Pyrolysis of Posidonia Oceanica balls: A comparative evaluation of conventional and microwave-assisted approaches.

Posidonia Oceanica is an abundant plant of the Mediterranean Basin, but its residues should be properly managed to minimize environmental, economic and social concerns. Posidonia Oceanica debris are wastes that could be advantageously exploited as source of biofuels and bioproducts. In this work, fibrous spheres of Posidonia Oceanica have been considered for studying the pyrolysis approach, comparing the traditional heating and the microwave-assisted one, aimed at the production of solid biochar, liquid bio-oil and gaseous syngas. The pyrolysis technologies were compared keeping similar pyrolysis conditions, e.g. heating rate, set-point temperature and residence time, all affecting the product distribution and the corresponding compositions. The comparison between the conventional and microwave-assisted approaches ascertained a reduction of secondary cracking reactions in this latter case, highlighting the improved control of the degradation mechanisms. The addition of a deflagmator unit increased the residence times of the hot vapors within the microwave reactor, favoring the secondary cracking reactions. These different set-ups lead to differences in the chemical compositions, mainly involving the bio-oil stream, in terms of the concentration of the oxygenates and the amount of water. Remarkably, the use of the same Posidonia Oceanica biochar both as an efficient microwave absorber and a reducing agent, was effective for improving the syngas yield, which selectively included H2 and CO (about 46 and 53 vol% of the microwave test versus 10 and 25 vol% of the conventional one, respectively). Such a significant difference was due to the occurrence of gasification reactions with microwave technology, enabled by local hotspots, making attractive the co-pyrolysis approach for the production of a syngas of higher quality. Finally, preliminary LCA evaluations revealed that the environmental impact of conventional pyrolysis remains lower than that of the microwave one, due to the still higher impact of the electricity cost from fossil resources and energy consumption of the latter technology, which represent the main bottlenecks to be overcome in order to reduce the total environmental impact of this technology, thus favoring its scale-up in the next future.