Chemical Kinetic Models for Enhancing Gas Turbine Flexibility: Model Validation and Application

Abstract

In recent years, market trends towards higher power generation flexibility are driving gas turbine requirements of operation at stable conditions and below environmental emission guarantees over a wide range of operating conditions, such as load, and for changing fuels. In order to achieve these targets, engine components and operation concept need to be optimized to minimise emissions (e.g. CO, NOx) and combustion instabilities, as well as to maximize component lifetime. Therefore the combination of field experience, experimental studies and theoretical modelling of flames with state of the art tools play a key role in enabling the development of such solutions. For many applications the relative changes of reactivity due to changes in operation conditions are important thus in this report a few examples are shown, where chemical kinetics simulations are used to determine the reactivity and to predict engine behaviour. The predicted trends are validated by correlating them to validation data from high pressure test rigs and real gas turbine operational data. With this approach the full operational range from highest reactivity (flashback) to lowest reactivity (blow out or CO emission increase) are covered. The study is focused on the sequential combustor (SEV) of reheat engines and addresses both the safety margins with respect to highly reactive fuels and achievable load flexibility with respect to part load CO emissions. The analysis shows that it is necessary to utilize updated kinetic mechanisms since older schemes have proved to be inaccurate. A version of the mechanism developed at NUI Galway in cooperation with Alstom and Texas A&M was used and the results are encouraging, since they are well in line with experimental test data and can be matched to GT conditions to determine, predict, and optimize their operational range. This example demonstrates nicely how a development over several years starting from fundamental basic research over experimental validation finally delivers a product for power plants. This report therefore validates the kinetic model in combination with the approach to use modelling for guidance of the GT development and extending it fuel capabilities. The GT24 / GT26 can not only be operated with H2 containing fuels, but also at very low part load conditions and with the integration of H2 from electrolysis (∼power to gas ∼PTG) the turndown capability can even be further improved. In this way the energy converted at low electricity prices can be stored and utilised at later times when it is advantageous to run the GT at lower loads increasing the overall flexibility. This development is well suited to integrate renewable energy at highly fluctuating availability and price to the energy provisioning by co-firing with conventional fuels.

Long-term carbon monoxide emission behavior of heavy-duty gas turbines: An approach for model-based monitoring and diagnostics

“…The dimensionless time reduced is a measure for the burnout progression, ranging from reduced ¼ 0 (no burnout) to reduced ¼inf, while the first occurrence of complete burnout (CO concentration sufficiently close to its equilibrium) is reached at reduced ¼ 1 by definition. 2 The oxidation of CO leads thus to a decline of the initial value CO peak of equation ( 6) to its equilibrium CO equil by oxidation of ÁCO peak ¼ CO peak À CO equil with (reduced) time and slope…”

“…More recently, Therkorn et al 17 assessed burner switch-off concepts for the auto-ignited sequential combustor in the GT24/GT26 gas turbine and successfully modeled its influence on CO emissions with a semiempirical model. Enriching this approach with accurate kinetics in a simple reactor model, Gu¨the et al 2 demonstrated the capability to capture the effect of fuel reactivity (i.e. hydrocarbon and hydrogen content) at lab and engine scale.…”

“…Due to the common reduction of firing temperatures with decreasing load, carbon monoxide (CO) emissions limit the achievable engine turndown. 2 Aging-induced CO emissions may further restrict the market access of a gas turbine, as the CO margin of a given combustion technology to the strict legal limits would be reduced during low load. In order to provide and extend low-load gas turbine operation, reliable and comprehensive gas turbine modeling approaches offer a possibility to understand gas turbine behavior, reduce maintenance costs, 3 and enable longterm emission compliance by optimized operation and monitoring.…”

“…Bainier et al 8 recently reported their progress on the development of such a predictive emission monitoring system, which however requires recalibration every quarter of the year. In contrast, physical models such as reactor networks 2,9 and CFD simulations [10][11][12] are able to predict spatially resolved CO formation and thus allow for detailed physical investigation of combustor design and operation aspects. The accuracy and success of these methods are highly dependent on the existence and quality of boundary conditions, such as oxidant and fuel composition and flow, temperature, pressure, and detailed geometry.…”