Mechanisms of Toluene Removal in Relation to the Main Components of Biosyngas in a Catalytic Nonthermal Plasma Process

In this study, a packed-bed dielectric barrier discharge reactor was built for the removal of biomass gasification tar. Two kinds of packing materials, glass pellets and a Ni/gamma-Al2O3 catalyst, were employed for the plasma-alone process and the plasma catalytic process, respectively. Toluene was used as the tar surrogate, and five typical gas combinations, N-2, N-2 + CO2, N-2 + CO2 + CO, N-2 + CO + H-2, and simulated gasification gas (SGG), were selected as carrier gases. The effects of the main components of the gasification gas on toluene removal under plasma and/or catalysis treatment were studied. The results indicated that the highest removal efficiency in the plasma-alone process was achieved in the N-2 atmosphere, and the addition of H-2, CO, or CO2 to N-2 led to a decrease in the removal performance. When Ni/gamma-Al2O3 was introduced into the plasma process, the best removal efficiency was obtained in the N-2 + CO2 atmosphere. However, the positive effect of CO2 on toluene removal switched to a negative effect when CO2 coexists with CO and H-2 in the gas mixture. In addition, both the terminating effect at low temperature and the occurrence of methanation induced by CO at high temperature could significantly inhibit the removal of toluene in the plasma catalytic process. Typically, in the SGG atmosphere, the highest toluene removal efficiencies under plasma-alone, catalysis-alone, and plasma catalysis treatments were 33.5, 38.6, and 92.5%, respectively. Furthermore, no CN-containing or O-containing compounds were identified as the major by-products in the plasma catalytic process, which is significantly different from what was observed in the plasma-alone process. Moreover, a detailed mechanism of toluene removal under plasma and plasma catalysis treatment was proposed after the analysis of gas and liquid products.