Gas-solid Contact Research Articles

Solar methane pyrolysis in a liquid metal bubble column reactor: Effect of medium type and gas injection configuration

Solar methane pyrolysis in different molten media and reactor configurations was experimented to improve hydrogen production. Pure Sn, Ni0.18Sn0.82, Cu0.45Bi0.55, and KCl melts were compared at three different temperatures (1030–1130–1230 °C), and no significant difference was observed except for KCl and only at 1230 °C (XCH4 = 72 % vs. 57 % for pure Sn). This enhanced performance was attributed to possible carbon dispersion in the salt, which probably modified the physical properties and enhanced hydrodynamics. A porous quartz sparger (downward injection) did not significantly enhance the bubbles hydrodynamics, mainly because bubbles were trapped and coalesced below the sintered disc. A custom-made sparger (lateral bubbling) did not either improve conversion due to non-uniform pores. A sparger with an upward injection should be preferred to generate small bubbles with longer residence time. When a solid bed of tungsten carbide particles was placed around the injector, overlaid by molten tin, the conversion was improved even at a relatively low temperature (XCH4 = 17 % at 1030 °C). The immersed bed likely behaved as a porous device and increased the gas-solid surface contact. Combining particle bed and liquid bubbling system is very promising for further optimization of methane pyrolysis in molten media. The carbon collected above molten metals showed mostly a sheet-like structure with significant metal contamination. In case of KCl, most carbon was entrained with the gas, while the remaining was mixed with KCl in the reactor. The density of KCl is close to that of carbon, which prevented a good separation.

A novel method for gas-solid contact visualization and efficiency quantification in dry fixed-bed desulfurization

A novel method for the visualization of gas–solid contact states and the quantitative evaluation of contact efficiency in the dry fixed-bed desulfurization process is proposed in this study. This method utilizes the chromogenic properties of phenolphthalein (PP) on the surfaces of various sodium salt products during the sodium bicarbonate dry desulfurization process, enabling visualization of gas–solid contact status. Samples of desulfurization products are photographed, and ImageJ software is employed for image processing to calculate color residual rates, thereby quantifying gas–solid contact efficiency. The specific operating conditions that produced the best visualization effect of gas–solid contact states were determined through experimentation, including: an appropriate PP solid content of ≤ 0.336 % for ultrafine NaHCO3 with D90 = 6.14 μm; a steam volume ratio of approximately 13.94 % to 24.46 %, a temperature inside the reactor of 110 °C–175 °C. Moreover, the fading effect of the indicative sodium-based absorbent was consistent across SO2 concentrations from 850 mg/m3 to 5100 mg/m3, with higher concentrations reducing the time needed for visualizing gas–solid contact and quantifying efficiency. The gas–solid contact efficiency (Egs = 87.85 %) calculated by the method showed a small deviation of only 0.55 % from the efficiency (E’gs = 88.4 %) calculated based on sulfur capacity, validating the method’s reliability. The study’s outcomes present a novel method for analyzing the gas–solid contact state, which can serve future efforts in the design optimization of dry fixed-bed desulfurization reactors.

Experimental investigation and CFD study of direct reduction of iron ore in the conical fluidized bed

The reduction behavior of iron ore is the fundamental process in ferrous metallurgy. Considering the gas–solid interface-structure relationship, heterogeneous transfer-reaction behavior, and multiple reaction kinetics, this work provides an effective analytical tool to investigate the fluidized reduction of iron ore. The conical fluidized bed is numerically examined to be capable of preventing de-fluidization induced by the sticking behavior, due to the steady circulation of flow pattern and full fluidization of coarse agglomerates. Compared with the hydrogen concentration, the gas velocity shows a stronger influence on the agglomerate reduction rate for the more efficient gas–solid contact. For the simulated transitions of Fe2O3 → Fe3O4 → Fe(1-x)O → Fe are assumed to overlap each other, there exist discrepancies between computed and theoretical reduction degrees. The experimental and numerical findings can be used to develop a methodology for optimizing the operational complexity and economic benefits of the fluidized reduction of iron ore.

Steel slag powder-CO2 contact state in static or rotary process of carbon fixation

The carbon fixation of steel slag can effectively improve its soundness and activity, increase its utilization rate, and reduce the emission of CO2, which has a significant development prospect. Currently, the effects of different carbon fixation processes for steel slag need further exploration. In this study, the ‘static-pressure’ and the’rotary’ process of the carbon fixation by steel slag powder were simulated, using the gas–solid contact between slag and CO2 as the indicator. The influence of different parameters was investigated, and experimental validation was conducted under optimized conditions. The results indicate that in the ‘static-pressure’ process, reducing the stack thickness and applying pressure to accelerate CO2 transfer are beneficial for gas–solid contact. In the ‘rotary’ process, reducing the filling coefficient to below 20.0 % and increasing the rotation speed to 1.0 rad/s are conducive to gas–solid interaction. Under the optimized conditions, the carbon fixation rate for the ‘static-pressure’ process after 2 h is approximately 6.4 %, while the carbon fixation rate for the ‘rotary’ process is approximately 9 %, indicating better carbon fixation performance in the latter. The addition of microorganisms can further accelerate carbon fixation and enhance its effectiveness. Under the ‘rotary’ process, the carbon fixation rate of microorganism-treated samples reaches 10.21 % after 1 h, surpassing the carbon fixation rate of 7.8 % in the’static-pressure’ process. A satisfied carbon fixation effect is achieved when the contact duration between steel slag particles and CO2 reaches 12.6 min.

Fuel reactor simulation for coal chemical looping process: Effects of ash on flow behavior

Chemical looping gasification (CLG) is an environmentally friendly strategy for using coal that has significant promise for application in reality. Nevertheless, the process of generating and accumulating ash from coal presents obstacles to the advancement of this technology. To fully comprehend the effects of ash from coal on the internal flow properties of a fuel reactor (FR). The Computational Particle Fluid Dynamics (CPFD) approach was applied to predict the CLG with coal at the FR, uncovering distinctive flow properties of gas-solid reactions. This study investigated the effects of ash particles content, polydispersity and sphericity on the flow behavior within the bed. The simulation accurately predicted the motion of the bubbles within the FR, and the gas phase composition distribution at the outlet is closer to the experimental data. Introducing coal ash creates a reduction in the concentration of ilmenite, resulting in a drop in the rates of the related reactions. The narrow particles diameter distribution among coal ash facilitates the particles arrangement in the bed and improves the overall reaction. The addition of moderately irregular-shaped coal ash particles has a certain positive effect on improving the gas-solid contact.

A novel liquid air energy storage system with efficient thermal storage: Comprehensive evaluation of optimal configuration

Liquid air energy storage (LAES) stands out as a highly promising solution for large-scale energy storage, offering advantages such as geographical flexibility and high energy density. However, the technology faces challenges inherent in the cold and heat storage processes. While systems with liquid-phase medium can offer high efficiency, it comes with drawbacks including environmental pollution, safety concerns, and high costs. Alternatively, systems with solid-phase media in packed beds can address these issues, yet it profoundly weakens system performance. Compounding these issues, many current economic evaluations of LAES rely on inaccurate or outdated data, resulting in skewed assessments. In response to these challenges and to drive scientific evaluation and engineering advancements in LAES, this study introduces an innovative LAES system with efficient thermal storage (ETS-LAES). An original thermal storage method is introduced for the first time, based on gravity-driven solid-phase particle flow and gas-solid direct contact heat transfer. The study establishes thermodynamic and heat transfer models, along with a universally applicable economic evaluation model. The ETS-LAES system is comprehensively assessed utilizing different operational modes introduced in the study. Research findings showcase a round-trip efficiency (RTE) of 58.76%, currently standing as the highest RTE record for cold and heat storage based on solid-phase media. The economic evaluation reveals substantial advantages for the ETS-LAES system during the transition from demonstration projects to commercial projects and guides the selection of the scale for the LAES system. The levelized cost of storage (LCOS) for the ETS-LAES system can decrease to 0.0982 USD/kWh as the capacity increases to 1000MW, due to the use of inexpensive solid-phase thermal storage media throughout. The investment payback period (IPP) is halved compared to conventional LAES systems, indicating a strengthening of the profitability. The proposed solution in this study holds great potential, particularly in offering valuable insights into the practical implementation and commercialization of LAES.

Flow Behavior of Nanoparticle Agglomerates in a Fluidized Bed Simulated with Porous-Structure-Based Drag Laws.

Fluidization bed reactor is an attractive method to synthesize and process quantities of functional nanoparticles, due to the large gas-solid contact area and its potential scalability. Nanoparticles fluidize not individually but as a form of porous agglomerates with a typical porosity above 90%. The porous structure has a significant effect on the hydrodynamic behavior of a single nanoparticle agglomerate, but its influence on the flow behavior of nanoparticle agglomerates in a fluidized bed is currently unclear. In the present study, a drag model was developed to consider the porous structure effects of nanoparticle agglomerates by incorporating porous-structure-based drag laws in the Eulerian-Eulerian two-fluid model. Numerical simulations were performed from particulate to bubbling fluidization state to evaluate the applicability of porous-structure-based drag laws. Results obtained for the minimum fluidization and bubbling velocities, bed expansion ratio, and agglomerate dispersion coefficient show that, compared with the drag law of solid sphere, the porous-structure-based drag laws, especially the drag law of fractal porous spheres, provide a closer fit to the experimental data. This indicates that the pore structures have a great impact on gas-solid flow behavior of nanoparticle agglomerates, and the porous-structure-based drag laws are more suitable for describing flows in nanoparticle agglomerate fluidized beds.

A numerical scale-up study of annular fluidized bed reactor with high centrifugal acceleration

A rotating fluidized bed imposes a strong centrifugal force to form the dense annular bed with high interphase slip velocity for significantly enhanced gas-solids contact. The fundamental fluidization behaviors in rotating fluidized beds have been well understood in small-scale systems. The current study focuses on the scale-up of a rotating fluidized bed from lab-scale to commercial-scale through computational fluid dynamics modeling. The computational model was first validated against a lab-scale horizontal rotating fluidized bed for the general flow behavior and pressure drop. The validated computational model with a column diameter of 0.12 m was then scaled up to 0.48 m and 1.92 m to investigate the flow hydrodynamics at larger scales. By comparing the solids distribution and other flow field variables, major limitations in the stable operation of a rotating fluidized bed at large scales were identified. The challenges faced in the scale-up of an annular fluidized bed with centrifugal acceleration are discussed and ways to overcome such challenges are explored.

Direct reduction of iron-ore with hydrogen in fluidized beds: A coarse-grained CFD-DEM-IBM study

Hydrogen metallurgy technology uses hydrogen as the reducing agent instead of carbon reduction, which is one of the important ways to reduce carbon dioxide emissions and ensure the green and sustainable development of iron and steel industry. Due to the advantages of high gas-solid contact efficiency and outstanding mass and heat transfer, direct reduction of iron ore in fluidized beds has attracted much attention. Based on the three-layer unreacted shrinking core model and considering the diversity of particle structure during the reaction process, a multi-reaction path (multi-resistance network) model was established. In this study, a coarse-grained CFD-DEM-IBM solver based on hybrid CPU–GPU computing is developed to simulate the direct reduction process of two kinds of iron ore with hydrogen in fluidized beds, where the developed unreacted shrinking core model is used to model the reduction reactions, a coarse-grained model and multiple GPUs enable the significant acceleration of particle computation, and the immersed boundary method (IBM) enables the use of simple mesh even in complex geometries of reactors. The predicted results of particle reduction degree are in good agreement with the experimental values, which proves the correctness of the CFD-DEM-IBM solver. In addition, the effects of reaction kinetic parameters and operating temperature on particle reduction degree are also investigated. Present study provides a method for digital design, optimization and scale-up of ironmaking reactors.

The Mechanism behind Vibration Assisted Fluidization of Cohesive Micro-Silica

Vibro-assisted fluidization of cohesive micro-silica has been studied by means of X-ray imaging, pressure drop measurements, and off-line determination of the agglomerate size. Pressure drop and bed height development could be explained by observable phenomena taking place in the bed; slugging, channeling, fluidization or densification. It was observed that channeling is the main cause of poor fluidization of the micro-silica, resulting in poor gas-solid contact and little internal mixing. Improvement in fluidization upon starting the mechanical vibration was achieved by disrupting the channels. Agglomerate sizes were found to not significantly change during experiments.

Dy-Modified Mn/TiO2 Catalyst Used for the Selective Catalytic Reduction of NO in Ammonia at Low Temperatures.

A novel Mn/TiO2 catalyst, prepared through modification with the rare-earth metal Dy, has been employed for low-temperature selective catalytic reduction (SCR) denitrification. Anatase TiO2, with its large specific surface area, serves as the carrier. The active component MnOx on the TiO2 carrier is modified using Dy. DyxMn/TiO2, prepared via the impregnation method, exhibited remarkable catalytic performance in the SCR of NO with NH3 as the reducing agent at low temperatures. Experiments and characterization revealed that the introduction of a suitable amount of the rare-earth metal Dy can effectively enhance the catalyst's specific surface area and the gas-solid contact area in catalytic reactions. It also significantly increases the concentration of Mn4+, chemisorbed oxygen, and weak acid sites on the catalyst surface. This leads to a notable improvement in the reduction performance of the DyMn/TiO2 catalyst, ultimately contributing to the improvement of the NH3-SCR denitrification performance at low temperatures. At 100 °C and a space velocity of 24,000 h-1, the Dy0.1Mn/TiO2 catalyst can achieve a 98% conversion rate of NOx. Furthermore, its active temperature point decreases by 60 °C after the modification, highlighting exceptional catalytic efficacy at low temperatures. By doubling the space velocity, the NOx conversion rate of the catalyst can still reach 96% at 130 °C, indicating significant operational flexibility. The selectivity of N2 remained stable at over 95% before reaching 240 °C.

Identification of HCl corrosion mechanism on Cu-based oxygen carriers in chemical looping combustion

In chemical looping combustion (CLC) of coal, HCl has been demonstrated as the primary gaseous products of the chlorine in coal. Therefore, a comprehensive characterization of Cu-based oxygen carriers (OC) by HCl corrosion was meticulously conducted, including the activity retention, composition distribution and apparent morphology. The experiment consisted of three parts: (I) evaluating of the OC performance for 100 redox cycles with 7.8 vol% CH4; (II) investigating HCl-corrosion of OC by adding 50 ppmv HCl with 7.1 vol% CH4 for the subsequent 50 cycles; (III) regenerating of OC without HCl addition during the last 50 cycles. The reactivity of OC was attenuating via the HCl addition during the 101st–150th cycles (part II) due to the competition adsorption between HCl and CH4, especially for the partly reduced OC. While, in part III, the reactivity performance of OC was partially recovered because of the abundant cracks on the surface of OC particles which ensured the sufficient gas–solid contact. It should be noted that the single particle hardness index and service life were irreversibly decreased for the HCl-treated OC. Moreover, OC particles were fragmented and the inside Al2O3 balls were exposed on the surface of the HCl-treated OC in SEM image. This work will contribute to understanding the mechanism of Cl-corrosive OC and designing the Cl-resistant OC.

Impact of gas-solid direct contact on gas-liquid-solid reaction performance in a flow reactor

Impact of gas-solid direct contact on gas-liquid-solid reaction performance in a flow reactor

Mixing nanoparticles in a ProCell type spouted bed

Mixing nanoparticles is critical in various industries, including pharmaceuticals, solar, and nanotechnology. When different nanopowders are mixed, hetero-aggregates consisting of hetero-contacts are formed, facilitating the exchange of charge, mass and moments between particles of different materials. This research paper presents a novel approach for mixing nanoparticles using special spouted bed equipment to produce hetero-aggregates. The paper explores the potential of special spouted bed equipment for mixing nanoparticles. High-velocity opposed inlet air streams in this equipment result in high gas-solid contact efficiency and enhanced particle circulation, which contribute to the breakage of aggregates and promote the mixing process. The study investigates the aggregate breakage and mixing quality in produced hetero-aggregates. Experimental results demonstrate that the special spouted bed equipment enables efficient mixing of nanoparticles, leading to higher dispersion in hetero-aggregates with many hetero-contacts. The modified homogeneity of mixing method is utilized to quantify the mixing inside the single hetero-aggregates. Visual confirmation of the assessed values of mixing quality is provided by EDX spectral mapping.

CFD-DEM simulation of high density particles fluidization behaviors in 3D conical spouted beds

The spouted bed has been used in the coating process of high-density nuclear fuel particle. The particle fluidization behaviors in pseudo-2D and 3D spouted beds were simulated and validated. The effects of four independent variables (cone angle, particle density, inlet gas velocity, and particle loading) on particle fluidization behaviors in the 3D spouted bed were investigated systematically. The cone angle effect on fluidization mechanism was studied quantitatively first time. A new fluidization quality index was proposed based on the particle entrainment principle, and an extreme value was obtained when the cone angle was 60°, considered to be the optimum value for the 3D conical spouted bed. It was indicated the gas–solid contact efficiency can be kept up if the gas velocity was proportional to ρp0.65 and Np0.78 when the particle density or loading was increased. These results will be useful for geometry and operation parameters design of the 3D conical spouted bed and helpful for developing the fluidization mechanism of high-density particles.

Effect of Furnace Structure on Burden Distribution and Gas Flow in Sinter Vertical Cooling Furnace

Sinter sensible heat recovery via a vertical cooling furnace is a new type of waste heat recovery process proposed based on coke dry quenching. However, the segregation of the burden in a vertical cooling furnace is serious, resulting in a large amount of cooling gas escaping from the short-circuit channel of the vertical cooling furnace, which seriously affects the uniform gas–solid heat transfer in the furnace. To improve the burden distribution and gas flow in such a furnace, this paper proposes a Venturi-type vertical cooling furnace. Based on the single silo of a vertical cooling furnace in Meishan Steel, a slot model was established, and the improvement effect of the Venturi furnace structure on the burden distribution and gas flow was studied using the DEM–CFD coupling method. The results show that compared with the existing furnace type, the inclined wall of the Venturi furnace changed the direction of the high Dnv (average diameter) channel from vertical to inclined-vertical and reduced the Dnv from >0.033 m to 0.028~0.03 m in the vertical part of the variable-diameter section, thus reducing the influence area of the high Dnv channel. The minimum and average values of the voidage in the contraction part of the variable-diameter section increased from 0.28 and 0.315 to 0.31 and 0.33, respectively, which caused the voidage distribution to change from U-shaped to W-shaped along the longitudinal direction while simultaneously reducing the longitudinal fluctuation range of the voidage from 0.28~0.39 to 0.298~0.37. The gas flow direction changed from vertical-upward to vertical-inclined-upward, which increased the gas–solid contact. The gas velocity increased significantly. In the vertical section, the average gas velocity was 2.34 m/s, which was 30.73% higher than the velocity of 1.79 m/s of the existing furnace type. In the variable-diameter section, the average gas velocity was 3.52 m/s, which was 72.55% higher than the velocity of 2.04 m/s of the existing furnace type. The high-speed gas channel basically only existed in the sidewall area and the center area of the vertical section, and the length was reduced from 3.11 m to 2.52 m, which reduced the influence area. In the variable-diameter section, the high-speed gas channel disappeared, and the uniformity of the gas velocity distribution was greatly improved. The gas pressure drop increased from 4140 Pa to 6410 Pa, with an increase of 54.83%. Therefore, when designing the Venturi furnace type, it was necessary to take into consideration the improvement in the gas velocity distribution and the increase in the pressure drop. The research results of this paper can provide guidance for the structure optimization of the sinter vertical cooling furnace.

Extending homogeneous fluidization flow regime of Geldart-A particles by exerting axial uniform and steady magnetic field

The homogeneous/particulate fluidization flow regime is particularly suitable for handling the various gas–solid contact processes encountered in the chemical and energy industry. This work aimed to extend such a regime of Geldart-A particles by exerting the axial uniform and steady magnetic field. Under the action of the magnetic field, the overall homogeneous fluidization regime of Geldart-A magnetizable particles became composed of two parts: inherent homogeneous fluidization and newly-created magnetic stabilization. Since the former remained almost unchanged whereas the latter became broader as the magnetic field intensity increased, the overall homogeneous fluidization regime could be extended remarkably. As for Geldart-A nonmagnetizable particles, certain amount of magnetizable particles had to be premixed to transmit the magnetic stabilization. Among others, the mere addition of magnetizable particles could broaden the homogeneous fluidization regime. The added content of magnetizable particles had an optimal value with smaller/lighter ones working better. The added magnetizable particles might raise the ratio between the interparticle force and the particle gravity. After the magnetic field was exerted, the homogeneous fluidization regime was further expanded due to the formation of magnetic stabilization flow regime. The more the added magnetizable particles, the better the magnetic performance and the broader the overall homogeneous fluidization regime. Smaller/lighter magnetizable particles were preferred to maximize the magnetic performance and extend the overall homogeneous fluidization regime. This phenomenon could be ascribed to that the added magnetizable particles themselves became more Geldart-A than -B type as their density or size decreased.

Characterization and mechanism of red-mud-catalyzed steam gasification of corn stover

Catalytic biomass gasification has received extensive attention from scholars worldwide. This study proposes steam gasification of corn stover using red mud (RM), an industrial solid waste, as a catalyst to produce high-quality reducing gas. Experiments were conducted at different reaction temperatures and gasification times. Characterization of RM and its three main components, Fe2O3, CaCO3, and Al2O3, in the catalytic steam gasification process of corn stover, was performed and its catalytic mechanism was explored by characterizing magnetic separation products. Results show that an increase in reaction temperature significantly promotes the production of CO, CO2, and H2. The promotion of steam to H2 production is caused by the gas-phase reaction and gasification of fixed carbon (FC). Fe2O3 promotes gas-phase reactions, such as the water–gas shift reaction, and effectively catalyzes the steam gasification of FC as an intermediate reactant. The addition of Al2O3 is beneficial for the production of CH4 and H2. The CO2 produced by the decomposition of CaCO3 inhibits the water–gas shift and methane-steam reforming reactions, thus reducing H2 output. The addition of RM is beneficial for the production of H2 and CO2 and inhibits the production of CO. Compared with pure Fe2O3, the superior pore structure and unique solid solution structure of RM provide Fe2O3 with more active surface lattice oxygen, higher redox performance, larger gas–solid contact area, superior ability to prevent carbon deposition, and better catalytic ability.

Diffusion combustion of NH3 in a single bubble of fluidized bed

Fluidized bed is a potential equipment for NH3 combustion due to its efficient gas–solid contact and high temperature particles as heat source for ignition. This paper studies the diffusion combustion of NH3 in a single bubble of fluidized bed, using an experimental approach and CFD-DEM simulation incorporated with a skeleton mechanism of 98 elementary reactions. The results show that the diffusion combustion of NH3 intermittently occurs in the bubble injection period and the oxygen is mainly consumed by H + O2 = OH + O. Slow oxidation, instead of combustion, takes place in the bubble rising period and the oxygen is mainly consumed by NNH + O2 = N2 + HO2. With the rupture of the bubble, unreacted NH3 re-burns, generating flames in the freeboard of the bed. The chemical reaction in the bubble is highly correlated to the mass transfer between the emulsion phase and bubble phase. Increasing the fluidization gas velocity promotes the accumulation of oxygen and enhances the reaction rate of slow oxidation of NH3 in the bubble due to the increasing inter-phase mass transfer. The increase of the bed temperature reduces the ignition delay time, and increases the chemical reaction rate and mass transfer coefficient. The changes in oxygen concentration of fluidizing gas not only changes the chemical reaction rate, but also affects the oxygen consumption path. The fraction of NH3 oxidized in the dense bed can be enhanced by increasing the fluidization gas velocity, bed temperature and oxygen concentration of the fluidizing gas, the latter exerting the greatest influence.

Methane and Carbon Dioxide Separation Characteristics of Quaternary Ammonium Salt Hydrates for Biogas Upgrading under Static Conditions of Gas–Solid Contact

This report describes the methane and carbon dioxide separation characteristics of stoichiometric quaternary ammonium salt (QAS) hydrates for gas–solid contact with simulated biogas (CH4, 0.613 mol fraction; CO2, 0.387 mol fraction). Tests for elucidating the separation characteristics were conducted using tetra-n-butylammonium fluoride (TBAF), tetra-n-butylammonium chloride (TBAC), and tetra-n-butylammonium bromide (TBAB) hydrates as solid phases. To ascertain system temperature and pressure effects on their gas separation performance, the tests were conducted at 253–293 K under the initial fed pressures of 3.0 and 0.9 MPa. The gas–solid contact between the simulated biogas with 0.387 mol fraction of CO2 and the QAS hydrates reduced CO2 in the gas phase to 0.26–0.37 mol fraction and enriched CO2 in the hydrate phases to 0.54–0.89 mol fraction. All QAS hydrates showed selective incorporation of carbon dioxide into the hydrate phase. Under all comparable conditions, TBAC hydrate exhibited the highest separation factor (15.2 ± 3.0 under initial pressure of 3.0 MPa at 253 K and 13.9 ± 2.5 under initial pressure of 0.9 MPa at 253 K), indicating that TBAC hydrate had the highest CO2 separation performance for biogas upgrading among the QAS hydrates we examined. At the same initial pressure, high gas separation performance was shown for low-temperature ranges. The separation factors of the QAS hydrates were maintained or improved by lowering the initial pressure at the same contact temperature, suggesting that the separation performance of the QAS hydrates, even at lower pressures, was nearly equal to or better than that achieved at higher pressures. The results contrasted against those reported for tetrahydrofuran hydrate, for which the separation factor decreases with lower initial pressure. The use of the QAS hydrates is expected to lower a system’s operating pressure without compromising separation performance.