Influence of the Carbonization Process on Activated Carbon Properties from Lignin and Lignin-Rich Biomasses

Lignin-rich biomass (beech wood, pine bark, and oak bark) and four lignins were tested as precursors to produce activated carbon (AC) via a two-step chemical activation with KOH. First, the precursors were carbonized via either pyrolysis or hydrothermal carbonization, with the purpose of evaluating the influence of the carbonization process on the AC properties. Pyrolysis chars (pyrochars) were thermally more stable than hydrothermal carbonization chars (hydrochars); thus, more AC was yielded from pyrochars (AC yield calculated from the char amount). The difference between ACs from hydrochars and pyrochars was small regarding the AC yield calculated from the initial amount of biomass or lignin. Additionally, no considerable differences in terms of total surface area and surface chemistry were found between both ACs. To understand this, the mechanism of the activation was explained as a local alkali-catalyzed gasification. In the case of hydrochar, carbonization reactions occurred simultaneously to the gasification because of their lower thermal stability. Thus, the carbon content and yields of hydrochar ACs were similar to pyrochar ACs, but their microporous surface areas were lower, likely due to condensation of volatile matter.