Abstract:
A combustion gas analyzer for measuring the concentration of a gas constituent in an exhaust gas stream is provided. The gas analyzer includes a sensor cell assembly coupled to a transmitter having electrical circuitry configured to provide an output of the concentration of the gas constituent as sensed by the sensor cell assembly. The combustion gas analyzer also includes a filter substantially enclosing the sensor cell assembly and a conduit coupled to the filter at a first end of the conduit and coupled to a valve assembly at a second end of the conduit. The conduit is used for supplying a calibration gas to the sensor cell assembly or for supplying a blow-back gas used to purge the filter.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to the field of gas analysis instrumentation, and more specifically to a combustion gas analyzer.  
           [0002]    Industrial processes are used in the manufacture or combustion of various materials. It is often desirable to monitor operation of a process such that the process can be controlled and adjusted accordingly. Exhaust gas from the combustion is vented through a stack.  
           [0003]    Combustion analyzers are used to measure the concentrations of a variety of exhaust gases in industrial combustion processes. For example, the exhaust gas in a combustion process consists of by-product and excess gases. The concentrations of exhaust gases, such as oxygen, oxides of nitrogen, sulfur dioxide and carbon monoxide, relate to the combustion efficiency of the process. Exhaust gas concentration measurements enable operators to adjust the amount of fuel supplied to the process to attain an efficient combustion.  
           [0004]    Combustion oxygen analyzers are designed to measure the net concentration of excess oxygen in a combustion process. Excess oxygen is the oxygen remaining after all oxygen has been oxidized in the process and is related to the efficiency of the combustion process. An example of such a device is the Oxymitter 4000 manufactured and sold by Rosemount Analytical, Inc. of Orrville, Ohio. Common applications for a combustion oxygen analyzer include: glass furnaces, coking ovens, catalytic crackers, utility coal pulverizers, sulfur paint incinerators, and other industrial incinerators.  
           [0005]    The combustion oxygen analyzer includes a sensor cell assembly which is positioned within an exhaust stack or duct which vents the exhaust gas from a combustion chamber. The sensor cell assembly includes a diffusion element and a sensing cell. As the exhaust gas is vented through the stack, it enters the sensor cell assembly and the diffusion element disperses the gas about the sensing cell. An electrical output from the sensor cell is indicative of oxygen concentration. Electrical circuitry in the transmitter reads the sensor cell output and provides an output related to oxygen concentration.  
           [0006]    The combustion oxygen analyzer must be periodically calibrated in order to maintain accuracy in measurements. For example, the sensitivity of the sensor cell can drift over time. Calibration is through a process of standardizing the analyzer by determining the deviation between actual oxygen concentration and measured oxygen concentration. The deviation is used to adjust the output of the analyzer to bring it back into calibration. For example, to calibrate an oxygen analyzer, a calibration gas containing a mixture of oxygen and other gases has a known concentration of oxygen and is applied to the sensor cell assembly. The sensor cell assembly senses the concentration of oxygen in the calibration gas. The electrical circuitry provides an output value for the measured oxygen concentration. The measured value of oxygen is compared to the known concentration of oxygen in the calibration gas. A correction factor is calculated and can be applied to all subsequent measurements of the exhaust gas until a future calibration is performed. The correction factor can be stored, for example, in a memory in the transmitter.  
           [0007]    In another calibration technique, the electrical circuitry in the transmitter measures impedance of the sensing cell to provide an indication of the accuracy of the sensing cell. An indication that the sensing cell is inaccurate can be used to indicate that calibration is required.  
           [0008]    The calibration process typically requires the process to be shut down so that the analyzer can be removed from the stack for application of the calibration gas. Further, in applications where exhaust gas contains a high particle content, the diffusion element can become plugged and damaged. This also requires the industrial process to be shut down so that the diffusion element can be cleaned or replaced. Diffusion element maintenance and other procedures requiring the sensor cell assembly to be removed from service are time consuming and costly.  
         SUMMARY OF THE INVENTION  
         [0009]    A combustion gas analyzer for measuring the concentration of a gas constituent in an exhaust gas stream includes a sensor cell assembly which is configured to sense the gas constituent. A filter substantially encloses the sensor cell assembly. A valve assembly is coupled to a conduit which connects to the filter. The conduit is used for supplying a calibration gas or for back washing dust particles in the filter. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 shows a combustion gas analyzer mounted on a stack in accordance with the prior art.  
         [0011]    [0011]FIG. 2 is a combustion gas analyzer system mounted on a stack in accordance with an embodiment of the present invention.  
         [0012]    [0012]FIG. 3 is an expanded view of the valve assembly in accordance with an embodiment of the present invention.  
         [0013]    [0013]FIG. 4 is an expanded view of the valve assembly in accordance with an embodiment of the present invention.  
         [0014]    [0014]FIG. 5 is an expanded view of the valve assembly in accordance with an embodiment of the present invention.  
         [0015]    [0015]FIG. 6 is a flow diagram showing a method of a controller operating a valve assembly in accordance with an embodiment of the present invention.  
         [0016]    [0016]FIG. 7 is a combustion gas analyzer system mounted on a stack in accordance with another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    [0017]FIG. 1 depicts gas analyzer  10  which is used to measure the concentration of gases in exhaust gas  17 . The exhaust gas  17  is vented through stack  18  from an industrial process. Measured concentration of gas, such as oxygen, oxides of nitrogen, sulfur dioxide and carbon monoxide, in exhaust gas  17  relate to the combustion efficiency of the industrial process. The amount of fuel supplied to the industrial process can be adjusted to attain efficient combustion based upon the concentration of the gases in exhaust gas  17 .  
         [0018]    Gas analyzer  10  includes sensor cell assembly  14  positioned and supported in stack  18  by flange  16 . Stack  18  is a flue or duct of an industrial combustion process and carries exhaust gas  17 . Sensor cell assembly  14  includes a diffusion element  15  and sensing cell  11 . Diffusion element  15  disperses exhaust gas  17  about the sensing cell  11  as exhaust gas  17  enters the sensor cell assembly  14 . Electrical circuitry  12  in transmitter  9  is coupled to and reads an electrical output from the sensor cell  11  indicative of gas concentration. Electrical circuitry  12  provides gas concentration output through output conductors  13 .  
         [0019]    If there is a high particle content in exhaust gas  17 , the diffusion element  15  can become plugged and damaged. To replace diffusion element  15 , the industrial process must be shut down so that diffusion element  15  can be cleaned or replaced. As a result of a high particle content in exhaust gas  17 , cleaning or replacing diffusion element  15  is time consuming and costly. Further, as discussed in the Background section, the gas analyzer  10  may need to be removed from the stack for calibrating.  
         [0020]    [0020]FIG. 2 depicts gas analyzer system  51  in accordance with an embodiment of the present invention. Gas analyzer system  51  includes filter  20 . For example filter  20  can be a porous metal filter such as those available from Mott Corporation of Farmington, Conn. Filter  20  encloses sensor cell assembly  14  and traps particulate matter in exhaust gas  17  as exhaust gas  17  moves past sensor cell assembly  14 . Filter  20  prevents particulate matter from reaching sensor cell assembly  14 .  
         [0021]    Gas analyzer system  51  also includes valve assembly  38  coupled to filter  20  by a conduit  26 . Conduit  26  is approximately 0.25 inches in diameter in this example and has a first end  28  connected to filter  20  and a second end  30  connected to an outlet port  32  of valve assembly  38 . Conduit  26  can be of any appropriate length such that valve assembly  38  can be positioned at the base of stack  18  where an operator can easily reach it. Valve assembly  38  also includes first inlet port  37  and second inlet port  39  as discussed below.  
         [0022]    As filter  20  traps particulate matter in exhaust gas  17 , filter  20  may become plugged and prevent a sufficient amount of exhaust gas  17  from entering the sensor cell assembly  14 . One technique to clean filter  20  is to shut down the industrial process and clean or replace filter  20  in the stack. This technique of cleaning or replacing filter  20  is time consuming and costly. Therefore, in one aspect of the invention, gas analyzer system  51  includes a blow-back operation to periodically purge and dislodge particulate matter in filter  20 .  
         [0023]    First inlet port  37  of valve assembly  38  is coupled to pressurized blow-back gas  46  which is set, for example, to more then  10  psig higher than the industrial process. When filter  20  becomes plugged with trapped particulate matter, pressurized blow-back gas  46  is directed from first inlet  37  of valve assembly  38  to exit outlet port  32  through valve assembly  38 . Valve assembly  38  can be manually operated, operated by gas analyzer  10  or operated by another controller such as controller  44 . Pressurized blow-back gas  46  travels through conduit  26  and enters filter  20 . For example, when gas analyzer  10  is a combustion oxygen analyzer, pressurized blow-back gas  46  can consist of dry pressurized air or dry pressurized nitrogen.  
         [0024]    In another aspect of the invention, gas analyzer  10  must be periodically calibrated in order to maintain accuracy in gas concentration measurements. Gas analyzer system  51  includes a calibration operation. Gas analyzer  10  is calibrated using calibration gas  48 . Second inlet port  39  is coupled to the pressurized calibration gas  48  which is at least 10 psig higher than the industrial process. Valve assembly  38  is operated to allow pressurized calibration gas  48  to enter second inlet  39  of valve assembly  38  and exit outlet port  32 . Valve assembly  38  can be manually operated, operated by gas analyzer  10  or operated by another controller such as controller  44 . Pressurized calibration gas  46  travels through conduit  26  and floods sensor cell assembly  14 . When gas analyzer  10  is a combustion oxygen analyzer, pressurized calibration gas  48  consists of, for example, a mixture of nitrogen and a known concentration of oxygen.  
         [0025]    During the calibration process, sensor cell assembly  14  senses the concentration of oxygen in the calibration gas  48 . The electrical circuitry  12  provides an output value representative of the measured oxygen concentration. The measured value of oxygen concentration is compared to the known concentration of oxygen in the calibration gas  48 . A correction factor is calculated and can be applied to all subsequent measurements of the exhaust gas  17  until a future calibration is performed. The correction factor can be stored, for example, in a memory in transmitter  9 .  
         [0026]    The particular implementation of valve assembly  38  can be configured as desired. FIGS. 3-5 are diagrams which show three example configurations for valve assembly  38  when manually operated, operated by gas analyzer  10  or operated by a controller such as controller  44 . In FIG. 3, first inlet port  37  is coupled to outlet port  32  when valve assembly  38  is in a first position  40 . First position  40  allows pressurized blow-back gas  46  to flow through conduit  26  and into filter  20  to purge filter  20  of particulate matter. In FIG. 4, second inlet port  39  is coupled to outlet port  32  when valve assembly  38  is in a second position  41 . Second position  41  also allows pressurized calibration gas  48  to flow through conduit  26  and flood sensor cell assembly  14  to calibrate the sensing cell  11 . In FIG. 5, valve assembly  38  is in a third position  43  in which neither first inlet port  37  nor second inlet port  39  are coupled to outlet port  32 . Both blow-back gas  46  and calibration gas  48  are blocked from flowing through conduit  26  in third position  43 .  
         [0027]    Regardless if the operation is manually operated, operated by gas analyzer  10  or oeprated by controller  44 , each position of valve assembly  38  relates to whether gas analyzer system  51  is purging filter  20 , calibrating the sensor cell  11 , or doing neither.  
         [0028]    Referring back to FIG. 2, when valve assembly  38  is operated by controller  44 , controller  44  includes input  52 . In some embodiments of the invention, input  52  is coupled to transmitter  9  through conductors  13 . Electrical circuitry  12 , in this configuration, measures impedence of the sensing cell  11  to determine if sensing cell  11  is drifting in accuracy. When the measured impedence indicates an inaccuracy of the sensing cell  11 , a signal is transmitted through conductor  13  to input  52 . This signal indicates that a calibration operation should be initiated. Electrical circuitry  12  can monitor response speed of sensing cell  11  during the application of calibration gas  48  and exhaust gas  17 . A slow response speed can be an indicator that the filter  20  is clogged. When a slow response speed is detected, controller  44  can initiate a blow-back operation.  
         [0029]    In other embodiments of the invention, valve assembly  38  is controlled by controller  44  which stores, for example in a memory of controller  44 , pre-programmed time intervals conveyed through input  52 . In this configuration, a clock periodically initiates valve assembly  38  to perform the blow-back operation or the calibration operation.  
         [0030]    In other embodiments of the invention, controller  44  has a user input  52 . In this configuration, input  52  receives a signal from an operator to initiate either a blow-back or calibration operation.  
         [0031]    All and/or some of all the above-identified inputs can be included in controller  44 . Controller  44  can be a programmable logic controller (PLC), digital controller (DC), a pneumatic controller or any other process controller or comparable device.  
         [0032]    [0032]FIG. 6 is a flow diagram which shows an example of a method with which controller  44  operates valve assembly  38 . The method begins at first step  54  where controller  44  sets valve assembly  38  in a nominal state. In the nominal state, calibration gas  48  and blow-back gas  46  are blocked from flowing through conduit  26 .  
         [0033]    At step  56 , controller  44  determines whether a calibration is required. If a calibration is required, the process advances to step  58 . At step  58 , the controller opens the valve assembly  38  to allow calibration gas  48  to flood the sensor cell assembly  14 . After calibration gas  48  is allowed to flood the sensor cell assembly  14 , the process passes control to step  60 . If a calibration is not required in step  56  the process passes control to step  60 .  
         [0034]    At step  60 , controller  44  determines whether a blow-back is required. If a blow-back is required, the process advances to step  62 . At step  62 , controller  44  opens the valve assembly  38  to allow blow-back gas  46  to purge filter  20 . After blow-back gas  46  is allowed to purge filter  20 , the method ends. If a blow-back is not required in step  60  the method also ends.  
         [0035]    [0035]FIG. 7 depicts gas analyzer system  51  in accordance with another aspect of the present invention. Valve assembly  38  includes first solenoid valve  34  and second solenoid valve  36 . For example, solenoid valves  34  and  36  can be two-way solenoid valves, three-way solenoid valves, and four-way solenoid valves. Solenoid valves  34  and  36  can be manually operated, operated by gas analyzer  10  or operated by controller  44  as in the method previously discussed.  
         [0036]    When solenoid valve  34  is open and solenoid valve  36  remains closed, pressurized blow-back gas  46  is allowed to flow through conduit  26  and purge filter  20  as discussed above. When solenoid valve  36  is open and solenoid valve  34  remains closed, pressurized calibration gas  48  is allowed to flow through conduit  26  and flood sensor cell assembly  14  to calibrate the sensor cell  11  as discussed above. Lastly, when solenoid valves  34  and  36  are both closed, blow-back gas  46  and calibration gas  48  are blocked from flowing through conduit  26 .  
         [0037]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.