Influence Of Coal Blending Methods On The Burnout And NO_x Emissions Of Coal Blends Combustion

In China, blending coal combustion is a common phenomenon. At this stage, most research has focused on the combustion characteristics of out-furnace blending methods,but the combustion characteristics of in-furnace blending methods are lack of study.Therefore, this study designs and builds a drop tube furnace experimental system and the influence of out-furnace and in-furnace blending method is analyzed for four kinds of coals and their binary blends used by Chinese power plants. Three factors such as blend ratio, blending coal type and excess oxygen coefficient affecting the blend coal combustion characteristics and NOX emission characteristics are also analyzed in this paper.Firstly, a drop tube furnace with two pulverized coal injectors is designed and built which can be used to achieve both two blending methods. In out-furnace blending method, before being injected into the reaction tube, two coals are mixed outside the furnace. In in-furnace blending method, two coals are injected into the reaction tube through separate injectors in proportion, and the blending time can be controlled by the distance between the two injectors of pulverized coal.Secondly, the influence of out-furnace blending method on the combustion characteristics of four kinds of coals and their blends is analyzed. The results show that when combustion characteristics of binary coals are in large difference, as the blending ratio of coal with higher volatile increases, the burnout of blending coal first increases and then decreases. When combustion characteristics of binary coals are in small difference, as the blending ratio of coal with higher volatile increases, the burnout increases. Coal with higher volatile has two opposite effect, which will affect the blending coal combustion at the same time. With the increasing of the difference between combustion characteristics of binary coals, excess oxygen coefficient and the ratio of coalwith lower volatile, the promote function(the coal with higher volatile burns first and enhance the local temperature to help the low volatile coal ignites) increases. With the increasing of the difference between combustion characteristics of binary coals, the coal with higher volatile and the decreasing of excess oxygen coefficient, the restrain function(the coal with higher volatile burns rapidly and consume much oxygen making the coal with lower volatile lack of oxygen) increases. The NOX emission of blending coals is similar to the theoretical linear value.Finally, the influence of in-furnace blending method on the combustion characteristics of four kinds of coals and their blends is analyzed. The results show that out-furnace blending method will weaken both the promote function and restrain function.With the increasing of the difference between combustion characteristics of binary coals and the decreasing of in-furnace blending time, the degree of the weaken function increases. The NOX emission of in-furnace blending coal combustion is lower than in out-furnace blending coal combustion. With the increasing of in-furnace blending time,the combustion characteristics of mixed coals, the blending ratio and the excess oxygen coefficient the NOX emission decreases.