O2-CO2 Mixtures Research Articles

Efficient Oxidative Dehydrogenation of Ethylbenzene over K/CeO2 with Exceptional Styrene Yield

Oxidative dehydrogenation (ODH) is an alternative for styrene (ST) production compared to the direct dehydrogenation process. However, ODH with O2 or CO2 suffers from either over-oxidation or endothermic property/low ethylbenzene conversion. Herein, we proposed an ODH process with a CO2-O2 mixture atmosphere for the efficient conversion of ethylbenzene (EB) into styrene. A thermoneutral ODH is possible by the rationalizing of CO2/O2 molar ratios from 0.65 to 0.66 in the temperature range of 300 to 650 °C. K modification is favorable for ethylbenzene dehydrogenation, and 10%K/CeO2 achieved the highest ethylbenzene dehydrogenation activity due to the enhanced oxygen mobility and CO2 adsorbability. The catalyst achieved 90.8% ethylbenzene conversion and 97.5% styrene selectivity under optimized conditions of CO2-4O2 oxidation atmosphere, a temperature of 500 °C, and a space velocity of 5.0 h−1. It exhibited excellent catalytic and structural stability during a 50 h long-term test. CO2 induces oxygen vacancies in ceria and promotes oxygen exchange between gaseous oxygen and ceria. The ethylbenzene dehydrogenation in CO2-O2 follows a Mars-van Krevelen (MvK) reaction mechanism via Ce3+/Ce4+ redox pairs. The proposed ODH strategy by using oxygen vacancies enriched catalysts offers an important insight into the efficient dehydrogenation of ethylbenzene at mild conditions.

Combustion characteristics of liquid carbon dioxide-dried coal at different pressures of CO2–O2 mixture

In this study, the coal combustion behavior was determined at different combustion pressures, including supercritical CO2–O2 conditions. The coal mass and mass ratio of oxygen-to-coal remained constant. The drying impact of liquid carbon dioxide (LCO2) on supercritical combustion was investigated using raw coal with a high moisture content (20%). The extent of combustion (i.e., coal conversion) and particle temperature were measured using a weighing method and a two-color pyrometer, respectively. The effects of oxygen diffusion and volatile release on the extent of combustion were explored as pressures varied from subcritical to supercritical conditions. The temperature was further increased at the same supercritical pressure to study the effect of oxygen diffusivity on combustion. The phase change behavior of CO2–O2 mixture is identified and visualized. The kinetic and diffusion data were used to determine the Thiele modulus and the resultant effectiveness factor. The role of the hydrophobic surface groups in coal in the LCO2 drying process was investigated using Fourier Transform Infrared (FTIR) spectroscopy. In addition, the time-resolved water contact angle with the LCO2-treated coal sample was determined using the in-situ pendent drop method at atmospheric pressure.

Study of the dielectric breakdown strength of CO2–O2 mixtures by considering ion kinetics in a spatial–temporal growth avalanche model

The gas mixture CO2–O2 has been considered as an insulation and arc-quenching medium in gas-insulated switchgears. In this paper, the dielectric breakdown properties of CO2–O2 mixtures at different O2 concentrations and gas pressures were studied theoretically by considering ion kinetics in a spatial–temporal growth avalanche model. A kinetic scheme that includes all the main reactions likely to occur in CO2–O2 mixtures is presented. An improved method to calculate the dielectric strength of the gas mixture is developed, based on an avalanche model that considers both spatial growth and temporal processes. Next, the reaction rates of ionization, attachment, detachment and ion conversion, the effective ionization Townsend coefficient αeff/N, and reduced critical electric field strength (E/N)cr in CO2–O2 mixtures at different mixing ratios and gas pressures are analyzed in detail. Finally, a pulsed Townsend experiment is performed to verify the validity and accuracy of the calculation method. Based on this, one detachment reaction rate is modified to yield more accurate results. Better consistency between the results and the experimental values supports the validity of the kinetic system, reaction rates, and the improved calculation method.

Primary Study on Medium and Low Carbon Ferromanganese Production by Blowing CO2-O2 Mixtures in Converter

The production of medium- and low-carbon ferromanganese (M-LCFeMn) using the converter method has not been industrialized to date in China due to the high manganese loss and serious erosion of the furnace lining. To solve the above problems and to improve the refining technology of M-LCFeMn, the introduction of CO2 gas into the traditional converter process is proposed. In this study, the oxidation behavior of C and Mn in various conditions was analyzed by blowing different proportions of CO2-O2 mixed gas into the high-carbon ferromanganese (HCFeMn) melt. The results showed that it is feasible to make M-CFeMn by blowing CO2-O2 mixtures, and the Mn loss can be effectively reduced during the decarburization process. It is considered that when the proportion of CO2 reaches 25%, the mixed gas has the best effect on the decarburization and manganese preservation under current experimental situation. Two hypotheses and corresponding rate formulas of decarburization kinetics by using pure oxygen are put forward, and the effect of CO2 on the kinetics of decarburization was studied according to different hypotheses.

CO-O₂ 예혼합화염의 후류상호작용에 관한 H₂O 첨가 효과

Numerical study is conducted to understand the effects of additional H₂O on downstream interaction in CO-O₂ counterflow premixed flames. Adding 1% H₂O to fuel side for freely-propagating premixed flame with CO-O₂ mixture increases laminar burning velocity from 5.3 cm/s to 81.8 cm/s in a way that the main oxidation reaction is changed from CO + 0.5O₂ → CO₂ to CO + OH → CO₂ + H. When global strain rate reaches 23.5 s<SUP>-1</SUP> for interacting CO-O₂ premixed flames, the flames cannot be sustained. While for (0.1% H₂O + CO)-O₂ premixed flames, the flame is extinguished at 2687 s<SUP>-1</SUP>. Because the fuel Lewis number and effective one are larger than unity, downstream interaction are mainly through chemical one. For lean-lean (rich-rich) flames, the production of O (H) radical is vigorous, resulting in the formation of OH via H₂O + O → OH + OH (H₂O + H → OH + H₂). These different chemical situations can influence downstream interactions in various flame configurations. Such interesting aspects in chemical interactions are presented and discussed in detail.

Theoretical study of the CO2-O2 van der Waals complex: potential energy surface and applications.

A four-dimensional-potential energy surface (4D-PES) of the atmospherically relevant carbon dioxide-oxygen molecule (CO2-O2) van der Waals complex is mapped using the ab initio explicitly correlated coupled cluster method with single, double, and perturbative triple excitations (UCCSD(T)-F12b), and extrapolation to the complete basis set (CBS) limit using the cc-pVTZ-F12/cc-pVQZ-F12 bases and the l-3 formula. An analytic representation of the 4D-PES was fitted using the method of interpolating moving least squares (IMLS). These calculations predict that the most stable configuration of CO2-O2 complex corresponds to a planar slipped-parallel structure with a binding energy of V ∼ -243 cm-1. Another isomer is found on the PES, corresponding to a non-planar cross-shaped structure, with V ∼ -218 cm-1. The transition structure connecting the two minima is found at V ∼ -211 cm-1. We also performed comparisons with some CO2-X van der Waals complexes. Moreover, we provide a SAPT analysis of this molecular system. Then, we discuss the complexation induced shifts of CO2 and O2. Afterwards, this new 4D-PES is employed to compute the second virial coefficient including temperature dependence. A comparison between quantities obtained in our calculations and those from experiments found close agreement attesting to the high quality of the PES and to the importance of considering a full description of the anisotropic potential for the derivation of thermophysical properties of CO2-O2 mixtures.

Experimental Density Data of Three Carbon Dioxide and Oxygen Binary Mixtures at Temperatures from 276 to 416 K and at Pressures up to 20 MPa

In the context of power-to-gas systems, including power-to-gas–oxyfuel, possible storage of a mixture of CO2 and O2 requires density data information and evaluation of equations of state. Densities of three CO2–O2 binary systems were measured using a vibrating tube densitometer, and the forced path mechanical calibration method in the gas, liquid, and supercritical regions between 276 and 416 K and at pressures up to 20 MPa (maximum expanded uncertainties U(p) = 0.0005 MPa, U(T) = 0.3 K, and U(ρ) = 15 kg·m–3). The mole fractions of the prepared CO2–O2 mixtures are 0.726:0.274, 0.517:0.483, and 0.872:0.128. The Peng–Robinson cubic equation of state (PR EoS) and the EoS-CG, based on GERG-2008 (and implemented in Refprop v10.0), were considered for the analysis of the data. Comparisons were carried out with literature data. It appears that the data are overall better predicted by the EoS-CG than the PR EoS

The Phenomenon of Flame Jump in Counter–current Flame Propagation in Biomass Packed Beds – Experiments and Theory

ABSTRACT In this paper the phenomenon of flame jump vis-a-vis steady propagation in biomass packed beds in counter-current mode is discussed. By analyzing the fuel flux and propagation rate data from experiments with a range of oxidizers, namely, air, O2-N2, O2-CO2, and O2-steam mixtures, parameter regimes of steady propagation, and flame jump are identified. A theoretical basis for this classification is developed by analyzing the thermo-chemical conversion of single particles subject to flow and thermal conditions in a packed bed. The ratio of the ignition () to devolatilization () times is shown to emerge as the controlling parameter in determining the flame propagation regimes. It is found from the theoretical analysis that steady propagation occurs for < 2 and transition to flame jump occurs if 2. Operational zones of a packed bed biomass system is mapped using the predicted ratio of as a function of volatiles-based equivalence ratio (). Implications of these results to practical ligno-cellulosic biomass combustion and gasification systems, especially using oxygen-steam mixtures for hydrogen generation, are brought out.

A novel self-sustained single step process for synthesizing activated char from ligno-cellulosic biomass

A gasification-based single step process is reported for synthesizing moderately activated char (surface area in the range of 100–600 m2/g) from ligno-cellulosic biomass. The novelty of the process lies in the use of mixtures of O2-CO2 and O2-steam as gasification agents to obtain significantly higher activation in a single-step as compared to gasification with air (<100 m2/g) in a self-sustained process. Experiments were conducted in a counter-current packed bed reactor with three biomass (agro-residue pellets, wood based pellets and coconut shells). Yield and Brunauer-Emmett-Teller (BET) surface area are reported for char obtained from experiments covering an overall equivalence ratio range of 3.9–1.2. The volatiles equivalence ratio (ϕv), identified as the unifying parameter for analysis from the earlier studies of the authors, emerges as the relevant parameter to characterize the relationship between yield and activation as well. With mixtures of O2-CO2, as ϕv decreases from 2.7 to 1.1 (fuel-rich to stoichiometric) the yield decreases from 100 to 10% (of fixed carbon content) with a corresponding increase in BET surface area from 10 to 570 m2/g. With mixtures of O2-steam, as ϕv decreases from 2.1 to 1.04 the yield decreases from 92 to 5% with a corresponding increase in BET surface area from 106 to 561 m2/g. The zone of transition of activation from reaction rate limited to diffusion limited conditions is identified from the experimental results. It occurs at around ϕv ~ 1.7 (T ~ 1450 K) for O2-CO2 mixtures and around ϕv ~ 2.1 (T ~ 1200 K) for O2-steam mixtures. The activation obtained with the current single step process is an order of magnitude higher compared to that from other single step processes like pyrolysis and air-gasification. Char obtained from the current process can be used as activated biochar and for other applications requiring only moderate activation (100<SBET<600 m2/g).

Competitive Oxidation of O2 and CO2 in Fe–C Melts Using Isotope Tracing Method

The application of an O2–CO2 mixture gas in the steel industry makes an important contribution to energy savings and emissions reduction. To clarify the reaction mechanisms of this gas mixture, isotope gases (18O2 and 13CO2) are used to study the competitive oxidation of O2 and CO2, for Fe–C melts at 1873 K. Changes in gas mix composition during the decarburization of Fe–C melts with O2–CO2 are monitored using an online mass spectrometer. The results show that when the O2/CO2 ratio at the inlet is 4:1, the O2 oxidation rate is approximately four times that of CO2. O2 and CO2 decarburization abilities are found to be basically the same, even when the O2 supply is more than sufficient. As for the ability to oxidize Fe in molten steel, when the ratio of O2/CO2 at the inlet is 4:1, the O2 oxidation rate is approximately seven times that of CO2, allowing us to conclude that CO2 is less able to compete with molten iron oxide.

Hydrogen concentration effects on swirl-stabilized oxy-colorless distributed combustion

Oxy-colorless distributed combustion is a novel combustion technique to achieve more uniform and controlled thermal field, enhance flame stability, and reduce emissions including combustion noise. Colorless distributed combustion (CDC) conditions was achieved through controlled entrainment of hot reactive gaseous species. This allowed for reduction of oxygen concentration to provide improved mixture preparation for distributed combustion with the reaction zone distributed across the entire volume of the combustor. In this study, oxy-colorless distributed combustion was examined for fuel flexibility using different mixtures of methane and hydrogen diluted with 10% each of N2 and CO2 to represent low heating value fuels. The oxidizer used was an O2-CO2 mixture that eliminated the formation of NOx from the combustor. The evolved OH* chemiluminescence signatures were recorded for various hydrogen concentrations in the fuel at equivalence ratios in the range of 0.6–0.9 under distributed combustion condition. At Φ = 0.9, transition to CDC initiated at oxygen concentrations of 17%, 19% and 21% for fuels having a hydrogen concentration of 60%, 50% and 40% (on volume basis), respectively. Differences in transition point were attributed to higher flame speed associated with more hydrogen in the fuel. Gaseous fuel having high hydrogen concentration provided more distributed OH* flame structures at lower oxygen concentration while the flame stability enhanced at higher oxygen concentration. Oxy-colorless conditions provided only 10–30 ppm CO even at high oxygen concentration and high (0.9) equivalence ratio. The results showed fuel flexibility from different heating value gaseous fuels having high hydrogen content. The results provided very low CO emission and enhanced flame stability under oxy-colorless distributed combustion for the low heating value fuels using O2-CO2 mixture as the oxidant.

Adsorption of interacting binary mixtures on heterogeneous surfaces: theory, Monte Carlo simulations and experimental results

The development of a method for the prediction of the amounts adsorbed on a surface is of interest due to its application in various areas such as the recovery of gases and the transport thereof in porous beds. Various scientific contributions have been carried out on the adsorption under equilibrium conditions of gas mixtures. In the present work, a theoretical cluster approximation (CA) is analyzed against the experimental and simulation behavior of the adsorbed phase of a binary mixture of gases. The substrate is modeled as an heterogeneous surface and a combination of repulsive lateral interactions between adsorbed particles of the same species is considered. Under these conditions, a rich variety of structural orderings were observed in the adlayer. CA results were compared with Monte Carlo simulations, showing a very well agreement over the range of parameters investigated. Finally, the theoretical formalism was used to model experimental data of CO–O2 mixtures on a template-synthesized 5A zeolite. The study presented here has shown that CA model is a good one considering the complexity of the physical situation, which is intended to be described, and could be more useful in interpreting experimental data of adsorption of interacting binary mixtures on heterogeneous surfaces.

Auto-ignition characteristics of diesel fuel in an O2–CO2 mixture

To study diesel fuel auto-ignition in an O2–CO2 mixture, a TZ (temperature zone) model is proposed. The effect of O2 and CO2 on reaction rate is considered. The relationship between temperature and ignition delay time is obtained. Different reduced mechanisms based on steady-state assumptions are applied in three temperature zones (T ≤ 800 K, 800 K &lt; T ≤ 1100 K, T &gt; 1100 K). The TZ model is coupled to KIVA-3V code for simulation calculations. To support the simulations, a constant-volume combustion bomb test bench is set up to visualize diesel fuel auto-ignition in air (21%O2–79%N2), a 53%O2–47%CO2 mixture, and a 61%O2–39%CO2 mixture. Ignition delay time and the flame image in these three conditions are compared and analyzed. Then the flame temperature contour and the flame lift-off length in a 53%O2–47%CO2 mixture and a 61%O2–39%CO2 mixture are analyzed. The results show that diesel fuel auto-ignition can be achieved in the tested O2–CO2 mixture. The TZ model can predict the auto-ignition characteristics of diesel fuel in a 53%O2–47%CO2 mixture and a 61%O2–39%CO2, with errors of 12% and 10%, respectively. In these two conditions, the ignition delay time and flame lift-off length are shorter than they are in air.

High Efficiency Dephosphorization by Mixed Injection during Steelmaking Process

A new dephosphorization technology with introduction of partial CO2 into O2 is proposed in this paper. Thermodynamic characteristics and dephosphorization mechanism by introduction of CO2 are analyzed and the results show that using CO2 as partial dephosphorization oxidant can change the selective oxidation condition of steelmaking system, control the temperature of bath, strengthen the stirring ability, and finally obtain a high efficiency of dephosphorization. Meanwhile, the relationship between selective oxidation temperature T of C and P, partial pressure of CO gas, slag activity, and compositions of melt is obtained as follows. Furthermore, dephosphorization trails are carried out in a 30t steelmaking converter and a 300t special dephosphorization converter, respectively. It is concluded that the new technology of injecting CO2–O2 mixture is beneficial to reduce phosphorus content in liquid steel, increase the partition ratio of phosphorus, and reduce iron loss of slag.

Investigation of coal entrained-flow gasification in O2-CO2 mixtures for oxy-fuel IGCC

Purpose of the study is to obtain fundamental knowledge about Kuznetsk bituminous coal entrained-flow gasification in O2-CO2 mixtures for oxy-fuel IGCC. To achieve this purpose, it is necessary to carry out experimental and numerical studies. To obtain universal knowledge, “CKTI” single-stage gasifier with fuel consumption of 5-15 kg/h was chosen as an experimental installation. In order to identify the most interesting and informative experimental regimes, a number of computational studies have been carried out, whose results are given in the paper. A numerical (CFD) model has been developed, which includes all the submodels necessary for the study. Two series of numerical calculations were carried out. In the first series, the composition of the blast (O2 = 0-100%, CO2 = 0-100%) and the oxygen ratio were varied at constant consumption of coal and blast. In the second series, the composition of the blast (O2 = 0-100%, CO2 = 0-100%) and the coal consumption were varied at constant oxygen ratio and blast flow rate. The study allowed predicting and analyzing the temperature distribution and the syngas components content along the gasifier height with different CO2 concentrations in the blast.

Effects of H2 Enrichment and Inlet Velocity on Stability Limits and Shape of CH4/H2–O2/CO2 Flames in a Premixed Swirl Combustor

The stability characteristics of swirl-stabilized hydrogen-enriched CH4–O2–CO2 premixed flames were experimentally investigated to determine the effects of hydrogen addition and inlet velocity on flame stability under stoichiometric conditions (φ = 1.0). The stability limits in terms of blowout and flashback were identified over wide ranges of hydrogen fraction (HF: %H2 in H2–CH4 mixture) and oxygen fraction (OF: %O2 in O2–CO2 mixture) at fixed inlet velocity of 6 m/s. The lines of stability limits were plotted against the contours of adiabatic flame temperature (AFT), power density (PD), and Reynolds number (Re) to understand the physics behind the flame extinction mechanism. The effects of inlet velocity on flame stability limits under hydrogen enrichment were investigated by comparing the stability maps of the combustor for different inlet velocities, namely, 4.4, 5.2, and 6.0 m/s. The results show a stable combustion zone in the OF–HF space in the ranges of OF from 16 up to 44% and HF from 0 (pure CH4) up to 90%. However, increasing HF restricts the range of both OF and mixture Re for stable flame operation. Flame shapes at different inlet velocities were captured using a high-definition camera and compared to investigate the effects of OF and HF on flame stability. Flame visualizations near flashback and blowout limits were recorded to explore the physics behind flame extinctions mechanisms. The effect of reaction kinetic rates on the flame stability was investigated by recording flame shapes at fixed adiabatic flame temperature. The results show that the flames become gradually shorter and more compact with the increase in HF because of the enhanced reaction rates within the combustion zone. Insignificant changes in the flame shape were observed at fixed AFT or fixed Re operation.

Oxygen-blown operation of the TwoStage Viking gasifier

The TwoStage Viking gasifier from the Technical University of Denmark is being further developed for biofuel synthesis applications. In order to optimize the gasification process, it is suggested to apply an O2-CO2 gas mixture as gasification medium, instead of air, to limit N2-dilution of the product gas. It is found through a modeling study that the system is expected to achieve operating conditions in the range of air-blown operation, when 21v% O2 in CO2 is applied, and nearly identical parameters as the concentration is increased to 30v%. An experimental campaign with the 80kWth Viking pilot plant using 21v% oxygen confirms this, as operation temperatures are seen to slightly decrease the partial oxidation (POX) temperature by 52–69 °C and grate temperature by 31–36 °C. Tests with 25v% oxygen were also carried out with slightly higher temperatures. Detailed gas analysis showed that N2 had effectively been reduced to a few percent and that tar and sulphur levels were similar to the very high standards of the air-blown operation: only a few mg/Nm3 of tar and <3 ppm sulphur were detected. The lone gas cleaning, a bag filter, was found to be virtually inactive for capturing these impurities. Hence, the gasifier had been successfully demonstrated with O2-CO2 mixtures and is expected to be able to maintain its simple design, whilst enabling very high system efficiency.

Study of the dielectric breakdown properties of CO2–O2 mixtures by considering electron detachments from negative ions

The dielectric breakdown properties of CO2-O2 mixtures at different O2 concentrations and gas pressures were studied in this paper, with electron detachments from negative ions taken into consideration. The influences of the electron detachment on the reduced effective ionization coefficients αeff/N, the critical reduced electric fields (E/N)cr, the critical electron temperature Tcr, the breakdown reduced electric fields (E/N)breakdown, and the breakdown electron temperature Tbreakdown were analyzed for the CO2–O2 mixture. Based on the results, it was found that an enhancement in αeff/N and a decrease in (E/N)cr and Tcr were caused by the electron detachment, which appeared to be more significant at relatively low E/N and low gas pressures. With the increase in the pd product, both (E/N)breakdown and Tbreakdown in the CO2–O2 mixture decreased first and then tended to be a constant at relatively high pd products.

Study of the synergistic effect in dielectric breakdown property of CO2–O2 mixtures

Sulfur hexafluoride, SF6, is a common dielectric medium for high-voltage electrical equipment, but because it is a potent greenhouse gas, it is important to find less environmentally harmful alternatives. In this paper we explore the use of CO2 and O2 as one alternative. We studied the synergistic effect in a mixture of CO2 and O2 from both macroscopic and microscopic perspectives. The effect leads to a dielectric strength of the mixture being greater than the linear interpolation of the dielectric strengths of the two isolated gases. We analyzed the critical reduced electric field strength, (E/N)cr, the breakdown gas pressure reduced electric field, E/p, and the breakdown electron temperature, Tb, and their synergistic effect coefficients for various CO2 concentrations and various products of the gas pressure times the gap distance (pd). A gas discharge and breakdown mechanism in a homogenous electric field is known to be controlled by the generation and disappearance of free electrons, which strongly depend on the electron temperature. The results indicate that adding a small amount of O2 to CO2 can effectively improve the value of (E/N)cr and bring a clear synergistic effect. In addition, significantly different variation trends of the synergistic effect in the E/p and Tb of CO2-O2 mixtures at various CO2 concentrations and pd values were also observed.

Bifunctional application of lithium ferrites (Li5FeO4 and LiFeO2) during carbon monoxide (CO) oxidation and chemisorption processes. A catalytic, thermogravimetric and theoretical analysis

The CO oxidation and subsequent CO2 chemisorption processes were evaluated using two lithium ferrites, Li5FeO4 and LiFeO2, as possible catalytic and captor materials. The analysis of the bifunctional process was dynamic and isothermally evaluated using catalytic and thermogravimetric techniques, while solid final products were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). These experiments were performed using a CO-O2 mixture or only CO as gas flows. In addition, different ab initio thermodynamic calculations were performed to elucidate the theoretical viability of these processes. Thermogravimetric and catalytic results clearly showed that both lithium ferrites were able to perform the CO oxidation and CO2 chemical capture. The efficiency and reaction mechanism varied as a function of the lithium ferrite (Li5FeO4 or LiFeO2), gas mixture and temperature. As it would be expected, Li5FeO4 sample presented better chemisorption properties than LiFeO2, regardless the gas mixture employed (CO or CO+O2). Moreover, catalytic tests showed that the reaction process was produced even in the oxygen absence. In such a case, both lithium ferrites released the oxygen necessary for the oxidation process with a consequent iron reduction, as it was observed by XRD. Based on the obtained experimental and theoretical results, reaction mechanisms were proposed for each lithium ferrite into this bifunctional process. Finally, the best catalytic behavior was obtained with the Li5FeO4-CO-O2 system, where high CO conversions (50–75%) were observed between 500–650°C and T>800°C. Also according to TGA results, this system at T>700°C presented the highest ability for capture the CO2 previously formed during the CO oxidation process (∼45%).