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Comminution Theory of Superfine Pulverized Coal Based on Fractal Analysis of aggregate structures Jiaxun Liu, Hai Zhang, and Xiumin Jiang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02097 • Publication Date (Web): 19 Oct 2016 Downloaded from http://pubs.acs.org on October 24, 2016
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Comminution Theory of Superfine Pulverized Coal Based on Fractal Analysis of aggregate structures Jiaxun Liu, Hai Zhang, Xiumin Jiang ∗ School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
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Corresponding author. Tel: +86 21 3420 5681. E-mail address:
[email protected] (X.M. Jiang) Notes: The authors declare no competing financial interest. 1
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Abstract The coal fragmentation and resulting particle size distribution (PSD) have significant influences on the physical and chemical properties, which play a crucial role in coal conversion and utilization processes. In this paper, a comprehensive comminution theory of superfine pulverized coal was proposed, in combination of particle fracture mechanisms, fractal dimension analysis and energy laws of comminution. The subtle crystalline changes of aggregate structures due to the coal comminution were validated through the synchrotron-based high-resolution X-ray diffraction (HRXRD), and the distribution patterns of submicron aggregate clusters were identified according to the fractal fragmentation theory. Finally, a novel energy dissipation assumption is proposed based on the molecular sliding mechanism, which is suitable for the evaluation of energy consumption of the comminution induced submicron particles. The results here can improve the interpretation and modeling of coal macromolecular networks, and offer a new way for predicting the particle size distribution of grinding products. The findings from this work provides some new insights into the phenomena of limit fineness of mechanical comminution from a molecular level perspective, which is helpful for the development of fragmentation methodology and equipment in the field of ultrafine grinding.
1. Introduction China becomes the largest coal importer and producer around the world, and faces serious energy-saving and emission-reduction issues associated with coal utilization. As a result, the development of clean and efficient coal conversion technologies becomes urgent. Superfine pulverized coal as a new coal utilization technology, has attracted much interest recently due to its advantages on the combustion performance.
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In addition, it can also provide a novel way to reduce the pollutant 2
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emissions,1 and a number of studies have since been performed on the NO reduction in air/fuel staged5-8 and O2/CO2 combustion.9,10 A better understanding of these laboratory and industrial applications can be guided by a legible description of the fragmentation mechanism due to the important size effect on the ignition and combustion process.11-13 The importance of coal fragmentation is not limited to the influence on the physical characteristics (e.g., mechanical, adsorption, wetting, and permeation properties) and the chemical reactivity (e.g., rate of reaction), but also determines the coal conversion and utilization processes such as dehydration, carbonization, pyrolysis, gasification, and combustion. The coal particle size distribution is a key factor in ensuring the stable and high efficiency of industrial performance, due to its direct association with the heat/mass transfer rates. Therefore, it is of great interest to acquire the knowledge about the comminution mechanism as well as the particle size distribution features as much as possible. Unfortunately, the superfine pulverized coal is a heterogeneous carbonaceous material with complex and irregular particle morphology, containing diverse organic elements and inorganic mineral impurities. This irregularity and disorder are recognized as the huge obscure for the application of traditional Euclidean geometry because of its inherent limitations.14 The fractal theory relying on the idea of self-similarity is an intrinsic and quantitative description of the geometrical properties of irregular settings or fragments.15 For recent years, it has been generalized in many fields including mechanics, soil, and energy and environment science, etc.16-20 Numerous researchers have reported that the micropore structure, surface morphology, and particle size distribution of coal can be analyzed by fractal geometry. Johnston et al.21 informed that the fractal analysis of small angle X-ray scattering (SAXS) data was a useful approach to describe the pore surfaces of Victorian brown coal. The potential of fractal analysis in helping understand the textural
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changes of carbonaceous material such as coal and char has been highlighted in Refs. 22 and 23. In addition, the fractal properties of different coals during the heat treatment were investigated by Nakagawa et al. who found that the Ds value decreased greatly with raising the temperatures.25 Furthermore, the particle-size distribution (PSD) plays a key role in understanding physical characteristics of grains such as hydrophobicity, hydraulic conductivity, heat and mass transfer, and flow behavior. For example, Tang et al.25 adopted the single power law fractal dimension to evaluate the PSD and compactability of coal gangues. Li et al.26 characterized the PSD of coal through a volume-based fractal model, and found a bi-fractal performance of coal particles. Moreover, the effects of coal water slurry particle size on the rheological behavior and combustion dynamics have been characterized by the fractal theory.27 The results suggested that the fractal dimensions increased with decreasing of particle size, leading to the increment in apparent viscosities and decline in ignition temperatures. However, the fractal dimension has rarely been reported in describing the comminution process. Comminution can be defined as the process where materials are reduced in size. Coal is a kind of complex high molecular polymer, consisting of numerous aggregate structures called basic structural units (BSU).28 These aggregates are composed of crystalline carbons with graphite-like structures, which is an intermediate state between amorphous and graphitic configurations.29 The individual crystallites with stacking and parallel aromatic layers are linked randomly through aliphatic side chains and cation bridges (e.g., hydrogen bonds), resulting in the anisotropic nature of coal. Therefore, the coal macromolecular network with turbostratic structure can be simplified as fragments of stacking crystalline carbons and saturated structures (sp3- hybridized carbons) on their edges. It is considered the disk-like aggregate structures act as the supporting walls of pore networks, giving birth to the
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slit-shaped micropores.30 As a consequence, the cracking of pores and breakage of particles are essentially the cleavage of weak cross-linkings and generation of crystalline assembles. The aggregate clusters become distorted under the stress of comminution forces until they are opened up, and thus smaller particles containing fewer assembles are produced. The morphology of aggregate structures plays a crucial role in coal conversion processes such as carbonization,31 plasticization,32 liquefaction,33 pyrolysis34 and combustion,35 which needs a more comprehensive elucidation. In our previous work, we performed a preliminary discussion based on the fractal theory about the comminution mechanism of superfine pulverized coal.36 According to the piecewise fractal model, two fractal dimensions were obtained within distinctive size ranges during coal comminution, corresponding to different particle fracture mechanisms. Unfortunately, the distribution pattern of small particles especially in the submicron region (
