Method and device for evaluating a fuel gas wobbe index

CA 02432895 2003-06-23 WO 02/052258 PCT / FRO1 / 04125 "Method and device for evaluating the wobbe index of a combustible gas" The invention relates, in general, to the field of measurement techniques the Wobbe index, this index being represented by the ratio of the calorific value of a combustible gas to the square root of the density of this gas.

More precisely, the invention, according to a first of its aspects, relates to a method for evaluating the Wobbe index of a fuel gas belonging to a family of gases defined by constituents of a determined chemical nature occurring in variable relative proportions. , this method comprising a measurement procedure during which a flow measurement of this combustible gas is carried out.

The Wobbe index constitutes the main characteristic quantity of a gaseous fuel and is therefore involved in the combustion settings of gas burners.

Thus, all other things being equal, the heat output of a burner is proportional to the Wobbe index, and its excess air depends directly on it.

As natural gas transport and distribution networks are increasingly networked and supplied by various energy sources, the Wobbe index of a given type of gas can vary in significant proportions, for example by + / - 5 ° s at a given point of a network.

However, certain industrial processes, in the glass and lime industries in particular, are sensitive to these variations to the point of requiring the implementation of specific combustion control solutions, one of these solutions consisting of integrating CA 02432895 2003 -06-23 WO 02/052258 PCT / FRO1 / 04125 2 the result of a local measurement of the Wobbe index in the regulation algorithms.

To date, all Wobbe index measuring devices available on the market are relatively complex, and therefore expensive.

Three principles are known to determine the Wobbe index.

The first principle consists in associating a measurement of calorific value, obtained by calorimetry or chromatography, with a measurement of density, obtained by densitometry or by chromatography.

The second principle consists in analyzing the combustion products of the gas concerned in a small furnace where a sample of this gas is burnt by storchiometry or in excess of air.

The third principle consists in measuring the physical characteristics of the gas, such as viscosity, heat capacity, etc., and in making a correlation between these measurements and the Wobbe index.

Two examples of implementation of this third principle are described in patent documents DE 41 18 781 and US Pat. No. 4,384,792.

Document DE-41 18 781 in fact describes a method aiming at other measurements in addition to that of the Wobbe index, using two correlation functions with four constants, using three flow measurements, one pressure measurement. differential and temperature measurement, and requiring methane calibration.

Document US Pat. No. 4,384,792 describes a method for measuring the Wobbe index, using a three-constant correlation function, using volume flow measurement, differential pressure measurement and temperature measurement, and requiring a pressure regulator and calibration using hydrogen gas.

These known techniques therefore exhibiting relative complexity, the aim of the invention is to provide a CA 02432895 2003-06-23 WO 02/052258 PCT / FRO1 / 04125 3 method for measuring the Wobbe index which is easier to implement. work and, correspondingly, a device that is significantly less complex and less expensive than the known devices.

To this end, the method of the invention, moreover in accordance with the generic definition given in the preamble above, is essentially characterized in that it comprises. an operation providing, as a measurement of the flow rate of the fuel gas, a measurement of a mass flow rate of this fuel gas in sonic flow through a fluidic restriction, such as an orifice or a micro-nozzle, carried out at an absolute pressure measurement and at an absolute measurement temperature;

a calibration procedure in which a measurement is made of a mass flow rate of a reference gas in sonic flow through the fluidic restriction, at an absolute reference pressure and at an absolute reference temperature; and an evaluation procedure during which the Wobbe index receives a value linked, by an empirical affine law previously established for the family of gases, to the product of three factors, the first factor representing the ratio of the flow rate measurement mass of the fuel gas to the measurement of the mass flow rate of the reference gas, the second factor representing the respective image ratio of the absolute reference and measurement pressures by a determined polynomial function, and the third factor representing the square root of the ratio of the absolute measurement and reference temperatures.

In the case where the fuel gas and the reference gas may have different pressures, the measurement procedure includes measurement of the measuring absolute pressure, and the calibration procedure includes measurement of the reference absolute pressure.

If the absolute measurement and reference pressures are maintained within a predetermined range, the polynomial function CA 02432895 2003-06-23 WO 02/052258 PCT / FRO1 / 04125 4 can be assimilated to the identity function, the image of a pressure (P) by the determined polynomial function thus taking the form.

F (P) - P.

Otherwise, the image of a pressure (P) by the determined polynomial function preferably takes the form.

F (P) - P - k. p ~ 1-r), where k and r are construction parameters of the fluidic restriction.

In the event that the fuel gas and the reference gas may have different temperatures, the measurement procedure includes measurement of the absolute temperature of measurement, and the calibration procedure includes measurement of the absolute reference temperature.

Under these conditions, the reference gas can be freely chosen from a set of gases comprising non-combustible gases and gas mixtures, such as air or nitrogen.

The method of the invention advantageously comprises a preliminary correlation procedure, during which the empirical affine law is established for at least two gases of the gas family.

For the evaluation of the Wobbe index of a gas of the natural gas family, the empirical affine law is defined by an ordinate at the origin equal to -18.40.

In this case, and if compressed air is used as the reference gas, the empirical affine law is defined by a slope equal to 19.40.

If, on the other hand, nitrogen is used as a reference gas, the empirical affine law is defined by a slope equal to 19.72.

If, still for the evaluation of the Wobbe index of a gas of the natural gas family, methane is used as reference gas, the empirical affine law is defined by a slope equal to 33.28.

CA 02432895 2003-06-23 WO 02/052258 PCT / FRO1 / 04125 The invention also relates to a device for implementing the method for evaluating the Wobbe index as described above, this device being.

characterized in that it comprises a pipe having an inlet and an outlet, inlet means for selectively guiding, to the inlet of the pipe, a flow of pressurized fuel gas or a flow of reference gas under pressure, a fluid restriction, such as an orifice or a micro-nozzle, having an inlet connected to the outlet of the pipe, and an outlet, and a. mass flowmeter connected to the output of the fluidic restriction and providing an output signal representative of the mass flow rate of the gas passing through this fluidic restriction in sonic flow.

In cases where the reference gas and the fuel gas can be used under different conditions of pressure and / or temperature, the device of the invention comprises an absolute pressure sensor and / or an absolute temperature sensor installed on the driving.

The previously mentioned admission means comprise for example a first main inlet for the fuel gas, a second main inlet for the reference gas, and first and second solenoid valves respectively interposed between the inlet of the pipe, and the first and second main entrances.

In this case, the device of the invention is advantageously equipped with a logic unit comprising at least three acquisition inputs respectively connected to the flowmeter, to the pressure sensor and to the temperature sensor, and two control outputs respectively connected to the solenoid valves. .

Other characteristics and advantages of the invention will emerge clearly from the description which is given below, by way of indication and in no way limiting, with reference to the single appended figure, CA 02432895 2003-06-23 WO 02/052258 PCT / FRO1 / 04125 6 constituted by the diagram of a device according to the invention.

To make the description of the invention more concrete, the present description will first of all refer to the appended figure, and to the device it represents.

This device comprises a pipe 1 whose inlet 11 is connected to a first main inlet 81 of the device by a first solenoid valve 21, and connected to a second main inlet 82 of the device by a second solenoid valve 22.

The first main inlet 81 of the device is permanently connected to a pressurized source of the combustible gas to be analyzed G1.

The second main inlet 82 of the device is permanently connected to a pressurized source of a reference gas G0, such as air, nitrogen or methane, among other possibilities.

The admission means that constitute the main inlets 81 and 82, in combination with the solenoid valves 21 and 22, each of which is controlled to be pass-through while the other is closed, therefore make it possible to convey at will, up to the 'inlet 11 of line 1, a flow of fuel gas G1 under pressure, or a flow of reference gas GO under pressure.

The device also comprises a fluidic restriction 3, the inlet 31 of which is connected to the outlet 12 of the pipe 1, and a mass flowmeter 4 connected to the outlet 32 ​​of the fluidic restriction 3.

The fluidic restriction 3, intended to be the seat of a sonic flow of the gas GO or G1 which crosses it, and to offer resistance to this flow, typically takes the form of an orifice or a micro-nozzle whose diameter is for example of the order of 0.4 millimeters.

The mass flowmeter 4, known per se and for example dimensioned for approximately 300 normal liters per hour, provides an output signal Qm representative of the CA 02432895 2003-06-23 WO 02/052258 PCT / FRO1 / 04125 7 mass flow rate of the gas GO or G1 which crosses the fluidic restriction 3 in sonic flow.

In the embodiment allowing the greatest measurement precision, the device of the invention also comprises an absolute pressure sensor 5 and an absolute temperature sensor 6, both installed on the pipe 1, that is to say arranged so as to provide respective measurement signals P and T, respectively representative of the pressure and the temperature prevailing in this pipe, the pressure P being typically less than 5 bar.

The control of the solenoid valves 21 and 22, and the use of the signals Qm, P and T, can be entrusted to a logic unit 7, which comprises three acquisition inputs 71, 72, and 73, respectively connected to the flowmeter 4, to the pressure sensor 5 and to the temperature sensor 6, and ~ two control outputs 74 and 75, respectively connected to the solenoid valves 21 and 22.

The method of the invention, which is implemented in this device, makes it possible to evaluate the Wobbe index of any combustible gas such as G1, provided that the family of gases to which this gas belongs is identified. , as defined by the main chemical constituents of this gas, even if they occur in variable proportions, and subject to having, on this family of gases, prior knowledge which will be specified later.

This method comprises the following operations.

First of all, any of the gases to be used, for example the fuel gas G1, is allowed to pass through the restriction 3 in sonic flow, and the signal Qml then delivered by the flowmeter 4 is taken into account as a flow measurement. mass of fuel gas G1 in restriction 3.

Correlatively, the signals P1 and Tl respectively delivered during the same time by the CAs 02432895 2003-06-23 WO 02/052258 PCT / FRO1 / 04125 8 sensors 5 and 6 are taken into account as respective measurements of absolute pressure of measurement and of absolute measurement temperature.

The second of the gases to be used, in this case the reference gas G0, is then authorized to cross the restriction 3 in sonic flow, and the signal Qm0 then delivered by the flowmeter 4 is taken into account as a measurement of the mass flow rate of the gas GO reference in restriction 3.

Correspondingly, the signals PO and TO respectively delivered during the same time by the sensors 5 and 6 are taken into account as respective measurements of absolute reference pressure and absolute reference temperature.

This measurement phase is followed by an evaluation phase during which at least three factors are evaluated, which will be denoted Za, Zb, and Zc.

The first factor Za is represented by the ratio Qml / Qm0 of the output signals of the flowmeter 4 for the gases G1 and G0, that is to say by the ratio of the measurement of the mass flow Qml of the fuel gas G1 to the measurement of the mass flow Qm0 of the reference gas G0.

The second factor Zb is represented by the ratio F (PO) / F (P1) of the image F (PO) of the absolute reference pressure PO by a determined polynomial function F, which will be specified later, in the image F (P1) of the absolute measurement pressure P1 by the polynomial function F.

In practice, if the pressures PO and P1 are close to each other, and for example if they only present between them a maximum relative difference of the order of 2 ° s at 3 ~, the polynomial function F can be assimilated to the identity function, that is to say that the ratio F (PO) / F (P1) is simply assimilated to the ratio PO / P1.

And the third factor Zc is represented by the square root (T1 / TO) ~ of the ratio T1 / TO of the CA 02432895 2003-06-23 WO 02/052258 PCT / FRO1 / 04125 9 absolute temperature of measurement T1 at the absolute temperature reference T0.

Under these conditions, the method of the invention attributes to the combustible gas G1, as Wobbe index, the value W defined by an empirical affine law of the shape.

W = A XY + B, in which the term Y represents the product Za x Zb x Zc of the three factors Za, Zb, and Zc, and in which the coefficient A, called "slope", and the coefficient B, called. "ordered at the origin", are established beforehand, for the gases of the gas family considered, in a way which will be specified later.

If the device includes means making it possible to make P1 equal to PO and T1 equal to T0, that is to say if (PO / P1) - 1 and (T1 / TO) ~ - 1, the factor Za can directly be assimilated to the term Y, as the above relations easily show and the fact that the factors Zb and Zc are then both equal to 1.

In the case where the pressures PO and P1 are on the other hand different from each other, and present for example between them a relative difference greater than 3 ~, the polynomial function F is no longer comparable to the identity function, c 'that is to say that the image F (P) of a pressure P by this function F is no longer assimilable to the pressure P itself, but must be corrected by a factor denoted CD and equal to 1 - k. P -r.

The factor noted CD in fact makes it possible to take into account the effect that the boundary layer has, due to the viscosity of the gas which passes through the restriction 3, on the flow characteristics of this gas, the parameters k and r intervening in the factor CD being parameters of construction of the fluidic restriction 3 which can either be supplied by the manufacturer of this restriction, or be determined by techniques well known to those skilled in the art.

CA 02432895 2003-06-23 WO 02/052258 PCT / FRO1 / 04125 In a particular embodiment of the invention, the CD factor for a given restriction could be assimilated to 1 - 0.881. P -4'9.

Whatever the particular values ​​of k 5 and r, the image of a pressure P by la-, polynomial function F therefore takes the form.

F (P) - P - k. P 1-r, and the second factor Zb, which is represented by the ratio F (PO) / F (P1), takes the form.

10 Zb = (PO - k. PO 1-r) / (P1 - k. P1 1-r).

In the case of the evaluation ~ of the Wobbe index of a gas of the natural gas family, the intercept B of the empirical affine lôi W = A x Y + B takes the value - 18 , 40.

If, moreover, for this same application, compressed air is used as reference gas G0, the slope A of the empirical affine law W = AXY + B takes the value 19.40, this law being therefore globally defined by the relation .

W = 19.40 XY - 18.40, that is to say again by.

W = 19.40 X Za X Zb X Zc - 18.40.

In the case of the evaluation of the Wobbe index of a gas of the natural gas family, and of the use of nitrogen as reference gas G0, the slope A of the empirical affine law W = A x Y + B takes the value 19.72, this law being thus globally defined by the relation.

W = 19.72 XY - 18.40, that is to say again by. .

W = 19.72 X Za X Zb x Zc - 18.40.

In the case of the evaluation of the Wobbe index of a gas of the natural gas family, and of the use of pure methane as reference gas G0, the slope A of the empirical affine law W - A x Y + B takes the value 33.28, this law being thus globally defined by the relation.

CA 02432895 2003-06-23 WO 02/052258 PCT / FRO1 / 04125 11 W = 33.28 XY - 18.40, that is to say again by.

W = 33.28 x Za x Zb x Zc - 18.40.

As shown in the preceding examples, the invention allows the free choice of the reference gas G0, so that it is possible to use, as reference gas, non-combustible and therefore inexpensive gases and gas mixtures, such as air or nitrogen.

In the case of the evaluation of the Wobbe index of a gas not belonging to the family of natural gases, and / or of the use, as reference gas G0, of a different gas of those for which the empirical affine law W = A x Y + B has been defined above, the method of the invention must include a preliminary correlation procedure, during which the empirical affine law W - A x Y + B is established for at least two gases G11 of the new family of combustible gases considered, and / or for at least one reference gas GO such as air, nitrogen or methane, or for another reference gas G00.

This preliminary correlation procedure can easily be implemented on the one hand by measuring the factors such as Zai, Zbi, and Zci, respectively obtained for different fuel gases Gli of the new family of gases with the same reference gas, on the other hand. on the other hand by measuring directly, by a technique different from that of the invention and for example by one of the known traditional techniques, the Wobbe indices such as wi of these same combustible gases, and finally by solving, in a manner well known per se, the system of first degree equations of the type Wi - A x Zai x Zbi x Zci + B to obtain the values ​​to be given to the parameters A and B, a priori unknown, which must be used in accordance with the process of the invention for this new family of gases.

From a physical point of view, the measurement of mass flow rate Qm for a gas such as GO or G1, carried out in CA 02432895 2003-06-23 WO 02/052258 PCT / FRO1 / 04125 12 the conditions set out previously with reference to the figure, is linked to a quantity called "normal mass flow" and denoted Q by the relation.

Qm = Q / C, in which C is a correction coefficient which depends on various physical properties of the real gas for which the measurement is carried out, and more precisely on its heat capacity, its viscosity, and its thermal conductivity.

For a gas of known composition, the correction coefficient C of this gas is linked to the various correction coefficients Cj of its constituents by the relation. ~.

(1 / C) - E (Xj / Cj), the various coefficients Cj being given in tables established by the manufacturers of mass flowmeters, and each Xj ~ representing the volume fraction of component j.

The normal mass flow Q itself satisfies the relation.

Q = kx CD x CR x P / (T xd) ~ in which.

k is a construction parameter of restriction 3, already mentioned, CD is a correction factor of the form 1 - k.

P -r, already mentioned, P is the absolute pressure of the gas considered, as measured by the sensor 5, T is the absolute temperature of the gas considered, as measured by the sensor 6, d is the density of the gas considered, and CR is the real gas coefficient of restriction 3, which depends, for the real gas which is the object of the measurements, only on the ratio y of the specific heat CP of this gas at constant pressure, at its specific heat Cv at constant volume, the coefficient CR taking the form.

- (Y) ~ X (2 / (Y + 1)) "Y + m ~ 2 X cY - l ~ CA 02432895 2003-06-23 WO 02/052258 PCT / FRO1 / 04125 13 the ratio y being typically l 'order of 0.67 for methane and natural gases, and 0.69 for air and nitrogen.

Under these conditions, the above relatiôns make it possible to show that.

Y = (Qml / Qm0) X (CDO / CD1) X (PO / P1) X (T1 / TO) ~, and that.

Y = (CO / C1) X (CR1 / CRO) X (d0 / dl) ~, where CO and C1 denote respectively the correction coefficient C for the reference gas GO and for the fuel gas G1, where CR1 and CRO denote respectively the real gas coefficient of the restriction 3 for the fuel gas G1 and for the reference gas G0, and where d0 and dl respectively denote the density of the reference gas GO and that of the fuel gas G1.

From a physical point of view, the process of the invention can therefore be analyzed as being based on the demonstration of an empirical affine law linking the Wobbe index W of each fuel gas to each of the above expressions of the term Y.