Methane oxidation reactions

Methane oxidations occur only by intermediate and high temperature mechanisms and have been reported not to support cool flames (104,105). However, others have reported that cool flames do occur in methane oxidation, even at temperatures >400 ° C (93,94,106,107). Since methyl radicals caimot participate in reactions 23 or 24, some other mechanism must be operative to achieve the quenching observed in methane cool flames. It has been proposed that the interaction of formaldehyde and its products with radicals decreases their concentrations and inhibits the whole oxidation process (93). 

The reported characteristics of methane oxidation at high pressures are interesting. As expected,the reaction can be conducted at lower temperatures eg, 262°C at 334 MPa (3300 atm) (100). However, the cool flame phenomenon is observed even under these conditions. At high pressures. 

Aldehydes are important products at all pressures, but at low pressures, acids are not. Carbon monoxide is an important low pressure product and declines with increasing pressure as acids increase. This is evidence for competition between reaction sequence 18—20 and reaction 21. Increasing pressure favors retention of the parent carbon skeleton, in concordance with the reversibiUty of reaction 2. Propylene becomes an insignificant product as the pressure is increased and the temperature is lowered. Both acetone and isopropyl alcohol initially increase as pressure is raised, but acetone passes through a maximum. This increase in the alcohoLcarbonyl ratio is similar to the response of the methanoLformaldehyde ratio when pressure is increased in methane oxidation. 

The overall process for producing a 1 1 CO to ratio by partial methane oxidation and the water gas shift reaction is represented by equation 5. 

In the Sabatier reaction, methane and water are formed over a nickel— nickel oxide catalyst at 250°C. The methane is recovered and cracked to carbon and hydrogen, which is then recycled ... 

Miscellaneous Reactions. Ethylene oxide is considered an environmental pollutant. A study has determined the half-life of ethylene oxide ia the atmosphere (82,83). Autodecomposition of ethylene oxide vapor occurs at - 500° C at 101.3 kPa (1 atm) to give methane, carbon monoxide, hydrogen, and ethane (84—86). 

The lower than expected yields can be explained by the nature of methane oxidation to methanol in these bacteria. This reaction, catalysed by methane mono-oxygenase, is a net consumer of reducing equivalents (NADH), which would otherwise be directed to ATP generation and biosynthesis. In simple terms the oxidation of methane to methanol consumes energy, lowering the yield. 

Interestingly, this situation is very different when we consider activation of NH3 or H2O by coadsorbed O. This would typically occur in the Ostwald reaction that oxidizes ammonia to NO or the methane reforming reaction in which CH4 reacts with O2 or H2O to give CO, CO2, and H2. 

From the results discussed above as well as from the literature data [5-10,12-14] it follows that an important role of O2 in the SCR process is to convert NO into NOj. The latter then initiates methane oxidation into CO, and is itself reduced into NO and N2. Both NO, and O2 may participate in CH4 oxidation (Fig. 1B) and the ratio between the rates of these competitive oxidation reactions will be critical for the selectivity of the SCR process. Hence, the absolute rates of CH4 oxidation by Oj were compared with those occurring in the SCR process. The rates of these reactions were determined under different reaction conditions (using the... 

Hoffmann, C., Schmidt, H.W. and Schuth, F. (2001) A multipurpose parallelized 49-channel reactor for the screening of catalysts methane oxidation as the example reaction. J. Catal., 198, 348. 

Figure 2.10 provides a thermodynamic equilibrium molar fraction of the products of CPO of methane as a function of temperature. It is evident that at temperatures above 800°C, hydrogen and CO (in molar ratio of 2 1) are two major products of the reaction. The oxidant (oxygen or air) and the hydrocarbon feedstock (e.g., methane) are premixed in a mixer... 

Hydrogen production by SIP can be accomplished through direct and indirect employment of hydrocarbon feedstocks (e.g., NG). In the direct employment method, iron oxide directly reacts with methane or other hydrocarbons to produce the reduced form of iron oxide and methane oxidation products, according to the following generic reaction ... 

For illustration, we consider a simplified treatment of methane oxidative coupling in which ethane (desired product) and CO, (undesired) are produced (Mims et al., 1995). This is an example of the effort (so far not commercially feasible) to convert CH, to products for use in chemical syntheses (so-called Q chemistry ). In this illustration, both C Hg and CO, are stable primary products (Section 5.6.2). Both arise from a common intermediate, CH, which is produced from CH4 by reaction with an oxidative agent, MO. Here, MO is treated as another gas-phase molecule, although in practice it is a solid. The reaction may be represented by parallel steps as in Figure 7.1(a), but a mechanism for it is better represented as in Figure 7.1(b). 

Application to Methane Oxidation. This selection of an appropriate initial model can be accomplished as shown here for the complete oxidation of methane. A general representation of the surface reaction model is (K12)... 

The fact that reaction (3.82) may not proceed as written at high temperatures may explain why methane oxidation is slow relative to that of other hydrocarbon fuels and why substantial concentrations of ethane are found [4] during the methane oxidation process. The processes consuming methyl radicals are apparently slow, so the methyl concentration builds up and ethane forms through simple recombination ... 

The previous discussion has focused on the properties of perovskite materials rather than on their performance as anodes. The number of actual fuel-cell studies is more limited, but this literature has been reviewed recently by Irvine. Various perovskites have been investigated as potential SOFC anode materials however, these early efforts were hampered by low electrochemical activity toward methane oxidation,poor anode structure,or insufficient electrode conductivity. Most recently, Tao and Irvine demonstrated that an anode based on (Lao.75Sro.25)o.9Cro.5Mno.503 can provide reasonable power densities at 1173 K in 3% humidified CH4. Barnett and co-workers also reported stable power generation with methane and propane fuels on an anode based on LaCr03 however, they reported that the addition of Ni, in levels too small to affect the conductivity, was crucial in providing activity for the electrochemical oxidation reactions. 

However, the combustion process for methane requires no fewer than 325 individual mechanistic steps (elementary reactions) to be accurately described, rather than the one-step route shown above. As such, incomplete combustion is a common occurrence and ROS are pervasive byproducts of that phenomenon, affecting an engine s fuel efficiency and producing atmospherically detrimental emissions. Moreover, combustion varies with system temperature, as different oxidative pathways become accessible, as well as fuel/oxidizer ratio (equivalence ratio). By examining the representative cases of methane oxidation at high and low temperatures, this phenomenon becomes clearer. 

Notably, the Gas Research Institute s mechanism (GRI-MECH) for methane combustion is well-established, drawing on research from several groups over several decades to define and calibrate kinetic and thermodynamic data for each elementary reaction step. Additional mechanisms" for methane oxidation are also available and updated periodically to include the most recent data. 

Write the reactor mass-balance equations that must be solved for the methane oxidation reaction... 

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