. The superheated steam enters the steam-water separator 15 and then the separator inlet flue tube screen 17. Then it enters the high-efficiency steam-cooled cyclone separator 18, and after being heated, it enters the steam-water separator 15 again. Then it enters the tail wall superheater 20. The steam coming out of the tail wall superheater 20 first enters the low-temperature superheater 21 in the tail flue 39, and then enters the first medium-temperature superheater 29 and the second medium-temperature superheater 30 in the external bed 13 in sequence. Finally, it enters the first screen-type high-temperature superheater 12 and the second screen-type high-temperature superheater 34 in the furnace 1 in sequence, and finally leaves the boiler system.

[0033] Step 2: The low-temperature reheat steam first enters the low-temperature reheater 23 in the tail flue, then enters the first-panel high-temperature reheater 14 and the second-panel high-temperature reheater 35 in the furnace 1, and finally leaves the boiler system. The flue gas regulating baffle 25 in the tail flue 39 can regulate the flue gas volume in the front and rear flues, and the cone valve 8 between the return valve 7 and the external bed 13 can adjust the amount of high-temperature material entering the external bed 13. Through these two regulation methods, the reheat steam temperature is regulated.

[0034] Step 3: After being heated by the rotary air preheater 6, the primary air enters the air duct burner 2 at the bottom of the furnace 1 through the primary air duct 9, and then enters the furnace 1 through the water-cooled air chamber 31 and the air distribution plate 32. After being heated by the rotary air preheater 6, the secondary air enters the annular air box 6 through the primary air duct 9, and then enters the bottom of the furnace 1 in two layers through the secondary air branch pipe 27.

[0035] Step 4: After being heated by the high-pressure fluidizing air heater 36, the high-pressure fluidizing air enters the return valve 7 and the external bed 13 respectively. Coal is fed into the furnace 1 through the coal feed port 4 on the front wall and the coal feed port 5 of the return valve. Coal slurry is fed into the furnace 1 through the coal slurry gun 28 arranged on the rear wall at the bottom of the furnace 1. Coal and coal slime entering the furnace 1 are mixed and burned with primary and secondary air. The resulting high-temperature flue gas flows upward in the furnace 1, exchanges heat with the heating surfaces in the furnace 1, and then enters four high-efficiency steam-cooled cyclone separators 18. The separated material returns to the furnace 1 through the return valve 7 and the external bed 13. The flue gas from the separator enters the tail flue 39 through the separator outlet flue 19, where it exchanges heat with the heating surfaces in the tail flue 39. Both the front and rear tail flues are equipped with flue gas regulating baffles 25, which can regulate the flue gas flow rate of the front and rear flues. After the flue gas passes through the tail flue 39, the temperature decreases. Before the rotary air preheater 26, it is divided into a main flue and a bypass flue. Most of the flue gas enters the rotary air preheater 26 through the main flue and exchanges heat with the primary and secondary air. A small portion of the flue gas enters the high-pressure fluidized air heater 36 through the bypass flue for heat exchange. The flue gas temperature is further reduced in the main flue and bypass flue, and finally, the flue gas is discharged from the boiler system after merging.

[0036] In the description of this application, it should be noted that the terms "upper," "lower," etc., indicating orientation and positional relationships are based on the orientation and positional relationships shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. Unless otherwise expressly specified and limited, the terms "installed," "connected," and "linked" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal communication between two elements. For those skilled in the art, the specific meaning of the above terms in this application can be understood according to the specific circumstances.

[0037] The embodiments of this utility model disclosed above are merely illustrative of the present utility model. The embodiments do not exhaustively describe all details, nor do they limit the utility model to the specific implementations described. Many modifications and variations can be made based on the content of this specification. These embodiments are selected and specifically described in this specification to better explain the principles and practical applications of this utility model, thereby enabling those skilled in the art to better understand and utilize it.

Claims

1. A 660MW ultra-supercritical CFB boiler with external bed and double flue baffle temperature regulation, characterized in that: It includes a furnace (1), a duct burner (2), an annular wind box (6), a primary air duct (9), a secondary air duct (10), a screen-type high-temperature superheater, an external bed (13), a screen-type high-temperature reheater, a separator inlet flue screen (17), a separator (18), a separator outlet flue (19), a low-temperature superheater (21), a high-temperature economizer (22), a low-temperature reheater (23), a low-temperature economizer (24), a flue gas regulating baffle (25), a rotary air preheater (26), a secondary air branch pipe (27), a medium-temperature superheater, a water-cooled air chamber (31), an air distribution plate (32), a water-cooled wall (33), a high-pressure fluidized air heater (36), an air preheater bypass flue (37), and a tail flue (39). The furnace (1) is connected to the separator (18) through the separator inlet flue screen (17). The separator (18) is connected to the tail flue (39) through the separator outlet flue (19). The tail flue (39) includes a tail front flue and a tail rear flue. A low-temperature reheater (23) is arranged in the tail front flue. A low-temperature superheater (21) and a high-temperature economizer (22) are arranged from top to bottom in the tail rear flue. A flue gas regulating baffle (25) is set at the bottom of the tail flue. A low-temperature economizer (24) is installed below the flue gas regulating baffle (25). A rotary air preheater (26) is installed below the low-temperature economizer (24). An air preheater bypass flue (37) is installed in the rotary air preheater (26). A high-pressure fluidized air heater (36) is installed in the air preheater bypass flue (37). The furnace (1) is provided with an air distribution plate (32) at the bottom, and a water-cooled air chamber (31) is provided at the bottom of the air distribution plate (32). A screen-type high-temperature superheater is provided on the rear wall of the furnace (1), and a screen-type high-temperature reheater is provided on the front wall of the furnace (1). A medium-temperature superheater is arranged in the external bed (13). The external bed (13) is connected to the furnace (1). The rotary air preheater (26) is connected to the primary air duct (9). The primary air duct (9) is connected to the air duct burner (2). The air duct burner (2) is connected to the water-cooled air chamber (31). The rotary air preheater (26) is also connected to the secondary air duct (10). The secondary air duct (10) is connected to the annular wind box (6). The annular wind box (6) is connected to the secondary air branch pipe (27). The secondary air branch pipe (27) is connected to the furnace (1) in two layers.

2. The 660MW ultra-supercritical CFB boiler with external bed and double flue baffle temperature regulation according to claim 1, characterized in that: The outer wall of the furnace (1) is a water-cooled wall (33), and the membrane wall structure of the water-cooled wall (33) and the air distribution plate (32) forms a fully enclosed furnace.

3. The 660MW ultra-supercritical CFB boiler with external bed and double flue baffle temperature regulation according to claim 1, characterized in that: Five independent headers (38) are set at the bottom of the water-cooled walls (33) on both sides, arranged in a stepped manner.

4. The 660MW ultra-supercritical CFB boiler with external bed and double flue baffle temperature regulation according to claim 1, characterized in that: It includes four external beds (13), two of which are equipped with a first medium-temperature superheater (29) and two of which are equipped with a second medium-temperature superheater (30).

5. The 660MW ultra-supercritical CFB boiler with external bed and double flue baffle temperature regulation according to claim 4, characterized in that: The separator (18) uses four parallel high-efficiency air-cooled cyclone separators and a return valve (7) is installed at its bottom. One end of the separator (18) is connected to the separator inlet flue screen (17), and the other end of the separator (18) is connected to the separator outlet flue (19). Each return valve (7) is connected to an external bed (13) through a cone valve (8). The external bed (13) is connected to the furnace (1), and each return valve (7) is connected to the bottom of the furnace (1).

6. The 660MW ultra-supercritical CFB boiler with external bed and double flue baffle temperature regulation according to claim 5, characterized in that: The furnace (1) has 12 front wall coal feed ports (4) at the bottom front wall, and the return leg of the return valve (7) has 4 return valve coal feed ports (5).

7. The 660MW ultra-supercritical CFB boiler with external bed and double flue baffle temperature regulation according to claim 5, characterized in that: The furnace (1) is equipped with 6 coal slurry guns (28) and 6 drum slag coolers (3) on the bottom rear wall.

8. The 660MW ultra-supercritical CFB boiler with external bed and double flue baffle temperature regulation according to claim 1, characterized in that: The furnace (1) is equipped with a first-panel high-temperature superheater (12) and a second-panel high-temperature superheater (34) on the rear wall of the furnace.

9. The 660MW ultra-supercritical CFB boiler with external bed and double flue baffle temperature regulation according to claim 1, characterized in that: The furnace interior front wall is equipped with a water-cooled screen (11), a first-screen high-temperature reheater (14), and a second-screen high-temperature reheater (35).

10. The 660MW ultra-supercritical CFB boiler with external bed and double flue baffle temperature regulation according to claim 9, characterized in that: The water-cooled wall (33) and water-cooled screen (11) are connected to the steam-water separator (15), and the steam-water separator (15) is connected to the separator inlet flue screen (17).

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