Method for producing synthesis gas for ammonia production

Abstract

In a process for producing ammonia synthesis gas by electrolysis, comprising feeding a mixture of steam and compressed air into a first of a series of electrolysis units and passing it together with air through an outlet of one electrolysis unit to an inlet of the next electrolysis unit, the electrolysis units operating in an endothermic mode, and providing a nitrogen portion of the syngas by combusting hydrogen produced by steam electrolysis with air within or between the electrolysis units. The electrolysis unit is preferably a solid oxide electrolytic cell (SOEC) stack.

Description

[0001] This application is a divisional application of Chinese invention patent application filed on October 1, 2018, with application number 201880066165.5 and invention title "Method for generating synthesis gas for ammonia production". Technical Field

[0002] This invention relates to a novel method for generating syngas for ammonia production. In one specific embodiment of the method, syngas is generated using a solid oxide electrolyzer (SOEC) stack. Background Technology

[0003] A typical ammonia plant first converts desulfurized hydrocarbon gases, such as natural gas (i.e., methane) or LPG (liquefied petroleum gas, such as propane and butane) or naphtha, into gaseous hydrogen through steam reforming. Then, the hydrogen is combined with nitrogen using the Haber-Bosch process to produce ammonia. 3 H2 + N2 → 2 NH3 Therefore, the synthesis of ammonia (NH3) requires syngas containing hydrogen (H2) and nitrogen (N2) in a suitable molar ratio of approximately 3:1.

[0004] Ammonia is one of the most widely produced chemicals, synthesized directly using gaseous hydrogen and nitrogen as reactants, without precursors or byproducts. In its gaseous state, nitrogen is primarily provided as N2, typically produced by separating it from the atmosphere. Hydrogen (H2) production remains challenging for the industrial synthesis of ammonia, which is usually obtained from steam methane reforming (SMR) of natural gas. Furthermore, N2 is also introduced when air is used in the reforming process, thus eliminating the need for an air separation unit, but a purification process is necessary to remove oxygen-containing substances such as O2, CO, CO2, and H2O to prevent catalyst poisoning in the ammonia converter. Carbon dioxide, a product of SMR, can be separated and recovered on-site. Therefore, hydrogen production is a critical process in ammonia synthesis, and sustainable ammonia production is desired to reduce the consumption of major sources such as natural gas and avoid CO2 emissions from the process. Summary of the Invention

[0005] The basic idea behind this invention is to produce ammonia syngas by electrolysis, for example, in an SOEC reactor, without the need for air separation. Of course, the necessary hydrogen can be produced using an SOEC, but this would subsequently require a separate air separation unit. Such units, especially small ones, are expensive. The idea then is to burn off the air inside or between the electrolysis unit (such as an SOEC reactor) and essentially utilize the unit's capacity to separate oxygen from the hydrogen.

[0006] Therefore, the present invention provides a method for generating syngas for ammonia production by electrolysis (preferably via an SOEC stack). This method avoids the use of any air separation units (cryotherapy, pressure swing adsorption, etc.) by utilizing the ability to operate in endothermic mode, and provides the necessary nitrogen by burning hydrogen produced from steam electrolysis in air. In a preferred embodiment using an SOEC stack, hydrogen combustion can occur within the stack or between individual stacks. Attached Figure Description

[0007] Figure 1 The layout of a specific implementation is shown. Detailed Implementation

[0008] More specifically, the present invention relates to a method for generating ammonia synthesis gas by electrolysis, the method comprising the following steps: - Feed the mixture of steam and compressed air into the electrolysis unit or the first in a series of electrolysis units, and - This allows it to pass through the outlet of one electrolysis unit to the inlet of the next, while adding air after each electrolysis unit, or adding air only after the last electrolysis unit. The electrolysis unit operates in a thermally neutral or endothermic mode and provides the nitrogen portion of the syngas by burning hydrogen produced by steam electrolysis in air within or between the electrolysis units.

[0009] The feature of adding air only before and after the electrolysis unit results in a slight increase in energy consumption, but on the other hand, it is easier to implement and air separation can still be avoided.

[0010] The preferred electrolysis unit is an SOEC stack. When using an SOEC stack as the electrolysis unit, the stack's operating voltage is preferably lower than the so-called thermal neutral voltage, which is the minimum thermodynamic voltage at which a fully adiabatic electrolysis unit will operate if there is no net inflow or outflow of heat. Combustion of hydrogen produced by steam electrolysis with air can be carried out within the SOEC stack or between individual SOEC stacks.

[0011] In a preferred embodiment, the steam used is steam from the ammonia synthesis loop, which is mixed with the recycled ammonia synthesis gas.

[0012] The fact that the operating voltage of the reactor is below the thermal neutral voltage means that the temperature across the adiabatic reactor will decrease. Then, by burning a portion of the hydrogen formed in the air, the inlet temperature of the subsequent reactor is raised again, thereby providing the nitrogen required for the ammonia synthesis reaction in a separate synthesis loop.

[0013] As is well known, the hydrogen required for ammonia synthesis can be provided by electrolysis, such as by water electrolysis, which has actually been practiced on an industrial scale.

[0014] The nitrogen required for ammonia synthesis is then produced by air separation (via cryogenic separation, pressure swing adsorption (PSA), or the use of a membrane). Such separate air separation units constitute an expensive investment, and for PSA or membranes, they require regular maintenance. This invention eliminates these problems.

[0015] The preparation of ammonia synthesis gas by electrolysis has been described in various patents and patent applications. Therefore, a method for the anodic electrochemical synthesis of ammonia is described in US 2006 / 0049063. This method includes providing an electrolyte between an anode and a cathode, oxidizing negatively charged nitrogen-containing substances and negatively charged hydrogen-containing substances present in the electrolyte at the anode to form adsorbed nitrogen and hydrogen substances, respectively, and reacting the adsorbed nitrogen with the adsorbed hydrogen to form ammonia.

[0016] In US 2012 / 0241328, ammonia is synthesized using electrochemical and non-electrochemical reactions. The electrochemical reaction occurs in an electrolytic cell with a lithium-ion conductive membrane that divides the electrochemical cell into an anolyte chamber and a cathode electrolyte chamber, the latter comprising a porous cathode tightly connected to the lithium-ion conductive membrane.

[0017] WO 2008 / 154257 discloses a method for producing ammonia, which includes producing nitrogen from the combustion of a hydrogen stream mixed with air. Hydrogen can be produced by the electrolysis of water, and this hydrogen can be used to produce nitrogen for the ammonia combustion process. The hydrogen produced by water electrolysis can also be combined with nitrogen to produce ammonia.

[0018] To date, little attention has been paid to the production of ammonia using syngas generated by electrolysis (particularly using SOEC reactors). Recently, the design and analysis of a system for producing “green” ammonia using electricity from renewable energy sources have been described (Applied Energy 192 (2017) 466-476). In this concept, a solid oxide electrolysis (SOE) system for hydrogen production is coupled with a modified Haber-Bosch reactor and includes an air separator to supply pure nitrogen. Zero CO2 emissions for ammonia production are claimed, while reducing power input by 40% compared to equivalent equipment.

[0019] A flexible concept for the synthesis of ammonia from intermittently generated H2 is described (Chem. Ing. Tech. 86 No.5 (2014), 649-657), and it is compared with the widely discussed power-to-gas concept in terms of both technical and economic aspects. The electrolytic synthesis of ammonia in molten salt at atmospheric pressure has been described (J. Am. Chem. Soc. 125 No.2 (2003), 334-335) using a novel electrochemical method that exhibits higher current efficiency and lower temperature than the Haber-Bosch process. In this method, nitride ions (N2) generated by nitrogen reduction at the cathode are used. 3- It is anoly oxidized and reacts with hydrogen to produce ammonia at the anode.

[0020] Frattini et al. (Renewable Energy 99 (2016), 472-482) describe a systems approach for energy assessment of different renewable energy sources integrated into ammonia production plants. Thermochemical simulations were used to investigate the impacts of three different strategies on renewable energy integration and large-scale sustainability in ammonia synthesis. To provide a comprehensive assessment of the benefits of the entire system, plant balancing, the use of other units, and equivalent greenhouse gas emissions were considered.

[0021] Pfromm (J. Renewable Sustainable Energy 9 (2017), 034702) describes and summarizes the latest developments in existing technologies, particularly the renewed interest in ammonia production without fossil feedstocks and potential alternatives to the Haber Bosch process.

[0022] Finally, Wang et al. (AIChE Journal 63 No.5 (2017), 1620-1637) described an ammonia-based energy storage system that utilizes a pressurized reversible solid oxide fuel cell (R-SOFC) for power conversion and is coupled with an external ammonia synthesis and decomposition process and a steam power cycle. Pure oxygen, produced as a byproduct in electrochemical water splitting, is used to drive the fuel cell.

[0023] The invention is described in more detail below with reference to the accompanying drawings, which illustrate the layout of a particular embodiment.

[0024] Example

[0025] Superheated steam (1) at 400°C and 40 barg, generated in the ammonia synthesis loop (2) and the SOEC electrolysis unit consisting of eight identical SOEC stacks (numbered 1-8), is mixed with recycled ammonia synthesis gas (3) (a mixture of hydrogen and nitrogen) preferably at a stoichiometric ratio of 3:1. The mixture is passed through first (A) and second (B) feed / effluent heat exchangers, where heat is exchanged using gases from the cathode (fuel) side and the anode (oxygen) side of the SOEC stack, respectively. Compressed air (4) at 40 barg is then added to the catalytic combustor (not shown), raising the temperature at the inlet of the first SOEC stack to 785°C. The stack operates at 1175 mV per cell, resulting in a temperature drop across the stack to 692°C at the outlet. Compressed air is added in a certain amount to the effluent of the first SOEC stack, resulting in a temperature of 785°C at the inlet of the second SOEC stack. The second stack operated at 1196 mV per cell, resulting in an outlet temperature of 722°C. This air addition was repeated five times between stacks (for a total of eight SOEC stacks).

[0026] After heat exchange with the incoming steam and recirculated gas in the first heat exchanger (A), makeup air is added to make the final gas composition a stoichiometric ammonia synthesis gas and meet the steam requirements of the SOEC unit. Makeup steam in addition to the amount generated in the ammonia synthesis loop is obtained by cooling the gas after the final air addition point.

[0027] Finally, the unconverted vapor is condensed in the condenser (C) and the gas is split into two streams: one stream (3) is recompressed and recycled to the inlet of the SOEC unit, while the other stream (5) is further compressed and dried and then sent to the ammonia synthesis loop.

[0028] To avoid catalyst poisoning in the ammonia synthesis loop, CO2 must be quantitatively removed from the air used. This can be done by known physical or chemical methods for CO2 removal and / or by methanating the CO2 and CO formed in the SOEC unit in a methanation reactor, and then allowing the synthesis gas (5) to enter the synthesis loop.

Claims

1. A method for producing ammonia synthesis gas by electrolysis, the method comprising the steps of: - feeding a mixture of steam and compressed air into an electrolysis unit or into the first of a series of electrolysis units, and - passing the mixture through the outlet of one electrolysis unit to the inlet of the next electrolysis unit, while adding air after each electrolysis unit or only after the last electrolysis unit, wherein the electrolysis units are operated in a heat neutral or endothermic mode and the nitrogen part of the synthesis gas is provided by combusting hydrogen produced by steam electrolysis with air within or between the electrolysis units.

2. The method according to claim 1, wherein air is added only before and after the electrolysis units.

3. The method according to claim 1 or 2, wherein the electrolysis units are a stack of solid oxide electrolysis cells (SOECs).

4. The method according to claim 2, wherein the operating voltage of the stack is below the so-called heat neutral voltage.

5. The method according to claim 3 or 4, wherein the combusting of hydrogen produced by steam electrolysis with air is performed inside the SOEC stack or between separate SOEC stacks.

6. The method according to any one of the preceding claims, wherein the steam used is steam from the ammonia synthesis loop.

7. The method according to claim 6, wherein the steam is mixed with recycled synthesis gas.

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