循环流化床二次风射流及床料粒径对流化特性的影响

NOx的生成控制与流化床流化特性及气固流动有关,而冷态试验能更直观地反映气固流动状态和流化效果,因此冷态试验的研究结果可为流化床脱硝反应热态试验的参数选取提供参考。前人研究大多使用窄筛分床料颗粒,且较多针对传统的墙式布置二次风,鲜有学者综合研究中心布置二次风的穿透性能及其对炉膛流化特性的影响。因此,在循环流化床冷态试验台上,研究了中心布置二次风和宽、窄筛分的床料对流化特性的影响,采用无量纲剩余温度、物料循环流率和表观颗粒体积分数来定量描述二次风穿透性能、物料循环效率和颗粒浓度的分布。结果表明:喷射高度附近的颗粒浓度会随二次风射流增大而增大。二次风喷射高度为15 cm时,在35 cm以下的密相区,二次风占比越大,颗粒浓度的增长越明显。增大二次风占比、提高二次风射流的速度、提高二次风射流的喷射位置等可以有效提高二次风射流的穿透性能。其中,当二次风喷射高度距炉膛底部5 cm处,射流穿透率为0.4;喷射高度为15 cm时,射流穿透率为0.84;当喷射高度继续上升10 cm后,射流穿透率达到1.0。造成这个现象的原因是越靠近炉膛底部,床料颗粒的浓度越大,二次风所受的阻力急剧增加。随着窄筛分床料的平均粒径减小,炉膛整体的压降上升,且压降会在炉膛更高位置趋于平稳,物料循环流率也随之提高。这说明更多的颗粒能够随流化风的扬析被带到炉膛外,进入分离器参与炉外循环。与窄筛分床料不同的是,床料组分中,细颗粒占比也决定了宽筛分床料的炉膛压降、颗粒浓度分布和物料循环流率等参数。细颗粒占比越高,炉膛压降和物料的循环流率越大。宽筛分中,平均粒径大的床料颗粒浓度分布和物料循环流率不一定小,这是由于试验风速下,<300μm颗粒更易随流化风的扬折作用,被携带至炉膛出口。这部分细颗粒占比越高,造成颗粒浓度分布和物料循环流率越大,而粗颗粒更倾向于聚集在炉膛底部。

Influence of secondary air jet and bed materials size of circulatingfluidized bed on the fluidization characteristics

The formation control of NOx is related to fluidized bed fluidization characteristics and gas-solid flow,and the cold state test can more directly reflect the gas-solid flow state and fluidization effect,so the results of the cold state test can provide reference for the parameter selection of the reaction thermal experiment of fluidized bed denitration.Previous researches mostly used narrow screening bed material particles,and mostly focused on the traditional wall type secondary air arrangement.Few scholars comprehensively studied the penetration performance of secondary air arrangement in the center and its influence on the fluidization characteristics of the furnace.Therefore,the influence of central secondary air distribution and bed material with wide and narrow screening on fluidization characteristics was studied on the cold experimental bench of circulating fluidized bed.The dimensionless residual temperature,material circulation rate and apparent particle volume fraction were used to quantitatively describe the secondary air penetration,material circulation efficiency and particle concentration distribution.The experimental results show that the particle concentration near the jet height increases with the increase of the secondary air jet.When the secondary air injection height is 15 cm,in the dense phase area below 35 cm,the larger the proportion of secondary air is,the more obvious the increase of particle concentration is.The penetration performance of the secondary air jet can be effectively improved by increasing the proportion of secondary air jet,increasing the velocity of the secondary air jet and improving the jet position of the secondary air jet.When the injection height of secondary air is 5 cm from the bottom of the furnace,the jet penetration rate is 0.4;When the injection height is 15 cm,the jet penetration rate is 0.84.When the injection height continues to rise by 10cm,the jet penetration rate reaches 1.0.The reason for this phenomenon is that the closer to the bottom of the furnace is,the greater the concentration of the bed material particles is,and the secondary wind resistance increases sharply.With the decrease of the average particle size of the narrow screening bed material,the overall pressure drop in the furnace increases,and the pressure drop tends to be stable at a higher position in the furnace,and the circulation flow rate of the material also increases.This indicates that more particles can be carried out of the furnace along with the uplift of fluidized wind and enter the separator to participate in the circulation outside the furnace.Different from the narrow sieve bed material,the proportion of fine particles in the bed material determines the furnace pressure drop,particle concentration distribution and material circulation rate of the wide sieve bed material.The higher the proportion of fine particles is,the greater the pressure drop in furnace and the circulating flow rate of materials are.In wide screening,the particle concentration distribution and material circulating flow rate of bed materials with large average particle size are not necessarily small,because under the experimental wind speed,particles with particle size less than 300μm are more likely to be carried to the furnace outlet along with the elutriation action of fluidized wind.The higher the proportion of these fine particles is,the greater the particle concentration distribution and the material circulation rateare.Coarse particles tend to concentrate at the bottom of the furnace.