A biogas slurry treatment system and method without the addition of alkali

Abstract

This invention relates to a biogas slurry treatment system and method without the addition of alkali. The steps include: after removing CO2 and increasing the pH of the biogas slurry through air aeration and stripping, ammonia stripping is performed by dripping the biogas slurry from the top of a convection ammonia stripping tower. The ammonia-rich gas stripped out enters an absorption tower and is washed with sulfuric acid to complete the self-circulation absorption of ammonia nitrogen. The deammoniated biogas slurry is further purified by biogas aeration and stripping, which completes the pH adjustment while absorbing CO2 from biogas. This invention achieves a single-pass ammonia removal rate of over 70% and an ammonia nitrogen recovery rate of over 98% without the addition of alkali, ultimately making the biogas slurry meet irrigation water discharge standards and yielding nitrogen fertilizer and pure biogas.

Description

Technical Field

[0001] This invention relates to the field of agricultural waste treatment and utilization technology, specifically to a biogas slurry treatment system and method without the addition of alkali. Background Technology

[0002] Low-carbon circular agriculture models that harmlessly treat livestock and poultry manure using biogas projects are widely applied. Biogas slurry, a byproduct of biogas projects, contains high concentrations of ammonia nitrogen; direct discharge inevitably causes serious environmental pollution and is a waste of recyclable resources. Therefore, ammonia removal and recycling of biogas slurry has become one of the biggest obstacles to the widespread application of biogas technology. Currently, ammonia removal from chicken manure biogas slurry mainly includes technologies such as struvite precipitation, membrane separation, and ammonia stripping. Chinese patent CN215756840U discloses an electrochemical precipitation method for struvite in chicken manure biogas slurry treatment, utilizing Mg... 2+ NH 4+ PO4 3- ion coprecipitation to achieve NH 4+ PO4 3- Simultaneous removal is possible. However, this method requires the addition of large amounts of chemical agents, and the treatment equipment accumulates a large amount of stones, undoubtedly increasing operating and maintenance costs. Chinese patent CN207435283U utilizes filter membranes to achieve the resource recycling of chicken manure fermentation slurry, but because the filter membranes require regular cleaning and have limited stability, chemical resistance, heat resistance, and solvent resistance, it is difficult to achieve widespread adoption. Furthermore, because membrane separation technology alone has limited capacity for treating slurry, it needs to be used in conjunction with other separation technologies. Therefore, it is also unsuitable for large-scale ammonia removal from slurry.

[0003] In comparison, ammonia stripping technology is considered the best option for removing ammonia from biogas slurry with high ammonia nitrogen concentrations. By alkalizing the biogas slurry, ammonium ions are converted into free ammonia, and under the influence of the free ammonia concentration gradient between the gas and liquid phases, ammonia migrates from wastewater to gas. Finally, ammonia removal from the biogas slurry is completed by acid washing. The tail liquid from the washing process is rich in ammonium salts and can be used as bio-fertilizer, effectively balancing the cost of biogas treatment. However, the addition of alkali significantly increases the cost of biogas treatment. Therefore, finding a cheap pH adjustment method to replace traditional alkali addition is crucial for reducing the cost of ammonia stripping in biogas slurry. Chinese patent CN117361597A discloses a method for improving the pH of biogas slurry through aeration stripping using purified biogas, thereby achieving ammonia recovery and biogas purification without the addition of alkali. However, biogas purification implies additional operating expenses or time costs. Furthermore, the ammonia nitrogen removal capacity of a single convection stripping is limited, and the stripped biogas slurry remains alkaline, failing to meet emission standards. Summary of the Invention

[0004] This invention provides a biogas slurry treatment system and method without the addition of alkali, and optimizes the design of the stripping-recovery device to improve the ammonia removal efficiency, while ensuring that the pH of the biogas slurry can be adjusted back after stripping without affecting the discharge. This solves the problems of high cost, low ammonia removal efficiency, and excessively high pH of the biogas slurry after stripping in biogas ammonia recovery and biogas purification processes.

[0005] The present invention provides the following solution to the above-mentioned technical problems: a method for treating biogas slurry without the addition of alkali, comprising the following steps:

[0006] 1) Heat the raw biogas slurry tank, and air enters the bottom of the raw biogas slurry tank through an air pump to aerate and blow off the biogas slurry in the tank, so that the carbon dioxide and ammonia in the biogas slurry are input from the top of the raw biogas slurry tank into the absorption liquid of the ammonia recovery tower.

[0007] 2) After aeration and stripping, the biogas slurry is dripped into the convection ammonia stripping tower by the biogas slurry water pump. Air is aerated and stripped from the bottom of the convection ammonia stripping tower by the gas circulation pump. The ammonia gas in the biogas slurry in the convection ammonia stripping tower is input into the absorbent filled in the ammonia recovery tower. The absorbent is self-circulated in the ammonia recovery tower by the liquid circulation pump. The biogas slurry stripped by the convection ammonia stripping tower flows into the deammonium biogas slurry tank from the bottom of the convection ammonia stripping tower.

[0008] 3) Biogas is pumped into the deammonium slurry tank to aerate and strip the biogas from the slurry, thus separating the biogas from the slurry.

[0009] Preferably, the method further includes: the input end of the gas circulation pump is connected to the top of the ammonia recovery tower, so that the ammonia gas that has not been absorbed in the ammonia recovery tower enters the convection ammonia stripping tower for further stripping.

[0010] Preferably, the method further includes: the biogas used for separation and the biogas from the deammoniation biogas slurry separation in the deammoniation biogas slurry pond enter the methane storage tank from the top of the deammoniation biogas slurry pond.

[0011] The original biogas slurry pond undergoes a first air stripping process to remove carbon dioxide and increase the pH. It then enters a convection ammonia stripping tower for a second stripping process to remove ammonia, which is then absorbed by the absorbent. Finally, it enters a deammoniation biogas pond for a third stripping process, where the stripped biogas and the biogas used as a gas source are fed into a methane storage tank. This process purifies the biogas in the biogas slurry and restores the pH of the biogas slurry to 6.0-8.5, meeting the irrigation water discharge standards.

[0012] Preferably, in step 1), the biogas slurry in the original biogas slurry tank is heated to 35-45°C, and the ratio of the air volume to the volume of the biogas slurry in the original biogas slurry tank is 300-400; aeration and stripping are performed every 2-6 minutes for 4-8 minutes.

[0013] When the temperature is below 35℃, the HCO3 in the biogas slurry 2-It is relatively stable and does not easily decompose into CO2 to increase the pH of the biogas slurry. However, at temperatures above 45°C, most of the nitrogen in the biogas slurry is converted into NH4+. + In its natural form, approximately 80% of CO2 is converted to NH3, easily leading to nitrogen loss. When the gas-liquid ratio is below 300, the gas turbulence time and intensity are insufficient to completely strip CO2 from the biogas slurry. At a gas-liquid ratio of 350, over 80% is stripped, and above 400, energy consumption is wasted. Short aeration and stripping times result in low CO2 removal rates, while short intervals lead to excessive foaming, hindering the transfer of CO2 from the liquid phase to the gas phase.

[0014] Preferably, in step 2), the pH of the biogas slurry after aeration and stripping in step 1) is 9-10, the operating temperature of the convective ammonia stripping tower is 40-50℃, and the ratio of air volume to biogas slurry volume in the convective ammonia stripping tower is 3000-4000.

[0015] When the pH is less than 9, the percentage of ammonium ions converted to NH3 is usually very low, while at pH 10, almost all ammonium ions are converted to NH3. Exceeding this pH does not increase the NH3 content; therefore, a pH above 10 is uneconomical. At a stripping temperature of 45℃, most of the nitrogen in the biogas slurry is in the form of NH4+. + While existing in its natural form, approximately 80% or more is converted to NH3. Temperatures exceeding 50°C do not significantly improve ammonia removal efficiency, thus reducing its economic benefits. The gas-liquid ratio is the same as above.

[0016] Preferably, in step 2), the absorbent is concentrated sulfuric acid with a mass concentration of 30-50%, and the circulation flow rate of the absorbent in the ammonia recovery tower is 6-10 m³ / s. 3 / h.

[0017] When the sulfuric acid concentration is too low, the solution cannot provide enough SO4. 2- The absorption efficiency of NH3 is low, while excessively high concentrations will result in excess SO4. 2- This results in excessively high reagent costs and low economic efficiency. When the circulation flow rate of the absorbent is too high, the residence time of NH3 in sulfuric acid is too short, which easily leads to incomplete reaction and low absorption efficiency; when the flow rate is too low, it will cause the system to work for too long and consume too much energy.

[0018] Preferably, in step 3), the biogas slurry in the deammonium-removing biogas slurry tank is heated to 40-60°C, the ratio of biogas aeration volume to biogas slurry volume in the heated deammonium-removing biogas slurry tank is 400-500, and aeration and stripping are performed every 2-6 minutes for 4-8 minutes.

[0019] When the temperature is below 40℃, biogas is not released sufficiently, affecting the stripping effect. If the temperature is above 60℃, it will increase energy consumption on the one hand, and avoid damage to the equipment caused by high temperature on the other hand.

[0020] A biogas slurry treatment system without the addition of alkali includes a raw biogas slurry tank, a convection ammonia stripping tower, an ammonia recovery tower, an ammonia-removed biogas slurry tank, and a methane storage tank.

[0021] The inlet pipe of the original biogas slurry tank is connected to an air pump, the outlet pipe is connected to the bottom of the ammonia recovery tower, the liquid outlet pipe is connected to the convection ammonia stripping tower, and the liquid outlet pipe of the original biogas slurry tank is equipped with a biogas slurry pump.

[0022] The outlet of the inlet pipe of the convective ammonia stripping tower is located at the bottom of the tower. A gas circulation pump is installed on the inlet pipe and the inlet is connected to the top of the ammonia recovery tower. The outlet pipe of the convective ammonia stripping tower is connected to the bottom of the ammonia recovery tower. The drain pipe of the convective ammonia stripping tower is connected to the ammonia removal digester.

[0023] The bottom of the ammonia recovery tower is filled with absorbent liquid. The outlet of the gas pipe of the original biogas slurry tank and the outlet of the gas pipe of the convection ammonia stripping tower are submerged in the absorbent liquid. A liquid circulation pump is installed on the liquid outlet pipe of the ammonia recovery tower. The outlet of the liquid outlet pipe of the ammonia recovery tower is located at the top of the ammonia recovery tower and is connected to a nozzle.

[0024] The inlet pipe of the deammonium-removing biogas slurry tank is connected to a biogas pump, and the outlet pipe is connected to a methane storage tank.

[0025] Preferably, the convection ammonia stripping tower is provided with a biogas overflow support, packing, packing pressure plate, packing support plate, and aeration disc from top to bottom; the packing, packing pressure plate, and packing support plate are connected to the inner wall of the convection ammonia stripping tower to form a packing zone; the output end of the gas circulation pump is connected to the aeration disc.

[0026] Preferably, the biogas slurry overflow support includes an upper overflow plate and a lower overflow plate. The upper overflow plate is annular and has multiple evenly spaced holes along its circumference. The lower overflow plate is circular, with the ratio of the diameter of the hollow circle to the diameter of the larger circle being 1:2. The lower overflow plate also has multiple evenly spaced holes along its circumference. The upper and lower overflow plates are connected by multiple support frames, the bottom of which penetrates the lower overflow plate and abuts against the packing support plate. The diameter of the holes in the upper and lower overflow plates is 5mm.

[0027] The biogas slurry overflow support has two layers. When biogas slurry drips onto the upper overflow plate, some of the biogas slurry flows to the surrounding area and through the small holes to the lower overflow plate. Some of the biogas slurry flows directly from the hollow of the ring to the lower overflow plate, making the biogas slurry flow evenly and preventing the biogas slurry from clogging the nozzles when spraying.

[0028] Preferably, the packing material in the packing zone is 25mm multifaceted polypropylene hollow spheres, and the packing height is 65%-75% of the height of the convective ammonia stripping tower.

[0029] Preferably, the ammonia recovery tower includes an absorbent filling zone 31 and an absorbent reflux zone 32. The outer diameter of the absorbent filling zone is larger than that of the absorbent reflux zone. The bottom end of the absorbent reflux zone is submerged in the absorbent of the absorbent filling zone and close to the bottom of the absorbent filling zone. The air inlet pipe at the bottom of the absorbent reflux zone is connected to the air outlet pipe of the original biogas slurry tank and the air outlet pipe of the ammonia stripping tower. The outlet of the liquid outlet pipe of the ammonia recovery tower is located at the top of the absorbent reflux zone and is connected to a nozzle.

[0030] The bottom of the absorbent reflux zone comes into contact with the absorbent, forming a liquid seal and increasing the contact between ammonia and sulfuric acid.

[0031] The beneficial effects of this invention are:

[0032] (1) Intermittent aeration helps to eliminate foam generated during aeration of biogas slurry. Using air instead of purified biogas as the stripping carrier can save 25 yuan / t compared with using traditional alkali agent NaOH to adjust the pH of biogas slurry, which has higher economic benefits.

[0033] (2) Using biogas to aerate the stripped biogas slurry not only effectively removes CO2 from the biogas and achieves the purpose of biogas purification, but also lowers the pH of the biogas slurry, so that the pH of the stripped biogas slurry returns to 6.0-8.5, which meets the irrigation water discharge standards.

[0034] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it according to the contents of the specification, the preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings. Specific embodiments of the present invention are given in detail below with reference to the accompanying drawings. Attached Figure Description

[0035] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, illustrate exemplary embodiments of the invention and, together with their description, serve to explain the invention and do not constitute an undue limitation thereof. In the drawings:

[0036] Figure 1 This is a process flow diagram of Example 1;

[0037] Figure 2 This is a process apparatus diagram for Example 2;

[0038] Figure 3 This is a structural diagram of the biogas slurry overflow support in Example 2;

[0039] Figure 4 This is a schematic diagram of the liquid seal formed between the gas outlet pipe and the absorbent liquid in the ammonia recovery tower in Example 2.

[0040] The attached diagram lists the components represented by each number as follows:

[0041] 1. Raw biogas slurry tank; 2. Convection ammonia stripping tower; 3. Ammonia recovery tower; 4. Air pump; 5. Biogas slurry water pump; 6. Liquid circulation pump; 7. Gas circulation pump; 8. Biogas pump; 9. Ammonia-removing biogas slurry tank; 10. Methane storage tank; 21. Biogas slurry overflow support; 22. Packing area; 23. Aeration disc; 211. Upper overflow disc; 212. Lower overflow disc; 31. Absorbent liquid filling area; 32. Absorbent liquid return area. Detailed Implementation

[0042] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0043] Example 1

[0044] like Figure 1 As shown, this embodiment provides a biogas slurry treatment method without the addition of alkali, including the following steps:

[0045] 1) Heat the original biogas slurry tank 1 until the biogas slurry reaches 35-45℃. Set the ratio of air volume to biogas slurry volume in the original biogas slurry tank 1 to 300-400. Air enters the bottom of the original biogas slurry tank 1 through air pump 4 and aerates and blows off the biogas slurry for 4-8 minutes every 2-6 minutes. This allows carbon dioxide and ammonia in the biogas slurry to be fed from the top of the original biogas slurry tank 1 into the 30-50wt% concentrated sulfuric acid absorption liquid in the ammonia recovery tower 3.

[0046] 2) After the first aeration and stripping, the pH of the biogas slurry reaches 9-10. It is then dripped from the top of the convection ammonia stripping tower 2 by the biogas slurry pump 5, maintaining the operating temperature of the convection ammonia stripping tower 2 at 40-50℃. The ratio of air volume to biogas slurry volume in the convection ammonia stripping tower 2 is set at 3000-4000. Air is aerated and stripped from the bottom of the convection ammonia stripping tower 2 by the gas circulation pump 7. The input end of the gas circulation pump 7 is connected to the top of the ammonia recovery tower 3, allowing the ammonia gas that has not been absorbed in the ammonia recovery tower 3 to enter the convection ammonia stripping tower 2 for further stripping. The convection ammonia stripping tower 2 is filled with 25mm multi-faceted polypropylene hollow spheres, and the packing height is set to 65-75% of the tower height.

[0047] Ammonia gas from the biogas slurry is fed from the top of the convection ammonia stripping tower 2 into the 30-50 wt% concentrated sulfuric acid absorbent at the bottom of the ammonia recovery tower 3. The absorbent is then pumped upwards by the liquid circulation pump 6 within the ammonia recovery tower 3 at a rate of 6-10 m. 3 The digester operates in a self-circulating manner, with the digester slurry flowing from the bottom of the convection ammonia stripping tower 2 into the ammonia removal digester slurry tank 9. After the second stripping, the ammonia nitrogen concentration of the digester slurry decreases from the initial 4864 mg / L to 1268 mg / L, achieving a removal rate of 74.7% and an ammonia absorption rate of over 98%.

[0048] 3) Heat the biogas slurry in the deammoniation biogas slurry tank 9 to a stripping temperature of 40-60℃. Set the ratio of biogas aeration volume to biogas slurry volume in the deammoniation biogas slurry tank 9 to 400-500. The biogas is introduced into the deammoniation biogas slurry tank 9 through the biogas pump 8. Aeration and stripping are performed every 2-6 minutes for 4-8 minutes to separate the biogas from the biogas slurry. The separated biogas and the biogas used for stripping enter the methane storage tank 10 from the top of the deammoniation biogas slurry tank 9 for storage. After stripping, the pH of the biogas slurry can be adjusted back to 6.0-8.5.

[0049] Example 2

[0050] like Figure 2 As shown, this embodiment provides a biogas slurry treatment system without the addition of alkali, including a raw biogas slurry tank 1, a convection ammonia stripping tower 2, an ammonia recovery tower 3, an ammonia-removed biogas slurry tank 9, and a methane storage tank 10;

[0051] The inlet pipe of the original biogas slurry tank 1 is connected to an air pump 4, the outlet pipe is connected to the bottom of the ammonia recovery tower 3 and is equipped with a flow meter, the outlet pipe is connected to the top of the convection ammonia stripping tower 2 and is connected to a nozzle, and the outlet pipe of the original biogas slurry tank 1 is equipped with a biogas slurry pump 5.

[0052] The outlet of the inlet pipe of the convective ammonia stripping tower 2 is located at the bottom of the tower. A gas circulation pump 7 is installed on the inlet pipe and its inlet is connected to the top of the ammonia recovery tower 3. The outlet pipe of the convective ammonia stripping tower 2 is connected to the bottom of the ammonia recovery tower 3. The drain pipe of the convective ammonia stripping tower 2 is connected to the ammonia removal slurry tank 9.

[0053] The convection ammonia stripping tower 2 is equipped with, from top to bottom, a biogas overflow support 21, packing material, packing pressure plate, packing support plate, and aeration disc 23; the packing material, packing pressure plate, and packing support plate are connected to the inner wall of the convection ammonia stripping tower 2 to form a packing zone 22; the output end of the gas circulation pump 7 is connected to the aeration disc 23. For example... Figure 3 As shown, the biogas slurry overflow support 21 includes an upper overflow plate 211 and a lower overflow plate 21. The upper overflow plate 211 is annular and has multiple evenly spaced holes along its circumference. The lower overflow plate 21 is circular, with the ratio of the diameter of the hollow circle to the diameter of the larger circle being 1:2. It also has multiple evenly spaced holes along its circumference. The upper and lower overflow plates 211 are connected by multiple support frames, the bottom of which penetrate the lower overflow plate 21 and abut against the packing support plate. The diameter of the holes in the upper and lower overflow plates 211 is 5mm. The packing material in the packing zone 22 is 25mm multi-faceted polypropylene hollow spheres, and the packing height is 65%-75% of the height of the convective ammonia stripping tower 2.

[0054] A liquid circulation pump 6 is installed on the outlet pipe of the ammonia recovery tower 3; the ammonia recovery tower 3 includes an absorbent filling zone 31 and an absorbent reflux zone 32, such as... Figure 4The absorbent filling zone 31 shown is integrally formed with the bottom of the ammonia recovery tower 3 and is filled with absorbent. Its outer diameter is larger than that of the absorbent return zone 32. The bottom end of the absorbent return zone 32 is submerged in the absorbent in the absorbent filling zone 31 and close to the bottom of the absorbent filling zone 31. The air inlet pipe at the bottom of the absorbent return zone 32 is connected to the air outlet pipe of the original biogas slurry tank 1 and the air outlet pipe of the ammonia stripping tower. The inlet of the liquid outlet pipe of the ammonia recovery tower 3 is located in the absorbent filling zone 31, and the outlet is located at the top of the absorbent return zone 32 and is connected to a nozzle.

[0055] The inlet pipe of the deammonium slurry digester 9 is connected to a biogas pump 8, and the outlet pipe is connected to a methane storage tank 10.

[0056] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Those skilled in the art can readily implement the present invention based on the accompanying drawings and the above description. However, any modifications, alterations, or variations made by those skilled in the art without departing from the scope of the present invention, utilizing the disclosed technical content, are equivalent embodiments of the present invention. Furthermore, any modifications, alterations, or variations made to the above embodiments based on the essential technology of the present invention are still within the protection scope of the present invention.

Claims

1. A method for treating biogas slurry without adding alkali, characterized in that, Includes the following steps: 1) Heat the original biogas slurry tank (1) to 35-45 ℃. Air enters the bottom of the original biogas slurry tank (1) through an air pump (4) to aerate and strip the biogas slurry in the original biogas slurry tank (1), so that carbon dioxide and ammonia in the biogas slurry are transferred from the original biogas slurry tank (1) into the absorbent in the ammonia recovery tower (3); wherein, the ratio of air volume to biogas slurry volume in the original biogas slurry tank (1) is 300-400; aeration and stripping are performed every 2-6 min for 4-8 min. 2) After aeration and stripping, the biogas slurry is dripped into the convection ammonia stripping tower (2) by the biogas slurry pump (5). Air is aerated and stripped from the bottom of the convection ammonia stripping tower (2) by the gas circulation pump (7). The ammonia in the biogas slurry of the convection ammonia stripping tower (2) is fed into the absorbent in the ammonia recovery tower (3). The absorbent is self-circulated in the ammonia recovery tower (3) by the liquid circulation pump (6). The biogas slurry stripped by the convection ammonia stripping tower (2) flows into the deammoniation biogas slurry tank (9) from the bottom of the convection ammonia stripping tower (2). 3) Biogas enters the deammonium-removing biogas slurry tank (9) through the biogas pump (8) to aerate and blow off the biogas slurry in the deammonium-removing biogas slurry tank (9) and separate the biogas from the biogas slurry in the deammonium-removing biogas slurry tank (9).

2. The biogas slurry treatment method without alkali additive according to claim 1, characterized in that, The method further includes: the input end of the gas circulation pump (7) is connected to the top of the ammonia recovery tower (3), so that the ammonia gas that has not been absorbed in the ammonia recovery tower (3) enters the convection ammonia stripping tower (2) for further stripping.

3. The biogas slurry treatment method without alkali additive according to claim 1, characterized in that, In step 2), the pH of the biogas slurry after aeration and stripping in step 1) is 9-10, the working temperature of the convective ammonia stripping tower (2) is 40-50 ℃, and the ratio of the air volume to the biogas slurry volume of the convective ammonia stripping tower (2) is 3000-4000.

4. The biogas slurry treatment method without alkali additive according to claim 1, characterized in that, In step 2), the absorbent is concentrated sulfuric acid with a mass concentration of 30-50%, and the circulation flow rate of the absorbent in the ammonia recovery tower (3) is 6-10 m³ / s. 3 / h.

5. The biogas slurry treatment method without alkali additive according to claim 1, characterized in that, In step 3), the biogas slurry in the deammonium-removing biogas slurry tank (9) is heated to 40-60 ℃, and the ratio of biogas ventilation volume to biogas slurry volume in the deammonium-removing biogas slurry tank (9) is 400-500. Aeration and blowing are performed every 2-6 minutes for 4-8 minutes.

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