Supercritical Water Gasification Research Articles

Experimental study on treatment of mixed ion exchange resins by supercritical water gasification

Nuclear power acts as a significant role in the current world energy supply system. The treatment of radioactive waste from nuclear station is a considerable issue and has attracted more and more attention. Spent ion exchange resins used in nuclear power plants are typical organic nuclear waste which are difficult to deal with. Supercritical water gasification could offer an efficient and clean treatment to spent ion exchange resins. In this study, Supercritical water gasification of mixed ion exchange resins was carried out in a batch reactor with various conditions. Effects of reaction parameters such as temperatures (550oC–750 °C), concentrations (5 wt%-15 wt%), catalysts (K2CO3, Na2CO3, KOH and NaOH) and residence times (10 min–60 min) were investigated. Results showed that high temperature, low concentration, K2CO3 addition and long residence time promote the gasification result. And the carbon gasification efficiency reached up to 97.2% with 10 wt% concentration and K2CO3 catalyst addition at 750 °C. The main intermediate organic component residues in liquid products were trimethylamine, benzene, toluene, ethylbenzene p-xylene styrene benzene, 1-ethyl-4-methyl- and naphthalene. Reaction pathways in supercritical water have been discussed and proposed. And the conversion pathways and distributions of sulfur and nitrogen were disclosed.

The integration of low temperature supercritical water gasification with continuous in situ nano-catalyst synthesis for hydrogen generation from biomass wastewater

A continuous hydrothermal process is demonstrated, for the first time, that can operate at low gasification temperature (430 °C) and residence time (20 s) by combining supercritical water gasification (SCWG) and partial oxidation with in situ synthesis of virgin metal oxide nano-catalyst. Using olive wastewater as the feedstock, this gasification study experimentally investigated the impact of multiple variables: (1) COD feed concentration, (2) the in situ synthesis of different metal oxide nano-catalysts, (3) the partial oxidation coefficient (ɳ) and (4) the nano-catalyst precursor solution concentration. The optimum conditions for the generation of hydrogen and methane from olive wastewater were a feed COD of 38.6 g/L, ɳ = 0.8, and 60 mM precursor concentration for the in situ synthesis of Fe2O3 nano-catalyst. These optimised conditions were further investigated using spent lees and stillage. The efficiency of hydrogen and methane yields and COD reduction were in the order of stillage > spent lees > olive wastewater. The highest hydrogen molar selectivity, hydrogen and methane yields at 18.8 %, 17 and 11.4 mol/(kg biomass) respectively were obtained with stillage feedstock. Gasification, COD and TOC reduction efficiencies were 68.8–71.7 %, 72.6–76.5 % and 53.9–55.7 % respectively, with this process. Importantly, this novel gasification approach prevents any performance drop or catalyst deactivation during continuous operation. This study exemplifies that the co-generation of catalyst during SCWG is a promising and economically feasible direction for large-scale continuous generation of hydrogen and methane from different types of biomass wastewater at < 450 °C, whilst lowering its COD and TOC. (249 words)

Energy, exergy and economic (3E) evaluations of a novel power generation system combining supercritical water gasification of coal with chemical heat recovery

Supercritical water gasification (SCWG) is a promising clean coal utilization technology. A proposed power system with integrated coal SCWG, supercritical turbine, gas turbine and chemical heat recovery is proposed in this paper. And the power system with integrated combined cycle and parallel heat recovery is re-proposed for structure simplification and efficiency enhancement. With a gasification condition of 660℃/250 bar, and a coal-water-slurry concentration of 11.3 %, the efficiencies of the proposed system and the system with parallel heat recovery are 49.06 % and 47.93 %, respectively. The power efficiency of the proposed system is greatly improved compared with the traditional systems. This is mainly due to the reasonable use of the pressure energy of the gasification product through the supercritical turbine, the release and use of high level chemical energy stored in the syngas in the gas turbine, and chemical heat recovery of the sensible heat of the flue gas from the gas turbine. Exergy and economic analyses are also performed. The levelized cost of electricity (LCOE) of the proposed power system is 0.2963 ¥/kWh. The power system proposed herein can be considered as one of the suitable options to achieve the carbon peak and carbon neutrality goals.

Thermodynamic assessment of hydrothermal combustion assisted fossil fuel in-situ gasification in the context of sustainable development

Developing countries cannot get rid of the fate of relying on fossil energy development in the short term, so the low-carbon transformation must be carried out in the development process to achieve the goal of net zero emissions. This work proposed a novel hydrothermal combustion assisted fossil fuel in-situ gasification (HC-ISG) system combining renewable energy, supercritical hydrothermal combustion technology and supercritical water gasification technology. The system has the advantages of high thermal efficiency, high hydrogen yield, carbon dioxide utilization and storage, and landfill of hazardous waste. Firstly, based on the concept of energy grade, the effects of renewable energy and auxiliary fuel on the characteristics of energy grade change in HC-ISG system is studied. Then, the Gibbs free energy minimization method is conducted to discuss the effects of fossil fuels, oxidants, sub- and supercritical water on the key parameters of the system. The results indicated that for HC-ISG system, the energy grade and input amount of renewable energy have the optimal values. Compared with coke and methane, the system with methanol as an auxiliary fuel for hydrothermal combustion owned better the matching between energy quantity and energy grade. Moreover, increasing the reaction temperature could reduce the exergy destruction of the system and increase the energy grade change of the input energy. When the combustion coefficient was 0.2–0.3, the molar fraction of H2 reached the maximum value. When fossil fuel with high carbon content was used, the maximum H2 yield could reach 89.4 mol/kg, and the carbon emission was 3.9 kg CO2/kg H2, so the HC-ISG system could produce clean hydrogen. The system proposed in this study may provide a promising method for efficient and low-carbon in-situ conversion of fossil energy to hydrogen production.

Experimental investigation on organic functional groups evolution in hydrogen production process by coal gasification in supercritical water

Supercritical water gasification (SCWG) of coal has great application prospect for converting coal into hydrogen-rich gas efficiently and cleanly. However, the previous study on the reaction mechanism for SCWG of coal is relatively macroscopic rather than reflects the reaction essence deeply. The evolution of organic functional groups in Zhundong lignite (ZD), Hongliulin bitumite (HLL) and Ningxia anthracite (NX) during SCWG, as well as the correlation with gaseous products were analyzed quantitatively in this paper. It was found that the lower rank coal contained more free radicals and produced more H2 with SCW. H2 yield of the three types of coal exceeded 2 times the hydrogen content in coal at 800 °C. The organic functional groups evolve in 2–4 stages during SCWG process. The decomposition and gasification of organic functional groups mainly took place in low or medium temperature range. About 95% of C=O groups and 90% of aromatic C=C groups cracked and were gasified. Aromatic ether (Car-O) groups were formed in high temperature range. The reasonable functional relationship between the parameters of gaseous products and organic functional groups was established, providing a new approach to predict organic functional groups through gaseous products. This research may lay the foundation for further optimization design of reactor.

Insight into the thermal conversion of corn stalk gasification in supercritical water based on reactive molecular dynamics simulations

Supercritical water gasification (SCWG) technology could be used for the clean and efficient utilization of waste biomass. This work first constructed a three-dimensional macromolecular model based on the characteristics of real corn stalks (CS). The reactive force field molecular dynamics (ReaxFF MD) method was then employed to study the gasification process in supercritical water. The depolymerization process of CS in SCW was observed by tracing the evolution of the carbon-containing products during the reaction. Temperature and water concentration effects on the SCWG of CS were analyzed. The results indicated that higher temperature caused more water to decompose into hydrogen and hydroxyl radicals. Meanwhile, the large number of reactive radicals favored the production of gases such as HCHO, CH4, CO, and H2. In addition, HCHO, CH4, and CO2 would continue to hydrolyze or decompose at high temperature to produce small molecule gases such as CO and H2. Furthermore, water-gas conversion and steam-reforming reactions would occur more frequently when the water concentration was higher. As a result, higher gasification rates and higher hydrogen yields were achieved.

A circular approach for the treatment of aqueous metal effluent and biomass to generate superparamagnetic nanometal carbon hybrid and hydrogen-rich gas mixture

In this study, simulated water effluent of transition metals (Ni, Zn, Fe, and Cu) and pine needles were integrated and treated hydrothermally for the generation of hydrogen and superparamagnetic carbon hybrid of metal/metal oxides of nano-size in a single-step. The temperature plays a significant role and decides the product yield and the same was varied (300-600 °C) to optimize the product yield. The highest yield of hydrogen was obtained with nickel metal effluent, 17.2 mmol. gram−1 of pine needles at 600 °C followed by other metals i.e., Fe(11.3 mmol.gram−1), Cu (10.64 mmol.gram−1), and Zn (8.86 mmol.gram−1) and greater than 99 % of the heavy metals (HMs) has been recovered from effluents (Ni, Zn, Fe, and Cu) in the form of nanometal carbon hybrid. The synthesized nanometal carbon hybrids were analyzed with the help of X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), which shows that the metals present in the effluent get transferred to the lignocellulosic matrix of biomass during the supercritical water gasification (SCWG) that eventually gets reduced to pure metal (M(o)) from metal oxide (M(+n)). Metal oxide reduction results in the release of active oxygen, which promote the decomposition of the organics of biomass. Furthermore, the in-situ generated pure metal (M(o)) catalyzes the steam reforming and water gas shift reaction (WGS). Moreover, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM) indicate that synthesized nanometal carbon nanometal hybrid was quasi-spherical or cubical has a size less than <30 nm and are superparamagnetic material.

On the green hydrogen production through gasification processes: A techno-economic approach

Green hydrogen rises in the world energy scenario as one of the most promising non-carbon-based fuels capable of replacing fossil fuels. Biomass gasification is emerging as a cleaner and sustainable process of producing green hydrogen. Green hydrogen can be obtained by resorting to different biomass gasification processes. Moreover, techno-economic studies are essential to determining the viability of these processes and are missing in the literature. The novelty of this study is the identification of the gasification process that produces the highest hydrogen yields and the presentation of the viability conditions for three gasification plants aiming to produce green hydrogen. The methodology followed is based on the development of three models of biomass gasification processes (conventional, plasma, and supercritical water) in Aspen Plus® to estimate maximum hydrogen yields through a parametric study. The economic study was conducted based on economic indicators (net present value and payback) to assess the viability of each gasification process for green hydrogen production. The results suggest that supercritical water gasification is the most suitable process for hydrogen production, with hydrogen yields of 0.844 Nm3/kgbiomass. Hydrogen yields of 0.828 Nm3/kgbiomass, and 0.758 Nm3/kgbiomass were achieved for the conventional gasification and plasma gasification processes, respectively. From the economic assessment, it was concluded that none of the gasification processes are viable under the determined optimal operating conditions. A sensitivity analysis, however, revealed that conventional gasification is viable for steam-to-biomass ratios <3. Process intensification techniques were applied, revealing that supercritical water gasification can be viable for feed concentrations between 15 and 25%. A minimum selling price of 7 €/kg, 10 €/kg, and 13 €/kg were obtained for green hydrogen generated by the conventional, supercritical water, and plasma gasification processes, respectively. The results obtained in this study are of paramount importance for the hydrogen economy. However, this research should be supplemented by further intensifying the gasification processes in order to bring green hydrogen production costs down to a more competitive level.

Experimental study on alkali catalytic gasification of oily sludge in supercritical water with a continuous reactor

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 (K2CO3 and Na2CO3) 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 Na2CO3 showed better performance due to its better stability. A maximum CE of 95.87% was achieved when 5 wt% Na2CO3 was added at 650 °C, 23 MPa with 5 wt% oily sludge concentration. Besides, according to XRD patterns of solid residues, Na2CO3 was more stable than K2CO3 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. Na2CO3 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. NH4–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.

Study on gasification characteristics and kinetics of polyformaldehyde plastics in supercritical water

Supercritical water (SCW) treatment technology has been applied in the field of waste plastics recycling due to its advantages of clean, high efficiency and low carbon. Previous research has focused on liquefaction of plastics to produce oil, mainly for some general-purpose plastics. Different plastics have different structures, which will affect their gasification behavior in SCW. As one of the five engineering plastics, polyoxymethylene (POM) plastics will also become one of the main sources of waste plastics recycling industry in the future. In this paper, the supercritical water gasification (SCWG) experiment of POM plastics was carried out. The effects of temperature, residence time, feedstocks concentration, pressure on the gasification reaction were discussed. The carbon balance of the products under different working conditions was investigated. The optimum condition for SCWG of POM plastics was determined. On this basis, the gasification kinetics of POM plastics were studied. It was found that the gasification efficiency of plastics was significantly improved by temperature increase. The total carbon content of gas and liquid phase products increased gradually. At 700 °C, the total conversion rate reached 99.79%. With the prolongation of the reaction time, the organic carbon in the liquid phase was gradually converted into gas and the gasification reaction proceeds more completely. The increase in concentration reduced the contact area between the raw material and the gasification agent. At the same time, the diffusion of organic matter was hindered, resulting in a lower gasification rate. The effect of pressure on the plastics gasification reaction was not significant. Under the optimum gasification reaction conditions, the carbon gasification efficiency reached 97.15%. Finally, a gasification kinetic model was established, and the activation energy of paraformaldehyde was 160.38 kJ/mol.

Performance comparison of black liquor gasification and oxidation in supercritical water from thermodynamic, environmental, and techno-economic perspectives

The black liquor poses a great threat to the environment and it is also a kind of available biomass resources. By the supercritical water gasification (SCWG) and supercritical water oxidation (SCWO) technology, the black liquid with high water content can be converted into hydrogen-rich gas or heat energy. In this study, a pulp mill integrated with black liquor SCWG and SCWO subsystems, which have a processing capacity of 11.2 t/h of black liquor with a concentration of 15.2 wt%, is designed. The thermodynamic analysis, techno-economic analysis (TEA), and life cycle assessment on the system are conducted. The gasification characteristics and thermodynamic performance of the SCWG and SCWO systems under different working conditions are studied. The thermodynamic analysis shows that the highest energy efficiencies of SCWG and SCWO systems are 41.53 % and 19.89 %, respectively. The lowest processing cost of black liquor is 398.68 RMB/t, which is obtained in the SCWO system at 600 °C. At 800 °C, the minimum hydrogen production cost of SCWG system is 23.28 RMB/kg H2. The LCA results show that the environmental benefits of SCW system at high temperatures are significantly better than those of integrated black liquor combustion systems. Finally, a multi-attribute decision making results of the SCWG and SCWO systems show that the optimal reaction temperature is 800 °C.

Molecular dynamics investigation on the co-gasification of various components of sewage sludge in supercritical water

The supercritical water gasification (SCWG) technology is a promising method for the disposal and utilization of sewage sludge. As a mixture with various components, detailed studies on the co-gasification mechanism of sewage sludge in supercritical water (SCW) are needed. In this paper, three typical components, namely, α-O-4 structured lignin (C20H24O6), triglyceride (C9H14O6), and alanine (C3H7O2N) are selected to present the sludge mixture. The detailed SCWG processes of these components under different conditions are simulated by using reactive force field (ReaxFF) molecular dynamics method. The three components first undergo their own cleavage in SCW, and the cleavage order is C-O-C bond breakage of triglyceride, ether bond detachment of lignin and deamination of alanine. The order of further gasification is triglyceride, alanine, and then benzene ring segments of lignin. The presence of alanine improves the gasification efficiency of lignin, and thus increases the overall gasification of the system. The SCW mass fraction increases the production of H2 but decreases that of CO. SCW mass fraction of 66 %–81 % is suggested for the simulation cases. In addition, the SCWG process is highly temperature-dependent because of the high energy needed for opening the benzene rings of lignin, but a much higher temperature above ring-opening temperature cannot further increase the gasification efficiency.

Synthesis and characterization of molybdenum trioxide with an orthorhombic crystal structure for supercritical water gasification application

Supercritical water (SCW) catalytic gasification is promising for the production of synthetic gas from biomass. However, maintaining the activity of heterogeneous catalysts under high pressure and hydrothermal conditions is the most important. Therefore, this study investigates the activity and stability of molybdenum trioxide (MoO3) for the gasification of formaldehyde in SCW. MoO3 with an orthorhombic crystal structure was synthesized by the sol-gel method and tested in the gasification of formaldehyde in SCW. Structural and morphological changes of MoO3 with supercritical water gasification were studied by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), a scanning electron microscope (SEM), N2 adsorption-desorption, thermogravimetric analysis (TGA), temperature-programmed desorption of oxygen (O2-TPD), temperature-programmed desorption with ammonia (TPD-NH3), and temperature-programmed reaction of hydrogen (H2-TPR). The XRD results show that MoO3 has an orthorhombic crystal structure. Its structure changed to a monoclinic structure upon gasification with supercritical water. SEM images showed that the rod-shaped catalyst particles transformed into thin planar plates at high pressure and temperature conditions in the presence of carbonaceous compounds. H2-TPR and XRD results showed that MoO3 was reduced to MoO2 during SCW gasification with hydrogen and carbonaceous compounds. Moreover, the oxygen stability of the catalyst changed during SCW gasification, and active oxygen species formed on the surface of the catalyst. Compared with homogeneous gasification, in the presence of the MoO3 catalyst, the fractions of CO2 and CH4 increased to 50% and 8.4%, respectively, and the fractions of H2 and CO decreased from 37.8% and 11.8% to 32.2% and 3.9%, respectively, due to the changed decomposition mechanism. Orthorhombic molybdenum oxide can be proposed as a promising catalyst for methane production.

Development of kinetic model for supercritical water gasification and dynamic characteristics investigation on tubular reactor

Supercritical water gasification (SCWG) technology is considered as a potential technology to realize resource utilization of biomass waste. Both experimental and modeling researches are performed to guide the development and scale-up of this technology. Among these researches, dynamic study of the reactor is an important aspect. However, related published literature is relatively limited. In this study, a one-dimensional dynamic model was developed, and a kinetic model of soybean stem containing detailed reaction pathway of biomass was established based on the lumped parameter method. It was calculated through fitting experiment data (obtained at 600, 650, and 700℃ three temperatures and pressure of around 25 MPa), after which it was applied to the tubular reactor for further analysis. Sensitivity analysis of the reactor displayed that H2 mole fraction was most sensitive to the inlet temperature, which increased or decreased by around 10 % when the temperature had a ± 5 % fluctuation. Dynamic simulation results showed that the gasifier responds quickly under a transient change of concentration. Gasification efficiency decreased from 83.8 % to 41.9 % at 500 s, kept constant for 240 s, and then increased to 83.4 % and stabilized at this value. Total response time was 900 s. Molar fraction of CO, CO2 and CH4 increased by 0.8 %, 0.66 %, 3.08 % respectively. While that of H2 decreased by 4.54 % within 660 s. As for fluctuation of flowrate, gasification efficiency increased by 3.4 % when residence time increased from 527 s to 584 s. Mole fraction of H2 and CO2 increased with time, while that of CH4 and CO decreased, and response time was 900 s.

Proposal and assessment of a solar-coal thermochemical hybrid power generation system

The thermochemical complementarity is an efficient path to improve the utilization efficiency of coal and address the intermittence of solar energy. In this paper, a solar-coal thermochemical hybrid power generation system based on supercritical water gasification is proposed, and the feature is that the low gasification temperature of 650 °C makes it feasible to use concentrated solar energy to provide reaction heat for the gasification process. The energy and exergy analysis methods are used to evaluate the thermodynamic performance of the system with a net power output of 500 MW. The results show that the net power generation efficiency and exergy efficiency of the proposed system can reach 53.06 % and 52.24 %, respectively, which are approximately 6.94 and 6.83 percentage points higher than those of the reference system. The significant role of thermochemical method is that the chemical energy of syngas produced by the proposed system is 851.57 MW, which is increased by approximately 29.70 %. Through the energy utilization diagram analysis, it is found that the key to performance improvement is that the exergy destruction of the fuel conversion process and heat exchange process are decreased by 10.36 % and 54.68 %, respectively. Finally, the proposed system has better performance and lower exergy destruction. The solar thermal energy of low energy level is converted into syngas chemical energy of high energy level, and the upgrading of solar energy is achieved. In addition, the proposed system is simpler, and eliminates the air separation unit and syngas purification unit. This work provides a promising method for the complementary utilization of solar energy and coal.

Insight into the internal relevance of three-phase products in supercritical water gasification of coal

• Internal relevance of three-phase products of coal gasification was investigated. • Insight into the formation mechanism of products in the micro dimension. • Solid-liquid reaction model in coal particle surface was established. • Control steps for decreasing the irreversibility of reaction process was summarized. Supercritical water gasification (SCWG) is a promising technology for coal utilization and has attracted more attentions. This paper is aimed to obtain coal gasification behavior and the internal relevance of products from the perspective of micro level. Evolution characteristics of three-phase products were employed in high heating rate batch reactor. As a result, removal of functional groups in solid particles led to the decrease of the gasification reactivity of char (R coal > R 20s > R 60s > R 100s > R 1200s ). The hydrogen bonds formed by water forced the coal structure decomposition, in which the CO and CO 2 gas were generated from the deetherization and decarboxylation reaction. H 2 and CH 4 primarily came from the collision of free radicals and polycondensation reaction. Liquid intermediate components were detected by high-performance liquid chromatography quantitatively. The content of phenol and naphthalene was up to 280.8 ppm and 3647.4 ppm, and the mutual conversion mechanism of organic matters and polycyclic aromatic hydrocarbons (PAHs) was represented. Effectively controlling the ring growth of PAHs and the polycondensation of solid fragments is the primary goal to inhibit char formation, which is of great significance to decrease the irreversibility of chemical reaction process and realize the large-scale industrial production.

ReaxFF molecular dynamics simulation of nickel catalysed gasification of cellulose in supercritical water

Reactive force field (ReaxFF) molecular dynamic simulation was performed to elucidate the mechanism of Ni-catalysed supercritical water gasification of cellulose considering the effects of temperature and cellulose to water ratio. Simulations showed that Ni could decrease the activation energy of C–C and C–O bond cleavage, promoting the depolymerisation and ring-opening process of cellulose. The yields of gaseous products increase with the increasing temperature. H2 yield mainly depends on H free radical number, which can be generated from cellulose dehydrogenation and water splitting reactions. These two reactions were promoted on Ni surface, leading to an increase in H2 yield. In the presence of Ni catalyst, water plays a limited role in providing H free radicals to produce H2, while the hydrogen atoms in cellulose are the primary source of H2 generation. Meanwhile, reducing the concentration of O•H could enhance H2 production as the combination of O•H and H• is a H radical consumption process. Small organic fragments would be absorbed on the Ni surface, where they undergo deoxygenation via the cleavage of C–O bonds, resulting in a decrease in CO and CO2 yields. The increase in water mass fraction would promote the H2 yield as more H radical would be produced due to water splitting reaction. Moreover, the addition of water would occupy the Ni active sites and prevent the adsorption of organic fragments. These dissociative fragments are prone to produce more CO. The carbon deposition on the Ni surface results in the deactivation of the catalyst. Simulation results suggested that carbon deposition and permeation increase with increasing temperature. In contrast, the increase in water mass fraction can favour carbon elimination from the catalyst surface.

Thermodynamic and environmental analysis of heat supply in pig manure supercritical water gasification system

Pig manure, a high water content waste with massive annual output, must be treated in a clean and effective way to protect the environment. Supercritical water gasification (SCWG) is a high-moisture waste utilization technology that is both efficient and pollution-free. The study developed a thermodynamic equilibrium model of the SCWG of pig manure. The effects of operating conditions were examined on heat production and system efficiency. In addition, life cycle assessment (LCA) was used to perform the environmental significance. The results indicated that decreasing the amount of preheating water, increasing the slurry concentration, and the gasification temperature can increase the heat output and system efficiency. The system efficiency is 95.53% at the ratio of water to the slurry is 1:1 and 94.90% at the slurry concentration is 70 wt%. The detailed exergy analysis results showed that the exergy efficiency of the entire system could reach 39.8%. The primary exergy loss occurs in heat exchange and oxidation processes. Adopting reasonable operating conditions and optimizing the layout of heat exchangers can effectively reduce the exergy loss of the system. Besides, GWP has been significantly reduced to 7.4 kg CO2-eq/t steam with carbon capture.

Resource utilization of waste HFC-134a refrigerant by supercritical gasification method: A reactive molecular dynamic study

Effective treatment of HFCs (hydrofluorocarbons) waste refrigerants is of great significance for reducing greenhouse gas emissions. In this study, the supercritical water gasification method was applied to the treatment of waste HFCs refrigerants in order to realize the harmless treatment and resource utilization at the same time. Taking widely used HFC-134a (1,1,1,2-Tetrafluoroethane, CF3CFH2) refrigerant as the research object, the gasification mechanism of HFC-134a in supercritical water was studied by the ReaxFF-MD, and the effect of temperature, reactant concentration and pressure on gasification was studied. The results show that the main products of HFC-134a gasification in supercritical water are HF, H2, CO and CO2. HF is mainly generated by the hydrogen extraction reaction between F radicals and H2O molecules, and the energy barrier is only 31.3 kJ·mol−1. H2 is mainly generated by the hydrogen extraction reaction between H radicals and H2O molecules and H atom-containing molecules or radicals. When the F, H radicals and H2O undergo hydrogen extraction reaction, a large number of OH radicals are generated, and the carbon-containing radicals combine with OH radicals to undergo a reforming reaction. After further removal of H and F atoms, CO and CO2 are ultimately generated. Parametric analyses show that higher reaction temperature can speed up the reaction rate and improve the yields of CO and H2; Lower reactant concentrations help to increase the yields of CO2 and H2 and have little effect on the yields of HF and CO; Higher pressure boosts the reaction rate to increase CO2 and CH4 yields but decrease the yields of HF, CO and H2.

Catalytic gasification of indole in supercritical water with non-noble bimetallic catalysts

Indole is commonly reported as a N-containing heterocyclic compounds in biomass supercritical water gasification (SCWG), and is difficult to be degraded to small molecule substances. In this work, Ni/AC, Ni–Cu/AC, Ni–Co/AC, Ni–Ce/AC were prepared and tested for catalytic SCWG of indole. The results showed that Ni–Cu/AC had the highest activity for converting indole and producing gases, leading to the highest hydrogen gasification efficiency (135.1%) and carbon gasification efficiency (33.1%). During the catalytic SCWG of indole, the addition of auxiliary metal Cu in multi-component catalyst showed synergistic effects by changing reaction pathways and reaction rates. It significantly promoted water-gas shift and steam reforming reactions, so forming the highest H2 yield of 2.62 mol mol−1. Furthermore, large specific surface, Ni reduction and high catalyst stability point to potential advantages of the Ni–Cu/AC catalyst for catalytic SCWG of recalcitrant N-containing compounds.