粒度对煤燃烧和热解动力学影响的量化研究

以六方石墨原子簇模拟纳米煤颗粒,采用量子化学AM1方法,研究了粒度对煤燃烧和热解动力学参数的影响规律.结果表明,粒度对煤粒燃烧和热解动力学参数有显著影响,Ea随着煤粒粒径的减小而减小,R则随粒径减小而增大,并且Ea和lnR均与粒径的倒数呈线性关系,这些影响规律与文献报道的实验结果一致.     

生物质颗粒度对燃烧特性影响

以稻壳为例考察了不同稻壳粒径对热解及燃烧特性、传热系数、焦炭燃烧的影响,利用TG/DTG6200热重差热分析仪对不同颗粒度稻壳粉做了热解实验,得到了不同的TG、DTG、DTA曲线.结果表明,稻壳颗粒度越小,其挥发分析出温度越低,析出时间越短,传热系数增大,升温速率增大,有利于燃烧,但过小的颗粒度同时增大了散热损失,当粒径低于临界尺度时,易出现熄火现象.焦炭燃烧过程受粒径大小的影响,粒径减小,燃烧反应速率加快,燃烧时间缩短,有利于燃烧.     

西部煤的热解特性及动力学研究

采用热重和红外分析法从升温速率与样品粒径的相关性等方面对平朔煤和神东煤的热解特性进行研究,并从动力学上进行分析.结果表明,随升温速率增大,煤样热解反应的初始温度、终止温度以及最大失重速率对应的温度都逐渐升高,但对粒径较小的煤样来说,这些特性温度增加的幅度较大,而最大失重率没有表现出一定的规律性;煤样粒径对热解也有一些影响,但最大失重速率与样品粒径的关系不大.随升温速率增大,热解活化能和频率因子呈现出先增大后减小的趋势.     

低阶煤热解条件对高炉喷吹半焦燃烧性能及动力学特性的影响

采用热重分析法研究了不同热解条件下半焦的燃烧性能和动力学特征，利用Ozawa法求取动力学参数。结果表明，热解温度越低、保温时间越短时，半焦的燃烧性能越好；热解升温速率对半焦燃烧过程的反应程度影响不大；粒度越大，燃烧性能差异性越明显。热解温度对半焦燃烧性能影响较大，550℃是本研究中制备高燃烧反应性半焦的适宜热解温度。两种不同粒度原煤制得的半焦均随转化率增大，活化能减小。1~3 mm原煤在热解温度为550℃时所得半焦在燃烧过程中符合反应级数模型，化学反应为限制性环节，反应最概然机理函数为f(α)=(1–α)2。     

煤颗粒燃烧过程中的热解及焦燃烧行为模拟研究

为研究煤颗粒在燃烧过程中的热解与焦燃烧行为,给解耦燃烧装置的设计和优化提供参考,基于煤燃烧过程的不同阶段(干燥阶段、热解阶段、焦燃烧阶段),结合传热传质过程和反应动力学机理,建立了单颗粒煤燃烧模型。利用该模型,分析了煤在燃烧过程中颗粒内部温度、气体组分的演化过程,以及热解阶段和焦燃烧阶段的耦合关系。结果表明,在煤燃烧过程中,靠近颗粒表面处焦的燃烧提高了颗粒内部焦燃烧的初始反应温度,导致颗粒可达到的最高温度出现在颗粒中心处;随着颗粒尺寸的增加,热解时间和总燃烧时间的差值增加,该趋势对于粒径大于4 mm的煤颗粒更加明显,意味着在解耦燃烧实际应用中应选用粒径大于4 mm的煤颗粒。     

吐哈盆地褐煤的热解和燃烧特性及动力学分析

吐哈盆地褐煤的热解和燃烧特性研究利于煤的清洁高效利用及煤炭地下气化的开展,为探究吐哈盆地褐煤煤粉颗粒的热解特性和燃烧特性及动力学特性,通过热重实验、热解特征指数计算、综合燃烧指数计算及动力学软件Kinetics Neo模型拟合法,研究了煤粉在不同升温速率(5℃/min, 10℃/min, 20℃/min)和不同粒径(大于0.8 mm, 0.2 mm～0.6 mm,小于0.1 mm)下分别在氮气气氛中的热解特性和空气气氛中的燃烧特性,获得了不同条件下煤粉颗粒热解和燃烧过程的动力学参数。结果表明：升温速率升高有利于煤粉颗粒热解和燃烧,显著提升了热解和燃烧性能;煤粉颗粒粒径增大有利于煤粉热解,不利于煤粉燃烧;不同粒径煤样热解和燃烧焦产率没有明显区别,粒径增加对于挥发分释放的影响不大;热解与燃烧过程中活化能与指前因子分别在一次热解阶段和干燥挥发阶段较高,说明在这两个阶段反应速率较慢,单位时间内化学反应程度较高。     

纳米粒子的粒度对化学反应的影响

综述了多相反应体系中纳米反应物的粒度对化学反应热力学和动力学的影响规律，预测了纳米粒子作为反应物在化学领域中的应用前景和发展趋势。目前的研究结果表明：多相反应体系中的反应物粒径对化学反应的热力学性质、平衡常数、电动势、反应速率和表观活化能均有明显的影响；随着反应物粒度的减小，反应的平衡常数增大，表观活化能降低，反应速率加快；对于分散电池，其电动势随着反应物电极分散度的增大而增大。     

精细水煤浆的颗粒分形特征对燃烧特性的影响规律

利用神华、兖州和淮南3种煤制备了精细水煤浆，并对精细煤粉颗粒的粒度分布与表面分形特征、精细水煤浆的流变特性和不同升温速率下的燃烧特性进行了研究。结果表明随着精细煤粉的表面分形维数增大，其比表面积增大，粒度、孔径均相应减小，相同浓度下的精细浆的表观黏度值上升，精细水煤浆的着火温度和燃烧反应活化能降低。其中表观黏度值以淮南精细浆与兖州精细浆增加较多，在100 s^-1下都增加了740 mPa·s。着火温度和燃烧反应活化能都以淮南精细浆降低最多，其着火温度降低了41.36℃，燃烧反应活化能降低了77.71 kJ·kmol^-1。     

热解条件对煤膨胀和球形度的影响

针对粉煤密相输运床气化技术,使用滴管炉对云南褐煤(YN)、山东次烟煤(SD)和内蒙烟煤(NM)进行快速升温热解实验,应用数字成像颗粒分析仪同时获得颗粒粒径和球形度,研究了不同煤阶、粒径、温度、预烘干除水对煤焦形貌参数的影响.结果表明:随着煤阶提高,热解膨胀度和球形度均提高,1 273 K热解后YN褐煤因收缩和破碎,膨胀度为0.66;SD煤和NM煤的膨胀度均大于1;焦的球形度因热解软化,较原煤均略有提高;NM烟煤随着热解温度的提高,膨胀度降低,球形度先增加后减小;小粒径NM煤颗粒的膨胀度和球形度均更高.干燥NM煤的膨胀度显著高于含水原煤,且随温度升高,膨胀度提高,变化规律与含水原煤相反;球形度随温度变化规律相似,但却低于原煤,预烘干除水对NM煤焦形貌的影响显著.     

非等温热重分析长焰煤热解过程与动力学特征

为提高煤热解过程中焦油的产率,用非等温热重分析方法研究了不同粒径、热解终温和升温速率条件下长焰煤的热解过程和机理,分析了20和100℃/min升温速率下长焰煤热解过程特征,并求解了热解动力学参数。结果表明,煤颗粒在2.8mm以下时,粒径对热解过程影响较小;热解终温越高,热解最终固体产物中挥发分产率越低;升温速率越快,挥发分的析出速率越快。在同一升温速率下,不同热解温度段得到的活化能呈现两头大中间小的特征,且指前因子随活化能的增大而增大。     

用无形孔模型研究影响煤粉燃烧的因素

用一种新的孔隙结构模型──无形孔模型来研究影响煤粉燃烧的因素。对该模型进行数值计算，跟踪燃尽过程中温度、氧浓度、燃烧速率等随时间的变化，得到有关曲线。定量探讨整个煤粒内部的燃烧过程。研究挥发分析出和燃烧对煤粉燃烧过程的影响。研究孔隙率、比表面积、粒径等因素对煤粉着火、燃尽的影响。结果表明孔隙率对燃尽的影响较大，而比表面对着火的影响较大。     

压力对煤气化反应的影响

为了提高煤气化效率,分析了影响产能的重要因素——压力。研究了压力对煤热解过程、煤焦燃烧速度及煤焦气化反应的影响。研究发现:加压热解情况下,挥发分和焦油产率均下降,但煤气产量增加,推测是因为焦油发生二次反应造成的。随着压力的增大,煤焦明显膨胀且比表面积下降。但过高的压力下,膨胀度减弱,易生成孔隙率高、薄壁的煤焦颗粒。提高O2分压,煤燃烧速度加快且生成的小颗粒较多。提高气化剂分压,煤气化速度加快,且蒸汽分解速度大于CO2还原速度,但生成的煤气对气化反应有抑制作用。     

Numerical study of coal particle cluster combustion under quiescent conditions

Behavior of ignition and combustion of coal particle cluster under a quiescent condition was numerically simulated by solving balance equations of mass and enthalpy with combustion kinetic models of volatiles and char. Two-flame structure, one flame penetrating into the cluster and the other moving out of the cluster, was predicted during the combustion of coal particle cluster. Effects of radiative heat transfer, group number, ambient temperature, coal particle size, and oxygen concentration on ignition and combustion of coal particle clusters were also analyzed. Simulations indicated that the gas volume fraction of coal particle cluster increases with time after devolatilization. Gas velocity passing through the cluster surface varied significantly at volatile liberation. The ignition time delay was reduced with the increase of ambient temperature. The cluster devolatilization rate and char burning rate increased while the ignition time delay decreased with the increase of ambient oxygen concentration.     

The devolatilisation of millimetre sized coal particles at high heating rate: the influence of pressure on the structure and reactivity of the char

Most studies on the influence of pressure on the combustion of coal particles have shown that for a constant oxygen concentration, an increase of pressure leads to a decrease of combustion rate. Among the different phenomena, which can explain this behaviour, the influence of the devolatilisation pressure on the structure and reactivity of the char formed may be important. The aim of this paper was to obtain a quantitative characterisation of the physical and chemical structure of chars formed during pyrolysis under a large range of pressure. Experiments of single coal particle pyrolysis were conducted in a laser reactor with pressure ranging from 0.014 to 2.1 MPa in a nitrogen atmosphere. As expected, an increase of pressure lead to a decrease of the volatile matter yield, which can be related to the secondary reactions of volatile matter. A characterisation of the char was performed by gas adsorption methods: nitrogen adsorption, carbon dioxide adsorption and active surface area (ASA) measurement. True and apparent densities, porosities and swelling of the particles were also investigated. Although the volatile matter yield decreases, the porosity and the swelling of the char increases with increasing pyrolysis pressure. We observed an increase in surface area and microporosity with increasing pressures up to 0.6 MPa. The ASA surface also increases in this temperature range, but the ratio of ASA to CO₂ surfaces shows that the intrinsic reactivity of the surface decreases with increasing pyrolysis pressure.     

Transient group combustion of the pulverized coal particles in spherical cloud

A transient group combustion model for the pulverized coal particles in a spherical cloud is developed to predict the transient group combustion characteristics. The submodels, which account for the detailed combustion process of both homogeneous and heterogeneous reactions, are also applied for the pulverized coal particles. The numerical simulation of the collective behaviors of ignition and its subsequent burning were carried out. The ignition mechanism is characterized by the heterogeneous process of the dilute cloud and the homogeneous process of the dense cloud. Two dominant flame structures are observed: one flame penetrating inside the cloud and the other moving outside the cloud. The effects of various parameters (i.e., radiation heat transfer, group combustion number, air temperature, air oxygen concentration, particle size distribution, and particle number density) on the particle mass burning rate and on the overall performance of the group combustion are examined. The results are in good agreement with existing experimental data.     

铝含量和含氟有机化合物对丁羟推进剂燃烧性能的影响

为进一步提高HTPB推进剂的能量并抑制铝粉在燃烧过程中的团聚,制备了铝粉质量分数为16%~22%的端羟基聚丁二烯(HTPB)推进剂,并分别加入含氟有机化合物(OF)作为铝燃烧促进剂,研究了铝含量和OF对HTPB推进剂燃烧性能的影响;使用氧弹量热仪测定了推进剂在氩气氛围下(3 MPa)的爆热;收集在3 MPa下推进剂燃烧的凝聚相产物,采用激光粒度仪、X射线光电子能谱仪(XPS)及X射线衍射仪(XRD)等分别对其进行粒度分布、元素和物相分析;通过线扫描摄像机和高压燃烧室系统测定推进剂的燃速;利用高速摄影系统观察推进剂燃面上熔铝粒子的团聚过程。结果表明,HTPB推进剂在铝粉质量分数为20%时实测爆热最大,含氟有机物OF的引入使得爆热有所下降;随着HTPB推进剂中铝含量的提高,燃面上熔铝粒子的团聚愈加严重,凝聚相燃烧产物的尺寸和残留铝含量均逐渐增加;加入含氟有机物OF能够促使-Al2O3和AlF3的生成,有效抑制铝颗粒在燃烧过程中的团聚,使凝聚相燃烧产物的尺寸和残留铝含量显著降低,当铝粉质量分数为20%时,OF的加入使得残留铝的生成率降低了50%;较低的铝含量和OF的添加有利于HTPB推进剂燃速的提高。     

Combustion of a solid fuel particle in a pulsating flow

Summary The increased burning rate of the fuel in PCC can be attributed to an increase in the flow over the particle; this probably also applies to a number of other combustion intensifiers. An increase in the pulsations is especially effective with increase in fuel flow temperatures, when the diffusion regime goes over into the intermediate and possibly the kinetic regime, i.e., the burning rate reserves are realized. As may be seen from the graphs, the flow puIsation effect (relative increase in burning rate as compared with combustion in a steady-state flow) increases with decrease in the constant velocity component; this obvious relationship does not apply to oscillating flow, when, as observations of the burning particle show, at small flow pulsations the particle is enveloped in a cloud of combustion products and its burning rate decreases. The heat and mass transfer between the burning particle and the pulsating gas flow is determined by the aerodynamic characteristics of the flow and by diffusion processes and chemical reactions at the surface of the particle. Chelyabinsk. Translated from Fizika Goreniya i Vzryva, Vol. 6, No. 2, pp. 157–162, April–June, 1970.     

An analytical model for the combustion of coal particles

A model for the combustion of coal particles is developed which accounts for the gasification and pyrolysis reactions throughout the particles, oxidation reactions at the particles' outer surface, distributed gas-phase reactions around the particles, and transport of heat and mass throughout the reaction zones. The model shows that the combustion of large particles is dominated by gasification and gas-phase reactions. As large particles burn and shrink, however, the mechanism changes to one dominated by surface oxidation. This change in mechanism can be gradual or very rapid (even discontinuous) depending on the combustion conditions. The particle size at which this transition takes place decreases as the bulk gas temperature or oxygen concentration increase. The proposed model is used to predict the conversion of pulverized coal as a function of time; the results agree with literature data for particles of 26-32 μm radius.     

Influence of aluminum particle size on ignition and nonstationary combustion of heterogeneous condensed systems

The results of studies of the effect of particle size of aluminum powder in condensed systems on the ignition, nonstationary combustion, and acoustic conductivity of the burning surface are presented. Analysis of the experimental data shows that the ignition delay and the temperature of burning surface of the heterogeneous condensed systems under study decrease with increasing particle size of aluminum powder, and the nature of the dependence of the nonstationary burning rate on the time of depressurization of the combustion chamber for compositions containing micron or ultrafine aluminum powders is in qualitative agreement with the phenomenological theory of nonstationary combustion. Replacement of micron aluminum powder by ultrafine powder in a heterogeneous condensed system increases acoustic conductivity.