• Energy Research

The hydrothermal gasification of dewatered sewage sludge converts waste into syngas fuel (gas containing H2, CH4, CO, and CO2), but the formation of char and tar by a side reaction restricts practical applications. In this study, the formation of char/tar by the hydrothermal gasification of dewatered sewage sludge at 300-400 °C for 5-90 min with a heating rate of 8-50 °C/min is investigated. The results showed that the formation of tar/char reached equilibrium after 30 min, and an increase in the temperature and heating rate reduced the char yield. However, a higher tar yield was achieved at the highest temperature. Experiments on sludge with different moisture contents were also conducted, and the formation of char/tar was inhibited at a higher moisture content. The addition of NaOH and H2O2 can effectively reduce the char and tar yields, respectively. A lumped kinetic model for describing carbon conversion during the hydrothermal gasification of sewage sludge was developed to elucidate the char/tar formation mechanism. The experimental data were used to determine the model parameters, and the fitting results showed that solid-solid conversion from organic matter in sludge was the predominant char/tar formation pathway, with an activation energy of 7.1 kJ/mol. The experiments using the model compounds indicated that humus and protein are the main precursors of char and tar, respectively. This study provides insights for understanding the formation of char/tar from sewage sludge following hydrothermal gasification, which is vital for controlling the formation of char and tar to improve the gasification efficiency.

Supercritical water gasification (SCWG) is considered a promising technology for sewage sludge (SS) treatment and utilization; however, char produced by a side reaction has become a bottleneck in SCWG. In this study, SS and its model compound (10% humic acid) were treated in an autoclave by SCWG at 400 °C for 30 min and by hydrothermal carbonization (HTC) at 250 °C for 300 min. The char yield was 15.4% in SCWG and 41.3% in HTC. The chars were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, Brunauer-Emmett-Teller (BET) analysis, and elemental analysis. By comparing the properties the char produced by SCWG and the hydrochar produced by HTC, which has been considered a valuable product, the feasibility of using char as an additional product in SCWG was explored. Compared with the char produced by HTC, the char generated in SCWG exhibits a lower BET specific surface area (8.257 and 15.782 m2/g) and combustion activity, a higher proportion of small pores (with pore sizes of 1-2 nm), and greater thermal stability. The formation pathway of the two types of chars is related to both dehydration and aromatization; decarboxylation also occurs in char formation during SCWG. Humus was proved to be related to char formation during the SCWG of SS based on experimental results obtained with the model compound. This work provides insights needed to guide follow-up treatments or utilization of the char produced during the SCWG of SS.

This study explored the feasibility of char and tar formation inhibition during supercritical water gasification of sewage sludge (SS) by additive addition. Experiments were conducted in autoclave with 5 wt% additives at 400 °C for 30 min. The non-additive gasification of SS resulted in a higher char yield (12.6%) and tar yield (16.4%). In contrast, the five additives reduced the char yield (3.4-11.2%), the inhibition of char yield by additives was in the order of NaOH > K2CO3 > H2O2 > acetic acid > NiCl2. The inhibition of tar formation was limited, tar yield were 13.3-18.8% with additives. Fourier-transform infrared spectroscopy was used to determine the functional groups of char/tar, and it was observed that the spectra of char were more similar to those of hydrochar obtained in a low-temperature experiment. Model compounds of potential precursors was also tested to study the mechanism of action of additives, the results reveal that additives have different effects on char/tar formation from various components, the inhibitory effects of additives on the yield of char from humus and tar from lignin were limited. Finally, the effects of additives on gasification were also studied. The addition of additives will have an impact on the hydrogen yield and gasification efficiency, which also needs to be considered when use additive to reduce the by-products yield.

Abstract Dewatered sewage sludge with different moisture contents was treated with a nickel catalyst in supercritical water in a batch quartz tube micro-reactor at different temperatures and reaction times. The catalytic activity of the nickel wire catalyst was investigated. Because of the gradual deactivation of the catalyst, the hydrogen yield decreased by 8–72% after five catalytic cycles. The temperature has the largest influence on the deactivation rate of the nickel catalyst. The decline rate of the hydrogen yield decreased from 67% to 8% when the temperature was increased from 400 °C to 500 °C. SEM, EDS and TPO were used to characterize the spent catalyst. The results showed that the deactivation of the nickel catalyst is caused by the deposition of char/coke on the catalyst surface. Finally, the relationship between the reaction conditions and catalyst deactivation was investigated from the viewpoint of the organic matter degradation pathway.