Oxygen concentration analyzer with pressure correction

Abstract

The process oxygen analyzer (10) includes a process probe (12) extendable into a process combustion exhaust stream, the process probe (12) having an oxygen sensor measurement cell (36). A measurement circuit (64) is coupled to the oxygen sensor measurement cell (36) and configured to obtain an uncorrected indication of an oxygen concentration for the combustion process based on an electrical characteristic of the oxygen sensor measurement cell (36). A controller (60) is operatively coupled to the measurement circuit (64) and configured to obtain an indication of a process pressure and to selectively provide a corrected oxygen concentration output based on the uncorrected indication of oxygen concentration and the indication of the process pressure. A method (200) of providing a process oxygen concentration using a process oxygen analyzer (10) coupled to an industrial combustion process is also disclosed.

Description

Technical Field

[0001] Background Art The industrial process industry mainly relies on energy sources that include one or more combustion processes. Such combustion processes include the operation of furnaces or boilers that generate energy from combustion and then use it in the process. Combustion provides relatively low-cost energy, but its use is generally regulated, and maximizing combustion efficiency is required. Therefore, one of the goals of the process management industry is to reduce greenhouse gas emissions by maximizing the combustion efficiency of existing furnaces and boilers.

[0002] For the monitoring, optimization, and control of combustion processes, in-situ or in-process flue gas analyzers are commonly used. Generally, these analyzers use sensors that operate relatively close to or directly above the combustion zone of a furnace or boiler, which is heated to a relatively high temperature. In-situ flue gas analyzers sold under the trade name Oxymitter or Model 6888 In-situ Flue Gas Oxygen Transmitter, available from Rosemount, Inc. (a business unit of Emerson Automation Solutions), often use high-temperature zirconia-based electrochemical oxygen sensors.

Summary of the Invention

[0003] The process oxygen analyzer includes a process probe that can extend into the exhaust flow of the process combustion, and the process probe has an oxygen sensor measurement cell. The measurement circuit is coupled to the oxygen sensor measurement cell and is configured to obtain an uncorrected indication value of the oxygen concentration regarding the combustion process based on the electrical characteristics of the oxygen sensor measurement cell. The control device is operably coupled to the measurement circuit, obtains an indication value of the process pressure, and is configured to selectively provide a corrected oxygen concentration output based on the uncorrected indication value of the oxygen concentration and the indication value of the process pressure. A method for providing the process oxygen concentration using a process oxygen analyzer coupled to an industrial combustion process is also disclosed.

Brief Description of the Drawings

[0004]

Figure 1

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Embodiments for Carrying Out the Invention

[0005] Detailed Description of Exemplary Embodiments Zirconia-based electrochemical oxygen sensors are widely used in industrial applications for oxygen measurement. Most of such industrial applications are for measuring the oxygen in flue gas discharged to the atmosphere. In this application, the differential pressure between the process and the surroundings is much smaller than 12 inches of water column (0.43 psi). A field oxygen analyzer (see Figure 1) in such an environment will function for years while controlling the oxygen level in the post-combustion flue gas. In actual combustion, it is very difficult to achieve the stoichiometric point for maximum efficiency and minimum emissions because the fuel energy density and the fuel / air flow rate vary, and the fuel / air uniformity is incomplete. The typical excess oxygen concentration in flue gas is about 2 - 3% for gas burners, and 3 - 6% for boilers and oil burners. The best operating point is thought to be similar between excess oxygen concentrations of 1 - 6%. This optimal operating point depends on the boiler load and the combustion rate. The curve of the function generator is generally created from test data to assign the ideal oxygen trimming control point based on the combustion rate index, fuel, or steam flow rate.

[0006] Figure 1 is a schematic diagram of a combustion process using an in-situ analyzer. The transmitter 10 can be any suitable analyzer including the above Model 6888 flue gas oxygen transmitter. The transmitter 10 is disposed in a chimney or a flue 14 and includes a probe assembly 12 that measures at least one parameter (such as oxygen concentration) related to the combustion generated by the burner 16. Generally, the transmitter 10 is an oxygen transmitter, but can be any device that measures any suitable parameter related to the combustion process. The burner 16 is operably coupled to a source 18 of air or oxygen and a source 20 of combustible fuel. Each of the sources 18 and 20 is preferably coupled to the burner via a valve of some kind to supply a controlled amount of oxygen and / or fuel to the burner 16 for controlling the combustion process. The transmitter 10 measures the amount of oxygen in the combustion exhaust stream and provides an indication value of the oxygen level to a combustion control device 22, which controls one or both of the valves 24, 26 to provide closed-loop combustion control. The transmitter 10 generally includes an oxygen sensor that uses a zirconia sensor to provide an electrical signal indicating the oxygen concentration, content, or percentage in the flue gas.

[0007] When the pressure difference of the in-situ oxygen analyzer increases beyond 12 inches of water column (0.43 psi), the signal of the sensor, as well as the accuracy and reliability of the information, will decrease. The response of the sensor to the difference in oxygen concentration when the partial pressure on the reference electrode is fixed (for example, using air) can be calculated using the well-known Nernst equation shown below:

[0008]

Equation

[0009] In the above equation, C is a constant related to the temperature change on the reference / process side and the thermocouple in the oxygen probe, R is the universal gas constant, T is the process temperature in Kelvin, and F is the Faraday constant.

[0010] Unfortunately, in many combustion applications, pressure fluctuations within the process can impair the regulation and control of oxygen and result in significant oxygen reading errors in measurements (e.g., between 0.7 and 2.55%). According to the embodiments shown below, a pressure sensor is disposed proximate to the measurement cell to measure the process pressure acting on the measurement cell. This measured value of the process pressure is then provided to a control device or other suitable computational circuitry to adjust and / or correct the oxygen sensor output based on the process pressure.

[0011] FIG. 2 is a schematic diagram of an in-situ process combustion oxygen analyzer according to an embodiment of the present invention. The probe assembly 12 is generally configured to house a sensor core assembly that includes a diffuser 32 disposed adjacent to the measurement cell 36. The measurement cell 36 and the heater assembly 38 are electrically coupled to the electronics circuitry included on an electronics circuit board 42 within the housing 44. The transmitter 10 also includes a plurality of gas inlets 46 and 48 for receiving reference air and calibration gas, respectively.

[0012] As shown in FIG. 2, the transmitter 10 includes a pressure sensor 50 fluidly coupled to a chamber or region 52 between the measurement cell 36 and the diffuser 32. In the embodiment shown in FIG. 2, the pressure sensor 50 is arranged to measure the process pressure during normal operation. The pressure sensor 50 can include a deflectable diaphragm, a capacitance-based pressure sensor, a deflectable diaphragm strain gauge, a resistance-based pressure sensor, or any other suitable type of pressure sensor. However, the pressure sensor should be configured to be exposed to relatively low pressures and relatively high temperatures that operate in a flue gas environment.

[0013] The above-described embodiments provide an oxygen sensor measurement cell and a pressure sensor proximate to the measurement cell such that the pressure sensor proximate to the measurement cell provides an indication of process pressure. However, the embodiments described herein can also receive process pressure measurement information from an external device such as a process pressure transmitter and use the received process pressure information to provide a corrected oxygen sensor output.

[0014] FIG. 3 is a block diagram of an electronic circuit board 42 of an oxygen transmitter 10 for process analysis according to an embodiment of the present invention. FIG. 3 shows additional components of the electronic circuit board 42. Specifically, the electronic circuit on the electronic circuit board 42 includes a control device 60 coupled to a communication circuit 62 and a measurement circuit 64. The control device 60 is also coupled to a UI / display device module 66. The control device 60 can be any suitable device that executes a sequence of instructions for performing one or more control functions. In one embodiment, the control device 60 is a microprocessor.

[0015] The communication module 62 is coupled to the control device 60 and enables the control device 60 to communicate with one or more process devices, such as a combustion control device 22 (shown in FIG. 1), in accordance with a wired process industry standard communication protocol. Examples of such protocols include the Highway Addressable Remote Transducer HART® protocol and the FOUNDATION® fieldbus protocol. Further, or alternatively, the communication module 62 can be a wireless communication module that enables the control device 60 to communicate in accordance with a wireless process communication protocol such as IEC62591. In an embodiment where the control device 60 uses process pressure information received via another process variable transmitter (i.e., a pressure transmitter), the communication module 62 provides such communication to the control device 60 to enable the control device 60 to acquire the process pressure information.

[0016] The measurement circuit 64, in one embodiment, includes an analog / digital converter configured to measure the electrical characteristics of the sensors connected thereto. As shown, the measurement circuit 64 is coupled to the oxygen sensor 36 to obtain an uncorrected oxygen sensor signal. Further, the measurement circuit 64 is also coupled to the pressure sensor 50 to measure the electrical characteristics such as the capacitance of the pressure sensor 50 indicating the process pressure and provide its digital indication value to the control device 60. The measurement circuit 64 can also include appropriate amplification, filtering, and / or linearization circuits, as required.

[0017] The control device 60 receives the uncorrected oxygen sensor signal and the information indicating the process pressure, and corrects the uncorrected oxygen sensor signal based on the measured process pressure using the known relationship between the process pressure and the error of the oxygen sensor signal. In embodiments where the process pressure information is received from an external source such as a process pressure transmitter or a user who inputs the process pressure information via the UI / display device module 66, the pressure sensor can be omitted.

[0018] FIG. 4 is a chart of gas concentration versus oxygen concentration showing combustion control. FIG. 4 is an example of a curve of a function generator generally developed from test data to assign an ideal oxygen trimming control point based on the combustion rate index, fuel, or steam flow rate. As described above, in many combustion applications, pressure fluctuations within the process can have an undesirable effect on the oxygen sensor signal.

[0019] FIG. 5 is a chart showing the effect of process pressure on the reading of the oxygen sensor at an oxygen concentration of 2.0 - 2.7%. As shown, at higher levels of oxygen concentration (i.e., 2.65%) and relatively high pressures (e.g., 5 PSI), the error can be significant.

[0020] Figure 6 is a chart showing the influence of pressure on the reading values of an oxygen sensor analyzer when the oxygen concentration is high (8.0% - 10.75%). In this example, since the percentage concentration of oxygen is relatively high, the error is less than that in the case of low concentration (for example, about 2%) shown in Figure 5.

[0021] Figure 7 is a chart showing various oxygen concentrations (2%, 3%, 4%, and 5%) at various pressures in the range of 0 to 5 psi. The effect of process pressure on the oxygen sensor readings is approximately 1.38 mV / psi. There are still some processes that require accurate oxygen measurement even in processes operating under pressure. In the past, some pressure balancing systems were used to counteract the effects of pressure imbalance. However, such an approach has been shunned because it requires approval for hazardous locations. According to the embodiments of the present specification, in the case of a pressurized process (i.e., exceeding 0.5 psi), the control device 60 applies a pressure correction to the uncorrected oxygen sensor measurement according to the relationship shown in Figure 7. The error caused by the pressure shown in Figure 7 was obtained experimentally using a standard oxygen cell. This correlation is similar for other zirconia-based oxygen analyzers, but does not always match exactly due to structural differences. Therefore, some experimentation may be required to associate the effect of pressure on a given zirconia-based oxygen sensor. However, once such a relationship is obtained, a relationship such as the linear relationship shown in Figure 7 (i.e., 1.38 mV / psi) can simply be input into the control device (e.g., during system manufacture or when testing / calibrating a particular sensor in the system) to provide a corrected oxygen sensor output. Further, for process pressures below the selected threshold (i.e., 0.5 psi), the control device 60 can simply provide the uncorrected output using the uncorrected oxygen sensor signal and the Nernst equation. Therefore, according to at least one embodiment described herein, the control device 60 of the oxygen analyzer 10 can receive an indication of the process pressure (either using a local pressure sensor placed close to the measurement cell or using process communication), and can determine whether the process pressure exceeds a threshold (i.e., 0.5 psi) to determine whether to apply a pressure-based correction.As described above, in some embodiments, the measured process pressure can also be input into the oxygen analyzer using the user interface module / display device 66.

[0022] The pressures used in the embodiments described herein may have the potential to be continuously updated pressure values, but this is not necessary. This is because the T90 response time of the oxygen sensor is generally on the order of minutes rather than seconds. Therefore, the pressure may simply be a one-time input and only needs to be updated each time the oxygen analyzer is calibrated.

[0023] FIG. 8 is a flowchart of a method for providing pressure-corrected oxygen output according to an embodiment of the present invention. Method 200 begins at block 202 where the process pressure is obtained. The process pressure can be obtained from a pressure sensor (e.g., as shown in FIG. 2) disposed within the probe of the oxygen analyzer, as indicated by reference number 204. Further, or alternatively, the process pressure can also be obtained from other sources. For example, the process pressure can be obtained from a remote process device using process communication 206 and / or can be input into the oxygen analyzer by the user via a suitable user interface such as the UI / display device module 66 (shown in FIG. 3). Thereafter, at block 210, an uncorrected oxygen sensor measurement is obtained from an oxygen sensor disposed within or in proximity to the process. At block 212, the process pressure obtained at block 202 is compared to a predetermined (i.e., 0.5 psi) threshold value, and it is determined whether the process pressure is greater than or equal to the predetermined threshold value. If not, control passes to block 214 where the uncorrected oxygen sensor signal is simply used and an oxygen concentration output is provided using the Nernst equation described above.

[0024] In block 212, if it is determined that the process pressure is equal to or greater than a predetermined threshold, control transfers to block 216 where a pressure-based correction is applied to the uncorrected oxygen sensor signal. This correction can add a correction value that varies with the process pressure to the uncorrected oxygen sensor signal. As an example, the correction value can be a voltage value obtained by multiplying the process pressure by a linear correction value (e.g., 1.38 mV / psi) as indicated by reference numeral 218. In other embodiments, as indicated by reference numeral 220, a more complex relationship between the process pressure (as with other variables) and the correction value can be modeled using curve fitting. Further, to reduce the complexity of the calculations, such a relationship can also be modeled using a look-up table as indicated by reference numeral 222. The corrected oxygen sensor signal is then used by the control device in block 214 to provide an oxygen concentration output. This method is repeated as indicated by reference numeral 224. Note that the iteration can return to block 202 substantially in real time to obtain the process pressure. However, since the process pressure does not need to be updated as frequently as the oxygen sensor measurements, the iteration can return to block 210 for a predetermined number of cycles or for the time before the iteration obtains an updated process pressure.

Claims

1. A process oxygen analyzer, comprising: A process probe extendable into the flow of process combustion exhaust, said process probe having an oxygen sensor measurement cell; A measurement circuit coupled to said oxygen sensor measurement cell and configured to obtain an uncorrected indication value of the oxygen concentration related to the combustion process based on the electrical characteristics of said oxygen sensor measurement cell; and A control device operably coupled to said measurement circuit, said control device configured to obtain an indication value of the process pressure and selectively provide a corrected oxygen concentration output based on the uncorrected indication value of the oxygen concentration and said indication value of the process pressure; A user interface (UI) / display device module operably coupled to said control device; Including; Said control device is configured to obtain an indication value of said process pressure via said UI / display device module; A process oxygen analyzer.

2. The control device is configured to compare the process pressure with a predetermined threshold value and selectively provide the corrected oxygen concentration output based on whether the indication value of the process pressure exceeds the predetermined threshold value. The process oxygen analyzer according to claim 1.

3. The process oxygen analyzer according to claim 2, wherein the predetermined threshold value is about 0.5 psi.

4. The process oxygen analyzer according to claim 1, further comprising a communication circuit coupled to said control device, said communication circuit configured to communicate according to a process industry standard communication protocol.

5. The control device is configured to generate a corrected oxygen concentration output based on a linear correction coefficient that relates the error of the uncorrected indication value of the oxygen concentration to the process pressure. The process oxygen analyzer according to claim 1.

6. The process oxygen analyzer according to claim 1, wherein said oxygen sensor measurement cell includes a zirconia oxygen sensor.

7. A method of providing a process oxygen concentration using a process oxygen analyzer coupled to an industrial combustion process, comprising: Obtaining an indication value of the process pressure; Using a measurement cell coupled to said industrial combustion process to obtain an uncorrected indication value of the oxygen concentration; and Comparing the indicated value of the process pressure with a predetermined threshold value, and providing a corrected oxygen concentration output if the process pressure exceeds the predetermined threshold value, and providing an uncorrected oxygen concentration output if the process pressure does not exceed the predetermined threshold value, A method comprising.

8. Obtaining the indicated value of the process pressure includes using the measurement circuit of the process oxygen analyzer to determine the electrical characteristics of a pressure sensor arranged to measure the process pressure. The method according to claim 7.

9. Obtaining the indicated value of the process pressure includes receiving the indicated value from a remote device using the process communication circuit of the process oxygen analyzer. The method according to claim 7.

10. Obtaining the indicated value of the process pressure includes receiving the indicated value from the user interface (UI) / display device module of the process oxygen analyzer. The method according to claim 7.

11. Providing a corrected oxygen concentration output includes multiplying the indicated value of the process pressure by a known correction factor that relates the error in the uncorrected indicated value of the oxygen concentration to the process pressure. The method according to claim 7.

12. The predetermined threshold value is 0.5 psi. The method according to claim 7.

13. The method is repeated. The method according to claim 7.

14. In the repeated method, the uncorrected indicated value of the oxygen concentration is obtained more frequently than the indicated value of the process pressure. The method according to claim 13.

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