Biogas synchronous edulcoration-CO2 electrocatalytic reduction integrated purification method

Abstract

The invention discloses a biogas synchronous edulcoration-CO2 electrocatalytic reduction integrated purification method, and belongs to the technical field of gas purification, a biogas purification device is adopted to treat biogas to be treated, the biogas to be treated is in full contact with an electrolyte through a microbubble aeration device, H2S and NH3 in the biogas to be treated are absorbed, and the rest of gas rises, so that the biogas to be treated is purified. The gas is in countercurrent contact with the electrolyte sprayed by the spraying head, H2S and NH3 in the gas are further removed, the gas passes through the position where the three electrodes are located, CO2 in the gas is selectively reduced into CH4 through an electrocatalytic reduction reaction, and the purified biogas is discharged into a methane collecting tank from an exhaust port I; the method is simple to operate and mild in reaction condition, purification of the biogas and purification of impurities are realized, and the purified biogas is convenient for subsequent collection and utilization.

Description

Technical Field

[0001] This invention belongs to the field of gas purification technology, specifically relating to an integrated purification method and device for simultaneous biogas deimpurification and CO2 electrocatalytic reduction. Background Technology

[0002] The decomposition of organic matter in landfills, the anaerobic digestion of sludge in sewage treatment plants, and the anaerobic fermentation of manure in livestock farms all produce large amounts of biogas. This gas is both a valuable renewable energy source and a pollutant posing potential safety and environmental hazards. Its core components are methane (CH4) and carbon dioxide (CO2), while also containing trace impurities such as hydrogen sulfide (H2S), ammonia (NH3), siloxanes, and volatile organic compounds (VOCs). Hydrogen sulfide and ammonia are typical malodorous pollutants in environments such as livestock farming, sewage treatment, and landfills. They both have strong, pungent odors and pose serious health risks at high concentrations. Hydrogen sulfide is also a highly toxic gas; its hydrosulfuric acid form when dissolved in water can severely corrode pipes, engines, boilers, and other equipment, shortening equipment lifespan and increasing maintenance costs.

[0003] Unlike biogas projects that can efficiently produce biogas under controlled conditions, biogas from livestock farming, wastewater treatment, and landfills has significant "uncontrolled" characteristics. Its complex composition, uneven stratification, and large temperature and humidity variations lead to drastic fluctuations in gas production rates and methane concentrations, making large-scale, stable applications extremely difficult. The actual biogas collection volume from landfills nationwide has reached 2.5-3 billion cubic meters annually, with methane content fluctuating between 45% and 60%. This means that approximately 1.125 billion to 1.8 billion cubic meters of methane gas can be extracted annually from the collected biogas alone. The actual biogas production from wastewater treatment plants nationwide is 1.5-1.8 billion cubic meters annually; due to relatively better anaerobic digestion conditions for sludge, the methane content is higher, typically between 60% and 70%. A pig farm with 10,000 pigs annually produces approximately 500-700 cubic meters of biogas per day. 3 Separating and purifying the methane from these biogases could replace millions of tons of standard coal and reduce emissions by tens of millions of tons of carbon dioxide equivalent. However, the current utilization rate of these biogases is still not ideal. Some of the collected biogas is simply burned through flares (directly vented into the air or ignited), which, while eliminating safety and some environmental hazards, results in a huge waste of energy.

[0004] To achieve the resource utilization of biogas, it is essential to overcome the technological and equipment challenges across the entire chain, from collection to purification. The technological approach should be tailored to different scenarios. First, a highly efficient biogas collection system needs to be built to ensure comprehensive collection at the source. Second, separation, purification, and impurity degradation technologies are the core of biogas treatment, aiming to remove CO2, increase methane concentration, and eliminate various harmful impurities. Currently, commonly used desulfurization technologies include biological desulfurization (using microorganisms such as thiobacillus), dry desulfurization (using adsorbents such as iron oxide and activated carbon), and wet desulfurization (using alkaline absorbents for chemical absorption). Common CO2 separation and purification technologies include pressure swing adsorption (PSA), membrane separation, and high-pressure water washing. The disadvantages of water washing include high energy consumption and secondary pollution. It requires high-pressure pumps to maintain system pressure, and the water circulation and regeneration (pressure reduction and stripping) processes consume significant energy. The water enriched with CO2 and H2S discharged from the absorption tower requires secondary treatment. The core disadvantage of pressure swing adsorption is its extremely high requirements for pretreatment and rapid adsorbent aging. The core drawback of membrane separation is the high cost and limited lifespan of membrane materials. Other trace impurities are often removed using deep condensation combined with activated carbon adsorption, biofiltration, or catalytic oxidation. These step-by-step processes are relatively complex and costly.

[0005] Electrocatalysis is an emerging environmentally friendly treatment method, currently transitioning from laboratory to industrialization. Compared to traditional technologies, electrocatalysis offers milder reaction conditions, reducing equipment requirements and safety risks. Furthermore, its core driving force is electricity, lowering operating costs. CN103551031A treats a mixture of gases containing phosphine, hydrogen sulfide, and hydrogen cyanide under normal pressure through a combination of electrochemical oxidation and catalytic oxidation, combined with electrodialysis to purify the mixed gas. CN109925850B uses corona discharge and dielectric discharge to charge the waste gas, followed by spraying with an absorbent liquid, which shares similarities with this invention. Therefore, how to apply liquid-phase absorption synergistic electrocatalytic conversion technology to biogas purification and upgrading, achieving simultaneous resource recovery and utilization, is a major research challenge. Summary of the Invention

[0006] This invention provides an integrated purification method for biogas simultaneous deimpurification and CO2 electrocatalytic reduction. In this method, the biogas to be treated first passes through a microbubble aeration device to ensure sufficient contact with the electrolyte. H2S and NH3 are absorbed, while unabsorbed gas rises and comes into countercurrent contact with the electrolyte sprayed from the top spray head, further intercepting H2S and NH3. CO2 is selectively reduced to CH4 in the cathode chamber of the electrocatalytic reaction section through an electrocatalytic reduction reaction. The purified biogas is then discharged from the top exhaust port to a methane collection tank.

[0007] The integrated biogas purification device with simultaneous decontamination and CO2 electrocatalytic reduction includes a biogas purification unit comprising a shell, spray heads, a working electrode, a counter electrode, a reference electrode, a proton exchange membrane, a microbubble aeration device, a circulating pump, a replenishment tank, a waste liquid collection tank, and a methane collection tank. The shell is divided into a cathode chamber and an anode chamber by the proton exchange membrane. Electrolyte is provided within the shell. The biogas inlet is located at the bottom of the cathode chamber and connected to the microbubble aeration device. Several spray heads are located at the top of the cathode chamber. A drain outlet on the lower side of the cathode chamber is connected to the replenishment tank via the circulating pump. The replenishment tank is connected to the spray heads via a pipe. The cathode chamber contains the working electrode and the reference electrode. Exhaust port I at the top of the cathode chamber is connected to the methane collection tank. The anode chamber contains the counter electrode. Exhaust port II is located at the top of the anode chamber, and a drain outlet at the bottom is connected to the waste liquid collection tank. The biogas to be treated enters the microbubble aerator through the biogas inlet. After passing through the microbubble aerator, it comes into full contact with the electrolyte, where H2S and NH3 are absorbed. The remaining gas rises and comes into countercurrent contact with the electrolyte sprayed from the spray head, further removing H2S and NH3. The gas then passes the three electrodes, where the CO2 in the gas is selectively reduced to CH4 via electrocatalytic reduction. The purified biogas is discharged from exhaust port I to the methane collection tank. In the anode chamber, water is oxidized to produce H2S. + H + Electrons selectively permeate from the anode chamber to the cathode chamber through the proton exchange membrane. Electrons travel from the counter electrode to the working electrode through the external circuit. Both electrodes participate in CO2 reduction at the working electrode, while the O2 product from the counter electrode is discharged from exhaust port II.

[0008] The biogas purification device also includes an online pH monitoring sensor for real-time monitoring of the electrolyte pH value. When the pH value deviates from 10.0~10.5, the electrolyte is replenished or replaced by adding fresh electrolyte to the replenishment tank through the fresh electrolyte filling port. If a complete replacement of the electrolyte is required, first close the valve on the connecting pipeline between the circulation pump and the replenishment tank, open the drain valve of the anode chamber to drain all the spent electrolyte to the waste liquid collection device, close the drain pipeline shut-off valve, and inject the pre-prepared fresh electrolyte into the replenishment tank through the fresh electrolyte filling port. Then, open the valve on the connecting pipeline between the circulation pump and the replenishment tank and start the circulation pump. When the electrolyte pH value does not fluctuate significantly and meets the set requirements, only the original electrolyte in the storage chamber is circulated to the spray head by the circulation pump.

[0009] The biogas purification device is a direct-flow, closed-loop structure with a total height of 4.0~5.0m, a diameter of 1.2~1.5m, and a total volume of 4.5~8.8m³. 3 A single unit can process 50-200 m³ of biogas. 3 / h, spray volume (m) 3 / h = 1.2~1.5 × biogas flow rate (m³) 3 / h, that is, 60~300m 3 / h, spray density is 0.08~0.15 L / (m 2 ·s), circulating pump circulation volume m 3 / h = 1.1~1.3 × spray volume (m) 3 / h, that is, 66~390m 3 / h.

[0010] The electrolyte is an aqueous solution containing 0.05~0.1 mol / L KOH, 0.3~0.5 mol / L KHCO3, 0.05~0.2 mol / L H3BO3, and 0.01~0.05 mol / L Fe(EDTA) complex. The pH value of the electrolyte is 10.0~10.5. The electrolyte serves as both an absorbent for H2S and NH3 in biogas and an electrolyte for the electrocatalytic reduction of CO2, achieving in-situ coupling of absorption and electrocatalytic reaction.

[0011] The electrocatalytic reaction section uses a three-electrode system. The reference electrode is a saturated calomel electrode, the counter electrode is a platinum mesh electrode or a graphite electrode, and the working electrode uses carbon paper containing polytetrafluoroethylene (PTFE) as a substrate. A copper-based catalyst is loaded onto the carbon paper using methods such as hydrothermal or electrodeposition. The copper-based catalyst is a Cu-based catalyst doped with at least one element selected from silver, lanthanum, palladium, lithium, cesium, zinc, and zirconium. The voltage applied to the working electrode is -1V to -3V vs SCE.

[0012] After being absorbed by the electrolyte, the H2S in the biogas reacts with Fe (EDTA). 3+ A redox reaction occurs, generating elemental sulfur solid particles. Under gravity, these particles naturally settle and eventually accumulate at the bottom of the storage chamber. Therefore, a filter assembly is installed on the inlet pipe of the circulating pump, allowing for online cleaning. Based on the observed sulfur accumulation, the reactor can be opened for discharge and flushing.

[0013] Advantages and technical effects of the present invention: (1) In the prior art, biogas pretreatment (removal of H2S, NH3, etc.) and CO2 electrocatalytic reduction to methane are usually two independent process units that need to be connected by pipelines. This results in problems such as complex equipment, large footprint, high losses during gas transfer, and low mass transfer efficiency. The present invention integrates the spray section, electrocatalytic reaction section, and circulating storage section into one unit, which can complete the simultaneous removal of H2S and NH3 and the in-situ electrocatalytic reduction of CO2 in the same reactor, achieving "purification-conversion" in one step. At the same time, the spray liquid is directly used as the electrolyte, and the spray process is coupled in-situ with the electrocatalytic reaction. This not only simplifies the equipment structure and reduces the solution preparation and transfer links, but also solves the technical pain points of equipment dispersion and poor synergy in the prior art. Furthermore, the heat absorbed by the spray can be used to maintain the micro-temperature required for the electrocatalytic reaction, while the OH generated by the electrocatalytic reaction can be used to maintain the micro-temperature required for the electrocatalytic reaction.- It can replenish the alkalinity consumed by absorbing acidic gases.

[0014] (2) In existing technologies, biogas pretreatment is mostly targeted at single pollutants. This invention optimizes the formulation of the spray liquid (electrolyte) by using a KOH-KHCO3 weakly alkaline buffer system, combined with H3BO3 as an NH3 scavenger, to achieve simultaneous and efficient removal of H2S, NH3, and CO2. Simultaneously, the addition of Fe(EDTA) complexes to the spray liquid prevents H2S from poisoning the electrodes, extending their lifespan. This buffer system maintains stable electrolyte pH, preventing pH fluctuations caused by CO2 dissolution, and improving the stability and product selectivity of the electrocatalytic reaction.

[0015] (3) The present invention sets up a circulating storage section and a circulating pump to realize the recycling of the spray liquid (electrolyte) and reduce the consumption of reagents; at the same time, the reaction products in the spray liquid (such as K2S, K2CO3) can be further converted or recycled during the electrolysis process. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the device structure of the present invention; In the diagram: 1-Shell, 2-Spray head, 3-Working electrode, 4-Counter electrode, 5-Reference electrode, 6-Proton exchange membrane, 7-Microbubble aeration device, 8-Circulating pump, 9-Supplementation tank, 10-pH online monitoring sensor, 11-Waste liquid collection tank, 12-Methane collection tank, 13-Biogas inlet, 14-Exhaust port I, 15-Drain outlet, 16-Fresh electrolyte filling port, 17-Exhaust port II. Detailed Implementation

[0017] The present invention will be further described in detail below through embodiments, but the scope of protection of the present invention is not limited to the content described.

[0018] Example 1: Biogas purification based on integrated biogas deimpurification-CO2 electrocatalytic reduction technology like Figure 1As shown, the integrated purification device used in this embodiment includes a shell, spray heads, working electrode, counter electrode, reference electrode, proton exchange membrane, microbubble aeration device, circulation pump, replenishment tank, waste liquid collection tank, and methane collection tank. The shell 1 is divided into a cathode chamber and an anode chamber by a proton exchange membrane 6. Electrolyte is provided in the shell. Biogas inlet 13 is located at the bottom of the cathode chamber and is connected to the microbubble aeration device 7. Several spray heads 2 are located at the top of the cathode chamber. The drain outlet 15 on the lower side of the cathode chamber is connected to the replenishment tank 9 through the circulation pump 8. The replenishment tank 9 is connected to the spray heads 2 through a pipe. The cathode chamber is equipped with a working electrode 3 and a reference electrode 5. The exhaust port I 14 at the top of the cathode chamber is connected to the methane collection tank 12. The anode chamber is equipped with a counter electrode 4. The anode chamber has an exhaust port II 17 at the top and a drain outlet 15 at the bottom that is connected to the waste liquid collection tank 11. The anode chamber is equipped with a pH online monitoring sensor 10. The replenishment tank is equipped with a fresh electrolyte filling port 16. The biogas to be treated (composed of 35% CO2, 60% CH4, 1% H2S, and 200ppm NH3) enters the microbubble aerator 7 through biogas inlet 13. After passing through the microbubble aerator 7, it comes into full contact with the electrolyte. H2S and NH3 in the biogas are absorbed, while the remaining gas rises and comes into countercurrent contact with the electrolyte sprayed by the spray head 2, further removing H2S and NH3. As the gas passes the three electrodes, the CO2 in the gas is selectively reduced to CH4 via electrocatalytic reduction. The purified biogas is discharged from exhaust port I14 to the methane collection tank 12. In the anode chamber, water is oxidized to produce H2S. + H + Electrons selectively permeate from the anode chamber to the cathode chamber through the proton exchange membrane. Electrons travel from the counter electrode to the working electrode through the external circuit. Both electrodes participate in CO2 reduction at the working electrode, while the O2 product from the counter electrode is discharged from the exhaust port II17.

[0019] The working electrode 3 uses carbon paper containing 20% ​​PTFE as the substrate and supports a copper-lanthanum composite catalyst (Cu:La molar ratio of 3:2). The counter electrode 4 is a graphite sheet electrode, and the reference electrode 5 is a saturated calomel electrode, providing a stable voltage of -2V. The electrolyte solution is an aqueous solution containing 0.05M KOH, 0.3M KHCO3, 0.05M H3BO3, and 0.01M Fe(EDTA) complex. The biogas flow rate is 60m³ / h. 3 / h; the test results showed that the methane concentration collected in the cathode chamber increased from 60% to 89%, and hydrogen sulfide and ammonia were completely removed.

[0020] Example 2: Biogas purification based on integrated biogas deimpurification-CO2 electrocatalytic reduction technology The device structure used in this embodiment is the same as in Embodiment 1, except that a platinum mesh electrode is used as the counter electrode, and carbon paper containing 20% ​​PTFE is used as the substrate for the working electrode, supporting a copper-lanthanum-silver ternary catalyst (Cu:La:Ag molar ratio of 4:6:1). The electrolyte is an aqueous solution containing 0.05M KOH, 0.5M KHCO3, 0.1M H3BO3, and 0.01M Fe(EDTA) complex, and the biogas inlet flow rate is controlled at 100m. 3 / h, the DC power supply applies a voltage of -1.75V.

[0021] The biogas processed in this embodiment has the following components: CO2 content 30%, CH4 content 58%, H2S concentration 200ppm, and NH3 concentration 200ppm.

[0022] Biogas enters the electrolyte solution from the microbubble aerator. Upon gas-liquid contact, H2S and NH3 are absorbed, while some of the uncaptured gas rises and comes into countercurrent contact with the electrolyte sprayed by the top spray nozzle array, further intercepting H2S and NH3. A voltage of -1.75V is applied to the electrocatalytic zone to initiate the electrochemical reaction. During electrolysis, carbon dioxide is reduced to methane at the cathode through electrocatalysis, accompanied by trace amounts of hydrogen, while oxygen is generated at the anode. Detection shows that the methane concentration collected in the cathode chamber increased from 58% to 92%, and hydrogen sulfide and ammonia were completely removed.

Claims

1. A method for integrated purification of biogas by simultaneous deimpurification and CO2 electrocatalytic reduction, characterized in that: A biogas purification device is used to treat the biogas to be treated. The biogas purification device includes a shell, spray heads, working electrode, counter electrode, reference electrode, proton exchange membrane, microbubble aeration device, circulation pump, replenishment tank, waste liquid collection tank, and methane collection tank. The shell (1) is divided into a cathode chamber and an anode chamber by a proton exchange membrane (6). Electrolyte is provided in the shell. The biogas inlet (13) is located on the bottom side of the cathode chamber and is connected to the microbubble aeration device (7). Several spray heads (2) are located on the top of the cathode chamber. The drain port (15) on the lower side of the cathode chamber is connected to the replenishment tank (9) through the circulation pump. The replenishment tank (9) is connected to the spray heads (2) through a pipe. The cathode chamber is equipped with a working electrode and a reference electrode. The exhaust port I (14) on the top of the cathode chamber is connected to the methane collection tank (12). The anode chamber is equipped with a counter electrode. The top of the anode chamber has an exhaust port II, and the bottom has an exhaust port (15) connected to the waste liquid collection tank. The biogas to be treated enters the microbubble aeration device from the biogas inlet (13). After passing through the microbubble aeration device and fully contacting the electrolyte, H2S and NH3 in the biogas to be treated are absorbed, and the remaining gas rises and comes into countercurrent contact with the electrolyte sprayed by the spray head, further removing H2S and NH3 from the gas. The gas passes through the location of the three electrodes, where the CO2 in the gas is selectively reduced to CH4 by the electrocatalytic reduction reaction. The purified biogas is discharged from exhaust port I to the methane collection tank. Water in the anode chamber is oxidized to produce H2S. + H + Electrons selectively permeate from the anode chamber to the cathode chamber through the proton exchange membrane. Electrons travel from the counter electrode to the working electrode through the external circuit. Both electrodes participate in CO2 reduction at the working electrode, while the O2 product from the counter electrode is discharged from exhaust port II.

2. The integrated purification method for simultaneous biogas deimpurification and CO2 electrocatalytic reduction according to claim 1, characterized in that: The biogas purification device also includes an online pH monitoring sensor, which is used to monitor the pH value of the electrolyte in real time. When the pH value is detected to deviate from 10.0~10.5, the electrolyte is added to or replaced through the fresh electrolyte filling port.

3. The integrated purification method for simultaneous biogas deimpurification and CO2 electrocatalytic reduction according to claim 1, characterized in that: The biogas purification device is a direct-flow, closed-loop structure, with a total height of 4.0~5.0m, a diameter of 1.2~1.5m, and a total volume of 4.5~8.8m³. 3 A single unit can process 50-200 m³ of biogas. 3 / h, spray volume (m) 3 / h = 1.2~1.5 × biogas flow rate (m³) 3 / h, that is, 60~300m 3 / h, spray density is 0.08~0.15 L / (m 2 ·s), circulating pump circulation volume m 3 / h = 1.1~1.3 × spray volume (m) 3 / h, that is, 66~390m 3 / h.

4. The integrated purification method for simultaneous biogas deimpurification and CO2 electrocatalytic reduction according to claim 1, characterized in that: The electrolyte is an aqueous solution containing 0.05~0.1 mol / L KOH, 0.3~0.5 mol / L KHCO3, 0.05~0.2 mol / L H3BO3, and 0.01~0.05 mol / L Fe(EDTA) complex, with a pH value of 10.0~10.

5.

5. The integrated purification method for simultaneous biogas deimpurification and CO2 electrocatalytic reduction according to claim 1, characterized in that: The reference electrode is a saturated calomel electrode, the counter electrode is a platinum mesh electrode or a graphite electrode, and the working electrode uses carbon paper containing polytetrafluoroethylene as a substrate. A copper-based catalyst is loaded onto the carbon paper using a hydrothermal method or an electrodeposition method. The copper-based catalyst is a Cu-based catalyst doped with at least one element selected from silver, lanthanum, palladium, lithium, cesium, zinc, and zirconium. The voltage applied to the working electrode is -1V to -3V vs SCE.

Images