生物质快速热解装置研究进展

当今化石能源日渐枯竭和环境压力日益加重是亟待解决的问题,而生物质热解液化技术被认为是解决能源紧张的潜在方法,尤其是生物质快速热解技术。随着生物质快速热解技术与工艺不断成熟,需要快速热解装置不断放大以提高处理量,以实现生物质快速热解的工业化。生物质快速热解装置复杂且多样化,在装置的放大过程中,各系统的合理选择是难点。本文首先对生物质热解机理、快速热解过程的粒径选择和前处理进行了简述,并对快速热解流程中的进料系统、供能系统、热解反应器和快速冷凝系统4个关键系统进行了综述,着重介绍了快速热解反应器的类型及其特点,提供了该4个关键系统的选择及研究趋势。流化床反应器具有易放大、可以较好地实现自热式快速热解的优点,本文总结出流化床式反应器是目前研究的热点。在保证产品品质下,设备易放大、稳定实现自热式、流程能耗低、运行稳定安全等是快速热解装置未来的研究方向。     

稻壳快速热解制取生物质油的试验研究

目前生物质快速热解高温热解气主要利用间壁式冷却器进行冷凝,容易造成冷却管道的结焦堵塞问题,本试验根据流化床稀相输送特点、生物质的热解特性以及生物质油的冷凝收集特点,设计了生物质快速热解反应装置,改进生物质物快速冷凝系统,以稻壳为原料进行快速热解制取生物质油的试验研究,分别考察单因素反应温度、流化气量以及进料速度对生物质油产率的影响。试验表明:稻壳热解气能够快速顺利地得到冷凝,反应系统能够连续顺利运行,随着反应温度、流化气量、进料速度的增大,生物质油的产率都呈现先增大后减小的趋势。另外对产出的生物质油用气质联用设备进行了成分分析,得出了生物质油的主要成分,其中酸类、酮类、脂类以及酚类的含量相对较高。     

Experiment and computational fluid dynamics investigation of biochar elutriation in fluidized bed

In fluidized bed biomass fast pyrolysis, the biomass is converted to biochar and elutriated. The elutriation rate is a key parameter in reactor designs and operations. This research presents a video-based continuous measurement of biochar elutriation rate in a fluidized bed with sands and biomass as bed materials. The fluidized bed is simulated with the computational fluid dynamics—coarse-grained discrete element method (CFD-CGDEM) in MFiX. The fluidization behavior of nonspherical sands can be more accurately captured when a rolling friction model is used. The predicted elutriation rate is close to the experimental measurement when the particle size distributions are considered and the filtered drag with a shape correction is used. These results validated the accuracy of the MFiX-based CFD framework for the prediction of biochar elutriations in the fluidized bed biomass fast pyrolysis reactor.     

Modelling fast pyrolysis of biomass in a fluidized bed reactor

A compartmental one-dimensional model of a fluidized bed pyrolytic converter of biomass is presented. Reference conditions are those of non-catalytic fast pyrolysis of biomass in a shallow fluidized bed with external regeneration of the bed material. The fate of biomass and of the resulting char has been modelled by considering elutriation of biomass and char particles, char attrition as well as bed drain/regeneration. The course of primary and secondary pyrolitic reactions is modelled according to a semi-lumped reaction network using well-established kinetic parameters taken from the literature. A specific focus of the present study is the role of the heterogeneous volatile–char secondary reactions, whose rate has been modelled by borrowing a kinetic expression from the neighbouring area of tar adsorption/decomposition over char. The results of computations highlight the relevance of heterogeneous volatile–char secondary reactions and of the closely associated control of char loading in the bed. The sensitivity of the reactor performance to char elutriation and attrition, to proper management of bed drain/regeneration, and to control of gas phase backmixing is demonstrated. Model results provide useful guidelines for optimal design and control of fluidized bed pyrolyzers and pinpoint future research priorities.     

反应器结构对多段气化炉内颗粒分布的影响

针对多段气化炉(上部快速床、下部鼓泡床),采用MP-PIC(Multi-Phase Particle In Cell)方法模拟了多粒径煤粉颗粒的三维全循环流化过程,考察了鼓泡床与快速床床径比及鼓泡床和快速床之间的过渡段高度对气化炉内流动特性的影响。结果表明,基本工况下,大颗粒主要存在于下部鼓泡床中,细颗粒主要存在于上部快速床内,但细颗粒会通过旋风分离器和回料管再次进入鼓泡床参与循环。进入旋风分离器的大部分为半径622 ?m以下的小颗粒,无1216 ?m以上的大颗粒。旋风分离器对小颗粒的分离效率为99.75%,分离效率良好。增大床径比(即减小快速床直径),快速床中气速增大,整个气化床更快达到稳定状态,被夹带到快速床中的颗粒增多,所夹带的颗粒粒径增大。过渡段高度存在一个适当值(炉高0.6~1.0 m),升高或降低过渡段高度,快速床中颗粒浓度均增大,颗粒通量均升高,旋风分离效率降低。     

高铝矾土改性对稻草热解与气化特性的影响

在固定床中研究了高铝矾土改性剂及其浓度和反应温度对稻草热解产气特性的影响,选取固定床中最佳实验条件,在流化床中研究了改性高铝矾土床料对稻草气化特性及焦油产率的影响。结果表明,不同物质改性的高铝矾土对稻草热解产气特性的影响不同,4种物质提高稻草热解产气能力的顺序为CaCl2相似文献   

糠醛渣热解特性及热解挥发产物对其燃烧烟气原位控氮作用

通常,具有高含氮资源禀赋生物质在能源化利用过程中需控制NO_x排放。解耦燃烧是可适用于高含水、高含氮燃料的低NO_x燃烧技术,其对NO_x生成的抑制效果优于其他燃烧技术。为揭示解耦燃烧中热解挥发产物的原位控氮潜力、发展双流化床解耦燃烧技术,以糠醛渣为原料,借助固定床装置和双流化床装置,分别开展其热解特性和双流化床解耦燃烧近实际工况模拟研究。具体地,首先在固定床反应器中考察糠醛渣在不同温度下的热解产物分布,继而借助双流化床反应器考察了热解在线挥发产物对热解半焦同步燃烧烟气中NO_x的还原效果。结果表明：在500~700℃热解温度区间内,随温度的升高,半焦产率逐渐减少,从45.2%下降到39.8%;气体产率呈明显上升趋势,从12.4%上升到22.5%,CO、CH₄、H₂等还原性组分产率增加显著;焦油产率略有降低,从15.9%降低到12.9%;水分产率变化不大。双流化床解耦燃烧实验中,糠醛渣热解挥发产物对热解半焦同步燃烧所产烟气控氮效果良好,热解挥发产物对半焦燃烧烟气NO_x减排效果主要受热解温度、二次风占比影响,总过量空气系数ER=1.3,热解温度600℃、二次风过量空气系数ER₂=0.5时,糠醛渣热解挥发产物对相同热解条件下生成的半焦燃烧（900℃,过量空气系数ER₁=0.8）所产烟气原位控氮效果达到最优,NO_x减排率为54.80%。这表明,可通过控制热解挥发分产物产率、氧化程度,充分发挥挥发分的NO_x还原能力,从而明显改善解耦燃烧原位控氮效果。     

CFD modelling of the fast pyrolysis of biomass in fluidised bed reactors. Part B: Heat, momentum and mass transport in bubbling fluidised beds

The fluid-particle interaction inside a 150 g/h fluidised bed reactor is modelled. The biomass particle is injected into the fluidised bed and the heat, momentum and mass transport from the fluidising gas and fluidised sand is modelled. The Eulerian approach is used to model the bubbling behaviour of the sand, which is treated as a continuum. Heat transfer from the bubbling bed to the discrete biomass particle, as well as biomass reaction kinetics are modelled according to the literature. The particle motion inside the reactor is computed using drag laws, dependent on the local volume fraction of each phase. FLUENT 6.2 has been used as the modelling framework of the simulations with the whole pyrolysis model incorporated in the form of user-defined function (UDF). The study completes the fast pyrolysis modelling in bubbling fluidised bed reactors.     

Hydrodynamics of a Novel Biomass Autothermal Fast Pyrolysis Reactor: Flow Pattern and Pressure Drop

A novel biomass, autothermal, fast pyrolysis reactor with a draft tube and an internal dipleg dividing the reactor into two interconnected beds is proposed. This internally interconnected fluidized beds (IIFB) reactor is designed to produce high‐quality bio‐oil using catalysts. Meanwhile, the pyrolysis by‐products, i.e., char, coke and non‐condensable gases, are expected to burn in the combustion bed to provide the heat for the pyrolysis. On the other hand, the catalysts can be regenerated simultaneously. In this study, experiments on the hydrodynamics of a cold model IIFB reactor are reported. Geldart group B and D sand particles were used as the bed materials. The effects of spouting and fluidizing gas velocities, particle size, static bed height and the total pressure loss coefficient of the pyrolysis bed exit, on the flow patterns and pressure drops of the two interconnected beds are studied. Six distinct flow patterns, i.e., fixed bed (F), periodic spouted/bubbling bed (PS/B), spouted bed with aeration (SA), spout‐fluidized bed (SF), spout‐fluidized bed with slugging (SFS) and spouted bed with backward jet (SBJ) are identified. The investigations on the pressure drops of the two beds show that both of them are seen to increase at first (mainly in the F flow pattern), then to decrease (mainly in the PS/B and SA flow patterns) and finally to increase again (mainly in the SA and SF flow patterns), with the increase of the spouting gas velocity. It is observed that a larger particle size and lower static bed height lead to lower pressure drops of the two beds.     

生物质流化床快速裂解模型

研究了流化床内的生物质快速裂解模型,其特点是考虑了原料粒子在下部密相区和上部稀相区的不同反应历程.模型的计算结果表明,原料粒子和产物气体在反应器内的停留时间有较大的区别,其变化情况对裂解产物的分布有很大影响.由该模型得到的计算结果能和实验值很好吻合,表明它能较好地描述流化床反应器内生物质快速裂解的反应过程.结合计算数据对影响裂解结果的一些因素进行了分析.     

Coarse grained computational fluid dynamic simulation of sands and biomass fluidization with a hybrid drag

The bubbling fluidized bed reactor is widely used in fast pyrolysis of biomass. Discrete simulation of this reactor is challenging due to many sand particles and lack of accurate drag corrections accounting for the interaction of two different solid particles with different properties. In this research, the computational cost is reduced by using the coarse-grained computational fluid dynamic-discrete element method, where many sand particles are lumped into a larger numerical parcel. The Syamlal–O'Brien drag model is used for sand, while Ganser correction coupled with Gidaspow model is used for the nonspherical biomass particles. This hybrid approach shows superior behavior over other drag models using pressure drops as a benchmark. The predicted bed height and pressure fluctuating frequencies compare well with experiment. The mixing of biomass is close to perfect if the superficial velocity is larger than four times the minimum fluidization velocity.     

Computational fluid dynamics modeling of biomass catalytic fast pyrolysis in bubbling fluidized reactor: Effects of catalyst parameters on process performance

In this study, a computational fluid dynamics mathematical model has been developed for catalytic fast pyrolysis (CFP) of biomass based on multiphase flow, transfer process, and biomass pyrolysis reactions in a bubbling fluidized bed reactor. The multiphase fluid flow, and the inter-phase momentum and energy transfer processes are modeled with Eulerian multiphase formulas, representing the flows of gases and solids (catalyst and biomass) within the reactor. The biomass CFP reactions are described by using a two-stage, semi-global model. Specified secondary tar catalytic cracking process, which considers both intrinsic reaction rates and mass-transfer process, is embedded to the developed model by user-defined function. The model simulation results of pyrolysis product yield and distribution are compared with the experimental data with close agreement. The model is then employed to investigate the effects of structural properties of catalyst, such as specific internal area, average size of active sites, pore diameter, and tortuosity, on products yields and composition. The tar cracking process by the selected catalyst is proposed and the influences of adsorption capability of tar molecule on catalyst surface and external film mass transfer are also analyzed. The developed model can be solved with short computational time and thus it can be employed for further research and engineering designs of the catalytic pyrolysis of carbonaceous materials.     

Reduction of the Volatile Matter Evolution Rate from a Plastic Pellet during Bubbling Fluidized Bed Pyrolysis by Using Porous Bed Material

Rapid volatile matter evolution from high‐volatile fuels such as wastes and biomass is one of problems associated with fluidized bed incinerators and gasifiers. When volatile matter evolves rapidly in the vicinity of the fuel feed point, the mixing of volatile matter with reactant gas is poor, and therefore, unreacted volatile matter is expected to be released from the reactor. In the present work, reduction of the volatile matter evolution rate was attempted by employing porous solids as bed materials instead of nonporous sand. The effect of bed material on the onset of devolatilization was measured by use of a bench‐scale bubbling fluidized bed reactor. Volatile matter capture by the porous solids (capacitance effect) and the heat transfer rate within the bed, both of which affect volatile matter evolution rate, were also measured. Four types of porous solids, both with and without capacitance effect, were employed as the bed material. By employing porous solids without capacitance effect, the contributions of reduced heat transfer rate and capacitance effect to the delay of volatile matter evolution can be evaluated separately. For porous bed materials with a moderate capacitance effect (volatile matter capture of up to 20 %), the delay of the onset of devolatilization, which was measured by detecting the flame combustion of the volatile matter, was explained by the lower heat transfer between the fuel and bed. However, for a porous particle with high capacitance effect (volatile matter capture of 30 %), the capacitance effect also affected the delay of the onset of the flame combustion.     

Open source implementation of glued sphere discrete element method and nonspherical biomass fast pyrolysis simulation

In this research, a glued-sphere discrete element method was implemented in the open-source, computational fluid dynamics software MFiX. The implementation was verified using a cylinder-wall collision and then validated by simulating the packing and fluidization of nonspherical particles. The validated code was applied to simulate fast pyrolysis of nonspherical biomass particles in a bubbling fluidized bed. The glued sphere occupancy ratio was proposed to quantify the quality of shape resolution using glued sphere. Shape resolution showed significant influence on the packing height in the simulation of particle packing and an occupancy ratio of 80% was recommended. Its influence is minor in fully fluidized bed but can be eight times higher in packed bed. Three tested heat transfer models predicted similar yields of elongated biomass fast pyrolysis. The solver developed in this research can be used to simulate other multiphase reacting flows involving nonspherical particles.     

生物质快速热解制油试验及流程模拟

使用自主研发的流化床热解反应器对生物质热解制油进行实验研究,通过对不同实验温度450、500、525、550、580、610℃下得到的目标产物进行分析,得到了反应温度对生物油产率的影响规律。实验表明：550℃时,最大液体产率为42.5%（质量）;实验得到的不可冷凝气体的组分以CO、CO₂、CH₄和H₂为主,气相产物产率约为37.7%（质量）。在实验基础上,利用Aspen Plus流程模拟软件,建立了生物质热解制油工艺模拟流程,模拟分析了热解温度对生物油产率的影响,结果表明该模型能准确模拟实际热解过程,具有较好的适用性和可靠性。     

生物质快速热解流化床反应器气力进料特性

为探究不同工况下热解流化床反应器的气力进料特性,设计并搭建了流化床反应器气力进料冷态试验装置。生物质原料和床料分别采用落叶松颗粒和石英砂颗粒,通过试验测得了本装置的最小流化速度,研究了流化气速、喷动气速、流量比、初始静床高、石英砂粒径、落叶松粒径对流化床反应器气力进料特性的影响。试验结果表明：流化气速和喷动气速的增加均会提高进料率;流化气使床料流化并为落叶松颗粒提供进料空间,喷动气为落叶松颗粒提供动能,并平衡一部分床层压力;落叶松与石英砂粒径的增加对进料效果不利;流量比在1.9~2.7范围内进料率高且稳定性好。本文构建了生物质、床料与气体的三相流物理及数学模型,开展试验对模型进行验证,结果表明其预测误差为±13%。     

Production of aromatics by catalytic fast pyrolysis of cellulose in a bubbling fluidized bed reactor

Catalytic fast pyrolysis of cellulose was studied at 500°C using a ZSM‐5 catalyst in a bubbling fluidized bed reactor constructed from a 4.92‐cm ID pipe. Inert gas was fed from below through the distributor plate and from above through a vertical feed tube along with cellulose. Flowing 34% of the total fluidization gas through the feed tube led to the optimal mixing of the pyrolysis vapors into the catalyst bed, which experimentally corresponded to 29.5% carbon aromatic yield. Aromatic yield reached a maximum of 31.6% carbon with increasing gas residence time by changing the catalyst bed height. Increasing the hole‐spacing in the distributor plate was shown to have negligible effect on average bubble diameter and hence did not change the product distribution. Aromatic yields of up to 39.5% carbon were obtained when all studied parameters were optimized. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1320–1335, 2014     

Performance evaluation of a pilot scale vortexing fluidized bed combustor

To understand vortexing fluidized bed combustor (VFBC) performances, an investigation was carried out in a 0.45 m diameter and 4.45 m height pilot scale VFBC. Rice husks, corn, and soybean were used as the biomass feedstock and silica sand serving as the bed material. The bubbling bed temperature was controlled by using water injected into the bed. The experimental results show that the excess air ratio is the dominant factor for combustion efficiency. The in-bed combustion proportion increases with the primary air flow rate and bed temperature, and decreases with the volatile/fixed carbon ratio. The stability constant is proposed to describe the inertia characteristics of the vortexing fluidized bed combustor. The experimental results indicate that the stability of the VFBC increases with bed weight and primary air flow rate, but decreases with bed temperature.     

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     

Model of combustion and dispersion of carbon deposited on porous bed material during bubbling fluidized bed combustion

Porous bed materials capture volatile matter as carbon deposits during fluidized bed combustion of high-volatile fuels such as biomass and wastes. Carbon deposits burn in a dense bed mixed with bed materials; thereby enhancing horizontal dispersion of carbonaceous materials. Commercial scaling-up requires a model that simultaneously assesses carbon deposit combustion and horizontal solid dispersion. This study measured the carbon deposit combustion rate using a fluidized bed. A one-dimensional model of carbon deposit combustion in a fluidized bed is based on the carbon deposit burning rate. A two-dimensional model incorporates reactions and solid dispersion to predict the horizontal concentration profile. Experiments using a bubbling fluidized bed validated that model.