Methane Diffusion Research Articles

Effect of fuel and oxygen concentration toward catalyst encapsulation in water-assisted flame synthesis of carbon nanotubes

Early catalyst deactivation through the encapsulation of catalyst nanoparticles by amorphous carbon layer due to the oversupply of carbon source is one of the key elements that inhibit efficient carbon nanotubes (CNT) synthesis in flame environment. The present study utilizes methane diffusion flame with water vapor additive to analyze the effectiveness of the novel water-assisted synthesis of CNT in reducing amorphous carbon formation. Flame shape, temperature, and CNT growth region within the water-assisted flame were analyzed at varying fuel and oxygen concentration. The amorphous carbon layer thickness (ACLT) is analyzed through cross-sectional analysis of the CNT growth region using a developed bend wire method. A 50% increase in fuel concentration results in the increase in axial extent of the growth region by 37.5% followed by an increase in the ACLT by 20%. Meanwhile, an 8% increase in oxygen concentration results in the decrease of growth region axial extent by 82% with a 35% reduction in ACLT. On average, water vapor additive produces an additional 17.3% reduction in ACLT in any flame composition. Both the reduction in ACLT as oxygen and water concentration is independently increased happen due to the carbon supply regulation. However, since the change in the growth temperature that happens at varying oxygen concentration is not observed at varying water concentration, the carbon supply regulation in both situations is remarkably different as explained by the water-gas shift reaction.

Pressure dependence of sooting characteristics of m-xylene and n-octane doped laminar methane diffusion flames from 2 to 10 bar

The work reported here contributes to a wider effort that focuses on the issue of whether single ring aromatics and their same carbon number alkane counterparts have the same pressure dependence of their sooting characteristics. Influence of doping methane base fuel with either m-xylene or n-octane in laminar high-pressure co-flow diffusion flames was studied from 2 to 10 bar pressure in current work. Two liquid hydrocarbons added to the base fuel methane contributed 3% of the total carbon mass flow in the fuel stream, which was kept fixed at 0.524 mg/s at all pressures considered, to have tractable measurements. The methane flames doped with either m-xylene or n-octane were stabilized on a co-flow diffusion burner installed inside of a high pressure combustion chamber with optical access. Temperatures and soot volume fractions were inferred from spectrally-resolved measurements of soot radiation. Line of sight soot emission intensities were inverted at each measurement plane along the flame axis to obtain radially-resolved temperature and soot volume fraction. Soot yields, calculated from soot volume fractions, of m-xylene doped flames were found to be higher than those of n-octane doped flames in the pressure range of 2–10 bar; however, the ratio of soot yields of m-xylene to n-octane decreased with pressure, from 2.9 at 2 bar to 1.2 at 10 bar. This finding implies that m-xylene and n-octane, both with eight carbons in their molecular structure, do not have the same sooting propensity with increasing pressure; m-xylene’s pressure dependence is weaker than that of n-octane within the pressure range of 2–10 bar.

Hydrocarbon Diffusion in Mesoporous Carbon Materials: Implications for Unconventional Gas Recovery

Methane diffusion in micro- and mesopores of carbonaceous materials is dominated by molecular interactions with the pore walls. As a consequence, the fluid molecules are mainly in a diffusive regime and the laws of fluid mechanics are not directly applicable. A method called the “free volume theory” has been successfully used by different authors to study the diffusion of n-alkanes into microporous carbons. However, we show in this paper that such a method fails to describe the dynamical properties of methane in porous hosts presenting both micro- and mesopores. We further evidence that this theory is limited to structures whose pore diameters are lower than ∼3 nm. We then propose a simple scaling method based on the micro- and mesoporous volume fraction in order to predict diffusion coefficients. This method only requires the knowledge of (i) the host microporous volume fraction and (ii) the self-diffusion coefficient in micropores smaller than 3 nm, which can be obtained using the “free volume theory”, quasi-elastic neutron scattering experiments, or atomistic simulations.

Adsorption kinetics and thermodynamics of CO2 and CH4 on activated carbon modified by acetic acid

AbstractIn order to improve the properties of activated carbons in the separation of carbon dioxide/methane mixture, organic acid, namely acetic acid, is used to modify the commercial coconut shell activated carbons. The modified sample 15H‐AC shows greater adsorption capacity than the raw material. To further explore the adsorption mechanism of carbon dioxide and methane on 15H‐AC, adsorption kinetics and thermodynamics are studied and compared with that on R‐AC. According to the results of adsorption kinetic study, it can be observed that the diffusion of both carbon dioxide and methane on 15H‐AC is faster than that on the raw material. The findings obtained in the adsorption thermodynamics indicate that the interaction of both carbon dioxide and methane with the modified sample is stronger and the spontaneity degree is higher. The physical adsorption of carbon dioxide on 15H‐AC indicates that the regeneration of the adsorbents is easy. The modified activated carbon, which possesses higher adsorption capacity, faster adsorption rate and good reproducing property, is promising in the separation of carbon dioxide / methane gas mixtures industrially.

Study of soot functional groups and morphological characteristics in laminar coflow methane and ethylene diffusion flames with hydrogen addition

The soot in laminar coflow methane and ethylene diffusion flames with different hydrogen addition ratios is sampled by a capillary soot sampling system and analyzed by Fourier transform infrared spectroscopy (FTIR) and field emission scanning electron microscopy (FESEM). Soot functional groups and morphological characteristics are studied. The results show that with hydrogen addition, the relative contents of aromatic and aliphatic functional groups in the soot of methane flames increase, while the relative contents of the same functional groups in the soot of ethylene flames mostly show different trends, indicating that H2 has a chemical promotion and inhibition on soot in methane and ethylene flames, respectively. The relative content of oxygen-containing functional groups in the soot of both methane and ethylene flames with hydrogen addition increases. By adding H2, the coagulation and condensation of soot low in the methane flames are greatly enhanced, indicating that H2 promotes the formation and growth of PAH molecules. This is an important reason for the chemical promotion effect of H2 in methane flames. For the soot of C2H4 hydrogen-added flames, oxidation is relatively thorough such that a large number of soot branches are oxidized. This is an important reason for the chemical inhibition of H2.

A Numerical Model to Estimate the Effects of Variable Sedimentation Rates on Methane Hydrate Formation—Application to the ODP Site 997 on Blake Ridge, Southeastern North American Continental Margin

AbstractSedimentation and burial rate can vary considerably at continental slopes, due to constantly changing tectonic processes on the geological timescale. This variation may affect the transport of methane in sediment, as manifested in the change of pore water flux, methanogenesis, methane diffusion and hydrate removal from hydrate stability zone (HSZ) and further affect the accumulation of hydrate in submarine sediments. Most of previous models assumed a constant sedimentation rate and thus may result in inaccurate estimates of total amount of hydrate within the HSZ. In this study, we developed a hydrate accumulation model that is capable of handling multiple sedimentation stages. Site 997 of ODP Leg 164, with data indicating its recent history of four sedimentation stages of different rates, is chosen to investigate the significance of sedimentation rate variation experienced at this location. We examined the effect of varying sedimentation rates on hydrate accumulation by taking in consideration sediment compaction, in situ methanogenesis, and composition and component transport processes. Simulation results suggest that the history of hydrate accumulation at Site 997 is characterized by a sequence of increase, decrease, and then increase till the present day. At present, the hydrate deposit has accumulated to 8.30 × 104 mol/m2, and the average hydrate saturation near the base of the HSZ is 6.3%, which is in general agreement with published estimates in the literature. The accumulation of hydrate at Site 997 was significantly affected by variable sedimentation rates, and nearly one half of the hydrate deposit was accumulated during the last 2.5 Myr.

Effects of tectonism on the pore characteristics and methane diffusion coefficient of coal

The diffusion coefficient of methane in coal is a key parameter that influences the methane extraction effect. Former studies on the methane diffusion coefficient mainly paid attention to the intact coal. The diffusion coefficient of tectonic coal is different from that of intact coal, which may be caused by the effect of tectonism on the pore characteristics. In this study, the pore characteristics of the intact coal and tectonic coal were tested using mercury intrusion porosimetry (MIP) and the physisorption method, while the counterdiffusion experiments were adopted to measure the methane diffusion coefficients of both coal samples. The results indicate that the methane diffusion coefficient of the tectonic coal is higher than that of the intact coal, and the diffusion coefficients decrease with the increasing methane pressure. The tectonism increases the volumes of mesopores and macropores and the porosity of coal, which is the main reason for the higher methane diffusion coefficient of the tectonic coal. The pore structure of coal becomes more winding or complicated by tectonism, thus showing a higher tortuosity of the tectonic coal. The tectonism makes it easier to extract methane, and coal mining in the tectonic region should receive more attention.

A non-monotonic blow-off limit of micro-jet methane diffusion flame at different tube-wall thicknesses

In order to provide guideline for choosing a suitable tube-wall thickness (δ) for the micro-jet methane diffusion flame, the effect of tube-wall thickness on the blow-off limit is investigated via numerical simulation in the present work. The results show that the blow-off limit of micro-jet methane diffusion flame firstly increases and then decreases with the increase of tube-wall thickness. Subsequently, the underlying mechanisms responsible for the above non-monotonic blow-off limit are discussed in terms of the flow filed, strain effect and conjugate heat exchange. The analysis indicates that the flow field is insignificant for the non-monotonic blow-off limit. A smaller strain effect can induce the increase of the blow-off limit from δ=0.1 to 0.2 mm, and a worse heat recirculation effect can induce the decrease of the blow-off limit from δ=0.2 to 0.4 mm. The non-monotonic blow-off limit is mainly determined by the heat loss of flame to the tube-wall and the performance of tube-wall on preheating unburned fuel. The smallest heat loss of flame to the tube-wall and the best performance of tube-wall on preheating unburned fuel result in the largest blow-off limit at δ=0.2 mm. Therefore, a moderate tube-wall thickness is more suitable to manufacture the micro-jet burner.

Pore structure and diffusion characteristics of intact and tectonic coals: Implications for selection of CO2 geological sequestration site

With the increasing amount of CO2 in the atmosphere, geologic sequestration becomes a promising measure to reduce the rising concentration of CO2. During the process of CO2 injection into coal seam to store CO2, it is critical to select the appropriate coal seam. In order to study whether the intact or tectonic coal is more suitable for CO2 geological sequestration, the pore structure characteristics and fractal dimensions of these coal samples were analyzed by mercury intrusion porosimetry, N2 (77 K)/CO2 (273 K) adsorption methods, and methane adsorption and diffusion properties of these coal samples were obtained. The results show that tectonic coals have greater pore volume and specific surface area of micropore, mesopore and macropore, while the fractal dimensions of the tectonic coals are smaller (that is, simpler pore structure, smaller pore surface roughness) than that of the intact coals, indicating that tectonic coal has more favorable pore structure to provide more adsorption sites for CO2 storage. Furthermore, better connectivity as well as the adsorption and diffusion capacity of tectonic coals have been significantly improved compared to intact coals, which indicates that CO2 is easier to migrate into the internal pore system of tectonic coal in a short time, reducing the time consumed by CO2 injection into the coal seam, thus saving the cost of CO2 storage. Interestingly, a fractal conceptual model is proposed to account for the evolution of original long and complex pores were converted into short and simple pores, and closed pores transformed into connectivity pores during the formation of tectonic coals. Therefore, the results of this study contribute to understand the advantages of tectonic coal reservoirs as target sites for CO2 geological sequestration.

Quantification of sorption, diffusion, and plasticization properties of cellulose triacetate films under mixed-gas CO2/CH4 environment

Membrane technology is employed in large-scale removal of acid gases from natural gas, and cellulose acetate is by far the most adopted material for this application. Because of its utmost industrial relevance, we analyzed the gas sorption behavior of CO2–CH4 mixtures in cellulose triacetate (CTA) at 35 °C. CO2 solubility in CTA was only slightly affected by the presence of methane, whereas competition effects sharply reduced CH4 uptake. Regardless of mixture concentration, CO2vs. CH4 solubility coefficients regressed linearly, which translated in solubility selectivities that increased as equilibrium pressures increased. Specifically, compared to other relevant glassy polymer membrane materials, CTA positioned very close to the solubility selectivity upper bound at infinite dilution and demonstrated the highest affinity to CO2 at all investigated pressures. The experimental solubility and permeability data were used in the framework of the solution-diffusion theory to determine pure- and mixed-gas concentration averaged diffusion coefficients of CTA. CO2 diffusion was essentially unaffected by mixture effects, whereas methane diffusivity was boosted by the CO2-induced plasticization of CTA. The ratio between the pure- and mixed-gas concentration averaged diffusion coefficients of methane was used to quantify the effect of plasticization on the mixed-gas performance of CTA and other relevant membrane materials previously analyzed in similar experimental studies. When we further extended this comparison in a mixed-gas diffusion analysis (at 10 atm partial pressure), we observed that CTA had lower diffusion selectivity due to an inferior size-sieving capability than a reference material, 6FDA-mPDA polyimide, but displayed superior solubility selectivity.

Diffusivities of an Equimolar Methane\u2013Propane Mixture Across the Two-Phase Region by Dynamic Light Scattering

The present contribution examines the accessibility of diffusivities across the two-phase region of an equimolar methane–propane mixture for dynamic light scattering (DLS) experiments. Heterodyne DLS experiments and theoretical calculations of the Rayleigh ratio were performed at 125 different thermodynamic states including the gas, liquid, supercritical, and the two-phase region. The present measurements document that two diffusivities can be determined simultaneously in the liquid state and saturated liquid phase for temperatures and pressures which correspond to densities larger than 1.15 times the critical density. Based on a rigorous assignment of the signals detected in this work, the slow and fast diffusivities could be associated with the Fick and thermal diffusivities. For all other thermodynamic states, a single hydrodynamic mode or signal was obtained experimentally. With the help of theoretical Rayleigh ratios as well as from the general behavior of the diffusivities as a function of temperature and pressure, the signals were identified to be related to the Fick diffusivity in the supercritical state and to a mixed diffusivity in the gas state and the saturated vapor phase. The results are discussed in connection with the behavior of the diffusivities along certain paths in the pressure–temperature projection of the phase diagram of the mixture.

REDIM reduced chemistry for the simulation of counterflow diffusion flames with oscillating strain rates

The dynamic behaviour and structures of laminar counterflow diffusion methane flames under oscillatory strain rates are investigated based on both detailed and reduced chemistry. It is known both from experiment and numerical studies that flame can deviate from its quasi-steady manner for high frequencies, which makes the well-known steady laminar flamelet model questionable. In this work, the Reaction-diffusion manifolds (REDIM) concept for simplified chemistry is applied to study the laminar counterflow diffusion flames with oscillating strain rates. In the construction of REDIM, one needs little knowledge of the studied system, and the REDIM reduced chemistry can cover both stable and unstable (extinction) regimes of the studied system. It is shown that the REDIM can capture both the steady and unsteady behaviours of the flame structures even for high frequencies, where the standard laminar steady flamelet approach is not appropriate.

The Magnitude and Drivers of Methane Ebullition and Diffusion Vary on a Longitudinal Gradient in a Small Freshwater Reservoir

Key Points CH4 ebullition rates decreased by 98% and CH4 diffusion decreased by 32% on a longitudinal gradient from reservoir inflow to dam Ebullition was driven by physical variables upstream and phytoplankton downstream; diffusion was most related to phytoplankton Despite large variation in CH4 emissions longitudinally, time series models well captured the dynamics of emissions from a small reservoir

Separation of methane from ethane and propane by selective adsorption and diffusion in MOF Cu-BTC: A molecular simulation study

Pure methane is an alternative source of cleaner energy. Although, natural gas contains around 90% of methane, there are other heavier alkanes such as ethane and propane. Presence of these heavier hydrocarbons affects the reusability of an adsorbed natural gas system (ANG). Thus, separation of these higher alkanes from methane is important. In the present study, we employed molecular simulation techniques to assess the performance of MOF Cu-BTC for separation of methane from ethane and propane at 298 K and for a range of pressure. The assessment was carried out on the basis of variety of performance metrics suitable for adsorption based separation. The performance metrics that we relied upon are adsorption selectivity, working capacity, regenerability (R%), adsorption performance score (APS), diffusion selectivity, and membrane selectivity. We investigated the performance for two equimolar binary mixtures – methane/ethane and methane/propane, and two ternary mixtures – one equimolar mixture and the other containing 90% methane, 7% ethane, and 3% propane. The adsorption selectivity of ethane over methane and propane over methane are really attractive indicating good performance of the MOF in separating the two binary mixtures. We also investigated the effect of the presence of ethane and propane on the mobility of methane. The diffusivities of methane, although, reduces by some factor in presence of ethane and propane, are sufficiently higher at all the pressures and at different compositions with ethane and propane. Finally, we assessed the performance of Cu-BTC as a membrane for separation of methane from ethane and propane.

Sorption and Diffusion of Methane and Carbon Dioxide in Amorphous Poly(alkyl acrylates): A Molecular Simulation Study.

Molecular simulations were carried out to understand the structural features and the sorption and diffusion behavior of methane and carbon dioxide in amorphous poly(alkyl acrylates) in the temperature range of 300-600 K. The hybrid Monte Carlo/molecular dynamics approach was employed to address the effects of polymer swelling and framework flexibility on the gas sorption. Simulations show that the glass-transition temperature decreases with the side-chain length of poly(alkyl acrylate), consistent with experiments. This is due to the fact that the shielding of the polar ester groups increases with the side-chain length. The simulated sorption isotherms for methane and carbon dioxide were in agreement with the experimental data. The polymer swelling becomes more pronounced, especially in the case of sorption of carbon dioxide. A significant swelling occurs, possibly because of the greater interaction between carbon dioxide and the polar ester groups in the polymers. The uptake of methane and carbon dioxide by poly(alkyl acrylates) generally increases with the side-chain length. Our simulations confirm the experimental findings that the diffusion coefficients of methane and carbon dioxide in poly(alkyl acrylates) increase with the side-chain length. Interestingly, the activation energies of gas diffusion decrease with the side-chain length. The diffusion coefficients of the penetrants have an exponential relationship with the void fraction, which is in agreement with the free volume theory.

Molecular Dynamics Simulations of Hydrophobic Nanoparticle Effects on Gas Hydrate Formation

To investigate the effect of particle-based hydrate inhibitors on hydrate crystallization, molecular dynamics (MD) simulations were performed at the interface of hydrate-forming liquids where 2 nm sized silica nanoparticles are stabilized. The hydrophobic nanoparticles were prepared by functionalization of CH3 groups on the amorphous SiO2 surface, and the three-phase contact angle of the oil (CH4 + decane), the aqueous phase (water + tetrahydrofuran (THF)), and the solid was 108.7°. Although the hydrates were not crystallized on the solid substrate, there was a trend that hydrate nucleation occurred on the side of the aqueous phase in the presence of nanoparticles. Through the four-body structural order parameter profiles, the formation of a low order parameter layer around the nanoparticles was observed, which originates from the reduced diffusivity by water binding. Adjacent to this layer, an elevation of the order parameter appeared in the same region where nucleation occurs because the less ordered layer acts as nucleation seeds. Indeed, a well of low free energy is formed in the presence of nanoparticles, in contrast to the non-nanoparticle system where no significant deviations were observed. Also, the driving force of hydrate nucleation is reduced with the increased nanoparticle injection. The nanoparticles prevented the diffusion of methane into the aqueous phase, resulting in a V-shape graph of induction time and delayed hydrate growth rate. These simulation results on the nanoparticle effect will provide a fundamental basis for improving the performance of particle-based inhibitors.

Sealing effects of marginal gas injection on oil shale in situ pyrolysis exploitation

Groundwater inrush and outward diffusion of pyrolysis products are two disadvantageous factors for oil shale in situ pyrolysis, leading to the loss of heat injected and groundwater pollution. Therefore, forming a closed in situ pyrolysis system is extremely important. The sealing effects of marginal gas injection were studied by numerical simulation. A two-dimensional model was established according to the engineering geology conditions of the National Pilot Project for Oil Shale in situ Exploitation of China (NPOSE), in which methane was regarded as the representative of pyrolysis products. The migration state of methane in dissolved and gas phases during in situ pyrolysis was revealed. The results showed that proper marginal gas injection pressure can effectively inhibit the outward diffusion of methane. Meanwhile, a marginal gas injection pressure of 0.60 or 0.61 MPa can effectively reduce water yield to ~0.5 m3/h. Consequently, a closed in situ pyrolysis system can be effectively formed by marginal gas injection, and the mass exchange inside and outside the pyrolysis zone is insulated, ensuring a clean and efficient pyrolysis process.

The effects of hydrate formation and dissociation on the water-oil interface: Insight into the stability of an emulsion

Understanding of the stability of oil-dominated emulsion, such as the oil-based drilling fluids and crude oils, is of great importance to control the gas hydrate risk management when gas hydrates formation and dissociation occurs in the emulsion. Here, a high-pressure optical capillary cell with micro-Raman spectroscopy is utilized to investigate the effects of hydrate formation and dissociation on the stability of a simulated oil-water emulsion and the micro kinetics near the oil/water interface. Both the hydrate formation and dissociation processes are stimulated in water phase near the oil-water interface. The results show that the oil can enter the water phase from the oil phase with the oil-water interface failure when hydrates rapidly growth very close to the water-oil interface, even resulting in a series of oil droplets in the water phase (oil-droplet trajectory). The interesting phenomenon are mainly caused by the concentration gradient of methane and the Marangoni effect. Subsequently, the hydrate phase dissociates, a new stable water-oil interface presents when the system becomes rebalanced. The methane diffusion behavior induced by the chemical potential gradient in hydrate formation and dissociation processes could affect the stability of the emulsion. In addition, a more profound conceptual hydrate-induced destabilization model for different types of oil-water emulsions is proposed. This study can provide a theoretical and experimental basis for the safe implementation for deep-water oil and gas drilling, hydrate drilling in permafrost and multiphase mixing transportation in pipelines.

Study on desorption and diffusion dynamics of coal reservoir through step-by-step depressurization simulation——an experimental simulation study based on LF-NMR technology

Pressure dropping control is one of the key technologies for coalbed methane surface development. Low-field nuclear magnetic resonance (LF-NMR) technology was adopted to study methane diffusion and pore dynamics of various pressure drop procedures. Calibration, isothermal desorption, pressure step-down desorption, and pore dynamic experiments were conducted with high-rank coal samples. The results include the following four aspects. 1) Calibration experiments of methane content and LF-NMR T2 spectrum in confined cylindrical coal samples show that adsorbed, porous medium-confined and bulk methane can be distinguished and quantitatively calculated using LF-NMR technology. 2) According to the desorption equation fitting using LF-NMR technology, ultimate desorption volume at different stages of depressurization is calculated and the overall desorption process has been divided into inefficiency, slow, rapid, and sensitive stage. 3) LF-NMR desorption experiments with constant confine pressure show that cumulative desorption volume of two and three step-down pressure reductions are higher than that of one-step and uniform pressure reductions. Synchronisation of rapid or sensitive desorption stage and large effective diffusion coefficient (De) are keys to achieve a high cumulative gas production. 4) De varies significantly along with the variation of equilibrium gas pressure at the inefficiency and slow desorption stage, which is mainly affected by the surface coverage. Stress compression and matrix shrinkage coupling influence the pore structure at rapid and sensitive stage, thereby on De. The different desorption results using various depressurization schemes are the reason that pore deformation behaviour causing dynamic variation of the diffusion process. The above results can provide a basis for optimising the drainage work of coalbed methane.

Improved activity of magnetite oxygen carrier for chemical looping steam reforming by ultrasonic treatment

Magnetite with high content of Fe3O4 is a promising low-cost oxygen carrier candidate for co-production of syngas and pure hydrogen via chemical looping steam reforming (CLSR). The present work investigated the effect of ultrasonic treatment on the structure of magnetite by using X-ray diffraction (XRD) technology and Energy Dispersive Spectrometer (EDS) mapping technologies, and the results were correlated to the activity of magnetite oxygen carrier for selective oxidation of methane and water splitting. The evolutions of phases and compositions of magnetite during the reaction with methane were also studied to discuss the reaction mechanism. The results showed that ultrasonic treatment significantly improves the activity of magnetite oxygen carrier for methane selective oxidation by destroying the dense structure of magnetite and promoting the interdiffusion of different elements (e.g., Fe, Al and Mg) that could enhance the interaction between the active species (Fe oxides) and inert spaces (Al and Mg oxides). The syngas production increased from 10.29 to 13.15 mmol/g and hydrogen production increased from 4.93 to 5.43 mmol/g after the ultrasonic treatment, and both the produced hydrogen and syngas show high purity (ca. 96.5% for syngas and ca. 98.9% for H2). During the reaction between methane and magnetite, the reduction of iron oxides firstly occurred in the boundary between the iron oxides and the inert components (e.g., Al/Mg oxides), which then diffused to the adjacent areas and created numbers of smaller Fe islands. The channels around the Fe islands provide pathways for methane diffusion, improving the further reduction of oxygen carrier.