Increasing Efficiency of Charcoal Production with Bio-Oil Recycling

Charcoal from biomass is a promising alternative for fossil coal. Although its quality increases at high pyrolysis temperature, charcoal yield decreases, meaning lower economic performances of charcoal production processes. This work aims at demonstrating potential methods to increase charcoal yield while keeping its quality at satisfying levels. We suggested the recycling of bio-oil from the pyrolysis process as a primary measure. In addition, we also investigated in detail the consequence of utilizing CO2 instead of N2 as reaction media under practical conditions (i.e., thick particles). An experimental investigation was carried out in a macro-thermogravimetric (macro-TG) reactor. The sample (woodchips, bio-oil, and woodchips embedded with bio-oil) was exposed to the reaction temperature either instantaneously (isothermal condition) or by slow heating (slow pyrolysis) in controlled gas flows of N2 and CO2. The results showed that charcoal yield increases with the bio-oil recycling on woodchips at all pyrolysis temperatures (300–700 °C). By 20% of bio-oil embedding on woodchips, charcoal yield increased by 18.3% on average. The increase of charcoal yield was not only because of the increase in reactants but also due to the synergetic effect between bio-oil and woodchips upon physical contact. Bio-oil recycling had negligible effects on the property of charcoal, such as carbon content and heating value. Although CO2 did not affect primary pyrolysis, it had effects on mass transfer processes. As a result, significantly higher charcoal yield was obtained from pyrolysis in CO2 than in N2 by ensuring a good contact of volatiles and solid surface (i.e., usage of thick particles and slow heating). This study suggests that we can achieve high charcoal yield while maintaining the similar charcoal property by bio-oil recycling, CO2 purging, use of thick particles, and slow heating.