Modern Gas Turbine Systems || Combustors in gas turbine systems

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  • Published by Woodhead Publishing Limited, 2013 151 5 Combustors in gas turbine systems P. FLOHR and P. STUTTAFORD , Alstom (Schweiz) AG, Switzerland DOI : 10.1533/9780857096067.2.151 Abstract : This chapter discusses combustion systems in gas turbines. It begins by reviewing basic design principles before discussing developments in technology such as advanced fuel staging and reheat combustion systems. The chapter also covers the impact of different natural gas types on combustor operations, including combustor design for low calorifi c gases and fuel oils. Key words : combustion systems, combustors, gas turbines, advanced fuel staging, reheat combustion systems. 5.1 Introduction Modern gas turbine combustion chambers have little in common with the combustion system that was used in the fi rst industrial gas turbine at Neuchatel in Switzerland, built in 1939 ( Fig. 5.1 ). The thermal combustion intensity has increased from 10 MW/m 3 up to levels of more than 200 MW/m 3 . At the same time, the requirement to meet ever-increasing environmental standards has brought nitric oxide (NO x ) emission levels down from more than 500 vppm to well below 25 vppm. In addition, todayâs power generation market requires highly reliable and robust combustor components, being able to achieve high fl exibility in engine power output, as well as handling a variety of different fuel qualities. Operating effi ciency improvement is a key development driver, and the associated higher fi ring temperatures, while minimizing emissions and parasitic pressure loss, require innovative combustor solutions. This chap- ter outlines some of the key features that are the building blocks for these requirements of modern gas turbine combustors. While describing the gen- eral elements of state-of-the-art combustion technologies, Alstomâs combus- tor systems will be used as examples for best practices applied today. The chapter is organized as follows: The fi rst section describes the basic design principles of a modern com- bustion system for power generation turbines. The fi rst element is the
  • 152 Modern gas turbine systems Published by Woodhead Publishing Limited, 2013 choice of a premix burner system with a small pressure drop. The second element is a suitable multi-burner arrangement. The third element is the selection of an appropriate cooling scheme. In the last step, the combus- tor operation concept, which is the key element for engine integration, is highlighted. In the next section we address one of the key topics in combustor devel- opment from the past decade, namely the development of advanced fuel- staging principles. The fundamental limitation of a premix burner is the lean fl ame stability limit, that is the amount of excess air which is permitted for stable combustion. During engine start-up and low loads this limit is typ- ically exceeded, and fl ame stability could be reached only by reverting to simple non-premixed combustion. Non-premixed combustion is very stable, but results in higher emission values. Therefore, the implementation of pre- mix combustion for the widest possible load range by means of fuel-staging principles has been a fi eld of active work for all manufacturers, and such staging concepts will be discussed here. An alternative way of fuel staging is given by reheat combustion. In a sep- arate section we discuss the basic working principle of this combustion con- cept, which affects the engine architecture. By explaining the complementary fl ame stabilization mechanisms of reheat combustion it will become clear why this concept is well suited to operate over an extreme range of load conditions with low emissions. An alternative is to stage the combustor in such a way that the burner is divided within one combustor and separately optimized for operations at minimum load and full load points. Starting motor Generator Compressor Gas turbine Single combustor The worldâs first industrial gas turbine set at Neuchâtel (1939) Power: Efficiency: Turbine inlet temp.: An international historic mechanical engineering landmark since 2 September 1988 4 MW 17.4% 550ºC In use until 1998 as emergency genset Layout of the single-stage gas turbine set without recuperator 5.1 The fi rst gas turbine for power generation.
  • Combustors in gas turbine systems 153 Published by Woodhead Publishing Limited, 2013 A modern combustor must also be able to handle a variety of fuels, ran- ging from fuel oil to more exotic fuels such as low calorifi c gases. The full range of fuels is explained in a separate chapter of this book, but we will touch briefl y on the implications of different natural gas qualities for com- bustor operation, and a brief overview of a typical combustor design for low calorifi c gases, as well as the use of fuel oil as backup fuel. We conclude with a brief overview of future trends in combustor development. 5.2 Design principles Gas turbines are designed for increasingly higher turbine inlet tempera- tures. This increasing temperature requires a step-by-step experience-based development procedure, and validated design principles, on all hot gas path components as far as fulfi lling lifetime requirements are concerned. In add- ition, it becomes increasingly diffi cult to achieve emissions targets, due to an exponential relationship between combustor fl ame temperature and NO x emissions. This section describes the basic design principles of a modern combustion system for power generation turbines, focusing on applications as imple- mented in the Alstom gas turbines (Eroglu et al ., 2009). The fi rst principle is the choice of a premix burner system with a small pressure drop. The second principle is a suitable multi-burner arrangement. The third principle is the selection of an appropriate cooling scheme. In the last step, the com- bustor operation concept, which is the key element for engine integration, is discussed. 5.2.1 Premix burner The main component of a gas turbine combustor is the burner itself. The key design requirements for a burner are: safe and reliable operation, ⢠good combustion effi ciency, and ⢠low emissions. ⢠The functional elements for such a burner are typically the fuel injector, a mixing device, and a fl ame holder. During the past 30 years, these burners have evolved from large, single diffusion burners, to compact premix burners in multi-burner arrangements, see Fig. 5.2 . Detailed descriptions of the development evolution can be found in D ö bbeling et al . (2005) and references therein. The actual design
  • 154 Modern gas turbine systems Published by Woodhead Publishing Limited, 2013 and shape of modern premix burners has varied signifi cantly, to the present day, between the different manufacturers, but the key design features are common: Distributed fuel ports in combination with highly swirling fl ow are used ⢠to achieve good fuelâair mixing for premix combustion. Multiple fuel injection systems, that is, two or more independent fuel ⢠control lines, are used to achieve stable combustion for the entire load range. High fl ow velocities inside the burner premixers are needed to avoid ⢠fl ame fl ashback. Flame anchoring is ensured by reversed fl ow, caused by central vortex ⢠breakdown in a highly swirling fl ow, or a reversed fl ow about a low swirl, robust stagnation point As a specifi c design example, we describe the premix burner design from Alstom, the EnVironmental (EV) burner. It has been developed through a number of evolutionary step-by-step modifi cations. The basic EV burner operating principle is shown in Fig. 5.3 . Air is fed into the slots generated by two radially displaced half cones. Inside the burner cone a swirling fl ow is generated. As the air passes through the slots, premix gas is evenly distributed in the air before being further mixed inside the cone section of the burner. When the vortex fl ow expands upon entering the combustion chamber, a central reverse fl ow fi eld is generated (called vortex breakdown) in which the fl ame front can stabilize without attaching to the metal surfaces of the burner. Single B GT13E GT8C GT11 N2 GT13E2 Seq. combustion GT24/GT26 Annular combustorSilo combustor Gas turbine combustor development from 20 to 200 MW/m3 power density Gas turbine burner development from 500 to 10 ppm NOx 1st generation Dry low NOx burners 2nd generation EV burners 5.2 Important milestones in the burner and combustor development at Alstom.
  • Combustors in gas turbine systems 155 Published by Woodhead Publishing Limited, 2013 A second fuel injection point is at the head or apex region of the burner where pilot gas is injected. The pilot gas remains concentrated on the burner axis so that a diffusion fl ame is formed for low load operation and gas turbine start-up. As the load is increased, premix fuel can be introduced, spreading the fuel in all the supplied air for low emissions and the pilot is deactivated. The EV burner was fi rst introduced in GT11N engines with silo com- bustors in 1991. It has been since then successfully implemented in GT8C, GT13E2, GT11N2, GT24 and GT26 engines. A number of evolutionary step- by-step modifi cations have been implemented in the EV burner, in order to reduce emissions and to improve overall performance. The design optimiza- tions include improvements of the swirler aerodynamics, optimization of the fuel injector distribution, and an adjustment of the central fuel lance position (Paschereit et al ., 2002), as shown in Fig. 5.4 . The most recent design variant, currently used in the GT26 engine fl eet, has eliminated completely the central diffusion fl ame (Zajadatz et al ., 2007), and this is described in more detail in the next section. A further design example is that of the Alstom FlameSheet Combustor (Stuttaford et al ., 2010). The FlameSheet combustor is a dry lean premixed combustion system, designed specifi cally for extended turndown and fuel fl exible operation. The name derives from the method used for injecting the fuelâair mixture as a continuous uninterrupted sheet into the reaction zone of the combustor, whereupon an aerodynamically generated trapped vortex is utilized to anchor and stabilize the fl ame. The combustor consists of two aerodynamic stages and multiple fuel stages. The stages are designed for specifi c operational issues, such as tran- sient loading and extended turndown operation. The two aerodynamic stages Combustion air Premix gas Pilot gas and oil Premix gas Atomization Premix gas injection holes Ignition Flame front Spray evaporation Vortex breakdown 5.3 The EV burner principle.
  • 156 Modern gas turbine systems Published by Woodhead Publishing Limited, 2013 consist of a pilot along the axis of the combustor and a main stage surround- ing the pilot. Figure 5.5 illustrates the overall structure of the FlameSheet system. The pilot and main stage are fed from a compressor discharge ple- num. Pilot air passes in the radially outermost circuit to the head end of the combustor where it enters a radial infl ow swirler. Fuel is mixed into the air stream through a row of vanes. The fuelâair mixture enters the combustor, and a fl ame is swirl stabilized behind a bluff body on the centerline of the combustor. The main stage air fl ows along the backside of the combustion liner, and then through a main fuel injector. The fuelâair mixture is then turned 180° and fl ows into the combustor. As the fl ow enters the combustor it separates and forms a strong recirculation, or aerodynamically trapped vortex, which stabilizes the fl ame. 5.2.2 Combustor design The combustor provides the geometrical boundary of the combustion space within the gas turbine. The key design requirements for a combustor include: 5.5 Overall fl ow design of the FlameSheet system. 5.4 The evolution of the EV burner.
  • Combustors in gas turbine systems 157 Published by Woodhead Publishing Limited, 2013 integration of the burner into a multi-burner arrangement; ⢠stable operation for an entire engine operation envelope; ⢠homogeneous hot gas temperature profi le for the turbine inlet plane; ⢠minimal losses (pressure drop and heat), and; ⢠robustness, maintainability and low cost. ⢠The functional elements for combustors include burners or groups thereof, hot gas liners, cold carrying and casing structures, and fuel distribution systems. As for the burners themselves, the actual design and shape of modern premix combustors has varied signifi cantly between the different manufac- turers until today, but many of the key design features are in fact common: A compact combustion space is chosen to minimize NO ⢠x emissions, with a minimum volume requirement given by CO burnout times. The injection of cooling fl ow downstream of the heat release zone is ⢠avoided, to maintain lowest possible fl ame temperatures. Thermal barrier coatings (TBCs) are applied to the hot gas liners. ⢠A number of parts of the combustor are retractable from the engine cas-⢠ing (fuel lances, burners, or cans) for ease of maintenance. The fl ame distribution and air management is such that the temperature ⢠distribution at the combustor exit is highly homogeneous. Traditionally, industrial and heavy-duty gas turbine combustors have been of the silo type. This type of design was preferred for robustness and ease of maintenance. However, the cooled surface per unit volume is known to be high for this geometry. Moreover, turbine inlet temperature distri- bution is highly non-uniform and the cost of the combustor and the space required was high. At Alstom, the combustors have evolved through var- ious steps since the early 1990s. Starting from single diffusion burners in a single silo combustor, multiple premix burners were implemented in a single silo combustor, and fi nally multiple burners were integrated in com- pact annular combustors. The development steps are shown in Fig. 5.6 . Todayâs compact combustors require signifi cantly less cooling air and pro- vide a much more uniform turbine inlet temperature distribution (Eroglu et al ., 2009), and other manufacturers have since adopted similar design principles. Today, essentially two types of annular combustors are used at Alstom. The EV and the Sequential EV (SEV) combustor (which will be described in a subsequent section in more detail) of the GT24/GT26 engines are typ- ical examples of single-row combustors, as shown in Fig. 5.7 . In the case of the GT26, 24 burners each for the EV and SEV are used. The second type of annular combustor is the EV combustor of the GT13E2, with a multi-row
  • 158 Modern gas turbine systems Published by Woodhead Publishing Limited, 2013 arrangement of 72 burners in four rows, similar to the sketch in Fig. 5.8 . In this case, two types of burners are mixed with clockwise and counter-clock- wise swirl direction. This confi guration has been selected during the concept phase, due to optimal mixing properties and most homogeneous fl ow pro- fi le within the annulus. In this multi-row annular combustor it is possible to form several staging patterns within different groups of burners, to cope with a variety of pulsation and extinction topics with very small deteriora- tion of the combustor exit temperature profi le. The introduction of annular combustors with the distribution of multi- ple burners around the circumference of the engine has brought signifi cant advantages, for example: 5.6 Transition from a single-burner combustor via a multi-burner silo to an annular combustor. 5.7 EV and SEV single-row annular combustors in the GT26 confi guration.
  • Combustors in gas turbine systems 159 Published by Woodhead Publishing Limited, 2013 The hot combustor surface (which is exposed to the fl ame) is minimized, ⢠and thereby the cooling air consumption required is automatically minimized. The annular combustor generates naturally a very homogeneous hot gas ⢠temperature distribution, which is key for long turbine lifetime. Cross-ignition between individual burners is ensured without the need ⢠for cross-fi re tubes or multiple ignition torches. Operation at part load or full load with burner groups at different fl ame ⢠temperatures is possible. Burners which are running below the extinc- tion point are stabilized by their neighbors, and the shearing fl ow in the combustor leads to homogenized exit temperature profi les. The annular design is compact and cost-effective compared to single or ⢠can combustors. The basic design of the EV and SEV annular combustors consists of a ring- shaped combustion volume between inner and outer combustor liners, as shown in Plate V (see colour section between pages 346 and 347) and Fig. 5.9 . A combustor front panel is used to install individual burners which are fi red in this volume. At the downstream end of the combustion volume, a contraction accelerates the fl ow into the fi rst vane of the turbine. 201 202 111 109 110 107 108 105 103 104 101 102 611 612 610 609 607 608 605 603 604 601602 512 511 509 510 507 508 505 506 504 503 502 501412 411 410 409 408 407 405 406 403 404401 402 311 309 310307 308 305 306 303 304 301 302 View against flow direction 4/4-Premixgroup 3/4-Premixgroup 18 bumers 54 bumers 211 212 208 210 207 206 205 206 204 203 312 606 106112 5.8 EV double-row combustor in the GT13E2 confi guration.
  • 160 Modern gas turbine systems Published by Woodhead Publishing Limited, 2013 Inner and outer liners are basically cold carrying structures and they are shielded against hot gases with discrete segments, which are cooled by the combustion airfl ow along the backside. These casted segments can be replaced and reconditioned at regular intervals. In addition to the backside cooling, the segments are protected on the hot side with a TBC. The number of segments is the same as the burner number, which ensures a repeating hot gas loading and cooling pattern for all segments. Both, burners and lances can be retracted in the EV combustor without opening the engine. In the SEV combustor, the fuel lance can be retracted, thus allowing easy inspection of both combustors. Multiple can combustors have additional advantages in their modularity for installation and removal as simple individual units, while allowing fl exible modular upgrades, all without opening the engine. Often, the number of cans required by a specifi c unit is adjusted depending on whether implementing the system on a 50 or 60 Hz machine while maximizing the use of identical hard- ware. The Alstom FlameSheet and LEC-III ® combustion systems are examples of such systems. In both cases, an array of combustion cans (typically 10â20 cans) is arranged around the engine centerline providing hot air via a transition piece to the turbine. The LEC-III ® , which is currently in operation on more than 40 machines, evolved from a 25 ppm NO x system to a sub-5 ppm guaranteed NO x system through a number of cooling and mixing development enhancements. An example of the LEC-III ® burner arrangement is shown in Fig. 5.10 . The engine arrangement of such a combustion system is shown in Fig. 5.11 . 5.2.3 Cooling The combustor liners are cooled with a serial cooling scheme, as shown in Fig. 5.9 . SEV inner segment SEV outer segment SEV outer liner SEV lance SEV outer carrier SEV burner SEV inner liner SEV inner carrier 5.9 Main components of the GT24/GT26 SEV combustor.
  • Combustors in gas turbine systems 161 Published by Woodhead Publishing Limited, 2013 The cooling air enters the cooling passage at the downstream end, through a number of impingement holes, ensuring locally high heat transfer rates through impingement cooling in this highly loaded area (Fig. 5.12). The rest of the liner segments are cooled convectively, augmented through various tur- bulators and rib combinations. The liner segments are installed on rails, with highly compliant seals, which reduce leakages down to only a few percent. Individual gaps between liner segments are purged against hot gas ingestion with precisely designed gap-purging holes. The front segments are cooled with Primary fuel nozzle and endcover LBTU gas Secondary fuel nozzle Transition piece 5.10 Components making up the sub-5 ppm NO x LEC-III® combustion system. Turbine Air from compressor Combustors Transition piece 5.11 A typical can combustor layout with the Alstom LEC-III ® combustor implemented on a Siemens 501D5 machine upgraded and now operating below 5 ppm NO x .
  • 162 Modern gas turbine systems Published by Woodhead Publishing Limited, 2013 impingement cooling combined with fi lm cooling and a TBC layer on the hot side. It is emphasized that testing, from feature to full engine tests, is key for successful combustor development. Combustion tests to understand sta- bility and emission behavior of premix combustion are well established. As an example for other advanced testing methods carried out at Alstom, it is reported that a series of full-scale, full-pressure sector tests has been carried out as part of the validation program for the cyclic lifetime of com- bustor liners with an improved cooling scheme. A schematic of the rig is shown in Fig. 5.13 . Thermal loading of the segments can be simulated in this rig with a cyclic pattern as indicated in Fig. 5.14 . Results of this type of cyclic test series, in combination with thermal paint test results as shown in Plate VI, were used to validate mechanical integrity calculations of the segments. Different sealing geometries are used for the sealing of combustor parts, ranging from rope seals to C- or double-E, or even brush seals. An example is given in Fig. 5.15 , showing the sealing of a longitudinal gap between two liner segments with a double-E seal. Several test rigs, such as impact-, fret- ting-, fatigue- and leakage-rigs, have been utilized to optimize the sealing and lifetime characteristics of different seal concepts. Finally, selected seals have been implemented in the GT26 test power plant in Birr, Switzerland, and validated before fi eld implementation. 5.3 Combustor operation As an example for a typical combustor operation concept of a standard gas turbine, the GT13E2 operation concept is described here. The discussion includes an upgrade that has been implemented in the existing fl eet. EV outer carrier EV front plate/EV front segment Cooling air EV out er segm ent EV inn er segm ent Impingement cooling Convective cooling EV inner carrier 5.12 Cooling of casted-metal liner segments.
  • Combustors in gas turbine systems 163 Published by Woodhead Publishing Limited, 2013 Figure 5.16 shows the basic elements of the operation concept. The GT13E2 engine is started with the EV burners in pilot mode (diffusion fl ame) operation. As soon as combustor stability is suffi cient for reliable operation, the main premix groups are taken into operation between 10% and 20% load. From medium load onwards, between 50% and 60% load, Pressure vessel, typical testing level: 15 barCooling air Exhaust Burner Combustor liners 5.13 High pressure rig for liner segments. 1.0 Te m pe ra tu re ( no rm al iz ed ) 0.8 0.6 0.4 0.2 0.0 08:24:00 09:36:00 10:48:00 12:00:00 13:12:00 Time TW30047 TW40050 Pilot High premix level 14:24:00 15:36:00 16:48:00 5.14 A typical cyclic thermal loading pattern in sector tests.
  • 164 Modern gas turbine systems Published by Woodhead Publishing Limited, 2013 the pilot mode is switched off completely and the engine is operated with the premix burners only. For optimal combustion control two premix groups are applied, a âmainâ premix group and a âleanâ premix group. The division between the groups is such that three-quarters of the burners are in the main group, and a quarter of the burners are in the lean group, Fig. 5.17 . The lean premix group operates at or below the lean stability limit, and its combustion is supported in the annular combustor by the main premix group. This addi- tional degree of freedom allows the combustor to operate stably at all load conditions. In 2005, an upgrade of the combustor was implemented by introducing a closed-loop (i.e., feedback) controller, which is used to automatically adjust the main and lean premixed groups for constant combustion stability Segment 1 Segment 2 Cooling airCooling air Hot gas 5.15 A typical sealing geometry for liner segments. 100 100 Lean group ration 20% Main premix group Lean premix group Pilot Relative GT load (%) Pilot operation Mixed operation Premix operation90 90 80 80 70 70 60 60 50 50 F ue l g ro up d is tr ib ut io n (% ) 40 40 30 30 20 20 10 10 0 0 5.16 GT13E2 operation concept.
  • Combustors in gas turbine systems 165 Published by Woodhead Publishing Limited, 2013 even if ambient conditions or fuel composition changes (D ö bbeling et al ., 2008). From fi eld experience it was found that across the entire range of ambient conditions, gas compositions, and gas turbine loads, the stability limits of the combustor can change, requiring time-consuming, regular local adjustments. To overcome this restriction, the correlation between the lean blow-out limit of the combustor and the occurrence of low frequency pulsations was used to implement an Advanced Pulsation Control Logic (APCL), which con- stantly monitors the fl ame stability by pulsation measurements and adjusts the gas distribution according to known characteristics, such that the com- bustor operates stably but with minimal emissions. Another example of an automatic tuning system is given in Stuttaford (2011). It has been developed and fi elded on multiple GT units, which ensures emissions compliancy in operation while maintaining acceptable levels of frequency pulsations to avoid unit trips and maximize the hard- ware operating life. 5.3.1 Staged premix combustion All manufacturers of power generation turbines have pursued the devel- opment of advanced fuel-staging principles over the past decade (Lefebvre, 1999). This work was motivated by the lean fl ame stability limit of premix combustion, that is, the amount of excess air which is permitted for stable combustion. During engine start-up and at low loads, this limit is typically exceeded. In principle, there are four ways to overcome the limit. First, one reverts to non-premixed combustion mode, accepting the high emissions associ- ated with it. Secondly, air to the combustion zone is restricted in such a way that the excess air in the fl ame zone is reduced. To some degree this can be achieved by variable guide-vanes in the compressor to limit the mass fl ow for a given power output; implementation of air staging within a combustor is, however, generally impracticable. Thirdly, one applies fuel staging within Control valve Main burner group Lean burner group Fueal distribution pipe work Gas GT13E2 Hardware Software A-PCL - Fuel to main burner group - Fuel to lean burner group Pulsation controllerPulsation measurement Control valve 5.17 GT13E2 APCL.
  • 166 Modern gas turbine systems Published by Woodhead Publishing Limited, 2013 a group of burners circumferentially, or even axially. Effectively, this stag- ing principle creates an âair by-passâ through a set of burners that are not in operation in the low load operation regime. An example of this confi gura- tion is described in the next section. Finally, the burner itself is optimized with fuel staging, such that the use of non-premixed diffusion combustion can be avoided. A case study is given here as one example where a burner system, in this case the Alstom EV burner, has been improved, such that the need for dif- fusion combustion could be avoided altogether. Staging concept In Fig. 5.18, the operation with the standard EV burner is shown schemat- ically. This burner is operating with pilot fuel gas injection at gas turbine start-up and low part load. The pilot gas injection with the central pilot fuel lance leads to a fuel rich core in the EV burner fl ow fi eld. Under those con- ditions, a broad operation range with respect to fl ame extinction and pul- sations can be generated, but emission levels are high when the pilot is in operation. To avoid high emission levels, the burner must be switched over with increasing load to premixed operation to provide an evenly distrib- uted fuel gas concentration at the burner exit. In this case, the fuel injec- tion is accomplished over the premix fuel gas injection system in the air slots of the EV burner, which is a pattern of distributed small gas injection holes. The pilot gas injection is turned off in order to achieve lowest NO x emissions. A development project was carried out with the target to eliminate the pilot system, while maintaining good combustion stability over the entire Premix mode for low NOx emissions Internal pilot mode for start up Gas concentration Premix gas Pilot gas 5.18 The operation modes of the standard EV burner with pilot and premix operation.
  • Combustors in gas turbine systems 167 Published by Woodhead Publishing Limited, 2013 load range. A number of variants were investigated (reference Martinâs paper), and the fi nal design solution with staged-fuel injection is described here. The EV burner with staged-fuel gas injection is shown in Fig. 5.19 . Fuel is injected over two separate gas injection patterns termed as stage 1 and stage 2. The stage 1 fuel is injected upstream of the lance tip in radial direc- tion, while stage 2 is injected at the air slot of the EV burner. Both stages are in use over the complete gas turbine acceleration phase and load range. During start-up and low load, a signifi cantly higher amount of fuel is fed in the stage 1 through the lance. Therefore, similar to the pilot/premix EV, a fuel rich core in the fl ow fi eld can be generated, which produces a stable fl ame. At higher load more relative fuel fl ow through stage 2 downstream in the EV air slot has to be generated to provide (in addition with the remain- ing stage 1) an even fuel distribution at the burner exit. The injection of fuel gas is still divided between the burner air slots and the center of the burner cone, but the additional fuelâair mixing in the core fl ow has multiple advantages. The staging layout allows a closer control of the homogeneity of the premix mixture at the fl ame front. In addition to the reduced NO x emissions, the staged concept gives the possibility of adjusting the burner characteristics on-line in response to changes of the operation, and it simplifi es the auxiliary systems required by the combustion system because both fuel systems are in continuous operation. The additional advantage of this staged system is that no redesign of the EV burner itself is required. This staged concept can be simply applied by modifying the gas injection hole pattern at the fuel lance and the EV burner. Stage 2 Premix mode Low NOx emissions conditions with stages 1 and 2 fuel flows evenly distributed Start up mode High stage 1 amounts at start up but both stages in use Stage 2 Stage 1 Stage 1 Gas concentration 5.19 The operation modes of the staged EV burner with premix stages 1 and 2.
  • 168 Modern gas turbine systems Published by Woodhead Publishing Limited, 2013 The key advantage of an EV burner with a staged premix gas injection is simplifi ed operation concept without premix pilot switchover procedures. This allows a fast and reliable reaction of the gas turbine in transient oper- ation such as frequency response and load rejection. As both fuel stages are kept in operation over the complete load range, the previously required nitrogen-based purge of the fuel lines, on switching from pilot to premix, can be avoided for gas operation. The LEC-III ® and FlameSheet can combustors also require fuel staging to ensure robust fl ame stability over the gas turbine load range. In both cases a centrally located fully premixed pilot is used for initial light-off and low load operation. The main burner is wrapped around the pilot and is fueled as the unit is ramped up in speed and load. The pilot burner fuelâ air ratio is maintained at a high enough level to ensure the combustor does not fl ameout until the main burner fl ames are fully lit. At this point the pilot burner fuelâair ratio may be reduced to optimize emissions without negatively impacting the stability of the combustor. In the FlameSheet the main burner stage is also circumferentially staged to provide greater fl exibility in ensuring the transient stability of the combustor while also allowing a further dimension for optimization of combustion pulsation and emissions. Mixing properties To improve the combustion performance of the EV burner, various stage 1 patterns have been designed, to vary penetration depth, mixing length, azimuthal orientation within the cone, etc. In a fi rst step, the mixing and aerodynamic properties of these variants were calculated by CFD. Plate VII shows the EV burner slots with the fuel injection ports, as well as a fuel lance for staged gas injection. The fuel lance has a multi-hole fuel injection pattern, which allows the calculation of the various design parameters. One such parameter was the defi nition of the appropriate fuel momentum of the injection on the fuel lance, in order to adjust the fuel penetration into the passing air stream. In order to minimize the risks and costs within the development phase, a funneling approach has been applied (see Fig. 5.20 ), where a large number of variants are tested in the fi rst development steps and only the most promising are promoted to the next steps, with the tar- get of obtaining one single variant for validation in the gas turbine itself. The fi rst step usually consists of cold fl ow tests for the fl ow fi eld and mix- ing investigations, then the burner is tested under atmospheric pressure conditions, and fi nally under high pressure conditions, before gas turbine validation. The water channel provides a cost-effective and fl exible way to investi- gate turbulent fl ow and mixing phenomena within a gas turbine burner, as
  • Combustors in gas turbine systems 169 Published by Woodhead Publishing Limited, 2013 several non-invasive laser-based measurement techniques can be applied in water, and its density allows for suffi ciently high Reynolds numbers. For mixing studies the laser-induced fl uorescence (LIF) technique is used where water containing a small concentration of a fl uorescent dye is injected in the burner to simulate fuel injection. An example of the mixing fi elds produced by LIF tests during the variants selection step of the staged EV burner development is presented in Plate VIII. It is clear that variant B has a fuel rich core which can possibly lead to high NO x emissions, while the other two variants, especially variant C, show a more uniform fuel distribution and therefore better potential for low NO x . To quantify mixing quality, a spatial unmixedness parameter is used, defi ned by average and root mean square values of pixel concentrations in the LIF. Figure 5.21 shows that the correlation between spatial unmixedness and emissions measured in atmospheric combustion experiments was well established. It was also instructive to compare the mixing fi elds of the staged EV burner with the mixing fi eld of the original design. It appeared that with appropriate choice of stage 1, it was possible to achieve a better mixing than in the standard case, that is, a more uniform concentration distribution, see Plate IX. Indeed, the NO x advantage that became apparent during early development could be later exploited in the fi nal design. A key mixing design feature of the LEC-III ® and FlameSheet can com- bustors is that the air is mixed into the fuel, rather than the fuel mixed into the air. Since around 96% of the working fl uid in an F-class premixer is air, such a strategy provides good mixing with low cycle parasitic pressure loss. A further benefi t is that, since the momentum of the fuel exiting the injector Variants Cold flow tests Atmospheric single burner tests Development process High pressure single burner tests Full engine tests 5.20 Experimental burner development process.
  • 170 Modern gas turbine systems Published by Woodhead Publishing Limited, 2013 is dependent on the constituents of the fuel, minimizing the effect of fuel jet momentum minimizes the changes observed in combustor performance as the properties of the fuel vary. Flame stability Atmospheric single-burner tests were used to evaluate design variants in terms of pulsations and emissions. For natural gas combustion, the behav- ior of the fl ame, regarding shape and position relative to the burner exit, is independent of the operation pressure as long as other parameters like pre- heat temperature, burner velocity, etc. are kept the same as under engine conditions. The rig length was, however, adjusted such that it matches the pulsation frequency spectrum that is typically observed in the gas turbine itself. Figure 5.22 shows NO x emissions and combustion forced pulsations as a function of fl ame temperature for the standard EV burner and the staged EV burner. For the plot shown here the burner is operated with a constant stage 1 ratio. For high fl ame temperatures, the staged EV burner shows lower NO x values; for low fl ame temperatures, the range of low pulsations is clearly extended. The additional degree of freedom for the staged EV burner is shown in Fig. 5.23 . For constant air and fl ame temperatures the stage 1 ratio is varied. The minimum NO emissions are reached with a stage ratio between 20% and 30%. With the variation of the stage ratio, the mixing of air and fuel can be changed. This means at higher engine load levels where low emissions are favored, the mixing is optimized regarding emissions, and at lower load 2 Variant A Variant B Variant C 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 0.9 S pa tia l u nm ix ed ne ss U ( % ) 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 1 2 3 4 Stage 1 ratio (a.u) 5 6 â3.5 â3 â2.5 â2 â1.5 â1 â0.5 0 0.5 N O x (N or m al iz ed ) 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 = U = NOx 1 5.21 Correlation between unmixedness of mixing fi eld (waterlab) and NO x emissions (atmospheric combustion tests).
  • Combustors in gas turbine systems 171 Published by Woodhead Publishing Limited, 2013 operation modes, the stage ratio is optimized for high fl ame stability and suf- fi cient blow-out margin. Emission characteristics To conclude this section, the emission characteristics of the staged EV burner are compared to the standard EV burner, as implemented in the GT26 engine, in Fig. 5.24 . This graph shows two effects: fi rstly, emission levels N O x at 1 5% O 2 (p pm ) P ul sa tio n pe ak s (m ba r) Adiabatic flame temperature (K) Staged EV burner, NOx Operation range Staged EV burner, Puls. EV burner, NOx EV burner, Puls. 5.22 NO x emissions and pulsations against adiabatic fl ame temperature. N O x at 1 5% O 2 (p pm ) P ul sa tio n pe ak s (m ba r) Stage 1 ratio (%) Staged EV Burner, NOx Staged EV Burner, Puls. 5.23 NO x emissions and pulsations against stage 1 ratio, for constant fl ame temperature.
  • 172 Modern gas turbine systems Published by Woodhead Publishing Limited, 2013 are reduced and thus confi rm the expected improvement of the staged com- bustion principle. Secondly, the emission gradient is changed over a wide load range (in this plot between the measured points between 65% and 100% load). This change in gradient is interesting, because it is a direct con- sequence of the optimization described earlier. The emission levels of the EV combustor are now so low that the total emissions of the engine start to become dominated by the SEV combustor. As will be shown in the next section, the fuel input in the SEV is increased with load, and, as a result, now visible in the engine emissions performance even if absolute values are very low. The low emissions engine operation of the LEC-III ® is shown in Fig. 5.25 . The ability to operate the combustor in fully premixed mode resulted emis- sions being reduced from diffusion operation with water injection at 42 ppm NO x , to premixed dry operation with 4 ppm NO x on this machine. 5.3.2 Reheat combustion An alternative route to fuel staging is given by reheat combustion. We start by summarizing the basic working principle of this combustion and, indeed, engine concept (Joos et al ., 1996; Eroglu et al ., 2001). Following a short overview of the SEV burner design principle, the generic operational and fuel fl exibility of the reheat system is explained. The fl exibility is based on the presence of two fundamentally different fl ame stabilization mechanisms, namely fl ame propagation in the fi rst combustor stage and auto-ignition in the second combustor stage. Only recently has it become clear that the possibility of operating only one combustor and shutting down the SEV combustor altogether can be attractive to some 40 N O x_ dr y at 1 5% O 2 (p pm ) 50 60 70 Relative load (%) 80 Non-staged EV burner Staged EV burner 90 100 EV burner, staged EV burner, non-staged Typical guarantee level 5.24 NO x emissions of the staged and standard EV burners, measured in the GT26.
  • Combustors in gas turbine systems 173 Published by Woodhead Publishing Limited, 2013 engine operators, and we conclude this section with this low load operation concept. Reheat concept Sequential combustion was fi rst introduced into the market in 1948 as a way of increasing effi ciency at low turbine inlet temperature levels. But only when the development of the sequential or reheat combustion technology for the GT24/GT26 engines started in 1990 did this concept become widely recognized and accepted in the industry (G ü the et al ., 2007, and references therein). Using a pressure ratio of 30:1, the compressor delivers nearly dou- ble the pressure ratio of a conventional compressor. The compressed air is heated in a fi rst, annular EV combustor, based on the well-proven EV burner principle. Approximately 50% of the total fuel (at base-load) is added in this combustor, the combustion gas expands through a single-stage high pressure turbine, which lowers the pressure by approximately a factor of 2. The remaining fuel is added together with some additional cool- ing air into the second combustion chamber, the SEV combustor. The combus- tion gas is heated a second time to the maximum turbine inlet temperature and fi nally expanded in the four-stage low-pressure turbine, Fig. 5.26 . Relative to a conventional non-reheat cycle the same specifi c power output is achieved at lower turbine inlet temperature, and this is illustrated in Fig. 5.27 . Combustor design The SEV combustor consists of 24 burners, each with four vortex gener- ators, as can be seen in Fig. 5.28 . These vortex generators are formed as 4.5 4.0 3.5 3.0 2.5 2.0 E m is si on ( 15 % O 2) /A m pl itu de ( ps i p k- pk ) 1.5 1.0 0.5 0.0 65 70 75 80 85 90 Load (MW) 95 100 NOx (15% O2) CO (15% O2) Max dyn 5.25 LEC-III ® emissions characteristic following successful retrofi t of the 501D5 machine (95 MW being the 100% load condition).
  • 174 Modern gas turbine systems Published by Woodhead Publishing Limited, 2013 delta-wing shaped ramps, which roll up the incoming fl ow into a pair of streamwise vortices. The burner itself, including the front panel is cooled via laser-drilled effusion holes to cope with the high temperatures within the burner, in excess of 1000°C. The operation principle of the SEV burner is shown in Fig. 5.29 . A cen- tral lance is used for the injection of gaseous or liquid fuel directly in the center of four pairs of vortices generated by the vortex generators. Four holes are used for fuel injection, each consisting of a central fuel injector surrounded by a shielding air fl ow. This coaxial injection scheme allows, fi rst, cooling of the SEV fuel lance itself, and subsequently protection of the fuel against premature ignition on the near fi eld of injection. Additionally, injec- tion momentum is increased, ensuring suffi cient penetration even under low SEV fuel lance Annular SEV combustor Annular SEV combustor EV = EnVironmental SEV = Sequential EnVironmental EV burners Compressor HP turbine LP turbine 5.26 The sequential combustion gas turbine. h T h TConventional combustion Sequential combustion Same spec work at lower temperatures q wc wT qEV wHPT qSEV wLPT ss wHPC + wLPC 5.27 Principle of the reheat cycle, compared to a standard gas turbine cycle.
  • Combustors in gas turbine systems 175 Published by Woodhead Publishing Limited, 2013 load conditions, where fuel mass fl ow is only a fraction of the base-load level. The fuel injection is aligned with the vortex pattern emanating from the vortex generators. Injection parameters, such as hole size and angle, have been carefully optimized in order to place the fuel in the vortices as quickly as possible. It is very important to achieve suffi cient mixing quality prior to auto- ignition, which takes place within a few milliseconds at SEV conditions. In other words while, in a conventional lean premix combustor, sponta- neous or auto-ignition must be avoided at all circumstances, the SEV com- bustor has been specifi cally designed to operate at auto-ignition conditions. The mixing system is designed with very high fl ow velocities to prevent any Single annular combustion zone SEV burners Fuel lances Vortex generators (Fuel lance removed) 5.28 A side view and an upstream view from the combustor into the SEV burner. Reaction zone Fuel injector Self ignition Mixing zone Vortex generators 5.29 Operation principle of the SEV burner.
  • 176 Modern gas turbine systems Published by Woodhead Publishing Limited, 2013 premature self-ignition during the process of fi ne-scale mixing between fuel and the hot EV combustor exhaust gases. The combustion takes place in the combustion space, where stabilization is supported by additional recircula- tion zones behind the inner and outer backward facing steps at the expan- sion. This expansion also retards the fl ow inside the vortex cores of the central fl ow, and as a result, auto-ignition reaction is simultaneously initiated at the outer recirculation zones and inner vortex cores. There is no need for a fl ame ignition torch or fl ame monitor for this robust fl ame behavior. Engine operation The introduction of reheat combustion allowed development of a completely new operation concept in the GT24/GT26 engines. The basic features of this operation concept are explained in Fig. 5.30 . The engine is ignited and operated up to approximately 10% load with the EV combustor alone. At this point the EV combustor is operating at the nominal premixed conditions. The fuel injected in the SEV combustor at these conditions will ignite spontaneously. Due to that, the turndown ratio of the SEV combustor is practically down to zero fuel fl ow. This ensures intrin- sically a smooth transition, from EV only to EV-plus-SEV operation, with- out reaction and load instabilities. After this point, the gas turbine is further loaded with constant EV combustor temperature, and the loading consists of several phases from the initial ignition of the EV burners, the subsequent ignition of the SEV burners, and then the loading of the gas turbine by opening the variable guide vane to allow a greater air mass fl ow through the gas turbine. When the guide vane is fully open, the fi ring temperature of the GT exhaust temperature VIGVIde position SEV lgnition ~10% loadFull speed no load 0% 25% 100% Base load Relative GT load Inlet air flow (Inlet guide vane position) SEV combustor temperature EV combustor temperature 5.30 Operation concept of the GT24/GT26 engines with reheat combustion.
  • Combustors in gas turbine systems 177 Published by Woodhead Publishing Limited, 2013 SEV combustor is increased further to achieve the base-load value. From the operation concept, two points become apparent. Firstly, the EV com- bustor temperature is kept high and constant across a wide low range â this promotes low emissions, also at low part load ranges. Secondly, the exhaust gas temperature of the engine exiting to the heat recovery steam genera- tor is kept high and constant, also across a wide load range. This allows the combined-cycle part load effi ciency to be maximized. Emissions The low emission levels, which can be achieved with a reheat system, are the combined effect of three key mechanisms. Firstly, a reheat combustor makes more effi cient use of the oxygen by burn- ing twice in lean premix mode. At the inlet of the second stage, the oxygen content is at approximately 15% (reduced from 21% by combustion in the EV combustor). The oxygen content at the engine exhaust is then approxi- mately 10%, yielding a benefi cial normalization factor against the correction to 15% oxygen that is commonly used. Emissions quantifi ed in the widely used emission index defi ned as g(NO 2 )/kg(fuel) are reduced accordingly. Secondly, there exists a fundamental chemical advantage of reheat com- bustion, which can be exploited. This is caused essentially by the combus- tion at reduced oxygen-levels and increased water content (approximately 5%), as a result of the combustion in the EV combustor. Essentially, this chemical advantage is linked to the internal fl ue-gas ârecirculationâ in the reheat combustor. This allows the SEV combustor to operate at higher fi r- ing temperatures and produce less NO x than an EV combustor would pro- duce at the same temperature. Thirdly, the fl ame stabilization by auto-ignition leads to increased fl exibil- ity, which allows for operating in low emission mode at a wide load range by avoiding high peak fl ame temperatures. The higher SEV inlet temperature results in much higher reactivity, and this high reactivity at high combus- tor inlet temperatures has the consequence that fl ame stabilization always occurs under premixed conditions by auto-ignition and no piloting is needed to stabilize the SEV fl ame. The fl ame temperatures of the two combustors are ideally optimized primarily with respect to performance and turbine lifetime, but also at a given load for equal amounts of NO x produced deriv- ing an absolute minimum in engine emissions. The reheat concept enables operating the two combustors at differ- ent temperatures, with only a little impact on overall power output. This increases the fl exibility of the combustion system allowing burning more or less reactive fuels in the same engine by adjusting the relative load between the two combustors and allows mitigation of ranges where operation would become diffi cult (see next section for more details).
  • 178 Modern gas turbine systems Published by Woodhead Publishing Limited, 2013 In general, the engines are adjusted so that the fi rst combustor runs at low fi ring temperatures, keeping the emissions as low as possible for conven- tional premix combustion, while the second combustor is operated at higher temperatures producing similar emissions (on absolute scale) due to the reasons mentioned above. Note that while the EV is running more or less at the constant conditions keeping low NO x even at very low loads, the loading is obtained by varying the fuel input in the SEV, starting as low as ~25% GT load (as well as increasing the mass fl ow of air). The high inlet temperature to the SEV burners allows operation without pilot for the entire operation range, entirely relying on auto-ignition for the SEV fl ame. 5.3.3 Low load operation concept The introduction of the staged EV burner, which was explained in the previ- ous section, in combination with the sequential combustion, enabled a new gas turbine feature altogether. The gas turbine can be âparkedâ at around 15% load of the engine, just before the ignition point of the SEV combustor where the staged EV burner is already operated in full premix mode, resulting in extremely low emis- sions, both for NO x and CO which would be diffi cult to achieve otherwise, Fig. 5.31 . At these conditions, the turbine exit temperature is suffi ciently high (despite the SEV combustor being switched off), and the steam cycle of a combined-cycle power plant can be kept in operation. This is attractive to 110 Period of low power demand and/or reduced power tariff General possibilites of reducing load Typical âGT minimum technical loadâ: Determined by emission regulations and GT emission characteristic GT26 low load operation: Reduced load at low emission levels and acceptable performance Plant shutdown and restart: Thermal stress cycles, starting reliability, no on-line power reserve 100 90 80 70 60 50 G T r el at iv e lo ad ( % ) 40 30 20 10 0 5.31 The low load operation concept of the GT24/GT26 engines, with a âparking pointâ at around 15% gas turbine load. Single-stage combustion turbines need to be shut-down to avoid excessive emission values.
  • Combustors in gas turbine systems 179 Published by Woodhead Publishing Limited, 2013 operators that need to respond to high fl uctuations in power demand, and who will benefi t from start-up times within minutes instead of hours (Lilley et al ., 2007). An alternate method for extended low load operation is used by the FlameSheet combustion system. By having radially staged premixers and combustors, which are able to operate independently at very low load and in tandem at high load, emissions can be optimized at both extremes of the operating envelope. Figure 5.32 compares the back-to-back CO emissions at 10â20% load on a conventional SW501F premixed combustor with that of the FlameSheet combustor able to improve the CO emissions by two orders of magnitude. Such a can combustor upgrade retrofi t dramatically improves the range of operation over which the gas turbine is able to remain within emissions compliance. 5.4 Fuel flexibility Todayâs power generation market requires considerable fl exibility in terms of gas turbine fuels. Market forecasts predict a signifi cant shift in these fuels over the next decades. Already today, pipeline gases have a fl uctuation in composition, which will increase with the introduction of LNG in the near future. In 20â30 years, other fuels such as syngases, might become more prominent. The full range of fuels is explained in a separate chapter of this book, but we will discuss a few case studies here, with a particular focus on the implication of different natural gas qualities on combustor operation, more details can be found in Pennell et al . (2010). 10000 501FD2 CO emission ~3500 ppm CO Target in the rig (corrected for rig to engine difference) Flame sheet 501FD2 OEM 1000 100 C O a t 1 5% O 2 (p pm ) 10 1 0 20 6040 NOx at 15% O2 (ppm) 80 100 120 5.32 FlameSheet and conventional 501FD2 emissions measured back- to-back on full-scale test rig at 10â20% load.
  • 180 Modern gas turbine systems Published by Woodhead Publishing Limited, 2013 5.4.1 Natural gases The range of natural gases for gas turbine combustion continues to become wider in terms of species composition, Wobbe index and heating value, and thus can be signifi cantly different from pure methane. The gas composition may include large fractions of rich gases, or higher hydrocarbons with more than one carbon atom. Those higher hydrocarbons are summarized as âC2+â, which is the sum of all mole fractions of hydrocarbons with more than one carbon atom. At the same time the inert content of natural gases, mainly CO 2 and N 2 , may be large and varying. Thus when talking about fuel fl exibil- ity of natural gases, this means that the gas turbine must handle: Different fuel gases without hardware changes. ⢠A large range of C2+ levels (high hydrocarbons). ⢠A large range of inert contents. ⢠Fast and wide variations in gas composition. ⢠Fuel fl exibility is essentially driven by the discovery of new gas fi elds with large variations in gas composition (including diluents and higher hydrocar- bons, and LNG) and the need to burn such fuels without additional treat- ment to limit fuel cost. This trend is becoming highly signifi cant to many operators in the near and mid-term future, and puts pressure on the gas turbine manufacturers to develop improvements to combustion systems to ensure maximum fl exibility while maintaining extremely low emissions. Operators may encounter sudden changes in fuel composition because, for example, a gas supplier is changing from one well to another, or a different LNG gas source is being used. Gas turbines must be able to handle such changes without interruption of power generation. The variation of C2+ levels changes the reactivity of the fuel. As the amount of higher hydrocarbons increases, fl ame propagation speed increases, and auto-ignition temperature decreases. These variations in reac- tivity lead to altered stability characteristics of fl ames as well as a shifted fl ame position. The variation in inert content is characterized by the Wobbe index of the fuel. It results in greater or smaller penetration of gas through the injector holes into the combustion air and therefore affects the premixing quality of the gaseous fuel. These variations in premixing can lead to altered emission behavior of the combustor. As an example for typical variations observed, Plate X summarizes the natural gasesâ qualities for some of todayâs GT26 fl eet engines, in terms of Wobbe index and C2+ content. The majority of units lie in the range of 0â16% C2+, but it is expected that higher C2+ concentrations will be observed in the future. Likewise, it is expected that values even lower than
  • Combustors in gas turbine systems 181 Published by Woodhead Publishing Limited, 2013 a Wobbe index of 30 MJ/m 3 will be reached in pipeline natural gases in future. To mitigate the effects of changing gas composition on combustion per- formance, one may strictly limit the allowable range of fuels and modify the combustor hardware for different ranges of gas qualities. However, this implies a severe restriction for the gas turbine operator who is likely to receive various gases during the lifetime of the power plant. It is therefore better to implement suffi ciently robust burner hardware, in combination with an appropriate gas turbine control system, to counter-act the changed combustion behavior. At Alstom, the second strategy has been adopted (Riccius et al ., 2005), and it led to the âC2+ operation conceptâ for the GT24/GT26 gas turbines. Three key elements form the basis of this operation concept. Firstly, the fundamental design of the EV and SEV combustor is suffi ciently robust and insensitive against variations in diluents; this is achieved fundamentally by making the fuel injector insensitive against variations in volumetric fuel fl ow. Secondly, a rapid fuel composition measurement system is available, which is able to monitor composition changes in C2+ within fractions of a second; this has been achieved by developing a sensor that is much faster than traditional gas chromatographs. Thirdly, the combustor operation concept is linked to the actual fuel composition. A schematic of the operation concept is shown in Fig. 5.33 . In the pre- vious section the relation between EV combustor fi ring temperature and SEV inlet temperature has been explained. It turns out that with increased C2+ content in the fuel, and therefore increased reactivity, the lean stability 0 S E V b ur ne r in le t t em pe ra tu re 1 2 3 Standard operating concept Adjusted operating concept Gas operating monitoring 9 6 4 5 6 7 SEV burner protection limit EV burner lean extinction limit 8 C2+ (%) 9 10 11 12 13 14 15 16 Operation window 5.33 The C2+ operation concept for reheat engines.
  • 182 Modern gas turbine systems Published by Woodhead Publishing Limited, 2013 limit of the EV combustor at lowered fi ring temperatures is improved. This allows the inlet temperature of the SEV combustor to be reduced to stay within the optimum operating window. However, the SEV fl ame itself is rather insensitive to changes in fuel composition. This implies that the C2+ operation concept has only little loss in overall power due to the reduced temperature in the EV, as more fuel can be added to the SEV. This opera- tion concept is unique to reheat combustion until today. Also the FlameSheet can combustor system has demonstrated the poten- tial for running on a wide range of fuels through full-scale high pressure rig tests. The nature of the premixers, mixing air into fuel, as well as high premixer exit velocities allowed operation with a range of fuels shown in Fig. 5.34 , including fuels containing up to 60% hydrogen. 9 8 7 6 5 4 N O x an d C O ( at 1 5% O 2) N O x an d C O ( at 1 5% O 2) 3 2 1 0 44 45 46 47 48 49 50 51 52 53 54 Modified Wobbe index 24% H2 12% H2 NG w/hot fuel 12% H2, 66%CH4, 11%C2H6, 12%C3H8 24% H2 40% H2 60% H2 NG w/warm fuel NOx with torch on 20%H2, 20%C2H4, 19%C2H6 CO with torch on 71%CH4, 15%C2H6, 14%C3H8 NG w/cold fuel 55 56 57 58 59 60 44 45 46 47 48 49 50 51 52 53 54 Modified Wobbe index 55 56 57 58 59 60 9 8 7 6 5 4 3 2 1 0 NOx CO 5.34 FlameSheet full-scale high pressure rig results operations on varying fuels.
  • Combustors in gas turbine systems 183 Published by Woodhead Publishing Limited, 2013 5.4.2 Fuel oil For completeness, it is stressed that most gas turbines are designed such that they are able to burn fuel oil in addition to natural gas, but this fuel is mostly used as backup fuel only. For the EV burner, shown in Fig. 5.3 , the fuel oil is injected via a central plain jet nozzle in the cone head. Water can be added to the fuel for emis- sion control. The liquid plain jet disintegrates into small droplets within the burner cone. The swirling fl ow distributes the droplets in the whole fl ow fi eld, and the fl ame stabilization occurs near to the burner outlet with the help of the central recirculation zone, similar to the combustion with natu- ral gas. 5.4.3 Low calorifi c gases All gas turbine manufactures have developed in the past tailored solutions both for medium and highly diluted fuels such as blast furnace gases or syngases. We show here the design examples from Alstom, which are able to burn these highly diluted and/or highly reactive gases. For syngas fuels the EV burner was further developed in order to burn fuels with hydrogen content of up to 50%, and heating values in the range of 6â15 MJ/kg. In order to account for the high reactivity of hydrogen, the fuel injection location was repositioned closer to the burner exit. The gas channel volume and injector size was increased to minimize the gas pres- sure requirements from the higher volume fl ows of the fuel. A sketch of this burner variant is shown in Fig. 5.35 . The burner has been installed in a GT13E2 gas turbine (Reiss et al ., 2002). Vortex breakdow Gas injection holes Combustion air MBTU fuelFlame front 5.35 EV burner for combustion of syngas fuels with hydrogen concentrations up to 50%.
  • 184 Modern gas turbine systems Published by Woodhead Publishing Limited, 2013 For low calorifi c gases such as blast furnace gases, the heating value can be as low as 2 MJ/kg. In this case, the volumetric fl ow of the fuel is of the same order as the air fl ow itself, and the engine (i.e., the matching of com- pressor and turbine) needs to be adjusted. For the combustor itself, one typically applies a design that is similar to the large silo combustors shown in Fig. 5.6 . A detail of the burner design is shown in Plate XI where it can be seen that the main burner swirler is built in a sandwich format with low cal- orifi c gas supplied through fi ns in each of the swirler blades. The gas channel is split into an inner and outer section to be able to control the fl ammability limits of the fuel with the incoming air, while maintaining optimum emis- sions. On the central axis of the burner a lance is installed for backup oil operation as well as an injection of more reactive gases containing hydrogen of natural gas. The vortex breakdown stabilizes the fl ame in the silo combustor similar to the principle employed for fl ame stabilization in the EV burner. 5.5 Future trends Progress in gas turbine combustion continues to be made not only by step improvements of existing concepts, but also through major innovations with novel combustion concepts. The key drivers for progress continue to be emission reduction, handling an increasing number of fuels or fuel blends, and generally extending the range of operation in low emission mode. 5.5.1 Emission reduction The need to fi nd new ways for emission reduction is driven by two confl ict- ing targets. Environmental legislation continues to drive allowable emis- sion levels of NO x further down to single digit values. At the same time, the hot gas temperatures continue to increase to achieve even higher engine effi ciencies, leading to increased NO x emissions for the existing technolo- gies. Perfecting fuelâair mixing within the burner, increasing the complex- ity of fuel-staging techniques (where cumbersome switchover or purging sequences are to be avoided to maintain reliability), or applying advanced cycles such as reheat combustion is pursued in the industry. One innovation area that has recently gained interest for various man- ufacturers is fl ue-gas recirculation. In the context of the development of CO 2 -free gas turbines, it has been identifi ed as an effective technique for post-combustion CO 2 sequestration in natural gas power plants. As an example, Fig. 5.36 shows a sketch of the Alstom GT26 gas turbine, inte- grated into a fl ue-gas recirculation cycle. The gas fl ow is split after the heat recovery steam generator with a certain ratio into a fl ow entering the CO 2 removal section and a recirculating fl ow. This appears on the one hand to
  • Combustors in gas turbine systems 185 Published by Woodhead Publishing Limited, 2013 be an effective means to enrich the CO 2 content in the exhaust, thereby reducing the cost of the CO 2 capture equipment. At the same time, fl ue-gas recirculation is a promising technique for further NO x emission reduction. Further reading can be found in G ü the et al . (2009, 2011). Another alterna- tive for CO 2 reduction is the removal of carbon from the fuel prior to burn- ing the fuel. The operating effi ciency of such systems is optimized by using the already existing gas turbine compressor to drive a fuel partial oxidation system as described in Stuttaford and Oumejjoud (2008). 5.5.2 Engine fi ring temperature increase The drive to continuously improve cycle effi ciency leads to ever-increas- ing operating temperatures. This is a challenge for the durability of the hardware, as well as the NO x emissions. Novel materials will be required to minimize the cooling air on combustor parts while operating those compo- nents at higher temperature. Novel techniques as mentioned in the previous paragraph will be required to control and even reduce NO x emissions even as the reaction zone temperatures are increased. 5.5.3 Fuel fl exibility Properties of natural gases are already varying today and this variation is expected to increase in the future. The degree of variation is diffi cult to pre- dict but already today fl uctuations in Wobbe index can easily exceed 10% Air Fuel (CH4) Fuel (CH4) HRSG removal Steam BFW CO2CO2 H2O EV combustor Gen. Cooling air SEV combustor 5.36 Flue-gas recirculation concept.
  • 186 Modern gas turbine systems Published by Woodhead Publishing Limited, 2013 for some power plants. It is clear that gas turbine combustors must be able to cope with such an increased variability of natural gas qualities preferably without change of hardware. Solutions have to be found where on the one hand the fuelâair mixing is inherently insensitive to changes in fuel compo- sition (volumetric fl ow and reactivity), and on the other hand the control of engine operation is linked to parameters of fuel composition. It is the view of the author that a stepwise extension of the capability to burn various fuels is also the right way towards co-fi ring of future fuels like biogases or hydrogen. 5.5.4 Operation fl exibility Through the lifetime of a power plant it is diffi cult to predict the expected load range where the plant will be operating for most of the time. Many plants will run extensively in the part load regime, due to increased use of fl uctuating renewable energy such as wind power. Others will continue to require highly effi cient operation at base-load with lowest possible emis- sions. Future combustors will have to cover both ends of the extreme, using only a single hardware. These requirements are not only key for new systems, but also when retrofi tting existing engines (Stuttaford et al ., 2010). 5.6 References D ö bbeling, K., Hellat, J. and Koch, H. (2005), 25 Years of BBC/ABB/ALSTOM lean premix combustion technologies, ASME Turbo Expo, GT2005-68269. D ö bbeling, K., Meeuwissen, T., Zajadatz, M. and Flohr, P. (2008), Fuel fl exibility of the Alstom GT13E2 medium sized gas turbine, ASME Turbo Expo, GT2008- 50950. Eroglu, A., Brunner, P., Hellat, J. and Flohr, P. (2009), Combustor design for low emissions and long lifetime requirements, ASME Turbo Expo, GT2009-59540. Eroglu, A., D ö bbeling, K., Joos, F. and Brunner, P. (2001), Vortex generators in lean- premix combustion, Transactions of the ASME, Journal of Engineering for Gas Turbines and Power , 123 , 41â49. G ü the, F., Hellat, J. and Flohr, P. (2007), The reheat concept: the proven pathway to ultra-low emissions and high effi ciency and fl exibility, ASME Turbo Expo, GT2007-27846. G ü the, F., de la Cruz Garcia, M. and Burdet, A. (2009), Flue gas recirculation in gas turbine: investigation of combustion reactivity and NO x emission, ASME Turbo Expo, GT2009-59221. G ü the, F., Stankovic, D., Genin, F., Syed, K. and Winkler, D. (2011), Flue gas recircu- lation of the Alstom sequential gas turbine combustor tested at high pressure, ASME Turbo Expo, GT2011-45379. Joos, F., Brunner, P., Schulte-Werning, B., Syed, K. and Eroglu, A. (1996), Development of the sequential combustion system for the ABB GT24/GT26 gas turbine family, ASME Turbo Expo, 1996-GT-315.
  • Combustors in gas turbine systems 187 Published by Woodhead Publishing Limited, 2013 Lefebvre, A.H. (1999), Gas Turbine Combustion , CRC Press (Taylor & Francis Group). Lilley, D., Ruecker, F., Lachner, R., Lindvall, A. and Rebhan, D. (2007), Low load operation concept for Alstomâs GT26 gas turbine, PowerGen Europe. Paschereit, O., Flohr, P., Kn ö pfel, H., Geng, W., Steinbach, C., Stuber, P., Bengtsson, K. and Gutmark, E. (2002), Combustion control by extended EV burner fuel lance, ASME, 2002-GT-30462. Pennell, D., Hiddemann, M. and Flohr, P. (2010), Alstom fuel fl exibility for todayâs and future market requirements, PowerGen Europe. Reiss, F., Griffi n, T. and Reyser, K. (2002), The Alstom GT13E2 medium BTU gas turbine, ASME Turbo Expo, 2002-GT- 30108. Riccius, O., Smith, R., G ü the, F. and Flohr, P. (2005), The GT24/GT26 combustion technology and high hydrocarbon (âC2+â) fuels, ASME Turbo Expo, GT2005- 68393. Stuttaford, P.J. and Oumejjoud, K. (2008), Low CO 2 Combustion System Retrofi ts for Existing Heavy Duty Gas Turbines, ASME GT2008-50814. Stuttaford, P.J., Rizkalla, H., Chen, Y., Copely, B. and Faucett, T. (2010), Extended Turndown, Fuel Flexible Gas Turbine Combustion System, ASME GT2010- 22585. Stuttaford, P.J. (2011), Extended Fuel Flexibility for E and F-class Gas. Turbine Combustion Systems, Power Gen Europe, ASME GT2010-22585, Milan, Italy, 2011. Zajadatz, M., Lachner, R., Bernero, S., Motz, C. and Flohr, P. (2007), Development and design of Alstomâs staged fuel gas injection EV burner for NO x reduction, ASME Turbo Expo, GT2007-2773.
  • © Woodhead Publishing Limited, 2013 EV Outer liner EV Outer segment EV Front plate EV Burner EV Lance EV Front segmentEV Inner segmentEV Inner liner Plate V (Chapter 5) Main components of the GT24/GT26 EV combustor.
  • © Woodhead Publishing Limited, 2013 0.82 0.76 0.71 0.66 0.61 0.56 0.51 0.41 0.36 0.46 1.00 0.87 Plate VI (Chapter 5) Calculated temperatures and thermal paint results from a liner segment. Follow streamlines from stage 1 injection to place fuel ports on lance Fuel penetration of premix holes at base load conditions x y zN or m al iz ed v el oc ity m ag ni tu de Plate VII (Chapter 5) Comparison of fuel streamlines for premixed and staged fuel gas injection.
  • © Woodhead Publishing Limited, 2013 Variant A Stage 1 ratio Fuel con- centration Rich core Uniform distribution Variant B Variant C Plate VIII (Chapter 5) Comparison of mixing fields between different staged EV variants. Stage 1 ratio Staged fuel injection Standard fuel injection (Full premix mode) Plate IX (Chapter 5) Comparison of mixing fields of the staged and standard EV burners.
  • © Woodhead Publishing Limited, 2013 60 55 Trend of Wobbe index versus fuel C2+ content at 15ºC Customer 1 Customer 2 Customer 4 Customer 5 Customer 6 Customer 7 Customer 8 Customer 9 Customer 10 Customer 11 Customer 12 Customer 13 Customer 14 Customer 15 Customer 16 Customer 17 Customer 18 Customer 19 Customer 20 GT26 Test facility 50 W ob be in de x (M J/ m 3 ) 45 40 35 30 25 20 0 5 10 15 20 25 C2+ content (vol %) Plate X (Chapter 5) GT26 fleet C2+ concentrations and Wobbe index variantions. Natural gas LBTU inner channel LBTU outer channel Air Plate XI (Chapter 5) Single burner design for combustion of low calorific gases and fuels with high hydrogen content.