The influence of pilot-scale pyro-gasification and activation conditions on porosity development in activated biochars

Few studies have examined the influence of pyro-gasification and activation conditions on porosity development in activated biochars. In this context, this study investigates the effects of pyro-gasification temperature (315, 399, and 454 °C), activation temperature (700, 800, and 900 °C), and activating agent (CO2 flow rate: 2, 3, and 5 L min−1) on porosity in materials made from wood residues (black spruce and white birch). Activated biochars were prepared in a two-step process: torrefaction/fast pyrolysis in a pilot-scale plant and activation using an in-house pilot-scale furnace. Results show that the physical properties of activated biochars improved over biochars and wood residues, with fivefold greater surface area for activated birch biochar over biochars, and threefold greater surface area for activated spruce biochars. Statistical analysis results reveal that pyro-gasification and activation temperature, CO2 gas flow rate, and wood residue type significantly affected the porosity of activated biochars (at p < 0.05). The main findings are as follows: i) Torrefaction or pyrolysis pre-treatment step had less impact on the porosity of activated biochars, so lower energy expenditure is required to improve product quality, i.e., porosity; ii) Activation temperature was the major variable to optimize specific surface area; by increasing from 700 to 900 °C, the average surface area for activated biochars made from both wood residues increased to nearly 120 m2 g−1; iii) pilot-scale technologies produced porous activated biochars comparable to laboratory-scale technologies which could boost incentives to use thermochemical biomass conversion, and increase the profitability with these diversified by-products in biorefinery industry.