Kraft lignin (KL) is a by-product from cellulose production typically treated as a waste or used as a low-value fuel in heat and power generation in the pulp and paper industry.

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The synthesis of effective heterogeneous catalysts is one of the main challenges toward hydrothermal processing of wood-derived biomass into marketable sustainable chemicals. Many of these catalysts are based on noble metals and are normally synthesized using multiple steps in time-consuming processes. Here, we have developed a one-pot catalyst synthesis method for Ni–CeO2–x supported on carbon nanofibers. In situ H2 production through formic acid decomposition enabled the synthesis of catalysts in their reduced form, with ceria as Ce3+ and presence of metallic Ni. This catalyst promoted Kraft lignin conversion in supercritical water at short reaction times with a 79 wt % yield of a bio-oil composed of nearly 69 wt % of monoaromatics. Thus, lignin breakdown was achieved without resourcing to noble metal catalysts, molecular H2, or cosolvents, with a decrease in catalyst synthesis time and unit operations and with an attractive yield of a chemically uniform product fraction.

Sewage sludge mixed with Yunnan low-rank coal was treated by hydrothermal treatment (HT) at mild temperature and high-pressure conditions. The fuel characteristics and chemical structure of HT solid products were investigated to explore the synergistic effects between low-rank coal and sludge. The results indicated that the fuel properties of HT solid products were negatively influenced as the amount of sludge increased, but there were synergistic positive effects on the content of volatile matter, fixed carbon, oxygen, and calorific value when the amount of sludge was more than 30 wt% in the HT at 300°C. More hydrogen groups in sludge were involved in the hydrogen transfer reactions between the low-rank coal and sludge, which promoted the occurrence of synergistic effects during the HT process. CO 2 and H 2 were the main gas products, but the yield of both gases produced by the HT of sewage sludge was significantly higher than that of low-rank coal. The aromaticity of HT solid products reduced from 60.65% to 51.83%, and the aliphaticity increased from 39.35% to 48.17% when the amount of sludge increased from 10 to 70 wt%.