Green hydrogen, synthesis gas, and flue gas with a reduced nitrogen content for the synthesis of ammonia and urea

Abstract

The invention relates to the synthesis of urea from ammonia and carbon dioxide, wherein the hydrogen required for ammonia synthesis is obtained both by steam reforming of feed natural gas (grey hydrogen) and by electrolysis of water using electricity from renewable energy sources (green hydrogen). As the proportion of green hydrogen increases, the amount of carbon dioxide formed in the synthesis gas during steam reforming is no longer sufficient for the synthesis of urea. Therefore, flue gas, which is formed during the firing of the steam reformer and also contains carbon dioxide, is additionally used. After reducing the nitrogen content, the flue gas is fed into the reforming process. The carbon dioxide from the synthesis gas and the flue gas is combined, separated using conventional carbon dioxide scrubbing, and used for the synthesis of urea.

Description

Green hydrogen, synthesis gas and flue gas with reduced nitrogen content for the synthesis of ammonia and urea

[0001] The invention relates to the synthesis of urea from ammonia and carbon dioxide, whereby the hydrogen required for the synthesis of ammonia is obtained both by steam reforming of feed natural gas (grey hydrogen) and by electrolysis of water using electricity from renewable energy sources (green hydrogen). As the proportion of green hydrogen increases, the amount of carbon dioxide produced in the synthesis gas during steam reforming is no longer sufficient for the synthesis of urea. Therefore, additional flue gas is used, which is produced during the firing of the steam reformer and also contains carbon dioxide. After reducing the nitrogen content, the flue gas is fed to the reforming process. Carbon dioxide from the synthesis gas and flue gas is combined, separated by conventional carbon dioxide scrubbing, and used to synthesize urea.

[0002] While purely green ammonia synthesis plants are desirable and touted as promising, they are currently not particularly cost-effective. A much more practical and significantly less cost-intensive option is to convert existing conventional ammonia synthesis plants to partially utilize green hydrogen.

[0003] The green hydrogen is produced through water electrolysis, with the required electricity being generated from renewable energy. This reduces natural gas consumption and the associated carbon dioxide emissions. The green hydrogen is mixed with make-up gas and fed into the ammonia synthesis process using the Haber-Bosch process. The resulting ammonia is then reacted with carbon dioxide to synthesize urea.

[0004] Conventional plants for the synthesis of ammonia and urea are designed for the case (normal case) where the entire required amount of hydrogen is produced through steam reforming using feed natural gas. If, instead, a portion of the hydrogen is produced through electrolysis and mixed with the make-up gas, the load on the so-called "front end" of the conventional plants is reduced, but the operating point also deviates from the design case (normal case) of the conventional plants. This deviation is particularly significant when the proportion of green hydrogen is higher, starting at approximately 15%.

[0005] The following four aspects should be given particular attention:

[0006] 1.) Synthesis gas: The nitrogen requirement for ammonia synthesis remains unchanged. However, the reduced use of feed natural gas means a lower oxygen demand and thus also a lower air requirement.

[0007] 2.) Carbon dioxide: Due to the reduced use of feed natural gas, there is no longer sufficient carbon dioxide in the process gas, which is required for the downstream flame fuel production.

[0008] 3.) Steam: The demand for high-pressure steam (HP) remains almost unchanged. However, due to the reduced use of fuel natural gas and feed natural gas, smaller quantities of flue gas and process gas are generated, thus also reducing the amount of steam (HP), which is generated using the heat contained in the flue gas and process gas.

[0009] 4.) Waste heat utilization: Due to the reduced use of natural gas (both feed natural gas and fuel natural gas), the primary reformer operates at lower power. This results in less usable waste heat, and the heat transfer coefficients (k-values) of the heat exchangers may decrease. This is unfavorable, and the outlet temperatures of the streams should remain as unchanged as possible from normal.

[0010] These four aspects are very closely interrelated, so that a separate consideration or search for solutions is not possible.

[0011] In principle, various solutions are conceivable:

[0012] a) Synthesis gas: By shifting the load from the primary reformer to the secondary reformer, an unchanged air feed is possible.

[0013] b) Carbon dioxide: Additional carbon dioxide is extracted from the flue gas. The flue gas scrubbers used for this purpose represent a significant part of existing solutions, but are very costly.

[0014] c) Steam (HP) and waste heat recovery: Various measures are discussed: (i) use of a reserve of an auxiliary boiler; (ii) use of additional burners (support burners), if available; (iii) use of an additional electric heater.

[0015] If the proportion of green hydrogen is comparatively low (up to about 10% of the total amount of hydrogen in the make-up gas), stable operation of conventional plants can be achieved with the solutions a) to c), even if the required flue gas scrubbing is very cost-intensive.

[0016] However, if the proportion of green hydrogen is increased (from approximately 15%), balancing conventional plants becomes significantly more complicated. In this case, the required amount of steam (HP) cannot be provided, even with costly electrical preheating.

[0017] Previous approaches to a comprehensive solution are not sufficient and there is a need for improved solutions that bring economic and / or technical advantages in the complex consideration.

[0018] It is an object of the invention to provide advantageous processes and plants for the production of ammonia and urea in which part of the grey hydrogen can be replaced by green hydrogen and which overcome or at least reduce the disadvantages of conventional processes and plants.

[0019] This problem is solved by the subject matter of the patent claims.

[0020] It was surprisingly found that flue gas can be advantageously used as a source of additional CO2 if its N2 content is reduced and the N2-depleted flue gas is compressed together with electrolytically obtained O2 and then fed to a secondary reformer. The N2-depleted flue gas does not have to be highly pure for this purpose, but can be used as such. A first partial stream of the CO2 required for the synthesis of urea is obtained during the steam reforming of feed natural gas, and a second partial stream is obtained during the combustion of fuel natural gas to form flue gas. Both partial streams are combined in the secondary reformer so that the amount of CO2 required for the urea synthesis can be separated from the synthesis gas using conventional CO2 scrubbing. Further measures, such as cost-intensive flue gas scrubbing, are not necessary.

[0021] The inventive solution enables N2 to be separated from the flue gas using comparatively less efficient and thus less complex measures, preferably through pressure swing adsorption or temperature swing adsorption, or a membrane. The separation of N2 from the flue gas does not have to be quantitative; residual amounts of N2 are unproblematic. The CO2 remains in the flue gas and does not need to be separated, for example, through flue gas scrubbing.

[0022] Furthermore, the inventive solution makes it possible to mix the N2-depleted flue gas as a CO2 stream with air and / or electrolytically produced O2. This mixture can then be compressed in the existing process air compressor instead of pure air, preheated by absorbing heat from the flue gas, and fed to the secondary reformer. The total amount of CO2 required for urea synthesis can then be separated from the synthesis gas in the existing CCF scrubbing device.

[0023] According to the invention, several advantages are achieved:

[0024] For example, the separation of N2 from the flue gas using comparatively less efficient methods is significantly more cost-effective than the separation of CO2 from the flue gas using a separate flue gas scrubber, which would have to produce highly pure CO2. All other components that remain in the N2-depleted flue gas according to the invention are either desirable (such as residual N2 and O2) or do not represent a deterioration compared to a conventional process (such as residual Ar).

[0025] A further advantage is that not only is complex flue gas scrubbing to separate CO2 from the flue gas eliminated, but also nitrogen oxides (NOx) do not need to be separated. Separate NOx separation from the flue gas is eliminated because the flue gas, along with the CO2 it contains, is fed into the secondary reformer, where all NOx is removed anyway.

[0026] An additional advantage is achieved if argon is removed from the flue gas along with the N2. This reduces the amount of argon entering the process gas and thus the synthesis circuit compared to normal. The reduced amount of inert gas improves the performance of the NH3 reactor and the NFL separator and also reduces the number of devices and piping in the synthesis circuit.

[0027] In addition, the mass flow downstream of the secondary reformer is closer to the normal case for which the plant was originally designed. This results in more superheated steam being generated in the process gas cooling section.

[0028] These advantages outweigh the disadvantages. A significant portion of the costs for the inventive separation of N2 from the flue gas is attributable to the compression of a portion of the flue gas, which is required for the respective measure (PSA, TSA, membrane). Due to the increased CO2 content, the compressor (process air compressor) is also subjected to somewhat greater load than under normal conditions, for which the system was originally designed. However, this can be counteracted with relatively simple measures, in particular by increasing the inlet pressure and / or reducing the temperatures upstream of the individual stages.

[0029] A first aspect of the invention relates to a process for the synthesis of NH3 comprising the steps: (a) Combustion of fuel natural gas and air in a primary reformer to obtain flue gas; (b) reforming feed natural gas with steam in the primary reformer to obtain primary synthesis gas; (c) reducing the N2 content in the flue gas using an N2 separation device to obtain treated flue gas; (d) Electrolyzing H2O with electric current from renewable energy in an electrolysis device to obtain - electrolytically produced O2 and - electrolytically obtained FE; (e) reforming a mixture comprising - primary synthesis gas, - treated flue gas and - electrolytically produced O2 and / or air in a secondary reformer to obtain secondary synthesis gas; (f) processing secondary synthesis gas comprising separating CO2 in a CCE separation device to obtain - separated CO2 and - processed synthesis gas; (g) synthesizing NH3 from a mixture comprising - processed synthesis gas and - electrolytically produced H2 in an NHs reactor to obtain product gas comprising NH3; and (h) separating NH3 from the product gas to obtain separated NH3 and cycle gas.

[0030] Industrial processes for the synthesis of ammonia and urea are known to those skilled in the art. In this context, reference can be made in full to A. Nielsen, Ammonia - Catalysis and Manufacture, Springer, 1995; V. Gowariker, The Fertilizer Encyclopedia, Wiley, 2009; H. Liu, Ammonia Synthesis Catalysts - Innovation and Practice, World Scientific Publishing, 2013; and K. Staszak et al., Chemical Technologies and Processes, Walter de Gruyter, 2020.

[0031] In step (a) of the process according to the invention, fuel natural gas and air are combusted in a primary reformer to obtain flue gas.

[0032] For the purposes of this description, "flue gas" refers to the gas mixture formed during the combustion of fuel natural gas with air. Flue gas typically includes N2 and CO2, as well as H2O, O2, and Ar.

[0033] The combustion of fuel natural gas serves primarily to generate heat, which is required for the steam reforming of feed natural gas, which also takes place in the primary reformer in step (b). This heat is also used, among other things, to preheat all streams involved in steps (b) and (e). High-pressure steam (HP steam) is preferably superheated by absorbing heat from the flue gas and then used to drive compressors.

[0034] In step (b) of the process according to the invention, feed natural gas is reformed with steam in the primary reformer to obtain primary synthesis gas.

[0035] In the primary reformer, carbon monoxide and hydrogen, among other things, are produced by steam reforming methane and steam over nickel catalysts at temperatures up to approximately 850°C and pressures up to approximately 40 bar. The reaction is endothermic and requires an energy input, which is provided by step (a): CH4+ H2O ^ CO + 3 H2AH = +206 kJ / mol (1)

[0036] In addition, hydrogen is formed by the exothermic shift reaction, carbon dioxide as a by-product: CO + H2O CO2+ H2AH = -41 kJ / mol (2)

[0037] For the purposes of this description, "primary syngas" refers to the process gas that exits the primary reformer and is fed to the secondary reformer, which is located downstream of the primary reformer in the process gas flow direction. The primary syngas typically comprises H2O, H2, CO, and CO2.

[0038] In step (c) of the process according to the invention, the N2 content in the flue gas is reduced using an N2 separation device to obtain treated flue gas.

[0039] For the purposes of this description, "treated flue gas" refers to the gas mixture leaving the N2 separation device, which has a reduced N2 content compared to the flue gas entering the N2 separation device. The treated flue gas is depleted in N2 compared to the flue gas.

[0040] In preferred embodiments, in step (c) the N2 content in the flue gas is reduced by pressure swing adsorption (PSA) or temperature swing adsorption (TSA) or with the aid of a membrane, ie the N2 separation device is preferably based on one of these technologies.

[0041] These technologies are well known to experts. In this context, reference can be made in full to DM Ruthven, Pressure Swing Adsorption, Wiley 1994; AJ Kidnay et al., Fundamentals of Natural Gas Processing, Third Edition, CRC Press, 2020.

[0042] In preferred embodiments, in step (c), at most 95% of the N2 contained in the flue gas is separated; preferably at most 90%, more preferably at most 85%, even more preferably at most 80%, most preferably at most 75%, and in particular at most 70%. Thus, the process according to the invention makes it possible to keep the efficiency of separating N2 from the flue gas comparatively low, thereby minimizing the effort required. This contributes significantly to economic efficiency.

[0043] In preferred embodiments, at least a portion of the Ar contained in the flue gas is also separated in step (c). Measures for separating N2 from the flue gas, such as pressure swing adsorption (PSA), preferably have this advantageous accompanying effect, which results in less inert gas entering the process circuit. In step (c), at least a portion of the O2 contained in the flue gas can also be separated.

[0044] In preferred embodiments, step (c) comprises the substeps: (ci) dividing the flue gas obtained in step (a) into a first partial stream and a second partial stream; (C2) reducing the N2 content in the first partial stream while preserving the treated flue gas; and (cs) optionally discarding the second partial stream.

[0045] In preferred embodiments, the mass ratio of the first partial stream to the second partial stream is in the range from 20:1 to 1:20, preferably 10:1 to 1:10, more preferably 3:1 to 1:3, even more preferably 2.5:1 to 1:2.5, most preferably 2:1 to 1:2, and in particular 1.5:1 to 1:1.5.

[0046] In step (d) of the process according to the invention, H2O is electrolyzed with electric current from renewable energy in an electrolysis device to obtain electrolytically produced O2 and electrolytically produced H2. The renewable energy is preferably selected from solar energy, wind power, and hydropower.

[0047] For the purpose of description, "electrolytically produced H" refers to the hydrogen that is produced by the electrolysis of water (green hydrogen).

[0048] For the purpose of description, "electrolytically produced O " refers to the oxygen obtained by electrolysis of water.

[0049] Industrial processes for the electrolysis of water are well known to experts. In this context, reference can be made, for example, to P. Cavaliere, "Water Electrolysis for Hydrogen Production," Springer, 2023; R. Srivastsva et al., "Solar-Driven Green Hydrogen Generation and Storage," Elsevier, 2023.

[0050] In step (e) of the process according to the invention, a mixture comprising primary synthesis gas, treated flue gas and electrolytically obtained O2 and / or air is reformed in a secondary reformer to obtain secondary synthesis gas.

[0051] For the purposes of this description, "secondary syngas" refers to the process gas exiting the secondary reformer, which is located downstream of the primary reformer in the process gas flow direction. Secondary syngas typically includes H2O, H2, N2, CO, and CO2.

[0052] With the conventional use of pure air (about 80% N2 and 20% O2), carbon monoxide, hydrogen and water are produced in the secondary reformer by partial oxidation, preferably at about 1000 to 1200°C, over nickel catalysts from the remaining methane (about 7.5%) not converted in the primary reformer: CH4 + O2CO + H2O + H2AH = -278 kJ / mol (3)

[0053] Hydrogen is also produced by shift reactions, which also produce carbon dioxide (see above).

[0054] According to the invention, however, no pure air is used in the secondary reformer, but rather a mixture is used, depending on the proportion of electrolytically produced H2 and the associated relief of the front end, which comprises primary synthesis gas, treated flue gas and electrolytically produced O2 and / or air. This mixture can basically be either outside the secondary reformer or within the secondary reformer. According to the invention, synthesis gas, treated flue gas, electrolytically obtained O2, and / or air are preferably fed separately to the secondary reformer and only mixed therein. According to the invention, treated flue gas is first mixed with electrolytically obtained O2 and / or air, subsequently compressed together, and then fed to the secondary reformer, where the mixture with the primary synthesis gas is finally formed.

[0055] In preferred embodiments, the mixture comprises primary synthesis gas, treated flue gas and electrolytically produced O2.

[0056] In other preferred embodiments, the mixture comprises primary synthesis gas, treated flue gas and air.

[0057] In further preferred embodiments, the mixture comprises primary synthesis gas, treated flue gas, electrolytically produced O2 and air.

[0058] The components to be mixed and their respective proportions depend on several factors, in particular (1) the ability of the plant to tolerate deviations from the normal conditions for which the plant was originally designed; (2) the proportion of electrolytically produced H2 and the associated relief of the front end,' and (3) the efficiency and selectivity of the N2 separation device with regard to the separation of N2 and possibly other components.

[0059] The greater the proportion of electrolytically produced H2, the more electrolytically produced O2 is preferentially added to the mixture.

[0060] The lower the efficiency of the N2 separation device with regard to the separation of N2, the more electrolytically produced O2 is preferentially added to the mixture.

[0061] In preferred embodiments, step (e) comprises the substeps: (ei) combining electrolytically produced O2 and / or air and treated flue gas to obtain a gas mixture; (02) Compressing the gas mixture in a compressor (process air compressor); (es) heating the compressed gas mixture by absorbing heat from the flue gas; and (04) Combining the heated compressed gas mixture with primary synthesis gas to obtain the mixture.

[0062] In preferred embodiments, a target amount of N2 S(N2) is specified for the secondary synthesis gas leaving the secondary reformer and in step (e) the mixture comprising primary synthesis gas, treated flue gas and electrolytically obtained O2 and / or air are adjusted so that the secondary synthesis gas, when leaving the secondary reformer, actually contains an actual amount of N2 I(N2) which deviates from the target amount S(N2) by at most 15%; preferably at most 10%, more preferably at most 8%, even more preferably at most 6%, most preferably at most 4%, and in particular at most 2%.

[0063] In preferred embodiments, a target amount of CO2 S(CO2) is specified for the secondary synthesis gas leaving the secondary reformer, and in step (e) the mixture comprising primary synthesis gas, treated flue gas and electrolytically obtained O2 and / or air is adjusted such that the secondary synthesis gas actually contains an actual amount of CO2 1(CO2) upon leaving the secondary reformer which deviates from the target amount S(CO2) by at most 15%; preferably at most 10%, more preferably at most 8%, even more preferably at most 6%, most preferably at most 4%, and in particular at most 2%.

[0064] In preferred embodiments, a target amount of CIL S(CIL) is specified for the secondary synthesis gas leaving the secondary reformer (so-called "methane slip", e.g., approximately 0.5% by volume), and in step (e), the mixture comprising primary synthesis gas, treated flue gas, and electrolytically obtained O2 and / or air is adjusted such that the secondary synthesis gas leaving the secondary reformer actually contains an actual amount of CIL I(CIL) that deviates from the target amount S(CIL) by at most 15%; preferably at most 10%, more preferably at most 8%, even more preferably at most 6%, most preferably at most 4%, and in particular at most 2%. For this purpose, a minimum target amount of O2 is required, with which CIL is partially combusted and the heat for steam reforming is provided. This target amount of O2 is minimal in that the combustion of CIL is ensured, but the additional combustion of the products (IL and CO) is to be prevented.The three target limits mentioned here would not be violated by this unwanted combustion.

[0065] Preferably, - the target amount S(N2) is the amount of N2 in the secondary synthesis gas, and / or - the target amount S(CO2) is the amount of CO2 in the secondary synthesis gas, and / or - the target quantity S(CIL) is the quantity of CIL in the secondary synthesis gas for which the primary reformer and the secondary reformer are designed when only feed natural gas is reformed with steam and no electrolysis of H2O takes place (normal case).

[0066] In preferred embodiments, a target amount of O2 S(C>2) is defined for the mixture comprising primary synthesis gas, treated flue gas and electrolytically obtained O2 and / or air (for the oxidation of the "methane slip") and in step (e) the mixture comprising primary synthesis gas, treated flue gas and electrolytically obtained O2 and / or air is adjusted such that the mixture actually contains an actual amount of O2 1(O2) which differs from the target amount S(02) deviates by at most 15%; preferably at most 10%, more preferably at most 8%, even more preferably at most 6%, most preferably at most 4%, and in particular at most 2%.

[0067] Preferably, the target amount S(O2) corresponds to the amount of O2 for which the secondary reformer is designed when only feed natural gas is reformed with steam and no electrolysis of H2O takes place (normal case).

[0068] In step (f) of the process according to the invention, the secondary synthesis gas is processed, which may comprise several steps in several devices. The processing comprises separating CO2 from the secondary synthesis gas in a CO2 separation device to obtain separated CO2 and processed synthesis gas.

[0069] In preferred embodiments, the processing of the secondary synthesis gas in step (f) comprises the substeps: (fi) converting CO contained in the secondary synthesis gas to CO2 in a conversion device to obtain converted secondary synthesis gas; (f2) separating CO2 from the converted secondary synthesis gas in the CO2 separation device to obtain the separated CO2 and the processed synthesis gas; and (f , ) Methanization of residual CO2 contained in the processed synthesis gas in a methanation device .

[0070] For the purposes of this description, the term "converted secondary synthesis gas" refers to the process gas leaving the conversion device. The converted secondary synthesis gas essentially comprises H2O, H2, N2, and CO2, as well as smaller amounts of CH4 and Ar. The converted secondary synthesis gas preferably contains only small amounts of carbon monoxide, typically approximately 0.3% by volume. It differs from the secondary synthesis gas introduced into the conversion device, particularly in the lower or non-existent amount of CO and the correspondingly larger amount of CO2.

[0071] For the purposes of this description, "upgraded synthesis gas" refers to the process gas leaving the CO2 separation device, preferably the gas scrubber. The upgraded synthesis gas preferably contains only small amounts of carbon dioxide (typically less than 500 ppmv) and differs particularly in this respect from the converted secondary synthesis gas, which is fed to the CO2 separation device, preferably the gas scrubber.

[0072] During the conversion of CO in step (fi), all of the carbon monoxide (10 to 50 vol%) is converted with steam to carbon dioxide (shift reaction, see above) preferably at 200 to 360°C over iron-chromium or copper-zinc catalysts, which is then removed in step (f2), preferably by gas scrubbing: CO + H2O co2+ H2AH = -41 kJ / mol (2)

[0073] In the methanation device, in sub-step (T), the remaining small amounts of carbon monoxide and carbon dioxide in the processed synthesis gas are preferably converted with hydrogen to methane.

[0074] In preferred embodiments, in step (f), the CCE separation device is a CCE washing device.

[0075] In step (g) of the process according to the invention, NH3 is synthesized from a mixture comprising processed synthesis gas and electrolytically obtained H2 in an NH3 reactor to obtain product gas comprising NH3.

[0076] For the purposes of this description, "product gas" refers to the process gas leaving the NH3 reactor (converter). In addition to the synthesized NH3, the product gas typically includes unreacted residual N2 and H2, as well as NH3 and inert gases, particularly CH4 and Ar.

[0077] The synthesis gas is preferably converted to NH2 in an exothermic reaction at 400 to 500°C and usually 150 to 250 bar over iron catalysts: \2+ 2 H22 NH3AH = -91.4 kJ / mol (4)

[0078] In preferred embodiments, step (g) comprises the substeps: (gi) combining electrolytically produced H2 and processed synthesis gas and / or cycle gas to obtain the mixture; and (gs) Compression of the mixture.

[0079] In preferred embodiments, in step (g), the molar fraction of electrolytically obtained H2 in the mixture is at least 5%, based on the total amount of H2 contained in the mixture; preferably at least 10%, more preferably at least 15%, even more preferably at least 20%, most preferably at least 25%, and in particular at least 30%.

[0080] In step (h) of the process according to the invention, NH3 is separated from the product gas to obtain separated NH3 and cycle gas.

[0081] The separation of NH3 is preferably carried out by condensation. For the purposes of this description, the "recycle gas" refers to the process gas remaining after the separation, preferably condensation, of NH3 from the product gas. The recycle gas typically comprises N2 and H2 as well as NH3 and inert gases, in particular CH4 and Ar, and is preferably recycled to the NH3 reactor according to the invention.

[0082] The NHs reactor is preferably designed in several stages.

[0083] A further aspect of the invention relates to a process for the synthesis of urea comprising the process according to the invention for the synthesis of NH3 described above and the additional step: (i) Synthesizing urea from separated NH3 and separated CO2.

[0084] To synthesize urea, ammonia and carbon dioxide are reacted preferably at approximately 200°C and 250 bar. This initially produces ammonium carabamate, which is in equilibrium with urea and water: CO2+ 2 NH3- NH4OCONH2 AH = - 117 kJ / mol (5) NH4OCONH2 NH2CONH2 + H2O AH = +15.5 kJ / mol (6)

[0085] A further aspect of the invention relates to a device which is configured to carry out the method according to the invention for the synthesis of NH3 described above.

[0086] A further aspect of the invention relates to a device which is configured to carry out the method according to the invention for the synthesis of urea described above.

[0087] The device according to the invention comprises a primary reformer, an N2 separation device, an electrolysis device, optionally a compressor, a secondary reformer, optionally a conversion device, a CO2 separation device, optionally a methanation device, and an NH3 reactor.

[0088] An essential aim of the inventive - Separation of N2 from the flue gas, - Mixing of the N2-depleted flue gas with electrolytically obtained O2 and / or air, and - The purpose of introducing the gas stream thus generated into the secondary reformer is to provide an alternative to conventionally used air, as used in the normal case for which the plant was originally designed. As a result of the addition of electrolytically produced H2 (green hydrogen), less feed natural gas needs to be reformed with steam, which changes the composition and quantity of the primary synthesis gas. To compensate for these changes, the composition of the gas stream consisting of N2-depleted flue gas mixed with electrolytically produced O2 and / or air is adjusted according to the invention so that the composition and quantity of the secondary synthesis gas after leaving the secondary reformer are as close as possible to the normal case.

[0089] For this purpose, the required amounts can be added individually according to the invention. This allows for flexible response to a changing amount of electrolytically produced H2. If the added amount of electrolytically produced H2 is comparatively large, the gas mixture fed to the secondary reformer will have a different composition than if the added amount of electrolytically produced H2 is comparatively small.

[0090] The advantages of the invention are illustrated by the following examples, which, however, are not to be construed as limiting.

[0091] With a feed of 20% electrolytically produced H2 relative to the total amount of H2 contained in the recycle gas, a typical plant with a normal capacity of 50 tNH3 / h and a CO2 demand for the downstream urea plant of 60 t / h lacks approximately -235 kmol / h of CO2 in the process gas for the subsequent synthesis of urea. This amount is then recovered from the flue gas according to the invention.

[0092] For example, the flue gas has the following typical composition (actual) and the secondary synthesis gas should have the following typical target composition after leaving the secondary reformer, which is close to the normal case: Example 1 - Maximum removal of N2, Ar, and O2 from flue gas by PSA:

[0093] In this example, no electrolytically produced O2 is used. The flue gas is divided into two substreams. A first substream, which accounts for 55% of the total flue gas generated, is fed to a pressure swing adsorption (PSA) process. A second substream, which accounts for the remaining 45% of the total flue gas generated, is discarded, i.e., after removal of environmentally harmful components, it is released into the environment via a chimney.

[0094] The PSA's N2 removal efficiency is 95%, and Ar and O2 are also completely removed. The flue gas produced after N2 removal by PSA is mixed with air to achieve the desired composition as closely as possible.

[0095] The composition of 1. the separated gas streams, 2. the gas streams separated by PSA and 3. the gas streams mixed with air are summarized in the following table:

[0096] By adding air, the required amount of N2 is exceeded (1722 kmol / h instead of 1590 kmol / h). Example 2:

[0097] In this example, the O2 offgas from the electrolysis is added to the PSA out stream instead of air. With respect to N2 removal, the PSA's efficiency is only 60%, and Ar and O2 are also completely removed.

[0098] The composition of 1. the separated gas streams, 2. the gas streams separated by PSA and 3. the gas streams mixed with air are summarized in the following table:

[0099] This significantly reduces the required efficiency of the PSA relative to the separation performance for N2 to just 60%. Furthermore, no additional argon is introduced, because electrolytically produced O2 does not contain argon.

[0100] The invention is illustrated schematically below with reference to figures, which are also not to be interpreted in a restrictive manner.

[0101] Figure 1 schematically illustrates a conventional process for the synthesis of urea from ammonia using grey hydrogen.

[0102] Fuel natural gas (C n H2n+2) and air (N2+O2) are fed as combustion gas to a primary reformer (1) and burned there to generate heat. The resulting flue gas (CO2+N2) is discarded. In addition, feed natural gas (C n H2n+2) is fed to the primary reformer (1), where it is reformed with steam (not shown) to primary synthesis gas (H2+CO+CO2).

[0103] The primary reformer (1) preferably comprises a furnace in which catalyst-filled tubes are heated by burners on the furnace roof. The required gas temperature at the outlet of the catalyst-filled tubes is preferably approximately 800°C at a pressure of preferably approximately 45 bar. The primary synthesis gas leaves the tubes and is preferably passed through a refractory-lined cold outlet manifold system to a secondary reformer (5). The primary synthesis gas leaving the primary reformer (1) is preferably passed through a central internal riser pipe into a combustion chamber at the upper end of the secondary reformer (2). Air is introduced into this combustion chamber, preferably via nozzles, which are preferably arranged around the circumference of the combustion chamber. This air (N2+O2) is previously compressed in compressor (4) and reformed together with the primary synthesis gas (H2+CO+CO2) in the secondary reformer (5) to form secondary synthesis gas (H2+N2+CO+CO2).The partially oxidized synthesis gas flows preferably from top to bottom through a catalyst bed of the secondary reformer (2), which is preferably supported by a ceramic sheet. Finally, the secondary synthesis gas leaves the secondary reformer (5), preferably at the bottom.

[0104] The secondary synthesis gas leaving the secondary reformer is preferably cooled and purified. For this purpose, the carbon monoxide contained in the secondary synthesis gas is preferably converted to carbon dioxide in a conversion device (6). The carbon dioxide is then preferably separated from the converted secondary synthesis gas (H2+N2+CO2) in a CCE separation device (7). Residual amounts of CO and CO2 from the thus treated synthesis gas are preferably converted to methane in a methanation device (8). The synthesis gas is then compressed and fed to an NH3 reactor (9), in which ammonia is synthesized from it (product gas). The ammonia is condensed from the product gas, and the remaining cycle gas is returned to the NH3 reactor (9). In a further synthesis, urea is then synthesized from the separated carbon dioxide and the condensed ammonia (only indicated).

[0105] Figure 2 schematically illustrates a preferred embodiment of the method according to the invention.

[0106] In an electrolysis device (3), H2 and O2 are electrolytically produced from H2O. The electrolytically produced H2 is fed to the NH3 reactor (9) and mixed with synthesis gas and / or recycle gas. Depending on the amount of electrolytically produced H2, the front end is relieved because less gray hydrogen needs to be produced. The flue gas is preferably divided into a first partial stream and a second partial stream. While the N2 content in the first partial stream is reduced in an N2 separation device (2), the second partial stream is discarded. The N2-depleted flue gas (CO2+N2) is mixed with electrolytically produced O2 and / or air (N2+O2), compressed in the compressor (4), and then fed to the secondary reformer (5).

[0107] All other process steps are essentially analogous to the conventional embodiment shown in Figure 1.

[0108] List of reference symbols: - Primary reformer - N2 separation device - Electrolysis device - Compressor - Secondary reformer - Conversion device - CO2 separation device - Methanization device - NH; reactor

Claims

Patent claims:

1. A process for the synthesis of NH3 comprising the steps: (a) Combustion of fuel natural gas and air in a primary reformer (1) to obtain flue gas; (b) reforming feed natural gas with steam in the primary reformer (1) to obtain primary synthesis gas; (c) reducing the N2 content in the flue gas with an N2 separation device (2) to obtain treated flue gas; (d) electrolyzing H2O with electric current from renewable energy in an electrolysis device (3) to obtain - electrolytically produced O2 and - electrolytically obtained FE; (e) reforming a mixture comprising - primary synthesis gas, - treated flue gas and - electrolytically obtained O2 and / or air in a secondary reformer (5) to obtain secondary synthesis gas; (f) processing secondary synthesis gas comprising separating CO2 in a CCE separation device (7) to obtain - separated CO2 and - processed synthesis gas; (g) synthesizing NH3 from a mixture comprising - processed synthesis gas and - electrolytically obtained H2 in an NFf reactor (9) to obtain product gas comprising NH3; and (h) separating NH3 from the product gas to obtain separated NH3 and cycle gas.

2. The process according to claim 1, wherein in step (c) the N2 content in the flue gas is reduced by pressure swing adsorption (PSA), temperature swing adsorption (TSA) or by means of a membrane.

3. The process according to claim 1 or 2, wherein in step (c) at most 95% of the N2 contained in the flue gas is separated; preferably at most 90%, more preferably at most 85%, still more preferably at most 80%, most preferably at most 75%, and in particular at most 70%.

4. The process according to any one of the preceding claims, wherein in step (c) at least a portion of the argon contained in the flue gas is additionally separated.

5. The method according to any one of the preceding claims, wherein step (c) comprises the substeps: (ci) dividing the flue gas obtained in step (a) into a first partial stream and a second partial stream; (C2) reducing the N2 content in the first partial stream while preserving the treated flue gas; and (cs) optionally discarding the second partial stream.

6. The method according to any one of the preceding claims, wherein step (e) comprises the substeps: (ei) Associations of - electrolytically produced O2 and / or air and - treated flue gas to obtain a gas mixture; (02) Compressing the gas mixture in a compressor (4); (es) heating the compressed gas mixture by absorbing heat from the flue gas; and (04) Combining the heated compressed gas mixture with primary synthesis gas to obtain the mixture.

7. The process according to any one of the preceding claims, wherein the processing of the secondary synthesis gas in step (f) comprises the substeps: (fi) converting CO contained in the secondary synthesis gas to CO2 in a conversion device (6) to obtain converted secondary synthesis gas; (f>) separating CO2 from the converted secondary synthesis gas in the CO2 separation device (7) to obtain the separated CO2 and the processed synthesis gas; and (T) Methanizing residual CO2 contained in the treated synthesis gas in a methanation device (8).

8. The method according to any one of the preceding claims, wherein in step (f) the CO2 separation device (7) is a CO2 scrubbing device.

9. The method according to any one of the preceding claims, wherein step (g) comprises the substeps: (gi) Associations of - electrolytically produced H2 and - processed synthesis gas and / or recycle gas while maintaining the mixture; and (g2) Compacting the mixture.

10. The process according to any one of the preceding claims, wherein in step (g) the molar fraction of electrolytically produced H2 in the mixture is at least 5%, based on the total amount of H2 contained in the mixture; preferably at least 10%, more preferably at least 15%, even more preferably at least 20%, most preferably at least 25%, and in particular at least 30%.

11. The method according to any one of the preceding claims, wherein for the secondary synthesis gas leaving the secondary reformer (5) - a target amount of N2 S(N2) and / or - a target amount of CO2 S(CC>2) and / or - a target amount of CH4 S(CH4) is determined and wherein in step (e) the mixture comprising - primary synthesis gas, - treated flue gas and - electrolytically obtained O2 and / or air is adjusted so that in the secondary synthesis gas when leaving the secondary reformer (5) actually - an actual amount of N2 1(N2) is contained, which differs from the target amount S(N2) and / or - an actual amount of CO2 1(CC>2) is contained, which differs from the target amount S(CC>2) and / or - an actual amount of CEE I(CH4) is contained which deviates from the target amount of S(CH4) independently of one another by at most 15%; preferably at most 10%, more preferably at most 8%, even more preferably at most 6%, most preferably at most 4%, and in particular at most 2%.

12. The method of claim 11, wherein - the target quantity S(N2) corresponds to the required quantity of N2 in the secondary synthesis gas, and / or - the target quantity S(CC>2) corresponds to the quantity of CO2 in the secondary synthesis gas, and / or - the target quantity S(CFh) corresponds to the quantity of CH4 in the secondary synthesis gas for which the primary reformer (1) and the secondary reformer (5) are designed when only feed natural gas is reformed with steam and no electrolysis of H2O takes place (normal case).

13. A process for the synthesis of urea comprising the process for the synthesis of NH3 according to any one of the preceding claims and the additional step: (i) Synthesizing urea from separated NH3 and separated CO2.

14. A device comprising a primary reformer (1), an N2 separation device (2), an electrolysis device (3), optionally a compressor (4), a secondary reformer (5), optionally a conversion device (6), a CCE separation device (7), optionally a methanation device (8), and an NH3 reactor (9), wherein the device is configured to carry out the process for the synthesis of NH3 according to any one of claims 1 to 12 or the process for the synthesis of urea according to claim 13.

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