焦緯洲,許承騁,劉有智,郭 亮,李 靜,劉文麗
(中北大學(xué) 山西省超重力化工工程技術(shù)研究中心,山西 太原 030051)
柴油機(jī)因具有良好的動(dòng)力性、經(jīng)濟(jì)性和耐久性等優(yōu)點(diǎn)而得到廣泛應(yīng)用[1],并且上世紀(jì)末出現(xiàn)了汽車(chē)柴油化的趨勢(shì),在運(yùn)輸車(chē)輛、農(nóng)用機(jī)械和工程機(jī)械等領(lǐng)域形成了主導(dǎo)地位。但是中國(guó)石油資源形勢(shì)不容樂(lè)觀(guān),2012年進(jìn)口原油2.8×108t石油,對(duì)外依存度58%左右;另外,柴油機(jī)的尾氣中顆粒物(PM)、氮氧化物(NOx)含量較高,可能引起呼吸以及心血管疾病。為了滿(mǎn)足能源消耗需求增長(zhǎng)和控制環(huán)境污染,學(xué)者們開(kāi)始尋求柴油機(jī)的清潔替代能源。研究表明,在燃油中摻燒含氧燃料,可以減少PM排放量,而不顯著影響NOx排放量[2-4]。在眾多含氧燃料中,醇類(lèi)[5-8]、酯類(lèi)[9-10]以及醚類(lèi)[11-13]最為常見(jiàn)。其中,在燃油中添加醇類(lèi)對(duì)油品的理化性質(zhì)、燃燒以及排放有顯著的影響。醇類(lèi)的排放性能與脂類(lèi)、醚類(lèi)以及其他含氧燃料相當(dāng)[14],并且可以提高燃油的辛烷值。因此,許多學(xué)者希望在柴油中添加醇類(lèi)含氧燃料來(lái)實(shí)現(xiàn)柴油的替代與清潔燃燒。盡管甲醇和乙醇在柴油中溶解性和穩(wěn)定性不理想,并且十六烷值低和吸濕性高,但由于其容易制備,價(jià)格低廉,含氧量高,可以不用改變傳統(tǒng)內(nèi)燃機(jī)結(jié)構(gòu),被認(rèn)為是最有前景的柴油添加含氧燃料之一[15]。不足的是,隨著儲(chǔ)存時(shí)間的增加或體系溫度的降低會(huì)出現(xiàn)相分離現(xiàn)象[16-17],造成內(nèi)燃機(jī)工作異常。近年來(lái),國(guó)外開(kāi)始關(guān)注生物質(zhì)丁醇燃料。它最早是通過(guò)發(fā)酵玉米芯獲得,理化性質(zhì)與柴油相似,能與柴油任意比例穩(wěn)定互溶[18],同樣被認(rèn)為是未來(lái)具有競(jìng)爭(zhēng)潛力的燃料之一[19-20]。筆者以甲醇、乙醇和丁醇柴油為對(duì)象,總結(jié)和評(píng)論了近年來(lái)的研究進(jìn)展,并預(yù)測(cè)未來(lái)醇類(lèi)柴油燃料的發(fā)展方向。
柴油機(jī)是基于柴油的理化性質(zhì)設(shè)計(jì)的,醇類(lèi)柴油燃料在柴油機(jī)上適用程度取決于其與柴油理化性質(zhì)的接近程度。醇類(lèi)與柴油理化性質(zhì)列于表1[21-23]。雖然醇類(lèi)燃料的低熱值(Lower heating value)、黏度、十六烷值等參數(shù)低于柴油,但也表現(xiàn)出比傳統(tǒng)石化燃料性能優(yōu)異的一面,如辛烷值高,可以適用于高壓縮比的柴油機(jī);空燃比低,著火極限寬,可以在較寬的混合氣濃度范圍內(nèi)工作。值得注意的是,隨著醇碳鏈的增長(zhǎng)其理化性質(zhì)越接近柴油。
作為替代能源必須具備原料來(lái)源廣泛以及生產(chǎn)技術(shù)成熟的特點(diǎn)。甲醇的工業(yè)化生產(chǎn)主要采用合成氣和ICI(Imperial chemical industries)低壓工藝[24]。合成氣來(lái)源廣泛,原料包括煤炭、天然氣、石油、焦?fàn)t氣、生物質(zhì)等[25]。中國(guó)主要采用煤炭為原料合成甲醇。乙醇主要通過(guò)生物質(zhì)原料發(fā)酵獲得。原料按種類(lèi)可以分為3類(lèi):(1)富淀粉植物[26-27],如玉米、小麥等;(2)富糖植物[28-30],如甘蔗、甜菜;(3)廢棄物[31-32]。丁醇早期主要通過(guò)生物質(zhì)原料發(fā)酵獲得,現(xiàn)在主要通過(guò)石油制備。隨著石油資源緊缺,有回歸生物質(zhì)制備的趨勢(shì)。目前制備工藝主要有羰基合成法、醇醛縮合法和生物質(zhì)法3種[33]。前2種工藝以石油為原料,投資大,技術(shù)設(shè)備要求高;生物質(zhì)法不需要依賴(lài)石油資源,目前已經(jīng)成為丁醇制備方法的研究熱點(diǎn)之一。
3種醇中,中國(guó)的甲醇生產(chǎn)技術(shù)和催化劑性能均達(dá)到國(guó)際領(lǐng)先水平[25],并且中國(guó)煤炭資源豐富。所以,甲醇是中國(guó)最有潛力的替代燃料之一。乙醇作為替代燃料已開(kāi)始大規(guī)模應(yīng)用。但是,工藝上主要通過(guò)富淀粉植物轉(zhuǎn)化,中國(guó)人多地少,會(huì)造成“與人爭(zhēng)糧”的困境。丁醇的理化性質(zhì)與柴油相似,也是優(yōu)良的替代燃料,但是制備成本高,制約了丁醇燃料的發(fā)展。
3種不含水的醇與柴油適當(dāng)比例混合制備的燃料具有良好的穩(wěn)定性與溫度適應(yīng)性。但由于甲醇和乙醇的吸濕性,一定量的水會(huì)從空氣轉(zhuǎn)移到混合燃料中,隨著儲(chǔ)存時(shí)間的增加或是體系溫度降低會(huì)出現(xiàn)相分離現(xiàn)象,造成發(fā)動(dòng)機(jī)工作異常。這可能是因?yàn)椴裼椭虚L(zhǎng)鏈的烷烴具有一些弱的電荷位點(diǎn)和中性電荷位點(diǎn)。弱的電荷位點(diǎn)對(duì)羥基有排斥作用,而中性的電荷位點(diǎn)可以與醇類(lèi)形成范德華力。中性電荷位點(diǎn)數(shù)量越多,單位體積的柴油溶解的醇類(lèi)就越多[34]。體系溫度降低時(shí),水更容易和烷烴上的電荷點(diǎn)位成鍵,造成相分離。因此,為了制備大比例醇類(lèi)混合燃料和避免油-醇兩相分離,一般將甲醇、乙醇與柴油制備成乳化液型醇類(lèi)柴油,而丁醇基本不具有吸濕性,并且可以與柴油任意比例互溶,可以直接混合制備成溶液型醇類(lèi)柴油。
Becher[35]定義乳化液為,一個(gè)非均相體系,其中至少有一種液體以液珠的形式分散在另一種液體中,液珠直徑一般大于0.1μm,這種體系有一個(gè)最低穩(wěn)定度,并且穩(wěn)定度在表面活性劑以及助表面活性劑存在下會(huì)增加。因此,形成乳化液的主要影響因素為能量和表面活性劑及助劑。外加能量可以使液珠分散在另一種液體中,表面活性劑及助劑可以提高體系穩(wěn)定度。筆者以這兩因素為主線(xiàn),總結(jié)了乳化設(shè)備(能量提供方式)、表面活性劑以及助劑的研究進(jìn)展。
2.1.1 乳化設(shè)備的研究進(jìn)展
乳化設(shè)備主要有靜態(tài)乳化器和動(dòng)態(tài)乳化器。靜態(tài)乳化器有乳化管、超聲波乳化器、SMV靜態(tài)混和器、SK靜態(tài)混和器等;動(dòng)態(tài)混和器有攪拌器、均質(zhì)器等。甲醇、乙醇與柴油極性差異大,需要外加能量(如機(jī)械攪拌、升溫等)增加乳液的穩(wěn)定性。為了節(jié)省能源與提高乳化效率,近年來(lái)一些科研機(jī)構(gòu)在超聲波乳化設(shè)備、膜乳化設(shè)備以及超重力乳化設(shè)備上取得了新進(jìn)展,為開(kāi)發(fā)高效乳化器奠定了基礎(chǔ)。
(1)超聲波乳化設(shè)備:1927年 Wood等[36]發(fā)表了第1篇關(guān)于采用超聲波乳化的文章,此后許多學(xué)者用各種形式的超聲波設(shè)備(哨音式、探頭式)達(dá)到乳化的效果,并研究了超聲波乳化機(jī)理。Li等[37]研究了油-水體系的超聲波乳化過(guò)程,指出超聲波乳化過(guò)程的2個(gè)階段,即,(a)超聲導(dǎo)致油-水界面層不穩(wěn)定;(b)空化作用產(chǎn)生微汽核,高壓震波(100MPa)和高溫(4000K)使分散相分散在連續(xù)相里。Cucheval等[38]用攝像機(jī)拍攝了此過(guò)程,如圖1所示[38]。超聲波乳化的影響因素主要有功率[39]、超聲波處理時(shí)間[40-42]、表面活 性劑含量[42]、油/水兩相含量比[42]、連續(xù)相的黏度[43]、預(yù)乳化效果[40]等。值得注意的是,預(yù)乳化效果也是參數(shù)之一,所以超聲波乳化設(shè)備一般不能單獨(dú)使用,需要預(yù)先外加機(jī)械能,將分散相分散在連續(xù)相中,然后采用超聲波進(jìn)一步乳化。Nasiri等[44]采用神經(jīng)元網(wǎng)絡(luò)的方法模擬了超聲波乳化過(guò)程,與實(shí)驗(yàn)對(duì)比可以精確地預(yù)測(cè)乳化液的尺寸,這為超聲波乳化的機(jī)理研究和超聲波乳化器的設(shè)計(jì)奠定了基礎(chǔ)。
圖1 油-水體系的超聲波乳化過(guò)程[38]Fig.1 The process of emulsification for oil-water system by ultrasound[38]
(2)膜乳化設(shè)備:在膜兩側(cè)壓差的驅(qū)動(dòng)下,分散相通過(guò)膜孔以小液滴的形式與連續(xù)相在膜表面直接接觸,從而制備出乳化液。1986年Nakashima等[45]制備出硅砂多孔玻璃膜,并制備出尺寸均勻的水包煤油及煤油包水乳狀液[46]。Van der graaf等[47]采用微孔篩,在乳化劑質(zhì)量分?jǐn)?shù)0.1%~10%條件下,將十六烷-水體系制備成粒徑分布在34~100μm 的水包油乳液。Yuyama等[48]采用SPG(Shirasu porous glass membrane)膜制備出粒徑分布在6.0~10.9μm 的水包油乳液。甄宗晴等[49]利用疏水陶瓷膜制備了乳化柴油,考察了各因素對(duì)乳化柴油穩(wěn)定性的影響,得到制備乳化柴油的最佳工藝條件,即乳液中乳化劑質(zhì)量分?jǐn)?shù)為2%,乳化溫度為303K。張春芳等[50]研究了UF膜法制備乳化柴油,結(jié)果表明,當(dāng)乳化劑質(zhì)量分?jǐn)?shù)高于0.5%、連續(xù)相流速在2.0~5.0m/s范圍內(nèi)、跨膜壓差為0.02MPa時(shí),可以得到液滴尺寸為30~65nm的乳化柴油。從上述各例可以看出膜乳化的效率高、乳化劑用量低、能耗低的優(yōu)點(diǎn)。在實(shí)驗(yàn)方面,研究者已經(jīng)對(duì)膜乳化影響參數(shù)進(jìn)行了詳細(xì)的研究。膜乳化影響參數(shù)主要有膜參數(shù)[45,51]、壓差[52]、分散相通量[53]、溫度[53]、黏度[54]等。但是理論研究還未提供液滴從膜表面形成、剝離諸多影響因素的充分描述。另外,膜乳化存在膜污染較嚴(yán)重、清洗膜困難的問(wèn)題。下一步工作應(yīng)該是開(kāi)發(fā)高孔隙率、高耐污染的膜乳化設(shè)備。
(3)超重力乳化設(shè)備:中北大學(xué)研制了一種超重力乳化設(shè)備[55]。Jiao等[56]用此設(shè)備制備的柴油乳液為透明、半透明狀接近純柴油外觀(guān)的乳液,產(chǎn)品放置30d以上不分層。焦緯洲[57]用此設(shè)備與高速分散器進(jìn)行對(duì)比。采用甲醇體積分?jǐn)?shù)為10%、乳化劑質(zhì)量分?jǐn)?shù)為4%的條件制備乳化柴油,結(jié)果表明,超重力乳化設(shè)備制備乳化柴油的分散相粒徑和能耗均優(yōu)于高速分散器。圖2為超重力乳化設(shè)備和高速分散器制備的乳化柴油的數(shù)碼顯微鏡照片[57]。
2.1.2 乳化劑研究進(jìn)展
表2列出了國(guó)內(nèi)外新能源生產(chǎn)公司公開(kāi)的商用乳化柴油乳化劑配方專(zhuān)利[34,58-63]。從表2可以看出,乳化劑一般以非離子型為主,離子型乳化劑為輔;使用最多的乳化劑是Span系列(失水山梨醇脂肪酸酯)和Tween系列(聚氧乙烯失水山梨醇脂肪酸酯)。這些乳化劑價(jià)格昂貴,均在1萬(wàn)元/t以上。尋求廉價(jià)、環(huán)保的乳化劑是乳化柴油推廣的關(guān)鍵。
圖2 超重力乳化設(shè)備與高速分散器制備乳化柴油的顯微鏡照片F(xiàn)ig.2 The micrographs of methanol-diesel emulsion prepared by high gravity device and high speed dispersion stirrer
表2 部分商用乳化柴油乳化劑配方專(zhuān)利Table 2 Some patents of commercial emulsifier formula for emulsified diesel
在新型乳化劑中,Gemini表面活性劑分子構(gòu)型特殊,如圖3[64]所示,它是由2個(gè)單頭基表面活性劑在頭基或接近頭基的位置被聯(lián)接基團(tuán)聯(lián)接起來(lái)形成的,乳化效果優(yōu)于傳統(tǒng)乳化劑,因此得到了研究者的關(guān)注[65]。Dreja等[66]研究發(fā)現(xiàn),在苯乙烯-水體系中雙子表面活性劑比傳統(tǒng)表面活性劑增容作用更大。Kunieda等[67]研究了油-水體系的乳化,發(fā)現(xiàn)雙子表面活性劑比單體表面活性劑效果更好。Chen等[68]發(fā)現(xiàn),Gemini表面活性劑比相應(yīng)的傳統(tǒng)表面活性劑能更高效地降低原油-水的界面張力,在一定的濃度范圍內(nèi)將原油-水的界面張力降到非常低的水平。Fan等[69]用二甲苯-水體系研究了雙子乳化劑濃度對(duì)乳化效果的影響。圖4為不同濃度雙子乳化劑制備的乳化液的顯微鏡照片[69]。由圖4可知,在乳化劑濃度很低情況下就可得到較好的乳化效果。趙田紅等[70]制備了1種聯(lián)接基含氮原子的磺酸鹽型Gemini表面活性劑,趙修太等[71]用此表面活性劑制備乳化柴油并進(jìn)行了性能研究,結(jié)果表明,當(dāng)表面活性劑質(zhì)量分?jǐn)?shù)為0.2%時(shí),具有比傳統(tǒng)表面活性劑更優(yōu)越的乳化性能。Nadeem等[72]將用Gemini表面活性劑和傳統(tǒng)表面活性劑制備的乳化柴油進(jìn)行了排放性能實(shí)驗(yàn),結(jié)果表明,Gemini表面活性劑制備的乳化柴油具有更好的排放性能。但是,目前未見(jiàn)復(fù)配Gemini表面活性劑制備乳化柴油的報(bào)道,未來(lái)可以開(kāi)展復(fù)配研究工作。
圖3 不同分子結(jié)構(gòu)的表面活性劑Fig.3 Different molecular architectures of surfactants
圖4 含不同濃度雙子乳化劑的乳化液的顯微鏡照片F(xiàn)ig.4 Optical micrographs of the gel-emulsions stabilized with different concentration of gimi-emulsifier
也有學(xué)者探索從天然植物提取有機(jī)化合物作為乳化劑制備乳化柴油。董英等[73]用大豆磷脂為乳化劑制備乙醇柴油,乳液中磷脂質(zhì)量分?jǐn)?shù)為1%,乙醇體積分?jǐn)?shù)為25%,乳化效果好于Span80,劣于復(fù)配乳化劑。Lei等[74]以從廢食物中回收的生物油和蓖麻油為主要成分,配制了一種新型復(fù)配乳化劑,用于制備乳化乙醇柴油,當(dāng)乳化劑質(zhì)量分?jǐn)?shù)為0.8%、乙醇體積分?jǐn)?shù)為10%時(shí),制備的乳化乙醇柴油能保持60d的穩(wěn)定性。隨著環(huán)保意識(shí)的提高和法規(guī)的完善,以天然有機(jī)化合物為主要成分的乳化劑具有優(yōu)良的降解性,對(duì)環(huán)境危害小,是未來(lái)具有發(fā)展?jié)摿Φ拇碱?lèi)柴油乳化劑之一。
2.1.3 助乳化劑的研究進(jìn)展
低相對(duì)分子質(zhì)量的醇、酸、胺等具有類(lèi)似乳化劑的親水、親油性,能降低油-水界面張力,它們常常與乳化劑組成乳化劑體系使用,被稱(chēng)作助乳化劑。Prince[75]在其著作中指出,Schulman等通過(guò)研究油-水體系發(fā)現(xiàn),在乳化劑和低鏈醇存在下,二者產(chǎn)生混合吸附作用,出現(xiàn)了負(fù)界面張力。所以,在柴油-醇體系中加入某種助乳化劑,同樣可以降低柴油-醇界面張力,更容易形成乳液。Bansal等[76]固定助乳化劑、改變油相種類(lèi)研究了增溶水量的變化規(guī)律。結(jié)果得到,醇的鏈長(zhǎng)La、油的鏈長(zhǎng)Lo及離子型乳化劑的非極性基鏈長(zhǎng)Ls具有相關(guān)性,當(dāng)微乳體系符合Ls=La+Lo時(shí),所得微乳增溶能力對(duì)于某個(gè)特定的醇微乳體系達(dá)到最大。他們還認(rèn)為乳化劑和助乳化劑在界面膜上的總量影響著微乳的增溶水量,當(dāng)其總量越多,則增溶的水量就越多,而乳化劑和助乳化劑在界面膜上的總量受乳化劑和助乳化劑在水和油相中溶解的情況而定??梢钥闯?,不同的助乳化劑對(duì)體系的增溶效果不同,選擇適用于特定的柴油-醇體系的助乳化劑,是未來(lái)研究的方向。李會(huì)芬等[77]以正丁醇、正庚醇和正癸醇為助乳化劑,研究了它們對(duì)乙醇-柴油體系的影響。結(jié)果發(fā)現(xiàn),對(duì)于碳原子數(shù)小于10的醇類(lèi),隨著碳原子數(shù)增加,其助溶效果基本呈增大趨勢(shì),但是他們未對(duì)異構(gòu)體的助溶效果進(jìn)行研究;溫度對(duì)助溶效果有影響,在283K時(shí),正癸醇、正庚醇、正丁醇的助溶效果依次降低;而在293K時(shí),助溶效果依次降低的順序變?yōu)檎?、正癸醇、正丁醇。另外,還研究了這3種醇對(duì)排放性能的影響,其中正丁醇效果最好,這可能是正丁醇含氧量比其他2種醇高的緣故。徐珊[78]研究了助乳化劑乙醇胺、二乙醇胺、三乙醇胺、乙二醇-甲醚、乙二醇、1,2-丙二醇以及一元醇(碳數(shù)l~8)各自在油-水體系中的增溶作用,結(jié)果表明,空間位阻小,親油、親水能力平衡的助乳化劑助溶效果明顯??傮w來(lái)講,助乳化劑的選擇要遵循的原則是,(1)空間位阻小,親水、親油能力平衡;(2)優(yōu)異的排放性能;(3)適合工況溫度;(4)價(jià)格低廉。
丁醇與柴油極性差異不大,有很好的互溶性。在柴油中直接添加一定比例的丁醇,適當(dāng)攪拌均勻,就能形成穩(wěn)定的丁醇柴油。杜標(biāo)[79]制備了丁醇體積分?jǐn)?shù)為40%的丁醇柴油,150d內(nèi)沒(méi)有發(fā)現(xiàn)明顯的分層現(xiàn)象,如圖5[79]所示。
圖5 丁醇體積分?jǐn)?shù)為40%的丁醇柴油150d內(nèi)的分層情況Fig.5 The phase separation of diesel-butanol blends(φ(Butanol)=40%)during 150d
李頂根等[80]在型號(hào)YC6105的柴油機(jī)上進(jìn)行了M15甲醇柴油實(shí)驗(yàn)。結(jié)果表明,與柴油相比,NOx排放大幅度降低,PM排放明顯減少,并對(duì)CO排放具有抑制作用,對(duì)HC排放影響不大,在高負(fù)荷運(yùn)行時(shí)會(huì)促進(jìn)HC生成。馮丹華等[81]在增壓中冷柴油機(jī)上進(jìn)行甲醇柴油(甲醇體積分?jǐn)?shù)為10%和15%)燃燒性能實(shí)驗(yàn)。結(jié)果表明,隨著混合燃料中甲醇含量的增加,NOx和碳煙排放降低,HC排放增加,CO排放在小負(fù)荷下大幅增加而在大負(fù)荷下略有降低。李芳等[82]研究了甲醇柴油在增壓柴油機(jī)內(nèi)的燃燒規(guī)律。結(jié)果表明,燃用甲醇柴油能降低CO和NO的排放,但會(huì)引起HC排放量的上升。可見(jiàn)甲醇的加入對(duì)柴油排放性能有顯著影響,其他學(xué)者[83-85]也得出相似的結(jié)論,燃用甲醇柴油可以降低PM、NOx和CO排放,但是HC排放增加。另外,甲醇柴油的非常規(guī)排放也是需要注意的問(wèn)題。張學(xué)敏等[86]研究了燃用甲醇柴油(甲醇體積分?jǐn)?shù)為20%時(shí))14種醛酮類(lèi)非常規(guī)排放物的排放特性。在標(biāo)定功率工況下,燃用甲醇柴油的醛酮類(lèi)排放物比燃用柴油時(shí)升高144.6%。柴油機(jī)噴射系統(tǒng)對(duì)排放和燃
油經(jīng)濟(jì)性有顯著的影響[85,87-88],增大噴射壓力和時(shí)間一般可以?xún)?yōu)化PM、CO和HC的排放,但是NOx排放惡化,并且存在1個(gè)最佳的噴射壓力和時(shí)間,此時(shí)燃油最經(jīng)濟(jì)。甲醇柴油和純柴油表現(xiàn)出不同燃燒特性。徐斌等[89]分析了甲醇柴油(甲醇體積分?jǐn)?shù)為5%和10%)混合燃料對(duì)直噴式柴油機(jī)燃燒特性的影響。結(jié)果表明,在相同的平均有效壓力和轉(zhuǎn)速下,相比于純柴油,甲醇柴油混合燃料滯燃期延長(zhǎng),燃燒持續(xù)期縮短,缸內(nèi)最大爆發(fā)壓力、最大壓力升高率及最高平均燃燒溫度上升;甲醇柴油混合燃料與純柴油放熱規(guī)律相似,最大瞬時(shí)放熱率比純柴油大,且峰值所對(duì)應(yīng)的時(shí)刻滯后;混合燃料預(yù)混燃燒部分比柴油略大,燃燒放熱重心向上止點(diǎn)后偏移。
國(guó)內(nèi)外學(xué)者對(duì)燃用乙醇柴油的排放性能和發(fā)動(dòng)機(jī)性能研究較多。其排放性能與甲醇柴油相似,降低PM、NOx和 CO 排 放,但 HC排 放 增 加[90-91]。一些學(xué)者得到不同結(jié)論,主要是柴油機(jī)結(jié)構(gòu)和操作條件不同造成的[92-94]。乙醇柴油也存在非常規(guī)排放問(wèn)題,張學(xué)敏等[95]研究表明,燃用乙醇-生物柴油-柴油混合燃料(乙醇體積分?jǐn)?shù)為20%)可以減少非常規(guī)排放量,在中低負(fù)荷時(shí),比燃用純柴油醛類(lèi)排放下降,在高負(fù)荷時(shí),醛類(lèi)排放升高。乙醇柴油的燃油經(jīng)濟(jì)性也與甲醇柴油類(lèi)似[96-98],其當(dāng)量燃油消耗在小負(fù)荷時(shí)與柴油相近,在中、大負(fù)荷時(shí)明顯低于柴油,而且基本上隨著乙醇比例增大而減小。與甲醇柴油類(lèi)似,發(fā)動(dòng)機(jī)噴射系統(tǒng)對(duì)排放和發(fā)動(dòng)機(jī)性能有顯著的影響[99-100]。乙醇柴油和純柴油也表現(xiàn)出不同燃燒特性。余紅東等[101]通過(guò)研究發(fā)現(xiàn),乙醇柴油(乙醇體積分?jǐn)?shù)為10%、15%、20%、25%)的滯燃期隨乙醇比例的增加而延長(zhǎng)。在各轉(zhuǎn)速的中、大負(fù)荷工況下,混合燃料的缸內(nèi)最大爆發(fā)壓力大于柴油,壓力峰值出現(xiàn)的時(shí)刻滯后于柴油,且隨著燃料中乙醇比例的增加,峰值滯后越明顯。在小負(fù)荷工況下,混合燃料的缸內(nèi)最大爆發(fā)壓力小于柴油。隨著燃料中乙醇比例的增加,缸內(nèi)最大爆發(fā)壓力逐步減小,所對(duì)應(yīng)的曲軸轉(zhuǎn)角明顯延遲。
關(guān)于丁醇柴油的燃燒性能?chē)?guó)內(nèi)報(bào)道較少。陳征等[102]指出,柴油輕型車(chē)可以燃燒體積分?jǐn)?shù)30%的丁醇柴油混合燃料,燃油經(jīng)濟(jì)性不超過(guò)7%;丁醇柴油混合燃料的使用不會(huì)導(dǎo)致柴油輕型車(chē)HC的過(guò)度排放,但使CO排放惡化,特別對(duì)質(zhì)量較小的輕型車(chē)。此外,丁醇的加入雖然使有些工況下NOx排放小幅度增加,但柴油機(jī)煙度排放明顯降低,為采用EGR(Exhaust gas recirculation)手段降低柴油輕型車(chē)原始NOx排放騰出了更大的EGR率提升空間。杜標(biāo)[79]的研究表明,隨著丁醇摻混比例(丁醇體積分?jǐn)?shù)為20%、30%、40%)的提高,缸內(nèi)最大爆發(fā)壓力呈現(xiàn)增大的趨勢(shì),在中、大負(fù)荷工況下尤其明顯,小負(fù)荷的差異不大;最大壓力升高率大致是隨著丁醇摻燒比例的增加而減小,而中、大負(fù)荷工況卻呈現(xiàn)相反的趨勢(shì)。丁醇柴油混合燃料的燃燒放熱速率比純柴油略高。中、高轉(zhuǎn)速,小負(fù)荷工況下,滯燃期較純柴油有所延長(zhǎng),隨負(fù)荷升高滯燃期較純柴油縮短,燃燒持續(xù)期較柴油有縮短的趨勢(shì)。國(guó)外有系統(tǒng)的研究報(bào)道稱(chēng),在排放性能上,丁醇柴油與其他醇類(lèi)柴油相似,能降低PM、NOx和CO排放,但HC排放增加[103-105]。有的學(xué)者發(fā)現(xiàn)其N(xiāo)Ox排放升高,Rakopoulos等[106]在渦輪增壓柴油機(jī)加速時(shí)進(jìn)行丁醇柴油(丁醇體積分?jǐn)?shù)為25%)試驗(yàn),發(fā)現(xiàn)碳煙減少,但是NOx排放增多。Mehta等[107]采用四缸、四沖程、水冷柴油機(jī)進(jìn)行混合燃料(丁醇體積分?jǐn)?shù)為5%、10%、15%、25%)臺(tái)架試驗(yàn),結(jié)果表明,在大負(fù)荷情況下,CO排放有明顯下降,最高達(dá)到42%;NOx排放升高的范圍為2.4%~11%。Chen等[108]采用乘用車(chē)柴油機(jī)研究了丁醇柴油(丁醇體積分?jǐn)?shù)為20%、30%、40%)燃燒與排放性能,結(jié)果表明,隨著丁醇比例的增大,在低負(fù)荷情況下CO排放有明顯上升,NOx排放下降;在大負(fù)荷情況下,CO排放下降,而NOx排放上升。出現(xiàn)這種現(xiàn)象的原因可能是柴油機(jī)結(jié)構(gòu)和操作的不同。在燃油經(jīng)濟(jì)性方面,丁醇柴油與其他醇類(lèi)柴油相似[109-110],燃油消耗率高于純柴油,并隨丁醇比例的增大燃油消耗率增加,這主要是因?yàn)槎〈嫉臒嶂当炔裼偷?,但是有效熱效率高于純柴油?/p>
優(yōu)良的醇類(lèi)替代燃料應(yīng)該具有來(lái)源廣泛、生產(chǎn)加工技術(shù)成熟、經(jīng)濟(jì)性良好、環(huán)境友好等特點(diǎn),可以滿(mǎn)足國(guó)家能源戰(zhàn)略的補(bǔ)充需求。更為重要的是,我國(guó)醇類(lèi)燃料的發(fā)展,還應(yīng)結(jié)合自身“人口大國(guó)”與“富煤貧油”的特點(diǎn),規(guī)劃自主的能源發(fā)展道路。
我國(guó)甲醇的生產(chǎn)以能源結(jié)構(gòu)中儲(chǔ)量比例最大的煤炭為生產(chǎn)原料,并且生產(chǎn)技術(shù)達(dá)國(guó)際領(lǐng)先水平;特別是利用低附加值的高硫煤合成甲醇,具有較高的經(jīng)濟(jì)性和環(huán)保意義。乙醇、丁醇雖然在合成工藝與生產(chǎn)成本上缺乏競(jìng)爭(zhēng)實(shí)力,但是能通過(guò)生物質(zhì)轉(zhuǎn)化獲得,具有可再生的優(yōu)勢(shì),是未來(lái)很有潛力的替代能源之一。需要特別指出的是,丁醇具有和傳統(tǒng)石化燃料相似的理化性質(zhì),在國(guó)際學(xué)術(shù)界具有較高的關(guān)注度。應(yīng)加快“非糧植物”轉(zhuǎn)化以及低成本生產(chǎn)工藝研究。利用基因技術(shù)培育高產(chǎn)量的含能植物(如高粱、木薯等)和選育高效的發(fā)酵菌種,并且有效利用農(nóng)林廢棄物,開(kāi)發(fā)低壓低酸水解纖維素轉(zhuǎn)化葡萄糖制備乙醇(丁醇)工藝也是未來(lái)的研究方向。
醇類(lèi)柴油對(duì)內(nèi)燃機(jī)性能有一定影響,主要體現(xiàn)在醇類(lèi)柴油穩(wěn)定性較差,特別是甲醇、乙醇柴油。未來(lái)的研究可以集中在以下方面:(1)進(jìn)一步研究乳化設(shè)備對(duì)醇、柴油的乳化行為機(jī)理,優(yōu)化設(shè)備乳化效果;(2)開(kāi)發(fā)適用于醇類(lèi)柴油體系的新型乳化劑,同時(shí)也要注重開(kāi)展乳化劑復(fù)配的研究,并且要兼顧乳化劑對(duì)排放性能以及環(huán)境(如可降解性、生物毒性)的影響;(3)丁醇柴油雖然在實(shí)驗(yàn)室條件下穩(wěn)定性?xún)?yōu)于甲醇、乙醇柴油,但是未來(lái)還需要進(jìn)一步考察丁醇柴油在實(shí)際工況下的穩(wěn)定性;(4)醇類(lèi)柴油較純柴油十六烷值低,直接影響內(nèi)燃機(jī)的冷啟動(dòng)、排放與噪音等性能,需要開(kāi)發(fā)適用于醇類(lèi)柴油的十六烷值改進(jìn)劑,并且其對(duì)醇類(lèi)柴油穩(wěn)定性、燃油經(jīng)濟(jì)性、排放性能的影響也需要系統(tǒng)研究;(5)更為重要的是醇含量直接影響內(nèi)燃機(jī)的動(dòng)力性、經(jīng)濟(jì)性、排放性,需要建立醇含量與這3個(gè)因素的評(píng)價(jià)方法與模型,確定最適宜的醇含量。
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