Method for converting a solid slagging steam drum boiler into a double U-shaped liquid slagging boiler

Abstract

A method for converting a solid slag-discharging steam drum boiler into a double-U liquid slag-discharging boiler, which relates to the technical field of liquid slag-discharging boilers. In order to solve the problems that after the existing solid slag-discharging boilers burn Zhundong coal, slagging and fouling occur to varying degrees, the water-cooled walls of the cold ash hoppers are frequently damaged, the slag removal system has operating failures, and the flue is blocked, and in severe cases, the boiler has to be shut down. This application converts the original solid slag-discharging boiler into a double-U liquid slag-discharging boiler. The modified double-U liquid slag-discharging boiler can burn Zhundong coal with extremely strong caking property or other coal types with low ash melting point, which is conducive to making full use of the rich Zhundong coal resources, reducing the fuel cost, and improving the operation economy. The present invention is applicable to the operation field of converting solid slag-discharging steam drum boilers into liquid slag-discharging boilers.

Description

Technical Field

[0001] The present invention relates to the technical field of liquid slag removal boilers, and specifically relates to a method for converting a solid slag removal steam drum boiler into a double U-shaped liquid slag removal boiler. Background Art

[0002] In China, the reserves of Zhundong coal are rich and the price is cheap. If existing boilers can be retrofitted to burn Zhundong coal, the coal consumption cost will be greatly reduced and the economy will be improved. Zhundong coal has a high alkali metal content and serious slagging and fouling problems. After many solid slag removal boilers burn Zhundong coal, varying degrees of slagging and fouling have occurred, and cold ash hopper water-cooled wall injuries, slag removal system operation failures, and flue blockages have frequently occurred. In severe cases, the boiler has to be shut down. Summary of the Invention

[0003] In order to solve the problems that after existing solid slag removal boilers burn Zhundong coal, varying degrees of slagging and fouling have occurred, cold ash hopper water-cooled wall injuries, slag removal system operation failures, and flue blockages have frequently occurred, and in severe cases, the boiler has to be shut down, the present invention proposes a method for converting a solid slag removal steam drum boiler into a double U-shaped liquid slag removal boiler.

[0004] The method for converting a solid slag removal steam drum boiler into a double U-shaped liquid slag removal boiler of the present invention is as follows:

[0005] Step 1: Cancel the burner, cold ash hopper and slag lifter of the original boiler;

[0006] Step 2: After canceling the burner, the furnace of the original boiler is used as the cooling chamber of the double U-shaped liquid slag removal boiler, and the front and back or the middle parts of the two side walls of the water-cooled wall are contracted inward, dividing the cooling chamber into an upper cooling chamber and a lower cooling chamber;

[0007] Step 3: Add a U-shaped combustion chamber to the front and back or both sides of the lower cooling chamber respectively. The lower part of the U-shaped combustion chamber has a liquid slag removal hopper, and the liquid slag removal hopper is communicated with the inside of the U-shaped combustion chamber. A slag removal port is provided at the bottom end of each liquid slag removal hopper;

[0008] Step 4: A saddle arch is provided at the connection between one end of the bottom of the lower cooling chamber and one end of the U-shaped combustion chamber. The lower parts of the front and back walls or the two side walls of the water-cooled wall are communicated with one end of the slag capture tube bundle. The slag capture tube bundle sequentially passes through the outlet flue of the U-shaped combustion chamber and the saddle arch horizontally;

[0009] Step 5: The slag capture tube bundle serves as the boundary between the U-shaped combustion chamber and the lower cooling chamber, forming a barrier at the outlet of the U-shaped combustion chamber to capture the liquid slag carried in the flue gas;

[0010] Step 6: When pulverized coal burns in the U-shaped combustion chamber through the swirl burner, the heat load of the U-shaped combustion chamber is much higher than that of a normal solid slagging boiler, which increases the temperature in the U-shaped combustion chamber. The ash formed is molten liquid slag, and the high-temperature flue gas generated by combustion enters the cooling chamber after passing through the slag-catching tube bundle;

[0011] Step 7: Change the connection of the original economizer of the boiler from being connected to the steam drum by the economizer outlet connecting pipe to being connected to the inlet header of the combustion chamber by the inlet connecting pipe of the combustion chamber; after the feed water passes through the economizer, it enters the inlet header of the combustion chamber through the inlet connecting pipe of the combustion chamber, and flows through the liquid slagging hopper, the saddle arch, the liquid slagging hopper and the four walls of the water-cooled wall of the combustion chamber in a once-through manner, and then enters the outlet header of the combustion chamber. The outlet connecting pipe of the combustion chamber enters the steam drum through the pipe hole of the original economizer outlet connecting pipe for connecting to the steam drum, and the water in the steam drum circulates through the downcomer and the water-cooled wall and then returns to the steam drum;

[0012] Further, the two U-shaped combustion chambers on the front and rear or both sides of the lower cooling chamber are symmetrically arranged;

[0013] Further, in Step 3, the swirl burner is inverted at the top of one end of the U-shaped combustion chamber, and the flue gas flows downward in the U-shaped combustion chamber. The cross-section of the U-shaped combustion chamber is rectangular;

[0014] Further, the outer walls of the U-shaped combustion chamber and the liquid slagging hopper in Step 3 are both wound with the water-cooled wall of the combustion chamber;

[0015] Further, the outer wall of the saddle arch in Step 4 is wound with the water-cooled wall of the combustion chamber;

[0016] Further, in Step 4, pins are welded on the fire-facing outer walls of the U-shaped combustion chamber, the slag-catching tube bundle, the saddle arch and the lower cooling chamber, and a layer of refractory castable is covered;

[0017] Further, the water-cooled wall of the combustion chamber adopts a once-through type;

[0018] Further, in Step 3, the front and rear walls or both side walls of the lower part of the U-shaped combustion chamber incline inward to form a constriction, and the angle between the inward-inclined surface of the front and rear walls or both side walls and the vertical direction is 15° to 40°; the shape of the liquid slagging hopper at the bottom of the U-shaped combustion chamber is a quadrangular pyramid, and the angle between each surface of the liquid slagging hopper and the horizontal plane is 0° to 25°;

[0019] Further, in Step 4, the slag-catching tube bundle adopts a staggered arrangement or an in-line arrangement type. The design method of the slag-catching tube bundle is determined according to the flue gas temperature drop. The best range of the flue gas temperature drop through the slag-catching tube bundle is 30°C to 100°C, and the longitudinal number of rows of the slag-catching tubes is two to six;

[0020] Further, the height of the saddle arch in Step 4 is 1 / 5 to 3 / 5 of the height of the U-shaped combustion chamber.

[0021] The present invention has the following beneficial effects compared with the prior art:

[0022] The present invention overcomes the shortcomings of the prior art. All burners of the original boiler are cancelled, and the furnace is transformed into the cooling chamber of a liquid slag removal boiler. Two U-shaped combustion chambers of the U-shaped liquid slag removal boiler are designed and added outside the furnace. The inner walls of the U-shaped combustion chambers, slag-catching tube bundles, and the lower cooling chamber are covered with refractory castables to increase the temperature inside the U-shaped combustion chambers and the lower cooling chamber, so that the ash slag melts to achieve liquid slag removal. A swirl burner is used to increase the effect of liquid slag hitting the wall. The U-shaped combustion chambers adopt a vertical inverted combustion method, and the flue gas forms a U-shaped turn at the bottom. Under the action of inertial force, most of the molten slag is thrown onto the liquid slag removal hopper and the slope of the saddle arch, increasing the separation effect of the liquid slag, improving the slag-catching rate, reducing the dust content in the flue gas at the furnace outlet, and preventing slagging and fouling of the furnace and other heating surfaces. The transformed double U-shaped liquid slag removal boiler can burn Zhundong coal with extremely strong coking property or other coal types with low ash melting point, which is beneficial to making full use of the rich Zhundong coal resources, reducing the fuel cost, and improving the operation economy;

[0023] In the present invention, the flue gas temperature inside the entire U-shaped combustion chamber is higher than the melting temperature of the ash slag. The ash slag melts and flows downward along the inner wall of the U-shaped combustion chamber into the liquid slag removal hopper, forming a liquid slag pool in the liquid slag hopper. In order to ensure that the liquid slag has sufficient fluidity throughout the process, the viscosity of the liquid slag should not exceed 25 Pa·s. As the temperature rises, the viscosity of the liquid slag decreases. When the viscosity of the liquid slag decreases to 25 Pa·s, the temperature at this time is determined as the lowest slag removal temperature. The lowest slag removal temperature is generally about 150 °C higher than the ash slag melting temperature. In this way, the liquid slag has good fluidity, can flow out smoothly from the slag discharge port, will not cause unsmooth slag discharge due to high viscosity, nor will it be blocked due to solidification during the slag discharge process, thus avoiding slagging and fouling, preventing damage to the water-cooled wall of the cold ash hopper and operation failures of the slag removal system and flue blockage, and further improving the operation stability of the boiler. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a schematic diagram of the original steam drum boiler;

[0025] Figure 2 is a schematic layout diagram of the transformed double U-shaped liquid slag removal boiler;

[0026] Figure 3 is a schematic diagram of the liquid slag removal hopper and the saddle arch structure;

[0027] Figure 4 is a schematic flow diagram of the liquid slag removal hopper and the saddle arch in the working state;

[0028] Figure 5It is a schematic diagram of the W-shaped structure formed by integrating the combustion chamber and the lower cooling chamber in the modified double-U type liquid slag removal boiler as an independent whole;

[0029] Figure 6 It is a layout schematic diagram of the modified double-U type liquid slag removal boiler in which the combustion chamber and the lower cooling chamber are integrated as an independent whole to form a W-shaped structure, and the original furnace is lifted as a whole;

[0030] Figure 7 It is a schematic diagram of the structure in which two water-cooled walls between the U-shaped combustion chamber and the lower cooling chamber in the modified double-U type liquid slag removal boiler are combined into one water-cooled wall. Specific implementation mode

[0031] Specific implementation mode one: Combined with Figures 1 to 5 This implementation mode is described. The method for converting a solid slag removal steam drum boiler into a double-U type liquid slag removal boiler is as follows:

[0032] Step one, cancel the burner 14, cold ash hopper 16 and slag extractor 17 of the original boiler;

[0033] Step two, after canceling the burner 14, the original furnace 15 of the boiler is used as the cooling chamber of the double-U type liquid slag removal boiler, and the front and back or the middle parts of the two side walls of the water-cooled wall 10 are contracted inward, dividing the cooling chamber into an upper cooling chamber 7 and a lower cooling chamber 8;

[0034] Step three, add a U-shaped combustion chamber 1 respectively in the front and back or on both sides of the lower cooling chamber 8. The lower part of the U-shaped combustion chamber 1 has a liquid slag removal hopper 4, and the liquid slag removal hopper 4 is communicated with the inside of the U-shaped combustion chamber 1. A slag discharge port 5 is provided at the bottom end of each liquid slag removal hopper 4;

[0035] Step four; a saddle arch 6 is provided at the connection between one end of the bottom of the lower cooling chamber 8 and one end of the U-shaped combustion chamber 1. The lower parts of the front and back walls or the two side walls of the water-cooled wall 10 are communicated with one end of the slag capture tube bundle 3, and the slag capture tube bundle 3 sequentially passes through the outlet flue of the U-shaped combustion chamber 1 and the saddle arch 6 horizontally;

[0036] Step five, the slag capture tube bundle 3 serves as the boundary between the U-shaped combustion chamber 1 and the lower cooling chamber 8, forming a barrier at the outlet of the U-shaped combustion chamber 1 to capture the liquid slag carried in the flue gas;

[0037] Step six, when pulverized coal burns in the U-shaped combustion chamber 1 through the swirl burner 2, the heat load of the U-shaped combustion chamber 1 is much higher than that of a normal solid slag removal boiler, increasing the temperature in the U-shaped combustion chamber 1. The formed ash slag is molten liquid slag, and the high-temperature flue gas generated by combustion enters the cooling chamber after passing through the slag capture tube bundle 3;

[0038] Step 7: Change the connection of the original economizer of the boiler from being connected to the steam drum 9 through the economizer outlet connecting pipe 18 to being connected to the combustion chamber inlet header 19 through the combustion chamber inlet connecting pipe 12; after the feed water passes through the economizer, it enters the combustion chamber inlet header 19 through the combustion chamber inlet connecting pipe 12, and flows through the liquid slag removal hopper 4, the saddle arch 6, the liquid slag removal hopper 4 and the four walls of the combustion chamber water wall 21 in a once-through manner, and then enters the combustion chamber outlet header 20. The combustion chamber outlet connecting pipe 13 enters the steam drum 9 through the pipe hole of the original economizer outlet connecting pipe 18 connecting to the steam drum 9. The water in the steam drum 9 circulates through the downcomer 11 and the water wall 10 and then returns to the steam drum 9;

[0039] In this specific embodiment, the slag capture rate of the solid slag removal boiler is only 10 - 15%, and most of the ash is carried away by the flue gas. When burning Zhundong coal, due to the low ash melting point of Zhundong coal and the large amount of alkali metal oxides contained in the ash, it is extremely easy to cause slagging, fouling and ash accumulation in the furnace outlet and subsequent heating surfaces. After converting to a double U-shaped liquid slag removal boiler, the slag capture rate can reach over 60%, greatly reducing the ash content in the flue gas. The low melting point alkali metal oxides form sulfates and are captured along with the liquid slag, which can effectively solve the coking, fouling and ash accumulation problems of Zhundong coal.

[0040] Specific embodiment two: Combining Figure 2 To illustrate this embodiment, this embodiment is a further limitation on the conversion method described in specific embodiment one. For a method of converting a solid slag removal steam drum boiler into a double U-shaped liquid slag removal boiler described in this embodiment, the two U-shaped combustion chambers 1 on the front and back or both sides of the lower cooling chamber 8 in step three are symmetrically arranged.

[0041] Specific embodiment three: Combining Figure 2 To illustrate this embodiment, this embodiment is a further limitation on the conversion method described in specific embodiment one. For a method of converting a solid slag removal steam drum boiler into a double U-shaped liquid slag removal boiler described in this embodiment, in step three, the swirl burner 2 is inverted at the top of one end of the U-shaped combustion chamber 1, and the flue gas flows downward in the U-shaped combustion chamber 1. The cross-section of the U-shaped combustion chamber 1 is rectangular;

[0042] In this specific embodiment, all the original burners 14 of the boiler are cancelled, and the pulverized coal pipeline is changed to be connected to the swirl burner 2 of the U-shaped combustion chamber 1. A matrix layout method is adopted on the ceiling of the U-shaped combustion chamber 1. Each U-shaped combustion chamber 1 has 8 swirl burners 2, and the number of swirl burners 2 can be changed according to needs.

[0043] Specific embodiment four: Combining Figure 2Describing this embodiment, this embodiment is a further limitation on the modification method described in the specific embodiment one. For the method of converting a solid slagging steam drum boiler into a double U-shaped liquid slagging boiler described in this embodiment, combustion chamber water walls 21 are wound around the outer walls of the U-shaped combustion chamber 1 and the liquid slagging hopper 4 in step three;

[0044] In this specific embodiment, by winding combustion chamber water walls 21 around the outer walls of the U-shaped combustion chamber 1 and the liquid slagging hopper 4 in step three, the heat absorption of the U-shaped combustion chamber 1, the slag-catching tube bundle 3, and the lower cooling chamber 8 is reduced, the flue gas temperature in these areas is increased, and the ash slag is ensured to be in a freely flowing liquid state in these areas, which is beneficial to the capture of liquid slag and smooth outflow.

[0045] Specific embodiment five: Combining Figure 2 Describing this embodiment, this embodiment is a further limitation on the modification method described in the specific embodiment one. For the method of converting a solid slagging steam drum boiler into a double U-shaped liquid slagging boiler described in this embodiment, combustion chamber water walls 21 are wound around the outer wall of the saddle arch 6 in step four.

[0046] Specific embodiment six: Combining Figure 2 Describing this embodiment, this embodiment is a further limitation on the modification method described in the specific embodiment one. For the method of converting a solid slagging steam drum boiler into a double U-shaped liquid slagging boiler described in this embodiment, studs are welded on the fire-facing outer walls of the U-shaped combustion chamber 1, the slag-catching tube bundle 3, the saddle arch 6, and the lower cooling chamber 8 in step four and covered with a layer of refractory castable;

[0047] In this specific embodiment, by welding studs on the fire-facing outer walls of the U-shaped combustion chamber 1, the slag-catching tube bundle 3, the saddle arch 6, and the lower cooling chamber 8 and covering with a layer of refractory castable, the heat absorption of the U-shaped combustion chamber 1, the slag-catching tube bundle 3, the saddle arch 6, and the lower cooling chamber 8 is reduced, the flue gas temperature in these areas is increased, and the ash slag is ensured to be in a freely flowing liquid state in these areas, which is beneficial to the capture of liquid slag and smooth outflow.

[0048] Specific embodiment seven: Combining Figure 2 Describing this embodiment, this embodiment is a further limitation on the modification method described in the specific embodiment five. For the method of converting a solid slagging steam drum boiler into a double U-shaped liquid slagging boiler described in this embodiment, the combustion chamber water wall 21 is of a once-through type;

[0049] In this specific embodiment, the combustion chamber water wall 21 is designed in a once-through manner, adopting a vertical tube coil or a spiral tube coil type, and the liquid slagging hopper 4 and the saddle arch 6 are combined to form an independent flow, adopting once-through or reciprocating flow.

[0050] Specific embodiment eight: CombiningFigure 2 Regarding this embodiment, this embodiment further defines the modification method described in the fourth specific embodiment. For the method of converting a solid slagging drum boiler into a double U-shaped liquid slagging boiler described in this embodiment, in step 3, the front and rear walls or the side walls at the lower part of the U-shaped combustion chamber 1 incline inward to form a constriction, and the angle between the inward-inclined surface of the front and rear walls or the side walls and the vertical direction is 15° to 40°; the shape of the liquid slagging hopper 4 at the bottom of the U-shaped combustion chamber 1 is a quadrangular pyramid, and the angle between each surface of the liquid slagging hopper 4 and the horizontal plane is 0° to 25°;

[0051] In this specific embodiment, the front and rear walls or the side walls at the lower part of the U-shaped combustion chamber 1 incline inward to form a constriction, and the angle between the inward-inclined surface of the front and rear walls or the side walls and the vertical direction is 15° to 40°, so as to increase the sectional heat load by reducing the cross-sectional area.

[0052] Specific embodiment nine: Combining Figure 2 Regarding this embodiment, this embodiment further defines the modification method described in the sixth specific embodiment. For the method of converting a solid slagging drum boiler into a double U-shaped liquid slagging boiler described in this embodiment, in step 4, the slag-catching tube bundle 3 is arranged in a staggered or in-line arrangement pattern, and the design method of the slag-catching tube bundle 3 is determined according to the flue gas temperature drop. The best range of the flue gas temperature drop through the slag-catching tube bundle 3 is 30°C to 100°C, and the longitudinal number of rows of the slag-catching tubes is two to six.

[0053] Specific embodiment ten: Combining Figure 2 Regarding this embodiment, this embodiment further defines the modification method described in the ninth specific embodiment. For the method of converting a solid slagging drum boiler into a double U-shaped liquid slagging boiler described in this embodiment, the height of the saddle arch 6 in step 4 is 1 / 5 to 3 / 5 of the height of the U-shaped combustion chamber 1;

[0054] In this specific embodiment, by adopting the height of the saddle arch 6 in step 4 to be 1 / 5 to 3 / 5 of the height of the U-shaped combustion chamber 1, the saddle arch 6 can be the watershed of the two U-shaped combustion chambers 1, preventing the flue gas from colliding, making the flue gas form a U-shaped turn. At the bottom of the U-shaped combustion chamber 1, the flue gas makes a nearly 180° turn, and the liquid slag carried in the flue gas is thrown onto the liquid slagging hopper 4 and the saddle arch 6 under the action of inertia, improving the liquid slag separation effect, and the liquid slag flows down along the saddle arch 6 into the liquid slagging hopper 4.

[0055] Specific embodiment eleven: Combining Figure 5 and Figure 6This embodiment will be described. This embodiment is another implementation of the transformation method described in the first specific embodiment. A method for converting a solid slag-discharging steam drum boiler into a double-U liquid slag-discharging boiler, in which the combustion chamber and the lower cooling chamber are taken as an independent whole to form a W-shaped structure. The water-cooling system of this part is a separate loop. The lower part of the original boiler furnace 7 is shortened upwards, and the corresponding downcomer 11 is also shortened upwards. The W-shaped structure is arranged at the lower part of the furnace and is connected by welding in the middle.

[0056] Specific Embodiment Twelve: In combination with Figure 7 This embodiment will be described. This embodiment is another implementation of the transformation method described in the first specific embodiment. A method for converting a solid slag-discharging steam drum boiler into a double-U liquid slag-discharging boiler, in which the two water walls between the U-shaped combustion chamber 1 and the lower cooling chamber 8 are combined into one water wall. The combined water wall is heated on both sides, which simplifies the structure and system process of the water wall and reduces the occupied space.

Claims

1. A method for converting a solid slag-discharging steam drum boiler into a double-U liquid slag-discharging boiler, characterized in that: The specific method is as follows: Step 1: Remove the burner (14), cold ash hopper (16) and slag extractor (17) of the original boiler; Step 2: After removing the burner (14), the furnace (15) of the original boiler serves as the cooling chamber of the double-U liquid slag-discharging boiler. The front and back or the middle parts of the two side walls of the water-cooled wall (10) contract inward, dividing the cooling chamber into an upper cooling chamber (7) and a lower cooling chamber (8); Step 3: Add a U-shaped combustion chamber (1) respectively in the front and back or on both sides of the lower cooling chamber (8). The lower part of the U-shaped combustion chamber (1) has a liquid slag-discharging hopper (4), and the liquid slag-discharging hopper (4) is communicated with the inside of the U-shaped combustion chamber (1). A slag-discharging port (5) is provided at the bottom end of each liquid slag-discharging hopper (4); Step 4: A saddle arch (6) is provided at the connection between one end of the bottom of the lower cooling chamber (8) and one end of the U-shaped combustion chamber (1), connecting the lower parts of the front and back walls or the two side walls of the water-cooled wall (10) with one end of the slag-catching tube bundle (3). The slag-catching tube bundle (3) sequentially passes through the outlet flue of the U-shaped combustion chamber (1) and the saddle arch (6) horizontally; Step 5: The slag-catching tube bundle (3) serves as the dividing line between the U-shaped combustion chamber (1) and the lower cooling chamber (8), forming a barrier at the outlet of the U-shaped combustion chamber (1) to capture the liquid slag carried in the flue gas; Step 6: When pulverized coal burns in the U-shaped combustion chamber (1) through the swirl burner (2), the heat load of the U-shaped combustion chamber (1) is much higher than that of a normal solid slag-discharging boiler, increasing the temperature in the U-shaped combustion chamber (1). The formed ash slag is molten liquid slag, and the high-temperature flue gas generated by combustion enters the cooling chamber after passing through the slag-catching tube bundle (3); Step 7: Change the connection of the original economizer of the boiler from the economizer outlet connecting pipe (18) to the steam drum (9) to connecting from the combustion chamber inlet connecting pipe (12) to the combustion chamber inlet header (19); The combustion chamber water-cooled wall (21) adopts a once-through type. After the feed water passes through the economizer, it enters the combustion chamber inlet header (19) through the combustion chamber inlet connecting pipe (12), and flows through the liquid slag-discharging hopper (4), saddle arch (6), liquid slag-discharging hopper (4) and the four walls of the combustion chamber water-cooled wall (21) in a once-through manner, and enters the combustion chamber outlet header (20). The combustion chamber outlet connecting pipe (13) enters the steam drum (9) using the pipe hole of the original economizer outlet connecting pipe (18) connecting to the steam drum (9). The water in the steam drum (9) circulates through the downcomer (11) and the water-cooled wall (10) and then returns to the steam drum (9); In step 3, the swirl burner (2) is inverted at the top of one end of the U-shaped combustion chamber (1), and the flue gas flows downward in the U-shaped combustion chamber (1). The cross-section of the U-shaped combustion chamber (1) is rectangular; The outer walls of the U-shaped combustion chamber (1) and the liquid slag-discharging hopper (4) in step 3 are both wound with the combustion chamber water-cooled wall (21); The height of the saddle arch (6) in step 4 is 1 / 5 - 3 / 5 of the height of the U-shaped combustion chamber (1).

2. A method for converting a solid slag-discharging steam drum boiler into a double-U liquid slag-discharging boiler according to claim 1, characterized in that: The two U-shaped combustion chambers (1) on the front and rear or both sides of the lower cooling chamber (8) in the third step are symmetrically arranged.

3. A method for converting a solid slag-discharging steam drum boiler into a double U-shaped liquid slag-discharging boiler according to claim 1, characterized in that: The outer wall of the saddle arch (6) in the fourth step is wound with a combustion chamber water-cooled wall (21).

4. A method for converting a solid slag-discharging steam drum boiler into a double U-shaped liquid slag-discharging boiler according to claim 1, characterized in that: In the fourth step, studs are welded on the fire-facing outer walls of the U-shaped combustion chamber (1), the slag-catching tube bundle (3), the saddle arch (6) and the lower cooling chamber (8), and a layer of refractory castable is covered.

5. A method for converting a solid slag-discharging steam drum boiler into a double U-shaped liquid slag-discharging boiler according to claim 1, characterized in that: In the third step, the front and rear walls or both side walls of the lower part of the U-shaped combustion chamber (1) are inclined inward to form a constriction, and the angle between the inward inclined surface of the front and rear walls or both side walls and the vertical direction is 15° to 40°; the shape of the liquid slag-discharging hopper (4) at the bottom of the U-shaped combustion chamber (1) is a quadrangular pyramid, and the angles of each surface of the liquid slag-discharging hopper (4) with the horizontal plane are 0° to 25°.

6. A method for converting a solid slag-discharging steam drum boiler into a double U-shaped liquid slag-discharging boiler according to claim 1, characterized in that: In the fourth step, the slag-catching tube bundle (3) is arranged in a staggered or in-line arrangement pattern. The design method of the slag-catching tube bundle (3) is determined according to the flue gas temperature drop. The best range for the flue gas temperature drop through the slag-catching tube bundle (3) is 30°C to 100°C, and the longitudinal number of rows of the slag-catching tubes is two to six.

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