Investigation On Directional Regulation Of Sulfur And Coking Process During Coal-blending Coking Of High-sulfur Coal

Based on the current situation of blast furnace operation in China,the status of metallurgical coke as an important raw material for iron-making will not change in the next few decades.In recent years,with the continued consumption of high-quality coking coal,the reserves of these scarce coking coal resources have been decreasing,and the proportion of high sulfur coal has increased significantly due to the increase in the depth of coal mining,and the sulfur form in these high-sulfur coals is mainly organic sulfur,which is difficult to remove by washing technique,this further restricts its utilization in a wider range.Based on its own characteristics,in order to reasonably and efficiently utilize high sulfur coal rich in organic-sulfur in the coking process,this thesis proposes a research approach regarding the investigations on correlation between coking coal properties and thermal transformation behavior of sulfur,directional regulation of sulfur and coking process analysis,correlation between sulfur content in coke and coal properties,and the prediction method for sulfur content in coke.Coking coals with various coal-rank,sulfur content,ash content and ash composition were selected,pyrolysis experiments and coking tests were carried out in different scale apparatus based on the amount of coal sample.Transformation and directional regulation of sulfur,effects of blending high sulfur coal on physicial and chemical changes of coke,and correlation between sulfur content in coke and coal properties during pyrolysis were studied.Main results are summarized as follows:(1)The chemical structure and content and distribution of sulfur forms in high sulfur coking coals with various coal-rank were characterized by FTIR,Raman,TG,and sulfur K-edge XANES technique,the effects on sulfur transformation during pyrolysis were also investigated.Results showed that the thermal decomposition of unstable aliphatic structures in high-sulfur coking coal with a lower rank released a large amount of volatiles with wider temperature release range,the hydrogen components in volatiles reacted with active sulfur species and promoted the release of sulfur-containing gases,and increased the sulfur removal during pyrolysis.As the degree of coalification increases,the content of thermally stable thiophene in coal increases,the amount of volatiles released during pyrolysis and the sulfur removal decreased.Based on the characteristics of chemical structure and sulfur forms in high sulfur coking coals,the ratio of high sulfur coking coals with lower rank could be increased to a certain extent due to the higher sulfur removal rate;on the premise of ensuring that the other quality indices of coke meet the requirements,high volatile coals,such as gas coal or long flame coal,could be used to regulate the sulfur transformation during blend-coking of high sulfur coal with medium or higher rank,more sulfur species would be transferred into gas or liquid phase and the sulfur content in coke could be further reduced.(2)High sulfur fat coal and coking coal were blended into a basic coal blend and subjected to coking,high volatile coals were selected to regulate the sulfur transformation behavior during coking process.Results showed that the amount of active sulfur produced by the decomposition of unstable organic sulfur increased and its interactions with the nascent coke caused more sulfur retained on the surface of coke,resulting in an increase in the sulfur content of coal blend coke.The volatiles produced by pyrolysis of high volatile coals contain a large number of active hydrogen-containing groups,which could inhibit the interaction between active sulfur and nascent coke,thus more sulfur was released into gas phase or transferred into liquid phase.Coals with a wider release range of volatiles,especially the CH₄,had a better overlap with the release temperature range of H₂S from coal blend pyrolysis,and the sulfur regulation was further enhanced.Excessive blending of high volatile coal would introduce more alkaline minerals into the pyrolysis system and participate in the interactions between volatiles and nascent coke,the secondary reactions between sulfur-containing radicals,sulfur-containing gases and alkaline minerals on the coke surface were enhanced,and further made partial sulfur species retained in the coke,which is not conducive to the directional regulation of sulfur.10kg-scale coke oven coking test showed that blending the optimal ratio of high volatile coal could make all coke quality indices meet the requirements,and the cost of coal blending would be reduced based on the current price gap between premium coking coal and the alternative coals.(3)A high sulfur coal rich in volatile content(HSC)was selected and blended with high-quality coking coal for coking.The evolution of the physical and chemical structure,and the transformation of sulfur at different stages of the coking process were characterized and investigated by FTIR,micro-CT and XPS.Results showed that the blend-coking of coking coal(C2),which possesses a wider plastic temperature range and a large fluidity,combined with 15%HSC would increase the thickness of plastic layer.The chemical structure of different coal blends changed during the movement of the plastic layer from the oven wall to the center of the reactor.From coal to coke,the degree of aromatic ring condensation gradually increased,and the ratio of aromatic CH structure to aliphatic CH structure increased first and then decreased.When the coal reached the coke closest to the oven wall through the semi-coke position,the sulfur content in coke first decreased and then increased;the inter-conversions between different sulfur forms mainly occurred during the semi-coke and coke phases,and the stable thiophene sulfur content in coke increased,the reaction of sulfur-containing gases with the minerals on the surface of coke would also cause some of the sulfur remain in the coke in the form of sulfates.Compared with the individual coking of C2,blending coking of C2 with 15%HSC reduced the coke reactivity and increased the strength after reaction.The high sulfur coal rich in volatile content itself should have a certain extent of maximum fluidity to utilize in the coking process,and the rank of the premium coal should be moderate together with contain a certain content of vitrinite and wider plastic range.As for the regulation of sulfur,the inherent volatiles should be firstly considered and the ratios in coal blend should be optimized based on the sulfur content in coal.(4)The correlation between sulfur content in coke and coal properties was investigated based on the changes of sulfur content during pyrolysis process.Results showed that among the coal quality indicators that characterize the degree of metamorphism,the volatile content had the best correlation with the sulfur removal rate during pyrolysis;the sulfur content in coal was the most direct factor affecting the sulfur content in coke,mineral matters in coal would also affect the sulfur content in coke in the meantime,which suggests that the accurate prediction of sulfur content in coke should be achieved by considering all the factors comprehensively.Organic sulfur was the main sulfur form in clean coking coal and its correlation coefficient with the sulfur content in coke was 0.962,the correlation was even stronger when eliminating other affecting factors.Changing the volatile content in coal blend solely,the obtained correlation coefficient between the predicted value and experimental value of sulfur content in coke reached 0.980.The content of Ca O and Fe₂O₃(m)and the alkaline index(AI)affected the sulfur transformation in coke,AI<0.10 or m<1.0%might have a catalytic effect on the decomposition of some sulfur forms in coal,while if the content exceeds a certain range(AI>0.10 or m>1.0%),the reactions of alkaline minerals with active sulfur or sulfur-containing gases would lead to increased sulfide retention in coke.