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Ultralow-NOx burner efficiency also is inseparably linked to the technology employed to generate lower NOx formation. One of the most common ways to reduce NOx formation in gas combustion is to operate the burner with premixed combustion and higher excess air. This method allows heat from the combustion process to be distributed over the increased air volume and significantly reduces peak flame temperature. For this reason, premixed gas burners are the simplest form of ultralow-NOx technology. Some burner geometries allow stable operation over large turndown ratios and in many furnace applications. An efficiency penalty is caused by the excess air that is employed to reduce the flame temperature. With higher excess-air levels, more air (oxygen and N₂) is passing through the flame, being heated to the flame temperature and exhausted out of the stack at a still relatively high exit-gas temperature. Combustion efficiency will increase with increased excess air until the excess air's heat loss is larger than the heat provided by more efficient combustion (Figure 1). The exit-gas temperature is a function of various factors, such as boiler efficiency and operating pressure, and can have a significant impact on efficiency (Table 2).

Many boiler and burner manufacturers have elected to utilize flue-gas recirculation (FGR) in place of higher excess air to achieve the same level of NOx emissions. This is a well-established design strategy that commonly is employed by manufacturers to achieve NOx emissions in the 20- to 40-ppm range. Reaching sub-10-ppm levels with this technology requires significant design considerations to support flame stability. Natural-gas flammability limits can be affected by the addition of hot surfaces and/or hot recirculated gases at the flame front because higher temperatures result in reduced lower explosive limits (LELs) and higher upper explosive limits (UELs)³. Most applications utilized to achieve ultralow-NOx combustion use more than one reduction strategy. Combinations of premixed combustion staging are incorporated with FGR to minimize the formation of NOx.

Because of the difficulty in achieving ultralow-NOx combustion, each type of burner technology must address efficiency and turndown. There is no one-size-fits-all burner design, and each burner type must be matched to the furnace geometry and boiler operating conditions. These burners can be extremely sensitive to back-pressure changes and alterations to combustion-air temperatures, making their operation very different from the old burner technologies. Additional controls often are added to burner packages to create more complex control of the combustion processes. Users must be ready to take a more involved approach to the boiler/burner operation process.

Conclusion

In our original article, we concluded that burner turndown was not a panacea for correcting low efficiency or compensating for poor equipment selection. With the current increased emphasis on energy efficiency — and emissions — we believe that is even truer now than it was then. Selecting the proper boiler and burner for any application requires careful consideration of many factors, of which turndown is just one.

References

1. Demirbas, A. (2007). Effects of moisture and hydrogen content on the heating value of fuels. Energy Sources, 27, 649-655.

2. API. (2001). API Publication 4261: Alcohols and esters: A technical assessment of their application as fuels and fuel components. Washington, DC: American Petroleum Institute.

3. Geerssen, T.M. (1988). Physical properties of natural gases. The Netherlands: NV Nederlandse Gasunie.

Did you find this article useful? Send comments and suggestions to Associate Editor Megan White at megan.white@penton.com.

Larry Clark, LEED AP, is director of corporate business development for air-conditioning-contractor Hill York. Previously, he served as president, vice president of sales and marketing, and regional sales manager for MEPCO and regional sales manager for Vapor Power. An EBR boiler and mechanical lead for Consumers Energy at J.H. Campbell Generating Plant, David Thornock, PE, is responsible for the economic-based reliability of three large utility boiler systems. His previous experience includes serving as director of engineering for a commercial-boiler manufacturer, designing and manufacturing boiler and burner packages to meet customer needs and environmental regulations.