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(山東科技大學 化學與環(huán)境工程學院,山東 青島 266590)
煤和生物質(zhì)經(jīng)熱解氣化,可以得到燃氣、焦油等產(chǎn)品,有利于提高含碳有機物的利用效率。焦油作為一種高芳香度的復雜混合物,含有苯系、酚系、萘系等有機化合物,烷烴類化合物,稠環(huán)芳香族化合物等,具有重要的加工利用價值。但在實際應用(特別是煤和生物質(zhì)的快速熱解)過程中,高溫重質(zhì)焦油蒸汽易與粉塵等粘結(jié)成焦油渣,堵塞和腐蝕管路,影響系統(tǒng)穩(wěn)定運行。另一方面,焦油中含有致癌物質(zhì),排放到環(huán)境中會嚴重危害人體健康。因此,在高資源需求量和嚴重環(huán)境污染情況下,高效利用焦油中的富氫資源,將重質(zhì)焦油組分輕質(zhì)化,轉(zhuǎn)化為富氫氣體和輕油,具有重要的意義。
催化裂解法是當前焦油輕質(zhì)化最高效、最經(jīng)濟的加工方法之一,高活性、高穩(wěn)定性、長壽命焦油裂解催化劑的研發(fā)已成為研究熱點。焦油裂解催化劑種類繁多,本研究以白云石類催化劑和Ni-Al2O3類催化劑為例,重點從焦油裂解原理、助劑種類及作用、工藝條件等方面進行分析。由于前者為天然原料,來源充足、成本低,無需再生利用,從工業(yè)應用方面出發(fā),可將復合白云石催化劑作為非再生型催化劑;后者催化效果好,但價格昂貴,成本較高,需再生利用,因此將以γ-Al2O3為載體的鎳基催化劑作為再生型催化劑。
周勁松等[1]發(fā)現(xiàn)白云石煅燒后表面形成一種具有極性活化位的酸堿型CaO-MgO絡(luò)合物,該絡(luò)合物不僅能吸附焦油中負電性電子體系的稠環(huán)化合物,而且能吸附脂肪側(cè)鏈氫原子,消除芳環(huán)和脂肪烴間的鍵能,提供電子對以脫除C—H中的氫離子,使C—C鍵、C—H鍵容易發(fā)生斷裂,降低裂解反應活化能。呂俊復等[2]的研究表明焦油轉(zhuǎn)化率的提高是由于CaO改變了甲苯和苯裂解的反應路徑,從而減小了裂解反應的活化能。另有研究者[3]認為鈣以原子態(tài)Ca2+與酸性位交換才能表現(xiàn)出較高的焦油裂解活性。此外添加鎳的白云石催化劑能進一步促進焦油中芳香族組分的開環(huán)裂解反應,從而提高芳香族組分的裂解率,同時氧化鎳能在白云石表面形成固溶體,提高鎳的分散度和催化劑的穩(wěn)定性[4]。
天然白云石催化劑具有較高的焦油裂解活性,但其機械強度較低,反應過程中易因磨損而粉碎,堵塞下游設(shè)備,危害系統(tǒng)安全。因此,在制備白云石催化劑時,既要考慮添加助劑提高催化劑活性、穩(wěn)定性,又要選擇合理的制備方法,以提高催化劑的機械強度。
通常采用添加金屬氧化物的方法制備白云石催化劑,主要有物理干混法、液體浸漬法、化學沉淀法等。Wang等[5]向白云石粉末中加入Fe2O3進行改性,焦油轉(zhuǎn)化率有所提高,但催化劑壽命、穩(wěn)定性沒有得到明顯改善。薛俊等[6]采用配位沉淀法制備Ni-白云石催化劑,實驗表明該催化劑活性較高,再生性能好,且失活催化劑在900 ℃下焙燒2 h可恢復到新鮮催化劑的性能,表現(xiàn)出良好的再生性。梁鵬等[7]采用浸漬法制備10%Ni/1%La-白云石催化劑,結(jié)果表明La(NiO3)晶相有利于增強催化劑的穩(wěn)定性。王鐵軍等[8]發(fā)現(xiàn)煅燒溫度對白云石類催化劑性能影響顯著,當煅燒溫度低于800 ℃,催化劑煅燒不完全,穩(wěn)定性差,且反應后氣體中二氧化碳含量較高。在白云石催化劑成型方面,繆冶煉等[9]通過添加粘結(jié)劑、擴孔劑等將白云石成型造粒,所得催化劑機械強度高,不易破碎。Li等[10]將硝酸鎂浸漬到白云石中,再進行煅燒,制備的MgO-白云石催化劑的強度較天然白云石提高250倍,但催化劑的活性略有下降。
白云石催化劑助劑主要包括Ni、Fe、La、Mg等,其中Ni是白云石催化劑最常用的助劑。研究表明添加助劑鎳,有利于提高白云石催化劑的活性及穩(wěn)定性。常見白云石催化劑助劑種類及作用如表1所示。
有研究[20]表明,Ni/Mg-Al2O3催化劑裂解苯、甲苯焦油模型化合物時,鎳含量從0增至5%,鎳基催化劑促進苯或甲苯的水蒸氣重整反應CnHm+nH2O=nCO+(n+m/2)H2,使氣體產(chǎn)品CO產(chǎn)率增加;鎳含量繼續(xù)增加至15%時,水蒸氣逐漸被消耗,以甲烷化反應CO+3H2=CH4+H2O為主,使CH4產(chǎn)率則升高。另一方面,CO2和H2O在鎳基催化劑活性中心會解離生成氧自由基,使焦油裂解[21]。
表1 常見白云石催化劑助劑種類及主要作用Tab. 1 The promoters and main effects of common dolomite catalysts
鎳基γ-Al2O3催化劑制備方法主要有沉淀法、浸漬法、溶膠凝膠法、原位合成法、燒結(jié)法等,其中前兩種是較為常用的制備方法。吳云芬[22]對比了共沉淀-負載法、溶膠凝膠法制備的Ni/CeO2-ZrO2-Al2O3催化劑,發(fā)現(xiàn)前者比表面積大、孔隙結(jié)構(gòu)豐富、活性較高,但積炭率亦相對較高。李翠平等[23]分別采用原位合成法、浸漬法制備鎳-介孔氧化鋁催化劑,發(fā)現(xiàn)兩種制備方法均能得到穩(wěn)定有序介孔結(jié)構(gòu)的催化劑,但前者負載的鎳大部分以偏鋁酸鎳的形式存在,對甲烷化反應催化活性不高;后者則是以易還原的氧化鎳的形式存在,使鎳得到很好分散。Wang等[14]研究表明,制備催化劑過程中,成型壓力會對催化劑的孔結(jié)構(gòu)產(chǎn)生影響??酌偷萚24]采用浸漬法制備鎳基催化劑,發(fā)現(xiàn)隨煅燒溫度(550~800 ℃)升高,活性組分鎳向催化劑載體相擴散,形成難以還原的Ni-MgO固溶體,同時催化劑活性、穩(wěn)定性相對減弱,并指出在600 ℃煅燒的催化劑活性最高。楊詠來等[25]通過H2-TPR(temperature programmed reduction)、XPS(X-ray photoelectron spectroscopy)等表征方法,發(fā)現(xiàn)隨著煅燒溫度升高(400~600 ℃),鎳基催化劑的還原峰溫度逐漸升高,金屬與載體間的作用愈來愈強,進而阻止Ni的還原,促進鎳鋁尖晶石形成。
復合載體的鎳基催化劑比單一載體催化劑具有更高的催化活性和穩(wěn)定性。岳寶華等[226]用共沉淀水熱法制備Ni/MgAl(O)復合載體催化劑時發(fā)現(xiàn),與普通共沉淀法相比,復合載體催化劑表現(xiàn)出較高的催化活性、穩(wěn)定性能和抗硫中毒性能。詹吉山等[26]采用溶膠凝膠法制備Ni/TiO2-Al2O3復合載體催化劑,發(fā)現(xiàn)適量TiO2能有效抑制鎳鋁尖晶石NiAl2O4生成,改善催化劑還原性能。另有研究者[27]指出,加入ZrO2會在一定程度上削弱Ni與Al2O3載體的相互作用。
助劑能促進催化劑表面電子轉(zhuǎn)移,提高活性組分分散度和抗積炭能力,通常助劑添加量通常不超過10%。鎳基γ-Al2O3催化劑所使用助劑通常分為3類:結(jié)構(gòu)助劑,如MgO;晶格助劑,如La2O3、CeO2等;電子助劑,如Mn、Fe、Mo等過渡金屬元素。常見助劑及其功能如表2所示。
表2 常見Ni-Al2O3催化劑助劑種類及主要作用Tab. 2 The promoters and main effects of Ni-Al2O3catalysts
裂解過程中常見的反應氛圍主要包括氧化(空氣或氧氣)氛圍、水蒸氣氛圍、氫氣氛圍、二氧化碳氛圍以及混合氣氛圍。文獻研究[41-42]發(fā)現(xiàn),部分氧化氛圍能降低催化劑表面的積炭量、減少催化劑積炭失活,提高產(chǎn)品氣中CO含量。Wises等[43]研究表明,調(diào)節(jié)反應氛圍中水蒸氣含量,提高水碳比,可一定程度上降低催化劑表面積炭,提高裂解氣熱值[21]。H2氛圍有助于催化劑的還原,產(chǎn)生更多催化劑活性位點,提高反應活性[44]。Simell等[45]在研究CO2氛圍下白云石催化劑裂解甲苯時發(fā)現(xiàn)氧化鈣易吸收CO2形成碳酸鈣,引起催化劑孔徑堵塞;但CO2能促進鎳基催化劑消炭反應的進行,間接提高催化劑的穩(wěn)定性[21]。煤和生物質(zhì)熱解焦油組分中,萘是熱穩(wěn)定性最好,也是最難分解的物質(zhì)[46]。已有研究表明,合理的反應氣氛可促進萘等重質(zhì)焦油組分的催化裂解。Aldén等[44]利用水蒸氣和水蒸氣/空氣兩種氣氛對萘進行重整,萘轉(zhuǎn)化率分別為72%和96%。
反應溫度對焦油裂解的產(chǎn)品組成及數(shù)量、催化劑壽命等方面有重要的影響。一方面,升高溫度能促進裂解反應正向進行,Kinoshita等[47]對生物質(zhì)氣化的研究表明,隨反應溫度的升高,燃氣中焦油含量和種類均有減少;當溫度高于800℃,幾乎檢測不到苯酚等含氧焦油化合物。另一方面,反應溫度也會顯著影響催化劑活性和選擇性。較低的反應溫度易使活化的焦油組分吸附于催化劑表面形成積炭[48],覆蓋活性中心,導致活性下降;升高溫度能改善催化劑表面的積炭現(xiàn)象,但溫度過高也容易導致催化劑水熱失活。Berrueco等[49]研究溫度對白云石裂解生物質(zhì)焦油的影響,發(fā)現(xiàn)隨著溫度從750 ℃升至850 ℃,高溫導致白云石結(jié)構(gòu)發(fā)生變化,裂解反應以熱裂解為主,使得裂解氣中H2、CO、CH4、CO2產(chǎn)率增大,C2+等烴類氣體產(chǎn)率略有下降。Josuinkas等[50]研究了反應溫度對鎳基催化劑上焦油裂解的影響,發(fā)現(xiàn)低溫時氣體產(chǎn)物以CO2和CH4為主,高溫時(500 ℃以上)以H2和CO為主。王鐵軍等[9]也發(fā)現(xiàn)相似實驗結(jié)果,鎳基催化劑對H2和CH4具有很好的選擇性。
焦油在反應器內(nèi)與催化劑的接觸反應時間是影響焦油裂解程度、產(chǎn)物組成的重要因素。Katheklakis等[51]研究發(fā)現(xiàn)高溫下焦油在稀相段的停留時間延長,可增加其二次裂解反應,促進焦油的水蒸氣重整反應,使小分子氣體轉(zhuǎn)化率增加。研究者[52]在650 ℃研究不同停留時間對氧化鈣裂解焦油的影響,發(fā)現(xiàn)氣相段停留時間從0增加至3 s時,H2和CO產(chǎn)率明顯增加,焦油組分中C6~C10的脂肪烴含量明顯減小,隨停留時間繼續(xù)增加,氧化鈣對焦油裂解轉(zhuǎn)化率的影響逐漸減小。岳寶華等[22]研究表明隨著空速增加,Ni/MgAl(O)催化劑裂解甲苯、苯轉(zhuǎn)化率降低,CH4收率減少,說明縮短停留時間會降低甲烷的生成速率。Dou等[53]發(fā)現(xiàn)鎳基催化劑在3 000~11 000 h-1空速范圍內(nèi),隨空速增大,1-甲基萘等焦油模擬化合物的平均轉(zhuǎn)化率約減小32%,C1~C4收率也明顯減小。Corella等[54]在流化床中的實驗也表明焦油裂解存在最優(yōu)停留時間,繼續(xù)增加停留時間,焦油轉(zhuǎn)化率沒有明顯提高。
焦油裂解催化劑種類繁多,其中天然白云石和鎳基γ-Al2O3催化劑是最具研究和開發(fā)價值的焦油裂解催化劑。白云石來源充足,活性相對較高,但對重質(zhì)焦油組分裂解度不高且機械強度低、易磨損。天然白云石催化劑添加助劑改性后,催化劑機械強度與反應活性等均有所提升。鎳基γ-Al2O3催化劑焦油裂解活性高,穩(wěn)定性好,壽命較長,但鎳基γ-Al2O3催化劑易因活性組分Ni燒結(jié)、表面積炭失活;經(jīng)Mg、Ce、Zr等助劑改性的鎳基催化劑能提高催化活性、增強抗燒結(jié)和抗積炭性能。
目前多數(shù)實驗研究是以苯、甲苯、萘等為焦油模型化合物為研究對象,在實驗室規(guī)模裝置上評價和研究催化劑性能,很少結(jié)合工業(yè)化設(shè)備及真實工況進行放大研究。因此,今后研究方向一方面是研發(fā)高活性、高穩(wěn)定性、長壽命、價格低廉的天然白云石類催化劑和人工合成鎳基γ-Al2O3催化劑,另一方面應重點結(jié)合煤與生物質(zhì)熱加工工藝中焦油的實際工況,探索優(yōu)化的工藝條件和反應器類型。
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