Co-gasification of municipal sewage sludge and cotton stalk enhanced by metal-enriched texture dyeing sludge additives for syngas production

[Display omitted] •Texture dyeing sludge additives were active catalysts for co-pyrolysis/gasification.•Activation energy for co-pyrolysis significantly decreased with sludge additives.•Sludge additives enhanced tar abatement and promoted syngas production.•Fe-containing species in sludge additives dominated the co-gasification performance. In this study, co-gasification characteristics of municipal sewage sludge (MSS) and cotton stalk (CS) for syngas production enhanced by using metal-rich texture dyeing sludge (TDS) pyrolysis residues as additives wereinvestigated in a TGA and a fixed bed reactor. Kinetic analysis results showed that the addition of TDS residue prepared at 600 °C (TDS-600) resulted in the reduction of activation energy of MSS/CS co-pyrolysis to the maximum extent. The syngas yield greatly increased with the rising temperature and steam volume fraction (fH2O). However, the excessive steam at high temperature may lead to the decrease of syngas yield due to the gas expansion effect. The maximum syngas yield of 0.808 m3/kg could be obtained at an optimal gasification temperature of 850 °C and a fH2O of 20 %. In the co-gasification of MSS and CS, the syngas yield increased from 0.683 without TDS-600 to 0.903 kJ/mol with 20 % TDS-600 and the tar content decreased from 40.15 % to 26.45 %, suggesting the outstanding catalytic effect of TDS-600 on promoting syngas yield and tar decomposition. The possible ecological risk of heavy metals in ashes after co-gasification was evaluated by the potential ecological risk index (RI). The calculated RI values of all samples were