Modelling and simulation of coal gasification process in fluidised bed

A one-dimensional steady state mathematical model and a numerical algorithm have been developed to simulate the coal gasification process in fluidised bed. The model incorporates two phases, the solid and the gas. The gaseous phase participates in the emulsion (with the solid phase) and forms the bubble. The solid phase is composed of carbonaceous material, limestone and/or inert bed material. The model can predict temperature, converted fraction, and particle size distribution for the solid phase. For the gaseous phase, in both emulsion and bubble, it can predict profiles of temperature, gas composition, velocities, and other fluid-dynamic parameters. In the feed zone, a Gaussian distribution for the solid particle size is considered. This distribution changes due to attrition, elutriation, consumption and drag inside the reactor. A system of 29 differential and 10 non-linear equations, derived from the mass, energy and momentum balances for each phase, at any point along the bed height, are solved by the Gear and Adams Method. Experimental data from the Universidad de Antioquia and Universidad Nacional-Medellin have been used to validate the model. Finally, the model can be used to optimise the gasification process by varying several parameters, such as excess of air, particle size distribution, coal type, and geometry of the reactor.     

The effect of coal particle size on pyrolysis and steam gasification

For future power generation from coal, one preferred option in the UK is the air-blown gasification cycle (ABGC). In this system coal particles sized up to 3 mm, perhaps up to 6 mm in a commercial plant, are pyrolysed and then gasified in air/steam in a spouted bed reactor. As this range of coal particle sizes is large it is of interest to investigate the importance of particle size for those two processes. In particular the relation between the coal and the char particle size distribution was investigated to assess the error involved in assuming the coal size distribution at the on-set of gasification. Different coal size fractions underwent different changes on pyrolysis. Smaller coal particles were more likely to produce char particles larger than themselves, larger coal particles had a greater tendency to fragment. However, for the sizes investigated in this study ranging from 0.5 to 2.8 mm, the pyrolysis and gasification behaviour was found not to vary significantly with particle size. The coal size fractions showed similar char yields, irrespective of the different char size distributions resulting from pyrolysis. Testing the reactivity of the chars in air and CO₂ did not reveal significant differences between size fractions of the char, nor did partial gasification in steam in the spouted bed reactor. From the work undertaken, it can be concluded that pyrolysis and gasification within the range of particle sizes investigated are relatively insensitive to particle size.     

BGL炉煤气化过程建模和模拟

建立了BGL气化炉的三维非稳态煤气化模型,模型考虑煤炭颗粒的收缩过程,应用收缩核模型集成煤热解模型、气相湍流模型、气固流动模型、气固异相反应模型、气相均相反应模型、能量守恒方程以及相间传热模型等。该模型充分考虑了气化炉内部三维空间的温度和组成分布,通过煤热解段模型化学计量参数优化,得到CO/H₂摩尔比在1.59左右,符合BGL炉热解段运行机制;然后对BGL炉气化段过程进行三维非稳态模拟,模拟出口气组成(CO,H₂,CO₂,CH₄,H₂O,O₂)与文献结果对比,误差均小于4%。证明了BGL模型的准确性。基于该模型,本文对煤气化过程的主要参数进行影响分析。分析结果表明:煤气化效率随汽氧比的增加而提高,当汽氧比确定在1～1.3之间可以满足工艺要求及生产的需要,适合本文研究所用褐煤的特点;氧煤比增加会降低煤气化效率,但合成气中有效气的含量呈现出先增大后减小的趋势,当氧煤比在0.17左右时有效气含量达到峰值;随着煤粒直径的增加,BGL炉内的温度呈降低趋势,最高温度从...     

两阶段煤炭地下气化温度场模型

引　言煤炭地下气化过程实际上是一个自热平衡过程 ,依靠煤燃烧产生的热量使地下气化炉内建立起理想的温度场 ,进而发生还原反应和分解反应 ,产出煤气 .因此 ,在地下气化过程中起关键作用的是炉内的温度场 ,尤其对于两阶段地下气化更是如此 .两阶段气化是一种循环供给空气和水蒸气的地下气化方法 ,每个循环由两个阶段组成 :第 1阶段为鼓空气燃烧蓄热 ,生产空气煤气 ;第 2阶段为鼓水蒸气生产地下水煤气 .只有在第 1阶段积蓄了足够量的热能以后 ,才能使第 2阶段水蒸气的分解反应得以顺利进行 ,从而产生高热值地下水煤气 ,同时煤层热分解的程度…     

An analytical model for the combustion of coal particles

A model for the combustion of coal particles is developed which accounts for the gasification and pyrolysis reactions throughout the particles, oxidation reactions at the particles' outer surface, distributed gas-phase reactions around the particles, and transport of heat and mass throughout the reaction zones. The model shows that the combustion of large particles is dominated by gasification and gas-phase reactions. As large particles burn and shrink, however, the mechanism changes to one dominated by surface oxidation. This change in mechanism can be gradual or very rapid (even discontinuous) depending on the combustion conditions. The particle size at which this transition takes place decreases as the bulk gas temperature or oxygen concentration increase. The proposed model is used to predict the conversion of pulverized coal as a function of time; the results agree with literature data for particles of 26-32 μm radius.     

等离子体煤气化反应器内部流动数值模拟及设计优化的初步研究

本文对一种等离子体煤气化反应器内部湍流、两相流动和燃烧化学反应等复杂流动现象进行了三维数值模拟 ,得出了其内部的温度分布、速度分布和组分浓度分布 ,在对计算结果进行分析的基础上 ,提出了等离子体煤气化反应器的一种优化设计方案 ,并对此方案进行了不同等离子体流速下的数值模拟和比较分析 ,以确定气流参数的最佳匹配 ;数值分析结果表明 :改进的设计方案和气流参数匹配提高了等离子体热源利用的效率和煤气转化的效率     

煤粉粒径对HNCERI气化炉碳转化率与固/液渣层分布的影响

以中国华能集团清洁能源技术研究院（Huaneng Clean Energy Research Institute,HNCERI）两段干粉加压气化炉为研究对象,采用考虑了焦炭颗粒表面气体组分扩散效应的随机孔模型计算焦炭气化反应速率以评估碳转化率。同时,耦合熔渣子模型计算气化炉一段壁面固液渣层分布特性和热损失,研究了煤粉粒径对HNCERI气化炉碳转化率和固液渣层分布特性的影响。结果表明所构建的模型可以准确预测气化炉出口主要气体组分组成、碳转化率和气化炉一段壁面热损失;气化炉一段碳转化率受固有气化速率和停留时间控制,二段主要受颗粒停留时间控制;因此,通过减小煤粉粒径可以减小气体在颗粒表面扩散阻力,有利于提高气化炉一段碳转化率,而适量增加煤粉粒径可以增加煤粉颗粒在气化炉二段的停留时间,有利于提高二段碳转化率。模拟结果显示煤粉颗粒粒径从20μm增加到200μm,一段碳转化率从99.68%降低到了95.06%,二段碳转化率从69.03%增加到了89%。煤粉粒径对气化炉上缩口和直段壁面液态渣层分布影响很小,但显著影响固态渣层厚度的发展。     

榆林煤作湿法加压气流床气化原料的分析

榆林煤属于高发热量、低灰、低灰熔点、高还原率的优质煤;通过成浆性试验,确定了原煤的粒度分布、添加剂加入量,并测定了料浆的流变特性;分析了原料煤反应活性和煤灰特性对气化操作的影响,并估计了原料煤的气化指标;指出榆林煤是适合湿法加压气化的优良煤种。     

Numerical simulation of 3D fluidized bed biomass gasification based on CPFD

Based on computational particle fluid dynamics (CPFD), a three-dimensional bubbling fluidized bed steam-air mixed gasification numerical model was established, and it was verified with experiment trials. The results show that the simulation and experiment have good consistency. Based on the model, the gas distribution and temperature distribution in the gasifier were studied; meanwhile, the biomass properties (particle size, water content, types) and operating conditions (gasification temperature, bed height) were investigated. The results show that there is an optimal value for the impact of biomass particle size on gasification performance, with an average particle size of 0.6 mm being the best; a higher water content will reduce the output of combustible gas and is not conducive to the gasification reaction. Among the four types of biomass, sawdust gasification has the highest efficiency, the largest combustible gas production, and the highest gas calorific value. Rice husk is second only to sawdust but its carbon conversion rate is higher than that of sawdust; increasing the gasification temperature can increase the proportion of combustible gas and increase gasification efficiency; while the change of initial bed height can change the ratio of H₂/CO. This experiment provides a theoretical reference for biomass steam/air gasification, which is helpful for the selection and processing of biomass raw materials, and also facilitates the amplification and optimization of the gasifier.     

基于CPFD方法的流化床生物质气化数值模拟

基于计算颗粒流体动力学(CPFD)建立了三维鼓泡流化床水蒸气-空气混合气化的数值模型,并进行了模型验证,结果表明模拟和实验具有良好的一致性。在该模型的基础上,研究了气化炉内气体分布以及温度分布;同时探究了生物质属性(颗粒粒径、含水率、种类)以及操作条件(气化温度、床料高度)对气化特性的影响。结果表明,生物质颗粒粒径对气化性能的影响存在一个最优值,平均粒径为0.6 mm是最佳的;较高的含水率会降低可燃气体产量,不利于气化反应的进行;四种生物质中,锯末气化的效率最高、可燃气体产量最大、气体热值最高,稻壳仅次于锯末但其碳转化率高于锯末;提高气化温度可以增加可燃气体的比例、提高气化效率;而初始床层高度的变化可以改变H₂/CO的比例。本实验为生物质水蒸气/空气气化提供了理论参考,有助于生物质原料的选取和处理,也有助于气化炉的放大和优化。     

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.     

气流床煤气化的Aspen Plus建模：平衡模型和动力学模型

基于Aspen Plus软件建立了GE气流床煤气化的平衡模型和动力学模型,计算了气化的煤气组成和碳转化率。模型分为热解、气化和气液分离三个阶段。其中,气化阶段又分为初步气化和气化重整,从而获得气化产物在恒定温度下的分布。平衡模型的气化阶段使用了吉布斯反应器RGIBBS,基于吉布斯自由能最小化原理对体系内的气化产物进行计算;动力学模型的气化阶段使用了全混流反应器RCSTR,基于煤气化反应的动力学机理对体系内的气化产物进行计算。模拟值与GE气化炉的实际工程数据进行了对比,结果表明,平衡模型可在一定程度上反映气化结果的变化趋势,但预测结果的准确性有所欠缺,而基于气化反应机理建立的动力学模型能很好地预测GE气化炉的气化结果。对动力学模型中的全混流反应器进行反应时间设定,可以对GE气化炉生产提供一定的指导,结果表明：反应停留时间为3.5s时就可以达到很好的气化效果。温度是影响气化反应速率及产物分布的重要因素,利用煤气化的动力学模型模拟了气化温度对气体组成及碳转化率的影响,结果表明：随着气化温度的升高,CO含量逐渐增加,H₂含量基本不变,CO₂含量逐渐减小,碳转化率逐渐升高。     

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.     

煤气化细渣燃烧技术研究进展

简述了煤气化细渣的基础燃烧特性、燃烧动力学和燃烧应用三方面的研究现状，分析表明：煤气化细渣基础燃烧特性较差，难以单独稳定燃烧，与其他燃料掺烧可提高其燃烧性能；煤气化细渣的粒径、掺烧比和种类等因素对其燃烧性能影响较大，应着重加以研究；煤气化细渣燃烧动力学多采用Coats-Redfern积分法进行研究，求解过程简单，但存在准确性相对较低的问题，采用多重扫描速率法等进行相关研究可提高准确性，具有研究价值，但鲜有报道。掺混燃烧是煤气化细渣燃烧应用的主要方式，具有一定的经济和环境效益，但多数会引起一些副作用；单独燃烧是煤气化细渣燃烧应用的另一方式，具有燃烧效率高的优点，但需对燃烧设备进行针对性的设计、开发。最后提出了煤气化细渣燃烧技术今后的研究方向，以期为相关领域科研工作者提供参考。     

Thermophoresis effects on gas-particle phases flow behaviors in entrained flow coal gasifier using Eulerian model

A numerical model based on the Eulerian–Eulerian two-fluid approach is used to simulate the gasification of coal char inside an entrained flow gasifier. In this model, effects of thermophoresis of coal char particles are thoroughly investigated. The thermophoresis is due to the gas temperature gradient caused by absorpted heat of coal char gasification. This work, firstly, calculates the gas temperature gradient and thermophoretic force at1100 °C,1200 °C,1300 °C and 1400 °C wall temperatures. Then, the changes of particle volume fraction and velocity in the gasifier are studied in the simulation with thermophoresis or not. The results indicate that considering the particle thermophoresis has some effects on the calculation of particle volume fraction in the gasifier, especially at wall temperature of 1400 °C, and the maximum particle volume fraction variance ratio reaches up to 1.38% on wall surface of the gasifier. These effects are mainly caused by large gas temperature gradient along the radial direction of the gasifier. For the particle velocity, the changes are small but can be observable along radial direction of the gasifier, which has good agreement with the distributions of radial gas temperature gradient and thermophoretic force. These changes above may have certain effects on gasification reaction rates in this Eulerian model. So the change of gasification reaction rates in the simulation with thermophoresis or not is studied finally.     

低热稳定性低阶煤在碎煤加压气化工艺中的应用

为提高低热稳定性低阶煤的碎煤加压气化效果,采用新疆、河南等地碎煤加压气化工艺,研究低阶煤热稳定性对碎煤加压气化效果的影响。结果表明:低热稳定性低阶煤的单台气化炉氧负荷仅达到设计的70%左右,比高热稳定性低阶煤低40%。煤的热稳定性越低,粗煤气中煤粉含量越高,影响煤气水分离、变换等后续工艺。最后提出可通过研究不同粒度煤的热稳定性、确定最小入炉煤粒度、测定碎煤加压气化煤粉的粒度、改进碎煤加压气化除尘系统、选择合理的气化压力、采用气化工艺组合等提高低热稳定性低阶煤碎煤加压气化效果。     

IGCC示范工程煤气化炉的数值模拟

采用Aspen Plus流程模拟软件对某拟建的IGCC示范工程的德士古煤气化炉进行数值模拟,通过考虑碳的不完全转换对计算流程进行了改进,并运用CPD模型预测煤热裂解的产物分布.研究了煤气化炉的重要操作参数(即水煤浆浓度、氧煤比、气化压力和气化温度)对气化结果的影响.在计算区间内,发现高浓度水煤浆浓度范围内,随浓度的增加,煤气的主要成分(H2+CO)的总含量增加.气化温度增大到1400℃左右时,煤气的主要成分随气化温度的进一步增加会趋于一个恒定值.     

Reaction kinetics and producer gas compositions of steam gasification of coal and biomass blend chars, part 2: Mathematical modelling and model validation

In gasification of biomass, coal and blended biomass and coal, there are two steps including an initial pyrolysis process followed by gasification of solid char. The latter process is a slow process and thus dominates the whole gasification. In our previous paper (Xu et al., in press), the differences between steam gasification of biomass chars and that of coal chars have experimentally been investigated and the results show that these differences are mainly due to the difference in microstructures of these two fuels. In this work, a mathematical model of char gasification is developed based on reaction kinetics and gas transportation of both the producer gas and the gasification agent (steam). The model also includes mass conservation equations for each of the gas components and solid carbon involved in the gasification process. This has resulted in a set of highly nonlinear differential equations which have been solved using a numerical technique to predict gas production rate, gas compositions and carbon consumption rate during the gasification.The developed mathematical model is validated using experimental results reported in previous paper (Xu et al., in press), and close agreement between the simulation results and the experimental values have been observed. From the modelling, it has been confirmed that the char gasification is mainly determined by the characteristics of char matrix including the exposed surface area and the micro-pore size. The former determines intrinsic reaction rate and the latter influences the intra-particle mass transportation. Biomass char has more amorphous structure, thus the intrinsic reaction rate is enhanced. For coal char, the larger pore size enables the high transport rate of the gasification agent (water vapour) into the char particles but the resultant gases have higher resistance to transfer through compact clusters. For simulation of the blended biomass and coal, the blend properties were determined based on the blend proportion of each fuel. The close agreement between the simulation results and experimental data suggests that the approach in this work can adequately quantify the gasification kinetics and the gas composition.     

Reaction kinetics and producer gas compositions of steam gasification of coal and biomass blend chars, part 1: Experimental investigation

Biomass and coal are important solid fuels for generation of hydrogen-rich syngas from steam gasification. In this work, experiments were performed in a bench-scale gasifier to investigate the effect of coal-to-biomass ratio and the reaction kinetics for gasification of chars of biomass, coal and coal–biomass blends. In the gasification of these chars, steam was used as the gasification agent, while nitrogen was used as a gas carrier. The gasification temperature was controlled at 850, 900 and 950 °C. Gas produced was analysed using a micro-GC from which carbon conversion rate was also determined. From the experiments, it is found that the coal and biomass chars have different gasification characteristics and the overall reaction rate decreases with an increase in the ratio of coal–to-biomass.The microstructure of the coal char and biomass char was examined using scanning electronic microscopy (SEM), and it was found that the biomass char is more amorphous, whereas the coal char has larger pore size. The former enhances the intrinsic reaction rate and the latter influences the intra particle mass transportation. The difference in mass transfer of the gasification agent into the char particles between the two fuels is dominant in the char gasification.     

高温烟气中煤焦气化行为

针对高温烟气中煤焦的气化行为,本文采用FactSage 6.1计算了煤焦在高温烟气下的高温反应特性,并利用热重分析仪分析了煤焦气化行为。通过沉降炉实验进一步研究了不同温度、气体配比、粒径条件下气体产物的动态析出特性,同时计算了评价指标α、β、LHV值。结果表明：随着温度的升高,气体产物H₂和CO的含量增加,β、α、LHV值增大,CH₄和CO₂的含量下降。在温度为1200℃时,β、α值分别由CO₂/CO比为10∶70时的10.80%、5.21%增加到CO₂/CO比为50∶30时的24.71%、41.06%。同时,随着CO₂/CO比值的增大,高温烟气对煤焦气化反应抑制减弱。通过对比反应温度和粒径对煤焦气化反应的影响,得出反应温度远大于粒径对煤焦气化反应的影响。通过实验验证了向高温烟气中喷吹煤焦制备高品质可燃气体方法的可行性。