DOC與POC耦合柴油機(jī)燃用調(diào)合生物柴油顆粒物的排放特性

杜家益，魏 松，張登攀，袁銀男，逄大慶

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DOC與POC耦合柴油機(jī)燃用調(diào)合生物柴油顆粒物的排放特性

杜家益1，魏 松1，張登攀1，袁銀男2，逄大慶3

（1. 江蘇大學(xué)汽車(chē)與交通工程學(xué)院，鎮(zhèn)江 212013； 2. 蘇州大學(xué)能源學(xué)院，蘇州 215006； 3. 常柴股份有限公司，常州 213001）

在一臺(tái)高壓共軌柴油機(jī)上進(jìn)行燃用調(diào)合生物柴油（B0、B10和B20）臺(tái)架試驗(yàn)，利用MOUDI顆粒分級(jí)采樣系統(tǒng)和氣相色譜-質(zhì)譜聯(lián)用儀（GC-MS）分別研究氧化催化器（diesel oxidation catalyst，DOC）結(jié)合顆粒氧化催化器（particle oxidation catalyst，POC）對(duì)顆粒物的粒徑質(zhì)量濃度分布和可溶性有機(jī)組分（SOF）的影響。結(jié)果表明：隨著生物柴油的摻混比增加，各粒徑范圍的排氣顆粒物質(zhì)量濃度均下降，質(zhì)量濃度峰值均在0.18～0.32m；顆粒物SOF中脂類(lèi)、酸類(lèi)質(zhì)量分?jǐn)?shù)增加，烷烴類(lèi)、芳香烴、酚類(lèi)物質(zhì)質(zhì)量分?jǐn)?shù)減少；B0和B20的碳原子數(shù)質(zhì)量分?jǐn)?shù)均呈現(xiàn)近似以C16為峰值的正態(tài)分布。加裝DOC+POC后，3種燃料顆粒物的質(zhì)量濃度均降低，聚集態(tài)顆粒的質(zhì)量濃度轉(zhuǎn)化率高于粗顆粒態(tài)，其中B20聚集態(tài)轉(zhuǎn)化率最高，為58.36%；隨著生物柴油的摻混比增加，DOC+POC對(duì)SOF的轉(zhuǎn)化率增大，其中B20顆粒中SOF轉(zhuǎn)化率達(dá)65.15%；DOC+POC對(duì)脂類(lèi)和酸類(lèi)物質(zhì)凈化作用明顯，加裝DOC+POC后，B20脂類(lèi)和酸類(lèi)物質(zhì)的質(zhì)量分?jǐn)?shù)降幅分別為55.45%和43.27%；DOC+POC對(duì)B20顆粒物中SOF的C12～C18氧化作用明顯。

柴油機(jī)；生物柴油；顆粒物；排放特性；氧化催化器；顆粒物氧化催化器

0 引 言

生物柴油作為柴油機(jī)比較理想的替代燃料，具有十六烷值高、含氧量高、含硫量低和可再生等優(yōu)點(diǎn)[1-2]。隨著全球石化能源的日益短缺，采用一定比例的生物柴油與石化柴油摻混燃燒的方法，對(duì)解決石油能源短缺具有重要意義[3]。研究表明柴油機(jī)燃燒生物柴油能明顯降低顆粒物排放，同時(shí)還可減少CO、HC的排放[4-10]。

隨著排放法規(guī)日趨嚴(yán)格，采用后處理裝置控制柴油機(jī)顆粒排放已必不可少。目前常見(jiàn)的降低柴油機(jī)顆粒物的后處理裝置有顆粒氧化催化器（particle oxidation catalyst，POC）和顆粒物濾清器（diesel particulate filter，DPF）。POC的結(jié)構(gòu)是一個(gè)多褶皺不堵塞的通道，通常與氧化催化器（diesel oxidation catalyst，DOC）組合應(yīng)用（以下簡(jiǎn)稱(chēng)DOC+POC），對(duì)顆粒物的凈化效率較好，最高質(zhì)量濃度轉(zhuǎn)化率達(dá)89.0%[11-14]，與DOC+DPF組合系統(tǒng)相比，DOC+POC具有低排氣背壓、成本低、標(biāo)定過(guò)程簡(jiǎn)單等優(yōu)點(diǎn)。

目前，國(guó)內(nèi)外研究DOC+POC對(duì)純柴油的排放特性影響較多[15-19]，對(duì)生物柴油的排放特性的影響研究較少[20-21]，且多集中在對(duì)常規(guī)排放以及顆粒物的凈化效率方面。本文研究重點(diǎn)是排放后處理技術(shù)對(duì)燃燒生物柴油顆粒物中可溶有機(jī)物組分（soluble organic fractions，SOF）的影響，分析DOC+POC對(duì)SOF中各組分的占比變化以及對(duì)碳原子數(shù)的變化規(guī)律。通過(guò)柴油機(jī)燃用調(diào)合生物柴油臺(tái)架試驗(yàn)，利用微孔均勻沉積沖擊式采樣器（micro-orifice uniform deposition impactor，MOUDI）對(duì)DOC+POC作用前后的顆粒物進(jìn)行采樣，利用微克天平進(jìn)行稱(chēng)質(zhì)量，獲得顆粒物的質(zhì)量濃度和粒徑分布，借助氣相色譜-質(zhì)譜聯(lián)用儀（gas chromatography-mass spectrometer，GC-MS）對(duì)顆粒物中SOF進(jìn)行分析。研究結(jié)果可為生物柴油排放顆粒物后處理技術(shù)提供基礎(chǔ)性數(shù)據(jù)，同時(shí)有助于DOC+POC裝置的改進(jìn)與優(yōu)化。

1 試驗(yàn)發(fā)動(dòng)機(jī)與方案

1.1 發(fā)動(dòng)機(jī)與燃料

試驗(yàn)所用發(fā)動(dòng)機(jī)為某型號(hào)直列四缸高壓共軌柴油機(jī)，該機(jī)缸徑和行程分別為84和90 mm，額定功率（轉(zhuǎn)速）為60 kW/（3 200 r/min），最大轉(zhuǎn)矩（轉(zhuǎn)速）為201(N·m)/（2 200 r/min），排量為1.995 L。

試驗(yàn)所用DOC和POC為安徽艾可藍(lán)節(jié)能環(huán)保科技公司提供，表1所列為DOC和POC后處理裝置的主要參數(shù)。

表1 DOC和POC后處理裝置的主要參數(shù)

注：DOC為氧化催化器，POC為顆粒氧化催化器。

Note: DOC is diesel oxidation catalyst; POC is particle oxidation catalyst.

試驗(yàn)所用柴油為市售0#柴油，生物柴油由常州悅達(dá)卡特新能源有限公司提供，生產(chǎn)原料為餐飲廢油。生物柴油是對(duì)餐飲廢油進(jìn)行酯化處理后得到的，酯化后的生物柴油十六烷值提高，著火性能改善[22-24]。按照一定的體積比將生物柴油和柴油摻混制備調(diào)合生物柴油，其中B0、B10和B20分別表示生物柴油的體積比為0、10%和20%，表2所列為柴油和生物柴油的主要理化參數(shù)。

表2 柴油和生物柴油的主要理化參數(shù)

1.2 試驗(yàn)設(shè)備

試驗(yàn)所用的MOUDI采樣器為美國(guó)MSP公司生產(chǎn)，其采樣粒徑分級(jí)為8級(jí)，分別為0.18～0.32、0.32～0.56、0.56～1.0、1.0～1.8、1.8～3.2、3.2～5.6、5.6～10和10～18m，所采用的濾膜為聚四氟乙烯濾膜，該濾膜具有耐高溫、不溶于有機(jī)溶劑的特點(diǎn)，便于后期對(duì)顆粒物中SOF組分萃取。

試驗(yàn)所用GC-MS為T(mén)hermo Scientific公司生產(chǎn)的ITQ1100型，分析條件設(shè)定為：采用全掃描方式；色譜柱為HP-5MS型；載體為高純度氦氣，流量為1 mL/min；進(jìn)樣方式為不分流進(jìn)樣，進(jìn)樣量為1L。

1.3 試驗(yàn)方案

試驗(yàn)選擇發(fā)動(dòng)機(jī)工況為3 200 r/min，100%負(fù)荷，保持噴油提前角不變，分別燃用B0、B10和B20調(diào)合生物柴油，利用MOUDI對(duì)DOC+POC作用前后的顆粒物進(jìn)行采樣，采樣時(shí)間為1 h，氣體流量為30 L/min。采樣前后對(duì)所用的聚四氟乙烯濾膜需置于干燥箱內(nèi)保持溫濕度平衡6 h，再利用微克天平對(duì)顆粒物進(jìn)行精確稱(chēng)質(zhì)量。

利用GC-MS檢測(cè)顆粒物中SOF組分，需要預(yù)先對(duì)濾膜上顆粒物進(jìn)行萃取，SOF溶液提取采用超聲波震蕩法結(jié)合索氏萃取法，將溶液旋轉(zhuǎn)蒸發(fā)至1 mL后冷凍保存。取樣1L濾液進(jìn)行GC-MS分析，升溫程序設(shè)定為：初始溫度為80 ℃，恒溫2 min，以20 ℃/min的升溫速率升溫至160 ℃，再以8 ℃/min的升溫速率升溫至280 ℃，恒溫14 min。

2 結(jié)果與分析

2.1 顆粒物質(zhì)量濃度與粒徑分布

柴油機(jī)排放的顆粒物主要以核態(tài)（5～50 nm）、聚集態(tài)（100～1 000 nm）和粗顆粒態(tài)（>1 000 nm）3種模態(tài)存在。圖1所示為燃用B0、B10和B20燃油在發(fā)動(dòng)機(jī)轉(zhuǎn)速為3 200 r/min下的顆粒物質(zhì)量濃度分布曲線(xiàn)。由圖可見(jiàn)，B0、B10和B20顆粒物質(zhì)量濃度皆呈單峰分布，且峰值均在0.18～0.32m范圍內(nèi)，B10和B20的顆粒物質(zhì)量濃度明顯低于B0。這是因?yàn)樯锊裼秃跆匦?、十六烷值高，改善缸?nèi)燃燒，減少顆粒物排放。隨著摻混比增加，B20較B10質(zhì)量濃度下降不明顯，這是因?yàn)檩^大的摻混比導(dǎo)致燃油運(yùn)動(dòng)黏度變大，影響了燃油霧化混合及燃燒過(guò)程。加裝DOC+POC后，3種燃料的顆粒物質(zhì)量濃度均明顯下降，B0、B10和B20排氣顆粒物的總質(zhì)量濃度分別由18.707、5.071和4.17 mg/m3降低至10.743、2.591和1.991 mg/m3，轉(zhuǎn)化率分別為42.57%、48.91%、52.25%。隨著生物柴油摻混比增加，顆粒物質(zhì)量濃度的轉(zhuǎn)化率逐漸增大。對(duì)比圖1曲線(xiàn)可以看出，DOC+POC對(duì)粒徑0.18～1m的顆粒物凈化效率更高，主要因?yàn)镾OF組分多數(shù)以小顆粒物存在，而DOC主要通過(guò)氧化SOF來(lái)降低顆粒物排放[25-26]，因此對(duì)于小粒徑顆粒物作用明顯。

注：B0，B10，B20分別為生物柴油的體積比為0，10%，20%。

由圖2可知，隨著生物柴油摻比增加，聚集態(tài)和粗顆粒態(tài)的轉(zhuǎn)化率均呈增大趨勢(shì)，且對(duì)聚集態(tài)的轉(zhuǎn)化率高于粗顆粒態(tài)。B20聚集態(tài)轉(zhuǎn)化率最高，達(dá)58.36%，主要因?yàn)殡S著生物柴油摻混比增加，燃料黏度變大，霧化效果變差，造成燃料的不充分燃燒，產(chǎn)生較多以SOF為主體的聚集態(tài)顆粒，DOC+POC對(duì)這部分顆粒的凈化效果較好。由圖2b可知，DOC+POC對(duì)B20的粗顆粒態(tài)轉(zhuǎn)化率最高，為38.5%。這是因?yàn)樯锊裼偷暮跆匦?，在高溫富氧的環(huán)境產(chǎn)生較多的NOx，經(jīng)過(guò)DOC將部分NO氧化成強(qiáng)氧化性氣體NO2，提高了POC對(duì)粗顆粒態(tài)中碳煙顆粒的轉(zhuǎn)化率[27-30]。

圖2 DOC+POC對(duì)聚集態(tài)與粗顆粒態(tài)的顆粒轉(zhuǎn)化率

2.2 顆粒物中SOF的研究

2.2.1 SOF的質(zhì)量濃度

圖3所示為DOC+POC對(duì)顆粒物中SOF的轉(zhuǎn)化率。加裝DOC+POC后，3種燃料的排氣顆粒中的SOF質(zhì)量濃度均降低，對(duì)B0，B10和B20排氣顆粒物中SOF的轉(zhuǎn)化率分別為53.27%，60.02%和65.15%。隨著生物柴油摻混比增加，DOC+POC對(duì)SOF的轉(zhuǎn)化率呈增大趨勢(shì)，B20的轉(zhuǎn)化率最高。雖然生物柴油的十六烷值高，在相同工況下滯燃期縮短，排氣溫度較柴油略有下降[31]，但本文試驗(yàn)工況為額定轉(zhuǎn)速3 200 r/min，100%負(fù)荷，燃用B0，B10和B20排氣溫度仍均較高，后處理裝置中催化劑活性均較強(qiáng)，同時(shí)生物柴油中含氧，氣缸內(nèi)消耗的氧氣量減少，排氣中的氧濃度增加，DOC+POC對(duì)SOF的轉(zhuǎn)化率提高[30]。

圖3 DOC+POC對(duì)顆粒物中SOF的轉(zhuǎn)化率

2.2.2 SOF的組分分析

通過(guò)對(duì)不同燃料顆粒物中SOF溶液試樣進(jìn)行GC-MS分析，得到SOF總離子流色譜，結(jié)合總離子流圖檢索NIST譜圖，對(duì)SOF組分進(jìn)行定量分析。表3所示為顆粒物中SOF經(jīng)GC-MS檢測(cè)，得到的各組分的質(zhì)量分?jǐn)?shù)，由表3可知，無(wú)論是原機(jī)還是加裝DOC+POC后，3種燃料的排放顆粒物中SOF組分皆以烷烴類(lèi)、芳香烴、酚類(lèi)、脂類(lèi)和酸類(lèi)為主。表中的其他組分為少量的醇類(lèi)、醛類(lèi)和醚類(lèi)等，因組分占比較少，本文不列入研究。未加裝DOC+POC時(shí)，隨著生物柴油摻混比增加，SOF組分中脂類(lèi)、酸類(lèi)物質(zhì)質(zhì)量分?jǐn)?shù)增加，烷烴類(lèi)、芳香烴和酚類(lèi)物質(zhì)質(zhì)量分?jǐn)?shù)減少，這是因?yàn)樯锊裼偷闹饕煞质侵舅峒字?，隨著摻混比增大，生物柴油的不完全燃燒造成脂類(lèi)和酸類(lèi)物質(zhì)增加；生物柴油不含苯環(huán)，因此隨著生物柴油摻混比增加，含有苯環(huán)結(jié)構(gòu)的芳香烴和酚類(lèi)物質(zhì)質(zhì)量分?jǐn)?shù)略有降低。加裝DOC+POC后，燃用B0，B10和B20排氣顆粒中SOF組分中烷烴類(lèi)、脂類(lèi)和酸類(lèi)物質(zhì)質(zhì)量分?jǐn)?shù)呈減少趨勢(shì)，且在燃用B20時(shí)脂類(lèi)和酸類(lèi)物質(zhì)質(zhì)量分?jǐn)?shù)降幅最大，分別為55.45%和43.27%，可以看出DOC+POC對(duì)燃用生物柴油排氣顆粒中SOF組分中脂類(lèi)和酸類(lèi)物質(zhì)凈化作用明顯。而芳香烴類(lèi)和酚類(lèi)物質(zhì)質(zhì)量分?jǐn)?shù)呈上升趨勢(shì)，主要因?yàn)榉枷銦N和酚類(lèi)物質(zhì)分子結(jié)構(gòu)中含有難以氧化的苯環(huán)，相對(duì)于其他物質(zhì)，芳香烴和酚類(lèi)物質(zhì)氧化速率低，導(dǎo)致質(zhì)量分?jǐn)?shù)的增加。

表3 柴油機(jī)排氣顆粒中SOF各組分質(zhì)量分?jǐn)?shù)

2.2.3 SOF的碳原子數(shù)

圖4所示為B0和B20顆粒中SOF組分的碳原子數(shù)分布圖。未加裝DOC+POC時(shí)，B0和B20的SOF碳原子數(shù)質(zhì)量分?jǐn)?shù)均呈現(xiàn)近似以C16為峰值的正態(tài)分布，分布區(qū)間為C3～C33。其中B0顆粒物中C15和C16占比最高，質(zhì)量分?jǐn)?shù)分別為18.62%和20.78%；B20顆粒物 C16占比最高，質(zhì)量分?jǐn)?shù)為28.29%。對(duì)比2種燃料排放顆粒物中SOF碳原子數(shù)可以發(fā)現(xiàn)，燃用B0和B20燃料的SOF碳原子數(shù)在C25～C33之間的質(zhì)量分?jǐn)?shù)分別為14.9%和7.72%，隨著生物柴油的添加，C25～C33的質(zhì)量分?jǐn)?shù)減少，這是因?yàn)樯锊裼偷难鹾扛撸龠M(jìn)高碳原子分子氧化成中低碳原子分子。加裝DOC+POC后，B0和B20顆粒中SOF的峰值碳原子數(shù)均下降，B20顆粒中SOF的峰值碳原子數(shù)C16下降明顯，由28.29%下降至14.03%，降幅為50.41%。與B0相比，B20顆粒中SOF碳原子數(shù)向C25～C33聚集，C12～C18質(zhì)量分?jǐn)?shù)降低。B20排氣顆粒物中SOF中C12～C18質(zhì)量分?jǐn)?shù)降幅由B0的9.89%增大為35.15%，C25～C33由B0降幅31.54%變?yōu)樵龇?12.69%。由此可知，DOC+POC對(duì)燃用B20顆粒物中SOF的C12～C18氧化作用明顯，對(duì)C25～C33轉(zhuǎn)化效率較差。一方面因?yàn)槿加肂20排放出較多的脂類(lèi)物質(zhì)，且脂類(lèi)物質(zhì)較多以低碳原子數(shù)存在，如鄰苯二甲酸二丁酯等，DOC+POC對(duì)脂類(lèi)物質(zhì)氧化作用明顯；另一方面燃用B20排放出較多的NOx，經(jīng)DOC后氧化成強(qiáng)氧化性氣體NO2，在后處理裝置中進(jìn)一步對(duì)SOF組分氧化，由于高碳原子分子氧化需要更多的活化能，相對(duì)于中低碳原子分子氧化速率較慢。

圖4 顆粒物中SOF碳原子數(shù)分布圖

3 結(jié) 論

1）隨著生物柴油的摻混比增加，DOC+POC對(duì)顆粒物的轉(zhuǎn)化率提高，B0，B10和B20總質(zhì)量濃度轉(zhuǎn)化率分別為42.57%、48.91%和52.25%。

2）DOC+POC對(duì)聚集態(tài)顆粒物轉(zhuǎn)化率優(yōu)于粗顆粒態(tài)，對(duì)B20聚集態(tài)的轉(zhuǎn)化率達(dá)58.36%。

3）隨著生物柴油摻混比增加，SOF組分中脂類(lèi)、酸類(lèi)物質(zhì)質(zhì)量分?jǐn)?shù)增加，烷烴類(lèi)、芳香烴、酚類(lèi)物質(zhì)質(zhì)量分?jǐn)?shù)減少。加裝DOC+POC后，DOC+POC對(duì)B0、B10和B20顆粒物中SOF的轉(zhuǎn)化率呈增大趨勢(shì)，其中對(duì)B20顆粒物中SOF轉(zhuǎn)化率達(dá)到65.15%；DOC+POC對(duì)脂類(lèi)和酸類(lèi)物質(zhì)凈化作用明顯，在燃用B20時(shí)脂類(lèi)和酸類(lèi)物質(zhì)質(zhì)量分?jǐn)?shù)降幅分別為55.45%和43.27%。

4）燃用B20可以明顯減少SOF中C25～C33的質(zhì)量分?jǐn)?shù)，DOC+POC對(duì)B20排氣顆粒中SOF的C12～C18氧化作用明顯，對(duì)C25～C33氧化速率較慢。

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Effects of DOC+POC on characteristics of particulate matter from diesel engine fueled with biodiesel blends

Du Jiayi1, Wei Song1, Zhang Dengpan1, Yuan Yinnan2, Pang Daqing3

(1.,,212013,; 2.,,215006,; 3.,213001,)

Particulate matter (PM) emissions from diesel engines are being recognized as the pollutants having adverse effects on the environment as well as on human health. Therefore, the combination of using clean alternative fuels and particulate matter after-treatment devices is one of the effective ways to reduce particulate emissions. Biodiesel as an alternative fuel can significantly reduce particulate emissions. The diesel oxidation catalysts (DOC) are commonly used to oxidize carbon monoxide (CO) and hydrocarbon (HC) emissions as well as partial particles. The particulate oxidation catalyst (POC) is considered as an alternative PM reduction aftertreatment technology to the wall-flow diesel particulate filter (DPF). The combination of DOC and POC is a commonly and widely used to reduce PM. In order to analyze the influence of biodiesel blending ratio, DOC+POC on PM emissions and components of soluble organic fractions (SOF), bench test was carried out on a high pressure common rail diesel engine fueled with diesel-biodiesel dual fuels (B0, B10 and B20). Particles were collected at rated condition. Particle samples with different size grades were achieved from micro-orifice uniform deposition impactor (MOUDI) and mass concentration was obtained by weighting the particle samples. Using the Soxhlet extraction method to extract SOF component from particulates. The effects of biodiesel blending ratio and DOC+POC on SOF content were studied by gas chromatography-mass spectrometer (GC-MS) analysis. The distribution of carbon atoms of B0 and B20 were obtained by analyzing the GC-MS data. The results showed that when the content of biodiesel percentage was increased, DOC + POC conversion rate of particulate matter was increased,the conversion of total mass concentration of B0, B10 and B20 were 42.57%, 48.91% and 52.25% respectively. The mass concentration within each size grade was decreased. The mass concentration peak value of particulate matter emitted from three fuels all ranged from 0.18 to 0.32m. The mass fraction of lipids and acids components in SOF were increased and alkanes, aromatic hydrocarbons and phenols compounds were decreased. Moreover, the increase of biodiesel percentage promotes the oxidation of high-carbon atoms into low-carbon atoms. the mass fraction of carbon atoms in SOF of B0 and B20 showed a normal distribution with a peak at C16. After the installation of DOC+POC, the mass concentration of particulate matter was decreased and the convert efficiency of accumulation state particles was higher than that of coarse particles. Meanwhile, the convert efficiency of accumulation state particles with B20 reached 58.36%. With the increase of biodiesel percentage, the convert efficiency of SOF was increased, and the convert efficiency of SOF reached 65.15% when B20 fuel was used. DOC+POC had a significant effect on the conversion of lipid and acid substances. The mass fraction of lipid and acid substances changed from 15.4% to 6.86% and 9.43% to 5.35% respectively. Moreover, DOC+POC had obvious effect on the oxidation of C12-C18 in SOF of B20.These results could provide a theoretical basis for the aftertreatment of biodiesel combustion particulates, and it is helpful to improve and optimize of diesel oxidation catalysts and particulate oxidation catalyst，according to the biodiesel emission characteristics.

diesel engines; biodiesel; particulate matter; emission characteristic; diesel oxidation catalyst; particle oxidation catalyst

10.11975/j.issn.1002-6819.2017.22.009

TK6

A

1002-6819(2017)-22-0069-06

2017-07-06

2017-10-28

國(guó)家自然科學(xué)基金資助項(xiàng)目（51376095）；江蘇省高校自然科學(xué)研究重大項(xiàng)目（13KJA470001)；江蘇省高校自然科學(xué)研究項(xiàng)目（15KJB470002）；江蘇高校優(yōu)勢(shì)學(xué)科建設(shè)工程資助項(xiàng)目（PAPD）

杜家益，男，博士，副教授，主要從事動(dòng)力機(jī)械工作過(guò)程仿真、代用燃料排放控制等研究。Email：jydu@ujs.edu.cn