METHOD AND ARRANGEMENT FOR OBSERVING A COMBUSTION PROCESS IN A HEATING APPLIANCE

Description

[0001] The invention relates to a method and an arrangement for observing a combustion process in a heating device operable with hydrogen and / or a hydrogen-containing fuel gas.

[0002] Hydrogen as a fuel gas or as an additive to fuel gases is becoming increasingly important, and great efforts are being made to adapt new or existing heating systems for operation with it. This applies not only to large systems, but also to wall-mounted water heaters and, more generally, to heating systems for heating buildings and / or providing hot water. Since such devices are long-lasting goods, there are also efforts to prepare them for operation with various fuel gases, although preparing them for operation with hydrogen poses special requirements.

[0003] Hydrogen combustion 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 generated with carbon-containing fuels, and requires different measurement systems than those used in heaters for hydrocarbon fuels. The present invention is therefore particularly, but not exclusively, suitable for heaters operated with pure hydrogen or with fuel gas that consists of more than 50%, in particular more than 95%, hydrogen.

[0004] Heating devices have generally used simple and robust sensors for ionization, temperature, light or heat radiation, pressure, volume flow, and the like to regulate the heaters and ensure their safe operation. However, with conventional sensors, some measurements cannot be reliably performed when hydrogen is used as the fuel gas. One important function is to detect the presence of a stable flame (a so-called flame detector), and another is to set a suitable ratio of combustion air to fuel gas (lambda value) for stable and environmentally friendly combustion.

[0005] The use of optical sensors using optical filters is already known for a specific application in heaters operated with hydrogen-containing fuel gas, for example, from DE 10 2019 101 329 A1. Optical measuring systems in heaters are also known from EP 2 223 016 B1, although these two documents primarily deal with the observation of flame radiation, i.e., radiation emissions in the combustion chamber. DE 10 2009 009 314 A1 also discloses the use of absorption spectroscopy in the infrared range of light to monitor flames or control a combustion process. In these considerations, water vapor is considered an interfering factor and not evaluated for the actual observation.

[0006] Due to the high temperatures and strong emissions in the area of ​​flames, known measuring methods and the evaluation of such measurements involve a certain amount of effort, since errors should be avoided in safety-relevant functions.

[0007] US 2011 / 067408 A1 discloses systems and methods for controlling combustion emission parameters in a gas turbine combustor. These include providing an optical path through a gas turbine exhaust duct, propagating light, particularly infrared light, along the optical path, measuring the absorption of the light by exhaust gas species within the gas turbine exhaust duct, and controlling a combustion parameter based on the measured absorption of the exhaust gas species. This allows for the reduction of undesirable exhaust gas components resulting from incomplete combustion.

[0008] US Pat. No. 7,414,726 B1 discloses an equivalent O2 analysis system for diatomic oxygen that measures CO2 and water vapor (H2O) in the combustion exhaust gas flowing through a furnace, chimney, or flue duct of a process heating or boiler system using a light source, e.g., a quantum cascade (QC) laser. The system processes the measurement signals to determine the oxygen dilution (relative to a stoichiometric concentration) and thus the O2 excess in the flue gas. This allows excess air during combustion to be detected and the air supply to be adjusted. The boiler system is also shut down if the excess air is too low, due to the risk of deflagration.

[0009] 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 methods are intended to be particularly suitable for heating devices operated with hydrogen as the fuel gas or with hydrogen-containing fuel gas.

[0010] To achieve this object, a method and an arrangement according to the independent claims serve. Advantageous embodiments and further developments of the invention are specified in the respective dependent claims. The description, particularly in conjunction with the drawing, illustrates the invention and provides an exemplary embodiment.

[0011] To achieve the object, a method for observing a combustion process of hydrogen or a hydrogen-containing fuel with air in a combustion chamber of a heater contributes to determining the presence of flames and, if necessary, also measuring the ratio of combustion air to fuel gas, wherein infrared light is radiated from a light source to a sensor sensitive to infrared light through a measuring area through which combustion products can flow, and wherein the sensor observes the intensity of the incident light and, from changes in the intensity, conclusions are drawn as to the presence and, if necessary, also the quality of combustion.

[0012] The main consideration here is that combustion products are produced during combustion, the detection of which in a measuring area outside the actual combustion zone allows just as reliable conclusions about the presence and, where applicable, the quality of combustion as, for example, direct observation of the flame. Only a delay required by the combustion products for their journey from the combustion zone to the measuring area may need to be taken into account. The closer the measuring area is to the combustion zone, the shorter the delay. Matter in a measuring area through which light shines attenuates this light in different ways, in particular through absorption in wavelength ranges that are different for different atoms or molecules.So-called absorption lines or bands are created, whose wavelengths and intensity decreases allow conclusions to be drawn about the amount and type of matter present in the measurement area. The present invention takes advantage of this.

[0013] For this purpose, at least one absorption line or band of water is observed. Water (in the gaseous state) is the quantitatively detectable combustion product of hydrogen with air and is particularly suitable for the desired application. In fact, the water content in the combustion gas is uniquely linked to the ratio of combustion air to fuel gas, so measuring the water content in the combustion gas has proven very suitable for controlling this ratio. A water molecule in the gaseous state exhibits three types of transitions in particular, which can lead to the absorption of electromagnetic radiation (including light): i. Rotational transitions, in which the molecule gains a quantum of rotational energy. Water vapor leads to absorption in the far-infrared region of the spectrum, from about 200 cm -1 (1 / centimeter) (50 µm (micrometer)) up to longer wavelengths toward the microwave range. ii. Vibrational transitions, in which a molecule gains a quantum of vibrational energy. The fundamental transitions lead to absorption in the mid-infrared region around 1650 cm -1 (µ-band, 6 µm) and 3500 cm -1 (so-called X-band, 2.9 µm). iii. Electronic transitions, in which a molecule is placed into an excited electronic state.

[0014] In the following, the term "absorption line" or "band" always refers to a narrow spectral range that can be selectively observed, particularly with a non-dispersive infrared spectrometer (NDIR). By observing the intensity of light that has passed through a gas volume (measurement range) in this spectral range, the proportion of certain gases (particularly water) in the gas volume can be determined. Since the combustion of hydrogen only produces water as a combustion product, and water is also formed when other combustible gases are burned, the ignition of a flame in the combustion chamber (i.e., the beginning of a combustion process) can be detected by the appearance of one or more absorption lines that were previously absent or less pronounced.

[0015] In a preferred embodiment, the sensor is part of a non-dispersive infrared spectrometer (NDIR). Non-dispersive infrared spectroscopy is highly developed and can be used cost-effectively for gas analysis, so an NDIR spectrometer can also be a practical part of a heater's control and regulation unit.

[0016] For calibration and accurate measurements, it is useful to observe at least one absorption line while the burner is (still) off and / or before it is ignited. Since some moisture is always present in the air, this component can be observed, for example, when a heater's fan is running but without any fuel gas supply (or at least before the combustion process is ignited), and thus eliminated from subsequent measurements.

[0017] After the heater has been started, a decrease in intensity in the area of ​​at least one absorption line of the water is used to conclude that combustion has ignited and / or an increase in intensity is used to conclude that combustion has extinguished.

[0018] Particularly preferably, the intensity of at least one water absorption line is observed during combustion and when the ratio of fuel gas to air changes, with the ratio of combustion air to fuel gas being deduced from the intensity profile in comparison to calibration curves and / or empirical values. In addition to simply detecting whether combustion is taking place, the air ratio can also be measured. The greatest decrease in intensity for water vapor absorption can be measured for an air ratio of 1 (lambda = 1). Due to dilution effects, the intensity decreases sharply at higher air ratios (superstoichiometric operation), whereas it decreases less sharply in substoichiometric operation because the degree of dilution is lower. For example,In a laboratory, characteristic curves for the intensity decrease for over-stoichiometric and under-stoichiometric operation are determined and stored in a control and regulation unit. By comparing measured values ​​with stored values, over- and under-stoichiometric operation can be determined. By adjusting the fuel gas / air mixture accordingly, over-stoichiometric operation can be achieved as the desired operating mode. Furthermore, further evaluation of measured values ​​can be used to determine the water vapor concentration and thus the air ratio once it is confirmed that over-stoichiometric operation is occurring. This enables control of the air ratio.

[0019] Preferably, light from at least the observed wavelength ranges of the observed absorption lines is sent through the measuring range to the sensor. To minimize interference, it may be advisable to send only infrared light from a few wavelength ranges through the measuring range or to filter it out before reaching the sensor. This can be achieved using a suitable light source and / or suitable optical filters. While modern spectrometers can also observe absorption lines or bands across a broad spectral range simultaneously, the analysis is faster, simpler, and / or more cost-effective if only selected spectral ranges are observed.

[0020] An arrangement for observing a combustion process of hydrogen or a hydrogen-containing fuel gas with air in a combustion chamber of a heater to determine the presence of flames and, if necessary, also to regulate the ratio of combustion air to fuel gas also contributes to solving the problem. A light source for infrared light and a sensor at least for infrared light in the region of at least one absorption line or band of water are provided, between which there is a measuring range through which combustion gases can flow, and the sensor is connected to evaluation electronics which are designed to observe the intensity of the incident light in the region of at least one absorption line or band of water and to infer from the course of the latter the presence of flames and, if necessary, also the ratio of combustion air to fuel gas.

[0021] Preferably, the measuring area is positioned so that the water produced during the combustion process is still gaseous upstream. Condensation of some of the water vapor contained in the combustion gases upstream of the sensor would distort the measurements, so this must be avoided wherever possible. The measuring area should therefore be located close enough to the flame area that no condensation occurs upstream, especially in the combustion chamber or near the combustion chamber in the exhaust system.

[0022] Preferably, the sensor is part of a non-dispersive infrared spectrometer. This can be part of the evaluation electronics and / or, together with the electronics, form part of a control and regulation unit of the heater.

[0023] While there are light sources and sensors that are also suitable for installation in a combustion chamber, the sensor and / or light source are preferably located outside the combustion chamber behind a window in a housing surrounding the combustion chamber. This simplifies maintenance and eliminates the need for electrical feedthroughs to the combustion chamber. However, arrangements using fiber optic cables are also possible, allowing even greater flexibility in the arrangement of the light source and sensor.

[0024] Particularly preferably, at least one wavelength-selective filter is arranged between the light source and the sensor. This allows interference to be reduced and / or a specific wavelength range to be observed without complex (electronic) spectral classification.

[0025] Preferably, the evaluation electronics (as part of a control and regulation unit) are configured to detect the presence and, if applicable, also the quality of combustion from the measured values ​​of the sensor and to use these for a safe start and / or operation of the heater.

[0026] A computer program product may be provided, comprising instructions that cause the described arrangement to execute the described method. The evaluation of the data measured by the sensor and its further use in the heater require a program and data for controlling the heater, both of which must be updated occasionally.

[0027] The explanations of the procedure can be used to further characterize the arrangement, and vice versa. The arrangement can also be designed to carry out the procedure.

[0028] A schematic embodiment of the invention, to which it is not limited, and the functioning of the method will now be explained in more detail with reference to the schematic figures. They depict: Fig. 1: a heater with a measuring system for the absorption of infrared radiation and Fig. 2: the dependence of the water content in the combustion gas on the lambda value during combustion of pure hydrogen.

[0029] Fig. 1 shows a schematic diagram of a heater 1, which is operable in particular with hydrogen or a hydrogen-containing fuel gas. Air (usually outside air / ambient air) is sucked in by a fan 3 via an air supply 2 and conveyed to a burner 7. Fuel gas is added to the air flow generated by the fan 3 in a mixer 6 (e.g., a Venturi nozzle) via a fuel gas supply 4 and a fuel gas valve 5. The mixture is combusted in a combustion chamber 9, which is surrounded by a housing 10, creating a flame region 8. Resulting combustion gases are discharged via an exhaust system 18. An optical sensor 11, sensitive to IR radiation, is arranged for measurement purposes such that it can observe a measuring region 19 in the combustion chamber 9 (or at least downstream of the flame region 8). The measuring region 19 lies outside the flame region 8, thereby reducing interference.It must be positioned so close to the flame region that water vapor cannot condense there, as condensation of some of the water contained in the combustion gases would distort the measurements. The sensor 11 can either be designed to be wavelength-sensitive (sensitive only to a specific wavelength range), or it can be preceded by an optical filter 13 which only allows a specific wavelength range through which the line(s) to be observed lie. This wavelength range lies in the infrared range, particularly where water produces absorption lines or bands. In general, such a sensor 11 should not be positioned inside a combustion chamber 9 due to 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 inside the window 15, outside the window 15, or preferably integrated with the window 15. Evaluation electronics 12, to which the sensor 11 is connected, evaluates the measurement signals from the sensor 11 and uses them to determine the presence of water and, if necessary, also quantitatively determines its concentration in the measuring area 19. For this purpose, a light source 14 radiates light in the IR range through the measuring area 19. This light source is preferably also arranged outside the housing 10 behind a second window 16 (possibly also with an optical filter 17). Light emitted by this light source 14 (at least in the observed wavelength range) reaches the sensor 11 through the measuring area 19. An optical filter 13, 17 can be arranged somewhere in the beam path to achieve sufficient measurement sensitivity.However, modern NDIR spectrometers can also observe a larger infrared range and evaluate several absorption lines or bands occurring there in parallel, thus eliminating the need for optical filters. However, it is advisable to perform a temperature correction, for which purpose a temperature sensor 23 is located in or near the measuring area 19. Since infrared radiation (heat radiation) also depends on the temperature in a measurement environment, correction is important. This can be done using stored laboratory data and / or empirical values ​​according to a correction curve in the evaluation electronics 12.

[0030] The evaluation electronics 12 is preferably part of a control and regulation unit 22, which controls the entire heater 1 and is connected to important components such as the fan 3 and the fuel gas valve 5 via control lines 21. It regulates these components and can shut them down in the event of safety-relevant 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 form a measuring system that can be used as a flame detector and, if necessary, also for adjusting the air-to-fuel gas ratio for various fuel gases.

[0031] It should be noted that other spatial arrangements are possible, especially when optical fibers are used to transmit light to a desired location. The arrangement described so far can measure absorption lines of water, which are created by the water vapor produced during combustion in measuring area 19.

[0032] Fig. 2illustrates why absorption measurements on water (vapor) 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 volume percent) as a function of the air / fuel ratio lambda shows a continuous decrease in the water concentration in the exhaust gas as a function of the ratio of air to fuel gas during combustion of pure hydrogen. Each water concentration can therefore be clearly assigned to a lambda value. The dependency is also strong enough, particularly in the important lambda range between 1 and 2, to enable reliable control even with measurement inaccuracies. This applies not only to the combustion of pure hydrogen, but also to fuel gases containing hydrogen, especially when the fuel gas contains high hydrogen content.

[0033] The present invention makes it possible to implement the functions of flame monitor and, if necessary, combustion control for pure hydrogen as fuel gas or hydrogen-containing fuel gases with a simple and robust instrumentation on a heater. List of reference symbols

[0034] 1Heater 2Air supply 3Blower 4Fuel gas supply 5Fuel gas valve 6Mixer 7Burner 8Flame area 9Combustion chamber 10Housing (of the combustion chamber) 11Sensor 12Evaluation electronics (with NDIR) 13First optical filter 14Light source 15First window 16Second window 17Second optical filter 18Exhaust system 19Measuring area 20Measuring line 21Control lines 22Control and regulation unit 23Temperature sensor

Claims

1. Method for observing a combustion process of hydrogen or a hydrogen-containing fuel gas with air in a combustion chamber (9) of a heating device (1) for detecting the presence of flames, wherein infrared light from a light source (14) is radiated to a sensor (11) sensitive to infrared light through a measurement area (19) through which combustion products flow, wherein the sensor (11) monitors the intensity of the incidental light, and from changes in the intensity at least the presence of combsution is inferred, and wherein at least one absorption line or band of water is observed.

2. Method according to claim 1, wherein the at least one absorption line or band is observed when the burner (7) is switched off and / or before its ignition.

3. Method according to claim 1 or 2, wherein after starting the heating device (1) a drop in the intensity in the region of at least one absorption line or band of water indicates an ignition of the combustion and / or an increase in the intensity indicates an extinguishing of the combustion.

4. Method according to any one of the preceding claims, wherein, during a combustion and with changes in a ratio of combustion air to fuel gas, the intensity of the at least one absorption line or band of water is observed, wherein the ratio of combustion air to fuel gas is inferred from the course of the intensity in comparison with calibration curves and / or empirical values.

5. Method according to any one of the preceding claims, wherein light in at least the observed wavelength ranges of the observed absorption lines or bands is sent through the measurement area (19) to the sensor (11).

6. Arrangement for monitoring a combustion process of hydrogen or a hydrogen-containing fuel gas with air in a combustion chamber (9) of a heating device (1) for detecting the presence of flames, wherein a light source (14) for infrared light and a sensor (11) at least for infrared light in the region of at least one absorption line or band of water are provided, between which there is a measurement area (19) through which the combustion gases can flow, and wherein the sensor (11) is connected to an electronic evaluation unit (12) which is configured to observe the intensity of the incidental light in the region of at least one absorption line or band of water and to draw conclusions about the presence of flames from its course.

7. Arrangement according to claim 6, wherein the measurement area (19) is arranged so that the water produced in the combustion process is still gaseous when it gets there.

8. Arrangement according to any one of claims 6 or 7, wherein the sensor (11) is part of a non-dispersive infrared (NDIR) spectrometer.

9. Arrangement according to any one of claims 6 to 8, wherein the sensor (11) and / or the light source (14) are arranged outside the combustion chamber (9) behind a window (15, 16) in a housing (10) around the combustion chamber (9).

10. Arrangement according to any one of claims 6 to 9, wherein at least one wavelength-selective filter (13, 17) is arranged between the light source (14) and the sensor (11).

11. Arrangement according to any one of claims 6 to 10, wherein the evaluation electronic unit (12) is configured to recognize the presence of combustion from the measurement values of the sensor (11) and to use them to safely start the heating device (1).

12. Heating device (1), configured for the combustion of hydrogen or a hydrogen-containing fuel gas with air, comprising an arrangement according to any one of claims 6 to 11 as a flame monitor.