Presently, climate change and the emission of greenhouse gases are among the main global challenges today. As part of mitigation measures, stringent regulations on carbon emissions have tremendously increased. This has favored the adoption and development of low carbon technologies. In particular, development and deployment of hydrogen, as a renewable energy carrier for both mobile and distributed applications, has a great potential of reducing carbon dioxide emissions. However, considering that hydrogen does not exist in nature, its extraction from water or hydrocarbon fuels are among the challenges hindering its full deployment as a clean energy carrier. Among the available techniques for hydrogen production, gasification of coal/biomass attracted significant attention of researchers. Generally, it involves a series of homogenous and heterogeneous reactions that results in the production of a mixture of gases including hydrogen, methane and carbon monoxide.
In an attempt to enhance hydrogen production, storage and utilization as clean energy, accurate modeling of the gasification process is highly desirable. Recent studies have shown a trend of inaccurate prediction of equilibrium models. This have been attributed to several factors such as not taking into consideration the physiochemical processes occurring in different gasifier zones as well as the assumption of reversible heat exchange between the system and the surrounding. Alternatively, in some models, the prediction of the syngas composition is based on the minimization of the Gibbs energy equation. Unfortunately, the inconsistency and discrepancy obtained between experimental and equilibrium calculations hinder efficient extraction and development of hydrogen as a source of clean energy.
To this note, Professor Yousef Haseli at Central Michigan University, College of Science and Engineering explored the variations between the experimental measurements and the equilibrium models. Indeed, the main aim of the study was to show the difference in predicting the chemical equilibrium through minimization of the Gibbs function and the kinetic modeling. The work is currently published in International Journal of Hydrogen Energy.
In brief, the author evaluated the existing prediction of equilibrium models to establish the main sources of discrepancies between the experimental measurements and the equilibrium predictions. Furthermore, a methane steam reforming was developed to distinguish the predicted chemical composition equilibrium by modified Gibbs function and kinetic modeling.
Professor Haseli correctly applied the energy conservation and entropy balance equation and observed the similarity between the minimized modified Gibbs function and maximized total entropy generation. For instance, from the methane steam reforming illustration confirmed the difference in the chemical composition equilibrium predicted by modified Gibbs function and kinetic modeling.
In summary, the study demonstrated the chemical equilibrium criteria based on methane steam reforming. In general, he noted that the chemical equilibrium state does not necessarily have to correspond to maximum entropy generation because, in most cases, the modified Gibbs function and the total entropy generation remain independent of time at chemical equilibrium. Altogether, the study enhances the accuracy of the prediction of equilibrium models that will further advance development of hydrogen as an alternative renewable energy for sustainability.
Haseli, Y. (2019). Criteria for chemical equilibrium with application to methane steam reforming. International Journal of Hydrogen Energy, 44(12), 5766-5772.Go To International Journal of Hydrogen Energy