Biogas desulfurization by adsorption on thermally treated sewage-sludge

Biogas is a renewable source for power production, but the H2S present must be removed because it is very corrosive and may damage the combustion engines. The adsorption using activated carbon is one of the most used desulfurization methods. The operational life of the activated carbon could be extended if the H2S concentration was reduced prior entering the activated carbon bed by using other cheaper adsorbent. Sewage sludge is a possible inexpensive precursor to obtain adsorbents, and thus it would be valorized. An experimental study was performed using three types of sludge from three Spanish locations, which were activated to increase their adsorption capacity. Two thermal treatments were tested using nitrogen (pyrolysis) and air (calcination), as well as three heating temperatures. The adsorption dynamics of the prepared adsorbents were investigated in a fixed-bed column, determining the breakthrough curves and adsorption capacity of adsorbents. Besides, both their surface properties and their chemical properties were analyzed to get more insight about the adsorbent behavior. In addition, the effect of the oxygen content, relative humidity and the chemical impregnation, using different procedures, were also studied. As a relevant result, the adsorbent obtained by calcination at 700 °C of one of the three kinds of sludge showed a capacity twice of that of a commercial activated carbon without impregnation, although somewhat lower than that of a commercial activated carbon impregnated with a NaOH solution. The results showed that the use of this kind of precursors is very attractive to achieve adsorbents with a relative high adsorption capacity valuable to apply them in an economically feasible pretreatment.