Thermal decomposition of methane

3 Nonoxidative Processing of Hydrocarbons 2.3.1 Thermal Decomposition of Methane 

When hydrocarbons are heated to a high temperature, they thermally decompose into their constituent elements hydrogen and carbon, 

The amount of energy required to carry out this process depends on the nature of the hydrocarbon it is the highest for saturated hydrocarbons (alkanes, cycloalkanes) and low for unsaturated and aromatic hydrocarbons (in fact, decomposition of acetylene and benzene are exothermic reactions). Methane is one of the most thermally stable organic molecules. 

Thermodynamic equilibrium data for methane decomposition reaction at atmospheric pressure. 

The dissociation energy for C-H bond in methane (E = 436 kj/mol) is one of the highest among all organic compounds. Its electronic structure (i.e., the lack of n- and n-electrons), lack of polarity, and any functional group makes it extremely difficult to thermally decompose the methane molecule into its constituent elements. 

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The steam reforming of natural gas process is the most economic near-term process among the conventional processes. On the other hand, the steam reforming natural gas process consists of reacting methane with steam to produce CO and H2. The CO is further reacted or shifted with steam to form additional hydrogen and CO2. The CO2 is then removed from the gas mixture to produce a clean stream of hydrogen. Normally the CO2 is vented into the atmosphere. For decarbonization, the CO2 must be sequestered[l,2]. The alternative method for hydrogen production with sequestration of carbon is the thermal decomposition of methane. 

Direct thermal decomposition of methane was carried out, using a thermal plasma system which is an environmentally favorable process. For comparison, thermodynamic equilibrium compositions were calculated by software program for the steam reforming and thermal decomposition. In case of thermal decomposition, high purity of the hydrogen and solidified carbon can be achieved without any contaminant. 

Gulyaev and Polak [Kinetics and Catalysis, 6 (352), 1965] have studied the kinetics of the thermal decomposition of methane with a view toward developing a method for the commercial production of acetylene in a plasma jet. The following differential equations represent the time dependence of the concentrations of the major species of interest. 

In most publications, Iijima is given credit for the discovery in 1991 of the nanotube structure of carbon (Iijima, 1991 Bethune et al., 1993 Iijima and Ichihashi, 1993). However, it has been said that Oberlin et al. (1976) also imaged carbon nanotubes, perhaps even SWNTs. Incredibly, nearly a century earlier, there was a study on the thermal decomposition of methane that resulted in the formation of long carbon strands, which were proposed at the time as a candidate for filaments in light bulbs (see Bacon and Bowman, 1957). 

Because methane decomposition reaction requires high temperatures, there have been attempts to use catalysts to reduce the temperature of thermal decomposition of methane. Figure 2.21 summarizes reported literature data on different catalysts for methane decomposition and the preferred temperature range. It can be seen that transition metals... 

Kobayashi, A. and Steinberg, M., The thermal decomposition of methane in tubular reactor, Technical Report BNL-47159, Brookhaven National Laboratory, Upton, NY, 1992. 

Chen, C., Back, M., and Back, R., Mechanism of the thermal decomposition of methane, in Symp. Industrial Lab. Pyrolysis, ACS Symp. Series, 1,1976. 

Dunker, A., Kumar, S., and Mulawa, P., Production of hydrogen by thermal decomposition of methane in a fluidized bed reactor—effect of catalyst, temperature and residence time, Int.. Hydrogen Energ., 31, 473, 2006. 

Fig. 6.1 (a) TEM image and (b) Raman spectrum of graphene prepared by the thermal decomposition of methane (70 seem), (c) FESEM image and (d) Raman spectrum of graphene obtained by benzene (argon passed through benzene with flow rate of 200 seem) at 1000 °C on a nickel sheet (from [8]). 

A final possibility for a type of reaction resulting in acetylene formation which must be considered is that of combination of radicals. This is not likely to be significant in most systems, but in the case of methane reactions it becomes of particular interest. The production of ethylene, and acetylene in the thermal decomposition of methane requires some sort of combination reaction (40, 65). Kassel suggested a mechanism involving methylene radicals ... 

There have been attempts to use catalysts in order to reduce the maximum temperature of thermal decomposition of methane. In the 1960s, Universal Oil Products Co. developed the HYPROd process for continuous production of hydrogen by catalytic decomposition of a gaseous hydrocarbon streams.15 Methane decomposition was carried out in a fluidized bed catalytic reactor from 815 to 1093°C. Supported Ni, Fe and Co catalysts (preferably Ni/Al203) were used in the process. The coked catalyst was continuously removed from the reactor to the regeneration section where carbon was burned off by air, and the regenerated catalyst returned to the reactor. Unfortunately, the system with two fluidized beds and the solids-circulation system was too complex and expensive and could not compete with the SR process. 

Besides the traditional markets for carbon, some novel applications for the carbon produced via methane decomposition are discussed in the literature. Kvaemer has initiated R D program to investigate the potential of novel grades of carbon black as a storage medium for hydrogen, and as a feedstock for the production of solar grade silicone.35 The production of carbon nanotubes and nanofibers via solar thermal decomposition of methane over supported Co and Ni catalysts, respectively, was also reported.36... 

A. Kobayashi, and M. Steinberg, The thermal decomposition of methane in a tubular reactor, BNL-47159,... 

C. Chen, M. Back, R. Back, Mechanism of die thermal decomposition of methane, Symp. Industrial and Laboratory Pyrolysis, (1976)... 

The silicon atoms nucleate a liquid droplet of silicon, and this silicon surface is a catal5dic surface for the thermal decomposition of methane, as follows ... 

Dimethyl ditellurium is formed when methyl radicals, generated by thermal decomposition of methane butane, diethyl ether, or dimethyl ether interact with tellurium mirrors. Because dimethyl ditellurium can be prepared more conveniently by other methods, these reactions are not of synthetic importance. However, this reaction is one of the two ways to prepare bis[trifluoromethyl] ditellurium. Thus, this compound was prepared through interaction of trifluoromethyl radicals with tellurium tetrachloride , tellurium tetrabromide " or tellurium "... 

Natural diamonds are uncommon. Synthetic diamonds are produced from graphite at high pressures (> 80 kbar) and temperatures (> 1500°C) and by thermal decomposition of methane. 

The direct thermal decomposition of methane or higher hydrocarbons represents the unique approach for a theoretical direct decarbonization strategy [34, 35]. 

Pinilla JE, Suelves I, Lazaro MJ, Moliner R (2008) Kinetic study of the thermal decomposition of methane decomposition of methane using carbonaceous catalysts. Chem Eng J 138 301-306... 

This study analyzed hydrogen production via thermal decomposition of methane using a solar reactor for two different applications (1) a fueling station and (2) power production. The analysis shows that for either application, the production of carbon black plays a key role in the economics of the process. In addition, the net greenhouse gas emissions and overall fossil energy consumption are lower for the solar processes than for the conventional fossil system. 

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