| Lignite resource is abundant in our country, but the forming time is short, metamorphic degree is low. Lignite has the characteristics of high moisture content, developed pore structure, and rich oxygen containing functional groups, which limit the wide application of lignite. Preparation into lignite coal water slurry(CWS) for chemical transformation is one of the important development directions, and it is necessary to dehydrate lignite before pulping to decrease the internal water, improve the interface properties of lignite, and improve the CWS solid concentration.Hydrothermal dehydration(HTD) is a highly efficient non-evaporative dehydration technology, which can reduce the internal moisture, remove the carboxyl oxygen containing functional groups, and increase heat value of lignite. The upgraded lignite can be beneficial to prepare lignite CWS with high solid concentration, but the energy consumption is higher relatively. Currently, the research on lignite HTD mainly focused on high temperature about 300℃ and high pressure, which has the characteristics of higher energy consumption and more difficult waste heat utilization, and the study on the interface properties and slurrying of upgraded lignite after HTD with lower temperature(<200℃) and pressure is less.This research used low temperature and pressure(160~200℃,0~2MPa) HTD technology to property-modify Xilingol lignite, and prepared the upgraded lignite to CWS. A series of work were performed around the interface properties and slurrying characteristics of lignite before and after HTD.Detailed analysis of Xilingol lignite coal quality characteristics was studied. Used the naphthalene series dispersant KY33, lignin series dispersant LS and humic acid series dispersant HAS to prepare lignite CWS respectively, and a deep analysis of the slurrying property, rheological property and stability of CWS were made. The solid concentration at 1000mPa·s of Xilingol lignite CWS was about 50%, and the surry had yield pseudoplastic fluid characteristics and good static stability, no drainage clarify layer and hard precipitation within 7 days. Dispersant could improve the slurrying characteristics of lignite in a certain extent, but its influence was less. To prepare CWS using upgraded lignite through some dehydration methods can fundamentally improve the slurrying characteristics.Enclosed low temperature and pressure reaction kettle was used to dewatering Xilingol lignite, the effect of terminal temperature, dry-coal/water ratio, initial pressure of reaction kettle and residence time on the HTD reaction were studed, and analyzed the composition and change law of solid, liquid and gaseous product after HTD. It was concluded that, with the rise of terminal temperature, the increase of initial pressure and the extension of residence time, the balance moisture, volatile matter and oxygen element content of lignite decreased obviously, the fixed carbon content increased, the heat value rised, the carbon element content increased, and the coal-rank parameter AO/C declined, namely coal rank rised, and associated with certain desulfurization and denitrification effect. In the liquid product, the organic and inorganic minerals dissolved in water increased, TOC and TIC content increased. Under the condition of low temperature and pressure, the main ingredient gaseous product was CO2, the content of CO, H2, CH4 and N2 was less. It showed that the carboxyl had higher reactivity, coule be break generates CO2 easily, while carbonyl and ether groups had lower reactivity relatively, more energy was needed to make it break. Dry-coal/water ratio influenced the HTD process little.Prepared the upgraded lignite into CWS, and made a deep analysis of the influence of terminal temperature, dry-coal/water ratio, initial pressure and residence time on the solid concentration at 1000mPa·s, rheological property and stability of lignite CWS. It coule be seen that, along with the rise of terminal temperature, the increase of initial pressure and the extension of residence time, the chemical reaction of lignite was more intense, which made the solid concentration at 1000mPa·s of the upgraded lignite CWS increased, yield stress τy reduced gradually, consistency coefficient K value increased, and the flow characteristic index n reduced gradually, namely, pseudoplastic characteristics enhanced obviously. Due to the stability of lignite before and after HTD were all good, static observation method could not distinguish the stability good or bad effectively, Turbiscan Lab stability analyzer based on the multiple light scattering principle could realize real-time analysis on the stability without destroying samples, analyzed the size of coal particle aggregate, calculated dynamic stability index TSI, which had more advantages for CWS with little stability difference. With the increase of the reaction intensity, TSI increased gradually, and the stability reduced, but the dry-coal/water ratio had little influence on slurrying characteristics.After hydrothermal dehydration, the interface properties of lignite were improved, the microscopic mechanism of the influence on the slurrying characteristics can be concluded as follows. The content of oxygen containing functional groups such as carboxyl group and phenolic hydroxyl decreased, the calculated coal rank index C=C/(C=O+C=C) measured by infrared spectrometer increased obviously, the carboxyl oxygen removal degree index C=O/C=C reduced gradually along with the the rise of terminal temperature, the increase of initial pressure and the extension of residence time, which showed that the long chain alkane of upgraded lignite had been decomposed, aromatic carbon increased, AO/C reduced to make coal rank rised. The contact angle of upgraded lignite increased, the lignite had strong surface hydrophobicity, which made anionic dispersant KY33 adsorbed on the surface of lignite more easily. The thickness of hydrated film reduced, making the proportion of free water in CWS increased relatively, interparticle frictional resistance reduced in the mixing process, then the apparent viscosity of upgraded lignite CWS decreased, enhanced the solid concentration at 1000mPa·s and improved its rheological property. The removal of oxygen containing functional groups made the Zeta potential on the surface of lignite lower, attraction between coal particles strengthen gradually, then the dispersed coal particles were easily gathered and settling, thus the stability of CWS reduced gradually. The results of XRD mineral composition analysis of lignite showed that, the relative content of quartz and white mica with larger density which were easy to sink in CWS to destroy the stability of the slurry increased, however, the gypsum and kaolinite which could improve the stability of CWS reduced, as a result, HTD had an adverse effect on the stability of CWS.Making use of atomic force microscope(AFM) to measure the change of the thickness of hydrated film on the surface of lignite before and after HTD, it was found that the thickness of hydrated film on the upgraded lignite surface became thinner, and the solid concentration at 1000mPa·s of CWS was negatively related with the thickness of hydrated film, namely, the thinner the thickness of hydrated file was, the higher solid concentration at 1000mPa·s of CWS was. This was minly because the tinnner hydration film needed less free water when the concentration was fixed, then the proportion of free water which played a flow effect role in the process of slurry shear movement increased, which could reduce the frictional resistance to decrease the viscosity of CWS, enhance the solid concentration at 1000mPa·s and improve its rheological property. Meanwhile, the thinner the thickness of hydrated film could produce smaller mechanical resistance and elastic resistance which could prevent particles from gathering, result in the decrease of steric-hindrance effect and coal particles were easy to gather and aggregation. As a consequence, the stability of CWS reduced gradually with the reaction intensity of HTD increased.Orthogonal experiments of HTD based on the four factor with 3 levels, including terminal temperature,initial reaction kettle pressure, residence time and dry-coal/water ratio were carried out, taking the content of total acid group, contact angle, solid concentration at 1000mPa·s and flow characteristics index n as evaluation indexes. The range analysis and variance analysis results of orthogonal experiments showed that, terminal temperature and initial pressure were the most significant influence to the four evaluation indexes, and they had interaction, then the residence time, while dry-coal/water ratio had no significant effect. The optimal level combination plan for 200℃ terminal temperature, 2MPa initial pressure, 120 min residence time and 1:1 was ultimately determined, the results were that the total acid group content was 2.645mol/kg, contact angel was 47.6°, the solid concentration at 1000mPa·s was 55.53%, and the flow characteristic index n was 0.602, namely the CWS had good pseudoplastic rheology characteristic. And the stability test results showed that the stability of the lignite CWS was good. |
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