Measuring and testing – Gas analysis
Reexamination Certificate
2000-04-27
2001-06-12
Williams, Hezron (Department: 2856)
Measuring and testing
Gas analysis
C073S023210, C073S023280, C073S030010, C374S036000, C702S030000, C702S136000
Reexamination Certificate
active
06244097
ABSTRACT:
The present invention relates to a non-combustive method for measuring the gross calorific value of fuel gas and to a use of the method for the non-combustive measurement and/or regulation of the amount of heat supplied to gas consumption devices, in particular to devices for consumption of natural gas.
The advantage of non-combustive methods for measuring the gross calorific value or measuring the amount of heat compared with calorimetric methods in which controlled combustion of a substream of the gas stream being transferred is carried out is that they are significantly cheaper. However, practical implementation is frequently complicated, quite apart from the difficulties which can occur in calibration.
Non-combustive methods of measurement include indirect and correlative methods. With the indirect methods, the gas composition is analysed. The composition of the gas together with the gross calorific values for the pure substances can then be used to determine the gross calorific value of the fuel gas. These methods (e.g. gas chromatography) give very accurate results, but the technology is complicated and therefore they are not very suitable for use in, for example, the residential sector. In addition, they are prone to faults.
With the correlative methods for measuring the gross calorific value or the amount of heat, a relationship between a readily measurable physical or chemical parameter and the gross calorific value is exploited. This makes them easier to perform from the technical point of view, but the reproducibility and the accuracy of the measurement of the gross calorific value or the amount of heat are restricted to an undesirable degree.
It is an object of the present invention to make the correlative non-combustive measurement of the gross calorific value or the measurement and/or regulation of the amount of heat supplied to consumption devices less complicated and, in particular, to provide a reliable and accurate measuring method.
This object is achieved according to the invention by the characterising features of claim
1
.
In practice there are different time-tested methods for measuring the density under reference or operating conditions. The speed of sound under reference or 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 ultrasonic flowmeters. The dielectric constant can be measured inexpensively and with high accuracy even under operating conditions.
Consequently, the two measurements required in each case can be carried out without complicated technology, reliably and accurately so that the combination of the measured values gives corresponding results for the gross calorific value. The gross 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 gross calorific value on a volume basis under reference or operating conditions, the specific gross calorific value (on a mass basis) or the molar gross calorific value.
There are a total of four variations of the method of the invention for measuring the gross calorific value of fuel gas. In the first variant, the density and the dielectric constant are measured under reference conditions and in the second variant the speed of sound and the dielectric constant are measured under reference conditions. These two variants have the advantage that the measuring apparatus working under reference conditions is cheap to buy and simple to maintain.
In the third variant, the density and the dielectric constant are measured under operating conditions and in the fourth variant the speed of sound and the dielectric constant are measured under operating conditions. The advantage of these last two variants is that no thermostats are required to establish the reference conditions. Finally, in the third and fourth variants no pressure reductions are required to establish normal reference conditions.
However, the measuring equipment to measure the dielectric constant and the density or speed of sound under operating conditions is somewhat more expensive to purchase than the measuring equipment for the first two variants. Furthermore, the evaluation of the parameters to determine the gross calorific value is more difficult in the last two variants of the inventive method for measuring the gross calorific value than in the first two variants.
To find a suitable correlation between the measured parameters and the gross 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 gross calorific value. The parameters required for the various variants of the method are then measured on the reference gas. In these reference cycles, a number of reference signal patterns corresponding to the number of measurement cycles determined from the ratio of the various signals measured is stored with assignment to the known gross calorific values. The signal pattern from a future measurement cycle on fuel gas of unknown gross calorific value is compared with reference signal patterns for the purposes of establishing a particular gross calorific value.
To increase the reference accuracy, a large number of reference cycles in which the various parameters are varied in succession over the expected measurement range should be carried out. An unambiguous and accurate assignment of a particular gross calorific value to a signal pattern of a fuel gas determined in a measurement cycle is achieved by interpolation of the various reference signal patterns.
A significant advantage is that the correlation between gross calorific value and measured parameters only has to be found once for a specific application by means of any desired number of reference cycles. The one-off work involved is comparatively small. The reference conditions should here be selected so as to correspond as closely as possible to the measurement conditions expected later. Thus, for all parameters only the measurement ranges which actually come into question should be determined with sufficient accuracy as reference signal patterns.
The dielectric constant and the density or the speed of sound are preferably measured under reference conditions in a common measuring environment. This makes only one temperature and pressure measurement and consequently only one thermostat necessary for setting and maintaining the reference conditions. Furthermore, uniform reference conditions for the measurement of the dielectric constant and the density increase the accuracy to which the amount of heat supplied can be determined.
One advantageous embodiment is characterised in that normal conditions are set as reference conditions for the measurement of the density or the speed of sound and/or the dielectric constant.
The dielectric constant can be measured particularly accurately at a reference pressure of at least 1 MPa.
The accuracy with which the gross calorific value is measured can be further increased by also measuring in step a) at least one of the parameters temperature, pressure or the proportion of at least one inert gas, preferably the proportion of carbon dioxide. Naturally, the highest measuring accuracy can be obtained if all three parameters are also measured.
A further embodiment of the present invention is characterised in that a substream of the fuel gas is taken off for the measurement of the density or the speed of sound under reference conditions and that the proportion of at least one inert gas, preferably the proportion of carbon dioxide, is measured on this substream, preferably after the density or the speed of sound have been measured.
The object of the invention is also achieved by the use of the novel non-combustive method of measuring and/or regulating the amount of heat supplied to gas consumption devices, in particular devices for consumption of natural gas, wherein, in step a),
Jaeschke Manfred
Kleinrahm Reiner
Rosmalen Renee Janssen-van
Schley Peter
Schouten Jan A.
Blakely & Sokoloff, Taylor & Zafman
Cygan Michael
N.V. Nederlandae Gasunie
Williams Hezron
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