MXPA97008933A - Measurement of the heating value using combustion catalit - Google Patents

Abstract

The heating value of a sample gas by a microcontroller (12) from the heating value of a reference gas, and from the flow rates determined as the gas is consumed by catalytic combustion. In a preferred embodiment, the fixed volume chambers (5, 14) are charged to a pressure previously determined with a reference gas and a sample gas, respectively, and are flowed into the catalyst at varying flow rates, changing according to the pressure decreases. During the discharge cycle, a pressure transducer (13) detects the diminishing preison and this information is introduced to the microcontroller (12), which calculates molar flow rates and also detects the energy level of the combustion through the circutio of bridge (24) in the catalytic apparatus (8, 10). Based on a proportion of the molar flow rates and a proportion of the corresponding energy levels, a heating value of the sample gas is calculated by the microcontroller (12) and output to a visual display or other output device.

Description

MEASUREMENT OF THE WARMING VALUE USING CATALYTIC COMBUSTION BACKGROUND OF THE INVENTION The field of the invention is the methods and apparatus for determining the heating value of gases. The measurement of the heating value of natural gas is important for the distribution and sale of natural gas. There are three commonly used methods to measure the heating value. One method is stoichiometry, in which combustion is • substantially complete. This type of combustion produces maximum flame temperature and minimum oxygen in the exhaust stream. In this case, the natural gases are burned with air and the fuel-air ratio is adjusted before the combustion results in either a maximum flame temperature or the stoichiometric point of perfect combustion, that is, the razor's edge where there is no oxygen remaining. Clingman, Patent of the United States of North America Number: 3,777,562, is an example of this method. In Clingman, the heating value is measured by the combustion of a gas with amounts of air that are adjusted to obtain the maximum flame temperature. This is also described in Clingman, Patents of the United States of North America Numbers: 4,062,236, 4,125,018 and 4,125,123. In each of these patents, the combustion flame at the top of a burner and with a temperature sensing device such as a thermocouple. A second method to measure the heating value is the constituent analysis. Using a chromatograph, the fraction of each chemical constituent of the gas is determined. Then, the heating value is determined by adding the heating value of the individual constituents. The third method is the calorimetric measurement in which a volume of the gas is sampled and then completely burned. The combustion can be by flame or by other methods that do not produce an open flame, such as passing the gas on a catalytic material. In the case of catalytic combustion, the amount of heat released can be measured either by temperature changes related to the catalytic reaction, by changes in the energy supplied to heat the catalyst or by measuring the temperature of the catalytic material. Catalytic combustion occurs at temperatures below a normal ignition temperature associated with hydrocarbons. For example, methane when mixed with air, in a stoichiometric ratio, will ignite at a temperature of approximately 630 ° C and reach an open flame temperature passing the 1600 ° C. Catalytic oxidation will take place at catalyst temperatures as low as 400 ° C although efficient catalysis is then achieved at a temperature close to 500 ° C. Therefore, for gaseous mixtures containing methane, catalytic oxidation is enabled below the ignition temperatures of the surrounding atmosphere. In the practice of catalytic combustion, it is usual to mix the gas sample with a fixed amount of air, usually an excess of air, where the proportion of air is more than sufficient to provide all the oxygen required for the oxidation of the gas. sample. In catalytic oxidation, the temperature of the catalyst must be limited to avoid overheating and temperature and exhaust reaction conditions. Goldberg, U.S. Patent Number: 5,012,432, discloses measurements of heating values using catalytic combustion. In Goldberg the measurement of the heating value requires measuring the temperature before and after the catalytic combustion to determine the heating value per unit volume of the gas. In Stetter, the precise constant volumes of the gas are sampled and then oxidized using a reaction with a catalyst to generate a signal representative of the heat released. A baseline signal for the air is produced, and then a reference gas flow and a sample gas flow are reacted with the catalyst to provide additional signals to be compared with the baseline signal. In the previous catalytic heating value measurements, extreme stability of air and gas volumes was required to achieve precision. The prior art uses elements to maintain constant gas flow regimes and elements to maintain fixed gas volumes. The present invention provides improved methods and apparatus for measuring heating values in a catalytic combustion apparatus using variable flow rates.

SUMMARY OF THE INVENTION The present invention measures the heating value of a gas using flameless catalytic combustion and an improved flow rate measurement apparatus. The invention establishes a variable fuel mixture within a general range and measures the combustion energy of gas introduced into the catalytic reactor and the associated molar flow rate of the gas. A reference gas and the sample gas are measured in respective cycles. The only requirement is that the molar flow rate of the gas is compared with its associated combustion energy levels that support catalytic combustion. In the present invention, an air flow is established which is well in excess of the air required to burn the gas. A reference gas is mixed with the air, and the gas flow rate is allowed to change slowly over time in an uncontrolled manner. The gas / air flow is directed on or through a catalytic layer or bed or where a portion of the fuel is oxidized. The level of energy that supports combustion varies with the flow regime of the gas. At a selected combustion energy level, the molar flow rate of the gas is measured by suitable sensors. This cycle is followed by the introduction of a flow of sample gas that passes through the same catalytic cycle and that varies with time. When the level of combustion energy in the catalyst reaches a selected energy level, the molar flow rate of the sample gas is measured. It will be shown that the heating value of the sample gas can be calculated from the ratio of the molar flow rates of the sample gas and the reference gas, the proportion of the combustion energy levels of the sample gas and the reference gas and a known heating value for the reference gas.

In the preferred embodiment, the fuel-air mixture is varied by lowering the pressure in a volume chamber to produce a decreasing flow of fuel that progressively changes the fuel-air mixture. Molar flow rates are measured for a sample gas and a reference gas within a catalytic combustion cycle. The heating value for a sample gas can then be calculated with a previously stored value for the heating value of the reference gas. For those skilled in the art, various objectives and advantages will become apparent from the description of the preferred embodiment that follows. In the description, reference is made to the accompanying drawings, which form a part thereof, and which illustrate examples of the invention. Those examples, however, are not exhaustive of the various embodiments of the invention, and, therefore, reference is made to the claims that follow the description to determine the scope of the invention.

Brief Description of the Drawings Figure 1 is a block diagram of an apparatus for practicing the method of the present invention, - Figure 2 is a schematic diagram in detail of an electrical circuit in the catalytic apparatus of Figure 1; Figure 3 is a graph of energy versus time illustrating the operation of the apparatus of Figure 1; and Figure 4 is a flow diagram of the operation of a microcontroller in the apparatus of Figure 1.

Detailed Description of the Preferred Modality of the Invention Referring to Figure 1, the apparatus 10 of the present invention includes a line 1 for supplying air from an external air supply (not shown) to a catalytic apparatus 8. The flow rate of air to catalytic apparatus 8 is not critical and can vary by ± 10 percent in a slow mode, but should always be in excess with respect to combustion requirements. The catalytic apparatus 8 includes a bed which is composed of material, such as platinum and / or palladium coated with fibrous material, which promotes and increases the oxidation of the gas without flame combustion. The apparatus also includes the heating element 9, which is located on, or within, the catalytic bed to provide an initial start temperature for the reaction. The heating element 9 will heat the catalytic material to a temperature of 400 ° C or more. The apparatus 8 also includes a temperature sensor 11 which provides a signal proportional to the temperature at the reaction surface of the catalytic material. The heating element 9 receives electrical energy from the power source 19. The temperature sensor 11 is embedded in the catalytic material to detect the temperature at the reaction surface of the catalytic material. The temperature sensor 11 generates a signal as an input to the power source 19. This signal is recognized by the power source 19 as representative of the catalytic temperature. Figure 2 shows details of the catalytic apparatus 8 and the power source 10 described above in relation to Figure 1. Within the elements 8, 19, a bridge circuit 24 is formed as seen in Figure 2. On the left side of the bridge resistors 9 and 20 are connected in series. Resistor 9 (Rh) also called a heating element in Figure 1, is typically a platinum spiral wire resistor. Platinum is selected due to its stable temperature coefficient over a wide temperature range. Its resistance value (Rh) is a function of temperature Rh = Rh0 (1 + a? T). The resistor 9 also acts as the temperature sensor 11 of the catalyst. Resistor 20 is a resistor whose value (R?) Is selected to be the desired resistance of 9 at the temperature selected for the operation of the catalyst. The resistors 21, 25 are connected in series to the right side of the bridge 24 to divide the applied voltage (+ V). In Figure 2, the resistors are shown of equal value (R0), but this is not a strict requirement. The operational amplifier 22 captures the difference between the voltages of the branch center on the right and left sides of the bridge 24 and amplifies the difference. The result is applied to the FET energy 23 to change the voltage on the bridge 24 until the voltages of the central branch of the two sections become equal. Therefore, the level of electrical energy within the heater / sensor 9 is that which is required to maintain the resistance and temperature of the heater / sensor 9 constant. If gas combustion takes place, the gas energy introduced to the catalyst associated with the heater / sensor will attempt to raise the temperature of the heater / sensor and the applied electrical power will be reduced in proportion to keep the temperature of the sensor heater constant. An exhaust stream 17 (Figure 1) escapes from the catalytic device 8. This exhaust stream 17 includes air, the products of combustion and any unburned gas. Additional steps must be followed to process the exhaust stream. The microcontroller 12 (Figure 1) is a microelectronic central processing unit (CPU) with interface circuits A-to-D and D-to-A. The microcontroller 12 operates by executing program instructions, some of which are represented by blocks in the flow chart in Figure 4, the instructions being stored in a memory also generally represented by the reference 12. The microcontroller 12 captures the energy level of combustion through an input connected to the power source 19. The microcontroller 12 also controls the flow of the reference gas and the sample gas to the catalytic apparatus in successive cycles by operating a series of valves and chambers. In one cycle, a reference gas flows through an on-off valve 4 into the chamber 5, and then through an on-off valve 6 to a gasket 18 leading to the flow restrictor 7 and finally , to the catalytic device 8. The actual flow regime is determined solely by the pressure in the volume chamber 5 and the flow properties of the flow restrictor 7. An objective of the present invention is to use uncontrolled gas flow rates but to introduce controlled flow rates would serve the same purpose with greater complication . The control valve 4 opens to fill the volume chamber 5 with the reference gas from the sample gas source 3. The flow within the volume chamber 5 increases the pressure in the volume chamber 5 until a pressure is reached previously determined, but not critical, usually determined by the pressure in the supply line 3, and then the flow control valve 4 is closed. After closing the valve 4, the microcontroller 12 opens the control valve 6 to establish the flow of the reference gas through the seal 18 and the limiter 7 to the catalytic apparatus 8 where a portion of the reference gas is burned. In another cycle, a sample gas flows through an on-off valve 16 into the volume chamber 14, and then through an on-off valve 2 into the seal 18 leading to the flow limiter 7. and finally, to the catalytic apparatus 8. The control valve 16 is closed to fill the volume chamber 14 with sample gas from the sample source 15. The flow within the volume chamber 14 increases the pressure in the volume chamber 14 until a predetermined pressure, but not critical, usually determined by the pressure in the supply line 15, then the inlet flow control valve 16 is closed. After closing the valve 16 the microcontroller 12 opens the control valve 2 to establish the flow of the sample gas through the limiter 7 and on the catalytic bed 8 where a portion of the sample gas is burned as was the case with the gas in question. reference As each cycle progresses, the gas trapped in the volume chamber 5 or 14 is removed and the pressure of the volume chamber 5 or 14 is reduced. The microprocessor unit 12 monitors the changes in pressure in the volume chamber 5 or 14 using the pressure transducer 13 to determine the molar flow rate. It should be noted that the measurement of the molar flow rate using the rate of change in pressure in volume chamber 5 or 14 is of the type described in Kennedy, US Pat. No. 4,285,245, to capture the regimen of molar flow in response to pressure changes due to gas flow out of a chamber. This eliminates the molecular weight of the gas from consideration in gas measurements. Such a flow meter is incorporated into a product commercially offered under the trade name "TRU-THERM". In addition to measuring the molar flow rate, the microcontroller 12 also monitors the energy level required for the catalytic oxidation of the sample gas or the reference gas. In the preferred embodiment, the power source 19 continuously adjusts the energy for the heater 9 by maintaining a constant temperature in the sensor 11. As the gas flow rate changes, the energy changes for the heater 9 represent the energy of the combustion of gas in the catalytic bed 8. These energy levels are captured by the microcontroller 12. At a predetermined combustion energy level, or a change in the energy level, the molar flow rate of the volume chamber 14 is calculates and stores using the microcontroller 12. It is not required to use two volume chambers, 5 and 14, but it is the preferred modality. Using a camera slows down the measurement process because a single chamber that uses two gases has the problem of residual gas residence and several exhaust cycles are required to completely change the gas. If the response speed is not the dominant target, the measurement can be modified to use reference gas infrequently and a single volume chamber can be used. The sample gas and the reference gas are alternately cycled through the catalyst bed. In Figure 3, the first cycle is a reference gas cycle and the reference gas flow changes over a period of 10 to 20 seconds. When the reference gas pressure reaches a low point, the connection is made to the sample gas and the sample gas decrease cycle begins. This is a repetitive sequence. The method is carried out at ambient temperatures of about -40 ° C to 54 ° C. As the gas flow regime decreases, the amount of the combustion energy level also, and the electric power for the heater / sensor in the catalyst increases in inverse proportion to the flow rate. In each flow cycle, there is a selected energy level for which the molar flow rate is measured and this value is used to calculate the energy level ratio "?" as well as the heating value of the sample gas. The molar heating value Hm is defined as the amount of heat that can be released by combustion of one mole of gas and has typical units of energy / mole. If the molar flow rate of a gas, ng, with units of moles / second, is multiplied by the molar heating value, the result is the combustion energy described as? P = Hm ng. If the combustion energy of the sample gas and the reference gas are identical at the selected point of measurement, then equalizing the two combustion energies results in: where the subscripts r and s refer to the reference and sample conditions. A desirable feature of this invention is that the response speed can be increased by terminating the individual measurement cycles before completion. If the change in the combustion energies of the reference and sample cycle are not equal but are in a known proportion, then the equation (l) can be modified by introducing a correction factor,?, Which is the proportion of the changes in the energy levels and (1) it is reset as: where ? is the ratio of the two energy levels. It should be clear that? it can have values, at the end, between zero and unity. A mole of gas contains a fixed number of molecules, known as the Avogadro number, and occupies a defined volume Vm that is a function of temperature and pressure. At 0 ° C and 14,696 psia, this volume, for an ideal gas is 22.4138 liters, the compressibility effect must be recognized and used to define the volume of a real gas as real Vm = Vm ¿eai zreai • with volume units per mole and where the compressibility, zreai 'is calculated at the temperature and pressure of the measurement. Therefore, the heating value of the volume (energy / volume) of the gas is: The heating value as defined in equation (3) will be set as the standard temperature and pressure and a user will select the standard values. This can be easily adjusted using the general gas law and is known to anyone skilled in the art of gas calculation. It is possible to capture the flow regime of the gas flowing from the volume chambers 5 or 14 by measuring the rate of change or the pressure change as the gas in the volume is removed. The relationship between the molar gas flow regime and the pressure change regime is obtained from the general gas law and is: p v n = Z2 R T where n is the molar flow regime and p is the rate of pressure change. Figure 4 shows the operation from the point of view of the microcontroller 12 to execute its control program. The start of the operation is represented by the initial pad 30. The microcontroller executes instructions to select either the reference gas cycle or the sample gas cycle, as represented by the process pad 31. If the gas is selected reference, the microcontroller 12 executes additional instructions, represented by the process pad 32, to open the valve 16 and let the sample gas fill the volume chamber 14 in preparation for the next cycle using the sample gas. Next, as represented by the process pad 33, the microcontroller 12 executes additional instructions to open the valve 6 and let the reference gas flow into the catalytic device 8. The microcontroller 12 then executes instructions represented by the process pad 34 to start the sample molar flow rate (n) and the changes in electrical power (? P) required by the catalytic device 8. The microcontroller 12 executes instructions represented by the decision block 35 to see if the requirements of the Molar flow regime and electrical energy. If the result is "NO", the cycle is returned to continue with another sample. If the result is "YES", proceed to execute the instructions represented by the pad 36 to finish the first cycle and prepare for the next cycle. As represented by the process pad 36, the microcontroller 12 executes instructions to stop the gas flow of the reference gas by closing the valve 6. The microcontroller 12 then executes the instructions represented by the process pad 37 to change the selection to the other gas cycle. The microcontroller 12 then executes the instructions represented by the process pad 38 to flood the camera 5. Next, the microcontroller 12 then executes the instructions represented by the process pad 38 to store the values of the final flow rate and energy for the cycle that has just been completed. Then a verification is made, as represented in decision block 40, to see if both the reference cycle and the sample gas cycle were completed in a recent period of time. If the result is "YES", the data can be used to calculate the heating value as represented by the process pad 41. The heating value is then taken to a visual display (not shown in Figure 1) or other type of output device. If the information is not complete, the result of decision block 40 is "NO", and the program returns to start a new gas measurement cycle in block 31. This was a description of examples of how it can be carried out. the invention. Those skilled in the art will recognize that various details can be modified to arrive at other detailed embodiments and these embodiments will fall within the scope of the invention. Therefore, to appreciate the public of the field of the invention and of the embodiments covered by the invention, the following claims are made.

Claims (16)

1. CLAIMS 1. A method for measuring the heating value of a fuel gas, the method comprising: flowing a reference gas and a sample gas in contact with a catalyst in separate cycles to cause a flameless oxidation of the reference gas and the sample gas, respectively; detect the energy level and a corresponding value for the flow rate of the sample gas; the method being characterized by: varying the flow rate of the reference gas for the catalyst to obtain a changing energy level for the catalyst; detecting the flow rate of the reference gas at a selected energy level less than a maximum level of electrical energy supplied to the catalyst; varying a flow rate of the sample gas to the catalyst to obtain an energy level either equal to or proportional to the level of electrical energy selected for the reference gas; detecting the flow rate of the sample gas at the energy level at which the flow regime of the reference gas was detected; and calculating the heating value of the sample gas in response to a ratio of the reference gas flow rate and the flow rate of the sample gas in relation to the selected electric power level. The method of claim 1, further characterized in that the heating value of the sample gas is calculated in response to a proportion of an energy level corresponding to the flow rate of the reference gas and an energy level corresponding to the flow rate of the sample gas. 3. The method of claim 1, characterized in that the flow rates of the sample gas and the reference gas are molar flow rates. The method of claim 1, further characterized in that the flow rate of the sample gas is varied by releasing a volume of gas from the pressurized chamber and allowing the pressure in the chamber to decrease. The method of claim 1, further characterized in that the flow rate of the reference gas is varied by releasing a volume of pressurized gas from a first chamber and allowing the pressure to decrease in a first chamber. The method of claim 5, further characterized by filling and pressurizing a second chamber with the sample gas as the reference gas is released from the first chamber. The method of claim 1, further characterized by capturing the energy level of the catalyst for the reference gas and the sample gas to obtain an energy level of equal value corresponding to the respective flow rates of the reference gas and of the sample gas. The method of claim 1, further characterized in that the method is carried out at ambient temperatures of from about -40 ° C to 54 ° C. The method of claim 1, further characterized by the detection of the temperature of the catalytic oxidation of the reference gas and the sample gas and in response to that the control of the energy levels for the catalyst to control the temperature of the the catalytic oxidation of the reference gas and the sample gas. The method of claim 1, further characterized in that the fluid passages, variation of the flow rate and detection of the flow rate of the reference gas are carried out in a first cycle, which is then followed by a second cycle that it includes the fluid passages, variation of the flow rate and detection of the flow rate of the sample gas. 11. An apparatus for determining the heating value of a combustible gas, the apparatus comprising: an element for flowing a sample gas in contact with the catalyst; and an element for detecting the flow rate of the sample gas, and the apparatus being characterized by: an element for establishing a variable flow rate of a reference gas flowing in contact with a catalyst under changing pressure conditions, an element for establishing a variable flow rate for the sample gas flowing in contact with the catalyst under changing pressure conditions, - an element for detecting the level of electrical energy supplied to the catalyst for the catalytic combustion of the reference gas; an element for detecting the flow rates of the reference gas and the sample gas corresponding to selected levels of electrical energy supplied to the catalyst for the catalytic combustion of the reference gas and the sample gas, respectively; and an element for calculating the heating value of the sample gas in response to the detection of the flow rates of the reference gas and the sample gas at selected levels of electrical energy supplied to the catalyst for the catalytic combustion of the reference gas and the sample gas, respectively; and an element for calculating the heating value of the sample gas in response to the detection of the flow rates of the reference gas and the sample gas at selected levels of electric power supplied to the catalytic heater for the catalytic combustion of the reference gas and the sample gas, respectively. The apparatus of claim 11, further characterized in that the element for establishing a variable flow rate of a reference gas includes a first chamber of a fixed volume, a first valve that controls the flow of the reference gas outside the first chamber, and an element to control the operation of the first valve and the second valve. The apparatus of claim 12, further characterized in that the element for establishing a variable flow rate of the sample gas includes a second fixed volume chamber, a third valve that controls the flow of the reference gas within the second chamber, a fourth valve that controls the flow of the reference gas out of the second chamber, and an element to control the operation of the third valve and the fourth valve. The apparatus of claim 13, further characterized in that the element for detecting the flow rates of the reference gas and the sample gas includes a pressure transducer for detecting the pressure decrease in the first chamber and in the second chamber, and a calculation element to calculate that decrease as a function of time to determine the molar flow rates of the reference gas and the sample gas. The apparatus of claim 11, further characterized in that the elements for detecting the level of electrical energy supplied to the catalytic heater further comprises a resistance bridge circuit including the catalytic heating element and further comprises an element for detecting the electric power supplied to the catalytic heating element in said bridge circuit. The apparatus of claim 11, further characterized in that the elements for calculating the heating value of the sample gas in response to the detection of the flow rates of the reference gas and the sample gas and in response to the detection of The electrical power supplied to the catalytic heater includes a microelectronic processor that is programmed to perform those calculations for the heating value of the sample gas.