Single particle ignition and combustion of pulverized pine wood,wheat straw,rice husk and grape pomace

This work examines the combustion behavior of single pulverized biomass particles from ignition to early stages of char oxidation. The biomass residues investigated were pine wood, wheat straw, rice husk and grape pomace. The biomass particles, in the size range 224–250 µm, were injected upward into a confined region with hot combustion products, produced by a flat flame McKenna burner, with a mean temperature of 1610 K and a mean O₂ concentration of 6.5 vol%. Temporally and spectrally resolved images of the single burning particles were recorded with an intensified charge-coupled device camera equipped with different band-pass spectral filters. Data are reported for CH*, C₂*, Na* and K* chemiluminescence, and thermal radiation from soot and char burning particles. The data on CH* and C₂* chemiluminescence and soot thermal radiation permits to identify important differences between the ignition delay time, volatiles combustion time and soot formation propensity of the four biomass residues, which are mainly affected by their volatile matter content. The Na* and K* emission signals follow the same trends of the CH* and C₂* emission signals until the end of the volatiles combustion stage, beyond which, unlike the CH* and C₂* emission signals, they persist owing to their release from the char burning particles. Moreover, during the volatiles combustion stage, the Na*/CH* and K*/CH* ratios present constant values for each biomass residue. The CH* and thermal radiation emission data suggest that all biomass char particles experienced heterogeneous oxidation at or immediately after the extinction of the homogeneous volatiles combustion.     

Insight into KOH and KCl release behavior of burning wood and straw pellets using quantitative in situ optical measurements

Two-dimensional laser-induced photofragmentation fluorescence (LIPF) was employed to quantitatively visualize the potassium hydroxide (KOH) and potassium chloride (KCl) vapor in the plume above burning wood and straw pellets. In the LIPF measurement, two excitation lasers at 266 and 193 nm were adopted to discriminate KOH and KCl. Meanwhile, tunable diode laser absorption spectroscopy (TDLAS), laser-induced breakdown spectroscopy (LIBS) and two-color pyrometry were used to measure the atomic potassium concentration, total elemental potassium concentration and surface temperature of the burning pellets, respectively. The combustion environment had a temperature of 1550 K and an oxygen concentration of 4.6 vol.%. Two peaks were observed from the temporal potassium release profile of the burning wood, corresponding to the devolatilization and char oxidation stage, while only a single release peak was observed from the burning straw attributed to its high ash content. During the char oxidation and ash cooking stages, KOH was observed to be the dominant potassium species released from the wood, while only KCl was observed for the straw which had a high content of chlorine. About 45% of the total potassium in the wood samples and about 10% in the straw samples were measured to be released during the combustion process. The high content of silicon in the straw retained a considerable amount of potassium in the ash. The wood had the potassium release mainly in the char oxidation stage (∼53% of the total release), while the straw had the main release during the ash cooking stage (∼49% of the total release). During the char oxidation and ash cooking stages, about 32% of Cl was released from the straw pellets in KCl, while the other part of Cl was considered to be released during the devolatilization stage in other Cl species form, such as HCl.     

The effect of bulk gas diffusivity on apparent pulverized coal char combustion kinetics

Apparent char kinetic rates are commonly used to predict pulverized coal char burning rates. These kinetic rates quantify the char burning rate based on the temperature of the particle and the oxygen concentration at the external particle surface, inherently neglecting the impact of variations in the internal diffusion rate and penetration of oxygen. To investigate the impact of bulk gas diffusivity on these phenomena during Zone II burning conditions, experimental measurements were performed of char particle combustion temperature and burnout for a subbituminous coal burning in an optical entrained flow reactor with helium and nitrogen diluents. The combination of much higher thermal conductivity and mass diffusivity in the helium environments resulted in cooler char combustion temperatures than in equivalent N₂ environments. Measured char burnout was similar in the two environments for a given bulk oxygen concentration but was approximately 60% higher in helium environments for a given char combustion temperature. To augment the experimental measurements, detailed particle simulations of the experimental conditions were conducted with the SKIPPY code. These simulations also showed a 60% higher burning rate in the helium environments for a given char particle combustion temperature. To differentiate the effect of enhanced diffusion through the external boundary layer from the effect of enhanced diffusion through the particle, additional SKIPPY simulations were conducted under selected conditions in N₂ and He environments for which the temperature and concentrations of reactants (oxygen and steam) were identical on the external char surface. Under these conditions, which yield matching apparent char burning rates, the computed char burning rate for He was 50% larger, demonstrating the potential for significant errors with the apparent kinetics approach. However, for specific application to oxy-fuel combustion in CO₂ environments, these results suggest the error to be as low as 3% when applying apparent char burning rates from nitrogen environments.     

Effect of gasification reactions on biomass char conversion under pulverized fuel combustion conditions

The effect of gasification reactions on biomass char conversion under pulverized fuel combustion conditions was studied by single particle experiments and modelling. Experiments of pine and beech wood char conversion were carried out in a single particle combustor under conditions of 1473-1723 K, 0.0-10.5% O₂, and 25-42% H₂O. A comprehensive progressive char conversion model, including heterogeneous reactions (char oxidation and char gasification with CO₂ and H₂O), homogeneous reactions (CO oxidation, water-gas shift reaction, and H₂ oxidation) in the particle boundary layer, particle shrinkage, and external and internal heat and mass transfer, was developed. The modelling results are in good agreement with both experimental char conversion time and particle size evolution in the presence of oxygen, while larger deviations are found for the gasification experiments. The modelling results show that the char oxidation is limited by mass transfer, while the char gasification is controlled by both mass transfer and gasification kinetics at the investigated conditions. A sensitivity analysis shows that the CO oxidation in the boundary layer and the gasification kinetics influence significantly the char conversion time, while the water-gas shift reaction and H₂ oxidation have only a small effect. Analysis of the sensitive parameters on the char conversion process under a typical pulverized biomass combustion condition (4% O₂, 13% CO₂, 13% H₂O), shows that the char gasification reactions contribute significantly to char conversion, especially for millimeter-sized biomass char particles at high temperatures.     

Characterizing char particle fragmentation during pulverized coal combustion

A particle population balance model was developed to predict the oxidation characteristics of an ensemble of char particles exposed to an environment in which their overall burning rates are controlled by the combined effects of oxygen diffusion through particle pores and chemical reactions (the zone II burning regime). The model allows for changes in particle size due to burning at the external surface, changes in particle apparent density due to internal burning at pore walls, and changes in the sizes and apparent densities of particles due to percolation type fragmentation. In percolation type fragmentation, fragments of all sizes less than that of the fragmenting particle are produced. The model follows the conversion of particles burning in a gaseous environment of specified temperature and oxygen content. The extent of conversion and particle size, apparent density, and temperature distributions are predicted in time.Experiments were performed in an entrained flow reactor to obtain the size and apparent density data needed to adjust model parameters. Pulverized Wyodak coal particles were injected into the reactor and char samples were extracted at selected residence times. The particle size distributions and apparent densities were measured for each sample extracted. The intrinsic chemical reactivity of the char to oxygen was also measured in experiments performed in a thermogravimetric analyzer. Data were used to adjust rate coefficients in a six-step reaction mechanism used to describe the oxidation process.Calculations made allowing for fragmentation with variations in the apparent densities of fragments yield the type of size, apparent density, and temperature distributions observed experimentally. These distributions broaden with increased char conversion in a manner that can only be predicted when fragmentation is accounted for with variations in fragment apparent density as well as size. The model also yields the type of ash size distributions observed experimentally.     

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.     

Experimental and numerical investigations on the interactions of volatile flame and char combustion of a coal particle

The model that takes chemical reactions, heat and mass transfers in the boundary layer of the particle into account simultaneously, is developed for simulating the combustion of a pulverized coal particle. The FTIR in situ temperature-measurements and the comparison between numerical simulations for the pulverized coal and the devolatilized char show that the volatile flame induces the combustion of the primary product of surface oxidation CO. Due to the influence of volatile flame, the char particle can be ignited at temperature lower than its heterogeneous ignition temperature, which elucidates the physical essence of joint hetero-homogeneous ignition mode discovered by Jüntgen.     

煤低温氧化的微区组分分析与反应性研究

煤的低温氧化对着火性能、反应性、煤焦质量以及煤的自燃有重要影响．本文通过TGA-DSC、FSEM和EBSP研究了烟煤和无烟煤的低温氧化特性,研究表明:活性较高的煤表面固相氧的浓度增量比热重低温段的表观增重量大得多;根据最大吸氧量确定的着火温度随煤变质程度的加深和升温速率的增加而增加;升温速率小于1／12 K／s时,最大氧化速率和最大吸氧量能代表煤的氧化活性;含灰量低的中等变质程度的煤氧化活性最高;化学吸氧经历了从临界活化能、到活化能减小再增加到极大最后吸附终止的过程．     

Investigating particle and volatile evolution during pulverized coal combustion using high-speed digital in-line holography

Devolatilization is an important process in pulverized coal combustion because it affects the ignition, volatile combustion, and subsequent char burning and ash formation. In this study, high-speed digital in-line holography is employed to visualize and quantify the particle and volatile evolution during pulverized coal combustion. China Shanxi bituminous coal particles sieved in the range of 105–154 µm are entrained into a flat flame burner through a central tube for the study. Time-resolved observations show the volatile ejection, accumulation, and detachment in the early stage of coal combustion. Three-dimensional imaging and automatic particle extraction algorithm allow for the size and velocity statistics of the particle and stringy volatile tail. The results demonstrate the smaller particle generation and coal particle swelling in the devolatilization. It is found that the coal particles and volatiles accelerate due to the thermal buoyancy and the volatiles move faster than the coal particles. On average, smaller particles move faster than the larger ones while some can move much slower possibly because of the fragmentation.     

Quantifying the effect of CO2 gasification on pulverized coal char oxy-fuel combustion

Previous research has provided strong evidence that CO₂ and H₂O gasification reactions can provide non-negligible contributions to the consumption rates of pulverized coal (pc) char during combustion, particularly in oxy-fuel environments. Fully quantifying the contribution of these gasification reactions has proven to be difficult, due to the dearth of knowledge of gasification rates at the elevated particle temperatures associated with typical pc char combustion processes, as well as the complex interaction of oxidation and gasification reactions. Gasification reactions tend to become more important at higher char particle temperatures (because of their high activation energy) and they tend to reduce pc oxidation due to their endothermicity (i.e. cooling effect). The work reported here attempts to quantify the influence of the gasification reaction of CO₂ in a rigorous manner by combining experimental measurements of the particle temperatures and consumption rates of size-classified pc char particles in tailored oxy-fuel environments with simulations from a detailed reacting porous particle model. The results demonstrate that a specific gasification reaction rate relative to the oxidation rate (within an accuracy of approximately +/- 20% of the pre-exponential value), is consistent with the experimentally measured char particle temperatures and burnout rates in oxy-fuel combustion environments. Conversely, the results also show, in agreement with past calculations, that it is extremely difficult to construct a set of kinetics that does not substantially overpredict particle temperature increase in strongly oxygen-enriched N₂ environments. This latter result is believed to result from deficiencies in standard oxidation mechanisms that fail to account for falloff in char oxidation rates at high temperatures.     

The effects of oxygen concentration and gas temperature on coal stream ignition and particle surface temperature in reducing-to-oxidizing environments

In the near-burner region of pulverized coal burners, two zones exist, with very different oxygen concentrations. The first zone is a locally reducing environment, caused by the fast release of volatiles from a region of dense coal particles, and the second zone, which is surrounding the first zone, is a hot oxidizing environment. The transition of coal particles from the reducing zone to the oxidizing zone affects early stage coal combustion characteristics, such as devolatilization, ignition and particle temperature history. In this work, we used a two-stage Hencken flat-flame burner to simulate the conditions that coal particles experience in practical combustors when they transition from a reducing environment to an oxidizing environments. The composition of the reducing environment was chosen to approximate that of a typical coal volatile. Three oxygen concentrations (5, 10 and 15 vol%) in the “ambient” oxidizing environment were tested, corresponding to those at different distances downstream from a commercial burner. The corresponding gas temperatures for the oxidizing environments were adjusted for the different oxygen concentrations such that the “volatile” flame temperatures were the same, as this is what would be expected in a commercial combustor. High speed videography was used to obtain the ignition characteristics, and RGB color pyrometry was used to measure particle surface temperatures. Two different sizes of coal particles were used. It is found that when particles undergo a reducing-to-oxidizing transition at high temperatures, the particles are preheated such that the critical factor for ignition delay is point at which the particle is in the presence of oxygen, not the concentration of oxygen. The ignition delay of large particles is found to be 53% longer than that of small particles due to their higher thermal mass and slower devolatilization. The oxygen concentration in the ambient have a negligible effect on early-stage particle temperatures.     

Ignition and combustion of a dense powder jet of micron-sized aluminum particles in hot gas

Aiming at the potential implementation of aluminum as a primary fuel in powder-fueled ramjets or engines, this work seeks to investigate the ignition and combustion characteristics of a dense gas-suspended jet of micron-sized aluminum particles in a hot flow with controlled temperature and compositions. Aluminum particles with a mean diameter of 40 µm are aerosolized using a custom-made feeder and carried into the burner by a nitrogen stream. The powder jet with a particle density of up to 1–3 kg/m³ can be ignited and burned violently at a surrounding gas temperature as low as 1500 K. The lowered ignition temperature of the powder jet can be attributed to a cooperative mechanism resulting in fast reactions. Meanwhile, the ignition delay time decreases from ∼25 to ∼5 ms when the surrounding temperature increases from 1500 to 2200 K. The burning powder jet generates strong luminance and AlO emission signals detected by a spectrometer. Particle image velocimetry (PIV) and camera pyrometry are used to derive the two-dimensional velocity and average projected temperature distribution, respectively. Furthermore, a high-speed camera with a microscopic lens captures the transition from dispersed combustion to group combustion that forms a large-scale flame column wrapping the entire powder jet. The aluminum oxide produced in the columnar flame forms a large number of nanosized smoke particles in the condensation region. Finally, a numerical model considering the collective effect of the powder jet is developed to predict the particle temperature history during the ignition stage, which shows good agreement with the temperature profiles derived from camera pyrometry and PIV techniques.     

基于简单碰撞理论煤粉燃烧动力学模型的研究-PART Ⅲ:氧气可达比表面积

根据不同温度下氧分子平均自由程的大小,比较了小孔、中孔和大孔中三种扩散速率与煤焦表面燃烧速度的大小.研究表明2000 K以内,颗粒表面分子扩散速率比氧化反应速率大1个数量级以上,过度扩散速率不小于氧化速率.温度小于1200K时,燃烧速率比Knudsen扩散速率小1～5个数量级,扩散孔径小于15～28 nm,反应主要在内外表面进行;1200～1600K时,燃烧速率与Knudsen扩散速率相当,扩散临界孔径28～38 nm,反应在外表面及浅层内表面进行;温度1600K以上时,Knudsen扩散速率比燃烧速率小1个数量级,孔径38～50 nm以下内表面上碳的氧化速度受扩散控制.煤焦的氧化主要发生在Knudsen扩散临界孔径10～50 nm以上的氧气可达表面上.     

Pressurized oxy-fuel combustion of a char particle in the fluidized bed combustor

Pressurized oxy-fuel combustion of coal in fluidized bed (FB) holds the potential to realize low-cost CO₂ capture. However, the fundamental study in this manner is still rare due to the difficult access to the pressurized oxy-FB combustion tests. In this work, the experimental study of single char combustion was firstly conducted in a visualized pressurized FB combustor under various operating conditions. Then an experimentally verified particle-scale char combustion model was developed to reveal the dependence of char combustion on parameters. Results showed that the char conversion was accelerated with the increase of pressure, mainly due to the high oxygen diffusion and char gasification. The gasification played a non-negligible role in pressurized oxy-fuel combustion, especially under high oxygen concentration and bed temperature. Increasing oxygen concentration and bed temperature not only promotes the char oxidation rate and particle temperature, but also increases the gasification rate and the share of char conversion via gasification, resulting in shortening the burnout time of char. In addition, a higher fluidization number lowered both the burnout time and peak temperature of char particle, due to the simultaneous improvement of mass and heat transfer. The influences of char size and fluidization number on char gasification conversion ratio are very weak. In addition, the quantitative analysis of the influence of different operating parameters on the combustion process was obtained by model sensitivity analysis.     

A multi-step reaction model for ignition of fully-dense Al-CuO nanocomposite powders

A multi-step reaction model is developed to describe heterogeneous processes occurring upon heating of an Al-CuO nanocomposite material prepared by arrested reactive milling. The reaction model couples a previously derived Cabrera-Mott oxidation mechanism describing initial, low temperature processes and an aluminium oxidation model including formation of different alumina polymorphs at increased film thicknesses and higher temperatures. The reaction model is tuned using traces measured by differential scanning calorimetry. Ignition is studied for thin powder layers and individual particles using respectively the heated filament (heating rates of 10³–10⁴ K s^?1) and laser ignition (heating rate ～10⁶ K s^?1) experiments. The developed heterogeneous reaction model predicts a sharp temperature increase, which can be associated with ignition when the laser power approaches the experimental ignition threshold. In experiments, particles ignited by the laser beam are observed to explode, indicating a substantial gas release accompanying ignition. For the heated filament experiments, the model predicts exothermic reactions at the temperatures, at which ignition is observed experimentally; however, strong thermal contact between the metal filament and powder prevents the model from predicting the thermal runaway. It is suggested that oxygen gas release from decomposing CuO, as observed from particles exploding upon ignition in the laser beam, disrupts the thermal contact of the powder and filament; this phenomenon must be included in the filament ignition model to enable prediction of the temperature runaway.     

Evaluation of char combustion models: measurement and analysis of variability in char particle size and density

Char samples representing a range of combustion conditions and extents of burnout were obtained from a well-characterized laminar flow combustion experiment. Individual particles from the parent coal and char samples were characterized to determine distributions in particle volume, mass, and density at different extent of burnout. The data were then compared with predictions from a comprehensive char combustion model referred to as the char burnout kinetics model (CBK). The data clearly reflect the particle-to-particle heterogeneity of the parent coal and show a significant broadening in the size and density distributions of the chars resulting from both devolatilization and combustion. Data for chars prepared in a lower oxygen content environment (6% oxygen by vol.) are consistent with zone II type combustion behavior where most of the combustion is occurring near the particle surface. At higher oxygen contents (12% by vol.), the data show indications of more burning occurring in the particle interior. The CBK model does a good job of predicting the general nature of the development of size and density distributions during burning but the input distribution of particle size and density is critical to obtaining good predictions. A significant reduction in particle size was observed to occur as a result of devolatilization. For comprehensive combustion models to provide accurate predictions, this size reduction phenomenon needs to be included in devolatilization models so that representative char distributions are carried through the calculations.     

Ignition and volatile combustion behaviors of a single lignite particle in a fluidized bed under O2/H2O condition

O₂/H₂O combustion, as a new evolution of oxy-fuel combustion, has gradually gained more attention recently for carbon capture in a coal-fired power plant. The physical and chemical properties of steam e.g. reactivity, thermal capacity, diffusivity, can affect the coal combustion process. In this work, the ignition and volatile combustion characteristics of a single lignite particle were first investigated in a fluidized bed combustor under O₂/H₂O atmosphere. The flame and particle temperatures were measured by a calibrated two-color pyrometry and pre-buried thermocouple, respectively. Results indicated that the volatile flame became smaller and brighter as the oxygen concentration increased. The ignition delay time of particle in dense phase was shorter than that in dilute phase due to its higher heat transfer coefficient. Also, the volatile flame was completely separated from particles (defined as off-flame) in dense phase while the flame lay on the particle surface (defined as on-flame) in dilute phase. The self-heating of fuel particles by on-flame in dilute phase was more obvious than that in dense phase, leading to earlier char combustion. At low oxygen concentration, the flame in the H₂O atmosphere was darker than that in the N₂ atmosphere because the heat capacity of H₂O is higher than that of N₂. With the increase of oxygen concentration, the flame temperature in the O₂/H₂O atmosphere was dramatically enhanced rather than that in the O₂/N₂ atmosphere, where the diffusion rate of oxygen in O₂/N₂ atmosphere became the dominant factor.     

Modes of particle combustion in iron dust flames

The so-called argon/helium test is proposed to identify the combustion mode of particles in iron dust flames. Iron powders of different particle sizes varying from 3 to 34 μm were dispersed in simulated air compositions where nitrogen was replaced by argon and helium. Due to the independence of the particle burning rate on the oxygen diffusivity in the kinetic mode, the ratio between the flame speeds in helium and argon mixtures is expected to be smaller if the particle burning rate is controlled by reaction kinetics rather than oxygen diffusion. Experiments were performed in a reduced-gravity environment on a parabolic flight aircraft to prevent particle settling and buoyancy-driven disruption of the flame. Uniform suspensions of the iron powders were produced inside glass tubes and a flame was initiated at the open end of the tube. Quenching plate assemblies of various channel widths were installed inside the tube and pass or quench events were used to measure the quenching distance. Flame propagation was recorded by a high-speed digital camera and spectral measurements were used to determine the temperature of the condensed emitters in the flame. The measured flame speeds and quenching distances were in good agreement with previously developed one-dimensional, dust flame model where the particles are assumed to burn in a diffusive mode and heat losses are described on a volumetric basis. However, a significant drop of the ratio of flame speeds in helium and argon mixtures was observed for finer 3 μm particles and was attributed to a transition from the combustion controlled by diffusion for larger particles to kinetically controlled burning of micron-size particles. In helium mixtures, the lower flame temperatures measured in suspensions of fine particles in comparison to larger particles reinforces this assumption.     

煤粉燃烧过程中灰膜形成比例实验研究

煤焦颗粒燃烧过程中,灰膜形成显著影响其燃烧特性。因此,本文借助高温沉降炉研究了61~75,75~90和90~125μm三种粒径黄陵烟煤在1273和1673 K温度下的燃烧特性与灰膜形成比例;借助扫描电镜(SEM)详细观测空心微珠颗粒内部结构,提出灰膜比例计算公式,并分析温度,粒径和碳转化率对灰膜比例的影响。结果表明,高温下大部分灰分在焦炭烧尽阶段以灰膜形式存在。灰膜比例随温度和碳转化率增加而增加,随煤粉粒径增大而减小。高温下灰分用于形成灰膜比例相对较高,这为煤焦燃尽阶段的低反应性提供了合理的解释。煤焦颗粒动态燃烧过程中灰膜形成比例随燃烧工况变化而变化。该研究为煤焦颗粒燃烧动力学模拟灰膜比例选择提供了关键数据支撑。     

Simultaneous measurements of the release of atomic sodium, particle diameter and particle temperature for a single burning coal particle

The temporal history of the release of volatile alkali species during coal combustion is a significant, but poorly understood factor in the fouling and corrosion of heat transfer surfaces within industrial coal-fired boilers. We present new results of the simultaneous measurement of particle temperature, particle size and the atomic sodium concentration in the plume of a burning coal particle. During the char phase, the sodium concentration in the plume was found to be linearly dependent on the inverse of particle diameter, but during the ash phase the sodium concentration was found to decay exponentially with decreasing particle temperature. The centreline decay of Na within the plume above the burning particle consists of one region controlled by a first order chemical reaction and a second region controlled by diffusion.