Advanced detection methods provide trace element gas analysis

Pipeline & Gas Journal, July, 2003 by Karen L. Crippen

Routine analysis processed natural gas for the calculation of heating value, specific gravity and compressibility is an established measurement practice by the gas industry worldwide. However, these routine analyses provide information only on major and minor components. They do not detect trace constituents at low concentration levels.

As detection technology improves, new analytical methods are constantly being developed to measure these trace level compounds. The primary motivation for this is the gas industry's ever-increasing environmental awareness along with federal and state requirements to regulate and monitor air quality emissions. Furthermore, these compounds could have deleterious effects on the gas distribution system, harm gas-processing operations or result in operational or end-user difficulties.

Natural gas is a colorless, highly flammable gaseous hydrocarbon mixture consisting primarily of methane and ethane with lesser amounts of inert gases and heavier hydrocarbons. Automated gas chromatographs provide gas composition with excellent precision and accuracy for components such as N2, C[O.sup.2], and paraffins from C1 through C5. especially if calibration gases of known uncertainty are used. These tests have detection limits of about 001 mole percent.

Below that 0.01 mole percent concentration level there may exist a whole population of gas components collectively classified as trace constituents. The major portion of these trace constituents are naturally occurring species such as paraffinic and aromatic hydrocarbons, organic sulfur compounds, hydrogen, and others. However, some compounds are those inadvertently introduced by gas processing or contamination.

Regardless of their origin, a number of advanced tests can be used to identify these trace components. Tests for certain specific compound classes are described below. As the leading research, development and training organization serving energy and environmental markets, the Gas Technology Institute (GTI) frequently uses many of these procedures.

Sulfur Analysis

Sulfur compounds are either naturally occurring or odorants that are artificially added to impart a smell for easy detection. Gas chromatographic analysis offers speciation information useful for gas quality monitoring. However, often the total sulfur information used for estimations of sulfur emissions for EPA permitting must be performed by one of the four methods cited in 40 CFR Part 60 Subpart GG Section 60.335 (d).

GC systems for sulfur speciation can be designed with several different detectors. As a rule, element selective detectors will provide the best data because the detector responds to sulfur contained in the separated components. The detectors used most generally include flame photometric, pulsed flame photometric, chemiluminescence, atomic emission, and UV-Fluorescence. Detection limits can be highly dependent on sample matrix and injection volume.

The flame photometric detector (FPD) combusts the GC eluent in a H2 rich flame. producing an excited $2* species. Energy emitted upon decay is directly related to the (approx) square root of the S concentration. It has a detection limit and range of 20-20,000 picograms, with generally throe orders of magnitude dynamic range. However hydrocarbon quenching from closely eluting HCs can interfere with the production of $2* species. The pulsed flame photometric detector (PFPD) uses the same excitation technology except the flame is pulsed. Because the background emissions decay more quickly than the excited $2* species, the detector response is optimized by delaying the signal monitoring to eliminate background emissions. This can improve the detection limit at least 10-fold.

The sulfur chemiluminescence detector (SCD) uses a different combustion chemistry to yield an excited SO* radical plus other reaction products. Reaction of SO* with ozone creates electronically excited S[O.sub.2]. The chemiluminescence signal emitted upon decay is directly (equimolar) related to S concentration This technique has a detection limit and range of about 10-1,000,000 picograms with generally five orders of magnitude dynamic range. The detector requires precise control of the hydrogen and air flow rates, and the vacuum system.

Another new detector uses UV fluorescence. The GC eluent gas is combusted with O2 producing S[O.sub.2]. Excitation with UV energy ultimately generates a fluorescence decay signal that is directly related to S content. It can detect from the ppm to the % level range, offering four orders of magnitude dynamic range. However, combustion byproducts can interfere by contributing to the fluorescence decay signal.

The atomic emission detector (AED) is an analytical tool that is not found in many laboratories. Instead of combusting the GC eluent, the gas is introduced into a microwave induced helium plasma-producing electronically excited atoms. A photodiode array measures the emitted energy upon decay back to the ground state. Generally, a 1 ppmv detection limit can be achieved with four orders of magnitude dynamic range. The technique can suffer from spectral interferences from hydrocarbon and other molecular emission bands, causing baseline upsets. While it is true that it is expensive to maintain and operate, it is a powerful tool for elemental speciation. not only for sulfur but also for many other element specific compounds. It offers a true compound independent calibration that gives a linear response regardless of file source molecule.