Municipal sewage sludge (MSS) production is rising globally, and its pyrolysis offers both volume reduction and resource recovery. However, the resulting pyrolysis gas often contains hydrogen sulfide (H₂S), which can poison catalysts in downstream processes like Fischer‑Tropsch synthesis or inhibit fermentation. In a study published in ENGINEERING Chemical Engineering, researchers systematically investigated how temperature and acid zeolite addition affect product yields, gas composition, and H₂S production.
Thermal pyrolysis of MSS was first performed at 350, 500, 650 and 800 °C. Biochar yield decreased from 58.4 wt% at 350 °C to 38.6 wt% at 800 °C, while pyrolysis gas yield increased from 7.6 wt% to 18.2 wt%. H₂S concentration in the gas dropped from 8.67 vol% at 350 °C to 2.40 vol% at 800 °C, but the H₂S formation ratio (moles H₂S per mole of feedstock sulfur) remained stable (0.40–0.55). Thus, the lower H₂S concentration at higher temperatures was purely due to dilution by increased gas yield.
Catalytic pyrolysis at 500 °C used a 1:1 mass ratio of MSS to zeolite. Three catalysts were tested: H‑mordenite (SAR = 220), H‑ZSM5 (SAR = 250) and H‑ZSM5 (SAR = 1880). All zeolites significantly reduced biochar yield and increased gas and bio‑oil yields. H‑mordenite gave the highest bio‑oil yield (55.3 wt%), 24 % higher than the non‑catalytic run. H‑ZSM5 with SAR 1880 increased gas yield by 55 % (from 11.4 to 17.7 wt%).
Most importantly, H‑ZSM5 (SAR 1880) and H‑mordenite (SAR 220) reduced H₂S concentration by 46 % and 42 %, respectively. However, the mechanisms differed: H‑ZSM5 (SAR 1880) acted solely through dilution (higher gas yield) with no change in H₂S formation ratio, whereas H‑mordenite directly suppressed H₂S formation, reducing the H₂S formation ratio by 28 %. Meanwhile, H‑ZSM5 (SAR 250) did not lower H₂S concentration but reduced overall sulfur migration to the gas phase by 33 %, suggesting selective retention of other sulfur compounds (e.g., SO₂, COS, CS₂, CH₃SH). Lower SAR favored retention of sulfur in solid and liquid phases.
The results demonstrate that in‑situ addition of acid zeolites can simultaneously enhance product yields and lower H₂S levels, with the optimum depending on zeolite type and SAR. This approach could reduce or eliminate the need for separate, energy‑intensive desulfurization steps, advancing the circular economy of sewage sludge treatment.
DOI
10.1007/s11705-026-2651-9