Gas Technology: Reduced boiler NOx from advanced burner designs
Meeting tightened standards with new solutions
For decades, it has been understood that man-made sources of nitrogen oxides (NOx) can contribute to diminished air quality. In the troposphere they combine with ozone and hydrocarbon vapors to create smog. Alone or in combination with other contaminants, they can aggravate asthmatic conditions, and in high levels can damage respiratory systems of humans, animals and even plants. Sources of nitrogen oxide emissions include transportation engines, stationary engines, thermal power plants, many industrial processes, home heating units, and commercial and industrial boilers.
Tightening Levels of Emission Control
The first U.S. Clean Air Act was passed in 1963, with increasing requirements for research, monitoring and emission reductions with each amendment in 1970, 1977 and 1990. With this legislation and other legislation and rulemaking, standards were established for NOx emissions from all major sources. The U.S. Environmental Protection Agency has given air standard-setting authority and emission enforcement responsibility to the state pollution control agencies, subject to the oversight of the EPA.
As major sources of NOx emissions, vehicles received the first emission standard, with other NOx emission sources subsequently being identified and limits set. It became apparent that certain districts of the country needed stricter standards because of population concentrations, meteorological conditions, and concentrations of point source and transportation emitters. For these areas, states and the EPA established more rigorous emission standards.
In older industrial boilers, NOx emissions were sometimes relatively high because of boiler design and burner performance characteristics. In boilers, high NOx emissions can be a result of two issues. NOx levels increase dramatically where temperatures in exhaust gas streams rise significantly above 2200°F. A second condition that favors creation of higher levels of NOx is incomplete mixing of combustion air and gaseous fuel.
Significant NOx emission reductions have been achieved by designing burners and boilers to avoid “hot spots”, where elevated temperatures can generate excess NOx. Low-NOx burners were designed with this in mind. Properly selected, installed and operated, these can achieve emissions below 30 ppm NOx with the standard 3% excess oxygen. These burners have been successfully installed in many areas in the U.S. and Canada.
However in many areas, state and federal regulators have continued to monitor ambient levels, and have used their regulatory authority to require boilers to meet even lower emissions levels. Thus, industry researchers and burner manufacturers have developed a class of so-called “ultra-low NOx” (ULN) burners. According to Rick Fiorenza, Vice President-Sales, Burner Applications for Cleaver-Brooks’ Natcom group, the definition of “Ultra-Low NOx” is not a fixed standard in the boiler industry today. He says, “ULN is generally defined by regional and local air regulation districts throughout the U.S. and Canada. Generally speaking, we consider burners achieving 15 ppm or less with reference to 3% oxygen when firing natural gas and incorporating flue gas recirculation technology (FGR) to be ULN burners.” In a number of cases local ULN performance requirements have been set as low as 9 ppm or even lower in a few California air districts. Even these levels are achievable with advanced designs.
According to Smith, Miura’s LXN series of modular boilers offers NOx emission rates below 9 ppm, as certified by a third party. Units are available in sizes of 50, 100, 150, 200 and 300 bhp. Available steam pressure ranges from 15 to 300 psi and the unit can come to steam production in less than five minutes, making it an attractive modular unit for variable steam requirements.
According to Smith, Miura has plans soon to offer a near-zero NOx boiler into the North American market. This unit will use a proprietary catalyzer to virtually eliminate NOx altogether. It will include a sophisticated pre-mix burner that produces a very low-temperature, “self-quenching” flame, thereby minimizing the thermal reaction that causes the formation of NOx.
Burners for large water-tube boilers are available from several manufacturers with NOx emission rates in the ULN range. Cleaver Brooks' NATCOM group offers burners for both OEM use and retrofitting with NOx emissions as low as 7 ppm. Coen and Alzeta Corporation also offer lines of both Low NOx and Ultra-Low NOx burners that can meet rigorous emissions standards. Burner manufacturers have used all the tools possible to meet demanding emission requirements without significantly sacrificing efficiency.
Because buying a boiler or a boiler burner is a long-term commitment, it is necessary to not only meet current emission standards, but to try to anticipate future requirements. This is causing many industrial owners to select systems with ULN capabilities. If you are entering this market, be sure to get information on current and anticipated emission standards, and guidance in the rapidly changing area of ULN boilers and burners. You can select from an expanding array of low NOx and ultra-low NOx burners and boilers.