摘要
目前,提高入炉黑液固含量是碱回收系统的发展趋势,采用计算流体力学(CFD)技术可以预测碱回收过程黑液的流动特性及其雾化燃烧性能。本文归纳了碱回收系统黑液雾化和燃烧数值模型,重点阐述了碱回收系统黑液雾化和燃烧的数值模拟研究进展,总结了等温和非等温模型在碱回收系统黑液燃烧中的应用,提出进一步完善黑液燃烧模型并开发简化子模型以实现黑液雾化和燃烧数值精准模拟的研究,以期为高固含量黑液碱回收研究提出新的攻关方向,最后对碱回收技术的未来发展进行了展望。
碱法制浆黑液含有大量无机物和有机物,通常在浓缩后被送入碱回收炉中燃烧,以回收热能和化学
黑液是一种特殊的有机物/无机物混合流体,高固含量黑液呈现出高黏度、非牛顿流体的流变学特
黑液的雾化性能是评价碱回收炉黑液燃烧效果的关键。Fluent软件自带雾化模型,包括平口喷嘴雾化模型、压力旋流雾化模型和气泡雾化模型等。其中,压力旋流雾化模型的原理是流体经过喷嘴内部的旋流片加速后进入中心旋流室,然后沿壁面流转,在流体中心形成空气柱,以不稳定的薄膜状喷出,进而破碎成液滴形
注 LEVM:线性涡流黏度模型;ARSM:代数雷诺应力模型;DRSM:微分雷诺应力模型;SST K-ω:SST湍流模型;VOF:多相流流体体积模型;DPM:离散相模型。
由
黑液燃烧过程分为4个阶段:干燥、热解、焦炭燃烧和无机残渣反
通常,碱回收炉黑液燃烧模型包括等温模型和非等温模型,等温模型易构建且便于计算,常用于研究黑液燃烧的各个阶段,但此模型忽略了燃烧过程中黑液颗粒内部的温度梯度;非等温模型改善了等温模型存在的缺陷,考虑了燃烧过程中黑液颗粒内部温度的变化,可以精准预测黑液液滴在碱回收炉内的飞行轨迹和燃烧行为,但非等温模型结构复杂、计算耗时。因此,等温模型仍是研究碱回收炉黑液燃烧最常用的模拟方法,对于非等温模型还需简化模型、开发适用于实际碱回收炉黑液燃烧的子模型。笔者对相关研究进行了总结和归纳,结果如
黑液雾化是为了增加黑液液滴的蒸发表面积,加强黑液液滴与助燃空气的混合,保证黑液液滴在炉膛内部能够迅速、完全干燥并炭化。由于入炉黑液固含量高、黏度大,需要直径大的喷嘴进行雾化,但是这样产生的压力较低,黑液液滴平均粒径较大,导致黑液雾化液滴含有大量非球形液

图1 (a)溅板式喷嘴3D模
Fig. 1 (a) 3D model of splash nozzl
Foust等
非木材原料,尤其是竹材(我国造纸原料的补充),其高硅含量会导致黑液黏度增大,给碱回收系统造成一系列困扰。刘宇
虽然众多研究者针对碱回收炉黑液雾化模拟研究提出了建议,但目前实际生产中,碱回收系统黑液入炉固含量通常大于78%,甚至大于80%,为提高高固含量黑液的雾化性能,还需进一步模拟研究高固含量黑液的雾化性能及其影响因素,以及高固含量黑液对碱回收系统运行产生的影响。
对黑液燃烧阶段进行数值模拟研究,可进一步指导黑液碱回收的实际过程,众多研究学者和工厂实践人员对此做了大量研究。贵州赤天化纸业有限公司开发了一种由黑液喷枪枪管、枪头和溅板组成的碱回收炉黑液燃烧专用喷枪,其枪头与溅板呈38°夹角,且在枪头前端设有二次喷口,以防止黑液大量飞散及溅射在炉壁上,有利于在炉底形成良好垫
在高温条件下,辐射是碱回收炉中非常重要的传热机制。Ferreira等
纪晓
综上可知,国内外研究人员在碱回收系统黑液燃烧模拟研究方面已做了一些初步工作,碱回收系统黑液燃烧通常与燃烧各个阶段相关,尤其是干燥和热解过程液滴的膨胀行为对其运动轨迹和燃烧阶段黑液颗粒内部温度梯度的影响。因此,进一步完善黑液单颗粒燃烧与运动轨迹模型和非等温模型的研究,为模拟研究碱回收炉黑液燃烧提供了一种可靠的预测方法,并为解决高固含量黑液在碱回收系统中产生的新问题提供详细数据和理论基础。
本文归纳了多个基于计算流体力学(CFD)所开发的碱回收系统黑液雾化与燃烧的数值计算模型,并综述了碱回收炉黑液雾化和燃烧模拟技术研究进展,这对我国黑液碱回收系统的实践应用产生了积极指导作用。目前,我国在黑液数值模拟研究领域取得了长足进展,但缺乏对非木浆制浆黑液的模拟研究。而竹浆是我国目前产量最大的非木浆种之一,碱回收系统竹浆黑液入炉固含量通常大于78%,甚至大于80%。因此,对竹浆黑液的模拟研究更为迫切,特别是高固含量条件下的竹浆黑液雾化和燃烧的模拟研究。在竹浆黑液的模拟研究过程中,需考虑高固含量下黑液的非牛顿流体特性,同时结合黑液燃烧阶段的膨胀性及非等温性,进一步开发黑液燃烧阶段的简化模型或用户定义函数(UDF)程序,研究管道中黑液的流动特性及黑液的雾化和燃烧等。因此,后续需要提出新的思路继续深入研究,优化模型并针对不同模型与实验结果进行对比验证,揭示高固含量竹浆黑液的特性及燃烧规律,为改善竹浆黑液流动性及雾化性提供详细的理论数据,为实现碱回收系统高固含量竹浆黑液燃烧奠定理论基础。笔者期望本文已综述的方法对我国研究人员进行非木浆黑液碱回收模拟研究能够提供一定参考。
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