For a newly built supercritical 350 MW unit with down-fired boiler applying multi-injection staging combustion technology

the fluid flow and combustion characteristics inside the furnace were numerically simulated

under conditions with the tertiary air declination angle of 0°

10°

20°

30°.The results show that

with the tertiary air declination angle of 0°and 10°

a deflected flow field with low furnace room utilization efficiency was observed

accompanied by high levels of gas temperature and NOx emissions at the furnace outlet.As the tertiary air declination angle increased to 20°

the flow filed turned into a relatively symmetrical pattern with great flame penetration depth

and the aforementioned high NOxemission at the furnace outlet decreased apparently to acceptable levels.However

excessively increasing the tertiary air declination angle to 30°resulted in an excessively large downward flame penetration depth

with the downward coal/air flow washing over the hopper wall and high gas temperatures in the hopper zone.Comprehensively considering the formation of symmetric combustion with low NOxemission and gas temperature at the furnace outlet

the optimal tertiary-air declination angle should be 20°.