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In the offshore petroleum industry, polymer-containing oily sludge (PCOS) hinders oil extraction and causes tremendous hazards to the marine ecological environment. In this paper, an effective pretreatment method is proposed to break the adhesive structure of PCOS, and the experiments of supercritical water gasification are carried out under the influencing factors including residence time (5–30 min) and temperature (400–750 °C) in batch reactors. The increase of time and temperature all show great promoting effects on gas production. Polycyclic aromatic hydrocarbons, including naphthalene and phenanthrene, are considered as the main obstacles for a complete gasification. Carbon gasification efficiency (CE) reaches maximum of 95.82% at 750 °C, 23 MPa for 30 min, while naphthalene makes up 70% of the organic compounds in residual liquid products. The highest hydrogen yield of 19.79 (mol H 2 /kg of PCOS) is observed in 750 °C for 25 min. A simplified reaction pathway is presented to describe the gaseous products (H 2 , CO, CO 2 , CH 4). Two intermediates are defined for describing the reaction process bases on the exhaustive study on organic matters in residual liquid products. The results show that the calculated data and the experimental data have a high degree of fit and tar formation reaction is finished within 10 min. [Display omitted] • Utilization of polymer-containing oily sludge via supercritical water gasification. • Optimal gasification condition is obtained. • Reaction mechanism of gasification is discussed. • Tar formation reaction can be finished within 10 min in quartz tubes. [ABSTRACT FROM AUTHOR]

Supercritical water gasification (SCWG) of oily sludge is a potential handling method for resource utilization and pollution reduction. Heavy metals (HMs) in oily sludge were assessed due to their threat to human health and environmental safety. This work investigated the distribution of four HMs (Cu, Zn, Ni and Cr) under different reaction conditions (550–700 °C, 1–60 min). The concentrations of HMs in liquid residues (LRs) decreased continuously at higher temperatures and longer residence times except for Ni. Besides, the concentrations of HMs in solid residues (SRs) increased after SCWG except for Zn, and they varied slightly under different conditions. Then, HMs in LRs and SRs were evaluated in terms of Nemerow index, geo-accumulation index and potential ecological risk. The results indicated that the pollution risks of HMs in LRs were minimum at 650 °C and 30 min, while that in SRs changed little. When Na 2 CO 3 was added, pollution risks changed slightly, but nearly complete stabilizations of Cu, Zn and Cr (96.2%, 84.2% and 98.6%) were achieved at 600 °C. Adding Na 2 CO 3 promoted the formation of aluminosilicate to combine with HMs and enhanced their stabilization notably. This work may demonstrate a promising clean way for oily sludge utilization and HMs stabilizing. [Display omitted] [ABSTRACT FROM AUTHOR]

Biomass supercritical water gasification is clean and renewable, which can convert biomass into hydrogen rich gas. Previous studies indicated that external recycle of liquid residual could improve gas yield and gasification efficiency. However, the influence mechanism of external recycle on energy and exergy efficiency is complicated and theoretical model for system optimization is insufficient. Thermodynamic model of external recycle of liquid residual was built in this paper. Exergy efficiency of the main components and exergy loss distribution were specified and the result showed that exergy loss of reactor and preheater accounted for 26.06% and 35.88% of the total exergy loss, which were the main exergy loss sources. Effective ways to reduce exergy loss of components with large exergy loss and to improve energy and exergy efficiency of the system were proposed. Moreover, life cycle assessment of biomass gasification process was carried out. The results indicated that the increase of gasification temperature, pressure and external recycle flow rate of liquid residual and decrease of biomass concentration could improve energy and exergy efficiency of the system. Energy and exergy efficiency reached 63.67% and 48.29% respectively at the condition of gasification temperature of 560 °C, pressure of 25 MPa, recycle flow ratio of 32.43%, biomass concentration of 2.78%. Besides, the increase of gasification temperature and decrease of biomass slurry concentration and pressure could decrease GWP. Thermodynamics and LCA analysis of biomass supercritical water gasification using external recycle system was conducted. Image 103 • Material, mass and energy balance of the system and each component was achieved. • Component with largest exergy loss was found and solutions were proposed. • Approaches to improve energy and exergy efficiency of the system were proposed. • LCA of biomass gasification in SCW was conducted. • Highest energy and exergy efficiency reached 63.67% and 48.29% respectively. [ABSTRACT FROM AUTHOR]

Supercritical water gasification (SCWG) is a promising clean technology for coal utilization due to high chemical reactivity and no tar emission. However, the wall-type heat exchange in the SCWG system demands a large heating surface area and an extreme high-temperature heat source. This paper puts forward an optimized design, namely direct mass transfer, in which the final product gas is partially recycled back into the gasification reactor to supplement the gasification agent and provide sensible heat energy. A whole system simulation was established to analyze the mass flow and exergy efficiency in the reference design and the optimized design. The process analysis shows that the optimized design has higher exergy efficiency (85.6%) than that (72.5%) of the reference design. In the optimized system, direct mass transfer significantly alters the mass flow and exergy flow, with less influent water (33.3%), less heat transfer rate (30.5%), and less oxygen consumption (51.5%). Furthermore, sensitivity analysis shows that increasing circulation ratio, feedstock concentration, and preheating temperature can reduce oxygen consumption and improve gas output and efficiency. The study indicates that direct mass transfer has the potential superiority to solve the challenge of heat integration and facilitate the application of the industrial-scale SCWG system. • Direct mass transfer for supercritical water gasification of coal was simulated. • The optimized cold gas and exergy efficiency increase by 15.5% and 13.1%. • The direct mass transfer design greatly reduces exergy loss in heat integreation. • Higher circulation ratio leads to more CH 4 , less heat flux and higher efficiency. • The direct mass transfer has superiority for disposing low-concentration feedstock. [ABSTRACT FROM AUTHOR]

Supercritical water gasification (SCWG) technology can convert oily sludge into hydrogen-rich gas. To achieve high gasification efficiency of oily sludge with a high oil concentration under mild conditions, a two-step method involving a desorption process and a catalytic gasification process using Raney-Ni catalyst was investigated. High oil removal efficiency (99.57%) and carbon gasification efficiency (93.87%) were achieved. The lowest wastewater total organic carbon, oil content, and carbon content in the solid residues were 4.88 ppm, 0.08% and 0.88%, respectively, using a gasification temperature of 600 °C, treatment concentration of 1.11 wt%, gasification time of 70.7 s, and the optimal desorption temperature of 390 °C. The main organic carbon component in the solid residues was cellulose, which is environmentally safe. As the treatment concentration increased, the two-step method outperformed the single-step method. The mechanism for the two-step SCWG of oily sludge was revealed. In the first step, supercritical water is used in the desorption unit to achieve a high oil removal efficiency with few liquid products generated. In the second step, the Raney-Ni catalyst promotes efficient gasification of high-concentration oil at a low temperature. This research provides valuable insights into the effective SCWG of oily sludge at a low temperature. [Display omitted] • A continuous two-step method for SCWG of oily sludge was proposed. • A SCWG mechanism of two-step method for oily sludge was proposed. • The primary substance of organic carbon in solid residues is cellulose. • With increased treatment concentration, the two-step method has more significant advantages. [ABSTRACT FROM AUTHOR]

• The respective maximum CE and H 2 selectivity of 95.34% and 0.99 were achieved. • The dependence of gasification efficiency on reaction temperature was apparent. • Bond-making of functional group was observed at a high feedstock concentration. • The disorderly conversion between organic matters triggered a low CE. • The improvement method on gasification performance was proposed. The clean and efficient utilization of biomass waste for hydrogen production conforms to social development in the future. In this work, hydrogen production from supercritical water gasification (SCWG) of fir bark (FB) was investigated systematically at various reaction temperatures (500–750 °C), feedstock concentrations (5 wt%–15 wt%), and retention time (0–40 min). The gaseous, liquid, and solid products were analyzed to master the gasification mechanism. The results demonstrated the maximum CE and H 2 selectivity of 95.34% and 0.99 were obtained at 700 °C-5 wt%-40 min. Reaction temperature was positively related to the gasification efficiency, and the effects of feedstock concentration and retention time were related to the reaction temperature. Increasing feedstock concentration contributed to the cyclisation and Diels-Alder reaction, with a cohesive aromatic structure. A benign conversion resulted in the enhancement of gasification efficiency with prolonging retention time in the high temperature SCWG. The vital reason for low gasification efficiency was due to the disorderly interconversions between organic matters. [ABSTRACT FROM AUTHOR]

Realizing the harmless resource utilization of oily sludge is urgent for petroleum industry and of great significance for environmental management. The treatment of oily sludge was investigated using supercritical water gasification (SCWG) with a continuous fluidized bed reactor. The effect of operating parameters on gasification efficiency and gas yield without catalyst was tested, and then the influences of catalyst type (K 2 CO 3 and Na 2 CO 3) and concentrations (1–8 wt%) were systematically studied. The results indicated that a medium mass flow ratio and low feedstock concentration were beneficial for gas production. Alkali catalyst improved carbon gasification efficiency (CE) prominently, and Na 2 CO 3 showed better performance due to its better stability. A maximum CE of 95.87% was achieved when 5 wt% Na 2 CO 3 was added at 650 °C, 23 MPa with 5 wt% oily sludge concentration. Besides, according to XRD patterns of solid residues, Na 2 CO 3 was more stable than K 2 CO 3 during SCWG. SEM-EDX results also revealed that more K was migrated into solid residues than Na. The analysis of pore structure demonstrated that alkali catalyst promoted the evolution of pore structure, resulting in higher specific surface areas and total pore volumes. Na 2 CO 3 has a more substantial destructive effect on solid matrix, causing the matrix structure to collapse and inhibiting pore structure development. The FTIR spectra of solid products exhibited a lower content of carbohydrates and aromatic structures than the initial oily sludge. NH 4 –N results demonstrated that SCWG was a potential green treatment process for oily sludge. This work can not only give an insight into the reaction mechanism of alkali catalytic gasification of oily sludge, but also help to guide the optimal design of reactor and the regulation of operating parameters. [Display omitted] • Oily sludge was gasified in supercritical water with a fluidized bed reactor. • Maximum carbon gasification efficiency was 95.87% at 650 °C with 5 wt% Na2CO3. • Na2CO3 exhibited a better gasification effect due to better stability than K2CO3. • NH4–N values of liquid residues increased with increasing alkali catalyst. [ABSTRACT FROM AUTHOR]

As an industrial organic waste, oily sludge can be transformed into valuable syngas by supercritical water gasification (SCWG) technology. The transformation behaviors of heavy metals in SCWG of oily sludge must be considered to prevent the secondary pollution attributed to the high contents of heavy metals in oily sludge. In this work, the gasification products distribution after SCWG of oily sludge in a batch reactor under different reaction conditions (550–700 °C, 1–15 min) was studied. The distributions of heavy metals (Cu, Cr and Zn) in different forms including acid-soluble and exchangeable fraction (F1), reducible fraction (F2), oxidizable fraction (F3), residual fraction (F4) and heavy metal ions in liquid residues (M+) were presented and the transformation trend of those five fractions during the SCWG process was demonstrated. A reaction pathway and quantitative kinetic model of Zn transformation in SCWG were proposed. The results indicated that gas yield was enhanced at higher reaction temperature and longer residence time. Heavy metals in oily sludge were mainly composed of F3 and M+, they tended to deposit in F4 with increasing reaction temperature and residence time, which was stable and environmentally harmless. F3 showed better reactivity than F2 and contributed more to the formation of F4. Enhancing the transformation of Zn ions in liquid residues to more stable forms was necessary. This study highlighted that SCWG of oily sludge could realize energy utilization and heavy metals stabilization. [Display omitted] • The transformation behaviors of heavy metals (Cu, Cr and Zn) were discussed. • Heavy metals transformed to more stable forms via supercritical water gasification. • Longer time and higher temperature promoted liquid heavy metals transformation. • A reaction pathway and kinetic model of Zn transformation were proposed. [ABSTRACT FROM AUTHOR]