A reaction mechanism for gasoline surrogate fuels for large polycyclic aromatic hydrocarbons

This work aims to develop a reaction mechanism for gasoline surrogate fuels (n-heptane, iso-octane and toluene) with an emphasis on the formation of large polycyclic aromatic hydrocarbons (PAHs). Starting from an existing base mechanism for gasoline surrogate fuels with the largest chemical species being pyrene (C₁₆H₁₀), this new mechanism is generated by adding PAH sub-mechanisms to account for the formation and growth of PAHs up to coronene (C₂₄H₁₂). The density functional theory (DFT) and the transition state theory (TST) have been adopted to evaluate the rate constants for several PAH reactions. The mechanism is validated in the premixed laminar flames of n-heptane, iso-octane, benzene and ethylene. The characteristics of PAH formation in the counterflow diffusion flames of iso-octane/toluene and n-heptane/toluene mixtures have also been tested for both the soot formation and soot formation/oxidation flame conditions. The predictions of the concentrations of large PAHs in the premixed flames having available experimental data are significantly improved with the new mechanism as compared to the base mechanism. The major pathways for the formation of large PAHs are identified. The test of the counterflow diffusion flames successfully predicts the PAH behavior exhibiting a synergistic effect observed experimentally for the mixture fuels, irrespective of the type of flame (soot formation flame or soot formation/oxidation flame). The reactions that lead to this synergistic effect in PAH formation are identified through the rate-of-production analysis.     

Modeling of autoignition and NO sensitization for the oxidation of IC engine surrogate fuels

This paper presents an approach for modeling with one single kinetic mechanism the chemistry of the autoignition and combustion processes inside an internal combustion engine, as well as the chemical kinetics governing the postoxidation of unburned hydrocarbons in engine exhaust gases. Therefore a new kinetic model was developed, valid over a wide range of temperatures including the negative temperature coefficient regime. The model simulates the autoignition and the oxidation of engine surrogate fuels composed of n-heptane, iso-octane, and toluene, which are sensitized by the presence of nitric oxides. The new model was obtained from previously published mechanisms for the oxidation of alkanes and toluene where the coupling reactions describing interactions between hydrocarbons and NO_x were added. The mechanism was validated against a wide range of experimental data obtained in jet-stirred reactors, rapid compression machines, shock tubes, and homogeneous charge compression ignition engines. Flow rate and sensitivity analysis were performed in order to explain the low temperature chemical kinetics, especially the impact of NO_x on hydrocarbon oxidation.     

Addition of toluene and ethylbenzene to mixtures of H2 and O2 at 773 K: Part I: Kinetic measurements for H and HO2 reactions with the additives and a data base for H abstraction by HO2 from alkanes, aromatics and related compounds

Relative rate constants have been determined for the reactions of H atoms and HO₂ radicals with toluene and ethylbenzene by adding traces of these compounds to mixtures of H₂ + O₂ at 773 K. The values k_24t = (5.5 ± 1.5) × 10⁴ and k_24e = (1.65 ± 0.63) × 10⁵ dm³ mol⁻¹ s⁻¹ are the first reliable kinetic data obtained for the abstraction of an H atom from any aromatic compound by HO₂ radicals. It is shown that the values are significantly lower than expected on the grounds of enthalpy of reaction, and it is concluded the explanation lies in a combination of a lower A factor than observed with alkanes (because of the loss of entropy of activation in the emerging electron-delocalized radicals), and a slightly higher activation energy. A comprehensive database for HO₂ abstraction reactions from alkanes, aromatics, alkenes, and related compounds has been assembled and recommended for use over the temperature range 600 to 1200 K.

(24t)     

Detailed numerical modeling of PAH formation and growth in non-premixed ethylene and ethane flames

A chemical kinetic mechanism for C₁ and C₂ fuel combustion and PAH growth, previously validated for laminar premixed combustion, has now been modified and applied to opposed flow diffusion flames. Some modifications and extensions have been made to the reaction scheme to take into account recent kinetic investigations, and to reduce the stiffness of the reaction model. Updates have been made to the cyclopentadienyl reactions, indene formation reactions, and aromatic oxidation and decomposition reactions. Reverse reaction rate parameters have been revised to account for numerical stiffness. Opposed flow diffusion flame simulation data for ethylene and ethane flames with the present mechanism are compared to data computed using two other mechanisms from the literature and to experimental data. Whereas the fuel oxidation chemistry in all three mechanisms are essentially the same, the PAH growth pathways vary considerably. The current mechanism considers a detailed set of PAH growth routes, and includes hydrogen atom migration, possible free radical addition schemes, methyl substitution/acetylene addition pathways, cyclopentadienyl moiety in aromatic ring formation, and numerous reactions between aromatic radicals and molecules. It is shown that while bulk flame properties and major species profiles are the same for the three mechanisms, the enhanced PAH growth routes in the present mechanism are necessary to numerically predict the correct order of magnitude of PAHs that were measured in the experimental studies. In particular, predicting concentrations of naphthalene, phenanthrene, and pyrene, to within the correct order of magnitude with the present mechanism show a significant improvement over predictions obtained using mechanisms in the literature. Sensitivity and production rate analyses show that this improvement is attributable to the enhanced PAH growth pathways and updated reaction rates in the present mechanism. The overreaching goal of this research is to generate and fully validate a detailed chemical kinetic mechanism, with as few fitted rates as possible, that can be applied to premixed or diffusion systems, and used with any type of soot model. To that end, in recently published works, the present mechanism has been used to simulate premixed flames, while coupled to a method of moments to determine soot formation, and to simulate diffusion flames, while coupled to a sectional representation for soot formation. The present work extends the validation of the mechanism by applying it to counterflow diffusion flames, for which measurements of large PAH molecules are uniquely available. The validation of PAH growth predictions are of key interest to soot modeling studies as soot inception from PAH combination and PAH condensation are often major constituents of soot production.     

A consistent chemical mechanism for oxidation of substituted aromatic species

Computational studies of combustion in engines are typically performed by modeling the real fuel as a surrogate mixture of various hydrocarbons. Aromatic species are crucial components in these surrogate mixtures. In this work, a consistent chemical mechanism to predict the high temperature combustion characteristics of toluene, styrene, ethylbenzene, 1,3-dimethylbenzene (m-xylene), and 1-methylnaphthalene is presented. The present work builds on a detailed chemical mechanism for high temperature oxidation of smaller hydrocarbons developed by Blanquart et al. [Combust. Flame 156 (2009) 588-607]. The base mechanism has been validated extensively in the previous work and is now extended to include reactions of various substituted aromatic compounds. The reactions representing oxidation of the aromatic species are taken from the literature or are derived from those of the lower aromatics or the corresponding alkane species. The chemical mechanism is validated against plug flow reactor data, ignition delay times, species profiles measured in shock tube experiments, and laminar burning velocities. The combustion characteristics predicted by the chemical model compare well with those available from experiments for the different aromatic species under consideration.     

A modelling study of aromatic soot precursors formation in laminar methane and ethene flames

A relatively short kinetic mechanism (93 species and 729 reactions) was developed to predict the formation of poly-aromatic hydrocarbons (PAH) and their growth of up to five aromatic rings in methane and ethane-fueled flames. The model is based on the C₀-C₂ chemistry with recent well-established chemical kinetic data. Reaction paths for mostly stable and well studied PAH molecules were delineated and the reaction rate constants for PAH growth were collected. These were obtained by analysing the data reported in the literature during the last 30 years, or by using the estimates and optimisations of experimentally measured concentration profiles for small and PAH molecules. These profiles were collected by 12 independent work groups in laminar premixed CH₄ and C₂H₄ flames under atmospheric pressure or in shock tube experiments under elevated pressure. The simulated flame speeds, temporal profiles of small and large aromatics and also soot particles volume fraction data are in good agreement with the experimental data received for different temperatures, mixing ratios and diluents. The extensive analysis of PAH reaction steps showed that the main reaction routes can be conditionally divided into “low temperature” reaction routes, dominating at T < 1550 K and “high temperature” reaction routes, which contribute mostly to PAH formation at T > 1550 K. The presented mechanism can be used as the basis for further extensions or reductions applied in kinetic schemes involving PAH and soot production in practical fuel combustion.     

Chemical mechanism for high temperature combustion of engine relevant fuels with emphasis on soot precursors

This article presents a chemical mechanism for the high temperature combustion of a wide range of hydrocarbon fuels ranging from methane to iso-octane. The emphasis is placed on developing an accurate model for the formation of soot precursors for realistic fuel surrogates for premixed and diffusion flames. Species like acetylene (C₂H₂), propyne (C₃H₄), propene (C₃H₆), and butadiene (C₄H₆) play a major role in the formation of soot as their decomposition leads to the production of radicals involved in the formation of Polycyclic Aromatic Hydrocarbons (PAH) and the further growth of soot particles. A chemical kinetic mechanism is developed to represent the combustion of these molecules and is validated against a series of experimental data sets including laminar burning velocities and ignition delay times. To correctly predict the formation of soot precursors from the combustion of engine relevant fuels, additional species should be considered. One normal alkane (n-heptane), one ramified alkane (iso-octane), and two aromatics (benzene and toluene) were chosen as chemical species representative of the components typically found in these fuels. A sub-mechanism for the combustion of these four species has been added, and the full mechanism has been further validated. Finally, the mechanism is supplemented with a sub-mechanism for the formation of larger PAH molecules up to cyclo[cd]pyrene. Laminar premixed and counterflow diffusion flames are simulated to assess the ability of the mechanism to predict the formation of soot precursors in flames. The final mechanism contains 149 species and 1651 reactions (forward and backward reactions counted separately). The mechanism is available with thermodynamic and transport properties as supplemental material.     

Ammonia and borane activation by Tantalum Carbide cluster anion Ta2C4−: A theoretical approach

Activation of C–H, B–H as well N–H sigma bonds has been a subject of fundamental interest due to potential feedstock in chemical industry. The co-operative effect of the metal centers in a di-nuclear carbide cluster Ta₂C₄^? for methane C–H activation and dissociation has recently been revealed, in which a molecule of hydrogen is evolved. Based on that, we have explored reaction pathways for ‘E?H’ sigma bond activation and dissociation processes of gaseous Ammonia (E: N) and Borane (E: B) using the stable form of the aforesaid complex at DFT levels. The geometries and energetics associated with the reactions are found to be method insensitive. The course of the reaction is initiated by a 1:1 precursor complex formed between the cluster and N (/B)H₃. This complex formation is found to be more exothermic than that of its methane counterpart. In case of BH₃, considerable lengthening of two B–H bonds in the first step is observed which implies that the two B–H bonds are activated simultaneously under the influence of Ta₂C₄^?. But for NH₃, N–H bond lengthening as well as activation is observed to be insignificant in this precursor complex. The chemical nature of the participating hydrogen atoms is inspected by NPA analysis (‘protic’ type in NH₃ and ‘hydride’ type in BH₃).The overall dehydrogenation procedures for both the molecules are found to be multi-step with high exothermicity. Highly negative net energy change in terms of Gibbs Free Energy (?31.03 kcal/mol for ammonia and ?31.36 kcal/mol for borane) as well as Enthalpy (?33.68 kcal/mol for ammonia and ?34.13 kcal/mol for borane), in forming a H₂ molecule with stable Ta₂C₄N(/B)H^? complex, with respect to the reactant pair govern the thermodynamic feasibility of the overall process. In a nutshell, this computational study provides a detail understanding of the activation and dissociation processes of the concerned gaseous molecules, which will be beneficial for further experimental studies.     

正庚烷甲苯混合物燃烧简化机理分析

提出了一个新的包括多环芳香烃(PAH)生成的正庚烷/甲苯混合物燃烧化学动力学简化机理.该机理包括64种物质,120个反应,与激波管内滞燃期实验结果吻合较好.在不同进气氧体积分数下,使用该机理对柴油机缸内燃烧过程进行了计算,其结果与缸内的实验结果吻合良好.通过机理的敏感性分析发现,PAH的重要前驱物乙炔主要是由甲苯反应路径中的C6H5及C6H4O2生成,说明在正庚烷中加入甲苯会对模拟柴油的燃烧特别是碳烟的生成有很大的影响;过氧化氢自由基HO2和羟自由基OH在甲苯、正庚烷的分解反应及小分子烃的裂解和氧化反应中都起着非常重要的作用.     

Pressure- and temperature-dependent combustion reactions

In combustion systems, many reactions are simple thermal unimolecular isomerizations or dissociations, or the reverse thereof. It is well understood that these reactions typically depend on temperature, pressure and the nature of the bath gas. These kinds of reactions are a subset of the more general behavior that can be described as free radical association reactions that produce highly energized intermediates, which can isomerize or dissociate via multiple chemical pathways. Each reaction rate depends on excitation energy and all of the competing reactions occur in competition with collisional activation and deactivation. These complicated multi-well, multi-channel reaction systems can only be simulated accurately by using master equation techniques. In this paper, master equation calculations are discussed for several examples of reactions important in combustion (and atmospheric chemistry). Current master equation codes are based on statistical RRKM reaction rate constants (including quantum mechanical tunneling) and simplified models for collisional energy transfer. A pragmatic semi-empirical approach is adopted in order to compensate for limited knowledge. The reaction energies needed for RRKM calculations are usually obtained from quantum chemistry calculations, which are often of limited accuracy and may be adjusted empirically. Energy transfer cannot be predicted accurately and must be parameterized by fitting experimental data. For combustion modeling, the master equation results are usually expressed as chemical reactions with rate constants fitted to empirical algebraic equations. However, the results may be expressed more accurately by interpolating from look-up tables. Several current research issues are also mentioned, including the effects of angular momentum conservation, vibrational anharmonicity, slow intramolecular vibrational energy redistribution, and assumptions surrounding the details of collisional energy transfer.     

Thermal runaway in VRLAB—Phenomena,reaction mechanisms and monitoring

During operation of the oxygen cycle water decomposes forming O₂ at the lead dioxide electrode, while at the lead electrode O₂ is reduced forming water. The mechanism of these processes is related with thermal phenomena as a result of which heat is released. When the cell temperature increases substantially the battery can be damaged. This phenomenon is often called thermal runaway (TRA). The present work investigates the changes in positive and negative plate potentials, temperature, current, and gassing rate during thermal runaway. It is established that during TRA maximums in the transient curves of positive plate potential, current, and finally cell temperature appear. These maximums mark four periods in the development of the TRA phenomenon. The processes that take place during each of these periods are elucidated. On ground of the experimental results a model of the electrochemical and chemical reactions that take place in the system is proposed. The thermal effects of these reactions lead to increase of the cell temperature. Water decomposition at the positive plate and water formation at the negative one cause changes in the concentration of H₂SO₄ at the plate interfaces. When the applied cell voltage is high the increase of the temperature and the changes in H₂SO₄ concentration lead to changes in the structure and phase composition of the electrodes interfaces. This results in changes of the type of the reactions that proceed at the two interfaces. Exothermic chemical reactions take place at the negative plate. Due to the increased temperature and H₂SO₄ concentration the positive plate partially passivates and the current goes through maximum and starts to decrease. The changes in Pb/solution interface and the decreased O₂ flow lead to a maximum in the cell temperature. Problems appear when the value of this maximum becomes higher than the temperature limit below which the battery operates normally.On ground of this model of the thermal phenomena during OxCy operation the limits in the cell current and voltage as well as the thickness and type of the separator for which appearance of TRA does not cause battery damage are determined.     

Reaction mechanism for the free-edge oxidation of soot by O2

The reaction pathways for the oxidation by O₂ of polycyclic aromatic hydrocarbons present in soot particles are investigated using density functional theory at B3LYP/6-311++G(d,p) level of theory. For this, pyrene radical (4-pyrenyl) is chosen as the model molecule, as most soot models present in the literature employ the reactions involving the conversion of 4-pyrenyl to 4-phenanthryl by O₂ and OH to account for soot oxidation. Several routes for the formation of CO and CO₂ are proposed. The addition of O₂ on a radical site to form a peroxyl radical is found to be barrierless and exothermic with reaction energy of 188 kJ/mol. For the oxidation reaction to proceed further, three pathways are suggested, each of which involve the activation energies of 104, 167 and 115 kJ/mol relative to the peroxyl radical. The effect of the presence of H atom on a carbon atom neighboring the radical site on the energetics of carbon oxidation is assessed. Those intermediate species formed during oxidation with seven-membered rings or with a phenolic group are found to be highly stable. The rate constants evaluated using transition state theory in the temperature range of 300–3000 K for the reactions involved in the mechanism are provided.     

Pore-scale simulation of coupled multiple physicochemical thermal processes in micro reactor for hydrogen production using lattice Boltzmann method

A general numerical scheme based on the lattice Boltzmann method (LBM) is established to investigate coupled multiple physicochemical thermal processes at the pore-scale, in which several sets of distribution functions are introduced to simulate fluid flow, mass transport, heat transfer and chemical reaction. Interactions among these processes are also considered. The scheme is then employed to study the reactive transport in a posted micro reactor. Specially, ammonia (NH₃) decomposition, which can generate hydrogen (H₂) for fuel of proton exchange membrane fuel cells (PEMFCs), is considered where the endothermic decomposition reaction takes place at the surface of posts covered with catalysts. Simulation results show that pore-scale phenomena are well captured and the coupled processes are clearly predicted. Effects of several operating and geometrical conditions including NH₃ flow rate, operating temperature, post size, post insert position, post orientation, post arrangement and post orientation on the coupled physicochemical thermal processes are assessed in terms of NH₃ conversion, temperature uniformity, H₂ flow rate and subsequent current density generated in PEMFC.     

Kinetic effects of toluene blending on the extinction limit of n-decane diffusion flames

The impact of toluene addition in n-decane on OH concentrations, maximum heat release rates, and extinction limits were studied experimentally and computationally by using counterflow diffusion flames with laser induced fluorescence imaging. Sensitivity analyses of kinetic path ways and species transport on flame extinction were also conducted. The results showed that the extinction strain rate of n-decane/toluene/nitrogen flames decreased significantly with an increase of toluene addition and depended linearly on the maximum OH concentration. It was revealed that the maximum OH concentration, which depends on the fuel H/C ratio, can be used as an index of the radical pool and chemical heat release rate, since it plays a significant role on the heat production via the reaction with other species, such as CO, H₂, and HCO. Experimental results further demonstrated that toluene addition in n-decane dramatically reduced the peak OH concentration via H abstraction reactions and accelerated flame extinction via kinetic coupling between toluene and n-decane mechanisms. Comparisons between experiments and simulations revealed that the current toluene mechanism significantly over-predicts the radical destruction rate, leading to under-prediction of extinction limits and OH concentrations, especially caused by the uncertainty of the H abstraction reaction from toluene, which rate coefficient has a difference by a factor of 5 in the tested toluene models. In addition, sensitivity analysis of diffusive transport showed that in addition to n-decane and toluene, the transport of OH and H also considerably affects the extinction limit. A reduced linear correlation between the extinction limits of n-decane/toluene blended fuels and the H/C ratio as well as the mean fuel molecular weight was obtained. The results suggest that an explicit prediction of the extinction limits of aromatic and alkane blended fuels can be established by using H/C ratio (or radical index) and the mean fuel molecular weight which represent the rates of radical production and the fuel transport, respectively.     

First-principles based microkinetic modeling of borohydride oxidation on a Au(1 1 1) electrode

Borohydride oxidation electrokinetics over the Au(1 1 1) surface are simulated using first-principles determined elementary rate constants and a microkinetic model. A method to approximate the potential dependent elementary step activation barriers based on density functional theory calculations is developed and applied to the minimum energy path for borohydride oxidation. Activation barriers of the equivalent non-electrochemical reactions are calculated and made potential dependent using the Butler-Volmer equation. The kinetic controlled region of the borohydride oxidation reaction linear sweep voltammogram over the Au(1 1 1) surface is simulated. The simulation results suggest that B-H bond containing species are stable surface intermediates at potentials where an oxidation current is observed. The predicted rate is most sensitive to the symmetry factor and the BH₂OH dissociation barrier. Surface-enhanced Raman spectroscopy confirms the presence of BH₃ as a stable intermediate.     

Progress report on the thermochemical hydrogen production by the Mg-S-I cycle-laboratory-scale demonstration

The Mg-S-I thermochemical water-splitting cycle, consisting of: (i) redox reaction of sulfur dioxide and iodine with magnesium oxide in aqueous phase; (ii) hydrolysis of magnesium iodide; (iii) thermal decomposition of magnesium sulfate; and (iv) thermal dissociation of hydrogen iodide, was studied. Based on the fundamental studies on each of the constituent reactions, the whole cycle was demonstrated on a laboratory scale by constructing an apparatus and by the repeated operations of chemical reactions and circulation of reactants through purely thermochemical processes below 1000°C. Electric furnaces and quartz glass reactors were used. With satisfactory performance of the system, 38 times of cycle operations were successfully repeated with roughly constant production of 0.3 liter of H₂ and 0.15 liter of O₂ per cycle. The time requirement for one cycle operation was about 1 h, where all the chemical reactions proceeded and all the reactants were circulated.     

Enhancing the aromatic hydrocarbon yield from the catalytic copyrolysis of xylan and LDPE with a dual-catalytic-stage combined CaO/HZSM-5 catalyst

The high concentration of oxygenated compounds in pyrolytic products prohibits the conversion of hemicellulose to important biofuels and chemicals via fast pyrolysis. Herein CaO and HZSM-5 was developed to convert xylan and LDPE to valuable hydrocarbons by thermogravimetric analysis (TGA) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and elucidate the reaction mechanism were also investigated in detail. The results indicated that xylan/LDPE copyrolysis was more complicated than pyrolysis of the individual components. LDPE hindered the thermal decomposition and aromatic hydrocarbon formation from xylan at temperatures under 350 °C and had a synergistic effect at high temperatures. 50% LDPE was proven to be more beneficial than other percentages for the formation of monocyclic aromatic hydrocarbons. Simultaneously, the addition of CaO/HZSM-5 significantly reduced the reaction Ea and increased the reaction rate. CaO can effectively improve the deoxygenation and aromatization reaction, enhancing the yield and selectivity of aromatics to a certain extent. The maximum yield of hydrocarbons (96.01%), mono-aromatic hydrocarbons (88.53%) and S_BTXE (85.79%) were obtained at a CaO/HZSM-5 ratio of 1:2, a pyrolysis temperature of 450 °C, a catalytic temperature of 550 °C, a catalyst dose of 1:2 and a xylan-to-LDPE ratio of 1:1 via an ex situ process. The system was dominated by toluene, xylene and alkyl benzene. Diels-Alder reactions of furans and hydrocarbon pool mechanism of nonfuranic compounds improved aromatic formation. This study provides a fundamental for recovering energy and chemicals from pyrolysis of hemicellulose.     

The distributed-energy chain model for rapid coal devolatilization kinetics. Part I: Formulation

The distributed-energy chain model (DISCHAIN) interprets coal devolatilization in terms of independent influences from chemical reaction rates and from macromolecular configuration. Coal is represented by three components: (1) aromatic units that are attached pairwise by (2) labile bridges to form nominally infinite linear chains, with (3) peripheral groups branching from the aromatic units. These components are the building blocks for unreacted coal, free monomers (mobile aromatic units), gas, tar, and char. Four chemical reactions represent bridge dissociation, peripheral group elimination, and tar and char formation. Analytic probability expressions and competitive reactions describe the conversion of bound aromatic units into free monomers, and enter into the formation of all products except gas. There are no hypothetical ultimate yields.The model is introduced in two parts. Here in Part I, the coal model, chemical reactions, and chain statistics are derived and formulated into rate equations. Mechanisms leading to major products are identified, including a novel mechanism for yield enhancement by faster heating. Whenever bridge dissociation and char formation occur concurrently, as for slow heating, the subsequent generation of monomers is inhibited. Aromatic units are thereby excluded from the competition between tar and char formation. Conversely, bridge dissociation and char formation occur consecutively for rapid heating, and a greater proportion of the original bound aromatic units become monomers and, ultimately, tar.     

Open circuit voltage mystery solved for proton-conducting fuel cells?

Hydrogen and oxygen are combined to water by a chain reaction. In proton-conducting fuel cells the electrolyte membrane is placed between the first and the second link. The formation of water is completed by chemical reactions not contributing to the voltage establishment, but only to the thermal balance. Thus the Equilibrium Potential “EP” cannot be based on the overall molecular reaction 2H₂ + O₂ => 2H₂O yielding an EP of 1.23 V, but another reactions must be considered. Hydrogen radicals are formed by platinum inside the anode chamber. Radicals reappearing at the cathode either recombine to hydrogen molecules or split oxygen molecules and form hydroxyl radicals. Platinum is needed at the cathode to secure a steady supply of hydrogen radicals. Water is obtained by chemical oxidation of hydroxyl radicals. Two EPs are obtained for hydrogen recombination (measurable) and hydrogen partial oxidation (virtual). Also, two polarity change steps must be considered in the EP equation. With these modifications an excellent agreement between theory and experiment is obtained.     

Thermal phenomena during operation of the oxygen cycle in VRLAB and processes that cause them

The present paper makes a summary of the results of the investigations on the oxygen cycle (OxCy) performed in this laboratory with the aim to elucidate the processes that take place at the two electrodes of VRLA cells during OxCy operation and the thermal phenomena caused by these processes. It has been established that on constant voltage polarization, the cell reaches a certain state after which its temperature (T) and current (I) begin to increase spontaneously and pass through maxima before reaching stationary values. This phenomenon is called thermal runaway (TRA). These maxima are a result of self-accelerating interrelation established between the rates of the reactions involved in the oxygen cycle at the two electrodes. At high polarization voltages and currents, electrochemical reactions proceed at the positive and negative plates leading to changes in the surface properties and the structures of the PbO₂ and Pb plates as well as in the composition of the electrolyte filling the pores of the active masses. The above changes result in passivation of the PbO₂ electrode, decrease of the rate of O₂ evolution at the positive plates and initiate new chemical exothermic reactions of O₂ reduction at the negative plates. The generated heat causes the cell temperature to rise. If the temperature is higher than 60 °C for a long period of time, this may impair the performance characteristics of the cell. This paper proposes a mechanism of the chemical and electrochemical reactions that proceed at the positive and negative plates during operation of the oxygen cycle (OxCy) and their evolution on constant voltage polarization of the cell. It has been established that there are critical values of T and φ above which the OxCy efficiency declines substantially and thermal phenomena proceed causing TRA.