Sustainable production of porous carbon from biomass: Influence of pre-carbonization on pore evolution and environment impact

Pre-carbonization is one frequently used pre-treatment for production of activated carbon (AC) from activation of biomass, which impacts not only nature of AC but also composition of bio-oil and gases in activation of biomass. In this study, activation of sawdust/cellulose/lignin directly or via pre-carbonization were conducted, with a particular focus on effects of pre-carbonization on evolution of the various products and environment. The results indicated that the pre-carbonization removed significant portion of aliphatic structures, creating difficulty for developing pores from the resulting biochar. This, however, enhanced overall yields of AC and gases at expense of bio-oil with K2C2O4, CO2 or H2O as activators. The high yield and high specific surface area of AC could not be obtained simultaneously and needs to be balanced in application. In converse, ZnCl2 or H3PO4 as activators generated more AC with developed pore structures via direct activation of biomass, as they catalyzed the dehydration of aliphatic structures in biomass, but not in biochar. The life cycle assessment (LCA) indicated that the pre-carbonization led to a higher impact to environment and energy consumption than the direct activation. ZnCl2 and CO2 showed the smallest influence on the fossil fuel depletion and global warming. Direct activation exhibited lower environmental impact in the production of AC with more developed pore structures, rendering it a more desirable route for production of AC.