Plasma assisted combustion for more eco-friendly fuels
A promising technology in development at GE Global Research is the application of plasma-enhanced flame stabilization for gas turbine combustors. It has been known for several decades that combustion performance, specifically flame stabilization, can be enhanced through the application of electric discharges in the flame or near the base of the flame. Plasma discharges can enhance combustion in a couple of ways. Thermal or “hot” plasmas such as a spark or arc are commonly used for ignition in many engine applications. The primary benefit of these plasmas is localized deposition of heat, and the very high gas temperatures and radicals generated initiate the ignition process, causing a flame front to propagate into the fuel-air mixture.On the other hand, non-equilibrium or “cold” plasmas such as glow and dielectric-barrier discharges are more commonly used for chemical excitation and transformation. The primary benefit of these plasmas is volumetric generation of energetically excited species, species fragments, reactive radicals, and ions. If the conditions are appropriate, these reactive species can initiate chain reactions and give the combustion reactions an effective “head-start” at the base of the flame – helping to stabilize the flame.
Whereas “hot” plasmas are characterized by low voltage (after initial breakdown), high current and high power “cold” plasmas are characterized by high voltage, low current and lower power. Cold plasmas are inherently more energy-efficient in the generation of reactive species, but at the same time are somewhat limited in their ability to transfer significant energy into the flow.
Much research has been published regarding plasma-enhancement of atmospheric-pressure flames and the fundamentals of plasma-combustion chemistry; however, essentially no research has been published regarding the application of plasma enhancement at the conditions of gas turbine combustors. The elevated pressure conditions in a gas turbine make it a challenging environment in which to generate electric discharges, particularly “cold” plasmas, due to the high voltages required. In addition, at such densities the discharges tend to always contract into filamentary discharges, and it becomes more difficult to generate plasma discharges that act upon a significant volume of the gas.
Recent research and development at GE Global Research in the area of plasma-enhancement of combustion has demonstrated the capability to ignite and stabilize a slightly modified production gas turbine fuel nozzle with integrated plasma. At temperature and pressure conditions relevant to low-load operation, the plasma was capable of extending the lean flammability limits of a full-scale gas turbine fuel nozzle by up to 300F in flame temperature, or roughly 17% lower fuel flow, with a plasma energy equivalent to ~0.1% of the thermal power of the combustor. The video shows this capability, where a lifted flame is stabilized under this condition after the plasma is switched on.
Video Caption: The first few seconds are the unstable flame without the plasma and the last few seconds are with the plasma “on”, showing a very robust flame. Plasma stabilizes combustion at 17% reduced fuel input.
Plasma-chemical kinetic mechanisms and reactor models have been assembled in order to study the plasma-combustion enhancement mechanism at high pressures and temperatures, and these models can also be utilized to examine the chemical effect of plasma discharge parameters on the stability limits of lean premixed flames. The team has also been active in the development of high voltage, high frequency power supplies required to generate the plasma at the elevated pressures of a gas turbine combustor.
Our research work on plasma-assisted combustion will continue under a new DOE-funded program, “Fuel Flexible Combustion Systems for High-Efficiency Utilization of Opportunity Fuels in Gas Turbines.”
During this development program, the team will research plasma-assisted fuel nozzle technology that will enable gas turbine operation using fuel streams with ultra-low heating values – such as very weak natural gas, highly diluted industrial process gases, or gasified waste streams – which are beyond the capability range of current product offerings.
Am I reading this right? Are you saying that the plasma technique can reduce fuel consumption by 17% for the same power output?