Coal Jet Flame Research Articles

Experimental Study on Flame Chemical Composition of Coal and Ammonia Gas-Solid Jet in Flat Flame Burner.

Ammonia as a fuel to partially or completely replace fossil fuels is one of the effective ways to reduce carbon dioxide, and the research on ammonia coal cocombustion is of great significance. The combustion characteristics of ammonia are very different from those of pulverized coal, resulting in the ignition and emission characteristics of ammonia and pulverized coal gas flow that is different from traditional pulverized coal flame. In this paper, the effect of pulverized coal concentration in coal and ammonia mixed combustion jet on the ignition distance and gas-phase components at different positions of the jet flame were studied experimentally on the flat flame burner, and the conditions of ignition and ignition stability of coal and ammonia gas-solid fuel were expounded. It was found that the ammonia mixed with pulverized coal changed the temperature field of the flat flame burner and therefore the ignition characteristics of the jet were changed. The ignition delay time at the same jet speed was positively correlated with the pulverized coal concentration, but when the pulverized coal concentration continued to decrease, the influence on the ignition delay time gradually became smaller. The composition of coal ammonia gas-solid fuel changed the heat transfer path and share during combustion, and finally, the flame temperature was negatively correlated with the concentration of pulverized coal. Therefore, the reduction of the pulverized coal concentration was conducive to the stable combustion of coal ammonia mixed fuel. When HAB = 100 mm, the conversion rate of fuel N to NOx per unit mass of coal ammonia mixture increased with the increase of pulverized coal concentration. The NOx production amount first increased and then decreased with the increase of pulverized coal concentration, and the amount of N2O and NO2 decreased rapidly with the increase of HAB. The proportion of NOx in NO exceeded 94%, which was conducive to achieving low nitrogen combustion of coal and ammonia gas-solid fuel. In general, the O2 concentration in the ammonia coal jet flame decreased, the flue gas temperature, and NOx and CO generation increased after mixing ammonia, and the optimal pulverized coal concentration in this experiment was 0.41 kgc/kga (mass ratio of pulverized coal to the sum of N2 and NH3).

Ignition characteristics of the high-velocity pulverized coal jet in MILD combustion mode: Experiments and prediction improvements

The influence of high jet velocity on the ignition characteristics of pulverized coal is very important for the Moderate or Intense Low-oxygen Dilution (MILD) combustion technology of pulverized coal. This paper conducts experimental and numerical simulation research on the ignition characteristics of the pulverized coal jet in a high-temperature environment based on the Hencken-type flat flame burner. The results show that when the ambient temperature is 1873 K and the ambient oxygen mole fraction is 5 %, the ignition delay distance of the pulverized coal jet decreases as the jet velocity increases from 15 m/s to 100 m/s for the formation of MILD combustion. The adoption of the turbulent particle heat transfer model and the non-spherical particle motion model effectively improves the prediction of particle dispersion, ignition delay, and reaction zone in the high-velocity coal jet flame. The enhancement of gas–solid heat transfer by turbulent fluctuation plays a dominant role in advancing the coal ignition in the high-velocity jet, while the non-spherical shape of coal particles mainly improves the uniformity of reaction distribution. The influence of ignition characteristics in the high-velocity jet on the MILD combustion formation of pulverized coal has been also analyzed in detail.

Non-gray gas and particle radiation in a pulverized coal jet flame

In pulverized coal combustion, the participating radiative media includes gas, soot, and coal/char particles; a fundamental understanding of their radiation behavior is essential for the prediction of radiation heat transfer and control of radiation in the furnace. In this study, the individual contributions of gas, soot, and coal particles to the total radiation were examined based on the large eddy simulation of the CRIEPI pulverized coal jet flame. The radiation transfer equation was solved using the discrete ordinate method with state-of-the-art non-gray radiative property models of multi-phase media. The full spectrum correlated k-distributions (FSCK) model was used to calculate the radiative property of the gas-soot mixture. A burnout-based particle radiative property model was applied for coal/char particles. The results showed that the predicted spatial distributions of coal particles, tar, and soot were in good agreement with the experimental data. The gray gas radiation model predicted temperatures approximately 50 K lower in this flame, and the peak soot volume fraction was predicted at approximately 22% lower. For the laboratory scale burner under examination, the individual radiative contributions of gas, soot, and coal/char were found to be 35%, 40%, and 25%, respectively, so that it was necessary to consider all components for accurate prediction. Moreover, these contribution ratios varied with the flame position. Because of the high particle concentration, the individual contribution of coal/char particles accounted for up to 65% in the devolatilization region. Soot radiation played an important role in the pulverized coal flame. In the region where the ensemble-averaged soot volume fraction reached the maximum of 2.7 ppm, the individual contribution of soot was 50% of the total radiation, which accounted for 70% of the total particles (soot and char) radiation.

Evaluation of ammonia co-firing in the CRIEPI coal jet flame using a three mixture fraction FPV-LES

Highly resolved Large Eddy Simulations (LES) are performed to investigate co-firing of coal and ammonia in a burner experiment conducted by the Central Research Institute of Electric Power Industry (CRIEPI) in Japan. The coaxial burner with a hydrogen supported pulverized coal flame is modeled using the in-house code PsiPhi. A three mixture fraction flamelet/progress variable (FPV) approach is employed to simulate coal particle conversion due to devolatilization, hydrogen combustion, and ammonia combustion. Three cases are investigated and compared to each other: 1) a coal combustion case, injecting air and coal particles, 2) an ammonia combustion case, injecting a mixture of ammonia and air, and 3) a co-firing combustion case, injecting a mixture of coal, ammonia and air in the center tube. Two mechanisms are used to build the chemistry table and are compared against each other: a reduced CRECK mechanism with 120 reaction species and 1551 elementary reactions, originally reduced for coal combustion modeling, and a newly introduced reduced CRECK mechanism with 129 reaction species and 1644 elementary reactions, including the detailed NH3 reaction paths in addition to the coal chemistry. Species are compared for the coal case and temperature fields are compared for both the coal and co-firing case. Normalized LIF signals for OH and NH are presented for all three cases. The gas composition profiles are in good agreement with the experiment and the temperature fields are consistent with previous results for the pure coal flames. For pure ammonia and for ammonia co-firing, the new mechanism shows an improved prediction of the reaction zone.

Application of flamelet/progress-variable approach to the large eddy simulation of a turbulent jet flame of pulverized coals

In this study, the flamelet/progress-variable (FPV) approach was applied to a large eddy simulation of a pulverized coal jet flame. The FPV approach considers the characteristics of the pulverized coal flame, e.g., non-adiabatic system and several types of fuel streams, via additional representative variables. First, the applicability of the FPV approach to a turbulent flame with pulverized coals was confirmed through a comparison of the numerical solutions and experimental data. In this study, the pure pilot case was also investigated to clarify the effects of pulverized coals on the flame. The flame structure changes significantly upon the injection of pulverized coals, and the flame index suggests the coexistence of premixed and diffusion combustion modes even in the downstream region. In particular, the combustion mode fluctuates with time in the middle region of the flame. The fuel gas released from the pulverized coals should increase in this region; therefore, the release and combustion behavior of the volatile matter must be involved in the combustion mode variation. The evaluation of the combustion modes of fuel gas in the coal flame is useful for the design and optimization of pulverized coal combustors with next-generation technologies.

Large eddy simulations and analysis of NO emission characteristics in a laboratory pulverized coal flame

Large eddy simulation of a lab-scale turbulent pulverized coal jet flame is conducted using the extended flamelet/progress variable (FPV) approach, where normalized total enthalpy, char conversion parameter, progress variable, coal mixture fraction and coal mixture fraction variance are selected as the control parameters for a five-dimensional flamelet library. Two ways of fuel-N partitioning between volatiles and char are applied for NO formation in two cases, respectively. The model performance is evaluated by comparing with the experimental data. The results indicate that the present model can well reproduce the pulverized coal flame. Reasonably good agreements on particle dispersion and velocity, temperature as well as major species distributions are obtained. In addition, the distributions of NO obtained with different treatments are analyzed qualitatively as well as quantitatively. The results indicate that the extended FPV approach shows an advantage over the conventional post-processing approach. When reasonable fuel-N partitioning is considered, a more accurate NO prediction can be achieved. Finally, combustion and NO emission characteristics are further discussed.

Soot Formation and Distribution in Coal Jet Flames over a Broad Range of Coal Concentration

The soot formation and distribution in coal jet flames were measured using Laser-Induced Incandescence (LII) technique over a wide range of coal mass concentration (Ccoal). Experiments were conduct...

Large eddy simulation of a 660 MW utility boiler under variable load conditions

Large eddy simulation (LES) has become a promising tool for pulverized coal combustion with the development of computational fluid dynamics (CFD) technologies in recent years. LES can better capture the unsteady features and turbulent structures of coal jet flame than Reynolds averaged Navier Stokes (RANS). The coal-fired power plants in China are now required to be operated in a wide load range and quickly respond to the electric grid. The boiler performance of variable loads should be evaluated in terms of flow, heat transfer, and combustion processes. In this paper, LES was applied to simulate a 660 MW ultra-supercritical boiler under BMCR (boiler maximum continue rate), 75%THA-100, and 50%THA-100 conditions. The predicted gas velocities agree well with the thermal calculation and the temperature error is less than 130 K. The simulation results show that the operation load has significant effects on the boiler performance. It is also proved that LES can provide guidance for the design and operation of advanced coal-fired boilers.

Identification of the initial particle size distribution for coal combustion simulations

AbstractPulverized coal generally features wide particle‐size distributions (PSD). How to set an initial PSD in coal combustion simulation remains an open question. To answer this, the Gaussian‐quadrature theory was applied to provide a discrete reconstruction of the PSD described by Rosin‐Rammler and Upper‐Limit functions. From the perspective of the moments of PSD, a theoretical analysis was conducted for mass, momentum, and energy exchange terms between gas and particle phases in pulverized coal jet flame. The analysis presented that the first, the second and the third moments of PSD were the most important to capture the flame ignition behavior. The large eddy simulations showed that the proposed method with two or more particle sizes could provide good predictions of flame ignition distances. And Sauter mean diameter (D32) could represent the complete PSD when predicting the ignition behavior of pulverized coal jet flame.

Numerical Investigation on the Detailed Structure of a Coaxial Coal Jet Flame Using Large-Eddy Simulation with Elementary Reactions

A numerical simulation of a pulverized coal jet flame was performed to investigate the physics and flame structure in detail by means of large-eddy simulation with an elementary reaction mechanism....

Assessment of Global and Network Models of Devolatilization for Numerical Analysis of Pulverized Coal Combustion

ABSTRACTMathematical modeling of devolatilization plays a significant role in the prediction of flame development in solid fuel combustion simulations. Commonly implemented devolatilization models available in the literature to simulate pulverized coal combustion are reviewed and evaluated. The effect of the particle heating rate on the onset of devolatilization and on final volatile yield is discussed. Selected devolatilization models are investigated with respect to their implementation in Computational Fluid dynamics (CFD) simulations of methane-assisted pulverized coal jet flame experiments. Differences in volatile release region, particle temperature and species concentration are presented and discussed with comparisons to the experimental data.

Statistics of reaction progress variable and mixture fraction gradients of a pulverised coal jet flame using Direct Numerical Simulation data

The statistical behaviour of the variances, covariance and gradients of the reaction progress variable (c), and the mixture fraction (ξ) have been analysed in a pulverised coal jet flame using a three-dimensional carrier phase direct numerical simulation (DNS) dataset. It has been observed that the Favre-probability density functions (PDFs) of c and ξ can be parametrised by the standard β function. Furthermore, the log-normal distribution has been found to accurately represent |∇c| and |∇ξ|. It is also found that ∇c and ∇ξ remain aligned throughout the flame brush. Finally the joint PDF of |∇c| and |∇ξ| has been compared with the product of the PDF of |∇c| and PDF of |∇ξ| extracted from carrier phase DNS, and it has been found that |∇c| and |∇ξ| are not statistically independent in the case investigated.The bivariate log-normal distributions with and without correlation have also been considered, and the former is found to be in better agreement with the carrier phase DNS data.

A three mixture fraction flamelet model for multi-stream laminar pulverized coal combustion

A three mixture fraction flamelet model is proposed for multi-stream laminar pulverized coal combustion. The technique of coordinate transformation is utilized to map the flamelet solutions from a unit pyramid space into a unit cubic space to improve the stability of the simulation. The validity of the three mixture fraction flamelet model was assessed on different configurations, including a laminar counterflow pulverized coal/methane flame and a laminar piloted pulverized coal jet flame. The flamelet predictions were compared to the reference results of the detailed chemistry solutions. For the counterflow flame, it was found that the flame temperature and major species mass fractions are correctly predicted by the three mixture fraction flamelet model. However, discrepancies are observed for combustion-mode-sensitive species such as CO and H2 in the premixed combustion region. The thermo-chemical quantities in the char surface reaction zone cannot be correctly predicted if the mixing between the char off-gas stream and other streams is neglected. For the piloted jet flame, it was shown that the stable thermo-chemical variables can be correctly predicted at the upper and middle stream locations. However, at the downstream location, discrepancies can be observed in certain regions. Overall, the validity of the three mixture fraction flamelet model for multi-stream pulverized coal combustion is confirmed and its performance in turbulent pulverized coal combustion will be tested in future work.

Experimental study and chemical reactor network modeling of the high heating rate devolatilization and oxidation of pulverized bituminous coals under air, oxygen-enriched combustion (OEC) and oxy-fuel combustion (OFC)

Experimental and numerical investigations of the combustion of 2 bituminous coals under air, oxygen-enriched and oxy-fuel combustion conditions have been performed. 7 pulverized coal jet flames have been stabilized using a specifically designed flat flame reactor (FFR) allowing industrial heating rates to be reproduced. The investigated reacting media have been fully characterized in terms of temperature fields, fuel residence time, devolatilization rates and concentration profiles of O2, CO2, CO, NO and SO2. Obtained data have then been compared with predictions derived from a chemical reactor network (CRN) modeling approach. This latter incorporates a detailed reaction mechanism the formulation of which has been defined by comparing the results obtained through the implementation of 3 different detailed gas-phase combustion reaction schemes. On the whole, obtained numerical results show a good agreement with experimental data. By analyzing the reaction paths relying on the CRN predictions, the present study allowed highlighting and further exploring the influence played by the combustion atmosphere on the chemical pathways leading to species like CO, NO, SO2 or soot precursors. The main findings drawn through this work especially led to trace the increased CO concentrations measured under oxy-fuel combustion (OFC) to heightened HCO production and tar oxidation. Alternative NO formation pathways arising under OFC conditions (like the HCN → HNCO → NH → NO and HCN → NCO → HNCO → HNO → NO routes) have, moreover, been highlighted. While the SO2 production has been found to mainly depend on the quantity of fuel-S emitted during the devolatilization and on the local oxygen partial pressure (the main SO2 formation pathway being indeed not truly affected by the nature of the combustion atmosphere), the analysis of the main routes involved in the oxidation of aromatics such as benzene, naphthalene and phenanthrene has been found well insightful in order to explain some reduced soot productions recently reported in the literature under OFC.

Optical Diagnostics for Pulverized Coal Jet Flame

Optical Diagnostics for Pulverized Coal Jet Flame

Eulerian LES simulation of coal jet flame with a simplified DQMOM model

Eulerian methods are popular for describing particle dispersion and reaction in gas-particle two-phase flow simulations. However, for problems with wide Particle Size Distributions (PSD), such as those associated with coal combustion, the PSD has to be represented by a limited number of particle sizes in order to decrease the computational costs of the transport equations. The objective of this study is to investigate the effects of the number of particle sizes on the Eulerian LES simulation of a pulverized coal jet flame. In this study, experimental work on a Hencken-type entrained flow reactor was carried out to provide the validation data of flame ignition distances. A simplified Direct Quadrature Method of Moments (DQMOM) method was proposed in order to decrease the computation cost in the Eulerian framework. Several simulation cases with different quadrature nodes (particle sizes) were conducted. The flame ignition distance was well predicted when two and more quadrature nodes were adopted in the simulation. But it was overestimated when only one quadrature node was considered. So one particle size is not enough to represent a wide PSD of coal in the Eulerian LES simulation of pulverized coal flames. When 5 particle sizes were adopted in the simulation, the ignition distances and time-variant fluctuations can be well predicted when compared with experimental results under different operating conditions. This study shows that LES with the simplified DQMOM method can describe the ignition distance in gas-solid two-phase reacting flows even with only a few quadrature nodes.

Direct Numerical Simulation Study on the Stabilization Mechanism of a Turbulent Lifted Pulverized Coal Jet Flame in a Heated Coflow

The stabilization mechanism of a turbulent lifted pulverized coal jet flame in a heated coflow is investigated by means of three-dimensional direct numerical simulation. The coal particles are tracked in the Lagrangian frame with experience of moisture evaporation, volatile releasing, and carbon combustion. The devolatilization process is modeled with a competing two-step model, and the carbon reaction is described by a single-film model. It is found that the mean velocities for the gas-phase and particles can develop into self-similar profiles, while the fluctuation velocities have not achieved the self-similar status. The turbulence can be enhanced by ignition and combustion processes. By investigating the correlations among the temperatures, heat release rate, and devolatilization rate, it is found that autoignition of the volatile is the key mechanism responsible for coal flame stabilization under the present condition. The heating effects from the stripe flames in the shear layers, heated coflow, and...

Numerical analysis of particle dispersion and combustion characteristics on a piloted coaxial pulverized coal jet flame

A numerical simulation was performed and validated on a piloted coaxial pulverized coal jet flame by means of large eddy simulation (LES) in which the dynamic Smagorinsky SGS model and the Lagrangian particle tracking model were employed. In the devolatilization process, a postulate substance, CaHbOc, was considered as the devolatilized gas. For gaseous phase reactions, a simple two-step global kinetic mechanism was implemented. For a solid phase reaction, a two-step char oxidation reaction was installed. Results were compared to an experimental result in terms of particle distribution, gaseous temperature, gas composition, and particle velocity. The particle distribution result shows that particle dispersion for the combustion case is narrower than that for the non-combustion case. This phenomena is discussed by observing an axial interphase momentum transfer. The axial momentum of fluids can be maintained as high or similar to the particles by combustion, so momentum transfer is limited in the combustion case. It is also confirmed in the particle and gas velocity results. Time-averaged temperature and mole fractions of gases capture the featured characteristics of the experiment well. From these results, an effect of devolatilized gas combustion on particle dispersion and stream flow are discussed. And it is revealed from instantaneous results that particles tends to form groups following the edge of the devolatilized gas combustion.

Large eddy simulation of piloted pulverised coal combustion using extended flamelet/progress variable model

An extended flamelet/progress variable (EFPV) model for simulating pulverised coal combustion (PCC) in the context of large eddy simulation (LES) is proposed, in which devolatilisation, char surface reaction and radiation are all taken into account. The pulverised coal particles are tracked in the Lagrangian framework with various sub-models and the sub-grid scale (SGS) effects of turbulent velocity and scalar fluctuations on the coal particles are modelled by the velocity-scalar joint filtered density function (VSJFDF) model. The presented model is then evaluated by LES of an experimental piloted coal jet flame and comparing the numerical results with the experimental data and the results from the eddy break up (EBU) model. Detailed quantitative comparisons are carried out. It is found that the proposed model performs much better than the EBU model on radial velocity and species concentrations predictions. Comparing against the adiabatic counterpart, we find that the predicted temperature is evidently lowered and agrees well with the experimental data if the conditional sampling method is adopted.

Effect of multiphase radiation on coal combustion in a pulverized coal jet flame

The accurate modeling of coal combustion requires detailed radiative heat transfer models for both gaseous combustion products and solid coal particles. A multiphase Monte Carlo ray tracing (MCRT) radiation solver is developed in this work to simulate a laboratory-scale pulverized coal flame. The MCRT solver considers radiative interactions between coal particles and three major combustion products (CO2, H2O, and CO). A line-by-line spectral database for the gas phase and a size-dependent nongray correlation for the solid phase are employed to account for the nongray effects. The flame structure is significantly altered by considering nongray radiation and the lift-off height of the flame increases by approximately 35%, compared to the simulation without radiation. Radiation is also found to affect the evolution of coal particles considerably as it takes over as the dominant mode of heat transfer for medium-to-large coal particles downstream of the flame. To investigate the respective effects of spectral models for the gas and solid phases, a Planck-mean-based gray gas model and a size-independent gray particle model are applied in a frozen-field analysis of a steady-state snapshot of the flame. The gray gas approximation considerably underestimates the radiative source terms for both the gas phase and the solid phase. The gray coal approximation also leads to under-prediction of the particle emission and absorption. However, the level of under-prediction is not as significant as that resulting from the employment of the gray gas model. Finally, the effect of the spectral property of ash on radiation is also investigated and found to be insignificant for the present target flame.