Combustion is a critical cross-cutting technology for industrial processes, electric power production, material synthesis, space and water heating, and transportation. Maximizing the benefit of combustion while minimizing emissions of harmful pollutants poses large technological challenges.
An EETD team has been conducting fundamental research into the nature of combustion and its interaction with turbulence that exists naturally in all combustion devices. It has patented two new concepts that may have a significant impact on the designs of next-generation, low-emission burners. Both technologies exploit lean premixed combustion, which is fast becoming the technology of choice for reducing oxides of nitrogen (NOx) emission and increasing combustion efficiency. Initial laboratory studies have shown the two new burner technologies to be highly effective, producing emissions far below some of the most stringent clean-air standards, with no particulate emissions.
One technology, an inner-ring flame stabilizer, was developed for a National Aeronautics and Space Administration project to investigate open-flame behavior in the absence of buoyancy. The other, a low-swirl burner, was developed as a versatile laboratory tool for studying turbulence-flame interaction processes.
The Ring Stabilizer
The ring stabilizer is an extremely simple device that can be fitted inside a burner port. This ring maintains a stable premixed flame under both fuel-rich and -lean conditions. Unaffected by turbulence, the stabilizer helps produce more complete combustion, thereby lowering emissions of carbon monoxide (CO) and hydrocarbons (HC), while its ability to support ultra-lean premixed combustion lowers the production of NOx.
The Low-Swirl Burner
All conventional swirl burners are high-swirl burners. In 1991, EETD's Combustion Group developed a prototype low-swirl burner that uses small air jets to swirl the premixed flow inside the burner tube. The low-swirl technology leaves the center core flow undisturbed and produces a divergent flow above the burner exit. Flow divergence provides a highly effective means for stabilizing lean premixed combustion because it enables the flame to settle where the local velocity equals the flame speed. A vane swirler (with angled guide vanes) has been developed that eliminates the need for air jets and is much more suitable for commercial use because of its simplicity.
Low-swirl burners with vane swirlers offer the same ability to support ultra-lean premixed combustion as their air-jet predecessors. A unique feature of the low-swirl burner is that the flame is completely detached from the burner such that the burner receives almost no thermal stresses. In the photograph, a plastic low-swirl burner is firing at about 16 kW.
Potential for Commercialization
The flame stabilizer ring insert and the low-swirl burner are in various stages of testing in laboratory studies to establish their potential for commercial use. When a multiport ring burner was fitted to a domestic forced-air furnace, the resulting emissions were well below air-quality standards for NOx and CO, and the system efficiency was increased by 5%. A low-swirl burner tested in a 15-kW spa heater produced the same results.
The low-swirl burner has been successfully scaled to furnace applications. In tests conducted in a furnace simulator at the University of California (Irvine) Combustion Laboratory, a 10-cm burner was operated with an output of up to 600 kW. Emissions were low: at 600 kW—typical burners operate from 10kW to 100 MW—the burner emitted only 12 parts per million (ppm) of NOx, 20 ppm of CO, and <1 ppm of HC. The test conditions will be extended beyond 1.5 MW. Recently, the potential of using the low-swirl flame stabilization method in gas turbine combustors was confirmed at the Solar Turbines test facility in San Diego. Successful firing of a "low-swirl injector" prototype under typical gas turbine conditions shows that this new combustion concept will also be applicable to power generation systems.
These combustion technologies offer direct and simple means to achieve ultra-low emissions. In addition to their low emissions and high efficiency features, the main economic advantage will be their potential for significant first-cost savings compared to current low-NOx burners that use complicated processes such as flue gas recirculation, selective catalytic reduction, or sophisticated burner materials. Add to this simple flow controls and the ability to support stable, lean premixed flames free of loud rumble and instabilities. Operating with high combustion efficiency, a high turndown of at least 15:1 (turndown is the ratio of the highest operating power to the lowest operating power), and low pressure drop, the burners can be scaled to different capacities to meet the varied needs of commercial and industrial production.
For more information, contact:
- Robert Cheng
- (510) 486-5438; fax (510) 486-7303
This research is supported by the U.S. Department of Energy, Laboratory Technology Research Program.