Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

The Reheat Concept: The Proven Pathway to Ultralow Emissions and High Efficiency and Flexibility

[+] Author and Article Information
Felix Güthe1

 Alstom, Brown-Boveri-Strasse 7, CH-5400 Baden, Switzerlandfelix.guethe@power.alstom.com

Jaan Hellat, Peter Flohr

 Alstom, Brown-Boveri-Strasse 7, CH-5400 Baden, Switzerland


Corresponding author.

J. Eng. Gas Turbines Power 131(2), 021503 (Dec 24, 2008) (7 pages) doi:10.1115/1.2836613 History: Received May 30, 2007; Revised October 08, 2007; Published December 24, 2008

Reheat combustion has been proven now in over 80units to be a robust and highly flexible gas turbine concept for power generation. This paper covers three key topics to explain the intrinsic advantage of reheat combustion to achieve ultralow emission levels. First, the fundamental kinetic and thermodynamic emission advantage of reheat combustion is discussed, analyzing in detail the emission levels of the first and second combustor stages, optimal firing temperatures for minimal emission levels, as well as benchmarking against single-stage combustion concepts. Second, the generic operational and fuel flexibility of the reheat system is emphasized, which is based on the presence of two fundamentally different flame stabilization mechanisms, namely, flame propagation in the first combustor stage and autoignition in the second combustor stage. This is shown using simple reasoning on generic kinetic models. Finally, the present fleet status is reported by highlighting the latest combustor hardware upgrade and its emission performance.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

GT24/GT26 sequential combustion system

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Figure 2

Principle of the reheat cycle compared to a standard GT cycle

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Figure 3

NOx emissions on log scale (normalized, noncalibrated) for a single combustor (EV), the SEV combustor, and a reheat combustion system. The factors calibrating to 15% O2 depend on Tflame and are shown in green and refer to the right hand scale.

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Figure 4

O-radical concentration profiles for EV and SEV flames (taken from PREMIX and PFR calculations) for similar Tflame. The arbitrary time scale extends over 4ms. The shaded area in the integral ∫t[O]dt determines the most important NOx formation routes and is approximately three times higher in the EV than in the SEV.

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Figure 5

Schematic diagram explaining the physics and chemistry of the EV flame (left) and the SEV flame (right) for natural gas. The intermediates indicating preflame zones (CH2O) are displayed in the upper graphs. The shaded areas indicate the most relevant regions of reactivity. The different time axes were given in reverence to the typical mixing times.

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Figure 6

EV and SEV reactivities as defined by Eqs. 3,4 for varied fuel contents and constant Tinlet (compare with Fig. 8). The shaded area defines an operational range.

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Figure 7

Probability of reactivity derived from a distribution of fuel for EV and SEV combustors at fixed inlet conditions. The histograms show the probability for a given reactivity, while the bubbles are calculated reactivities for a given fuel-mass ratio.

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Figure 8

EV reactivity for varied Tflame̱EV (upper graph) for two different fuels on logarithmic scale. In the lower graph, the reactivity of the SEV for varied inlet conditions (TinleṯSEV) is presented, referenced to a standard inlet temperature Tiṉref. A reactivity increase due to the fuel is compensated by reduced TinleṯSEV. Note that the RRSEV is plotted versus TflameSEV in Fig. 6.

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Figure 9

GT26/GT24 operation concept as a function of fuel reactivity, here given by the content of higher hydrocarbon content (C2+) in the fuel

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Figure 10

Schematic of the operating concept of the GT24/GT26 with load variation done through the SEV combustor fuel flow

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Figure 11

Staged EV-burner principle. Details of its development are described in Ref. 7.

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Figure 12

Emission levels achieved with the staged EV burner on logarithmic scale



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