考虑热解热的大颗粒煤热解过程计算模拟

流化床中大颗粒煤的热解与粉煤炉中偻煤的热解有很大差别，其过程由煤粒内部的传热和化学动力学联合控制，除必须考虑热解热效应对热解过程的影响外，还应考虑温度对热解动力学的影响。本文应用双方程化学动力学模型，考虑了热解热效应，对经床大颗粒煤的热解规律进行计算模拟，并与实验结果比较，取得较好结果。     

A study on devolatilization of large coal particles

A generalized devolatilization model for large coal particles is proposed in this paper. The difference between the present model and previous ones is that the equivalent activation energy, E, and the equivalent frequency factor, K, of coal particle devolatilization are independent of coal type, and depend only on the final temperature, T_∞, of the coal particles. The relationship between E/K and T_∞ has been obtained and is found to be universal for all coal types. If the final volatile yield, V_∞, is determined, the devolatilization processes of a large coal particle for any kind of coal can be predicted by using the present model. This model has been tested with five Chinese coals with quite different properties, diameters ranging from 3 mm to 9 mm, and temperatures ranging from 1173K to 1733K. The calculated results agree with experimental data.     

A new framework for modeling coal devolatilization and combustion in boiler furnaces

This paper presents a new framework for the modeling of coal‐fired boiler furnaces. The input required for the model is the ultimate analysis of a coal sample. The model accounts for devolatilization followed by gas‐phase combustion. The devolatilization model used in this work is taken from published literature with slight modifications to match the numerical predictions with experimental measurements. This work also develops a reactor network model for simulating the performance of boiler furnaces. For the seamless integration of kinetic models of coal devolatilization and combustion with furnace numerical model, the thermochemistry data of several hypothetical and intermediate species involved in devolatilization chemistry are evaluated in the form of 14 coefficient National Aeronautics and Space Administration polynomials. The capability of the model for predicting the furnace temperature and product composition is demonstrated by simulating a single‐zone model. Copyright © 2017 John Wiley & Sons, Ltd.     

燃煤链条炉排工业锅炉燃烧数值模型研究

针对燃煤链条炉排工业锅炉的燃烧过程中床层内部存在复杂的传热、传质及物理化学反应过程等特点开发了三维床层和炉膛耦合的燃烧数值计算模型,模型包含了煤燃烧过程中水分蒸发、挥发析出、气相反应、焦炭燃烧和传热传质等基本的化学物理过程,同时考虑了大粒径煤颗粒内部的非等温传热特性,并通过实验测试与数值模拟对数值模型进行校核,实验结果与模型计算吻合得较好,从而验证了计算模型的准确性。燃煤链条炉排工业锅炉燃烧数值模型的建立为实现燃煤工业锅炉的优化设计和运行指导提供了先进的设计手段和理论支持。     

A numerical simulation of pulverized coal combustion employing a tabulated-devolatilization-process model (TDP model)

A new coal devolatilization model employing a tabulated-devolatilization-process model (TDP model) is developed, and its validity is investigated by performing a numerical simulation of a pulverized coal combustion field formed by an industrial low-NO_x burner in a 100 kg-coal/h test furnace. The predicted characteristics of the pulverized coal combustion field obtained from the simulation employing the TDP model are compared with those employing the conventional devolatilization model, those employing the two competing reaction rate model, and the experiments. The results show that drastic differences in the gas flow patterns and coal particle behavior appear between simulations. In particular, the recirculation flow behavior is strongly affected by the difference in the coal devolatilization model because of the difference in the volatile matter evolution rate. The TDP model captures the observed behavior of the coal particles in the experiment better than the other models. Although it is considered that by adjusting the devolatilization parameters the prediction similar to the TDP model is also possible by the other models, appropriate devolatilization parameters are automatically set to particles depending on the particle heating rate without trial–error method by employing the TDP model.     

Spouted and spout-fluid bed combustors 1: Devolatilization and combustion of coal volatiles

The devolatilization and volatile combustion of a single coal particle in spouted and spout-fluid beds have been studied. The results showed that the flame extinction time increases with the particle diameter, and decreases with the bed temperature. When the bed temperature and the air flow rate were fixed, the operation modes (spouted or spout-fluid bed) showed less effect on the mean flame extinction time. A mathematical model of the spouted bed mode for preignition and postignition periods has also been developed assuming the devolatilization rate to be controlled by heat transfer and multireaction pyrolysis kinetics based on volatile products. Ignition, heat transfer back from the volatile flame to the particle surface, variation in flame temperature, and the hydrodynamics of SB are taken into account. The model predictions, with some adjusting parameters, were in good agreement with experimental results.     

Combustion of millimeter sized coal particles in convective flow

Continuous mass versus time data for single coal particles from 5.3 to 9.9 mm with gas temperatures of 900–1200K, Reynolds numbers of 63–126, and oxygen concentrations of 4.5–21 % are presented. Devolatilization and char burn models for millimeter sized particles are formulated and compared with the experimental results. The devolatilization rate is most sensitive to particle size and gas temperature. The char reactivity depends on initial size, Reynolds number, and oxygen concentration. The devolatilization rate agrees with the model of Anthony and Howard when volatile yields are provided from experimental data. The char burning rate follows a diffusion controlled shrinking sphere model when a diffusion screening factor is included.     

煤催化热解过程动力学的研究

由ＷＲＴ－２型热天平测定上海市工业用煤在添加不同类型催化剂（或氧化剂）时的热解曲线，由此确定不同催化剂添加量时的热解动力学参数（表观活化能Ｅ，频率因子Ｋ０，反应级数ｎ），并根据动力学参数的变化规律建立催化热解模型。试验结果表明：催化剂可促进煤热解过程的发生，但不能使挥发份产量提高；氧化剂的效果不如催化剂；热解过程中，热重微分曲线上的峰值点所对应的温度Ｔｍａｘ可作为判断催化剂效能的特征参数；Ｔｍａｘ     

Kinetics of devolatilization and oxidation of a pulverized biomass in an entrained flow reactor under realistic combustion conditions

In this paper the results of a complete set of devolatilization and combustion experiments performed with pulverized (∼500 μm) biomass in an entrained flow reactor under realistic combustion conditions are presented. The data obtained are used to derive the kinetic parameters that best fit the observed behaviors, according to a simple model of particle combustion (one-step devolatilization, apparent oxidation kinetics, thermally thin particles). The model is found to adequately reproduce the experimental trends regarding both volatile release and char oxidation rates for the range of particle sizes and combustion conditions explored. The experimental and numerical procedures, similar to those recently proposed for the combustion of pulverized coal [J. Ballester, S. Jiménez, Combust. Flame 142 (2005) 210-222], have been designed to derive the parameters required for the analysis of biomass combustion in practical pulverized fuel configurations and allow a reliable characterization of any finely pulverized biomass. Additionally, the results of a limited study on the release rate of nitrogen from the biomass particle along combustion are shown.     

气流温度脉动对煤粉颗粒瞬时热解挥发速率的影响

研究了热气流中气相温度脉动对煤粉颗粒瞬时热解挥发速率的影响．计算结果表明,在不同的气相平均温度条件下,不同粒径煤粉颗粒的瞬时热解挥发速率均受到气相温度脉动的影响．考虑气相温度脉动与未考虑气相温度脉动相比,煤粉颗粒的瞬时热解挥发速率有明显的不同,其峰值增加的幅度超过了谷值降低的幅度．因而气相温度脉动会导致煤粉颗粒热解与挥发分释放的加快．     

A method of determining thermal parameters of granular coal particles during devolatilization

Fluidized bed combustion offers great potential for the utilization of high-sulphur coal and low-grade coal in an environmentally acceptable manner. Utilization of fluidized bed technology, especially for the combustion of low quality lignites, enables pollutant emission control as well as efficient combustion. The most important stage during the combustion of coal particles is devolatilization, in which various factors such as heat transfer from the surroundings to the particle surface, heat conduction within the particle, the chemicals involved, the kinetics and the transport of volatile compounds within the particle play significant roles. The heat transfer coefficient, thermal diffusivity, thermal conductivity, heating coefficient and lag factor are the most significant thermal parameters in this process. In this study, a 1-D transient heat transfer analysis is carried out for a granular coal particle during devolatilization in a fluidized bed. The particle is idealized as a spherical solid body. Models are developed to determine the thermal parameters of such particles, and are verified using experimental centre temperatures of a 10 mm granular coal particle subjected to devolatilization at a medium temperature of 960 K. The data are taken from the literature. The results show that the thermal parameters determined here are in good agreement with experimental findings.     

A comparison of various models in predicting ignition delay in single-particle coal combustion

In this paper, individual coal particle combustion under laminar conditions is simulated using models with various levels of complexity for the particle and gas phase chemical kinetics. The mass, momentum and energy governing equations are fully coupled between the particle and the gas phase. In the gas phase, detailed chemical kinetics based on GRI3.0 and infinitely-fast chemistry are considered and compared. For the particle phase, models for vaporization, devolatilization and char oxidation/gasification are considered, and the Kobayashi–Sarofim devolatilization model is compared to the Chemical Percolation Devolatilization (CPD) model. Ignition delay is used as a quantitative metric to compare the simulation prediction with experimental data, with careful attention given to the definition of ignition delay in the simulations. The effects of particle size, coal type and gas-phase temperature on the ignition delay are studied and compared with experimental data.     

A review on devolatilization of coal in fluidized bed

Devolatilization is an important step in fluidized bed combustion and gasification of coal. ‘Devolatilization’ is a general term that signifies the removal of volatile matters from the coal matrix. It is an extremely important step because the combustion of volatile matter can account for 50% of the specific energy of fluidized bed combustion of a high‐volatile coal. Significant insights into the complex physicochemical phenomena that occur during devolatilization have been obtained in the recent years. This review focuses on the devolatilization of coal in an inert gas, air, and oxygen‐enriched air, with emphasis on the effects of the operating parameters (e.g. temperature, heating rate, pressure, and gas velocity) on the yield of volatile matter. Particle size, oxygen content of the fluidizing gas, volatile content of coal and specific heat are some of the other important parameters for the devolatilization of coal. This review also explains the development and application of structural and empirical models. The structural models (e.g. FG‐DVC and CPD models) are fairly complex. However, they can accurately predict the yields of gas and tar. It is observed from the review of the literature that the mechanism of coal devolatilization needs further study. Although the shrinking‐core model can describe the devolatilization in the beginning and toward the end of the process, major deviations are often observed. The economic studies reveal that the capital cost of fluidized bed combustion reduces upon doubling the capacity. Some problems associated with bubbling fluidized bed combustion (e.g. the increase in freeboard temperature) have been explained with the present knowledge of devolatilization. Copyright © 2011 John Wiley & Sons, Ltd.     

Formation of the structure of chars during devolatilization of pulverized coal and its thermoproperties: A review

The paper provides an overview of current studies on the behaviour of coal during devolatilization, especially the experimental studies and modelling efforts on the formation of char structure of bituminous coals in the open literature. Coal is the most abundant fossil fuel in the world. It dominates the energy supply in the future and plays an increasing role particularly in the developing countries. Coal utilization processes such as combustion or gasification generally involve several steps: i.e., the devolatilization of organic materials, homogeneous reactions of volatile matter with the reactant gases and heterogeneous reactions of chars with the reactant gases. The devolatilization process exerts its influence throughout the life of the solid particles from the injection to the burnout, therefore is the most important step which needs to be understood. When volatile matter is generated, the physical structure of a char changes significantly during the devolatilization, some accompanying a particle's swelling. The complexity of a char's structure lies in the facts that the structure of a char itself is highly heterogenous inside an individual particle and between different particles and the chemistry of a char is strongly dependent on the raw coal properties. A char's structure is strongly dependent on the heating conditions such as temperature, heating rate and pressure. Understanding the swelling of coal and the formation of char's pore structure during the devolatilization of pulverized coal is essential to the development of advanced coal utilization technologies. During combustion and gasification of pulverized coal, the behaviour of individual particles differs markedly due to the variation of their maceral composition. Particles with different maceral constituents generate different types of char structure. The structure of a char has a significant impact on its subsequent heterogeneous reactions and ash formation. The review also covers the most recent studies carried out by the authors, including the experimental observations of the thermoplastic behaviour of individual coal particles from the density fractions using a single-particle reactor, the experimental analysis on chars prepared in a drop tube furnace using the density-separated coal samples, the development of a mathematical model for the formation of char's pore structure based on a simplified multi-bubble mechanism and the investigation on the effect of pressure on char formation in a pressurized entrained-flow reactor.     

旋风炉内气相燃烧及两相流动的数值模拟

在有反应两相流动及煤粉燃烧的全双流体模型（ＰＴＦ模型,ｐｕｒｅ ｔｗｏ－ｆｌｕｉｄ ｍｏｄｅｌ）基础上,采用修正的ｋ－ε－ｋｐ两相湍流模型,对旋风炉内的湍流气相燃烧（甲烷和一氧化碳的燃烧）及在气相燃烧条件下的两相流动进行了数值模拟研究,模拟结果表明,在有燃烧的情况下,在旋风炉的底部存在近壁回流区,该回流区有利于火焰稳定,气粒两相切向速度分布具有类似的Ｒａｎｋｉｎｅ涡结构,该研究为煤粉燃烧的数值模拟     

A model for the devolatilization of a coal particle sufficiently large to be controlled by heat transfer

This study follows previous experimental work showing that the shrinking-core model applies to the pyrolysis (i.e., heating in the absence of oxygen) of particles (diam. ≈ 14 mm) of a bituminous coal or a lignite in a fluidized bed at 700-950 °C. These experimental facts are in accord with the production of volatile matter being endothermic and not thermoneutral, as often assumed. Also, the rate at which volatile matter is produced in the presence of oxygen (i.e., devolatilization) or in its absence (pyrolysis) is demonstrated here to be controlled not at all by mass transfer, but by heat conduction to a moving reaction front inside a coal particle, provided its diameter exceeds ∼3 mm. The resulting steady-state model of devolatilization indicates that six dimensionless groups are required to describe the rate of

(I)     

Study of devolatilization during chemical looping combustion of large coal and biomass particles

Chemical Looping Combustion (CLC) is one of the emerging technologies for carbon capture, with less energy penalty. The present way of using pulverized coals in a fluidized bed (FB)-CLC have limitations like loss of unconverted char and gaseous combustibles, which could be mitigated by use of coarser fuel particles. Devolatilization time is a critical input for the effective design of FB-CLC systems, primarily when large fuel particles are used. The present study investigates the devolatilization time and the char yield of three coals of two shapes, namely, two high ash Indian coals and a low ash Indonesian coal and a wood (Casuarina equisetifolia) in the size range of +8–25 mm, at different fuel reactor temperatures (800–950 °C) of a hematite based CLC unit. The devolatilization times of single fuel particles during CLC are determined using a visual method called ‘Color Indistinction Method’. Indonesian coal has the longest devolatilization time among the fuels, and biomass has the least. Increasing the bed temperature enhances the rate of volatile release, whereas this effect is less pronounced in larger particles. Devolatilization of Indonesian coal is found to be strongly influenced by the changes in operating conditions. With the decrease in sphericity, a maximum of 56% reduction in devolatilization time is observed for the +20–25 mm slender particles of Indonesian coals when compared to the near-round particles. The maximum average char yields at the end of the devolatilization phase for coal and biomass are about 55–76% and 16% respectively. Char yield in coal particles increases with an increase in particle size, whereas biomass particles show relatively consistent yield across all experimental conditions. Increase in bed temperature reduces the char yields of coal up to 12% and in biomass up to 30%. High volatile Indian coal is the most influenced fuel by the changes in fuels shape. A correlation for determining devolatilization time under CLC environment is presented, and it successfully fits most of the experimental values within ±20% deviation for coals (R² = 0.95) and within ±15% deviation for biomass (R² = 0.97).