Thermodynamics carbon-steam systems

One example is the formation of carbon in high flux reformers [389] operating far from thermodynamic carbon limits. It means that methane may decompose to carbon instead of reacting with steam to form the required syngas in spite of no potential for carbon in the equilibrated gas. This is of course not possible in a closed system, but in an open system carbon may be stable in a steady state and the accumulation of carbon may continue [389], This risk may be assessed by the so-called criteria of actual gas, which for the methane decomposition reaction as in Equation (5.5) can be written as ... 

Carbon produced by these latter reactions is formed in the catalyst pores, making it much more difficult to remove, and potentially causing physical breakage. Operating steam to carbon ratios are chosen above the minimum required in order to make carbon formation by these reactions thermodynamically impossible (3). Steam is another potential source of contaminants. Chemicals from the boiler feedwater or the cooling system are poisons to the reformer catalyst, so steam quality must be carefully monitored. 

This paper surveys the field of methanation from fundamentals through commercial application. Thermodynamic data are used to predict the effects of temperature, pressure, number of equilibrium reaction stages, and feed composition on methane yield. Mechanisms and proposed kinetic equations are reviewed. These equations cannot prove any one mechanism however, they give insight on relative catalyst activity and rate-controlling steps. Derivation of kinetic equations from the temperature profile in an adiabatic flow system is illustrated. Various catalysts and their preparation are discussed. Nickel seems best nickel catalysts apparently have active sites with AF 3 kcal which accounts for observed poisoning by sulfur and steam. Carbon laydown is thermodynamically possible in a methanator, but it can be avoided kinetically by proper catalyst selection. Proposed commercial methanation systems are reviewed. 

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. 

MRG [Methane rich gas] A catalytic steam-reforming system, similar to the classic syngas reaction of steam with a hydrocarbon mixture, but yielding hydrogen, methane, and carbon monoxide in different proportions. The system is thermodynamically balanced,... 

Whether a driving force for carbon formation exists is dictated by thermodynamics, and so it is dependant on reaction temperature and pressure, and the H/C and O/C ratios in the system (Fig. 14.6). By removing hydrogen from the reaction zone, the H/C ratio decreases and the system moves closer to the thermodynamic area where carbon formation is likely. The arrow in Fig. 14.6 labeled SMR is equal to a reforming mixture with a steam-to-carbon ratio of 3. When hydrogen... 

A process that is similar to carbon activation is coal gasification. Here coal, consisting primarily of carbon, is reacted at high temperatures with various mixtures of air, oxygen, and steam to produce a fuel gas. We have applied some of the techniques used to analyze the thermodynamics of coal gasification systems to develop a technical data base. 

Generally, in a conventional WGS system a two-step shift is used to obtain high CO conversion rates. In the first high-temperature shift reactor the major part of the CO is converted at high activity, whereas in the second shift reactor the rest of the CO (closely up to the thermodynamic equilibrium) is converted at low temperature and also low activity. Steam to carbon monoxide ratios above the stoichiometric ratio (higher than 2) are generally being used to attain the desired carbon monoxide conversion, but also to suppress carbon formation on certain catalysts. 

The temperature during reduction has also a lot of influence on the extent of carbon deposition and the conversion of the fuel in case NiO is used as OC. In the packed bed case, the beds are at 450-600 °C during reduction, and, therefore, the selectivity is not an issue, but additional steam is required to prevent carbon deposition. Because of the latter, the process efficiency is decreased. In this case, a process efficiency of 40.9% of LHV can be achieved. The fuel reactor in the circulating fluidized bed system is at 1200 °C. Hence, carbon deposition is not an issue, but here 1-2% of the fuel leaves the reactor unconverted, because of the thermodynamics. In this case, some additional oxygen is fed to the outlet stream to convert the remaining fuel, leading to a process efficiency of 41.6% of LHV. 

For higher CH4 content, more steam and CO2 at the anode reduce power and fuel utilization of the stack, thus a small (0/C)Ref ratio should be preferred for high system efficiency. On the other hand, the risk of carbon formation in the reformer increases with lower (0/C)Ref, so a compromise between system efficiency and a safe operation has to be found. Thermodynamic calculation indicated that for soot free operation an (0/C)Rer above 2.5 at temperature > 625 °C has to be assured. ... 

Seo et al. investigated thoroughly the interplay of the 0/C ratio and S/C ratio on carbon formation and hydrogen yield for methane oxidative steam reforming by thermodynamic calculations [66]. A minimum S/C ratio of 1.4 had been identified previously as being necessary to prevent carbon formation in the absence of oxygen [66] (see Section 3.1). When air was fed to the system on top of steam and methane, this minimum value decreased, as shown in Figure 3.19. 

The minimum amount of steam that needs to be added to a hydrocarbon fuel gas to avoid carbon deposition may be calculated. The principle here is that it is assumed that a given fuel gas/steam mixture reacts via reactions 8.3, 8.4, and 8.5 to produce a gas that is at equilibrium with respect to reactions 8.3 and 8.5 at the particular temperature of operation. The partial pressures of carbon monoxide and carbon dioxide in this gas are then used to calculate an equilibrium constant for the Boudouard reaction 8.9. This calculated equilibrium constant is then compared with what would be expected from the thermodynamic calculation at the temperature considered. If the calculated constant is greater than the theoretical one, then carbon deposition is predicted on thermodynamic grounds. If the calculated constant is lower than theory predicts, then the gas is said to be in a safe region and carbon deposition will not occur. In practice, a steam/carbon ratio of 2.0 to 3.0 is normally employed in steam reforming systems so that carbon deposition may be avoided with a margin of safety. 

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