Physical and Chemical Structure Characterisation of Biomass for Biocoke Production

Partial substitution of coking coal with renewable biomass is identified as a promising approach to decrease emissions associated with BF ironmaking. Biomass derived materials are understood to negatively impact the reactivity and strength of metallurgical coke, primarily due to unfavourable chemistry and the porous and isotropic nature of these materials. It is imperative to select suitable biomass species and appropriate pre-treatment methods for rendering the physical and chemical structure of biomass suitable for cokemaking applications.

This project forms an initial part of an integrated program of research to better understand the biomass and coal quality requirements for biocoke production for blast furnace ironmaking. The overall objective of this scoping study was to study the impacts of parent species and pre-treatment methods on the physical and chemical structures and properties of biomass. Five lignocellulosic (woody and crop waste) and microalgae biomass samples which are readily available in Australia, were selected. Torrefied and pyrolyzed char samples were prepared in a custom designed fixed bed pyrolysis reactor heated in an electric oven, followed by a comprehensive structural characterisation using a range of analytical techniques.

Results showed that biomass samples in their raw form differ significantly. Proximate and ultimate analysis results indicated that, unlike coking coals, biomass samples have high contents of volatiles and oxygen, and in some cases, high ash content reaching approximately 23 wt.%. One key objective was to understand the impact of torrefaction on the chemical and physical properties of the biomass samples and to assess the resistance of different biomass species to volatile and oxygen loss. It was found that bagasse and bamboo had lower thermal decomposition temperatures compared to hardwood acacia and mallee and lost oxygen functional groups at lower temperatures.

The impact of thermal treatment was investigated on the surface area using N₂ adsorption-desorption isotherms. It was found that all biomass in raw form exhibit low surface areas (<3 m²/g). This remained true for the torrefied samples across all species studied. After complete pyrolysis at 800 °C, bamboo and mallee did not develop significant surface area and remained largely unchanged. In contrast, acacia and bagasse developed relatively higher surface areas at 800 °C, suggesting extensive microporosity development at high temperatures.

In summary, the findings of this project showed that the inherent differences in biomass species impact their thermal decomposition behaviour during pre-treatment.