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生物质双流化床解耦燃烧系统的CFD模拟; CFD modeling of biomass decoupling combustion in dual fluidized bed system
耿淑君
学位类型博士
导师许光文
2017-01
学位授予单位中国科学院研究生院
学位专业化学工程
关键词Cfd模拟 颗粒混合/分级 停留时间分布 双流化床 鼓泡流化床
摘要双流化解耦燃烧是一种综合处理高含水含氮生物质残渣(本文以白酒糟为例)的高值化应用技术,该工艺主要包括鼓泡流化床热解器和提升管燃烧器两个核心设备。白酒糟首先通过螺旋加料器进入热解器,发生干燥和热解,生成的半焦再进入提升管燃烧器燃烧。同时,热解过程产生的热解气进入燃烧器,以还原燃烧反应过程中产生的NOx,降低NOx的生成和排放。该工艺的设计过程中,需要保证白酒糟在热解器中与固体热载体充分混合,并具有一定的停留时间,以确保热解转化率;而提升管燃烧器内部的气固流动结构决定了半焦的燃烧效率和NOx的还原效率。本论文通过开展计算流体力学(Computational Fluid Dynamics, CFD)数值模拟,对以上两个问题进行了探讨,以期为该工艺技术的放大、优化和工程设计提供理论指导。本论文的具体研究内容和主要结论如下:(1)鼓泡流化床中颗粒混合/分级特性研究。本论文第二章通过对比模拟与实验数据,系统研究了CFD模拟中的模型参数对模拟结果的影响,以为后期的数值模拟奠定基础。研究结果表明,数值模拟中所采用的颗粒温度方程、颗粒-壁面镜面反射系数、颗粒-颗粒弹性恢复系数和颗粒摩擦粘度对模拟结果有重要的影响。对流动结构的分析结果表明,气泡尺寸和颗粒轴向速度是影响双组分颗粒混合程度的关键因素。当鼓泡流化床尺寸较小时,壁面对大气泡的生成和颗粒的轴向运动存在明显的抑制作用,导致颗粒混合特性与大尺寸鼓泡流化床存在显著差异。研究发现存在临界尺寸(40dp),当鼓泡床厚度大于该值时,颗粒混合/分级特性与壁面条件无关。这一结果为研究鼓泡床内颗粒混合特性放大效应的实验设计提供了理论指导。(2)错流鼓泡流化床中颗粒停留时间分布研究。本论文第三章通过分析不同床层高度、颗粒循环流量和操作气速下的数值模拟结果发现,错流鼓泡流化床中颗粒停留时间分布与床内储料量和颗粒循环流量密切相关。通过适当的数据处理,不同操作条件下颗粒停留时间分布曲线的下降段可以归一为一个具有唯一参数的指数函数,且该参数与操作条件无关。基于这一结果,论文首次提出了颗粒停留时间分布上升段由CFD模拟得到、下降段由该指数函数预测的半经验预测方法。采用此半经验方法对不同尺寸错流鼓泡流化床中颗粒停留时间分布特性进行了预测,所得结果与实验结果很好吻合。进一步的研究结果表明,此经验方法可推广到双组分颗粒的鼓泡流化床系统。由于错流鼓泡流化床中颗粒停留时间分布存在长拖尾现象,直接基于CFD模拟研究颗粒停留时间分布特性计算量极大,采用该预测方法可缩短大量计算时间。该半经验方法的建立为计算双流化床解耦燃烧系统中大尺寸鼓泡流化床热解器内颗粒停留时间分布、定量设计连续颗粒流鼓泡流化床反应器提供了有效方法。(3)双流化床解耦燃烧系统中鼓泡流化床热解器的模拟研究。针对双流化床解耦燃烧工艺进一步优化的需求,本论文第四章对不同结构和布气条件下热解器内部的流动传热现象进行了模拟研究。研究表明,随着热解器壁面与水平线角度(60°、77°和90°)的增加,热解器中的流动死区逐渐减少,循环灰与白酒糟的混合程度逐渐增加,颗粒的停留时间逐渐减小;热解器底部采用非均匀气速分布时,增加靠近白酒糟入口一侧的操作气速有利于强化白酒糟与循环灰的混合。数据分析表明,尽管热解器内整体循环灰与白酒糟呈近似均匀混合状态,但它们的停留时间分布仍存在显著差异,循环灰的平均停留时间要显著大于白酒糟停留时间。对传热机理的分析结果表明,热解器内气固相间传热为主要的热量传递方式,而且循环灰与气体之间的传热速率远大于白酒糟与气体之间。(4)双流化床解耦燃烧系统中提升管燃烧器的模拟研究。本论文第五章对提升管中的气体混合和颗粒流动特性进行了分析,并系统考察了热解气入口条件、颗粒循环量及循环灰粒径对流动特性的影响,以期对提升管的操作参数进行优化。研究表明,提升管燃烧器处于非常稀疏的气力输送状态,平均固含率小于0.002;矩形截面提升管的边壁效应明显,边角处形成向下流动的颗粒层。对热解气入口条件的研究表明,随着高度增加,热解气分布逐渐均匀;保证总气量相同的条件下,增加热解气气量,可加速热解气的均匀分布。当热解气与一次风气量比大于0.144时,颗粒速度分布均匀性与热解气的分布规律一致;当风量较小时,一次风主导颗粒的运动特性。随着颗粒循环量和循环灰粒径的增加,颗粒停留时间增长
其他摘要Dual fluidized bed decoupling combustion is a technology of efficiently utilizing biomass wastes with high contents of moisture and nitrogen (spirits lees in this study). The system contains a bubbling fluidized bed pyrolyzer and a riser combustor. The raw spirits lees are continuously fed into the pyrolyzer through a screw feeder for drying and pyrolysis. The produced biomass char is then transported into the riser combustor for combustion. The pyrolysis gas is fed into the riser so as to reduce the generated NOX during combustion and decrease the emissions of NOX. In this technology, it is critical to guarantee that in the pyrolyzer the raw spirits lees and heat carrier ash are well mixed. Meanwhile, the residence time of spirits lees is long enough to ensure the sufficient progresses of drying and pyrolysis. At the same time, the combustion efficiency of biomass char and the reduction of NOX are decided by the fluidization characteristics of gas and solids in the riser. With regard to these requirements, in this work Computational Fluid Dynamics (CFD) simulations are conducted to investigate the hydrodynamic characteristics of the combustor and riser so as to guide the future scale-up and further optimization of this technology. The main contents and conclusions of this work are as follow: (1) Mixing/segregation behavior of binary particle mixture in bubbling fluidized bed. In Chapter 2, the influences of modeling parameters settings on the CFD simulation results are systematically investigated and discussed through comparing the simulation and experimental data. It is found that the predicted mixing/segregation behavior is closely related to the boundary wall condition, particle-particle restitution coefficient, frictional viscosity, as well as the transport equation for granular temperature. The analysis of flow structure shows that the size of gas bubble and particle vertical velocity play critical roles on the mixing extent of the binary mixture. For small-scale bubbling fluidized bed, the side walls suppress the size increase of gas bubble and hinder the vertical movement of particles. Such wall boundary effects lead to the results that mixing behavior of particle mixture is clearly different from that in large-scale bubbling fluidized bed. The simulation results suggest that there exist a critical bed depth (around 40 particle diameter) larger than which the predicted mixing behavior is independent from wall boundary conditions. Such results provide theoretical guidance for future experimental design on investigating the scale-up effect on particle mixing behavior in rectangular bubbling fluidized bed. (2) Solids residence time distribution (RTD) in rectangular cross-flow bubbling fluidized bed. In Chapter 3, the influences of bed height, solids flux and superficial gas velocity on solids residence characteristics have been studied. The simulation results show that, in the investigated rectangular cross-flow bubbling fluidized bed, solids residence time is closely related to solids inventory and solids flux. Further data analysis indicate that, through proper data processing, the descending parts of solids RTD profiles obtained from different operation conditions can be uniquely fitted by an empirical exponential function with only one parameter. And the parameter value is independent from operation conditions. Based on such result, a semi-empirical approach for predicting solids RTD is then proposed for the first time in the literature. In this approach, the ascending part of solids RTD profile is obtained through CFD simulation, and the descending part is given by the fitted empirical exponential function. This semi-empirical approach is then used to investigate solids RTDs in rectangular cross-flow bubbling fluidized beds with different sizes and validated by experimental data. Further investigation shows that this semi-empirical approach can also be applied to binary particle mixture system. It is worth noting that, due to the long tail nature of solids RTD in cross-flow bubbling fluidized bed, predicting solids RTD through CFD simulation is extremely time-consuming. The development of the above mentioned semi-empirical approach thus makes it be possible to predict the RTDs of solids in industrial-scale bubbling fluidized beds with a continuous particle flow and provides a feasible approach to design the pyrolyzer of the dual fluidized bed decoupling combustion system. (3) CFD modeling of the bubbling fluidized bed pyrolyzer. For further optimization of pyrolyzer, in Chapter 4 the influences of geometrical structure of pyrolyzer and the gas in the gas inlet conditions on the hydrodynamic and thermal-transfer characteristics are investigated. Simulation results show that increases the slope of pyrolyzer side wall leads to the strengthening of mixing between heat carrier ash and spirits lees, and the decreasing of average residence times of both solids. When the strategy of non-uniform gas distribution at gas inlet is adopted, increasing the gas velocity at the side close to spirits lees inlet is in favor of the mixing of heat carrier ash and spirits lees. It is found that, though the two solids are nearly well mixed, their residence time distributions present clear difference. And the average residence time of heat carrier ash is larger than that of spirits lees. The analysis of the heat transfer properties indicates that the thermal energy is mainly transferred through gas-solid inter-phase heat transfer. Meanwhile, the heat transfer rate between gas and ash is far more than that between gas and spirits lees.(4) CFD modeling of the riser combustor. In Chapter 5, the influences of pyrolysis gas inlet conditions, solids flux and the size of ash particle on gas mixing and particle flow characteristics are systematically investigated so as to guide the future optimization of the operation conditions in the riser. Simulation results demonstrate that in the riser the solids flow is in a very sparse pneumatic conveying state with the average solid volume fraction less than 0.002. Due to the rectangular cross-section characteristics, there exist obvious boundary wall effects and a layer of down-flow particles is formed at the cross-section corners. It is found that the distribution of pyrolysis gas tends to be uniform with the increase of bed height. For given total gas flux, increasing the pyrolysis gas flux leads to fast evening of cross-section distribution of pyrolysis gas. When the ratio of of pyrolysis gas flux to primary gas flux is larger than 0.144, the solids vertical velocity distribution is similar to that of pyrolysis gas. Whereas, if the ratio is smaller, the solids flow behavior is dominated by the primary gas. The simulation results also show that the solids residence time increases with the increases of solids fluxes and solids diameter. 
语种中文
文献类型学位论文
条目标识符http://ir.ipe.ac.cn/handle/122111/24312
专题研究所(批量导入)
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耿淑君. 生物质双流化床解耦燃烧系统的CFD模拟, CFD modeling of biomass decoupling combustion in dual fluidized bed system[D]. 中国科学院研究生院,2017.
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