Meeting Gas Quality Challenges

By Teresa Hansen, Senior Editor

The United States has enjoyed not only an ample supply of natural gas for many years, but natural gas of consistent quality. Demand, however, is growing rapidly and experts predict that it will soon outpace the consistent supply that has been available for decades. To make up the growing difference between supply and demand, liquefied natural gas (LNG) from other countries, “unconventional” natural gas and synthetic gases will be added to the gas pipelines in growing quantities.

The introduction of LNG and unconventionally sourced gas into North American gas pipelines presents gas quality issues and challenges that combustion turbine operators, as well as OEMs, must address. Click here to enlarge image

The American Gas Association predicts that 22 percent of the natural gas in pipelines will be supplied by LNG in 2020. And unconventional sources, such as gas from the Barnett Shale natural gas field (which had only one well in 1981 and today is one of the hottest natural gas producing fields in the United States) are growing. The introduction of LNG and unconventionally sourced gas into the nation’s pipeline has some end users–especially combustion turbine owners/operators–concerned about gas composition and interchangeability.

In a few areas in the United States and Mexico, gas being sourced from multiple wells is already beginning to cause some problems for combustion turbine operators, said Carlos Koeneke, a technical service manager for Mitsubishi Power Systems. “Our clients don’t usually experience variations in gas composition due to diverse LNG supplies, but they are experiencing issues caused by the introduction of natural gas from different wells,” he said.

Natural Gas Composition and Measurement

Natural gas is a naturally occurring hydrocarbon mixture found throughout the world. It contains mostly methane (usually more than 90 percent) and other hydrocarbons, such as ethane, propane and butane, as well as inert gases, water vapor and trace compounds.

According to the Gas Technology Institute (GTI), the composition of natural gas at a given point of use depends on many factors, including proximity to supply, number of pipelines serving the point of use, characteristics of the producing gas field and the techniques used to process and clean the gas. As a result, gas transported to a particular utility or customer can vary with time and location.

As different sources of natural gas are mixed into the pipeline in greater quantities and as overall composition begins to vary, many combustion turbine owners/operators are concerned that these variations will affect their machines’ reliability and efficiency. Although today’s combustion turbines are designed to burn a variety of fuels, once they are tuned to a particular gas composition they have difficulty tolerating large variations and/or rapid changes in that composition. Changes in gas type that go beyond the allowed range can affect flame stability, NOX formation and other operating factors and parameters.

“Gas composition variation is a legitimate challenge many of our customers will face,” said Ryan Obenhoff, GE Energy’s product line leader for its OpFlex Wide Wobbe solution.

Not only does a shift in fuel composition cause concerns for large-scale combustion turbines operators, but the rate at which the composition changes is an even bigger issue, said Obenhoff. As LNG supply increases, gas likely will be injected into the pipeline infrastructure at multple points from beginning to end, meaning that some users may experience rapid changes in gas quality. A rapid change in composition is likely to cause greater operational problems than, for example, a larger but more gradual composition change. “This means system responsiveness is important,” Obenhoff said.

Most combustion turbine owners/operators and original equipment manufacturers (OEMs) measure gas composition and interchangeability using the “Wobbe Index.” The Wobbe Index defines the heating value of a quantity of gas that will flow though a hole (combustion turbine burner tip) of a given size in a given amount of time; the higher the index, the higher the heating value. The Wobbe Index is calculated by dividing the high heating value of the gas–expressed in British thermal units per standard cubic foot (scf)–by the square root of its specific gravity with respect to air. In practice, this means that all gas mixtures having the same Wobbe Index number will deliver the same amount of heat. Once tuned to a particular number, most turbines can tolerate only a 4 percent to 5 percent Wobbe Index shift without affecting operations.

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The amount of methane in natural gas can greatly affect its Wobbe Index. Most natural gas contains about 90 percent to 95 percent methane by volume after processing. According to GTI, pure methane has a Wobbe number of 1363; natural gas piped into homes in the United States typically has a Wobbe number between 1310 and 1390. GTI conducted a comprehensive natural gas composition survey and found that the U.S. gas methane content average is 92.3 percent. Figure 1 illustrates average U.S. natural gas composition, while Figure 2 illustrates the average composition of various LNG imports.

Source: Gas Technology Institute. The data are based on higher heating values for natural gas. Click here to enlarge image

Table 1 further compares the U.S. average with various LNG import natural gas properties. The table illustrates how higher heating value can affect the Wobbe Index. As the table shows, international LNG resources have natural gas properties higher than those typically found in the United States. Because of these differences, international LNG often can be characterized as “rich” natural gas.

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The table helps explain why combustion turbine owners/operators are concerned about variations in gas quality. The difference between the average U.S. gas Wobbe Index and that of LNG from Oman (the highest Wobbe Index) is more than 7.5 percent. For the many owners/operators who are running combustion turbines that can tolerate a shift of only 4 percent or 5 percent, this could be reason to worry.

Pipeline Quality Standards

In 2004, in an attempt to mitigate the effects of varying gas composition on end users, the National Gas Council convened two task forces–which included combustion turbine owners/operators–to independently address gas quality issues. These task forces, operating under the title of NGC+, produced white papers on their findings. They presented these papers along with recommendations on gas standards and specifications to FERC in 2005, hoping the Commission would initiate a Notice of Proposed Rulemaking (NOPR). They wanted the NOPR to include criteria for defining Wobbe number limits, along with limits on some specific constituents and a means of defining interchangeability where gas interchangeability specifications have already been set.

In June 2006, FERC declined to initiate a NOPR. Instead, it recommended that every natural gas company subject to its jurisdiction include tariffs, special terms and conditions of service to address gas quality and interchangeability issues. At the time, the Commission said “it is not apparent that natural gas quality and interchangeability is a national problem that lends itself to a national solution.”

OEMs Prepare for Variations

Jacqueline Engel, global 60 Hz product line marketing at Siemens, agrees that gas quality and interchangeability are not yet a national problem, but she believes that situation will change and the combustion turbine industry must be ready to address it.

“In the North American market, fuel quality variations have historically been infrequent and would be considered isolated, to date. But our operational experience in other countries, where fuel supplies vary across a much wider range, has demonstrated that a robust combustion system is necessary to meet the operability requirements of those customers,” she said.

To prepare for the impending influx of LNG and unconventional gas in the U.S. natural gas market, Engle said Siemens proactively identified and developed technologies and hardware configurations, and paired them with protective systems and control methodologies. In addition to Siemens, large-scale combustion turbine manufacturers Alstom, GE Energy and Mitsubishi have also developed technologies and solutions to deal with gas quality variations and Wobbe Index changes. The major issues stemming from large variations in Wobbe Index numbers for F class turbines are: auto-ignition, flashback, emissions variation (specifically for NOX and CO), combustion dynamics and combustion blowout. “Combustion dynamics emissions variation and combustion blowout are usually the biggest issues when a gas composition changes outside the specified levels,” Obenhoff said.

Fuel gas quality essentially has two effects on the combustion process, according to Peter Flohr, Alstom Turbomachines group combustor development department manager. First, the heating value of a fuel is linked directly to the volumetric flow rate and has a direct impact on the mixing process inside the burner. Second, fuel components such as higher hydrocarbons or hydrogen have an impact on the reactivity. Both these effects will lead to a modification of the flame structure inside the combustor. If not adjusted, it can lead to combustion dynamics or increased emissions. Flohr said that the reverse can be true as well. “One combustion system may suffer from increased emissions if higher hydrocarbons are added to the fuel, another combustion system may run more stable (and at lower emissions) if it is prone to extinction pulsations otherwise,” he said.

As a fuel source, LNG or unconventional gas by itself does not typically pose issues with combustion operability for a combustion turbine, provided the gas meets the turbine’s fuel gas specification. Basically, all later-model large-scale combustion turbines can burn a variety of gas.

According to Flohr, all of Alstom’s combustion turbines can burn various gas compositions. Field experience has proven that Alstom’s high-end GT24/GT26 combustion turbines (which feature two-stage combustion systems) are well-suited to burning a wide range of natural gas qualities without compromising performance. One reason is the turbine’s ability to change the relative fuel input between the two combustion chambers, as a function of the fuel composition.

The Wobbe Index (based on lower heating value) that Alstom’s gas turbines are designed to handle using standard burners is 965 to 1290. Once tuned, the issue of how large a fluctuation range the engine will experience is important. According to Flohr, Alstom’s gas turbines can accept Wobbe Index variations of ±10 percent without adjusting the burner hardware or fuel control.

Alstom turbines installed in Thailand were some of the company’s first machines to operate on gas compositions with high C2+ (high hydrocarbons). “We ran a test campaign in 2001 on our GT26 engine at the Alstom Test Power Plant in Switzerland,” said Stephen Philipson, Alstom’s Turbomachines group senior product specialist. “With the aid of a purpose-built fuel gas blending plant, we were able to operate with differing levels of inerts and C2+ and so confirm operating concepts for these non-standard gases.”

In addition, to adjust to rapid changes in gas composition, Alstom developed a fast-acting optical measurement system to replace the conventionally used gas chromatographs, which are too slow. The fast input allowed Alstom to develop a protection and control system for gas turbines that can automatically adjust within seconds the operating parameters that consider both fast and large swings in fuel gas composition, thus avoiding engine trips and increasing availability. This system is available for new gas turbine plants where fuel quality might be an issue. It can also be retrofitted to existing plants that are having problems with fuel composition variations.

Alstom has field experience with C2+, up to 16 percent and inert gases, up to 20 percent. The company has 22 engines in its GT24/GT26 fleet running on non-standard gases with more than 780,000 fired hours to date, said Flohr.

“Although we first started seeing gases with ‘nonstandard’ compositions in Southeast Asia, over the last few years such gases have become more widespread,” said Philipson. Alstom now has engines running with these types of gases on a number of continents, including North America. “The Poryong plant in Korea has eight GT24 units that have been successfully operating on LNG since first going into commercial operation in 1997 and 1998,” Flohr said.

GE combustion turbines, like Alstom’s, typically do not have issues with LNG or unconventional gas as a fuel source. “LNG by itself does not pose issues with combustion operability for a GE heavy-duty gas turbine provided the LNG meets the GE fuel gas specification,” said Obenhoff. However, as mentioned earlier, potential issues arise when natural gases of varying compositions are injected into the same supply line.

Although GE heavy-duty gas turbines have the ability to burn a wide variety of gas fuels, to protect the gas turbine and burn these fuels in an efficient and reliable manner, GE recommends its customers adhere to its publication “Specification for Fuel Gases for Combustion in Heavy-Duty Gas Turbines.” GE uses the Modified Wobbe Index in its fuel specifications because it incorporates the effect of fuel temperature on the interchangeability of gas fuels for a given fuel system design. The publication’s latest revision, released in April 2007, states that the absolute limits for the Modified Wobbe Index are 40 to 54.

Over the years, GE has used two methods to help customers address the issue of significant variation of gas fuel composition. Initially, GE offered a closed-loop control system using a gas chromatograph and gas fuel heaters that maintained the Modified Wobbe Index before the gas entered into the gas turbine. Although this methodology is effective on non-dry low NOX GE frame sizes, it had limitations with respect to the system’s responsiveness, as well as potentially having a significantly negative effect on plant heat rate, according to Obenhoff. Therefore, for all dry low NOX (DLN) 2.6 systems, GE’s method of control is the OpFlex Wide Wobbe control system for both new units and existing gas turbines.

This offering can accommodate variations in fuel quality without the need to directly measure the fuel’s composition or add additional combustion system hardware. Rather than trying to modify the fuel’s characteristics prior to entry into the gas turbine, the OpFlex Wide Wobbe system uses a comprehensive model-based control (MBC) structure that controls directly the operability boundaries that are affected by fuel quality, namely combustion dynamics, emissions and combustion blowout. This MBC methodology has been used by aircraft gas turbines for years due to the broad operating envelope they experience.

To accommodate issues that arise from a rapid shift in Modified Wobbe Index, the OpFlex Wide Wobbe solution uses the Adaptive Real-time Engine Simulation (ARES) technology, which is a real-time, aero-thermal model of the gas turbine that resides in the turbine control system. The ARES technology gives the OpFlex Wide Wobbe solution the capability to tune the gas turbine in real time (at a rate of four to five times a second) for optimum emission and combustion dynamics. Simulation results show that the system has the ability to handle Wobbe Index change rates in excess of 18 percent per minute, Obenhoff said.

The OpFlex Wide Wobbe offering was launched in 2007 on four units at two sites in the Southeastern United States that anticipated experiencing a Wobbe Index variation due to introduction of LNG from a pipeline expansion.

“The OpFlex Wide Wobbe solution has been able to accommodate the initial fuel variability,” said Obenhoff. “The system also gives the operator peace of mind that the units will be able to handle an increase in Wobbe variation in the future, since the system is capable of handling up to plus or minus 5 percent in Modified Wobbe Index variation.”

Eight units currently have accumulated more than 12,000 operating hours with the OpFlex Wide Wobbe solution. These units have experienced no operability issues as a result of fuel variation, he said.

Mitsubishi combustion turbines can also operate with LNG and natural gas at various Wobbe Index ratings. The OEM’s turbines can handle Wobbe Index ratings of 1200 ±15 percent (between 1020 and 1380) with standard fuel gas equipment. The unit can be tuned for a specific Wobbe Index.

Mitsubishi developed a self-tuning combustion dynamics system that can cope with relatively wide Wobbe Index oscillations. When the system is combined with the company’s latest combustion system, Wobbe Index changes on the order of ±5 percent can be overcome without interrupting the turbine operation. The self-tuning system can be retrofitted onto existing turbines or installed on new turbines.

Mitsubishi’s system deals effectively with auto-ignition and flashback, said Koeneke. Emission compliance can be maintained with some brief excursions in case of rapid swings in gas composition. If the combustion tuning parameters are not automatically adjusted, the control system response to increased levels of combustor dynamics will initiate a load runback or a unit shutdown to prevent equipment damage.

System development began in the late 1990s when Mitsubishi developed a combustion dynamics protection system called CPFM (combustion pressure fluctuation monitoring). This system was upgraded with self-tuning capabilities. This improved version is called Advanced CPFM (A-CPFM) and became available commercially in 2003. A newer version, recently released, can sustain even wider and more aggressive Wobee Index oscillations, according to Koeneke.

The ACPFM system auto tunes the combustion to minimize the effect of fuel gas characteristic changes. Instead of initiating a trip or runback, the system makes combustion adjustments that minimize generation losses. Mitsubishi began testing and using the technology in Japan, then moved into Egypt and Malaysia with it. Currently, 64 units are in service around the world with an accumulated 320,000 operating hours.

Some of Mitsubishi’s clients using the system are in the United States and Mexico, where Koeneke said they have experienced rather aggressive Wobbe Index changes. “In some severe cases, the fuel gas experiences not only aggressive Wobbe Index oscillation, but the gas is contaminated with liquid carryover contamination,” he said. The ACPFM has been able to minimize the effect of the Wobbe Index fluctuations, which has helped not only the owner, but Mitsubishi as well. The self-tuning system can reduce or eliminate the need to dispatch tuning engineers to remote locations, Koeneke said.

Siemens, another major manufacturer of large-scale combustion turbines, also offers a combustor system designed to accomodate varying gas composition. Its premixed DLN F-class combustors can operatin within a range of gas index applications.

Siemens’ ultra low NOX (ULN) design is an evolution beyond the DLN system that has proven to be a perferred design for wide range gas index applications. “Thus, combustor system selection is a combination of the lowest level of NOx emission produced by the gas turbine, the class of engine and the expected variation of gas constituents in the supply,” Engel said.

Both Siemens’ DLN and ULN combustion systems use a staged fuel mixing process that provides a leaner overall fuel mixture and flame, which drives down emissions. The gas variability solutions can be retrofitted onto existing units, as well as incorporated into new units.

To prepare for the impending influx of LNG and unconventional gas in the United States, Siemens has worked to identify optimal hardware configurations, paired with protective systems and control methodologies. “The expanded capability of LNG operation lies in our integrated fuel gas characterization (IFGC) system,” Engel said. It employs a Wobbe Index meter with an integrated gas chromatograph, combustor dynamics protection system (CDPS), fuel buffer system, and integrated, feed-forward control methodology. The IFGC meter, which includes the Wobbe meter and gas chromatograph installed upstream of the gas turbine boundary, feeds signals forward to the engine control system. A buffer tank is installed in the fuel line to allow the control system sufficient time to react to fuel quality changes. In addition, a CDPS and readings from the turbine exhaust’s continuous emissions monitoring system are integrated to provide a closed loop control methodology that tunes the engine to operate safely and reliably within combustor dynamics and emissions compliance. The IFGC system enables the Siemens DLN and ULN combustion turbines to accommodate a Wobbe Index variation as high as ±5 percent.

Although there have been isolated events where U.S. customers of Siemens have seen fuel quality variations outside the anticipated rate of change, most of North American customers have a stable and consistent quality of natural gas supply. Therefore, Siemens has simulated the effects of gas quality variation by running rig tests on various combustor configurations, as well as a full scale F-class DLN field test, over wide Wobbe Index ranges. To address potential emissions swings associated with those variations and provide optimal engine protections, the previously mentioned IFGC system was developed.

Siemens currently has several units across its product line operating in countries with widescale LNG use. In these applications, the technology has met customer operability requirements. This operational information contributed to the IFGC system development, which not only provides engine protection but also helps ensure emissions and operability reach optimal convergence.

“One key difference between these markets and the U.S. market is that U.S. customers are typically subject to more stringent environmental requirements than those required in other countries,” Engel said. It is the drive to lower NOX emissions with premixing that affects the gas turbine’s range of operability on different gas compositions.

“Operability in one part of the world with LNG,” she said, “does not necessarily translate to the same fuel flexibility in North America.

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