Attrition of spherical electrode carbon particles during batch fluidized combustion

Spherical electrode carbon particles prepared from carbon rods of dry cell batteries have been used to study the attrition behaviour in a bubbling fluidized bed combustor. Experiments have been conducted in a 40 mm I.D. and 1 m high fluidized bed combustor operated at 1 m/s superficial velocity. The bed was operated with nitrogen and with two different oxygen concentrations at 850°C to study the effect of combustion on attrition of these particles. The experimental technique used allowed the time resolution of attrited fines generation, providing detailed curves of attrition rates as a function of time. Attrition rate constants have been evaluated. Results show an enhancement of attrition due to combustion even for spherical, homogeneous and smooth particles.     

煤燃烧过程中碳、氧官能团演化行为

采用X射线光电子能谱（XPS）系统研究燃烧过程中准格尔褐煤及其煤焦颗粒表面中碳、氧的存在形态及演化过程.研究结果表明，准格尔褐煤颗粒表面含碳官能团中石墨化碳含量非常低，但同时具有一定含量的双键碳;含氧官能团中无机氧含量较低，有机氧含量相对较高.随着燃烧的进行，颗粒表面碳、氧官能团呈现不同的变化趋势.     

The kinetic rate of combustion of electrode carbon in the temperature range of 1000-1200 k

Kinetic rates of combustion of electrode carbon particles were measured in a temperature range of 1000-1200 K. Spherical carbon particles of size 7-9 mm are burnt in hot streams of nitrogen-oxygen mixtures. The velocity and the temperature of the mixture were varied. To compensate for the mass transfer resistance to the flow of oxygen to the carbon surface a mass transfer experiment was carried out in the same apparatus. By using mass transfer coefficients from the above experiment kinetic rates are analysed statistically to give the following Arrhenius type correlation of the kinetic rate: where n is in the range of 0.3 to 0.5.     

洗涤塔内相变脱除燃煤超细颗粒的试验研究

为研究洗涤塔内相变对燃煤超细颗粒的脱除,在洗涤塔液相进口上方注入蒸汽使烟气达到过饱和,由高效除雾器脱除凝并长大的含尘液滴。系统研究了蒸汽添加量、洗涤塔入口气液温差、液气体积比、烟气在相变区域的停留时间等操作参数对燃煤超细颗粒脱除效率的影响规律。结果表明:在塔内添加少量蒸汽,可显著促进燃煤超细颗粒的脱除,蒸汽添加质量浓度为0.03 kg/m3时,数量浓度脱除效率由10%增至60%以上;液气体积比的提高有利于燃煤超细颗粒的脱除,特别是当洗涤塔入口气液温差较大时。研究结果表明利用洗涤塔内相变脱除燃煤细颗粒是一种有应用前景的新方法。     

Dual role of activated carbon as fuel and template for solution combustion synthesis of porous zinc oxide powders

In this work, nanosized zinc oxide (ZnO) powders were fabricated by urea–nitrate solution combustion synthesis using activated carbon as a structure-directing template and secondary fuel at different fuel–oxidant ratios. The as-synthesized powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N₂ adsorption–desorption measurements, UV–Vis diffuse reflectance spectroscopy, and photoluminescence. The effect of fuel amount on photocatalytic activity of ZnO powders was evaluated by the degradation of an azo dye Orange G. It was observed that combustion synthesis with activated carbon as a secondary fuel had a profound effect on reducing crystallite size and enhancement of specific surface area. The crystallite size of the as-synthesized powders varied from 46 to 26 nm. The ZnO powder prepared at a fuel–oxidant ratio of 1.8 possessed the small crystallite size and high specific surface area of 69 m²/g. It correspondingly resulted in the highest dye removal percentage of 99% with a rate constant of 0.027 min⁻¹. The improvement in dye degradation can be due to the synergistic interaction and interplay of enhanced surface area and catalytic ability of the photocatalyst. This study provides a simple single-step synthesis methodology to produce metal oxide nanopowders with tunable surface properties for high potential applications in catalysis, optoelectronics, and gas sensors.     

往复热循环多孔介质“超焓燃烧”特性

对稀薄低热值气体在往复式热循环多孔介质燃烧系统的“超焓燃烧”特性进行了研究。在模型验证的基础上,阐明了系统“超焓燃烧”产生机理,分析了换向半周期、燃气热值、二次风比等工况参数与气固两相的热容比、对流传热系数、光学厚度等物性参数对系统“超焓燃烧”特性的影响,并给出了相应的变化规律;着重指出“超焓燃烧”在燃气热值较低（H₀<3.48）才能产生,且随着燃气热值降低,超焓现象越明显;稳定燃烧时,燃烧效率受各参数影响相对较小,基本保持在非常高的水平,验证了多孔介质燃烧的高效性。     

Stabilized gas combustion wave in an inert porous medium

A stabilized gas combustion wave in an inert porous medium with intense internal interphase heat exchange (at low velocities) is considered to elucidate the mechanism underlying the increase in the burning velocity of the homogeneous gas mixture due to the porous medium. It is shown that the major factor of the increase in the burning velocity is conductive heat recuperation from the postflame zone to the region of the fresh mixture through the solid skeleton of the porous medium. Analytical dependences of the degree of increase in the burning velocity of the mixture in the stabilized wave on the determining parameters are obtained. The possibilities and restrictions of the use of the results to analyze the operation of porous burners are discussed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 5, pp. 8–15, September–October, 2008.     

Mathematical modeling of heterogeneous detonation in gas suspensions of aluminum and coal-dust particles

Results of investigations performed by the authors in the field of theoretical and numerical modeling of heterogeneous detonation of reacting gas suspensions since 2005 are systematized. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 4, pp. 166–177, July–August, 2009.     

Numerical study of combustion in a cylindrical porous burner

Hybrid combustion waves in a two-section apparatus with cylindrical symmetry were studied using a two-temperature mathematical model. A program was written and a large series of numerical calculations was performed using an algorithm developed earlier. For one-and two-section apparatuses with various flow rates, characteristics of the process such as the position of the steady-state point, the maximum gas temperature, the gas velocity at the steady-state point, the width of the combustion zone, etc., were determined. It is shown that, in the two-section apparatus, there is a stabilization of the position of the combustion front near the interface. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 3, pp. 17–21, May–June, 2008.     

多孔介质内低热值乙烯燃烧的污染物排放特性试验研究

近年来低热值气体的利用慢慢进入大众视野。为了加大对低热值气体的利用,实现低热值气体的清洁排放,自行搭建了多孔介质实验台,探究乙烯在自由堆积氧化铝小球中的预混燃烧的污染物排放规律。在改变当量比、流速以及空隙率等工况下进行乙烯的燃烧并对出口烟气进行数据采集,分析在不同工况时的污染物排放情况以及乙烯转换率。实验结果表明多孔介质燃烧技术是一种可以高效清洁处理热值气体的燃烧方式,在较好的实验工况下CO的排放可以降低至125~187 mg/m³,NO的排放与当量比有关,和流速以及空隙率关系不大,整个实验过程中的NO出口浓度低于16 mg/m³。稳定燃烧情况下乙烯的转化率在80%～90%之间。实验结果对于高效清洁地处理低热值气体具有一定指导意义。     

Combustion of porous carbon particles in oxygen

A model is developed for the combustion of porous carbon particles in oxygen in a high-temperature furnace. Conditions for the existence of solutions to the equations of the model were found. A collation between the found region of existence of the solutions and experimental data shows that only some portions of the experimental curve fall within the region of existence of the solutions. These portions correspond to four different modes of particle combustion, and the realization of one or another mode depends primarily on the internal surface area of the particle. The mechanisms and some features of the modes of combustion of porous carbon particles in oxygen were considered.     

铁基载氧体辅助无烟煤焦富氧燃烧动力学分析

流化床铁基载氧体辅助富氧燃烧下传统石英砂床料被铁基载氧体替代,铁基载氧体扩展了传统床料的“热载体”的功能,另承担了“氧载体”的角色,为调节炉内氧分布与煤燃烧过程匹配提供了新思路。本文在热重实验平台探究了10%O₂/90%CO₂气氛下分析纯Fe₂O₃、赤铁矿及钢渣三种铁基载氧体辅助无烟煤焦燃烧特性及动力学。结果表明,相较于纯无烟煤焦燃烧,铁基载氧体辅助燃烧下无烟煤焦的燃烧特性得到显著改善,其中燃烧速率提高29%以上,燃尽温度降低65℃以上,综合燃烧指数提升2倍以上,活化能与指前因子同步增加且表现出“补偿效应”。三种铁基载氧体中分析纯Fe₂O₃对无烟煤焦燃烧特性的改善略优于赤铁矿和钢渣,钢渣可作为流化床铁基载氧体辅助富氧燃烧的床料替代石英砂。     

Kinetics of surface reactions in carbon deposition from light hydrocarbons

Carbon deposition from ethene, ethine and propene as a function of pressure was studied at various temperatures and two different surface area/volume ratios. Deposition rates as a function of pressure of all hydrocarbons indicate Langmuir-Hinshelwood kinetics which suggests that the deposition process is controlled by the heterogeneous surface reactions (growth mechanism). These kinetics are favored at decreasing reactivity (C₃H₆>C₂H₂>C₂H₄), decreasing temperature and residence time as well as increasing surface area/volume ratio. A linear rate increase at high pressures suggests that carbon is additionally or preferentially deposited by aromatic condensation reactions between polycyclic aromatic hydrocarbons large enough to be physisorbed or condensed on the substrate surface (nucleation mechanism). The results completely agree with earlier results obtained with methane.     

典型尺寸燃煤颗粒富氧燃烧特性及燃烧本征动力学研究

利用微型流化床加热速度快、温度分布均匀以及气体近平推流等优势,在直径20 mm自动控温的微型流化床反应分析仪中研究了粒度分布为1.7～3.35 mm和0.12～0.23 mm两种典型尺寸燃煤颗粒在790～900℃温度范围内的富氧燃烧行为。通过快速响应过程质谱对燃烧产生的烟气进行实时监测,成功地识别和记录了粗颗粒燃烧过程中经历的挥发分燃烧和原位新生半焦燃烧两个主要阶段。挥发分析出速度最快,然后快速燃烧,而半焦燃烧速度较慢。相比之下,细颗粒燃烧的这两个阶段具有几乎相同的速率,因而相互耦合而难以区分。根据实验结果,挥发分析出和燃烧为快速反应,煤颗粒燃烧过程速率受原位新生半焦燃烧过程控制。进一步研究了挥发分和原位新生半焦燃烧动力学行为,获得其本征动力学的活化能分别为107.2和143.9 kJ/mol。     

化学链燃烧铁基载氧体还原反应积炭趋势

利用热重分析仪对采用机械混合法自行制备的铁基载体还原过程中的积炭现象进行了实验研究。根据实验获得的热重曲线对铁基载氧体的CH₄还原特性进行了分析,实验结果表明,CH₄与铁基载氧体的还原反应过程中存在较为严重的积炭影响,且气体的浓度对反应有较大的影响。通过检测载氧体氧化过程中生成的CO₂量对这种影响进行了定量分析,结果表明积炭随着循环次数的增多而略有下降。XRD和SEM分析结果显示还原反应生成的C部分与载体反应生成Fe₃C,另一部分以碳丝的形式存在于载体表面以及颗粒之间。     

基于过剩焓函数多孔介质中超绝热燃烧的分析

分析了稀预混气体多孔介质中超绝热燃烧反应区的能量集中现象。通过一维双温模型理论上推导出量纲1过剩焓函数。分析表明,超绝热燃烧温度是修正的Lewis数、固体和气体热导率的比值、填充床孔隙率以及量纲1燃烧波波速的综合函数。通过对量纲1焓方程4个源项的分析表明,修正的Lewis数主导源项的大小并决定焓值的分布。     

Distinctive burning characteristics of carbon particles

Several reaction mechanisms exist in the literature for the gasification of char/carbon particles. The general procedure for modelling particle gasification is to assume a mechanism of combustion, obtain the theoretical burning rate and particle temperature, and then validate the mechanism by comparing the results with experimental data. The present work shows that the burning rate and particle temperature are independent of heterogeneous and homogeneous reaction mechanisms and their rate kinetics as long as the oxygen (O₂) and carbon dioxide (CO₂) concentrations at the burning surface are low compared to carbon monoxide (CO) concentration. Experimental data are cited which are in agreement with the model's predictions for both particle temperatures and burning times. The relevance of the results to industrial combustors is discussed.     

燃煤超细颗粒物团聚促进机制的实验研究

煤粉燃烧排放大量超细颗粒物对大气环境和人类健康造成了严重的危害。提出一种燃后区烟尘团聚促进新方法，通过在燃后区烟道喷射团聚促进剂，利用絮凝理论，增加超细颗粒之间的液桥力和固桥力，促使超细颗粒物团聚长大，进而被现有除尘装置捕获，减少超细颗粒的排放。在自行设计搭建的团聚实验台上进行了系统的实验研究，分析了团聚促进剂溶液的pH值、流量、浓度，团聚室的温度以及模拟烟气中粉尘浓度等因素对超细颗粒物团聚效率的影响。结果表明，化学团聚对于超细颗粒物的脱除具有显著的作用，喷入团聚促进剂后的烟尘排放浓度远比无团聚和喷水的情况要低；团聚促进剂的高分子链对超细颗粒的吸附絮凝作用促进超细颗粒团聚，显著降低烟尘排放浓度，实现超细颗粒物经济高效脱除，达到国家新的烟尘排放标准。     

Kinetics of redox reactions of ilmenite for chemical-looping combustion

The objective of this study was to establish the kinetic of both reduction and oxidation reactions taking place in the chemical-looping combustion (CLC) process using ilmenite as an oxygen carrier. Because of the benefits of using of pre-oxidized ilmenite and the activation of the ilmenite during the redox cycles, the reactivity of both the pre-oxidized and activated ilmenite was analyzed. The experimental tests were carried out in a thermogravimetric analyzer (TGA), using H₂, CO or CH₄ as reducing gases, and O₂ for the oxidation step. Thus, the reactivity with the main reacting gases was analyzed when natural gas, syngas or coal are used as fuels in a CLC system. The changing grain size model (CGSM) was used to predict the evolution with time of the solid conversion and to determine the kinetic parameters. In most cases, the reaction was controlled by chemical reaction in the grain boundary. In addition, to predict the behaviour of the oxidation during the first redox cycle of pre-oxidized ilmenite, a mixed resistance between chemical reaction and diffusion in the solid product was needed. The kinetic parameters of both reduction and oxidation reactions of the pre-oxidized and activated ilmenite were established. The reaction order for the main part of the reduction reactions of pre-oxidized and activated ilmenite with H₂, CO, CH₄ and O₂ was n=1, being different (n=0.8) for the reaction of activated ilmenite with CO. Activation energies from 109 to 165 kJ mol⁻¹ for pre-oxidized ilmenite and from 65 to 135 kJ mol⁻¹ for activated ilmenite were found for the different reactions with H₂, CO and CH₄. For the oxidation reaction activation energies found were lower, 11 kJ mol⁻¹ for pre-oxidized and 25 kJ mol⁻¹ for activated ilmenite.Finally, simplified models of the fuel and air reactors were used to do an assessment of the use of ilmenite as an oxygen carrier in a CLC system. The reactor models use the reaction model in the particle and the kinetic parameters obtained in this work. Taking into account for its oxygen transport capacity, the moderated solids inventory and the low cost of the material, ilmenite presents a competitive performance against synthetic oxygen carriers when coal or syngas are used as fuel.