Gas Interchangeability Defined
AGA Staff Paper:
Technical Background and Issues of Gas Interchangeability
Prepared for:
Building Energy Codes and Standards Committee American Gas Association
Prepared by:
Ted A. Williams Director, Codes, Standards & Technical Support American Gas Association
April 2006
Disclaimer
This report was prepared by American Gas Association (AGA) Staff for the purpose of summarizing prior technical work, research, and technical application of gas interchangeability principles and practices. Neither AGA, its members, nor any person acting on behalf of these organizations:
• Makes any warranty or representation with respect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any information disclosed in this report may not infringe privately owned rights; or
• Assumes any liability with respect to the use of, or for damages resulting from the use of, any information disclosed in this report.
2 Table of Contents
1. Gas Interchangeability Defined ...... 5.
1.1 The NGC+ Definition ...... 5.
1.2 Historical Definitions from the Gas Industry Literature ...... 6.
1.3 Implications of Gas Interchangeability as Defined ...... 6.
2. The Objectives of Gas Interchangeability Criteria ...... 7.
2.1 Anticipation and Avoidance of Adverse Combustion Behavior ...... 7.
2.2 Drivers for Gas Interchangeability Specifications in the U. S...... 8.
2.3 Appliance Focus of Gas Interchangeability Studies and Specifications . . . . . 8.
2.4 Relevance of Historical Studies to Current End Uses ...... 11.
3. Combustion Issues in Classical and Contemporary Gas Interchangeability ...... 12.
3.1 General Combustion Issues ...... 12.
3.2 Combustion Failure Modes ...... 13.
3.2.1 Elevated Pollutant Generation ...... 13.
3.2.1.1 CO Production ...... 13.
3.2.1.2 Particulate Matter (Soot) Production ...... 16.
3.2.1.3 NOX Production ...... 18.
3.2.2 Altered Heat Rates ...... 19.
3.2.2.1 Durability of Heat Exchangers and Components ...... 19.
3.2.2.2 Jacket/Vent Overheating and Fire Hazards ...... 20.
3.2.2.3 Changes in Efficiency ...... 20.
3.2.3 Combustion Stability ...... 20.
3.2.3.1 Flame Lifting ...... 20.
3.2.3.2 Flashback ...... 21.
3.2.3.3 Oxygen Depletion Sensor (ODS)/Pilot Reliability ...... 21.
4. Appliance/Gas Consumer Issues ...... 21.
4.1 Installation/Maintenance Issues...... 21. 3
4.2 Unattended Operation ...... 22.
4.3 Changing Means of Adjustment ...... 22.
4.4 Vented versus Unvented Design and Operation ...... 22.
5. Considerations from U. S. Historical Gas Interchangeability Studies ...... 23.
6. Gas Interchangeability Indices ...... 26.
7. Activities and Gas Industry Requirements Outside the U. S...... 29.
7.1 Overseas Specifications ...... 29.
7.2 Current U. K. Activities...... 30.
8. Criteria for Acceptable Appliance Performance ...... 32.
8.1 CO Criteria Are Most Critical ...... 32.
8.2 Standards for Acceptable CO Generation in Appliances ...... 32.
9. Issues Beyond the Scope of This document ...... 33.
9.1 Adjustment at End Use versus Gas Supply Modification ...... 33.
9.2 Economic Efficiency and Equity ...... 33.
9.3 Similarities and Differences of Requirements Among End Users ...... 34.
9.4 Consumer Risk Under Alternative Actions ...... 34.
References ...... 35.
4
The objective of this Staff Paper is to discuss technical aspects of gas interchangeability including its meaning, the objective of gas interchangeability criteria, combustion issues and failure modes associated with non- interchangeable gases, technical history and background, use of indices as metrics, examples of interchangeability requirements in other countries as of this writing, and criteria for acceptability. Because of the wealth of technical documentation of gas interchangeability science and applications, references to important work are provided, and descriptions of these studies are not provided here in most cases. Emphasis is placed upon domestic appliances because, despite the large customer class using these devices and the wealth of information available on testing and gas interchangeability studies on them, recent debate over impacts upon appliance function and residential consumers may not be receiving equitable consideration, in the authors opinion. This Staff Paper does not address issues of gas composition as it relates to potential issues of U. S. gas supply, distribution of costs that may be associated with bringing interchangeable gases to market, the diversity of end uses and potential inconsistencies of gas interchangeability requirements among end uses, or issues of implementation and policy associated with gas interchangeability criteria and requirements.
1. Gas Interchangeability Defined
1.1 The NGC+ Definition
In 2004, the Natural Gas Council (NGC), a coalition of natural gas trade associations composed of the Interstate Natural Gas Association of America (INGAA), the Natural Gas Supply Association (NGSA), the Process Gas Consumers Group (PGCG), and AGA, recognized the need for the U. S. natural gas industry and its customers to develop industry based recommendations for addressing changes in gas composition brought on by changes in domestic gas and anticipated LNG importation. The NGC recognized the need to expand participation on issues of gas quality, including gas interchangeability, to other stakeholders concerned with gas supply, transportation, and end use. This expanded ad hoc group was convened in 2004 as "NGC+." The Gas Interchangeability Task Group was organized by NGC+ representing over 40 stakeholder organizations including gas producers, pipelines, local distribution companies (LDCs), end use equipment manufacturers, trade associations, and regulators. In development of its "White Paper on Natural Gas Interchangeability and Non-Combustion End Use," the Task Group developed the following definition of gas interchangeability:
“The ability to substitute one gaseous fuel for another in a combustion application without materially changing operational safety, efficiency, performance or materially increasing air pollutant emissions.”1
1 Gas Interchangeability Task Group. White Paper on Natural Gas Interchangeability and Non- Combustion End Use. Natural Gas Council Interchangeability Task Group, 2005, p. 3. 5
1.2 Historical Definitions from the Gas Industry Literature
Prior to the NGC+ work, the LDC industry had developed a variety of definitions of gas interchangeability as part of its seven decades of work in this area (i.e., beginning formally in the 1930s). The following are examples of these definitions, which are different but consistent:
"Two gases may be interchangeable if flame characteristics are satisfactory after substitution of one gas for another,”2
"The two basic tenets for the gas industry are:
• Suppliers must furnish a fuel gas that burns safely and performs adequately in the appliances and equipment connected to their lines.
• Manufacturers must furnish appliances and equipment that operate properly on the fuel gas furnished.
Thus, as fuel gases become more varied and maintaining a uniform product becomes increasingly difficult or impossible, the definition of acceptable variations in the composition of the gas becomes more vital. A responsive change is now taking place in the was the gas industry looks at interchangeability."3
1.3 Implications of Gas Interchangeability As Defined
These definitions clearly identify gas interchangeability as fundamentally an end use issue in terms of the interaction of gases with end use equipment. Specifically, it is concerned with gas combustion. Granted that the gas interchangeability criteria and requirements may have far-reaching implications, the focus of gas interchangeability science since the 1930s in the U. S. has remained on how end use appliance combustion responds to changes in fuel gas composition. This focus is reflected in appliance and equipment testing and development of interchangeability indices and parameter limits for indices.
In addition, these definitions point to the interaction of appliances and equipment and changes in gas composition as appliances and equipment are manufactured and installed, not as appliances might be modified by manufacturers or by installers in the field. This condition is documented in gas industry literature:
2 American Gas Association. Gas Engineers Handbook. Segler, C. George, Editor-in-Chief, The Industrial Press: New York, NY, 1965. p. 12/239. 3 American Gas Association. Utilization, Volume V: Gas Engineering and Operating Pracitce, Book U-1 Residential/Commercial, American Gas Association: Arlington, VA, 1994, p. 24. 6 "…for a substitute gas to be interchangeable with the base gas, the base settings of primary air and gas input rate must be within the flame limits of the substitute gas."4
Using an analogy from mathematics, gas interchangeability as it is studied and applied directly addresses the fixed environment of the appliance and equipment stock as a constant and gas composition as an independent variable. What might be done in terms of modifying appliances and equipment in the future, or even how appliances and equipment might be modified in the field to better address a known change in gas composition, are technically beyond the initial issue of whether gases are interchangeable.
2. The Objectives of Gas Interchangeability Criteria
2.1 Anticipation and Avoidance of Adverse Combustion Behavior
Gas interchangeability criteria emerged in the early days of the natural gas industry as a means of avoiding end use combustion problems before they occurred in widespread fashion in the field. Specific statements of the need for gas interchangeability criteria are rare in the early industry literature, probably because of the obvious importance addressing the suitability of gases before they were introduced as substitutes for other supplies and the need to avoid problems in the field. Two statements were offered in 1946 that suggest this:
"The ultimate objective of the investigation was to develop a method that would reliably predict [emphasis added] what gases could be substituted for natural gases or high heating value mixed gases, or supplement an inadequate supply of high heating value base gases during peak load periods."
and
"The matter of satisfactory interchangeability is obviously of extreme importance since no value can be attched [sic] to any supplemental gas which, if mixed with the base natural gas in any substantial proportion, will not permit customers to continue to utilize their appliances in a normal manner."5
Concerns of the gas industry to questions about gas supply and end use can be traced to a number of aspects of the early gas industry. However, the presence of significant concentrations of carbon monoxide (CO) in manufactured or "town
4 American Gas Association. Gas Engineers Handbook. Segler, C. George, Editor-in-Chief, The Industrial Press: New York, NY, 1965. p. 12/239. 5 American Gas Association Laboratories, Interchangeability of Other Fuel Gases with Natural Gases, Research Bulletin Number 36. AGA Committee on Mixed Gas Research, Joint Committee of Natural Gas Department and Technical Section, American Gas Association : Cleveland, Ohio, 1946, p. 2. 7 gas" and poisoning incidents from unburned gas undoubtedly contributed to these concerns as well as the need to better understand and anticipate problems of end use before they occurred.
2.2 Drivers for Gas Interchangeability Specifications in the U. S.
Questions of interchangeability have been driven by specific, anticipated changes in the U. S. gas market. In general, the following factors contribute to peaks in general interest in gas interchangeability and have driven technical activities:
• In the 1930s, the introduction of natural gases and issues of interchangeability with locally produced town gases.
• In the 1940s, development of peakshaving supply approaches, including manufactured gases and propane-air system sendout, and interchangeability with base load supplies.
• In the 1970s, introduction of imported LNG in local market areas and interchangeability with domestic supplies.
• Present, renewed interest in imported LNG locally and nationally, changes in processing of domestically produced gas, and interchangeability with traditional domestic supplies.
Exceptions are obvious to this characterization of the peaks (and valleys) in interest in gas interchangeability such as activity in the 1980s in the Western states addressing domestic sources such as Williston Basin gases and their interchangeability with other sources. However, interest in gas interchangeability spans decades, has been driven by previous concerns associated with new supplies including imported LNG, and has shared the common feature of evaluating gases before widespread experience with these gases in the field and potential combustion problems.
The study of gas interchangeability, briefly described later in this paper, has been the legacy of the U. S. gas utility industry. The need to anticipate issues associated with new supplies through testing and the development of interchangeability indices as tools demonstrates the commitment of the utility industry to address these issues before problems affecting gas consumers occur. In this respect, work on gas interchangeability and development of interchangeability limits may infer a "standard of care" by which the industry has operated for several decades.
2.3 Appliance Focus of Gas Interchangeability Studies and Specifications
8 While gas interchangeability can be determined for any combustion- related end use, historical focus of interchangeability studies have focused almost exclusively on appliances and equipment comprising residential and light commercial end uses. As discussed in Section 5, historical U. S. testing data covers several thousand appliances and appliance-type gas burners. While no limitation exists for conducting similar tests on industrial burners and gas turbines, the U. S. gas industry emphasized appliance applications for several reasons:
• Residential and commercial end uses represent the largest customer classes potentially affected by gas interchangeability issues. Even today, natural gas consumption by the residential sector is only 23.5% of total U. S. consumption, but residential consumers represent 92% of natural gas consumers.6
• Residential and many commercial end uses operate unattended or without direct operational control. As a result, normal use of residential and commercial appliances and equipment are subject to changes in combustion behavior without the benefit of an operator to either alter operation or adjustment or shut down equipment in the event of unacceptable combustion performance.
• Residential and commercial appliances would require extensive and highly labor intensive field modifications to adjust combustion performance for even a single significant change in gas composition. As was noted in 1994:
"Gross changes in gas composition that assuredly will require physical modification of appliances and equipment are another facet of interchangeability. When and if they come, these changes will require large-scale conversion programs similar to those associated with the change from manufactured gas to natural gas in the first half of the century."7
Current U. S. market conditions and the prospect of increased LNG importation from a variety of supply sources make this analogy imperfect. End users may be facing supplies from a variety of sources with diverse compositions, not a simple monotonic change from one general supply to another. As a result, the adequacy of field adjustment and modification approaches may not be sufficient in many cases. In addition, with changes to the gas utility industry and its decreased role in gas appliance servicing nationally, it is unclear what resources would accomplish large- scale appliance modification and adjustment programs, even where monotonic changes in gas supply were anticipated. The experience of
6 U. S. Department of Energy, Energy Information Administration , "Natural Gas Statistics for 2004," http://tonto.eia.doe.gov/dnav/ng/. 7 American Gas Association. Utilization, Volume V: Gas Engineering and Operating Pracitce, Book U-1 Residential/Commercial, American Gas Association: Arlington, VA, 1994, p. 28. 9 Questar discussed in the Gas Interchangeability White Paper summarizes unique issues and challenges of such programs to accommodate a monotonic change in gas supply.8
• Residential appliance maintenance and operating condition may be very important since annual maintenance and adjustment of appliances is not required in the U. S. This phenomenon is less likely in other end uses. As expressed in 1994:
"…it is reasonable to believe that there will always be a body of appliances that are unable to cope with changes in fuel-gas composition simply because of their poor condition. This is another complication that must be considered when fuel-gas changes are contemplated."9
Under these conditions, it can be argued that residential appliances are the most sensitive end use category and should drive the setting of general gas Interchangeability specifications. Whether it is the role of gas interchangeability specifications to account for poorly maintained or maladjusted appliances is a policy question. Some studies and activities to set gas compositional requirements have been undertaken such as the work leading to the tariff specifications for send out from the Cove Point LNG terminal.10 The end result of such approaches is likely to be the setting of tighter gas interchangeability specifications than if appliance design performance was used. Such approaches also require careful local study of appliance stocks and quality assurance that appliance testing is representative of the stock, reproducible in the laboratory, and repeatable.
• Residential gas appliances, as a category of end uses, represent a diverse set of technologies and potentially high level of uncertainty with respect to responses to changes in supply. In part, the diversity of these end uses is due to the longevity of residential appliances and the resulting relatively slow turnover in the installed appliance stock and their long-term operating condition under continuous use:
"Such longevity raises several concerns about appliance condition:
The burners, combustion chambers, pilots, vents, flues, and other items will vary widely in their designs.
Use and misuse will have created a wide diversity of physical conditions.
8 Gas Interchangeability Task Group. White Paper on Natural Gas Interchangeability and Non- Combustion End Use, Appendix D. Natural Gas Council Interchangeability Task Group, 2005, pp. 27-30. 9 Op cit, p. 29. 10 Rana, H., and Johnston, D. "An Empirical Approach to Evaluating Gas Interchangeabiiity," 2003 AGA Operations Conference Paper, 03-OP-41, May 2003. 10
Some makes and models of equipment many not be able to cope with a change in fuel gas."11
2.4 Relevance of Historical Studies to Current End Uses
The last point listed above suggests a number of issues with respect to changes in appliance technology and the robustness of past gas interchangeability testing. Recently, increasing attention has been given to concerns over more advanced end use burner technologies designed for higher combustion efficiencies and lower atmospheric emissions, specifically oxides of nitrogen (NOx). Arguments have been put forward that the current data base on so-called "legacy appliances" does not represent these technologies adequately. While this may be true, per se, it does not dismiss the relevance of the existing published gas interchangeability research. This research, which focuses on conventional "atmospheric" or Bunsen-type burners used in appliances, is generally applicable to the vast majority of burners used in the U. S. gas appliance stock.
Atmospheric burners share common design and operating features that lend to similar performance. Most of these burners use partial premixing of combustion air and fuel gas upstream of the burner head (i.e., primary aeration) and are designed to hold the flame at the burner head by equalizing the burning velocity with the flow velocity at the burner head. Rates of primary aeration vary but typically represent 20-60% of the air required for complete combustion. Limiting factors for acceptable burner performance include the delivery of the fuel gas at premixing through the burner orifice and availability of sufficient air to the flame beyond the burner head to complete combustion (e.g., full secondary aeration). These are common features of atmospheric burners and are widely represented in the interchangeability research literature.
Features of appliances other than general burner design may have even more importance to gas interchangeability. These include, but are not limited to, the following:
• Burner material
• Secondary air chamber and heat exchanger design
• Requirements for burner turn down in the application.
However, these variables have not been characterized in detail in gas interchangeability studies. In the final analysis, the existing gas interchangeability testing literature is relevant to the gas appliances that were tested and the "representativeness" of these appliances to the existing appliance stock. However, design age of the appliance is not, in of itself, a disqualifying
11 Op cit, p. 28. 11 characteristic. With gas furnaces lasting from an average of 18 years to "high" lifetime of 25 years,12 many older designs are certainly well represented in the current appliance stock. As a result, what some may call a "legacy appliance" may be more appropriately called a "conventional appliance."
3. Combustion Issues in Classical and Contemporary Gas Interchangeability
3.1 General Combustion Issues
Combustion issues associated with gas interchangeability are well documented in the industry literature included in the references to this paper and include the following general end user concerns:
• Safety
• Reliability of Performance
• Appliance and Equipment Durability
Combustion safety concerns include control of pollutant (i.e., CO) generation in vented and unvented appliances and equipment, overheated appliance surfaces and maintaining clearances to combustible materials, fire hazards due to flame rollout, and overpressures due to delayed ignition.
Reliability of performance concerns include poor flame stability leading to outages due to activation of safety devices in over-firing or under-firing, flame blowoff, and flashback from the burner head. Reliability concerns also include reduced efficiency due to input rate changes and flame length and shape changes relative to heat exchanger surfaces, altered appliance capacity, and reduced turn-down capability.
Durability concerns include thermal degradation, fatigue and warping of appliance components including heat exchangers, and fouling due to soot production . Durability concerns may lead to progressive declines in reliability and safety performance that may not be recognized from initial investigations.
Specific combustion "failure modes" are associated with most of these concerns and relate to observable combustion phenomena in appliances and equipment. Gas interchangeability criteria incorporate these observable phenomena into numerical indices for evaluating interchangeability. However, the relationship of these combustion failure modes to safety, reliability, or durability may not be obvious from simply reviewing the specification of the indices.
12 "28th Annual Portrait of the U. S. Appliance Industry," Appliance, September 2005, p. 7. 12 3.2 Combustion Failure Modes
The following combustion failure modes are described in terms of their causes, relative importance in gas interchangeability criteria, and coverage in gas interchangeability indices.
3.2.1 Elevated Pollutant Generation
Pollutants associated with combustion problems and gas interchangeability include elevated CO, soot, and NOX production. Particular attention is paid in the discussion to follow on carbon monoxide production since it is viewed as, perhaps, the most sensitive appliance behavior with respect to gas interchangeability and consumer safety and wellbeing. However, technical discussions often ignore causes for elevated carbon monoxide production. Other emission issues are well covered by other, cited sources.
3.2.1.1 CO Production. CO production is perhaps the most sensitive combustion behavior with respect to conventional gas interchangeability (i.e., aside from generation of atmospheric pollutants at or near their regulated limits). CO generation and exposure is of concern because of the gas's role as an acute hypoxic poison in high dosages (i.e., concentration over time) and is of concern for both unvented gas appliances and vented gas appliances operating under vent failure conditions.
Design certification standards recognized by the American National Standards Institute (ANSI) for gas-fired appliances and equipment in many standards set limits for CO generation for both vented and unvented appliances. The ANSI standards design certify appliances based on a limited range of gases, and for natural gas, certify safety performance with respect to CO production and other behaviors on a single test gas specification, referred to as "Test Gas A." Generally speaking, seafety performance is based on this test gas, and acceptable performance with respect to CO generation is based on non- excedence of the applicable standard's CO limit. Application of these numerical limits to field operation and acceptable performances is not supported by the design certification standards but has been implemented in various ways and under various sets of conditions. CO production, per se, is not evaluated directly in U. S. gas interchangeability indices, with the exception of the Weaver Incomplete Combustion Index, which permits development of index limits for acceptable performance based on a change in fuel gas. However, these index limits do not relate directly to the ANSI design performance limits.
The most common cause of increased CO production is insufficient secondary air for complete combustion. Figure 1 shows a simple Bunsen burner in which primary combustion air is introduced and mixed in the burner body following the primary shutter where combustion air is first introduced conceptually the introduction of combustion air in a partially premixed burner. Here, premixing occurs at levels of air below the minimum combustible mixture to decrease the likelihood of the burning rate exceeding the follow rate of the mixture and “flashing back” into the burner body.
13
Figure 1. Bunsen Burner Example.13
Secondary air completes combustion in the case of the Bunsen burner through entrainment of air into the outer flame cone. In more realistic appliance burners, secondary air is introduced through secondary air openings as shown in Figure 2, although in most cases, full secondary aeration requires additional entrainment into the outer flame cones.
Figure 2. Simple Appliance Burner.14
13 American Gas Association Laboratories. Fundamentals of Gas Combustion, Revised. Prepared for American Gas Association and Gas Appliance Manufacturers Association, 1996, p. 20. 14 Ibid, p. 29. 14
Complete combustion that results from primary aeration and full secondary aeration to its complete stoichiometric end products is shown in Equation 1 for methane:
CH4 + 2O2 → CO2 + 2H2O Eq. 1.
However, natural gas combustion is, in fact, more complex than as represented in Equation 1. Oxidation of methane and accompanying hydrocarbons, mostly ethane, undergoes intermediate reactions as shown in simplified form in Equation 2:15,16,17
CxHy → CH3 → CH2 → CH → HCO → CH2O → CO → CO2 Eq. 2.
The formation of unstable hydroxyl (OH) radicals (not shown for simplicity) facilitates this sequence through the formation of CO in the inner reaction zone of these flames. Subsequently CO is recombined in the outer reaction zone to complete combustion to CO2.
In full stoichiometric combustion, these reactions occur in general proximity so that the simplification of Equation 1 is a reasonable description. However, only fuel rich combustion can occur in partially premixed regions of the flame, and if entrainment of secondary air is required to complete combustion, the persistence of these free radicals may persist until entrainment of oxygen reaches stoichiometric levels. Factors that may lead to a disruption of sufficient secondary air may include:
• Insufficient gross air supply to the flame available in the secondary combustion space or chamber
• Obstruction of idealized entrainment flow due to burner or combustion chamber geometry
• Impingement of flames on surfaces and subsequent alteration of flame geometry, and
• Inherent changes in oxygen requirements for burner throughput associated with higher hydrocarbons in fuel and consequent higher oxygen requirements for complete combustion.
The last of these factors can be a first order impact of introducing non- interchangeable fuels. However, other flame changes due to longer flame
15 Peeters,J. and Mahnen,G., "Reaction Mechanisms and Rate Constants of Elementary Steps in Methane-Oxygen Flames," Fourteenth Symposium (International) on Combustion, pp. 133-146, 1973. 16 Smoot, L., Hecker, W., and Williams, G., "Prediction of Propagating Methane-Air Flames," Combuston and Flame, Vol. 26, pp. 323-342, 1976. 17 Tsatsaronic, G., "Prediction of Propagating Laminar Flames in Methane, Oxygen, Nitrogen Mixtures," Combustion and Flame, Vol. 33, pp. 217-239, 1978. 15 lengths and other altered shapes, contributing to the other three factors, may be sources of increased frequency of incomplete combustion as well. In fact, and as will be discussed later, specific design features of burners in combustion chambers may have much to do with differences in gas interchangeability performance of among appliances. However, historical data from interchangeability tests suggests that these effects are second order impacts on CO generation.
Elevated CO production in appliance operation has been inferred in the laboratory and field by observed “yellow tipping” of flames. Direct measurement of CO production was first accomplished in laboratory experiments and later, with the availability of less expensive field instrumentation, by direct field measurement.
3.2.1.2 Particulate Matter (Soot) Production. Soot production is most important in appliance emissions as a source of combustion fouling over time and diminished overall emissions performance and reliability. It is less significant as a direct source of particulate matter pollutant in the environment. Soot production is represented by a complex series of reactions leading to the production of elemental carbon and carbonaceous solids, which may or may not be visible under ordinary circumstances. Soot production is mainly a function of combustion at elevated temperatures and shortages of oxygen. Appliance factors leading to these conditions include some of the influences discussed above and, as a result, soot production may accompany elevated production of CO.
As pointed out most recently by Levinsky in his analysis of appliance sooting, chemical reactions leading to soot production are not well understood such that:
"…the prediction of the actual occurrence of soot in practical appliances is not yet possible. We thus follow a different route to compare gases for their relative soot-forming tendencies."18
The Levinsky paper provides the most current and complete discussion of practical issues of appliance sooting.
Prediction of soot formation is desirable because soot formation and deposition changes appliance combustion characteristics over time. As a consequence, an appliance with a sooting flame may, at an early point in its operating history, exhibit no operation issues. However, over time the accumulation of soot and fouling of combustion chambers, venting systems, and components can lead to hazardous operating conditions. This accumulation may
18 Levinsky, H. "Report of 'Identification of the Concentration and Combination of Higher Hydrocarbons in Natural Gas Likely to Cause Sooting in Gas Appliances," Report to the Department of Trade and Industry, United Kingdom, p. 3, 2005. 16 not be obvious where soot is not visible or obvious, and fouling may occur in areas of the appliance not readily detected by the consumer.
Sooting caused by changes in gas supply is addressed through various means. In the U. K., the Dutton "soot index" is used to describe the tendencies of gases to develop soot. In the U. S., yellow tipping of flames has been used to empirically evaluate flames for development of visible and non-visible soot. Again, yellow tipping is also used to characterize flames that may be producing elevated levels of CO, but yellow tipping may not be a strong predictor of simultaneous production of CO and soot.
Yellow tipping as well as other flame behaviors are used to both diagnose appliance operational problems and for empirical research on performance and interchangeability. In fact, the gas interchangeability indices discussed later in this paper, for the most part, associate gas compositions and properties with these directly observable behaviors. Gas utility training and service organizations have, for many years, emphasized identifying abnormal flame behaviors including flame lifting, flash back, and yellow tipping and adjustment to proper appearance as part of training of service personnel. Training has included use of training media including slide presentations, videos, and printed materials. One established objective source for visual interpretation of normal and abnormal flame behaviors, published in 1950, is the AGA Flame Code classification method19 represented in Table 1 and recently applied in interchangeability studies by GTI.20
Table 1. AGA Flame Code Classifications
Code Flame Description
+5 Flames lifting from ports with no flame on 25% or more of the ports +4 Flames tend to lift from ports, but become stable after short period of operation +3 Short inner cone, flames may be noisy +2 Inner cones distinct and pointed +1 Inner cones and tips distinct 0 Inner cones rounded, soft tips -1 Inner cones visible, very soft tips -2 Faint inner cones -3 Inner cones broken at top, lazy wavering flames -4 Slight yellow streaming in the outer mantles, or yellow fringes on tops of inner cones. Flames deposit no soot on impingement -5 Distinct yellow in outer mantles or large volumes of luminous yellow tips on inner cones. Flames deposit soot on impingement.
19 Interchangeability of Various Fuel Gases with Manufactured Gases, AGA Research Bulletin No. 60, American Gas Association Laboratories, 1950. 20 Gas Interchangeability Tests: Evaluating the Range of Interchangeability of Vaporized LNG and Natural Gas, Final Report, Gas Technology Institute, April 2003. 17 Another source depicting normal and abnormal flames with color photography is a U. S. Environmental Protection Agency publication, “Guidelines for Adjustment of Atmospheric Gas Burners for Residential and Commercial Space Heating and Water Heating,” published in 1979.21 While burner adjustment to flame appearance depends upon subjective interpretation of installers, service technicians, and even gas interchangeability researchers, relatively consistent performance in terms of more objective criteria (e.g., calculated air-free CO emissions based on CO measurements in combustion products) is generally achieved.
3.2.1.3 NOX Production. Elevated NOX emissions are not directly characterized as a form of combustion failure. In appliances, NOX emissions are more closely associated with the inherent emission characteristics of appliance. However, development of low- NOX emitting appliances, primarily for the State of California where NOX limits are imposed on appliances, has led to increasing concern about sensitivity of NOX performance of these appliances in response to changes in gas supply. Across end uses, NOX emissions concerns are dominated by regulatory compliance of combustion applications with ambient air quality emission limits. Particular public attention has been paid to issues of gas turbines and, in particular dry low NOX (DLN) turbines and sensitivity of response to gas supply, particularly high Wobbe gases and various combinations of non- methane constituents in gas supply. Those issues will not be discussed here but are covered at length in the Gas Interchangeability White Paper.
NO2 is a more general issue of concern associated with emissions of gas appliances, primarily as it can contribute to indoor air quality. As a respiratory irritant, NO2 emissions are, themselves, a concern. In contrast, NOX emissions are of concern primarily as a contributing factor to ozone formation, ozone being a "criteria pollutant" under the National Ambient Air Quality Standards. As a result of the differences in interest between NO2 indoors and NOX outdoors as it contributes to ozone, the U. S. interchangeability literature sometimes lacks the speciation of NOX required to determine how changes in gas supply affect NO2 levels and consumer health and safety. Details on NOX and NO2 formation and reaction chemistry are provide in Appendix C of the Gas Interchangeability White 22 Paper. However, the discussion of NO2 as it relates to gas turbines focuses on visual impacts from atmospheric concentrations of this species.
The United Kingdom Department of Trade and Industry (DTI) sponsored appliance tests in order to investigate a range of gas interchangeability issues in the 2003-05 timeframe. The report of Advantica to DTI covering its gas interchangeability tests of twenty appliances has a detailed discussion of NO2 formation and results for appliances. Generally, Advantica found that while total NOX increased as a function of Wobbe number increases in the gas supply,
21 Guidelines for Adjustment of Atmospheric Gas Burners for Residential and Commercial Space Heating and Water Heating, U. S. Environmental Protection Agency, February 1979, EPA-600/8-79-005. 22 Gas Interchangeability Task Group. White Paper on Natural Gas Interchangeability and Non- Combustion End Use, Appendix C. Natural Gas Council Interchangeability Task Group, 2005, pp. 66-77. 18 appliances produced inconsistent results for the NO2 fraction of total NOX. In some appliances, the rise was dominated by NO, a highly reactive species that interacts with building elements and quickly decays. In other appliances, 23 included unvented appliances, NO2 dominated the increase.
3.3.2 Altered Heat Rates
Heat rate of an appliance can be represented by the volumetric throughput based on manifold pressure and flow limitation (such as flow through an orifice) and the gaseous fuel Wobbe number:
Btu/scf W = Eq. 3. (specific gravity)0.5
The heat rate at the burner is more properly expressed as the Wobbe number alone. For a given volumetric throughput adjustment, the appliance heat rate will change proportionally by the change in the Wobbe number of the fuel supply alone. Interchangeability issues are associated, at the appliance level, with such a fuel change and absent any modification of the volumetric flow rate or burner orifices.
The impact of such changes in appliance heat rate may be significant and may exceed "overfire" limits specified in the Z21/83 design standards. Such overfiring may lead to appliance problems.
3.2.2.1 Durability of Heat Exchangers and Components. Overheating of heat exchangers and components, including controls, has been associated with overfiring of appliances due to introduction of abnormally high Wobbe gases. No data is available on this, and measurements of heat exchanger and other surface temperatures measured in gas interchangeability tests by Sempra Utilities/Southern California Gas Company do not suggest, for the gases it analyzed, temperature limits in product standards. However, such elevated temperatures are associated with the Wobbe numbers of the gases and, therefore, are specific to the gases tested. In addition, comments from the Gas Appliance Manufacturers Association (GAMA) at Gas Interchangeability Task Force meetings suggest that analysis of heat exchanger lifetimes may be impacted by continuous operation on higher than normal Btu gases. Data on this was not presented. Suggested heat exchanger damage is most frequently associated with warping, deformation, and accelerated corrosion. Such impacts would occur over time, potential over months or years. Controls related issues suggested include control failures which, due to the fail safe design of most appliance systems, would occur as nuisance outages of appliances and not immediate safety-related problems.
23 Williams, T. Assessment of Changes to the Performance of Gas Appliances in Relation to Variations in Gas Quality, prepared for Department of Trade and Industry, URN 05/1938, October 2005, pp. 66-77.
19
3.2.2.2 Jacket/Vent Overheating and Fire Hazards. Surface temperature limits are imposed on appliances through the Z21/83 standards and are accompanied, where needed, by clearance minimums from combustible materials. Higher appliance throughputs from overfiring may lead to the exceeding of these limits, and clearance requirements may no longer be sufficient. Such high temperatures may pose consumer hazards from touching hot surfaces or, at an extreme, lead to fire hazards.
3.2.2.3 Changes in Efficiency. It has been suggested that gas compositional changes can affect appliance efficiency. In most ways, one would expect this to be a secondary impact of a gas supply change since changes in useful heat tend to be proportional to the input rate, and reasonable changes in merchantable gases will not lead to venting of unburned gas. Thermal efficiency, heat energy out divided by heat energy in, should remain relatively constant.
Second order effects on efficiency may be associated with flames inefficiently interacting with heat exchanger surfaces and other influences of changes in flame length and shape. Reduced input due to introduction of a very low Wobbe gas may lead to very short flames and inefficient heat transfer. (Cooler flames on low Wobbe gas may even contribute to elevated CO production due to insufficient reaction zone temperatures to promote full oxidation.) Very high Wobbe gases may impinge the outer reaction zone at the heat exchanger and product incomplete combustion. However, in the latter instance, reduced efficiency would likely be the least important consumer issue. Investigators for DTI found not strong issue on appliance efficiency associated with testing on wide Wobbe ranges of gas.24
3.2.3 Combustion Stability
Stability of flames is needed to maintain useful heating function, emissions performance, and efficiency while reducing the potential for nuisance shutdown or outages of appliances. The following are potential problems associated with loss of flame stability.
3.2.3.1 Flame Lifting. Flame lifting is due to the burner flame front leaving the burner head leading to inherent instability, or the ability to stay lit, and changes in the temperature profile affecting heated surfaces. Lifting of the flame front is fundamentally due to the burning rate decreasing below the flow rate through the burner, and may be due to overly high burner port loadings and overly rich mixtures at the burner head, too much primary air (overly lean) conditions, or high levels of inerts in the fuel gas. Lifting flames may produce excessive levels of CO.
24 Williams, T. Assessment of Changes to the Performance of Gas Appliances in Relation to Variations in Gas Quality, prepared for Department of Trade and Industry, URN 05/1938, October 2005, p. 87. 20 3.2.3.2 Flashback. An early flame behavior covered in gas interchangeability studies, flashback was mainly a behavior associated with concentrations of hydrogen in fuel gases. Hydrogen's high burning rate in the fuel gas mixture could facilitate moving the flame front from the burner heat upstream into the burner body and tubing. Flame pulsation is a common behavior associated with flashback. Hydrogen as a fuel gas constituent was an issue when manufactured gas represented a significant portion of local gas supplies. With the transition to natural gas in the U. S. system and the relative absence of hydrogen from natural gas, this concerned lessened. In rare instances, flashback can occur when a partial premixed burner is maladjusted to an overly lean condition and stoichiometric combustion mixtures are achieve through primary aeration.
3.2.3.3 Oxygen Depletion Sensor (ODS)/Pilot Reliability. Flame stability is important to reliability of pilot lights, both in staying lit and providing sufficient energy to ignite appliance main burners. While newer gas appliances make less use of pilot lights, notable exceptions are new flammable vapor ignition resistant (FVIR) storage water heaters, which in most designs use a pilot in conjunction with a shutoff system. For these designs and older appliances, retrofit for electronic ignition is not a reasonable option. Flame stability issues discussed above apply to pilots generally. The general concern, here, is nuisance outages of the pilots or heating failure due to insufficient pilot activity.
However, ODS shut off systems, required on unvented heating appliances in the U. S., depend upon flame instability as a shutoff mechanism. These devices use a low input flames to heat and energize an electronic element that, when so energized, keeps the gas valve operating and the appliance operational. If the flame diminishes or extinguishes, the gas valve closes, and the appliance shuts down. ODS flames are intended to diminish or go out when room air concentrations reach approximately 17% oxygen, an oxygen depletion condition. In doing so, the ODS shuts down the appliance before unacceptable concentrations of CO, which is produced at rates lagging oxygen depletion, are reached due to combustion in the oxygen depleted room. Two issues are faced by ODS functions in response to significant gas supply changes. With newer low Wobbe gases, the potential for nuisance outages are possible. More importantly, at newer higher Wobbe gases, the ODS flames may not diminish as intended. This may occur because a higher CO generation rate and room accumulation may occur without depletion of oxygen in the room. In other words, the higher Wobbe gas may change the intrinsic emission factor for CO from the appliance without this depletion. In fact, Advantica found this behavior in its tests of ODS function in U. K. appliances.25
4. Appliance/Gas Consumer Issues
4.1 Installation/Maintenance Issues
25 Ibid, pp. ii, 75-76. 21 U. S. requirements for installation are embedded in design standard requirements, installation codes, and manufacturers installation instructions. Maintenance of residential combustion appliances is not required for private homes, although product information and public service information promotes annual service and adjustment. Appendix C of the Gas Interchangeability White Paper lists detailed considerations for the installation and operation of residential combustion appliances.26 The prevailing situation in the U. S. cannot support extensive field adjustment, including checking firing rate, for U. S. residential appliance, assuming that targets for adjustment could be agreed to. Current service infrastructure limitations and costs of contractor labor to implement a major program in this area are likely to be prohibitive. The field situation as assessed by U. K. Department of Trade and Industry, discussed below, holds important potential parallels to the options facing the U. S.
4.2 Unattended Operation
As discussed earlier, domestic appliances (unlike many other types of gas combustion equipment) operate without supervision of owners or building occupants. As a result, consumer response to changes in gas supply relying upon consumer notification and response are not reasonable and, in most cases, not advisable. Gas interchangeability is intended to provide assurance of operational consistency to consumers to maintain safety and reliability of performance without residential occupants having to respond.
4.3 Changing Means of Adjustment
Modern appliances generally have fewer adjustable settings for installation and adjustment in the field. While input settings are still provided (and required), burner adjustments for primary air, in particular, are rarer incorporated in designs. The use of "air shutters," a common feature in many older appliances, are no longer used or are fixed. As a result, field adjustment of combustion is generally limited to adjustment of the firing rate by means of manifold pressure adjustment or orifice replacement. These are largely indirect means of adjusting burners for observed flame behavior in that they do not directly address stoichiometric combustion by changing primary air. The adequacy of these approaches as a replacement to primary air adjustment has not be evaluated in addressing gas interchangeability. However, appliance manufacturers have commented at the Gas Interchangeability Task Group meetings and elsewhere that move away from primary air adjustment at the burner is a concern for addressing new gas supplies where appliance adjustment is required.
4.4 Vented versus Unvented Design and Operation
26 Gas Interchangeability Task Group. White Paper on Natural Gas Interchangeability and Non- Combustion End Use, Appendix C. Natural Gas Council Interchangeability Task Group, 2005, pp. 35-38. 22 While appliance design standards differ for vented and unvented appliances, all Z21 design certified residential appliances incorporate requirements addressing performance safety, including air free CO limits. Some outside of the gas utility industry have commented that CO emissions from vented appliances do not represent consumer risks comparable to unvented appliances. However, in practice, vented appliances generally receive the same scruitiny as unvented appliances in terms of emissions performance. Several factors lead to this response.
• CO fatalities statistics involving residential appliances demonstrate that vented appliances contribute significantly to total CO risks from residential appliances. Problems due to venting system failures and other problems show that CO emissions from the appliance are important, whether or not the appliance is design certified for vented operation.
• Vented appliances, due to their generally higher input rates than unvented appliances and their usage patterns, require scrutiny for combustion malfunction under a condition of venting system failure.
• The Z21 standards were developed under a philosophy of redundant requirements to address potential venting system failures. Furthermore, field application of CO limits on appliances in many cases use the air free CO limits from the appliance's design standard, or fractions thereof, as the basis for evaluating appliances and "red tagging" policies.
5. Considerations from U. S. Historical Gas Interchangeability Studies
Historical accounts of gas interchangeability studies in the U. S. are exhaustively covered in major gas industry references27,28,29,30 and most recently in a forthcoming paper by Halchuk-Harrington and Wilson.31 These studies and the chronology of research does not need to be repeated here. However, several points coming out of this research need to be raised.
First, the appliance tests comprising the data base used for derivation of gas interchangeability indices and evaluation of appliance populations is extensive and covers thousands of data points. The following table from the
27 American Gas Association. Gas Engineers Handbook. Segler, C. George, Editor-in-Chief, The Industrial Press: New York, NY, 1965. pp. 12/239-12/252. 28 American Gas Association Laboratories, Interchangeability of Other Fuel Gases with Natural Gases, Research Bulletin Number 36. AGA Committee on Mixed Gas Research, Joint Committee of Natural Gas Department and Technical Section, American Gas Association : Cleveland, Ohio, 1946. 29 American Gas Association. Interchangeability: What It Means. Operating Section Topical Technology Report, May 1978, pp 24-26 (from Weaver,1951). 30 American Gas Association. Utilization, Volume V: Gas Engineering and Operating Pracitce, Book U-1 Residential/Commercial, American Gas Association: Arlington, VA, 1994, pp. 23-67. 31 Halchuk-Harrington, R. and Wilson, R. "AGA Bulletin #36 and Weaver Interchangeability Methods: Yesterday's Research and Today's Challenges," AGA Operations Conference Paper (not yet published), May 2006. 23 Halchuk-Harrington and Wilson paper is illustrative and does not include a number of industry studies that apply gas interchangeability techniques.
Table 2. Summary of Major U. S. Interchangeability Test Programs Prior to 1950.32
Second, while the data covers many older appliances, very few of the appliances tested are completely out of the U. S. appliance stock. As a result, generalizations based on appliance type remain valid to support the use of the current indices, if not the specific index limits proposed by the studies themselves. However, within appliance types, specific design and construction changes may make the historical studies less general to the current appliance stock. In addition, most of these changes may make today's appliances more sensitive to gas compositional changes. Such changes include:
• Changes in burner material from cast materials to stamped metal with lower total heat capacities
• Fixed premixing for primary aeration, which limits means of adjustment
• Higher levels of premixing and lower dependency for entrainment of secondary air, requiring more precise primary aeration
• Tighter combustion chamber designs and proximities to heat exchangers to increase efficiency, reducing "excess air" and opportunities to increase secondary aeration when needed to complete combustion
• "Lean burn" approaches with leaning based on fixed assumptions of fuel composition.
Trends that may reduce sensitivity tend to require active combustion controls, such as on modern instantaneous gas water heaters, but such controls are not common in appliance design and appliances so equipped are not common in the U. S. appliance stock.
32 Ibid, p. 3. 24
Third, researchers have been almost uniformly conservative in extrapolating appliance test results to appliance populations outside of their test program. Weaver, for example, cautions the users of his work to avoid overly broad generalizations based on his results.33 Likewise, interchangeability studies addressing specific interchangeability questions, such as importation of LNG into Northeastern U. S. markets, have avoided suggestions of extending index limits for interchangeability to other markets and even other nearby service territories.
Finally, conservatism in this area tends to avoid suggesting acceptability of gases beyond the scope of the studies themselves. This is due to a fundamental limit of inference from limited testing. Laboratory data indicating that a gas yields acceptable performance for a set of appliances is suggestive but is limited fundamentally to the appliances themselves. Without a credible statistical sampling design, such tests cannot produce inferences about the population as a whole. No gas interchangeability studies published to date have been based on a thorough statistical sampling design, mainly due to resource limitations on appliance testing and the costs of achieving a sufficient sample size for statistical validity. On the other hand, one appliance failure on a test gas may suggest difficulties for a significant (although uncertain) fraction of the appliance in the population. After all, the likelihood of a testing program sampling the one appliance from the population that would have problems with the test gas is infinitesimally small. As a result, the caution exhibited by researchers and policy makers is based on good scientific practice, not risk aversion on the part of the researcher, per se.
Additional complications exist for representative sampling of appliances to make statistical inferences. In the end, sampling approaches should meet three general requirements, summarized by Reuther34 among others:
• Representativeness. Appliances tested should be statistically representative of the appliances in the stock in terms of type, design, construction, approximate age and condition, and where the program design requires, adjustment.
• Reproducibility. Appliance tests should reproduce, to the extent possible, their installation in the field (if inferences to field operation are sought) or in design certification tests (if inferences to adherence to certification performance are sought). Without the emphasis on reproducible installation and other conditions, inferences from the test program may not be valid.
• Reliability. Appliance tests must consider real conditions in operation, particularly with respect to the test gases introduced to gain confidence
33 Weaver, E. "Formulas and Graphs of Representing the Interchangeability of Fuel Gases," Journal of Research of the National Bureau of Standards, Vol. 46, No. 3, March 1951, pp. D321- D322. 34 Reuther, J. Critique of ANSI Z21.1 Standard for CO Emissions From Gas-Fired Ovens and Ranges, Gas Research Institute, September 1996, pp. 11-17. 25 that results are applicable to operation and over time. For example, testing appliances on LNG vapor as assayed at liquefaction or the import terminal may not give sufficient information on how the appliance would operate on the same gas source following weathering of the liquid product.
6. Gas Interchangeability Indices
As with discussions of historical aspects of gas interchangeability, technical discussion of gas interchangeability indices is covered exhaustively in major gas industry references. Review of these reference materials may cause some confusion since the many empirical studies conducted over different time periods and addressing different analytical questions led to development of new indices and index limits. The paper by Halchuk-Harrington and Wilson provides the most recent account of this literature and is recommended as an initial source of information on the indices.35
Currently, three of these indices are used in the U. S. and are likely to continue to be used in the near future:
• Wobbe Index
• AGA Index, including Lifting Index, IL Flashback Index, IF Yellowtipping, IY
• Weaver Index, including Flashback Index, JF Yellowtipping Index, JY Incomplete Combustion Index, JI Lifting Index, JL Primary Air Ratio, JA Heat Rate Ratio, JH
Both the AGA and Weaver indices were formulated to account for observed atmospheric burner behaviors not directly addressed by the simpler Wobbe index. These behaviors could be observed by field technicians as well as researchers, making the resulting indices practical for use based on observation of flame characteristics. The AGA and Weaver indices are fundamentally empirical in their derivation and application to combustion appliances. The Wobbe Index is extended to appliances based more on its theoretical underpinnings. A more extensive critique of simple methods was offered in 1978
35 Halchuk-Harrington, R. and Wilson, R. "AGA Bulletin #36 and Weaver Interchangeability Methods: Yesterday's Research and Today's Challenges," AGA Operations Conference Paper (not yet published), May 2006.
26 and is documented in the report, "Interchangeability: What it Means."36 However, all three indices require the setting of index limits in practical application. These limits are based on observed behaviors and may vary among studies. Potential reasons for this variation are discussed below.
The AGA and Weaver indices require the specification of a baseline or "adjustment" gas in application and for calculation of index values for comparison to defined limits. The adjustment gas must represent the conditions under which a new supply is assessed. While one can calculate a "Wobbe Number" for a given gas using Equation 3 above, a given gas Wobbe Number is only meaningful in comparison to Wobbe numbers of other gases, such as baseline gases. Without this comparison, the Wobbe Number's dimensionless quality is of no practical value in assessing interchangeability. In some applications, where the Wobbe number of a new "substitute" gas falls with a range of known acceptable gases is assessed. In other applications, the "Wobbe Ratio" (the ratio of the substitute Wobbe Number over the adjustment Wobbe Number) is used to express interchangeability limits and where the substitute gas lies relative to the range of acceptable gases.
The Wobbe Index has its origins in the 1920's with Italian physicist Goffredo Wobbe.37 English translations of Wobbe's original work are rare, but his work was quickly adopted by practical combustion investigators because of the index's simplicity and intuitive reasonableness. In concept, the Wobbe Index captures not only the volumetric energy value of the gas but also the ability of that gas to be delivered to a burner as a function of its density. It, therefore, describes the "heat rate" to the burner provided by the gas. As the Gas Interchangeability White Paper states, the Wobbe Index is the most efficient and robust interchangeability parameter for describing gases for the widest range of end uses.38 Wobbe Index limits are used in conjunction with the AGA and Weaver indices, in the case of the Weaver Heat Rate Ratio, JH (itself a Wobbe Ratio) making up a portion of the overall index.
A valuable illustration of the relationship of Wobbe Number to "equivalence ratio" (i.e. the ratio of stoichiometric air for combustion to air actually supplied for combustion) is provided in Appendix G of the White Paper. The discussion illustrates that for combustion of alkanes, including methane, ethane, propane, butane, etc., air requirements to combust 1,000 Btus of energy are remarkably similar, even though the stoichiometric air/fuel requirements vary significantly among these fuels.39 Since the density of the fuel limits the amount of energy that can be delivered to a burner (i.e., through an orifice), the equivalence ratio for different mixtures of alkanes is highly correlated to the Wobbe Number of the mixtures.
36 American Gas Association, Interchangeability: What It Means," Operating Section Topical Technology Report, May 1978, pp. 1-2. 37 Wobbe, G. "A New Definition of the Quality of Gas," Ind. Gas Acquedolli, 1926. 38 Gas Interchangeability Task Group. White Paper on Natural Gas Interchangeability and Non- Combustion End Use,. Natural Gas Council Interchangeability Task Group, 2005, p. 18. 39 Gas Interchangeability Task Group. White Paper on Natural Gas Interchangeability and Non- Combustion End Use, Appendix D. Natural Gas Council Interchangeability Task Group, 2005, pp. 1-6. 27
However, the White Paper and other sources point out that changes in gas supply Wobbe Numbers alone do not account for changes in gas combustion. Other, secondary effects are important to explain the changes in combustion. In the case of efforts to develop more complete descriptions such as the AGA and Weaver indices, these secondary effects are address directly through empirical descriptions and index limits. In cases where Wobbe based limits are retained, supplemental criteria are used. For example, the Interim Guidelines for gas interchangeability proposed by the White Paper use Btu limits on the gas to capture behaviors that would be more precisely calculated by the more complex indices, specifically the Weaver Incomplete Combustion Index, JI, which was shown to be the most sensitive index within the Weaver system.40 Use of factors such as a Btu limit in the case of the Interim Guidelines avoids the need to calculate the full Weaver Index values and the need to specify an adjustment gas, which in the case of applying the Interim Guidelines would have been impractical. An alternative of tightening the Wobbe Index limits might exclude the gases shown to be unacceptable using the Weaver Index, but it would likely exclude a number of other gases that would be otherwise acceptable. Therefore, using supplemental criteria to account for secondary effects may help to maintain supply options.
However, the relationship of equivalence ratios for alkanes to Wobbe Index strongly depends upon the efficiency of total aeration (primary and total secondary aeration). As a result of different volumetric air requirements to burn different gases, a stronger dependence upon secondary air may develop for mixtures with heavier alkanes and greater stoichiometric requirements. This dependence may or may not be met within actual appliances due to physical limitations described in Section 3.2.1.1. Since these behaviors may be appliance specific, empirical studies may reach somewhat different conclusions about appropriate index limits based on the types of appliances tested. This factor is understood by researchers and has resulted in generally conservative extrapolations of research results to more general problems of gas interchangeability as discussed in Section 5.
Application index limit ranges presumes consensus on the adjustment gas used as a baseline. In practical terms, an appropriate adjustment gas would be the assumed gas on which appliances were design certified, installed, and put on rate for operation. The adjustment gas would need to be representative for the geographical area of interest and the time period under which the existing appliance stock was installed. These assumptions put use of the AGA and Weaver indices, which are compositionally dependent, at a disadvantage for use in large regional or national analyses since historical gases and appliance stock subpopulations vary. Use of Wobbe based criteria have fewer problems since physical attributes of the gas are better characterized and have not varied as widely. Nevertheless, application of Wobbe index limits to national scale issues tends to force consideration of the "least common denominator" in gas acceptability by encompassing more adjustment gas conditions and wider ranges
40 Gas Interchangeability Task Group. White Paper on Natural Gas Interchangeability and Non- Combustion End Use, Appendix G. Natural Gas Council Interchangeability Task Group, 2005. 28 appliance populations. However, in the development of the Interim Guidelines, this was not found to be an insurmountable problem, and it was addressed by incorporating local historical Wobbe Numbers as the basis for defining ranges of interchangeable gases.
One concern in development of this approach was that, in rolling up local requirements to large geographical areas, a "least common denominator" approach would result and very tight ranges would result. However, due to historical conditions in the U. S. gas industry, it was found that contiguous regions of the U. S. did not experience great changes in gas supplies historically, suggesting that the problems of introducing new supplies in these contiguous regions might not be problematic (i.e., overly tightened by local histories).
These are just some of the issues associated with use of gas interchangeability indices in the U. S. Similar questions have arisen in the U. K., which uses the Dutton diagram as a fundamental tool of describing gas interchangeability.41 This graphical approach to describing interchangeability limits uses Wobbe Index in conjunction with the Incomplete Combustion Factor (ICF) and Sooting Index (SI) to bracket gases in a manner similar to the Interim guidelines. While derivation of the limits is done differently (e.g., the U. S. does not use the SI parameter), the results in the U. K are surprisingly similar to those resulting from the Interim Guidelines when similar adjustment gases are used.
7. Activities and Gas Industry Requirements Outside the U. S.
7.1 Overseas Specifications
Many sources provide summaries of gas quality specifications used by other countries for cross boarder trade and internal purposes. One summary is provided by BP, shown in the table below:
Table 3. Summary of International Gas Specifications.42
41 Dutton, B, "A New Dimension to Gas Interchangeability, The Institute of Gas Engineers, 50th Annual Meeting, November 1984. 42 Ho. B. "Gas Interchangeability/Quality Management & Their Impact on Technology," presented at Platts Gas Interchangeability and Gas Quality Forum, Houston, TX, November 2005. 29 7.2 Current U. K. Activities
As discussed above, the U. K. Department of Trade and Industry (DTI) sponsored appliance tests in order to investigate a range of gas interchangeability issues in the 2003-05 timeframe. The U. K. experience and ongoing activity provides a clear, carefully organized approach to addressing gas interchangeability, especially in a situation where maintaining stable supply is critical.
Overall, the U. K. ahead of the U. S. on gas interchangeability issues as a matter of regulatory and public policy, and while its economic and regulatory situations are different from the U. S. situation, its findings and "lessons learned" may help the U. S. avoid duplicating efforts. The U. K. position may also help the U. S. in better understand where the world market for gas is going (as opposed to where it is today) and help clarify factors of how the U. S. will participate.
DTI is recommending "no change" from the U. K.'s current gas quality specifications for imported gas, which includes a Wobbe range of 47.2 to 51.41 MJ/m3 (~1,267 to 1,380 Btu/scf) representing a range of +/- 4.3% of the midpoint Wobbe. These specifications are currently in the U. K. Gas Safety Management Regulations (GS(M)R) administered by the Health and Safety Executive and are augmented by additional limits on incomplete combustion factor (ICF), an index for carbon monoxide generation, and soot index (SI). These Wobbe limits, including consideration of carbon monoxide (CO) generation in the ICF and sooting, are consistent with the NGC Interim Guidelines with respect to implications for gas suppliers even while they appear to be numerically somewhat more conservative than the Interim Guidelines.
DTI had considered "change" options of increasing the Wobbe baseline and range in the U. K. as high as +/- 7% (to 1,449 Wobbe on the high end) to allow introduction of unprocessed pipeline supplies from the European Union and imported LNG and to approach consistency with EU EASEE-gas/Marcogaz specifications.
DTI calculated that adopting such a change from the GS(M)R requirements would cost the U. K. from 5 to 15 times (NPV basis) the cost of the "no change" option, which would include costs of nitrogen ballasting costs to maintain current specifications in imported LNG. The high cost of the change option includes estimated costs of appliance modification and adjustment within the current stock and redesign of new appliances. Any future "early" changes in the GS(M)R specifications would be unlikely to take effect before the end of the 2010's due primarily to slow turnover in the appliance stock, which has similar turnover rates as the stock of U. S. appliances.
While gradations between the "change" and "no change" options can be developed, DTI recognizes that, with respect to the near term, natural turnover of appliances and the assumption of availability of new and more robust appliances would not control for uncertainty of "at-risk" appliances remaining in the stock. As a result, the benefits of intermediate policies between the "change" and "no change" are, themselves, uncertain. The time scale for the DTI "no change" 30 approach (i.e., no change before 2020) allows for more comprehensive approaches and appliance development to take place so that end use needs could be matched with supplies.
DTI findings on the inadequacies of the "change" approach hinge upon appliance testing with gases at and around the GS(M)R limits and found that failure most often was due to exceedence of CO and NOx certification limits. While U. K. appliances are different from U. S. appliances in design and certification, the U. K. limits tend to be higher of CO (1,000 ppm verses 400 ppm air-free for U. S. appliances). DTI tests showed that approximately two-thirds of appliances tested to broader limits exceeded the U. K. appliance standard limits. The tests also showed that some of the appliance tested under as-installed condition but within normal adjustment and maintenance also exceeded these limits. Furthermore, DTI found that teardown, modification and adjustment of appliances operating outside the standard limits produced worse-performing appliances at least one of these cases. The implication of this in the U. S. is that field modification of appliances to achieve design performance may not be a simple matter and, according to DTI, "may not be advisable."
DTI bases its analysis fundamentally upon domestic appliances based on the size of the customer class and the inability of these customers to monitor and adjust combustion behavior in real time. DTI found that new appliance technologies, as a group, may be more sensitive to changes in gas supply, particularly for NOX production and limits.
Under U. K. law, GS(M)R requirements apply to gas suppliers and must be met by suppliers before gases are introduced to the domestic transport system. Any gas conditioning and blending to meet the GS(M)R must be accomplished upstream of the transport systems in the U. K.
DTI discounts arguments that broadening gas quality specifications is needed for energy security reasons in the U. K. Despite that domestic production of gas will decline rapidly (decreasing below U. K. consumption sometime this year), DTI sees pipeline and LNG imports meeting U. K. requirements from a variety of sources, even with retention of the current GS(M)R limits. Gas conditioning to U. K. limits may cost consumers more, but DTI staff commented that the world gas market is likely to accommodate U. K. requirements through various means. DTI staff have commented that the world LNG market might take on features of the world oil market to accomplish this. In his model, prices could be benchmarked to a given composition and discounted and/or applied premiums for specific compositional needs and markets.
While a major effort is underway to get the U. K. to accept the EASEE-gas specifications as a replacement for the GS(M)R limits, DTI sees retention of the GS(M)R limits as essential to protection of consumers. DTI also recognizes a change in the EU's advocacy of the EASEE-gas specifications from implied end user specifications to "cross-boarder" specifications. This change adds confusion to what the specifications mean or how they are to be met (i.e., who would be responsible in the U. K. for meeting requirements). This confusion adds to the problem of acceptance of the EASEE-gas specifications as must-take 31 (e.g., "safe harbor") protection for the exporting country and transfer of risk to U. K. stakeholders.
The full record of DTI policy documents and research reports is available through its website:
http://www.dti.gov.uk/energy/domestic_markets/gas_market/gas_quality.shtml
8. Criteria for Acceptable Appliance Performance
In applying gas interchangeability test results and indices, specific criteria for acceptability must be selected from the wide range of results produced by these tests and computations. The following discusses CO, the most sensitive parameter in the Weaver series of indices and the most critical safety issue facing gas consumers.
8.1 CO Criteria Are Most Critical
Analysis of interchangeability data and calculations with interchangeability indices consistently show that, for domestic appliances in reasonably proper operating condition, that CO production in excess of design certification standards and field acceptability criteria is the most sensitive criterion for acceptable performance. Weaver found this to be true in his analysis of laboratory data and development of indices.43 Numerical experiments using indices demonstrated this in the development of the NGC+ Gas Interchangeability Interim Guidelines.
8.2 Standards for Acceptable CO Generation in Appliances
ANSI recognized Z21/83 consensus standards for design and performance of domestic and commercial appliances provide the clearest basis for acceptability. The U. S. does not have national standards for acceptable CO performance of appliances and equipment in the field. As a result, development of more quantitative criteria, along with the availability of less expensive field instrumentation, has led to the development of field acceptability criteria based on the associated Z21/83 design certification standards. This application of the standards was never intended, and the standard test conditions under which the CO emission levels are applied are not typically applicable in field installations. Nevertheless, the CO limits in the standards, or explicit fractions of these limits, are used by servicing organizations, gas utilities, and some governmental agencies for qualifying appliances in the field. For example, the California Low Income Energy Efficiency Program implemented by gas utilities in conjunction with the California Public Utilities Commission uses field measured air-free CO
43 Weaver, E. "Formulas and Graphs of Representing the Interchangeability of Fuel Gases," Journal of Research of the National Bureau of Standards, Vol. 46, No. 3, March 1951, pp. D315- D322. 32 emissions measurements in comparison with the Z21/83 design standard limits as part of its qualification of low income housing appliances.44
Association of Z21/83 standard limits for CO with consumer safety is problematic, again principally since the design certification conditions are not replicated in the field. However, the single study attempting to do this, conducted for gas ovens and ranges does show that the applicable Z21.1 standard is protective of consumer safety.45
9. Issues Beyond the Scope of this Document
While the following issues are beyond the scope of this document, comments are offered based on recent work in gas interchangeability and public policies regarding introduction of new supplies.
9.1 Adjustment at End Use versus Gas Supply Modification
The alternatives of adjusting end use equipment versus modifying composition of a supply to meet end use equipment has precedence in the U. S. experience. Questar's experience and associated costs are discussed in detail in Appendix D of the Gas Interchangeability White Paper.46 The U. K. experience, which is different in a number of respects, is discussed in Section 7.2 and below. It needs to be understood that with the Questar experience, at least, the decision between alternatives is based on a monotonic change in gas supply, from one range of compositions to another, and would need to be repeated if a new range of compositions would be proposed. While the ranges represented by feasible supplies to a market area may not be heterogeneous, policies to address new supplies in this way must anticipate future conditions (i.e., perhaps beyond the new supply being considered in the near term).
9.2 Economic Efficiency and Equity
Economic efficiency suggests approaches to gas interchangeability that minimize total societal costs. The U. K. DTI has taken on this issue in its consideration of broadening U. K. gas specifications while maintaining consumer safety through its analysis of the alternatives of modifying imported supply versus modifying domestic end uses. In the case of the DTI analysis, the difference in the economic efficiency between these mutually exclusive approaches is
44 Joint Motion of Pacific Gas and Electric Company, San Diego Gas & Electric Company and Southern California Gas Company Requesting Commission Approval of Proposed Settlement Establishing Uniform Low Income Energy Efficiency Gas Appliance Flue Testing Carbon Monoxide Threshold Levels, presented to California Public Utilities Commission, November 2004. 45 Reuther, J. Critique of ANSI Z21.1 Standard for CO Emissions From Gas-Fired Ovens and Ranges, Gas Research Institute, September 1996. 46 Gas Interchangeability Task Group. White Paper on Natural Gas Interchangeability and Non- Combustion End Use, Appendix D. Natural Gas Council Interchangeability Task Group, 2005, pp. 27-30 33 immense.47 Equity refers to the allocation of costs of a specific alternative. Allocation may be divided among stakeholders receiving new supply or other formulae. Such an approach has been proposed in a recent case in Florida.48
9.3 Similarities and Differences of Requirements Among End Users
The work of the Gas Interchangeability Task Group revealed, to a significant degree, that gas composition requirements across major end uses (i.e., beyond appliances and residential and commercial markets) are roughly the same in the U. S. While certain applications might generally require tighter specifications for operation without installation of additional controls (e.g., gas turbines and internal combustion engines), development of all of these applications to run on "natural gas" has guided technical efforts toward maximizing performance, efficiency, and safety on what has been reasonably consistent with U. S. historical gas compositions. New pressures on end uses through emissions limitations have suggested a compression of the historical variability, and some specific concerns related to compositional requirements as a result have suggested that these may become the "most sensitive end uses" for consideration in the future.
9.4 Consumer Risk Under Alternative Actions
Alternative actions for addressing residential end uses (and intrinsic vulnerabilities associated with unattended operation, lack of maintenance requirements, etc.) suggest that policies address the relative risks of each alternative. For example, if a segment of the appliance population is maladjusted to the extent that introducing a new gas supply would lead to excessive CO generation, analysis might be focused on what efforts might be taken reduce the potential for CO incidents such as through expanded service programs and even service requirements. Alternatively, gas compositional specifications might be set to minimize the risk of maladjusted appliances producing hazardous conditions, in other words very tight gas interchangeability requirements. Approaches would be assessed on a basis of economic efficiency but would also require consideration of residual risks to consumers such as proportion of appliances not addressed by the alternatives. The U. K. DTI analysis considered this as well and found that such risk-oriented approaches were not only costly but left, in the judgment of DTI, unacceptable residual risk.49
47 Department of Trade and Industry. Partial Regulatory Impact Assessment, URN 05/1903, December 2005. 48 Federal Energy Regulatory Commission Trial Staff, Initial Brief of the Commission Trial Staff, AES Ocean Express, LLC v. Florida Gas Transmission Company, Docket No. RP04-249-001, January 26, 2006. 49 Department of Trade and Industry. Partial Regulatory Impact Assessment, URN 05/1903, December 2005, pp. 14-17, 18-20.
34 References
The following references were used in development of this report and are cited, but they do not represent an exhaustive listing of relevant gas industry or other literature on gas interchangeability:
"28th Annual Portrait of the U. S. Appliance Industry," Appliance, September 2005.
American Gas Association Laboratories, Interchangeability of Other Fuel Gases with Natural Gases, Research Bulletin Number 36. AGA Committee on Mixed Gas Research, Joint Committee of Natural Gas Department and Technical Section, American Gas Association : Cleveland, Ohio, 1946.
American Gas Association, Interchangeability of Various Fuel Gases with Manufactured Gases, AGA Research Bulletin No. 60, American Gas Association Laboratories, 1950.
American Gas Association. Gas Engineers Handbook. Segler, C. George, Editor-in-Chief, The Industrial Press: New York, NY, 1965.
American Gas Association. Interchangeability: What It Means. Operating Section Topical Technology Report, May 1978.
American Gas Association. Utilization, Volume V: Gas Engineering and Operating Pracitce, Book U-1 Residential/Commercial, American Gas Association: Arlington, VA, 1994.
American Gas Association Laboratories, Interchangeability of Other Fuel Gases with Natural Gases, Research Bulletin Number 36. AGA Committee on Mixed Gas Research, Joint Committee of Natural Gas Department and Technical Section, American Gas Association : Cleveland, Ohio, 1946.
American Gas Association Laboratories. Fundamentals of Gas Combustion, Revised. Prepared for American Gas Association and Gas Appliance Manufacturers Association, 1996.
Department of Trade and Industry (U. K.). Partial Regulatory Impact Assessment, URN 05/1903, December 2005.
Dutton, B, "A New Dimension to Gas Interchangeability, The Institute of Gas Engineers, 50th Annual Meeting, November 1984.
Federal Energy Regulatory Commission Trial Staff, Initial Brief of the Commission Trial Staff, AES Ocean Express, LLC v. Florida Gas Transmission Company, Docket No. RP04-249-001, January 26, 2006.
Gas Interchangeability Task Group. White Paper on Natural Gas Interchangeability and Non-Combustion End Use: Report and Appendices. Natural Gas Council Interchangeability Task Group, 2005.
35 Halchuk-Harrington, R. and Wilson, R. "AGA Bulletin #36 and Weaver Interchangeability Methods: Yesterday's Research and Today's Challenges," AGA Operations Conference Paper (not yet published), May 2006.
Ho. B. "Gas Interchangeability/Quality Management & Their Impact on Technology," presented at Platts Gas Interchangeability and Gas Quality Forum, Houston, TX, November 2005.
Joint Motion of Pacific Gas and Electric Company, San Diego Gas & Electric Company and Southern California Gas Company Requesting Commission Approval of Proposed Settlement Establishing Uniform Low Income Energy Efficiency Gas Appliance Flue Testing Carbon Monoxide Threshold Levels, presented to California Public Utilities Commission, November 2004.
Levinsky, H. "Report of 'Identification of the Concentration and Combination of Higher Hydrocarbons in Natural Gas Likely to Cause Sooting in Gas Appliances," Report to the Department of Trade and Industry, United Kingdom, p. 3, 2005.
Peeters,J. and Mahnen,G., "Reaction Mechanisms and Rate Constants of Elementary Steps in Methane-Oxygen Flames," Fourteenth Symposium (International) on Combustion, pp. 133-146, 1973.
Rana, H., and Johnston, D. "An Empirical Approach to Evaluating Gas Interchangeabiiity," 2003 AGA Operations Conference Paper, 03-OP-41, May 2003.
Reuther, J. Critique of ANSI Z21.1 Standard for CO Emissions From Gas-Fired Ovens and Ranges, Gas Research Institute, September 1996.
Rue, et. al., Gas Interchangeability Tests: Evaluating the Range of Interchangeability of Vaporized LNG and Natural Gas, Final Report, Gas Technology Institute, April 2003.
Smoot, L., Hecker, W., and Williams, G., "Prediction of Propagating Methane-Air Flames," Combuston and Flame, Vol. 26, pp. 323-342, 1976.
Tsatsaronic, G., "Prediction of Propagating Laminar Flames in Methane, Oxygen, Nitrogen Mixtures," Combustion and Flame, Vol. 33, pp. 217-239, 1978.
U. S. Department of Energy, "Natural Gas Statistics for 2004," Energy Information Administration, http://tonto.eia.doe.gov/dnav/ng/.
U. S. Environmental Protection Agency, Guidelines for Adjustment of Atmospheric Gas Burners for Residential and Commercial Space Heating and Water Heating, February 1979, EPA-600/8-79-005.
Weaver, E. "Formulas and Graphs of Representing the Interchangeability of Fuel Gases," Journal of Research of the National Bureau of Standards, Vol. 46, No. 3, March 1951
Williams, T. Assessment of Changes to the Performance of Gas Appliances in Relation to Variations in Gas Quality, prepared for Department of Trade and Industry (U. K.), URN 05/1938, October 2005.
Wobbe, G. "A New Definition of Gas Quality," Ind. Gas Acquedolli, 1926. 36