Catalytic role of low-cost natural minerals in sewage sludge gasification using different gasifying agents for hydrogen-enriched syngas production

Abstract

Sewage sludge, a by-product of wastewater treatment plant operations, presents a growing environmental challenge due to increasing urbanization, industrial activities and population growth. The urgent demand for cost-effective, energy-efficient and sustainable management strategies for sewage sludge has driven the search for alternative treatment technologies. Among them, thermal conversion methods offer promising solutions, especially gasification emerges as the most advantageous thermochemical process compared to incineration and pyrolysis. The valorization of sewage sludge through gasification plays a critical role in sustainable waste management. Converting sludge into high value-added products not only reduces landfill dependency and greenhouse gas emissions but also aligns with circular economy principles by enabling resource recovery and energy generation. This study focuses on the gasification of sewage sludge for syngas production, specifically investigating the catalytic effects of natural minerals (dolomite, olivine and limonite) and nickel-impregnated biochar. Gasification experiments were conducted in a fixed-bed tubular reactor at laboratory scale and a fluidized-bed reactor at pilot scale. Six different gasifying agents (air, steam, CO2, steam-air, steam-CO2 and CO2-air) were tested in the fixed-bed reactor, while fluidized-bed experiments were performed under air and steam-air conditions. Additionally, the effect of incorporating hazelnut shell into sewage sludge at varying ratios was examined under air gasification in the fluidized-bed reactor to identify an optimal blending ratio. Sewage sludge samples, collected from municipal wastewater treatment plants, were characterized using analytical techniques, including proximate and ultimate analysis, calorimetry, ash fusion tests, X-ray fluorescence, thermogravimetric analysis and fourier transform infrared spectroscopy analysis. Based on the characterization results, sewage sludge was identified as a suitable feedstock for gasification processes due to its high organic content and calorific value. In the fixed bed gasification, the best results regarding the carbon conversion of 88% were obtained in steam-air condition whereas carbon conversion exceeded 99% in the presence of catalytic minerals. The low calorific value of the char samples in 0.1-0.2 MJ/kg and high syngas yields of 15.4-17.0% in total upon catalytic gasification supported the improved efficiencies in comparison to the noncatalytic situation with 1.9 MJ/kg and 14.4%, respectively. CH4 and C2-C5 compounds in the product gas have been reduced in the presence of catalytic minerals as an indication of the increased tar reforming activity favoring hydrogen production. Gasification efficiency can be also assessed by the percentage of syngas consisting of H2, CO, CO2, CH4 and C2-C5s in the total gas yield. The highest syngas percentage of 19.95% was observed under CO2-air condition, while CO2 alone resulted in the lowest value of 11.00%. The addition of steam to CO2 improved this value to 14.50%, which is consistent with the higher reactivity of the char-H2O(g) reaction compared to char-CO2. This study demonstrated that the incorporation of catalysts into sewage sludge enhances syngas quality and gasification performance in the fluidized-bed reactor, while also effectively decreasing bed agglomeration and associated operational challenges. Under air gasification conditions, all catalysts improved process performance to varying degrees. The Ni-based catalyst addition provided the highest H2 content of 13.03% and H2/CO ratio of 1.57, whereas olivine achieved the highest CH4 formation (6.39%) and resulted in the highest heating value of 6.73 MJ/m3 and cold gas efficiency (CGE) of 71% , indicating its strong contribution to overall energy recovery. The introduction of steam to air improved gasification performance in terms of syngas quality, consistent with the enhancement of reforming and water-gas shift reactions. Steam addition inceased the H2 production from 8.36% to 19.28%, H2/CO from 0.99 to 2.14 and higher heating value (HHV) of syngas from 5.44 MJ/m3 to 8.31 MJ/m3. Under steam-air gasification, dolomite assisted gasification yielded the highest H2 content of 21.72%, while the olivine-catalyzed gasification produced the highest CH4 fraction of 8.87%. In terms of gasification efficiency, the olivine-mixed gasification exhibited the most favorable performance under steam-air conditions, achieving an HHV of 7.08 MJ/Nm3 (13.41 MJ/Nm3, N2-free), a CGE of 85% and a CCE of 79.25%. These results highlight the critical role of catalyst selection and gasifying atmosphere in optimizing sewage sludge gasification performance in fluidized-bed systems. In terms of chemical composition of fly ash and bottom ash from sewage sludge gasification, the major components were SiO2, P2O5, CaO, and Al2O3 of under air and steam-air atmospheres. Considering the proximate analysis results of the ash samples obtained in both gasification atmospheres, among the catalysts tested, olivine demonstrated the most efficient performance by achieving the lowest volatile and fixed carbon contents in both of bottom and fly ashes, suggesting superior carbon conversion. In contrast, the Ni-based catalyst resulted in higher volatile and fixed carbon contents. Regarding tar analysis, among all catalysts, olivine enabled the most efficient tar transformation, exhibiting the minimal overall GC-MS peak area and indicating its strong ability for degrading and reforming tar compounds. Among all the catalysts evaluated in the fluidized gasification, olivine proved to be the most effective catalyst in promoting tar reforming, resulting in the highest syngas yield and improved overall gasification efficiency. In the fluidized bed gasification, blending sewage sludge with hazelnut shell influenced gasification performance. The gasification of blended sewage sludge and hazelnut shell was investigated at varying blending ratios 100:0, 75:25, 50:50, 25:75 and 0:100. The 75:25 SS:HS ratio yielded the best results in syngas quality in terms of the highest H2 (9.17%), CH4 (4.84%) and CO (9.91%) contents and energy efficiency (CGE:67.7 % and HHV:5.88 Mj/m3). In the comparison of the two reactor types, sludge gasification yielded syngas with a greater combustible gas content and higher higher heating value in the fluidized-bed reactor than in the fixed-bed reactor, attributable to enhanced gas-solid interaction and superior heat and mass transport. Steam addition markedly enhanced hydrogen production in the fluidized-bed reactor, while its impact was negligible in the fixed-bed system, likely due to steam-induced catalyst deactivation under continuous exposure.