Dynamic experiment of biogas desulfurization by activated carbon

Abstract: The world suffers the fossil energy crisis, and biogas gains more and more attention for it is a kind of reproducible, clean and environmental-friendly energy. Biogas comes from the anaerobic fermentation of organic materials in agriculture, industry and household waste at certain temperature and pressure. The concentration of H2S in biogas varies from 10 to 2 000 ×10-6 or even more, which is different with the type, concentration of organic materials and the operation of anaerobic digestion process. In order to use biogas safely, the H2S in biogas which is hazardous to the equipment or human health should be removed from the biogas. Purification methods of H2S vary from simple physical or chemical technology to complex process including chemical, physical or biological treatment units, which depended on the use purpose of biogas. Activated carbon is a highly porous material, and known as an efficient media for low-concentration H2S removal by adsorption process. Moreover, activated carbon is easily available and cheap in price, because it is produced from biomass such as agricultural waste, wood, bamboo, coconut shells, and almond shells, which is abundant in China. Currently, most efforts are made in the study on H2S adsorption by modified activated carbon such as impregnated activated carbon with alkali or heavy metal salts. The modified active carbon has the better performances in most cases, but it is proved to have negative effects on the removal of H2S. For example, active carbon’s absorption capacity of H2S will decrease when the crystals of alkali or metal salts block the pores which act as adsorption site. The price of impregnated active carbon is higher than unmodified activated carbon and the corrosion often occurs inevitably. It is necessary to study the most efficient way to improve the performance of unmodified active carbon. Thus, the effects of the inflow rate, the concentration of H2S, the size of unmodified activated carbon and the length of the fixed adsorption bed on activated carbon’s adsorption capacity of H2S and the breakthrough time through fixed adsorption column were investigated. The adsorption capacity of H2S was 1.20 and 1.86 mg/g when the effluent H2S concentration was 0.0124% and 0.0454% respectively. The adsorption capacity of H2S with the effluent rate of 0.15 L/min was 1.6 times that with the effluent rate of 0.30 L/min, and the adsorption capacity of H2S with the particle size of 0.84-2.00 mm was only 58% of that with the particle size of 0.42-0.84 mm. Therefore, it could enhance activated carbon’s adsorption capacity of H2S by increasing the inlet concentration of H2S, decreasing the inflow rate and utilizing the small-size particles. The adsorption process was modeled by Bangham equation. The H2S adsorption capacity of unmodified activated carbon could be well described by Bangham equation.