Optical sensor for natural gas composition analysis

Composition analysis of C1-C5 alkane gases (methane, ethane, propane, butanes and pentanes) is important in various applications within the LNG industries - with applications ranging from quality monitoring during production and custody transfer to its use in power generation. Advanced process control technologies providing fast and accurate feed and product analysis are critical in optimising efficiency and payback in various processing stages. In power generation applications, online and fast sensors are required to ensure optimum combustion efficiency and acceptable emission levels. In LNG transport applications, fast and easy-to-use analysers are desired to ensure quality at transfer points.

The LNG industry has generally focused on large base-load plants on land for many years. However, in recent years, smaller plants have received considerable investments including floating liquefied natural gas (FLNG) plants. In these plants, simple, low-cost and maintenance-free online analysis is key, due to capex/opex tradeoff shifts. In addition, some of these plants are built in areas where access is limited, making complex analyser system support cost prohibitive.

There are multiple measurement points through the supply chain where gas composition and calorific value are sought, including gas pre-treatment facilities, export and import LNG locations, storage tanks and vaporisation/condensing facilities. The feedstock natural gas for the liquefaction plants may have different chemical compositions, yielding a different calorific value of the output LNG. The decision to remove the heavier hydrocarbons depends on the market for by-products (such as butane or propane) at the site of the liquefaction. The blending of multiple sources, or the desire to increase or reduce the calorific value with injection, requires careful consideration of not only the total energy content but also the implications of a constantly changing composition of C1 through C5 alkanes will have for downstream refining - and particularly in power generation applications (solid oxide fuel cell, gas turbine and high horsepower stationary combustion engines).

20th-century separation technique challenged by the optical approach

The prevalent instrumentation technology used to date for hydrocarbon gas speciation and composition measurement is gas chromatography (GC). It works by physically separating the hydrocarbon compounds such as methane, ethane, propane, through long thin columns. The sample gas is injected at one end of the column, pushed by the carrier gas, and exits at the other end of the column where a detector measures the intensity of the signal corresponding to the concentration of the compounds. Measurement time for a C1-C5 using a GC instrument varies from 90 seconds to 5 minutes, depending on the type of instrumentation, accuracy requirements and the configuration used. A continuous flow of a dry, high-purity carrier gas such as hydrogen or helium is also needed with a GC. Due to its comparative nature, a calibration mixture(s) is generally needed to recalibrate the GC analyser regularly.

Infrared absorption is an optical technology that has been well accepted for various online process industrial gas/liquid measurement applications from combustion analysis to beverage process monitoring. Infrared absorption spectroscopy analyses the absorption of light as it interacts with the sample. It can provide fast measurement (seconds or sub-second) and a simple flow-through sampling without the need for carrier gas or any other consumables. In addition, it is a direct measurement based on first-principle (not a comparative or correlative technique), providing robust and stable measurement.

In general, the system consists of a light source, a sample cell (for gases, liquids or solids), a wavelength separating element (spectrometer) and a photodetector, as depicted in Figure 1. The wavelength separating element ‘slices’ the wavelength components of the broadband light source, which then interacts with the sample molecules. Some of the wavelength components are absorbed and some are transmitted through without any absorption. The resulting spectrum is called an absorption spectrum (Figure 2), which acts as ‘fingerprints’ that can then be used to identify the sample components or quantify the composition of the sample.

Figure 1: Simplified schematic of light absorption spectroscopy.

Infrared absorption spectroscopy for natural gas composition measurement has long been envisioned and investigated by various groups [1-4]. Its advantages from operational perspectives over gas chromatography are clear. However, several technical challenges have prevented its use in industrial settings. The biggest challenge has to do with the overlapping nature of hydrocarbon spectra. As seen in Figure 2, although the spectra of C1-C4 alkanes are unique, they are overlapping. There are not isolated peaks that make it possible to use a simple ‘peak analysis’ algorithm, but rather, a more sophisticated ‘pattern recognition’ algorithm or chemometrics is required.

Figure 2: Absorption spectra of C1-C4 alkane gases.

Tunable Filter Spectroscopy with chemometrics

Infrared spectrometers utilising discrete optical filter elements, generally called non-dispersive infrared (NDIR) instruments, are widely used in various online monitoring applications today. They are proven to be robust and effective due to their simple design and high optical throughput. However, they are not effective in differentiating or speciating similar chemical entities with overlapping spectra such as the different compounds of hydrocarbons.

This article presents a novel and powerful improvement of the NDIR instrumentation where, rather than using discrete optical filters with a limited amount of wavelength bands selected to correspond to required measurements, a tunable element is introduced to enable a degree of continuous spectral coverage, which leads to the capability to deconvolute complex spectra. Tunable Filter Spectroscopy (TFS) has now been introduced very successfully to rapidly speciate light hydrocarbons in natural gas, enabling fast and repeatable calorific value measurement.

A TFS analyser uses a proprietary implementation of a tunable Fabry-Perot assembly that enables high-throughput and high-precision wavelength scanning in preselected region(s). One or more wavelength bands are scanned in a continuous manner such that the digital representation of the spectral features is captured. A chemometric-based pattern recognition algorithm deconvolves the multicomponent spectra and quantifies the concentration of the individual hydrocarbon compounds. The advanced algorithm also compensates for nonlinearities and other unwanted spectral behaviours that are unavoidably present in real-world measurements.

An optical hydrocarbon analyser for unattended monitoring

An all-optical hydrocarbon gas analyser utilising a TFS ‘engine’ has been developed and used in various natural gas processing applications from drilling to pipeline gas analysis in power generation.

The Precisive 5 analyser, a standalone TFS analyser packaged in a NEMA4X, IP66 rated, Div2/Zone2-certified box is an attractive alternative to the traditional GC technique in various LNG measurements. It provides speciated concentration values of methane, ethane, propane, iso-butane and n-butane as well as pentanes. From these values, calorific value and Wobbe index are computed and reported. Optional carbon dioxide and hydrogen sulfide direct measurement channels are also available.

Figure 3: Precisive 5 gas analyser - standalone analyser utilising a tunable filter spectroscopy sensor platform (left) and analyser unit with zero/cal sampling system (right).

A TFS-based analyser is permanently calibrated at the factory and does not require any consumable gases. Gas phase samples can flow through the analyser at wide ranges of pressure, temperature and flow rates, minimising sample conditioning requirements.

Case study: Downstream LNG quality analysis

A Precisive 5 gas analyser was installed on an LRSU (LNG rundown system upgrade) application at one of the largest LNG producers in Indonesia, where the LNG final product is being recirculated prior to being loaded onto a ship. This measurement is critical to ensure that the final LNG product meets the intended specification and is used as a last checkpoint prior to the custody transfer metering point.

The LNG final product sample is heated using an existing sample handling system to reach the ambient temperature of 24°C, where it becomes a vapour before entering the analyser.

The analyser is calibrated to provide the following measurement channels:

• CH₄ (methane): 0-100%
• C₂H₆ (ethane): 0-25%
• C₃H₈ (propane): 0-25%
• C₄H₁₀ (iso-butane): 0-10%
• C₄H₁₀ (n-butane): 0-10%
• C₅H₁₂ (pentanes): 0-5%
• N₂ (nitrogen) calculated balance
• Calculated CV (MJ/m³)
• Calculated Wobbe Index (MJ/m³)

Note that the CV and Wobbe Index calculations are performed according to ISO 6976:1995 standard in particular using Table 3, Hs[25C, V(0C; 101325 kPa)].

At the beginning of the installation, the continuous real-time measurement from the Precisive 5 gas analyser was compared against a daily measurement with a laboratory GC over a period of 28 days. Figure 4 shows the continuous real-time data from the Precisive 5 gas analyser with an update rate of every 5 s, while Figure 5 shows the daily GC reports.

As can be seen, the TFS-based infrared measurements track the daily GC reading very well, quite noticeably the low methane readings on around 2/8 and 15/8, higher C4 readings around 15/8 and high C3 reading around 2/8.

More importantly, the infrared reading captures the fluctuations that otherwise would be missed by the daily GC measurements.

Figure 4: Precisive measurement (5 s updates over 28 days). For a larger image click here.

Figure 5: Gas chromatograph measurement (daily over 28 days). For a larger image click here.

Summary

The LNG industry has been seeking a simple, low cost, field-deployable alternative to traditional gas chromatography for various measurements points, particularly over the last few years, due to the widespread increase of smaller plants including FLNG. Moreover, the cost of servicing, calibration gases, carrier gases and maintenance of sampling systems of the traditional gas chromatograph-type instruments is significant.

A real-time, all-optical analyser is an attractive alternative to gas chromatographs, and infrared absorption spectroscopy is a well proven technology in various online process industries. In this article, its successful application in LNG composition and calorific value analysis is demonstrated. It is being evaluated around the world as a robust 21st-century alternative to the array of applications considered exclusive to gas chromatography.

References

1. Brown, C 1993, Optical BTU Sensor Development - Final Report, Technical report no. GRI-93/0083, Gas Research Institute, Chicago, IL under Contract No. 5034-271-1197.
2. Van Agthoven, MA, Fujisawa, G, Rabbito, P & Mullins, O 2002, ‘Near-Infrared Spectral Analysis of Gas Mixtures’, Applied Spectroscopy, 56, 593-598.
3. Goldstein, N, Gersh, M, Bien, F, Richtsmeier, S, Gruninger, J & Adler-Golden, S et al 1999, Real-time optical BTU measurement of natural gas at line pressure, 4th International Symposium on Fluid Flow Measurement, Denver, Colorado, USA, 27-30 June.
4. Zelepouga, S, Gnatenko, V, Pratapas, J, Jangale, V & Saveliev, A 2010, ‘Gas Quality Sensor To Improve Biogas Fueled CHP/DG’, Proceedings of the ASME Internal Combustion Engine Division 2010 Fall Technical Conference, 12-15 September.

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