Role of Microporosity and Nitrogen Functionality on the Surface of Activated Carbon in the Process of Desulfurization of Digester Gas

Microporous coconut-shell based activated carbon was modified by heating with urea or melamine at 450 and 950 °C. The materials were characterized using adsorption of nitrogen, thermal analysis, elemental analysis, and potentiometric titration to obtain detailed information about surface chemistry and texture. Then they were used as adsorbents of hydrogen sulfide from digester gas where CO 2 and CH 4 are present in predominant quantities. The experiments were run in dry conditions and with preadsorbed water on the surface. The results revealed the importance of water, which, by its adsorption in small pores, eliminated the adsorption of methane, and thus caused the accessibility of adsorption centers for H 2 S. That water also helps in H 2 S dissociation. The introduction of nitrogen groups, which provide basicity needed for hydrogen sulfide dissociation, enhanced significantly the removal process. Since oxygen is not supplied to the system and only that oxygen which is chemisorbed on the surface can be used, elemental sulfur is a predominant product of surface reaction. The importance of nitrogen is in providing basicity, which is not deactivated by the presence of CO 2 in the challenge gas.