Experimental study on optimization of over-fire air in modified combustion condition with selective catalytic reduction

Abstract

Recently, the use of NOx control retrofits is significantly increasing due to further tightening of regulations caused by worldwide environmental concerns. In order to reduce NOx emissions, most of the generators are equipped with SCR (Selective Catalytic Reduction) or OFA (Over-Fire Air) systems, while some generators being equipped with both systems. Here we present experimental evidence of higher boiler efficiency with efficient NOx control with consideration for influences on unburned carbon, under the condition that it should meet local and federal emissions requirements. The higher boiler efficiency has been achieved for a newly installed SCR system by optimizing OFA and reducing excess air quantity. The test was conducted over 6 months with a 500 MW coal-fired boiler. Stepwise closing of OFA dampers was carried out with and without simultaneous excess air optimization at a 500 MW nominal rating. We confirmed that our new operation leads to the following benefits: a) Reduction of UBC due to higher temperature in the furnace’s main combustion zone, b) Improved fly-ash recycling ratio, c) Reduction of spray water into re-heater due to lower temperatures in the second (upper) combustion zone, and d) Reduction of exhaust gas loss. As a result, the boiler efficiency has increased by up to 0.4% and UBC (unburned carbon) has decreased by 0.8%. In conclusion, we confirmed that our new operation mode yields better boiler efficiency for newly installed SCR systems and mixed coal firing operating conditions.