EP4050260A1 - Method and arrangement for observing a combustion process in a heating device - Google Patents

Abstract

The invention relates to a method and an arrangement for observing a combustion process of hydrogen or fuel gas containing hydrogen with air in a combustion chamber (9) of a heating device (1) to determine the presence of flames and/or to regulate the ratio of combustion air to fuel gas, with a measurement area (19) through which combustion products can flow, infrared light is radiated from a light source (14) to a sensor (11) that is sensitive to infrared light, and the sensor (11) measures the intensity of the incident light in the area of at least one absorption line or band of Water is observed and the presence and/or quality of combustion is concluded from changes in intensity. The present invention makes it possible to implement the flame monitor and/or combustion control functions for pure hydrogen as fuel gas or hydrogen-containing fuel gases with simple and robust instrumentation on a heater.

Description

The invention relates to a method and an arrangement for monitoring a combustion process in a heating device that can be operated with hydrogen and/or a fuel gas containing hydrogen.

Hydrogen as a fuel gas or as an admixture to fuel gases is becoming more and more important, and great efforts are being made to upgrade new or existing heating devices for operation with it. It is not only a question of large systems, but also of wall-mounted units for heating water and, in general, heaters for heating buildings and/or providing hot water. Since such devices are durable goods, efforts are also being made to prepare such devices for operation with various fuel gases, with the preparation for operation with hydrogen making special demands.

The combustion of hydrogen differs from previously used fuel gases in several respects, in particular a hydrogen flame is almost invisible to the human eye, radiates less heat than flames produced with carbonaceous fuels and requires different measuring systems than hydrocarbon fuel heaters. The present invention is therefore particularly, but not only suitable for heaters with pure hydrogen or with Fuel gas that consists of more than 50%, in particular more than 95%, of hydrogen can be operated.

Hitherto, simple and robust sensors for ionization, temperature, light or thermal radiation, pressure, volume flow and the like have generally been used in heaters in order to regulate the heaters and ensure their safe operation. However, when using hydrogen as the fuel gas, some measurements cannot be carried out reliably with the sensors that have been customary up to now. An important function is the detection of the presence of a stable flame (a so-called flame monitor), another is the setting of a ratio of combustion air to fuel gas (lambda value) that is suitable for stable and environmentally friendly combustion.

A use of optical sensors using optical filters is for a special application in heaters that are operated with hydrogen-containing fuel gas, for example from the DE 10 2019 101 329 A1 known. Also from the EP 2 223 016 B1 optical measuring systems in heaters are known, these two documents mainly dealing with the observation of the radiation from flames, ie with radiation emissions in the combustion chamber. From the DE 10 2009 009 314 A1 the use of absorption spectroscopy in the infrared range of light is already known to monitor flames or regulate a combustion process. In the considerations there, water vapor is regarded as a disruptive factor and is not evaluated for the actual observation.

Because of the high temperatures and strong emissions in the area of flames, known measuring methods and an evaluation of such measurements are with a certain Effort involved, as no errors should occur in safety-related functions.

The object of the present invention is to at least partially solve the problems described with reference to the prior art. In particular, a method and an arrangement are to be created with which a combustion process can be observed without using the radiation generated by flames, namely by absorption spectroscopy. These should be particularly suitable for heaters that are operated with hydrogen as the fuel gas or hydrogen-containing fuel gas.

A method and an arrangement as well as a computer program product according to the independent claims serve to solve this task. Advantageous refinements and developments of the invention are specified in the respective dependent claims. The description, particularly in connection with the drawing, illustrates the invention and provides an exemplary embodiment.

A method for observing a combustion process of hydrogen or a hydrogen-containing fuel with air in a combustion chamber of a heater to determine the presence of flames and/or to measure the ratio of combustion air to fuel gas contributes to solving the problem, with a measuring area through which combustion products can flow infrared light is radiated from a light source to a sensor sensitive to infrared light and the sensor observes the intensity of the incident light and conclusions are drawn from changes in the intensity to the presence and/or quality of a combustion.

In particular, this is based on the consideration that combustion products are produced during combustion, the detection of which in a measuring area outside of an actual combustion zone allows just as reliable conclusions to be drawn about the presence and/or quality of combustion as, for example, direct observation of the flame. There may only be a delay that the combustion products need to travel from the combustion zone to the measurement area. The delay is the smaller, the closer the measuring range is to the combustion zone. Matter in a measurement area that is penetrated by light weakens this light in different ways, in particular through absorption in different wavelength ranges for different atoms or molecules. So-called absorption lines or bands are created, whose wavelengths and decreases in intensity allow conclusions to be drawn about the amount and type of matter present in the measurement area. The present invention makes use of this.

1. i. Rotationsübergänge, bei denen das Molekül ein Quantum Rotationsenergie gewinnt. Wasserdampf führt zu einer Absorption im fernen Infrarotbereich des Spektrums von etwa 200 cm^-1 [1/Zentimeter] (50 µm [Micrometer]) bis zu längeren Wellenlängen in Richtung Mikrowellenbereich.
2. ii. Schwingungsübergänge, bei denen ein Molekül ein Quantum Schwingungsenergie gewinnt. Die fundamentalen Übergänge führen zu einer Absorption im mittleren Infrarot in den Bereichen um 1650 cm^-1 (µ-Bande, 6 µm) und 3500 cm^-1 (sogenannte X-Bande, 2,9 µm).
3. iii. Elektronische Übergänge, bei denen ein Molekül in einen angeregten elektronischen Zustand versetzt wird.

For this purpose, at least one absorption line or band of the water is observed. Water (in the gaseous state) is the quantitatively easily detectable product of combustion of hydrogen with air and is particularly suitable for the desired purpose. In fact, the water content in the combustion gas is linked in a unique way to the ratio of combustion air to fuel gas, so that the measurement of the water content in the combustion gas has turned out to be very suitable for controlling this ratio. In the gaseous state, a water molecule has three types of transitions in particular, which can lead to the absorption of electromagnetic radiation (including light):

1. i. Rotational transitions where the molecule gains a quantum of rotational energy. Water vapor leads to absorption in the far-infrared region of the spectrum about 200 cm ^-1 [1/centimeter] (50 µm [microns]) to longer wavelengths towards the microwave range.
2. ii. Vibrational transitions in which a molecule gains a quantum of vibrational energy. The fundamental transitions lead to an absorption in the mid-infrared in the ranges around 1650 cm ^-1 (µ-band, 6 µm) and 3500 cm ^-1 (so-called X-band, 2.9 µm).
3. iii. Electronic transitions in which a molecule is brought into an excited electronic state.

In the following, absorption line or band always means a narrow spectral range that can be selectively observed, especially with a non-dispersive infrared spectrometer (NDIR). By observing the intensity of light that has passed through a gas volume (measuring range) with this spectral range, the proportion of certain gases (in particular water) in the gas volume can be determined. Since only water is produced as a combustion product when hydrogen is burned and water is also formed when other combustible gases are burned, the ignition of a flame in the combustion chamber (i.e. the start of a combustion process) can already be signaled by the occurrence of one or more absorption lines that were not previously present or less pronounced.

In a preferred embodiment, the sensor is part of a non-dispersive infrared spectrometer (NDIR). Non-dispersive infrared spectroscopy is well developed and can be used inexpensively to analyze gases, so that an NDIR spectrometer can also be an everyday part of a control and regulation unit of a heating device.

For calibration and correct measurement, it makes sense to observe at least one absorption line when the burner is (still) switched off and/or before it is ignited. Since there is always some moisture in the air, this proportion can e.g. B. when the fan of a heater is running but still without fuel gas supply (or at least before ignition of the combustion process) and can thus be eliminated from the following measurements.

After the heater has been started, a drop in intensity in the area of at least one absorption line of the water indicates ignition of the combustion and/or an increase in intensity indicates that the combustion has been extinguished.

The intensity of the at least one absorption line of water is particularly preferably observed during combustion and when the ratio of fuel gas to air changes, with the ratio of combustion air to fuel gas being inferred from the course of the intensity compared to calibration curves and/or empirical values. In addition to simply detecting whether combustion is present, the air ratio can also be measured. The highest decrease in intensity with water vapor absorption can be measured for the air ratio 1 (lambda = 1). Due to dilution effects, the intensity drops sharply at higher air ratios (over-stoichiometric operation), while it drops less sharply in under-stoichiometric operation because the degree of dilution is lower. It can e.g. B. characteristic curves for the decrease in intensity for the over-stoichiometric and under-stoichiometric operation are determined in a laboratory and stored in a control and regulation unit. By comparing measured values with stored values, over- and under-stoichiometric operation can be deduced. By appropriately adjusting the fuel gas/air mixture, more than stoichiometric operation can be achieved as desired Mode of operation are generated. In addition, by further evaluating measured values, conclusions can be drawn about the concentration of water vapor and thus about the air ratio as soon as it is ensured that over-stoichiometric operation is present. In this way, the air ratio can be regulated.

Preferably, light of at least the observed wavelength ranges of the observed absorption lines is sent through the measurement area to the sensor. In order to keep interference as low as possible, it can make sense to only send infrared light from a few wavelength ranges through the measuring area or to filter them out in front of the sensor. This can be achieved by an appropriate light source and/or by suitable optical filters. Although modern spectrometers can also simultaneously observe absorption lines or bands in a broad spectral range, the evaluation becomes faster, simpler and/or cheaper if only selected spectral ranges are observed.

An arrangement for observing a combustion process of hydrogen or a hydrogen-containing combustible gas with air in a combustion chamber of a heater to determine the presence of flames and/or to regulate the ratio of combustion air to combustible gas also contributes to solving the problem, with a light source for infrared light and a sensor at least for infrared light in the area of at least one absorption line or band of water is present, between which there is a measurement area through which combustion gases can flow, and wherein the sensor is connected to evaluation electronics that are set up to measure the intensity of the incident light in the area observed by at least one absorption line or band of water and from their progression the presence of flames and/or the ratio of combustion air to combustible gas can be deduced.

The measuring area is preferably arranged in such a way that the water produced in the combustion process is still gaseous up to that point. A condensation of parts of the water vapor contained in the combustion gases in front of the sensor would falsify the measurements, which is why this must be avoided as far as possible. The measuring range should therefore be close enough to the flame area that no condensation takes place up to that point, especially in the combustion chamber or close to the combustion chamber in the exhaust system.

The sensor is preferably part of a non-dispersive infrared spectrometer. This can belong to the evaluation electronics and/or be part of a control and regulation unit of the heating device.

Although there are light sources and sensors which are also suitable for arrangement in a combustion chamber, the sensor and/or the light source are preferably arranged outside the combustion chamber behind a window in a housing around the combustion chamber. This simplifies maintenance and no electrical feedthroughs to the combustion chamber are required. However, arrangements with optical waveguides are also possible, which allow even more flexibility in the arrangement of the light source and sensor.

At least one wavelength-selective filter is particularly preferably arranged between the light source and the sensor. In this way, interference can be reduced and/or a certain wavelength range can be observed without complex (electronic) spectral classification.

The evaluation electronics (as part of a control and regulation unit) are preferably set up to detect the presence and/or the quality of combustion from the measured values of the sensor and to use these for reliable starting and/or operation of the heater.

A further aspect also relates to a computer program product comprising instructions which cause the arrangement described to carry out the method described. The evaluation of the data measured by the sensor and their further use in the heater require a program and data for controlling the heater, both of which must be updated from time to time.

The explanations for the method can be used for a more detailed characterization of the arrangement, and vice versa. The arrangement can also be set up in such a way that the method is carried out with it.

Fig. 1:

ein Heizgerät mit Messsystem für die Absorption von Infrarot-Strahlung und

Fig. 2:

die Abhängigkeit des Wassergehaltes im Verbrennungsgas vom Lambda-Wert bei Verbrennung von reinem Wasserstoff.

A schematic exemplary embodiment of the invention, to which it is not limited, and the mode of operation of the method will now be explained in more detail with reference to the schematic figures. They represent:

Figure 1:

a heater with a measuring system for the absorption of infrared radiation and

Figure 2:

the dependence of the water content in the combustion gas on the lambda value when burning pure hydrogen.

1 shows schematically a heater 1, which can be operated in particular with hydrogen or a hydrogen-containing fuel gas. Air (usually outside air/ambient air) is sucked in by a blower 3 via an air supply 2 and conveyed to a burner 7 . Via a fuel gas supply 4 and a fuel gas valve 5, fuel gas is added to the air flow generated by the blower 3 in a mixer 6 (eg a Venturi nozzle). The mixture is burned in a combustion chamber 9 surrounded by a housing 10, with a flame region 8 being formed. Combustion gases that arise are discharged via an exhaust system 18 . An optical sensor 11 sensitive to IR radiation is arranged for measurement in such a way that it can observe a measurement area 19 in the combustion chamber 9 (or at least downstream of the flame area 8). The measuring range 19 is outside the flame range 8, which means that interference can be reduced. It is located so close to the flame area that water vapor cannot condense up to there, as condensing parts of the water contained in the combustion gases would falsify the measurements. The sensor 11 can either be designed to be wavelength-sensitive itself (sensitive only to a specific wavelength range), or it can be preceded by an optical filter 13 which only lets through a specific wavelength range in which the line(s) to be observed lies or . lie. This wavelength range is in the infrared range, particularly where water produces absorption lines or bands. In general, such a sensor 11 should not be arranged inside a combustion chamber 9 simply because of its supply lines and its temperature sensitivity, which is why it is preferably located behind a first window 15 arranged in the housing 10 . The filter 13 can be arranged on the inside of the window 15, on the outside or preferably integrated with the window 15. Evaluation electronics 12, to which sensor 11 is connected, evaluates the measurement signals from sensor 11 and concludes therefrom the presence of water and, if necessary, also quantitatively determines its concentration in measurement area 19. For this purpose, a light source 14 radiates light in the IR range through the Measurement area 19, which is preferably also arranged outside of the housing 10 behind a second window 16 (possibly also with an optical filter 17). Light emitted by this light source 14 (in any case in the observed wavelength range) is reached the sensor 11 through the measuring area 19. An optical filter 13, 17 can be arranged somewhere in the beam path in order to achieve sufficient sensitivity of the measurement. However, with modern NDIR spectrometers, a larger infrared range can also be observed and several absorption lines or bands occurring there can be evaluated in parallel, so that no optical filters are required. However, it makes sense to carry out a temperature correction, for which purpose a temperature sensor 23 is arranged in the measuring area 19 or in its vicinity. Since infrared radiation (heat radiation) also depends on the temperature in a measurement environment, a correction is important. This can be done with stored laboratory data and/or empirical values according to a correction curve in evaluation electronics 12 .

The electronic evaluation system 12 is preferably part of a control and regulation unit 22 that controls the entire heater 1 and is connected via control lines 21 to important components such as the blower 3 and the fuel gas valve 5 and regulates them and can switch them off in the event of safety-related events. The infrared-sensitive sensor 11 and the temperature sensor 23 (possibly also the light source 14) are connected to the evaluation electronics 12 via measuring lines 20 and together with them form a measuring system which can be used as a flame monitor and/or for setting the air/fuel gas ratio for various fuel gases can be used.

It should be mentioned that other spatial arrangements are possible, in particular if optical waveguides are used to transmit light to a desired location. The arrangement described up to this point can measure absorption lines of water, which arise in the measurement area 19 as a result of water vapor occurring during combustion.

2 illustrates why the absorption measurements of water (vapour) are particularly suitable for measuring and thus controlling the ratio of combustion air to fuel gas. The curve shown of the water content (gaseous in percent by volume) as a function of the lambda air ratio shows a steady decrease in the water concentration in the exhaust gas as a function of the ratio of air to fuel gas when burning pure hydrogen. Each water concentration can thus be clearly assigned to a lambda value. The dependency is also strong enough, especially in the important lambda range between 1 and 2, to still be able to regulate reliably even with measurement inaccuracies. This applies not only to the combustion of pure hydrogen, but also to fuel gases containing hydrogen, in particular when there is a high proportion of hydrogen in the fuel gas.

The present invention makes it possible to implement the flame monitor and/or combustion control functions for pure hydrogen as fuel gas or hydrogen-containing fuel gases with simple and robust instrumentation on a heating device.

Reference List

1

heater

2

air supply

3

fan

4

fuel gas supply

5

fuel gas valve

6

mixer

7

burner

8th

flame area

9

combustion chamber

10

housing (of the combustion chamber)

11

sensor

12

Evaluation electronics (with NDIR)

13

First optical filter

14

light source

15

First window

16

second window

17

Second optical filter

18

exhaust system

19

measuring range

20

Measurement line

21

control lines

22

control and regulation unit

23

temperature sensor

Claims (12)

Method for observing a combustion process of hydrogen or a hydrogen-containing combustible gas with air in a combustion chamber (9) of a heating device (1) to determine the presence of flames and/or to measure the ratio of combustion air to combustible gas, with a measuring area ( 19) infrared light is radiated from a light source (14) to a sensor (11) sensitive to infrared light, the sensor (11) observing the intensity of the incident light and from changes in intensity at least the presence or quality of a burn is closed and at least one absorption line or band of the water is observed. Method according to Claim 1, in which the at least one absorption line or band is observed when the burner (7) is switched off and/or before it is ignited. Method according to Claim 1 or 2, in which, after the heater (1) has been started, a drop in intensity in the region of at least one absorption line or band of the water indicates ignition of the combustion and/or an increase in intensity indicates that the combustion has been extinguished becomes. Method according to one of the preceding claims, wherein during combustion and when a ratio of combustion air to fuel gas changes, the intensity of the at least one absorption line or band of water is observed, with the ratio of combustion air to combustible gas being inferred from the progression of the intensity in comparison to calibration curves and/or empirical values. Method according to one of the preceding claims, in which light of at least the observed wavelength ranges of the observed absorption lines or bands is sent through the measuring area (19) to the sensor (11). Arrangement for observing a combustion process of hydrogen or a hydrogen-containing combustible gas with air in a combustion chamber (9) of a heating device (1) to determine the presence of flames and/or to regulate the ratio of combustion air to combustible gas, with a light source (14) for infrared Light and a sensor (11) at least for infrared light in the area of at least one absorption line or band of water are present, between which there is a measuring area (19) through which combustion gases can flow, and wherein the sensor (11) is connected to evaluation electronics (12). is set up to observe the intensity of the incident light in the area of at least one absorption line or band of water and to conclude from the course of the presence of flames and/or the ratio of combustion air to fuel gas. Arrangement according to Claim 6, in which the measuring area (19) is arranged in such a way that up to there the water produced in the combustion process is still gaseous. Arrangement according to one of Claims 6 or 7, in which the sensor (11) is part of a non-dispersive infrared spectrometer (NDIR). Arrangement according to one of claims 6 to 8, wherein the sensor (11) and/or the light source (14) is arranged outside the combustion chamber (9) behind a window (15, 16) in a housing (10) around the combustion chamber (9). are. Arrangement according to one of Claims 6 to 9, at least one wavelength-selective filter (13, 17) being arranged between the light source (14) and the sensor (11). Arrangement according to one of Claims 6 to 10, in which the evaluation electronics (12) are set up to detect the presence and/or the quality of combustion from the measured values of the sensor (11) and to use these to ensure reliable starting and/or operation of the heating device ( 1) to use. A computer program product comprising instructions which cause the arrangement according to any one of claims 6 to 11 to carry out the method according to any one of claims 1 to 5.

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