Carbide Slag-Assisted Steam Gasification of Sewage Sludge for Hydrogen-Rich Syngas Production and Heavy Metal Stabilization

Carbide slag (CS), a Ca-rich industrial residue, was investigated as a calcium source for enhancing steam gasification of sewage sludge (SS) and mitigating heavy metal risks in bottom residues. Gasification performance and heavy-metal behavior across Ca/C ratios and temperatures were assessed using apparent retention, Community Bureau of Reference (BCR) sequential extraction, the toxicity characteristic leaching procedure (TCLP), and the risk assessment code (RAC). At 700 °C, increasing Ca/C from 0 to 0.75 raised the H2 volume fraction from 57.73 to 68.02 vol %, H2 yield from 207.40 to 332.85 mL g–1, and cold gas efficiency from 28.09 to 42.24%. CS addition increased the apparent retention of Zn, Pb, and Cd and was accompanied by redistribution from the F1/F2 fractions toward the F3/F4 fractions and by lower TCLP leaching concentrations; the RAC risk levels of Zn and Cd decreased from high to medium. However, CS was largely ineffective for As immobilization because its apparent retention, BCR fraction distribution, and TCLP leaching behavior changed only slightly, while As remained at medium RAC risk. At Ca/C = 0.50, increasing temperature from 650 to 800 °C increased H2 yield and cold gas efficiency but decreased the H2 volume fraction and H2/CO ratio. Higher temperature reduced the potential mobility and TCLP leachability of metals retained in the collected bottom residues but decreased the apparent retention of Zn, Pb, Cd, and As. Overall, under the investigated conditions, Ca/C = 0.50 and 700–750 °C provided a representative compromise between hydrogen-rich gas production, CS dosage, operating-temperature requirements, and the environmental response of the collected bottom residues.