随机孔模型应用于煤焦燃烧破碎机理的研究

煤焦颗粒是典型的多孔介质,在燃烧过程中会发生复杂的物理变化,如内部孔隙结构的变化、比表面积的变化等,最终完成碳的燃烧发生破碎,这一系列的变化过程很难进行热态的直接观察,但是可以选择合适的模型进行数值模拟.采用随机孔模型在孔结构变化的模拟过程中取值更贴近实际反应,对平顶山烟煤在恒定温度下的燃烧过程进行模拟后,与实验结果进行比较,发现二者的拟合程度较高.     

富氧气氛下水蒸气气化对煤焦燃烧的影响

采用自制的恒温高升温速率热重实验台,研究了富氧气氛下水蒸气气化对煤焦燃烧特性的影响,并使用低温氮吸附仪和环境扫描电子显微镜分析燃烧过程中煤焦孔隙结构。结果表明:在低氧浓度下,水蒸气气化作用对煤焦燃烧影响显著,可失重速率增大,燃尽时间提前,且随氧气浓度的增加而减弱;随着温度升高R0.5指数逐渐增加,当环境温度为1 000℃、水蒸气浓度为20%时,R0.5增长速度最大;煤焦燃烧过程中,加水后煤焦比表面积增大,孔隙结构丰富。     

不同工况和粒径下飞灰孔隙结构的实验研究

通过氮气等温吸附(77K)方法对不同燃烧工况下燃煤电厂排放的不同粒径飞灰的孔隙结构进行了测试、分析,并通过BET法计算了其比表面积.低压下的吸附等温线和高压下的脱附等温线分别用HK方法和BJH方法解析.结果表明:飞灰的比表面积和孔容积随着氧浓度和钙硫比的增加而增大,随着煤焦比的减小而增大;随着粒径的减小,飞灰孔隙结构变得发达,特别是粒径小于2.5μm时,比表面积、孔容积明显增大.     

煤粉中内在矿物对煤焦燃烧反应活性的影响

用浮沉法分选出中等密度级别的煤粉,在马弗炉上以900℃制焦,用HCl-HF-HCl法对煤焦进行脱灰处理,用TGA研究煤焦脱灰前后燃烧反应活性的变化,用N2吸附法研究孔结构的变化,用可溶离子萃取法研究可溶金属离子含量的变化.结果发现,含内在矿物的无烟煤煤焦和褐煤煤焦脱灰后低温反应速率明显降低,而含内在矿物的烟煤煤焦脱灰后低温反应速率变化不大,所有样品脱灰后高温反应速率都明显提高.机理研究表明,这些变化是煤焦比表面积、孔容、颗粒粒径变化和可溶离子含量在不同温度下综合影响的结果.     

影响煤焦比表面和孔隙特性因素的灰色关联分析

煤焦比表面积和孔隙特性直接决定着煤焦的反应性和煤焦的燃尽时间。影响煤焦比表面积和孔隙特性的因素很多，其关系也相当复杂。本文对实验测得的不同煤种煤焦的比表面积和平均孔径与各影响因素之间的关系进行了灰色关联分析。结果发现，水分、挥发分和矿物质对煤焦的比表面积影响较大，而镜质组含量、固定碳和灰分对煤焦的孔隙特性有较大影响。     

热解时加热速率对煤粉孔隙结构变化的影响

为研究热解时煤粉颗粒孔隙结构的变化,采用随机行走的方法生成煤粉颗粒的孔隙结构,采用碎片化和扩散煤热解模型描述热解时的化学反应,基于分子运动论描述气体在分形孔隙中的扩散,构建了一个描述孔隙结构变化的数学模型.通过对模型计算结果的拟合,得到描述孔隙结构变化的微分方程组,并用沉降炉热解实验对方程组进行了验证.应用该微分方程组研究了加热速率对孔隙结构变化的影响,结果表明,在膨胀阶段和收缩阶段,4个孔隙结构参数的变化速率都存在一个峰值.随着加热速率的提高,煤焦的膨胀率、比表面积和分形维数先增大后减小,而煤焦的孔隙率是一直增大的.     

O_2_CO_2气氛快速升温煤焦低温氮吸附等温线形态分析

利用热力工况与实际煤粉炉相似的沉降炉实验装置,在快速升温条件下制备了两种燃烧气氛(O2/CO2和O2/N2)、不同O2浓以及不同停留时间下的煤焦试样,采用AS-AP2020M型全自动比表面积及孔隙度分析仪测定了各煤焦试样的低温氮吸附等温线.分析表明,各不同条件下获取的煤焦试样的吸附等温线形态均为典型的Ⅱ型吸附等温线(即反S型),煤焦具有小至分子级的孔(孔径约0.86nm)大至无上限孔(相对)的较连续的完整的孔体系.吸附回线形态的分析可知,煤焦试样的孔隙可能包含大量不产生吸附回线的盲孔(一端封闭的圆筒孔、一端封闭的平行板状或劈尖状孔)以及部分的裂缝孔隙.结果表明,燃烧气氛的改变并未使得煤焦燃烧过程孔隙的形成与发展发生显著的变化,但与O2/N2气氛相比,在O2/CO2气氛下所取焦样的等温线与之存在着细微的差别,其表明不同条件下的焦样存在着不同的孔径分布.实验结果为进一步研究高浓度CO2气氛下煤粉的燃烧过程及其与常规燃烧模式的差异提供了一定的帮助.     

烟煤煤焦的CO2气化反应

采用TG-FTIR方法,在反应温度为950～1300℃时,研究了几种典型煤种及其在高温下慢速和快速热解煤焦的CO2气化反应特性.对4种原煤及其1200℃快、慢速热解条件下煤焦气化产物CH4和CO进行了实时检测和分析.同时对煤焦的孔隙结构和化学组成进行了分析.结果表明,各种热解煤焦的反应速率随气化温度的升高而增大,当达到最大值后随温度的升高而下降;4种煤焦的活化能随热解和气化温度的升高而增大;煤焦气化过程释放CH4和CO的特性与原煤的趋势相似,但原煤热解气化过程中释放CH4的质量浓度比不同热解速率制得煤焦的热解气化释放CH4的质量浓度高出2个数量级,快焦相比慢焦释放出更高质量浓度的CH4;各种煤焦的BET比表面积都较小(除神府慢焦外都小于2 ㎡/g);快焦的气化活性比慢焦的好.     

煤粉颗粒热解时的膨胀与其物理化学性质的关系

通过建立数学模型来研究煤粉颗粒热解时的膨胀规律,用分形孔隙模型生成煤粉的孔隙结构,描述热解过程发生的化学反应、气体在孔隙中的扩散和煤粉颗粒的膨胀与收缩.对模型计算结果进行拟合,得到描述煤粉颗粒膨胀的微分方程,通过与其他研究者模型计算结果的对比验证该微分方程,分析了煤种的官能团含量、煤粉颗粒的孔隙率和比表面积对其膨胀的影响,并定义了一个综合了煤的物理化学性质的参数X.结果表明:官能团含量上升,煤粉颗粒的最大膨胀率和煤焦的膨胀率增大;孔隙率或比表面积增大,煤粉颗粒的最大膨胀率和煤焦的膨胀率减小;参数X越大,煤粉颗粒在热解过程中会发生更大的膨胀.     

半焦孔隙结构的影响因素

用氮气等温吸附(77K)方法测量了原煤及其加压、常压部分气化后半焦的BET比表面积,并通过BJH法计算了孔比表面积、孔容积、孔径和孔分布。根据测试结果,从气化操作条件、半焦颗粒粒径、半焦工业分析3方面分析了影响半焦孔隙结构的因素。常压喷动流化床气化中,挥发分析出或热解对半焦孔隙的生成和发展起到主导作用;而加压气化过程中,炭发生的气化反应对半焦孔隙的生成和发展有更加重要的影响。实验中发现在一定的气化工况下,煤焦存在一个合适的颗粒尺寸范围,能形成比较大的孔比表面积和孔容积,有利于增强煤焦的气化反应。     

煤焦的微观结构对N2O的生成与分解特性的影响

选取５种煤焦，在一个小型循环流床上进行燃烧，研究Ｎ２Ｏ的生成，并就这几种煤焦对Ｎ２Ｏ的分解特性进行了实验，用液氮吸附法对５种煤焦的微观结构进行了分析，就Ｎ２Ｏ在煤焦内部的扩散机理、煤焦的微观特性对煤焦燃烧生成Ｎ２Ｏ或分解Ｎ２Ｏ的特性的影响进行了分析和讨论。     

混煤煤质及燃烧特性研究

针对混煤的煤质特性和燃烧特性开展实验研究,以指导燃煤电站科学合理的燃用混煤。研究结果表明,混煤的元素分析、工业分析及发热量满足质量加权平均,但混煤的可磨性和灰熔融特性不满足加权平均,低灰熔点煤中掺烧高灰熔点煤能显著提高混煤灰熔点,改善锅炉燃烧过程中的结渣问题,混煤灰熔点变化受到单煤灰成分的影响。热重实验分析表明,混煤的剧烈燃烧阶段与单煤存在明显差异,混煤的燃烧特性介于参与掺混的单煤之间,但不满足线性叠加,其燃烧过程存在着不同程度的交互作用。混煤的着火特性接近于易燃煤,而燃尽特性与难燃煤相近。除此以外,随着氧浓度的降低,混煤的燃烧特性明显变差,易燃煤对氧浓度的变化更加敏感。     

Combustion behavior in air of single particles from three different coal ranks and from sugarcane bagasse

Comparative combustion studies were performed on particles of pulverized coal samples from three different ranks: a high-volatile bituminous coal, a sub-bituminous coal, and two lignite coals. The study was augmented to include observations on burning pulverized woody biomass residues, in the form of sugarcane bagasse. Fuel particles, in the range of 75–90 μm, were injected in a bench-scale, transparent drop-tube furnace, electrically-heated to 1400 K, where they experienced high-heating rates, ignited and burned. The combustion of individual particles in air was observed with three-color pyrometry and high-speed high-resolution cinematography to obtain temperature–time–size histories. Based on combined observations from these techniques, in conjunction to morphological examinations of particles, a comprehensive understanding of the combustion behaviors of these fuels was developed. Observed differences among the coals have been striking. Upon pyrolysis, the bituminous coal chars experienced the phenomena of softening, melting, swelling and formation of large blowholes through which volatile matter escaped. Combustion of the volatile matter was sooty and very luminous with large co-tails forming in the wake of the particle trajectories. Only after the volatile matter flames extinguished, the char combustion commenced and was also very luminous. In contrast, upon pyrolysis, lignite coals became fragile and experienced extensive fragmentation, immediately followed by ignition of the char fragments (numbering in the order of 10–100, depending on the origin of the lignite coal) spread apart into a relatively large volume. As no separate volatile matter combustion period was evident, it is likely that volatiles burned on the surface of the chars. The combustion of the sub-bituminous coal was also different. Most particles experienced limited fragmentation, upon pyrolysis, to several char fragments, with or without the presence of brief and low-luminosity volatile flames; other particles did not fragment and directly proceeded to char combustion. Finally combustion of bagasse was once again very distinctive. Upon pyrolysis, long-lasting, low-luminosity, nearly-transparent spherical flames formed around slowly-settling devolatilizing particles. They were followed by bright, short-lived combustion of the chars. Both volatiles and chars experienced shrinking core mode of burning. For all fuels, flame and char temperature profiles were deduced from pyrometric data and burnout times were measured. Combustion rates were calculated from luminous carbon disappearance measurements, and were compared with predictions based on published kinetic expressions.     

非线性优化掺烧模型及应用

为满足电站燃煤锅炉在安全稳定运行的前提下降低燃料成本,提出多种煤优化掺烧研究。采用线性回归方法建立掺烧煤的发热量、挥发分、硫分、水分和灰分的预测公式,应用径向基人工神经网络建立掺烧煤的软化温度预测模型,从而建立了新的非线性优化掺烧模型。珠海电厂掺烧试验表明,所建模型能指导电厂多煤种优化掺烧,取得显著的经济效益。     

NOx and H2S formation in the reductive zone of air-staged combustion of pulverized blended coals

Low NO_x combustion of blended coals is widely used in coal-fired boilers in China to control NO_x emission; thus, it is necessary to understand the formation mechanism of NO_x and H₂S during the combustion of blended coals. This paper focused on the investigation of reductive gases in the formation of NO_x and H₂S in the reductive zone of blended coals during combustion. Experiments with Zhundong (ZD) and Commercial (GE) coal and their blends with different mixing ratios were conducted in a drop tube furnace at 1200°C–1400°C with an excessive air ratio of 0.6–1.2. The coal conversion and formation characteristics of CO, H₂S, and NO_x in the fuel-rich zone were carefully studied under different experimental conditions for different blend ratios. Blending ZD into GE was found to increase not only the coal conversion but also the concentrations of CO and H₂S as NO reduction accelerated. Both the CO and H₂S concentrations inblended coal combustion increase with an increase in the combustion temperature and a decrease in the excessive air ratio. Based on accumulated experimental data, one interesting finding was that NO and H₂S from blended coal combustion were almost directly dependent on the CO concentration, and the CO concentration of the blended coal combustion depended on the single char gasification conversion.Thus, CO, NO_x, and H₂S formation characteristics from blended coal combustion can be well predicted by single char gasification kinetics.     

混煤燃烧过程中NOx排放特性试验研究

针对山东动力用煤,研究了混煤燃烧NOx的生成及煤质、温度、掺混比、掺混煤种对NOx生成的影响.研究结果表明,混煤NOx的生成量,主要取决于混煤中的氮含量;在试验条件下温度越高生成的NOx反而减少;HT贫煤随掺混FF无烟煤的增大,NOx浓度升高;掺混煤种对NOx释放有较大的影响.混煤燃烧过程中NOx释放往往出现"双峰",同时燃烧工况、掺混比例及煤种影响"双峰"的形成.     

Effect of temperature on Lu’an bituminous char structure evolution in pyrolysis and combustion

In the process of pyrolysis and combustion of coal particles, coal structure evolution will be affected by the ash behavior, which will further affect the char reactivity, especially in the ash melting temperature zone. Lu’an bituminous char and ash samples were prepared at the N₂ and air atmospheres respectively across ash melting temperature. A scanning electron microscope (SEM) was used to observe the morphology of char and ash. The specific surface area (SSA) analyzer and thermogravimetric analyzer were respectively adopted to obtain the pore structure characteristics of the coal chars and combustion parameters. Besides, an X-ray diffractometer (XRD) was applied to investigate the graphitization degree of coal chars prepared at different pyrolysis temperatures. The SEM results indicated that the number density and physical dimension of ash spheres exuded from the char particles both gradually increased with the increasing temperature, thus the coalescence of ash spheres could be observed obviously above 1100°C. Some flocculent materials appeared on the surface of the char particles at 1300°C, and it could be speculated that β-Si₃N₄ was generated in the pyrolysis process under N₂. The SSA of the chars decreased with the increasing pyrolysis temperature. Inside the char particles, the micropore area and its proportion in the SSA also declined as the pyrolysis temperature increased. Furthermore, the constantly increasing pyrolysis temperature also caused the reactivity of char decrease, which is consistent with the results obtained by XRD. The higher combustion temperature resulted in the lower porosity and more fragments of the ash.     

恒温下准东煤及其混煤燃烧过程中NO释放特性研究

基于恒温热重-燃烧污染物在线监测系统,通过对NO瞬时释放曲线的分析,并结合燃烧反应动力学计算,研究了恒温条件下燃烧环境温度对准东煤燃烧过程中NO释放特性的影响。结果表明：随温度升高,单煤和混煤燃烧过程中NO的释放时间会显著降低,释放速度显著提高;煤种成分的差异会导致NO释放特性的差异,固定碳挥发分的质量分数对NO释放有影响,而灰分的质量分数对NO释放无影响,当高固定碳、高挥发分的煤与低固定碳、低挥发分的煤进行掺混燃烧时,混煤的NO释放量和释放速率会降低。当燃烧过程中掺混北山煤时,混煤NO释放量低,释放速率慢,可以看作是一个较为优良的混煤掺烧方案。     

O2/CO2/N2气氛下玉米秸秆混煤燃烧特性及动力学分析

以玉米秸秆和煤粉为原料,在不同原料配比,不同升温速率下,利用热重分析仪在模拟锅炉气氛下进行燃烧实验,采用Flynn-Wall-Ozawa法建立动力学模型,研究模拟锅炉气氛下玉米秸秆及其混煤燃烧的燃烧特性及其动力学,对比相同实验条件空气气氛下的燃烧工况结果表明,燃料的综合燃烧特性指数SN随升温速率的增大而成倍增长,因掺入煤粉的比例加倍而减半;随着掺混煤比例的增大,失重速率(DTG)曲线上固定碳燃烧阶段逐渐分化为2个失重峰,模拟锅炉气氛下分化现象更为明显;煤粉的掺入会使燃烧过程所需表观活化能波动增大。     

600MW机组锅炉配煤掺烧实验研究

随着燃煤价格的攀升,燃料成本在火力发电厂运行成本中所占比重也逐步上升。在保证机组正常运行的前提下,研究、推广配煤掺烧,是节约能源、降低成本的有效措施之一。以600 MW机组为研究对象进行配煤掺烧实验,并对实验数据进行分析。为今后配煤掺烧的研究提供理论及数据参考。