July 28, 2003
Berkeley Lab Science Beat Berkeley Lab Science Beat
Low-Swirl Combustion
Clears the Air
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A laboratory prototype of an ultraclean, low-swirl burner (UCLSB) has an internal diameter of five centimeters, shown firing at a rate of 15 kilowatts. This burner is made entirely out of plastic components to showcase its unique lifted-flame feature.
A unique type of clean-burning combustion technology called ultraclean, low-swirl combustion (UCLSC), developed by Berkeley Lab combustion researcher Robert Cheng, is now entering the marketplace after years of research and development. Burners using this technology produce 10 to 100 times lower emissions of nitrogen oxides than conventional burners, making it easier and more economical for industries to meet clean air requirements. Because conventional theory does not predict its features, new advances in combustion theory have been developed to explain the principles of this new combustion technology.

Natural gas is the primary energy source for manufacturing, industrial processing, commercial and residential hot air and hot water supplies, and industrial space heating. Natural gas burners are used in homes, in boilers and furnaces; natural-gas-burning turbines are producing more and more electricity as well. In the year 2000, homes, businesses, industry, and power generators burned nearly 23 trillion cubic feet of natural gas, generating 22.6 quadrillion Btus (British thermal units) of energy.

During the same period there were U.S. emissions of nearly 22 million metric tons of nitrogen oxides (NOx) from all sources. Nitrogen oxides are a family of gases that generate photochemical smog and haze; controlling them is a major priority for air quality management districts throughout the U.S.

Although much NOx comes from vehicle emissions, stationary sources are important as well, and reducing emissions from these would be a great help in the fight against photochemical smog. In large urban regions on the West Coast, East Coast, Louisiana, and Texas, stationary sources of NOx must be in compliance with stringent emissions regulations.

Genesis of a new technology

For more than 10 years combustion expert Robert Cheng, a scientist in Berkeley Lab's Environmental Energy Technologies Division, has been studying the unique, clean-burning combustion technology known as UCLSC: ultraclean, low-swirl combustion. The research had its origins in an experimental program of the Department of Energy's Office of Science to investigate the intricate coupling between fluid mechanical turbulence and combustion heat release -- but the work is leading to numerous practical applications.

The new technology not only burns cleanly, it is as cheap or cheaper than many existing burners. UCLSC could be scaled for devices as small as home furnaces and boilers or as large as gas-fired power generators. The Department of Energy's Office of Energy Efficiency and Renewable Energy now funds the research to adapt this technology to heating and power generation.

"Currently, natural-gas industrial equipment emits on the order of 100 parts per million of NOx. Ultraclean, low-swirl combustion for industrial processes can reduce the average emission to well below 10 parts per million NOx," Cheng says. "In the U.S. alone, this would remove 340,000 tons of NOx per year from our atmosphere." That is equivalent to the NOx emissions of 45 thousand-megawatt coal-fired power plants. Adapting this technology to power generation, and to residential and commercial applications as well, could remove an additional 400,000 tons per year of NOx.

The ultraclean, low-swirl burner (UCLSB) operates according to a novel combustion method that uses lean, premixed flames -- a type of combustion in which the appropriate ratios of air and fuel are mixed to burn completely when the mixture reaches the flame. Its operating principle, overall flame behavior, and turbulent-flow features have been studied using lasers.

A 12.7-centimeter UCLSB designed for water and steam boilers. In this design the flame is highly lifted to optimize performance in boiler tubes.  

"The most distinct characteristic of the burner is a detached flame that is lifted above the burner," says Cheng. "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 flameout. 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 than conventional burners. "We are conducting research to develop a broader theoretical foundation to explain this combustion," he says.

According to Cheng, the UCLSB provides the most stable platform for lean premixed turbulent flames to propagate in their natural state without interference from the flame interacting with the burner components. With the flame not touching the burner, the UCLSB is also highly efficient in energy conversion because there is no energy lost to the burner.

Growing attention from the marketplace

The Maxon MPAKT Ultra Low NOx Burner is the first product using ultraclean, low-swirl combustion technology.

Depending on the application, low-swirl combustion emits 10 to 100 times fewer oxides of nitrogen than conventional combustion systems. The Maxon Corporation of Muncie, Indiana, has licensed the ultraclean, low-swirl combustion technology for industrial process heaters. These devices are used in many industrial baking and drying ovens, industrial processes which consume more than 9.8 quadrillion Btus of natural gas per year in the U.S.

Maxon's ultra-low-NOx burner, scheduled to appear in 2003, will meet stringent air quality regulations requiring NOx emissions of less than nine parts per million (at three percent oxygen). Maxon is also developing larger capacity low-swirl burners for other industrial heating processes.

Another manufacturer recently evaluated a UCLSB five centimeters (one inch) in diameter, for domestic appliances in a 15-kilowatt spa heater. The results show that the UCLSB reduces NOx emissions from 150 parts per million to less than 12 ppm while maintaining the same overall thermal efficiency.

Cheng is working with several boiler manufacturers to engineer and adapt UCLSB for their products. They tested a UCLSB 12.7 centimeters (five inches) in diameter in six different boiler configurations and found potential for its use in industrial hot water and steam generation.

Swirlers for gas turbines

With his industrial partner, Solar Turbines of San Diego, California, Cheng recently demonstrated the technology's potential for gas turbines, which are being used more and more to generate electricity for the power grid. Cheng and his partners successfully fired a "low-swirl injector" (LSI) prototype -- a version of the UCLSB designed for power turbines -- and showed that it can match the emissions of much more expensive and less durable catalytic combustors, currently considered the best technology available.

The LSI reduced the NOx emissions by a factor of five to ten and maintained less than five parts per million of carbon monoxide emissions, comparable to catalytic technology. Because demand is increasing for electricity generation through cleaner gas turbines, these field tests suggest that ultraclean, low-swirl technology could have an impact on gas turbine and microturbine development.

A UCLSB's relative dimensions can be varied to an extent without affecting the burner's performance, and the burners can be made using standard stock materials. This gives engineers many options for choosing economical fabrication and manufacturing methods allowing them to adapt the technology to everything from household boilers to gas power turbines. They can also use low-temperature materials in some applications: because the flame is detached, the burner does not receive or retain a lot of heat from the flame.

Key components of a UCLSB are, at top, the vane swirler, with an open center channel and a screen. Shown at bottom, left to right, are UCLSBs from 2.54 centimeters to 12.7 centimeters in diameter. Variations in the number of swirl vanes and center-body sizes show this to be a robust and easily adaptable technology.  

Cheng expects that, over the life of the burner, none of the UCLSB components will degrade substantially from excessive heating, making the cost to maintain and operate a UCLSB comparatively low. UCLSC 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."

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