Object of the invention

The present invention relates to ammonia recovery from pretreated feedstock from an ammonification reactor of a biogas plant. The biogas plant is a two-phase fermentation plant which has at least one ammonification reactor and at least one biogas reactor.

Background of the invention

Biogas production is generally operated in one-stage units where the four phases of anaerobic digestion (hydrolysis, acidogenesis, acetogenesis, methanogenesis) are carried out in the same digester. Biogas production can also be carried out in two-stage units: the first two phases of anaerobic digestion that occur in acidic conditions are operated in a separate hydrolyser-reactor that feeds a second reactor, the anaerobic digester, where the last two phases are carried out in near neutral conditions. Determination of volatile organic acids (FOS) and total inorganic carbon (TAC) is a widely used way to monitor biogas fermentation process conditions in a biogas plant. US8759052 describes measuring and controlling of FOS/TAC ratio, carbonate content and pH level in a biogas reactor. EP3517505 describes stripping of ammonia from fermented digestate discharged from a biogas reactor and how release of gaseous carbon dioxide elevates the pH value of stripped separated liquid. Reducing ammonia content by stripping allows recirculation of liquid to diluting feedstock without enrichment of ammonia within the process and facilitates production of ammonia/nitrogen fertilizers.

A biogas producing system which operates in two fermentation stages and types of reactors is disclosed in publication W02015151036. The first stage is performed in an ammonification reactor which pretreats nitrogen rich feedstock in near neutral conditions. The ammonification fermentation phase converts nitrogen of feedstock to ammonia. Pretreated feedstock is then separated to liquid and solids parts. The solid component is fed to biogas producing reactor of the second biogas fermentation stage. Ammonia is stripped and recovered from separated liquid by a stripper of an ammonia recovery unit before the ammonia reduced liquid and pretreated feedstock are fed to the biogas reactor.

Summary of the invention

Typically, nitrogen rich biomass feedstock, which has carbon to nitrogen (C/N) ratio below 15 to 20, depending on process conditions and feedstock, at least a part of the feedstock should be first fermented in a pretreatment ammonification stage to prevent ammonia inhibition of the second, biogasification phase. In the ammonification stage most of organic nitrogen content of the feedstock is converted to ammonia and ammonium ions by microbes. If ammonia content of pretreated feedstock which is discharged from the ammonification reactor is sufficiently reduced by an ammonia recovery unit, the subsequent biogas fermentation stage will not have excess ammonia content. Before recovering ammonia from the pretreated feedstock by e.g. stripping in the ammonia recovery unit, liquid and solid phases of feedstock may be separated and ammonia is recovered or stripped from the liquid phase. The ammonification phase takes place in a first reactor that is preferably continuously or sequentially fed and discharged. Feasible average pretreatment time duration is from 3 to 7 days at thermophilic conditions over 41 °C. A longer treatment time may not be economical as it incurs higher investment and operating costs. If the ammonification phase produces relevant amount of methane, the duration may be extended to 10, 15 or even up to 20 days. Ammonification will also occur within mesophilic conditions from 20 to 45 °C within about the same periods.

Ammonia and ammonium ions are mainly suspended in liquid phase of the pretreated feedstock. Resulting content of ammonia in the liquid phase may be too high for methanogenic microbes to effectively and stably convert carbon content of the pretreated feedstock to methane in the ammonification and biogas reactors where the ammonia content may rise to an inhibiting level. Proportion of ammonia in the pretreated feedstock or the liquid separated from it should in such cases be reduced in an ammonia recovery unit before the pretreated feedstock and the separated liquid can be supplied to the biogas reactor. Without ammonia reduction, the separated liquid also cannot be used to dilute fresh feedstock to be fed to the reactors without resulting to too high enrichment of ammonia within the ammonification reactor and/or within the biogas reactor. Ammonia stripping is a method for transferring of volatile ammonia of a liquid into gas phase. Ammonia stripping happens most effectively at high pH levels and elevated temperatures. Ammonia reduction and recovery from the pretreated feedstock may also be performed in a biomass stripper without liquid/solid separation.

Methane production is normally low or completely inhibited during the short pretreatment in the ammonification reactor, but other first phase transformations such as formation of ammonia and fatty acids will happen quite fast in the continuously or sequentially operated ammonification reactor. The separately performed ammonification stage enables efficient and reliable biogas fermentation, but reducing ammonia content of the pretreated feedstock usually faces problems related to the two-stage fermentation process.

It has now been discovered that fatty acids formed during ammonification strongly hinder vaporization of ammonia in the stripping phase. The fatty acids lower the pH of the pretreated feedstock and further hinder ammonia stripping by binding ammonium ions. Reducing the ammonia/ammonium content of the pretreated feedstock by stripping in the ammonia recovery unit has previously only been feasible by adding sufficient amounts of pH elevating additives such as sodium hydroxide or potassium hydroxide to cause transformation of soluble ammonium ion to volatile ammonia. The addition of pH elevating chemicals leads to high operating costs that reduce economic availability of this environmentally effective technology that can produce energy and fertilizers from organic waste and manures. Sodium from sodium hydroxide would also enrich in water recirculation of the process and is an unwanted material in the fermented digestate which is used as a fertilizer. One aim of the ammonia recovery is that nitrogen fertilizer product formed from the recovered ammonia has increased value if it can be verified to be organic fertilizer for organic cultivation. Introducing non-organically created additives to the processes of the biogas plant will destroy the organic status of not only the nitrogen containing fertilizer byproduct but also the organic status of potassium and phosphorus rich digestate discharged from the biogas reactor. In our extensive studies concerning stripping of ammonia from pretreated feedstock discharged from an ammonification reactor, it has surprisingly been found out that FOS/TAC ratio of a material to be stripped correlates with removal rate of ammonia at stripping phase. The FOS/TAC ratio, also known as FOS/TAC value, determines the ratio of free organic acids (FOS) to total inorganic carbonate (TAC). The FOS value indicates content of volatile fatty acids, while the TAC value is a measure of the buffering capacity of the sample. Herein, we report that the FOS/TAC ratio has an even more significant correlation to removal rate of ammonia than pH value that is conventionally used to assess stripping efficiency.

It was also found out that the FOS/TAC ratio in the pretreated feedstock and/or the liquid separated from pretreated feedstock can be adjusted to a desired level by mixing with liquid separated from the fermented digestate discharged from the biogas reactor. At low FOS/TAC ratio levels, recovery rate of ammonia at an elevated temperature of for example 80 degrees Celsius may be about 80 % without any of the pH elevating additives. FOS/TAC ratio of 1 .3 or below enables relevant stripping of ammonia without further additives. The lower the ratio is, the better the recovery ratio of ammonia. A low ratio also facilitates use of a lower stripping temperature and thus a smaller heat input. Initial FOS/TAC ratio of discharged pretreated feedstock from the ammonification reactor is usually between 1 .5 and 4 depending on feedstock and other process conditions. Initial FOS/TAC ratio of discharged fermented digestate from the biogas reactor is usually well below 0.5. Mixing returned liquid to pretreated feedstock can enable efficient stripping results without adding additives. As used herein, initial FOS/TAC ratio refers to the FOS/TAC ratio of the pretreated feedstock and the fermented digestate before any later process phase that may reduce carbon dioxide content of the materials. In an embodiment, an optimum FOS/TAC value for material to be stripped is 0.8 or below.

In the ammonification phase, fresh feedstock is normally diluted from 8% to 15% (weight/volume) total solids. This means that a significant amount of liquid needs to be heated up and a large amount of heat energy is required to increase the temperature of the stripped material to achieve sufficient reduction of ammonia content within the stripping phase. Further increasing the amount of liquid in the material to be stripped is contrary to economical operation principle of an ammonia recovery unit as more liquid to be stripped means that larger and more expensive installations are required with higher capacity and higher consumption of heat and electric energy. Even though a biogas plant often has a combined heat and power (CHP) unit for producing electricity and heat, the amount of heat available for the processes of the plant is often restricted. Any generated heat energy would also have more value if it could be utilized externally. Still, the reduced or omitted need of pH adjusting additives leads to more economic process and more valuable fertilizer products for sale. The mixing of the liquid from biogas reactor to the material to be stripped also reduces ammonia enrichment in the internal circulation of water within the biogas plant and reduces the need to add fresh water and to clean effluent discharged from the plant.

Here, a novel solution is developed to solve the problems related to recovery of ammonia formed during the ammonification phase. The purpose of the invention is achieved when a biogas plant and/or a method for recovery of ammonia within a biogas plant are implemented as defined in the independent claims. Preferred embodiments of the invention correspond to dependent claims.

The solution of the invention is based on that separated returned liquid that is the liquid phase of fermented digestate discharged from the biogas reactor is mixed with pretreated feedstock discharged from the ammonification reactor and/or with liquid separated from the pretreated feedstock. Fatty acids are decomposed during biogas fermentation, so their content in the fermented digestate is very low. Liquid separated from the fermented digestate instead has a high carbon dioxide content that is formed in connection with biogas fermentation in the biogas reactor. Thus, the separated liquid from the fermented digestate has very low initial FOS/TAC ratio. Initial pH of the fermented digestate is also normally above neutral and may be higher than pH of the pretreated feedstock. Mixing the returned liquid separated from the fermented digestate with the pretreated feedstock and/or liquid fraction separated from the pretreated feedstock will lead to a remarkably better ammonia recovery rate and/or ability to use lower stripping temperature without introducing pH elevating additives. Notably, feeding the returned liquid to the ammonification reactor as dilution water does not lead to the same stripping enhancing effect of the pretreated feedstock. In order to improve stripping conditions, the returned liquid must be mixed i) with the pretreated feedstock after it is discharged from the ammonification reactor and/or ii) with the liquid fraction separated from the pretreated feedstock before feeding the mixed material(s) into the ammonia recovery unit for stripping. Mixing fresh water instead of the returned water does not help either as this will not change the FOS/TAC ratio. The improvement of stripping conditions is based mainly on releasing out carbon dioxide from the liquid which leads to raised pH of the material to be stripped.

Mixing at least a part of the returned liquid with the pretreated feedstock results also in lower ammonia content of solid fraction separated from the pretreated feedstock. Then, the ammonia content of ammonia reduced liquid discharged from the ammonia recovery unit can advantageously be higher in order to keep the combined ammonia feed to the biogas reactor below a safe value that does not cause inhibition of biogas production. This enables use of lower stripping temperature. The returned liquid also contains ammonia, and it can be simultaneously recovered. Stripping all of the returned liquid would not be economically feasible. If at least part of the returned liquid is directly used to dilute fresh feedstock without stripping, it’s ammonia content will be recovered from the pretreated feedstock later and ammonia enrichment in internal circulation of fluids is avoided.

Optimum or minimum mixing ratio of the returned liquid with the pretreated feedstock and/or the separated liquid can be adjusted during production of the biogas plant to create desired ammonia recovery from material to be stripped within the ammonia recovery unit. A mixture controller and/or to a controlling system of the biogas plant may be configured to adjust mixing ratio of the pretreated feedstock mixing arrangement and/or the liquid mixing arrangement according to measured properties of mixed materials. The samplings and/or measurements may be taken from streams of mixed materials before and/or after the mixing arrangements and from discharged material from the ammonia recovery unit. The preferred measured properties are initial FOS/TAC ratio, pH, temperature, ammonia content and recovered amount of ammonia from discharged ammonia water or salt from the ammonia recovery unit. An important value is also recovery rate of ammonia which rate is calculated from the ammonia content of fed and discharged materials of the ammonia recovery unit. Initial FOS/TAC ratios should be measured from the pretreated feedstock and the fermented digestate before any treatment including separation if separation allows escape of carbon dioxide. Then the targeted FOS/TAC ratio and corresponding mixing ratio is not affected by optional decarbonization of materials to be stripped downstream of the sampling points. The targeted FOS/TAC ratio defining the mixing ratio of mixed materials should be between 0.5 to 1.3. The mixing ratio can be linearly calculated from the initial FOS/TAC ratios. Measured flow rates of supplied mixed materials are easy to use for adapting and controlling the mixing ratios of the mixing arrangements. The main target parameter that affects the targeted mixture is the ammonia contents within the feedstocks and liquids that are supplied to the biogas reactor. If the ammonia recovery rate is not sufficient to accomplish low enough ammonia content, fresh water can be mixed to the supplied materials fed to the bigas reactor. Separated liquid discharged from the ammonia recovery unit may then ,be at least partially supplied to dilute fresh feedstock instead of pumping all of it to the biogas reactor. Normally water circulation within the biogas plant is anyway not closed in order to avoid harmful enrichment of unwanted materials.

The measured operating values will in practice stay quite constant as normal operation of a biogas plant should be very stable. Thus, active sensors located at the measurement locations may not be needed for taking the measurements. Measurements can be taken for example daily, weekly or even monthly from samples of the materials. The installation should have sample collection ports for taking the samples at relevant locations before and after relevant process phases that changes properties of the treated materials. Adjustable valves and pumps for adjusting desired mixture may not need to be remote controlled. Automated controls could still enable remote control of the whole plant with less or no local employees. Preferably, the mixing ratio should be between 75% and 20 % of the returned liquid from the biogas reactor within combined flows of liquid to the ammonia recovery unit. A larger ratio that the range may lead to excess capacity and costs of the installations. A lower ratio may not lead to relevant changes to stripping conditions.

Ammonification is a quite a rapid process compared to biogas fermentation. The controlling system of the biogas plant should be configured to adjust average retention time of the pretreated feedstock within the ammonification reactor between 4 to 10 days. More preferably, the retention time is between 4 to 7 days. If relevant methane production within the ammonification phase takes place, the retention time may be longer, and the retention time of the biogas reactor can thus be shorter to achieve full fermentation of the feedstock.

Evaporation of gaseous carbon dioxide i.e. decarbonization during stripping results in higher pH value of stripped material and enables efficient ammonia removal. Carbon dioxide evaporates from the stripped material during stripping, but it can also be removed before stripping. If carbon dioxide of material to be stripped is mainly released before entering the ammonia recovery unit, decarbonization of the stripped material may be performed at lower temperature than ammonia stripping. Some decarbonization may also happen within extended time period during storage of the returned liquid. Preferably, decarbonization is performed to the returned liquid. Decarbonization of the separated liquid or the mixed materials may also be performed. Decarbonization liquids before stripping can be beneficial for designing an optimum arrangement of an ammonia recovery unit. Cheaper lower temperature heating media sources can be utilized for the decarbonization phase. The decarbonization may be boosted by ultrasonic waves or other agitation means within a decarbonization vessel. Decarbonization before stripping is very beneficial in enhancing stripping phase. Then there is less need to vent or wash out released carbon dioxide gas from the ammonia recovery unit in order to keep partial pressure of carbon dioxide within recirculated stripping gas adequately low.

At least a part of exhaust gas from absorber of the ammonia recovery unit may be led to a preheater of stripping gas and/or to the reactors for recovery of water and heat of the exhaust gas. The absorption within the absorber is exothermic and the applied heat should be recovered to processes of the biogas plant, preferably to preheat fresh stripping gases. List of drawings

In the following, examples of the embodiments of the invention are disclosed in more detail with reference to the appended drawing.

Fig. 1 illustrates a biogas plant according to various embodiments of the invention.

Detailed description of the invention

Fig. 1 illustrates a biogas plant according to various embodiments of the invention. The biogas plant stores feedstock materials in storages. Feeding means conveys and comminutes and dilutes and feeds the feedstock to an ammonification reactor

1 . A part of the feedstock may also be fed directly to a biogas reactor 2 wherein main methane production takes place. Conversion of most of carbon of the feedstock to methane happens typically within 30 days within the biogas reactor. The biogas plant may have several ammonification reactors 1 and biogas reactors

2. Nitrogen rich feedstock which contains too high portion of nitrogen versus to the portion of carbon (too low C/N molar ratio) is mainly fed for pretreatment to an ammonification reactor 2 for converting the nitrogen to ammonia by ammonification fermentation. The ammonification fermentation is accomplished by biological organisms. Such process is disclosed in detail in publication W02015151036. The ammonification fermentation is preferably performed in thermophilic anaerobic conditions.

Pretreated feedstock discharged from the ammonification reactor 1 is fed to a pretreated feedstock separator 3 via a feedstock conduit 7. The feedstock conduit 7 may be led through a pretreated feedstock mixing arrangement 12. Separated pretreated solid feedstock from the pretreated feedstock separator 3 is transferred via another feedstock conduit to the biogas reactor 2. Separated liquid from the pretreated feedstock separator 3 is fed through a liquid conduit 8 to ammonia recovery unit 4 for reducing ammonia content of the separated liquid in elevated temperature. The liquid conduit 8 may be led through a liquid mixing arrangement 13. Ammonia reduced liquid is pumped via a liquid conduit 9 to the biogas reactor 2 and/or to the ammonification reactor 1 in order to dilute fed the pretreated feedstock or fresh feedstock. Recovered ammonia gas from a stripping chamber of the ammonia recovery unit 4 may be directed to react with an acid in within absorber of the ammonia recovery unit 4 for creating ammonia salt fertilizer or it is concentrated to ammonia water. Citric acid is normally produced organically, so ammonium citrate can be an organic fertilizer product of the ammonia recovery unit 4.

The pretreated feedstock separator 3 may be omitted, and ammonia of the pretreated feedstock can be recovered within a biomass stripper of the ammonia recovery unit 4. Then the pretreated feedstock is directly transferred from the pretreated feedstock mixing arrangement 12 to the ammonia recovery unit 4 via a mixed pretreated feedstock conduit 19, optionally through a decarbonization vessel 17. The invention would be highly useful also with that embodiment.

Discharged fermented digestate from the biogas reactor 2 is transferred to digestate separator 5. Solids fraction from the digestate separator 5 is transferred to biomass storages for further transport optionally through a dryer. Liquid fraction of the fermented digestate i.e. returned liquid is transferred via a returned liquid conduit 10 to the pretreated feedstock mixing arrangement 12 and/or the liquid mixing arrangement 13, optionally via a returned liquid storage 16. Within the pretreated feedstock mixing arrangement 12 the pretreated feedstock from the feedstock conduit 7 is mixed with liquid from the returned liquid conduit 10. Within the liquid mixing arrangement 13 the separated liquid from liquid conduit 8 is mixed with liquid from the returned liquid conduit 10. Within the mixing arrangements 12 and/or 13 materials to be stripped from the reactors 1 and 2 are mixed in order to enhance stripping conditions. The mixing arrangements 12, 13 may comprise adjustable pumps and/or valves for controlling mixing ratio of mixed materials. The mixing ratios of mixing arrangements 12 or 13 may be controlled by a mixture controller 14 or they may be controlled by a controlling system 15 or operator of the biogas plant. The returned liquid conduit 10 may lead the returned liquid through a decarbonization vessel 17 and/or storage vessel 16 for releasing carbon dioxide from the returned liquid. The pretreated feedstock may not contain relevant amount of carbon dioxide for relevant decarbonization. The decarbonization may also take place after mixing arrangements 12, 13 in a decarbonization vessel 17 before ammonia recovery unit 4. The separators 3 and 5 may be any type of filters and separators dependent of the separated material. A preferred type, decanter centrifuge may release carbon dioxide during operation.

FOS/TAC ratios of the pretreated feedstock and the fermented digestate are measured by titration. Automatic titrators are available for the task. Initial FOS/TAC ratio of the pretreated feedstock depends on supplied feedstock and process conditions. The FOS/TAC ratio is normally above 1 ,5 when pretreating chicken manure. Initial FOS/TAC ratio of the fermented digestate is normally from 0,1 to 0,5.

The mixture controller 14 or the controlling system 15 of the biogas plant may be configured to optimize or minimize the mixture ratio so that there is no or minimal need to add pH raising additives to materials to be stripped. The controlling system 15 may also direct supplied fresh water to the biogas reactor, especially if ammonia recovery is not sufficiently effective. The recovery ratio of ammonia may also be maximized. Minimized liquid feed to the ammonia recovery unit is often preferable as higher liquid throughflow will cause higher need of heat energy of the ammonia recovery unit 4 and heat availability may be limited. The optimization tasks may be based on for example measurements of FOS/TAC ratios of mixed materials and/or percentage of recovered ammonia versus amount of inputted ammonia and/or measured ammonia content of ammonia reduced liquid exiting the ammonia recovery unit 4. Operating temperature of the stripping phase of the ammonia recovery unit 4 is also a critical parameter that affects to the optimal mixing ratio of the fluids and to reaching required low ammonia content of the ammonia reduced liquid. For example, ammonia content of the pretreated feedstock may be around 6 g/liter and targeted ammonia recovery should result to 2 g/l in ammonia reduced liquid. An optimal situation may also be overall economic optimization of the overall biogas production plant. That may mean for example optimized direction of different temperature fluids to optimize operation temperatures of all units of the biogas plant. Average treatment times within the ammonification reactor and/or biogas reactor may also affect the properties of the materials to be stripped as well as economy of the biogas plant and they are also relevant parameters to be optimized. Also, recovery cost and price of recovered ammonia salt or water will affect financial optimization. The critical value for the operation of the biogas plant is the combined ammonia content of ammonia reduced liquid from the liquid conduit 9 and the pretreated feedstock fed to the biogas reactor in order to keep the ammonia content of the biogas reactor at desired optimal level. Mentioned other parameters should fulfill that initial task and other adaptations are just economical and other adjustments to cope with other operational limitations. The mixture rate may also be manually adjusted.

The ammonia stripping phase within the ammonia recovery unit 4 may be performed by any known stripping process, for example air, steam or flash stripping or by distillation. If the gases exhausted from an ammonia absorbing phase of the ammonia recovery unit 4 are returned to the stripping phase, there should be means to keep partial pressure of carbon dioxide of the circulated gas so low that it does not hinder releasing of carbon dioxide from the material to be stripped. The carbon dioxide may be washed or bled out from the circulation. Effective releasing of carbon dioxide from the material to be stripped within ammonia recovery unit 4 and/or prior decarbonization is required to achieve the stripping enhancement effect. At least a part of exhaust gas from absorber of the ammonia recovery unit may be led through preheater 18 of fresh stripping gas and then to the reactors 1 , 2 for recovery of water and heat of the exhaust gas. The decarbonization temperature is preferably from 45 to 55 degrees. Air bubbles may be blown to vessels 16, 17 for enhancing the decarbonization. Within so low temperatures much of ammonia will not be released. The decarbonization vessel 17 may be vented out to the biogas reactor 2. The throughput time of the decarbonization vessel 17 should be below 30 minutes.

Generated biogas from ammonification and biogas reactors 1 , 2 may fed to a CHP unit 6 for producing electricity and heat. The heat energy is mainly in the form of hot cooling water and hot flue gas. Those fluids can be used to apply heat to any of the process phases of the biogas plant. Excess heat can be used for other external heating purposes too. Biogas may also or instead be locally burned in a boiler to heat processes of the biogas plant. A boiler or other heat source may be needed especially if produced biogas is purified to methane fuel.