Mechanism of catalytic tar reforming over biochar: Description of volatile-H2O-char interaction

Abstract

The two-stage fluidized bed/fixed bed reaction system was used to study the evolution characteristics of catalytic tar reforming process over biochar. The effect of different interaction time (feeding time varies within 10–50 min) between volatiles and char on the characteristics of char/tar is considered. InVia-Reflex Raman spectrometer (Raman), Fourier transform infrared spectrometer (FTIR), automatic specific surface and porosity analyzer, X-ray photoelectron spectrometer (XPS), inductively coupled plasma optical emission spectrometer (ICP-OES) were used to characterize the structural characteristics and alkali metal and alkaline earth metal species (AAEMs) content of biochar. The tar components were analyzed by gas chromatography-mass spectrometry (GC–MS) and the gas components were monitored by MCA 100 SYN synthetic gas analyzer. The results indicate that the tar removal efficiency of gasification char is over 70% when the interaction time is within 30 min. When corn straw is fed for 40 min, the carbon deposition on the surface of biochar reaches saturation. The size of coke particles is mainly in the range of 2–5 nm, and the structure is mainly small aromatic ring within five rings. During catalytic tar reforming over biochar, the content number of O-containing functional groups and surface K element decrease rapidly. The main removal path of toluene, styrene, phenol and indene in tar is heterogeneous reforming on biochar surface. Carbon deposition results in the increase of biochar external surface area (73.8 m2/g to 158.1 m2/g). The homogeneous reforming of styrene and toluene on biochar surface is enhanced. Under the catalysis of biochar surface, the self-gasification of biochar, water–gas shift reaction and methane steam reforming reaction are all promoted. This results in the increase of CO, CO2 and H2 yield (9.68%, 52.38% and 69.63%), as well as the decrease of CH4 yield (25.23%) in the syngas of corn straw gasification.