Coal Particle Streams Research Articles

Application of plasma burners for char combustion in a pulverized coal-fired (PC) boiler – Experimental and numerical analysis

This study considers the combustion of pulverized coal with unburned char recovered from ash of the industrial boiler. The concept of char combustion with coal is implemented by installing two plasma burners on the front wall of the combustion chamber. The main objective is to improve the boiler's efficiency by reducing the amount of unburned fuel. The behavior of the combustion process is investigated using CFD modeling of the combustion chamber with tangentially-fired burners and two plasma burners. Thermogravimetric analysis and laboratory-scale tests in a drop tube furnace are performed to obtain the combustion characteristics of the investigated char and, eventually, to assess the validity of co-firing with coal the unburned char. The CFD simulation of the non-retrofitted boiler with twelve pulverized-fired burners results in the excessive movement of coal particle streams toward the chamber walls outside of the designed swirl diameter caused by a high centrifugal force, thereby contributing to a decreased level of fuel burnout. The CFD results with plasma burners indicated that the swirl diameter is reduced improving the degree of mixing resulting in a more uniform temperature and velocity distributions. The average temperature at the combustion chamber outlet increases by approximately 40 °Creaching 1260 °C.

Investigation on ignition behaviors of pulverized coal particles in a tubular swirl burner

This paper intensively investigated the ignition of turbulent coal flames in a novel fully-mixed tubular swirl burner. The Nikon D300s digital camera was used to capture the statistical ignition behavior of dispersed coal particle streams in different ambiences. Meanwhile, the combustion dynamics of individual coal particles were also recorded by means of high-speed photography. Two low-rank coal samples, Hulunbel lignite and Zhundong coal, were tested in this study. The ignition delay times of coal particles in the swirl burner were compared with those in a flat-flame burner. In contrast to previous work on a laminar flat-flame burner, the current experimental results show that the turbulent ambience significantly enhances the ignition of all coal samples, which is exceptionally pronounced under high temperature and low oxygen conditions. In addition, the sensitivity analysis suggests that both the enhanced heat and mass transfer contribute to the early ignition in turbulence. The effect of elevated mass transfer coefficient turns prominent in low oxygen fraction ambience, wherein the volatile barrier effect is suppressed by the enhanced mixing process. The combined effect of turbulence favors the shifting of ignition modes to the heterogeneous-dominant region. Last but not least, the ceased volatile flame that visualized in turbulent low oxygen ambience further confirms the important role of heterogeneous ignition.

Study on the combustion behaviours of two high-volatile coal particle streams with high-speed OH-PLIF

An optical entrained flow reactor, based on a downward flat-flame, was established to explore the devolatilization and volatile combustion behaviours of coal particle streams with surrounding temperatures from 1600 K to 1800 K and oxygen mole fractions in the range of 10%–30%. The coal particle streams with particle sizes of 53–80 μm are two kinds of high-volatile pulverized bituminous coal and lignite. Using High-speed Planar Laser-Induced Fluorescence to diagnose OH (OH-PLIF), the planar sheet-imaging of OH-radicals was obtained to visualize the coal particle flames, and then, the normalized fluorescence signal intensity along the distance downstream from the burner and along the radial distance was obtained to investigate the volatiles combustion process. From the planar sheet-imaging, the ignition of tested high-volatile bituminous coal and lignite is dominated by the combustion of their volatiles. The combustion processes of volatiles can be divided into 4 stages: ignition, accelerating burning, stable burning and feeble burning, to investigate the intrinsic combustion correlation phenomena. The relative standard deviation (RSD) changes of the OH radial profiles can provide a new insight into flame stabilization, wherein the flame stability of the bituminous coal particle stream is higher than that of lignite under the same combustion conditions. The volatiles combustion of lignite is more sensitive to the oxygen concentration and temperature than that of bituminous coal, and the volatile releasing ratio of bituminous coal (65%–81%) was substantially less than that of lignite (92%–100%) by the analysis of the char residues.

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.

Characterization on Ignition and Volatile Combustion of Dispersed Coal Particle Streams: In Situ Diagnostics and Transient Modeling

In this paper, applying OH-PLIF diagnostics, we explored the ignition and volatile combustion of dispersed coal particle streams in a Hencken flat-flame burner. The effects of ambient temperature and oxygen mole fraction on coal combustion behaviors have been systematically examined. First, different coal combustion stages and ignition modes (i.e., heterogeneous ignition, homogeneous ignition, and hetero-homogeneous joint ignition) in different ambiences are characterized by the OH profiles within the coal flame domain, with ignition delay time and volatile combustion time being quantitatively determined. Then, an improved transient model, integrating the gas-phase flame-sheet reaction model and the chemical percolation devolatilization (CPD) model, is established to reveal the underlying physicochemical mechanisms during the early stage of coal combustion. This transient model well captures the effects of ambiences on both the characteristic ignition delay time and volatile combustion time. Finally, base...

Investigation of steam effect on ignition of dispersed coal particles in O2/N2 and O2/CO2 ambiences

In this paper, we intensively investigate the influence of steam on the hetero-homogeneous joint ignition of dispersed coal particle streams in both O2/N2 and O2/CO2 conditions of an optically accessible flat-flame burner. Two differently ranked coals and one bituminous char were tested for a comparison. The effects of moisture content in the raw coal, steam content or diluents (N2/CO2) in the ambience, and fuel types were systematically examined under isothermal conditions. First, it is seen that the moisture content in raw coal delays its ignition because of endothermic vaporization, which becomes weakened at the high ambient temperature. Secondly, three main mechanisms are categorized to explore the profound effects of ambient steam content on ignition of coal particles, i.e., (i) the promotion effect of OH radical on the oxidation of volatile hydrocarbons, (ii) the inhibition effect of gaseous H2O molecule on the equilibrium of hydrocarbon oxidation, (iii) the negative effect of endothermic surface gasification reaction (C-CO2/C-H2O). In O2/N2 ambience (1500 or 1800 K), as the steam content increases, the ignition delay of coal particles is rapidly reduced in an early stage of 0%–5% due to the positive effect of OH radicals, followed by an inappreciable reduction in 5%–13% stage because of both the steam inhibition effect and the endothermic surface gasification. However, in O2/CO2 ambience with sufficiently high steam content (5%–18%), the ignition delay even becomes longer with the further increment of steam. It is because of the synergistic effect between C-H2O and C-CO2 reaction at high temperatures (1500 K and 1800 K). Finally, for fuel types, the OH radical promotion effect of two lignite samples is obviously higher than that of low-volatile char samples, whereas with steam addition the sodium-rich Zhundong coal exhibits lower ignition delay than Hulunbel lignite due to catalytic effect.

Experimental and theoretical analyses on ignition and surface temperature of dispersed coal particles in O2/N2 and O2/CO2 ambients

An experimental and theoretical investigation is conducted on the combustion of pulverized coal particle streams in either conventional or oxy-fuel conditions. The laboratory setup consisted of a Hencken flat-flame burner operated at the same temperatures of 1200, 1500 and 1800K in N2 and CO2 environments. The visible light detection technique and the calibrated three-color pyrometry are separately used to characterize the ignition delay time and the luminous char surface temperature. First, experimental results indicate that the ignition delay time is relatively longer in CO2 environment in heterogeneous mode or even heterogeneous-controlled joint mode. The ignition delay is obviously enhanced in O2/CO2 ambient when the volatile-flame controlled joint mode is prevalent. The model predictions of particle temperature history further reveal that the extra gas flame radiation of CO2 and the declined difference between the thermal conductivity are two main reasons. Then, during char combustion, the measurement shows that the char surface temperature is lower in an O2/CO2 environment than in an O2/N2 environment. It is noted that the temperature difference between N2 and CO2 environments enlarges with the increasing ambient temperature. In contrast to conventional O2/N2 conditions, the lower O2 diffusivity in CO2, the endothermic carbon reaction with CO2 and the higher heat capacity of CO2, which are all temperature dependent, synergistically contribute to the enlargement of char surface temperature differences at high ambient temperature.

Investigation of mechanisms in plasma-assisted ignition of dispersed coal particle streams

The plasma-assisted ignition enhancement of pulverized lignite particles is intensively studied in a laminar, upward Hencken flat-flame burner. By applying a pair of pin-to-pin electrodes with a high-voltage DC power supply, the apparent reductions of ignition delay time, about 0.8–3.5ms, are observed for all three ambient temperatures, 1200K, 1500Kand 1800K, across which the ignition mode transits from heterogeneous ignition (HI) to hetero-homogeneous combined one. Concentrated on 1200Kambience where HI mode prevails, the plasmas-induced reduction of ignition delay time as a function of the applied voltage (0–20kV) is explored, in which the chemical effect is found to play a key role in cases with voltage less than 3kV. Then, under the conditions of different oxygen mole fractions, the discharge mechanisms and the reductions of ignition delay time are further examined to distinguish the chemical and thermal effects on the ignition enhancement. Finally, the gas temperature rise, both measured and predicted at a point located 6mmabove the electrodes, verifies the thermal effect. This paper provides a preliminary understanding of plasma-assisted ignition mechanisms on dispersed coal particles that may be applied in practical combustion system.

Characterization on hetero-homogeneous ignition of pulverized coal particle streams using CH∗ chemiluminescence and 3 color pyrometry

Two in situ optic methods on CH∗ chemiluminescence and coal particle temperature, are newly applied to investigate the devolatilization/homogeneous and heterogeneous ignition behaviors of dispersed coal particle streams, with ambient temperature from 1200K to 1800K and oxygen mole fractions from 10% to 30%. The dispersed coal particles of 65–74μm are injected into an optical Hencken flat-flame burner by a novel de-agglomeration feeder. First, the consecutive images through multiple filters near CH∗ emission band, centered at 420 430 and 440nm, are exquisitely processed to subtract the interferences from continuum blackbody radiation of particles and soots in the CH∗ band. Then, the signal of CH∗ chemiluminescence is capable of providing a good indicator of the coal devolatilization process. Secondly, 3 color pyrometry is adopted to detect particle surface temperature and determine the heterogeneous ignition delay time. The prevalent devolatilization/homogeneous-ignition are observed at high temperatures (1500 and 1800K) with apparent CH∗ signals, whereas CH∗ emission can rarely be detected in the initial stage of 1200K case that implies a dominated heterogeneous ignition mode. The effect of oxygen concentration on devolatilization is further discussed. The particles temperatures history after injected into Hencken burner are represented by 3 color pyrometry based on a two stage principle, and thus the role of oxygen in determining particle temperature and heterogeneous ignition is finally examined.

The transition of heterogeneous–homogeneous ignitions of dispersed coal particle streams

Abstract This work is assessing a study of the collective ignition behaviors of dispersed coal particle streams, with ambience temperature from 1200 K to 1800 K and oxygen mole fractions in the range of 10–30%. The dispersed coal particles of 65–74 μm are injected into an optical Hencken flat-flame burner by a novel de-agglomeration feeder. Three kinds of pulverized coals from different ranks, Hulunbel lignite, high-ash-fusion bituminous and low-ash-fusion bituminous, are considered. The normalized visible light signal intensity, deleting the background noise, is established to characterize the ignition delay of coal particle streams. Firstly, the prevalent transition from heterogeneous ignition to hetero–homogeneous ignition due to ambience temperature is observed. The pure homogeneous ignition rarely occurs, with an exception under high temperature and low oxygen for high-volatile coal. By comparing time scales between pyrolysis and heating processes, the competition of the volatile evolution and heterogeneous surface reaction are discussed. Then, the effects of ambience temperature, oxygen mole fraction and coal rank on the characteristic ignition delay are examined. Finally, the transient mode is developed, which not only well interprets the observed ignition transition phenomena, but also approximately predicts a variation of heterogeneous ignition time as a function of oxygen fraction.

Ignition and combustion of coal particle streams

This paper presents a model for group combustion of a cylindrical stream of coal particles and preliminary experimental data. Results are obtained for transient ignition and combustion behaviour. It is found that homogeneous ignition (ignition of volatiles) occurs for a dense cloud, while heterogeneous ignition (ignition of carbon) occurs for a dilute cloud. Once ignited, the flame moves towards the cloud, reaches the cloud surface and then bifurcates into two flames, called the inner and outer flames. The inner flame propagates into an unburnt volatile-air mixture in the cloud, while the outer diffusion flame moves away from the cloud. The inner flame propagates at the laminar burning velocity and rapidly heats the particles. There is only a diffusion flame outside the cloud after all the oxygen in the cloud is consumed. The flame moves toward the cloud as the volatiles are depleted. The cloud, with the volatiles exhausted and starved of oxygen, burns slowly, controlled by diffusion into the cloud rather than to each particle in the cloud. Qualitative comparisons are given with preliminary experimental data obtained using a digital imaging system.

Ignition and combustion of coal particle streams

Ignition and combustion of coal particle streams

Pyrolysis of a lignite in an entrained flow reactor: 2. Effect of metal cations on decarboxylation and tar yield

The role of exchangeable cations in the release of carboxylic functional groups and tars from a Montana lignite has been studied. Pyrolysis was carried out in an entrained flow reactor using a dilute stream of coal particles, and high heating and cooling rates. Direct determination of the carboxylic functional groups on the coal and char samples by ion exchange with barium acetate was employed to study both the amount and kinetics of decarboxylation. These results indicate that the presence of metal cations decreases the extent of decarboxylation at temperatures of 973, 1073 and 1173 K when compared with the behaviour of an acid washed lignite. It was also possible to determine first order activation energy values using this direct measure of decarboxylation. The activation energies determined for decarboxylation were 100 and 97 kJ mol −1 for the raw and acid washed samples, respectively. Furthermore, it is reported that the presence of metal cations on the coal resulted in a decrease of tar release of 70–94%. Analysis of the tars by Fourier transform infrared spectroscopy indicated that the tars released during pyrolysis were more aliphatic when metal cations were present.

Flash pyrolysis of coals: influence of cations on the devolatilization behaviour of brown coals

The influence of cations on the pyrolysis behaviour of brown coals under flash heating conditions was investigated by means of a small fluidized-bed pyrolyser. A stream of coal particles in nitrogen was injected at rates of 1–3 g coal/h directly into a heated bed of sand fluidized by nitrogen. Yields of tar, C1–C3 hydrocarbons and total volatile matter from four Gelliondale brown coals and a Montana lignite were determined as a function of pyrolysis temperature. With all coals the maximum tar yield was obtained at 600 °C. Removal of cations present in the coals markedly increased the yields of tar and total volatile matter, with little effect on the yields of hydrocarbon gases. The converse was also observed in that the addition of Ca2+ to a cation-free coal decreased the yields of tar and total volatile matter. The extent of the reduction in tar yield at 600 °C in the presence of cations was found to be similar for all coals. After acid washing, tar yields appear to correlate with the atomic HC ratios of the coals in a manner similar to that observed previously with bituminous coals.

The measurement of oxidation of coal by static electrification

The electrostatic charge acquired by a stream of fine coal particles after making contact with a metal surface has been found to be related to the degree of oxidation of the coal. The effect forms the basis of an instrument for quality control in an oxidation plant.