Synergies and progressive effects of H2O/CO2 and nascent tar on biochar structure and reactivity during gasification

This work investigates the heterogeneous reactions of biochar involved activation/gasification and tar reforming in H2O and CO2 atmospheres. The effects of simultaneous and stepwise H2O/CO2-activation and the addition of nascent tar on the biochar's structure and reactivity during gasification are studied using a two-stage fluidized/fixed bed reactor. FTIR, XPS and Raman analyses are used to investigate changes in the biochar's structure while thermogravimetric analysis is used to characterize its reactivity. Simultaneously activating the biochar in an H2O atmosphere and adding nascent tar inhibits biochar's gasification efficiency, while carrying out the same tests in a CO2 atmosphere promotes it. The nascent tar is mainly reformed in a sequence of coking and coke-gasification reactions, rather than via direct tar reforming over biochar. More O-functional groups are formed on the biochar's surface for the simultaneous process with CO2, while there is little difference between the synergies and progressive processes in an H2O atmosphere. For both types of treatment, the biochar surface's C-O content was greater in CO2 atmospheres than in H2O atmospheres. The synergies and progressive effects of H2O and nascent tar on biochar aromatic structure are similar in many ways to that in CO2. When nascent tar is present, CO2 has a stronger positive impact than H2O on biochar reactivity (maintaining it or even increasing it throughout the reaction).