Coke Reactivity Test: Critical Parameters

A Draft International Standard ISO/DIS 18894 has been prepared titled "Coke - Determination of coke reactivity index (CRI) and coke strength after reaction (CSR)". The technical committee has the role of standardising common practice. The Australian members of that committee do not have sufficient data to form a position with regards this draft standard. The objective of this project was to examine the standard, with particular reference to a set of variables which were possible sources of variation within the coke reactivity test. The aim of this was to establish whether these variables were critical or non-critical in controlling reactivity.

A series of cokes were tested with controlled variation in:

• The reaction retort diameter
• Screen size used to prepared the sample and the screen size used to determine the CSR
• Reaction temperature
• Sample heat-up time
• Sample soaking time

The variation between results was compared to the claimed repeatability limits of the test.

The repeatability limits prescribed in the proposed standard were found to not match the measured repeatability limits of the test. The proposed repeatability limits are suitable for superior quality cokes (CRI <30, CSR >60). However, for inferior cokes the repeatability limits diverge from measured results.

Within the limits of the experimentation, the following parameters were found to influence CRI results:

• Retort diameter - 33% of results were outside the draft ISO prescribed repeatability limits, with narrower retorts producing positively biased results. Non-parametric testing also indicated a bias at 99% confidence limit with narrower retorts producing positively biased results.
• Screen size - 29% of results were outside the prescribed repeatability limits with neither the NSC nor ASTM screens producing superior or inferior results. However, additional external data indicated negatively biased results for cokes screened below a mean coke size of ~15mm.
• Furnace recovery time - non-parametric testing indicated a bias at 80% confidence limits, with longer recovery times producing positively biased results.
• Reaction temperature - non-parametric testing indicated a bias at 95% confidence limits, with lower reaction temperatures producing positively biased results.

Within the limits of the experimentation, the following parameters were found to influence CSR results:

• Retort diameter - 17% of results were outside the draft ISO prescribed repeatability limits, with narrower retorts producing positively biased results. Non-parametric testing also indicated a bias at 99% confidence limits with narrower retort producing positively biased results
• Furnace recovery time - non-parametric testing indicated a bias at 99% confidence limits with longer recovery time producing positively biased results.
• Soaking time - 14% of results were outside the draft ISO prescribed repeatability limits with longer soak times producing positively biased results.
• Reaction temperature - 22% of results were outside prescribed repeatability limits but there was no trend of negatively or positively biased results with higher temperatures. However, additional external data indicated inferior results when testing at higher temperatures. Also, non-parametric testing recognised a bias at 99% confidence limits with lower coking temperatures producing positively biased results.
• Screen size - 29% of results were outside prescribed repeatability limits with neither the NSC nor ASTM screens producing superior or inferior results. However, additional external data indicated inferior results for cokes screened below a mean coke size of ~15mm.

Thus it may be concluded that the proposed standard has sufficient variability in operating parameters to produce biased results depending upon furnace operating conditions and the type of screens that are used to undertake coke sizing.

It was also considered that the NSC Reactivity Test is more fundamentally flawed. Some of the variation that was observed in the testing program is not due to either lack of rigor in the testing procedure or sampling method. The probable root cause of this variation is sample non-representivity due to the coke lump size being too large for the sample mass. While the current coke size distribution and sample mass is adhered to non-representative samples will be tested.

An e-newsletter has also been published for this project, highlighting its significance for the industry.