Abstract
Tars present in syngas pose major challenges to the development of thermochemical processes. Catalytic cracking of tars using biochar is a promising option, both technically and economically. To elucidate the role of biochar porosity, heterogeneous tar cracking tests were carried out in an inert atmosphere. The variation in biochar porosity during long-term tar cracking tests was studied to gain a deeper understanding of biochar deactivation. Physically activated biochar derived from beech wood biomass was used as a catalyst, with toluene serving as a model molecule for tars. The results showed that the activated biochar exhibited good initial catalytic activity for toluene cracking, compared to homogeneous thermal cracking. However, due to polymerization reactions during toluene cracking, coke was deposited on the inner pore surfaces of the biochar, leading to its deactivation. After a while, the toluene conversion rate decreased to levels comparable to those achieved during thermal cracking. This was accompanied by a decrease of the specific surface area of the biochar. It has been shown that heterogeneous toluene conversion primarily occurs in the micropores, which serve as the active sites, while mesopores are essential for the diffusion of toluene into the micropores. Biochar deactivation was exacerbated by the obstruction of mesopores. Furthermore, increasing the mesopore content in biochar could slow down the process of biochar deactivation.