Coal and biomass grading pyrolysis method for biomass external combustion heat supply

Abstract

The invention provides a coal and biomass staged pyrolysis method and system for biomass external combustion heat supply. According to the method, a staged pyrolysis mode is adopted, biomass is combusted to generate first heat, heat is supplied to a low-temperature pyrolysis furnace, and biomass and coal are pyrolyzed to generate semi-coke and low-temperature pyrolysis gas; the low-temperature pyrolysis gas is rich in carbon dioxide released by low-temperature pyrolysis; then, the semi-coke enters a high-temperature pyrolyzing furnace, the biomass is combusted to generate second heat to supply heat to the high-temperature pyrolyzing furnace, and the semi-coke is pyrolyzed to generate coke and high-temperature pyrolyzing gas; the second heat is greater than the first heat. The pyrolysis process of biomass and coal is divided into a low-temperature stage and a high-temperature stage, carbon dioxide is released in the low-temperature stage, carbon monoxide and hydrogen are released in the high-temperature stage, enrichment of carbon dioxide at the low pyrolysis temperature and enrichment of hydrogen at the high pyrolysis temperature are achieved, the energy consumption of carbon dioxide separation is reduced, and the production cost is reduced. And directional separation of hydrocarbon components is realized.

Description

Technical Field

[0001] The invention belongs to the field of energy and power, and mainly relates to a coal and biomass graded pyrolysis method and system for biomass external combustion heating. Background Art

[0002] The use of coal inevitably leads to an increase in carbon dioxide emissions. As the fourth largest energy source after fossil fuels, biomass is a carbon-neutral fuel. Rational use of biomass energy can effectively reduce carbon dioxide emissions.

[0003] Pyrolysis technology is a thermochemical process that decomposes fuels such as coal and biomass into gas, liquid and solid products under high temperature. Usually, the pyrolysis process is carried out in the absence of oxygen or under an inert gas atmosphere, and the temperature range of the pyrolysis process is roughly between 300℃ and 1100℃. The properties of the pyrolysis products are mainly affected by factors such as temperature, reaction time, type of raw materials and reactor type.

[0004] For traditional pyrolysis technology, fuel is usually treated at high temperature in one continuous step to complete all pyrolysis reactions at once to generate gas, liquid and solid products. Traditional solutions do not analyze the carbon dioxide production process and simply use a single-stage pyrolysis step, which cannot achieve precise control of the products.

[0005] In addition, in the existing pyrolysis technology, electric heating, infrared heating and microwave heating are usually used to provide heat for the pyrolysis process. The above heating methods have the disadvantages of high energy cost, low energy conversion efficiency and high carbon dioxide emissions. Summary of the invention

[0006] In view of this, the present invention provides a method and system for graded pyrolysis of coal and biomass for biomass external combustion heating, which adopts a graded pyrolysis method to achieve the enrichment of carbon dioxide at a lower pyrolysis temperature and the enrichment of hydrogen at a higher pyrolysis temperature, thereby reducing the energy consumption of carbon dioxide separation.

[0007] In order to solve the above technical problems, the present invention is implemented as follows.

[0008] A method for graded pyrolysis of coal and biomass for biomass external combustion heating, comprising:

[0009] Step 1: The combustion of biomass generates first heat, which is used to heat the low-temperature pyrolysis furnace, and the coal is pyrolyzed to generate semi-coke and low-temperature pyrolysis gas; the semi-coke enters the high-temperature pyrolysis furnace; the low-temperature pyrolysis gas is rich in carbon dioxide released by low-temperature pyrolysis, and the volume fraction of carbon dioxide is higher than the volume fraction of carbon dioxide released by high-temperature pyrolysis;

[0010] Step 2: The biomass combustion generates a second heat to provide heat for the high-temperature pyrolysis furnace, pyrolyzing the semi-coke to generate coke and high-temperature pyrolysis gas; the second heat is greater than the first heat.

[0011] Preferably, step 1 is further to introduce biomass into the low-temperature pyrolysis furnace, and the low-temperature pyrolysis furnace utilizes the first heat to pyrolyze the mixture of biomass and coal.

[0012] Preferably, before step 1, the method further comprises: using the heat of flue gas generated by biomass combustion to preheat air and dry biomass; and the preheated hot air is used for biomass combustion.

[0013] Preferably, after step 2, the method further comprises: removing tar in the low-temperature pyrolysis gas and the high-temperature pyrolysis gas to generate pure pyrolysis gas.

[0014] Preferably, the operating temperature range of the low-temperature pyrolysis furnace supported by the first heat is 200-1000°C; the operating temperature range of the high-temperature pyrolysis furnace supported by the second heat is 600-1500°C; and at the same time, the operating temperature of the low-temperature pyrolysis furnace is lower than the operating temperature of the high-temperature pyrolysis furnace.

[0015] The present invention also provides a coal and biomass graded pyrolysis system with biomass external combustion heating, the system comprising a biomass external combustion heating component, a low-temperature pyrolysis furnace, and a high-temperature pyrolysis furnace; the biomass external combustion heating component supplies heat to the low-temperature pyrolysis furnace and the high-temperature pyrolysis furnace through biomass combustion; the low-temperature pyrolysis furnace is connected to the high-temperature pyrolysis furnace;

[0016] When the biomass external combustion heating component provides heat for the low-temperature pyrolysis furnace, the first heat is generated by the combustion of biomass; when the biomass external combustion heating component provides heat for the high-temperature pyrolysis furnace, the second heat is generated by the combustion of biomass; the second heat is greater than the first heat;

[0017] The low-temperature pyrolysis furnace pyrolyzes the coal to generate semi-coke and low-temperature pyrolysis gas under the heating of the first heat. The low-temperature pyrolysis gas is rich in carbon dioxide released by low-temperature pyrolysis, and the volume fraction of carbon dioxide is higher than the volume fraction of carbon dioxide released by high-temperature pyrolysis. The semi-coke enters the high-temperature pyrolysis furnace.

[0018] The high temperature pyrolysis furnace pyrolyzes the semi-coke to generate coke and high temperature pyrolysis gas under the heat supply of the second heat.

[0019] Preferably, the system further comprises a tar separation device connected to the low-temperature pyrolysis furnace and the high-temperature pyrolysis furnace to remove tar in the low-temperature pyrolysis gas and the high-temperature pyrolysis gas to generate pure pyrolysis gas.

[0020] Preferably, the biomass external combustion heating component includes a dryer, a burner, and an air preheater;

[0021] The dryer is used to dry the biomass; there are conveying channels between the dryer, the burner and the low-temperature pyrolysis furnace, and the dried biomass enters the burner through the first conveying channel; according to the required blending ratio of biomass and coal, the dried biomass enters the low-temperature pyrolysis furnace through the second conveying channel, and the low-temperature pyrolysis furnace uses the first heat to pyrolyze the blend of biomass and coal; the blending ratio is greater than or equal to 0; the first exhaust flue gas after heat exchange in the dryer is discharged into the environment;

[0022] The burner burns the dried biomass and the hot air provided by the air preheater to generate heat and high-temperature flue gas, wherein the heat includes first heat and second heat; the high-temperature flue gas is provided to the air preheater;

[0023] The air preheater uses the high-temperature flue gas generated by the burner to heat the air to produce hot air, which is provided to the burner for combustion; the second exhaust flue gas after heat exchange enters the dryer to provide drying heat.

[0024] Preferably, the system further comprises a coal pre-processing unit for drying and pulverizing the coal entering the low-temperature pyrolysis furnace.

[0025] Preferably, the operating temperature range of the low-temperature pyrolysis furnace is 200-1000°C; the operating temperature range of the high-temperature pyrolysis furnace is 600-1500°C; and the operating temperature of the low-temperature pyrolysis furnace is lower than the operating temperature of the high-temperature pyrolysis furnace.

[0026] Beneficial effects:

[0027] (1) Compared with the traditional pyrolysis method, the present invention adopts a graded pyrolysis method, which achieves the enrichment of carbon dioxide at a lower pyrolysis temperature and the enrichment of hydrogen at a higher pyrolysis temperature, reduces the energy consumption of carbon dioxide separation, and achieves the directional separation of hydrocarbon components.

[0028] (2) The existing heating methods such as electric heating, infrared heating and microwave heating have the disadvantages of high energy cost, low energy conversion efficiency and high carbon dioxide emissions. The present invention uses the heat generated by the combustion of biomass in an external burner to provide heat for the pyrolysis process. Since biomass is a carbon-neutral fuel, compared with traditional heating methods, biomass external combustion heating achieves zero carbon dioxide emissions in the heating process.

[0029] (3) In a preferred embodiment, waste heat recovery technology is also used. The flue gas generated by biomass combustion has a higher temperature and has a higher utilization potential. Therefore, the preferred solution uses the high-temperature flue gas generated by biomass combustion to preheat the air, and the low-temperature flue gas is used to dry the biomass. This process maximizes the use of flue gas heat, realizes the cascade utilization of flue gas heat, and improves the overall energy efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 A schematic diagram of the steps of a method for graded pyrolysis of coal and biomass for biomass external combustion heating proposed by the present invention;

[0031] Figure 2 This is a block diagram of the composition of a coal and biomass graded pyrolysis system for biomass external combustion heating proposed by the present invention.

[0032] In the above-mentioned Figure 2 In the figure, the components and their corresponding marks are: 1-dryer; 2-burner; 3-air preheater; 4-coal pretreatment unit; 5-low-temperature pyrolysis furnace; 6-high-temperature pyrolysis furnace; 7-tar separation device; 8-biomass; 9-first flue gas; 10-first dried biomass; 11-second dried biomass; 12-heat; 13-first heat; 14-second heat; 15-high-temperature flue gas; 16-air; 17-hot air; 18-second flue gas, i.e., flue gas used for biomass drying; 19-coal; 20-pretreated coal; 21-low-temperature pyrolysis gas; 22-semi-coke; 23-high-temperature pyrolysis gas; 24-coke; 25-pure pyrolysis gas; 26-tar. DETAILED DESCRIPTION

[0033] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0034] Among the conversion and utilization methods of coal, pyrolysis is a potential low-energy method for reducing carbon dioxide emissions. In conventional pyrolysis, different gas components are generated at different stages. At lower pyrolysis temperatures, carbon dioxide and methane are generated first. As the pyrolysis temperature increases, the yields of hydrogen and carbon monoxide gradually increase, causing the concentration of carbon dioxide components to continue to decrease, increasing the energy consumption for separating carbon dioxide. The present invention is based on this discovery. In order to reduce the energy consumption for separating carbon dioxide, the present invention separately collects gaseous products at different pyrolysis temperature stages of coal to avoid mixing of hydrocarbon components.

[0035] In view of the above analysis, the present invention provides a scheme for the graded pyrolysis of coal and biomass with biomass external combustion for heating. The basic idea is: adopting the graded pyrolysis method, firstly, the biomass combustion generates the first heat, which provides heat for the low-temperature pyrolysis furnace, and pyrolyzes the coal to generate semi-coke and low-temperature pyrolysis gas; the low-temperature pyrolysis gas is rich in carbon dioxide released by low-temperature pyrolysis; then, the semi-coke enters the high-temperature pyrolysis furnace, the biomass combustion generates the second heat, which provides heat for the high-temperature pyrolysis furnace, and pyrolyzes the semi-coke to generate coke and high-temperature pyrolysis gas. The second heat is greater than the first heat.

[0036] It can be seen that the present invention divides the pyrolysis process of coal into two stages, low temperature and high temperature, so that carbon dioxide is released in the low temperature stage, and carbon monoxide and hydrogen are released in the high temperature stage, thereby realizing the enrichment of carbon dioxide at a lower pyrolysis temperature and the enrichment of hydrogen at a higher pyrolysis temperature, reducing the energy consumption of carbon dioxide separation, and realizing the directional separation of hydrocarbon components.

[0037] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings.

[0038] Example 1

[0039] This embodiment provides a preferred process for a method for pyrolysis of coal and biomass using biomass as an external combustion heating source. Figure 1 As shown, the method comprises the following steps:

[0040] S1. The heat of flue gas generated by the burner is used to preheat the air and dry the biomass. A part of the dried biomass enters the burner, and the other part of the dried biomass and the pretreated coal enter the low-temperature pyrolysis furnace.

[0041] This process can be considered as a pretreatment process of biomass and coal, which can realize the functions of biomass drying, coal crushing and drying, and air preheating. The heat required for biomass drying and air preheating comes from the flue gas generated by the combustion of biomass and hot air in step S2. The flue gas first enters the air preheater to heat the air, and then enters the dryer to dry the biomass. This process realizes the cascade utilization of flue gas heat and reduces the heat loss of flue gas.

[0042] Here, biomass refers to all types of biomass; the type of coal is not limited; the blending ratio of biomass and coal entering the low-temperature pyrolysis furnace is not limited, and the biomass ratio can be 0.

[0043] In a preferred embodiment, the biomass drying temperature may be in the range of 100-150°C.

[0044] S2. Dry biomass and hot air are burned to generate first heat.

[0045] The part of the dried biomass and hot air generated in step S1 are burned to generate heat and high-temperature flue gas. The heat released by the combustion in this step is the first heat required for step S3. The flue gas released by the combustion provides heat for the air preheater and the dryer.

[0046] S3. Use the first heat to heat the low-temperature pyrolysis furnace. The pyrolysis temperature of the low-temperature pyrolysis furnace is maintained at a relatively low level, and biomass and coal are pyrolyzed. The products mainly include semi-coke and low-temperature pyrolysis gas. The main components of the low-temperature pyrolysis gas are carbon dioxide, methane, carbon monoxide, hydrogen and tar. The volume fraction of carbon dioxide is greatly increased compared with that of high-temperature pyrolysis gas.

[0047] This process can be considered as a low-temperature pyrolysis process of biomass and coal, which can achieve carbon dioxide enrichment at a relatively low temperature. The required heat is provided by step S2, and under the pyrolysis condition of a relatively low temperature, semi-coke and low-temperature pyrolysis gas with a relatively high carbon dioxide content can be obtained.

[0048] In this step, the operating temperature range of the low temperature pyrolysis furnace is preferably 200-1000°C.

[0049] S4, burning the dried biomass and the hot air to generate a second heat quantity which is greater than the first heat quantity.

[0050] The heat released by combustion in this step is the second heat required for step S5, which can achieve high-temperature pyrolysis of semi-coke. The flue gas released by combustion still provides heat for the air preheater and dryer.

[0051] S5. The second heat is used to heat the high-temperature pyrolysis furnace. The pyrolysis temperature of the high-temperature pyrolysis furnace is maintained at a high level to pyrolyze the semi-coke. The main products include coke and high-temperature pyrolysis gas. The main components of the high-temperature pyrolysis gas are hydrogen, carbon monoxide, methane, carbon dioxide and tar. The main components of the high-temperature pyrolysis gas and the low-temperature pyrolysis gas are the same.

[0052] This process can be considered as a high-temperature pyrolysis process of semi-coke, which generates hydrogen and carbon monoxide at a relatively high temperature. The semi-coke is provided by step S3, and the required heat is provided by step S4. Under the pyrolysis condition of a relatively high temperature, coke and high-temperature pyrolysis gas mainly composed of hydrogen and carbon monoxide can be obtained.

[0053] In this step, the operating temperature range of the high temperature pyrolysis furnace is preferably 600-1500° C. At the same time, the operating temperature of the low temperature pyrolysis furnace is lower than the operating temperature of the high temperature pyrolysis furnace.

[0054] S6. The low-temperature pyrolysis gas and the high-temperature pyrolysis gas enter the tar removal device to remove the tar and generate pure pyrolysis gas.

[0055] This process can be considered as a tar separation process, which separates the tar from the low-temperature pyrolysis gas and the high-temperature pyrolysis gas to produce a pure mixed gas with a low tar content. The tar removal method can be a water washing method or an adsorption method.

[0056] The above process can produce: pure pyrolysis gas, tar and coke. The main components of pure pyrolysis gas are hydrogen, carbon monoxide and carbon dioxide.

[0057] In the hierarchical pyrolysis method proposed in the present invention, compared with the traditional pyrolysis method, the pyrolysis process in the present invention is divided into two stages: low-temperature pyrolysis and high-temperature pyrolysis, so that carbon dioxide is released in the low-temperature stage, and carbon monoxide and hydrogen are released in the high-temperature stage, thereby realizing the separation of hydrocarbon components. Secondly, the heat required by the pyrolysis furnace is provided by an external burner, which reduces energy consumption and carbon dioxide emissions compared with traditional heating methods. In addition, the high-temperature flue gas generated by the combustion of biomass in the external burner provides heat for the air preheater and dryer. This process maximizes the use of flue gas heat and realizes the cascade utilization of flue gas heat.

[0058] Example 2

[0059] In order to better illustrate the method proposed in Example 1 of the present invention, this embodiment provides a coal and biomass graded pyrolysis system for biomass external combustion heating. Figure 2 As shown, the system includes a biomass external combustion heating component, a low-temperature pyrolysis furnace 5, and a high-temperature pyrolysis furnace 6. In this embodiment, the biomass external combustion heating component specifically includes a dryer 1, a burner 2, and an air preheater 3. The system also includes a tar separation device 7, a coal pretreatment unit 4, wherein:

[0060] The dryer 1 can implement the above step S1, and is used to dry the original biomass 8. A part of the dried biomass is called the first dried biomass 10 and enters the burner 2, and the other part is called the second dried biomass 11 and the pretreated coal 20 and enters the low-temperature pyrolysis furnace 5. The mixing ratio of the coal 20 and the second dried biomass 11 is not limited, and the ratio of the second dried biomass 11 can be 0. In this embodiment, the source of the drying heat of the dryer 1 is the second exhaust flue gas 18 discharged by the air preheater 3. The second exhaust flue gas is the waste heat of the high-temperature flue gas discharged by the air preheater 3 after the burner is used. The first exhaust flue gas 9 after heat exchange in the dryer 1 is discharged into the environment.

[0061] The burner 2 can implement the above steps S2 and S4, and is used to burn the first dried biomass 10 and the hot air 17 to generate heat 12 and high-temperature flue gas 15, wherein the heat 12 includes the first heat 13 and the second heat 14. When supplying heat to the low-temperature pyrolysis furnace 5, the first heat 13 is generated by burning the biomass; when supplying heat to the high-temperature pyrolysis furnace 6, the second heat 14 is generated by burning the biomass. The high-temperature flue gas 15 is provided to the air preheater 3. The hot air 17 comes from the air preheater 3.

[0062] The air preheater 3 can implement the above step S1, and is used to utilize the high-temperature flue gas 15 generated by the burner 2 to heat the air 16 to produce hot air 17, and the second exhaust flue gas 18 after heat exchange enters the dryer 1.

[0063] The coal pretreatment unit 4 can implement the above step S1, and is used to dry and crush the coal 19 into coal 20 for pyrolysis.

[0064] The low-temperature pyrolysis furnace 5 can implement the above step S3, and is used to use the first heat 13 to pyrolyze the coal 20 and the second dry biomass 11 to produce semi-coke 22 and low-temperature pyrolysis gas 21. The low-temperature pyrolysis gas 21 is rich in carbon dioxide.

[0065] The high-temperature pyrolysis furnace 6 can implement the above step S5, and is used to pyrolyze the semi-coke 22 to generate coke 23 and high-temperature pyrolysis gas 23 using the second heat 14.

[0066] The tar separation device 7 can implement the above step S6, and is used to separate the tar 26 in the low-temperature pyrolysis gas 21 and the high-temperature pyrolysis gas 23, and generate a pure pyrolysis gas 25 with a low tar content.

[0067] In this embodiment, the dryer 1 may be an airflow dryer. The coal pretreatment unit 4 may use existing equipment, such as a coal mill, etc. The low-temperature pyrolysis furnace 5 and the high-temperature pyrolysis furnace 6 may be a heating furnace, a soaking furnace or a calcining furnace.

[0068] Examples

[0069] In order to further illustrate the feasibility of the present invention, a typical operating condition was selected and the pyrolysis process was simulated and calculated. In the selected operating condition, coal is used as the fuel for graded pyrolysis, and biomass is only used as the fuel for the burner and does not participate in the pyrolysis process. Table 1 shows the basic operating parameters of the example.

[0070] In order to illustrate the difference between the components of low-temperature pyrolysis gas and high-temperature pyrolysis gas, Table 2 shows the mole fractions of the main components of low-temperature and high-temperature pyrolysis gas in the simulation calculation results.

[0071] Table 3 shows the system energy analysis results of the example. From the perspective of energy input, the coal energy used for graded pyrolysis in the example accounts for 94.04% of the total input energy, and the biomass energy used for external heating accounts for 5.96% of the total input energy. From the perspective of energy output, the energy carried by coke accounts for 79.80% of the total output, the energy carried by low-temperature pyrolysis gas accounts for 4.47% of the total output, and the energy carried by high-temperature pyrolysis gas accounts for 15.73% of the total output. From the perspective of energy efficiency, the pyrolysis efficiency of the example is 94.17%, which is higher than the efficiency of traditional coal pyrolysis.

[0072] Table 1 Basic parameters of the example

[0073] Basic parameters Numeric unit Low temperature pyrolysis temperature 600 ℃ High temperature pyrolysis temperature 950 ℃ Biomass combustion temperature 1050 ℃ Pyrolysis pressure 1.1 bar Heat transfer loss 6 ％ Coal consumption 5 kg / s Biomass usage 0.495 kg / s

[0074] Table 2 Mole fractions of main components of low-temperature pyrolysis gas and high-temperature pyrolysis gas

[0075]

[0076]

[0077] Table 3 Energy analysis results of the example

[0078]

[0079] In summary, traditional pyrolysis technology has the disadvantages of high heating energy consumption, high carbon dioxide emissions, and high carbon dioxide separation energy consumption. The present invention proposes a coal and biomass graded pyrolysis method for biomass external combustion heating. Through the external combustion of biomass, zero carbon dioxide emissions in the heating process can be achieved. In addition, recovering the heat of biomass combustion flue gas also realizes energy cascade utilization and improves overall energy efficiency. At the same time, through the graded pyrolysis method, the energy consumption of carbon dioxide separation can be reduced, and the directional separation of hydrocarbon components can be achieved. Therefore, the invention has good social benefits and industrial application scenarios.

[0080] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for graded pyrolysis of coal and biomass for biomass external combustion heating, characterized in that: include: Step 1: The combustion of biomass generates the first heat, which is used to heat the low-temperature pyrolysis furnace, and the coal is pyrolyzed to generate semi-coke and low-temperature pyrolysis gas; the semi-coke enters the high-temperature pyrolysis furnace; The low-temperature pyrolysis gas is rich in carbon dioxide released by low-temperature pyrolysis, and the volume fraction of carbon dioxide is higher than that of carbon dioxide released by high-temperature pyrolysis; Step 2: The biomass combustion generates a second heat to provide heat for the high-temperature pyrolysis furnace, pyrolyzing the semi-coke to generate coke and high-temperature pyrolysis gas; the second heat is greater than the first heat.

2. The method according to claim 1, characterized in that Step 1 is further to introduce biomass into the low-temperature pyrolysis furnace, and the low-temperature pyrolysis furnace utilizes the first heat to pyrolyze the mixture of biomass and coal.

3. The method according to claim 1, characterized in that Before step 1, the method further comprises: using the heat of flue gas generated by biomass combustion to preheat air and dry biomass; and the preheated hot air is used for biomass combustion.

4. The method according to claim 1, characterized in that After step 2, the method further includes: removing tar in the low-temperature pyrolysis gas and the high-temperature pyrolysis gas to generate pure pyrolysis gas.

5. The method according to claim 1, characterized in that The first heat supports the low-temperature pyrolysis furnace with an optional operating temperature range of 200-1000°C; the second heat supports the high-temperature pyrolysis furnace with an optional operating temperature range of 600-1500°C; and at the same time, the operating temperature of the low-temperature pyrolysis furnace is lower than the operating temperature of the high-temperature pyrolysis furnace.

6. A coal and biomass graded pyrolysis system for biomass external combustion heating, characterized in that: The system comprises a biomass external combustion heating component, a low-temperature pyrolysis furnace (5), and a high-temperature pyrolysis furnace (6); the biomass external combustion heating component supplies heat to the low-temperature pyrolysis furnace (5) and the high-temperature pyrolysis furnace (6) through biomass combustion; the low-temperature pyrolysis furnace (5) is connected to the high-temperature pyrolysis furnace (6); When the biomass external combustion heating component provides heat for the low-temperature pyrolysis furnace (5), the biomass combustion generates a first heat (13); when the biomass external combustion heating component provides heat for the high-temperature pyrolysis furnace (6), the biomass combustion generates a second heat (14); the second heat (14) is greater than the first heat (13); The low-temperature pyrolysis furnace (5) pyrolyzes the coal under the heat of the first heat (13) to generate semi-coke and low-temperature pyrolysis gas (21), wherein the low-temperature pyrolysis gas is rich in carbon dioxide released by low-temperature pyrolysis, and the volume fraction of carbon dioxide is higher than the volume fraction of carbon dioxide released by high-temperature pyrolysis; the semi-coke (22) enters the high-temperature pyrolysis furnace (6); The high-temperature pyrolysis furnace (6) pyrolyzes the semi-coke under the heat supply of the second heat (14) to generate coke (24) and high-temperature pyrolysis gas (23).

7. The system according to claim 6, characterized in that The system further comprises a tar separation device (7) connected to the low-temperature pyrolysis furnace (5) and the high-temperature pyrolysis furnace (6) to remove tar from the low-temperature pyrolysis gas and the high-temperature pyrolysis gas to generate pure pyrolysis gas (25).

8. The system according to claim 6, characterized in that The biomass external combustion heating component comprises a dryer (1), a burner (2), and an air preheater (3); The dryer (1) is used to dry biomass; there are conveying channels between the dryer (1), the burner (2) and the low-temperature pyrolysis furnace (5); the dried biomass enters the burner (2) through the first conveying channel; according to the required blending ratio of biomass and coal, the dried biomass enters the low-temperature pyrolysis furnace (5) through the second conveying channel; the low-temperature pyrolysis furnace (5) pyrolyzes the blend of biomass and coal using a first heat; the blending ratio is greater than or equal to 0; the first exhaust flue gas (9) after heat exchange in the dryer (1) is discharged into the environment; The burner (2) burns the dried biomass and the hot air (17) provided by the air preheater (3) to generate heat (12) and high-temperature flue gas (15), wherein the heat includes a first heat (13) and a second heat (14); the high-temperature flue gas (15) is provided to the air preheater (3); The air preheater (3) utilizes the high-temperature flue gas (15) generated by the burner (2) to heat air to produce hot air (17), which is provided to the burner (2) for combustion; the second exhaust flue gas (18) after heat exchange enters the dryer (1) to provide drying heat.

9. The system according to claim 6, characterized in that The system further comprises a coal pre-treatment unit (4) for drying and pulverizing the coal entering the low-temperature pyrolysis furnace (5).

10. The system according to claim 6, characterized in that The operating temperature of the low-temperature pyrolysis furnace can be selected in the range of 200-1000°C; the operating temperature of the high-temperature pyrolysis furnace can be selected in the range of 600-1500°C; and the operating temperature of the low-temperature pyrolysis furnace is lower than the operating temperature of the high-temperature pyrolysis furnace.

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