柴油機排放顆粒物的熱重特性分析

第29卷第16期農業(yè)工程學(xué)報Vol 29 No. 1502013年8月Transactions of the Chinese Society of Agricultural EngineeringAug.2013柴油機排放顆粒物的熱重特性分析梅德清',趙翔1,王書(shū)龍1,袁銀男2,孫平1(1.江蘇大學(xué)汽車(chē)與交通工程學(xué)院,鎮江212013;2.南通大學(xué)機械學(xué)院,南通226019)摘要:為了解不同粒徑顆粒物在特定氛圍下的氧化特性,該文利用 MOUDI采樣器收集到的柴油機顆粒物,在純N2及純O2環(huán)境下,對0.18~0.32、0.32~0.56、0.56~1.00和1.00~1.80mm4個(gè)粒徑級顆粒分別進(jìn)行熱重分析試驗。結果表明,隨著(zhù)粒徑級的增大,顆粒物中水分和SOF( soluble organic fraction)的含量下降,而干碳煙(soot)和無(wú)機鹽的含量增加。在純N2氛圍下,顆粒在SOF揮發(fā)階段隨著(zhù)顆粒物粒徑級減小,其SOF含量和失重峰值速率隨之增加;但在soot熱解階段不同粒徑級的失重速率趨同。程序升溫終了時(shí),各粒徑級顆粒的熱重曲線(xiàn)在純N2氛圍下緩滯停在不同位置,而在純O2氛圍下則漸趨歸一。隨著(zhù)顆粒物粒徑級的減小,熱重曲線(xiàn)呈下降趨勢,顆粒越細則越易升溫(氧化)失重:而在純O2氛圍下,各粒徑級在SOF揮發(fā)階段表現出與純N2氛圍下一致的規律但失重速率峰值明顯增加。在soot熱解階段,隨著(zhù)顆粒物粒度減小,比表面積増加使其吸附氧的能力増加發(fā)生化茡反應的活性増大,顆粒氧化的起燃溫度降低,且起燃時(shí)刻對應的失重速率增加:各粒徑級顆粒物的失重速率峰值出現在600~640℃之間,隨著(zhù)顆粒物粒徑増大其soυt所占含量隨之増多,熱解失重速率峰值亦顯著(zhù)増加。研究結果可為顆粒物處理的技術(shù)措施提供基礎物性數據,有助于推動(dòng)顆粒物處理裝置的改進(jìn)和優(yōu)化。關(guān)鍵詞:柴油機,顆粒物,氧化,熱重分析doi:10.3969/s.1002-68192013.16.007中圖分類(lèi)號:TK421.5文獻標志碼:A文章編號:1002-6819(2013)-16-0050-0梅德清,趙翔,王書(shū)龍,等.柴油機排放顆粒物的熱重特性分析[J].農業(yè)工程學(xué)報,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引言氛圍下其成分及含量基本不發(fā)生改變?？蓳]發(fā)性組分主要為少量水分及碳氫化合物,在惰性氣氛氛圍柴油機排出的顆粒物作為大氣污染源之一,其中會(huì )隨溫度升高而被蒸發(fā)掉。對顆粒中可溶性有機粒徑絕大多數分布在PM1(粒徑小于或等于1{m)成分(SOF, soluable organic fraction)的研究是顆范圍內,屬PM25(粒徑小于或等于2.5m)范疇,粒毒性機理研究的主要組成部分1,因此對顆粒其粒度特性及后繼處理受到密切重視。一般來(lái)說(shuō),組成中SOF的研究尤為重要柴油機排氣顆粒粒徑呈單峰或雙峰分布,單峰時(shí)粒柴油機燃燒過(guò)程產(chǎn)生的顆粒物在排向大氣徑峰值在60~120nm之間,主要為積聚模態(tài)顆粒;前,通過(guò)氧化使顆粒物消減已成為主要控制手部分情況下呈雙峰分布,另一峰值為10~30nm之段。而熱重法則是研究顆粒物氧化的一個(gè)重要方間的核模態(tài)3。而空氣中不同粒徑的顆粒在人體內法1315。熱重分析法通過(guò)樣品質(zhì)量隨溫度的變化沉積部位及難易程度不同,其誘發(fā)疾病的類(lèi)型也不關(guān)系來(lái)表征樣品的物理、化學(xué)變化過(guò)程,可以獲同4。通常認為顆粒物粒徑越小對人體的健康危害得樣品質(zhì)量與溫度的關(guān)系曲線(xiàn)(TG,更大6。thermogravimetry)和微商熱重曲線(xiàn)(DTG根據顆粒物組分的揮發(fā)性質(zhì)可將顆粒中的成 derivative thermogravimetry)。分級顆粒物的熱分分為可揮發(fā)組分和不可揮發(fā)組分。其中不可揮發(fā)重試驗將分析不同粒徑級顆粒物氧化過(guò)程中氧組分主要為干碳煙(soot)和無(wú)機鹽,在惰性氣氛化著(zhù)火開(kāi)始時(shí)刻、最大燃燒速率及燃燒結束時(shí)刻等一系列特征點(diǎn)處的變化規律,評定不同粒徑顆收稿日期:2013-05-23修訂日期:2013-07-18粒物在特定氛圍下的氧化特性,為顆粒物減排技基金項目:國家自然科學(xué)基金(50976051);江蘇省交通運輸廳科技創(chuàng )術(shù)措施提供基礎物性數據新公關(guān)計劃項目(10Y25):江蘇高校優(yōu)勢學(xué)科資助項目(蘇政發(fā)辦[20l16號)現有文獻中,可與分級顆粒物類(lèi)比的是不同粒作者簡(jiǎn)介:梅德清(1974-),男,副教授,主要從事發(fā)動(dòng)機排放控制度煤粉的熱重與新能源研究。鎮江江蘇大學(xué)汽車(chē)與交通工程學(xué)院,212013中國煤化工煤粉顆粒粒徑的減小,煤Email:meideqing@ujs.edu.cnCNMHG明顯,最大第16期梅德清等:柴油機排放顆粒物的熱重特性分析燃燒速率出現得越早,著(zhù)火溫度也會(huì )相應降低。粒微孔均勻沉積沖擊式采樣器( MOUDI,度是影響煤粉熱重特性的重要因素1618,但研究 micro- orifice uniform deposition impactor)是基于空對象是較大尺度的粒度分級(7.0~35.9m)。盡氣動(dòng)力學(xué)方法測量大氣氣溶膠粒徑分布的重要儀管柴油杋顆粒物與煤粉的相似之處在于兩者的主器。該裝置不僅可以獲得顆粒粒徑分布的規律,而要成分同為碳,但針對更細粒徑區間(0.18且能收集分級后的各粒徑級顆粒232?？捎糜诜旨?.8μm)顆粒物的氧化特性研究還很少。本文利用樣品的進(jìn)一步化學(xué)組分、微觀(guān)形貌、氧化特性和生熱重分析法,對 MOUDI采樣器收集到的不同粒徑物毒性的研究。柴油機顆粒物進(jìn)行氧化特性分析,旨在綜合評定不MOUDI顆粒取樣系統結構如圖1所示。為同粒徑顆粒物在特定氛圍下的氧化特性,以期為了更加真實(shí)地模擬排氣在實(shí)際環(huán)境中的擴散過(guò)DPF( diesel particulate filters)中顆粒物的氧化再程以及滿(mǎn)足儀器入口氣流的溫度條件,在取樣生提供機理依據。管道前柴油機原始排氣需引入潔凈新鮮空氣進(jìn)1材料與方法行充分稀釋并冷卻。試驗開(kāi)始前根據上下壓差對流量進(jìn)行標定。在真空抽氣泵的作用下,稀1.1試驗樣機釋后的發(fā)動(dòng)機排氣以30L/min的恒體積流量進(jìn)試驗用發(fā)動(dòng)機為D754G增壓柴油機,其主要入沖擊器,顆粒物在慣性作用下逐階分級并被技術(shù)參數見(jiàn)表1置于沖擊板上的沖擊介質(zhì)鋁箔(Φ47mm,MSP表1D754G柴油機的主要性能參數公司)所捕獲。Table I Technical parameters of D754G diesel engine參數 Parameters數值 values增壓 Turbo-charged型式Type沖程 Four -st氣缸數 Number of cylinders缸徑 Cylinder bore/mm行程 Stroke/mm連桿 Connecting rod/mm縮比 Compression ratiol8.0供油提前角Fuel supply advance angle/o)a. MOUDI顆粒采集系統現場(chǎng)圖噴油泵 Fuel pumpL659-3MOUDI particle collection sy油嘴 Nozzle38125標定功率 Nominal power/kw標定轉速 Rated speed/(rmin)2200最大扭矩 Maximum torque(Nm)最大扭Maximum torqu1.2排氣顆粒的采集1.2.1顆粒采集裝置目前,用于測量尾氣按粒徑區分的顆粒數目濃度分布的裝置主要分為2類(lèi)。一是掃描電遷移率粒b. MOUDI顆粒采集系統示意圖b Sketch of MOudI particle collection system子分析儀(SMPS, scanning mobility particle sizers),1.柴油機顆粒排氣2.稀釋空氣流量調節閥4.浮子流量計基于不同大小顆粒具有不同電遷移率的原理,具有5.慣性沖擊器6.壓差表7.上壓差表儀表盤(pán)8.下壓差表儀表盤(pán)9.流量調節閥10.抽氣泵較高的瞬態(tài)響應,測量范圍為9.6~352nm19201. Diesel engine exhaust gas 2. Dilution air 3. Flow control valve 4.Float flow meter 5. Inertial impactor 6. Differential pre另一個(gè)是電子低壓沖擊器(ELPI, electrical low Up board of differential pressure 8. Down board of diffepressurepressure impactor),既可以實(shí)時(shí)瞬時(shí)測量粒子數目9. Flow control valve 10 Air pump圖1 MOUDI顆粒取樣系統分布,也可以作為慣性沖擊器使用獲得質(zhì)量尺寸分Fig 1 MOUDI particle collection system布212這2類(lèi)儀器在測量過(guò)程中都需要對原始排氣進(jìn)行高倍稀釋,電遷移率粒子分析儀不帶有顆粒1.2.2顆粒采集試驗方案分級捕獲裝置,ELPⅠ可以獲得分級顆粒物,但捕獲在采集中國煤化工動(dòng)機轉速高收集的量較小。低和負荷大HCNMHG的影響,為農業(yè)工程學(xué)報2013年使樣品具有普遍代表性,按8工況(非道路)標2試驗結果與分析準循環(huán)及國標(GB20891-2007)對應的權重分配采集顆粒物,如表2所示。采集樣品直至獲2.1分級顆粒物揮發(fā)特性分析取足夠質(zhì)量(2mg以上)的0.18~1.8m區間4圖2為純N2氣氛下不同粒徑級顆粒的TG及個(gè)粒徑級的分級顆粒物,顆粒樣品分類(lèi)放置于潔DTG曲線(xiàn)圖。在純N2環(huán)境下只有樣品的揮發(fā)和分凈密閉的器皿中。由于柴油機排出尾氣中大于解,而沒(méi)有氧化促進(jìn)分解失重的行為,可以更好地1.8/m粒徑級段顆粒物含量很少,難以收集足夠表征顆粒物各個(gè)升溫失重階段的區分。由圖2中質(zhì)量用于試驗,因此對粒徑大于1.8m的顆粒TG曲線(xiàn)可以看出,在程序溫度范圍內,顆粒質(zhì)量未納入研究范圍隨溫度升高而不斷減少,相應的質(zhì)量變化率DTG曲線(xiàn)處于負值,其絕對值表示失重速率。顆粒在表2顆粒采集測試工況升溫過(guò)程中主要發(fā)生了3個(gè)階段的物理化學(xué)變Table 2 Test modes for particulates samplin化,即水分的揮發(fā)、SOF組分的揮發(fā)和soot的熱測試工況計權因子轉速扭矩Test modeSpeed/(r-min) Torque/(N- m)解26。結合圖2的曲線(xiàn)數據及各個(gè)組分失重階段2200221的區分,表3列出了各粒徑級別通過(guò)熱重分析得到的組分含量。若將105℃以?xún)葮悠返氖е刂饕狪11考慮為水的蒸發(fā),在TG曲線(xiàn)上此階段失重約為2.0%,且由表3可見(jiàn)隨著(zhù)粒徑級的增大顆粒物中含水的百分率在下降。第二階段為可溶14001400性有機物SOF的揮發(fā),在150~400℃之間有一個(gè)明顯的失重速率峰,峰值出現在200℃C附近。由圖2b可見(jiàn),在這一階段的失重速率峰值出現以1.3熱重特性試驗前,隨著(zhù)溫度的增加,顆粒物粒徑級越小,失重利用瑞士梅特勒-托利多公司的 TGA/DSCI熱分速率越大。較小的粒徑級0.18-0.32m具有最大析儀器,對4個(gè)粒徑區間的分級顆粒進(jìn)行純N2及的失重速率峰值,而且隨著(zhù)顆粒粒徑級的增大純O2氛圍下的熱重分析試驗。該峰值減小,與此同時(shí)出現失重速率峰的對應的首先對0.18~0.32、0.32~0.56、0.56~1.0溫度也前移。這說(shuō)明隨著(zhù)顆粒粒徑減小,比表面和1.0~1.8m4個(gè)粒徑級顆粒分別進(jìn)行惰性氛積增加,其表面官能團和表面活性增加,這都促?lài)碌臒嶂胤治?。各級樣品量約1mg,試驗氛進(jìn)顆粒吸附力的增加2x28。因而較小粒徑級顆粒圍為純N2,流量為50mL/min,保護氣為物SOF含量更高。第三階段為顆粒物中的soot20m/min的純N2,程序溫度區間40~750℃,組分在純N2氛圍高溫下的分解失重,由于溫度持升溫速率20℃/min。續上升,soot的熱分解稍有加快,DTG曲線(xiàn)上約其后進(jìn)行分級顆粒在氧化氛圍下的熱重分在520℃之后失重速率曲線(xiàn)出現持續下降。但和析。顆粒樣品為原始樣品,未經(jīng)揮發(fā)或干燥處理。前一階段SOF的揮發(fā)相比,soot分解的失重速率進(jìn)樣量、各氣路流量及升溫程序等與前述純N2總體很低。不同粒徑級顆粒經(jīng)過(guò)前述的水分蒸發(fā)氛圍熱重試驗一致,將熱重爐腔內的氣氛氛圍變和SOF揮發(fā)后,在soot熱解階段剩下多為碳,其為純O2。失重速率趨同。50粒徑 Particle size01R-032>0.32~0.56->0.56-1.00600溫度 Temperature/C溫度 Temperature℃a.TG曲線(xiàn)b.DTG曲線(xiàn)a. TG curves圖2分級顆粒純N2氛圍下的TG和DTG中國煤化工Fig 2 TG and DTG curves of divided particles underCNMHG第16期梅德清等:柴油機排放顆粒物的熱重特性分析表3分級顆粒各組分含量峰值出現時(shí)刻及各粒徑級顆粒的失重規律與純N2氛Table 3 Contents of each components in divided particles圍下對應的熱重特性總體上一致。只是由于溫度已無(wú)機鹽近200°C,純O2的氧化氛圍極大地促進(jìn)了顆粒物中ParticleH2O/%SOF% Soot/% Inorganic saltSOF的揮發(fā)與氧化,見(jiàn)表4中2種氛圍下各個(gè)粒徑0.18~0,3232.40級顆粒物的失重速率峰值發(fā)生時(shí)刻及質(zhì)量變化率的>0.56~1.0025.83列表比較。純O2氛圍下造成的失重速率峰值變化率4.71也隨著(zhù)顆粒物粒徑級的減小而呈現増加的趨勢。在2.2分級顆粒氧化特性分析圖3b中,其后出現的失重峰表征著(zhù)顆粒物中soo圖2a中當臨近程序升溫終了時(shí),soot組分在的氧化失重特性。由于純O2的強烈氧化氛圍促使顆N2氛圍下失重漸趨緩滯,由于各粒徑級顆粒中soot粒中的soot發(fā)生煙煤基元氧化反應,使得soot的熱組分含量的差異導致TG曲線(xiàn)以不同的位置結束。而解失重與純N2氛圍相比呈現2個(gè)區別。一是與純在圖3a對應的氧化氛圍中,sot組分會(huì )繼續被氧化N2氛圍下出現的微小的失重速率特性曲線(xiàn)明顯不失重,因而各粒徑級顆粒物的TG曲線(xiàn)盡管中間過(guò)程同,在純O2氛圍下sot熱解階段呈現一個(gè)更大的有明顯差異,但終了時(shí)會(huì )出現歸一現象。從圖2a和失重峰。其次,從N2氛圍的顆粒物DTG曲線(xiàn)上從圖3a均可見(jiàn),隨著(zhù)顆粒物粒徑級的減小,TG曲線(xiàn)SOF揮發(fā)到soot分解這一段出現的拐點(diǎn)可以看出均有下降,說(shuō)明顆粒越細則越易在升溫中失重soot分解約在520℃之后進(jìn)行;而在純O2氛圍下由圖3b可見(jiàn),在純O2氛圍下顆粒物的熱重特DTG曲線(xiàn)上在400℃之后就可見(jiàn)失重速率曲線(xiàn)略有性明顯呈現兩個(gè)失重峰。第一個(gè)失重峰表征顆粒物増加,說(shuō)明soot分解已經(jīng)進(jìn)行,熱解開(kāi)始時(shí)刻對應中SOF揮發(fā)特性。在該階段,純O2氛圍下失重速率的溫度明顯提前。82令50粒徑 Particle size/um0.18~0.32>0.32~0.56>0.56~1.00>1.00~1.80200溫度 Temperature/℃cTG曲線(xiàn)b.DTG曲線(xiàn)a. TG curvesb, DTG curves圖3分級顆粒純O2氛圍下的TG和DTG曲線(xiàn)Fig3 TG and DTG curves of divided particles under O: atmosphere表4不同氛圍下顆粒物SOF失重特性比較Table 4 Analysis about ignition and peak of particles oxidation reaction純N2氛圍N2 atmosphere純O2氛圍O2 atmosphere重率峰值失重率峰值對應溫度失重率峰值對應溫度增加率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氧化階段,各個(gè)粒徑級顆粒的起燃時(shí)附一些氣體分子,溫度很高時(shí)碳晶體通過(guò)絡(luò )合作刻及質(zhì)量變化率峰值分析見(jiàn)表5,結合圖3b可見(jiàn)用化學(xué)吸附周界的氣體分子。顆粒的氧化反應在隨著(zhù)顆粒粒徑減小顆粒氧化的起燃溫度降低,且其內部表面上進(jìn)行,粒度減小(顆粒物更疏松)起燃時(shí)刻對應的失重速率增加;各粒徑級顆粒物時(shí)發(fā)生化學(xué)反應的活性增大9-30。自進(jìn)入soot熱的失重速率峰值出現在600~640℃之間,隨著(zhù)顆解階段始,0.189粒徑級甄粒物的失重速粒物粒徑級增大其soot熱解失重速率峰值也在增率曲線(xiàn)處于最中國煤化工達到失重速加。常溫下soot組分的碳粒子晶體表面會(huì )物理吸率峰值,說(shuō)明CNMHG被氧化。由農業(yè)工程學(xué)報2013年前述可知,隨著(zhù)粒徑級的變大,顆粒物中soot的熱解階段可以有越多的原料參與反應,因此其在組分含量就越多,這意味著(zhù)在純O2氛圍的 soot soot熱解階段的失重速率就越大表5soot氧化階段起燃時(shí)刻及質(zhì)量變化率峰值分析Table 5 Analysis about ignition and peak of mass change rate for particles oxidation reactie起燃時(shí)刻 Beginning of ignition峰值時(shí)刻 Timing of peak粒徑 Particle size/un大重率峰值對應溫度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結論[2 Raux S, Forti L, Barbusse S, et al. 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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(責任編輯:鮑洪杰)中國煤化工CNMHG