The Research of the Influence of Primary Air Ratio on the Combustion in a Lignite-Fired Ultra Supercritical Boiler

The computational fluid dynamics (CFD) code PHOENICS was applied to evaluate the combustion process in the furnace of a 1000 MW dual circle tangential firing single furnace lignite-fired ultra supercritical (USC) boiler. The influence of different primary air ratios (35%, 39% and 43%) on the flow and mixing characteristics of the gas-solid two-phase flow and the combustion process in the furnace was focused on. The results indicate that in the furnace with double tangential firing, the flow field shows two well-symmetrical ellipses at different primary air ratios. The surface temperatures of the burners at which, the long axis of the ellipses pointed, are much higher than those in the other four corners. Thus the phenomena of ´Hot corners´ and ´Cold corners´ arise. In practical operating, the flow erodes the walls in the hot corner which may lead to high temperature corrosion and slagging. With the increase of primary air ratio, the average concentration of NO_x at the outlet of furnace rises while the char distributions in the furnace are similar. By the comparisons of the characteristics of the airflow, the temperature distributions, the NO_x formation amounts and the char burnout rates, the situation with the 35% primary air ratio is preferable. The results of this paper have great value because of the support they lend to the design of USC lignite-fired boilers.