張德俐,王 芳,易維明,李志合,李永軍,柳善建
·農(nóng)業(yè)生物環(huán)境與能源工程·
木質纖維素生物質厭氧發(fā)酵沼渣熱化學轉化利用研究進展
張德俐,王 芳※,易維明,李志合,李永軍,柳善建
(1. 山東理工大學農(nóng)業(yè)工程與食品科學學院,淄博 255000; 2. 山東省清潔能源工程技術研究中心,淄博 255000)
厭氧發(fā)酵技術可以將木質纖維素生物質轉化為沼氣,并伴隨副產(chǎn)物沼渣產(chǎn)生。隨著大型沼氣工程的發(fā)展,大量沼渣排放已成為厭氧發(fā)酵技術推廣應用的主要限制因素之一,亟須對沼渣進行快速有效處理。其中,沼渣的熱化學轉化利用符合大型沼氣工程發(fā)展趨勢,是當前的研究熱點之一。首先分析木質纖維素沼渣的原料特性與熱化學轉化潛力;再對沼渣成型燃料、熱解以及水熱炭化等領域的研究現(xiàn)狀進行分析,著重對沼渣衍生產(chǎn)物特性、熱化學轉化過程中存在的問題以及與厭氧發(fā)酵結合的潛在優(yōu)勢等方面進行討論;最后,對沼渣熱化學轉化的發(fā)展趨勢進行了展望。木質纖維素生物質厭氧發(fā)酵與沼渣熱化學轉化結合的應用模式研究對大型沼氣工程推廣應用具有一定的科學意義。
生物質;厭氧發(fā)酵;熱解;沼渣;燃燒;水熱炭化
生物質資源的開發(fā)利用是實現(xiàn)循環(huán)農(nóng)業(yè)和低碳經(jīng)濟的重要途徑,更是達成碳中和目標的重要助力,因此,中國大力發(fā)展生物質綜合利用技術[1]。其中,通過厭氧發(fā)酵技術,可以對生物質廢棄物進行有效的資源化利用[2-3]。隨著中國“十三五”期間沼氣工程轉型升級戰(zhàn)略實施,大規(guī)模沼氣工程得到了快速發(fā)展,出現(xiàn)了一批日產(chǎn)1萬m3以上生物天然氣的大型沼氣工程[4]。以秸稈為代表的纖維素類生物質具有甲烷產(chǎn)量高、含硫量低、價格低廉、資源豐富、便于儲存等優(yōu)勢,能夠滿足大型沼氣工程對原料供應的需求,是未來厭氧發(fā)酵技術產(chǎn)業(yè)化發(fā)展的方 向[5]。然而,隨著沼氣工程迅速發(fā)展,伴隨產(chǎn)生大量沼渣的處置問題已成為厭氧發(fā)酵技術推廣應用的主要限制因素之一[6]。
由于富含植物生長所需的營養(yǎng)元素并含有大量小分子腐殖質,現(xiàn)階段沼渣最主要的利用方式為土地施肥與土壤改良[7]。但是,隨著厭氧發(fā)酵殘余物總量的提升,沼渣供給與農(nóng)用土地消納能力的時空矛盾凸顯:一方面殘余物連續(xù)產(chǎn)出,而土地施肥、土壤改良等屬于間歇式需求。一旦進入肥料需求淡季,沼渣、沼液等儲存會占用大量空間并存在潛在環(huán)境污染。研究表明,沼渣、沼液儲存期間會釋放N2O、CO2、CH4、NH3等溫室氣體與有害氣體[8-9],而且,CH4的析出也降低了生物質能的利用效率[10];另一方面,如若大型沼氣工程周邊沒有足夠的土地對其進行消納,高含水率沼渣的遠途運輸將會耗費大量的人力和物力資源[11]。
此外,沼渣施肥對周邊環(huán)境以及農(nóng)作物產(chǎn)品的潛在危害也不能忽視,包括氮磷富集、重金屬沉積、病原體附著等問題[12-14]。尤其隨著人們對食品安全及環(huán)境保護等方面的逐漸重視,許多國家與地區(qū)提高了土地施肥或土壤改良的準入標準,這也在一定程度上限制了沼渣作為有機肥料與土壤改良劑的應用[15]。因此,在大型沼氣工程的發(fā)展背景下,亟需一種新的技術手段對厭氧發(fā)酵過程伴隨產(chǎn)生的沼渣進行快速有效的處理,緩解沼渣處置問題。
其中,沼渣的熱化學轉化利用符合大型沼氣工程的發(fā)展趨勢,可能實現(xiàn)木質纖維素沼渣的無害化處理與資源化利用,已成為當前的研究熱點之一。以沼渣為紐帶,可以將厭氧發(fā)酵技術與熱化學轉化技術有機結合。因此,本文將基于國內外木質纖維素生物質厭氧發(fā)酵沼渣熱化學轉化技術的最新研究成果,對沼渣熱化學轉化的研究情況進行歸納。分析木質纖維素沼渣的熱轉化潛力,分別對沼渣成型燃料、沼渣熱解以及沼渣水熱炭化等領域的研究現(xiàn)狀進行綜述,分析沼渣熱化學轉化過程中存在的問題及其與厭氧發(fā)酵技術相結合的潛在優(yōu)勢,并展望沼渣熱化學轉化技術的發(fā)展趨勢。該項研究對于以木質纖維素生物質為主要原料的沼氣工程推廣應用具有一定的科學指導意義。
厭氧發(fā)酵產(chǎn)甲烷的過程實際上是微生物物質代謝和能量轉換的過程[16]。由于纖維素的結晶性與木質素的存在,甲烷細菌無法對木質纖維素生物質進行有效降解,降低了生物質的整體轉化效率[17]。由表1[18-22]所示,木質纖維素生物質在厭氧發(fā)酵后,原料中僅有23.2%~45.8%的能量流入到沼氣,其物質和能量利用效率較低。相比較,厭氧發(fā)酵后沼渣的質量產(chǎn)率較高,在70%~90%左 右[18-19,23],碳元素和能量所占比例也較高,在60%左右。同時,沼渣本身碳元素含量與熱值也比較可觀,一般約在35%~50%和14~22 MJ/kg之間[18-27]??梢?,木質纖維素沼渣中仍保留著發(fā)酵前原生物質中的大部分能量,有必要對沼渣進行合理的利用。
在木質纖維素生物質發(fā)酵沼渣中,纖維組分的含量也非??捎^。由表2[24-28]可見,秸稈類沼渣中纖維組分含量在67.55%~84.80%之間,牛糞沼渣中纖維組分含量也達到了52.67%。與發(fā)酵前的原料相比,沼渣中纖維素與半纖維素相對含量降低明顯,木質素與灰分含量則有明顯增加。但是,從纖維組分含量來看,沼渣仍具有較大的熱化學轉化潛力。
表1 木質纖維素生物質厭氧發(fā)酵能量與沼渣碳元素分布
表2 木質纖維素生物質及其發(fā)酵沼渣纖維組分含量
根據(jù)國內外厭氧發(fā)酵沼渣熱化學轉換技術的文獻調研,相關研究內容主要集中在沼渣成型燃料燃燒、沼渣熱解和沼渣水熱炭化3個領域,如圖1所示。
圖1 木質纖維素生物質沼渣熱轉化技術路線
生物質成型燃料是指干燥粉碎后的生物質在成型設備中被加工成一定形狀、一定密度的固體燃料[29]。研究表明,沼渣的成型性能明顯優(yōu)于其發(fā)酵前原材料[30-31]。在不添加粘結劑的條件下,木質纖維素發(fā)酵沼渣即可實現(xiàn)顆粒化成型,且表現(xiàn)出良好的機械耐久性[30]。楊世關等[31]將厭氧發(fā)酵過程作為一種秸稈類生物質制備成型燃料的預處理手段,相比于玉米秸稈原料,發(fā)酵沼渣制備的成型燃料松弛密度提高了12.86%,同時,也降低了成型過程中設備的磨損損耗。閆芳等[32]對比研究了玉米秸稈及其沼渣與褐煤的混合成型特性,結果表明,在相同制備條件下,沼渣型煤的抗壓強度最高是玉米秸稈型煤的3倍。王雅君等[33]研究表明,僅以沼渣與生物炭摻混部分水即可直接成型,并表現(xiàn)出良好的抗跌落強度和疏水性。沼渣成型性能提升的主要原因是沼渣中木質素相對含量的增加。木質素作為一種天然黏結劑,能夠增強成型過程中的固體架橋作用,提高機械強度[34]。此外,木質素相對含量的增加也使得秸稈類沼渣的成型燃料熱值有所提高,其低位熱值能夠達到15.8 MJ/kg(9.2%含水率),接近松木成型顆粒的熱值[30]。Li等[35]在沼渣與褐煤混合成型研究中表明,沼渣摻混比例為20%時可制備高品質的成型燃料,熱值能夠達到20.2 MJ/kg。
在燃燒污染氣體排放特性方面,發(fā)酵沼渣也與其發(fā)酵原料有所不同。表3列出了幾種沼渣以及典型生物質成型燃料燃燒的氣體排放水平[30,36-38],并對比了國內相關的排放標準。其中,GB 13271-2014為鍋爐大氣污染物排放標準,NB/T 34006-2011為國內小于50 kW的生物質爐具燃燒排放的行業(yè)標準[39]。與原料相比,沼渣基成型燃料燃燒過程中SO2的排放非常少,幾乎檢測不到[30]。這是由于在發(fā)酵過程中會伴隨部分H2S的析出,降低了沼渣中S元素含量。但是,沼渣燃燒過程中NOx排放量明顯升高,是玉米秸稈等典型生物質成型燃料的3~4倍,超出了新建鍋爐以及小型生物質爐具的標準要求。這是由于發(fā)酵過程中需要補充N源調控發(fā)酵底物的C/N比,導致沼渣中N元素含量升高。因此,沼渣基成型燃料的氣體排放指標中應重點檢測與關注NOx的排放水平。此外,沼渣燃燒的煙塵排放值也較高,但通過增設靜電除塵裝置可使其濃度降低到40~43 mg/m3[30],滿足標準要求。
表3 幾種沼渣與典型生物質成型燃料燃燒的排放水平
注:①為經(jīng)過靜電除塵裝置處理后的煙塵濃度。
Note:①is the dust concentration after treatment by electrostatic filter.
在燃燒灰分特性方面,沼渣基成型燃料的結渣現(xiàn)象較為明顯。纖維素類原料經(jīng)過厭氧發(fā)酵后,其灰分相對含量增加,而且,灰分中含有較大比例的堿/堿土金屬,可能會在較低溫度下發(fā)生熔融現(xiàn)象。Chen等[40]分析纖維素類沼渣的灰熔融軟化溫度為1 180℃,Kratzeisen等[30]分析得到沼渣基成型燃料的灰熔融軟化溫度在1 090~ 1 110℃之間,都遠低于松木燃燒灰分的軟化溫度 (1 370~1 430℃)。Pedrazzi等[36]研究表明,由于聚團、結渣等原因,無法利用小型燃燒爐對木質纖維素沼渣成型燃料進行超過1 h的穩(wěn)定燃燒。付成果[41]等研究表明,即使經(jīng)過水洗處理,秸稈類發(fā)酵殘渣的燃燒仍處于嚴重結渣水平。因此,降低沼渣灰分中非水溶性的低熔點組分含量是其進行燃燒應用的關鍵因素之一。研究表明,將沼渣與木材按照一定比例制備成型燃料或與煤混燒可以降低堿/堿土金屬在燃燒過程中的影響,緩解聚團、結渣等現(xiàn)象[36]。此外,通過酸洗或水熱等預處理手段也可以有效降低生物質灰分中堿/堿土金屬含量,有助于提高其燃燒特性[42-44]。
整體來看,沼渣的成型性能好,具有較高的制備固體燃料的轉化潛力。但是,需要在NOx排放與結渣控制等方面作進一步深入研究,以提高沼渣基成型燃料的燃燒特性。
生物質熱解是在惰性氣氛下使生物質發(fā)生熱分解生成可冷凝揮發(fā)分、固體產(chǎn)物和不可冷凝氣體的技術[45]。通常改變熱解參數(shù),可以對不同的目標產(chǎn)物(生物油、熱解炭和熱解氣)進行調控。木質纖維素沼渣的熱解研究現(xiàn)狀如表4所示[25,40,46-61]。
在制取生物油液體燃料方面,沼渣表現(xiàn)出一定的優(yōu)勢。為了提高生物油的品質,楊昌炎等[46]將固態(tài)發(fā)酵作為一種預處理手段,對發(fā)酵后的小麥秸稈進行了快速熱解液化試驗,結果表明其產(chǎn)出的生物油熱值由發(fā)酵前的16~17 MJ/kg提升到22~24 MJ/kg,并降低了49%的乙酸含量;Neumann等[47]通過異位催化熱解木質纖維素發(fā)酵沼渣制備的生物油熱值達到了35.2 MJ/kg,黏度與總酸值也都有所降低。Hossain等[48]對沼渣進行催化熱解得到的生物油,可以與丁醇以及餐廚廢油等進行混合直接應用于柴油機中,最大配比為30%。此外,厭氧發(fā)酵過程還可以明顯增加熱解生物油中酚類化合物的含量,尤其是4-乙烯基苯酚。Wang等[50]利用PY-GC-MS,對厭氧發(fā)酵前后的玉米秸稈進行了熱解生物油的對比分析,在250℃時,發(fā)酵后其酚類化合物含量由42.25%增加到79.32%,4-乙烯基苯酚的含量則由28.6%增加到60.9%。但隨著熱解溫度逐漸升高到500℃,其生物油中酚類化合物的含量不斷降低。Liang等[25]對稻桿發(fā)酵沼渣的研究也得到了類似的規(guī)律,在330℃時,生物油中4-乙烯基苯酚的含量由發(fā)酵前的29.33%增加到發(fā)酵后的34.93%;而在650℃時,4-乙烯基苯酚的含量則僅從5.76%增加到7.68%。
在厭氧降解過程中,隨著纖維素與半纖維素的去除,木質素相對含量增加,有助于生物油中酸類化合物含量的降低與酚類化合物含量的升高,提高生物油燃料品質。同時,經(jīng)過發(fā)酵后,三組分交聯(lián)結構變得松散,使得更多的木質素結構暴露出來,在相對較低的溫度即可降解產(chǎn)生大量4-乙烯基苯酚[25]??梢姡?jīng)過厭氧發(fā)酵后的沼渣在熱解轉化制取生物油液體燃料和提取酚類化合物等方面具有一定的優(yōu)勢。
在沼渣基熱解炭方面,對其吸附特性的研究相對較多,吸附目標包含無機污染物(氨氮、磷)、有色染料(剛果紅)、抗生素(四環(huán)素)以及重金屬(Pb2+、Cu2+、 Ni2+、Cd2+)等[54-57, 62]。由表4可見,許多沼渣基熱解炭能夠媲美甚至優(yōu)于商業(yè)活性炭的吸附效果。根據(jù)文獻,沼渣基熱解炭優(yōu)良的吸附特性主要歸因于其與吸附質的絡合反應或共沉淀作用。Yao等[54]研究認為甜菜發(fā)酵沼渣對磷酸鹽優(yōu)異的吸附特性是由其表面存在的納米方鎂石膠質結構導致的;Inyang等[55]研究認為沼渣基熱解炭中富含的碳酸鹽和磷酸鹽能夠通過共沉淀反應實現(xiàn)重金屬離子的吸附。可見,沼渣基熱解炭的吸附特性與其自身的灰分結構、組分等密切相關,有必要加強發(fā)酵過程中無機組分的演變規(guī)律研究,進一步明確其與吸附行為之間的關聯(lián)性。
表4 木質纖維素厭氧發(fā)酵沼渣熱解研究現(xiàn)狀
此外,鄭楊清等[57]以沼渣基熱解炭為前驅體,通過氫氧化鉀活化方式制備的活性炭(KOH-CC)對氨氮的最大吸附容量達到120 mg/g。以此為依據(jù),一個500 m3的生物產(chǎn)甲烷示范裝置產(chǎn)生沼渣制備的KOH-CC足夠用于處理其每天排放的沼液,循環(huán)工藝如圖2所示[54]。這為沼渣基熱解生物炭在沼氣工程中的循環(huán)利用途徑提供了一種新的思路,會更加直觀地提高沼氣工程的能量、經(jīng)濟以及環(huán)境效應。
圖2 KOH-CC處理沼液循環(huán)工藝示意[57]
沼渣基熱解炭在土壤改良方面也展現(xiàn)出一定的優(yōu)勢,包括較高的pH值、表面負電荷、離子交換能力以及營養(yǎng)元素含量等。Monlau等[63]對沼氣工程中的沼渣及其熱解炭進行了對比分析,結果表明熱解炭中營養(yǎng)元素P和K的含量大幅增長,能夠部分取代無機肥料的使用;而且其孔隙結構更加發(fā)達(49~88 m2/g),具有更好的持水能力,表現(xiàn)出優(yōu)良的土壤改良性能。但是,由于厭氧發(fā)酵底物源頭復雜,沼渣基熱解炭的施用對生態(tài)環(huán)境的影響也引起了學者們的關注[64]。Stefaniuk等[65]研究表明,木質纖維素沼渣基熱解炭中鎘金屬含量最高達到 8.8 mg/kg,超標嚴重,需進行重點防控。Garlapalli等[66-67]研究表明沼渣基熱解炭中多環(huán)芳烴(PAHs)含量隨著熱解溫度的升高而升高,在800℃制得的沼渣基熱解炭中PAHs的含量達到4.7 mg/kg,遠高于中國農(nóng)用污染物控制標準。因此,在關注沼渣基熱解炭在土壤改良領域正向效應的同時,也應在重金屬與多環(huán)芳烴的遷移規(guī)律等方面做重點研究,嚴格評估其潛在的生態(tài)風險。
在生物質熱解氣化領域,纖維素類沼渣的轉化也引起了眾多學者們的關注,如表4所示。相較于生物質原料,發(fā)酵沼渣的揮發(fā)分含量較低,導致其熱解氣低位熱值(LHV)與冷氣化效率(CGE)相對較低[40,58-60]。研究表明,共氣化技術可有效改善沼渣的熱解氣化性能。Chang等[61]研究表明,沼渣與褐煤共氣化,可有效提高其熱解氣LHV。Yao等[68]將沼渣以20%摻混比例與木屑進行氣化,其能量轉化效率可達70.8%。此外,Chen等[69]研究表明,適度調控發(fā)酵時間等參數(shù)也能夠有效提高氣化產(chǎn)物LHV和CGE。而且,輕度的發(fā)酵預處理后,熱解氣中H2含量增幅達63%,提高了H2/CO摩爾比,這將有利于其作為合成氣原料做進一步化工合成。Marchese等[70]的對比研究也表明,相較于生物質原料,發(fā)酵沼渣制備的熱解氣更適合作為費托合成等合成原料。
在氣化副產(chǎn)物方面,沼渣的應用顯著降低了氣化過程中初級焦油的產(chǎn)生。Chen等[40]研究表明,沼渣氣化過程中的焦油含量可低至1.61 g/Nm3,約為常規(guī)生物質原料氣化焦油產(chǎn)量的1/3。即使經(jīng)過輕度的厭氧發(fā)酵預處理,其焦油含量降幅也能夠達到30%~35%[69],這將有利于其簡化下游凈化過程。而且,沼渣氣化的灰渣中P2O5含量(26.96%)明顯高于常規(guī)生物質氣化灰渣中的含量(3.48%~9.76%),其在緩釋肥應用方面具有一定潛 力[69]。但是,沼渣氣化灰渣的農(nóng)田應用與熱解炭相似,也同樣面臨著潛在的生態(tài)風險。郭祥[71]研究表明,沼渣氣化灰渣中鉻含量(296.48 mg/kg)遠遠高于有機肥的農(nóng)用標準(150 mg/kg),需要進行重點關注??梢姡釉鼩饣诤铣蓺饣ず铣?、焦油控制和灰渣農(nóng)用等方面表現(xiàn)出一定的獨特優(yōu)勢。但還需要進一步優(yōu)化設計沼渣氣化工藝以提高熱解氣LHV和CGE等關鍵指標。
整體來看,木質纖維素沼渣通過熱解制取生物油燃料、酚類化合物、碳基吸附劑、土壤改良劑和合成氣等方面均呈現(xiàn)出一定的應用潛力。而且,在反應動力學方面,厭氧發(fā)酵過程也降低了沼渣的熱解活化能,有利于沼渣的熱解轉化[24,72-74]。但是,有研究表明,由于厭氧發(fā)酵工藝的要求,即使經(jīng)過固液分離的沼渣中含水率依然達到了 70%[75]。沼渣高濕特性導致的干燥能耗在一定程度上制約了其熱解轉化應用。同時,由于發(fā)酵底物多元化,組分極為復雜,相關農(nóng)藥、重金屬以及多環(huán)芳烴等有害組分在沼渣中具有一定的累積效應,也需要進一步探究沼渣熱解過程中這些有害組分的演變規(guī)律。
生物質水熱炭化(Hydrothermal Carbonization,HTC)技術是指生物質與水在一定的溫度(180~250℃)與自生壓力(或高于自生壓力)下,生成富碳固體產(chǎn)物的過程[76]。同時,還伴隨部分有機相轉移為水相產(chǎn)物,小部分物質轉化為氣體。由于HTC過程中水的存在,其非常適合處理高含水率的生物質廢棄物,無需在反應前對原料進行干燥處理。而且,水熱炭也呈現(xiàn)出較大的應用潛力。因此,木質纖維素發(fā)酵沼渣的水熱炭化處理是現(xiàn)階段的研究熱點之一,如表5[21,26,66,77-81]所示。
表5 木質纖維素厭氧發(fā)酵沼渣水熱炭化研究現(xiàn)狀
許多學者對不同水熱炭化工況下,沼渣基水熱炭產(chǎn)率與理化特性的變化規(guī)律進行了研究,其關鍵參數(shù)主要包括水熱溫度、反應時間、初始pH值與液固比等[82]。水熱溫度無疑是影響HTC行為的最主要因素。根據(jù)Arrhenius公式,化學反應速率常數(shù)與反應溫度和反應時間分別呈正比例指數(shù)關系和線性直線關系。水熱溫度相較于反應時間,對水熱炭產(chǎn)率具有更加明顯的影響。pH值則主要通過催化作用影響水熱反應。但是,由于多數(shù)研究中用于調控pH值的檸檬酸在高壓高溫下會降解為丙酮和乙酸等物質,影響了其催化效果[81]。同時,半纖維素等組分在水熱炭化過程中也會產(chǎn)生乙酸等酸性物質,降低了初始pH值對HTC特性的影響[81]。因此有必要加強初始pH值以及反應過程中pH值的變化對沼渣HTC催化機制的研究。此外,液固比不僅能影響水熱炭的性質,還與反應的能耗以及水相副產(chǎn)物的排放有直接關系[83]。較高的液固比有利于提高生物質的溶解性,但也導致能耗升高、水相副產(chǎn)物排放增加等問題。因此,需要綜合考慮水熱炭性能、反應能耗、副產(chǎn)物排放與處置等因素,調節(jié)HTC液固比。
現(xiàn)階段沼渣基水熱炭的應用研究主要集中在固體燃料與土壤改良方面。木質纖維素沼渣經(jīng)過HTC處理后,其燃料品質有顯著提升。一方面,HTC降解以脫水和脫羧為主,獲得的水熱炭熱值高于相同溫度慢速熱解制備的生物炭[78,83]。Oliveira等[79]在180℃下獲得的沼渣基水熱炭熱值即接近于褐煤,Mumme等[80]在270℃的水熱溫度下得到的水熱炭熱值甚至達到了35.7 MJ/kg(干燥無灰基);另一方面,HTC對生物質中堿/堿土金屬有一定的脫除效果[84]。作者對玉米秸稈發(fā)酵沼渣在190~240℃的HTC試驗研究表明,K和Na的去除率均超過80%,Ca與Mg的去除率在40%~60%之間[77]。堿/堿土金屬的降低能夠有利于緩解本文2.1中所述沼渣燃燒過程中潛在的結渣現(xiàn)象[85]。同時,沼渣經(jīng)過水熱炭化處理后,其疏水性、可磨性、脫水性與可流化性等均有明顯提 升[26,79,86-87],能夠有效降低其作為固體燃料在粉碎、儲存以及干燥等階段中的物質損耗與能量消耗。研究表明,相較于傳統(tǒng)的生物質烘焙預處理,水熱預處理能夠節(jié)約30%~50%左右的能耗[88]。此外,作者在玉米秸稈發(fā)酵前后的HTC對比研究中發(fā)現(xiàn),厭氧發(fā)酵過程相對富集了秸稈中的不溶性灰分,同時,增加了秸稈沼渣在水熱轉化體系中的水解傾向,從而在一定程度上降低與抑制了HTC在較高水熱炭化溫度下的脫灰性能與縮合反應[77]。因此,從固體燃料角度看,推薦選擇較低的水熱溫度對秸稈沼渣進行水熱炭化處理。
在農(nóng)田施用方面,與熱解炭相似,沼渣基水熱炭也具有兩面性。一方面,沼渣基水熱炭富含N、P等營養(yǎng)元素,具有良好的土壤改良特性。Funke等[21]分析了小麥秸稈發(fā)酵沼渣水熱炭化過程中N、P等營養(yǎng)元素的分布規(guī)律,60%~65%的N元素和77%~80%的P元素留存在水熱炭中,其自身含量遠高于沼渣中的N、P含量。Mumme等[80]通過分析比表面積與孔隙結構等沼渣基水熱炭結構特性,也肯定了其土壤改良潛力。另一方面,水熱炭的施用對周邊環(huán)境也存在著一定的潛在生態(tài)風險。Busch 等[89]研究表明水熱炭中存在一定的植物性毒素,不利于植物萌芽。Garlapalli等[66]的研究也表明水熱炭含有較多的酚類物質和PAHs,直接施用不利于植物的生長。然而,水熱炭經(jīng)過生物處理或者高溫熱解等方式處理后,都可以有效消除其毒性[59, 89-90]??梢?,水熱炭的后處理過程是其在農(nóng)田中安全施用的重要環(huán)節(jié)之一。
此外,也有學者對沼渣基水熱炭作為活性炭前驅體的性能進行了研究。Catalina等[81]通過對秸稈、牧草和牛糞混合發(fā)酵沼渣進行水熱炭化與KOH高溫活化試驗,獲得的炭材料在CH4/CO2吸附試驗中表現(xiàn)出極佳的選擇吸附性,對CO2吸附量達到8.8 mol/kg(30℃/1.48 MPa),可以滿足沼氣中去除CO2的工藝需求。這也為沼渣基水熱炭在沼氣工程的“就地應用”策略提供了一種新的方式,用于沼氣提純。
整體來看,木質纖維素沼渣的HTC具有一定獨特的優(yōu)勢,可忽略熱轉化前的干燥能耗,能夠改善沼渣燃料品質和土壤改良特性,并展現(xiàn)出一定制備功能化炭材料的潛力。因此,有必要進一步加強木質纖維素沼渣HTC轉化的相關理論研究,以指導優(yōu)化厭氧發(fā)酵與HTC工藝參數(shù)。同時,HTC過程還會產(chǎn)出大量的水相產(chǎn)物,富含有機酸、糠醛等有機組分,也應重視這些副產(chǎn)物的應用研究,包括水相循環(huán)利用[91]、生物轉化[92]等方式。
根據(jù)上述文獻調研,沼渣在燃燒、熱解以及水熱炭化等領域均表現(xiàn)出一定的應用潛力。針對木質纖維素生物質廢棄物,可以很好的以沼渣為紐帶將厭氧發(fā)酵技術與熱化學轉化技術進行有機結合。兩種技術在能源利用效率、規(guī)?;幚硪约爱a(chǎn)物內部循環(huán)等方面具有明顯的互補優(yōu)勢。
對木質纖維素沼渣進行合理的熱化學轉化利用可以有效提高生物質的能量利用效率,如圖3[21,26,93]所示。厭氧發(fā)酵結合沼渣熱解的整體利用效率較高,達到了85%[93]。然而,在熱解轉化過程中,沼渣中含有的大量水分顯著增加了干燥能耗。Kratzeisen等[30]對兩種不同沼渣制備成型燃料的能耗進行了估算,包括機械脫水、干燥與顆粒成型等階段。其中,干燥階段能耗所占比例達到了92%左右,兩種沼渣基成型燃料的制備能耗與其低位熱值的比值分別為0.74與0.78。Monlau等[75]則基于一個運營中的沼氣熱電聯(lián)產(chǎn)項目,對沼渣熱解的可行性進行了分析。結果顯示,耦合沼渣熱解工藝后,可以提高該沼氣工程42%的產(chǎn)電能力。但是,沼渣(23.8 t/d)的日干燥能耗達到了13 249 kW·h,需要消耗大量的熱能,不利于產(chǎn)電能力的進一步提高。相較于燃燒或熱解,厭氧發(fā)酵結合沼渣HTC的整體能量利用效率相對較低,在65%~72%之間。然而,HTC轉化過程中可完全避免前處理過程中的干燥能耗。而且由于水熱炭化反應較溫和、水熱炭脫水性能較好等原因,HTC反應以及后續(xù)的水熱炭干燥能耗較小,一般占比在水熱炭總能量的7%~20%之間[21,94]。此外,Reza等[26]研究表明小麥秸稈厭氧發(fā)酵結合沼渣發(fā)酵的能量效率比單獨HTC處理增加了20%,從能量利用效率來看,相較于原物料,小麥秸稈發(fā)酵沼渣更加適合作為水熱炭化的原料。
圖3 厭氧發(fā)酵與熱轉化技術結合的生物質能利用效率[21,26,93]
高含水率在一定程度上制約了木質纖維素沼渣的燃燒與熱解應用。但是,沼渣經(jīng)過干燥、成型等預處理后,燃燒和熱解技術仍然可以實現(xiàn)其能量增益。而HTC技術,由于能夠忽略沼渣干燥工藝,在能耗方面具有巨大優(yōu)勢。3.2 厭氧發(fā)酵沼渣規(guī)?;幚?/p>
隨著大型沼氣工程的發(fā)展,沼渣的產(chǎn)量非常巨大。Ravina等[95]研究表明,以牛糞和青貯玉米秸稈為發(fā)酵原料,日產(chǎn)14 000 m3左右沼氣工程的沼渣日產(chǎn)量達到了80.5 t。傳統(tǒng)的農(nóng)用土地很難對如此巨量的沼渣進行消納,亟需其他技術手段進行快速有效的處理。其中,燃燒是最簡便有效的生物質能產(chǎn)業(yè)化應用方式,相關燃燒技術已經(jīng)非常成熟;在熱解方面,固定床、流化床以及下降管等熱解工藝的規(guī)?;D化技術也已經(jīng)相對成熟[96-97]。同時,HTC技術也表現(xiàn)出足夠的規(guī)?;D化潛力[97-98]。
此外,由于木質纖維素生物質分布廣泛、能量密度低,使得纖維素原料熱化學轉化技術產(chǎn)業(yè)化過程中的“收、儲、運”成本較高,從而導致原本價格低廉的廢棄生物質原料成本提高。以生物質熱解液化技術為例,生物質收集半徑的增加直接制約了熱解液化設備的規(guī)模,原料供應問題凸顯[99-100]。如果以大型沼氣工程產(chǎn)生的沼渣為生物質熱化學轉化原料,則可大幅降低傳統(tǒng)秸稈等木質纖維素生物質的收集與運輸成本。趙勝雪等[101]研究表明,與常規(guī)生物質相比,沼渣成型燃料的生產(chǎn)成本可節(jié)省46.4%,銷售凈利潤增加1倍。節(jié)省的成本主要來自于生物質原料的收集、運輸與粉碎等環(huán)節(jié)。可見,在原料供應方面與生物質熱化學轉化技術能夠很好地實現(xiàn)互補,有效提高經(jīng)濟效益。
由于沼氣工程是對包括玉米秸稈、畜禽糞便等在內的生物質廢棄物進行厭氧處理,其產(chǎn)生的沼渣中含有大量的抗生素、農(nóng)藥、細菌、寄生蟲和各種病原菌等。如果直接施用于農(nóng)田中,這些有害物質會污染相關農(nóng)業(yè)產(chǎn)品并對周邊環(huán)境造成危害[14-15]。熱化學轉化過程中高溫處理可以有效去除沼渣中具有生物毒性的物質[79]。目前,沼渣的利用主要在農(nóng)業(yè)方面,如若能夠通過熱化學轉化技術,在消除有害物質的同時,還能進一步提高其肥料或土壤改良特性,將對現(xiàn)階段的沼渣利用具有重要的現(xiàn)實意義。
對沼渣進行合理的熱化學轉化,其產(chǎn)物具有極大的潛力可重新應用于沼氣工程中。溫度作為影響厭氧發(fā)酵產(chǎn)氣特性的主要參數(shù)之一,嚴重制約了沼氣工程在中國北方寒冷地區(qū)的正常運行[102]。通過對沼渣的熱轉化處理,其產(chǎn)生的高溫煙氣可循環(huán)應用于發(fā)酵罐的保溫,以提高沼氣工程的穩(wěn)定性[103];沼渣燃燒灰渣以及熱解生物炭對于沼液的凈化作用也十分顯著,這將有助于沼液的無害化處理[57,104];沼渣熱解以及HTC過程中的水相產(chǎn)物也表現(xiàn)出良好的厭氧發(fā)酵產(chǎn)甲烷潛力[79,105]。這些沼渣燃燒、熱解以及水熱轉化過程中的產(chǎn)物應用與厭氧發(fā)酵技術的結合,減少了生物質能源轉化的二次污染,符合現(xiàn)階段低碳循環(huán)的發(fā)展趨勢,也將會極大地推動沼渣熱化學轉化技術的發(fā)展。
如何實現(xiàn)沼渣快速有效的規(guī)模化應用是目前推廣大型沼氣工程運營的關鍵問題之一。木質纖維素生物質產(chǎn)甲烷技術符合大型沼氣工程的發(fā)展趨勢,其產(chǎn)生的沼渣也具有極高的熱化學轉化潛力,在成型燃料燃燒、熱解以及水熱轉化等領域均表現(xiàn)出一定的優(yōu)勢。而且,沼渣的熱化學轉化一旦與大型沼氣工程形成有機結合,可顯著提高生物質能利用效率,實現(xiàn)厭氧發(fā)酵過程中過量沼渣的無害化處理與資源化利用。這種生物質廢棄物厭氧發(fā)酵與沼渣熱化學轉化相結合的應用模式將有利于促進中國生物質能源的產(chǎn)業(yè)化發(fā)展。
但是沼渣熱轉化應用,還面臨著燃燒結渣與NOx排放、重金屬和PAHs等有害組分積累以及大量干燥能耗等問題。今后建議在以下幾點對沼渣的熱化學轉化應用進行研究:
1)優(yōu)化木質纖維素生物質厭氧發(fā)酵的相關參數(shù)及工藝流程,以有機結合厭氧發(fā)酵過程與熱化學轉化過程。譬如,干發(fā)酵工藝可有效降低沼渣中的含水率,將有利于其燃燒或熱解應用;不同的沼氣發(fā)酵工藝及沼渣收集方式對沼渣生物炭特性也有顯著影響[105]。針對不同發(fā)酵工藝的沼渣,在應用策略上應該有所改變,以提高厭氧發(fā)酵與沼渣熱化學轉化應用的適配性。
2)拓展木質纖維素類沼渣的熱化學轉化應用途徑。建議開展沼渣的水熱處理制備液體燃料或提取精細化工產(chǎn)品等相關研究,一方面能夠解決熱解中的干燥能耗問題,另一方面可以提高熱轉化產(chǎn)物的附加值。但是,目前沼渣水熱液化研究還相對較少。Biller等[106]研究認為由于沼渣水熱的生物油產(chǎn)率非常低(25%左右),厭氧發(fā)酵與水熱液化技術的結合還存在一定問題。因此,需要加強沼渣的水熱液化機理與反應過程調控等方面研究,提高沼渣水熱處理的液相轉化率。
3)對厭氧發(fā)酵技術與沼渣熱轉化技術的結合進行生命周期評估,全面掌握該模式下生物質轉化過程中的環(huán)境影響,包含燃燒污染氣體或HTC水相產(chǎn)物等廢棄物的排放、物料和能源的消耗以及對環(huán)境可能造成的破壞作用。
4)基于產(chǎn)業(yè)化應用,開發(fā)新型規(guī)?;b置,加大專用設備研發(fā)力度。包括專用于木質纖維素沼渣沼液的分離裝置,高效便捷的沼渣干燥、成型裝置以及沼渣基生物質燃燒鍋爐、連續(xù)式高溫高壓反應器等,并重點關注相關技術工業(yè)化產(chǎn)業(yè)鏈上下游的發(fā)展。同時,加深沼渣熱化學轉化產(chǎn)物的應用研究,包括將其轉化為高品質的清潔能源、提取高附加值的化學品以及制備高性能的生物炭基材料等。
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Thermochemical conversion and utilization of digestates from anaerobic digestion of lignocellulosic biomass
Zhang Deli, Wang Fang※, Yi Weiming, Li Zhihe, Li Yongjun, Liu Shanjian
(1.255000; 2255000)
Anaerobic digestion can be widely used to convert the lignocellulosic biomass into biogas, particularly with the by-product digestates. A large amount of digestate discharge has been one of the most limiting factors for the promotion and application of anaerobic digestion, with the development of large-scale biogas engineering in recent years. It is highly urgent to rapidly and effectively treat the digestate. Alternatively, the thermochemical conversion can be selected to realize the harmless treatment and resource utilization of lignocellulosic digestates. The digestates still retain most of the carbon elements and energy in the original material before digestion. The content of lignocellulosic is also very considerable for a large potential of thermochemical conversion. Therefore, this review aims to focus on the digestates forming fuel, pyrolysis, and hydrothermal carbonization. The forming performance of digestates was better than that before digestion, but the NOx emission and slagging phenomenon during the combustion were outstanding to be monitored and controlled. In pyrolysis, the lignocellulosic digestates presented the application potential in the preparation of liquid fuels, phenolic compounds, carbon-based adsorbents, soil amendments, and syngas synthesis. Specifically, the digestate derived bio-oil behaved a much higher calorific value while a lower acid content, where the relative content of 4-vinylphenol reached 60.9%. The pore structure of the biochar was also developed to contain more nutrients, such as P and K. The gaseous product presented a more suitable H2to CO molar ratio with less tar. However, the conversion application in the combustion and pyrolysis was confined to the drying energy consumption caused by the high water content of digestates. In comparison, the drying energy consumption was ignored before hydrothermal carbonization. At the same time, hydrothermal carbonization was used to improve the quality of digestates fuel, including the removal of alkali and alkaline earth metals, the higher calorific value, as well as the improved hydrophobicity, grindability, and fluidizability. Another potential was to prepare the functionalized carbon materials for soil improvement. But, there were still some challenges to the disposal of water phase products after hydrothermal carbonization. In addition, the potential ecological hazards of biochar and hydrochar derived from digestates for farmland application also needed to be paid enough attention, including heavy metals, and polycyclic aromatic hydrocarbons. Overall, the anaerobic digestion and thermochemical conversion presented complementary advantages in energy utilization efficiency, large-scale treatment, and the removal of biotoxicity. Additionally, the thermochemical conversion products of digestates also showed great potential for recycling in anaerobic digestion processes. For instance, the high-temperature flue gas produced by combustion can be recycled to the insulation of the biogas engineering, while the ash residue and biochar can be used to purify the biogas slurry, and the water-phase by-products also have a certain methane production potential. Consequently, a combination of lignocellulosic biomass anaerobic digestion and digestates thermochemical conversion can greatly contribute to the promotion and application of large-scale biogas engineering.
biomass; anaerobic digestion; pyrolysis; digestate; combustion; hydrothermal carbonization
10.11975/j.issn.1002-6819.2021.21.026
TK6
A
1002-6819(2021)-21-0225-12
張德俐,王芳,易維明,等.木質纖維素生物質厭氧發(fā)酵沼渣熱化學轉化利用研究進展[J]. 農(nóng)業(yè)工程學報,2021,37(21):225-236.doi:10.11975/j.issn.1002-6819.2021.21.026 http://www.tcsae.org
Zhang Deli, Wang Fang, Yi Weiming, et al. Thermochemicalconversion and utilization of digestates from anaerobic digestion of lignocellulosic biomass[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2021, 37(21): 225-236. (in Chinese with English abstract) doi:10.11975/j.issn.1002-6819.2021.21.026 http://www.tcsae.org
2021-06-21
2021-10-17
國家重點研發(fā)計劃項目(2019YFD1100602);國家自然科學基金(51536009);山東省自然科學基金(ZR2019BEE049)
張德俐,博士,講師,研究方向為生物質能源與材料。Email:zhangdeli@sdut.edu.cn。
王芳,博士,講師,研究方向為生物質能生物化學與熱化學轉化技術。Email:wangfang1987711@126.com