Devolatilization Of Coal Particles Research Articles

Coal Particle Devolatilization and Soot Formation in Pulverized Coal Combustion Fields

In this paper, recent developments of the devolatilization model and soot-formation model for the numerical simulations of pulverized-coal combustion fields, and the technology used to measure soot particles in pulverized-coal combustion fields are reviewed. For the development of new models, the validation of the developed models using measurement is necessary to check the accuracy of the models because new models without validation have a possibility to make large errors in simulations. We have developed the tabulated devolatilization process model (TDP model) that can take into account the effect of particle heating rate on the volatile matter amount and the devolatilization-rate parameters. The accuracy of the developed TDP model was validated by using the laser Doppler velocimetry data for the bench-scale coal combustion test furnace. The soot-formation model combined with TDP model for the large eddy simulation (LES) has been also developed. The spatial distributions of both the soot-volume fraction and the polycyclic aromatic hydrocarbons were measured by virtue of laser-induced incandescence (LII) and laser-induced chemiluminescence (PAHs-LIF). The accuracy of the developed soot-formation model was validated by using the measured data.

Numerical study of interaction of coal dust with premixed fuel-lean methane-air flames

Interaction of combustible particles with flames occurs in fire scenarios. In this study, numerical investigation of interaction of coal dust or micron-sized particles with lean premixed methane-air flames is presented. A two-dimensional axisymmetric domain is employed to simulate conical premixed flames from lab-scale Bunsen burner. A chemical kinetic mechanism having 25 species and 121 elementary reactions, temperature dependent thermo-physical properties, multi-component diffusion with Soret effect and radiation model accounting for gas and soot radiation are used. Discrete Phase Model (DPM) is used to simulate the transport of coal particles. Coal particles in varies size ranges and concentrations are injected into the premixed reactant mixture at equivalence ratio between 0.75 and 0.85. Multiple species from devolatilization of coal particles are considered to enter the gas-phase. Laminar flame speeds are predicted using numerical shadowgraphs and validated against the experimental data from literature. Injection of coal particles affects the laminar burning velocity and flame structure. The numerical model is able to predict the variation trends in the laminar flame speed data quite reasonably. A detailed analysis of injection of coal particles on the resultant flame dynamics are presented using the fields of temperature, flow, species, net reaction rate, heat release rate and DPM.

Numerical investigation of coal particle stream ignition in oxy-atmosphere

The ignition of a stream of coal particles under oxy-fuel atmosphere is studied numerically in a laminar flow. Devolatilization of coal particles is modeled with the chemical percolation devolatilization (CPD) method coupled with a detailed chemistry model for homogeneous reactions. The particle and gas phase interactions are modeled with a fully coupled Euler-Lagrange code. A parametric study is performed to investigate the influence of particle number density, gas temperature, and velocity on ignition. We found that increasing particle number density delays the onset of ignition. Delayed ignition in denser streams is due to the lower gas temperature, which is caused by the higher energy required for particle heating. In addition, increasing particle number density leads to a more continuous and narrower flame front. Particle heating and ignition induce velocity variations in the gas phase through the coupling with particles. In the denser stream, velocity variations become significant and compromise the validity of a constant velocity assumption that is usually made in computing ignition delay time from the observed ignition location. It is also found that high particle slip velocities lead to a locally low volatile concentration and low temperature, and consequently increase ignition delay time.

Resolved flow simulation of pulverized coal particle devolatilization and ignition in air- and O2/CO2-atmospheres

A resolved laminar flow simulation approach is used to investigate the effect of enhanced oxygen levels on single coal particle ignition, comparing the numerical results against experimental data for well-defined conditions (Molina and Shaddix, 2007). Devolatilization is described by a generic boundary condition at the particle surface that accounts for both convective and diffusive phenomena during pyrolysis. The heating rate history of the particle is obtained by solving for intra-particle heat transfer and heat exchange between the particle and its surroundings. The time evolution of volatile release is captured by using the particle mean temperature to calculate the devolatilization rate from a single kinetic rate law with CPD-fitted parameters. The assumed volatile composition includes both light gases and larger hydrocarbons to represent tars. A skeletal kinetic mechanism for pyrolysis and oxidation of hydrocarbon and oxygenated fuels containing 52 species and 452 reactions is used to accurately describe homogeneous chemistry. Particle heat-up, pyrolysis, ignition and envelope flame stabilization are characterized in four gas atmospheres differing in oxygen content and the use of either N2 or CO2 as balance gas. In agreement with the experimental evidence, enhanced oxygen levels shorten ignition delay time τign and result in a higher intensity of the combustion process according to temperature and radical production peaks for all studied mixtures. For the studied oxy-mixtures the presence of CO2 in substitution of N2 delays ignition. The observed behavior is coherent with the different thermo-physical properties of the gas mixtures. The sensitivity of predicted ignition delay to a set of uncertainties is also discussed. It is found that while the absolute values of predicted ignition delay time are functions of potential particle preheating, particle Reynolds number and the chosen criterion to extract ignition delay, the relative trends among the gas mixtures remain in line with the experimental evidence.

Non-intrusive optical diagnostics of co- and counter-swirling flames in a dual swirl pulverized coal combustion burner

Optical non-intrusive measurements are performed to a 10kWth laboratory-scale dual swirl pulverized coal combustion burner in order to elucidate the behaviors of internal recirculation zone (IRZ) and heat release region. The pulverized coal flame is operated with four different swirl combinations: co-swirling (low- and high-swirl) and counter-swirling (low- and high-swirl). The flow field is measured using particle image velocimetry (PIV). The IRZ area increases for the high-swirl conditions than that in the low-swirl conditions. With changing from co- to counter-swirl combination, the IRZ appearance changes from a heart- to an elongate-shape IRZ because double stagnation points for the co-swirling flames merges to one stagnation point for the counter-swirling flames. For the co-swirling conditions, a tube-type vortex is detected at the end of the IRZ near the double stagnation points. However, it is not present in the counter-swirling conditions. From two-color pyrometry, the overall temperature in the high-swirl flames is lower than the low-swirl flames. For the counter-swirling flames, higher temperature is observed due to the better mixing, which allows for a more intense combustion reaction of pulverized coal particles. Flame color measurements (OH and CH chemiluminescence band light emission) shows that the devolatilization of coal particles is observed at more upstream region comparing with the heat release reaction for volatile gas combustion. For the high-swirl flames, the CH∗ band intensity for heat release decreases, however its overall dimension increases. For the counter-swirling flames, the higher heat release is observed.

Process Development and Reactor Analysis of Coal Pyrolysis to Acetylene in Hydrogen Plasma Reactor

Coal pyrolysis in hydrogen plasma opens up a direct means for producing acetylene. This process is operated under high temperature conditions (e. g., ~3000 K) with the residence time in milliseconds. To better understand the complex reaction behavior in a 2 MW pilot-scale reactor, a comprehensive CFD model was established to describe the gas–particle reacting flows for the process of coal pyrolysis in hydrogen plasma. Equilibrium chemistry in the gas phase was introduced and the mixture fraction approach with β-shape probability density function was used. The coal particles were modeled by discrete phase model approach so that the heating and devolitilization processes of coal particles can be well quantified in the reactor. The 3-dimensional simulations revealed the detailed flow field, temperature field, devolitilization process of coal and the spatial distribution of product gases. The unique structure of the V-shape plasma torch leads to the non-uniformity of temperature distribution in the reactor as well as the dissimilar courses of devolatilization of coal particles injected from different inlets. The results indicated that coal released its volatile matter in the first third of the reactor. Increasing the coal feeding rate results in the temperature drop of the reactor and an increase of the concentration of methane. The concentration of acetylene may reach its maximum in a good match between the plasma power input and the feeding rate of coal. All the results showed good accordance with the operation experience on the 2 MW pilot-plant reactor.

The Chemical Percolation Devolatilization Model Applied to the Devolatilization of Coal in High Intensity Acoustic Fields

The chemical percolation devolatilization model (CPD) was extended for the prediction of drying and devolatilization of coal particles in high intensity acoustic fields found in Rijke tube reactors. The acoustic oscillations enhance the heat and mass transfer processes in the fuel bed as well as in the freeboard, above the grate. The results from simulations in a Rijke tube combustor have shown an increase in the rate of water evaporation and thermal degradation of the particles. The devolatilization model, based on chemical percolation, applied in pulsating regime allowed the dynamic prediction on the yields of CO, CO2, CH4, H2O, other light gases as well as tar which are important on ignition and stabilization of flames. The model predicted the quantity and form of nitrogen containing species generated during devolatilization, for which knowledge is strategically indispensable for reducing pollutant emissions (NOx) in flames under acoustic excitation .

99/00905 Numerical simulation of large coal particle devolatilization in a fluidized bed allowing for devolatilization heat : Yu, H. et al. Zhongguo Kexue Jishu Daxue Xuebao, 1998, 28, (3), 351–355. (In Chinese)

99/00905 Numerical simulation of large coal particle devolatilization in a fluidized bed allowing for devolatilization heat : Yu, H. et al. Zhongguo Kexue Jishu Daxue Xuebao, 1998, 28, (3), 351–355. (In Chinese)

97/05156 Comparison of experimental methods for determining mining coal particle devolatilization times under fluidized bed combustor conditions: Stubington, J. F. et al. Fuel, 1997, 76, (3), 233–240

97/05156 Comparison of experimental methods for determining mining coal particle devolatilization times under fluidized bed combustor conditions: Stubington, J. F. et al. Fuel, 1997, 76, (3), 233–240

Comparison of experimental methods for determining coal particle devolatilization times under fluidized bed combustor conditions

Numerous studies have measured the devolatilization times of coal particles ‘under fluidized bed conditions’, either in a hot fluidized bed or by holding the particle(s) stationary in a hot reactor. Here, two fluidized bed methods and one stationary particle method were compared, using the same three coal samples. Devolatilization times and fragmentation were studied. A new devolatilization endpoint definition from CO 2 evolution profiles gave good agreement with flame extinction time measurements, so either of these methods may be used in fluidized bed experiments to measure devolatilization time. The different environment around the coal particle in thermogravimetric experiments gave substantially longer devolatilization times, so the results from stationary particle experiments are not relevant to fluidized bed combustion. Coal particle fragmentation behaviour differed for the three coals studied, but no effect of coal properties on the devolatilization time in fluidized beds was observed, and an earlier correlation fitted these results well.

96/01857 On the timing of primary fragmentation during bituminous coal particle devolatilization in a fluidized bed combustor

96/01857 On the timing of primary fragmentation during bituminous coal particle devolatilization in a fluidized bed combustor

96/00679 Devolatilization of coal particles in a flat flame — Experimental and modeling study

96/00679 Devolatilization of coal particles in a flat flame — Experimental and modeling study

A general model for devolatilization of large coal particles

There have been several studies on the devolatilization of large coal particles under laboratory conditions simulating large-scale fluidized or fixed-bed combustion conditions, all of which demonstrate that the process of devolatization of large coal particles occurs over a much longer timescale than that of pulverized coals under the same conditions. This paper reports the development of a model for large coal particle devolatilization under fluidized and fixed-bed combustion conditions. The model combines a heat transfer model with a general and comprehensive coal devolatilization model, FG-DVC. It inherits the generality of FG-DVC and can be applied to coals of various types without extensive prior knowledge of the coals. This model includes a detailed treatment of coal pyrolysis reactions including the yields of individual gas species, tar yields, and tar molecular weight distributions. The changes in coal physical properties are modeled with the application of Merrick's coal physical property submodels for specific heat and thermal conductivity. Both the convective and radiative heat transfer between the gas phase and coal particles are considered and Gunn's correlation for the surface heat transfer coefficient is used. Model predictions are compared with the data of large coal particle devolatilization measured in fluidized beds and tubular reactors, for particle thermal response, total weight loss, and individual gas evolution rates. In general, the agreement with the data is very good for the particle center temperature and weight loss curves and is fair for the individual gas evolution curves.

Devolatilization of coal particles in a flat flame—experimental and modeling study

Pulverized coals have been tested under the conditions of industrial flames, with high heating rate and high temperatures. Coal particles were injected (5 g/h) into a flat, air-propane flame at 1400°C. The chars were collected after different pyrolysis times. The thermal history of the particles, as monitored by two color pyrometry, shows that particles undergo very large heating rates (6 × 10 6 K/s) and reach a peak temperature of 1100°C. For eight coals (volatile matter 1–57% d.a.f.), the devolatilized fraction of coal has been measured, as well as those of carbon, hydrogen and nitrogen. In every case, the devolatilized fraction of coal was greater than the predicted A.S.T.M. value, but proportional to it. The devolatilized fraction of hydrogen seems to be a more sensitive parameter than the coal's weight loss. During pyrolysis, the evolution of the texture of the grains has been studied by measurement of their microporous surface area, which undergoes a large increase, depending on coal rank. The composition of the volatiles, as deduced from the ultimate and proximate analysis of chars, showed high volatile bituminous coals to essentially produce tars with an aromatic structure. In that case, the surface area passed through a maximum at the beginning of devolatilization. The subsequent decrease could be due to recondensation of heavy molecular weight volatile matter at the char's surface. Low and medium volatile bituminous coals produced light hydrocarbons on devolatilization and the char's surface area continued increasing slowly during the whole of devolatilization, according to the slow increase of the fraction of hydrogen devolatilized. The char's reactivity with oxygen was followed by measurements of Active Surface Area (A.S.A.). It was shown that the A.S.A. continuously decreases during devolatilization. This may be due to the departure of prompt volatiles made of aliphatic groups, which are potential active sites for oxygen. Five models of devolatilization in the literature were tested and compared to the experimental results, assuming first-order reactions with respect to the remaining volatile matter. Badzioch's model correctly fitted the experimental results and values of the rate constant obtained by computer trial and error adjustment were higher for lower ranks of the four bituminous coals. Anthony's model also fits the measurements, provided an adjustment of the preexponential factor and activation energy ( k 0, E 0), for which it is shown that an infinite number of such pairs is suitable. If the model is run isothermally at the flame's peak temperature, it also correctly fits the experimental results.

Experimental investigation of single coal particle devolatilization by laser heating

A facility for high intensity laser heating of coal particles is described. The following characteristics of coal flash pyrolysis are given: time of volatile yield; specific molar yield; and total yield of volatiles. The theoretical pattern of volatile yield through the coal particle pores is discussed.

EXPERIMENTAL FACTORS AFFECTING COAL DEVOLATILIZATION TIME IN FLUIDIZED BED COMBUSTION

Abstract Coal particle devolatilization times were determined by analysis of the flue gas from an air-fluidized bed at 850 °C, when single coal particles were injected. The CO2concentration versus time profile was corrected for gas flow and dispersion in the sampling line and analyser, and the devolatilization time was measured as the time from particle injection to the time corresponding to the minimum CO2concentration separating volatiles and char combustion. No differences in devolatilization time were observed for the three Australian bituminous coals studied, and similar times were measured for both spherical and irregularly-shaped particles, provided the equivalent-mass definition of diameter was used. Comparison with literature data showed that this method for measuring coal particle devolatilization time gave similar values to the visual flame extinction time and greater values than visual flame period. From a critical analysis of the literature, it was deduced that batch size (i.e. number of panicles in each batch) could affect the measured devolatilization time, but that no such effect was present for some of the data for small numbers of large diameter particles in larger diameter beds. A single correlation was fitted to the present data and acceptable literature data, for individual coal particles from 1 to 30 mm in diameter in fluidized bed combustors at 850 °C in air: tv = 1.84 dp 1.5particles in a fluidized bed, using FET and/or CO evolution measurements, and that sufficient particles be studied to identify the variability of the measurements, in order to assess whether any effect observed is significant compared to the particle-to-particle variability.

A general model of pulverized coal devolatilization

A general model for pulverized coal devolatilization is proposed in this paper. The difference between the present model and previous models is that the equivalent activation energy, E, and the equivalent frequency factor, K 0, of coal particle devolatilization are independent of coal type, and depend only on the final temperature, T ∞, of the coal particles. This is a new conclusion that is contrary to previous theories. The relationship between E K 0 and T ∞ has been obtained and is found to be universal for all types of coals. If the final volatile yield, V ∞, is determined under certain conditions, the devolatilization processes of fine coal particles for any kind of coal can be predicted by using the model. This model has been tested with three Chinese coals, and five American coals. The coal devolatilization process was studied in a drop tube furnace heated by an arc plasma jet using Ar N 2 as an operating gas and by electrical heating elements. The experimental conditions were as follows: the diameters of the coal particles range from 35 μm to 100 μm and temperature from 973 K to 2100 K. The volatile yield was obtained by means of an ash tracer. The gas flow inside the tube is laminar, so that the dispersion of coal particles in the tube is quite small.

A study on devolatilization of large coal particles

A generalized devolatilization model for large coal particles is proposed in this paper. The difference between the present model and previous ones is that the equivalent activation energy, E, and the equivalent frequency factor, K, of coal particle devolatilization are independent of coal type, and depend only on the final temperature, T ∞, of the coal particles. The relationship between E K and T ∞ has been obtained and is found to be universal for all coal types. If the final volatile yield, V ∞, is determined, the devolatilization processes of a large coal particle for any kind of coal can be predicted by using the present model. This model has been tested with five Chinese coals with quite different properties, diameters ranging from 3 mm to 9 mm, and temperatures ranging from 1173K to 1733K. The calculated results agree with experimental data.

Model for devolatilization of coal particles in fluidized beds

A model for the devolatilization of coal in a non-combusting fluidized bed is proposed. Previous studies have either considered devolatilization as a non-rate process or assumed the devolatilization coal particle as isothermal. The assumption of an isothermal particle requires the heat transfer Biot number ⪝0.02. In view of the larger Biot numbers predicted using existing fluidized bed gas-solid heat exchange correlations and reported values for thermophysical properties, the present model considers the devolatilizing particle to be, in general, non-isothermal. The temperature profiles are computed from the analytical solution of the one-dimensional spherical coordinate unsteady heat transport equation with a convective boundary condition. The temperatures are then used in the non-isothermal coal decomposition kinetic expression proposed by Anthony et al., integrated over the particle to obtain the fractional volume average devolatilization at any given time. Parametric studies show a chemical kinetics controlled regime for small particles, a heat transfer controlled regime for larger particles and a mixed regime for intermediate particle sizes. The extent of the mixed regime depends on the type of coal as well as the operating conditions. The model results are also compared with the fluidized CH 4 and CO evolution data reported in the literature for various particle sizes and different temperatures.