Continuous Supercritical Water Gasification Research Articles

Investigating Salt Precipitation in Continuous Supercritical Water Gasification of Biomass

The formation of solid deposits in the process of supercritical water gasification (SCWG) is one of the main problems hindering the commercial application of the process. Seven experiments were conducted with the grass Reed Canary Grass with different preheating temperatures, but all ended early due to the formation of solid deposits (maximum operation of 3.8 h). The position of solid deposits in the lab plant changed with the variation in the temperature profile. Since the formation of solid deposits consisting of salts, coke, and corrosion products is a severe issue that needs to be resolved in order to enable long-time operation, inner temperature measurements were conducted to determine the temperature range that corresponds with the zone of solid formation. The temperature range was found to be 400 to 440 °C. Wherever this temperature was first reached solid deposits occurred in the system that led to blockage of the flow. Additional to the influence of the temperature, the influence of the flow direction (up-flow or down-flow) on the operation of the continuous SCWG plant was examined. If salts are not separated from the system sufficiently, up-flow reactors should be avoided because they amplify the accumulation of solid deposits leading to a shortened operation time. The heating concept coupled with the salt separation needs to be redesigned in order to separate the salts before entering the gasification reactors. Outside of the determined temperature zone no deposition was visible. Thus, even though the gasification efficiency was low it could be shown that the operation was limited to the deposits forming in the heating section and not by incomplete gasification in the reactor where T > 600 °C.

Comparison study of supercritical water gasification for hydrogen production on a continuous flow versus a batch reactor

This study compares batch and continuous supercritical water gasification (SCWG) processes for green hydrogen production from biomass. It offers insights for optimizing processes, enhancing yields, quality, and energy efficiency, assessing scale-up feasibility, and supporting techno-economic analyses. Glucose, glycerol, and black liquor were SCWG-treated at 500 °C with K2CO3 catalyst in a self-built continuous-flow reactor (150 g/h) and a batch reactor (75 mL). Comparisons primarily focused on gas product yields. Batch reactors outperformed continuous-flow reactors in hydrogen (glucose: 1.53 to 0.9 mmol/g, glycerol: 7.22 to 1.14 mmol/g, black liquor: 2.88 to 1.74 mmol/g) and total gas yields due to differences in reaction time and heating behavior. Temperature effects on continuous operation (450–600 °C) were studied, with glycerol showing the highest hydrogen yield increase (from 1.21 to 4.30 mmol/g). The study discusses the applicability of both reactors for biomass SCWG processes and their implications for sustainable green hydrogen production from renewable feedstocks.

Particle size effects in Ru/CNF catalysts during supercritical water gasification of glycerol

Ru/CNF catalysts of different Ru nanoparticle (NP) sizes (0.9–2.7 nm) were assessed for their performance in continuous supercritical water gasification (SCWG) of glycerol. A structure sensitivity of Ru was demonstrated, with high initial turnover frequencies (TOF) for Ru NPs smaller than 1.2 nm. Deactivation happens essentially through: 1) coking, which occurs more readily at low Ru surface densities and is limited to nanometric layers, and 2) sintering, which led to final NP sizes ranging from 2 to 3 nm independent of the catalyst loading (1–30 wt% Ru). A correlation between TOF and the density of surface Ru atoms was found, with an optimal surface density (0.4–0.7 atomRu,sfc nm−2) resulting in high initial and high steady-state TOFs, allowing longer lifetimes through the delay of coke formation. CNF as a support enables high metal loadings with optimal performance, which could help in decreasing the volume of high-pressure vessels and hence the cost of SCWG plants, making this technology even more attractive.

Investigating active phase loss from supported ruthenium catalysts during supercritical water gasification.

Active phase loss mechanisms from Ru/AC catalysts were studied in continuous supercritical water gasification (SCWG) for the first time by analysing the Ru content in process water with low limit-of-detection time-resolved ICP-MS. Ru loss was investigated alongside the activity of commercial and in-house Ru-based catalysts, showing very low Ru loss rates compared to Ru/metal-oxides (0.2–1.2 vs. 10–24 μg gRu−1 h−1, respectively). Furthermore, AC-supported Ru catalysts showed superior long-term SCWG activity to their oxide-based analogues. The impact on Ru loss of several parameters relevant for catalytic SCWG (temperature, feed concentration or feed rate) was also studied and was shown to have no effect on the Ru concentration in the process water, as it systematically stabilised to 0.01–0.2 μgRu L−1 for Ru/AC. Looking into the type of Ru loss in steady-state operation, time-resolved ICP-MS confirmed a high probability of finding Ru in the ionic form, suggesting that leaching is the main steady-state Ru loss mechanism. In non-steady-state operation, abrupt changes in the pressure and flow rate induced important Ru losses, which were assigned to catalyst fragments. This is directly linked to irreversible mechanical damage to the catalyst. Taking the different observations into consideration, the following Ru loss mechanisms are suggested: 1) constant Ru dissolution (leaching) until solubility equilibrium is reached; 2) minor nanoparticle uncoupling from the support (both at steady state); 3) support disintegration leading to the loss of larger amounts of Ru in the form of catalyst fragments (abrupt feed rate or pressure variations). The very low Ru concentrations detected in process water at steady state (0.01–0.2 μgRu L−1) are close to the thermodynamic equilibrium and indicated that leaching did not contribute to Ru/AC deactivation in SCWG.

Stability and activity maintenance of sol-gel Ni-MxOy (M=Ti, Zr, Ta) catalysts during continuous gasification of glycerol in supercritical water

The long-term stability of activities and physicochemical properties for nickel nanoparticles supported on TiO2, ZrO2 and Ta2O5 prepared by sol-gel methods, was evaluated for continuous supercritical water gasification (SCWG) of 3 wt.% glycerol at 425 °C and 25.2 MPa. The Ni/TiO2 catalyst bed gradually deactivated while the Ni/ZrO2 and Ni/Ta2O5 catalyst beds showed persistent and high carbon gasification efficiency for 80 h operation. Characterization results verified that property stabilization of the sol-gel Ni/ZrO2 and Ni/Ta2O5 catalysts occurred in the long-term SCWG flow, indicating a “catalyst in-situ activation” effect that enhanced the activity and coking resistance of the as-prepared sol-gel catalysts for continuous SCWG. Results also indicate that sintering of Ni with the supports to form new phases and metal leaching of the catalyst materials in the SCW flow may be key deactivation factors of the catalyst beds.

Experimental and fluid dynamic study of continuous supercritical water gasification of glucose

The supercritical water gasification (SCWG) of glucose as a model compound for H2 production is studied in term of process development at 25MPa and 650°C. Gasification has been investigated using a laboratory scale continuous plant comprising a continuous down flow reactor with a volume of 200cm3, a heat recovery section and pre-heat of the biomass to simulate more closely an industrial design. Feed streams (pure water and glucose solution) are introduced at the top of the vessel and products are extracted from the bottom. The reactor is equipped with various inlet points that allow to modify the inlet position of reacting streams in order to study the effect of stream mixing on reaction performance. Moreover, in order to obtain a description of the fluid flow inside the reactor, computational fluid dynamics (CFD) simulations were carried out. The simulations were conducted by the ANSYS CFX 15 code which uses a finite volume method. Simulation allowed not only to understand the complex fluid dynamics that describes the system, but also to optimize the reactor configuration in order to attain good process performance.Experimental results coupled with CFD analysis, allowed to find that the reactor has the top section that behaves like a mixed reactor and the bottom section that behaves like a plug flow reactor. The two jet streams (water and glucose solution) entering the reactor cause back mixing in the top-side of the reactor, so contributing to rapid initiation of reaction and down-flow of species through the chamber. Using the best reactor configuration in term of mixing and temperature distribution allowed to maximize the gasification efficiency and H2 yield up to 72% and 74% by mole, respectively.

High-yield hydrogen production by supercritical water gasification of various feedstocks: Alcohols, glucose, glycerol and long-chain alkanes

Continuous supercritical water gasification (SCWG) of various feedstocks of C1–C16 was conducted to produce hydrogen-rich gas. These feedstocks represent model compounds of biomass such as methanol/ethanol (alcohol-type), glucose and glycerol (byproducts of biodiesel synthesis), and model compounds of petroleum fuels such as iso-octane/n-octane (gasoline), n-decane/n-dodecane (jet fuels) and n-hexadecane (diesel). Almost complete gasification of all the feedstocks was achieved at 25MPa, 740°C and 10wt% with low total organic carbon values of their liquid effluents. The hydrogen gas yields of each feedstock were very similar to the theoretical equilibrium yields estimated by Gibbs free energy minimization. SCWG at different gasification temperatures (650 and 740°C) and concentrations (10 and 20wt%) revealed that methanol and ethanol (alcohols), the simple oxygenated hydrocarbons, were easier to be gasified, producing negligible amounts of liquid products, when compared with long-chain hydrocarbons (iso-octane and n-decane) under the identical conditions. When the feedstock concentration was increased from 10 to 20wt%, the equilibrium hydrogen ratio from iso-octane gasification decreased from 1.02 to 0.79 while that of n-decane increased from 1.12 to 1.50, implying that a branched hydrocarbon may be more resistant to gasification in supercritical water.

O-402 麦焼酎残渣の24時間連続ガス化試験について(ガス・ケミカル,Oral Session4)

O-402 麦焼酎残渣の24時間連続ガス化試験について(ガス・ケミカル,Oral Session4)

3-2-4 超臨界水ガス化プロセスによる焼酎残渣の長時間連続ガス化試験結果(3-02 ガス化2,Session 3 バイオマス等)

3-2-4 超臨界水ガス化プロセスによる焼酎残渣の長時間連続ガス化試験結果(3-02 ガス化2,Session 3 バイオマス等)

High-yield hydrogen production from glucose by supercritical water gasification without added catalyst

Continuous supercritical water gasification of glucose is investigated with a recently developed updraft gasification apparatus under various conditions: temperatures of 600–767 °C, residence times of 15–60 s, glucose concentrations of 1.8–15 wt% and without added a catalyst. The experimental gas yields are compared with predicted values at equilibrium that are estimated via Gibbs free energy minimization. Total gas yields and hydrogen gas yield increase with temperature. At 740 °C and 1.8 wt%, hydrogen gas yields become very high (10.5–11.2 mol/mol glucose). The hydrogen gas yields do not vary significantly with different residence times. The hydrogen gas yield decreases to 5.7 mol/mol glucose at 15 wt%, a value very close to the predicted value (6.3 mol/mol glucose). Only acetic acid is detected in the liquid effluents at temperatures above 740 °C, while 42 products are detected at 600 °C. The highest hydrogen gas yield obtained in this study is 11.5 mol/mol glucose at 25 MPa, 767 °C, and 1.8 wt%, for 60 s; this value is very close to the theoretical equilibrium hydrogen yield of 11.9 mol/mol glucose. Under these conditions, the carbon efficiency is very high (91%) and total organic carbon (TOC) in the liquid product is very low (23 ppm), indicating that glucose is almost completely converted to gaseous products. Comparison with other work under similar operating conditions shows that the current reactor is capable of attaining higher hydrogen gas yields at temperatures above 650 °C. Possible explanations for the higher hydrogen gas yields are presented.

Turbulent Operation of a Continuous Reactor for Gasification of Alcohols in Supercritical Water

Supercritical water gasification of ethanol and butanol in a continuous supercritical water gasification reactor was explored over a range of residence times from 0.7 to 34 s. Residence time was decreased until turbulence was achieved (τ ≤ 1.2 s). As fluid transitioned from laminar to turbulent flow, the gasification rate was maximized while gasification efficiency was reduced. Gasification rates up to 107 g/L s were observed; this is nearly an order of magnitude higher than previously reported. Arrhenius parameters were estimated to model the intrinsic kinetics of gasification of model compounds in supercritical water without any transport limitations (i.e., turbulence); this technique has never before been applied to supercritical water gasification. The effects of feed, enthalpy of gasification, and residence time on product composition were also explored.

Noncatalytic gasification of isooctane in supercritical water: A Strategy for high-yield hydrogen production

Continuous supercritical water gasification of isooctane, a model gasoline compound, is investigated using an updraft gasification system. A new reactor material, Haynes® 230® alloy, is employed to run gasification reactions at high temperature and pressure (763 ± 2 °C; 25 MPa). A large-volume reactor is used (170 mL) to enable the gasification to be run at a long residence time, up to 120 s. Various gasification experiments are performed by changing the residence time (60–120 s), the isooctane concentration (6.3–14.7 wt%), and the oxidant concentration (equivalent oxidant ratio 0–0.3). The total gas yield and the hydrogen gas yield increase with increasing residence time. At 106 s and an isooctane concentration of 6.3 wt%, a very high hydrogen gas yield of 12.4 mol/mol isooctane, which is 50% of the theoretical maximum hydrogen gas yield and 92% of the equilibrium hydrogen gas yield under the given conditions, is achieved. Under these conditions, supercritical water partial oxidation does not increase the hydrogen gas yield significantly. The produced gases are hydrogen (68 mol%), carbon dioxide (20 mol%), methane (9.8 mol%), carbon monoxide (1.3 mol%), and ethane (0.9 mol%). The carbon gasification efficiency is in the range 75–91%, depending on the oxidant concentration. A comparison of supercritical water gasification with other conventional methods, including steam reforming, autothermal reforming, and partial oxidation, is also presented.

Exploration of the effect of process variables on the production of high-value fuel gas from glucose via supercritical water gasification

A new continuous supercritical water gasification reactor was designed to investigate glucose gasification in supercritical water at high temperatures and low residence times. A 2(3) full factorial experiment was performed to determine the effects of feed concentration, temperature, and residence time on glucose gasification. The temperature levels (750°C and 800°C) were higher than ever used, while the residence times (4 and 6.5s) were shorter than ever used in previous supercritical water gasification studies. The reactor proved capable of attaining higher gasification rates than previously shown with high efficiencies and yields. In addition, the glucose gasification reaction was modeled by estimating activation energy and reaction order of glucose gasification in supercritical water.

Continuous supercritical water gasification of isooctane: A promising reactor design

A new design of supercritical water gasification system was developed to achieve high hydrogen gas yield and good gas–liquid flow stability. The apparatus consisted of a reaction zone, an insulation zone and a cooling zone that were directly connected to the reaction zone. The reactor was set up at an inclination of 75° from vertical position, and feed and water were introduced at the bottom of the reactor. The performances of this new system were investigated with gasification of isooctane at various experimental conditions – reaction temperatures of 601–676 °C, residence times of 6–33 s, isooctane concentrations of 5–33 wt%, and oxidant (hydrogen peroxide) concentrations up to 4507 mmol/L without using catalysts. A significant increase in hydrogen gas yield, almost four times higher than that from the previous up-down gasifier configuration (B. Veriansyah, J. Kim, J.D. Kim, Y.W. Lee, Hydrogen Production by Gasification of Isooctane using Supercritical Water, Int. J. Green Energy. 5 (2008) 322–333) was observed with the present gasifier configuration. High hydrogen gas yield (6.13 mol/mol isooctane) was obtained at high reaction temperature of 637 °C, a low feed concentration of 9.9 wt% and a long residence time of 18 s in the presence of 2701.1 mmol/L hydrogen peroxide. At this condition, the produced gases mainly consisted of hydrogen (59.5 mol%), methane (14.8 mol%) and carbon dioxide (22.0 mol%), and a small amount of carbon monoxide (1.6 mol%) and C 2–C 3 species (2.1 mol%). Reaction mechanisms of supercritical water gasification of isooctane were also presented.

Industrial emergy evaluation for hydrogen production systems from biomass and natural gas

Fossil fuel resources are the main source for hydrogen production, and hydrogen production by renewable energy, such as biomass, is under development. To compare the performance in natural resource utilization for different hydrogen production systems, in this paper, two laboratorial hydrogen production systems from biomass and one industrial hydrogen production system from natural gas are analyzed by using industrial emergy evaluation indices. One of the laboratorial systems is a continuous supercritical water gasification system from glucose, and the other is a batch supercritical water gasification system from sawdust. The industrial system adopts American Brown technology. The evaluation results show that although the industrial emergy efficiency (IEE) of the industrial system from natural gas is higher than that of the laboratorial systems from biomass, the industrial emergy index of sustainability (IEIS) of the two laboratorial systems are higher than that of the industrial system. To make the laboratorial biomass system become an industrial system, the system should improve its yield, and reduce its capital investment.