EP4715029A1

EP4715029A1 - Method and apparatus for steam cracking

Info

Publication number: EP4715029A1
Application number: EP24020289.5A
Authority: EP
Inventors: Tobias SINN; Gunther Schmidt; Gunther Kracker; Rainer Kemper; Michael HOERENZ; Martin HOFSTAETTER; Torben Hoefel; Juergen Feigl
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2024-09-18
Filing date: 2024-09-18
Publication date: 2026-03-25
Also published as: WO2026061961A1

Abstract

A method for steam cracking is proposed, the method comprising operating a steam cracking furnace (100) with a radiant section (110) and a convection section (120), the radiant section (110) comprising a plurality of reaction tubes (111) and a plurality of burners (112) and the convection section (120) comprising a plurality of heat recovery units (121, 122). The proposed method includes operating the steam cracking furnace (100) using a group of gases to provide heat to the reaction tubes (111) and/or the heat recovery units (121, 122), the group of gases consisting of one or more combustible gases and a subgroup of further gases, the subgroup of further gases consisting of oxygen and one or more incombustible gases. In the proposed method, the oxygen is provided in the subgroup of further gases in a proportion lower than a proportion of oxygen in atmospheric air. The subgroup of further gases, or one or more gas components used in forming the subgroup of further gases, is subjected to a preheating step to a temperature above 500 °C upstream of the burners (111), and the preheating step is performed using electric energy in an energy amount which is at least 20% of an energy amount provided by the lower heating value of the one or more combustible gases (111). A corresponding apparatus (1000) is also proposed herein.

Description

Field

The present disclosure relates to a method and an apparatus for steam cracking.

Background

Steam cracking is a technology for the production of olefins and other base chemicals, as, e.g., described in the article "Ethylene" in Ullmann's Encyclopedia of Industrial Chemistry, online publication 15 April 2009, DOI: 10.1002/14356007.a10_045.pub2.
Presently, the thermal energy required for initiating and maintaining the endothermic steam cracking reactions is provided by the combustion of fuel gas. A process gas containing steam and the hydrocarbons to be cracked is passed through reaction tubes or cracking coils which are placed inside a furnace region usually referred to as radiant zone or section. On its flow path through the cracking coils in the radiant section, the process gas is continuously heated, enabling the desired cracking reactions to take place inside the cracking coils.
In addition to the radiant section, fired cracking furnaces comprise a further region usually referred to as convection zone or section. The convection section may be positioned above the radiant section and typically comprises various heat recovery units to be heated by flue gas withdrawn from the radiant section and passed through the convection section. The main function of the convection section is to recover as much energy as possible from the hot flue gas leaving the radiant section. The flue gas heat recovered in the convection section of a fired steam cracking furnace is typically used for process duties such as preheating or vaporization of boiler feed water and/or hydrocarbon feeds, and superheating of steam.
In order to reduce greenhouse gas emissions, there is the desire to reduce the demand of carbonaceous fuels for steam cracking. Recently, substituting carbonaceous fuels with alternative fuels such as hydrogen or ammonia and electrification of steam cracking furnaces have been proposed. Both options, however, come with certain technical problems as further discussed below.
In view of the above, there is a need for improved instrumentalities for steam cracking, particularly in view of revamping existing steam cracking apparatus.

Summary

Against this background, methods and apparatus including the features of the independent claims are proposed. Embodiments are the subject of the dependent claims and of the description that follows.
The proposed method for steam cracking comprises operating a steam cracking furnace with a radiant section and a convection section, the radiant section comprising a plurality of reaction tubes and a plurality of burners and the convection section comprising a plurality of heat recovery units. The method further comprises operating the steam cracking furnace using a group of gases to provide heat to the reaction tubes and/or the heat recovery units, the group of gases consisting of one or more combustible gases and a subgroup of further gases, the subgroup of further gases consisting of oxygen and one or more incombustible gases.
The one or more combustible gases may be used to operate the burners in the method proposed herein. The one or more incombustible gases may also, partly or completely, be sent to the burners, either in a mixture with the one or more combustible gases or the oxygen, or respective parts thereof, or in the form of separate gas streams. Alternatively or additionally, the one or more incombustible gases may, partly or completely, be heated by other means than the burners and be passed to the radiant and/or convection section for providing heating additional to the burners. No further gases than those included in the group of gases are, in the proposed method, sent to the steam cracking furnace externally to the reaction tubes.
In the proposed method, oxygen is provided in the subgroup of further gases in a proportion lower than a proportion of oxygen in atmospheric air. The subgroup of further gases, a part thereof, or one or more components used in forming the subgroup of further gases, is subjected to a preheating step to a temperature above 500 °C upstream of the steam cracking furnace, particularly the burners, and the preheating step is performed using electric energy in an energy amount which corresponds to at least 20% of an energy amount provided by the lower heating value of the one or more combustible gases.
The term "group" or "subgroup" is used herein in connection with gas components or gases to express that such gas components or gases may, or may not, be provided in a mixture of gas components or gases. They are, however, provided at the same time. For example, the one or more combustible gases may be premixed with one or more of the subgroup of further gases in order to be supplied to the burners, or the one or more combustible gases may be supplied to the burners separately from the one or more of the subgroup of further gases, and the latter may likewise be supplied in a mixture or separately. As mentioned, some gases, or parts thereof, particularly the incombustible gases or parts thereof, may als be supplied to the steam cracking furnace, i.e., the radiant section and/or the convection section, externally from the burners. Herein, a "part" of a group of gases may either relate to one of the gases forming the group, partial amounts of several gases forming the group, or any combinations thereof.
Different fuels can be applied for providing heat in a steam cracking furnace, having an impact on certain combustion properties in terms of, e.g., adiabatic flame temperature and/or certain radiation properties, among others. Such fuels or mixtures thereof may include hydrogen, methane, various other hydrocarbons, and ammonia. Likewise, different oxidants can be applied which may includepure oxygen, oxygen enriched air, and atmospheric air.
The proposed method and embodiments disclosed herein solves certain problems arising when substituting carbonaceous fuels in conventional steam cracking methods as will now be explained in further detail.
Typically, alternative fuels like hydrogen or ammonia are more expensive and/or less available than conventional, carbonaceous fuels, or their use may be limited for other reasons. A reduction of the fuel demand is therefore desired. This can be achieved by introducing further heat into the system, which can be realized, e.g., via preheating of the fuel and/or the oxidant. However, existing cracking furnaces are not designed for alternative fuels and/or for being retrofit with fuel and/or oxidant preheating. One key reason is that the overall combustion properties are significantly altered by such amendments, resulting in, e.g., reduced available duty within the convection section.
This conflict is solved, according to the present disclosure, by performing a combustion of one or more fuel gases with a gas mixture which is, as compared to atmospheric air, provided with a reduced oxygen content. This mixture, or one or more components thereof or one or more components or parts in forming the same, is preheated to high temperatures by electrical heating. The method provided herein significantly reduces the fuel demand within the cracking furnace and simultaneously reaches the same heat distribution across radiation and convection zone, reducing the need for revamp of an existing steam cracking furnace. That is, embodiments as proposed herein allow for a continuation of operations of an existing steam cracking furnace even when a conventional fuel is substituted by an alternative fuel or when the amount of fuel is reduced and partly substituted by electric energy.
Therefore, embodiments as provided herein significantly reduce the fuel demand within the cracking furnace and allow for simultaneously reaching the same heat distribution across radiation and convection zone of a cracking furnace, reducing the need for revamp of an existing cracking furnace.
In certain embodiments of the method proposed herein, a flue gas formed by operating the burners, or a part thereof, is passed from the radiant section through the convection section to provide heat to the heat recovery units. Embodiments as disclosed herein therefore pertain to classical steam cracking arrangements in which a furnace with a radiation and a convection zone is used.
In certain embodiments of the method proposed herein, subgroup of further gases may include a part of the flue gas having been passed through the convection section as the incombustible gas or one of the incombustible gases. This is typically a flue gas with a temperature of 100 to 150 °C, such as 120 to 140 °C, e.g., about 130 °C. Therefore, embodiments disclosed herein may be used to conventionally heat certain media as typical for heat recovery units in a convection zone.
As an alternative to using a flue gas, the subgroup of further gases may also include an inert gas or gas mixture provided from a source different from the cracking furnace as the incombustible gas or one of the incombustible gases. This may be, e.g., an exhaust gas of a gas turbine or a gas from an air separation unit. Such embodiments allow for advantageously utilizing such gases when available.
In certain embodiments of the method proposed herein, the oxygen in the subgroup of further gases of the group of gases is provided using atmospheric air or using a gas or gas mixture comprising an oxygen content higher than atmospheric air. Certain embodiments may relate to an "oxyfuel" combustion, reducing the amount of inert gases such as nitrogen in a flue gas.
In certain embodiments of the method proposed herein, the combustible gas or gases is, or includes, one or more of hydrogen and one or more hydrocarbon gases. As further explained below, embodiments of the present invention allow for reducing the fuel gas in such cases and nevertheless operate a cracking furnace with a heat distribution comparable to a conventional furnace.
In certain embodiments of the method proposed herein, the subgroup of further gases is mixed upstream or downstream of the preheating step upstream of the steam cracking furnace, i.e., upstream of the burners and/or before being passed into the steam cracking furnace otherwise. Also parts thereof may be mixed. In alternative embodiments, mixing may take place after preheating, which may allow adjusting a mixing temperature.
In certain embodiments of the method proposed herein, the heat recovery units are used for heating at least one of boiler feed water, process steam, high pressure steam and one or more reaction feeds for the steam cracking furnace.
In certain embodiments of the method proposed herein, the preheating step is performed to a temperature in a range from 500 to 1,500 °C, from 750 to 1,250 °C, or from 800 to 1,200 °C. As shown below, an adjustment of the preheating temperature may directly correlate with fuel gas usage.
In certain embodiments of the method proposed herein, further electric heating is performed in the radiation zone by using radiative heating and/or direct resistive heating of the reaction tubes. This may allow for an additional adjustment of the parameters in the radiant and convection zone.
The apparatus for steam cracking as proposed herein comprises a steam cracking furnace with a radiant section and a convection section, the radiant section comprising a plurality of reaction tubes and a plurality of burners and the convection section comprising a plurality of heat recovery units. The apparatus is configured to operate the steam cracking furnace using a group of gases to provide heat to the reaction tubes and/or the heat recovery units, the group of gases consisting of one or more combustible gases and a subgroup of further gases, the subgroup of further gases consisting of oxygen and one or more incombustible gases. As proposed herein, the apparatus is configured to provide the oxygen in the subgroup of further gases in a proportion lower than a proportion of oxygen in atmospheric air, to subject the subgroup of further gases, a part thereof, or one or more components used in forming the subgroup of further gases, to a preheating step to a temperature above 500 °C upstream of the burners, and to perform the preheating step using electric energy in an energy amount which is at least 20% of an energy amount provided by the lower heating value of the one or more combustible gases.
As to further details and advantages of such an apparatus, reference is made to the explanations above in regarding the method proposed herein and its different embodiments. Particularly, such an apparatus may, in embodiments as proposed herein, comprise means adapted to perform a method according to any of the embodiments as discussed herein.

Figures

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

Figure 1 illustrates an apparatus for steam cracking;
Figure 2 illustrates an apparatus for steam cracking;
Figure 3 illustrates an apparatus for steam cracking;
Figure 4 illustrates an apparatus for steam cracking according to an embodiment;
Figure 5 illustrates an apparatus for steam cracking; and
Figure 6 shows a correlation between a preheating temperature and a fuel demand.

Embodiments

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.
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.
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.
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.
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.
As mentioned above, substituting a part of a conventional fuel gas in a steam cracking furnace by electric energy which is used to preheat one or more gases supplied to the burners, such as fuel gas or oxidant, causes significant changes in availability and/or distribution of energy in a steam cracking furnace. This will now be explained in connection with Figures 1 to 5.
Figure 1 illustrates a steam cracking apparatus not forming part of the present invention. The steam cracking apparatus shown in Figure 1 includes a steam cracking furnace 100 with a radiant section 110 and a convection section 120. A plurality of reaction tubes 111, of which one reaction tube is illustrated in Figure 1 in a simplified manner, is passed through the radiant section 110. A plurality of burners 112, of which only two burners 112 are illustrated for reasons of clarity, is provided in the radiant section 110 to heat the reaction tubes 111 for performing steam cracking reactions.
In the apparatus shown in Figure 1, a plurality of heat recovery units 121, 122 is provided in the convection section 120. It will be understood that further heat recovery units may be present, and the two heat recovery units 121, 122 shown in Figure 1 are only provided as examples. In heat recovery unit 121, a reaction feed 1 is heated before being passed through the reaction tubes 111 in the radiant section 110. It will be understood that reaction feed 1 may be combined at any position technically possible or advantageous with reaction steam which also may be produced or heated in the convection section 120, or a steam or boiler feed water injection can be performed. Furthermore, boiler feed water 2 may be converted into steam 3 in the example shown. It will be understood that further means for producing or superheating steam, such as a steam drum and further heat recovery units, may be present.
A crude gas 4 withdrawn from the reaction tubes 111 is quenched in a quench cooler 130 to provide a quenched crude gas 5 which may be sent to a crude gas compression and fractionation sequence not illustrated for reasons of conciseness. Using quench cooler 130, further steam 6 may be provided and a combined steam stream 7 may be formed. Burners 112 may be operated using a methane-rich fuel gas 8 and air 9.
In the example shown in Figure 1, air 9 may be provided at a temperature of 25 °C and a lower calorific value of fuel gas 8 may be 73.6 MW, of which 30 MW may be transferred in radiant section 110 to the reaction tubes 111 and of which 40.2 MW may be transferred in the convection section 120 to the heat recovery units 121, 122. A flue gas 10 withdrawn from the convection section 120 by means of a blower 140 may have a temperature of 130 °C, corresponding to an energy content of 3.4 MW.
Figure 2 illustrates a steam cracking apparatus not forming part of the present invention. The steam cracking apparatus shown in Figure 2 essentially comprises the elements already explained in connection with Figure 1. Instead of the methane-rich fuel gas 8, in the steam cracking apparatus shown in Figure 2, a hydrogen-rich fuel gas 11 or essentially pure hydrogen is used as a fuel.
In the example shown in Figure 2, air 9 may be provided at a temperature of 25 °C and a lower calorific value of hydrogen-rich fuel gas 11 may be 63.7 MW, i.e. lower than in the example shown in Figure 1 for a methane-rich fuel gas 8, of which 30 MW, as required, and as also the case in the example shown in Figure 1 may be transferred in radiant section 110 to the reaction tubes 111, but only 31 MW remain to be transferred in the convection section 120 to the heat recovery units 121, 122. The flue gas 10 withdrawn from the convection section 120 by means of blower 140 may have a temperature of 130 °C, corresponding to an energy content of 2.7 MW.
That is, when keeping the heat or energy amount constant in the radiant section 110, which is required for properly controlling the steam cracking reaction or to provide a similar product distribution in the example shown in Figure 2 as in the example shown in Figure 1, a lower energy input in hydrogen-rich fuel gas 11 is required. This, however, results in a lower availability of energy in the convection section 120.
Figure 3 illustrates a steam cracking apparatus not forming part of the present invention. The steam cracking apparatus shown in Figure 3 essentially comprises the elements already explained in connection with Figure 1 and 2. As in the steam cracking apparatus shown in Figure 2, in the steam cracking apparatus shown in Figure 3, a hydrogen-rich fuel gas 11 or essentially pure hydrogen is used as a fuel. Furthermore, a partial stream 13 of flue gas 10 withdrawn from the convection section 120 is combined with the air 9 used to operate the burners 112.
In the example shown in Figure 3, air 9 may be provided at a temperature of 25 °C and a lower calorific value of hydrogen-rich fuel gas 11 may be 73.2 MW, i.e. higher than in case of the steam cracking apparatus shown in Figure 2 and similar to the steam cracking apparatus shown in Figure 1, of which 30 MW may be transferred in radiant section 110 to the reaction tubes 111 and of which, such as in the steam cracking apparatus shown in Figure 1, 40.2 MW may be transferred in the convection section 120 to the heat recovery units 121, 122. The flue gas 10 withdrawn from the convection section 120 by means of blower 140 may have a temperature of 130 °C, corresponding to an energy content of 3 MW.
That is, by increasing the energy input via hydrogen-rich fuel gas 11, and by recycling a part of the flue gas, the energy or heat distribution of the steam cracking apparatus shown in Figure 1 may be approximated, but in this case the energy input using the hydrogen-rich fuel gas 11 cannot be reduced, which is however desired for the reasons already explained above.
Figure 4 illustrates a steam cracking apparatus 1000 according to an embodiment proposed herein. The steam cracking apparatus shown in Figure 4 essentially comprises the elements already explained in connection with Figures 1 to 3. As in the steam cracking apparatus shown in Figures 2 and 3, in the steam cracking apparatus shown in Figure 4, a hydrogen-rich fuel gas 11 or essentially pure hydrogen is used as a fuel. Furthermore, as in the steam cracking apparatus shown in Figure 3, a partial stream 13 of flue gas 10 withdrawn from the convection section 120 is combined with the air 9 used to operate the burners 112. The air 9 is provided using a blower 150 and a combined stream of air 9 and the partial stream 13 of the flue gas is heated in an electric heater 160.
In the example shown in Figure 4, air 9 may be provided at a temperature of 25 °C and a lower calorific value of hydrogen-rich fuel gas 11 may be 31,5 MW, i.e. significantly lower than in the steam cracking apparatus shown in Figures 1, 2 and 3. An energy amount of 40 MW is provided using heater 160, which heats the combined stream of air 9 and the partial stream 13 to a temperature of 1,250 °C. 30 MW of energy may be transferred in radiant section 110 to the reaction tubes 111, such as in the steam cracking apparatus shown in Figures 1, 2 and 3, and 40.2 MW may be transferred in the convection section 120 to the heat recovery units 121, 122, such as in the steam cracking apparatus shown in Figures 1 and 3. The flue gas 10 withdrawn from the convection section 120 by means of blower 140 may have a temperature of 130 °C, corresponding to an energy content of 1.3 MW.
That is, additional preheating of non-combustible gases supplied to the burners 112 may be performed to reduce the energy input required by using hydrogen-rich fuel gas 11 significantly, and this therefore represents an advantageous option for generally reducing the fuel gas demand or using electric energy.
Figure 5 illustrates a steam cracking apparatus not forming part of the present invention. The steam cracking apparatus shown in Figure 5 comprises some elements already explained in connection with Figures 1 to 4. In the steam cracking apparatus shown in Figure 5, radiant section 110 does not comprise burners 112, and therefore no flue gas, but nevertheless hot gas 14, is withdrawn from the convection section 120. Hot gas 14 is essentially recirculated to radiant section 110. No air 9 is used in the apparatus shown in Figure 5. A significant amount of energy provided to the cracking furnace 100 is provided by an electric heater 160 and, as illustrated with an arrow 15, by electric heating in the radiant section 110. The hot gas 14 can be routed to the radiant section 110, or alternatively directly to the convection section 120, as indicated by stream 16,in order to adjust heat distribution.
In the example shown in Figure 5, an energy amount of 40 MW is provided using heater 160 which heats the recycled hot gas 14 from convection section 120 to a temperature of 1,250 °C. 30 MW of electric energy 15 may be provided to the radiant section 110 by direct resitive heating of the coils or by resitive heating elements therein. 40.2 MW may be transferred in the convection section 120 to the heat recovery units 121, 122.
It could be of benefit to combine two electric heating methods as described above. For example, it could be less expensive to provide heat up to a certain temperature level by electrically heating up a gas outside the cracking furance in an electric heater 160, allowing to reduce the specific investment for the direct resitive heating of the coils or the resitive heating elements within the cracking furnace 100. In another example, special ceramic heating elemtens could be utilized within the cracking furnace 100, which only work at a certain minimum temperature. The minimum temperature could be provided by the hot gas from the electric heater 160.
That is, the measures provided according to Figure 5 represent an alternative option to provide essentially the same heat distribution, but due to the complete omission of burners and fuel gas heating, a substantial reconstruction is required, such that the steam cracking arrangement shown in Figure 5 is a less desired option for revamp.
In the steam cracking arrangement shown in Figure 4, an oxygen-depleted stream is used in form of the recycled flue gas 13, and the components of the flue gas, together with air 9, represent "a subgroup of further gases" referred to hereinbefore. This subgroup of further gases has, due to the flue gas being part thereof, a reduced oxygen content as compared to atmospheric air. For reasons of conciseness, reference is made to an "oxygen-deplete stream".
Instead of flue gas 13, an oxygen-depleted stream may also be provided from other sources, provided that this is largely composed of inert material neither acting as fuel or oxidant. Examples include, besides the flue gas from the furnace stack, gas turbine exhaust gas, any media depleted in oxygen and enriched in nitrogen, water, or carbon dioxide, as compared to air, e.g., unused "offgas" (nitrogen) from an air separation unit, and inert streams like water, carbon dioxide, and nitrogen, or a mixture thereof.
In embodiments disclosed herein, the oxygen-depleted stream is preheated. The preheating of the oxygen-depleted stream can be done alone, in combination with the oxidant, or in combination with the fuel gas 11. As to the temperatures for preheating, reference is made to the explanations above.
In certain embodiments of the invention, the preheated oxygen-depleted stream may not, or only in part, be sent directly to the burners 112. Instead, the preheated oxygen-depleted stream, or parts thereof, could also be sent separately to the radiant section 110 and/or convection section 120. Any of these options could serve the purpose of ideally balancing the heat supply to a radiant section 110 and convection section 120 of a new or existing furnace in a particular individual case.
In certain embodiments of the method proposed herein, the oxygen-containing stream, shown as air stream 9 before, may be an oxygen-enriched stream comprising an oxygen content higher than atmospheric air. In the context of the embodiment described in figure 4, the application of an oxygen-enriched stream ("oxyfuel" combustion) would reduce the amount of nitrogen in the flue gas, which would further reduce the amount of flue gas eventually discharged to atmosphere or downstream processing units like carbon dioxide capture units (in case carbonaceous fuels are applied). Despite using highly oxygen-concentrated streams, embodiments of the invention would allow for adjustment of the combustion properties by variation of flue gas recycle flow and preheating temperature and hence enable suitable heat release profiles in radiation and convection zone.
It was surprisingly found that the instrumentalities proposed herein resolve the technical problems described above simultaneously. Firstly, as, in the form of the preheating, a large amount of heat from an external source is introduced into the combustion, the demand of fuel (and oxidant) is significantly reduced, therefore also reducing the generation of carbon dioxide if carbonaceous materials are used as fuel. Secondly, for existing furnace designs, flows and temperatures of streams can be selected in such a way that the combustion properties are very similar to the previous combustion without preheat and/or fuel switch, enabling the same heat distribution across radiation and convection zone and thus reducing the need for revamp of an existing cracking furnace.
Previous solutions do not explicitly describe the possibility that flow and temperature of an oxidant depleted stream, or other streams, can be adjusted in such a way that the combustion properties and heat distribution within an existing furnace design is kept similar to the previous operating conditions.
Again referring to Figures 1 to 5, and to summarize what was said above, switching to a carbon-free fuel 11 like hydrogen (Figure 2) would reduce fuel demand in terms of fuel lower heating value, reducing however the heat supply the convection section 120. This would likely require a major revamp of the cracking furnace 100.
Introducing a flue gas recycle 13 (Figure 3) could re-establish the heat balance within the furnace 100, which would then however require the same fuel demand in terms of lower heating value as the original operating case
According to Figure 4, the oxygen-depleted stream in form of the flue gas recycle 13 is combined with the air 9 sent to the combustion in the burners 112. Both streams are preheated to 1,250 °C, e.g., by an electrical preheater. As consequence, the demand of hydrogen-rich fuel 11 is significantly reduced, but at the same time, the heat balance is the same as for the original operating case.
It should be noted that fuel demand is reduced over-proportionally with increasing preheating temperature (see Figure 6), an effect which might not be noticed immediately by a skilled person. One particularly suitable method for reaching high temperatures is electrical preheating of the oxygen-depleted stream 14 (together with air 9), enabling efficient use of low-carbon renewable power.
Using a preheating scheme as particularly shown in Figure 4, the resulting system can actually be described as a "hybrid system", where a part of the heat for the reaction is delivered by fuel and another (substantial) part is delivered by electricity.

Table 1 summarizes the operating conditions discussed above. Bold values indicate the specific values discussed above for Figure 4.

Table 1

		Fig. 1 Base case, methane-rich fuel	Fig. 2 Hydrogen-rich fuel	Fig. 3 Hydrogen-rich fuel, flue gas recycle	Fig. 4 Hydrogen-rich fuel, flue gas recycle, preheating
Radiant Section	MW	30	30	30	30	30	30	30	30	30	30	30
Convection Section	MW	40.2	31	40.2	40.2	40.2	40.2	40.2	40.2	40.2	40.2	40.2
Flue Gas losses	MW	3.4	2.7	3	3	2.8	2.5	2.1	1.8	1.3	0.8	0.3
Preheat, air and flue gas	°C				100	250	500	750	1000	1250	1500	1750
	MW				1.6	5.7	13	21.1	30.1	40	50.8	62.8
Fuel lower heating value	MW	73.6	63.7	73.2	71.6	67.3	59.7	51.2	41.9	31.5	20.2	7.7
	%	100	86.5	99.5	97.3	91.4	81.1	69.6	56.9	42.8	27.4	10.5

In Figure 6, a diagram is shown in which, on the basis of the steam cracking apparatus 1000 shown in Figure 4, a preheating temperature in °C is indicated on the horizontal axis versus a relative lower heating value in percent on the vertical axis. As can be seen from Figure 6, fuel savings are a function of the preheating temperature.
Aspects of embodiments proposed herein will now be summarized, partly repeating what was discussed above, and pointing out alternative and/or additional options provided in certain embodiments.
As mentioned above, the preheating of the oxygen-depleted stream can be done separately or together with the oxidant or fuel. Preheating together with oxidant significantly increases the flow and therefore enables higher amounts of heat transferred at a given preheating temperature. However, separate preheating, possibly to different maximum temperatures, may avoid certain difficulties at high temperatures with certain components or mixtures thereof, e.g. in terms of nitrous oxides generation.
As mentioned above, the preheated oxygen depleted stream, or parts thereof, may, or may not be sent directly to the burners 112. Instead, the preheated oxygen-depleted stream, or parts thereof, could also be sent separately to the radiant section 110 and/or convection section 120. Any of these options could serve the purpose of ideally balancing the heat supply to a radiant section 110 and convection section 120 of a new or existing furnace in a particular individual case.
For preheating, electricity is intended for certain embodiments. However, alternatively or in addition, preheating could be done against any other typical heat carrier, particularly on the low-temperature side, such as, but not limited to, quench water, flue gas, and steam. If electricity is applied, an electrical preheater can be designed in different ways. Options may include resistive heating in various designs, with direct contact to heating elements or indirectly via radiation, using metallic or ceramic heating elements, or a mixture thereof. A different option is so-called rotodynamic heating, so-called shockwave heating, electric arc or plasma heating, or virtually any other type of industrial electric heating.
Embodiments proposed herein may be suitable for designing a flexible operation, where a system can switch between combustion operation and hybrid (electricity supported) operation, enabling an agile response to future power markets ("Peak Shaving"), particularly in connection with renewable energy sources.
Furthermore, more than one electrical heating concept could be combined with fuel firing. For example, direct resitive heating of the cracking coils could be combined with fuel firing as well as heating an oxygen depleted gas in an external heater, sending the hot gas through the radiation and/or convection zone for further heat supply, which eventually combines different elements as described above for Figure 4 and Figure 5.
Embodiments proposed herein are proposed for steam cracking. It shall be noted that similar concepts might also be considered for other heating equipment traditionally operated with fuel firing and being divided into a first heat consumption zoe ("radiation zone"), and a second heat consumption zone ("convection zone"). Such processes and associated equipment may include but not limited to steam methane reformer, steam boiler and high-temperature hydrocarbon flow superheater.

Claims

A method for steam cracking, comprising:
operating a steam cracking furnace (100) with a radiant section (110) and a convection section (120), the radiant section (110) comprising a plurality of reaction tubes (111) and a plurality of burners (112) and the convection section (120) comprising a plurality of heat recovery units (121, 122); and

operating the steacm cracking furnace (100) using a group of gases to provide heat to the reaction tubes (111) and/or the heat recovery units (121, 122), the group of gases consisting of one or more combustible gases and a subgroup of further gases, the subgroup of further gases consisting of oxygen and one or more incombustible gases,

characterized in that
(a) the oxygen is provided in relation to the subgroup of further gases in a proportion lower than a proportion of oxygen in atmospheric air,

(b) the subgroup of further gases, a part thereof, or one or more components used in forming the subgroup of further gases, is subjected to a preheating step to a temperature above 500 °C, and

(c) the preheating step is performed using electric energy in an energy amount which is at least 20% of an energy amount provided by the lower heating value of the one or more combustible gases.
The method according to claim 1,
wherein the one or more combustible gases and the oxygen is sent to the burners (112).
The method according to claim 2,
wherein the one or more incombustible gases or a part thereof is sent to the burners (112), and/or

wherein the one or more incombustible gases or a part thereof is sent to the radiation section (110) and/or to the convection section (120) separately from the burners (112).
The method according to any of the preceding claims,
wherein a flue gas (10) formed by operating the burners (111), or a part thereof, is passed from the radiant section (110) through the convection section (120) to provide heat to the heat recovery units (121, 122).
The method according to claim 4,
wherein at least one of the incombustible gases is provided using a part of the flue gas (13) having been passed through the convection section (120) as the incombustible gas or one of the incombustible gases.
The method according to claim any one of claims 1 to 4,
wherein the group of gases is provided using an inert gas or gas mixture from a source different from the cracking furnace (100) as the incombustible gas or one of the incombustible gases.
The method according to any one of the preceding claims,
wherein the oxygen in the subgroup of further gases of the group of gases is provided using atmospheric air or using a gas or gas mixture comprising an oxygen content higher than atmospheric air.
The method according to any one of the preceding claims,
wherein the combustible gas or gases is, or includes, hydrogen and/or one or more hydrocarbon gases.
The method according to any one of the preceding claims,
wherein the subgroup of further gases is mixed upstream or downstream of the preheating step and upstream of the burners (111).
The method according to any one of the preceding claims,
wherein the heat recovery units (121, 122) are used for heating at least one of boiler feed water, process steam, high pressure steam and one or more reaction feeds for the steam cracking furnace (100).
The method according to any one of the preceding claims,
wherein the preheating step is performed to a temperature in a range from 500 to 1,500 °C, from 750 to 1,250 °C, or from 800 to 1,200 °C.
The method according to any one of the preceding claims,
wherein further electric heating is performed in the radiation zone (110) by using resistive heating elements and/or direct resistive heating of the reaction tubes (111).
An apparatus (1000) for steam cracking, comprising:
a steam cracking furnace (100) with a radiant section (110) and a convection section (120), the radiant section (110) comprising a plurality of reaction tubes (111) and a plurality of burners (112) and the convection section (120) comprising a plurality of heat recovery units (121, 122),

wherein the apparatus (1000) is configured to operate the steam cracking furnace using a group of gases to provide heat to the reaction tubes (111) and/or the heat recovery units (121, 122), the group of gases consisting of one or more combustible gases and a subgroup of further gases, the subgroup of further gases consisting of oxygen and one or more incombustible gases,

characterized in that the apparatus (1000) is configured to
(a) provide the oxygen in the subgroup of further gases in a proportion lower than a proportion of oxygen in atmospheric air,

(b) subject the subgroup of further gases, or one or more gas components used in forming the subgroup of further gases, to a preheating step to a temperature above 500 °C, and

(c) to perform the preheating step using electric energy in an energy amount which is at least 20% of an energy amount provided by the lower heating value of the one or more combustible gases.
The apparatus (1000) according to claim 11,
wherein the apparatus (1000) is configured to perform a method according to any one of claims 1 to 10.