Semi-global Reaction Research Articles

Thermal conversion potential of tea stems: Experimental investigation, reaction mechanism, and kinetic modeling

Recent studies on tea waste pyrolysis have revealed significant gaps, including a lack of experimental data regarding the types of tea waste and a scarcity of comprehensive studies devoted to kinetic models and well-established reaction pathways tailored to this process. This study systematically investigates tea stem pyrolysis, employing comprehensive physicochemical, thermogravimetric, and kinetic analyses. By Identifying the promising characteristics of tea stem, such as its low ash content, high volatile matter, substantial phenolic compounds, and impressive calorific value, this study underscores its potential and delves deep into the complex stages of tea stem pyrolysis. The decomposition process of pseudo-components related to cellulose, hemicellulose, and lignin is an integral part of the comprehensive description of the thermal degradation of tea stems. This description includes the analysis of the contribution factors of its components and their interactions, which significantly influence the reaction rates. These aspects constitute key focal points of our investigation. Furthermore, a comprehensive comparative analysis, incorporating four semi-global reaction mechanisms, was conducted to identify the most optimal reaction mechanism for the thermal decomposition of tea stems. The present study employed a multivariate optimization technique to intricately characterize the kinetics of the process, with a final objective of determining the most appropriate reaction mechanism. Additionally, during the optimization process, it was found that incorporating nucleation-growth reaction model into the reaction mechanism substantially enhances the accuracy. Based on the analysis of kinetic triplets and a comparison of the reaction mechanisms, the mechanism with two independent parallel reactions containing an intermediate consecutive step demonstrates the best agreement with experimental results, with a fit quality of 4.396% and an R-squared (R2) value of 0.997. The findings of this study contribute to a better understanding of the thermal conversion potential of tea stems and provide a deeper understanding of their pyrolysis behavior and kinetic characteristics.

A pyrolysis model for the thermal decomposition of low-density polyethylene blended with ammonium polyphosphate and pentaerythritol

In this work, a semi-empirical pyrolysis model is developed to predict the thermal decomposition behavior of an intumescent fire retardant (IFR) system consisting of ammonium polyphosphate/pentaerythritol (APP/PER) with the ratio 3:1 (wt/wt) in a low-density polyethylene (LDPE) matrix. This work is based on a previously developed semi-global reaction mechanism for the degradation of LDPE and APP/PER, which consists of two consecutive first-order reactions for the LDPE and five first and second-order reactions for the APP/PER combination. The apparent properties that define heat transport inside the pyrolyzing solid are determined via inverse modeling of cone calorimeter experiments for the pure LDPE and then for the complete IFR system. This is achieved by using the Shuffled Complex Evolution optimization algorithm. The flame heat flux in the cone calorimeter experiments is also evaluated by targeting the heat release rate (HRR) data in the optimization process of one of the cone radiative heat setups. The optimized parameters and flame heat feedback are in line with the literature and the robustness of the model is assessed by a comparison with the experimental data for a wide range of cone heat fluxes. Ignition times and peaks of HRR and total HRs are within the engineering accuracy whatever the flux conditions for both pure LDPE and the IFR system.

A semi-global reaction mechanism for the thermal decomposition of low-density polyethylene blended with ammonium polyphosphate and pentaerythritol

A semi-global reaction mechanism has been developed to capture the decomposition of a polymer, i.e. low-density polyethylene (LDPE) that contains a condensed-phase-active intumescent flame retardant, i.e. ammonium polyphosphate (APP) mixed with pentaerythritol (PER) with ratio 3:1 (wt/wt). The methodology combines thermogravimetric analysis (TGA) in anaerobic environment over a wide range of heating rates and inverse modeling of these experiments based on the Shuffled Complex Evolution (SCE) optimization algorithm. The decomposition of neat LDPE can be described by a single first-order reaction. In contrast, the decomposition of the APP/PER additive is characterized by strong interactions between the reactants and a 5-step reaction mechanism was designed driven by the mechanisms of the intumescence process. The direct coupling of the 1-step reaction mechanism for LDPE and the 5-step reaction mechanism for APP/PER was found to provide decent predictions for the decomposition of the 90%LDPE/10% APP/PER blend. The slight interactions between the LDPE matrix and the APP/PER additive were taken into account by updating the kinetic parameters of two reactions, leading to a final robust and accurate 6-step reaction model.

A spectral method for fast sensitivity analysis: Perfectly stirred reactors

A new spectral method is proposed to speedup sensitivity analysis (SA) for ignition and extinction of perfectly stirred reactors (PSR). The method takes advantage of the linear dependency in the stoichiometric coefficient vectors of the elementary reactions in detailed kinetic mechanisms. Compared with the conventional SA that needs to perturb each elementary reaction, the spectral method lumps the elementary reactions into a smaller set of semi-global reactions that are basis vectors for the elementary reaction set, and perturbs each semi-global reaction instead. The sensitivities for the elementary reactions are then computed as linear combinations of the sensitivities for the semi-global reactions. As a result, the computational cost of the spectral method scales as the number of species, while that of the conventional method scales as the number of reactions. The proposed method is demonstrated for PSR extinction using a detailed H2 mechanism.

LES simulation of char particle gasification at Reynolds numbers up to 1000

The interplay of turbulent flow and heterogeneous gasification reactions of char particles is studied using Reynolds-averaged and large-eddy simulations. Currently available submodels for char gasification are typically developed for laminar flow conditions. For that reason, the simulation results are used to assess the applicability of these models at higher particle Reynolds numbers. Two representative gasification conditions were chosen on the basis of simulation data obtained for two different entrained-flow gasifiers: a 2 mm particle at atmospheric pressure in an O2/CO2/H2O atmosphere at 2006 K and a 263 µm particle at 30 bar in another O2/CO2/H2O atmosphere at 1480 K. The Navier–Stokes equations for the flow field were solved and were coupled with energy and species conservation equations. Six gaseous chemical species, H2, O2, CO, CO2, H2O, N2, and solid carbon, were considered and a semi-global reaction mechanism was applied for the homogeneous gas-phase reactions. Three heterogeneous gas-solid reactions were modeled: the water gas reaction, the Boudouard reaction and the oxidation of carbon to create carbon monoxide. The modification of the reaction zone due to a changing wake structure is presented in this work and the effect of turbulent structures on the overall carbon conversion rate is outlined. The impact of flow oscillations on char consumption and the particle temperature distribution at particle Reynolds numbers of 500 and 1000 is shown to be moderate.

Development of a Semiglobal Reaction Mechanism for the Thermal Decomposition of a Polymer Containing Reactive Flame Retardants: Application to Glass-Fiber-Reinforced Polybutylene Terephthalate Blended with Aluminum Diethyl Phosphinate and Melamine Polyphosphate.

This work details a methodology for parameterization of the kinetics and thermodynamics of the thermal decomposition of polymers blended with reactive additives. This methodology employs Thermogravimetric Analysis, Differential Scanning Calorimetry, Microscale Combustion Calorimetry, and inverse numerical modeling of these experiments. Blends of glass-fiber-reinforced polybutylene terephthalate (PBT) with aluminum diethyl phosphinate and melamine polyphosphate were used to demonstrate this methodology. These additives represent a potent solution for imparting flame retardancy to PBT. The resulting lumped-species reaction model consisted of a set of first- and second-order (two-component) reactions that defined the rate of gaseous pyrolyzate production. The heats of reaction, heat capacities of the condensed-phase reactants and products, and heats of combustion of the gaseous products were also determined. The model was shown to reproduce all aforementioned experiments with a high degree of detail. The model also captured changes in the material behavior with changes in the additive concentrations. Second-order reactions between the material constituents were found to be necessary to reproduce these changes successfully. The development of such models is an essential milestone toward the intelligent design of flame retardant materials and solid fuels.

Numerical study of the influence of particle reaction and radiative heat transfer on the flame velocity of gas/nanoparticles hybrid mixtures

Abstract A one-dimensional model was developed to determine the flame velocity of a gas mixture explosion through a two-phase media containing nanoparticles. The mass and energy balances, which take into account a semi-global reaction mechanism with 10 reactions for methane and one carbon nanoparticles combustion, were solved by the finite volume method. The flame propagation model shows a good agreement with commercial software (Premix) to estimate the final temperature, the mass fraction of burnt gases and the flame velocity. For methane/carbon black nanoparticles hybrid mixtures, the numerical model evidences that the insertion of 10 μm particles (agglomerates diameter) does not influence significantly the flame velocity. Nevertheless, if the particle diameter of the dispersed dust is equals to 75 nm (diameter of the primary particles), a considerable increase of 23% of the flame propagation velocity is obtained when only 6 g m−3 are added to the combustible mixture. Hence, the results of the numerical model suggest that the heat radiation contribution has a promoting effect on the flame propagation and it is consistent with the experimental increase on the explosion severity for some methane/carbon black hybrid mixtures.

Development of a semi-global reaction mechanism for thermal decomposition of a polymer containing reactive flame retardant

To enable intelligent design of fire resistant polymeric materials, it is highly important to have a quantitative understanding of the relation between the composition of these materials and chemical and physical properties that control the process of fire growth. This paper presents a methodology that provides a capability to determine the core subset of these properties including the kinetics and thermodynamics of the thermal decomposition of the condensed-phase constituents and enthalpy of combustion of gaseous pyrolyzates. This methodology is based on three experimental techniques – Thermogravimetric Analysis, Differential Scanning Calorimetry and Microscale Combustion Calorimetry – and inverse numerical modeling of the results of these tests. The material system analyzed in this study is polyamide 66 reinforced with chopped glass fiber and flame retarded with red phosphorous. This system shows a complex decomposition behavior that is highly dependent on the phosphorus concentration. First, a semi-global thermal decomposition mechanism consisting of a set of first- and second-order (two components) reactions was developed using three material specimens containing different phosphorus concentrations. Subsequently, the extrapolative power of the developed reaction model was demonstrated by accurately predicting the experimental measurements obtained for several compositions not used in the model development process.

A compact skeletal mechanism for n-dodecane with optimized semi-global low-temperature chemistry for diesel engine simulations

A skeletal mechanism with 54 species and 269 reactions was developed to predict pyrolysis and oxidation of n-dodecane as a diesel fuel surrogate involving both high-temperature (high-T) and low-temperature (low-T) conditions. The skeletal mechanism was developed from a semi-detailed mechanism developed at the University of Southern California (USC). Species and reactions for high-T pyrolysis and oxidation of C5-C12 were reduced by using reaction flow analysis (RFA), isomer lumping, and then merged into a skeletal C0-C4 core to form a high-T sub-mechanism. Species and lumped semi-global reactions for low-T chemistry were then added to the high-T sub-mechanism and a 54-species skeletal mechanism is obtained. The rate parameters of the low-T reactions were tuned against a detailed mechanism by the Lawrence Livermore National Laboratory (LLNL), as well as the Spray A flame experimental data, to improve the prediction of ignition delay at low-T conditions, while the high-T chemistry remained unchanged. The skeletal mechanism was validated for auto-ignition, perfectly stirred reactors (PSR), flow reactors and laminar premixed flames over a wide range of flame conditions. The skeletal mechanism was then employed to simulate three-dimensional turbulent spray flames at compression ignition engine conditions and validated against experimental data from the Engine Combustion Network (ECN).

Pyrolysis model of single biomass pellet in downdraft gasifier

By coupling the heat transfer equation with semi-global chemical reaction kinetic equations, a onedimensional, unsteady mathematical model is developed to describe the pyrolysis of single biomass pellet in the pyrolysis zone of downdraft gasifier. The simulation results in inert atmosphere and pyrolysis zone agree well with the published experimental results. The pyrolysis of biomass pellets in pyrolysis zone is investigated, and the results show that the estimated convective heat transfer coefficient and emissivity coefficient are suitable. The mean pyrolysis time is 15.22%, shorter than that in inert atmosphere, and the pellet pyrolysis process in pyrolysis zone belongs to fast pyrolysis. Among the pyrolysis products, tar yield is the most, gas the second, and char the least. During pyrolysis, the temperature change near the center is contrary to that near the surface. Pyrolysis gradually moves inwards layer by layer. With the increase of pyrolysis temperature and pellet diameter, the total pyrolysis time, tar yield, char yield and gas yield change in different ways. The height of pyrolysis zone is calculated to be 1.51–3.51 times of the characteristic pellet diameter.

Modeling and simulation of biomass fast pyrolysis in a fluidized bed reactor

This paper presents a phenomenologically based mathematical model for biomass fast pyrolysis process in a bubbling fluidized-bed. The model was developed in transient state, is one-dimensional and is based on the two-phase theory. A semi-global reaction mechanism in two stages was used, considering the primary formation of products and the secondary reactions of the vapors. Also, population balances for the density/temperature distribution of the particles and distribution of the bubble size were proposed. The model can predict the temperature of the phases, the distribution and yields of the products, the heating rate of particles, and the residence time of gases. Finally, a solution algorithm for the model was proposed, which was programmed in MATLAB 7.0, being able to find a good fit between simulation results and the experimental data.

On lumped-reduced reaction model for combustion of liquid fuels

A systematic approach to developing compact reduced reaction models is proposed for liquid hydrocarbon fuels using n-dodecane and n-butane as the model fuels. The approach has three elements. Fast fuel cracking reactions are treated by the quasi-steady state approximation (QSSA) and lumped into semi-global reactions to yield key cracking products that are C1–C4 in size. Directed relation graph (DRG) and sensitivity analysis reduce the foundational fuel chemistry model to a skeletal model describing the oxidation of the C1–C4 compounds. Timescale-based reduction using, e.g., QSSA, is then employed to produce the final reduced model. For n-dodecane, a 24-species reduced model is derived from JetSurF and tested against the detailed model for auto-ignition, perfectly stirred reactors (PSR), premixed flame propagation, and extinction of premixed and non-premixed counterflow flames. It is shown that the QSSA of fuel cracking reactions is valid and robust under high-temperature conditions from laminar flames, where mixing is controlled by molecular diffusion, to perfectly stirred reactors, which correspond to the limit of fast turbulent mixing. Bifurcation analysis identifies the controlling processes of ignition and extinction and shows that these phenomena are insensitive to the details of fuel cracking. To verify the applicability of the above finding to turbulent flames, 2-D direct numerical simulation (DNS) of a lean turbulent premixed flame of n-butane/air with Karlovitz number of 250 was carried out using a reduced model developed from USC-Mech II. The results show that QSSA for fuel cracking remains valid even under intense turbulence conditions. Statistical analysis of the DNS data shows that fuel cracking is complete before the flame zone, and for the conditions tested, turbulent transport does not bring any significant fuel molecules into the flame zones, thus further substantiating the validity of the approach proposed.

Heat transfer-limited flash pyrolysis of woody biomass: Overall reaction rate and time analysis using an integral model with experimental support

High temperature flash pyrolysis takes place on the second to sub-second time-scale depending on particle size. This particle size dependency conveys an overall heat transfer-limited reaction process as shown by characteristic time analysis. For processes that employ rapid pyrolysis, such as gasification, global or semi-global reaction models are used and are confounded with competing physical phenomena. A heat transfer-limited integral model was used to resolve characteristic pyrolysis times and rates fitted to experimental results from a drop-tube reactor and woody biomass at temperatures in the range 1773–2273K. Two model derivations were compared and little difference was observed. Particle size, biomass density, reactor wall temperature, and heat of reaction were the most significant parameters from a sensitivity analysis. Experimentally, it was found that increasing initial particle temperature above the boiling point of water changed the thermophysical properties of the reacting particle such that the reaction time was accelerated up to 30%, affecting the calculated heat of reaction and thermal diffusivity. Halving the particle size resulted in up to 75% decreased pyrolysis reaction time, and increasing reactor wall temperature from 1773K to 2273K resulted in up to 50% faster reaction times.

Fast Pyrolysis of Wood for Biofuels: Spatiotemporally Resolved Diffuse Reflectance In situ Spectroscopy of Particles.

Fast pyrolysis of woody biomass is a promising process capable of producing renewable transportation fuels to replace gasoline, diesel, and chemicals currently derived from nonrenewable sources. However, biomass pyrolysis is not yet economically viable and requires significant optimization before it can contribute to the existing oil-based transportation system. One method of optimization uses detailed kinetic models for predicting the products of biomass fast pyrolysis, which serve as the basis for the design of pyrolysis reactors capable of producing the highest value products. The goal of this work is to improve upon current pyrolysis models, usually derived from experiments with low heating rates and temperatures, by developing models that account for both transport and pyrolysis decomposition kinetics at high heating rates and high temperatures (>400 °C). A new experimental technique is proposed herein: spatiotemporally resolved diffuse reflectance in situ spectroscopy of particles (STR-DRiSP), which is capable of measuring biomass composition during fast pyrolysis with high spatial (10 μm) and temporal (1 ms) resolution. Compositional data were compared with a comprehensive 2D single-particle model, which incorporated a multistep, semiglobal reaction mechanism, prescribed particle shrinkage, and thermophysical properties that varied with temperature, composition, and orientation. The STR-DRiSP technique can be used to determine the transport-limited kinetic parameters of biomass decomposition for a wide variety of biomass feedstocks.

From Detailed Description of Chemical Reacting Carbon Particles to Subgrid Models for CFD

This work is devoted to the development and validation of a sub-model for the partial oxidation of a spherical char particle moving in an air/steam atmosphere. The particle diameter is 2 mm. The coal particle is represented by moisture- and ash-free nonporous carbon while the coal rank is implemented using semi-global reaction rate expressions taken from the literature. The submodel includes six gaseous chemical species (O 2 , C O 2 , CO, H 2 O , H 2 , N 2 ). Three heterogeneous reactions are employed, along with two homogeneous semi-global reactions, namely carbon monoxide oxidation and the water-gas-shift reaction. The distinguishing feature of the subgrid model is that it takes into account the influence of homogeneous reactions on integral characteristics such as carbon combustion rates and particle temperature. The sub-model was validated by comparing its results with a comprehensive CFD-based model resolving the issues of bulk flow and boundary layer around the particle. In this model, the Navier-Stokes equations coupled with the energy and species conservation equations were used to solve the problem by means of the pseudo-steady state approach. At the surface of the particle, the balance of mass, energy and species concentration was applied including the effect of the Stefan flow and heat loss due to radiation at the surface of the particle. Good agreement was achieved between the sub-model and the CFD-based model. Additionally, the CFD-based model was verified against experimental data published in the literature (Makino et al. (2003) Combust. Flame 132, 743-753). Good agreement was achieved between numerically predicted and experimentally obtained data for input conditions corresponding to the kinetically controlled regime. The maximal discrepancy (10%) between the experiments and the numerical results was observed in the diffusion-controlled regime. Finally, we discuss the influence of the Reynolds number, the ambient O 2 mass fraction and the ambient temperature on the char particle behaviour.

A semi-global reaction rate model based on experimental data for the self-hydrolysis kinetics of aqueous sodium borohydride

This work studied the self-hydrolysis kinetics of aqueous sodium borohydride (NaBH4) for hydrogen generation and storage purposes. Two semi-global rate expressions of sodium borohydride and hydrogen ion consumption were derived from an extensive series of batch process experiments where the following parameters were systematically varied: solution temperature (298 K–348 K), NaBH4 concentration (0.5 wt% to 25.0 wt%), and sodium hydroxide (NaOH) concentration (0.0 wt% to 4.0 wt%). Transient hydrogen generation rates and transient solution pH were measured during the hydrolysis experiments. Given initial conditions (temperature, NaBH4 concentration, and H+ concentration), the two coupled semi-global rate equations can be integrated to obtain the transient time history of H2 generation (or NaBH4 consumption) and solution pH (or H+ concentration). Comparing analytical results of transient hydrogen generation rate and transient solution pH with experimental data, good agreement was reached for many conditions, especially for elevated solution pH values, levels at which NaBH4 solutions are used practically.

Numerical study of the influence of heterogeneous kinetics on the carbon consumption by oxidation of a single coal particle

This work is devoted to the numerical study of the influence of heterogeneous kinetics on the oxidation rates of a single carbon particle in quiescent and non-quiescent environments. The coal particle is represented by moisture- and ash-free nonporous carbon while the coal rank is implemented using several kinetic rate expressions. The model includes six gaseous chemical species (O2, CO2, CO, H2O, H2, N2). Three heterogeneous reactions are employed (C+O2, C+CO2 and C+H2O), along with two homogeneous semi-global reactions, namely carbon monoxide oxidation and the water–gas shift reaction. Several semi-global reaction rate expressions taken from the literature are utilized. The Navier–Stokes equations coupled with the energy and species conservation equations are used to solve the problem by means of the pseudo-steady state approach. At the surface of the particle, the balance of mass, energy and species concentration is applied including the effect of the Stefan flow and heat loss due to radiation at the surface of the particle. The model and the code used are validated against an analytic two-film model. Good agreement is observed. The numerical simulations performed reveal that the origin of heterogeneous kinetics has a significant effect on the carbon consumption rates of the particle. In particular, the maximal discrepancy between results is achieved in a kinetically controlled regime and is proportional by a factor of 10 in respect to carbon mass flux on the particle surface. Additionally, the influence of the particle Reynolds number referring to the laminar flow regime, the ambient O2 mass fraction and the temperature on the regimes of combustion and gasification is discussed.

A reduced mechanism for ethylene/methane mixtures with excessive NO enrichment

A detailed mechanism for methane–ethylene mixtures enriched with excessive amount of NO was systematically reduced for efficient numerical simulations of flames in arc-heated co-flowing air. Methane and ethylene were selected as the surrogate fuel in the present study due to their drastically different features of ignition and extinction properties and flame propagation speeds, such that the mixtures of them may be utilized to mimic practical hydrocarbon fuels with various kinetic properties in experiments. The recently released USC Mech-II for C1–C4 was grafted with the NO x sub-mechanism in GRI-Mech 3.0 with updated reaction parameters for prompt NO formation. The resulting detailed mechanism with 129 species and 900 reactions was first validated against experiments involving NO x enrichment and reasonably good agreements were observed. The detailed mechanism was then employed as the starting mechanism for the reduction. A skeletal mechanism with 44 species and 269 reactions was derived using the methods of directed relation graph (DRG) and DRG-aided sensitivity analysis (DRGASA); a 39-species reduced mechanism with 35 semi-global reaction steps was further obtained using the linearized quasi steady state approximations (LQSSA). Five species related to prompt NO were retained in the reduced mechanism because of their significant impacts on the fuel oxidation. The reduced mechanism closely agrees with the detailed mechanism for ignition and extinction of homogenous mixtures, as well as selected 1-D flames over a wide range of parameters with NO concentrations between 0% and 3%. The observed worst-case relative error of the reduction is approximately 20%. The reduced mechanism was further validated with experiments involving excessive NO x enrichment.

Numerical study of fast pyrolysis of woody biomass in a gravity‐driven reactor

AbstractIn the present study, computational fluid dynamics is applied to investigate fast pyrolysis characteristics of woody biomass in a gravity‐driven reactor. A Eulerian‐Eulerian approach is adopted, including three gas species and three solid phases, for multiphase reacting flow fields between solid and gas. Two‐stage semiglobal reaction kinetics considering the secondary tar cracking mechanism is used for simulation of the pyrolysis reaction. The flow and pyrolysisreaction characteristics of the reactor are investigated, and the reaction rates of woody biomass pyrolysis are evaluated for streamwise locations in the reactor. Also, the inlet condition and inclination angle are changed to study their effects on the reaction. From the predicted results, it is seen that the heat transfer to biomass is governed mainly by mixing of particles, and this influences the final pyrolysis reaction. © 2009 American Institute of Chemical Engineers Environ Prog, 2009