Desulfurization of Digester Gas on Wood-Based Activated Carbons Modified with Nitrogen: Importance of Surface Chemistry

Activated carbon of wood origin with micro- and mesoporosity was modified with either melamine or urea by impregnation followed by heat treatment at 450 and 950 °C. The materials obtained were used as media for desulfurization of digester gas in the dynamic conditions. The initial and exhausted samples were characterized using elemental analysis, adsorption of nitrogen, Boehm titration, potentiometric titration, FTIR, and thermal analysis. The adsorbents revealed high capacities for H2S removal, and the presence of CO2 was not found detrimental for surface activity. Introduction of basic nitrogen functionalities, especially those thermally stable quaternary and pyridinic nitrogen, significantly increased the performance. It is proposed that these centers activate chemisorbed oxygen, which oxidizes H2S to elemental sulfur, in the first step. That sulfur can be further oxidized providing that enough oxygen is present in the system. Prehumidification of the carbon bed besides resulting in adsorption of water increases the performance by enhancing hydrogen sulfide dissociation in the basic environment, which increases the efficiency of its oxidation. In such conditions, besides elemental sulfur, SO2/sulfurous acid is deposited on the surface. Selectivity of oxidation to SO2 depends upon the availability of oxygen and pore sizes of the materials. When small pores are present, more SO2 is formed.