Effect of Blend Characteristics on the High-Temperature Strength Evolution and Relevant Mechanisms in Cokes

Coals are blended to overcome the increasing scarcity of high-quality coal and coke resources. The resultant properties of blended coals are not always a proportional sum of the starting blend constituents, which makes it difficult to predict the degradation under reactive conditions and at high temperatures in the blast furnace. In this project, researchers conducted high temperature mechanical tests of unreacted and reacted coke samples.

This project focussed on determining the effects of gasification and high temperature (simulating blast furnace conditions) on the strength evolution and mineralogical, chemical, and microstructural characteristics of coke blends (binary C1 and ternary C2) fabricated using a lab-scale oven (L) and pilot oven (P). Coke C1 was derived from a high rank and an intermediate rank coal while C2 was derived from the same components in the same proportion plus a low rank coal. The fabricated cokes were cored to cylinders and ~200 g of each coke was then gasified to 1400°C under a profile simulating a blast furnace.

The C1-P and C1-L cokes showed similar weight losses after gasification while the C2-P sample showed a significantly greater weight loss compared to the C2-L. This suggests that coke C2-L may not have used the same coal precursor and this would have affected the properties and performance of this coke. Comparing the lab-scale cokes, C2-L showed generally higher strengths compared to C1-L at all temperatures while for the pilot oven cokes, C1-P showed greater strengths than C2-P. However, it should be noted that the strengths are in the similar range of values for all coke sets considering variability, with these values generally being similar or higher than those seen at room temperatures. The IMDC strength of C2-L was generally higher than that for C1-L while the IMDC strength of C1-P was generally higher than that for C2-P. RMDCs for the pilot and lab-scale oven cokes were similar in hardness and degraded with increasing temperature. Variations in the RMDC/IMDC strengths and microstructural and mineralogical changes are believed to contribute to the differences in strengths. Samples showed increased graphitisation and Fe/Fe-Si and SiC formation at high-temperature, with the iron formation occurring to a greater extent in C2 cokes.

The results show that lab-scale oven cokes show similar macrostructural and microstructural strengths at room and high-temperatures and confirm the potential of using the lab oven cokes as a simpler route to obtain samples for testing coke properties consistently. However, issues in homogenisation particularly for complex ternary blend consistency need to be resolved.