外文翻譯-- 最新關(guān)于生物質(zhì)預(yù)處理探究-烘焙工藝與方法

1、<p><b>　　附錄3 譯文</b></p><p>　　最新關(guān)于生物質(zhì)預(yù)處理探究-烘焙工藝與方法 </p><p><b>　　摘要 </b></p><p>　　生物質(zhì)與生物質(zhì)資源利用，在最近這些年與可再生能源項(xiàng)目緊密相連，然而生物質(zhì)中含有的一些不良物質(zhì)例如：高度受潮物質(zhì)和其自然含有的不良物質(zhì)在生物質(zhì)與其

2、在能源市場中的競爭對(duì)手的競爭中形成了障礙。解決這種有關(guān)生物質(zhì)燃料供給的辦法有 采用一種叫做烘焙的預(yù)處理進(jìn)程。烘焙處理是一種在惰性氣體中加熱到200—300度的相對(duì)溫和的處理過程，在這里，你將看到最新有關(guān)生物質(zhì)烘焙處理研究的進(jìn)程。烘焙前后生物質(zhì)成分的變化也將清晰呈現(xiàn)。各種有關(guān)生物質(zhì)烘焙研究實(shí)驗(yàn)的數(shù)據(jù)也有清晰的匯總，有關(guān)運(yùn)動(dòng)引起的生物質(zhì)研究也在這片論文中有大致說明。同樣的，在生物質(zhì)烘焙進(jìn)程的商業(yè)化應(yīng)用的研究的一些方面，本文也有所探討。 &l

3、t;/p><p><b>　　1、引言： </b></p><p>　　生物質(zhì)可以大致北分為從動(dòng)植物殘骸和排泄物中得到的生物材料，就一些發(fā)達(dá)國家的人口基數(shù)而言，生物質(zhì)能源例如農(nóng)業(yè)廢物和谷物殘留物是他們主要生物質(zhì)能源的來源，能源消耗的大拇指規(guī)則與經(jīng)濟(jì)增長緊密相連。隨著人口與經(jīng)濟(jì)的飛速增長，能源需求也將飛速遞增。然而不可再生能源例如化石能源是會(huì)被消耗殆盡的。在尋求可再生能源資

4、源的路上，人們發(fā)現(xiàn)生物質(zhì)是一個(gè)可行的選擇。在2005年國際能源組織(IEA)建立了一個(gè)科技工業(yè)發(fā)展路線規(guī)劃圖，希望建立一個(gè)清潔，智能，富有競爭力的未來能源市場。在這個(gè)藍(lán)圖的背景下，生物質(zhì)利用將會(huì)在2050年增長三倍。 </p><p>　　具有代表性的生物質(zhì)，被稱作“碳中性”的燃料，并成為了生物自然循環(huán)的一部分。生物質(zhì)燃料被使用時(shí)產(chǎn)生的二氧化碳也會(huì)被新生作物吸收利用。生物質(zhì)燃料是一種相對(duì)低碳的，而且更加可持續(xù)利用

5、的燃料，并且相比燃燒化石燃料而言，也以顯著降低碳排放。在過去的7年里，巴西就因在機(jī)動(dòng)車上利用生物質(zhì)燃料而減少了8300萬噸的二氧化碳排放。 </p><p>　　無論是在發(fā)達(dá)國家還是在發(fā)展中國家，在國內(nèi)對(duì)生物質(zhì)燃料的利用的意圖已經(jīng)在逐漸顯現(xiàn)。在2009年，歐洲和美國都對(duì)外解釋自己新安裝的發(fā)電裝機(jī)容量有一半以上都采用了可再生能源。在同一年，瑞典在能源生產(chǎn)中生物質(zhì)燃料所占的比重已經(jīng)超過了化石燃料，而且巴西也在清潔能源

6、利用方面投資了78億。這些全球趨勢(shì)都說明了可再生能源擁有廣闊而光明的發(fā)展前景。 </p><p>　　可再生能源政策是克服可再生能源利用障礙的金鑰匙之一。在2010年初，預(yù)計(jì)有83個(gè)國家已經(jīng)有了在發(fā)電領(lǐng)域提升可再生能源的利用比率的政策，歐盟希望借可再生能源政策和氣候政策(CCP)在2020年將最終能源消耗中的可再生能源的利用比提升到20%。中國也在可再生能源中長期發(fā)展計(jì)劃(MLTPRE)中將2020年底把基礎(chǔ)能源

7、消耗中可再生能源的利用提升至15%作為目標(biāo)。新西蘭宣布本國目標(biāo)為：到2025年底，本國發(fā)電能量的90%為可再生能源。 </p><p>　　世界上每年的生物質(zhì)產(chǎn)量預(yù)計(jì)有1460億噸，其中的大部分是野生植物。然而，在利用生物質(zhì)燃料過程中有一定不利條件，導(dǎo)致只有其中的一小部分被用作能源燃料利用。由于生物質(zhì)中含有的一些不理想成分，如高含水量成分并要考慮到生態(tài)環(huán)境破壞，可想而知利用生物質(zhì)的成本相對(duì)有點(diǎn)高。在例如發(fā)電和進(jìn)程

8、選擇中的技術(shù)革新，將會(huì)在推進(jìn)世界國家在可再生能源利用上的承諾與進(jìn)步中扮演重要角色。在最近幾十年來，預(yù)處理技術(shù)，如在熱轉(zhuǎn)換進(jìn)程中的烘焙處理技術(shù)顯示了出積極的進(jìn)步。本文旨在比較和對(duì)比在不同研究過程中的烘焙處理熱處理工作。 </p><p><b>　　2、生物質(zhì) </b></p><p>　　生物質(zhì)可以大體上分為木質(zhì)生物質(zhì)與非木質(zhì)生物質(zhì)，木質(zhì)生物質(zhì)是森林，林地與樹叢中主要

9、的產(chǎn)品與副產(chǎn)品。非木質(zhì)生物質(zhì)包括農(nóng)業(yè)作物，農(nóng)業(yè)林業(yè)廢物，草本類植物，動(dòng)物糞便乃至第三代動(dòng)物糞便。由于與化石能源有相似的轉(zhuǎn)換過程，生物燃料相比化石燃料而言具有潛在競爭優(yōu)勢(shì)。然而，一些關(guān)于木質(zhì)纖維的固有屬性問題也亟待解決。 </p><p><b>　　3、烘焙 </b></p><p>　　烘焙處理是一種在無氧環(huán)境中把生物質(zhì)進(jìn)料加熱到200—300度的相對(duì)溫和的熱處理

10、過程。烘焙的定義一般用于干燥加熱，溫和與緩慢的高溫分解以及熱預(yù)處理過程。根據(jù)它的利用歷史，烘焙一開始是被用于諸如木條和鋸末的木基材料。最近幾年，包含莊稼，農(nóng)業(yè)林余渣的研究也在進(jìn)行。表1總結(jié)了在不同生物質(zhì)利用中得到的不同燃料產(chǎn)品。盡管各種各樣來源的生物質(zhì)都在被進(jìn)行研究，通過烘焙處理都可以得到例如提升燃料價(jià)值，增強(qiáng)疏水性和易碎性的制品性能。這也是在熱化學(xué)處理進(jìn)程中一個(gè)理想趨勢(shì)。 </p><p>　　在烘焙過程前后的

11、物理和化學(xué)參數(shù)的變化在下表中有討論 </p><p>　　3.1 質(zhì)量與能量變化 </p><p>　　在烘焙中，未加工的生物質(zhì)顯得十分服從這一有限的轉(zhuǎn)換過程。在人工合成的進(jìn)程中，有價(jià)值的成分在獲取后待用。在烘焙過程中，溫度范圍是200-300度，質(zhì)量損失主要是脫水和脫揮的反應(yīng)機(jī)制的半纖維素的組成部分。質(zhì)譜分析表明，質(zhì)量下降的同時(shí)，減少的是半纖維素和木質(zhì)素。在處理初期質(zhì)量的顯著變化表明了被

12、處理的生物質(zhì)水分的顯著減少。 </p><p>　　由表2可以看出，不同生物質(zhì)的質(zhì)量與能量變化因烘焙進(jìn)程而不同。烘焙過程生物質(zhì)發(fā)生的質(zhì)量變化的幅度可達(dá)原重的24%-95%。由于木質(zhì)生物質(zhì)含有更多的纖維素成分，農(nóng)業(yè)殘?jiān)霓D(zhuǎn)換率更高而木質(zhì)生物質(zhì)質(zhì)量變化也更小。研究表明，給料成分結(jié)構(gòu)的不同也會(huì)影響烘焙過程的反應(yīng)的進(jìn)行。在主要部分半纖維素中，木聚糖含量較高，這會(huì)加快反應(yīng)速率。提高烘焙處理溫度和時(shí)間也會(huì)提升生物質(zhì)的高位發(fā)熱

13、量(HHV)。表2可以看出不同生物質(zhì)的高位發(fā)熱量(HHV)的增加范圍可以達(dá)到1%-58%。木質(zhì)和非木質(zhì)生物質(zhì)的凈熱量值的范圍分別是18–26 MJ/kg和12–25焦MJ/kg </p><p>　　能量變化是基于質(zhì)量變化，熱值變化的，并和可以被看做是烘焙過程中能量損失的單位。木質(zhì)生物質(zhì)在低于250度的烘焙過程中的能量變化可以超過95%，除了盧塞恩木（88%）。當(dāng)烘焙溫度高于250度時(shí)，能量變化范圍是55%-98

14、%。由于含有更高的易揮發(fā)成分和半纖維素成分，非木質(zhì)生物質(zhì)相比木質(zhì)生物質(zhì)有更廣的能量變化范圍，可達(dá)29%-98%。烘焙時(shí)間的影響相比烘焙溫度要小，而且，理想的處理環(huán)境是在低溫下處理或者是在高溫下短時(shí)處理以保證最小能量損失。 </p><p>　　3.2 最終和近因分析 </p><p>　　取自文獻(xiàn)的受烘焙處理的生物質(zhì)式樣最終分析數(shù)據(jù)在表3中給出。大體上看，元素分析表明，隨著烘焙過程越來越強(qiáng)

15、烈，固定碳成分也在增加。與碳相比，氫和氧的損失率更加與生物質(zhì)能量值有關(guān)。在生物燃料應(yīng)用方面的調(diào)查研究中，生物燃料相比化石燃料在元素描述方法上的提升至關(guān)重要。Van Krevelen圖是一個(gè)表明生物質(zhì)中元素變化的一個(gè)簡潔明了的圖，在這個(gè)圖中，原子氫碳比指數(shù)與原子氧碳比指數(shù)針鋒相對(duì)。 </p><p>　　圖2表明了煤樣和未處理生物質(zhì)樣品中的原子比率。圖中的虛線表示脫水反應(yīng)過程。在烘焙過程之前，木質(zhì)生物質(zhì)有1.6的氫

16、碳比和0.75的氧碳比，在200-250度的烘焙溫度下，氫碳比下降到1.6，氧碳比下降到0.6；在高于250度的烘焙環(huán)境下，Van Krevelen圖認(rèn)為烘焙將生物質(zhì)的元素比向碳轉(zhuǎn)化。在烘焙過程中，這些變化可以由二氧化碳和水蒸氣的釋放，順利的氣化和燃燒過程來解釋。比較一下圖2中的三個(gè)過程這樣認(rèn)為：烘焙過程中的分解作用包括顯著的脫水過程和脫水過程中的氫碳，氧碳原子比的變化。</p><p>　　表4說明，在不同生物

17、質(zhì)實(shí)驗(yàn)中，隨著烘焙溫度和烘焙時(shí)間的升高和加長，生物質(zhì)中固定碳成分含量增加，同時(shí)易變成分減少。氧官能團(tuán)的裂變被用來解釋化合物近似分析中的變化。相比其他生物質(zhì)而言，麥稈，谷殼，木條，松木和甘蔗渣中的揮發(fā)物減少量大約是25%，也是相對(duì)較高的。生物質(zhì)中無機(jī)礦物質(zhì)的催化作用也被認(rèn)可會(huì)導(dǎo)致更高的揮發(fā)物質(zhì)損失。在烘焙之后，盡管相比固定碳的變化0。9-29%，灰分的變化較小，大致會(huì)增加0.1-12%。隨著灰分附著在烘焙給料上，進(jìn)料的最初灰分會(huì)顯著影響烘

18、焙結(jié)果。 </p><p>　　3.3 減少親水性 </p><p>　　水分的出現(xiàn)是生物質(zhì)在熱化學(xué)處理進(jìn)程中性能下降的一個(gè)主要原因。烘焙后產(chǎn)品疏水性的檢驗(yàn)方式主要有：a,浸沒實(shí)驗(yàn)；b,平衡含水率(EMC)研究。在浸沒實(shí)驗(yàn)中，處理前后的生物質(zhì)都被完全浸沒在水中一段時(shí)間，式樣疏水性的判斷依據(jù)是基于重量變化的吸水量。平衡含水率研究則用靜態(tài)干燥技術(shù)，用飽和鹽溶液來得到需要的濕度，再加上水浴，一種

19、恒溫恒濕的環(huán)境就得到了。被測式樣在一定時(shí)間內(nèi)質(zhì)量恒定即可認(rèn)為達(dá)到平衡態(tài)。 </p><p>　　在浸沒試驗(yàn)中，經(jīng)過熱處理的式樣相比沒有經(jīng)過熱處理的式樣水分吸收率要更低， 然而，水分吸收的趨勢(shì)會(huì)因烘焙條件的不同而變化。在高溫下烘焙的生物質(zhì)，水分吸收率會(huì)更低。烘焙對(duì)平衡含水量的影響在11.3-97%的相對(duì)濕度范圍內(nèi)都有檢測，可以得到與浸沒實(shí)驗(yàn)相似的結(jié)論，即經(jīng)過預(yù)處理的生物質(zhì)相比沒有處理過的生物質(zhì)具有更低的吸水性。關(guān)于

20、測量數(shù)據(jù)我們可以建立一個(gè)平衡含水率(EMC)的模型和吸水型的總結(jié)，并用來降低烘焙過程溫度。在烘焙的溫度進(jìn)程中，水分是隨著揮發(fā)分一起散出的主要成分。物理干燥的生物質(zhì)被首先用于討論，在大約100度的時(shí)候，生物質(zhì)進(jìn)料中的自由水就揮發(fā)出去了。隨著后處理的進(jìn)行，生物分子脫水，其中含有的輕有機(jī)揮發(fā)物也加入了干燥進(jìn)程。當(dāng)干燥溫度逐漸超過200度時(shí)，生物質(zhì)中的結(jié)合水也得到釋放。主要在半纖維素結(jié)構(gòu)中的長的多糖鏈的解聚使生物質(zhì)中化合物結(jié)構(gòu)縮短，這與木質(zhì)素和

21、細(xì)胞膜中有限的脫揮和碳化過程有關(guān)。在烘焙過程中，烘焙過的生物質(zhì)由于細(xì)胞膜質(zhì)的微纖維結(jié)構(gòu)中羥基的破壞從而降低了其的吸水性能。這個(gè)不可逆的反應(yīng)國過程使得生物質(zhì)分子變得非極性，從而可以保證生物質(zhì)材料的質(zhì)量并延長了生物質(zhì)材料的儲(chǔ)藏時(shí)間。另外也因此使</p><p><b>　　4、易磨性 </b></p><p>　　隨著半纖維素的分解伴隨著有細(xì)胞膜質(zhì)的解裂和木質(zhì)素的軟化，在

22、烘焙過程之后，生物質(zhì)材料式樣的細(xì)胞壁強(qiáng)度顯著減弱。這種由烘焙帶來的脆性提升對(duì)生物質(zhì)材料的可磨性有顯著提升。關(guān)于烘焙過程提升易磨性的研究十分復(fù)雜，需要對(duì)烘焙之后研磨出的不同大小的碎片進(jìn)行分類研究和討論。大體上來說，隨著烘焙過程的進(jìn)行，生物質(zhì)中具有良好可磨性顆粒的部分比例增加，可磨性提升。另一種方法，即研究粒度分布的研究加上磨能源消耗在研究的易磨性。在文獻(xiàn)中，將可磨性的提升和能源消耗的降低歸納成兩個(gè)機(jī)械階段。 </p><

23、;p>　　生物質(zhì)可磨性的提升對(duì)脫水性和低溫下木質(zhì)素的物理可變性的提高也有幫助。緊接而來的第二階段，是上面談到有助于烘焙之后具有良好可磨性顆粒形成的細(xì)胞壁熱分解過程。在煤處理中的標(biāo)準(zhǔn)哈氏可磨性指數(shù)(HGI)在文獻(xiàn)中被用來描述烘焙之后生物質(zhì)式樣的可磨性。由于生物質(zhì)相比煤具有更低的密度，所以修改標(biāo)準(zhǔn)哈氏可磨性指數(shù)(HGI)的研究是測量一片被碾磨過的容積和質(zhì)量。盡管經(jīng)過超參數(shù)烘焙處理過的生物質(zhì)可以達(dá)到跟煤一樣的可磨性，由于在預(yù)磨過程中大尺

24、寸的生物質(zhì)材料被剔除，文獻(xiàn)指出定體積哈氏可磨性指數(shù)(HGI)可能低估了生物質(zhì)的可磨性。結(jié)果是，盡管烘焙后材料的可磨性提高確實(shí)唄觀察到了，但是從文獻(xiàn)中總結(jié)的定體積哈氏可磨性指數(shù)(HGI)的研究并不能代表全部的式樣。 </p><p><b>　　4.1 動(dòng)力學(xué) </b></p><p>　　反應(yīng)動(dòng)力學(xué)研究，以及化學(xué)反應(yīng)速率影響因素的反應(yīng)速度。在適宜溫度條件下設(shè)計(jì)好的操作

25、設(shè)備熱的化學(xué)反應(yīng)和的初期，反應(yīng)機(jī)理和動(dòng)力學(xué)的基本知識(shí)是至關(guān)重要的。烘焙動(dòng)力學(xué)研究研究的數(shù)學(xué)模型，主要是生物質(zhì)熱解過程產(chǎn)生的模型。生物質(zhì)通常被視為主要是半纖維素，纖維素和木質(zhì)素組成的。有研究表明，生物質(zhì)熱解過程可分為四個(gè)階段。水分的演變是主要的反應(yīng)過程，在低溫度低于220度的環(huán)境下進(jìn)行。隨著溫溫度高于200度，木質(zhì)素的分解(在160度-900度的范圍里慢慢進(jìn)行)也帶來了半纖維素的解裂。在200度到400度的范圍里，纖維素分解作用持續(xù)進(jìn)行。

26、在文獻(xiàn)中，烘焙唄描述成能夠提高生物質(zhì)燃料性能的一種溫和的高溫分解過程?；仡櫤姹旱臏囟确秶?00-300度，主要的反應(yīng)是水蒸氣成分的主要演變，伴隨著木質(zhì)素分解的半纖維素的解裂過程。表5總結(jié)了在烘焙過程中的動(dòng)力學(xué)模型。熱解動(dòng)力學(xué)模型的最簡單的形式是能夠正確描述生物質(zhì)熱分解進(jìn)程初期系列反應(yīng)的。一步通用的模型在表5中檢驗(yàn)了兩種木質(zhì)生物質(zhì)(云杉和山毛櫸)因受到烘焙處理所引起的其中結(jié)晶水損失帶來的動(dòng)力學(xué)能變化?？梢钥吹?，計(jì)算與實(shí)驗(yàn)良好符合，結(jié)晶水

27、損失量大約是(R2 of 0.961–0</p><p>　　還有一些研究采用了兩部通用模型，木質(zhì)生物質(zhì)質(zhì)量損失中的Di Blasi–Lanzetta模型，在模型中引入中間反應(yīng)為二級(jí)分解反應(yīng)的產(chǎn)物統(tǒng)計(jì)。在230-300度的溫度范圍內(nèi)，烘焙反應(yīng)動(dòng)力學(xué)可以被描述成兩個(gè)連續(xù)的一階反應(yīng)，描繪了木質(zhì)素分解帶來的細(xì)胞膜質(zhì)的熱解過程。相比單步通用模型，(R2 取值 0.986–0.987)Di Blasi–Lanzetta被認(rèn)

28、為是更加符合與硬質(zhì)與軟質(zhì)木的模型。這個(gè)改進(jìn)模型的擬合是由于連續(xù)兩個(gè)步驟的模型考慮到中間的偽組件。另外，在柳樹樣本得出的動(dòng)力學(xué)參數(shù)的基礎(chǔ)上，第一反應(yīng)率是高于第二反應(yīng)的。在烘焙溫度制度，熱分解下繼續(xù)延長時(shí)間。文獻(xiàn)中這一現(xiàn)象由無機(jī)物質(zhì)的分解纖維素和木質(zhì)素可能催化或液態(tài)和氣態(tài)的副產(chǎn)品。木質(zhì)纖維素的熱降解提出了三個(gè)獨(dú)立的重疊反應(yīng)，半纖維素，細(xì)胞膜質(zhì)和木質(zhì)素。在山毛櫸和云杉的烘焙過程中，這個(gè)反應(yīng)過程有詳細(xì)的研究。在這個(gè)反應(yīng)中，半纖維素隨著Di Bl

29、asi–Lanzetta模型中預(yù)測的進(jìn)行而被分解。木質(zhì)素也按照單一步通用模型開始瓦解。細(xì)胞膜質(zhì)則順應(yīng)改進(jìn)的Di Blasi–Lanzetta模型開始了表5中寫出的兩個(gè)并行反應(yīng)。實(shí)驗(yàn)和模型的結(jié)果吻合良好。但是</p><p>　　當(dāng)烘焙處理的是大型生物質(zhì)，例如木料和煤球時(shí)，我們采用Shafizadeh與 Chin 模型來處理熱分解反應(yīng)過程。該模型包括三個(gè)平行競爭的主要反應(yīng)裂解途徑，包括木質(zhì)素被熱解成焦炭，焦油的產(chǎn)生

30、和揮發(fā)性物質(zhì)溢出。這個(gè)模型在230-260度的溫度范圍都與實(shí)驗(yàn)吻合相對(duì)較好。但是一旦超過260度，這個(gè)模型就不能與實(shí)驗(yàn)符合。這可能是由于這個(gè)模型中沒有考慮到的碳化反應(yīng)開始發(fā)生的原因。關(guān)于TORSYPD專利反應(yīng)器的優(yōu)化數(shù)學(xué)模型也被建立起來。TORSYPD 一個(gè)由法國工程公司的Thermya為烘焙生產(chǎn)生物燃料而設(shè)計(jì)的可以持續(xù)工作的移動(dòng)床反應(yīng)器。在這個(gè)工作中，作者選擇了Shafizadeh與 Chin 模型來處理生物質(zhì)烘焙反應(yīng)過程。另外還加上

31、干燥和運(yùn)輸?shù)牧Ｗ幽Ｐ鸵约皻饬鞯挠绊?。研究者們能夠預(yù)測TORSYPD反應(yīng)模型的良好的溫度曲線與連續(xù)試驗(yàn)工廠的實(shí)驗(yàn)數(shù)據(jù)相比較。在Agrawal和Sivasubramanian的個(gè)自單獨(dú)工作中，溫度積分近似建議通過的烘焙參數(shù)估計(jì)下的非等溫生物質(zhì)的分解反應(yīng)熱處理的生物量研究而取得。經(jīng)過烘焙處理的生物質(zhì)燃燒曲線展現(xiàn)出兩個(gè)階段。衍生的動(dòng)力學(xué)參數(shù)表明，第一階段的活化能停留時(shí)間變化而第</p><p><b>　　附錄

32、4 譯文原文</b></p><p>　　Recent advances in biomass pretreatment – Torrefaction fundamentals and technology</p><p><b>　　Abstract:</b></p><p>　　Biomass is generally defi

33、ned as the biological material derived from plant or animals as well as their waste and residues [1]. For the population in developing countries, biomass energy such as agricultural waste and crop residue is one of their

34、 prime energy sources[2]. Energy consumption by rule of thumb is closely related to economic growth. The energy demand will progressively increase with the rapid population growth and economic development. However non-re

35、newable energy source such as fossi</p><p>　　Biomass is typically acclaimed as a ‘carbon neutral’ fuel as biomass is part of the bio-cycle. The carbon dioxide produced from biomass combustion is consumed by

36、cultivation of new crops. Biomass is a low carbon fuel and a form of sustainable fuel that offers significant reduction in net carbon emissions compared with fossil fuels [6]. In the past seven years, Brazil has avoided

37、an estimated 83 million tons of carbon dioxide emission from the utilization of biofuel in its motor vehicles [7].</p><p>　　The utilization of biomass for domestic purpose has gradually expanded in both deve

38、loped and developing nations in the recent years. Renewable energy source in 2009 accounted for more than half of the newly installed power capacity in Europe and USA. In the same year, biomass share in energy production

39、 exceeded oil in Sweden while Brazil invested $7.8 billion in clean energy [8]. These global trends show an encouraging future for renewable energy resources.Renewable energy policies are among th</p><p>　　

40、The world production of biomass is estimated at 146 billion metric tons a year, mostly wild plant growth [13]. However, only as a small fraction is utilized for energy generation, as there are certain drawbacks in the us

41、e of raw biomass as a fuel source.Logistic cost of biomass is relatively high; due to its undesirable characteristics such as high moisture content and biological attack[14]. Technology advancement in areas such as power

42、 generation system and process selection will play a major r</p><p>　　2. Biomass</p><p>　　Biomass can generally be classified as woody biomass and nonwoody biomass. Woody biomass comprise mainly

43、 of products and by-products derived from the forest, woodland and trees sector.Non-woody biomass includes agricultural crops, a gro-forestry residue, herbaceous products, animal waste as well as tertiary waste [2]. Biof

44、uel have the potential to compete with fossil fuel as they share similar conversion processes. However, several issues related to the inherent properties of lignocelluloses bio</p><p>　　2.1. Issues faced wit

45、h biomass</p><p>　　Biomass like any other energy source has its advantages and disadvantages. One of the most obvious drawbacks is the heterogeneous nature of biomass. Biomass feedstock can differ considerab

46、ly in term of physical, chemical and morphological characteristics.Biomass has relatively low energy density and high moisture content in its untreated form compared to fossil fuel.Higher load of biomass is required to g

47、enerate the same amount of energy when compared to fossil fuel. Majority of the plant based </p><p>　　The world production of biomass is estimated at 146 billion metric tons a year, mostly wild plant growth

48、[13]. However, only as a small fraction is utilized for energy generation, as there are certain drawbacks in the use of raw biomass as a fuel source.Logistic cost of biomass is relatively high; due to its undesirable cha

49、racteristics such as high moisture content and biological attack[14]. Technology advancement in areas such as power generation system and process selection will play a major r</p><p>　　2.2. Biomass to energy

50、 conversion processes</p><p>　　Bio-fuel can be classified into three main types namely wood fuels, agro fuels and municipal by-products which is based on the source of the biomass used. Fig. 1 summarizes the

51、 technological options to convert raw biomass into convenient energy carriers such as bio-gas, liquid fuel or processed solids. The technologies can be classified into three main categories: biochemical, mechanical and t

52、hermochemical conversion. Biofuel synthesized can be grouped as three main types namely the wood fuels, </p><p>　　Biochemical conversion utilizes biological organism and biological catalyst to convert biomas

53、s into convenient fuel such as bio-ethanol, biogas and bio diesel. Centuries-old technology of mechanical extraction is another option to obtain plant oil by physical rolling and crushing of seeds, kernel and fruits.<

54、/p><p>　　Thermochemical processing relies on heat and chemical catalyst to synthesize useful secondary energy. This is an attractive option for conversion of biomass to energy due to its higher efficiencies,gre

55、ater versatility as well as wider range of fuel feedstock. Thermochemical conversion of biomass compared to biological conversion is a faster process. Gasification technology offers advantages such as reduced emissions,

56、improved thermal efficiency, and the ability to generate hydrogen and other hig</p><p>　　In view of the problems associated with the undesirable characteristics of raw biomass, pre-treatment offers a promisi

57、ng solution to enhance process efficiency prior to the main energy conversion step [20]. Torrefaction, a pre-treatment technology that requires lower treatment temperature is reported to be highly efficient for thermoche

58、mical processing and will be the main pretreatment method discussed in this paper.</p><p>　　3.Torrefaction</p><p>　　Torrefaction is a thermolysis process that subjects the feedstock to thermal t

59、reatment at relatively low temperatures of 200–300 ?C in the absence of oxygen. Definition for torrefaction is commonly associated with roasting, mild pyrolysis, slow pyrolysis, and thermal pretreatment, according to its

60、 utilization. Early research work on torrefaction was mainly on wood based material such as woodchips</p><p>　　and sawdust. In the recent years, more studies incorporate agricultural crops and a gro-forestry

61、 residue. Table 1 summarize the fuel properties of different biomass. Although various sources of biomass material were investigated, similar product properties can be attained through torrefaction process such as improv

62、ed energetic value, enhanced hydrophobicity and friability; which is a favored trend for thermochemical processing.</p><p>　　The physical and chemical properties of biomass before and after torrefaction are

63、analyzed for the following (a) yield, (b) energy content,(c) elemental composition, (d) change in major components,</p><p>　　(e) hydrophobicity, and (f) ease of comminution.</p><p>　　3.1. Mass y

64、ield and energy yield</p><p>　　Raw biomass is deliberately subjected to limited conversion in the torrefaction process. The valuable intermediates synthesized in the process are used for energy recovery at a

65、 later stage [41].In the torrefaction temperature range of 200–300 ?C, mass loss is dominated by dehydration and de volatization in the reaction regime of hemicelluloses component [26].Mass spectrometry analysis indicate

66、s that weight loss is accompanied by reduction in the hemicelluloses and primary lignin sections [42]. T</p><p>　　Mass yield and energy yield of different biomass subjected to torrefaction process is illustr

67、ated in Table 2. The mass yield of torrefied biomass can vary from 24% to 95% of its original weight.Conversion rate of agricultural residues is comparatively higher than woody biomass due to its higher hemicelluloses co

68、ntent, thus resulting in lower mass yield [33,39,44]. Studies show that the polymeric structure of the feedstock will affect the reactivity of torrefaction reaction [44,45]. Higher conten</p><p>　　Energy yie

69、ld based upon the mass yield and calorific value and can be viewed as an indicator of the amount of energy lost during torrefaction. Energy yield for woody biomass subjected to torrefaction temperatures below 250 ?C is a

70、bove 95% except for Lucerne wood (88%). As torrefaction temperature increase to above 250 ?C,energy yield spreads from 55% to 98%. Non-woody biomass generally has a wider spread in energy yield compared to woody biomass,

71、ranging from 29% to 98%, due to the higher variati</p><p>　　3.2. Ultimate and proximate analysis</p><p>　　The ultimate analyses data of biomass samples subjected to torrefaction were obtained fr

72、om the literature and presented in Table 3. Generally, the elemental analysis demonstrates an increase in fixed carbon content as torrefaction conditions intensifies. The higher loss in oxygen and hydrogen compared to ca

73、rbon is highly related to the increase in energy value of the biomass [16,24]. The characterization of the elemental improvement in biomass relative to fossil fuel is vital in the investigatio</p><p>　　chang

74、es in biomass. The atomic hydrogen to carbon ratio index is plotted against the atomic oxygen to carbon ratio in Van Krevelen diagram.</p><p>　　Fig. 2(a) illustrates the atomic ratio of coal samples and untr

75、eated biomass samples. The dotted straight lines in the diagram represents the dehydration reaction pathway. Prior to torrefaction,the woody biomass samples have a H:C ratio of 1.6 and O:C ratio of 0.75. For torrefaction

76、 temperature range of 200–250 ?C in Fig. 2(b), the H:C ratio drops to approximately 1.5 and O:C ratio is</p><p>　　0.6. At torrefaction temperature above 250 ?C in Fig. 2(c), the van Krevelen plot suggests th

77、at torrefaction shifts the elemental ratios of biomass towards that of coal. During the process of torrefaction,changes have been accounted to the release of carbon dioxide as well as water, favorable for gasification an

78、d combustion [14,36,47].Comparing the three plot in Fig. 2, it can be noted the decomposition mechanism of torrefaction involves significant dehydration as the changes in the H:C and O:C </p><p>　　Table 4 sh

79、ows that fixed carbon content increases while volatile content decreases as torrefaction temperature and residence time intensifies across different biomass. The disintegration of oxygen functional group has been account

80、ed for the change of the proximate analysis compounds [28]. Volatile loss for wheat straw, rice husk, logging wood chip, pine and sugarcane bagasse are around 25% which is relatively higher compared to the remaining biom

81、ass. Catalytic effect of inorganic mineral matter </p><p>　　3.3. Reduced moisture affinity</p><p>　　One of the main is advantages of biomass has been related to the presence of moisture which pe

82、nalizes its performance especially in thermochemical processes. Hydrophobic property of torrefied product is generally examined via: (a) immersion test, or (b) equilibrium moisture content (EMC) study. For immersion test

83、, treated and untreated biomass is submerged in water for a fixed duration of time. Hydrophobicity is judged based on the total moisture absorption of sample in weight basis. Equilibrium</p><p>　　Moisture ab

84、sorption of thermal treated sample is comparatively lower than untreated biomass using immersion test [34,36]. However the trend moisture absorption of torrefied sample with respect to torrefaction parameter differs in b

85、oth cases. Moisture absorption is lower for biomasses that were torrefied at higher temperature[34,36]. The effect of torrefaction on equilibrium moisture content</p><p>　　Was examined across relative humidi

86、ty range of 11.3–97.0% [29,49].Analogous results are obtained for the immersion test, whereby pretreated biomass has reduced affinity for moisture compared to raw sample. An EMC model was fitted to the measured data and

87、the moisture adsorption capacity was concluded to reduce with increased torrefaction temperature [49,50]. In the torrefaction temperature regime, water is one of the main products release along with volatiles. Physical d

88、rying of biomass is first</p><p>　　at approximately 100 ?C whereby the free water in biomass feedstock is liberated. Light organic volatiles are evolved in the post drying steps as the organic molecules dehy

89、drate. As the temperature gradually increases to the excess of 200 ?C, bound water in biomass is released [16,33]. Depolymerization of long polysaccharide chains shortens the polymeric structure of biomass, mainly from t

90、he hemicelluloses fractions [16]. This is coupled with limited devolatization and carbonization of the ligni</p><p>　　because of its increased stability and durability.</p><p>　　Grindability<

91、/p><p>　　Through decomposition of the hemicelluloses coupled with depolymerization of cellulose and thermal softening of lignin, the orientation of microfibrils is displaced during torrefaction. The cell wall i

92、n the biomass sample is greatly weakened after torrefaction[37]. The increased brittleness and friability introduced by torrefaction improves the grindability of biomass. The ease of comminution</p><p>　　in

93、torrefaction studies is widely examined through the particle distribution of milled samples after being distributed according to its size range. Generally, grindability of biomass improves after torrefaction based on the

94、 increased percentages of fine particle as torrefaction condition are raised [26,27,39]. An alternative method is the particle distribution study is coupled with grinding energy</p><p>　　consumption in exami

95、ning the grindability [47,52]. Specific energy consumption for treated biomass are reduced as much as 10 times after torrefaction [31]. Literature defines the improved grindability</p><p>　　and reduced energ

96、y consumption in comminution to a two stage mechanism [52].</p><p>　　The improved ease to grind biomass is attributed to the dehydration and physical transformation of lignin at lower temperature.Subsequentl

97、y, the second stage is the thermal degradation of the cell wall biomass as discussed earlier that contributes to the higher percentage of fine particle after torrefaction [52]. The standard Hardgrove Grindability Index (

98、HGI) used to analyze the grindability</p><p>　　of coal had been studied in literature for torrefied biomass sample[47]. The modified HGI study adopted volumetric measurement for the sample to be milled in pl

99、ace of mass measurement as biomass are of lower density compared to coal. Although treated sample achieves similar grindability to reference coal samples for extended torrefaction parameter, literature indicate that volu