Flue Gas Recirculation in Gas Turbine: Investigation of Combustion Reactivity and NOX Emission

Abstract

Flue gas recirculation (FGR) is a promising technology for the optimization of post-combustion CO2 capture in natural gas combined cycle (NGCC) plants. In this work, the impact of FGR on lean gas turbine premix combustion is predicted by analytical and numerical investigations as well as comparison to experiments. In particular the impact of vitiated air condition and moderate increase of CO2 concentration into combustion reactivity and NOx emission is studied. The influence of inlet pressure, temperature and recirculated NOx are taken as parameters of this study. Two different kinetic schemes are used to predict the impact that FGR has on the combustion process: the GRI3.0 and the RDO6_NO, which is a newly compiled mechanism from the DLR Stuttgart. The effects of the FGR on the NOx emissions are predicted using a chemical reactor network including unmixedness as presumed probability density function (PDF) to account for real effects. The magnitude and ratio of prompt to post-flame thermal NOx changes with the FGR-ratio producing less post flame NOx at reduced O2 content. For technical mixtures (i. e. an industrial fuel injector), NOx emission can be expected to be lower with the vitiation of the oxidizer. This is due to several effects: at low O2 concentration, the highest possible adiabatic flame temperatures for stoichiometric conditions decreases resulting in lower NOx when averaged over all mixing fractions. Further effects result from lower post flame NOx production and the role of “reburn” chemistry, actually reducing NOx (recirculated from the exhaust), which might become relevant for the high recirculation ratios, where parts of the flame would operate at rich stoichiometry at given unmixedness. Therefore in general for each combustor technical mixing could decrease NOx with respect to perfect mixing at high FGR-ratio assuming the engine can still be operated. Although the findings are quite general for gas turbines the advantage that reheat engines have in terms of operation are highlighted. For reheat engines this can be understood as an extension of the “reheat concept” and used as the next step in the goal to achieve minimal emissions at increasing power. In addition, NOx emission obtained in FGR combustion reduces even further when the engine pressure ratio increases, making the concept particularly well suited for reheat engines.

Flue Gas Recirculation of the Alstom Sequential Gas Turbine Combustor Tested at High Pressure

“…This indicates the potential to increase the operational flexibility by operating at locally higher firing temperatures. This observation is according to expectations and nicely confirms the experimental and theoretical findings of several groups [6,8,9,13]. With a model as described in [8] the NO x emissions can also be described semi-quantitatively with good accuracy.…”

“…The NO x emissions are plotted as emission index EMI_NO x [g NO 2 per kg fuel] from the net SEV NO x values and SEV fuel as discussed in [8]. This enables the evaluation of the SEV as a combustor component and allows accurate predictions of the engine behavior by using an appropriate model.…”

“…This work investigates to what extent the usage of FGR is limiting the operational flexibility with respect to non FGR operation. The impact of FGR on a GT power plant and especially its combustion behavior had been described before by several authors [3,4] including several manufacturers [5,6,7,8] In general the effect on most components including the power outputs and efficiency is not very large. The effect on combustion is however significant with an advantage for lower NO x emissions and more challenging conditions for blow out stability and CO emissions [8,9].…”

“…An example of a reheat system is the Alstom reheat engine as is sketched for an FGR application in Figure 1 (from [8]): After the heat recovery steam generator (HRSG) part of the flue gas is recirculated and fed back into the GT inlet after passing through a cooler. The gas at the GT-inlet is a mixture of air and flue gas with reduced O 2 and increased CO 2 -content, which is after the compression mixed and burned with fuel in the combustor resulting also in altered exhaust gas composition with respect to the non-FGR engine.…”

Gas turbine operation and combustion performance issues

“…This section will discuss the natural gas fi red gas turbine cycle with EGR. This concept has been under evaluation for some years, with a focus on thermodynamical cycle studies and the resulting impact on combustion systems (ElKady et al ., 2008;Guethe et al ., 2009). Pre-combustion capture cycles, which are based on hydrogen rich fuels -similar to fuel from IGCC cycles with carbon capture -are not covered in this chapter.…”