The Evolution Mechanisms Of Oxygen,Sulfur,and Moisture In Lignite Upgraded By Hydrothermal Dewatering And Its Slurryability

Given coal resources are rich while oil and gas resources are deficient in China,the emissions of pollutants and greenhouse gases from coal exploitation and utilization become a huge issue for China.Therefore,developing an efficient and feasible upgrading technique for low rank coals?LRCs?is imperative.Hydrothermal dewatering?HTD?technology has attracted considerable research attention because of its ability to upgrade LRCs comprehensively through irreversible dewatering,improving calorific value,carbonization,and surface modification.In this study,two typical Chinese lignites were upgraded by HTD,and some key issues such as deoxygenation,dehydration,and desulfurization were investigated.Moreover,the effect of HTD on the properties of coal water slurry?CWS?was also studied.XiMeng lignite was hydrothermally treated,and gas-,solid-,and liquid-phase products obtained after HTD were analyzed.XiMeng lignite was upgraded by HTD via dewatering,volatile matter decomposition,and oxygen functional group removal.The carbon-holding capacity of solid products was as high as 96%.Solid matter migrated into the gaseous and liquid products.This phenomenon was especially notable at high HTD temperatures.Solid-state 13C NMR spectra showed that the signal intensity of aliphatics decreased whereas that of aromatics increased,indicating that coal rank was improved after HTD.Oxygen-containing groups,such as carboxyls,hydroxyls,and methoxyls,significantly decreased in number after HTD.Production of CO₂ and CO,the main gaseous products,was attributed to decomposition of carboxyl and carbonyl groups,respectively.Evident changes in phenolic hydroxyl content were not observed because of the high bonding energy of this functional group.A simplified molecular model of lignite was constructed,and the deoxygenation during HTD was inveatigated using density functional theory?DFT?.The oxygen functional groups contributed to regions with a large absolute ESP value.These regions also exhibited strong hydrophilicity because of the formation of hydrogen bonds with water.Bond order and bond dissociation enthalpy?BDE?analyses indicated that the hydrothermal treatment began with the cleavage of phO-CH3,followed by the cleavage of C-C bonds in carbonyl and carboxyl,and then with the cleavage of the C-0 bonds in alcoholic hydroxyl?phCH2-OH?and CH3O-CH3.The cleavage of C-0 bond in phenolic hydroxyl?ph-OH?was the most difficult because it has the highest BDE value.A molecular-level description of the non-covalent interactions between water and sites with both hydrophilicity and hydrophobicity was provided using DFT method.Seven typical lignite…water complexes were constructed for topology analysis based on the atoms in molecules?AIM?method.The typical hydrogen bonds O-H…O were formed between water and oxygen functional groups,whereas the weaker hydrogen bonds C-H…O were formed between water and the skeleton components of lignite,such as benzene ring,methyl,and methylene.Oxygen functional groups were revealed to exhibit more hydrophilic than skeleton structures of lignite,including benzene rings and aliphatic chains.The hydrophilicity sequences for the different oxygen functional groups in lignite model were determined as carboxyl>phenolic hydroxyl>carbonyl>alcoholic hydroxyl>ether.The color-mapped reduced density gradient?RDG?isosurface can be used to accurately demonstrate the non-covalent interactions of lignite…·water complexes.The effects of pore structure and oxygen functional group content in lignite on the moisture distribution during HTD were investigated.The dewatering mechanism of HTD was proposed:The removal of free water was associated with the collapse of macropore structures,which was caused by shrinkage forces.The bound water was remarkably reduced because of the removal of oxygen functional groups,such as phenolic hydroxyl and carboxyl,rather than the developed mesopore structure.During HTD,the oxygen functional groups were decomposed,and the hydrogen bonding between water and hydrophilic sites was destroyed,thereby leading to a weakening of the water-holding capacity of lignite.Based on the analysis of sulfur in gas-,solid-,and liquid-phase products and mass balance on the sulfur in coals after HTD,a sulfur evolution mechanism of lignite during HTD is proposed.HTD promoted the liberation of aliphatic sulfur from the coals and release of H2S and CH3SH.Afterwards,CH3SH was decomposed to form H2S as the temperature increased.The gaseous product,H2S,reacted with the organic matter in coal to form thiophenes,which is a thermally stable sulfur structure.As a result,organic sulfur significantly decreased,whereas the sulfate sulfur increased with increasing HTD temperature.Nevertheless,the amount of pyrite and sulphone was almost unchanged.The vast majority of sulfur?exceeding 87%?remained in upgraded coals after HTD,and only a minimal amount of sulfur was released into gaseous and liquid products.Xiaolongtan lignite was upgraded by HTD,and the main factors affecting the slurry-ability of the lignite including oxygen functional groups,surface hydrophilicity,and particle size distribution were analyzed.The effect of HTD on the solid concentration,rheology,and stability of the upgraded coal was also investigated.The contact angle between coal and water increased after the removal of oxygen functional groups by HTD,thereby improving the surface property of lignite after HTD.A typical bimodal distribution of the lignite particle size was observed.After HTD,the mean particle diameter of lignite decreased and the lignite particles became more regular.HTD upgrading significantly improved the slurry-ability of lignite.The solid concentration of raw coal was 44.09%,while that of the upgraded coal after HTD was increased to 61.94%.The consistency coefficient K decreased,while the rheological index n increased for the CWS prepared from the upgraded coals.HTD upgrading decreased the apparent viscosity,and maintained the shear-thinning behavior of pseudo-plastic fluid.Moreover,the water separation ratio decreased,and the stability of CWS was enhanced after HTD upgrading.