颗粒尺寸对生物质粉阴燃过程影响的实验研究

实验研究颗粒尺寸对红松锯末和玉米秸秆粉自然向下阴燃过程的影响。结果表明:颗粒尺寸对不同物料影响不同,玉米秸粉床层最高温度随颗粒尺寸的减小而升高,阴燃传播速率基本不受尺寸影响;红松锯末阴燃传播速度随颗粒尺寸的减小而减小,床层最高温度变化不明显。随颗粒尺寸的减小,两种物料床层均有出现大裂纹的趋势。     

焙烧温度对球状Ni-Al纳米催化剂结构及生物质催化热解的影响研究

采用水热法制备三维Ni-Al纳米结构催化剂,并利用多种表征手段和稻壳催化热解实验研究焙烧温度（500~800 ℃）对催化剂的整体结构及催化性能的影响。结果表明：催化剂为球状结构,活性位点分布均匀,焙烧温度对材料结构有显著影响,800 ℃焙烧条件下球状结构有向内塌陷的趋势。相较于无催化条件下的稻壳热解产物,催化热解后焦油产率明显减小,产气量大幅提高。500 ℃焙烧制备的Ni-Al催化剂作用条件下,稻壳热解气体产物中H₂/CO最大可达2.66,600 ℃焙烧条件下可获得最大合成气产量737 mL/g,而700 ℃焙烧条件下可获得最低的焦油产率（13.5%）。材料表征发现,反应后的催化剂仍具有稳定的球状结构与活化性能。     

玉米秸秆成型生物炭燃烧特性与动力学分析

采用热重法对玉米秸秆成型颗粒及生物炭燃烧特性及动力学进行研究,并考察不同升温速率下(10、20、40℃/min)对320、500℃热解生物炭燃烧特性的影响,分析其燃烧特性及动力学参数。结果表明:玉米秸秆成型颗粒炭化后其燃烧热重分析(DTG)曲线呈现多峰状态,峰整体向高温区转移;玉米秸秆成型颗粒的着火温度和燃尽温度均小于生物炭,综合燃烧特性指数(SN)大于各温度热解炭化后样品。随着炭化温度的升高,成型生物炭着火温度和燃尽温度升高,SN减小;一级反应动力学能很好地描述各样品的燃烧动力学,相关系数(R~2)均高于0.9。     

柠条燃烧特性及燃烧动力学研究

应用热分析仪对柠条生物质燃料的燃烧过程进行分析,研究颗粒度、升温速率和风量对燃烧特性与动力学参数的影响。结果表明:(1)颗粒度为0.16 mm试样在升温速率为20 K/min,风量为40 mL/min的工况下,着火温度为221.1℃,最大燃烧速率温度为336.2℃,燃尽温度为559.4℃,最大燃烧速率0.6 mg/min,平均燃烧速率为0.129mg/min,相对于10 K/min和30 K/min升温速率,20 K/min工况下的燃料动力学参数最优,活化能为39.094 kJ/mol,频率因子为2.175×10~7L/min;(2)升温速率的增大会使平均燃烧速率和燃烧特性指数增大,着火温度降低;风量对燃烧速率无影响,但较大风量不利于挥发分析出和燃烧稳定性;颗粒度对挥发分析出有显著影响,颗粒度较大时需较高升温速率和风量才可充分燃烧,而颗粒度较小时即使风量较小也能充分燃烧。     

隔板式内循环流化床的流动特性研究

以石英砂和稻壳为实验床料,在隔板式内循环流化床气化炉冷态实验装置上对颗粒的内循环流动特性进行了研究,考察了高速区和低速区的流化速度、结构尺寸和侧风量等对颗粒内循环流动的影响.结果表明:在保持低速区流化速度一定的条件下,随着高速区风速增大,颗粒循环量先增大后减小;流化速度不变的条件下,颗粒循环量随孔口和侧风量的增大而增加,但增加趋势逐渐变缓.实验给出了合理的运行设计参数.通过实验数据回归,得到了石英砂和稻壳通过隔板式内循环流化床孔口的颗粒循环量关联式,计算结果与实验值误差分别小于6%和14%,能较好地预测孔口颗粒流动.     

神华烟煤煤粉中低温热解半焦的粒径和形态特性

选择挥发分较高的神华烟煤煤粉用固定床反应器进行300℃至700℃的中低温热解,使用筛分法研究半焦颗粒粒径变化,结合扫描电子显微镜、傅里叶红外光谱和低温氮吸附考察半焦颗粒的微观形态、官能团和孔隙结构变化,讨论颗粒尺寸变化的机理。结果表明,颗粒粒径随温度升高而减小。在温度低于400℃时,活泼的热解反应尚未进行,由于水分和少量挥发物的析出导致部分中孔孔隙收缩,此时半焦粒径轻微减小,且主要是粒径较大的发生轻微变化;热解温度升高到500℃及以上,在加剧的缩聚反应作用下,颗粒粒径剧烈减小,内部孔径分布变化加剧,同时收缩产生的裂痕促使更多碎块产生,进一步减小了颗粒的粒径。     

不同粒径稻壳粉堆积密度试验研究

生物质物料的堆积密度是确定料斗尺寸和设计进料装置的基础。通过对稻壳粉进行筛分,测量了不同粒径稻壳粉的松散堆积密度和振实堆积密度。当稻壳粉粒径d逐渐减小时．2种堆积密度均先增大后减小,当0．074mm〈d≤0．105mm时达到最小值。当d≤0．074mm时,振实堆积密度达到最大值。采用生物显微镜对稻壳粉进行观察,从堆积形态上解释了稻壳粉堆积密度随粒径变化的原因,为稻壳的工业化利用提供了必要的数据。     

不同热解温度下稻壳炭的理化特性分析

以稻壳为原料,分别在400,500,600℃条件下热解制备生物炭,通过工业分析和对粒径分布、炭得率、碘吸附值、亚甲基蓝吸附值等指标的测定,以及扫描电镜(SEM)的观察和红外光谱的分析,系统研究了热解温度对稻壳炭理化特性的影响。研究表明,随着热解温度的升高,稻壳炭中的小粒径颗粒所占比例增大;从其组成来看,炭得率和挥发分含量不断降低,而灰分和固定碳含量呈上升趋势。从碘和亚甲基蓝吸附值的变化和扫描电镜观察结果可以看出,随着热解温度的升高,稻壳炭的孔隙结构更为发达。红外光谱分析证实,随着温度的升高,稻壳炭中的官能团数量下降,种类逐渐减少,低温热解所获炭中各类官能团更为丰富。     

煤粉颗粒群着火和燃烧过程的数值模拟

建立了一维非稳态球形煤粉颗粒团的群燃烧模型.数值模拟煤粉颗粒团的着火和燃烧过程,获得了颗粒团燃烧火焰随时间的变迁.分析了煤粉颗粒团内部参数和外部环境参数对颗粒团着火和燃烧的影响.随着颗粒团内煤粉浓度的增加,颗粒团的均相着火延迟先减小后增加.增加煤粉颗粒尺寸和降低外部温度都会明显延迟均相着火.环境氧气含量的增加会减小着火延迟,同时增加颗粒团的燃烧速率.模拟计算和文献试验结果的变化趋势相吻合.     

酸洗预处理对稻壳快速热解产物的影响

配制不同浓度盐酸溶液对稻壳进行酸洗预处理,采用热裂解-气相色谱质谱联用装置对样品快速热解并分析产物,结果表明:经过酸洗预处理后,热解温度低于600℃时,稻壳快速热解得到的蒸汽产物总峰面积减小,而热解温度高于600℃时总峰面积增大。将热解蒸气产物分类,研究造成总峰面积变化的原因并分析盐酸溶液浓度及热解温度对产物的影响。结果表明:酸洗处理可显著促进左旋葡萄糖和糠醛的生成,而乙酸、1,2-环戊二酮及苯酚的产率明显降低;部分产物的酸洗效果随热解温度的变化呈相反的变化。总产物峰面积的增大主要由纤维素及半纤维素的热解产物贡献。     

A comparison of various models in predicting ignition delay in single-particle coal combustion

In this paper, individual coal particle combustion under laminar conditions is simulated using models with various levels of complexity for the particle and gas phase chemical kinetics. The mass, momentum and energy governing equations are fully coupled between the particle and the gas phase. In the gas phase, detailed chemical kinetics based on GRI3.0 and infinitely-fast chemistry are considered and compared. For the particle phase, models for vaporization, devolatilization and char oxidation/gasification are considered, and the Kobayashi–Sarofim devolatilization model is compared to the Chemical Percolation Devolatilization (CPD) model. Ignition delay is used as a quantitative metric to compare the simulation prediction with experimental data, with careful attention given to the definition of ignition delay in the simulations. The effects of particle size, coal type and gas-phase temperature on the ignition delay are studied and compared with experimental data.     

煤颗粒热膨胀破碎特性的试验研究

煤颗粒的热膨胀破碎特性直接影响流化床锅炉的运行效率。利用热机械分析仪(TM A)测定了不同种类不同密度的型煤受热膨胀的特性,并对部分破碎微观形貌利用场发射扫描电子显微镜(FSEM)进行了观察;通过马弗炉内的燃烧试验研究了热膨胀特性和燃烧破碎的关系。研究表明,煤颗粒在燃烧过程中其热膨胀破碎主要发生在挥发分析出阶段;内部挥发分的析出会使颗粒内压增大而产生膨胀,进而产生细小裂纹并破碎;挥发分越高,颗粒密度越大,其热膨胀形变率越大,越容易发生破碎现象;主要挥发分析出后热膨胀引起的破碎可以忽略。     

Combustion of single coal particles in a jet

Fluidized bed combustion has attracted much interest in recent years, but there is very little data on the behavior of coal particles at these new conditions. Coal of much larger diameter (1–10 mm), much lower furnace temperatures (~850 °C), and different fluid mechanical conditions exist compared to pulverized coal furnaces. This paper presents experimental data on the behavior and combustion rates of individual coal particles aerodynamically suspended in a heated jet, to stimulate flow conditions in a fluidized bed.Tests of bituminous, sub-bituminous and lignite coals from 2 to 12 mm at jet temperatures of 705 and 816 °C in air and air diluted with equal parts of nitrogen were conducted. The ignition delay time varied from 2 to 44 sec. The devolatilization time extended up to 80 sec and was dependent mainly on particle size. The total burn time was independent of coal type and temperature, and varied as the square of the size and inversally with the oxygen concentration. The total turn time varied from 25 to 740 sec independently of coal type. The square law for the char burning rate was investigated.     

Kinetics of devolatilization and oxidation of a pulverized biomass in an entrained flow reactor under realistic combustion conditions

In this paper the results of a complete set of devolatilization and combustion experiments performed with pulverized (∼500 μm) biomass in an entrained flow reactor under realistic combustion conditions are presented. The data obtained are used to derive the kinetic parameters that best fit the observed behaviors, according to a simple model of particle combustion (one-step devolatilization, apparent oxidation kinetics, thermally thin particles). The model is found to adequately reproduce the experimental trends regarding both volatile release and char oxidation rates for the range of particle sizes and combustion conditions explored. The experimental and numerical procedures, similar to those recently proposed for the combustion of pulverized coal [J. Ballester, S. Jiménez, Combust. Flame 142 (2005) 210-222], have been designed to derive the parameters required for the analysis of biomass combustion in practical pulverized fuel configurations and allow a reliable characterization of any finely pulverized biomass. Additionally, the results of a limited study on the release rate of nitrogen from the biomass particle along combustion are shown.     

燃煤链条炉排工业锅炉燃烧数值模型研究

针对燃煤链条炉排工业锅炉的燃烧过程中床层内部存在复杂的传热、传质及物理化学反应过程等特点开发了三维床层和炉膛耦合的燃烧数值计算模型,模型包含了煤燃烧过程中水分蒸发、挥发析出、气相反应、焦炭燃烧和传热传质等基本的化学物理过程,同时考虑了大粒径煤颗粒内部的非等温传热特性,并通过实验测试与数值模拟对数值模型进行校核,实验结果与模型计算吻合得较好,从而验证了计算模型的准确性。燃煤链条炉排工业锅炉燃烧数值模型的建立为实现燃煤工业锅炉的优化设计和运行指导提供了先进的设计手段和理论支持。     

Black liquor devolatilization and swelling—a detailed droplet model and experimental validation

In this paper, we present results from a new detailed physical model for single black liquor droplet pyrolysis and swelling, and validate them against experimental data from a non-oxidizing environment using two different reactor configurations.

In the detailed model, we solve for the heat transfer and gas phase mass transfer in the droplet and thereby, the intra-particle gas–char and gas–gas interactions during drying and devolatilization can be studied. In the experimental part, the mass change, the swelling behaviour, and the volume fraction of larger voids, i.e. cenospheres in the droplets were determined in a non-oxidizing environment. The model gave a good correlation with experimental swelling and mass loss data. Calculations suggest that a considerable amount of the char can be consumed before the entire droplet has experienced the devolatilization and drying stages of combustion. Char formed at the droplet surface layer is generally consumed by gasification with H₂O flowing outwards from the droplet interior. The extent of char conversion during devolatilization and the rate of devolatilization are greatly affected by swelling and the formation of larger voids in the particle. The more the particle swells and the more homogeneous the particle structure is, the larger is the conversion of char at the end of devolatilization.     

Formation of the structure of chars during devolatilization of pulverized coal and its thermoproperties: A review

The paper provides an overview of current studies on the behaviour of coal during devolatilization, especially the experimental studies and modelling efforts on the formation of char structure of bituminous coals in the open literature. Coal is the most abundant fossil fuel in the world. It dominates the energy supply in the future and plays an increasing role particularly in the developing countries. Coal utilization processes such as combustion or gasification generally involve several steps: i.e., the devolatilization of organic materials, homogeneous reactions of volatile matter with the reactant gases and heterogeneous reactions of chars with the reactant gases. The devolatilization process exerts its influence throughout the life of the solid particles from the injection to the burnout, therefore is the most important step which needs to be understood. When volatile matter is generated, the physical structure of a char changes significantly during the devolatilization, some accompanying a particle's swelling. The complexity of a char's structure lies in the facts that the structure of a char itself is highly heterogenous inside an individual particle and between different particles and the chemistry of a char is strongly dependent on the raw coal properties. A char's structure is strongly dependent on the heating conditions such as temperature, heating rate and pressure. Understanding the swelling of coal and the formation of char's pore structure during the devolatilization of pulverized coal is essential to the development of advanced coal utilization technologies. During combustion and gasification of pulverized coal, the behaviour of individual particles differs markedly due to the variation of their maceral composition. Particles with different maceral constituents generate different types of char structure. The structure of a char has a significant impact on its subsequent heterogeneous reactions and ash formation. The review also covers the most recent studies carried out by the authors, including the experimental observations of the thermoplastic behaviour of individual coal particles from the density fractions using a single-particle reactor, the experimental analysis on chars prepared in a drop tube furnace using the density-separated coal samples, the development of a mathematical model for the formation of char's pore structure based on a simplified multi-bubble mechanism and the investigation on the effect of pressure on char formation in a pressurized entrained-flow reactor.     

Spouted and spout-fluid bed combustors 1: Devolatilization and combustion of coal volatiles

The devolatilization and volatile combustion of a single coal particle in spouted and spout-fluid beds have been studied. The results showed that the flame extinction time increases with the particle diameter, and decreases with the bed temperature. When the bed temperature and the air flow rate were fixed, the operation modes (spouted or spout-fluid bed) showed less effect on the mean flame extinction time. A mathematical model of the spouted bed mode for preignition and postignition periods has also been developed assuming the devolatilization rate to be controlled by heat transfer and multireaction pyrolysis kinetics based on volatile products. Ignition, heat transfer back from the volatile flame to the particle surface, variation in flame temperature, and the hydrodynamics of SB are taken into account. The model predictions, with some adjusting parameters, were in good agreement with experimental results.     

Influence of functional group structures on combustion behavior of pulverized coal particles

The ignition and combustion behavior of pulverized coal was studied with respect to coal rank in a custom-designed visual drop tube furnace. The results showed that low-rank coals were ignited in a shorter time, mainly due to the presence of larger amounts of functional groups, while the ignition delay time of high-rank coals was longer. With increasing temperature and particle size, the ignition mode of coals shifted from heterogeneous into homogeneous, which was related to the increased yield of volatile matter. The chemical percolation devolatilization analysis results showed a clear relationship between the yield and composition of volatile matter and the amount and type of functional groups in coal. In addition, the tar yield was consistent with the amount of aliphatic hydrocarbons and the length of aliphatic chains, which explained the tailing combustion mode of the bituminous coal. The findings of the study showed that the yield and composition of volatiles in coal had a significant impact on the ignition behavior, which depended on the composition of functional groups, particle size, and the combustion environment.     

The devolatilization of coal and a comparison of chars produced in oxidizing and inert atmospheres in fluidized beds

This is a study of the devolatilization of coal in a laboratory-scale bed of silica sand, fluidized with either air or N₂ and electrically heated to 750 or 900°C. Coal particles (diameter 1.4–1.7 or 2.0–2.36 mm) were fed in batches to the surface of the bed and allowed to devolatilize in either an oxidizing atmosphere of air or inert N₂. In the first case, combustion of the volatiles occurred, but there was only thermal decomposition (pyrolysis) in the second situation. The resulting chars were recovered and analyzed for composition and structure, so that comparisons could be made between the effects of devolatilization with combustion and of pyrolysis in an inert atmosphere. It was found that the fractions of C and H in the char were only slightly sensitive to the type of fluidizing gas used. The amount of nitrogen in the recovered char and also the devolatilization time showed no dependence on the type of fluidizing gas, whereas BET areas were slightly larger after combustion in air. It is concluded that these effects are small relative to other errors, inherent in experiments on coal combustion, so that chars prepared in a bed fluidized by hot N₂ are very similar to those formed during coal combustion at nominally the same temperature. Equally, the overall composition of the volatile matter released during combustion in a fluidized bed is the same as in pyrolysis in nitrogen. The effects of other parameters, such as the temperature of the bed, the flow rate of the fluidizing gas and the size of the coal particles are also discussed in detail. It is concluded that most of the volatiles burn in a fluidized bed (at or less than 900°C) far away from the original coal particle. Also, NO_x is in effect a primary product of devolatilization, being produced in appreciable amounts when coal is heated in inert N₂. The ratio of C/N in the volatiles is found to be a constant during the latter stages of devolatilization, but beforehand at lower temperatures, carbon species are preferentially released. Overall, devolatilization of small particles (< 2.4 mm) in a fluidized bed at 900°C is kinetically controlled. The activation energy is small, being 15 ± 6 kJ/mol.