Microstructure Characterisation and Simulation of Bio-additives in Coke

The objectives of this project were to:

• Develop a capability, based on statistical characterisation of coke microstructures, to relate the properties of bio-additives in coke to the mechanical properties of coke (i.e. coke strength);
• Use the capability to identify the specific properties of bio-additives that affect coke strength; and to
• Investigate the range of possible microstructures that could be created using bio additives to optimise the coke strength, for specified bio-additive demand (e.g. mass fraction added).

The project was conducted in a series of steps which are detailed within the report:

• Sample preparation, coal samples were prepared under three different conditions to determine the impact of biomass on the resulting coke microstructure;
• Cokes underwent industry-standard testing, with a specific focus on the coke strength indices;
• Resulting digital images of the microstructures were assessed using a variety of microstructure characterisation methods, the results were concatenated and compared.

Key Outcomes

The addition of bamboo char and acacia char biomass (10% by weight, sub-0.5mm) to the coal blend negatively affects the strength characteristics of the resulting coke. The impact of acacia char was extreme, with a reduction from a favourable CSR of 67.9 (control) to 2.9 (acacia). In addition to the measured impact on CSR, significant challenges were encountered when preparing (cored) acacia char blend samples for imaging, as they were prone to fracture, highlighting a brittle microstructure compared to both the control and bamboo samples.

The addition of biochar introduces several changes to the coke microstructure, and these changes were assessed using quantitative techniques for the pore, IMDC (inert maceral derived component) and the combined coke-matrix phase (reactive derived maceral component and biochar). Biochar addition alters pore structure by suppressing the typical formation of medium-sized pores (75-125 micron diameter) and increasing the formation of small pores (40-75 micron diameter) compared to the base coke. Bamboo showed the formation of excess small pores and fewer medium pores than acacia. In addition to the excess small-pore formation, acacia biochar addition resulted in a significant difference in larger pores (over 150 microns in diameter). Biochar addition also increased the mean pore coordination number, demonstrating increased pore connectivity compared to the base coke. Acacia addition also increased pore throat mean size compared to both the base and bamboo biochar-derived cokes. The impact on the size distributions and inert carbon density was largely unchanged with the addition of biomass, suggesting that the impact on microstructure due to biochar addition is largely isolated to the coke-matrix phase.

A thorough investigation into the impact of biochar addition on the coke matrix indicated that the bamboo and acacia biochar particles are integrated into the coke matrix differently, with bamboo biochar resulting in the formation of thinner coke matrix features (less than 50 microns) and acacia producing thicker coke matrix features (over 50 microns). Conversely to what would be expected in the analysis of non-bio char cokes, this increase in pore wall thickness does not correlate with enhanced coke strength as measured by tumble drum tests. The coke matrix exhibits a lower mean density and an overall shift towards lower density values with the addition of biochar into the coal. This shift is more substantial for acacia than for bamboo. The change in coke matrix density is positively correlated with coke strength (I600, ASTM, CSR) and negatively correlated with reactivity (CRI), suggesting a significant relationship between changes in carbon density and overall coke quality.

In addition to overall correlations between sample condition and industry measures, linear regression was performed using individual-sample-adjusted industry measures, both to examine the overall trend and relationships within each sample condition. These results revealed that, generally, higher mean greyscale values within the coke matrix correlate with higher strength metrics for biochar samples. Both CSR and CRI show robust relationships with mean greyscale values.

Analysis of mean coke matrix width reveals that model variance for CSR across all samples is low. However, predictive power rises significantly when focusing on three samples per coke type, highlighting negative relationships in which increased width correlates with lower CSR values, indicating that the amalgamation of biochar into the matrix is not increasing the matrix strength. Lastly, the regression for mean pore size and quality metrics indicates that the linear model does not effectively capture the variability in CSR and CRI across all sample types. Overall, the analyses emphasise the complex interactions among biomass types, physical properties, and coke strength metrics, as well as the variability and sensitivity of the different materials.

This research has developed a robust analytical methodology that allows for the separate consideration of the coke matrix and coke inserts during analysis. It has yielded fundamental insights into alterations in the coke microstructure upon incorporation of bamboo and acacia char.