Gasification of porous carbon particle by steam

Diffusive-kinetic model of porous carbon particle gasification by steam is developed. The model considers the processes of heat and mass transfer both inside the porous particle and above it. Heat losses by radiation to the particle from furnace wall are taken into account. Heterogeneous reactions of carbon with steam and carbon with carbon dioxide and homogeneous reaction of carbon monoxide with steam are considered. Pressure variation caused by gas mass increasing inside the particle is considered too. The analysis of the model inside the porous particle made possible determining the correlation between the reaction rate of carbon with steam and the reaction rate of carbon with carbon dioxide. The homogeneous reaction is supposed to be equilibrium. It is considered that the kinetics of heterogeneous reactions is known, than the equations of the model may be solved; and consequently the dependences of the particle gasification rate and the composition of the gasification products vs. composition, pressure and temperature of ambient gas and the internal surface of the porous particle are determined.     

An analysis of the transient combustion and burnout time of carbon particles

The various coupled and transient processes controlling the gasification mechanism and burnout time of carbon particles were analyzed, with emphasis on the influence of the initial particle size for the size range that is relevant to the firing of pulverized solid fuels. The formulation recognizes the suppression of the envelop gas-phase CO flame because of the small particle size, and allows for the three surface reactions of C + O₂, C + CO₂, and C + H₂O, as well as radiation heat transfer because of the potential high temperature attainable by the carbon particle. Results show that while the particle temperature continuously increases during the combustion of sufficiently large particles, the gasification actually consists of three phases: namely an initial particle heating period, an activation period for the surface reactions, and a diffusion-controlled, d²-law gasification period characterized by perpetually maximized surface reaction rates in spite of the continuously decreasing particle size. Radiation heat transfer is shown to have the same magnitude as those of reaction heat release and conduction, and actively affects the particle gasification response. For smaller particles, activation of the surface reactions is either substantially delayed subsequent to the initial heating period, or is completely suppressed, which respectively leads to either long burnout times or incomplete particle gasification. Influences due to the ambient oxygen concentration and the presence of CO₂ and H₂O as the oxidizer were also studied. Comparisons with literature experimental data show adequate agreement.     

Numerical Analysis of Unsteady Magneto-Biphase Williamson Fluid Flow with Time Dependent Magnetic Field

Numerical investigation of the dusty Williamson fluid with the dependency of time has been done in current disquisition. The flow of multiphase liquid/particle suspension saturating the medium is caused by stretching of porous surface. The influence of magnetic field and heat generation/absorption is observed. It is assumed that particle has a spherical shape and distributed uniformly in fluid matrix. The unsteady two-dimensional problems are modeled for both fluid and particle phase using conservation of mass, momentum and heat transfer. The finalized model generates the non-dimensioned parameters, namely Weissenberg number, unsteadiness parameter, magnetic parameter,heat generation/absorption parameter, Prandtl number, fluid particle interaction parameter, and mass concentration parameters. The numerical solution is obtained. Locality of skin friction and Nusselt number is deliberately focused to help of tables and graphs. While inferencing the current article it is clearly observed that increment of Williamson parameter, unsteadiness parameter, magnetic parameter, volume fraction parameter, and mass concentration parameter reduces the velocity profile of fluid and solid particles as well. And increment of Prandtl number, unsteadiness parameter,volume fraction parameter, and mass concentration parameter reduces the temperature profile of fluid and solid particles as well.     

Survey on nucleate pool boiling of nanofluids: the effect of particle size relative to roughness

Pool boiling heat transfer using nanofluids (which are suspensions of nano-sized particles in a base fluid) has been a subject of many investigations and incoherent results have been reported in literature regarding the same. In the past, experiments were conducted in nucleate pool boiling with varying parameters such as particle size, concentration, surface roughness etc. and all sort of results ranging from heat transfer enhancement, deterioration and no effect were reported. This work tries to segregate a survey on pool boiling of nanofluids with respect to particle concentration. This is due to the fact that a major drift in heat transfer behavior is observed at higher and lower particle concentration. But upon deep perusal it has been found that deterioration in heat transfer coefficient are mainly observed at higher particle concentrations (4–16% by weight) and enhancements mainly at lower particle concentrations (0.32–1.25% by weight). Moreover, the relative size of the particle with respect to the surface roughness of the heating surface seems to play an important role in understanding the boiling behaviour. Also, recent works have reported that change in ‘surface wetting’ of the heating surface due to nanofluids and the formation of a porous layer modifiying nucleation site density can be of importance in predicting nucleate pool boiling characteristics of nanofluids. In the present paper, attempts are made to make systematic analysis of results in literature and try to bring out a common understanding of the results in literature.     

Kinetics of coal char gasification in a carbon dioxide medium

Solid fuel samples with different carbon contents are gasified by successively subjecting to pyrolysis in argon and oxidation in carbon dioxide at various temperatures to determine the rate of the chemical reactions and the activation energy required for simulating and optimizing the operation of gas generators. The samples were prepared from bituminous coal, lignite, and anthracite of the Kuznetsk and Kansk-Achinsk coal basins. The gasification of coal char samples in a carbon dioxide medium at 900–1200°C is analyzed by thermogravimetry. The temperature dependences of the weight change rate and gasification time of coal char samples are measured and used to calculate the preexponential factor and activation energy of the carbon oxidation reaction. It is found that, with increasing oxidizing medium temperature from 900 to 1200°C, the gasification time of the coal char samples obtained from anthracite and bituminous coal decrease 8- and 22-fold, respectively. A physicomathematical model of coal char gasification in a fixed bed, with the oxidizing gas diffusing through the ash layer formed, is proposed.     

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.     

Modeling of radiative heat transfer in 3D complex boiler with non-gray sooting media

The radiative heat transfer problem is solved for 3D complex industrial boiler with five baffles containing a mixture of carbon dioxide and water vapor for non-uniform temperature fields. A numerical formulation using the FTn finite volume method coupled with the bounded high-order resolution CLAM scheme, the blocked-off-region procedure and the narrow-band based weighted-sum-of-gray-gases (WSGG) [Kim OJ, Song T-H. Data base of WSGGM-based spectral model for radiation properties of combustion products, JQSRT 2000; 64: 379-94] model is adapted. The effect of soot volumetric fraction, particle temperature and uniform particle concentration on the radiative heat flux and radiative heat source is investigated and discussed. Also the advantages, in non-gray media, of the FTnFVM compared to the classical FVM are highlighted.     

三维有序排列多孔介质对流换热的数值研究

利用CFD软件数值研究了颗粒三维有序堆积多孔介质的对流换热问题. 采用颗粒直径分别为14 mm,9.4 mm和7 mm的球形颗粒有序排列构成多孔介质骨架,在多孔骨架的上方有一恒热流密度的铜板. 采用流固耦合的方法研究了槽通道内温度分布和局部对流换热系数的分布以及对流换热的影响因素. 研究结果表明:热渗透的厚度和温度边界层的厚度在流动方向上逐渐增大,并且随流量的增加而减小;当骨架的导热系数比较高时,对流换热随颗粒直径的减小而略有增大;对流换热系数随聚丙烯酰胺溶液浓度的增大而减小,黏性耗散减弱了对流换热. 关键词： 多孔介质 温度场 局部对流换热系数 数值模拟     

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.     

超临界CO2在烧结多孔圆管中换热的实验研究

本文对超临界压力二氧化碳在烧结多孔介质的竖直圆管中的对流换热进行了实验研究。分析了入口温度超过准临界温度、颗粒直径为0．2-0．28 mm的多孔圆管中,压力、流量、热流密度以及流动方向对超临界二氧化碳对流换热的影响。结果表明,准临界点附近剧烈变物性的影响使得超临界二氧化碳在多孔结构中的对流换热非常复杂。对流换热随着温度远离准临界温度和热流密度的增加不断减弱;流量对对流换热的影响比较复杂。在准临界温度附近,浮升力对换热有一定的影响。     

高热流密度激光作用下含湿多孔材料的传热传质分析

建立了快速高热流密度激光作用下含湿多孔材料传热传质数学模型,并采用恰当的数值处理方法进行了数值计算.结果表明,快速高热流密度激光作用下含湿多孔材料的传热传质具有经典热质传递理论难以解释的超常效应-非傅立叶与非费克效应,且非费克效应比非傅立叶效应更明显,较高含湿量的材料在高热流密度激光作用下的非费克效应应予以足够重视.     

Preparation of activated carbons from used tyres by gasification with steam and carbon dioxide

Activated carbons were prepared from waste tyres by gasification with steam and carbon dioxide and their characteristics were investigated. A two-stage activation procedure (pyrolysis at 800 °C in N₂ atmosphere, followed by steam or carbon dioxide activation) was used for the production of activated samples. The effect of the activation temperature (750-900 °C) and the activation time (1-3 h) on the surface characteristics of the prepared carbon was investigated. Carbons produced to different degrees of burn-off were characterized by means of their nitrogen adsorption isotherms at 77 K. In both sets of experiments, the mesopore, micropore volume, and BET surface area increased almost linearly with the degree of activation. For burn-off values lower than 53%, the steam activation produced carbons with a narrower and more extensive microporosity and higher BET and external surface area than the carbon dioxide activation. As the activation proceeds (burn-off > 53%), a strong development of the mesoporosity in the carbons was observed and the micropores size distribution revealed broader micropores, that is, a more heterogeneous distribution.     

烧结/堆积床多孔介质流动沸腾换热实验研究

本文实验对比研究了0.3 mm、0.5mm、0.7 mm三种粒径的铜颗粒烧结与堆积床多孔介质中的流动沸腾换热,主要研究了入口流速、热流密度、加热方位及粒径对流动沸腾换热的影响,以及多孔介质中的沸腾滞后。实验结果表明:大入口流速、低热流密度、下方加热以及小粒径时加热壁面的过热度较低,即有利于沸腾换热;本实验所用烧结多孔介质壁面过热度高于堆积床多孔介质,其原因是内部含有闭孔。     

二维固定床双组分气体吸附的流动传热传质的数值模拟

本文建立固定床吸附过程中流动传热和传质模型,二氧化碳占20%的二氧化碳氮气双组分气体在吸附剂沸石13X上吸附过程进行了数值模拟,并对结果进行了分析.结果表明吸附过程中的温度变化是显著的,对吸附过程的传质影响巨大.通过改变吸附颗粒的大小和传热性能及改变流动过程,从而改变传热过程对吸附分离的影响是显著的.     

Heat transfer between tungsten surface and glow discharge plasmas in argon and CO<Subscript>2</Subscript>

A hot-filament method is used to study the heat transfer between tungsten surface and hollow-cathode glow discharge plasmas in argon and CO₂. The dependence of the electric power supplied to a tungsten wire on the discharge current is determined for argon and carbon dioxide in the temperature range between 1000 and 1700 K. A difference in heat transfer at the tungsten wire surface is found between experiments on argon and carbon dioxide. The difference is attributed to heterogeneous recombination in CO₂ plasma.     

Heat and mass transfer on MFD flow through porous media over an accelerating surface in the presence of suction and blowing

The heat and mass transfer of electrically conducting fluid through porous media over an accelerating surface subject both to power law surface temperature and power law heat flux variations with a temperature-dependent heat source in the presence of a transverse uniform magnetic field is studied. A series solution to the energy and species concentration equation in terms of Kummer’s function is studied. The effect of Prandtl number and Schmidt number is studied with the help of graphs.     

二氧化碳超临界流体的管内对流换热研究

随着自然工质ＣＯ２研究的进一步发展，系统中气体冷却器的换热问题越来越受到人们的重视，这是因为高的换热效率是提高系统ＣＯＰ的重要因素．本文首先阐明了超临界流体换热研究的处理原则和分类方法，并重新定义了ＣＯ２临界区范围．利用修改的 ＢＷＲ方程计算得出了 ＣＯ２临界区的物性变化规律，并分析了获得超临界换热关联式的理论求解方法．最后，建立了超临界ＣＯ２管内冷却过程的数学模型，为求解其换热规律提供了方法和依据．     

气流床气化炉壁面熔渣行为模拟

本文对气流床煤气化炉内高温合成气和壁面渣层的流动与传热传质过程进行了分析,建立了渣层流动、传热传质和相变数理模型,采用VOF方法对渣层和气体之间自由界面进行追踪,采用射线追踪方法(DTRM)计算辐射换热,采用焓法计算渣层相变,熔融态渣滴在渣层表面的沉积通过源项引入.应用所建立的数学模型对实验室规模的Texaco煤气化炉进行了模拟.结果表明:气化炉壁面换热系数从上到下先增加再减少最后再增加;炉膛温度升高,固态渣层厚度减少;壁面温度升高,固态渣层厚度减少.     

激光对碳纤维增强复合材料的热烧蚀数值模拟

高能激光对复合材料的辐照效应研究,可以拓展激光技术的应用范围。为了预测激光辐照下碳纤维增强复合材料的瞬态热响应,提出了一个简化计算模型。采用隐式有限体积方法求解控制方程,边界条件包括激光辐照加热、对流换热、辐射换热以及材料表面烧蚀。考虑了激光辐照过程中基体热分解、质量迁移、比热容变化情况。基于该烧蚀模型,预测了激光辐照下碳纤维增强复合材料的瞬态温度场和表面烧蚀速率,计算结果与文献试验数据一致。最后,通过修正烧蚀模型分析了高速气流剥蚀对激光辐照复合材料热效应的影响。     

Ultrahigh heat transfer enhancement using nano-porous layer

Heat transfer enhancement is one of the key issues of saving energies and compact designs for mechanical and chemical devices and plants. We discover an ultrahigh convective heat transfer performance compared to the well-known heat transfer correlations caused by a nano-particle porous surface: the maximum increase of heat transfer coefficient was around 180%. This nano-particle porous layer can be formed on the substrate surface by an etching with some acids or alkalis including around 100 nm size nano-particles made from copper oxide, carbon nano-tube and aluminium oxide. Moreover, we have done some experiments using a co-current flow heat exchanger consisting of hot and cold water-channels and obtained an ultrahigh heat transfer performance: over 200% increase compared to the conventional correlation. On the other hand, in order to theoretically investigate effects of nano-particle porous layer structures on the surface energy transfer, the energy transfer from fluid to the heat transfer surface is calculated by a classical molecular dynamics method. Energy transfer to the surface from the fluid strongly depends on the surface structures in nanometre scale that affect the static structure and dynamic behaviours of fluid molecules in the vicinity of the surface.