流化床反应器内碳纳米管流态化技术的研究进展

碳纳米管作为一种战略新型碳材料以其独有的高强度、高导热、高导电等诸多优异性能,可应用于航空、电子器件、新能源汽车等国防军事及民生领域,而限制其广泛应用主要受制于高质高产制备。流化床反应器作为制备碳纳米管的核心部件,研究其内碳纳米管制备过程的流态化技术已愈发重要。综述了从碳纳米管的制备技术、生长机理及操作条件对其制备过程的影响到不同流化床内碳纳米管的流态化研究等这一系列围绕宏量制备碳纳米管的技术,提出了反应器内碳纳米管流态化特性研究中存在及未解决的问题,为将来更详细深入地研究碳纳米管的流态化技术提供了指导与方向。     

大同煤循环流化床的O_2-CO_2预热特性

利用循环流化床高温预热技术,对大同煤在O_2-CO_2环境下预热前后的颗粒特性、燃料特性和燃料氮的转化特性开展研究。结果表明,循环流化床可将大同煤稳定预热至850℃,预热所需的热量全部由煤粉自身的部分燃烧提供。预热后的半焦颗粒粒径减小,比表面积和孔容积分别增大10和2倍,孔隙结构更加发达;大同煤在循环流化床O_2-CO_2预热中,煤气中CO、H_2、CH_4和CO_2体积分数分别为11.5%,9.7%,2.11%和79.6%,煤气热值为2 848.7 kJ·m~(-3);预热过程,煤中的N-Q和N-X向N-6和N-5发生了转化,N-6质量分数由原煤的17.28%增大到32.11%。     

鼓泡流化床内颗粒旋转速度的实验研究

为了获得鼓泡流化床内颗粒的旋转特性,利用双脉冲激光器与跨帧相机构成的测试系统对冷态鼓泡流化床内颗粒的旋转过程进行研究。结果表明:颗粒粒径越小,旋转速度越大;颗粒的水平分速度增大时,平均转速增大;颗粒的垂直分速度增大时,平均转速呈一定幅度的上下波动趋势;相对于垂直分速度大的颗粒,垂直分速度小的颗粒的转速有一定幅度的减小;在x轴方向上颗粒平均转速的分布趋势为先增大、后减小。     

循环流化床锅炉二次送风结构影响的数值模拟

利用CFD软件Fluent,基于颗粒动力学理论的双流体模型对循环流化床锅炉的3种二次风送风结构对炉膛内气-固两相流宏观流动特性的影响进行数值模拟,将3种结构下的计算结果进行对比分析。结果表明:二次风通常采用90°垂直入射的情况下,炉膛内颗粒的环-核分布结构及上稀下浓的分布不均匀性达到最大;二次风进口入射角为与轴向方向成60°向上入射情况下,炉膛内颗粒主要分布在炉膛中上部区域,不均匀程度减小,最有利于炉膛内的燃烧和提高传热效率,同时得到二次风的射流深度最大;二次风送入结构为渐缩型入射口的情况时,二次风射流深度最小,炉膛内偏流现象明显,在本文的操作条件下,并没有达到喷射的设计效果,还有待于进一步的研究。     

Flow pattern transition and coal beneficiation in gas solid fluidized bed with novel secondary distributor

Gas solid fluidized bed (GSFB) is an effective method of dry coal separation. In this study, porous sponge was introduced into a typical gas solid fluidized bed as secondary air distribution layer (PSFB) to stabilize the fluidized bed layer. The difference between PSFB and GSFB in flow pattern transition process was studied. Compared with GSFB, the minimum gas velocity and bed density fluctuation decreased while bed expansion ratio increased in PSFB. Furthermore, the distribution of bubble phase and emulsion phase were more homogeneous in PSFB. Under the operational conditions, the results of coal preparation in a PSFB showed that the ash content of clean coal was 10.25% .The probable error (E) was 0.095?g/cm³, indicating that PSFB could provide a novel way for a good performance of dry coking coal beneficiation.     

Unstable sinking of spheres at higher air velocity in a gas-solid fluidized bed

Float-sink of large objects (on order of cm) in a gas-solid fluidized bed of powder (on order of 100 s of microns) based on density difference has been utilized for dry density separation in industry. The air velocity u₀/u_mf is one of the important factors operating the fluidized bed, where u₀ and u_mf are the superficial air velocity and the minimum fluidization air velocity, respectively. It is empirically known that the sinking of heavy objects is “occasionally” unstable in the fluidized bed combustor, for which the higher air velocity u₀/u_mf > 4 is used. Unstable sinking means heavy objects that are expected to sink but sometimes do not. However, the precise conditions at which the unstable sinking occurs are not clear. In this study, we investigated the float-sink characteristics at a given air velocity u₀/u_mf = 2–7 using glass beads of size D_gb = 425–600 μm and 600–850 μm as the fluidized powder bed media. The float-sink experiments were carried out at the bed height h_gb = 150 mm and 75 mm using density adjusted spheres (diameter = 30 mm). We found that the spheres stably float or sink based on density difference at D_gb = 425–600 μm & h_gb = 150 mm and at D_gb = 600–850 μm & h_gb = 75 mm. However, the unstable sinking does occur at u₀/u_mf > 4 at D_gb = 600–850 μm & h_gb = 150 mm. These results indicate that the powder size and the bed height are key factors to induce the unstable sinking at the higher air velocity.     

Filtration of nano-particles by a gas-solid fluidized bed

The filtration of 80 nm SiO2 and Al(2)O(3) particles in a gas stream using fluidized beds was studied. Silica sand and activated carbon (A.C.) were adopted as bed materials to filtrate 80 nm SiO2 and Al(2)O(3) particles. The collected particles were elutriated from the fluidized bed, so the filtration was a dynamic process and the variations of the removal efficiency with time were studied. Experimental results showed that the filtrations of 80 nm SiO2 and Al(2)O(3) particles with a bed material of silica sand were not dynamic processes but the filtration by A.C. was. The removal efficiencies for SiO2 and Al(2)O(3) particles using silica sand as bed material were held steady and found to be equal, between 86 and 93%. A.C. is considered to be more efficient than silica sand because it has a high specific surface area. However, the experimental data yield conflicting results. The removal efficiency of Al(2)O(3) particles fell from 92% initially to 80% at the end of test-a little lower than that obtained by filtration using silica sand. A higher voidage of A.C. than silica sand weakens the removal of nanoparticles since the diffusion mechanism dominates. The removal efficiency of SiO2 by A.C. decayed from 83 to 40% with time passed. The huge differences between the filtration efficiency of SiO2 and that of Al(2)O(3) particles by A.C. was associated with the extensive segregation of SiO2 and A.C. particles, which caused more SiO2 particles to move to the top of the bed, where they were elutriated. The weak inter-particle force for SiO2 decreased the removal efficiency also.     

Experimental investigation of bubble behavior in gas-solid fluidized bed

In this work, to investigate the source of pressure fluctuations, behavior of a single bubble in a two-dimensional gas–solid fluidized bed was studied. Pressure sensors located at different heights of the bed measured presure fluctuations, and simultaneously a high speed camera was used to pursue all steps from formation to eruption of bubbles. Two types of particles were applied with different sizes and densities. Experiments showed that the maximum amplitude of formation was independent of the bubble diameter. But, it depended on density of particles, velocity of injection and the distance from bed surface. When injection stopped, there was a minimum in pressure profile related to the higher dense phase voidage for a higher superficial gas velocity after injection. Also, the maximum pressure fluctuation of bubble eruptions was related to the bubble diameter, density and size of particles. It was concluded that pressure fluctuations of formation, passing and eruption of bubbles in fluidized beds are originated due to changes in dense phase voidage, bed voidage and movement of particles during bubble eruption.     

Novel method of air distributor design for enhancing bed stability and reducing impurities in gas–solid fluidized bed system

Coal plays a key role in the economic development of China. It is greatly significant to improve the efficient use of coal through high-efficiency dry separation. In this paper, a porous sponge was used to optimize the air distributor, and its fluidization characteristics were studied. Response surface methodology was used to study the collaborative optimization effects of gas velocity, separation time, and bed height on the fine coal separation. Gas velocity was the main factor which affects separation efficiency. When separation is in operation parameters, the yield and ash content were 65.48 and 10.89% as to the clean coal product and 9.24 and 80.47% as to the gangue product, respectively. Furthermore, the yield and ash content of middlings were 25.29 and 20.32%, respectively. The probable error, E, values were between 0.085 and 0.100?g/cm³. Using XRF and FTIR analysis, it was observed that the harmful elements and impurities in coal were reduced during the separation process.     

Numerical simulation of gas-solid flows in fluidized bed gasification reactor

A numerical model for simulating a fluidized bed gasifier should include appropriate parameters to capture the dynamics of gas-solid flows, gasification kinetics and the interaction between these two. The focus of the present study is to analyze the effects of coal gasification chemistry models reported in literature on the prediction of product gas composition in a fluidized bed gasification reactor. Numerical results are validated against the experimental data available in literature. The validated model is used to examine the available chemical kinetics schemes for water gas shift reaction, steam methane reforming reaction and char heterogeneous reactions. It is also used to assess the effects of hydrodynamic models parameters such as drag model, particle-particle restitution coefficient and specularity coefficient on exit gas composition. Results show that the predictions of product gas composition are notably affected by the choices of the kinetics schemes for water gas shift and steam methane reforming reactions. Systematic analysis using the available choices to simulate initial processes such as moisture removal, volatile and tar cracking is reported. Drag models and the value of specularity coefficient are shown to have no effect on product gas composition, and the particle-particle restitution coefficient slightly influences the predicted gas composition.     

Evaluation of the separation performance of an air dense medium gas-solid fluidized bed for coal cleaning: Effect of the binary dense media

The binary dense media plays a significant role in the air dense medium gas–solid fluidized bed for coal separation. Magnetite powder with a narrow size fraction of ?0.20?+?0.15?mm and ?0.35?+?0.25?mm quartz sand were mixed in proportion to form a binary dense media. The separation performance of ?13?+?6?mm coal was evaluated through the combination of various indicators of the yield and ash content of products, combustible material recovery, relative segregation degree, and probable error. Results suggested that the optimal separation performance was achieved by an adjustment of the appropriate operational gas velocities of the bed and use of the binary dense media with a suitable composition as the separating medium. The proportion of the quartz sand mixed in the binary dense media should be adjusted as 10% or 20% and the fluidization number could be varied from 1.6 to 1.8. The satisfactory ash content and yield of clean coal were 10.12% and 68.29%, respectively, with a combustible material recovery of 92.09%. The largest relative segregation degree was 0.95, and the minimum probable error was 0.085?g/cm³ with a standard deviation of 0.024?g/cm³. These findings indicated efficient coal cleaning.     

Characterization of temporal and spatial distribution of bed density in vibrated gas-solid fluidized bed

This study uses a Φ 200?mm?×?900?mm vibrated gas-solid fluidized bed (VGFB) with ?0.3?+?0.074?mm magnetite powder was utilized to characterize the temporal and spatial distribution of bed density in VGFB and the influence of bubble movement on fluctuations in bed density. The results indicate that the bed density decreases with an increase in gas velocity (U) and the frequency (f) and amplitude (A) of vibration and that the bed density spatial distribution is lower in the central region but higher in the border regions. The standard deviation of the density first increases then decreases and finally tends to stabilize with an increase in apparent gas velocity. Moreover, when A?=?2?mm, f?=?25?Hz and U?=?14?cm/s, the density distribution is 1.82–1.88?g/cm³ and the fluidization state is improved. The energy of the pressure signal increases with an increase in gas velocity and vibration amplitude. In particular, the low-frequency band of the pressure signal exhibits the highest amplitude and energy, which reveals that bubbles are the main cause of pressure fluctuation. Furthermore, the bed density decreases with an increase in bubble generation frequency, and the relationship between these follows the ExpDec 2 mathematical equation.     

Combustion of agro-waste with coal in a fluidized bed

In this study, a review of the studies done on the co-combustion of some agro-waste in a bubbling fluidized bed combustor (BFBC) having an inside diameter of 102 mm and a height of 900 mm is given. The agro-waste used to investigate the co-combustion characteristics were peach and apricot stones produced as a waste from the fruit juice industry, and olive cake produced as a waste from the olive oil industry. These are typical wastes for a Mediterranean country. A lignite coal was used for co-combustion. On-line concentrations of O₂, CO, CO₂, SO₂, NO _x and total hydrocarbons (C _m H _n ) were measured in the flue gas during combustion experiments. Variations of emissions of various pollutants were studied by changing the operating parameters (excess air ratio, fluidization velocity and fuel feed rate). Temperature distribution along the bed was measured with thermocouples. Emissions were also monitored from the exhaust. Various combinations of coal and biomass mixtures were tested. During the combustion tests, it was observed that the volatile matter from the biomass quickly volatilizes and mostly burns in the freeboard. The temperature profiles along the bed and the freeboard also confirmed this phenomenon. It was found that as the volatile matter of the biomass increases, combustion takes place more in the freeboard region. Better combustion conditions occur at higher excess air ratios. The results showed that co-combustion with these three proposed biomasses lowers the SO₂ and NO _x emissions considerably. CO and hydrocarbon emissions are lower at the higher excess air ratios.     

Stable-combustion temperature regime of coal in a fluidized bed

The system of energy, fuel, and oxidizer balance equations is analyzed and the temperature of the fluidized layer determined for ignition of the fuel and at the moment the ash component begins to soften.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 52, No. 3, pp. 429–433, March, 1987.