炉膛内煤焦颗粒燃烧速率的简易求法

以傅维镳教授提出的煤焦非均相反应动力学新思想为基础,将Fb数和Fz数引入计算,建立了炉膛的一维模型,推导出一种计算煤焦颗粒燃烧速率的简易表达式,采用划分网格的方式可将这种表达式应用在燃烧速率的求解过程中。     

Group combustion of char/carbon particles

Extensive literature exists for the experimental data on coal/char ignition and combustion. While most of the experiments are performed with a cloud or stream of particles, the theoretical modeling used to compare and interpret the experimental data is based on the individual particle combustion (IPC) model. As opposed to individual particle modeling, a group combustion (GC) theory is proposed for the combustion modeling of char/carbon particles. For a cloud of liquid drops, the group behavior implies the formation of a flame (group flame) around a large number of drops rather than a flame around each drop. More generally, the group behavior for a cloud of particles represents the change in the burning characteristics due to collective behavior of particles with or without a group flame. To gain a basic understanding of the group behavior, a model such as the analysis of a spherically symmetric cloud of particles burning in quiescent air is presented here. Each particle within the cloud produces CO, due to both the oxidation of C to CO and the reduction of CO₂ to CO which subsequently oxidizes to CO₂ in the homogeneous gas phase.

Generalized results for the burning rate and the flame structure are given as a function of group combustion number (G). Predicted results show unexpected results including the independence of the burning rate of CO kinetics. Quantitative results for both the cases of frozen and fast CO kinetics are given. There is a group flame for the case of fast CO kinetics. It is shown that the group flame occurs at G > 5 while for a cloud of liquid drops, the group flame occurs at G > 0.1. The higher critical group combustion number is attributed to the lower burning rate of particle inside the cloud compared to the burning rate of liquid drops inside the cloud. The results show that there exists mainly three modes of combustion: (i) Individual Particle Combustion (IPC, low G), (ii) Group Combustion (GC, intermediate G) and (iii) Sheath Combustion (SC, high G). Criteria are given for identifying the mode of combustion from the experimental conditions. The criteria and the establishment of modes of combustion are independent of the extent of CO kinetics. It is found that the experimental data, obtained with a stream of particles and commonly interpreted with the IPC model, indicate the combustion modes to vary from IPC to SC modes. These data are now reinterpreted with the group theory.     

Mutual effects of porosity and reactivity in char oxidation

The motivation for this review is the need to understand the interdependence of porous structure and reactivity of highly porous carbonaceous materials during oxidation. These materials can be oxidized in three regimes: regime I, kinetically controlled conditions; regime II, partial diffusion-controlled conditions; regime III, diffusion controlled conditions. Since the emphasis here is on the porous structure and its influence on reactivity, conditions where transport processes are dominant were not included for they mask the view of interest. Therefore, the review discusses only physicochemical processes occurring during oxidation of highly porous chars in regime I. Furthermore, reactivity is influenced by many factors, such as catalysis, volatile matter, and water content. To avoid the effect of these factors, highly porous synthetic chars with nothing but elemental carbon and residual hydrogen and oxygen was chosen. Mainly, we discuss a commercial product known as Spherocarb which consists of spherical particles with specific surface area of about 1000 m² g⁻¹ and porosity of about 0.6. These particles are well defined and reproducible in their properties. They serve well as model materials for various synthetic chars, coal chars, and other carbonaceous materials. The review presents in a systematic manner macroscopic properties and processes that shed light on different aspects of porosity and reactivity. These are presented both from experimental observation as well as modeling view. An attempt was made to present a porous structure model that can reconstruct all available experimental data on these particles during oxidation. In the review the following processes and properties are discussed: shrinkage, fragmentation, and porosity. All are directly connected to porous structure and reactivity.     

The combustion of large particles of char in bubbling fluidized beds: The dependence of Sherwood number and the rate of burning on particle diameter

Particles of char derived from a variety of fuels (e.g., biomass, sewage sludge, coal, or graphite), with diameters in excess of , burn in fluidized bed combustors containing smaller particles of, e.g., sand, such that the rate is controlled by the diffusion both of O₂ to the burning solid and of the products CO and CO₂ away from it into the particulate phase. It is therefore important to characterize these mass transfer processes accurately. Measurements of the burning rate of char particles made from sewage sludge suggest that the Sherwood number, Sh, increases linearly with the diameter of the fuel particle, dchar (for ). This linear dependence of Sh on dchar is expected from the basic equation Sh=2εmf(1+dchar/2δdiff)/τ, provided the thickness of the boundary layer for mass transfer, δdiff, is constant in the region of interest (). Such a dependence is not seen in the empirical equations currently used and based on the Frössling expression. It is found here that for chars made from sewage sludge (for ), the thickness of the boundary layer for mass transfer in a fluidized bed, δdiff, is less than that predicted by empirical correlations based on the Frössling expression. In fact, δdiff is not more than the diameter of the fluidized sand particles. Finally, the experiments in this study indicate that models based on surface renewal theory should be rejected for a fluidized bed, because they give unrealistically short contact times for packets of fluidized particles at the surface of a burning sphere. The result is the new correlation

     

反应温度对木屑炭水蒸气气化产物的影响

以木屑炭为原料,在固定床反应器中进行了水蒸气气化试验。试验在水蒸气流量为0.854 g/min,温度为800～1 000℃条件下,反应15 min。主要考查气化反应温度对碳转化率、合成气产率、燃气热值及燃气组成的影响。研究结果表明,在高温条件下木屑炭与水蒸气具有很高的反应活性,燃气产率为0.9～3 L/g;在气化温度为1 000℃时,碳转化率最高达到80%;燃气热值为8.9～9.4 MJ/m3,合成气(H2+CO)比例为68%～79%,H2/CO为4.02～6.32。     

Catalytic effect of KOH on the reaction of NO with char

Experimental results are presented on the reduction of NO by char particles prepared from a Chinese low-volatile coal. The experiment was conducted in a drop-tube furnace at 1173 to 1323 K. Kinetic parameters of the global reaction of NO with char were determined, and the effects on these kinetic parameters of adding the catalyst KOH to the char particles and also of [O₂] in the flue gas were studied. The results show that KOH can increase the frequency factor and reduce the activation energy of the reduction of NO by char. However, the benefit of increasing the KOH content in char particles decreases when there is more than 1.0 wt % of the catalyst in the char. The activation energy of the global NO-char reaction is shown to be independent of [O₂], but the frequency factor strongly depends on the equivalence ratio under oxygen-lean conditions.     

Influence of pyrolysis pressure on structure and combustion reactivity of Zhundong demineralized coal char

The investigation on evolution of coal char structure during pressurized pyrolysis can reveal the combustion reactivity of coal char in thermal utilization at elevated pressure. In this study, Zhundong subbituminous coal was demineralized and a pressurized drop tube reactor (PDTR) was used to prepare coal char under different temperature and pressure conditions. The physicochemical structures of raw and demineralized coal chars were characterized by the application of nitrogen adsorption analyzer, automatic mercury porosimeter, and Fourier transform infrared spectroscopy (FTIR). The change mechanism of char infrared structure with pyrolysis pressure is revealed on the molecular level in this paper. The results show that the N₂ adsorption quantity of raw coal char increases with the increase of pyrolysis temperature, while that of demineralized coal char decreases. Because of the difference in molecular volume and steric hindrance between aliphatic and aromatic structure in char, the increasing pressure has less inhibition effect on the escape of the former than the latter. With the increase of pyrolysis pressure, the combustion reactivity of char is related to the infrared structure at 700 and 800 °C while to macropore structure at 900 and 1000 °C.     

Modeling and countermeasures of combustion characteristics of char particles in CFBC

INTRAODUCTIONAsahigh-efficiencyandcleancoalcombustiontechnology,circulatingfluidizedbed(CFB)combustiontechnologyachievesrapiddevelopmentinChinaforburningvariouslow--gradefuels.ThescalerupofCFBboilersbecomesakeypointconcernedbytheCFBboilerdesigners.At...     

燃烧反应干熄炉内传热模拟的浅析

建立了干熄炉内焦炭层床循环气体的传热模型,并耦合燃烧反应计算,分析了干熄炉冷却室内的温度变化,计算结果与实测数据基本一致。本文还进一步分析了化学反应活化能、风料比和燃烧反应对干熄炉内传热的影响,以期为干熄炉的优化设计提供一定参考。     

Entropy generation during the quasi-steady burning of spherical fuel particles

Entropy generation during the quasi-steady combustion of spherical liquid fuel particles has been presented in detail. The effects of freestream velocity, particle diameter, ambient temperature and gravity, on the entropy generation rate, have been discussed in detail. In the range of sub-critical freestream velocity, where an envelope flame is present, the entropy generation rate presents a minimum value. At a critical velocity, where the flame transition occurs, the entropy generation rate reaches a maximum value. Flame transition significantly affects the entropy generation rate, which suffers a sharp decrease in its value after the transition. Heat transfer and chemical reaction contribute almost equally to the total entropy generation rate. When normal gravity is considered in an upward flow configuration, there is an increase in the entropy generation rate as compared to the zero gravity case. The effect of gravity poses a complex variation pattern in the entropy generation rate, for a downward flow configuration. The entropy generation rate decreases with increasing ambient temperature. The entropy generation rate increases with the particle diameter. A correlation has been presented for the non-dimensional entropy generation number as a function of Froude number.     

不同热解温度下落叶松半焦特性的演变规律

为探究热解温度对生物质半焦特性的影响规律,文章以落叶松为原料,采用管式电阻炉制取200～1 000℃的热解半焦,利用元素分析、XRD、BET、SEM等测试手段,结合碳-氢-氧相图及Scherrer方程,深入分析了热解温度对生物焦元素组成、石墨化程度、孔隙结构及表观形貌的影响。结果表明:热解温度升高,热解半焦的H/C,O/C原子比减小,芳构化程度加深,碳微晶结构更趋于有序化,片层状碳骨架结构逐渐凸显,石墨化程度增加;400℃下的半焦比表面积最高,微孔对比表面积的贡献大;300℃和500℃是热解半焦结构发生明显变化的两个特殊的温度点。     

Effect of temperature fluctuations of fluid on thermal stability of particles with exothermic chemical reaction

Effect of temperature fluctuations in liquid phase on heat explosion of individual particles with internal heat release is investigated. Heat emission due to chemical reaction inside particles describes as Arrhenius low. Fluctuations of temperature of carrier phase are modeled as random statistically stationary Gaussian process uniformly distributed in space. Method of functional derivations is used for obtaining closed equation for probability density function of temperature fluctuations of particles. Closed system of moments of particles temperature fluctuations is derived. Reasons leading to loss of thermal stability of particles are studied by numerical and analytical methods. Comparison between various scenarios of heat explosion of particles with temperature fluctuations is carried out. On the base of reverse Kolmogorov equation for probability density function of transition new effect of stochastic drift of particle temperature to the critical value of heat explosion was discovered. Results of numerical integration of the closed equation for probability density function of temperature fluctuations of particles are also presented.     

Effect of multiple particle interactions on burning droplets

The interactions between burning droplets are analyzed on the basis of the quasi-steady assumption. A generalized treatment for burning rates of droplets in an array has been developed using a modified Laplace equation with point sources. This treatment is applied to two droplets of different sizes, as well as finite arrays containing up to eight symmetrically arranged monodisperse droplets. Particle interactions are shown to be a function of particle size, number density, and geometry of the array. Results are presented in terms of correction factors from which multiple particle burning rates can be calculated from single particle burning rates. The correction factors are shown to be in close agreement with results available in the literature.     

Mechanism and kinetics of sodium release from brown coal char particles during combustion

A model for the release of sodium during the combustion of single Loy Yang brown coal char particles is presented. The model is combined with further analysis of recently published measurements of the release of sodium from single brown coal particles burning in a flat flame to estimate the rate constant for sodium release as a function of burnout time for these experiments. A char combustion and heat transfer model is also used to predict the char burnout behaviour and surface temperature of the particle as a function of time during combustion for each of the experiments. By combining the predicted time–temperature history of the particles with the estimated rate constant for sodium release, an Arrhenius expression for the release of sodium during char combustion is determined as:A full mechanism for sodium release during the various stages of coal combustion is also proposed. Utilising the proposed mechanism, the rate-determining step for sodium release during char combustion is proposed to be the formation of a reduced form of sodium in the char which subsequently leads to the rapid loss of sodium from the particle.     

Kinetic study for the reduction of residual char particles using oxygen and air

This study characterizes the chemical kinetics for the reduction and elimination of char particles using air and pure oxygen as the oxidants. Commercial carbon black was used as char for the experimental studies reported here. Different oxidant injection flow rates have been examined at various injection temperatures between 400 and 700 °C under atmospheric pressure conditions to obtain the intrinsic kinetic parameters. The char conversion rates have been measured in a laminar flow hot stream using 0.2 g and 0.3 g mass of initial char samples. The kinetic parameters are obtained by fitting the available experimental data into the derived one-film model. The proposed one-film model is compared and analyzed for consistency and reliability for the calculated intrinsic kinetic parameters.     

Physicochemical structure characteristics and intrinsic reactivity of demineralized coal char rapidly pyrolyzed at elevated pressure

This study aims to investigate the effect of pyrolysis pressure on the physical and chemical structure characteristics and reactivity of subbituminous demineralized coal char. The pyrolysis experiments were studied under different pressures using a pressurized drop tube reactor (PDTR). Structural properties of coal chars were investigated by the application of scanning electron microscopy (SEM), nitrogen adsorption analyzer, automatic mercury porosimeter, and Raman spectroscopy, respectively. The Random Pore Model was used to determine kinetic parameters and intrinsic reactivity of chars. The specific pore volume of chars pyrolyzed at 900–1000 °C increased first and then decreased with pyrolysis pressure increasing, and the maximum value of the specific pore volume of chars occurred at 1.0 MPa. The degree of graphitization of chars deepened with the increase of temperature or pressure. Intrinsic activation energy of char-O₂ was within the range of 126–134 kJ/mol. The intrinsic reactivity of char-O₂ reaction showed strong correlation the Raman parameters with the change of pyrolysis conditions, and it suggested that the intrinsic reactivity of char-O₂ reaction was mainly affected by aromatic ring structures rather than pore structures.     

Impacts of char structure evolution and inherent alkali and alkaline earth metallic species catalysis on reactivity during the coal char gasification with CO2/H2O

In this work, we studied the effects of char structural evolution and alkali and alkaline earth metallic species (AAEMs) catalysis on the reactivity during the char gasification with CO₂, H₂O, and their mixture. The gasified chars with different carbon conversion levels were prepared, and their physicochemical structures were characterized via nitrogen adsorption and FT‐Raman techniques. The concentrations of AAEMs in different modes were obtained by the sequential chemical extraction method. The reactivities of the raw and gasified chars were analyzed by the thermogravimetric analysis. The gasification atmospheres had varied effects on the physicochemical structure of coal char. The gasified char obtained in the CO₂ atmosphere had a lower aromatic condensation degree compared with that obtained in the H₂O atmosphere, irrespective of the temperature. The impact of the atmospheres on the specific surface area of the char varied with the temperature because H₂O and CO₂ have different routes of development of pore structure with coal char. A large specific surface area facilitates the exposure and dispersion of more AAEMs on the surface of the channel, which is conducive to their contact with the gasification agent to play the catalytic role. Thus, the reactivity of the gasified char is well correlated with its specific surface area at different gasification temperatures. In the absence of AAEMs, the chemical structure of coal char becomes the dominant factor affecting the reactivity.