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.     

Model for devolatilization of coal particles in fluidized beds

A model for the devolatilization of coal in a non-combusting fluidized bed is proposed. Previous studies have either considered devolatilization as a non-rate process or assumed the devolatilization coal particle as isothermal. The assumption of an isothermal particle requires the heat transfer Biot number ?0.02. In view of the larger Biot numbers predicted using existing fluidized bed gas-solid heat exchange correlations and reported values for thermophysical properties, the present model considers the devolatilizing particle to be, in general, non-isothermal. The temperature profiles are computed from the analytical solution of the one-dimensional spherical coordinate unsteady heat transport equation with a convective boundary condition. The temperatures are then used in the non-isothermal coal decomposition kinetic expression proposed by Anthony et al., integrated over the particle to obtain the fractional volume average devolatilization at any given time. Parametric studies show a chemical kinetics controlled regime for small particles, a heat transfer controlled regime for larger particles and a mixed regime for intermediate particle sizes. The extent of the mixed regime depends on the type of coal as well as the operating conditions. The model results are also compared with the fluidized CH₄ and CO evolution data reported in the literature for various particle sizes and different temperatures.     

Devolatilization and combustion of large coal particles in a fluidized bed

Devolatilization and combustion of large particles of Eastern Canadian coals (Evans and Minto), 5-50 mm dia., were studied in a bench-scale atmospheric fluidized bed reactor at 1023-1173 K with 0.5 mm sand particles as the bed material. The devolatilization time, mass loss history, changes in proximate volatiles content and C/H mass ratio, and temperature history at the centre of the particle during devolatilization were determined. The mass loss during devolatilization is correlated with the proximate volatiles content of the parent coal. The devolatilization time is correlated with the initial particle diameter by a power-law relation with an exponent of 1.54-1.64. The results show insignificant effect of superficial velocity on devolatilization.     

Devolatilization of coals of North-Eastern India under fluidized bed conditions in oxygen-enriched air

Oxygen-enriched air can increase the combustion efficiency, boiler efficiency, and sulfur absorption efficiency of atmospheric fluidized bed combustion (AFBC) boilers which use high-sulfur coal, and other combustion systems that use coal. Devolatilization is the first step in the gasification or combustion of coal. In this work, devolatilization characteristics of five run-of-mine (ROM) coals of North-Eastern India having particle-size between 4 mm and 9 mm are reported. The experiments were performed under fluidized bed conditions at 1123 K in enriched air containing 30% oxygen. The devolatilization time was correlated with the particle diameter by a power law correlation. The variation of mass with time was correlated by an exponential correlation. It was observed that the average ratio of yield of volatile matter to the proximate volatile matter decreased with the increase in volatile-content of the coals. A shrinking-core model was used to determine the role of film-diffusion, ash-diffusion and chemical reaction. The experimental results indicate the likelihood of film-diffusion to be the rate-controlling mechanism in presence of oxygen-enriched air. A cost-analysis was carried out to study the economy of the process.     

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     

Temperature field within a devolatilizing coal particle

The aim of the present research was to test a mathematical model of coal devolatilization. Verification of the thermal conductivity, specific heat capacity and endothermic heat of pyrolysis was carried out by measurement of the coal particle temperature at the outer surface and in the center of the particle. The model, with recommended thermal and transport data, was then tested on the basis of gaseous products generation rates and total mass loss during experiments with coal granules of size 4-20 mm and coal grains 0.08-5 mm dropped into a hot fluidized bed.     

Fluidized bed combustion of wet brown coal

The behaviour of very wet Victorian brown coal was examined in a bed of sand fluidized, at temperatures around 1000 K, with either air or nitrogen. Small batches of coal with a narrow particle size range were added to the 76 mm diameter bed and the times required for devolatilization and total combustion were recorded. Changes in particle water content, volatiles level and particle size distribution were also measured. All the particles tested, up to 8.4 mm in diameter, dried rapidly and remained substantially intact throughout carbonization and combustion. Devolatilization was complete after about 60 s but extensive freeboard combustion of volatiles was evident. The water content of the coal had very little influence on burnout time. Char combustion dominated the overall combustion process and took place under kinetic control with significant pore burning.     

Devolatilization and char burning of coal particles in a fluidized bed combustor

Devolatilization and char burning were studied in an electrically heated bench-scale fluidized-bed reactor at 750 to 900°C bed temperature, gas oxygen mole fractions ranging from zero to 0.21, superficial gas velocities from 0.3 to 0.7 m/s and coal particle diameters 5 to 35 mm. The coals investigated include lignite, bituminous and anthracite. The coal devolatilization and char burning times, H/C ratio histories, and particle fragmentation were measured. Statistical correlations with the operating variables were developed for the devolatilization time. A mathematical model is given for the combustion of char. Most predictions of the model agree quite well with the experimental results.     

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 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.     

流化床粉煤气体热载体快速热解反应器的计算

在冷态模拟实验和煤热解动力学计算的基础上,对粉煤气体热载体快速热解提升管反应器的高度进行了计算。利用高速摄像粒子测速法结合互相关算法研究了不同气体流量和不同颗粒粒径时固体颗粒在热解提升管中的运动速度,通过求解神府煤热解动力学方程,得到了不同粒径神府煤颗粒热解挥发分析出的时间,从而确定了快速热解提升管反应器的高度。研究结果表明:当气体流量在850 m3/h,粉煤的粒径主要集中在0.7—3.0 mm时,提升管的高度应选择在10.0 m。     

Comminution characteristics of Korean anthracite in a CFB reactor

Comminution characteristic of Korean anthracite has been determined with operation conditions in a laboratory scale circulating fluidized bed (CFB) combustor. The fragmentation of the anthracite occurs explosively, and generates lots of fine particles at an early stage of devolatilization. The fragmented particles continue to be reduced with generation of the fine particles during an attrition stage in the CFB combustor. With an increase of operation temperature, the coal shows a high degree of fragmentation and generation of fine particles in the CFB reactor. The particle fragmentation occurs actively as its size and Hard Grove Index (HGI) increase. The attrition is also affected with particle size and HGI of the coal. The initial surface crack and the fine clusters on the particle surface are found to be reasons for explosive fragmentation and for generation of fine particles during devolatilization and combustion in the CFB reactor.     

煤在流化床中的热解——实验研究及数学模型

本研究利用一个改进的流化床热解装置对神木煤的脱挥发分行为进行了实验研究及数学模拟，得到了煤粒在流化床中的升温及分层热解的规律。     

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.     

煤炭流化床燃烧与热天平燃烧动力学关系研究

通过对不同煤种在热天平中的单颗粒燃烧过程的分析发现:煤种、颗粒大小、氧气浓度对颗粒燃烧完全所需要的时间影响特别显著;而在一般流化床生产运行温度范围内,温度对颗粒燃烧完全所需要时间的影响有限;同一煤种不同颗粒煤焦孔隙率的影响很小.通过对单颗粒热天平燃烧实验,考查不同煤种及颗粒大小等各因素对颗粒燃烧过程的影响,可方便地指导流化床的设计以及生产运行过程中工艺的调整.     

Three‐stage well‐mixed reactor model for a pressurized coal gasifier

Three Canadian coals of different rank were gasified with air‐steam mixtures in a 0.1 m diameter spouted bed reactor at pressures to 292 kPa, average bed temperatures varying between 840 and 960°C, and steam‐to‐coal feed ratios between 0.0 and 2.88. In order to analyze gasifier performance and correlate data, a three‐stage model has been developed incorporating instantaneous devolatilization of coal, instantaneous combustion of carbon at the bottom of the bed, and steam/carbon gasification and water gas shift reaction in a single well mixed isothermal stage. The capture of H₂S by limestone sorbent injection is also treated. The effects of various assumptions and model parameters on the predictions were investigated. The present model indicates that gasifier performance is mainly controlled by the fast coal devolatilization and char combustion reactions, and the contribution to carbon conversion of the slow char gasification reactions is comparatively small. The incorporation of tar decomposition into the model provides significantly closer predictions of experimental gas composition than is obtained otherwise.     

Depth of combustion inside char from evidence of oxidized pyrite

In coal combustion processes the rate controlling mechanism varies from external diffusion and pore diffusion to chemical kinetics. The particle zone where combustion takes place therefore changes from the outer layer, for large particles at high temperatures, to the total internal pore volume, for small particles at low temperatures. Partial penetration of oxygen occurs in intermediate cases. Recent publications report on fluidized bed experiments where, according to the model used, combustion takes place at or near the outer surface of the particle (‘shrinking particle model’) in a narrow boundary layer. Knowledge of the depth of this layer could contribute to combustion modelling. This paper shows that during fluidized bed combustion at 900 °C oxygen penetrates into the char to a depth of 50–100 μm. This is concluded from the width of the zone where pyrite particles in the char are oxidized. The presence of open pores may increase the depth of the internal combustion layer up to several hundreds of micrometres.     

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.     

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.     

A model for volatiles release into a bubbling fluidized-bed combustor

A volatiles release model has been developed to predict the location and quantity of coal volatiles that are released into a bubbling fluidized-bed combustor, using overbed, in-bed or underbed feed systems. This model not only considers time resolution of the simultaneous processes of coal devolatilization and coal particle movement after injection, but also takes account of the stochastic nature of this particle movement. Volatiles release is nonuniform, occurring throughout an industrial-scale bed, but only in restricted parts of a laboratory-scale bed. For the industrial-scale bed, 32% of volatiles are released directly into the freeboard while the particle is devolatilizing at the surface, whereas 24% are released there for the laboratory scale bed. The model predicts the formation of numerous discrete volatiles regions within the bed, in agreement with a new interpretation of experimental measurements from an in-bed oxygen probe. The model is shown to be consistent with other experimental in-bed measurements obtained during combustion of “simulated” volatiles.