Chemical Reactions in Carbon Gasification Processes

Abstract

A thermodynamic analysis of carbon gasification process in the presence of moisture has been performed. The chemical process has been represented by system C–O–H with a ratio between the elements therein amounting to 1 : 1 : 2 and 1 : 2 : 2. To refine the methodology of the studies and to verify the results, a well-studied C–O subsystem has been used. The initial array of the processed data has been represented by a content of such chemical components as C, CO, CO₂, CH₄, H₂, and H₂O calculated using a Terra software package. There is no unit chemical reaction in the С–О–Н system; therefore, the entire operating temperature range from 298 to 1400 K has been divided into three characteristic subranges to analyze each of them separately. By comparing the numerical values of component content at the boundaries of the subranges, a change in the values going from one subrange to another has been determined. These values are multiples of stoichiometric coefficients for the proposed chemical reactions. Thus, the problem of establishing the type of chemical reactions has been solved. However, in two of the three subranges, the identified reactions exhibit a complicated character, that is, they contain more than four components. Therefore, we have decomposed them in the basis of three more simple reactions inherent in these thermal subranges. As a result, the total number of reaction types has been reduced to four: two main reactions of carbon gasification (С + 2H₂О = СО₂ + 2H₂, С + СО₂ = 2СО) and two reactions of methane formation and decomposition (2С + 2H₂О = СН₄ + СО₂, СН₄ = С + 2H₂). At the same time, with the use of balance coefficients β, the fraction of each reaction in the general chemical process has been determined. However, the type of chemical reactions gives all the necessary information concerning the content of system components only at the boundaries of the subranges. The quantitative assessment of the chemical process in the temperature subranges can be obtained by determining the temperature dependence of the reaction coordinates ξ(Т) as a function of Gibbs energies and pressure, i.e., ξ(Т) = f [Δ_rG°(T), Р]. The reaction coordinates ξ in combination with balance coefficients β make it possible to calculate not only the content of the reactants and reaction products, but also the conditional temperature values for the beginning and termination of the reactions themselves. In this case, no fitting coefficients and parameters of a fitting should be used in the calculations. The average absolute error in the quantitative description of computer simulation results for system C–O–H is less than 0.02 mol (per 1 mol of carbon), whereas for subsystem C–O it is almost zero.