The effect of air preheating on the combustion of solid fuels on a grate

Combustion of solid fuels on a grate is widely used. Mostly, the combustion behaviour is explained by the classical theory of Rogers. However, that theory cannot explain the combustion process when primary air preheating is applied. Solid fuel grate combustion is studied by experiments in a pot furnace. Experiments with and without primary air heating are described. These are compared with conclusions learnt from real plant experiments. It was found that the pot furnace experiments have a limited value in explaining the combustion behaviour of solid fuels on a grate. In order to be able to explain the results from practice an quantitative extension of Rogers' theory for the case with air preheating is presented.     

Numerical simulation of grate firing systems using a coupled CFD/discrete element method (DEM)

The objective of this study is to simulate the motion and chemical conversion of solid fuels in a packed bed moving on a forward acting grate. The approach considers a packed bed to be composed of a large number of individual (currently spherical) particles, which have different properties and sizes. Each of these particles undergoes a sequence of processes such as heating, drying, pyrolysis and char combustion. Chemical species and energy are released from the solid phase mix and react in the cross flowing gas phase.This approach for the simulation of a reacting moving bed on a forward acting grate is coupled to the numerical simulation of the reacting flow in the combustion chamber above the grate. For the reacting granular flow the 2D-version of the discrete element code of the Department of Energy Plant Technology (LEAT) is used. For the reacting flow in the combustion chamber the 3D-CFD code ANSYS-CFX has been applied. The concept of discrete element modelling with special emphasis on the multi-physics coupling and the coupling of DEM with ANSYS-CFX is introduced, and the necessary framework is presented. Results are shown for the fluid flow in the combustion chamber and the solid flow on the grate.     

Discrete element analysis of experiments on mixing and stoking of monodisperse spheres on a grate

Understanding the details of the mixing and stoking process on grate firing systems is crucial for the optimization of the combustion process in waste or biomass incineration plants. The Discrete Element Method (DEM) can help to obtain further information on the mixing process within a bed of fuel particles. Especially the influence of a change in operational parameters can be examined avoiding large experimental effort. In the current paper five simulations for a generic grate are compared with the corresponding experiments. The experiments were carried out throughout an anterior parameter study on mixing and stoking on a grate [Sudbrock F.; Simsek E.; Wirtz S.; Scherer V.: “An experimental analysis of the influence of operational parameters on mixing and stoking of a monodisperse granulate on a grate”, Powder Technology 198, Issue 1, 29-37, 2010] [19]. The system considered is equipped with vertically moving bars which induce stoking. In a first approach monodisperse plastic spheres are used. The grate is encased by a transparent polycarbonate housing which provides optical access to the movement of the particles in the wall planes. The mixing process is measured and quantified by image analysis of the front wall of the grate. The mixing behaviour of the particle assembly observed in experiments and simulation appears to be very similar indicating that DEM is able to predict the particle mixing in the bed. In order to quantify the visual observations the mixing behaviour has been evaluated by different mixing parameters. They are compared in dependence of the number of strokes of the grate bars. A good agreement between measurements and simulations could be observed.     

An experimental analysis on mixing and stoking of monodisperse spheres on a grate

A better understanding of the mixing and stoking process is crucial for an optimization of the combustion process on grate firing systems. Thus experimental studies were carried out to analyse the response of a particle assembly on varying grate operational parameters. To reduce the number of variables which affect the system a generic grate design was chosen and a material of monodisperse spheres was selected. The grate system applied uses vertically moving parallel bars to induce mixing. Different patterns of bar motion were created by linking the bars in groups of uniform movement. A transparent polycarbonate side wall gives optical access to front layer of spheres. The mixing process was measured and quantified by image analysis of this visible layer. When applying a constant number of bar strokes it is found that the mixing performance is independent of the bar velocity. However, mixing performance increases nearly linearly with the stroke length. It turned out that specific “movement patterns” could be identified which show improved mixing behaviour. The results provided here may also be used for comparison with simulations of the particle mixing with the Discrete Element Method (DEM).     

Co-current and counter-current fixed bed combustion of biofuel—a comparison

A generalised model is developed for combustion of solid fuel in a fixed bed on a grate. The model can be applied to co-current and counter-current combustion and treats a bed consisting of thermally large fuel particles of optional shape (spheres, finite cylinders and parallelepipeds) of any solid fuel. The result of model calculations agrees well with the measurements available in the literature, and the validity of the model is also shown to be satisfactory for the investigation of the differences between co- and counter-current combustion in a fuel bed, simulated by a fuel batch. The results show how the different phases, drying, devolatilisation and char combustion interact during conversion.     

A diffusion model for particle mixing in a packed bed of burning solids

Particle mixing caused by grate movement in a packed bed of solids is an important process for biomass combustion and waste incineration. In this paper, a diffusion model for particle mixing in a burning bed is proposed and the related diffusion coefficient is measured. The diffusion model was incorporated into a combustion model for waste incineration in an actual full-scale bed and numerical calculations were carried to assess the effect of different mixing levels on the burning characteristics of the furnace. In-bed measurement of temperature, oxygen concentration and particle movement was also made using a special electronic device. It is found that the modelled flame front reaches the bed bottom at an earlier stage for a higher level of particle mixing; the average burning rate ranges from 0.05 to 0.13 kg/m² s and the mass loss rate for a higher mixing level can be twice of that for a lower mixing level. However, excessive mixing can cause significant delay in ignition or even extinction of the bed combustion; the obtained local air to fuel stoichiometric ratio covers a range from sub-stoichiometric (0.6 for the highest mixing level) to super-stoichiometric (1.6 for the lowest mixing level); the carbon in ash ranges from 3.5 to 10.5%; the most reasonable range of the particle-mixing (diffusion) coefficient is from 1.8 to 6.0 cm²/min for a full-scale bed, according to the calculation.     

Numerical modeling of straw combustion in a fixed bed

Straw is being used as main renewable energy source in grate boilers in Denmark. For optimizing operating conditions and design parameters, a one-dimensional unsteady heterogeneous mathematical model has been developed and experiments have been carried out for straw combustion in a fixed bed. The straw combustion processes include moisture evaporation, straw pyrolysis, gas combustion, and char combustion. The model provides detailed information of the structure of the ignition flame front. Simulated gas species concentrations at the bed surface, ignition flame front rate, and bed temperature are in good agreement with measurements at different operating conditions such as primary air-flow rate, pre-heating of the primary air, oxygen concentration, moisture content in straw, and bulk density of the straw in the fixed bed. A parametric study indicates that the effective heat conductivity, straw packing condition, and heat capacity of the straw have considerable effects on the model predictions of straw combustion in the fixed bed.     

偏心组合桨搅拌槽内层流混合过程的数值模拟

目前,处理高黏流体和对剪切敏感介质的层流搅拌槽的报道并不多见。本文建立了描述双层组合桨搅拌槽内高黏非牛顿流体层流流动、混合过程的数学模型,利用Laminar模型、多重参考系法（MRF）和示踪剂浓度法对其流场特性、示踪剂扩散过程进行数值模拟,分析搅拌槽内轴向速度曲线、示踪剂浓度响应曲线和混合时间。结果表明：中心搅拌中间面将介质阻隔在各自的半层内运动,偏心搅拌介质作全局运动,轴向混合能力突出;转轴中心搅拌依靠上下半层浓度差的增大向下扩散,转轴偏心搅拌通过不对称结构扩散示踪剂,叶轮相对转轴偏心搅拌则利用叶片的不对称分布;距离加料点较近和较远的监测点浓度响应曲线因振荡和调整,混合时间较长,处于中间面的监测点拥有最短的混合时间。     

Measuring and modelling lateral solid mixing in a three-dimensional batch gas—solid fluidized bed reactor

A 0.27 m diameter fluidized bed reactor has been designed to allow experimental measurement of the axial and radial mixing behaviour of the solids. A unique method has been developed which permits the continuous determination of solid tracer concentration with time at different radial and axial positions within the fluidized bed. Solids mixing has been described by a model in which vertical mixing is instantaneous and lateral mixing occurs by dispersion. The lateral solids dispersion coefficients have been evaluated at various operating conditions from the experimental results of tracer concentration versus time. Based on the results, a modification of an existing correlation is proposed.     

CFB密相区内颗粒横向扩散对燃烧的影响

循环流化床(CFB)床内燃料颗粒的扩散、混和，特别是复杂的密相区内的混和特性在很大程度上影响了燃烧状况，密相区颗粒横向扩散的规律，对于循环流化床的设计具有重要意义，在循环流化床密相区颗粒横向扩散实验研究的基础上，总结了密相区内颗粒横向扩散系数的经验公式，以此为基础，研究了密相区内碳的分布规律，并建立了相应的燃烧模型，模型包括两个子模型，即密相区二维流动及燃烧子模型、稀相区一维流动及燃烧子模型。通过模型定性模拟了流化风速、给料点布置对床内燃烧的影响，有效地反映了实际情况，并确认了将密相区颗粒横向扩散规律引入现有循环流化床燃烧模型的重要意义。     

Numerical investigation of gas mixing in gas‐solid fluidized beds

Gas mixing in a tall narrow fluidized bed operated in the slugging fluidization regime is simulated with the aid of computational fluid dynamics. In the first part, a parametric study is conducted to investigate the influence of various parameters on the gas mixing. Among the parameters studied, the specularity coefficient for the partial‐slip solid‐phase wall boundary condition had the most significant effect on gas mixing. It was found that the solid‐phase wall boundary condition needs to be specified with great care when gas mixing is modeled, with free slip, partial slip and no‐slip wall boundary conditions giving substantial differences in the extent of gas back mixing. Axial and radial tracer concentration profiles for different operating conditions are generally in good agreement with experimental data from the literature. Detailed analyses of tracer back mixing are carried out in the second part. Two parameters, the tracer backflow fraction and overall gas backflow fraction, in addition to axial profiles of cross‐sectional averaged tracer concentrations, are evaluated for different flow conditions. Qualitative trends are consistent with reported experimental findings. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

A model for two-phase turbulent mixing in a jet bubble column

Mixing behavior of the two phase air-water turbulent flow in a jet bubble column is examined. The time evolution of the mixing behavior of a liquid tracer in a turbulent air-water flow within a jet bubble column is predicted using a model based on the fundamental governing equations of fluid motion. The predictions of the model are compared with experimental measurements. Measured residence time distributions (RTD) of the liquid tracer within the cone agree well with the predicted values given by the model. For the range of parameters considered in the study, lack of radial mixing and large axial mixing are evident within the cone of the jet bubble column. Use of fundamental mathematical models for the study of hydrodynamics in a two-phase conventional bubble column has been reported earlier (Torvik, 1990; Jakobsen et al., 1993). The present paper extends the use of such models to predict the mixing characteristics in a jet bubble column.     

热解燃烧链条炉低NOx排放特性的数值模拟

利用Fluent软件,对功率为1.4 MW的新型热解燃烧链条炉的NOx排放特性进行了数值模拟,其中,煤热解产生的还原性可燃气简化为CH4,采用添加元素N的乙烯-空气混合物模拟链条炉排半焦层燃烧及其生成的NO. 数值计算结果表明,在过量空气系数为1.2、再燃比为30%的燃烧条件下,热解燃烧比传统燃烧可降低NO排放14.6%. 热解燃烧链条炉由于热解气的再燃作用,在炉膛中形成一局部还原区,可较有效地降低NOx排放,证明了热解燃烧技术的可行性. 增大再燃比和减小炉排前段风室配风量可提高出口NO还原率,减小炉膛前拱长度和前后拱间距会使NO还原作用增强.     

Application of Central Composite Rotatable Design for Mixing Time Analysis in Mechanically Agitated Vessels

Central composite rotatable design (CCRD) was applied for analyzing and optimizing the effects of impeller rotational speed, gas flow rate, probe location, and tracer injection point on the gas‐liquid two‐phase mixing time in an agitated vessel with a dual six‐blade Rushton turbine. Knowledge of the effects of independent factors on the mixing time is necessary in order to optimize the mixing process. The mathematical relationship between mixing time and four significant independent variables can be approximated by a nonlinear polynomial model. The obtained results demonstrate that CCRD could efficiently be applied for modeling mixing time. It requires fewer experimental runs and provides sufficient information as compared to a factorial design.     

The use of two different models to describe the axial mixing of solids in fluidised beds

Two widely used models to describe axial solid mixing in fluidised beds (the dispersion model and the countercurrent backmixing (CCBM) model) are evaluated against identical sets of experimental data. Experimental work has been obtained at different conditions (gas velocity, particle properties and two column diameters) using an image analysis technique. Previously published data by other authors are also compiled to enlarge the experimental database for model development and validation. It is shown that both models are capable to fit the majority of experiments well, in agreement with a well-known relation between the models in some extreme conditions. This relation is further explored by incorporating independent measurements of the tracer rise velocities during the mixing experiments. It is concluded that, although a simple correlation for the solid dispersion coefficients compiled in this work is useful, the CCBM model is a much more reliable idealisation in describing and scaling up axial solid mixing in fluidised beds.     

An approach to qualify the intensity of mixing on a forward acting grate

The objective of this study is to derive methods on a statistical basis to describe quantitatively the mixing process of a packed bed on a forward acting grate. The packed bed was represented by spherical particles, whereby a varying size accounts for the variety of particle geometries in a combustion chamber. In order to describe accurately the motion of a packed bed e.g. its particles, the discrete element method (DEM) was applied. Thus, detailed data on all the particle's paths and velocities are available. These data were used for the two proposed methods based on particles’ velocities and trajectories to assess mixing and segregation of a packed bed. Both methods describe the mixing process very satisfactorily, whereby the trajectory based method is more favourable to describe segregation.     

Analysis of lurgi gasification of two U.S. Coals

A mathematical model of moving bed coal gasification is used to determine the performance of two U.S. coals in a pressurized Lurgi gasifier. Air and oxygen blasts are considered.Optimum feed conditions are highly dependent on coal type and oxidant, with high activity Wyoming coal requiring less oxygen and less steam to gasify a given amount of fixed carbon than low activity Illinois coal. The different steam to oxygen ratios for the two coals result in markedly different H₂/CO ratios in the product gas.Optimum operation requires that the combustion zone be maintained at a given distance above the ash grate.Calculations in the neighborhood of the optimum define process variables that can be used to infer deviations from optimum conditions.     

循环流化床密相区内颗粒的横向扩散的研究

Lateral solid mixing was investigated experimentally in the dense zone of a 900mm×100mm×5.2m rectangular circulating fluidized bed riser. Using heated tracer injection, the lateral solid dispersion was determined by measuring the temperature response at different lateral positions. Furthermore, a one-dimensional dispersion model, which describes the solid mixing in the dense zone, is presented. The experimental results were used to determine the lateral particle dispersion coefficient under various operating conditions. A correlation of dispersion coefficient with bed height, gas velocity, and particle size is also proposed.     

Mathematical modeling of combustion of a frozen suspension of nanosized aluminum

A mathematical model of combustion of a composite solid propellant called ALICE (frozen suspension of nanosized aluminum in water) is presented. The model takes into account the combustion of aluminum nanoparticles in water vapor, the motion of combustion products, and the smaller velocity of particles as compared to the gas. The calculated burning rate is consistent with available experimental data on the burning rate of ALICE as a function of pressure.     

Combustion wave structure in heterogeneous solid propellants

On the basis of describing physicochemical processes in a mixed solid propellant combustion wave, mathematical techniques are suggested for heterogeneous media which make it possible to consider the heterogeneous and gas-phase combustion regimes for individual components in generalized chemical reaction kinetics and to consider their effect on combustion wave velocity. The agglomeration process for Al powder on a hot surface is studied experimentally by high-speed photorecording. A mathematical model is constructed for Al agglomeration in a combustion wave based on agglomeration mechanisms observed in experiments. The dependence of combustion rate on deformation is determined on the basis of an improved method for optical recording of combustion front movement for loaded specimens. The mechanism of the effect of stress on mixed solid propellant combustion rate is connected with activation of chemical bonds of the polymer matrix, which increases its destruction rate. Use of kinetic theory for the durability of polymers made it possible to obtain an analytical equation expressing the dependence of relative combustion rate on measured specimen deformation.Novosibirsk. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 3, pp. 8–16, May–June, 1993.