Ignition Of Single Coal Particles Research Articles

Mathematical Simulation of the Influence of Volatile Matter Cloud on Heterogeneous Ignition of Single Coal Particles

Mathematical Simulation of the Influence of Volatile Matter Cloud on Heterogeneous Ignition of Single Coal Particles

An experimental study on ignition of single coal particles at low oxygen concentrations

An experimental study on the ignition of single coal particles at low oxygen concentrations ( $${X_{{{\rm{O}}_2}}} < 21\% $$ ) was conducted using a tube furnace. The surface temperature (Ts) and the center temperature (Tc) of the coal particles were obtained from the images taken by an infrared camera and thermocouples respectively. The ignition processes were recorded by a high-speed camera at different $${X_{{{\rm{O}}_2}}}$$ values and furnace temperatures Tw. Compared with literature experimental data obtained at a high $${X_{{{\rm{O}}_2}}}$$ value, the ignition delay time τi decreases more rapidly as $${X_{{{\rm{O}}_2}}}$$ increases at the low $${X_{{{\rm{O}}_2}}}$$ region. The responses of Ts and Tc to the variation of $${X_{{{\rm{O}}_2}}}$$ are different: Ts decreases while Tc remains nearly constant with increasing $${X_{{{\rm{O}}_2}}}$$ at a low $${X_{{{\rm{O}}_2}}}$$ value. In addition, τi is less sensitive to Tw while the ignition temperature Ti is more sensitive to Tw at a low $${X_{{{\rm{O}}_2}}}$$ value than in air. Observations of the position of flame front evolution illustrate that the ignition of a coal particle may change from a homogeneous mode to a heterogeneous or combined ignition mode as $${X_{{{\rm{O}}_2}}}$$ decreases. At a low $${X_{{{\rm{O}}_2}}}$$ value, buoyancy plays a more significant role in sweeping away the released volatiles during the ignition process.

Numerical study of heat and mass transfer during ignition of thermal decomposition products of a coal particle

A numerical research was executed for macroscopic regularities determination of heat and mass transfer processes under the conditions of phase transformation and chemical reaction at the ignition of vapour coming from fabrics impregnated by typical combustible liquid into area at the local power supply. Limit conditions of heterogeneous system fabric - combustible liquid - oxidant ignition at the heating of single metal particle was established. Dependences of ignition delay time on temperature and rates of local power source were obtained. The mathematical modeling of interrelated processes of heat and mass transfer and chemical reacting was carried out at ignition of single coal particles with different sizes from 0.05 mm to 0.5 mm in air flow heated to moderate temperatures (T<500 K). A physical model of ignition process describes the more common scheme of investigating phenomena (Figure 1).

A Numerical Modeling Study of Ignition of Single Coal Particles Under Microgravity Conditions

The ignition characteristics of single coal particles under microgravity conditions were investigated by means of numerical modeling, considering the effects of particle size, ambient temperature, and oxygen concentration as well as intraparticle thermal conduction. The predicted particle surface temperature and flame displacement agreed well with the experimental data observed under microgravity. It was revealed that when the coal particle size was greater than 100 µm, the intraparticle temperature distribution becomes significant. The primary ignition mechanism was heterogeneous for smaller particles but homogeneous for larger particles. The transition diameter for the ignition mechanism to switch from heterogeneous to homogeneous was found to depend on the ambient temperature, O2 concentration, and whether the intraparticle thermal conduction was considered or not. At a relatively low ambient temperature of 1123 K in air, the predicted transition diameter changed from 800 µm to 600 µm when intraparticle thermal conduction was considered. However, at 1500 K, the intraparticle thermal conduction had little effect on the ignition temperature and the transition diameter predicted was about 700 µm. It was shown that homogeneous ignition occurred when O2 concentration was low while heterogeneous ignition occurred when O2 concentration was high. It was also shown that the homogeneous ignition temperature was slightly lower than the heterogeneous ignition temperature. At high O2 concentrations, the predicted ignition temperature and ignition delay decreased with increasing O2 concentration, consistent with the literature findings, including those from the authors own previous studies. However, at low O2 concentrations, the predicted ignition temperature and ignition delay were less variant. The discrepancies of the predicted transition diameters for the isothermal particles and those with intraparticle thermal conduction considered were attributed to the heating rate and the influence of volatile matter release.

Ignition of single coal particle in a hot furnace under normal- and micro-gravity condition

An experimental study on ignition and combustion of single particles was conducted at normal gravity (1-g) and microgravity (μ-g) for three high volatile coals with initial diameter of 1.5 and 2.0 mm, respectively. The non-intrusive twin-color pyrometry method was used to retrieve the surface temperature of the coal particle through processing the images taken by a color CCD camera. At the same time, a mathematical model considering thermal conduction inside the coal particle was developed to simulate the ignition process. Both experiments and modeling found that ignition occurred homogeneously at the beginning and then heterogeneously for the testing coal particles burning at μ-g. Experimental results confirmed that ignition temperature decreased with increasing volatile content and increasing particle size. However, contradicted to previous studies, this study found that for a given coal with certain particle size, ignition temperature was about 50–80 K lower at μ-g than that at 1-g. The model predictions agreed well with the μ-g experimental data on ignition temperature. The criterion that the temperature gradient in the space away from the particle surface equaled to zero was validated to determine the commence of homogeneous ignition. Thermal conduction inside the particle could have a noticeable effect for determining the ignition temperature. With the consideration of thermal conduction, the critical size for the phase transient from homogeneous to heterogeneous is about 700 μm at ambient temperature 1500 K and oxygen concentration 0.23.

Ignition of single coal particles in high-temperature oxidizers with various oxygen concentrations

In this investigation, coal pellets were combusted using a high temperature oxidizer with varying oxygen concentration, using a small scale batch reactor able to preheat the oxidizer to 1273 K. In base of the experimental results, the influence of oxygen concentration on the ignition mechanism, the solid temperature inside the particle at the moment of ignition, the mass lost at the moment of the ignition and ignition time is analyzed and discussed. A theoretical basis for the division of the conditions tested into three ignition regimes is developed and a formula for the prediction of the ignition time directly from the material and oxidizer temperature and oxygen concentration is proposed.

A model for devolatilization and ignition of an axisymmetric coal particle

A two-dimensional numerical model is developed to predict shape-dependent devolatilization and ignition of single coal particles exposed to radiant heat flux in an ambience at initially constant temperature. Intraparticle temperature gradients and radiation absorption are taken into account as well as the local release of volatiles and therefore the reduction of the raw coal. During pyrolysis, internal pressure gradients occur, which cause a viscous flow of gas through the pores described by Darcy's law. The model considers intrinsic char combustion depending on the local oxygen concentration and internal and external volatile oxidation. Ignition is identified either by an increase of gas-phase temperature or the inflection condition of the particle surface temperature for the homogeneous and the heterogeneous ignition mode, respectively. Comparison with a published one-dimensional model, which assumes an isothermal particle, provides reasonable agreement for the ignition delays of particles with diameters of 50–1000 μm inserted into hot air. In contrast to the published data, the calculated results show a different primary ignition phase for particles larger and equal to 300 μm due to intraparticle temperature gradients. Former experiments on coal particle ignition by radiation in cold air under microgravity conditions show different homogeneous ignition delays for spherical, cylindrical, and flat particles. These experimental results are confirmed by the present model: in addition, heterogeneous ignition points are detected in the calculations. Although the homogeneous ignition delays increase in the order of slabs, cylinders, and spheres, which is the order where specific surface decreases, ignition delays can not be sufficiently described in terms of particle-specific surface, especially not for the calculated heterogeneous ignition.

Modeling of Effects of Volatile Matter Cloud on Heterogeneous Ignition of Single Coal Particles.

A simple model has been developed to predict the heterogeneous ignition temperature for single coal particles under conditions of negligible, natural and forced convections. These conditions cause different sizes of the volatile matter cloud surrounding the coal particle. The model is based on the ignition of a single coal particle heated by irradiation from spot heaters in cold (ambient) surroundings. Energy from the particle surface oxidation, volatile matter combustion and physical heating from the spot heaters are all included in the particle energy balance. General ignition criterion is used to determine the ignition point. The model results agree quite well with experimental findings obtained earlier as far as the effects of particle size and volatile matter are concerned. Gas temperature emerged as the most effective model parameter influencing the ignition temperature. In the absence of volatiles in the particle vicinity, as for the forced convection case, the gas temperature remains almost constant and so does the ignition temperature. For cases where volatiles surround the particle, high gas temperatures as well as ignition temperatures are obtained. This is mainly attributed to the combustion of the volatiles contributing to the gas temperature rise, and possibly raising the particle temperature.

Mechanism of Single Coal Particle Ignition under Microgravity Condition.

In order to simplify the complex coal ignition process, a microgravity condition (10–3g), in which natural convection is negligible, has been employed to observe ignition of single coal particles. The particle was heated and ignited by radiation from spot heaters. Three coals with different volatile matter contents were employed, having particle sizes in a range of 0.8 mm to 1.2 mm.As expected, under microgravity, a nearly spherical volatile cloud was found to form around the particle. Results clearly show that regardless of the particle size, the ignition is preceded by homogeneous gas-phase volatile ignition, followed by heterogeneous solid-phase char ignition after burn-out of the volatiles. The heterogeneous ignition temperature and time were higher and longer, respectively, under microgravity than under normal gravity condition. The homogeneous ignition also took place at a higher particle temperature under microgravity than under normal gravity. Further, the temperature for heterogeneous ignition increased with the absolute amount of volatiles contained in a coal particle, whereas the particle temperature for the homogeneous ignition decreased.

97/00041 Mechanism of ignition of single coal particle: Effect of heating rate on particle-size dependence of ignition temperature

97/00041 Mechanism of ignition of single coal particle: Effect of heating rate on particle-size dependence of ignition temperature

Mechanism of ignition of single coal particle: effect of heating rate on particle-size dependence of ignition temperature.

Experimental work has been undertaken using a radiation method to ignite single coal particles and to study their ignition behaviour. Particle sizes used were in the range of 600 μm–1800 μm. Effect of particle size on ignition temperature and ignition time was investigated for three types of coal under conditions of constant heating rates. Results show that under these conditions, ignition temperature increases with particle size contrary to the widely reported trend of decreasing ignition temperature with increasing particle size. It was also found that ignition temperature increases with heating rate. When particles of different sizes are heated by the same power, smaller particles experience higher heating rates than larger ones. The effect of particle size on ignition time was found to be the same as reported by other researchers where ignition time increases with particle size.

Factors influencing the ignition of coal particles studies with a pressurized heated-grid apparatus

The influence of coal rank, maceral composition, particle size, pressure, temperature and heating rate on the ignition of single coal particles was studied with a heated-grid apparatus (1400K, 10 MPa) working in connection with a high-speed camera. Experimental conditions were chosen to simulate the conditions of p.f. combustion and to provide data for a kinetic description. The temperatures of the homogeneous and heterogeneous ignition are strongly influenced by oxygen partial pressure and heating rate as well as rank and maceral composition. The oxygen partial pressure is an important parameter during combustion, and also influences the interplay of pyrolysis and combustion.

Experimental and theoretical study of ignition of single coal particles at zero gravity

In the coal combustion process one of the most important problems is the knowledge of chemical reaction principles concerning combustion of single particles and the knowledge of mutual interaction between the burning particles. In this paper theoretical results are presented, which demonstrate the characteristic ways of ignition between coal particles depending both on volatile content in the coal and oxygen content in the atmosphere. The influence of the particles diameter and the distance between them are also discussed. Based on these experiments, a mathematical and physical model of the process have been made. Due to numerical calculation, the radiation and conduction portion in overall balance of heat exchange between the particle and its surrounding in the course of ignition process for various kinds of cool and oxygen contents in the air have been determined. The presented calculation results show an accordance with data results of experiments carried out at zero gravity conditions.