Use of thermogravimetric analyzer and high temperature furnace as an alternative to the coke reactivity index test and study on effect of binders in metallurgical coke making

Abstract

Metallurgical coke, a porous carbon material produced by carbonization of coking coal at high temperature, generally above 900 ○C, is an integral part of iron-making in the blast furnace. During iron making, coke serves several important roles in the blast furnace such as a source of energy, as a reducing agent, to provide a permeable bed for gas flow and provides enough strength to sustain the load of the charge within the furnace. A coke is said to be of good quality if the CRI is lower and CSR is higher. Reaction of coke with carbon dioxide increases the porosity of the coke and, hence, the coke strength decreases. However, tests involving CRI and CSR are very laborious and time-consuming; this study aims to propose TGA and HTF experiments as an alternative for the CRI test.In the first part of this study, a series of Thermo-gravimetric analysis (TGA) and high temperature furnace (HTF) experiments with various cokes was conducted in CO2 atmosphere to observe kinetics of the CO2 gasification; in particular, the mass loss during the isothermal period at 1100 ○C. The results from TGA and HTF showed a linear relation with the actual CRI values. The results from this study were helpful to predict the reactivity of the cokes and to decide the quality of different cokes. In a wider perspective, this study has high potential in assessing the reactivity index of the metallurgical cokes for iron-making industry. Another part of this study describes the attempts to produce the carbonization products from different bituminous coals and a sub-bituminous coal by adding binders such as petcoke and a blend of petcoke and asphaltene. Also, a study was conducted by adding minerals such as calcite, siderite, magnesite and dolomite. Coking experiments were carried out in a lab-scale horizontal tube furnace at a temperature of about 900 ○C in an inert atmosphere (N2). The effect on carbonization products by addition of binders to the coal samples was studied. Porosity of the produced cokes was determined using optical microscopy by obtaining the 2D stitched images. Raman spectroscopy was used to determine the amount of graphitic carbon and disorientated carbon, which was used to determine the extent of graphitization. Further, Thermo-Gravimetric Analysis (TGA) in CO2 atmosphere was also conducted to observe kinetics of the CO2 gasification in terms of Coke Reactivity Index (CRI) - mass loss during isothermal period at 1100 ○C for two hours. This study suggests the potential usage of petcoke as binder to bituminous coals and blend of asphaltene and petcoke as binder to improve the quality of cokes from sub-bituminous coals.Keywords: Coke, carbonization, binders, minerals, CRI, reactivity, porosity, graphitization