An integrated method and system for simultaneous ammonia and hydrogen sulfide removal in-situ by internal circulation of biogas while enhancing mass transfer

Abstract

The present application relates to the field of waste management and renewable energy generation, in particular, to an integrated method and system for in-situ removal of ammonia and hydrogen sulfide while enhancing mass transfer through biogas internal circulation. The core innovation lies in the synergy of the mixing technology based on biogas internal circulation and the in-situ ammonia and hydrogen sulfide removal process, achieving integrated integration. The system includes an anaerobic digestion unit and a biogas circulation subsystem, which is used to circulate the biogas produced by anaerobic digestion, thereby enhancing the mass transfer effect. Compared with non-mixed systems, this mixing method can significantly improve substrate uniformity and process stability. The improvement of the present application is that only through the mixing mechanism of biogas reflux, the mixing effect of the digestion tank, the process stability, the removal level of hydrogen sulfide and ammonia can be realized at the same time. The biogas stream containing hydrogen sulfide and ammonia gas from the anaerobic digester passes through the hydrogen sulfide absorption unit containing ferric chloride solution, and then passes through the sulfuric acid absorption liquid to absorb ammonia and generate ammonium sulfate. This integrated technology can effectively alleviate the inhibition of hydrogen sulfide and ammonia without additional energy. The new method has multiple advantages, including significantly improving mass transfer efficiency, methane yield, biogas quality, improving digester stability, and efficiently alleviating the hydrogen sulfide and ammonia inhibition problems related to anaerobic digestion of high-nitrogen substrates.

Description

Technical Field

[0001] This invention generally relates to the fields of waste management and renewable energy generation. More specifically, it relates to an integrated method and system for enhancing mass transfer through biogas internal circulation to promote anaerobic digestion and achieve in-situ removal of hydrogen sulfide and ammonia. Background Technology

[0002] Anaerobic digestion is a widely used method for organic waste management and energy recovery. However, this process still faces several significant challenges, such as inadequate mixing, scum and foam formation, hydrogen sulfide production in biogas, and ammonia accumulation. These problems can lead to system imbalances and increase the need for post-treatment steps. This imbalance can ultimately poison methanogens, inhibit the entire process, and reduce efficiency. Co-digestion with carbon-rich waste (e.g., rice husks) has proven to be an effective and well-structured strategy for enhancing anaerobic digestion (CN201910657639). An authorized invention patent (CN201910657639) discloses a method for anaerobic fermentation of chicken manure mixed with rice husk biochar (produced through pyrolysis at 550°C) to improve biogas production performance. This method includes a post-treatment step: treating the biochar at 105–110°C for 24 to 36 hours; although this process is energy-intensive, it helps stabilize the fermentation process and increase biogas yield. Furthermore, the high operating energy consumption required for mixing reduces the feasibility of this process. Furthermore, patent US202436082A1 discloses a two-stage continuous / semi-continuous method for anaerobic digestion of high-nitrogen feedstocks. In this method, ammonia removal is achieved through post-hydrolysis in the first stage, followed by further methanogenic treatment of the deammoniated slurry. However, this method uses a separate ammonia removal unit operating at 55°C and pH 9.5, resulting in higher energy consumption and costs for the ammonia recovery process. In addition, the toxicity and corrosiveness of hydrogen sulfide necessitate post-treatment of the generated biogas for purification. Summary of the Invention

[0003] The purpose of this invention is to provide a system for anaerobic co-digestion that enhances the mixing effect of the anaerobic digester, increases biogas yield, improves the quality of the produced biogas, and achieves in-situ hydrogen sulfide removal and ammonia suppression by employing a biogas recirculation mixing strategy. In one embodiment, the invention evaluates the mixing effect by comparing and evaluating anaerobic digestion of manure without initial mixing with anaerobic digestion using biogas recirculation mixing. Furthermore, co-digestion of animal manure with a predetermined amount of pretreated rice husks was performed. The rice husks underwent a physical pretreatment step to disrupt their lignocellulose structure, thereby improving their biodegradability. Finally, in-situ ammonia removal via recirculated biogas was employed in the co-digestion of manure and rice husks to evaluate the system's hydrogen sulfide and ammonia removal efficiency. This invention describes a method and system for enhancing anaerobic digestion processes by combining optimized biogas recirculation mixing with in-situ hydrogen sulfide and ammonia recovery. The specific process and system configuration include the following steps: 1. A method for enhancing mass transfer and stabilizing a reactor through biogas internal circulation to promote anaerobic digestion and achieve in-situ removal of hydrogen sulfide and ammonia, characterized by comprising the following steps: Step 100: The pretreated and homogenized substrate is fed into an anaerobic digester that operates under mesophilic conditions and the temperature and pressure are monitored in real time. Step 200: Under anaerobic conditions, the biogas produced is circulated at the bottom of the digester using a gas nozzle and a guide pipe to achieve thorough mixing inside the substrate. Parameters such as biogas circulation intensity and duration can be adjusted as needed. Step 300: To achieve hydrogen sulfide removal and precipitation, the generated biogas is passed through a ferric chloride solution column to effectively convert the hydrogen sulfide in the biogas into sulfur precipitate. Step 400: In order to achieve ammonia removal and absorption, biogas is passed through the ammonia absorption unit and then absorbed by the H2SO4 solution. When the pH value of the sulfuric acid absorption solution exceeds 4, it needs to be replaced with fresh sulfuric acid solution. Step 500: After ammonia absorption, sulfuric acid droplets entrained in the biogas flow are condensed using a room temperature condenser. The biogas then passes further through a silica gel and molecular sieve bed to further remove residual moisture or liquid, and is then stored in a biogas collection bag for recirculation.

[0004] 2. The biogas recycling, hydrogen sulfide removal, and ammonia removal method according to claim 1, characterized in that step 200 further includes the following sub-steps: Step 210: Pass a portion of the biogas produced through a gas nozzle surrounded by a guide pipe to generate bubbles and induce substrate turbulence; Step 220: The circulating biogas discharged from the gas nozzle slightly above the bottom of the guide pipe causes the liquid level in the guide pipe to drop, which in turn promotes the scum or foam to enter the jet pipe from the top, thereby achieving effective management of scum / foam and full mixing of materials. Step 230: The generated biogas bubbles form turbulence in the substrate and rise to the liquid surface, during which free ammonia is removed.

[0005] 3. An integrated system that enhances mass transfer and stabilizes the reactor through biogas internal circulation to promote anaerobic digestion and achieve in-situ removal of hydrogen sulfide and ammonia: First, the animal manure, which is the main substrate, is pretreated and homogenized to obtain a uniform particle size distribution. For the co-digestion batch, the manure is mixed with pretreated rice husks and stored in a substrate tank (1). The homogenized mixture with a total solids content of about 12% is introduced into the anaerobic digester (2) with a water jacket through the substrate inlet (21). The digestion process is carried out under mesophilic conditions, and the temperature is regulated by a thermostatically controlled water jacket (22) and matching warm water inlets and outlets (23, 24). To evaluate the mixing effect, the non-mixed batch is used as a reference, and then the biogas circulation mixing batch is used. The circulation is controlled by a time relay (11), and the circulation intensity is precisely regulated by a rotor flow meter (31). Intermittent mixing is performed according to the settings. Biogas is drawn from the biogas collection bag (9) and reintroduced into the reactor from the bottom through the gas nozzle (28) and the guide pipe (29). The system is equipped with a temperature sensor (210) and a pressure sensor (211) to monitor the temperature and pressure in real time, and samples are collected periodically from the biogas liquid outlet (212) at the bottom of the reactor. The biogas was analyzed for multiple parameters, including total solids, volatile solids, pH, chemical oxygen demand removal rate, total organic carbon, volatile fatty acids and ammonia concentration. At the same time, the methane and carbon dioxide content in the biogas was analyzed using a gas chromatograph. The circulating biogas gas containing denutrients such as hydrogen sulfide, ammonia and carbon dioxide first passed through a ferric chloride solution with a nozzle (5) to remove hydrogen sulfide and precipitate it as sulfur. This hydrogen sulfide removal unit was further extended to remove sulfur and used oxygen to regenerate the ferric chloride solution, so that Fe²⁺ was converted back to Fe³⁺. In addition, after hydrogen sulfide removal, there was an ammonia removal unit, which contained a container (6) containing sulfuric acid solution to absorb ammonia and generate ammonium sulfate. When the pH value of the acid solution exceeded 4.0, fresh acid solution needed to be replaced to maintain absorption efficiency. Subsequently, the biogas passed through a condenser (7) to remove moisture and steam, and then passed through a silica gel and molecular sieve bed (8) for drying. The dried biogas can be returned to the reactor for mixing or introduced into a collection bag (9) for storage.

[0006] The present invention has the following beneficial effects: a. Mixing is achieved solely through biogas internal circulation, and hydrogen sulfide and ammonia are removed simultaneously using the same circulation system, reducing operating energy consumption without the need for additional units; b. In addition to effectively reducing hydrogen sulfide inhibition, controlling ammonia concentration and improving digestion stability, it can also improve biogas quality in situ without consuming additional resources. Attached Figure Description

[0007] Figure 1 A schematic flowchart illustrating an integrated biogas recirculation mixing system that simultaneously achieves enhanced mixing to promote in-situ biogas quality improvement, in-situ hydrogen sulfide suppression, and in-situ ammonia removal.

[0008] Figure 2 Mind map of this invention.

[0009] Figure label: 1 – Substrate tank; 2 – Digester; 21 – Substrate inlet; 22 – Water jacket; 23 – Circulating water inlet; 24 – Warm circulating water outlet; 25 – Anaerobic digester; 26 – Biogas inlet; 27 – Biogas outlet; 28 – Gas nozzle; 29 – Flow guide pipe; 210 – Temperature sensor; 211 – Pressure sensor; 212 – Biogas slurry outlet; 3 – Control panel; 31 – Rotor flow meter; 4 – Water bath heater; 5 – Ferric chloride solution; 6 – Sulfuric acid solution; 7 – Condenser; 8 – Silica gel and molecular sieve bed; 9 – Biogas collection bag; 10 – Biogas circulation pump; 11 – Time relay; Arrows indicate flow direction. Detailed Implementation

[0010] The present invention will be further described below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.

[0011] Reference Figure 1 The digester system (2) used to enhance anaerobic co-digestion includes the following key components: a. Substrate inlet (21) leading to an anaerobic digester (25) equipped with a water jacket (22) for maintaining the mixture in the reactor at a mesophilic temperature of 37 ± 1°C; b. Circulating water inlet (23) and outlet (24) leading to the water bath heater (4); c. Gas nozzle (28) and guide pipe (29) for efficient biogas circulation and mixing; d. Temperature sensor (210) and pressure sensor (211) for real-time monitoring; e. Pipelines leading to / from the anaerobic digester’s slurry outlet (212), biogas inlet (26) and biogas outlet (27).

[0012] also, Figure 1 It also includes other components that enable the boot process to achieve its goals: a. A control console (3) is used to display the real-time temperature and pressure inside the reactor. A rotor flow meter (31) is also installed on the control console to regulate the biogas flow rate to the reactor; b. Water bath heater (4), used to supply warm water to the reactor water jacket, controlled by the control console; c. A biogas circulation pump (10), whose operation is regulated by a time relay (11) and operates at specific time intervals; d. A 0.5 mol / L ferric chloride (FeCl3·6H2O) solution (5), which is further extended for solution regeneration to convert Fe²⁺ back to Fe³⁺; e. A 1 mol / L sulfuric acid (H2SO4) solution (6), followed by a small condenser (7) for collecting and condensing the entrained vapor carried by the biogas; f. A silica gel and molecular sieve bed (8) is used to capture the moisture contained in the biogas before it is collected into the biogas collection bag (9).

[0013] The specific process of running this system Figure 2 As shown, pretreated animal manure and rice husks, thoroughly mixed in a known ratio, are introduced into a reactor under mesophilic conditions through a substrate inlet pipeline. Anaerobic digestion is performed on the manure material, either as a single substrate or co-digested with rice husks. The resulting biogas is then treated by removing hydrogen sulfide, ammonia, and water vapor to improve its quality, and is subsequently recycled back to the anaerobic digester to enhance the mass transfer process.

[0014] On the other hand, such as Figure 1 The system described in claim 1 can also be used directly in ammonia-free systems without the need for ammonia removal and absorption sections, aiming to achieve enhanced mixing and biogas upgrading. Furthermore, if hydrogen sulfide is not involved in the system, the hydrogen sulfide removal system can be removed. In addition, the carbon-enriching additive is not limited to rice husks and can be selected from the group consisting of rice straw, wheat straw, corn stalks, sawdust, or other lignocellulosic materials. Similarly, the nitrogen-enriching substrate is not limited to animal manure and can be chicken manure, dairy cow manure, or sewage sludge. Furthermore, the pretreatment of the substrate and rice husks can also be a combination of chemical and physical methods. Example

[0015] The following describes the implementation process of the present invention using animal manure collected from a pig farm (and its co-digestion with rice husks) as an example. The initial characteristics of the collected manure included: density (973 kg / m³), pH (8.34), total solids (6.84%), average VS / TS ratio (67.4), soluble chemical oxygen demand (sCOD) (20390 mg / L), and total chemical oxygen demand (tCOD) (55500 mg / L). The collected manure was homogenized by gentle stirring and passed through a 2 mm (10-mesh standard) sieve to remove large solids and prevent pipe blockage. [The following text also mentions using...] Figure 1 The custom reactor shown was compared with other treatments performed in a conventional sample bottle reactor to evaluate the impact of reactor design on mixing and digestion efficiency. Based on biogas yield (ml / g vs. added ) and methane yield (ml CH4 / g VS added Digestion efficiency was compared. All batches used a hydraulic retention time of 45 days and a mesothermal temperature of 37 ± 1°C. The substrate was pumped into the reactor vessel, taking care to avoid air entering the system. When biogas production began, the CH4 and CO2 content of the biogas produced was measured three times daily using gas chromatography. Similarly, samples were collected every 3 days to measure pH, COD, TOC, and VFAs. At the end of each batch, TS and VS were also measured to assess VS removal efficiency. In addition, all co-digestion treatments were performed at a biogas recirculation flow rate of 2 L / min for 5 min / h. In all co-digestion treatments, pretreated rice husks (reduced in size and passed through a 2 mm (10 mesh) sieve) were added to the manure to achieve a total solids content of 12%. The hydrogen sulfide treatment system used 300 ml of a 0.5 mol / L ferric chloride (FeCl3·6H2O) solution, which was introduced into the circulating biogas at the bottom of the unit through a microbubble nozzle to convert hydrogen sulfide into sulfur precipitate. Using the same co-digestion parameters, an ammonia absorption system (using 500 ml of 1 mol / L H2SO4 solution) was applied to evaluate the system's ammonia removal efficiency. The single digestion system based on biogas recirculation achieved a 12.86% higher COD removal rate compared to the unmixed treatment. The ammonia stripping system in co-digestion achieved a 13.80% higher COD removal rate compared to a system without ammonia removal. Furthermore, compared to conventional reactors, the customized reactor integrating biogas recirculation and ammonia stripping achieved an 11% increase in methane yield and a 13.2% improvement in biogas quality. According to component testing, the hydrogen sulfide removal system achieved a hydrogen sulfide removal rate exceeding 98%. In-situ ammonia removal in the integrated ammonia removal system achieved an ammonia removal rate of 70%. Despite the low biodegradability, lignocellulose structure, and recalcitrant nature of the co-substrate (rice husk), the integrated in-situ hydrogen sulfide and ammonia removal biogas recirculation system further improved the process and digestion efficiency.

[0016] Although the invention has been described with reference to preferred embodiments, various modifications can be made and components can be replaced with equivalents without departing from the scope of the invention. In particular, the technical features mentioned in the embodiments can be combined in any manner as long as there is no structural conflict. The invention is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. An integrated method for enhancing mass transfer and stabilizing the reactor through biogas internal circulation to promote anaerobic digestion and achieve hydrogen sulfide removal and in-situ ammonia removal, characterized in that... Includes the following steps: Step 100: The pretreated and homogenized substrate is fed into an anaerobic digester that operates under mesophilic conditions and the temperature and pressure are monitored in real time. Step 200: Under anaerobic conditions, the biogas produced is circulated at the bottom of the digester using a gas nozzle and a guide pipe to achieve thorough mixing inside the substrate. Parameters such as biogas circulation intensity and duration can be adjusted as needed. Step 300: To achieve hydrogen sulfide removal and precipitation, the generated biogas is passed through a ferric chloride solution column to effectively convert the hydrogen sulfide in the biogas into sulfur precipitate. Step 400: To achieve ammonia removal and absorption, biogas is passed through the ammonia absorption unit and then absorbed by the H2SO4 solution. When the pH value of the sulfuric acid absorption solution exceeds 4, it needs to be replaced with fresh sulfuric acid solution. Step 500: After ammonia absorption, sulfuric acid droplets entrained in the biogas flow are condensed using a room temperature condenser. The biogas then passes through a silica gel bed to further remove residual moisture or liquid, and is subsequently stored in a biogas collection bag for recirculation.

2. The integrated method for biogas recycling, hydrogen sulfide removal, and ammonia removal according to claim 1, characterized in that... Step 200 further includes the following sub-steps: Step 210: Pass a portion of the biogas produced through a gas nozzle surrounded by a guide pipe to generate bubbles and induce substrate turbulence; Step 220: The circulating biogas discharged from the gas nozzle slightly above the bottom of the guide pipe causes the liquid level in the guide pipe to drop, which in turn promotes the scum or foam to enter the jet pipe from the top, thereby achieving effective management of scum / foam and full mixing of materials. Step 230: The generated biogas bubbles form turbulence in the substrate and rise to the liquid surface, during which free ammonia is removed.

3. An integrated system that enhances mass transfer and stabilizes the reactor through biogas internal circulation to promote anaerobic digestion and achieve in-situ removal of hydrogen sulfide and ammonia: First, the animal manure, which is the main substrate, is pretreated and homogenized to obtain a uniform particle size distribution. For the co-digestion batch, the manure is mixed with pretreated rice husks and stored in a substrate tank (1). The homogenized mixture with a total solids content of about 12% is introduced into the anaerobic digester (2) with a water jacket through the sample inlet (21). The digestion process is carried out under mesophilic conditions, and the temperature is regulated by a thermostatically controlled water jacket (22) and matching warm water inlets and outlets (23, 24). To evaluate the mixing effect, the non-mixed batch is used as a reference, and then the biogas circulation mixing batch is used. The circulation is controlled by a time relay (11), and the circulation intensity is precisely regulated by a rotor flow meter (31). Intermittent mixing is performed according to the setting. Biogas is drawn from the biogas collection bag (9) and reintroduced into the reactor from the bottom through the gas nozzle (28) and the guide pipe (29). The system is equipped with a temperature sensor (210) and a pressure sensor (211) to monitor the temperature and pressure in real time, and samples are collected periodically from the biogas liquid outlet (212) at the bottom of the reactor. The biogas was analyzed for multiple parameters, including total solids, volatile solids, pH, chemical oxygen demand removal rate, total organic carbon, volatile fatty acids and ammonia concentration. At the same time, the methane and carbon dioxide content in the biogas was analyzed using a gas chromatograph. The circulating biogas gas containing denutrients such as hydrogen sulfide, ammonia and carbon dioxide first passed through a ferric chloride solution with a nozzle (5) to remove hydrogen sulfide and precipitate it as sulfur. This hydrogen sulfide removal unit was further extended to remove sulfur and used oxygen to regenerate the ferric chloride solution, so that Fe²⁺ was converted back to Fe³⁺. In addition, after hydrogen sulfide removal, there was an ammonia removal unit, which contained a container (6) containing sulfuric acid solution to absorb ammonia and generate ammonium sulfate. When the pH value of the acid solution exceeded 4.0, fresh acid solution needed to be replaced to maintain absorption efficiency. Subsequently, the biogas passed through a condenser (7) to remove moisture and steam, and then passed through a silica gel and molecular sieve bed (8) for drying. The dried biogas can be returned to the reactor for mixing or introduced into a collection bag (9) for storage.

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