熱解油水相酸洗-烘焙二級(jí)預(yù)處理改善玉米秸稈熱解特性

胡志超，梅艷陽(yáng)，楊 晴，王賢華，楊海平，邵敬愛，陳漢平

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熱解油水相酸洗-烘焙二級(jí)預(yù)處理改善玉米秸稈熱解特性

胡志超，梅艷陽(yáng)，楊 晴※，王賢華，楊海平，邵敬愛，陳漢平

（華中科技大學(xué)煤燃燒國(guó)家重點(diǎn)實(shí)驗(yàn)室，武漢 430074）

烘焙和酸洗都是可提升生物質(zhì)品質(zhì)的預(yù)處理方法。烘焙可以脫除生物質(zhì)中的氧，酸洗則可有效脫除堿金屬及堿土金屬，而氧和AAEMs對(duì)熱解油的品質(zhì)和產(chǎn)率均具有影響。該文研究了酸洗-烘焙兩級(jí)耦合預(yù)處理對(duì)玉米秸稈熱解特性的影響。試驗(yàn)用酸液取自熱解聯(lián)產(chǎn)聯(lián)供示范項(xiàng)目熱解油的水相部分，烘焙溫度選取230、260、290 ℃。研究發(fā)現(xiàn)，酸洗預(yù)處理能夠有效脫除AAEMs，對(duì)K、Na、Mg脫除率分別達(dá)到97.53%、81.38%、84.86%。兩級(jí)預(yù)處理能明顯降低O/C；酸洗-290 ℃烘焙半焦相比玉米秸稈原樣，O/C降低了25.32%。兩級(jí)預(yù)處理能明顯削弱烘焙對(duì)熱解油產(chǎn)率的不利影響，酸洗-290 ℃烘焙半焦相比290 ℃烘焙半焦，其熱解油產(chǎn)率提高127.66%；兩級(jí)預(yù)處理顯著提高了熱解油中糖類的含量，同時(shí)降低了酚類和酸類的含量；對(duì)酸洗-290 ℃烘焙半焦，其熱解油中糖類含量高達(dá)45.89%，酚類和酸類則低至9.76%和6.31%。其他化學(xué)組成如酮類和呋喃類的含量存在一定程度的下降，醛類含量則有小幅度的提升。該文提出的利用熱解聯(lián)產(chǎn)聯(lián)供示范項(xiàng)目熱解油的水相部分對(duì)秸稈進(jìn)行酸洗，并結(jié)合烘焙的兩級(jí)預(yù)處理方法可為對(duì)生物質(zhì)熱解提供參考。

烘焙；秸稈；熱解；酸洗；堿金屬及堿土金屬

0 引 言

生物質(zhì)熱解是一種重要的生物質(zhì)熱轉(zhuǎn)化技術(shù)，可以把低品位生物質(zhì)轉(zhuǎn)化為品質(zhì)相對(duì)較高的熱解油、氣體、焦炭等產(chǎn)品。但是，生物質(zhì)的高含氧量會(huì)導(dǎo)致熱解油水分含量大、氧含量高、酸性強(qiáng)、黏度大、穩(wěn)定性差等問題[1]。因此，采取適當(dāng)?shù)念A(yù)處理方法降低生物質(zhì)的氧含量十分有必要。烘焙預(yù)處理是一種在惰性氛圍下，溫度介于200～300 ℃的熱化學(xué)轉(zhuǎn)化過程[2]，是一種能夠降低生物質(zhì)氧含量、增強(qiáng)疏水性和易磨性的預(yù)處理方法[3-5]。同時(shí)，烘焙可以降低生物油的水含量和酸含量[6]。

但烘焙預(yù)處理之后，生物質(zhì)中仍殘留有不少的AAEMs（alkali and alkaline earth metals，AAEMs）。AAEMs會(huì)催化熱解揮發(fā)分中的大分子發(fā)生重聚反應(yīng)，致使熱解油產(chǎn)率降低而產(chǎn)生更多的氣體和焦炭[7]。同時(shí)，熱解油中的AAEMs會(huì)導(dǎo)致后續(xù)提質(zhì)過程的催化劑中毒[8-9]。所以，有必要采取適當(dāng)?shù)念A(yù)處理方法脫除AAEMs。酸洗預(yù)處理則能有效脫除AAEMs，理論上能夠增加熱解油產(chǎn)率，降低氣體和焦炭產(chǎn)率[10]。此外，酸洗還有利于提升糖類產(chǎn)量，同時(shí)降低生物油中羧酸，酮類及酚類含量[11]。且酸洗后熱解油中無機(jī)礦物質(zhì)元素含量很低，理論上后續(xù)利用過程中催化劑中毒或者灰沉積影響將被減弱。因此，采取有效的預(yù)處理手段提升熱解油的品質(zhì)十分必要。

目前，關(guān)于烘焙預(yù)處理及酸洗預(yù)處理的研究已有很多，但是將二者結(jié)合起來研究對(duì)熱解特性的影響還少有報(bào)道。在現(xiàn)有的報(bào)道中，Wigley等在烘焙預(yù)處理之前，分別利用烘焙液體產(chǎn)物和熱解油的輕質(zhì)油部分進(jìn)行酸洗，認(rèn)為2種預(yù)處理的結(jié)合可以有效脫除AAEMs、乙?；八郑⒖娠@著提升熱解油中糖類的含量[12-13]。張理等分別利用水和水溶相生物油對(duì)生物質(zhì)洗樣，并聯(lián)合烘焙預(yù)處理，發(fā)現(xiàn)熱解油中糖類含量有所增加[14-15]。熱解聯(lián)產(chǎn)聯(lián)供示范項(xiàng)目熱解油的水相部分富集了熱解過程產(chǎn)生的酸性物質(zhì)，如果能將這部分酸類利用起來進(jìn)行酸洗，對(duì)增加系統(tǒng)的經(jīng)濟(jì)性和實(shí)現(xiàn)產(chǎn)業(yè)物質(zhì)內(nèi)部循環(huán)均有益處。因此，本文提出一種利用熱解聯(lián)產(chǎn)聯(lián)供示范項(xiàng)目熱解油的水相部分對(duì)玉米秸稈進(jìn)行酸洗，并結(jié)合烘焙的兩級(jí)預(yù)處理方法，同時(shí)研究二者耦合對(duì)生物質(zhì)熱解特性的影響。

1 試驗(yàn)樣品及方法

1.1 原料選取與制備

試驗(yàn)用玉米秸稈收集自河南洛陽(yáng)，經(jīng)粉碎和篩分之后選取60～120目的粉末顆粒備用，記作CS。試驗(yàn)之前在105 ℃的烘箱中烘48 h至恒質(zhì)量。

試驗(yàn)用酸液取自鄂州萬噸級(jí)生物質(zhì)熱解聯(lián)產(chǎn)聯(lián)供示范項(xiàng)目的熱解油中的水相部分（含水率91%左右），有機(jī)成分為酸類69%（乙酸67.88%、丙酸1.12%）、酚類12.9%、酮類2.56%、酯類1.77%等。鄂州萬噸級(jí)生物質(zhì)熱解聯(lián)產(chǎn)聯(lián)供示范項(xiàng)目由華中科技大學(xué)煤燃燒國(guó)家重點(diǎn)實(shí)驗(yàn)室與湖北藍(lán)焰生態(tài)能源有限公司合作建設(shè)，以秸稈為原料，通過熱解技術(shù)，連續(xù)生產(chǎn)生物燃?xì)?、生物炭和生物油，?shí)現(xiàn)供氣、供電、供熱，為新農(nóng)村集中居住區(qū)提供高品位清潔能源。

酸洗過程為稱取30 g玉米秸稈粉末與600 mL酸液（pH值3.4）混合均勻，在30 ℃下，連續(xù)攪拌4 h[10]；然后水洗過濾，直至沖洗后的去離子水接近中性，水洗后的玉米秸稈粉末放置于105 ℃的烘箱中烘至恒質(zhì)量。酸洗后的玉米秸稈粉末記作WCS。

表1是玉米秸稈酸洗前后主要的AAEMs的電感耦合等離子體質(zhì)譜儀（ICP-MS）分析?？梢钥闯鯳SC相比SC，酸洗對(duì)K、Na、Ca、Mg、Fe脫除率分別達(dá)到97.53%、81.38%、27.16%、84.86%、43.05%，這表明酸洗對(duì)AAEMs有很好的脫除效果[16]。同時(shí)，在酸洗過程中酸液中的其他有機(jī)成分也可能起到了一定的作用，Karnowo等認(rèn)為酚類會(huì)彌漫到有機(jī)質(zhì)中和大分子形成H鍵，形成疏水作用使得乙酸和水更容易通過有機(jī)質(zhì)，進(jìn)而提高對(duì)附著在有機(jī)質(zhì)上的AAEMs的脫除效率[17]。如表2所示，酸洗對(duì)灰分的脫除也十分明顯，WCS相比CS，灰分含量降低65.89%，這是由于酸洗脫除了玉米秸稈原樣中的部分AAEMs及可溶性雜質(zhì)。

表1 酸洗前后玉米秸稈堿金屬及堿土金屬結(jié)果分析

1.2 烘焙及熱解試驗(yàn)

烘焙試驗(yàn)在臥式爐上進(jìn)行，采用高純N2為載氣，N2流量300 mL/min；選取的烘焙溫度分別為：230、260、290 ℃[18]；臥式爐達(dá)到目標(biāo)溫度后，迅速將3 g的玉米秸稈粉末推至反應(yīng)器中央恒溫段，在此溫度下恒溫30 min。CS和WCS在230、260、290 ℃烘焙溫度下得到的半焦分別記作CS230、CS260、CS290、WCS230、WCS260、WCS290。

熱解試驗(yàn)系統(tǒng)為固定床熱解系統(tǒng)。熱解試驗(yàn)中，采用高純N2為載氣，N2流量設(shè)定為400 mL/min。玉米秸稈粉末質(zhì)量為3 g，待反應(yīng)器溫度升高到550 ℃時(shí)，快速推至反應(yīng)器中央恒溫段，恒溫時(shí)間為30 min[19]；熱解揮發(fā)分經(jīng)冰水混合冷凝后，不凝性氣體采用氣袋收集。分別對(duì)試驗(yàn)前后樣品瓷舟和冷凝瓶進(jìn)行稱量，確定固體產(chǎn)率和液體產(chǎn)率，氣體產(chǎn)率通過差減法得到。烘焙及熱解的每組試驗(yàn)重復(fù)3次，重復(fù)性良好，標(biāo)準(zhǔn)偏差在5%以內(nèi)，最終結(jié)果取3次結(jié)果平均值。

1.3 熱解產(chǎn)物特性分析方法

元素分析采用的是美國(guó)珀金埃爾默公司生產(chǎn)的2400系列II型CHNS/O元素分析儀，測(cè)定過程利用純氧燃燒的方法。工業(yè)分析使用的是SDTGA5000a型工業(yè)分析儀，其測(cè)試的依據(jù)是利用樣品的熱失重原理。

AAEMs的測(cè)定采用美國(guó)珀金埃爾默公司生產(chǎn)的ELAN DRC-e型電感耦合等離子體質(zhì)譜儀（ICP-MS）進(jìn)行測(cè)定。檢測(cè)之前，需要將固體樣品消解成液態(tài)，采用HG08Z-8/10/12/16型高通量智能微波消解儀對(duì)樣品進(jìn)行消解。

熱解氣體的測(cè)定使用安捷倫公司的7890B氣相色譜儀，利用Agilent OpenLAB CDS軟件進(jìn)行運(yùn)行。利用氣相色譜儀可以測(cè)定氣體中的CO2、H2、CH4和CO的相對(duì)體積含量。

熱解油組分的測(cè)定選用安捷倫公司的1909IS- 433UI型氣相色譜質(zhì)譜聯(lián)用儀。每次進(jìn)樣1L，流量設(shè)定為1mL/min，運(yùn)行45 min。MS的試驗(yàn)參數(shù)為：加熱器的試驗(yàn)溫度300 ℃，進(jìn)樣模式為分流的方式，分流比例10:1，即分流流量10 mL/min。

1.4 試驗(yàn)數(shù)據(jù)處理方法

對(duì)于工業(yè)分析和元素分析數(shù)據(jù)，采用專業(yè)統(tǒng)計(jì)分析軟件SPSS（Statistical Product and Service Solutions），版本為22.0，對(duì)數(shù)據(jù)之間的顯著性差異進(jìn)行了分析。檢驗(yàn)方法選擇檢驗(yàn)中的配對(duì)樣本檢驗(yàn)法，置信區(qū)間百分比設(shè)為95%。

2 結(jié)果與討論

2.1 酸洗對(duì)玉米秸稈烘焙三態(tài)產(chǎn)率的影響

酸洗對(duì)玉米秸稈烘焙三態(tài)產(chǎn)率的影響如圖1所示。從圖1中可以看出，酸洗前后，烘焙三態(tài)產(chǎn)率的變化趨勢(shì)基本一致：隨烘焙溫度提高，半焦產(chǎn)率不斷下降，液體和氣體產(chǎn)率逐漸增加；但是，相比酸洗前，酸洗后烘焙半焦產(chǎn)率的下降更加緩慢；290 ℃烘焙半焦產(chǎn)率降至49.67%，而酸洗-290 ℃烘焙半焦產(chǎn)率仍可達(dá)68.50%，差異顯著（<0.05）。這可歸因于具有降低熱解溫度和促進(jìn)揮發(fā)分析出的作用AAEMs的脫除[20]。酸洗后，隨烘焙溫度提高，烘焙氣體產(chǎn)物和液體產(chǎn)物的產(chǎn)率依然呈上升趨勢(shì)，在酸洗-290 ℃烘焙條件下，氣體產(chǎn)物和液體產(chǎn)物產(chǎn)率分別為17.67%和13.59%。但是，酸洗后的烘焙氣體產(chǎn)物和液體產(chǎn)物產(chǎn)率的上升速率相比酸洗前二者的上升速率更為緩慢。

2.2 兩級(jí)預(yù)處理對(duì)玉米秸稈特性的影響

兩級(jí)預(yù)處理前后玉米秸稈的工業(yè)分析和元素分析結(jié)果如表2所示。從表2中可以看出，對(duì)于烘焙預(yù)處理，隨著烘焙溫度的提高，揮發(fā)分含量明顯減少，固定碳含量和灰分含量不斷增加；290 ℃烘焙半焦相比CS，揮發(fā)分質(zhì)量分?jǐn)?shù)降低30.91%，固定碳含量和灰分含量分別增加151.32%和50.25%，差異性不顯著（>0.05）。這可歸因于烘焙過程中揮發(fā)分的析出、碳化作用的不斷增強(qiáng)及灰分的殘留。對(duì)于兩級(jí)預(yù)處理，隨著烘焙溫度的提高，揮發(fā)分含量整體上呈降低趨勢(shì)，WCS290相比CS降低2.14%；固定碳含量整體上呈增加趨勢(shì)，WCS290相比CS增加20.24%（<0.05）；灰分含量整體上有所增加，WCS290相比WCS，差異不顯著（>0.05）。

圖1 酸洗對(duì)玉米秸稈烘焙三態(tài)產(chǎn)率的影響

表2 兩級(jí)預(yù)處理前后玉米秸稈工業(yè)分析、元素分析結(jié)果

注：氧含量由差減法計(jì)算得出；表中數(shù)據(jù)均以干基計(jì)。

Note: The oxygen content was determined by difference; Value in table were calculated based on dry basis.

預(yù)處理后，玉米秸稈主要元素有不同程度的變化，其中C元素和O元素變化最為明顯。從表2中可以看出，對(duì)于烘焙預(yù)處理，隨著烘焙溫度的提高，C元素含量逐漸升高，O元素含量和O/C分子摩爾比逐漸降低，這表明烘焙預(yù)處理具有很好的脫氧效果[21]；CS290相比CS，C元素含量增加23.39%，O元素含量和O/C分子摩爾比分別降低20.85%和25.35%，差異性顯著（<0.05）。這可歸因于烘焙過程中發(fā)生了脫水反應(yīng)、脫羧基、羰基化反應(yīng)，生成了大量的H2O、CO2、CO和較多的含氧化合物[12-13, 22]。對(duì)于兩級(jí)預(yù)處理，隨著烘焙溫度的提高，C元素含量整體上呈升高趨勢(shì)，WCS290相比CS增加15.91%，差異性顯著（<0.05）；O元素含量和O/C分子摩爾比整體上呈降低趨勢(shì)，WCS290相比CS，O元素含量和O/C分子摩爾比分別下降14.35%和25.32%，差異性顯著（<0.05）。H元素整體上有所降低，這是因?yàn)椴糠諬元素以H2O、CH4、C2H6等形式釋放出去。

2.3 兩級(jí)預(yù)處理對(duì)熱解三態(tài)產(chǎn)率的影響

兩級(jí)預(yù)處理對(duì)玉米秸稈熱解三態(tài)產(chǎn)率的影響如圖2所示。酸洗預(yù)處理之后，WCS熱解三態(tài)產(chǎn)率相比CS熱解三態(tài)產(chǎn)率，熱解油產(chǎn)率提升了10%。這可能是因?yàn)樗崽幚硎共糠諧-O斷裂，半纖維素、纖維素及木質(zhì)素的鏈單元變得更短，因此更容易轉(zhuǎn)化為熱解油[23]。而氣體產(chǎn)率降低了4%，可能是因?yàn)槊摮舜龠M(jìn)揮發(fā)分發(fā)生二次裂解反應(yīng)的AAEMs[24]。

圖2 兩級(jí)預(yù)處理對(duì)玉米秸稈熱解三態(tài)產(chǎn)物產(chǎn)率的影響

從圖2中可以看出，隨著烘焙溫度升高，酸洗-烘焙半焦熱解過程中熱解油產(chǎn)率不斷降低，焦炭產(chǎn)率逐漸增加。這可以歸因于烘焙過程中揮發(fā)分的析出、交聯(lián)反應(yīng)及碳化反應(yīng)隨烘焙溫度升高而不斷增強(qiáng)[25-27]。但是，相比烘焙半焦，酸洗-烘焙半焦的熱解油產(chǎn)率降低和焦炭產(chǎn)率增加的趨勢(shì)都變得更加緩慢；WCS290相比CS290，其熱解油產(chǎn)率增加了127.66%，而焦炭產(chǎn)率下降了46.27%。這可以解釋為酸洗脫除了AAEMs，抑制了熱解過程中纖維素-木質(zhì)素及纖維素-堿金屬之間的反應(yīng)，改變了熱解路徑，促進(jìn)了熱解油的生成和抑制了焦炭的形成[28]。

2.4 兩級(jí)預(yù)處理對(duì)熱解氣體產(chǎn)物特性的影響

兩級(jí)預(yù)處理對(duì)玉米秸稈熱解氣體產(chǎn)物組分的影響如圖3所示。從圖3中可以看出，對(duì)于烘焙預(yù)處理，隨著烘焙溫度的提高，CO含量呈明顯下降趨勢(shì)，CS290相比CS下降了37.77%。這可歸因于CO主要來自半纖維素中羰基的脫除以及含氧分子的裂解，半纖維素相對(duì)含量隨烘焙溫度提高逐漸降低[29]。隨烘焙溫度的提高，熱解氣體中CO2含量逐漸增加，CS290相比CS增加了40.01%。這是因?yàn)閷?duì)于500℃以上的熱解過程，CO2的生成主要與木質(zhì)素有關(guān)，烘焙過程中半纖維素與纖維素的部分分解造成了木質(zhì)素的富集，因此CO2含量呈升高趨勢(shì)。CH4與H2的含量較低，整體上呈現(xiàn)輕微的增長(zhǎng)趨勢(shì)。

對(duì)于酸洗-烘焙預(yù)處理，隨著烘焙溫度的提高，CO、CO2、CH4與H2含量均沒有明顯的變化趨勢(shì)。這可能是因?yàn)樵谒嵯催^程中脫除了玉米秸稈中的部分的堿金屬及堿土金屬，而堿金屬及堿土金屬具有促進(jìn)揮發(fā)分發(fā)生二次裂解反應(yīng)的催化作用，堿金屬及堿土金屬的脫除使得這種催化作用被弱化，進(jìn)而抑制了揮發(fā)分中小分子物質(zhì)的生成[24]。

圖3 兩級(jí)預(yù)處理對(duì)玉米秸稈熱解氣體組分的影響

2.5 兩級(jí)預(yù)處理對(duì)熱解液體產(chǎn)物特性的影響

預(yù)處理對(duì)玉米秸稈熱解液體產(chǎn)物組分的影響如圖4所示。從圖4中可以看到，兩級(jí)預(yù)處理明顯的改變了熱解液體產(chǎn)物的化學(xué)組成，主要是促進(jìn)了糖類含量的提升，同時(shí)降低了酚類、酸類、呋喃類、酮類的含量。

CS及烘焙半焦的熱解油中都沒有檢測(cè)到糖類的含量。這可能是因?yàn)锳AEMs對(duì)脫水反應(yīng)、碳架裂解反應(yīng)及葡萄糖單元的分解具有促進(jìn)作用，使得一些中間體如糖類被立即分解或發(fā)生了二次反應(yīng)[30-32]。酸洗預(yù)處理之后，熱解油中糖類相對(duì)含量達(dá)到20.80%；且隨烘焙溫度升高，糖類含量不斷提升，WCS290達(dá)到45.89%。這可能是因?yàn)锳AEMs以羧酸鹽或以交聯(lián)點(diǎn)的形式存在于木質(zhì)素、纖維素及半纖維素之間，酸洗過程中AAEMs被H+替代，導(dǎo)致了相關(guān)交聯(lián)點(diǎn)的分解，形成了一種松散的結(jié)構(gòu)，這些結(jié)構(gòu)變化會(huì)提升對(duì)糖類選擇性[33-35]。此外，半纖維素和纖維素的交互作用會(huì)抑制糖類的生成，烘焙則會(huì)減弱這種交互作用，因此能夠強(qiáng)化熱解油中糖類的生成[36-37]。

CS熱解的液體產(chǎn)物中，酚類相對(duì)含量最高，為25.92%。對(duì)烘焙半焦，隨著烘焙溫度提高，其熱解油中酚類含量不斷增加，CS290達(dá)到47.32%。這可歸因于酚類主要來源于木質(zhì)素的分解，而木質(zhì)素相對(duì)含量隨烘焙溫度升高不斷增加[22]。對(duì)酸洗-烘焙半焦，隨烘焙溫度提高，其熱解油中酚類含量逐漸降低，由WCS的18.42%降低至WCS290的9.76%。Mourant等認(rèn)為AAEMs以弱鍵形式存在于木質(zhì)素三維結(jié)構(gòu)、木質(zhì)素-纖維素及木質(zhì)素-半纖維素之間，熱分解時(shí)這些鍵會(huì)斷裂生成小分子量一元酚。而AAEMs的脫除，使得這些弱鍵密度降低，抑制了木質(zhì)素的解聚，增強(qiáng)了木質(zhì)素的碳化程度，導(dǎo)致酚類含量的降低[20, 38-39]。

CS熱解的液體產(chǎn)物中，酸類相對(duì)含量達(dá)到20.79%。對(duì)烘焙半焦和酸洗-烘焙半焦，其熱解油中酸類含量都隨烘焙溫度提高而不斷降低，CS290和WCS290分別降至16.81%和6.31%。這是因?yàn)樗犷愔饕獊碓从诎肜w維素和纖維素的熱解，而在烘焙過程中半纖維素和纖維素逐漸減少和不斷碳化。其他化學(xué)組成如酮類和呋喃類的含量存在一定程度上的下降，而醛類含量則有小幅度的提升。

3 結(jié) 論

本文主要研究了熱解油水相洗-烘焙兩級(jí)預(yù)處理對(duì)玉米秸稈熱解產(chǎn)物的影響，結(jié)論如下：

1）酸洗預(yù)處理能夠有效脫除堿土及堿土金屬，對(duì)K、Na、Mg脫除率分別達(dá)到97.53%、81.38%、84.86%；同時(shí)，灰分含量降低65.89%。

2）兩級(jí)預(yù)處理能夠明顯降低O/C，且O/C隨烘焙溫度提高而不斷低；酸洗-290 ℃烘焙半焦相比玉米秸稈原樣，O/C降低了25.32%。

3）酸洗-烘焙半焦相比烘焙半焦，熱解油產(chǎn)率降低和焦炭產(chǎn)率增加的趨勢(shì)更加緩慢。酸洗-290 ℃烘焙半焦相比290 ℃烘焙半焦，熱解油產(chǎn)率增加了127.66%，而焦炭產(chǎn)率下降了46.27%。這表明兩級(jí)預(yù)處理相比烘焙預(yù)處理，既達(dá)到了脫氧的效果，又提高了熱解油的產(chǎn)率。

4）兩級(jí)預(yù)處理顯著提升了熱解液體產(chǎn)物中糖類的含量，降低了酚類和酸類的含量。酸洗-290 ℃烘焙半焦熱解油中糖類相對(duì)含量高達(dá)45.89%，而酚類和酸類則分別降至9.76%和6.31%。

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Effect of two-stage pretreatment combined acid-washing with aqueous portion of pyrolysis oil and torrefaction on pyrolysis characteristics of corn stalk

Hu Zhichao, Mei Yanyang, Yang Qing※, Wang Xianhua, Yang Haiping, Shao Jingai, Chen Hanping

(430074,)

Torrefaction and acid-washing are both the pretreatment methods to improve the quality of biomass. Torrefaction can remove oxygen in the biomass, and acid-washing can effectively remove alkali metals and alkaline earth metals (AAEMs). Both oxygen and alkali metals and alkaline earth metals have effect on the quality and yield of pyrolysis oil. However, after torrefaction, there are still a lot of alkali metals and alkaline earth metals remaining in the biomass. Therefore, it is necessary to take appropriate pretreatment methods to remove alkali metals and alkaline earth metals. But acid-washing can not remove oxygen from the biomass. Therefore, for improving the quality of biomass, torrefaction and acid-washing have some limitations. At present, there are many researches on torrefaction and acid-washing, but the the coupling effect of the torrefaction and acid-washing on pyrolysis characteristics is rarely reported. In this paper, a method of two-stage pretreatment by acid-washing and torrefaction was proposed and the coupling effect of the torrefaction and acid-washing on pyrolysis characteristics of corn stalk was studied. The acid liquor was taken from the aqueous portion of the pyrolysis oil from a demonstration project. If the aqueous portion can be used for washing biomass, it is beneficial to increase the benefit of the system and realize the internal circulation of industrial materials. The torrefaction temperature is 230, 260 and 290 ℃. It is found that the AAEMs can be effectively removed by acid-washing, and the removal rates of K, Na, and Mg are 97.53%, 81.38% and 84.86%, respectively. Two-stage pretreatment can significantly reduce O/C ratio; compared to the original corn stalk, the O/C of semi-coke with washing-torrefaction under 290 ℃ reduces by 25.32%. Two-stage pretreatment can obviously reduce the unfavorable effects of torrefaction on oil yield; compared to 290 ℃ torrefaction semi-coke, the oil yield of semi-coke with washing ?290 ℃ torrefaction increases by 127.66%. Compared to original corn stalk, there is still a 3.88% increase in the oil yield. Two-stage pretreatment can significantly enhance the carbohydrate content in pyrolysis oil, while reducing the content of phenols and acids. There is no carbohydrate in the pyrolysis oil of corn stalks and torrefaction semi-coke. With the increasing of torrefaction temperature, the relative content of carbohydrate in the pyrolysis oil of washing-torrefaction semi-coke gradually increases. For semi-coke with washing ?290 ℃ torrefaction, the content of the carbohydrate in the pyrolysis oil is 45.89%. The relative content of phenols in the pyrolysis oil of corn stalks is the highest, reaching 25.92%. With the increasing of torrefaction temperature, the content of phenols in the pyrolysis oil decreases gradually from 18.42% of sampling with acid-washing to 9.76% of sampling with acid-washing and 290℃ torrefaction. For straw without pretreament, the relative content of the acids in the pyrolysis oil is 20.79%. With the increasing of torrefaction temperature, the relative content of acids in the pyrolysis oil of the torrefaction semi-coke and the washing-torrefaction semi-coke decreases. For 290 ℃torrefaction semi-coke and the washing and 290℃torrefaction semi-coke, the content falls to 16.81% and 6.31%, respectively. Contents of other chemical components such as ketones and furans reduce to a certain extent, but there is a small increase in aldehydes content. On the whole, the two-stage pretreatment not only improves the yield and quality of the pyrolysis oil, but also finds a good way for the use of the aqueous portion.

torrefaction; straw; pyrolysis; acid-washing; alkali metals and alkaline earth metals

10.11975/j.issn.1002-6819.2017.12.029

TK6

A

1002-6819(2017)-12-0224-06

2016-12-15

2017-05-15

國(guó)家自然科學(xué)基金：生物質(zhì)低溫脫氧及其與熱解過程的關(guān)聯(lián)耦合機(jī)制研究（51306067）；生物質(zhì)低溫強(qiáng)化脫氧與催化熱解共耦合制備單環(huán)芳烴的機(jī)理研究（51576087）；基于組分的生物質(zhì)分級(jí)液化機(jī)理及產(chǎn)物分離方法研究（51676075）

胡志超，男，河南許昌人，主要從事生物質(zhì)熱化學(xué)轉(zhuǎn)化技術(shù)研究。武漢 華中科技大學(xué)煤燃燒國(guó)家重點(diǎn)實(shí)驗(yàn)室，430074。 Email：zc.hu@outlook.com

楊 晴，女，湖北京山人，副教授，博士生導(dǎo)師，主要從事生物質(zhì)熱轉(zhuǎn)化理論及技術(shù)研究。武漢 華中科技大學(xué)煤燃燒國(guó)家重點(diǎn)實(shí)驗(yàn)室，430074。Email：qingyang@g.harvard.edu

胡志超，梅艷陽(yáng)，楊 晴，王賢華，楊海平，邵敬愛，陳漢平. 熱解油水相酸洗-烘焙二級(jí)預(yù)處理改善玉米秸稈熱解特性[J]. 農(nóng)業(yè)工程學(xué)報(bào)，2017，33(12)：224－229. doi：10.11975/j.issn.1002-6819.2017.12.029 http://www.tcsae.org

Hu Zhichao, Mei Yanyang, Yang Qing, Wang Xianhua, Yang Haiping, Shao Jingai, Chen Hanping. Effect of two-stage pretreatment combined acid-washing with aqueous portion of pyrolysis oil and torrefaction on pyrolysis characteristics of corn stalk[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2017, 33(12): 224－229. (in Chinese with English abstract) doi：10.11975/j.issn.1002-6819.2017.12.029 http://www.tcsae.org