Evolution of coke properties while descending through a blast furnace

Abstract: Due to increasing price and economic pressures, there is a need to minimise coke consumption. The lesser amount of coke used has indirectly set higher standards for coke quality and led to a wish for even more knowledge about its function in the blast furnace. Over the last 20 years, coke quality has been strongly dictated by the so- called CSR value because it was believed that a higher CSR leads to improvement in productivity and more stable operation. Due to lack of suitable coals, often cokes are made from coals with relatively inferior coals leading to coke with lower values of the so-called CRI indicia. Because of this, there was an indirect focus on cokes with lower CRI values. Therefore, this thesis will address some of the important issues of coke strength and focus on changes occurring with coke when it passes through a blast furnace. The main aim of this study is to understand the degradation mechanisms and reactivity changes of coke in order to investigate the factors that affect coke quality. Cokes excavated from LKAB's Experimental Blast Furnace (EBF) are used as a basis for the research. Two campaigns with similar coke (low CRI/high CSR) but different blast furnace injection material have been studied. The coke is supplied from SSAB Tunnplåt Luleå AB. Physical and chemical properties of cokes samples from the EBF were measured. Evolution of coke properties particularly carbon structure and alkali uptake were related to CO2 reactivity as well as coke behaviour (e.g. CSR/abrasion). In addition to this, a trial with very high CRI coke was studied. On the basis of this study, following conclusions were made. The order of carbon structure and concentration of alkali species were increased and these were the most notable changes in the coke properties as it passed through shaft to the cohesive zone of the EBF. The degree of graphitisation was increased while amorphous carbon content was decreased in the hotter zones of the EBF. A linear correlation between the height of the carbon crystallite (Lc) values and the coke bed temperature was established to demonstrate the strong effect of temperature on the carbon crystallite value (Lc) of coke in the EBF. The alkali concentration of coke increased with increasing temperature of the coke bed such that most of the alkali content was evenly distributed in the bulk of the coke rather than in the periphery of the coke matrix. The CO2 reactivity of coke was found to increase during progressive movement of the coke from shaft to cohesive zone of the EBF, and was related to the catalytic effect of increased alkali concentration in coke. The deterioration of coke quality in the EBF, particularly coke strength (CSR) and abrasion propensity (I drum test), was related to coke graphitisation, alkalization and reactivity to demonstrate the strong effect of the coke graphitisation on the propensity of coke degradation. Differential Thermal Analysis indicated that reactions with CO2 are enhanced as coke descends through the EBF. In addition, a trial period with poor coke quality was studied by extensive sampling. The results from this study gave the following additional conclusions: Comparison between high and poor quality coke indicate structure to be connected with alkali uptake, reaction with CO2 and degradation. Isotropic coke carbon components are more resistant than anisotropic components when passing through the EBF. Both cokes develop a more ordered structure as they descend through the EBF.