Until the early 1980s, most boilers were configured with open flues to vent the waste gases burned by industrial equipment. Emissions from boiler burners were one of the biggest safety concerns at the time. This necessitated the need to reduce NOx emissions by various means, including minor design changes that could affect thermal NOx levels, such as using special gas element drilling patterns. This led to the development of low NOx burners.
Industrial Boiler Controls—NOx Control
Reducing NOx emissions involves various process changes, such as modifying the burner combustion process, implementing new boiler technologies, or using various NOx control strategies, such as:
Fuel switching
Staged combustion
Flue gas recirculation (FGR)
Installation of low NOx burners
Selective catalytic reduction (SCR), etc.
FGR (Flue Gas Recirculation)
While there are many ways to reduce and control NOx emissions from boiler burners, flue gas recirculation (FGR) quickly became a major concern in the mid-to-late 1980s when California's South Coast Air Quality Management District mandated NOx emissions of less than 30 ppm for gas-fired industrial boilers. In most cases, this requirement is met by introducing FGR. As stringent low NOx requirements become the norm in the coming years, new burner designs will help achieve even lower NOx levels.
While FGR can help significantly reduce NOx emissions, it has its drawbacks: increasing FGR rates often result in increased mass flow, which starves the burner of oxygen and causes unstable combustion.
As a result, boiler suppliers have begun developing new boiler technologies and more sophisticated burner control systems to accommodate high FGR rates. In some states, such as California, where single-digit low NOx emissions have been mandated for many years, low NOx gas burner systems have been used.
However, in most of the United States, extreme NOx reductions are not required. Perhaps the biggest challenge in reducing NOx levels during combustion is rapid NOx. This is a common problem with all conventional burners. Since it is formed instantly, the use of FGR cannot help reduce rapid NOx. Therefore, ultra-low NOx burners use higher FGR rates to address thermal and rapid NOx emissions.