Device and method for utilizing high-temperature flue gas waste heat of iron ore rotary kiln

Abstract

The invention relates to the technical field of metallurgical engineering, and particularly discloses an iron ore rotary kiln high-temperature flue gas waste heat utilization device and method. The problems that in the prior art, a kiln body forms rings, temperature distribution is uneven, and waste heat is not utilized are mainly solved. The core of the device comprises an air heat exchanger arranged in a gravity settling chamber, a discharge end sleeve type burner, and a blowpipe array (each pipe is provided with a spherical air nozzle) arranged along the kiln wall of the high-temperature reduction section at an interval of 0.8-1.2 m. The method comprises the following steps: pressurizing air to 20-60kPa, introducing the pressurized air into an air heat exchanger, and carrying out heat exchange with flue gas at 600-700 DEG C to produce preheated air at 580 DEG C; then, preheated air is introduced into the sleeve type burner to form diffusion flames larger than 10 m; 2-3 kPa of air is blown into the blowpipe through the partition kiln back fan, and the air is radially sprayed into the kiln through the spherical spray head. The purposes of uniformizing the temperature of a high-temperature section in the kiln, increasing the temperature of a cooling section and preventing ring formation of the rotary kiln are achieved while waste heat of high-temperature flue gas is recycled.

Description

Technical Field

[0001] The present invention relates to the technical field of metallurgical mineral engineering, and in particular to a method for utilizing waste heat of high-temperature flue gas from an iron ore rotary kiln. Background Art

[0002] As the leading equipment for direct reduction of iron ore, rotary kiln has the advantages of large-scale, continuous, closed operation, strong adaptability to raw materials and stable product quality. However, the problem of kiln ring formation is common, which seriously affects continuous production. Figure 1 This is a schematic diagram of the existing iron ore direct reduction rotary kiln system. The system primarily includes the following components: rotary kiln body, kiln back draft fan, granular coal lance, premix burner, granular coal charging port, Roots blower, vaporizer cooler, gravity settling chamber, flue gas cooler, bag filter, fume extractor, and chimney. The rotary kiln is divided into three zones along its length: the material drying and preheating zone, the high-temperature reduction zone, and the cooling zone, from the inlet to the outlet.

[0003] In the direct reduction process in an iron ore rotary kiln, a mixture of iron ore and reducing agent is added at the feed end. Driven by the rotation of the rotary kiln, it moves along the kiln body toward the discharge end, exchanging heat with the countercurrent flow of high-temperature flue gas. When the material temperature rises above 450°C, the iron oxides begin to react with the reducing agent. The reaction accelerates with increasing temperature, producing large quantities of metallurgical gas. This gas, combined with the combustion-supporting air introduced by the kiln's back-draft fan, releases heat, continuously heating the mixture. The reduction reaction accelerates significantly as the material enters the middle and rear sections of the kiln (900-1200°C). The addition of excess reducing coal ensures that the metallurgical gas combustion heat meets production requirements. As the reaction proceeds, the content of iron oxides and reducing coal in the material gradually decreases. When the material enters the cooling section, the generated metallurgical gas combustion heat is not enough to maintain the temperature. At this time, in order to increase the temperature of the rotary kiln cooling section, a premixed burner is set at the discharge end of the rotary kiln. The fuel and air are premixed in the burner and then burned. The flame length generated by the fuel combustion is 4-6m. The shorter flame length can generate local high temperature in the cooling section of the kiln. The roasting material is prone to produce liquid phase in the high temperature area, which causes ringing in the kiln body.

[0004] During the process, in addition to air supplied by the kiln back draft fan, the kiln head burners and granular coal injection lances also blow room-temperature air into the kiln. This cold air absorbs heat and heats up, creating localized low-temperature zones at the outlet, impacting reduction quality. To address the localized high- and low-temperature zones generated by fuel combustion in the rotary kiln, existing technologies adjust the air-fuel ratio at the discharge end burners to achieve incomplete combustion and reduce flame temperatures. However, secondary combustion of unburned fuel in the blowpipe area still fails to address the low-temperature zone issue. When the granular coal injection lance is in operation, the blowpipe changes the point at which the granular coal lands, causing it to accumulate there, further impacting reduction efficiency.

[0005] FeO generated during iron ore reduction easily combines with SiO2 to form low-melting-point FeSiO4, which not only reduces reduction quality but also adheres to the kiln walls, causing ringing. While anti-ringing technologies, such as adding kiln back draft fans, have shown some effectiveness, rotary kilns, as equipment with high thermal inertia, experience lags in temperature control, making it difficult to quickly eliminate localized high-temperature zones. Furthermore, existing rotary kilns typically have one kiln back draft fan for each blowpipe, with 2-4 blowpipes spaced 3-4 meters apart and operating at temperatures of 1150-1250°C. The oxidizing atmosphere and granular coal injection within the kiln can easily damage these blowpipes, resulting in a short service life and failing to meet the long-term production requirements of rotary kilns. Furthermore, kiln back draft fans are centrally located in the high-temperature reduction section (with an air pressure of 2-3 kPa). The air they blow creates a high-temperature zone within the kiln (2-3 meters behind the blowpipe outlet), which can easily cause the material to liquidize. Furthermore, areas where the metallurgical gas is not fully burned experience insufficient temperatures, both of which affect reduction quality. In addition, in the existing system, the exhaust gas with a temperature of 600-700℃ is cooled by the gravity settling chamber and the flue gas cooler, and then discharged through dust removal and pressurization, and the waste heat is not effectively utilized. Summary of the Invention

[0006] The purpose of the present invention is to provide a device and method for utilizing the waste heat of high-temperature flue gas from an iron ore rotary kiln, which can recover and utilize the waste heat of high-temperature flue gas while uniformizing the temperature of the high-temperature section in the kiln, preventing the rotary kiln from ringing, increasing the temperature of the cooling section, and reducing energy consumption.

[0007] To this end, the technical solution adopted in the present invention is as follows: 1. Flue gas waste heat utilization device Based on the traditional iron ore direct reduction rotary kiln process, the original kiln back air supply system and the gasification cooler in the gravity settling chamber of the rotary kiln were cancelled. An air heat exchanger was set in the gravity settling chamber of the rotary kiln, a sleeve burner was set at the discharge end, and multiple blowpipes were set in the high-temperature reduction section on the kiln back. The air was preheated by the air heat exchanger to recover the waste heat of the high-temperature flue gas, and the preheated high-temperature air was blown into the kiln through the sleeve burner to control the incomplete combustion of the sleeve burner fuel and the secondary supplementary combustion of the kiln back fan, so as to realize the diffuse combustion of the fuel in the kiln.

[0008] In addition to the rotary kiln body, gravity settling chamber, Roots blower, and kiln back fan, it also includes: Air heat exchanger: It is installed inside the gravity settling chamber, with its air inlet pipe connected to the outlet of the Roots blower, and the air outlet pipe extends to the discharge end of the rotary kiln; Sleeve burner: installed on the kiln head cover at the discharge end of the rotary kiln, with its air inlet connected to the air outlet pipe of the air heat exchanger, and its fuel inlet connected to the fuel supply pipeline; Blowing pipe array: It consists of multiple blowing pipes, and the blowing pipes are arranged at intervals of 0.8-1.2m along the kiln wall of the high-temperature reduction section of the rotary kiln body. The length of the kiln extension section of each blowing pipe is 1 / 3 of the kiln body diameter, and the air inlet of the blowing pipe is connected to the outlet of the kiln back fan through an integrated pipe. Wherein: the nozzle of the blowing pipe is provided with a spherical air nozzle, the surface of which is evenly distributed with circular holes of 3-5mm in diameter; the sleeve-type burner is composed of a coaxially arranged central fuel pipe and an external air pipe.

[0009] The blowing pipe array is equally divided into a front section group and a rear section group along the length of the kiln; all the blowing pipes in the front section group are connected to the first kiln back draft fan, and all the blowing pipes in the rear section group are connected to the second kiln back draft fan; the front section group and the rear section group are symmetrically distributed at 180° along the radial direction of the kiln body.

[0010] The circular hole spacing on the surface of the spherical air nozzle is 15-20 mm; the base material of the blowing pipe is heat-resistant steel, and the outer surface is covered with a high-temperature wear-resistant ceramic layer.

[0011] The kiln section covered by the blowing pipe array is: starting from 6m away from the discharge end and ending at the junction of the drying and preheating section and the high-temperature reduction section; and there are no blowing pipes set in the cooling section and the drying and preheating section within 6m from the discharge end.

[0012] Each blow pipe is equipped with a regulating valve to facilitate dynamic adjustment of local air flow.

[0013] The above device structure functions as follows: An air heat exchanger is installed in the gravity settling chamber of the rotary kiln. Normal temperature combustion air is pressurized to 20-60 kPa by a Roots blower and then introduced into the air heat exchanger. The air flows inside the heat exchanger tubes, while the high temperature flue gas flows outside the heat exchanger tubes. The heat exchange between the normal temperature air and the high temperature flue gas is achieved through indirect heat exchange. The high temperature combustion air preheated to about 600°C is discharged from the air heat exchanger outlet and then transported through a pipeline to the sleeve burner at the discharge end of the rotary kiln for use as combustion air. A blow pipe is installed every 0.8-1.2m on the back of the high-temperature section of the rotary kiln. The length of each blow pipe extending into the kiln is about 1 / 3 of the kiln diameter. The blow pipe assembly is divided into two parts of equal weight along the length of the kiln: the front section and the rear section. These two parts are installed at an angle of 180 degrees along the radial direction of the kiln body. The front section and the rear section each have a kiln back fan. The air supply pressure of the kiln back fan is 2-3KPa. The combustion-supporting air is pressurized by the kiln back fan and blown into the air main pipe. Multiple blow pipes are connected to the air main pipe. The combustion-supporting air is sprayed into the kiln through the blow pipes to support combustion. The blowpipe extends from the kiln back through the kiln wall, refractory materials, and kiln materials, then into the kiln. A spherical air nozzle is connected to the blowpipe's outlet. Combustion-supporting air flows from the blowpipe into the spherical air nozzle, which then blows it into the kiln. This uniformly distributes the combustion-supporting air within a radius centered on the spherical structure. The air injected into the kiln continuously mixes with the metallurgical gas in the kiln gas as it flows, creating a diffuse combustion process during which the mixture and combustion occur simultaneously.

[0014] The sleeve-type burner used in a rotary kiln consists of a central tube and an outer sleeve. Fuel is blown out of the central tube, and high-temperature air is blown out through the annular gap between the central tube and the outer sleeve. The high-temperature air and fuel flow in parallel and, under the action of the jet, the air and fuel mix and undergo diffuse combustion. During the heating, insulation, and cooling processes of the rotary kiln, fuel is introduced into the kiln through the sleeve-type burner. The fuel combustion maintains the kiln temperature at a certain level. During normal operation of the rotary kiln, a large amount of metallurgical gas overflows from the material bed. The metallurgical gas can meet the heat needs of the high-temperature reduction section of the rotary kiln. At this time, the sleeve-type burner is used as a supplementary heat source for the cooling section.

[0015] 2. A method for utilizing waste heat from flue gas, using the above device, comprises the following steps: (a) The combustion air at room temperature is pressurized to 20-60 kPa by a Roots blower and then introduced into the air heat exchanger in the gravity settling chamber; (b) The high-temperature flue gas at 600-700°C enters the gravity settling chamber and fully exchanges heat with the air in the tube of the air heat exchanger to produce high-temperature preheated air; (c) supplying preheated air to the outer air pipe of the sleeve burner and simultaneously supplying fuel to the central fuel pipe; (d) Preheated air is injected through the external air pipe and mixed with the fuel injected from the central fuel pipe under the action of the jet to form a diffuse combustion flame; (e) Combustion-supporting air at 2-3 kPa is blown into the blowpipe array through the kiln back draft fan, and then sprayed into the kiln through the radial holes of the spherical air nozzles, where it mixes with the metallurgical gas and burns.

[0016] In step (e), the air volume ratio of the front section group and the rear section group blowing pipes is controlled to be 1:1; and the combustion air flow rate of each blowing pipe is independently adjusted by adjusting the valve.

[0017] Compared with the prior art, the present invention has the following beneficial effects: (1) In the gravity settling chamber of the existing rotary kiln, the vaporization cooler is replaced with an air heat exchanger, and the waste heat of the high-temperature flue gas is recovered by air. The preheated air is then blown into the kiln from the discharge end of the rotary kiln through a sleeve-type burner, which can save vaporization cooling water while recovering the waste heat of the high-temperature flue gas; (2) The present invention sets a blow pipe every 0.8-1.2 m on the wall of the high-temperature section of the rotary kiln. The blow pipe covers an area of ​​1 / 3-1 / 2 of the kiln length. The increase in the number of blow pipes and the extension of the coverage area make the temperature distribution in the kiln more uniform, avoiding the problem of ring formation in the kiln due to the existence of the high-temperature area in the kiln; (3) The present invention blows high-temperature air into the cooling section of the rotary kiln, thereby increasing the temperature of the cooling section and the reduction quality and output of the iron ore; (4) The present invention shortens the length of the rotary kiln blowpipe extending into the kiln, so that the granular coal sprayed from the granular coal spray gun can fall evenly along the length of the kiln to the high-temperature reduction section in the kiln during the jet process in the kiln, thereby improving the reduction quality of the iron ore; (5) The high-temperature air pressure supplied from the discharge end of the rotary kiln into the kiln of the present invention is 20-60KPa. During the jet process, the air gradually mixes with the combustible gas in the kiln gas and produces diffuse combustion, thereby making the temperature of the high-temperature section in the kiln uniform. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a structural diagram of an existing iron ore direct reduction rotary kiln.

[0019] Figure 2 for Figure 1 Schematic diagram of the structure of the high-temperature flue gas dissipation system of a medium rotary kiln.

[0020] Figure 3 It is a structural schematic diagram of the iron ore direct reduction rotary kiln of the present invention.

[0021] Figure 4 for Figure 3 Schematic diagram of the structure of the high-temperature flue gas utilization system of the medium rotary kiln.

[0022] Figure 5 for Figure 3 Schematic diagram of the structure of the medium spherical air nozzle.

[0023] Figure 6 This is the temperature distribution curve inside the existing iron ore direct reduction rotary kiln.

[0024] Figure 7 This is the temperature distribution curve inside the iron ore direct reduction rotary kiln of the present invention.

[0025] In the figure, 1-rotary kiln body, 2-kiln back draft fan, 2a-first kiln back draft fan, 2b-second kiln back draft fan, 3-blowing pipe, 301-spherical air nozzle, 4-sleeve burner, 401-external air pipe, 402-central fuel pipe, 5-gravity settling chamber, 6-Roots blower, 7-air heat exchanger, 8-gasification cooling water tank, 9-flue gas cooler, 10-bag dust collector, 11-gas discharge tower. DETAILED DESCRIPTION

[0026] The present invention will be further explained below with reference to the accompanying drawings.

[0027] Combined with attachment Figure 3-7 The implementation process of the present invention is described using a Φ4.8×60m iron ore rotary kiln as an example. In practical applications, the parameters can be scaled proportionally according to the kiln size.

[0028] 1. Device structure implementation 1.1 Core Component Configuration Air heat exchanger 7: Installed inside gravity settling chamber 5, replacing the original vaporization cooler. It adopts shell and tube structure: the shell side passes 650℃ high temperature flue gas (inlet cross-sectional area 1.2m×1.5m), the tube side passes combustion air (168 Φ38×3mm Incoloy800H heat exchange tubes, total heat exchange area 320m 2 ); heat exchange efficiency ≥85%, air preheated to 580±20℃.

[0029] The sleeve burner 4 is installed on the kiln head cover at the discharge end and includes: External air pipe 401: inner diameter 200mm, connected to the preheated air pipe; Central fuel pipe 402: inner diameter 80mm, for natural gas / coal gas, annular gap designed to be 15mm, forming a high-speed jet (flow velocity > 30m / s).

[0030] Blowpipe array: Quantity: 48 (total length of high-temperature reduction section 24m, spacing 1.0m); Structure: Φ89×8mm heat-resistant steel pipe, extending into the kiln for 1.6m (1 / 3 of the kiln diameter of 4.8m); Surface treatment: plasma spraying Al2O3-40%ZrO2 composite ceramic layer (thickness 1.8mm); Nozzle: spherical structure (Φ120mm), with 256 Φ4.2mm circular holes evenly distributed on the surface (opening rate 32%).

[0031] Fan system: Roots blower 6: output pressure 40kPa (range 20-60kPa); Kiln back fan grouping: The first kiln back draft fan 2a: supplies 24 blowing pipes in the front section; Second kiln back draft fan 2b: supplies 24 blowpipes in the rear section; The two groups are symmetrically distributed 180° along the radial direction of the kiln body (to offset the rotational eccentricity).

[0032] Each blowpipe 3 is equipped with an electric regulating valve at the air inlet end (flow range 50-150Nm 3 / h) Layout key points: Blowing pipe coverage area: starting from 6m away from the discharge end and ending at the junction of the drying preheating section and the high-temperature reduction section.

[0033] Prohibited areas: cooling section (0-6m), drying and preheating section (to avoid interference with countercurrent heat exchange).

[0034] 2. Waste heat utilization and temperature control methods 2.1 Flue gas waste heat recovery ( Figure 4 ) Pressurized preheating: Normal temperature air is pressurized to 40kPa by Roots blower 6 → enters the air heat exchanger 7 tube pass; 650℃ flue gas enters the 5th shell side of gravity settling chamber → after countercurrent heat exchange, the temperature is reduced to 250℃; Air heat exchanger 7 produces 580°C preheated air (heat recovery rate 68%); 2.2 Dispersed combustion control ( Figure 4 ) Sleeve burner operation: Preheated air is introduced into the external air pipe 401, fuel (natural gas 800Nm 3 / h) into the central fuel pipe 402, adjusting the air-fuel ratio to 1.1:1 (stoichiometric ratio 0.9:1), achieving a 25% incomplete combustion rate; the jets mix to form a dispersed flame >10m (compared to the 4-6m flame of the original premixed burner).

[0035] Partitioned air distribution enhancement: The air volume ratio of the front and rear groups is 1:1 (each air supply is 12000Nm 3 / h), dynamically adjust the air volume of the single blowing pipe through the electric valve (to cope with local temperature fluctuations).

[0036] Air is injected radially through the spherical nozzle 301 (covering a radius of 1.2 m).

[0037] 2.3 Temperature uniformity mechanism Cooling section temperature increase: diffuse flame extends the hot zone, raising the cooling section temperature from 750°C to 950°C; High-temperature section temperature uniformity: dense blowing pipes (1.0m spacing) form a continuous combustion zone, and spherical nozzles achieve progressive air-gas mixed combustion; the temperature fluctuation in the high-temperature reduction section is reduced from ±150°C to ±50°C (1150±50°C); Anti-ringing effect: eliminate local high temperature areas (FeSiO4 formation temperature > 1100℃), and the ceramic coated blowpipe resists liquid phase adhesion.

[0038] 3. Effect demonstration contrast Figure 6 and7 , as shown in Table 1 below: Table 1 Comparison between the existing technology and the method of the present invention It can be seen that the present invention not only achieves efficient waste heat recovery, but also completely solves the core problems of rotary kiln ring formation, uneven temperature distribution, and insufficient temperature rise in the cooling section, meeting the requirements of large-scale industrial application. Taking a Φ4.8×60m rotary kiln as an example, the daily output increased from 2200t to 2600t (+18.2%), and the annual recovery of flue gas waste heat was 1.2×10 8 MJ (equivalent to 12,000 tons of standard coal), the system thermal efficiency increased from 54% to 68%.

Claims

1. A device for utilizing waste heat from high-temperature flue gas of an iron ore rotary kiln, comprising a rotary kiln body (1), a gravity settling chamber (5), a Roots blower (6), and a kiln back fan (2); characterized in that Also includes: Air heat exchanger (7): arranged inside the gravity settling chamber (5), with its air inlet pipe connected to the outlet of the Roots blower (6), and the air outlet pipe extending to the discharge end of the rotary kiln; Sleeve-type burner (4): installed on the kiln head cover at the discharge end of the rotary kiln, its air inlet is connected to the air outlet pipe of the air heat exchanger (7), and its fuel inlet is connected to the fuel supply pipeline; Blowing pipe array: composed of a plurality of blowing pipes (3), and the blowing pipes (3) are arranged at intervals of 0.8-1.2 m along the kiln wall of the high-temperature reduction section of the rotary kiln body (1), the kiln extension length of each blowing pipe (3) is 1 / 3 of the kiln body diameter, and the air inlet of the blowing pipe (3) is connected to the outlet of the kiln back fan (2) through a pipeline integration; Wherein: the nozzle of the blowing pipe (3) is provided with a spherical air nozzle (301), and the surface of the nozzle is uniformly distributed with circular holes of 3-5 mm in diameter; The sleeve-type burner (4) is composed of a coaxially arranged central fuel pipe (402) and an external air pipe (401).

2. The device for utilizing waste heat from high-temperature flue gas of an iron ore rotary kiln according to claim 1, characterized in that: The blowing pipe array is equally divided into a front section group and a rear section group along the kiln length direction; All the blowing pipes (3) of the front section group are connected to the first kiln back draft fan (2a), and all the blowing pipes (3) of the rear section group are connected to the second kiln back draft fan (2b); The front section group and the rear section group are symmetrically distributed at 180° along the radial direction of the kiln body.

3. The device for utilizing waste heat from high-temperature flue gas of an iron ore rotary kiln according to claim 1, characterized in that: The circular holes on the surface of the spherical air nozzle (301) have a spacing of 15-20 mm. The base material of the blowing pipe (3) is heat-resistant steel, and the outer surface is covered with a high-temperature wear-resistant ceramic layer.

4. The device for utilizing waste heat from high-temperature flue gas of an iron ore rotary kiln according to claim 1, characterized in that: The kiln section covered by the blowing pipe array is: starting from 6m away from the discharge end and ending at the junction of the drying and preheating section and the high-temperature reduction section; and there are no blowing pipes set in the cooling section and the drying and preheating section within 6m from the discharge end.

5. The device for utilizing waste heat from high-temperature flue gas of an iron ore rotary kiln according to claim 1, characterized in that: Each blowing pipe (3) is provided with a regulating valve.

6. A method for utilizing waste heat from high-temperature flue gas in an iron ore rotary kiln, using the device according to any one of claims 1 to 5, characterized in that: The following steps are involved: (a) The combustion-supporting air at room temperature is pressurized to 20-60 kPa by a Roots blower (6) and then introduced into an air heat exchanger (7) in a gravity settling chamber (5); (b) allowing the high-temperature flue gas of 600-700°C to enter the gravity settling chamber (5) and fully exchange heat with the air in the tube of the air heat exchanger (7) to produce high-temperature preheated air; (c) delivering preheated air to the outer air pipe (401) of the sleeve burner (4) and simultaneously supplying fuel to the central fuel pipe (402); (d) preheated air is injected through the external air pipe (401) and mixed with the fuel injected from the central fuel pipe (402) under the action of the jet to form a diffuse combustion flame; (e) Combustion-supporting air at 2-3 kPa is blown into the blowing pipe array through the kiln back draft fan (2), and is sprayed into the kiln through the radial holes of the spherical air nozzle (301) to mix with the metallurgical gas and burn.

7. The method for utilizing waste heat from high-temperature flue gas of an iron ore rotary kiln according to claim 6, characterized in that: In step (e), the air volume ratio of the front section group and the rear section group blowing pipes is controlled to be 1:1; and the combustion air flow rate of each blowing pipe (3) is independently adjusted by adjusting the valve.

Images