Systems and methods for optically measuring properties of hydrocarbon fuel gases

A method for determining properties of a hydrocarbon fuel gas comprising the steps of:

• acquiring an absorption spectrum in a region between 700 and 1000 nm of a hydrocarbon fuel gas, by irradiating the gas with a light source and by dispersing light transmitted through the gas with a spectrograph;
• detecting the dispersed light with a silicon sensor array;
• determining an absorbance spectrum of the gas;
• decomposing the spectrum into a plurality of hydrocarbon components;
• and
• determining properties of the gas from the components.

An optical system for measuring energy content and other proper ties of a natural gas comprising:

• a light source generating light over a range of wavelengths to provide a measurement light beam and a reference light beam;
• a pipe having a natural gas flow at a gas pressure above one atmosphere;
• a fixed optical path in fluid communication with the natural gas in the pipe such that natural gas in the optical path is optically coupled to the measurement light beam, the light being partially transmitted by the natural gas;
• a temperature sensor that measures a temperature of the natural gas;
• a pressure sensor that measures the gas pressure of the natural gas;
• a spectrograph that receives the measurement light beam that is transmitted by the natural gas and the reference light beam, the spectrograph dispersing the measurement and reference light beams onto a silicon sensor array that detects at least some portion of the light in the range between 700-1000 nm and generates spectral data,
• calibration means that calibrates the spectrograph using spectral features from the spectral data, the spectrograph having a wavelength resolution such that a wavelength scale of the spectrograph can be calibrated using the spectral features;
• and
• a processing circuit that processes the spectral data to determine the energy content of the natural gas.

A method for determining the energy content and other properties of a natural gas comprising the steps of:

• acquiring an absorption spectrum of the natural gas by irradiating the gas with a light source and by dispersing light transmitted through the gas with a spectrograph;
• detecting at least some portion of the light in the range between 700-1000 nm with a silicon sensor array;
• determining the absorbance spectrum of the natural gas by comparing the spectrum of light transmitted through the gas with a reference spectrum obtained by coupling light from the light source to the spectrograph without transmitting the light through the gas;
• calibrating a wavelength scale of the spectrograph using spectral features of the natural gas;
• decomposing the absorbance spectrum into a plurality of spectral elements corresponding to hydrocarbon components of the natural gas;
• measuring the temperature and pressure of the natural gas;
• determining the hydrocarbon composition of the natural gas from the decomposed spectral elements and the temperature and pressure data;
• determining the energy content of the natural gas at a state of pressure and temperature using the determined hydrocarbon composition.

A method for determining properties of a hydrocarbon fuel gas comprising the steps of:

• acquiring an absorption spectrum in a region between 700 and 1000 nm of a hydrocarbon fuel gas at a line measurement pressure above one atmosphere, by irradiating the gas with a light source and by dispersing light transmitted through the gas with a spectrograph;
• detecting the dispersed light with a silicon sensor array;
• determining an absorbance spectrum of the gas;
• and
• determining energy content per unit volume at about the measurement pressure above one atmosphere.