Source of Variability of Reactivity of Coke in the CSR Test

The Coke Strength after Reaction (CSR) test is used worldwide to determine the reactivity of coke. The repeatability of the Coke Reactivity Index (CRI) test result as defined in the ISO18894 standard is between 2.5 and 4%, over a typical range of 20-40% of CRI. In project C12004 several approaches were investigated to identify the critical test parameters whose control could improve repeatability. However, when these experimental factors were controlled, repeatability was not improved substantially. The width of these repeatability limits may limit the discrimination of small changes in coke quality that occur due to blend changes, if the variability of test results was able to be narrowed the impact of these changes could be quantified.

The mass of individual coke particles before and after reaction in a CSR test furnace was previously measured. Major variations in weight loss of individual coke particles were found, even for a low reactivity (low CRI) coke. The extent of variation was great enough to account for the variability in results from an CSR test. The objective of this project is to identify the source of variation in mass loss of individual coke particles when subjected to the CSR test. If the source of variation was identified, then perhaps it could be controlled.

In this project three possibilities were examined as a potential source of variation in reactivity of individual coke particles:

• The method of crushing the coke particles. If incipient fractures were introduced into some coke particles, then they could introduce variability in reaction rates. No evidence was found that the crushing method affected the reactivity of the coke.

• Textural and structural examination of coke particles to see if either was related to the overall reactivity of the product coke particle. While it was clear that textural and structural examination provided insights into how the constituents of the coke particle reacted, it did not identify any cause of the variability of the overall coke particle reactivity.

• Non-random clustering of inertinite maceral derived component. If some coke particles were 'enriched' in IMDC, that could lead to the variations in reactivity of individual coke particles. No evidence for such clustering was found.

With the elimination of these factors, the cause of the variation remains unknown. During this project other findings were made:

• On reaction the pores measurable by optical microscopy become much more irregular in shape.
• The maximum temperature a coke is exposed to during the coking process is 1050°C and often less. There was evidence that the CSR test temperature of 1100°C annealed the coke nanostructure independently of reaction. This annealing manifested itself as an increase in numbers of pores at around 10 nm in size. This annealing effect could modify the coke properties and therefore complicate comparisons between CSR and cold strength tests.