Combustion Of Single Coal Particles Research Articles

Fully-resolved simulations of volatile combustion and NO[formula omitted] formation from single coal particles in recycled flue gas environments

The interaction of coal particles and recycled/recirculated flue gas (RFG) with elevated temperatures and low levels of oxygen occurs in various pulverised coal combustion scenarios. In this work, the effect of oxygen level and temperature on single coal particle combustion characteristics and NOx formation in N2 diluent is studied by means of fully-resolved particle simulations. Comprehensive gas-phase kinetics are utilised to consider the critical pathways of NOx formation including tar-N. Results show that higher RFG temperatures decrease the time to reach the peaks of temperature and species profiles and increase the corresponding peak values. When decreasing O2, irrespective of the RFG temperature, the fuel release period is prolonged, the volatile combustion time increases and the combustion process becomes overall less intense. The reduction of O2 in RFG results in a significant decrease of NO production, while the reduction of the RFG temperature has a smaller effect. The analysis of the key reactions that contribute to NO production in the region around stoichiometry shows that fuel-NOx is the major contributor. Both NH3 and HCN in fuel-N play a major role, while tar-N only contributes in the case with the lowest temperature and O2 concentration. The classical NOx formation pathways are negligible and the initiation reaction of the Zeldovich mechanism is even reversed, i.e. NO⟶+NN2 is dominant and contributes to NO destruction. The destruction of NO mainly occurs in a rich region close to the particle surface where abundant tar species and their derivatives play a major role for NO destruction via the re-burn mechanism. The prompt mechanism is also active in this region and eventually contributes to NO reduction via production of HCN which is the feed to the re-burn mechanism.

Analysis of the Single Coal Particle Combustion Process under O2/CO2 Atmosphere Based on Spectral Diagnostics Technology: Combination of Spectroscopic Characteristics and Flame Temperature

This work studies the single coal particle combustion process in an O2/CO2 atmosphere based on spectral diagnostics technology in a visual drop tube furnace (VDTF). The chemiluminescence characteristics of OH*, CH*, Na*, and K* in single coal particles at two reaction stages and different oxygen fractions (Xi,O2) are investigated. The results show that both flame temperature and alkali metal spectral intensity increase first and then decrease, and the trend of temperature variation is consistent with that of alkali metal spectral intensity. Meanwhile, with Xi,O2 increases, Na* and K* peak intensities are enhanced because of the improvement in the oxygen flow rate. In the volatile reaction stage, the coal particle shows a typical envelope flame with high soot generation and luminosity. As the reaction time increases from 10 to 30 ms, the flame size increases and the flame temperature increases from 1550 to 1950 K. The spectroscopic results corresponding to the volatile reaction stage show that, as Xi,O2 increases from 30 to 50, the OH* and CH* intensity peaks increase linearly. Moreover, the intensities of the Na* and K* peaks increase by approximately 50.2 and 89.2%, respectively. During the volatile–char reaction stage, the coal particle exhibits a brighter luminous characteristic. As the reaction time increases from 40 to 70 ms, the flame size reduces and the flame temperature decreases from 1900 to 1700 K. The spectroscopic results corresponding to the volatile–char reaction stage indicate that, with the increase in Xi,O2, the positions of the OH* and CH* peaks change little and the intensities of the Na* and K* peaks increase by 21.3 and 75.1%, respectively. Our results prove that the flame temperature and alkali metal atomic emission spectroscopy exhibit the same trends as a function of the reaction time, and the alkali metal atomic emission spectroscopy can be used to characterize the flame temperature.

Investigation on the OH*and CH* chemiluminescence characteristics of single coal particle flames under O2/CO2 atmosphere

Here, the OH* and CH* chemiluminescence characteristics of a single coal particle flame combustion in a visual drop-tube furnace (VDTF) for different oxygen fraction, Xi,O2, in O2/CO2 atmosphere were investigated using an optical diagnostic technique. The results indicate that the combustion of single coal particles is composed of a volatile reaction stage followed by a volatile-char reaction stage. During volatile reaction stage, the single coal particle burns in an enveloped flame. The flame becomes brighter and the flame length increases gradually. In addition, the OH* and CH* intensity shows peak values with axial distance along the furnace, with that of OH* appearing earlier as the oxygen fraction increases. In the volatile-char reaction stage, the flame length fluctuates slightly, and OH* and CH* peak position are spread over a smaller range of distance. The value of Xi,O2 affects the reaction rate as it promotes the diffusion of oxygen to the interior of coal particle, as the intensity of OH* and CH* increasing rapidly for Xi,O2 > 0.4. This indicates that the rate of reaction is controlled by Xi,O2. Furthermore, the peak intensity ratio of OH* and CH* increases linearly with Xi,O2.

Homogeneous ignition and volatile combustion of single solid fuel particles in air and oxy-fuel conditions

The ignition and volatile combustion of single coal particles were investigated under laminar conditions. Relevant physico-chemical processes were analyzed under conventional and oxy-fuel atmospheres with varying O2 contents in experiments and simulations. An optically accessible laminar flow reactor with well-defined boundary conditions measured with PIV and quantitative OH-LIF was employed. Multi-parameter optical diagnostics were conducted, including OH-LIF, luminescence imaging, and backlight illumination. Simultaneously acquired experimental data allowed for the evaluation of particle size, ignition delay time, and volatile combustion duration for individual particles. A statistical analysis revealed the improved accuracy of OH-LIF compared to luminescence imaging regarding ignition detection. Simulations within an Eulerian–Lagrangian framework were introduced and validated against experiments. On this basis, particle temperatures, local gas temperatures, and fuel mass fraction were evaluated, providing insights into the devolatilization. Both experimental and numerical results indicated that increasing particle sizes significantly retarded homogeneous ignition and volatile consumption. When increasing the O2 content, a shorter ignition delay time and volatile combustion duration were observed experimentally, which was more significant for larger particles. High slip velocities accelerated convective transport resulting in an earlier ignition and faster volatile combustion. An atmosphere change from N2 to CO2 showed an earlier ignition and increased volatile combustion duration for larger particles, whereas the differences were insignificant for small particles. Simulation results suggested that the local heat transfer was improved by CO2, mainly due to the lower temperature sink close to particles and hence higher volatile release rates. As the initial ambient temperatures were similar, the introduction of CO2 favored homogeneous ignition and slowed down the volatile consumption.

Oscillation characterization of volatile combustion of single coal particles with multi-species optical diagnostic techniques

This paper investigated the oscillation characterization of volatile combustion of single coal particles using high-speed OH-PLIF and CH* chemiluminescence, meanwhile advanced data-driven algorithm as post-processing. The measurement region of in-situ 500 Hz OH-PLIF system was set to 53mm × 35 mm with a spatial resolution of ~23 µm per pixel, which was used to capture the continuous and whole volatile combustion process of single particle in a group of pulverized coal particles (SPGCP). A high-volatiles bituminous coal (30 wt%) burned at a gas temperature of 1700 K with oxygen mole fractions in the range of 10%–30%. The morphology of volatile flame and dynamic evolution was obtained. Dynamic mode decomposition (DMD) method can provide a new insight into combustion oscillation of pulverized coal. The oscillation frequency characteristics of pulverized coal flame were captured, and the red-shift phenomenon of dominant frequency with the increase of oxygen concentration was found. The particle aggregation might lead the low-frequency oscillation of pulverized coal combustion. On the contrary, individual or separated particle combustion could generate larger oscillation frequency.

Modeling of single coal particle combustion in O2/N2 and O2/CO2 atmospheres under fluidized bed condition

A one-dimensional transient single coal particle combustion model was proposed to investigate the characteristics of single coal particle combustion in both O2/N2 and O2/CO2 atmospheres under the fluidized bed combustion condition. The model accounted for the fuel devolatilization, moisture evaporation, heterogeneous reaction as well as homogeneous reactions integrated with the heat and mass transfer from the fluidized bed environment to the coal particle. This model was validated by comparing the model prediction with the experimental results in the literature, and a satisfactory agreement between modeling and experiments proved the reliability of the model. The modeling results demonstrated that the carbon conversion rate of a single coal particle (diameter 6 to 8 mm) under fluidized bed conditions (bed temperature 1088 K) in an O2/CO2 (30:70) atmosphere was promoted by the gasification reaction, which was considerably greater than that in the O2/N2 (30:70) atmosphere. In addition, the surface and center temperatures of the particle evolved similarly, no matter it is under the O2/N2 condition or the O2/CO2 condition. A further analysis indicated that similar trends of the temperature evolution under different atmospheres were caused by the fact that the strong heat transfer under the fluidized bed condition overwhelmingly dominated the temperature evolution rather than the heat release of the chemical reaction.

Effect of particle distance on combustion behaviors through 1-D model with Neumann boundary condition

Moderate or intense low oxygen dilution (MILD) combustion is a promising technology for coal utilization due to the uniform heat flux and low NOx emission. In such coal cloud combustion, it is essential to investigate the effect of surrounding particles on the single coal combustion behaviors. In this work, the effect of particle distance on ignition, volatile flame, soot volume fraction and combustion modes are studied through a 1-D model with Neumann boundary condition. For pulverized coal combustion, heterogeneous ignition is dominating rather than homogeneous ignition. When particle number density is small, the ignition time of coal jet is dominated by that of large particles; while when particle number density is large, the ignition time of coal jet is dominated by that of small particles. According to the transient oxygen distribution at different particle distances, when the particle distance is large, the combustion is diffusion controlled; while when the particle distance is small, the combustion is kinetics controlled. At low oxygen mole fraction, soot emission increases with increasing particle distance because the yields-promoting effect dominates. At high oxygen mole fraction, the consumption-strengthening effect competes with the yields-promoting effect, thus the soot emission increases first and then decreases as particle distance increases. The numerical method in this work is able to predict the combustion modes at different particle number density. At least 90 times of the particle diameter is required for the size of the calculating domain when the Dirichlet boundary condition is used for the modeling of a single coal particle combustion.

Soot cloud size of a single coal particle in air/oxy combustion under forced convection

Soot cloud size is important in radiative heat transfer and particulate matter emission since it determines the integral effect in the sooty region; meanwhile, study on soot cloud size during single coal particle combustion is essential to develop theories of soot-relevant reaction pathways and mass transport. In this work, a 1-D transient coal combustion model with an empirical soot model was adopted and validated to study the effect of gas physicochemical properties and forced convection on soot cloud size (ls/r0). Tar gasification results in a larger ls/r0 at any xO2, while soot gasification has a limited effect at low xO2 and leads to a larger soot cloud size at high xO2. The effect of tar and soot diffusion becomes more important at higher oxygen mole fraction; a larger heat capacity leads to a larger soot cloud size. In O2/N2 environments, soot cloud size increases when Rep increases from 0 to 10 and decreases when Rep increases from 10 to 16. In O2/CO2 environments, soot cloud size increases dramatically when Rep increases from 0 to 13 and slightly decreases when Rep increases from 13 to 16. The ls/r0 in O2/CO2 is always larger than that in O2/N2. The effect of convection on soot cloud size has two sides. On one hand, the blow away effect tends to increase the ls/r0. On the other hand, convection may increase or decrease total soot mass, leading to an increase or decrease of ls/r0, respectively.

Numerical simulation investigation of ignition and combustion process for single pulverized particle of anthracite

The transient model on fast pyrolysis and combustion of single coal particle is developed, taking into consideration the various pyrolysis products containing tar, the heat transfer, the mass transfer, and the chemical reactions. The findings of this study are shown as follows: the single particle ignition of anthracite begins from volatiles at high temperature; the volatile flame gradually parts from the particle surface due to the increasing intense combustion of volatiles and the oxygen diffusion restriction, especially for the smaller particle, the flame of which could reach a position departing from the coal center by about 20 times radius of the coal; the char combustion is governed by the surface oxidation reaction after the volatile release completion, because the CO from the char surface oxidation has been ignited and the flame has returned to the char surface; then the char combustion begins to be governed by the char surface reduction reaction because the oxygen near the char surface is consumed sharply by the CO flame, which then leaves char surface and reaches a position departing from the char center by about 2 times radius of the char; after that the CO combustion weakens gradually with the decreasing char size, and the surface oxidation begins to dominate the surface reaction again; meanwhile, generally the highest combustion temperature increases greatly and the burnout time shortens with the increase of ambient oxygen content; in air, the variation laws of the particle temperatures only change slightly and the highest combustion temperatures all approximate with the increasing coal size; in the meantime, the burnout time of bigger coal particle shortens due to the more obvious heat transfer from the gaseous flame to solid phase.

Multi-parameter diagnostics for high-resolution in-situ measurements of single coal particle combustion

To study volatile combustion processes of single coal particles non-intrusive simultaneous multi-parameter measurements were performed. The experiment was carried out in a fully premixed flat flame burner with well-defined boundary conditions. For flame visualization high-speed luminescence imaging was combined with high-resolution high-speed OH-PLIF. To address particle size and shape a stereoscopic high-resolution backlight-illumination system was set up. Due to simultaneous recording of individual particle events the volatile combustion duration related to particle size, shape and velocity was measured. A comparison of luminescence imaging and OH-PLIF for flame visualization was investigated to define their application areas in coal combustion. The stereoscopic backlight-illumination setup was benchmarked to a well characterized bituminous coal. With a pixel resolution of ∼2.5 µm fine particle contours were resolved. The particle diameter and eccentricity were evaluated by an ellipse approximation. The experimental setup can be used to investigate different coal ranks and biomass in N2/O2 and CO2/O2 atmospheres in future.

Coal particle volatile combustion and flame interaction. Part I: Characterization of transient and group effects

The heat-up, devolatilization, ignition and volatile combustion of single coal particles and particle arrays in laminar and turbulent flow are investigated by direct numerical simulation (DNS). The first part of the paper considers the transient evolution and group effects in laminar flows, the second part will report the effects of particle Reynolds number and turbulence. The conditions for DNS are extracted from an accompanying large eddy simulation (LES) of a semi-industrial coal furnace. The DNS fully resolves the particle boundary layers, the flame thickness and the smallest flow scales. Particle heat-up is captured by solving for intra-particle heat transfer, while devolatilization and volatile combustion are described by specific particle boundary conditions and detailed homogeneous chemistry. The transient physico-chemical processes around the particle(s) are characterized for a single particle first, before the interactions of several particles are studied. The analysis of arrays of infinite particle layers shows a strong dependence of the flame interaction on the inter-particle distance Lx. In particular, different combustion regimes are observed for different Lx (for a fixed particle Reynolds number Rep), ranging from isolated burning of the particles for large Lx to group combustion for small Lx, and spanning a wide range of global equivalence ratios from very lean (large Lx) to very rich (small Lx). The regime transition affects the surface temperature and devolatilization rate history of the particles. Models for the mixture fraction distributions in the particle wake are provided, based on the analogous problem of droplet evaporation.

Investigation of ignition and volatile combustion of single coal particles within oxygen-enriched atmospheres using high-speed OH-PLIF

To study devolatilization, ignition, and volatile combustion processes of single coal particles at a fundamental level, a flat flame burner was designed to provide oxygen-enriched exhaust gas environments at elevated temperatures and varying gas compositions. In one set of experiments coal particles were carried by a central jet using a gas mixture of the same composition as the flat flame. By this means a homogenous and connected flame was generated. At the instant when single coal particles transited this stationary flame, start of particle heat-up was well defined. This allows studying start of ignition more accurately compared to the other set of experiments where an inert carrier gas jet was used for transportation of coal particles. Heat-up and ignition were retarded temporally due to precedent mixing between inert gas and ambient flue gases. Coal particles with a high percentage of volatiles (36wt%) and a particle distribution ranging from 90 to 125µm were used. Coherent anti-Stokes Raman spectroscopy (CARS) measurements were performed to characterize ambient gas phase temperatures. By high-speed laser-induced fluorescence, imaging of OH-radicals ignition and volatile combustion was visualized for individual coal particles. Reaction zones surrounding each coal particle and the corresponding distances to coal particles were identified. This distance depended strongly on oxygen concentration of the ambient flue gas atmosphere. In addition to an improved phenomenological understanding the data may serve for model validation.

Transient model for soot formation during the combustion of single coal particles

A transient mathematical model was developed to describe soot formation during the combustion of single coal particles based on the static semi-empirical model presented by Fletcher and coworkers. Sensitivity analyses of the model parameters show that soot emissivity and mass diffusivity of tar play an important role in predicting soot volume fraction (fv) and flame temperature (Tf). The model was applied to simulate the combustion of single bituminous coal particles with initial diameter (2r0) of 83µm in a drop tube furnace and air atmosphere. It was found that soot is only formed within the first ∼ 5ms after the appearance of the volatile flame. Although most of the soot is oxidized during the volatile flame phase, a small portion of soot still remains during the char combustion. Due to the soot presence, the volatile flame duration is extended by 2.6ms. Compared with the soot-free flame, the sooting flame has remarkable lower Tf and its peak Tf value is ∼ 410K lower. As a consequence, char combustion starts at a temperature that is ∼ 125K lower than that of the soot-free case. Spatially, the peak fv at 16.6ms appears at 4.5 r0 and soot oxidation zone spans to ∼ 10 r0. The model was validated by comparing the predicted Tf and fv under different O2 mole fractions (xO2) with recent experimental results reported by Khatami and coworkers. The predicted trends are consistent with those of the experimental results. With increasing xO2, Tf increases, but the increase rate becomes more gradual at a large xO2. While for fv, a non-monotonic variation is observed, where soot first increases and then decreases with a peak value occurring at xO2 ≈ 40%.

Devolatilization and volatiles reaction of individual coal particles in the context of FGM tabulated chemistry

The method of Flamelet Generated Manifolds (FGM) is coupled with a coal devolatilization model to perform transient simulations of a well-defined single coal particle combustion experiment for the first time. The gas phase chemistry is mapped onto a three-dimensional manifold controlled by the mixture fraction, a reaction progress variable and the enthalpy. A simulation of an electrically heated inert pyrolysis reactor is performed in order to evaluate transferability and applicability of experimentally obtained devolatilization kinetic parameters to Large Eddy Simulation (LES) codes for combustion configurations. Finally, the FGM modeling approach is applied to a premixed flat flame configuration, in which the coal particles cross a laminar flame front and are exposed to the hot gases. Numerical results regarding the volatiles reaction are compared to experimental findings. Particle and gas phase states are studied. Overall, good agreement between numerical results and experimental findings regarding the volatiles ignition range could be observed.

Volatile Release and Ignition Behaviors of Single Coal Particles at Different Oxygen Concentrations Under Microgravity

An experimental study on ignition and combustion of single coal particles under different O 2 concentrations was conducted at both normal (1-g) and microgravity (μ-g) in the first time. The surface and centre temperatures of the bituminous coal particle with initial diameter of ∼ 2.0mm were measured by the monochromatic imaging technique using a short wavelength infrared (SWIR) camera and an embedded fine thermocouple respectively. Results revealed that at μ-g, ignition of the tested coal particles was homogeneous. O 2 concentration significantly affects the shape, ignition temperature and ignition delay time of the volatile flames. A mathematical model considering thermal conduction inside the coal particle was developed to describe the ignition process of single particle, adopting the volatile matter flammability limit as the homogeneous ignition criterion. The predicted ignition temperatures were slightly lower but closer to μ-g data. And the predicted variation trends of ignition temperature and delay time under different O 2 concentrations agreed well with the μ-g experimental results.

Characterization of single coal particle combustion within oxygen-enriched environments using high-speed OH-PLIF

This work presents first-of-its-kind high-speed planar laser-induced fluorescence measurements of the hydroxyl radical in the boundary layer of single coal particles. Experiments were performed in a laminar flow reactor providing an oxygen-enriched exhaust gas environment at elevated temperatures. Single coal particles in a sieve fraction of 90–125 µm and a significant amount of volatiles (36 wt%) were injected along the burner’s centerline. Coherent anti-Stokes Raman spectroscopy measurements were taken to characterize the gas-phase temperature. Time-resolved imaging of the OH distribution at 10 kHz allowed identifying reaction and post-flame zones and gave access to the temporal evolution of burning coal particles. During volatile combustion, a symmetric diffusion flame was observed around the particle starting from a distance of ~150 µm from the particle surface. For subsequent char combustion, this distance decreased and the highest OH signals appeared close to the particle surface.

Conversion of Fuel-N to N2O and NOx during Coal Combustion in Combustors of Different Scale

With focus on investigating the effect of combustor scale on the conversion of fuel-N to NOx and N2O, experiments are carried out in three combustors, including single coal particle combustion test rig, laboratory scale circulating fluidized-bed boiler (CFB) and full scale CFB in this work. For single coal particle combustion, the majority of fuel-N (65%–82%) is released as NOx, while only a little (less than 8%) fuel-N yields N2O. But in laboratory scale CFB, the conversion of fuel-N to N2O is increases, but the conversion of fuel-N to NOx is quite less than that of single coal particle combustion. This is because much char in CFB can promote the NOx reduction by increasing N2O formation. In full scale CFB, both of the conversion of fuel-N to NOx and the conversion of fuel-N to N2O are smaller than laboratory scale CFB.

Experimental and modeling study of single coal particle combustion in O2/N2 and Oxy-fuel (O2/CO2) atmospheres

Coal particle combustion experiments were performed in a drop tube furnace (DTF) with oxygen concentration from 21% to 100%, in N2 and CO2 mixtures, under quiescent flow conditions. Small particles (75–90μm) of a high-volatile bituminous coal (PSOC-1451) and a lignite coal (DECS-11) are analyzed with particular attention to the particle burnout times and the particle surface temperatures. These experimental measurements are compared with the predictions of a comprehensive model of coal combustion. Combustion of coal particles is a multi-scale process where both chemical and physical phenomena are involved, thus it requires a coupled and accurate description of the kinetics as well as of the heat and mass transport phenomena. Important features of the model are a multistep kinetic scheme of coal volatilization and detailed kinetics of the successive gas-phase reactions and of the heterogeneous reactions of both char oxidation and gasification. The achieved overall agreement between the experimental data and the numerical predictions, in terms of particle temperature and burnout times, highlights the capability of the model to simulate the effect of different operating conditions in the coal combustion processes.

Mathematical model for heat transfer and combustion in a pulverized coal flame

A mathematical model for the heat transfer and combustion of a single coal particle is presented. The model correctly predicts the time of the reaction steps and the temperature of the coal particle during ignition and combustion. Results are given of calculations of the combustion of single coal particles of the hard coal and brown Nazarovo coal at various oxygen concentrations and ambient temperatures. The results of combustion experiments with Irsha-Borodino coal on a combustion test facility are in good agreement with the results of three-dimensional numerical modeling of a pulverized coal flame.

On the deduction of single coal particle combustion temperature from three-color optical pyrometry

Temperature–time histories of burning single coal particles can be obtained with multi-color (multi-wavelength) optical pyrometry. With this method, a number of different temperatures can be deduced from the resulting number of two-color ratios. However, these two-color temperatures do not always agree, causing considerable uncertainty in the temperature measurement. This work used a three-color pyrometer and focused on identifying and minimizing the causes of disparity among the three deduced temperatures. Components of the pyrometer (such as dichroic filters, interference filters and photo-detectors) were modeled mathematically, taking into account their wavelength-dependent properties. The pyrometer was calibrated with both a high-temperature pre-calibrated tungsten lamp, and a moderately-high temperature blackbody cavity, to span the temperature range of interest in pulverized coal combustion. Temperatures were deduced based not only on a suitably-modified pyrometric signal ratio method but also, on a similarly modified pyrometric signal non-linear least-square method, to provide comparison. Results are exemplified by presenting radiation-signal-time and temperature–time profiles of single particles burning in air. The variation of the projected luminous area of burning particles was also computed using both methods, and area–time profiles are presented herein. The char particle emissivity was either treated as a quantity independent of the wavelength (i.e., assuming gray-body behavior), or as a quantity assumed to depend linearly on the wavelength and using pertinent published emissivity data. Finally, a sensitivity analysis was performed to investigate individual effects of parameters, such as the calibration method, the wavelength dependencies of filter transmissivities, and the photo-detector responsivities on the pyrometric signal ratio method temperature consistency.