Technical Market Support » Metallurgical Coal
The suitability of cokes for use in a blast furnace is determined by a range of factors such as strength and reactivity, both of which are critically dependent on the coke microstructure. The objective of this project was to improve the ability to predict coke behaviour through a better understanding of the impact of the microstructure on these properties and how the microstructure is affected by reaction with carbon dioxide.
This project used Xenon gas K-edge subtraction in synchrotron micro-CT imaging for probing the fine scale porosity and gas transport behaviour in coking coals and in cokes before and after reaction with CO2. This included investigating the differences in open and closed pore structure at fine scales for a range of cokes and one coal and how this varies within coke particles. Xenon gas K-edge subtraction allowed the determination of the distribution of xenon within the sample in 3D. The absence of xenon in a region indicates that the region is impenetrable, and in regions where the density of xenon in the coal or coke is much greater than in the free gas, xenon must be sorbed onto surfaces, and indicates that the region has a high surface area that is accessible by xenon. Thus the xenon density distribution in the image provides a measure of the distribution of accessible surface area in the sample.
Synchrotron micro-CT scans of coke samples were carried out pressurised with xenon gas before reaction and after reaction to 20-30% mass loss with CO2. The high visibility of the gas in an x-ray image enabled the gas uptake into the coke samples to be observed. Scans were also made of xenon uptake in coal samples over a few days after being pressurised with gas and again after one month. The scans of xenon uptake in coal showed a large number of microcracks distributed throughout the coal that were almost immediately penetrable by xenon. This was followed by a steady diffusion of gas from surfaces and micro-cracks into the bulk material. The images showed that the density of such microcracks were highly variable even within different parts of the same sample. The two-step penetration into coal shows that in order to model diffusion into coal, it is clear that single component systems that assume one diffusion rate are insufficient to describe the diffusion behaviour in coal.
The coke specimens showed intriguing and complex behaviour. The xenon gas readily penetrated nearly all of the visible pore network which was largely interconnected. However, prior to reaction relatively little xenon was taken up into most of the coke material itself. There was a small minority of inerts taking up significant quantities of xenon. Following reaction the RMDC components took up little gas, as they had before reaction, but most of the inerts took up large quantities of xenon gas reaching peak xenon densities many time that seen in the gas phase. The behaviour of different inerts was also quite variable in terms of the extent of gas uptake.
These observations indicated that much of the surface area of unreacted coke comes from rare high surface area components. This provided an explanation of why the surface area of unreacted cokes, if it is low, does not provide a good indicator of coke reactivity. Furthermore, porosity is introduced by reaction or (more probably) pre-existing nanoporosity is made accessible by reaction, and nearly all of this porosity occurs in IMDC. This provide most of the reacting surface during early stages of reaction with carbon dioxide.
However there was not a simple relationship found between xenon uptake after reaction and the rate of reaction seen in these samples.