Method for operating a furnace and corresponding furnace

Abstract

Method for operating a furnace and corresponding furnace A method for operating a furnace (100, 200) for performing an endothermic gas phase reaction or heating a fluid is provided, the furnace (100, 200) comprising a firebox (110) and a heat recovery duct (120), a plurality of fluid passages (101) and a plurality of firebox burners (102) being provided in the firebox (110). The method comprises passing a reaction fluid (3) through the fluid passages (101), combusting ammonia with oxygen using the firebox burners (102) to form a flue gas containing uncombusted ammonia, passing the flue gas (5) containing uncombusted ammonia from the firebox (110) towards the heat recovery duct (120), and post-combusting the uncombusted part of ammonia contained in the flue gas (5) in a post-combustion region including an outlet zone of the firebox (110) and an inlet zone of the heat recovery duct (120). A corresponding furnace (100, 200) is also provided.

Description

[0001] Description

[0002] Method for operating a furnace and corresponding furnace

[0003] Field

[0004] The present disclosure relates to a method for operating a furnace at least partly heated by combusting a fuel comprising ammonia and to a corresponding furnace.

[0005] Background

[0006] Ammonia, a key ingredient in many fertilizers, could play a key role as a carbon-free fuel due to its ability to transport and store clean hydrogen conveniently. Comprised of hydrogen and nitrogen, ammonia can be used as a zero-carbon fuel.

[0007] However, as discussed in an article by M.B. Bertagni et al., Proc. Natl. Acad. Sci. USA 120, 2023, e2311728120, while ammonia itself does not contribute to carbon dioxide pollution, an improperly managed ammonia-based economy could significantly increase emissions of nitrous oxides. Nitrous oxides are are potent greenhouse gases with an atmospheric impact about 300 times stronger than carbon dioxide and a major factor in ozone layer depletion. Nitrous oxides are also known as pollutants that contribute to smog and acid rain formation.

[0008] Pure or high concentration ammonia combustion is still in its infancy, and information as to combustion chemistry is mainly available from research papers. As traces of ammonia and ammonia radicals are also released from nitrogen sources in conventional fuels, ammonia combustion chemistry has so far mostly been investigated with a focus on nitrous oxides formation pathways in such fuels. The results of these investigations, however, allow conclusions as to the emission problems probably occuring with high ammonia containing fuels and essentially pure ammonia.

[0009] Various reaction paths lead from ammonia to undesired nitrous oxides, but also to desired nitrogen gas. For the reasons given above, there is a desire for solutions suitable to reduce nitrous oxides emissions. Summary

[0010] Against this background, a method for operating a furnace at least partly heated by combusting a fuel comprising ammonia and a corresponding furnace comprising the features of the independent claims is proposed herein. Embodiments are the subject of the dependent claims and of the description that follows hereinbelow.

[0011] The method proposed herein may be applied for operating a furnace for performing an endothermic gas phase reaction, or for heating fluids without chemical reactions. In any case, said furnace comprises a firebox and a heat recovery duct. The firebox is elswere also referred to as a radiant zone and the heat recovery duct is elsewhere also referred to as a convection zone. The firebox may comprise a plurality of reaction tubes, which may be filled with a catalyst in case of a furnace for performing an endothermic gas phase reaction, or other fluid passages passed through the firebox in case furnaces used for heating a fluid. For reasons of generality, reference is made herein to “fluid passages”, and this term shall include reaction tubes which may be filled with a catalyst or not. A plurality of firebox burners is provided in the firebox. The term “firebox burners” is used herein mainly for indicating the location of the burners. As explained below, there may be further burners arranged in the heat recovery duct, which are referred to as “duct burners”. Firebox burners and duct burners, however, may in some embodiments proposed herein be provided in an essentially identical design.

[0012] The proposed method may comprise supplying a reaction feed to the reaction tubes and withdrawing a product stream from the reaction tubes in case of a furnace for performing an endothermic gas phase reaction, or passing a fluid to be heated through the fluid passages. Again, for reasons of generality, reference is made herein to a fluid passed through fluid passages. The method further comprises combusting ammonia, which may particularly be part of a fuel gas containing ammonia, with oxygen using the firebox burners to form a flue gas containing uncombusted ammonia. The method further comprises passing the flue gas containing uncombusted ammonia from the firebox towards the heat recovery duct, and post-combusting the uncombusted ammonia contained in the flue gas in a post-combustion region including an outlet zone of the firebox and an inlet zone of the heat recovery duct. The post-combustion region is particularly a region extending from a position downstream of a most downstream fluid passage in the firebox to a position upstream of a most upstream heat recovery bundle or heat recovery arrangement in the heat recovery duct. A particular advantage of selecting the post-combustion region accordingly is that the flue gas is still hot at such a position and therefore postcombustion is particularly effective. It goes without saying that means for said postcombusting may be provided anywhere in said region.

[0013] The reason for the flue gas containing uncombusted ammonia may particularly be an incomplete combustion due to incomplete mixing of oxygen or an oxygen-containing gas and ammonia, and may rather not be a result of an understoichiometric oxygen amount. For combustion in the firebox burners, oxygen may be provided in a slightly understoichiometric to a slightly overstoi ch iometric amount, for example in an amount from 0.9 to 1.1 or from 0.95 to 1.05 of a stoichiometric amount.

[0014] The proposed method therefore implements engineering measures for nitrous oxides reduction. In embodiments, these measures include air and fuel staging making use of or promoting the above-mentioned reactions paths toward nitrogen gas, consuming some of the formed nitrogen monoxide on the way.

[0015] As shown in Figure 1 below, it is expected that combustion air temperatures at 700 °C may result in nitrous oxides emissions approaching 1,000 ppmvd, which unit is explained below. Previous measures, such as disclosed in European patent application No. 23020441.4 and International application No. PCT / EP2024 / 025219 are suitable steps to reduce these nitrous oxides emissions. However, since these measures can be implemented at or close to the burners, flue gas homogenization and residence time is limited for achieving very low nitrous oxides emissions. The measures as proposed herein allow for an increase in residence time.

[0016] In certain embodiments, for said post-combusting the uncombusted part of ammonia contained in the flue gas, an oxygen containing gas is introduced into the postcombustion region. This oxygen containing gas may particularly be air, oxygen-enriched air, or any other mixture comprising oxygen such as oxygen-enriched flue gas, but also pure oxygen or essentially pure oxygen or any mixture thereof. This embodiment therefore corresponds to an “air staging” in the furnace. Such air staging downstream of the firebox, particularly in the post-combustion region, as performed in embodiments proposed herein, allows for a reduction of the excess air rate in the firebox to a value close to zero. This decrease in the excess air rate in the firebox allows for good mixing of the flue gases and ample residence time compared to an additional air stage at or close to the burner, and thus lower nitrous oxides emissions can be achieved. The observations made by the present inventors is corroborated in the literature, and reference is made, e.g., to Figure 3 from an article by A.M. Elbazet al., Fuel Commun. 10, 2022, 100053.

[0017] In certain embodiments, the oxygen containing gas is introduced into the postcombustion region using a gas injection grid arranged in the post-combustion region. In such embodiments, the gas injection grid may be made of or comprising certain stainless steels, nickel based alloys and / or an oxide ceramic matrix composite.

[0018] In the embodiments proposed herein, a part of the combustion air, e.g. 9 to 10%, may be piped or bypassed around the firebox and injected into a refractory lined duct leading to the heat recovery duct or convective section, particularly to the postcombustion region. Since the firebox is typically operated at negative pressure, no flue gases containing unburned fuel will exit at normal operation. Additionally, the fuel gases may, for a stoichiometry greater than 1 , at least 99% oxidized, and therefore the flue gas is well below the lower explosive limit. Due to the low temperature at the exit of the firebox, the air can be injected using an injection grid made out of stainless steel 3101.4845 or similar or oxide ceramic matrix composite. This grid distributes the combustion air evenly over the cross section of the duct, like in a duct burner. Since there is no or nearly no heat release at this location, the flue gas temperature will actually drop due to the amount of injected combustion air.

[0019] In certain embodiments as proposed herein, for said post-combusting the uncombusted part of ammonia contained in the flue gas in the post-combustion region, duct or postcombustion burners may be arranged and operated in the post-combustion region and downstream thereof. A plurality of heat recovery bundles is, in certain embodiments, serially arranged in a streaming direction in the heat recovery duct, i.e. , downstream of the post-combustion region, wherein at least one of the post-combustion burners is arranged upstream of a first one of the heat recovery bundles in the streaming direction, i.e. , in the post combustion region, and at least a further one of the postcombustion burners is arranged downstream of a first one of the heat recovery bundles in the streaming direction, i.e., downstream of the post-combustion region.

[0020] In such alternatives to mere air staging, duct burners are installed into the convective section which can be used to inject (the same or a different ammonia-containing) fuel and air before and after the first heat heat recovery bundle, thus implementing additional stages in the combustion for potential further nitrous oxides reduction, but also for temperature control in the first heat recovery bundle and heat recovery bundles downstream thereof. These burners can also be used for potential debottlenecking for an operating overload case. In certain embodiments, the duct burners may be selected from any type of primary burners in the firebox, such as jet burners, swirl burners or even flameless oxidation burners.

[0021] Embodiments of the method proposed herein may be used for a large number of endothermic gas phase reactions, particularly catalytic reactions selected from a reforming reaction, a dehydrogenation reaction, and ammonia cracking, or as a process gas heater like in direct reduction iron applications.

[0022] In certain embodiments, the ammonia is provided to the firebox burners, and the duct burners, as the case may be, as part of a fuel gas stream or fuel gas mixture comprising an amount from 0 to 30 vol.% of ammonia. In certain embodiments, pure or essentially pure ammonia may be used.

[0023] In certain embodiments, oxygen is provided to the firebox burners as part of an oxidator gas stream comprising oxygen in an atmospheric amount or above the atmospheric amount, as already discussed above.

[0024] As to the furnace proposed herein, reference is, reference is made to the corresponding independent claim.

[0025] As to further details and advantages of such a furnace, reference is made to the explanations above regarding the method proposed herein and its different embodiments. Particularly, such a furnace may, in embodiments as proposed herein, comprise means adapted to perform a method according to any of the embodiments as discussed herein.

[0026] Figures

[0027] Embodiments as disclosed herein will now be described, by way of example only, with reference to accompanying drawings, in which

[0028] Figure 1 shows a diagram showing a correlation between nitrous oxygen formation and ammonia combustion air temperatures;

[0029] Figure 2 shows a diagram showing a correlation between nitrous oxygen formation and an excess oxygen amount used for combustion;

[0030] Figure 3 schematically illustrates a furnace according to an embodiment; and

[0031] Figure 4 schematically illustrates a furnace according to a further embodiment.

[0032] Embodiments

[0033] In the Figures, elements of identical, essentially identical, functionally comparable, or technically compatible function and / or purpose 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 certain embodiments disclosed herein likewise may apply to methods, processes, procedures, etc. according to corresponding embodiments.

[0034] 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 as disclosed herein may suitably comprise, consist of, or consist essentially of, appropriate and technically sensible 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.

[0035] The following explanations and definitions relating to some of the principles of the invention may apply to all or some of the embodiments presented herein, and the explanation of certain aspects in connection with only some or one of the embodiments should not be taken to mean that these aspects cannot also be realised with other or all embodiments, as far as technically possible and reasonable.

[0036] All percentages used herein may refer to molar, quantitative or volumetric proportions. Pressure specifications in bar are, unless otherwise explained, to be understood in particular as absolute pressures.

[0037] The conjunction “and / or”, when used in a list or in an enumeration before the last element of the list or enumeration, should be understood to mean that all terms previously mentioned in the enumeration can be combined with each other in any way. In other words, “A, B and / or C” means “A and / or B and / or C” or “at least one of the elements A, B, C in any combination”. The terms used in the context of the present disclosure generally have the meanings recognised in the art.

[0038] While the embodiments described hereinbelow may include reactors for performing gas phase reactions, it will be understood by the skilled person that the present invention is not limited to such apparatus but may likewise be used in mere heating applications, as already described above.

[0039] Figure 1 is a diagram illustrating some aspects underlying embodiments disclosed herein. In the diagram of Figure 1, nitrous oxides emissions in parts per million by volume, dry (ppmvd), as generally used as a measure of emissions in the field of combustion technology, are indicated on the vertical axis. A value of 100 ppmvd, for example, refers to (100 / 1,000,000) x 100 = 0.01% of the volume of the flue gas. The value is expressed on the basis of the dry flue gas, i.e., the part of the flue gas which is not formed by water. In the diagram of Figure 1 , the combustion air temperature in °C is indicated on the horizontal axis.

[0040] Combustion air temperatures tested are in the range of 100 °C to 375 °C (limited by the test site), and a quadratic relationship to the nitrous oxides formation was found in this temperature range. In other words, the resulting nitrous oxides emissions increase with the square of the combustion air temperature. Therefore, an extrapolation is possible. In the diagram of Figure 1, results from combusting a fuel comprising 17% ammonia are indicated with triangles and results from combusting a fuel comprising 26% ammonia are indicated with squares. The respective extrapolations are indicated with dashed lines contiguous to the solid lines of the respective results.

[0041] Figure 2 is a diagram illustrating further aspects underlying embodiments disclosed herein. In the diagram of Figure 2, nitrous oxides emissions in ppmvd are indicated on the vertical axis, and an excess air rate in % is indicated on the horizontal axis. Figure 2 shows the strong dependence of nitrous oxides emissions emissions on excess air rate. These are results from the same burner tests shown in Figure 1 but different test points, wherein results from combusting 4% ammonia at a combustion air temperature of 375 °C are indicated with triangles and results from combusting 17% ammonia at a combustion air temperature of 375 °C are indicated with squares. Both combustion test have been performed at a furnace outlet temperature of 800 °C. An overall excess air rate of 8 % is indicated with a dashed line, which is the value used according to certain embodiments as disclosed herein. As indicated by an arrow in Figure 2, without limitation, the nitrous oxides emissions appear to converge at a value significantly below 100 ppmvd at an excess air rate of zero.

[0042] In Figure 3, a furnace according to an embodiment proposed herein is schematically illustrated and generally referred to with 100. The setup of furnace 100 is typical for a steam reformer furnace, a furnace as generally known to the person skilled in the art. However, furnace 100 may also configured differently and may be used for other endothermic reactions such as dehydrogenation of alkanes or cracking of ammonia.

[0043] Furnace 100 comprises a firebox 110 and a heat recovery duct 120. In the firebox 110, as illustrated by a cross hatched box, an arrangement of reaction tubes 101 is provided in one or more groups. For heating the reaction tubes 101, an arrangement of burners 102 is provided, in the example illustrated, at a roof of the firebox 110. For reasons of conciseness, only one burner 102 is illustrated, and any other configuration and arrangement of burners 102 may be present. The burners 102 are supplied with a fuel stream 1 and a combustion air stream 2. The burners 102 are referred to as “firebox burners” herein.

[0044] A reaction feed 3 which, in case of steam reforming, comprises a hydrocarbon such as methane and steam and which may, separately and / or commonly, preheated in the heat recovery duct 120, for example, and which may have undergone a pretreating step such as a prereforming step or preheating, is supplied to the reaction tubes 101. A product stream 4 may be withdrawn from the reaction tubes 101 and may be supplied to subsequent treatment steps as generally known in the field.

[0045] A flue gas stream 5 is withdrawn from firebox 110 via the heat recovery duct 120 using a blower 104. In the heat recovery duct 120, an arrangement of heat recovery bundles 105 to 108 is provided, which may be used as known in the field, e.g., for boiler feed water preheating, boiler feed water evaporation, steam heating, steam superheating, feed preheating, combustion air preheating, fuel preheating, etc. After heat recovery a low temperature flue gas stream 6 is released to the atmosphere via stack.

[0046] An air stream 7 may be introduced into firebox 110, in order to provide an extra air stage for the combustion process in the firebox and give additional momentum to the flue gas in the fire box for better mixing. An air injection grid 109 is, in the embodiment illustrated, arranged at an inlet of the heat recovery duct 120, or more generally in a region referred to as “post-combustion region” above, and an airstream 8 is introduced at this position therewith. This establishes an additional combustion stage (air staging) downstream of the firebox burners 102 in the firebox 110, particularly at the entrance of the heat recovery duct, or more generally in the “post-combustion region”, which allows for a reduction of the excess air rate in the fire box close to 0% (with a target lambda value of 0.99 to 1.05, particularly 1.01). This decrease in the excess air rate in the firebox 110 allows for good mixing of the flue gases and ample residence time compared to an additional air stage at or close to the burner (air stream 7), and thus lower nitrous oxides emissions can be achieved. Figure 6 shows an alternative implementation of the proposed furnace 200 wherein, instead of injecting only a combustion air stream 8 into the post-combustion region, duct burners 201 and 202 are installed which can be used to inject further fuel streams 10 and 11 , as well as air stream 8 and an air stream 89, before and after the heat recovery bundle 105 thus implementing additional stages in the combustion for potential further nitrous oxides reduction but also for temperature control on the cold side for the first and second heat heat recovery bundles 105 and 106. These burners 201 , 202 can also be used for potential debottlenecking for an operating overload case.

Claims

Patent Claims1. A method for operating a furnace (100, 200) for performing an endothermic gas phase reaction or heating a fluid, said furnace (100, 200) comprising a firebox (110) and a heat recovery duct (120), a plurality of fluid passages (101) and a plurality of firebox burners (102) being provided in the firebox (110), the method comprising: passing a fluid through the fluid passages (101);combusting ammonia with oxygen using the firebox burners (102) to form a flue gas containing uncombusted ammonia;passing the flue gas (5) containing uncombusted ammonia from the firebox (110) towards the heat recovery duct (120); andpost-combusting the uncombusted ammonia contained in the flue gas (5) in a post-combustion region including an outlet zone of the firebox (110) and an inlet zone of the heat recovery duct (120).

2. The method according to claim 1,wherein for said post-combusting the uncombusted part of ammonia contained in the flue gas (5) in the post-combustion region, an oxygen containing gas is introduced into the post-combustion region.

3. The method according to claim 2,wherein the oxygen containing gas is introduced into the post-combustion region using a gas injection grid (109) arranged in the post-combustion region.

4. The method according to claim 3,wherein the gas injection grid (109) is made of or comprises stainless steel, a nickel based alloy, and / or an oxide ceramic matrix composite.

5. The method according to claim 1 ,wherein duct burners (201, 202) are operated in the heat recovery duct (120).

6. The method according to claim 5,wherein a plurality of heat recovery bundles (105, 106, 107, 108) is serially arranged in a streaming direction in the heat recovery duct (120), andwherein at least one of the duct burners (201) is arranged upstream of a first one of the heat recovery bundles (105, 106, 107, 108) in the streaming direction and at least a further one of the duct burners (202) is arranged downstream of a first one of the heat recovery bundles (105, 106, 107, 108) in the streaming direction.

7. The method according to claim 6,wherein the endothermic gas phase reaction is a catalytic reaction selected from a reforming reaction, a dehydrogenation reaction and an ammonia cracking reaction, or heating the fluid includes heating a fluid used for direct reduction of iron.

8. The method according to any one of the preceding claims,wherein the ammonia is provided to the firebox burners (102) as part of a fuel gas stream (1) comprising an amount from 0 to 50 vol.% of ammonia.

9. The method according to any one of the preceding claims,wherein the oxygen is provided to the firebox burners (102) as part of an oxidator gas stream (1) comprising oxygen in an atmospheric amount or above the atmospheric amount.

10. A furnace (100, 200) for performing an endothermic gas phase reaction or heating a fluid, said furnace (100, 200) comprising a firebox (110) and a heat recovery duct (120), a plurality of fluid passages (101) and a plurality of firebox burners (102) being provided in the firebox (110), the furnace (100, 200) being configured for:passing a fluid through the fluid passages (101);combusting ammonia with oxygen using the firebox burners (102) to form a flue gas containing uncombusted ammonia;passing the flue gas (5) containing uncombusted ammonia from thefirebox (110) towards the heat recovery duct (120); andpost-combusting the uncombusted part of ammonia contained in the flue gas (5) in a post-combustion region including an outlet zone of the firebox (110) and an inlet zone of the heat recovery duct (120).

11. The furnace (100, 200) according to claim 10,wherein the furnace (100, 200) is configured to perform a method according to any one of claims 1 to 9.

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