Method and system for improving methane production of livestock and poultry manure by CO2 pressurized hydrothermal pretreatment

Abstract

The application discloses a method and system for improving the methane yield of livestock and poultry manure anaerobic fermentation based on CO2 pressurized hydrothermal pretreatment, and belongs to the field of environmental engineering and biomass energy cross technology.The application introduces CO2 as a "green, reversible and in-situ" chemical additive into the hydrothermal system.Under the conditions of medium and low temperature and pressurization, the substrate modification is realized through the physical and chemical synergistic effects of in-situ acid catalytic depolymerization, ammonia nitrogen form regulation, blocking of side reactions and physical structure blasting.The methane yield of the anaerobic digestion of cow dung can be improved by 30% to 60%.The pathogenic bacteria can be efficiently killed, the residual antibiotics (such as oxytetracycline removal rate of 96.5%) can be degraded, and the risk of drug resistance gene diffusion can be reduced.The whole process does not use strong acid and alkali, and there is no secondary pollution;CO2 can be recycled in a closed loop by using the by-product of biogas purification, and the system has positive net energy output by cooperating with heat recovery technology, so the system has broad industrialization prospect.

Description

Technical Field

[0001] This invention belongs to the interdisciplinary technical field of environmental engineering, biomass energy, and waste resource utilization. Specifically, this invention relates to an advanced pretreatment technology for high-cellulose and high-nitrogen livestock and poultry manure (such as dairy cow manure, beef cow manure, and livestock and poultry manure containing bedding). This invention introduces CO2 as a "green, reversible, and in-situ" chemical additive to construct an acidic hydrolysis system under pressurized and low-temperature hydrothermal conditions. The aim is to simultaneously solve three core problems: the anti-degradation barrier of lignocellulose, ammonia nitrogen inhibition during anaerobic digestion, and the generation of inhibitory byproducts in traditional thermal pretreatment. This significantly improves the biochemical methane yield of livestock and poultry manure and the stability of the anaerobic digestion system. Background Technology

[0002] With the rapid development of intensive livestock farming, livestock and poultry manure has become one of the world's major sources of organic solid waste. Statistics show that China alone produces over 3.8 billion tons of livestock and poultry manure annually. If not properly treated, this nitrogen- and phosphorus-rich waste will lead to severe eutrophication of water bodies, soil compaction, and the uncontrolled emission of greenhouse gases (methane and nitrous oxide). Anaerobic digestion technology, because it can convert organic waste into clean energy (biogas) and produce organic fertilizers (biogas residue and biogas slurry), is widely recognized as the mainstream technological route for achieving the resource utilization, reduction, and harmless treatment of livestock and poultry manure.

[0003] However, in practical engineering applications, the anaerobic digestion efficiency using livestock and poultry manure as a substrate is often far lower than the theoretical value, mainly due to the following key technological bottlenecks:

[0004] (1) The degradation barrier of lignocellulose

[0005] Livestock and poultry manure, especially ruminant manure (cow manure) and manure mixed with bedding materials such as straw and sawdust, contains a large amount of undigested lignocellulose. Microscopically, cellulose forms highly ordered crystalline regions through intramolecular and intermolecular hydrogen bonds, and is encapsulated by a dense three-dimensional network structure formed by hemicellulose and lignin. This complex physicochemical barrier severely restricts the contact between extracellular enzymes (such as cellulase and xylanase) secreted by hydrolytic bacteria in the early stages of anaerobic fermentation and the substrate, resulting in an extremely slow hydrolysis rate, becoming the rate-limiting step in anaerobic digestion. The anaerobic digestion cycle of untreated cow manure can be as long as 20–40 days, and the organic matter degradation rate is usually less than 50%, seriously affecting the economic viability of the project.

[0006] (2) Limitations and side effects of traditional pretreatment techniques

[0007] To overcome these barriers, existing research and applications mainly focus on physical, chemical, and thermal treatment methods, but all of them have significant shortcomings:

[0008] Acid / alkali pretreatment: Although dilute sulfuric acid or sodium hydroxide can effectively dissolve hemicellulose or lignin, it requires a large amount of chemical reagents, and the resulting high-salt wastewater has an inhibitory effect on methanogens. pH neutralization is also required afterward, which is costly and poses a risk of secondary pollution.

[0009] High-temperature hydrothermal treatment: This is typically carried out at 160℃ to 220℃. While it promotes the dissolution of organic matter, at this high temperature, the dissolved reducing sugars are easily dehydrated and degraded, generating furan derivatives (such as 5-hydroxymethylfurfural (HMF) and furfural) and phenolic compounds. These substances are potent microbial inhibitors, poisoning methanogens and causing delayed or even halted gas production. Furthermore, the high temperature can induce the Maillard reaction, where the carbonyl group of sugars condenses with the amino group of proteins, generating recalcitrant melanoidins, resulting in a double loss of both carbon and nitrogen sources.

[0010] Mechanical processing, such as ball milling and ultrasonic treatment, can reduce particle size, but it consumes a lot of energy and is difficult to destroy the molecular-level encapsulation of lignin in cellulose.

[0011] (3) The problem of ammonia nitrogen suppression

[0012] For raw materials rich in protein and uric acid, such as chicken and pig manure, high concentrations of total ammonia nitrogen (TAN) are released during anaerobic digestion. When the pH is alkaline (>7.5), the proportion of free ammonia (FAN) in TAN increases significantly. FAN has strong membrane permeability and can inhibit the enzyme activity and proton balance of methanogenic bacteria. Traditional heat treatment often accelerates protein hydrolysis, causing a sudden spike in ammonia nitrogen concentration in the pretreatment solution, making the subsequent anaerobic digestion system highly susceptible to "ammonia poisoning" and collapse.

[0013] (4) Shortcomings of existing CO2-related technologies

[0014] In recent years, research on CO2-assisted biomass pretreatment has gradually emerged. Current technologies mainly focus on supercritical CO2 extraction or ethanol fermentation pretreatment of agricultural straw. However, for the complex substrate of livestock and poultry manure (high buffering capacity, high ash content, high protein), a systematic technical solution has yet to be developed to utilize the chemical properties of CO2 to synergistically address the three mutually constraining issues of "cellulose depolymerization," "inhibitor control," and "ammonia nitrogen buffering." In particular, in-depth research is lacking on the inhibitory mechanism of in-situ acidity on the Maillard reaction in CO2 pressurized hydrothermal systems, and the mitigation mechanism of ammonia inhibition by bicarbonate systems.

[0015] In conclusion, developing a green pretreatment technology that can efficiently disrupt the structure of lignocellulose, inhibit the formation of toxic byproducts, and alleviate ammonia inhibition is an urgent need in the field of livestock and poultry manure energy utilization. Summary of the Invention

[0016] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for treating livestock and poultry manure based on CO2 pressurized hydrothermal pretreatment. This method utilizes the unique phase behavior and chemical equilibrium of CO2 in a hydrothermal environment to achieve targeted regulation of the physicochemical properties of livestock and poultry manure.

[0017] This invention is based on the synergistic effect of the following four core scientific principles:

[0018] A. In-situ acid-catalyzed depolymerization: Under high pressure (1-5 MPa) and medium-low temperature (100-200℃) conditions, the solubility of CO2 in water increases significantly, forming carbonic acid (…). Because high temperatures promote the dissociation constant of water (Kw) and the ionization of carbonic acid, the in-situ pH of the reaction system can be maintained between 3.5 and 5.0. This acidity range constitutes the "golden window" for hemicellulose hydrolysis—sufficient to break the glycosidic bonds and acetyl groups of hemicellulose, causing it to depolymerize into soluble xylooligosaccharides and monosaccharides, thereby disintegrating the LCC network; but the acidity is insufficient to cause cellulose to undergo violent hydrolysis to generate a large amount of HMF, thus achieving selective depolymerization with "high hydrolysis rate and low inhibitors".

[0019] B. Physical Structure Explosion and Swelling: Under pressure, small-molecule CO2 easily penetrates into the micropores and interstitial spaces of biomass and cellulose crystals. When the pressure is released instantly at the end of the reaction, the adsorbed CO2 gas expands instantly, generating enormous tensile stress, which "explodes" the dense fecal particles, forming a loose, porous honeycomb structure (significantly reducing the crystallinity of cellulose), thus increasing the accessibility of enzymatic hydrolysis.

[0020] CO2 ammonia nitrogen form regulation and buffering: For high-nitrogen feces, CO2 is not only an acid catalyst but also an ammonia "capturer." Ammonia gas released during hydrolysis ( It reacts rapidly with CO2 and water to produce ammonium bicarbonate. The reaction formula is:

[0021]

[0022] This reaction converts volatile, highly toxic FAN into less toxic ammonium ions. This also increases the alkalinity of the system, providing a strong pH buffer for subsequent anaerobic digestion and preventing acidification.

[0023] D. Maillard Reaction Inhibition: The Maillard reaction is a polymerization reaction between carbonyl groups (reducing sugars) and amino groups (proteins / amino acids) at high temperatures. This reaction is extremely rapid under alkaline conditions. This invention, through the introduction of CO2 to create a weakly acidic environment (pH < 5.0), significantly inhibits the nucleophilic addition step in the initial stage of the Maillard reaction, thereby reducing the formation of melanoidins, retaining more available sugars and amino acids, and improving the biodegradability of the substrate.

[0024] The technical solution of the present invention:

[0025] A method for increasing methane production from anaerobic fermentation of livestock and poultry manure based on CO2 pressurized hydrothermal pretreatment includes the following steps:

[0026] Step 1: Raw material conditioning: Remove impurities and crush the livestock and poultry manure, and adjust the total solids content to the range of 8% to 20% to form a homogenized manure slurry;

[0027] Step 2, Gas-liquid mixing and pressurization: The fecal slurry is pumped into a high-pressure hydrothermal reactor. Carbon dioxide gas is continuously introduced into the high-pressure hydrothermal reactor until the initial pressure inside the reactor reaches 1.0 MPa to 5.0 MPa. Stirring is maintained to ensure that the carbon dioxide is fully dissolved in the slurry to form a carbonic acid saturated system.

[0028] Step 3, Low-Temperature Hydrothermal Depolymerization: While maintaining carbon dioxide pressure, the high-pressure hydrothermal reactor is heated to the target temperature of 100℃~200℃ at a heating rate of 2℃ / min~10℃ / min, and held at this temperature for 10 minutes~60 minutes. During this process, the in-situ acidic environment formed by the carbon dioxide-water system under high temperature and pressure is used to directionally catalyze the hydrolysis of hemicellulose, destroying the cross-linking network of cellulose-lignin-hemicellulose. At the same time, the weakly acidic environment can also inhibit the Maillard reaction, reducing the formation of melanoidins and furan inhibitors. Synergistic inhibition and blocking: Utilizing the chemical equilibrium mechanism of the reaction between carbon dioxide atmosphere and ammonia during the hydrothermal process to produce ammonium bicarbonate, a bicarbonate buffer system is generated, reducing the concentration of free ammonia.

[0029] Step 4, Anaerobic Digestion: The slurry treated in Step 3 is cooled to 35℃~55℃, and anaerobic sludge is inoculated for anaerobic digestion to produce methanogens. The bicarbonate buffer system in the slurry maintains the pH stability of the digestion process.

[0030] Furthermore, an instantaneous pressure relief explosion is performed before the anaerobic digestion in step four: after the hydrothermal reaction is completed, the quick-opening valve is opened to depressurize the material in the high-pressure hydrothermal reactor into the atmospheric pressure gas explosion tank within 5 to 30 seconds. The rapid volume expansion effect of supercritical or subcritical carbon dioxide fluid in the micropores of biomass is used to physically tear apart the cell wall and cellulose crystal structure.

[0031] Furthermore, in step one, the livestock and poultry manure is one or more of the following: cow manure, chicken manure with bedding material, and pig manure; the cellulose content in the livestock and poultry manure is 15% to 40% of the dry weight, and the lignin content is 5% to 25% of the dry weight.

[0032] Furthermore, the target temperature mentioned in step three is 160℃~180℃, and the constant temperature holding time is 20 minutes~40 minutes; under these reaction conditions, the hydrolysis rate of hemicellulose is greater than 60%, and the concentrations of furfural and 5-hydroxymethylfurfural (HMF) generated are lower than the anaerobic digestion inhibition threshold.

[0033] Furthermore, the carbon dioxide source mentioned in step two is the carbon dioxide captured after purification and separation of biogas produced by the anaerobic digestion system, thereby realizing carbon cycling within the system; the initial pressure is 2.0 MPa to 4.0 MPa, which is sufficient to form an acidic buffer environment with a pH of 3.5 to 5 in the aqueous phase.

[0034] Furthermore, in step three, there is also an antibiotic degradation mechanism: using the carbon dioxide pressurized hydrothermal conditions described in step three, the residual veterinary antibiotics in livestock and poultry manure are thermochemically degraded, with a degradation rate of not less than 85%, eliminating their biotoxicity to the subsequent anaerobic digestion microbial community.

[0035] Furthermore, in step three, there is also an ammonia inhibition and mitigation mechanism: for chicken manure raw materials with a total nitrogen content higher than 3.0 g / L, carbon dioxide reacts with the ammonia nitrogen produced by hydrolysis in step three to generate ammonium bicarbonate, which buffers the pH value of the reaction system between 7.5 and 8.2, preventing the high concentration of free ammonia (FAN) at high pH value from being toxic to methanogens.

[0036] A system for implementing the above-mentioned method for increasing methane production from anaerobic fermentation of livestock and poultry manure based on CO2 pressurized hydrothermal pretreatment includes:

[0037] Raw material pretreatment unit: includes a crusher and a mixing tank, used to adjust the particle size and total solids content of livestock and poultry manure;

[0038] Carbon dioxide pressurized hydrothermal reaction unit: includes a high-pressure resistant hydrothermal reactor with a stirrer, a heating jacket and a pressure control system, and is connected to a carbon dioxide injection pipeline;

[0039] Gas explosion pressure relief unit: includes a gas explosion tank and a gas-liquid separator, used to receive high-pressure materials and separate out small amounts of water vapor and carbon dioxide;

[0040] Heat and gas recovery unit: includes a condenser and a carbon dioxide compressor, which recovers the carbon dioxide released from the gas explosion into a storage tank for recycling;

[0041] Anaerobic digestion unit: includes a CSTR reactor or UASB reactor, connected to a biogas collection and purification device.

[0042] Furthermore, the inner wall of the high-pressure hydrothermal reactor is lined with acid-resistant material to resist corrosion from high-temperature and high-pressure carbonic acid solution; the agitator is designed as an anchor type or a ribbon type to meet the mass transfer requirements of high-viscosity fecal slurry.

[0043] The beneficial effects of this invention are:

[0044] (1) Significant increase in methane production: Through the dual effects of chemical depolymerization and physical modification, the methane yield of difficult-to-degrade matrices such as cow manure is increased by 30% to 60%.

[0045] (2) Environmentally friendly: It does not use strong corrosive chemicals such as sulfuric acid and alkali, thus avoiding secondary pollution and equipment corrosion.

[0046] (3) Stable process: It effectively alleviates the ammonia inhibition problem of high nitrogen feedstock and controls the inhibitory byproducts (furans) at extremely low levels.

[0047] (4) It also has the function of harmlessness: under medium and low temperature hydrothermal conditions (150-180℃) and in the acidic environment of CO2, it can efficiently degrade residual antibiotics (such as tetracycline degradation rate >85%) and kill pathogens. Attached Figure Description

[0048] Figure 1 This is a process flow diagram of the CO2 pressurized hydrothermal pretreatment system of the present invention.

[0049] Figure 2 The diagram shows the methanogenic potential of hydrothermal products from cow manure. (A) shows the biomethane potential of CO2 pressurized hydrothermal pretreatment of high-cellulose dairy cow manure in Examples 1, 1-1, 1-2, and 1-3; (B) shows the biomethane potential of hydrothermal pretreatment of high-cellulose dairy cow manure under different initial CO2 pressures in Comparative Example 1-1, Examples 1, 2, and 3; and (C) shows the hydrothermal pretreatment and biomethane potential of high-cellulose dairy cow manure at different hydrothermal temperatures in Comparative Example 1-1, Examples 1, 4, and 5.

[0050] Figure 3 This is a schematic diagram of the reaction mechanism. It illustrates the microscopic process of CO2 molecules penetrating into the interstices of cellulose, forming carbonic acid hydrolysate hemicellulose, and capturing ammonia.

[0051] Figure 4The concentrations of 5-HMF, furfural, and ammonia nitrogen in the hydrothermal products of cow manure are as follows: (A) represents the concentrations of 5-HMF, furfural, and ammonia nitrogen in Example 1, Comparative Example 1-1, Comparative Example 1-2, and Comparative Example 1-3; (B) represents the concentrations of 5-HMF, furfural, and ammonia nitrogen in Comparative Example 1-1, Example 1, Example 2, and Example 3; and (C) represents the concentrations of 5-HMF, furfural, and ammonia nitrogen in Comparative Example 1-1, Example 1, Example 4, and Example 5. Detailed Implementation

[0052] To enable those skilled in the art to better understand the technical solution of the present invention, the following detailed description is provided in conjunction with specific embodiments and experimental data.

[0053] Example 1: CO2 pressurized hydrothermal pretreatment and biochemical methane potential of high-cellulose dairy cow manure (150℃-3MPa CO2 group)

[0054] Raw material source and characteristics: The raw materials used in the experiment were obtained from a dairy farm, and contained a small amount of sawdust bedding. Initial characteristic analysis showed that the total solids (TS) content was 16.5%, the volatile solids (VS) content was 13.8%, the cellulose content was 32.5%, the hemicellulose content was 18.4%, and the lignin content was 14.2%. This type of raw material is a typical organic waste with high lignocellulose content and poor biodegradability.

[0055] Experimental setup and procedures:

[0056] A high-temperature and high-pressure reactor with an effective volume of 5L is used (equipped with a magnetically coupled stirrer and a PID temperature control system).

[0057] Step A: After removing impurities and crushing the cow dung, dilute it with water to adjust the total tidal volume (TS) to 10%, stir well, and then add it to the reaction vessel.

[0058] Step B: Seal the reactor, purge the air inside the reactor with industrial-grade CO2 gas three times, and then pressurize it to the initial pressure of 3.0 MPa.

[0059] Step C: Turn on the stirrer (300 rpm) and heat to 150°C at a rate of 5°C / min. At this point, due to the temperature increase, the total pressure inside the vessel rises to approximately 4.5 MPa (including water vapor partial pressure and CO2 partial pressure). Maintain this temperature for 30 minutes.

[0060] Step D: After the reaction is complete, open the ball valve and release the slurry into the gas explosion tank through the pressure relief pipe to achieve instantaneous pressure relief. Collect the slurry for subsequent analysis and anaerobic digestion.

[0061] Control group 1-1 (untreated group, i.e. CM group): raw cow dung slurry.

[0062] Control groups 1-2 (unpressurized group, i.e., 150℃-unpressurized group): The operation is the same as in Example 1, except that no pressurization is performed in step B. This is used to eliminate the influence of simple physical pressure and thermal effects.

[0063] Control group 1-3 ( Pressurized group, i.e., 150℃-3Mpa Group): The operation is the same as in Example 1, except that in step B, it is filled with... Initial pressure was set to 3.0 MPa. This was used to verify the effect of CO2.

[0064] Table 1: Comparison of component dissolution and inhibitor formation under different pretreatment conditions (after step AC).

[0065]

[0066] Data Analysis: As shown in Table 1, the CO2 group showed the largest increase in SCOD, reaching 4.1 times that of the untreated group, and significantly higher than... This demonstrates that the carbonic acid formed from CO2 in water effectively catalyzes the hydrolysis of solid organic matter. More importantly, despite the high hydrolysis efficiency, the concentrations of inhibitors (HMF and furfural) in the CO2 group were lower than those in the CO2 group. Group. This is because in In an unsuitable atmosphere, the local pH may be uneven due to the release of organic acids, leading to excessive degradation of some sugars; while the CO2 / carbonic acid system provides a uniform buffer acid environment, reducing the occurrence of side reactions.

[0067] Anaerobic digestion performance (BMP test):

[0068] A batch anaerobic digestion experiment was conducted for 30 days under mesophilic (37°C) conditions.

[0069] Methane production potential of untreated groups: .

[0070] Methane production potential without pressurization: (Increased by 185.6%).

[0071] Hydrothermal methane production potential: (Increased by 209.6%).

[0072] Methane production potential of CO2 hydrothermal reactors: (Increased by 247.0%).

[0073] The results show that the method of this invention can significantly increase methane production from high-cellulose cow manure. The CO2 group not only produced high amounts of gas but also had a lag period of almost zero, indicating excellent substrate biocompatibility.

[0074] Energy consumption analysis and economic evaluation:

[0075] To verify the industrial feasibility of the present invention, energy consumption and output simulation calculations were performed based on a processing scale of 100 tons / day.

[0076] Energy input:

[0077] Heating 100 tons of slurry from 25°C to 170°C requires approximately [amount missing] heat. .

[0078] Waste heat from the discharge is recovered via a high-efficiency plate heat exchanger (recovery rate 80%), requiring only a small amount of additional heat. .

[0079] Energy output:

[0080] Methane production in the untreated group: Methane.

[0081] Methane production without pressurization: Methane.

[0082] Methane production from the N2 hydrothermal unit: Methane.

[0083] CO2 hydrothermal group methane production: Methane.

[0084] Compared to the untreated group, the methane increments in the unpressurized group, the N2 hydrothermal group, and the CO2 hydrothermal group were 2071 m³, 2339 m³, and 2756 m³ m³, respectively. Based on the methane calorific value of 36 MJ / m³, the energy increases were respectively... Energy inputs come from hydrothermal pretreatment and anaerobic digestion, respectively, according to... Calculations are performed. The net energy output of the CO2 pressurized unit is... .

[0085] The results show that although pretreatment consumes thermal energy, the system as a whole exhibits a positive net energy output due to the significant increase in methane production and the application of waste heat recovery technology. Furthermore, the CO2 is derived from a byproduct of biogas purification within the system, eliminating the need for additional purchase and resulting in extremely low operating costs.

[0086] Aperture distribution analysis:

[0087] The porosity of the treated solid slag was determined using a mercury porosimeter.

[0088] Untreated: Porosity 15%, average pore size 5nm.

[0089] Unpressurized group: porosity 28%, average pore size 12nm.

[0090] N2 hydrothermal group: porosity 45%, average pore size 29nm.

[0091] CO2 hydrothermal group: porosity 54%, average pore size 35nm.

[0092] Data confirms that the "molecular wedging" effect of CO2 and the "expansion burst" effect during depressurization significantly clear the internal pores of biomass, which is crucial for the entry of large cellulase molecules into the substrate.

[0093] Carbon balance and element flow:

[0094] In traditional combustion or aerobic composting processes, a large amount of carbon from livestock and poultry manure is emitted as CO2. In the system of this invention:

[0095] Carbon sequestration: Some CO2 combines with ammonia and enters the liquid phase, eventually being returned to the field as biogas slurry fertilizer, which not only reduces greenhouse gas emissions but also increases the carbon pool in the soil.

[0096] Carbon cycle: The CO2 used in pretreatment comes from biogas produced by anaerobic digestion (usually containing 40% CO2). Reusing this portion of CO2, which is usually considered waste gas, in pretreatment achieves a closed-loop cycle of "treating waste with waste".

[0097] Example 2: Hydrothermal pretreatment of high-cellulose dairy cow manure and its biochemical methane potential under different initial CO2 pressures (150℃-5MPa CO2 group)

[0098] The operation steps are the same as in Example 1, except that the pressure condition of pressurizing to the initial pressure of 3.0 MPa in Example 1 is changed to pressurizing to the initial pressure of 5.0 MPa.

[0099] Example 3: Hydrothermal pretreatment of high-cellulose dairy cow manure and its biochemical methane potential under different initial CO2 pressures (150℃-1MPa CO2 group)

[0100] The procedure is the same as in Example 1, except that CO2 is introduced to an initial pressure of 1.0 MPa in step B. This is used to examine the effect of pressure and explore the optimal pressure.

[0101] Table 2: Comparison of component dissolution and inhibitor formation under different pretreatment conditions (after step AC).

[0102]

[0103] Data Analysis: Table 2 shows that the SCOD concentration of the hydrothermal products of cow dung first increases and then decreases with increasing initial CO2 pressure. The carbonic acid formed by CO2 in water effectively catalyzes the hydrolysis of solid organic matter; however, excessive pressure leads to over-hydrolysis of the products, resulting in a decrease in SCOD concentration. When the pressure reaches 5 MPa, hemicellulose is completely degraded.

[0104] Anaerobic digestion performance (BMP test):

[0105] A batch anaerobic digestion experiment was conducted for 30 days under mesophilic (37°C) conditions.

[0106] Methane production potential of 1 MPa CO2 hydrothermal group: (Increased by 223.5%).

[0107] Methane production potential of the 5 MPa CO2 hydrothermal group: (Increased by 260.4%).

[0108] The results showed that the 5MPa CO2 group achieved the highest methane yield.

[0109] Example 4: Hydrothermal pretreatment of high-cellulose dairy cow manure and its biochemical methane potential at different hydrothermal temperatures (100℃-3MPa CO2 group)

[0110] The operation steps are the same as in Example 1, except that the hydrothermal temperature in Example 1 is changed from 150 ℃ to 100 ℃.

[0111] Example 5: Hydrothermal pretreatment of high-cellulose dairy cow manure and its biochemical methane potential at different hydrothermal temperatures (200℃-3MPa CO2 group)

[0112] The procedure is the same as in Example 1, except that the hydrothermal temperature is changed from 150 °C to 200 °C. This is used to investigate the effect of temperature and explore the optimal hydrothermal temperature.

[0113] Table 3: Comparison of component dissolution and inhibitor formation under different pretreatment conditions (after step AC).

[0114]

[0115] Data Analysis: Table 3 shows that as the hydrothermal temperature increases, hemicellulose is gradually degraded, and at 200 ℃, it is completely degraded. The carbonic acid formed by CO2 in water effectively catalyzes the hydrolysis of solid organic matter, but excessively high temperatures lead to over-hydrolysis of the products, resulting in a decrease in SCOD concentration.

[0116] Anaerobic digestion performance (BMP test):

[0117] A batch anaerobic digestion experiment was conducted for 30 days under mesophilic (37°C) conditions.

[0118] Methane production potential of the 100℃ CO2 hydrothermal group: (Increased by 19.8%).

[0119] Methane production potential of the 200 ℃ CO2 hydrothermal group: (Increased by 144.4%).

[0120] The results showed that excessively high temperatures were not conducive to improving the biochemical methane potential, and the 150 ℃ CO2 group achieved the highest methane production.

[0121] Example 6: Relief of ammonia inhibition and control of Maillard reaction in high-nitrogen chicken manure

[0122] Raw material characteristics: Fresh laying hen manure is used, with a total sulfide (TS) content of 18%, a total nitrogen (TN) content as high as 68 g / kg TS, and a C / N ratio of only 9.5. This type of raw material is highly susceptible to ammonia poisoning during direct anaerobic digestion.

[0123] Experimental steps:

[0124] Step A: After removing impurities and crushing the chicken manure, adjust the TS to 12%, stir evenly, and then add it to the reaction vessel.

[0125] Step B: Seal the reactor, introduce industrial-grade CO2 gas to replace the air inside the reactor three times, and then pressurize the reactor with CO2, with the initial pressure set at 2.0 MPa.

[0126] Step C: The stirring speed of the reactor is 300 rpm, and the temperature is increased to 160℃ at a rate of 5℃ / min, with a residence time of 40 min.

[0127] Step D: After the reaction is complete, open the ball valve and release the slurry into the gas explosion tank through the pressure relief pipe to achieve instantaneous pressure relief. Collect the slurry for subsequent analysis and anaerobic digestion.

[0128] Control group 6-1: The operation is the same as in Example 6, except that CO2 is not introduced in step B, only CO2 is introduced. The air is replaced by atmospheric pressure (or slightly positive pressure), i.e., a control group is set up that is heated but not pressurized.

[0129] Control group 6-2 (untreated group): raw chicken manure slurry.

[0130] Control group 6-3: The operation was the same as in Example 6, except that CO2 was not introduced in step B, only CO2 was introduced. The pressure was adjusted to normal (or slightly positive) to replace the air, i.e., a control group was set up with heating but no pressurization, and an exogenous acid (HCl) was added to adjust the pH to 5.0.

[0131] Table 4: Ammonia Nitrogen Speciation and Color Analysis of Pretreated Solution

[0132]

[0133] The results showed that CO2 pressurized hydrothermal pretreatment significantly alleviated ammonia inhibition and blocked the Maillard reaction. While simple heat treatment promoted protein hydrolysis and increased TAN levels, the decarboxylation reaction led to a pH increase to 8.8, causing a surge in the concentration of toxic FAN to 1150 mg / L, far exceeding the tolerance threshold of methanogens (approximately 200-500 mg / L). In contrast, although the CO2 group of this invention also had higher TAN levels, the pH remained stable at around 7.8 due to the formation of an ammonium bicarbonate buffer system. Calculations based on the Henderson-Hasselbach equation showed that the FAN concentration at this point was only 185 mg / L, within the safe range for microorganisms. The hydrolysate from the simple heat-treated group was dark black with extremely high A420 absorbance, indicating a vigorous Maillard reaction and the generation of a large amount of melanoidins. This not only consumed available sugars and amino acids, but melanoidins themselves also inhibited enzyme activity. A weakly acidic environment was established in the early stages of the reaction, which inhibited the initiation of the Maillard reaction. As ammonia nitrogen was released, CO2 formed an ammonium bicarbonate buffer pair with it, which eventually stabilized the pH of the system at 7.8. At the same time, the toxic FAN was converted into a non-toxic ammonium salt, which effectively inhibited the occurrence of the Maillard reaction and preserved the nutritional value of the substrate.

[0134] Anaerobic gas production: The heat-only group showed acidification on day 5 of digestion, and gas production ceased (a typical characteristic of ammonia inhibition). In contrast, the CO2 group operated stably, achieving a methane yield of [missing information]. It improved by more than 40% compared to the untreated group, and showed no signs of inhibition.

[0135] Example 7: Deep treatment of antibiotic-containing pig manure to achieve harmlessness

[0136] Raw material characteristics: Tetracycline (TC) and sulfonamide antibiotic residues are frequently detected in pig manure. These antibiotics inhibit microbial activity when they enter anaerobic digesters, and lead to the spread of antibiotic resistance genes when they enter farmland.

[0137] Experimental parameters:

[0138] Step A: After removing impurities and crushing the pig manure, adjust the total sulfide (TS) to 10%, with oxytetracycline content of 2 mg / L. Stir well and add to the reaction vessel.

[0139] Step B: Seal the reactor, purge the air inside the reactor with industrial-grade CO2 gas three times, and then pressurize it to the initial pressure of 4.0 MPa.

[0140] Step C: Set the stirring speed in the reactor to 300 rpm, heat to 170℃ at a rate of 5℃ / min, and hold for 20 min.

[0141] Step D: After the reaction is complete, open the ball valve and release the slurry into the gas explosion tank through the pressure relief pipe to achieve instantaneous pressure relief. Collect the slurry for subsequent analysis and anaerobic digestion.

[0142] Antibiotic removal rate: HPLC analysis showed that the removal rate of oxytetracycline reached 96.5% after treatment by the method of the present invention.

[0143] Mechanism: Tetracycline antibiotics are sensitive to high temperatures and acidic conditions. At 170°C, OTCs undergo epimerization and dehydration; the acidic environment provided by CO2 further accelerates their degradation kinetics.

[0144] Microbial community response: Subsequent metagenomic sequencing of anaerobic digested sludge showed that the abundance of sequences associated with antibiotic resistance genes in the CO2 pretreated group was reduced by two orders of magnitude compared with the untreated group, demonstrating the significant advantages of this method in controlling biosafety risks.

Claims

1. A method for increasing methane production from anaerobic fermentation of livestock and poultry manure based on CO2 pressurized hydrothermal pretreatment, characterized in that, Includes the following steps: Step 1: Raw material conditioning: Remove impurities and crush the livestock and poultry manure, and adjust the total solids content to the range of 8% to 20% to form a homogenized manure slurry; Step 2, Gas-liquid mixing and pressurization: The fecal slurry is pumped into a high-pressure hydrothermal reactor. Carbon dioxide gas is continuously introduced into the high-pressure hydrothermal reactor until the initial pressure inside the reactor reaches 1.0 MPa to 5.0 MPa. Stirring is maintained to ensure that the carbon dioxide is fully dissolved in the slurry to form a carbonic acid saturated system. Step 3, Low-Temperature Hydrothermal Depolymerization: While maintaining carbon dioxide pressure, the high-pressure hydrothermal reactor is heated to the target temperature of 100℃~200℃ at a heating rate of 2℃ / min~10℃ / min, and held at this temperature for 10 minutes~60 minutes. During this process, the in-situ acidic environment formed by the carbon dioxide-water system under high temperature and pressure is used to directionally catalyze the hydrolysis of hemicellulose, destroying the cross-linking network of cellulose-lignin-hemicellulose. At the same time, the weakly acidic environment can also inhibit the Maillard reaction, reducing the formation of melanoidins and furan inhibitors. Synergistic inhibition and blocking: Utilizing the chemical equilibrium mechanism of the reaction between carbon dioxide atmosphere and ammonia during the hydrothermal process to produce ammonium bicarbonate, a bicarbonate buffer system is generated, reducing the concentration of free ammonia. Step 4, Anaerobic Digestion: The slurry treated in Step 3 is cooled to 35℃~55℃, and anaerobic sludge is inoculated for anaerobic digestion to produce methanogens. The bicarbonate buffer system in the slurry maintains the pH stability of the digestion process.

2. The method for increasing methane production from anaerobic fermentation of livestock and poultry manure based on CO2 pressurized hydrothermal pretreatment according to claim 1, characterized in that, Before the anaerobic digestion in step four, instantaneous pressure relief and explosion are performed: After the hydrothermal reaction is completed, the quick-opening valve is opened to depressurize the material in the high-pressure hydrothermal reactor into the atmospheric pressure gas explosion tank within 5 to 30 seconds. The rapid volume expansion effect of supercritical or subcritical carbon dioxide fluid in the micropores of biomass is used to physically tear the cell wall and cellulose crystal structure.

3. A method for increasing methane production from anaerobic fermentation of livestock and poultry manure based on CO2 pressurized hydrothermal pretreatment, as described in claim 1 or 2, characterized in that... In step one, the livestock and poultry manure is one or more of the following: cow manure, chicken manure with bedding, and pig manure; the cellulose content in the livestock and poultry manure is 15% to 40% of the dry weight, and the lignin content is 5% to 25% of the dry weight.

4. The method for increasing methane production from anaerobic fermentation of livestock and poultry manure based on CO2 pressurized hydrothermal pretreatment according to claim 1, characterized in that, The target temperature mentioned in step three is 160℃~180℃, and the constant temperature holding time is 20 minutes~40 minutes; under these reaction conditions, the hydrolysis rate of hemicellulose is greater than 60%, and the concentrations of furfural and 5-hydroxymethylfurfural (HMF) generated are lower than the anaerobic digestion inhibition threshold.

5. The method for increasing methane production from anaerobic fermentation of livestock and poultry manure based on CO2 pressurized hydrothermal pretreatment according to claim 1, characterized in that, The carbon dioxide source mentioned in step two is the carbon dioxide captured after purification and separation of biogas produced by the anaerobic digestion system, thus realizing carbon cycling within the system; the initial pressure is 2.0 MPa to 4.0 MPa, which is sufficient to form an acidic buffer environment with a pH of 3.5 to 5 in the aqueous phase.

6. The method for increasing methane production from anaerobic fermentation of livestock and poultry manure based on CO2 pressurized hydrothermal pretreatment according to claim 1, characterized in that, In step three, there is also an antibiotic degradation mechanism: using the carbon dioxide pressurized hydrothermal conditions described in step three, the residual veterinary antibiotics in livestock and poultry manure are thermochemically degraded, with a degradation rate of not less than 85%, eliminating their biotoxicity to the subsequent anaerobic digestion microbial community.

7. The method for increasing methane production from anaerobic fermentation of livestock and poultry manure based on CO2 pressurized hydrothermal pretreatment according to claim 1, characterized in that, In step three, there is also an ammonia inhibition and mitigation mechanism: for chicken manure raw materials with a total nitrogen content higher than 3.0 g / L, carbon dioxide reacts with the ammonia nitrogen produced by hydrolysis in step three to generate ammonium bicarbonate, which buffers the pH value of the reaction system between 7.5 and 8.2, preventing the high concentration of free ammonia (FAN) at high pH value from being toxic to methanogens.

8. A system for implementing the method for increasing methane production from anaerobic fermentation of livestock and poultry manure based on CO2 pressurized hydrothermal pretreatment as described in any one of claims 1 to 7, characterized in that, Including those connected sequentially: Raw material pretreatment unit: includes a crusher and a mixing tank, used to adjust the particle size and total solids content of livestock and poultry manure; Carbon dioxide pressurized hydrothermal reaction unit: includes a high-pressure resistant hydrothermal reactor with a stirrer, a heating jacket and a pressure control system, and is connected to a carbon dioxide injection pipeline; Gas explosion pressure relief unit: includes a gas explosion tank and a gas-liquid separator, used to receive high-pressure materials and separate out small amounts of water vapor and carbon dioxide; Heat and gas recovery unit: includes a condenser and a carbon dioxide compressor, which recovers the carbon dioxide released from the gas explosion into a storage tank for recycling; Anaerobic digestion unit: includes a CSTR reactor or UASB reactor, connected to a biogas collection and purification device.

9. The system according to claim 8, characterized in that, The inner wall of the high-pressure hydrothermal reactor is lined with acid-resistant material to resist corrosion from high-temperature and high-pressure carbonic acid solution.

10. The system according to claim 8, characterized in that, The agitator is designed with either an anchor or ribbon type to meet the mass transfer requirements of high-viscosity fecal slurry.

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