Gas Calorimeter and Wobbe Index Meter

This invention relates in general to fuel gas analysis and more particularly to an apparatus and method for measuring the Wobbe index of fuel gas, together with the calorific value of the fuel gas.

There are a number of situations in which it is required to make a determination of the Wobbe index of a fuel gas as an indication of the efficiency of that mixture for combustion purposes. The well known Wobbe Index has been defined as the amount of heat released by a burner of constant orifice, McGraw-Hill Dictionary of Science, 1979. It is also desirable in many of these same situations to measure the calorific value of the fuel gas for determination of appropriate billing amounts. The calorific value has been defined as the quantity of heat liberated in a complete combustion of a unit weight or unit volume of fuel, Dictionary of Scientific and Technical Terms, McGraw Hill Book Company, 1974. The Wobbe index is inversely related to the square root of the density of the fuel gas mixture and directly proportional to the calorific value. The measurement of both the Wobbe index and the calorific value of fuel gases is done in a number of conventional systems employing conventional sensors. However, in addition to the obvious requirements of accuracy and reliability of such measurements, speed of response is important, but difficult to achieve.

It is therefore a primary object of the present invention to provide for measurement of Wobbe index and of calorific value of a fuel gas in a system which provides for rapid response time for measuring variations in the fuel. If these measured values are employed in a feedback system to control a process or if the calorific value is being used to provide a billing basis, then it is apparent that this rapid response time renders the entire system more efficient.

Broadly speaking in the present invention a fuel gas is provided through an orifice at a regulated pressure to a mixing chamber. The mixing chamber also receives ambient air from each of two capillaries. One capillary is provided at a constant pressure, while the air pressure in the other is controlled by a feedback loop. The constant pressure source provides the major portion (perhaps 90% of the total) of the air to the mixer, while the air from the second source is a controlled variable. The fuel input and air inputs to the mixer are adjusted initially with a known fuel gas to provide a stoichiometric mixture. This mixture is stoichiometrically balanced, that is, the quantity of reactants to be reacted in a chemical reaction are balanced such that there will be no residual reactants after that chemical reaction. This mixture is provided to an oxygen sensor of the high temperature zirconium oxide type. The sensor burns the stoichiometric mixture at high temperature and provides an output indication of the residual oxygen. This signal is converted in a controller/transducer to a pressure value controlling the variable air pressure supplied to one of the capillaries coupled to the mixer chamber. If the input fuel gas in the mixer chamber varies from the initial calibrated value so that it is no longer stoichiometric, then the change in residual oxygen sensed varies the air pressure applied to the mixer chamber, until the original stoichiometric balance is reachieved. This variation in air pressure or of the signal from the residual oxygen sensor is directly proportional to the Wobbe index.

A second oxygen sensor, in this instance not of a combustion type, but rather one which will measure the total amount of oxygen in the stoichiometric mix, is placed in parallel with the residual oxygen sensor to provide an output indication of the calorific value of the continuing flow of fuel gas.

In the drawings:

FIG. 1 is an illustration in graphic form of the relationship between calorific value and stoichiometric air in the fuel mixture; and

FIG. 2 is an illustration in generally diagrammatic form of a measurement apparatus for determining the Wobbe index and the calorific value of a fuel gas mixture supply on an ongoing basis.

FIG. 1 is an illustration of the relationship between stoichiometric combustion oxygen and the calorific value for some specific fuels. As shown this relationship is basically linear.

FIG. 2 is a diagrammatic illustration of an apparatus for measuring the Wobbe index for a gaseous fuel sample and also for measuring the calorific value (the heating value) for that same gas sample. A sample of the fuel gas of interest is provided at inlet A and is coupled through a flashback arrestor 13 and a gas regulator 15 to orifice 11. A pressure gauge 17 in the line between the regulator and the orifice provides an indication of the input pressure to the orifice.

The output fuel gas from the orifice 11 is provided as one input to a mixing chamber 19. A second input to the mixing chamber 19 is supplied from air capillary 27. This capillary receives air at a substantially constant pressure through a line which originates at an air inlet B for ambient air, and is passed through air regulator 47 to a second stage air regulator 23. The pressure from air regulator 47 is indicated at gauge 45. The second stage regulator 23 may be adjusted to provide a flow of air through the capillary 27 to the mixer 19 so that this air supply, together with that supplied through capillary 29 provides a stoichiometric fuel/air mixture in the chamber 19. The output from the chamber 19 is supplied to a flowmeter 31 controlled by a valve to oxygen sensor 35. Oxygen sensor 35 is of the high temperature zirconium oxide type. The sensor burns the stoichiometric mix from the mixer 19 and senses the residual oxygen as a measure of any imbalance in either direction in the stoichiometric mix. Such sensors are characterized by a very fast response time (for example 90% of a step change is realized in less than one second) and can measure oxygen reliably and with high accuracy from proportions as small as a 0.10 ppm up to 100%. One commercially available oxygen sensor suitable for this purpose is a Series 350 Zirconia Oxygen Analyzer available from Panametrics, Inc. in Waltham, Mass., which unit provides an output electrical signal indicative of the oxygen excess in air/fuel mixtures.

The output signal from the oxygen sensor 35 is coupled to process controller 37, which in turn provides an output current signal, varying in response to variations in the sensed residual oxygen, to current/pressure converter 39. Such process controllers are well known in the art, suitable controllers are manufactured under the designation 300 series by Love Controller Corporation of Wheeling, Ill. The current to pressure converter 39, which is conventional in the art, receives a current signal from the process controller 37 and is supplied pressure from the pressure inlet B through the primary regulator 47 and secondary pressure regulator 43, the pressure from regulator 43 being indicated on gauge 41. The process controller 37 and the converter 39 together form a controller means for controlling the process of the first source of air in response to the residual oxygen measurement at sensor 35. The output pressure from converter 39 is then controlled to vary with variations in the input current from processor 37. This output pressure is provided through capillary 29 as an input to the mixer 19. Thus, the feedback loop formed by the elements from residual oxygen sensor 35 through the controller, converter and capillary, acts to maintain the mixture in the air fuel mixer 19 as a stoichiometric mixture by varying the air input in response to variations in the residual oxygen.

In the apparatus of FIG. 2 the flow through the gas orifice 11 can be expressed as Q.sub.g =K.sub.g .sqroot.P/d where K.sub.g is an appropriate proportionality constant, P is the pressure of the fuel gas as indicated by the gauge 17 and d is the density of fuel. The flow through capillary 27, is ##EQU1## where K.sub.a is a suitable proportionality constant, P is the pressure sensed from regulator 23 as indicated by the gauge 25, and .eta. is the viscosity of the air.

As discussed above the air pressure supplied through capillary 29 is variable, being controlled through the feedback loop, but at a typical value for the relative flow of air from capillary 29 and that from capillary 27 the air from capillary 29, Qw, equals 0.1 Q.sub.a at a nominal 15 PSIG.

The Wobbe index is indicated either by the pressure value at the output of converter 39, or the current signal from process controller 37 at the input to converter 39.

The apparatus of FIG. 2 is shown with a second oxygen sensor 21 to which the stoichiometric mix is provided from mixer 19 through flow meter 38. The oxygen sensor 21 measures the total oxygen in the stoichiometric mixture. While there are many acceptable oxygen sensors to perform this function, a suitable one is commercially available from Panametrics, Inc. of Waltham, Mass. under the designation TM02D Oxygen Analyzer. The output signal from this second sensor 21 is a measure of the calorific value of the gaseous fuel.

A typical method of operation for this particular apparatus involve an initial calibration step in which fuel of a known composition is provided from fuel inlet A and the air supplied through capillaries 27 and 29 is adjusted so that a specific value of residual oxygen is detected in sensor 35, corresponding to a stoichiometric mix in chamber 19. The regulators 23 and 43 may be adjusted to provide for a desirable ratio between them for this initial setting.

Other specific apparatus may be employed to implement the method described herein for determining the Wobbe index and calorific value of a gaseous fuel. If, at the fuel input, the orifice 11 and regulator 15 are replaced with a suitable means for producing a constant flow, such as a volumetric pump, the air pressure from the converter 39 or the signal from the sensor/analyzer 35 represents directly the calorific value of the fuel, rather than the Wobbe index.

The invention having been described and specific embodiments having been set forth other implementations will be realized by those skilled in the art.