Biogas power plant internal combustion engine waste heat and lng cold energy comprehensive utilization power generation system and method

Abstract

The application provides a biogas plant internal combustion engine waste heat and LNG cold energy comprehensive utilization power generation system and method, the system comprises a transcritical CO2 cycle device, an LNG heating expansion device and a biogas internal combustion engine device, CO2 generated by combustion of the biogas internal combustion engine device is stored in a CO2 liquefaction storage tank after liquefaction treatment, and biogas generated by the biogas plant is stored in an LNG liquefaction storage tank after treatment; CO2 output by the CO2 liquefaction storage tank is pressurized to supercritical pressure, then heated and expanded in a first turbine expander to generate power, and LNG output by the LNG liquefaction storage tank is pressurized, then heated and expanded in a second turbine expander to generate power, and the heat for heating comes from flue gas generated by the biogas internal combustion engine device. The application effectively utilizes multi-grade energy existing in the biogas plant, improves energy efficiency and power output capacity, increases power production, reduces CO2 emission and has good economic benefits.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of comprehensive utilization of multi-grade energy in biogas plants, in particular, to a biogas plant internal combustion engine waste heat and LNG cold energy comprehensive utilization power generation system and method. BACKGROUND

[0002] The increasing living standards lead to a large consumption of non-renewable energy, serious environmental pollution problems worldwide and energy crisis. Therefore, renewable energy plays an increasingly important role in the future energy structure. Compared with other renewable energy, biomass energy has the advantages of large reserves, easy access and broad prospects. Anaerobic fermentation technology is an effective method for turning waste into treasure by utilizing biomass energy. Organic waste, such as agricultural straw and livestock wastewater, can be converted into biogas and organic fertilizer through anaerobic fermentation technology. China is rich in biomass resources, and the government encourages centralized and distributed biogas plants to harmlessly treat organic waste. Therefore, biogas plants have developed rapidly in recent years, but they still have problems such as high cost, long investment recovery period, and extensive energy management. How to comprehensively utilize the various different grades of energy in the biogas plant to improve the economic performance of the biogas plant is a key issue. Existing researches are mostly based on simple power cycles such as organic Rankine cycle to recover the waste heat of biogas internal combustion engine exhaust gas or based on simple cogeneration systems, which cannot comprehensively utilize various grades of energy to maximize the output of electric energy.

[0003] A part of the biogas produced in the biogas plant is used for combustion power generation, and another part is purified into bio-natural gas through a purification process. The byproduct of the purification process is CO2, which has the advantages of high stability, non-toxicity and low cost, and is very suitable as a working medium for power cycles. The transcritical CO2 cycle has the advantages of simple structure, strong stability and low investment cost, but needs a suitable cold source. In recent years, China has imported an increasing amount of LNG, and the gasification process of LNG needs additional heat, while LNG itself has a considerable amount of cold energy. Liquefied Natural Gas (LNG) is mainly composed of methane and is recognized as the cleanest fossil energy on earth. It is colorless, odorless, non-toxic and non-corrosive, and its volume is about 1 / 625 of the same amount of gaseous natural gas, and the mass of liquefied natural gas is only about 45% of the same volume of water. The exhaust gas of the biogas plant internal combustion engine generally has a high exhaust gas temperature. How to comprehensively utilize the waste heat of the internal combustion engine exhaust gas, the cold energy of the CO2 working medium and the cold energy of the LNG is a technical problem. Solving this technical problem is of great significance to improve the energy utilization efficiency and power generation capacity of the biogas plant. SUMMARY

[0004] In view of the defects in the prior art, the purpose of the present application is to provide a biogas plant internal combustion engine waste heat and LNG cold energy comprehensive utilization power generation system and method.

[0005] The application provides a biogas plant internal combustion engine waste heat and LNG cold energy comprehensive utilization power generation system, which comprises a transcritical CO2 cycle device, an LNG heating expansion device and a biogas internal combustion engine device.

[0006] The transcritical CO2 cycle device is provided with a CO2 liquefaction storage tank and a first turbine expander, the LNG heating expansion device is provided with a second turbine expander and an LNG liquefaction storage tank, CO2 generated by combustion of the biogas internal combustion engine device is stored in the CO2 liquefaction storage tank after being liquefied, and biogas generated by the biogas plant is stored in the LNG liquefaction storage tank after being processed to form LNG.

[0007] CO2 output by the CO2 liquefaction storage tank is pressurized to supercritical pressure, obtains first heat, and is expanded in the first turbine expander to generate power, and LNG output by the LNG liquefaction storage tank is pressurized, obtains second heat, and is expanded in the second turbine expander to generate power.

[0008] Part of the first heat and the second heat is from flue gas waste heat generated by the biogas internal combustion engine device.

[0009] Preferably, the transcritical CO2 cycle device is further provided with a first high-pressure pump, an air heat exchanger and a first high-temperature flue gas heat exchanger, CO2 output by the CO2 liquefaction storage tank is pressurized to supercritical pressure by the first high-pressure pump, and then sequentially passes through the air heat exchanger and the first high-temperature flue gas heat exchanger to enter the first turbine expander, wherein the heat exchange medium in the air heat exchanger is air, and the heat exchange medium in the first high-temperature flue gas heat exchanger is flue gas generated by the biogas internal combustion engine device.

[0010] Preferably, the LNG heating expansion device is further provided with a second high-temperature flue gas heat exchanger, a low-temperature LNG preheater and a second high-pressure pump, LNG output by the LNG liquefaction storage tank is pressurized by the second high-pressure pump, and then sequentially passes through the low-temperature LNG preheater and the second high-temperature flue gas heat exchanger to enter the second turbine expander, wherein the heat exchange medium in the second high-temperature flue gas heat exchanger is flue gas generated by the biogas internal combustion engine device, and the heat exchange medium in the low-temperature LNG preheater is CO2 discharged from the first turbine expander.

[0011] Preferably, the biogas internal combustion engine device comprises a biogas internal combustion engine group and a flue gas diverter, flue gas generated after combustion of the biogas internal combustion engine group is separated by the flue gas diverter, wherein one of the separated flue gas enters the first high-temperature flue gas heat exchanger, and the other enters the second high-temperature flue gas heat exchanger.

[0012] Preferably, the temperature of the outlet flue gas of the first high-temperature flue gas heat exchanger and the second high-temperature flue gas heat exchanger is greater than 80℃.

[0013] Preferably, the pressure ratio of the first high-pressure pump and the second high-pressure pump can be adjusted to match the power generation demand of the system.

[0014] Preferably, the proportion of flue gas entering the first high-temperature flue gas heat exchanger and the second high-temperature flue gas heat exchanger can be adjusted to match the dynamic fluctuations of the system.

[0015] Preferably, the pressure of the CO2 discharged from the first turbine expander is higher than the pressure in the CO2 liquefaction storage tank, and the pressure of the natural gas discharged from the second turbine expander is higher than the operating pressure of the natural gas pipeline network.

[0016] Preferably, the air heat exchanger, the first high-temperature flue gas heat exchanger, and the second high-temperature flue gas heat exchanger all use printed circuit board finned heat exchangers, and the low-temperature LNG preheater uses a plate heat exchanger.

[0017] According to the application, a power generation method based on the comprehensive utilization of biogas plant internal combustion engine waste heat and LNG cold energy is provided, which comprises the following steps:

[0018] S1: According to the daily biogas production of the biogas plant and the flow of biogas entering the biogas internal combustion engine device for combustion, the flow and heat exchange amount of the tail gas of the biogas internal combustion engine device are determined, the CO2 circulation flow of the transcritical CO2 circulation device and the LNG heating expansion device is set, the LNG expansion flow is determined, and the optimal flue gas split ratio is determined;

[0019] S2: Determine the pressure boosting ratio range of CO2 and LNG in the transcritical CO2 circulation device and the LNG heating expansion device;

[0020] S3: The flue gas from the biogas internal combustion engine device is split by a flue gas splitter and enters the heat exchangers of the transcritical CO2 circulation device and the LNG heating expansion device, respectively, and exchanges heat with the working medium to reach the specified temperature range and then is discharged;

[0021] S4: The inlet temperature range of the turbine expander working medium of the transcritical CO2 circulation device and the LNG heating expansion device is set.

[0022] Compared with the prior art, the application has the following beneficial effects:

[0023] 1. The present application effectively utilizes the multi-grade energy existing in the biogas plant, including the high-temperature waste heat of the biogas engine exhaust gas, the cold energy of CO2 and LNG byproducts in the biogas purification process, and utilizes the high-temperature flue gas heat energy and the low-temperature internal energy of LNG to simultaneously attack the environmental state, increasing the heat exchange energy temperature difference and improving the system work capacity; the transcritical CO2 power system and the LNG heating expansion system are used, and the CO2 working medium is derived from the purification system of the biogas plant, reducing the system cost; the flue gas diverter is used to flexibly match the flue gas heat energy demand of the two power systems, improve the power output and heat energy utilization rate, improve the energy efficiency and power output capacity, compared with the traditional cogeneration system, the system has higher energy utilization efficiency, increases the power production, has greater output work, reduces the CO2 emission, and has better economic benefit.

[0024] 2. The present application utilizes the combined system of the supercritical CO2 cycle and the LNG heating direct expansion system, has less equipment, low cost and high stability.

[0025] 3. The present application can flexibly match the flue gas flow of the comprehensive power system according to the biogas yield of the biogas plant, increasing the adaptability and operation stability of the system and the biogas plant. BRIEF DESCRIPTION OF DRAWINGS

[0026] Other characteristics, objects and advantages of the present application will become more apparent from the following detailed description of non-limiting embodiments, made with reference to the accompanying drawings:

[0027] Figure 1 The figure shows the principle of the present application.

[0028] In the figure:

[0029] CO2 liquefied storage tank 1

[0030] First high-pressure pump 2

[0031] Air heat exchanger 3

[0032] First high-temperature flue gas heat exchanger 4

[0033] First turbine expander 5

[0034] Second turbine expander 6

[0035] Second high-temperature flue gas heat exchanger 7

[0036] Low-temperature LNG preheater 8

[0037] Second high-pressure pump 9

[0038] LNG liquefied storage tank 10

[0039] Biogas internal combustion engine set 11

[0040] Flue gas diverter 12 Detailed implementation manners

[0041] The present invention will be described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that those of ordinary skill in the art can make several changes and improvements without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0042] To solve the deficiencies in the prior art and comprehensively utilize the waste heat of the biogas internal combustion engine exhaust gas, the cold energy of the CO2 working medium, and the cold energy of LNG for power generation, the present invention provides a comprehensive utilization power generation system for the waste heat of the biogas plant internal combustion engine and the cold energy of LNG, which improves the energy utilization efficiency and economic benefits of the biogas plant, as Figure 1 shown, including a transcritical CO2 cycle device, an LNG heating and expansion device, and a biogas internal combustion engine device; the transcritical CO2 cycle device is configured with a CO2 liquefied storage tank 1, a first high-pressure pump 2, an air heat exchanger 3, a first high-temperature flue gas heat exchanger ④, and a first turbine expander 5 to form a closed-loop transcritical CO2 power cycle. The LNG heating and expansion device is configured with an LNG liquefied storage tank 10, a second high-pressure pump 9, a low-temperature LNG preheater 8, a second high-temperature flue gas heat exchanger 7, and a second turbine expander 6 to form an open-loop LNG heating and expansion system. The biogas internal combustion engine device includes a biogas internal combustion engine group 11 and a flue gas diverter 12.

[0043] As Figure 1 shown, the CO2 generated by the combustion of the biogas internal combustion engine device is liquefied and stored in the CO2 liquefied storage tank 1. The CO2 in the CO2 liquefied storage tank 1 is in a liquid state, with a temperature of approximately -30°C to -25°C and a pressure of approximately 1.43 MPa to 1.55 MPa. Its flow rate is controlled by a regulating valve supporting the storage tank. The biogas generated by the biogas plant is processed to form LNG and stored in the LNG liquefied storage tank 10; the CO2 output from the CO2 liquefied storage tank 1 is pressurized to a supercritical pressure and then expands and does work to generate electricity in the first turbine expander ⑤ after obtaining the first heat. The LNG output from the LNG liquefied storage tank 10 is pressurized and then expands and does work to generate electricity in the second turbine expander 6 after obtaining the second heat; wherein, part of the first heat and the second heat comes from the waste heat of the flue gas generated by the biogas internal combustion engine device.

[0044] Specifically, the CO2 output from the CO2 liquefied storage tank 1 is pressurized to supercritical pressure by the first high-pressure pump 2, and then sequentially exchanges heat with the air heat exchanger 3 and the first high-temperature flue gas heat exchanger 4 before entering the first turbine expander 5 to expand and generate power. The CO2 discharged from the first turbine expander 5 finally flows back into the CO2 liquefied storage tank 1, wherein the supercritical pressure of the CO2 is greater than 8 MPa, the heat exchange medium used in the air heat exchanger 3 is space air, the CO2 after heat exchange in the air heat exchanger 3 changes from liquid to gas, and the temperature slightly increases. Subsequently, the CO2 enters the first high-temperature flue gas heat exchanger 4, and the heat exchange medium in the first high-temperature flue gas heat exchanger 4 is the flue gas generated from the biogas internal combustion engine device. The CO2 absorbs the waste heat of the flue gas to rise to a high-temperature and high-pressure state.

[0045] The LNG output from the LNG liquefied storage tank 10 is pressurized by the second high-pressure pump 9, and then sequentially exchanges heat with the low-temperature LNG preheater 8 and the second high-temperature flue gas heat exchanger 7 before entering the second turbine expander 6 to expand and generate power. The LNG pressurized by the second high-pressure pump 9 reaches a supercritical pressure, wherein the heat exchange medium used in the second high-temperature flue gas heat exchanger 7 is the flue gas generated from the biogas internal combustion engine device, and the heat exchange medium used in the low-temperature LNG preheater 8 is the CO2 discharged from the first turbine expander 5. The CO2 working medium discharged from the first turbine expander 5 enters the low-temperature LNG preheater 8 to exchange heat with the LNG, transferring the waste heat to the LNG. The LNG changes from gas to liquid after heat exchange in the low-temperature LNG preheater 8, and the CO2 after heat exchange in the low-temperature LNG preheater 8 flows back into the CO2 liquefied storage tank 1, completing a closed cycle. The second high-temperature flue gas heat exchanger 7 heats the natural gas from the low-temperature LNG preheater 8, increasing its temperature and improving its expansion work capacity. The gaseous natural gas output from the second high-temperature flue gas heat exchanger 7 reaches the second turbine expander 6 to expand and generate power. It should be noted that the pressure of the natural gas NG at the outlet of the second turbine expander 6 is slightly higher than the operating pressure of the natural gas pipeline network, and the temperature is within the specified range at the inlet of the natural gas pipeline network, facilitating direct connection to the pipeline network to save the cost of a flow pump. The pressure of the CO2 working medium at the outlet of the first turbine expander 5 is preferably slightly higher than the pressure of the CO2 liquefied storage tank 1 to save the cost of a backflow low-pressure pump.

[0046] Further, the flue gas generated after combustion of the biogas internal combustion engine group 11 is separated by the flue gas flow divider 12, and the separated flue gas enters the first high-temperature flue gas heat exchanger 4 for heat exchange and the second high-temperature flue gas heat exchanger 7 for heat exchange. It should be noted that, in order to avoid low-temperature corrosion of equipment, the temperature of the flue gas at the outlets of the first high-temperature flue gas heat exchanger 4 and the second high-temperature flue gas heat exchanger 7 is greater than 80℃.

[0047] It should be noted that the pressure ratio of the first high-pressure pump 2 and the second high-pressure pump 9 can be adjusted to match the power generation demand of the system, and the proportion of the flue gas entering the first high-temperature flue gas heat exchanger 4 and the second high-temperature flue gas heat exchanger 7 can be adjusted to match the dynamic fluctuations of the system, so that the system can achieve the best operating state.

[0048] The air heat exchanger 3, the first high-temperature flue gas heat exchanger 4, and the second high-temperature flue gas heat exchanger 7 all adopt printed circuit board fin heat exchangers, which have excellent high-temperature and high-pressure resistance, compact structure, and small footprint; the low-temperature LNG preheater 8 adopts a plate heat exchanger, which has good cooling performance and is commonly used in fluid condensers.

[0049] Based on the system in the present application, the present application also provides a power generation method based on comprehensive utilization of biogas plant internal combustion engine waste heat and LNG cold energy, comprising the following steps:

[0050] S1: According to the daily biogas production of the biogas plant and the flow of the biogas entering the biogas internal combustion engine device for combustion, the flow and heat exchange amount of the tail gas of the biogas internal combustion engine device are determined, the CO2 circulation flow of the transcritical CO2 circulation device and the LNG heating expansion device and the LNG expansion flow are set, and the optimal flue gas split ratio is determined; experiments show that, based on a biogas plant with a daily biogas production of 50000m 3 / day, when the flue gas split ratio is increased from 3:10 to 3:5, the system efficiency is increased from 47% to 57%, and in the original system, the flue gas generated by the biogas internal combustion engine device is directly discharged into the atmosphere after being cooled by circulating water, and the present application not only eliminates the circulating water cooling link, but also recycles the waste heat of the flue gas for power generation, and the utilization mode of the present application is compared with the original system, the power generation efficiency of the biogas internal combustion engine in the present application is increased from 38% to 46%; the excess power is converted into standard coal, which can save about 2300 tons of standard coal per year, and the corresponding CO2 emission reduction amount is 5100 tons per year, which has good energy efficiency and environmental benefits, not only utilizes the waste heat energy in the flue gas, but also ingeniously utilizes the cold energy of the LNG, creates a cold and hot hedging efficient collaborative utilization energy utilization strategy, and provides a new idea for the development and progress of subsequent biogas plant energy management strategies.

[0051] S2: Determine the pressure ratio range of CO2 and LNG in the transcritical CO2 circulation device and the LNG heating expansion device, and the pressure ratio of CO2 and LNG is preferably 6-15, so as to save electric energy consumption and increase net power output;

[0052] S3: The flue gas from the biogas internal combustion engine device is divided by the flue gas diverter 12 and enters the heat exchangers of the transcritical CO2 cycle device and the LNG heating expansion device respectively, exchanges heat with the working medium, and is discharged after reaching the limited temperature range, which must meet the requirement that the tail gas discharge temperature is always higher than the dew point temperature to avoid acid dew point and acid corrosion;

[0053] S4: The inlet temperature range of the turbine expander working medium of the transcritical CO2 cycle device and the LNG heating expansion device is set respectively to obtain the best power output and improve the system efficiency.

[0054] It should be noted that the temperature of CO2 before entering the first turbine expander 5 is set to be preferably 400-500 DEG C, and the flow rate is determined according to the flow rate of the flue gas entering the first high-temperature flue gas heat exchanger 4. The temperature range of CO2 at the outlet of the air heat exchanger 3 needs to be set, and the flow rate of the air flowing into the air heat exchanger 3 is determined by the temperature range. The temperature of the CO2 working medium at the outlet of the low-temperature LNG preheater 8 is close to the temperature of the CO2 liquefied storage tank 1. The flow rate at the outlet of the LNG liquefied storage tank 10 depends on the heat exchange amount in the low-temperature LNG preheater 8, and the purpose is to make the LNG fully absorb the heat energy of the CO2 working medium. The temperature of LNG at the inlet of the second turbine expander 6 depends on the flow rate of the flue gas entering the second high-temperature flue gas heat exchanger 7, and the greater the flow rate of the flue gas, the higher the temperature of LNG at the inlet of the second turbine expander 6.

[0055] The working principle of the present application is as follows:

[0056] As shown in Figure 1 , when the system is running, the CO2 working medium flows out of the CO2 liquefied storage tank 1, is pressurized by the first high-pressure pump 2, is preheated by the low-temperature air heat exchanger 3, absorbs the waste heat of the flue gas in the first high-temperature flue gas heat exchanger 4 of the transcritical CO2 cycle system, becomes high-temperature and high-pressure at the outlet, then enters the first turbine expander 5 to do work, and after leaving the first turbine expander 5, enters the low-temperature LNG preheater 8, transfers heat to LNG, and the CO2 changes from gas to liquid at the outlet of the low-temperature LNG preheater 8, returns to the CO2 liquefied storage tank 1, and completes the cycle. LNG flows out of the LNG liquefied storage tank 10, is pressurized by the second high-pressure pump 9, enters the low-temperature LNG preheater 8, absorbs the waste heat of CO2, becomes low-temperature gaseous natural gas at the outlet of the low-temperature LNG preheater 8, is heated by the flue gas in the second high-temperature flue gas heat exchanger 7, and finally enters the second turbine expander 6 to do work, and after flowing out of the second turbine expander 6, is introduced into the natural gas pipeline network to complete the cycle.

[0057] In the description of the present application, it should be understood that the terms "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer" and the like refer to the orientation or positional relationship shown in the drawings, and are only intended to facilitate the description of the present application and simplify the description, and are not intended to indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application.

[0058] The specific embodiments of the present application are described above. It should be understood that the present application is not limited to the above specific embodiments, and various changes or modifications can be made by those skilled in the art within the scope of the claims, which do not affect the essential content of the present application. The embodiments of the present application and the features in the embodiments can be arbitrarily combined with each other without conflict, provided that the combinations do not conflict.

Claims

1. A biogas plant internal combustion engine waste heat and LNG cold energy comprehensive utilization power generation system, characterized in that, The device comprises a transcritical CO2 cycle device, an LNG heating expansion device and a biogas internal combustion engine device; The transcritical CO2 cycle device is configured with a CO2 liquefaction storage tank (1) and a first turbine expander (5), the LNG heating expansion device is configured with a second turbine expander (6) and an LNG liquefaction storage tank (10), the CO2 produced by the biogas internal combustion engine device is liquefied and stored in the CO2 liquefaction storage tank (1), and the biogas produced by a biogas plant is processed to form LNG and stored in the LNG liquefaction storage tank (10); The CO2 output by the CO2 liquefaction storage tank (1) is pressurized to supercritical pressure, then passes through the first heat obtained and expands in the first turbine expander (5) to generate power, and the LNG output by the LNG liquefaction storage tank (10) is pressurized, then passes through the second heat obtained and expands in the second turbine expander (6) to generate power; Part of the first heat and the second heat comes from the flue gas waste heat generated by the biogas internal combustion engine device; The transcritical CO2 cycle device is further configured with a first high-pressure pump (2), an air heat exchanger (3) and a first high-temperature flue gas heat exchanger (4), the CO2 output by the CO2 liquefaction storage tank (1) is pressurized to supercritical pressure by the first high-pressure pump (2), then sequentially passes through the air heat exchanger (3) and the first high-temperature flue gas heat exchanger (4) for heat exchange, and then enters the first turbine expander (5), wherein the heat exchange medium in the air heat exchanger (3) is air, and the heat exchange medium in the first high-temperature flue gas heat exchanger (4) is flue gas generated by the biogas internal combustion engine device; The LNG heating expansion device is further configured with a second high-temperature flue gas heat exchanger (7), a low-temperature LNG preheater (8) and a second high-pressure pump (9), the LNG output by the LNG liquefaction storage tank (10) is pressurized by the second high-pressure pump (9), then sequentially passes through the low-temperature LNG preheater (8) and the second high-temperature flue gas heat exchanger (7), and then enters the second turbine expander (6), wherein the heat exchange medium in the second high-temperature flue gas heat exchanger (7) is flue gas generated by the biogas internal combustion engine device, and the heat exchange medium in the low-temperature LNG preheater (8) is CO2 discharged from the first turbine expander (5); The pressure ratio of the first high-pressure pump (2) and the second high-pressure pump (9) can be adjusted to match the power generation demand of the system; The proportion of flue gas entering the first high-temperature flue gas heat exchanger (4) and the second high-temperature flue gas heat exchanger (7) can be adjusted to match the dynamic fluctuations of the system; The pressure of CO2 discharged from the first turbine expander (5) is higher than the pressure in the CO2 liquefaction storage tank (1), and the pressure of natural gas discharged from the second turbine expander (6) is higher than the operating pressure of the natural gas pipeline network; The air heat exchanger (3), the first high-temperature flue gas heat exchanger (4) and the second high-temperature flue gas heat exchanger (7) all adopt printed circuit board fin heat exchangers, and the low-temperature LNG preheater (8) adopts a plate heat exchanger.

2. The biogas plant internal combustion engine waste heat and LNG cold energy combined utilization power generation system according to claim 1, characterized in that, The biogas internal combustion engine device comprises a biogas internal combustion engine group (11) and a flue gas shunt (12), the flue gas produced after combustion of the biogas internal combustion engine group (11) is separated by the flue gas shunt (12), wherein the separated flue gas enters the first high-temperature flue gas heat exchanger (4) and the second high-temperature flue gas heat exchanger (7).

3. The biogas plant internal combustion engine waste heat and LNG cold energy combined utilization power generation system according to claim 1, characterized in that, The temperature of the flue gas at the outlet of the first high-temperature flue gas heat exchanger (4) and the second high-temperature flue gas heat exchanger (7) is greater than 80℃.

4. A method for generating electricity based on comprehensive utilization of waste heat of a biogas plant internal combustion engine and LNG cold energy, characterized in that, The biogas plant internal combustion engine waste heat and LNG cold energy comprehensive utilization power generation system based on any one of claims 1 to 3 comprises the following steps: S1: determining the flow rate of the flue gas of the biogas internal combustion engine device and the total heat exchange amount according to the daily biogas production of the biogas plant and the flow rate of the biogas entering the biogas internal combustion engine device for combustion, setting the CO2 circulation flow rate of the transcritical CO2 cycle device and the LNG heating expansion device and the LNG expansion flow rate, and determining the optimal flue gas shunt ratio; S2: determining the pressurization ratio range of CO2 and LNG in the transcritical CO2 cycle device and the LNG heating expansion device; S3: the flue gas from the biogas internal combustion engine device is shunted by the flue gas shunt (12) and enters the heat exchangers of the transcritical CO2 cycle device and the LNG heating expansion device respectively, and is exchanged with the working medium to reach the limited temperature range and then is discharged; S4: setting the inlet temperature range of the working medium of the turbine expander of the transcritical CO2 cycle device and the LNG heating expansion device respectively.

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