Abstract:
An example method of detecting components of a gas includes detecting substantially all components of a gas using distinct infrared wavelengths within a portion of the infrared spectrum, the portion being less than the entire infrared spectrum.

Description:
BACKGROUND 
       [0001]    This disclosure relates generally to determining a quality of a gas. 
         [0002]    High-quality gas is typically worth more than low-quality gas. If gas is offered for sale, its price may depend on its quality. Determining the quality of other gases, such as atmospheric gas, indoor air, etc., may be useful for environmental reasons. 
         [0003]    Components are the chemically independent constituents of a gas. Natural gas, an example type of gas, is made of several components, some of which are hydrocarbons. The quality of natural gas may be based on the enthalpy of combustion of its individual components. 
         [0004]    Methane and the other components of natural gas vary with time in a pipeline and it may be necessary to understand the nature and extent of this variation in composition. Natural gas comprises mainly methane (CH4), with a small proportion of higher hydrocarbons such as ethane (C2H6), propane (C3H8), butane (C4H10) and so on. Inert gases such as nitrogen (N2) and carbon dioxide (CO2) are present at the level of a few percent volume, and various compounds can be present in parts per million (ppm) quantities, including the odorant. 
         [0005]    One technique for determining the quality of gas involves separation of individual components of the gas. The separation technique is not suitable for use in some environments, such as when measuring gas within a pipeline. Another technique for determining the quality of gas measures changes in light intensity that has been directed through, and not absorbed by, the gas. The light not absorbed by the gas is spatially dispersed (by wavelength) and forms a light spectrum that is projected onto a detector. The modified light spectrum is compared to the light&#39;s actual light spectrum to determine the absorbance spectrum of the fluid. Cross-interference may undesirably distort these measurements making quality analysis difficult. 
       SUMMARY 
       [0006]    An example method of detecting components of a gas includes detecting substantially all components of a gas using distinct infrared wavelengths within a portion of the infrared spectrum, the portion being less than the entire infrared spectrum. 
         [0007]    An example method of detecting components of a gas includes identifying a group of wavelengths associated with components that distort measurements, and then detecting components of the gas utilizing wavelengths that are not in the group of wavelengths. 
         [0008]    An example gas component meter includes a filter that is configured to limit detection of a first group of wavelengths associated with components that distort measurements to a detector. A controller is configured to determine a quality of the gas utilizing a second group of wavelengths different than the first group of wavelengths. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0009]    The various features and advantages of the disclosed examples will become apparent to those skilled in the art from the detailed description. The figures that accompany the detailed description can be briefly described as follows: 
           [0010]      FIG. 1  shows an example gas component meter. 
           [0011]      FIG. 2A  shows a plot of transmission percentages from the  FIG. 1  meter for some wavelengths in the mid-infrared spectrum. 
           [0012]      FIG. 2B  shows a plot of transmission percentages from the  FIG. 1  meter for other wavelengths in the mid-infrared spectrum. 
           [0013]      FIG. 3  shows a plot of transmission percentages from the  FIG. 1  meter for wavelengths that are from about eight to eleven microns. 
           [0014]      FIG. 4  shows an example method of identifying components utilizing the  FIG. 1  meter. 
           [0015]      FIG. 5  shows a highly schematic view of how the transmission percentages are used to determine quality. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Referring to  FIG. 1 , an example gas component meter assembly  10  includes an infrared light source  14 , a filter  18 , and a detector  22  within a housing  26 . The housing  26 , in this example, is secured to a gas pipeline  30 . Apertures  34  within the pipeline  30  and the housing  26  permit gas to communicate between an interior  38  of the housing  26  and the pipeline  30 . 
         [0017]    Gas G communicates through the pipeline  30  from a supply  42  to a destination  46 . The example gas G is natural gas. The supply  42  is a utility company. The destination  46  is a home or business. 
         [0018]    The example meter  10  determines the quality of the natural gas within the interior  38  (and thus the composition of gas within the pipeline  30 ). The composition is used to determine the quality of the natural gas within the interior  38  and the pipeline  30 . 
         [0019]    In one example, a provider of the supply  42  utilizes the quality information when determining how much to charge the destination for the gas G. The meter  10  is mounted to the pipeline  30  between the supply  42  and the destination  46 . In other examples, the meter  10  may be utilized at location of the supply  42 , at the location of the destination  46 , or at some other location. 
         [0020]    The meter  10  includes a controller  50  that is operably linked to the infrared light source  14 , the filter  18 , and the detector  22 . To monitor the components of the natural gas within the interior  38 , the controller  50  adjusts the filter  18  to select a group of infrared light waves  54  emitted by  14  within the meter  10 . The waves  54  propagate from the infrared light source  14 . The waves  54  are mid-infrared spectrum waves ranging, in this example, from three microns to twelve microns. The waves  54  pass through the gas G within the interior  38 . 
         [0021]    In this example, the filter  18  allows some of the waves  54  to reach the detector  22 , and blocks some of the waves  54  from reaching the detector  22 . The example filter  18  is a cross-interference/broadband filter device that ensures only waves having lengths from eight to ten microns are detected by the detector  22 . 
         [0022]    In another example, the infrared light source  14  generates some, rather than all, the waves in the mid-infrared spectrum such as only waves having lengths from eight to ten microns. In such an example, the filter  18  is not used. Another filter, such as a cross-interference filter, may still be used however. 
         [0023]    The distance L between the infrared light source  14  and the detector  22  is the optical path length of the waves  54 . As the waves  54  move through the gas G toward the detector  22 , alkanes in the gas G absorb some of the light. For the wavelengths that pass through the filter  18 , the detector  22  detects the light that has not been absorbed by alkanes in the gas G. The controller  50  utilizes this information to determine the percentage of the waves  54  that have been transmitted through the gas G to the detector. The percentages detected by the detector  22  represent the percentages of the waves  54  that have not been absorbed by alkanes in the gas G. 
         [0024]    Certain wavelengths are associated with the detection of certain components within the gas. For example, as shown in  FIGS. 2A and 2B , an amount of a CO2 component within the gas G may be revealed by the transmission percentages of the wavelengths in area  60 . As shown, these wavelengths are slightly greater than four microns. Also, an amount of an H2O component within the gas G is typically measured using wavelengths in area  64  that are near six microns. In prior art systems, transmission percentages were determined for substantially all wavelengths in the mid-infrared spectrum. At least groups of some wavelengths within the mid-infrared spectrum provide transmission percentages that tend to distort measurements of components within the gas G. 
         [0025]    In the example system, wavelengths from eight microns to ten microns are detected by the detector  22 . That is, transmission percentages for the gas G are only determined within this range of wavelengths. This example range includes three distinct wavelengths at eight microns, nine microns, and ten microns. The transmission percentages of wavelengths within this range are shown in  FIG. 3 . 
         [0026]    Example components associated with wavelengths in the range of eight to eleven microns include CH4 in area  66 , C3H8 in area  74 , and C4H10+C5H12+C6H14 in area  78 . Multiple alkane species, C2nH2n+2, are associated with wavelengths in area  70 . Using the transmission percentages within this range of wavelengths, the example meter  10  is able to detect substantially all the component absorbances within the gas G that are critical to determining quality of the gas G. 
         [0027]    Referring to  FIG. 4 , an example method  100  of determining components within a gas includes a step  102  of identifying wavelengths associated with components that distort. In this example, those wavelengths would be wavelengths in areas  60  and  64  ( FIG. 2A ). 
         [0028]    The method  100  next, at a step  104 , detects components without using the wavelengths identified in the step  102 . In an example, the components are detected utilizing wavelengths within the range of eight to ten microns, which does not include the wavelengths in areas  60  and  64 . 
         [0029]    Referring to  FIG. 5 , a method  200  utilizes the transmission percentages to determine the quality of natural gas. The method  200  inputs the transmission percentages (or intensities) from a step  202  to a step  204 . 
         [0030]    The step  204  utilizes Beer&#39;s law to determine the concentrations of components using equation 1. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0031]    In Equation 1, α is the absorption coefficient of a single component of the natural gas mixture at a given wavelength, x is the alkane concentration, and L is the optical path length of the measurement cell. This information is the input to Equation 1 along with absorption coefficients for specific gas species and optical path length in step  205 . These absorption coefficients are calculated from accepted spectral infrared databases. 
         [0032]    The output of Beer&#39;s law is the concentration of the components at step  206 . This information is then used as the input to step  208 , the Gibb&#39;s rule summarized in Equation 2. In Equation 2, the sum of the heats of combustion for each hydrocarbon component is scaled by the concentration of the particular alkane; x is the alkane concentration, and ΔH combustion  is the alkane heat of combustion from step  210 . In principle, the simple molar addition of the individual heats of combustion gives rise to the Higher Heating Value,  212 . This input in conjunction with a database having the heats of combustion for hydrocarbons,  210 , are used to compute the Higher Heating Value,  212 . 
         [0000]    
       
                 
         
             
             
         
       
     
         [0033]    In some examples, the energy flow rate of this mixture of gases is given as: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0034]    Where Q is the energy flow rate given as a function of: V the volumetric flow rate; Z the compressibility factor; ρ the density and ΔH the heat of combustion, energy content or higher heating value. The energy content of natural gas is an intensive thermodynamic property. A volume of natural gas has N+1 degrees of freedom, where N is the number of constituents that make up the gas mixture. In order to calculate, the exact energy content value, N+1 measurements would need to be taken. In a typical natural gas sample this would mean greater than nine independent measurements. This measurement of nine or more wavelengths corresponds to monitoring the composition of natural gas components from methane (CH3) to octane (C8H18) or higher. 
         [0035]    Specifically, the system of linear equations corresponding to the components of the gas need to be solved. The algorithmic development for calculating the High Heating Value of a multispecies natural gas mixture is as follows. The absorption of infrared light at a particular wavelength for a natural gas mixture can be explained using Beer&#39;s law. The expansion of Beer&#39;s law at a given wavelength to take into account multiple gas species is given below as Equation 4. 
         [0000]        O.D.   λ1   =α   a1   x   a   L+α   b1   x   b   L+α   c1   x   c   L+. . . +α   j1   x   j   L   Equation 4
 
         [0036]    Notice that from the expansion of Beer&#39;s law that the absorption of infrared light at a particular wavelength is the summation of absorption from individual components. 
         [0037]    The expansion of Beer&#39;s law at a different given wavelength to take into account multiple gas species is given below as Equation 5. 
         [0000]        O.D.   λ2   =α   a2   x   a   L+α   b2   x   b   L+α   c2   x   c   L+. . . +α   j2   x   j   L   Equation 5
 
         [0038]    Both these equations are linear. The optical density and absorption coefficients are unique and different for each wavelength and gas mixture. However, the concentration of the gas species remains constant in each equation. Thus, a system of linear equations can be compiled to convert absorption to concentration. The system of linear equations can be converted to matrix form as shown below: 
         [0000]    
       
         
           
             
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         [0039]    A simpler representation of the matrix is: 
         [0000]    
       
      
         O.D. =  αLx )  
      
     
         [0040]    In order to solve for concentration, x, two methods are available. If the matrix is square than the solution to the equation above is dependent on inverting the operator: 
         [0000]          x =  O.D.   (   αL     −1 ) 
         [0041]    The solution above exists for a well defined system. In practice, a system of equations is either over or under determined. In this case an approximation of the solution needs to be made to fit the observed data. This method is normally referred to as the least squares method and is shown below (The superscript T refers to the transpose of the matrix  αL ): 
         [0000]          x = (   αL     T     αL   ) −1     αL     T     O.D.     
         [0042]    Approaches in the past have relied on determining regions of the infrared spectra that could be speciated. In other words, concentrations of all species within a natural gas were determined individually. Only then was the higher heating value calculated. By contrast, methods disclosed here remove this limitation. Specifically, this method is applicable to convoluted spectral ranges. Convolution is due to multiple alkane absorption coefficients at a particular wavelength contributing to the overall absorption coefficient at a particular wavelength. In this region or with an apparatus that measures a convoluted spectrum, speciation is difficult. However, gas quality still can be determined. This is accomplished by taking the dot product and minimizing the Euclidean Normal, ∥  αLx −  O.D. ∥ instead of determining gas species. The higher heating value for the mixture is then the dot product between  x , and the heats of combustions of hydrocarbon components. 
         [0043]    The higher heating value for natural gas mixture can be determined to an arbitrary accuracy by calculating the Euclidean Normal. 
         [0044]    The use of the method described above and minimizing the Euclidean Normal to calculate natural gas quality are features of the disclosed examples. These features were used when evaluating a set of wavelengths in the range of eight to eleven microns. 
         [0045]    The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. Thus, the scope of legal protection given to this disclosure can only be determined by studying the following claims.