准东煤灰的动态沉积与脱落特性数值模拟研究
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Study on Numerical Simulation of Dynamic Deposition and Shedding Characteristics of Zhundong Coal Ash
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收稿日期: 2024-10-16 修回日期: 2025-01-03
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Received: 2024-10-16 Revised: 2025-01-03
作者简介 About authors
目的 电站锅炉在燃用准东煤时会出现严重的受热面沾污结渣问题,为此,对准东煤灰在受热面上的沉积和脱落特性进行了研究。 方法 建立了基于颗粒黏附能的沉积灰侵蚀模型,对准东煤灰在单管上的动态沉积和脱落过程进行了数值模拟研究。 结果 耦合侵蚀机理的模型模拟结果与实验结果之间的误差仅为3.3%,展现出较高的精确度。此外,沉积灰的脱落速率呈先增大后趋于平缓的趋势,但在所模拟的时间范围内仅能使25%的沉积灰脱落。与此同时,沉积灰的累积使受热面的传热损耗速率逐渐增大,6 h时间段内的平均传热损耗速率为1 h时间段的1.76倍。另外,6 h沉积灰的黏附能为1 h的6.11倍,相应的吹灰出口空气质量流率需要提高1.85倍才能有望清除全部的积灰。 结论 研究结果加深了对准东煤灰动态沉积和脱落过程的认识,为工业上的吹灰优化提供了重要研究数据和理论支撑。
关键词:
Objectives Power plant boilers often experience severe fouling and slagging issues on the heating surfaces when burning Zhundong coal. Therefore, the deposition and shedding characteristics of Zhundong coal ash on the heating surface were studied. Methods A deposited ash erosion model based on particle adhesion energy is established, and numerical simulations are then conducted to examine the dynamic deposition and shedding processes of Zhundong coal ash on a single tube. Results The model incorporating the coupled erosion mechanisms has a deviation of only 3.3% compared to the experimental results, demonstrating high accuracy. Besides, the shedding rate of the deposited ash increases initially but then levels off. However, within the simulated timeframe, only 25% of the deposited ash can be removed. In addition, the accumulation of deposited ash leads to a gradual increase in the heat transfer loss rate of the heating surface, with the average heat transfer loss rate during a 6-hour period being 1.76 times that of a 1-hour period. Furthermore, the adhesion energy of the deposited ash after 6 hours is 6.11 times that after 1 hour, and the corresponding mass flow rate of air at the soot-blowing outlet needs to be increased by 1.85 times to effectively remove all the deposited ash. Conclusions These findings enhance the understanding of the dynamic deposition and shedding processes of Zhundong coal ash and offer important research data and theoretical support for optimizing soot-blowing operations in industrial applications.
Keywords:
本文引用格式
郭前鑫, 李建波, 王虎, 梁银堂, 韩新建, 阮雄伟, 卢啸风.
GUO Qianxin, LI Jianbo, WANG Hu, LIANG Yintang, HAN Xinjian, RUAN Xiongwei, LU Xiaofeng.
0 引言
与实验手段相比,通过数值模拟可以对沉积灰的沉积、脱落等动态过程以及沉积机理的作用效果进行分析,因此成为研究灰沉积的主要手段[18]。截至目前,国内外学者在考虑不同沉积机理的情况下,开发出了临界黏度模型[19]、熔体分数模型[20]、临界速度模型[21]和冷凝沉积模型[22]等,各模型的优缺点详见文献[23-24]。近期,Liang等[23]和Guo等[24]采用修正后的黏度模型很好地预测了准东煤灰的沉积过程。与此同时,已沉积的灰颗粒会在飞灰和吹灰介质的冲击侵蚀下发生脱落。对此,Bouris等[25]开发建立了一个耦合脱落机理的数值模型,并对某燃褐煤电站锅炉的灰沉积过程进行了数值模拟研究。此外,Wang等[21]通过能量守恒研究了沉积灰的流动特性与实时污垢特性。Zhou等[26]引入了一种新的侵蚀模型,并对颗粒撞击过程消耗的能量和沉积表面的熔体分数进行了系统分析,结果显示,耦合侵蚀模型后沉积灰层的厚度与实验结果更为接近。然而,目前对准东煤灰沉积和脱落过程的认识仍然不足,如何根据其脱落特性对吹灰过程进行优化更鲜有报道。
本文通过数值模拟的方法,对准东煤灰在受热面上的沉积和脱落过程进行了研究,并结合颗粒黏附能的思想对积灰脱落所需要的条件进行了分析,旨在加深对灰沉积和脱落过程的认识,为吹灰优化等工业过程提供指导和参考。
1 物理模型
表1 准东煤的工业分析、元素分析以及煤灰化学成分 (%)
Tab. 1
| 化学成分分析(干燥基) | 工业分析 | 元素分析 | ||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| w(SiO2) | w(Al2O3) | w(Fe2O3) | w(CaO) | w(MgO) | w(TiO2) | w(SO3) | w(P2O5) | w(K2O) | w(Na2O) | w(C) | w(H) | w(O) | w(N) | w(S) | ||||||
| 11.71 | 6.69 | 5.93 | 32.51 | 7.56 | 0.39 | 27.93 | 0.09 | 0.44 | 4.9 | 15.6 | 30.07 | 64.84 | 5.09 | 75.45 | 3.51 | 14.69 | 0.69 | 0.57 | ||
表2 模拟输入的条件
Tab. 2
2 数学模型及求解
2.1 侵蚀模型
在灰的沉积过程,沉积灰的脱落主要来自灰颗粒以及蒸汽、激波等介质这2个部分对沉积灰层的冲击侵蚀作用。
2.2.1 灰颗粒的侵蚀作用
式中:
灰颗粒撞击后的能量耗散主要由颗粒塑性变形以及弹性波在沉积灰层中的传播和耗散引起。因此以校正系数
对于沉积灰层内部的灰颗粒黏附能量,基于Liu等[37]的研究,应用碱涂层表面黏附模型进行计算。上述研究认为,沉积灰层中的灰颗粒黏附能是由沉积灰颗粒表面黏性层与沉积灰层表面黏性层的表面张力作用主导的。基于此,黏性涂层的厚度计算如下:
式中:
对于一个沉积灰颗粒,通过碱涂层表面黏附模型可计算去除它所需的能量
式中
将碰撞灰颗粒的侵蚀能量
2.2.2 吹灰介质的侵蚀作用
基于侵蚀模型计算沉积灰层黏附能,具体计算公式如下:
式中:
吹灰过程需要提供足够的冲击能量才能将沉积灰清除。本研究中,假设在换热管道正前方50 mm处设置一直径为5 mm的空气吹灰器,通过空气射流对换热管道沉积灰进行吹扫。
空气射流在传播过程中存在能量耗散的问题,本文结合气体射流公式与物理模型几何特征对其进行计算。从射流出口到换热壁面的断面平均空气流速变化以及断面空气质量流率变化计算式分别为:
式中:
空气射流到达换热管道处时,其覆盖范围会大于换热管道面积。本文在计算冲击能量时,仅对换热管道截面内的冲击能量进行了计算,将高速空气动能等价于吹灰冲击能量,使其恰好与换热管道平均黏附能相同。
2.2 模型求解
在数值计算前,本文还进行了网格无关性分析,共生成了网格总数为52 381、109 884和136 921的网格系统,并在109 884与136 921网格数下得到了相似的结果。为节约运算时间,选择网格数为109 884的网格系统进行计算[23]。
3 结果与讨论
3.1 模型验证
图1
图1
积灰厚度及其分布特性模拟结果与实验结果对比
Fig. 1
Comparison between simulated and experimental results of deposited ash thickness and its distribution characteristics
3.2 沉积灰的动态增长与脱落特性
图2给出了有无侵蚀模型时的沉积灰形貌,可知,随着时间的推移,沉积灰的高度整体呈增高趋势。但是由于灰颗粒的侵蚀作用,引入侵蚀模型的沉积灰在同一时刻比未引入侵蚀模型时偏低。此外,驻点处法向碰撞的颗粒数最多,侵蚀脱落现象也最为严重,采用侵蚀模型后驻点处沉积灰厚度的减小更加明显。
图2
图2
有无侵蚀模型时的沉积灰形貌
Fig. 2
Morphology of deposited ash with and without erosion model
图3
图3
换热管道沉积灰质量及脱落质量随时间的变化规律
Fig. 3
Variation patterns of deposited ash mass and shedding amount in heat exchange tube over time
耦合侵蚀模型后,沉积灰的质量在前1.5 h内与无侵蚀模型时基本一致,但随着时间的推移,沉积灰的质量逐渐降低,在6 h处已经与无侵蚀模型有了明显的差异,表明由飞灰颗粒侵蚀导致的脱落现象越来越显著。而就侵蚀脱落速率而言,沉积灰侵蚀脱落速率在初始1.5 h内基本维持在0.000 12 kg/(m⋅h);随后在1.5~3.5 h阶段逐渐变大并在3.5~4.5 h阶段达到峰值,且在最后的4.5~6 h保持此水平。此外还可以发现,脱落的沉积灰质量约占总沉积量的25%,说明由飞灰颗粒本身冲击造成的积灰脱落量相对较少,还需要其他手段[27]来清除受热面上的积灰。
在入口灰颗粒恒定的情况下,沉积灰侵蚀脱落质量由灰颗粒的平均黏附能控制,而这又由不同网格面的沉积灰颗粒平均粒径与沉积灰层表面温度(表面张力系数)决定[37]。图4给出了沉积灰平均粒径随时间的变化。其中,驻点处的平均粒径曲线为换热管道迎风侧驻点处对应网格面内的沉积灰颗粒平均粒径,也就是驻点处网格面内的平均黏附能计算粒径;而整个换热管道表面的平均粒径由整个换热管道上的沉积灰颗粒粒径进行平均得到。由图4可得,换热表面总沉积灰颗粒的平均粒径随时间的变化趋势较平缓,虽然在灰沉积初期(2 h内)有小部分下降趋势,但在2 h后平均粒径呈缓慢上升趋势。由此可以发现,侵蚀脱落的沉积灰质量与沉积灰颗粒平均粒径并不是完全一一对应。
图4
图4
沉积灰平均粒径随时间的变化
Fig. 4
Variation of average particle size of deposited ash over time
图5给出了沉积灰表面温度随时间的变化规律,可知,沉积灰层表面温度在沉积初期(4 h内)变化较快,随着灰沉积时间越久,其变化趋势越趋于平缓。这是因为温度越高,颗粒的黏附能越大,因此沉积灰的侵蚀脱落倾向在积灰后期逐渐减缓。
图5
图5
沉积灰表面温度随时间的变化规律
Fig. 5
Variation patterns of surface temperature of deposited ash over time
3.3 积灰传热损耗
准东煤灰在受热面上的沉积会影响管外烟气与管内工质的传热,使换热效率降低,进而造成能量损失。因此,本文计算了积灰动态沉积过程的传热损耗变化,计算结果如图6所示。可知,在受热面无积灰的情况下,管道表面与烟气间的温差为238 K。在这种情况下,管外烟气与管内工质的热流量为恒定值7 200 W。随着灰的沉积,管外烟气与管内工质换热的热流量呈抛物线下降趋势,与文献[25]研究结果一致。积灰1 h后,积灰程度相对较轻,但此时管外烟气与管内工质的热流量已经降为6 000 W左右,出现了10%的传热损耗。积灰6 h后,受热面形成高度为3.1 mm的灰锥,此时的热流量降低至3 500 W,积灰的存在导致50%的传热损耗,管外烟气与管内工质的换热效率大大降低。
图6
图6
管内外热流量随时间的变化关系
Fig. 6
Variation of heat flux inside and outside tube over time
表3 传热损耗率随时间的变化关系
Tab. 3
| 灰沉积时间/h | 传热损耗/kJ | 平均传热损耗速率/(kJ/h) |
|---|---|---|
| 1 | 7 145.41 | 7 145.41 |
| 2 | 17 236.44 | 8 618.22 |
| 3 | 29 597.16 | 9 865.72 |
| 4 | 43 663.31 | 10 915.83 |
| 5 | 59 052.21 | 11 810.44 |
| 6 | 75 547.71 | 12 591.28 |
3.4 积灰黏附能与吹灰强度计算
通过计算沉积灰颗粒的平均黏附能,还可以对不同积灰阶段沉积灰脱除所需要的条件进行计算。表4给出了不同积灰时间段沉积灰颗粒的平均黏附能和所需的吹灰质量流率。可知,随着灰沉积时间的延长,换热管道沉积灰黏附能逐渐增大,6 h灰沉积时间段的换热管道沉积灰黏附能为1 h灰沉积时间段的6.11倍;吹灰装置所需空气出口质量流率也相应变大,6 h灰沉积时间段的吹灰装置出口质量流率为1 h灰沉积时间段的1.85倍。因此,在不同的积灰时间段吹灰时,还应当相应地调整吹灰的压力和空气(蒸汽)流量,以最大程度避免吹灰过度或吹灰不足现象。值得说明的是,现有研究暂时没有考虑沉积灰烧结使黏附能增大的情况,后续工作可结合灰的烧结以及烧结前后颗粒间的黏性变化[19, 31],进一步改进现有的侵蚀模型,确保模型的准确性和工程实用性。
表4 沉积灰的平均黏附能和所需的吹灰质量流率
Tab. 4
| 灰沉积时间/h | 换热管道沉积灰黏附能/J | 吹灰装置出口质量流率/(kg/s) |
|---|---|---|
| 1 | 302.39 | 0.003 9 |
| 2 | 597.01 | 0.004 9 |
| 3 | 884.62 | 0.005 6 |
| 4 | 1 201.73 | 0.006 2 |
| 5 | 1 525.69 | 0.006 7 |
| 6 | 1 847.79 | 0.007 2 |
4 结论
建立了耦合侵蚀机理的灰沉积脱落数学模型,对准东煤灰在受热面上的动态沉积和脱落特性进行了数值模拟研究,主要得到以下结论:
1)耦合侵蚀机理的灰沉积模型与未耦合的相比,预测精度更高(与实验误差仅为3.3%),预测结果更为可靠。
2)灰颗粒的黏附沉积速率相对稳定,但沉积灰的侵蚀脱落速率呈先增加后趋于平缓的趋势。准东煤灰自身的侵蚀作用仅能使25%的沉积灰脱落。
3)沉积灰的积累使得传热损耗和颗粒的黏附能逐渐增大,吹灰出口质量流率需要同步提高才能有望清除全部积灰。
参考文献
“双碳”目标下清洁高效灵活煤电技术现状及煤电前景展望
[J].
Techniques status and perspective of efficient-flexible coal fired power generation under carbon peak and neutrality targets
[J].
基于“双碳”目标的中国火力发电技术发展路径研究
[J].
Research on the development path of China’s thermal power generation technology based on the goal of “carbon peak and carbon neutralization”
[J].
广东电力低碳高质量发展下的电煤需求影响分析
[J].
Influence analysis of thermal coal demand under the low-carbon and high-quality development of electric power in Guangdong Province
[J].
“双碳” 目标下煤电机组节能改造技术发展与实践
[J].
Development and practice of energy-saving retrofit technologies for coal power units under carbon peaking and carbon neutrality goals
[J].
新疆能耗双控向碳排放双控转变分析和预测
[J].
Analysis and forecast of the shift from double control of energy consumption to double control of carbon emissions in Xinjiang
[J].
燃准东煤电站锅炉沾污结渣特性及防治措施研究进展
[J].
Research progress on the characteristics and countermeasures of ash slagging and fouling in boilers burning Zhundong coal
[J].
Distribution,occurrence and leaching dynamic behavior of sodium in Zhundong coal
[J].
燃用准东煤过程中碱/碱土金属迁移规律及锅炉结渣沾污研究进展
[J].
Transformation of AAEM and ash deposition characteristics during combustion of Zhundong coal
[J].
The morphological and mineralogical characteristics and thermal conductivity of ash deposits in a 220 MW CFBB firing Zhundong lignite
[J].
Size distribution of fine particles from coal combustion
[J].
添加石灰石对准东煤CFB燃烧过程床料团聚和受热面积灰的影响
[J].
Effect of limestone addition on bed particle agglomeration and ash deposition on heat transfer surface during Zhundong coal combustion in a circulating fluidized bed
[J].
高碱煤钠赋存形态及其燃烧过程中迁移转化的研究进展
[J].
Occurrence of sodium in high alkali coal and its transformation during combustion
[J].
Measurement of atomic sodium release during pyrolysis and combustion of sodium-enriched Zhundong coal pellet
[J].
Ash formation and deposition in coal and biomass fired combustion systems:progress and challenges in the field of ash particle sticking and rebound behavior
[J].
Shedding of ash deposits
[J].
Fine particulate formation and ash deposition during pulverized coal combustion of high-sodium lignite in a down-fired furnace
[J].
Experimental measurement of the effective thermal conductivity of ash deposit for high sodium coal (Zhun Dong coal) in a 300 KW test furnace
[J].
准东煤飞灰沉积特性及数值模拟研究
[J].
Sedimentation characteristics and numerical simulation of Zhundong coal fly ash
[J].
A temperature-history based model for the sticking probability of impacting pulverized coal ash particles
[J].
Towards a CFD-based mechanistic deposit formation model for straw-fired boilers
[J].
Real-time fouling characteristics of a typical heat exchanger used in the waste heat recovery systems
[J].
Prediction of potassium chloride sulfation and its effect on deposition in biomass-fired boilers
[J].
A numerical simulation study of ash deposition in a circulating fluidized bed during Zhundong lignite combustion
[J].
A numerical simulation study into the effect of longitudinal and transverse pitch on deposition of Zhundong coal ash on tube bundles
[J].
Design of a novel,intensified heat exchanger for reduced fouling rates
[J].
Numerical simulation of ash deposition behavior with a novel erosion model using dynamic mesh
[J].
An experimental investigation into mineral transformation,particle agglomeration and ash deposition during combustion of Zhundong lignite in a laboratory-scale circulating fluidized bed
[J].
Dynamic simulation on ash deposition and heat transfer behavior on a staggered tube bundle under high-temperature conditions
[J].
Mechanistic modeling,numerical simulation and validation of slag-layer growth in a coal-fired boiler
[J].
Dynamic CFD modeling evaluation of ash deposition behavior and morphology evolution with different tube arrangements
[J].
Deposition of bituminous coal ash on an isolated heat exchanger tube:effects of coal properties on deposit growth
[J].
The effects of wetting and surface roughness on liquid metal droplet bouncing
[J].
Surface energy and the contact of elastic solids
[J].
Particle deposition model for particulate flows at high temperatures in gas turbine components
[J].
Spread and rebound of liquid droplets upon impact on flat surfaces
[J].
Submodel for predicting slag deposition formation in slagging gasification systems
[J].
Numerical investigation on development of initial ash deposition layer for a high-alkali coal
[J].
煤电机组运行灵活性提升技术研究与应用
[J].
Research and application of operation flexibility improvement technology for coal-fired power unit
[J].
一种T型吹灰装置在锅炉空气预热器上的应用
[J].
Application of a T-type soot blowing device on bolier air preheater
[J].
适应新型电力系统的调峰火电机组空气预热器安全评估策略
[J].
Flexibility-oriented safety assessment strategy for air preheater in thermal power units adapting to the advanced power system
[J].
Numerical simulation and experiment of gas-solid two phase flow and ash deposition on a novel heat transfer surface
[J].
A practical numerical approach for prediction of particulate fouling in PC boilers
[J].
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