Methane total combustion

The results of catalytic activity tests carried out with the INOCERMIC membrane at 1 atm are reported in Fig. 6.9 in terms of CH4, CO, CO2, H2 concentration (vol%, dry basis) at both retentate and permeate sides as function of time. During the start-up phase only CO2 was detected at the reactor outlet at the retentate side, due to the methane total combustion, while at the permeate side any gaseous component was detected. After the reactor start-up the more reducing conditions allowed to obtain a mixture of CH4, CO, CO2 and H2 at both retentate and permeate sides, with a remarkable amount of H2 with respect to the other components. The H2 concentration at the permeate side was about 2 vol%, while the temperature close to the membrane surface was about 490°C. 

However, the selectivities towards H2 and CO can be strongly influenced by the simultaneous occurrence of methane total combustion and/or secondary oxidation reactions of CO and H2, such as the formation of water and carbon dioxide, which can be favored at high reaction temperatures. 

In industry many selective oxidations are carried out in a homogeneously catalyzed process. Heterogeneous catalysts are also applied in a number of processes, e.g. total combustion for emission control, oxidative coupling of methane, the synthesis of maleic acid from butanes, the epoxidation of ethylene. Here we focus upon heterogeneous catalysis and of the many examples we have selected one. We will illustrate the characteristics of catalytic oxidation on the basis of the epoxidation of ethylene. It has been chosen because it illustrates well the underlying chemistry in many selective oxidation processes. 

By convention, the amount of excess reactant in a reaction is always defined on the basis of the reaction going to 100 percent completion for the limiting reactant. The degree of completion is not a factor in determining or specifying the excess of reactants. For example, if methane is burned with 10 percent excess air, the volume of air needed to burn the methane is calculated as though there is total combustion of methane to carbon dioxide and water. 

In this work the performance of Cr-Co spinels as catalysts for total combustion of methane is studied. The spinels were prepared from nitrate precursors. The effect of temperature and time of calcining was studied using x-ray diffraction (XRD) in order to check the crystalline structure and the absence of other phases, N2 physisorption (BET) in order to study the porous structure of these solids and Temperature Programmed Reduction (TPR) in order to determine the presence of different metallic species. 

Another application could be in the treatment of the emissions from small-scale wood burners, containing pollutants such as particulate, CO, and unbumed hydrocarbons (methane, naphthalene, etc.). Jaris and co-workers [37] arc actually studying catalysts for the total combustion of such hydrocarbons. Such a catalyst might be coupled with high-temperature ceramic filters for solving at once the entire range of pollution problems entailed by the considered emissions. 

This example can be applied to a broad class of catalytic reactions but it is much more obvious for partial oxidation reactions where secondary reactions (total combustion) result in a dramatic decrease of selectivity. This is the case with methanol decomposition and methane conversion, where the intensification of gas-phase catalytic operations in micro- or nanochannels clearly appears. 

Two mechanisms have been proposed for the POM reaction (i) The Combustion and Reforming Reactions mechanism (CRR). In this, the methane is combusted in the absence of oxygen in the first part of the catalytic bed, producing CO2 and H2O. Along the rest of the bed, and after total oxygen conversion, the remaining methane is converted to CO + H2 by SMR and CO2 reforming (reaction (2)). (ii) The Direct Partial Oxidation mechanism (DPO). CO + H2 is produced directly from methane by recombination of CHX and O species at the surface of the catalysts. 

However, it should be noted that the total combustion of methane to C02 also is favored. Therefore, the basic challenge is to find a system including a catalytic system that kinetically favors the formation of hydrocarbons compared to the total combustion reaction. 

In contrast, surface sodium was only and significantly detected on the two samples for which the NaOH base was used. Thus, the method using Au(CH3COO)3 and urea (hereafter named the ureacetate method) leads to highly-loaded samples with well-dispersed gold particles, no Cl and no Na surface contaminants. The catalytic performances of the Au/Ti02 catalysts were evaluated in the total combustion of methane. Au/Ti02 samples exhibited very poor activity in this test reaction. 

Structured Pd/y AI2O3 catalysts on FeCrAlloy fibers for total combustion of methane... 

A dip coating method is applied in order to allow the deposition of Pd/y-AlaOa on FeCrAlloy-type fibers. All samples are investigated by means of XRD, XPS, CO chemisorption and nitrogen physisorption. The catalytic fibers are tested in the total combustion of methane showing that O2/H2 gaseous pretreatment, which takes place before the catalytic reaction, is strongly beneficial as confirmed by a better dispersion of Pd particles. 

The catalytic fibers, presented in this work, are prepared by dip-coating of FeCrAlloy-type fibers into a slurry made of Pd/y-AlaOs. The catalytic activities of the samples are tested in total combustion of methane and, in order to correlate the catalytic performances with the solid state properties, they are characterized by XRD, XPS, CO chemisorption, specific surface area and porosity measurements. The importance of O2/H2 pretreatment and the implications on both physico-chemical properties and catalytic activity of the catalysts is discussed. 

The 9A2B1 sample was used for supporting palladium (2 wt %). The resulting catalyst and the 2 wt % Pd/Al203 reference sample were studied in the methane catalytic combustion. Catalytic tests were performed in the temperature range of 300°C-900°C using 1% CH4, 4 % O2 and N2 balance at total flow rate of 36 L/h. Prior to catalytic activity measurements, samples were treated in the reaction mixture up to 900°C for stabilisation. Catalytic run consisted of heating from 300°C to 900°C at l°C/min. 

The R-based mixed oxides with p-type electronic conductivity as well as the superconducting oxides were found to be inactive for the OCM reaction, and only total combustion of methane was observed. The only exception was Lao Sro MnOj 5, which is probably an n-type conductor under the experimental conditions used. This solid is likely to be n-type under reducing atmosphere due to the stability of the Mn2+ ion. LaftsSro.2Mn03and LaFeo.sNdo.2O3 5, both considered n-type conductors, had low selectivity to C2, whereas La-Sr-Y-O systems, either ionic conductors or insulators, presented a good selectivity to those compounds (table 8). 

Conventional combustion processes generally proceed at high temperatures and lead to formation of undesired nitrous oxides. Combustion catalysts are intended to achieve fast total combustion of the fuel at lower temperatures. Catalytic combustion of methane in a gas turbine has already been developed by a company in lapan, where a research society for catalytic combustion has also been established. However, the complex metal oxide catalysts do not yet have sufficient temperature stability and resistance to catalyst poisons. 

The majority of the oxides tested totally combusted methanol below 400°C. Over the oxide Sb203, methanol showed exceptional stability, as only 3% was converted at 500°C, whereas the oxides M0O3, Nb20s, Ta20s, and WO3 did not readily combust methanol. However, methanol conversion was high and major reaction products were formaldehyde and dimethyl ether, which are desirable byproducts from a methane partial oxidation process. 

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