By the LBNL-EETD Combustion Group
A research team headed by Dr. Robert K. Cheng at the Lawrence Berkeley National Laboratory in California is working on development of a low swirl combustion (LSC) concept for fuel-flexible gas turbines of all sizes.
The low-swirl burner uses a swirler that is deceptively simple. Its annulus vane swirler resembles the conventional high-swirl burners. But the circular openings at its center allow a portion of the fuel/air mixture to by pass the swirl annulus to promote a non-recirculating divergent flow that is a stable aerodynamic configuration for the premixed flame to self-propel. Turbulence intensity provides the feedback for the flame to burn faster or slower as load changes.
The researchers are collaborating with a gas turbine manufacture to develop an ultra-low emissions LSC burner for natural gas-fired 250+ MW engines and will soon begin the development of a LSC combustor for an 80kW recuperated microturbine operating on low-Btu digester-gas produced from waste water treatment plant. Prior to these endeavors, LSC have been adapted to Solar Turbine’s Taurus 70 engines and demonstrated well below 9 ppm NO X and 10 ppm CO emissions. The LBNL/Solar team garnered a 2007 RD100 award for this achievement. For natural gas fueled microturbines, a LSC combustor for an Elliott 80 kW engine achieved low single NO X from full to 50 percent load. The researchers are also engaging in laboratory studies to adapt LSC to gas turbines that operate on syngases and very high hydrogen (90 percent+) fuels.
The classic approach described in combustion texts and gas turbine handbooks stipulates that a stable flame can only be held by a strong recirculating flow usually generated by high-swirl. But Cheng discovered in 1991 that a turbulent premixed flame which is the heart of DLN technology can self-propel in a divergent flow generated when the swirl intensity of the burner is lowered to below the critical recirculation point. This radical concept was first thought of as an academic curiosity; its origin as a research burner did little to help dispel such notion. But the use of laser diagnostics to probe and characterize the flame stabilization mechanism proved that the concept is robust. The insights gained from the laboratory studies have been the key for the transfer of the technology to industrial burners. Beginning in 2003, Maxon Corp. of Muncie Indiana commercialized two lines of ultra-low emissions low-swirl burner products ranging in sizes from 0.3 to over 100 MMBtu/hr and the development and field experiences set the stage for the adaptation to gas turbines.
Application of sophisticated laser diagnostics on laboratory flames provided much of the basic knowledge needed to scale the technology to gas turbines. Here, Cheng demonstrates that the burner stays cool to the touch because the flame is completely lifted from its body.
The basic configuration of the LSC burner has no moving parts and has the same size and similar appearance as the high-swirl burners of a typical gas turbine combustor. The key component is a swirler that allows a central portion of the fuel/air mixture to bypass the swirl vanes. This bypass effectively lowers the swirl intensity and promotes the formation of flow divergence. Propelling against the divergent flow, the turbulent premixed flame settles at a position where the flow velocity is equal and opposite to its turbulent flame speed. The flame is detached from the burner and imparts very little thermal stresses to the hardware. It remains very stable even at the very fuel lean conditions where the flame temperatures are well below the high NO X limit. To explain how the detached LSC flame remains stationary during load changes, Cheng and his colleagues developed an analytic model that describes the flame position in terms of the coupling between the flow divergence rate and the turbulent flame speed.
With increasing concern regarding the impact of energy use on global climate change, fuel-flexibility has become a critical requirement of next-generation gas turbines. The US Department of Energy’s Office of Fossil Energy is supporting the LBNL research group to extend the LSC technology to a variety of fuels including syngas and hydrogen. Theirs is one of many projects in a large effort to develop Near-Zero Emissions Integrated Gasification Combined Cycle (IGCC) Coal Power Plants. For the hydrogen fueled gas turbines in these power plants, DOE sets a very aggressive goal of less than 2 ppm NOX operation on syngas and up to 90 percent pure hydrogen at 2500-2600 degrees F turbine firing temperatures. Burning of hydrogen in a gas turbine presents significant technical and engineering challenges because of the high reactivity of hydrogen, its fast turbulent flame speed, and the propensity of the H2/air mixture to auto-ignite. Many conventional DLN approaches may not work without diluting the fuel/air mixture with inert gases or exhaust gas NOX cleanup with catalysts.
The low-swirl injector developmend for Solar Turbines T70 (bottom) is engineered to be a drop-in replacement for the SoLoNOx injector (top).
Cheng approaches these challenges from a scientific perspective and is using the analytical model to help to understand the changes in the LSC flame when switching from natural gas to hydrogen. Preliminary laboratory tests results are very encouraging and show that the basic LSC design accepts fuels from pure natural gas to over 90 percent H 2 at simulated gas turbine conditions while meeting the < 2 ppm NO X target. Another important finding is that the model predicts the changes in the LSC flames with fuel contents to confirm its validity as a basis for scaling and system adaptation. However, significant amount of R&D effort is still required to make LSC a reality in hydrogen fueled gas turbines. It will require a gas turbine manufacturer to lead the development and address the operational and safety issues. Cheng is optimistic that the basic knowledge on LSC will be an advantage to help the OEM to overcome the challenges associated with the very energetic hydrogen flames.