Devolatilization Model Research Articles

Pyrolysis and Devolatilization of High-Density Polyethylene

A simple model for combined pyrolysis and devolatilization of high-density polyethylene (HDPE) is developed and studied. The model parameters are estimated from available literature data, and the resulting predictions are compared with a single thermogravimetric experiment. It is found that the bubble loss model makes a better prediction than a critical carbon number loss model. As expected, the bubble loss model is seen to lose validity when the sample becomes predominantly composed of volatile species.

Pyrolysis and Devolatilization of High-Density Polyethylene

A simple model for combined pyrolysis and devolatilization of high-density polyethylene (HDPE) is developed and studied. The model parameters are estimated from available literature data, and the resulting predictions are compared with a single thermogravimetric experiment. It is found that the bubble loss model makes a better prediction than a critical carbon number loss model. As expected, the bubble loss model is seen to lose validity when the sample becomes predominantly composed of volatile species.

Comprehensive Kinetic Modeling Study of Bio-oil Formation from Fast Pyrolysis of Biomass

The aim of this work is to analyze the optimal operating conditions for fast biomass pyrolysis. The operating conditions required to maximize the yield of liquid products are investigated and discussed on the basis of a comprehensive mathematical model of wood/biomass devolatilization. Crucial issues are the fast and complete heating of biomass particles to reduce char formation and the rapid cooling of released products to reduce the role of secondary gas-phase pyrolysis reactions. Chemical kinetics as well as heat- and mass-transfer phenomena play an important role in this process; thus, a comprehensive kinetic model is applied. The proposed model, when compared to the majority of other devolatilization models, attempts to characterize pyrolysis reactions with a lumped stoichiometry using a limited number of equivalent components to describe not only gaseous products but also tar species. Model predictions are compared to experimental measurements not only with further validation in mind but also princi...

Large eddy simulation of a pulverized coal jet flame ignited by a preheated gas flow

Large eddy simulation (LES) is applied to a pulverized coal jet flame ignited by a preheated gas flow. The simulation results are compared to experimental data obtained for the inlet stoichiometric ratios of 0.14, 0.22, and 0.36. An accurate and computationally inexpensive devolatilization model suitable for combustion simulation in LES is proposed and incorporated into the LES. The numerical results of gas temperature and coal burnout on the centerline show good agreement with the experimental data. Two kinds of lift-off heights are introduced to verify the combustion simulation. One is the height from the primary nozzle exit to the starting point of the growing flame region. The other is the height from the primary nozzle exit to the starting point of the continuous flame region. The calculated results of the two lift-off heights show good agreement with the experimental data. In contrast to LES, the standard k– ε model overestimates the lift-off heights because it calculates time-averaged temperature which does not contain information about local flame structure. The stoichiometric ratio in the gas phase at the starting point of the growing flame region is found to be independent of the inlet stoichiometric ratio in the range from 0.14 to 0.36.

Formation of gold deposits: a metamorphic devolatilization model

AbstractA metamorphic devolatilization model can explain the enrichment, segregation, timing, distribution and character of many goldfields such as those found in Archean greenstone belts, slate‐belts and other gold‐only provinces. In this genetic model, hydrated and carbonated greenschist facies rocks, particularly metabasic rocks, are devolatilized primarily across the greenschist–amphibolite facies boundary in an orogenic setting. Devolatilization operates on the scale of individual mineral grains, extracting not just H2O and CO2 but also S and, in turn, Au. Elevated gold in solution is achieved by complexing with reduced S, and by H2CO3 weak acid buffering near the optimal fluid pH for gold solubility (the buffering is more important than being at the point of maximum gold solubility). Low salinity ensures low base metal concentrations in the auriferous metamorphic fluid. Migration of this fluid upwards is via shear zones and/or into hydraulic fracture zones in rocks of low tensile strength. The geometry of the shear zones dictates the kilometre‐scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of gold. Deposition of gold from solution necessitates breakdown of the gold–thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon‐bearing host rocks and/or by sulphidation of wallrocks to generate iron‐bearing sulphide and precipitated gold. As such, black slate, carbon seams, banded iron formation, tholeiitic basalt, magnetite‐bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major goldfields. Gold deposition is accompanied by carbonation, sulphidation and muscovite/biotite alteration where the host rock is of suitable bulk composition. The correlation of major gold deposits with rock type, even when the gold is primarily in veins, argues for rock‐dominated depositional systems, not fluid‐dominated ones. As a consequence, a general role in gold deposition for fluid mixing, temperature decrease and/or fluid pressure decrease and boiling is unlikely, although such effects may be involved locally. Several geological features that are recorded at gold‐only deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process. Overprinting by higher‐grade metamorphism and deformation, and/or (palaeo)‐weathering may provide many of the most‐obvious features of goldfields including their mineralogy, geochemistry, geometry, small‐scale timing features, geophysical response and even mesoscopic gold distribution.

CFD simulation of entrained-flow coal gasification: Coal particle density/sizefraction effects

Computational Fluid Dynamics (CFD) simulation of commercial-scale two-stage upflow and single-stage downflow entrained-flow gasifiers was conducted to study effects of simulating both the coal particle density and size variations. A previously-developed gasification CFD model was modified to account for coal particle density and size distributions as produced from a typical rod mill. Postprocessing tools were developed for analysis of particle–wall impact properties. For the two-stage upflow gasifier, three different simulations are presented: two (Case 1 and Case 2) used the same devolatilization and char conversion models from the literature, while Case 3 used a different devolatilization model. The Case 1 and Case 3 solutions used average properties of a Pittsburgh #8 seam coal ( d = 108 μm, SG = 1.373), while Case 2 was obtained by injecting and tracking all of the series of 28 different coal particle density and size mass fractions obtained by colleagues at PSU as a part of the current work, for this same coal. Simulations using the two devolatilization models (Case 1 and Case 3) were generally in reasonable agreement. Differences were observed between the single-density solution and the density/size partitioned solution (Case 1 and Case 2). The density/size partitioned solution predicted nominally 10% less CO and over 5% more H 2 by volume in the product gas stream. Particle residence times and trajectories differed between these two solutions for the larger density/size fractions. Fixed carbon conversion was 4.3% higher for the partitioned solution. Particle–wall impact velocities did not vary greatly. Grid independence studies for the two-stage upflow gasifier geometry showed that the grid used in the comparison studies was adequate for predicting exit gas composition and wall impact velocities. Validation studies using experimental data for the Pittsburgh #8 coal from the SRI International pressurized coal flow reactor (PCFR) at 30 atmospheres indicated adequate agreement for gasification and combustion cases, but poor agreement for a pyrolysis case. Simulation of a single-stage downflow gasifier yielded an exit gas composition that was in reasonable agreement with published data.

A Mass Transfer Model for Devolatilization of Highly Viscous Media in Rotating Packed Bed

A mass transfer model for devolatilization process of highly viscous media in rotating packed bed (RPB) was developed based on penetration theory and mass conservation. Before establishing the model, some mass transfer experiments of thin film were conducted in a designed diffusion cell including vacuum and feeding system. In this study, acetone was used as the volatile organic compound (VOC) and syrup as the highly viscous media. The thickness of thin film was changed by using different liquid distributor. It was found that bubbling played an important role in the devolatilization. The correlation of diffusion coefficient of acetone in highly viscous dilute solution was proposed. The relative error between predicted and experimental data was within the range of ±30% for diffusion coefficient of acetone in syrup. A comparison of experimental data of RPB with model indicated that the relative error was within ±30% for efficiency of acetone removal.

Pyrolysis of wheat straw in a thermogravimetric analyzer: Effect of particle size and heating rate on devolatilization and estimation of global kinetics

The influence of different parameters such as particle size, initial weight of the sample, and heating rate on the devolatilization of wheat straw particles have been studied using thermogravimetric analysis. In addition, the variations in proximate analysis with different particle sizes of wheat straw have also been investigated. Results show that the curves corresponding to the third stage of pyrolysis differ for variations in particle size, initial weight, and heating rate of the pyrolysis process. A devolatilization model with three parallel nth-order reactions has been considered to determine the global kinetic parameters using thermogravimetric data. The kinetic parameters such as activation energy (kJ/mol), frequency factor (1/min), and order of the reaction for the three stages considered in devolatilization model were E 1 = 69, E 2 = 78, E 3 = 80; k 01 = 2.57 × 10 12, k 02 = 3.97 × 10 7, k 03 = 3.17 × 10 6; and n 1 = 2.3, n 2 = 0.65, n 3 = 2.7, respectively. It was noted from the order of the reaction that the second stage of the pyrolysis curve corresponds to the degradation of cellulose and hemicellulose, and the third stage to the lignin degradation.

Effect of turbulence and devolatilization models on coal gasification simulation in an entrained-flow gasifier

Numerical simulations of the oxygen-blown coal gasification process inside a generic entrained-flow gasifier are carried out. The Eulerian–Lagrangian approach is applied to solve the Navier–Stokes equations and the particle dynamics. Seven species transport equations are solved with three heterogeneous global reactions and two homogeneous reactions. Finite rates are used for the heterogeneous solid-to-gas reactions. Both finite rate and eddy-dissipation combustion models are calculated for each homogeneous gas-to-gas reaction, and the smaller of the two rates is used. Four different devolatilization models are employed and compared. The Kobayashi model produces slower devolatilization rate than the other models. The constant rate model produces the fastest devolatilization rate. The single rate model and the chemical percolation model produce moderate and consistent devolatilization rate. Slower devolatilization rate produces higher exit gas temperature and higher CO and CO 2 mass fractions, but lower H 2 and heating value, and hence, achieves lower gasification efficiency. Combustion of volatiles is modeled with two-stage global reactions with an intermediate stage via benzene. Turbulence models significantly affect the simulated results. Among five turbulence models employed, the standard k– ε and the RSM models give consistent results. The time scale for employing stochastic time tracking of particles also affects simulated result. Caution has to be exerted to select the appropriate time constant value. Smaller particles have a higher surface/volume ratio and react faster than larger particles. However, large particles possessing higher inertia could impinge on the opposing jet and change the thermal-flow filed and the reaction rates.

Detailed kinetic modeling of the thermal degradation of lignins

The aim of this kinetic work is to provide a better understanding of the pyrolysis of lignin and biomasses not only in terms of devolatilazation rate but also of the volatile species released. The complexity of both lignin structure and its degradation mechanism meant that a lumping approach suitable for handling the huge amount of initial, intermediate and final products had to be used. Despite these simplifications, the proposed semi-detailed kinetic scheme involves about 100 molecular and radical species in 500 elementary and lumped reactions. It has already been proved that this lignin devolatilization model correctly predicts the degradation rates and the detail of the released products. This work constitutes an initial yet significant step towards deriving a complete kinetic scheme of biomass devolatilization.

Predictions of NOX in a Laboratory Pulverized Coal Combustor Operating under Air and Oxy-Fuel Conditions

A new approach to modeling NOX under oxy-fuel combustion conditions in a simple staged-oxidizer flow field is presented. The approach is centered on the combination of devolatilization and char oxidation models with a detailed kinetic mechanism for light hydrocarbon combustion. NOX chemistry is included by the user's selection of the detailed mechanism, while the devolatilization model consists of the chemical percolation devolatilization (CPD) model modified to be independent of oxidizer composition. Literature-based correlations provide elemental composition of the volatiles. Model predictions were compared to experimental measurements with good agreement in several respects. The model provides insights for the interpretation of experimental oxy-fuel combustion NOX results, and recommendations are given for computational fluid dynamics (CFD) modeling of NOX in oxy-fuel combustion.

A predictive multi-step kinetic model of coal devolatilization

Devolatilization is the first step in coal combustion and gasification, thus an accurate kinetic modeling is relevant for the optimal design of these processes. In this work a relatively simple but flexible kinetic model is used to predict the thermal degradation of different coals in a wide range of operating conditions. The main feature of the model lies in its predictive capability: the elemental composition of the starting coal and the operating conditions are the only information required. Three reference coals are used to characterize the devolatilization process. The pyrolysis of each reference coal is described with a multi-step kinetic mechanism effective both at high and low heating rates. The devolatilization of the actual coal is simply obtained as a linear combination of the thermal degradation of the reference coals. The complete kinetic model refers to ∼30 reactions and lumped species, which makes this scheme suitable for being adopted in fluidynamic computations. A wide collection of comparisons between model prediction and experimental data validates this model both in terms of residual char and in terms of detailed gas and tar composition. The importance of secondary gas-phase reactions, mainly at high pressure, is also discussed and verified on the basis of an existing detailed kinetic scheme of pyrolysis and oxidation of hydrocarbon fuels.

Mathematical modeling of MILD combustion of pulverized coal

MILD (flameless) combustion is a new rapidly developing technology. The IFRF trials have demonstrated high potential of this technology also for N-containing fuels. In this work the IFRF experiments are analyzed using the CFD-based mathematical model. Both the Chemical Percolation Devolatilization (CPD) model and the char combustion intrinsic reactivity model have been adapted to Guasare coal combusted. The flow-field as well as the temperature and the oxygen fields have been accurately predicted by the CFD-based model. The predicted temperature and gas composition fields have been uniform demonstrating that slow combustion occurs in the entire furnace volume. The CFD-based predictions have highlighted the NO x reduction potential of MILD combustion through the following mechanism. Before the coal devolatilization proceeds, the coal jet entrains a substantial amount of flue gas so that its oxygen content is typically not higher than 3–5%. The volatiles are given off in a highly sub-stoichiometric environment and their N-containing species are preferentially converted to molecular nitrogen rather than to NO. Furthermore, there exists a strong NO-reburning mechanism within the fuel jet and in the air jet downstream of the position where these two jets merge. In other words, less NO is formed from combustion of volatiles and stronger NO-reburning mechanisms exist in the MILD combustion if compared to conventional coal combustion technology.

Prediction of Light Gas Composition in Coal Devolatilization

The chemical percolation devolatilization (CPD) model describes the devolatilization behavior of rapidly heated coal based on the chemical structure of the coal. It predicts the overall char, tar, and light gas yields. This paper presents an improved CPD model with improved capability for predicting light gas composition. This is achieved by incorporating a kinetic model that simulates the release of various light gas species from their respective sources/functional groups in coal. The improved CPD model is validated using experiments with a wire mesh reactor and published experimental observations.

Biomass pyrolysis: Kinetic modelling and experimental validation under high temperature and flash heating rate conditions

This work analyzes and discusses the general features of biomass pyrolysis, both on the basis of a new set of experiments and by using a detailed kinetic model of biomass devolatilization that includes also successive gas phase reactions of the released species and is therefore able to predict the main gases composition. Experiments are performed in a lab-scale Entrained Flow Reactor (EFR) to investigate biomass pyrolysis under high temperatures (1073–1273 K) and high heating fluxes (10–100 kW m −2). The influence of particle dimensions and temperature has been tested versus solid residence time in the reactor. The particle size appeared as the most crucial parameter. The pyrolysis of 0.4 mm particles is nearly finished under this range of temperatures after a reactor length of 0.3 m, with more than 75 wt% of gas release, whereas the conversion is still under evolution until the end of the reactor for larger particles up to 1.1 mm, due to internal heat transfer limitations. The preliminary comparisons between the model and the experimental data are encouraging and show the ability of this model to contribute to a better design and understanding of biomass pyrolysis process under severe conditions of temperature and heating fluxes typically found in industrial gasifiers.

Chemical Kinetics of Biomass Pyrolysis

This paper analyzes the main kinetic features of biomass pyrolysis, devolatilization, and the gas phase reactions of the released species. Three complex steps are faced in sequence: the characterization of biomasses, the description of the release of the species, and finally, their chemical evolution in the gas phase. Biomass is characterized as a mixture of reference constituents: cellulose, hemicellulose, and lignin. This assumption is verified versus experimental data, mainly relating to thermal degradation of different biomasses. Devolatilization of biomasses is a complex process in which several chemical reactions take place in both the gas and the condensed phase alongside the mass and thermal resistances involved in the pyrolysis process. A novel characterization of the released species is applied in the proposed devolatilization models. The successive gas phase reactions of released species are included into an existing detailed kinetic scheme of pyrolysis and oxidation of hydrocarbon fuels. Comparisons with experimental measurements in a drop tube reactor confirm the high potentials of the proposed modeling approach.

Modelling of pulverised wood combustion using a functional group model

Modelling of pulverised wood flames in a laboratory vertical furnace was carried out. The aim was to gain deeper understanding of the combustion process and to validate a mathematical model to simulate the process. Pulverised wood combustion involves many different processes such as two-phase flow dynamics, drying and devolatilisation of the particles, oxidation of the volatile and formation and oxidation of char. It is desirable to know which are the most dominating/sensitive processes that control the combustion behaviour and in particular the emissions of unburned hydrocarbons and carbon monoxide. To achieve this goal, a comprehensive devolatilisation model based on the functional group concept is applied to predict the details of the devolatilisation products including tar. The solid-gas coupling is made using the Eulerian/Lagrangian approach. A ‘rocket force’ model is developed to account for the influence of drying and devolatilisation on the particle motion. The present mathematical model successfully simulated the flame temperature and detailed species distributions including CH4 and CO. These two species were shown to be sensitive to the fate of tar. Major paths for the CO formation were identified as the devolatilisation of the wood particles and the char oxidation. Influences of the initial functional group yield, the char oxidation and gasification reactions, the turbulence mixing rate and the fuel particle size on the flame structures andb emissions were examined.

Prediction of Pyrolysis Process for Wood and Grass Biomass by CPD Model

This paper presents new pyrolysis model of wood and grass biomass. It is named biomass-CPD model. A scheme of the Chemical Percolation Devolatilization (CPD) model which was used in numerical analysis of coal pyrolysis was applied to description of wood and grass biomass pyrolysis. Cellulose, hemicellulose and lignin which are three main components of wood and grass were modeled at first. A molecular structure of each component was determined from that of its monomer, and a rate constant of each component was determined from both the conventional model and the pyrolysis curve from experiment. The pyrolysis process of wood and grass biomass was expressed by a summation of that of cellulose, hemicellulose and lignin calculated from the developed model. The content of each component in wood and grass was predicted from the ultimate analysis and the proximate analysis of wood and grass. By using above-mentioned new method, the pyrolysis process and the yields of total volatiles and tar of wood and grass biomass was predicted by only values of biomass properties and pyrolysis conditions. From a comparison between calculation and experimental results, the predicted pyrolysis process and yields of total volatiles and tar of various wood and grass biomass pyrolyzed under various conditions are in good agreement with the experimental ones, and we show the validity of the biomass-CPD model.

The importance of talc and chlorite “hybrid” rocks for volatile recycling through subduction zones; evidence from the high-pressure subduction mélange of New Caledonia

The transfer of fluid and trace elements from the slab to the mantle wedge cannot be adequately explained by simple models of slab devolatilization. The eclogite-facies melange belt of northern New Caledonia represents previously subducted oceanic crust and contains a significant proportion of talc and chlorite schists associated with serpentinite. These rocks host large quantities of H2O and CO2 and may transport volatiles to deep levels in subduction zones. The bulk-rock and stable isotope compositions of talc and chlorite schist and serpentinite indicate that the serpentinite was formed by seawater alteration of oceanic lithosphere prior to subduction, whereas the talc and chlorite schists were formed by fluid-induced metasomatism of a melange of mafic, ultramafic and metasedimentary rocks during subduction. In subduction zones, dehydration of talc and chlorite schists should occur at sub-arc depths and at significantly higher temperatures (∼ 800°C) than other lithologies (400–650°C). Fluids released under these conditions could carry high trace-element contents and may trigger partial melting of adjacent pelitic and mafic rocks, and hence may be vital for transferring volatile and trace elements to the source regions of arc magmas. In contrast, these hybrid rocks are unlikely to undergo significant decarbonation during subduction and so may be important for recycling carbon into the deep mantle.

Modeling and Practice of Ethanol-devolatilization of Silica-silane Rubber Compoundsin an Internal Mixer

Abstract During mixing of a rubber compound containing silica and silane, the mixer is not only used for the dispersion of the filler and other ingredients, but also for a chemical reaction. These two functionalities of the mixer result in opposite processing requirements: A good dispersion is reached by high shearing forces, increasing the compound temperature. The silanization is not dependent on high shearing forces; it is positively influenced by high temperatures, but with an increasing risk of scorch. Another drawback is the equilibrium between the ethanol concentration in the vapor phase in the void space of the mixing chamber and in the rubber phase, which is limiting the reaction rate of the silanization. Devolatilization of the compound is a crucial factor for the efficiency of the silanization reaction. In this article a model for devolatilization of a rubber compound in an internal mixer is developed, including a chemical reaction replenishing the volatile component during the devolatilization process. The model is based on the penetration theory, with the main contribution to the devolatilization being convective mass transfer. The main influencing factors, theoretically deducted and practically verified, are temperature, mixer volume, fill factor, rotor speed, reaction time and partial pressure of the volatile component in the void volume of the mixer.