Characterization of the reactivity of limestones with SO2 in a fluidized bed reactor

The reaction between SO₂ and calcined limestone particles has been studied in a fluidized bed combustor. Measurements of sorbent reactivity with SO₂ were made for small batches of limestone injected into the combustor. Simultaneous continuous combustion of bituminous coal provided conditions like those of a boiler for study of the sulphation reaction. A semi-empirical rate model of the CaO-SO₂ reaction has been developed. External mass transfer of SO₂, diffusion within the particles and chemical reaction are taken into account. The limestone reactivity with SO₂ is characterized by two parameters which are dependent on the temperature and sorbent particle size. A model for predicting the limestone requirements in a fluidized bed boiler has been developed. Parameters from the batch experiments are included. The predictions for sulfur retention agree with the experimental results. In addition, effects of operating conditions (gas velocity, recycle, limestone particle size) on the retention of SO₂ were simulated using the model.     

Combustion modelling of coal volatiles in the freeboard of a calorimetric fluidized bed combustor

A simplified kinetic approach, based on functional groups of the parent coal, was coupled with the bed hydrodynamics and a volatiles evolution region within the bed to conduct a parametric study with the experimental results obtained from a calorimetric fluidized bed combustor (FBC). The model results revealed that, for high-volatile coals with particle diameters of 1–3 mm, the fraction of the original volatiles burnt above the bed may be as high as 0.44-0.20, 0.36-0.09 and 0.30-0.02 for excess air levels ranging from 0 to 40% and bed temperatures of 800, 850 and 900°C respectively. For a low-volatile coal, the computed fractions were found to be in the ranges 0.35-0.08, 0.29-0.01 and 0.25-0.00 for similar operating conditions to the above. Good agreement between the model and experimental data suggests that the evolution of volatiles for coal particle diameters <5 mm is mechanistically controlled by both diffusion and chemical kinetics, while their combustion is largely governed by the mixing of volatiles and oxygen in the bed region.     

Analysis of combustion efficiency in CFB coal combustors

Fluidized bed technology is well known for its high combustion efficiency and is widely used in coal combustion. In this study, the combustor efficiency has been defined and investigated for CFB coal combustor based on the losses using a dynamic 2D model. The model is shown to agree well with the published data. The effect of operating parameters such as excess air ratio, bed operational velocity, coal particle diameter and combustor load and the effect of design variables such as bed height and bed diameter on the mean bed temperature, the overall CO emission and the combustion efficiency are analyzed for the small-scale of CFBC in the presently developed model. As a result of this analysis, it is observed that the combustion efficiency decreases with increasing excess air value. The combustion efficiency increases with the bed operational velocity. Increasing coal particle size results in higher combustion efficiency values. The coal feed rate has negative effect on the combustion efficiency. The combustor efficiency considerably increases with increasing combustor height and diameter if other parameters are kept unchanged.     

High temperature desulphurization by fine limestone during staged fluidized‐bed combustion

This paper reviews the SO₂ emission from a 0.3 m² stainless‐steel fluidized‐bed combustor. Fine coal was premixed with fine limestone and fed pneumatically under the bed. The SO₂ emission was found to depend largely on air staging ratio and bed temperature, which agrees with previous observations. The SO₂ emission observed in sorbent‐free tests (reported earlier by Khan and Cibbs, 1995) was found to be proportional to the sulphur content of the fuel when limestone was added, the sulphur capture at a fixed Ca/S molar ratio was dependent on oxygen stoichiometry and bed temperature. Finely sized limestone enhanced the effectivity of the sorbent at low bed temperature and air staging ratio. During staged combustion, the combustion efficiency depended largely on primary air to coal ratio. Around 90% combustion efficiency was observed at 1 m/s fluidizing velocity which was reduced when fluidizing velocity was increased to 1.5 and 2 m/s. This reduction is due to increased elutriation of finer coal particles from the combustor.     

Modeling a fluidized bed reactor by integrating various scales: Pore,particle, and reactor

This work proposes a novel population-balance based model for a bubbling fluidized bed reactor. This model considers two continuum phases: bubble and emulsion. The evolution of the bubble size distribution was modeled using a population balance, considering both axial and radial motion. This sub-model involves a new mathematical form for the aggregation frequency, which predicts the migration of bubbles from the reactor wall toward the reactor center. Additionally, reacting particles were considered as a Lagrangian phase, which exchanges mass with emulsion phases. For each particle, the variation of the pore size distribution was also considered. The model presented here accurately predicted the experimental data for biochar gasification in a lab-scale bubbling fluidized bed reactor. Finally, the aggregation frequency is shown to serve as a scaling parameter.     

A simplified model of SO2 capture by limestone in 71 MWe pressurized fluidized bed combustor

A mathematical model of SO₂ capture by uncalcined limestone particles with solid attrition under pressurized fluidized bed combustion conditions was developed based on the shrinking unreacted-core model. Since the thickness of the product layer is sufficiently much smaller than the particle size, a flat surface model was employed. The difference in SO₂ capture behavior between continuous solid attrition and intermittent attrition was investigated. The reaction rate for intermittent solid attrition was found to be lower than that for continuous attrition mode under low SO₂ concentration conditions. A simple mathematical expression to calculate reaction rate of SO₂ capture per unit external surface area of limestone is proposed.The present simplified mathematical model of SO₂ capture by single limestone particle under periodical attrition conditions was applied to the analysis of a large-scale pressurized fluidized bed combustor. By giving the period of attrition as a parameter, the experimental results agreed well with the model results. From the vertical concentration profile of SO₂ concentration, the emission of SO₂ was found to be governed by the balance between SO₂ formation rate from char and SO₂ capture by limestone at the upper surface of the dense bed. A simplified expression to estimate SO₂ emission from pressurized fluidized bed combustors was proposed.     

Combustion characteristics of a two-stage swirl-flow fluidized bed combustor

In order to investigate the combustion characteristics of a two-stage swirl-flow fluidized bed combustor, combustion experiments of low-grade anthracite coal were performed. Experimental parameters were the fluidizing air velocity, coal feed rates, bed temperature, stoichiometric air ratio, swirl nozzle diameter and rotational diameter. The experimental results showed that, due to the swirl flow, the elutriation rates of fines were lower than those of the single-stage fluidized bed combustor. The combustible contents of the ash in the outflow streams were also reduced. Therefore, the combustion efficiency of the two-stage swirl-flow fluidized bed combustor was 20% greater than that of the single-stage fluidized bed combustor under the same operating conditions.     

Effects of operational parameters on emission performance and combustion efficiency in small-scale CFBCs

A well-designed CFBC can burn coal with high efficiency and within acceptable levels of gaseous emissions. In this theoretical study effects of operational parameters on combustion efficiency and the pollutants emitted have been estimated using a developed dynamic 2D (two-dimensional) model for CFBCs. Model simulations have been carried out to examine the effect of different operational parameters such as excess air and gas inlet pressure and coal particle size on bed temperature, the overall CO, NO_x and SO₂ emissions and combustion efficiency from a small-scale CFBC. It has been observed that increasing excess air ratio causes fluidized bed temperature decrease and CO emission increase. Coal particle size has more significant effect on CO emissions than the gas inlet pressure at the entrance to fluidized bed. Increasing excess air ratio leads to decreasing SO₂ and NO_x emissions. The gas inlet pressure at the entrance to fluidized bed has a more significant effect on NO_x emission than the coal particle size. Increasing excess air causes decreasing combustion efficiency. The gas inlet pressure has more pronounced effect on combustion efficiency than the coal particle size, particularly at higher excess air ratios. The developed model is also validated in terms of combustion efficiency with experimental literature data obtained from 300 kW laboratory scale test unit. The present theoretical study also confirms that CFB combustion allows clean and efficient combustion of coal.     

Modelling of circulating fluidized bed combustion of a char

The combustion of a char in the 41 mm ID riser of a laboratory circulating fluidized bed combustor has been investigated at different air excesses and rates of solids (char and sand) circulating in the loop. Riser performance was characterized by an axial oxygen concentration profile as well as by the overall carbon content and particle size distribution. The proposed model accounts for carbon surface reaction, intraparticle and external diffusion, and attrition. External diffusion effects were relevant in the riser dense region where char was potentially entrapped in large clusters of inert solids. Experimental data and results of the model calculations are in satisfactory agreement.     

NO emission on pulverized coal combustion in high temperature air from circulating fluidized bed – An experimental study

High temperature air was adopted by combustion in high excess air ratio in a circulating fluidized bed. Experiments on pulverized coal combustion in high temperature air from the circulating fluidized bed were carried out in a down-fired combustor with the diameter of 220 mm and the height of 3000 mm. The NO emission decreases with increasing the residence time of pulverized coal in the reducing zone, and the NO emission increases with excess air ratio, furnace temperature, coal mean size and oxygen concentration in high temperature air. The results also revealed that the co-existing of air-staging combustion with high temperature air is very effective to reduce nitrogen oxide emission for pulverized coal combustion in the down-fired combustor.     

A method of evaluating limestone reactivity with SO2 under fluidized bed combustion conditions

The reactivity of limestone with sulphur dioxide has been evaluated during conditions similar to those existing in a fluidized bed combustor using a fixed-bed quartz reactor. The reactivity of up to 11 particle sizes of 2 limestones were evaluated, and an exponential decay function was found to best describe the rate behaviour versus conversion of CaO to CaSO₄. The two constants in the exponential decay approximation could both be expressed as functions of particle size. Subsequently, the limestone reactivity as a function of both size and conversion could be reasonably well described by a total of 4 to 6 constants. An analytical sulphur capture model for fluidized bed boilers (FBB) that incorporates this type of reactivity function is proposed.     

Modelling and simulation of a coal gasification process in pressurized fluidized bed

A coal gasification mathematical model that can predict temperature, converted fraction and particle size distribution for solids have been developed for a high pressure fluidized bed. For gases in both emulsion and bubble phase, it can predict temperature profiles, gas composition, velocities and other fluid-dynamic parameters. In the feed zone, it could be considered a Gaussian distribution or any other distribution for the solid particle size. Experimental data from literature have been used to validate the model. Finally, the model can be used to optimize the gasification process changing several parameters, such as excess of air, particle size distribution, coal type and reactor geometry.     

循环流化床燃煤过程NOx和N2O产生-控制研究进展

循环流化床(CFB)燃烧技术因其燃料适应范围广、污染物排放低等优点,近几十年得到广泛应用. 随着当前环保要求的日益提高,CFB燃煤过程N2O排放浓度较高成为其应用的瓶颈问题. 因此系统总结CFB燃煤过程中NOx和N2O排放的研究现状对开发新型CFB燃煤技术具有重要意义. 本工作首先讨论了CFB燃煤过程中NOx和N2O的均相和异相反应机理,然后应用这些机理分析了床温、过剩空气系数、分级燃烧,以及煤种对CFB燃煤过程NOx和N2O排放的影响. 在此基础上,对常见的抑制NOx和N2O排放的工艺从机理角度进行了归纳总结. 最后,对2种本工作认为有应用前景的CFB燃煤减排NOx和N2O新技术?反向分级燃烧技术及CFB解耦燃烧技术进行了简要论述.     

CFB锅炉燃烧福建无烟煤的工业性试验

用石灰石做脱硫剂,在一台35 t/h循环流化床(CFB)锅炉上进行工业试验.采用炉内添加石灰石混烧的脱硫工艺,测试钙硫比和石灰石颗粒径变化对脱硫及锅炉运行效果的影响.试验结果表明(1)时均脱硫效率与瞬态最高脱硫效率均随Ca/S比的增加而增大,但在Ca/S＜2.5时增加缓慢.当Ca/S摩尔比为3.15时,时均脱硫效率有53.5%,瞬态最高脱硫效率可达77%;(2)石灰石的平均颗粒径越细,脱硫效率越高.当平均颗粒径为0.36 mm时,时均脱硫效率接近于50%;(3)加入石灰石脱硫会引起炉床温度波动,但波动幅度不大.当钙硫比较小时,炉床温度先升后降;而在钙硫比较大情况下,炉床温度则是先降后升;(4)在Ca/S比较小的情况下,添加石灰石混烧会使锅炉炉床平均温度下降,而使炉膛中部、上部和出口处的温度上升;在高Ca/S比情况下(＞3.0)却使锅炉整体的运行温度都下降.从试验结果推断,当钙硫比取2.5～3.0,石灰石平均粒径取0.2 mm～1.0 mm时,可以得到较好的脱硫运行效果.     

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。     

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.     

Analysis of cyclic combustion of solid fuels

The cyclic nature of coal particles combustion results from the movement of loose material in the flow contour of the circulating fluidized bed (CFB): the combustion chamber, the cyclone, the downcomer.The experimental results proved that the cyclic change of the oxygen concentration around coal particles, led to the vital change of both mechanism and combustion kinetics. The mathematical model of the process of coal combustion has been scientifically described whose original concept is based on the allowance for cyclic changes of concentrations of oxygen around the char particle. It enables the prognosis for change of the surface and the centre temperatures and a mass loss of the char particles during the cyclic combustion. It allows to appoint mass-rate of combustion of a char particle in the above conditions.     

流化床煤燃烧过程不同气氛下的气态氮释放特征

利用微型流化床反应装置,结合快速过程质谱仪,在850~940℃操作温度下,研究了三种不同粒度分布烟煤和无烟煤在热解、气化和燃烧反应条件下四种主要气态氮产物HCN、NH₃、NO和NO₂的释放规律。结果表明,微型流化床可以实时检测挥发分氮和焦炭氮的动态释放序和类型,热解、气化和燃烧反应气氛的改变主要影响HCN和NH₃的释放量。热解产物的气态氮主要是来自于挥发分,燃烧反应的HCN和NH₃的释放量与温度有明显关系,而气化反应的各类气态氮释放量随温度变化波动不大。煤颗粒尺寸和温度变化对烟煤和无烟煤中各类气态氮释放量产生影响比较复杂,其中NH₃的释放特性是区分挥发分N释放和半焦N释放的重要特征。     

Modeling of sulphur retention in atmospheric fluidized bed combustors. Sensitivity analysis and simulation

For the design, simulation and optimization of sulphur retention in atmospheric fluidized bed coal combustors, a mathematical model is needed that would be able to predict the behaviour of the combustor in a wide range of operating conditions. In this work, a sensitivity analysis of the sulphur retention predictions of the different hypotheses, equations and parameters, which define the different submodels and phenomena occurring in the combustor, has been carried out. It has been found that the hypotheses related to the gas flow, devolatilization type and sulphur distribution in the pyrolysis products imply an important division among models. The greatest effect on sulphur retention predictions is exercised by the parameters defining the fines elutriation and sorbent sulphation capacity. However, those corresponding to the bed hydrodynamics (minimum fluidization velocity and bed expansion) do not have a significant effect on the sulphur retention predictions. The sulphur retentions obtained in the combustion of high sulphur lignites with eight different limestones were used for model validation. A good fit of the experimental sulphur retentions was found, without using any adjustable parameter. Finally, a simulation of the process was made. The great effect of the bed height, air velocity and the particle size distribution of the limestone must be pointed out, as well as the effect of its reactivity through the maximum conversion attainable by each particle size.     

Pressure fluctuation properties in combustion of mixture of anthracite and bituminous coal in a fluidized bed

The combustion characteristics of a mixture of anthracite and bituminous coal were studied in a 0.155 m i.d. fluidized bed combustor (FBC). The properties of the pressure fluctuation for the bed such as the standard deviation, cross-correlation function, dominant frequency and the power spectral density function were obtained through statistical analysis. To interpret the combustion characteristics in the FBC with uniform or multi-sized particles of anthracitebituminous coal mixture, the properties of pressure fluctuation were determined as a function of the particle size distribution and anthracite mixing fraction. In the present work, it is known that the combustion region could be obtained from the analysis of pressure fluctuation properties, and the mixed-firing of anthracite-bituminous coal is related to the reaction models of both coals and particle size distribution. Moreover, the relation between coal size distribution and static mean pressure, and the ignition region could be obtained from the mean pressure profile.