Solar pyrolysis of biomass - part I: Volatile evolution mechanism

Abstract

To study the volatile evolution mechanism during solar pyrolysis of biomass, willow pellets with thicknesses of 3 mm, 1 mm and 0.5 mm were used to investigate the dynamic pyrolysis characteristics at heat flux densities of 0.5 and 1.1 MW/m2. The results showed that the larger thickness significantly reduced oil yield (by 23%–35%) and increases gas yield (by 19%–20%), while increasing heat flux density showed a similar trend. These changes in solar pyrolysis behavior and product composition were governed by intra-particle secondary reactions due to the heat and mass transfer conditions. The evolution of heavy compounds (molecular weight ≥ 200 Da) was integrated into the volatile evolution mechanism of solar pyrolysis. Results showed the secondary reactions promoted the cracking, deoxygenation and condensation of sugars and phenolic species, and also led to the growth of condensed aromatic hydrocarbons. A novel dimensionless number (Bi∙Prmass) was proposed in this study to estimate the solar pyrolysis behavior of wood pellets. A smaller Bi∙Prmass indicates that the conversion process is dominated by primary pyrolysis, while a higher value indicates that it is dominated by intra-particle secondary reactions. Depending on the degree of condensation, a critical value of Bi∙Prmass around 0.1 was adopted in this study. This work can provide guidance about operating conditions optimization for solar gasification of biomass.