Method for Producing a Nitrogenous Fertilizer from Nitrogen-Containing Material of Biogenous and Industrial Origin

The present invention relates to the field of biogenous and industrial material processing. More in particular, it relates to a method for the processing of nitrogen- containing biogenous material and of nitrogen-containing residual material from industrial processes, wherein a nitrogenous fertilizer is prepared.

The term "biogenous material" as used herein relates to products - processed or non-processed - from biologic sources, such as sewage, sewage sludge, fermented substrate from a biogas plant, liquid manure, dung, offal, slaughterhouse waste, grass, maize or Leguminosae. These biogenous materials contain nitrogen in the form of ammonia, ammonium and/or organically bound.

Alternatively, nitrogen-containing residual material from industrial processes, here-below called "industrial residual material", can be processed to obtain a nitrogenous fertilizer. These nitrogen-containing industrial residual materials include, for instance, residual materials originating from meat production, fermentation or composting processes, or industrial fertilizer production.

In many cases, the ammonia and/or ammonium content causes problems during the processing (e.g. evaporation or

fermentation) , disposal and/or deployment of the nitrogen- containing biogenous or industrial residual material:

In sewage plants, ammonia and/or ammonium present in the sewage has to be removed due to its toxicity for water organisms, thus necessitating nitrification and denitrification steps.

In the field of stock farming, large amounts of liquid manure and/or dung can have a harmful environmental impact by means of over-fertilization, malodor, animal diseases, or the transport of large amounts of liquid manure and/or dung.

In many cases, fermented substrate from biogas plants has a harmful environmental impact by means of over- fertilization, malodor, or the transport of large amounts of fermented substrate.

In the field of meat production or the composting industry, nitrogen-containing waste gas and/or water lead to malodor, corrosion of equipment and fish die-off.

Currently, the ammonia and/or ammonium contained in biogenous materials is, for instance, oxidized in an automatic high-temperature oxidation system and discharged into the atmosphere, as described in WO-A-2007/015598.

Sewage plants typically make use of biological nitrification and denitrification processes using bacteria to remove ammonia and/or ammonium from the sewage. Thereby, elemental nitrogen is produced, which is released into the atmosphere.

Alternatively, the ammonia and/or ammonium content of biogenous materials is used for the preparation of a

fertilizer. In this context, US-B-6, 409, 788 discloses a method for treating organic waste. To conserve nitrogen, an ammonia retaining agent (zeolite) is added to one or several of the different organic waste generation steps. Additionally, the capture of ammonia can be increased by reacting ammonia with an acid, such as carbonic (see US-B- 4,710,300), sulfuric, or phosphoric acid. The retained ammonia is available for use in a derived fertilizer product.

The method described in US-B-6, 409, 788 leads to reduction of ammonia emission from organic waste materials. The use of zeolites or acids for capturing the ammonia, however, causes the need for purchase, transportation to the site of processing, and storage of the retaining agent or acid. Furthermore, the acids mentioned are highly reactive and bear a large risk potential. In addition, the nitrogen contained in the thus produced fertilizer is supplied in the form of ammonium ions, which are far less bioavailable than nitrate ions.

US-B-5, 656, 059 discloses an alternative method for processing a liquid nitrogen-rich organic waste product to an aqueous fertilizer solution using a biological conversion process including at least a nitrification step, wherein ammonium is converted to nitrate by means of aerobic nitrifying bacteria, and optionally a denitrifaction process. During the nitrification step, it is essential that the pH of the solution is kept at a value which enables the nitrifying bacteria to be sufficiently active. Furthermore, a denitrification step for limiting the nitrate content is necessary if the material to be nitrified has a content of nitrifiable nitrogen of at least 3000 mg per liter. This step requires

the addition of a carbon and an energy source for the denitrifying bacteria. Due to its need for sophisticated means, the described method is rather laborious.

WO-A-2007/097612 describes a process for the treatment of liquid waste biomass, which includes a first biological conversion stage, wherein ammonium is converted to nitrite using nitrifying bacteria in an aerated reactor, and a subsequent chemical oxidation stage, wherein nitrite is converted to nitrate by heating liquid waste biomass in an aerated reactor under acidic conditions.

For the biological nitrification, a well aerated tank with a large surface area of biogenous material is needed. In order to meet the nutritional requirements of the bacteria, an additional source of carbon has to be supplied. Furthermore, continuous processing is preferred. Since the bacteria are directly applied to the biogenous residual material, recycling and maintenance is rather cost-intensive. In addition, it is not possible to obtain ammonium nitrate in pure form as a highly concentrated fertilizer. Furthermore, it is rather difficult to determine the exact bioavailable nitrogen content of the end product.

Altogether, the process described in WO-A-2007/097612 is rather impractical for application on a relatively small scale and directly at the source site of the biogenous material, for instance a sewage plant or a farm - which may or may not be equipped with a biogas plant.

Ammonia-containing waste water and gas from industrial sources is typically subjected to filtration using a biofilter, combustion, SCR (selective catalytic

reduction) , biological nitrification/denitrification, membrane separation, stripping or acid washing.

Thus, there is still a need for an economically and ecologically useful process for the treatment of nitrogen- containing biogenous and industrial residual materials directly at their source site, which allows for the preparation of an effective nitrogenous fertilizer in pure form. The present invention provides a solution to this problem.

The problem is solved by the method according to the independent claim 1 of the present invention. Preferred embodiments of the invention are defined in the dependent claims .

The present invention is characterized by a method for producing a nitrogenous fertilizer in a stepwise process, in the course of which ammonia, that is extracted from a nitrogen-containing biogenous material or a nitrogen- containing industrial residual material, is converted to a nitrogenous fertilizer, that is an ammonium nitrate- and/or urea-based fertilizer.

According to the present invention, an easily applicable, highly efficient, and environmentally friendly method for the conversion of ammonia and/or ammonium contained in biogenous material and residual material from industrial processes to a nitrogenous fertilizer is provided. Furthermore, the described method can suitably be carried out on a small scale directly at the source site of the

nitrogen-containing biogenous or industrial residual material, such as a farm, sewage plant, biogas plant, or industrial installation.

In general, a nitrogenous fertilizer is produced from a nitrogen-containing biogenous material, such as, for example, sewage sludge, fermented substrate from a biogas plant, liquid manure, dung, offal, slaughterhouse waste, grass, maize or Leguminosae, or from a nitrogen-containing residual material from an industrial process, such as waste water or gas originating from meat production, fermentation or composting processes, or industrial fertilizer production. In a first step, ammonia is extracted from the nitrogen-containing material. Hence, the unwanted by-product ammonia is made a valuable starting material for the production of a high-quality nitrogenous fertilizer.

In a first embodiment (I.), the ammonia extracted in the first step is reacted with carbon dioxide to obtain a urea-based, high-quality fertilizer. Urea can be used as a component of fertilizers or animal feed, providing a relatively cheap source of fixed nitrogen to promote growth. Urea and urea-based fertilizers have generally a very high nitrogen content.

In a second embodiment (II.), the ammonia extracted in the first step is split into two parts. A first part of the ammonia is oxidized to nitrogen dioxide by reaction with oxygen in the presence of a noble metal catalyst, the nobel metal being selected from a group consisting of platinum, rhodium, gold, palladium, and mixtures thereof. Preferably, the noble metal catalyst is superimposed on a

carrier. More preferably, a honeycomb-like carrier comprising a ceramic material is used, the noble metal catalyst being superimposed on the carrier and ammonia being conducted through the honeycomb-like matrix.

The nitrogen dioxide thus obtained is subsequently reacted with water to obtain nitric acid, which is reacted with a second part of the ammonia to produce an ammonium nitrate- based fertilizer.

Preferably, the biogenous or industrial residual material is filtered or decanted prior to the extraction of ammonia, in order to remove solid material and facilitate the extraction of pure ammonia. Furthermore, a higher grade and hence more valuable fertilizer can be obtained by subjecting the biogenous or industrial residual material to aerobic or anaerobic fermentation before the extraction of ammonia. In the case of materials containing organically bound, non-mineralized nitrogen, fermentation prior to the extraction leads to a better nitrogen recovery.

In a preferred realization of the present invention, the extraction of ammonia is brought about by means of ammonia stripping, which directly supplies gaseous ammonia from ammonia and/or ammonium contained in the biogenous or industrial residual material. This method allows for an efficient and inexpensive recovery of ammonia.

In a preferred setting, the recovery of ammonia gas from the extraction is improved by adjusting the pH of the nitrogen-containing material to ≥ 8 by addition of a base, such as the hydroxides, carbonates or bicarbonates of

sodium, lithium, potassium, calcium or magnesium, by increasing the temperature to 50-90 ⁰ C, by lowering the pressure to ≤ 800 mbar, by fine dispersion of the substrate, by means of a stripping gas, or by a combination of two or more of the above measures.

Additionally, purification of the extracted ammonia is preferably brought about by filtration, using, for example, a cyclone filter for removal of foreign material, such as solids, water, or foam, etc., and/or a HEPA- or ULPA-filter for removal of viruses, bacteria, or the like.

According to the first embodiment, the ammonia extracted in the first step is reacted with carbon dioxide to obtain an urea-based fertilizer. The reaction of ammonia with carbon dioxide is typically conducted in a two-step process, involving a) reaction of ammonia with carbon dioxide at 30-58 ⁰ C, optionally in the presence of water, to obtain ammonium carbamate or ammonium carbonate, and b) heating of the ammonium carbamate or carbonate obtained in the previous step to ≥ 130 ⁰ C under a pressure of 35-40 bar to obtain the urea-based fertilizer.

In a preferred realization, the reaction of ammonia with carbon dioxide according to the first embodiment is conducted in a single-step one-pot process at a temperature of ≥ 130 ⁰ C and under a pressure of 35-40 bar.

Preferably, the concentration of urea in the aqueous suspension or solution obtained according to the first embodiment is monitored by measuring its conductivity and pH. Accordingly, it is possible to separate batches of the urea suspension or solution having a certain ion

concentration, which allows for the determination of the nitrogen content of the derived nitrogenous fertilizer. Alternatively, the urea suspension or solution is collected continuously.

In a preferred realization of the first embodiment, the product obtained by reaction of ammonia with carbon dioxide is crystallized, filtered, and dried prior to use. Pure urea being a white odorless solid, the dry product can easily be stored and does not have the unpleasant smell known from ammonia.

The urea-based fertilizer obtained according to the first embodiment is directly applied as a fertilizer. Alternatively, said urea-based fertilizer may be further processed by addition of ammonium nitrate, water, phosphorous salts, potassium salts, magnesium salts, boron salts, sulfur salts, lime, or other fertilizer ingredients or additives known to a person skilled in the art.

According to the second embodiment (II.), the ammonia extracted in the first step is split into two parts.

Preferably, the ammonia is partitioned by means of a valve. In a second step (step II. b), a first part of ammonia is oxidized to nitrogen dioxide by reaction with oxygen in the presence of a noble metal catalyst. In a third step (step II. c), the nitric acid obtained in step

II. b is combined with a second part of ammonia to produce an ammonium nitrate-based fertilizer. In a preferred arrangement, the pH of the solution obtained in the reaction between ammonia and nitric acid is kept in the range of 4.0-6.0, preferably in the range of 4.0-5.0, by variation of the size of the first and the second part of

ammonia. In general, steps II. b and II. c are carried out consecutively in a two-step process.

The catalytic oxidation of the first part of ammonia to nitrogen dioxide is typically conducted at a temperature of 300-1000 ⁰ C, preferably at 700-900 ⁰ C. This catalytic oxidation supersedes the use of stoichiometric amounts of expensive and/or dangerous reagents and hence represents a very practical, economically and ecologically useful method for the treatment of ammonia from biogenous or industrial residual materials directly at their source site.

Preferably, the concentration of ammonium and nitrate ions in the aqueous suspension or solution obtained in step II. c is monitored by measuring its conductivity and pH. Accordingly, it is possible to separate batches of the ammonium nitrate suspension or solution having a certain ion concentration, which allows for the determination of the nitrogen content of the derived nitrogenous fertilizer. Alternatively, the ammonium nitrate suspension or solution is collected continuously. Additionally, it is possible to control the pH of the resulting fertilizer product by adjusting the sizes of the first and the second part of ammonia.

In a preferred realization, the product obtained by reaction of ammonia with nitric acid is crystallized, filtered and dried, prior to use. In this way, pure ammonium nitrate is obtained, which is ideal for use as a fertilizer component, since its exact nitrogen content is easily determined.

The ammonium nitrate-based fertilizer obtained according to the second embodiment is directly applied as a

fertilizer. Alternatively, said ammonium nitrate-based fertilizer may be further processed by addition of calcium carbonate, magnesium carbonate, ammonium sulfate, urea, or water, phosphorous salts, potassium salts, magnesium salts, boron salts, sulfur salts, lime, or other fertilizer ingredients or additives known to a person skilled in the art.

The use of ammonia extracted from a biogenous or an industrial residual material for the preparation of a nitrogenous fertilizer not only allows for a highly- efficient fertilizer production, but also provides an economically and ecologically useful method for processing materials containing ammonia and/or ammonium, which are otherwise potentially harmful to the environment.

Examples

Example 1

According to a first example (see Fig. 1), the ammonia gas 10 obtained in the first step is reacted with carbon dioxide 11 to obtain a urea-based fertilizer 14 according to the first embodiment. The ideal stoichiometric ratio of NH ₃ :Cθ2 for this process is 2:1.

In general, the reaction of ammonia with carbon dioxide occurs at an elevated temperature and proceeds via an ammonium carbamate or carbonate intermediate. Conversion of ammonia and carbon dioxide to the ammonium carbamate or carbonate intermediate is typically performed at a temperature of 30-58 ⁰ C in a first reactor 12, while a

temperature of 130 ⁰ C or higher and a pressure of 35- 40 bar is necessary for the subsequent formation of urea and water in a second reactor 13. Accordingly, it is possible to prepare the urea-based fertilizer in two separate reactors 12, 13 and reaction steps using two different temperatures and pressure conditions. Alternatively, a one-step, one-pot process at a temperature of 130 ⁰ C or higher and a pressure of 35- 40 bar can be carried out.

Preferably, the carbon dioxide 11 is supplied from a preceding biogas production or from liquid manure storage, where carbon dioxide is formed by respiration of bacteria during hydrolysis and is dissolved in the substrate. Typically, the fermented substrate from a biogas plant contains an excess of carbon dioxide in comparison to the nitrogen contained as ammonia and/or ammonium. Hence, enough carbon dioxide can be extracted along with the ammonia during the stripping process. Alternatively, it is also possible to supply carbon dioxide from a different source, for example from combustion of fossil fuels or of renewable energy sources, such as biogas or wood.

The product obtained by reaction of ammonia with carbon dioxide can be directly applied as a nitrogenous fertilizer. Alternatively, the urea can be crystallized, filtered, and dried prior to application. Additionally, the dry product can be further processed by addition of ammonium nitrate, water, phosphorous salts, potassium salts, magnesium salts, boron salts, sulfur salts, lime, or other fertilizer ingredients or additives known to a person skilled in the art.

Example 2

According to a second example (see Fig. 2), the ammonia gas 10 obtained in the first step is used for the preparation of an ammonium nitrate-based fertilizer 19 according to the second embodiment. For this purpose, the ammonia obtained in the first step is divided into two parts by means of a valve 15.

In step II. a, the first part of ammonia is catalytically oxidized to nitrogen dioxide. Typically, this first part of ammonia is introduced into a reactor 16 containing a noble metal catalyst, the nobel metal being selected from a group consisting of platinum, rhodium, gold, palladium, and mixtures thereof. In reactor 16, the ammonia is combined with oxygen gas or a gaseous mixture containing oxygen, for example air, and is catalytically oxidized to nitrogen dioxide. In general, the oxidation is carried out at a temperature of 300-1000 ⁰ C, preferably at 700-900 ⁰ C. The oxidation of ammonia to nitrogen dioxide being an exothermic process, heat is generated in the course of the reaction, which can be used as an energy source.

In step II. b, the nitrogen dioxide obtained in step II. a is introduced into a scrubber 17, where it is reacted with water to obtain nitric acid.

According to step II. c, the nitric acid obtained in step II. b is reacted with the second part of ammonia 10 obtained in the first step in a reactor 18 to produce an aqueous suspension or solution of ammonium nitrate, that is an ammonium nitrate-base fertilizer 19. In general, steps II. b and II. c are carried out consecutively, in a two-step process. Preferably, the concentration of ammonium and nitrate ions in the aqueous suspension or

solution is monitored by measuring its conductivity and pH. Accordingly, it is possible to separate batches of the ammonium nitrate suspension or solution having a certain ion concentration, which allows for the determination of the nitrogen content of the derived nitrogenous fertilizer. Alternatively, the ammonium nitrate suspension or solution is collected continuously. Additionally, it is possible to control the pH of the resulting fertilizer product by adjusting the ammonia valve 15, thus regulating the amount of nitric acid and ammonia introduced into the scrubber.

The aqueous ammonium nitrate obtained in step II. c is directly applied as a nitrogenous fertilizer. Alternatively, the product is crystallized, filtered, and dried to obtain pure solid ammonium nitrate. Furthermore, the dry product can be further processed by addition of calcium carbonate, magnesium carbonate, ammonium sulfate, urea, water, or other fertilizer ingredients or additives known to a person skilled in the art.

In a preferred setting, steps II. b and II. c are performed concurrently in a one-pot reaction (see Fig. 3), wherein both the nitrogen dioxide obtained in step II. a and the second part of ammonia from the first step are simultaneously introduced into a scrubber 20 and reacted with water.