Khawar Syed

Development of the Sequential Combustion System for the ABB GT24/GT26 Gas Turbine Family

The 60Hz, 165MW gas turbine GT24 and the 50Hz, 240MW gas turbine GT26 are the first two members of ABB’s Sequential Combustion System gas turbine family. These turbines are designed to offer increased output at up to 4% efficiency advantage over today’s engines. Whereas the first combustor is based on the proven EV-combustor technology, an extensive research and development program has been carried out in developing the lean premixed, self-igniting second combustor.This paper reports the basic research work concerning the lean premixing burners with self-ignition. The development of the burner and the combustor was based on wind tunnel and water channel experiments, CFD-calculations and combustion tests at atmospheric and high pressure.Moreover an innovative cooling technology was developed to fullfill all conditions of the self-igniting premix combustor requiring minimal cooling air consumption. Special attention was paid both to a low sensitivity of the cooling effectiveness to variations of the imposed boundary conditions and to a robust hardware construction.Tests of real engine parts at real engine conditions will be demonstrated in detail. Finally the paper demonstrates the potential of the sequential combustion system to reach single digit NOx levels by unveiling the results of the extensive testing program.Copyright

FLUE GAS RECIRCULATION OF THE ALSTOM SEQUENTIAL GAS TURBINE COMBUSTOR TESTED AT HIGH PRESSURE

Concerning the efforts in reducing the impact of fossil fuel combustion on climate change for power production utilizing gas turbine engines Flue Gas Recirculation (FGR) in combination with post combustion carbon capture and storage (CCS) is one promising approach. In this technique part of the flue gas is recirculated and introduced back into the compressor inlet reducing the flue gas flow (to the CCS) and increasing CO2 concentrations. Therefore FGR has a direct impact on the efficiency and size of the CO2 capture plant, with significant impact on the total cost. However, operating a GT under depleted O2 and increased CO2 conditions extends the range of normal combustor experience into a new regime. High pressure combustion tests were performed on a full scale single burner reheat combustor high-pressure test rig. The impact of FGR on NOx and CO emissions is analyzed and discussed in this paper. While NOx emissions are reduced by FGR, CO emissions increase due to decreasing O2 content although the SEV reheat combustor could be operated without problem over a wide range of operating conditions and FGR. A mechanism uncommon for GTs is identified whereby CO emissions increase at very high FGR ratios as stoichiometric conditions are approached. The feasibility to operate Alstom’s reheat engine (GT24/GT26) under FGR conditions up to high FGR ratios is demonstrated. FGR can be seen as continuation of the sequential combustion system which already uses a combustor operating in vitiated air conditions. Particularly promising is the increased flexibility of the sequential combustion system allowing to address the limiting factors for FGR operation (stability and CO emissions) through separated combustion chambers. Nomenclature FGR Flue gas recirculation FGR-ratio exhaust ion recirculat

Sensitivity Analysis of Auto-Ignition Simulation at Gas Turbine Operating Conditions

The demand to reduce CO2 emissions favors the use of alternative hydrogen-rich fuels, which can stem from precombustion carbon capture or power-to-gas technologies. These fuels are characterized by a higher reactivity and reduced ignition delay time compared to natural gas. Therefore, current combustor designs need to be adapted to the new requirements. Numerical modeling greatly assists the further development of such systems. The present study aims to determine how far a sophisticated computational fluid dynamics (CFD) combustion method is able to predict auto-ignition at real engine conditions. Scale-resolving computations of auto-ignition were performed at elevated pressure (15 bar) and intermediate temperatures (>1000 K). The conditions are similar to those occurring in premixing ducts of reheat combustors. A nitrogen-diluted hydrogen jet is injected perpendicularly into a stream of hot vitiated air. The scale-adaptive simulation (SAS) method as proposed by Menter and coworkers has been applied. The chemistry is captured by direct inclusion of detailed kinetics. Subgrid fluctuations of temperature and species are considered by an assumed probability density function (PDF) approach. The results are compared with appropriate experimental reference data. The focus of the present work is set on the identification of the major sources of uncertainty in the simulation of auto-ignition. Despite the very challenging operating conditions, satisfactory agreements could be obtained within experimental uncertainties.

Co-Firing of Hydrogen and Natural Gases in Lean Premixed Conventional and Reheat Burners (Alstom GT26)

Addition of hydrogen (H2), produced from excess renewable electricity, to natural gas has become a new fuel type of interest for gas turbines. The addition of hydrogen extends the existing requirements to widen the fuel flexibility of gas turbine combustion systems to accommodate natural gases of varying content of higher hydrocarbons (C2+). The present paper examines the performance of the EV and SEV burners used in the sequential combustion system of Alstom’s reheat engines, which are fired with natural gas containing varying amounts of hydrogen and higher hydrocarbons. The performance is evaluated by means of single burner high pressure testing at full scale and at engine-relevant conditions.The fuel blends studied introduce variations in Wobbe index and reactivity. The latter influences, for example, laminar and turbulent burning velocities, which are significant parameters for conventional lean premixed burners such as the EV, and auto-ignition delay times, which is a significant parameter for reheat burners, such as the SEV. An increase in fuel reactivity can lead to increased NOx emissions, flashback sensitivity and flame dynamics. The impact of the fuel blends and operating parameters, such as flame temperature, on the combustion performance is studied. Results indicate that variation of flame temperature of the first burner is an effective parameter to maintain low NOx emissions as well as offsetting the impact of fuel reactivity on the auto-ignition delay time of the downstream reheat burner. The relative impact of hydrogen and higher hydrocarbons is in agreement with results from simple reactor and 1D flame analyses. The changes in combustion behaviour can be compensated by a slightly extended operation concept of the engine following the guidelines of the existing C2+ operation concept and will lead to a widened, safe operational range of Alstom reheat engines with respect to fuel flexibility without hardware modifications.Copyright

Numerical Simulations of Turbulent Mixing and Autoignition of Hydrogen Fuel at Reheat Combustor Operating Conditions

Turbulent mixing and autoignition of H2 -rich fuels at relevant reheat combustor operating conditions are investigated in the present numerical study. The flow configuration under consideration is a fuel jet perpendicularly injected into a crossflow of hot flue gas (T > 1000 K, p = 15 bar). Based on the results of the experimental study for the same flow configuration and operating conditions two different fuel blends are chosen for the numerical simulations. The first fuel blend is a H2 /natural gas/N2 mixture at which no autoignition events were observed in the experiments. The second fuel blend is a H2 /N2 mixture at which autoignition in the mixing section occurred. First, the non-reacting flow simulations are performed for the H2 /natural gas/N2 mixture in order to compare the accuracy of different turbulence modeling methods. Here the steady-state Reynolds-averaged Navier-Stokes (RANS) as well as the unsteady scale-adaptive simulation (SAS) turbulence modeling methods are applied. The velocity fields obtained in both simulations are directly validated against experimental data. The SAS method shows better agreement with the experimental results. In the second part of the present work the autoignition of the H2 /N2 mixture is numerically studied using the 9-species 21-steps reaction mechanism of O’Conaire et al. [1]. As in the reference experiments, autoignition can be observed in the simulations. Influences of the turbulence modeling as well as of the hot flue gas temperature are investigated. The onset and the propagation of the ignition kernels are studied based on the SAS modeling results. The obtained numerical results are discussed and compared with data from experimental autoignition studies.Copyright

HYDROGEN COMBUSTION WITHIN A GAS TURBINE REHEAT COMBUSTOR

This paper describes a novel lean premixed reheat burner technology suitable for Hydrogen-rich fuels. The inlet temperature for such a combustor is very high and reaction of the fuel/oxidant mixture is initiated through auto-ignition, the delay time for which reduces significantly for Hydrogen-rich fuels in comparison to natural gases. Therefore the residence time available for premixing within the burner is reduced. The new reheat burner concept has been optimized to allow rapid fuel/oxidant mixing, to have a high flashback margin and to limit the pressure drop penalty. The performance of the burner is described, initially in terms of its fluid dynamic properties and then its combustion characteristics. The latter are based upon full-scale highpressure tests, where results are shown for two variants of the concept, one with a pressure drop comparable to today’s natural gas burners, and the other with a two-fold increase in pressure drop. Both burners indicated that Low NOx emissions, comparable to today’s natural gas burners, were feasible at reheat engine conditions (ca. 20 Bars and ca. 1000C inlet temperature). The higher pressure drop variant allowed a wider operating window. However the achievement of the lower pressure drop burner shows that the targeted Hydrogen-rich fuel (70/30 H2/N2 by volume) can be used within a reheat combustor wit hout any penalty on gas turbine performance.