硬木熱解過(guò)程中顆粒內部二次反應的數值研究Ⅰ. 單顆粒熱解模型的構建

第30卷第4期燃料化學(xué)學(xué)報Vol.30 No.42002年8月JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGYAug.2002文章編號:0253-2409 2002 )4-0336-06硬木熱解過(guò)程中顆粒內部二次反應的數值研究I .單顆粒熱解模型的構建余春江,周勁松,廖艷芬,駱仲泱,岑可法(浙江大學(xué)能源潔凈利用與環(huán)境工程教育部重點(diǎn)實(shí)驗室, 浙江杭州310027 )商要:為了研究生物質(zhì)(硬木熱解過(guò)程中顆粒內部的二次反應彌補常規實(shí)驗研究在該領(lǐng)域內的不足構建合適的數學(xué)模型進(jìn)行數值模擬是.--個(gè)很好的途徑。本文介紹了硬木單顆粒熱解綜合模型的構建方法。針對所研究的問(wèn)題模型中細致考慮了包含一次焦油二 次裂解的生物質(zhì)熱解動(dòng)力學(xué)過(guò)程。為了準確描述熱解產(chǎn)物在熱解顆粒內部的傳遞和二次反應過(guò)程模型包含了對熱解過(guò)程中顆粒內部各種氣相產(chǎn)物在孔隙率不斷變化的生物質(zhì)顆粒內的生成、消耗、積累以及在壓力驅動(dòng)下的輸運等行為的描述。模型參數的選取盡可能真實(shí)地模擬了實(shí)際過(guò)程引入轉化率或溫度對涉及到的各種物理性質(zhì)參數進(jìn)行了修正。構建的模型在理論上較全面地描述了單顆粒生物質(zhì)熱解的復雜過(guò)程可以利用它對顆粒內部的二次反應過(guò)程進(jìn)行進(jìn)-步的研究。關(guān)鍵詞:生物質(zhì);硬木;熱解;二次裂解;數學(xué)模型中圖分類(lèi)號:TK6 .文獻標識碼:A作為清潔的可再生能源，生物質(zhì)能的開(kāi)發(fā)利用要。越來(lái)越受到重視生物質(zhì)能高效轉化利用技術(shù)的研1熱解基本過(guò)程究正在全世界范圍內迅速展開(kāi)。鑒于生物質(zhì)原料具典型的生物質(zhì)原料,如硬木顆粒在熱解反應器有特別高的揮發(fā)分含量熱解反應是各種生物質(zhì)熱中的基本熱解過(guò)程如下隨著(zhù)木顆粒進(jìn)入惰性、高溫化學(xué)轉化過(guò)程中非常關(guān)鍵的一個(gè)環(huán)節另一方面熱的反應環(huán)境熱量通過(guò)各種方式傳遞到顆粒表面在解是一種部分氣化工藝,它的可貴之處在于能提供顆粒溫度從外到內逐漸升高的過(guò)程中,硬木中的水低成本的高品質(zhì)燃氣。因此生物質(zhì)熱解過(guò)程的研分首先開(kāi)始蒸發(fā),由處于吸附狀態(tài)的液體水變?yōu)樗吭谏镔|(zhì)轉化利用技術(shù)領(lǐng)域內具有獨特的地位。蒸汽進(jìn)入顆?？障吨械臍庀?并隨氣相流動(dòng)離開(kāi)顆熱解-次產(chǎn)物在特定的條件下會(huì )進(jìn)-步的裂解粒。當顆粒溫度升高到-定程度后相應的生物質(zhì)生成二次反應產(chǎn)物這是普遍存在的現象。這種二熱分解反應隨之發(fā)生反應生成了固、液、氣態(tài)的各次反應有時(shí)會(huì )顯著(zhù)地改變熱解產(chǎn)物的分布影響產(chǎn)種產(chǎn)物。其中固態(tài)產(chǎn)物駐留在原地構成并改變顆物的品質(zhì)因此針對該現象展開(kāi)研究有重要的實(shí)際粒的多孔結構而氣、液態(tài)產(chǎn)物則在不斷發(fā)展的孔隙意義。二次反應可以劃分為顆粒內反應和顆粒外反結構中擴散、積累并在壓力的驅動(dòng)下流動(dòng)。其中一應兩種。顆粒外二次反應受一次熱解產(chǎn)物在熱解反些具有反應活性的產(chǎn)物還將在流動(dòng)過(guò)程中經(jīng)歷進(jìn)一應器內的停留時(shí)間反應器的溫度等外部因素影響,步的二次反應,生成相應的二次產(chǎn)物。在上述各過(guò)相對比較易于評估或控制;顆粒內部的二次反應由程發(fā)生發(fā)展的同時(shí)蒸發(fā)吸熱、熱解反應熱、氣體、液于涉及到生物質(zhì)熱解過(guò)程中顆粒內部的一系列錯綜體產(chǎn)物的流動(dòng)帶來(lái)的焓流會(huì )改變顆粒內部不同位置復雜、相互關(guān)聯(lián)的變化在不同情況下反應進(jìn)行的速處的局部熱量平衡影響顆粒內的溫度分布這反過(guò)度與程度都很難估計。鑒于在一些實(shí)際情況下,如來(lái)又影響著(zhù)熱解化學(xué)反應或蒸發(fā)過(guò)程的進(jìn)行速率。流化床熱解反應器中,顆粒的外部傳熱傳質(zhì)強度非顯然這是一個(gè)物理和化學(xué)過(guò)程相互影響的多層面常大此時(shí)顆粒內部傳質(zhì)過(guò)程有可能會(huì )成為過(guò)程的的復雜歷程。由于目前技術(shù)水平下通過(guò)儀器實(shí)時(shí)控制因素，因此顆粒內部的二次反應對熱解的影響測量顆粒內部的物理量還非常困難建立一個(gè)能夠不容忽略對顆粒內部二次反應的研究顯得尤為必收稿日期:2002-01-07 ;修回日期:2002-07-04基金項目:國家重點(diǎn)基礎研究專(zhuān)項經(jīng)費( 2001CB409600 );國家自然科學(xué)基金( 29976039 );國家杰出青年自然科學(xué)基金( 50025618 )。338燃料化學(xué)學(xué)報30卷水占據的空間V和氣相占據的孔隙體積Vi。 其中空氣k ]和水蒸氣[ m ]的質(zhì)量守恒方程。式左邊第V.和V。構成控制體積里的固相,V; 構成氣相,V構-項表示氣相組份在控制體內的積累,左邊第二項成被束縛的液相。代表由于氣流流動(dòng)引起的控制體內該組份質(zhì)量變在建模時(shí)我們引入了如下假設:化,右邊的項則體現了氣相組份的產(chǎn)生和消耗?！耦w粒內部的氣、固、液相處于局部熱平衡狀為了描述氣體在原料孔隙中的流動(dòng)模型中引入了Darcy定理。Darcy定理的本質(zhì)是動(dòng)量方程的●忽略由物質(zhì)濃度擴散引起的焓流。-個(gè)特殊的簡(jiǎn)化形式,主要用于計算流體在多孔介●忽略動(dòng)能、勢能和體積力。質(zhì)內的流動(dòng)過(guò)程。式9)表述了各種氣體組份組成●沒(méi)有顆粒收縮、破裂現象。的氣相的表觀(guān)流速< uy >與壓力差、介質(zhì)穿透率( )熱解氣態(tài)產(chǎn)物認為是理想氣體具有恒定的K,和相應動(dòng)力粘度puy的關(guān)系。平均分子量。 =K王(9)( )熱解開(kāi)始前原料的初始孔隙結構中的空氣是惰性氣體。.

= (p,YR,T( 10)●物性參數(孔隙率、穿透性、導熱系數、比熱M容和粘度)是溫度、轉化率的函數。由于假設所有氣態(tài)產(chǎn)物都是理想氣體,可以用●二次反應是單相氣相反應。理想氣體方程( 10 )來(lái)計算控制容積內的氣相壓力以基于以上假設,以球形顆粒沿半徑的球面層作求得Darey 定理中所需的壓力差。其中M,是氣相為微分控制體積可以列微分方程如下:平均分子量,< pr >/是氣相真實(shí)密度通用氣體常i<ρw>= -(h+h2+ k:zXρ。>(1)數R, =8314} kmol-+ K-。氣相壓力實(shí)際上是四種氣體組份的分壓力之和。p.) = ak;+ ks;(p.)(2)式11 )是控制體積的能量平衡方程,左邊第-式112 )是生物質(zhì)w和半焦c的質(zhì)量守恒方項代表能量在控制體內的積累第二項是液相和氣程。式右邊表示控制體內固體物質(zhì)的消耗或產(chǎn)生速相流動(dòng)引起的焓流右邊第一項是由有效導熱系數表征的導熱和輻射傳熱造成的能量傳遞,式中右邊率。最后一項考慮了熱解中的各步反應的熱效應。日影)= k:(p,)-(r27(uX(p,Yc, +<ρ, Yer, +(4)+= (2(6)c ))+ k<ρ。XOH2 +( T-TolCc-(ac. +Bc。+( 1-a-是+專(zhuān)錄(2)=0 (7) β)e; ))+ k,s(3)此熱容模型中的能量平衡計算中需要物( 14)〈p?！?°質(zhì)比熱容數據,-般認為純物質(zhì)的比熱容是溫度的( 函數。根據文獻,本模型中涉及的各種氣、固、液態(tài)Mj =(Kp)M, + .+<定:P,>)(<ρ>++++< Pmφ= nP.. +(1-n)中。其中φm是初始原料性質(zhì)重。是半焦的性質(zhì)轉化>)( 18)(5)其它參數 模型中涉及的其它參數還有各率η由<ρw>/<ρ。>初始計算。物質(zhì)的導熱系數、硬木和半焦的密度、氣相組分平均(2 )本征穿透率本征穿透率 K是表征多孔介分子量、硬木和半焦的平均孔隙直徑等,見(jiàn)表3。340燃料化學(xué)學(xué)報30卷表3模型中其它參數的選取Table 3 Selection of other parameters involvedParameterValueReferenceHeat conductvity of Hardwood /W mK 10.13+0.0003x( T -273)C. A. Koufopano5’]Heat conductivity of Char /W mK~0.08-0.0001x( T-273)C.A. KoufopanoS77Heat conductivity of gas phase /W mK-0.02577Colomba Di Blasilf 8Apparent density of hardwood /kg m540EstimatedReal density of hardwood /kg m'1100Boutin , 0.[91Real density of char /kg m'.1100Average diameter of void of hardwood /m -!0.00005M.C. Melaaert 10]Average diameter of void of char /m^0.0001Average molecular weight of gas /g mol"8Average molecular weight of volaile /g mol - !76Average molecular weight of secondary tar /g mol- L110C.A. Koufopanos']4結論數的選取盡可能真實(shí)地模擬了實(shí)際過(guò)程,引入轉化在對生物質(zhì)熱解反應過(guò)程進(jìn)行細致分析的基礎率或溫度對涉及到的各種物理性質(zhì)進(jìn)行了修正。該上構建了生物質(zhì)單顆粒熱解綜合模型。為了研究熱模型在理論上較全面地描述了單顆粒生物質(zhì)熱解的解顆粒內部的二次反應數學(xué)模型中包含了一次焦復雜過(guò)程,可以利用它對顆粒內部的二次反應過(guò)程油二次裂解的動(dòng)力學(xué)過(guò)程描述了氣相熱解產(chǎn)物在進(jìn)行進(jìn)-步的研究?；谠撃Ｐ蛯τ材驹诹骰矡犷w粒內部的傳遞和二次反應過(guò)程考慮了各種氣相解過(guò)程中顆粒內部二次反應的數值模擬研究將在本產(chǎn)物在孔隙率不斷變化的生物質(zhì)顆粒內的生成、消文的下篇中詳細介紹。耗、積累以及在壓力驅動(dòng)下的輸運等行為。模型參參考文獻[1 ] Blasi Colomba Di. Comparison of semi-global mechanisms for primary pyrolysis of lignocellulosic fuelC[ J]. J Anal Appl Pyrol ,1998 ,47 1 ):43-64.[2] Liden A G. A kinetic model for the production of liquid from the flash pyrolysis of biomas{[J]. Chem Eng Comm , 1988 ,65( 2):207-221.[3] Chan W C R , Kelbon M , Krieger Barbara B. Modelling and experimental verification of physical and chemical processes during py-rolysis of a large biomass particlC J] Fuel , 1985 , 64( 11): 1505-1513.[ 4] Teng Hsisheng , Wei Yun-Chou. Thermogravimetric studies on the kinetics of rice hull pyrolysis and the influence of water treatment[J] Ind Eng Chem Res , 1998 ,37 10): 3806-3811.[5] Koch P. Specific heat of ovendry spruce pine wood and bark{ J] Wood Sci , 1969 , K 2 ): 203-214.[6] Raznjevik K. Handbook of Thermodynamic Tables and Charts[ M]. 2nd edition. New York : Hemisphere Publishing Corp. ,McGraw-Hill Book Company , 1976.[ 7 ] Koufopanos C A , Papayannakos N , Maschio G ,et al . Modelling of the pyrolysis of biomass particles. Studies on kinetics , yhermaland heat transfer efect[J] Can J Chem Eng , 1991 ,6% 4):907-915.[8] Blasi Colomba Di. Kinetic and heat transfer control in the slow and flash pyrolysis of solidJ] Ind Eng Chem Res , 1996 ,35 1):37-46. .[9] Boutin O , Ferrer M , Lede J. Radiant flash pyrolysis of ellulose一evidence for the formation of short life time intermediate liquidspecies[J] J Anal Appl Pyrol , 1998 ,47 1): 13-31.[ 10] Melaaen M C , Gronle M G. Modelling and simulation of moist wood drying and pyrolysi[ A] In : Bridgewater A V , Boocock DG B. Development in Thermochemical Biomass conversior[ C] UK. :1EA BIOENERGY , 1996.132- 148.4期余春江等:硬木熱解過(guò)程中顆粒內部二次反應的數值研究I .341NUMERICAL SIMULATION OF SECONDARYREACTIONS INSIDE HARDWOOD PARTICLE DURING PYROLYSISI . CONSTRUCTION OF SINGLE PARTICLE PYROLYSIS MODELYU Chun-jiang , ZHOU Jin-song ,LIAO Yan-fen , LUO Zhong yang , CEN Ke-fa( Clean Energy and Environment Engineering Key Lab of Minitry of Education , Zhejiang University , Hangzhou 310027 ,China )Abstract : The experimental research on secondary reactions inside particle during hardwood pyrolysis is greatly limit-ed by the level of up-to-date measure technology ，while numerical simulation based on comprehensive mathematicalmodel provides an excellent tool in this field. The construction of a single particle pyrolysis model is introduced in thispaper. The comprehensive model includes the kinetic sub-model of secondary reactions of primary pyrolytic product.To fulfill the exploration of secondary reactions inside particle , the model also carefully includes the processes of theyielding , consuming , accumulating or escaping of the major pyrolysis products in developing porous matrix of reactinghardwood. The parameters of the model are also selected carefully to simulate the real situation ; the conversion rateand temperature are introduced to correct the involved physical properties during pyrolysis. This comprehensive modelis capable of simulating the complex pyrolysis process in enough detail to do further research on secondary reaction.Key words : biomass ; hardwood ; pyrolysis ; secondary reaction ; mathematical modelFoundation item : China National Key Basic Research Special Funds project ( NKBRSF )( 2001CB409600 ); National Natural ScienceFoundation of China( NSFC )( 29976039 ); National Science Fund for Distinguished Young Scholars( 50025618 ).Author introduction : YU Chun jiang( 1973- ), male , Ph.D , Lecturer , major in Engineering Thermophysics , interesting field includesbiomass pyrolysis , biomass gasification , coal combustion and coal combined cycle system.Email : chunjiang @ cmee. zju. edu. cn[上接327頁(yè)][ 1 ] Robert G Jenkins , Satyendra P Nandi , Philip L , et al. Reactivity of heat-treated coals in air at 500 C[ J]. Fu-el ,1973 ,52( 4):288-293.[ 2 ] Levenspiel O. Chemical Reaction Engineering[ M ]. New Delhi : Wiley Eastern , 1974 ,368-371.[3]&[ 4] Strake P , Buchtele J , Nahunkora. Chemical structure of maceral groups of coa[ A] LiBQ. LiuZ Y.Proceedings of the 10th Intemational Conference on Coal Science[ C ] Taiyuan : Shanxi Science Technology ,1999 :113.[5] Haag W 0 , Olson D H , Rodewald P G. Hydrogen regeneration of coke-selectivated crystalline aluminosilicatecatalys[ P ] US Patent :4358395 , 1982-11-09.[ 6] Yan Shirun. Supercritical-phase Fischer- Tropsch synthesis for producing heavy hydrocarbons[ D ]. Institute ofCoal Chemistry , Chinese Academy of Sciences , Taiyuan , China , 1997.6. Publication of papers in the journal is free of charge. Authors will receive 10 free re-prints of his/her paper and 2copies of the joumal on which his/her paper is published.7. Please send the manuscript to the following address :Editorial OfficeJourmal of Fuel Chemistry and TechnologyP. O. Box 165 , TaiyuanShanxi ,030001