Method for recovering ammonia in a biogas plant and biogas plant

Abstract

The present invention is intended to lower the FOS / TAC ratio of the material to be stripped, thereby enabling a higher pH of the material, and to enhance the stripping conditions of the material in an ammonia recovery unit (4) of a biogas plant having at least one ammonification reactor (1) for processing an input raw feedstock into a pretreated feedstock, and at least one biogas reactor (2) for fermenting the pretreated feedstock into biogas and a fermentation digestate, the fermentation digestate being separated into a solid fraction and a liquid fraction in a fermentation digestate separator (5). At least a portion of the separated liquid fraction is directed as a return liquid having a low FOS / TAC ratio via a return liquid conduit (10) to a pretreated feedstock blending device (12) where the return liquid is mixed with a pretreated feedstock having a higher FOS / TAC ratio, and / or to a liquid blending device (13) where the return liquid is mixed with a separated liquid from the pretreated feedstock, and this mixed material having an intermediate FOS / TAC ratio is then fed to the ammonia recovery unit (4).

Description

[Technical Field]

[0001] Object of the invention The present invention relates to the recovery of ammonia from pretreated feedstock from an ammonification reactor of a biogas plant, which is a two-stage fermentation plant having at least one ammonification reactor and at least one biogas reactor. [Background technology]

[0002] Biogas production is typically performed in a single-stage unit, where the four stages of anaerobic digestion (hydrolysis, acidogenesis, acetogenesis, and methanogenesis) are carried out in the same digester. Biogas production can also be performed in a two-stage unit, where the first two stages of anaerobic digestion, which occur under acidic conditions, are operated in a separate hydrolysis reactor fed by a second reactor, the anaerobic digester, and the remaining two stages are carried out under near-neutral conditions. Measurement of volatile organic acids (FOS) and total inorganic carbon (TAC) is a widely used method for monitoring biogas fermentation process conditions in biogas plants. U.S. Patent No. 8,759,052 describes the measurement and control of FOS / TAC ratios, carbonate content, and pH levels in biogas reactors. European Patent No. 3,517,505 describes the stripping of ammonia from the fermentation digestate discharged from a biogas reactor and how the release of carbon dioxide from the gaseous form increases the pH value of the stripped effluent. Reducing the ammonia content through stripping allows the liquor to be recycled to dilute the feedstock without concentrating ammonia in the process, facilitating the production of ammonia / nitrogen fertilizer.

[0003] A biogas production system operating with two fermentation stages and two types of reactors is disclosed in WO2015151036. The first stage is carried out in an ammonification reactor, where nitrogen-rich feedstock is pretreated under near-neutral conditions. The ammonification fermentation stage converts the feedstock's nitrogen to ammonia. The pretreated feedstock is then separated into a liquid and a solid fraction. The solid fraction is fed to a biogas-producing reactor in the second biogas fermentation stage. After ammonia is stripped and recovered from the separated liquid by a stripper in an ammonia recovery unit, the ammonia-reduced liquid and pretreated feedstock are fed to the biogas reactor. Summary of the Invention

[0004] Typically, nitrogen-rich biomass feedstocks with carbon-to-nitrogen (C / N) ratios of less than 15 to 20 should first be fermented in a pretreatment ammonification stage to prevent ammonia inhibition in the second biogasification stage, depending on the process conditions and feedstock. In the ammonification stage, most of the organic nitrogen content of the feedstock is converted by microorganisms to ammonia and ammonium ions. If the ammonia content of the pretreated feedstock discharged from the ammonification reactor is sufficiently reduced by an ammonia recovery unit, the subsequent biogas fermentation stage will not have excessive ammonia content. Before recovering ammonia from the pretreated feedstock, for example by stripping in an ammonia recovery unit, the liquid and solid phases of the feedstock can be separated, and the ammonia is recovered or stripped from the liquid phase. The ammonification stage is preferably carried out in a first reactor, with continuous or sequential feed and discharge. A feasible average pretreatment time is 3 to 7 days under thermophilic conditions above 41°C. Longer treatment times may not be economical because they incur higher investment and operating costs. If the ammonification step produces adequate amounts of methane, the period can be extended to 10, 15, or even up to 20 days. Ammonification will occur within roughly the same period within mesophilic conditions of 20-45°C.

[0005] Ammonia and ammonium ions are primarily suspended in the liquid phase of the pretreated feedstock. As a result, the ammonia content in the liquid phase can rise to inhibitory levels in the ammonification and biogas reactors, becoming so high that methanogenic microorganisms cannot efficiently and stably convert the carbon content of the pretreated feedstock into methane. In such cases, the ammonia content in the pretreated feedstock or the effluent from the pretreated feedstock should be reduced in an ammonia recovery unit, and the pretreated feedstock and effluent can then be fed to the biogas reactor. Without ammonia reduction, using the effluent to dilute the raw feedstock fed to each reactor would inevitably result in excessive ammonia concentration in the ammonification and / or biogas reactors. Ammonia stripping is a method of transferring volatile ammonia from the liquid to the gas phase. Ammonia stripping is most effective at high pH levels and high temperatures. Ammonia reduction and recovery from the pretreated feedstock can also be performed in a biomass stripper without solid-liquid separation.

[0006] During the short pretreatment in the ammonification reactor, methane production is usually low or completely inhibited, while other first-stage conversions, such as the formation of ammonia and fatty acids, occur very rapidly in continuously or sequentially operated ammonification reactors. Although a separately conducted ammonification stage allows for efficient and reliable biogas fermentation, reducing the ammonia content of the pretreated feedstock is often a challenge for two-stage fermentation processes.

[0007] It has now been discovered that fatty acids formed during ammonification strongly hinder ammonia evaporation during the stripping stage. Fatty acids lower the pH of the pretreated feedstock and further hinder ammonia stripping by binding to ammonium ions. Reducing the ammonia / ammonium content of the pretreated feedstock through stripping in an ammonia recovery unit has previously only been possible by adding pH-raising additives, such as sodium hydroxide or potassium hydroxide, in amounts sufficient to convert soluble ammonium ions to volatile ammonia. The addition of pH-raising chemicals leads to high operating costs, which reduces the economic viability of this environmentally effective technology for producing energy and fertilizer from organic waste and compost. Sodium from sodium hydroxide also concentrates during the process water recycle, which is an undesirable substance in the fermentation digestate used as fertilizer. One goal of ammonia recovery is to increase the value of the nitrogen fertilizer product formed from the recovered ammonia, if it can be demonstrated to be an organic fertilizer for organic farming. Introducing inorganically produced additives into the biogas plant process destroys the organic matter content of not only the nitrogen-containing fertilizer by-product, but also the potassium- and phosphorus-rich digestate discharged from the biogas reactor.

[0008] In our extensive research on ammonia stripping from pretreated feedstock discharged from an ammonification reactor, we have surprisingly found that the FOS / TAC ratio of the material being stripped correlates with the ammonia removal rate during the stripping stage. The FOS / TAC ratio, also known as the FOS / TAC value, determines the ratio of free organic acids (FOS) to total inorganic carbonates (TAC). The FOS value indicates the content of volatile fatty acids, while the TAC value is a measure of the buffering capacity of the sample. We report herein that the FOS / TAC ratio correlates more closely with ammonia removal rate than the pH value, which is traditionally used to evaluate stripping efficiency.

[0009] It has also been found that the FOS / TAC ratio of the pretreated feedstock and / or the effluent from the pretreated feedstock can be adjusted to a desired level by mixing with the effluent from the fermentation digestate discharged from the biogas reactor. At low FOS / TAC ratio levels, ammonia recovery at high temperatures, e.g., 80°C, can be approximately 80% without any pH-raising additives. An FOS / TAC ratio of 1.3 or less allows adequate ammonia stripping without additional additives. The lower this ratio, the better the ammonia recovery. A lower ratio also facilitates the use of lower stripping temperatures, thus reducing heat input. The initial FOS / TAC ratio of the pretreated feedstock discharged from the ammonification reactor is typically 1.5-4, depending on the feedstock and other process conditions. The initial FOS / TAC ratio of the fermentation digestate discharged from the biogas reactor is typically well below 0.5. Mixing the return liquid with the pretreated feedstock allows for efficient stripping without the addition of additives. As used herein, the initial FOS / TAC ratio refers to the FOS / TAC ratio of the pretreated feedstock and fermented digestate prior to later process steps that may reduce the carbon dioxide content of the material. In one embodiment, the optimal FOS / TAC value for the material to be stripped is 0.8 or less.

[0010] In the ammonification stage, the raw feedstock is typically diluted to 8%–15% (wt / vol) total solids. This means that a significant amount of liquid must be heated, and achieving sufficient reduction of ammonia content within the stripping stage requires significant thermal energy to raise the temperature of the stripped material. Further increasing the amount of liquid in the stripped material runs counter to the economical operating principles of the ammonia recovery unit, since more liquid to be stripped means a larger, more expensive facility with higher capacity and greater thermal and electrical energy consumption. While biogas plants often have combined heat and power (CHP) units to produce electricity and heat, the amount of heat available for the plant's processes is often limited. The generated thermal energy would be more valuable if it could be utilized externally. Furthermore, reducing or eliminating the need for pH-adjusting additives leads to a more economical process and a more valuable fertilizer product for sale. Mixing the liquid from the biogas reactor with the material to be stripped also reduces ammonia concentration in the internal water circulation within the biogas plant and reduces the need to add fresh water and purify the effluent discharged from the plant.

[0011] A novel solution has now been developed to solve the problem of recovering the ammonia formed during the ammonification stage. The object of the invention is achieved when a biogas plant and / or a method for recovering ammonia in a biogas plant is implemented as defined in the independent claims. Preferred embodiments of the invention correspond to the dependent claims.

[0012] The solution of the present invention is based on mixing the separated return liquid, which is the liquid phase of the fermentation digestate discharged from the biogas reactor, with the pretreated feedstock and / or the separated liquid from the pretreated feedstock discharged from the ammonification reactor. Because fatty acids are decomposed during biogas fermentation, the fatty acid content in the fermentation digestate is very low. The separated liquid from the fermentation digestate instead contains a high carbon dioxide content formed in association with biogas fermentation in the biogas reactor. Therefore, the separated liquid from the fermentation digestate has a very low initial FOS / TAC ratio. The initial pH of the fermentation digestate is also typically higher than neutral and may even be higher than the pH of the pretreated feedstock. Mixing the separated return liquid from the fermentation digestate with the pretreated feedstock and / or the liquid fraction separated from the pretreated feedstock leads to a significant improvement in ammonia recovery and / or the ability to use lower stripping temperatures without introducing pH-raising additives. Note that feeding the return liquid to the ammonification reactor as dilution water does not result in the same enhanced stripping effect of the pretreated feedstock. To improve stripping conditions, the return liquid must be mixed with i) the pretreated feedstock after it is discharged from the ammonification reactor and / or ii) the liquid fraction separated from the pretreated feedstock, and then the mixed material is fed to an ammonia recovery unit for stripping. Mixing fresh water instead of return water is also useless, as it does not change the FOS / TAC ratio. The improvement in stripping conditions is mainly due to the release of carbon dioxide from the liquid, which leads to an increase in the pH of the material being stripped.

[0013] Mixing at least a portion of the return liquid with the pretreated feedstock also reduces the ammonia content of the solid fraction separated from the pretreated feedstock. Therefore, the ammonia content of the ammonia-reducing liquid discharged from the ammonia recovery unit can be advantageously higher, allowing the total ammonia feed to the biogas reactor to be kept below a safe level that will not inhibit biogas production. This allows for the use of lower stripping temperatures. The return liquid also contains ammonia, which can be simultaneously recovered. Stripping all of the return liquid would not be economically feasible. If at least a portion of the return liquid is used directly to dilute the raw feedstock without stripping, the ammonia content is later recovered from the pretreated feedstock, avoiding ammonia concentration in the internal fluid circulation.

[0014] The optimum or minimum mixing ratio of return liquid to pretreated feedstock and / or separated liquid can be adjusted during biogas plant production to achieve the desired ammonia recovery from the material being stripped in the ammonia recovery unit. The mixing controller and / or biogas plant control system can be configured to adjust the mixing ratio of the pretreated feedstock mixer and / or the liquid mixer according to the measured characteristics of the mixed material. Sampling and / or measurements can be performed from the mixed material stream before and / or after the mixer, as well as from the material discharged from the ammonia recovery unit. Preferred measured characteristics are the initial FOS / TAC ratio, pH, temperature, ammonia content, and the amount of ammonia recovered from the aqueous ammonia or ammonia salt discharged from the ammonia recovery unit. Another important value is the ammonia recovery rate, which is calculated from the ammonia content of the material fed to and discharged from the ammonia recovery unit. The initial FOS / TAC ratio should be measured from the pretreated feedstock and fermentation digestate before any processing, including separation if the separation releases carbon dioxide. This ensures that the target FOS / TAC ratio and the corresponding blend ratio are not affected by the optional decarbonization of the stripped material downstream of the sampling point. The target FOS / TAC ratio, which determines the blend ratio of the blended material, should be between 0.5 and 1.3. The blend ratio can be calculated linearly from the initial FOS / TAC ratio. The measured feed blend flow rate can be easily used to adjust and control the blend ratio of the blending device. The main target parameter affecting the target blend is the ammonia content in the feedstock and liquor fed to the biogas reactor. If the ammonia recovery rate is insufficient to achieve a sufficiently low ammonia content, fresh water can be mixed into the feedstock fed to the biogas reactor. Furthermore, the separated liquor discharged from the ammonia recovery unit can be at least partially fed to the biogas reactor rather than being sent entirely to the biogas reactor to dilute the raw feedstock. The water circulation in a biogas plant is usually not closed in any way to avoid harmful concentrations of undesirable substances.

[0015] Normal operation of a biogas plant should be very stable, so measured operating values ​​will likely be quite constant in practice. Therefore, active sensors placed at measurement locations may not be necessary to perform measurements. Measurements can be made from material samples, for example, daily, weekly, or monthly. The facility should have sample collection ports for taking samples at appropriate locations before and after relevant process steps that alter the characteristics of the treated material. Remote control of adjustable valves and pumps may not be necessary to adjust the desired blend. Automatic control may allow remote control of the entire plant with few or no on-site personnel. Preferably, the blend ratio in the combined fluid stream to the ammonia recovery unit should be 75% to 20% return liquid from the biogas reactor. Ratios greater than this range may lead to excess facility capacity and costs. Lower ratios may not result in adequate changes to stripping conditions.

[0016] Ammonification is a much faster process compared to biogas fermentation. The biogas plant's control system should be configured to regulate the average retention time of the pretreated feedstock in the ammonification reactor to 4-10 days. More preferably, the retention time is 4-7 days. If adequate methane production occurs within the ammonification stage, the retention time can be increased, thus shortening the retention time in the biogas reactor to achieve complete fermentation of the feedstock.

[0017] The evaporation of gaseous carbon dioxide during stripping, i.e., decarbonization, results in a higher pH value in the stripped material, enabling efficient ammonia removal. Carbon dioxide evaporates from the stripped material during stripping, but it can also be removed before stripping. If the carbon dioxide in the stripped material is primarily released before entering the ammonia recovery unit, decarbonization of the stripped material can be performed at lower temperatures than ammonia stripping. Some decarbonization can also occur during extended storage of the return liquid. Decarbonization is preferably performed on the return liquid. Decarbonization of separated liquid or mixed materials can also be performed. Decarbonizing the decarbonized liquid before stripping can be useful for optimal equipment design of the ammonia recovery unit. The decarbonization step can utilize less expensive low-temperature heat transfer sources. Decarbonization can be promoted by ultrasound or other agitation means in the decarbonization vessel. Decarbonization before stripping is very useful for enhancing the stripping step. This reduces the need to vent or wash effluent carbon dioxide gas from the ammonia recovery unit to maintain a sufficiently low carbon dioxide partial pressure in the recycled stripping gas.

[0018] At least a portion of the exhaust gas from the absorber of the ammonia recovery unit may be directed to a stripping gas preheater and / or to a reactor to recover water and heat from the exhaust gas. The absorption in the absorber is exothermic and the added heat should be recovered for the biogas plant process, preferably for preheating unused stripping gas. [Brief explanation of the drawings]

[0019] In the following, exemplary embodiments of the invention are disclosed in more detail with reference to the accompanying drawings. [Figure 1] FIG. 1 illustrates a biogas plant according to various embodiments of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] FIG. 1 illustrates a biogas plant according to various embodiments of the present invention. In a biogas plant, raw materials are stored in various storage compartments. A supplying means conveys, crushes, dilutes, and supplies the raw materials to an ammonification reactor 1. A portion of the raw materials may be directly supplied to a biogas reactor 2, where the primary methane production occurs. Conversion of most of the raw material's carbon to methane typically occurs within 30 days in the biogas reactor. A biogas plant may have several ammonification reactors 1 and 2. Nitrogen-rich raw materials containing too high a nitrogen content relative to their carbon content (too low a C / N molar ratio) are primarily supplied to the ammonification reactor 2 for pretreatment, where the nitrogen is converted to ammonia by ammonification fermentation. Ammonification fermentation is accomplished by living organisms. Such a method is disclosed in detail in WO2015151036. Ammonification fermentation is preferably carried out under thermophilic anaerobic conditions.

[0021] The pretreated feedstock discharged from the ammonification reactor 1 is supplied to the pretreated feedstock separator 3 via a feedstock conduit 7. The feedstock conduit 7 may be routed through a pretreated feedstock blender 12. The separated solid pretreated feedstock from the pretreated feedstock separator 3 is transferred to the biogas reactor 2 via another feedstock conduit. The separated liquid from the pretreated feedstock separator 3 is supplied to the ammonia recovery unit 4 via a liquid conduit 8, where the ammonia content of the separated liquid is reduced at elevated temperatures. The liquid conduit 8 may be routed through a liquid blender 13. The ammonia-reducing liquid is sent to the biogas reactor 2 and / or the ammonification reactor 1 via a liquid conduit 9 to dilute the supplied pretreated or raw feedstock. Ammonia gas recovered from the stripping chamber of the ammonia recovery unit 4 may be directed to react with an acid in the absorber of the ammonia recovery unit 4 to produce ammonia salt fertilizer, or to be concentrated into aqueous ammonia. Since citric acid is usually produced organically, the organic fertilizer product of ammonia recovery unit 4 may be ammonium citrate.

[0022] Pretreated feedstock separator 3 may be omitted and the ammonia from the pretreated feedstock may be recovered in the biomass stripper of ammonia recovery unit 4. In that case, the pretreated feedstock is transferred directly from pretreated feedstock blending device 12 via blended pretreated feedstock conduit 19, optionally through decarbonization vessel 17, to ammonia recovery unit 4. The present invention would also be highly useful in that embodiment.

[0023] The fermentation digestate discharged from biogas reactor 2 is transferred to digestate separator 5. From digestate separator 5, the solid fraction is transferred to biomass storage and, optionally, further transported through a dryer. The liquid fraction of the fermentation digestate, i.e., return liquid, is transferred via return liquid conduit 10 to pretreated feedstock blending device 12 and / or liquid blending device 13, optionally via return liquid storage 16. In pretreated feedstock blending device 12, pretreated feedstock from feedstock conduit 7 is mixed with liquid from return liquid conduit 10. In liquid blending device 13, separated liquid from liquid conduit 8 is mixed with liquid from return liquid conduit 10. In blending device 12 and / or blending device 13, the materials to be stripped from reactor 1 and reactor 2 are mixed to enhance stripping conditions. Blending device 12, blending device 13 may include adjustable pumps and / or valves to control the mixing ratio of the mixed materials. The mixing ratio of mixer 12 or mixer 13 may be controlled by a mixing controller 14, or by the biogas plant's control system 15 or an operator. Return liquid conduit 10 may direct the return liquid through decarbonization vessel 17 and / or storage vessel 16 to release carbon dioxide from the return liquid. The pretreated feedstock may not contain an adequate amount of carbon dioxide for proper decarbonization. Decarbonization may occur in decarbonization vessel 17 after mixer 12, mixer 13, and before ammonia recovery unit 4. Separators 3 and 5 may be any type of filter and separator, depending on the separation material. A preferred type of decanter centrifuge may release carbon dioxide during operation.

[0024] The FOS / TAC ratios of the pretreated feedstock and fermentation digestate are determined by titration. Automated titrators are available for this purpose. The initial FOS / TAC ratio of the pretreated feedstock depends on the feedstock and process conditions. The FOS / TAC ratio is typically greater than 1.5 when pretreating chicken manure. The initial FOS / TAC ratio of the fermentation digestate is typically between 0.1 and 0.5.

[0025] The blending controller 14 or the biogas plant's control system 15 can be configured to optimize or minimize the blending ratio so that the addition of pH-raising additives to the material being stripped is unnecessary or minimized. The control system 15 can also direct fresh water feed to the biogas reactor, particularly if ammonia recovery is not sufficiently effective. The ammonia recovery ratio can also be maximized. It is often preferable to minimize the liquid feed to the ammonia recovery unit 4 because a higher liquid flow increases the thermal energy demand of the ammonia recovery unit 4 and can limit heat availability. The optimization effort can be based, for example, on the measured FOS / TAC ratio of the blended material, the percentage of recovered ammonia relative to the input ammonia amount, and / or the measured ammonia content of the ammonia-reducing liquid exiting the ammonia recovery unit 4. The operating temperature of the stripping stage of the ammonia recovery unit 4 is also an important parameter that influences the optimal fluid blending ratio and achieving the required low ammonia content of the ammonia-reducing liquid. For example, the ammonia content of the pretreated feedstock can be approximately 6 g / liter, and the target ammonia recovery should be 2 g / liter in the ammonia-reducing liquid. The optimal situation can also be the overall economic optimization of the entire biogas production plant. This can mean, for example, optimizing the direction of fluids with different temperatures to optimize the operating temperatures of all units in the biogas plant. The average processing time in the ammonification reactor and / or biogas reactor is also a relevant parameter to optimize, as it can affect the characteristics of the stripped material and the economics of the biogas plant. The cost and price of recovered ammonia salts or aqueous ammonia also affect financial optimization. An important value for the operation of a biogas plant is the total ammonia content of the ammonia-reduced liquid from the liquid conduit 9 supplied to the biogas reactor and the pretreated feedstock, in order to maintain the ammonia content of the biogas reactor at the desired optimal level. The other mentioned parameters should fulfill this primary task; other adaptations are simply economical, and other adjustments are made to address other operational constraints. The mixing ratio can also be adjusted manually.

[0026] The ammonia stripping step in the ammonia recovery unit 4 can be carried out by any known stripping process, such as air, steam, or flash stripping, or by distillation. If the exhaust gas from the ammonia absorption step in the ammonia recovery unit 4 is returned to the stripping step, there should be a means to maintain the carbon dioxide partial pressure of the cycle gas low enough to not interfere with the release of carbon dioxide from the material being stripped. Carbon dioxide may be scrubbed or extracted from the cycle. To achieve an enhanced stripping effect, effective carbon dioxide release from the material being stripped in the ammonia recovery unit 4 and / or prior decarbonization is required. At least a portion of the exhaust gas from the absorber of the ammonia recovery unit can be passed through a preheater 18 for unused stripping gas to recover water and heat from the exhaust gas before being directed to reactors 1 and 2. The decarbonization temperature is preferably 45-55°C. Air bubbles can be blown into vessels 16 and 17 to enhance decarbonization. At such low temperatures, most of the ammonia is not released. The decarbonization vessel 17 may discharge into the biogas reactor 2. The processing time for the decarbonization vessel 17 should be less than 30 minutes.

[0027] The biogas generated in the ammonification reactor 1 and biogas reactor 2 can be fed to a CHP unit 6 for the production of electricity and heat. The thermal energy is primarily in the form of hot cooling water and hot flue gas. These fluids can be used to add heat to any of the biogas plant's process stages. Excess heat can also be used for other external heating purposes. The biogas can also, or instead, be burned in an on-site boiler to heat the biogas plant's processes. A boiler or other heat source may be required, especially if the resulting biogas is to be refined into methane fuel.

Claims

1. 1. A method for ammonia stripping in an ammonia recovery unit (4) of a biogas plant, comprising: - processing the feedstock into a pretreated feedstock in at least one ammonification reactor (1); - fermenting said pretreated feedstock into biogas and fermentation digestate in at least one biogas reactor (2); - separating the fermentation digestate into a solid fraction and a liquid fraction in a fermentation digestate separator (5), and - at least a part of said separated liquid fraction is returned as return liquid via a return liquid conduit (10) i) mixing the return liquid with the pretreated raw material into a pretreated raw material mixing device (12); and / or ii) mixing the return liquid with the liquid fraction from the solid-liquid separation of the pretreated feedstock towards a liquid mixer (13); and - feeding the mixed material from step i) and / or step ii) to said ammonia recovery unit (4) for ammonia stripping; A method comprising:

2. 2. The method according to claim 1, wherein the return liquid is led through a storage vessel (16) and / or a decarbonization vessel (17), the operating temperature of which is lower than the operating temperature of the stripping stage of the ammonia recovery unit (4), preferably between 45 and 55 degrees.

3. 3. The method of claim 1 or 2, wherein the predetermined target or measured FOS / TAC ratio of the mixed material is less than 1.3 or less than 0.

8.

4. 4. The method according to claim 1, wherein the mixing ratio of the mixed materials in the mixing device (12, 13) is adjusted according to a measured FOS / TAC ratio of the initial or mixed materials, and / or a calculated percentage of recovered ammonia relative to the amount of ammonia input, and / or a measured ammonia content of the ammonia-reduced liquid leaving the ammonia recovery unit (4).

5. A biogas plant, at least one ammonification reactor (1) for processing a feedstock into a pretreated feedstock; - at least one biogas reactor (2) for fermenting said pretreated feedstock into biogas and fermentation digestate, and - Ammonia recovery unit (4) Including, the inlet of the digestate separator (5) is connected to a digestate conduit connected to the outlet of said biogas reactor (2), and the liquid outlet of the digestate separator (5) is connected via a return liquid conduit (10) to the inlet of the ammonia recovery unit (4) via a pretreated feed mixer (12) for mixing the return liquid with pretreated feed from a pretreated feed conduit (7), the pretreated feed conduit (7) being connected to the ammonification reactor (1); and / or the return liquid conduit (10) is connected to the inlet of the ammonia recovery unit (4) via a liquid mixer (13), the liquid mixer (13) is connected to the liquid outlet of the pretreated feed separator (3) via a liquid conduit (8), and the inlet of the pretreated feed separator (3) is connected to the pretreated feed conduit (7); This is a biogas plant that features:

6. 6. The biogas plant according to claim 5, wherein the pretreated raw material mixer (12) and / or the liquid mixer (13) are connected to a mix controller (14) and / or a control system (15) of the biogas plant.

7. 7. The biogas plant according to claim 6, wherein the mixing controller (14) and / or the control system (15) of the biogas plant are configured to adjust the mixing ratio of the pretreated feedstock mixing device (12) and / or the liquid mixing device (13) according to information received about the percentage of ammonia in the ammonia-reduced liquid discharged from the ammonia recovery unit (4).

8. 8. The biogas plant according to claim 6 or 7, wherein the mixing controller (14) and / or the control system (15) of the biogas plant are configured to adjust the mixing ratio of the pretreated feedstock mixer (12) and / or the liquid mixer (13) according to information received about the measured FOS / TAC ratio of the pretreated feedstock or the separated liquid from the pretreated feedstock, and preferably according to the measured FOS / TAC ratio of the fermentation digestate or the separated liquid from the fermentation digestate and / or according to the measured FOS / TAC ratio of the mixed material fed to the ammonia recovery unit (4).

9. 9. The biogas plant according to claim 6, wherein the mixing controller (14) and / or the control system (15) of the biogas plant are configured to adjust the mixing ratio of the pretreated feedstock mixer (12) and / or the liquid mixer (13) according to information received about a measured amount of ammonia recovered from the ammonia recovery unit (4).

10. 10. The biogas plant according to any one of claims 6 to 9, wherein the mixing controller (14) and / or the control system (15) of the biogas plant are configured to adjust the mixing ratio of the pretreated feedstock mixer (12) and / or the liquor mixer (13) according to information received about the measured and calculated proportions of ammonia recovered from the pretreated feedstock or the mixed liquor fed to the ammonia recovery unit (4).

11. 11. The biogas plant according to claim 6, wherein the mixing controller (14) and / or the control system (15) of the biogas plant are configured to adjust the mixing ratio of the pretreated feedstock mixing device (12) and / or the liquid mixing device (13), wherein the mixing ratio is between 50% and 20% return liquid in the mixed liquid supplied to the ammonia recovery unit (4).

12. 12. The biogas plant according to claim 6, wherein the mixing controller (14) and / or the control system (15) of the biogas plant are configured to adjust the mixing ratio of the mixed streams of the pretreated feedstock mixing device (12) and / or the liquor mixing device (13) according to a measured pH of the pretreated feedstock or the centrate of the pretreated feedstock, and preferably according to a measured pH of the fermentation digestate or return liquid and / or according to a measured pH of the mixed liquor fed to the ammonia recovery unit (4).

13. 13. Biogas plant according to any one of claims 5 to 12, wherein the control system (15) of the biogas plant is configured to adjust the average retention time of the pretreated feedstock in the ammonification reactor (1) to between 3 and 10 days, more preferably between 4 and 7 days.

14. 14. Biogas plant according to any one of claims 5 to 13, wherein the liquid return conduit (10) and / or the liquid conduit (8) are led through a storage vessel (16) and / or a decarbonization vessel (17) for releasing carbon dioxide from the transport liquid.

15. The biogas plant according to any one of claims 5 to 12, wherein the control system (15) of the biogas plant is configured to supply fresh water when a total ammonia content of the ammonia-reduced liquid from the liquid conduit (9) supplied to the biogas reactor and the pretreated feedstock exceeds a predetermined ammonia amount.

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