Devolatilization of large coal particles in fluidized beds

Analysis of devolatilization of predried large coal particles in fluidized beds requires consideration of both the chemical kinetics of coal decomposition and transport processes. Models available either assume the devolatilization particle to be isothermal (whereas it may be shown that, in general, large temperature gradients may exist within the particle) or require extensive numerical integration procedures. This Paper describes a model which permits formulation of analytical and easy-to-use equations for the estimation of the devolatilization history of a large predried coal particle in a fluidized bed. The model predictions are compared with experimental data collected for Mississippi lignite. A correlation is proposed for the estimation of the total devolatilization time. The analytical solutions presented may be used with ease in coupling the devolatilization process to the other phenomena, such as drying and/or combustion of volatiles and residual char, occuring during fluidized bed combustion of coal.     

Modeling and experimental studies on single particle coal devolatilization and residual char combustion in fluidized bed

Single particle devolatilization followed by combustion of the residual coal char particle has been analyzed in a batch-fluidized bed. The kinetic scheme with distributed activation energy is used for coal devolatilization while multiple chemical reactions with volume reaction mechanism are considered for residual char combustion. Both the models couple kinetics with heat transfer. Finite Volume Method (FVM) is employed to solve fully transient partial differential equations coupled with reaction kinetics. The devolatilization model is used to predict the devolatilization time along with residual mass and particle temperature, while the combined devolatilization and char combustion model is used to predict the overall mass loss and temperature profile of coal. The computed results are compared with the experimental results of the present authors for combustion of Indian sub-bituminous coal (15% ash) in a fluidized bed combustor as well as with published experimental results for coal with low ash high volatile matter. The effects of various operating parameters like bed temperature, oxygen mole fraction in bulk phase on devolatilization time and burn-out time of coal particle in bubbling fluidized bed have been examined through simulation.     

Coupled drying and devolatilization of wet coal in fluidized beds

A single particle model is proposed for the coupled drying and devolatilization (under inert conditions) of coals in fluidized beds. The model assumes that heat transfer to and through the particle is the rate-controlling mechanism for drying. It is also assumed to be the rate-controlling mechanism for devolatilization in conjunction with the kinetics of overall coal decomposition. The drying is assumed to take place from a moving boundary (the drying front) and an unsteady-state heat conduction equation (in spherical coordinates) with a convective boundary condition is solved using approximate analytical techniques. The resultant temperature profile in the dried shell is used in a numerical integration over the volume of particle, using the non-isothermal coal decomposition kinetic expression proposed by Anthony et al. (1975) Proc. 15th Symp. Combustion, to determine the devolatilization characteristics. After the drying is complete, the remaining possible volatile evolution at any particular bed temperature is obtained from a similar integration with the temperature profile determined from the analytical solution for the unsteady-state heat conduction equation with a convective boundary condition and a non-uniform initial temperature condition. Results of parametric analyses are presented along with the comparison of model predictions with data reported in the literature.     

Devolatilization characteristics of large particles of tyre rubber under combustion conditions

A systematic experimental study has been performed in order to investigate the effect of particle size and temperature on the devolatilization rate of large tyre rubber particles. Cylindrical tyre particles with diameters between 7.5 and 22 mm were devolatilised in a macro-TGA reactor, at temperatures between 490 and 840 °C in an inert atmosphere. The effect of particle size and surrounding temperature on the rate of devolatilization was observed to be significant, i.e. larger particle diameters and lower temperatures increased the devolatilization time. A detailed mathematical model for the devolatilization process including internal and external heat transfer, three parallel independent devolatilization reactions and reaction enthalpy effects has been developed and solved using an implicit finite difference method. Comparison of the model predictions with experimental data, reveals that the devolatilization process of large tyre rubber particles at temperatures above 490 °C can be considered to be controlled mainly by heat transfer and reaction kinetics. The model analysis further shows that exothermic devolatilization reaction enthalpy effects cannot be neglected in the prediction of the intra particle temperature rise. A sensitivity analysis of the model parameters, demonstrates that the specific heat capacity of the virgin fuel and the activation energies of the devolatilization reactions is the most important model parameters in the prediction of devolatilization times of large tyre rubber particles.     

Release of volatiles from large coal particles in a hot fluidized bed

Coal particles with diameters of 3–11 mm were injected into a small, hot bed of sand fluidized by nitrogen. Volatiles evolution was followed by sampling the exit gas stream and subsequent analysis by gas chromatography. Three Australian coals covering a range of volatile matter were studied and the effects of coal particle size and bed temperature were determined. The yields of gaseous components, char and tar are explained by consideration of the competitive reactions for coal hydrogen and oxygen and secondary reactions of the volatile species within the coal particle. The pore structure developed during devolatilization has a significant effect on the extent of these secondary reactions. It is concluded that heat transfer is the main process controlling the volatilization time in fluidized bed combustors. The time required for heat transfer into the coal particle, determined by calculation and experiment, agrees with the measured volatilization time. Significant factors are external heat transfer to the surface of the particle, internal conduction through the coal substance and radiation through the pores, and the counterflow of volatiles out of the coal particle. For different coals, variations in the volatilization time appear to be caused by the development of different pore structures, which affect radiant heat transfer through the pores.     

Modeling of biomass devolatilization in a fluidized bed reactor

A simple model that simulates a single biomass particle devolatilization is described. The model takes into account the main physical and chemical factors influencing the phenomenon at high temperatures (>700 K), where the production of gaseous components far outweighs that of liquids. The predictions of the model are shown to be in good agreement with published data. The model is then applied to the devolatilization of biomass in a fluidized bed, in which attention is focused on heat transfer, particle mixing and elutriation, and gas production. Predictions on the overall devolatilization time for a biomass particle are compared with experimental results obtained in a fluidized bed reactor in which the process was monitored by continuous measurement of the bed pressure. Good correspondence of predicted with calculated values was obtained, supporting the validity of the many approximations made in the derivation of the governing relationships for the pyrolysis process.     

Kinetic studies of the pyrolysis of sewage sludge by tga and comparison with fluidized beds

The pyrolysis of sewage sludge was examined by thermogravimetric analysis using different heating rates, particle sizes and final temperatures. A semi-empirical model was developed with four global consecutive/competitive reactions to volatile and solid products including also a heat balance. This model was extrapolated to isothermal reactor conditions and compared to experimental data on fluidized bed pyrolysis. Depending on the particle size and reactor temperature, about 90% of the maximum conversion in a fluidized bed takes place under external heat transfer control, the rest under kinetic control. Incomplete conversion occurs at short particle residence times, which were calculated approximately by the model.     

Modelling and experimental investigation of devolatilizing wood in a fluidized bed combustor

A two-dimensional model is developed for the determination of devolatilization time and char yield of cylindrical wood particles in a bubbling fluidized bed combustor. By using the concept of shape factor, the model is extended to particles of cuboid shape. The model prediction of the devolatilization time agrees with the measured data (present and those reported in the literature) for cylindrical and cuboidal shaped particles within ±20% while the char yield is predicted within ±17%. Influence of some important parameters namely, thermal diffusivity, external heat transfer coefficient and shrinkage, on the devolatilization time and char yield are studied. Thermal diffusivity shows noticeable influence on devolatilization time. The external heat transfer coefficient shows little influence beyond a value of 300 W/(m² K). However particle shrinkage shows negligible effect on the devolatilization time but has a significant influence on the char yield.     

鼓泡流化床内高灰煤气化过程的颗粒尺度特性研究

基于多相流体质点网格方法(MP-PIC)对高灰煤在三维鼓泡流化床气化过程进行了数值模拟研究。在欧拉-拉格朗日框架下将气相和固相分别视作连续介质和离散相处理。首先,将模拟得到的出口处气体组分结果与实验数据进行对比,实验数据与模拟结果具有良好的一致性。其次,研究了煤颗粒在气化炉内的温度、传热系数、速度和停留时间,从颗粒尺度揭示了鼓泡流化床气化炉内的颗粒分布特性和气固流动特征。结果表明:在气化炉入口附近煤颗粒与床层温差最大,传热系数最大;由于流化床内强非线性的气固流动,床中煤温度和传热系数的空间分布不均匀;煤颗粒和床料的瞬时速度具有稳定的波动幅度,其中垂直方向速度波动最明显,且煤颗粒的瞬时速度比床料的瞬时速度略大;由于颗粒间的剧烈碰撞,延长了煤颗粒停留时间。此外,对鼓泡流化床中煤气化过程颗粒尺度的研究,有助于深入了解固体颗粒的流动行为以及气固相相互作用特性,对鼓泡流化床反应器的设计优化具有重要意义。     

Modeling the temperature in coal char particle during fluidized bed combustion

The temperatures of a coal char particle in hot bubbling fluidized bed (FB) were analyzed by a model of combustion. The unsteady model includes phenomena of heat and mass transfer through a porous char particle, as well as heterogeneous reaction at the interior char surface and homogeneous reaction in the pores. The parametric analysis of the model has shown that above 550 °C combustion occurs under the regime limited by diffusion. The experimental results of temperature measurements by thermocouple in the particle center during FB combustion at temperatures in the range 590-710 °C were compared with the model predictions. Two coals of different rank were used: lignite and brown coal, with particle size in the range 5-10 mm. The comparisons have shown that the model can adequately predict the histories of temperatures in char particles during combustion in FB. In the first order, the model predicts the influence of the particle size, coal rank (via porosity), and oxygen concentration in its surroundings.     

Generalized model of heat transfer and volatiles evolution inside particles for coal devolatilization

Devolatilization is acknowledged as the first important step in coal conversion techniques. A comprehensive heat transfer and devolatilization model was established, with special consideration of the particle‐scale physics and chemistry, to predict the internal heat transport and pyrolysis behavior of particles. The chemical percolation devolatilization model with corrected kinetic parameters and structure parameters was validated with a lot of experimental data and then adopted to describe the devolatilization behaviors under a broader range of temperatures, heating rates, and coal types. The newly achieved understanding of the integrated effect of heating rate and coal type on coal devolatilization could help to provide a preliminary coal rank selection method for industrial processes. In particular, in‐depth discussion of the influences of heat conduction, volatiles diffusion, and endothermic heat of devolatilization inside particle indicated the dominant roles of these factors when the intensity of heat transfer was strong or the release of volatiles was rapid. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2893–2906, 2014     

Heat transfer in a swirling fluidized bed with geldart type-D particles

A relatively new variant in fluidized bed technology, designated as the swirling fluidized bed (SFB), was investigated for its heat transfer characteristics when operating with Geldart type D particles. Unlike conventional fluidized beds, the SFB imparts secondary swirling motion to the bed to enhance lateral mixing. Despite its excellent hydrodynamics, its heat transfer characteristics have not been reported in the published literature. Hence, two different sizes of spherical PVC particles (2.61 mm and 3.65 mm) with the presence of a center body in the bed have been studied at different velocities of the fluidizing gas. The wall-to-bed heat transfer coefficients were measured by affixing a thin constantan foil heater on the bed wall. Thermocouples located at different heights on the foil show a decrease in the wall heat transfer coefficient with bed height. It was seen that only a discrete particle model which accounts for the conduction between the particle and the heat transfer surface and the gas-convective augmentation can adequately represent the mechanism of heat transfer in the swirling fluidized bed.     

EFFECT OF OPERATING VARIABLES ON BED TO WALL HEAT TRANSFER IN A CIRCULATING FLUIDIZED BED

Heat transfer coefficients were measured for 227 and 87 micron size particles of silica sand in a 102 mm dia. and 5.5 m tall circulating fluidized bed made of plexiglass. Effects of solid circulation rate, fluidization velocity, bed density and particle sizes were studied. The heat transfer coefficients were calculated from the temperatures measured along the axis of a 25 mm dia 100 mm long cylindrical heat flux meter.     

Characterization of the devolatilization rate of solid fuels in fluidized beds by time‐resolved pressure measurements

The characterization of volatile matter (VM) release from solid fuel particles during fluidized‐bed combustion/gasification is relevant to the assessment of the reactor performance, as devolatilization rate affects in‐bed axial fuel segregation and VM distribution across the reactor. An experimental technique for the characterization of the devolatilization rate of solid fuels in fluidized beds is proposed. It is based on the analysis of the time series of pressure measured in a bench‐scale fluidized‐bed reactor as VM is released from a batch of fuel particles. A remarkable feature of the technique is the possibility to follow fast devolatilization with excellent time‐resolution. A mathematical model of the experiment has been developed to determine the time‐resolved devolatilization rate, the devolatilization time and the volume‐based mean molecular weight of the emitted volatile compounds. Devolatilization kinetics has been characterized for different solid fuels over a broad range of particle sizes. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Devolatilization of coals of northeastern India in inert atmosphere and in air under fluidized bed conditions

Devolatilization of five coals having volatile matter in the range of 31 to 41% was studied in argon and in air under fluidized bed conditions. The diameter of the coal particles varied between 4 and 9.5 mm. The variation of devolatilization time with particle diameter was expressed by the correlation, t_v = Ad_vⁿ. The superficial gas velocity was found to have a significant effect on the rate of devolatilization. The devolatilization rate increased with the increase in the oxygen concentration in the fluidizing gas. The correlations developed in this study fitted the mass versus time profiles of the coal particles satisfactorily. The same correlations were found to be appropriate for predicting devolatilization of a batch of coal particles. The correlations developed in the present study will be useful for the design of fluidized bed combustors.     

Mathematical Model for Batch Drying in an Inert Medium Fluidized Bed

A modified three-phase model is proposed for batch drying of fine powders in an inert medium fluidized bed. The overall heat and mass transfer coefficients between the interstitial gas and solid phases have been determined by the proposed surface-stripping model in which the Biot number is a governing parameter. The effects of gas velocity, inlet gas temperature and mass ratio of starch to inert particles on the drying characteristics of starch in a 0.083 m ID × 0.80 m high medium fluidized bed have been determined. Based on the proposed model, the internal resistance of mass transfer at the powder is equal to the external resistance. The model predicts well the bed temperature, humidity of outlet gas, moisture content of solid particles, heat and mass transfer in an inert medium fluidized bed.     

A non-isothermal model of a bubbling fluidised-bed coal gasifier

A previous one-dimensional numerical isothermal model of a fluidised-bed coal gasifier has been further improved to consider the non-isothermal behaviour of gases and heat transfer mechanisms in the fluidised-bed. Comparisons were made with data simulating the performance of a full-scale fluidised-bed coal gasifier, a 2 MWth pilot plant and a laboratory-scale bed. Simulation results indicate that comparisons of overall carbon conversions, operating bed temperatures and individual gas species predicted from both the non-isothermal and isothermal models with experimental data are favourable. The temperature profile of the gas phase predicted from the non-isothermal model follows the trend of the bubble temperature due to a large majority of the product gas flowing through the bed as bubbles. The temperature of cold feed gas is predicted to experience a heating up period at the lower part of the bed and homogeneous combustion in the gas phase results in a peak temperature in the gas phase. As a consequence of the higher solid temperature predicted from the non-isothermal model than from the isothermal model, the final product gas molar flow rate and fractional carbon conversion due to gasification predicted from the non-isothermal model are all higher giving better agreeing results compared with experimental data than those predicted from the isothermal model. In addition, these comparisons show that improvement in the predictive capabilities of the model depends on ability to predict the product CO/CO₂ ratio which is particularly important for low volatile coals or char.     

管式炉中燃煤一次破碎特性的实验研究

燃煤在循环流化床锅炉中的破碎特性极大地改变了物料的粒度分布,对床内颗粒浓度、物料传热传质及煤颗粒的燃烧过程都有重要影响.由于循环流化床锅炉本身的复杂性及实验现象难于观察,在1台管式炉中研究了各种煤的一次破碎特性.实验发现,烟煤、贫煤、无烟煤和煤矸石的破碎形式并不相同.烟煤颗粒遵循环核分层破碎;贫煤、无烟煤既有表面破碎也有中心破碎,且少数颗粒因热爆性而迅速变为细小颗粒;煤矸石沿着颗粒层面发生破碎,破碎为一些碎片;此外,深入研究了颗粒粒径、炉膛温度和加热气氛对西山贫煤和阳泉无烟煤一次破碎特性的影响.     

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。     

Modeling a pilot-scale fluidized bed coal gasification reactor

A steady-state model has been developed to simulate the North Carolina State University pilot-scale fluidized bed coal gasification reactor. The model involves instantaneous devolatilization of coal at the top of the gasifier (freeboard region) and char combustion and gasification in the fluidized bed. A two-phase (emulsion-dilute gas) representation of the fluidized bed incorporates the phenomena of jetting, bubbling, slugging, and mass and heat transfer between phases, and enables the prediction of individual species flow rates and temperature profiles within the bed. The model has been successfully used to simulate the gasification of a devolatilized Western Kentucky bituminous coal and a New Mexico subbituminous coal and to predict effects of various operating parameters on key gasifier performance variables.