Influence of bio-gas addition on the flame stability of lean premixed gas-turbine combustors (EWO.5874)

Research
The use of biomass related fuels in energy production is considered as CO2 neutral and therefore not contributing to the green-house effect. Therefore it is financially stimulated by governments to use "green electricity" and the demand is growing faster than the capacity. An alternative way to increase the capacity is to synthesize the bio-fuels to produce bio-gas, mixtures including H2 and CO as combustibles, and to apply it in the existing natural gas fired gas-turbine generator infrastructure. Therefore bio-gas has to be added to natural gas in increasing amounts.

Addition of bio-gas to natural gas may influence the stability and safety of the generating units considerably. This is due to the change in combustion properties, especially the turbulent burning velocity. Even for arbitrary mixtures the laminar burning velocities are not well known and the influence of turbulence can not be very accurately described yet. Therefore laminar burning velocities will be determined for the mixtures considered and improved turbulent combustion models for circumstances in lean-burn gas-turbines will be developed.

Three new techniques will be incorporated. The chemistry will be reduced by FGM (flamelet generated manifolds) without loss of accuracy. The turbulence modelling will be improved by taking into account the flame stretch, flame curvature and gradient and counter-gradient diffusion with CFM (coherent flamelet model). Furthermore the influence of turbulent fluctuations will be diminished by explicitly taking into account the large fluctuating scales in LES (large eddy simulation). These techniques will be implemented in a commercial CFD-code and an assessment will be made of the combustion phenomena and the stabilization in an actual gas-turbine.

Utilization
The present project will be carried out in close collaboration with Electrabel Nederland. Electrabel is one of the first electricity producers in The Netherlands with plans to use bio-gas in their General Electric gas-turbines in the Eemscentrale, one of their electricity plants. However, the addition of bio-gas might cause stability problems in the gas-turbines. Electrabel is prepared to carry out some very expensive test experiments with bio-gas additions, but not before they are firmly confident that there is no flash-back or blow-off danger. Therefore, recently, Electrabel started a joint project with NRG to assess the possibilities of biogas addition.

First computations were carried out at NRG with a mixed is burnt model for Norwegian gas. Since the burning velocity, which should be of key importance, is no parameter in this model, NRG switched to a simplified G-equation model. First computational results for the burning velocities and flamelet databases needed for these computations are delivered by TU/e. Besides the continuing support of TU/e to these calculations, with new data for burning velocities for bio-gas addition, new flamelet libraries for premixed conditions and post-processing strategies to determine NOx emissions, TU/e will develop more efficient and accurate strategies for the calculations.

A year after the initiation of the project an educated assessment can be made of the improvements made with the different levels of modelling and simulation strategies. At that point a symposium will be organized, at which these results will be reported. Besides academic fellows, electricity producers and gas-turbine manufacturers will be invited to attract the attention to the new possibilities.

Other fields concerned with turbulent premixed combustion, like direct-injection engines, would benefit from the improved modelling as well.

More information can be found on the website of the research group: http://www.combustion.tue.nl

Three companies are involved in this project.