Coal Jet Flame Research Articles

LES of pulverized coal combustion with a multi-regime flamelet model

It is well known that premixed and non-premixed flamelets coexist in the combustion system, especially for multiphase combustion (Luo et al., 2011; Bai et al., 2016). In this work, a multi-regime combustion model for partially premixed multiphase combustion (2PMC) is developed in the framework of large eddy simulation (LES). In this model, the multi-regime combustion mode is decomposed into two pure combustion regimes (i.e. premixed and non-premixed) through combustion regime indicator. Different combustion models are chosen for different combustion regimes. For example, for premixed combustion regime, the flamelet generated manifold (FGM) tabulation method in combination with the artificially thickened flame (ATF) approach is used. For non-premixed combustion regime, on the other hand, the thermo-chemical quantities will be extracted from the non-premixed chemtable generated by the flamelet/progress variable (FPV) approach. The proposed multi-regime flamelet model is then extended to adapt to the pulverized coal combustion (PCC) and applied to a laboratory-scale pulverized coal jet flame. The simulation results show that the percentage of the volume-weighted premixed combustion regime of this studied flame is up to 18%. Quantitative comparisons between the experimental data and the numerical results with the present multi-regime flamelet model, the FPV model, and the Eddy Break Up (EBU) model shows that the proposed multi-regime flamelet model has several advantages. It performs better than the FPV model in the regions where premixed combustion mode prevails and also much better than the EBU model in species concentration prediction.

A DNS study on effect of coal particle swelling due to devolatilization on pulverized coal jet flame

The effects of particle swelling and shrinkage during devolatilization and char combustion on the flame characteristics are investigated by performing DNS of a lab-scale turbulent pulverized-coal straight-jet flame. The obtained characteristics are compared with those in the previous DNS (Hara et al., 2015), in which particle swelling and shrinkage are not taken into account. The results show that the changes in coal-particle diameter because of swelling and shrinkage during devolatilization and char combustion do not affect significantly the gas temperature distribution of the turbulent pulverized-coal straight-jet flame, although it explicitly affects the coal-particle diameter distribution in the flame.

Large-eddy simulation of pulverized coal combustion using flamelet model

A flamelet model for pulverized coal combustion, which considers three mixture fractions for coal moisture, volatile matter and gasified char, has been coupled with a large-eddy simulation. Its prediction accuracy for ignition and extinction has been investigated in the configurations of a lab-scale coal jet flame and a large-scale test furnace with an actual coal burner. For the lab-scale coal jet flame, the profile of the ratio of coal burnt on the central axis of the coal jet was compared with the measurement. The profile by the flamelet model well agreed with the experimental data within the measurement error. For the large-scale test furnace with the actual burner, the prediction ability for the ignition limit in a decreasing coal feed rate was examined. The present simulation reproduced an unstable flame state occurring at a lower coal feed rate. The ignition stability index, which was evaluated from the flame images, was compared with the test result. The sharp decline of the ignition stability index near the ignition limit was well-captured by the present simulation. The prediction error for the coal feed rate at the ignition limit was estimated to be within 10%.

Numerical investigation of two-phase flame structures in a simplified coal jet flame

The coal flame structures are investigated based on a three-dimensional compressible point-source direct numerical simulation (DNS) of a turbulent simplified coal jet flame. The coal particles are treated as point sources and tracked in the Lagrangian frame considering the interphase transfers. A comprehensive coal combustion model, including the evaporation, devolatilization and char combustion, is applied. The reaction mechanism of CH4 is adopted for gas combustion. In this paper, the general flame features and particle distributions are investigated and three typical flame structures of coal jet flames, namely interspersed flame, stripe flame and stable continuous flame, are identified. By analyzing the temporal evolution of the flame structures, the stable flame formation process is obtained intuitively. It is also found that these three typical flame zones have similar gaseous reaction and the non-premixed flame dominates. The strong reaction is more likely to appear at the locations with small vorticity and high scalar dissipation rate. Compared with purely gaseous flames, the scalar dissipation rate is not only related to turbulent micro-mixing between the fuel and oxidizer flows but also the devolatilization process. By testing the heat release rate proportion of the three zones, it is found that the contribution of interspersed flame is less. The stripe flame contributes about 26% and is important to the whole stable flame formation.

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The coal flame structures are investigated based on a three-dimensional compressible point-source direct numerical simulation (DNS) of a turbulent simplified coal jet flame. The coal particles are treated as point sources and tracked in the Lagrangian frame considering the interphase transfers. A comprehensive coal combustion model, including the evaporation, devolatilization and char combustion, is applied. The reaction mechanism of CH4 is adopted for gas combustion. In this paper, the general flame features and particle distributions are investigated and three typical flame structures of coal jet flames, namely interspersed flame, stripe flame and stable continuous flame, are identified. By analyzing the temporal evolution of the flame structures, the stable flame formation process is obtained intuitively. It is also found that these three typical flame zones have similar gaseous reaction and the non-premixed flame dominates. The strong reaction is more likely to appear at the locations with small vorticity and high scalar dissipation rate. Compared with purely gaseous flames, the scalar dissipation rate is not only related to turbulent micro-mixing between the fuel and oxidizer flows but also the devolatilization process. By testing the heat release rate proportion of the three zones, it is found that the contribution of interspersed flame is less. The stripe flame contributes about 26% and is important to the whole stable flame formation.

Measurements and CFD modeling of a pulverized coal flame with emphasis on ash deposition

Measurements of fly ash deposition in a 15kW pulverized coal jet flame and CFD-based mathematical modeling have been performed. The deposits have been collected at two ports at particle Stokes numbers in the 0.02–0.34 range and particle kinetic energies not larger than 2×10-9J. Inertial impaction and thermophoresis have been identified as main mechanisms of particle transport towards the deposition surfaces. Deposition rates on air-cooled probes (600–700°C surface temperature) have been measured to be 24% (Port 2) and 79.4% (Port 3) larger than those measured on uncooled probes (1150°C surface temperature) due to the enhanced role of thermophoresis. Complex dependencies of the deposition rate on the probe surface temperature and the probe location have been observed. The CFD-model predictions are able to reproduce these dependencies after adjustments to the particle sticking sub-model. The paper contains estimations of both the impaction and sticking efficiencies.

Direct numerical simulation of a pulverized coal jet flame employing a global volatile matter reaction scheme based on detailed reaction mechanism

A two-step global reaction scheme for the volatile matter of coal is proposed, and the unsteady coal particle and combustion behaviors in a turbulent pulverized coal jet flame are investigated by performing a direct numerical simulation (DNS) employing the proposed global reaction scheme. The two-step global reaction scheme is constructed to take into account the properties of the volatile matter such as transport coefficients, laminar flame speed and unburned gas temperature and to be applicable to various coal types, and it is validated by comparing the results with those obtained by the detailed reaction mechanism which includes 158 chemical species and 1,804 reactions. The validity of the DNS is also assessed by comparing the results with those in the previous experiment (Hwang et al., 2005) [16], and the unsteady coal particle motions and combustion characteristics are examined in detail. The results show that the proposed two-step global reaction scheme for the volatile matter of coal can precisely predict the laminar flame speed and burned gas temperature for various coal types from bituminous to low-rank coals over wide ranges of conditions of equivalence ratios, pressures and unburned gas temperatures. In addition, it can correctly take into account the effects of dilutions by H2O and CO2 which compromise the evaporated moisture from coal and products of char reaction. It is also verified that a lab-scale turbulent pulverized coal jet flame is well predicted by the DNS employing the proposed global reaction scheme. That is, the pulverized coal particles’ velocity and its fluctuation and the characteristics of particle preferential motions are in general agreement with those observed in the experiment. The DNS reveals that in the turbulent pulverized coal jet flame, there appear premix and diffusion flame layers inside and outside, respectively. In addition, the reaction of the volatile matter and O2 in coal-carrier air occurs in the inner premixed flame layer, whereas the reactions of the volatile matter and CO and O2 in surrounding air occur in the outer diffusion flame layer.

Large Eddy Simulation of piloted pulverized coal combustion using the velocity-scalar joint filtered density function model

Abstract A piloted pulverized coal jet flame was simulated by Large Eddy Simulation (LES). For the sub-grid scale (SGS) effects of the turbulence velocity and the scalar fluctuation of the gas flow on the coal particles, the velocity-scalar joint filtered density function (VSJFDF) model was used. Other models such as the governing equations for particles and coal combustion sub-models were also presented. Simulation results with the VSJFDF model, non-VSJFDF model and those by Franchetti et al. [7] were compared with the experimental data, including the mean and RMS (root mean square) velocity, temperature and species concentrations. The results showed that VSJFDF can distinctly improve the predictions, both in quantity and quality. The location where coal particles start burning could be roughly predicted along the centerline of the flame by VSJFDF model.

Study of the high heating rate devolatilization of a pulverized bituminous coal under oxygen-containing atmospheres

This study deals with the analysis of pulverized coal devolatilization in oxygen-containing atmospheres. To do so, a newly developed flat flame reactor allowing coal jet flames to be stabilized with fuel heating rates of the order of 106Ks−1 has been used to mimic operating conditions close to those met in practical combustors. A high volatile bituminous coal milled in an industrial grinder has been fluidized by carrier gases containing various amounts of oxygen (air and pure oxygen) and then injected in hot gases (temperatures ranging up to ∼1240K) generated by propane/air flat flames. The thermal history of coal particles has been monitored by coupling particle image velocimetry (PIV) and pyrometric measurements to be integrated in constant-rate, single-kinetic-rate, two-competing-rate and DAEM devolatilization models. Devolatilization profiles derived from the analysis of the char collected at different residence times have then been compared with simulated data and a simplified fitting procedure has been implemented to find kinetic parameters leading to reproduce experimental data so as to evaluate the relative influence of the surrounding atmosphere on the devolatilization kinetics. Obtained results confirm that devolatilization is faster and more complete under oxygen enriched environments which has been related to an enhanced combustion of volatiles inducing a rise of the temperature of fuel particles. On the other hand, apparent devolatilization rates appeared to be only indirectly affected by the surrounding atmosphere through the way this one influences the heating of the coal. Furthermore, results obtained in this work tend to indicate the absence of overlapping between devolatilization and char oxidation stages. Measurements of gaseous species released during or formed after the devolatilization process finally confirm that the combustion of volatiles is more complete under oxygen enriched combustion (OEC). More CO2 is thus produced while CO concentrations significantly decrease. An enhanced fuel-N conversion has moreover been related to the higher temperatures and oxygen partial pressures met under OEC while SO2 concentrations ∼50% higher have been measured due the higher quantities of fuel-sulfur and oxygen available in the medium.

J0110102 微粉炭噴流火炎の直接数値シミュレーション : 当量比の影響

A direct numerical simulation (DNS) is applied to a laboratory scale pulverized coal jet flame. Pulverized coal particles are injected with primary air in the double stream burner and the effects of equivalent ratio in the primary stream on the coal jet flame are investigated. In the DNS, the number of grid is about 0.17 billion, and the coal particle motions are tracked by a Lagrangian approach. The results show that the gas temperature of the flame annularly formed downstream the burner increases as the equivalent ratio increases. While, the spatial distribution of the gas temperature inside the flame becomes uniform under the low equivalence ratio condition.

Large-eddy simulation of pulverized coal jet flame – Effect of oxygen concentration on NOx formation

Large-eddy simulation is applied to a laboratory-scale open-type pulverized coal flame generated by a triple stream burner, and the NO production and reduction in oxy-fuel condition are investigated for the first time. Pulverized Cerrejon coal which is classified as bituminous coal is used as a fuel. The results show that regardless of the equivalence ratio, as the O2 concentration increases from 21% to 40%, O2 consumption becomes marked because gas temperature rises and oxidation reaction is enhanced by the higher concentration of O2. Also, NO is formed rapidly due to the oxidation reaction of nitrogen from volatile matter of coal, and its concentration reaches a few hundred ppm further downstream. After the rapid formation, in the case of equivalence ratio larger than unity, NO decreases, because the reducing atmosphere becomes dominant due to the lack of O2. The trend becomes significant as the O2 concentration in the carrier gas increases from 21% to 40%. In the case of equivalence ratio less than unity, on the other hand, NO does not decrease clearly, because the oxidizing atmosphere contributes to the further formation of NO. Present study shows the usefulness of the large-eddy simulations for predicting the characteristics of pulverized coal flames.

Experimental study and kinetic modeling of pulverized coal devolatilization under air and oxycombustion conditions at a high heating rate

This work aims to study the devolatilization of pulverized coal particles under air and oxycombustion conditions. To do so, a newly developed experimental test bench has been used to stabilize coal jet flames with fuel heating rates similar to those found in industrial combustors. The thermal history of coal particles has been experimentally monitored by coupling pyrometric and particle image velocimetry (PIV) measurements. Char samples have been collected at different residence times to obtain devolatilization profiles that have been compared with data issued from 4 empirical models. New sets of kinetic parameters have been proposed to simulate coal devolatilization under high heating rate (>106K/s) with N2- and CO2-based atmospheres. The composition of the burnt gases has also been characterized at various heights above the burner (HAB). The analysis of the obtained results confirmed that an oxygen enrichment of the combustion atmosphere enhances the devolatilization process by favoring an increase of the coal particle temperature. From the comparison between measured and modeled data, it has been observed that apparent devolatilization rates and kinetics were influenced only by the thermal history of the fuel particles with no char-CO2 gasification or CO2-cross linking reaction at the surface of the char in the conditions investigated here. CO releases were found to depend on the devolatilization rates and on the temperatures of the flames. An increase of the SO2 emissions has been strongly correlated to the oxygen concentration in the medium for a given devolatilization yield. NO emissions were not significantly reduced during experiments conducted under oxycombustion conditions compared to those performed under air which is due to the fact that NO emissions have been mainly related to the formation of fuel-NO in this work.

Transported PDF modeling of pulverized coal jet flames

A transported composition probability density function (PDF) method is developed for pulverized coal combustion. A consistent hybrid Lagrangian particle/Eulerian mesh algorithm is used to solve the modeled PDF transport equation for the gas phase, with finite-rate gas-phase chemistry. The model includes k–ε turbulence, gradient transport for scalars, and a Euclidean minimum spanning tree (EMST) mixing model. A separate Lagrangian description is used to solve for the coal particle phase, including particle tracking, coal devolatilization and surface reaction models. Interphase coupling models are developed to handle the interaction between the gas phase and the solid phase. Radiative heat transfer is modeled by a P1 model for a gray absorbing emitting and scattering gas–particle system. Two independent laboratory-scale pulverized coal jet flames (“flame A” and “flame B”) are simulated using the new model. For flame A, the baseline model reproduces the measured mean and rms particle axial velocity reasonably well. Some discrepancies are found in particle temperature and gas-phase concentrations, which may in part be due to the uncertainties in the experimental data. Sensitivities of model results to coal-related model variations, turbulence–chemistry interactions, different interphase coupling strategies, and finite-rate chemistry are explored to establish sensitivities and to determine which aspects of the models are most important. The same model is applied to a second flame (flame B), the only change being in parameters related to the different coal composition. It is found that experimental standoff heights cannot be reproduced for three different stoichiometric ratios using a single model. Time scales for chemical reactions, devolatilization and turbulence are extracted and compared, to study the level of turbulence–chemistry–particle interactions in flame A and to test the popular assumption of equilibrium chemistry in coal combustion modeling. Mixture-fraction statistics for flame B are explored to test assumptions that have been proposed for mixture-fraction-based coal models. While the usual assumption of Beta distributions is found to be appropriate, assumptions of statistical independence of two mixture fractions are not valid.

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Large Eddy simulation (LES) has been applied to the pulverised coal jet flame studied at the Japanese Central Research Institute of Electric Power (CRIEPI). A working set of models to represent coal combustion, Lagrangian particle transport and radiative heat transfer in an LES framework has been implemented and tested. The simulation results of the flow field were compared to experimental data for both a reactive and non-reactive case, and an overall good agreement emerged. A simple method for replicating pyrometer measurements was developed for the LES and results obtained from the method were compared to the experimental data. Finally the species concentrations were compared to experimental results for CO2, O2 and N2. The results show the potentials of using LES for pulverised coal combustion and open the way for further developments on the coal combustion models and the applications to more complex burners.

Numerical investigation of pulverized coal jet flame characteristics under different oxy-fuel conditions

Pulverized coal combustion in mixture of oxygen and recycled flue gasses, known as oxy-fuel combustion, is considered as one of the several possible alternatives to conventional pulverized coal combustion. Switching from conventional pulverized-coal combustion to oxy-fuel combustion brings significant changes in flame properties among which the most important are ignition properties and flame stability. This paper presents the results of experimental and numerical analysis of ignition phenomena under O2/CO2 mixtures with different oxygen content. The main focus of the presented paper is to suggest novel ignition sub-model which can describe all possible ignition mechanisms. Proposed ignition sub-model together with Large Eddy Simulation (LES) turbulence modeling enables accurate prediction of main flame characteristics: ignition point position, ignition temperature, and flame stability.

Large Eddy Simulation of Pulverized Coal Jet Flame Ignition Using the Direct Quadrature Method of Moments

The goal of this study is to show that large eddy simulation (LES) combined with the direct quadrature method of moments (DQMOM) can accurately predict coal flame ignition mechanisms and more generally be a valuable tool for predicting and understanding coal flame characteristics. A validation study of the ARCHES LES tool is performed. LES is applied to a pulverized coal jet flame ignited by a preheated gas flow. A DQMOM has been developed to track the particle phase in an Eulerian framework. The simulation results are compared qualitatively and quantitatively to experimental observations for different inlet stoichiometric ratios. LESs are able to capture the various combustion regimes observed during flame ignition and to accurately model the flame stand-off distance sensitivity to the stoichiometric ratio. The gas temperature and coal burnout predictions are also examined and show good agreement with experimental data. Overall, this study shows that high-fidelity LESs combined with DQMOM can yield a deeper understanding of complex coal flames and be a valuable tool for the design, retrofit, and scale-up of oxy-coal boilers.

Direct Numerical Simulation of Pulverized Coal Combustion in a Hot Vitiated Co-flow

A compressible direct numerical simulation (DNS) solver for pulverized coal combustion has been developed and used to study a pulverized coal jet flame with a Reynolds number of 28 284 based on the nozzle diameter. An eighth-order center differential scheme combined with an explicit tenth-order filter is used for spatial discretization. The classical fourth-order Runge–Kutta method is used for time integration. The characteristic non-reflecting boundary conditions are used to describe the boundary conditions. A comprehensive model for coal combustion is applied, and the reaction mechanism of CH4 with five species and two-step reactions is adopted for gas-phase combustion. The grid system has been carefully designed to make sure that turbulent scales and chemical reaction scales are reasonably resolved and the point-source assumptions of particles are valid. The simulation is partially validated against the experiment, and the particle behavior is investigated. It is found that, in the upstream region, the reaction rate is quite scattered and a single particle is found inside the burning flame to form an individual particle combustion mode. While in the downstream region, the reaction zone is more continuous, with a large number of particles enclosed, which characterizes the group combustion mode. Conditional statistics with respect to the mixture fraction are also obtained to provide insights into coal combustion and the related models.

Large Eddy simulation of a pulverised coal jet flame

Large Eddy simulation (LES) has been applied to the pulverised coal jet flame studied at the Japanese Central Research Institute of Electric Power (CRIEPI). A working set of models to represent coal combustion, Lagrangian particle transport and radiative heat transfer in an LES framework has been implemented and tested. The simulation results of the flow field were compared to experimental data for both a reactive and non-reactive case, and an overall good agreement emerged. A simple method for replicating pyrometer measurements was developed for the LES and results obtained from the method were compared to the experimental data. Finally the species concentrations were compared to experimental results for CO2, O2 and N2. The results show the potentials of using LES for pulverised coal combustion and open the way for further developments on the coal combustion models and the applications to more complex burners.

Numerical simulation of pulverized coal jet flame employing the TDP model

The effect of the devolatilization model on the coal particle behavior is investigated in detail by performing numerical simulations of a simple pulverized coal jet flame formed by a small jet burner (0.5kg-coal/h) [S. Hwang et al., Energy & Fuels 2005;19:382–92]. As the devolatilization model, widely-used conventional devolatilization model, two competing reaction rate model and newly-proposed tabulated-devolatilization-process model (TDP model) [N. Hashimoto et al., Combust Flame 2012;159:353–366] are used. The results show that the coal particle velocities predicted by the TDP model are in better agreement with the experiments than those by the other models, with a slight increase in computation time. The difference in the mean axial particle velocity between the simulations is caused by the difference in the axial gas velocity, which is ultimately caused by the difference in the volatile matter evolution rate. It is also found that the devolatilization model has great influence on particle velocity prediction compared to the turbulence model, the gas-phase combustion model and the radiation model.

Measurement of Flame Height by Image Processing Method

In order to measure the flame height (Lf), the image processing method was employed. The jet flame image was captured by the common-used digital camera. The resolution (C, with the unit of mm/pixel) of the pixel of the digital camera was calibrated by metric scale. The jet flame image was then successively processed by ROI (region of interest) processing, gray processing, binarization and edge detection. Through the image processing, the pixels where the flame root and the flame tip were located was detected. The flame region expressed in pixels can be easily obtained by the difference between the flame root (X1) and the flame tip(X2). The flame height (Lf) was calculated by multiplying the difference in pixels (Lp= X2-X1) with the resolution of pixel (in millimetres per pixel). Using the proposed method, the flame heights of the pulverized coal jet flame at the high temperature air combustion condition were measured. The effect of experimental parameters on the flame heights of the coal jet flame was discussed.