Combustion system and combustion method for pulverized coke and coal dual fuel boiler

Abstract

The present application relates to a combustion system of a pulverized coke and pulverized coal dual fuel boiler and a combustion method thereof, and belongs to the technical field of boiler production design. The present application solves the technical problems of the existing pulverized coke utilization mode, i.e. mixing and burning pulverized coke as a fuel of a thermal power plant with coal, which requires special structural design or modification of the mixing and burning system, and the mixing and burning mode also has the problems of poor combustion stability, increased fly ash content and the like. In the pure pulverized coke mode, hot primary air passing through an air preheater carries high-temperature pulverized coke added by a rotary feeding valve into a pulverized coal burner, and hot secondary air and the pulverized coke primary air are combusted through the burner. In the pure pulverized coal mode, pulverized coal dried and ground by a coal mill is sent into the burner through a pulverized coal pipeline, and hot secondary air and the pulverized coal primary air are combusted through the burner. The present application can quickly switch between the two combustion modes of pure pulverized coke and pure pulverized coal, thereby improving the flexibility of the fuel of a power station boiler.

Description

Technical Field

[0001] This invention relates to the field of boiler manufacturing and design technology, specifically to a combustion system and combustion method for a dual-fuel boiler consisting of pulverized coke and pulverized coal. Background Technology

[0002] In recent years, coal chemical industry, with coal gasification as its leading sector, has developed rapidly, with projects such as coal-to-gas, coal-to-oil, coal-to-olefins, and coal-to-methanol emerging one after another, generating a large amount of chemical ash and slag.

[0003] Pulverized coal pyrolysis technology produces coal tar and coal gas, while also generating a large amount of fine coke. Coal tar can be hydrogenated to produce diesel, naphtha, and other petroleum products, and can also be upgraded to produce high-grade lubricating oils and aviation fuels. However, the large amount of fine coke generated during the production process needs to be utilized as a resource.

[0004] According to coal quality analysis, the dry ash-free volatile matter (Vdaf) of pulverized coke is approximately 10%–13%, close to that of anthracite. If pulverized coke is co-processed in a pulverized coal boiler in a thermal power plant, special technical designs are typically required for its ignition and stable combustion. Currently, a common method for utilizing pulverized coke is as a fuel in thermal power plants, blending it with coal to achieve coke utilization. However, this method also presents certain technical challenges: due to the significant difference in combustion characteristics between pulverized coke and the main fuel pulverized coal, there is air competition between the coke and pulverized coal in the furnace, leading to increased carbon content in fly ash and affecting boiler thermal efficiency. Furthermore, the primary air temperature for coke combustion is selected based on the characteristics of the main fuel pulverized coal, while the primary air temperature selected for pulverized coke is relatively low, which is detrimental to its ignition and stable combustion in the boiler furnace.

[0005] In summary, the existing method of utilizing pulverized coke involves blending it with coal as fuel in thermal power plants. This method requires special structural design or modification of the blending system and also presents technical problems such as decreased combustion stability and increased fly ash content. Summary of the Invention

[0006] The purpose of this invention is to address the existing methods of utilizing pulverized coke, which involve blending pulverized coke with coal in thermal power plants. These methods suffer from issues such as the need for special structural design or modification of the blending system, decreased combustion stability, and increased fly ash content. The invention provides a combustion system and method for a dual-fuel boiler using both pulverized coke and pulverized coal. This combustion system enables efficient and stable pure combustion of pulverized coke in power plant pulverized coal boilers. Furthermore, the system allows for rapid switching between two combustion modes: pure pulverized coke and pure pulverized coal, thus achieving a dual-fuel design for power plant boilers.

[0007] The technical solution of this invention is:

[0008] A combustion system for a dual-fuel boiler (coke and pulverized coal) includes a pulverized coal boiler 1, an air preheater 2, a secondary air fan 3, a primary air fan 4, a pulverized coal supply device, a coke supply device, an air volume measuring device A8, a primary cold air regulating damper 9, a primary hot air regulating damper A10, a primary hot air regulating damper B11, an air volume measuring device B12, a burner 13, a secondary air box 14, and a secondary air regulating damper 15; the air preheater 2 is installed in the tail flue of the pulverized coal boiler 1, and the burner 13 is installed on the side wall of the furnace of the pulverized coal boiler 1; Secondary air fan 3 is connected to the cold secondary air inlet of air preheater 2 via a cold secondary air duct. The secondary hot air outlet of air preheater 2 is connected to the secondary hot air inlet of secondary air box 14 via a secondary hot air duct. The secondary hot air outlet of secondary air box 14 is connected to the secondary hot air inlet of burner 13 via a secondary hot air duct. A secondary air regulating damper 15 is installed on the secondary hot air duct between burner 13 and secondary air box 14. Primary air fan 4 is connected to the cold primary air inlet of air preheater 2 via a cold primary air duct. Air preheater 2... The primary hot air outlet B is connected to the primary hot air inlet of the burner 13 via the primary hot air pipe B. A primary hot air regulating damper B11 and an air volume measuring device B12 are sequentially installed on the primary hot air pipe between the air preheater 2 and the burner 13. A coke feed inlet is provided on the primary hot air pipe between the air volume measuring device B12 and the burner 13. The outlet of the coke supply device is connected to the coke feed inlet on the primary hot air pipe via a coke pipe. The primary hot air outlet A of the air preheater 2 is connected to the pulverized coal supply device via the primary hot air pipe A. The device is connected to the primary hot air inlet. A primary hot air regulating valve A10 and an air volume measuring device A8 are installed sequentially on the primary hot air pipe A between the pulverized coal supply device and the air preheater 2. The outlet of the pulverized coal supply device is connected to the pulverized coal inlet of the burner 13 through a pulverized coal pipe. A primary cold air inlet is opened on the primary hot air pipe A between the primary hot air regulating valve A10 and the air volume measuring device A8. The primary blower 4 is connected to the primary cold air inlet through a primary cold air pipe. A primary cold air regulating valve 9 is installed on the primary cold air pipe.

[0009] Furthermore, the pulverized coal supply device includes a coal mill 5 and a coal feeder 6. The primary hot air outlet A of the air preheater 2 is connected to the primary hot air inlet of the coal mill 5 through a primary hot air pipe A. A primary hot air regulating valve A10 and an air volume measuring device A8 are installed sequentially on the primary hot air pipe A between the coal mill 5 and the air preheater 2. The discharge port of the coal feeder 6 is connected to the feed port of the coal mill 5. The discharge port of the coal mill 5 is connected to the primary air inlet of the burner 13 through a pulverized coal pipe.

[0010] Furthermore, it also includes an isolation door 7, which is installed on the pulverized coal pipeline between the coal mill 5 and the burner 13.

[0011] Furthermore, the coke feeding device includes a coke silo 16 and a rotary feeder valve 17. The outlet of the coke silo 16 is connected to the coke inlet on the primary hot air pipe through a coke pipe, and the rotary feeder valve 17 is installed on the coke pipe.

[0012] A combustion method for a combustion system based on a dual-fuel boiler (coke and pulverized coal) in pure coke combustion mode. In pure coke combustion mode, the primary air fan 4 sends cold primary air into the air preheater 2 at the tail end of the pulverized coal boiler 1. After heat exchange with the flue gas at the tail end of the boiler, the air becomes primary hot air. In this mode, the primary hot air regulating damper B11 is open. After the primary hot air flow is measured by the air volume measuring device B12, the coke in the coke silo 16 is fed into the primary hot air pipe through the rotary feed valve 17 to mix with the primary hot air. The mixture of coke and primary hot air is uniformly mixed and enters the burner 13 as the primary air of the pulverized coal boiler 1. The secondary air fan 3 sends the cold secondary air to the air preheater 2 at the tail of the pulverized coal boiler 1. After heat exchange with the flue gas at the tail of the boiler, the air becomes secondary hot air. The secondary hot air is introduced into the secondary air box 14 and the flow rate is regulated by the secondary air regulating valve 15. The hot secondary air and the coke primary air are injected into the furnace of the pulverized coal boiler 1 through the combustion organization of the burner 13. The coke ignites and burns, releasing heat.

[0013] Furthermore, the primary hot air temperature in the pure pulverized coke combustion mode is 300℃~330℃.

[0014] Furthermore, the temperature of the coke powder inside the coke powder silo 16 is approximately 320℃~350℃.

[0015] Furthermore, after the primary hot air is mixed with the high-temperature coke powder, the temperature of the mixture can still reach above 300°C, and the air-to-coke ratio of the primary air flow is 1.0 to 1.2.

[0016] A combustion method for a combustion system based on a dual-fuel boiler of pulverized coke and pulverized coal in pure pulverized coal mode. In pure pulverized coal mode, the primary hot air regulating valve B11 is closed, the primary hot air is regulated by the primary hot air regulating valve A10, and the primary cold air is regulated by the primary cold air regulating valve 9. The primary hot air and primary cold air after the air volume is regulated are mixed and the total primary air volume is measured by the air volume measuring device A8 before entering the coal mill 5. After the coal is ground and dried in the coal mill 5, it is sent to the burner 13 through the pulverized coal pipeline. The hot secondary air and the pulverized coal primary air are injected into the furnace of the pulverized coal boiler 1 through the combustion organization of the burner 13, and the pulverized coal ignites and burns, releasing heat.

[0017] Furthermore, in the pure pulverized coal mode, the temperature of the primary air of pulverized coal passing through burner 13 is 70℃~80℃, and the air-to-pulverized coal ratio of the primary air flow is 1.8~2.1.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] 1. The combustion system of the dual-fuel boiler of coke and pulverized coal of the present invention directly utilizes the mixture of high-temperature coke and primary hot air to organize combustion. The primary air-coke ratio is beneficial to ignition. The high primary air temperature and the appropriate air-coke ratio can achieve pure combustion of coke, ensuring excellent ignition and stable combustion characteristics of the burner.

[0020] 2. The combustion system of the dual-fuel boiler of the present invention can quickly switch between two combustion modes of pure coke combustion and pure pulverized coal combustion, thereby improving the fuel flexibility of power plant boilers.

[0021] 3. The combustion system layout scheme of the dual-fuel boiler of pulverized coke and pulverized coal of the present invention is relatively simple, can be used to retrofit existing boiler units, has low retrofit investment cost, high overall return on investment, and can be promoted for engineering application. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the combustion system of the dual-fuel boiler for pulverized coke and pulverized coal of the present invention. Detailed Implementation

[0023] Specific implementation method one: Combining Figure 1This embodiment describes a combustion system for a dual-fuel boiler (coke and pulverized coal), comprising a pulverized coal boiler 1, an air preheater 2, a secondary air fan 3, a primary air fan 4, a pulverized coal supply device, a coke supply device, an air volume measuring device A8, a primary cold air regulating damper 9, a primary hot air regulating damper A10, a primary hot air regulating damper B11, an air volume measuring device B12, a burner 13, a secondary air box 14, and a secondary air regulating damper 15. The air preheater 2 is installed in the tail flue of the pulverized coal boiler 1, and the furnace side of the pulverized coal boiler 1... A burner 13 is installed on the wall; a secondary air fan 3 is connected to the cold secondary air inlet of the air preheater 2 through a cold secondary air duct, the secondary hot air outlet of the air preheater 2 is connected to the secondary hot air inlet of the secondary air box 14 through a secondary hot air duct, the secondary hot air outlet of the secondary air box 14 is connected to the secondary hot air inlet of the burner 13 through a secondary hot air duct, and a secondary air regulating damper 15 is installed on the secondary hot air duct between the burner 13 and the secondary air box 14; a primary air fan 4 is connected to the cold primary air inlet of the air preheater 2 through a cold primary air duct. The primary hot air outlet B of the air preheater 2 is connected to the primary hot air inlet of the burner 13 via a primary hot air duct B. A primary hot air regulating damper B11 and an airflow measuring device B12 are sequentially installed on the primary hot air duct between the air preheater 2 and the burner 13. A coke feed inlet is provided on the primary hot air duct between the airflow measuring device B12 and the burner 13. The outlet of the coke supply device is connected to the coke feed inlet on the primary hot air duct via a coke pipe. The primary hot air outlet A of the air preheater 2 is connected to the primary hot air inlet of the burner 13 via a primary hot air duct A. The primary hot air inlet of the pulverized coal supply device is connected, and a primary hot air regulating valve A10 and an air volume measuring device A8 are installed sequentially on the primary hot air pipe A between the pulverized coal supply device and the air preheater 2. The outlet of the pulverized coal supply device is connected to the pulverized coal inlet of the burner 13 through a pulverized coal pipe. A primary cold air inlet is opened on the primary hot air pipe A between the primary hot air regulating valve A10 and the air volume measuring device A8. The primary blower 4 is connected to the primary cold air inlet through a primary cold air pipe, and a primary cold air regulating valve 9 is installed on the primary cold air pipe.

[0024] Specific Implementation Method Two: Combining Figure 1 This embodiment describes a pulverized coal supply device comprising a coal mill 5 and a coal feeder 6. The primary hot air outlet A of the air preheater 2 is connected to the primary hot air inlet of the coal mill 5 via a primary hot air pipe A. A primary hot air regulating damper A10 and an air volume measuring device A8 are sequentially installed on the primary hot air pipe A between the coal mill 5 and the air preheater 2. The discharge port of the coal feeder 6 is connected to the feed port of the coal mill 5. The discharge port of the coal mill 5 is connected to the primary air inlet of the burner 13 via a pulverized coal pipe. This configuration is as described above. Other components and connections are the same as in specific embodiment one.

[0025] Specific implementation method three: Combining Figure 1This embodiment further includes an isolation door 7, which is installed on the pulverized coal pipeline between the coal mill 5 and the burner 13. With this configuration, other components and connections are the same as in specific embodiments one or two.

[0026] Specific implementation method four: Combination Figure 1 This embodiment describes a coke feeder that includes a coke silo 16 and a rotary feeder valve 17. The outlet of the coke silo 16 is connected to the coke inlet on a primary hot air pipe via a coke pipe, and the rotary feeder valve 17 is installed on the coke pipe. With this configuration, other components and connections are the same as in specific embodiments one, two, or three.

[0027] Specific Implementation Method Five: Combining Figure 1 This embodiment describes a combustion method in pure coke combustion mode of a dual-fuel boiler (coke and pulverized coal) as described in embodiment four. In pure coke combustion mode, the primary air fan 4 sends cold primary air into the air preheater 2 at the tail of the pulverized coal boiler 1. After heat exchange with the flue gas at the tail of the boiler, the air becomes primary hot air. In this mode, the primary hot air regulating valve B11 is open. After the primary hot air is measured by the airflow measuring device B12, the coke in the coke silo 16 is fed into the coke silo through the rotary feed valve 17. The coke is uniformly mixed with the primary hot air in the primary hot air duct. The mixture of coke and primary hot air is then used as the primary air for the pulverized coal boiler 1 and enters the burner 13. The secondary air fan 3 sends cold secondary air to the air preheater 2 at the tail end of the pulverized coal boiler 1. After heat exchange with the flue gas at the tail end of the boiler, the secondary air becomes secondary hot air. The secondary hot air is introduced into the secondary air box 14, and its flow is regulated by the secondary air regulating damper 15. The hot secondary air and the coke primary air are then injected into the furnace of the pulverized coal boiler 1 through the combustion mechanism of the burner 13. The coke ignites and burns, releasing heat. Other components and connections are the same as in specific embodiments one, two, three, or four.

[0028] In this embodiment, the coke powder is produced by a coke pyrolysis process and conveyed into the coke powder silo 16. The temperature of the coke powder in the coke powder silo 16 is approximately 320°C to 350°C. After the primary hot air is mixed with the high-temperature coke powder, the temperature of the mixture can still reach >300°C. Furthermore, the temperature of the primary hot air is higher than that of the primary air in the direct-blown pulverizing system, and the density of the primary hot air is lower, making the air-to-coke ratio (approximately 1.0 to 1.2) more suitable for the ignition and combustion of the coke powder.

[0029] Specific Implementation Method Six: Combination Figure 1 This embodiment describes a pure pulverized coke combustion mode where the primary hot air temperature is 300℃~330℃. Other components and connections are the same as in specific embodiments one, two, three, four, or five.

[0030] Specific implementation method seven: Combination Figure 1 In this embodiment, the temperature of the coke powder in the coke powder silo 16 is approximately 320℃ to 350℃. Other components and connections are the same as in specific embodiments one, two, three, four, five, or six.

[0031] Specific implementation method eight: Combination Figure 1 This embodiment describes a method where, after the primary hot air is mixed with high-temperature coke powder, the temperature of the mixture can still reach above 300°C, and the air-to-coke ratio of the primary air flow is 1.0 to 1.2. Other components and connections are the same as in specific embodiments one, two, three, four, five, six, or seven.

[0032] Specific Implementation Method Nine: Combining Figure 1 This embodiment describes a combustion method in pure pulverized coal mode for the combustion system of a dual-fuel boiler (coke and pulverized coal) as described in embodiment four. In pure pulverized coal mode, the primary hot air regulating valve B11 is closed, the primary hot air volume is regulated by the primary hot air regulating valve A10, and the primary cold air volume is regulated by the primary cold air regulating valve 9. The regulated primary hot air and primary cold air are mixed, and after the total primary air volume is measured by the air volume measuring device A8, the mixture enters the coal mill 5. After the coal is ground and dried in the coal mill 5, it is sent to the burner 13 through the pulverized coal pipeline. The hot secondary air and the pulverized coal primary air are injected into the furnace of the pulverized coal boiler 1 through the combustion mechanism of the burner 13, where the pulverized coal ignites and burns, releasing heat. Other components and connections are the same as in embodiments one, two, three, four, five, six, seven, or eight.

[0033] In this embodiment, the pure pulverized coal mode is arranged in the same way as the direct-fired pulverizing system. The primary air temperature is selected according to the characteristics of pulverized coal, and is generally only 70℃~80℃. The air-to-pulverized coal ratio of the primary air flow is usually about 1.8~2.1.

[0034] When switching between pure pulverized coke combustion mode and pure pulverized coal combustion mode, it is necessary to switch each air damper or isolation door to isolate the equipment of the two systems from each other.

[0035] Specific Implementation Method Ten: Combining Figure 1 In this embodiment, under the pure pulverized coal combustion mode, the primary air temperature of the pulverized coal passing through the burner 13 is 70℃~80℃, and the air-to-pulverized coal ratio of the primary air flow is 1.8~2.1. Other components and connections are the same as in embodiments one, two, three, four, five, six, seven, eight, or nine.

[0036] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A combustion system for a dual-fuel boiler (coke and pulverized coal), characterized in that: It includes a pulverized coal boiler (1), an air preheater (2), a secondary air fan (3), a primary air fan (4), a pulverized coal supply device, a coke supply device, an air volume measuring device A (8), a primary cold air regulating damper (9), a primary hot air regulating damper A (10), a primary hot air regulating damper B (11), an air volume measuring device B (12), a burner (13), a secondary air box (14), and a secondary air regulating damper (15); the tail flue of the pulverized coal boiler (1) is equipped with an air preheater (2), and the furnace sidewall of the pulverized coal boiler (1) is equipped with a burner (13); the secondary air fan (3) The air preheater (2) is connected to the cold secondary air inlet via a cold secondary air duct. The secondary hot air outlet of the air preheater (2) is connected to the secondary hot air inlet of the secondary air box (14) via a secondary hot air duct. The secondary hot air outlet of the secondary air box (14) is connected to the secondary hot air inlet of the burner (13) via a secondary hot air duct. A secondary air regulating damper (15) is installed on the secondary hot air duct between the burner (13) and the secondary air box (14). The primary air fan (4) is connected to the cold primary air inlet of the air preheater (2) via a cold primary air duct. The air preheater ( 2) The primary hot air outlet B is connected to the primary hot air inlet of the burner (13) through the primary hot air pipe B. A primary hot air regulating valve B (11) and an air volume measuring device B (12) are installed sequentially on the primary hot air pipe between the air preheater (2) and the burner (13). A coke feed inlet is opened on the primary hot air pipe between the air volume measuring device B (12) and the burner (13). The outlet of the coke supply device is connected to the coke feed inlet on the primary hot air pipe through the coke pipe. The primary hot air outlet A of the air preheater (2) is connected to the pulverized coal through the primary hot air pipe A. The primary hot air inlet of the supply device is connected, and a primary hot air regulating valve A (10) and an air volume measuring device A (8) are installed sequentially on the primary hot air pipe A between the pulverized coal supply device and the air preheater (2). The outlet of the pulverized coal supply device is connected to the pulverized coal inlet of the burner (13) through the pulverized coal pipe. A primary cold air inlet is opened on the primary hot air pipe A between the primary hot air regulating valve A (10) and the air volume measuring device A (8). The primary blower (4) is connected to the primary cold air inlet through the primary cold air pipe. A primary cold air regulating valve (9) is installed on the primary cold air pipe. The pulverized coal supply device includes a coal mill (5) and a coal feeder (6). The primary hot air outlet A of the air preheater (2) is connected to the primary hot air inlet of the coal mill (5) through a primary hot air pipe A. A primary hot air regulating valve A (10) and an air volume measuring device A (8) are installed sequentially on the primary hot air pipe A between the coal mill (5) and the air preheater (2). The discharge port of the coal feeder (6) is connected to the feed port of the coal mill (5). The discharge port of the coal mill (5) is connected to the primary air inlet of the burner (13) through a pulverized coal pipe. The coke feeder includes a coke silo (16) and a rotary feeder valve (17). The outlet of the coke silo (16) is connected to the coke feed inlet on the primary hot air pipe through a coke pipe. The rotary feeder valve (17) is installed on the coke pipe.

2. The combustion system of the dual-fuel boiler (coke and pulverized coal) according to claim 1, characterized in that: It also includes an isolation door (7), which is installed on the pulverized coal pipeline between the coal mill (5) and the burner (13).

3. A combustion method for a combustion system based on a dual-fuel boiler (coke and pulverized coal) as described in claim 2, in pure coke combustion mode, characterized in that: In the pure pulverized coke combustion mode, the primary air fan (4) sends cold primary air into the air preheater (2) at the tail of the pulverized coal boiler (1). After heat exchange with the flue gas at the tail of the boiler, it becomes primary hot air. In this mode, the primary hot air regulating valve B (11) is opened. After the primary hot air is measured by the air volume measuring device B (12), the pulverized coke in the pulverized coke silo (16) is fed into the primary hot air pipe through the rotary feed valve (17) and mixed evenly with the primary hot air. The mixed pulverized coke and primary hot air are mixed The compound is used as the primary air of the pulverized coal boiler (1) and enters the burner (13); the secondary air fan (3) sends the cold secondary air into the air preheater (2) at the tail of the pulverized coal boiler (1). After heat exchange with the flue gas at the tail of the boiler, it becomes secondary hot air. The secondary hot air is introduced into the secondary air box (14). After the flow rate is regulated by the secondary air regulating valve (15), the hot secondary air and the pulverized coke primary air pass through the combustion organization of the burner (13) and are jointly injected into the furnace of the pulverized coal boiler (1). The pulverized coke ignites and burns, releasing heat. The primary hot air temperature in the pure pulverized coke combustion mode is 300℃~330℃; The temperature of the coke powder inside the coke powder silo (16) is approximately 320℃~350℃; After the primary hot air is mixed with the high-temperature coke powder, the temperature of the mixture can still reach above 300℃, and the air-to-coke ratio of the primary air flow is 1.0 to 1.

2.

4. A combustion method for a combustion system based on a dual-fuel boiler (coke and pulverized coal) as described in claim 2, in pure pulverized coal combustion mode, characterized in that: In pure pulverized coal mode, the primary hot air regulating valve B (11) is closed, the primary hot air is regulated by the primary hot air regulating valve A (10), and the primary cold air is regulated by the primary cold air regulating valve (9). After the primary hot air and primary cold air are regulated, they are mixed and the total primary air volume is measured by the air volume measuring device A (8) before entering the coal mill (5). After the coal is ground and dried in the coal mill (5), it is sent to the burner (13) through the pulverized coal pipeline. The hot secondary air and the pulverized coal primary air are injected into the furnace of the pulverized coal boiler (1) through the combustion organization of the burner (13), and the pulverized coal ignites and burns to release heat. In the pure pulverized coal mode, the temperature of the primary air of pulverized coal passing through the burner (13) is 70℃~80℃, and the air-to-pulverized coal ratio of the primary air flow is 1.8~2.1.

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