Hydrochar-Supported, in Situ-Generated Nickel Nanoparticles for Sorption-Enhanced Catalytic Gasification of Sewage Sludge

Conversion of sewage sludge into hydrogen-rich syngas is highly attractive but is frequently hampered by low hydrogen selectivity and low gasification efficiency. In the present study, a new strategy was developed that could greatly promote hydrogen production and markedly suppress the formation of undesired byproducts. A hydrochar (HC)-supported Ni nanoparticles (NPs) composite was initially synthesized via a facile, one-step hydrothermal carbonization of Ni-preloaded sewage sludge. A sorption-enhanced steam gasification process was then probed to shift the equilibrium of the water–gas shift (WGS) reaction toward hydrogen production using calcined eggshell as the natural CO2 sorbent. During the catalytic gasification process, uniformly dispersed NiO NPs were generated in situ and provided the active sites for the adsorption of tar molecules for catalytic conversion. The resulting Ni0.1@HC composite showed the highest catalytic activity in promoting hydrogen production and tar reduction under mild conditions due to the strong interactions between the metal cations and the carbonaceous support. Excellent catalytic results (72.5% selectivity for hydrogen and 78.7 g H2/kg hydrochar for hydrogen yield) were achieved, and little or no tar was formed, even at moderately low temperatures (700–800 °C). Successful utilization of sewage sludge via the proposed catalytic strategy has the potential to enhance hydrogen-rich syngas production and simplify cleanup of the syngas without extra energy consumption.