sulfur-containing fertilizer

The present invention relates to a process for cleaning a biogas and simultaneously producing a sulfur and nitrogen containing fertilizer product. More specifically, the pro cess of the invention is a process for producing a ferti lizer from sulfur and nitrogen originating from the di gester feedstock by extracting or stripping ammonia from the digester feedstock, from the anaerobic digester slurry itself or from the digestate originating from an anaerobic digestion process and subsequently reacting the extracted or stripped ammonia with sulfur compounds also ultimately derived from the digester feedstock in a series of reac- tions.

It is known to extract ammonia (NH ₃₎ from either feedstock, the digester itself or in the digestate after the digester. The purpose of doing so is mainly that feedstocks with high nitrogen content (such as chicken manure) can be utilized, but it is very rarely done in actual digesters. However, it is not known to combine it with sulfur from the process with the purpose of making a fertilizer product containing sulfur. Today, biogas is typically cleaned by removing sul- fur in a biological process.

The invention enables cleaning of biogas from anaerobic di gestion while simultaneously making a fertilizer product containing nitrogen and sulfur. This way, ammonia (NH ₃₎ and hydrogen sulfide (¾S), which are pollutants in the biogas, can be made into a fertilizer product containing nitrogen and sulfur. These pollutants also have an inhibitory effect on the gas production in the digester, meaning that more gas can be produced and a much wider range of feedstocks can be used in the digester.

Regarding prior art, US 7,014,768 describes a multi-step process for removal and recovery of nutrients and recycling of water from digested manure or other organic wastes. The first step involves separating waste from an anaerobic di gester into digested liquids, digested solids and biogas; the second step involves precipitating solids from the di gested liquids; the third step involves stripping ammonia from the digested liquids; the fourth step involves inject ing a stream of CO2 into the digested liquids to reduce pH; the fifth step involves recycling the digested liquids back to the anaerobic digester for use in diluting incoming solid wastes, and the sixth step involves passing the NH ₃ stripped from the digested liquid through both the digested solids and the precipitated solids to recover nitrogen through absorption with the resultant solids being usable as a biofertilizer with a high nitrogen content. In US 9,339,760, a method for removing contaminants from a biogas comprises heating and aerating an anaerobic digester effluent in an aeration reactor to produce an alkaline ef fluent, treating an input biogas with the alkaline effluent in a reactor to produce a treated biogas and a resulting effluent with a lower pH as compared to the pH of the alka line effluent, and recovering the treated biogas from the previous step, wherein the treated biogas has less ¾S as compared to the input biogas .

EP 2 398 737 B is directed to a process for the production of ammonium, in particular in the form of a concentrated ammonium salt solution, from an aqueous liquid comprising ammonia (NH ₃₎ . The process comprises feeding a heated feed stream comprising water and NH ₃ into an evaporator, com pressing the produced vapor stream, bringing the compressed vapor stream and the feed stream in heat exchanging contact in a heat exchanger, thereby condensing at least a part of the compressed vapor stream into an ammonium solution, and contacting at least part of said ammonium solution with an acid, thereby obtaining an ammonia salt solution.

In US 8,491,798 B2, hydrogen sulfide (¾S) is selectively eliminated from liquid ammonia (either anhydrous or aque ous) by contacting an anhydrous or aqueous liquid stream comprising NH ₃ and ¾S with a solution comprising SO2 to convert the ¾S to thiosulfate.

Finally, WO 2006/042551 A1 discloses a biogas producing method and facility with anaerobic hydrolysis, in which or ganic waste is converted into biogas, i.e. a gas containing methane, with improved efficiency and economy. The method comprises three consecutive steps of i) digestion of the organic waste in a first reactor, ii) hydrolysis of the di gested organic waste in an anaerobic hydrolysis tank, and iii) digestion of the hydrolyzed organic waste in a second reactor, wherein evolved gases are removed from the anaero bic hydrolysis tank. In a previous patent application, WO 2019/158474 Al, the Applicant has described the production of high value ferti lizers from various off-gases. More specifically, that in vention related to using the ammonium thiosulfate (ATS) process to produce a high value fertilizer from the sulfur and ammonia content in gases such as landfill gas, digester gas, off-gas from geothermal power production or coke oven gas . The ATS process is a referenced technology from the Appli cant which is used to clean refinery off-gases from sour water stripper (SWS), amine regenerator off-gas and/or Claus plant tail gas for H ₂ S and NH ₃ . The product is a 50- 60% aqueous solution of ammonium thiosulfate, which can be used directly as a fertilizer because it is consistent with the standards for sale and distribution of ATS fertilizers.

The ATS process is based on the three following main reac tions :

1. Combustion of a gas rich in H ₂ S with atmospheric air in a combustor:

4 H ₂ S + 6 0 ₂ -> 4 ¾0 + 4 S0 ₂ (1)

2. Formation of ammonium hydrogen sulfite (AHS) by absorp tion of S0 ₂ and NH ₃ in water:

4 S0 ₂ + 4 NH ₃ + 4 ¾0 -> 4 NH ₄ HS0 ₃ (2)

3. Reaction of the AHS from reaction (2) with H ₂ S and NH ₃ to form an aqueous solution of ammonium thiosulfate (ATS) : 4 NH4HSO3 + 2 H ₂ S + 2 NH ₃ -> 3 (NH ₄₎ 2S ₂ 03 + 3 ¾0 (3)

It can be seen from the above reactions ( 1 )— ( 3 ) that the stoichiometric ratio between H ₂ S and NH ₃ is 1:1 and that 2/3 of the H ₂ S is used for formation of S0 ₂ and 1/3 is used for ATS formation. Likewise, 2/3 of the NH ₃ is used for AHS formation and 1/3 is used for ATS formation.

The main advantages of the ATS process are that the product is a high value fertilizer and that the process can utilize off-gas containing H ₂ S and NH ₃ , such as the SWS (sour water stripper) gas and Claus feed gas normally processed in re fineries, as feedstock. In addition, due to the preferred design with two S0 ₂ scrubbers in series connection, very low levels of sulfur emission can be accomplished.

The present invention is based on the fact that there is a need for sulfur removal from gases coming from anaerobic digestion processes. When considering anaerobic digestions based on agricultural feedstock, some kinds of feedstock (such as chicken manure) have a too high ammonium content, which inhibits the bacteria in the digester that are making the methane for the biogas. For that reason, chicken manure is typically not used for anaerobic digestion processes. In some places, the manure is burned because there is a need to get rid of it. By extracting the ammonia, the ammonium content is lowered, and there is less inhibition of the bacteria in the digester. This means that more gas can be produced (or the residence time in the digester can be shorter) and that a wider range of feedstock can be uti- lized. By reacting some of this ammonia that has been ex tracted with sulfur coming from hydrogen sulfide from the anaerobic digestion process, it is possible to create a fertilizer product that contains sulfur, such as ammonium thiosulfate (ATS) or ammonium sulfate, thereby transforming the sulfur from a pollutant into a product.

It is believed that there are two different preferred solu tions for the ammonia extraction: a. Performing the extraction from the feedstock upstream the digester, which means that it can be combined with the sanitation (typically keeping the feedstock at around 70°C for one hour to kill bacteria) , which for instance is done for digesters using waste water treatment sludge as feed stock . b. Extracting the ammonia directly from the slurry in the digester, by having a recirculating side stream process where the slurry is heated and/or by increasing the pH by external means. This is believed to be preferred if sanita tion is not needed and/or ¾S can be removed in the same process . One way of reacting the extracted ammonia with sulfur is to use it in Applicant's ATS process to produce a high value fertilizer from the sulfur and ammonia content in gases such as landfill gas, digester gas, off-gas from geothermal power production or coke oven gas.

The gas flows as well as the content of sulfur and ammonia are much lower in the landfill gas and digester industry. The efforts of the Applicant within processes relating to removal of siloxanes and transformation of landfill gas (LFG) to renewable natural gas (RNG) have shown that for gases with a significant sulfur content, the current sulfur removal technologies used in the industry are absorption techniques, Lo-Cat type technology, biological units or caustic ¾S scrubbing. Regarding the digester industry, a technique comprising several steps of water scrubbing at elevated pressure is often used.

So the present invention relates to a process for producing a fertilizer, wherein:

- ammonia is extracted or stripped from a feedstock, the anaerobic digester itself or the digestate coming from an anaerobic digestion process, and

- the ammonia is used in a process where it reacts with sulfur compounds originating from hydrogen sulfide or other inorganic or organic sulfur compounds coming from the an aerobic digestion process, whereby

- a fertilizer product is obtained, said product containing nitrogen and sulfur.

In the product, the nitrogen and sulfur preferably both originate from the feedstock of the biogas product.

It is preferred that excess stripped ammonia is exported. The stripping of ammonia from a feedstock is preferably done only by raising the temperature, possibly through heat integration with digester and/or amine wash. The heat for the stripping process preferably comes from an amine wash treating of the biogas from the digester.

Preferably at least some of the heat input required for the digester is provided by the stripping process. But since the stripping process in itself does not produce any heat, it requires a heat input which can then be "re-used" as heat for the digester.

The ammonium thiosulfate can be prepared by combustion or catalytic oxidation of the PhS-rich gas with oxygen to con vert ¾S to SO2, formation of ammonium hydrogen sulfite (AHS) by absorption of SO2 and NH ₃ in water, and reaction of the AHS with ¾S and NH ₃ to form an aqueous solution of ammonium thiosulfate (ATS) .

The ammonia extraction/stripping can be combined with sani tation of the feedstock.

The stripped ammonia is preferably reacted with sulfur com- pounds originating from a stream rich in CO2 and ¾S coming from an amine wash used for cleaning the biogas from the digester .

The ammonia from the stripping process can be removed from the stripping gas by condensing water present in the strip ping gas, whereby the ammonia is absorbed in the condensed water. Further ammonia is preferably removed by subse quently contacting the stripping gas with water.

The invention presents a number of advantages over the prior art, such as a higher gas production in the digester and a wider range of usable feedstocks, which are high in ammonium and/or with a high sulfur content. Another ad vantage is that the removal of nitrogen from the digestate means that less land is required to distribute the diges- tate on. This is often a limiting factor, e.g. for swine farming .

Acidification is a well-known method to reduce the ammonia evaporation from farms. The reduced ammonia evaporation has two big advantages; firstly, the environmental benefits from reduced ammonia emissions and, secondly, more nitrogen is kept in the manure/slurry which can then later be used by plants as nitrogen is a key nutrient. However, acidified manure cannot be accepted by anaerobic digesters as sulfu- ric acid is the typical way of acidifying the manure, and the increased sulfur levels have a significant negative im pact on the bacteria in the anaerobic digestion process and thereby in the biogas production. The present invention presents a solution where acidified manure can be used in anaerobic digestion processes, at least to some extent. By extracting/stripping the hydrogen sulfide (¾S) from the slurry in the digester in a side stream process, the inhibiting effect on the bacteria is reduced resulting in an increased biogas production. As acidification also results in a higher nitrogen content in the manure because less nitrogen evaporates as ammonia, there is also the risk of ammonia inhibiting the bacteria in the anaerobic digestion process. But as the present in vention extracts/strips ammonia from the biogas process as well, such inhibiting effects are removed or at least re- duced, making it feasible to utilize acidified manure from e.g. cattle, chicken or swine production in anaerobic di gestions processes producing biogas.

Stripping of NH ₃ and ¾S in one process is well known from the sour water strippers for instance found in refineries.

However, doing this on feedstock, digester slurry or diges- tate from an anaerobic process and combining them into a fertilizer product is not known. In case the feedstock for the anaerobic digestion is of or ganic origin, the present invention can create an organic fertilizer containing nitrogen and sulfur. This is valuable as conventional fertilizers cannot be utilized in organic farming .

In an embodiment of the method according to the invention, NH ₃ and/or ¾S is/are stripped by only applying heating and not external control of pH. This is easier to operate as no external chemicals are needed for the pH control and be- cause altering the pH can affect the digester operation.

In another embodiment, the biogas from the digester is treated using an amine wash, where this amine wash creates a stream of cleaned biogas and a stream containing CO2 and ¾S originating from the biogas. Such amine wash requires heat input to strip off the CO2 and ¾S from the amine. The amine regeneration temperature is typically around 140°C, whereas the temperatures for NH ₃ and/or ¾S stripping are lower. By using the heat from the amine wash as heat source for the NH ₃ and/or ¾S stripping, significant energy and cost savings can be achieved. By further using the heat in- put for the NH ₃ and/or ¾S stripping process as heat input for the digester, further energy and cost savings can be achieved .

In a further embodiment, NH ₃ from the stripping process is extracted from the stripping gas by condensing water pre sent in the stripping gas. Ammonia will be absorbed in this condensed water, resulting in an ammonia rich stream which can be used to control ammonia input for the process of making a fertilizer product containing nitrogen and sulfur. If there is more ammonia than what is needed for the ferti lizer product, this ammonia excess can be exported.

In a still further embodiment, NH ₃ present in the stripping gas is extracted from the stripping gas by passing the stripping gas through a water bath, where the NH ₃ is ab sorbed in the water. Thereby, the ammonia input for the process of making a fertilizer product containing nitrogen and sulfur can be controlled. If there is more ammonia than what is needed for the fertilizer product, this ammonia ex- cess can be exported.

The stripping unit may comprise several stages, for in stance stages at different temperature levels in order to obtain gases with different contents of NH ₃ and ¾S .

The invention is described in more detail in the following examples, using the appended figures 1-4. The feedstock can for instance be any waste water treatment sludge, cattle, chicken and/or swine manure, organic waste fractions and combinations thereof. Hydrogen sulfide (¾S) is formed in the digester as sulfur containing proteins that are decomposed under anaerobic conditions. ¾S can also be formed from inorganic sulfur (particularly sul fate) . ¾S is corrosive and therefore unwanted, and for use in energy applications or biogas upgrading there are strict limitations as to the ¾S content of the biogas. Further, ¾S inhibits the bacteria responsible for the formation of methane (CH ₄ ) which is the key component of the biogas.

The invention is described in more detail in the examples which follow.

Example 1

In Fig. 1, NH ₃ is stripped from the digester. The digestate from the digester is pumped to a stripping unit. The equi librium between ammonium and ammonia depends on the temper ature and the pH value. Heat and/or pH control can be ap plied to the stripping unit to create conditions where am monium in the digestate reacts to form ammonia which can then be removed by passing a stripping gas over the surface of the digestate. This gas could e.g. be air, but it could also be a gas stream found elsewhere in the biogas plant, such as the biogas itself or off-gas of an amine wash for upgrading the biogas to natural gas quality. The ammonia (NH ₃₎ passes from the digestate to the stripping gas. The ammonia in the stripping gas is then contacted with sulfur from the biogas plant to form a fertilizer product. This can e.g. be done as described in Applicant's above-men tioned previous patent application WO 2019/158474 A1.

Example 2

In Fig. 2, NH ₃ is stripped from the anaerobic digester. The slurry from the digester is pumped to a stripper unit and heated. The slurry can either be heated before the stripper unit or in a combined unit. In Fig. 2, a separate slurry heater is shown. The slurry is heated to approximately 70°C, whereby the equilibrium between ammonium and ammonia in the slurry is shifted. The higher share of ammonia leads to a higher vapor pressure of ammonia, and by blowing a stripper gas over or through the heated slurry, the ammonia is stripped off.

As shown in the figure, a portion of the biogas from the anaerobic digester is used as stripping gas. The advantage of using biogas as stripping gas is that anaerobic condi tions can be maintained because there is not introduced any additional oxygen into the system. The slurry, from which NH ₃ is stripped off, is recirculated to the digester, where it can be used as a heat input to maintain the temperature at the optimal level in the digester. This slurry can also be recirculated to the feedstock before entering the di gester if that is preferred for temperature control in the digester. In this example, the NH ₃ rich gas from the slurry stripper is cooled, resulting in an ammonia water solution being condensed. This NH ₃ solution can then be used to re act with the sulfur in the biogas. If there is an excess of ammonia present, it can be exported as an ammonia water so lution. The biogas leaving the condenser can be bubbled through an absorption unit with water serving as the ab sorption media. This means that more NH ₃ can be removed from the stripping gas.

Example 3

Fig. 3 shows that NH ₃ and ¾S are stripped in one process in a slurry stripper using steam (about 90-95°C) . The NH ₃ and ¾S rich gas can then be used to react NH ₃ and ¾S, or it can be passed through condensing and absorption stages to separate the majority of the NH ₃ and ¾S from the gas. This gas will contain some methane from the stripping pro cess. The gas can be reintroduced into the biogas stream for further cleaning in order to recover this methane as biogas . The slurry stripped of NH ₃ and ¾S is recirculated to the feedstock for the digester in order to reduce temperature gradients in the digester. However, it can also be returned to the digester, optionally after being cooled.

Example 4

In Fig. 4, NH ₃ is stripped from the feedstock to the di gester in a combination with sanitation of the feedstock. An NH ₃ solution is condensed from the NH ₃ rich gas. This solution can be dosed to the ATS reactor as needed, or it can be exported if the NH ₃ is in excess. As NH ₃ is ex tracted, the inhibition effects from ammonia on the bacte ria is reduced, and it is possible to use feedstock with a high nitrogen content, such as chicken manure. An amine wash removes ¾S, CO2 and remaining NH ₃ from the biogas. This gas can then be used in Applicant's ATS process de scribed above.