The multistep decomposition of boric acid

• Published in: Energy science & engineering Vol. 8; no. 5; pp. 1650 - 1666
• Main Authors: Huber, Clemens, Jahromy, Saman Setoodeh, Birkelbach, Felix, Weber, Jakob, Jordan, Christian, Schreiner, Manfred, Harasek, Michael, Winter, Franz
• Format: Journal Article
• Language: English
• Published: London John Wiley & Sons, Inc 01.05.2020; Wiley
• Subjects: Acids; Alternative energy sources; boric acid; Boron; Chemical reactions; Decomposition; Decomposition reactions; Deconvolution; Emissions; Energy resources; Energy storage; Fossil fuels; Fraser‐Suzuki; Kinetics; nonparametric kinetics; Nonparametric statistics; Process controls; R&D; Reaction kinetics; Reaction mechanisms; Research & development; Storage systems; Thermal analysis; Thermal energy; thermal energy storage; thermogravimetric analysis; waste heat trading
• ISSN: 2050-0505; 2050-0505
• DOI: 10.1002/ese3.622

Due to its high potential for thermal energy storage systems (Huber, Setoodeh Jahromy, Jordan, et al, Energies. 2019;12:17) the decomposition of boric acid is of particular interest in the field of applied research. The complexity of the reaction mechanism, with its multiple partial‐overlapping reaction steps, hitherto prevented a clear identification and analysis of each stoichiometric reaction step. So far, various research teams performed different kinetic analyses of boric acid, which led to various reaction mechanisms and stoichiometric reaction steps with yet inconclusive results for process modeling. Thus, a deeper examination of the process was desirable, to validate whether a proposed reaction is reasonable or not. For this purpose, experimental data were used for a deconvolution of the reaction sequence, using the Fraser‐Suzuki function, which clearly revealed the respective single reactions. The results of the deconvolution were compared with the proposed reaction steps in consideration of the stoichiometric ratio and thereby illustrated that the decomposition of polycrystalline boric acid more likely consists of three reaction steps. In contrary to the two‐step mechanism, the three‐step mechanism showed a very good correlation (r > 99%). Based on these outcomes, kinetic analyses were performed for each reaction step, by means of the nonparametric kinetics 2 (NPK2) method with subsequent determination of kinetic parameters. Additionally, for a deeper insight into the reaction, analyzing techniques like X‐ray diffraction (XRD), scanning electron microscopy (SEM) and simultaneous thermal analysis (STA) were applied. The complexity of the boric acid decomposition so far prevented a clear identification of the reaction sequence of this promising thermal energy storage system and led to various reaction mechanisms (2‐ and 3‐step). The application of an enhanced deconvolution procedure on experimental data with multiple partial‐overlapping reaction steps revealed that the boric acid decomposition more likely consists of three reaction steps. In addition, a subsequent kinetic analysis via the nonparametric kinetics method was performed and yielded the kinetic triplets of each reaction step.