Why it matters.
In southern California and the north central valley of California, some burner applications require NOx emissions below 5 ppmv to obtain a permit to operate by the local air quality district. NOx is typically represented by the compound NO2, though other less stable compounds are included such as NO, N2O and N2O4. The molecular weight of NO2 (46 g/g-mole) is used for conversion of volume to mass levels of NOx. NOx will convert to ground level ozone (O3) in the presence of gaseous hydrocarbons and sunlight. In turn, ozone is an irritant for those with respiratory issues. NO2 also contributes to acid rain. In ozone non-attainment areas, where ambient ozone exceeds clean air standards, NOx regulations are the strictest and are strictly enforced.
The ALZETA approach
There are three primary mechanisms for NOx formation: prompt, fuel-bound, and thermal. Prompt NOx formation can occur with fuel-rich combustion where nitrogen can dissociate into NOx compounds in the presence of excess hydrocarbons. Fuel-bound nitrogen, such as ammonia (NH3), will also form NOx by combustion with air. Thermal NOx occurs when air and hydrocarbons are burned at flame temperatures exceeding 2400 F. The concentration of NOx depends on flame temperature in an exponential way. This is the predominant mechanism for most commercial burners operating on air and natural gas, propane, or butane.
- The strategy used for ALZETA burner design, then, is to limit the flame temperature, so the flame is cool enough to avoid thermal NOx formation. In addition, the flame temperature must be sufficiently high to maintain a stable flame. To achieve this accurate control of flame temperature for low NOx and flame stability, ALZETA burners incorporate the following:
- Fully premixed air and fuel gas, with a high degree of mixing of both gases. This provides uniformity of chemistry and flame temperature.
- Uniform distribution of the premix to the burner surface, again to provide uniform conditions at the combustion zone. Flame stability will depend on flame temperature (well mixed) and premix velocity (well distributed).
- Careful design of the air-fuel ratio controls, incorporating feedback of the excess stack oxygen to position the air and fuel valve positions to maintain flame temperature with minimum error.
ALZETA Ultra-Low NOx experience
ALZETA has supplied natural gas burners that have met 9 ppm NOx permit requirements for over 25 years, and 5 ppm NOx permit requirements for over 8 years in air heating and commercial/industrial boiler applications. These installations range in firing rate from 5 to 180 MMBtu/hr, and include ALZETA burners for new equipment installations as well as retrofit of old boilers with ALZETA burners. For more information about our experience, customer referrals, and how we can help you meet strict NOx limits for your application, please contact us at CSBsales@alzeta.com.
