Impregnação alcalina e caracterização textural de carbono ativado para a dessulfurização de biogás

Abstract

The biogas is a renewable source of energy that represents an alternative to the use of fossil fuels (from example, natural gas) due to the high calorific value of its main constituent, methane, in addition to reducing emissions of greenhouse gases. However, this gaseous mixture presents contaminants in its composition, which make it impossible to use it as a high-quality fuel. One of these contaminants is hydrogen sulfide (H2S), which causes harmful effects to health and corrosion in equipment. Hence, it must be removed from biogas. One of the technologies most used in the removal of H2S is the adsorption on activated carbons. The objective of this work was to evaluate the capacity of H2S retention in fixed bed of four activated carbons, one being the base matrix and the other impregnated with alkaline solutions of sodium hydroxide, potassium hydroxide and sodium carbonate. The adsorbents were characterized by pH measurement and N2 and CO2 adsorption/desorption isotherms at -196 °C at 0 °C, respectively. It was observed that all the samples showed high specific surface areas and microporosities, with a slight decrease of these properties in relation to the base matrix, indicating influence of the impregnation in the porous structure of the material. The results of H2S adsorption tests at room temperature showed that the KOH impregnated carbon presented the highest H2S retention capacity, probably due to its high alkalinity and percentage of ultramicropores, evidenced by the characterization, which confirms that the chemical impregnation with consequent modification of the porous structure was fundamental in the H2S retention. However, NaOH and Na2CO3 based materials presented equal and lower capacities than those obtained for the base matrix, respectively. The tests at different temperatures showed that the H2S retention capacities increased with temperature, indicating chemisorption or H2S access to the ultramicropores of the materials, whose diffusion resistance might have been overcome at high temperatures. Finally, the H2S-saturated samples were subjected to further characterization tests and a decrease of the textural properties was observed, indicating that the products of the reaction of H2S with the metals present on the surface of the samples could have been deposited onto the porous or surface structure of the samples as an indicative of chemisorption.