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热解是利用长焰煤、褐煤等低阶煤的重要技术之一，对我国尤其重要，因为其占我国煤炭资源总量的55%以上，但是国内外仍没有同时技术和经济可行的成功产业化低阶煤热解技术，是当前煤转化科学与技术研究的热点之一。中国科学院过程工程研究所通过在外热式煤热解固定床/移动床中加装内构件，以提高对煤颗粒的传热效率、调控热解反应，有效提高了焦油和热解气的产率和品质，形成了一种新型的外热式内构件固定床/移动床煤热解技术。在该工艺的设计放大过程中，需要对热解反应器中的流动、传热和热解反应等关键问题进行深入探索。本课题通过开展外热式固定床煤热解反应器的数值模拟研究，旨在探索煤热解反应机理、深入了解反应器内的传热、流动、反应特征，认识内构件对煤热解反应的调控机制，以期为该工艺的放大、优化和工程设计提供理论指导。 本论文主要研究内容和结论如下： （1）煤热解机理研究。基于Shinn次烟煤分子结构模型，采用反应分子动力学（ReaxFF MD）方法进行了煤分子的热解模拟。首先考察了温度、升温速率对热解产物分布的影响，发现随热解温度升高，热解程度加深，高温下焦油二次反应显著，呈现出焦油产率先增大后减小的趋势；升至相同的终温，升温速率低时高温下焦油二次反应明显，升温速率高时热解尚不完全，使得随升温速率的增大，焦油产率先增大后减小。通过将煤的升温热解过程分成三个阶段，分析了每个阶段的断键规律，探索了各类产物的生成路径，总结出煤热解机理模型：煤初始热解阶段，煤大分子结构中的-C-O-等桥键、杂原子官能团断裂生成CO2、H2O、CH4等小分子气体以及重质焦油。快速热解阶段，预热解后的煤分子片段中大量的芳香侧链及环烃断裂生成大量焦油、热解气，在该阶段焦油产率最大。第三阶段为焦油二次反应阶段，焦油优先发生裂解，生成热解气和更轻质焦油。温度更高时，芳香环结构发生交联缩合反应生成半焦。各热解产物的生成量及生成顺序则由生成该产物对应的官能团的数量及键能决定。 （2）传热模型的评价和选择。系统分析了适用于密相颗粒填充床的各种导热、辐射模型。通过模型预测值与文献中的实验数据对比，分析不同模型在颗粒粒径、温度、发射率及床层空隙率等变化时的预测能力，选择出预测性更好，适用性更广的导热、辐射传热模型：Zehner-Bauer-Schlunder（ZBS）有效导热系数模型和Breitbach-Barthels（B-B）有效辐射导热系数模型。并且通过耦合入传热模型的计算流体力学（Computational Fluid Dynamics, CFD）模拟和本文进行的石英砂传热实验的温度数据的对比，验证了传热模型的准确性。 （3）煤热解固定床反应器的CFD模拟研究。基于欧拉-欧拉双流体模型框架，耦合入传热模型、水分蒸发-冷凝模型、煤热解反应动力学模型，建立了初步的煤热解固定床反应器CFD模型。通过模拟结果与实验数据的对比验证了模型的合理性。模拟结果表明，加热壁面附近蒸发的水蒸气在反应器内部床层的冷凝，导致床层中心的恒温平台出现，推迟了热解过程；煤层的升温速率越大，挥发分的释放强度越大；由于水分的蒸发以及挥发分的释放，床层空隙率逐渐增大，而床层温度分布的不均匀性导致床层空隙率分布的不均匀，使得生成的挥发分倾向于穿过高温壁面，这将导致热解焦油产率的降低。表现出煤热解床层中温度-反应-流动间的耦合影响。 （4）内构件调控机制的研究。基于煤热解固定床反应器的CFD模型，同时考虑煤热解反应和焦油的二次裂解反应，对内构件的影响机制进行考察。模拟结果表明，中心集气管通过加强气体对流传热增强了床层的径向传热，传热板通过提高床层内的辐射传热增强了床层内的传热；相比集气管，传热板强化传热的效果更显著。提高升温速率更有利于焦油的生成，因此，在973 K较低的加热炉温时，由于焦油的二次裂解反应较弱，传热板的加入更有效地提高焦油的产率。而在1173 K较高的炉温时，焦油的二次裂解反应剧烈，传热板的加入虽然显著提高升温速率，但由于挥发分产物由床层“低温区”向壁面及传热板附近的“高温区”流动时焦油发生裂解，焦油产率变低；而当反应器中央加入集气管时，热解挥发分的流动路径发生改变，由“高温区”向“低温区”流动，极大地避免了焦油的高温裂解，显著提高了焦油产率。集合了传热板和集气管的反应器，既通过升温速率的显著升高增加了初焦油的产率，又通过挥发分流动路径的调控，降低了焦油的二次裂解，使得最终焦油产率变高。 （5）中试规模内构件移动床煤热解反应器模拟优化研究。通过CFD模拟，对内构件移动床反应器内的温度场、反应场、气体流场及产物分布等细节进行了分析。考察了颗粒粒径、炉温、煤质量流率等操作参数对模拟结果的影响，发现适当增大小颗粒碎煤的颗粒粒径、提高加热炉温、降低煤质量流率更有利于移动床内的传热，增加焦油的产率。考察内构件的设置影响的结果表明，相比传统移动床，内构件（同时加入传热板和集气通道）移动床的传热增强、床高降低、焦油产率显著增加，进一步验证了内构件的技术优势。而对于内构件移动床内传热板的数量及宽度，需要合理设计，传热板数量过多或宽度过大时，可能会导致焦油产率降低。 ;Pyrolysis is one of the important technologies for the use of low-rank coals such as sub-bituminous and lignite, which is particularly important for China. For more than 55% of China’s total coal reserve is low-rank coal. However, there are still no successful low-rank coal pyrolysis technologies industrially and economically feasible at home and abroad, which is one of the hottest topics in the research of coal conversion science and technology. A novel indirectly heated moving bed pyrolyzor with specially designed internals is being developed in Institute of Process Engineering, Chinese Academy of Sciences. In this pyrolyzor, internals are mounted to increase the heating rate and regulate the flow of the gaseous products so as to achieve high tar yield and good qualities. A better understanding of the flow behavior of gaseous products, the heat transfer characteristics and the pyrolysis reaction details is critical to the design and scale-up of this pyrolyzor technology. In this work, numerical simulations were conducted to explore the mechanisms of coal pyrolysis reaction, characterize the thermal, hydrodynamic and reaction performance, and understand the regulating principle, so as to guide the future scale-up, optimization and engineering design of this technology. The main research contents and conclusions of this work are as follows: (1) Investigation of the mechanisms of coal pyrolysis. Based on the molecular model of Shinn sub-bituminous coal, the pyrolysis process was simulated though reactive molecular dynamic (ReaxFF MD). The effects of pyrolysis temperature and heating rate on the distribution of pyrolysis products were firstly investigated. It was found that the degree of pyrolysis increased with the increase of pyrolysis temperature. Increasing pyrolysis temperature also strengthened the cracking of tar, leading the fact that tar yield first increased and then decreased. For same targeted pyrolysis temperature, lower heating rate led to serious secondary reaction of tar at later high temperature stage, whereas higher heating rate might lead to incompletion of the primary pyrolysis reaction. Thus, with the increase of heating rate, the tar yield presented a fist increase and then decrease trend. By dividing the temperature-rising process into three stages, the bond-breaking rules and the formation detail of each product was analyzed. It was found that the pyrolysis process of the investigated coal model could be summarized as follow: At the initial pyrolysis stage, the bridge bond such as -C-O- and heteroatom functional groups broke and formed heavy tar and smaller moleculars such as CO2, H2O, CH4. At the fast pyrolysis stage, a large amount of aromatic side chains and cyclic hydrocarbons of the coal model broken and formed large amount of tar and pyrolysis gas. The tar yield at this stage is highest. The third stage is the tar secondary reaction stage. At this stage, the cracking of tar dominated and formed pyrolysis gas and lighter tar. If the temperature was high enough, cross-linking condensation reactions of aromatic ring structure took place, leading to the generation of semicoke. The amount of each pyrolysis product and the generation order were determined by the amount and the bond energies of the corresponding functional group. (2) Evaluation and identification of heat transfer models. A variety of thermal conduction and radiation models for particle-packed beds reported in literature were evaluated and discussed. By comparing the results predicted by the models with the experimental data collected from literature, the sensitiveness of each model to particles size, temperature, emissivity and bed voidage was evaluated. It was found that the Zehner-Bauer-Schlunder (ZBS) model for thermal conduction and Breitbach-Barthels (B-B) model for radiative heat transfer presented the best prediction and broader application region. The accuracy of the identified heat transfer models was illustrated through the comparisons between the CFD simulation results and the temperature rising data measured from the quartz sand heating experiments. (3) CFD modeling of coal pyrolysis in fixed-bed reactor. By facilitating with heat transfer models, water evaporation-condensation model and coal pyrolysis reaction kinetics model, an Eulerian-Eulerian two-fluid simulation framework was built to simulate the coal pyrolysis behavior in fixed bed reactor. The simulation was first validated by comparing the predicted temperature evolution with experimental results. Simulation results indicated that the condensation of vapor formed in the near wall high temperature region led to the appearance of constant temperature platform in the radial interior region, and subsequently postponed the pyrolysis of coal in that region. Increasing the heating rate led to the intenser release of volatiles. The porosity of the bed increased with the evaporation of water and the release of volatile. Due to nonuniform distribution of bed temperature, the bed porosity presented clear nonuniform distribution in the radial direction, which subsequently led to the phenomenon that the generated gaseous products tended to flow through the porous zone adjacent to the heating wall, this would lead to a decrease of pyrolysis tar yield. The results showed the coupling effects of temperature-reaction-flow in the coal pyrolysis bed. (4) Influences of internals. The influences of internals on the pyrolysis performance were investigated by considering the secondary reaction of produced tar in the CFD simulations. The simulation results showed that installing central gas gathering pipe strengthened the contribution of convection heat transfer, and mounting heat transfer plates enhanced the contribution of radiative heat transfer. And both were in favor of increasing the heating rate along radial direction. Relatively, mounting heat transfer plate was more efficient in increasing heating rate. Higher heating rate favored the produce of tar. Thus, for furnace temperature of 973 K, due to the relatively weaker secondary reaction of tar, mounting heat transfer plates was more efficient in increasing tar yield. Nevertheless, for furnace temperature of 1173 K, compared with central gas gathering pipe, the tar yield was notably lower when heat transfer plates were mounted. This was due to the fact that the cracking of tar was intense under this temperature. Because the primary gaseous products generated in the relatively lower temperature zone tended to flow though the higher temperature near heating wall and plates zone before flowing out of the reactor, the yield of tar was significantly reduced. Installing central gas gathering pipe changed the flow direction of primary gaseous products: they tended to flow from the near wall high temperature zone to central low temperature zone, thus notably alleviated the cracking of tar and increased tar yield. The reactor integrated with the heat transfer plates and the gas gathering pipe not only increased the yield of primary tar through a significant increase in the heating rate, but also reduced the secondery cracking of tar through the regulation of primary gaseous products's flow path. Thus, this type of pyrolyzor gave the highest tar yield. (5) Numerical modeling and optimization of pilot scale moving bed pyrolyzor with internals. The temperature field, reaction field, flow structure inside the reactor and the distribution of pyrolysis products were detailedly investigated though CFD simulations. it was found that slightly increasing coal particle size and heating furnace temperature and decreasing the mass flow rate of coal charge were in favor of strengthening heat transfer and increasing tar yield. The influence of the internals was investigated and the results showed that compared with the traditional moving bed pyrolyzor, mounting internals (heat transfer plates and central gas gathering channel) could strengthen heat transfer, decrease bed height and increase tar yield. All these results clearly illustrate the advantage of the newly developed pyrolyzor. Simulation results also suggest that the number of and the distance between heat transfer plates need to be carefully optimized. Too many plates or closer distance might lead to the decrease of tar yield.
|钱亚男. 外热式内构件固定床/移动床煤热解反应器的数值模拟[D]. 中国科学院研究生院,2018.|
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