Measurement of hydrocarbon fuel gas composition and properties from tunable diode laser absorption spectrometry

Patent metadata

Field	Value
Publication number	US-10495570-B2
Application number	US-201615283965-A
Country	US
Kind code	B2
Filing date	Oct 3, 2016
Priority date	Oct 3, 2016
Publication date	Dec 3, 2019
Grant date	Dec 3, 2019

Abstract

A tunable diode laser absorption spectrometer and a method of processing absorption spectra is used to measure concentrations of selected fuel gas components and calculate several fuel gas parameters, including heating value, relative density, compressibility, theoretical hydrocarbon liquid content and Wobbe index. In the described incarnation, a tunable laser diode directs near-infrared light into an optical cavity through a sample of fuel gas. A sensor measures intensity of light exiting the cavity as the laser wavelength is tuned over a specified range to construct a cavity-enhanced absorption spectrum for the fuel gas. A set of basis spectra for expected component species is used to analyze the spectrum and determine component concentrations, including methane, ethane, carbon dioxide, and other discrete and structured absorbers. Critically, a generic broadband absorption is used to model higher hydrocarbons that present themselves as nearly featureless absorption spectra. The fuel gas parameters are then calculated directly from determined component concentrations and the broadband absorption representing the higher hydrocarbons.

First claim

What is claimed is:

1. A tunable diode laser absorption spectrometry system for fuel gas measurement, comprising:
an off-axis integrated cavity output spectroscopy (ICOS) instrument comprising a first tunable laser diode with a first tunable wavelength range providing laser light coupled off-axis into a high-finesse optical cavity, a second tunable laser diode with a second tunable wavelength range providing laser light coupled off-axis into the cavity, a gas inlet and a gas outlet arranged to flow a fuel gas through the cavity, and an optical sensor arranged to measure intensity of laser light exiting the optical cavity, wherein the laser diodes are tuned over their respective wavelength ranges so that the sensor measurement provides a wavelength-dependent optical absorption spectrum; and
a computer processor to receive the sensor measurement from the optical sensor, access a database of basis spectra for fuel gas components, process the absorption spectrum with a chemometric fitting routine to determine concentrations of selected fuel gas components, and calculate at least a heating value (F) and an additional value (X) selected from relative density, compressibility, theoretical hydrocarbon liquid content, and Wobbe index from the determined fuel gas component concentrations on the basis of determined concentrations for methane (C CH 4 ) and ethane (C C 2 H 6 ) and the determined concentration (C BB ) of an offset basis spectrum representing higher hydrocarbons such that:
X=C CH 4 ·X CH 4 +C C 2 H 6 ·X C 2 H 6 +C BB ·E,

 where X CH 4 and X C 2 H 6 are respective coefficients for methane and ethane, and E is an empirical factor for a composite relative density, compressibility, theoretical hydrocarbon liquid content, or Wobbe index of all expected higher hydrocarbons in the fuel gas mixture.

2. The system as in claim 1 , wherein the database includes basis spectra for at least methane, ethane, together with a broadband offset basis to represent the higher hydrocarbons.

3. The system as in claim 2 , wherein the heating value (F) is calculated by the processor on the basis of the determined concentrations for methane and ethane and the determined concentration of the offset basis spectrum representing the higher hydrocarbons, such that:
F=C CH 4 ·F CH 4 +C C 2 H 6 ·F C 2 H 6 +C BB ·E,

where F CH 4 and F C 2 H 6 are respective heating values for methane and ethane, and E is an empirical factor for a composite heating value of all the expected higher hydrocarbons in the fuel gas mixture.

4. The system as in claim 1 , wherein the Wobbe index (I w ) is calculated by the processor on the basis of the determined concentrations for methane and ethane and the determined concentration of the offset basis spectrum representing the higher hydrocarbons, such that:
I w =C CH 4 ·I wCH 4 +C C 2 H 6 ·I wC 2 H 6 +C BB ·E,

where I wCH 4 and I wC 2 H 6 are respective Wobbe indices for methane and ethane, and E is an empirical factor for a composite Wobbe index of all the expected higher hydrocarbons in the fuel gas mixture.

5. The system as in claim 1 , wherein the relative density (G) is calculated by the processor on the basis of the determined concentrations for methane and ethane and the determined concentration of the offset basis spectrum representing the higher hydrocarbons, such that:
G=C CH 4 ·G CH 4 +C C 2 H 6 ·G C 2 H 6 +C BB ·E,

where G CH 4 and G C 2 H 6 are respective relative densities for methane and ethane, and E is an empirical factor for a composite relative density of all the expected higher hydrocarbons in the fuel gas mixture.

6. The system as in claim 1 , wherein the compressibility (Z) is calculated by the processor on the basis of the determined concentrations for methane and ethane and the determined concentration of the offset basis spectrum representing the higher hydrocarbons, such that:
X=C CH 4 ·Z CH 4 +C C 2 H 6 ·Z C 2 H 6 +C BB ·E,

where Z CH 4 and Z C 2 H 6 are respective compressibility factors for methane and ethane, and E is an empirical factor for a composite compressibility of all expected higher hydrocarbons in the fuel gas mixture.

7. The system as in claim 1 , wherein the theoretical hydrocarbon liquid content (L) is calculated by the processor on the basis of the determined concentrations for methane and the ethane and the determined concentration (C BB ) of the offset basis spectrum representing the higher hydrocarbons, such that:
L=C CH 4 ·L CH 4 +C C 2 H 6 ·L C 2 H 6 +C BB ·E,

where L CH 4 and L C 2 H 6 are respective theoretical liquid content values for methane and ethane, and E is an empirical factor for a composite theoretical liquid content of all the expected higher hydrocarbons in the fuel gas mixture.

8. The system as in claim 1 , wherein concentrations of the selected fuel gas components to be determined include specified fuel gas contaminant species.

9. The system as in claim 8 , wherein specified fuel gas contaminant species are selected from any one or more of H 2 S, H 2 O, O 2 , CO 2 , or OCS.

10. The system as in claim 1 , wherein the first wavelength range is a near-infrared wavelength range encompassing absorption bands of the selected fuel gas components with minimal cross-interference.

11. The system as in claim 10 , wherein the first wavelength range is in a vicinity of 1.58 μm.

12. The system as in claim 11 , wherein the second wavelength range is in a vicinity of 1.27 μm.

13. A method of measuring a heating value for a fuel gas, comprising:
coupling laser light from first and second tunable laser diodes off-axis into a high-finesse optical cavity of an off-axis integrated cavity output spectroscopy (ICOS) instrument, wherein the first and second laser diodes have respective tunable wavelength ranges,
flowing a fuel gas from a gas inlet through the cavity to a gas outlet,
measuring intensity of laser light exiting the optical cavity using an optical sensor while the laser diodes are tuned over their respective wavelength ranges so that the sensor measurement provides a wavelength-dependent optical absorption spectrum,
receiving, with a computer processor, the sensor measurement from the optical sensor,
accessing, with the computer processor, a database of basis spectra for fuel gas components,
employing, with the computer processor, a chemometric fitting routine to process the absorption spectrum so as to determine concentrations of selected fuel gas components, and
calculating, with the computer processor, a heating value (F) and an additional value (X) selected from relative density, compressibility, theoretical hydrocarbon liquid content, and Wobbe index from the determined fuel gas component concentrations on the basis of determined concentrations for methane (C CH 4 ) and ethane (C C 2 H 6 ) and the determined concentration (C BB ) of an offset basis spectrum representing higher hydrocarbons such that:
X=C CH 4 ·X CH 4 +C C 2 H 6 ·X C 2 H 6 +C BB ·E,

 where X CH 4 and X C 2 H 6 are respective coefficien

Assignees

• Abb Schweiz Ag

Inventors

• Owen kyle
• Gupta Manish X

Classifications

• F23N2221/10

  Analysing fuel properties, e.g. density, calorific · CPC title

• G01N2201/129

  Using chemometrical methods · CPC title

• G01N21/05

  Flow-through cuvettes (G01N21/09 takes precedence; handling fluid samples G01N1/10) · CPC title

• G01N21/031

  Multipass arrangements · CPC title

• G01J3/42

  Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry (beam switching arrangements G01J3/08) · CPC title