Method and arrangement for combusting ammonia

Abstract

A method for combusting ammonia provided in a combustion feed (1) using a combustion arrangement (100) comprising a combustion section (110), a flue gas section (120), and a plurality of burners (111, 112) is provided, wherein the plurality of burners (111, 112) are operated using the combustion feed, and wherein a flue gas (2) formed in the combustion section (110) is passed through the flue gas section (120). A first group of the plurality of burners (111) are operated using first ratio of oxygen to ammonia and a second group of the plurality of burners (112) are operated with a second ratio of oxygen to ammonia lower than the first ratio of oxygen to ammonia. A corresponding combustion arrangement (100) is also provided.

Description

[0001] Description

[0002] Method and arrangement for combusting ammonia

[0003] The present invention relates to a method and a combustion arrangement for combusting ammonia.

[0004] Background

[0005] Ammonia may be an attractive fuel and a “storage form” for hydrogen less difficult to handle than compressed or liquefied hydrogen gas. As summarized in an article by Alexander H. Tullo, “Is ammonia the fuel of the future?”, Chemical and Engineering News 2021 , Volume 99, Issue 8, ammonia has a higher energy density, at 12.7 MJ / L, than even liquid hydrogen, at 8.5 MJ / L. Liquid hydrogen has to be stored at cryogenic conditions of -253 °C, whereas ammonia can be stored at a much less energy- intensive -33 °C. Furthermore, ammonia, though hazardous to handle, is much less flammable than hydrogen. Due to the wide use of ammonia in agriculture, an ammonia infrastructure already exists. Worldwide, about 180 Mt of ammonia are produced annually, and 120 ports worldwide are already equipped with ammonia terminals.

[0006] Ammonia may be used for co-firing in thermal power plants, for example, in order to provide additional energy, but also to provide better control of nitrous oxides formation. For example, JP 2020-112280 A discloses a furnace comprising a plurality of burners which are adapted to use pulverized coal as a fuel, the coal being introduced into the burners by being entrained by combustion air in a combustion air tube. For co-firing ammonia, an inner tube in the combustion air tube is provided. A similar arrangement is disclosed in JP 2018-096680 A.

[0007] Combustion of pure or high concentration ammonia is still in its infancy and corresponding information is mainly availably from research papers. As traces of ammonia and ammonia radicals (NHi) are also released from nitrogen sources in other fuels, ammonia combustion chemistry has been investigated in nitrous oxides (NOX) formation pathways for the combustion and fuels and give an idea on the emission problems which might occur with high ammonia containing fuels. Various reaction paths lead from ammonia to undesired nitrous oxides but also to desired nitrogen gas. Using ammonia as fuel, nitrous oxide formation will increase, and additional effort must be taken to reduce nitrous oxides emissions.

[0008] Summary

[0009] In view of the above, a method and a combustion arrangement comprising the features of the independent claims are provided. Embodiments are the subject of the dependent claims and of the description that follows.

[0010] Herein, a method for combusting ammonia provided in a combustion feed using a combustion arrangement comprising a combustion section, a flue gas section, and a plurality of burners is proposed, wherein the plurality of burners are operated using the combustion feed, and wherein a flue gas formed in the combustion section is passed through the flue gas section.

[0011] As proposed herein, a first group of the plurality of burners are operated using first ratio of oxygen to ammonia and a second group of the plurality of burners are operated with a second ratio of oxygen to ammonia, wherein the second ratio of oxygen to ammonia is lower than the first ratio of oxygen to ammonia. Particularly, the first ratio of oxygen to ammonia is a super-stoichiometric ratio and the second ratio of oxygen to ammonia is a stoichiometric or slightly sub-stoichiomatric ratio.

[0012] The proposed method solves a main challenge for selective catalytic and non-catalytic nitrous oxide reduction steps, which is a good distribution of ammonia in the flue gas and the added capital and operating cost of the injection system, as also discussed further below in connection with certain embodiments.

[0013] In embodiments of the proposed method, a selective catalytic and / or non-catalytic nitrous oxide reduction may be performed in the flue gas section. That is, one or more zones configured for selective catalytic and / or non-catalytic nitrous oxide reduction may be provided in the flue gas section.

[0014] While selective catalytic reduction requires a catalyst bed in a certain zone of the flue gas section in which a temperature window matches the requirements for catalytic reduction, i.e. a dedicated catalytic unit must be provided, this is not the case for selective non-catalytic reduction wherein no further apparatus may be required. Therefore, a “zone” for non-catalytic nitrous oxide reduction as referred to herein may be a sub-section of the flue gas section corresponding to an appropriate temperature window with optionally no specific apparatus features.

[0015] Aspects as proposed herein include tweaking some of the burners to produce an ammonia slippage (i.e., leave an amount of ammonia in uncombusted state) which can react in the temperature window for a selective non-catalytic nitrous oxide reduction zone and / or further downstream also in a selective catalytic nitrous oxide reduction zone or system. This can be achieved by adjusting the supply of air or another oxygencontaining gas mixture, or even pure oxygen, to the burners which can be provided as state-of-the-art burners.

[0016] Some burners will, as proposed herein, run on higher excess air so that the overall complete combustion in the furnace is achieved. These are the burners of the first group of burners as mentioned before. Other burners will run close to stoichiometry or slightly sub-stoichiometric to leave over some unreacted ammonia. These are the burners of the second group of burners as mentioned before.

[0017] In embodiments, the first ratio of oxygen to ammonia and the second ratio of oxygen to ammonia may be expressed as lambda or air-fuel equivalence values, as generally known to the skilled person. The first ratio of oxygen to ammonia may correspond to a lambda value in a range from 1.1 to 2, particularly from 1.1 to 1.5, and / or the second ratio of oxygen to ammonia may correspond to a lambda value in a range from 0.5 to 1.1 , particularly 0.9 to 1.1. The values may also be expressed in fuel-air equivalence ratios or phi values, as also known to the skilled person. For information as to the ammonia slippage at different stoichiometric ratios, reference is made to literature, e.g. Elbaz et al., “Review on the recent advances on ammonia combustion from the fundamentals to the applications”, Fuel Communications 2022, Volume 10, Page 100053, e.g., Figure 31.

[0018] In embodiments, the combustion feed comprises ammonia in a content of 1% to 100% on a molar, mass or volume basis. The present invention may therefore be used in connection with a large type of different feeds. The burners of the second group (operated at lower ratios of oxygen to ammonia) may, in embodiments as proposed herein, be distributed among the burners of the first group in the combustion section according to a predefined distribution pattern, particularly at a roof of the combustion section. While operating some burners at low and some at burner at high stoichiometry leads to air staging on furnace scale resulting already in lower nitrous oxides emissions compared to operating all burners equally, distributing the burners of the second group (operated at lower ratios of oxygen to ammonia) among the burners of the first group (operated at higher ratios of oxygen to ammonia) may particularly result in a good and even distribution of ammonia in the flue gas without additional injection or mixing equipment as necessary according to the prior art. This is particularly realized in a burner array, in which each n-th burner is from the second group and the other burners are from the first group.

[0019] The burners of the second group may, in certain embodiments, be distributed among the burners of the first group in the combustion section according to a predefined distribution pattern to reach such a good and even distribution. This may be a regular or irregular distribution pattern which may be selected by the skilled person accordingly and depending on local concentrations observed, for example.

[0020] In embodiments as proposed herein, the first group of burners may comprise a first number of burners, particularly a first total number of burners, and the second group of burners may comprise a second number of burners, particularly a second total number of burners, wherein the first (total) number of burners may be equal to or larger than the second (total) number of burners. The first (total) number of burners may particularly be an integer multiple of the second (total) number of burners, wherein an integer may particularly be 1 , 2, 3, 4, 5, 6 or an integer larger than 6.

[0021] In certain embodiments, the plurality of burners may be assigned to the first group of burners and to the second group of burners depending on the combustion feed, i.e., they may be operated flexibly with higher or lower stoichiometric ratios, particularly depending on an ammonia content of the combustion feed or any other parameter.

[0022] In certain embodiments, at least some of the plurality of burners may be arranged in the combustion section and / or at least some of the burners may be arranged in the flue gas section. While the subsequent explanations mainly relate to roof burners in the combustion section, embodiments of the invention may also be applied to ammonia reheat burners for a tail end selective catalytic reduction system which are arranged not in the combustion section but in the flue gas section.

[0023] At least some of the plurality of burners may, in embodiments of the present invention, comprise dedicated ports for the combustion feed and / or for an oxidator gas to adapt the first ratio of oxygen to ammonia and / or the second ratio of oxygen to ammonia. These may also be additional ports such that the main combustion of a burner is not disturbed. The burners can be modified with some having extra air nozzles while others have extra fuel nozzles or even dedicated ammonia nozzles for furnaces running on dual fuel systems.

[0024] In embodiments of the present invention, an ammonia slip reduction unit may be arranged, particularly downstream of the selective catalytic and / or non-catalytic nitrous oxide reduction zone(s), in the flue gas section, in order to reduce ammonia emissions.

[0025] One or more heat recovery bundles may, in embodiments of the present invention, be arranged, particularly upstream and / or downstream of the selective catalytic and / or non-catalytic nitrous oxide reduction zone(s), in the flue gas section. This enables for an effective heat recovery but also has the effect of a further mixing of the ammonia in the flue gas.

[0026] The combustion arrangement is operated at a maximum temperature of less than 1.100 °C, particularly from 900 to 1.100 °C, in embodiments proposed herein. Since ammonia has relatively low flame temperature, operating close to stoichiometry does not result in excessive temperatures in the furnace.

[0027] The combustion arrangement for combusting ammonia provided in a combustion feed, as proposed herein, comprises a combustion section, a flue gas section, and a plurality of burners. In embodiments, a selective catalytic and / or non-catalytic nitrous oxide reduction unit is arranged in the flue gas section. The combustion arrangement is configured to operate the plurality of burners using the combustion feed, and to pass a flue gas formed in the combustion section through the flue gas section. As proposed, the combustion arrangement is configured to operate a first group of the plurality of burners using first ratio of oxygen to ammonia and a second group of the plurality of burners with a second ratio of oxygen to ammonia lower than the first ratio of oxygen to ammonia.

[0028] As to further features and advantages of an inventive arrangement and its possible embodiments, particular reference is made to the description of the method according to the present invention and its embodiments as described before. An arrangement according to the present invention is particularly adapted to perform a corresponding method or an embodiment thereof.

[0029] The present invention will further be described with reference to the appended drawings illustrating embodiments of the present invention.

[0030] Brief description of the drawings

[0031] Figure 1 illustrates an arrangement according to an embodiment.

[0032] Figure 2 illustrates aspects of an arrangement according to an embodiment.

[0033] Detailed description

[0034] In the Figures, elements of identical, essentially identical, functionally comparable, or technically compatible function and / or purpose and / or construction may be identified with identical reference numerals, and repeated explanations may be omitted for reasons of conciseness. Explanations herein relating to devices, apparatus, arrangements, systems, etc., according to embodiments of the present invention likewise may apply to methods, processes, procedures, etc. according to embodiments of the present invention and vice versa.

[0035] The various embodiments described herein are presented only to assist in understanding and teaching the claimed features. These embodiments are provided as a representative sample of embodiments only, and are not exhaustive and / or exclusive. It is to be understood that advantages, embodiments, examples, functions, features, structures, and / or other aspects described herein are not to be considered limitations on the scope of the invention as defined by the claims or limitations on equivalents to the claims, and that other embodiments may be utilised and modifications may be made without departing from the scope of the claimed invention. Various embodiments of the invention may suitably comprise, consist of, or consist essentially of, appropriate combinations of the disclosed elements, components, features, parts, steps, means, etc., other than those specifically described herein. In addition, this disclosure may include other inventions not presently claimed, but which may be claimed in future, particularly when encompassed by the scope of the independent claims.

[0036] Reaction paths of ammonia and ammonia radicals to nitrogen monoxide and nitrogen are discussed elsewhere, for example in T. Kolb, “Experimentelle und theoretische Untersuchungen zur Minderung der NOx-Emission technischer Feuerungen durch gestufte Verbrennungsfiihrung”, VGB-Kraftwerkstechnik 1990, Volume 70, Issue 8, and M.J. Murer, “Numerical methods for efficient power generation from municipal solid waste”, Technische Universitat Miinchen, Dissertation 2014, particularly Figure 3.4.

[0037] Combustion engineering measures for the reduction of nitrous oxides formation like air and fuel staging make use of reaction paths toward nitrogen gas, consuming some of the formed nitrogen monoxide on the way.

[0038] Post combustion nitrous oxides reduction measures like selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR) apply ammonia or urea injection to reduce the emissions from a combustion process to the required limits. After the selective catalytic reduction catalyst, an ammonia slip catalyst (ASC) can be installed to decompose unreacted ammonia to nitrogen gas and water.

[0039] Switching from hydrocarbon-based fuels to ammonia results in an order of magnitude increased production of nitrous oxides for the same burner design. Additionally, unburned ammonia is a pollutant by itself with much stricter emission limits than for nitrous oxides. Emissions from unreacted ammonia must therefore be prevented.

[0040] To meet an emission limit of 40 ppm of nitrous oxides or lower is close to impossible with a selective catalytic nitrous oxides reduction system when starting from raw nitrous oxides concentration of higher than 1 .500 ppm, due to reduction efficiencies in the range of 90 to 95%. Selective non-catalytic nitrous oxides reduction systems have an even lower nitrous oxides reduction performance (50 to 80%) and are therefore not an attractive option for industrial applications either.

[0041] Combined systems use ammonia injection at the lower end of the selective non- catalytic nitrous oxides reduction temperature window so that some ammonia remains as slippage, which can than react in a downstream selective catalytic or non-catalytic nitrous oxides reduction zone at lower temperatures. Using such an approach, the reduction rate of both technologies may be advantageously combined.

[0042] In selective catalytic and non-catalytic nitrous oxides reduction, an injection equipment of ammonia is conventionally necessary. For selective non-catalytic nitrous oxides reduction systems, these are typically provided as lances for an injection of an ammonia solution surrounding the fire box, sometimes using steam or plant air for better distribution and mixing. Sometimes additional water is added to form bigger droplets for further penetration depth.

[0043] However, for big furnaces getting a good coverage with the injections becomes challenging and evaporating the ammonia solution reduces the available high temperature heat and therefore the process efficiency of the fired heater.

[0044] During turndown or changed fuel composition, the temperature profile can change and a second or even third level of lances could be necessary since the selective non- catalytic nitrous oxides reactions work in the narrow temperature window of 850 to 1.050 °C. Above this temperature window, ammonia reacts to nitrogen monoxide in oxygen-containing flue gas. Below this window, ammonia will not react and become an emission or can unwantedly react in a downstream selective catalytic nitrous oxides reduction catalyst.

[0045] For selective catalytic nitrous oxides reduction, the ammonia is injected at lower temperature, so that the injection nozzles can be also located inside the flue gas duct. An additional static mixer can help evenly distribute the ammonia in the flue gas before or upstream of the catalyst.

[0046] Alternatively, the ammonia may be injected in a recirculated flue gas stream, which is extracted from the heat recovery system at high enough temperatures to evaporate the injected ammonia. The mixture of flue gas with high concentration of ammonia may afterwards injected in the main flue gas duct for good distribution of the ammonia.

[0047] As mentioned, the main challenges for nitrous oxide reaction is a good distribution of ammonia in the flue gas and the added capital and operating cost of the injection system. These challenges are addressed by aspects of the invention.

[0048] Figure 1 illustrates an arrangement 100 according to an embodiment. Arrangement 100 comprises a combustion section 110 and a flue gas section 120. The combustion section may also be referred to as a “furnace”, a “firebox”, etc. The flue gas section is sometimes also referred to as a “convection section”.

[0049] As shown in a strictly simplified manner, a combustion feed 1 , such as an ammonia- containing gas mixture of pure ammonia, may be supplied to arrangement 100, i.e., to a plurality of burners 111 , 112 which are, in the example illustrated, but not limiting the scope of the invention, arranged at a roof of combustion section 110.

[0050] A selective catalytic or non-catalytic nitrous oxide reduction unit 121 , or a combination of such units, is arranged in the flue gas section 120 in a manner generally also known to the skilled person. The plurality of burners 111 , 112 are operated using the combustion feed 1 , and a flue gas 2 formed in the combustion section 110 is passed through the flue gas section 120.

[0051] As illustrated with “A » 1” and “A < 1”, a first group of the plurality of burners, which are referred to with reference numeral 111 , are operated using first ratio of oxygen to ammonia and a second group of the plurality of burners, which are referred to with reference numeral 112, are operated with a second ratio of oxygen to ammonia lower than the first ratio of oxygen to ammonia. Further details are explained above. This approach has the effect that, as indicated with “NO” and “NH3” in Figure 1 , that the burners 111 of the first group produce a flue gas containing nitrogen monoxide and the burners 112 of the second group produce a flue gas containing a considerable amount of unreacted ammonia.

[0052] That is, by adjusting the air supply to the burners 111 , 112, some of these burners 111 will run on higher excess air so that the overall complete combustion in the furnace is achieved. Other burners 112 will run close to stoichiometry or slightly sub- stoichiometric to leave over some unreacted ammonia.

[0053] Since ammonia has relatively low flame temperature operating close to stoichiometry does not result in excessive temperatures in the furnace. As mentioned, the burners 111 , 112 can be modified with some having extra air nozzles while others have extra fuel nozzles or even dedicated ammonia nozzles for furnaces running on dual fuel systems. This is not shown for reasons of generality.

[0054] Upstream and downstream of the catalytic or non-catalytic nitrous oxide reduction unit 121 , a plurality of heat recovery bundles 122 may be provided. Any type of heat recovery bundles can be used, and these may particularly be adapted for reaction feed heating, boiler feed heating, steam heating or superheating, and and heating of media used in processing of certain materials, such as typically the case in a convection zone of a steam cracking furnace. Furthermore, between the burners 111 , 112, or some of them, steam tubes 113 or any other type of units for heating certain media are arranged. Again, these may be coils for heating or reacting process gas like in steam cracker furnaces or direct reduction iron heaters or reaction coils filled with catalyst like in steam methane reformers or ammonia crackers.

[0055] Operating some burners 112 at low and some burners 111 at high stoichiometry leads to air staging on furnace scale. This is further improved by a certain distribution of the burners 111 , 112 in the combustion section 110.

[0056] Figure 2 illustrates different options for burner distributions according to certain embodiments in a schematic top (or bottom) view of a combustion section, such as the combustion section 110 shown in Figure 1. Three different options are illustrated as vertical rows A to C and a vertical double row D which are each enclosed by dashed lines. All options shown may be provided in a combustion section 10, either alone or in any combination advantageous.

[0057] Figure 2 illustrates how the burners 112 of the second group (which are illustrated as large black or filled circles and only partly indicated by reference numerals) may be distributed among the burners 111 of the first group (which are illustrated as large white or unfilled circles and are and only partly indicated by reference numerals) in the combustion section 110 according to a predefined distribution pattern.

[0058] In each case, i.e., in each vertical row, the first group of burners 111 comprises a first number of burners 111 and the second group of burners 112 comprises a second number of burners 112 wherein, the first number of burners is, in row A, equal to the second number of burners 112 and larger in rows B and C as well as in double row D. The first number of burners 111 is, in rows B and C as well as in double row D, an integer multiple of the second number of burners 112, wherein the integer is 3 in row B, 6 in row C, and 4 in double row D.

Claims

Patent Claims1 . A method for combusting ammonia provided in a combustion feed (1) using a combustion arrangement (100) comprising a combustion section (110), a flue gas section (120), and a plurality of burners (111 , 112), wherein the plurality of burners (111 , 112) are operated using the combustion feed, and wherein a flue gas (2) formed in the combustion section (110) is passed through the flue gas section (120), characterized in that a first group of the plurality of burners (111) are operated using a first ratio of oxygen to ammonia and a second group of the plurality of burners (112) are operated with a second ratio of oxygen to ammonia lower than the first ratio of oxygen to ammonia.

2. The method according to claim 1 , wherein a a selective catalytic and / or non- catalytic nitrous oxide reduction is performed in the flue gas section.

3. The method according to claim 1 or 2, wherein the first ratio of oxygen to ammonia corresponds to a lambda value in a range from 1.1 to 2 and / or the second ratio of oxygen to ammonia corresponds to a lambda value in a range from 0.5 to 1.1.

4. The method according to any of the preceding claims, wherein the combustion feed comprises ammonia in a content of 1% to 100% on a molar, mass or volume basis.

5. The method according to any of the preceding claims, wherein the burners (112) of the second group are distributed among the burners (111) of the first group in the combustion section (110) according to a predefined distribution pattern.

6. The method according to claim 5, wherein the predefined distribution pattern is a regular or irregular distribution pattern.

7. The method according to any of the preceding claims, wherein the first group of burners (111) comprises a first number of burners (111) and the second group of burners (112) comprises a second number of burners (112), the first number of burners (111) being equal to or larger than the second number of burners (112).

8. The method according to any of the preceding claims, wherein the plurality of burners (111 , 112) are assigned to the first group of burners (111) and to the second group of burners (112) depending on the combustion feed (1).

9. The method according to any of the preceding claims, wherein at least some of the plurality of burners (111 , 112) are arranged in the combustion section (110) and / or at least some of the burners (111 , 112) are arranged in the flue gas section (120).

10. The method according to any of the preceding claims, wherein at least some of the plurality of burners (111 , 112) comprise dedicated ports for the combustion feed and / or for an oxidator gas to adapt the first ratio of oxygen to ammonia and / or the second ratio of oxygen to ammonia.11 . The method according to any of the preceding claims, wherein an ammonia slip reduction unit is arranged in the flue gas section (120).

12. The method according to any of the preceding claims, wherein one or more heat recovery bundles (122) are arranged in the flue gas section (120).

13. The method according to any of the preceding claims, wherein the combustion arrangement (100) is operated at a maximum temperature of less than 1.100 °C.

14. A combustion arrangement (100) for combusting ammonia provided in a combustion feed (1), the combustion arrangement (100) comprising a combustion section (110), a flue gas section (120), and a plurality of burners (111 , 112), wherein the combustion arrangement (100) is configured to operate the plurality of burners (111 , 112) using the combustion feed, and to pass a flue gas (2) formed in the combustion section (110) through the flue gas section(120), characterized in that the combustion arrangement (100) is configured to operate a first group of the plurality of burners (111) using first ratio of oxygen to ammonia and a second group of the plurality of burners (112) with a second ratio of oxygen to ammonia lower than the first ratio of oxygen to ammonia.

15. The arrangement (100) according to claim 14, wherein the arrangement (100) is configured to perform a method according to any of claims 1 to 13.

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