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。     

油页岩半焦燃烧动力学研究

燃烧动力学是研究油页岩半焦颗粒燃烧特性的基础。利用热重分析仪对油页岩半焦进行了恒温燃烧实验研究,在排除外扩散影响的基础上,分析了燃烧温度、氧气浓度对油页岩半焦燃烧过程的影响。在实验范围内,氧气浓度和燃烧温度均能对油页岩半焦燃烧速率产生重要影响,更高的氧气浓度和燃烧温度可以加快油页岩半焦燃烧速率。结合实验结果,建立了考虑氧气浓度影响的油页岩半焦燃烧动力学模型,发现油页岩半焦燃烧速率与氧气浓度的0.97次方呈线性关系。模型计算结果与实验结果符合较好,为进一步研究油页岩半焦大颗粒燃烧特性提供了燃烧动力学基础。     

油页岩半焦燃烧动力学研究

燃烧动力学是研究油页岩半焦颗粒燃烧特性的基础。利用热重分析仪对油页岩半焦进行了恒温燃烧实验研究,在排除外扩散影响的基础上,分析了燃烧温度、氧气浓度对油页岩半焦燃烧过程的影响。在实验范围内,氧气浓度和燃烧温度均能对油页岩半焦燃烧速率产生重要影响,更高的氧气浓度和燃烧温度可以加快油页岩半焦燃烧速率。结合实验结果,建立了考虑氧气浓度影响的油页岩半焦燃烧动力学模型,发现油页岩半焦燃烧速率与氧气浓度的0.97次方呈线性关系。模型计算结果与实验结果符合较好,为进一步研究油页岩半焦大颗粒燃烧特性提供了燃烧动力学基础。     

油页岩矿物质催化半焦燃烧特性及机理

利用微型流化床反应分析仪(MFBRA)研究了油页岩矿物质催化半焦燃烧特性,重点考察了半焦内部矿物质和外部页岩灰床料对半焦燃烧的催化作用,揭示了流化床反应器中半焦燃烧过程和机理。结果表明:内部矿物质和外部床料对半焦燃烧均具有明显催化作用,而两者共同催化效果最为显著。矿物质中CaO和Fe₂O₃对半焦燃烧具有催化活性,CaO催化作用强于Fe₂O₃。油页岩半焦燃烧反应活化能在60.41～78.97 kJ/mol之间,矿物质的催化作用会明显降低反应活化能。流化床反应器中,矿物质对半焦燃烧的催化作用主要表现在四个反应,即:挥发分裂解和燃烧、半焦表面炭燃烧、半焦内部炭燃烧以及一氧化碳燃烧。     

吐哈盆地褐煤的热解和燃烧特性及动力学分析

吐哈盆地褐煤的热解和燃烧特性研究利于煤的清洁高效利用及煤炭地下气化的开展,为探究吐哈盆地褐煤煤粉颗粒的热解特性和燃烧特性及动力学特性,通过热重实验、热解特征指数计算、综合燃烧指数计算及动力学软件Kinetics Neo模型拟合法,研究了煤粉在不同升温速率(5℃/min, 10℃/min, 20℃/min)和不同粒径(大于0.8 mm, 0.2 mm～0.6 mm,小于0.1 mm)下分别在氮气气氛中的热解特性和空气气氛中的燃烧特性,获得了不同条件下煤粉颗粒热解和燃烧过程的动力学参数。结果表明：升温速率升高有利于煤粉颗粒热解和燃烧,显著提升了热解和燃烧性能;煤粉颗粒粒径增大有利于煤粉热解,不利于煤粉燃烧;不同粒径煤样热解和燃烧焦产率没有明显区别,粒径增加对于挥发分释放的影响不大;热解与燃烧过程中活化能与指前因子分别在一次热解阶段和干燥挥发阶段较高,说明在这两个阶段反应速率较慢,单位时间内化学反应程度较高。     

煤的流化燃烧

本文综述了流化燃烧的基础和工程方面的问题,主要讨论内容包括: ——流化两相理论应用于煤燃烧; ——将挥发分的析出、挥发分的燃烧动力学以及碳一氧和碳一二氧化碳反应动力学等方面有用的信息与流化床中煤的燃烧关连; ——用添加吸咐剂到床层固体中的方法抑制硫同时转化燃料氮成NO,然后用半焦使之部分还原成氮; ——热传递到埋于床中管子的速度;以及——在提高压力时操作的效果。最后,从流化燃烧的实际应用中讨论需要研究的问题。     

海藻生物质颗粒流化床燃烧试验研究

在小型流化床试验台上研究了海藻颗粒(条浒苔与马尾藻)的流化床燃烧。海藻在流化床内的挥发分析出燃烧时间都在1 min左右。条浒苔颗粒在流化床中燃烧先进行脱水和挥发分的燃烧,接着发生焦炭燃烧,其燃烧过程符合缩核模型,炭核由外向内逐层燃烧,而灰层半径几乎不变。但马尾藻颗粒由于挥发分的大量快速释放而迅速膨胀破碎成屑。另外通过对条浒苔颗粒及不同燃烧时间后收集的焦炭颗粒剖面的SEM扫描电镜观察,发现随着燃烧的进行,颗粒内孔隙增大,微孔表面粗糙。进一步详细研究了两种海藻颗粒(条浒苔与马尾藻)在流化床内单次投料下的燃烧。随着床温的升高,条浒苔释放NO_x相对浓度增加,CO相对浓度减少。而马尾藻释放气体中SO₂与NO_x含量相对条浒苔有所增加;随着床温的升高,CO相对浓度减少。床温的升高使得床内传热速率加快,两种海藻挥发分的析出提前,燃尽时间缩短。风速、床高的升高使得两种海藻燃烧容易,燃尽时间缩短。     

不连沟煤热解半焦燃烧特性研究

煤热解产生具有高利用价值的煤气和焦油,并伴随产生大量的热解半焦,燃烧是半焦的主要利用途径之一。本文采用非等温热重分析法研究了热解条件(热解温度和停留时间)、热解气氛和燃烧升温速率对热解半焦燃烧行为的影响,并利用Coats-Redfern积分法对半焦燃烧过程进行动力学计算。结果表明:热解温度对甲烷二氧化碳重整与煤热解耦合过程半焦的燃烧反应特性有重要影响。随热解温度升高,半焦燃烧反应性呈下降趋势,反应活化能逐渐增加,这与半焦中较低的挥发分成正相关。热解停留时间和热解气氛对半焦燃烧影响较小。与在氮气中热解半焦相比,加氢热解和耦合热解半焦表现出几乎相同的燃烧特征和反应活化能。燃烧升温速率显著影响半焦的燃烧特性,提高燃烧升温速率促使半焦燃烧反应在更高温度下进行。     

麦渣与制浆废液共混制备成型颗粒燃料及其燃烧特性分析

为研究麦渣与制浆废液共混制备的成型颗粒燃料的燃烧特性,通过热重分析法对其燃烧热力学及燃烧动力学进行了研究。结果表明：制浆废液的添加使颗粒燃料出现固定碳的二次燃烧阶段,有利于降低成型颗粒燃料的挥发分、固定碳燃烧阶段的点火温度及最大燃烧速率温度,对颗粒燃料的燃烧有正向协同作用;制备的颗粒燃料的一阶动力学模型拟合曲线的相关系数在0.95以上,颗粒燃料在挥发分燃烧和固定碳燃烧阶段的活化能和指前因子均随制浆废液的添加而降低。当废液固形物质量分数为53%时制备的成型颗粒燃料,其挥发分燃烧阶段和固定碳燃烧阶段的活化能为72.85和83.52 kJ/mol,指前因子为2.82×10⁶和3.73×10⁵ min^-1。制浆废液的添加使颗粒燃料更易燃烧,且燃烧过程稳定不易爆燃。     

煤粉着火燃烧过程光学诊断研究发展

煤炭储量丰富,尽管新能源和可再生能源快速发展,煤炭资源在未来几十年仍将作为我国一次能源重要组成部分。同时煤炭利用带来很多环境污染问题,因此未来煤炭资源的利用逐渐向高效、低碳、低污染物排放利用方式转变。随着光学技术的不断发展,涌现出多种适用于煤粉燃烧诊断的原位非接触式光学诊断技术,极大地促进了燃烧学的发展,为煤炭清洁高效利用提供了更多试验手段。介绍了国内外煤粉着火、不同方式下燃烧特性的光学诊断研究进展,对煤粉单颗粒和煤粉颗粒流的着火燃烧过程的光学诊断研究进行总结。目前常用的煤粉燃烧光学诊断技术主要包括全光谱成像、CH*/C2*化学发光成像、平面激光诱导荧光(PLIF)、双色/三色高温计、米氏散射、激光诱导白炽光、相干反斯托克斯-拉曼光谱、激光诱导击穿光谱等多种先进的光学诊断技术,可对煤粉单颗粒、颗粒流的着火延迟、脱挥发分、挥发分燃烧、着火模式、环境因素(环境温度、氧浓度、气氛)、富氧燃烧、水-氧燃烧、煤中碱金属释放等多方面关键问题进行光学诊断研究,为煤炭清洁高效利用提供了理论和试验基础。采用OH-PLIF和三色高温计对热解半焦和神华烟煤混合燃料共燃的着火和燃烧特性进行研究。综合考虑着火延迟和混合物的燃尽率,热解半焦的最佳掺混比为20%,为热解半焦的实际工业应用提供了参考。同时采用500 Hz、5 k Hz高时间、空间分辨率的OH-PLIF技术探究煤粉颗粒流中单颗粒挥发分燃烧的发展过程和挥发分着火的时序演变过程,通过二者的结合获得煤粉颗粒流从着火到挥发分燃烧的时间特性。采用OH-PLIF技术对烟煤和褐煤煤粉颗粒流燃烧火焰的脱挥发分和挥发分燃烧行为进行探究,提出采用OH信号径向分布的相对标准偏差探究火焰稳定性的方法。相同燃烧条件下,烟煤煤粉颗粒流燃烧的稳定性高于褐煤。基于OH-PLIF和CH*化学发光诊断技术,提出一种用于探究煤粉颗粒流中颗粒挥发分燃烧振荡特性分析方法——动态模态分解方法(DMD)。随着氧浓度的增加,挥发分火焰振荡增强。颗粒的聚集可能导致煤粉挥发分燃烧的低频振荡。相反,单独或分离的颗粒燃烧会产生较大的振荡频率。但目前取得的成果还不够完善,需要继续深入开展煤粉燃烧的光学诊断试验研究,对污染物NOx的生成及排放、新型水氧燃烧技术中水蒸气作用机理等方面深入探索,开发出新型清洁煤燃烧技术,为我国煤炭资源清洁高效利用做出贡献。     

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.     

A single particle model for the evolution and combustion of coal volatiles

A single particle model is proposed for the evolution and combustion of coal volatiles. The analysis is divided into preignition and postignition stages. The rate limiting steps for devolatilisation are assumed to be heat transfer (both to and through the coal particle) as well as chemical reaction (overall decomposition represented by the distributed activation energy kinetics). Approximate expressions are proposed for the estimation of volatile burnout times. The model results are compared with the experimental data reported for single coal particles in stagnant as well as convective oxidizing environments. The application of the model to fluidized beds is discussed. Model predictions are also compared with the volatile burnout times in fluidized beds.     

Fluidized combustion of char and volatiles from coal

This paper reviews the literature about fluidized combustion of char and volatiles from coal. While the rate of combustion of particles bigger than about 2 mm is mainly limited by the rate of diffusion of oxygen, there is usually an effect of chemical rate. As particle diameter decreases, the influence of chemical rate increases. Char particles are porous and combustion occurs in pores near their exterior. Char reactivity can be modelled by an effective pore area for combustion. At high bed temperatures (>1,150 K), the rate of combustion of volatiles is limited by the rate of mixing of fuel and oxygen. At low bed temperatures (<1,000 K), combustion reactions are inhibited by the inert particles in the bed.     

Relative roles of chemical and mass transfer rate control in fluidized-bed combustion of coal chars

The relative roles of mass transfer and chemical reaction in controlling combustion rates in fluidized beds are examined in the light of measurements of char-particle combustion kinetics and correlated data on gas—particle mass transfer in fluidized beds. It is concluded that at 1200 K mass transfer is the main rate-control influence for materials having reactivities the same as, or higher than, a char from a swelling bituminous coal. Mass transfer has little effect on the combustion rate of materials whose reactivity to oxygen is one-tenth that of the bituminous coal char. At 800 K, chemical reaction control is dominant for all reactivities considered. The main uncertainty in the present calculations arises from the lack of a suitable analysis of mass-transfer rates in conditions appropriate to fluidized-bed combustion.     

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.     

Fluidization of extremely large and widely sized coal particles as well as its application in an advanced chain grate boiler

A pyrolysis combustion technology (PCT) was developed for high-efficiency and environment-friendly chain grate boilers (CGBs). The realization of the PCT in a CGB requires that extremely large and widely sized coal particles should be first pyrolyzed in a semi-fluidized state before being transported into the combustion chamber of the boiler. This article was devoted first to investigating the fluidization of 0-40 mm coal particles in order to demonstrate the technical feasibility of the PCT. In succession, through mixing 0-10 mm and 10-20 mm coal particles in different proportions, multiple pseudo binary mixtures were prepared and then fluidized to clarify the effect of particle size distribution. With raw steam coal used as the feedstock, the superficial gas velocity of about 2.0 m/s may be suitable for stable operation of the fluidized-bed pyrolyzer in the CGB with the PCT. In the fluidization of widely sized coal particles, approximately half of the coal mass is segregated into the bottom section of the bed, though about 15% of 10-20 mm large particles are broken into 0-10 mm small particles because of particle attrition. The experimental results illustrate that an advanced CGB with the PCT has a high adaptability for various coals with different size distributions.     

Characteristics and mechanism of catalytic effect of inner minerals on combustion of oil shale coke

The micro fluidized bed reaction analyzer (MFBRA) was used to study the combustion characteristics of oil shale mineral catalytic semi-coke. This study compared the effect of minerals inside char and bed material (oil shale ash) outside char on char combustion, and the process and mechanism of char combustion in the fluidized bed were further revealed. Both of minerals inside char and bed material outside char had a marked catalysis for char combustion and their combined catalysis was most notable. It is found that the CaO and Fe₂O₃ were the major active components in oil shale minerals for catalytic combustion of char, and the catalysis of CaO was stronger than that of Fe₂O₃. The activation energy of char combustion ranged from 60.41 kJ/mol to 78.97 kJ/mol, and it would significantly decrease with presence of the catalysis by minerals in oil shale. For char combustion in a fluidized bed, the contribution of minerals to catalytic combustion was mainly reflected in four reactions, such as volatiles cracking and combustion, surface carbon combustion, internal carbon combustion and CO combustion.     

粒径对福建无烟煤在CFB锅炉中燃尽的影响

建立了福建无烟煤细颗粒燃烧模型,计算了其在容量35 t/h循环流化床锅炉炉膛内的燃尽时间和一次通过炉膛的停留时间,分析了不同粒径煤颗粒在不同燃烧温度和不同烟气流速时在CFB锅炉内的燃尽时间和停留时间的变化差异. 实验研究了福建无烟煤粒径对飞灰碳含量的影响及燃尽的影响. 结果表明,细煤颗粒的燃尽时间与停留时间均随粒径增大而增长,但燃尽时间增幅更明显,颗粒一次通过炉膛完全燃尽的临界粒径约为0.15 mm;粒径越大的颗粒其停留时间和燃尽时间对烟气流速和燃烧温度变化越敏感;无烟煤入炉粒径明显影响CFB锅炉飞灰含碳量,选用粒度为3~8 mm的偏粗颗粒为宜.     

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.