柴油机排放颗粒物的热重特性分析

第29卷第16期农业工程学报Vol 29 No. 1502013年8月Transactions of the Chinese Society of Agricultural EngineeringAug.2013柴油机排放颗粒物的热重特性分析梅德清',赵翔1,王书龙1,袁银男2,孙平1(1.江苏大学汽车与交通工程学院,镇江212013;2.南通大学机械学院,南通226019)摘要:为了解不同粒径颗粒物在特定氛围下的氧化特性,该文利用 MOUDI采样器收集到的柴油机颗粒物,在纯N2及纯O2环境下,对0.18~0.32、0.32~0.56、0.56~1.00和1.00~1.80mm4个粒径级颗粒分别进行热重分析试验。结果表明,随着粒径级的增大,颗粒物中水分和SOF( soluble organic fraction)的含量下降,而干碳烟(soot)和无机盐的含量增加。在纯N2氛围下,颗粒在SOF挥发阶段随着颗粒物粒径级减小,其SOF含量和失重峰值速率随之增加;但在soot热解阶段不同粒径级的失重速率趋同。程序升温终了时,各粒径级颗粒的热重曲线在纯N2氛围下缓滞停在不同位置,而在纯O2氛围下则渐趋归一。随着颗粒物粒径级的减小,热重曲线呈下降趋势,颗粒越细则越易升温(氧化)失重:而在纯O2氛围下,各粒径级在SOF挥发阶段表现出与纯N2氛围下一致的规律但失重速率峰值明显增加。在soot热解阶段,随着颗粒物粒度减小,比表面积増加使其吸附氧的能力増加发生化茡反应的活性増大,颗粒氧化的起燃温度降低,且起燃时刻对应的失重速率增加:各粒径级颗粒物的失重速率峰值出现在600~640℃之间,随着颗粒物粒径増大其soυt所占含量随之増多,热解失重速率峰值亦显著増加。研究结果可为颗粒物处理的技术措施提供基础物性数据,有助于推动颗粒物处理装置的改进和优化。关键词:柴油机,颗粒物,氧化,热重分析doi:10.3969/s.1002-68192013.16.007中图分类号:TK421.5文献标志码:A文章编号:1002-6819(2013)-16-0050-0梅德清,赵翔,王书龙,等.柴油机排放颗粒物的热重特性分析[J].农业工程学报,2013,29(16):50-56Mei Deqing, Zhao Xiang, Wang Shulong, et al. Thermogravimetric characteristics analysis of particulate matter ofemission of divided diesel[J]. Transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSae2013, 29(16): 50-56 ( in Chinese with English abstract)0引言氛围下其成分及含量基本不发生改变。可挥发性组分主要为少量水分及碳氢化合物,在惰性气氛氛围柴油机排出的颗粒物作为大气污染源之一,其中会随温度升高而被蒸发掉。对颗粒中可溶性有机粒径绝大多数分布在PM1(粒径小于或等于1{m)成分(SOF, soluable organic fraction)的研究是颗范围内,属PM25(粒径小于或等于2.5m)范畴,粒毒性机理研究的主要组成部分1,因此对颗粒其粒度特性及后继处理受到密切重视。一般来说,组成中SOF的研究尤为重要柴油机排气颗粒粒径呈单峰或双峰分布,单峰时粒柴油机燃烧过程产生的颗粒物在排向大气径峰值在60~120nm之间,主要为积聚模态颗粒;前,通过氧化使颗粒物消减已成为主要控制手部分情况下呈双峰分布,另一峰值为10~30nm之段。而热重法则是研究颗粒物氧化的一个重要方间的核模态3。而空气中不同粒径的颗粒在人体内法1315。热重分析法通过样品质量随温度的变化沉积部位及难易程度不同,其诱发疾病的类型也不关系来表征样品的物理、化学变化过程,可以获同4。通常认为颗粒物粒径越小对人体的健康危害得样品质量与温度的关系曲线(TG,更大6。thermogravimetry)和微商热重曲线(DTG根据颗粒物组分的挥发性质可将颗粒中的成 derivative thermogravimetry)。分级颗粒物的热分分为可挥发组分和不可挥发组分。其中不可挥发重试验将分析不同粒径级颗粒物氧化过程中氧组分主要为干碳烟(soot)和无机盐,在惰性气氛化着火开始时刻、最大燃烧速率及燃烧结束时刻等一系列特征点处的变化规律,评定不同粒径颗收稿日期:2013-05-23修订日期:2013-07-18粒物在特定氛围下的氧化特性,为颗粒物减排技基金项目:国家自然科学基金(50976051);江苏省交通运输厅科技创术措施提供基础物性数据新公关计划项目(10Y25):江苏高校优势学科资助项目(苏政发办[20l16号)现有文献中,可与分级颗粒物类比的是不同粒作者简介:梅德清(1974-),男,副教授,主要从事发动机排放控制度煤粉的热重与新能源研究。镇江江苏大学汽车与交通工程学院,212013中国煤化工煤粉颗粒粒径的减小,煤Email:meideqing@ujs.edu.cnCNMHG明显,最大第16期梅德清等:柴油机排放颗粒物的热重特性分析燃烧速率出现得越早,着火温度也会相应降低。粒微孔均匀沉积冲击式采样器( MOUDI,度是影响煤粉热重特性的重要因素1618,但研究 micro- orifice uniform deposition impactor)是基于空对象是较大尺度的粒度分级(7.0~35.9m)。尽气动力学方法测量大气气溶胶粒径分布的重要仪管柴油杋颗粒物与煤粉的相似之处在于两者的主器。该装置不仅可以获得颗粒粒径分布的规律,而要成分同为碳,但针对更细粒径区间(0.18且能收集分级后的各粒径级颗粒232。可用于分级1.8μm)颗粒物的氧化特性研究还很少。本文利用样品的进一步化学组分、微观形貌、氧化特性和生热重分析法,对 MOUDI采样器收集到的不同粒径物毒性的研究。柴油机颗粒物进行氧化特性分析,旨在综合评定不MOUDI颗粒取样系统结构如图1所示。为同粒径颗粒物在特定氛围下的氧化特性,以期为了更加真实地模拟排气在实际环境中的扩散过DPF( diesel particulate filters)中颗粒物的氧化再程以及满足仪器入口气流的温度条件,在取样生提供机理依据。管道前柴油机原始排气需引入洁净新鲜空气进1材料与方法行充分稀释并冷却。试验开始前根据上下压差对流量进行标定。在真空抽气泵的作用下,稀1.1试验样机释后的发动机排气以30L/min的恒体积流量进试验用发动机为D754G增压柴油机,其主要入冲击器,颗粒物在惯性作用下逐阶分级并被技术参数见表1置于冲击板上的冲击介质铝箔(Φ47mm,MSP表1D754G柴油机的主要性能参数公司)所捕获。Table I Technical parameters of D754G diesel engine参数 Parameters数值 values增压 Turbo-charged型式Type冲程 Four -st气缸数 Number of cylinders缸径 Cylinder bore/mm行程 Stroke/mm连杆 Connecting rod/mm缩比 Compression ratiol8.0供油提前角Fuel supply advance angle/o)a. MOUDI颗粒采集系统现场图喷油泵 Fuel pumpL659-3MOUDI particle collection sy油嘴 Nozzle38125标定功率 Nominal power/kw标定转速 Rated speed/(rmin)2200最大扭矩 Maximum torque(Nm)最大扭Maximum torqu1.2排气颗粒的采集1.2.1颗粒采集装置目前,用于测量尾气按粒径区分的颗粒数目浓度分布的装置主要分为2类。一是扫描电迁移率粒b. MOUDI颗粒采集系统示意图b Sketch of MOudI particle collection system子分析仪(SMPS, scanning mobility particle sizers),1.柴油机颗粒排气2.稀释空气流量调节阀4.浮子流量计基于不同大小颗粒具有不同电迁移率的原理,具有5.惯性冲击器6.压差表7.上压差表仪表盘8.下压差表仪表盘9.流量调节阀10.抽气泵较高的瞬态响应,测量范围为9.6~352nm19201. Diesel engine exhaust gas 2. Dilution air 3. Flow control valve 4.Float flow meter 5. Inertial impactor 6. Differential pre另一个是电子低压冲击器(ELPI, electrical low Up board of differential pressure 8. Down board of diffepressurepressure impactor),既可以实时瞬时测量粒子数目9. Flow control valve 10 Air pump图1 MOUDI颗粒取样系统分布,也可以作为惯性冲击器使用获得质量尺寸分Fig 1 MOUDI particle collection system布212这2类仪器在测量过程中都需要对原始排气进行高倍稀释,电迁移率粒子分析仪不带有颗粒1.2.2颗粒采集试验方案分级捕获装置,ELPⅠ可以获得分级颗粒物,但捕获在采集中国煤化工动机转速高收集的量较小。低和负荷大HCNMHG的影响,为农业工程学报2013年使样品具有普遍代表性,按8工况(非道路)标2试验结果与分析准循环及国标(GB20891-2007)对应的权重分配采集颗粒物,如表2所示。采集样品直至获2.1分级颗粒物挥发特性分析取足够质量(2mg以上)的0.18~1.8m区间4图2为纯N2气氛下不同粒径级颗粒的TG及个粒径级的分级颗粒物,颗粒样品分类放置于洁DTG曲线图。在纯N2环境下只有样品的挥发和分净密闭的器皿中。由于柴油机排出尾气中大于解,而没有氧化促进分解失重的行为,可以更好地1.8/m粒径级段颗粒物含量很少,难以收集足够表征颗粒物各个升温失重阶段的区分。由图2中质量用于试验,因此对粒径大于1.8m的颗粒TG曲线可以看出,在程序温度范围内,颗粒质量未纳入研究范围随温度升高而不断减少,相应的质量变化率DTG曲线处于负值,其绝对值表示失重速率。颗粒在表2颗粒采集测试工况升温过程中主要发生了3个阶段的物理化学变Table 2 Test modes for particulates samplin化,即水分的挥发、SOF组分的挥发和soot的热测试工况计权因子转速扭矩Test modeSpeed/(r-min) Torque/(N- m)解26。结合图2的曲线数据及各个组分失重阶段2200221的区分,表3列出了各粒径级别通过热重分析得到的组分含量。若将105℃以内样品的失重主要I11考虑为水的蒸发,在TG曲线上此阶段失重约为2.0%,且由表3可见随着粒径级的增大颗粒物中含水的百分率在下降。第二阶段为可溶14001400性有机物SOF的挥发,在150~400℃之间有一个明显的失重速率峰,峰值出现在200℃C附近。由图2b可见,在这一阶段的失重速率峰值出现以1.3热重特性试验前,随着温度的增加,颗粒物粒径级越小,失重利用瑞士梅特勒-托利多公司的 TGA/DSCI热分速率越大。较小的粒径级0.18-0.32m具有最大析仪器,对4个粒径区间的分级颗粒进行纯N2及的失重速率峰值,而且随着颗粒粒径级的增大纯O2氛围下的热重分析试验。该峰值减小,与此同时出现失重速率峰的对应的首先对0.18~0.32、0.32~0.56、0.56~1.0温度也前移。这说明随着颗粒粒径减小,比表面和1.0~1.8m4个粒径级颗粒分别进行惰性氛积增加,其表面官能团和表面活性增加,这都促围下的热重分析。各级样品量约1mg,试验氛进颗粒吸附力的增加2x28。因而较小粒径级颗粒围为纯N2,流量为50mL/min,保护气为物SOF含量更高。第三阶段为颗粒物中的soot20m/min的纯N2,程序温度区间40~750℃,组分在纯N2氛围高温下的分解失重,由于温度持升温速率20℃/min。续上升,soot的热分解稍有加快,DTG曲线上约其后进行分级颗粒在氧化氛围下的热重分在520℃之后失重速率曲线出现持续下降。但和析。颗粒样品为原始样品,未经挥发或干燥处理。前一阶段SOF的挥发相比,soot分解的失重速率进样量、各气路流量及升温程序等与前述纯N2总体很低。不同粒径级颗粒经过前述的水分蒸发氛围热重试验一致,将热重炉腔内的气氛氛围变和SOF挥发后,在soot热解阶段剩下多为碳,其为纯O2。失重速率趋同。50粒径 Particle size01R-032>0.32~0.56->0.56-1.00600温度 Temperature/C温度 Temperature℃a.TG曲线b.DTG曲线a. TG curves图2分级颗粒纯N2氛围下的TG和DTG中国煤化工Fig 2 TG and DTG curves of divided particles underCNMHG第16期梅德清等:柴油机排放颗粒物的热重特性分析表3分级颗粒各组分含量峰值出现时刻及各粒径级颗粒的失重规律与纯N2氛Table 3 Contents of each components in divided particles围下对应的热重特性总体上一致。只是由于温度已无机盐近200°C,纯O2的氧化氛围极大地促进了颗粒物中ParticleH2O/%SOF% Soot/% Inorganic saltSOF的挥发与氧化,见表4中2种氛围下各个粒径0.18~0,3232.40级颗粒物的失重速率峰值发生时刻及质量变化率的>0.56~1.0025.83列表比较。纯O2氛围下造成的失重速率峰值变化率4.71也随着颗粒物粒径级的减小而呈现増加的趋势。在2.2分级颗粒氧化特性分析图3b中,其后出现的失重峰表征着颗粒物中soo图2a中当临近程序升温终了时,soot组分在的氧化失重特性。由于纯O2的强烈氧化氛围促使颗N2氛围下失重渐趋缓滞,由于各粒径级颗粒中soot粒中的soot发生烟煤基元氧化反应,使得soot的热组分含量的差异导致TG曲线以不同的位置结束。而解失重与纯N2氛围相比呈现2个区别。一是与纯在图3a对应的氧化氛围中,sot组分会继续被氧化N2氛围下出现的微小的失重速率特性曲线明显不失重,因而各粒径级颗粒物的TG曲线尽管中间过程同,在纯O2氛围下sot热解阶段呈现一个更大的有明显差异,但终了时会出现归一现象。从图2a和失重峰。其次,从N2氛围的颗粒物DTG曲线上从图3a均可见,随着颗粒物粒径级的减小,TG曲线SOF挥发到soot分解这一段出现的拐点可以看出均有下降,说明颗粒越细则越易在升温中失重soot分解约在520℃之后进行;而在纯O2氛围下由图3b可见,在纯O2氛围下颗粒物的热重特DTG曲线上在400℃之后就可见失重速率曲线略有性明显呈现两个失重峰。第一个失重峰表征颗粒物増加,说明soot分解已经进行,热解开始时刻对应中SOF挥发特性。在该阶段,纯O2氛围下失重速率的温度明显提前。82令50粒径 Particle size/um0.18~0.32>0.32~0.56>0.56~1.00>1.00~1.80200温度 Temperature/℃cTG曲线b.DTG曲线a. TG curvesb, DTG curves图3分级颗粒纯O2氛围下的TG和DTG曲线Fig3 TG and DTG curves of divided particles under O: atmosphere表4不同氛围下颗粒物SOF失重特性比较Table 4 Analysis about ignition and peak of particles oxidation reaction纯N2氛围N2 atmosphere纯O2氛围O2 atmosphere重率峰值失重率峰值对应温度失重率峰值对应温度增加率Particle size/umPeak rate of weightCorrespondinPeak rate of weightCorrespondingIncrease rate of Peakloss(%.℃')temperature/℃rate of weight loss/%0.18-0.320.1940.2860.17719870.24800~1.800.104181.8在soot氧化阶段,各个粒径级颗粒的起燃时附一些气体分子,温度很高时碳晶体通过络合作刻及质量变化率峰值分析见表5,结合图3b可见用化学吸附周界的气体分子。颗粒的氧化反应在随着颗粒粒径减小颗粒氧化的起燃温度降低,且其内部表面上进行,粒度减小(颗粒物更疏松)起燃时刻对应的失重速率增加;各粒径级颗粒物时发生化学反应的活性增大9-30。自进入soot热的失重速率峰值出现在600~640℃之间,随着颗解阶段始,0.189粒径级甄粒物的失重速粒物粒径级增大其soot热解失重速率峰值也在增率曲线处于最中国煤化工达到失重速加。常温下soot组分的碳粒子晶体表面会物理吸率峰值,说明CNMHG被氧化。由农业工程学报2013年前述可知,随着粒径级的变大,颗粒物中soot的热解阶段可以有越多的原料参与反应,因此其在组分含量就越多,这意味着在纯O2氛围的 soot soot热解阶段的失重速率就越大表5soot氧化阶段起燃时刻及质量变化率峰值分析Table 5 Analysis about ignition and peak of mass change rate for particles oxidation reactie起燃时刻 Beginning of ignition峰值时刻 Timing of peak粒径 Particle size/un大重率峰值对应温度Peak rate of weight lossCorrespondingPeak rate of weight lossorresponding temperatureemperature/07,3>0.32~0.5>0.56~1.000.665622.71.00~1.800.02532580.792630.33结论[2 Raux S, Forti L, Barbusse S, et al. 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Technol, 1998English abstract)32(14):2033-2042[30] Nugroho Y S, McIntosh A C, Gibbs B M21 Matti Maricq M, Diane H Podsiadlik, Richard E Chase, etLow-temperal. size distributions of motor vehicle exhaust pm:aFuel 2000H中国煤化工 lended coalCNMHG农业工程学报2013年31]GB20891-2007.非道路移动机械用柴油机排气污染物exhaust pollutants from diesel engines of non-road放限值及测量方法(中国Ⅰ、Ⅱ阶段)S].北京:中匡mobile machinery(I、Ⅱ)ISl. Beijing: China标准出版社,2007Standard Press, 2007. (in Chinese with EnglishGB20891-2007. Limits and measurement methods foabstract)Thermogravimetric characteristics analysis of particulate matter ofemission of divided dieselMei Deqing, Zhao Xiang, Wang Shulong, Yuan Yinnan, Sun Ping(1. School of Automotive and Traffic Engineering, Jiangsu University, Zhenjiang 212013, china,2. School of Mechanical Engineering, Nantong University, Nantong 226019, China)Abstract: Particulate matter(PM), which contains soluble organic fraction(SOF), soot and inorganic salt, hasbeen one of the main pollutions from diesel engine. Longtime exposure to the particulate matters especially thosesmaller than 2.5 micrometers(PM2. 5) which can go directly to the alveoli of the lungs, is a major health hazardIn addition, many studies have revealed that the particles of smaller size would bring about greater harm to thehuman body. Due to the potential health risk in urban areas, the elimination of fine and ultrafine particulatesemissions from diesel engine has attracted much attention in recent years. Meanwhile, the strict regulations forPM emission have been enforced in many developed countries. The micro-orifice uniform deposition impactor(MOUDIis a favorable apparatus for measuring particles size distribution of atmospheric aerosol base onaerodynamic method. It can not only obtain the particle size distribution but also collect the particles in thedifferent size ranges after classification, which extremely facilitates further research for chemical componentsmicrostructure, oxidation characteristic and biological toxicity of the divided particle samples. Thermogravimetricanalysis (TGA)has been widely used as an analytical method for kinetic of chemical reactions. Thermalgravimetric analysis means to investigate the relationship between the material weight and temperature under thecondition of programming temperature rise. The curve of sample weight then can be obtained with thetemperature Derivative thermal gravimetry(DTG) curve, the first order differential to TG curve, can reveal thefeatures of mass variation with the temperature. To study the ignition and oxidation properties of particulates,thermogravimetric method would be an effectual option. The particulates collected by mouDI were divided intore ranges which were 0. 18-0.32um, 0.32-0.56um, 0.56-1 Oum and 1.0-1 8um Thermogravimetricexperiments on the divided particulates were carried out in the atmosphere of N2 and then O2. The experimentalresults showed that the moisture and sof contents in Pm increased with the increase of particle size, while thesoot and inorganic salt contents decreased conversely. During the SoF volatilation phase in the atmosphere of Nthe sof content and the peak weight loss rate of particles decreased with the decrease of particle size. However,the weight loss rates of the divided particles converged during the soot pyrolysis phase. At the end of programmedtemperature rise, the TG curves of particles of each size in N2 atmosphere slowly stopped at various positionswhile those were almost overlapping in O2 atmosphere. With the decrease of particle size, the TG curves showedan apparent downward trend, which means the smaller the particle size was the easier the losing of weight wadue to heating or including oxidation. In O2 atmosphere, the derivative thermal gravimetry (DTG) curves ofdifferent sizes of particles in the SOF volatilation phase were consistent with those in N2 atmosphere but the peakweight loss rates augmented evidently. In the soot pyrolysis phase, the soot content increased with the increase ofparticle size, and the peak weight loss rate also boosted significantly by oxidation of O2. This study aimed atassessing the oxidation features of different sizes of particles in the specified atmosphere and providingfundamental data for the after-treatment technology of diesel particulate matteKey words: diesel engines, particles, oxidation, thermogravimetric analysis(责任编辑:鲍洪杰)中国煤化工CNMHG