Search

• Abrasion resistance of near-matched blast furnace bosh and feed cokes was examined. • High temperature abrasion resistance of cokes was lower than that at room temperature. • Differences in abrasive wear between RMDC and IMDC were observed for the bosh coke. • Difference in RMDC and IMDC wear decreased with increasing severity of conditions. • Tribochemical wear of IMDC was observed during in-situ testing in a CO 2 atmosphere. In this second in a series of two papers, the results of tribological testing of surfaces of coke samples retrieved from an operating blast furnace were compared with those of the corresponding feed coke, to assess the impact of the conditions in the blast furnace on the abrasion resistance of coke. Tribological tests were carried out at temperatures of up to 950 °C under a controlled inert (argon) or reactive (CO 2) atmosphere. Coke wear characteristics were quantified via (i) analysis of the coefficient of friction (COF) during tribological testing, and (ii) the application of microscopy and imaging techniques to the abraded specimens. The blast furnace coke sample was from the underside of the cohesive zone and is referred to as bosh coke in this paper. A near-matched feed coke was also examined. Under ambient testing conditions, the bosh coke had a lower abrasion resistance than the unreacted feed coke samples, indicating that the conditions coke is subjected to during its descent in the blast furnace effectively reduces its resistance to abrasion. Increasing the measurement temperature to 950 °C lowered the abrasion resistance of both the reactive maceral derived components (RMDC) and the inertinite maceral derived components (IMDC) in both samples. The bosh coke RMDC showed more severe damage than the IMDC, using a subjective damage severity scale. The difference in damage severity between these two phases in the bosh coke was reduced as the severity of the tribological testing conditions increased from ambient to elevated temperature (950 °C) to a reactive CO 2 environment. Feed coke samples that had been pre-reacted with CO 2 displayed a mean COF over time trend that was similar to that obtained from the bosh coke samples. During in-situ testing in a CO 2 environment, tribo-chemical wear of the IMDC was detected, due to the surface of the IMDC reacting with the CO 2 in the atmosphere. The observed tribo-chemical wear was due to the indenter and the coke surface rubbing against each other in this CO 2 environment, resulting in the continuous formation and removal of reaction products. Similar trends in COF over time were observed for the bosh and feed cokes during in-situ reaction with CO 2. The substantial decrease in abrasion resistance in coke at high temperature suggests that abrasion may be a more significant degradation pathway for coke in the blast furnace than hitherto expected. [ABSTRACT FROM AUTHOR]

• In-situ abrasion resistance of cokes was examined at elevated temperatures. • Tests were carried out in a controlled gas atmosphere (reactive (CO 2) or inert). • Abrasion resistance decreased with increase in temperature, even at 400 °C. • Difference between room and high temperature test results depended on parent coals. • Wear severity was quantified using a combination of image analysis techniques. A novel approach was developed to examine coke abrasion resistance in-situ at temperatures of up to 950 °C and in a controlled gas atmosphere using rotational tribological testing. The originality of this approach lay in the ability to apply tribological testing to the porous coke surface at elevated temperatures under a controlled inert or CO 2 reactive atmosphere. Coke wear characteristics were quantified via (i) the application of advanced microscopy and image analysis techniques, and (ii) analysis of the coefficient of friction (COF) during tribological testing. Two pilot oven cokes were examined in this study, which were generated from single coking coals of similar rank but different petrographic composition. The cokes tested showed a deterioration in abrasion resistance even at 400 °C, and this was accentuated at 950 °C. The COF and the severity of the wear as a function of temperature and gas atmosphere were markedly different between the two cokes. Coke C396 from a mid-to-low vol, medium vitrinite coal showed no statistically significant differences between the wear severity results recorded following room temperature testing and high temperature testing in a CO 2 environment. Conversely, the more reactive coke C426 with a higher parent coal inertinite content showed clear differences in the extent of abrasive wear at room temperature and at high temperature in both inert and reactive gas atmospheres. This suggests that the reduction in abrasion resistance at elevated temperatures is accentuated in the coke with the highest coke reactivity index (CRI) and the highest content of inertinite maceral derived components (IMDC). Our research has shown that at room temperature, the IMDC show greater resistance to abrasion than the reactive maceral derived components (RMDC). This has implications for understanding the relevance of room temperature testing to the abrasion resistance of coke under practical blast furnace operation conditions. For example, a coke from a coal or blend with a comparatively high inertinite content may show lower resistance to abrasion in the blast furnace than would be expected from tumble drum indices. Larger sample numbers and cokes from a variety of single coals need to be tested in further work to verify these assertions. Finally, a novel method using micro-CT image analysis was developed to examine the wear path generated at the coke surface during tribological testing. The purpose of this was to assess the extent of the abrasive wear by comparing the rendered micro-CT images before and after tribological testing. A colour gradient scale was used to facilitate the quantitative assessment, whereby the colour of each voxel was based on the distance from the nearest voxel that is associated with solid phase. As expected, the specific surface area (SSA) calculated for the upper 1 mm of each sample decreased following tribological testing due to the generation of a wear track which is smoother than the initially rough, porous coke surface. In the C426 coke type examined, the SSA decreased the most for the sample tested in an argon atmosphere at 950 °C, matching the COF and quantified wear severity results, which showed there was a notable increase in abrasive wear to this coke as the temperature was raised from room to elevated temperature. Coupling this method of analysis with 3D visualisation techniques in further work would provide a powerful approach to examine and quantify mechanisms of coke surface breakage and the microstructural and/or microtextural features responsible for the breakage. [ABSTRACT FROM AUTHOR]