Cost-Effective NOx Control for Air-Preheated Systems
New advancements in combustion control technologies, such as flue gas recirculation (FGR) and low NOx burners (LNB), have made it possible to achieve ultra low NOx levels without using expensive post-combustion flue gas clean-up technology. For some natural draft units it may be possible to meet the most stringent of the NOx regulations with cost-effective combustion control technologies. For forced draft (mechanical draft) units it may not be possible to achieve efficient NOx control with LNB. This is because most forced draft systems have air preheat that results in a higher baseline NOx emissions. Increased air preheat temperature results in higher adiabatic flame temperatures and LNB technology is ineffective in reducing flame temperatures. NOx formation is an exponential function of adiabatic flame temperature (see Chemical Engineering, Feb 2001). Any increase in combustion air temperature results in increasing the adiabatic flame temperature, which in turn increases NOx formation.