Insights into heavy components evolution in the condensed volatiles from amino acids pyrolysis

Heavy components, comprised of molecules with a substantial molecular mass, are essential constituents within volatiles. Within these molecules, nitrogen-containing compounds have a tendency to undergo transformation into coke during thermal treatment, exerting a substantial impact on the efficient and secure utilization of biomass. Three amino acids- alanine with an aliphatic structure, phenylalanine with additional aromatic features, and tryptophan with an extra heterocyclic ring were selected as model compounds, individually subjected to pyrolysis within the temperature range of 450 to 650 °C. The evolution of heavy components and the associated reaction mechanisms were comprehensively explored through extensive characterization and network analysis. The results showed that alanine transformed into heavy components mainly through dehydration followed by dissociation of CH2 and CH. In contrast, phenylalanine and tryptophan tended to undergo fragmentation into C8H7N· and C12H10N2· derivatives, subsequently recombining to form heavy molecules. As the temperature increased, heavy components in alanine-derived volatiles monotonously decreased due to elimination reactions and the breaking of C–C bonds. Conversely, heavy components in phenylalanine and tryptophan initially increased and then decreased due to the enhanced cracked fragments, along with the competitive relationship between aromatization and polymerization reactions. For all three amino acids, there is a positive correlation among the average mass of heavy components, tar yield, and the intensity of permanent free radicals in the char. This implies that we may rapidly predict the evolutionary patterns of heavy components and tar through free radical detection. HCN dissociation is mathematically identified as a pivotal reaction in heavy component evolution, irrespective of amino acid type, possibly unveiling an important pathway for NOx formation. This study shows substantial theoretical significance for the eco-friendly and efficient conversion of nitrogen-rich biomass.