Steam/oxygen gasification of dried sewage sludge in a two-stage gasifier: Effects of the steam to fuel ratio and ash of the activated carbon on the production of hydrogen and tar removal

•Stable steam/O2 gasification of dried sewage sludge was performed in a two-stage gasifier.•The role of ash of activated carbon was investigated.•A H2-rich (52.2 vol%) and tar-free producer gas was produced with activated carbon.•The effect of steam to fuel ratio on syngas quality was investigated.•The minimum NH3 content in syngas was 20 ppm.

Steam/oxygen gasification of dried sewage sludge was performed in a two-stage gasifier to produce an H2-rich and tar-free syngas. The experiment mainly investigated the effects of activated carbon, ash of activated carbon, steam to fuel ratio and the combination of additives on syngas quality. In the results, all the syngases obtained with activated carbon did not contain any tar. Activated carbon increased the H2 production and decreased the NH3 content in syngas. Acid-treated activated carbon, which has less ash content than the original activated carbon, was less active in tar cracking and H2 production. The steam to fuel ratio had a strong influence on syngas quality, causing a significant rise in the H2 and NH3 contents in syngas at a high steam to fuel ratio. The extra addition of CaO and activated carbon to the base additive (activated carbon) led to increased H2 production and active tar cracking. The maximum H2 content in syngas (52.2 vol%) was obtained with 2.5 kg of activated carbon at a steam to fuel ratio of 0.52. The minimum NH3 content in syngas was 20 ppm.