Redox Properties of Gas Phases

Abstract

In the H₂–O₂–C system, in the general case, two reversible reactions of carbon gasification and the water gas reaction, the gas mixture H₂–H₂O–CO–CO₂ is formed at high temperatures. In this mixture, the very low content of oxygen formed by the dissociation of H₂O and CO₂ is represented by the oxygen potential log (${p_{{O_2}}}$, atm). Thus, the redox properties may be assessed in terms of the oxygen potential. In any gas mixture containing H₂O and/or CO₂, it may be calculated from the equations

$${\log [{p_{{O_2}}},atm] = 2\log (\frac{{{x_{{H_2}O}}}}{{{x_{{H_2}}}}}) - \frac{{25708}}{T} + 5.563}$$

;

$$\log [{p_{{O_2}}},atm] = 2\log (\frac{{{x_{C{O_2}}}}}{{{x_{CO}}}}) - \frac{{29529}}{T} + 9.149$$

. In the present work, possible compositions of the H₂–O₂–C system at 700–1500 K and a total pressure of 1 atm are considered: H₂–H₂O, CO–CO₂, CO–CO₂–C, H₂O–CO₂–O2, H₂–CO–C, H₂–H₂O–CO–CO₂, and H₂–H₂O–CO–CO₂–C. Analysis yields two nomograms in the following coordinates: log(${x_{{H_2}O}}$/${x_{{H_2}}}$)–log${p_{{O_2}}}$–T and log(${x_{C{O_2}}}$/xCO)–log${p_{{O_2}}}$–T. Using the nomograms and reference information regarding the dissociation pressure of metal oxides, the redox properties of the gas mixtures with respect to those oxides may be assessed. In CO–CO₂ systems without hydrogen that are obtained in the combustion of CO, carbon may be formed as soot. This explains the existence of a limited region of gas-phase compositions and log${p_{{O_2}}}$ in the corresponding nomogram and hence the limited potential for the reduction of some metal oxides in CO–CO₂–C systems. The introduction of hydrogen permits the creation of gas mixtures with extremely low oxygen pressure and hence increases the thermodynamic probability of reduction for any metal oxide. Hydrogen may be introduced in the system by methods that differ in economic expediency: from the use of pure hydrogen to the production of gas mixtures as a result of the reaction between water vapor and carbon. In the first case, the reduction of the oxide by hydrogen in the MeO–C–H₂ system activates the gasification of carbon by water vapor, the water gas reaction, the reduction of carbon monoxide, and the gasification of carbon dioxide. In the second case, practically pure H₂–CO mixture may be obtained above 1300 K. The utility of representing the results on a three-dimensional diagram based on the H₂–O₂–C concentration triangle is analyzed. If methane formation is taken into account, the equilibrium parameters of gas mixtures are changed markedly only at temperatures below about 900 K.