| Under the background of "emission peak" and "carbon neutrality",the domestic blast furnace ironmaking injection fuel system is currently facing the challenges of high injection cost,shortage of high-quality coal resources,and continuous increase in consumption.If the low-rank coal with large reserves,wide distribution and easy mining can be properly modified,the obtained product has the potential to replace part of bituminous coal or anthracite for blast furnace injection.Hydrothermal carbonization is a typical high-efficiency,low-consumption and energy-saving technology for processing carbonaceous materials such as coal,and due to its low energy consum ption and pollution,it has the conditions for largescale industrial application.In this work,the typical low-rank coal is selected for hydrothermal upgrading.Through experimental research combined with quantum chemical simulation calculation,the product distribution behavior of low-rank coal after hydrothermal carbonization,the evolution law of physical and chemical structure characteristics,the molecular chemical reaction mechanism and the key issues of the change of blast furnace injection characteristics are deeply revealed.It provides theoretical support for the feasibility of applying hydrothermal ly upgraded low-rank coal to blast furnace injection.The compositional changes of low-rank coal and the distribution characteristics of different phases under different hydrothermal upgrading temperatures were systematically studied by using a hydrothermal reactor.The results showed that after hydrothermal upgrading,volatile content of coal decreased,fixed carbon content increased,and calorific value increased correspondingly.With the increase of hydrothermal temperature,the deashing effect of the process is obvious.Since the impurity elements in coal are removed in the form of gas or liquid phase,the proportion of oxygen element in the coal molecule decreases significantly.The carbon-oxygen groups in the raw coal and upgraded coal gradually decreased,while the carbon-carbon and hydrocarbon groups increased with the increase in the upgrading temperature.The gas released during the upgrading process includes three categories:conventional gas,sulfur-containing gas and hydrocarbon compounds.CO2 and CO are mainly distributed in the released conventional gas.The hydrocarbon compounds are mainly alkanes such as methane and ethane,while the sulfur in coal escapes in the form of gases such as H2S.Hydrothermal carbonization can remove harmful elements including potassium and sodium in coal,and the generated sulfate ions indicate that sulfur in coal also enters the liquid phase product in the form of ions.Through.scanning electron microscope,infrared and Raman spectrometer,and specific surface area analyzer,the structural characteristics of coal before and after hydrothermal carbonization were deeply studied.The micro-morphological analysis shows that the staggered crack structure can be clearly seen on the coal surface when treated at 250℃,which is due to the rupture and separation of the pore structure caused by the escape of volatiles in the coal.With the increase in the temperature,the cracks on the coal surface gradually deepened,the pore distribution became extremely uneven,and the surface melting loss increased at 340℃.The research on the characteristics of nitrogen adsorption and desorption shows that the gas adsorption and desorption of upgraded coal is significantly lower than that of raw coal,and the overall pore structure develops toward densification.The specific surface area and pore volume showed a decreasing trend,but the pore size became long and narrow,and the pore size distribution increased.From the infrared analysis results,it is found that the overall peak shape of the coal still maintains the profile characteristics of bituminous coal.The ratio of methyl and methylene in coal changed,the condensation degree of aromatic benzene ring was deepened,and the aromatic structure was strengthened to a certain extent,which proved that the degree of coalification was improved.Raman analysis of the structure shows that the degree of carbon ordering of coal has been further developed,the carbon structure of coal tends to be more stable,and the degree of graphitization is further deepened.Combined with the methods of quantum chemical calculation and wave function analysis,a low-rank coal macromolecular model was constructed and simulated.Through the calculation results of bond order analysis,it is found that the Laplacian bond order in the coal molecule is a better reflection of the bond polarity,and the corresponding unstable chemical bond first breaks and dissociates during the hydrothermal process.Based on the high-precision thermodynamic combination method,the dissociation energies of different molecular fragments were calculated.It was found that the closer to the benzene ring structure,the higher the dissociation energy and the more stable the structure,The most easily broken and dissociated parts include methyl benzoate and carbonic acid.The dissociation energies of the methyl-oxygen bonds in the hydrogen methyl ester are all less than 100kcal/mol,and the cracking reaction occurs first during the upgrading process.The phenolic hydroxyl group in phenol has the highest dissociation energy value.Through the analysis of the electrostatic potential of the molecular surface,it is found that there are minimal electrostatic potential points in the regions near the alcohol hydroxyl oxygen,carbonyl oxygen,phenolic hydroxyl oxygen and benzene ring.These regions are more vulnerable to the attack of electrophiles.The electrostatic potential ranges from-10kcal/mol to+15kcal/mol.The results of weak interaction analysis showed that carbonyl.methyl and alcohol hydroxyl groups easily form strong hydrogen bonds with water,while weak van der Waals interactions were mainly formed between phenolic hydroxyl groups and water molecules.The basic characteristics of coal were studied comprehensively.Compared with the raw coal,the ignition point shows an increasing trend,and the explosive decreases gradually,and the grindability increases gradually with the increase in temperature.The change of ignition point and explosiveness indicates that the safety performance of the upgraded coal is improved,the change in grindability is also within the range to meet blast furnace injection requirements.The combustion characteristic parameters and activation energy were calculated by the equal conversion method.With the increase in the upgrading temperature,the thermogravimetric curve gradually moved to the high temperature region.When the temperature reaches 340℃,the reaction rate is the largest,and the average reaction rate of different treatment temperatures is close to that of the raw coal,indicating that the combustion performance of the upgraded coal is still strong.The activation energy gradually decreased when the conversion ratio was between 0.2 and 0.85,and in the last stage of the.reaction,the activation energy increased slightly due to the influence of coal ash.Through the energy-mass balance calculation,it is found that when the upgraded coal proportion is gradually increased to 20%,the fuel ratio is reduced by 4kg/tHM,the gas utilization rate is increased by 0.24%,the air volume is reduced by 6.13m3/tHM,and the reduction of belly gas volume is 8.22m3/tHM.the theoretical combustion temperature is reduced by 4℃,and the amount of slag is reduced by 2.13kg/t compared with the original.In conclusion,appropriately increasing the mixed injection ratio of upgraded coal and changing the fuel structure can reduce the fuel ratio and injection cost,improve the indirect reduction and the gas utilization rate,and also ensure the stable smelting of the blast furnace.Therefore,the hydrothermally upgraded coal has broad application prospects in the field of blast furnace injection. |
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