Research Of Lignite Hydro-modification In CO/Subcritical Water System

The resource of lignite and other low metamorphic is abundant in our country,while that of coking coal is scarce.Improving the caking property of lignite to be used as coking coal blending will expand the resource of coking coal,which is of great significance.The methods of modification to improve the caking property of coal mainly includes preheating at low temperature treatment,hydro-thermal treatment and plasticity and quality improvement hydro-modification.For preheating at low temperature and hydro-thermal treatment,part of oxygen containing functional groups can be removed,thus reducing the oxygen content and improving the caking property of coal after the treatment.But,the effect of modification is poor.Similarly,not only the toxic solvent is required as the hydrogenation medium but the process of hydro-modification is complicated.Taking the Inner Mongolia lignite as the main target of modification,hydrogen generated from water-gas shift reaction（WGSR）as the hydrogen source and Na₂C03 as the catalyst,the research of hydro-modification to improve the caking property of lignite was conducted in CO/H₂O system.In this thesis,the influences of technological conditions which includes reaction temperature,CO initial pressure,water-coal ratio,mineral substance,catalyst and it’s additive amount on the caking index（G）of modified coal was studied in CO/H₂0 system.Then,the technological conditions were optimized by orthogonal experiment.Moreover,the composition properties of modified coal was discussed on the basis of the analysis results which characterized by proximate analysis,ultimate analysis,TG-DTG,FT-IR,XPS,SEM and solvent extraction analysis of the modified coal.Finally,according to the analysis results above,the mechanism of improving caking property of lignite in CO/H₂O system was speculated.The main research conclusions of this thesis are as follows:1.The catalyst namely sodium carbonate showed a good catalytic effect on the hydrogenation modification which was used the WGSR as system.The coal hydro-modification will be complete with the enhance of CO initial pressure,the increase of additive amount of catalyst and the water-coal ratio,the G of modified coal was rapidly increased and then stabilized.With the increase of reaction temperature,the G was increased at first and then decreased,the inflection point appears at 350℃.The optimal technological conditions of the coal hydro-modification were water coal ratio of 1:1,reaction temperature of 340 ℃ and CO initial pressure of 4.5MPa.Under these conditions,the modified coals with G above 97 could be prepared.2.The inherent minerals has promote the pyrolysis of raw coal,while the added minerals slightly affect the pyrolysis of demineralized coal.The effect of modification is different when added Al2O₃,K2CO₃,CaO into the dem-coal separately.The results indicate that Al2O₃ does not affect the hydro-modification of dem-coal,the G of the modified coal is 0.On the contrary,K2CO₃ and CaO has shown a catalytic effect and their catalytic performance is similar to that of Na₂C03,the G is above 90.3.The volatile of the modified coal is lower than that of raw coal,while the addition of catalyst Na₂CO₃ has increased the ash.The oxygen containing functional groups with low thermal stability and other heteroatoms contained sulfur of lignite can be removed in the process of coal hydro-modification,lowering the Od and St,d content of coal and increasing the Cd and Hd content.4.The main organic structure of coal cannot be changed by the coal hydro-modification.The intensity of absorption peak of modified coal related to oxygen containing functional groups in the range of 3600～3200 cm-1 is less than that of raw coal,indicating the deoxygenation effect of hydro-modification is obvious.At the same time,the relative content of C-O and COO-in the surface structure of modified coal decreased by 8.93%and 0.07%respectively.The free radicals produced by coal pyrolysis can react with the active hydrogen H· to generate hydrocarbon containing-CH₂ and-CH3 groups,enhancing the intensity of absorption peak around 2924～2854cm-1 of modified coal.The relative content of C-C（C-H）in the surface structure of modified coal has increased from 69.34%of raw coal to 77.74%.5.In the process of coal hydro-modification,the free radicals from coal pyrolysis can be stabilized by the active hydrogen H· from WGSR,reducing the free radicals concentration of modified coal.And a series of removal of alkyl and heteroatom has increased the aromatic ring condensation degree of coal,stabilizing the unpaired electrons on higher level,extending the relaxation time,so as to reduce the line width of the modified coal.6.The overall structure arrangement of raw coal is relatively loose,and the surface is smooth.Part of the surface presents the "plate" form.Compared with the raw coal,the structure of the modified coal sample was compact and solid.Plenty of "spots" and floc were distributed in the surface which were considered as the macromolecular materials such as asphaltene generated in the process of modification.7.The G of modified coal extracted by n-hexane is rarely unchanged.While,the G of benzene residues fall to 79 from 92 when the asphaltene has been extracted out.The G of THF residues down to 0 and the G has picked up to 55 when asphaltene mixes with the THF residues.It is indicated that the coal caking property is not only relative to the macromolecules such as asphaltene and preasphaltene,but also associated with the junction and dispersed state of coals.8.In CO/H₂O system,the hydrogen source of coal hydro-modification is not required from the gas phase hydrogen,but the active hydrogen from WGSR catalyzed by sodium carbonate.The catalyst first promoted the WGSR to produce intermediate product formate.Then,the formate can be easily decomposed to the high activity H·,which tended to react with the free radicals generated from coal pyrolysis and formed gaseous hydrocarbon,small molecule oil and macromolecules with strong caking property,thereby,the caking property of modified coal has been improved.