System and method for preparing liquid fuel by coupling biological methane with biomass pyrolysis gas

Abstract

The invention belongs to the technical field of utilization of biomass energy, and particularly relates to a system and a method for preparing liquid fuel by coupling biological methane with biomass pyrolysis gas, the system comprises a methane generation device, a purification device, a pyrolysis device, a separation device, a melting device, a steam generation device, a reforming device, a conversion device and a synthesis device; biomass is subjected to an anaerobic fermentation reaction in the methane generation device to generate biogas and biogas residues, and the biomass is subjected to a thermal cracking reaction in the cracking device to obtain biochar and cracked gas; biogas is fed into the synthesis device after being subjected to purification and reforming treatment, cracked gas enters the synthesis device after being treated by the separation device and the conversion device, liquid fuel is obtained through catalytic reaction of the two gases, and biogas residues and biochar are combusted in the melting device to supply heat and steam. According to the invention, preparation processes of biological methane and biomass pyrolysis gas are coupled, and the difference of hydrogen-carbon ratios in different products is fully utilized, so that efficient and low-carbon preparation of liquid fuel from biomass is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of biomass energy utilization, and in particular to a system for preparing liquid fuel by coupling biomass cracking gas with biomethane, and a method for preparing liquid fuel. Background Art

[0002] Under the global trend of reducing carbon emissions from energy, biomass energy has attracted widespread attention for its clean and low-carbon properties, especially in the production of methane and liquid fuels from biomass. At present, biomethane is mainly produced through anaerobic fermentation of biomass, and biomass liquid fuel is mainly synthesized through biomass gasification gas or pyrolysis gas.

[0003] Usually, the ratio of hydrogen to carbon monoxide in biomass pyrolysis gas is less than 1, but when used to synthesize liquid fuel, the ratio needs to reach 2.5 or above. Although the water-gas shift reaction can be used to adjust the ratio of hydrogen to carbon monoxide, the process will produce a large amount of carbon dioxide emissions; and in the process of preparing biomethane by anaerobic fermentation, biomethane is limited due to the difficulty of grid connection, and the produced biogas residue has high content of difficult-to-degrade lignin, excessive heavy metals, and low fertilizer efficiency, which also becomes a processing problem; in the process of synthesizing liquid fuel by gasification or pyrolysis of biomass, since the carbon-hydrogen ratio in the gasification gas or pyrolysis gas is much higher than that required for synthesizing liquid fuel, the carbon-hydrogen ratio is often adjusted by the conversion section, which increases the cost of liquid fuel. The present invention intends to develop a system and process to solve the above problems. Summary of the invention

[0004] The purpose of the present invention is to solve the problems of high cost, large amount of carbon dioxide emissions and difficulty in handling biogas residue in the anaerobic fermentation process for preparing methane in the prior art, and to propose a biomethane coupled biomass pyrolysis gas preparation system and a method for preparing liquid fuel.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] The present invention discloses a system for preparing liquid fuel by coupling biomass cracking gas with biomethane, comprising: a methane generating device, a purification device, a cracking device, a separation device, a melting device, a steam generating device, a reforming device, a conversion device, and a synthesis device;

[0007] The biomass undergoes an anaerobic fermentation reaction in the methane generating device to produce biogas and biogas residue, the biogas outlet of the methane generating device is connected to the purification device, and the biogas residue outlet of the methane generating device is connected to the biogas residue inlet of the melting device;

[0008] The biogas is purified in the purification device to obtain biomethane, and the purification device is connected to the reforming device;

[0009] The biomass undergoes a thermal cracking reaction in the cracking device, the cracking device is heated by an external source, and produces biochar and cracking gas, the biochar outlet of the cracking device is connected to the biochar inlet of the melting device, and the cracking gas outlet of the cracking device is connected to the separation device;

[0010] The cracked gas is separated from methane in the separation device, the cracked gas alkane outlet of the separation device is connected to the reforming device, and the other cracked gas outlets are respectively connected to the conversion device, and the conversion device is connected to the synthesis device;

[0011] The biogas residue and biochar are burned and melted in the melting device to generate high-temperature flue gas and molten slag, and the high-temperature flue gas outlet of the melting device is connected to the steam generating device;

[0012] The steam outlet of the steam generating device is connected to the reforming device and the conversion device respectively;

[0013] Biomethane, cracked gas alkanes and steam are catalytically converted into a mixed gas of carbon monoxide and hydrogen in the reforming device, and the mixed gas outlet of the reforming device is connected to the synthesis device;

[0014] Other cracked gas and steam are catalytically converted into a mixed gas of carbon monoxide and hydrogen in the conversion device, and the mixed gas outlet of the conversion device is connected to the synthesis device.

[0015] As a further preferred solution, the cracking device is wrapped by a jacket for heating, and the jacket inlet is connected to the high-temperature flue gas outlet of the melting device.

[0016] As a further preferred embodiment, the jacket outlet of the cracking device is also connected to a drying device, which is respectively connected to the sludge outlet of the methane generating device and the sludge inlet of the melting device; the drying device also has a flue gas outlet, which is also connected to a flue gas treatment device.

[0017] As a further preferred solution, the smoke outlet of the steam generating device is connected to the smoke treatment device.

[0018] As a further preferred embodiment, the melting device has a two-stage structure, the front section is the melting zone, and the rear section is the combustion zone; the melting zone is provided with an inlet for adding flux, and the melting zone is respectively connected to the cracking device and the methane generating device for receiving dried sludge and biochar, and the melting zone is connected to the quenching device through the slag outlet; the combustion zone is provided with a high-temperature flue gas outlet, which is connected to the cracking device and the steam generating device; the flux is an organic flux, and its components are a hydrocarbon mixture and an alkali metal oxide.

[0019] As a further preferred embodiment, a purification device is provided between the cracking gas outlet of the cracking device and the separation device; and a charcoal conveying device is provided between the biochar outlet of the cracking device and the biochar inlet of the melting device.

[0020] As a further preferred embodiment, the separation device is connected to the synthesis device.

[0021] A method for preparing a liquid fuel system by coupling biomethane with biomass pyrolysis gas, comprising the following steps:

[0022] Step 1: Biomass is fed into a methane generator and a cracking unit respectively;

[0023] Step 2: The biomass undergoes anaerobic fermentation reaction in the methane generating device to generate biogas and biogas residue; the biogas is purified to remove the water vapor and carbon dioxide carried by it, and the obtained biomethane is sent to the reforming device; the drying device is connected to the jacket of the cracking device, and the biogas residue is dried in the drying device under the action of the high-temperature flue gas from the jacket of the cracking device, and the dried biogas residue is sent to the melting device;

[0024] Step 3: The biomass undergoes thermal cracking reaction in the cracking device, and the heat comes from the high-temperature flue gas in the jacket, thereby obtaining biochar and cracking gas; the biochar is sent to the melting device via the carbon conveying device; the cracking gas is removed from the impurities of tar, dust, and water vapor by the purification device, and the cracking gas is purified and separated from the cracking gas alkanes by the separation device. The cracking gas alkanes obtained are sent to the reforming device, and the other cracking gases are sent to the conversion device and the synthesis device respectively;

[0025] Step 4: The biochar, dried biogas residue and added flux are burned and melted in a melting device to obtain high-temperature flue gas and high-temperature slag; part of the high-temperature flue gas is sent to the jacket of the cracking device to provide heat for the cracking device, and the other high-temperature flue gas is sent to the steam generating device to generate steam, and the steam is sent to the reforming device and the conversion device respectively; the high-temperature slag enters the quenching device for rapid cooling to obtain vitrified slag;

[0026] Step 5: In the reforming unit, biomethane and cracked gas alkanes are reformed into a mixed gas of carbon monoxide and hydrogen under the action of steam and catalyst, and the mixed gas is sent to the synthesis unit;

[0027] Step 6: Steam and other cracked gases from the separation device undergo a shift reaction in the shift device through the action of a catalyst, converting carbon monoxide in other cracked gases into hydrogen, which is then transported to the synthesis device to ensure a suitable carbon-hydrogen ratio in the subsequent synthesis device;

[0028] Step 7: In the synthesis device, the mixed gas of carbon monoxide and hydrogen from the reforming device, the separation device, and the conversion device can be converted into the corresponding liquid fuel under different reaction conditions and the action of catalysts;

[0029] Step 8: The flue gas in the drying device and the flue gas in the steam generating device are concentrated in the flue gas treatment device for treatment to meet the emission standards.

[0030] As a further preferred solution, the reaction temperature of the inner cylinder of the cracking device is controlled at 500-700°C; and the excess air coefficient of the melting zone of the melting device is <1.

[0031] Beneficial Effects

[0032] 1. Since the methane steam reforming reaction can convert methane into carbon monoxide and hydrogen, and the ratio of hydrogen to carbon monoxide is close to 3, the biomethane and biomass cracking gas preparation processes are coupled to make full use of the difference in hydrogen-carbon ratios in their different products. The biomethane is subjected to a steam reforming reaction, which can not only realize the consumption of biomethane, but also increase the hydrogen / carbon monoxide ratio of the mixed gas for synthetic liquid fuel, thereby reducing the proportion of cracking gas shift reaction, reducing energy consumption and carbon dioxide emissions, and realizing efficient and low-carbon preparation of liquid fuel from biomass.

[0033] 2. The biochar produced by biomass pyrolysis has a high calorific value, and with the assistance of flux, the dried biogas residue can be melted. The slag is rapidly cooled and vitrified, and harmful substances such as heavy metals are wrapped in the slag, which can be recycled; the heat generated by the melting of biogas residue is absorbed by the steam generator, realizing the energy utilization of biogas residue. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 A schematic diagram of a system for preparing liquid fuel by coupling biomethane with biomass pyrolysis gas according to the present invention;

[0035] In the figure: 1. Purification device; 2. Separation device; 3. Conversion device; 4. Synthesis device; 5. Methane generation device; 6. Purification device; 7. Reforming device; 8. Drying device; 9. Cracking device; 10. Carbon transportation device; 11. Melting device; 12. Steam generation device; 13. Flue gas treatment device; 14. Refrigeration device. DETAILED DESCRIPTION

[0036] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0037] A biomethane coupled biomass cracking gas preparation liquid fuel system of the present invention comprises: a methane generating device 5, a purification device 6, a cracking device 9, a separation device 2, a melting device 11, a steam generating device 12, a reforming device 7, a conversion device 3, and a synthesis device 4;

[0038] The biomass undergoes an anaerobic fermentation reaction in the methane generating device 5 to produce biogas and biogas residue. The biogas outlet of the methane generating device 5 is connected to the purification device 6, and the biogas residue outlet of the methane generating device 5 is connected to the biogas residue inlet of the melting device 11.

[0039] The biogas is purified in the purification device 6 to obtain biomethane, and the purification device 6 is connected to the reforming device 7;

[0040] The biomass undergoes a thermal cracking reaction in the cracking device 9, and the cracking device 9 is heated by an external source to produce biochar and cracking gas. The biochar outlet of the cracking device 9 is connected to the biochar inlet of the melting device 11, and the cracking gas outlet of the cracking device 9 is connected to the separation device 2;

[0041] The cracked gas is separated from methane in the separation device 2, the cracked gas alkane outlet of the separation device 2 is connected to the reforming device 7, and the other cracked gas outlets are respectively connected to the conversion device 3, and the conversion device 3 is connected to the synthesis device 4;

[0042] The biogas residue and biochar are burned and melted in the melting device 11 to generate high-temperature flue gas and molten slag. The high-temperature flue gas outlet of the melting device 11 is connected to the steam generating device 12;

[0043] The steam outlet of the steam generating device 12 is connected to the reforming device 7 and the conversion device 3 respectively;

[0044] Biomethane, cracked gas alkanes, and steam are catalytically converted into a mixed gas of carbon monoxide and hydrogen in the reforming device 7, and the mixed gas outlet of the reforming device 7 is connected to the synthesis device 4;

[0045] Other cracked gas and steam are catalytically converted into a mixed gas of carbon monoxide and hydrogen in the conversion device 3 , and the mixed gas outlet of the conversion device 3 is connected to the synthesis device 4 .

[0046] The cracking device 9 is wrapped in a jacket for heating, and the jacket inlet is connected to the high-temperature flue gas outlet of the melting device 11; the hot flue gas in the jacket is used to achieve thermal cracking of biomass, and the methane and hydrogen content in the cracking gas is high, which is easy to subsequently synthesize liquid fuel.

[0047] The jacket outlet of the cracking device 9 is also connected to a drying device 8, which is respectively connected to the sludge outlet of the methane generating device 5 and the sludge inlet of the melting device 11; the drying device 8 also has a flue gas outlet, which is also connected to a flue gas treatment device 13.

[0048] The flue gas outlet of the steam generating device 12 is connected to the flue gas treatment device 13 .

[0049] The melting device 11 is a two-stage structure, the front section is the melting zone, and the rear section is the combustion zone; the excess air coefficient in the front section is controlled to be less than 1, and the biomass ash is easier to melt under the action of reducing atmosphere and flux, and the rear section is used to burn the combustible components in the flue gas in the front section.

[0050] The melting zone is provided with an inlet for adding flux, and the melting zone is respectively connected to the cracking device 9 and the methane generating device 5 to receive dried biogas slag and biochar, and the melting zone is connected to the burnout zone through the slag outlet; the burnout zone is provided with a high-temperature flue gas outlet, which is connected to the cracking device 9 and the melting device 11; the flux is an organic flux, and its components are a hydrocarbon mixture and an alkali metal oxide; the organic flux can supplement part of the energy for the melting device 11 when the ash and moisture content of the biogas slag are high, thereby ensuring the complete melting of the biomass ash.

[0051] A purification device 1 is further provided between the cracking gas outlet of the cracking device 9 and the separation device 2 ; a charcoal conveying device 10 is further provided between the biochar outlet of the cracking device 9 and the biochar inlet of the melting device 11 .

[0052] The slag outlet of the melting device 11 is also connected to a quenching device 14 .

[0053] The separation device 2 is connected to the synthesis device 4 .

[0054] A method for preparing a liquid fuel system by coupling biomethane with biomass pyrolysis gas of the present invention comprises the following steps:

[0055] Step 1: Different biomasses are added to the methane generator 5 and the cracking device 9 respectively. Conventionally, the biomass used in the methane generator is mainly livestock manure and straw; the biomass used in the cracking device is mainly sawdust, wood chips and straw.

[0056] Step 2: The biomass undergoes anaerobic fermentation reaction in the methane generating device 5 to generate biogas and biogas residue; the biogas is purified by the purification device 6 to remove the water vapor and carbon dioxide carried by it, and the obtained biomethane is sent to the reforming device 7; the drying device 8 is connected to the jacket of the cracking device 9, and the biogas residue is dried in the drying device 8 under the action of the high-temperature flue gas from the jacket of the cracking device 9, and the dried biogas residue is sent to the melting device 11.

[0057] Step 3: The biomass undergoes thermal cracking reaction in the cracking device 9, and the high-temperature heat comes from the high-temperature flue gas of the jacket to obtain biochar and cracking gas; the biochar is sent to the melting device 11 through the carbon conveying device 10; the cracking gas is removed from the tar, dust and water vapor by the purification device 1, and the cracking gas obtained is separated from the cracking gas alkane by the separation device 2. The cracking gas alkane is sent to the reforming device 7, and the other cracking gas is sent to the conversion device 3 and the synthesis device 4 respectively.

[0058] Step 4: The biochar, dried sludge and added flux are burned and melted in the melting device 11 to obtain high-temperature flue gas and high-temperature slag; the high-temperature flue gas is sent to the jacket of the cracking device 9 to provide heat for the cracking device 9, and the high-temperature flue gas is sent to the steam generating device 12 to generate steam, and the steam is sent to the reforming device 7 and the conversion device 3 respectively; the high-temperature slag enters the quenching device 14 for rapid cooling to obtain vitrified slag.

[0059] Step 5: In the reforming device 7, biomethane and cracked gas alkanes are reformed into a mixed gas of carbon monoxide and hydrogen under the action of steam and catalyst, and the mixed gas is sent to the synthesis device 4. Conventional methane steam reforming catalysts include nickel-based catalysts, copper-based catalysts, and precious metal catalysts such as platinum and palladium.

[0060] Step 6: Steam and other purified gas from the separation device 2 undergo a conversion reaction in the conversion device 3 through the action of a catalyst, converting a large amount of carbon monoxide in the purified gas into hydrogen, which is then transported to the synthesis device 4 to ensure a suitable carbon-hydrogen ratio in the subsequent synthesis device 4;

[0061] The reforming unit mainly produces reforming reactions of steam and methane, and its main reactions are:

[0062] CH4+H2O=CO+3H2

[0063] The shift device mainly produces the shift reaction of steam and CO, and its main reaction is:

[0064] CO+H2O=H2+CO2

[0065] It can be seen from the reaction that 1 mol of methane can produce 1 mol of CO and 3 mol of H2, while the shift reaction can only achieve a one-to-one shift of CO and H2 and produce 1 mol of CO2.

[0066] Step 7: In the synthesis device 4, the mixed gas of carbon monoxide and hydrogen from the reforming device 7, the separation device 2, and the conversion device 3 can be converted into corresponding liquid fuel under different reaction conditions and the action of catalysts;

[0067] In the synthesis device, different proportions of CO and H2, at different reaction temperatures and pressures, and under the action of different catalysts, can synthesize different liquid fuels (alcohols, alkanes) (common methanol and biodiesel belong to this category), and the product may not be a fixed component, but may also be a mixture of multiple substances. The methanol synthesis reaction formula is CO+2H2=CH30H, and the synthesis of alkanes can be expressed by the formula (2n+1)H2+nCO→CnH(2n+2)+nH2O.

[0068] One of the focuses of the present invention is to adjust the carbon-hydrogen ratio, thereby providing the possibility for synthesizing various liquid fuels.

[0069] Step eight: The flue gas in the drying device 8 and the flue gas in the steam generating device 12 are concentrated in the flue gas treatment device 13 for treatment to meet emission standards.

[0070] The reaction temperature of the inner cylinder of the cracking device 9 is controlled at 500-700°C; the excess air coefficient of the front section of the melting device 11 is less than 1; the temperature of the cracking device 9 is controlled within 700°C to prevent hydrogen from being converted into carbon monoxide in large quantities by carbon dioxide.

[0071] Example 1

[0072] Step 1: livestock and poultry manure and biomass straw are added to the methane generating device 5, and the crushed wood chips are added to the cracking device 9;

[0073] Step 2: The biomass undergoes anaerobic fermentation reaction in the methane generating device 5 to generate biogas and biogas residue, the main components of the biogas are: biomethane: 50-70%, CO2: 30-45%, H20 (g): 10-15%, and others (N2, O2, H2S): 1-3%; the biogas is purified by the purification device 6 to remove the water vapor, carbon dioxide and other impurities carried, and the obtained biomethane (> 95%) is sent to the reforming device 7; the drying device 8 is connected to the jacket of the cracking device 9, and the biogas residue is dried in the drying device 8 under the action of the high-temperature flue gas from the jacket of the cracking device 9, and the dried biogas residue (calorific value of about 4-5MJ / kg) is sent to the melting device 11;

[0074] Step 3: The biomass undergoes thermal cracking reaction in the cracking device 9, and the reaction temperature is controlled at 500-700℃ to obtain biochar and cracking gas; the biochar (calorific value of about 7000kcal / kg) is sent to the melting device 11 through the carbon conveying device 10; the cracking gas is removed from the impurities of tar, dust, and water vapor through the purification device 1, and the main components of the purified cracking gas are: CO: 26-33%, H2: 23-30%, CO2: 15-25%, CH4: 12-18%, C2H4: 3-6%, and others (N2, O2, etc.): 5-10%. The cracking gas alkanes (CH4, C2H4) are separated by the separation device 2, and the cracking gas alkanes are sent to the reforming device 7, and the other cracking gases are sent to the conversion device 3 and the synthesis device 4 respectively;

[0075] Step 4: The biochar, dried biogas residue and flux (dried wheat straw) are burned and melted in the melting device 11, the reaction temperature is controlled to be about 850°C, and the melting zone is controlled to be a reducing atmosphere, that is, high-temperature flue gas and high-temperature slag are obtained; the high-temperature flue gas is sent to the jacket of the cracking device 9 to provide heat for the cracking device 9, and the high-temperature flue gas is sent to the steam generating device 12 to generate steam, and the steam is sent to the reforming device 7 and the conversion device 3 respectively; the high-temperature slag enters the quenching device 14 for rapid cooling to obtain vitrified slag;

[0076] Step 5: In the reforming device 7, biomethane and cracked gas alkanes are reformed into a mixed gas of carbon monoxide and hydrogen under the action of steam and catalyst. The ratio of hydrogen to carbon monoxide in the mixed gas is close to 3, and the mixed gas is sent to the synthesis device 4;

[0077] Step 6: Steam and other cracked gases from the separation device 2 undergo a shift reaction in the shift device 3 through the action of a catalyst, converting part of the carbon monoxide into hydrogen, which is then transported to the synthesis device 4, and the CO shift ratio is adjusted so that the ratio of H2 to CO in the synthesis device 4 is maintained at 2.5;

[0078] Step 7: In the synthesis device 4, the mixed gas of carbon monoxide and hydrogen from the reforming device 7, the separation device 2, and the conversion device 3 is synthesized into CH3OH at 270°C, 5.0MPa, and a copper-based catalyst;

[0079] Step 8: The flue gas in the drying device 8 and the flue gas in the steam generating device 12 are concentrated in the flue gas treatment device 13 for treatment and emission up to standard. The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, shall be covered within the protection scope of the present invention.

Claims

1. A system for preparing liquid fuel by coupling biomethane with biomass pyrolysis gas, characterized in that: include: Methane generating device (5), purification device (6), cracking device (9), separation device (2), melting device (11), steam generating device (12), reforming device (7), conversion device (3), synthesis device (4); The biomass undergoes an anaerobic fermentation reaction in the methane generating device (5) to generate biogas and biogas residue, the biogas outlet of the methane generating device (5) is connected to the purification device (6), and the biogas residue outlet of the methane generating device (5) is connected to the biogas residue inlet of the melting device (11); The biogas is purified in the purification device (6) to obtain biomethane, and the purification device (6) is connected to the reforming device (7); The biomass undergoes a thermal cracking reaction in the cracking device (9), the cracking device (9) is heated by an external source, and produces biochar and cracking gas, the biochar outlet of the cracking device (9) is connected to the biochar inlet of the melting device (11), and the cracking gas outlet of the cracking device (9) is connected to the separation device (2); The cracked gas is separated from methane in the separation device (2), the cracked gas alkane outlet of the separation device (2) is connected to the reforming device (7), and the other cracked gas outlets are respectively connected to the conversion device (3), and the conversion device (3) is connected to the synthesis device (4); The biogas residue and biochar are burned and melted in the melting device (11) to generate high-temperature flue gas and molten slag, and the high-temperature flue gas outlet of the melting device (11) is connected to the steam generating device (12); The steam outlet of the steam generating device (12) is connected to the reforming device (7) and the conversion device (3) respectively; Biomethane, cracked gas alkanes, and steam are catalytically converted into a mixed gas of carbon monoxide and hydrogen in the reforming device (7), and the mixed gas outlet of the reforming device (7) is connected to the synthesis device (4); Other cracked gas and steam are catalytically converted into a mixed gas of carbon monoxide and hydrogen in the conversion device (3), and the mixed gas outlet of the conversion device (3) is connected to the synthesis device (4).

2. A system for preparing liquid fuel from biomethane coupled with biomass pyrolysis gas according to claim 1, characterized in that: The cracking device (9) is wrapped by a jacket for heat supply, and the jacket inlet is connected to the high-temperature flue gas outlet of the melting device (11).

3. A system for preparing liquid fuel from biomethane coupled with biomass pyrolysis gas according to claim 2, characterized in that: The jacket outlet of the cracking device (9) is also connected to a drying device (8), and the drying device (8) is respectively connected to the sludge outlet of the methane generating device (5) and the sludge inlet of the melting device (11); the drying device (8) is also provided with a flue gas outlet, and the flue gas outlet is also connected to a flue gas treatment device (13).

4. A system for preparing liquid fuel from biomethane coupled with biomass pyrolysis gas according to claim 3, characterized in that: The smoke outlet of the steam generating device (12) is connected to the smoke treatment device (13).

5. The system for preparing liquid fuel by coupling biomass pyrolysis gas with biomethane according to claim 1, characterized in that: The melting device (11) is a two-stage structure, the front section is a melting zone, and the rear section is a combustion zone; the melting zone is provided with an inlet for adding a flux, and the melting zone is respectively connected to the cracking device (9) and the methane generating device (5) for receiving dried biogas residue and biochar, and the melting zone is connected to the quenching device (14) through a slag outlet; the combustion zone is provided with a high-temperature flue gas outlet, which is connected to the cracking device (9) and the steam generating device (12); the flux is an organic flux, and its components are a hydrocarbon mixture and an alkali metal oxide.

6. The system for preparing liquid fuel from biomethane coupled with biomass pyrolysis gas according to claim 3, characterized in that: A purification device (1) is also provided between the cracked gas outlet of the cracking device (9) and the separation device (2); and a charcoal conveying device (10) is also provided between the biochar outlet of the cracking device (9) and the biochar inlet of the melting device (11).

7. A system for preparing liquid fuel from biomethane coupled with biomass pyrolysis gas according to claim 6, characterized in that: The separation device (2) is connected to the synthesis device (4).

8. The method for preparing a liquid fuel system by coupling biomethane with biomass pyrolysis gas according to claim 7, characterized in that: The following steps are involved: Step 1: biomass is added to a methane generating device (5) and a cracking device (9) respectively; Step 2: The biomass undergoes an anaerobic fermentation reaction in the methane generating device (5) to generate biogas and biogas residue; the biogas is purified by the purification device (6) to remove the water vapor and carbon dioxide carried by the biogas, and the obtained biomethane is sent to the reforming device (7); the drying device (8) is connected to the jacket of the cracking device (9), and the biogas residue is dried in the drying device (8) under the action of the high-temperature flue gas from the jacket of the cracking device (9), and the dried biogas residue is sent to the melting device (11); Step 3: The biomass undergoes a thermal cracking reaction in the cracking device (9), and the heat is obtained from the high-temperature flue gas in the jacket, thereby obtaining biochar and cracking gas; the biochar is sent to the melting device (11) via the carbon conveying device (10); the cracking gas is passed through the purification device (1) to remove the impure tar, dust and water vapor, and the cracking gas obtained after purification is separated from the cracking gas alkane by the separation device (2). The cracking gas alkane obtained is sent to the reforming device (7), and the other cracking gas is sent to the conversion device (3) and the synthesis device (4) respectively; Step 4: The biochar, dried biogas residue and added flux are burned and melted in a melting device (11) to obtain high-temperature flue gas and high-temperature slag; part of the high-temperature flue gas is sent to the jacket of the cracking device (9) to provide heat for the cracking device (9), and the rest of the high-temperature flue gas is sent to the steam generating device (12) to generate steam, and the steam is sent to the reforming device (7) and the conversion device (3) respectively; the high-temperature slag enters the quenching device (14) to be quenched to obtain vitrified slag; Step 5: In the reforming device (7), biomethane and cracked gas alkanes are reformed into a mixed gas of carbon monoxide and hydrogen under the action of steam and catalyst, and the mixed gas is sent to the synthesis device (4); Step 6: Steam and other cracked gases from the separation device (2) undergo a conversion reaction in the conversion device (3) under the action of a catalyst, and the carbon monoxide in the other cracked gases is converted into hydrogen, which is then transported to the synthesis device (4) to ensure a suitable carbon-hydrogen ratio in the subsequent synthesis device (4); Step 7: In the synthesis device (4), the mixed gas of carbon monoxide and hydrogen from the reforming device (7), the separation device (2) and the conversion device (3) can be converted into corresponding liquid fuel under different reaction conditions and the action of catalysts; Step 8: The flue gas in the drying device (8) and the flue gas in the steam generating device (12) are concentrated in the flue gas treatment device (13) for treatment to meet emission standards.

9. The method for preparing liquid fuel by coupling biomethane with biomass pyrolysis gas according to claim 8, characterized in that: The reaction temperature of the inner cylinder of the cracking device (9) is controlled at 500-700°C; the excess air coefficient of the melting zone of the melting device (11) is less than 1.

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