Removal of tar derived from biomass gasification via synergy of non-thermal plasma and catalysis

In this study, the reforming of toluene, as a surrogate for tar, was investigated in plasma-alone (PA) and plasma-catalytic (PC) systems. The effects of feed gas oxygen content (O₂/(O₂ + N₂) = 0, 3, 12, 21, or 30 vol%) and the discharge power (30, 75, or 90 W) on toluene conversion, the selectivity of syngas (H₂ + CO), and undesirable liquid by-products were evaluated using the PA system. A maximum toluene conversion of 87.9% and a minimum selectivity of undesirable liquid by-products of 0.53% for ethylbenzene, and 1.24% for benzene, were obtained when the discharge power was 90 W and the oxygen content in the carrier gas was 3 vol%. However, a maximum gas selectivity of 48.4% for H₂ and 19.4% for CO was attained when the discharge power was 75 W and the oxygen content was 3 vol% and 12 vol%, respectively. The effect of the steam/carbon molar ratio (S/C) on toluene reforming was investigated using the PC system with Ni/ZSM-5 catalyst under a discharge power of 75 W. The addition of steam to the feed gas significantly enhanced the conversion of toluene to syngas. A maximum toluene conversion of 88.5% was reached with a minimum selectivity of liquid by-products (1.9% for ethylbenzene and 5.2% for benzene) when S/C was 2. However, the highest selectivity of syngas (69.8% for H₂ and 21.2% for CO) was achieved when S/C was 2.5. The catalyst employed in the plasma reforming of toluene exhibited excellent anti-carbon deposition performance. A possible reaction mechanism and pathway of toluene destruction was proposed based on analysis of both gaseous and liquid products.