不同负荷下循环流化床锅炉粉煤灰的理化性质研究

为拓展循环流化床锅炉粉煤灰的利用途径,开发粉煤灰综合利用技术,研究了不同负荷下循环流化床(CFB)锅炉粉煤灰的粒径分布、化学组成、物相组成、Al_2O_3溶出特性和微观形貌等理化性质,考察了锅炉负荷对粉煤灰理化性质的影响。结果显示,CFB锅炉负荷对粉煤灰的粒径分布、化学组成和物相组成等性质影响较小,对粉煤灰中Al_2O_3溶出率影响较大,Al_2O_3溶出率与样品颗粒的微观形貌有关,颗粒表面孔隙和裂缝越多,Al_2O_3溶出率越高。CFB锅炉粉煤灰中的无定形相含量较高,在70%以上;CaO在粉煤灰中的主要存在形式有3种:硬石膏、生石灰和无定形CaO。Al_2O_3均是以无定形氧化物的形式存在,煤样中的Al_2O_3在燃烧过程中更容易以飞灰的形式排出;石英和硬石膏更容易以底渣的形式排出。     

Devolatilization of coals of North-Eastern India under fluidized bed conditions in oxygen-enriched air

Oxygen-enriched air can increase the combustion efficiency, boiler efficiency, and sulfur absorption efficiency of atmospheric fluidized bed combustion (AFBC) boilers which use high-sulfur coal, and other combustion systems that use coal. Devolatilization is the first step in the gasification or combustion of coal. In this work, devolatilization characteristics of five run-of-mine (ROM) coals of North-Eastern India having particle-size between 4 mm and 9 mm are reported. The experiments were performed under fluidized bed conditions at 1123 K in enriched air containing 30% oxygen. The devolatilization time was correlated with the particle diameter by a power law correlation. The variation of mass with time was correlated by an exponential correlation. It was observed that the average ratio of yield of volatile matter to the proximate volatile matter decreased with the increase in volatile-content of the coals. A shrinking-core model was used to determine the role of film-diffusion, ash-diffusion and chemical reaction. The experimental results indicate the likelihood of film-diffusion to be the rate-controlling mechanism in presence of oxygen-enriched air. A cost-analysis was carried out to study the economy of the process.     

Influence of ash agglomerating fluidized bed reactor scale‐up on coal gasification characteristics

To study the influence of fluidized‐bed reactor scale‐up on coal gasification characteristics, a model of the ash agglomerating fluidized‐bed reactor has been developed using an equivalent reactor network method. With the reactor network model, the scale‐up effects of a gasifier were studied in terms of the characteristics of the chemical reactions in the jet zone, the annulus dense‐phase zone and the freeboard zone. Results showed that the changes occurred in the inequality proportion of the volume of the jet zone during the reactor scale‐up. Taking into consideration the utilization of a portion of the backflow gas, the expansion of the jet zone volume and the coal particle residence time, the temperature of the jet zone was increased from 1592 to 1662 K. Also, both the annulus dense‐phase zone temperature and the freeboard zone temperature decreased, causing subsequent decrease in the carbon conversion efficiency. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1821–1829, 2014     

流化床煤气化试验研究

为研究温度和煤的煤焦反应性对流化床煤气化特性的影响,在小型电加热鼓泡床试验装置上,对4种高灰劣质粉煤——石沟驿煤、大塘煤、洋江煤和玉带煤进行空气为气化剂的煤气化试验研究。研究结果表明:随着温度的升高,各煤种气化得到冷煤气的热值、产气率、冷煤气效率和碳转化率均升高,在1 000℃时,石沟驿煤气化得到煤气的热值、产气率、冷煤气效率和碳转化率分别达到4.65 MJ/m3,2.21 m3/kg,47.8%和65.7%;4种煤的煤焦反应性均较差,其中石沟驿煤相对较高,其他煤的煤焦反应性低且相近;在相同温度水平下,气化得到煤气有效成分和热值与煤的煤焦反应性成正相关关系,煤的煤焦反应性最高的石沟驿煤气化得到的冷煤气的有效成分、热值均明显高于其他煤种。     

Devolatilization of coals of northeastern India in inert atmosphere and in air under fluidized bed conditions

Devolatilization of five coals having volatile matter in the range of 31 to 41% was studied in argon and in air under fluidized bed conditions. The diameter of the coal particles varied between 4 and 9.5 mm. The variation of devolatilization time with particle diameter was expressed by the correlation, t_v = Ad_vⁿ. The superficial gas velocity was found to have a significant effect on the rate of devolatilization. The devolatilization rate increased with the increase in the oxygen concentration in the fluidizing gas. The correlations developed in this study fitted the mass versus time profiles of the coal particles satisfactorily. The same correlations were found to be appropriate for predicting devolatilization of a batch of coal particles. The correlations developed in the present study will be useful for the design of fluidized bed combustors.     

Effect of alkali concentrations and operating conditions on agglomeration/defluidization behavior during fluidized bed air gasification

Biomass gasification plays an important role in finding a solution to the energy crisis, and the fluidized bed (FB) is recognized as an important technique for using biomass. However, this process significantly tends toward bed material agglomeration. Accordingly, the aim of this study is to emphasize the effects of operating conditions and different Na concentrations on agglomeration behavior during FB air gasification. Defluidization time decreases as Na concentration increases from 0.8 to 3%. Defluidization time decreases as temperature and the amount of bed materials increase. In addition, no significant change in the trend for this process occurs as the equivalence ratio increases. Adding CaO and Al₂O₃ can significantly prolong this process, and the inhibition level follows the sequence: Al₂O₃ > CaO. The same observations are noted during both incineration and air gasification in the FB at various operating conditions.     

Prediction of optimum operating conditions of liquid fluidized bed systems

An important advantage of fluidized beds applied in various chemical and physical operations is the high rate of heat transfer between the bed and the heat transfer surfaces. However, the design of these systems for optimum conditions remains uncertain and essentially empirical. In this study a model for bed voidage and a model for heat transfer are presented which can be used to predict the maximum attainable heat transfer coefficient and the corresponding bed voidage of liquid fluidized bed systems with good accuracy.     

Oxygen-steam gasification of coals in a spouted bed

A bituminous and a sub-bituminous coal from Western Canada have been gasified in oxygen-steam and air-steam mixtures in a 0.30-m diameter, 50 kg coal/h continuous spouted bed reactor. Results are presented to show the effects of the blast composition and reactor temperature on gas heating value and carbon conversion. Operation in the ash agglomeration mode is illustrated, and the role of K₂CO₃ as catalyst explored. Results from a wide range of experimental gasification conditions are compared with predictions of an equilibrium model.     

灰熔聚粉煤气化技术运行中出现的问题及措施

灰熔聚循环流化床粉煤气化技术是我国具有自主知识产权的新型煤气化技术。介绍了该技术的特点、工艺流程,总结了在应用中出现的问题及一些整改措施。     

循环流化床锅炉煤灰成分对其磨耗特性的影响

采用静态燃烧加冷态振筛磨耗方法对4种煤灰的磨耗速率常数进行了测量,发现同类煤灰的磨耗速率常数随其粒径的增加呈指数衰减的规律, 而相同粒径的不同煤灰的磨耗速率常数不同。通过分析煤灰组分,认为煤灰成分差异是造成这一现象的主要原因。基于煤灰成分主要由偏高岭土、石英、氧化铁和氧化钙组成的假设,采用灰色关联和神经网络分析了煤灰主要成分对磨耗速率常数的影响以及趋势,认为氧化钙、氧化铁以及偏高岭土含量的增加能够增大煤灰的磨耗速率常数,而煤灰的磨耗速率常数随氧化硅含量的增加而降低。     

Modelling coal char gasification in a fluidized bed

Predictions obtained by using the accepted reaction model together with three different reactor models — plug-flow, complete-mixing and bubble-assemblage — are compared. In general, the three reactor models gave significantly different results. The bubble-assemblage model, therefore, represents a valid alternative when modelling fluidized-bed gasifiers. For small inverse space velocities, meaning high steam feed rates and shallow beds, the bubble-assemblage model predicts the lowest (among the three) steam conversions. For deep beds and low steam feed rates, the conversion predicted by the bubble-assemblage model becomes greater than that predicted by the completely mixed model and approaches the values for the plug-flow model. However, this occurs only at very large height/diameter ratios which are not generally used in operating fluidized beds. Increase in the reaction rate constant associated with the partial pressure of water, k₁, caused a marked increase in conversion. The effect of k₂,-associated with the partial pressure of hydrogen, was not so great, conversion decreasing as k₂ was increased. Inclusion of the water-gas shift reaction when calculating the gas composition in the kinetic rate expression may be of significance in predicting steam conversion by reactor model 3. However, varying the value of K_eq did not have a significant effect.     

Effect of entrainment on combustion efficiency high ash anthracites in fluidized bed combustors

The characteristics of the freeboard combustion of Korean low grade anthracites were studied in two fluidized bed combustors, which employed over-bed feeding and non-recycle of cyclone captured solids. The total combustion efficiency was ruled mainly by entrainment of fed coal particles and more or less by the freeboard combustion of entrained coal particles. The conversion of entrained coal particles increased with the bed temperature and decreased with the higher coal feed rate per bed area.     

鼓泡流化床内粉煤气化的数值模拟

将应用欧拉双流体模型对鼓泡气化炉内的气化过程进行研究。摒弃传统颗粒动理学理论中颗粒光滑无旋转的假设,引入颗粒的旋转运动,构建粗糙颗粒动理学理论来封闭双流体模型。基于燃烧理论建立粉煤热解、气化模型以及鼓泡床内气固之间以及气体和气体之间的传热、传质模型。采用该模型进行数值模拟计算,分析床内的气固反应过程,对比实验结果表明粗糙颗粒动理学理论适用于模拟鼓泡床气化炉内的反应。     

生物质流化床气化炉气化过程的实验研究

在流化床生物质气化炉内 ,采用空气作气化剂 ,对七种农、林废弃物进行了气化实验研究。生成的燃气成分 :CO在 1 4 %～ 1 7%之间 ,H2 含量一般低于 1 0 % ,甲烷含量为 5%～ 1 0 %。燃气热值多数在 53 0 0～ 6 50 0 k J/ Nm3 ,气化效率 72 .6 %。实验结果表明 ,流化床生物质气化炉可用于生物质气化。     

A mathematical model of fluidized bed biomass gasification

A mathematical model of biomass gasification in a fluidized bed has been developed. It considers axial variations of concentrations and temperature in the bubble and emulsion phases. The mass balance involves instantaneous oxidation and equilibrium devolatilization of the biomass, kinetics of solid-gas gasification reactions as well as of gaseous phase reactions and interphase mass transfer and gas convection. The energy balance is solved locally for each vertical volume element, and globally on the reactor by iteration on the temperature at the bottom of the bed. Three parameters have been adjusted based on the experimental results: the heat transfer coefficient at the wall, the weighting of the kinetics of the water-gas shift reaction and the fraction of biomass carbon remaining as char after devolatilization. The model is used to simulate a pilot scale (50 kg/h) biomass gasifier, and its predictions compared to experimental measurements. The temperature and gaseous concentrations are estimated with good accuracy for the experiments using a wood feedstock, except for the concentration of hydrogen which is overestimated.     

射流流化床体积效应对煤气化特性的影响

根据射流流化床内流体流动特性,在流体动力学行为相似的基础上,利用化学动力学软件Chemkin构建了射流床煤气化炉的动力学模型,通过体积放大引起物料停留时间及流体力学相似性的改变,研究其对射流床中物料特性的影响.分析了射流流化床几何相似放大与高径比减小放大对反应器特性的影响,认为等高径比放大停留时间较长,反应器内流体力学相似性较好,在后续工艺对出口组成要求较高时,采用此放大原则能满足要求.     

串行流化床煤气化试验

针对串行流化床煤气化技术特点,以水蒸气为气化剂,在串行流化床试验装置上进行煤气化特性的试验研究,考察了气化反应器温度、蒸汽煤比对煤气组成、热值、冷煤气效率和碳转化率的影响。结果表明,燃烧反应器内燃烧烟气不会串混至气化反应器,该煤气化技术能够稳定连续地从气化反应器获得不含N₂的高品质合成气。随着气化反应器温度的升高、蒸汽煤比的增加,煤气热值和冷煤气效率均会提高,但对碳转化率影响有所不同。在试验阶段获得的最高煤气热值为6.9 MJ•m^-3,冷煤气效率为68％,碳转化率为92％。