Methods and systems for nitrogen oxides recovery

Abstract

The present invention relates to a method for recovery of NOx, said method comprising: producing a flue gas stream comprising NOx in a gas turbine, cooling said flue gas stream, and feeding the cooled flue gas stream to a NOx recovery unit to obtain a purified NOx flue gas stream. The invention further relates a system configured to carry out said method and to the use of said system.

Description

[0001] METHODS AND SYSTEMS FOR NITROGEN OXIDES RECOVERY

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the field of recovery of nitrogen-based compounds from flue gas. More concretely, the present invention relates to the field of nitrogen oxides recovery.

[0004] BACKGROUND OF THE INVENTION

[0005] The ongoing energy transformation, which is fueled by environmentally cautious policies, demands a full synergy with existing back-up gas turbines. Renewable energy sources, such as wind and solar, are intermittent by nature and present large variations across the span of the day, seasons, and geographies. In this context, the gas turbine is seen as an essential part of the energy transition because of its superior operational flexibility over non-renewable counterparts, such as hydropower and nuclear energy (Farhat H. et al, Energies 2022, 15, 5474).

[0006] Gas turbine-based power plants serve a pivotal role in the energy sector by providing essential baseload and peak power generation capabilities. To enhance the sustainability of existing gas turbine plants, improving their overall efficiency stands as a crucial objective to curtail greenhouse gas emissions and optimize energy production.

[0007] Recent studies have focused on enhancing gas turbine systems to achieve higher efficiency and lower emissions. Researchers have explored various methods to improve these systems, including advanced cooling techniques (Unnikrishnan U. et al, Propulsion and Power Research 2022, 11 (3), 293-310), combustion optimization (Barakat A. A et al., Energy Conversion and Management 2020, 226, 113538), and waste heat recovery systems (Faqihi B.et al, International Journal of Thermofluids 2023, 18, 100347).

[0008] However, one of the main limitations in power production using gas turbines is the generation of nitrogen oxides. Nitrogen oxides are polluting compounds whose emissions are regulated by environmental protection agencies. To adjust the nitrogen oxides content to the emissions limit, the combustion temperature in the gas turbines is strictly controlled to avoid exceeding the emissions limit, although other pollution control techniques are also used. Briefly, the higher the combustion temperature, the higher the nitrogen oxides production meaning that, by reducing the combustion temperature in the turbine to adjust the nitrogen oxides production and meet the current regulations, the amount of fuel required in the turbine is increased.

[0009] Therefore, there is a need for efficient methods to reduce nitrogen oxides emissions related to flue gas produced by gas turbines, without reducing the combustion temperature, thereby achieving the highest thermodynamic efficiency of the turbine. BRIEF DESCRIPTION OF THE INVENTION

[0010] The inventors have found a solution to the above problem by feeding a cooled flue gas stream from a gas turbine to a nitrogen oxides (NOx) recovery unit to obtain a purified nitrogen oxides (NOx) flue gas stream. In particular, the solution to the problem resides in feeding the cooled flue gas stream from a gas turbine to a NOx recovery unit working on the principle of adsorption-desorption of NOx in a solution of nitric acid. This solution can be applied to gas turbines, either conventional or modified to accommodate the recovery unit within. The utilization of a NOx recovery unit as the one described above in the treatment of a flue gas stream generated in a gas turbine primarily allows for increasing the combustion temperature in the turbine up to the limits dictated by the turbine materials, thereby reducing the amount of fuel required in the turbine while maximizing the production of nitrogen oxides. The higher the combustion temperature, the higher the thermodynamic efficiency of the turbine and, therefore, the lower the amount of fuel required per kWh of electricity produced. Increasing the NOx production by applying the method of the invention does not represent an environmental problem, since these nitrogen oxides are not released into the atmosphere but are rather efficiently recovered in a secondary stream, which can be used to produce nitric acid or other useful nitrogen-based compounds.

[0011] Therefore, the present invention provides a reliable solution for the recovery of NOx from flue gas of gas turbines, especially at the highest combustion temperatures permitted by the turbines.

[0012] Thus, in a first aspect the method of the invention relates to a method for recovery of NOx, said method comprising: a) producing a flue gas stream comprising NOx in a gas turbine, wherein the gas turbine comprises: a first gas compressor, a turbine combustor, and a turbine expander, b) cooling said flue gas stream, and c) feeding the cooled flue gas stream of step b) to a NOx recovery unit to obtain a purified NOx flue gas stream, wherein the first gas compressor, the turbine combustor, the turbine expander and the NOx recovery unit are fluidically connected.

[0013] In a second aspect, the invention relates to a system (also denoted as “system of the invention”) configured to carry out the method of the first aspect, said system comprising: a) a gas turbine (A) comprising: i. a first gas compressor (1), ii. a turbine combustor (2), and iii. a turbine expander (3), wherein the gas turbine (A) is configured so as the combustor (2) is located downstream the first gas compressor (1) and upstream the turbine expander (3); b) a first heat exchanger (B); c) a NOx recovery unit (C) comprising:

[0014] - an absorption column (4) comprising an aqueous solution of nitric acid,

[0015] - a second heat exchanger (5),

[0016] - a desorption column (6), and

[0017] - an optional separation unit (7). wherein the NOx recovery unit (C) is configured so as the second heat exchanger (5) is located downstream the absorption column (4) and upstream the desorption column (6), and wherein the first gas compressor (1), the turbine combustor (2), the turbine expander (3), the first heat exchanger (B), the absorption column (4), the second heat exchanger (5), the desorption column (6) and the optional separation unit (7) are adapted to be fluidically connected.

[0018] In a further aspect, the invention relates to the use of the system according the second aspect of the invention in the recovery of NOx from flue gas streams of a gas turbine.

[0019] DESCRIPTION OF THE FIGURES

[0020] Figure 1. Schematic view of the system of the invention according to one of its embodiments.

[0021] Figure 2. Schematic view of the system of the invention according to one of its embodiments. The NOx recovery unit is downstream a second gas compressor.

[0022] Figure 3. Schematic view of the system of the invention according to one of its embodiments. The NOx recovery unit is fed between two compression stages of a multistage gas compressor.

[0023] Figure 4. Schematic view of the system of the invention according to one of its embodiments. The NOx recovery unit is located between the turbine gas compressor and the turbine combustor.

[0024] Figure 5. Schematic view of the NOx recovery unit of the method / system of the invention.

[0025] Figure 6. Schematic view of the NOx recovery unit of the method / system of the invention according to one of its embodiments.

[0026] Figure 7. Schematic view of the NOx recovery unit of the method / system of the invention according to one of its embodiments.

[0027] DETAILED DESCRIPTION OF THE INVENTION In a first aspect, the invention relates to a method for recovery of NOx, said method comprising: a) producing a flue gas stream comprising NOx in a gas turbine, wherein the gas turbine comprises: a first gas compressor, a turbine combustor and a turbine expander, b) cooling said flue gas stream, and c) feeding the cooled flue gas stream of step b) to a NOx recovery unit to obtain a purified NOx flue gas stream, wherein the first gas compressor, the turbine combustor, the turbine expander and the NOx recovery unit are fluidically connected.

[0028] In the context of the present invention, the term “gas turbine” refers to a continuous flow internal combustion engine, typically comprising a gas compressor, a combustor and an expander. The working fluid of a gas turbine is gas, also denoted as “fuel” or “fuel gas” throughout the description. This gas can be particularly hydrogen, natural gas, LPG (propane, butane or a mixture thereof), syngas, biogas (such as biogas comprising methane, H2S and mixtures thereof), ammonia or a mixture thereof. In the gas turbine, the fuel is contacted with the comburent, which is a comburent comprising oxygen, such as air, oxygen-enriched air or mixtures thereof. A flue gas stream is produced in the turbine combustor from the combustion between fuel and comburent. The use of the term "gas turbine" herein is intended to be broad, and does not limit the applicability of the invention to certain types of turbine engines.

[0029] In the context of the present invention, the term “compressed” refers to a gas which has been passed through a compressor to increase its pressure to an optimal value, being said optimal value higher than the initial pressure value of the gas before being passed through the compressor. The person skilled in the art will know which value of pressure is optimal for a compressed gas, considering the application desired for said gas.

[0030] In the context of the present invention, the term “flue gas” refers to the gas which leaves the gas turbine combustor after the combustion process. In a preferred embodiment, this “flue gas” comprises dinitrogen, CO2, NOx and water.

[0031] In the context of the present invention, the terms “NOx” or “nitrogen oxides” are used indistinctly, and both term refer to a group of gases, such as nitric oxide or nitrogen dioxide that contain nitrogen and oxygen in variable proportions. In particular, NOx or “nitrogen oxides” refer to NO, NO2 or a mixture thereof.

[0032] In the context of the present invention, the terms “molecular nitrogen” or “dinitrogen” are used indistinctly. Both terms refer to the diatomic homonuclear molecule N2, composed by two nitrogen atoms bound together. In the context of the present invention, the terms “molecular oxygen” or “dioxygen” are used indistinctly. Both terms refer to the diatomic homonuclear molecule O2, composed by two oxygen atoms bound together.

[0033] In the context of the present invention, the terms “molecular hydrogen” or “dihydrogen” are used indistinctly. Both terms refer to the diatomic homonuclear molecule H2, composed by two hydrogen atoms bound together.

[0034] In the context of the present invention, the term “fluidically connected” or “in fluidic communication” refers to two or more elements connected in an appropriate manner such that a fluid or at least a portion of a fluid can directly or indirectly pass or flow from one to another of the two or more elements.

[0035] In the context of the present invention, the term “directly connected” refers to physical and immediate connection between two components or elements (e.g. through pipes, tubes, welded or flanged joints, and the like).

[0036] In the context of the present invention, the term “purified NOx flue gas stream” refers to a gas stream which has been treated according to the method of the first aspect, particularly steps b) and c) of the method of the first aspect, to increase the NOx content. Preferably, the “purified NOx flue gas stream” has an increased NOx content at least by 2 times, more preferably at least by 5 times, when compared to the non-treated flue gas stream comprising NOx (e.g. such as the one produced in step a) of the method of the method of the first aspect).

[0037] Throughout the present specification, when a numeric range is indicated herein, both the lower limit and the upper limit are meant to be included in said range. The term “about” indicate a variation of ±10%, preferably ±5%, of the value it is referred to.

[0038] Throughout the present specification, the unit “% v / v” refers to the ratio of volume / volume solution, that is, the number of milliliters of solute dissolved per 100 mL of solution.

[0039] Throughout the present specification, the unit “% mol” refers to the number of moles of a certain gas present in a mixture of gases, per 100 moles of the total gases present in said mixture of gases.

[0040] In a particular embodiment of the first aspect, a comburent, preferably air, enters the first gas compressor. The first gas compressor increases the pressure of the comburent, preferably air, up to 40 bar, preferably up to 30 bar, even more preferably up to 20 bar.

[0041] In another particular embodiment, the first gas compressor increases the pressure of the comburent, preferably air, up to a pressure comprised between 5 bar and 40 bar, preferably between 5 bar and 30 bar, even more preferably between 5 bar and 20 bar.

[0042] After being compressed, the comburent, particulary air, is fed into the turbine combustor wherein is contacted with the fuel gas of the gas turbine, and reacts with the molecular oxygen of the comburent, preferably molecular oxygen of air, leading to combustion products (such as CÜ2, water, etc). The type of combustion products will depend on the specific gas fueling the gas turbine. Further, part of the molecular nitrogen of the air reacts with the molecular oxygen, leading to NOx. The gas stream which exits the turbine combustor is herein referred as flue gas stream comprising NOx. The content of NOx (expressed as % mol) in the flue gas stream comprising NOx will primarily depend on combustion temperature, fuel type, air-to-fuel ratio, and combustion design among other variables. In a particular embodiment, the content of NOx in the flue gas stream comprising NOx is at least 0.1% mol, at least 0.2% mol, at least 0.3% mol, at least 0.4% mol.

[0043] In a particular embodiment, the content of NOx in the flue gas stream comprising NOx is from 0.1 % mol to 1.2% mol, preferably from 0.2% mol to 1.2% mol, more preferably from 0.3% mol to 1.2% mol, even more preferably from 0.4% mol to 1.2% mol

[0044] In a particular embodiment, the turbine combustor operates at a temperature comprised between 1000 °C and 2500 °C, preferably at a temperature comprised between 1300 °C and 2000 °C, even more preferably at a temperature comprised between 1400 and 1700 °C.

[0045] In a particular embodiment, the turbine combustor operates at a pressure comprised between 5 bar and 40 bar, preferably between 5 bar and 30 bar, even more preferably between 5 bar and 20 bar.

[0046] In a particular embodiment, the turbine combustor can be, among others, a can combustor, annular combustor and cannular combustor or any other turbine combustor known in the state of the art.

[0047] The flue gas stream comprising NOx exiting the turbine combustor is then fed into the turbine expander, wherein the pressure and the temperature of said flue gas stream is decreased. In a preferred embodiment, the turbine combustor and the turbine expander are fluidically and directly connected.

[0048] In a particular embodiment, the flue gas stream comprising NOx enters the turbine expander at a temperature comprised between 1000 °C and 2500 °C, preferably at a temperature comprised between 1300 °C and 2000 °C, even more preferably at a temperature comprised between 1400 and 1700 °C.

[0049] In a particular embodiment, the flue gas stream comprising NOx enters the turbine expander at a pressure comprised between 5 bar and 40 bar, preferably between 5 bar and 30 bar, even more preferably between 5 bar and 20 bar.

[0050] In a particular embodiment, the flue gas stream comprising NOx exits the turbine expander at a temperature comprised between 300 °C and 1500 °C, preferably at a temperature comprised between 500 °C and 1000 °C.

[0051] In a particular embodiment, the flue gas stream comprising NOx exits the turbine expander at a pressure comprised between 1 bar and 2 bar, preferably between 1.5 bar and 2 bar, even more preferably between 1.6 and 1.8 bar. The temperature of the flue gas stream comprising NOx which exits the gas turbine expander is high and unsuitable for the NOx recovery unit of the method of the invention. Thus, before feeding said flue gas stream comprising NOx into the NOx recovery unit of step c), a cooling step must be performed.

[0052] In a particular embodiment, the temperature of the flue gas stream comprising NOx is decreased in step b) to a temperature comprised between 0 and 85 °C, preferably comprised between 20 °C and 65 °C, more preferably at a temperature comprised between 30 and 40 °C, even more preferably at a temperature comprised between 32 and 37 °C .

[0053] In a more particular embodiment, the cooling of step b) is performed stepwise, preferably in two steps: a first and a second step. Preferably, the cooling is gradual from the first to the second cooling step.

[0054] In the most particular embodiment, the cooling of step b) comprises: b1) a first temperature decrease to between 105 °C and 200 °C, preferably to between 105 °C and 150 °C, even more preferably to 105 °C to 120°C, and b2) a second temperature decrease to a temperature comprised between 0 and 85 °C, preferably at a temperature comprised between 20 °C and 65 °C, more preferably at a temperature comprised between 30 and 40 °C, even more preferably at a temperature comprised between 32 and 37 °C.

[0055] The time for the cooling of step b) or the cooling of steps b1) and b2) is not particularly limited as it would be apparent to a person skilled in the art, depending on the technique for cooling in each of the indicated step, how long the cooling should take (this could be monitored, for example, by a devices measuring the decrease of pressure and / or temperature),

[0056] The cooling step b), preferably comprising the steps b1) and b2), can be performed by means of any suitable technique known in the state of the art.

[0057] The cooling step b1) can be performed by means of any suitable technique known in the art which allows the flue gas stream to be cooled at a temperature comprised between 105 °C and 200 °C. Non-limiting examples are the cooling by a steam generator, a reheater, a reboiler or a combination thereof. In a preferred embodiment, the cooling step b1) is performed by a steam generator.

[0058] The cooling step b2) can be performed by means of any suitable technique known in the art which allows to the flue gas stream to be cooled at a temperature comprised between 0 °C and 85 °C. Non-limiting examples are the use of a shell and-tube heat exchanger, a plate heat exchanger or a combination thereof. In a preferred embodiment, the cooling step b2) is performed in a shell-and-tube heat exchanger.

[0059] Each of the methods for the cooling steps b1) and b2) would apply to the step b) with no gradual cooling. In a particular embodiment, the cooled flue gas stream comprising NOx is fed into the NOx recovery unit at a temperature comprised between 0 and 85 °C, preferably comprised between 20 °C and 65 °C, more preferably at a temperature comprised between 30 and 40 °C, even more preferably at a temperature comprised between 32 and 37 °C.

[0060] In an embodiment, the NOx recovery unit of the method of the invention comprises an absorption column comprising an aqueous solution of nitric acid, a heat exchanger, a desorption column and, optionally, a separation unit, wherein the absorption column, the heat exchanger, the desorption column and the optional separation unit are fluidically connected.

[0061] In a more particular embodiment, the NOx recovery unit further comprises an additional heat exchanger located upstream the absorption column.

[0062] The method of the present invention can be performed according to different alternative ways which couple the use of a turbine gas with a NOx recovery unit, depending on the characteristics and on the relative position of the elements which are or are not fluidically connected.

[0063] In a particular embodiment, the method of the invention comprises feeding the cooled flue gas stream of step b) sequentially into a second gas compressor and the NOx recovery unit, preferably being the second gas compressor and the NOx recovery unit fluidically connected, more preferably being the second gas compressor and the NOx recovery unit fluidically and directly connected. In a more particular embodiment, the second gas compressor is a single- step gas compressor or a multistage compressor, preferably the second gas compressor is a multistage gas compressor.

[0064] In a particular embodiment, the second gas compressor increases the pressure of the cooled flue gas stream from step b) up to a pressure comprised between 1 bar and 80 bar, preferably between 5 bar and 60 bar, even more preferably between 10 bar and 40 bar.

[0065] The person skilled in the art would know that when a multistage gas compressor is used, a cooler and a vessel, wherein the vessel is preferably a knockout drum (KO drum), are added between two compression stages. This is due to the fact that after a compression stage the gas is heated, increasing its temperature to a value that can be prejudicial to the gas compressor. Therefore, there is a need to cool down the temperature prior to the next compression stage. As a consequence of the compression and cooling down steps, some liquid drops can appear. These drops are required to be collected in a vessel, since liquids such as liquid drops are prejudicial to the gas compressor.

[0066] In a particular embodiment, the NOx recovery unit is located downstream the first gas compressor and upstream the turbine combustor. In another particular embodiment, the NOx recovery unit is located between the first gas compressor and the turbine combustor of the gas turbine.

[0067] In a particular embodiment, the NOx recovery unit is directly connected to the first gas compressor, thus preferably downstream and directly connected to the first gas compressor. When the NOx recovery unit is directly connected to the first or to the second gas compressor, the pressure of the flue gas stream comprising NOx is increased before entering the NOx recovery unit.

[0068] Using a gas compressor before the NOx recovery unit (for example, see particular embodiments in figures 2-4) has the following advantages:

[0069] Increase of the pressure of the flue gas stream comprising NOx. If the pressure of said stream is increased enough, some water can be recovered in an additional vessel. If the amount of this recovered water is high enough, the flow rate of the inlet flue gas stream comprising NOx to be treated in the absorption columns is lower. Therefore, the size of the absorption column can be reduced.

[0070] Increase of the operating pressure of the absorption column. As a result of this increase, the solubility of NOx in the aqueous solution of nitric acid is higher, thus reducing the required height of the absorption column and the required flow rate of the aqueous nitric acid solution. In addition, the density of the flue gas stream comprising NOx is also increased, resulting in a reduction in the volumetric flow rate of the flue gas stream comprising NOx to be treated. This implies a reduction in the required diameter of the absorption column.

[0071] Reduction of the required size of other components of the NOx recovery unit, such as desorption column and reboiler, if used. The greater the pressure difference between the absorption column and the desorption column, the greater the desorption produced at the entrance of the desorption column. In addition, the increase of the NOx solubility in the aqueous nitric acid solution means that the desorption performed in the reboiler, if used, is easier. As a result, the size and the duty of the reboiler are reduced and, since the liquid and vapor traffic in the desorption column has been reduced, the size of said desorption column has also been reduced.

[0072] In other words, using a gas compressor before the NOx recovery unit reduces the required size of both the absorption column and the desorption column.

[0073] In an embodiment, when a desorption column further comprises a reboiler, said reboiler is located at the bottom of the desorption column.

[0074] When the reboiler is present, it is located at the bottom of the desorption column, being the reboiler a heat exchanger which produces vapor to be reintroduced into said desorption column.

[0075] In a more particular embodiment, the NOx recovery unit is directly connected to the first gas compressor and to the turbine combustor, being the first gas compressor upstream the NOx recovery unit and the turbine combustor downstream the NOx recovery unit.

[0076] In a particular embodiment, the first gas compressor is a multistage gas compressor comprising two or more compression stages. In a particular embodiment, the NOx recovery unit is fed between two compression stages of the first multistage gas compressor.

[0077] In a particular embodiment of the first aspect, the method for recovery of NOx comprises the steps of: a) introducing a comburent, preferably air, into a first gas compressor of a gas turbine to obtain compressed comburent, preferably compressed air, b) introducing the compressed comburent, preferably compressed air, into a turbine combustor, c) producing a flue gas stream comprising NOx from the combustion of the compressed comburent, preferably compressed air, and fuel gas, d) introducing the flue gas stream comprising NOx into a turbine expander, e) cooling the flue gas stream comprising NOx exiting the turbine expander, and f) feeding the cooled flue gas stream of step e) into a NOx recovery unit to obtain a purified NOx flue gas stream, wherein the first gas compressor, the turbine combustor, the turbine expander and the NOx recovery unit are fluidically connected.

[0078] In a particular embodiment of the first aspect, the method for recovery of NOx comprises the steps of: a) introducing a comburent, preferably air, into a first gas compressor of a gas turbine to obtain compressed comburent, preferably compressed air, b) introducing the compressed comburent, preferably compressed air, into a turbine combustor, c) producing a flue gas stream comprising NOx from the combustion of the compressed comburent, preferably compressed air, and fuel gas, d) introducing the flue gas stream comprising NOx into a turbine expander, e) cooling the flue gas stream comprising NOx exiting the turbine expander, f) introducing the cooled flue gas into a second gas compressor to obtain a compressed flue gas stream, and g) feeding the compressed flue gas stream of step f) into a NOx recovery unit to obtain a purified NOx flue gas stream, wherein the first gas compressor, the turbine combustor, the turbine expander, the second gas compressor and the NOx recovery unit are fluidically connected.

[0079] In a particular embodiment of the first aspect, the method for recovery of NOx comprises the steps of: a) introducing a comburent, preferably air into a first gas compressor of a gas turbine to obtain compressed comburent, preferably compressed air, b) introducing the compressed comburent, preferably compressed air, into a turbine combustor, c) producing a flue gas stream comprising NOx from the combustion of the compressed comburent, preferably compressed air, and fuel gas, d) introducing the flue gas stream comprising NOx into a turbine expander, e) cooling the flue gas stream comprising NOx exiting the turbine expander, f) optionally, performing a purge to eliminate inert gases comprised in the cooled flue gas stream, g) introducing the cooled flue gas stream from step e) or f) into the first gas compressor of the gas turbine to obtain a compressed flue gas stream, and h) feeding the compressed flue gas stream of step g) into a NOx recovery unit to obtain a purified NOx flue gas stream, wherein the first gas compressor, the turbine combustor, the turbine expander and the NOx recovery unit are fluidically connected, wherein the first gas compressor is a multistage gas compressor, and wherein the NOx recovery unit is fed between two compression stages of the first gas compressor.

[0080] In a particular embodiment of the first aspect, the method for recovery of NOx comprises the steps of: a) introducing a comburent, preferably air into a first gas compressor of a gas turbine to obtain compressed comburent, preferably compressed air, b) introducing the compressed comburent, preferably compressed air, into a turbine combustor, c) producing a flue gas stream comprising NOx from the combustion of the compressed comburent, preferably compressed air, and fuel gas, d) introducing the flue gas stream comprising NOx into a turbine expander, e) cooling the flue gas stream comprising NOx exiting the turbine expander, f) optionally, performing a purge to eliminate inert gases comprised in the cooled flue gas stream, g) introducing the cooled flue gas stream from step e) or f) into the first gas compressor of the gas turbine to obtain a compressed flue gas stream, and h) feeding the compressed flue gas stream of step g) into a NOx recovery unit to obtain a purified NOx flue gas stream, wherein the first gas compressor, the turbine combustor, the turbine expander and the NOx recovery unit are fluidically connected, wherein the first gas compressor is a multistage gas compressor, and wherein the NOx recovery unit is located between the first gas compressor and the turbine combustor.

[0081] In a particular embodiment, the method of the invention comprises a step of feeding the flue gas stream into a NOx recovery unit comprising an absorption column comprising an aqueous solution of nitric acid, a heat exchanger, a desorption column and, optionally, a separation unit, wherein the absorption column, the heat exchanger, the desorption column and the optional separation unit are fluidically connected, said step comprising the sub-steps of: i). introducing a cooled flue gas stream into the absorption column of the NOx recovery unit to obtain a first gas stream and a first liquid stream, ii). heating the first liquid stream to obtain a pre-heated first liquid stream, iii). introducing the pre-heated first liquid stream into the desorption column to obtain a second gas stream and a second liquid stream, iv). optionally, condensing the second gas stream of step iii) to obtain a liquid phase and a gas phase, and, v). optionally, separating said gas phase from said liquid phase to obtain a purified

[0082] NOx flue gas stream and an aqueous stream.

[0083] In a particular embodiment the pressure in the absorption column of step i) is from 1 to 80 bar, preferably from 1 bar to 60 bar, more preferably from 1 bar to 50 bar, even more preferably froml bar to 30 bar.

[0084] In a particular embodiment, the temperature in the absorption column of step i) is from 20 °C to 85 °C, preferably from 20 °C to 65 °C, more preferably from 30 °C to 40 °C, even more preferably from 32 °C to 37 °C.

[0085] In a particular embodiment, the concentration of the aqueous solution of nitric acid of the absorption column of step i) is from 5% to 60% v / v, preferably from 10 to 40% v / v, even more preferably from 20 to 40% v / v.

[0086] In a particular embodiment, the absorption column of step i) is tray or packed, preferably tray.

[0087] In a particular embodiment, the first gas stream, also referred herein as “clean stream”, comprises dinitrogen, water and NOx, wherein the concentration of the NOx is comprised between 0.1 and 100 ppm, preferably between 0.1 and 50 ppm, even more preferably between 0.1 and 10 ppm.

[0088] In a particular embodiment, a fuel comprising carbon is used as fuel gas in the gas turbine, thus the first gas stream comprises CO2. Therefore, in a more particular embodiment, the first gas stream is further treated to capture the CO2. In a more particular embodiment, the first gas stream comprising CO2 is further treated by means of an amine-based absorption process to trap CO2, although any suitable CO2 recovery process known in the state of the art can be used. In the absorption column of step i) the nitric oxide is contacted with the aqueous solution of nitric acid, leading to the formation of nitrous acid according to the following reaction:

[0089] HNO3+ 2 NO + H2O - ► 3 HNO2

[0090] Whereas the NO2 comprised in the NOx reacts with water, according to the following reaction:

[0091] NO2+ H2O - ► 2 HNO3

[0092] Therefore, in a particular embodiment, the first liquid stream obtained in step i) comprises nitrous acid at a concentration comprised between 0.020% mol and 0.050% mol, preferably at a concentration comprised between 0.030% mol and 0.040% mol.

[0093] In a particular embodiment, the first liquid stream obtained in step i) is heated in step ii) to obtain a pre-heated liquid stream. In a more particular embodiment, the first liquid stream obtained in step i) is heated up to a temperature comprised between 60 °C and 110 °C, preferably between 80 °C and 110 °C, even more preferably between 85 °C and 100°C

[0094] In a particular embodiment, the pre-heated first liquid stream obtained in step ii) is introduced into the desorption column. In a more particular embodiment, the desorption column of step ¡ii) is tray or packed, preferably tray.

[0095] In another particular embodiment, the temperature of the desorption column of step ¡ii) is between 60 °C and 130 °C, preferably between 85 °C and 100°C.

[0096] In another particular embodiment, the pressure of the desorption column of step ¡ii) is from 1 bar to 20 bar, preferably from 1 to 10 bar, even more preferably from 1 to 5 bar. In the most preferred embodiment, the pressure of the desorption column of step ¡ii) is from 1 bar to 2 bar.

[0097] In a preferred embodiment, the desorption column of step ¡ii) further comprises a reboiler. In a preferred embodiment, the reboiler can be a shell-and-tube heat exchanger, thermosiphon reboiler, kettle reboiler and plate exchanger reboiler, although any suitable reboiler known in the state of the art can be used for the purposes of the invention. In a more preferred embodiment, the reboiler is a shell-and-tube heat exchanger.

[0098] In the desorption column, nitrous acid is converted back to nitric acid and NOx by desorption. Therefore, in an embodiment, the second gas stream obtained in step ¡ii) comprises NOx. In another particular embodiment, the second liquid stream obtained in step ¡ii) comprises nitric acid.

[0099] The desorption process takes place by reversal of the absorption reaction and / or by similar reactions, operating in place of or in addition to this absorption reaction. Without wanting to be bounded to a particular theory, it is believed that some of the reactions implied in the desorption process, apart from the reverse reactions of the absorption process mentioned above, are as follows: HNO2+ HNO3- ► H2O + 2 NO2(or

[0100] 2 HNO2- ► H2O + N2O3 or

[0101] 2 HNO2- ► H2O + NO + NO2

[0102] In a particular embodiment, the second gas stream obtained in step iii) is condensed to obtain a liquid phase and a gas phase in optional step iv). In a more particular embodiment, the gas phase obtained in the step iv), when carried out, is separated from the liquid phase to obtain a purified NOx flue gas stream and an aqueous stream. In an even more particular embodiment, the separation of step v), when carried out, is performed by condensation.

[0103] In a more particular embodiment, the second liquid stream of step iii) is reintroduced into the absorption column of step i). In an even more particular embodiment, the second liquid stream of step iii) is reintroduced into the absorption column at a temperature between 20 °C and 85 °C, preferably between 20 °C and 65 °C, more preferably between 30 °C and 40 °C, even more preferably between 32 °C to 37 °C.

[0104] The optional reintroduction of the second liquid stream of step iii) into the absorption column of step i) is highly advantageous from an environmental and economic approach, since the aqueous solution of nitric acid comprised in said second liquid stream can be reused in the absorption process, with concomitant waste and cost reduction of the method. The second liquid stream of step iii) can be reintroduced into the absorption column of step i) as many times as desired. Nevertheless, fresh aqueous solution of nitric acid should be added when required to compensate de nitric acid consumed in both absorption and desorption processes. Preferably, the concentration of nitric acid in the nitric acid aqueous solution is kept constant while carrying the method of the invention.

[0105] As previously mentioned, the method of the invention can be implemented in any gas turbine known in the state of the art. However, when a carbon-based fuel is used in the gas turbine, such as methane or natural gas, a carbon dioxide recovery unit can be located upstream the NOx recovery unit in order to recover carbon dioxide. Therefore, in a particular embodiment, the method of the invention further comprises treating the cooled flue gas stream of step b) in a carbon dioxide recovery unit located upstream the NOx recovery unit. In a more particular embodiment, this carbon dioxide recovery unit comprises an absorption process, preferably a solvent-based absorption process, more preferably an amine-based absorption process, although any suitable dioxide recovery unit known in the state of the art can be used.

[0106] In an embodiment, the method of the first aspect of the invention is performed in the system of the second aspect of the invention. In another embodiment, the method of the first aspect of the invention is not performed in the system of the second aspect of the invention.

[0107] A second aspect of the invention is related to a system configured to carry out the method of the first aspect, said system comprising: a) a gas turbine (A) comprising: i. a first gas compressor (1), ii. a turbine combustor (2), and iii. a turbine expander (3), wherein the gas turbine (A) is configured so as the combustor (2) is located downstream the first gas compressor (1) and upstream the turbine expander (3), b) a first heat exchanger (B), c) a NOx recovery unit (C) comprising:

[0108] - an absorption column (4) comprising an aqueous solution of nitric acid,

[0109] - a second heat exchanger (5),

[0110] - a desorption column (6), and

[0111] - an optional separation unit (7). wherein the NOx recovery unit (C) is configured so as the second heat exchanger (5) is located downstream the absorption column (4) and upstream the desorption column (6), and wherein the first gas compressor (1), the turbine combustor (2), the turbine expander (3), the heat exchanger (B), the absorption column (4), the second heat exchanger (5), the desorption column (6) and the optional separation unit (7) are adapted to be fluidically connected.

[0112] The person skilled in the art will know where to locate control valves for the optimal control of the pressure, stream flow or any other parameter that may need to be monitored in order to ensure proper operation of the system of the second aspect of the invention.

[0113] In a particular embodiment, the optional separation unit (7) is located downstream the desorption column (6). In a more particular embodiment, the optional separation unit (7) is located downstream every element comprised in the system of the second aspect.

[0114] In a particular embodiment, the turbine combustor (2) is configured for operating at a temperature comprised between 1000 °C and 2500 °C, preferably at a temperature comprised between 1300 °C and 2000 °C, even more preferably at a temperature comprised between 1400 °C and 1700 °C.

[0115] In a particular embodiment, the turbine combustor (2) is configured for operating at a pressure comprised between 5 bar and 40 bar, preferably comprised between 5 bar and 30 bar, even more preferably comprised between 5 bar and 20 bar.

[0116] In a particular embodiment, the turbine combustor (2) can be, among others, a can combustor, annular combustor and cannular combustor, or any combustor known in the state of the art. In a particular embodiment, the turbine combustor (2) and the turbine expander (3) are fluidically and directly connected. In a particular embodiment, the turbine expander (3) is configured for operating at a temperature comprised between 1000 °C and 2500 °C, preferably at a temperature comprised between 1300 °C and 2000 °C, even more preferably at a temperature comprised between 1400 and 1700 °C.

[0117] In a particular embodiment, the turbine expander (3) is configured for operating at a pressure comprised between 1 bar and 2 bar, preferably between 1.5 bar and 2 bar, even more preferably between 1.6 bar and 1.8 bar.

[0118] In a particular embodiment, the first heat exchanger (B) is any heat exchanger, or combination of heat exchangers known in the art, which allows to cool down the flue gas stream which comes from the turbine expander (3), preferably to a temperature comprised between 0 °C and 85 °C. A non-limiting example of heat exchanger is steam generators, any type of reheater, reboilers, shell-and-tube heat exchangers, air-air heat exchangers or a combination thereof.

[0119] In a particular embodiment, the first heat exchanger (B) is located downstream the turbine expander, and optionally directly connected to the turbine expander (3).

[0120] In a particular embodiment, the first gas compressor (1) and the first heat exchanger (B) are directly connected.

[0121] In a particular embodiment, the system further comprises a second gas compressor (8) located downstream and, optionally, directly connected to the first heat exchanger (B). In a more particular embodiment, the second gas compressor (8) is a multistage gas compressor.

[0122] In a particular embodiment, the second heat exchanger (5) is any heat exchanger, or combination of heat exchangers known in the art, which allows to heat the first liquid stream which comes from the absorption column (4) or to cool the second liquid stream which comes from the desorption column (6) to a temperature comprised between 0 °C and 100 °C. Nonlimiting examples of heat exchangers are steam generators, any type of reheater, reboilers, shell-and-tube heat exchangers, plate heat exchangers or a combination thereof.

[0123] In a particular embodiment, the second gas compressor (8) is located upstream and directly connected to the NOx recovery unit (C). A schematic view is illustrated in Fig. 2.

[0124] In a more particular embodiment, the system comprises, preferably consists of: a) a first gas compressor (1), b) a turbine combustor (2), c) a turbine expander (3), d) a heat exchanger (B), e) a second gas compressor (8), and f) a NOx recovery unit (C), wherein all of these elements are fluidically connected. In the latter, the first gas compressor (1) is directly connected to the turbine combustor (2), which is directly connected to the turbine expander (3), which is directly connected to the heat exchanger (B), which is directly connected to the second gas compressor (8), which is directly connected to the NOx recovery unit (C).

[0125] In a particular embodiment, the first gas compressor (1) is a multistage gas compressor (T) comprising two or more compression stages. A schematic view is illustrated in Fig. 3. In a more particular embodiment, the NOx recovery unit (C) is located downstream the multistage gas compressor (T) and is configured for being fed between two stages of compression. In an even more particular embodiment, the NOx recovery unit (C) is directly connected to the multistage gas compressor (1’).

[0126] In an even more particular embodiment, the system comprises, preferably consists of: a) a first multistage gas compressor (T), b) a turbine combustor (2), c) a turbine expander (3), d) a heat exchanger (B), and e) a NOx recovery unit (C) wherein all of these elements are fluidically connected wherein the NOx recovery unit (C) is directly connected to the first multistage gas compressor (T) and is configured for being fed between two stages of compression.

[0127] In another particular embodiment, the system comprises, preferably consists of: a) a first multistage gas compressor (T), b) a NOx recovery unit (C), c) a turbine combustor (2) d) a turbine expander (3), e) a heat exchanger (B), and f) a NOx recovery unit (C) wherein all of these elements are fluidically connected wherein the NOx recovery unit (C) is located downstream the multistage gas compressor (T) and is configured for being fed after said multistage gas compressor (T) and before the turbine combustor (2). In another particular embodiment, the NOx recovery unit (C) is located upstream and directly connected to the turbine combustor (2). A schematic view is illustrated in Fig. 4

[0128] In a particular embodiment, the absorption column (4) is tray or packed, preferably tray. In a particular embodiment, the absorption column (4) is configured for operating at a pressure comprised between 1 and 80 bar, preferably from 1 to 60 bar, more preferably from 1 bar to 50 bar, even more preferably from 1 to 30 bar.

[0129] In a particular embodiment, the absorption column (4) is configured for operating at a temperature comprised between 20 °C to 85 °C, preferably from 20 to 65 °C, more preferably from 30 °C and 40 °C even more preferably from 32 °C to 37 °C.

[0130] In a particular embodiment, when the system of the second aspect is functioning, the absorption column (4) comprises an aqueous solution of nitric acid, preferably at a concentration comprised between 5% and 60% v / v, more preferably between 10% and 40% v / v, even more preferably between 20 to 40% v / v.

[0131] In a particular embodiment, the desorption column (6) is tray or packed, preferably tray.

[0132] In another particular embodiment, the desorption column (6) is configured for operating at a temperature comprised between 60 °C and 130 °C, preferably between 85 °C and 100 °C.

[0133] In another particular embodiment, the desorption column (6) is configured for operating at a pressure comprised between 1 and 20 bar, preferably between 1 bar and 10 bar even more preferably at between 1 bar and 5 bar. In the most preferred embodiment, the pressure of the desorption column (6) is between 1 bar and 2 bar.

[0134] In a particular embodiment, the absorption column (4) and the second heat exchanger (5) are directly connected

[0135] In a particular embodiment, the second heat exchanger (5) and the desorption column (6) are directly connected.

[0136] Level control valves may be found between the second heat exchanger (5) and, for example, the absorption column (4) and / or the desorption column (6).

[0137] In a preferred embodiment, the desorption column of step (6) further comprises a reboiler (9), preferably a shell-and-tube heat exchanger.

[0138] In another preferred embodiment, the desorption within the desorption column (6) is performed by steam stripping, that is, by directly introducing at the bottom of the desorption column (6) a stream of superheated steam.

[0139] In a particular embodiment, the optional separation unit (7) comprises a condenser (10) and a vessel (11). In a more particular embodiment, the condenser (10) and the vessel (11) are directly connected.

[0140] In a particular embodiment, the system further comprises an additional heat exchanger (12) located upstream the absorption tower (4). In a more particular embodiment, the system further comprises an additional heat exchanger (12) and a vessel (13) located upstream the absorption tower (4). In an even more particular embodiment, the vessel (13) is directly connected to the additional heat exchanger (12) and to the absorption tower (4).

[0141] As previously mentioned, the invention can be implemented in any gas turbine. However, when a carbon-based fuel is used in the gas turbine, such a methane or natural gas, a carbon dioxide recovery unit can be located upstream the NOx recovery unit (C), to recover carbon dioxide. Therefore, in a particular embodiment, the system of the invention further comprises a carbon dioxide recovery unit located upstream the NOx recovery unit (C). In a more particular embodiment, this carbon dioxide recovery unit comprises an amine-based absorbent, although any suitable dioxide recovery unit known in the state of the art can be used.

[0142] A further aspect of the invention is related to the use of the system according to the second aspect of the invention in the recovery of NOx from flue gas streams of a gas turbine. In a particular embodiment, the recovery of NOx is combined with the production of nitric acid, ammonia, nitrous acid, nitrogen oxides, nitrosonium, nitrites, nitrates and nitrosyl halides. Thus, the system according to the second aspect of the invention can be further used in the production of nitric acid, ammonia, nitrous acid, nitrogen oxides, nitrosonium, nitrites, nitrates and nitrosyl halides.

[0143] In another particular embodiment, the gas turbine uses hydrogen, a nitrogen-based fuel or a carbon-based fuel.

[0144] The present invention will be described in further detail with reference to the following examples. These examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

[0145] EXAMPLES

[0146] Example 1

[0147] The system described in this Example is shown in figure 1. An air stream entered the turbine gas compressor (1) at 1 atm and 20 °C and was compressed up to 20 bar. The compressed stream was later mixed with 149 kg / h of hydrogen, the fuel consumed in the turbine combustor (2). The flue gas, containing nitrogen (72.1% mol), oxygen (10.5% mol), water (17.0% mol), and NOx (0.4% mol), was expanded in the turbine expander (3) up to a pressure of 1.7 bar, being this pressure enough to cover the frictional losses from the turbine to the outlets of the absorption unit. The airflow (11516 kg / h) and the hydrogen flow (149 kg / h) had been calculated to obtain 1 MW of power in the gas turbine (turbine expander (3) power minus turbine gas compressor (1) power) considering an adiabatic efficiency of 75% and a combustion temperature at the outlet of the turbine combustor (2) of 1600 °C.

[0148] After the turbine expander (3), the flue gas comprising NOx was sent to a heat exchanger (B), which in this Example is a steam generator, to recover heat (3600 kW), and to be cooled down to 110 °C. After cooling, the flue gas stream was sent to the NOx recovery unit (C), where it was first further cooled to 35 °C. Due to the low partial pressures of both nitric oxide and oxygen, the conversion of nitrogen oxides to nitrogen dioxide was minimal (118 ppm). After being cooled down, the vapor fraction of the flue gas stream was 0.858 mol / mol. The liquid phase of this stream is separated from the vapor phase in a vessel and had a composition of 99.8% mol H2O, 0.1% N2, 0.02% mol O2, and 0.01% mol NO2.

[0149] The gas stream was sent to the absorption column (4), which was a 14-tray tower with a diameter of 1.5 m, where 188.4 m3 / h of a 30% v / v nitric acid solution was fed at the top tray. The absorption column (4), operated at 1.5 bar, had two outlets one located at the top and the other located at the bottom.

[0150] The first gas stream exited the absorption column (4) by the outlet located at the top and comprised nitrogen (84.5% mol), oxygen (12.3% mol), water (2.3% mol), nitric acid (0.1 % mol), and minimum content of nitrogen oxides (6 ppm of NO, traces of NO2). In this Example, after the absorption column (4), a second washing column was added to clean the outlet gas stream and remove the nitric acid. This second washing column had 4 trays, a diameter of 1.4m, and consumed 176.7 m3 / h of water as washing fluid.

[0151] The first liquid stream obtained in the absorption column (4), exited said column by an outlet located at the bottom. This first liquid stream had the following composition 0.1 %mol N2, 0.02% mol O2, 0.035% mol HNO2, 69.9% mol H2O, 29.9% mol HNO3, and was sent to the heat exchanger (5) located upstream the desorption column (6).

[0152] After being heated in the heat exchanger (5), the temperature of this first liquid stream got increased from 35 °C to 94 °C (vapor fraction equals 0.15% mol / mol). This pre-heated liquid stream entered the desorption column (6), where the dissolved gases were stripped out from the liquid. This desorption column (6) was a 3-tray column which operated at 1 .4 bar having a diameter of 1.7 m. It also had a reboiler (9) and a condenser. While the boilup ratio in the reboiler was equal to 0.07, resulting in a duty consumption of 5335 kW, the reflux ratio was equal to 9.8 (R / V), resulting in 1247 kW of duty consumed in the condenser. In this example, cooling water was used as the cooling fluid in the condenser. The resulting purified NOx flue gas stream has the following composition: 2.4% mol H2O, 68.7% mol N2, 12.4% mol O2, 0.1 % mol HNO3, 15.5% mol NOx.

[0153] Table 1. Comparison of the composition of the flue gas stream before and after being processed in the NOx recovery unit.

[0154] Gas% mol in flue gas stream % mol in purified flue gas comprising NOx stream comprising NOx

[0155] N272.1 68.7

[0156] O210.5 12.4

[0157] H2O 17 2.4

[0158] NOx 0.4 15.5

[0159] HNO3 - 0.1 Example 2

[0160] The system was similar to that described in Example 1 , with the difference of a second gas compressor (8) located upstream and directly connected to the NOx recovery unit (C). The system described in Example 2 is shown in figure 2.

[0161] In this example, the outlet stream of the heat exchanger (B) located immediately after the gas turbine (A) was sent to a two-stage gas compressor (8) with intercooling. Before the first compression stage, the inlet stream was cooled to 35°C, separating 1132 kg / h of liquid in a vessel, a KO drum. Between the first and second stages, thw intercooler reduced the temperature again to 35 °C, recovering 120 kg / h more of liquid in a second KO drum. The gas compressor operated with a compression ratio between stages of 2.5 and a discharge pressure of 10 bar, consuming 734 kW. After exiting the two-stage gas compressor (8), the resulting gas stream is sent to the NOx recovery unit (C), where it was cooled down to 35 °C recovering 50 kg / h of liquid in the third KO drum. Because of the increased pressure in comparison with Example 1 , this alternative recover 170 kg / h more of liquid in the KO drums located before the absorption column. Additionally, the increase in the partial pressure of oxygen and nitric oxide resulted in a higher nitric oxide conversion to nitric dioxide, so the nitric dioxide concentration increases up to 1232 ppm in the gas stream sent to the absorption column (4). The resulting gas stream from the separation vessel (10363 kg / h) was sent to the absorption column (4), which in this case was a 14-tray tower of 0.77 m of diameter, which consumed 50 m3 / h of nitric acid solution (30% v / v).

[0162] Due to the lower amount of nitric acid required in the absorption column (4) and the higher operating pressure, 10 bar, the amount of nitric acid contained in the first gas stream of the absorption tower (4) was lower than in Example 1 (30 kg / h vs 206 kg / h). As in Example 1 , after the absorption column (4), a second washing column (4 trays, 0.66 m of diameter, and 18 m3 / h of water) was added to clean the outlet first gas stream and remove the nitric acid. This second washing column had 4 trays, a diameter of 1.4m, and consumed 176.7 m3 / h of water as washing fluid.

[0163] On the other hand, the first liquid stream exiting the absorption column (4) contained a higher amount of dissolved gases, as the partial pressure in the absorption column (4) is higher in this Example than in Example 1 . Thus, the composition of this first liquid stream was 0.7% mol N2, 0.12% mol O2, 0.107% mol HNO2, 69.4% mol H2O, and 29.7% mol HNO3.

[0164] The performance of the heat exchanger (5) is like in Example 1. However, the duty in the heat exchanger (5) (3390 kW vs 12715 kW) is lower than in Example 1 , since the liquid flow was also lower (50 m3 / h vs 189 m3 / h in the case of the cold fluid). Because of the higher expansion before the column desorption (6), from 10 bar to 1.4 bar, a higher amount of gases were desorbed before entering the desorption column (6). Moreover, due to the lower flow of nitric acid solution compared with Example 1 , the desorption column (6) is also smaller in this case, with a calculated diameter of 0.96 m. For the same reason, the duty in the reboiler (9) decreased from 5335 kW to 1817 kW. The boilup ratio remained in the same range as in the previous alternative: boilup ratio (0.09 vs 0.07), but the reflux ratio decreased (3.41 vs 9.8), resulting in a duty reduction from 1247 kW to 752 kW. Finally, the composition of the purified NOx flue gas stream is 78.4% mol N2, 13.6% mol O2, 2.4% mol H2O, 7.8% mol NOx, 0.1% mol HNO3.

[0165] Table 2. Comparison of the composition of the flue gas stream before and after being processed in the NOx recovery unit.

[0166] Gas% mol in flue gas stream % mol in purified flue gas comprising NOx stream comprising NOx

[0167] N272.1 78.4

[0168] O210.5 13.6

[0169] H2O 17 2.4

[0170] NOx 0.4 7.8

[0171] HNO3- 0.1

[0172] Example 3

[0173] The process flowsheet configuration of Example 3 is shown in Figure 3. In this alternative, the single-stage first gas compressor (1) of the gas turbine was replaced by a three stages gas compressor (T). The compressed flue gas stream obtained after the second stage of the compressor (T) was sent to the nitric oxide recovery unit (C). The first gas stream obtained at the top of the absorption column (4) were sent to the third stage of the turbine gas compressor (T), and the flue gases were recycled to the first stage of the gas turbine gas compressor (T) after being cooled down in the heat exchanger (B), located downstream the gas turbine (A). More specifically, 6232 kg / h of flue gas stream at 35 °C and with the following composition: 80.2% mol N2, 15.6% mol O2, 2.7% mol H2O, 0.46% mol NO, and 240 ppm NO2, 1% mol inerts were recycled from the outlet of the heat exchanger (B) located downstream the gas turbine (A) to the first stage of the gas compressor (T), being mixed with 385 kg / h of air. Air was used as a makeup to replace the nitrogen that is lost in the process, as part of it reacted with oxygen yielding nitrogen oxides and another part is dissolved in the nitrogen oxides recovery unit (C).

[0174] The flue gas stream comprising NOx was compressed in the first two stages of the gas compressor (T) up to 7.5 bar and sent to the nitrogen oxides recovery unit (C). The operating pressure of the absorption column (4), 7.5 bar, allowed for a balanced gas compressor ratio between the different stages of the gas compressor (2.71 bar / bar). As in Example 2, the gas coming from the first stage of the gas compressor is cooled down to 35°C, and then, the resulting liquid (35 kg / h) is separated in the KO drum. Then, in this configuration, the cooler and the KO drum located upstream of the absorption column (4) replace the intercooler and KO drum of the second stage of the gas compressor, so the gas coming from the second stage of the gas compressor (T) is cooled down to 35°C and 45 kg / h of liquid were recovered in the KO drum. In this alternative the absorption column (4), with 14 trays, only requires 26 m3 / h of nitric acid solution to recover nitrogen oxides. Thus, the resulting tower diameter is 0.61 m, less than in Examples 1 and 2 (1.5 m for Example 1 , and 0.77 m for Example 2). The first gas stream obtained at the top of the absorption column (4) is sent to the third compression stage of the gas compressor (T). The composition of this first gas stream is 82.1 % mol N2, 16.2% mol O2, 0.45% mol H2O, and 0.2% mol HNO3, 1 % mol inerts.

[0175] One of the main advantages of the system described in Example 3, is the fact that, since the gases are constantly recycled to the gas turbine (A), there is no need to clean the resulting first gas stream from the absorption column (4) to recover nitric acid, as this compound simply decomposes into NO2 and water when raising the temperature in the turbine combustor (2).

[0176] In the third compression stage of gas compressor (T), the gas pressure increases up to 20 bar. The resulting gas stream is sent to the gas turbine combustor (2). In the turbine combustor (2), the gas stream was mixed with the fuel, which was hydrogen in this example (123 kg / h), and the comburent, which was oxygen in this example (957 kg / h). It was considered that the turbine was operated using hydrogen as a fuel produced from water electrolysis at high pressure (around 30 bar). Thus, relatively pure oxygen was available at the same pressure. The implementation of this alternative allowed for the consumption of the oxygen produced in the electrolyzer in the gas turbine (A) when the fuel of the gas turbine is hydrogen produced by electrolysis. If air were used instead of oxygen as the comburent, the flowrate of air fed in the first stage of the gas compressor (T) increased, and a purge is needed in the recycling stream, before the first stage of the gas compressor (T). The resulting flue gas stream comprising NOx obtained after the combustion was expanded in the turbine expander (3) and then cooled down in the heat exchanger (B) located downstream of the gas turbine recovering 2363 kW. As in Examples 1 and 2, all the flowrates had been adjusted to yield a total power of 1 MW in the gas turbine (A) and a turbine combustor (2) outlet temperature of 1600 °C.

[0177] Replacing the single-stage gas compressor (1) with a three-stage gas compressor (T) reduced the amount of power consumed in the gas compressor (T) and the hydrogen flow required. Regarding the liquid first stream from the absorption column (4), its composition was as follows (0.5% mol N2, 0.11% mol O2, 0.15% mol HNO2, 69.5% mol H2O, and 29.7% mol HNO3).

[0178] As in Examples 1 and 2, this first gas stream was sent to a heat exchanger (5) where was heated up to 88°C. Then, after being expanded to 1.4 bar, it entered the desorption column (6) at the first tray. The desorption column (6) operated with a reflux ratio of 3.3 (R / V), and a boilup ratio of 0.08, consuming 840 kW in the reboiler (9) and 296 kW in the condenser. The diameter of the desorption column (6) is 0.65 m, which is lower than in the two previous examples. After condensation, 216 kg / h of purified NOx flue gas stream were obtained with the following composition: 65.6% mol N2, 16.2% mol O2, 2.4% mol H2O, 13.9% mol NOx, 1%mol HNO3, 1 % mol inerts. Table 3. Comparison of the composition of the flue gas stream before and after being processed in the NOx recovery unit.

[0179] Gas% mol in flue gas stream % mol in purified flue gas comprising NOx stream comprising NOx

[0180] N280.2 65.6

[0181] O215.6 16.2

[0182] H2O 2.7 2.4

[0183] NOx 0.46 13.9

[0184] HNO3- 1

[0185] Example 4

[0186] The process flowsheet configuration of Example 4 is shown in Figure 4. Example 4 was a variation of the system described in Example 3, where the nitrogen oxides recovery unit (C) was located between the gas compressor (T), downstream of the last compression stage, and the gas turbine combustor (2). In this way, the absorption column (4) of the nitrogen oxides recovery unit (C) operated at a higher pressure than in Example 3, which is 20 bar, balanced with the operating pressure of the turbine combustor (2).

[0187] In this example, 6309 kg / h of flue gas (81.6% mol N2, 14.1 %mol O2, 2.7% mol H2O, 0.45% mol NO, and 203 ppm NO2, 1% mol inerts) were recycled from the heat exchanger (B) located downstream of the gas turbine (A), to the first stage of the gas compressor (T). The flue gas stream was mixed with 231 kg / h of air to compensate for the nitrogen converted into nitrogen oxides and dissolved in the absorption column (4), and part of the oxygen consumed in the process.

[0188] The gas compressor (T) defined in this example as a three-stage gas compressor, pressurized the gas up to 20 bar with a compressor ratio of 2.71 between stages. Both the first and the second compression stages had intercooling to 35 °C and KO drums, removing 81 kg / h of liquid. After pressurization, the resulting gas was sent to the nitrogen oxides recovery unit (C), where it was first cooled down to 35 °C and then, the resulting liquid (16 kg / h), was separated in the separation vessel.

[0189] The gas stream obtained at the top of the separation vessel was sent to the absorption column (4), which had 14 trays and operated at 20 bar, consuming 11 m3 / h of nitric acid solution (at 30% v / v. The diameter of the absorption column (4) is 0.44 m (less than Examples 1-3), and 6218 kg / h the first gas stream were obtained at the top with the following composition (N2: 84.0% mol, O2: 14.7% mol, 0.2% mol H20, 656 ppm HNO3, 1 % mol inerts). This first gas stream was sent to the combustor (2) where it was mixed with the fuel, in this case 130 kg / h of hydrogen, and the comburent, 1030 kg / h of oxygen. As in Example 3, it was considered that both the oxygen and the hydrogen came from an electrolyzer and were available at high pressure. If air is used as a comburent, the flowrate of air to the gas compressor's (T) first stage increases, and an additional purge is required before this stage. The resulting flue gas stream comprising NOx was expanded in the expander (3) and cooled down in the heat exchanger (B) located downstream the gas turbine (A), recovering 2370 kW.

[0190] The flowrates of all the gases sent to the combustor (2) had been adjusted to obtain 1 MW of power in the turbine and a combustion temperature of 1600°C. The first liquid stream from the nitrogen oxides recovery unit (C) (12990 kg / h) had the following composition (1.4% mol N2, 0.3% mol O2, 0.23% mol HNO2, 68.7% mol H2O, and 29.3% mol HNO3). As in the previous Examples, the first liquid stream was sent to the heat exchanger (5) where it was heated up to 89°C, and then entered the first tray of the desorption column (6), which operated at 1 .4 bar.

[0191] In this Example, the desorption column (6) operated with a reflux ratio of 1.63, consuming 171 kW in the condenser, and a boilup ratio of 0.09, consuming 393 kW in the reboiler (9). The resulting desorption column (6) diameter is 0.43 m. The overhead product stream was cooled down to 35 °C, being later separated in the overhead accumulator vessel obtaining a purified NOx gas stream of 238 kg / h with the following composition (68.7% mol N2, 15.2% mol O2, 2.4% mol H2O, 11.7% mol NOx, 1 %mol HNO3, 1 %mol inerts).

[0192] Table 4. Comparison of the composition of the flue gas stream before and after being processed in the NOx recovery unit.

[0193] Gas% mol in flue gas stream % mol in purified flue gas comprising NOx stream comprising NOx

[0194] N281.6 68.7

[0195] O214.1 15.2

[0196] H2O 2.7 2.4

[0197] NOx 0.45 11.7

[0198] HNO3 - 1

[0199] Example 5

[0200] Tables 5 to 9 summarize the key performance indicators of the different systems described in Examples 1 to 4.

[0201] Table 5: Key performance indicators: Gas turbine (A)

[0202] Hydrogen Gas Heat Post consumption compressor exchanger Compressor (kg / h) stages duty (kW) Power (MW)

[0203] Example 1 1 149 1 3600 0

[0204] Example 2 1 149 1 3600 0.73

[0205] Example s 1 123 3 2363 0

[0206] Example 4 1 130 3 2370 0 Table 6: Key performance indicators: Absorption column (4)

[0207] Operatingr.x. . Solution Nitric acid in

[0208] , Trays Diameter (m) . ,.. . .. . .. .. . pressure (bar) (nr / h) outlet gas (kg / h)

[0209] Example 1 1.5 14 1.5 188.4 206

[0210] Example 2 10 14 0.77 50 29

[0211] Example s 7.5 14 0.61 26 24

[0212] Example 4 20 14 0.44 11 9

[0213] Table 7: Key performance indicators: Desorption column (6)

[0214] NOx NOx

[0215] Trays Diameter Reboiler Condenser recovery concentration

[0216] (m) duty (kW) duty (kW) (kg / h) (%mol)

[0217] Example 1 3 1.4 5335 1247 49 15.5

[0218] Example 2 3 0.96 1817 752 41 7.8

[0219] Example s 3 0.65 840 296 30 13.9

[0220] Example 4 3 0.43 393 171 29 11.8

[0221] Table 8: Key performance indicators: relative basis .x. Total , Heat , ,x. _xAbsorbent

[0222] Combustion _ , NOx / _ , Heating Duty _ .x. ,

[0223] -j-xPower / . . Exchanger , ®}Solution /

[0224] TemperatureuH2, / NOx .

[0225] 7n_. H2, (B) Duty / ... ... . NOx

[0226] ( C)(MJ / kg)(kg / k9)H2(MJ / kg)(MJ / kg)(m3 / kg)

[0227] Conventional140Q 2Q d,89, / / gas turbine*

[0228] Example 1 1600 24.2 0.33 87.0 127 3.84

[0229] Example 2 1600 6.5 0.28 87.0 -157 1.22

[0230] Example s 1600 29.3 0.24 69.2 -183 0.87

[0231] Example 4 1600 27.7 0.22 65.6 -245 0.38

[0232] *Gas turbine without a NOx recovery unit

[0233] As shown in Tables 1 and 4, Examples 3 and 4 resulted in the lowest consumption of hydrogen per kg of electricity produced in the process (about a 15% less of hydrogen than in Examples l and 2).

[0234] Example 2 presented the highest fuel consumption per unit of final power produced, as part of the power produced in the gas turbine (A) was consumed in the second gas compressor (8) that raised the pressure before the NOx recovery unit (C). As it can be seen from Table 8, increasing the combustion temperature in the gas turbine (A), when comparing with conventional gas turbine allowed for reducing the amount of fuel required per unit of power in the gas turbine (A). Moreover, reducing the hydrogen consumption per unit of power also decreased the NOx production.

[0235] Replacing the single stage turbine gas compressor (1) of Examples 1 and 2 by the three-stage gas compressor (T) used in Examples 3 and 4 improved the compression efficiency, reducing the amount of power required in the gas compressor (T). However, the main disadvantage, apart from the potential increase in the capital costs, was the decrease in the exhaust temperature, as the gas fed to the turbine combustor (2) in Examples 1 and 2 was at 522°C, much higher than the temperatures in Examples 3 and 4 (167 °C and 35 °C respectively). This decrease in the fed temperature reduced the amount of heat that was recovered in the heat exchanger (B) located downstream the gas turbine (A) (around a 25%), as it is shown in Tables 5 and 8. Nevertheless, when accounting for the duty required in the reboiler (9) of the desorption column (6), the system described in Examples 3 and 4 clearly outperformed the system described in Example 1 , as in this example there was a net positive consumption of heat, while in Examples 3 and 4 heat was produced.

[0236] Regarding the design and operation of the nitrogen oxides recovery unit (C), as shown in Tables 6 and 7, increasing the operating pressure of the absorption column (4) decreased the nitric acid solution consumption, the diameter of both absorption and desorption columns. Furthermore, the nitric oxide that is evaporated in the absorption unit (4), and the duties at both the reboiler (9) and the condenser are also reduced. Thus, depending on the capacity of the plant, the operating pressure at this point was optimized to reduce the total annual costs of the plant. It is reminded in this point that the use the system described in Examples 3 and 4, apart from reducing the nitric acid content, allowed for recycling the absorber column (4) outlet gas (with a minimum purge to remove the inert gases). This resulted in the elimination of the washing tower unit required to avoid nitric acid being sent to the atmosphere as described in Examples 1 and 2.

Claims

CLAIMS1 . A method for recovery of NOx, said method comprising: a) producing a flue gas stream comprising NOx in a gas turbine, wherein the gas turbine comprises: a first gas compressor, a turbine combustor, and a turbine expander, b) cooling said flue gas stream, and c) feeding the cooled flue gas stream of step b) to a NOx recovery unit to obtain a purified NOx flue gas stream. wherein the first gas compressor, the turbine combustor, the turbine expander and the NOx recovery unit are fluidical ly connected.

2. The method according to claim 1 , wherein the NOx recovery unit comprises an absorption column comprising an aqueous solution of nitric acid, a heat exchanger, a desorption column and, optionally, a separation unit; wherein the absorption column, the heat exchanger, the desorption column and the optional separation unit are fluidically connected.

3. The method according to claim 2, wherein the method further comprises the steps of: i. introducing the cooled flue gas stream of step b) into the absorption column to obtain a first gas stream and a first liquid stream, ii. heating the first liquid stream to obtain a pre-heated first liquid stream, iii. introducing the pre-heated first liquid stream into the desorption column to obtain a second gas stream and a second liquid stream, iv. optionally, condensing the second gas stream of step iii) to obtain a liquid phase and a gas phase, and, v. optionally, separating said gas phase from said liquid phase to obtain a purified NOx flue gas stream and an aqueous stream.

4. The method according to claims 1 to 3, wherein said method comprises feeding the cooled flue gas stream of step b) sequentially into a second gas compressor and the NOx recovery unit of step c); preferably being the second gas compressor and the NOx recovery unit fluidically and directly connected.

5. The method according to claims 1 to 3, wherein the NOx recovery unit is located downstream the first gas compressor and upstream the turbine combustor.

6. The method according to claim 5, wherein the NOx recovery unit is directly connected to the first gas compressor.

7. The method according to claim 5 or 6, wherein the first gas compressor is a multistage gas compressor comprising two or more compression stages.

8. The method according to claim 7, wherein the NOx recovery unit is fed between two compression stages of the multistage gas compressor.

9. The method according to any one of claims 2 to 8 wherein the pressure of the absorption column is from 1 to 80 bar.

10. The method according to any one of claims 2 to 9, wherein the temperature of the absorption column is from 20 °C to 85 °C.

11. The method according to any one of claims 2 to 10, wherein the concentration of the aqueous solution of nitric acid of the absorption column is from 5% to 60% v / v, preferably from 10 to 40% v / v.

12. The method according to any one of claims 2 to 11 , wherein the absorption column is tray or packed, preferably tray.

13. The method according to any one of claims 1 to 12, further comprising treating the cooled flue gas stream of step b) in a carbon dioxide recovery unit located upstream the NOx recovery unit.

14. A system configured to carry out the method according to any of claims 1 to 13, said system comprising: a) a gas turbine (A) comprising: i. a first gas compressor (1), ii. a turbine combustor (2), and iii. a turbine expander (3), wherein the gas turbine (A) is configured so as the combustor (2) is located downstream the first gas compressor (1) and upstream the turbine expander (3), b) a first heat exchanger (B), c) a NOx recovery unit (C) comprising:- an absorption column (4) comprising an aqueous solution of nitric acid,- a second heat exchanger (5),- a desorption column (6), and- an optional separation unit (7). wherein the NOx recovery unit (C) is configured so as the second heat exchanger (5) is located downstream the absorption column (4) and upstream the desorption column (6), andwherein the first gas compressor (1), the turbine combustor (2), the turbine expander (3), the first heat exchanger (B), the absorption column (4), the second heat exchanger (5), the desorption column (6) and the optional separation unit (7) are adapted to be fluidically connected.

15. The system according to claim 14, wherein the optional separation unit (7) is located downstream the desorption column (6).

16. The system according to claims 14 or 15, further comprising a second gas compressor (8) located downstream and, optionally, directly connected to the first heat exchanger (B).

17. The system according to claim 16, wherein the second gas compressor (8) is located upstream and, optionally, directly connected to the NOx recovery unit (C).

18. The system according to claims 14 or 15, wherein the first gas compressor (1) is a multistage gas compressor (T) comprising two or more compression stages.

19. The system according to claim 18, wherein the NOx recovery unit (C) is located downstream the multistage gas compressor (T) and is configured for being fed between two stages of said multistage gas compressor (T).

20. The system according to any one of claims 14-15 or 18-19, wherein the NOx recovery unit (C) is located upstream and directly connected to the turbine combustor (2).

21. The system according to any one of claims 14 to 20, wherein the absorption column (4) is configured for operating at a pressure comprised between 1 and 80 bar.

22. The system according to any one of claims 14 to 21 , wherein the absorption column (4) is configured for operating at a temperature comprised between 20 °C and 85 °C.

23. The system according to any one of claims 14 to 22, wherein the absorption column (4) comprises an aqueous solution of nitric acid at a concentration comprised between 5% and 60% v / v, preferably between 10% and 40% v / v.

24. The system according to any one of claims 14 to 23, wherein the absorption column (4) is tray or packed, preferably tray.

25. The system according to any one of claims 14 to 24, further comprising a carbon dioxide recovery unit located upstream the NOx recovery unit (C).

26. Use of the system according to any one of claims 14-25 in the recovery of NOx from flue gas streams of a gas turbine.

27. Use according to claim 26, wherein the recovery of NOx is combined with the production of nitric acid, ammonia, nitrous acid, nitrogen oxides, nitrosonium, nitrites, nitrates and nitrosyl halides.

28. Use according to any one of claims 26 or 27, wherein the gas turbine uses hydrogen, a nitrogen-based fuel or a carbon-based fuel.

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