Autocatalytic properties of biochar during lignocellulose pyrolysis probed using a continuous reaction system

The autocatalytic role of the biochar formed during the pyrolysis of oak woodchips has been in situ assessed using a continuous reaction system. To that end, the evolution of the product distribution along the time on stream was evaluated at different temperatures ranging between 300 and 550 degrees C. When working at low tem-peratures (300 and 350 degrees C), the system operates under almost steady state conditions with little variations in the yield and composition of the pyrolysis fractions. However, at higher temperatures, a progressive reduction in the bio-oil* yield (bio-oil on a water-free basis) is observed along the time on stream, which is accompanied by a significant increase in the gas production, mainly as a result of the enhanced generation of CO and CO2. Within the liquid organic fraction, these transformations cause a gradual reduction in its oxygen content and affect to a greater extent the heavy/oligomeric components. Likewise, noticeable variations are denoted along the time on stream for other bio-oil* components, such as carboxylic acids, sugars, and oxygenated aromatics. Additional pyrolysis tests, performed using two biochars with different K content as external catalysts, confirm that the origin of the biochar catalytic effects can be linked with its relatively high AAEMs (alkali and alkaline earth metals) content. Nevertheless, the contribution of the biochar carbonaceous matter should be also taken into account since it may retain many of the species present in the pyrolysis vapours, in particular the heaviest components, thus facilitating their subsequent conversion.