Ultraclean Low-swirl Combustion Will Help Clear the Air
A unique type of clean-burning combustion technology called ultra-clean low-swirl combustion (UCLSC), developed by Lawrence Berkeley National Laboratory (Berkeley Lab) researcher Robert Cheng, is entering the marketplace after years of research and development. UCLSC emits 10 to 100 times less nitrogen oxide (NOx) than conventional burners do. This new technology will reduce air pollution and allow industry to meet clean air requirements easily and economically. Because conventional combustion theory does not account for the features of the new technology, UCLSC has prompted advances in theoretical studies to explain its underlying principles.
Nitrogen oxides generate photochemical smog and haze, so controlling these gases is a high priority for air quality management districts throughout the United States. Although vehicle emissions are responsible for a large part of NOx emissions, stationary sources that burn natural gas also emit a significant amount of NOx; UCLSC's contribution to reducing emissions from these sources will greatly help the fight against photochemical smog.
U.S. homes, businesses, industry, and power generators burned nearly 23 trillion cubic feet of natural gas in 2000, generating 22.6 quadrillion British Thermal Units (Btus) of energy and emitting nearly 22 million metric tonnes of NOx. Natural gas burners used in boilers and furnaces are the primary energy source for manufacturing, industrial processing and space heating, and commercial and residential space heating and hot water. Natural-gas-burning turbines are also increasingly being used to produce electricity.
Combustion expert Cheng, a scientist in Berkeley Lab's Environmental Energy Technologies Division, has been studying UCLSC for more than 10 years. The research originated in a U.S. Department of Energy (DOE) Office of Science experimental program to investigate the intricate coupling between fluid mechanical turbulence and combustion heat release; that work has led to numerous practical applications.
UCLSC not only burns cleanly, but it is also as cheap as (or cheaper than) many existing burners. UCLSC could be scaled for use in devices as small as home furnaces and boilers and as large as gas-fired power generators. DOE's Office of Energy Efficiency and Renewable Energy is currently funding research to adapt this technology to heating and power generation.
"Currently, natural gas industrial equipment emits on the order of 100 parts per million (ppm) of NOx. Ultra-clean low-swirl combustion for industrial processes can reduce the average mission to well below 10 ppm NOx," Cheng says. "In the U.S. alone, this would remove 340,000 tons per year of NOx from our atmosphere." That is equivalent to the NOx emissions of forty-five 1,000-MW, coal-fired power plants. Adapting this technology to residential and commercial applications as well as to power generation could remove an additional 400,000 tons of NOx per year.
The ultra-clean low-swirl burner (UCLSB) (Figure 1) employs a novel combustion method that uses lean premixed flames; air and fuel are combined in ratios that burn completely when they reach the flame. The burner's operating principle, overall flame behavior, and turbulent flow features have been studied using laser methods.
"The most distinct characteristic of the burner is a detached flame that is lifted above the burner," Cheng says. "This feature defies a long-held notion that a lifted flame is inherently unstable. Until the discovery of low-swirl combustion, flame detachment was considered a prelude to combustion instability and flame-out. Therefore, a burner that generated a lifted flame was deemed unsuitable for commercial use." Through laboratory experiments, Cheng has proven that low-swirl combustion operates on a new and entirely different principle from the one that underlies conventional burners. "We are conducting research to develop a broader theoretical foundation to explain this combustion," says Cheng.
According to Cheng, the UCLSB provides the most stable platform for lean premixed turbulent flames to propagate in their natural state without interference resulting from flame interaction with burner components. Because the flame does not touch the UCLSB, no energy is lost to the burner, so the technology converts energy very efficiently.
Marketplace Attention Grows
Depending on the application in which it is used, low-swirl combustion emits 10 to 100 times less NOx than is produced by conventional combustion systems. Maxon Corporation (Muncie, Indiana) has licensed the UCLSC technology for industrial process heaters, which are used in many industrial baking and drying ovens. These ovens consume more than 9.8 quadrillion BTUs of natural gas per year in the United States.
Maxon's ultra-low NOx burner, which will come out later this year, will meet stringent air quality regulations requiring NOx emissions of less than nine parts per million at three percent oxygen. (See Figure 3.) Maxon is also developing larger-capacity low-swirl burners for other industrial heating processes.
Another manufacturer recently evaluated a UCLSB with a diameter of five centimeters for domestic use in a 15-kilowatt spa heater. The results show that the UCLSB reduces NOx emissions from 150 ppm to less than 12 ppm, with the same overall thermal efficiency as standard technology.
Cheng is working with several boiler manufacturers to engineer and adapt UCLSBs for their products. They tested a UCLSB with a 12.7-centimeter diameter in six different boiler configurations and determined that UCLSBs could be used in industrial hot water and steam generation (Figure 4).
With industrial partner Solar Turbines of San Diego, California, Cheng recently demonstrated the technology's potential in gas turbines, which are increasingly being used to generate electricity for the power grid. Cheng and his partners successfully fired a "low-swirl injector" (LSI) prototype — a version of UCLSB designed for power turbines — and showed that it can match the emissions of the much more expensive and less durable catalytic combustors that are currently considered the best available technology. The LSI reduced NOx emissions by a factor of five to 10 and emitted less than five ppm of carbon monoxide, which is comparable to the performance of the catalytic technology. These tests suggest that this ultra-clean technology could have an impact on gas turbine and micro-turbine development as demand is increasing for electricity generation using ultra-clean gas turbines.
The UCLSB's relative dimensions can be varied to some degree without affecting the burner's performance, and the burner can be made using standard, stock materials. These characteristics mean that engineers have many options for choosing economical fabrication and manufacturing methods, which could allow the technology to be adapted to products ranging from household boilers to gas power turbines. Low-temperature materials can be used in some applications because the detached flame means that the burner itself does not receive or retain much heat. Cheng expects that the lack of exposure to excessive heating will protect the UCLSB's components from degrading substantially over the life of the burner, so the UCLSB will be comparatively inexpensive to operate and maintain. This technology is also more energy-efficient than current technologies because it requires less fan power to push the fuel mix through the burner.
"We are continuing our studies of the theoretical underpinnings of the technology," says Cheng, "but field demonstrations have already proven its potential to reduce pollution emissions from the burning of natural gas. We hope to see these burners become a useful tool in the marketplace for reducing emissions."
For more information, contact:
- Robert Cheng
- (510) 486-5438; fax (510) 486-7303
For information on licensing this technology, see Technology Transfer: Success Stories: UltraClean Low Swirl Combustion