EP4105297A1 - Method and measuring system for determining an oxygen content in a furnace, furnace and processing system - Google Patents

Abstract

The invention relates to a method for determining an oxygen content in a furnace (10) of a process plant (100), in which a heating gas (X) is burned with the supply of an oxygen-containing gas (L) and with the formation of flue gas (Z), using several lambda probes (114, 116) in each case a measured value is recorded in the flue gas (Z) and an oxygen content in the furnace (10) is determined from the measured values by means of a computing system (112), a measuring system (110) for this, a furnace (10) and a processing plant (100).

Description

The invention relates to a method and a measuring system for determining an oxygen content, in particular at multiple points in a furnace such as a cracking furnace or reformer, a process engineering system in which a fuel gas is burned with the supply of an oxygen-containing gas such as air, a furnace with such a measuring system and a process plant with such a furnace.

State of the art

Furnaces in which the oxygen content in the flue gas is a relevant parameter are used in various process engineering systems (or process systems). For example, in the so-called steam cracking (steam cracking, thermal cracking, steam cracking, etc.), which is used for the production of olefins and other basic chemicals, and which is described, for example, in the article " Ethylene" in Ullmann's Encyclopedia of Industrial Chemistry, online publication of April 15, 2009, DOI: 10.1002/14356007.a10_045.pub2 , is described, one or more so-called. Cracker furnaces (also referred to as cracking furnaces) for use. With regard to the terms used below, reference is also made to the relevant specialist literature. In cracker furnaces, feedstocks such as ethane, liquefied petroleum gas (LPG), naphtha, atmospheric gas oil (AGO) and hydrocracker bottoms are converted into ethylene and valuable by-products. A furnace is also used in a steam reformer for the production of synthesis gas, hydrogen and carbon monoxide (steam reforming) and the oxygen content is relevant.

For the initiation and maintenance of the endothermic reactions in steam cracking, the thermal energy required is typically provided by the combustion of heating gas in a combustion chamber (combustion chamber), which forms the so-called radiant zone of the cracking or cracking furnace, and guided through the so-called coils (cracking tubes). are, through which a hydrocarbon vapor mixture to be converted to obtain a product mixture, the so-called raw or cracked gas. In the most common applications, the combustion air required for combustion is fed into the radiation zone without preheating (so-called natural draft) and burned there together with the heating gas. Air preheating, possibly with gas turbine exhaust gas, is also increasingly being considered, and with it a need to determine the oxygen content in the exhaust gas.

When operating a cracker furnace, the fuel gas should be burned under strict conditions to ensure safe and efficient operation. This includes, for example, measuring the oxygen content in the combustion chamber in order to be able to detect any excess oxygen (sub-stoichiometric combustion). However, this generally requires complex and/or expensive measuring devices.

Against this background, the present invention sets itself the task of providing a possibility of determining the oxygen content in cracker furnaces or other furnaces in process engineering systems as simply and inexpensively as possible, in particular at as many points as possible, in particular in order to obtain the most comprehensive information possible about the combustion in the combustion chamber obtain.

This object is achieved by a method, a measuring system, a furnace and a process engineering system with the features of the independent patent claims. Configurations are the subject of the dependent patent claims and the following description.

Advantages of the Invention

The present invention generally deals with the operation of a furnace (e.g. cracking furnace or reformer, generally a device with a furnace) in a process engineering (in particular petrochemical) plant, in which a heating gas with the supply of oxygen-containing gas such as air (among gas can generally also a gas mixture, but in principle in this special case, for example, also pure oxygen) is burned, with a flue gas being produced, and an oxygen content, for example after the combustion, then in the flue gas, is to be recorded. In this respect, the invention deals in particular with determining the oxygen content in or at such a furnace.

In particular, a cracker furnace (or cracking furnace) can be considered as such a furnace, as has already been explained in more detail at the beginning and is used in particular in steam cracking. However, it should be pointed out that other furnaces - e.g. reformers - can also be used in which (especially on an industrial scale) fuel gases are burned, e.g. in steam reforming (so-called "steam reforming"). Ultimately, all types of burns come into consideration.

One way of determining or measuring the oxygen content in such a furnace or in the flue gas is, for example, a so-called "tunable diode laser", i.e. a tunable laser diode. The so-called "Tunable Diode Laser Absorption Spectroscopy" (TDLAS, English, in German roughly "absorption spectroscopy using tunable laser diodes") is a method with which the concentration or density of the gas or gas component to be examined (e.g. methane or water vapor or even oxygen) is determined. However, such lasers are very expensive and deliver (only) an average value over the measuring section or a locally limited value that does not provide information about the entire combustion chamber.

In the context of the invention, it is now proposed that a corresponding measured value in the flue gas is recorded or measured by means of a plurality of lambda probes--and in particular also by a plurality of sampling points--and that by means of a computer system--particularly common to all lambda probes--a computer system (e.g. a (high-performance) ) computing system, so-called "edge computing") from the measured values, an oxygen content (or oxygen concentration) in the furnace is determined. This preferably takes place continuously. A special computing unit suitable for acquiring and/or evaluating measured values from lambda probes or a control unit or the like is also expedient as the computing system. In this case, a measured value in the flue gas can be recorded in particular by means of the multiple lambda probes at multiple different points of the furnace and the oxygen content can be determined at each of the multiple different points. In particular, this allows a local course, ie a profile, of the oxygen content in the determine oven. Preferred points at which measurements should be taken are, for example, along the firebox wall at the transition of the flue gas from the combustion chamber (closed combustion) into the so-called convection zone.

It is also useful if the oxygen content at one point in the furnace is determined from the readings from at least two or three lambda probes, i.e. the readings from the at least two, in particular at least three, lambda probes are used for this purpose. With three lambda probes, for example, mutual monitoring and indication of a error possible, with an error tolerance of two out of three), to determine a (single) value for the oxygen content. The at least two lambda probes can therefore be used in particular redundantly, i.e. if one sensor fails, there are still two probes out of a total of three, the mean value of which can be used, or the measured values of the three probes can be compared with one another in order to eliminate an inaccurate measured value and replace it to indicate the probe. For example, three lambda probes can be used, from whose measured values only the measured values of the two best lambda probes and/or the middle value (median) are used to determine the oxygen content. For example, the measured values from the lambda probe that deviate the furthest from an average value can be excluded. The (e.g. arithmetic) mean value can then be selected from the remaining measured values. This allows a particularly accurate determination of the oxygen content. For this purpose, the relevant lambda probes are arranged at the same sampling point on the furnace. In the case of a total of several points, three or more lambda probes can therefore be used per point.

The computing system ("edge device") measures, for example, the voltage applied to the individual probes, which is in relation to the oxygen. A local measurement, for example, always consists of three measured values (error tolerance), which are then checked for the consistency of the data before the oxygen concentration is calculated from the measured voltages using a formula. A profile is then in turn determined from the various measuring points. The edge device takes over the communication with the user (data transfer) and also the monitoring of the sensors (see above), checks any existing drift and alarms if necessary.

This is where the particular advantage of lambda probes (or also lambda sensors) as they are used in motor vehicles to measure the oxygen content in the exhaust gases becomes evident. These lambda probes are particularly cost-effective—at least in comparison to the oxygen measuring systems previously used for furnaces in process engineering systems—and can therefore also be used in large numbers. This in turn allows redundant use, which allows a particularly accurate and reliable determination of the oxygen content in the furnace by averaging the individual measured values despite possibly poorer measurement quality of the individual lambda probes compared to other oxygen measurement systems.

The lambda probes are arranged in particular in a line in which the flue gas is led out of the furnace and, in particular, is then fed back into the furnace again. For example, a pipe installation with two or three inch diameter pipes (conduit) can be used. A pressure of e.g. 0.01 bar for the flue gas is usually sufficient. Such lines can be arranged on the roof of the furnace, for example, and then also at different points there. A draft in a convection section can then be used, whereby the line can be connected, for example, to an existing nozzle or a new one to be added in the convection section or in front of a fan or blower. Alternatively, the flue gas can also be extracted from the furnace (e.g. via an ejector or a gas ring blower) and fed back into the combustion chamber.

A lambda probe works in particular in such a way that a measured value is recorded which indicates a ratio of an oxygen content in a gas to be measured--in this case the flue gas of the furnace--to a reference value. In particular, the oxygen content of an ambient air can then be considered as a reference value. There are in particular the so-called Nernst probes as lambda probes.

The Nernst probe, for example, uses zirconium dioxide (zirconium(IV) oxide) as the permeable material. The property of zirconium dioxide is exploited to transport oxygen ions electrolytically at temperatures above approx. 350 °C, which creates a voltage between the external electrodes. Due to this property, zirconium-based lambda probes (or Oxygen sensors) the difference in oxygen partial pressure (this corresponds to a difference in oxygen concentration) of two different gases. In the case of the lambda probe, one side of the material is then exposed to the flue gas or flue gas flow, while the other side is at an oxygen reference, for example the ambient air. The ambient air (or another reference gas) can be fed in through an opening directly on the lambda probe or via a separate supply line, which makes it more difficult for the reference gas to be contaminated by other gases or water. If the reference gas is contaminated, the oxygen content of the reference is reduced, which reduces the probe voltage.

Lambda probes or sensors that work with a pumped reference also come into consideration; no separate reference gas such as ambient air, which can become contaminated, is required here. Rather, the oxygen reference is produced independently in the lambda probe. For example, a current can be passed through the material and oxygen can be pumped out of the flue gas. This creates a reference of pure oxygen at the inner electrode.

It is also conceivable that zirconium could be used as YSZ ceramic (yttria-stabilized zirconia), which, among other things, would noticeably reduce the operating temperature. The yttrium-doped zirconium dioxide material of the probe becomes penetrable for negative oxygen ions at temperatures of around 300 °C and above.

With Nernst probes, the difference in concentration (or difference in partial pressure) leads to ion diffusion of the oxygen, as a result of which oxygen ions migrate from the high concentration (usually in the ambient air) to the low concentration (usually the flue gas). The oxygen atoms can therefore diffuse through the zirconium ceramic as doubly negatively charged ions. The electrons required to ionize the oxygen atoms are supplied by the electrically conductive electrodes. As a result, an electrical voltage can be measured between the internal and external electrodes (eg platinum electrodes), the probe voltage. This voltage is then forwarded to the computing system (Edge Device) via a cable, for example.

A suitable calibration can thus be used to determine the oxygen content in the furnace or in the flue gas of the furnace using a lambda probe. Even with the simple lambda probe, an exact measurement can be carried out, at least in certain areas. The invention makes it possible to detect an oxygen value that is locally too low (local sub-stoichiometry, λ<1), although the average oxygen content of the flue gas is in the acceptable range. It can often also be sufficient to detect an oxygen content that is too high or too low, regardless of the specific value. Rather, the furnace can then be controlled or adjusted in such a way that the oxygen content remains within a desired range. Ultimately, in this way, the greatest possible reduction in undesirable exhaust gases such as carbon monoxide and soot can be achieved.

In this sense, it is also preferred if the operation of the furnace is in particular automatically controlled or also regulated based on the determined oxygen content. For example, a volume flow of supplied fresh air (or other oxygen-containing gas) can be increased, possibly also at individual points if the oxygen content in the flue gas is too low. However, mere monitoring of the operation is also conceivable. Then, for example, a measure can also be initiated if the oxygen content leaves a specified range; a hint to an operator to intervene manually is conceivable, for example.

The invention further relates to a measuring system for determining an oxygen content in a furnace or in the flue gas of a furnace of a process engineering plant, in which a fuel gas is burned with the supply of an oxygen-containing gas. For more detailed explanations of the measuring system, to avoid repetition, reference is made to the above statements. The measuring system is set up in particular to carry out a method according to the invention.

The invention also relates to a furnace for a process plant, in which a heating gas is burned with the supply of an oxygen-containing gas, in particular an exhaust gas (e.g. from a gas turbine, there for determining the residual oxygen content), and to which a measuring system according to the invention is assigned, as well as a process engineering plant with such a furnace including measuring system.

For more detailed explanations (e.g. also the possibility of positioning or arranging the lambda probes) as well as further configurations and advantages, to avoid repetition, reference is made to the above statements on the method, which apply here accordingly.

The invention is explained in more detail below with reference to the attached drawing, which shows various parts of the system, with the aid of which the measures according to the invention are explained.

Figur 1

zeigt eine verfahrenstechnische Anlage mit Crackerofen, bei dem ein erfindungsgemäßes Verfahren durchführbar ist.

Figur 2

zeigt schematisch einen Ablauf eines solchen erfindungsgemäßen Verfahrens in einer bevorzugten Ausführungsform.

Figur 3

zeigt eine Ansicht des Crackerofens aus Figur 1 zur näheren Erläuterung der Erfindung.

Brief description of the drawing

figure 1

shows a process plant with a cracker furnace, in which a method according to the invention can be carried out.

figure 2

shows schematically a sequence of such a method according to the invention in a preferred embodiment.

figure 3

shows a view of the cracker furnace figure 1 for a more detailed explanation of the invention.

Detailed description of the drawing

In figure 1 a process engineering plant 100 designed as a steam cracking arrangement with a cracker furnace 10 is shown, on the basis of which the invention is to be explained. It should already be mentioned at this point that this system 100 is only used as an example to explain the invention and that the system can also be configured differently or it could be a different system with a furnace (ie a device with a furnace).

The cracker furnace 10 or a corresponding furnace unit (here also referred to as a cracking furnace or furnace for short) has a radiation zone 11 and a convection zone 12 . The plant 100 for steam cracking can also include several corresponding cracker furnaces 10 . System components or units referred to below as central several cracker ovens 10 are available, decentralized units are provided separately for each cracker oven 10.

By means of a central feed preheater 20 shown as an example and a central process steam generation 30, a hydrocarbon feed H is heated and process steam P is provided, which is further heated in the convection zone 12 in a manner known per se (not relevant to the present case), combined to form a feed stream F and then the radiation zone 11 are supplied. The representation according to figure 1 is greatly simplified and only an example. For example, in a so-called pass control, a corresponding feed stream can already be divided into several partial streams in the area of the convection zone, which can then be preheated separately from one another and finally passed through groups of, for example, six or eight cans in the radiation zone 11. Centralized units can be replaced here and subsequently by decentralized units and vice versa at any time.

The cleavage gas C is removed from the radiation zone 11, which is cooled by means of one or more quench gas coolers 13, which can in particular be designed as known quench coolers or can include such quench coolers and which can also function as steam generators at the same time, and then undergo a central cleavage gas separation and cleavage gas treatment 90 is supplied. The invention is not limited by a specific embodiment.

A central feed water system 40 provides feed water W, which in the example shown is also heated in the convection zone 12 and then further heated by means of one or more cracked gas coolers 13 to obtain high-pressure or super-high-pressure saturated steam S (hereinafter also referred to as saturated steam for short) and finally is vaporized. In the example shown, the saturated steam S is superheated in the convection zone 12 to obtain superheated high-pressure steam or superheated superhigh-pressure steam T (also referred to as superheated steam below) and fed into a central steam system 50 .

For example, by means of a central heating gas system 60, which is followed by a possible central heating gas preheating 65, in which process or auxiliary means such as superheated steam at high, medium or low pressure, washing water and/or quench oil, but also electric power is used as heating media or heat sources, feed heating gas Y is heated to form preheated heating gas X and is fed to the radiation zone 11 or to the burners in this zone, which are not separately illustrated, and thus to the furnace 10.

In the embodiment illustrated here, combustion air L—and thus an oxygen-containing gas—passes via an air intake 79 into the radiation zone 11 or the burners located there. Flue gas Z is discharged from the radiation zone 11, which passes through the convection zone 12 and is then discharged into a flue gas treatment system or to a central or decentralized chimney 80, e.g. with a blower, and via this to the atmosphere.

In the figure 1 illustrated central heating gas 65 is optional. A decentralized heating gas preheating (ie separately for the individual cracker ovens 10 or oven units) is also possible. The same applies to the preheating of the insert and the generation of process steam, which can also be carried out decentrally as an alternative to the central design.

As mentioned, the heating gas X is burned in the furnace 10 with the supply of the combustion air L, with flue gas Z (ie a type of exhaust gas) being produced, which is ultimately discharged to the environment. For reasons of environmental protection and energy efficiency, it is desirable to achieve the best possible combustion in the furnace 10. For this purpose, the combustion of the fuel gas X should take place under strict conditions, also to enable safe and efficient operation. As already mentioned, this includes in particular the measurement of the oxygen content in the furnace or in the combustion chamber in order to be able to detect any excess oxygen. This is where the invention comes in.

Within the scope of the invention, a measuring system 110 that is provided for the furnace 10 is used for this purpose. The measuring system 110 comprises, for example, a computing system 112 (eg processor with memory) and several lambda probes, with two lambda probes 114, 116 being shown here by way of example, by means of which measurements of the oxygen content in the flue gas Z can be undertaken. In the example shown, the lambda sensors 114, 116 are arranged at two different locations on the furnace 10 at which a measurement is taken. For example, for this purpose flue gas can be diverted--as already mentioned--and fed back into the furnace 10 or into the regular course after the measurement by means of the respective lambda probe. As already mentioned, several lambda probes can also be provided at one point, each recording a measured value.

The measured values recorded by the lambda probes 114, 116 are transmitted, for example, via signal lines (indicated with dashed lines with arrows) to the measuring system 112, where the measured values can then be offset in order to determine the oxygen content in the flue gas or in the furnace, possibly also at different places to determine.

In figure 2 a sequence of such a method according to the invention is shown schematically in a preferred embodiment. For this purpose, the lambda probe 114 and the computing system 112 are off figure 1 shown. The lambda probe 114 contains a measuring element 120 which is exposed to the flue gas Z of the furnace on one side and to a reference, for example ambient air, on the other side. In the flue gas there is an oxygen content Oz, in the ambient air or in the reference, on the other hand, an oxygen content O _R . If these two oxygen contents differ, as explained above, there is a voltage drop across the measuring element 120, which is transmitted to the computing system 112 as a measured value M, for example.

By way of example, five further measured values M from lambda probes (not shown here) are shown, which are also transmitted to the computing system 112 . Provision is made for three lambda probes to be arranged at (at least approximately) the same point (extraction point) of the furnace and to record the measured value. These three measured values are then processed, e.g. the (objectively) worst measured value can be excluded from the calculation. In this way, an oxygen content O is determined for each of the two locations on or in the furnace.

As mentioned, initially only a ratio of the oxygen content in the flue gas to the oxygen content in the ambient air is determined by means of the lambda probe. if, for example, the oxygen content in the ambient air is known, the Oxygen content in the flue gas can be determined. This can be done, for example, as part of the calculation mentioned, so that the oxygen content O for a specific point can correspond to the (possibly averaged) oxygen content Oz in the flue gas. It goes without saying that it is also possible to work only with relative values.

In figure 3 FIG. 14 is yet another view of cracker furnace 10. FIG figure 1 shown to explain the invention in more detail, namely with four zones 10A, 10B, 10C and 10D by way of example. In each of the zones in the wall 140 (firebox wall) is a tap 130, only one of the taps being shown in more detail.

At an extraction point 130, a pipe 132 is guided through the wall 130, on which, for example, three lambda probes 114 are attached on the outside (only one probe is shown). In addition, cooling fins 134, for example, can be provided in order to ensure sufficient cooling of the gas after the measurement, in order to prevent any damage to materials. Here a measurement like in terms of figure 2 (There for only two places) explained, take place in order to be able to determine the oxygen content O mentioned in the sense of an oxygen profile.

Claims (12)

Method for determining an oxygen content in a furnace (10) of a process plant (100), in which a heating gas (X) is burned with the supply of an oxygen-containing gas (L) and with the formation of flue gas (Z),
a measured value (M) in the flue gas (Z) being recorded by means of a plurality of lambda probes (114, 116) and an oxygen content (O) in the furnace (10) being determined from the measured values by means of a computer system (112). Method according to Claim 1, in which a measured value (M) in the flue gas (Z) is recorded at a number of different points (130) of the furnace (10) by means of the number of lambda probes (120114, 116) and the oxygen content ( O) is determined, and with it in particular a local course of the oxygen content (O), in particular an oxygen profile, in the furnace (10) being determined. The method according to claim 1 or 2, wherein the oxygen content (O) at a point of the furnace (10) in the flue gas (Z) is determined from the measured values (M) of at least two, in particular at least three lambda probes, and wherein the at least two lambda probes in this case, in particular, be used redundantly and/or in a fault-tolerant manner. Method according to one of the preceding claims, wherein each lambda probe (114, 116) is arranged in a line in which the flue gas (Z) is conducted out of the furnace (10) and in particular subsequently returned to the furnace (10). Method according to one of the preceding claims, wherein a measured value (M) is recorded by means of the lambda probes (114, 116), which indicates a ratio of an oxygen content (Oz) in a flue gas (Z) of the furnace to a reference value (O _R ), wherein as Reference value in particular the oxygen content of ambient air is used. A method according to any one of the preceding claims, wherein Nernst sensors and/or resistance step sensors are used as the lambda sensors (114, 116). Method according to one of the preceding claims, wherein based on the determined oxygen content (O) an operation of the furnace (10) is monitored and / or controlled. Method according to one of the preceding claims, in which a cracker furnace for steam cracking is used as the furnace (10). Measuring system (110) for determining an oxygen content in a furnace (10) of a process plant (100), in which a fuel gas (X) is burned with the supply of an oxygen-containing gas (L) with the formation of flue gas (Z), with a plurality of lambda probes (114, 116) which are intended to be arranged in or on the furnace (10) and are set up to record measured values (M) in the flue gas (Z), and with a computing system (112) which is set up to receive the measured values (M) from the lambda probes (114, 116) and to determine an oxygen content (O) in the furnace (10) from them. Measuring system (110) according to Claim 9, which is set up to carry out a method according to one of Claims 1 to 8. Furnace (10) for a process plant (100), in which a heating gas (X) is burned with the supply of an oxygen-containing gas (L), with a measuring system (110) according to Claim 9 or 10 Process engineering plant (!00) with a furnace (10) according to Claim 11.

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