Coal Devolatilization Research Articles

Prediction of product distributions in coal devolatilization by an artificial neural network model

Currently most coal combustion simulations treat the devolatilization products as a mixture of light gases with a given proportion or a postulate substance, which is obviously different from the reality. To obtain a more accurate treatment on the product distribution from coal devolatilization, an artificial neural network (ANN) model is innovatively developed based on a training database constructed from diverse experimental data for a wide range of coal types under a wide range of heating conditions. The accuracy and applicability of the developed ANN model are validated and compared with that of the chemical percolation devolatilization coupled with the functional group (FG-CPD) model for the validation database, and the relative impact of each input parameter on the evolution of each devolatilization product is evaluated. The results show that the detailed product distributions of coal devolatilization predicted by the proposed ANN model are in good agreement with the experimental data for both the training and validation database, and the ANN model can give a more accurate prediction on both the yield of each component and its evolution compared with the FG-CPD model. The coal composition accounts for the most impact (above 60%) on the product distribution, and the relative impact of Cdaf, Hdaf, Odaf, coal particle diameter, instantaneous heating rates, particle residence time and particle temperature decrease successively. This ANN model has great potential to be coupled into coal combustion simulations to improve efficiency and accuracy, which will be studied in the future.

A deep insight into carbon conversion during Zhundong coal molten salt gasification

Molten salt gasification is a promising way for collaborative use of solar energy and Zhundong coal, with in situ capture of Cl/S-pollutants as well as alkali metals. The present study provides a deep insight into carbon conversion during molten salt gasification of a typical Zhundong coal in ternary carbonate eutectics (Li2CO3-Na2CO3-K2CO3). The results demonstrated that the molten salt gasification of Zhundong coal underwent a rapid devolatilization of raw coal and then the gasification of char. The interactions between CO2 and volatiles promoted the cracking of macromolecular compounds, increasing the carbon conversion efficiency during the devolatilization process. The gasification of char was mainly determined by the direct reaction with molten salt, which was slightly affected by the CO2 concentration in the carrier gas. The results of char characterization evidenced that the condensation/graphitization of char was stimulated in the molten eutectics. Meanwhile, a large amount of O-containing groups, especially carbonyl groups, were formed in the molten salt treated char with carbonates as oxygen supplier. Under the influence of molten salts, some carbonyl groups would be formed inside the char. However, the decomposition of those carbonyl groups was suppressed by the mass transfer resistance of the released CO and/or CO2. Applying proper feeding mode, such as feeding coal in pulverized form, could enhance the contact of coal, molten salt and CO2, which remarkably promoted the interactions between molten salts and char containing functional groups.

FLASHCHAIN Theory for Rapid Coal Devolatilization Kinetics. 10. Extents of Conversion for Hydropyrolysis and Hydrogasification of Any Coal

This paper extends FLASHCHAIN theory with mechanisms for (i) hydrogenation of labile bridges in the condensed coal phase which enhance tar yields during primary devolatilization under some, but not all, conditions and (ii) heterogeneous hydrogasification of char into CH4. Under the H2 pressures associated with entrained coal gasification technology, coal devolatilization at heating rates faster than 103 °C/s does not provide sufficient time for appreciable bridge hydrogenation so tar yields are not enhanced. Conversely, tar yields under slow heating conditions are strongly enhanced. The mechanisms proposed for bridge hydrogenation and its associated impact on fragment recombination in FLASHCHAIN accurately depict the joint impact of heating rate and H2 pressure on tar yields. Whereas tar yields from rapid devolatilization diminish for progressively higher pressures, total weight loss remains steady or passes through a minimum at some H2 pressure around 1 MPa, because char hydrogasification counteracts the lower weight loss associated with diminished tar yields at progressively higher pressures. A single, half-order methanation reaction within the CBK/G framework gave results within the measurement uncertainties through 15 MPa H2. In combination, the mechanisms for hydropyrolysis and hydrogasification accurately interpreted a database representing coals of rank from lignite to medium volatile bituminous; heating rates from 1 to 103 °C/s; temperatures from 550 to 1150 °C; reaction times to 180 s; and particle diameters to 1 mm. The assigned hydrogasification reactivities are far less variable than those for char gasification by steam and CO2, and show no consistent trend with rank.

Devolatilization and Combustion of Coarse-Sized Coal Particles in Oxy–Fuel Conditions: Experimental and Modeling Studies

A generalized unsteady-state kinetic model, coupled with all modes of heat transfer, was developed to describe the combined coal devolatilization and the subsequent combustion of the residual char under oxy–fuel condition in both O2–CO2 and O2–N2 environments. Experiments were conducted to validate the model, which was also found to predict the experimental data published in the literature well. The effect of coal particle diameter, temperature of the reactor, and oxygen concentration on devolatilization time was investigated. Peaks in devolatilization and char combustion rates and particle center temperature were studied, and the effect of different parameters assessed. Higher reaction time was observed in an O2–CO2 environment compared to that in an O2–N2 environment due to lower particle temperatures resulting from endothermic gasification reaction and the difference in thermo-physical properties. Simulation studies were carried out to generate temperature, carbon, O2, CO, and CO2 contours to understan...

Development of mathematical model for co-firing pulverized coal and biomass in experimental furnace

A comprehensive mathematical model for prediction of turbulent transport processes and reactions during co-combustion of pulverized fuels in furnace fired by 150 kW swirl stabilized-burner has been developed. Numerical simulations have been carried out by using an in-house developed computer code, with Euler-Lagrangian approach to the two-phase flow modelling and sub-models for individual phases during complex combustion process: evaporation, devolatilization, combustion of volatiles, and char combustion. For sub-model of coal devolatilization the approach of Merrick is adopted, while for biomass devolatilization the combination models of Merrick, and of Xu and Tomita are selected. Products of devolatilization of both the pulverized coal and biomass are considered to contain the primary gaseous volatiles and tar, which further decomposes to secondary gaseous volatiles and residual soot. The residual soot in tar and carbon in coal and biomass char are oxidized directly, with ash remaining. For volatiles combustion the finite rate/eddy break-up model is chosen, while for char oxidation the combined kinetic-diffusion model is used. The comprehensive combustion model is validated against available experimental data from the case-study cylindrical furnace. The agreement of the simulations with the data for the main species in the furnace is quite good, while some discrepancies from experimental values are found in the core zone. The presented model is a good basis for further research of co-combustion processes and is able to provide analysis of wide range of pulverized fuels, i. e. coal and biomass. At the same time, the model is relatively simple numerical tool for effective and practical use.

FLASHCHAIN Theory for Rapid Coal Devolatilization Kinetics. 9. Decomposition Mechanism for Tars from Various Coals

This paper extends FLASHCHAIN theory with a mechanism for tar decomposition with any coal at any temperature under inert gases at atmospheric pressure, including (i) continuous elimination of heteroatoms as noncondensables, which transforms primary tar into polynuclear aromatic hydrocarbons at moderate temperatures; (ii) disintegration of tar monomers with attached hydrogen sources into oils and additional noncondensables; and (iii) nucleation and addition of oils and tars to a nascent soot phase at elevated temperatures. The aromatic nuclei, labile bridges, char links, and peripheral groups in FLASHCHAIN describe tar decomposition without modification, and 7 of 11 proposed reactions were also transferred from FLASHCHAIN, albeit with markedly different kinetic parameters. Only the channel for oil production and the three for soot production are new and distinctive. All stoichiometric coefficients can be evaluated from a primary tar composition from FLASHCHAIN, and the associated variations in these coeffi...

Collaboration of simulations and experiments for development and uncertainty quantification of a reduced char combustion model

Computational fluid dynamics (CFD) plays a decisive role in the development of cost-effective oxy-coal combustion technologies to improve process efficiency and to decrease pollutant emissions. The implementation of detailed physical models describing coal devolatilization, char oxidation, gas-phase reactions and pollutant formation ensures accurate CFD simulations of coal combustion but is still challenging for large-scale combustors due to the significant computational efforts required, especially in the framework of Large Eddy Simulation (LES). The development of reduced physics model with quantified model-form uncertainty is needed to overcome the challenges of performing LES of industrial coal-fired boilers. Reduced models must reproduce the main features of the detailed models and the capability of bridging scales and being predictive. A tight coupling of simulation and experiments is necessary to ensure predictivity with uncertainty quantification for a reduced model. This work proposes a combined experimental/numerical methodology that uses global sensitivity analysis to rank fundamental input parameters of a reduced char oxidation and gasification model describing reactions between char carbon and O2, CO2 and H2O reagents in both air and oxy-coal conditions. A careful evaluation of uncertainty in the data, in the model form and in the model parameters is performed. The reference dataset, consisting of the experiments carried out in a laminar entrained flow reactor operated by Sandia National Laboratories, has been exploited. The methodology is based on the use of so-called instrument models to include all the physical sub-models and the sources of uncertainty considered in the experiments and in the numerical simulations and affecting the main quantities of interest, e.g. reaction rates. The quantified uncertainty in the instrument models provides the range of uncertainty for the reduced char combustion model. Then, the reduced model with quantified uncertainty will be validated against the experimental data. The reduced model capability to address heterogeneous reaction at the particle surface, mass transport of species in particle boundary layer, pore diffusion and internal surface area changes will be assessed.

Characterization and non-isothermal kinetics of Shenmu bituminous coal devolatilization by TG-MS

The devolatilization characteristics and non-isothermal kinetics of gaseous volatile evolution of bituminous coal were investigated through simultaneous thermogravimetry-mass spectrometry (TG-MS). The TG-MS results indicated that the three types of coals possess similar devolatilization characteristics in terms of mass loss and gas evolution. A high heating rate slightly shifted the TG, derivative thermogravimetric analysis (DTG), and MS curves to a high temperature range and promoted the release of evolved gases. The kinetic parameters of gaseous volatile evolution were determined through a novel procedure. In this procedure, the kinetic model function f(α) was identified using the Málek method, and the activation energy E and the pre-exponential factor A were calculated with common isoconversional methods or a distributed activation energy model (DAEM). With the Málek method, the reaction order model (n=2) was the most probable mechanism to describe the gaseous formation of H2 and C2H2. This finding implied that chemical reaction is a limiting factor of gas evolution. The evolved rate curve of CO2 and CO could be fitted and identified by subjecting the spectral peaks to multi-Gaussian analysis. A JMA model (n=3) and a standard Z-T-L model were applied to describe various kinetic stages. Results revealed that the evolved CO2 and CO were mainly controlled by diffusion. All of the standard models failed to fit the gaseous evolution of CH4 and H2O. Therefore, CH4 and H2O possibly underwent a complex process involving different reactions and stages.

Catalytic pyrolysis of coal particles in a fluidized bed: Experiments and modeling

ABSTRACTCoal as a form of fossil fuel may be utilized in two thermochemical techniques: directly by combustion of coal particles and indirectly by converting it into valuable products. In this research, a kinetic model of coal devolatilization has been developed based on the Arrhenius expression. To validate the model, an experimental study also has been performed in a drop-tube fluidized bed. There was reasonably good agreement between modeling data and experimental data obtained from single-tube fluidized bed. The effects of reaction temperature on the pyrolysis products and fuel conversion were studied. As a result, the maximum tar yield obtained was 28.3 (%) at 400° C. An increase in activation energy was led to a relative decrease in the fuel conversion maybe due to a decrease of heating rate.

Universal Devolatilization Process Model for Numerical Simulations of Coal Combustion

A universal devolatilization process (UDP) model, considering the effect of particle instantaneous heating rates and coal types, is developed for an accurate prediction of the devolatilization process in numerical simulations of coal combustion. This model is developed in the framework of a competing two-step model based on the fitting and correlation works from the database, which is constructed from the devolatilization process of 23 coal types under 40 heating rates predicted by the chemical percolation devolatilization (CPD) model. The kinetic parameters are determined in terms of coal types and instantaneous heating rates with the correlations obtained. The thorough validations of the UDP model, including the internal and external validations, are carried out through comparing the devolatilization process of the 23 selected coals and 10 other coals predicted by the UDP model and the CPD model. It is found that the UDP model can accurately predict the coal devolatilization process for a wide range of ...

Collaborative simulations and experiments for a novel yield model of coal devolatilization in oxy-coal combustion conditions

Oxy-coal combustion is an emerging low-cost “clean coal” technology for emissions reduction and Carbon Capture and Sequestration (CCS). The use of Computational Fluid Dynamics (CFD) tools is crucial for the development of cost-effective oxy-fuel technologies and the minimization of environmental concerns at industrial scale. The coupling of detailed chemistry models and CFD simulations is still challenging, especially for large-scale plants, because of the high computational efforts required. The development of scale-bridging models is therefore necessary, to find a good compromise between computational efforts and the physical-chemical modeling precision. This work presents a procedure for scale-bridging modeling of coal devolatilization, in the presence of experimental error, that puts emphasis on the thermodynamic aspect of devolatilization, namely the final volatile yield of coal, rather than kinetics. The procedure consists of an engineering approach based on dataset consistency and Bayesian methodology including Gaussian-Process Regression (GPR).Experimental data from devolatilization tests carried out in an oxy-coal entrained flow reactor were considered and CFD simulations of the reactor were performed. Jointly evaluating experiments and simulations, a novel yield model was validated against the data via consistency analysis. In parallel, a Gaussian-Process Regression was performed, to improve the understanding of the uncertainty associated to the devolatilization, based on the experimental measurements. Potential model forms that could predict yield during devolatilization were obtained. The set of model forms obtained via GPR includes the yield model that was proven to be consistent with the data. The overall procedure has resulted in a novel yield model for coal devolatilization and in a valuable evaluation of uncertainty in the data, in the model form, and in the model parameters.

A new framework for modeling coal devolatilization and combustion in boiler furnaces

This paper presents a new framework for the modeling of coal-fired boiler furnaces. The input required for the model is the ultimate analysis of a coal sample. The model accounts for devolatilization followed by gas-phase combustion. The devolatilization model used in this work is taken from published literature with slight modifications to match the numerical predictions with experimental measurements. This work also develops a reactor network model for simulating the performance of boiler furnaces. For the seamless integration of kinetic models of coal devolatilization and combustion with furnace numerical model, the thermochemistry data of several hypothetical and intermediate species involved in devolatilization chemistry are evaluated in the form of 14 coefficient National Aeronautics and Space Administration polynomials. The capability of the model for predicting the furnace temperature and product composition is demonstrated by simulating a single-zone model. Copyright © 2017 John Wiley & Sons, Ltd.

Investigation of Tabas anthracite coal devolatilization: Kinetics, char structure and major evolved species

The pyrolysis of low-volatile Tabas anthracite coal was investigated by thermogravimetric technique (TGA) in the temperature range from ambient to 1100°C under non-isothermal heating conditions (1.5, 3, and 7°C/min heating rates). Higher heating rates showed a small retarding effect on devolatilization toward higher temperatures. Iso-conversional method was used for the kinetic study of non-isothermal Thermogravimetric data. Activation energy calculated for coal conversion of 20–80% was about 319kJ/mol which may be a result of stable ordered structure of this type of coal. Analysis of evolved gases by Fourier transform infrared spectrometry (FTIR) in 7°C/min heating rate was conducted for the identification of the gaseous species and their evolution profiles during coal thermal degradation. H2O, CO, SO2, CO2, CH4 were respectively major gases detected in FTIR while C2H6 and H2 were also detected. From X-ray photoelectron spectroscopy (XPS) Cls spectrum, major functional group changes on raw and devolatilized coal surfaces are found to be an increase in ether and carbonyl bond intensity and a decrease in carboxylic bond. It was also showed in XPS wide energy spectra that C atomic percent increased after pyrolysis while O and S decreased.

Fragmentation model of coal devolatilisation in fluidised bed combustion

ABSTRACTDevolatilisation is the release of volatile compounds from coal matrix by thermal decomposition. During coal devolatilisation, fragmentation could occur due to pressure build-up of accumulated volatiles. A fragmentation model is necessary to ensure the safe operation of coal combustion and optimum process condition. In this study, the refinement of fragmentation model of coal devolatilisation was done. In order to obtain a comprehensive model, a fluidised bed combustion experiment was conducted using two Indonesian coals (Musi Banyuasin and Berau) and the results were then compared with the model simulation. Using a coal diameter of 0.8–17 mm at a combustion temperature of 850°C, it shows that the fragmentation probability and number of fragments could be affected by the coal particle diameter, convective pore diameter, and porosity. Predictions made by the developed model were close to the experimental fragmentation data, with an error range of less than 5.1%.

Experimental Design for Discrimination of Chemical Kinetic Models for Oxy-Methane Combustion

The concept of combustion under oxy-fuel conditions has the potential to reduce greenhouse gas emissions. For the design of combustion devices operating under these conditions, a good understanding of fuel oxidation behavior in terms of chemical kinetic mechanisms is useful. For the oxidation of the main component of natural gas and coal devolatilization products, i.e., methane, various chemical mechanisms are available in the literature validated mostly with experiments using air and none of them is developed particularly or has been validated extensively for oxy-methane combustion. An important prerequisite for model assessment is high-quality data typically obtained from resource- and time-consuming measurements. The aim of this study is to identify the best methane mechanism for oxy-fuel combustion from a set of models available in the literature with a minimum number of measurements. Five chemical models, which have been validated for the oxidation of methane/air mixtures, are compared in terms of their performance for extinction strain rates, ignition delay times, and laminar burning velocities of oxy-methane mixtures. A model-based experimental design method, i.e., Akaike weights design criterion, is applied to determine the optimal potential measurements. Ideally, at the conditions of designed experiments, model predictions are nicely separated and, thus, the best model can be identified by comparison to these measurements. It is shown that the employed experimental design strategy identifies informative experiments for model discrimination efficiently. While measurements of extinction strain rates are proposed to be carried out for flames with small methane mass fractions of the fuel stream and oxygen mass fractions of the oxidizer stream, shock tube experiments are evaluated as equally useful for model discrimination over the investigated range of conditions. Measurements of flame speeds are designed at very small and very large equivalence ratios particularly at relatively high pressures.

CFD simulation of fuel reactor for chemical looping combustion of Indian coal

Chemical looping combustion (CLC) is an energy efficient technology to separate CO2 from the flue gases. It has been accepted globally as one of the promising methods to control the concentration of CO2 in the atmosphere and there by helps to provide a sustainable solution to the environmental crisis. CLC basically consists of two reactors – fuel and air reactors. Fuel reactor of the CLC process is considered for the present study and the computational fluid dynamics (CFD) studies of the fuel reactor for high ash (Indian) coal are presented. Various oxygen carriers like Fe2O3, CuO and the combination of these metal oxides are considered for the study. The reactions involving coal devolatilization, gasification and metal oxides reduction are incorporated. The conversion and performance of the reactor with each metal oxide is analysed and compared. The ash present in the coal is considered as non-reacting component and its effect on the flow and concentration of the species is analysed. The simulations are carried out at various temperatures. The obtained results are found to be justifiable and are discussed in detail.

FLASHCHAIN Theory for Rapid Coal Devolatilization Kinetics. 8. Modeling the Release of Sulfur Species from Various Coals

This modeling study expands FLASHCHAIN theory to interpret the release rates and yields of H2S, COS, and SO2 and the sulfur contents of tar and char from any coal and thereby completes the theoretical framework for the devolatilization of all five major organic elements in any coal. The proposed reaction set accurately interprets a database that represents heating rates from 10 to almost 104 °C/s, temperatures to 1500 °C, contact times to 7 min, and S product distributions from 27 different coal samples representing ranks from lignites to anthracites, albeit with persistent uncertainties on the split between tar S and noncondensable S products. A single distributed-energy reaction with the same parameters for all coals accurately describes the release of H2S from pyrite during primary devolatilization. Additional kinetic rate data on sulfate decomposition under rapid heating is needed to close the analysis for inorganic S species. By associating aliphatic and aromatic sulfide functional groups with labile...

Analyzing the effects of turbulence and multiphase treatments on oxy-coal combustion process predictions using LES and RANS

Numerical studies of pulverized coal swirl combustion in oxy-fuel atmosphere are carried out. Thereby two issues are especially addressed: (1) how LES and RANS impact differently the predictions of combustion properties even though, in both approaches, the same kinetic rates are used to represent the coal combustion processes; (2) how the numerical multiphase treatments may affect the prediction of micro-process interaction as well as the range in which these processes are not negligible. For that purpose a methodology is developed based on an Eulerian-Lagrangian oxy-coal combustion module which is designed relying on the state of the art models as implemented in the commercial code ANSYS Fluent 17. This especially includes three kinetic rates for the description of coal combustion, namely coal devolatilization, volatile combustion and char combustion. Based on an appropriate Stokes number consideration, a full two-way inter-phase coupling has been numerically adopted.To assess the prediction capability of the overall model, a new set of experimental data from a 60kWth oxy-coal test facility is employed. First, the model validation is ensured by comparison of results in terms of flow field and products from volatile and char combustion. Then, an analysis is performed to elucidate how the two-phase turbulence modeling impacts the thermal flow predictions along with the evolution of multiphase oxy-coal combustion properties.Finally, it is demonstrated how the numerical multiphase treatments affect the prediction of micro-process interaction in terms of coal devolatilization, coal particle distribution due to turbulent particle dispersion, and of gaseous heat release as well as char burnout. The range in which these interphase processes (subgrid scale particle dispersion) are not negligible is also pointed out in terms of subgrid scale Stokes number.

Extension of the chemical percolation devolatilization model for predicting formation of tar compounds as soot precursor in coal gasification

This paper proposed and validated a new coal devolatilization model that can predict yields of respective gas and tar components, and be directly coupled with an elementary step-like reaction model. The new model was an extension of the chemical percolation devolatilization (CPD) model. The CPD model is one of the existing primary pyrolysis models that consider the coal chemical structure. The mole fraction of labile bridges, cross links, peripheral groups, and average molecular weight of monomer in coals examined in this study were determined by a 13C NMR analysis. The aromatic ring clusters size distribution was determined by a coal pyrolysis test using a Curie point pyrolyzer. The thermal decomposition process of the coal chemical structure was expressed by nine elementary reactions. From the above, the extended CPD model can predict gas and tar components as respective chemical species (H2O, CO2, CO, CH4, benzene, naphthalene and phenanthrene) and consequently capture the secondary decomposition and polymerization in gas phase by coupling the detailed gas-phase chemistry. The proposed model was validated by comparing with experiments using a pressurized drop tube furnace (PDTF). The results showed that the trend of light gases and soot yield in the experiments could be successfully reproduced by the extended CPD model.

Preliminary Research on the Effects of Coal Devolatilization and Char Combustion Processes on the Emission of Particulate Matter during Lignite Combustion under Air and Oxy-fuel Conditions

The pulverized coal combustion in both air and oxy-fuel conditions generally experiences successive coal devolatilization and char combustion processes. The effects of these two processes have important influence on the emission characteristics of particulate matter. With some different characteristics of lignite from the higher rank coal, including the high contents of organically bound cations and the apparent char–CO2 gasification reaction upon oxy-fuel combustion, the effects of both coal devolatilization and char combustion processes on the emission characteristics of particulate matter during oxy-fuel combustion of lignite would accordingly be distinctive. A typical Chinese lignite was devolatilized in a N2 or CO2 atmosphere to generate N2–char and CO2–char, respectively. Raw coal and the two chars were burned in a drop-tube furnace under both air and oxy-fuel conditions. Afterward, the yields and inorganic compositions of segregated PM10 were carefully analyzed. The results have shown that the devo...