RU2836840C1 - Method of operating solid oxide fuel cell (sofc) for combined production of electrical energy and nitrogen (ii) oxide - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: invention relates to electrical engineering, specifically to a method for combined production of electrical energy and nitrogen oxide in a solid oxide fuel cell (SOFC), oxygen-containing gas is fed into cathode chamber while ammonia-containing gas stream is fed into anode chamber to produce electric current and nitrogen oxide. Anode material contains a composition A_1-xA'_xB_1-yB'_yO_3-δ, in which A is selected from a group consisting of La, Y, Sm, Pr, Nd and Gd, A' is selected from a group consisting of Ca, Sr and Ba, B and B' is selected from a group consisting of Co, Fe, Mn, Cr and V or mixtures thereof; x is in range from 0.05 to 0.6 and, in particular, from 0.1 to 0.4, y is in range from 0 to 1 and δ is in range from 0.025 to 0.3. Present invention also discloses a solid oxide fuel cell and a stack of solid oxide fuel cells for implementing the method and a method of producing nitric acid during operation of solid oxide fuel cells.

EFFECT: increased specific power of SOFC.

19 cl, 6 dwg

Description

Field of technology to which the present invention relates

The present invention relates to the field of solid oxide fuel cells for the combined production of electrical energy and nitrogen(II) oxide, as well as for the integration of related applications, such as nitric acid production, water electrolysis and air separation.

Prior art of the present invention

A solid oxide fuel cell (SOFC) is a high-temperature device that is a device for directly converting chemical energy into electrical energy. A solid oxide fuel cell is a highly dense ceramic electrolyte membrane capable of transporting ions, which in most cases are oxygen ions ( ^O2- ), when a partial pressure gradient of the species being transported is applied across the membrane. For a membrane that transports oxygen ions, such a gradient is created when, for example, air on one side of the membrane and a gas with a low oxygen content on the other side of the membrane are in contact with the membrane. In a solid oxide fuel cell, the cause of the low partial pressure of oxygen is the gaseous fuel that is oxidized by the oxygen ions transported across the membrane, as shown schematically in Fig. 1.

On the side of the membrane with high partial pressure of oxygen, oxygen molecules are reduced and converted into oxygen ions, which are introduced into the electrolyte membrane.

O ₂ +4e ^- → 2O ^2-

The oxygen partial pressure gradient across the membrane causes oxygen ions to move into the fuel space through the oxygen ion vacancies in the crystal lattice of the electrolyte membrane. When the oxygen ions reach the surface of the membrane fuel space, they react with the gaseous fuel. A variety of gaseous fuels such as hydrogen, carbon monoxide, methane, and ammonia can be oxidized, and examples of the reactions are shown below:

H ₂ +O ^2- → H ₂ O+2e ^-

CO+O ^2- → CO ₂ +2e ^-

CH ₄ +4O ^2- → 2H ₂ O+CO ₂ +8e ^-

4NH ₃ +3O ^2- → 3H ₂ O+2N ₂ +6e ^-

From the above reactions, it can be seen that the oxidation of the gaseous fuel by the transported oxygen ions results in the release of electrons. In order to achieve the process in which the oxygen and fuel surfaces are not polarized and thus stop the transport of oxygen ions, electrons must be transported from the fuel space to the oxygen space. The electrolyte transports the oxygen ions, but it is an electrical insulator, and thus electrons cannot pass through the electrolyte. However, when suitable electrodes are used on each of the oxygen and fuel surfaces, that is, an anode on the fuel surface and a cathode on the oxygen surface, and there is an electrical connection between them, an electric current passes from the anode to the cathode. The driving force is an electric potential, that is, a voltage, the source of which is the difference in oxygen concentrations across the membrane, according to the formula below, and electric energy is generated according to the formula below:

where E _SOFC (V) is the voltage of the solid oxide fuel cell, E _max is the maximum voltage (V) given by the Nernst equation, i _max is the maximum electric current density (for a given fuel flow), η _f is the fuel utilization factor, and r ₁ and r2 are the ionic and electronic resistivities of the electrolyte, respectively. This equation has been tested and found to be suitable for optimization and sensitivity studies at the plant level of modeling a variety of systems containing solid oxide fuel cells.

In order to obtain a high oxygen flux through the membrane and, consequently, a high electric current, the electrolyte and the electrodes are heated to relatively low temperatures in the range of from about 400°C to about 500°C, or to moderate temperatures in the range of from about 500°C to about 750°C, or to high temperatures in the range of from about 750°C to about 1000°C.

The electrodes in a solid oxide fuel cell must fulfill several functions. First of all, they must exhibit good electrical conductivity under operating conditions, in particular at high temperature and high oxygen partial pressure at the cathode and low oxygen partial pressure at the anode when the solid oxide fuel cell contains an oxygen ion-conducting electrolyte. In addition, they must exhibit good agreement with the selected electrolyte in terms of the coefficient of thermal expansion. In addition, they must not form reaction products with the electrolyte that exhibit insulating or resistive properties. In addition, under operating conditions, they must neither melt nor evaporate to an excessive degree, and they must also provide sufficient stability so that they do not require excessively frequent replacement and so as to reduce operating costs to a minimum. The electrodes must be mixed ionic and electronic conductivity (MIEC) conductors, i.e. they must conduct both oxygen ions in an oxygen ion conducting membrane or protons in a proton electrolyte membrane, and electrons.

As for the anode, the anode must be a good oxidation catalyst.

Solid oxide fuel cells based on electrolytes containing zirconia and ceria typically employ a nickel-based composite anode, with the second phase being yttria-stabilized zirconia (YSZ) or gadolinium-doped ceria (CGO). The metallic nickel provides the electronic conductivity, and the YSZ or CGO provides some ionic conductivity.

Two other components of the flat plate solid oxide fuel cell are interconnected plates and electric current collectors. These components are particularly important in the context of mounting solid oxide fuel cells together to form a stack of solid oxide fuel cells. The interconnected plates and electric current collectors, as well as the anodes, cathodes and electrolytes, are illustrated in the schematic representation of the solid oxide fuel cell shown in Fig. 2. For interconnecting the electrodes, such as the cathodes and anodes, a good match with the selected electrolyte is required in terms of the coefficient of thermal expansion.

The connectors are gas-impermeable plates with electrical conductivity that separate and guide the oxygen and gaseous fuel flows. For high-temperature operation in excess of 900°C, these plates are made of an electrically conductive ceramic material such as La _1-x Sr _x CrO ₃ . The problem that led to the development of electrolytes with high oxygen conductivity and thin-film electrolytes is to ensure operation at temperatures at which metallic connectors can be used. In order to ensure particularly good electrical contact between the electrodes and the connectors, an electric current collector consisting of a metal grid or mesh is most often used.

The solid oxide fuel cell, which uses pure hydrogen as fuel and air as oxidizer during operation, can achieve an efficiency of approximately 60% and can exhibit a high maximum power density of 2 W/ ^cm2 , and can be operated at a temperature of 400°C to 500°C (Minh, “High-Temperature Solid Oxide Fuel Cells for the 21st Century,” 2016).

A number of the above-mentioned basic SOFC units can be installed together to form a stack, as shown schematically in Fig. 3; in this case, a number of units from 50 to 100 is not unusual. A large SOFC system has a modular design, which is made up of numerous individual stacks. Each stack can generate power of up to several kilowatts. Currently, the largest SOFC modules generate power of approximately 300 kW.

As is well known, ammonia is used as a fuel in a solid oxide fuel cell. It has some positive aspects as a fuel. Compared to hydrocarbon fuels, ammonia requires only minor processing such as purification, reforming, or humidification. There is no risk of carbon deposits forming on the anode as is the case with hydrocarbons. When operated in a typical solid oxide fuel cell with nickel-based anodes, the same electrical power output is achieved as when operating a solid oxide fuel cell using hydrogen as a fuel because ammonia undergoes a cracking reaction to form hydrogen and nitrogen at the nickel-based anode before the ammonia is oxidized. Essentially, the standard ammonia solid oxide fuel cell is a hydrogen solid oxide fuel cell due to the cracking of ammonia to form nitrogen and hydrogen (Dekker & Rietveld, Sixth SOFC Forum, 28 June - 02 July 2004, Lucerne (Switzerland)). This cracking reaction is so efficient that the standard ammonia solid oxide fuel cell is considered a low-nitrogen oxide producing power generation system.

If three possible products of ammonia oxidation are considered, namely nitrogen, nitric oxide(I) and nitric oxide(II), according to the following reactions:

_4NH3 + _3O2 → _2N2 + _6Н2О - 1267 kJ/mol

_4NH3 + _4O2 → _2N2O + _6H2O - 1103 kJ/mol

4NH ₃ +5O ₂ →4NO+6H ₂ O - 906 kJ/mol

It can be concluded that nitrogen is the most favorable product thermodynamically, and thus if the objective of the solid oxide fuel cell is to produce maximum power, then nitrogen is the desired product of ammonia oxidation. However, the production of nitric acid requires selective oxidation of ammonia to form nitric oxide (II). This means that thermal cracking of ammonia to form nitrogen and hydrogen when introducing ammonia into the solid oxide fuel cell, as well as catalytic cracking of ammonia at the anodes, must be prevented. As such, the new anode must be designed to oxidize ammonia to form nitric oxide (II) rather than nitrogen or nitric oxide (I).

Prior Art Documents

Document US 4,272,336 (Massachusetts Institute of Technology) discloses methods and devices for producing nitrogen(II) oxide (NO) from ammonia ( _NH3 ) using electrolytic cells and simultaneously directly producing electrical energy. This document describes that typical solid electrolytes that conduct oxygen ions are solid solutions that form oxides containing divalent and trivalent cations, such as _{CaO, Sc2O3 , Y2O3} , _La2O3 or _similar oxides, and oxides containing tetravalent cations, such as _ZrO2 , _ThO2 and _CeO2 . Catalysts containing a noble metal, such as platinum, _are proposed as electrodes. However, document US 4,272,336 does not contain any information regarding the achieved degree of conversion of ammonia into nitrogen(II) oxide. In the work of S.E. Teague "Highly efficient ammonia fuel cell" (Massachusetts Institute of Technology, 1981, p. 39) it is indicated that the achieved degree of conversion of ammonia into nitrogen(II) oxide is approximately 24% with a corresponding specific power of approximately 0.2 mW/cm ² .

A paper by Dekker & Rietveld (Sixth SOFC Forum, 28 June - 02 July 2004, Lucerne (Switzerland), p. 1524) discloses the oxidation of ammonia to nitrogen using a solid oxide fuel cell in which only minor amounts of nitrogen oxides are produced as by-products. Lanthanum strontium manganite (LSM) was used as the cathode but not as the anode; the anode was a nickel-containing Ni/YSZ composite anode.

Document US 20160079623 (Korea Institute of Industrial Technology) discloses methods for producing a solid electrolyte material for solid oxide fuel cells. Hydrogen was used as a fuel. This document describes solid oxide fuel cells comprising a NiO/YSZ anode, a solid electrolyte (YSZ), and a LSM/YSZ cathode. Document US 20160079623 proposes replacing the YSZ electrolyte with a solid electrolyte material with high ion conductivity. In particular, ytterbium and scandium stabilized zirconia (YbScSZ) is proposed as an alternative solid electrolyte material.

Cao et al. (ChemCatChem, 2014, 94, 6, 1190-1194) disclose the use of a highly dense membrane containing lanthanum strontium cobalt ferrite (LSCF) with mixed conductivity for the oxidation of ammonia to form nitric oxide (II). According to the invention of Cao et al., since electrons are transported from the fuel space or membrane to the air space inside, no electrical energy is produced.

Document US 2017/062855 (Kceracell Co. Ltd.) discloses a solid oxide fuel cell comprising a ceria-based electrolyte. The ceria-based electrolyte comprises gadolinium (Gd) or samarium (Sm), and is further doped with ytterbium (Yb) and bismuth (Bi), and exhibits low-temperature sintering properties. The LSCF material is described as a conventional mixed ionic and electronic conductivity (MIEC) cathode material. The ceria-based electrolyte finds application, in particular, as a buffer layer between a zirconia-based electrolyte and a mixed ionic and electronic conductivity electrode. Zirconia-based electrolytes are typically used in combination with mixed ionic and electronic conductivity cathode materials such as LSCF because they do not react with MIEC cathodes, unlike zirconia-based electrolytes. In particular, a cerium dioxide-based electrolyte (gadolinium- or samarium-doped cerium dioxide) containing CeO ₂ and 5 to 10 mol % Gd ₂ O ₃ or Sm ₂ O ₃ has high oxygen ion conductivity and does not react with MIEC cathodes such as LSCF, and as a result, it finds wide application as a material for a buffer layer, which is located between an electrolyte membrane of a zirconium dioxide-based solid oxide fuel cell (ZrO ₂ ) and a cathode layer with mixed ionic and electronic conductivity. In particular, a solid oxide fuel cell comprising a samarium-doped cerium dioxide-based electrolyte is described herein.

Hartley et al. (Catalysis Today, 2000, 55, 197-204) reported the catalytic activity for methane oxidation and susceptibility to deactivation during carbon deposition by comparing the perovskite material La _0.6 Sr _0.4 Co _0.2 Fe _0.8 O ₃ (LSCF-6428) used as anode and cathode with a conventional Ni/YSZ anode material.

In the paper by Jamale et al. (Journal of Materials Science. Materials in Electronics, 2016, 27, 1, 795-799), the fabrication and characterization of a thick film of La _0.6 Sr _0.4 Co _0.2 Fe _0.8 O _3-δ (LSCF)-Ce _0.9 Gd _0.1 O _1.95 (GDC) composite material for deposition on solid oxide fuel cell anodes are disclosed.

Lai et al. (Journal of Power Sources, 2011, 196, 4, 1826-1832) reported nanostructured La _0.6 Sr _0.4 Co _0.8 Fe _0.2 O ₃ /Y _0.08 Zr _0.92 O _1.96 /La _0.6 Sr _0.4 Co _0.8 Fe _0.2 O ₃ (LSCF/YSZ/LSCF) symmetric thin film solid oxide fuel cells.

Xu et al (Journal of Materials Science & Technology, 2017, 33, 11, 1329-1333) reported the fabrication and optimization of the La electrode._0.4Sr_0.6Co_0.2Fe_0.7Nb_0.1O_3-δ For symmetrical solid oxide fuel cell containing a zirconium dioxide-based electrolyte.

Lu et al. (Electrochimica Acta, 2019, ₃₂₃ , 134857) reported an efficient and stable _{symmetrical La0.6Sr0.4Co0.2Fe0.7Mo0.1O3 -δ} electrode for direct hydrocarbon _{solid oxide} fuel cells.

Document JP 2017082670 A discloses an internal combustion engine, including an engine operated by burning hydrocarbon fuel and comprising a combustion device for burning combustible substances in an exhaust gas in an exhaust gas flow path through which the exhaust gas produced by the engine passes, and for oxidizing nitrogen to remove nitrogen oxides.

Document US2019/284052 discloses a method for producing nitric acid, which provides for synthesizing ammonia by catalytically converting a make-up gas containing hydrogen and nitrogen in an ammonia synthesis loop, wherein the off-gas is used as a source of nitrogen to produce the aforementioned make-up gas.

Document WO 2020/035521 A1 discloses a new concept of a method and system for producing nitric acid with high energy and material efficiency, according to which, in particular, the method and system for producing nitric acid are integrated with a method and system for producing ammonia and are designed to use ammonia to produce a large amount of energy, which is consumed, in particular, in the form of electrical energy, while maintaining a high degree of recovery of nitric acid in the process of converting ammonia into nitric acid. This document does not disclose a solid oxide fuel cell (SOFC).

Brief description of the figures

Fig. 1 shows a schematic illustration of a solid oxide fuel cell.

Fig. 2 shows a SOFC unit that can be reproduced to form a solid oxide fuel cell stack.

Fig. 3 shows a stack of solid oxide fuel cells.

Fig. 4 is a schematic illustration of a method according to an embodiment of the present invention, in which steam is introduced into the cathode space.

Fig. 5 is a schematic illustration of a method according to an embodiment of the present invention, in which, alternatively, the solid oxide fuel cell is operated in an oxygen consumption mode (SOFC mode) using air and in a water electrolysis mode as a solid oxide electrolytic cell (SOEC) using steam.

Fig. 6 shows the nitrogen(II) oxide yield as a function of specific power for different anode materials.

Brief summary of the present invention

According to a first aspect of the present invention, a method of operating a solid oxide fuel cell for the combined production of electrical energy and significant amounts of nitrogen oxide (II) (NO) is disclosed. The method comprises the following steps:

(a) providing a solid oxide fuel cell (SOFC) comprising an anode space comprising a solid gas-permeable anode, a gas inlet and a gas outlet; a cathode space comprising a solid gas-permeable cathode and a gas inlet and a gas outlet; and a maximally dense solid electrolyte separating the cathode space and the anode space;

b) introducing an oxygen-containing gas into the inlet of the cathode space of the solid oxide fuel cell;

c) introducing an ammonia-containing gas stream into the inlet of the anode space of the solid oxide fuel cell;

d) collecting nitrogen(II) oxide at the outlet of the anode space; and

e) collect the electric current passing between the anode space and the cathode space;

wherein the method is carried out at a temperature in the range from 500 to 800°C, and

wherein the anode comprises a composition A _1-x A' _x B _{1-y B} ' _y O _3-δ , wherein A is selected from the group consisting of La, Y, Sm, Pr, Nd and Gd; A' is selected from the group consisting of Ca, Sr and Ba; B and B' are in each case independently selected from the group consisting of Co, Fe, Mn, Cr and V or mixtures thereof; x is in the range from 0.05 to 0.6 and, in particular, from 0.1 to 0.4; y is in the range from 0 to 1; and δ is in the range from 0.025 to 0.3, and wherein the cathode comprises a composition A _1-x A' _x B _1-y B' _y O _3-δ , wherein A is selected from the group consisting of La, Y, Sm, Pr, Nd and Gd; A' is selected from the group consisting of Ca, Sr and Ba; B and B' are in each case independently selected from the group consisting of Co, Fe, Mn, Cr and V or mixtures thereof; x is in the range from 0.05 to 0.6 and, in particular, from 0.1 to 0.4; y is in the range from 0 to 1; and δ is in the range from 0.025 to 0.3.

According to a second aspect of the present invention, a solid oxide fuel cell for the combined production of electrical energy and significant amounts of nitrogen oxide (II) (NO) is disclosed. This solid oxide fuel cell comprises:

- an anode space containing a solid gas-permeable anode, a gas inlet and a gas outlet;

- a source of gaseous ammonia, which is in communication with the possibility of transferring the fluid medium or is connected to the gas inlet in the anode space;

- a cathode space containing a solid gas-permeable cathode, a gas inlet and a gas outlet;

- a maximally dense electrolyte separating the anode space and the cathode space, in particular containing a composition selected from the following:

(a) Ce _1-x Gd _x O _2-δ , where x is in the range from 0.1 to 0.2, and where 5 is in the range from 0.05 to 0.1, and

(b) Zr _1-(x+y) Sc _x M _y O _2-δ , where M is Al, Yb, Ce, where x is in the range from 0.03 to 0.2, where y is in the range from 0.001 to 0.01, and where 5 is in the range from 0.01 to 0.06;

- a device for heating a solid oxide fuel element to a temperature in the range of 550 to 800°C; and

- a device for collecting electric current passing between the anode space and the cathode space.

The anode comprises a composition A _1-x A' _x B _{1-y B} ' _y O _3-δ , wherein A is selected from the group consisting of La, Y, Sm, Pr, Nd and Gd; A' is selected from the group consisting of Ca, Sr and Ba; B and B' are in each case independently selected from the group consisting of Co, Fe, Mn, Cr and V or mixtures thereof; x is in the range from 0.05 to 0.6 and, in particular, from 0.1 to 0.4; y is in the range from 0 to 1; and δ is in the range from 0.025 to 0.3. The cathode comprises a composition A _1-x A' _x B _1-y B' _y O _3-δ , wherein A is selected from the group consisting of La, Y, Sm, Pr, Nd and Gd; A' is selected from the group consisting of Ca, Sr and Ba; B and B' are in each case independently selected from the group consisting of Co, Fe, Mn, Cr and V or mixtures thereof; x is in the range from 0.05 to 0.6 and, in particular, from 0.1 to 0.4; y is in the range from 0 to 1; and δ is in the range from 0.025 to 0.3.

According to a third aspect of the present invention, a solid oxide fuel cell stack is disclosed comprising at least two or more solid oxide fuel cells according to the present invention.

According to a fourth aspect of the present invention, a system for producing nitric acid is disclosed. This system comprises:

- a Haber-Bosch unit containing an outlet for the delivery of ammonia produced by the unit;

- a device for heating ammonia produced by a Haber-Bosch plant to a temperature in the range of 500 to 800°C, so that the ammonia is collected in a gaseous state at the outlet of the heating device;

- a stack of solid oxide fuel cells according to the present invention, in which the inlets of the anode spaces of the cells are in fluid communication with the outlet of the heating device;

- a device for transferring nitrogen oxide (II) (NO), entering at the outlets of the anode spaces of the stack elements, to the oxidation section of the nitric acid production plant, into which oxygen-containing gas enters or which contains an ammonia oxidation catalyst, and in which nitrogen oxide (II) is oxidized to gaseous nitrogen oxides, which are, in particular, nitrogen dioxide and dinitrogen tetroxide; and

- a device for transferring gaseous nitrogen oxides to a nitric acid absorption column in which the gaseous nitrogen oxides are absorbed by an aqueous solution, resulting in the production of nitric acid.

According to a fifth aspect of the present invention, a method for producing nitric acid is disclosed. This method comprises successive stages in which:

a) produce ammonia in a Haber-Bosch plant;

b) collect the produced ammonia at the outlet of the Haber-Bosch unit;

c) heating the ammonia produced by the Haber-Bosch unit to a temperature in the range of 500 to 800°C in such a way that the ammonia is collected in a gaseous state at the outlet of the heating device;

d) transferring ammonia into the inlets of the anode spaces of the elements of the solid oxide fuel cell stack according to the present invention, in particular into the inlets of the anode spaces of the elements of the solid oxide fuel cell stack according to the present invention;

e) producing nitrogen(II) oxide using the method according to the present invention for the combined production of electrical energy and nitrogen(II) oxide;

f) collecting the produced nitrogen(II) oxide at the outlet of the solid oxide fuel cell according to the present invention, in particular at the outlets of the anode spaces of the stack elements;

g) transferring the produced nitrogen(II) oxide to the oxidation section of a nitric acid production plant, which is fed with oxygen-containing gas or which contains an ammonia oxidation catalyst;

h) oxidize nitrogen(II) oxide to produce gaseous nitrogen oxides, which are, in particular, nitrogen dioxide and dinitrogen tetroxide;

i) transfer gaseous nitrogen oxides to the absorption section of the nitric acid production plant; and

j) absorb gaseous nitrogen oxides in an aqueous solution to produce nitric acid.

According to a sixth aspect of the present invention, there is disclosed the use of a solid oxide fuel cell according to the present invention or a stack of solid oxide fuel cells according to the present invention for the combined production of electrical energy and significant amounts of nitrogen oxide (II) (NO).

According to a seventh aspect of the present invention, there is disclosed the use of an anode comprising a composition A _1-x A' _x B _{1-y B} ' _{y O 3-δ} , wherein A is selected from the group consisting of La, Y, Sm, Pr, Nd and Gd; A' is selected from the group consisting of Ca, Sr and Ba; B and B' are in each case independently selected from the group consisting of Co, Fe, Mn, Cr and V or mixtures thereof; x is in the range from 0.05 to 0.6 and, in particular, from 0.1 to 0.4; y is in the range from 0 to 1; and δ is in the range of 0.025 to 0.3, in the solid oxide fuel cell according to the present invention, in the solid oxide fuel cell stack according to the present invention, in the nitric acid production system according to the present invention, in the method for combined production of electric power and nitric oxide (II) according to the present invention, or in the method for producing nitric acid according to the present invention.

Detailed disclosure of the present invention

Throughout the description and claims presented herein, the words "include" and variations thereof mean "including, but not limited to," and are not intended to (and do not) exclude other parts, additives, components, integers, or steps. Throughout the description and claims presented herein, the singular forms may include the plural unless the context otherwise requires. In particular, when the indefinite article "a" or "the" is used in the description of the present invention, it should be understood that both the plural and the singular may be intended unless the context otherwise requires.

Features, integers, characteristics, chemical compounds, moieties or groups that are described in connection with a particular aspect, embodiment or example of the present invention shall be understood as applicable to any other aspect, embodiment or example that are described herein, unless there is incompatibility. All of the features that are disclosed in the description of the present invention (including in any of the accompanying claims, in the abstract and in the figures) and/or all of the steps of any method or process described in this manner may be combined in any combination, except for combinations in which at least some of such features and/or steps are mutually exclusive. The present invention is not limited by the detailed description of any of the embodiments presented above. The present invention extends to any new feature or any new combination of features that are disclosed in the description of the present invention (including any accompanying claims, abstract and figures), or to any new step or any new combination of steps of any method or process that are so described.

The presentation of numerical values by means of value ranges applies to all integer and fractional values within the stated ranges, as well as to the stated end values. The terms "ranges from ... to ..." or "from ... to ..." or "between ... and ...", which are used to describe the range of a measurand such as a parameter, quantity, period of time, etc., are intended to include the corresponding end values of the described range.

According to the definition in this document, the aqueous solution is selected from the group consisting of water, an aqueous solution containing up to 0.5 wt. % nitric acid, an aqueous solution containing up to 1 wt. % ammonium nitrate, or combinations thereof.

As defined herein, air is a mixture of gases containing approximately 78 vol.% nitrogen and approximately 21 vol.% oxygen, and additionally containing approximately 0.9 vol.% argon, approximately 0.04 vol.% carbon dioxide, and other gases in small quantities. Air typically also contains some water vapor.

According to a first aspect of the present invention, a method of operating a solid oxide fuel cell for the combined production of electrical energy and significant amounts of nitrogen oxide (II) (NO) is disclosed. This method includes the following steps: (a) providing a solid oxide fuel cell (SOFC) comprising an anode space comprising a solid gas-permeable anode, a gas inlet and a gas outlet; a cathode space comprising a solid gas-permeable cathode and a gas inlet and a gas outlet; and a maximally dense solid electrolyte separating the cathode space and the anode space; (b) introducing an oxygen-containing gas into the inlet of the cathode space of the solid oxide fuel cell; (c) introducing an ammonia-containing gas stream into the inlet of the anode space of the solid oxide fuel cell; (d) removing a gas stream containing nitrogen(II) oxide in significant quantities at the outlet of the anode space, or collecting nitrogen(II) oxide, in particular a gas stream containing nitrogen(II) oxide, at the outlet of the anode space; and (e) collecting the electric current passing between the anode space and the cathode space. The advantage of using ammonia as a fuel is that it is a clean fuel: unlike other fuels, such as hydrocarbon fuels, no formation of carbon deposits is observed during the oxidation of the fuel and no carbon dioxide is produced, which limits greenhouse gas emissions from the process. Potential ammonia-containing gases are, but are not limited to, pure ammonia gas, or a mixture of ammonia gas and steam, or a mixture of ammonia gas and an inert gas such as nitrogen or argon, or a mixture of ammonia gas and several inert gases, or a mixture of ammonia gas, steam and one or more inert gases. It should be understood that, depending on the composition of the ammonia-containing gas, together with nitrogen(II) oxide, steam and/or other inert gases, as well as ammonia gas that is not converted to nitrogen(II) oxide, can be collected at the outlet of the anode space.

The method is carried out at a temperature in the range from 500 to 800°C. The cathode comprises a material suitable for reducing oxygen. The anode comprises a composition A _1-x A' _x B _{1-y B} ' _y O _3-δ , in which A is selected from the group consisting of La, Y, Sm, Pr, Nd and Gd; A' is selected from the group consisting of Ca, Sr and Ba; B and B' are independently selected in each case from the group consisting of Co, Fe, Mn, Cr and V or mixtures thereof, the first variable x is in the range from 0.05 to 0.6 and, in particular, from 0.1 to 0.4, the first variable y is in the range from 0 to 1 and the first variable 5 is in the range from 0.025 to 0.3. In particular, A is La.

Surprisingly, the inventors of the present invention have found that a conversion rate of ammonia to nitrogen(II) oxide of at least 80% can be achieved using the method of the present invention. In addition, the method of the present invention allows the solid oxide fuel cell to be operated at a low electrical resistance of 1 ohm/cm ² , which means that oxygen ions are efficiently transported from the cathode to the anode, and ammonia is efficiently converted to nitrogen(II) oxide. When the electrical resistance is high and is in the range of 10 to 15 ohm/cm ² , it becomes possible to introduce additional energy to increase the transport of oxygen ions from the cathode to the anode to achieve a conversion rate of ammonia to nitrogen(II) oxide of at least 80%. Accordingly, a person skilled in the art may not necessarily introduce additional energy into the cell.

The anode comprises a perovskite composition of the general formula A _1-x A' _x B _1-y B' _y O _3-δ . A and A' are large cations. A is selected from the group consisting of La, Y, Sm, Pr, Nd and Gd, and A' is selected from the group consisting of La, Ca, Sr or Ba. In particular, A is La, and the stability of the anode increases when the reduction potential of the perovskite composition contained in the anode decreases, and as a result of this, the transport of oxygen ions through the composition is accelerated, and, consequently, the degree of conversion of ammonia into nitrogen oxide (II) increases. B and B' are smaller cations. B and B' are independently selected in each case from the group consisting of Co, Fe, Mn, Cr and V or mixtures thereof.

The oxidation state of the cations A, A', B and B' affects the electronic structure of the perovskite and thus the ionic and electronic properties of the solid oxide fuel cell used in the method according to the present invention. In order to maintain charge neutrality, a decrease in the oxidation state of A' can lead, for example, to an increase in the oxidation state of B or to a decrease in the number of oxygen ion vacancies in the composition, as a result of which the ionic conductivity decreases.

It has been found that with an increase in the value of the first variable x in the perovskite structure described above, the concentration of oxygen vacancies increases, as a result of which the mobility of oxygen ions can increase; this increase in ionic conductivity is a positive effect that should be taken into account when considering the perovskite composition. However, with an increase in the value of x, the stability of the perovskite composition with respect to reduction also decreases. Thus, x is in the range from 0.05 to 0.6 and, in particular, from 0.1 to 0.4 in order to achieve optimal ionic and electrical conductivity, but at the same time sufficient stability of the perovskite composition.

B and B' are each independently selected from the group consisting of Co, Fe, Mn, Cr and V or mixtures thereof. It has been found that by increasing the amount of Co and Fe, the ionic conductivity increases, but the stability with respect to reduction of the perovskite composition also decreases. By partially replacing Co and/or Fe and introducing Cr, Mn, V or a combination thereof, the stability of the perovskite composition increases. By increasing the content of Co and/or Mn and/or Cr, the electrical conductivity can increase.

Ionic conductivity increases with increasing oxygen partial pressure gradient between the anode and cathode. Therefore, the higher the oxygen concentration in the oxygen-containing gas introduced into the cathode inlet, the higher the electrical conductivity.

In particular, the cathode comprises a composition A _1-x A' _x B _{1-y B} ' _y O _3-δ , in which A is selected from the group consisting of La, Y, Sm, Pr, Nd and Gd; A' is selected from the group consisting of Ca, Sr and Ba; B and B' are in each case independently selected from the group consisting of Co, Fe, Mn, Cr and V or mixtures thereof; x is in the range from 0.05 to 0.6 and, in particular, from 0.1 to 0.4; y is in the range from 0 to 1; and δ is in the range from 0.025 to 0.3. In particular, A' represents Sr and Ca in order to weaken the reaction of the cathode with an oxygen-containing gas introduced into the inlet of the cathode space.

In a method according to one embodiment of the present invention, nitrogen(II) oxide constitutes at least 10% and preferably from 10 to 80% of the mass of the product or gas stream that is collected or removed at the outlet of the anode space.

According to one embodiment of the present invention, the method is carried out at a temperature that is in the range of 700 to 750°C.

According to one embodiment of the present invention, in the method for the combined production of electrical energy and nitrogen(II) oxide, the anode and cathode comprise a composition of La _0.6 oSr _0.40 Co _0.20 Fe _0.80 O ₃ . It was found that by using this particular perovskite composition, a conversion rate of ammonia to nitrogen(II) oxide of 76% was achieved, compared to a conversion rate of only 5%, which is achieved using a platinum anode and cathode of the prior art. In addition, at a temperature of 750 ° C, a specific power of 80 mW / cm ² can be produced. In addition, by using the same perovskite compositions for the anode and cathode, operating costs are reduced due to a decrease in the number of chemical elements in the fuel cell, and as a result, a simplification of the fuel cell occurs.

According to one embodiment of the present invention, in the method for the combined production of electrical energy and nitrogen(II) oxide, the oxygen-containing gas is air, steam, oxygen or a mixture of nitrogen and oxygen. By introducing air or a mixture of nitrogen and oxygen, as a result of reduction in the cathode space, a gas with a reduced oxygen content is obtained, which can be sent to an air separation unit, where the air is separated into its main components, which are nitrogen and oxygen; and since the oxygen content of the gas that enters the air separation unit is reduced, the work that the air separation unit must perform to separate gaseous oxygen and gaseous nitrogen is reduced.

According to one embodiment of the present invention, the method for the combined production of electrical energy and nitrogen(II) oxide is carried out at a temperature in the range of 500 to 750°C, and the electrolyte is an electrolyte based on cerium dioxide. The advantage of operating at such moderate temperatures is that the stability of the perovskite composition increases, and therefore the costs associated with the maintenance of the anode are reduced. There are a number of advantages to operating the cell at a low temperature in the range of 350 to 500°C or at a moderate temperature in the range of 500 to 750°C. The first advantage is that less expensive materials can be used, rather than special alloys such as stainless steel. In addition, the stability of the electrodes is increased. In addition, with a decrease in temperature, the thermal expansion of the electrolyte decreases, and the formation of resistive layers at the interface between the electrolyte and the electrodes and the electrodes and connecting devices is reduced. Finally, the low-temperature range operation allows the use of a low-cost electrolyte based on cerium dioxide. In particular, the electrolyte composition is Ce _1-x Gd _x O _2-δ , wherein the first variable x is in the range from 0.1 to 0.2 and wherein the second variable δ is in the range from 0.05 to 0.1. This cerium dioxide electrolyte turns out to be particularly suitable in the case of a cell that is operated at a temperature in the range from 500 to 750 °C, since with increasing temperature some electrical conductivity may occur, which reduces the efficiency of the cell.

According to one embodiment of the present invention, the method for the combined production of electrical energy and nitrogen(II) oxide is carried out at a temperature in the range of 750 to 800°C, and the electrolyte is a zirconium dioxide-based electrolyte. As the temperature increases, the conductivity of oxygen ions in the electrolyte increases, and, consequently, the electrical resistance of the electrode decreases, resulting in the production of additional energy.

In particular, the electrolyte composition is Zr _1-(x+y) Sc _x M _y O _2-δ , where M is Al, Yb, Ce, the first variable x is in the range from 0.03 to 0.2, where the second variable y is in the range from 0.001 to 0.01, and wherein the third variable δ is in the range from 0.01 to 0.06. Such an electrolyte ensures satisfactory transport of oxygen ions and is particularly suitable at elevated temperatures ranging from 750 to 800 °C. The auxiliary alloying substance M is added to suppress the transition of the cubic phase to the rhombohedral phase, which occurs at a temperature of 750 to 900 °C and causes mechanical instability and a decrease in ionic conductivity. In particular, the electrolyte is an independent membrane with a thickness of 50 to 200 μm on the cathode or anode. Since the oxygen transport through the electrolyte is directly related to the oxygen potential gradient, the oxygen flow increases with decreasing membrane thickness. In particular, the independent electrolyte film has a thickness of 100 to 150 μm and is applied to the cathode or anode at a thickness of approximately 20 to 50 μm. In particular, the thin membrane electrolyte is applied to the cathode.

Alternatively, the thickness of the anode or cathode is more than 500 μm, and thus the corresponding electrode can act as a physical carrier of a thin-film electrolyte whose thickness is in the range of 5 to 30 μm. The rare element scandium (Sc) has a high cost. However, at a temperature of 750 °C and above, such a self-contained thin film, which is a scandium-doped electrolyte, provides satisfactory oxygen ion transport at a moderate cost.

According to one embodiment of the present invention, in the method for the combined production of electrical energy and nitrogen oxide (II), the steam is an oxygen-containing gas, which is introduced into the inlet of the cathode space in step (a), and hydrogen gas is collected at the outlet of the cathode. As a result of the formation of hydrogen at the cathode, a usable fuel is produced, which can subsequently be used, for example, in the production of ammonia in a Haber-Bosch plant. In fact, as shown in Fig. 4, as a result of the production of hydrogen, additional ammonia can be produced (for example, using a Haber-Bosch plant), which can be sent as fuel to the anode space of the cell. Thus, this method provides for the combined production of nitrogen oxide (II) and hydrogen gas, which are necessary for the production of nitric acid and ammonia, respectively.

In addition, both the use of steam as an oxygen-containing gas and the production of hydrogen gas are functions that are carried out in the process of water electrolysis using a water electrolyzer. This means that the method according to the present invention additionally provides water electrolysis, and that the system used in this method can be used not only as a solid oxide fuel cell, but also as a water electrolyzer using steam as an oxygen-containing gas. In addition, the respective use of steam and ammonia as a feed gas at the inlets of the cathode and anode spaces of the solid oxide fuel cell makes it possible to produce hydrogen using less energy than in a traditional electrolyzer: the oxygen gradient created by introducing the gas into the inlet of the cathode space provides a higher oxygen content compared to the gas that is introduced into the inlet of the anode space, and as a result of this, a natural flow of oxygen ions from the cathode to the anode is created, which ensures the production of hydrogen, nitrogen oxide (II) and electrical energy.

In particular, when steam is used as the oxygen-containing gas, it is recommended to apply a voltage in the range of 0.5 to 0.6 V to the cell in order to facilitate the flow of oxygen ions from the cathode and thereby promote the production of nitrogen(II) oxide in the anode space.

In particular, the steam is an oxygen-containing gas introduced into the inlet of the cathode space in step (a), hydrogen gas is collected at the outlet of the cathode, the oxygen-containing gas introduced into the inlet of the cathode space is an alternative of air and steam, and the gas that is collected at the outlet of the anode space is an alternative of oxygen-depleted air and hydrogen, respectively.

Let us consider Fig. 5. As mentioned above, the method according to the present invention additionally provides water electrolysis, and the system used in this method can be used not only as a solid oxide fuel cell but also as a water electrolyzer using steam as the oxygen-containing gas. In the case of a reversible installation, the same installation can be switched between the fuel cell mode and the electrolyzer mode by simply measuring the air with steam as the starting material, which is supplied to the cathode. Such switching is possible since the cathode used is able to regulate both the conditions of intense oxidation when introducing air and the conditions of intense reduction in the production of hydrogen gas. The main problem associated with the electrolysis of water for the production of hydrogen gas without the formation of carbon dioxide is the dynamic load and the possibility of switching modes, the so-called flexibility, which means that the installation must be started and stopped according to the energy input, i.e. environmentally friendly renewable energy produced from solar or wind power plants. Due to the limited flexibility of the plant, energy storage is required, for example using batteries or hydrogen accumulators, which are the most realistic options, but which are very expensive. In addition, the reduction in the use of the electrolyzer makes the conventional concept very expensive and reduces the service life of the electrolyzer, in particular when it is operated at a temperature of 750 to 900 ° C. Now, with the use of the method according to the invention, only ammonia and steam storage are required in order to simultaneously produce nitrogen(II) oxide and hydrogen; both ammonia and steam are relatively inexpensive and a constant amount of nitrogen(II) oxide is produced, regardless of the variability of the energy input. In addition, in the method according to the invention, due to the continuous production in the presence of ammonia storage, the problems of destruction associated with the high temperature at which water electrolyzers are operated in a batch mode are overcome: with full use of the equipment of the plant, the operating costs are reduced.

In particular, the air is an oxygen-containing gas introduced into the inlet of the cathode space in step (a), and as a result of this, oxygen-depleted air is produced at the outlet of the cathode, and the oxygen-depleted air produced at the outlet of the cathode is sent to the air separation unit. In particular, the mixture containing nitrogen gas and oxygen gas is an oxygen-containing gas introduced into the inlet of the cathode space in step (a), and as a result, a mixture containing nitrogen gas and oxygen gas is produced at the outlet of the cathode, and the content of oxygen gas is reduced compared to the mixture of gases introduced into the inlet of the cathode. This mixture containing nitrogen gas and oxygen gas, which is collected at the outlet of the cathode, is sent to the air separation unit.

As mentioned above, by introducing air after reduction in the cathode space, a gas with reduced oxygen content is obtained, which can be introduced into the air separation plant; when the oxygen content of the gas introduced into the air separation plant is reduced, the air separation plant must perform less work to separate gaseous oxygen and gaseous nitrogen. In addition, by processing air with reduced oxygen content in the air separation plant, a larger amount of gaseous nitrogen is produced for the Haber-Bosch plant.

According to a second aspect of the present invention, a solid oxide fuel cell for the combined production of electrical energy and significant amounts of nitrogen oxide (II) (NO) is disclosed. This solid oxide fuel cell comprises an anode space comprising a solid gas-permeable anode, a gas inlet and a gas outlet; a source of gaseous ammonia in fluid communication with or connected to the gas inlet of the anode space; a cathode space comprising a solid gas-permeable cathode, a gas inlet and a gas outlet; a maximally dense electrolyte separating the anode space and the cathode space, in particular comprising a composition selected from the following: (i) Ce _1-x Gd _x O _2-δ , wherein x is in the range from 0.1 to 0.2, and wherein δ is in the range from 0.05 to 0.1, and (ii) Zr _1-(x+y) Sc _x M _y O _2-δ , wherein M is Al, Yb, Ce, wherein x is in the range from 0.03 to 0.2, wherein y is in the range from 0.001 to 0.01, and wherein δ is in the range from 0.01 to 0.06; as well as a device for heating the solid oxide fuel cell to a temperature in the range from 500 to 800°C; and a device for collecting the electric current passing between the anode space and the cathode space.

The cathode comprises a material suitable for reducing oxygen in an oxygen-containing gas, and the anode comprises a composition A _1-x A' _x B _1-y B' _y O _3-δ , in which A is selected from the group consisting of La, Y, Sm, Pr, Nd and Gd; A' is selected from the group consisting of Ca, Sr and Ba; B and B' are in each case independently selected from the group consisting of Co, Fe, Mn, Cr and V or mixtures thereof; x is in the range from 0.05 to 0.6 and, in particular, from 0.1 to 0.4; y is in the range from 0 to 1; and δ is in the range from 0.025 to 0.3.

In particular, the cathode comprises a composition A _1-x A' _x B _1-y B' _y O _3-δ , in which A is selected from the group consisting of La, Y, Sm, Pr, Nd and Gd; A' is selected from the group consisting of Ca, Sr and Ba; B and B' are in each case independently selected from the group consisting of Co, Fe, Mn, Cr and V or mixtures thereof; x is in the range from 0.05 to 0.6 and, in particular, from 0.1 to 0.4; y is in the range from 0 to 1; and δ is in the range from 0.25 to 0.3.

According to one embodiment of the present invention, in a solid oxide fuel cell, the anode and cathode comprise a composition of La _0.60 Sr _0.40 Co _0.20 Fe _0.80 O ₃ .

In particular, the electrolyte composition is Ce _1-x Gd _x O _2-δ , wherein x is in the range from 0.1 to 0.2, and wherein δ is in the range from 0.05 to 0.1, and the device for heating the element allows heating to a temperature in the range from 500 to 750°C.

In particular, the electrolyte is Zr _1-(x+y) Sc _x M _y O _2-δ , wherein M is Al, Yb, Ce, wherein x is in the range from 0.03 to 0.2, wherein y is in the range from 0.001 to 0.01, and wherein δ is in the range from 0.01 to 0.06, and the device for heating the element allows heating to a temperature in the range from 750 to 800°C.

Suitable sources of gaseous ammonia are known in the art. For example, any ammonia container or ammonia tank can be used.

According to one embodiment of the present invention, the solid oxide fuel cell further comprises a means for reacting nitrogen(II) oxide in fluid communication with the outlet of the anode space.

According to one embodiment of the present invention, the solid oxide fuel cell further comprises a device for reacting nitric oxide (II) in fluid communication connected to the outlet of the anode space, and the device for reacting nitric oxide (II) is an oxidation section of a nitric acid production plant that receives an oxygen-containing gas or that contains an ammonia oxidation catalyst and in which nitric oxide (II) is oxidized to gaseous nitrogen oxides, which are, in particular, nitrogen dioxide and dinitrogen tetroxide.

According to one embodiment of the present invention, the solid oxide fuel cell further comprises an oxidation section of the nitric acid production unit, into which an oxygen-containing gas is supplied or which contains an ammonia oxidation catalyst, and in which nitrogen(II) oxide is oxidized to gaseous nitrogen oxides, which are, in particular, nitrogen dioxide and dinitrogen tetroxide, wherein this section is in fluid communication with the outlet of the anode space, and a nitric acid absorption column is also present, in which the gaseous nitrogen oxides produced in the oxidation section are absorbed by an aqueous solution. Thus, with the use of the solid oxide fuel cell according to the present invention, it is possible to produce nitric acid without using an ammonia oxidation burner, which is present in traditional nitric acid production units.

According to one embodiment of the present invention, in a solid oxide fuel cell, the cathode outlet is in fluid communication or is connected to an air separation unit.

According to a third aspect of the present invention, a solid oxide fuel cell stack is disclosed, comprising at least two or more elements according to the present invention. In particular, in addition to combining solid oxide fuel cells to obtain a solid oxide fuel cell stack, a person skilled in the art can directly combine solid oxide fuel cell stacks in such a way as to ensure the production of nitrogen(II) oxide and electrical energy in the required quantities.

The package may contain, for example, 50 elements. It may be sealed by melting a glass-ceramic seal or welding metal-reinforced elements. Once sealed, the package cannot be opened or repaired if one element fails. In order to prevent problems such as high electrical resistance of the entire package due to delamination or coking of the electrode in only one element, it is necessary, as described above, to ensure sufficient stability of the components of the installation elements, in particular the electrodes, which in the plural form the package.

According to a fourth aspect of the present invention, a system for producing nitric acid is disclosed. The system comprises a Haber-Bosch unit with an outlet for collecting ammonia produced by the unit; a device for heating the ammonia produced by the Haber-Bosch unit to a temperature in the range of 500 to 800°C, so that the ammonia is collected in a gaseous state at the outlet of the heating device; a solid oxide fuel cell according to the present invention, in particular a stack of solid oxide fuel cells according to the present invention, in which the inlets of the anode spaces of the cells are in fluid communication or are connected to the outlet of the ammonia heating device; a device for transferring nitrogen oxide (II) collected at the outlets of the anode spaces of the stack elements to an oxidation section of a nitric acid production plant, into which an oxygen-containing gas is supplied or which contains an ammonia oxidation catalyst, and in which nitrogen oxide (II) (NO) is oxidized to gaseous nitrogen oxides, which are, in particular, nitrogen dioxide and dinitrogen tetroxide; and a device for transferring gaseous nitrogen oxides to a nitric acid absorption column, in which gaseous nitrogen oxides are absorbed by an aqueous solution, and as a result of this nitric acid is produced. In particular, the gaseous ammonia produced by the Haber-Bosch plant is heated using a heat exchange system in which heat is provided by exothermic reactions occurring in the ammonia production system to which the Haber-Bosch plant belongs, either in the oxidation section or as a result of the absorption of gaseous nitrogen oxides in the absorption column. Thus, the system is energy efficient, and the heat released in the production of ammonia or nitric acid is used to heat the gaseous ammonia produced by the Haber-Bosch plant to the required temperature for its conversion into nitrogen(II) oxide in the solid oxide fuel cell.

In particular, a stack of solid oxide fuel cells finds application in the system according to the present invention for producing nitric acid, since this allows the production of nitric acid on an industrial scale, with a capacity of 100 to 2800 tons of nitric acid per day. However, it is obvious to a person skilled in the art that the use of a single solid oxide fuel cell also results in the production of nitric acid.

Therefore, by using a system for simultaneously producing nitrogen(II) oxide and hydrogen, the system according to the present invention allows a combination of a high-temperature water electrolyzer and an ammonia oxidation burner, thereby obtaining a nitric acid production plant in which ammonia is oxidized to nitrogen(II) oxide in the same plant. The oxidation and absorption sections of the nitric acid production plant are then integrated for the purpose of producing nitric acid from nitrogen(II) oxide, which is produced in this single plant. In addition, as described above, the air separation plant can also be integrated by returning the oxygen-depleted air at the outlet of the cathode space to the air separation plant, and the air separation plant will save energy in operation as described above. Alternatively, it may be recommended to use an air separation plant with a smaller capacity.

According to a specific embodiment, the system further comprises an ammonia combustion device, which is located downstream and, therefore, in fluid communication with the outlets of the anode spaces of the stack elements, as well as a device for transferring the gas collected at these outlets to the ammonia combustion device. The ammonia combustion device is designed to oxidize the ammonia that has not been converted into nitrogen oxide(II) in the stack elements. Essentially, as indicated above, the ammonia that has not been converted in the stack elements is collected together with nitrogen oxide(II) at the outlets of the anode spaces of the stack elements. In order to minimize ammonia losses during the nitric acid production process and to increase the amount of nitric acid produced, the ammonia that has not been converted is oxidized to nitrogen oxide(II) in the ammonia combustion device. The ammonia combustion device may contain a well-known ammonia oxidation catalyst, such as a platinum-based catalyst or a perovskite-based catalyst. Alternatively, the ammonia combustion device may contain an oxide-based catalyst. In addition, the ammonia combustion device may be equipped with a device into which an oxygen-containing gas, such as oxygen, air or oxygen-enriched air, is supplied. The gases that are formed as a result of oxidation in the combustion device are then transferred to the oxidation section of the nitric acid production plant, and thus, a device for carrying out such a transfer is additionally present in the system.

According to a fifth aspect of the present invention, a method for producing nitric acid is disclosed. The method comprises the successive steps of: (a) producing ammonia in a Haber-Bosch unit; (b) collecting the produced ammonia at the outlet of the Haber-Bosch unit; (c) heating the ammonia produced by the Haber-Bosch unit to a temperature in the range of from 500 to 800°C, so that the ammonia is collected in a gaseous state at the outlet of the ammonia heating device; (d) transferring the ammonia to the inlet of the anode spaces of the elements of the solid oxide fuel cell stack according to the present invention; (e) producing nitric oxide (II) using the method for the combined production of electrical energy and nitric oxide (II) according to the present invention; (f) collecting the produced nitric oxide (II) (NO) at the outlets of the anode spaces of the elements of the stack; (g) transferring the produced nitrogen(II) oxide to an oxidation section of a nitric acid production plant which receives an oxygen-containing gas or which contains an ammonia oxidation catalyst; (h) oxidizing the nitrogen(II) oxide to produce gaseous nitrogen oxides, which are in particular nitrogen dioxide and dinitrogen tetroxide; (i) transferring the gaseous nitrogen oxides to an absorption section of the nitric acid production plant; and (j) absorbing the gaseous nitrogen oxides with an aqueous solution to produce nitric acid.

In particular, the solid oxide fuel cell stack is used in the method for producing nitric acid according to the present invention, since its use allows the production of nitric acid to be carried out on an industrial scale. However, it is obvious to a person skilled in the art that even a single solid oxide fuel cell also produces nitric acid.

According to a specific embodiment, the method additionally comprises a stage in which the gases collected at the outlets of the anode spaces of the stack elements are transferred to an ammonia combustion device. The method additionally comprises a stage in which ammonia that has not been converted in the stack elements is oxidized to nitrogen(II) oxide in the ammonia combustion device. The gases that are formed as a result of oxidation in the combustion device are then transferred to the oxidation section of the nitric acid production plant. Optionally, the method additionally comprises a stage in which an oxygen-containing gas, such as oxygen, air or oxygen-enriched air, is introduced into the ammonia combustion device using a suitable device. By introducing this oxygen-containing gas, the oxidation of ammonia in the ammonia combustion device is improved.

According to a sixth aspect of the present invention, there is disclosed the use of a solid oxide fuel cell according to the present invention or a stack of solid oxide fuel cells according to the present invention for the combined production of electrical energy and significant quantities of nitrogen(II) oxide.

According to a seventh aspect of the present invention, there is disclosed the use of an anode comprising a composition A _1-x A' _x B _1-y B' _y O _3-δ , wherein A is selected from the group consisting of La, Y, Sm, Pr, Nd and Gd; A' is selected from the group consisting of Ca, Sr and Ba; B and B' are in each case independently selected from the group consisting of Co, Fe, Mn, Cr and V or mixtures thereof; x is in the range from 0.05 to 0.6 and, in particular, from 0.1 to 0.4; y is in the range from 0 to 1; and δ is in the range of 0.025 to 0.3, in the solid oxide fuel cell according to the present invention, in the solid oxide fuel cell stack according to the present invention, in the nitric acid production system according to the present invention, in the method for combined production of electric power and nitric oxide (II) according to the present invention, or in the method for producing nitric acid according to the present invention.

Examples

Example 1

Materials

Elements with a diameter of 20 mm containing a self-contained 150 μm thick scandium oxide-doped zirconium dioxide (SSZ) electrolyte were fabricated. The anode and cathode were fabricated using a mixed oxide La _0.6 Sr _0.4 Co _0.2 Fe _0.8 O _3-δ (LSCF) with a rhombohedrally distorted perovskite structure.

For comparison purposes with the LSCF anode, a platinum anode was also fabricated from conductive platinum paste. After drying at 100°C, the anode was sintered for 2 hours at 1150°C. Initial studies using a pure platinum paste anode demonstrated very high cell resistance, which was attributed to the pore adhesion of the platinum layer to the smooth electrolyte surface.

The cell performance was measured in a test setup that contained an aluminum oxide tube, an internal gas distribution tube, a gold grid, and a conductor as an electric current collector for the anode. A second gold grid and conductor acted as an electric current collector for the cathode.

An aluminum oxide fixture allowed the element to be uniformly compressed to seal the element, and the element was sealed by heating in a tube furnace for 12 hours at 950°C. The fixture also contained a quartz tube that provided a flow of oxygen and argon mixture over the cathode. The gaseous fuel, in this case dilute ammonia, was introduced over the anode by means of an inner aluminum oxide tube. To improve the electrical contact of the gold grid, which is the collector of the electric current, the anode and the cathode, a layer of gold paste was applied to the grid and the electrodes.

Experimental design

Argon, oxygen, and helium were fed to the anode. Ammonia was fed to the anode from a gas reservoir containing 10% NH ₃ in argon. After the cell, the gas from the anode was further diluted to obtain a composition containing NH ₃ , NO, NO ₂ , N ₂ O, and H ₂ O within the calibrated ranges of the Fourier transform infrared analyzer.

The conversion of ammonia and the selectivity of its oxidation to NO, NO ₂ and N ₂ O were determined using a Gasmet CR4000 Fourier Transform Infrared Spectrometer. The gas lines between the cell anode and the Fourier Transform Infrared Analyzer, as well as argon as a diluent gas, were provided with accompanying heating to 180 °C to prevent condensation of the acid. The infrared analyzer was calibrated for NH ₃ , NO, NO ₂ and N ₂ O using calibration gas mixtures. An Agilent Technologies 490 Gas Micro Chromatograph was used to detect oxygen and helium in the gas leaving the anode to verify successful sealing of the cell.

A Gamry Reference 3000 potentiostat/impedance analyzer with ZView software from Scribner Associates Inc. provided the potential across the cell as a function of the current through the cell and the power produced as a function of the current through the cell.

Research procedure

After sealing and leak testing, the cell was cooled from the sealing temperature of 950°C to 700°C. After testing at 700°C, the cell temperature was increased to 750°C for testing. Ammonia was used at concentrations of 1 and 10% for testing at each temperature. The flow rate of the ammonia-argon gas mixture ranged from 9 ml/min up to 150 ml/min; lower flow rates were used for the feed gas containing 10% ammonia in argon.

The electrical current through the cell was controlled using a galvanostat. The LSCF-SSZ-LSCF cells exhibited a low enough electrical resistance that, in combination with the oxygen potential gradient due to the oxygen-containing gas flow by the cathode and the ammonia-containing gas flow over the anode, no applied potential was required to provide oxygen flow or electrical current. Thus, the cells produced electrical energy.

The cell exhibited an electrical resistance of 1 ohm/ ^cm2 at 800°C. This resistance was low enough to operate the cell as a fuel cell. An electrical current of 200 mA/ ^cm2 was passed through the cell before the initial ammonia-argon mixture was introduced. This was done to prevent reduction of the anode before the study began. The ammonia-argon mixture flows were then introduced and the electrical current through the cell was adjusted using a galvanostat.

Results

Consider Fig. 6. The LSCF anode was capable of producing NO in yields as high as 77% at 700°C and a power density of 4 mW/ ^cm2 . A cell study with a platinum composite anode demonstrated high ammonia conversion but very low selectivity for the desired NO product due to cracking of the ammonia at the platinum anode. When a voltage was applied to obtain a current density greater than approximately 55 mA/ ^cm2 , only 15% of the converted ammonia was converted to nitric oxide.

Conclusion

The performance of this platinum anode in fuel cell mode was clearly inferior to that of the LSCF anode.

Claims (49)

1. A method of using a solid oxide fuel cell (SOFC) for the combined production of electrical energy and a gas stream containing nitrogen(II) oxide (NO), comprising the steps of: (a) providing a solid oxide fuel cell (SOFC) comprising an anode space comprising a solid gas-permeable anode, a gas inlet and a gas outlet; a cathode space comprising a solid gas-permeable cathode and a gas inlet and a gas outlet; and a solid electrolyte of maximum density separating the cathode space and the anode space; b) introducing an oxygen-containing gas into the inlet of the cathode space of the solid oxide fuel cell; c) introducing an ammonia-containing gas stream into the inlet of the anode space of the solid oxide fuel cell; d) collecting nitrogen(II) oxide at the outlet of the anode space, wherein the nitrogen(II) oxide constitutes at least 10% by weight, preferably from 10 to 80%, of the gas stream containing NO collected at the outlet of the anode space; and e) collect the electric current passing between the anode space and the cathode space; wherein the method is carried out at a temperature in the range from 500 to 800°C, and wherein the anode comprises a composition A _1-x A' _x B _1-y B' _y O _3-δ , in which A is selected from the group consisting of La, Y, Sm, Pr, Nd and Gd; A' is selected from the group consisting of Ca, Sr and Ba; B and B' are in each case independently selected from the group consisting of Co, Fe, Mn, Cr and V or mixtures thereof; x is in the range from 0.05 to 0.6 and, in particular, from 0.1 to 0.4; y is in the range from 0 to 1; and δ is in the range from 0.025 to 0.3, and wherein the cathode comprises a composition A _1-x A' _x B _1-y B' _y O _3-δ , in which A is selected from the group consisting of La, Y, Sm, Pr, Nd and Gd; A' is selected from the group consisting of Ca, Sr and Ba; B and B' are in each case independently selected from the group consisting of Co, Fe, Mn, Cr and V or mixtures thereof; x is in the range from 0.05 to 0.6 and, in particular, from 0.1 to 0.4; y is in the range from 0 to 1; and δ is in the range from 0.025 to 0.3. 2. The method according to claim 1, wherein the temperature is in the range from 700 to 750°C. 3. The method according to any one of claims 1, 2, wherein the anode and/or cathode contain the composition La _0.60 Sr _0.40 Co _0.20 Fe _0.80 O ₃ . 4. The method according to any one of claims 1 to 3, wherein the oxygen-containing gas is air, steam, oxygen or a mixture of nitrogen and oxygen. 5. The method according to any one of claims 1 to 4, wherein the method is carried out at a temperature in the range from 500 to 750°C, and wherein the electrolyte is a cerium-based electrolyte, wherein the electrolyte is preferably Ce _1-x Gd _x O _2-δ , wherein x is in the range from 0.1 to 0.2, and wherein δ is in the range from 0.05 to 0.1. 6. The method according to any one of claims 1 to 4, wherein the method is carried out at a temperature in the range from 750 to 800°C, and wherein the electrolyte is a zirconium-based electrolyte, wherein the electrolyte is preferably Zr _1-(x+y) Sc _x M _y O _2-δ , wherein M is Al, Yb, Ce, wherein x is in the range from 0.03 to 0.2 and wherein y is in the range from 0.001 to 0.01, and wherein δ is in the range from 0.01 to 0.06, and wherein the electrolyte is preferably an independent membrane with a thickness of 50 to 200 μm on the cathode or anode. 7. The method according to any one of claims 1 to 6, wherein the oxygen-containing gas introduced into the inlet of the cathode space in step (b) is steam, optionally alternating with air, and hydrogen gas, optionally alternating with oxygen-depleted air, is collected at the outlet of the cathode, and wherein preferably the oxygen-depleted air is sent to an air separation unit. 8. A solid oxide fuel cell for the combined production of electrical energy and a gas stream containing nitrogen(II) oxide (NO), where NO constitutes at least 10% by weight of the gas stream containing NO, comprising: - an anode space containing a solid gas-permeable anode, a gas inlet and a gas outlet; - a source of gaseous ammonia in communication with the possibility of transferring the fluid medium with a gas inlet in the anode space; - a cathode space containing a solid gas-permeable cathode, a gas inlet and a gas outlet; - an electrolyte of maximum density separating the anode space and the cathode space and containing, in particular, a composition selected from: (i) Ce _1-x Gd _x O _2-δ , wherein x is in the range from 0.1 to 0.2, and wherein δ is in the range from 0.05 to 0.1, and (ii) Zr _1-(x+y) Sc _x M _y O _2-δ , wherein M is Al, Yb, Ce, wherein x is in the range from 0.03 to 0.2, wherein y is in the range from 0.025 to 0.01, and wherein δ is in the range from 0.01 to 0.06; - a device for heating the solid oxide fuel element to a temperature in the range of 550 to 800°C; and - a device for collecting electric current passing between the anode space and the cathode space; wherein the anode comprises a composition A _1-x A' _x B _1-y B' _y O _3-δ , wherein A is selected from the group consisting of La, Y, Sm, Pr, Nd and Gd; A' is selected from the group consisting of Ca, Sr and Ba; B and B' are in each case independently selected from the group consisting of Co, Fe, Mn, Cr and V or mixtures thereof; x is in the range from 0.05 to 0.6 and, in particular, from 0.1 to 0.4; y is in the range from 0 to 1; and δ is in the range from 0.025 to 0.3, and wherein the cathode comprises a composition A _1-x A' _x B _1-y B' _y O _3-δ , wherein A is selected from the group consisting of La, Y, Sm, Pr, Nd and Gd; A' is selected from the group consisting of Ca, Sr and Ba; B and B' are in each case independently selected from the group consisting of Co, Fe, Mn, Cr and V or mixtures thereof; x is in the range from 0.05 to 0.6 and, in particular, from 0.1 to 0.4; y is in the range from 0 to 1; and δ is in the range from 0.025 to 0.3. 9. A solid oxide fuel cell according to claim 8, wherein the anode and/or cathode comprise a composition of La _0.60 Sr _0.40 Co _0.20 Fe _0.80 O ₃ . 10. A solid oxide fuel cell according to any one of paragraphs. 8, 9, in which: (i) the electrolyte composition is Ce _1-x Gd _x O _2-δ , wherein x is in the range of from 0.1 to 0.2, and wherein δ is in the range of from 0.05 to 0.1, and wherein the device for heating the element allows heating to a temperature in the range of from 500 to 750°C, or (ii) the electrolyte is Zr _1-(x+y) Sc _x M _y O _2-δ , wherein M is Al, Yb, Ce, and wherein x is in the range from 0.03 to 0.2, wherein y is in the range from 0.001 to 0.01, wherein δ is in the range from 0.01 to 0.06, and wherein the device for heating the element allows heating to a temperature in the range from 750 to 800°C. 11. A solid oxide fuel cell according to any one of claims 8 to 10, further comprising a device for reacting nitrogen(II) oxide in fluid communication with the gas outlet of the anode space. 12. A solid oxide fuel cell according to claim 11, wherein the device for reacting nitric oxide (II) is an oxidation section of a nitric acid production plant into which an oxygen-containing gas is introduced or which contains an ammonia oxidation catalyst, and in which nitric oxide (II) is oxidized and gaseous nitrogen oxides, in particular nitrogen dioxide and dinitrogen tetroxide, are formed. 13. The solid oxide fuel cell of claim 12, further comprising a nitric acid absorption column in which gaseous nitrogen oxides produced in the oxidation section are absorbed by an aqueous solution. 14. A solid oxide fuel cell according to any one of claims 8 to 13, wherein the cathode outlet is in fluid communication with the air separation unit. 15. A stack of solid oxide fuel cells for the combined production of electrical energy and a gas stream containing nitrogen oxide (II) (NO), where NO is at least 10% by weight of the gas stream containing NO, comprising at least two or more solid oxide fuel cells according to any one of paragraphs. 8-14. 16. A system for producing nitric acid, comprising: - a Haber-Bosch unit containing an outlet for the delivery of ammonia produced by the unit; - a device for heating ammonia produced by a Haber-Bosch plant to a temperature in the range of 500 to 800°C, so that the ammonia is collected in a gaseous state at the outlet of the heating device; - a stack of solid oxide fuel cells according to claim 15, in which the inlets of the anode spaces of the cells are in fluid communication with the outlet of the heating device; - a device for transferring nitrogen oxide (II) (NO), entering at the outlets of the anode spaces of the stack elements, to the oxidation section of the nitric acid production plant, into which oxygen-containing gas enters or which contains an ammonia oxidation catalyst, and in which nitrogen oxide (II) is oxidized to gaseous nitrogen oxides, in particular nitrogen dioxide and dinitrogen tetroxide; and - a device for transferring gaseous nitrogen oxides to a nitric acid absorption column in which the gaseous nitrogen oxides are absorbed by an aqueous solution, resulting in the production of nitric acid. 17. A method for producing nitric acid, comprising successive stages in which: a) produce ammonia in a Haber-Bosch plant; b) collect the produced ammonia at the outlet of the Haber-Bosch unit; c) heating the ammonia produced by the Haber-Bosch unit to a temperature in the range of 500 to 800°C so that the ammonia is collected in a gaseous state at the outlet of the heating device; d) transferring ammonia into the inlets of the anode spaces of the elements of the solid oxide fuel cell stack according to paragraph 15; e) producing nitrogen(II) oxide using the method according to any one of paragraphs 1-7; f) collect the produced nitrogen(II) oxide (NO) at the outlets of the anode spaces of the stack elements; g) transferring the produced nitrogen(II) oxide to the oxidation section of a nitric acid production plant, which is supplied with oxygen-containing gas or which contains an ammonia oxidation catalyst; h) oxidize nitrogen(II) oxide to produce gaseous nitrogen oxides, in particular nitrogen dioxide and dinitrogen tetroxide; i) transfer gaseous nitrogen oxides to the absorption section of the nitric acid production plant; and j) absorb gaseous nitrogen oxides in an aqueous solution to produce nitric acid. 18. Use of a solid oxide fuel cell according to any one of claims 8 to 14 for the combined production of electrical energy and a gas stream containing NO, where NO constitutes at least 10% by weight of the gas stream containing NO. 19. Use of a solid oxide fuel cell stack according to claim 15 for the combined production of electrical energy and a gas stream containing NO, where NO constitutes at least 10% by weight of the gas stream containing NO.