An apparatus and a method for producing nitric acid

Abstract

An apparatus comprising an electrolyser subsystem (20), a Haber Bosch subsystem (22), and an Ostwald subsystem (24), and a method for producing nitric acid. Fluid passageways are configured to: route hydrogen produced by the electrolyser subsystem (20) to the Haber Bosch subsystem (22) via a first path (26) for use in a Haber process at the Haber Bosch subsystem (22) to produce ammonia; route at least a portion of the ammonia produced by the Haber Bosch subsystem (22) to the Ostwald subsystem (24) via a second path (28) for use in an Ostwald process at the Ostwald subsystem (24) to produce nitric acid; and route at least a portion of steam produced using heat from the Ostwald subsystem (24) to the electrolyser subsystem (20) via a third path (30) for use as at least a part of an infeed gas for the electrolyser subsystem (20).

Description

[0001] An apparatus and a method for producing nitric acid

[0002] The present invention relates to an apparatus and a method for producing nitric acid, and optionally ammonium nitrate. It can be incorporated into an industrial plant.

[0003] Ammonium nitrate or fertiliser plants commonly utilise the Ostwald process for making nitric acid (HNO3, or hereinafter NHO3) - the main raw material for the most common types of fertiliser production. That Ostwald process is generally closely associated with the Haber process, which provides the requisite raw material for the Ostwald process - i.e. ammonia (NH3 or hereinafter NH3).

[0004] In the Ostwald process, ammonia is converted to nitric acid in two stages. It is first oxidised by heating it with oxygen in the presence of a catalyst to form nitric oxide and water - a highly exothermic reaction. It is then processed in an absorption apparatus containing water, wherein the nitric oxide is again oxidised to yield nitrogen dioxide, which gas is readily absorbed by the water, yielding diluted nitric acid. A portion, however, reduces back to nitric oxide, which can be recirculated, and the diluted nitric acid can be concentrated to a required strength by distillation.

[0005] The chemical formula for the Ostwald process is effectively:

[0006] NH3+ 2O2-* H2O + HN03(water plus nitric acid)

[0007] In the Haber process, also known as the Haber-Bosch process, nitrogen is reacted with hydrogen using a catalyst under high temperatures and pressure to produce ammonia - a slightly exothermic reaction.

[0008] The chemical formula for the Haber process is effectively:

[0009] 3H2+ N2= 2NH3(ammonia)

[0010] Due to the exothermic nature of the Ostwald process, ammonium nitrate plants often produce a significant quantity of superheated steam, and it is commonly used to drive a turbine, thus utilising the heat, which would otherwise be a waste product, to produce electricity.

[0011] The present inventors have realised that a more efficient implementation of such a plant is possible.

[0012] According to a first aspect of the present invention, there is provided an apparatus comprising: an electrolyser subsystem; a Haber Bosch (HB) subsystem; and an Ostwald subsystem; wherein the apparatus is configured with fluid passageways to: a) route hydrogen produced by the electrolyser subsystem to the Haber Bosch subsystem via a first path for use in a Haber process at the Haber Bosch subsystem to produce ammonia; b) route at least a portion of the ammonia produced by the Haber Bosch subsystem to the Ostwald subsystem via a second path for use in an Ostwald process at the Ostwald subsystem to produce nitric acid; and c) route at least a portion of steam produced using heat from the Ostwald subsystem to the electrolyser subsystem via a third path for use as at least a part of an infeed gas for the electrolyser subsystem.

[0013] The apparatus may take the form of a series of pieces of equipment, usually in the form of the various subsystems, all installed within or across a plant or an industrial complex. The present invention may provide a system or apparatus for producing and processing ammonia, or for producing ammonium nitrate, or for producing (and optionally processing) nitric acid.

[0014] The electrolyser subsystem is for producing hydrogen (H2) and oxygen (O2) in separate off-gas streams.

[0015] The Haber Bosch subsystem is for carrying out the Haber (or Haber-Bosch ) process, i.e. to take the hydrogen from a first off-gas stream from the electrolyser subsystem, and to then produce ammonia (NH3) with it by reacting it with nitrogen (e.g. from air - such as via an air separation unit). Typically, the hydrogen from the electrolyser subsystem provides all the required hydrogen for the Haber process, although it might provide only part of the required hydrogen if another hydrogen source is available.

[0016] The Ostwald subsystem is then for receiving oxygen (O2), for example at least partially from the other off-gas stream from the electrolyser subsystem, and for example with any balance coming from another oxygen supply, such as the above air separation unit, and the ammonia (NH3) from the Haber Bosch subsystem, and to then produce nitric acid (HNO3) with it, along with water in the form of steam (H2O).

[0017] In some embodiments, the heat from the Ostwald subsystem is used to distil the nitric acid, e.g. via a heat exchanger or boiler, with a steam by-product then being fed to the electrolyser subsystem for use in the electrolyser subsystem. Alternatively, or additionally, that heat can be used to produce steam from another water source. Using that heat, and recirculating the water from the diluted nitric acid exiting the Ostwald subsystem, reduces the apparatus’ water demand, the apparatus’ energy demand, and when combined, the apparatus’ need to preheat any externally sourced water supplies for the electrolyser subsystem. These arrangements thus improve the efficiency of the apparatus.

[0018] In some embodiments, the first path is between a first exhaust of the electrolyser subsystem and a first inlet of the HB subsystem.

[0019] In some embodiments, the electrolyser subsystem is configured to produce hydrogen and the apparatus is configured to route all hydrogen produced by the electrolyser subsystem via the first path to the Haber Bosch subsystem.

[0020] In some embodiments, the second path is between a first exhaust of the Haber Bosch subsystem and a first inlet of the Ostwald subsystem. In some embodiments, the Haber Bosch subsystem is configured to produce ammonia and the apparatus is configured to route all ammonia produced by the Haber Bosch subsystem via the second path to the Ostwald subsystem. More usually, however, some of the ammonia is redirected to storage or a downstream process, for example for producing ammonium nitrate with it and the nitric acid produced by the Ostwald subsystem.

[0021] In some embodiments, the third path is at least between a first heat exchanger and a first inlet of the electrolyser subsystem. The first heat exchanger (HX1) may be configured to exchange heat from the Ostwald subsystem (or Ostwald process) to water or steam, or to distil the outputted dilute nitric acid from the Ostwald process to produce steam and more concentrated nitric acid.

[0022] In some embodiments, the apparatus is configured to route only a portion of the steam produced using heat produced by the Ostwald subsystem via the third path for use by the electrolyser subsystem. The electrolyser subsystem then can process that steam to produce the hydrogen and oxygen off gases. The rest of the steam can be used for other purposes, albeit potentially as a heat source for heat exchangers of the apparatus, or within a turbine for producing electrical power, or at further electrolysers that are superfluous for the current process, i.e. for producing additional hydrogen and oxygen for other purposes, such as in other chemical processes elsewhere in the plant (which may even have a demand for steam, whereby that excess steam can be used directly in those other chemical processes).

[0023] It is typically the case that the Haber process and the Ostwald process, and where present the ammonium nitrate process, together produce more excess heat, for example converted to steam, than the electrolysers of the apparatus can usefully use, i.e. for the purposes of producing ammonia, nitric acid and ammonium nitrate. In other words, there is some maximum amount of steam from the waste heat that is worthwhile routing to the electrolyser subsystem for these purposes. Hence, some of the heat / steam produced can be used elsewhere if desired. In some cases, some of the heat / steam produced can be used by the electrolyser subsystem or a further electrolyser subsystem, to produce hydrogen and oxygen (or oxygen enriched gas) as a product.

[0024] In some embodiments, at least 90% of the hydrogen used or consumed in the Haber Bosch process is produced by the electrolyser subsystem. In some embodiments, it may be as much as 100% - i.e. the apparatus operates with no other hydrogen supply for the Haber Bosch subsystem.

[0025] In some embodiments, the electrolyser subsystem is supplied electrical power from an external power source. In some embodiments, some electrical power may be supplied to the electrolyser subsystem from a turbine of the apparatus, which generates electricity using heat from the apparatus, such as heat from either the Haber process or the Ostwald process, or from renewable energy sources such as solar, wind, geothermal or hydroelectric power.

[0026] The inventors have recognised that the apparatus, via the Haber process and the Ostwald process will together produce enough heat and steam, e.g. from the waste heat produced by the exothermic Ostwald and Haber processes, and as a by-product of the Ostwald process, to meet the electrolyser subsystem’s need for steam (i.e. the “fuel” for the electrolysers) to produce all the hydrogen typically needed for the Haber Bosch process. To use the heat by-product, a heat exchanger for each of these subsystems may thus be provided to exchange that heat with water or steam due to enter the electrolyser subsystem. There can even potentially be enough excess heat and / or steam from the apparatus such that it could be used to power a turbine as well, for example to produce a part of the electrical demand of the electrolysers within the electrolyser subsystem. However, it is unlikely that all the electrical demand can be provided from waste heat and steam from just those two processes. Indeed, it is expected that more than 90% of the electrical demand will be externally sourced, preferably from one or more of the above-mentioned renewable energy sources, or from (or supplemented by) an external power grid.

[0027] In the Ostwald process, the ammonia is oxidised, and that oxidation reaction produces a lot of steam. That steam needs to be condensed out, and thus it is good for raising the steam for the electrolyser. In particular, the oxidation reaction results in high quality heat (as the reaction usually occurs at 800-900 degrees C. This also, or alternatively, lends itself well to power generation using steam turbines. There is also a possibility to use it to run the electrolyser stack endothermically and to thus offset stack power requirements with high temperature heat / steam.

[0028] In some embodiments, at least 40%, and preferably about 50%, albeit generally less than 75%, or less than 50%, of the steam used by the electrolyser subsystem (i.e. as fuel in hydrogen / oxygen production) is produced by, or using heat from, the Ostwald subsystem.

[0029] In some embodiments, at least 25%, preferably at least 40%, or about 50%, or more than 50%, of the steam used by the electrolyser subsystem (i.e. as fuel in hydrogen / oxygen production) is produced by using heat from the Haber Bosch subsystem.

[0030] In some embodiments, about 50% of the heat for producing steam for the electrolyser subsystem comes from the Haber process and the balance (or about 50%) of the heat for producing steam for the electrolyser subsystem comes from the Ostwald process.

[0031] In some embodiments, heat or steam from other subsystems, or nearby industrial infrastructure, can also be used.

[0032] By using waste heat or waste water or steam, it is possible to increase the overall efficiency of the apparatus, or the plant / industrial complex operating the apparatus.

[0033] In some embodiments, the apparatus is configured to route oxygen produced by the electrolyser subsystem to the Ostwald subsystem via a fourth path for use / consumption of oxygen in the Ostwald process.

[0034] In some embodiments the fourth path is between a second exhaust of the electrolyser subsystem and a second inlet of the Ostwald subsystem. In some embodiments, the electrolyser subsystem is configured to produce oxygen and the plant / system is configured to route oxygen produced by the electrolyser subsystem via the fourth path to the Ostwald subsystem.

[0035] In some embodiments, this oxygen is in the form of oxygen enriched air (e.g. a gas mixture with higher than 23% oxygen by mass within the gas mixture) - for example if air is used as a sweep gas in the electrolyser, but it may also be substantially pure oxygen (e.g. a gas containing at least 90% oxygen by mass). Using oxygen, or oxygen enriched air, instead of air, makes the Ostwald process more efficient.

[0036] In some embodiments, the apparatus further comprises an air separation unit (ASU) for separating nitrogen from air, the nitrogen being for feeding to the Haber Bosch subsystem. In other embodiments, nitrogen for the Haber process may come from a separate supply of nitrogen.

[0037] In some embodiments, the ASU also supplies oxygen for the Ostwald process. Oxygen for the Ostwald process may thus come from both the electrolyser subsystem and the ASU.

[0038] In some embodiments, at least 50%, and preferably at least 75%, by mass of fluid exhausted from the electrolyser subsystem at the second exhaust of the electrolyser subsystem (i.e. on an anode side of the electrolyser subsystem) is oxygen produced by the electrolyser subsystem. The remaining percentage by mass will be an optional sweep gas provided at a second inlet of the electrolyser subsystem. By using the electrolyser subsystem as well as the ASU to supply the oxygen for the Ostwald process, less energy will be required to operate the ASU versus prior art systems where the ASU provides all the oxygen, because the ASU will not need to supply as much oxygen.

[0039] In some embodiments, the oxygen produced by the electrolyser subsystem is exhausted from the electrolyser subsystem at an oxidant (second) exhaust.

[0040] In some embodiments, less than 50% by mass of the fluid exhausted from the electrolyser subsystem at the second exhaust of the electrolyser subsystem is the sweep gas used in the electrolyser subsystem as an optional sweep gas provided at a second inlet of the electrolyser subsystem. By keeping the volume of sweep gas lower than the mass of produced oxygen by the electrolyser subsystem, the purity of the oxygen can be reliably maintained, which both reduces the power usage of the electrolyser subsystem, and improves the efficiency of the Ostwald subsystem.

[0041] The Ostwald subsystem typically comprises two stages. These are a first stage (Ostwaldl) that is configured to receive ammonia from the Haber Bosch subsystem and to oxidise that ammonia to produce nitric oxide (NO). The second stage (Ostwald2) is then configured to receive the nitric oxide from the first stage, and to oxidise said nitric oxide using oxygen from the electrolyser subsystem (and optionally the ASU), and to absorb the oxidation product in water to produce nitric acid (NHO3).

[0042] In some embodiments, in the first stage the ammonia is oxidized (in a sense "burned") by heating it with oxygen in the presence of a catalyst such as platinum, with 10% rhodium, to form nitric oxide and water. This step is strongly exothermic, making it a useful heat source once initiated. In some embodiments the specific enthalpy may be -950 kJ / mol. The chemical equation for this stage of the reaction is:

[0043] 4NH3(g) + 502(g) 4N0(g) + 6H2O(g)

[0044] In some embodiments, the second stage, i.e. the oxidization of the nitric oxide (NO) and the absorption by water, is carried out in the presence of water in an absorption apparatus. Initially nitric oxide is oxidized again to yield nitrogen dioxide: 2N0(g) + 02(g) — > 2NO2(g). This gas is then readily absorbed by the water, yielding the desired product (nitric acid, albeit in a dilute form), while also reducing a portion of it back to nitric oxide: 3NO2(g) + H2O(I) — > 2HNO3(aq) + NO(g).

[0045] In some embodiments, the nitric oxide (NO) is recycled, and the acid is concentrated to the required strength by distillation.

[0046] In some embodiments, when the last step is carried out in air, the following reaction occurs: 4NO2(g) + 02(g) + 2H2O(I) 4HNO3(aq).

[0047] Typical conditions for the first stage, which contribute to an overall yield of about 96%, are: a) a pressure between 4 and 10 atmospheres (approx. 400-1010 kPa or 60-145 psig); and b) a temperature of about 1173 K (approx. 900°C or 1652°F). The second stage (the NO reaction) is instead heavily favoured at a low temperature both kinetically and thermodynamically (i.e. 100C or below is good). It is also favoured at higher pressure, and there is typically a compression stage for achieving this. The reaction generally does not need a reactor or a catalyst. Given the desire to use a lower temperature for the second stage, it is beneficial for the process to raise steam for the electrolyser based on the second reaction, thus taking or absorbing the heat generated in the second process by way of making steam for the electrolyser - e.g. via a heat exchanger.

[0048] As mentioned above, the final product, nitric acid, is formed in an absorption process as 3NO2 + H2O -> 2HNO3 + NO, with the NO typically being recycled. This is also exothermic, but typically below 100C, and therefore less likely to be useful for steam generation.

[0049] In some embodiments a first heat exchanger (HX1) is provided between the first stage and the second stage, and the heat exchanger is configured to transfer heat to the third path to produce at least a portion of the steam for use in the electrolyser subsystem - i.e. for the hydrogen production by the electrolyser subsystem. This also keeps the temperature down ready for the second stage.

[0050] As the first stage is exothermic, a large quantity of heat energy can be recovered in the third path through heating water or steam therewith.

[0051] In some embodiments, a fifth path is configured to route nitrous oxide (NOX) produced by the Ostwald process’ second stage to combine it with the third path. The Ostwald process’ second stage may additionally or alternatively produce steam and the fifth path is configured to route said steam to combine it with the third path. The NOX and / or steam produced in the Ostwald process’ second stage can then be supplied to the electrolyser subsystem alongside the steam for use as fuel. This steam, or the steam and NOX mixture, from the Ostwald process’ second stage can potentially be used directly in the electrolyser. The electrolyser will then produce oxygen from the NOX and the steam, nitrogen from the NOX, and hydrogen from the steam, the nitrogen and the hydrogen being mixed together in the cathode output stream. The oxygen (from the anode output stream) can then be routed to the Ostwald subsystem for the second stage process (Ostwald 2), and the nitrogen and the hydrogen can be routed to the Haber Bosch subsystem via first path for use in the Haber process. Allowing some NOX in the fuel for the electrolyser subsystem can help to further increase the efficiency of the apparatus as less nitrogen is required from the ASU. Furthermore, this would make use of the NOX, rather than allowing it to vent to atmosphere - or otherwise requiring it to be subsequently processed to avoid it being vented. NOX, after all, is a harmful greenhouse gas, and its venting to atmosphere may be controlled by regulatory restrictions.

[0052] In some embodiments, the apparatus further comprises an ammonium nitrate (AN) subsystem, wherein the ammonium nitrate subsystem is configured to produce ammonium nitrate. The chemical equation for this ammonium nitrate or AN process is as follows:

[0053] NH3+ HNO3NH4N03

[0054] In some embodiments, the apparatus is configured to route a portion of the ammonia produced by the Haber Bosch subsystem to the ammonium nitrate subsystem via a sixth path and to route nitric acid produced by the Ostwald subsystem’s second stage to the ammonium nitrate subsystem via a seventh path.

[0055] The AN process is also exothermic - similar to the Haber process.

[0056] The present inventors realised that the efficiency of an ammonium nitrate plant can be increased by locating it close to both ammonia and nitric acid supplies, with the Haber Bosch subsystem providing the former and the Ostwald subsystem providing the latter. Connecting this with the electrolyser subsystem then further improves the overall efficiency by enabling hydrogen and oxygen supplies to be available through reuse of waste heat and steam from those processes.

[0057] In some embodiments, the apparatus is further configured to produce steam using heat from the ammonium nitrate subsystem (i.e. from the AN process) and to route that steam to the electrolyser subsystem. By using the waste heat from the AN process, the overall apparatus efficiency can be increased.

[0058] In some embodiments, a third heat exchanger (HX3) may be used to exchange this heat with the third path. For example, the heat exchanger can be downstream of the AN process, or integrated with the ammonium nitrate subsystem such that the third path passes over, against or through the ammonium nitrate subsystem for heat exchanging with the ammonium nitrate (NH4NO3).

[0059] In some embodiments the heat from the ammonium nitrate subsystem is transferred to the water or steam in the third path before heat is transferred to the water or steam from the Ostwald process or from the Haber process. However, it may instead be transferred to the water or steam after heat from the Ostwald process or from the Haber process is transferred to the water. Typically, the operating temperatures will dictate the order, with the hottest source being used last and the coolest first.

[0060] In some embodiments, as previously suggested, the apparatus is further configured to produce at least some of the steam, or to pre-heat the steam, using heat from the Haber Bosch subsystem. As previously discussed this increases the efficiency of the apparatus as it is reusing waste heat. This may be via a second heat exchanger (HX2).

[0061] In some embodiments, the second heat exchanger is connected to the second path to exchange heat between the second path (containing ammonia produced by the Haber Bosch subsystem) and the third path (containing steam / water). As it is on the second path, in some embodiments it only receives a portion of the ammonia produced in the Haber process, as some instead passes along the sixth path to the optional ammonium nitrate subsystem. Therefore, it only transfers a corresponding portion of the heat to the third path. A further heat exchanger may thus be provided for the sixth path, connected to the third path, to utilise the heat in the sixth path. Alternatively, the supply of ammonia produced in the Haber process may split after the heat exchanger such that after the heat exchanger, some of the ammonia goes to the Ostwald subsystem (via the second path) and some of the ammonia goes to the ammonium nitrate subsystem (via the sixth path).

[0062] In some embodiments, the second heat exchanger is located along the third path in a position subsequent to the first heat exchanger so that the steam is already preheated by the Ostwald process’ waste heat before it gets heated by the Haber process’ waste heat. This is beneficial as the Ostwald process’ waste heat is usually hotter than that of the Haber process.

[0063] In some embodiments, a steam turbine is positioned along the third path, for example after the first heat exchanger, so that the steam is heated and thus expanded by the first heat exchanger to drive it through the steam turbine subsequent to the first heat exchanger. In some embodiments the steam turbine is positioned between the first and second heat exchangers. In some embodiments it is positioned after the second heat exchanger - for example to benefit from both heat sources, although preferably it is between the first and second heat exchangers along the third path. The steam turbine is configured to produce electrical power and is preferably electrically coupled to the electrolyser subsystem to directly provide electrical power to the electrolyser subsystem for powering (or part powering) at least some of the electrolysers thereof.

[0064] In some embodiments, the steam turbine provides at least 5%, preferably about 10%, albeit usually less than 50%, of the power demand used by the electrolyser subsystem, with the remaining power requirement being sourced from an external power supply. By using the turbine, there is a significant increase in the efficiency of the apparatus versus just using external power for the electrolyser subsystem, as it utilised waste heat from the Ostwald process (e.g. the Ostwald2 step), plus potentially other waste heat supplies from the Ostwald process, the Haber process and the AN process, to produce part of that required electrical power for hydrogen and oxygen (and potentially nitrogen) production.

[0065] In some embodiments, the steam turbine receives through it more than 90% of the steam produced using the heat from the Ostwald process, and the steam for the electrolyser subsystem is produced or pre-heated from the steam or water (steam) exiting the steam turbine, ready for feeding into the electrolyser subsystem, using heat from the Haber Bosch subsystem and / or the ammonium nitrate subsystem.

[0066] In some embodiments, at least 75% and preferably at least 90% of the steam used as fuel in the electrolyser subsystem is produced using heat from the Ostwald subsystem, although as mentioned above this is commonly less than 75%, with the Haber process providing around 50% of the steam.

[0067] Although a large proportion of the steam used as fuel in the electrolyser subsystem is preferably sourced from waste water from the Ostwald subsystem, some of it may come from a common water supply to that used by the Ostwald subsystem.

[0068] In some embodiments, some or potentially all of the heat from the Ostwald process’ first stage is used to produce steam which is routed to the steam turbine. Such steam is relatively high grade. Steam output from the steam turbine may be used as fuel for the electrolyser subsystem. In such cases, fuel for the electrolyser subsystem may also be steam raised using heat from one or more of the Ostwald process’ second stage, the Haber process, and the ammonium nitrate process. Fuel for the electrolyser may additionally or alternatively be sourced as steam or NOX produced as by-products in the various subsystems. In some embodiments, oxygen from the ASU is combined with oxygen from the electrolyser subsystem for use in Ostwald process’ first stage. In some embodiments, oxygen from the electrolyser subsystem is at least 25% (preferably less than 75%) of the oxygen fed to the Ostwald subsystem for use in the Ostwald process. As previously discussed, this mixing in of oxygen from the electrolyser subsystem reduces the amount of oxygen being needed to be supplied by the ASU, which in turn increases the efficiency of the apparatus.

[0069] In some embodiments, oxygen only from the ASU is used in the first stage of the Ostwald process and only oxygen from the electrolyser subsystem is used in the second stage of the Ostwald process. In some embodiments air may be fed to the first stage of the Ostwald process and oxygen from the electrolyser and ASU may be fed to the second stage of the Ostwald process. This improves the yield and efficiency of the second stage.

[0070] In some embodiments the electrolyser subsystem comprises at least one solid oxide electrolyser cell (SOEC), preferably a metal supported solid oxide electrolyser cell (MS-SOEC). In other words, the electrochemically active region of the electrolyser cell is a solid oxide. Such cells typically operate in the 400-900 degrees C range, although preferably for this apparatus the electrolyser cell operates at a temperature between 500 and 800 degrees C.

[0071] There are many possible forms of solid oxide electrolyser cell, each using different electrochemically active electrolyte chemistries. For example, three well known electrolyte materials are yttria-stabilized zirconia (YSZ), scandia stabilized zirconia (ScSZ) and gadolinium doped ceria (GDC or CGO). Anode and cathode materials can also vary. The present invention can be used with any of these, along with many other forms of electrolyser cell.

[0072] For some chemistries, the cells operate between 400 and 700 degrees C, or more particularly in the 450-650 degrees C temperature range. Such electrolyser cells may be referred to as intermediate temperature solid oxide electrolyser cells, or IT-SOECs. When operating above 700 degrees C, they are instead commonly referred to as high temperature solid oxide electrolyser cells, or HT-SOECs.

[0073] Other forms of electrolyser may also be applicable to the present invention.

[0074] Typically the electrolyser cells are configured in one or more stacks of cells, for example stacks of 20 or more cells, or even 100 or more cells. In some embodiments the electrolyser cells in the electrolyser subsystem operate at a thermoneutral voltage.

[0075] In some embodiments, the apparatus is aimed to be operated such that the cells or each stack thereof within the electrolyser subsystem are in a thermoneutral state such that the temperature within the cells or the stacks remain substantially constant.

[0076] In some embodiments, the electrolyser subsystem operates substantially under galvanostatic conditions, i.e. with a constant current. In such an electrolyser subsystem, a temperature difference across the electrolyser subsystem may be detected by comparing an infeed gas temperature for the fuel (i.e. water / steam) with an outflow temperature for the off-gas (i.e. hydrogen or hydrogen and steam mixture). In some embodiments the oxygen off-gas’s temperature is instead or additionally used. The temperature difference, or temperature delta can be compared against a temperature delta threshold, and an adjustment to a target input temperature can then be applied.

[0077] In some embodiments, when the temperature delta is positive, the adjustment to the target input temperature is also positive, and when the temperature delta is negative, the adjustment to the target input temperature is also negative. This is counterintuitive as typically in the prior art a heating stack wants to be cooled, whereas a cooling stack wants to be heated. However, the present invention can utilise a resistance response to stack temperature, and that resistance’s interaction with the voltage across the stack under galvanostatic conditions: as the input temperature increases, the stack warms. This reduces the electrical resistance of the stack, which in turn reduces the voltage across the stack. That reduction of voltage, in galvanostatic conditions (i.e. with a constant current), reduces the power draw of the stack, and thus, if now lower than the thermoneutral voltage, due to the endothermic nature of electrolysis, the stack actually avoids heating up by as much as the increased input temperature, and can even cool. Conversely, as the input temperature reduces, the stack cools. This increases the electrical resistance of the stack, which in turn increases the voltage across the stack. That increase of voltage, in galvanostatic conditions (i.e. with a constant current), increases the power draw of the stack, and thus, when it exceeds the thermoneutral voltage, due to the now exothermic nature of the configuration (due to the increase in power input from the constant current source), the stack actually avoids cooling down by as much as the reduced input temperature, and can even heat up. The present invention thus provides a regulated internal temperature control for the stack using external fluid temperature control, galvanostatic conditions across the stack, and the power draw characteristics of the stack arising from fluctuations in voltage either side of a thermoneutral voltage for the stack.

[0078] Since the present invention uses the efficiencies of the integration with the Haber process and the Ostwald process, and since it can also provide for use of galvanostatic conditions within the stack, i.e. a constant electrical current (constant amps), while also having the capability of adopting thermoneutral voltages across the stack to avoid heat wastage in the stack, as at a thermoneutral condition the electrolyser is in an adiabatic state, i.e. it is balanced energetically which effectively means no heat is consumed or released, the present invention can provide a highly efficient apparatus, as wastage of heat can be avoided throughout the process.

[0079] According to a second aspect of the present invention there is provided an apparatus comprising an electrolyser subsystem; a Haber Bosch subsystem; an Ostwald subsystem; and an ammonium nitrate subsystem;

[0080] - wherein the apparatus is configured to: a) route hydrogen produced by the electrolyser subsystem to the Haber Bosch subsystem via a first path for use in a Haber process at the Haber Bosch subsystem; b) route at least a portion of ammonia produced by the Haber Bosch subsystem to the Ostwald subsystem via a second path for use in an Ostwald process at the Ostwald subsystem; and c) produce steam using heat from at least one of the Ostwald subsystem, the ammonium nitrate subsystem and the Haber Bosch subsystem; and to

[0081] ■ route at least a first portion of said steam to the electrolyser subsystem via a third path for use as a fuel in hydrogen production by the electrolyser subsystem;

[0082] ■ route at least a second portion of said steam to a steam turbine for use in generation of electrical power.

[0083] In some embodiments, the second portion of said steam is routed to the steam turbine via the third path. In some embodiments, the second portion may comprise also the first portion, both portions routing through the steam turbine prior to the electrolyser subsystem.

[0084] In some embodiments, the steam turbine is electrically coupled to the electrolyser subsystem to thus directly provide electrical power to the electrolyser subsystem for use therein.

[0085] This apparatus may also incorporate any of the features of the apparatus of the first aspect of the present invention.

[0086] The present invention also provides a method of using the above-mentioned apparatuses. For example, according to a third aspect of the present invention there is disclosed a method for producing nitric acid (HNO3) comprising

[0087] • producing hydrogen and oxygen using an electrolyser subsystem;

[0088] • producing ammonia using a Haber process using hydrogen from the electrolyser; and

[0089] • producing nitric acid using an Ostwald process using ammonia from the Haber process and oxygen from the electrolyser subsystem;

[0090] • wherein the method further comprises using heat from the Ostwald process and / or the Haber process at the electrolyser subsystem to produce said hydrogen and oxygen at the electrolyser subsystem.

[0091] In some embodiments the method involves routing the hydrogen from the electrolyser subsystem to a Haber Bosch subsystem of the apparatus, and routing oxygen from the electrolyser subsystem to an Ostwald subsystem of the apparatus.

[0092] As for using heat from the Ostwald process and / or the Haber process at the electrolyser subsystem to produce said hydrogen and oxygen at the electrolyser subsystem, that may be, for example, by using the heat to power a steam turbine to generate electricity. In another embodiment, it may be by using the heat to pre-heat or generate steam for passing through to the electrolyser subsystem. In another embodiment, the method produces hydrogen and oxygen at the electrolyser subsystem by producing steam using said heat from the Ostwald process and / or the Haber process and using said steam as fuel for the electrolyser subsystem, and by supplying electrical power to the electrolyser.

[0093] In some embodiments, the method uses oxygen produced by the electrolyser subsystem in the Ostwald process. In some embodiments, the Ostwald process comprises a first stage (Ostwaldl) to oxidise ammonia from the Haber process to produce nitric oxide (NO) and a second stage (Ostwald2) to oxidise the nitric oxide from the first stage using oxygen from the electrolyser subsystem, and to then produce nitric acid by absorbing the oxidation product in water.

[0094] In some embodiments, the method uses nitrous oxide (NOX) produced by the Ostwald process as a component of the fuel for the electrolyser subsystem (for example by mixing it with steam).

[0095] In some embodiments the method further produces ammonium nitrate in a subsequent ammonium nitrate subsystem using an ammonium nitrate process using ammonia from the Haber process and nitric acid from the second stage of the Ostwald process.

[0096] In some embodiments the method comprises producing steam using heat from the ammonium nitrate process.

[0097] In some embodiments the method comprises producing steam using heat from the Haber process.

[0098] In some embodiments the method comprises producing steam using heat from the Ostwald process.

[0099] In some embodiments, the method comprises superheating the steam using heat from the other process(es).

[0100] In some embodiments, the method comprises generating electrical power using steam produced using heat from one or more of the Haber process, the Ostwald process or the AN process, and using said electrical power at the electrolyser subsystem to produce hydrogen and oxygen. That electrical power will typically be supplemented by electricity from a further power supply.

[0101] In some embodiments, an air separation unit is also provided for separating air into a stream of oxygen and nitrogen and providing the oxygen to the Ostwald process, alongside oxygen from electrolyser subsystem, and providing the nitrogen to the Haber process. In some embodiments, nitrogen is also supplied by the electrolyser subsystem - for example if nitrous oxide from the Ostwald process (NOX) is also fed to the electrolyser subsystem with the steam. As with the apparatus defined above, the electrolyser subsystem preferably comprises at least one SOEC, which is preferably a MS-SOEC that preferably has an operating temperature of between 500 and 800 degrees C. It may be part of a stack thereof.

[0102] In an aspect there is provided a plant or system comprising: an electrolyser subsystem having a first electrolyser inlet and a first electrolyser exhaust; a Haber Bosch subsystem having a first Haber Bosch inlet and a first Haber Bosch exhaust; and an Ostwald subsystem having a first Ostwald inlet, a first Ostwald exhaust and a second Ostwald exhaust; wherein: the first electrolyser exhaust is in fluidic communication with the first Haber Bosch inlet via a first path; the electrolyser subsystem is configured to produce hydrogen and the apparatus is configured to route hydrogen produced by the electrolyser subsystem via the first path to the Haber Bosch subsystem; the first exhaust of the Haber Bosch subsystem is in fluidic communication with the first inlet of the Ostwald subsystem via a second path; the Haber Bosch subsystem is configured to produce ammonia and the apparatus is configured to route ammonia produced by the Haber Bosch subsystem via the second path to the Ostwald subsystem; the Ostwald subsystem is configured to produce nitric acid and steam and the Ostwald subsystem is configured to exhaust the nitric acid at the second exhaust and the steam at the first exhaust; the first exhaust of the Ostwald subsystem is in fluidic communication with the first inlet of the electrolyser subsystem via a third path; and the plant or system is configured to route at least a part of the steam produced by the Ostwald subsystem via the third path for use by the electrolyser subsystem in production of hydrogen by the electrolyser subsystem.

[0103] In some embodiments, the electrolyser subsystem comprises a second electrolyser inlet. In some embodiments, the electrolyser subsystem comprises a second electrolyser exhaust. In some embodiments, the Haber Bosch subsystem comprises a second Haber Bosch inlet. In some embodiments, the Haber Bosch subsystem comprises a second Haber Bosch exhaust. The plant or system may comprise any one or more features of the above-mentioned apparatus, or the described below apparatus, plant or system. Likewise, the above mentioned apparatus may comprise any one or more features of the above-mentioned plant or system. As such the plant or system may comprise the apparatus as described above, wherein the electrolyser subsystem, the Haber Bosch subsystem and the Ostwald subsystem of the plant or system are the electrolyser subsystem, the Haber Bosch subsystem and the Ostwald subsystem of the apparatus.

[0104] The plant or subsystem may operate the methods as discussed above.

[0105] The present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:

[0106] Figure 1 schematically shows a typical electrolyser cell, multiples of which may be stacked in a stack within an electrolyser subsystem;

[0107] Figure 2 is a flow diagram showing optional control steps within a galvanostatic electrolyser subsystem;

[0108] Figure 3 is a flow diagram showing the interaction between the various subsystems of an apparatus in accordance with an embodiment of the present invention;

[0109] Figure 4 is a flow diagram showing the interaction between the various subsystems of an apparatus in accordance with another embodiment of the present invention;

[0110] Figure 5 is a flow diagram showing the interaction between the various subsystems of an apparatus in accordance with a further embodiment of the present invention with an alternative turbine configuration; and

[0111] Figure 6 is a flow diagram showing the interaction between the various subsystems of an apparatus in accordance with an embodiment of the present invention.

[0112] An electrolyser subsystem 20 typically comprises an electrolyser comprising an electrolysis stack 12, which itself comprises a stack of electrolyser cells 11. Generally, a stack will have tens or even hundreds of cells in parallel, and multiple stacks 12 may be provided within the electrolyser, all electrically connected in series or in parallel.

[0113] Referring now to Figure 1, the basic structure and operation of a typical electrolyser cell 11 within an electrolyser of an electrolyser subsystem 20 is shown by reference to one fuel / electrolyser cell 11 of a stack. It should be noted that other ancillary components related to the electrolyser cell 11 are included in an electrolyser. These usually include heat exchangers, heaters, valves and sensors.

[0114] The electrolyser cell 11 comprises an anode 33, a cathode 34 and an electrolyte 35. Such a structure for an electrolyser cell 11 is well known in the art. With the present invention, the fuel for the electrolyser subsystem will typically be water in the form of steam, although as will be discussed below, and as mentioned earlier in this document, nitrous oxide can also be passed through the electrolyser subsystem.

[0115] As the fuel passes through the electrolyser cell, the fuel will be electrolysed to split it into component parts, such as hydrogen and oxygen in the case of water, or nitrogen and oxygen in the case of nitrous oxide.

[0116] Water - typically in the form of steam - is passed over the cathode 34 via a first electrolyser inlet 41 and a sweep gas, e.g., hot air, 42 can be passed over the anode 33 via a second electrolyser inlet 40. To power the electrolyser cell, an electric current is applied across the electrolyser cell 11 via electric terminals / connections 36, 37 at the anode and cathode sides of the electrolyser cell 11.

[0117] As a consequence of the electrical current, an electrolytic reaction occurs across the electrolyte 35, with, in the case of steam and an oxygen ion conducting electrolyte, oxygen ions passing across the electrolyte 35 from the cathode 34 to the anode 33, whereby some of the steam brakes down into hydrogen on the cathode side of the electrolyser cell 11 and oxygen is produced at the anode side.

[0118] The oxygen can be extracted via an air flow or sweep flow provided by the hot air 42, or without a sweep flow through its own generated backpressure, thus venting it out of an off-gas outlet 38 on the anode side of the electrolyser cell 11. That output is generally oxygen enriched air (with the oxygen enriching the hot gas flow), but if self venting, it may be pure oxygen.

[0119] The hydrogen can be extracted and vented out of another off-gas outlet 39 on the cathode side of the electrolyser cell 11 for use in a subsequent subsystem, as will be discussed below. This off gas typically will be “wet hydrogen” as it will still be mixed with the remaining steam, as the splitting of the steam into oxygen and hydrogen is usually only in respect of a proportion of the supplied steam. Such operational characteristics of electrolyser cells, including SOECs, are well known in the art.

[0120] The anode and cathode off-gases coming from the various electrolyser cells or stacks can be routed together to common cathode and anode outlets for downstream use.

[0121] The off-gases will usually be at a similar temperature to the operational temperature of the electrolyser cell 11 from which it came. However, it can be at a different temperature to the infed (infeed) fluids (i.e. fuel). The specific delta from the input temperature will depend upon the amount of electrical power supplied to the electrolyser, and the internal resistance of the cells.

[0122] Due to the high operating temperature of electrolyser cells 11 (i.e. above 100 degrees C for a steam electrolyser, and in the case of a solid oxide electrolyser cell (SOEC) it is usually in excess of 400 degrees C), the heat from the off-gases from the off-gas outlets 38, 39 is often able to be usefully used by the electrolyser subsystem 20, rather than being wasted, for example to provide at least some of the heat for the steam generation on the input side of the stack, and likewise for heating the hot air entering the electrolyser, in advance of the inlets 40, 41. This may be achieved via heat exchangers. However, as shown in Figures 3 to 5, these off-gases are also instead useable in other industrial processes where the heat can be useful. The present invention concerns such use and this will be discussed now with reference to Figures 3 to 5.

[0123] Figure 3 is a flow diagram showing the interaction between various subsystems of an apparatus 10 in accordance with an embodiment of the present invention. These subsystems include, within the apparatus, or within a plant, such as an industrial plant, or a system, three primary subsystems, them being an electrolyser subsystem 20, a Haber Bosch (HB) subsystem 22 and an Ostwald subsystem 24. The apparatus can additionally include, in some embodiments, a secondary subsystem also integrated into the apparatus, plant or system, comprising an ammonium nitrate subsystem 50 - in the form of an ammonium nitrate reactor. Then, as shown in Figures 3, 4 and 5, the apparatus, plant or system can produce, as an end product, ammonium nitrate.

[0124] The electrolyser subsystem 20 can comprise one or more electrolyser cells 11 , or one or more stacks of cells 11 , such as those discussed above with reference to Figure 1. Each cell 11 will be for producing hydrogen (H2) and oxygen (02) in separate off-gas streams from supplied water / steam using electricity across the cells 11 and electrolysis of the water across the electrolyte 35.

[0125] The Haber Bosch subsystem 22 is for carrying out a Haber (or Haber-Bosch) process by taking the hydrogen from a first off-gas stream 44 from the electrolyser subsystem 20, and to then produce ammonia (NH3) with it by reacting it with nitrogen (e.g. from air - such as via an air separation unit 48). Typically, the hydrogen from the electrolyser subsystem 20 provides all the required hydrogen for the Haber process, although it might provide only part of the required hydrogen if another hydrogen source is available.

[0126] The Ostwald subsystem 24 is then for receiving oxygen (02), for example at least partially from the other - a second - off-gas stream 46 from the electrolyser subsystem 20, and for example with any balance of oxygen coming from another oxygen supply, such as the above air separation unit 48, and the ammonia (NH3) from the Haber Bosch subsystem 22, and to then produce nitric acid (HNO3) with it, along with water in the form of steam (H2O).

[0127] Finally, the ammonium nitrate reactor 50 is configured to produce ammonium nitrate. It achieves this by using a portion of the ammonia produced by the Haber Bosch subsystem 22 and the nitric acid produced by the Ostwald subsystem 24.

[0128] The apparatus 10 is configured with fluid passageways to route the various fluids between the various subsystems.

[0129] In this embodiment, the apparatus 10 comprises a first path 26 to route hydrogen produced by the electrolyser subsystem 20 to the Haber Bosch subsystem 22 for use in the Haber process at the Haber Bosch subsystem to produce ammonia. The apparatus also comprises a second path 28 to route at least a portion of the ammonia produced by the Haber Bosch subsystem 22 to the Ostwald subsystem 24 for use in the Ostwald process at the Ostwald subsystem 24 to produce nitric acid. The apparatus also comprises a third path 30 to route at least a portion of steam produced using heat from the Ostwald subsystem 24 to the electrolyser subsystem 20 for use as at least a part of an infeed gas (fuel) for the electrolyser subsystem 20.

[0130] As shown in Figure 3, the steam that is routed through the third path 30 may directly be carrying steam to the electrolyser subsystem, or it may be conveying heat (e.g. superheated steam or a mixture thereof with other fluids) from the Ostwald system to an evaporator 52 to evaporate water into steam (or to superheat steam) ready for that steam to enter the electrolyser subsystem 20. In Figure 3, a liquid water (54) is fed into the evaporator 52, whereat the water is converted to steam using heat from the Ostwald process - and also heat from the Haber process - so that the steam can be pre-heated to a suitable temperature for use as the fuel 32 for the electrolyser subsystem 20 - for example around 400 to 500 degrees C for a solid oxide fuel cell 11.

[0131] The use of this steam or heat in such an apparatus, plant or system avoids wastage of the heat produced in the Haber Bosch and Ostwald processes. These processes within, for example an ammonium nitrate plant (e.g. where the secondary subsystem is provided) commonly produces a significant quantity of superheated steam. Indeed, there is commonly enough that the steam from the Haber process, and particularly the seam off the endotherm of the Ostwald process, can also be used to drive a turbine to produce electricity that can be utilised in the electrolyser subsystem as well, thus further improving the efficiency of the apparatus. When there is a both a relatively clean, low grade (temperature and pressure) steam supply available (for use in the evaporator, or for directly feeding into the electrolyser cells 11 , as well as a power source (from the turbine - see Figures 4 and 5), all of which can be directly coupled to the H2 production facility (the electrolyser subsystem), a high efficiency integration is possible, even if the electrical supply from that steam is only covering a relatively small fraction of the electrical demand of the stacks 12.

[0132] In practice, steam or heat from the Haber Bosch reaction alone can cover approximately 50% of the steam or heat demand of the electrolyser subsystem (or the evaporator and the electrolyser subsystem together), and it is potentially possible to use the steam from the Ostwald process directly in an SOEC, and potentially with no or only minimal cleaning or filtering.

[0133] The present inventors also realise that the Ostwald process produces more heat and steam than the Haber Bosch subsystem, so there is an excess of heat and steam for supplying steam to the electrolyser subsystem, thus allowing that excess to drive the turbine.

[0134] Another possibility is that nitrous oxide gases, a by-product of the Ostwald process, can be part- recirculated back through the electrolyser subsystem 20 as part of the fuel circuit of the stack 12, instead of needing to separate it from the steam, and thus preventing it from being emitted into the atmosphere. Nitrous oxide gases are hazardous and are particularly troublesome as a greenhouse gas, and thus if they were to be produced and released, they would command a regulatory cost, so the recirculation can help to reduce or avoid that. In addition, if the electrolyser cells 11 can convert the nitrous oxide into oxygen and nitrogen, then the separate outputs thereof within the first and second off-gas streams 44, 46 from the electrolyser subsystem 20 can also put the nitrogen gas with the hydrogen gas in the second path 26 into the Haber process, thus reducing the nitrogen demand on the air separation unit 48.

[0135] Still referring to Figure 3, oxygen from the electrolyser subsystem 20 can be passed to the Ostwald subsystem 24 via a fourth path 56, a fifth path (not shown in Fig 3 - see Figs 4 and 5 instead) can be configured to route nitrous oxide (NOX) and / or steam produced by the Ostwald process to combine it with the third path 30 (i.e. with steam for the electrolyser subsystem), ammonia can be passed from the Haber Bosch subsystem 22 to the ammonium nitrate reactor 50 via a sixth path 60, nitric acid can be passed from the Ostwald subsystem 24 to the ammonium nitrate reactor 50 via a seventh path 64, nitrogen can be passed from the air separation unit 48 to the Haber Bosh subsystem 22 via an eighth path 66 and oxygen can be passed from the air separation unit 48 to the Ostwald subsystem 24 via a ninth path 68 - which in this embodiment joins the fourth path 56. Further, air 62 is fed into the air separation unit 48 for separation into nitrogen and oxygen streams for the eighth and ninth paths 66, 68. Other gases from the air can be captured and used in other processes as known in the art of air separation units.

[0136] Referring next to Figure 4 there is shown a flow diagram showing an alternative embodiment of the present invention, again with the figure showing the interaction between the various subsystems of the apparatus 10 of the present invention.

[0137] Once again the apparatus 10 comprises an electrolyser subsystem 20, a Haber Bosch subsystem 22 and an Ostwald subsystem 24, although the Ostwald subsystem is shown in its two component parts (it is a two stage process). Furthermore, this apparatus again has the secondary subsystem of an ammonium nitrate subsystem comprising an ammonium nitrate reactor 50.

[0138] As with Figure 3, this apparatus routes hydrogen produced by the electrolyser subsystem 20 to the Haber Bosch subsystem 22 via a first path 26 for use in the Haber process. It also routes at least a portion of ammonia produced by the Haber Bosch subsystem 22 to the Ostwald subsystem 24 via a second path 28 for use in an Ostwald process and it routes nitrous oxides (NOX) and / or steam produced in the Ostwald process (a by-product thereof) back to the electrolyser subsystem 20 via a fifth path 58 for use as a fuel in hydrogen production by the electrolyser subsystem 20. However, this figure also shows a second supply of water being routed through two heat exchangers 72, 73 and an optional steam turbine 74 for use in generation of electrical power, with the turbine 74 and heat exchangers 72, 73 being on the third path 30.

[0139] As briefly mentioned above, the Ostwald subsystem typically comprises two stages. These are a first stage (Ostwaldl) in which ammonia from the Haber Bosch subsystem is oxidised to produce nitric oxide (NO) and a second stage (Ostwald2) where the nitric oxide is oxidised using oxygen from the electrolyser subsystem (and optionally the ASU), with the oxidation product being absorbed in water to produce nitric acid (NHO3).

[0140] In the first stage the ammonia from the second path 28 is oxidized (in a sense "burned") by heating it with oxygen from the ninth path 68 in the presence of a catalyst such as platinum, with 10% rhodium, and potentially also a source of air 62, to form nitric oxide and water. This step is exothermic, making it a useful heat source once initiated. Its product - the nitric acid and water (steam) - can then be passed through a first heat exchanger 72 before that product is passed to the second stage.

[0141] In some embodiments the specific enthalpy may be -950 kJ / mol and the chemical equation for this stage of the reaction is:

[0142] 4NH3(g) + 502(g) 4NO(g) + 6H2O(g)

[0143] Given the strong exothermic nature of that first stage, the heat exchanger 72 will be able to exchange heat from the product with water from a liquid water source 54, to create steam, which steam can power a steam turbine 74. Electricity from the turbine can then partially power the electrolysers within the electrolyser subsystem 20 (or some of them), and the now cooled steam (or water) can pass through a second heat exchanger 73 to reheat it.

[0144] The second heat exchanger 73 takes heat from the Haber Bosch process via the ammonia in the second path 28 to reheat the water / steam from the turbine (74) to pre-heat it ready for passing into the electrolyser subsystem 20 as the fuel therefor.

[0145] In the second stage (of the Ostwald process), the oxidization of the product, i.e. the nitric oxide (NO) and some steam, and its absorption by water, is carried out first by the NO combining with oxygen from the fourth path 56 in an oxidation apparatus 78 to produce NO2, and that NO2 is then combined with water (and any remaining steam) in an absorption apparatus 76. For this purpose, the water supply 54 is again used. There is thus a first part where nitric oxide is oxidized again to yield nitrogen dioxide: 2NO(g) + 02(g) — > 2NO2(g). This gas is then readily absorbed by the surrounding water, yielding the desired product (nitric acid, albeit in a dilute form), although a portion of it will also reduce back to nitric oxide: 3NO2(g) + H2O(I) — > 2HNO3(aq) + NO(g). In some embodiments (not shown), the resulting nitric oxide is recycled back through the oxidisation apparatus 78.

[0146] In a preferred apparatus 10, the nitric acid is then subsequently concentrated to a desired strength by distillation (not shown).

[0147] In some embodiments, there is surplus oxygen. Then the following reaction instead occurs: 4NO2(g) + 02(g) + 2H2O(I) 4HNO3(aq).

[0148] Thus no NO needs recycling.

[0149] Typical conditions for the first stage, which contribute to an overall yield of about 96%, are: a pressure between 4 and 10 atmospheres (approx. 400-1010 kPa or 60-145 psig); and a temperature of about 1173 K (approx. 900°C or 1652°F).

[0150] Given these high temperatures, the off-gases from the electrolyser are beneficially used without any cooling, whereas in prior art electrolyser system, the heat in the off-gases is often used to preheat the steam for the electrolyser.

[0151] In some embodiments, the fifth path 58 from the Ostwald process is configured instead (or additionally) to route nitrous oxide (NOX) and / or steam produced by the Ostwald process’ second stage to route it back through the electrolyser subsystem. The NOX and / or steam can then be supplied to the electrolyser subsystem alongside the steam (e.g., raised from waster heat) for use as fuel. The electrolyser will then produce oxygen from the NOX and the steam, nitrogen from the NOX, and hydrogen from the steam, the nitrogen and the hydrogen being mixed together in the second off-gas stream 46. The nitrogen and the hydrogen can then together be routed to the Haber Bosch subsystem 22 via first path 26 for use in the Haber process. This can further increase the efficiency of the apparatus 10 as less nitrogen is required from the ASU 48 for the Haber process. Furthermore, this would make use of the NOX, rather than allowing it to vent to atmosphere - or otherwise requiring it to be subsequently processed to avoid it being vented. NOX, after all, is a harmful greenhouse gas, as mentioned previously, and thus its venting to atmosphere can be controlled by regulatory restrictions. Still referring to Figure 4, this apparatus 10 also comprises an ammonium nitrate (AN) subsystem 50 that is configured to produce ammonium nitrate. The chemical equation for the ammonium nitrate (AN) process is as follows:

[0152] NH3 + HNO3 NH4NO3

[0153] Therefore, the sixth path 60, as in Figure 3, passes a portion of the ammonia produced by the Haber process to the ammonium nitrate subsystem 50, and the seventh path 64 passes the dilute nitric acid from the Ostwald process to the ammonium nitrate subsystem 50. This fourth process is also exothermic, although the ammonium nitrate may want to be processed with that heat to concentrate ammonium nitrate solution before heat is removed (which can be used to raise steam for the electrolyser subsystem) to form a solid product.

[0154] In some embodiments, the apparatus is further configured to produce steam using heat from the ammonium nitrate subsystem and to route that steam to the electrolyser subsystem, or the turbine. This can further improve the overall apparatus efficiency.

[0155] In some embodiments, a third heat exchanger (HX3) may be used to exchange this heat with a suitable path. For example, the heat exchanger can be downstream of the output path from the ammonium nitrate subsystem, or it may be integrated with the ammonium nitrate subsystem. Indeed any of the heat exchangers may be integrated with one or more of the subsystems to utilise heat thereof.

[0156] Referring finally to Figure 5, a modified version of Figure 4 is provided in which the steam turbine is moved. In this embodiments, the steam turbine 74 is positioned along the third path 30, downstream of both the first and the second heat exchangers 72, 73, so that the steam is heated and thus expanded by both the first heat exchanger 72 and the second heat exchanger 73 and thus both the Haber process and the Ostwald process to maximise the power output from the steam turbine 74.

[0157] Therefore, in some embodiments the steam turbine is positioned between the first and second heat exchangers, whereas in other embodiments it is positioned after the second heat exchanger - to benefit from both heat sources, although preferably it is between the first and second heat exchangers to cool the ammonia before it enters the first stage of the Ostwald process. Figure 5 also shows an alternative configuration for the steam recycling. Instead of directly taking steam from the Ostwald process, only the heat from the Ostwald process is taken and that heat is used to generate steam for the fuel of the electrolyser subsystem. In this embodiment, therefore, the fifth path 58 recirculates gases such as oxygen, nitrous oxide and nitrous dioxide from the second stage of the Ostwald process back to the first stage of the Ostwald process. Instead only water from the water supply passes through the third path to the input for the electrolyser - past the two heat exchangers 72, 73 and the steam turbine.

[0158] Furthermore, ammonia for the ammonium nitrate reactor 50 comes through the sixth path 60 via a part of the second path 28 and the second heat exchanger 73, it thus being cooled versus the exit temperature from the Haber process, and a greater amount of heat is available for transfer from the ammonia in the second path to the water / steam in the third path. Both approaches can be used in either embodiment.

[0159] Yet further, oxygen from the electrolyser subsystem 20 is fed into the Ostwald process at the first stage, along with oxygen from the ASU. In Figure 4, the oxygen from the electrolyser subsystem is only fed into the Ostwald process at the second stage, with the ASU instead providing all the oxygen at the first stage. Both approaches can be utilised in either embodiment.

[0160] Given the importance of ammonium nitrate for agriculture and other industries, the present inventors realised that significant benefits can be achieved through the present invention’s integration of these processes. For example, ammonium nitrate is one of the most common fertiliser chemicals. The global production volume in 2018 was about 48.6 million tonnes. The overall process provided by the present invention, by way of its integration of the primary subsystems, and the interactions they have between themselves, and similarly the interaction with the secondary subsystem of the ammonium nitrate reactor, is highly advantageous when using an intermediate or high temperature electrolyser - particularly an SOEC, due to the ability to use the products and by products of each process within the other processes to result in minimal product wastage and minimal heat loss. Significant cost savings are thus achievable through these efficiencies. Indeed, an almost ideal mass balance is achieved through the combined process between Nitrogen, hydrogen and oxygen.

[0161] Ammonium nitrate is an intended final product. However, nitric acid (as a liquid) is also a highly valuable commodity too, and as such the present invention initially comprises the primary subsystems, but can also comprise the secondary subsystem. The energy balance is also highly useful for this integration:

[0162] Furthermore, the use of oxygen instead of air in the Ostwald process makes the Ostwald reaction more efficient, less polluting and less power intensive. That is because few or no driers are needed and there is less gas to move around. Furthermore, the electrolyser process can be more efficient as less reliance on heaters, or heat exchange with the off-gases, is needed for pre-heating the water into steam, ready for passage through the stack. Furthermore, that hot off-gas (both the hydrogen and the oxygen) can be utilised in the Haber process or the Ostwald process, both of which are at high temperatures, thus needing little or no heat exchange prior to use in those processes. Furthermore, the excess heat from the exothermic Haber and Ostwald processes can be used to generate power (albeit only a part of the power demand) for the electrolyser subsystem.

[0163] The present invention is thus highly efficient simply through its integration of the various processes.

[0164] Figure 6 also shows an alternative configuration for the steam recycling from the Haber process and Ostwald process. The ammonium nitrate process is not shown in Fig. 6 but may be integrated with the system of Fig. 6 in a similar manner to that described previously. Only differences between Fig. 6 and the previous Figures will be described.

[0165] In the case of Fig. 6, the source of oxygen for first stage of the Ostwald process - ammonia oxidation - is air 62, and may in some cases be oxygen-depleted air. This enables oxidation and may be required in order to meet safety limits for ammonia oxidation. The second stage of the Ostwald process is supplied wholly with 02 from the electrolyser and optionally from the air separation unit. This improves efficiency and yield of the NO oxidation at the second stage.

[0166] Steam raised from the first stage (Ostwaldl) of the Ostwald process is used to drive the steam turbine 74 since said steam is high quality and high pressure. This can be via the first heat exchanger 72, in which water from a water supply 54 gets converted to steam for the turbine 74. Electrical power and exhaust steam (lower quality and pressure, but suitable as fuel for electrolyser subsystem) from the turbine 74 is then routed to the electrolyser subsystem 20 for use therein.

[0167] Steam and optionally or alternatively NOX by-products of the second stage (Ostwald 2) of the Ostwald process can also be routed via the third and fifth paths 30, 58 to be used as fuel in electrolyser subsystem 20, for example combining with the path exiting the turbine 74.

[0168] Further steam, if needed can be provided via the second heat exchanger 73, which heats water - again from the water supply 54 using heat from the Haber Bosch subsystem 22, taken from the ammonia exiting the Haber Bosch subsystem 22 and entering the first stage of the Ostwald process.

[0169] Figure 6 also shows oxygen from the air separation unit 48 passing through the fourth path 56 both to the electrolyser subsystem 20 and the second stage (Ostwald 2) of the Ostwald process. Furthermore, waste (or by-product) oxygen, NO and NO2 from the Nitric acid process of the Ostwald2 stage is recycled back through the second stage of the Ostwald process.

[0170] Again, therefore, the present invention is highly efficient simply through its integration of the various processes of its various subsystems.

[0171] Referring now back to Figure 1, and Figure 2, a further enhancement of the efficiency of the apparatus can be achieved through the beneficial use of galvanostatic conditions in the electrolyser subsystem.

[0172] One beneficial feature of the present invention is that the fuel (steam) can be supplied to the stack at operational temperatures, as the Haber process and the Ostwald process produce an abundance of heat (they are both exothermic). That then allows the operation of the stack(s) to be in an adiabatic state, i.e. balanced energetically, which effectively means no heat is consumed or released by the stack (it can be neither exothermic or endothermic at optimal power usage). This is an efficient state for the stack(s), especially when the stack is being operated in galvanostatic conditions (i.e. with a constant current). In galvanostatic operation, the electrical resistance of the stack controls the voltage applied across the stack, and there will be a variable power draw from the stack as the resistance changes.

[0173] Off-gases exiting the electrolyser cells (or the stacks) will usually be at a similar temperature to the operational temperature of the electrolyser cell 11. However, the specific difference (delta) between the off gas temperature and the input temperature of the steam for the electrolyser will depend upon the amount of electrical power supplied to the electrolyser, and the internal resistance of the cells. In an adiabatic state, the delta will be small or zero.

[0174] Operational efficiency can be best improved by reducing the amount of external heat needing to be supplied to the system through the use of heaters. Where that external heat is provided for free - for example as a waste product of another industrial process, then the amount of use of heaters can be reduced, potentially to zero. This increases the efficiency of the system.

[0175] Instead of heaters, additional electrical power across the stack can also be applied, which also heats the stack and thus the off-gases. Although using more power can increase the output rate of the off-gases, there is a limit to this, and likewise with cell temperatures - there is an optimum temperature and an optimum power draw for that temperature. When using off-gases and heat exchangers to heat the infeed fluids, that increased current can then replace heaters. However, the additional power usage is detrimental to the efficiency of the stack, if the system becomes exothermic, just as electric heaters would be. Likewise if it becomes endothermic due to a lack of power, additional heating becomes necessary, thus again reducing efficiency. With the present invention, such stack and infeed gas heating is not necessary as steam at operational temperatures can be provided by the Haber process and the Ostwald process. The present invention can thus operate the stack using both galvanostatic conditions within the stack, i.e. a constant electrical current (constant amps), and by seeking to adopt thermoneutral voltages across the stack to avoid heat wastage in the stack: at a thermoneutral condition, the electrolyser is in an adiabatic state.

[0176] A further feature of the present invention can also be utilised when there are variations in load on the system, and thus the absolute value (i.e. modulus) of the temperature delta can change. The present invention can utilise a control regime to compensate and correct the stack’s operation if this occurs. In this respect, if the delta is more than a temperature delta threshold (positive or negative), an adjustment to the target input temperature is applied. In the preferred regime, when the temperature delta is positive, the adjustment to the target input temperature is also positive, and when the temperature delta is negative, the adjustment to the target input temperature is also negative. This is counterintuitive as typically in the prior art a heating stack wants to be cooled, whereas a cooling stack wants to be heated. However, the present invention relies upon the resistance response to stack temperature changes (which is fast). By doing this, the invention can use the stack’s electrical resistance response for interaction with the voltage across the stack due to the stack’s galvanostatic operational condition: as the input temperature increases, the stack warms. This reduces the electrical resistance of the stack, which in turn reduces the voltage across the stack. That reduction of voltage, in galvanostatic conditions (i.e. with a constant current), reduces the power draw of the stack, and thus, if now lower than the thermoneutral voltage, due to the endothermic nature of electrolysis, the stack actually avoids heating up by as much as the increased input temperature, and can even cool. Conversely, as the input temperature reduces, the stack cools. This increases the electrical resistance of the stack, which in turn increases the voltage across the stack. That increase of voltage, in galvanostatic conditions (i.e. with a constant current), increases the power draw of the stack, and thus, when it exceeds the thermoneutral voltage, due to the now exothermic nature of the configuration (due to the increase in power input from the constant current source), the stack actually avoids cooling down by as much as the reduced input temperature, and can even heat up.

[0177] The present invention thus provides a regulated internal temperature control for the stack using external fluid temperature control, galvanostatic conditions across the stack, and the power draw characteristics of the stack arising from fluctuations in voltage either side of a thermoneutral voltage for the stack.

[0178] The present invention has therefore been described above purely by way of example. Modifications in detail may be made to the invention within the scope of the claims as appended hereto.

Claims

CLAIMS1. An apparatus comprising: an electrolyser subsystem; a Haber Bosch (HB) subsystem; and an Ostwald subsystem; wherein the apparatus is configured with fluid passageways to: a) route hydrogen produced by the electrolyser subsystem to the Haber Bosch subsystem via a first path for use in a Haber process at the Haber Bosch subsystem to produce ammonia; b) route at least a portion of the ammonia produced by the Haber Bosch subsystem to the Ostwald subsystem via a second path for use in an Ostwald process at the Ostwald subsystem to produce nitric acid; and c) route at least a portion of steam produced using heat from the Ostwald subsystem to the electrolyser subsystem via a third path for use as at least a part of an infeed gas for the electrolyser subsystem.

2. The apparatus of claim 1 , wherein heat from the Ostwald subsystem is used to produce steam from another water source.

3. The apparatus of claim 2, wherein a first heat exchanger is configured to exchange the heat from the Ostwald subsystem to produce the steam.

4. The apparatus of any one of the preceding claims, wherein at least 90% of the hydrogen used or consumed in the Haber Bosch process is produced by the electrolyser subsystem.

5. The apparatus of any one of the preceding claims, wherein the hydrogen from the electrolyser subsystem provides all the required hydrogen for the Haber process.

6. The apparatus of any one of the preceding claims, wherein some electrical power is supplied to the electrolyser subsystem from a turbine of the apparatus, which turbine generates electricity using heat from the apparatus.

7. The apparatus of any one of the preceding claims, wherein a heat exchanger for each of the Ostwald and Haber Bosch subsystems are provided to exchange heat from those processes with water or steam for entering the electrolyser subsystem.

8. The apparatus of any one of the preceding claims, wherein at least 40% of the steam used by the electrolyser subsystem is produced by, or using heat from, the Ostwald subsystem.

9. The apparatus of any one of the preceding claims, wherein at least 25% of the steam used by the electrolyser subsystem is produced by using heat from the Haber Bosch subsystem.

10. The apparatus of any one of the preceding claims, wherein the apparatus is configured to route oxygen produced by the electrolyser subsystem to the Ostwald subsystem via a fourth path for use of oxygen in the Ostwald process.

11. The apparatus of any one of the preceding claims, wherein the apparatus further comprises an air separation unit (ASU) for separating nitrogen from air, the nitrogen being for feeding to the Haber Bosch subsystem.

12. The apparatus of claim 11 , wherein the ASU also supplies oxygen for the Ostwald process.

13. The apparatus of any one of the preceding claims, wherein at least 50% by mass of fluid exhausted from the electrolyser subsystem at an anode side of the electrolyser subsystem is oxygen produced by the electrolyser subsystem.

14. The apparatus of any one of the preceding claims, wherein a fifth path is configured to route nitrous oxide produced by a second stage of the Ostwald process to combine it with the third path.

15. The apparatus of claim 14, wherein the second stage additionally produces steam and the fifth path is configured to route said steam to combine it with the third path.

16. The apparatus of claim 14 or claim 15, wherein the apparatus is configured to route a portion of the ammonia produced by the Haber Bosch subsystem to an ammonium nitrate subsystem via a sixth path and to route nitric acid produced by the second stage to the ammonium nitrate subsystem via a seventh path.

17. The apparatus of any one of the preceding claims, wherein a steam turbine is positioned along the third path.

18. The apparatus of claim 17, when dependent upon claim 3, wherein the steam is heated and thus expanded by the first heat exchanger and is driven through the steam turbine subsequent to the first heat exchanger.

19. The apparatus of claim 17, when dependent upon claim 7, wherein the steam turbine is positioned after the second heat exchanger.

20. The apparatus of any one of the preceding claims, wherein the electrolyser subsystem comprises at least one solid oxide electrolyser cell.

21. The apparatus of any one of the preceding claims, wherein electrolyser cells in the electrolyser subsystem operate at a thermoneutral voltage and / or wherein the electrolyser subsystem operates substantially under galvanostatic conditions.

22. An apparatus comprising: an electrolyser subsystem; a Haber Bosch subsystem; an Ostwald subsystem; and an ammonium nitrate subsystem;- wherein the apparatus is configured to: a) route hydrogen produced by the electrolyser subsystem to the Haber Bosch subsystem via a first path for use in a Haber process at the Haber Bosch subsystem; b) route at least a portion of ammonia produced by the Haber Bosch subsystem to the Ostwald subsystem via a second path for use in an Ostwald process at the Ostwald subsystem; and c) produce steam using heat from at least one of the Ostwald subsystem, the ammonium nitrate subsystem and the Haber Bosch subsystem; and to■ route at least a first portion of said steam to the electrolyser subsystem via a third path for use as a fuel in hydrogen production by the electrolyser subsystem;■ route at least a second portion of said steam to a steam turbine for use in generation of electrical power.

23. The apparatus of claim 22, being further in accordance with any one or more of claims 124. A method for producing nitric acid (HNO3) comprising: producing hydrogen and oxygen using an electrolyser subsystem; producing ammonia using a Haber process using hydrogen from the electrolyser; and producing nitric acid using an Ostwald process using ammonia from the Haber process and oxygen from the electrolyser subsystem;- wherein the method further comprises using heat from the Ostwald process and / or the Haber process at the electrolyser subsystem to produce said hydrogen and oxygen at the electrolyser subsystem.

25. The method of claim 24, wherein the method involves routing the hydrogen from the electrolyser subsystem to a Haber Bosch subsystem of the apparatus, and routing oxygen from the electrolyser subsystem to an Ostwald subsystem of the apparatus.

26. The method of claim 25, carried out using an apparatus in accordance with any one of claim 1 to 21.

27. The method of any one of claims 24 to 26, comprising using the heat to power a steam turbine to generate electricity and / or comprising using the heat to pre-heat or generate steam for passing through to the electrolyser subsystem.

28. The method of any one of claims 24 to 27, wherein the method produces hydrogen and oxygen at the electrolyser subsystem by producing steam using said heat from the Ostwald process and / or the Haber process and using said steam as fuel for the electrolyser subsystem, and by supplying electrical power to the electrolyser.

29. The method of any one of claims 24 to 28, wherein the method further produces ammonium nitrate in a subsequent ammonium nitrate subsystem using an ammonium nitrate process using ammonia from the Haber process and nitric acid from a second stage of the Ostwald process.

30. The method of any one of claims 24 to 29, wherein an air separation unit is provided for separating air into a stream of oxygen and nitrogen and providing the oxygen to the Ostwald process, alongside oxygen from electrolyser subsystem, and providing the nitrogen to the Haber process.

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