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CRI/CSR CO2 combustion data are key metrics of coke quality and performance in the blast furnace. Recently adapted 3D analysis techniques applied to coke and a new laboratory tool, the coke analogue, offer opportunities for generating an improved understanding of CRI/CSR coke gasification that could be used to inform coal blending for coke making or in steel industry support for coke behaviour in use.
Both coke combustion in CO2 (gas) and coke carbon reactions with the coke ash are heterogeneous reaction processes and therefore their reaction rates are highly dependent on the contact areas or interface between the coke and gas and carbon and coke ash respectively.
The key objective of this project was to utilise CSIRO's micro-CT 3D characterisation tool to evaluate and understand the mineral and porosity evolution during CO2 gasification of coke under coke strength after reaction (CSR)/coke reactivity index (CRI) conditions. This was be achieved by:
- Measurement of the 3D pore structure evolution of the coke and coke analogue under CSR/CRI conditions;
- Measurement of the 3D pore structure evolution of the coke analogue at a temperature greater than the 1100°C of the CSR/CRI test;
- Where possible, validation of the 3D data generated using classical 2D approaches.
A brief review of 3D measurement techniques applied to coke was also undertaken early in the research program to inform/aid achieving the project objective. This review is appended to this report and offers the reader a useful reference for what has gone before.
From a qualitative perspective, the general findings from the study were positive. Minerals, carbon and pore structures in both the coke and coke analogue were readily discernable using micro CT and it was possible to track their changes on CO2 gasification.
When the assessment of changes as a result of CO2 gasification through classical 2D laboratory techniques, such as optical microscopy porosity distributions and mineral phase distributions, were compared with the micro CT 3D measurements, there was a general consistency of trends between the techniques. Again, this was true for the coke and coke analogue.
Through both 3D and 2D techniques under CRI conditions it was found that coke A and coke analogues A and E showed mixed control kinetics where both chemical reaction and gas pore diffusion were rate limiting whereas coke E was chemical reaction controlled.
It was also found that there was significant pore connectivity with both cokes and the coke analogues and that the connectivity increased on reaction with CO2 in the CRI test. This is significant as it dictates gas access and therefore reaction rates of the coke under reactive gases.
While there was good consistency with trends, there was generally poor agreement with absolute values. This tends to indicate that there are calibration issues associated with the micro-CT measurements that need to be resolved. This was true for both the coke and the coke analogue, though the higher uncertainty associated with 2D measurements of coke (optical porosity and mineral size distribution) made its assessment of more difficult.
The primary advantage of micro CT analysis is that it offers possibility of giving a more meaningful description of the pore connectivity of a coke a coke analogue that is not possible from other techniques. This is of profound importance in understanding coke reactivity and developing more accurate predictive models on coke performance and expand the relevance of key metrics such as CRI/CSR. From a qualitative perspective, the 3D micro CT connectivity assessment carried out at both CSIRO and UOW was successful and trends were in reasonable agreement. Quantitatively there were significant differences that were at least part due to the different approaches used to describe the pore structure. These differences are likely an artefact of the analysis approaches and would be ready identifiable if the micro CT measurements were carried out at a higher resolution/smaller voxel size. In any future study, a more detailed micro CT analysis that deals with higher resolution imaging and sample representation is required to address this problem.
With its more controlled properties, the coke analogue offered a potential approach to calibration of key micro CT measurement and analysis parameters. This proved significant in this study as it allowed setting of image analysis parameters independent of the researcher carrying out the analyses. For example, the mineral component of the analogue can be controlled with respect to size and content, and can be considered as a known in the analogue. These data can then be used to vary micro CT threshold parameters until the measured values match the known data. Such “tuning” approaches to aid measurement objectivity were applied with some success in establishing total porosity and total volume of minerals. Unfortunately, this tuning approach was not fully realized in the study. The intended follow up measurements with the micro CT at higher resolution were not possible due to equipment failure issues. Further work is required to address the measurement objectivity issues in the data analysis.
A significant finding of this study was confirmation of the suitability of the Jenkins et al approach, using greyscale distributions, in assessing coke and coke analogue gasification in CO2. The use of this technique appeared able to distinguish simple chemical reaction control and mixed control (chemical reaction and pore diffusion) regimes. This offers possibilities in reducing assessment times for characterizing coke behaviour in use, as less experiments would be required to define rate controlling regimes. Using this approach, it was found that at the 1100°C of the CSR/CRI type test that the coke analogues (both analogue A and E) and coke A's reactivity appeared to be in the mixed control (pore diffusion and reaction control) regime and coke E in the reaction control regime.
The effect of temperature on the reactivity with both CO2 and the mineral matter was evaluated on only coke analogue A. It was reacted in a similar manner to that of the CSR/CRI type testing except at 1550°C. With respect to the performance of the micro CT 3D characterisation, the results were essentially the same as that of the 1100°C testing i.e. generally qualitative rather than quantitative. The primary difference was that there was significant mineral-carbon reaction taking place. Post reaction XRD analysis indicated SiC formation, likely formed from a reaction with quartz containing ash. There was also indication that the gasification reaction mechanism had changed from mixed control at 1100°C to mass transfer in the gas phase at 1550°C.