Evolution of Char Structure during the Steam Gasification of Biochars Produced from the Pyrolysis of Various Mallee Biomass Components

This study reports the evolution of char structure during the steam gasification of biochars under chemical-reaction-controlled conditions. Partially gasified samples were collected at various conversion levels during the steam gasification of both the raw biochars and the acid-treated biochars that had been prepared via acid-washing of the raw biochars. Results from FT-Raman spectroscopy show that the biochars have highly heterogeneous and disordered structures, which are selectively consumed with progress of steam gasification, leading to enrichment of larger aromatic ring systems, hence the so-called selective gasification. Selective gasification of biochar can be significantly influenced by the inherent alkali and alkaline earth metallic (AAEM) species in the biochars. The abundant catalysts present in the raw biochars can alter the gasification reaction pathway, but such an alteration appears to have little effect on the evolution of pore surface area, which increases significantly with conversion. While the wood biochar has too low a content of AAEM species to have an apparent effect on selective gasification, for the raw leaf and bark biochars with high contents of AAEM species, selective gasification is considerably less significant in comparison with the respective acid-treated biochars. For acid-treated biochars, gasification seems to take place slowly throughout the biochar on carbon active sites to consume the smaller rings selectively; the reactivity is controlled by the biochar carbon structure. However, for the raw leaf and bark biochars, gasification would be more focused or localized on the catalytic sites so that the activity of carbon active sites becomes less important. The catalytic effect of the inherent AAEM species seems to in turn depend on the carbon structure that probably affects the catalyst dispersion.