Fundamental Study On The Structural Characteristics Of Low Quality Coking Coal And Its Modification For Coke Production

Coke is an essential fuel in blast furnace.With the development of upscaling blast furnace,strengthening smelting and oxygen-enriched and large-dose coal injection,more demands on the function of stock column skeleton and thermal intensity of coke has been put forward.Rare storages of fat coal and coking coal and lower proportion of these coals（about 30 percent）impossibly satisfy real production in the way that current coke-making enterprises produce coke whose thermal intensity is higher than 60 percent by usingplus 50 percent superior coking coal,such as fat coal and coking coal.It is estimated that coking coal would be exhausted in decades if keeping the current mining intensity.Therefore,how to use rare coking coal effectively is one of the most important tasks on which those working on iron-making and coke-making are concentrating.Numerous coal resources riched in Xinjiang of China contain all kinds of coking coal which have low ash and sulfur content.Xinjiang coking coal mainly at the early and middle Jurassic period rapidly formed with the regional magmatic thermal metamorphism and its metamorphic time is around 100 million years shorter than common Carboniferous-Permian coking coal’s metamorphic time in mainland,making the coke produced from Xinjiang Jurassic coking coal show high reactivity and low coke strength after reaction and this kind of coke can not be used in medium and large blast furnace.Focused on the low-quality coking coal mainly including Xinjiang Jurassic coal,this study initially analyzes the difference between Jurassic coking coal and Carboniferous coking coal,then explores the crucial factors effecting the quality of coke produced from Jurassic coking coal and the solutions to solve above detrimental factors through modifying coke-making experiments,next narrates modification mechanism from macroscopic,submicroscopic and microscopic perspectives,and finally,under the action of original modifier,explores the method about how to make superior metallurgical coke using the whole Xinjiang Jurassic coking coal or high-proportion gas coal and proves its feasibility.The conclusions are following:（1）The analyisises on a series of Xinjiang Jurassic coking coal and common Carboniferous coking coal’s physical and chemical properties,coal rock,expansion characteristics,coal ash composition,the quality of coke produced from single coal and Fourier transform infrared spectrum（FTIR）show that coke quality is effected not only by indicators to evaluate the grade of coking coal,such as caking index,height index of colloid layer,and volatile component content,but also closely by the rank and chemical structure of coking coal.The specificity of Xinjiang Jurassic coking coals is mainly reflected on the fat coal and coking coal.Due to the short metamorphism time of these coals,the metamorphism degree and the condensation degree of aromatic nucleus are low,resulting excessive aliphatic side chains and oxygen-containing functional groups dominated by hydroxy groups in the coals.（2）According to the analysis mentioned in conclusion（1）,the thermogravimetric（TG）analysis of coal samples and the coking mechanism,radical reason for the poor coke quality of Jurassic fat coal and coking coal lies not on the specificity of the ash composition in the coal but on the excessive oxygen-containing functional groups which makes the condensation and crosslinking reactions in the main pyrolysis stage too intense.Similarly the coke produced by coking coal with high volatile has the same character and reasons leading to it with the coke produced by Jurassic fat coal and coking coal.（3）The study on the chemical structure and the features shown in pyrolyzation of Xinjiang Jurassic coking coal and normal Carboniferous coking coal suggests that aromaticity（f_a）can be used as an important index to evaluate metamorphic degree of the two types of coals,weight loss rate of coal largely depends on the content of aliphatic carbon during pyrolysis,and the temperature at maximum mass loss rate can better reflect the initial curing temperature of plastic phase.（4）A high-quality modifier（HQM）,which can damatically improve pyrolysis performance of low quality coal,has been originally invented through numerous coke-making experiments involved in one or multiple kinds of coal.The addition of HQM can not only use whole Xinjiang Jurassic coking coal to produce first-grade metallurgical coke（CRI<30%,CSR>60%）but also use the blending coal containing 35%gas coal and 35%medium volatile coking coal to manufacture superior metallurgical coke with CSR>60%.（5）The modification mechanism was investigated by combinating TG,In situ X-ray Diffractometer（in-situ XRD）,Raman Spectroscopy（RAMAN）,FTIR,High Resolution Transmission Electron Microscopy（HRTEM）,Scanning Electron Microscopy（SEM）,and so on.1)The main component（B_xC_y）in HQM can react with reactive oxygen species generated in the main pyrolysis stage of coal to form B₂O₃ with high viscosity,which reduces the consumption of free mobile hydrogen in reaction with active oxygen species,and further provides more free mobile hydrogen to stabilize free radical fragments,reducing condensation and crosslinking reactions and forming more stable liquid phase.This is conducive to the growth of liquid crystal molecules and their ordered arrangement and condensation polymerization reaction in the late pyrolysis period,which ultimately improves the microcrystalline structure of coke.2)With the addition of HQM,the content of amorphous carbon and porosity in coke decreased,but the thickness of pore wall,volume and compactness of microcrystalline structure unit increased.In addition,the product B₂O₃ formed after the modifier reacted can block pores in the coke,and has a negative catalytic effect on the carbon dissolution reaction.Finally,the boron atoms in the boron-containing compounds at high temperature can replace active carbon atom sites in the coke by the means of solid solution reaction,resulting in the formation of boron-containing carbon mesh layers which can resist oxide and have fewer defects.Through the improvement of the above macro and microstructure of coke,the reactivity of coke obtained from gas coal with HQM can be significantly reduced,and the coke strength after reaction can be obviously enhanced.（6）Through large-scale industrial modification coking experiments,it has been proved that it is feasible to produce first-class metallurgical coke with CRI<30%and CSR>58%using 100%Xinjiang coal under HQM.Simultaneously,under the action of HQM,when the content of gas coal in the coal blending is as high as 35%,and the content of fat coal and coking coal is only 35%,it can produce first-class metallurgical coke with M₂₅ of 92.1%,M₁₀ of 6.3%,CRI of 27.6%,CSR of 61.5%,sulfur content of0.52%,and ash content of 11.65%,which helps to significantly reduce the cost of coal blending.（7）This study breaks through the technical bottleneck that high strength metallurgical coke cannot be produced from 100%Xinjiang coal and provides a new process and technology for coke-making by 100%Xinjiang coal.