Technical Market Support » Metallurgical Coal
Stamp charged cokemaking has emerged as an effective technique to improve the quality of coke produced from weakly coking coals or blends of premium and low quality coking coals. Stamping can be used to either improve coke quality of poorly coking coals or allow the introduction of inferior coking coals in the blend. With the continuous growth in the adoption of this technology, especially in Asian markets, improved fundamental knowledge of the behaviour of Australian coals under stamped conditions and the mechanism of formation of coke structure is essential to improve the understanding of the carrying capacity of Australian coals and design suitable blends for stamp-charged cokemaking applications.
The overall objective of this project was to gain insights into the coke formation mechanism under stamped conditions and understand how higher charge densities affect the development of thermoplasticity, microstructure and microtexture, and ultimately coke reactivity and strength. This was achieved by a comprehensive investigation of the coal-to-coke transformation of an Australian premium coal, two weakly coking coals, and binary blends of these coals.
Coke samples from single coals and blends were prepared using the coke oven at University of Newcastle at around 8kg capacity under top-charged and stamped conditions, followed by coke CSR and CRI testing to determine the uplift in coke quality with stamping. Then, the coking performance of samples was studied under top-charged and stamped conditions. This included IGP measurements in the 4kg coke oven and the dilatation and permeability measurements using the University of Newcastle test facility. Lastly, the plastic layer and coke samples were characterised to evaluate the impact of charge density on the transformation of coke microstructure and microtexture.
Results showed that stamping improves coke quality across all single coals and blends, however, the uplift in CSR was more effective on samples producing weaker cokes. In addition, the CRI values of cokes decreased with stamping, however, the relative changes were less than CSR. The impact of stamping on the mechanism of reactivity and post reaction strength of coke requires further investigation.
One key objective of the project was to understand the impacts of coal properties on the quality of cokes made under stamped conditions. The uplift in CSR (ΔCSR) was correlated with the properties of single coals and blends used. While the dataset is limited, a positive correlation was found between ΔCSR and the inertinite content in coal samples. In addition, samples with lower initial dilatation were found to benefit more from stamping.
The impact of stamping on thermoplasticity and IGP development, coke microstructure transformation, and coke microtexture were studied in detail to understand the key drivers of coke quality improvement under stamped conditions.
The influence of stamping on the development of thermoplasticity was studied. Lower rank and higher vitrinite samples were less affected by stamping and their dilatation, permeability, and internal gas pressure remained relatively unchanged at elevated densities. This was related to a greater post-resolidification shrinkage of these coals, which hinders the compaction of the plastic layer under stamped conditions. In contrast, the thermoplasticity of higher inertinite coals improved significantly.
The impact of bulk density on coke microstructure was investigated using the micro-CT image analysis technique. Using the quenched plastic layer sample images, a new method was developed to calculate the transformation of porosity through different stages of coke formation. It was found that stamping reduced the porosity of most cokes, except for cokes made from the lower rank and higher vitrinite coal which showed the least CSR uplift with stamping. Conversely, the porosity of cokes made from coals with higher inertinite content decreased by around 10-15%. Lower coke porosity is understood to greatly contribute to its mechanical strength. Stamping was also found to significantly improve the interactions between RMDC and IMDC. Increasing the charge density to 1015 kg/m3 reduced the porosity at the IMDC-RMDC interface by 15-20%.
A further key finding was the positive influence of bulk density on the anisotropy of carbon in coke, suggesting that larger carbon domains were formed under stamped conditions. Moreover, increasing the charge density led to a significant decrease in the proportion of isotropic carbon forms and a corresponding increase in anisotropic carbon forms. The combined effect of these changes is an effective increase in the rank of stamped cokes, which leads to decreased reactivity and improved mechanical strength. The uplift in anisotropy of cokes showed a strong positive correlation with the inertinite content in coal, while samples with lower initial dilatation showed a greater uplift.