基于绝热氧化的煤自燃倾向性鉴定研究

本文首先指出了我国现行的煤自燃倾向性色谱吸氧鉴定法的不足之处,同时给出了煤自燃倾向性鉴定标准的基本原则。根据绝热氧化理论和绝热氧化方程,设计了相应绝热氧化实验装置。通过理论和实验研究,提出了能够反映煤自燃关键阶段,即低温氧化阶段动力学参数活化能E作为煤自燃倾向性鉴定指标,并测试了五个典型煤样的低温氧化活化能E,同时根据该指标对煤自燃倾向性等级进行了初步划分。最后,通过现场煤自燃发火期与煤物理吸附氧量和低温氧化活化能E的对比,对该指标的实用性和科学性进行了检验。     

不同升温速率下微米级铝粉的氧化特性分析

采用热重-质谱联用仪(TG-MS)、X射线衍射仪(XRD)及扫描电子显微镜(SEM)等测试手段,研究了不同升温速率下1～2μm铝粉的氧化特性,并分析了氧化膜厚度和反应活化能与升温速率的关系。结果表明,多晶相变机理同样适用于1～2μm铝粉,其氧化可分为五个阶段。氧化铝晶形转变、铝核心熔化膨胀等引起的氧化膜破裂是快速氧化的主要原因;第三阶段因θ-Al_2O_3出现导致DTG曲线出现较小的增重峰,该峰值温度随着升温速率的减慢而提前。结果还发现扩散驱动力的选择对计算活化能产生的影响比较小,各个阶段的活化能与升温速率关系不完全统一。     

O_2/CO_2气氛下污泥与煤混燃特性研究

利用热重分析研究了O_2/CO_2气氛下污泥与煤混燃特性,基于热重(TG)和微商热重(DTG)分析曲线,分析了泥煤比、升温速率和气氛对污泥与煤混燃特性的影响。结果表明;在21%O_2/79%CO_2气氛下,污泥与煤混合物的着火温度随泥煤比的上升而显著降低,最大失重速率随着升温速率的提高而提高。随着氧浓度由21%提高到50%,燃尽温度从708.3℃降低至599.8℃,污泥与煤混合物的燃尽时间也明显缩短。21%O_2/79%N_2气氛下污泥与煤混合物的燃烧特性介于21%O_2/79%CO_2和30%O_2/70%CO_2气氛之间.     

低氧浓度下煤燃烧特性的热重实验研究

煤燃烧的部分过程是在低氧浓度下进行的,本文利用热重实验研究煤在低氧浓度下燃烧特性的变化,重点研究着火特性、燃尽特性和燃烧速率的变化;同时计算分析低氧浓度下,煤燃烧反应动力学参数的变化。实验结果表明,低氧浓度下煤燃烧反应的TG和DTG曲线均向高温区靠近,着火温度基本不变,燃尽温度提高,燃烧速率下降;低氧浓度下燃烧反应的动力学参数活化能E和频率因子k_0之间存在着补偿效应。     

化学脱灰对低灰煤粉性质的影响

本文研究了HCl-HF-HCl方法脱灰对低灰煤粉性质的的影响.研究发现,酸处理不仅使煤的灰分脱除,也使其挥发分降低,煤变质程度越低,挥发分降低幅度越大;通过对脱灰滤液的检测,发现脱落的有机质主要是短链烷烃及其同系物,也有少量的含氧物质;酸处理后煤粉中金属离子也将全部脱除;燃烧特性研究发现,煤粉脱灰后高变质程度煤反应活性提高,低变质程度煤反应活性下降,取决于有机质和金属离子脱除与比表面积增加之间的平衡.因此,化学脱灰不能证明矿物对煤粉燃烧的影响.     

半焦燃烧特性的热重试验研究

针对目前提倡的煤部分气化燃烧系统集成优化联合生产煤气和热能的新概念,在不同温度下制得四种煤的半焦,通过热天平燃烧试验研究了半焦的燃烧特性,考察了煤种和制备温度对半焦燃烧特性的影响.试验结果表明:煤种不同,所制得半焦燃烧特性不同;相同煤种制得半焦,随制备温度升高,半焦着火温度上升,燃烧活化能增加,燃烧反应活性降低.     

硼颗粒着火阶段氧化性能研究

硼具有比镁、铝更高的能量密度,是一种具有很大应用潜力的理想固体添加剂。但硼颗粒着火过程机理尚不明确,尤其是着火过程中关键阶段:氧化层增厚的过程,更需要深入研究。本文利用热重和管式炉等实验方法,对微米硼颗粒着火阶段的氧化膜增厚过程,即氧化性能进行了研究。发现在氧气/氮气或空气的气氛中,微米硼颗粒的氧化过程分为脱水、快速氧化和慢速氧化三个阶段。利用多个升温速度法求得快速氧化阶段的活化能为131.74 kJ/mol。微米硼颗粒的着火温度约为929~969 K。着火温度随着加热速度的增加而升高,随着氧气浓度的增加而降低。硼颗粒氧化后会黏结在一起,形成类似"切糕"状产物:未氧化的单质硼镶嵌在氧化产物B_2O_2及B_2O_3中。这种产物结构说明了(BO)n扩散模型具有合理性。     

煤尘层最低着火温度变化规律实验研究

为研究煤尘层最低着火温度随煤样变质程度、煤尘粒径及煤尘层厚度的变化规律,采用煤尘层最低着火温度测定系统进行实验研究,结果表明:随着煤样由褐煤到无烟煤变质程度逐渐增大,煤尘层最低着火温度由290℃上升到400℃以上,同时,褐煤、长焰煤、不粘煤、气煤煤尘层着火时观察到明显的火焰。随着煤尘粒径不断减小,不同煤质的煤尘层最低着火温度明显减小,煤尘层厚度为15mm时,随着煤尘粒径由0.5mm减小至0.075mm,不同煤质煤尘层最低着火温度分别减小了31.0%、26.7%、28.1%、25.8%、28.6%、27.8%、18.9%和15.0%,煤尘粒径影响作用十分显著。随着煤尘层厚度的增大,不同煤质在不同粒径下的煤尘层最低着火温度都减小,其中无烟煤的变化最不明显。     

用多个速率升温热重法测定MgB_2的氧化活化能

用热分析仪研究了 Mg B2 在高温空气气氛下的热稳定性和氧化情况 ,结果表明 Mg B2 在 4 0 0℃开始氧化 ,70 0℃后氧化非常强烈 ,用多个速率升温热重法测定 Mg B2 的氧化活化能为 80 k J/ mol左右。     

O3对甲烷／空气着火过程的影响

本文基于零维均质着火系统研究了非平衡等离子中臭氧（O3）对甲烷／空气着火过程的影响，并通过化学反应路径分析揭示了O3促进甲烷／空气着火的化学反应机理。研究结果表明，O3促进甲烷／空气着火手要是通过化学反应动力学效应来实现的，热效应的影响十分有限。在不添加O3时，甲基的氧化速率缓慢，从而导致甲烷／空气不易着火；在添加O3...     

Particle temperature and potassium release during combustion of single pulverized biomass char particles

This work investigated the combustion characteristics of single pulverized biomass-derived char particles. The char particles, in the size range 224–250 µm, were prepared in a drop tube furnace at pyrolysis temperatures of 1273 or 1473 K from four types of biomass particles – wheat straw, grape pomace, kiwi branches and rice husk. Subsequently, the char particles were injected upward into a confined region of hot combustion products produced by flat flames stabilized on a McKenna burner, with mean temperatures of 1460, 1580 and 1670 K and mean O₂ concentrations of 4.5, 6.5 and 8.5 vol%. The data reported include particle temperature, obtained using a two-color pyrometry technique, and potassium release rate, measured using a laser-induced photofragmentation fluorescence imaging technique. In addition, particle ignition delay time and burning time, obtained from the temporal evolution of the thermal radiation intensity of the burning char particles, are also reported. The results indicated that ignition of the char particles occurs simultaneously with the starting of the potassium release, then the particle burning intensity increases rapidly until it reaches a maximum, after which both the particle temperature and the potassium release rate remain approximately constant until the end of the char oxidation process. The char ignition process is temperature controlled, and the char oxidation process is oxygen diffusion controlled, with the total potassium release being independent of the oxygen concentration and the temperature of the combustion products. The combustion behavior of the chars studied is more affected by the char type than by the conditions used to prepare them.     

The influence of thermal annealing on oxygen uptake and combustion rates of a bituminous coal char

The effect of thermal annealing on the combustion reactivity of a bituminous coal char has been investigated with a focus on the role of the formation of surface oxides by oxygen chemisorption. The combined use of thermogravimetric analysis and of analysis of the off-gas during isothermal combustion of char samples enabled the determination of the rate and extent of oxygen uptake along burn-off. Combustion was carried out at temperatures between 350 and 510 °C. Char samples were prepared by controlled isothermal heat treatment of coal for different times (in the range between 1 s and 30 min) at different temperatures (in the range 900–2000 °C). Results indicate that oxygen uptake is extensive along burn off of chars prepared under mild heat treatment conditions. The maximum oxygen uptake is barely affected by the combustion temperature within the range of combustion conditions investigated. The severity of heat treatment has a pronounced effect on char combustion rate as well as on the extent and rate at which surface oxides are built up by oxygen chemisorption. Chars prepared under severe heat treatment conditions show negligible oxygen uptake and strongly reduced combustion rates. Altogether it appears that a close correlation can be established between the extent and the accessibility of active sites on the carbon surface and the combustion rate. Despite the investigation has been carried out at temperatures well below those of practical interest, results provide useful insight into the relationship existing between thermal annealing, formation of surface oxide and combustion reactivity which is relevant to the proper formulation of detailed kinetic models of char combustion.     

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.     

Oxygen consumption and chemisorption in low-temperature oxidation of sub-bituminous pulverized coal

Studying the effect of oxygen in coal oxidation is very important for understanding and controlling coal spontaneous combustion. However, the oxygen effect is not very easy to determine clearly due to the large effect of heat source on coal oxidation in temperature rising experiments. Here, focused on sub-bituminous coal, the oxygen effect was separated from coal oxidation by continuously measuring FTIR spectra of coal with respect to varying temperatures and under oxygen and nitrogen. The active groups’ real-time changes of coal oxidation, thermal treatment and oxygen effect were measured. The carboxylic ester and carboxyl units are the main functional groups that increase with temperatures increasing under oxygen and nitrogen, while the other functional groups decrease in quantity. The oxygen effect promoted the consumption of aliphatic hydrocarbons and hydroxyl groups and also promoted the formation of oxygen-containing groups (except hydroxyl). Four characteristic temperature stages involved in the oxygen effect and their key functional groups were identified. Simultaneously, the relationship of oxygen consumption and chemisorption in oxygen effect was analyzed. The starting temperature of oxygen chemisorption is between 50 and 60°C. The maximum contribution of oxygen effect was observed in methyl and methylene groups. These results are important for chemical control of coal spontaneous combustion. The oxidation of aliphatic hydrocarbon should be controlled before oxygen chemisorption. The value of oxygen consumption between 70 and 80°C can be measured accurately due to the constant chemisorption rate, which help to identify the tendency for spontaneous combustion. These results will help in better understanding of the reaction mechanism of coal oxidation, especially the oxygen effect.     

Glowing ignition of wood: the onset of surface combustion

A theoretical model for wood pyrolysis including char surface oxidation is presented. The main objective is to expose the physical mechanisms governing glowing ignition. By “glowing ignition,” we mean the onset of surface combustion. The char surface oxidation, which can lead to glowing ignition, is considered at the surface boundary condition. Two regimes of char surface oxidation, namely, kinetic and diffusion-controlled, are distinguished. Depending on the char surface oxidation resistances, the char surface oxidation as either kinetic- or diffusion-controlled can be identified. A criterion for glowing ignition is developed based on a surface energy balance. A numerical result shows that according to the present glowing ignition criteria, an inflection point of the surface temperature history can indicate glowing ignition. Generally, a good agreement between theoretical and experimental results at glowing ignition is obtained.     

The influence of char surface oxidation on thermal annealing and loss of combustion reactivity

Thermal annealing associated with heat treatment of coal chars affects gasification reactivity and levels of unburned carbon in residual ash from coal-fired furnaces. The present study addresses the effect of char surface oxidation, occurring upon exposure to oxygen, on the course of thermal annealing, and related loss of combustion reactivity. This goal is pursued by comparing the extent of thermal annealing suffered by coal char upon heat treatment in a nitrogen atmosphere with that of chars that underwent oxidation prior to or during heat treatment. Oxidation of char was accomplished by supplying single or multiple pulses of air during the heat treatment, which were sufficient to oxidize the char surface but small enough to limit carbon gasification to less than 5%. The extent of thermal annealing was characterized both in terms of the loss of combustion reactivity and of the development of structural anisotropy of char samples, investigated by HRTEM. Results of the present study confirm that heat treatment reduces oxyreactivity of char samples, the effect being more pronounced at temperatures exceeding 1200 °C. Oxidation of samples mitigates the effects of heat treatment, as demonstrated by the smaller loss of gasification reactivity and by the more limited development of structural anisotropy of oxidized samples. Correspondingly, elemental analysis of samples indicates the formation of stable surface oxides upon oxidation, that are subsequently desorbed upon heat treatment. At temperatures exceeding 1200 °C, the effect of oxidation vanishes. Results are analysed and discussed in the light of the possible hindrance of thermal annealing due to the formation of stable surface oxides and of the parallel modifications occurring to the ash constituents.     

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.     

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.     

Particle imaging of ignition and devolatilization of pulverized coal during oxy-fuel combustion

Oxy-fuel combustion of coal is a promising technology for cost-effective power production with carbon capture and sequestration that has ancillary benefits of emission reductions and lower flue gas cleanup costs. To fully understand the results of pilot-scale tests of oxy-fuel combustion and to accurately predict scale-up performance through CFD modeling, fundamental data are needed concerning coal and coal char combustion properties under these unconventional conditions. In the work reported here, the ignition and devolatilization characteristics of both a high-volatile bituminous coal and a Powder River Basin subbituminous coal were analyzed in detail through single-particle imaging at a gas temperature of 1700 K over a range of 12–36 vol % O₂ in both N₂ and CO₂ diluent gases. The bituminous coal images show large, hot soot cloud radiation whose size and shape vary with oxygen concentration and, to a lesser extent, with the use of N₂ versus CO₂ diluent gas. Subbituminous coal images show cooler, smaller emission signals during devolatilization that have the same characteristic size as the coal particles introduced into the flow (nominally 100 μm). The measurements also demonstrate that the use of CO₂ diluent retards the onset of ignition and increases the duration of devolatilization, once initiated. For a given diluent gas, a higher oxygen concentration yields shorter ignition delay and devolatilization times. The effect of CO₂ on coal particle ignition is explained by its higher molar specific heat and its tendency to reduce the local radical pool. The effect of O₂ on coal particle ignition results from its effect on the local mixture reactivity. CO₂ decreases the rate of devolatilization because of the lower mass diffusivity of volatiles in CO₂ mixtures, whereas higher O₂ concentrations increase the mass flux of oxygen to the volatiles flame and thereby increase the rate of devolatilization.     

基于简单碰撞理论煤粉燃烧动力学模型的研究--PART Ⅱ:颗粒表面的氧气浓度分布模型

为了较准确地预报炉内煤粉燃烧速率,正确区分TGA中滞止煤粉表面与炉内载流煤粉表面氧气浓度的变化规律是非常关键的。从TGA中非稳态条件下坩埚内颗粒表面氧气浓度分布的数理解知,煤样的氧化过程是同时进行的,只是上部的氧化速率大一些,底部的氧化速率小一些;同一样品,同一升温速率,试样的堆积厚度的差异,会影响实验结果的重复性。分析表明,在初始和反应结束时,坩埚内颗粒表面氧气浓度等于环境浓度;反应速率达到最大值时,颗粒表面氧气浓度达到最小值。颗粒在炉内流动燃烧过程中,环境中氧气浓度值是单调减少的,煤焦表面氧的浓度是非线性变化的。