Pulverized coal fluidized bed gasifier capable of improving circulation ratio and operation method

Abstract

The invention relates to a pulverized coal fluidized bed gasifier and an operation method, in particular to a pulverized coal fluidized bed gasifier capable of improving circulation ratio and an operation method. The invention aims to solve the problems of high specific coal consumption and low carbon conversion rate caused by the fact that gas in the gasification furnace flows back into a blanking pipe and the circulating ratio of fly ash is too low. The material return unit is installed in the material return pipe, the pressure is reduced and the speed is increased after a water vapor laval nozzle is sprayed, and circulating ash at the inlet of the gradually-shrinking inlet section is injected into the second throat pipe. Mixed working media are violently mixed in the second throat pipe, and the speed is reduced after energy and momentum exchange, so that the pressure in the material return pipe is increased, and airflow in the furnace is prevented from reversely flowing to the cyclone separator; meanwhile, more fly ash particles are introduced into the gasification furnace, so that the carbon content of fly ash is reduced, the amount of ash circulating back to the gasification furnace is increased, and the carbon conversion rate is improved. The method is applied to the fields of production of chemical synthesis or fuel oil synthesis feed gas, industrial fuel gas, civil gas, metallurgical reducing gas, combined cycle power generation fuel gas and the like.

Description

Technical Field

[0001] This invention relates to a pulverized coal fluidized bed gasifier and its operating method, specifically to a pulverized coal fluidized bed gasifier and its operating method with improved circulation ratio, belonging to the technical field of fluidized bed gasifiers. Background Technology

[0002] Coal has always been my country's primary energy source, accounting for 60% to 70% of the country's energy consumption, and this trend is unlikely to change in the short term. Therefore, developing coal gasification technology is essential.

[0003] Current coal gasification technologies are mainly classified into four categories: moving bed gasification, fluidized bed gasification, entrained gasification, and molten bed gasification. Circulating fluidized bed gasifier technology, as a clean coal gasification technology, has significant advantages in reducing pollutant emissions and improving the environment. Due to its advantages such as the ability to utilize low-quality coals like lignite for gasification, high gasification intensity, and the absence of tar and phenols, it is widely used in the production of industrial fuel gas, city fuel gas, and chemical syngas.

[0004] The Ende furnace is one of the most representative circulating fluidized bed gasifiers, with advantages such as a wide range of raw coal applications, low production costs, strong cleaning capabilities, and minimal environmental pollution.

[0005] like Figure 1 As shown, the Ende gasifier currently has the following defects in use:

[0006] 1. The pressure at the bottom of the gasifier is greater than the pressure inside the feed pipe. Gas inside the gasifier flows back into the feed pipe, causing most of the fly ash particles to be unable to return smoothly from the feed pipe to the bottom of the gasifier, resulting in incomplete gasification.

[0007] Second, the circulating material relies solely on gravity to return naturally through the return pipe, resulting in poor return efficiency and an excessively low fly ash circulation rate. Consequently, the syngas produced by current pulverized coal fluidized bed gasifiers contains a high amount of fly ash, with a high carbon content (30%~40%), affecting the stable operation of the gasification system. Furthermore, not all emitted fly ash can be reused, leading to high specific coal consumption and low carbon conversion rate in the gasifier, resulting in significant energy waste.

[0008] In summary, how to propose a novel pulverized coal fluidized bed gasifier and its operation method to address the aforementioned technical problems has become a pressing issue for those skilled in the art. Summary of the Invention

[0009] To address the shortcomings of the prior art, this invention provides a pulverized coal fluidized bed gasifier with an increased circulation ratio.

[0010] The technical solution of the present invention is: a pulverized coal fluidized bed gasifier with improved circulation ratio, comprising a gasifier, a cyclone separator and a steam pipeline.

[0011] The lower conical wall of the gasifier has pulverized coal inlet and gasifying agent inlet sequentially from top to bottom; the outlet at the top of the gasifier is connected to the feed inlet on the side wall of the cyclone separator.

[0012] The discharge port at the bottom of the cyclone separator is connected to the cone at the bottom of the gasifier through a return pipe, and the outlet of the steam pipe is inserted into the return pipe.

[0013] Furthermore, it also includes a material return unit, which comprises a Laval nozzle and a Venturi nozzle.

[0014] Both the Laval nozzle and the Venturi tube are coaxially arranged inside the return pipe. The inlet of the Laval nozzle is coaxially fixed to the outlet of the steam pipe, and there is a distance L between the outlet of the Laval nozzle and the inlet of the Venturi tube.

[0015] Furthermore, a top pressure measuring hole is opened on the conical wall at the top of the gasifier.

[0016] Furthermore, a bottom pressure measuring hole is opened on the cone wall at the bottom of the gasifier.

[0017] Furthermore, a pressure testing hole for the return pipe is provided on the return pipe.

[0018] Furthermore, the Laval nozzle includes a converging section, a first throat, and an expanding section that are coaxially fixed together in sequence, with the converging section being coaxially fixed to the outlet of the steam pipe.

[0019] Furthermore, the venturi tube includes a converging inlet section, a second throat, and a diverging outlet section that are coaxially fixed together in sequence, with a distance L between the converging inlet section and the diverging section of the Val nozzle.

[0020] This invention also provides an operation method for a pulverized coal fluidized bed gasifier with improved circulation ratio, the method specifically comprising the following steps:

[0021] Step 1: The steam volume in the steam pipeline accounts for 2% to 3% of the total steam volume of the gasifier during operation. The steam velocity ejected from the expansion section is maintained at 500 m / s to 1000 m / s, and the steam pressure is maintained at 0.5 MPa to 1 MPa.

[0022] Step 2: Monitor the pressure P at the pressure test hole of the return pipe. 13 And the pressure P2 at the bottom pressure measuring hole, ensure P 13 >P2;

[0023] Step 3: The velocity of the mixture of steam and circulating ash at the outlet of the return pipe is maintained at 3m / s to 5m / s.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] 1. The pressure inside the return pipe 10 is greater than the pressure at the bottom of the gasifier 4, which effectively prevents the airflow inside the gasifier 4 from flowing back into the return pipe 10 and the cyclone separator 8.

[0026] In the existing Ende coal gasifier, pulverized coal is fed into the cone section at the bottom of the gasifier through the coal feed inlet. Superheated steam (temperature around 250℃, pressure around 0.7MPa) and gasifying agent (temperature around 130℃, pressure around 0.05MPa) enter the gasifier through the gasifying agent inlet. The mixture of gasifying agent and pulverized coal undergoes gasification during its flow from the bottom to the top of the gasifier. The gasifying agent needs to overcome the gravity of the material and the resistance of particle collision, which makes the pressure at the pressure measuring point at the top of the gasifier 500~1000Pa lower than the pressure at the pressure measuring point at the bottom of the gasifier.

[0027] The gas, carrying a large amount of ash, enters the cyclone separator from the top of the gasifier. During this process, there is a certain pressure drop in the bed and pressure loss inside the separator. At the same time, the friction between the rotating gas and ash mixture and the wall further reduces the pressure of the gas and ash mixture, resulting in a pressure drop of 400-500 Pa in the return pipe. This makes the pressure at the pressure measuring point in the return pipe 900-1500 Pa lower than the pressure at the pressure measuring point at the bottom of the gasifier. Ultimately, the pressure at the discharge port at the bottom of the cyclone separator is 900-1500 Pa lower than the pressure at the pressure measuring point at the bottom of the gasifier, causing the gas in the gasifier to backflow into the return pipe.

[0028] With this invention, a return unit 12 is installed inside the return pipe 10. In the return unit 12, the potential energy of the high-temperature, high-pressure steam from the Laval nozzle 20 is converted into kinetic energy, causing a decrease in pressure and temperature, while the velocity increases instantaneously, raising the steam velocity to 500-1000 m / s. The steam ejected from the expansion section 16 has a pressure less than P. 10 (The pressure in the area between the inlet of the return pipe 10 and the Laval nozzle 20) guides the circulating ash at the inlet of the tapered inlet section 17 into the second throat 18.

[0029] The working fluid mixes violently within the second throat 18, exchanging energy and momentum. At the outlet of the second throat 18, the velocity of the working fluid decreases due to the effect of the gradually expanding outlet section 19, increasing the pressure within the return pipe 10 by 1000-1700 Pa. This results in the pressure at the pressure measuring hole 13 of the return pipe being 100-200 Pa higher than the pressure at the bottom pressure measuring hole 2. Ultimately, this causes the pressure at the discharge port of the cyclone separator 8 to be 100-200 Pa higher than the pressure at the bottom pressure measuring hole 2, effectively preventing the airflow in the furnace from flowing back into the cyclone separator 8. This ensures that the airflow direction within the return pipe 10 is from the discharge port at the bottom of the cyclone separator 8 to the outlet of the return pipe 10.

[0030] 2. While improving the separation efficiency of the cyclone separator 8, the problems of intermittent ash entering the gasifier 4 and blockage of the return pipe 10 are eliminated.

[0031] In the existing pulverized coal gasifier in Ende, the pressure at the discharge port at the bottom of the cyclone separator is 900~1500Pa lower than the pressure at the pressure measuring point at the bottom of the gasifier. As a result, the gas in the gasifier flows back into the return pipe, causing most of the fly ash particles to be unable to return smoothly to the bottom of the gasifier from the return pipe, thus reducing the particle return rate.

[0032] Large fly ash particles separated by the cyclone separator accumulate at the bottom of the return pipe, causing the material layer height inside the pipe to continuously increase. As the weight of the material increases, the pressure also increases. Only when the pressure exceeds the pressure at the bottom of the gasifier can the particles enter the gasifier. At this point, the gas inside the gasifier flows back into the return pipe, causing the particles to re-aggregate and circulate again under gravity, resulting in intermittent entry of circulating ash into the gasifier. This causes pressure fluctuations inside the furnace, unstable particle flow, and affects the normal operation of the gasifier.

[0033] After adopting this invention, under the action of the return material unit 12, the pressure in the return material pipe 10 is increased by 1000~1700Pa, and the pressure at the pressure measuring hole 13 of the return material pipe is 100~200Pa higher than the pressure at the bottom pressure measuring hole 2. Ultimately, the pressure at the discharge port of the cyclone separator 8 is 100~200Pa higher than the pressure at the bottom pressure measuring hole 2, preventing the gas flow in the furnace from flowing back into the cyclone separator 8, effectively improving the efficiency of the cyclone separator 8. This ensures that the fly ash particles separated by the cyclone separator 8 continuously and stably enter the bottom of the gasifier 4, eliminating the problems of intermittent entry of circulating material into the gasifier 4 and blockage of the return material pipe 10.

[0034] 3. Introduce more fly ash particles into the gasifier 4 for regasification, thereby reducing the carbon content of the fly ash discharged from the gasifier 4, increasing the amount of ash recycled back to the gasifier 4, reducing the specific coal consumption of the gasifier 4, and improving the carbon conversion rate.

[0035] In the existing pulverized coal gasifier in Ende, the separation efficiency of the cyclone separator is about 85%. The coal gas generated in the gasifier carries ash into the cyclone separator. 15% of the ash with a particle size of 0~20μm flows out from the gas phase outlet of the cyclone separator with the coal gas, while the remaining 85% of the ash with a particle size between 20~1200μm is separated and enters the return pipe 10 from the discharge port at the bottom of the cyclone separator 8.

[0036] However, because the pressure at the discharge port at the bottom of the cyclone separator is 900-1500 Pa lower than the pressure at the pressure measuring point at the bottom of the gasifier, the gas at the bottom of the gasifier flows back into the cyclone separator through the return pipe. This part of the gas carries ash particles with a diameter of 20-400 μm and flows out from the gas phase outlet of the cyclone separator. The remaining ash particles with a diameter of 400-1200 μm are able to enter the bottom of the gasifier for recirculation and gasification through the return pipe because their gravity is greater than the buoyancy of the gas. This results in a reduction in the amount of ash circulating back into the gasifier, which increases the specific coal consumption of the gasifier and reduces the carbon conversion rate, resulting in the carbon content of the fly ash discharged from the gasifier being around 30%-40%.

[0037] After adopting the present invention, the gas generated by the gasifier 4 carries ash into the cyclone separator 8. The ash with a particle size of 0~20μm flows out from the gas phase outlet of the cyclone separator with the gas. Under the action of the return unit 12, the pressure at the discharge port of the cyclone separator 8 is 100~200Pa higher than the pressure at the bottom pressure measuring hole 2. The phenomenon of gas carrying ash backflow from the bottom of the gasifier 4 to the cyclone separator 8 through the return pipe 10 will not occur. This allows the remaining ash with a particle size of 20~1200μm to enter the gasifier 4 for circulating gasification through the return pipe 10. This increases the amount of circulating ash. Through circulating gasification, the combustibles in the fly ash further participate in combustion, and finally the carbon content of the fly ash discharged from the gasifier 4 is reduced to 10%~20%.

[0038] 4. It has no impact on the gasification characteristics within gasifier 4 or on the safe operation of gasifier 4.

[0039] In this invention, the working medium flowing through the Laval nozzle 20 is water vapor from the gasifying agent, rather than air or nitrogen, and the amount of steam accounts for 2% to 3% of the total operating steam of the gasifier. This has little impact on the gasifying agent in the gasifier 4. At the same time, the steam velocity at the outlet of the return pipe 10 is maintained at 3m / s to 5m / s, which is relatively low and will not affect the combustion of pulverized coal at the bottom of the gasifier. Therefore, this invention has no impact on the gasification characteristics and safe operation of the gasifier 4.

[0040] 5. The return material unit 12 can assist in the start-up of the gasifier 4.

[0041] During the ignition process of the existing gasifier, the gas flow will flow from its outlet to the cyclone separator 8 in the return pipe, causing the gas flow to backflow during the ignition process. It is necessary to install a gate valve in the return pipe to control the backflow of the gas flow.

[0042] After adopting the present invention, under the action of the return unit 12, the velocity of the mixed working fluid is reduced at the outlet of the second throat 18 due to the action of the gradually expanding outlet section 19, which increases the pressure in the return pipe 10 by 1000~1700Pa, and the pressure value P at the pressure measuring hole 13 of the return pipe is increased. 13The pressure value P2 at the bottom pressure measuring hole 2 is 100~200Pa higher than that at the bottom pressure measuring hole 2, which ultimately makes the pressure at the discharge port at the bottom of the cyclone separator 8 100~200Pa higher than that at the bottom pressure measuring hole 2. This effectively prevents the airflow in the furnace from flowing back into the cyclone separator 8. As a result, during the gasifier ignition process, there is no need for a gate valve, and there will be no backflow of airflow in the return pipe 10. Attached Figure Description

[0043] Figure 1 This is a schematic diagram illustrating the defects of the existing Ende gasifier technology;

[0044] Figure 2 This is a schematic diagram of the present invention;

[0045] Figure 3 This is a schematic diagram of the material return unit 12 in this invention.

[0046] In the diagram: 1. Gasifying agent inlet; 2. Bottom pressure measuring hole; 3. Pulverized coal inlet; 4. Gasifier; 5. Top pressure measuring hole; 8. Cyclone separator; 10. Return pipe; 12. Return unit; 13. Return pipe pressure measuring hole; 14. Converging section; 15. First throat; 16. Expanding section; 17. Gradually narrowing inlet section; 18. Second throat; 19. Gradually expanding outlet section; 20. Laval nozzle; 21. Sealing sleeve; 22. Steam pipe; 23. Venturi tube. Detailed Implementation

[0047] To make the objectives, features, and advantages of the present invention more apparent and understandable, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0048] Specific implementation method one: Combining Figure 2 and Figure 3 This embodiment describes a pulverized coal fluidized bed gasifier with an increased circulation ratio, comprising a gasifier 4, a cyclone separator 8, and a steam pipe 22.

[0049] The lower conical wall of the gasifier 4 has a pulverized coal inlet 3 and a gasifying agent inlet 1 sequentially opened from top to bottom; the outlet at the top of the gasifier 4 is connected to the feed inlet on the side wall of the cyclone separator 8.

[0050] The discharge port at the bottom of the cyclone separator 8 is connected to the cone at the bottom of the gasifier 4 through the return pipe 10, and the outlet of the steam pipe 22 is inserted into the return pipe 10.

[0051] Furthermore, it also includes a return unit 12, which includes a Laval nozzle 20 and a Venturi nozzle 23.

[0052] Both the Laval nozzle 20 and the Venturi tube 23 are coaxially arranged inside the return pipe 10. The inlet of the Laval nozzle 20 is coaxially fixed to the outlet of the steam pipe 22, and there is a distance L between the outlet of the Laval nozzle 20 and the inlet of the Venturi tube 23.

[0053] Specific Implementation Method Two: Combining Figure 2 and Figure 3 In this embodiment, a sealing sleeve 21 is fitted onto the steam pipe 22. This is done because the steam pipe 22 has bends and its internal steam pressure is high. To prevent steam from squeezing and impacting the steam pipe 22, the sealing sleeve 21 is installed to prevent damage to the steam pipe 22 and steam leakage.

[0054] The other components and connections are the same as in Specific Implementation Method 1.

[0055] Specific implementation method three: Combining Figure 2 and Figure 3 To illustrate this embodiment, a top pressure measuring hole 5 is provided on the conical wall at the top of the gasifier 4.

[0056] Furthermore, a bottom pressure measuring hole 2 is opened on the cone wall at the bottom of the gasifier 4, and the bottom pressure measuring hole 2 is located between the gasifying agent inlet 1 and the pulverized coal inlet 3.

[0057] Furthermore, a pressure testing hole 13 for the return pipe 10 is provided.

[0058] The other components and connections are the same as in specific implementation method one or two.

[0059] Specific implementation method four: Combination Figure 2 and Figure 3 This embodiment describes a Laval nozzle 20 comprising a converging section 14, a first throat 15, and an expanding section 16 that are coaxially fixed together in sequence. The converging section 14 is coaxially fixed to the outlet of the steam pipe 22.

[0060] Furthermore, the venturi tube 23 includes a tapered inlet section 17, a second throat section 18, and a tapered outlet section 19 that are coaxially fixed together in sequence, with a distance L between the tapered inlet section 17 and the expansion section 16 of the Val nozzle 20.

[0061] The other components and connections are the same as those in specific implementation methods one, two, or three.

[0062] Specific Implementation Method Five: Combining Figure 2 and Figure 3 This embodiment describes an operation method for a pulverized coal fluidized bed gasifier with an increased circulation ratio, which is carried out according to the following steps:

[0063] Step 1: The steam volume in steam pipe 22 accounts for 2% to 3% of the total steam volume of the gasifier during operation. The steam velocity ejected from expansion section 16 is maintained at 500m / s to 1000m / s, and the steam pressure is maintained at 0.5MPa to 1MPa.

[0064] Step 2: Monitor the pressure P at pressure test hole 13 on the return pipe. 13 And the pressure P2 at the bottom pressure measuring hole 2, ensure P 13 >P2; Preferably, the pressure value P at the pressure measuring hole 13 of the return pipe is... 13 The pressure value P2 at the bottom pressure measuring hole 2 is 100~200 Pa higher.

[0065] This configuration ensures that the pressure at the discharge port at the bottom of the cyclone separator 8 is 100-200 Pa higher than the pressure at the bottom pressure measuring hole 2.

[0066] Step 3: The velocity of the mixture of steam and circulating ash at the outlet of return pipe 10 is maintained at 3m / s to 5m / s.

[0067] Example

[0068] A coal chemical company uses an Ende coal gasifier with a designed gas production capacity of 20,000 Nm3 / h to produce coal gas. The gasifier operates for approximately 8,000 hours per year. During operation, the gasifier produces 3 tons of ash per hour, with a carbon content of 35% in the fly ash.

[0069] After adopting the present invention, the steam volume of the Laval nozzle 20 is 2% of the total steam volume of the gasifier, the steam pressure is 0.8 MPa, and the steam flow rate is 800 m / s.

[0070] The mixture of steam and circulating ash (hereinafter referred to as the mixed working fluid) is maintained at 3 m / s at the outlet of the return pipe 10 after passing through the Venturi tube 23. The pressure at the bottom pressure measuring hole 2 is 1000 Pa lower than the pressure at the top pressure measuring hole 5. The pressure at the return pipe pressure measuring hole 13 is 150 Pa higher than the pressure at the bottom pressure measuring hole 2. There is no backflow of air in the return pipe 10. Finally, the gasifier operates stably, producing 2.294 tons of ash per hour. The carbon content of the fly ash is 15%, and the coal saving per hour is 1.177 tons.

[0071] Therefore, the present invention effectively solves the problems of high carbon content in fly ash, resulting in high specific coal consumption and low carbon conversion rate in the operation of the Ende pulverized coal gasifier.

[0072] Working principle

[0073] Pressure gauges are connected to the bottom pressure measuring hole 2, the top pressure measuring hole 5, and the return pipe pressure measuring hole 13 to measure the pressure at the corresponding positions.

[0074] Pulverized coal enters the gasifier 4 through pulverized coal inlet 3. A gasifying agent composed of air (or oxygen) and superheated steam is injected into the gasifier 4 through gasifying agent inlet 1. The pulverized coal and gasifying agent mix, burn, and gasify inside the gasifier 4. After combustion, smaller fly ash particles, along with syngas, enter the cyclone separator 8. Ash particles with a diameter of 0-20 μm flow out from the gas phase outlet of the cyclone separator 8 with the coal gas. The remaining ash particles with a diameter of 20-1200 μm are separated and enter the return pipe 10 through the discharge port at the bottom of the cyclone separator 8. Under its own gravity and pressure, the ash circulates and burns within the gasifier 4 through the outlet of the return pipe 10 (hereinafter referred to as circulating ash). During normal operation of the gasifier 4, the pressure in the area between the inlet of the return pipe 10 and the Laval nozzle 20 is P. 10 .

[0075] In the return section 12, high-temperature, high-pressure steam flows into the converging section 14. As the cross-sectional area of ​​the converging section 14 gradually decreases, the steam velocity increases with the decrease in nozzle cross-sectional area, converting potential energy into kinetic energy, causing a pressure drop, a temperature decrease, and consequently, a sudden increase in velocity. The steam continuously accelerates into the first throat 15, where its velocity increases from subsonic to supersonic. The supersonic steam then passes through the expansion section 16, where the steam velocity increases with the increase in the cross-sectional area of ​​the expansion section 16, while the steam pressure continues to decrease. The steam velocity ejected from the expansion section 16 is 500 m / s to 1000 m / s, and the steam pressure is 0.5 MPa to 1 MPa. Therefore, the steam pressure ejected from the expansion section 16 is less than P. 10 The circulating ash at the inlet of the tapering inlet section 17 is injected into the second throat tube 18.

[0076] The working fluid mixes violently within the second throat 18, exchanging energy and momentum. At the outlet of the second throat 18, the velocity of the working fluid decreases due to the effect of the gradually expanding outlet section 19, increasing the pressure in the return pipe 10 by 1000~1700 Pa. Simultaneously, the pressure value P at the pressure measuring port 13 of the return pipe also increases. 13 The pressure value is greater than that at the bottom pressure measuring hole 2; this effectively prevents the airflow in the gasifier 4 from flowing back to the cyclone separator 8, ensuring that the airflow in the return pipe 10 flows from the discharge port at the bottom of the cyclone separator 8 to the outlet of the return pipe 10. The mixed working fluid then flows out from the gradually expanding outlet section 19 and subsequently enters the gasifier 4 through the outlet of the return pipe 10.

[0077] During the above operation, the working medium flowing through the Laval nozzle 20 is water vapor, not air or nitrogen, which has no impact on the safe operation of the gasifier 4. Moreover, the water vapor flowing through the Laval nozzle 20 comes from the gasifying agent, and the amount of steam accounts for 2% to 3% of the total steam volume of the gasifier. This has a very small impact on the gasifying agent in the gasifier 4. At the same time, the steam velocity at the outlet of the return pipe 10 is maintained at 3m / s to 5m / s, which is relatively low and will not affect the combustion of pulverized coal at the bottom of the gasifier.

[0078] The present invention has been disclosed above with reference to preferred embodiments, but it is not intended to limit the present invention. Any simple modifications, equivalent changes and alterations made by those skilled in the art to the above embodiments without departing from the technical solution of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A pulverized coal fluidized bed gasifier with improved circulation ratio, comprising a gasifier (4), a cyclone separator (8), and a steam pipe (22); The lower conical wall of the gasifier (4) has a pulverized coal inlet (3) and a gasifying agent inlet (1) from top to bottom. The outlet at the top of the gasifier (4) is connected to the feed inlet on the side wall of the cyclone separator (8). The discharge port at the bottom of the cyclone separator (8) is connected to the cone at the bottom of the gasifier (4) through the return pipe (10), and the outlet of the steam pipe (22) is inserted into the return pipe (10); Its features are: It also includes a recycling unit (12); The return unit (12) includes a Laval nozzle (20) and a Venturi tube (23); The Laval nozzle (20) and the Venturi tube (23) are both arranged coaxially inside the return pipe (10). The inlet of the Laval nozzle (20) is coaxially fixed to the outlet of the steam pipe (22), and there is a distance L between the outlet of the Laval nozzle (20) and the inlet of the Venturi tube (23).

2. The pulverized coal fluidized bed gasifier with improved circulation ratio according to claim 1, characterized in that: A sealing sleeve (21) is fitted onto the steam pipe (22).

3. The pulverized coal fluidized bed gasifier with improved circulation ratio according to claim 2, characterized in that: The gasifier (4) has a top pressure measuring hole (5) on the conical wall at the top.

4. A pulverized coal fluidized bed gasifier with improved circulation ratio according to claim 3, characterized in that: The gasifier (4) has a bottom pressure measuring hole (2) on the conical wall at the bottom.

5. A pulverized coal fluidized bed gasifier with improved circulation ratio according to claim 4, characterized in that: The bottom pressure measuring hole (2) is located between the gasifying agent inlet (1) and the pulverized coal inlet (3).

6. A pulverized coal fluidized bed gasifier with improved circulation ratio according to claim 5, characterized in that: The return pipe (10) has a return pipe pressure test hole (13).

7. A pulverized coal fluidized bed gasifier with improved circulation ratio according to claim 6, characterized in that: The Laval nozzle (20) includes a converging section (14), a first throat (15) and an expanding section (16) that are coaxially fixed together in sequence. The converging section (14) is coaxially fixed to the outlet of the steam pipe (22).

8. A pulverized coal fluidized bed gasifier with improved circulation ratio according to claim 7, characterized in that: The Venturi tube (23) includes a tapered inlet section (17), a second throat (18), and a tapered outlet section (19) that are coaxially fixed together in sequence. The tapered inlet section (17) is spaced L apart from the expansion section (16) of the Val nozzle (20).

9. A method for operating a pulverized coal fluidized bed gasifier with increased circulation ratio as described in claim 8, characterized in that: This method is specifically carried out in the following steps: Step 1: The amount of steam in the steam pipe (22) accounts for 2% to 3% of the total steam volume of the gasifier. The steam velocity ejected from the expansion section (16) is maintained at 500m / s to 1000m / s, and the steam pressure is maintained at 0.5MPa to 1MPa. Step 2: Monitor the pressure P at the pressure test hole (13) of the return pipe. 13 And the pressure P2 at the bottom pressure measuring hole (2), ensure P 13 >P2; Step 3: The velocity of the mixture of steam and circulating ash at the outlet of the return pipe (10) is maintained at 3m / s to 5m / s.

10. The method for operating a pulverized coal fluidized bed gasifier with improved circulation ratio according to claim 9, characterized in that: In step two, the pressure value P at the pressure measuring hole (13) of the return pipe is... 13 The pressure value P2 at the bottom pressure measuring hole (2) is 100~200Pa higher than that at the bottom pressure measuring hole (2).

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