| Pulverized coal gasification process,wide adaptability to various kinds of coal,high efficiency of carbon conversion,and low operating cost,has been recognized as one of the advanced and mature coal gasification technologies.In the gasification process,the fine slag is inevitably produced as a by-product with high content of residual carbon and a low utilization rate,which could pollute the environment and result in a waste of resources.Thus,taking feed coal as a reference,X-ray photoelectron spectroscopy(XPS),Scanning electron microscopy-energy dispersive X-ray spectroscopy(SEM-EDX),and X-ray diffraction(XRD)have been used to analyze the gasification fine slag obtained from different pulverized coal gasification units.Based on the investigation of the microstructure of fine slag,the chemical compositions of different structures in fine slag,and the distribution of main components in different structures,the formations of the carbon-rich structure and inorganic composition-rich structure in fine slag were mainly explored.To analyze the fine slag from whole to part,the screening,different acid treatments with different concentrations,and flotation were employed.Moreover,combined with the properties of gasification fly ash and coarse slag,the formation and mechanism of gasification fine slag were revealed from a macrocosmic to a microcosmic viewpoint.Based on the study of the properties of fine slag and residual carbon,the residual carbon-based electromagnetic wave absorbing materials were prepared,and the performances were optimized and evaluated.The main research and conclusions are as follows.Firstly,the characteristics of fine slag from different gasification processes were studied.The results show that the particle size distribution of the fine slag is of multi-peaks.The content of residual carbon and morphology both differ in the fine slag with different particle sizes,and the content of residual carbon rises with the increase of particle size.Compared with the feed coal,the microstructure and element distribution of fine slag change significantly.Two distinct morphologies are observed in the fine slag:spherical particles and irregular shape particles both with different sizes,and small spherical particles are attached on the surface and interior of the irregular particles.According to the differences in the microstructure of the substrate,the irregular particles consist of at least four types:dense particles,layer structure,porous irregular particles,and wrinkle particles.Moreover,the amount of attached spherical particles and the chemical constitution of substrates are different and vary from substrate to substrate.The elemental carbon primarily distributes on the substrate,while elemental silicon and aluminum are mainly found on the spherical particles.In addition,the existences of silicon and aluminum are in the form of amorphous structures.Secondly,the separation and enrichment of the inertinite and vitrinite in feed coal were conducted by positive flotation.The results show that the chemical states of carbon remain the partial original chemical states in the feed coal,and is partly derived from the incomplete unreacted inertinite,which leads to the structure of inertinite has been preserved essentially unaltered in the fine slag.The results of XPS,XRD,and SEM tests show that the microcrystalline structure of carbon tends to be ordered,and the content of graphitized carbon is about 28.83%-39.05%.The spherical particles mainly with inorganic elements were corroded to varying degrees by adjusting the concentration of different acids(acetic acid,hydrochloric acid)exposing the internal structure.An obvious core-shell structure could be seen in large spherical particles.Small spherical particles and irregular flocculated structures are wrapped in the global shell.The shell is not formed directly from uniform matter,and the chemical composition is not homogeneous.Meanwhile,the high iron content promotes the carbon matrix used with the spherical particle,which leads to the contents of elemental carbon and iron in the larger spherical particles are higher than those in the smaller spherical particles.During the formation of spherical particles,taking the fine spherical particles as core,the molten inorganic constituents gradually coat on the surface of the core,or fine spherical particles fuse with each other,which leads to the volume of the spherical particles gradually increases.However,volatile elements,like K and Na,are easy to deposit in spherical particles with smaller particle sizes.The high calcium content promotes the fine spherical particles partly aggregate with each other and partly adhere to the surface of the large ones.In addition,the matrix with high carbon content is wrapped by the molten inorganic components and form the high carbon matrix with different structures,which leads to the content of elemental carbon decreasing,density increasing,and structure compacting.The formation mechanism of fine slag from pulverized coal gasification was revealed.Finally,partly graphitized carbon of the fine slag with a certain amount of oxygen functional groups can improve the dielectric properties.Therefore,the residual carbon(RC)from gasification fine slag was prepared via a facile two-step acid leaching process.The RC shows good thermal stability,electromagnetic wave absorption,and impedance matching properties.The outstanding electromagnetic wave absorbing capability may be caused by dielectric loss and polarization relaxation.Significantly,the RC content in the mixture has a noticeable impact on its electromagnetic wave absorbing properties.The maximum reflection loss of the RC achieves-52.27 dB at 4.8 mm when the adding content was only 20%in a paraffin matrix.Whereas,the effective absorption bandwidth is only 1.7 GHz.To effectively solve the problem of thick thickness and narrow effective absorption bandwidth,the electromagnetic parameters are optimized,and the multiple dielectric loss mechanisms of the material are introduced through the compounding of ZnSnO3 particles and RC.The modified ZnSnO3@RC has a typical core-shell structure.When the mass adding of ZnSnO3@RC is 50%in a paraffin matrix,the maximum reflection loss value of obtained ZnSnO3@RC is-47.8 dB at 2.5 mm,showing the enhanced dielectric loss ability.Meanwhile,the widest effective absorption bandwidth(RL≤-10 dB)can cover 7.0 GHz(11.0-18.0 GHz)with a thickness of only 2.2 mm.Under the same paraffin ratio condition,the electromagnetic wave absorbing capability of ZnSnO3/RC is similar to it of synthesized ZnSnO3/rGO composite.It provides a theoretical basis for improving the utilization of coal and the resource utilization of residual carbon in fine slag.Figure[88]table[23]reference[228]... |
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