Heterogeneous Ignition Research Articles

Numerical analysis of the ignition and gas-phase flame evolution of pulverized coal based on online experimental diagnostics

A transient ignition model employing a reduced chemical mechanism was developed to investigate the ignition characteristics and the gas-phase flame evolution of pulverized coal particles. The chemical percolation devolatilization (CPD) model was chosen to simulate the devolatilization process, and its accuracy was validated using a high-temperature entrained-flow reactor. Additionally, a novel method was introduced to cross-validate the single-particle simulation results with real-time OH-PLIF experimental measurements of particle streams, particularly at a large particle spacing ratio. The ignition mode was determined using the ignition delay time and volatile burnout time. Results show that as the oxygen volume fraction increases from 5% to 50% at a temperature of 1800 K, the ignition mode transitions from homogeneous ignition (GI) to heterogeneous ignition (HI). Notably, the same ignition mode was observed regardless of whether GI was defined using gas-phase temperature or OH levels. In the homo-heterogeneous ignition mode, the gas-phase flame intensity, characterized by OH levels, increases rapidly, then decreases, and re-increases slightly. The sequence of gas-phase reactions initiates with volatile combustion, followed by the co-combustion of residual volatiles and newly generated CO, and culminates in the combustion of CO itself. Online experimental findings confirmed that CO originates from char oxidation. Throughout this process, the gas-phase flame front extends outward until the volatiles are consumed.

Physical characteristics and combustion behavior of pellets from sawdust and refuse-derived fuel

Experimental research findings are reported on the ignition and combustion behavior, as well as mechanical properties of pellets from wood biomass (pine sawdust) and additives of refuse-derived fuel (cardboard, plastic and their mixture). The addition of RDF was found to increase the vibration durability and moisture resistance of pellets by 3.7–45.6% % compared to samples without additives. Cardboard and its mixture with plastic contributed to 2.7–5.8% greater density of pellets % compared to wood pellets. The impact resistance coefficient of pellets with waste decreased by 0.3– 2.3% compared to samples without additives. An increase in the proportion of cardboard and plastic led to longer delay times of the gas-phase and heterogeneous ignition. The ignition delay times decreased by a factor of 1.3–2.2 as a result of mixing cardboard with plastic compared to pellets with only cardboard or plastic. On the environmental side, the use of cardboard in pellets reduced the concentrations of carbon dioxide and carbon monoxide by 10–24 % and 5–46%, respectively, compared to samples without additives. By contrast, the addition of plastic waste increased CO2 and CO by 4–15% and 5–63%, respectively. The combustion of pellets with RDF was characterized by 22–78% lower concentrations of NOx. The results of a multi-criteria decision analysis revealed that the use of RDF in wood pellets increased the efficiency indicator by 1 to 12% compared to wood pellets.

Modeling of the heterogeneous hypergolic ignition with particle packing model

The ignition process of a hypergolic hybrid rocket fuel presents a significant challenge due to its complex and interconnected nature. However, understanding its underlying physics is crucial for accurately estimating the delay time and ignition performance. In this study, we employed an approximate analytical method to directly address the transient conduction problem with nonlinear surface heat generation. By using the Arrhenius equation and considering dual heat flux directions in both the liquid and solid domains, we propose a model for determining the delay time of heterogeneous hypergolic ignition. Our solution is straightforward and computationally efficient. Notably, when subjected to identical initial conditions and property settings, our results exhibit similarity with the solutions derived from other models. Moreover, we advance our approach by developing a particle packing model through the utilization of a well-established algorithm. By simulating the particle arrangement within the pellet, the surface layer properties and the thermal time constant of certain fuel designs can be modeled. This simulation serves the purpose of incorporating the particle size effect of the fuel pellet into our ignition model. The results generated by our heterogeneous hypergolic ignition model, in conjunction with the particle packing model, demonstrated impressive agreement with the experimental data derived from droplet tests. Novelty and significance statementThis study utilized a novel solving method to address a transient conduction problem with non-linear heat generation on the surface during a heterogeneous hypergolic ignition process. The study successfully derived an analytical solution, providing a simpler method for calculating ignition delay times. A scenario involving different initial temperatures was also examined. Additionally, the research integrated particle packing model data and thermal time constants to assess the effect of particle size. The model results were in agreement with the experimental data, confirming its accuracy and offering a reliable method for estimating the heterogeneous hypergolic ignition delay time.

Parametric study on reaction characteristics of methane/air mixture in microchannels

Micro-combustion is a promising technology for addressing global energy consumption challenges; however, its practical implementation is hindered by a limited understanding of the underlying reaction characteristics. To address this, a comprehensive numerical investigation was conducted using a well-established two-dimensional (2D) steady model to examine the influence of key operating parameters (e.g., inlet velocity, equivalence ratio, and inlet temperature) on catalytic methane/air combustion in micro-channels. The results show that the heterogeneous reaction exhibits limited sensitivity to changes in inlet velocity, whereas the homogeneous reaction shows substantial enhancement with increasing inlet temperature. With the increment of inlet velocity, the peak value of stage III- heterogeneous ignition (conversion of CO to CO2 and H2 to H2O) decreases and levels off once uin ≥ 0.50 m/s. Simultaneously, the minimum value of S (the ratio of heat released by the heterogeneous reaction to that of the total reaction) decreases as well. As the equivalence ratio is increased, the combustion zone shifts towards the upstream direction. The value of S initially increases and then decreases, reaching a maximum of 17.61 % at Φ = 0.95. Raising the inlet temperature results in the upstream movement of the flame, accompanied by reduced combustion heat release, lower combustion temperature, and diminished intensity of homogeneous combustion. The influence of the heterogeneous reaction on combustion is more pronounced under operating conditions with lower inlet velocities, an equivalence ratio approaching 0.95, and higher inlet temperature. Additionally, under the condition of higher inlet velocities, an equivalence ratio close to 1.0, and elevated inlet temperature, the heterogeneous reaction aids in igniting the homogeneous reaction more effectively.

Analysis of the possibility of solid-phase ignition of coal fuel

The article presents the results of theoretical and experimental studies of the ignition process of coal fuel particles in a pelletized state during high-temperature radiation-convective heating in an oxidizing environment. A new, different from the known, “two-temperature” mathematical model of the ignition process of a cylindrical coal pellet under intense heating conditions is presented. During the modeling, a detailed kinetic scheme of the reaction of gaseous products of thermal decomposition of coal with atmospheric oxygen was considered. The mathematical model was tested through a comparative analysis of the theoretical and experimental values of the ignition delay times of coal pellet fuel with experimental data. A wide range of heating conditions that could potentially lead to solid-phase ignition were analyzed. The research results showed that even under conditions of high oxygen content in the original coal, solid-phase ignition of coal pellets does not occur during intense heating. Based on the results of numerical modeling, it was established that the ignition process occurs in the gas phase. The oxygen released during the thermal decomposition of coal is not enough for stable gas-phase or heterogeneous ignition of pyrolysis products in the intrapore space of the fuel pellet. Based on the results of the numerical modeling, it was established that the oxygen released during the thermal decomposition of coal is significantly deficient in the intrapore space. The O2 concentration is not enough for a significant (in terms of heat flow) thermochemical intrapore reaction with gaseous and solid products of thermal decomposition.

Autoignition of two-phase n-heptane/air mixtures behind an oblique shock: Insights into spray oblique detonation initiation

Autoignition of n-heptane droplet/vapor/air mixtures behind an oblique shock wave are studied, through Eulerian-Lagrangian method and a skeletal chemical mechanism. The effects of gas/liquid equivalence ratio (ER), droplet diameter, flight altitude, and Mach number on the ignition transient and chemical timescales are investigated. The results show that the ratio of chemical excitation time to ignition delay time can be used to predict the oblique detonation wave (ODW) transition mode. When the ratio is relatively high, the combustion heat release is slow and smooth transition is more likely to occur. In heterogeneous ignition, there are direct interactions between the evaporating droplets and the induction/ignition process, and the chemical explosive propensity changes accordingly. The energy absorption of evaporating droplets significantly retards the ignition of n-heptane vapor. In the two-phase n-heptane mixture autoignition process, the ignition delay time decreases exponentially with flight Mach number and increases first and then decreases with the flight altitude. As the liquid ER increases, both ignition delay time and droplet evaporation time increase. With increased droplet diameter, the ignition delay time decreases, and the evaporation time increases. Besides, when Mach number is less than 10, the ratio of the chemical excitation time to ignition delay time generally increases with the flight altitude or Mach number. It increases when the liquid ER decreases or droplet diameter increases. When the Mach number is sufficiently high, it shows limited change with fuel and inflow conditions. The ODW is more likely to be initiated with a smooth transition at high altitude or Mach number. Abrupt transition mode tends to happen when fine fuel droplets are loaded. The results from this work can provide insights into spray ODW initiation.

Characterizing potassium release under turbulent conditions during biomass stream combustion: A comparative analysis of different pretreatments

The release rate of potassium during biomass combustion significantly influences slagging, fouling, and corrosion in industrial equipment. Given that dispersed biomass stream combustion predominantly occurs under turbulent ambient conditions, we investigate the combustion characteristics and potassium release behavior of rice husk and its derived fuels (water-washed, torrefied, and char) under high-turbulent conditions in a tubular flame burner (TFB). Tunable diode laser absorption spectroscopy (TDLAS) technology, combined with high-speed imaging, was employed to online monitor the release of atomic potassium (K(g)) and the ignition characteristics of biomass stream combustion. The results showed that the rice husk stream followed a joint hetero-homogeneous ignition mechanism, whereas the biochar stream underwent heterogeneous ignition in the TFB. The K(g) peak, occurring ahead of the maximum spontaneous radiation position, was observed at approximately 25 mm from the injectors, with a concentration of 0.067 ppmv during rice husk stream combustion. In contrast, the peak K(g) concentrations for biochar and torrefied rice husk were delayed, reaching 0.026 ppmv and 0.021 ppmv, respectively, compared to the raw biomass samples combusted in the TFB. Distinct differences in activation energy for K(g) release were identified, with pulverized rice husk exhibiting the lowest activation energy (75.53 kJ/mol) and rice husk-char showing the highest (94.20 kJ/mol).

The ignition modes transformation and critical parameters for single coal particle under O2/CO2 atmospheres

Oxy-coal combustion with the substitution of CO2 for N2 is a state-of-the-art technology for CO2 capture and storage. Pulverized coal particles show various ignition and combustion characteristics under O2/CO2 atmospheres due to various physicochemical properties between CO2 and N2. A modified transient model with an augmented reduced mechanism (ARM) was developed to predict the ignition delays and ignition modes of the coal particle. Ignition modes transformation and critical parameters for single coal particle under O2/CO2 atmospheres were determined by using the modified transient model and the ignition criteria based on different characteristic ignition time scales. The predicted ignition delays were compared with experimental values and the predicted results obtained from the sophisticated model in literature. The ignition processes of the coal particle under O2/CO2 and O2/N2 atmospheres were analyzed in detail. Then, the contributions of the physical properties of CO2 including thermal conductivity and thermal sink, and the chemical properties of CO2 including gas mixture reactivity and char gasification to the variation of homogeneous and heterogeneous ignition delays and the transformation of ignition modes were separated at 1200–1800 K and 5%–60% O2. Finally, the ignition modes map under O2/CO2 atmospheres was firstly depicted and the critical parameters for ignition mode transformation were compared with O2/N2 atmospheres. The results indicated the larger thermal sink of CO2 and the reduced gas mixture reactivity played the dominant roles of the prolonged homogeneous ignition delays under O2/CO2 atmospheres. The critical gas temperature for the appearance of homogeneous ignition increased by 71 K at 27% O2 whereas the critical O2 concentrations for the ignition mode transformation from HI-GI to HI-coal changed insignificantly with the substitution of CO2 for N2.

Combustion of Coal and Coal Slime in Steam-Air Environment and in Slurry Form

One of the ways to minimize anthropogenic emissions from coal combustion is to replace conventional schemes used for the introduction of coal dust into the furnaces of power plants through the injection of water-containing fuels. In this research, the three most promising schemes for fuel combustion were implemented: (i) the simultaneous introduction of coal particles and water droplets into the combustion chamber; (ii) steam injection into the fuel particle combustion zone; and (iii) the introduction of coal–water slurries into the furnace. Three methods of supplying water to the combustion zone were evaluated using the multi-criteria decision-making technique. Experimental research was conducted to record a range of process characteristics: the time of the gas-phase and heterogeneous ignition, the time of complete combustion, minimum ignition temperatures, maximum combustion temperatures, the completeness of the fuel burnout and the concentrations of the main gaseous emissions. It has been found that the most favorable scheme for coal particle combustion in water-steam environments is to produce fuel slurries. The cumulative indicator integrating the energy and environmental characteristics is 7–47% higher for slurries than for the other examined schemes for burning coal particles and slime.

The role of cool-flame fluctuations in high-pressure spray flames, studied using high-speed optical diagnostics and Large-Eddy Simulations

This work investigates the low- and high-temperature ignition and combustion processes, applied to the Engine Combustion Network Spray A flame, combining advanced optical diagnostics and large-eddy simulations (LES). Simultaneous high-speed (50 kHz) formaldehyde (CH2O) planar laser-induced fluorescence (PLIF) and line-of-sight OH* chemiluminescence imaging were used to measure the low- and high-temperature flame, during ignition as well as during quasi-steady combustion. While tracking the cool flame at the laser sheet plane, the present experimental setup allows detection of distinct ignition spots and dynamic fluctuations of the lift-off length over time, which overcomes limitations for flame tracking when using schlieren imaging [Sim et al.Proc. Combust. Inst. 38 (4) (2021) 5713–5721]. After significant development to improve LES prediction of the low-and high-temperature flame position, both during the ignition processes and quasi-steady combustion, the simulations were analyzed to gain understanding of the mixture variance and how this variance affects formation/consumption of CH2O. Analysis of the high-temperature ignition period shows that a key improvement in the LES is the ability to predict heterogeneous ignition sites, not only in the head of the jet, but in shear layers at the jet edge close to the position where flame lift-off eventually stabilizes. The LES analysis also shows concentrated pockets of CH2O, in the center of jet and at 20 mm downstream of the injector (in regions where the equivalence ratio is greater than 6), that are of similar length scale and frequency as the experiment (approximately 5–6 kHz). The periodic oscillation of CH2O match the frequency of pressure waves generated during auto-ignition and reflected within the constant-volume vessel throughout injection. The ability of LES to capture the periodic appearance and destruction of CH2O is particularly important because these structures travel downstream and become rich premixed flames that affect soot production.

Combustion dynamics of droplets of aqueous slurries based on coal slime and waste oil

This paper shows the analysis of burnout characteristics of droplets of the following slurry fuels: (i) based on coal slime with or without the addition of waste turbine oil; (ii) based on coal dust with or without the addition of waste turbine oil. The oil share in the mixtures varied from 0% to 15%. The solid component share varied in the range of 40–60%. Single droplets with an initial size of 1–5 mm were burned in a tubular muffle furnace at temperatures of 700–900 °C. The inertia of gas-phase and heterogeneous ignition and the combustion time of mixtures were estimated. Based on experimental data, the values of the linear and mass burnout rates of fuel droplets were calculated. At a furnace temperature of less than 800 °C, the characteristics of ignition and burnout of droplets of water-containing fuel mixtures and solid particles deteriorated significantly. The most preferred is the combustion of coal slime in the composition of aqueous slurry with the addition of high-reactive components (for example, waste oil of petroleum origin or vegetable oils). To intensify ignition and combustion, it is recommended to use no more than 50% of the solid component in the mixture. The maximum linear burnout rate was typical for droplets about 3 mm in size and reached values of 0.5–0.6 mm2/s. The mass burnout rates of droplets of the investigated fuels with a size of 5 mm varied in the range of 0.0005–0.0018 g/s.

The determination method of the ignition modes of single coal particle with a transient coal ignition and combustion model

A transient model for the ignition and combustion of a quiescent coal particle was developed by coupling the mass, energy and species governing equations between the particle phase and the gas phase. The ignition delay times of single coal particle were calculated by using the transient ignition and combustion model with Kobayashi-Sarofim devolatilization model and Chemical Percolation Devolatilization (CPD) model, respectively. The predicted results were compared with the experimental data to further validate better performance of CPD model. The new ignition criteria of single coal particle were proposed to determine the onset of the ignition and subsequently to define the ignition modes of single coal particle by comparing the magnitude of typical ignition time scales, including the start time and the end time of devolatilization, homogeneous ignition delay time and the heterogeneous ignition delay time based on the thermal explosion theory (TET). The ignition modes map of single coal particle under various ambient oxygen molar fractions (O2 = 0–100%) and gas temperatures (Tg = 800–2000 K) was firstly depicted by using the transient ignition and combustion model with the new ignition criteria. The ignition modes of single coal particle transferred from homogeneous gas-phase ignition (GI) to homo-heterogeneous ignition (GI-HI) when the ambient oxygen molar fraction was increased from 5% to 21% under Tg = 1340 K condition, and then they transformed into hetero-homogeneous ignition (HI-GI) and heterogeneous ignition of coal (HI-coal) when the ambient oxygen molar fraction was further increased to 40% and 70%, respectively. Moreover, the simulated results showed that the critical oxygen molar fraction XO2,cr for the transition of ignition mode from homogeneous to heterogeneous ignition was 15% for the particle size of 50 μm, XO2,cr = 20% for the particle size of 100 μm, and XO2,cr = 30% for the particle size of 200 μm, respectively. The critical parameters for the transition of ignition mode of single coal particle were quantitatively determined to give a deep understanding on coal particle ignition process and to provide the guidance for improving the combustion performance of coal particles.

Delay time of gases's catalytic heterogeneous ignition on various sizes's catalyst particles

In this work, catalytic ignition delay time of combustible gas's small impurities in air on a spherical metal particle of various diameters is analytically determined by the example of gas-air mixtures's flameless combustion with hydrogen impurities on a platinum particle. It is shown that stable flameless combustion is observed after an induction period for particles of a certain range. It has been established that catalytic ignition time of gases is divided into three stages: 1. inert heating, the duration of which still depends on the combustible gas concentration; 2. the stage of self-acceleration and catalyst temperature rise during the course of the catalytic reaction in the transition region; 3. stage of diffusion inhibition and reaching stable catalytic combustion. The characteristic relaxation time was used in a dimensionless form. To determine the duration of the second stage, a modified Frank-Kamenetsky approach is applied. The duration of diffusion inhibition stage in the dimensionless form is practically independent of catalyst particle's diameter, although the catalytic combustion temperature decreases with an increase in the catalyst diameter. Heat transfer by radiation, the role of which increases with the growth of the catalyst size, is included in the effective heat transfer coefficient, which allows maintaining the classical ideology to solving the problem of the induction period.

Experimental and theoretical study on the inhibition effect of CO2/N2 blends on the ignition behavior of carbonaceous dust clouds

Gaseous inhibitors are used in many industries for the explosion prevention of combustible dusts, mitigating the potential hazard to humans, properties and environments. This work experimentally and theoretically studied the inerting effect of gaseous inhibitors on the ignition process of dust clouds in O2/N2/CO2 atmospheres, with an emphasis on the role of the CO2/N2 ratio. 10 different combustible carbonaceous dusts were selected, including grain dust, biomass dust and coal dust. Experimental results showed that the inhibition effect of CO2/N2 is closely related to the ignition mechanism of dust clouds. Specifically, a higher ratio of CO2/N2 yields a stronger inhibition effect on the ignition process of dust samples with relatively low volatile matter contents predominated by heterogeneous ignition. In addition, two novel steady-state ignition mechanism models were developed to interpret the experimental observations. Maxwell-Stefan equations were used to describe the diffusivity in the ternary O2/N2/CO2 gas mixtures. The analytical results were in good agreement with the experimental data of the minimum ignition temperature of dust cloud (MITC) in oxygen-lean atmospheres. The mechanism modelling can be used to estimate the critical ignition temperature of all carbonaceous dust clouds with a wide range of volatile matter content under different inert atmospheres, which will provide a reference for the explosion hazard assessment of dust posed by a hot surface in the process industries.

High-speed contactless measurements of temperature evolution during ignition and combustion of coal-based fuel pellets

The temperature evolution of coal pellets, ignited and burned in an initially motionless high-temperature air medium, was analyzed. The fuel pellet surface temperature was recorded using contactless two-color pyrometry with a time increment of 2 ms by means of a high-speed color video camera and an original video frame processing algorithm developed in Wolfram Mathematica. The obtained results made it possible to explore the patterns and characteristics of coal ignition, considering the detected contribution that the gas-phase combustion of released volatiles makes to the pellet heating. The effect of the heating source temperature on the intensity of the fuel surface temperature evolution during the induction period was shown. When the fuel was heated to temperatures above 1073 K, local extrema were identified on the corresponding temperature trend. These characterize the emergence of hot spots – the sites of volatile ignition, preceding the heterogeneous ignition of the solid carbonaceous residue forming as a result of the thermal decomposition of the near-surface layer of the fuel pellet. The time intervals between consecutive local flashes of volatiles in their series from the first recorded occurrence to the last one, corresponding to the moment of the fuel's heterogeneous ignition, reduce five- to sixfold. The uniformity of temperature distribution over the surface of the fuel pellet at the moment of its heterogeneous ignition allows us to determine the moment when the gas-phase spot ignition of volatiles changes to the heterogeneous combustion of the solid carbonaceous residue at heating source temperatures over 1100 K.

Augmentation of Aluminum and Boron Ignition

Effect of K2CO3 and KCl additives on the process of high-temperature oxidation and ignition of conglomerates of powdered boron and aluminum particles and on the critical conditions for the ignition of particle–gas mixtures of these metals is studied experimentally. It is revealed that the mixtures of boron and aluminum particles covered (encapsulated) with salts ignite at temperatures lower than the ignition temperatures of mechanical mixtures of metals with these salts in appropriate proportions and significantly lower ignition temperatures of pure boron and aluminum. This indicates the active role of salts in the process of heterogeneous ignition. It is established that, for boron conglomerates, the accelerating effect of salt additives on the oxidation of conglomerates is noticeable at all investigated concentrations and compositions starting from 1% (by weight). For aluminum, an increase in the reaction rate is noticeable only for KCl additives at concentrations of 3% and above.

Numerical investigation of physicochemical effects of CO2 on coal particle ignition in O2/CO2 ambience

Coal combustion is a complex phenomenon involving multiscale chemical and physical processes. In this paper, the physical and chemical effects of CO2 on the ignition behaviors of dispersed coal particles are investigated based on a self-developed transient ignition model. Attention has been paid on the separate effects of CO2, namely the thermal conductivity, the species diffusion rate, the heat capacity, and the gasification reaction, on the ignition behaviors in oxy-fuel ambience. Differences of coal ignition delay time and ignition modes in air and oxy-fuel ambience are compared. The heterogeneous ignition (HI) is significantly delayed in CO2 ambience, with the most significant contribution from the lower thermal conductivity of CO2. The negative effects of slower species diffusion rates and the endothermic C-CO2 gasification reaction in CO2 are less important; however, they both become prominent with increasing ambient temperature up to 1800 K. By contrast, the gas phase homogeneous ignition (GI) is less sensitive to CO2 enrichment. The positive effects of slower species diffusion rates, together with the negligible effects of higher heat capacity and extra gasification reaction, impair the negative effect from lower thermal conductivity. In summary, the CO2 substitution makes the ignition mode move from the hetero-homogeneous joint regime towards the homogeneous dominant regime. Thus, coal particle ignition under high temperature (1500–1800 K) and low oxygen fraction (~0.1) transits to the homogeneous dominant case, yielding a reduced difference of ignition delay between air and oxy-fuel combustion. This feature has been observed for coal samples of different ranks from several independent macroscopic experiments, and can be well interpreted by the transition mechanism of ignition mode.

Ignition temperature and mechanism of carbonaceous dust clouds: The roles of volatile matter, CH4 addition, O2 mole fraction and diluent gas

Minimum ignition temperature of dust clouds (MITC) was studied experimentally and theoretically in different atmospheres. Three carbonaceous dusts were tested in both air and O2/CO2 atmospheres with CH4 mole fraction from 0% to 2%. Results showed that the ignition risk of the three dusts significantly increases (decrease of MITC by ~100 ℃) with increasing XO2 from 21% to 50%, but significantly decreases replacing N2 in air with CO2. The inhibition effect of CO2 on MITCs could be diminished by increasing XO2 or adding CH4. The addition of small amount of CH4 has different effects on the MITCs of different dust samples, following the opposite order of volatile matter content: anthracite>bituminous coal>starch. Two modified steady-state ignition models, considering the density of mixture gas and dust cloud, XO2 and its diffusivity, were developed to interpret the experimental observations. The analysis revealed that the global heterogeneous ignition model suits well for the hybrid mixtures of anthracite or bituminous coal dusts. In contrast, the proposed global homogeneous ignition model was found to be only valid for the pure starch dust, and the extra CH4 addition could strongly affect the ignition process of starch, particularly in O2/CO2 atmospheres with higher XO2.

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

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.