Effect of Ash Layer Peeling on Nitric Oxide Emissions During Char Combustion in Local Flue Gas Recirculation

Local flue gas recirculation (LFGR) is an optimization of the traditional flue gas recirculation (FGR), which can effectively reduce nitric oxide (NO) emissions from coal-combustion grate-fired industrial boilers. The application effect of LFGR was evaluated scientifically and fairly by rederiving the NO concentration conversion formula under FGR (including LFGR) conditions. The industrial experiment results show that an appropriate amount of circulating flue gas entering wind room 2 can increase boiler load and reduce NO emissions. A low NO emission effect remains after stopping the circulating flue gas, which is speculated to be related to the peeling of the ash layer wrapped around the large-particle char. The ash layer peeling can be equivalent to the increase in oxygen (O₂) concentration, NO concentration, and temperature during char combustion. With the increase in temperature and the decrease in O₂ concentration, the char-N conversion rate of Neimenggu char decreases. The char-N conversion rate of Shuozhou char first increases and then decreases with the increase in temperature; it is inversely proportional to the O₂ concentration at low temperatures and may be proportional to the O₂ concentration at high temperatures. A small increase in NO concentration in the environment can effectively reduce NO emissions during char combustion, which may exceed the effects of temperature and O₂ concentration. This research deeply explores the reasons for the low nitrogen effect of FGR. Meanwhile, the new NO concentration conversion formula is likely to affect the environmental evaluation of boilers using FGR and attract renewed attention from the government.