High Resolution Micro-CT 3D Assessment of Coke Gasification

Coke reactivity index (CRI)/coke strength after reaction (CSR) measurements are considered key metrics of coke quality and performance in a blast furnace. Improved knowledge of the reaction interface/contact area between the gas and coke and the mineral matter and coke is needed to apply CRI and CSR data to a blast furnace. Techniques such as micro CT have the potential to provide this information, however, most studies have had a special resolution limit of ~20-30µm, which has made it difficult to obtain quantitative data.

The contact areas between coke (carbon) and gas, and coke (carbon) and minerals were quantitatively assessed using high resolution micro-CT measurements. These reaction interfaces/contact area data are vital for an improved understanding and application of CRI/CSR data to coke performance in the blast furnace.

High resolution micro-CT scans (voxel size 2.35 μm) of samples of coke and coke analogue were taken before and after reaction with CO2 under CRI conditions. The coke used (designated coke A) was an industrial coke produced by a coke user, while the coke analogue sample was prepared to mimic the metallurgical coke with respect to its ash composition. Analyses of the 3D micro-CT, where possible, were validated against standard 2D measurements and other known sample data, and used to quantitatively assess the porosity and mineral distribution in the samples.

The results from the high resolution micro-CT were highly encouraging, with the increase in resolution making the different components (carbon, porosity and minerals) and their structures readily distinguishable, down to ~9 μm, for both coke and coke analogue samples. Determination of thresholds for segmentation of the data based on comparison with traditional 2D techniques was found to be simpler for the high resolution micro-CT data.

The main findings and conclusions of the project were:

• The porosity of the coke A and coke analogue A samples increased on reaction with CO2. Most of this increase in porosity occurred in the smaller pore sizes (< 60 μm). This result is different to that typically reported in the literature for 2D analysis that pore size increases with reaction. This difference is caused by the different ways that pore size is defined.
• For both samples, as expected, it was found that the principal carbon loss of the sample in the pseudo-CRI test was from the pore walls during reaction with the CO2 gas. The larger proportion of smaller pores after reaction measured by the micro-CT analysis leads to an increase in the surface area of the pore, increasing the surface area available for gas-carbon reaction.
• IMDC regions in the coke A sample were more reactive than the RMDC regions, and could be related to a larger increase in the smaller sized pore (< 60 μm) compared to the RMDC.
• Pore connectivity increased on reaction with CO2 under CRI conditions. As pore connectivity can be related to gas access and penetration into the material, this parameter can be related to the reaction mechanism (and consequently reaction rate) of coke. This is key in understanding and predicting coke reaction behaviour in use.
• The reaction mechanism of both the coke and coke analogue samples was consistent with chemical reaction control (regime i). In the previous study (C26045), the reaction mechanism on larger samples was found to be mixed control (regime ii), where both chemical reaction and pore diffusion control the reaction rate. The difference in the reaction mechanism was a consequence of the small samples (2 x 2 x 15 mm) required for the high resolution micro-CT scans. These samples were smaller than the reaction layers observed in C26045. This finding demonstrates that the high and lower resolution micro -CT datasets should be considered complementary.
• Minerals were readily identified in the samples as the brightest features in the micro -CT data. Some issues with an excess of fine mineral particles found from the micro-CT data, especially near the surface of the samples. It is likely that there are some limitations on the fully quantitative analysis of the minerals in the coke and coke analogue samples at these very fine mineral particle size fractions, while being more robust at the larger mineral particle size fractions.