Technical Market Support » Metallurgical Coal
This project contains a study on the interactions occurring between coal macerals from single coals and between the macerals from different coals in blends. The goal was to detect interactions and to determine their nature, i.e. whether they occur through physical or chemical mechanisms. To achieve this goal we combined a number of tools. We used rheometry to characterise the viscoelastic properties, thermogravimetric analysis (TGA) to obtain total volatile release behaviour, and Dynamic Elemental Thermal Analysis (DETA) to obtain the volatile release behaviour of Carbon and Hydrogen. To study interactions we took the approach of firstly characterising the pure components. The predicted behaviour of those pure components in blends was then calculated using common additive rules. The predictions assume no interactions are occurring. We then compared the predictions to actual measured results for the blends. Any deviation between predicted and actual results indicated an interaction and from the nature of the deviation we attempted to deduce the nature of the interaction, i.e. whether the deviation could be explained by a physical or chemical mechanism. To study interactions between macerals we firstly produced maceral concentrates using a reflux classifier. We studied maceral interactions for three coals and we studied interactions occurring in 3 coal blend sets.
For the maceral interactions study, it is hypothesised that inertinite plays an important role in volatile release behaviour and this has a number of important effects. More specifically, it is proposed that inertinite can either trap or aid volatile release which affects the driving force for liquid evaporation and leads to either an increase or decrease in the amount of liquid, respectively. An increase or decrease in the amount of liquid will decrease or increase the viscosity of the melt, and the viscosity of the melt controls bubble growth and coalescence behaviour. At the two extremes, if viscosity is very high, bubble nucleation and growth do not occur which means that adhesion will not occur. If viscosity is too low, excessive pore wall thinning occurs. Therefore, it is proposed that inertinite may indirectly influence adhesion and pore structure for a given coal in either a positive or negative way. This interaction has been labelled a Type 5 interaction. It is speculated that the pore properties of the inertinite may dictate whether these events occur, and that pore properties of the inertinite can change during carbonisation as they themselves devolatilise. It is also proposed that inertinite can trap liquid matter in its porous structure, which can aid the bonding of inertinite with the melt.
For the blends, enhanced volatile release was also observed, which resulted in reduced expansion and improved strength. The improved strength may have been due to improved pore structure resulting from the reduced expansion. The DETA tests for the blends revealed the carbon yield to be lower than expected which may suggest that enhanced volatile release also results in an overall greater loss of carbon from the solid to the vapour phase. This may be an important area of study if cokemakers are trying to maximize the conversion of carbon into coke for economic and environmental reasons.
It follows that a greater understanding of the factors controlling volatile mass transfer could provide unique insights into coking behaviour from which a more complete understanding of the factors controlling adhesion and pore structure development could be gained. There is still a significant lack of knowledge on the behaviour of inertinite during coking and the extent that inertinite is detrimental or advantageous leading to the ideal level of inertinite in a coal product. Such knowledge would be useful for preparing coals with maximum value for the market.
This project did provide some fundamental insights into the effect of particle size on volatile mass transfer and led to an improvement in our approaches for studying these system, involving increasing sample size for thermogravimetric studies.
This study did not find strong signs of chemical interactions. Increasing the number of coals in the study is necessary to confirm their occurrence.
The main recommendation for further work is to continue studying interactions between macerals by mixing the inertinite rich fraction from one coal with vitrinite rich fractions from various different coals to see whether it shows common behaviour that can be directly attributed to its properties. The properties of the inertinite also needs to be obtained, in particular, its surface and pore properties (size and connectivity). Such a study should elucidate the extent that inertinite is able to prevent or aid volatile release behaviour.
This research feeds directly into the development of a fundamental mechanistic model of coke strength development. It is envisaged that it will be used to devise better models to predict strength and elucidate ways to improve the performance of a particular coal during coking, and hence improve its market acceptance. Therefore, this research is expected to provide both short-term and long-term gains.