Fast redox kinetics of a perovskite oxygen carrier measured using micro-fluidized bed thermogravimetric analysis

Redox kinetics of oxygen carrier in chemical looping combustion (CLC) is important for reactor design and its oxidation enthalpy is important in order to establish auto thermal demonstration. Most published redox kinetics of oxygen carrier has been measured by thermogravimetric analysis (TGA) which can include additional diffusion limitations and thus underestimate the overall kinetics. In this study, the redox kinetics of a new perovskite oxygen carrier (CaMn_0.375Ti_0.5Fe_0.125O_3-δ) was measured by a novel micro-fluidized bed thermogravimetric analysis (MFB-TGA) method which can achieve real-time weight measurement of oxygen carrier in a fluidizing state with similar mass and heat transfer characteristics as in a CLC reactor. The experimental data from MFB-TGA were analyzed with a reactor model. The redox kinetics was described by a two-stage model of gas-solid reaction. The effect of temperature, O₂ concentrations and reducing gas type (H₂ and CH₄) on the redox kinetics in MFB-TGA was investigated and compared with other oxygen carriers such as natural manganese ore and ilmenite. It is observed that the oxidation of both manganese ore and ilmenite can be divided into two stages, a fast initial stage followed by a second slower stage, resulting in slower total oxidation rates. A very interesting finding is that there is only the fast initial stage for the oxygen carrier of CaMn_0.375Ti_0.5Fe_0.125O_3-δ, and the full oxidation of CaMn_0.375Ti_0.5Fe_0.125O_3-δ can be finished within ～4 s which is ～7.5 and ～30 times faster than that of manganese ore and ilmenite. The reduction kinetics of CaMn_0.375Ti_0.5Fe_0.125O_3-δ by H₂ is also ～5 and ～2.2 times faster than that of manganese ore and ilmenite, respectively. The kinetic parameters of three oxygen carriers were compared and the redox mechanism of CaMn_0.375Ti_0.5Fe_0.125O_3-δ was discussed.     

A DFT-based microkinetic theory for Fe2O3 reduction by CO in chemical looping

Redox kinetics of oxygen carrier in chemical looping is an important component for material preparation, reactor design and process demonstration. How to bridge the gap between the microscale density functional theory (DFT) and the macroscale redox kinetics and develop a first-principle-based theoretical model is still a challenge in the field of chemical looping. This study addresses this challenge and proposes a DFT-based microkinetic rate equation theory to calculate the heterogeneous kinetics of Fe₂O₃ reduction by CO in chemical looping. Firstly, the DFT calculation is adopted to search the reaction pathways and to obtain the energy barriers of elementary reactions. Secondly, the DFT results are introduced into the transition state theory (TST) to calculate the reaction rate constants and build the rate equations of elementary surface reactions. Finally, by considering the bulk diffusion, a rate equation is developed to bridge the gap between the elementary surface reactions and the grain conversion. In the theory, the reaction mechanism obtained from DFT and kinetic rate constants obtained from TST are directly implemented into the rate equation to predict the reduction kinetics of oxygen carriers without fitting experimental data. The accuracy of the developed theory is validated by experimental data of two Fe₂O₃ oxygen carriers obtained from the thermogravimetric analyzer (TGA). The microkinetic rate equation theory is based on the first principles calculation and can predict directly the redox kinetics of oxygen carriers without depending on the experimental kinetic data, therefore, it provides a powerful theoretical tool to screen the oxygen carrier materials and optimize the microstructure of oxygen carriers.     

Uncertainty propagation of chemical kinetics parameters and binary diffusion coefficients in predicting extinction limits of hydrogen/oxygen/nitrogen non-premixed flames

A comprehensive investigation of the uncertainties associated with the experimental and numerical evaluation of the extinction strain rate in hydrogen/oxygen/nitrogen non-premixed flames is presented in this work. The reported new experimental uncertainties of the extinction strain rate include several sources of uncertainties that typically affect the characterisation of velocity and boundary conditions of counterflow flames via particle image velocimetry. The uncertainties associated with the numerical determination of the extinction strain rate not only depend upon the selected chemical kinetics parameters but also on the binary diffusion coefficients. In order to identify the major sources of uncertainties in the chemical and diffusion models, a Monte Carlo based high-dimensional model representation analysis of the extinction curve was performed. Independent and simultaneous perturbations of relevant chemical kinetics and diffusion parameters have shown that the uncertainties associated with the binary diffusion coefficients are about a factor of 10 smaller than the uncertainty due to chemical kinetics parameters. Since the experimentally well known binary diffusion coefficient for hydrogen and nitrogen, , accounts for most of the propagated uncertainty of the diffusion model, it is shown here that only a reduction of the uncertainty of chemical kinetics parameters will have a significant impact in improving the accuracy of the extinction strain rate predictions.     

Analysis of RDX-TAGzT pseudo-propellant combustion with detailed chemical kinetics

A detailed model of steady-state combustion of a pseudo-propellant containing cyclotrimethylene trinitramine (RDX) and triaminoguanidinium azotetrazolate (TAGzT) is presented. The physicochemical processes occurring within the foam layer, comprised of a liquid and gas bubbles, and a gas-phase region above the burning surface are considered. The chemical kinetics is represented by a global thermal decomposition mechanism within the liquid by considering 18 species and eight chemical reactions. The reactions governing decomposition of TAGzT were deduced from separate confined rapid thermolysis experiments using Fourier transform infrared spectroscopy and time-of-flight mass spectrometry. Within the gas bubbles and gas-phase region, a detailed chemical kinetics mechanism was used by considering up to 93 species and 504 reactions. The pseudo-propellant burn rate was found to be highly sensitive to the global decomposition reactions of TAGzT. The predicted results of burn rate agree well with experimental burn-rate data. The increase in burn rate by inclusion of TAGzT is due in part from exothermic decomposition of the azotetrazolate within the foam layer, and from fast gas-phase reactions between triaminoguanidine decomposition products, such as hydrazine, and oxidiser products from the nitramine decomposition.     

Phase transition kinetics of vanadium pentoxide II. Theoretical results

Experimental data on the phase transformation kinetics in vanadium pentoxide due to surface oxygen loss are analyzed theoretically. A model for the process as a one-dimensional problem with oxygen loss from the surface and coupled interface and diffusion controlled growth modes is described. This model appears to match well the experimental data with reasonable numbers for the surface loss rate and diffusion constant. In particular, the model reproduces changes in the number of phase fronts as a function of electron beam flux. In addition, the analysis confirms that the effective diffusion constant is electron beam flux dependent.     

The effect of bulk gas diffusivity on apparent pulverized coal char combustion kinetics

Apparent char kinetic rates are commonly used to predict pulverized coal char burning rates. These kinetic rates quantify the char burning rate based on the temperature of the particle and the oxygen concentration at the external particle surface, inherently neglecting the impact of variations in the internal diffusion rate and penetration of oxygen. To investigate the impact of bulk gas diffusivity on these phenomena during Zone II burning conditions, experimental measurements were performed of char particle combustion temperature and burnout for a subbituminous coal burning in an optical entrained flow reactor with helium and nitrogen diluents. The combination of much higher thermal conductivity and mass diffusivity in the helium environments resulted in cooler char combustion temperatures than in equivalent N₂ environments. Measured char burnout was similar in the two environments for a given bulk oxygen concentration but was approximately 60% higher in helium environments for a given char combustion temperature. To augment the experimental measurements, detailed particle simulations of the experimental conditions were conducted with the SKIPPY code. These simulations also showed a 60% higher burning rate in the helium environments for a given char particle combustion temperature. To differentiate the effect of enhanced diffusion through the external boundary layer from the effect of enhanced diffusion through the particle, additional SKIPPY simulations were conducted under selected conditions in N₂ and He environments for which the temperature and concentrations of reactants (oxygen and steam) were identical on the external char surface. Under these conditions, which yield matching apparent char burning rates, the computed char burning rate for He was 50% larger, demonstrating the potential for significant errors with the apparent kinetics approach. However, for specific application to oxy-fuel combustion in CO₂ environments, these results suggest the error to be as low as 3% when applying apparent char burning rates from nitrogen environments.     

Approach to a Stationary State in an External Field

We study relaxation towards a stationary out-of-equilibrium state by analyzing a one-dimensional stochastic process followed by a particle accelerated by an external field and propagating through a thermal bath. The effect of collisions is described within one-dimensional formulation of Boltzmann’s kinetic theory. We present analytical solutions for the Maxwell gas and for the very hard particle model. The exponentially fast relaxation of the velocity distribution towards the stationary form is demonstrated. In the reference frame moving with constant drift velocity the hydrodynamic diffusive mode is shown to govern the distribution in the position space. We show that the exact value of the diffusion coefficient for any value of the field is correctly predicted by the Green-Kubo autocorrelation formula generalized to the stationary state.     

Redox kinetics of NiO/YSZ for chemical-looping combustion and the effect of support on reducibility

This paper explores the reaction kinetics of NiO supported on YSZ (Yttria Stabilized Zirconia) as an oxygen carrier for chemical looping combustion. Nickel particles with size less than 1 μm mixed with YSZ nano-powders are used to prepare the solid mixture, with 45% mol of NiO. Redox reactivity and oxygen carrying capacity are measured in a laboratory scale fixed bed reactor in the temperature range 500–1000 °C with different concentrations of the reactive gasses. Samples are subjected to repeated redox cycles using synthetic air (O₂+Ar) for oxidation, and H₂/H₂O/Ar mixtures for reduction. NiO/YSZ demonstrates superb cyclic regenerability starting with the 2nd cycle, with full utilization of its oxygen carrying capacity. Compared to pure nickel, pronounced improvement is achieved in the kinetics and oxygen utilization. Full reduction is achieved, and the presence of H₂O does not affect the reduction rate. Reactivity is also determined as a function of conversion. Global models of redox conversion are developed, in which surface chemistry and solid diffusion are considered. Oxidation exhibits the characteristics of a shrinking-core model with internal reactions at the Ni/NiO interface being the rate limiting step, and it is weakly temperature dependent. Reduction with H₂ generally exhibits surface chemistry limitation (adsorption-desorption), with surface product formation being the rate limiting step. YSZ significantly enhances ionic transport during oxidation and reduction. Reaction rate dependencies on conversion during the two steps suggest an optimal range for the oxygen carrying capacity of the material.     

Numerical investigation of the partial oxidation process in porous media based reformer

Numerical investigation of the thermal partial oxidation process of Methane in porous media based reformer is performed. A finite volume based CFD code, including radiation modeling, in combination with a detailed chemical kinetics scheme is used to perform the numerical simulation. A heterogeneous approach for the heat transport modeling in porous media (separate coupled energy equations for the gas and solid phases) was used. Validation of the model with experimental data is also performed. The model was able to predict the temperature behavior in the reformer reasonably well. However, the concentrations of H₂ and CO were under predicted while the H₂O concentration was over predicted.     

多孔颗粒内气体传递反应双阶段描述

本文采用双阶段模型描述多孔颗粒与气体的传递反应。反应分为两个阶段:气体反应物在扩散进入颗粒内部同时与颗粒发生反应的第一阶段;形成产物层后,扩散和反应用双区域模型来描述的第二阶段。分析发现,蒂勒数是描述过程特性的重要参数,同时反映反应和扩散速度的影响,且与总阻力存在一定关系。利用TGA进行了脱硫反应实验,分析了反应温度、颗粒大小等对反应的影响,实验结果很好地证实了理论分析结论。     

Chemical diffusion in calcium titanate

This work reports the gas/solid equilibration kinetics for the O₂/CaTiO₃ system. The electrical conductivity measurement was applied for monitoring the kinetics in the ranges of temperature 973-1323 K and oxygen partial pressure 10 Pa-72 kPa. It was found that the gas/solid equilibration kinetics for the polycrystalline CaTiO₃ specimen in the above experimental conditions is determined by bulk diffusion rather than by grain boundary conditions. The obtained data of the electrical conductivity vs. time were used for the determination of the chemical diffusion coefficient as a function of temperature at low and high p(O₂), respectively:

(1)     

Modeling the process of chemical regeneration of air in airtight habitable facilities

Physical experiments and mathematical modeling are used to study the kinetics of the reactions of carbon dioxide and water with potassium superoxide accompanied by oxygen release at various values of the temperature and humidity of the breathing gas mixture. The kinetics of the chemisorption is demonstrated to be limited by the rate of air regeneration in an airtight habitable facility. Experimental and analytical approaches are applied to determine the kinetic coefficients of the chemical reactions using the experimental data and a mathematical model of chemisorption kinetics. To perform the above chemical reactions, an original-design chemisorption reactor was developed, which contains plates with potassium superoxide nanocrystalline fixed on the fibers and pore surface of a fibrous polymer matrix. A mathematical model of chemical air regeneration is developed to calculate the guaranteed values of the parameters of the reactor and the protective effect time of the chemisorbent during which, at a given load, the reactor provides the appropriate concentrations of oxygen and carbon dioxide in the breathing gas mixture in an airtight habitable.     

Diffusion of nanoparticles in a rarefied gas

It is suggested to describe the diffusion of nanoparticles in rarefied gases in terms of the kinetic theory. For this purpose, the potential of interaction between a carrier gas molecule and a dispersed particle is constructed by summing the interactions of the given gas molecule with all atoms (molecules) of the dispersed particle. With this potential, a formula for the diffusion coefficient of the dispersed nanoparticle is derived. The dependence of the diffusion coefficient on the radius and temperature is studied. Analytical results are compared with experimental data. The well-known experimental Cunningham-Millikan correlation is shown to apply only in the range of near-room temperatures, for which the parameters of this correlation were determined.     

Monte Carlo PDF modelling of a turbulent natural-gas diffusion flame

A piloted turbulent natural-gas diffusion flame is investigated numerically using a 2D elliptic Monte Carlo algorithm to solve for the joint probability density function (PDF) of velocity and composition. Results from simulations are compared to detailed experimental data: measurements of temperature statistics, data on mean velocity and turbulence characteristics and data on OH. Conserved-scalar/constrained-equilibrium chemistry calculations were performed using three different models for scalar micro-mixing: the interaction by exchange with the mean (IEM) model, a coalescence/dispersion (C/D) model and a mapping closure model. All three models yield good agreement with the experimental data for the mean temperature. Temperature standard deviation and PDF shapes are generally predicted well by the C/D and mapping closure models, whereas the IEM model gives qualitatively incorrect results in parts of the domain. It is concluded that the choice of micro-mixing model can have a strong influence on the quality of the predictions. The same flame was also simulated using reduced chemical kinetics obtained from the intrinsic low-dimensional manifold (ILDM) approach. Comparison with the constrained-equilibrium results shows that the shape of the OH concentration profiles is recovered better in the ILDM simulation, and that the ILDM reduced chemical kinetics can correctly predict super-equilibrium OH.     

Validation of a mixture-averaged thermal diffusion model for premixed lean hydrogen flames

The mixture-averaged thermal diffusion model originally proposed by Chapman and Cowling is validated using multiple flame configurations. Simulations using detailed hydrogen chemistry are done on one-, two-, and three-dimensional flames. The analysis spans flat and stretched, steady and unsteady, and laminar and turbulent flames. Quantitative and qualitative results using the thermal diffusion model compare very well with the more complex multicomponent diffusion model. Comparisons are made using flame speeds, surface areas, species profiles, and chemical source terms. Once validated, this model is applied to three-dimensional laminar and turbulent flames. For these cases, thermal diffusion causes an increase in the propagation speed of the flames as well as increased product chemical source terms in regions of high positive curvature. The results illustrate the necessity for including thermal diffusion, and the accuracy and computational efficiency of the mixture-averaged thermal diffusion model.     

Reaction kinetics of the Pt, O2(g)|c-ZrO2 system: precursor-mediated adsorption

A micro kinetic model of the Pt, O₂(g)|c-zirconia electrode/electrolyte system was developed in state space form (model M3). The oxygen adsorption/desorption process was modeled as a precursor-mediated surface reaction. The surface diffusion of atomic oxygen and the electrochemical reduction of atomic oxygen near the three-phase boundary (tpb) were considered. It was shown that the simulated charge-transfer behavior of M3 is significantly different from models with ordinary Langmuir kinetics (model M2). The electrochemical rate constant was estimated from selected experimental data as k₁₀=(6.05±0.25)·10⁶ m³/(mol·s). From experimental results it was concluded that only one adsorbed oxygen species is relevant for the dynamic behavior. In porous Pt electrodes binary gas phase diffusion of oxygen in O₂/N₂ gas mixtures becomes relevant at oxygen partial pressures below 10⁻³ atm. The general procedure for state and parameter estimation can be well adopted for the investigation of further reaction mechanisms.     

Numerical analysis of chemical reaction processes in different anode attachments of a high-intensity argon arc

The attachment mode of arc on anode is closely related to the non-equilibrium chemical kinetics process of the anode region of arc. In this paper, the detailed chemical reaction mechanisms in the flow-affected region for both diffuse and constricted argon arc attachments are investigated by means of one-dimensional discharge coupled with a single-fluid, two-temperature model. The collisional-radiative model is used to examine the chemical reaction processes occurring in the anode region, including the arc centreline and fringe region. The numerical results are validated by comparison with available experimental data. The obtained radial distributions of electron temperature, electron density, excited states densities, ionization, and recombination processes reveal that different mechanisms dominate the diffuse and constricted arc-anode attachments.     

Role of vacancy and metal doping on combustive oxidation of Zr/ZrO2 core-shell particles

We studied self-propagated combustion synthesis of transition-metal-doped tetragonal ZrO₂ (t-ZrO₂) with first principles-based one-dimensional diffusion reaction model. The optimal reaction condition for the combustion process was investigated by calculating energetic stability and surface reactivity of oxygen vacancy defects on (101) surface termination of t-ZrO₂ using first-principles density functional methods. In the first-principles model, the surface was doped with 14 different metal impurities in the 4th and 5th row of the periodic table to examine the role of transition-metal doping on the combustion process. Results indicate that there are clear trends in the defect stability and reactivity depending upon the type of metal impurity and their relative location with respect to the oxygen vacancy. Surface density of states and charge density information also show that there is a trade-off between the vacancy stability and chemical activity of the surface defect states. Based on the thermodynamic information obtained from first principles, we analyze the combustion process of a Zr metal particle by using a one-dimensional diffusion-reaction model. The competition between the vacancy-assisted chemisorption and the vacancy diffusion results in an optimal point for rate of combustion reaction with respect to the vacancy stability. From this, we suggest a plausible screening strategy for metal-doping which can be applied at different temperatures and pressures, as well as with different particle sizes. Our analysis indicates that first-principles calculation provides key information that can be subsequently used for an optimization of the reaction rate for a self-sustained combustion process. An explicit inclusion of rates of defect and ionic transport will be introduced into our model in future work.     

煤粒均相着火规律的研究

煤粒均相着火规律的研究张军，傅维标（清华大学工程力学系北京１０００８４）关键词均相着火，简化模型，预报１前言煤粉颗粒既能发生均相着火，又能发生非均相着火山。对非均相着火，许多学者进行了研究，取得了很多有价值的成果。均相着火由于比较复杂，一直发展缓慢。...     

基于简单碰撞理论煤粉燃烧动力学模型的研究 -PARTⅠ:理论建模与热重实验

本文根据反应动力学的简单碰撞理论(SCT),建立了气固两相反应通用模型,进一步研究了煤焦燃烧和燃尽的统一动力学模型;粉煤悬浮燃烧时挥发分的析出模型也可包含在该模型中;该模型充分考虑了粉煤在热天平中与在炉内燃烧条件下氧气浓度和氧气可达比表面积变化规律的差异,并给出了计算活化能函数和氧气可达比表面积的新方法,可提高利用热天平获取的动力学参数对炉内煤粉燃烧速率预报的准确性。通过热重分析和已经报道的试验数据对模型的合理性进行了检验。