【作者】 张楠；

【导师】 张建良；

【作者基本信息】 北京科技大学 ， 冶金工程， 2023， 博士

【摘要】 在“碳达峰,碳中和”的低碳时代背景下,目前国内高炉炼铁喷吹燃料系统面临着喷吹成本较高,优质煤资源短缺,消耗量持续增加的挑战。如果将储量大,分布广且易开采的低阶煤经过适当的改质处理,得到的产品具有替代部分烟煤或者无烟煤进行高炉喷吹的潜力。水热提质技术是典型的处理煤等碳质材料的高效低耗节能技术,并且由于能耗和污染较低,具备大规模工业应用的条件。本文选取典型的低阶煤进行水热提质处理,通过实验研究结合量子化学模拟计算等研究方法,深入揭示了水热提质处理后低阶煤的产物分布行为,物化结构特征演变规律,煤分子化学反应机制,高炉喷吹特性变化的关键问题,为水热提质低阶煤应用于高炉喷吹的可行性提供理论支撑。利用水热反应釜系统研究了不同水热提质温度处理条件下低阶煤的成分组成变化和不同物相的分布特征,研究结果表明,水热提质后煤的挥发分减少,固定碳含量增加,热值相应增加。随着水热提质处理温度的升高,水热提质过程呈现明显的脱灰效果。由于煤中的杂质元素以气相或液相的形式脱除,氧元素在煤分子内的比重显著下降。原煤和提质煤中的碳氧基团逐渐减少,碳碳和碳氢基团则随着提质处理温度的升高而增加。提质过程释放的气体包括常规气体,含硫气体和烃类化合物三大类,CO2和CO在释放的常规气体中占主要分布。烃类化合物中以甲烷、乙烷等烷类物质为主,而煤中的硫部分以H2S等气体形式逸出。水热提质处理能够脱除煤中包括钾,钠等有害元素,生成的硫酸盐离子表明煤中硫也以离子的形式进入到了液相产物中。通过扫描电镜,红外和拉曼光谱分析仪,比表面积分析仪对水热提质前后煤的结构特性变化进行了深入研究。微观形貌分析显示250℃处理时煤表面可以明显看到交错分布的裂纹结构,这是煤中的挥发分逸出造成的孔结构的破裂分离所导致。随着处理温度的升高,煤表面裂纹逐渐加深,孔隙分布变得极不均匀,在340℃时表面熔损现象加剧。氮气吸脱附行为特征研究表明,提质煤的气体吸脱附量较原煤均有明显降低,整体孔隙结构向致密化发展,比表面积和孔容的数值均呈现减少的趋势,但孔径变得狭长,孔径分布数值增加。从红外分析结果发现,煤的整体峰形仍然保持烟煤的轮廓特征。煤中甲基和亚甲基比值发生变化,芳香苯环的缩合度加深,芳香化结构得到一定增强,证明了煤化程度的提高。拉曼分析结构表明煤的碳质有序化程度得到进一步发展,煤的碳结构更加趋向于稳定状态,石墨化程度进一步加深。结合量子化学计算和波函数分析的方法,构建了低阶煤大分子模型并进行了模拟研究。通过键级分析的计算结果发现煤分子中Laplacian键级是对键极性的更好反映,对应的不稳定化学键在水热过程中最先发生断键解离。基于高精度热力学组合方法计算不同分子片段的解离能,结果发现距离苯环结构越近的部位的解离能越高,结构更加稳定,最容易断裂解离的部位包括苯甲酸甲酯和碳酸氢甲酯中的甲基氧键,对应的解离能均小于1 00kcal/mol,在水热提质过程中最先发生裂解反应。而苯酚中的酚羟基的解离能数值最高。通过分子表明静电势分析,发现在醇羟基氧,羰基氧,酚羟基氧和苯环附近区域存在静电势极小点,这些区域更容易受到亲电试剂的攻击,静电势范围分布在-10kcal/mol到+15kcal/mol的区间内。弱相互作用分析的结果表明羰基,甲基以及醇羟基这几类基团极易和水形成强氢键,酚羟基和水分子之间则主要形成弱范德华作用。对水热提质前后煤的基础特性进行了综合研究。和原煤相比,提质煤的着火点呈现增加的趋势,爆炸性逐渐降低,可磨性随着处理温度的增加逐步增加,着火点和爆炸性的变化说明提质煤的安全性能得到改善,而提质煤可磨性的变化也处于满足高炉喷吹要求的范围内。采用等转化率方法计算了原煤的燃烧特性参数及活化能,随着提质处理温度的升高,煤的热重曲线逐渐向高温区移动,在310℃时和原煤的燃烧曲线温度区间相近,温度达到340℃时的反应速率最大,不同处理温度的平均反应速率和原煤接近,说明提质煤的燃烧性能仍然较强。当转化率在0.2到0.85之间时活化能逐渐降低,在反应的最后阶段,由于煤灰的影响,活化能略有升高。结合能质平衡计算,发现提质煤配比逐渐提高到20%配比时,燃料比降低了 4kg/tHM,煤气利用率提高了0.24%,吨铁风量减少6.13m3/tHM,炉腹煤气量减少8.22m3/tHM,理论燃烧温度减少了 4℃,渣量和原来相比降低了2.13kg/t。综上,适当提高喷吹水热提质煤的混喷比例,改变燃料结构,能够起到降低燃料比和喷吹成本,提高炉内间接还原和煤气利用率作用,也确保了高炉的稳定冶炼,因此水热提质煤在高炉喷吹领域具有广阔的应用前景。更多还原

【Abstract】 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.更多还原