European Gas Quality, Interchangeability Issues Reflect Regional Diversity
By Williams, Terry
This article will look at gas quality trom the perspective of downstream interchangeability and concentrate on current issues surrounding increased dependence of many countries around the world on imported natural gas.
The rise in gas trading across international borders through new pipeline interconnectors and LNG shipping brings with it concerns for the variability of gas quality delivered from different sources. To trade natural gas as a truly international commodity, industry needs to agree on the definition and measurement of natural gas.
It may be a surprise that these are not already in place, given the maturity of most natural gas industries. Each gas market, however, has been almost isolated from other markets until now, with its own indigenous reserves. The need to secure gas supplies from many different sources is quite new.
The views are based on Advantica’s experience in recent years of working closely with the UK Department of Trade & Industry (DTI), the Technical Association of the European Natural Gas Industry (Marcogaz), and the American Gas Association (AGA).
Despite government initiatives to promote energy efficiency, increase energy supply from renewable energies, and reduce energy consumption from fossil fuels, annual demand for natural gas across Europe continues to rise (Fig. 1). The concern is that as demand increases, production capability across Europe will be declining. Imports of natural gas across the 25 European Union countries 2000- 30 are predicted to nearly treble.
Many European countries, including the UK, are constructing new LNG importation terminals (Fig. 2) and increasing gas imports via additional pipeline interconnectors. These moves raise the issue of gas interchangeability because the source and quality of natural gas across Europe will not be constant.
LNG supplies and Norwegian pipeline imports will generally be of a higher calorific value than UK Continental Shelf North Sea gas. Dutch pipeline imports, however, may possibly provide low calorific value gas supplies.
Of additional concern is that the large volumes of gas from LNG terminals and from new pipeline interconnectors will change the traditional flow and mixing patterns within existing gas networks so that many more parts of Europe may see a wider change in gas quality over short time and possibly at the extremes of gas specification limits.
The issue is therefore how to ensure security of gas supply at reasonable cost, knowing that gas-quality parameters of much imported gas may be at the extremes of, or outside, existing fication limits. The options are either to adjust the gas quality at entry points to meet existing limits or to consider widening the limits with the impact that might have on operation of downstream gas-fired equipment.
Fig. 1
EU natural gas: past, future
Interchangeability
This discussion employs a definition of gas interchangeability recognized internationally: The ability to substitute one gaseous fuel for another in a combustion application without materially changing the operational performance of the application (its safety, efficiency, or emissions).
This would include the need for any gas-fired equipment or appliance firing on the substitute gas to continue to meet the performance standard for which it was originally approved.
Gas interchangeability is not a new concept with considerable work completed, particularly in the US and Europe in the 1960-70s. This work was triggered by the need for step changes from a manufactured (town) gas to natural gas.
UK’s Isle of Grain LNG terminal began operations in July 2005 and has current capacity of 3.3 million tonnes/year (Fig. 2; photo from Grain LNG).
And, as might be expected, different researchers concluded their work on the gas appliances of their region and time with different definitions and measures. The result has been no consistent, internationally recognized way of interpreting gas interchangeability. Different countries, and even different regions within the same country, use a variety of dissimilar parameters and limits of acceptable performance to assess the impact of variable gas quality on gas supply and consumer operations.
Also, in pursuit of increased equipment efficiency and lower emissions, we have seen development of a new generation of gas- fired appliances, with premixed and staged combustion, that may not adjust readily to wide variations in gas quality and may not be appropriately represented by existing interchangeability parameters, found empirically with appliances popular more than 30 years ago.
Different parameters
Many parameters are in use worldwide to represent gas interchangeability. In many cases, including Europe, more than one factor may be used to set maximum and minimum thresholds that form an envelope of acceptable operation for downstream plant and equipment. A selection of the following factors is typically used:
* Heating value or calorific value.
* Wobbe Number or Wobbe Index.
* Relative density or specific gravity.
* Weaver indices.
* AGA indices.
* Incomplete combustion factor.
* Lift index.
* Soot index.
Heating value or calorific value represents the energy content of a gas usually given in units of MJoules/std. cu m or btu/scf and can be expressed as higher heating value (same as gross calorific value in which water vapor in combustion is assumed to be entirely condensed and the heat recovered) or lower heating value (same as net calorific value in which heat of vaporization is not recovered).
Heating value is not the best parameter to represent interchangeability because it does not account for flow through a burner.
Since the volumetric flow of gas in a pipe is inversely proportional to the square root of gas density (as is the definition of Wobbe Index), thermal input through a burner nozzle is proportional to Wobbe Index and not to heating value. It can also be shown that Wobbe Index is proportional to the equivalence ratio or stoichiometric air requirement for a burner. And, it therefore gives a good indication of gas combustion and energy flow through a burner nozzle.
Relative density or specific gravity, as a measure of gas density relative to air at reference conditions, is used for interchangeability specifications to limit the higher hydrocarbon content of the gas. An increased higher hydrocarbon content could lead to such combustion problems as increased carbon monoxide emissions, soot formation, engine knock, or spontaneous ignition on gas turbines even at the same Wobbe Index value.
Incomplete combustion factor, lift index, and sooting index are parameters developed for the UK appliance market, as are Weaver and AGA indices for the US. AGA is currently reviewing and updating AGA Bulletin 36 (Interchangeability of Other Fuel Gases with Natural Gases).
UK position
The UK gas specification is set by Gas Safety (Management) Regulations, which use the Wobbe Index as the main parameter of interchangeability. The GSMR set the limits of Wobbe at between 47.20 MJ/cu m and 51.41 MJ/cu m. This is a narrower band of acceptable Wobbe, however, than many other countries specify, including those in mainland Europe.
In response to the likely harmonization of future European gas- quality specifications, discussed later, the UK’s DTI has led a program to identify the issues for the UK, review options for gas processing at entry points, assess appliance performance under various gas-quality conditions, and consult with industry on future policy.
The interchangeability diagram (Fig. 3) shows the UK range of acceptable appliance operation of Wobbe between 51.41 and 47.20 MJ/ cu m and shows the 15 sample gases used as part of the DTI test program to test some 25 different UK appliance types representing more than two-thirds of the UK appliance population.
The test program was designed to be as representative as possible of the UK’s existing gas appliance population. Testing was on both new and used appliances with the objectives of establishing the impact on combustion performance and emissions of operating on test gases at, and beyond, GSMR limits, measuring emissions and efficiency performance. The program also examined the effect of diluting natural gases with an inert gas such as nitrogen.
It might be expected that modern appliances would be less influenced than older models by a change in the Wobbe index. The drive for higher efficiency and reduced emissions, however, has led to new appliances tending to have burners with a narrower Wobbe operating range, in effect being tuned to the expected gas-quality specification.
A summary of the program’s results reported that:
* Ignition was OK for all test gases.
Fig. 3
Dutton interchangeability
* Flame lift was not generally a problem.
* Little or no soot measured in the flue gas, but some was deposited on the decorative coals of the fires (for the higher Wobbe Number test gases).
* NOx emissions increased as Wobbe index increased.
* Little change in efficiency with Wobbe index change.
* CO emissions increased with Wobbe Number.
Industrial and commercial gas-fired equipment was not tested as part of the UK program but in general should be more tolerant to a wider range of Wobbe index and calorific value due to the investment in more sophisticated processcontrols such as air-fuel ratio and flue-gas feedback and trim control systems. Gas turbines for power generation, however, would be particularly sensitive to rapid changes in gas quality.
Other examples of processes and equipment that could be adversely affected include:
* Float glass and fiberglass production.
* Furnaces with controlled atmospheres.
* Ceramics and glazing processes.
* Gas engines.
* Direct-fired textile processes.
* Fertilizer manufacturing with natural gas as a feedstock.
Changes in flame shape and radiation temperatures could affect most of these industrial processes.
It is most likely, as a result of the 2-year gas quality program managed and recently completed by the DTI, that UK government will wish to retain the GSMR specifications for a time, requiring off- specification imports to be blended or processed to comply with existing Wobbe limits.1
European position
European Commission Directives 98/30/EC and 2003/55/EC aim to ereate a fully operational internal natural gas market with common rules for transmission, distribution, supply, and storage of natural gas. The differences in gas-quality specifications across EU member states have been a barrier to cross-border trade.
Most European countries currently operate a narrow band of acceptable Wobbe Index. There is agreement, however, that the Wobbe Index is the primary interchangeability factor. Several European countries operate completely separate networks for high and low calorific natural gas (Fig. 4; photo from Marcogaz).
The European Union Gas Regulatory Forum (Madrid Forum) consists of representatives of member states, national regulatory authorities, gas industry operators and suppliers, large industrial gas consumers, traders, and producers. In 2002, the forum established EASEE-gas (European Association for the Streamlining of Energy Exchange), with a group objective of giving all participants in the gas chain a forum voluntarily to agree on a set of common business practices. One part of this has been to establish such a practice for harmonization of natural gas quality. The EASEE-gas specification for gas quality includes a proposed Wobbe range of 13.60 kw-hr/std, cu m to 15.81 kw-hr/std. cu m (47.0 MJ/cu m to 54 MJ/cu m).
Fig. 4, widely published via Marcogaz,2 shows that most European countries currently operate a narrower band of acceptable Wobbe than the proposed EASEE-gas specification, but there is agreement that the Wobbe Index is the primary interchangeability factor.
It is also worth noting that several European countries operate completely separate networks for high and low calorific natural gas.
The European Commission has also mandated CEN, the European standards organization,3 to create a European standard for gas quality giving the broadest limits of acceptable performance as possible within reasonable cost. The CEN mandate identifies the need for a survey of all gas-fired equipment and appliances across all European Union countries and the need for a Europe-wide testing program of a representative sample of appliances.
This would be similar in activity to, although larger in numbers than, the UK Gas Quality Programme. This effort will provide data to analyze the impact on safety, efficiency, and environment, particularly for domestic appliances.
Following this work, CEN will consider creating new test standards and appliance performance standards to build up a population of new appliances that will be able to tolerate and work effectively across a wider variation of gas quality.
US; rest of the world
The US has both state-by-state regulation and federal regulation related to gas quality and gas interchangeability. In an attempt to harmonize this position the Natural Gas Council (NGC) has generated a guidance White Paper that reviews the issues in great detail and concludes that a Wobbe range of +/- 4% around the typical or historical local gas value would be acceptable.
This finding has been submitted to the US Federal Energy Regulatory Commission. Inputs were provided to this work by Advantica, Gasunie, and Gaz de France demonstrating a genuine enthusiasm to gain international consistency.
Within the last year, there has been increasing activity in sharing knowledge and awareness of gas quality and interchangeability development work because it is being raised as an issue across the Far East, particularly Japan, China, and Korea, where LNG imports are significant, and in India, where the impact of gas-quality fluctuations on natural gas vehicle operations is becoming important.
References
1. More information on the UK gas-quality program and test results can be found at http://www.dti.gov.uk/energy/markets/gas- quality/phase-2/page21044. hrml.
2. www.marcogaz.org.
3. CEN = Comit Europen de Normilasation.
Terry Williams
Advantica
Loughborough, UK
The author
Terry Williams (terry. williarns@advantica graup.com) is business leader for Adiuntica, Loughborough, UK, and Houston. He has also served as project manager, Foseco international Heat Treatment. He holds a bachelor of science in chemical engineering from Aston University and a masters in control engieering from Coventry University. Williams is a UK chartered engineer, a member of the institution of Gas Engineers & Managers, and a member of the Institute of Measurement & Control.
Copyright PennWell Publishing Company Apr 2, 2007
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