Methane slow combustion

According to a recent review (63), the important features of the slow combustion of methane are ... 

The elimination of these secondary reactions from the slow combustion of methane greatly simplifies the problem. 

Since, if once formed, it would be impossible for them to be oxidised away in accordance with equations (n), (m), or (vi), their detection and isolation would be an easy matter, and hence it may be postulated that under normal conditions of slow combustion the methane is not first dissociated into its constituent elements. It is equally clear that the carbon monoxide and water which were always found when the supply of oxygen was insufficient to completely oxidise the methane, are two of the primary disintegration products of the partial oxidation of the methane molecule at these temperatures, for these are too low for reaction (vi) to take place. 

In the slow combustion of ethane, on the other hand, ethyl alcohol has actually been detected amongst the oxidation products,1 and an analogous scheme is suggested 2 to that for methane. Thus ... 

Early studies of slow combustion and ignition characteristics of organic compounds showed that although methanol, ethanol and propanol are more readily oxidized than methane, ethane and propane, the situation is reversed with butanol and the higher homologues [1—4]. 

The slow combustion [93] is measurable at 380 °C, but there is no low temperature mechanism, nor have cool flames been observed [45]. At 560 °C, in a flow system, mixtures of air and methyl formate ignite with explosive violence [47(a)]. The preflame reaction produces methane and methanol. 

Harkness and Murray [109], while agreeing that in slow combustion all the sulphur is converted to sulphur dioxide, failed to find methanol. Carbon dioxide and methane were also absent, but a little formaldehyde was present in the products. 

Bone WA, Allum RE. The slow combustion of methane. Proc R Soc London 1932 A134 578—91. 

Newitt DM, Gardner JB. The initial formation of alcohols during the slow combustion of methane and ethane at atmospheric pressure. Proc R Soc 1936 A154 329—35. 

Tripathy N. Slow combustion of methane at elevated pressures. Isr J Chem 1975 13 190—6. 

Bui-Pham, M., K. Seshadri, and F. A. Williams. 1992. The asymptotic structure of premixed methane-air flames with slow CO oxidation. Combustion Flame 89 343-62. 

In combustion systems it is generally desirable to minimize the concentration of intermediates, since it is important to obtain complete oxidation of the fuel. Figure 13.5 shows modeling predictions for oxidation of methane in a batch reactor maintained at constant temperature and pressure. After an induction time the rate of CH4 consumption increases as a radical pool develops. The formaldehyde intermediate builds up at reaction times below 100 ms, but then reaches a pseudo-steady state, where CH2O formed is rapidly oxidized further to CO. Carbon monoxide oxidation is slow as long as CH4 is still present in the reaction system once CH4 is depleted, CO (and the remaining CH2O) is rapidly oxidized to CO2. 

During the strike, the sulfur plant was shut down for minor repairs. I had to supervise its start-up. Mainly, I had to reheat the adiabatic-combustion chamber to 1800°F, before restoring the flow of H2S. This was done by burning a controlled amount of methane or natural gas, with a carefully regulated flow of air. The idea was to slowly heat up the combustion chamber with hot flue gas by 100 to 200°F per hour. This slow reheat was needed to avoid cracking the refractory bricks, because of uneven heating. To carry away a portion of the heat of combustion of the natural gas, we used pipeline nitrogen. 

Equilibrium concentrations of carbon or ammonia are not found in short combustion chambers used in rocket motors. The reason for this non-equilibrium situation is that the rate of formation of soot is very slow and carbon does not have time to form. Similarly the dissociation of NH3 is very slow. Thus in ethylene oxide monopropellant rocket motors one finds very little carbon, whereas equilibrium considerations predict carbon as a predominant product and in hydrazine decomposition chambers one finds an excess of NH3 over that predicted by equilibrium considerations. In ethylene oxide motors carbon forms from the decomposition of methane, not the reaction represented above, thus both non-equilibrium situations give higher performance than expected, since the endothermic reactions do not have time to take place. Of course, carbon also could form in cool reactions which take place in boundary layers along the walls where velocities are slow. 

Combustion is defined as a rapid, high-temperature oxidation reaction. What happens in the reactor just described after the reaction rate accelerates dramatically is combustion, whereas the initial slow oxidation reaction between methane and oxygen to form CO2 and H2O and other reactions between these species, such as the formation reaction of... 

These reactions increase the heating value of the gas product, since methane has a high heat of combustion. However, these reactions are very slow except under high pressure and in the presence of a catalyst. Another source of the methane in the syngas is the pyrolysis process. Reaction R-4.11 is the reverse steam methane reforming reaction. All reactions that produce methane are exothermic reactions. 

Non-catalytic combustions conducted at still higher temperatures of about 1000° C. have resulted in the formation of considerable carbon monoxide, very little formaldehyde, and small amounts of carbon dioxide. Bari decomposed 36.5 per cent of the methane in a methane-air mixture to obtain 32.5 per cent carbon monoxide, 1.8 per cent formaldehyde, and 2.2 per cent of carbon dioxide 10 at a reported temperature of 1000° C. The fact that some formaldehyde was able to withstand decomposition at this temperature indicates that either its time of contact with this high temperature was exceedingly short or that its rate of decomposition is slow even at 1000° C. The former. seems to be nearer the truth since the two oxides of carbon are not present in the ratio demanded by the equilibrium for this temperature. Consideration of the experimental results of Rhead... 

The slow variation of the maximum position as a function of C/O mixing ratio renders OH virtually useless as a marker for the actual reaction zone. The shift in the position of the reaction zone is more pronounced for the other two radicals, HCO and CH2 this is exemplified for CH2 in the lower left panel of Figure 29.8. Note that the methylene radical, CH2, shown here in its singlet metastable state, is an important, highly reactive intermediate it constitutes part of the hydrogen abstraction pathway in (methane) combustion. Among others. 

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