Green hydrogen, synthesis gas with a reduced nitrogen content, and flue gas 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 combustion of a fuel gas composed of fuel natural gas and combustion air and which also contains carbon dioxide, is additionally used. The oxygen formed during the electrolysis of water is introduced into the flue gas, and the modified flue gas is fed to a secondary reformer; and / or the fuel natural gas is combusted together with combustion air and the oxygen formed during electrolysis. Excess nitrogen is preferably separated from the synthesis gas before it is used for the synthesis of ammonia.

Description

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

[0001] Priority is claimed from Luxembourg patent application No. LU103226 of 14 December 2023.

[0002] The invention relates to the synthesis of urea from ammonia and carbon dioxide, wherein 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 (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. Flue gas is therefore also used, which is formed during the combustion of a combustion gas from fuel natural gas and combustion air and also contains carbon dioxide. The oxygen produced during the electrolysis of water is introduced into the flue gas and fed to a secondary reformer as modified flue gas; and / or fuel natural gas is combusted together with combustion air and the oxygen produced during the electrolysis.Preferably, excess nitrogen is separated from the synthesis gas before it is used to synthesize ammonia.

[0003] 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.

[0004] 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.

[0005] 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%.

[0006] The following four aspects should be given particular attention:

[0007] 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.

[0008] 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 urea production.

[0009] 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.

[0010] 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.

[0011] These four aspects are very closely interrelated, so that a separate consideration or search for solutions is not possible.

[0012] In principle, various solutions are conceivable:

[0013] a) Synthesis gas: By shifting the load from the primary reformer to the secondary reformer, an unchanged air feed is possible.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] However, if the share of green hydrogen is increased (from about 15%), balancing conventional plants is significantly more complicated. In this case, in particular, the The required amount of steam (HP) can be provided, even if the steam is preheated electrically at great expense.

[0018] WO 2016 / 083434 A1 relates to a process for producing synthesis gas, comprising treating a hydrocarbon feedstock in a primary reformer, compressing at least a portion of the flue gas from the primary reformer in a compressor, and feeding the compressed flue gas together with the effluent from the primary reformer to a secondary reformer. Enriched air is fed to either the primary reformer, the secondary reformer, or both.

[0019] WO 2022 / 157223 A1 relates to a process for producing a synthesis gas suitable for the synthesis of ammonia or methanol. The process comprises the step of supplying oxygen-enriched air, obtained by mixing air with an oxygen stream generated by water electrolysis, to the radiation section of a primary reformer.

[0020] WO 2023 / 139175 A1 relates to a system for synthesizing ammonia, the system comprising at least one reformer for converting a hydrocarbon into hydrogen, the system comprising a converter for converting hydrogen and nitrogen into ammonia, the converter being integrated into a circulation system, a first carbon dioxide separator being located between the reformer and the circulation system, and the circulation system comprising an ammonia separator.

[0021] 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, especially with regard to the aspects 1.) and 2.) mentioned above.

[0022] 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.

[0023] This problem is solved by the subject matter of the patent claims.

[0024] It was surprisingly discovered that flue gas can be advantageously used as a source of additional CO2. A first substream of the CO2 required for urea synthesis is obtained during the steam reforming of LEED natural gas, and a second substream is obtained during the combustion of LEED natural gas to produce flue gas. Both substreams are combined in the secondary reformer, allowing the amount of CO2 required for urea synthesis to be separated from the synthesis gas via conventional CCL scrubbing. Further measures, such as costly flue gas scrubbing, are unnecessary.

[0025] According to the invention, the flue gas can be generated in two different processes, which serve in particular to generate heat.

[0026] On the one hand, flue gas is preferentially produced when firing a primary reformer with combustion gas comprising fuel natural gas and combustion air (referred to as "first flue gas" for the purposes of description). In this primary reformer, feed natural gas is reformed with steam, and the heat required for this is provided by the firing.

[0027] On the other hand, flue gas is preferentially generated when firing an auxiliary boiler with combustion gas comprising natural gas and combustion air (referred to as "second flue gas" for the purposes of description). This auxiliary boiler is primarily used to generate steam, which can be used in a variety of ways. During plant start-up, the steam can be used to drive the turbine, and during normal plant operation, any steam deficits can be compensated for in steam reforming. Furthermore, the steam can be used to generate electricity, which is often more cost-effective than using the local power grid and also creates independence from it.

[0028] For the purpose of description, unless expressly stated otherwise, preferred embodiments with respect to the "flue gas" refer independently to both flue gases, ie the first flue gas as well as the second flue gas.

[0029] The invention makes it possible to advantageously use the O2 produced during the electrolysis of H2O with electrical power from renewable energy.

[0030] On the one hand, the O2 content in the flue gas can be increased by adding electrolytically produced O2. This mixture can then be compressed 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 during the CCE scrubbing process, which is already performed.

[0031] On the other hand, fuel natural gas can be burned with the combustion gas from combustion air and electrolytically obtained O2 to produce flue gas.

[0032] For this purpose, electrolytically obtained O2 from the combustion air is preferably used - added before the primary reformer and this mixture is then mixed with fuel natural gas and burned as combustion gas in the primary reformer to form first flue gas; and / or - added before the auxiliary boiler and this mixture is then mixed with fuel natural gas and burned as combustion gas in the auxiliary boiler to form second flue gas.

[0033] This allows the excess air to be reduced when generating flue gas by burning fuel natural gas with combustion air, thus reducing the input of excess N2 from the combustion air.

[0034] For the purpose of the description, for the generation of flue gas, preferably independently of each other for the generation of first flue gas and / or the generation of second flue gas, the mixture of fuel natural gas and combustion air is characterized by an excess air X (Combustion air ratio). This key figure indicates the mass ratio of combustion air to natural gas fuel relative to the stoichiometrically ideal ratio for a theoretically complete combustion process. Any electrolytically extracted O2 contained in the combustion gas or combustion air is not taken into account when determining the excess air X, i.e., the numerical value of the excess air X results exclusively from the ratio of combustion air to natural gas fuel.

[0035] The inventive solution enables the separation of excess N2 from the synthesis gas using comparatively inefficient and therefore inexpensive measures, preferably after methanation by pressure swing adsorption. Only the excess N2 needs to be separated from the synthesis gas; the remaining amount of N2 is required for the synthesis of NH3.

[0036] According to the invention, several advantages are achieved:

[0037] Thus, the CCL conditioning in the synthesis gas according to the invention is significantly more cost-effective (both in terms of CAPEX and OPEX) than separate flue gas scrubbing. A further advantage is that not only is a complex flue gas scrubbing process for separating CO2 from the flue gas eliminated, but also that nitrogen oxides (NOx) do not need to be separated. Separate separation of NOx 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.

[0038] An additional benefit is achieved when argon is removed from the CO2-depleted synthesis gas along with the N2. This results in less or no argon entering the process gas and thus the synthesis circuit compared to normal. Furthermore, by partially replacing the combustion air with electrolytically produced O2, less argon is introduced into the system. The reduced proportion 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.

[0039] Furthermore, 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.

[0040] Furthermore, the existing plant equipment is utilized to its maximum, and flexible switching to conventional operation (normal case) and back is easily possible. For conventional operation, the existing compressor (process air compressor) is preferably additionally relieved by a pre-compressor (booster compressor).

[0041] In addition, the amount of flue gas to the chimney is significantly reduced and the nitrogen separated from the CO2-depleted synthesis gas can be used for other purposes.

[0042] The solution according to the invention comprises the following measures or their combinations:

[0043] A.) Flue gas is split into two streams, one of which contains exactly the missing amount of CO2, while the other is discarded. This can be done using only the first flue gas, only the second flue gas, or a combination of the first and second flue gases.

[0044] B.) Flue gas is modified by adding electrolytically produced O2 so that the modified flue gas contains the O2 content required for reforming the primary synthesis gas in the secondary reformer.

[0045] C.) The modified flue gas is compressed instead of air, preheated by absorbing heat from the flue gas and fed to the secondary reformer.

[0046] D.) The complete amount of CO2 required for the synthesis of urea is separated in a CO2 separation device, which is already present (CCL scrubbing).

[0047] E.) Excess N2 is separated from the CO2-depleted synthesis gas using an N2 separation device, preferably by PSA. The resulting N2-depleted synthesis gas is fed to an NH3 reactor (converter) together with electrolytically produced H2. The excess N2 is separated from the CO2-depleted synthesis gas using the N2 separation device, preferably either before or after methanation.

[0048] In the technology according to WO 2023 / 139175 A1, oxygen for the secondary reformer can come from any source. However, the residual oxygen still present in the flue gas or elsewhere is far from sufficient to meet the oxygen demand of the secondary reformer. In contrast, according to the invention, the electrolytically produced oxygen can be utilized. Preferably, the majority of the electrolytically produced oxygen is fed to the flue gas in order to supply the desired amount of oxygen to the secondary reformer. Preferably, the remaining portion of the electrolytically produced oxygen is fed to the combustion air in order to reduce the nitrogen content in the flue gas. A further advantage of the invention compared to the technology according to WO 2023 / 139175 A1 is the alternating addition of individual streams of electrolytically produced oxygen instead of the process air, depending on the amount of electrolytically produced H2.

[0049] A first aspect of the invention relates to a process for the synthesis of NH3 comprising the steps: (a) Electrolyzing H2O with electric current from renewable energy in an electrolysis device to obtain - electrolytically produced O2 and - electrolytically obtained H2; (b) burning a combustion gas comprising - Fuel natural gas, Combustion air and optionally electrolytically obtained O2 preferably in a primary reformer and / or in an auxiliary boiler; with the production of flue gas; (c) compressing a gas comprising - at least part of the flue gas, - optional electrolytically produced O2; preferred not only optional but mandatory; and - optionally air in at least one compressor to obtain modified flue gas; (d) reforming feed natural gas with steam in the primary reformer to obtain primary synthesis gas; (e) reforming a mixture comprising - primary synthesis gas and - modified flue gas in a secondary reformer to obtain secondary synthesis gas; (f) processing secondary synthesis gas comprising separating CO2 in a CCE separation device (7) to obtain - separated CO2 and - CO2-depleted synthesis gas; (g) optionally reducing the N2 content in the CO2-depleted synthesis gas in an N2 separation device to obtain N2-depleted synthesis gas; (h) synthesizing NH3 from a mixture comprising - CO2-depleted synthesis gas or N2-depleted synthesis gas and - electrolytically produced H2 in an NHs reactor to obtain product gas comprising NH3; and (i) Separating NH3 from the product gas to obtain separated NH3 and cycle gas.

[0050] 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.

[0051] In step (a) 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.

[0052] For the purpose of description, "electrolytically produced Hf" refers to the hydrogen obtained by electrolysis of water (green hydrogen).

[0053] For the purpose of description, "electrolytically produced oxygen" refers to the oxygen obtained by electrolysis of water.

[0054] 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.

[0055] In preferred embodiments, step (a) comprises the substeps (ai) dividing the electrolytically obtained O2 into a first partial stream and a second partial stream; (a2) feeding the first partial stream of electrolytically obtained O2 to step (b); and (as) feeding the second partial stream of electrolytically obtained O2 to step (c).

[0056] In preferred embodiments, the first partial flow is smaller than the second partial flow.

[0057] In other preferred embodiments, the first partial stream is larger than the second partial stream. This is particularly preferred when a comparatively large amount of CO2 is missing due to a high proportion of electrolytically produced H2 and the correspondingly lower use of natural gas for urea production. In such cases, it is advantageous to supply a larger amount of electrolytically produced O2 to the combustion in step (b), possibly even the entire available amount of electrolytically produced O2.

[0058] In step (b) of the process according to the invention, a combustion gas comprising fuel natural gas, combustion air and optionally electrolytically obtained O2 is combusted to obtain flue gas; preferably in a primary reformer and / or in an auxiliary boiler.

[0059] For the purposes of this description, "flue gas" refers to the gas mixture formed during the combustion of fuel natural gas with combustion air and, if necessary, electrolytically produced O2. The flue gas typically includes N2 and CO2, as well as H2O, O2, and Ar.

[0060] According to the invention, the flue gas can be generated in two different processes, both of which serve in particular to generate heat.

[0061] On the one hand, flue gas is preferentially produced when firing a primary reformer with combustion gas comprising fuel natural gas and combustion air (referred to as "first flue gas" for the purposes of description). In this primary reformer, feed natural gas is reformed with steam, and the heat required for this is provided by the firing.

[0062] On the other hand, flue gas is preferably produced when firing an auxiliary boiler with combustion gas comprising fuel natural gas and combustion air (referred to as "second flue gas"). This auxiliary boiler primarily serves to generate steam, which can be used in a variety of ways. During plant start-up, the steam can be used to drive the turbine, and during normal plant operation, it can compensate for steam deficits for steam reforming. Furthermore, the steam can be used to generate electricity, which is often more cost-effective than using the local power grid and also creates independence from it.

[0063] For the purpose of description, unless expressly stated otherwise, preferred embodiments with respect to the "flue gas" refer independently to both flue gases, i.e., the first flue gas and the second flue gas.

[0064] The combustion of fuel natural gas in the primary reformer 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 (d). This heat is preferably used, among other things, to preheat all streams involved in steps (b), (d), and (e). High-pressure steam (HP steam) is preferably superheated by absorbing heat from the flue gas and then used to drive compressors.

[0065] Depending on the design and operation of a plant with a primary reformer and auxiliary boiler, the amount of secondary flue gas generated in the auxiliary boiler is usually only slightly smaller than the amount of primary flue gas generated in the primary reformer. The excess air in the two processes is usually identical (preferably X = 1.1), as are the flue gas outlet temperatures (preferably 140 to 160°C). Depending on the situation, the output of the auxiliary boiler can be throttled more effectively than the output of the primary reformer during plant operation. Throttled secondary flue gas reduces the proportion of secondary flue gas in the total flue gas volume.

[0066] In preferred embodiments, the flue gas, at least a part of which is compressed in step (c), is obtained exclusively by burning combustion gas in the primary reformer (first flue gas).

[0067] In other preferred embodiments, the flue gas, at least a part of which is compressed in step (c), is obtained exclusively by burning combustion gas in an auxiliary boiler (second flue gas).

[0068] In further preferred embodiments, the flue gas, at least a part of which is compressed in step (c), is obtained both by burning combustion gas in the primary reformer (first flue gas) and by burning combustion gas in an auxiliary boiler (second flue gas).

[0069] The use of second flue gas is preferred according to the invention, because in the flow direction of the second flue gas downstream of the auxiliary boiler, fewer heat exchangers are preferably installed in the plant for the use of the waste heat, so that the use of the second flue gas in intervenes to a lesser extent in the operation of the plant for which it is designed, i.e. there is then less feedback to the process. In addition, due to the overall smaller amount of second flue gas, the limit above which the entire amount of flue gas is used as an additional source of CO2 is reached more quickly, so that when this limit is reached, splitting the flue gas into two partial streams is no longer necessary. For example, if the second flue gas accounts for 46% and the first flue gas 54% of the total amount of flue gas, this limit is reached when the proportion of electrolytically produced H2 is around 20 to 30%. From a proportion of around 20 to 30% of electrolytically produced H2, both flue gases must be used as an additional source of CO2. When the proportion of electrolytically produced H2 is around 50%, the entire amount of CO2 in both flue gases is used up in the urea synthesis, and there is then no additional, freely available CO2.

[0070] The use of only one of the two flue gases as an additional source of CO2, i.e. either exclusively the first flue gas or exclusively the second flue gas, is preferred according to the invention, provided that the amount of CO2 contained therein can compensate for the additional requirement for urea synthesis. If the first flue gas can compensate for the need for additional CO2, which is required at a proportion of X% electrolytically produced H2, and the second flue gas can compensate for the need for additional CO2, which is required at a proportion of Y% electrolytically produced H2, with X > Y, then according to the invention, the use of a proportion of electrolytically produced H2 in the range of - 0 to Y% only the second flue gas; - more than Y% to X% exclusively the first flue gas; and - more than X% to X%+Y% (and possibly even more) of both the first flue gas and the second flue gas are used as an additional source of CO2.

[0071] If the entire amount of first flue gas or second flue gas is used as an additional source of CO2, X can preferably be reduced to a value of 1.0, since it is harmless if a small part of the natural gas is not reformed - at the latest in the secondary reformer, this part of the natural gas is reformed and thus converted into CO or CO2.

[0072] In preferred embodiments, in step (b), the excess air X of combustion air and fuel natural gas is at least 1.0; preferably at least 1.1, more preferably at least 1.2, even more preferably at least 1.3, most preferably at least 1.4, and in particular at least 1.5.

[0073] In preferred embodiments, in step (b), the excess air X of combustion air and fuel natural gas is at most 1.9; preferably at most 1.7, more preferably at most 1.5, even more preferably at most 1.3, most preferably at most 1.1, and in particular at most 1.0.

[0074] In step (c) of the process according to the invention, a gas comprising at least a part of the flue gas, optionally electrolytically obtained O2 (preferably not only optionally, but mandatory) and optionally air is compressed in at least one compressor to obtain modified flue gas.

[0075] For the purposes of this description, the term "modified flue gas" refers to the gas compressed in step (c). The composition of the gas may vary.

[0076] In preferred embodiments, the gas consists essentially of flue gas.

[0077] In other preferred embodiments, the gas comprises, in addition to the flue gas, electrolytically obtained O2.

[0078] In further preferred embodiments, the gas comprises air in addition to the flue gas.

[0079] In other preferred embodiments, the gas comprises, in addition to the flue gas, electrolytically obtained O2 and air.

[0080] In preferred embodiments, step (c) comprises the substeps: (ci) dividing the flue gas obtained in step (b) into a first partial stream and a second partial stream; (C2) Providing the gas comprising - the first partial flow of the flue gas, - optional electrolytically produced O2; preferably not only optional but mandatory, and - optional air; (cs) optionally pre-compressing the gas in a pre-compressor; (C4) compressing the gas in a compressor; (cs) optionally heating the gas by absorbing heat from the flue gas; and (ce) optionally discarding the second partial stream of flue gas.

[0081] In preferred embodiments, in sub-step (ci), the mass ratio of the first partial stream of the flue gas to the second partial stream of the flue gas is in the range from 3:1 to 1:3, more preferably 2.5:1 to 1:2.5, even more preferably 2:1 to 1:2, most preferably 1.5:1 to 1:1.5.

[0082] In other preferred embodiments, the entire flue gas generated in step (b) is compressed in step (c) to utilize the maximum amount of CO2 contained therein. This is particularly preferred when the proportion of electrolytically produced H2 is comparatively large, so that a comparatively large amount of CO2 is missing as a result of the front-end relief.

[0083] In preferred embodiments, a target amount of CO2 S(CC>2) is defined for the secondary synthesis gas leaving the secondary reformer, wherein in step (c) the amount of flue gas is adjusted such that the secondary synthesis gas actually contains an actual amount of CO21(CC>2) when leaving the secondary reformer which deviates from the target amount S(CC>2) 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%.

[0084] Preferably, the target quantity S(CC>2) corresponds to the quantity of CO2 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).

[0085] In step (d) of the process according to the invention, feed natural gas is reformed with steam in the primary reformer to obtain primary synthesis gas.

[0086] In the primary reformer, carbon monoxide and hydrogen are produced from methane and steam through steam reforming 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)

[0087] In addition, hydrogen is formed by the exothermic shift reaction, carbon dioxide as a by-product: CO + H2O CO2+ H2AH = -41 kJ / mol (2)

[0088] 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.

[0089] In step (e) of the process according to the invention, a mixture comprising primary synthesis gas and modified flue gas is reformed in a secondary reformer to obtain secondary synthesis gas.

[0090] 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.

[0091] 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)

[0092] According to the invention, however, no pure air is used in the secondary reformer, but a mixture is used depending on the proportion of electrolytically obtained H2 and the associated relief of the front end, which contains primary synthesis gas and modified flue gas wherein the modified flue gas may optionally contain electrolytically obtained O2 and optionally air in addition to flue gas. This mixture can in principle be formed either outside the secondary reformer or within the secondary reformer. According to the invention, synthesis gas and modified flue gas are preferably fed separately to the secondary reformer and only mixed therein. According to the invention, flue gas is preferably 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.

[0093] In preferred embodiments, the mixture comprises primary synthesis gas and modified flue gas, which comprises flue gas and electrolytically produced O2.

[0094] In other preferred embodiments, the mixture comprises primary synthesis gas and modified flue gas, which comprises flue gas and air

[0095] In further preferred embodiments, the mixture comprises primary synthesis gas and modified flue gas, which comprises flue gas, electrolytically obtained O2 and air.

[0096] 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,' (3) the available amount of electrolytically produced O2; (4) the excess air in the combustion gas; and (5) or the efficiency of the N2 separation device resulting from these factors with regard to the separation of N2.

[0097] 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 CO2-depleted synthesis gas.

[0098] For the purposes of this description, "CO2-depleted synthesis gas" refers to the process gas leaving the CO2 separation device, preferably the gas scrubber. The CO2-depleted synthesis gas preferably contains only small amounts of carbon dioxide and differs particularly in this respect from the (converted) secondary synthesis gas that is fed to the CO2 separation device, preferably the gas scrubber.

[0099] Preferably, in step (f), the CO2 separation device is a CCE scrubbing device.

[0100] 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; (£2) separating CO2 from the converted secondary synthesis gas in the CO2 separation device to obtain the separated CO2 and the CO2-depleted synthesis gas; and (f3) optionally methanizing residual CO and CO2 contained in the CO2-depleted synthesis gas in a methanation device, unless the residual CO and CO2 have already been separated together with excess N2 in step (g) (see below).

[0101] For the purposes of this description, "converted secondary synthesis gas" refers to the process gas leaving the conversion device. The converted secondary synthesis gas 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 vol.%. 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.

[0102] During the conversion of CO in step (fi), all of the carbon monoxide (10 to 50 vol%) is converted with steam to carbon dioxide, preferably at 200 to 360°C over iron-chromium or copper-zinc catalysts, which is then removed in step (£2), preferably by gas scrubbing: CO + H2O CO2+ H2AH = -41 kJ / mol (2)

[0103] In the methanation device, in sub-step (T), the remaining small amounts of carbon monoxide and carbon dioxide in the CO2-depleted synthesis gas are preferably converted with hydrogen to methane.

[0104] In optional step (g) of the process according to the invention, the N2 content in the CO2-depleted synthesis gas is reduced in an N2 separation device to obtain N2-depleted synthesis gas.

[0105] For the purpose of description, "N2-depleted synthesis gas" refers to the process gas leaving the N2 separation device.

[0106] Preferably, in optional step (g), the N2 content in the CO2-depleted synthesis gas is reduced by pressure swing adsorption (PSA).

[0107] These technologies are well known to experts. In this context, reference can be made in full to DM Ruthven, "Pressure Swing Adsorption," New York 1994; AJ Kidnay et al., "Fundamentals of Natural Gas Processing," Third Edition, CRC Press, 2020.

[0108] In the optional step (g), only excess N2 is separated, which is not required for the subsequent synthesis of NH3. The separated N2 can then be used as a raw material for other purposes.

[0109] In preferred embodiments, in step (g), at most 40% of the N2 contained in the CO2-depleted synthesis gas is separated; preferably at most 35%, more preferably at most 30%, even more preferably at most 25%, most preferably at most 20%, and in particular at most 15%. Thus, the process according to the invention makes it possible to keep the efficiency of separating N2 from the CO2-depleted synthesis gas comparatively low, thereby minimizing the effort involved. This contributes significantly to economic efficiency.

[0110] In preferred embodiments, the processing of the secondary synthesis gas in step (f) comprises the substeps (f1), (f2) and optionally (T). Step (g) is carried out after substep (f2) and before the optionally carried out substep (f, ). (fi) converting CO contained in the secondary synthesis gas to CO2 in a conversion device to obtain converted secondary synthesis gas; (f>) separating CO2 from the converted secondary synthesis gas in the CO2 separation device to obtain the separated CO2 and the CO2-depleted synthesis gas; (g) reducing the N2 content in the CO2-depleted synthesis gas in an N2 separation device (2) to obtain N2-depleted synthesis gas; wherein preferably in the N2 separation device (2), in addition to N2, at least a portion of the CH4 and / or residual CO and / or residual CO2 contained in the CO2-depleted synthesis gas is separated; preferably practically the entire amount of CH4, residual CO and residual CO2; and (f3) optionally methanizing residual CO and CO2 contained in the CO2-depleted synthesis gas in a methanation device.

[0111] Whether or not the optional methanation in sub-step (T) is carried out depends on the amount of CO and / or CO2 still present in the CO2-depleted synthesis gas after step (g). If the remaining amounts are negligible, meaning they do not cause problems in the subsequent synthesis of NH3, the methanation in sub-step (T) may be omitted.

[0112] The virtually complete removal of residual CEE, CO and CO2 prior to the optional methanation brings several advantages: (1) no or less H2 is consumed for methanation; (2) no or less CH4 enters the cycle gas; (3) the methanation device can be bypassed if necessary, thus reducing the pressure drop; (4) no or less cooling water is required, which would otherwise be needed for the hot synthesis gas after methanation to cool from about 350°C to about 40°C before the Synthesis gas is compressed in a synthesis gas compressor to the pressure required for the synthesis of NH;

[0113] In step (h) of the process according to the invention, NH3 is synthesized from a mixture comprising CO2-depleted synthesis gas or N2-depleted synthesis gas and electrolytically obtained H2 in an NH3 reactor to obtain product gas comprising NH3.

[0114] 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.

[0115] The synthesis gas is preferably converted to NH2 in an exothermic reaction at 400 to 520°C and usually 150 to 250 bar over iron catalysts: \2+ 2 H22 NH3AH = -91.4 kJ / mol (4)

[0116] Preferably, step (h) comprises the substep (hi) Associations of - electrolytically obtained H2; - CO2-depleted synthesis gas and / or N2-depleted synthesis gas; and - optionally recycle gas while maintaining the mixture and compressing the mixture.

[0117] Preferably, in step (h), the molar proportion of electrolytically obtained H2 in the mixture is at least 5%, 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%, based on the total amount of H2 contained in the mixture.

[0118] In step (i) of the process according to the invention, NH3 is separated from the product gas to obtain separated NH3 and cycle gas.

[0119] 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.

[0120] The NHs reactor is preferably designed in several stages.

[0121] A further aspect of the invention relates to a process for the synthesis of urea comprising the above-described process according to the invention for the synthesis of NH3 and the additional step: (j) Synthesizing urea from separated NH3 and separated CO2.

[0122] 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 NH3NH4OCONH2 AH = - 117 kJ / mol (5) NH4OCONH2 NH2CONH2 + H2O AH = +15.5 kJ / mol (6)

[0123] 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.

[0124] 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.

[0125] The device according to the invention comprises a primary reformer, optionally a ^-separation device, an electrolysis device, optionally a pre-compressor, a compressor, a secondary reformer, optionally a conversion device, a CO2 separation device, optionally a methanation device, and an NH3 reactor.

[0126] An essential objective of the inventive introduction - of electrolytically produced O2 into the combustion gas and / or flue gas, as well as - of modified flue gas into the secondary reformer is to provide an alternative to conventionally used air, as it is normally used in the secondary reformer for which the plant was originally designed.

[0127] 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 amount of the primary synthesis gas. Preferably, (i) when the amount of electrolytically produced H2 obtained in step (a) is increased, the amount of primary synthesis gas is reduced accordingly by reforming feed natural gas with steam in step (d); and (ii) when the amount of electrolytically produced H2 obtained in step (a) is reduced, the amount of primary synthesis gas is increased accordingly by reforming feed natural gas with steam in step (d); which changes the amount and possibly also the composition of the primary synthesis gas.

[0128] In order to compensate for these changes, according to the invention the composition of the modified flue gas is changed or adjusted 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 or preferably correspond to it.

[0129] 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 modified flue gas fed to the secondary reformer will have a different composition than if the added amount of electrolytically produced H2 is comparatively small.

[0130] The following examples serve to illustrate preferred embodiments of the invention, but are not to be construed as limiting.

[0131] In the following, variants 1, 2 and 3 as well as a combination of variants 1 and 3 (variant 4) and a combination of variants 1, 2 and 3 (variant 5) are explained using the example of a synthesis of ammonia and urea (NHs-urea complex).

[0132] The plant chosen as an example has a capacity of 1200 tpd of NH3 and 1925 tpd of urea. For example, 20% of electrolytically produced H2 (green hydrogen) is fed in, which corresponds to a quantity of 890 kmol / h, which is required for the synthesis of 240 tpd of NH3. The feed-in of electrolytically produced H2 reduces the consumption of feed natural gas for the production of grey hydrogen by 235 kmol / h, thus eliminating a corresponding amount of 235 kmol / h of CO2 required for urea production. The plant generates first flue gas in the primary reformer and second flue gas in an auxiliary boiler. In the chosen example, 470 kmol / h of fuel natural gas are normally used in the primary reformer and 400 kmol / h of fuel natural gas in the auxiliary boiler at full load. Normally, the excess air in both processes is identical (X=1, 1), as are the outlet temperatures (typically 140 to 160°C). Accordingly, the amount of the first flue gas is, for example,54% and the amount of the second flue gas corresponding to 46%, each based on the total amount of flue gas.

[0133] The essential objective of all variants according to the invention is (1) to provide the amount of N2 required for the synthesis of NH3 of 1590 kmol / h at the latest in the cycle gas; (2) to provide the remaining amount of CO2 of 235 kmol / h from the flue gas as an additional source of CO2 for the synthesis of urea; and (3) to provide the amount of O2 of 427 kmol / h required for the partial oxidation of the primary synthesis gas in the secondary reformer. Variant 1:

[0134] Variant 1 according to the invention is schematically illustrated in Figure 2.

[0135] Variant 1 combines the measures A.), B.), C ), D.) and E.) described above.

[0136] For the combustion of fuel natural gas in step (b), in which the flue gas is generated, the excess air X (combustion air ratio) in variant 1 is, for example, 1.44. This comparatively high value for X makes it possible to increase the amount of flue gas and thus the amount of combustion heat in order to ensure the production of a sufficient amount of water vapor in the flue gas duct.

[0137] According to the invention, however, this is not absolutely necessary, since sufficient steam is available when the production of electrical current is carried out using steam-driven driven turbines is reduced to a minimum, i.e., if steam is saved there. According to the invention, the electrical power then missing is preferably provided by renewable energy. Such a process is significantly more efficient in terms of natural gas consumption than using electrical power for the electrolysis of water to produce H2.

[0138] The actual composition of the flue gas produced during combustion (actual, 100%) as well as the target values ​​for the modified flue gas (target) are summarized in the following table:

[0139] To obtain the required amount of CO2 of 235 kmol / h from the flue gas, it is split into two partial streams: 45% and 55%. The 55% partial stream is reused, while the 45% partial stream is discarded.

[0140] In variant 1, the missing amount of O2 (427 - 220 = 207 kmol / h) in the reused flue gas stream is added as electrolytically produced O2. The modified flue gas then contains the desired amount of CO2 (235 kmol / h) and O2 (427 kmol / h), but too much N2 (2260 - 1590 = 970 kmol / h):

[0141] In variant 1, this excess amount of N2 of 970 kmol / h is therefore separated from the CO2-depleted synthesis gas in step (g) in the N2 separation device, preferably by PSA.

[0142] Variant 1 offers significant advantages over cost-intensive flue gas scrubbing because the additional devices required for the implementation of Variant 1, typically pre-compressors (booster compressors) and N2 separation devices (preferably pressure swing adsorption devices, PSA), are both technically more sophisticated and economically more advantageous.

[0143] A further significant advantage can be achieved by reducing the N2 content in the flue gas. This reduces the excess N2, allowing the N2 separation device (preferably PSA) to be significantly smaller or even unnecessary. Variant 2:

[0144] Variant 2 according to the invention is also schematically illustrated in Figure 2.

[0145] Variant 2 aims to dispense with step (g), i.e. no N2 has to be separated from the CO2-depleted synthesis gas.

[0146] To this end, the excess air X is reduced for the combustion of fuel natural gas in step (b), which generates the flue gas. However, variant 2 is limited in its effectiveness, as X typically cannot fall below 1.1. To completely dispense with step (g), however, X would have to be reduced to a value of 0.86, which would be unacceptable. Therefore, in variant 2, the excess air is reduced to the minimum value of 1.1. This also reduces the amounts of N2 and O2 in the flue gas, while the amount of CO2 remains unchanged.

[0147] The actual composition of the flue gas produced during combustion (actual, 100%) as well as the target values ​​for the modified flue gas (target) are summarized in the following table:

[0148] To obtain the required amount of CO2 of 235 kmol / h from the flue gas, it is split into two partial streams: 45% and 55%. The 55% partial stream is reused, while the 45% partial stream is discarded.

[0149] Due to the reduced amount of O2 in the flue gas partial stream, more electrolytically extracted O2 must be added. In variant 2, the missing amount of O2 in the flue gas partial stream (427 - 50 = 377 kmol / h) is added as electrolytically extracted O2. The modified flue gas then contains the desired amount of CO2 (235 kmol / h) and O2 (427 kmol / h), but still contains too much N2 (2026 - 1590 = 436 kmol / h):

[0150] In variant 2, this excess amount of N2 of 436 kmol / h is therefore separated from the CO2-depleted synthesis gas in the N2 separation device in step (g), preferably by PSA. However, the efficiency of the N2 separation device can be significantly lower compared to variant 1 and only needs to be 55%. Variant 3:

[0151] Variant 3 according to the invention is schematically illustrated in Figure 3.

[0152] Variant 3 offers an improvement in which electrolytically obtained O2 is not added to the flue gas, but directly to the combustion gas comprising fuel natural gas and combustion air, from which the flue gas is then generated through combustion. This allows X to be significantly increased (e.g., to a value of 1.87) and, at the same time, the N2 content to be reduced.

[0153] The actual composition of the flue gas produced during combustion (actual, 100%) as well as the target values ​​for the modified flue gas (target) are summarized in the following table:

[0154] To obtain the required amount of CO2 of 235 kmol / h from the flue gas, it is split into two partial streams: 45% and 55%. The 55% partial stream is reused, while the 45% partial stream is discarded.

[0155] The amount of combustion air is adjusted so that the amount of N2 contained in the reused partial stream after the flue gas has been divided into substreams exactly corresponds to the target amount of N2 (1590 kmol / h). The amount of electrolytically extracted O2 added to the combustion gas is adjusted so that the amount of O2 after the flue gas has been divided into substreams in the reused partial stream exactly corresponds to the target amount of O2 (427 kmol / h). The modified flue gas has the same composition as the reused partial stream, i.e., neither air nor electrolytically extracted O2 is added:

[0156] In order to avoid step (g) in variant 3, electrolytically produced O2 must be added to the combustion gas in an amount of 900 kmol / h, of which 45% is subsequently discarded.

[0157] However, with the exemplary proportion of electrolytically produced H2 of 20%, electrolytically produced O2 is only available in an amount of 445 kmol / h. Since no other source of O2 is normally available, variant 3 is preferably varied according to the invention. Variant 4:

[0158] Variant 4 according to the invention combines variant 2 (minimization of excess air X to 1.1) and variant 3 (feeding of electrolytically obtained O2 into combustion gas) and is schematically illustrated in Figure 4.

[0159] The amount of N2 in the combustion gas from the combustion air is the same as in variant 3, but the amount of O2 from the combustion air is minimized (X = 1.1). The reduced O2 content in the combustion air is compensated with an initial partial flow of electrolytically produced O2 (214 kmol / h).

[0160] The actual composition of the flue gas produced during combustion (actual, 100%) as well as the target values ​​for the modified flue gas (target) are summarized in the following table:

[0161] To obtain the required amount of CO2 of 235 kmol / h from the flue gas, it is split into two partial streams: 45% and 55%. The 55% partial stream is reused, while the 45% partial stream is discarded (1590 kmol / h of CO2 remains in the reused partial stream).

[0162] In addition, a second partial flow of electrolytically produced O2 (377 kmol / h) is added to the used partial flow of flue gas before the compressor, so that ultimately the total amount of O2 in the secondary reformer corresponds exactly to the target amount of 427 kmol / h:

[0163] Although the demand for electrolytically produced O2 is significantly reduced compared to variant 3, at a total of 591 kmol / h it still remains above the maximum available amount of 445 kmol / h.

[0164] One could argue that with a lower N2 content, the excess air may be slightly below the value of 1.1. However, a possible further reduction of X to a value of 1.0 would reduce the demand for electrolytically produced O2 to only 550 kmol / h, which would also be above the maximum available amount of 445 kmol / h. Variant 5:

[0165] If no additional source of O2 is available or the demand for O2 in the secondary reformer cannot be reduced accordingly, step (g) cannot be completely omitted. This corresponds to the combination of all three variants 1, 2, and 3 and is schematically illustrated in Figure 5.

[0166] With a fixed excess air (e.g. X = 1.1), the proportion of O2 in the flue gas and in the reused part of the flue gas is fixed. The required amount of O2 for the The secondary reformer's O2 output (427 kmol / h) is achieved by adding electrolytically produced O2 to the used flue gas stream (377 kmol / h). Considering the maximum available amount of electrolytically produced O2 of 445 kmol / h, the maximum possible amount of electrolytically produced O2 that can be fed into the combustion gas for flue gas generation is 68 kmol / h. The proportion of combustion air is increased to achieve the predetermined excess air (e.g., X = 1.1). This also increases the proportion of N2, with excess N2 (1889 - 1590 = 299 kmol / h) later separated from the CO2-depleted synthesis gas in step (g):

[0167] The amount of N2 to be removed, only 299 kmol / h, represents an improvement of approximately 70% compared to variant 1 and an improvement of approximately 30% compared to variant 2, i.e. the efficiency of N2 separation in step (g) only needs to be correspondingly lower or the N2 separation device needs to be smaller.

[0168] Reducing the excess air X to a value below 1.1 would further reduce the amount of N2 to be removed, but even then step (g) could not be completely dispensed with; at X = 1.0, the amount of N2 to be removed would still be 216 kmol / h.

[0169] A further reduction in O2 demand (total for the primary and secondary reformers) can be achieved by shifting the load between the two reformers. Varying the load distribution between the primary and secondary reformers can further reduce O2 demand, although not necessarily to an extent that would completely eliminate the need for N2 separation from the synthesis gas. Nevertheless, such a load shift can make an important contribution to minimizing the efficiency of the N2 separation device, preferably a PSA, required for N2 separation without additional effort.

[0170] Preferred embodiments of the invention are also explained with reference to the figures, which, however, are also not to be interpreted in a restrictive manner.

[0171] Figure 1 schematically illustrates a conventional process for the synthesis of urea from ammonia using grey hydrogen.

[0172] Fuel natural gas (C n H2n+2) and combustion air (N2+O2) are fed as combustion gas with an excess of air X to a primary reformer (1) and burned therein to generate heat. Alternatively, fuel natural gas (C n H2n+2) and combustion air (N2+O2) as combustion gas with an excess air X are fed to an auxiliary boiler and burned therein to generate heat (not shown). The resulting flue gas (CO2+N2+O2) is discarded. In addition, feed- Natural gas (Cn hn+2) is fed to the primary reformer (1), where it is reformed with steam (not shown) to primary synthesis gas (H2+CO+CO2).

[0173] 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.

[0174] 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). From the synthesis gas thus depleted of CO2, residual amounts of CO2 are preferably converted to methane in a methanation device (8). The synthesis gas is then 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 NPh reactor (9). In a further synthesis, urea is then synthesized from the separated carbon dioxide and the condensed ammonia (only indicated).

[0175] Figure 2 schematically illustrates a preferred embodiment of the process according to the invention, with which variants 1 and 2 according to the invention can preferably be implemented. In an electrolysis device (3), H2 and O2 are electrolytically extracted from H2O. The electrolytically extracted H2 is fed to the NH3 reactor (9) and mixed with synthesis gas and / or recycle gas. Depending on the amount of electrolytically extracted H2, the front end is thus relieved because less gray hydrogen needs to be produced. The flue gas is mixed with electrolytically extracted O2 and / or air (N2+O2), preferably first pre-compressed in the pre-compressor (10), then compressed in the compressor (4), and then fed to the secondary reformer (5). The N2 content of the CO2-depleted synthesis gas is reduced in an N2 separation device (2), preferably after methanation, preferably by pressure swing adsorption (PSA).The N2-depleted synthesis gas is then fed to the NH3 reactor. All remaining process steps are essentially analogous to the conventional design shown in Figure 1.

[0176] Figure 3 schematically illustrates a preferred embodiment of the process according to the invention, with which variant 3 according to the invention can preferably be implemented. The O2 obtained in the electrolysis device (3) is not added to the flue gas, but to the combustion gas. The excess air X can thereby be minimized. If a sufficient amount of O2 is available, the N2 content in the flue gas can be kept so low that no excess N2 is produced, and thus the N2 separation device (2) downstream of the methanation device (8) can possibly be completely dispensed with.

[0177] Figure 4 schematically illustrates a preferred embodiment of the process according to the invention, with which variant 4 according to the invention can preferably be implemented. The O2 obtained in the electrolysis device (3) is divided into two partial streams. A first partial stream of electrolytically obtained O2 is metered into the combustion gas. A second partial stream of electrolytically obtained O2 is metered into the flue gas. If a sufficient amount of O2 is available, the N2 content in the flue gas can be kept so low that no excess N2 is produced, and thus the N2 separation device (2) downstream of the methanation device (8) can possibly be completely dispensed with.

[0178] Figure 5 schematically illustrates a preferred embodiment of the process according to the invention, with which variant 5 according to the invention can preferably be implemented. The O2 obtained in the electrolysis device (3) is divided into two partial streams. A first partial stream of electrolytically obtained O2 is metered into the combustion gas. A second partial stream of electrolytically obtained O2 is metered into the flue gas. Excess N2 is separated in an N2 separation device (2) downstream of the methanation device (8).

[0179] List of reference symbols: 1 - Primary Reformer 2 - N2 separation device 3 - Electrolysis device 4 - Compressor 5 - Secondary reformers 6 - Conversion device 7 - CCE separator 8 - Methanization device 9 - NH3 reactor 10 - Pre-compressor

Claims

1. A process for the synthesis of NH; comprising the steps: (a) Electrolyzing H2O with electric current from renewable energy in an electrolysis device (3) to obtain - electrolytically produced O2 and - electrolytically obtained H2; (b) burning a combustion gas comprising - Fuel natural gas, - combustion air and - optionally electrolytically produced O2 with flue gas; (c) compressing a gas comprising - at least part of the flue gas, - optional electrolytically produced O2; preferably not only optional but mandatory, and - optionally air in at least one compressor (4) to obtain modified flue gas; (d) reforming feed natural gas with steam in a primary reformer (1) to obtain primary synthesis gas; (e) reforming a mixture comprising - primary synthesis gas and - modified flue gas 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 - CO2-depleted synthesis gas; (g) optionally reducing the N2 content in the CO2-depleted synthesis gas in an N2 separation device (2) to obtain N2-depleted synthesis gas; (h) synthesizing NH3 from a mixture comprising - CO2-depleted synthesis gas or N2-depleted synthesis gas and - electrolytically obtained H2 in an NHs reactor (9) to obtain product gas comprising NH3; and TI (i) separating NH2 from the product gas to obtain separated NH; and cycle gas.

2. The method according to claim 1, wherein (i) when the amount of electrolytically produced H2 obtained in step (a) is increased, the amount of primary synthesis gas is reduced accordingly by reforming feed natural gas with steam in step (d); and (ii) when the amount of electrolytically produced H2 obtained in step (a) is reduced, the amount of primary synthesis gas is increased accordingly by reforming feed natural gas with steam in step (d); thereby changing the amount and, where appropriate, also the composition of the primary synthesis gas.

3. The method according to claim 2, wherein the changed amount and optionally also composition of the primary synthesis gas is compensated by changing the composition of the modified flue gas.

4. The process according to claim 3, wherein the composition of the modified flue gas is adjusted so that the secondary synthesis gas after leaving the secondary reformer has a composition and quantity as similar as possible, preferably the same as when the entire amount of hydrogen is obtained by steam reforming using feed natural gas (normal case).

5. The method according to any one of the preceding claims, wherein step (a) comprises the substeps (ai) dividing the electrolytically obtained O2 into a first partial stream and a second partial stream; (a2) feeding the first partial stream of electrolytically obtained O2 to step (b); and (as) feeding the second partial stream of electrolytically obtained O2 to step (c).

6. The process according to any one of the preceding claims, wherein the flue gas is obtained (i) by burning combustion gas in the primary reformer (1); (ii) by burning combustion gas in an auxiliary boiler; or (iii) both by burning combustion gas in the primary reformer (1) and by burning combustion gas in an auxiliary boiler.

7. The process according to any one of the preceding claims, wherein in step (b) the excess air X of combustion air and fuel natural gas is at least 1.0; preferably at least 1.1, more preferably at least 1.2, even more preferably at least 1.3, most preferably at least 1.4, and in particular at least 1.

5.

8. The process according to any one of the preceding claims, wherein in step (b) the excess air X of combustion air and fuel natural gas is at most 1.9; preferably at most 1.7, more preferably at most 1.5, even more preferably at most 1.3, most preferably at most 1.1, and in particular at most 1.

0.

9. The process according to any one of the preceding claims, wherein step (c) comprises the substeps: (ci) dividing the flue gas obtained in step (b) into a first substream and a second substream; (C2) Providing the gas comprising - the first partial flow of the flue gas, - optional electrolytically produced O2; preferably not only optional but mandatory, and - optional air; (cs) optionally pre-compressing the gas in a pre-compressor (10); (C4) compressing the gas in a compressor (4); (cs) optionally heating the gas by absorbing heat from the flue gas; and (ce) optionally discarding the second partial stream of flue gas.

10. The method according to any one of the preceding claims, wherein 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 CO2-depleted synthesis gas; and (f3) optionally methanizing residual CO2 contained in the CO2-depleted synthesis gas in a methanation device (8).

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

12. The process according to any one of the preceding claims, wherein in optional step (g) the N2 content in the CO2-depleted synthesis gas is reduced by pressure swing adsorption (PSA).

13. The method according to any one of the preceding claims, wherein a target amount of CO2 S(CC>2) is specified for the secondary synthesis gas leaving the secondary reformer (5), and wherein in step (c) the amount of flue gas is adjusted such that the secondary synthesis gas actually contains an actual amount of CO2 1(CC>2) upon leaving the secondary reformer (5) which deviates from the target amount S(CC>2) 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%.

14. The method according to claim 13, wherein the target amount S(CC>2) corresponds to the amount of CO2 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).

15. The method according to any one of the preceding claims, wherein step (h) comprises the substep: (hi) Associations of - electrolytically obtained FE; - CO2-depleted synthesis gas and / or N2-depleted synthesis gas; and - optional recycle gas while maintaining the mixture.

16. The process according to any one of the preceding claims, wherein in step (h) the molar proportion of electrolytically obtained H2 in the mixture is at least 5%, 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%, based on the total amount of H2 contained in the mixture.

17. 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: (j) Synthesizing urea from separated NH3 and separated CO2.

18. A device comprising a primary reformer (1), optionally an N2 separation device (2), an electrolysis device (3), optionally a pre-compressor (10), a compressor (4), a secondary reformer (5), optionally a conversion device (6), a CCE separation device- device (7), optionally a methanation device (8), and an NH3 reactor (9), wherein the device is configured to carry out the process for synthesizing NH3 according to any one of claims 1 to 16 or the process for synthesizing urea according to claim 17.

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