Steam/oxygen gasification of dried sewage sludge was performed in a two-stage gasifier to produce an H-2-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 H-2 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 H-2 production. The steam to fuel ratio had a strong influence on syngas quality, causing a significant rise in the H-2 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 H-2 production and active tar cracking. The maximum H-2 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. (C) 2015 Elsevier Ltd. All rights reserved.