Solar pyrolysis of biomass- part II: The physicochemical structure evolution of char

Dynamic solar pyrolysis experiments of willow pellets with different thicknesses (3 mm, 1 mm and 0.5 mm) were conducted at heat flux densities of 0.5 and 1.1 MW/m2 to study the physicochemical structure evolution of char. Characterizations of the physical structure showed that the reflectivity of char surface firstly reached the min-imum value of 7.3 % and then rebounded to 15.3 %. The specific surface area decreased with reaction time from 13.2 to 2.0 m2/g, indicating its poor microscale pore structure (pore size <= 500 nm). The 3D scanning conducted by computed tomography (micro-CT system) showed that the residue pellet has unique hierarchical layers with different density and porosity. The lamellar structure can be also found inside the char layer which may be due to the increasing temperature gradient and the internal pressure caused by the volatile release. Concerning the chemical structure evolution, the secondary coking of the heavy compounds in volatiles introduced more oxygen -containing groups and alkyl side chains into the char layer. The aromatic rings system in the inner part of char was developed through removal of side-chains and functional groups. Further structure ordering of char surface was contributed by dehydrogenation and condensation. These differences between two parts of char can be attributed to the poor heat and mass transfer properties of lamellar structure. This work reveals the unique structural characteristics of solar pyrolysis char, which complements the solar pyrolysis mechanism and provides a new insight into the structural evolutions.

Automated summary

In this study, dynamic solar pyrolysis experiments were conducted on willow pellets to investigate the physicochemical structure evolution of char. The results showed that the reflectivity of char surface initially decreased and then rebounded, while the specific surface area decreased with reaction time. Three-dimensional scanning revealed unique hierarchical layers in the char structure. The chemical structure evolution resulted in the formation of aromatic rings and enhanced surface ordering. The differences in structure can be attributed to the poor heat and mass transfer properties of the lamellar structure.