Measuring and testing – Gas analysis – By thermal property
Reexamination Certificate
1999-05-20
2001-08-28
Williams, Hezron (Department: 2856)
Measuring and testing
Gas analysis
By thermal property
C073S023310, C073S597000
Reexamination Certificate
active
06279380
ABSTRACT:
The invention relates to a noncombustive method of measuring the calorific value of fuel gas and to a use of the method for the noncombustive measurement and/or regulation of the amount of heat supplied to gas consumption devices, in particular to devices for consumption of natural gas. The noncombustive methods of measuring the calorific value or measuring the amount of heat have, compared with calorimetric methods in which controlled combustion of a substream of the gas stream being transferred is carried out, the advantage that they are significantly cheaper. However, practical implementation is frequently complicated, even without taking account of the difficulties which can occur in calibration.
Noncombustive methods of measurement include indirect and correlative methods. In the indirect methods, the gas composition is analysed. The composition of the gas together with the calorific values for the pure substances can then be used to determine the calorific value of the fuel gas. These methods (e.g. gas chromatography) give very accurate results, but are technically complicated and are therefore not very suitable for use in, for example, domestic premises. In addition, they are susceptible to faults. In the correlative methods of measuring the calorific value or the amount of heat, a relationship between a readily measurable physical or chemical parameter and the calorific value is exploited. This makes them technically easier to carry out, but the reproducibility and the accuracy of the measurement of the calorific value or of the amount of heat are restricted to an undesirable degree.
It is an object of the invention to further reduce the outlay in the correlative noncombustive measurement of the calorific value or the measurement and/or regulation of the amount of heat supplied to consuming devices and, in particular, to provide a reliable and accurate measurement method. This object is achieved according to the invention by, in the method mentioned at the outset,
a) determining the speed of sound in the gas under first and under second reference conditions and measuring one of the parameters-dielectric constant, speed of sound under third reference conditions or proportion of carbon dioxide in the fuel gas and
b) deriving the calorific value from these three parameters.
Particularly accurate results can be achieved for fuel gases whose calorific value at STP is from 20 to 48 Mj/m3, whose relative density compared with dry air is from 0.55 to 0.9, whose proportion of carbon dioxide is less than or equal to 0.3 and whose proportion of hydrogen and carbon monoxide is less than 0.1 and 0.03 respectively. Particularly suitable measurement conditions are temperatures in the range from 225 to 350 K and pressures of less than or equal to 60 MPa.
The first reference conditions set are preferably STP (273.15 K and 1 bar) or a pressure of from 1 to 10 bar, more preferably from 3 to 7 bar. Although the temperature is not very critical and can be selected within a wide range, for technical reasons the temperature is above 225 K, for example from 270 K to 295 K. For the second reference conditions, a pressure of above 30 bar is preferably set. Although the temperature is not very critical and can be selected within a wide range, for technical reasons the temperature is from 225 K to 350 K. Greatest preference is given to the operating conditions for this parameter. The preferred values for the third reference conditions are a pressure of over 150 bar, more preferably over 175 bar, for example 200 bar. When using a third reference condition, the pressure of the second reference condition should preferably be set below 70 bar.
The speed of sound at the reference conditions mentioned, including operating conditions, can be determined in a separate measuring unit, for example via the resonant frequency of pipes or of hollow bodies or a distance travelled-time measurement, e.g. in ultrasound meters. The dielectric constant can be measured inexpensively and with high accuracy even under operating conditions. The proportion of carbon dioxide is simple to determine under all conditions mentioned using known measuring instruments, e.g. by measurement of the light absorption in the infrared region.
Consequently, the three measurements required in each case can be carried out without great technical outlay, reliably and accurately so that the combination of the measured values gives corresponding results for the calorific value. The calorific value determined in this way can be used, for example, for controlling combustion processes.
Depending on the application, it is possible to determine the calorific value on a volume basis under reference or operating conditions, the specific calorific value (on a mass basis) or the molar calorific value. There are a total of three variations of the method of the invention for measuring the calorific value of fuel gas.
In all three variants, the speed of sound is determined under first and under second reference conditions.
In the first variant, the speed of sound is additionally measured under third reference conditions. The determination of two and, in this first variant, even three speeds of sound has the advantage that all measurements can be carried out in one measuring apparatus. The pressure in the apparatus can be varied by compressing the measurement volume or allowing it to expand. On compression or expansion, the temperature of the fuel gas also changes, which can make the setting of altered reference conditions easier. If desired, the measuring apparatus for determining the speeds of sound can be provided with additional means of setting an altered temperature.
In the second variant, not only are the two speeds of sound measured but the dielectric constant is also measured, preferably at a pressure of at least 10 bar to achieve high accuracy, e.g. under second reference conditions, e.g. operating conditions.
In the third variant, the proportion of carbon dioxide in the fuel gas is determined in addition to the two speeds of sound.
The determination of the dielectric constant and the proportion of carbon dioxide can be carried out in the same measuring environment as that in which the speeds of sound are determined. This makes a very compact measuring apparatus possible.
Advantageously, the speed of sound is measured under second reference conditions and the dielectric constant or the proportion of carbon dioxide are measured under the same reference conditions, preferably under operating conditions, in a joint measuring environment. This makes only one temperature and pressure measurement and consequently only one thermostat necessary for setting and maintaining the reference conditions. In addition, uniform reference conditions for the different measurements increase the accuracy to which the amount of heat supplied can be determined.
The determination of at least two speeds of sound offers the further advantage that it is not necessary to determine the density of the fuel gas. Density measurements are, particularly under operating conditions, expensive and complicated. In the method according to the invention, preference is given to not carrying out any additional density measurement, particularly when the proportion of carbon dioxide is determined as a third parameter.
To establish the reference conditions, the parameters temperature and pressure are required. These can be additionally measured in step a). If a lower measurement accuracy is permissible, the values estimates from practice can also be used for these parameters. The measurement accuracy can be further increased by additionally determining the proportion of nitrogen in step a).
To find a suitable correlation between the measured parameters and the calorific value, it is advantageous to precede the respective steps a) and b) at least once by a plurality of measurement cycles in which step a) is carried out using a plurality of reference gases of known calorific value. The parameters required for the various variants of the method are then measured on the reference gas. In thes
Jaeschke Manfred
Schley Peter
Schouten Johannes A.
Van Wesenbeeck Petrus J. M. M.
N. V. Nederlandese Gasunie
Pillsbury & Winthrop LLP
Wiggins David J.
Williams Hezron
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