O_2/CO_2气氛下单颗粒焦炭燃烧竞争效应的数值模拟

基于O_2/CO_2气氛下焦炭燃烧的复杂气固反应,提出多种碳基随机孔模型(Various Char-RPM)模拟焦炭颗粒燃烧过程,对影响其竞争效应的因素进行分析.结果表明,焦炭燃烧竞争效应主要发生在动力扩散控制区,此时颗粒孔隙中的气体浓度明显波动,燃烧不稳定.提高环境中O_2浓度和减小焦炭颗粒粒径均可弱化和消除竞争效应,减小粒径的效果更明显.     

O_2/CO_2气氛下多种碳基随机孔模型的建立

传统随机孔模型基于简单一步反应建立,不适用于处理O_2/CO_2气氛下焦炭颗粒复杂气固反应。针对此问题,基于焦炭本身具有多种碳基的特点,以及焦炭颗粒在O_2/CO_2气氛下燃烧的特性,建立复杂气固反应下的多种碳基随机孔模型和孔隙结构模型。模拟直径为100μm的焦炭颗粒在O_2/CO_2气氛下燃烧的过程,使用FORTRAN语言自主编程计算并分析结果。研究表明,燃烧初期颗粒呈现竞争效应,孔隙内部气体浓度产生剧烈波动。波动的生成原因是化学反应与物理扩散之间的竞争,可以通过增加环境氧浓度和减小焦炭颗粒粒径来改善。所提出的多种碳基随机孔模型对于表征O_2/CO_2气氛下焦炭颗粒的燃烧特性有着良好的适应性。     

O2/CO2气氛下多种碳基随机孔模型的建立

传统随机孔模型基于简单一步反应建立,不适用于处理O₂/CO₂气氛下焦炭颗粒复杂气固反应。针对此问题,基于焦炭本身具有多种碳基的特点,以及焦炭颗粒在O₂/CO₂气氛下燃烧的特性,建立复杂气固反应下的多种碳基随机孔模型和孔隙结构模型。模拟直径为100 μm的焦炭颗粒在O₂/CO₂气氛下燃烧的过程,使用FORTRAN语言自主编程计算并分析结果。研究表明,燃烧初期颗粒呈现竞争效应,孔隙内部气体浓度产生剧烈波动。波动的生成原因是化学反应与物理扩散之间的竞争,可以通过增加环境氧浓度和减小焦炭颗粒粒径来改善。所提出的多种碳基随机孔模型对于表征O₂/CO₂气氛下焦炭颗粒的燃烧特性有着良好的适应性。     

O2/CO2气氛下基于反应面积的Char-N转化模型的建立

目前,研究者普遍认为焦炭氮（Char-N）向NO转化时Char-N反应速率与焦炭燃烧速率成正比,这种处理方式忽略了焦炭颗粒燃烧过程中焦炭和Char-N反应面积变化的差别,与实际情况相差较大。针对此问题,本文充分考虑了焦炭燃烧时碳基和Char-N由于存在形式不同导致的反应面积变化的差别,在通过随机孔模型处理焦炭颗粒燃烧的同时采用收缩模型的思维处理Char-N的转化,建立了一个具有两种模型优势的综合模型。模拟了粒径为100μm的焦炭颗粒在O₂/CO₂气氛下的燃烧过程,以及颗粒内部Char-N向NO的转化过程。通过Fortran自主编程并进行计算。结果表明,新建立的模型计算结果能够与实验数据吻合,具有较好的适用性。同时新模型具有表征Char-N转化过程中各参数变化的能力。环境O₂浓度的增大和焦炭颗粒粒径的增大都会导致反应过程中NO生成量增多,同时由于焦炭和Char-N反应面积变化不同,NO的生成曲线存在二次升高的现象。     

焦炭颗粒气化表面积变化结构因子的研究

孔结构及其变化对焦炭颗粒的气化行为有重要的影响,如颗粒的比表面积变化与反应速率有相同的变化趋势.推导得出了一个新的反映焦炭颗粒气化过程中表面积变化的结构因子表达式,该结构因子显示表面积的变化趋势与初始孔隙率有重要的关系.随着初始孔隙率的不同,比表面积或是随转化率减小,或是出现最大值.这一特征与随机孔模型很相象.与随机孔模型作了对比研究,发现所提出的该表达式模型除了形式简单外,还具有物理意义明确、精度好的特点.     

不同升温速率下石油焦燃烧特性的热重分析

利用热天平（TGA）对石油焦的燃烧特性进行研究，根据不同升温速率下石油焦燃烧特性曲线，分析了石油焦在不同升温速率下的燃烧特性，计算出石油焦燃烧反应动力学参数，为该石油焦的燃烧提供了较为可靠的基础数据．由于石油热值高，尽管其着火较难，但一旦着火燃烧剧烈，燃烧过程处在动力区的时间很短．实验表明，石油焦的化学动力控制区在900K以下．在TGA上测量石油焦的化学动力学参数时，推荐采用较低的升温速率以延长在动力区的燃烧时间．     

铁基载氧体纤维素化学链解聚试验及分子模拟

为探索纤维素在铁基载氧体作用下的化学链解聚机理及过程。通过热重分析试验研究不同升温速率下纤维素的化学链燃烧特性；通过化学反应动力学计算纤维素化学链燃烧过程中的活化能并揭示其动力学机制；利用ReaxFF MD模拟综合技术从微观原子尺度阐释纤维素化学链燃烧过程微观反应网络。热分析结果表明，铁基载氧体的加入可降低纤维素化学链解聚的起始温度，其释放的晶格氧有助于促进纤维素的化学链解聚。纤维素化学链燃烧过程分为3个阶段：挥发分析出燃烧、半焦转化燃烧和焦炭燃烧阶段。反应动力学研究显示，纤维素在热转化过程中不同转化率下的活化能为220～405 kJ/mol,其中第3个阶段的反应活化能最高。ReaxFF MD模拟结果显示，纤维素化学链燃烧过程整体遵循自由基链反应理论。纤维素裂解产生的活性自由基与载氧体释放的晶格氧反应生成2-羟基丙酮等中间体，然后进一步发生自由基反应生成CO₂。最终获得了载氧体作用下纤维素化学链解聚过程中CO₂生成释放的复杂反应网络。     

氢气氛下半焦有机硫的脱除动力学

研究了半焦颗粒在氢气氛下有机硫脱除的本征动力学行为.实验采用微分反应器,研究结果表明,半焦颗粒的脱硫与气氛、温度和半焦硫含量有关.通过比较粒子反应模型和随机孔模型,粒子反应模型仅适合于处理半焦颗粒氢气气氛脱硫的初始阶段,而随机孔模型符合整个脱硫过程.实验和理论分析揭示了脱硫过程中煤中有机硫形态的转化.     

大颗粒煤反应过程中灰层有效扩散系数的研究

在大颗粒煤燃烧或者气化过程中,气体穿过灰层的有效扩散系数是非常重要的参数.对有效扩散系数的研究主要有两种思路:一方面,灰层本质上是多孔介质,因此在研究有效扩散系数时,沿用气体在多孔介质中扩散的研究思路;另一方面,焦炭的燃烧或者气化本质上是气固反应,气体在灰层也即产物层中的扩散是气固反应的重要影响因素,因此应用气固反应的理论对有效扩散系数进行研究.根据以上两种思路综述了有效扩散系数的实验和模型的研究现状,并对今后的研究重点进行了展望.     

高温煤焦/CO2气化反应的动力学研究

对煤气化随机孔模型的动力学控制区的假设进行了改进,建立了高温煤焦/CO2气化反应碳转化率(X)与反应时间(t)的修正随机孔模型:X=1-exp[-kt(a+bkt+k2t2)],并在950℃～1 400℃气化温度范围内,用修正随机孔模型模拟淮南慢速热解煤焦和淮南快速热解煤焦/CO2气化反应,所得表观活化能范围分别为121.99kJ/mol～153.75kJ/mol和88.57kJ/mol～121.39kJ/mol.结果表明,修正随机孔模型的拟合效果优于随机孔模型和收缩未反应芯模型的拟合效果,能很好地体现煤焦气化反应的动力学特征,且该模型适用于不同煤焦的气化反应模拟.     

随机孔模型研究煤焦O2/CO2燃烧动力学特征

在热重分析仪上对焦作无烟煤焦和云浮烟煤焦O₂/CO₂条件下燃烧特性进行研究。确定在不同温度下不同煤焦O₂/CO₂燃烧的特征。利用随机孔模型(RPM)表征两种煤焦反应速率与碳转化率的关系,同时与未反应缩核模型(Model Ⅰ)和混合模型(Model Ⅱ)的拟合结果进行比较。研究表明,在不同反应条件下,随机孔模型具有最佳的拟合效果,相关系数都在0.986以上。比较RPM、ModelⅠ和Model Ⅱ计算结果发现,焦作无烟煤焦的O₂/CO₂等温燃烧的活化能比云浮烟煤焦的高,且同一煤种燃烧反应温度越高反应速率常数越大。由于随机孔模型的结构参数ψ可以很好地表现孔结构变化对煤焦燃烧反应的影响,因此随机孔模型能更加准确地描述煤焦O₂/CO₂燃烧特征。     

The random pore model with intraparticle diffusion for the description of combustion of char particles derived from mineral- and inertinite rich coal

An investigation was undertaken to determine the applicability of the random pore model with intraparticle diffusion for the determination of the reaction kinetics for the combustion of chars particles derived from coals rich in minerals and inertinites. The char particles which were pyrolysed at 900 °C consisted of a dense carbon-containing fraction originating from the inert macerals (mainly inertinites), a high concentration of minerals and carbominerites, pores generated by the devolatilisation of the reactive macerals and cracks as a result of the presence of minerals. Combustion experimentation was carried out with a thermogravimetric analyser using 1 mm particles with 20% (mole) oxygen in nitrogen at 287.5 kPa and low temperatures (450-600 °C) and with high gas flow rates. The random pore model with intraparticle diffusion (pores and cracks) was solved numerically according to a method consisting of a step-wise regression procedure. This was achieved by using carbon conversion and reduced time relationships to calculate the structural parameter and the initial Thiele modulus followed by determination of an initial lumped reaction rate and validation with conversion versus real time results. The model is characterised by a decreasing Thiele modulus (increasing porosity) occurring during the reaction period which gives rise to a transition to a chemically controlled reaction system. It was found that the initial overall reaction rate was controlled by intraparticle diffusion with an increasing influence with increasing temperature.     

包含反应阶数变化的分形煤焦颗粒燃烧模型的建立与实验验证

为了深入研究煤焦燃烧的机理并提高对煤焦燃烧过程的预测精度,建立了一个综合的煤焦燃烧模型。该模型考虑了煤焦颗粒孔隙内二次反应与扩散的耦合作用、煤焦燃烧反应阶数的变化和反应过程中CO/CO₂比例等问题。使用热天平(TGA)对11种煤焦的燃烧特性进行分析,测得各种煤焦的表观活化能与指前因子,以确定模型中的待定参数。在沉降炉(DTF)中对这11种煤焦做燃烧实验,用TGA基于灰分守恒测得DTF炉管出口处的煤焦样品的转化率。运用建立的模型模拟这些煤焦的燃烧过程,预测的转化率与实验结果有较好的吻合度,相比传统的本征动力学模型,该模型预测的精度有了较大提高,证明了该模型能适用于从褐煤到无烟煤的较广煤焦范围。研究还发现,煤焦燃烧的表观反应阶数在燃烧过程中不断减小并最终趋于稳定。     

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.     

粉煤气化条件下不同粒径半焦的表征与气化动力学

利用可模拟真实粉煤气化条件的平流火焰反应器制取了同一煤种的2种不同平均粒径的半焦,并对其进行了物理表征和气化反应动力学研究. 其物理表征显示,小颗粒半焦的平均比表面积远小于大颗粒半焦. 半焦的CO2气化实验表明,含很多小孔的大颗粒半焦的气化反应受内扩散影响显著,比表面积较小的小颗粒半焦具有更高的气化反应速率. 利用随机孔模型导出了反应速率Ri. 根据小颗粒半焦在不同温度下平衡态时的反应速率Ri,s值获得半焦气化的本征气化反应速率表达式为Ri=1.243′103exp(-19243.5/T) g/(m2×min).     

Properties of high ash coal-char particles derived from inertinite-rich coal: II. Gasification kinetics with carbon dioxide

The reaction rate of carbon dioxide-nitrogen gas mixtures with a well-characterised high ash char derived from an inertinite-rich coal discard was investigated by experimentation and reaction rate modelling. Experimentation with a thermogravimetric analyzer at 87.5 kPa and 287.5 kPa between 850 °C and 900 °C and with 1mm diameter particles, similar to operating conditions used for bubbling fluidised bed gasification, was carried out. The char consisted of a large proportion of dense char formed from the inertinite in the parent coal, fine pores from the low concentration of reactive macerals and cracks formed as a result of thermal deflagration. The effects of carbon dioxide concentration, temperature and pressure on the carbon conversion with time were found to follow expected trends with long reaction times. The random pore model, which accounts for intraparticle structural changes, was examined to predict the overall reaction rate. For this evaluation, a new procedure was developed to determine the structural parameter which could not be calculated directly from initial characterisation results. This procedure consists of defining a reduced time parameter, which conveniently eliminates the effect of the intrinsic kinetics when conversion versus the reduced time results is used. Thus, the structural parameter, which characterises the pore growth and coalescence, can be evaluated by a regression procedure using experimental results obtained at all temperatures and pressures. Intrinsic reaction parameters based on the power rate law were also calculated from carbon conversion versus real time results using a stepwise regression procedure. It was found that the random pore model predictions for the carbon conversion with time using the determined parameters correlated very well with experimental results, thus confirming that the reaction rate is chemical-reaction controlled.     

Characterization of porous structure of coal char from a single devolatilized coal particle: Coal combustion in a fluidized bed

The combustion characteristics of coal char are highly dependent on initial pore structure of devolatilized char as well as on the structural evolution during the combustion of char. The development of pore structure also throws light on the mechanism of the combustion process. In the present work evolution of pore structure of partially burnt coal char of Indian origin has been investigated experimentally in a batch-fluidized bed and analyzed. The BET surface area, micropore surface area and porosity of char at various levels of carbon burn-off have been determined. Experimental specific surface area has been found to agree well with theoretical prediction using random pore model. Modified random pore model is used to determine the active surface area. Char combustion mechanism based on shrinking unreacted core and shrinking reacted core models are delineated during the course of reaction at various bed temperatures. This is substantiated with the proportional representation of ash and carbon matrix in scanning electron microscope images. It is also concluded that in the present investigation the mean pore size is much smaller and hence the Knudsen diffusion predominates. Analysis based on similar experimental observations and models for pore structure evolution to investigate char combustion reaction regime has not been reported in literature.     

随机孔模型应用于煤焦燃烧的动力学研究

采用热重法研究煤焦在变温和等温条件下的燃烧过程,分析升温速率和温度对煤焦燃烧行为的影响,用随机孔模型(RPM)研究煤焦的燃烧失重过程,得到了煤焦燃烧变温和等温动力学方程. 实验结果表明,变温实验中,随着升温速率的增加,煤焦燃烧的失重曲线向高温方向移动,最大燃烧速率增加,升温速率由5℃/min增加到20℃/min时,最大燃烧速率由3.2%/min增加到11.3%/min;等温实验中,随着燃烧温度的提高,煤焦最大燃烧速率增加,燃烧温度由510℃增加到630℃时,最大燃烧速率由2.1%/min增加到8.3%/min,煤焦燃烧性能得到改善. 动力学计算结果表明,RPM能较好描述煤焦变温和等温燃烧过程中煤焦转化率与温度和时间的关系,煤焦变温和等温燃烧的表观活化能分别为84.27和64.16 kJ/mol.     

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。