A phenomenological model of the mechanisms of lignocellulosic biomass pyrolysis processes

A comprehensive particle scale model for pyrolysis of biomass has been developed by coupling the reaction mechanisms and transport phenomena. The model, which also accounts for the combined effect of various parameters such as particle shrinkage and drying, was validated using available experimental data from the literature. The validated model was then used to study the effect of operating temperature and biomass particle size, both of which strongly influenced the rate of biomass conversion. For example, for particle sizes less than 1 mm, a uniform temperature throughout the particle was predicted, thus leading to higher conversion rates in comparison to those in the larger particles. On the other hand, any increase in moisture content led to considerable decrease in the rate of biomass conversion. For the operating conditions considered in this study, the volumetric particle shrinkage also increased the decomposition of biomass to end products.     

Pyrolysis of large wood particles: a study of shrinkage importance in simulations

Shrinkage models have been developed and included in a model for the pyrolysis of large wood particles. Shrinkage is modelled in three different ways: uniform shrinkage, shrinking shell and shrinking cylinders. These models and a reference model without shrinkage are compared with experimental data for mass loss versus time during pyrolysis of birch cylinders at different temperatures. In the experiments a wood particle was introduced into a pyrolysis furnace held at constant temperature. The particle mass and volume were recorded using a balance and a video camera. Uniform shrinkage slows down the pyrolysis whereas shrinking shell and cylinder models enhance the pyrolysis rate. The effect was sufficiently small to be neglected given the uncertainty about some wood physical properties.     

Modeling the Pyrolysis of a Centimeter‐Sized Biomass Particle

Finite element modeling was performed to study the pyrolysis of centimeter‐sized biomass particles. The model proved to be accurate in predicting the intraparticle heat transfer during the pyrolysis of biomass particles at different operating conditions and for various particle properties. It was further applied to find the dominant factors of biomass physical properties influencing biomass pyrolysis, especially when the product distribution was considered. Simulation results indicate that biomass density and initial moisture content mainly affect the conversion time while have a limited effect on product distribution. With larger particle size the conversion time increases significantly. Higher tar yield with lower yields of char and non‐condensable gases were predicted at smaller particle size, indicating that the product distribution can be adjusted partially based on appropriate selection of particle size.     

Heat transfer and kinetics in the pyrolysis of shrinking biomass particle

The impact of shrinkage on pyrolysis of biomass particles is studied employing a kinetic model coupled with heat transfer model using a practically significant kinetic scheme consisting of physically measurable parameters. The numerical model is used to examine the impact of shrinkage on particle size, pyrolysis time, product yields, specific heat capacity and Biot number considering cylindrical geometry. Finite difference pure implicit scheme utilizing tri-diagonal matrix algorithm (TDMA) is employed for solving heat transfer model equation. Runge-Kutta fourth-order method is used for chemical kinetics model equations. Simulations are carried out for radius ranging from 0.0000125 to , temperature ranging from 303 to and shrinkage factors ranging from 0.0 to 1.0. The results obtained using the model used in the present study are in excellent agreement with many experimental studies, much better than the agreement with the earlier models reported in the literature. Shrinkage affects both the pyrolysis time and the product yield in thermally thick regime. However, it is found that shrinkage has negligible affect on pyrolysis in the thermally thin regime. The impact of shrinkage reflects on pyrolysis in several ways. It includes reduction of the residence time of gases within the particle, cooling of the char layer due to higher mass flux rates of pyrolysis products and thinning the pyrolysis reaction region.     

Modeling the thermochemical degradation of biomass inside a fast pyrolysis fluidized bed reactor

A fast pyrolysis process in a bubbling fluidized bed has been modeled, thoroughly reproduced and scrutinized with the help of a combined Eulerian/Lagrangian simulation method. The 3‐D model is compared to experimental results from a 100 g/h bubbling fluidized bed pyrolyzer including such variables as particle composition at the outlet and gas/vapor/water yields as a function of fluidization conditions, biomass moisture concentrations, and bed temperatures. Multiprocessor simulations on a high‐end computer have been carried out to enable the tracking of each of the 0.8 million individual discrete sand and biomass particles, making it possible to look at accurate and detailed multiscale information (i.e., any desired particle property, trajectory, particle interaction) over the entire particle life time. The overall thermochemical degradation process of biomass is influenced by local flow and particle properties and, therefore, accurate and detailed modeling reveals unprecedented insight into such complex processes. It has been found, that the superficial fluidization velocity is important while the particle moisture content is less significant for the final bio‐oil yield. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3030–3042, 2012     

Simplified model for calculation of devolatilization in fluidized beds

A devolatilization model based on simplification of the earlier model has been developed for fluidized bed conditions. It is simple and computationally fast enough to be incorporated as a submodel into a CFD code, but accurate enough to be suitable for different fuels including biomass with varying particle size, moisture, reactivity and shape. In this new model, the partial differential equation describing heat and mass transfer inside the particle is approximately converted to two differential equations. Drying is described to take place on a shrinking core and pyrolysis, which can take place simultaneously with drying, is described to take place at a specific “characteristic pyrolysis temperature”. The dependence of this temperature on parameters for the kinetics of pyrolysis, bed temperature and particle size can be determined. The model can be extended to include the case, where pyrolysis is considered to consist of parallel reactions of different components.     

生物质热裂解实验和模型研究(英文)

The analysis on the feedstock pyrolysis characteristic and the impacts of process parameters on pyrolysis outcomes can assist in the designing,operating and optimizing pyrolysis processes.This work aims to utilize both experimental and modelling approaches to perform the analysis on three biomass feedstocks—wood sawdust,bamboo shred and Jatropha Curcas seed cake residue,and to provide insights for the design and operation of pyrolysis processes.For the experimental part,the study investigated the effect of heating rate,final pyrolysis temperature and sample size on pyrolysis using common thermal analysis techniques.For the modelling part,a transient mathematical model that integrates the feedstock characteristic from the experimental study was used to simulate the pyrolysis progress of selected biomass feedstock particles for reactor scenarios.The model composes of several sub-models that describe pyrolysis kinetic and heat flow,particle heat transfer,particle shrinking and reactor operation.With better understanding of the effects of process conditions and feedstock characteristics on pyrolysis through both experimental and modelling studies,this work discusses on the considerations of and interrelation between feedstock size,pyrolysis energy usage,processing time and product quality for the design and operation of pyrolysis processes.     

烘焙提升纤维素类生物质热解气化性能的研究进展

生物质作为唯一含碳的可再生能源受到广泛关注。由于生物质具有含水率高、氧含量高、热值低等特性,在生物质热解气化中存在热转化效率低、焦油含量高、产品气热值低等问题。烘焙预处理对于改善生物质原料特性和提升热解气化性能具有积极的影响。本文阐述了烘焙预处理技术对于纤维素类生物质原料的疏水性、可磨性、元素组成、能量密度以及热解气化中产生的产品气组分、焦油组分、产品气热值等方面的影响。原料经烘焙预处理后疏水性、可磨性增强,热值增加,提升了原料品质。同时,经烘焙预处理的纤维素类生物质原料可明显提高热解气化性能,产品气中可燃气体组分含量、产量以及热值得到提升,焦油含量明显下降,提高了热解气化的产品气燃烧性能和利用品质。下一步应开展烘焙与热解气化耦合工艺及应用模式研究,提高生物质热解气化的整体经济性和产品附加值。     

响应面法优化竹材热裂解制备生物油的工艺研究

为了提高竹材生物质流化床快速热裂解制备生物油产率,利用响应面法优化其最佳工艺条件.试验选择热裂解温度(450～550℃)、气相停留时间(1.5～2.5 s)和物料粒径(0.18 ～0.22 mm)三因素作为独立变量,采用中心组合设计建立模型和考察上述因素对生物油收率的影响.结果表明三因素对生物油收率具有显著影响而它们之间的交互作用均不显著.依据所得到的模型,在各因素设定范围内获得的最佳工艺条件为:热裂解温度519.0℃、气相停留时间2.1s、物料粒径0.18mm,生物油理论收率为58.17％.在该条件下进行的三次重复试验,竹材生物油的实际平均收率为57.85％,与模型预测值58.17％无显著差异.响应面法简便、高效,优化结果能给生物质流化床快速热裂解制备生物油制备工艺提供一定的参考.     

富氮生物质热解气的分级冷凝特性研究

采用固定床热解反应系统对稻壳负载尿素进行了热解耦合分级冷凝的研究，实验采用三级冷凝的方法，对比了热解温度(400、500、600℃)和冷凝温度(30、60、90℃)对产物分布和富集的影响，研究了生物质富氮热解和分级冷凝的机理。结果表明：富氮热解促进了Maillard反应产生含氮杂环物；分级冷凝富集规律明显，一级生物油富集了高露点的酚类，二级生物油富集了低露点的含氮杂环物；提高热解温度可以增加二级生物油中含氮杂环物的含量和降低二级生物油水分含量，热解温度为500℃时，液体产物产率和酚类产物产率最大；提高冷凝温度能增强各级油组分的富集效果，并降低一级生物油水分含量，一级冷凝温度为90℃时，水分几乎完全富集在第二级中，且一级生物油酚类产物含量最高。     

Pyrolysis of single large biomass particle: Simulation and experiments

Pyrolysis and heat transfer characteristics of single large biomass particle were investigated using three-dimensional unsteady heat transfer model coupled with chemical reactions. The consumption of biomass and the production of products were simulated. Some experiments were designed to provide model parameters for simulation calculations. The simulation was verified by pyrolysis experiments of large biomass particle in a vertical tube furnace. The simulation results show the internal heat and mass transfer law during the pyrolysis of large biomass particle. When the biomass particle diameter is between 10 and 30 mm, for every 5 mm increase in particle diameter, the time required for complete pyrolysis will increase on average by about 50 s. When the pyrolysis temperature is between 673 K and 873 K, a slight decrease in the pyrolysis temperature will cause the time required for the biomass to fully pyrolyze to rise significantly. And the phenomenon is more obvious in the low temperature range. The results indicate that the numerical simulation agrees well with the experimental results.     

Effects of particle size on the fast pyrolysis of oil mallee woody biomass

This study aims to investigate the effects of biomass particle size (0.18-5.6 mm) on the yield and composition of bio-oil from the pyrolysis of Australian oil mallee woody biomass in a fluidised-bed reactor at 500 °C. The yield of bio-oil decreased as the average biomass particle size was increased from 0.3 to about 1.5 mm. Further increases in biomass particle size did not result in any further decreases in the bio-oil yield. These results are mainly due to the impact of particle size in the production of lignin-derived compounds. Possible inter-particle interactions between bio-oil vapour and char particles or homogeneous reactions in vapour phases were not responsible for the decreases in the bio-oil yield. The bio-oil samples were characterised with thermogravimetric analysis, UV-fluorescence spectroscopy, Karl-Fischer titration as well as precipitation in cold water. It was found that the yields of light bio-oil fractions increased and those of heavy bio-oil fractions decreased with increasing biomass particle size. The formation of pyrolytic water at low temperatures (<500 °C) is not greatly affected by temperature or particle size. It is believed that decreased heating rates experienced by large particles are a major factor responsible for the lower bio-oil yields from large particles and for the changes in the overall composition of resulting oils. Changes in biomass cell structure during grinding may also influence the yield and composition of bio-oil.     

水分对神东煤热解产物分布的影响

为考察原料煤中水分对神东煤热解产物分布的影响,通过格金试验和固体热载体小试试验研究了神东煤中水分对热解特性的影响,得到神东煤在不同水分下热解产物的分布规律。格金试验表明,水分对神东煤热解产物中焦油和热解气收率有显著影响。随着水分降低,神东煤热解产物中焦油收率从9.98%降至4.92%,热解气收率从8.47%上升至11.07%,热解水收率从2.74%上升到5.94%。小试试验结果与格金试验趋势基本相同。随着原料煤中水分的降低,焦油收率下降,热解气收率上升;未经干燥的原煤在不同温度下热解的焦油收率比干燥后煤样平均高2.17%,热解气收率平均低1.58%。热解温度对H2和CO比例影响较大,对其他气体比例影响较小。研究结果表明,水分对神东煤的热解过程及其热解产物分布有显著影响,热解原料煤中水分的增加有利于抑制神东煤热解水和热解气的生成,提高焦油收率,因此有望通过控制原料煤中的水分来调节热解产物的分布。     

Coupling particle scale model and SuperDEM-CFD for multiscale simulation of biomass pyrolysis in a packed bed pyrolyzer

An efficient biomass pyrolysis process requires a comprehensive understanding of the chemical and physical phenomena that occur at multi-length and time scales. In this study, a multiscale computational approach was developed and validated for biomass pyrolysis in a packed-bed reactor by integrating pyrolysis kinetics, a particle scale model, and Superquadric Discrete Element Method-Computational Fluid Dynamics (SuperDEM-CFD) in open-source code MFiX. A one-dimensional particle–scale model that discretizes the characteristic length of biomass particle into layers was developed to predict the intraparticle phenomena inside a single particle. The 1D model was validated by comparing it with a single biomass particle pyrolysis experiment. A recently developed SuperDEM-CFD model was employed to simulate the non-spherical particle–particle contact and fluid-particle interaction. The coupled model was applied to simulate the pyrolysis of cubic biomass particles in a packed bed and validated by comparing with experimental data. Simulation with and without particle–scale model was compared, and the effect of the gas–solid heat transfer models was also investigated.     

微波热裂解木屑的基础研究

利用微波热裂解的方法将木屑转化为生物能源,是一种非常有前景的处理和利用废弃生物质转化为能源的工艺,考察了多模谐振腔和单模谐振腔对热裂解的影响,并研究了含水率和加热速率在微波加热下对木屑热裂解的影响,讨论了微波加热与传统加热下生物质热解机理。研究发现单模谐振腔比多模谐振腔更有助于生物质的快速热解。孔隙中的水分是微波热解生物质的主要因素,可以提高加热速率。生物质热解在微波加热与传统加热下的最大差别在于前者是由里及外的加热,可以减少二次反应的发生,提高生物油的收率和质量,固体产物炭的性质也得到了改善。热解油主要是由脂肪族含氧化合物和芳香烃类物质的复杂混合物,热解气体产物主要为CO、CO₂、甲烷和乙烷。     

Modelling spatial distributions of moisture and quality during drying of granular baker's yeast

A distributed parameter model was developed to predict the drying behaviour of granular baker's yeast by setting up material and heat balances at the particle level. Temperature and moisture gradients were calculated for cylindrical and spherical granules. The performance of the model with two granule sizes was compared with experimental measurements. The model was initially used for non‐shrinking granules but later modified to take shrinkage into account. The reduction in granule size during the course of drying was estimated and good correspondence with experimental measurements was obtained. In addition to temperature and moisture gradients, the product quality was predicted during drying and compared to experimental results. The accuracy of the model was better than the lumped parameter model.     

Fuel pellets from biomass: The importance of the pelletizing pressure and its dependency on the processing conditions

The aim of the present study was to identify the key factors affecting the pelletizing pressure in biomass pelletization processes. The impact of raw material type, pellet length, temperature, moisture content and particle size on the pressure build up in the press channel of a pellet mill was studied using a single pellet press unit. It was shown that the pelletizing pressure increased exponentially with the pellet length. The rate of increase was dependent on biomass species, temperature, moisture content and particle size. A mathematical model, predicting the pelletizing pressure, was in good accordance with experimental data. It was shown that increasing the temperature resulted in a decrease of the pelletizing pressure. Infrared spectra taken from the pellets surface, indicated hydrophobic extractives on the pellet surface, for pellets produced at higher temperatures. The extractives act as lubricants, lowering the friction between the biomass and the press channel walls. The effect of moisture content on the pelletizing pressure was dependent on the raw material species. Different particle size fractions, from below 0.5 mm up to 2.8 mm diameter, were tested, and it was shown that the pelletizing pressure increased with decreasing particle size. The impact of pelletizing pressure on pellet density was determined, and it was shown that a pelletizing pressure above 200 MPa resulted only in minor increase in pellet density.