The FLOX® gas turbine burner concept is a promising alternative to conventional swirl burners for gas turbine combustion chambers.
This combustion concept is characterised by the injection of air-fuel mixtures into the combustion chamber in the form of jets with high flow velocities. The arrangement of the jets results in a pronounced recirculation zobe in the combustion chamber, which means that fuel, air and recirculated burnt gas are intensively mixed before combustion reactions start. This prevents local temperature peaks and strong nitrogen oxide formation in the flame.
When the concept was originally used in industrial burners, the flames were barely visible, hence the name FLOX (flameless oxidation). However, with the high power densities and flame temperatures of modern gas turbines, the combustion is not flameless in the original sense, but the jet concept has proven to be very successful here too.
Low emissions and high load flexibility
The concept of jet-stabilised combustion with recirculation has been further developed at the institute in cooperation with industrial partners and optimised for gas turbines of all sizes. Burners of this design are characterised not only by low pollutant emissions, but also by great fuel flexibility and high stability. These properties are in demand in the energy system of the future.
In jet-stabilised combustion chamber systems, the air-fuel mixture is fed into the combustion chamber through nozzles, which are usually arranged on a ring, without swirl, partially premixed and with a high axial momentum. This creates a strong recirculation area on the burner axis. This recirculation transports the hot exhaust gases from the area behind the flame zone back to the combustion chamber inlet.
Mixing of the unburnt air-fuel mixture with the recirculating exhaust gases takes place in the turbulent shear layers of the jet flow. The balanced recirculation rates and the associated transport of heat and radicals result in effective flame stabilisation. At the same time, the dilution of the fresh gas with burnt gas reduces the reaction rates. A relatively homogeneous temperature field is achieved in the combustion chamber, in which the temperature peaks only slightly exceed the adiabatic flame temperature of the global mixture.
This homogeneous temperature distribution is decisive for the comparatively low emissions of this type of burners. NOx emissions in particular are therefore very low, as the formation of nitrogen oxides strongly increases with the combustion temperature. The number and dimensions as well as the arrangement of the air and fuel nozzles are of great importance for the design of such a burner system. The properties of the burner system are also significantly influenced by the geometric design of the nozzles, their contours and possible mixing devices.
In addition to the possibility of varying the thermal power of the burner system via the air and fuel flow rates through the nozzles, a configuration with a number of individual nozzles also offers the possibility of varying the power over a wide range by switching off individual nozzles or nozzle groups. This enables the desired load flexibility. The optimisation of the aforementioned variable parameters is an active field of research at the institute, which contributes to the understanding of combustion phenomena and to industrial development.
Jet-stabilized burner concept
Schematic illustration of the jet-stabilised burner concept (top sketch, bottom numerical simulation of the flow field).
Another advantage of jet-stabilised combustion chamber systems is that they are less susceptible to combustion instabilities. This makes them ideal for use with pure hydrogen and fuel mixtures in which the hydrogen content accounts for more than half. The risk of flashback is minimised with this combustion chamber concept due to the very high inlet flow velocities. The fuel flexibility enables operation with variable fuel compositions or a wide variety of gaseous mixtures and liquid fuels.
Video: A study in operating limits for the combustion of hydrogen
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Video: A study in operating limits for the combustion of hydrogen
In order to understand the complex flow and combustion processes in detail and to further optimise the design, the institute combines numerical simulation techniques with experiments on the institute's own atmospheric and high-pressure test benches (link) using optical and laser-based measurement technology.
If we assume that future combustion systems will increasingly - depending on availability - be operated with hydrogen in a climate-neutral manner, jet-stabilised burners offer an ideal option in the transitional period when hydrogen is not yet available everywhere to the desired extent. Due to the proven fuel flexibility, this applies equally to sustainably produced liquid fuels and the conceivable application in aviation.
Flame of a jet-stabilised combustion chamber system in the HBK-S high-pressure combustion chamber test rig
The HBK-S high-pressure combustion chamber test rig offers the possibility of analysing combustion chamber systems under typical gas turbine conditions.
