徐功迅，陳 偉，方 真

生物質(zhì)超臨界水制氫研究進(jìn)展

徐功迅，陳 偉，方 真※

（南京農(nóng)業(yè)大學(xué)工學(xué)院，南京 210031）

生物質(zhì)超臨界水制氫（supercritical water gasification，SCWG）以超臨界水為介質(zhì)通過(guò)熱化學(xué)方式將生物質(zhì)中的有機(jī)物轉(zhuǎn)化為氫氣等能源氣體。相較于傳統(tǒng)制氫方式，SCWG過(guò)程具有反應(yīng)速度快、氫氣選擇性好、副產(chǎn)物少等優(yōu)點(diǎn)，是一種高效、經(jīng)濟(jì)、清潔的生物質(zhì)處理技術(shù)。該研究主要圍繞SCWG過(guò)程中的影響因素進(jìn)行系統(tǒng)地分析，介紹了超臨界水特殊的物理化學(xué)性質(zhì)，詳細(xì)闡述了生物質(zhì)主要成分如纖維素、半纖維素和木質(zhì)素在SCWG過(guò)程中的反應(yīng)機(jī)理，以及試驗(yàn)裝置、原料類(lèi)型和濃度、反應(yīng)溫度、停留時(shí)間、壓力等工藝因素對(duì)SCWG的影響。研究發(fā)現(xiàn)纖維素占比較高的作物氣化效果更好，低濃度的進(jìn)料有利于氣化效率和碳?xì)饣实奶嵘?，提高裝置升溫速率、適當(dāng)增加反應(yīng)溫度和停留時(shí)間能夠增加氫氣產(chǎn)率，過(guò)大的壓力會(huì)形成“溶劑籠”效應(yīng)降低氫氣產(chǎn)量。對(duì)不同類(lèi)型反應(yīng)系統(tǒng)研究表明，間歇式反應(yīng)裝置雖然結(jié)構(gòu)簡(jiǎn)單、操作方便但也存在物料與催化劑混合不均勻、不能實(shí)現(xiàn)連續(xù)化生產(chǎn)而不適用于工業(yè)化推廣，連續(xù)式反應(yīng)裝置雖面領(lǐng)著堵塞等問(wèn)題，但具有性能好、效益高的優(yōu)點(diǎn)，是工業(yè)化推廣的發(fā)展方向。對(duì)SCWG主要應(yīng)用的催化劑進(jìn)行討論發(fā)現(xiàn)，均相催化劑雖然具有催化效果但具有較強(qiáng)腐蝕性，非均相催化劑因其具有高催化活性、易回收、穩(wěn)定性好等優(yōu)點(diǎn)更適用于大規(guī)模SCWG生產(chǎn)過(guò)程。同時(shí)還研究了金屬催化劑酸度在催化過(guò)程中的影響，酸度越高，在SCWG過(guò)程中積碳會(huì)越明顯，通過(guò)添加Cu、Ce、Co、La等合適的第二金屬作為促劑可以改變催化劑性能，增加催化劑使用壽命，提高氫氣選擇性。未來(lái)應(yīng)針對(duì)SCWG的試驗(yàn)裝置、高效催化劑及經(jīng)濟(jì)性分析等核心技術(shù)開(kāi)展研究，加速SCWG的工業(yè)化推廣，實(shí)現(xiàn)經(jīng)濟(jì)、安全、綠色、高效的氫能供給。該研究期望加深對(duì)生物質(zhì)SCWG理解，為日后研究提供理論指導(dǎo)。

生物質(zhì)；氫氣；催化劑；超臨界水

0 引 言

當(dāng)前國(guó)內(nèi)外仍以化石燃料為主要能源[1]，造成了嚴(yán)重的環(huán)境污染問(wèn)題。自2015年《聯(lián)合國(guó)氣候變化框架公約》提出以來(lái)各國(guó)紛紛提出碳減排措施。歐盟出臺(tái)《歐洲氣候法》提出在2050年前實(shí)現(xiàn)碳中和[2]；美國(guó)承諾2050年前達(dá)到碳凈零排放；中國(guó)作為碳排放大國(guó)，宣布力爭(zhēng)在2030年前完成碳達(dá)峰，爭(zhēng)取2060年實(shí)現(xiàn)碳中和[3]。目前世界正處于能源發(fā)展的交替時(shí)代，急需尋找一種科學(xué)技術(shù)上可行、環(huán)境友好的可替代能源形式[4]。

生物質(zhì)能是將生物質(zhì)轉(zhuǎn)化為有用的能源[5]，是自然界中唯一一種可再生的碳源，不僅可以用于燃燒還可以作為原料轉(zhuǎn)化為各種液體或氣體燃料[6]。氫能作為一種清潔高效的可再生二次能源，可以通過(guò)燃燒或者燃料電池的方式利用[7]。氫能在使用中有諸多優(yōu)點(diǎn)：1）氫在燃燒過(guò)程中唯一產(chǎn)物是水，可以做到零碳排放[8]；2）氫的熱值達(dá)到142.3 MJ/kg，是同等質(zhì)量化石能源的3～4倍[9]； 3）氫是很多化學(xué)產(chǎn)品的重要原料，氫能產(chǎn)業(yè)發(fā)展將推動(dòng)工業(yè)的發(fā)展[10]。通過(guò)主要制氫方式對(duì)比發(fā)現(xiàn)，傳統(tǒng)化石能源制氫雖成本較低但產(chǎn)生污染較大，工業(yè)副產(chǎn)制氫和水電解制氫成本相對(duì)較高，而生物質(zhì)制氫具有能耗低、原料廣泛等優(yōu)點(diǎn)，具有很大發(fā)展?jié)摿7,9,11-13]。

1 生物質(zhì)超臨界水制氫機(jī)理研究

1.1 超臨界水及性質(zhì)

當(dāng)溫度超過(guò)374 ℃，壓力超過(guò)22.1 MPa時(shí)，水就會(huì)變成超臨界水（supercritical water，SCW）。當(dāng)水的溫度和壓力超過(guò)其沸點(diǎn)，但在臨界點(diǎn)以下仍然保持液態(tài)時(shí)稱(chēng)為亞臨界水。表1總結(jié)了不同條件下水的性質(zhì)[14-15]：當(dāng)水處于超（亞）臨界狀態(tài)時(shí)物理化學(xué)性質(zhì)會(huì)發(fā)生巨大變化，主要包括以下幾點(diǎn)：

1）隨著氫鍵斷裂、介電常數(shù)降低，SCW更像非極性溶劑[16]，主要表現(xiàn)為可以溶解大多數(shù)有機(jī)溶劑并且可以與氣體互溶，但是無(wú)機(jī)物溶解度大大降低[17]。SCW能夠?yàn)榉磻?yīng)物提供均相反應(yīng)條件[18]，是一種很好的反應(yīng)媒介[19]；

2）SCW超低的黏度有更好的分子遷移率，可以加速溶質(zhì)擴(kuò)散[20]，防止碳沉積和催化劑中毒[21]；

3）SCW會(huì)抑制離子反應(yīng)，增強(qiáng)自由基反應(yīng)，有利于氣體產(chǎn)物生成[22]；

4）在臨界點(diǎn)附近，水中H3O+和OH-離子濃度增加，此時(shí)非常有利于酸性或者堿性催化反應(yīng)的進(jìn)行，SCW本身也可以替代一些酸和堿催化劑促進(jìn)反應(yīng)[18,23]。

表1 不同條件下水的性質(zhì)

分子動(dòng)力學(xué)常用于模型的預(yù)測(cè)，LIEW等[24]用四位柔性模型對(duì)亞臨界和超臨界區(qū)域的純水進(jìn)行了分子動(dòng)力學(xué)模擬，通過(guò)空間分布函數(shù)研究了水的三維溶解結(jié)構(gòu)和氫鍵，發(fā)現(xiàn)除了線性氫鍵的水外，超臨界水還包括分叉氫鍵的水；BOERO等[25]利用Car-Parrinello分子動(dòng)力學(xué)構(gòu)建出超臨界點(diǎn)附近水的氫鍵網(wǎng)絡(luò)結(jié)構(gòu)和介電特性，發(fā)現(xiàn)在低密度下水主要分裂成三聚體、二聚體和單分子。SASAKI等[26]利用連續(xù)流動(dòng)裝置發(fā)現(xiàn)纖維素在350 ℃和25 MPa的條件下僅4 s就可以完全溶解和轉(zhuǎn)化為葡萄糖和果糖等小分子物質(zhì)，并將快速溶解的原因歸結(jié)于超臨界流體形成的均相反應(yīng)條件，因此可以看出超臨界狀態(tài)非常有利于生物質(zhì)的溶解和進(jìn)一步反應(yīng)。

1.2 生物質(zhì)超(亞)臨界氣化反應(yīng)機(jī)理

生物質(zhì)主要由纖維素、半纖維素和木質(zhì)素構(gòu)成，纖維素是一種由葡萄糖單體通過(guò)（1,4）糖苷鍵連接而成的多糖，這使得纖維素分子內(nèi)和分子間形成強(qiáng)氫鍵，使其結(jié)晶、耐水溶脹[27-28]。半纖維素由木糖、半乳糖和葡萄糖等多種糖單體組成（圖1），不同作物糖單體的比例也不同。

圖1 水的狀態(tài)[29]

半纖維素結(jié)構(gòu)較不穩(wěn)定，更容易水解[30-31]。木質(zhì)素是由對(duì)香豆醇、松柏醇和芥子醇3種單體組成的一種復(fù)雜高分子量化合物，化學(xué)性質(zhì)相對(duì)穩(wěn)定。超臨界水氣化的過(guò)程復(fù)雜，涉及水解、熱解、焦油重整、水煤氣轉(zhuǎn)化、甲烷化反應(yīng)等[32-33]，主要反應(yīng)方程式[34-36]如下：

CHO+(2-)H2O → CO2+ (2-+/2)H2（1）

CHO+(1-)H2O → CO + (1-+/2)H2（2）

纖維素的水解：

(C6H10O5) +H2O → nC6H12O6（3）

葡萄糖重整反應(yīng)：

C6H12O6→ 6CO + 6H2（4）

木質(zhì)素水解：

(C10H10O3)+H2O →C10H12O4→ 酚醛樹(shù)脂 （5）

蒸汽重整反應(yīng)：

酚醛樹(shù)脂+H2O → CO + CO2+ H2（6）

水煤氣轉(zhuǎn)化反應(yīng)：

CO + H2O → CO2+ H2（7）

CO的甲烷化反應(yīng)：

CO + 3H2→ CH4+ H2O （8）

CO2的甲烷化反應(yīng)：

CO2+ 4H2→ CH4+ 2H2O （9）

加氫反應(yīng)：

CO + 4H2→ CH4+ 0.5O2（10）

焦油重整反應(yīng)：

CHO(tar) + CO2→ 2CO +/2H2（11）

式中和分別表示生物質(zhì)中H/C和O/C的元素摩爾比，產(chǎn)物中氣體組分的比例很大程度上取決于和的大小。生物質(zhì)在超臨界水狀態(tài)下主要轉(zhuǎn)化為氫氣、甲烷、二氧化碳、一氧化碳等氣體,在這個(gè)過(guò)程中超臨界水既作為反應(yīng)介質(zhì)又作為反應(yīng)物參與反應(yīng)過(guò)程。

反應(yīng)中主要過(guò)程及主要產(chǎn)物如圖2所示。在最初的水解過(guò)程中，生物質(zhì)主要分解為糖類(lèi)、酚類(lèi)和醇類(lèi)物質(zhì)，其中纖維素和半纖維素水解的主要成分是五碳糖和六碳糖，木質(zhì)素主要水解為酚類(lèi)，包括愈創(chuàng)木酚、酚醛樹(shù)脂等。在超臨界水的狀態(tài)下這些中間產(chǎn)物進(jìn)一步轉(zhuǎn)化為更簡(jiǎn)單的化合物，例如酸（羧酸、琥珀酸等）、醇（豆香醇、芥子醇等）、酚類(lèi)化合物、醛和芳香烴化合物。其中，酸類(lèi)化合物主要來(lái)自于糖類(lèi)物質(zhì)的分解，醇、芳烴、醛等物質(zhì)主要來(lái)自于木質(zhì)素（苯酚）的變性產(chǎn)物[37-38]。

圖2 生物質(zhì)超臨界水制氫反應(yīng)路線圖

纖維素有更大的氫含量因此在SCWG過(guò)程中比半纖維素對(duì)產(chǎn)氫的貢獻(xiàn)更大[39]，葡萄糖是纖維素水解過(guò)程中的主要產(chǎn)物。KABYEMELA等[40]研究發(fā)現(xiàn)在水熱條件下葡萄糖與木糖會(huì)發(fā)生互相異構(gòu)化反應(yīng)，研究證實(shí)果糖在SCWG中具有更高活性。CASTELLO等[41]以不同濃度的葡萄糖和苯酚為模型化合物在400 ℃和25 MPa的連續(xù)式反應(yīng)器中試驗(yàn)提出了反應(yīng)機(jī)理（圖3），并指出在400 ℃離子反應(yīng)條件下葡萄糖主要分解成赤蘚糖和乙醇醛。赤蘚糖具有呋喃環(huán)，進(jìn)一步反應(yīng)易形成糠醛，乙醇醛則進(jìn)一步分解為乙醇；葡萄糖與果糖之間會(huì)相互轉(zhuǎn)化，果糖脫水形成5-羥甲基糠醛（5-HMF），5-HMF部分通過(guò)縮合形成重質(zhì)化合物，部分進(jìn)一步降解為酸類(lèi)從而形成氣體產(chǎn)物。

圖3 葡萄糖降解機(jī)理[41]

CASTELLO等[41]提出苯酚也是葡萄糖的降解產(chǎn)物之一，因?yàn)槠咸烟墙到猱a(chǎn)生的糠醛會(huì)產(chǎn)生苯酚[42]，大量研究表明在水熱條件下葡萄糖會(huì)分解成5-HMF、苯酚等酚類(lèi)化合物以及酸類(lèi)化合物等，這些物質(zhì)進(jìn)一步分解為呋喃、芳烴、有機(jī)酸、醇、酮等物質(zhì)，其中有機(jī)酸、醇、酮進(jìn)一步分解為氫氣等氣體，呋喃、芳烴等化合物更多的轉(zhuǎn)化為焦炭[43-44]，而烴類(lèi)的重整反應(yīng)和水煤氣轉(zhuǎn)化過(guò)程中易在催化劑表面發(fā)生結(jié)焦造成催化劑失活[45]。酚類(lèi)物質(zhì)被認(rèn)為是生物質(zhì)氣化過(guò)程中的巨大障礙，因?yàn)樵诔R界狀態(tài)下酚類(lèi)物質(zhì)可以通過(guò)加氫脫氧等反應(yīng)生成難以氣化的芳烴，產(chǎn)生的中間產(chǎn)物如呋喃、酚類(lèi)和芳烴等物質(zhì)易促進(jìn)聚合反應(yīng)轉(zhuǎn)化生成焦炭導(dǎo)致氣化效率降低[46]。

半纖維素是由C5和C6糖組成的一種無(wú)定型的生物聚合物，GOODWIN等[47]以木糖為模型化合物探究半纖維素在SCWG過(guò)程中降解機(jī)理（圖4），認(rèn)為木糖反應(yīng)主要有2條途徑：第1種途徑是通過(guò)脫水生成糠醛，糠醛分解成水溶性腐殖質(zhì)（water soluble humus, WSHS）和脂類(lèi)化合物，再進(jìn)一步分解成氣體產(chǎn)物；第2種途徑是木糖分解成甲酸甲酯和甘油醛，進(jìn)一步分解為酸類(lèi)化合物進(jìn)而降解為氣體產(chǎn)物，在此過(guò)程中生成的氣體產(chǎn)物還會(huì)通過(guò)水煤氣變換和一氧化碳的甲烷化反應(yīng)進(jìn)行轉(zhuǎn)化。

木質(zhì)素的結(jié)構(gòu)較為復(fù)雜，是生物質(zhì)中最難氣化的組分。木質(zhì)素的存在會(huì)抑制氣體形成的速率[32]，在氣化過(guò)程中主要產(chǎn)生4個(gè)相：油相（如酚類(lèi)、PAH等）、水相（如醇類(lèi)、醛類(lèi)）、氣相（如氫氣、一氧化碳、二氧化碳、甲烷等）、固相（焦炭等物質(zhì)）[32,48]。木質(zhì)素氣體產(chǎn)物主要是二氧化碳和甲烷[49]，較高的氫氣產(chǎn)率需要高溫才能實(shí)現(xiàn)。在水熱降解過(guò)程中木質(zhì)素的醚鍵斷裂形成甲氧基酚等酚類(lèi)物質(zhì)，甲氧基苯酚通過(guò)脫甲氧基反應(yīng)產(chǎn)生烷基苯酚和甲醇。其中甲醇通過(guò)2種方式轉(zhuǎn)化為氣體產(chǎn)品，一種是直接分解成H2和CO，另一種是通過(guò)與水反應(yīng)生成CO2和H2，進(jìn)一步發(fā)生水煤氣轉(zhuǎn)化反應(yīng)和加氫反應(yīng)生成CH4和氫氣等氣體[50]（圖5）。烷基苯酚則通過(guò)自由基反應(yīng)轉(zhuǎn)化為酚和烷烴自由基，烷烴通過(guò)自由基反應(yīng)與氫自由基結(jié)合生成甲烷等氣體，酚類(lèi)物質(zhì)則通過(guò)氫化等反應(yīng)分解，或者通過(guò)縮聚反應(yīng)生成生物炭。木質(zhì)素在水解過(guò)程中還會(huì)形成有苯環(huán)的高分子化合物，在超臨界狀態(tài)下，苯酚通過(guò)脫羥基反應(yīng)生成苯環(huán)，再通過(guò)聚合反應(yīng)生成聯(lián)苯和苯基苯酚等物質(zhì)，這兩種物質(zhì)是生成焦炭的主要原料之一[51]。

圖4 木糖降解機(jī)理[47]

圖5 甲醇反應(yīng)路徑圖[50]

2 生物質(zhì)超臨界水制氫影響因素

2.1 試驗(yàn)裝置

試驗(yàn)裝置的升溫速率會(huì)影響SCWG的產(chǎn)物，在低溫區(qū)停留過(guò)久會(huì)導(dǎo)致焦炭等副產(chǎn)物生成[52]，高加熱速率會(huì)促進(jìn)中間產(chǎn)物（甲酸和乙酸等）形成，抑制酚類(lèi)化合物形成[53]，防止焦油焦炭產(chǎn)生[54-55]。目前應(yīng)用于SCWG的試驗(yàn)裝置主要有間歇式和連續(xù)式2種[34]，圖6展示了典型的間歇式與連續(xù)式反應(yīng)裝置。間歇式反應(yīng)器具有結(jié)構(gòu)簡(jiǎn)單、操作方便、原料適應(yīng)性強(qiáng)、周期性使用過(guò)程中易于清理反應(yīng)裝置內(nèi)部產(chǎn)生的焦炭等優(yōu)點(diǎn)，但溫度、壓力等因素不易精準(zhǔn)控制，易產(chǎn)生較大誤差，同時(shí)也存在物料與催化劑混合不均勻等缺點(diǎn)[46,56]。連續(xù)式反應(yīng)裝置可以較為精準(zhǔn)的控制試驗(yàn)參數(shù)，并實(shí)現(xiàn)連續(xù)化商業(yè)生產(chǎn)，在氣化中生成的焦炭易堵塞反應(yīng)器是目前面臨的主要問(wèn)題[57]。FANG等[18,37,58]利用可視鉆石反應(yīng)器（圖7）發(fā)現(xiàn)木質(zhì)纖維素易溶于高壓熱水中，并設(shè)計(jì)出連續(xù)裝置使生物質(zhì)在均相條件下快速溶解、水解和分解為小分子化合物，為大規(guī)模連續(xù)氣化快速產(chǎn)氫奠定了試驗(yàn)基礎(chǔ)。流化床反應(yīng)器不僅具有克服反應(yīng)器堵塞的能力還能夠增強(qiáng)傳熱傳質(zhì)，在過(guò)去幾年中被認(rèn)為是解決SCWG焦炭堵塞的有效方案[59-60]，YAKABOYLU等[60]以干淀粉為原料使用流化床反應(yīng)裝置在600 ℃條件下反應(yīng)，發(fā)現(xiàn)產(chǎn)生的焦油等副產(chǎn)物量較少并且運(yùn)行2 h均沒(méi)有發(fā)生堵塞問(wèn)題。CHEN等[61]對(duì)SCWG-Solar系統(tǒng)進(jìn)行評(píng)估提出將太陽(yáng)能利用于SCWG預(yù)熱過(guò)程可以減少能耗，是未來(lái)大規(guī)模高效應(yīng)用SCWG的可行方案之一。

a. 典型間歇式（高壓反應(yīng)釜）反應(yīng)器

a. Typical batch (high-pressure reactor) reactor

b. 典型連續(xù)反應(yīng)器

圖7 微型可視鉆石反應(yīng)器[18]

2.2 原料類(lèi)型及濃度

農(nóng)作物秸稈中纖維素約占質(zhì)量的35%～55%，半纖維素占20%～40%，木質(zhì)素占10%～25%[62-63]。不同農(nóng)作物類(lèi)型的纖維素、半纖維素和木質(zhì)素含量不同，在SCWG中產(chǎn)物也有很大不同，通常來(lái)說(shuō)纖維素的生物可降解性大于木質(zhì)素，因此纖維素占比高的作物氣體轉(zhuǎn)化率大于木質(zhì)素占比高的作物[64]。生物質(zhì)中還有蛋白質(zhì)、礦物質(zhì)、灰分等其他組分，其中蛋白質(zhì)的存在會(huì)降低氣體產(chǎn)率，因?yàn)榈鞍踪|(zhì)降解形成氨基酸與葡萄糖水解反應(yīng)會(huì)相互干擾。蛋白質(zhì)分解過(guò)程中的丙氨酸會(huì)產(chǎn)生自由基清除劑，抑制自由基反應(yīng)，而SCWG過(guò)程中大部分氣體通過(guò)自由基反應(yīng)生成[65]，此外生物質(zhì)中部分礦物質(zhì)可以充當(dāng)催化劑的效果[66]。

YANIK等[67]以玉米秸稈、棉花秸稈等8種不同生物質(zhì)作為反應(yīng)原料，使用間歇式高壓反應(yīng)釜在500 ℃進(jìn)行氣化，氫氣產(chǎn)量在4.05～4.65 mol/kg生物質(zhì)，發(fā)現(xiàn)不同作物的氨基酸、蛋白質(zhì)、油脂含量等都會(huì)影響氫氣產(chǎn)量。WILLIAMS等[68]以纖維素、淀粉、葡萄糖和木薯廢棄物為研究對(duì)象，在間歇式高壓反應(yīng)裝置中試驗(yàn)發(fā)現(xiàn)葡萄糖的氫氣產(chǎn)率最高，木薯廢棄物的氫氣產(chǎn)率最低。淀粉和纖維素雖然都是葡萄糖的聚合物，但在氣化過(guò)程中纖維素產(chǎn)出的烴類(lèi)化合物產(chǎn)出較多，淀粉產(chǎn)出的氫氣、一氧化碳和油類(lèi)物質(zhì)較多，試驗(yàn)表明相同單體的聚合物連接方式不同在SCWG中效果也不同。郭烈錦院士團(tuán)隊(duì)以不同種陸生植物、水生植物以及生物質(zhì)廢料等為原料氣化，發(fā)現(xiàn)陸生禾本植物具有較高的活性，其次為水生植物、有機(jī)廢料等[69]。YOSHIDA等[70]使用木質(zhì)素、纖維素及其混合物在400 ℃、25 MPa條件下利用鎳催化劑進(jìn)行氣化，發(fā)現(xiàn)當(dāng)原料中有軟木質(zhì)素時(shí)氣化效率較低，纖維素與軟木質(zhì)素在超臨界狀態(tài)下相互作用會(huì)生成焦油使催化劑失活，相較于木質(zhì)素、纖維素等模型化合物，真實(shí)生物質(zhì)在氣化過(guò)程中各組分之間也會(huì)相互作用影響氣化效果。

進(jìn)料濃度也會(huì)影響氣化效果，物料濃度高會(huì)導(dǎo)致物料與水的接觸面積減小，阻礙蒸汽重整過(guò)程，過(guò)高的進(jìn)料濃度會(huì)使生物質(zhì)形成較多焦炭、焦油等中間副產(chǎn)物，容易導(dǎo)致反應(yīng)器堵塞等問(wèn)題[57]。GUO等[71]使用連續(xù)流動(dòng)管式反應(yīng)器以甘油為原料進(jìn)行研究，在567 ℃下甘油濃度由10%增加至25%過(guò)程中，氫氣產(chǎn)率和氣化效率均降低；WANG等[72]使用間歇式反應(yīng)器在600 ℃條件下以堿性木質(zhì)素為原料進(jìn)行試驗(yàn)，發(fā)現(xiàn)當(dāng)進(jìn)料濃度由1%增加到6%時(shí)氣化效率由79.86%下降到42.20%，表明增加木質(zhì)素原料濃度抑制了氣化反應(yīng)。提高物料濃度反而會(huì)降低氣化效率，低濃度的進(jìn)料反而有益于氣化率和碳?xì)饣实奶岣遊49]，然而高濃度的進(jìn)料有利于對(duì)污水、廢棄生物質(zhì)的批量處理，能有效提高超臨界水處理廢棄物的效率，實(shí)現(xiàn)高濃度進(jìn)料水平下高氫氣產(chǎn)率是未來(lái)的研究方向。

2.3 反應(yīng)溫度

提高溫度可以增加氣化效率和氫氣產(chǎn)率，但在實(shí)際應(yīng)用過(guò)程中也就意味著能耗增加[73]，對(duì)試驗(yàn)器材也提出了更高要求，不符合綠色生產(chǎn)理念。因此在較低溫度下實(shí)現(xiàn)較高氣化效率是未來(lái)研究方向。

溫度在SCWG過(guò)程中起著至關(guān)重要的作用。從熱力學(xué)角度分析，生物質(zhì)分解需要大量熱量，因此高溫對(duì)SCWG非常有利[74]。隨著溫度升高生物質(zhì)會(huì)加速分解，較高的溫度能提高碳?xì)饣什⑶以黾託錃猱a(chǎn)量[57]。升高溫度會(huì)增加自由基的濃度，促進(jìn)離子積反應(yīng)向自由基轉(zhuǎn)化，有利于氣體產(chǎn)物生成[43]。OSADA等[75]將SCWG分為3個(gè)溫度區(qū)域：1）500 ～700 ℃的超臨界水區(qū)域，生物質(zhì)分解迅速，可以使用活性炭催化劑避免結(jié)焦或堿性催化劑促進(jìn)水煤氣轉(zhuǎn)化；2）374 ～500 ℃的超臨界水區(qū)域，可以利用金屬催化劑促進(jìn)生物質(zhì)水解；3）當(dāng)溫度低于374 ℃時(shí)，為亞臨界水狀態(tài)，此時(shí)生物質(zhì)水解非常緩慢，需要催化劑才能促進(jìn)氣體形成。PROMDEJ等[76]利用連續(xù)式流化床管式反應(yīng)裝置在300 ～460 ℃、25 MPa條件下，以1.5%葡萄糖為原料進(jìn)行試驗(yàn)，發(fā)現(xiàn)在亞臨界水狀態(tài)中產(chǎn)生的焦炭量明顯比超臨界水狀態(tài)下要多，在亞臨界條件下葡萄糖的分解以離子積反應(yīng)為主，在超臨界水狀態(tài)下以自由基反應(yīng)為主。KARAGOZ等[77]使用間歇式高壓反應(yīng)釜分別在180、250 和280 ℃條件下氣化鋸末，發(fā)現(xiàn)會(huì)產(chǎn)生2-呋喃甲醛、2-甲氧基苯酚等中間產(chǎn)物，這些產(chǎn)物不能氣化會(huì)導(dǎo)致反應(yīng)器堵塞，而較快的加熱速度能緩解這種現(xiàn)象的出現(xiàn)。大量研究表明，在較低溫度下氣化主要產(chǎn)物為甲烷，隨著溫度升高氫氣成為主要產(chǎn)物。從反應(yīng)機(jī)理上來(lái)解釋?zhuān)S著溫度升高，重整反應(yīng)的吸熱特性有利于氫氣和二氧化碳的生成，水煤氣轉(zhuǎn)化反應(yīng)的微弱放熱性質(zhì)會(huì)傾向于產(chǎn)生氫氣和二氧化碳，從而抑制一氧化碳的產(chǎn)生[38,78-80]。

2.4 停留時(shí)間

反應(yīng)物在反應(yīng)器內(nèi)停留的時(shí)間或者持續(xù)的時(shí)間稱(chēng)為停留時(shí)間，在一定時(shí)間范圍內(nèi)氫氣產(chǎn)率會(huì)隨著停留時(shí)間的增加而增加[81]。YOUSSEF等[82]以豬糞為原料氣化，發(fā)現(xiàn)反應(yīng)時(shí)間為60 min時(shí)甲烷和二氧化碳的產(chǎn)量最高，反應(yīng)時(shí)間由30 min變化到60 min時(shí)氫氣產(chǎn)量幾乎不變，當(dāng)反應(yīng)時(shí)間達(dá)到90 min時(shí)，氫氣產(chǎn)量反而降低。OSADA等[83]以甘蔗渣廢料為原料氣化，發(fā)現(xiàn)反應(yīng)時(shí)間為15 min時(shí)氣體產(chǎn)物的碳產(chǎn)量為10%，且不隨著時(shí)間的增加而增加，反應(yīng)30 min時(shí)甘蔗渣完全氣化氣且氫氣產(chǎn)量幾乎恒定。RESENDE等[80]建立了纖維素和木質(zhì)素SCWG的動(dòng)力學(xué)模型，發(fā)現(xiàn)短時(shí)間內(nèi)氫氣主要通過(guò)重整反應(yīng)生成，較長(zhǎng)時(shí)間主要通過(guò)水煤氣轉(zhuǎn)化反應(yīng)產(chǎn)生，而一氧化碳、二氧化碳和甲烷等氣體會(huì)由中間產(chǎn)物水熱反應(yīng)產(chǎn)生。CHEN等[84]利用石英管反應(yīng)器將食品廢棄物用于SCWG，通過(guò)建立模型發(fā)現(xiàn)碳?xì)饣孰S反應(yīng)溫度的升高而升高，反應(yīng)活性在氣化初期急劇升高，隨后隨反應(yīng)時(shí)間的延長(zhǎng)而降低。

2.5 壓 力

在超臨界水狀態(tài)下，氫氣產(chǎn)量和氣化效率與壓力之間的關(guān)系非常復(fù)雜。為達(dá)到超臨界狀態(tài)，反應(yīng)壓力需要大于22 MPa，在反應(yīng)過(guò)程中適合的反應(yīng)壓力范圍為22～30 MPa[85]。

溶質(zhì)在溶劑中的擴(kuò)散速率取決于溶劑黏度，溶劑黏度是壓力和溫度的函數(shù)，因此壓力可以影響溶質(zhì)的溶解程度進(jìn)而影響反應(yīng)速率。在SCW狀態(tài)下溶劑的“籠效應(yīng)”得到增強(qiáng)，此時(shí)溶劑籠可以通過(guò)隔離反應(yīng)物來(lái)降低溶質(zhì)與溶質(zhì)之間的反應(yīng)速率，加強(qiáng)溶質(zhì)與溶劑之間的反應(yīng)速率。因此高壓有利于水煤氣轉(zhuǎn)化，但是阻礙了原料的分解速率。GOKKAYA等[86]在500 ℃條件下對(duì)苯酚進(jìn)行氣化，發(fā)現(xiàn)當(dāng)壓力增大時(shí)碳?xì)饣史炊陆怠U等[87]使用流化床間歇式高壓反應(yīng)器對(duì)生物質(zhì)模型化合物（纖維素和葡萄糖）和真實(shí)生物質(zhì)（稻稈、麥稈、花生殼等）進(jìn)行氣化，發(fā)現(xiàn)高壓有利于水煤氣轉(zhuǎn)化進(jìn)而促進(jìn)氫氣產(chǎn)量。MADENOGLU等[88]在間歇式高壓反應(yīng)釜中以葡萄糖為原料氣化，發(fā)現(xiàn)相同溫度下增大壓力會(huì)促進(jìn)葡萄糖分解稱(chēng)為醛和酮類(lèi)化合物，隨著壓力增加碳?xì)饣屎蜌錃猱a(chǎn)量均減小。

2.6 催化劑

傳統(tǒng)的SCWG過(guò)程需要高溫高壓條件，不僅功耗大而且對(duì)于反應(yīng)裝置要求也非常高。向反應(yīng)體系中加入催化劑可以降低反應(yīng)的活化能，使反應(yīng)在較低溫度下進(jìn)行，能有效減少運(yùn)營(yíng)成本、提高氫氣選擇性[89]、減少SCWG過(guò)程焦油和焦炭產(chǎn)生[90]。催化劑主要是通過(guò)催化生物質(zhì)水解、脫水等形成中間體，快速實(shí)現(xiàn)C-C鍵斷裂，尤其對(duì)于酚類(lèi)化合物而言，氣化過(guò)程必須足夠快速才能避免產(chǎn)生的中間體聚合形成焦炭等物質(zhì)。同時(shí)催化劑會(huì)解離H2O，在催化劑表面產(chǎn)生活性自由基，這些自由基將與分解的小分子CHO結(jié)合，最終釋放出CO和CO2，從水分解和裂解的CHO中吸附的氫原子將結(jié)合形成H2。催化劑主要分為均相催化劑和非均相催化劑2種。

2.6.1 均相催化劑

均相催化劑便于操作，能夠與原料均勻混合。堿類(lèi)催化劑具有低成本和高活性的特性，是最廣泛應(yīng)用的均相催化劑。堿性催化劑可以吸收CO2促使水煤氣轉(zhuǎn)化向生成氫氣方向移動(dòng)[91-92]。CHEN等[93]使用流化床反應(yīng)裝置對(duì)污水污泥SCWG進(jìn)行了研究，得到催化劑的活性順序?yàn)橛纱蟮叫∫来螢椋篕OH、K2CO3、NaOH、Na2CO3、KOH，通過(guò)對(duì)比發(fā)現(xiàn)堿性催化劑不會(huì)顯著改變產(chǎn)物中碳的分布情況，而氫氣產(chǎn)率增加主要是堿性催化劑促進(jìn)超臨界水狀態(tài)下的水煤氣轉(zhuǎn)化反應(yīng)。ONWUDILI等[94]用間歇式高壓反應(yīng)釜在550 ℃、36 MPa下以葡萄糖為原料進(jìn)行研究，發(fā)現(xiàn)高濃度的NaOH對(duì)氫氣選擇性更高。FENG等[95]通過(guò)分析模擬軌跡并跟蹤目標(biāo)原子，指出C-H-O自由基是產(chǎn)生氫氣或氫離子的主要反應(yīng)物，解釋了堿金屬鹽催化劑（NaOH和KOH）可以提高氫氣產(chǎn)量的原因。雖然堿性催化劑降低了纖維素降解的起始溫度，有利于氣體產(chǎn)物生成[96]，但在實(shí)際操作過(guò)程中需要持續(xù)添加新鮮催化劑，并且反應(yīng)過(guò)后產(chǎn)生的含堿催化劑廢液難以處理[17,97]，因此使用范圍受到限制。

2.6.2 非均相催化劑

非均相催化劑具有易于回收、化學(xué)性質(zhì)穩(wěn)定等優(yōu)點(diǎn)而被廣泛研究，主要包括有金屬催化劑、金屬氧化物催化劑和碳基催化劑等。

ELLIOTT等[98]發(fā)現(xiàn)在SCWG過(guò)程活性最好的金屬是Ru、Rh、Ni，它們可以改善甲烷化反應(yīng)。Ni基催化劑因催化活性與貴金屬相當(dāng)，但是價(jià)格較為低廉因而得到大量研究[99]。Ni能有效促進(jìn)C-O鍵斷裂，同時(shí)促進(jìn)生物質(zhì)分解的中間體（苯酚、酸類(lèi)、醛類(lèi)）降解，進(jìn)而增加氣化效率[100-101]。ADAMU等[102]發(fā)現(xiàn)Ni基催化劑具有金屬/載體協(xié)同效應(yīng)，特別是當(dāng)浸漬在金屬氧化物載體（如Al2O3）上。Ni具有很強(qiáng)的催化C-C鍵斷裂能力，從而提高碳轉(zhuǎn)化率[101,103]，還能有效催化水溶性產(chǎn)物的蒸汽重整反應(yīng)和甲烷化反應(yīng)[104]。SINAG等[100]提出在葡萄糖SCWG過(guò)程中沉積的碳主要通過(guò)2個(gè)路徑實(shí)現(xiàn)，一是通過(guò)中間液體產(chǎn)物分解，二是副產(chǎn)氣；在超臨界狀態(tài)下葡萄糖分解為糠醛和有機(jī)酸，糠醛容易分解為焦炭沉積，Ni基催化劑能夠促進(jìn)有機(jī)酸分解為氣體產(chǎn)物。ZHU等[105]以不同濃度的硝酸鎳和硝酸鋯為原料，采用超臨界水合成法成功制備了一系列ZrO2負(fù)載的Ni納米催化劑（Ni、0.8Ni-0.2ZrO2、0.6Ni-0.4ZrO2、0.4Ni-0.6ZrO2和ZrO2），以甘油為原料試驗(yàn)表明0.4Ni-0.6ZrO2獲得了最高的氫氣產(chǎn)率，因?yàn)樗谒簹廪D(zhuǎn)化反應(yīng)中具有將CO轉(zhuǎn)化為H2的氣體的高活性，表征發(fā)現(xiàn)超臨界水合成法制備的Ni/ZrO2催化劑表現(xiàn)出優(yōu)異的晶體結(jié)構(gòu)和形態(tài)穩(wěn)定性，以及對(duì)甘油的SCWG具有良好的抗結(jié)焦能力。

盡管Ni基催化劑具有很多優(yōu)點(diǎn)，但是不可避免的面臨著易燒結(jié)、水熱性質(zhì)不穩(wěn)定、形成的焦油焦炭等會(huì)影響Ni的選擇性和穩(wěn)定性等問(wèn)題[17,106-107]。改變載體材料和選擇合適的促進(jìn)劑可以有效提高Ni基催化劑的穩(wěn)定性，良好的載體材料能夠?yàn)榇呋瘎┍ＷC機(jī)械強(qiáng)度、提供活化中心并增加活性金屬的分散性[108]（圖8）。研究表明-Al2O3作為催化劑載體會(huì)在高溫水熱條件下發(fā)生相變轉(zhuǎn)化為-Al2O3，這種相變會(huì)導(dǎo)致表面積和孔體積損失以及平均孔徑增加[109]，使活性元素溶解在水中從而造成催化劑失活[110]。LI等[111]利用溶膠-凝膠法制備了ZrO2、TiO2和Ta2O5負(fù)載的Ni基催化劑（分別表示為Ni-Zr、Ni-Ti和Ni-Ta），以甘油為原料在連續(xù)流動(dòng)反應(yīng)裝置中進(jìn)行SCWG試驗(yàn)發(fā)現(xiàn)Ni-Zr和Ni-Ta利用SCW的結(jié)晶環(huán)境可以實(shí)現(xiàn)“催化劑原位活化”效果，是用于長(zhǎng)期催化SCWG的良好候選者，LI認(rèn)為Ni與載體的燒結(jié)和催化劑材料的浸出是SCW環(huán)境中可能會(huì)導(dǎo)致催化劑失活的原因。XIE等[112]采用浸漬法制備了一系列不同沸石的鎳催化劑，使用高壓間歇式反應(yīng)器以微藻為原料進(jìn)行SCWG試驗(yàn)發(fā)現(xiàn)沸石的強(qiáng)酸性位點(diǎn)可以增加碳?xì)饣剩C明沸石也可以作為SCWG的催化劑載體。

圖8 鎳基催化劑SCWG催化纖維素生物質(zhì)反應(yīng)路徑圖[109]

酸度是決定催化劑活性、穩(wěn)定性和抗積碳能力的重要參數(shù)，因?yàn)樘穷?lèi)化合物會(huì)在催化劑酸性位點(diǎn)上發(fā)生脫水反應(yīng)，使用具有氫化活性金屬和強(qiáng)酸性位點(diǎn)的雙功能催化劑會(huì)導(dǎo)致連續(xù)脫水/氫化，因此會(huì)產(chǎn)生高級(jí)烷烴。酸性位點(diǎn)還會(huì)促進(jìn)聚合反應(yīng)導(dǎo)致焦油狀物質(zhì)的形成[109]。催化劑的酸度越高在SCWG中積碳會(huì)越明顯[113]，Ni基催化劑的強(qiáng)酸性加速了焦炭形成[114]。雖然焦炭對(duì)催化劑活性位點(diǎn)覆蓋會(huì)導(dǎo)致的催化劑失活，但可以通過(guò)高溫煅燒和還原的方式恢復(fù)催化劑活性。而大量試驗(yàn)表明在催化劑使用過(guò)程中失活是不可逆的，因此推斷在SCWG催化劑催化過(guò)程中燒結(jié)是導(dǎo)致催化劑失活最主要的原因，尤其集中在高負(fù)載的金屬催化劑上。催化劑燒結(jié)將導(dǎo)致微晶長(zhǎng)大、催化劑表面孔隙和孔徑分布發(fā)生變化、比表面積和活性位點(diǎn)減少、氫氣選擇性降低[45,115]。選擇合適的促劑將會(huì)提高Ni基催化劑氫氣選擇性和抗燒結(jié)能力。表2總結(jié)了不同金屬元素作為Ni基催化劑促進(jìn)劑對(duì)氣化效率的影響。

表2 不同金屬促進(jìn)劑對(duì)Ni基催化劑影響

活性炭（activated carbon，AC）催化劑可以使用作物秸稈、貝殼等天然生物質(zhì)在惰性氣體條件下制備。AC可以作為催化劑載體或者單獨(dú)作為催化劑使用，在SCWG過(guò)程中AC可以促進(jìn)碳?xì)饣?。MATSUMURA等[121]使用填充床式反應(yīng)器以葡萄糖為原料，加入椰殼AC在600 ℃條件下幾乎可以實(shí)現(xiàn)100%的碳?xì)饣?。LEE等[122]以葡萄糖為原料在575 ～725 ℃、28 MPa條件下通過(guò)對(duì)使用前后的催化劑進(jìn)行表征發(fā)現(xiàn)AC的結(jié)構(gòu)幾乎沒(méi)有改變，證明AC是一種不受超臨界水狀態(tài)影響的穩(wěn)定材料。QUAN等[123]發(fā)現(xiàn)將Ce引入催化劑顯著提高了炭基催化劑的穩(wěn)定性，表征發(fā)現(xiàn)在使用過(guò)的催化劑上形成了無(wú)定形碳和絲狀碳兩種碳，無(wú)定形碳比絲狀碳更容易氧化，Ce的加入可以減少反應(yīng)過(guò)程中非晶碳的生成，增強(qiáng)碳基催化劑穩(wěn)定性。催化劑的負(fù)載量和載體材料不同都會(huì)影響氣體的組分比率和產(chǎn)率[124]。YAO等[125]以麥秸、稻殼和棉稈為原料在500 ℃管式爐中快速熱解得到3種生物炭用作催化劑載體，采用工業(yè)品AC進(jìn)行對(duì)照試驗(yàn)，當(dāng)氣化溫度為800 ℃，Ni負(fù)載量為15%時(shí)，Ni/AC催化劑的性能最佳，棉花生物炭為載體的Ni氣化產(chǎn)氫活性最高，為64.02%，而水稻生物炭為載體的Ni氣化產(chǎn)氫活性最低。

碳納米管（carbon nanotubes，CNT）具有比表面積大、熱穩(wěn)定性好、機(jī)械強(qiáng)度高等優(yōu)點(diǎn)而被廣泛應(yīng)用于催化劑載體[126]。RASHIDI等[127]利用間歇式高壓反應(yīng)裝置以甘蔗渣為原料，采用浸漬法制備具有不同鎳負(fù)載量的Ni/CNT納米催化劑，發(fā)現(xiàn)由于CNT巨大的內(nèi)表面，在SCWG過(guò)程中使用Ni/CNTs納米催化劑可顯著提高氫氣產(chǎn)量。LI等[126]采用沉淀法制備了Ni負(fù)載在Mg促進(jìn)的-Al2O3、α-Al2O3和CNT催化劑，使用連續(xù)式流動(dòng)裝置以甘油為原料進(jìn)行SCWG試驗(yàn)，發(fā)現(xiàn)在SCW的高壓下，CNT表現(xiàn)出優(yōu)異的晶體穩(wěn)定性，減少了活性金屬流失，能在長(zhǎng)期SCWG過(guò)程中保證機(jī)械穩(wěn)定。

金屬氧化物催化劑具有耐高溫、價(jià)格低廉、易于氧化再生、易于運(yùn)輸儲(chǔ)存等優(yōu)點(diǎn)。CAO等[128]在600 ℃條件下以葡萄糖為模型化合物，對(duì)12種金屬氧化物（V2O5、Cr2O3、MnO2、Fe2O3、Co2O3、CuO、ZnO、ZrO2、MoO3、SnO2、WO3和CeO2)進(jìn)行SCWG，發(fā)現(xiàn)大多數(shù)氧化物對(duì)氫氣形成的影響很小，但對(duì)CO形成的影響很大，其中Cr2O3，CuO、WO3和V2O5是最有效的制氫催化劑。ZHANG等[129]研究了不同價(jià)態(tài)的鐵基催化劑在催化木質(zhì)素方面的表現(xiàn)，發(fā)現(xiàn)鐵基催化劑在SCWG中有得天獨(dú)厚的優(yōu)勢(shì)，尤其零價(jià)態(tài)的鐵H2產(chǎn)量明顯高于不同價(jià)態(tài)的氧化鐵催化體系，低價(jià)態(tài)的鐵基催化劑（Fe和FeO）具有低配位數(shù)活性鐵原子，有利于CO產(chǎn)生，高價(jià)態(tài)的鐵（Fe2O3和Fe3O4）有足夠的晶格氧，容易將CO氧化成CO2。CAO等[130]以黑液為原料測(cè)試了14種金屬氧化物在SCWG中效果，發(fā)現(xiàn)金屬氧化物不僅可以提供催化活性位點(diǎn)并且氧含量也是影響氣體組成的重要因素。

3 總結(jié)與展望

本文通過(guò)對(duì)生物質(zhì)SCWG（supercritical water gasification）機(jī)理總結(jié)和影響因素系統(tǒng)的梳理得到以下結(jié)論：1）在SCWG過(guò)程中有機(jī)酸、醛類(lèi)、酮類(lèi)等小分子中間體是形成氣體的主要來(lái)源，而酚類(lèi)和呋喃則主要是通過(guò)聚合反應(yīng)生成固體碳顆粒；2）間歇式反應(yīng)器適用于研究相行為和反應(yīng)機(jī)制，而連續(xù)式反應(yīng)裝置適合參數(shù)研究，提高升溫速率可以抑制焦油、焦炭等副產(chǎn)物產(chǎn)生；3）適當(dāng)提高進(jìn)料濃度有利于提高氫氣產(chǎn)量，但進(jìn)料濃度過(guò)高易形成較多的焦炭、焦油等中間產(chǎn)物導(dǎo)致氫氣產(chǎn)量降低，還易導(dǎo)致反應(yīng)器堵塞等問(wèn)題；4）低溫高壓水熱環(huán)境有利于離子反應(yīng)，隨著溫度升高氫鍵數(shù)目減少，自由基反應(yīng)增強(qiáng)，氣化效率會(huì)提高；5）在高壓下溶劑籠會(huì)導(dǎo)致分解反應(yīng)速率降低，有利于碳的形成，不利于氫氣產(chǎn)生，選擇合適的壓力對(duì)于SCWG過(guò)程十分重要；6）催化劑的酸性是影響SCWG過(guò)程的重要因素，酸性強(qiáng)會(huì)加速焦炭的形成造成催化劑失活，Ce、La、Ru等金屬元素作為合適的促劑可以增強(qiáng)催化劑活性。

為了能夠進(jìn)一步加深對(duì)生物質(zhì)SCWG的理解并投入于日常生產(chǎn)生活，未來(lái)還需要在以下幾個(gè)方向加強(qiáng)研究：1）現(xiàn)階段對(duì)于反應(yīng)路徑的研究主要集中在對(duì)于生物質(zhì)模型化合物，對(duì)于實(shí)際生物質(zhì)的反應(yīng)路徑和反應(yīng)機(jī)理還需要進(jìn)一步研究；2）高溫高壓的反應(yīng)環(huán)境對(duì)反應(yīng)裝置提出了更高要求，目前需要解決反應(yīng)裝置腐蝕、堵塞、鹽沉積等問(wèn)題；3）需要進(jìn)一步優(yōu)化催化劑的添加量，探索催化機(jī)理，同時(shí)進(jìn)一步探究催化劑失活原因；4）為進(jìn)一步降低成本，要對(duì)催化劑的穩(wěn)定性和回收利用性進(jìn)行研究，開(kāi)發(fā)高選擇性可重復(fù)使用的催化劑；5）目前沒(méi)有實(shí)際應(yīng)用的超臨界水制氫系統(tǒng)，因此對(duì)于實(shí)際生產(chǎn)中的能耗建模、經(jīng)濟(jì)性分析等相關(guān)工作較少。盡管以上的諸多問(wèn)題還需要不斷的研究和探索，但是生物質(zhì)SCWG能夠清潔高效的將生物質(zhì)廢棄物轉(zhuǎn)化為清潔的氫能，既實(shí)現(xiàn)了廢棄物的無(wú)害化處理，又能生產(chǎn)能源，是一種非常有前景的技術(shù)。

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Research progress of supercritical water hydrogen production from biomass

XU Gongxun, CHEN Wei, FANG Zhen※

(,,210031,)

Green, safe, and reliable clean energy is ever increasing under the "double carbon" goal. Among them, the waste biomass can be converted into green fuel, particularly with the application of many thermochemical and biochemical technologies. Supercritical Water Gasification (SCWG) can be a promising potential to convert the organic matter in the biomass into hydrogen using supercritical water as a medium. The feedstock can be used as a resource for biomass and the waste. SCWG process shares the fast reaction speed, excellent hydrogen selectivity, and fewer by-products, compared with traditional hydrogen production. In addition, water as the reactant in the SCWG process can avoid high energy consumption during drying, and thus reduce the cost. Previous systematic analysis has been made on the SCWG influence factors. In this review, the special physical and chemical properties of supercritical water were introduced to expound the main components (such as cellulose, hemicellulose, and lignin biomass) in the SCWG process reaction, and the experimental device, reaction temperature, residence time, and pressure in the influence factors. It was found that the batch reactor was suitable for the phase behavior and reaction mechanism, due to the simple structure and strong applicability of raw materials. By contrast, the continuous reaction device was used to more accurately control the experimental parameters, and then to realize the continuous commercial production, thus suitable for the parameter research. The hydrogen yield was improved to increase the heating rate of the device during operation, the reaction temperature, and residence time, but to reduce the feed concentration within a certain range. However, there was the complicated influence of pressure on the hydrogen yield. The solvent cage was often used under high pressure, leading to the reduction of the decomposition reaction rate unsuitable for the production of hydrogen. It was necessary to select the appropriate pressure, according to the actual situation. The homogeneous and heterogeneous catalysts were utilized in the SCWG. Specifically, the homogeneous catalyst performed a better catalytic effect on the water gas conversion was reaction, but the strong corrosion was caused the equipment to clog. The heterogeneous catalyst presented high catalytic activity, easy recovery, and excellent stability, more suitable for large-scale SCWG production. At the same time, there were some influences of the acidity of the metal catalyst in the catalytic process. The strong acidity of the catalyst accelerated the formation of carbon deposition, resulting in the catalyst deactivation. Appropriate secondary metals were added, such as Ce, La, and Ru. The performance of the catalyst was accelerated to increase the service life of catalyst for the better hydrogen selectivity. Future research can be focused on the equipment with corrosion resistance and salt deposition resistance, or constantly optimizing operating parameters, while the deactivation mechanism of catalysts, even to optimize the number of catalysts, and the catalysts with high activity and reusable. The existing technical barriers and development prospects of SCWG were analyzed to combine with the current technical development of SCWG. The finding can also provide theoretical guidance for the biomass SCWG in the future.

biomass; hydrogen; catalysts; supercritical water

2023-01-11

2023-02-15

國(guó)家自然科學(xué)基金面上項(xiàng)目（21878161）；南京農(nóng)業(yè)大學(xué)高層次人才引進(jìn)項(xiàng)目（68Q-0603）

徐功迅，研究方向?yàn)樯镔|(zhì)能源轉(zhuǎn)化。Email：wsywxns@126.com

方真，博士，教授，博士生導(dǎo)師，研究方向?yàn)樯锶剂霞霸趦?nèi)燃機(jī)中的利用。Email：zhenfang@njau.edu.cn

10.11975/j.issn.1002-6819.202301053

S216

A

1002-6819(2023)-07-0024-12

徐功迅，陳偉，方真. 生物質(zhì)超臨界水制氫研究進(jìn)展[J]. 農(nóng)業(yè)工程學(xué)報(bào)，2023，39(7)：24-35. doi：10.11975/j.issn.1002-6819.202301053 http://www.tcsae.org

XU Gongxun, CHEN Wei, FANG Zhen. Research progress of supercritical water hydrogen production from biomass[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2023, 39(7): 24-35. (in Chinese with English abstract) doi：10.11975/j.issn.1002-6819.202301053 http://www.tcsae.org