Conference Paper POWER
Paper presented at PowerGen Asia in Bangkok, Thailand (3 rd -5 th October 2012) Fuel Flexibility capabilities of Alstom s Klaus Knapp, Khawar Syed, Mark Stevens Alstom Power, Baden, Switzerland 1 Abstract... 3 2 Introduction... 3 3 Varying Natural Gas Composition... 4 4 Alstom s GT24 and GT26 Combustion Technology... 6 5 Burner Technology used in the GT24 and GT26 Gas Turbine... 7 6 Natural Gas and Hydrogen co-firing capability... 12 7 Summary and Conclusions... 17 8 References... 17
1 Abstract In order to ensure secure and cost effective natural gas supplies, there is a growing trend towards the utilization of LNG, as it allows natural gas to be procured from a number of sources. Depending upon the source of the natural gas, its composition will change. A gas fired power plant should therefore be flexible enough to allow such fuel variations, while meeting all other constraints such as operability, emissions and life. This paper describes the combustion systems of Alstom s GT24 and GT26 reheat gas turbines with sequential combustion and their operation with emphasis on the features that allow enhanced fuel flexibility. The range in Wobbe Index, higher hydrocarbons content and hydrogen content that can be handled will be demonstrated by way of both engine and rig test results. 2 Introduction The utilization of natural gas for power generation is expected to take an increasing share of the fossil fuel market. This is due to substantial natural gas reserves, the fact that natural gas is a relatively clean fuel and that combined cycle power plants are both highly efficient and can be erected and commissioned within short timescales. Using LNG allows natural gas to be procured from a number of sources. For example, within Asia the majority of LNG had been sourced from Malaysia and Indonesia, which recently has been supplemented by rising supplies from Australia and Qatar. In the future there could be an even greater diversification based upon expectations of LNG being received from Russia and the USA. A consequence of the widening use of natural gas from different sources is relatively rapid changes in fuel composition that can result. There is therefore a growing need for gas turbines to allow this. The critical fuel parameters that affect the performance and operation of gas turbines are summarised in figure 1. The Wobbe Index and the reactivity are of particular interest, the first correlates mainly to the inert content, the latter typically being a function of the content of higher hydrocarbons and for future applications the hydrogen content in the fuel. These factors modify the performance of lean premixed combustion systems. As such, systems are optimized to give low emissions, low combustion dynamics and good stability over the entire operating range of the machine. 3
Figure 1 Natural Gas parameters In this paper we describe the burner technologies used within Alstom s GT24 and GT26 gas turbines, specifically with respect to their fuel flexibility capabilities, operating in a wide range of Wobbe Index and fuel reactivity, due to changes in the fuel s inert, higher hydrocarbon and hydrogen contents. 3 Varying Natural Gas Composition Larger changes in Wobbe Index are often associated with changes of the inert components of a gas, namely nitrogen and carbon dioxide. The Wobbe Index reduces with increasing inert contents. The widening use of LNG will see the reduction in inert contents, as these are typically removed during the liquefaction process. However, for sites which are supplied by pipelines with high CH4 gas and low CH4 gas, with LNG and industrial gases, strong variations in Wobbe Index may occur. The reactivity of the fuel will fluctuate due to varying C2+ contents of the different LNG s and in some cases due to the addition of LPG (liquefied petroleum gas: propane, butane). In future applications the co-firing of natural gas and hydrogen that is for instance produced by renewable energy is likely to come. This hydrogen could be mixed to the fuel gas at different locations. A natural gas grid might be able to carry and supply H2 concentrations up to 10% vol., at a specific power plant H2 might be stored or produced online leading to even higher contents in the natural gas. Figure 2 shows a sketch of such a complex gas supply with varying gas sources. 4
Figure 2 Varying gas sources The resulting fuel composition range to be handled by a flexible gas turbine combustion system is shown in table 1. The Wobbe Index is expected to change by +/-15% around a site specific design gas. The higher end of the Wobbe Index range correlates to gases that consist mainly of CH4, with almost no inert content, but with C2+ contents of 0% to 18%. The lower end of the Wobbe Index range reflects diluted natural gases with inert contents of more than 10%. Table 1 fuel gas requirements Alstom s reheat gas turbines are particularly suited to meet such fuel flexibility demands. This is partly due to burner design, where the EV and SEV burners, used within the first and the reheat combustors respectively, are proper to changes in fuel volume flow rates, without penalties on emissions and combustion stability. Additionally, the reheat system of the GT24 and GT26 gas turbines allow additional flexibility to address fuel reactivity changes, through modulating the fuel flow to each of the two combustors, while maintaining the power demand. 5
4 Alstom s GT24 and GT26 Combustion Technology Alstom s GT24 and GT26 are reheat gas turbines operating a sequential combustor system. The hot section of the engine is shown in figure 3. The compressor air is delivered to the first combustor, the EV combustor. The hot gases are then expanded through a single-stage high pressure turbine where the pressure is reduced by almost 50%. The gases then enter the Sequential EV-combustor, the SEV combustor, at a temperature of approximately 1000 C. The inlet temperature to the SEV burners is sufficiently high enough for auto-ignition after a characteristic delay time of about 1 millisecond. Figure 3 also shows the key combustor operating parameters over the load range. From ignition up to approximately 10% load, only the EV combustor is in operation. It therefore reaches its design hot gas temperature at a low load, and thus is able to operate in low emissions mode over almost the entire load range. As the load is increased, the SEV is also fuelled. The load is then controlled through opening the Variable Inlet Guide Vanes (VIGVs), maintaining the EV hot gas temperature and steadily increasing the SEV hot gas temperature. At base load the fuel to both combustors is approximately equal. Both EV and SEV combustion systems operate in a lean premixed mode in fuel gas operation and therefore deliver low NOx emissions without the need for water/steam injection. Gas-only machines have two fuel gas stages for the EV combustor and a single gas stage for the SEV combustor. Figure 3 - GT24 and GT26 hot section and operation concept 6
5 Burner Technology used in the GT24 and GT26 Gas Turbine The EV combustor is of the annular type and is fitted with a single row of EV burners. The burners operate in a staged premixed mode in fuel gas operation which enables low emissions over a wide operating range and stability over the complete operating range. Figure 4 shows photographs of the EV burner flame near the design point flame temperature, the lean premix mode, and at low flame temperatures, associated with low load operation, the rich premix mode. At high flame temperatures the two fuel stages are optimized to give a uniform fuel distribution at the burner exit. This results in low NOx emissions at high flame temperatures. At low loads, the staging ratio is adjusted to bias the fuel towards the central part of the flow, which ensures stability at low flame temperatures. Figure 4 EV staged burner concept The SEV combustor is also of an annular type and is fitted with a single row of SEV burners. Each SEV burner, see figure 5, contains a single fuel injection lance with a single fuel gas stage and, in the dual fuel case, a single fuel oil stage with its associated single water stage. Rapid premixing between fuel and oxidant (i.e. the exhaust gas from the high pressure turbine) is achieved through injecting the fuel into vortices initiated by vortex generators that are integrated in the burner body. This combustor does not have an ignition system as the inlet temperatures are such that auto-ignition occurs. Crucial to the operation is that the auto-ignition delay time is within a certain window. 7
Figure 5 SEV burner concept In both, EV and SEV burner, the premixing of the fuel with air (EV) and of fuel with combustion gases (SEV) is similar to a jet in a cross-flow configuration. The mixing quality depends on an optimal fuel gas jet penetration. The Wobbe Index for his part correlates heat input, gas supply pressure and pressure loss characteristic of a burner fuel gas system (kv) and so reflects in a way also the gas jet momentum flux, see figure 6. To compare gas compositions the Wobbe Index is calculated at ISO-conditions (WI iso ), for GT applications it is more useful to consider the actual fuel gas temperature (WI net ). Figure 6 Wobbe Index definition 8
Large variation in Wobbe Index leads to large changes in the required fuel pressure and can therefore violate maximum supply pressure at the low Wobbe Index end. At very high Wobbe Index, minimal fuel pressure limits may be violated leading to combustion dynamic issues as fuel flows into the burners respond strongly to pressure fluctuations within the combustor. For such extreme Wobbe Index changes a hardware modification could be required, which alters the effective area of the burner fuel injection. More moderate changes in Wobbe Index require no hardware change but can alter the fuel distribution within the burner, due to fuel jet momentum changes. Additionally, changes in fuel reactivity modify, for example, burning rates and can therefore later the location of the flame zone. Both these can influence the combustion and emissions performance of the burner. The extent to which these affect the burner performance depends upon the burner concept. The Wobbe Index can also be affected by the fuel temperature. Pre-heating the fuel is desirable as it enhances significantly the efficiency of a combined cycle power plant. This fuel gas pre-heating has the effect of reducing the Wobbe Index (WI net ). The content of higher hydrocarbons (C2+) influences the reactivity of a fuel gas. Higher reactivity leads to a faster flame propagation, which is important for EV and SEV burners, and auto-ignition delay time, which is more important for the SEV burner. Flame propagation changes result in variation of the reaction zone location, which in turn influences the emissions and thermo-acoustic aspects of the combustion system. In order to limit this effect both the EV and SEV burners have been designed to have strong flow velocity gradient in the region of flame stabilization. This moderates the change in flame location, due to changes in flame propagation velocity. In the EV burner the flame positions shifts only very slightly with increasing reactivity of the fuel. For the SEV burner the auto-ignition delay times are significant for correct burning in the sequential combustor of the GT24 and GT26. If delay-time is far too short, combustion occurs too quickly and the flame appears within the burner. If it is too long, the flame is located too far downstream and combustion efficiency issues arise. Figure 7 shows the sensitivity of auto-ignition delay time to the mixture inlet temperature and the C2+ content and it shows an example for the possible wide range of C2+ fluctuation measured on a GT26. 9
Figure 7 Auto-ignition delay times as function of inlet temperature and C2+ content For the EV and SEV burners, insensitivity to Wobbe Index changes has been achieved through optimization of fuel injection and burner aerodynamics during the development process. In the EV burner the fuel is evenly distributed via multi-point injection along the air slots and the mixing quality is mainly depending on the swirling air-flow. On the SEV burner the radial-distributed fuel jets are surrounded by a co-axial air flow that has a triple-functionality: cooling the lance, shielding the fuel jet from too rapid mixing with the hot gas and enhancing the jet penetration as carrier air. This carrier air provides more than 50% of the jet momentum. Regarding fuel gas reactivity the reheat concept used in the GT24 and GT26 allows additional flexibility through altering the EV/SEV fuel split as function of the C2+ content, see figure 8. Shifting fuel from EV to SEV combustor reduces the EV combustor temperature and hence reduces the SEV inlet temperature, but keeps the SEV combustor temperature constant. This is used to counter the effects of fuel reactivity on the flame position within both the first and second combustor. By this the emissions can be optimized and the SEV flame position is controlled. As the inlet temperature to the LP turbine with its four stages stays constant there is almost no debit on the power of the gas turbine. Figure 8 EV/SEV fuel split control as function of C2+ 10
The Alstom experience in C2+ range and Wobbe Index (WI net ) is indicated in figure 9, which shows fuels compositions at a number of customer sites as well as fuels tested at Alstom s Power Plant in Birr, Switzerland. There are power plants that have been operated with C2+ contents from the low end, 0% to 5%, to the high end, 10% to 16%. And, there are sites that have been operated with cold high methane gases and resulting in high Wobbe Index numbers and sites that had gases with a high inert content and WI net in the lower range. For the complete range of 0-18% C2+ and a Wobbe Index interval of 21 MJ/m3 one EV and one SEV burner hardware is used. If WI net shall be extended by another 4 MJ/m3 at the lower range, the EV burner gas injection could be slightly adapted; the SEV fuel injection system could be used as is, both validated in single burner high pressure tests. Figure 9 GT24 and GT26 fuel gas flexibility, fleet experience range In Europe two GT26 power plants were commissioned with high and low CH4 contents in the gas, covering a total WInet range of 15 MJ/m3. The GT operation is robust with one EV/SEV burner hardware and one combustor operation concept (OPC) over the complete load range. 11
Figure 10 Two GT26 in Europe with a WI range of more than +/-15% 6 Natural Gas and Hydrogen co-firing capability It is likely that hydrogen, produced by renewable energy sources via electrolysis, is added to the natural gas in varying volumetric amounts of 5%, 15% or more. The co-firing of this highly reactive gas is influencing the burning velocity of a flame and the ignition delay time. In figure 11 relative changes to calculated laminar and turbulent flame speeds are shown for 100% Methane and a natural gas with 18% vol. C2+ (NG2) and different hydrogen contents [5]. Both, C2+ and H2 addition are accelerating the flame propagation. In ranges of maximum C2+ 18% and maximum H2 15% the effect on the turbulent flame speed is very similar. Adding 15% H2 to natural gas that contains 18% C2+ leads to the same turbulent flame speed as adding about 30% H2 to pure methane. Adding 5% H2 to either CH4 or to a natural gas with C2+ 18% has only a minor effect on the calculated flame speed. 12
Figure 11 Relative changes in burning velocity as function of C2+ and H2 contents In figure 12 calculated ignition delay times as function of the pressure are depicted for CH4 and NG2 with 5% vol. H2 and very high hydrogen contents. The reduction of the ignition delay time by C2+ 18% compared to 0% is much higher than the impact of adding 5% hydrogen. The conclusion is that for given EV and SEV burner designs that have proven to be very flexible, a moderate co-firing of hydrogen is possible. Based on these results single burner high pressure tests were conducted. Figure 12 Changes in ignition delay time as function of pressure, C2+ and H2 contents 13
The EV burner high pressure tests showed that the NOx/flame temperature characteristic and the lean stability limit is shifted to lower temperatures as the volumetric hydrogen content is increasing. Figure 13 shows that for H2 15% vol. / C2+ 6% vol. the EV NOx increase was measured to be similar than if C2+ was increased from 6% to 18% without any H2. For H2 contents of 10% the NOx impact is neglectable. For higher H2 concentrations of 20% and more the NOx emissions are significantly increased, but nevertheless the EV hardware as is could be operated with H2 contents up to 45% or a mixture of 30% H2 and 21% C2+. Figure 13 EV single burner high pressure tests Results from hydrogen/natural gas co-firing tests of the SEV burner are shown in figure 14. The heat input specific NOx formation is plotted versus the SEV flame temperature. For zero and 15% hydrogen addition the NOx emissions are shown for the nominal SEV inlet temperature, for 28% H2 and 33% C2+ addition the presented results include a reduction of the SEV inlet temperature by 50K and 100K, respectively. The NOx formation is increasing with the addition of more reactive components. The impact of C2+ addition on the SEV NOx formation is higher than when adding Hydrogen. To keep the NOx emissions at a low level the SEV inlet temperature has to be reduced. 14
Figure 14 SEV single burner high pressure tests For GT24 and GT26 applications this means that if hydrogen was added to the natural gas the EV/SEV fuel split would have to be adjusted based on the actual gas composition, considering C2+ and H2. For H2 concentrations up to 5% vol. the standard C2+ closed loop controller can be used, that measures the C2+ concentration with a fast infrared gas analyser (FIRGA) and adjusts the SEV inlet temperature accordingly, the impact of H2 on combustion in EV and SEV is small and covered by the safety margin. For H2 contents up to 15% vol. the system would have to be extended by an H2 concentration input signal, based on this an equivalent C2+ concentration (C2+equ) could be calculated (C2+equ = Sum {a C2+; b H2}). Such an operation concept is outlined in figure 15. 15
Figure 15 - Operation concept for co-firing of hydrogen and natural gas Finally, the next SEV burner generation was also tested in H2/Natural gas co-firing mode on the high pressure rig. The results are very promising and are shown in figure 16. For natural gas with a C2+ content of 6% the specific SEV NOx formation of this improved burner is only about 25% of the original one. The sensitivity to H2 addition has been significantly reduced. When adding 15% vol. Hydrogen to the natural gas the SEV NOx formation stays almost constant, increasing the H2 content to 28% results in a NOx increase, but the levels are still very low and the slight increase could be compensated by a small reduction of the SEV inlet temperature. Figure 16 Next SEV burner generation high pressure tests 16
7 Summary and Conclusions In the future the compositions of natural gases used for power generation are expected to fluctuate more than today in their reactivity and in their inert content. A wider volumetric variation in C2+ concentrations as well as wider Wobbe Index ranges have to be catered for by the gas turbine combustion systems. Alstom s GT26 and GT24 reheat engines with the EV and SEV annular combustors have proven that they can handle the expected natural gas range, because the burners are insensitive to the inert content and the EV/SEV fuel split can be adjusted via a closed loop C2+ operation concept. Co-firing of hydrogen and natural gas is also a likely scenario with H2 contents of 5% vol. that might come over the gas grid and with up to 15% vol. that might be added on site. Single burner high pressure tests have shown that EV and SEV can handle such fuels. H2 contents of 5% vol. can be covered by the standard C2+ closed loop control concept, H2 contents of 15% vol. can be handled by the EV and SEV combustors, low NOx emissions can be obtained using an adapted C2+ operation concept. The next SEV burner generation was designed for lower NOx and higher fuel flexibility. High pressure tests have proven this capability. Alstom s GT24 and GT26 with their EV and SEV combustors are ready for the growing LNG market, our fleet has shown superior fuel flexibility and co-firing of hydrogen and natural gas is feasible. 8 References [1] Increased operational flexibility from the latest GT26 (2011) upgrade Mark Stevens, Frank Hummel, Ralf Jakoby, Volker Eppler, Chr. Ruchti (Alstom) Paper presented at PowerGen Europe 2012 [2] The next generation GT26 The pioneer of operational flexibility Matthias Hiddemann, Frank Hummel, Juerg Schmidli (Alstom) Paper presented at PowerGen Europe 2011 [3] Superior fuel flexibility for today s and future market requirements Douglas Pennell, Matthias Hiddemann, Peter Flohr (Alstom) Paper presented at PowerGen Europe 2010 17
[4] The next generation KA24/GT24 from Alstom, The pioneer in operational flexibility Sasha Savic, Karin Lindvall, Tilemachos Papadopoulos, Michael Ladwig (Alstom) Paper presented at PowerGen International 2011, Las Vegas, USA [5] Ignition delay time and laminar flame speed calculations for natural gas/hydrogen blends at elevated pressures M.Brower, E.Petersen, W.Metcalfe, H.J.Curran, M.Fueri, G.Bourque, N.Aluri, F.Guethe ASME paper GT2012-69310, 2012, Copenhagen, Denmark 18
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