Mechanism of in-situ dynamic catalysis and selective deactivation of H2O-activated biochar for biomass tar reforming

Biochar, produced during gasification, is effective for tar catalytic reforming, finally with carbon deposition for deactivation. It explores the mechanism of in-situ dynamic catalysis and selective deactivation of H2O-activated biochar for tar reforming. The results indicate that ensuring the syngas quality, the gasification time can be extended for the high catalytic reactivity of biochar. The carbon deposit from tar on biochar surface causes the complete deactivation of H2O-activated biochar after 75 min-tar-reforming. In early stage (< 45 min), the O containing tars are selectively reformed on biochar surface with the formation of O-containing coke, while latter stage (45-75 min) is mainly dominated by coke carbonization. The surface functional groups could react directly with hydrocarbons, and metal species would act as active sites for tar reforming with their transformation. The coke leads to permanent deactivation of the active sites inside micropores of biochar, while meso/macropores become viable options to reform tars. The coke initially exists in dispersed single-layer ones, and then begins to grow, superimpose, and agglomerate to connecting into pieces, finally covering the entire surface of biochar. The amount/structure-difference of nascent tar ultimately determines the difference in the catalytic tar-reform ability of biochar. The tar containing heteroatoms such as oxygen has a high ability of addition and polymerization during coke formation. The higher porosity and hierarchical pore structure, more defects in graphene-like carbon structure and oxygen containing groups on the surface contribute to the better catalytic reactivity of biochar for tar reforming.