The invention concerns a device for direct measurement of the calorific energy contained in a combustible gas transported in a pipe comprising a gas meter to measure the volume V of gas under the pressure P and temperature T conditions of the gas circulating in said pipe and an apparatus for determination of the calorific value H of the gas.
Processes exist for the measurement of the energy contained in a combustible gas, such as natural gas for example, transported in a pipe.
To this end, an apparatus such as a chromatograph or a calorimeter is generally placed at a given point in the pipe and used to determine the calorific value H of the gas from a gas sample taken in said pipe and adjusted to normal pressure P0 and temperature T0 conditions.
The calorific value H is therefore determined under these conditions.
A gas meter placed at another point in the pipe, generally downstream to the apparatus, measures a volume V of gas under the pressure P and temperature T conditions of the gas circulating in said pipe at the point where measurement of volume takes place.
Moreover, an apparatus for correction of pressure and temperature and, in some cases, of the compressibility coefficient, is attached to the gas meter and transforms the gas volume V measured under pressure P and temperature T conditions to a volume V0 adjusted to normal pressure P0 and temperature T0 conditions.
A pressure sensor and a temperature sensor are required to carry out this correction for the volume V.
The calorific energy contained in a gas is therefore equivalent to the multiplication H.V0.
However, this technique for measuring the calorific energy of a gas has a number of disadvantages as it requires the use of a pressure and temperature correction apparatus, as well as pressure and temperature sensors, in addition to the apparatus for measurement of calorific value (chromatograph, calorimeter . . . ) which can be costly.
Consequently, it would be useful to be able to measure the calorific energy of a combustible gas using a method that is simpler than previous methods.
The object of this invention is therefore a device for direct measurement of the calorific energy contained in a combustible gas transported in a pipe comprising a gas meter measuring the volume V of gas under pressure P and temperature T conditions of the gas circulating in said pipe and an apparatus for determination of the calorific value H of the gas, characterised in that said apparatus for determination of the calorific value measures at least one physical quantity proportional to the number of molecules of the various constituents of the gas in a given volume and is placed as close to the gas meter as possible in order to determine the calorific value H(P,T) under the same pressure P and temperature T conditions as those in which the volume V(P,T) of the gas is measured, said device then being used to determine the calorific energy H(P,T) V(P,T) contained in the gas.
The device according to the invention does not require the use of a pressure and temperature correction apparatus and therefore measurement of pressure and temperature is no longer necessary in order to determine the calorific value of a gas.
This is possible, on the one hand, since determination of calorific value is carried out at a point where the volume of gas measured under the same temperature and pressure conditions and, oil the other hand, by measuring a physical quantity proportional to the various constituents of the gas in a given volume.
Measurement of a physical quantity directly gives the number of molecules present in a given volume at a given pressure and temperature.
If pressure or temperature vary, the number of molecules in a given volume varies according to the formula n=PV/ZRT where Z is the compressibility coefficient of the gas and R is the Boltzmann constant, and the physical quantity also varies in the same proportion.
Consequently, this physical quantity measures the number of molecules of the various constituents of a gas independently of pressure and temperature.
Advantageously, the physical quantity measured is the absorbance of electromagnetic radiation by at least one combustible constituent present in a large proportion in the gas and for at least one wavelength of said radiation.
The apparatus then deduces the calorific value of this absorbance measurement.
Moreover, the choice of this particular physical quantity is very useful since it does not require contact with the gas.
The choice of radiation, in other words a range of wavelengths, which neutral gases (N2, O2, CO2) do not absorb is especially favourable since such constituents do not contribute to the calorific value of a gas in any way.
For example, the constituents N2 and O2 do not absorb in the infrared range and the constituent CO2 does not absorb in one area of the infrared range.
It is therefore highly advantageous to use this physical quantity since it makes it possible to be concerned only with the constituents which contribute to the calorific value of a gas which is far simpler than having several different quantities to measure.
Depending on the composition of the gas and the required degree of accuracy for the calorific value, it may be sufficient to concerned only with the combustible constituent present in the highest proportion in the gas, for example methane or ethane or propane or butane or pentane.
To increase the accuracy of calorific value measurements, the apparatus used to determine the calorific value of the gas measures absorbance of electromagnetic radiation by other combustible constituents present in the gas.
Thus, for natural gas, in addition to measurement of the combustible constituent present in the largest quantity in the gas, for example methane, it is possible to measure, the absorbance of one or more other combustible constituents present in small quantities chosen from among ethane, propane, butane and pentane.
The various radiation wavelength(s) can be chosen such that the contribution made by a single combustible constituent of the gas or by several of these corresponds to each wavelength.
More particularly, the apparatus for determination of the calorific value of a gas comprises, over at least one area of flow of the gas:
at least one source of emission of electromagnetic radiation across said area of flow of gas,
means for filtering said radiation,
means for detecting said radiation attenuated by absorption due to the combustible constituent(s) of the gas for the corresponding wavelength(s), producing an electrical signal representative of this radiation for each wavelength in question, and
electronic means for deducting the calorific value of the gas as well as the energy H(P,T) V(P,T) contained in the gas.
The means for filtering radiation can include one or more interferential filters, each of which being adapted to a different radiation wavelength for which at least one of said gas constituents shows absorption.
According to another possibility, the means for filtering radiation can include a filter that can be electrically tuneable on a wavelength range including at least one wavelength for which said gas constituent(s) shows absorption.
Electromagnetic radiation is, for example, situated in the infrared.
The range of wavelengths includes a wavelength(s) for which said gas constituent(s) shows absorption, for example between 1 and 12 xcexcm.
When the principal constituent of the gas is methane, it is for example possible to concentrate on a wavelength range of 1.6 to 1.3 xcexcm.
The invention also concerns a process for measurement of the calorific energy contained in a combustible gas transported in a pipe consisting in measuring a volume V of gas under pressure P and temperature T conditions of the gas circulating in said pipe and determining the calorific value H of the gas, characterised in that said process consists in measuring, under pressure P and temperature T conditions that are those under which the volume V(P,T) of gas is measured, at least one physical quantity proportional to the number of molecules of the various constituents of the gas in a given volume, and directly deducing the calorific value H(P,T) from this measurement under the same pressure P and temperature T conditions and then determining the calorific energy H(P,T) V(P,T) contained in the gas.
More particularly, the process according to the invention consists in measuring a physical quantity, which is the absorbance of electromagnetic radiation by at least one combustible constituent present in a large proportion in the gas for different wavelengths of said radiation.
The process according to the invention can also consist in measuring the absorbance of electromagnetic radiation by other combustible constituents present in the gas at different wavelengths of said radiation.
The process according to the invention consists in selecting the radiation wavelength(s) such that each wavelength corresponds to the contribution made by a single combustible constituent of the gas or by several of these.
More precisely, the process according to the invention consists in
emitting electromagnetic radiation across at least one area of flow of gas,
filtering said radiation,
detecting the radiation attenuated by absorption due to the combustible constituent(s) of the gas for the corresponding wavelength(s), and producing an electrical signal representative of this radiation for each wavelength in question, and
determining, from the signal or signals, the calorific value of the gas and the calorific energy H(P,T) V(P,T) contained in the gas.
The electromagnetic radiation is chosen such that the neutral gases (N2, O2, CO2) do not absorb this radiation.
The radiation is, for example, chosen in the infrared range.