Experimental investigation of the synergy effect of partial oxidation and bio-char on biomass tar reduction

•The new idea of catalytic partial oxidation on tar reduction by bio-char is proposed.•The synergy effect of partial oxidation and bio-char on biomass tar reduction, including the tar selective removal by char and oxygen, the influence of tar and oxygen on the char evolution (pore structure and surface functional groups).•High oxygen concentration coupling with char may lead to carbon deposition and bring down tar conversion rate at 800 °C.•The coupling of partial oxidation and char catalysis is a feasible method for tar reduction of biomass tar.

In order to reveal the synergy effect of partial oxidation and bio-char on tar reduction and develop more efficient tar removal method, the tar and bio-char evolution properties were investigated on a bench-scaled fixed-bed reactor. The tar components, tar conversion rates, physical and chemical structure of bio-char after reaction at the second stage were sampled and analyzed. Results showed that at 700 °C, the coupling of char and oxygen could result in the significant improvement of tar conversion rate (89.32%) than both two separated method (85.1% and 86.14%). At 900 °C, the synergy effect could reach the highest conversion rate of 95.84%. High oxygen concentration coupling with char may lead to carbon deposition and bring down tar conversion rate at 800 °C. But a light amount of oxygen greatly promoted the formation of porosity. The reaction between tar and bio-char at high temperature (800 °C above) was in favor of toluene conversion. The coupling of char and partial oxidation benefited the elimination of larger PAHs tar compounds as well as toluene. BET analysis results showed that oxygen promoted the development of bio-char porosity at 700 °C and 900 °C under all oxygen concentrations. Slight amount of oxygen would benefit the char pore development, but high oxygen concentration (5%) would lead to the carbon deposition on char pore surface at 800 °C. FTIR results indicated that temperature and oxygen promoted the aromatic or graphitization of bio-char. The IR band peak of 1060 cm−1 showed the similar tendency with aromatic ring band peak, which meant more carbon deposition on the surface of char pore but not graphitization. The coupling of partial oxidation and char catalysis is a feasible method for tar reduction of biomass tar.