Carbonaceous gas

The question of the compatibility of metals and alloys with carbon and carbonaceous gases has assumed considerable importance in connection with the development of the gas-cooled nuclear reactor in which graphite is used as a moderator and a constituent of the fuel element, and carbon dioxide as the coolant. Tests of up to 1 000 h on a series of metals and nickel-containing alloys under pressure contact with graphite at 1 010°C" showed that only copper was more resistant than nickel to diffusion of carbon and that the high-nickel alloys were superior to those of lower nickel content. The more complex nickel-chromium alloys containing titanium, niobium and aluminium were better than the basic nickel-chromium materials. 

Fig. 11-19 Paleorecord of carbonaceous gases together with temperature. (After Jouzel et ah, 1993.)...

The chemistry of carbon, and radiocarbon, in the atmosphere represents one of the most important areas of environmental research today. The primary practical reason for this is the increasing attention which must be paid to the critical balance between energy and the environment, especially from the viewpoint of man s perturbations of natural processes and his need to maintain control. Probably more than other species, carbonaceous molecules play a central role in this balance. Some of the deleterious effects of carbonaceous gases and particles in the atmosphere are set down in Table 3. The potential effects of increased local or global concentrations of these species on health and climate have led to renewed interest in the carbon cycle and the "C02 Problem". It should be evident from the table, however, that carbon dioxide is not the only problem. In fact, the so-called "trace gases and particles" in the atmosphere present an important challenge to our interpretation of the climatic effects of carbon dioxide, itself [20]. 

The desorption of CO (m/e=28) and CO2 (m/e=44) was recorded by MS during the CO-TPD experiments for La(Co, Mn, Fe)i i(Cu, Pd)i03 samples. A quantitative analysis of the various carbonaceous gases desorbed from perovskites is summarized in Table 7. Both CO and CO2 desorptions were observed indicating that the oxidation of CO into CO2 occurs over those lanthanum perovskites during CO-TPD experiments. 

For carbonaceous gases such as CO and CH4 at relatively high temperatures (ca. > 800°C), carburization of steel surfaces takes place in the form of brittle interstitial carbides that may cause surface cracking. Cementite may also form on the surface of steel since its melting point is lower than the underlying metal, it may cause melting of the steel surface that is subsequently eroded by the gas stream. 

Unfortunately, KOH electrol3rte does not bear traces of carbonaceous gases (CO and CO2) in the reactants even at percentages below 0.1 per cent, which rules out not only commercial-fuel-reforming-gas as a stack-fuel, but also air as an oxidant. Higher CO or CO2 percentages bring about formation of... 

Firedamp An explosive mixture of carbonaceous gases, mainly methane, formed in coal mines by the decomposition of coal. 

The carbonaceous gases used include C2H2, C2H4, CH4, natural gas and benzene. Methane is rather unreactive, does not dissolve readily in the Fe particles and is converted to a more reactive species (C2H ) with hydrogen. The addition of CO increases the yield and is optimum when the CO/(CO + H2) volume fraction is 93 95% (Figure 8.11). This is believed to be due to the CO reducing any iron oxides, although in the presence of H2, CO can react ... 

The two corrosion phenomena caused by carbonaceous gases, carburisation and metal dusting, are both a consequence of carbon transfer into the metal matrix of steels and alloys. In hydrocarbons, the carbon is transferred by the reaction... 

There is, however, more to this story. The fullerenes were essentially a stepping stone in the realization that pyrolyses of carbonaceous gases, under different conditions, would provide a totally new family of carbon structures, now known as nanotubes. These take different forms, such as nano-homs, necklaces, bamboo and sausage formats, and others which will no doubt be found in future literature. These materials have a potential for applications because they are graphitic and filamentous and as such have an ability to transmit heat and electricity, over and above the mechanical properties of a near-perfect graphitic filament (fiber). 

PYROLYTIC GRAPHITE. A high purity form of carbon (C) produced by thermal decomposition of carbonaceous gases. 

Accelerated deterioration of metals in carbonaceous gases at elevated temperatures to form a dust-fike corrosion product a unique form of high-temperature corrosion that forms a dust-hke corrosion product and sometimes develops hemispherical pits on a susceptible metal surface simultaneous carburization is generally observed. 

Formation of carbon (coking) has been studied in terms of catalytic activity in which carbonaceous gases such as hydrocarbons or CO react with transition metals [15-19]. Figueiredo has suggested that the mechanism of carbon formation from hydrocarbon includes the following steps [18] ... 

According to step (b), the chemisorption of carbon atoms decomposed from the carbonaceous gases leads to carbon diffusion into the metal, i.e. initiation of metal dusting. This means that metal dusting may be prevented if the reaction between CO and metals is suppressed at their smface. 

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