On the kinetics of the thermal decomposition of sulfates related with hydrogen water splitting cycles

The purpose of this paper is to present a set of kinetic results on the decomposition of MgSO₄, NiSO₄, ZnSO₄ and to discuss these in relation to the theoretical models. Isothermal and non-isothermal thermogravimetric studies as functions of temperature, gas flowrate, shape and solid additives have been performed. The gas effluent analysis in the three cases show that the products are SO₂ and O₂. For NiSO₄ and ZnSO₄ the contracting sphere model, chemically controlled, gives a good representation of the results. In the case of MgSO₄ a diffusion shrinking-core model had been derived. The reaction is controlled by the mass transfer in the product layer.     

Kinetic and thermodynamic studies of hydrogen adsorption on titanate nanotubes decorated with a Prussian blue analogue

In this paper, the kinetic and thermodynamic hydrogen adsorption characteristics of a novel composite comprising TiNT decorated with the Prussian blue analogue Cd₃Fe^III are investigated at high pressures and different temperatures. It is shown that boundary-layer (film) diffusion does not play a limiting role in the mass transport of hydrogen inside the composite material. The diffusion coefficient and time constant at different temperatures and pressures are calculated using an intra-particle diffusion model. The results suggest that molecular diffusion dominates Knudsen diffusion in the composite material. There are clear improvements in the mass transport characteristics compared to bulk Cd₃Fe^III. The Gibb's free energy is estimated by fitting isotherm equilibrium data to the Dubinin–Astakhov model and is used to calculate the enthalpy and the entropy of adsorption. The calculated value of enthalpy is characteristic of a physisorption process and is considerably higher than the activation energy for intraparticle diffusion, suggesting that the rate-limiting step of hydrogen is not mass transport to the adsorption sites.     

Numerical study of heat transfer in process of gas adsorption in a Cu-benzene- 1, 3, 5-tricarboxylic acid particle adsorption bed

The heat transfer mechanism in the gas adsorption of a Cu-benzene- 1, 3, 5-tricarboxylic acid particle adsorption bed is investigated by a model combining the lattice Boltzmann method with the grand canonical Monte Carlo method. The effects of distribution and thermal conductivity of the adsorption particle and types of adsorbates (CO₂, CH₄, and H₂) on gas adsorption are discussed. The results indicate that in the fluid region, the temperature peak in the particle random range, low particle thermal conductivity, and CH₄ adsorption are higher than those in other cases. In the solid region, the temperature peak is independent of the particle range, whereas the temperature peak in low particle thermal conductivity and CO₂ adsorption are higher than those in other cases. In the case of high thermal conductivity of the adsorbate and particle, the adsorbent with low adsorption heat is recommended to improve the adsorption performance of the adsorption bed.     

The desorption kinetics of the Mg(NH2)2 + LiH mixture

The kinetics of hydrogen desorption of the Mg(NH₂)₂ + LiH mixture has been studied by measuring desorption rates at various temperatures. A desorption kinetic model based on the Gauss-diffusion equation derived from Fick's second law is proposed to interpret the dehydriding reaction. X-ray diffraction (XRD) and transmission electron microscopy (TEM) are carried out to assist the foundation of the model. Results show that the kinetic model obtained can basically describe the curvature of the experimental data and the dehydriding activation energy can be represented by the diffusion activation energy (104.3 KJ/mol) of H^δ+ in the matrix. These indicate that the dehydrogenation can be described by H^δ+ diffusing through the product layer between reactants. Based on the results, the methods of exploring suitable dopants to create more vacancies in the matrix and activating the N–H with an electromagnetic field are suggested to improve the desorption kinetics.     

Modeling of particulate carbon and species formation in coflowing diffusion flames of ethylene

A model of species and particulate formation in laminar diffusion flames is presented. The kinetic model is based on the chemistry of fuel oxidation and pyrolysis, the formation of aromatics and their growth into particle nuclei, particle growth by surface reactions, coagulation, and finally particle oxidation. A sectional model is used for the particle phase. The sectional method divides the particle mass range into classes of species each with a rate equation for surface growth, coagulation, and oxidation. An inception model links the gas-phase mechanism with the smallest particle section. Predictions are compared with experimental data in two laminar coflowing diffusion flames of ethylene for which experimental profiles of stable species, aromatic compounds, high-molecular-mass precursor species, and soot are available. The predictions show good agreement with data for total particulates, defined as the sum of soot plus nano-organic carbon particles. The model has a continuous size distribution and is able to address nanoparticles which comprise a significant part of the total particle loading. A conclusion from the sensitivity analysis is that the inception process, the molecular growth process by aromatic addition on particle nuclei, and surface addition of C₂H₂ all play important roles which need to be studied in greater detail to predict the right size distribution and volume fraction of particulates formed in flames.     

Analysis of thermal effects in a spherical adsorbent pellet

A new model of nonisothermal adsorption is presented. The model is applicable to systems with nonlinear adsorption isotherm. Both intraparticle and film mass-transfer resistances are accounted in the model with the intraparticle mass-transfer rate being controlled by pore diffusion. An analytical solution of the model equations has been developed and a criterium for the existence of a maximum in the temperature uptake curves has been presented. A simple algorithm for the calculation of the maximum particle temperature has been illustrated. A detailed sensitivity analysis has shown the influence of various parameters on the temperature uptake curves. The model has been applied to the system water vapour–alumina and the predicted temperature uptake curves were compared with experimental results.     

A combined GCMC and LBM simulation method for CH4 capture in Cu-BTC particle adsorption bed

Two-dimensional Lattice Boltzmann Method (LBM) combined with grand canonical Monte Carlo (GCMC) approach is proposed to investigate the heat and mass transfer of methane (CH₄) adsorption in Cu-BTC particle adsorption bed. In interfacial boundary, saturation adsorption capacities are obtained by GCMC method to replace empirical values. The diffusion and adsorption heat in particle interior are fully considered. Langmuir-Freundlich model and linear fitting formula are used to calculate the saturation adsorption capacities in Langmuir adsorption kinetics model and the adsorption heat in heat transfer in LBM model at mesoscopic level, respectively. At micro-scale level, GCMC method is used to obtain parameters of Langmuir-Freundlich and linear fitting formula. The effects of porosity and particle size on CH₄ adsorption are discussed. Results show that the time for the saturation adsorption decreases with an increase in porosity and increases with increased particle size. In fluid region and particle interior, temperature peak decreases with increasing porosity and particle size. Nonuniformity of temperature exists both in fluid region and solid particles. The adsorption properties in Cu-BTC adsorption bed, such as the relation between external and internal heat and mass transfer, is predicted intuitively by GCMC and LBM combined simulation method.     

采用欧拉和拉格朗日混合模型对浓相颗粒流的研究

论述了一种组合了欧拉／欧拉和欧拉／拉格朗日方法的研究浓相气固流的新数值模拟。模型用基于Chapman-Enskog浓相气体理论的微元流体动理学方法计及了欧拉坐标系中的颗粒间相互作用。该模型在计算拉格朗日坐标系中颗粒湍流扩散时采取颗粒随机分布模型。用该模型得出的计算结果与已发表的实验结果进行了比较,能较好地吻合。图7参12     

Numerical modeling of an anode-supported SOFC button cell considering anodic surface diffusion

A two-dimensional isothermal mechanistic model of an anode-supported solid oxide fuel cell was developed based on button-cell geometry. The model coupled the intricate interdependency among the ionic conduction, electronic conduction, gas transport, and the electrochemical reaction processes. All forms of polarizations were included. The molecular diffusion, Knudsen diffusion, as well as the simplified competitive adsorption and surface diffusion were also considered. An electric analogue circuit was used to determine the effective hydrogen diffusivity. The model results showed good agreement with the published experimental data in different H₂–H₂O mixtures without any other calibrations after the parameter estimation according to the experimental data in baseline operating condition. The distributions of species concentration and current density were predicted and the effects of cathode area, gas components, and anode thickness on the cell performance were studied.     

A computational simulation of a hydrogen/chlorine single fuel cell

A two-dimensional, isothermal mathematical model of an H₂–Cl₂ single fuel cell with an aqueous HCl electrolyte is presented. The model focuses on the electrode reactions in the chlorine cathode and also includes the mass and momentum balances for the electrolyte and cathode gas diffusion layer. There is good agreement between the model predictions and experimental results. Distributions of physical parameters such as reactant and product concentrations, solution and solid phase potentials and local current densities and overpotentials as a function of cell voltage are presented. Effects of varying the initial electrolyte concentration and operating pressure are analysed. It was found that an electrolyte inlet concentration of 6 mol dm⁻³ gave the best cell performance and that an increase of operating pressure gave a steady increase of the fuel cell performance.     

Evaluation of different biomass gasification modeling approaches for fluidized bed gasifiers

To develop a model for biomass gasification in fluidized bed gasifiers with high accuracy and generality that could be used under various operating conditions, the equilibrium model (EM) is chosen as a general and case-independent modeling method. However, EM lacks sufficient accuracy in predicting the content (volume fraction) of four major components (H₂, CO, CO₂ and CH₄) in product gas. In this paper, three approaches—MODEL I, which restricts equilibrium to a specific temperature (QET method); MODEL II, which uses empirical correlations for carbon, CH₄, C₂H₂, C₂H₄, C₂H₆ and NH₃ conversion; and MODEL III, which includes kinetic and hydrodynamic equations—have been studied and compared to map the barriers and complexities involved in developing an accurate and generic model for the gasification of biomass.This study indicates that existing empirical correlations can be further improved by considering more experimental data. The updated model features better accuracy in the prediction of product gas composition in a larger range of operating conditions. Additionally, combining the QET method with a kinetic and hydrodynamic approach results in a model that features less overall error than the original model based on a kinetic and hydrodynamic approach.     

Modelling and experimental investigations on gasification of coarse sized coal char particle with steam

The steam gasification characteristics of coal char produced two sub-bituminous coals of different origin have been investigated through modelling and experiments. The gasification experiments are carried out in an Isothermal mass loss apparatus over the temperature range of 800–900 °C using a gas mixture of 65% steam and 35% N₂. A fully transient single particle gasification model, based on the random pore model, is developed incorporating reaction kinetics, heat and mass transport inside the porous char particle and the gas film. Stefan-Maxwell equation and Knudson diffusion are incorporated in the multi-component diffusion of species and pore diffusion. The model is validated with the experimental data of the present authors as well as that reported in the literature. The particle centre temperature is found to increase, then decrease and increase again to reach the reactor temperature finally, and the trend is more prominent for the larger particles. The pore opening phenomenon is more evident in SBC2 char, leading to a final char porosity of 0.65 vis-à-vis 0.52 in SBC1 and making it more reactive. Temporal evolution of contours of carbon conversion and concentration of other gaseous species like steam, H₂O, H₂, CO and CO₂ in the particle are computed to investigate the gasification process. A higher temperature is found to favour both the rate peak and the total production of H₂ for both the chars. The total H₂ production from SBC2 char is found to be 0.0189 mol and 0.0236 mol at 800 and 850 °C, while the same for SBC1 char is0.0232 mol and 0.0290 mol respectively. The reaction follows the shrinking core model at the outset, shifting to the shrinking reactive core model subsequently.     

Hydrogen crossover in high-temperature PEM fuel cells

In this paper, hydrogen crossover was measured in an environment of high-temperature proton exchange membrane (PEM) fuel cells using a steady-state electrochemical method at various temperatures (T) (80–120 °C), backpressures (P) (1.0–3.0 atm), and relative humidities (RH) (25–100%). An H₂ crossover model based on an MEA consisting of five layers – anode gas diffusion layer, anode catalyst layer, proton exchange membrane (Nafion 112 or Nafion 117), cathode catalyst layer, and cathode gas diffusion layer – was constructed to obtain an expression for H₂ permeability coefficients as a function of measured H₂ crossover rates and controlled H₂ partial pressures. The model analysis suggests that the dominant factor in the overall H₂ crossover is the step of H₂ diffusing through the PEM. The H₂ permeability coefficients as a function of T, P, and RH obtained in this study show that the increases in both T and P could increase the H₂ permeability coefficient at any given RH. However, the effect of RH on the permeability coefficient seems to be more complicated. The T effect is much larger than that of P and RH. Through experimental data simulation an equation was obtained to describe the T dependencies of the H₂ permeability coefficient, based on which other parameters such as maximum permeability coefficients and activation energies for H₂ crossover through both Nafion 112 and 117 membranes were also evaluated. Both Nafion 112 and Nafion 117 showed similar values of such parameters, suggesting that membrane thickness does not play a significant role in the H₂ crossover mechanism.     

The mechanism and kinetics study of Fischer‒Tropsch reaction over iron-nickel-cerium nano-structure catalyst

The kinetic of Fischer‒Tropsch reaction was investigated using the iron‒nickel‒cerium nano-structure catalyst synthesized by the hydrothermal method in the fixed-bed micro reactor for the first time. The kinetic tests carried out under operating conditions including the pressure of 2–10 bar, temperature of 230–250 °C, molar ratio H₂/CO of 1, and the gas space velocity of 3000 h⁻¹. Twenty-two set of reaction mechanisms were proposed on the basis of the adsorption nature of carbon monoxide, and hydrogen using the Langmuir‒Hinshelwood‒Hougen‒Watson, and Eley Rideal adsorption theories for the FT reaction. The rate equations of CO consumption were obtained based on the proposed reaction mechanisms. The best kinetic model was chosen by non-linear regression analysis and its kinetic parameters including activation energy, adsorption enthalpies of H₂, and CO were estimated 60.4, −3.24, and −65.7 kJ/mol respectively. The nanocatalyst was characterized by various techniques such as XRD, FESEM, and Brunauer–Emmett–Teller surface area measurements.     

Three-dimensional transport model of PEM fuel cell with straight flow channels

In this work, an isothermal, steady-state, three-dimensional (3D) multicomponent transport model is developed for proton exchange membrane (PEM) fuel cell with straight gas channels. The model computational domain, includes anode flow channel, membrane electrode assembly (MEA) and cathode flow channel. The catalyst layer within the domain has physical volume without simplification. A comprehensive set of 3D continuity equation, momentum equations and species conservation equations are formulated to describe the flow and species transport of the gas mixture in the coupled gas channels and the electrodes. The electrochemical reaction rate is modified by an agglomerate model to account for the effect of diffusion resistance through catalyst particle. The activation overpotential is predicted locally in the catalyst layer by separately solving electric potential equations of membrane phase and solid phase. The model is validated by comparison of the model prediction with experimental data of Ticianelli et al. The results indicate the detailed distribution characteristics of oxygen concentration, local current density and cathode activation overpotential at different current densities. The distribution patterns are relatively uniform at low average current density and are severely non-uniform at higher current density due to the mass transfer limitation. The local effectiveness factor in the catalyst layer can be obtained with this model, so the mass transport limitation is displayed from another point of view.     

Modeling study on a two-stage hydrogen purification process of pressure swing adsorption and carbon monoxide selective methanation for proton exchange membrane fuel cells

A two-stage hydrogen purification process based on pressure swing adsorption (PSA) and CO selective methanation (CO-SMET) is proposed to meet the stringent requirements of H₂-rich fuel for kW-scale skid-mounted or distributed proton exchange membrane fuel cell systems. The reforming gas is purified using dynamic adsorption model of PSA with activated carbon for initial purification and then kinetic model of CO-SMET with 50 wt% Ni/Al₂O₃ for CO deep removal. Sensitive analyses of the gas hourly space velocity, adsorption time and adsorption pressure etc. are studied. The results show that excellent H₂ purity and CO concentration below 1000 ppm for the initial target using the three-bed and four-bed PSA system at shorter adsorption time and higher pressure, and then CO concentration below 10 ppm with H₂ purity over 99.94% on CO-SMET. This work provides a small-scale and hydrogen-saving process for hydrogen purification can be achieved by the two-stage process.     

Investigations on the hydrolysis step of copper-chlorine thermochemical cycle for hydrogen production

Detailed investigations of CuCl₂ hydrolysis step of Cu–Cl thermochemical cycle were carried out on various aspects: (a) characterization and thermal properties of reactants/products using X-ray diffraction (XRD), thermogravimetry–mass spectrometry (TG-MS), scanning electron microscopy (SEM), temperature-programmed desorption (TPD), and extended X-ray absorption fine structure (EXAFS); (b) performance evaluation of fixed bed hydrolysis; (c) parametric optimization with respect to S/Cu, flow rate (gas hourly space velocity, GHSV), reaction duration, temperature, and particle size; and (d) monitored hydrolysis using isothermal TG experiments at 360°C, 370°C, 380°C, 390°C, and 400°C to derive kinetic parameters rate constant (k) and activation energy (E_a) on the basis of the shrinking-core model. 97% conversion to Cu₂OCl₂ at 17 630 h⁻¹ of GHSV, 400°C was achieved using ball-milled CuCl₂ (BM6), as compared with that of 55% over commercial un–ball-milled reactant, CuCl₂ (UBM). Correspondingly, higher k value of 2.84 h⁻¹ over BM6 as compared with 0.97 h⁻¹ over UBM reactant at 400°C was achieved. E_a for hydrolysis of BM6 was 93 kJ/mol, while it was 106 kJ/mol for UBM as derived from the Arrhenius plot. A probable pathway for CuCl₂ hydrolysis is proposed here. It was found to be diffusion controlled, and the particle size of reactant molecules affects the packing and diffusion length. Based on our investigations, it is very unlikely to get >99% phase pure product (Cu₂OCl₂). Cu₂OCl₂ is labile in nature and tends to transform into structurally similar and stable compounds CuO and CuCl₂.     

Adsorption of H2 in porous solid sorbents using a two-phase modelling approach

The increasing energy demand, depleting fossil fuel reserves and increasing greenhouse gas emissions have initiated the search for cleaner and more affordable energy sources. Hydrogen is considered a major energy source due to its high calorific value. The physisorption of H₂ on a porous material is an attractive alternative for its storage. This paper presents a two-fluid equation-based model benchmarked against an experimental work on hydrogen storage in an activated carbon packed bed reported in the literature. The computational model is implemented in Ansys Fluent 21.1.0 to solve the hydrodynamic and thermal interaction between the gas and solid phases along with the mass transfer due to adsorption. Adsorption isotherm is calculated from the modified Dubinin-Astakhov (D-A) equation, while the resultant mass transfer is determined from the Linear Driving Force (LDF) model. These equations are incorporated into the model with the help of User Defined Functions (UDF). The predicted pressure, local bed temperature, and adsorption values show a good match with the experimental results. In addition, the sensitivity of different thermal and material properties revealed that a denser adsorbent bed and a high value of thermal properties improve the performance of the H₂ storage in the tank.     

Steam methane reforming on LaNiO3 perovskite-type oxide for syngas production,activity tests,and kinetic modeling

The kinetic experiments at various working conditions (ie, temperature: 500°C-900°C, pressure: 1-15 bar, H₂O/CH₄ ratio [S/C ratio] of 1-2.5, and gas hourly space velocity of 900-1700 1/h) in the presence of LaNiO₃ perovskite-type oxide were done to consider and assess the outcome of steam methane reforming (SMR) and to build up its kinetic models depending on Langmuir-Hinshelwood method in a fixed bed reactor. The outcomes demonstrate, the methane conversion, H₂ and CO yields and formed CO₂ are affected by the working parameters. Elevated temperature is profitable for more methane conversion, H₂ and CO yield. While high temperature has a negative effect on mol% of CO₂ in outlet products. The high working pressure will not profit SMR respect to CH₄ conversion and products distribution. The efficacy of S/C ratio on the CH₄ conversion and CO yield depended on temperature range. H₂ yield considerably diminishes with an increment in S/C ratio, while the trend was reverse for CO₂ value in outlet gas. The accuracy of suggested kinetic model was evaluated by correlation and statistical tests. Outcomes exhibited that the obtained data were well anticipated through the suggested model, owning to presumption of nonideal gas phase and by utilizing reasonable equation of state of PPR78.     

An improved hybrid nanocomposites of rice husk derived graphene (GRHA)/Zeolitic imidazolate framework-8 for hydrogen adsorption

In this work, hybrid nanocomposites rice husk derived graphene (GRHA) and zeolitic imidazolate framework-8 (ZIF-8) were prepared for hydrogen adsorption. The main contribution of this study is the simplification of the synthesized GRHA/ZIF-8 hybrid nanocomposites. Besides that, the use of synthesized graphene from rice husk (RH) could help in overcoming environmental issue since disposal of RH is rather challenging. GRHA was obtained through calcining rice husk ash (RHA) at 900 °C for 2 h in a muffle furnace at atmospheric condition while the nanocomposite of GRHA/ZIF-8 was produced in free solvent condition using deionized water at room temperature for only 1 h. The N₂ adsorption-desorption indicated a type I isotherm. Interestingly, it was found that the BET specific surface area (BET_SSA) of GRHA/ZIF-8 showed enhancement up to 3 times higher as compared to pristine GRHA with the addition of 0.2 g of GRHA. From the experimental data, the adsorption of H₂ by nanocomposite GRHA/ZIF-8 obeyed the pseudo-second order kinetic model and intraparticle diffusion model with R² value up to 0.9890 and 0.8087 respectively at 12 bar. Moreover, the GRHA/ZIF-8 possessed highest hydrogen adsorption of 31.84 mmol/g at 12 bar. These impressive results are justified by the combination of ZIF-8's microporosity and GRHA's mesoporosity.