1. Why is Characterisation of Gaseous Fuels Needed?

Gaseous fuels such as [GLOSS]natural gas[/GLOSS] are not single substances but complex mixtures of [GLOSS]hydrocarbons[/GLOSS] such as [GLOSS]methane[/GLOSS] or[GLOSS]ethane[/GLOSS], together with other gases such as nitrogen and carbon dioxide.  The composition of natural gas will vary depending on how it has been generated, and further discussion on the geological and other factors affecting the formation of natural gases may be found in a linked Combustion File (CF216).

The complex composition of natural gas potentially creates a number of problems for the pipeline companies that transport the gas to industrial, commercial and residential users, and for the users themselves.  Firstly, gas is now generally sold by the amount of energy it contains, measured in units such as kilowatt-hours (kWh).  This market requirement means that the [GLOSS]gross calorific value[/GLOSS] of the gas per unit volume must be determined.

A second reason for detailed characterisation of the gas arises from the need for different gas supplies to be interchangeable.  In many countries, customers could potentially receive gas that has originated from a number of sources, including one or more local gas fields, gas piped over long distances from remote fields, or gas imported in liquefied form by ship.  Gas appliances and burners are generally designed to function safely and efficiently within a relatively narrow range of gas compositions, and whilst some large-scale gas-fired processes are capable of manual or automatic adjustment to deal with a wider range of composition, the majority of appliances are not.  Typical combustion problems that can arise if a gas of unsuitable composition is burned include flame instability, sooting and incomplete combustion, all of which have safety and energy efficiency consequences.

Thirdly, the pipeline companies themselves will want to ensure that the gas can be transported safely:  the gas must not contain corrosive components that can degrade pipeline materials and equipment; nor contain solids or components that will liquefy if the pressure and/or temperature of the gas are reduced.  A typical set of composition acceptance criteria is shown below:

q         Hydrogen sulphide:  not more than 3.3ppm

q         Total Sulphur:    not more than 15ppm

q         Hydrogen:  not more than 0.1 mole%

q         Oxygen:  not more than 10ppm

q         [GLOSS]Hydrocarbon dew point[/GLOSS]:  not more than –2 degree C

q         [GLOSS]Water dew point[/GLOSS]:  not more than –10 degree C

q         Carbon dioxide:  not more than 2.0 mole%

q         Nitrogen:  not more than 5.0 mole%

q         Inert gases: not more than 7.0 mole%

q         Contaminants:  no solid or liquid material

2. The Characterisation of Natural Gas

The need to characterise gaseous fuels has arisen since the earliest days of the gas industry in the mid 19^th century.  Traditionally, the composition of gas mixtures was determined by ‘wet’ chemical methods, based on selectively absorbing components into liquids and noting volume changes, or through analysis of the resultant solution.  Such methods were slow – taking many hours, required technicians with considerable skill and experience, and even then produced inaccurate results for minor and trace components.  In contrast, modern methods typically take less than 5 minutes for a full gas analysis, use reliable automated instrumentation, and deliver results with high accuracy (e.g. calorific value is measured to well within less than 0.1%).

This radical change came about as a result of the development of [GLOSS]gas chromatography[/GLOSS] in the 1950s and 60s.  Chromatography is the separation of a mixture into its individual components, achieved by the repeated partitioning of the components between two phases, one stationary, the second moving.  In gas chromatography, the mobile phase is an inert gas such as helium or nitrogen; the stationary phase may be a solid such as silica or alumina used essentially in their natural states or coated with an inert non-volatile liquid.  When analysing natural gas, the amount of each separated component is measured accurately.

In its simplest form, a gas chromatograph will consist of a coiled metal or glass column containing the stationary phase, a device for introducing a small amount of the gas to be analysed onto the top of the column, and one or more detectors to detect and measure the amount of each separated component as they emerge from the end of the column.  Injector, column and detectors will be housed within a suitable container, parts of which may be maintained at elevated temperatures to improve separation or decrease analysis time.  The mobile phase is supplied from a high-pressure gas cylinder, and its flow controlled by suitable regulators.  The output from the detector(s) will be captured by an integral computer, which will calculate the amounts of each component and derive a range of physical properties for the gas.  The computer will present reports on gas composition and properties either locally or to a centralised location via suitable communications equipment.  In addition, the integral computer will control the operation of the chromatography, calibrating the system using a gas mixture of known composition, and alerting operators should equipment malfunction occur.

The [GLOSS]process gas chromatograph[/GLOSS]s used for natural gas analysis follow the same principles as other gas chromatographs, but have many refinements and modifications to improve reliability and enable unattended operation.  The instruments are designed to operate on remote, often inhospitable sites, and to be fully compliant with electrical and other regulations to ensure safe operation in all environments.  The instruments will usually be dedicated to a specific analysis, with the column and detectors selected to optimise the requirements for the application.  In cases such as natural gas analysis where many components must be determined, more than one column will often be used in parallel, with several detectors also being employed.

Although gas chromatography is capable of determining the majority of the components of natural gas, some trace components (including hydrogen, oxygen and water) are determined by separate dedicated instruments.   The trend, however, is towards increased integration of all instruments into a single process analysis package, and the latest instrument offered by one manufacturer provides a comprehensive solution incorporating:

·          Gas chromatographic analysis of nitrogen, carbon dioxide and hydrocarbons from methane up to and including a combined peak for hydrocarbons having six or more carbon atoms;

·          Separate gas chromatographic analysis of heavier hydrocarbons having six to twelve carbon atoms;

·          Separate gas chromatographic analysis of hydrogen sulphide and other sulphur-containing compounds including [GLOSS]odorants[/GLOSS];

·          Individual dedicated detectors providing moisture, hydrogen and oxygen measurement.

3. Gas Properties and Gas Interchangeability

As soon as the composition of the gas has been determined, the computer within the process analysis package will calculate a range of gas properties.  The calculations rely on specialist algorithms and software routines that incorporate extensive details on the physical properties of the individual components found in natural gas, and their behaviours within mixtures, including phase changes from gaseous to liquid state.  The calculation methodologies have been extensively tested and approved by international organisations such as the International Standards Organisation and the American Gas Association.

Gas properties that are typically reported include:

·          Gross calorific value, relative density and [GLOSS]Wobbe Index/Number[/GLOSS] (e.g. in accordance with the standard ISO6976);

·          Additional interchangeability parameters such as [GLOSS]incomplete combustion factor[/GLOSS] and [GLOSS]sooting index[/GLOSS] (e.g. in accordance with the UK’s Gas Safety (Management) Regulations); 

·          Hydrocarbon and water dew point temperatures.

From the compositional analysis and the calculated gas properties, the pipeline company can decide whether to accept a gas into its transportation network or whether further treatment or blending with other gases is required.  The pipeline company will usually retain the right to cut off a gas supply if the gas composition deviates from the acceptance specification. 

By measuring gas properties and volumes at network entry and major exit points, an accurate account can be made of the energy flows within the system.  In a fully liberated market where the pipeline network services the business needs of a large number of gas producers, shippers, traders and users, the accuracy and timeliness of information on gas quality and volumes is paramount.  There is little doubt that the modern process gas chromatograph has greatly facilitated the development of vibrant gas markets such as those in the UK and other countries.

Acknowledgements

The author thanks Nigel Bryant and his colleagues at Advantica Ltd, Loughborough, UK for their support during the preparation of this paper.

Sources

[1]        ISO 6976:1995, Natural Gas – Calculation of calorific values, density, relative density and Wobbe Index from composition, International Standards Organisation, 1995.

[2]        ISO 10723:1995, Natural Gas – Performance evaluation of on-line analytical systems, International Standards Organisation, 1995.

[3]        Gas Safety (Management) Regulatons (Statutory Instrument 1996 No 551), Her Majesty’s Stationery Office, UK, ISBN 0110541847, 1996.

[4]        Dutton B C, A new dimension in gas interchangeability, Communication 1246, 50^th Autumn Meeting at Eastbourne, Institution of Gas Engineers, 1984.

[5]        Dutton B C and Gimzewski E, Gas Interchangeabiliy: prediction of flame lift, Journal of the Institute of Energy, June 1983, p107.

[6]        Dutton B C and Wood S W, Gas Interchangeabiliy: prediction of soot deposition on domestic gas appliances with aerated burners, Journal of the Institute of Energy, September 1984, p381.

[7]        Dutton B C and Souchard R J, Gas Interchangeabiliy: prediction of incomplete combustion, Journal of the Institute of Energy, December 1985, p210.