DE102016014151A1 - Method for determining a calorific value and / or a Wobbe index of a gas mixture - Google Patents

Abstract

Method for determining a result parameter describing a calorific value and / or a Wobbe index of a gas mixture, comprising the steps carried out by a determination device (1): determining parameter values of the gas mixture for exactly two gas parameters from the group thermal conductivity, thermal diffusivity, heat capacity and density, calculation of the result parameter as a result of a given function, which depends exclusively on the parameter values of the exactly two gas parameters.

Description

The invention relates to a method for determining a result parameter describing a calorific value and / or a Wobbe index of a gas mixture. In addition, the invention relates to a determination device for determining this result parameter.

In natural gas networks, typically only one gas volume supplied has been recorded for billing purposes on the consumer side. This is sufficient if a substantially constant gas quality can be provided permanently. However, if natural gas is supplied to consumers from various deposits, or if other gases are added to this natural gas, the calorific value may vary, making calorific value-based billing of the gas supplied more expedient. This becomes increasingly relevant because, for example, biogas or hydrogen produced from green electricity or gases produced therefrom can be buried in natural gas in order to enable the use of regenerative energies.

In addition, it is advantageous for the operation of combustion devices, if it is ensured that not too much energy and thus heat is given off during the combustion of the natural gas in order to prevent damage or heavy wear of these combustion devices. Therefore, it may also be advantageous to take into account a determined calorific value or a Wobbe index of the gas mixture in the control of corresponding combustion devices. The Wobbe index is proportional to the calorific value of the gas mixture and inversely proportional to the root of the relative density of the gas mixture in relation to the density of air. For example, fuel gases having a similar Wobbe index are interchangeable in cases where the fuel gas is supplied to the constant nozzle pressure combustion process.

So far, the calorific value or the Wobbe index has been determined primarily by the utility. For this purpose, it is known, for example, to use gas chromatographs in order to separate the constituents of the gas mixture and thus to determine the composition of the gas mixture. From the composition of the gas mixture and the known properties of the individual gases, the calorific value or the Wobbe index can be determined. However, gas chromatographs are relatively large and expensive, so that use by the consumer is usually not appropriate. Therefore, several alternative approaches have been developed to determine the calorific values of a gas mixture.

For example, it is known to determine the calorific value of a gas mixture as a function of its thermal conductivity, its heat capacity and its density. These measures can be determined by passing the gas mixture through a critical nozzle through which the gas mixture flows at the speed of sound to a CMOS sensor. The CMOS sensor can be used to determine the thermal conductivity or a quantity dependent on the heat capacity. With the help of the critical nozzle, the density can be deduced with knowledge of the pressure of the gas. The disadvantage here is that due to the passage of the nozzle at the speed of sound, this measurement method is possible only for a defined volume flow. If gas mixtures are to be measured during operation with variable volumetric flow, this results in considerable additional expenditure in order to produce the measuring conditions for calorific value determination. Furthermore, a plurality of further sensors, e.g. Pressure sensors necessary.

Another approach is to measure the speed of sound or density and, in addition, an absorption capacity for infrared radiation of the fuel gas. However, both the measurement of infrared absorption and the measurement of the speed of sound require relatively expensive sensors.

There are also known approaches to measure the thermal conductivity of a gas mixture at different temperatures and to determine therefrom, for example, the calorific value or the Wobbe index. However, measuring at different temperatures requires the provision of two spaced sensors or alternating operation of a sensor at different ambient temperatures, thereby increasing the measurement overhead.

The invention is therefore based on the object to provide a method for determining a calorific value or a Wobbe index, which can be implemented with less technical effort and thus is particularly suitable for use by the consumer.

• - Ermitteln von Parameterwerten des Gasgemischs für genau zwei Gasparameter aus der Gruppe Wärmeleitfähigkeit, Temperaturleitfähigkeit, Wärmekapazität und Dichte,
• - Berechnen des Ergebnisparameters als Ergebnis einer vorgegebenen Funktion, die ausschließlich von den Parameterwerten der genau zwei Gasparameter abhängt.

The object is achieved according to the invention by a method of the type mentioned at the outset, which comprises the following steps performed by a determination device:

• Determining parameter values of the gas mixture for exactly two gas parameters from the group heat conductivity, thermal conductivity, thermal capacity and density,
• - Calculating the result parameter as a result of a given function that depends exclusively on the parameter values of the exactly two gas parameters.

According to the invention it is thus proposed to determine only exactly two gas parameters of the gas mixture and to determine from these the result parameter. In the context of the invention, it has been recognized that with a choice of the gas parameters from the group heat conductivity, thermal diffusivity, heat capacity and density, the calorific value or the Wobbe index can already be determined with good accuracy from parameter values of two of these gas parameters. In other words, for gas mixtures that are relevant to combustion processes, in particular for natural gases and mixtures of natural gas with hydrogen or other non-combustible gases, a clear relationship between the calorific value and the Wobbe index and parameter values of any two of the gas parameters thermal conductivity, thermal diffusivity, thermal capacity and density exist. The thermal conductivity is also referred to as thermal conductivity and the thermal conductivity as thermal diffusivity. In a three-dimensional parameter space whose first dimension is assigned to the calorific value or the Wobbe index and whose other dimensions are respectively defined by one of the parameters from the group mentioned, all relevant gases or gas mixtures are thus approximately on a generally curved surface. This area is defined by a corresponding predetermined function. The determination of a corresponding function will be explained in detail later.

In the method according to the invention, the result parameter can be determined in particular for natural gases or mixtures of natural gases with gases other than gas mixtures. For example, the gas mixture may be a natural gas to which hydrogen, higher proportions of at least one hydrocarbon and / or ambient air are added.

A calorific value determined in accordance with the method according to the invention can be further utilized in order to determine energy consumption from the calorific value and a volume flow of the gas. Thus, the method according to the invention makes it possible, with relatively little technical effort, to provide gas meters which detect a quantity of energy provided or which, in addition to a volume, can provide a calorific value assigned to the consumed volume. It is particularly advantageous if the determination device can detect a flow in addition to the parameter values of the gas parameters. This is possible, for example, when using a thermal measuring principle for flow detection, as will be explained in more detail later.

The function can be specified in the method according to the invention as a function of a training data record which describes parameter values for the two gas parameters and the result parameter for a plurality of gases and / or gas mixtures. The training data set may preferably comprise parameter values for more than one hundred, in particular for several hundred gases and / or gas mixtures. The function can be determined by fitting a function with a given structure to the parameter values. The function can also be determined in the form of an interpolation between a plurality of interpolation points, which are specified by the training data set. Certain scope areas can be defined for the function. For example, the function may be defined as valid only in those areas whose parameter values do not deviate too much from parameter values of a gas or gas mixture defined by the training data set. If it is determined in the method according to the invention that the parameter values are outside the range defined as valid in the function, an error signal can be provided. This can be used, for example, to mark certain measurement points in a measurement data store as invalid, to provide information about a malfunction locally or via a data network, or the like.

The function may, for example, be a polynomial in the parameter values of the two gas parameters as variables, the coefficients of the polynomial being predetermined as a function of the training data set. The individual coefficients can be determined, for example, by fitting the polynomial, for example by minimizing a cost function that depends on the deviations of the result parameters specified by the training data set from a respective result parameter calculated for the corresponding gas or gas mixture.

For example, a polynomial with a total of 16 coefficients can be used, each associated with products of different powers of the two gas parameters. For example, coefficients for all combinations of the different powers of the gas parameters can be specified, the powers being integer and having an exponent between 0 and 3. With such a polynomial, a training dataset approximate area has been computed that includes parameter values for the gas parameters and calorific value of 370 natural gases as well as for nitrogen and air, where the calorific values of nitrogen and air are zero. It was found that when using the thermal conductivity and the Thermal conductivity as a gas parameter of the calculated by the function of the values of the gas parameters calorific value by a maximum of 6.2% of the predetermined by the training data set for the corresponding gas or gas mixture calorific value. The deviation was greater than 4% for only three gas mixtures and less than 2% for 90% of the gas mixtures. The calorific values determined by the function thus agree with the actual calorific values with good accuracy. This is relevant because these two gas parameters can be determined with little technical effort in the context of a flow measurement.

By evaluating parameter values of density and heat capacity, the accuracy can be further improved. For the training data set explained above, a maximum error of 2.4% was determined. Apart from a gas mixture, the error is less than 2%. For more than 90% of the gas mixtures even an error of less than 1% was found.

The training data set may preferably describe parameter values for the two gas parameters and the result parameter for at least one gas or gas mixture that is not exothermic combustible. For such a gas, a calorific value of 0 kWh / m ^{3 can be} assumed. In this case, in particular, air as gas mixture and / or nitrogen as gas can be taken into account. Air and nitrogen differ significantly from most natural gases with respect to their gas parameters evaluated in the method according to the invention. Thus, in the process according to the invention, it is also possible to detect influences of, in particular, high amounts of burial of air or nitrogen to the natural gases on the calorific value or the Wobbe index.

As the preceding discussion of the 370 tested natural gases shows, the method according to the invention is also suitable for determining calorific values or Wobbe indices for complex, ie not only binary, gas mixtures. Therefore, in the method according to the invention, the result parameter for a gas mixture comprising at least three different gases can be determined. For example, the gas mixture may include methane, ethane, propane, other hydrocarbon-based gases, hydrogen gas, air and / or nitrogen.

In the method according to the invention, a result parameter for a gas mixture which comprises hydrogen is particularly preferably determined. The gas mixture may thus comprise gaseous molecular hydrogen H ₂ . A determination of calorific values or Wobbe indices for gas mixtures comprising hydrogen is highly relevant. Hydrogen is a particularly easy artificially produced gas. For example, in the context of a power-to-gas process, hydrogen can be produced by electrolysis when excess energy is produced by nonuniformly renewable energy producing devices, such as wind turbines or solar cells. This hydrogen can be fed into a natural gas grid. Due to the distinctly different combustion properties of the hydrogen gas compared to typical natural gas mixtures, this can result in a significantly different calorific value or a significantly different Wobbe index. If the hydrogen content of the supplied gas mixture is varied, it is thus advantageous to record the calorific value or the Wobbe index in order to enable energy consumption-based billing and / or to control a burning process of the gas mixture as a function of the Wobbe index.

In the method according to the invention, the parameter values for the thermal conductivity and the thermal diffusivity can be detected, the function depending exclusively on these parameter values. The parameter values of these two gas parameters are particularly easy to detect by sensors which can also be used for the thermal determination of a flow rate, whereby flowmeters which additionally determine the calorific value or the Wobbe index can be realized particularly easily.

In the method according to the invention, the parameter values for the two gas parameters are preferably recorded at the same temperature and / or the same pressure. This allows a particularly simple detection of the parameter values.

The parameter values can be detected by the detection device by a thermal measuring principle. In this case, a sensor can be used, which comprises a heating element for targeted heating of the gas mixture, and at least two, preferably at least three, temperature sensors. A first temperature sensor can be arranged on the heating element or in the region of the heating element. The further temperature sensors may be spaced from the heating element. If the heating element is operated in such a way that a constant heat output is made available, a temperature distribution results in the area around the heating element whose shape depends on the gas parameters of the gas mixture.

The height of this temperature distribution, that is to say in particular the height of the temperature at the first temperature sensor arranged in the region of the heating element, is in this case a measure of the thermal conductivity. If the gas mixture has a high thermal conductivity, then a large amount of heat can be transported away from the heating element, so that the temperature is in the range of Heating element with increasing thermal conductivity decreases. The thermal conductivity can be detected for example by a transit time measurement. By the heating element, a pulse-like heating can take place. The respective time duration from the heating pulse to the detection of a temperature maximum resulting from the heating pulse at one or more of the temperature sensors spaced from the heating element can be detected and from this the temperature conductivity can be determined.

If the heating element is arranged in a flow channel of a flowmeter, and if temperature sensors spaced from the heating element are used upstream and downstream of the heating element, a flow velocity of the gas mixture can additionally be determined on account of the asymmetry of the temperature distribution.

In addition to the method according to the invention, the invention relates to a determination device for determining a result parameter describing a calorific value and / or a Wobbe index of a gas mixture, which is designed to carry out the method according to the invention. The detection device can be designed as a flow meter or integrated into such a flow meter. In this case, a volume flow of the gas mixture can be determined by the flow meter and additionally a calorific value and / or a Wobbe index. This can be used to determine an energy flow through this counter by multiplying the calorific value by the volume flow rate. Such determined energy consumption and / or the measured volume and / or the calorific values and / or the Wobbe index can be provided to a central device, which can be operated, for example, by a natural gas supplier. If a flow meter is used as the detection device, the detection of the volume as described above can take place according to a thermal measuring principle, but also mechanically, for example by means of a diaphragm gas meter.

• 1 ein Ablaufdiagramm eines Ausführungsbeispiels des erfindungsgemäßen Verfahrens, und
• 2 ein Ausführungsbeispiel einer erfindungsgemäßen Erfassungseinrichtung.

Further advantages and details of the invention will become apparent from the following embodiments and the accompanying drawings. Here are shown schematically:

• 1 a flowchart of an embodiment of the method according to the invention, and
• 2 An embodiment of a detection device according to the invention.

1 shows the use of a method for determining a calorific value and a Wobbe index of a gas mixture in the context of determining a power consumption of a combustion device and the control of this combustion device. The procedure is divided into two sections. In steps S1 to S3, a function is specified, which is used in the subsequent steps in the context of determining the result parameter. While steps S4 to S11 constitute a measurement and control loop used in the operation of the combustor, steps S1 to S3 may be independently performed at a previous time. By way of example, these steps can already be carried out prior to the production of a determination device which is used to determine the result parameter, and the resulting function can then be stored in each case in a storage device of the determination device in order to be used by the following steps.

The function determined in steps S1 to S3 serves to calculate a calorific value or a Wobbe index from parameter values of exactly two gas parameters, in the example shown, from parameter values for the gas parameters thermal conductivity and thermal conductivity. In the exemplary embodiment, both of these result parameters are to be calculated, so that two different functions are predetermined, each of which depends on the thermal conductivity and the thermal diffusivity. In alternative embodiments of the method, the function could depend on any two of the gas parameters of thermal conductivity, thermal conductivity, heat capacity, and density.

In step S1, first a structure for these functions is given. This can be done in such a way that initially a respective set of functions is predefined, which additionally depends on a plurality of further parameters in addition to the thermal conductivity and the thermal conductivity. These additional parameters are determined in the following procedure to specify the function. A corresponding set of functions can, for example, be specified in the form of a polynomial in the parameter values of the two gas parameters as variables, the coefficients being determined as further parameters in the further course of the process.

The further parameters, that is, for example, the coefficients of the polynomial, are determined as a function of a training data set, which is specified in step S2.

For several gases and / or gas mixtures, the training data set describes parameter values for the two gas parameters, ie in the example for the thermal conductivity and the thermal conductivity, and the result parameter, ie the calorific value and / or the Wobbe index. The training record may include this data for more than 100, in particular for several hundred gases and / or gas mixtures. A training dataset is used that includes the parameter values and the result parameter for both hydrogen and air and nitrogen, two gases that are not exothermic. In addition, the training data set describes the parameter values and result parameters for a large number of gas mixtures which correspond to available natural gases, ie gas mixtures which contain primarily hydrocarbons. The resulting function can thus be parameterized so that it can determine calorific values and Wobbe indices with high accuracy for natural gases, to which higher amounts of air, nitrogen and / or hydrogen can be added.

In step S3, the further parameters of the set of functions given in step S1, that is to say for example the coefficients of the respective polynomial, are determined as a function of the training data set. This can be done by fitting the function to the training data set, ie by calculating a compensation level as a function. For this purpose, a large number of approaches is known, which is why the corresponding procedure should be presented only roughly. For each gas or gas mixture in the training data set, an error value can be defined which indicates how much the result parameter determined by applying the function to the two gas parameters deviates from the result parameter specified by the training data set for a specific parameterization of the function. From these error values, a cost function can be composed, for example by calculating an Euclidean norm for these error values, and this cost function can be minimized to determine an optimal approximation area for the training data sets. The corresponding parameterization specifies the function to be used.

The determining function can then be stored on a storage device of a detection device, which is used in the following method steps. A corresponding detection device 1 is in 2 shown. The detection device 1 serves, first, the thermal conductivity and the thermal conductivity of a through the detection device 1 to determine flowing gas mixture. In addition, a volume flow through the detection device 1 certainly.

The detection device 1 includes a measuring section 2 , which is located inside a tube 3. Within the measuring section 2 is centrally a heating element 4 arranged, which has a control 5 can be controlled. The control 5 may be formed as a separate controller, but it may also be in a computing device 10 be integrated. The activation of the heating element 4 through the controller 5 typically occurs such that the heating power of the heating element 4 is constant. A corresponding detection device 1 can be realized especially easily if a heating element 4 is used with a substantially constant resistance in the relevant temperature range. In this case, the controller can 5 the heating element 4 Charge with constant voltage or constant current to achieve a constant heat output. However, it is also possible that the controller 5 regulates the heat output to a constant value.

Next to the heating element 4 are in the test section 2 three temperature sensors 6 . 7 and 8 arranged. The temperature sensor 7 is in the range of the heating element 4 arranged. The two temperature sensors 6 . 8th are upstream and downstream of the heating element, respectively 4 arranged. The arrows 9 indicate the flow direction of the gas. The measured values of the temperature sensors 6 . 7 and 8th are determined by the calculation device 10 added. From these measured values, as will be explained in more detail below, the thermal conductivity is determined in step S4, the thermal diffusivity in step S5, and the flow velocity of the gas mixture in step S6.

In step S4, the thermal conductivity of the gas mixture is determined. This exploits that the heating element 4 operated with constant heating power. Is the thermal conductivity of the through the measuring section 2 flowing gas, this means that a large amount of energy is removed from the heating element. Thus, the temperature drops at the heating element, if this is heated with the same heating power. A high temperature at the temperature sensor 7 thus corresponds to a low thermal conductivity, a low temperature of high thermal conductivity. The temperature value at the temperature sensor 7 can thus be used to determine the thermal conductivity. The determination can be made by using a value table with calibration values. It should be noted that the temperature at the temperature sensor 7 obviously in addition to the temperature of the measuring section 2 flowing gas mixture depends. Will the investigator 1 thus used in a gas stream in which the temperature of the incoming gas mixture can vary greatly, it is advantageous, the temperature of the incoming gas by another temperature sensor, not shown, which significantly further than the temperature sensors 6 . 8th from the heating element 4 is to be detected and taken into account in the above-described determination of the thermal conductivity.

The temperature conductivity can be extracted from a transit time measurement in step S5. In this case, for example, a short heat pulse is applied to the heating element and the time is measured until the temperature of the two temperature sensors 6 . 8th reached. It is advantageous here if both temperature sensors have mutually different distances from the heating element. If the temperature conductivity of the gas is high, the temperature reaches the two temperature sensors 6 . 8th in a less time than a gas mixture with lower thermal conductivity. The difference between the two transit times can be used to accurately determine the thermal conductivity. A value table with calibration values can be used to determine the thermal conductivity from the transit time or the transit time difference or a variable derived therefrom.

In step S6, the flow velocity is determined. A flow of the gas mixture through the measuring section 2 leads to an increase in the flow velocity asymmetry of the temperature distribution curve. The flow rate can thus be calculated from the temperature difference at the temperature sensors 6 . 8th be determined.

In step S7, the functions provided in step S3, each depending on the thermal conductivity and the thermal conductivity and calculating the calorific value and the Wobbe index, respectively, are applied to the thermal conductivity determined in step S4 and the thermal conductivity determined in step S5. Thus, after step S7, both the Wobbe index of the gas mixture and its calorific value are available.

The Wobbe index is further processed in step S8 to control a combustion device to which the gas mixture is supplied. In this case, for example, a gas pressure with which the gas mixture is introduced into a combustion chamber can be adjusted in order to react to a change in the supplied gas mixture. In this way, it can be prevented, in particular, that too high energies are provided in the combustion device when the Wobbe index rises, which can damage them. In addition, variations in the combustion process, for example temperature changes and / or changes to a provided mechanical energy, can be reduced by controlling the combustion device, in particular the gas pressure, as a function of the Wobbe index.

The calorific value determined in step S7 is used within the scope of calculating a quantity of energy provided. For this purpose, a volume flow is first calculated from the flow velocity of the gas mixture determined in step S6 in step S9. This volume flow is then multiplied by the calorific value in step S10 in order to obtain a provided amount of energy. In step S11, this determined amount of energy via a not separately shown communication device of the detection device 1 to an external device, which may for example be assigned to the utility, transmitted. Subsequently, the method is repeated from the detection of the parameter values of the two gas parameters and the flow velocity of the gas mixture in steps S4 to S6.

LIST OF REFERENCE NUMBERS

1

detector

2

measuring distance

3

tube

4

heating element

5

control

6

temperature sensor

7

temperature sensor

8th

temperature sensor

9

arrow

10

calculator

S1 - S11

step

Claims (10)

Method for determining a result parameter describing a calorific value and / or a Wobbe index of a gas mixture, comprising the steps carried out by a determination device (1): Determining parameter values of the gas mixture for exactly two gas parameters from the group heat conductivity, thermal conductivity, thermal capacity and density, Calculation of the result parameter as a result of a given function, which depends exclusively on the parameter values of exactly two gas parameters. Method according to Claim 1 , characterized in that the function is predetermined as a function of a training data set which describes parameter values for the two gas parameters and the result parameter for a plurality of gases and / or gas mixtures. Method according to Claim 2 , characterized in that the function is a polynomial in the parameter values of the two gas parameters as variables, wherein the coefficients of the polynomial are predefined in dependence on the training data set. Method according to Claim 2 or 3 characterized in that the training data set describes parameter values for the two gas parameters and the result parameter for at least one gas or gas mixture that is not exothermally combustible. Method according to one of the preceding claims, characterized in that the result parameter for a gas mixture is determined which comprises at least three different gases. Method according to one of the preceding claims, characterized in that the result parameter is determined for a gas mixture comprising hydrogen. Method according to one of the preceding claims, characterized in that the parameter values for the thermal conductivity and the temperature conductivity are detected, wherein the function depends exclusively on these parameter values. Method according to one of the preceding claims, characterized in that the parameter values for both gas parameters are recorded at the same temperature and / or pressure. Method according to one of the preceding claims, characterized in that the parameter values are detected by the sensor device by a thermal measuring principle. Determining device for determining a calorific value and / or a Wobbe index of a gas mixture descriptive result parameter, characterized in that it is designed to carry out the method according to one of the preceding claims.

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