Using xenon K-edge subtraction to image the gas-accessible porosity distribution within metallurgical cokes and their partially reacted products

Abstract

The performance and reactivity of coke in a blast furnace is critically dependent on the accessibility of the coke structure to carbon dioxide (CO₂) gas. We used xenon gas K-edge subtraction in synchrotron micro-CT imaging to probe the extent to which gas could penetrate the microstructure of six different metallurgical cokes made from Australian coals. We compared the distribution of the xenon sorbed by the coke samples before and after reaction with CO₂ at 1100 °C to 20–30% mass loss. Xenon is as strongly sorbed onto surfaces as carbon dioxide and can thus be used as an x-ray-visible analogue of CO₂. Aside from traces of pyrolysis ash, coke comprises two major components; the reactive maceral derived component (RMDC), which passes through a molten state during coke manufacture to form a foam-like structure, and the inertinite maceral derived component (IMDC), which are particles ranging from a few microns to a few millimetres in size, embedded in the RMDC. These components were found to behave very differently in this study. Prior to reaction, the RMDC component sorbed only a small amount of xenon and most of the IMDC sorbed little to no xenon. However, a small fraction of the IMDC took up significant quantities of xenon in high concentration. This suggests that a significant fraction of the surface area of unreacted coke comes from rare, high-surface-area IMDC components.

Imaging of the coke after reaction showed the RMDC still sorbed only small amounts of xenon, indicating that the surface area in these components was largely unchanged. However, the previously xenon-inaccessible IMDC regions sorbed large quantities of xenon after reaction, reaching peak xenon densities many times that seen in the free xenon gas. Thus, surface area is produced by reaction with CO₂ or (more probably) much of the pre-existing surface area is made accessible by reaction. This shows that IMDC provide most of the reacting surface during early stages of reaction of coke with CO₂. This was confirmed by the corresponding loss of mass seen in these IMDC particles relative to the RMDC.