Influence of increased primary air ratio on boiler performance in a 660 MW brown coal boiler

A computational fluid dynamics model was established based on a 660 MW brown coal boiler, to study the effects of primary air ratio (PAR) on boiler performance. To improve model prediction, moisture content in brown coal is specially considered. Confidence in the model was established by carrying out mesh independence test and validation against real life data and theoretical calculations. Then it was used to simulate 10 cases with different PAR. Results show that with PAR increasing from 0.325 to 0.55, overall boiler performance deteriorates and total radiative heat flux decreases by 35.2 MW. However, the temperature distribution and heat flux in main burners region and above separated over fire air region show a parabolic trend. The results indicate that boiler performance deteriorates with PAR increasing, and well explain why boiler thermal load is still reduced, even if PAR is increased to maintain the drying capacity in high moisture content cases. Momentum ratio of primary and secondary air is pointed out to be the main cause of this phenomenon. At last, a feasible solution is proposed to raise primary air temperature, not the ratio of it, to maintain the drying capacity. Therefore, the adverse effects caused by increased PAR can be avoided.