Andrea Ciani

Experimental investigation of a generic, fuel flexible reheat combustor at gas turbine relevant operating conditions

Fuel flexibility in stationary gas turbines (GT) is becoming increasingly important due to the use of a broader spectrum of primary energy sources, particularly H2 -rich fuels derived from the gasification of coal or biomass. GTs also must be able to operate at extremely low emission levels, which is currently achieved with lean-premixed burner designs. To investigate the performance of highly reactive fuels in the reheat combustion concept, mainly with respect to autoignition and flashback limits, a generic reheat combustor with excellent optical access has been developed. The first objective of this work was to carefully characterize the mixing section in order to derive well-defined boundary conditions for the subsequent autoignition studies. Initial autoignition results at T > 1000 K and p = 15 bar are presented for natural gas (NG) and H2 -rich fuels. No autoignition was detected for NG at the investigated operating conditions. For H2 /NG/N2 blends with a constant volumetric N2 concentration of 20% and H2 concentrations higher than 76%, autoignition in the mixing section was detected.Copyright

Development of GT24 and GT26 (Upgrades 2011) Reheat Combustors, Achieving Reduced Emissions and Increased Fuel Flexibility

The recent development of the Alstom’s sequential combustion system for the GT24 (60Hz) and GT26 (50Hz) upgrades 2011 is a perfect example of evolutionary design optimizations. Better overall performance is achieved through improved SEV burner aerodynamics and fuel injection, while keeping the main features of the sequential burner technology. In particular this results in further reduced NOx and CO emissions over widest possible load range and allows operation with fuel gases with up to 18% of higher hydrocarbons (C2+) or a low Wobbe index.An extensive validation of the new sequential burners for GT24 and GT26 has been conducted, with a wide range of validation tools. This has included high pressure sector rig testing and full-engine tests at the Alstom Power Plant Birr, Switzerland.This paper presents the development and validation process, in terms of evolutionary design modifications and successful burner scaling, of the GT24 and GT26 (upgrades 2011) reheat combustors from concept phase to engine validation.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.

Effect of Mixing Quality on NOx Emissions in Reheat Combustion of GT24 and GT26 Engines

Alstoms GT24 and GT26 engines feature a unique sequential combustion system [1, 2]. This system consists of a premixed combustor (called EV), which is followed by a high pressure turbine, a reheat combustor (called SEV) and a low pressure turbine (Figure 1). Recently improvements in NOx performance of the SEV have been demonstrated. Starting with relatively simple methods numerous design variants have been tested and down selected. Further down-selection has been done with methods of increased complexity. Overall a fast and cost effective development process has been assured. During the development process the variation coefficient and unmixedness measured and calculated for mixing only systems (CFD and water channel) has proven to be a reliable indicators for low NOx emissions for the real combustion system on atmospheric and high pressure test rigs. To demonstrate this a comparison of both quantities against NOx emissions is shown. The paper focuses on the NOx results achieved during this development and its relation to mixing quantities. Using this relation, together with a detailed understanding of the flow characteristic in the SEV burner, reductions in NOx emissions for GT24 and GT26 SEV burner and lance hardware can be reached using relatively simple methods.Copyright