(197c) Treatment of Biomass Gasification Tars with Non-Thermal Plasmas

In this study, a non-thermal plasma dielectric barrier discharge (“DBD”) reactor was used to reduce the concentration of tar in the product gas, and its performance was evaluated at different reaction conditions. Toluene and benzene were used as tar model compounds. The effects of reaction parameters such as the residence time, concentration, wall temperature and plasma power on tar removal were studied in a tubular dielectric barrier discharge (DBD) plasma reactor at ambient pressure. The DBD plasma reactor proved to be an efficient tool to remove/decompose tar compounds in the product gas. The percentage removal of tar increased with increasing plasma power and residence time to as high as 99% in various carrier gases (CO₂, H₂, and N₂) and gas mixtures. However, the decomposition of tar analogue compounds decreased with increasing concentration. It was found that most of the toluene converted into solid residue due to the polymerization of hydrocarbon radicals produced in the plasma system at ambient temperature in all carrier gases (CO₂, H₂, N₂, and mixtures). The other products were lower hydrocarbons, CO, and H₂, depending upon the type of carrier gas. The synergetic effect of power and temperature was investigated to decrease the unwanted solid deposition. It was observed that selectivity to lower hydrocarbons increased to 99% at 400 ^oC and 40 W, with the non-thermal plasma. In these conditions solid formation was completely prevented. The maximum selectivities to methane were 60 % and 81% for toluene and benzene, respectively. However, in other carrier gases (N₂ and CO₂), the selectivity did not increase beyond 15 %, even with increasing temperature, and solid formation was observed even at elevated temperatures. However, in the gas mixtures, solid formation was significantly reduced when increasing the temperature due to presence of H₂. Therefore, the plasma power and surrounding temperatures can be used to control the product distribution in the presence of H₂ carrier gas. Hence, the installation of a DBD reactor at a suitable location after the gasifier exit, where the temperature was high enough, could substantially reduce the tar concentration.