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Supercritical water gasification (SCWG) is a promising technology to treat oil-containing wastewater. This study reports a Ni/ZrO2 catalyst with mixed phase of tetragonal ZrO2 and monoclinic ZrO2 synthesized by a one-step sol-gel method, which shows good catalytic activity and stability in SCWG of oil mixture. The evolution of particle size, crystal structure, and pore size caused by increasing calcination temperature is demonstrated by X-ray diffraction and transmission electron microscopy. When calcined at 700 °C, Ni/ZrO2 shows superior catalytic activity and stability in SCWG reaction, with a high carbon gasification efficiency of 91.3% at 500 °C. The characterizations of fresh and spent catalysts suggest that Ni/ZrO2 catalyst with the fraction of monoclinic ZrO2 around 75% would be highly active and stable in SCWG reaction. In addition, two sets of optimal gasification conditions of SCWG of oil are obtained by varying operating parameters, which provides guidance for different concentrations of oil-containing wastewater treatment.

Recently, promising multi-fluid thermal stimulation technology has been studied and applied successfully to the offshore heavy oil recovery process. However, there are still some shortcomings, including heavy reliance on diesel, heavy and bulky water pretreatment plant, etc., which hinders the further industrialization process of the current generation system of thermal multi-fluid. A novel thermal multi-fluid generation technology to decrease reliance on diesel was presented in this paper. In this technology, oil-containing wastewater can be used directly as the material and energy source. Increased attention should be given to the supercritical water gasification (SCWG) process as the core of this novel technology. An experimental study on SCWG of diesel, a representative of oil-containing wastewater, was described detailly in this paper. Experiments were conducted in a continuous SCWG system under different experimental conditions, as follows: pressure of 23 MPa; reaction temperature of 600 °C–680 °C; mass ratios of water to diesel in emulsification of 1:1, 1:2.5, and 1:3.5; and reactor diesel concentration (RDC) of 4.70 wt% to 7.70 wt%. The effects of alkali catalyst (K2CO3) on gaseous and liquid products were also investigated. A simplified gasification mechanism in SCW was obtained, which may provide a theory basis for the novel thermal multi-fluid generation technology based on oil-containing wastewater gasification in supercritical water.

Supercritical water gasification (SCWG) technology is a clean and cost-effective conversion technology due to its unique chemical and physical properties. However, the unique properties also lead to instability and inactivity for the pure Ni/ZrO2 catalyst in SCWG process. In this work, we investigated the effect of second metal addition on the catalytic performance by modifying Ni/ZrO2 catalysts with different promoters (Co, Ce, La, Y, Mg), which prepared by a single-step sol-gel method. The analysis results of catalysts by XRD, SEM and automatic micropore & chemisorption analyzer showed that Ce, Y, La may be helpful promoters to stabilize the structure of ZrO2. Compared to the non-catalytic experiment, all the catalysts showed significantly higher activities in the SCWG reaction. Among all catalysts, Ni-Co/ZrO2 exhibited excellent activity, which achieved the highest carbon gasification efficiency (CE) and highest hydrogen yield. Additionally, two key factors, concentration and temperature, were also investigated for the optimum conditions, and the maximum carbon gasification efficiency (CE) of 98.8% was achieved at 600 °C with the Ni-Co/ZrO2 catalyst.

Unsymmetrical dimethylhydrazine (UDMH) is a high N-containing (as much as nearly 50%) substance. Traditional treatment methods such as incineration will inevitably cause the formation of nitric oxide and secondary pollution. Supercritical water is a preferred transformation medium due to its unique physicochemical properties. However, at present most of studies are limited to supercritical water oxidation (SCWO) which tends to produce hydrogen nitrate resulting in corrosion to the reactor. To conquer this problem, we propose supercritical water gasification (SCWG) technology which is in a reducing environment, realizing both harmless treatment and resource utilization. In order to promote its industrialization process, the reaction pathways and kinetic parameters should be studied. In this paper, the reaction pathways and kinetics of UDMH in supercritical water were conducted under the conditions of 400 °C–550 °C in quartz reactor, which avoids the catalytic effect on the reaction kinetics. From the resource utilization perspective, the most abundant quantitatively detectable gaseous product is methane, together with less hydrogen, carbon monoxide and ethane orderly. All these gaseous products are combustible. The maximum of carbon efficiency is 90.25% at 550 °C, 10 min. In the point of view of harmless treatment, the organic compounds contained in the residual liquid are detected with 1H NMR, FTIR and GC/MS. Results show that UDMH could be fully degraded within 3 min and the ultimate organic compounds in the residual liquid are mainly dimethylamino acetonitrile and trimethylamine. In addition, a reaction pathway for UDMH disposed in supercritical water is developed. Finally, the quantitative kinetic model for describing the gaseous products and ammonia-nitrogen in the residual liquid is brought forward. The pyrolysis activation energy for UDMH in supercritical water is 49.98 ± 7.38 kJ/mol.

Supercritical water gasification (SCWG) is a novel thermochemical technique to convert wet feedstock into hydrogen-rich gas products with near zero emission. However, char formation during SCWG of coal could reduce production efficiency and operational stability. In this paper, the influence of recycled intermediate phenol on coal gasification and coal char inhibition mechanism was investigated. The results indicated that phenol had positive influence on the gas yield and char inhibition. Carbon gasification efficiency of coal increased from 65.18% to 99.36% and H2 yield increased by 59% with the optimal experimental parameters (phenol concentration of 1 wt% and temperature of 750 °C. The experimental results demonstrated that phenol suppressed char formation effectively and strengthened the char reactivity since no solid produced in the reactor after reaction. Coal char microstructure characteristics were detected to analyze the migration of functional groups. The results of Fourier transform infra-red spectroscopy showed the presence of phenol enhanced the exchange of –OH between the char and supercritical water circumstances and promoted the conversion of –CH3 in char to methane. The suppression mechanism of char was further studied, which could be beneficial for regulating the product distribution and promote the oriented conversion of coal in SCW.

Supercritical water gasification of coal (SCWGC) has great potential for converting coal into CO2 and H2 effectively and environmentally. For a better technological competitiveness, the conversion mechanism of carbon structure needs to be investigated in depth to achieve 100% carbon conversion for SCWGC under mild conditions. Ningxia anthracite (NX), Hongliulin bituminous coal (HLL) and Zhundong lignite (ZD) were selected as feedstock to investigate the carbon microstructure evolution for different ranks of coal gasification in supercritical water (SCW). Raman and XRD spectra were employed to investigate the carbon microstructure information of solid residues. The experimental results illustrated that coal rank had slight effect on the evolution law of carbon microstructure in coal. The process of SCWGC included three stages depending on the characteristic reactions. Inhibiting the condensation of aromatic rings under 550–750 °C in the third stage was the key to effectively control the ordered conversion and achieve complete conversion of carbon in the coal. Besides, the Raman and XRD spectral parameters had close relationship with carbon gasification efficiency, respectively.

Coal supercritical water gasification (SCWG) is famous for generating clean gas without SOx pollutant. Study of sulfur transformation characteristics can provide the basis of sulfur removal during hydrogen production by coal gasification in supercritical water (SCW) at the source. In this work, two coals produced from Linfen and Zhangjiamao in China (hereinafter to be referred as L-coal and Z-coal), were chosen as experimental feedstocks to investigate sulfur transformation characteristics during hydrogen production by coal gasification in SCW (550–750 °C, 20 min, 25 MPa). Sulfur transformation pathway and sulfur forms in the products were complex but detected comprehensively. H2S was the only gaseous product instead of SOx, whereas SO42– was the main liquid–sulfur product. Inorganic and organic sulfur compounds were used to investigate sulfur transformation mechanisms. H2S had three sources as follows. First, among inorganic sulfur of raw coal, FeS2 (Pyrite) was chemically stable in SCW lacking of hydrog...

Supercritical water gasification (SCWG) technology shows huge advantages to achieve efficient and clean utilization of coal. Ash agglomeration caused by K2CO3 addition makes ash discharging process more difficult and inhibits gasification reaction of carbon in agglomerated ash, which prevents the constant operation of the system and decreases gasification efficiency. This study investigated the formation mechanism of ash agglomeration in potassium carbonate-catalyzed SCWG of coal and find a solution to inhibit this problem. Experiments were conducted in an autoclave to figure out the effect of K2CO3 (0 wt%-10 wt%) and Al2O3 (0 wt%-20 wt%) on ash agglomeration level at 750 °C. After every single experiment, solid residue was dried and classified into different sizes (0–100 μm, 100–1000 μm, >1000 μm). The ash agglomeration characteristics was examined by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDS). Results indicated that K2CO3 reacted with clay and quartz in raw coal, then formed K2Si2O5, KAlSiO4, KAlSi2O6 and KAlSi3O8. K2Si2O5 plays a cohesive role between different ash particles. Adding Al2O3 can effectively solve this problem by forming KAlSiO4 instead of K2Si2O5. Besides, Al2O3 enhances carbon gasification efficiency by increasing heating rate of feedstock temperature. Simulation work was also done to investigate the boundary condition for ash agglomeration.