Preparation of adsorbents from sewage sludge pyrolytic char by carbon dioxide activation

•Sewage sludge char is valorized for the production of adsorbents.•HCl and Na2CO3 sequential washing steps to extract most of the ashes.•Lower inorganic content favors the accessibility of CO2 into the pores network.•Maximum BET surface area (440m2g−1) is attained after 15min of activation.•Promising results for the viability of sewage sludge flash pyrolysis. This work focuses the valorization by CO2 activation of the sewage sludge char obtained in the fast pyrolysis in a conical spouted bed reactor at 500°C. In order to improve the quality of the activated material for use as adsorbent, the sewage sludge char was subjected to two sequential steps of washing by using first a solution of HCl (in order to remove part of the ashes) and then a solution of Na2CO3 (to extract most of the silica remaining), thereby comparing the properties of the char washed with only HCl with that treated with both HCl and Na2CO3. The materials treated with one or two steps of washing were activated with CO2 in a fixed bed reactor at 800°C, which allowed proving that the step of Na2CO3 washing contributes significantly to increasing the gasification rate. Furthermore, the properties of interest for use as adsorbents were greatly improved, attaining a BET surface area of 440m2g−1 with a notorious presence of meso and macropores for an activation time of 15min and a burn-off value of 42%.