摘要
快速热解液化是将固态的农林剩余物转化为液态的生物油一种热化学转化手段,生物油可以作为燃料提供能量,也可以作为原料提供高附加值化学成分,具有巨大的发展潜力。快速热解是一个物理和化学反应相耦合的复杂过程,虽然目前建立了很多热解模型,如一维、二维、三维、湿材、缩核模型等,但模型还有许多地方需要完善,如模型方程中反应热源项的表达内容、模型适用情况等。本文结合实验室制备的搅拌式快速热解反应器,采用动力学热分析方法和偏微分数值求解有限元软件,研究了一维、二维、三维数学模型的适用情况;平板、圆柱体两种几何模型对颗粒温度分布的影响;第一类和第三类两种边界条件对颗粒温度分布影响;采用热解过程4个阶段动力学方程表示不同温度阶段的反应热源项对颗粒温度分布的影响;热解气体在x、y轴方向扩散对颗粒温度分布的影响;5种不同尺寸对热解过程中颗粒温度分布的影响。本文主要获得了如下成果:
(1)落叶松树皮和实木的热失重曲线有明显的区别;颗粒尺寸对热解主要阶段起始温度有影响;在一定范围内,失重率随含水率增大而增大。
(2)采用积分法和微分法及40种动力学机理函数,对4种不同尺寸和含水率的落叶松树皮进行了动力学分析,并分别求解获得了其热解过程中4个阶段的动力学参数和方程,含水率为15%,大小为0.8mm的树皮颗粒适合热解。
(3)粉碎后的落叶松树皮颗粒主要呈现片状和针状外形,在建立热解过程中几何模型时,平板和圆柱体几何模型比较符合实际情况。通过对两种几何模型的模拟计算,表明圆柱体几何模型热解时间比平板几何模型要减少约1倍;圆柱体几何模型受气体扩散影响较大,需要考虑各向异性和扩散压力。
(4)建立的平板三维传热模型如下,并可以对其进行简化得到一维和二维模型。建立的圆柱体二维模型方程为:
(5)树皮颗粒尺寸小于1mm时,可以采用一维或二维模型模拟其热解过程的温度分布;颗粒尺寸小于0.8mm时,不适合采用三维模型模拟温度分布;三维模型模拟大颗粒温度分布时,应尽量考虑物性参数的各向异性及热解气体的扩散阻碍等情况。
(6)对于尺寸小于0.8mm的树皮颗粒,采用第一类和第三类边界条件对模拟获得的温度分布没有显著影响。
(7)热解气体扩散方向对颗粒温度分布有显著的影响,沿着扩散方向,温度呈现由高到低趋势。
(8)采用动力学方程参与表达反应热(∑i=14(H*dα/dt)i)比Arrhenius常数K参与表达反应热(∑i=14(H*K)i)更加准确。
Fast pyrolysis is a thermochemical conversion method that transform agricultural and forestry residues into bio-oil, the bio-oil can supply enegy as fuel and the high added value chemical compositions also can be utilized by extraction. As it's a complex process coupled with the physical and chemical reactions, authorities in general use pyrolysis model to study the pyrolysis process. Some heat transfer models for pyrolysis has established at here and abroad, however, the models need to be improved, such as the expression of the reaction heat and use what kind of model in different conditions. In view of the above problems, we do researches in some aspects, including different dimensions, shapes, boundary conditions, pyrolysis gas diffusion, particle size and reaction heat expression. We has obtained four step kinetic equations of larch bark pyrolysis process and use the equations as the reaction heat expression.Then, we use the partial differential software do numerical solution for the heat transfer model equation of larch bark. Through the results of temperature distribution, we analysis the effects of dimension, gas diffusion, expression of reaction heat and particle size to pyrolysis process. The main results in this paper are as followed:
(1) The thermo-gravimetric curves of larch and larch bark have obvious differences; particle size can effect the starting temperature of pyrolysis steps; the weightlessness rate increases with the moisture content increases in a certain range.
(2)We study the kinetics of different moisture and particle size larch bark by integral and differential methods and forty mechanism functions, obtain the four pyrolysis steps kinetic parameters and equations,0.8mm larch bark with15%moisture content is suitable for pyrolysis.
(3)The larch bark particles after crushed mainly presenting flake and acicular; plate and cylinder geometrical model are more suitable for pyrolysis heat transfer model; pyrolysis time of cylinder geometrical model is half to plate model; however, we need consider gas diffusion pressure and anisortropy for the effect of diffusion to cylinder model.
(4) The three dimension heat transfer model equation for larch bark is:
Cylinder two dimension heat transfer model equation is:
(5)When the particle size (?)1mm, we can use both one and two dimension model simulate the temperature distribution of larch bark particle; three dimension is not suitable for the simulate the temperature distribution of the particle size which less-than0.8mm; we need to consider the anisortropy and gas diffusion when we use three dimension model for large particle.
(6)For the particle with0.8mm, the first and three boundary conditions have significant influence of the temperature distribution.
(7)The direction of gas diffusion is important to the temperature distribution, temperature has a decrease trend along the gas diffusion direction.
(8) The expression of heat reaction by kinetic equation (∑i=14=1(H*α/dt)i) is more accuate than the expression of Arrhenius rectant (∑i=14k(H*K)i)
引文
[1]蔡安明,岑可法.粗颗粒流化床内颗粒传热的试验研究[J].浙江大学学报,1985,19(4):82-91.
[2]陈海翔,刘乃安,范维澄.基于差示扫描量热技术的生物质热解两步连续反应模型研究[J].物理化学学报,2006.22(7):786-790.
[3]蔡均猛,易维明,何芳等.模式搜索法在生物质热解动力学中的应用[J].动力工程,2005.25(5):737-741.
[4]戴佳昆,杨立中,王亚飞等.热解分阶段反应模型中转换率问题的分析与讨论[C].中国工程热物理学会2008年燃烧学学术会议2008:西安.
[5]杜晓.落叶松原花色素的分级及精细化利用研究[D].四川大学,2006.
[6]高翔,周劲松,骆仲决,等.气固两相流中颗粒运动强化器壁对流传热的机理[J].化工学报,1998,49(3):294-302.
[7]景亮晶.木材热解过程中单颗粒内部传热模型的建立与研究[D].北京林业大学,2011.
[8]郭治,杜铭华,杜万斗.固体热载体褐煤热解过程的数学模型与模拟计算[J].神华科技,2010.08(2):69-72.
[9]贺海升,马纯燕,卜军等.针叶树皮土有机肥料发酵及理化性质检验[J].沈阳师范大学学报(自然科学版),2010.28(3):425-429.
[10]黄占华.落叶松单宁及其水处理剂制备技术[M].北京:化学工业出版社,2008.
[11]刘安源,刘石.流化床内单个颗粒传热特性的模拟[J].工程热物理学报,2006,27(5):835-837.
[12]刘乃安,王海晖,夏敦煌,等.林木热解动力学研究[J].中国科学技术大学学报,1998.28(1):40-47.
[13]林强.二氢槲皮素的生产工艺改进[D].西北大学,2009.
[14]赖艳华,马春元,施明恒.生物质燃料层热解过程的传热传质模型研究[J].热科学与技术,2005,4(3):219-223.
[15]赖艳华,吕明新,马春元,等.生物质多孔燃料层辐射加热热解过程的影响因素分析[J].太阳能学报,2011,32(5):735-740.
[16]刘一楠,敖宇佳,徐兰英等,酚醛树脂胶制作落叶松树皮板的初步研究[J].林业科技,2010.35(6):44-46.
[17]马隆龙.生物质能利用技术的研究及发展[J].化学工业,2007,25(8):9-14.
[18]米铁,陈汉平,杨国来等.农林生物质废弃物的热重实验研究[J].太阳能学报,2008,29(1):109-113.
[19]孙丰文,张齐生,孙达旺.落叶松单宁酚醛树脂胶粘剂的研究与应用[J].林业科技开发,2006.20(6):50-52.
[20]武锦涛,陈纪忠,阳永荣.移动床中颗粒接触传热的数学模型[J].化工学报,2006,57(4):719-725.
[21]王新运,陈明强,王君等.生物质热解动力学模型的研究[J].化学与生物工程,2009.26(7):61-63,74.
[22]项友谦,鞠睿,朱小星,等.垃圾焦自热式流化床气化平衡组成计算[J].煤气与热力,2010.30(1):B5-B8.
[23]余春江,周劲松,廖艳芬,等.硬木热解过程中颗粒内部二次反应的数值研究Ⅰ.单颗粒热解模型的构建[J].燃料化学学报,2002,30(4):336-341.
[24]余春江,张文楠,骆仲泱,等.流化床中单颗粒纤维素热解模型研究[J].太阳能学报,2002,23(1):87-95.
[25]赵建辉.生物质快速热裂解液化传热模型与试验研究[D].重庆大学,2006.
[26]朱孔远,谌伦建,马爱玲等.生物质与煤热解特性及动力学研究[J].农机化研究,2010.32(3):202-206.
[27]赵明,吴文权,卢玫,等.稻草热裂解动力学研究[J].农业工程学报,2002.18(1):107-110.
[28]Alves S, Figueiredo J L. A model for pyrolysis of wet wood[J]. Chem. Eng. Sci.1989,44: 2861-2869.
[29]Babu B Y, Chaurasia A S. Modeling and simulalion of pyrolysis:Effect of convective heat transfer and orders of reactions[A]. Proceedings of International Symposium and 55th Annual Session of IIChE, December 19-22, Indian Institute of Chemical Engineers, Hyderabad,2002a: 105-108.
[30]Bamford C H, Crank J, Malan K H. The combustion of wood. Part I. Proceedings of the Cambridge Philosophical Society[J].1946, University of Cambridge, UK.1946:166-182.
[31]Benkoussas B, Consalvi J L, Porterie B, et al. Modelling thermal degradation of woody fuel particles[J]. International Journal of Thermal Sciences,2000,46:319-327.
[32]Bernhard P, Elisabeth S, Christian B, et al. Measurements and particle resolved modelling of heat-up and drying of a packed bed[J]. Biomass and Bioenergy,2002 23:291-306.
[33]Bilbao R, Mastral J F, Ceamanos, J, et al. Modelling of the pyrolysis of wet wood [J]. J. Anal. Applied Pyro.1996,36:81-97.
[34]Bradbury A, Sakai Y, Shafizadeh F. A kinetic model for pyrolysis of cellulose[J]. J. Applied pol. Sci.1979,23:3271-3280.
[35]Capucine D, Chen L, Julien C, et al. Biomass pyrolysis:Kinetic modelling and experimental validation under high temperature and flash heating rate conditions[J]. J. Anal. Appl. Pyrolysis, 2009,85:260-267.
[36]Chan W R, Kelbon M, Krieger B B. Modeling and experimental verification of physical and chemical processes during pyrolysis of large biomass particle[J]. Fuel.1985,64:1505-1513.
[37]Chaurasia A S, Kulkarni B D. Most sensitive parameters in pyrolysis of shrinking biomass particle[J]. Energy Conv. Manage,2007,48:836-849.
[38]Di Blasi C. Physico-chemical processes occurring inside a degrading two-dimensional anisotropic porous medium[J]. Int. J. Heat Mass Transfer,1998,41:4139-4150.
[39]Fan L T, Fan L S, Miyanami K, Chen T Y, et al. A mathematical model for pyrolysis of a solid particle-effects of the lewis number[J]. Can.J.Chem. Eng.,55:47-53.
[40]Forest Products Laboratory 1987. Wood Handbook:Wood as an Engineering Material Agricultural Handbook 72. U.S. Department of Agriculture, Washington, DC.
[41]Freudlund B. Modelling of heat and mass transfers in wood structures during fire[J]. Fire Safe. J.1993,20:39-69.
[42]Galagano A, Di Blasi C. Modeling the propagation of drying and decomposition fronts in wood[J]. Combust. Flame,2004,139:16-27.
[43]Galagano A, Di Blasi C. Modeling wood degradation by the unreacted core-shrinking approximation[J]. Ind. Eng. Chem. Res.,2003,432:2101-2111.
[44]Grenli M G, Melaaen M C, Mathematical Model for Wood PyrolysissComparison of Experimental Measurements with Model Predictions[J]. Energy & Fuels,2000,14:791-800.
[45]Kansa E J, Perlee H E, Chaiken R F. Mathematical model of wood pyrolysis including internal forced convection[J]. Combust. Flame,1977,29:311-324.
[46]Kung H C. A mathematical model of wood pyrolysis[J]. Combust. Flame,1972,18:185-195.
[47]Kong Y Y, Hong W W, Dong K Z. Mathematical modelling of Collie coal pyrolysis considering the effect of steam produced in situ from coal inherent moisture and pyrolytic water[J]. Proceedings of the Combustion Institute,2009,32:2675-2683.
[48]Koufopanos C A, Papayannakos N, Maschio G, et al. Lucchesi,. Modeling of the pyrolysis of biomass particles. Studies on kinetics, thermal and heat transfer effects[J]. Canadain J. Chem. Eng., 1991,69:907-915.
[49]Lee C K, Chaiken R F, Singer J M. Charring pyrolysis of wood in fires by laser simulation[A]. Sixteenth Symposium (International) on Combustion,1976,1459-1470.
[50]Lee C K, Chaiken R F, Singer J M. Charring pyrolysis of wood in fires by laser or sphere[J]. Biomass and Bioenergy,1976,15:503-509.
[51]Liu N A, Fan W C, Dobashi R. Kinetics modeling of thermal decomposition of natural cellulosic materials in air atmosphere[J]. Journal of Analytical and Applied Pyrolysis 63,2002,303-325.
[52]Maa P S, Bailie R C. Influence of particle sizes and environmental conditions on high temperature pyrolysis of cellulosic material-1 [J]. Combust. Sci. Technol,1973,7:257-269.
[53]Mathew J H. A numerical model for biomass pyrolysis[M]. Iowa State,2005.
[54]Mathew J H, Kenneth M B. Modeling the impact of shrinkage on the pyrolysis of dry biomass[J]. Chemical Engineering Science,2002,57:2811-2823.
[55]Melaaen M C, Gronli M G. A generalized biomass pyrolysis model based on superim-posed cellulose, hemicelluloses and liginin kinetics[J]. Combust. Sci. Technol.1997,126:97-137.
[56]Miller R S, Bellan J. A generalized biomass pyrolysis model based on superimposed cellulose, hemicellulose and lignin kinetics[J]. Combust Sci Technol,1997,126:97-137.
[57]Miyanami K, Fan L S, Fan L T, et al. A mathematical model for pyrolysis of a solid particle-effects of the heat of reaction [J]. Can.J. Chem.Eng.1977,55:317-325.
[58]Moghtaderi B, Novozhilov V, Fletcher and D, et al. An integral model for the transient pyrolysis of solid materials[J]. Fire Mater.1997,21:7-16.
[59]Morten B L, Lars S, Peter G, et al. Devolatilization characteristics of large particles of tyre rubber under combustion conditions [J]. Fuel,2006,85:1335-1345.
[60]Prakash N, Karunanithi T. Advances in Modeling and Simulation of Biomass Pyrolysis[J]. Asian Journal ofScientific Research,2009,2 (1):1-27.
[61]Pyle K L, Zaror C A. Heat transfer and kinetics in the low temperature pyrolysis of solids[J]. Chem. Eng. Sci.1984,39:147-158.
[62]Ragland K W, Aerts D J. Properties of Wood for Combustion Analysis[J]. Bioresource Technology, 1991,37:161-168.
[63]Ragland K W, Boerger J C, Baker A J. A model of chunkwood combustion. [J] The Forest Products Journal,1988,38(2):27-32.
[64]Razvan V, Luc G, Mohand T, Cathy C, et al. Dimensional modelling of wood pyrolysis using a nodal approach[J]. Fuel,2008,87:3292-3303.
[65]Roberts A F, Clough G. Thermal degradation of wood in an inert atmosphere[A].9th International Symposium on Combustion, August 27-September 1, Academic Press Incorporated, New York, 1963:158-166.
[66]Saastamoinen J, Richard J R. Simultaneous drying and pyrolysis of solid fuel particles[J]. Combust. Flame,1996,106:288-300.
[67]Shen K K, Fang M X, Luo Z Y, et al. Modeling pyrolysis of wet wood under external heat flux[J]. Fire Safe. J.2007,42:210-217.
[68]Spearpoint M J, Quintiere J G. Predicting the piloted ignition of wood in the cone calorimeter using an integral model-effect of species, grain orientation and heat flux[J]. Fire Safe. J.2001,36: 391-415.
[69]Sreekanth M, Ajit K K, Leckner J. A semi-analytical model to predict primary fragmentation of wood in a bubbling fluidized bed combustor[J]. J. Anal. Appl. Pyrolysis,2008,83:88-100.
[70]Srivastava V K, Sushil, Jalan R K. Prediction of concentration in the pyrolysis of biomass materials-II [J]. Energy Conv. Manage.1996,37:473-483.
[71]Thurner F, Mann U. Kinetic investigation of wood pyrolysis[J]. Ind, Eng, Chem. Proc. Des. Dev. 1981,20:482-488.
[72]Yuen R K K, Yeoh G H, Vahl D G, Leonardi E. Modelling the pyrolysis of wet wood-Ⅰ Three-dimensional formulation and analysis[J]. Int. J. Heat Mass Transfer,2007a,50:4371-4386.