Steam oxygen equilibria

Figure 12. Carbon—steam-oxygen equilibria (T = 1100°K, no CHk output)...

The gasification is performed using oxygen and steam (qv), usually at elevated pressures. The steam—oxygen ratio along with reaction temperature and pressure determine the equilibrium gas composition. The reaction rates for these reactions are relatively slow and heats of formation are negative. Catalysts maybe necessary for complete reaction (2,3,24,42,43). 

A closer analysis of die equilibrium products of the 1 1 mixture of methane and steam shows the presence of hydrocarbons as minor constituents. Experimental results for die coupling reaction show that the yield of hydrocarbons is dependent on the redox properties of the oxide catalyst, and the oxygen potential of the gas phase, as well as die temperamre and total pressure. In any substantial oxygen mole fraction in the gas, the predominant reaction is the formation of CO and the coupling reaction is a minor one. 

Of course, there is no methane at exit from the PO reactor, and no oxygen. The hydrogen content is quite high, over 15% and comparable to that in Lloyd s example of the steam/TCR cycle, but the CO content is also nearly 8%. It is interesting to note that the calculated equilibrium concentrations of these combustible products from the reactor are reduced through the PO turbine (because of the fall in temperature) before they are supplied to the gas turbine combustor where they are fully combusted, but it is more likely that the concentrations would be frozen near the entry values. 

The composition of the gas produced is determined by the thermodynamic equilibrium of these reactions at the exit temperature, which is given by the adiabatic heat balance based on the composition and flow of the feed, steam and oxygen added to the reactor. 

The hydrocarbon feedstock is reacted with a mixture of oxygen or air and steam in a sub-stoichiometric flame. In the fixed catalyst bed the synthesis gas is further equilibrated. The composition of the product gas will be determined by the thermodynamic equilibrium at the exit pressure and temperature, which is determined through the adiabatic heat balance based on the composition and flows of the feed, steam and oxygen added to the reactor. The synthesis gas produced is completely soot-free [28]. 

In this example we have assumed a sufficient depth for the coal bed equilibrium is approached by the gases while they are in contact with the incandv carbon. This need not be the case if oxygen and steam are supplied at too rate, the reactions may not attain equilibrium or may reach equilibrium after have left the coal bed. In this event, carbon is not present at equilibrium, and problem must again be reformulated. 

This objection, however, is not valid, for, whatever the temperature, the law of mass action requires that definite, even if small, amounts of carbon dioxide and hydrogen shall exist in equilibrium with the other gases in the system. Hence, if for any reason the partial pressure of the carbon dioxide or hydrogen falls below that required for equilibrium, it is always possible for reaction (i) to proceed, even at high temperatures, in the direction of left to right. Traube explained the reaction, however, on the assumption that the function of the steam is to unite with one atom of the oxygen molecule, the second atom being occupied in the oxidation of the carbon monoxide. Thus... 

Dissociation of Steam.—From physico-chemical considerations it is probable that even under ordinary conditions liquid water contains an exceedingly minute though definite proportion of uncombined hydrogen and oxygen in equilibrium with the compound molecules. This state of equilibrium is outside the scope of experimental detection unless disturbed in some -way, as by the influence of ultra-violet light, when the decomposition may become appreciable. 

For methane oxidation, at equilibrium, the two products within the fuel cell, are CO2 at about 0.97 bar and H2O (steam/water) at about. 0.03 bar, the latter being the saturation pressure corresponding to 298.15 K. There is a reversible reaction between the two products and the two reactants, methane and oxygen. The result of the right-handed... 

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