Global Kinetic Model Research Articles

The effect of seed electrons on the repeatability of atmospheric pressure plasma plume propagation. II. Modeling

One of the distinguishable features of plasma jets compared with the traditional streamers is their repeatable propagation. As an initial objective, the effect of seed electrons on the repeatability of plasma plume propagation is investigated numerically. Besides residual electrons left from previous pulses, the electrons detached from O2− ions could also be a significant source of the seed electrons to affect the repeatability of plasma plume propagation when an electronegative gas admixture is presented. In this investigation, a global plasma chemical kinetics model is developed to investigate the temporal evolution of the electron and O2− ions in the afterglow of a plasma plume driven by microsecond pulse direct current voltages, at a total gas pressure of 2 × 104 Pa or 4 × 103 Pa in helium or helium-oxygen mixtures with an air impurity of 0.025%. In addition, a Monte Carlo technique has been applied to calculate the O2− detachment rate coefficient. Accordingly, the seed electron density due to detachment from O2− ions for different percentages of oxygen is obtained. Finally, the minimum seed electron density required for the plasma bullets to propagate in a repeatable mode is obtained according to the critical frequency from the experiments. It is found that the order of minimum seed electron number density required for repeatable propagation mode is independent of oxygen concentrations in the helium-oxygen mixture. It is 108 cm−3 for 20 kPa and 107 cm−3 for 4 kPa. Furthermore, for the helium with an air impurity of 0.025%, the residual electrons left over from previous discharges are the main source of seed electrons. On the other hand, when 0.5% of O2 is added, the detachment of O2− is the main source of the seed electrons.

Application of a global kinetic model on an SCR coated on Filter (SCR-F) catalyst for automotive applications

Selective Catalytic Reduction (SCR) catalysts coated on diesel particulate Filter, SCR-F, have been introduced for automotive applications in the last years due to the capability of reducing NOx and PM simultaneously below the limits imposed by emission regulations. In this context, the performance of a commercial silicon carbide Cu/zeolite SCR-F for controlling NOx emissions in an automotive diesel engine was analyzed both experimentally and numerically for different soot loading levels with the aim to investigate its catalytic properties and to build a simulation model of the aftertreament device capable of predicting NOx conversion efficiency, NH3 storage capacity and soot conversion due to passive regeneration.

Development of a Global Kinetic Model for a Commercial Lean NOx Trap Automotive Catalyst Based on Laboratory Measurements

We summarize a global kinetic model for a commercial automotive Lean NOx Trap (LNT) catalyst derived from laboratory flow reactor measurements at Oak Ridge National Laboratory (ORNL). The experimental measurements were made according to a modified version of a publicly shared protocol developed for characterizing the dynamic responses of LNT catalysts over a temperature range of 150–550 °C under lean-rich cycling with CO, H2, and C3H6 as the reductants. The resulting model includes three NOx storage sites. The present model also includes reactions for oxygen storage, water gas shift, steam reforming, NOx reduction, and N2O and NH3 generation. When implemented in 1D integral reactor simulations of long-period cycling, the model was found to accurately predict the observed outlet concentrations of CO, H2, C3H6, NO, NO2, NH3, and N2O.

Impact of PtOx formation in diesel oxidation catalyst on NO2 yield during driving cycles

Operation of a platinum-based diesel oxidation catalyst under lean conditions leads to the partial transformation of metallic Pt sites into platinum oxides (PtOx) with considerably lower NO oxidation rate. The varying NO2 yield depending on PtOx coverage significantly influences the performance of other devices following in a diesel exhaust aftertreatment line: particulate filter (soot oxidation) and SCR or LNT catalyst (NOx reduction).In this paper, we present a global kinetic model of a diesel oxidation catalyst, including PtOx formation induced by reactions with O2 and NO2, PtOx reduction by CO, hydrocarbons and NO, and PtOx thermal decomposition, and use it to reveal the extent of NO2 yield variation in four standard driving cycles for passenger car emission tests: NEDC, FTP, US06 and SC03. During a single driving cycle, the NO2 yield decreases by 3–10% relative to the original level of the reduced catalyst. The PtOx formation is a slow process and stabilizes only after approximately four repeated driving cycles. The stabilized NO2 yield is 7–27% (relative) lower than with the reduced catalyst, depending mainly on the history of operating temperatures. The largest variation is observed around 250–300°C. At lower temperatures, PtOx are partly reduced during CO and hydrocarbon peaks in the engine exhaust during dynamic operation. At higher temperatures, PtOx start to decompose and NO oxidation becomes limited by equilibrium.

Single-Molecule Analysis beyond Dwell Times: Demonstration and Assessment in and out of Equilibrium

We present a simple and robust technique for extracting kinetic rate models and thermodynamic quantities from single-molecule time traces. Single-molecule analysis of complex kinetic sequences (SMACKS) is a maximum-likelihood approach that resolves all statistically relevant rates and also their uncertainties. This is achieved by optimizing one global kinetic model based on the complete data set while allowing for experimental variations between individual trajectories. In contrast to dwell-time analysis, which is the current standard method, SMACKS includes every experimental data point, not only dwell times. As a result, it works as well for long trajectories as for an equivalent set of short ones. In addition, the previous systematic overestimation of fast over slow rates is solved. We demonstrate the power of SMACKS on the kinetics of the multidomain protein Hsp90 measured by single-molecule Förster resonance energy transfer. Experiments in and out of equilibrium are analyzed and compared to simulations, shedding new light on the role of Hsp90’s ATPase function. SMACKS resolves accurate rate models even if states cause indistinguishable signals. Thereby, it pushes the boundaries of single-molecule kinetics beyond those of current methods.

Modelling hydrogen production by the rich combustion of heavy fuel oil

This paper reports on a modelling study of the rich combustion of heavy fuel oil in a reactor packed with an inert porous medium. Decalin is used as a model compound to represent the heavy fuel oil. A computational model is developed for the reactor, based on a two space dimensional transient heterogeneous description, and a kinetic global model for partial oxidation reforming of decalin is proposed. Also, equilibrium calculations were performed for the experimental conditions of the study. The experimental and simulation results show that heavy fuel oil is a potential fuel to produce hydrogen. It is concluded that notable characteristics of the process can be observed by simulation: the presence of an axial maximum in the production of H2, and positive effects of the equivalence ratio and filtration velocity increase and heat losses reduction on the fuel conversion to H2.

Numerical study on the ignition behavior of coal dust layers in air and O2/CO2 atmospheres

Understanding the fundamental basis of coal dust layers ignition behavior is of interest for the mitigation of industrial fires and explosions. This paper develops a 2D numerical method to investigate the self-ignition of coal dust layers with an emphasis on the roles of the oxygen mole fraction and diluent gas. A global one-step 2nd-order reaction kinetic model considering both coal density and oxygen density is used to estimate the reaction rate of self-heating process until ignition. In addition, the overall heat transfer coefficient is evaluated based on a steady state method and the kinetic parameters are estimated by fitting experimental and numerical temperature evolution figures. This model has been found to be valid to predict the transient temperature and concentration profiles until ignition. The numerical results of the minimum ignition temperature of dust layers (MITL) are in close agreement with experimental observations. The ignition delay time and the most sensitive ignition position (MSIP) of dust layer are found to vary with varying ambient gas conditions, hot plate temperatures and ambient temperatures using the verified model. The model provides a satisfactory explanation for the dependence of ignition characteristics of coal dust layer in oxygen-enriched oxy-fuel atmospheres.

A comparison of simple global kinetic models for coal devolatilization with the CPD model

Simulations of coal combustors and gasifiers generally cannot incorporate the complexities of advanced pyrolysis models, and hence there is interest in evaluating simpler models over ranges of temperature and heating rate that are applicable to the furnace of interest. In this paper, six different simple model forms are compared to predictions made by the Chemical Percolation Devolatilization (CPD) model. The model forms included three modified one-step models, a simple two-step model, and two new modified two-step models. These simple model forms were compared over a wide range of heating rates (5×103 to 106K/s) at final temperatures up to 1600K. Comparisons were made of total volatiles yield as a function of temperature, as well as the ultimate volatiles yield. Advantages and disadvantages for each simple model form are discussed. A modified two-step model with distributed activation energies seems to give the best agreement with CPD model predictions (with the fewest tunable parameters).

Mars‐solar wind interaction: LatHyS, an improved parallel 3‐D multispecies hybrid model

AbstractIn order to better represent Mars‐solar wind interaction, we present an unprecedented model achieving spatial resolution down to 50 km, a so far unexplored resolution for global kinetic models of the Martian ionized environment. Such resolution approaches the ionospheric plasma scale height. In practice, the model is derived from a first version described in Modolo et al. (2005). An important effort of parallelization has been conducted and is presented here. A better description of the ionosphere was also implemented including ionospheric chemistry, electrical conductivities, and a drag force modeling the ion‐neutral collisions in the ionosphere. This new version of the code, named LatHyS (Latmos Hybrid Simulation), is here used to characterize the impact of various spatial resolutions on simulation results. In addition, and following a global model challenge effort, we present the results of simulation run for three cases which allow addressing the effect of the suprathermal corona and of the solar EUV activity on the magnetospheric plasma boundaries and on the global escape. Simulation results showed that global patterns are relatively similar for the different spatial resolution runs, but finest grid runs provide a better representation of the ionosphere and display more details of the planetary plasma dynamic. Simulation results suggest that a significant fraction of escaping O+ ions is originated from below 1200 km altitude.

Pyrolysis characteristics and kinetics of biomass torrefied in various atmospheres

This study aims at investigating into the pyrolysis characteristics and kinetics of torrefied Norway spruce obtained from our previous work on development of a biomass torrefaction process integrated with oxy-fuel combustion. Thermogravimetric analysis technique was employed to study the effects of different torrefaction conditions (including various torrefaction atmospheres and different temperatures) relevant to oxy-fuel combustion on the pyrolysis behavior of the torrefied biomass. Thereafter, a global kinetic model with an assumption of a three-pseudocomponent mechanism was used for kinetic modeling and evaluation. The results showed that torrefaction temperature and oxygen concentration in torrefaction gas have significant effects on the pyrolysis peak height and peak temperature of the torrefied biomass. In addition, more hemicellulose was removed during torrefaction in CO2, compared with torrefaction in N2 and CO2/H2O. Moreover, high oxygen concentration in torrefaction gas and high torrefaction temperature can completely remove hemicellulose. The kinetic evaluation revealed that non-oxidative torrefaction atmospheres do not influent the pyrolysis kinetics of three main biomass components, but only their contribution factors. However, the presence of oxygen in torrefaction gas and the torrefaction temperature have significant effects on the kinetic data.

Pyrolytic characteristics and kinetic studies of agricultural wastes—Four kinds of grasses

ABSTRACTSeveral grasses are among the agricultural wastes generated annually in a large quantity during farming activities. In this study, thermogravimetric analysis was used to evaluate the fuel properties of a number of grasses, including Aeluropus sinensis, Conyza canadensis, Imperata cylindrica, and Setaria viridis. The pyrolysis behavior was also compared with the other biomasses. The thermal reaction systems fitted well with the distributed activation energy model and the global kinetic model and it is obvious that the values of E and k0 calculated by the distributed activation energy model were much higher than that of the global kinetic model.

Kinetic analysis of the thermal decomposition of Polygonum lapathifolium

ABSTRACTThe optimal reaction model for analysis of thermal decomposition of various types of biomasses is currently under debate. Four established models for the kinetic analysis of Polygonum lapathifolium were employed in this study: double extrapolation method, Flynn–Wall–Ozawa method (FWO), distributed activation energy model (DAEM), and global kinetic model. The values of activation energy and frequency factor calculated by global kinetic model are much lower than that of the other three methods. Moreover, it has been demonstrated that the activation energy analyzed by FWO method and DAEM is widely distributed and is not directly related to the heating rates, which are more objective than double extrapolation method and global kinetic model.

Effect of the n-butanol addition on propargyl radical issued from the combustion of benzene

A global kinetic model was constructed from the combination of two existing and broadly validated kinetic schemes for benzene and butanol. Then the combined model, which was able to predict the combustion behavior of benzene-butanol fuel mixtures, was implemented in the PREMIX code in conjunction with CHEMKIN II in order to investigate the effect of n-butanol addition on the formation-depletion of propargil radical C3H3, which is known to be soot precursor, in fuel-rich n-butanol-benzene flames. It was treated the dependence of the soot precursor amounts on n-butanol percentage in the fuel mixture. Then was considered the causes of the variation of this species mole fractions with benzene replacement percentage by oxygenate additive. The principal objective of the current study was to obtain fundamental understanding of the mechanisms through which the oxygenate compound affects the soot precursor amounts. It was found that n-butanol-benzene fuel mixture displayed lower propargyl radical concentrations as compared to the pure benzene flame.

Sub/supercritical water oxidation of high concentration wastewater from tall oil rosin production base

The sub/supercritical water oxidation (SCWO) of wastewater with high concentrations of pollutants from tall oil rosin production base was studied under the temperature range of 320°C–420°C and the pressure range of 11–25 MPa. The results showed that after pretreatment with Ca(OH)2 for ions removal, around 385 mg/L remained which inhibited the oxidation activity by reducing the concentration of hydroxyl radicals (•OH). Since the wastewater compounds were mainly cyclic organic materials, higher O2 contents (stoichiometric ratio over 2.5) and temperatures (over 380°C) are favorable for TOC removal. With O2 SR = 2.5 and T = 420°C, the TOC removal reached almost 100% (99.89%) in 30 min. A global kinetic model for TOC removal during SCWO treatment was built during the research. The kinetic data acquired through the model were fitted to a power‐law function, and the data correlated well with the experimental data. © 2016 American Institute of Chemical Engineers Environ Prog, 35: 975–981, 2016

Effect of Surface Stoichiometry on Blinking and Hole Trapping Dynamics in CdSe Nanocrystals

We measure the photoinduced carrier dynamics as the surface composition of CdSe nanocrystals is systematically varied from metal rich (∼80% surface Cd) to nearly stoichiometric (∼50% surface Cd). Using time-resolved optical spectroscopy, we determine that the luminescence lifetime is controlled by the rate of hole trapping at the newly exposed surface selenium atoms. However, the increased rate of the photoluminescence decay is not sufficient to explain the decreased photoluminescence quantum yield, and requires a growing proportion of nanocrystals in a dark or “OFF” state to explain the data. A global kinetic model is proposed that relates the fraction of selenium sites to the rate of hole trapping. A linear relationship between the rate of hole trapping and the fraction of exposed Se sites (xSe) is observed within the range of accessible stoichiometries (xSe = 0.5–0.2). Extrapolation to higher surface cadmium fractions suggests that not all Se sites serve as effective hole traps. These results explain t...

Experimental, detailed, and global kinetic reaction model for NO oxidation over platinum/alumina catalysts

The oxidation of nitric oxide (NO) is a critical step when promoting lean NOx trap and selective catalytic reduction conversion effectiveness; hence, it is important to understand the fundamental reaction mechanism. This awareness of the detailed reaction mechanism allows for the determination of an appropriate global reaction rate expression for numerical simulation efforts in the automotive field. This work succinctly reviews the literature in this area and derives a global expression for NO oxidation from stoichiometry using the detailed reactions steps. In this version, the parameters used to simulate the global expression are derived from the kinetic theory of gases and the detailed study of each NO reaction step; ensuring the correct functional dependency of each reaction parameter. Moreover, the global model is adapted to simulate different surface morphologies. The effect of temperature and surface changes (dispersion, particle size) are included while determining the parameters, so that the same variables can be used over wide range of catalysts effectively. This work summarizes the literature of experimentally observed NO oxidation in terms of surface coverage, species concentration dependency, and temperature variation. Finally, the authors verify the global model by simulating experimental data obtained for catalysts with varying noble metal particle size. As a result, the adaptive global model predicts NO oxidation conversion with changes in surface morphologies at greater than 93 % accuracy for the experimental data taken.

The impact of CO and C3H6 pulses on PtOx reduction and NO oxidation in a diesel oxidation catalyst

A reversible deactivation of a diesel oxidation catalyst (DOC) due to the platinum oxide (PtOx) formation is observed under lean conditions. Among others, the gradual build-up of PtOx leads to a decreasing activity for NO oxidation. It is known that PtOx formation is caused by O2 and NO2 present in the gas mixture and that platinum oxide can be reduced by NO at lower temperatures or thermally decomposed at high temperatures. In this paper we present experimental and modeling results showing that PtOx can be reduced also by CO and C3H6 pulses.First, temperature ramps from 120 to 420°C and back to 120°C were performed with a constant inlet gas composition starting with a reduced catalyst. A higher NO oxidation activity was observed during the heat-up phase than during the subsequent cool-down phase, indicating formation of PtOx. The catalytic activity during the cool-down ramp was partially restored by pulses with an increased CO and C3H6 concentration (while still keeping lean conditions). The maximum effect of pulses was observed around the light-off temperature for CO and C3H6 oxidation.A global kinetic model was developed, considering PtOx existence and its reduction by CO and C3H6 pulses. The model was further validated by isothermal experiments, showing gradual decrease of NO oxidation activity and its recovery by CO and C3H6 at several different temperatures.

Charge Carriers in Planar and Meso-Structured Organic-Inorganic Perovskites: Mobilities, Lifetimes, and Concentrations of Trap States.

Efficient solar cells have been obtained using thin films of solution-processed organic-inorganic perovskites. However, there remains limited knowledge about the relationship between preparation route and optoelectronic properties. We use complementary time-resolved microwave conductivity (TRMC) and photoluminescence (PL) measurements to investigate the charge carrier dynamics in thin planar films of CH3NH3PbI(3-x)Cl(x), CH3NH3PbI3, and their meso-structured analogues. High mobilities close to 30 cm(2)/(V s) and microsecond-long lifetimes are found in thin films of CH3NH3PbI(3-x)Cl(x), compared to lifetimes of only a few hundred nanoseconds in CH3NH3PbI3 and meso-structured perovskites. We describe our TRMC and PL experiments with a global kinetic model, using one set of kinetic parameters characteristic for each sample. We find that the trap density is less than 5 × 10(14) cm(-3) in CH3NH3PbI(3-x)Cl(x), 6 × 10(16) cm(-3) in the CH3NH3PbI3 thin film and ca. 10(15) cm(-3) in both meso-structured perovskites. Furthermore, our results imply that band-to-band recombination is enhanced by the presence of dark carriers resulting from unintentional doping of the perovskites. Finally, our general approach to determine concentrations of trap states and dark carriers is also highly relevant to other semiconductor materials.

Steady-state and dynamic hysteresis effects during lean co-oxidation of CO and C3H6 over Pt/Al2O3 monolithic catalyst

A global kinetic model is developed for the co-oxidation of CO and C3H6 over a Pt/Al2O3 diesel oxidation monolithic catalyst based on a recent bench flow reactor study (Abedi et al., 2012). A monolith reactor model containing these kinetics is used to elucidate the dynamic and steady state hysteresis behavior observed during oxidation of a CO+C3H6 mixture on Pt/Al2O3. Dynamic hysteresis is observed during temperature-programmed oxidation which involves ramping the feed gas temperature up and down at a constant ramp rate. The predicted solid temperature spatial profile during ramp-up and ramp-down shows how the catalyst temperature lags behind the inlet temperature change, especially during the ramp-down. The dynamic hysteresis loop, whose width increases with the ramp rate and effective heat Peclet number (Peh,eff), is mainly a result of disparate time scales of thermal front propagation and temperature ramp and will be present even for reactions with negligible heat release. Steady state hysteresis can also be explained based on adiabatic temperature rise (ΔTad) and Peh,eff, i.e. thermokinetic multiplicity. The region of steady state hysteresis loop expands with an increasing ΔTad which is proportional to the feed concentrations of CO and C3H6. For fixed ΔTad, the steady state multiplicity disappears as Peh,eff increases or when there is negligible thermal feedback due to intra or interphase heat transfer. Finally, the model is used to construct a map that may be used to assess the light-off features of multiple reaction systems in monolith reactors.

ChemInform Abstract: Critical Review of the Global Chemical Kinetics of Cellulose Thermal Decomposition

AbstractReview: 86 refs.