颗粒间距对煤粉颗粒着火和燃烧行为影响的理论研究

Theoretical study on the effect of particle spacing on ignition and combustion behavior of pulverized coal particles

为了探究复杂燃烧组织形式下煤粉颗粒间距变化所带来的颗粒着火和燃烧行为的差异,以典型烟煤为研究对象,开展了不同颗粒间距下煤粉颗粒着火与燃烧特性的理论模拟研究,主要考察着火延迟时间、着火模式及关键流场、组分场特性随颗粒间距D的演变规律。结果表明,当颗粒间距较大,煤粉颗粒群的燃烧行为与单颗粒燃烧行为相似;对于粒径为70μm的烟煤颗粒,在1500 K-0.2O_(2)的条件下,均相着火滞后于非均相着火发生,且该现象对于气相反应机理的选取不敏感。当颗粒间距缩小(≤8d),煤粉颗粒的气相燃烧行为与单个煤粉颗粒有较大的差异。由于颗粒之间的相互作用,挥发分在下游颗粒边界处发生富集,气相着火点在下游颗粒边界产生,并逐步发展至上游颗粒,促使气相连续火焰的形成。由于上述颗粒间的相互作用,上游和下游颗粒的挥发分着火均有所提前,使得颗粒的着火模式向均相着火倾斜,且下游颗粒着火模式倾斜程度较上游颗粒更为明显。当颗粒间距缩小,多颗粒燃烧形成火焰包络面,导致颗粒边界层富燃贫氧区域的扩大,这对燃烧过程中氮氧化物等的生成具有重要影响。

In order to explore coal combustion behaviors under complex combustion organization conditions,the influence of particle spacing on ignition and combustion behavior of typical bituminous coal is investigated.By using Computational Fluid Dynamics(CFD),the interaction of pulverized coal particles is simulated and analyzed.The evolution of ignition delay time,ignition mode,as well as the key flow/species field with the variation of particle spacing were intensively studied.The results show that,when the particle spacing is large,the combustion behavior of particle group is similar to that of single particle;for a 70μm bituminous coal particle,homogeneous ignition occurs later than heterogeneous ignition in the ambience of 1500 K-0.2O_(2),and this phenomenon is not sensitive to the adoption of gas-phase reaction mechanism.However,when the particle spacing is small(≤8d),the gas phase combustion behavior of pulverized coal particles is quite different from that of single coal particles.Due to the interactions between particles,the volatiles accumulate around the boundary of downstream particle.Therefore,the gas-phase ignition occurs at the boundary of downstream particle,and then gradually develops to the upstream particle.This promotes the formation of gas-phase continuous flame.The change of the flame pattern leads to the decrease of homogeneous ignition time for both the upstream and downstream particles.It will further lead to the shift of ignition mode towards homogeneous ignition,which is much prominent for the downstream particle.Moreover,when the flame envelope is formed in multi-particle combustion,the fuel-rich region in the particle vicinity extends.It will bring significant impact on the subsequent formation of nitrogen oxides during combustion.