Application of Non- or Slightly Coking Coal to the Production of Metallurgical Coke

Abstract

The effort for utilizing non- or slightly coking coals have been extensively made to produce high quality coke although they do not have sufficient thermoplastic properties to form the coke suitable for the blast furnace operation. One of the possible ways of using non- or slightly coking coals is to improve their thermoplastic properties by rapid heating. The current research investigated the thermoplastic properties under rapid heating condition in view of viscoelastic properties and appropriate ways of utilizing non- or slightly coking coals to produce high quality coke. The strength of coke is one of the most important factors for stable operation and high efficiency. The thermoplastic properties during carbonization are largely related to the strength of coke. For producing high strength of coke, the development of thermoplastic properties for non- or slightly coking coals are needed. The development of thermoplastic properties at rapid heating rate is observed in terms of two viscoelastic parameters, complex viscosity and tan δ. Minimum complex viscosity decreases and maximum tan δ increases with increasing heating rate. As a result of changing thermoplastic properties, the formation of pore structure is extensively changed. During the pore structure formation, swelling and contraction of coal are observed. Therefore, the formation of pore structure during pyrolysis is assumed by the plate displacement profile of rheometer. The plate displacement profiles at various heating rates are largely related to viscoelastic properties and devolatilization rate. According to the plate displacement profiles, the significant difference between low and high heating rates lies in the stages of the growth and coalescence of pore. Although the open pore network was not formed for non- or slightly coking coals at lower heating rates, the open pore network and sufficiently strong bonded matrix are formed with increasing heating rate. For maintaining high permeability which could allow the flow of molten metal and slag and the passage of gas, the strength of coke is not only important but the sufficient size of coke should be also maintained. In the case of coal briquette, the fissuring and breakage during carbonization should be minimized for maintaining the sufficient size for stable operation. First of all, the mechanism of fissure generation is investigated in order to prevent the fissuring and breakage of coal briquette. Two possible mechanisms, irregular contraction of coal briquette and internal pressure build-up, are suggested for fissure generation of coal briquette. In this work, the generation of fissure by internal pressure build-up is dominant at the early stage of carbonization and irregular contraction is a primary factor after the carbonization proceeds to some extent. The addition of coke breeze is effective way for preventing the fissuring and breakage of coal briquette. Internal pressure build-up inside coal briquette and tensile stress induced by irregular contraction of coal briquette considerably decrease with 5 or 10wt% addition of coke breeze. Even though coke breeze is added to raw coal, the thermoplastic properties of blended coals are enough to maintain high strength of coke matrix. Therefore, the addition of coke breeze is greatly helpful to produce high strength coke with sufficient size.