(12b) Adsorptive Removal of Aromatic Compounds from Fuel Produced by Biomass Gasification for Fuel Cells

Organic wastes such as woody biomass are converted into the fuel gas by the methane fermentation or the high temperature gasification. The research of the waste gasification processes is paid attention to produce fuels for fuel cells. The fuel gas generated in these processes contains generally unsaturated hydrocarbon such as aromatic compounds in the order of hundreds ppm. The existence of such compounds causes the carbon deposition on electrode catalysis in fuel cells or reforming of the fuel. Therefore, the process to remove those compounds from the gasification gas becomes essentially necessary. In this work, the adsorption removal process using the activated carbons was proposed. The benzene adsorption characteristic of various activated carbons was evaluated, and the adsorption removal experiment under an actual gasification gas composition was carried out. The activated carbons made from the coconut shell, coal and the phenolic resin were employed for this examination. The adsorption experiment with the thermogravimetric analysis was carried out to clarify the adsorption characteristics of benzene as a representative sample of aromatic compounds on the various activated carbons. The characteristics of benzene adsorption on the activated carbons were evaluated at 303 K of adsorption temperature and 500 ppm of initial concentration. For preparation of the activated carbons, degassing of them had been carried out at 473 K for 60 minutes. From the results of equilibrium amount of adsorbed benzene, it was found that the activated carbons made from the coconut shell and the phenolic resin had same level of high adsorption ability. However, the activated carbon made from the phenolic resin is too expensive to apply to this removal process. Therefore, the activated carbon made from the coconut shell was used to the following investigation. To obtain adsorption isotherm of the activated carbon, equilibrium amount of adsorbed benzene at 303 K was measured in the range of 100-1000 ppm. The adsorption isotherm could be expressed as the Freundlich isotherm equation, such as q = 1.45xC^0.235. Here, q is the amount of adsorbed benzene [kg/kg] and C is the concentration of benzene [-]. Using fixed-bed adsorber of the activated carbon, breakthrough of benzene adsorption was measured. Simulated gasification gas, of which composition was H2: 15%, CH4: 3%, CO: 25%, CO2: 25%, benzene: 200 ppm and N2 balance, was supplied into the fixed-bed adsorber at 1.5 l/min and transient behavior of benzene concentration at the outlet of the bed was measured. Adsorption temperature was 303 K and the height of fixed-bed was about 100 mm in a cylindrical glass tube of 22.5 mm in diameter. The benzene concentration in the exit gas was maintained at about 4 ppm for 4 hours or more. It was indicated that removal of benzene from the gasification gas was possible even if other gases such as CO and CO2 coexistent with benzene.