煤燃烧过程中一次破碎的影响因素分析

本文从理论上分析了煤燃烧过程中的一次破碎过程,同时考虑热应力和内部压力积聚的作用,建立了简化的一次破碎模型,通过此模型分析了煤颗粒的粒径、炉膛温度、炉膛对煤颗粒的传热系数、煤颗粒的热扩散系数、煤颗粒脱挥发分的化学动力学参数、煤中对流孔直径、煤的孔隙率、挥发分的粘性系数以及挥发分含量这9个参数对燃烧过程中煤颗粒一次破碎的影响,分析所得结果与近年来报道的实验结论基本保持一致。     

温度对生物质热解过程中传热特性的影响

基于欧拉-拉格朗日方法,考虑热解过程中挥发分的析出和颗粒体积的变化,以上海市常见的绿化树种香樟树的树枝为实验材料,对固定床中生物质颗粒热解过程中温度对传热特性的影响进行了详细研究,利用开源软件OpenFOAM获得了反应器内不同热解阶段不同温度范围内颗粒数的分布,给出了不同温度下的有效传热系数及不同传热方式所占的比例.结果 表明:热解温度越高,有效传热系数降低,固体颗粒的平均比热容也逐渐降低;同时热解温度越高,辐射的换热量占比越大,而对流提供的热量占比则不断下降.     

煤粉挥发分火焰行为的理论分析与简化模拟方法

考虑热解的扩展连续膜模型,可以详细预报煤粉挥发分析出、焦碳燃烧的全过程.模型以CH4燃烧的反应动力学特性,近似地描述了挥发分火焰.提出新的简化模拟方法均相着火后挥发分燃烧的移动火焰锋面(MFFVC)模型,弥补了挥发分现有计算方法中未考虑颗粒边界层内挥发分均相着火及燃烧的不足.与挥发分现有计算方法、扩散控制的挥发分燃烧(DLVC)模型相比, MFFVC模型预报与考虑热解的扩展连续膜模型符合较好.     

热解条件对煤中微量元素挥发性影响的实验研究

针对神府、北宿煤,在箱式高温电阻炉和快速热解设备上进行了慢速、快速高温热解实验,采用ICP-MS法对实验样品进行了微量元素含量测定,计算了Se,As,Cd,Ga,Pb,Zn,Ba,Cr,Be,Ni,Sr,Hf等元素的挥发率.研究了高温制焦过程中有害微量元素的挥发性规律.在950～1400℃温度范围内,部分微量元素的挥发率随热解终温的升高而增大;神府煤慢速、快速热解过程中,元素的挥发性规律较相似;慢速升温热解时微量元素的挥发率较大.北宿煤变质程度高于神府煤,热解过程中微量元素的挥发性降低.     

煤颗粒热膨胀量与破碎特性的研究

煤颗粒的热膨胀破碎特性直接影响流化床锅炉的运行效率.本文利用热机械分析仪(TMA)测定了不同种类不同密度的型煤和原煤的热膨胀特性,并对部分破碎微观形貌进行了FSEM观察;通过旋转炉内的燃烧试验研究了热膨胀特性和破碎的关系.研究表明,煤颗粒在燃烧过程中其热膨胀破碎主要发生在挥发分析出阶段;内部挥发分的析出会使颗粒内压增大而产生膨胀,进而产生细小裂纹并破碎;挥发分越高,颗粒密度越大,其热膨胀形变率越大,越容易发生破碎现象;主要挥发分析出后热膨胀引起的破碎可以忽略.     

高超声速再入体表面热解烧蚀效应数值模拟

表面防热材料热解与烧蚀效应研究在高超声速飞行器总体设计中具有重要应用价值。以热解烧蚀效应对飞行器目标特性及通信性能影响的预测评估为背景,从化学非平衡气体动力学方程及固体热传导方程出发,建立了气-固交界面上热解烧蚀壁面边界条件的一般形式及热物理化学模型,发展了高超声速再入体绕流流场与表面材料内部温度场耦合求解的数值模拟方法,并对计算模型和数值方法的可靠性进行了验证分析。在此基础上针对复杂外形再入体及表面硅基防热材料,开展了典型再入条件下再入体绕流及尾流流场的数值模拟,重点分析了表面材料热解烧蚀效应对流场等离子体分布的影响。研究表明：在表面材料中不含碱金属杂质的情况下,热解与烧蚀效应对流场中等离子体分布影响较小,而在含有微量碱金属杂质的情况下,热解与烧蚀效应对流场中等离子体分布及化学组分分布具有很大影响,由此对再入目标特性与电磁通信性能带来的影响不容忽视。     

空气在多孔介质中对流换热的数值模拟

本文对空气在多孔结构中的对流换热进行了数值研究．数值模拟与实验的比较表明,对空气在玻璃或轴承钢颗粒多孔结构中的对流换热进行数值模拟时,应采用考虑热弥散效应的局部非热平衡模型．本文还研究了颗粒直径、颗粒导热系数、空气物性随压力的变化及粘性耗散等对换热的影响．     

中美典型煤种的脱挥发分特性

本文以中美典型煤种为研究对象,研究了不同煤阶煤种在接近于工业炉膛的O_2/N_2和O_2/CO_2气氛下的脱挥发分特性。详细对比探究了煤阶和脱挥发分气氛对表观挥发分产率、元素释放率和挥发分热值的影响。研究结果表明;高挥发分煤在O_2/CO_2气氛下的高温快速脱挥发分产率低于O_2/N_2气氛;煤粉在O_2/N_2和O_2/CO_2气氛下脱挥发分过程中元素的释放率没有明显差异;单位kg煤粉中含有挥发分的热值随煤阶的增加而降低;单位kg高挥发分的烟煤、次烟煤和褐煤在O_2/CO_2气氛下的挥发分热值要低于O_2/N_2气氛。     

受振颗粒“毛细”系统中的对流与有序化

将球形颗粒倒入内径较窄的管状容器时,管壁的曲率会对颗粒的堆积结构产生影响,存在壁效应. 实验表明通过连续的竖直方向的振动,壁效应可以被强化,颗粒可以经由对流由无序排列转变为稳定的同轴筒形“壳层”结构. 每一壳层内,颗粒是二维的六角密堆积, 在这一转变过程中,颗粒堆积率的径向分布由初始的衰减振荡转变为等幅振荡. 分析了堆积率的不均匀性及空气在对流中的作用,以及形成“壳层”结构的动力学过程,对“壳层”结构的稳定性亦进行了讨论. 关键词： 颗粒物质 对流 有序化 毛细     

基于TG-FTIR的圆柏心、边材热解研究

生物质热解是实现生物质废弃物有效处理及生物质材料高效利用的重要途径之一,因此对生物质热解过程及其机理进行研究具有极大的现实意义。木材作为一种来源广泛的可再生材料也是众多生物质热解原料中的一种,由于组成成分等多方面的差别,不同树种的热解特性具有明显差异。然而,同一种木材的心材与边材由于在组织构造、化学组成及其含量等方面有着明显不同,因此在热解特性和产物方面也存在一定的差异,该工作即对此进行深入研究。试验以心、边材区分明显的常见园林绿化物圆柏为试验材料,通过TG-FTIR联用技术分别得到其心、边材热解过程中的热重曲线和挥发分的红外光谱谱图并对其进行分析。结果表明,纤维素、半纤维素、木质素和抽提物含量对心、边材热解特性影响显著。由于圆柏心材半纤维素、木质素含量较高,在热解前期和热解后期具有比边材更大的失重率,而较高的抽提物含量则在一定程度上增加了其在反应中期的失重率,减小了该范围内心、边材失重率的差异;边材纤维素含量较高,因此其在300~380 ℃,失重明显,在DTG曲线中的最大失重速率也较高。在红外图谱中,心、边材热解所产生的挥发分种类几乎相同,但在数量上有差异,当热解达到最大失重速率时,所产生的挥发分明显增加,且整个过程中边材产生的有机酸类化合物更多,而心材相对产生更多的水和CO₂。     

激光点火煤粒着火的模型分析

本文对颗粒煤在激光加热条件下的着火和燃烧进行了数值模拟。采用的是一个简单的煤粒着火与燃烧的一维模型。该模型采用了热解和双平行反应模型,考虑了煤粒表面的多相反应和气相的基元反应以及气相中的传热与传质。从获得的煤粒表面和气相空间的温度随时间的变化规律,可以判断不同煤种的着火方式。     

Heat and mass transfer at gas-phase ignition of grinded coal layer by several metal particles heated to a high temperature

Characteristics of gas-phase ignition of grinded brown coal (brand 2B, Shive-Ovoos deposit in Mongolia) layer by single and several metal particles heated to a high temperature (above 1000 K) have been investigated numerically. The developed mathematical model of the process takes into account the heating and thermal decomposition of coal at the expense of the heat supplied from local heat sources, release of volatiles, formation and heating of gas mixture and its ignition. The conditions of the joint effect of several hot particles on the main characteristic of the process–ignition delay time are determined. The relation of the ignition zone position in the vicinity of local heat sources and the intensity of combustible gas mixture warming has been elucidated. It has been found that when the distance between neighboring particles exceeds 1.5 hot particle size, an analysis of characteristics and regularities of coal ignition by several local heat sources can be carried out within the framework of the model of “single metal particle / grinded coal / air”. Besides, it has been shown with the use of this model that the increase in the hot particle height leads, along with the ignition delay time reduction, to a reduction of the source initial temperatures required for solid fuel ignition. At an imperfect thermal contact at the interface hot particle / grinded coal due to the natural porosity of the solid fuel structure, the intensity of ignition reduces due to a less significant effect of radiation in the area of pores on the heat transfer conditions compared to heat transfer by conduction in the near-surface coal layer without regard to its heterogeneous structure.     

The influence of heat transfer conditions at the hot particle-liquid fuel interface on the ignition characteristics

The problem of ignition in the conditions of nonideal contact between liquid fuel and a single metallic particle heated to high temperatures is numerically solved. A gas-phase ignition model is created with regard to the heat-and-mass transfer processes in the gas region near the ignition source and the layer separating the particle and the fuel. The scale of the impact of the heat source surface roughness upon the ignition characteristics in a hot particle-liquid fuel-oxidant system is determined.     

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.     

基于分子运动对流换热和热辐射下煤粉颗粒群粒子的加热分析

本文建立了在烟气对流加热和辐射状态下的煤粉颗粒群加热模型,通过数值模拟,模拟研究了不同锅炉炉膛尺寸下不同煤粉粒子粒径的煤粉群粒子加热时间以及粒子温升的关系,对对流换热和辐射换热在着火热源中所占比重进行了分析,模型很好的模拟了粒子的升温,能够较好的反映出煤粉粒子加热升温机理,为煤粉射流微元加热及着火提供了计算方法。     

超细煤粉热解时轻质烃的析出规律

将不同的煤种在管式炉中,分别进行程序升温热解,利用气相色谱对热解气中轻质烃的析出规律进行了在线分析。分析结果表明:热解产物轻质烃的成分主要由以CH4为主的混和气态烃组成;在相同的热解条件下,轻质烃的热解析出速率随着煤种碳化程度的增高而明显降低,随着煤粉粒径的减小而逐渐升高;轻质烃热解析出高峰的温度随着煤种碳化程度的增高而明显增高,随着煤粉粒径的减小而逐渐降低。     

Heat and mass transfer at the ignition of a liquid substance by a single “Hot” particle

The results of a numerical solution to the problem of heat and mass transfer at the ignition of a liquid flammable substance by a single particle heated to a high temperature located on its surface are presented. The problem is solved within the framework of a gas phase model of ignition. A mathematical model is formulated. It describes the following processes in a two-dimensional statement: the heat conduction and evaporation of a flammable liquid and the diffusion and convection of the combustible vapors in the oxidizer medium in the system “particle heated to a high temperature-liquid flammable substance-air.” The numerical investigations established the relation between the ignition delay time, the particle temperature and sizes, and the particle minimum temperature and sizes at which ignition of a combustible liquid is possible.     

Numerical investigation of pulverized coal particle group combustion using tabulated chemistry

The ignition and combustion of coal particle groups are investigated numerically in a laminar flow reactor. The Flamelet Generated Manifold method is extended to account for the complex mixture of gases being released during devolatilization, which is calculated with a competing two-step model. A second mixture fraction is introduced to include the mixing with the second methane fuel stream. The interactions of the gas phase with particles are modeled within a fully coupled Euler-Lagrange framework. To investigate the influence of particle groups on ignition and combustion, successively increasing densities of particle streams have been analyzed. The ignition delay time is increased significantly by higher particle densities. This delay is validated successfully with the available measurements. Moreover, the shape of the volatile flame was found to be strongly influenced by the particle number density inside the flame. A transition from spherical flames around single particles to a conical flame around the particle cloud could be found in numerical results as well as in experiments. As the primary mechanism for the substantial ignition delay and the formation of the flame, the increased heat transfer from the gas-phase to the particle group, resulting in lower gas-phase temperatures, was identified.     

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.