Enhanced combined H2O/CO2 reforming of sludge to produce high-quality syngas via spiral continuous microwave pyrolysis technology: CO2 reforming mechanism and H2/CO directional regulation

Abstract

Emerging spiral continuous microwave pyrolysis technology could enhance the H2O/CO2 steam reforming of sludge to directionally optimize the quality of syngas. Based on the results of a response surface model analysis, an increase in the moisture content of sludge decreased the biogas yield but increased the H2 content. The synergistic effect of temperature and CO2 concentration helped to increase the biogas yield and CO content, but had a negative effect on the H2 generation. Using the model to optimize key parameters, the syngas concentration produced from the spiral continuous microwave pyrolysis of sludge increased to 62.59 vol%, and the directional regulation of the syngas ratio (H2/CO = 0.68–1.50) was achieved. The increase in the syngas concentration was primarily attributed to the reforming reactions of water molecules with light hydrocarbon compounds and CO2 with bio-oil. Based on the reaction path simulated by the density functional theory (DFT), CO2 could be used as a weak oxidant to promote the thermal decomposition of hexadecanoic acid, generating more H2 and CO. In addition, the use of the appropriate sludge moisture content as the feedstock and the introduction of the CO2 reaction atmosphere reduced the energy required for microwave pyrolysis of wet sludge (20.9–35.3 MJ/kg) and improved the quality of syngas (55.91–62.59 vol%). This study evaluated the potential of spiral continuous microwave pyrolysis of sludge to produce syngas with a high heating value for small-scale power generation plants, providing new inspiration for the low consumption and efficient energy utilization of sludge.