Method for continuously monitoring chemical species and temperature in hot process gases

Abstract

Methods and apparatus are presented using tunable diode lasers for monitoring and/or controlling a high temperature process using an oxidizer containing O 2 and organic fuel. Real-time monitoring of key species such as O 2 , CO, and H 2 O allow determination of the global or local stoichiometry, gas temperature, particulate concentration, and air entrainment levels into the process. Coupling the measured information with a control system provides a means for optimizing and controlling the process.

What is claimed is:  
     
         1 . A method of monitoring or controlling the global or local stoichiometry of a process operating at lean conditions, the method comprising the steps of: 
 a) launching an initial collimated beam of radiation emitted by tunable diode laser along a line-of-sight path through the process, and, for O 2 , monitoring a selected rotational line near the B-X (0,0) band of O 2 , corresponding to a wavelength near 763 nm, which can be accessed using commercially available AlGaAs diode lasers that are fiber optically compatible;    b) collecting and transporting the transmitted radiation at a site substantially opposite the launch position to a photo detector having a filter element, the photo detector having sensitivity in the O 2  spectral region;    c) processing the optical signal by observing the amount of attenuation observed from the initial beam as the laser is tuned over a resonance absorption line of O 2  the integrated area of the absorption line being directly proportional to the O 2  number density for a given temperature; and    d) producing an electrical signal based on the O 2  number density for use in controlling one or more of the following variables: process pressure, fuel inlet flow rate, and oxidant inlet flow rate.    
     
     
         2 . Method in accordance with  claim 1  wherein the electrical signal produced alerts an operator of the process operating condition, allowing manual adjustment of the process by adjusting oxidant, fuel or process pressure or any combination of these three process variables.  
     
     
         3 . Method in accordance with  claim 2  wherein the electrical signal is used to control the operation of one or more actuators, which in turn allows manipulation of one or more of the process variables.  
     
     
         4 . A method of monitoring or controlling the global or local stoichiometry of a process operating at fuel rich conditions, the method comprising the steps of: 
 a) launching an initial collimated beam of radiation emitted by tunable diode laser along a line-of-sight path through the process, and, for CO, monitoring a selected rotational line is used from the CO overtone band near 1.56 μm, which can be accessed using commercially available InGaAsP/lnP diode lasers that can be fiber optically coupled;    b) collecting and transporting the transmitted radiation at a site substantially opposite the launch position to a photo detector with a filter element, the photo detector having sensitivity in the CO spectral region;    c) processing the optical signal by observing the amount of attenuation observed from the initial beam as the laser is tuned over a resonance absorption line of CO, the integrated area of the absorption line divided by the line strength and pathlength being directly proportional to the CO number density for a given temperature; and    d) producing an electrical signal based on the CO number density for use in controlling one or more of the following variables: process pressure, fuel inlet flow rate, and oxidant inlet flow rate.    
     
     
         5 . Method in accordance with  claim 4  wherein the electrical signal produced alerts an operator of the process operating condition, allowing manual adjustment of the process by adjusting oxidant, fuel or process pressure or any combination of these three variables.  
     
     
         6 . Method in accordance with  claim 5  wherein the electrical signal to control the operation of one or more actuators, which in turn allows manipulation of one or more process variables.  
     
     
         7 . Method in accordance with  claim 6  wherein the process pressure may be decreased by increasing the stack damper area thereby increasing air infiltration, which will decrease the measured CO concentration due to combustion with air.  
     
     
         8 . Method in accordance with  claim 6  wherein the oxidant input flow rate which can consist of air, oxygen enriched air or oxygen can be adjusted to reach the desired CO concentration for the process.  
     
     
         9 . A method for monitoring both CO and O 2  number density simultaneous or nearly simultaneous at a specified location on a process, the method comprising the steps of: 
 a) launching a plurality of collimated beams of radiation emitted by a plurality of tunable diode lasers along a line-of-sight path through the process, the collimated beams containing a plurality of wavelengths, e.g., 763 nm and 1.5 μm radiation for O 2  and CO monitoring respectively;    b) simultaneously launching a plurality of tunable beams by spatially introducing the radiation through a fiber optic network, or alternatively, nearly simultaneously introducing beams by temporally separating the multiple input beams, i.e., tune the lasers at different times;    c) simultaneous transmission of multiple frequencies separated using dispersion elements and/or narrowband filters and detector combination allowing discrimination between the different wavelengths, and in the case of temporal separation, a single detector to demodulate the input signal thereby resolving all wavelengths tuned;    d) processing the optical signal by observing the amount of attenuation observed from a plurality of signals tuned over a resonance absorption line for a specific species, the integrated area of the absorption line being directly proportional to the number density for a given temperature; and    e) producing an electrical signal based on the O 2  number density for use in controlling one or more of the following variables: process pressure, fuel inlet flow rate, and oxidant inlet flow rate.    
     
     
         10 . A method for monitoring temperature of gas phase of a process comprising the steps of: 
 a) launching an initial collimated beam of radiation emitted by tunable diode laser along a line-of-sight path through the gas phase of the process;    b) tuning the diode laser over two or more rotational lines of the selected species selected from the group consisting of CO, O 2 , H 2 O or another detectable process gas, such as HCl, that is not strongly dependent on the process stoichiometry;    c) collecting and transporting the transmitted radiation at a site substantially opposite the launch position to a photo detector (sensitive at the wavelength of interest) having a filter element, the photo detector having sensitivity in the spectral region of interest, to produce an optical signal;    d) processing the optical signal by observing the amount of attenuation observed from the initial beam as the laser is tuned over two or more resonance absorption lines, the temperature being obtained by applying the following expression            R   =         (       S   1       S   2       )       T   o       ×     exp        [           -   hc                   Δ                 E     k          (       1   T     -     1     T   o         )       ]                           where R is the ratio of the integrated absorbance of each transition at the unknown temperature T, (S 1 /S 2 ) T     o    is the ratio of the linestrength values at some reference temperature, To, ΔE is the energy separation of the absorbing states, h is Planck's constant, k is Boltzmann's constant and c is the speed of light; and    e) producing an electrical signal based on the for use in regulation of one or more of the following variables: process pressure, fuel inlet flow rate, and oxidant inlet flow rate.    
     
     
         11 . A method for monitoring particulate concentration level in a monitored gas phase region of a process comprising the steps of: 
 a) launching an initial collimated beam of radiation emitted by tunable diode laser along a line-of-sight path through the gas phase;    b) opposite the launch position the transmitted radiation is collected and transported to a photo detector (sensitive at the wavelength of interest) having a filter element;    c) tuning the laser off any resonance absorbing species and monitor the attenuated radiation detected;    d) processing the optical signal by observing the amount of attenuation observed from the initial beam as the laser is tuned away from all resonance absorption lines, the observed attenuation being related to the particle density along the line-of-sight optical path by the following expression              I     I   o       =     e       -     α   ext          l                         where I o  is the beam initial irradiance and I is the measured irradiance after the beam propagates a distance 1 through the process, and, in the presence of particles, the measured attenuation can be related to the particle number density if the extinction coefflcient,α ext , is known; and    e) producing an electrical signal based on the particle number density that can be used in regulation of one or more of the following variables: process pressure, fuel inlet flow rate, and oxidant inlet flow rate.    
     
     
         12 . A method for monitoring the rate of air entrainment into a combustion process comprising the steps of: 
 a) measuring the concentration of the major combustion product species such as CO 2  or H 2 O by launching an initial collimated beam of radiation emitted by tunable diode laser along a line-of-sight path through the combustion process;    b) opposite the launch position the transmitted radiation is collected and transported to a photo detector (sensitive at the wavelength of interest) with a filter element;    c) processing the optical signal by observing the amount of attenuation observed as the laser is tuned over a resonance absorption line, the integrated area of the absorption line divided by the line strength and pathlength is directly proportional to the number density for a given temperature;    d) estimating the air entrainment rate into the process using the fuel and oxidizer inlet composition and flow rates along with the ambient air composition, and using a difference between the theoretical value for complete combustion and the measured species concentration; and    e) producing an electrical signal based on the estimated air entrainment rate for use in regulating one or more of the following variables: process pressure, fuel inlet flow rate, and oxidant inlet flow rate.    
     
     
         13 . A method for indirectly monitoring H 2  concentration and/or hydrocarbons in a high temperature gas phase comprising the steps of: 
 a) measuring the concentration of H 2 O by launching an initial collimated beam of radiation by a tunable diode laser along a line-of-sight path through a gaseous phase (preferably flue gas of a combustion process) in the spectral region where H 2 O absorption transitions are found.    b) opposite the launch position the transmitted radiation is collected and transported to a photo detector with a filter element;    c) processing the optical signal by observing the amount of attenuation observed from the initial beam as the laser is tuned over a resonance absorption line of H 2 O;    d) a second measurement point is used either within the gas phase region or in a diverted stream of gas;    e) measuring the difference between H 2 O downstream and H 2 O upstream of the O 2  gas introduction to back calculate the amount of H 2  and/or unburned hydrocarbons in the gaseous phase; and    f) producing an electrical signal based on the amount of unburned H 2  and/or hydrocarbons.    
     
     
         14 . A method to monitor pollutant X in gas phase where X is a pollutant of specific interest to a process, the method for monitoring comprising the steps: 
 a) launching an initial collimated beam of radiation emitted by tunable diode laser along a line-of-sight path through a gas phase of the process;    b) opposite the launch position the transmitted radiation is collected and transported to a photo detector having a filter element, the detector having sensitivity in the spectral region of species X;    c) processing the optical signal by observing the amount of attenuation observed from the initial beam as the laser is tuned over a resonance absorption line of species X; and    d) producing an electrical signal based on the number density of species X.    
     
     
         15 . A method of monitoring or controlling the global or local stoichiometry of an oxy-fuel combustion process operating at lean conditions, the method comprising the steps of: 
 a) launching an initial collimated beam of radiation emitted by tunable diode laser along a line-of-sight path through combustion products of the process, and, for O 2  monitoring a selected rotational line near the B-X (0,0) band of O 2 , corresponding to a wavelength near 763 nm, which can be accessed using commercially available AlGaAs diode lasers that are fiber optically compatible;    b) collecting and transporting the transmitted radiation at a site substantially opposite the launch position to a photo detector having a filter element, the photo detector having sensitivity in the O 2  spectral region;    c) processing the optical signal by observing the amount of attenuation observed from the initial beam as the laser is tuned over a resonance absorption line of O 2 , the integrated area of the absorption line being directly proportional to the O 2  number density for a given temperature; and    d) producing an electrical signal based on the O 2  number density for use in controlling one or more of the following variables of the oxy-fuel combustion process: process pressure, fuel inlet flow rate, and oxidant inlet flow rate.    
     
     
         16 . Method in accordance with  claim 15  wherein the electrical signal produced alerts an operator of the process operating condition, allowing manual adjustment of the process by adjusting oxidant, fuel or process pressure or any combination of these three process variables.  
     
     
         17 . Method in accordance with  claim 16  wherein the electrical signal is used to control the operation of one or more actuators, which in turn allows manipulation of one or more of the process variables.  
     
     
         18 . A method of monitoring or controlling the global or local stoichiometry of an oxy-fuel combustion process operating at fuel rich conditions, the method comprising the steps of: 
 a) launching an initial collimated beam of radiation emitted by tunable diode laser along a line-of-sight path through combustion products of the process, and, for CO, monitoring a selected rotational line is used from the CO overtone band near 1.56 μm, which can be accessed using commercially available InGaAsP/InP diode lasers that can be fiber optically coupled;    b) collecting and transporting the transmitted radiation at a site substantially opposite the launch position to a photo detector with a filter element, the photo detector having sensitivity in the CO spectral region;    c) processing the optical signal by observing the amount of attenuation observed from the initial beam as the laser is tuned over a resonance absorption line of CO, the integrated area of the absorption line divided by the line strength and pathlength being directly proportional to the CO number density for a given temperature; and    d) producing an electrical signal based on the CO number density for use in controlling one or more of the following variables of the oxy-fuel combustion process: process pressure, fuel inlet flow rate, and oxidant inlet flow rate.    
     
     
         19 . Method in accordance with  claim 18  wherein the electrical signal produced alerts an operator of the process operating condition, allowing manual adjustment of the process by adjusting oxidant, fuel or process pressure or any combination of these three variables.  
     
     
         20 . Method in accordance with  claim 20  wherein the electrical signal to control the operation of one or more actuators, which in turn allows manipulation of one or more process variables.  
     
     
         21 . Method in accordance with  claim 20  wherein the process pressure may be decreased by increasing stack damper area thereby increasing air infiltration, which will decrease the measured CO concentration due to combustion with air.  
     
     
         22 . Method in accordance with  claim 20  wherein the oxidant input flow rate which is selected from the group consisting of oxygen enriched air or oxygen can be adjusted to reach the desired CO concentration for the process.  
     
     
         23 . A method for monitoring both CO and O 2  number density simultaneous or nearly simultaneous at a specified location on an oxy-fuel combustion process, the method comprising the steps of: 
 a) launching a plurality of collimated beams of radiation emitted by a plurality of tunable diode lasers along a line-of-sight path through combustion products of the process, the collimated beams containing a plurality of wavelengths, e.g., 763 nm and 1.5 μm radiation for O 2  and CO monitoring respectively;    b) simultaneously launching a plurality of tunable beams by spatially introducing the radiation through a fiber optic network, or alternatively, nearly simultaneously introducing beams by temporally separating the multiple input beams, i.e., tune the lasers at different times;    c) simultaneous transmission of multiple frequencies separated using dispersion elements and/or narrowband filters and detector combination allowing discrimination between the different wavelengths, and in the case of temporal separation, a single detector to demodulate the input signal thereby resolving all wavelengths tuned;    d) processing the optical signal by observing the amount of attenuation observed from the plurality of tunable beams tuned over a resonance absorption line for a specific species, the integrated area of the absorption line being directly proportional to the number density of a species for a given temperature; and    e) producing an electrical signal based on the number density of a species for use in controlling one or more of the following variables: process pressure, fuel inlet flow rate, and oxidant inlet flow rate.    
     
     
         24 . A method for monitoring temperature of gas phase of oxy-fuel combustion process comprising the steps of: 
 a) launching an initial collimated beam of radiation emitted by tunable diode laser along a line-of-sight path through combustion products of the combustion process;    b) tuning the diode laser over two or more rotational lines of the selected species selected from the group consisting of CO, O 2 , H 2 O or another detectable process gas, such as HCl, that is not strongly dependent on the process stoichiometry;    c) collecting and transporting the transmitted radiation at a site substantially opposite the launch position to a photo detector (sensitive at the wavelength of interest) having a filter element, the photo detector having sensitivity in the spectral region of interest, to produce an optical signal;    d) processing the optical signal by observing the amount of attenuation observed from the initial beam as the laser is tuned over two or more resonance absorption lines, the temperature being obtained by applying the following expression            R   =         (       S   1       S   2       )       T   o       ×     exp        [           -   hc                   Δ                 E     k          (       1   T     -     1     T   o         )       ]                           where R is the ratio of the integrated absorbance of each transition at the unknown temperature T, (S 1 /S 2 ) T     o    is the ratio of the linestrength values at some reference temperature, To, ΔE is the energy separation of the absorbing states, h is Planck's constant, k is Boltzmann's constant and c is the speed of light; and    e) producing an electrical signal based on the for use in regulation of one or more of the following variables of the oxy-fuel combustion process: process pressure, fuel inlet flow rate, and oxidant inlet flow rate.    
     
     
         25 . A method for monitoring particulate concentration level in a monitored gas phase region of an oxy-fuel combustion process comprising the steps of: 
 a) launching an initial collimated beam of radiation emitted by tunable diode laser along a line-of-sight path through the gas phase combustion products of the process;    b) opposite the launch position the transmitted radiation is collected and transported to a photo detector (sensitive at the wavelength of interest) having a filter element;    c) tuning the laser off any resonance absorbing species and monitor the attenuated radiation detected;    d) processing the optical signal by observing the amount of attenuation observed from the initial beam as the laser is tuned away from all resonance absorption lines, the observed attenuation being related to the particle density along the line-of-sight optical path by the following expression              I     I   o       =     e       -     α   ext          l                         where I o  is the beam initial irradiance and I is the measured irradiance after the beam propagates a distance I through the process, and, in the presence of particles, the measured attenuation can be related to the particle number density if the extinction coefficient,α ext , is known; and    e) producing an electrical signal based on the particle number density that can be used in regulation of one or more of the following variables of the oxy-fuel combustion process: process pressure, fuel inlet flow rate, and oxidant inlet flow rate.    
     
     
         26 . A method for monitoring the rate of air entrainment into an oxy-fuel combustion process comprising the steps of: 
 a) measuring the concentration of the major combustion product species such as CO 2  or H 2 O by launching an initial collimated beam of radiation emitted by tunable diode laser along a line-of-sight path through the oxy-fuel combustion process;    b) opposite the launch position the transmitted radiation is collected and transported to a photo detector (sensitive at the wavelength of interest) with a filter element;    c) processing the optical signal by observing the amount of attenuation observed as the laser is tuned over a resonance absorption line, the integrated area of the absorption line divided by the line strength and pathlength is directly proportional to the number density for a given temperature;    d) estimating the air entrainment rate into the process using the fuel and oxidizer inlet composition and flow rates along with the ambient air composition, and using adifference between the theoretical value for complete combustion and the measured species concentration; and    e) producing an electrical signal based on the estimated air entrainment rate for use in regulating one or more of the following variables: process pressure, fuel inlet flow rate, and oxidant inlet flow rate.    
     
     
         27 . A method for indirectly monitoring H 2  concentration and/or hydrocarbons in a high temperature gas stream of an oxy-fuel combustion process comprising the steps of: 
 a) measuring the concentration of H 2 O by launching an initial collimated beam of radiation by a tunable diode laser along a line-of-sight path through a gaseous phase region of the process (preferably flue gas of the combustion process) in the spectral region where H 2 O absorption transitions are found.    b) opposite the launch position the transmitted radiation is collected and transported to a photo detector with a filter element;    c) processing the optical signal by observing the amount of attenuation observed from the initial beam as the laser is tuned over a resonance absorption line of H 2 O;    d) a second measurement point is used either within the gas phase region or in a diverted stream of gas;    e) measuring the difference between H 2 O downstream and H 2 O upstream of the O 2  gas introduction to back calculate the amount of H 2  and/or unburned hydrocarbons in the process stream; and    f) producing an electrical signal based on the amount of unburned H 2  and/or hydrocarbons.    
     
     
         28 . A method to monitor species X where X is a species of specific interest to an oxy-fuel combustion process, the method for monitoring comprising the steps: 
 a) launching an initial collimated beam of radiation emitted by a tunable diode laser along a line-of-sight path through combustion products of the combustion process;    b) opposite the launch position the transmitted radiation is collected and transported to a photo detector having a filter element;    c) processing the optical signal by observing the amount of attenuation observed from the initial beam as the laser is tuned over a resonance absorption line of species X; and    d) producing an electrical signal based on the number density of species X.    
     
     
         29 . An apparatus for monitoring a species X where X is a species of specific interest to a process, the apparatus comprising: 
 a) a tunable diode laser for launching an initial collimated beam of radiation along a line-of-sight path through a gas phase of a process;    b) a collector to collect the launched radiation, the collection positioned substantially opposite the tunable diode laser, the collector capable of transporting the collected radiation to a photo detector, the photo detector having a filter element;    c) an optical processor for observing attenuation of the initial beam as it is tuned over a resonance absorption line of species X; and    d) means for producing an electrical signal based on the number density of species X.