Implications of Coking Conditions on CSR

Coking is one of the main contributors to the quality of the resultant coke and therefore will influence the CSR of the coke. Laboratories worldwide use their own devised methodology for coke making, which is cause for concern since CSR results cannot be compared without a measure of error. Therefore, there is a need for the Australian coal industry to understand the implications of coking conditions on potential differences in coke quality when using different bench or pilot scale methods to produce coke for testing.

In the first stage of this project three Australian coals were selected based on their differences in rank, these were a high volatile soft coking coal (B), typical prime hard coking coal (A) and a low volatile semi hard coking coal (C). The coals were coked in ALS Coal's small scale coke oven using the ALS Coal standard conditions, which has a high heating rate and 0.5 hour of soak time at 900ºC. The coals were also coked at two lower heating rates to 900ºC. At each of these heating rates the soak time was 0 and 1 hour. The first stage was reported by Wimalaratne and Bennett (2013).

In the second stage of this project three additional coals (D, E and K) were examined. Coals D and E were tested under the same conditions as in stage 1 but also at bulk densities of 690, 815 and 900 kg/m3. Coal K was tested under limited coking conditions and at bulk densities of 690 and 815 kg/m3.

The resultant coke was stabilised using a modified Micum test before the I Drum test, the same drum test used for the CSR determination, and the porosity of the coke was determined on the unreacted coke. The NSC Reactivity test was conducted on the stabilised coke to determine the CRI and CSR.

In addition, the method of processing the coke prior to testing, e.g. degree of stabilisation, was also examined in this project using data from ACARP Project C23056.

This project has clearly demonstrated that coking conditions do impact on the CSR of the coke produced. The heating rate when the coal undergoes plastic deformation was found to be a key process variable that impacts on coke quality. It is expected that for most coals the coking conditions influence the microstructure of the coke via variations in the heating rate at the time of plastic layer formation. Due to the design of the ALS 8kg small scale coke oven the full range of different heating rates that would occur in the various designs of small scale coke ovens used around the world cannot be easily tested. A better determination of the influence of coking rate on the heating rates at different positions within the current small scale oven will provide a better basis to clarify how these heating rates may impact of coke formation. This will require more detailed temperature measurements during the coking tests. This understanding of how coking conditions coupled with appropriate modelling, such as that being undertaken by CSRIO, of the different ovens used by organisations around the world should provide the Australian coal industry the tools required to interpret the results from different organisations.

The results of this study clearly show the inherent strength of the coke has a strong influence on the resulting CSR. A better understanding of the breakage characteristics of a small scale coke before the NSC Reactivity test is required to allow for the interpretation of CSR on cokes produced under different coking conditions. This understanding will assist in the recommendation of standard procedures for the coking and processing before NSC Reactivity testing of coke produced at small scale for determination of coke quality. The breakage characteristics of coke produced in smaller coke ovens (less than 20 kg charge) is being studied in ACARP project C23056.