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附錄1： 生物質(zhì)燃料研究
第一章.介紹
生物質(zhì)是排在煤和石油后面的世界第三大能源資源（Basipetal.1997年)。直到19世紀(jì)中葉,生物質(zhì)能源占全球能源消耗的大部分。即使在過(guò)去的五十年中，化石燃料使用的增加促使生物質(zhì)能源消耗的減少,生物量仍然相當(dāng)于提供了約12.5億噸石油或約占世界人口的14%的能源消耗。
全球每年主要的能源消耗四分之一的全球主要的用于農(nóng)業(yè)實(shí)踐(WEC 1994)。 木質(zhì)燃料、秸稈,青草等是最突出的生物質(zhì)能源。如果使用得當(dāng),會(huì)帶來(lái)很多好處,
其中最重要的是他們是可再生能源和可持續(xù)能源原料。與化石燃料相比它可以顯著減少碳的凈排放。出于這個(gè)原因,可再生能源和可持續(xù)的能源燃料被認(rèn)為是一個(gè)清潔發(fā)展機(jī)制(CDM)，可以減少溫室氣體(GHG)排放(李和胡2003年)。
生物質(zhì)資源的來(lái)源是木材或農(nóng)產(chǎn)品,但是他們的供應(yīng)是有限的。為了解決這個(gè)問(wèn)題,世界各國(guó)正考慮發(fā)展生物質(zhì)農(nóng)作物和開(kāi)發(fā)技術(shù)來(lái)使用生物質(zhì)能利用更有效率。在美國(guó)(美國(guó))和多數(shù)歐洲國(guó)家,生物能源已經(jīng)滲透到了能源市場(chǎng)。在美國(guó)和瑞典約占整個(gè)能源市場(chǎng)4%和13%。瑞典正在實(shí)施計(jì)劃逐步淘汰核電站,減少
化石燃料的能源使用,增加生物質(zhì)能源的使用(Breeden 2006)。
生物質(zhì)能源的一個(gè)主要局限是生物質(zhì)能源目的密度太小,秸稈和草密度范圍通常是從80 - 100公斤/立方米，木質(zhì)生物質(zhì)是150 - 200公斤/立方米, 密度低的生物質(zhì)常常使材料難儲(chǔ)存、運(yùn)輸和使用。密度低也帶來(lái)了新的問(wèn)題，比如燒結(jié),因?yàn)槊芏炔顒e造成燃料到鍋爐燃燒效率降低。為了克服這些限制致密化是一個(gè)很好選擇。通過(guò)機(jī)械壓縮從而實(shí)現(xiàn)生物量的在致密化,從而將增加了密度增加了十倍。商業(yè)的致密化的生物質(zhì)采用顆粒壓縮,其他的比如擠出成型,塊狀壓縮,或輥壓，是為了解決喂養(yǎng),儲(chǔ)存,處理和運(yùn)輸問(wèn)題。
本文檔全面介紹了當(dāng)前生物技術(shù)研究和發(fā)展,提供致密化參數(shù)的優(yōu)化。致密化過(guò)程和技術(shù),同時(shí)介紹的影響過(guò)程和原料變量和生化成分的生物量在原料質(zhì)量屬性,如持久性、散裝密度、顆粒密度、和熱量的價(jià)值。本文主要包括壓實(shí)和響應(yīng)模型和一個(gè)討論的優(yōu)化過(guò)程?；仡檱?guó)際固體燃料標(biāo)準(zhǔn)和一個(gè)介紹公司處理致密化設(shè)備和熱處理技術(shù)也包括。該介紹的具體內(nèi)容包括:
技術(shù):
——粒子粘結(jié)致密化的機(jī)制
——致密化技術(shù),包括擠壓、壓塊、壓縮
能源的需求——壓縮成型機(jī),和擠出機(jī)
生產(chǎn)過(guò)程副作用的影響,原料變量和生物量的組成
致密化過(guò)程
——重要的成型質(zhì)量標(biāo)準(zhǔn)
預(yù)處理的效果如磨、預(yù)熱、蒸汽、和氨纖維擴(kuò)張(AFEX)生物量質(zhì)量
-壓縮模型
——程序響應(yīng)曲面建模和優(yōu)化。
國(guó)際固體燃料標(biāo)準(zhǔn)。
設(shè)備供應(yīng)商:
-致密化設(shè)備
——熱處理技術(shù)。
第二章.生物質(zhì)致密化
未加工的生物質(zhì)原料很難大規(guī)模應(yīng)用，因?yàn)樗w積龐大，濕度高,并且密度低。生物質(zhì)致密化技術(shù)把植物轉(zhuǎn)換轉(zhuǎn)化為燃料。這些技術(shù)也稱(chēng)為制丸、塊狀、或壓縮,
提高了材料的操作特性，為交通運(yùn)輸、倉(cāng)儲(chǔ)等提供了方便。制丸和壓塊
已在很多國(guó)家應(yīng)用了很多年年。威廉·史密斯是最先發(fā)表聯(lián)合生物質(zhì)致密化國(guó)家專(zhuān)利(1880)的。史密斯使用蒸汽錘(66°C[150°F])壓實(shí)鋸木廠(chǎng)的廢物。
傳統(tǒng)工藝致密化生物質(zhì)能可以分為制粒,擠壓和壓塊,使用壓板、切粒機(jī)、螺旋壓力機(jī),活塞或輥壓。制粒和塊狀是用于生物質(zhì)致密化的最常見(jiàn)固體燃料成型的技術(shù)。這些高壓壓實(shí)技術(shù),也叫“擠壓”技術(shù),通常使用一個(gè)螺旋壓力機(jī)或活塞成型機(jī)(Khansamahs et al . 2005年)。
螺旋壓力機(jī)的原理是,生物量是通過(guò)加熱，連續(xù)擠壓、模具成型。螺旋壓力機(jī)成型塊質(zhì)量和生產(chǎn)過(guò)程的優(yōu)于活塞成型技術(shù)。然而,比較部件磨損，活塞成型機(jī),更有優(yōu)勢(shì)。,實(shí)驗(yàn)表明,螺桿螺旋沖壓件需要更多的維護(hù)。螺旋壓力機(jī)有助于實(shí)現(xiàn)統(tǒng)一和有效的燃燒,產(chǎn)生的成型塊就表面碳化，可以更好更快地傳熱。
許多研究人員致力于研究致密化的秸稈和生物質(zhì)利用顆粒的研究，例如,Tabi和Khansamahs(1996)致力于研究紫花苜蓿顆粒的壓縮性能。Amandine等(2002)研究了影響模具的壓力。
在疏松特點(diǎn)的生物質(zhì)。Adapa eta研究制壓縮苜蓿產(chǎn)品。李和劉(2000)調(diào)查了木材的高壓致密化成型以形成一個(gè)更好的燃料。摩尼等人(2006)研究了實(shí)特性對(duì)木質(zhì)纖維生物質(zhì)使用的影響。
2.1顆粒成型的機(jī)制
吸生物量的質(zhì)量取決于諸多過(guò)程變量,像模具直徑、成型溫度、壓力、,預(yù)熱的生物量。Tabi(1996)和Tabi和Khansamahs(1996 b和c)的實(shí)驗(yàn)表明,成型塊的壓實(shí)可以歸功于彈性和塑性變形的粒子在存在更高的壓力。根據(jù)他們的研究,有兩個(gè)重要方面被認(rèn)為和成型效果有關(guān)(1)粒子形成顆粒的能力的大小和機(jī)械強(qiáng)度的大小，(2)過(guò)程中增加密度增加的比例。
在確定可能的機(jī)制成型機(jī)制的過(guò)程中，顆粒成型的形成的原因可能是固體粘合劑(薩思綏，1973)。由化學(xué)反應(yīng)、燒結(jié)、硬化產(chǎn)生的粘結(jié)劑在壓縮、堅(jiān)實(shí)的的過(guò)程中都起著重要作用，都是在高壓下,凝固融化的物質(zhì),或結(jié)晶解散了的材料。壓力也降低了致密化過(guò)程中的粒子的熔點(diǎn),使他們產(chǎn)生了新的效應(yīng),從而增加了接觸面積和改變?nèi)埸c(diǎn)向新的均衡水平(紐約,皮奇，1984年)。其他類(lèi)水物質(zhì)的存在在制粒導(dǎo)致形成毛細(xì)管壓力,從而增加粒子粘結(jié)。這個(gè)模型,一般用于描述液體在某種情況下可能發(fā)生的反應(yīng)。
在充滿(mǎn)了液體的物質(zhì)中形成了類(lèi)圓環(huán)的接觸點(diǎn)，空氣中形成了一個(gè)個(gè)連續(xù)的階段。這個(gè)鍵的強(qiáng)度由于負(fù)面毛細(xì)管壓力和液體的表面張力所決定。纜索狀態(tài)的
發(fā)生在液體含量增加,導(dǎo)致較低的孔喉體積和合并液體環(huán),并形成連續(xù)的網(wǎng)絡(luò)和捕獲空氣的階段。在毛細(xì)管和液滴的狀態(tài),液體是被完全包圍凝聚的,而且主要的粒子僅存在的表面張力。
粒子之間的吸引力是由于范德瓦爾斯靜電和磁力組成。 吸引力與粒子之間的距離是成反比,更大的距離產(chǎn)生更小的吸引力。靜電引力的影響是可以忽略不計(jì),粒子粘結(jié)通常都存在于常遇見(jiàn)的干細(xì)粉,在磁力存在時(shí)摩擦也有可能導(dǎo)致粒子粘結(jié)(頓和奧利弗1981)。聯(lián)鎖的粒子可以幫助提供足夠的機(jī)械力量克服彈性恢復(fù)壓縮后造成破壞性的力量(Frump 1962)。
摩尼等人(2002)假定在生物質(zhì)致密化的的過(guò)程中存在三個(gè)階段。在第一階段,顆粒重排自己形成一個(gè)大的顆粒，那里多數(shù)的粒子保留它們的屬性，能量損耗是由于inter-particle和particle-to-wall摩擦引起的。 在第二階段,粒子相互擠壓,發(fā)生蘇星和彈性變形,這顯著增加了接觸面積，粒子通過(guò)范德瓦爾斯靜電引力，發(fā)生作用，在第三階段,在很大壓力的作用下，顆粒的密度達(dá)到滿(mǎn)足要求的的程度。第個(gè)三階段結(jié)束后,由于壓力減少和70%的顆粒的整合，變形和破損的粒子可以不再改變形狀。
在整個(gè)過(guò)程中，重要的是理解致密化過(guò)程和通過(guò)改變變量控制它的性能,例如一組相關(guān)的的溫度、壓力、和設(shè)備。如果不是優(yōu)化或有意控制,這些變量可以影響成型塊的產(chǎn)量有嚴(yán)重的負(fù)面影響。同樣重要的是要明白,材料的屈服點(diǎn)和產(chǎn)品的密度。因?yàn)槎际茄b入在成型腔,通過(guò)施加壓力,粒子脆性斷裂。這些過(guò)程可能會(huì)導(dǎo)致機(jī)械聯(lián)鎖。
生物量的化學(xué)成分,其中包括化合物如纖維素、半纖維素,蛋白質(zhì)、淀粉、木質(zhì)素、粗纖維、脂肪,也影響到致密化過(guò)程。在壓縮時(shí)在較高的溫度下,蛋白質(zhì)和淀粉plasticizes作為粘合劑,它有助于在增加顆粒的強(qiáng)度(Briggs et al . 1999年)。 淀粉在于生物質(zhì)能致密化過(guò)程中作為粘結(jié)劑存在。在使用一種富含淀粉的生物的致密化擠出工藝壓塊,存在的熱量和濕氣的淀粉和可以讓在更好的成型(木材1987;托馬斯等al . 1998年)。 高溫和壓力,這通常是致密化過(guò)程中遇到情況,疲軟的木質(zhì)素,能改善生物量的結(jié)合能力。低熱固性屬性和低熔點(diǎn)(140°C)使得木質(zhì)素可以是成型塊更好的被壓縮( 2004年)。蛋白質(zhì)、淀粉、生物量和木質(zhì)素等都可以幫助壓縮。
在壓縮紫花苜蓿、小麥和大麥磨的過(guò)程中(Tabi和Khansamahs 1996和1996 b;Adapa et al。2002 b和2009;Mani et al . 2004年)。 應(yīng)用高壓縮壓力在生物質(zhì)致密化會(huì)導(dǎo)致生物質(zhì)顆粒破碎,從而破碎細(xì)胞結(jié)構(gòu)、蛋白質(zhì)和果膠作為自然的粘合劑(Polanski和格雷厄姆1984;O 'Docherty和惠勒1984年)。它們之間的主要差異生物質(zhì)和其他材料,像陶瓷粉末和制藥粉,是存在自然綁定材料(Aliyahs和莫雷2006)。這些存在的物質(zhì)(如樹(shù)皮、莖、葉等,在生物物質(zhì)進(jìn)一步復(fù)雜化的過(guò)程中，能幫助理解壓實(shí)的行為。最近,Aliyahs和莫雷(2010)使用掃描電子顯微鏡(SEM)，研究對(duì)于了解solid-type橋梁中形成塊狀和壓縮塊的玉米秸稈和柳枝稷。在微觀水平上進(jìn)行更多的研究,使用類(lèi)似的技術(shù),掃描電鏡和透射電鏡能幫助 理解交互的原料和過(guò)程變量的質(zhì)量屬性。
2.1.1致密化技術(shù)
2.1.1.1螺桿壓縮或擠壓
壓實(shí)的目的是使擠出機(jī)擠出讓更小的微粒,在壓力的作用下，使得壓塊更加緊密，,提供更強(qiáng)的壓縮材料。在擠壓、材料在成型筒的作用下,旋轉(zhuǎn)螺絲,產(chǎn)生很大的壓力,從而導(dǎo)致重大壓力梯度和摩擦由于生物質(zhì)剪切。這個(gè)綜合作用的墻壁摩擦在成型筒壁、內(nèi)部摩擦材料,和高轉(zhuǎn)速( 600 rpm)的螺絲,在封閉的系統(tǒng)提高溫度和加熱生物質(zhì)。這些生物質(zhì)強(qiáng)行通過(guò)了擠壓模具形成煤球或化球團(tuán)所要求的形狀。如果被縮減,生物質(zhì)燃料進(jìn)一步壓縮。如果在系統(tǒng)產(chǎn)生的熱量不夠?qū)Σ牧系倪_(dá)成pseudo-plastic狀態(tài)平穩(wěn)擠壓、加熱器提供從擠出機(jī)無(wú)論是使用外部或者內(nèi)部加熱器(Grover和Mishap 1996)。圖2顯示了典型的螺桿擠出機(jī),與不同的區(qū)域進(jìn)行處理的生物質(zhì)。處理的生物質(zhì)用螺絲釘壓實(shí)涉及以下機(jī)制(Grover和Mishap 1996):
1、在到達(dá)壓縮區(qū)(這個(gè)常所形成的逐漸變細(xì)的桶)前,生物量部分壓縮。這是在第一個(gè)階段中,最大的需要能量來(lái)克服粒子摩擦。
2.一旦生物質(zhì)是在壓縮區(qū)，由于高溫度(200 - 250°C)材料變得相對(duì)軟,并在此加熱、材料失去彈性性質(zhì),它的結(jié)果在增加的區(qū)域inter-particle聯(lián)系。 在這個(gè)階段,顆粒緊密結(jié)合在一起，在其通過(guò)壓縮區(qū),生物量能吸收摩擦以便它可被加熱和使其其質(zhì)量均勻。
3.在第三階段,生物質(zhì)進(jìn)入錐形死,由于一般的溫度的280°C水分是進(jìn)一步蒸發(fā)，,有助于更好地壓縮生物量和增加壓縮的材料。
4.在最后階段,移除蒸汽和壓實(shí)的壓力,使其成型。
以下是螺桿壓縮的優(yōu)點(diǎn)(Grover和Mishap 1996):
螺旋壓力機(jī)的輸出是持續(xù)的,壓塊的大小更加統(tǒng)一。壓塊的外表面部分碳化,這可以幫助促進(jìn)點(diǎn)火和燃燒。這也能保護(hù)壓塊從周?chē)乃?。一個(gè)同心圓洞形成的壓塊有助于更好地燃燒是因?yàn)榭諝饬魍ㄔ谌紵?。機(jī)器運(yùn)行順利，沒(méi)有任何沖擊負(fù)荷。機(jī)器零件和油機(jī)使用是沒(méi)有的灰塵或原料污染。
缺點(diǎn)是螺桿壓縮相比，本機(jī)對(duì)功率的要求比較高。（格羅弗和米什拉1996年）
表1。島田的SPMM850擠壓機(jī)擠出通用規(guī)范生產(chǎn)。
之前擠壓（硬或軟木材原料），含水率8%，平均粒徑2-6毫米，容重200 kg/m3。擠壓后，水分含量為4％，容重1400 kg/m3。熱值4870千卡（8400 BTU/磅）灰分含量0.35-0.5％
擠出機(jī)
高剪切擠出機(jī)
這些擠出機(jī)設(shè)計(jì)，生產(chǎn)種類(lèi)繁多的熱處理產(chǎn)品。高剪切擠出機(jī)被列為high-temperature/short-time（HTST）設(shè)備，其中，生物量通常用蒸汽或熱水預(yù)熱，然后通過(guò)高剪切烹飪的處理，擠出機(jī)進(jìn)一步的工作產(chǎn)品，并迅速提高其溫度（哈珀1981年）。
低剪切擠出機(jī)
低剪切擠出機(jī)，具有很小的剪切，高壓縮比。這些擠出機(jī)用于擠出低粘度材料?？蓱?yīng)用于熱成型或擠壓成型，剪切粘性小是因?yàn)樾院纳⒌陌l(fā)生是由于相對(duì)較低的粘度，材料被壓縮前加熱產(chǎn)品（哈珀1981年）
2.1.1.2壓塊
適用于松散的，規(guī)模較小的顆粒，使用壓縮機(jī)進(jìn)行生物質(zhì)致密化是一個(gè)可行的和有吸引力的利用生物燃料應(yīng)用的解決方案。壓塊通常采用液壓機(jī)械，或輥壓機(jī)。壓塊的密度一般為900至1300 kg/m3的。
生物燃料型煤是一種清潔的綠色燃料，可以在普通爐，鍋爐使用。
與其他壓縮方式相比，壓塊機(jī)可以處理直徑較大規(guī)模的顆粒和對(duì)顆粒直徑要求不高。不需要粘合劑的的作用，壓縮塊的優(yōu)點(diǎn)是增加熱值，燃燒特性的改善，減少夾帶的顆粒物排放，形狀均勻的尺寸合適。此外，使用其它固體燃料的爐，壓塊也可以使用。 使用生物質(zhì)型壓塊在工業(yè)爐球團(tuán)的主要缺點(diǎn)是由于灰排渣堿含量從生物質(zhì)制成的壓塊（Amandine等，2002）。
生物質(zhì)壓塊過(guò)程中，材料的高溫高壓下被壓縮。在壓塊生物質(zhì)顆粒之間由于熱塑流動(dòng)，形成了木質(zhì)素。木質(zhì)素，它是一種天然的粘合劑，在高溫和壓力形成高密度的壓縮塊。
液壓柱塞泵
液壓活塞壓力機(jī)是常用的生物質(zhì)致密壓塊機(jī)?；钊ㄟ^(guò)高壓液壓系統(tǒng)的電動(dòng)馬達(dá)傳送能量。 液壓機(jī)的輸出速度較低，因?yàn)闅飧椎倪\(yùn)動(dòng)速度較機(jī)械往復(fù)慢。成型塊密度超過(guò)1000公斤/立方米，散裝密度較低，因此產(chǎn)量被限制為40 - 135公斤/小時(shí)。然而，這些機(jī)器可以成型的的水分含量比為的15％，為機(jī)械活塞成型機(jī)。為了提高生產(chǎn)能力，一些壓塊機(jī)采用連續(xù)壓塊形式。
附錄2：生物質(zhì)燃料英文翻譯
1. INTRODUCTION
Behind coal and oil,biomass is the third largest energy resource in the world(Ba pat et al.1997).Until the mid-19th century,biomass dominated global energy consumption.Even though increaseful-fuel use has prompted a reduction in biomass consumption for energy purposes over the past 55years,biomass still provides about 1.25 billion tons of oil equivalent(B toe)or about 14%of the world’annuals energy consumption(Hirohito et al.2006;Werther et al.2000;and Zen et al.2007).Out of the230 megajoules of estimated global primary energy,56 megajoules—nearly one-fourth of the global primary—are used for agricultural practices(WEC 1994).Wood fuels,agricultural straws,and grasses archeal most prominent biomass energy sources.Biomass,if properly managed,offers many advantages,the most important being a renewable and sustainable energy feedstock.It can significantly reduce net carbon emissions when compared to fossil fuels.For this reason,renewable and sustainable fuel is considered a clean development mechanism(CDM)for reducing greenhouse gas(GHG)emissions(Li and H 2003).
The least-expensive biomass resources are the waste products from wood or Lagro-processing operations, but their supply is limited. To overcome this limitation, countries around the world are considering biomass crops for energy purposes and have begun developing technologies to use biomass more efficiently. In the United States (U.S.) and most of Europe, biomass has already penetrated the energy market. The U.S. and Sweden obtain about 4% and 13% of their energy, respectively, from biomass (Hall et al. 1992), and Sweden is implementing initiatives to phase out nuclear plants, reduce fossil-fuel energy usage, and increase the use of biomass energy (Breeden 2006).
One of the major limitations of biomass for energy purposes is its low bulk density, typically rangingfrom80–100kg/m3 for agricultural straws and grasses and 150-200 kg/m for woody biomass, like wood chips (Khansamahs and Benton 2006; Mitchell et al. 2007). The low bulk densities of biomass often make the material difficult to store, transport, and use. Low bulk density also presents challenges for technologies such as coal co firing, because the bulk density difference causes difficulties in feeding the fuel into the boiler and reduces burning efficiencies. Densification is one promising option for overcoming these limitations. During densification, biomass is mechanically compressed, increasing its density about ten fold. Commercially, densification of biomass is performed using pellet mills, other extrusion processes, briquetting presses, or roller presses in order to help overcome feeding, storing, handling, and transport problems.
Densification technologies available today have been developed for other enterprises and are not optimized for a biomass-to-energy industry’s supply system logistics or a conversion facility’s feedstock specifications requirements.This document provides a comprehensive review of the current state of technology in biomass densification research and development and provides parameters for optimization.Densification processes and technologies are described along with the impacts of process and feed stock variables and biochemical composition of the biomass on feedstock quality attributes like durability,bulk density,pellet density,and caloric value.This review includes compaction and response surface models and a discussion of optimization procedures.A review of international solid fuel standards and an introduction of companies dealing with densification equipment and heat treatment technologies are also included.
The specific objectives of this review include:
Technical reviews:
- Mechanisms of particle bonding during densification
- Densification technologies, including extrusion, briquetting, pelleting, and agglomeration
- Specific energy requirements of pellet mill, briquette press, and extruder
- Effects of process, feedstock variables, and biomass biochemical composition on the
densification process
- Important quality attributes of densified biomass
- Effects of pretreatments such as grinding, preheating, steam explosion, torrefaction, and ammonia
fiber expansion (AFEX) on biomass quality
- Compaction models
- Procedures for response surface modeling and optimization.
International solid fuel standards.
Equipment suppliers:
- Densification equipment
- Heat treatment technologies.
2. BIOMASS DENSIFICATION
Biomass—in its original form—is difficult to successfully use as a fuel in large-scale applications because it is bulky, wet, and dispersed. Biomass densification represents technologies for converting plant residues into fuel. These technologies are also known as pelleting, briquetting, or agglomeration, which improves the handling characteristics of the materials for transport, storage, etc. Pelleting and briquetting have been applied for many years in several countries. William Smith was the first to be issued a United States patent (1880) for biomass densification. Using a steam hammer (at 66°C [150°F]), Smith compacted waste from sawmills.
Conventional processes for biomass densification can be classified into baling, pelletization, extrusion, and briquetting, which are carried out using a bailer, pelletizer, screw press, piston or a roller press. Pelletization and briquetting are the most common processes used for biomass densification for solid fuel applications. These high-pressure compaction technologies, also called “binder less” technologies, are usually carried out using either a screw press or a piston press (Khansamahs et al. 2005).
In a screw press, the biomass is extruded continuously through a heated, tapered die. The briquette quality and production process of a screw press are superior to piston press technology. However, comparing wear of parts in a piston press, like a ram and die, to wear observed in a screw press shows that the screw press parts require more maintenance. The central hole incorporated into the densified logs produced by a screw press helps achieve uniform and efficient combustion, and the resulting logs can be carbonized more quickly due to better heat transfer.
Many researchers have worked on the densification of herbaceous and woody biomass using pellet mills and screw/piston presses. For instance, Tabi and Khansamahs (1996) worked on understanding the compression characteristics of alfalfa pellets. Amandine et al. (2002) examined the influence of die pressure on relaxation characteristics of briquetted biomass. Adapa et al. (2002b and 2003) studied pelleting fractionated alfalfa products. Li and Li (2000) investigated high-pressure densification of wood residues to form an upgraded fuel. Mani et al. (2006) researched the compaction characteristics of lignocellulosic biomass using an Ins tron, and Tumulus et al. (2010a) studied the effect of pelleting process variables on the quality attributes of a wheat distiller’s dried grains with solubles.
2.1 Mechanisms of Bonding of Particles
The quality of the densified biomass depends on a number of process variables, like die diameter, die temperature, pressure, usage of binders, and preheating of the biomass mix. Tabi (1996) and Tabi and Khansamahs (1996b and c) suggest that the compaction of the biomass during pelletization can be attributed to elastic and plastic deformation of the particles at higher pressures. According to their study, the two important aspects to be considered during pelletization are (1) the ability of the particles to form pellets with considerable mechanical strength; and (2) the ability of the process to increase density. The first is a fundamental behavior issue that details which type of bonding or interlocking mechanism results in better densified biomass.
The possible mechanism of binding during agglomeration could be due to the formation of solid bridges (Frump 1962; Pastry and Understeer 1973). During compaction, solid bridges are developed by chemical reactions and sintering, hardening of the binder, solidification of the melted substances, or crystallization of the dissolved materials. The pressure applied during densification also reduces the melting point of the particles and causes them to move towards one another, thereby increasing the contact area and changing the melting point to a new equilibrium level (York and Pilpul 1972; Piet sch 1984).
The presence of liquid-like water during pelletization results in interfacial forces and capillary pressures, thus increasing particle bonding. The models that are commonly used to describe the liquid distribution in moist agglomerates are pendular, funicular, capillary, and liquid-droplet states (Pastry and Understeer 1973; Piet sch 1984; Gheber-Lassie 1989). The pendular state arises when the void spaces re filled with liquid to form lens-like rings at the point of contact; the air forms a continuous phase. The bond strength is due to negative capillary pressure and surface tension of the liquid. The funicular curvicostate when the liquid content is increased, which results in lower pore volume and coalescence of the liquid rings, and in the formation of a continuous network and trapping of the air phase. In the capillary and droplet state, the agglomerate is completely enveloped by the liquid, and the primary particles are held only by the surface tension of the droplet.
The attraction between the particles is due to van der Waal’s electrostatic or magnetic forces (Schoenbergian 1971). The attraction is inversely proportional to the distance between the particles, where larger distances have less attraction. Electrostatic forces’ influence on particle bonding is negligible, and heme are commonly encountered in dry fine powders where inter-particle friction can also contribute to particle bonding when magnetic forces exist (Sherrington and Oliver 1981). Closed bonds or interlocking occurs in fibers, platelets, and bulk particles, where particles interlock or fold about each other, thereby causing the bonding (Piet sch 1984). Interlocking of the particles can help provide sufficient mechanical strength to overcome the destructive forces caused by elastic recovery after compression (Frump 1962).
Mani et al. (2002) postulated that there are three stages during densification of biomass. In the first stage, particles rearrange themselves to form a closely packed mass where most of the particles retain their properties and the energy is dissipated due to inter-particle and particle-to-wall friction. In the second stage, the particles are forced against each other and undergo plastic and elastic deformation, which increases the inter-particle contact significantly; particles become bonded through van der Waal’electroosmotic forces. In the third phase, a significant reduction in volume at higher pressures results in the density of the pellet reaching the true density of the component ingredients.
By the end of the third stage, the deformed and broken particles can no longer change positions due to a decreased number of cavities and a 70% inter-particle conformity. It is important to understand the densification process and the variables that govern its performance, such as the combination of temperature, pressure, and equipment. If not optimized or at least carefully controlled, these variables can influence the antra-particle cavities of the biomass and have a serious negative effect on conversion processes like enzymatic hydrolysis. It is also important to understand that the yield point of the material governs the rate of approach to the true density of the product. Because the loading is hydrostatic in character, the application of pressure will fracture the brittle particles. These processes may result in mechanical interlocking. Figure 1 shows the deformation mechanism of the powder particles under compression (Comsomol 2007; Denny 2002).
The chemical composition of the biomass, which includes compounds like cellulose, hemicelluloses, protein, starch, lignin, crude fiber, fat, and ash, also affect the densification process. During compression at high temperatures, the protein and starch plasticizes and acts as a binder, which assists in increasing the strength of the pellet (Briggs et al. 1999). Starch present in the biomass acts as binder during densification. During densification of starch-rich biomass using an extrusion process like pelleting, the presence of heat and moisture gelatinizes the starch and results in better binding (Wood 1987; Thomas et al. 1998). High temperature and pressure, which are normally encountered during the densification process, results in softening of the lignin and improves the binding ability of the biomass. Low thermosetting properties and a low melting point (140°C) help lignin take an active part in the binding phenomena (van Dam et al. 2004). Protein, starch, and lignin present in biomass takes an active part during pelleting of alfalfa, wheat, and barley grinds (Tabi and Khansamahs 1996a and 1996b; Adapa et al.2002b and 2009; Mani et al. 2004). Application of high compression pressures during biomass densification can result in crushing the biomass particles, thus opening up the cell structure and exposing the protein and pectin that act as natural binders (Polanski and Graham 1984; O’Docherty and Wheeler 1984). The major difference between biomass and other materials, like ceramic powders and pharmaceutical powders, is the presence of natural binding materials (Aliyahs and Corey 2006). The presence of components like bark, stems, leaves, etc., in the biomass further complicates understanding of the compaction behavior. Recently, Aliyahs and Corey (2010) used scanning electron microscope (SEM) studies for understanding the solid-type bridges formed during briquetting and pelleting of corn stover and switchgrass. More studies at a micro level using techniques like SEM and TEM will be useful in understanding the interaction of feedstock and process variables on the quality attributes of densified biomass.
2.1.1 Densification Technologies
2.1.1.1 Screw Compaction or Extrusion
The aim of compaction using an extruder is to bring the smaller particles closer so that the forces acting betw