Structure Characterization And Metallurgical Properties Of The Chars Formed By Devolatilization Of Lump Coals For Corex Process

Corex process uses non-coking coal instead of cokes to produce hot metal, which is a very potential ironmaking technology in the 21 st century. The prereduction and the final reduction are conducted in two separate reactors, respectively, thus the load pressing on the skeleton is reduced by nearly half compared with the BF and the requirement for cokes is not so rigor. In Corex process, however, the thermal decrepitation occurs when the lump coals are put into the high temperature gasifier. Char particle size distribution will affect permeability of the char bed. In addition, annealing time of the coal char is very short, resulting in a high reactivity. Furthermore, the height of the char bed is low and the temperature is high, thus the gasification rate is faster than cokes. So the coal char will undergo more complex physical and chemical changes, which could directly affect the normal production and energy consumption. Therefore, the study on the thermal decrepitation behavior of lump coal and its coal char property is very crucial. The study takes XLZ and DT coal as the research object, and the pyrolysis of lump coal were carried out using a silicon molybdenum furnace and a coking furnace. The thermal decrepitation behavior, the metallurgical property, pore structure, the crystalline structure, the functional group and char reactivity were comprehensively analyzed. The specific research work and conclusions are as follows:① The high temperature pyrolysis of lump coals was carried out, and the results show that: With increasing annealing temperature and particle size, the decrepitation index gradually increases, and prolonged annealing time increases the mean particle size for thermoplasticity coal. The high temperature decrepitation can be attributed to the huge temperature gradient. Thermal decrepitation could be occurred once the coal particle size exceeds 20 mm. Compared with adhesive property coal, the influence of temperature on non-adhesive property is stronger. With the increasing temperature, the pore size and total pore volume of the chars increase. The crystalline structure of carbon becomes more ordered with increasing annealing time and temperature, and the effect of temperature on the structure is stronger. The study further shows that changes in physical structure of the carbon are the main factor affecting the reactivity of XLC char, while the critical factor affecting the reactivity could be the deactivation of DLC char.② The lower heating rate pyrolysis of 1 kg lump coal was carried out, and the results show that: The mechanical strength of the two chars is enhanced with increasing time and temperature. However, the crushing strength is deteriorated by the fissures for DLC and the abrasion resistance is independent of the fissures. The SBET decreases with increasing annealing time. With increasing temperature, the SBET increases firstly, and then decreases. When the temperature exceeds 1000 ℃,more gases could discharge from the XLC char and produce more micropores. Fissure is bad for producing the micropores. The reactivity of DLC char decreases with increasing annealing temperature and time, however, the reactivity of XLC char changes with the change of SBET. There is a linear correlation between CRI and CSR for XLC and DLC gasified chars. A higher CRI results in a lower CSR.③ XLZ pulverized coal was pressed in briquette coal and the pyrolysis was carried out, and the results show that: Decrepitation degree of briquette increases with increasing temperature, but increases firstly and then decreases with increasing moulding pressure. The influence of temperature on decrepitation is stronger than moulding pressure, and decrepitation degree significantly decreases after adding the pitch. Mechanical strength of briquette char with a low heating rate is very low, but pitch can improve this property. The pitch and a higher moulding pressure leads to a better coking process, but they have a slight influence on crystalline structure, which is significantly influence by the temperature. The reactivity of the coal char increases with increasing temperature and decreasing moulding pressure, and it could be restricted after adding the pitch.④ Comparison of various chars, it is found that decrepitation degree of briquette coal is higher than the others. Its pore wall consists of rough particles, and the coking property is relatively poor. However, XLC char contains more pores and the pore wall is smooth, which is similar to that of coke. The appearance of pore structure could be improved after adding the pitch. Crystalline order of high temperature char is lower than that of lower heating rate, and the degree of briquette coal is the lowest and coke is the highest. The reactivity of XLZ chars is briquette char>high temperature lump coal char>briquette char with pitch>low heating rate char>coke. The reactivity of DT chars is high temperature lump coal char>coke>low heating rate char. The pitch in briquette coal can not only restrict the thermal decrepitation, but also increase the mechanical strength and decrease the char reactivity, which imply that briquette coal used in the Corex process is possible using proper technological methods.