Single Global Reaction Research Articles

Kinetic models of HMX decomposition via chemical reaction neural network

1,3,5,7-Tetranitro-1,3,5,7-tetrazocane (HMX) is commonly used in solid propellants and explosives as energetic materials. The study of its kinetics and decomposition mechanism is of great significance to its application in the aerospace industries. This work investigates the thermal decomposition of HMX based on the combined thermogravimetric (TG) measurements and chemical reaction neural network (CRNN). Two compact kinetic models for HMX are introduced, with one consisting of four substances and a single global reaction (4–1 model) and the other consisting of four substances and four reactions (4–4 model). In the 4–1 model, the calculated activation energy is 328.44 kJ·mol−1, which agrees with the experimental value. As for the 4–4 model, the substances and reactions are assigned based on a skeleton mechanism involving reactions of N-N and C-N bond cleavage and HONO elimination. Moreover, the catalytic effects of TiO2 and Al2O3 on HMX are well simulated using the aforementioned kinetic models. The CRNN models can reproduce the peak temperature with a reduced activation energy, but the initial decomposition temperature is overestimated owing to the complex nature of catalytic impact. This work presents the application of the CRNN model to obtain a decomposition mechanism of HMX, highlighting its efficacy in accurately capturing the thermal decomposition behavior. The potential extension of CRNN to kinetic modeling of other energetic materials is anticipated in future studies.

Thermal decomposition mechanism of 1,3,5-trinitroperhydro-1,3,5-triazine: Experiments and reaction kinetic modeling

This study investigates the thermal decomposition of 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) based on combined thermogravimetric (TG) and chemical reaction neural network (CRNN). Two compact kinetic models for RDX are introduced, with one consisting of three substances and a single global reaction (one-step model) and the other consisting of three substances and four reactions (3–4 model). In the one-step model, the calculated activation energy is 193.67 kJ mol−1, which agrees with the experimental value. As for the 3–4 model, the substances and reactions are assigned based on a skeleton mechanism involving reactions of N-N and C-N bond rupture and HONO elimination. This work presents the first application of the CRNN to obtain a reaction mechanism of RDX decomposition, which is further validated against experiments. Furthermore, effects of sublimation and vaporization phenomena are also included in the model. Extension of the CRNN to kinetic modeling of other energetic materials is anticipated in future studies.

Polydisperse spray flame ignition by a pulsed heat flux

A mathematical analysis of the ignition of a polydisperse spray/air mixture by an infinite surface heated in a pulsed manner is presented. In contrast to previous work in the literature, the entire history of the ignition process is accounted for starting from the flame-embryo progenitor stage, through the thermal runaway stage to the final flame propagation stage. For tractability at the current stage, the chemical kinetics is taken to be that of a single global reaction. The spray is modeled using the sectional approach and the influence of fuel spray characteristics on ignition is determined. Good agreement was found between the theoretical predictions and full numerical simulations. Delay in ignition due to the build-up of vapor from the fuel droplets as well as heat loss to the droplets for evaporation are found to play a significant role under certain operating conditions. Comparison between the critical energy flux and the initial spray polydispersity revealed small differences for larger values of the pulse duration but more significant minor differences for smaller pulse durations. Despite these seemingly minor differences, it was shown that the initial spray polydispersity can have a critical influence on whether flame ignition will occur or fail, even for sprays having the same initial SMD.

Kinetic modeling of Indian lignites pyrolysis in the context of underground coal gasification (UCG)

Pyrolysis is an important step in combustion and gasification of any solid fuel. In this work, we have studied the kinetics of the pyrolysis of three Indian lignite samples via thermogravimetric experiments and mathematical modeling. Applicability of various kinetic models viz. the single global reaction model, the consecutive reaction model and the distributed activation energy model (DAEM) are evaluated. The kinetic parameters of the models were estimated based on the experimental data. Kinetics of the evolution of the volatile matter was studied independently by using a specially designed fixed-bed reactor and the relevant kinetic parameters were estimated.

High‐Fidelity Microstructural Characterization and Performance Modeling of Aluminized Composite Propellant

AbstractImage processing and stereological techniques were used to characterize the heterogeneity of composite propellant and inform a predictive burn rate model. Composite propellant samples made up of ammonium perchlorate (AP), hydroxyl‐terminated polybutadiene (HTPB), and aluminum (Al) were faced with an ion mill and imaged with a scanning electron microscope (SEM) and x‐ray tomography (micro‐CT). Properties of both the bulk and individual components of the composite propellant were determined from a variety of image processing tools. An algebraic model, based on the improved Beckstead‐Derr‐Price model developed by Cohen and Strand, was used to predict the steady‐state burning of the aluminized composite propellant. In the presented model the presence of aluminum particles within the propellant was introduced. The thermal effects of aluminum particles are accounted for at the solid‐gas propellant surface interface and aluminum combustion is considered in the gas phase using a single global reaction. Properties derived from image processing were used directly as model inputs, leading to a sample‐specific predictive combustion model.

Global Reaction Model to Describe the Kinetics of Catalytic Pyrolysis of Coffee Grounds Waste

In view of the constant search for new sources of renewable energy, the particulate agro-industrial waste reuse emerges as an advantageous alternative. However, despite the advantages of using the biomass as an energy source, there is still strong resistance as the large-scale replacement of petroleum products due to the lack of scientifically proven efficient conversion technologies. In this context, the pyrolysis is presented as one of the most widely used thermal decomposition processes. The knowledge of aspects of chemical kinetics, thermodynamics these will, heat and mass transfer, are so important, since influence the quality of the product. This paper presents a kinetic study of slow pyrolysis of coffee grounds waste from dynamic thermogravimetric experiments (TG), using different powder catalysts. The primary thermal decomposition was described by the one-step reaction model, which considers a single global reaction. The kinetic parameters were estimated using nonlinear regression and the differential evolution method. The coffee ground waste was dried at 105°C for 24 hours. The sample in nature was analyzed at different heating rates, being 10, 15, 20, 30 and 50 K/min. In the catalytic pyrolysis, about 5% (w/w) of catalyst were added to the sample, at a heating rate of 30 K/min. The results show that the one-step model does not accurately represent the data of weight loss (TG) and its derivative (DTG), but can do an estimative of the activation energy reaction, and can show the differences caused by the catalysts. Although no one can say anything about the products formed with the addition of the catalyst, it would be necessary to micro-pyrolysis analysis, we can say the influence of the catalyst in the samples, based on the data obtained in thermogravimetric tests.

Pyrolysis kinetics of regional agro-industrial wastes using isoconversional methods

ABSTRACTThe thermochemical conversion, under inert atmosphere, at high heating rate of agro-industrial wastes was studied using thermogravimetric analysis. Two reaction mechanisms were supposed: (a) The thermal solid decomposition is carried out through a single reaction; or (b) this occurs through several independent parallel reactions based on its main components. Kinetic isoconversional methods were applied to both proposed mechanisms. The best fit was obtained for the single-reaction models. Nevertheless, to study the influence of pseudocomponent decomposition in the global process, the kinetic behavior of each of them was analyzed. The R2 and D3 models represent the thermal decomposition of biomass wastes and their pseudocomponents. The first model supposes that the reactions tend at first order, and the second assumes that the diffusion is the predominant phenomenon. In all cases, the model with the best fit for the cellulose decomposition is the same for the single global reaction model. The enthalpy variations are positive, indicating that the reactions are endothermic. The entropy variations have negative values, signifying that these processes are slow. The Gibbs free energy variations exhibited positive values, revealing that the total system energy increased at the approach of the reagents and the formation of the activated complex.

Thermal degradation analysis of innovative PEKK-based carbon composites for high-temperature aeronautical components

Nowadays, composite materials find a large application in several engineering fields, spanning from automotive to aerospace sectors. In the latter, especially in aircraft civil transportation, severe fireproof requirements must be accomplished, taking into account that the second most frequent cause of fatal accidents involving airplanes, was the post-impact fire/smoke, as reported by the European Aviation Safety Agency (EASA) in 2014. In the light of this, experimental research is of crucial importance in the understanding thermal behavior of composites for aircraft components, when exposed to high-temperature and fire conditions. In this context, a thermal degradation study is carried out for two carbon-reinforced resins: the well known thermosetting phenolic and a thermoplastic polyether–ketone–ketone (PEKK), recently developed specifically for this kind of application. The aim is to evaluate the PEKK behavior and to understand the impact of composite nature in terms of structural strength under fire. To this end, thermogravimetric analyses were performed for three different non-isothermal heating programs, between 30 and 1000 °C. Under inert atmosphere one single global reaction is observed for carbon-PEKK between 500–700 °C, while two for carbon-phenolic, whose pyrolysis begins around 200 °C. This better PEKK strengthening is attributed to the ether and ketone bonds between the three aromatic groups of the monomer. As expected, under oxidative atmosphere, the kinetic process becomes more complex, making more difficult the detecting of single-step reactions, especially for carbon-phenolic. Nevertheless, the oxidative process of carbon-PEKK seems to be driven by three consecutive global reactions. The activation energy is estimated by means of both integral (Starink) and differential (Friedman) isoconversional methods, as a function of the extent of conversion, corresponding to the identified reaction intervals. For carbon-PEKK in inert conditions, with Starink a mean value of 207.71 ± 6.57 kJ/mol was estimated, while 213.88 ± 20.04 kJ/mol with Friedman. This expected slight difference depends on the nature of the considered mathematical approaches. The difficulty in activation energy estimation for polymeric materials prefers the use of at least two different methods, allowing for the identification of an activation energy range, for a resin of which no data are available in the literature. For the decomposition model evaluation, the so-called compensation effect method was implemented, as well as the single-step-based approach proposed by Friedman. The evaluation of a possible decomposition expression has been achieved only for carbon-PEKK under inert conditions, since the considered methods are valid and applicable only for well defined single-step reactions. In fact, the three reactions of the oxidative case cannot be considered as single-step processes. Moreover, the higher difference in the estimated activation energy between Starink and Friedman suggests to check the achieved results by implementing further isoconversional methods, to understand the most reliable for polymer-based carbon composites degradation analysis under oxidative atmosphere. However, the observed higher thermal performance of PEKK resin, attributed to its chemical structure, increases the interest toward its use as matrix for aerospace composite materials, that can be subjected to hazardous environments.

Autocatalysis in thermal decomposition of polymers

Possibility of replicating polymer decomposition by a single global reaction greatly simplifies pyrolysis modeling. Apparent kinetic parameters are normally derived from the microscale experiments with linear heating program, and the n-th order reaction is routinely assumed thereby strongly affecting the numerical values of the kinetic parameters. In this work, we demonstrate inconsistency of the n-th order reaction assumption and reveal the autocatalytic behavior in thermal degradation of polyethylene, polystyrene and polycarbonate. The autocatalysis manifests itself in non-monotonicity of the conversion function, which markedly increases in a wide range of conversions. Although the iso-conversional approach makes it possible to explicitly recover the conversion function from the measurements, this option has not been used in most of the previous studies. Meanwhile, proper approximation of the experimentally derived conversion function results in excellent replication of the measured reaction rates, with the same kinetic parameters, in a range of the heating rates. Thus developed thermal decomposition kinetic models are provided in this paper for three kinds of polyethylene (LDPE, HDPE, and UHMWPE), seven kinds of polystyrene, polycarbonate, and two kinds of polymethylmethacrylate with different molecular weights. Although the pyrolysis of the polymers with different molecular weights proceeds differently, no systematic correlation of the pyrolysis characteristics (conversion-averaged apparent activation energy, heat of combustion, peak reaction rates and temperatures etc.) with the molecular weight has been observed for polystyrene. Peak reaction rates and temperatures varied in opposite directions for polyethylene and polymethylmethacrylate.

Generalized approach to model pyrolysis of flammable materials

Abstract Generalized approach is suggested to model pyrolysis of solid materials. Generalization implies use of conversion-dependent, rather than constant, formal kinetic parameters. Within this approach, two functions characterizing thermochemical stability of a material are evaluated: the apparent activation energy and the product of pre-exponential factor and conversion function. Both functions are obtained by the iso-conversional Friedman method and tabulated depending on the global conversion variable. The key advantage of this approach is application of a single global reaction to model pyrolysis with multi-step reaction mechanism of arbitrary complexity. This approach has been applied to five flammable materials (polystyrene, polycarbonate, polymethyl methacrylate, polyvinyl chloride, and wood) decomposing in ether single- or multi-step manner. To derive generalized pyrolysis kinetic models, material decomposition was studied by microscale combustion calorimetry and thermogravimetry. The advantage of MCC is highlighted. The proposed kinetic model is shown to replicate MCC measurements to a very good accuracy in a range of the heating rates from 0.25 to 1.5 °C/s. The generalized global kinetic models can be readily applied to predict ignition and burning of flammable materials with no need to develop multi-step reaction schemes. Favorable performance of the generalized approach has been demonstrated when it was applied as a component of the Pyropolis model to predict thermal decomposition, flaming ignition, and burning of the material (PVC) with complex multi-step decomposition chemistry.

A Numerical and Experimental Investigation of Performance of a Nonsprayed Porous Burner

The performance of a nonsprayed porous burner (NSPB) is investigated through both numerical and experimental studies. The major requirement of liquid fuel combustion systems is excellent fuel vaporization, which is accomplished by using porous medium. Instead of heterogeneous combustion, which occurs in free space of a conventional sprayed burner, a homogeneous combustion of vaporized kerosene and air takes place within a porous medium. The liquid kerosene is preheated and completely vaporized in the first porous medium before being mixed with preheated air in the mixing chamber (i.e., a small space between two porous media). Then the combustion occurs in the second porous medium. A subcooled boiling, single global reaction combustion, and local nonthermal equilibrium between fluid and solid phases with phase change under complex radiative heat transfer are considered. The model accuracy is validated by the experimental data before parametric study—that is, equivalence ratio and firing rate are performed. Result show that a self-sustaining evaporation without atomization and matrix-stabilized flame can be achieved in the NSPB by providing the radiant output efficiency in the same range as a conventional premixed gaseous porous burner. This indicates that the NSPB is one possible technology to replace conventional spray burners for future requirements.

Effect of Physical Properties of Porous Combustor on Radiant Output and Fuel-Preheated Efficiency of a Non-Sprayed Porous Burner

The effect of physical properties of porous combustor on performance of a non-sprayed porous burner (NSPB) in term of radiant output and fuel-preheated efficiency is studied through numerical modeling. The homogeneous combustion of vaporised kerosene and combustion air within a porous combustor (PC) is instead of the heterogeneous combustion occurs in free space of a conventional sprayed burner. The excellent fuel evaporation without atomisation is accomplished by using porous evaporator (PE). Single global reaction combustion, sub-cooled boiling, local non-thermal equilibrium between fluid and solid phases with phase change under complex radiative heat transfer and sub-cooled boiling are considered. The parametric study of porosity and optical thickness are investigated. The study shows that the radiant output efficiency is decreased with higher porosity while the fuel-preheated efficiency quite constant. The optical thickness has no significant effect on radiant output efficiency whereas the fuel-preheated efficiency is reduced with higher optical thickness. In addition, the evaporation front zone moves deep to downstream of PE when optical thickness increases.

Experimental Method for Kinetic Studies of Gas–Solid Reactions: Oxidation of Carbonaceous Matter

A methodology comprising careful consideration of sample preparation, reactor design, experimental procedures, and data evaluation routines for precise analysis of the kinetics of gas–solid reactions, specifically the oxidation of carbonaceous materials, has been developed and validated. The well-controlled solvent-free deposition of the carbonaceous material onto cordierite monolith substrates ensures experimental studies in the absence of diffusion limitations, temperature gradients, and hot zones. These critical aspects are supported also by theoretical considerations. Temperature-programmed oxidation and isothermal oxygen step-response experiments in a continuous gas-flow reactor using a homogeneous synthetic carbon-black material demonstrate excellent reproducibility, and the conversion profiles agree well with previously reported data. An independent set of global kinetic parameters was estimated for each 5% subconversion interval using linear regression such that the conversion dependence of each parameter could be analyzed separately and compared to previously published data. The results show that the evolution of reactive carbons cannot be described with a single global reaction order. This is supported by intermittent ex situ measurements of the specific surface area of the carbon-black material during the course of isothermal oxidation, which reveals a developing microporous structure at high conversions. Physically the changes in carbon reaction order are interpreted as changes in fraction of accessible reactive carbon atoms during progressing oxidation. Moreover, at high conversions, the carbon reaction order approaches 0.7 implying not only that the evolution of the concentration of reactive carbon atoms is proportional to the external surface area of shrinking spheres but also that these spheres have approximately the same size.

The influence of combustion on the fluid structure in a counter-flow diffusion flame

In outlining their experiments on counter-flow diffusion flames in the upstream region of a porous cylinder, Tsuji and Yamaoka [1] used a normal gradient of normal velocity as the single most important parameter in characterising the flow in each experiment. These normal gradients were calculated from ‘cold’ potential flow conditions. More recently, these same normal gradients have been used in attempted simulations of the experiments by way of a similarity solution of the boundary-layer equations. The outer boundary conditions for the fluid were derived from the potential flow solution approximated close to the stagnation point. Such solutions assume a Hiemenz type flow with a constant gradient throughout. The analysis relies on the assumption that the entire combustion and heat conduction region is confined to a thin layer ‘blown off’ a porous cylinder. A comparison with experiment and simple calculation of the order of magnitude of the thickness of the ‘boundary-layer’ shows that it is inappropriate to use this type of analysis. A numerical solution is presented which models the fluid dynamic and chemical characteristics of a counter-flow diffusion flame surrounding a porous cylinder. The model relies on the technique of direct co-ordinate expansions. Closure of the conservation equations is effected by assuming that the chemical heat release gives rise to large normal gradients. It is shown that the fluid mechanics differ radically from those encountered in the ‘cold flow’ calculations. The boundary conditions in the present case are taken from the potential solution of fluid flowing over a cylinder. The existence of a sheart layer is shown separating the combustion zone from the unheated outer flow and this leads to the calculation of a displacement thickness providing an ‘effective radius’ for the cylinder seen by the outer unheated fluid as it approaches the combustion zone. The ‘effective radius’ may be then used as a new boundary condition for the reactive flow. The finite difference equations are solved by a Newton-iteration scheme utilising an adaptive mesh. Profiles are presented for normal and tangential velocities, temperature, chemical species (for a single global reaction mechanism) and mixture density versus radial distance measured from the cyclinder surface. Extension to more complicated kinetics is not a difficulty. It is also proposed that in the light of these results extinction limits be defined at the flame ‘position’ defined with reactive flow parameters rather than using the assumption of undisturbed ‘cold flow’ conditions.

A model for the biological degradation of recent sedimentary organic matter

Biological degradation in a given environment is considered to be the sum of numerous elementary reactions. If degradation is to be followed in a sedimentary core, the steady state hypothesis is required, and the time scale in the core is t= h/ V h being the depth in the core, and V the sedimentation rate. If, for the sake of simplicity, one single global reaction is assumed, then its control parameters depend not only on V, but also on the depths of the measurements. Now first order kinetics is assumed for both the elementary reactions and the single global reaction. Control parameters, apart from q 0 (concentration at water/sediment interface) are the rate constants, r for the global reaction, r ij for the individual reactions, and α ij fraction of total organic matter involved in the ijth reaction. Using data from this work and from the literature, it is shown that consistent trends result when r is plotted against V/ H ( H being the depth of the deepest sample), but not against V alone. Comparisons of actual trends vs numerical simulations with several sets of r ij , α ij , show that sulfate reduction is dominated by rapid reactions, but total organic matter decay is dominated by slow reactions, compatible with geologic times. Thus bacteria can be geologically significant. Both sulfate reduction and microbiological degradation of total organic matter appear as processes of moderate efficiency. The high efficiency of biological degradation in oxic sediments relies on benthic activity at the boundary layer and in the bioturbated zone.