Measuring energy in fuel gases

Abstract: A device (40) for measuring energy in fuel gas comprises a solid electrolyte oxygen pump (10) for generating an amount of oxygen in response to current being supplied to the pump (10). A sample of the gas is mixed with the oxygen from the pump (10) to produce a mixture which is subjected to complete combustion in a catalytic combuster (12). An oxygen sensor (14) is connected to the catalytic combuster (12) for measuring excess oxygen in the exhaust gas from the combuster (12). A signal from the oxygen sensor (14) is compared in a feedback control unit (22) to a known and desired amount of excess oxygen in the exhaust gas. If there is too much or too little oxygen in the exhaust gas, a feedback signals used to adjust the value of current being supplied to the oxygen pump (10) until the actual amount of excess oxygen is equal to the selected amount of excess oxygen. The selected amount of excess oxygen is indicative of complete combustion of the sample mixture. With the excess oxygen equalling the desired oxygen in the exhaust gas, the value of current being supplied to the pump (10) is used to calculate the energy in the fuel sample.

Description: This invention relates to methods of and devices for measuring energy in fuel gases. Energy flow is an important parameter when a fuel gas such as natural gas is being sold or purchased. The customer is interested in how much heat energy he can obtain from the gas he is purchasing. The seller typically has a contractual agreement to supply a known minimum energy content with each unit volume of gas. In order to meet this content the producer typically provides a higher value to compensate for errors in the measurement system. Several parameters or variables must be measured in the present state of measurement technology. Each measurement has its own error contribution. Energy flow is currently not measured directly but is computed by taking an off-line sample to a calorimeter or gas chromatograph for a specific energy measurement. This is used in conjunction with volume flow, temperature, pressure and other measurements to calculate an energy flow. This is an involved process, requiring large, expensive instrumentation. This implementation utilises a tapped fraction of the total flow, which fraction is introduced into a combustion system along with a controlled amount of air. A stoichiometric combustion sensor monitors the output. Its output is used by an element which adjusts the air delivery system. The combustion air flow is measured and the energy flow is calculated from the air flow and the calibration of the split-off fraction or ratio. (Griffis, C.H., et al. "Development of an Accurate Energy Flowmeter", Gas Quality , edited by van Rossum, G.J., Elsevier Science Publishers, Amsterdam, April 22-25, 1986, pages 121-127.) On pages 77 and 78 of Heyne, L., "Some Properties and Applications of Zirconia-based Solid-Electrolyte Cells", Proc Interdisp Symposium , 1974, pages 65-88, the use of a combination of an oxygen pump and sensor as an analyser for combustible gases is discussed. This depends on combustion at the pump cell. The oxygen introduced is regulated so that the amount is just what is needed for complete combustion of the combustible gases present. The current or charge used to pump the oxygen is thus a measure of the combustible gases. An apparatus is described in Vizethum, F., Bauer, G., and Tomandl, G., "Computer-Control of Oxygen Partial Pressure", Science and Technology of Zirconia II (Proceedings of the Second International Conference) pages 631 to 635, in which oxygen partial pressure is controlled by a zirconia solid electrolyte cell and an oxygen sensor of the same material. This partial pressure control is used in a gas titration system. Haaland, D.M., "Internal-Reference Solid-Electrolyte Oxygen Sensor", Analytical Chemistry , Vol. 49, No. 12, October 1977, pages 1813-1817, discusses an electrochemical pumping oxygen sensor with a monitor section and a pumping section. This type essentially uses a small leak or sample from the gas being measured and a pump to evacuate all the oxygen that enters the cell. A measure of the pumping current gives a measure of the oxygen that leaks in and thus a measure of the oxygen in the stream being measured. This type of sensor was developed for use in automotive applications. US Patent US-A-4 841 934 discusses using an oxygen pumping device in conjunction with an oxygen sensor to control the air-fuel ratio of an engine. This uses a combination of an oxygen pumping device and an oxygen sensing device, a double ZrO₂ configuration. This essentially modifies the oxygen concentration operating region of the sensor to improve the signal characteristics of the sensor at its desired operating point. This technique accordingly provides the high sensitivity available at stoichiometry only for other mixtures or concentrations. Solid oxygen sources are available. These may be metal oxides where the oxygen is liberated upon heating the material. Speidel, R., and Weidlich, E-R, "A solid state oxygen source for UHV", Vacuum , No. 2, 1988, pages 89 to 92, discusses a device for employing CuO as the solid source. Here a constant partial pressure of oxygen is generated by applying heat to the material, causing the material to decompose. The partial pressure is a function of the heat applied to the material. This action thus liberates a controlled, small amount of oxygen. This therefore provides a self-contained source for the oxygen pump to use as it delivers a controlled, known amount of oxygen to the flow stream inside the energy sensor arrangement. An on-line, real time single sensor method would be preferable over the one that uses a number of measurements with one or more of the measurements being made off line. The indirect method described above requires that many parameters be measured. Each measurement has an error associated with it which contributes to the error level of the overall measurement system (Wilde, D., and Arcara, S., "Modern Energy Flow Measurements", Advances in Instrumentation, Proceedings of the ISA International Conference and Exhibit , 1984, Oct. 22-25, pages 1345-1349). According to one aspect of the invention there is provided a method of measuring the energy in a fuel gas, the method comprising: (a) taking a real-time sample from a total flow of the gas, the sample being a known fractional amount of the total flow and having the same temperature and pressure as the total flow; (b) generating an actual amount of combustion oxygen using a solid-electrolyte oxygen pump which is powered by a current that is proportional to the actual amount of oxygen; (c) mixing the sample with the actual amount of combustion oxygen to form a mixture; (d) supplying the mixture to a catalytic combuster for complete combustion of the sample to form an exhaust gas having an actual amount of exhaust oxygen therein; (e) measuring the actual amount of exhaust oxygen in the exhaust gas; and (f) if the actual amount of exhaust oxygen is approximately equal to a selected amount of exhaust oxygen which is indicative of complete combustion in the catalytic combuster, using the value of current powering the oxygen pump to calculate the energy in the gas, and if the actual amount of exhaust oxygen is not approximately equal to the selected amount of exhaust oxygen, changing the current in step (b) and repeating steps (c) to (e) until the actual amount of exhaust oxygen is approximately equal to the selected amount of the exhaust oxygen. According to another aspect of the invention there is provided a device for measuring the energy in a fuel gas, the device comprising: a solid-electrolyte oxygen pump for receiving a sample of the gas and for generating an actual amount of combustion oxygen when the pump is powered by a value of current which is proportional to the actual amount of combustion oxygen; a catalytic combuster connected to the pump for receiving a mixture of the sample plus the actual amount of combustion oxygen for combustion of the sample to form an exhaust gas having an actual amount of exhaust oxygen therein; an oxygen sensor connected to the catalytic combuster for sensing the actual amount of exhaust oxygen in the exhaust gas; and a feedback control unit connected between the oxygen sensor and the pump for sensing the actual amount of exhaust oxygen in the exhaust gas and for generating the value of current based on the actual amount of exhaust oxygen sensed, the control unit being operable to compare the actual amount of exhaust oxygen to a selected amount of exhaust oxygen which is indicative of complete combustion in the combuster and to adjust the value of current so that the actual amount of exhaust oxygen approximately equals the selected amount of exhaust oxygen, the control unit including calculating means for calculating the amount of energy in the fuel gas as a function of the actual value of current being supplied to the pump. A preferred embodiment of the invention, described more fully below, does not use the large and expensive equipment required in the above-mentioned references, but rather involves a more direct measurement technique. The actual flow of energy in a controlled fraction of the total flow is measured, as contrasted to measuring several flow parameters, such as volume flow, pressure and temperature, and calculating the energy flow as in the previously proposed techniques. The operation in a single element mode such as a fuel cell, results in electrode problems caused by the complete depletion of oxygen at the surface of the electrolyte (Logothetis, E.M., Vassell, W.C., Hetrick, R.E., and Kaiser, W.J., "A High-Sensitivity Sensor for the Measurement of Combustible Gas Mixtures", Transducers '85: 1985 International Conference on Solid State Sensors and Actuators, IEEE, 1985, pages 330-332). Also fuel cells operate at less than 100% efficiency. Using them requires multiple stages in order to reach a point where all of the combustible material is burned and measured. One implementation utilises a two-element configuration with a pump and a sensor (see Heyne, at pages 77 and 78). This implementation adds a catalytic converter to the system to ensure that all of the material is combusted. This ensures a high accuracy measurement of the gas being combusted by eliminating the error term associated with incomplete combustion. When operated at the stoichiometric point, a zirconia sensor has a high accuracy and reliability in practical use (Takeuchi, Takashi, "Oxygen Sensors", Sensors and Actuators , Vol. 14, 1988, pages 109-124). There is some combustion in the oxygen pump region of the device. However, the completeness of combustion in the pumping area or in the fuel cell operation is not 100% a priori. Use of a catalytic converter section assures 100% combustion and the following oxygen sensor is operated in a highly accurate region. This configuration combines to provide a high accuracy in the operation. Thus, as embodied by the present invention, a continuous, real-time portion or sample of the total flow of combustion gas is passed through the sensing system. This sample has a predetermined, known ratio to the total flow and has the same temperature, pressure and other parameters as the flow being measured. The sample is mixed with a controlled amount of oxygen and introduced into a catalytic combustion device so that complete combustion occurs. The output exhaust gas of the combustion stage or device is measured to determine if the oxygen-gas mixture supplied to the combustion stage was as desired, that is either stoichiometric or with a preset value of excess oxygen, dependent on the mode of operation of the system. If the mixture was not as desired, the oxygen introduced into the sample flow is changed so that the desired mixture is obtained. The oxygen is introduced into the sample flow using a solid-electrolyte, typically zirconia, oxygen pump. The oxygen being pumped is directly related to the current passed through the cell. This is known as the Faraday effect, namely that in an electrolyte, the quantity of electricity that flows is directly related to the quantity of electronic charge on the ions entering into the reaction at the electrodes ( Handbook of Physics , Second Edition, edited by Condon, E.U., and Odishaw, H., McGraw-Hill, New York, 1967, pages 4-147 to 4-148). The current in the electrochemical pump is thus directly related to the oxygen being pumped plus a small leakage term. Faraday efficiency (one oxygen ion transported per two electrons of current flow) is very close to unity, when the pumping cell and the measuring cell do not share common electrodes (Logothetis, E.M., and Hetrick, R.E., "High-Temperature Oxygen sensors based on Oxygen Pumping", Chapter 8 In Fundamentals and Applications of Chemical Sensors , the American Chemical Society, 1986, pages 136-154). The separate electrodes eliminate the effect of polarisation, at the electrodes and at grain boundaries, on the characteristics of the cell. The current in the cell flows primarily by the transport of the oxygen ions in the electrolyte. There is some leakage or resistive current flow but this is very low. A controlled amount of oxygen is supplied to attain complete combustion. The energy content of the gas so combusted is directly related to the quantity of oxygen required for combustion. The oxygen required varies linearly with flow rate and with calorific content of the gas. The value of current through the oxygen pump is directly related to the oxygen pumped. Thus the current is directly related to the heat or energy flow through the device. In instances where the flow through the device is a constant sub-multiple of the flow in a large conduit, as in a gas pipeline, the output of the sensor gives a measure of the energy flow in the large conduit. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a graph plotting the relationship between oxygen required for a stoichiometric combustion of combustible compounds commonly found in natural gas, and energy or heat content of the combustion gases; Figure 2 is an exploded schematic view of a device according to an embodiment of the present invention; Figure 3 is a block diagram showing the electronic components in an electronic control circuit which can be used in the device of Figure 2; Figure 4 shows a device embodying the present invention utilised in-situ for directly measuring energy available in a flow of fuel gas; Figure 5 illustrates a device embodying the present invention which is positioned outside the main flow of fuel gas; and Figure 6 is a schematic illustration of another arrangement for utilising a device embodying the present invention. Referring to Figure 1 of the drawings, there is shown a plot of the heat or energy content of natural gas plotted against the proportion of oxygen required to combust completely its constituents ( Steam , 38th edition, by the Babcock and Wilcox Company, New York, 1975, page 6-2) at a temperature of 16°C (60°F) and a pressure of 10⁵Pa (30 ins of mercury, wet). It will be apparent that the oxygen required to combust completely a given volume of the majority of mixtures of natural gas is related to the energy content of that same volume of the gas. This approach is very sensitive to flow and energy and insensitive to variations in temperature, gas density, and moisture. Figure 2 functionally displays an implementation of this approach. Although Figure 2 shows a cylindrical geometry for instructional purposes, the actual geometries would include a long narrow cylindrical section, honeycombed cross sections, sections with parallel sides and very small separations, and similar, all of which serve to maximise the area presented to the flow in order to maximise the efficiency of the cell. The factor of importance is that sufficient oxygen should be introduced into the gas flow to provide for complete combustion. The quantity of oxygen is varied from stoichiometry to excess oxygen so as to verify operability of the device. This variation is applied periodically to ensure that the device is operating properly. A variable fraction of the oxygen can be pumped to give varying levels of combustion rather than stoichiometric mixtures. Changing the levels can be used to check performance by determining if the output oxygen indication increases with a call for a higher level of oxygen to be pumped. The sensor has a very high gain at the stoichiometric combustion point and a slight change in the oxygen level causes a large change in output when it is functioning properly. This test serves to verify the operation of the oxygen pump and the oxygen sensor. It also indicates whether combustion is taking place at the catalytic combuster since the oxygen sensor indicates no excess oxygen for a level of oxygen being pumped into the gas stream. The measuring assembly 40 according to one embodiment of the invention is shown schematically in Figure 2. The flow is from left to right of the drawing. The geometry in practice has a small area opening relative to the cross sectional area of the interior of the device. This provides a large area which is exposed to the flow for oxygen pumping and for combustion in order to ensure that the combustion process is complete. This is necessary for accuracy of measurement. Gas flowing from left to right enters the first component of the system which is a device capable of providing a controllable source of oxygen, in the form of a miniature fuel cell 10 using a solid electrolyte such as a stabilised zirconia. It produces or pumps an amount of oxygen to its interior directly related to the current passed through it. The oxygen thus introduced into the flow will mix with the gases in the flow stream. The mixture is then combusted in a catalytic combuster 12, the second component of the arrangement. The output of the catalytic combuster 12 will be a mixture of carbon dioxide, water vapour, oxygen and possibly some unburned combustible gases. These output gases are then passed through the third component of the arrangement, a zirconia oxygen sensor 14. The reference oxygen for the sensor 14 is supplied either from atmosphere or from a solid sealed oxygen source 16. The voltage output 18 of the oxygen sensor 14 makes a large, sudden increase when going from the condition of there being some oxygen in the sensing volume to the condition where all of the oxygen has been removed from the sample. The point where this occurs is independent of the reference pressure unless the reference pressure is very low (see the above-mentioned Heyne reference at page 84). Thus the oxygen source is not critical in the accuracy of the control when the system is operated at stoichiometric mixtures. The oxygen pump or fuel cell 10, the catalytic converter or combuster 12, and the oxygen sensor 14 are operated in the range of 600 to 700°C. The solid oxygen source 16 could be operated by the same heater system that controls the temperature of the other parts of the system. The oxygen source for the oxygen pump 10, the first component of the assembly, is likewise supplied typically either from the atmosphere, or from the solid source 16 as shown in Figure 2 (see the Speidel et al. reference). The voltage output 18 of the oxygen sensor 14 is used to control the current 20 supplied to the oxygen pump 10. A feedback control circuit 22 compares the output of the oxygen sensor 14 with a setpoint value and either increases or decreases the current 20 accordingly. This causes the amount of oxygen being pumped to change as the current changes. The control loop operates either at a stoichiometric mixture or one which is slightly oxygen rich, either increasing or decreasing the flow of oxygen so that the oxygen measured by the sensor is held constant. The current through the oxygen pump 10 is a measure of the oxygen needed for total combustion of the gas flowing through the sensor arrangement. The current used to control the oxygen pump 10 is quantitatively related to the oxygen required for complete combustion. This current is converted to a digital value and used in a calculation of energy value, using constants from the flow ratio and the data plotted in Figure 1 or the like. Figure 3 diagrammatically shows the electronic feedback control circuit 22 for the energy flowmeter, except for the heater control for maintaining the parts at the necessary temperature. The voltage output 18 from the oxygen sensor 14 is converted to a digital value by an analog-to-digital (A/D) converter 24. This digital value is compared to a setpoint by a current control unit 26. An output 28 of the current control unit 26 operates a current generator 30 which in turn supplies a drive current 20 to the oxygen pump 10. The current generator 30 is preferably in the form of a digital-to-analog (D/A) converter. When the oxygen signal is lower than the setpoint or desired value, the current generator 30 is instructed to increase its output by a small amount. Conversely, when the oxygen signal is above the setpoint, the current generator 30 is instructed to decrease the current by a small amount. This action continually repeats with a time interval between actions and an amount of change selected together to keep the control loop stable. The voltage compliance of the output of the current generator 30 is limited to +/- 2.5 volts (see the above-mentioned Heyne reference at page 71) to protect the sensor. A digital output 29 of the current control unit 26 is also used for the energy flow calculation in a circuit 32. This circuit 32, as well as the other control circuitry, is preferably microprocessor-based with the constants necessary for its calculations stored in the microprocessor memory. The calculated output 34 of the calculation circuit 32 gives the energy measurement. The measuring assembly 40, including the oxygen pump 10, the catalytic combuster 12, the oxygen sensor 14, the solid oxygen source 16, and the heater (not shown), is mounted in a flow conduit 42, for example as shown in Figure 4, in order to obtain a measure of the total energy flow in the conduit of a size which may range from a few inches (2.5 cm) to one with a diameter of 30 inches (75 cm) or larger. One mounting arrangement involves a package including the measuring assembly 40 mounted on the inside wall of the conduit 42 using a strut assembly as shown in Figure 4. An alternative implementation shown in Figure 5 uses a flow sampling technique in which a small sample of the stream is routed via a sample conduit 44 out of the primary flow conduit 42 and to an energy flow sensor 46 which is similar to the assembly 40. The flow sensor gas stream output is exhausted back into the primary flow via a return conduit 48 so that there is no net pressure drop through the sensing assembly. Isokinetic sampling is preferred to maintain the accuracy of the ratio of flow between the sample and the main flow. Another arrangement involves mounting the measuring assembly 40 in a probe which is inserted into the flow conduit 42. A single energy flow measuring assembly can be mounted in the probe and inserted into the conduit, or an array of assemblies 40 may be mounted in the probe 50, as shown in Figure 6, to measure across a flow conduit 42 in the case of a flow having a non-uniform cross-section.

Claims: A method of measuring the energy in a fuel gas, the method comprising: (a) taking a real-time sample from a total flow of the gas, the sample being a known fractional amount of the total flow and having the same temperature and pressure as the total flow; (b) generating an actual amount of combustion oxygen using a solid-electrolyte oxygen pump (10) which is powered by a current that is proportional to the actual amount of oxygen; (c) mixing the sample with the actual amount of combustion oxygen to form a mixture; (d) supplying the mixture to a catalytic combuster (12) for complete combustion of the sample to form an exhaust gas having an actual amount of exhaust oxygen therein; (e) measuring the actual amount of exhaust oxygen in the exhaust gas; and (f) if the actual amount of exhaust oxygen is approximately equal to a selected amount of exhaust oxygen which is indicative of complete combustion in the catalytic combuster (12), using the value of current powering the oxygen pump (10) to calculate the energy in the gas, and if the actual amount of exhaust oxygen is not approximately equal to the selected amount of exhaust oxygen, changing the current in step (b) and repeating steps (c) to (e) until the actual amount of exhaust oxygen is approximately equal to the selected amount of the exhaust oxygen. A method according to claim 1, wherein the selected amount of exhaust oxygen comprises a selected amount of excess oxygen in the exhaust gas. A method according to claim 2, including measuring the actual amount of excess oxygen using an oxygen sensor (14) connected to the catalytic combuster (12), and supplying calibrating oxygen to the pump (10) and to the oxygen sensor (14). A method according to claim 3, including supplying the oxygen to the pump (10) and the sensor (14) from a solid oxygen source (16). A method according to any one of the preceding claims, wherein the solid-electrolyte oxygen pump comprises a zirconia oxygen pump (10). A device (40) for measuring the energy in a fuel gas, the device comprising: a solid-electrolyte oxygen pump (10) for receiving a sample of the gas and for generating an actual amount of combustion oxygen when the pump (10) is powered by a value of current which is proportional to the actual amount of combustion oxygen; a catalytic combuster (12) connected to the pump (10) for receiving a mixture of the sample plus the actual amount of combustion oxygen for combustion of the sample to form an exhaust gas having an actual amount of exhaust oxygen therein; an oxygen sensor (14) connected to the catalytic combuster (12) for sensing the actual amount of exhaust oxygen in the exhaust gas; and a feedback control unit (22) connected between the oxygen sensor (14) and the pump (10) for sensing the actual amount of exhaust oxygen in the exhaust gas and for generating the value of current based on the actual amount of exhaust oxygen sensed, the control unit (22) being operable to compare the actual amount of exhaust oxygen to a selected amount of exhaust oxygen which is indicative of complete combustion in the combuster (12) and to adjust the value of current so that the actual amount of exhaust oxygen approximately equals the selected amount of exhaust oxygen, the control unit (22) including calculating means (32) for calculating the amount of energy in the fuel gas as a function of the actual value of current being supplied to the pump (10). A device (40) according to claim 6, wherein the pump comprises a zirconia pump (10). A device (40) according to claim 6 or claim 7, including a solid oxygen source (16) connected to the pump (10) and to the oxygen sensor (14) for supplying calibrating oxygen to the pump (10) and to the sensor (14). A device (40) according to claim 6, claim 7 or claim 8, wherein the oxygen sensor (14) is operable to produce an analog signal which is proportional to the actual amount of oxygen in the exhaust gas, the control unit (22) including an A/D converter (24) connected to the oxygen sensor (14) for producing a digital signal, current control means (26) connected to the A/D converter (24) for producing digital current signals which are proportional to the amount of oxygen, a D/A current generator (30) connected to the current control means (26) for generating the actual current for supply to the oxygen pump (10), and calculator means (32) connected to the current control means (26) for calculating an energy signal which represents a measurement of the energy in the gas, the energy signal being calculated as a function of the digital signal from the current control means (26).

Inventors: THOMPSON WILLIAM LEE, THOMPSON, WILLIAM LEE
Applicants: INT CONTROL AUTOMATION FINANCE, INTERNATIONAL CONTROL AUTOMATION FINANCE S.A.
Application year: 1990
Main classification: G01N27
IPC codes: G01N27, G01N33, G01N25