Extended Fuels Capability of Siemens’ SGT-400 DLE Combustion System Andy Stocker

Extended Fuels Capability of Siemens’ SGT-400 DLE Combustion System Brian M Igoe Expert Proposal Manager (FEED) Siemens Industrial Turbomachinery Ltd Ruston House PO Box 1, Waterside South Lincoln LN5 7FD England T: +44 1522 584418 [email protected]

Andy Stocker Product Manager SGT-400 Siemens Industrial Turbomachinery Ltd Ruston House PO Box 1, Waterside South Lincoln LN5 7FD England T: +44 1522 586895 [email protected]

Abstract To meet the growing demand to operate on gaseous fuels with little or no treatment, or use fuels derived from a variety of waste conversion processes Siemens Energy has extended the fuels capability of its’ product range, especially the Dry Low Emissions combustion system. Fuels containing high levels of inert species, nitrogen or carbon dioxide, lower the effective Wobbe Index of the fuels, thus needing increased fuel mass flow to achieve the same energy content. This paper presents the development process along with the results achieved to accommodate a wide range of fuels. Discrete changes were required in the DLE burner hardware allowing fuel flows to be achieved at similar supply pressures and combustor pressure drop as for standard fuels thus ensuring combustion characteristics were not compromised. Some applications are presented and discussed covering both on-shore and off-shore duty, including the most recent application on the SGT-400 using a weak bio-gas derived from an ethanol production plant. A gas only solution was applied, requiring careful management of the control parameters to achieve acceptable starting and transient operation through to the application of load. This programme demonstrates the ability of the Siemens DLE combustor to accommodate a wide range of fuels.

1. Fuel Flexibility Introduction In many parts of the world the use of gas turbines is a new or growing market and the availability of premium pipeline quality gas fuels can be non-existent or very low. The ability to operate on poorer quality fuels offers an alternative route in such evolving markets. The Siemens Dry Low Emissions (DLE) combustion system, which is available across the small gas turbine products, has demonstrated successful operation with over 17million hours of service experience in a wide variety of Oil & Gas and Industrial Power Generation applications. The system meets stringent emission legislation limits over a wide operating range and ambient conditions together with the ability to operate over the full load range. This was therefore the combustion system of choice as the basis for developing an extended fuels capability to cover a much wider range and type of fuels as shown in figure 1. These include wellhead gas fuels with little or no treatment, or gas derived from industrial processes, which have qualities and heat values below those of typical pipeline quality gas fuels.

Figure 1: Range of gaseous fuels The SGT-300 was one of the first products, configured with DLE, to operate with gaseous fuels outside of standard pipeline quality gas fuels and has since been followed with extended fuels capability on the SGT-400. Before embarking on the changes necessary in the combustion hardware and fuel delivery system to accommodate the wide range of possible fuels, it is necessary to understand some of the key parameters used to assess a fuel’s suitability. 2. Gas fuel Quality Assessment of fuels proposed for use in a GT defines the critical features that need to be considered with Wobbe Index, Calorific Value and Dew Point some of the key parameters applied. Wobbe Index (WI) and Temperature Corrected Wobbe Index (TCWI) are two of many parameters used to assess fuel and allows a direct comparison of different fuels to be made based on the heat content. Wobbe Index (number) is the Net (lower) calorific value of the fuel divided by the square root of the fuels specific gravity.

WobbeIndex

WI 0

CVv 0 / SG 0

Where CVv0

= net calorific value (MJ/m3) at standard conditions (288K, 1.013bara)

SG0

= specific gravity at standard conditions

The Temperature Corrected Wobbe Index (TCWI) applies a simple temperature correction. As the supply temperature of a gaseous fuel is increased the TCWI falls. The range of acceptable temperatures for use in a gas turbine varies from OEM to OEM and for Siemens Industrial turbines the range is +2.5oC to +120oC. The lower figure prevents freezing in the vent of double block and bleed part of the fuel system, whilst the upper limit, 120oC, is determined by some of the components used in the fuel system. =

×

_

_

Where W it_ref and Tt_ref refer to Wobbe Index and temperature at a known reference condition WIt is the revised Wobbe Index at the new, known, temperature Tt TCWI becomes important when fuels contain some higher hydrocarbons, and, or water. Assessing fuels with such additions to the composition results in determining the dew point of the fuel. Gas Turbine OEMs apply a degree of superheat above the dew point to ensure liquid condensate is eliminated. Siemens apply a 20oC superheat margin for the SGT-100 through to SGT-400 product range. 3. Weak or Diluted Gas Fuels Wellhead gas fuels often contain high levels of nitrogen and or carbon dioxide, which would require specialist treatment to remove (or reduce to low levels) in order to create pipeline quality gas fuels. Decomposition of waste, either from landfill sites or from controlled anaerobic digestion processes, will produce high methane content gas fuels but with a WI significantly lower than for natural gas fuels. Gaseous fuels diluted with inert species result in a reduced Wobbe Index, with both Nitrogen and Carbon dioxide the major constituents influencing the fuels Wobbe Index, WI. Figure 2 shows the impact of inert species nitrogen or carbon dioxide has on the fuel WI, with such weakened fuels considered during the extended fuels development programmes. It should be noted for the same WI a higher proportion of nitrogen can be accommodated. 50 UK Natural Gas

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MCV development

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Figure 2: Influence of inert diluents, CO2 or N2, on fuel Wobbe Index Pipeline quality gas fuels contain mostly methane, with small quantities of ethane, and tend to fall into the 3 range 37 – 49MJ/m Wobbe Index. Weak, or Medium Calorific Value (MCV), fuels that include increasing

levels of inert species, such as nitrogen and carbon dioxide have a reduced WI, and can be as low as 17.5MJ/m3 (less than half the CV of natural gas), figure 3.

Figure 3: Wobbe Index Classification and SGT-300 Fuels Expansion Range The lower WI gas requires an increased volume flow to achieve the same energy content as for typical natural gas. The resulting increase in turbine power output could be seen as a benefit. However this increased volume flow does increase the temperature of downstream turbine components. Therefore the turbine entry temperature operating limit is reduced to maintain a similar or lower level of component temperature, whilst providing a power output equivalent to that normally achieved with standard natural gas. 4. Fuels Extension Programme Besides design and analytical methods, Siemens gas turbine development facilities include a comprehensive range of component test facilities, one of which is a high pressure combustion rig facility, figure 4. As the Siemens DLE and Conventional combustion systems are of a Can-Annular design, this permits testing of of a single combustion assembly at full operating conditions of the turbine model (ie full engine pressure and temperature).

Figure 4: 4th generation DLE combustion test in the HP rig Facility Siemens Lincoln. Supporting the high pressure combustion rig is a facility which can blend a range of fuels together with inert species, thus providing the means to achieve a wide range of fuels likely to be encountered. This is shown in figure 5 below.

Figure 5: Gas mixing facility arrangement – schematic and physical arrangement Comprehensive testing across a wide range of fuels was completed, with detailed measurements made including component temperatures, combustor exit profile (ensures gas temperature profile seen by turbine blades is acceptable) and exhaust emission levels, particularly oxides of nitrogen (NOx) and carbon monoxide (CO). Benchmarking of both existing and new burner designs were completed allowing coverage over the Wobbe Fuel range from approximately 50MJ/m3 to approximately 15MJ/m3. Siemens Industrial Gas Turbines configured with DLE combustion systems have operated on a wide range of fuels. In recent years further extensions to fuel flexibility has been achieved (applying results from development work described above) with many additional units now in commercial operation. In order to verify and release an extended fuels capability, a full range of test methods have been used, from the dedicated high pressure (HP) combustion rigs figure 4 through to part and full engine tests prior to release. Necessary design changes are subject to rigorous design methods and tools to ensure the final design achieves the lifing requirements. All development methods used follow the rigorous Siemens Product Development Process (PDP) to ensure the changes and improvements comply with stringent requirements and that risks are managed. Take, for example, the case of the LNG plant, the “off-gas” can have both a high content of nitrogen and also a high content of heavy hydrocarbons. Using the Wobbe Index parameter, as described earlier: as the nitrogen content in the fuel increases, the Wobbe Index decreases such that at approximately 40 mol % nitrogen the WI is halved. Fuel flow to the gas turbine increases accordingly, and for a standard engine/combustion configuration this can be handled by increased pressure in the fuel-feeding system, or by minor modifications to the burner fuel passage and injection geometry to maintain similar supply pressure and pressure drop across the combustor hardware when compared to standard fuels, figure 6.

Gas Pressure upstream of burner (Pa)

2050000

With no burner geometry Standard changes Yadana increased fuel supply pressure required as 30-37MJ??? fuelTGCI Wobbe Index decreases

2000000

1950000

1900000

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1750000 15000

20000

25000

30000

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Wobbe Index (kJ/m3)

Figure 6: Effect of burner pressure as Wobbe Index falls – indicates when change in geometry is required 5. Fuel Flexibility Applications One of the first applications extending the capability of the DLE hardware beyond the standard range of gas fuels as shown in figure 3 above was for an off-shore duty, for 3 off * SGT-300 power generation sets for the platform, shown in figure 7. In addition to a gas containing both CO2 and N2 high levels of Hydrogen Sulphide, H2S was present. The SGT-300 configured with standard DLE (and turbine) hardware was able to operate with no concerns on this gas fuel. High Pressure rig testing of different fuels at true engine temperatures and pressures in a single combustor, permitted the release of the standard DLE combustion hardware over a much wider range of fuel compositions, including the fuel for this application which contained 17-20 mol% CO2 and N2 combined and a TCWI circa 32-34MJ/m 3.

Figure 7: 3 * SGT-300 operates on fixed platform Mediterranean Sea The success of this application provided the design basis for the SGT-300 gas turbine at University of New Hampshire, where the ability to operate on a gas fuel with a lower WI was fully utilised, figure 8. The

fuel is a processed landfill gas (PLG), typically with a WI of circa 28-34MJ/m3. When assessed a minimum set point for operation was agreed, 32MJ/m3, thus when the WI dropped below 32MJ/m3, or was insufficient quantity for the GT duty, pipeline quality gas was blended as required, figure 9. Further details are provided later.

Figure 8: SGT-300 at University of New Hampshire, operates with a Processed LandFill Gas PLG)

Figure 9: SGT-300 – UNH process screen shot (courtesy UNH) Subsequent benchmarking of the SGT-300 DLE burner hardware, including further HP rig testing, has 3 confirmed 2 burner variants covering the Temperature Corrected Wobbe Index range from 17.5MJ/m 3 through to 49MJ/m . The lower limit for each burner range is dictated by supply pressure constraints as well as pressure drop across the burner (maintains optimum combustion, ref figure 6 above). The 3 standard DLE combustion system is capable of operating to approximately 25MJ/m , with limiting factors identified of supply pressure requirements and pressure drop across the burner. Increasing burner delivery gas ejection hole size to maintain acceptable supply pressure and pressure drop has also been 3 evaluated, again in the combustion rig and a capability down towards 15MJ/m with commercial release 3 set at a minimum WI 17.5 MJ/m . The same extensive programme was completed on the SGT-400 combustor with 3 burner derivatives released to cover the wide WI fuel range 17.5MJ/m3 through to 49MJ/m3. Commercial opportunities have been developed and experience gained at fuels circa 27MJ/m3 (offshore platform – well head gas);

34MJ/m3 (onshore - weak wellhead gas), and most recently an application at 22MJ/m3, (biogas from Ethanol Industry waste). 6. CASE STUDIES 6.1. University of New Hampshire – SGT-300 in co-generation application Initially sold as a dual fuel, natural gas and No2 distillate, the customer requested an operation with a weak gaseous fuel to be investigated. Access to a landfill derived gas, LFG, was confirmed along with a request to permit operation using the same DLE combustion configuration as supplied to the GT. Investigation concluded it was not possible to operate on the raw LFG, but it would be possible to operate on a gas fuel with a WI based on combustion testing, and commercial experience of the offshore application described earlier, ie down to approximately 30MJ/m3. To achieve this, the LFG had to be processed to remove CO2 content, leaving the residue nitrogen content as the dominant inert species. The gas fuel, processed landfill gas (PLG), typically with a WI range 28-34MJ/m 3, required a minimum set point for operation, 32MJ/m3. When the WI dropped below 32MJ/m3, or there was insufficient flow for the GT duty then pipeline quality gas was blended to achieve the minimum setpoint. This was seen as a significant technological advance offering genuine tri-fuel capability for this industrial product, natural gas, processed landfill gas and No2 distillate liquid (diesel fuel) whilst still meeting stringent local exhaust emissions regulations across the varying ambient and load conditions. 6.2. Off-shore Application SGT-400 SE Asia Applying DLE configurations to off-shore duty has been successfully achieved, with numerous projects completed on both fixed and floating projects. One of the early projects for an off-shore duty was described earlier. At a similar time frame a multiple application for SGT-400 was undertaken, for a fixed off-shore platform in SE Asia (off-shore Myanmar), figure 10. Adopting the control product development procedure, PDP, the high pressure combustion rig was fully used to assess the necessary changes in combustion hardware to accommodate a weal wellhead fuel with high N2 and CO2 content, with a resultant Temperature corrected Wobbe Index of circa 27MJ/m3. The burner changes made allowed similar supply pressures as well as maintaining similar pressure drop across the combustion system to be maintained commensurate with standard fuels, figure 6. SGT-400 proving tests were witnessed by the customer and included ignition and pull await tests during a full package test with site equivalent gas provided in bottle form.

Figure 10: SGT-400 fixed platform duty off-shore Myanmar – includes fuel flexibility benefit 6.3. SGT- 400 power generation application in China using the bio-gas derived from the production of Ethanol. This example demonstrates the need to link all aspects of turbine capability and control to achieve a satisfactory outcome without resorting to an alternative fuel. An Ethanol processing plant produced a waste biogas containing high amounts of inert species,

nitrogen and carbon dioxide and this biogas was evaluated for the main fuel for an SGT-400 GT, rated at 12.9MWe. A typical fuel composition and analysis of the biogas is shown in table 1 below. Siemens extended fuels capability released on the SGT-400 covered fuels as low as 17.5MJ/m 3 Wobbe Index (WI), and for this particular application the WI was determined at 21MJ/m3, at a required supply temperature of >50OC. A gas only solution was required. Installation and commissioning was completed during April and May 2013. Species Vol % O2 0.21 N2 3.11 CO2 36.73 CH4 59.85 H2 0.02 H2S 0.08 Total 100% Saturated at ambient conditions Table 1: Biogas composition (2013)

Ethanol Production Plant Product: SGT-400 Location: China Operation on biogas: Composition: CH4 60mol%; CO2 37mol%

0

TCWI 21-22MJ/m3 @ ~55 C Commissioned May 2013 Starting on biogas No alternative fuel required

Figure 11: SGT-400 during commissioning (May 2013)- (Note: GT Exhaust is the shorter stack on left side of photo.) Start-up and turbine Running Although the combustion rig testing confirmed ignition and load operation were satisfactory, it was recognized that the transient operation and maintaining sufficient fuel flow (sufficient heat input to sustain stable combustion) would need to be developed and optimized on the complete packaged unit. Therefore, it was expected that numerous parameter adjustments would be required as part of the commissioning phase. Turbine Ignition Attempts to light the turbine were made applying standard parameters, such as ignition speed of 3000 rpm. Using multi-light mapping tool (software coding), which allows fuel demand to be stepped in increments for light up tests, it was not possible to light all 6 combustors Lowering the ignition speed and ensuring gas was correctly conditioned for dew point control (achieved by a degree of re-circulation), resulted in successful ignition with a speed of 2000rpm and 80% fuel flow passing through pilot burner (80% pilot). Other parameter changes were introduced, such as the gas generator purge speed (also reduced to 2000 rpm) with a corresponding increase in purge time. Turbine Run Up To Full Speed

With a satisfactory ignition window achieved the next area to address was maintaining a stable combustion and eliminating burner blowing out during acceleration from ignition to idle condition. Changes to starter motor power and ramp rate were only partially successful, due to close proximity with the compressor surge line. Resolution came with changes to the acceleration map (software code) in order to maintain a near constant air fuel ratio for combustion. To compensate for the slow acceleration of the gas generator more control parameter changes were necessary such as the link between compressor discharge pressure and fuel flow. Idle conditions were now achievable, allowing synchronizing and loading to be evaluated next. Turbine Synchronizing and Load Application Synchronizing was completed allowing load to be applied, leading to the next series of adjustment iterations and optimization, initially in regard to flame stability within the burners. As with starting, the standard fuel split maps were applied, as no experimental data from using contract fuel composition was available. Load was applied steadily in 1Mw increments until 8Mw was reached. At this point combustion dynamics (band 1 - indicator of an unstable flame) increased, requiring adjustment to the running split map (increasing pilot fuel schedule). Continuing to increase load again resulted in band 1 combustion dynamics re-appearing. Evaluating the data recorded during these runs allowed Siemens Combustion Engineering to recommend the necessary changes in fuel scheduling, including modifications to some parameter absolute maximum limits from those used for natural gas running. With these adjustments in place it was possible to load the turbine to maximum capacity. It is clear when the GT is operating on very weak fuels, significant changes to the various control parameters are required to achieve a satisfactory ignition window and loading regime. 6.4. SE Asia SGT-400 industrial GT utilizing multiple fuel streams of different calorific value 2 gas streams were identified for this application, the first containing inert content and the second a normal pipeline quality gas fuel. Initial assessment confirmed both fuels could be used, but with differing burner configuration (standard and one of the two MCV types). This was not practical as both fuels needed to be accommodated in the same engine hardware configuration, to be able to responding to a potential upset condition when the main, weak, fuel supply could be suspended. The burner variant covering 30 – 37MJ/m3 fuel range was benchmarked, and confirmed to operate on the wider range defined for this project. Rig and core engine testing were planned and completed prior to contract release. A full package test, including customer witness, was also completed to demonstrate the operation on the full range of fuels by introducing varying amounts of nitrogen in the gas supply to the turbine, figure 12. The series of tests also included the turbine response to changes in fuel calorific value, where a rate of change of 10%/minute with margin was successfully validated.

Figure 12: Nitrogen generator feeding N2 to gas supply for full engine test

Conclusion An extended gas fuels capability range has been successfully developed and implemented for the Siemens DLE Combustion system on the SGT-300 and SGT-400, to address the growing gas turbine market in areas where the availability of premium fuels is low or non-existent. The service experience with these fuels has been achieved on a number of applications in both on-shore and off-shore duties. The most recent application in China utilizes the weakest fuel ever used in an SGT-400 of circa WI 21MJ/m3 and has now been successfully commissioned as a gas only installation. The expanded Dry Low Emissions combustion capability into diluted gas fuels has been subject to rigorous R&D programmes completed under the Siemens Product Development Process. Besides fundamental design and analytical work, comprehensive combustion rig testing along with both core engine and full packaged unit testing has been completed allowing the SGT-300 and SGT-400 increased capability products to be released for commercial operation. The Siemens DLE combustion systems has been developed to operate on a wide range of gases, including gas fuels available in an LNG Liquefaction plant, from minimally processed weak wellhead type fuels, even to biogas derived from landfill or anaerobic digestion processes.