基于树形分叉结构的微通道甲醇重整制氢

针对甲醇蒸汽的微通道重整催化反应过程,建立了三维稳态多组分传输反应模型,利用数值模拟分析,分别研究了平行矩形微通道和树形分叉微通道网络在Zn_Cr/CeO2/ZrO2催化剂下的反应情况。通过双速率模型考察这两种流道中操作条件对甲醇水蒸汽重整制氢输运规律的影响,发现这两种微通道反应器促进了甲醇转化率和氢气产率的提高,且有助于反应器内温度分布均匀;同时相较矩形平行微通道,树形分叉微通道可以进一步提高甲醇的转化率、减小出口CO的含量,是一种理想的适用于质子交换膜燃料电池的制氢流道。     

微细腔内甲烷-湿空气重整特性试验与数值模拟

针对甲烷-湿空气在微细腔内的自热重整,建立了直径为2mm的微细直管试验系统,采用试验研究和三维数值计算两种方法分析了微反应器内甲烷-湿空气自热重整转化特性,并重点分析了反应温度、原料气组分比例变化对重整反应中甲烷转化率和氢气产率的影响.结果表明：在相同的甲烷流量工况下,随着温度的升高以及空/碳比和水/碳比的增大,甲烷转化率和氢气产率均增大,其中空-碳比对甲烷转化率的影响比对氢气产率的影响大;进气质量流量较小的体系比较大的体系在重整产氢方面的效果好.试验和数值模拟得到的结果比较一致.     

微通道尺寸对甲烷蒸汽重整性能的影响

联合使用可计算表面反应的化学反应动力学软件CHEMKIN4.0和CFD软件,对平板微反应器中Ni催化剂涂层上的甲烷蒸汽重整制合成气进行了数值计算,并结合表面活性组分的分布分析了微通道长度、高度对蒸汽重整性能的影响.计算结果表明:甲烷蒸汽重整受CO(S)的解吸速率控制;反应通道高度减小,从而减少反应物和产物在通道中扩散所需要的时间并增大反应控制组分CO(S)的表面覆盖率,使得甲烷的转化率和产物中的氢含量提高;反应通道长度增大,反应物与催化剂的接触时间延长,甲烷的转化率和氢含量提高.这对进行微通道甲烷蒸汽重整的实验研究以及平板微通道反应器的设计和优化提供了理论依据.     

微波辅助下CO_2/水蒸气联合重整CH_4

在微波辐照生物半焦作用下,对CO_2/水蒸气联合重整CH_4开展实验研究,考查联合重整反应特性,讨论联合重整反应对合成气品质、生物炭损耗、表面特性和官能团的影响规律.结果表明,联合重整反应可以促进反应气转化,使得90,min反应时间内合成气的H_2/CO气体体积比平均值升至0.923,H_2/CO气体体积比值更接近1;与CH_4干重整反应相比,联合重整过程生物炭损耗问题更突出,90,min后生物炭余重率为93.18%,,但联合重整反应可以延迟生物炭表面特性的恶化;联合重整过程主要消耗内酯基和羰基官能团,其中消耗较多的内酯基推测是CH_4蒸汽重整所致.     

二甲醚水蒸气重整制氢的模型和数值模拟

为研究二甲醚的水蒸气重整制氢过程,设计了一种带有隔热套、瓦片式加热通道和催化反应床的重整反应器.建立了反应器的数学模型,并利用COMSOL软件对其仿真.试验研究了反应气体温度、水蒸气与二甲醚的物质的量比和反应器结构参数对二甲醚转化率、氢产率的影响.模拟结果显示了二甲醚水蒸气重整制氢过程中的各组分质量分布及不同温度、不同...     

缅甸DAPEIN水电站溢流坝泄流三维数值模拟

以k-ε湍流模型封闭Reynods方程,采用VOF法追踪自由水面,对缅甸DAPEIN水电站表孔溢流坝的水流流动进行了三维数值模拟.结果表明,溢流坝的泄流能力、水面线、压力分布与流速分布等数值模拟结果与模型试验实测结果吻合,数值模拟结果可信,选择的紊流模型合理,求解方法可靠,可为工程设计提供全面、合理的参考.     

天然气重整SOFC/MGT混合分布式供能系统性能分析

针对固体氧化物燃料电池(SOFC)与微型燃气轮机(MGT)构成的混合分布式供能系统,首先建立了一种管式SOFC准二维数值模型,优化了辐射计算,提高了热传递模型的准确性;考虑了CO及H2同时作为燃料参加电化学反应,并完善了损失计算模型;最后采用所发展的系统性能预测模型,分别在内部重整和外部重整情况下,预测比较了两种SOFC/MGT混合系统的性能,结果表明外部重整系统在系统输出功率、CO2排放以及热应力分布方面都比内部重整系统具有优势,然而这种轻微的优势是需要额外增加外部重整器的设备投资换取的。     

甲烷在多孔介质中过滤燃烧制氢的数值模拟

对燃料极富条件下(φ>2.0)甲烷在多孔介质中的部分氧化重整制取氢气的工艺过程进行了数值模拟.建立了一维定常双温度模型,采用详细化学反应机理GRI1.2,着重研究燃烧区峰值温度、主要化学组分的分布规律和氢气的转化效率.讨论了混合气体流速、当量比等参数对甲烷转化特性的影响.数值模拟结果与文献[4]的实验结果基本吻合.     

基于复相催化反应动力学的电站SCR系统三维数值模型

选择性催化还原(SCR)系统的三维数值模型通常不单独考虑内、外扩散.本文建立描述电站SCR反应器内烟气流动、传热传质、复相催化反应的三维数值模型.在复相催化反应的模拟中,对内、外扩散及本征动力学分别进行了考虑.计算结果与试验数据吻合良好.建立的模型可用于电站SCR系统中,氨氮摩尔比大于1的状态下,组分浓度场、温度场、速度场的分析.研究为SCR系统三维数值模型的进一步完善提供了新的思路.     

燃煤锅炉选择性催化还原脱硝反应器结构的模拟优化

以某600 MW燃煤锅炉选择性催化还原(SCR)反应器为研究对象建立数学模型,对SCR反应器的结构进行了数值模拟优化,并对优化结果进行了物理模拟分析及验证.结果表明:导流板对均匀流场具有明显的作用;物理模拟试验中进入催化剂第1层前的COx浓度分布标准偏差小于2%,速度分布标准偏差接近14%,与数值模拟结果较吻合,从而验证了优化方案的合理性;物理模拟系统的风道在催化剂层前拥有5级调节装置,可对脱硝反应器的优化设计及运行起到参考作用.     

焦炭体系下添加剂对CO2重整CH4特性的影响

利用固定床试验装置,以SiO2、MgO和CaO为添加剂,在焦炭体系下进行了CO2重整CH4活性的试验研究,并分析了反应温度和CaO含量对重整反应的影响.利用热重分析仪和扫描电镜分别对重整过程中焦炭失重和反应后焦炭微观形貌进行研究,探讨了反应机理.结果表明:加入SiO2后,制得的焦炭催化活性降低,加入CaO和MgO后,制得的焦炭催化活性提高,且CaO优于MgO;温度对重整反应影响显著,随着温度升高,反应气转化率增加;CaO添加剂含量控制在15%~20%之间对反应速率的提高较合适,且CaO的添加使焦炭失重速率也有所提高.     

A numerical analysis of heat and mass transfer processes in a macro-patterned methane/steam reforming reactor

The presented paper focuses on a numerical analysis of a heat and mass transfer process in a novel type of methane/steam reforming reactor. The novelty of the macro-patterned reactor design lies in dividing a reformer into segments of various lengths and reactivity. Precisely, splitting the catalyst and filling the created empty volume with porous, non-reactive, thermal conducting material such as metallic foam. This approach allows for moderating a sharp temperature drop at the inlet of the reactor typical for the endothermic methane/steam reforming process. To analyze the considered system, the mathematical and numerical models of transport phenomena and the reaction kinetics were developed and implemented into an in-house solver. The kinetics of methane/steam reforming was taken from the literature. The outlet composition obtained from the kinetic model was compared with the experimental measurements and good agreement was found. The conducted numerical analysis includes cases that differ from a number and lengths of catalytic and non-catalytic segments. The obtained results indicate that the macro-patterned design is a promising strategy that allows for a significant improvement of temperature distribution in a reforming reactor. It was shown that the proposed approach could help to cut the cost of the catalyst material by allowing for the conversion of methane with a smaller amount of the catalyst close to the reference case.     

Effect of oxidation-reduction cycles on steam-methane reforming kinetics over a nickel-based catalyst

The paper presents a detailed analysis of the effect of a high number of oxidation-reduction cycles on the steam-methane reforming process over a commercial Ni-based catalyst under conditions of intrinsic kinetics. These cycles take place within the sorption-enhanced steam-methane reforming process for hydrogen production including sorbent regeneration step. Experiments of steam-methane reforming and reverse water-gas shift reactions were carried out to obtain a complete characterisation of the system kinetics. After 80 cycles, methane conversion increased, while carbon dioxide conversion remained fairly constant. After 125 cycles, methane showed no significant changes in conversion compared to the increase exhibited by carbon dioxide. The kinetic model of Xu and Froment fitted the experimental results to provide the activation energies of the main reactions, which presented an evolution with redox cycles in line with the trends observed for conversion. The reactions with methane initially reduced their activation energy, facilitating conversion, while the reactions with carbon dioxide initially kept the activation energy constant, subsequently reducing it.     

Hydrogen production by supercritical water gasification of coal: A reaction kinetic model including nitrogen and sulfur elements

Researches on reaction kinetics and mechanism are crucial to the application of hydrogen production technology by supercritical water gasification of coal from experiment to industrialization. Based on the migration mechanisms of nitrogen and sulfur in the process, this paper developed a general model including nitrogen and sulfur to study the generation path, consumption path and reaction rate of the gasification products. The parameters of the kinetic model were obtained by fitting the experimental data of the gasification products, and the activation energy of each reaction was obtained by the Arrhenius equation. By comparing the reaction rates among the various reactions, the reaction steps for controlling the production or digestion of the product could be obtained. The main source of ammonia production was pyrolysis of coal followed by steam reforming reaction of fixed carbon. The rate of ammonia contribution from ammonia synthesis was extremely low and could be ignored. The consumption path of ammonia was the decomposition reaction of ammonia though its rate was also slow. The pyrolysis reaction of coal was the main source of hydrogen sulfide, followed by the steam reforming reaction of fixed carbon. The difference of the concentration and reactivity between organic sulfur and inorganic sulfur caused the difference in the generation source of hydrogen sulfide in early and late stage of the gasification. The kinetic model can predict not only the production of hydrogen, methane, carbon dioxide, carbon monoxide, ammonia and hydrogen sulfide under different operating conditions, but also the products for different coal types, which may provide a theoretical basis for the targeted regulation of nitrogen and sulfur elements in supercritical water.     

Simulation of methane conversion to syngas in a membrane reactor: Part I A model including product oxidation

A one-dimensional dense membrane reactor (DMR) model has been developed to simulate the partial oxidation of methane to syngas. A combustion–reforming mechanism was adopted and the oxidation of reforming products, i.e. H₂ and CO, was considered. The performance of the DMR and a conventional fixed-bed reactor was compared and discussed. The results show that the incorporation of the product oxidation steps has a significant effect on the simulation results of a DMR and provides a reasonable explanation of the experimental data. The model is therefore more reasonable than those ignoring the product oxidation reactions.     

Modeling of fluidized bed membrane reactors for hydrogen production from steam methane reforming with Aspen Plus

Hydrogen production via steam methane reforming with in situ hydrogen separation in fluidized bed membrane reactors was simulated with Aspen Plus. The fluidized bed membrane reactor was divided into several successive steam methane sub-reformers and membrane sub-separators. The Gibbs minimum free energy sub-model in Aspen Plus was employed to simulate the steam methane reforming process in the sub-reformers. A FORTRAN sub-routine was integrated into Aspen Plus to simulate hydrogen permeation through membranes in the sub-separator based on Sieverts' law. Model predictions show satisfactory agreement with experimental data in the literature. The influences of reactor pressure, temperature, steam-to-carbon ratio, and permeate side hydrogen partial pressure on reactor performances were investigated with the model. Extracting hydrogen in situ is shown to shift the equilibrium of steam methane reactions forward, removing the thermodynamic bottleneck, and improving hydrogen yield while neutralizing, or even reversing, the adverse effect of pressure.     

Theoretical modeling of a directly heated solar-driven chemical reactor

A theoretical formulation for calculating the performances of a solar-driven catalytic chemical reactor was developed. It accounts for the spatial distribution of the deposition of primary energy within the receiver, the heat transfer into the catalytic bed and the thermochemical endothermic reaction, chemical composition and flow distribution within the reactor. The theory set forth was applied to analyze results obtained in a solar furnace with a directly heated U-shaped tubular reactor, wherein catalytic carbon dioxide reforming of methane occurred. We find that the receiver/reactor assembly acts as a self-regulating system. Beyond a fractional catalytic bed length of 0.14, solar energy can be converted primarily into chemical enthalpy. The fluid temperature gradient monitors the heat balance by adjusting the overall rate of conversion to the rate at which energy is being transferred through the reactor walls. Under certain circumstances, the process may be heat-transfer limited or controlled by chemical thermodynamics. A good fit between theory and experiment and accountability of all the intricate details in the various calculated performances of the receiver/reactor system support the theoretical model set forth in this study. We offer it as a tool for simulating future experimental results and for designing solar-driven reactors.     

Simultaneous production of two types of synthesis gas by steam and tri‐reforming of methane using an integrated thermally coupled reactor: mathematical modeling

In this work, tri‐reforming and steam reforming processes have been coupled thermally together in a reactor for production of two types of synthesis gases. A multitubular reactor with 184 two‐concentric‐tubes has been proposed for coupling reactions of tri‐reforming and steam reforming of methane. Tri‐reforming reactions occur in outer tube side of the two‐concentric‐tube reactor and generate the needed energy for inner tube side, where steam reforming process is taking place. The cocurrent mode is investigated, and the simulation results of steam reforming side of the reactor are compared with corresponding predictions for thermally coupled steam reformer and also conventional fixed‐bed steam reformer reactor operated at the same feed conditions. This reactor produces two types of syngas with different H₂/CO ratios. Results revealed that H₂/CO ratio at the output of steam and tri‐reforming sides reached to 1.1 and 9.2, respectively. In this configuration, steam reforming reaction is proceeded by excess generated heat from tri‐reforming reaction instead of huge fired‐furnace in conventional steam reformer. Elimination of a low performance fired‐furnace and replacing it with a high performance reactor causes a reduction in full consumption with production of a new type of synthesis gas. The reactor performance is analyzed on the basis of methane conversion and hydrogen yield in both sides and is investigated numerically for various inlet temperature and molar flow rate of tri‐reforming side. A mathematical heterogeneous model is used to simulate both sides of the reactor. The optimum operating parameters for tri‐reforming side in thermally coupled tri‐reformer and steam reformer reactor are methane feed rate and temperature equal to 9264.4 kmol h^?1 and 1100 K, respectively. By increasing the feed flow rate of tri‐reforming side from 28,120 to 140,600 kmol h^?1, methane conversion and H₂ yield at the output of steam reforming side enhanced about 63.4% and 55.2%, respectively. Also by increasing the inlet temperature of tri‐reforming side from 900 to 1300 K, CH₄ conversion and H₂ yield at the output of steam reforming side enhanced about 82.5% and 71.5%, respectively. The results showed that methane conversion at the output of steam and tri‐reforming sides reached to 26.5% and 94%, respectively with the feed temperature of 1100 K of tri‐reforming side. Copyright © 2013 John Wiley & Sons, Ltd.     

Fundmental aspects of catalysed coal char gasification

A brief review of the basic aspects of catalysed coal char gasification is presented. Kinetics and mechanisms of catalysed and uncatalysed gasification reactions of coal char with steam, carbon dioxide and hydrogen are discussed. Mass transport effects and internal structure of coals are shown to be important in determining rates of these reactions. the importance of the type of catalyst used is also discussed. Such factors as catalyst cations and anions, the method by which the catalyst is contacted with the coal char, and physical and chemical states of the catalyst both prior to and during reaction are shown to be important in the gasification process. Finally, research instruments and equipment used recently for studies in catalysed gasification are reviewed. These include various types of reactor systems for following the course of these reactions and analytical instruments for assessing the physical and/or chemical state of the catalysts and/or coal char both prior to and during the gasification reactions.     

Utilizing carbon dioxide as a regenerative agent in methane dry reforming to improve hydrogen production and catalyst activity and longevity

The study evaluates the effect of forced periodic cycling between methane dry reforming and carbon regeneration using a gasifying agent, such as carbon dioxide. The activity of Ce-promoted Ni–C_O/Al₂O₃ catalyst was evaluated in a methane dry reforming process using a fixed-bed reactor under steady-state and periodic operation. Forced cycling reactions (reforming and regeneration) were conducted by manipulating the reactor feed between methane dry reforming and catalyst gasification using CO₂ at cycle periods of 10, 20, and 30 min, and cycle splits of 0.8, 0.6, and 0.4. The physicochemical properties of fresh and spent catalysts were evaluated using several characterization techniques, such as the BET surface area, H₂-chemisorption, and XRD. The results confirmed that methane dry reforming under periodic cycling provides an opportunity to improve methane conversion and increase the catalyst activity and longevity because of the periodic interruption of coke deposition. In particular, methane conversion deteriorated from 68% to 37% under steady-state within five hours of reforming, whereas a modest decrease in methane conversion (from 68% to 63% for a cycle period of 10 min and cycle split 0.8) was observed under periodic operation conditions. The results of catalyst characterization also demonstrated that the on-line removal of carbon during CO₂ regeneration did not lead to any structural effect on the catalyst properties, and it absolutely restored the catalyst properties up to the values measured for the fresh catalyst.