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煤粉燃烧全粒径飞灰生成机理研究
Mechanistic Studies on the Formation of Fly Ash Particles in Full Sizes during Pulverized Coal Combustion
【作者】 高琦；
【导师】 姚强；
【作者基本信息】 清华大学 ， 动力工程及工程热物理， 2019， 博士
【摘要】 煤粉燃烧产生的飞灰颗粒物,既是燃煤电站颗粒物排放的主要来源,也是造成炉内积灰结渣的重要隐患。工程实践表明,近年来大量高碱煤种的燃用具有加重炉内积灰结渣的倾向性。随着煤电机组智能化升级与灵活性调峰改造,深入认知煤粉燃烧飞灰生成过程对于变负荷条件下受热面灰渣防控与颗粒物排放治理具有重要意义。本文基于以上背景开展煤粉燃烧全粒径飞灰颗粒物生成机理研究,旨在为煤电机组燃烧优化与灰渣控制提供关键理论支撑。首先,采用清华大学25kW高温一维炉开展Na含量为6%左右的高碱大南湖煤燃烧细颗粒物PM10演化特性研究。通过分析不同采样口颗粒物样品特性,获得了大南湖煤挥发分燃烧阶段、焦炭燃烧阶段和焦炭燃尽阶段PM10质量分布和元素分布的演化规律。重点探究了煤粉燃烧各阶段AAEM元素在超细颗粒物PM0.1、亚微米颗粒物PM0.1-1和微米级颗粒物PM1-10中扮演的重要角色。针对高碱煤挥发分燃烧阶段Na元素大量存在于超细颗粒物中的实验现象,选用一种Na含量更高的准东煤,采用可视化多元平焰燃烧器（又名Hencken燃烧器）开展煤粉燃烧初期超细颗粒物生成机理研究。基于时间尺度分析方法,发现了准东煤热解、Na元素相态转变和超细颗粒物生成在特征时间上的一致性。类比多组分金属火焰合成过程,揭示了煤粉燃烧初期超细颗粒物生成的“热解-核化-聚并”新机制,进而建立了基于群平衡理论的超细颗粒物生成预测模型。进一步观测了不同煤种燃烧中的动态破碎特性,总结出煤粉颗粒在不同燃烧阶段破碎行为的主要特征。基于硬球模型的思想,发展了接近真实三维情况的煤粉燃烧内在矿物质向飞灰颗粒物转化模型,并分别采用序列破碎模型和边缘破碎模型描述煤粉热解和焦炭燃烧阶段破碎对内在矿物质迁移转化的影响。计算结果表明,准东煤热解阶段发生的破碎显著提高了PM2-10的量,并最终导致粗模态颗粒物质量分布曲线呈现双峰形状。最后,本文开发了一种可预测煤粉燃烧全粒径飞灰颗粒物生成的机理模型,成功预测了传统神华烟煤、高钠准东煤和高钠大南湖煤一维炉燃烧实验中飞灰颗粒物的质量分布结果。借助模型敏感性分析方法,划分出由矿物质前驱物、矿物质破碎、内在矿物质和外在矿物质控制的飞灰颗粒物粒径区间。定量表征了颗粒碰撞聚并、入炉煤粉细度和燃烧条件波动对飞灰颗粒物生成的影响。
【Abstract】 Fly ash particles generated during pulverized coal combustion are not only the main source of particulate matter（PM）emissions from coal-fired power plants,but also an hidden danger causing severe slagging and fouling problems in coal boilers.The practice shows that the massive usage of high-alkali coals in recent years tends to arouse more serious slagging/fouling in boilers.With the intelligent upgrading and flexibility promotion of coal plants,a deep understanding of the fly ash formation process is of great significance to prevent the ash/slag on heat exchangers and control the PM emissions under variable loads.Based on the above background,this paper conducts mechanistic studies on the formation of fly ash in full sizes during pulverized coal combustion,aiming to provide key theoretical support for the combustion optimization and ash/slag control in coal plants.Firstly,the progressive formation of fine PM10 during the combustion of high-alkali Dananhu coal with a Na content of～6%was investigated by using the Tsinghua 25 k W one-dimensional（1-D）furnace.Through analyzing the properties of PM samples from different sampling ports,the mass particle size distribution（PSD）of PM10 and size partitioning of mineral elements at volatile combustion,char combustion,and char burnout were obtained.The role of alkali and alkaline earth metals（AAEMs）in the formation of ultrafine PM0.1,submicron PM0.1-1,and supermicron PM1-10 in different coal combustion stages were intensively explored.Based on the experimental phenomenon of the massive existence of Na in ultrafine PM during the volatile combustion of high-alkali coal,we selected Zhundong coal with a higher Na content to study the ultrafine PM formation in the early coal combustion stage using an optically-accessible multi-element flat-flame burner（termed as Hencken burner）.With a time-scale analysis,a consistency on the characteristic time was discovered among coal devolatilization,Na phase transition,and ultrafine particle formation.Analogized with the multi-component metal flame synthesis process,the ultrafine PM formation in the early stage of coal combustion was interpreted by revealing a new"devolatilization-nucleation-coagulation"mechanism.Further,a prediction model based on the population balance theory was established for ultrafine PM formation.Besides,observations on the fragmentation characteristics during combustion were conducted on different coals.The main features of the coal fragmentation in different combustion stages were summarized.Based on the idea of hard-sphere model（HSM）,a three-dimensional（3-D）model close to the real transformation from included minerals to fly ash particles during coal combustion was developed.A sequence fragmentation model and a perimeter fragmentation model were applied to describe the effects of fragmentation during coal devolatilization and during char combustion on the transformation of included minerals,respectively.The calculation results were demonstrated that the fragmentation of Zhundong coal during devolatilization significantly increases the fraction of PM2-10,and ultimately leads to the mass PSD of coarse ash particles presenting bi-modal shape.Finally,this paper developed a mechanistic model for predicting the formation of fly ash particles in all sizes during pulverized coal combustion.The model successfully predicted the mass PSDs of fly ash from traditional high-ash-fusion（HAF）bituminous,high-sodium Zhundong coal,and high-sodium Dananhu coal in the combustion experiments of 1-D furnace.With the model sensitivity analysis,the size fractions of fly ash particles contributed by mineral precursors,mineral fragmentation,included minerals,and excluded minerals were classified.The effects of particle coagulation,coal fineness,and condition variations on fly ash formation were quantitatively measured.