DE102021110818A1 - Method and arrangement for increasing the charge carrier concentration in the vicinity of an ionization electrode in a heating device - Google Patents

Abstract

The invention relates to a method and an arrangement for increasing the charge carrier concentration in the vicinity of an ionization electrode (5) for controlling a gas-air mixture and/or monitoring flames (4) in a combustion chamber (1) of a heating device which is heated with a mixture (G) is operated from air (L) and a fuel, in particular fuel gas with more than 50% hydrogen content, with at least one substance promoting ionization being provided at least in the area (13) of the ionization electrode (5) or the proportion of this substance being increased becomes. The invention allows economical ionization measuring systems to be used in a heater for monitoring flames (4) and/or for controlling a combustion process, even when fuels with a low charge carrier concentration are used.

Description

The invention relates to a method and an arrangement for increasing the charge carrier concentration in the vicinity of an ionization electrode, which can be operated in a heating device that can be operated with a (solid, liquid and/or gaseous) fuel, such as oil, hydrogen and/or a fuel gas containing hydrogen and is used in particular for controlling a gas-air mixture and/or monitoring flames.

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.

Some fuels, such as B. hydrogen, differ in the combustion in several respects from previously used fuel gases, in particular, the ionization is much lower in the flame generated with it. There are far fewer charge carriers that can be used for an ionization measurement. As a result, the proven measuring principle of ionization measurement for detecting flames in a combustion chamber, which is often used with conventional fuel gases, cannot be used without further ado for heaters if they are operated with fuels that form a lower charge concentration in the area of the ionization electrode. This applies, for example, to the use of pure hydrogen or with a fuel gas that consists of more than 50%, in particular more than 95%, hydrogen. For reliable evaluation of ionization measurements, a minimum concentration of charge carriers (ions and electrons) in the vicinity of the ionization electrode is required in all operating states, which can be moved by applying a voltage and then generate an ionization current. In hydrogen flames this is not always the case.

An important function of ionization measuring devices is the determination 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 that is suitable for reliable, stable and environmentally friendly combustion (lambda value control). Certain minimum ion currents are required for both, which are not always available when hydrogen is used as the fuel gas.

The object of the present invention is to at least partially solve the problems described with reference to the prior art and in particular to provide a method and an arrangement which increase the charge carrier concentration in the vicinity of an ionization electrode when flames are to be observed in a combustion chamber, wherein the arrangements should be simple and suitable for everyday operation of a heater.

A method and an arrangement according to the independent claims are used to solve this problem. Advantageous refinements and developments of the invention are specified in the respective dependent claims. The description, in particular in connection with the drawing, illustrates the invention and specifies further exemplary embodiments.

The method explained below for increasing the charge carrier concentration in the vicinity of an ionization electrode contributes to this, which method is used in particular to regulate a gas-air mixture and/or monitor flames in a combustion chamber of a heating device. The heater is operated with a mixture of air and a fuel, in particular with a fuel gas with a hydrogen content of more than 50%. It is further provided that at least one substance promoting ionization is provided at least in the vicinity of the ionization electrode, possibly added, and/or the proportion of this substance is increased.

Upgrading of known ionization measurement systems for fuels with low charge carrier concentration in the area of the flame, such as e.g. B. with hydrogen as the fuel gas, is possible if during operation at least in the vicinity of the ionization electrode sufficient charge carriers (permanent or temporary) can be provided and/or generated that are not (immediately) during or from the combustion of the actual air -Combustible gas mixtures are formed. It is proposed here to provide an additional substance that makes (additional or non-combustion) charge carriers available under the conditions prevailing in the vicinity of the ionization electrode (flames of more than 1,000 K [Kelvin]). For a higher yield of charge carriers and thus a larger ion current that can be better evaluated, it is sufficient if an increase in the charge carriers is achieved in the vicinity of the ionization electrode, but the density of charge carriers can also be increased everywhere in the flames if this is technically easier can be reached.

In the context of the present context, in the vicinity of or in the vicinity of an ionization electrode does not necessarily apply to the entire combustion chamber or to the entire area of the combustion chamber filled with flames, but in particular means the area that can be influenced by an ionization electrode and a voltage applied to it. This is in particular an area between the ionization electrode and counter-electrode (e.g. a porous burner body, possibly formed with a metallic or ceramic material) and/or a (in particular cylindrical) area of up to 10 cm [centimetres], in particular up to 5 cm to the ionization electrode. In principle, the environment can also include the surface of the ionization electrode.

The method proposed here, in particular increasing the proportion of the substance, can also (only) be carried out if the heater is (temporarily) operated in predetermined conditions (unfavorable in terms of ionization), for example at (a) low heat loads (e.g. < 20% Qn, where Qn represents the normal heat load or the primary operating point of the heater) and/or (b) lean gas-air mixture (e.g. lambda > 1.6). If these operating ranges are left again, addition of the substance can be reduced or stopped again if necessary.

In fact, air is also such a substance, especially its nitrogen and carbon dioxide components. This applies in particular to hydrogen or fuel gas with more than 50% hydrogen content, because a higher proportion of carbon dioxide and nitrogen increases ionization, which an increase in temperature would not cause. An additional supply of air z. B. fed only with air openings in a burner body is therefore advantageous. As a result, the lambda value changes locally, but this only has a minor effect on the entire combustion process. The use of additional carbon dioxide (possibly instead of air) is also very effective because this substance can provide many charge carriers. If enough carbon dioxide is available, it can simply be mixed with the air and/or the fuel. To reduce consumption, it can make sense to only provide an additional supply in the vicinity of the ionization electrode. Correspondingly effective substances can be used with other fuels, such as suitable petroleum gases (propane, butane, etc.).

Another possibility is that the substance promoting ionization is a substance ionizable under ultraviolet radiation, which is placed in the vicinity of the ionization electrode. Hydrogen flames produce strong ultraviolet radiation that can be used to generate charge carriers when a substance is irradiated by it, which releases electrons under this irradiation. This substance is not used up, or only very slowly, and eventually captures electrons again and is therefore permanently available for the emission of charge carriers. Certain metal oxides that are stable under the conditions prevailing in a combustion chamber are particularly suitable.

A coating with titanium dioxide (as a substance promoting ionization) is preferably applied to existing components in the vicinity of the ionization electrode, or an additional component with titanium dioxide is applied. Preferably (alternatively or cumulatively) in the vicinity of the ionization electrode a coating with aluminum oxide (as a substance promoting ionization) is applied to existing components or an additional component with aluminum oxide. Titanium dioxide or aluminum oxide has the desired property of releasing electrons under ultraviolet radiation and is suitable as a coating for metallic components. It can therefore be applied to existing components in the vicinity of the ionization electrode and/or an additional component, e.g. B. a rod or sheet metal attached.

Another possibility is to supply a substance with low ionization energy, which releases valence electrodes at flame temperature, in the vicinity of the ionization electrode. Suitable substances of this type that promote ionization are, for example, some salts which are present in solid form (as crystals) at the temperatures occurring here. They decompose (albeit very slowly) under the influence of flame, requiring periodic refilling or continuous feeding.

Sodium chloride (NaCl) is particularly suitable as a substance with a low ionization energy. This is available inexpensively and its crystals can be shaped into any shape and are therefore easy to supply.

To this end, it is advantageous if the substance with low ionization energy is supplied in solid form via a supply mechanism (continuously, at least over a specifiable operating period). The mechanism can have a very simple structure and, for example, replace material that has been used up on a tip of a rod by pushing it up with a spring.

An arrangement for increasing the charge carrier concentration in the vicinity of an ionization electrode (for controlling a gas-air mixture and/or monitoring flames) in a combustion also contributes to the solution of the object space of a heating device which can be operated with a mixture of air and a fuel, in particular with a hydrogen content of more than 50%, with means being present at least in the vicinity of the ionization electrode for providing or increasing a proportion of a substance which promotes ionization.

The means addressed here can be a device for making the substance available as needed. These means may include storage, delivery, and/or delivery components. It is possible for at least one means to be assigned permanently or permanently to the ionization electrode. It is possible for the agent to be a component or coating made from the substance that promotes ionization, which is provided in particular on or on the ionization electrode. The means are positioned in particular on the ionization electrode up to a predefinable boundary of the environment. The environment is in particular an area between the ionization electrode and counter-electrode and/or a (in particular cylindrical) area of up to 10 cm [centimetres], in particular up to 5 cm, around the ionization electrode.

In a preferred embodiment, the means are at least one additional inlet for a gaseous substance, such as in particular air or carbon dioxide or a petroleum gas (propane, butane, etc.). If a pre-mixing of air and fuel only takes place behind a fan, air can be branched off before the mixture and fed to an area in the vicinity of the ionization electrode. This increases the proportion of nitrogen and carbon dioxide there, which intensifies ionization. The same can also be achieved by feeding in carbon dioxide.

In another preferred embodiment, the agents are components of or are coated with a substance that can be ionized under ultraviolet radiation, in particular titanium dioxide.

In another embodiment, a low ionization energy solid material delivery mechanism that releases valence electrodes at flame temperature is provided. In particular, this can be sodium chloride (NaCl).

Particularly preferably, the feed mechanism is arranged between the ionization electrode and a torch body and has a metal tube for feeding rod-shaped sodium chloride crystal, the tube being connected to a housing of the combustion chamber in a thermally conductive manner for cooling. As will be explained in more detail with reference to the drawing, such a feed mechanism can be of simple construction and cause sodium chloride to be continuously fed in by means of spring force, with sodium chloride not being exposed to the flames being protected by the metal tube and this being cooled via a connection to a housing.

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. It should also be pointed out that the different measures to increase ionization can be used individually, but also in any combination. They have an additive effect and can therefore also be used together in difficult cases.

• 1: schematisch einen Ausschnitt aus einem Längsschnitt durch einen Verbrennungsraum eines Heizgerätes im Bereich einer Ionisationselektrode,
• 2: schematisch die Anordnung eines zusätzlichen Einlasses in einen Verbrennungsraum für einen die Ionisation begünstigenden gasförmigen Stoff und
• 3: schematisch die Anordnung einer die Ionisation begünstigenden Komponente in der Umgebung einer Ionisationselektrode.

Schematic exemplary embodiments of the invention, to which it is not limited, and how the method works are explained in more detail with reference to the attached drawing. They represent:

• 1 : schematically a section of a longitudinal section through a combustion chamber of a heater in the area of an ionization electrode,
• 2 : schematic arrangement of an additional inlet into a combustion chamber for a gaseous substance that promotes ionization and
• 3 : Schematic of the arrangement of a component that promotes ionization in the vicinity of an ionization electrode.

1 shows schematically a section of a longitudinal section through a combustion chamber 1 of a heater, which can be operated with hydrogen or a hydrogen-containing fuel gas. A mixture G of air L and fuel gas from a burner interior 15 enters the combustion chamber 1 through openings 3 in a burner body 2 and burns there with the formation of flames 4. The flames 4 ionize the mixture and/or the gaseous combustion products produced therefrom. However, since only water is produced during the combustion of hydrogen, and both can hardly be ionized under the given conditions, the yield of charge carriers when hydrogen is burned is very low, so that reliable monitoring of the flames 4 by means of an ionization measuring system is not readily possible. In order to still be able to generate a sufficiently large ion current using an ionization electrode 5, there are means for the Increase in the concentration of charge carriers present. In the present embodiment, this is a Feeding mechanism 9, 10, 11, 12 for a substance that under the influence of flames 4 releases charge carriers (essentially by thermal ionization). Salts are particularly suitable for this purpose, in particular sodium chloride (NaCl). Thus, in a guide tube 9, a salt crystal rod 8 is pressed against a cover 10 by means of a compression spring 12, a small part of the salt crystal rod 8 being exposed to the flames 4 through at least one window 11. This part is used up over time and the remaining salt crystal rod 8 is then pushed in by the compression spring 12 . The salt crystal rod 8 can be renewed from outside the combustion chamber 1 at certain maintenance intervals. The ionization electrode 5 is preferably attached to a so-called burner door 7 by means of an electrically insulating bushing 6 . The burner door 7 is part of a housing (not shown) surrounding the combustion chamber. The burner body 2 is also fastened to it, with the supply of relatively cold mixture G keeping the burner door 7 at a much lower temperature than that in the combustion chamber 1 (and in particular in the flames 4) even during operation. This can be used to keep the salt crystal rod 8 also at a lower temperature, except in the area of the window 11, which extends its service life. For this purpose, the guide tube 9 (which generally consists of metal) can be coupled to the burner door 7 by a heat-conducting connection 18 .

Another embodiment of the invention is 2 shown schematically. Since an increase in the charge carrier concentration can also be achieved by increasing the concentration of carbon dioxide and/or nitrogen at least in the area surrounding 13 of an ionization electrode 5, an additional inlet 14 for air L is provided, so that mixture G of air and fuel gas can flow through the interior of the burner 15 is directed into the entire combustion chamber 1, but additional air reaches the surroundings 13 of the ionization electrode 5. This air can be diverted from a fan or supplied in some other way. Additional carbon dioxide can also be supplied instead of air. This can optionally be added to the air L, the fuel gas or the mixture G. However, the least carbon dioxide is required if it is only supplied through the additional inlet 14 . Carbon dioxide can be stored in a pressure vessel (not shown) and metered in when the heater is started, which means that, depending on the size of the pressure vessel, the carbon dioxide only has to be refilled at maintenance intervals that are required anyway. A system with pressure cartridges similar to that used in the preparation of carbonated beverages can also be envisaged.

3 FIG. 12 shows schematically another embodiment of the invention, in which a surface of an existing or additionally installed component 17 in the vicinity of the ionization electrode 5 consists of or is coated with a material which releases electrons under ultraviolet radiation. A suitable coating 16 can consist of titanium dioxide, for example. Since hydrogen flames emit strong ultraviolet radiation, additional charge carriers are released, which intensify ionization.

The present invention makes it possible to use cost-effective ionization measuring systems even when using fuel gas containing hydrogen or pure hydrogen as fuel gas in a heating device for monitoring flames and/or for controlling a combustion process.

reference list

1

combustion chamber (of a heater)

2

torch body

3

openings

4

Flames

5

ionization electrode

6

Electrically insulating bushing

7

burner door (part of an enclosure)

8th

Salt Crystal Wand

9

guide tube

10

lid

11

window

12

compression spring

13

vicinity

14

inlet for air

15

burner interior

16

Coating (Titanium Dioxide)

17

(additional) component

18

Thermally conductive connection

G

Mixture (air and fuel gas)

L

Air

Claims (15)

Method for increasing the charge carrier concentration in the vicinity of an ionization electrode (5) in a combustion chamber (1) of a heating device which is operated with a mixture (G) of air (L) and a fuel, at least in the vicinity (13) of the ionization electrode (5) at least one promoting ionization substance is made available or the proportion of this substance is increased. procedure after claim 1 , wherein the substance is made available after setting a low thermal load of the heater and/or a leaner gas-air mixture. procedure after claim 1 or 2 , whereby the heater is operated with fuel gas with more than 50% hydrogen content. A method according to any one of the preceding claims, wherein the substance is air (L) or carbon dioxide. A method as claimed in any one of the preceding claims, wherein the substance is an ultraviolet ionisable substance which is placed in the vicinity (13) of the ionisation electrode (5). procedure after claim 4 , wherein in the vicinity (13) of the ionization electrode (5) a coating (16) with titanium dioxide or aluminum oxide is applied to existing components or an additional component (17) with titanium dioxide or aluminum oxide is applied. procedure after claim 1 , wherein a substance (8) with low ionization energy, which releases valence electrodes at flame temperature, is supplied to the ionization electrode (5) in the environment (13). procedure after claim 1 , wherein the substance (8) with low ionization energy is sodium chloride (NaCl). procedure after claim 5 or 6 , wherein the substance (8) is fed in solid form via a feeding mechanism (9, 10, 11, 12). Arrangement for increasing the charge carrier concentration in the vicinity of an ionization electrode (5) in a combustion chamber (1) of a heating device which can be operated with a mixture (G) of air (L) and a fuel, at least in the vicinity (13) of the ionization electrode (5) There are means for providing or increasing a proportion of an ionization promoting substance. arrangement according to claim 10 , wherein the means comprise at least one additional inlet (14) for a gaseous substance. arrangement according to claim 10 or 11 wherein the compositions comprise components of or coated with a substance ionizable under ultraviolet radiation. arrangement according to claim 12 , where the substance is titanium dioxide. Arrangement according to one of Claims 10 until 13 , wherein there is a delivery mechanism for a solid with low ionization energy, the solid releasing valence electrodes at flame temperature. arrangement according to Claim 14 , wherein the feeding mechanism is arranged between the ionization electrode and a burner body and has a metallic tube for feeding rod-shaped sodium chloride crystal, the tube being connected to a housing of the combustion chamber in a thermally conductive manner for cooling.

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