A system and method for co-production of bio-natural gas and liquid carbon dioxide by dry anaerobic fermentation of straw

Abstract

This invention relates to the field of biomass energy and waste resource utilization technology, specifically a system and method for the co-production of biogas and liquid carbon dioxide through dry anaerobic fermentation of straw. The system integrates a pretreatment module, an anaerobic fermentation and biogas collection and storage module, an anaerobic digestion liquid solid-liquid separation module, a biogas purification and upgrading module, a carbon capture and liquefaction module, and an energy cascade utilization and by-product treatment module. A non-clogging vacuum feeding and discharging system for transporting the mixed materials is installed between the pretreatment module and the anaerobic fermentation and anaerobic digestion liquid solid-liquid separation module. This invention innovatively integrates multiple independent process units, forming a closed loop from straw to various high-value products. It solves the problems of short chains and fragmented processes in traditional processes. The final products include biogas, electricity, liquid CO2, biomass fuel, and organic fertilizer, achieving "five outputs" (biogas, electricity, liquid CO2, biomass fuel, and organic fertilizer), maximizing the value of straw and possessing extremely high market adaptability and risk resistance.

Description

Technical Field

[0001] This invention relates to the field of biomass energy and waste resource utilization technology, specifically to a system and method for the co-production of biogas and liquid carbon dioxide through dry anaerobic fermentation of straw. Background Technology

[0002] With the development of biogas attracting much attention, crop straw, as an important biomass resource, is primarily utilized for energy through anaerobic fermentation. Currently, the mainstream technology is wet fermentation (solid content <15%), which suffers from problems such as high water consumption, large amounts of biogas slurry produced, high subsequent treatment costs, and high energy consumption.

[0003] Dry anaerobic fermentation (20-30% solids content) can effectively reduce water consumption and biogas slurry volume, but it faces the following technical bottlenecks: ① Pretreatment and transportation challenges: Straw is loose and has a high fiber content, making it prone to tangling and bridging even after crushing, leading to severe blockage of conveying equipment (such as screw conveyors) and poor system stability during continuous operation. ② High biogas purification costs: The biogas produced by straw fermentation has a high H2S concentration (usually >2000ppm), requiring stringent desulfurization processes, frequent replacement of conventional agents, and high operating costs; single wet desulfurization requires significant investment and has limited adaptability to fluctuating loads. ③ Incomplete utilization of carbon: Traditional processes directly emit CO2 separated during purification, resulting in carbon resource waste and greenhouse gas emissions, failing to achieve true "carbon neutrality." ④ System energy efficiency needs optimization: The fermentation process requires continuous heat preservation; relying entirely on external energy sources will significantly reduce the project's energy efficiency and economic viability. Based on these considerations, a system and method for the combined production of biogas and liquid carbon dioxide through dry anaerobic fermentation of straw is proposed. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a system and method for the dry anaerobic fermentation of straw to co-produce biogas and liquid carbon dioxide. It innovatively integrates multiple independent process units to form a closed loop from straw to various high-value products, solving the problems of short process chains and fragmented links in traditional processes. The final products include biogas, liquid CO2, electricity, biomass fuel, and organic fertilizer liquid, maximizing the value of straw and possessing extremely high market adaptability and risk resistance.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a system for the co-production of biogas and liquid carbon dioxide by dry anaerobic fermentation of straw, comprising an integrated pretreatment module, an anaerobic fermentation and biogas collection module, a biogas purification and upgrading module, a carbon capture and liquefaction module, and an energy cascade utilization and by-product treatment module; A non-clogging vacuum feeding system for conveying the mixed materials is provided between the integrated pretreatment module and the anaerobic fermentation and biogas collection module; The carbon capture and liquefaction module is used to liquefy and recover the CO2-rich tail gas discharged from the biogas purification and upgrading module. The energy cascade utilization and by-product treatment module achieves efficient self-use of internal energy through power generation and the utilization of flue gas waste heat.

[0006] Preferably, the integrated pretreatment module includes a twin-shaft shredder, a closed scraper conveyor, and a mixing tank, with an agitator and a steam heater installed in the mixing tank.

[0007] Preferably, the anaerobic fermentation and biogas collection module includes an anaerobic fermenter and a gas storage tank connected thereto, and a gas-water heat exchanger is installed on the anaerobic fermenter.

[0008] Preferably, the biogas purification and upgrading module includes a desulfurization device, a compressor, and a PSA pressure swing adsorption purification device to separate biogas and CO2-rich tail gas.

[0009] Preferably, the carbon capture and liquefaction module includes a liquefaction device connected to the tail gas outlet of the PSA pressure swing adsorption purification device, which converts CO2-rich tail gas into liquid CO2.

[0010] Preferably, the energy cascade utilization and by-product treatment module includes a generator set that generates electricity using biogas, and a waste heat boiler that uses the waste heat from the generator set's flue gas to provide heat to the mixing tank and anaerobic digester.

[0011] Preferably, the energy cascade utilization and by-product treatment module further includes a solid-liquid separator connected to the anaerobic digester for separating the digestate after fermentation. The solid-liquid separator is connected to a deep dewatering device for dewatering the biogas residue and a buffer tank for storing the biogas slurry.

[0012] This invention also provides a method for the co-production of biogas and liquid carbon dioxide through dry anaerobic fermentation of straw, comprising the following steps: S1. After being crushed by a twin-shaft shredder, the straw is fed into a mixing tank by a closed scraper conveyor, where it is evenly mixed with biogas slurry and digestion liquid. The resulting mixture is preheated with steam. S2. The mixture is transported to the anaerobic digester through a vacuum feeding system and fermented at 55±1℃ for 25±5 days. The biogas produced is temporarily stored in a gas storage tank. S3. Biogas is purified by pressurization, wet desulfurization with complexed iron, dry deep desulfurization, compression and PSA pressure swing adsorption to obtain biomethane; the CO2-rich tail gas generated during the purification process is liquefied to obtain liquid CO2. S4. After purification, part of the biogas in the gas storage tank is sent to the generator set for power generation. The high-temperature flue gas generated during the power generation process enters the waste heat boiler for heat recovery. Part of the generated hot steam enters the steam heater to preheat the mixed materials, and the other part enters the steam-water heat exchanger to exchange heat into hot water for heat preservation of the anaerobic fermentation tank. S5. Perform solid-liquid separation treatment on the digestate flowing out of the anaerobic digester to obtain biogas slurry and biogas residue; S6. The biogas slurry is stored in a biogas slurry buffer tank, and the biogas residue is processed into biomass fuel after deep dewatering.

[0013] Preferably, in step S1, the solids content of the mixture is 20-30%.

[0014] Preferably, in step S3, the wet desulfurization is complexed iron wet desulfurization.

[0015] This invention provides a system and method for the co-production of biogas and liquid carbon dioxide through dry anaerobic fermentation of straw, which has the following advantages compared with the prior art: 1. It innovatively integrates multiple independent process units to form a closed loop from straw to various high-value products, solving the problems of short process chains and fragmented links in traditional processes.

[0016] 2. The adoption of the "vacuum power conveying" solution fundamentally overcomes the bottleneck of blockage in the continuous industrial operation of high solids straw materials, ensuring the stability and reliability of the system.

[0017] 3. Innovatively, PSA tail gas (CO2), which is usually regarded as waste gas, is used as raw material to produce liquid CO2 with commercial value. This not only creates additional economic benefits, but also makes the carbon utilization rate of the entire process reach an extremely high level, achieving "negative carbon" or "near-zero carbon" emissions.

[0018] 4. By adopting a cascade utilization model of "power generation and grid connection + waste heat recovery", the dependence on external energy has been significantly reduced, and the energy self-sufficiency and economic efficiency of the project have been improved.

[0019] 5. The final products include biogas, electricity, liquid CO2, biomass fuel and organic fertilizer, achieving "five outputs" and maximizing the value of straw, with extremely high market adaptability and risk resistance. Attached Figure Description

[0020] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings: Figure 1This is a schematic diagram of the system for the co-production of biogas and liquid carbon dioxide by dry anaerobic fermentation of straw according to the present invention; Figure 2 This is a process flow diagram of the dry anaerobic fermentation of straw to produce biogas and liquid carbon dioxide according to the present invention. Detailed Implementation

[0021] The following embodiments are provided to illustrate the implementation of this application in detail, so that the process of how this application uses technical means to solve technical problems and achieve technical effects can be fully understood and implemented accordingly.

[0022] Example 1 A system for the combined production of biogas and liquid carbon dioxide through dry anaerobic fermentation of straw includes an integrated pretreatment module, an anaerobic fermentation and biogas collection module, a biogas purification and upgrading module, a carbon capture and liquefaction module, and an energy cascade utilization and by-product treatment module.

[0023] The aforementioned integrated pretreatment module includes a twin-shaft shredder, a closed scraper conveyor, and a mixing tank. An agitator and a steam heater are installed in the mixing tank, which heats the mixture within. The twin-shaft shredder crushes the straw to a particle size ≤6cm. The crushed straw enters the mixing tank via the closed scraper conveyor, while biogas slurry and digestive liquid are simultaneously injected into the tank for temperature-appropriate fermentation to form the mixed material.

[0024] The aforementioned anaerobic fermentation and biogas collection module includes an anaerobic fermenter and a connected gas storage tank. A steam-water heat exchanger is installed on the anaerobic fermenter to maintain the fermentation reaction temperature. The inlet of the vacuum feeding system is connected to the mixing tank, and the outlet is connected to the anaerobic fermenter. The pressure difference is used to transport the mixed materials in the mixing tank to the anaerobic fermenter without clogging, effectively avoiding the blockage problem of traditional straw transportation. The biogas produced by anaerobic fermentation can flow into the gas storage tank for buffering and temporary storage.

[0025] The aforementioned biogas purification and upgrading module includes a desulfurization unit, a compressor, and a PSA (Pressure Swing Adsorption) purification unit to separate biogas and CO2-rich tail gas. Biogas flowing from the storage tank enters the desulfurization unit for desulfurization treatment. After desulfurization, the biogas is compressed by the compressor and then injected into the PSA purification unit to separate biogas (biogas (product gas) conforming to the GB 17820-2018 standard for natural gas) and CO2-rich tail gas (concentration typically >80%).

[0026] The aforementioned carbon capture and liquefaction module includes a liquefaction unit connected to the tail gas outlet of the PSA pressure swing adsorption purification device. After compression, dehydration, impurity removal, and liquefaction, the CO2-rich tail gas is produced into commercial-grade liquid CO2, realizing the comprehensive resource utilization of carbon components in biogas and turning waste into treasure.

[0027] The aforementioned energy cascade utilization and by-product treatment module includes a generator set that generates electricity using biogas, and a waste heat boiler that uses the waste heat from the generator set's flue gas to provide heat to the mixing tank and anaerobic digester. The generator set generates electricity using partially purified biogas, and the waste heat from the generated flue gas is recovered through the waste heat boiler to produce steam. Part of this steam is used for the steam heater in the mixing tank, and the other part provides a heat source for the insulated hot water system of the anaerobic digester through a steam-water heat exchanger, achieving efficient self-use of energy within the system.

[0028] Furthermore, the energy cascade utilization and by-product treatment module also includes a solid-liquid separator connected to the anaerobic digester to separate the digestate after fermentation. The solid-liquid separator is connected to a deep dewatering device to dewater the biogas residue. The biogas residue is dewatered to a moisture content of <35% and made into biomass briquettes. The solid-liquid separator is also connected to a buffer tank for storing the biogas slurry. The separated biogas slurry is temporarily stored in the buffer tank and undergoes secondary anaerobic digestion. The small amount of biogas produced is fed into the main biogas system, and the stabilized biogas slurry is used for farmland fertilization.

[0029] Example 2 A method for co-producing biogas and liquid carbon dioxide through dry anaerobic fermentation of straw includes the following steps: S1. After being crushed by a twin-shaft shredder, the straw is sent to a mixing tank by a closed scraper conveyor and mixed evenly with biogas slurry and digestion liquid. The mass ratio between them is about 1:6. The resulting mixture with a solid content of 20-30% is preheated with steam. S2. The mixture is transported to the anaerobic digester through a vacuum feeding system and fermented at 55±1℃ for 25±5 days. The biogas produced is temporarily stored in a gas storage tank. S3. Biogas is purified by pressurization, wet desulfurization with complexed iron, dry deep desulfurization, compression and PSA pressure swing adsorption to obtain biomethane; the CO2-rich tail gas generated during the purification process is liquefied to obtain liquid CO2. S4. After purification, part of the biogas in the gas storage tank is sent to the generator set for power generation. The high-temperature flue gas generated during the power generation process enters the waste heat boiler for heat recovery. Part of the generated hot steam enters the steam heater to preheat the mixed materials, and the other part enters the steam-water heat exchanger for fermentation and heat preservation. S5. Perform solid-liquid separation treatment on the digestate flowing out of the anaerobic digester to obtain biogas slurry and biogas residue; S6. The biogas slurry is stored in a biogas slurry buffer tank, and the biogas residue is processed into biomass fuel after deep dewatering.

[0030] Example 3 Using the system for dry anaerobic fermentation of straw in Example 1 to co-produce biogas and liquid carbon dioxide, and the method in Example 2, 100,000 tons of straw were processed.

[0031] Integrated pretreatment module: Two twin-shaft shredders with a processing capacity of 12-15 t / h are selected. The mixing tank has a volume of 2×500 m³, equipped with 4 horizontal agitators (N=18.5kW) and a steam heater. Two sets of vacuum feeding and discharging systems (Q=100 m³ / h) are also included.

[0032] Anaerobic fermentation and biogas collection module: Construct 8 anaerobic fermentation tanks with a volume of 5000m³.

[0033] Biogas purification and upgrading module: Equipped with two sets of complexed iron wet desulfurization units with a capacity of Q=50000 Nm³ / d and two sets of Q-PSA pressure swing adsorption units of the same specifications. The complexed iron wet desulfurization units remove H2S from biogas from ≤3000 mg / m³ to ≤10 ppm.

[0034] Carbon capture and liquefaction module: Equipped with one CO2 liquefaction unit with an annual capture capacity of 20,000 tons.

[0035] Energy cascade utilization and by-product treatment module: equipped with two 1.0MW biogas generator sets. Equipped with three deep dehydration units (processing capacity 120t / d).

[0036] After the system is operational, it will produce 11.7 million standard cubic meters of biogas, 20,000 tons of liquid CO2, and 54,000 tons of biomass fuel annually. Calculations show that the system's energy self-sufficiency rate exceeds 80%, and it will reduce CO2 emissions by approximately 150,000 tons equivalent annually.

[0037] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A system for the co-production of biogas and liquid carbon dioxide through dry anaerobic fermentation of straw, characterized in that, It includes an integrated pretreatment module, an anaerobic fermentation and biogas collection and storage module, an anaerobic digestion liquid solid-liquid separation module, a biogas purification and upgrading module, a carbon capture and liquefaction module, and an energy cascade utilization and by-product treatment module. A non-clogging vacuum feeding and discharging system for conveying the mixed materials is provided between the integrated pretreatment module and the anaerobic fermentation and anaerobic digestion liquid solid-liquid separation module. The carbon capture and liquefaction module is used to liquefy and recover the CO2-rich tail gas discharged from the biogas purification and upgrading module. The energy cascade utilization and by-product treatment module achieves efficient self-use of internal energy through power generation and the utilization of flue gas waste heat.

2. The system for co-producing biogas and liquid carbon dioxide by dry anaerobic fermentation of straw according to claim 1, characterized in that, The integrated pretreatment module includes a twin-shaft shredder, a closed scraper conveyor, and a mixing tank, in which an agitator and a steam heater are installed.

3. The system for co-producing biogas and liquid carbon dioxide through dry anaerobic fermentation of straw according to claim 2, characterized in that, The anaerobic fermentation and biogas collection module includes an anaerobic fermenter and a gas storage tank connected thereto, and a steam-water heat exchanger is installed on the anaerobic fermenter.

4. The system for co-producing biogas and liquid carbon dioxide through dry anaerobic fermentation of straw according to claim 1, characterized in that, The biogas purification and upgrading module includes a desulfurization device, a compressor, and a PSA pressure swing adsorption purification device, used to separate biogas and CO2-rich tail gas.

5. The system for co-producing biogas and liquid carbon dioxide through dry anaerobic fermentation of straw according to claim 1, characterized in that, The carbon capture and liquefaction module includes a liquefaction device connected to the tail gas outlet of the PSA pressure swing adsorption purification device, which converts CO2-rich tail gas into liquid CO2.

6. The system for co-producing biogas and liquid carbon dioxide by dry anaerobic fermentation of straw according to claim 3, characterized in that, The energy cascade utilization and by-product treatment module includes a generator set that generates electricity using biogas, and a waste heat boiler that uses the waste heat from the generator set's flue gas to provide heat to the mixing tank and anaerobic digester.

7. The system for co-producing biogas and liquid carbon dioxide by dry anaerobic fermentation of straw according to claim 6, characterized in that, The energy cascade utilization and by-product treatment module also includes a solid-liquid separator connected to the anaerobic digester to separate the digestate after fermentation. The solid-liquid separator is connected to a deep dewatering device for dewatering the biogas residue and a buffer tank for storing the biogas slurry.

8. A method for co-producing biogas and liquid carbon dioxide through dry anaerobic fermentation of straw, using the system described in any one of claims 1-7, comprising the following steps: S1. After being crushed by a twin-shaft shredder, the straw is fed into a mixing tank by a closed scraper conveyor, where it is evenly mixed with biogas slurry and digestion liquid. The resulting mixture is preheated with steam. S2. The mixture is transported to the anaerobic digester through a vacuum feeding system and fermented at 55±1℃ for 25±5 days. The biogas produced is temporarily stored in a gas storage tank. S3. Biogas is purified by pressurization, wet desulfurization with complexed iron, dry deep desulfurization, compression and PSA pressure swing adsorption to obtain biomethane; the CO2-rich tail gas generated during the purification process is liquefied to obtain liquid CO2. S4. After purification, part of the biogas in the gas storage tank is sent to the generator set for power generation. The high-temperature flue gas generated during the power generation process enters the waste heat boiler for heat recovery. Part of the generated hot steam enters the steam heater to preheat the mixed materials, and the other part enters the steam-water heat exchanger to exchange heat into hot water, which is then used for heat preservation of the anaerobic fermentation tank. S5. Perform solid-liquid separation treatment on the digestate flowing out of the anaerobic digester to obtain biogas slurry and biogas residue; S6. The biogas slurry is stored in a buffer tank, and the biogas residue is processed into biomass fuel after deep dewatering.

9. The method for co-producing biogas and liquid carbon dioxide by dry anaerobic fermentation of straw according to claim 8, characterized in that, In step S1, the solids content of the mixture is 20-30%.

10. The method for co-producing biogas and liquid carbon dioxide by dry anaerobic fermentation of straw according to claim 8, characterized in that, In step S3, the wet desulfurization is complexed iron wet desulfurization.

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