The present invention relates to a novel method for gener- ating synthesis gas for ammonia production. In a specific embodiment of the method, the synthesis gas is generated by using solid oxide electrolysis cell (SOEC) stacks.

A typical ammonia-producing plant first converts a desulfu- rized hydrocarbon gas, such as natural gas (i.e. methane) or LPG (a liquefied petroleum gas, such as propane and bu ^¬ tane) or petroleum naphtha into gaseous hydrogen by steam reforming. The hydrogen is then combined with nitrogen to produce ammonia via the Haber-Bosch process

3 H ₂ + N ₂ → 2 NH ₃

Thus, the synthesis of ammonia (N¾) requires a synthesis gas (syngas) comprising hydrogen (¾) and nitrogen ( ₂ ) in a suitable molar ratio of about 3:1.

Ammonia is one of the most widely produced chemicals, and it is synthesized directly using gaseous hydrogen and ni ^¬ trogen as reactants without precursors or by-products. In its gaseous state, nitrogen is largely available as N ₂ , and it is normally produced by separating it from atmospheric air. The production of hydrogen (H ₂ ) is still challenging and, for industrial synthesis of ammonia, it is most often obtained from steam methane reforming (SMR) of natural gas. Moreover, when air is used for reforming processes, ₂ is also introduced, thus rendering the need for an air separa ^¬ tion unit superfluous, but a clean-up process is necessary to remove oxygen-containing species, such as 0 ₂ , CO, CO ₂ and H ₂ O, in order to prevent the catalysts from being poi ^¬ soned in the ammonia converter. Carbon dioxide is a product of SMR and can be separated and recovered inside the plant. Hydrogen production is therefore a critical process in am ^¬ monia synthesis, and a sustainable production of ammonia is desirable to reduce the consumption of a primary source, such as natural gas, and to avoid CO ₂ emissions from the process .

The basic idea underlying the present invention is to make ammonia synthesis gas by electrolysis, e.g. in SOEC stacks, without having to use air separation. Of course SOEC can be used to produce the necessary hydrogen, but then a separate air separation device would be necessary. Such devices, especially small-scale, are expensive. The idea is then to burn off the air inside the electrolysis unit, such as the SOEC stack, or between the units and basically utilize the ability of the unit to separate oxygen from hydrogen.

So the present invention provides a method for generating synthesis gas for ammonia production by electrolysis, pref ^¬ erably by means of SOEC stacks. The method avoids any use of an air separation unit (cryogenic, pressure swing ad- sorption or the like) by taking advantage of the ability of being operated in an endothermal mode, and it provides the necessary nitrogen by burning the hydrogen produced by steam electrolysis by air. In the preferred embodiment, in which SOEC stacks are used, the combustion of hydrogen can take place inside the stacks or between separate stacks.

More specifically, the invention relates to a method for generating ammonia synthesis gas by electrolysis, said method comprising the steps of:

- feeding a mixture of steam and compressed air into the electrolysis unit or into the first of a series of elec ^¬ trolysis units and

- passing the outlet from one electrolysis unit to the in ^¬ let of the next electrolysis unit, either together with air added after each electrolysis unit or only adding air after the last electrolysis unit, wherein the electrolysis units are run in thermoneutral or endothermal mode and the nitrogen part of the synthesis gas is provided by burning the hydrogen produced by steam elec ^¬ trolysis by air in or between the electrolysis units.

The feature of only adding air before and after the elec ^¬ trolysis unit(s) leads to a slightly increased energy con- sumption, but on the other hand it is much easier to implement, and still the air separation can be avoided.

The electrolysis units are preferably SOEC stacks. When SOEC stacks are used as electrolysis units, the operating voltage of the stacks is preferably below the so-called thermoneutral voltage, which is the minimum thermodynamic voltage at which a perfectly insulated electrolysis unit would operate, if there is no net inflow or outflow of heat. The burning of the hydrogen produced by steam elec- trolysis by air can be done inside the SOEC stacks or be ^¬ tween separate SOEC stacks. In a preferred embodiment, the steam used is steam from an ammonia synthesis loop, which is mixed with recycle ammonia synthesis gas. The operating voltage of the stacks being below the ther- moneutral voltage means that the temperature will decrease across an adiabatic stack. The inlet temperature to the subsequent stack is then increased again by combusting a fraction of the formed hydrogen in air, thus providing the nitrogen needed for the ammonia synthesis reaction that is carried out in a separate synthesis loop.

It is well known that the hydrogen needed for ammonia syn ^¬ thesis can be provided electrolytically, e.g. by water electrolysis, which has indeed been practiced in industrial scale .

The required nitrogen for the ammonia synthesis is then generated by air separation, either by cryogenic separa- tion, by pressure swing adsorption (PSA) or by the use of membranes. Such separate air separation units constitute a costly investment, and they require regular maintenance in the case of PSA or membranes. The present invention elimi ^¬ nates these problems.

The preparation of ammonia synthesis gas by electrolysis has been described in various patents and patent applica ^¬ tions. Thus, a method for the anodic electrochemical syn ^¬ thesis of ammonia gas is described in US 2006/0049063. The method comprises providing an electrolyte between an anode and a cathode, oxidizing negatively charged nitrogen-con ^¬ taining species and negatively charged hydrogen-containing species present in the electrolyte at the anode to form ad ^¬ sorbed nitrogen species and hydrogen species, respectively, and reacting the adsorbed nitrogen species with the ad ^¬ sorbed hydrogen species to form ammonia.

In US 2012/0241328, ammonia is synthesized using electro ^¬ chemical and non-electrochemical reactions. The electro ^¬ chemical reactions occur in an electrolytic cell having a lithium ion-conductive membrane that divides the electro- chemical cell into an anolyte compartment and a catholyte compartment, the latter including a porous cathode closely associated with the lithium ion-conductive membrane.

WO 2008/154257 discloses a process for the production of ammonia that includes the production of nitrogen from the combustion of a stream of hydrogen mixed with air. Hydrogen used to produce the nitrogen for an ammonia combustion pro ^¬ cess may be generated from the electrolysis of water. Hy ^¬ drogen produced by electrolysis of water may also be com- bined with nitrogen to produce ammonia.

So far, little attention has been paid to ammonia produc ^¬ tion using synthesis gas produced by electrolysis, espe ^¬ cially generated using SOEC stacks. Recently, the design and analysis of a system for the production of "green" ammonia using electricity from renewable energy sources has been described (Applied Energy 192 (2017) 466-476) . In this concept, solid oxide electrolysis (SOE) for hydrogen pro ^¬ duction is coupled with an improved Haber-Bosch reactor, and an air separator is included to supply pure nitrogen.

An ammonia production with zero CO ₂ emission is said to be obtainable with a 40% power input reduction compared to equivalent plants.

A flexible concept for the synthesis of ammonia from inter ^¬ mittently generated ¾ is described (Chem. Ing. Tech. 86 No .5 (2014), 649-657) and compared to the widely discussed power-to-gas concepts on a technical and economical level. The electrolytic synthesis of ammonia in molten salts under atmospheric pressure has been described (J. Am. Chem. Soc. 125 No .2 (2003), 334-335), in which a new electrochemical method with high current efficiency and lower temperatures than in the Haber-Bosch process is used. In this method, nitride ion (N ^3~ ) , produced by the reduction of nitrogen gas at the cathode, is anodically oxidized and reacts with hydrogen to produce ammonia at the anode.

Frattini et al . (Renewable Energy 99 (2016), 472-482) de ^¬ scribe a system approach in energy evaluation of different renewable energy sources integrated in ammonia production plants. The impact of three different strategies for renew- ables integration and scale-up sustainability in the ammo ^¬ nia synthesis process was investigated using thermochemical simulations. For a complete evaluation of the benefits of the overall system, the balance of plant, the use of addi ^¬ tional units and the equivalent greenhouse gas emissions have been considered.

Pfromm (J. Renewable Sustainable Energy 9 (2017), 034702) describes and sums up the most recent state of the art and especially the renewed interest in fossil-free ammonia pro- duction and possible alternatives to the Haber Bosch pro ^¬ cess. Finally, Wang et al . (AIChE Journal 63 No. 5 (2017), 1620- 1637) deal with an ammonia-based energy storage system uti ^¬ lizing a pressurized reversible solid oxide fuel cell (R- SOFC) for power conversion, coupled with external ammonia synthesis and decomposition processes and a steam power cy ^¬ cle. Pure oxygen, produced as a side product in electro ^¬ chemical water splitting, is used to drive the fuel cell.

The invention is described in more detail in the following example with reference to the appended Figure, showing the layout of one particular embodiment.

Example Superheated steam at 400°C and 40 barg (1), generated in the ammonia synthesis loop (2) and in an SOEC electrolysis unit consisting of eight identical SOEC stacks (numbered 1- 8), is mixed with recycle ammonia synthesis gas (3), which is a mixture of hydrogen and nitrogen, preferably in the stoichiometric 3:1 ratio. The mixture is sent through a first (A) and a second (B) feed/effluent heat exchanger, where it is heat exchanged using the gas coming from the cathode (fuel) side of the SOEC stacks and from the anode (oxygen) side of the SOEC stacks, respectively. Compressed air (4) at 40 barg is then added to a catalytic burner (not shown in the figure) , and the temperature increases to 785°C at the inlet to the first SOEC stack. The stack is operated at 1175 mV per cell, resulting in a temperature drop across the stack to 692°C at the outlet. Compressed air is added to the effluent from the first SOEC stack in an amount resulting in a temperature of 785°C at the second SOEC stack inlet. The second stack is operated at a voltage of 1196 mV per cell, resulting in an outlet temperature of 722°C. This air addition is repeated five time between stacks (the total number of SOEC stacks being eight) . After heat exchange with incoming steam and recycle gas in the first heat exchanger (A) , supplementary air is added so that the final gas composition is stoichiometric ammonia synthesis gas and the need for steam to the SOEC unit is covered. The supplementary steam in addition to the amount generated in the ammonia synthesis loop is obtained by cooling the gas after the final air addition point.

Finally, the non-converted steam is condensed out in a con ^¬ denser (C) , and the gas is split into two streams: One stream (3) is re-compressed and recycled to the inlet of the SOEC unit, while the other stream (5) is further com ^¬ pressed and dried and then sent to the ammonia synthesis loop . To avoid catalyst poisoning in the ammonia synthesis loop, CO ₂ must be removed quantitatively from the air used. This can be done by known physical or chemical methods for CO ₂ removal and/or by methanation of CO ₂ and CO, which will be formed in the SOEC unit, in a methanation reactor prior to passing the syngas (5) to the synthesis loop.