The Feasibility of Using Near-Infrared Spectrometer to Estimate Selected Properties of Natural Gas

It has been demonstrated that the composition of compressed natural gas can have a marked effect on the operational characteristics of engines that bum natural gas as a fuel. For this reason, a quick and accurate method of determining natural gas composition is needed. Near-infrared (NIR) spectroscopy has the potential of meeting the requirements for a natural gas analyzer.

This limited scope study was conducted as a preliminary demonstration of the feasibility of using NIR as an on-line natural gas analyzer at a distribution site or perhaps as an on-board vehicle fuel sensor in a natural gas vehicle (NGV). Reference gases were analyzed and used to develop calibration models for three selected properties of natural gas. These properties were heat of combustion (higher heating value), Wobbe Index (heat of combustion per unit volume divided by the specific gravity of the gas), and hydrogen/carbon ratio. Transmission spectra were collected over the region of 900 to 1800 nanometers. All spectra were measured at a gas pressure of 206.8 kPa (30 psig) and a temperature of 22.8 to 23.4°C in the gas cell. Spectra were collected with both flowing gas and static gas in the gas cell. Multivariate calibration techniques were used to develop calibration models for each of the properties investigated.

This study shows that NIR, using a fiber-optic probe, has the potential to measure accurately several properties of natural gas. Calibration model validations were also conducted with mixed results. Further work is required to expand the calibration models.