Compressed Natural Gas (CNG) Composition Testing: Standards, Methods, and Applications

Compressed natural gas (CNG) composition testing is the analytical process of quantifying the hydrocarbon and non-hydrocarbon constituents of natural gas compressed for use as vehicle fuel, industrial energy, or custody transfer. Using gas chromatography and complementary techniques in accordance with ISO, ASTM, and GB standards, accredited third-party laboratories verify calorific value, Wobbe index, sulfur impurities, and water content to ensure safety, combustion performance, and regulatory compliance. This article outlines the scope, test items, analytical methods, and typical applications of CNG composition analysis.

Technical Overview

Compressed natural gas (CNG) is natural gas that has been compressed, typically to a pressure of 20 to 25 MPa, for efficient storage on board vehicles or in station cascades. Its composition is dominated by methane, generally in the range of 85 to 98 mol %, accompanied by ethane, propane, butanes, pentanes, hexanes and heavier hydrocarbons, as well as non-combustible and reactive components such as nitrogen, carbon dioxide, oxygen, hydrogen sulfide, mercaptans, and water vapor.

Accurate compositional analysis is essential because even minor constituents have a disproportionate effect on safety and performance. Trace hydrogen sulfide, in combination with condensed water, can corrode cylinders, valves, and fuel lines; water vapor can form hydrates or ice plugs under high-pressure expansion; carbon dioxide and nitrogen dilute the fuel and reduce heating value; and variations in hydrocarbon distribution shift the Wobbe index, affecting engine combustion. Composition testing therefore underpins conformity assessment against specifications such as GB 18047 for CNG used as vehicle fuel and ISO 15403-1 for natural gas designated for use as a compressed vehicle fuel.

The complete testing chain typically comprises representative sampling from the high-pressure source, laboratory or on-site chromatographic analysis, calculation of derived physical properties, and evaluation of results against contractual or regulatory limits.

Testing Samples

CNG Vehicle Fuel

Samples are drawn from refueling station dispensers, priority panels, or storage cascades to verify that the dispensed fuel meets the quality requirements of GB 18047, including heating value, sulfur limits, carbon dioxide content, oxygen content, and water dew point.

Pipeline Natural Gas

Gas sampled at custody transfer points, city gate stations, or compressor station inlets is analyzed to confirm composition consistency and to support energy-based metering and billing between trading parties.

Upstream and Processed Gas

Wellhead gas, gas after dehydration or desulfurization treatment, and gas immediately upstream of compression are tested to monitor process efficiency and to ensure feed gas entering the compression and dispensing system is within specification.

Alternative-Source Gases

Upgraded biomethane and coal-bed methane routed into CNG supply chains are characterized to demonstrate equivalence to conventional natural gas specifications.

Sampling is performed in accordance with ISO 10715 and GB/T 13609, using high-pressure sampling cylinders with inert or passivated internal surfaces when reactive sulfur species are to be determined. Careful pressure letdown is applied to prevent condensation and fractionation of heavier components during sample acquisition.

Testing Items

Main Composition Analysis

The full component breakdown is determined on a molar or volume basis, including:

• Methane, ethane, propane, iso-butane, n-butane
• Neo-pentane, iso-pentane, n-pentane, and hexanes-plus (C6+) as a composite peak
• Nitrogen, carbon dioxide, and oxygen
• Hydrogen, helium, and argon where an extended analysis is required

Trace and Impurity Analysis

• Hydrogen sulfide and carbonyl sulfide
• Individual mercaptans and odorant compounds such as tetrahydrothiophene
• Total sulfur content
• Water content and water dew point
• Hydrocarbon dew point

Calculated Physical Properties

• Gross and net calorific value (higher and lower heating value)
• Wobbe index (higher and lower)
• Relative density and density at reference conditions
• Compressibility factor at metering conditions

For CNG supplied as vehicle fuel under GB 18047, the principal acceptance limits include a gross calorific value of at least 31.4 MJ/m³, total sulfur not exceeding 200 mg/m³, hydrogen sulfide not exceeding 15 mg/m³, carbon dioxide not exceeding 3.0 mol %, oxygen not exceeding 0.5 mol %, and a water dew point below the lowest expected service temperature at the maximum operating pressure.

Testing Methods

Gas Chromatographic Determination of Composition

The core technique is multi-column gas chromatography equipped with thermal conductivity detection (TCD) for permanent gases and light hydrocarbons, and flame ionization detection (FID) for enhanced sensitivity toward hydrocarbons, with column backflushing used to elute hexanes and heavier components as a single composite peak. Instrument calibration is performed against certified gravimetric reference gas mixtures prepared in accordance with ISO 6142 or ISO 6143. Recognized method standards include ASTM D1945, GB/T 13610, and the ISO 6974 series for natural gas analysis by gas chromatography.

Calculation of Derived Properties

Calorific value, density, relative density, and Wobbe index are calculated from the molar composition using ISO 6976, GB/T 11062, or ASTM D3588. The compressibility factor for volume conversion at metering conditions is computed using the equations of state described in ISO 12213 and GB/T 17747.

Sulfur Compound Determination

Individual sulfur species are separated and quantified by gas chromatography with sulfur chemiluminescence detection in accordance with ISO 19739 and ASTM D5504. Total sulfur is determined by oxidative microcoulometry (ASTM D3246), ultraviolet fluorescence (ASTM D6667), or the titration-based methods of the GB/T 11060 series, while hydrogen sulfide may also be measured by iodometric or coulometric titration.

Water Content and Dew Point Measurement

Water dew point is measured directly by chilled-mirror hygrometry following GB/T 17283 or ASTM D1142, or inferred from water content determined with electrolytic phosphorus pentoxide cells, capacitive polymer sensors, or laser absorption analyzers. Hydrocarbon dew point is obtained by controlled chilled-mirror cooling or by calculation from the measured extended composition using a validated equation of state.

Applications

Composition testing results are applied across the CNG value chain, including:

• Incoming gas acceptance and routine quality control at CNG mother, daughter, and standard refueling stations
• Conformity assessment of vehicle fuel against GB 18047 and ISO 15403-1 requirements
• Custody transfer settlement and energy-based billing between gas suppliers, distributors, and fleet operators
• Engine calibration and fuel interchangeability studies based on Wobbe index and methane number control
• Materials integrity programs protecting high-pressure cylinders, valves, and fuel system components from corrosion
• Arbitration analysis, import and export inspection, and resolution of contractual quality disputes
• Emission control programs protecting exhaust oxidation catalysts from sulfur poisoning

FAQ

Why is hydrogen sulfide strictly limited in CNG?

Hydrogen sulfide is toxic, generates sulfur dioxide on combustion, poisons exhaust aftertreatment catalysts, and promotes sulfide stress corrosion of steel cylinders and piping when free water is present. GB 18047 therefore restricts its concentration in vehicle-grade CNG to no more than 15 mg/m³.

What is the Wobbe index and why does it matter?

The Wobbe index is the ratio of the gross calorific value to the square root of relative density. It is the key indicator of fuel interchangeability because gases with similar Wobbe indices deliver similar heat release at constant supply pressure, allowing engines and burners to operate without recalibration.

How are samples safely collected from a 20 to 25 MPa CNG system?

Sampling is performed through pressure letdown regulators into high-pressure cylinders following the guidance of ISO 10715 and GB/T 13609. Cylinders with passivated or inert-coated interiors are used for sulfur species, and flow rates and temperatures are controlled to prevent condensation that would bias the measured composition.

How frequently should CNG composition testing be performed?

Testing frequency is risk-based: each new supply source or major process change warrants full compositional verification, stations typically perform periodic scheduled checks, and online process gas chromatographs provide continuous monitoring with laboratory analysis used for validation and calibration.

How is the reliability of results assured?

Analyses are conducted by ISO/IEC 17025 accredited third-party laboratories using traceable certified reference gas mixtures, validated method standards, documented quality control procedures, and reported measurement uncertainties consistent with ISO 6976, ensuring results are defensible for regulatory and commercial purposes.