Expander for SOEC applications

Abstract

In a method for producing various syngases by electrolysis, the method comprises feeding steam and compressed air to a cathode and an anode of an electrolysis unit, respectively, or to a cathode and an anode in a first one of a series of electrolysis units, into the first one of the series of electrolysis units, the electrolysis units operating at elevated air pressure, the oxygen-rich gas exiting the anode is then expanded as low as approximately ambient pressure using a gas expander. The electrolysis unit is preferably a solid oxide electrolytic cell (SOEC) stack.

Description

[0001] This application is a divisional application of the invention patent application with the application date of July 9, 2019, application number: 201980039909.9, and name: "Expander for SOEC Applications". Technical Field

[0002] The present invention relates to electrolysis units, in particular solid oxide electrolysis cell (SOEC) systems, which operate at elevated pressure while producing a synthesis gas comprising hydrogen; carbon monoxide; or a mixture of hydrogen, carbon monoxide and carbon dioxide. More specifically, the present invention relates to the use of an expander in a SOEC system. Background of the Invention

[0004] The synthesis gas produced in the SOEC system may be synthesis gas for the production of, for example, ammonia, methane, methanol or dimethyl ether (DME).

[0005] The basic idea on which the present invention is based is that the SOEC system operates at elevated pressure while producing synthesis gas. The oxygen content at the outlet of the anode side of the SOEC system must be controlled to be below about 50 vol%, which can be achieved by diluting with a stream of compressed air and / or steam. The key to the present invention is to apply an expander to the stream to recover energy by expanding the gas to a pressure close to ambient pressure. This is feasible due to the high operating temperature of the SOEC (or other high temperature electrolyzers, such as proton conducting solid oxide cells).

[0006] For all applications using such synthesis gas, it is advantageous to use the gas under pressure, ie to keep the SOEC system pressurized.

[0007] It is highly beneficial for all SOEC applications if the stack is operated under pressure, as in this case, capital and maintenance intensive as well as energy consuming syngas compressors can be eliminated. Preliminary laboratory tests have shown that power consumption in the stack will remain constant at operating pressures up to 20 barg, as the improved electrode kinetics will offset the thermodynamic disadvantages of increased pressure.

[0008] However, one problem with SOEC technology is that the individual cells in a SOEC system can only withstand a very limited pressure differential (<0.2-0.5 bar). This disadvantage can be overcome by running the oxygen side of the system in dead-end mode, which means there will be no feed flow on the anode side of the cell.

[0009] However, this solution will result in pure oxygen leaving the anode at high operating temperatures of 650-850°C and pressures of up to 40 bar, which will place stringent demands on the materials of construction of the stack and downstream of the stack. In addition, there will be serious safety risks associated with this operating mode.

[0010] The only SOEC system known to date that operates under pressure was manufactured by sunfire GmbH in Dresden and used by HELMETH (which stands for H igh temperature ELectrolysis and METH anation to achieve efficient power-to-gas conversion) project, but details on the air-side operation have not yet been published. Idaho National Laboratory (INL) has published papers dealing with the safety of oxygen handling and recommends a maximum oxygen content of 50% in the exhaust gas. This operating mode is also the mode applied in the Danish biogas upgrading project in Foulum. This is achieved by blowing air into the anode side, thereby diluting the produced oxygen so that the outlet concentration does not exceed 50 vol%. High-pressure steam can also be used for dilution, provided that steam-resistant anodes are used.

[0011] Since the operating mechanism of SOEC is the transfer of oxygen ions through the electrolyte membrane and recombination into molecular oxygen on the anode side, a major or significant portion of the mass flow entering the SOEC stack remains on the anode side in the case of steam or carbon dioxide electrolysis, respectively.

[0012] Therefore, the expander will recover more energy than the investment of compressing the diluted air or generating diluted steam. Summary of the invention

[0013] The present invention therefore relates to a method for producing synthesis gas comprising hydrogen, carbon monoxide, or a mixture of hydrogen, carbon monoxide and carbon dioxide by electrolysis, the method comprising supplying steam and compressed air to the cathode and anode, respectively, of an electrolysis unit or to the cathode and anode of the first of a series of electrolysis units, wherein

[0014] - the electrolysis unit or the series of electrolysis units is operated at elevated gas pressure, and

[0015] - The oxygen-rich gas leaving the anode is then expanded down to approximately ambient pressure using an expander.

[0016] The electrolysis unit is preferably a SOEC stack.

[0017] To date, little attention has been paid to the production of ammonia using syngas produced by electrolysis, particularly using SOEC stacks. Recently, the design and analysis of a system for producing "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 production is combined with a modified Haber-Bosch reactor and includes an air separator to supply pure nitrogen. A typical ammonia plant first converts a desulfurized hydrocarbon gas, such as natural gas (i.e., methane) or LPG (liquefied petroleum gas, such as propane or butane) or naphtha, into gaseous hydrogen by steam reforming. The hydrogen is then combined with nitrogen to produce ammonia by the Haber-Bosch process.

[0018] 3H 2 +N 2 →2NH 3

[0019] Therefore, ammonia (NH 3 ) requires the presence of hydrogen (H 2 ) and nitrogen (N 2 ) is preferably syngas in a molar ratio of about 3:1.

[0020] Ammonia is one of the most widely produced chemicals and is synthesized directly using gaseous hydrogen and nitrogen as reactants without precursors or by-products. In its gaseous state, nitrogen is primarily present as N 2 Hydrogen (H 2 The production of ammonia remains challenging, and for industrial synthesis, it is usually obtained from steam methane reforming (SMR) of natural gas. In addition, when air is used in the reforming process, N 2 , so no air separation unit is required, but a purification process must be carried out to remove oxygen-containing substances, such as O 2 ,CO,CO 2 and H 2 O to prevent poisoning of the catalyst in the ammonia converter. Carbon dioxide is a product of the SMR and can be separated and recycled within the plant. Therefore, hydrogen production is a key process in ammonia synthesis, and sustainable production of ammonia is needed to reduce the consumption of major sources such as natural gas and avoid carbon dioxide emissions in the process.

[0021] The preparation of ammonia synthesis gas by electrolysis has been described in various patents and patent applications. Thus, in US2006 / 0049063 a method for anodic electrochemical synthesis of ammonia is described. The method comprises providing an electrolyte between an anode and a cathode, oxidizing negatively charged nitrogen-containing species and negatively charged hydrogen-containing species present in the electrolyte at the anode to form adsorbed nitrogen species and hydrogen species, respectively, and reacting the adsorbed nitrogen species with the adsorbed hydrogen species to form ammonia.

[0022] In US2012 / 0241328, ammonia is synthesized using electrochemical and non-electrochemical reactions. The electrochemical reaction occurs in an electrolytic cell having a lithium ion conducting membrane, which divides the electrolytic cell into an anolyte chamber and a catholyte chamber, the latter comprising a porous cathode closely connected to the lithium ion conducting membrane.

[0023] WO 2008 / 154257 discloses a method for producing ammonia, the method comprising producing nitrogen from the combustion of a hydrogen stream mixed with air. The hydrogen used to produce nitrogen in the ammonia combustion process can be produced from the electrolysis of water. The hydrogen produced by the electrolysis of water can also be combined with nitrogen to produce ammonia.

[0024] Compared with similar equipment, it is said to achieve zero CO 2 emissions from ammonia production and reduces power input by 40%.

[0025] describes a method for generating H 2 Flexible concepts for the synthesis of ammonia (Chem. Ing. Tech. 86 No. 5 (2014), 649-657) and compared with the widely discussed power-to-gas concept at the technical and economic level. The electrolytic synthesis of ammonia in molten salts at atmospheric pressure has been described (J. Am. Chem. Soc. 125 No. 2 (2003), 334-335), in which a new electrochemical method is used that has a higher current efficiency and lower temperature than that used in the Haber-Bosch process. In this method, nitride ions (N) produced by reducing nitrogen at the cathode are 3- ) is oxidized at the anode and reacts with hydrogen to produce ammonia at the anode.

[0026] US2014 / 0272734 describes a method for producing a gas comprising H by electrolysis using a solid oxide electrolysis cell (SOEC). 2 A method for producing a synthesis gas stream of 1,000 t and CO. The method comprises supplying steam to the cathode and supplying a compressed air stream to the anode without using a gas expander.

[0027] In DE 10 2015 007 732, a method for pressurized electrolysis of water to form an oxygen stream and a hydrogen stream is described. In order to provide an energy-saving process, the oxygen stream is relaxed down to ambient pressure in an expander. A similar method is described in WO 2017 / 118812.

[0028] Frattini et al. (Renewable Energy 99 (2016), 472-482) describe a systematic approach to evaluate the energy of different renewable energy sources integrated in an ammonia production plant. The impact of three different strategies on the sustainability of renewable energy integration and scale-up in the ammonia synthesis process was studied using thermochemical simulations. In order to fully evaluate the benefits of the entire system, the balance of the plant, the use of additional units and the equivalent greenhouse gas emissions have been considered.

[0029] Pfromm (J. Renewable Sustainable Energy 9 (2017), 034702) describes and summarizes the latest state of the art, especially the renewed interest in fossil-free ammonia production and possible alternatives to the Haber-Bosch process.

[0030] Wang et al. (AIChE Journal 63No.5(2017),1620-1637) are concerned with an ammonia-based energy storage system that utilizes a pressurized reversible solid oxide fuel cell (R-SOFC) for power conversion and is used in conjunction 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.

[0031] In a recent patent application, the applicant has disclosed a method for producing synthesis gas for ammonia production by electrolysis, preferably by SOEC stacks. The method avoids any use of air separation units (cryogenic, pressure swing adsorption, etc.) by exploiting the ability to operate in endothermic mode, and it provides the necessary nitrogen by burning hydrogen produced by steam electrolysis of air. In a preferred embodiment using a SOEC stack, the combustion of hydrogen can occur inside the stack or between separate stacks.

[0032] The present invention is described in more detail in the following examples. In this example, reference is made to a schematic diagram illustrating the principle of the present invention. Figure 1 . BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 Schematic diagram showing a method according to the principles of the present invention Example

[0035] This example shows one embodiment of the invention and represents a SOEC plant delivering hydrogen to produce 1 ton of ammonia.

[0036] High pressure steam is introduced from the ammonia synthesis and is also generated within the SOEC unit. The steam is mixed with recycled hydrogen and preheated within the feed / effluent heat exchanger Hex1 on the cathode (fuel) side. It is further preheated to the operating temperature of the SOEC using an electrically heated preheater ph1. In this example, the SOEC is operated in the so-called thermoneutral mode, so the outlet temperature of the stack is equal to the inlet temperature.

[0037] On the cathode side, steam is electrolyzed into hydrogen and oxygen is transported to the anode side through the electrolyte. The hydrogen stream mixed with steam is then passed through the above-mentioned feed / effluent heat exchanger Hex1 and then cooled by generating additional high-pressure steam. Finally, the stream is further cooled and all unconverted steam is condensed out. At this stage, the stream is split into a recycled hydrogen stream and residual steam, the latter of which is sent to the ammonia synthesis.

[0038] On the air side, the air is compressed to 40 barg in compressor C, which is sufficient to obtain 50% (v / v) oxygen at the outlet of the SOEC stack. The air is preheated to 765°C in the feed / effluent heat exchanger Hex2 before entering the electric preheater ph2 to further increase the temperature to 785°C (which is the inlet temperature of the stack). The oxygen-enriched air leaves the stack and recovers heat in the feed / effluent heat exchanger Hex2, and then enters the expander E at a temperature of 424°C. The gas is expanded to a pressure as low as 0.2 barg, thereby reducing the temperature to 91°C.

[0039] Using a polytropic efficiency of 85% and a working loss of 5% for the air compressor, a polytropic efficiency of 78% and a working loss of 4% for the expander, the work used and recovered are 311 kW and 356 kW respectively. It can therefore be seen that more power can be recovered (45 kWh for each tonne of ammonia equivalent synthesis gas produced) than is consumed in compressing the dilution air.

[0040] exist Figure 1 In the UPS, the compressor and expander are connected to different lines. However, they can be connected to a common line, which can achieve higher energy efficiency. It can also reduce pressure fluctuations inside the battery.

Claims

1. A method of producing synthesis gas comprising hydrogen, carbon monoxide, or a mixture of hydrogen, carbon monoxide and carbon dioxide by electrolysis, the method comprising supplying steam and compressed air to the cathode and anode, respectively, of the first of a series of electrolysis units, wherein: - the series of electrolysis cells are operated at elevated pressure and temperature, - subsequently, expanding the oxygen-rich gas leaving the anode down to about ambient pressure using a gas expander, wherein the electrolysis unit is a solid oxide electrolysis cell (SOEC) stack; wherein the SOEC stack is operated in a so-called thermoneutral mode; and The oxygen-rich gas comprises at most 50% (v / v) oxygen.

2. The method according to claim 1, wherein the synthesis gas is selected from the group consisting of methanol synthesis gas, methane synthesis gas, ammonia synthesis gas and dimethyl ether (DME) synthesis gas.

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